Effects of As8 structure formation on the surface morphology and internal microstructure of GaAs thin films

Gallium arsenide (GaAs) materials have the advantages of high electron mobility, electron saturation drift rate, and other irreplaceable semiconducting properties. They play an important role in the electronics, solar and other fields. However, during GaAs film sedimentary growth, As atoms can undergo segregation to form As8 clusters because of the influence of external factors, which affect the surface morphology and internal structure of these films. In this study, a series of investigations on the deposition and growth of GaAs crystal films were performed. Additionally, the deposition and growth of GaAs thin films were simulated using molecular dynamics. The influence of As8 clusters on the surface morphology and internal structure of GaAs films at different incidence angles, velocities and substrate temperatures was studied by using "defect analysis technology" and "diamond structure identification" in open source software, along with surface roughness and radial distribution function. Results show that with increasing incident angle, the number of As8 clusters decreases and film density increases. Increasing incident velocity increases the irregular movement of As8 clusters in air, and their deposition on the film surface affects the morphology of the film, the surface roughness increases first and then decreases. Additionally, we investigated the effect of different substrate temperatures on the film surface. Results show that at a substrate temperature of 1173 K, the number of As8 clusters in the film decreases or the As8 clusters disappear, heterogeneous nucleation occurs in the film, and the crystallization rate increases. Although the dislocation line associated with nucleation may affect the mechanical and optical properties of the film, it considerably reduces the annealing effort after the deposition and growth.

TCAD simulation study of heavy ion radiation effects on hetero junctionless tunnel field effect transistor

Semiconductor devices used in radiation environment are more prone to degradation in device performance. Junctionless Tunnel Field Effect Transistor (JLTFET) is one of the most potential candidates which overcomes the short channel effects and fabrication difficulties. In this work, 20 nm JLTFET is proposed with Silicon in the drain/channel region whereas source uses different materials, Silicon Germanium (SiGe), Gallium Nitride (GaN), Gallium Arsenide (GaAs), Indium Arsenide (InAs). The device performance is examined by subjecting it to heavy ion radiation at a lower and higher dose of linear energy transfer (LET) values. It can be seen that the most sensitive location is the source/channel (S/C) interface for SiGe, GaN and GaAs whereas the drain/channel (D/C) interface for InAs. Further analysis is carried out at these vulnerable regions by matching ION of all materials. The parameters, transient peak current (Ipeak), collected charge (QC), threshold voltage shift (ΔVth) and bipolar gain (β) are extracted using transient simulations. It is observed that for a lower dose of LET, Ipeak of SiGe is 27% lesser than InAs and for higher dose of LET, SiGe shows 56% lesser Ipeak than InAs. SiGe is less sensitive at lower and higher dose of LET due to reduced ΔVth, tunneling and electron density.

Excitation-wavelength-dependent photoluminescence in GaAs nanowires under high-pressure

GaAs nanowires (NWs) have wide application potential as near-infrared optical devices and the high-pressure strategy has been applied to modulatetheir crystal and electronic structures. As another typical thermodynamic parameter, temperature can also affect the optical performance of semiconductors. Here we report the excitation-wavelength-dependent photoluminescence in GaAs nanowires under high-pressure conditions. The pressure for achieving the maximum photoluminescence (PL) intensity and bandgap transition from direct to indirect of GaAs NWs varies (1.7-2.5 GPa) with the wavelength of the incident lasers (473-633 nm). The Raman peak of GaAs NWs shifts towards higher frequency with increasing excitation wavelengths at the same high-pressure conditions, revealing the stronger heating effect induced by incident laser with the shorter wavelength. The relative temperature difference in GaAs NWs induced by two different lasers can be estimated up to 537.5 K, and the strong heating effect suppresses the light-emission efficiency in GaAs NWs. With increasing the pressure, the relative temperature difference presents a gradual declining trend and PL intensity presents an opposite trend, which relates to the pressure-induced suppression of nonradiative recombination in GaAs NWs. Our study -provides insights into the mechanisms for the excitation-wavelength dependent photoluminescence (EWDP) effect and an alternative route to modulate the high-pressure performance of nanodevices. .

Towards quantification of doping in gallium arsenide nanostructures by low-energy scanning electron microscopy and conductive atomic force microscopy

We calculate a universal shift in work function of 59.4 meV per decade of dopant concentration change that applies to all doped semiconductors and from this use Monte Carlo simulations to simulate the resulting change in secondary electron yield for doped GaAs. We then compare experimental images of doped GaAs layers from scanning electron microscopy and conductive atomic force microscopy. Kelvin probe force microscopy allows to directly measure and map local work function changes, but values measured are often smaller, typically only around half, of what theory predicts for perfectly clean surfaces.

GaAs Mid-IR Electrically Tunable Metasurfaces

In this work, we explore III-V based metal-semiconductor-metal structures for tunable metasurfaces. We use an epitaxial transfer technique to transfer a III-V thin film directly on metallic surfaces, realizing III-V metal-semiconductor-metal (MSM) structures without heavily doped semiconductors as substitutes for metal layers. The device platform consists of gold metal layers with a p-i-n GaAs junction. The target resonance wavelength can be tuned by modifying the geometry of the top metal grating on the GaAs, while systematic resonance tunability has been shown through the modulation of various carrier concentration injections in the mid-IR range. Electrically tunable metasurfaces with multilevel biasing can serve as a fundamental building block for electrically tunable metasurfaces. We believe that our demonstration can contribute to understanding the optical tuning of III-V under various biased conditions, inducing changes in metasurfaces.

A Highly Integrated C-Band Feedback Resistor Transceiver Front-End Based on Inductive Resonance and Bandwidth Expansion Techniques

This paper presents a highly integrated C-band RF transceiver front-end design consisting of two Single Pole Double Throw (SPDT) transmit/receive (T/R) switches, a Low Noise Amplifier (LNA), and a Power Amplifier (PA) for Ultra-Wideband (UWB) positioning system applications. When fabricated using a 0.25 μm GaAs pseudomorphic high electron mobility transistor (pHEMT) process, the switch is optimized for system isolation and stability using inductive resonance techniques. The transceiver front-end achieves overall bandwidth expansion as well as the flat noise in receive mode using the bandwidth expansion technique. The results show that the front-end modules (FEM) have a typical gain of 22 dB in transmit mode, 18 dB in receive mode, and 2 dB noise in the 4.5-8 GHz band, with a chip area of 1.56 × 1.46 mm2. Based on the available literature, it is known that the proposed circuit is the most highly integrated C-band RF transceiver front-end design for UWB applications in the same process.

