Amorphous Transparent Cu(S,I) Thin Films with Very High Hole Conductivity

Amorphous transparent conductors (a-TCs) are key materials for flexible and transparent electronics but still suffer from poor p-type conductivity. By developing an amorphous Cu(S,I) material system, record high hole conductivities of 10³-10⁴ S cm^-1 have been achieved in p-type a-TCs. These high conductivities are comparable with commercial n-type TCs made of indium tin oxide and are 100 times greater than any previously reported p-type a-TCs. Responsible for the high hole conduction is the overlap of large p-orbitals of I^- and S^2- anions, which provide a hole transport pathway insensitive to structural disorder. In addition, the bandgap of amorphous Cu(S,I) can be modulated from 2.6 to 2.9 eV by increasing the iodine content. These unique properties demonstrate that the Cu(S,I) system holds great potential as a promising p-type amorphous transparent electrode material for optoelectronics.

Growth of κ-([Al,In]xGa1-x)2O3 Quantum Wells and Their Potential for Quantum-Well Infrared Photodetectors

The wide band gap semiconductor κ-Ga₂O₃ and its aluminum and indium alloys have been proposed as promising materials for many applications. One of them is the use of inter-sub-band transitions in quantum-well (QW) systems for infrared detectors. Our simulations show that the detection wavelength range of nowadays state of the art GaAs/Al_xGa_1-xAs quantum-well infrared photodetectors (QWIPs) could be substantially excelled with about 1-100 μm using κ-([Al,In]_xGa_1-x)₂O₃, while at the same time being transparent to visible light and therefore insensitive to photon noise due to its wide band gap, demonstrating the application potential of this material system. Our simulations further show that the QWIPs efficiency critically depends on the QW thickness, making a precise control over the thickness during growth and a reliable thickness determination essential. We demonstrate that pulsed laser deposition yields the needed accuracy, by analyzing a series of (In_xGa_1-x)₂O₃ QWs with (Al_yGa_1-y)₂O₃ barriers with high-resolution X-ray diffraction, X-ray photoelectron spectroscopy (XPS) depth profiling, and transmission electron microscopy (TEM). While the superlattice fringes of high-resolution X-ray diffraction only yield an average combined thickness of the QWs and the barrier and X-ray spectroscopy depth profiling requires elaborated modeling of the XPS signal to accurately determine the thickness of such QWs, TEM is the method of choice when it comes to the determination of QW thicknesses.

Suppression of Rotational Domains of CuI Employing Sodium Halide Buffer Layers

The occurrence of rotational domains is a well-known issue for copper iodide (CuI) that naturally occurs for growth on popular substrates like sapphire. However, this has detrimental effects on the thin film quality like increasing surface roughness or deteriorated transport characteristics due to grain boundary scattering. Utilizing pulsed laser deposition and the in situ growth of sodium chloride (NaCl) and sodium bromide (NaBr) template layers, studies were performed on their potential on suppressing the formation of rotational domains of CuI on c-plane sapphire and SrF₂(111) substrates. Corresponding samples were investigated concerning their epitaxial properties and further characterized regarding (volume) crystalline, morphological, and electrical properties. Particularly for NaBr template layers, fully single-crystalline growth of CuI thin films was obtained and resulted in significantly reduced surface roughness of the CuI layer.

Mid- and far-infrared localized surface plasmon resonances in chalcogen-hyperdoped silicon

Plasmonic sensing in the infrared region employs the direct interaction of the vibrational fingerprints of molecules with the plasmonic resonances, creating surface-enhanced sensing platforms that are superior to traditional spectroscopy. However, the standard noble metals used for plasmonic resonances suffer from high radiative losses as well as fabrication challenges, such as tuning the spectral resonance positions into mid- to far-infrared regions, and the compatibility issue with the existing complementary metal-oxide-semiconductor (CMOS) manufacturing platform. Here, we demonstrate the occurrence of mid-infrared localized surface plasmon resonances (LSPR) in thin Si films hyperdoped with the known deep-level impurity tellurium. We show that the mid-infrared LSPR can be further enhanced and spectrally extended to the far-infrared range by fabricating two-dimensional arrays of micrometer-sized antennas in a Te-hyperdoped Si chip. Since Te-hyperdoped Si can also work as an infrared photodetector, we believe that our results will unlock the route toward the direct integration of plasmonic sensors with the on-chip CMOS platform, greatly advancing the possibility of mass manufacturing of high-performance plasmonic sensing systems.