On-Chip Circularly Polarized Circular Loop Antennas Utilizing 4H-SiC and GaAs Substrates in the Q/V Band

This paper presents a comprehensive assessment of the performance of on-chip circularly polarized (CP) circular loop antennas that have been designed and fabricated to operate in the Q/V frequency band. The proposed antenna design incorporates two concentric loops, with the outer loop as the active element and the inner loop enhancing the CP bandwidth. The study utilizes gallium arsenide (GaAs) and silicon carbide (4H-SiC) semiconductor wafer substrates. The measured results highlight the successful achievement of impedance matching at 40 GHz and 44 GHz for the 4H-SiC and GaAs substrates, respectively. Furthermore, both cases yield an axial ratio (AR) of less than 3 dB, with variations in bandwidths and frequency bands contingent upon the dielectric constant of the respective substrate material. Moreover, the outcomes confirm that utilizing 4H-SiC substrates results in a significantly higher radiation efficiency of 95%, owing to lower substrate losses. In pursuit of these findings, a 4-element circularly polarized loop array antenna has been fabricated for operation at 40 GHz, employing a 4H-SiC wafer as a low-loss substrate. The results underscore the antenna's remarkable performance, exemplified by a broadside gain of approximately 9.7 dBic and a total efficiency of circa 92%. A close agreement has been achieved between simulated and measured results.

A 10-20 GHz 6-Bit High-Accuracy Digital Step Attenuator with Low Insertion Loss in 0.15 µm GaAs p-HEMT Technology

In a beamforming circuit for a modern broadband phased-array system, high accuracy and compactness have received sufficient attention as they are directly related to side lobe level and fabrication cost, respectively. In order to meet the low phase error required, this paper proposed an ultra-broadband 6-bit digital step switched-type attenuator (STA) with capacitive/inductive compensation networks. Compared to the conventional methods, the proposed technique employs an improved simplified T-structure with capacitive compensation networks, which simultaneously achieves low insertion loss and high-accuracy amplitude/phase control. In addition, on-chip level shifting circuit is integrated to avoid complex control schemes. The strategy of prioritizing return loss is adopted to alleviate the performance degradation caused by impedance mismatch after cascade. As a proof-of-principle demonstration, a wideband 6-bit STA with core area of only 0.5 mm × 1.8 mm was designed via 0.15-micrometer GaAs pHEMT technology. It exhibits ultra-broadband operation with a 31.5 dB amplitude tuning range and a 0.5 dB tuning step. The insertion loss of the reference state is 4-5.3 dB. The return loss is better than 15 dB for all the 64 states. The RMS amplitude and phase errors are less than 0.2 dB and 2° over the 10 to 20 GHz.

Enhanced Light Trapping in GaAs/TiO2-Based Photocathodes for Hydrogen Production

Photoelectrochemical cells (PEC) are appealing devices for the production of renewable energy carriers. In this context, III-V semiconductors such as GaAs are very promising materials due to their tunable band gaps, which can be appropriately adjusted for sunlight harvesting. Because of the high cost of these semiconductors, the nanostructuring of the photoactive layer can help to improve the device efficiency as well as drastically reduce the amount of material needed. III-V nanowire-based photoelectrodes benefit from the intrinsically high aspect ratio of nanowires, their enhanced ability to trap light, and their improved charge separation and collection abilities and thus are particularly attractive for PECs. However, III-V semiconductors often suffer from corrosion in aqueous electrolytes, preventing their utilization over long periods under relevant working conditions. Here, photocathodes of GaAs nanowires protected with thin TiO2 shells were prepared and studied under simulated sunlight irradiation to assess their photoelectrochemical performances in correlation with their structural degradation, highlighting the advantageous nanowire geometry compared to its thin-film counterpart. Morphological and electronic parameters, such as the aspect ratio of the nanowires and their doping pattern, were found to strongly influence the photocatalytic performances of the system. This work highlights the advantageous combination of nanowires featuring a buried radial p-n junction with Co nanoparticles used as a hydrogen evolution catalyst. The nanostructured photocathodes exhibit significant photocatalytic activities comparable with previous noble-metal-based systems. This study demonstrates the potential of a GaAs nanostructured semiconductor and its reliable use for photodriven hydrogen production.

Compact Bandwidth-Enhanced 180-Degree Phase Shifter Using Edge-Coupled Multi-Microstrip and Artificial Transmission Line

Compactness has obtained sufficient importance in wideband phase shifter design considerations, as it is directly related to fabrication cost. In this paper, a novel structure was presented to create compact broadband 180-degree phase shifter, which has the advantages of enhanced bandwidth and significantly reduced chip area. The proposed configuration consists of edge-coupled multi-microstrip lines (ECMML) and an artificial transmission line (ATL) with dual-shorted inductors, both of which have the periodic shunt load of capacitors. The ECMML can provide a high coupling coefficient, leading to an increase in the bandwidth, while the introduced capacitors can greatly reduce the line length (35.8% of the conventional method). To verify the relevant mechanisms, a wideband switched network with compact dimensions of 0.67 × 0.46 mm2 was designed via 0.15-micrometer GaAs pHEMT technology. Combined with the measured switch transistor, it was shown that the proposed phase shifter exhibits an insertion loss of less than 2 dB, a return loss of greater than 12 dB, a maximum phase error of less than 0.6° and a channel amplitude difference of less than 0.1 dB in the range of 10 to 20 GHz.

Low Dark Current Operation in InAs/GaAs(111)A Infrared Photodetectors: Role of Misfit Dislocations at the Interface

We demonstrate an extended short-wave infrared (e-SWIR) photodetector composed of an InAs/GaAs(111)A heterostructure with interface misfit dislocations. The layer structure of the photodetector consists simply of an n-InAs optical absorption layer directly grown with a thin undoped-GaAs spacer layer on n-GaAs by molecular beam epitaxy. The lattice mismatch was abruptly relaxed by forming a misfit dislocation network at the initial stage of the InAs growth. We found high-density threading dislocations (1.5 × 10⁹ cm^-2) in the InAs layer. The current-voltage characteristics of the photodetector at 77 K had a very low dark current density (<1 × 10^-9 A cm^-2) at a positive applied voltage (electrons flow from n-GaAs to n-InAs) of up to ∼+1 V. Simulation of the band structure revealed that the direct connection of GaAs and InAs and the formation of interfacial states by the misfit dislocations play significant positive roles in suppressing dark current. Under illumination with e-SWIR light at 77 K, a clear photocurrent signal was observed with a 2.6 μm cutoff wavelength, which is consistent with the bandgap of InAs. We also demonstrated e-SWIR detection at room temperature with a 3.2 μm cutoff wavelength. The maximum detectivity at 294 K exceeds 2 × 10⁸ cm Hz^0.5 W^-1 for the detection of e-SWIR light at 2 μm.