Mechanical Stress Stability of Flexible Amorphous Zinc Tin Oxide Thin-Film Transistors

Due to their low-temperature processing capability and ionic bonding configuration, amorphous oxide semiconductors (AOS) are well suited for applications within future mechanically flexible electronics. Over the past couple of years, amorphous zinc tin oxide (ZTO) has been proposed as indium and gallium-free and thus more sustainable alternative to the widely deployed indium gallium zinc oxide (IGZO). The present study specifically focuses on the strain-dependence of elastic and electrical properties of amorphous zinc tin oxide thin-films sputtered at room temperature. Corresponding MESFETs have been compared regarding their operation stability under mechanical bending for radii ranging from 5 to 2 mm. Force-spectroscopic measurements yield a plastic deformation of ZTO as soon as the bending-induced strain exceeds 0.83 %. However, the electrical properties of ZTO determined by Hall effect measurements at room temperature are demonstrated to be unaffected by residual compressive and tensile strain up to 1.24 %. Even for the maximum investigated tensile strain of 1.26 %, the MESFETs exhibit a reasonably consistent performance in terms of current on/off ratios between six and seven orders of magnitude, a subthreshold swing around 350 mV/dec and a field-effect mobility as high as 7.5 cm2V−1s−1. Upon gradually subjecting the transistors to higher tensile strain, the channel conductivity steadily improves and consequently, the field-effect mobility increases by nearly 80 % while bending the devices around a radius of 2 mm. Further, a reversible threshold voltage shift of about −150 mV with increasing strain is observable. Overall, amorphous ZTO provides reasonably stable electrical properties and device performance for bending-induced tensile strain up to at least 1.26 % and thus represent a promising material of choice considering novel bendable and transparent electronics.

Hot-phonon effects in photo-excited wide-bandgap semiconductors

Carrier and lattice relaxation after optical excitation is simulated for the prototypical wide-bandgap semiconductors CuI and ZnO. Transient temperature dynamics of electrons, holes as well as longitudinal-optic (LO), transverse-optic (TO) and acoustic phonons are distinguished. Carrier-LO-phonon interaction constitutes the dominant energy-loss channel as expected for polar semiconductors and hot-phonon effects are observed for strong optical excitation. Our results support the findings of recent time-resolved optical spectroscopy experiments.

Indium Gallium Oxide Alloys: Electronic Structure, Optical Gap, Surface Space Charge, and Chemical Trends within Common-Cation Semiconductors

The electronic and optical properties of (In_xGa_1-x)₂O₃ alloys are highly tunable, giving rise to a myriad of applications including transparent conductors, transparent electronics, and solar-blind ultraviolet photodetectors. Here, we investigate these properties for a high quality pulsed laser deposited film which possesses a lateral cation composition gradient (0.01 ≤ x ≤ 0.82) and three crystallographic phases (monoclinic, hexagonal, and bixbyite). The optical gaps over this composition range are determined, and only a weak optical gap bowing is found (b = 0.36 eV). The valence band edge evolution along with the change in the fundamental band gap over the composition gradient enables the surface space-charge properties to be probed. This is an important property when considering metal contact formation and heterojunctions for devices. A transition from surface electron accumulation to depletion occurs at x ∼ 0.35 as the film goes from the bixbyite In₂O₃ phase to the monoclinic β-Ga₂O₃ phase. The electronic structure of the different phases is investigated by using density functional theory calculations and compared to the valence band X-ray photoemission spectra. Finally, the properties of these alloys, such as the n-type dopability of In₂O₃ and use of Ga₂O₃ as a solar-blind UV detector, are understood with respect to other common-cation compound semiconductors in terms of simple chemical trends of the band edge positions and the hydrostatic volume deformation potential.

The principal axes systems for the elastic properties of monoclinic gallia

We discuss the principal axes systems of monoclinic and triclinic crystals regarding their elastic properties. Explicit formulas are presented for the orientation of these coordinate systems for monoclinic crystals. In this context, theoretical results from literature on the elastic properties of monoclinic (space group C2/m) gallia and alumina are critically discussed.

Toward three-dimensional hybrid inorganic/organic optoelectronics based on GaN/oCVD-PEDOT structures

The combination of inorganic semiconductors with organic thin films promises new strategies for the realization of complex hybrid optoelectronic devices. Oxidative chemical vapor deposition (oCVD) of conductive polymers offers a flexible and scalable path towards high-quality three-dimensional inorganic/organic optoelectronic structures. Here, hole-conductive poly(3,4-ethylenedioxythiophene) (PEDOT) grown by oxidative chemical vapor deposition is used to fabricate transparent and conformal wrap-around p-type contacts on three-dimensional microLEDs with large aspect ratios, a yet unsolved challenge in three-dimensional gallium nitride technology. The electrical characteristics of two-dimensional reference structures confirm the quasi-metallic state of the polymer, show high rectification ratios, and exhibit excellent thermal and temporal stability. We analyze the electroluminescence from a three-dimensional hybrid microrod/polymer LED array and demonstrate its improved optical properties compared with a purely inorganic microrod LED. The findings highlight a way towards the fabrication of hybrid three-dimensional optoelectronics on the sub-micron scale.

Though integrating functional organic materials with semiconductor nanostructures is attractive for 3D chip processing, realizing these hybrids remains a challenge. Here, the authors report an oxidative chemical vapor deposition-based process for designing novel 3D hybrid optoelectronic structures.

Experimental exploration of the amphoteric defect model by cryogenic ion irradiation of a range of wide band gap oxide materials

The evolution of electrical resistance as function of defect concentration is examined for the unipolarn-conducting oxides CdO,β-Ga₂O₃, In₂O₃, SnO₂and ZnO in order to explore the predictions of the amphoteric defect model. Intrinsic defects are introduced by ion irradiation at cryogenic temperatures, and the resistance is measured in-situ by current-voltage sweeps as a function of irradiation dose. Temperature dependent Hall effect measurements are performed to determine the carrier concentration and mobility of the samples before and after irradiation. After the ultimate irradiation step, the Ga₂O₃and SnO₂samples have both turned highly resistive. In contrast, the In₂O₃and ZnO samples are ultimately found to be less resistive than prior to irradiation, however, they both show an increased resistance at intermediate doses. Based on thermodynamic defect charge state transitions computed by hybrid density functional theory, a model expanding on the current amphoteric defect model is proposed.

Controlled formation of Schottky diodes on n-doped ZnO layers by deposition of p-conductive polymer layers with oxidative chemical vapor deposition

We report the controlled formation of organic/inorganic Schottky diodes by depositing poly(3,4-ethylenedioxythiophene) (PEDOT) on n-doped ZnO layers using oxidative chemical vapor deposition (oCVD). Current-voltage measurements reveal the formation of Schottky diodes that show good thermal and temporal stability with rectification ratios of 107 and ideality factors of ∼1.2. In the frame of a Schottky model, we identify a mean barrier height at the hybrid inorganic-organic interface of 1.28 eV, which is consistent with the difference between the work function of PEDOT and the electron affinity of ZnO. The findings highlight the strength of oCVD to design high-quality hybrid PEDOT/ZnO heterojunctions with possible applications in electronic and optoelectronic devices.

Band Offsets at κ-([Al,In]xGa1-x)2O3/MgO Interfaces

Conduction and valence band offsets are among the most crucial material parameters for semiconductor heterostructure device design, such as for high-electron mobility transistors or quantum well infrared photodetectors (QWIP). Because of its expected high spontaneous electrical polarization and the possibility of polarization doping at heterointerfaces similar to the AlGaN/InGaN/GaN system, the metastable orthorhombic κ-phase of Ga₂O₃ and its indium and aluminum alloy systems are a promising alternative for such device applications. However, respective band offsets to any dielectric are unknown, as well as the evolution of the bands within the alloy systems. We report on the valence and conduction band offsets of orthorhombic κ-(Al_xGa_1-x)₂O₃ and κ-(In_xGa_1-x)₂O₃ thin films to MgO as reference dielectric by X-ray photoelectron spectroscopy. The thin films with compositions x_In ≤ 0.27 and x_Al ≤ 0.55 were grown by pulsed laser deposition utilizing tin-doped and radially segmented targets. The determined band alignments reveal the formation of a type I heterojunction to MgO for all compositions with conduction band offsets of at least 1.4 eV, providing excellent electron confinement. Only low valence band offsets with a maximum of ∼300 meV were observed. Nevertheless, this renders MgO as a promising gate dielectric for metal-oxide-semiconductor transistors in the orthorhombic modification. We further found that the conduction band offsets in the alloy systems are mainly determined by the evolution of the band gaps, which can be tuned by the composition in a wide range between 4.1 and 6.2 eV, because the energy position of the valence band maximum remains almost constant over the complete composition range investigated. Therefore, tunable conduction band offsets of up to 1.1 eV within the alloy systems allow for subniveau transition energies in (Al_xGa_1-x)₂O₃/(In_xGa_1-x)₂O₃/(Al_xGa_1-x)₂O₃ quantum wells from the infrared to the visible regime, which are promising for application in QWIPs.

Enhanced Magnetoelectric Coupling in BaTiO3-BiFeO3 Multilayers-An Interface Effect

Combining various (multi-)ferroic materials into heterostructures is a promising route to enhance their inherent properties, such as the magnetoelectric coupling in BiFeO3 thin films. We have previously reported on the up-to-tenfold increase of the magnetoelectric voltage coefficient α ME in BaTiO3-BiFeO3 multilayers relative to BiFeO3 single layers. Unraveling the origin and mechanism of this enhanced effect is a prerequisite to designing new materials for the application of magnetoelectric devices. By careful variations in the multilayer design we now present an evaluation of the influences of the BaTiO3-BiFeO3 thickness ratio, oxygen pressure during deposition, and double layer thickness. Our findings suggest an interface driven effect at the core of the magnetoelectric coupling effect in our multilayers superimposed on the inherent magnetoelectric coupling of BiFeO3 thin films, which leads to a giant α ME coefficient of 480 V c m -1 Oe-1 for a 16 × (BaTiO3-BiFeO3) superlattice with a 4 . 8 nm double layer periodicity.