Novel octa-graphene-like structures based on GaP and GaAs

CONTEXT

The discovery of graphene gave way to the search for new two-dimensional structures. In this regard, octa-graphene is a carbon allotrope consisting of 4- and 8-membered rings in a single planar sheet, drawing the research community's attention to study their inorganic analogs. Considering the promising properties of octa-graphene-like structures and the role of GaAs and GaP in semiconductor physics, this study aims to propose, for the first time, two novel inorganics buckled nanosheets based on the octa-graphene structure, the octa-GaAs and octa-GaP. This work investigated the structural, electronic, and vibrational properties of these novel octa-graphene-based materials. The octa-GaP and octa-GaAs have an indirect band gap transition with a valence band maximum between M and Г points and a conduction band minimum at Г point with energy of 3.05 eV and 2.56 eV, respectively. The QTAIMC analysis indicates that both structures have incipient covalent in their bonds. The vibrational analysis demonstrates the occurrence of Γ_Raman = 6A_g + 6B_g and Γ_Raman = 12A' + 12B″ for octa-GaP and octa-GaAs, respectively. The symmetry reduction of octa-GaAs leads to activating inactive modes observed in the octa-GaP structure. The frontier crystalline orbitals are composed by Ga(p_x) and P(p_y and p_z) orbitals for octa-GaP and Ga(p_x and p_y) and As(s, p_y, and p_z) for octa-GaAs in the valence bands while in the conduction bands by Ga(p_y, p_z, and s) for both compounds and P(p_x and p_z) and As(p_y). The phonon bands demonstrate the absence of the negative frequency modes and the structural stability of these new nanosheets. This report aims to reveal the fundamental properties of both newfound materials for stimulating experimental research groups in the search for synthesis routes to obtain this structure.

METHODS

This work used the DFT/B3LYP approach implemented in the CRYSTAL17 computational package. Ga, As, and P atomic centers were described by triple-zeta valence with polarization (TZVP) basis set. The vibrational analysis was carried out via coupled-perturbed Hartree-Fock/Kohn Sham (CPHF/KS) method, and the chemical bonds were evaluated via the quantum theory of atoms in molecules and crystals (QTAIMC).

A 2D Bismuth-Induced Honeycomb Surface Structure on GaAs(111)

Two-dimensional (2D) topological insulators have fascinating physical properties which are promising for applications within spintronics. In order to realize spintronic devices working at room temperature, materials with a large nontrivial gap are needed. Bismuthene, a 2D layer of Bi atoms in a honeycomb structure, has recently attracted strong attention because of its record-large nontrivial gap, which is due to the strong spin-orbit coupling of Bi and the unusually strong interaction of the Bi atoms with the surface atoms of the substrate underneath. It would be a significant step forward to be able to form 2D materials with properties such as bismuthene on semiconductors such as GaAs, which has a band gap size relevant for electronics and a direct band gap for optical applications. Here, we present the successful formation of a 2D Bi honeycomb structure on GaAs, which fulfills these conditions. Bi atoms have been incorporated into a clean GaAs(111) surface, with As termination, based on Bi deposition under optimized growth conditions. Low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/S) demonstrates a well-ordered large-scale honeycomb structure, consisting of Bi atoms in a √3 × √3 30° reconstruction on GaAs(111). X-ray photoelectron spectroscopy shows that the Bi atoms of the honeycomb structure only bond to the underlying As atoms. This is supported by calculations based on density functional theory that confirm the honeycomb structure with a large Bi-As binding energy and predict Bi-induced electronic bands within the GaAs band gap that open up a gap of nontrivial topological nature. STS results support the existence of Bi-induced states within the GaAs band gap. The GaAs:Bi honeycomb layer found here has a similar structure as previously published bismuthene on SiC or on Ag, though with a significantly larger lattice constant and only weak Bi-Bi bonding. It can therefore be considered as an extreme case of bismuthene, which is fundamentally interesting. Furthermore, it has the same exciting electronic properties, opening a large nontrivial gap, which is the requirement for room-temperature spintronic applications, and it is directly integrated in GaAs, a direct band gap semiconductor with a large range of (opto)electronic devices.

Coherence Characteristics of a GaAs Single Heavy-Hole Spin Qubit Using a Modified Single-Shot Latching Readout Technique

We present an experimental study of the coherence properties of a single heavy-hole spin qubit formed in one quantum dot of a gated GaAs/AlGaAs double quantum dot device. We use a modified spin-readout latching technique in which the second quantum dot serves both as an auxiliary element for a fast spin-dependent readout within a 200 ns time window and as a register for storing the spin-state information. To manipulate the single-spin qubit, we apply sequences of microwave bursts of various amplitudes and durations to make Rabi, Ramsey, Hahn-echo, and CPMG measurements. As a result of the qubit manipulation protocols combined with the latching spin readout, we determine and discuss the achieved qubit coherence times: T1, TRabi, T2*, and T2CPMG vs. microwave excitation amplitude, detuning, and additional relevant parameters.

Speckle Measurement for Small In-Plane Vibration Using GaAs

In this study, the measurement characteristics of speckles based on the photoinduced electromotive force (photo-emf) effect for high-frequency, small-amplitude, and in-plane vibration were theoretically and experimentally studied. The relevant theoretical models were utilized. A GaAs crystal was used as the photo-emf detector for experimental research, as well as to study the influence of the amplitude and frequency of the vibration, the imaging magnification of the measuring system, and the average speckle size of the measuring light on the first harmonic of the induced photocurrent in the experiments. The correctness of the supplemented theoretical model was verified, and a theoretical and experimental basis was provided for the feasibility of using GaAs to measure in-plane vibrations with nanoscale amplitudes.

Fabrication and Characterization of In0.53Ga0.47As/InAs/In0.53Ga0.47As Composite Channel Metamorphic HEMTs (mHEMTs) on a GaAs Substrate

In this work, we successfully demonstrated In_0.53Ga_0.47As/InAs/In_0.53Ga_0.47As composite channel metamorphic high electron mobility transistors (mHEMTs) on a GaAs substrate. The fabricated mHEMTs with a 100 nm gate length exhibited excellent DC and logic characteristics such as V_T = -0.13 V, g_m,max = 949 mS/mm, subthreshold swing (SS) = 84 mV/dec, drain-induced barrier lowering (DIBL) = 89 mV/V, and I_on/I_off ratio = 9.8 × 10³ at a drain-source voltage (V_DS) = 0.5 V. In addition, the device exhibited excellent high-frequency characteristics, such as f_T/f_max = 261/304 GHz for the measured result and well-matched modeled f_T/f_max = 258/309 GHz at V_DS = 0.5 V, which is less power consumption compared to other material systems. These high-frequency characteristics are a well-balanced demonstration of f_T and f_max in the mHEMT structure on a GaAs substrate.