Voigt Exceptional Points in an Anisotropic ZnO-Based Planar Microcavity: Square-Root Topology, Polarization Vortices, and Circularity

Voigt points represent propagation directions in anisotropic crystals along which optical modes degenerate, leading to a single circularly polarized eigenmode. They are a particular class of exceptional points. Here, we report the fabrication and characterization of a dielectric, anisotropic optical microcavity based on nonpolar ZnO that implements a non-Hermitian system and mimics the behavior of Voigt points in natural crystals. We prove the exceptional-point nature by monitoring the complex-square-root topology of the mode eigenenergies (real and imaginary parts) around the Voigt points. Polarization state analysis shows that these artificially engineered Voigt points behave as vortex cores for the linear polarization and sustain chiral modes. Our findings apply to any planar microcavity with broken cylindrical symmetry and, thus, pave the way for exploiting exceptional points in widespread optoelectronic devices such as vertical cavity surface emitting lasers and resonant cavity light emitting diodes.

Processing Strategies for High-Performance Schottky Contacts on n-Type Oxide Semiconductors: Insights from In2O3

Preparation of rectifying Schottky contacts on n-type oxide semiconductors, such as indium oxide (In₂O₃), is often challenged by the presence of a distinct surface electron accumulation layer. We investigated the material properties and electrical transport characteristics of platinum contact/indium oxide heterojunctions to define routines for the preparation of high-performance Schottky diodes on n-type oxide semiconductors. Combining the evaluation of different Pt deposition methods, such as electron-beam evaporation and (reactive) sputtering in an (O and) Ar atmosphere, with oxygen plasma interface treatments, we identify key parameters to obtain Schottky-type contacts with high electronic barrier height and high rectification ratio. Different photoelectron spectroscopy approaches are compared to characterize the chemical properties of the contact layers and the interface region toward In₂O₃, to analyze charge transfer and plasma oxidation processes as well as to evaluate the precision and limits of different methodologies to determine heterointerface energy barriers. An oxygen-plasma-induced passivation of the semiconductor surface, which induces electron depletion and generates an intrinsic interface energy barrier, is found to be not sufficient to generate rectifying platinum contacts. The dissolution of the functional interface oxide layer within the Pt film results in an energy barrier of ∼0.5 eV, which is too low for an In₂O₃ electron concentration of ∼10¹⁸ cm^-3. A reactive sputter process in an Ar and O atmosphere is required to fabricate rectifying contacts that are composed of platinum oxide (PtO_x). Combining oxygen plasma interface oxidation of the semiconductor surface with reactive sputtering of PtO_x layers results in the generation of a high Schottky barrier of ∼0.9 eV and a rectification ratio of up to 10⁶. An additional oxygen plasma treatment after contact deposition further reduced the reverse leakage current, likely by eliminating a surface conduction path between the coplanar Ohmic and Schottky contacts. We conclude that processes that allow us to increase the oxygen content in the interface and contact region are essential for fabrication of device-quality-rectifying contacts on various oxide semiconductors.

Applicability of the constitutive equations for the determination of the material properties of optically active materials

For the description of the optical response of optically active media, different constitutive equations have been proposed and are still used in literature. Here, we demonstrate on potassium titanyl phosphate (KTiOPO₄) by means of ellipsometry that only the symmetric constitutive equation describes the observed spectra properly and allows the unique determination of the gyration tensor.

Spatiotemporal Evolution of Coherent Polariton Modes in ZnO Microwire Cavities at Room Temperature

Tunable waveguides for propagating coherent quantum states are demanded for future applications in quantum information technology and optical data processing. We present coherent whispering gallery mode polariton states in ZnO-based hexagonal microwires at room temperature. We observed their propagation over the field of view of about 20 μm by picosecond time-resolved real space imaging using a streak camera. Spatial coherence was proven by time integrated Michelson interferometry superimposing the inverted spatial emission pattern with its original one. We furthermore show that the real and momentum space evolution of the coherent states can not only be described by the commonly used model developed for ballistically propagating Bose-Einstein condensates based on the Gross-Pitaevskii equation but equivalently by classical ray optics considering a spatially varying particle density dependent refractive index of the cavity material, not yet considered in literature so far. By changing the excitation spot size, the refractive index gradient and thus the propagation velocity is changed.

Defect Manipulation To Control ZnO Micro-/Nanowire-Metal Contacts

Surface states that induce depletion regions are commonly believed to control the transport of charged carriers through semiconductor nanowires. However, direct, localized optical, and electrical measurements of ZnO nanowires show that native point defects inside the nanowire bulk and created at metal-semiconductor interfaces are electrically active and play a dominant role electronically, altering the semiconductor doping, the carrier density along the wire length, and the injection of charge into the wire. We used depth-resolved cathodoluminescence spectroscopy to measure the densities of multiple point defects inside ZnO nanowires, substitutional Cu on Zn sites, zinc vacancy, and oxygen vacancy defects, showing that their densities varied strongly both radially and lengthwise for tapered wires. These defect profiles and their variation with wire diameter produce trap-assisted tunneling and acceptor trapping of free carriers, the balance of which determines the low contact resistivity (2.6 × 10^-3 Ω·cm^-2) ohmic, Schottky (Φ ≥ 0.35 eV) or blocking nature of Pt contacts to a single nano/microwire. We show how these defects can now be manipulated by ion beam methods and nanowire design, opening new avenues to control nanowire charge injection and transport.