A sleeve and bulk method for fabrication of photonic structures with features on multiple length scales

Traditional photonic structures such as photonic crystals utilize (a) large arrays of small features with the same size and pitch and (b) a small number of larger features such as diffraction outcouplers. In conventional nanofabrication, separate lithography and etch steps are used for small and large features in order to employ process parameters that lead to optimal pattern transfer and side-wall profiles for each feature-size category, thereby overcoming challenges associated with reactive ion etching lag. This approach cannot be scaled to more complex photonic structures such as those emerging from inverse design protocols. Those structures include features with a large range of sizes such that no distinction between small and large can be made. We develop a sleeve and bulk etch protocol that can be employed to simultaneously pattern features over a wide range of sizes while preserving the desired pattern transfer fidelity and sidewall profiles. This approach reduces the time required to develop a robust process flow, simplifies the fabrication of devices with wider ranges of feature sizes, and enables the fabrication of devices with increasingly complex structure.

Selective area epitaxy of GaAs: the unintuitive role of feature size and pitch

Selective area epitaxy (SAE) provides the path for scalable fabrication of semiconductor nanostructures in a device-compatible configuration. In the current paradigm, SAE is understood as localized epitaxy, and is modelled by combining planar and self-assembled nanowire growth mechanisms. Here we use GaAs SAE as a model system to provide a different perspective. First, we provide evidence of the significant impact of the annealing stage in the calculation of the growth rates. Then, by elucidating the effect of geometrical constraints on the growth of the semiconductor crystal, we demonstrate the role of adatom desorption and resorption beyond the direct-impingement and diffusion-limited regime. Our theoretical model explains the effect of these constraints on the growth, and in particular why the SAE growth rate is highly sensitive to the pattern geometry. Finally, the disagreement of the model at the largest pitch points to non-negligible multiple adatom recycling between patterned features. Overall, our findings point out the importance of considering adatom diffusion, adsorption and desorption dynamics in designing the SAE pattern to create pre-determined nanoscale structures across a wafer. These results are fundamental for the SAE process to become viable in the semiconductor industry.

In situoff-axis electron holography of real-time dopant diffusion in GaAs nanowires

Off-axis electron holography was used to reveal remote doping in GaAs nanowires occurring duringin situannealing in a transmission electron microscope. Dynamic changes to the electrostatic potential caused by carbon dopant diffusion upon annealing were measured across GaAs nanowires with radial p-p+ core-shell junctions. Electrostatic potential profiles were extracted from holographic phase maps and built-in potentials (V_bi) and depletion layer widths (DLWs) were estimated as function of temperature over 300-873 K. Simulations in absence of remote doping predict a significant increase ofV_biand DLWs with temperature. In contrast, we measured experimentally a nearly constantV_biand a weak increase of DLWs. Moreover, we observed the appearance of a depression in the potential profile of the core upon annealing. We attribute these deviations from the predicted behavior to carbon diffusion from the shell to the core through the nanowire sidewalls, i.e. to remote doping, becoming significant at 673 K. The DLW in the p and p+ regions are in the 10-30 nm range.

Investigation of Radiation Effect on Structural and Optical Properties of GaAs under High-Energy Electron Irradiation

A systematic investigation of the changes in structural and optical properties of a semi-insulating GaAs (001) wafer under high-energy electron irradiation is presented in this study. GaAs wafers were exposed to high-energy electron beams under different energies of 10, 15, and 20 MeV for absorbed doses ranging from 0-2.0 MGy. The study showed high-energy electron bombardments caused roughening on the surface of the irradiated GaAs samples. At the maximum delivered energy of 20 MeV electrons, the observed root mean square (RMS) roughness increased from 5.993 (0.0 MGy) to 14.944 nm (2.0 MGy). The increased RMS roughness with radiation doses was consistent with an increased hole size of incident electrons on the GaAs surface from 0.015 (0.5 MGy) to 0.066 nm (2.0 MGy) at 20 MeV electrons. Interestingly, roughness on the surface of irradiated GaAs samples affected an increase in material wettability. The study also observed the changes in bandgap energy of GaAs samples after irradiation with 10, 15, and 20 MeV electrons. The band gap energy was found in the 1.364 to 1.397 eV range, and the observed intense UV-VIS spectra were higher than in non-irradiated samples. The results revealed an increase of light absorption in irradiated GaAs samples to be higher than in original-based samples.

The Role of Spin-Flip Collisions in a Dark-Exciton Condensate

We show that a Bose-Einstein condensate consisting of dark excitons forms in GaAs coupled quantum wells at low temperatures. We find that the condensate extends over hundreds of micrometers, well beyond the optical excitation region, and is limited only by the boundaries of the mesa. We show that the condensate density is determined by spin-flipping collisions among the excitons, which convert dark excitons into bright ones. The suppression of this process at low temperature yields a density buildup, manifested as a temperature-dependent blueshift of the exciton emission line. Measurements under an in-plane magnetic field allow us to preferentially modify the bright exciton density and determine their role in the system dynamics. We find that their interaction with the condensate leads to its depletion. We present a simple rate-equations model, which well reproduces the observed temperature, power, and magnetic-field dependence of the exciton density.

A 66-76 GHz Wide Dynamic Range GaAs Transceiver for Channel Emulator Application

In this study, we developed a single-channel channel emulator module with an operating frequency covering 66–67 GHz, including a 66–76 GHz wide dynamic range monolithic integrated circuit designed based on 0.1 µm pHEMT GaAs process, a printed circuit board (PCB) power supply bias network, and low-loss ridge microstrip line to WR12 (60–90 GHz) waveguide transition structure. Benefiting from the on-chip multistage band-pass filter integrated at the local oscillator (LO) and radio frequency (RF) ends, the module’s spurious components at the RF port were greatly suppressed, making the module’s output power dynamic range over 50 dB. Due to the frequency-selective filter integrated in the LO chain, each clutter suppression in the LO chain exceeds 40 dBc. Up and down conversion loss of the module is better than 14 dB over the 66–67 GHz band, the measured IF input P1 dB is better than 10 dBm, and the module consumes 129 mA from a 5 V low dropout supply. A low-loss ridged waveguide ladder transition was designed (less than 0.4 dB) so that the output interface of the module is a WR12 waveguide interface, which is convenient for direct connection with an instrument with E-band (60–90 GHz) waveguide interface.

Cathodoluminescence mapping of electron concentration in MBE-grown GaAs:Te nanowires

Cathodoluminescence mapping is used as a contactless method to probe the electron concentration gradient of Te-doped GaAs nanowires. The room temperature and low temperature (10 K) cathodoluminescence analysis method previously developed for GaAs:Si is first validated on five GaAs:Te thin film samples, before extending it to the two GaAs:Te NW samples. We evidence an electron concentration gradient ranging from below 1 × 10¹⁸cm^-3to 3.3 ×10¹⁸cm^-3along the axis of a GaAs:Te nanowire grown at 640 °C, and a homogeneous electron concentration of around 6-8 × 10¹⁷cm^-3along the axis of a GaAs:Te nanowire grown at 620 °C. The differences in the electron concentration levels and gradients between the two nanowires is attributed to different Te incorporation efficiencies by vapor-solid and vapor-liquid-solid processes.