Combinatorial Material Science and Strain Engineering Enabled by Pulsed Laser Deposition Using Radially Segmented Targets

Vertical composition gradients of ternary alloy thin films find applications in numerous device structures. Up to now such gradients along the growth direction have not been realized by standard pulsed laser deposition (PLD) systems. In this study, we propose an approach based on a single elliptically segmented PLD target suited for the epitaxial growth of vertically graded layers. The composition of the thin films can be varied by a simple adjustment of the position of the PLD laser spot on the target surface. We demonstrate this principle for the Mg _xZn_{1- x}O alloy system. Such vertically composition-graded Mg _xZn_{1- x}O thin films exhibit high optical quality and a well-defined Mg-content for each layer. No signs of interdiffusion of Mg-atoms between the layers have been found. Further, this method is capable to deposit homogeneous thin films with any desired, well-defined cation composition having the same high optical and structural quality as films grown by conventional PLD.

Electrical conductivity of In2O3 and Ga2O3 after low temperature ion irradiation; implications for instrinsic defect formation and charge neutrality level

The evolution of sheet resistance of n-type In₂O₃ and Ga₂O₃ exposed to bombardment with MeV ¹²C and ²⁸Si ions at 35 K is studied in situ. While the sheet resistance of Ga₂O₃ increased by more than eight orders of magnitude as a result of ion irradiation, In₂O₃ showed a more complex defect evolution and became more conductive when irradiated at the highest doses. Heating up to room temperature reduced the sheet resistivity somewhat, but Ga₂O₃ remained highly resistive, while In₂O₃ showed a lower resistance than as deposited samples. Thermal admittance spectroscopy and deep level transient spectroscopy did not reveal new defect levels for irradiation up to [Formula: see text] cm^-2. A model where larger defect complexes preferentially produce donor like defects in In₂O₃ is proposed, and may reveal a microscopic view of a charge neutrality level within the conduction band, as previously proposed.

Low-Temperature PLD-Growth of Ultrathin ZnO Nanowires by Using Zn x Al1-x O and Zn x Ga1-x O Seed Layers

ZnO nanowires (NWs) are used as building blocks for a wide range of different devices, e.g. light emitters, resonators, and sensors. Integration of the NWs into such structures requires a high level of NWs' diameter control. Here, we present that the doping concentration of Zn _x Al_1-x O and Zn _x Ga_1-x O seed layers has a strong impact on the NW growth and allows to tune the diameter of the NWs by two orders of magnitude down to less than 7 nm. These ultrathin NWs exhibit a well-oriented vertical growth and thus are promising for the investigation of quantum effects. The doping of the ZnO seed layers has also an impact on the deposition temperature which can be reduced down to T≈400^∘C. This temperature is much smaller than those typically used for the fabrication of NWs by pulsed laser deposition. A comparison of the NWs indicates a stronger impact of the Ga doping on the NW growth than for the Al doping which we attribute to an impact of the size of the dopants. The optical properties of the NWs were investigated by cathodoluminescence spectroscopy which revealed a high crystalline quality. For the thin nanowires, the emission characteristic is mainly determined by the properties of the surface near region.

Dynamical Tuning of Nanowire Lasing Spectra

Realizing visionary concepts of integrated photonic circuits, nanospectroscopy, and nanosensing will tremendously benefit from dynamically tunable coherent light sources with lateral dimensions on the subwavelength scale. Therefore, we demonstrate an individual nanowire laser based device which can be gradually tuned by reversible length changes of the nanowire such that uniaxial tensile stress is applied to the respective semiconductor gain material. By straining the device, the spontaneous excitonic emission of the nanowire shifts to lower energies caused by the bandgap reduction of the semiconductor. Moreover, the optical gain spectrum of the nanolaser can be precisely strain-tuned in the high excitation regime. The tuning of the emission does not affect the laser threshold of the device, which is very beneficial for practical applications. The applied length change furthermore adjusts the laser resonances inducing a redshift of the longitudinal modes. Thus, this concept of gradually and dynamically tunable nanolasers enables controlling and modulating the coherent emission on the nanoscale without changing macroscopic ambient conditions. This concept holds therefore huge impact on nanophotonic switches and photonic circuit technology.