Surface Acoustic Wave (SAW) Sensors: Physics, Materials, and Applications

Surface acoustic waves (SAWs) are the guided waves that propagate along the top surface of a material with wave vectors orthogonal to the normal direction to the surface. Based on these waves, SAW sensors are conceptualized by employing piezoelectric crystals where the guided elastodynamic waves are generated through an electromechanical coupling. Electromechanical coupling in both active and passive modes is achieved by integrating interdigitated electrode transducers (IDT) with the piezoelectric crystals. Innovative meta-designs of the periodic IDTs define the functionality and application of SAW sensors. This review article presents the physics of guided surface acoustic waves and the piezoelectric materials used for designing SAW sensors. Then, how the piezoelectric materials and cuts could alter the functionality of the sensors is explained. The article summarizes a few key configurations of the electrodes and respective guidelines for generating different guided wave patterns such that new applications can be foreseen. Finally, the article explores the applications of SAW sensors and their progress in the fields of biomedical, microfluidics, chemical, and mechano-biological applications along with their crucial roles and potential plans for improvements in the long-term future in the field of science and technology.

Epitaxial Growth of GaAs Nanowires on Synthetic Mica by Metal-Organic Chemical Vapor Deposition

The epitaxial growth of III-V nanowires with excellent optoelectronic properties on low-cost, light-weight, and flexible substrates is a key step for the design and engineering of future optoelectronic devices. In our study, GaAs nanowires were grown on synthetic mica, a two-dimensional layered material, via vapor-liquid-solid growth using metal-organic chemical vapor deposition. The effect of basic epitaxial growth parameters such as temperature and V/III ratio on the vertical yield of the nanowires is investigated. A vertical yield of over 60% is achieved at an optimum growth temperature of 400 °C and a V/III ratio 18. The structural properties of the nanowires are investigated using various techniques including scanning electron microscopy, high-resolution transmission electron microscopy, and high-angle annular dark-field imaging. The vertical nanowires grown at a low temperature and a high V/III ratio are found to have a zincblende phase with a [111] B polarity. The optical properties are investigated by photoluminescence (PL) and time-resolved PL measurements. First-principles electronic structure calculations within the framework of density functional theory elucidate the van der Waals nature of the nanowire/mica interface. Our results also show that these nanowires can be easily lifted off the bulk 2D mica template, providing a pathway for flexible nanowire devices.

GaAs diodes for TiT2-based betavoltaic cells

The GaAs semiconductor structures for the application as betavoltaic power sources were investigated. Three types of structures underwent a comparative study: a Schottky diode, a p-n junction and Schottky structure modified by deposition of a carbon layer. The power characteristics were estimated by Monte-Carlo simulation and collected current calculation using parameters obtained from the electron beam induced current technique. It was shown that carbon deposition on the top of n-GaAs allows passivating the surface states and thus improving betavoltaic performance.

High-Performance Laterally Oriented Nanowire Solar Cells with Ag Gratings

A laterally oriented GaAs p-i-n nanowire solar cell with Ag gratings is proposed and studied via coupled three-dimensional optoelectronic simulations. The results show that the gratings significantly enhance the absorption of nanowire for both TM and TE polarized light due to the combined effect of grating diffraction, excitation of plasmon polaritons, and suppression of carrier recombination. At an optimal grating period, the absorption at 650-800 nm, which is an absorption trough for pure nanowire, is substantially enhanced, raising the conversion efficiency from 8.7% to 14.7%. Moreover, the gratings enhance the weak absorption at long wavelengths and extend the absorption cutoff wavelength for ultrathin nanowires, yielding a remarkable efficiency of 13.3% for the NW with a small diameter of 90 nm, 2.6 times that without gratings. This work may pave the way toward the development of ultrathin high-efficiency nanoscale solar cells.

Monolithic lateral p-n junction GaAs nanowire diodes via selective lateral epitaxy

Semiconductor p-n junctions are essential building blocks of electronic and optoelectronic devices. Although vertical p-n junction structures can be formed readily by growing in sequence, lateral p-n junctions normal to surface direction can only be formed on specially patterned substrates or by post-growth implantation of one type of dopant while protecting the oppositely doped side. In this study, we report the monolithic formation of lateral p-n junctions in GaAs nanowires (NWs) on a planar substrate sequentially through the Au-assisted vapor-liquid-solid selective lateral epitaxy using metalorganic chemical vapor deposition. p-type and n-type segments are formed by modulating the gas phase flow of p-type (diethylzinc) and n-type (disilane) precursorsin situduring nanowire growth, allowing independent sequential control of p- and n-doping levels self-aligned in-plane in a single growth run. The p-n junctions formed are electrically characterized by fabricating arrays of p-n junction NW diodes with coplanar ohmic metal contacts and two-terminalI-Vmeasurements. The lateral p-n diode exhibits a 2.15 ideality factor and a rectification ratio of ∼10⁶. The electron beam-induced current measurement confirms the junction position. The extracted minority carrier diffusion length is much higher compared to those previously reported, suggesting a low surface recombination velocity in these lateral NWp-n diodes.

Integration of the Rhombohedral BiSb(0001) Topological Insulator on a Cubic GaAs(001) Substrate

Bismuth-antimony alloy (Bi_1 - xSb_x) is the first reported 3D topological insulator (TI). Among many TIs reported to date, it remains the most promising for spintronic applications thanks to its large conductivity, its colossal spin Hall angle, and the possibility to build low-current spin-orbit-torque magnetoresistive random access memories. Nevertheless, the 2D integration of TIs on industrial standards is lacking. In this work, we report the integration of high-quality rhombohedral BiSb(0001) topological insulators on a cubic GaAs(001) substrate. We demonstrate a clear epitaxial relationship at the interface, a fully relaxed TI layer, and the growth of a rhombohedral matrix on top of the cubic substrate. The antimony composition of the Bi_1 - xSb_x layer is perfectly controlled and covers almost the whole TI window. For optimized growth conditions, the sample generates a semiconductor band structure at room temperature in the bulk and exhibits metallic surface states at 77 K.

Limits of Applicability of the Composite Fermion Model

The popular model of composite fermions, proposed in order to rationalize FQHE, were insufficient in view of recent experimental observations in graphene monolayer and bilayer, in higher Landau levels in GaAs and in so-called enigmatic FQHE states in the lowest Landau level of GaAs. The specific FQHE hierarchy in double Hall systems of GaAs 2DES and graphene also cannot be explained in the framework of composite fermions. We identify the limits of the usability of the composite fermion model by means of topological methods, which elucidate the phenomenological assumptions in composite fermion structure and admit further development of FQHE understanding. We demonstrate how to generalize these ideas in order to explain experimentally observed FQHE phenomena, going beyond the explanation ability of the conventional composite fermion model.

Single-Shot Readout of a Driven Hybrid Qubit in a GaAs Double Quantum Dot

We report a single-shot-based projective readout of a semiconductor hybrid qubit formed by three electrons in a GaAs double quantum dot. Voltage-controlled adiabatic transitions between the qubit operations and readout conditions allow high-fidelity mapping of quantum states. We show that a large ratio both in relaxation time vs tunneling time (≈50) and singlet-triplet splitting vs thermal energy (≈20) allows energy-selective tunneling-based spin-to-charge conversion with a readout visibility of ≈92.6%. Combined with ac driving, we demonstrate high visibility coherent Rabi and Ramsey oscillations of a hybrid qubit in GaAs. Further, we discuss the generality of the method for use in other materials, including silicon.

GaAs Nanomembranes in the High Electron Mobility Transistor Technology

A 100 nm MOCVD-grown HEMT AlGaAs/InGaAs/GaAs heterostructure nanomembrane was released from the growth GaAs substrate by ELO using a 300 nm AlAs layer and transferred to sapphire. The heterostructure contained a strained 10 nm 2DEG In_0.23Ga_0.77As channel with a sheet electron concentration of 3.4 × 10¹² cm⁻² and Hall mobility of 4590 cm²V⁻¹s⁻¹, which was grown close to the center of the heterostructure to suppress a significant bowing of the nanomembrane both during and after separation from the growth substrate. The as-grown heterostructure and transferred nanomembranes were characterized by HRXRD, PL, SEM, and transport measurements using HEMTs. The InGaAs and AlAs layers were laterally strained: ~−1.5% and ~−0.15%. The HRXRD analysis showed the as-grown heterostructure had very good quality and smooth interfaces, and the nanomembrane had its crystalline structure and quality preserved. The PL measurement showed the nanomembrane peak was shifted by 19 meV towards higher energies with respect to that of the as-grown heterostructure. The HEMTs on the nanomembrane exhibited no degradation of the output characteristics, and the input two-terminal measurement confirmed a slightly decreased leakage current.

Effect of heteroepitaxial growth on LT-GaAs: ultrafast optical properties

Epitaxial low temperature grown GaAs (LT-GaAs) on silicon (LT-GaAs/Si) has the potential for terahertz (THz) photoconductive antenna applications. However, crystalline, optical and electrical properties of heteroepitaxial grown LT-GaAs/Si can be very different from those grown on semi-insulating GaAs substrates ('reference'). In this study, we investigate optical properties of an epitaxial grown LT-GaAs/Si sample, compared to a reference grown under the same substrate temperature, and with the same layer thickness. Anti-phase domains and some crystal misorientation are present in the LT-GaAs/Si. From coherent phonon spectroscopy, the intrinsic carrier densities are estimated to be 10¹⁵ cm^-3for either sample. Strong plasmon damping is also observed. Carrier dynamics, measured by time-resolved THz spectroscopy at high excitation fluence, reveals markedly different responses between samples. Below saturation, both samples exhibit the desired fast response. Under optical fluences ⩾54μJ cm^-2, the reference LT-GaAs layer shows saturation of electron trapping states leading to non-exponential behavior, but the LT-GaAs/Si maintains a double exponential decay. The difference is attributed to the formation of As-As and Ga-Ga bonds during the heteroepitaxial growth of LT-GaAs/Si, effectively leading to a much lower density of As-related electron traps.

Experimental Demonstration of Light Focusing Enabled by Monolithic High-Contrast Grating Mirrors

We present the first experimental demonstration of a planar focusing monolithic subwavelength grating mirror. The grating is formed on the surface of GaAs and focuses 980 nm light in one dimension on the high-refractive-index side of the mirror. According to our measurements, the focal length is 475 μm (300 μm of which is GaAs) and the numerical aperture is 0.52. The intensity of the light at the focal point is 23 times larger than that of the incident light. To the best of our knowledge, this is the highest value reported for a grating mirror. Moreover, the full width at half-maximum (FWHM) at the focal point is only 3.9 μm, which is the smallest reported value for a grating mirror. All of the measured parameters are close to or very close to the theoretically predicted values. Our realization of a sophisticated design of a focusing monolithic subwavelength grating opens a new avenue to technologically simple fabrication of the gratings for use in diverse optoelectronic materials and applications.

Describing angular momentum conventions in circularly polarized optically pumped NMR in GaAs and CdTe

The physical phenomena governing hyperpolarization through optical pumping of conduction electrons continue to be explored in multiple semiconductor systems. One early finding has been the asymmetry between the optically pumped nuclear magnetic resonance (OPNMR) signals when generated by different circular polarizations (i.e., light helicities). Because these resonances are asymmetric, the midpoint between the signals prepared with each of the two circular polarizations is either a positive or negative value, termed an "offset" that is representative of an optical Overhauser enhancement. Both negative offsets (in GaAs) and positive offsets (in CdTe) have been observed. The origins of these offsets in semiconductors are believed to arise from thermalized electrons; however, to the best of our knowledge, no study has systematically tested this hypothesis. To that end, we have adopted two configurations for OPNMR experiments-one in which the Poynting vector of the laser light and magnetic field are parallel, and one in which they are antiparallel, while other experimental conditions are kept the same. We find that the OPNMR signal response to a fixed helicity of light depends on the experimental configuration, and this configuration needs to be accounted for in order to properly describe the OPNMR results. Further, studying the offsets as a function of field strength shows that the optical Overhauser enhancement (the offset) increases in magnitude with field strength. Finally, by describing all angular momentum and phasing conventions unambiguously, we are able to determine that the absorptively-phased appearance of ¹¹³Cd (and ¹²⁵Te) OPNMR in CdTe is a consequence of the sign of the nuclear gyromagnetic ratios for these isotopes.

Improving the yield of GaAs nanowires on silicon by Ga pre-deposition

GaAs nanowire (NW) arrays were grown by molecular beam epitaxy using the self-assisted vapor-liquid-solid method with Ga droplets as seed particles. A Ga pre-deposition step is examined to control NW yield and diameter. The NW yield can be increased with suitable duration of a Ga pre-deposition step but is highly dependent on oxide hole diameter and surface conditions. The NW diameter was determined by vapor-solid growth on the NW sidewalls, rather than Ga pre-deposition. The maximum NW yield with a Ga pre-deposition step was very close to 100%, established at shorter Ga deposition durations and for larger holes. This trend was explained within a model where maximum yield is obtained when the Ga droplet volume approximately equals the hole volume.

Optical Bandgap Definition via a Modified Form of Urbach's Rule

We are reporting an esoteric method to determine the optical bandgap of direct gap materials by employing Urbach’s rule. The latter, which describes the slope of the band tail absorption in semiconductors, in its original version, cannot be employed to pinpoint the optical bandgap. Herein, however, we show that a modified form of Urbach’s rule defines the optical bandgap, and therefore, enables the accurate determination of the optical bandgap energy, which turns out to be identical with the threshold energy for the band tail absorption. The model further produces an explicit expression for the absorption coefficient at the optical bandgap energy.