Vital Role of Oxygen for the Formation of Highly Rectifying Schottky Barrier Diodes on Amorphous Zinc-Tin-Oxide with Various Cation Compositions

We present electrical properties of Schottky barrier diodes on room-temperature deposited amorphous zinc-tin-oxide (ZTO) with Zn/(Zn + Sn) contents between 0.12 and 0.72. A combinatorial approach with continuous composition spread pulsed laser deposition is used to achieve the wide range of compositions with four samples each on 50 × 50 mm² glass substrates. The Schottky barrier contacts were fabricated by the reactive direct-current sputtering of platinum. Best diode properties (rectification ratio S_V = 2.7 × 10⁷, ideality factor η = 1.05, and effective barrier height ϕ_B,eff = 1.25 eV) are obtained for a composition of 0.63 Zn/(Zn + Sn). Aging on the timescale of days and months is observed that leads to improved device properties (higher rectifications and lower ideality factors). In particular, the diodes with the lowest performance in the as-prepared state show the biggest improvements. The best diode properties after the aging process (S_V = 3.9 × 10⁷, η = 1.12, and ϕ_B,eff = 1.31 eV) were also observed for 0.63 Zn/(Zn + Sn).

Correlation of Interface Impurities and Chemical Gradients with High Magnetoelectric Coupling Strength in Multiferroic BiFeO3-BaTiO3 Superlattices

The detailed understanding of magnetoelectric (ME) coupling in multiferroic oxide heterostructures is still a challenge. In particular, very little is known to date concerning the impact of the chemical interface structure and unwanted impurities that may be buried within short-period multiferroic BiFeO₃-BaTiO₃ superlattices during growth. Here, we demonstrate how trace impurities and elemental concentration gradients contribute to high ME voltage coefficients in thin-film superlattices, which are built from 15 double layers of BiFeO₃-BaTiO₃. Surprisingly, the highest ME voltage coefficient of 55 V cm^-1 Oe^-1 at 300 K was measured for a superlattice with a few atomic percent of Ba and Ti that diffused into the nominally 5 nm thin BiFeO₃ layers, according to analytical transmission electron microscopy. In addition, highly sensitive enhancements of the cation signals were observed in depth profiles by secondary ion mass spectrometry at the interfaces of BaTiO₃ and BiFeO₃. As these interface features correlate with the ME performance of the samples, they point to the importance of charge effects at the interfaces, that is, to a possible charge mediation of ME coupling in oxide superlattices. The challenge is to provide cleaner materials and processes, as well as a well-defined control of the chemical interface structure, to push forward the application of oxide superlattices in multiferroic ME devices.

Raman tensor elements of β-Ga2O3

The Raman spectrum and particularly the Raman scattering intensities of monoclinic β-Ga₂O₃ are investigated by experiment and theory. The low symmetry of β-Ga₂O₃ results in a complex dependence of the Raman intensity for the individual phonon modes on the scattering geometry which is additionally affected by birefringence. We measured the Raman spectra in dependence on the polarization direction for backscattering on three crystallographic planes of β-Ga₂O₃ and modelled these dependencies using a modified Raman tensor formalism which takes birefringence into account. The spectral position of all 15 Raman active phonon modes and the Raman tensor elements of 13 modes were determined and are compared to results from ab-initio calculations.

Contacting ZnO Individual Crystal Facets by Direct Write Lithography

Many advanced electronic devices take advantage of properties developed at the surface facets of grown crystals with submicrometer dimensions. Electrical contacts to individual crystal facets can make possible the investigations of facet-dependent properties such as piezoelectricity in ZnO or III-nitride crystals having noncentrosymmetric structure. However, a lithography-based method for developing contacts to individual crystal facets with submicrometer size has not yet been demonstrated. In this report we study the use of electron beam-induced deposition (EBID), a direct write lithography method, for contacting individual facets of ZnO pillars within an electron microscope. Correlating structural and in situ deposition and electrical data, we examine proximity effects during the EBID and evaluate the process against obtaining electrically insulated contact lines on neighboring and diametrically opposite ZnO facets. Parameters such as incident beam energy geometry and size of the facets were investigated with the view of minimizing unwanted proximity broadening effects. Additionally, we show that the EBID direct write method has the required flexibility, resolution, and minimized proximity deposition for creating prototype devices. The devices were used to observe facet-dependent effects induced by mechanical stress on single ZnO pillar structures.

Carrier density driven lasing dynamics in ZnO nanowires

We report on the temporal lasing dynamics of high quality ZnO nanowires using the time-resolved micro-photoluminescence technique. The temperature dependence of the lasing characteristics and of the corresponding decay constants demonstrate the formation of an electron-hole plasma to be the underlying gain mechanism in the considered temperature range from 10 K to 300 K. We found that the temperature-dependent emission onset-time ([Formula: see text]) strongly depends on the excitation power and becomes smallest in the lasing regime, with values below 5 ps. Furthermore, the observed red shift of the dominating lasing modes in time is qualitatively discussed in terms of the carrier density induced change of the refractive index dispersion after the excitation laser pulse. This theory is supported by extending an existing model for the calculation of the carrier density dependent complex refractive index for different temperatures. This model coincides with the experimental observations and reliably describes the evolution of the refractive index after the excitation laser pulse.

Defect segregation and optical emission in ZnO nano- and microwires

The spatial distribution of defect related deep band emission has been studied in zinc oxide (ZnO) nano- and microwires using depth resolved cathodoluminescence spectroscopy (DRCLS) in a hyperspectral imaging (HSI) mode within a UHV scanning electron microscope (SEM). Three sets of wires were examined that had been grown by pulsed laser deposition or vapor transport methods and ranged in diameter from 200 nm-2.7 μm. This data was analyzed by developing a 3D DRCLS simulation and using it to estimate the segregation depth and decay profile of the near surface defects. We observed different dominant defects from each growth process as well as diameter-dependent defect segregation behavior.

Influence of the Cation Ratio on Optical and Electrical Properties of Amorphous Zinc-Tin-Oxide Thin Films Grown by Pulsed Laser Deposition

Continuous composition spread (CCS) methods allow fast and economic exploration of composition dependent properties of multielement compounds. Here, a CCS method was applied for room temperature pulsed laser deposition (PLD) of amorphous zinc-tin-oxide to gain detailed insight into the influence of the zinc-to-tin cation ratio on optical and electrical properties of this ternary compound. Our CCS approach for a large-area offset PLD process utilizes a segmented target and thus makes target exchange or movable masks in the PLD chamber obsolete. Cation concentrations of 0.08-0.82 Zn/(Zn + Sn) were achieved across single 50 × 50 mm(2) glass substrates. The electrical conductivity increases for increasing tin content, and the absorption edge shifts to lower energies. The free carrier concentration can be tuned from 10(20) to 10(16) cm(-3) by variation of the cation ratio from 0.1 to 0.5 Zn/(Zn + Sn).

Raman Tensor Formalism for Optically Anisotropic Crystals

We present a formalism for calculating the Raman scattering intensity dependent on the polarization configuration for optically anisotropic crystals. It can be applied to crystals of arbitrary orientation and crystal symmetry measured in normal incidence backscattering geometry. The classical Raman tensor formalism cannot be used for optically anisotropic materials due to birefringence causing the polarization within the crystal to be depth dependent. We show that in the limit of averaging over a sufficiently large scattering depth, the observed Raman intensities converge and can be described by an effective Raman tensor given here. Full agreement with experimental results for uniaxial and biaxial crystals is demonstrated.

Room-temperature Domain-epitaxy of Copper Iodide Thin Films for Transparent CuI/ZnO Heterojunctions with High Rectification Ratios Larger than 10(9)

CuI is a p-type transparent conductive semiconductor with unique optoelectronic properties, including wide band gap (3.1 eV), high hole mobility (>40 cm(2)V(-1)s(-1) in bulk), and large room-temperature exciton binding energy (62 meV). The difficulty in epitaxy of CuI is the main obstacle for its application in advanced solid-state electronic devices. Herein, room-temperature heteroepitaxial growth of CuI on various substrates with well-defined in-plane epitaxial relations is realized by reactive sputtering technique. In such heteroepitaxial growth the formation of rotation domains is observed and hereby systematically investigated in accordance with existing theoretical study of domain-epitaxy. The controllable epitaxy of CuI thin films allows for the combination of p-type CuI with suitable n-type semiconductors with the purpose to fabricate epitaxial thin film heterojunctions. Such heterostructures have superior properties to structures without or with weakly ordered in-plane orientation. The obtained epitaxial thin film heterojunction of p-CuI(111)/n-ZnO(00.1) exhibits a high rectification up to 2 × 10(9) (± 2 V), a 100-fold improvement compared to diodes with disordered interfaces. Also a low saturation current density down to 5 × 10(-9)Acm(-2) is formed. These results prove the great potential of epitaxial CuI as a promising p-type optoelectronic material.

Correlation of High Magnetoelectric Coupling with Oxygen Vacancy Superstructure in Epitaxial Multiferroic BaTiO₃-BiFeO₃ Composite Thin Films

Epitaxial multiferroic BaTiO₃-BiFeO₃ composite thin films exhibit a correlation between the magnetoelectric (ME) voltage coefficient αME and the oxygen partial pressure during growth. The ME coefficient αME reaches high values up to 43 V/(cm·Oe) at 300 K and at 0.25 mbar oxygen growth pressure. The temperature dependence of αME of the composite films is opposite that of recently-reported BaTiO₃-BiFeO₃ superlattices, indicating that strain-mediated ME coupling alone cannot explain its origin. Probably, charge-mediated ME coupling may play a role in the composite films. Furthermore, the chemically-homogeneous composite films show an oxygen vacancy superstructure, which arises from vacancy ordering on the {111} planes of the pseudocubic BaTiO₃-type structure. This work contributes to the understanding of magnetoelectric coupling as a complex and sensitive interplay of chemical, structural and geometrical issues of the BaTiO₃-BiFeO₃ composite system and, thus, paves the way to practical exploitation of magnetoelectric composites.