Elastocapillary Force Induced Alignment of Large Area Planar Nanowires

Achieving large scale precise positioning of the vapor-liquid-solid (VLS) nanowires is one of the biggest challenges for mass production of nanowire-based devices. Although there have been many noteworthy progresses in postgrowth nanowire alignment method development over the past few decades, these methods are mostly suitable for low density applications only. For high density applications such as transistors, both high yield and density are required. Here, we report an elastocapillary force-induced nanowire-aligning method that is extremely simple, clean, and can achieve single/multiple nanowire arrays with up to 98.8% yield and submicron pitch between the nanowires.

Characterization of Chromium Compensated GaAs Sensors with the Charge-Integrating JUNGFRAU Readout Chip by Means of a Highly Collimated Pencil Beam

Chromium compensated GaAs or GaAs:Cr sensors provided by the Tomsk State University (Russia) were characterized using the low noise, charge integrating readout chip JUNGFRAU with a pixel pitch of 75 × 75 µm2 regarding its application as an X-ray detector at synchrotrons sources or FELs. Sensor properties such as dark current, resistivity, noise performance, spectral resolution capability and charge transport properties were measured and compared with results from a previous batch of GaAs:Cr sensors which were produced from wafers obtained from a different supplier. The properties of the sample from the later batch of sensors from 2017 show a resistivity of 1.69 × 109 Ω/cm, which is 47% higher compared to the previous batch from 2016. Moreover, its noise performance is 14% lower with a value of (101.65 ± 0.04) e- ENC and the resolution of a monochromatic 60 keV photo peak is significantly improved by 38% to a FWHM of 4.3%. Likely, this is due to improvements in charge collection, lower noise, and more homogeneous effective pixel size. In a previous work, a hole lifetime of 1.4 ns for GaAs:Cr sensors was determined for the sensors of the 2016 sensor batch, explaining the so-called "crater effect" which describes the occurrence of negative signals in the pixels around a pixel with a photon hit due to the missing hole contribution to the overall signal causing an incomplete signal induction. In this publication, the "crater effect" is further elaborated by measuring GaAs:Cr sensors using the sensors from 2017. The hole lifetime of these sensors was 2.5 ns. A focused photon beam was used to illuminate well defined positions along the pixels in order to corroborate the findings from the previous work and to further characterize the consequences of the "crater effect" on the detector operation.

Phase-shifting electron holography for accurate measurement of potential distributions in organic and inorganic semiconductors

Phase-shifting electron holography (PS-EH) is an interference transmission electron microscopy technique that accurately visualizes potential distributions in functional materials, such as semiconductors. In this paper, we briefly introduce the features of the PS-EH that overcome some of the issues facing the conventional EH based on Fourier transformation. Then, we present a high-precision PS-EH technique with multiple electron biprisms and a sample preparation technique using a cryo-focused-ion-beam, which are important techniques for the accurate phase measurement of semiconductors. We present several applications of PS-EH to demonstrate the potential in organic and inorganic semiconductors and then discuss the differences by comparing them with previous reports on the conventional EH. We show that in situ biasing PS-EH was able to observe not only electric potential distribution but also electric field and charge density at a GaAs p-n junction and clarify how local band structures, depletion layer widths and space charges changed depending on the biasing conditions. Moreover, the PS-EH clearly visualized the local potential distributions of two-dimensional electron gas layers formed at AlGaN/GaN interfaces with different Al compositions. We also report the results of our PS-EH application for organic electroluminescence multilayers and point out the significant potential changes in the layers. The proposed PS-EH enables more precise phase measurement compared to the conventional EH, and our findings introduced in this paper will contribute to the future research and development of high-performance semiconductor materials and devices.

Influence of Silver as a Catalyst on the Growth of β-Ga2O3 Nanowires on GaAs

A simple and inexpensive thermal oxidation process was performed to synthesize gallium oxide (Ga₂O₃) nanowires using Ag thin film as a catalyst at 800 °C and 1000 °C to understand the effect of the silver catalyst on the nanowire growth. The effect of doping and orientation of the substrates on the growth of Ga₂O₃ nanowires on single-crystal gallium arsenide (GaAs) wafers in atmosphere were investigated. A comprehensive study of the oxide film and nanowire growth was performed using various characterization techniques including XRD, SEM, EDS, focused ion beam (FIB), XPS and STEM. Based on the characterization results, we believe that Ag thin film produces Ag nanoparticles at high temperatures and enhances the reaction between oxygen and gallium, contributing to denser and longer Ga₂O₃ nanowires compared to those grown without silver catalyst. This process can be optimized for large-scale production of high-quality, dense, and long nanowires.

A Hierarchical 3D TiO2 /Ni Nanostructure as an Efficient Hole-Extraction and Protection Layer for GaAs Photoanodes

Photoelectrochemical (PEC) water splitting is a promising clean route to hydrogen fuel. The best-performing materials (III/V semiconductors) require surface passivation, as they are liable to corrosion, and a surface co-catalyst to facilitate water splitting. At present, optimal design combining photoelectrodes with oxygen evolution catalysts remains a significant materials challenge. Here, we demonstrate that nickel-coated amorphous three-dimensional (3D) TiO₂ core-shell nanorods on a TiO₂ thin film function as an efficient hole-extraction layer and serve as a protection layer for the GaAs photoanode. Transient-absorption spectroscopy (TAS) demonstrated the role of nickel-coated (3D) TiO₂ core-shell nanorods in prolonging photogenerated charge lifetimes in GaAs, resulting in a higher catalytic activity. This strategy may open the potential of utilizing this low-cost (3D) nanostructured catalyst for decorating narrow-band-gap semiconductor photoanodes for PEC water splitting devices.

Anodically Induced Chemical Etching of GaAs Wafers for a GaAs Nanowire-Based Flexible Terahertz Wave Emitter

A generic top-down approach for the preparation of extended arrays of high-aspect ratio GaAs nanowires (NWs) with different crystallographic orientations (i.e., [100] or [111]) and morphologies (i.e., porous, nonporous, tapered, or awl-like NWs) is reported. The method is based on the anodically induced chemical etching (AICE) of GaAs wafers in an oxidant-free aqueous HF solution at room temperature by using a patterned metal mesh and allows us to overcome the drawbacks of conventional metal-assisted chemical etching (MACE) processes. Local oxidative dissolution of GaAs in contact with a metal is achieved by externally injecting holes (h⁺) into the valence band (VB) of GaAs through the metal mesh. It is found that injection of holes (h⁺) through direct GaAs contact, rather than the metal mesh, does not yield uniform nanowires but porosify GaAs wafers due to the high cell potential. On the basis of experiments and numerical simulation for the spatial distribution of an electric field, a phenomenological model that explains the formation of GaAs NWs and their porosification behaviors is proposed. GaAs NWs exhibit excellent terahertz (THz) wave emission properties, which vary with either the length or the shape of the nanowires. By taking advantage of controlled porosification and easy transfer of GaAs NWs to foreign substrates, a flexible THz wave emitter is realized.