Properties of Schottky Barrier Diodes on (In(x)Ga(1-x))₂O₃ for 0.01 ≤ x ≤ 0.85 Determined by a Combinatorial Approach

We investigated properties of an (In(x)Ga(1-x))2O3 thin film with laterally varying cation composition that was realized by a large-area offset pulsed laser deposition approach. Within a two inch diameter thin film, the composition varies between 0.01 ≤ x ≤ 0.85, and three crystallographic phases (cubic, hexagonal, and monoclinic) were identified. We observed a correlation between characteristic parameters of Schottky barrier diodes fabricated on the thin film and its chemical and structural material properties. The highest Schottky barriers and rectification of the diodes were found for low indium contents. The thermal stability of the diodes is also best for Ga-rich parts of the sample. Conversely, the series resistance is lowest for large In content. Overall, the (In(x)Ga(1-x))2O3 alloy is well-suited for potential applications such as solar-blind photodetectors with a tunable absorption edge.

Study of the negative magneto-resistance of single proton-implanted lithium-doped ZnO microwires

The magneto-transport properties of single proton-implanted ZnO and of Li(7%)-doped ZnO microwires have been studied. The as-grown microwires were highly insulating and not magnetic. After proton implantation the Li(7%) doped ZnO microwires showed a non-monotonous behavior of the negative magneto-resistance (MR) at temperature above 150 K. This is in contrast to the monotonous NMR observed below 50 K for proton-implanted ZnO. The observed difference in the transport properties of the wires is related to the amount of stable Zn vacancies created at the near surface region by the proton implantation and Li doping. The magnetic field dependence of the resistance might be explained by the formation of a magnetic/non-magnetic heterostructure in the wire after proton implantation.

Inhibition and enhancement of the spontaneous emission of quantum dots in micropillar cavities with radial-distributed Bragg reflectors

We present a micropillar cavity where nondesired radial emission is inhibited. The photonic confinement in such a structure is improved by implementation of an additional concentric radial-distributed Bragg reflector. Such a reflector increases the reflectivity in all directions perpendicular to the micropillar axis from a typical value of 15-31% to above 98%. An inhibition of the spontaneous emission of off-resonant excitonic states of quantum dots embedded in the microcavity is revealed by time-resolved experiments. It proves a decreased density of photonic states related to unwanted radial leakage of photons out of the micropillar. For on-resonance conditions, we find that the dot emission rate is increased, evidencing the Purcell enhancement of spontaneous emission. The proposed design can increase the efficiency of single-photon sources and bring to micropillar cavities the functionalities based on lengthened decay times.

Interface recombination current in type II heterostructure bipolar diodes

Wide-gap semiconductors are often unipolar and can form type II bipolar heterostructures with large band discontinuities. We present such diodes with very high rectification larger than 1 × 10(10). The current is assumed to be entirely due to interface recombination. We derive the ideality factor for both symmetric and asymmetric diodes and find it close to 2 in agreement with experimental data from NiO/ZnO and CuI/ZnO type II diodes. The comparison with experimental results shows that the actual interface recombination rate is orders of magnitude smaller than its possible maximum value.

(55)Mn pulsed ENDOR spectroscopy of Mn(2+) ions in ZnO thin films and single crystal

(55)Mn pulsed electron nuclear double resonance (ENDOR) experiments were performed at X-band on high spin S=5/2 Mn(2+) ions incorporated at zinc lattice sites in heteroepitaxial ZnO thin films. The films have been prepared by pulsed laser deposition and the manganese ions were doped during the growth process. We examine how the c/a lattice axes ratio of the ZnO films influences the (55)Mn hyperfine (hf) and nuclear quadrupole (nq) coupling parameters of the Mn(2+) probe ions. The results are compared with those obtained for Mn(2+) ions present as impurities in ZnO single crystals and revealed that the (55)Mn nq coupling monitors sensitively the structural distortions in the bonding environment of the Mn(2+) ions. The experiments provided the full axially symmetric (55)Mn hf and nq interaction tensors. The latter is found to be very sensitive to small axial distortions of the MnO4 tetrahedrons. In particular, the (55)Mn pulsed ENDOR spectra of the ZnO:Mn thin films are strongly subjected to strain effects in the nq coupling parameter indicating a variation of the local structural parameters for the heteroepitaxial films. In the analysis of the (55)Mn pulsed ENDOR spectra the axial and cubic zero field splitting of the Mn(2+) ions was taken into account and intensity effects in the ENDOR spectra due to the zero field splitting effects were discussed.

Amorphous zinc-tin oxide thin films fabricated by pulsed laser deposition at room temperature

For a cost-efficient fabrication of homogeneous oxide thin films the usage of amorphous materials is favorable. They can be deposited at room temperature (RT) and represent an interesting alternative to amorphous silicon in electronics. Zinc-tin oxide is a promising n-type channel material for thin film transistors and consists of abundant elements, only, in contrast to the well-explored indium gallium zinc oxide. Here, the electrical and optical properties of room temperature deposited ZTO thin films are discussed. These films were fabricated via pulsed-laser deposition on glass substrates by ablating a ceramic target composed of ZnO and SnO2 in a 1:2 ratio. The resistivity has been controlled over seven orders of magnitude via the oxygen growth pressure. Further, the optical transmittance tends to be higher for higher oxygen growth pressures.