A Low-Threshold Miniaturized Plasmonic Nanowire Laser with High-Reflectivity Metal Mirrors

A reflectivity-enhanced hybrid plasmonic GaAs/AlGaAs core-shell nanowire laser is proposed and studied by 3D finite-difference time-domain simulations. The results demonstrate that by introducing thin metal mirrors at both ends, the end facet reflectivity of nanowire is increased by 30–140%, resulting in a much stronger optical feedback. Due to the enhanced interaction between the surface charge oscillation and light, the electric field intensity inside the dielectric gap layer increases, resulting in a much lower threshold gain. For a small diameter in the range of 100–150 nm, the threshold gain is significantly reduced to 60–80% that of nanowire without mirrors. Moreover, as the mode energy is mainly concentrated in the gap between the nanowire and metal substrate, the output power maintains >60% that of nanowire without mirrors in the diameter range of 100–150 nm. The low-threshold miniaturized plasmonic nanowire laser with simple processing technology is promising for low-consumption ultra-compact optoelectronic integrated circuits and on-chip communications.

Low-Temperature In-Induced Holes Formation in Native-SiOx/Si(111) Substrates for Self-Catalyzed MBE Growth of GaAs Nanowires

The reduction of substrate temperature is important in view of the integration of III-V materials with a Si platform. Here, we show the way to significantly decrease substrate temperature by introducing a procedure to create nanoscale holes in the native-SiOx layer on Si(111) substrate via In-induced drilling. Using the fabricated template, we successfully grew self-catalyzed GaAs nanowires by molecular-beam epitaxy. Energy-dispersive X-ray analysis reveals no indium atoms inside the nanowires. This unambiguously manifests that the procedure proposed can be used for the growth of ultra-pure GaAs nanowires.

Periodic Microstructures Fabricated by Laser Interference with Subsequent Etching

Periodic nanostructures have wide applications in micro-optics, bionics, and optoelectronics. Here, a laser interference with subsequent etching technology is proposed to fabricate uniform periodic nanostructures with controllable morphologies and smooth surfaces on hard materials. One-dimensional microgratings with controllable periods (1, 2, and 3 μm) and heights, from dozens to hundreds of nanometers, and high surface smoothness are realized on GaAs by the method. The surface roughness of the periodic microstructures is significantly reduced from 120 nm to 40 nm with a subsequent inductively coupled plasma (ICP) etching. By using laser interference with angle-multiplexed exposures, two-dimensional square- and hexagonal-patterned microstructures are realized on the surface of GaAs. Compared with samples without etching, the diffraction efficiency can be significantly enhanced for samples with dry etching, due to the improvement of surface quality.

Carrier Recombination Processes in GaAs Wafers Passivated by Wet Nitridation

As one of the successful approaches to GaAs surface passivation, wet-chemical nitridation is applied here to relate the effect of surface passivation to carrier recombination processes in bulk GaAs. By combining time-resolved photoluminescence and optical pump-THz probe measurements, we found that surface hole trapping dominates the decay of photoluminescence, while photoconductivity dynamics is limited by surface electron trapping. Compared to untreated sample dynamics, the optimized nitridation reduces hole- and electron-trapping rate by at least 2.6 and 3 times, respectively. Our results indicate that under ambient conditions, recovery of the fast hole trapping due to the oxide regrowth at the deoxidized GaAs surface takes tens of hours, while it is effectively inhibited by surface nitridation. Our study demonstrates that surface nitridation stabilizes the GaAs surface via reduction of both electron- and hole-trapping rates, which results in chemical and electronical passivation of the bulk GaAs surface.

Effect of the Uniaxial Compression on the GaAs Nanowire Solar Cell

Research regarding ways to increase solar cell efficiency is in high demand. Mechanical deformation of a nanowire (NW) solar cell can improve its efficiency. Here, the effect of uniaxial compression on GaAs nanowire solar cells was studied via conductive atomic force microscopy (C-AFM) supported by numerical simulation. C-AFM I–V curves were measured for wurtzite p-GaAs NW grown on p-Si substrate. Numerical simulations were performed considering piezoresistance and piezoelectric effects. Solar cell efficiency reduction of 50% under a −0.5% strain was observed. The analysis demonstrated the presence of an additional fixed electrical charge at the NW/substrate interface, which was induced due to mismatch between the crystal lattices, thereby affecting the efficiency. Additionally, numerical simulations regarding the p-n GaAs NW solar cell under uniaxial compression were performed, showing that solar efficiency could be controlled by mechanical deformation and configuration of the wurtzite and zinc blende p-n segments in the NW. The relative solar efficiency was shown to be increased by 6.3% under −0.75% uniaxial compression. These findings demonstrate a way to increase efficiency of GaAs NW-based solar cells via uniaxial mechanical compression.

Absorption-Enhanced Ultra-Thin Solar Cells Based on Horizontally Aligned p-i-n Nanowire Arrays

A horizontally aligned GaAs p–i–n nanowire array solar cell is proposed and studied via coupled three-dimensional optoelectronic simulations. Benefiting from light-concentrating and light-trapping properties, the horizontal nanowire array yields a remarkable efficiency of 10.8% with a radius of 90 nm and a period of 5 radius, more than twice that of its thin-film counterpart with the same thickness. To further enhance the absorption, the nanowire array is placed on a low-refractive-index MgF₂ substrate and capsulated in SiO₂, which enables multiple reflection and reabsorption of light due to the refractive index difference between air/SiO₂ and SiO₂/MgF₂. The absorption-enhancement structure increases the absorption over a broad wavelength range, resulting in a maximum conversion efficiency of 18%, 3.7 times higher than that of the thin-film counterpart, which is 3 times larger in GaAs material volume. This work may pave the way for the development of ultra-thin high-efficiency solar cells with very low material cost.

Formation Mechanism of Twinning Superlattices in Doped GaAs Nanowires

Recent investigations of III-V semiconductor nanowires have revealed periodic zinc-blende twins, known as twinning superlattices, that are often induced by a high-impurity dopant concentration. In the present study, the relationship between the nanowire morphology, crystal structure, and impurity dopant concentration (Te and Be) of twinning superlattices has been studied in GaAs nanowires grown by molecular beam epitaxy using the self-assisted (with a Ga droplet) vapor-liquid-solid process. The contact angle between the Ga droplet and the nanowire top facet decreased linearly with the dopant concentration, whereas the period of the twinning superlattices increased with the doping concentration and was proportional to the nanowire radius. Our model, which is based entirely on surface energetics, is able to explain a unified formation mechanism of twinning superlattices in doped semiconductor nanowires.