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Bridging the gap between surface physics and photonics

Use and performance criteria of photonic devices increase in various application areas such as information and communication, lighting, and photovoltaics. In many current and future photonic devices, surfaces of a semiconductor crystal are a weak part causing significant photo-electric losses and malfunctions in applications. These surface challenges, many of which arise from material defects at semiconductor surfaces, include signal attenuation in waveguides, light absorption in light emitting diodes, non-radiative recombination of carriers in solar cells, leakage (dark) current of photodiodes, and light reflection at solar cell interfaces for instance. To reduce harmful surface effects, the optical and electrical passivation of devices has been developed for several decades, especially with the methods of semiconductor technology. Because atomic scale control and knowledge of surface-related phenomena have become relevant to increase the performance of different devices, it might be useful to enhance the bridging of surface physics to photonics. Toward that target, we review some evolving research subjects with open questions and possible solutions, which hopefully provide example connecting points between photonic device passivation and surface physics. One question is related to the properties of the wet chemically cleaned semiconductor surfaces which are typically utilized in device manufacturing processes, but which appear to be different from crystalline surfaces studied in ultrahigh vacuum by physicists. In devices, a defective semiconductor surface often lies at an embedded interface formed by a thin metal or insulator film grown on the semiconductor crystal, which makes the measurements of its atomic and electronic structures difficult. To understand these interface properties, it is essential to combine quantum mechanical simulation methods. This review also covers metal-semiconductor interfaces which are included in most photonic devices to transmit electric carriers to the semiconductor structure. Low-resistive and passivated contacts with an ultrathin tunneling barrier are an emergent solution to control electrical losses in photonic devices.

Decreasing Interface Defect Densities via Silicon Oxide Passivation at Temperatures Below 450 °C

Low-temperature (LT) passivation methods (<450 °C) for decreasing defect densities in the material combination of silica (SiO_x) and silicon (Si) are relevant to develop diverse technologies (e.g., electronics, photonics, medicine), where defects of SiO_x/Si cause losses and malfunctions. Many device structures contain the SiO_x/Si interface(s), of which defect densities cannot be decreased by the traditional, beneficial high temperature treatment (>700 °C). Therefore, the LT passivation of SiO_x/Si has long been a research topic to improve application performance. Here, we demonstrate that an LT (<450 °C) ultrahigh-vacuum (UHV) treatment is a potential method that can be combined with current state-of-the-art processes in a scalable way, to decrease the defect densities at the SiO_x/Si interfaces. The studied LT-UHV approach includes a combination of wet chemistry followed by UHV-based heating and preoxidation of silicon surfaces. The controlled oxidation during the LT-UHV treatment is found to provide an until now unreported crystalline Si oxide phase. This crystalline SiO_x phase can explain the observed decrease in the defect density by half. Furthermore, the LT-UHV treatment can be applied in a complementary, post-treatment way to ready components to decrease electrical losses. The LT-UHV treatment has been found to decrease the detector leakage current by a factor of 2.

Direct Epitaxial Growth of Polar (1 - x)HfO2-(x)ZrO2 Ultrathin Films on Silicon

Ultrathin Hf_1-x Zr _x O₂ films have attracted tremendous interest since they show ferroelectric behavior at the nanoscale, where other ferroelectrics fail to stabilize the polar state. Their promise to revolutionize the electronics landscape comes from the well-known Si compatibility of HfO₂ and ZrO₂, which (in amorphous form) are already used as gate oxides in MOSFETs. However, the recently discovered crystalline ferroelectric phases of hafnia-based films have been grown on Si only in polycrystalline form. Better ferroelectric properties and improved quality of the interfaces have been achieved in epitaxially grown films, but these are only obtained on non-Si and buffered Si(100) substrates. Here, we report direct epitaxy of polar Hf_1-x Zr _x O₂ phases on Si, enabled via in situ scavenging of the native a-SiO _x layer by Zr (Hf), using pulsed laser deposition under ballistic deposition conditions. We investigate the effect of substrate orientation and film composition to provide fundamental insights into the conditions that lead to the preferential stabilization of polar phases, namely, the rhombohedral (r-) and the orthorhombic (o-) phases, against the nonpolar monoclinic (m-), on Si.

Oxidation-Induced Changes in the ALD-Al2O3/InAs(100) Interface and Control of the Changes for Device Processing

InAs crystals are emerging materials for various devices like radio frequency transistors and infrared sensors. Control of oxidation-induced changes is essential for decreasing amounts of the harmful InAs surface (or interface) defects because it is hard to avoid the energetically favored oxidation of InAs surface parts in device processing. We have characterized atomic-layer-deposition (ALD) grown Al₂O₃/InAs interfaces, preoxidized differently, with synchrotron hard X-ray photoelectron spectroscopy (HAXPES), low-energy electron diffraction, scanning tunneling microscopy, and time-of-flight elastic recoil detection analysis. The chemical environment and core-level shifts are clarified for well-embedded InAs interfaces (12 nm Al₂O₃) to avoid, in particular, effects of a significant potential change at the vacuum-solid interface. High-resolution As 3d spectra reveal that the Al₂O₃/InAs interface, which was sputter-cleaned before ALD, includes +1.0 eV shift, whereas As 3d of the preoxidized (3 × 1)-O interface exhibits a shift of -0.51 eV. The measurements also indicate that an As₂O₃ type structure is not crucial in controlling defect densities. Regarding In 4d measurements, the sputtered InAs interface includes only a +0.29 eV shift, while the In 4d shift around -0.3 eV is found to be inherent for the crystalline oxidized interfaces. Thus, the negative shifts, which have been usually associated with dangling bonds, are not necessarily an indication of such point defects as previously expected. In contrast, the negative shifts can arise from bonding with O atoms. Therefore, specific care should be directed in determining the bulk-component positions in photoelectron studies. Finally, we present an approach to transfer the InAs oxidation results to a device process of high electron mobility transistors (HEMT) using an As-rich III-V surface and In deposition. The approach is found to decrease a gate leakage current of HEMT without losing the gate controllability.

Ultrathin silicon oxynitride layer on GaN for dangling-bond-free GaN/insulator interface

Despite the scientific and technological importance of removing interface dangling bonds, even an ideal model of a dangling-bond-free interface between GaN and an insulator has not been known. The formation of an atomically thin ordered buffer layer between crystalline GaN and amorphous SiO₂ would be a key to synthesize a dangling-bond-free GaN/SiO₂ interface. Here, we predict that a silicon oxynitride (Si₄O₅N₃) layer can epitaxially grow on a GaN(0001) surface without creating dangling bonds at the interface. Our ab initio calculations show that the GaN/Si₄O₅N₃ structure is more stable than silicon-oxide-terminated GaN(0001) surfaces. The electronic properties of the GaN/Si₄O₅N₃ structure can be tuned by modifying the chemical components near the interface. We also propose a possible approach to experimentally synthesize the GaN/Si₄O₅N₃ structure.

The impact of thickness and thermal annealing on refractive index for aluminum oxide thin films deposited by atomic layer deposition

The aluminum oxide (Al2O3) thin films with various thicknesses under 50 nm were deposited by atomic layer deposition (ALD) on silicon substrate. The surface topography investigated by atomic force microscopy (AFM) revealed that the samples were smooth and crack-free. The ellipsometric spectra of Al2O3 thin films were measured and analyzed before and after annealing in nitrogen condition in the wavelength range from 250 to 1,000 nm, respectively. The refractive index of Al2O3 thin films was described by Cauchy model and the ellipsometric spectra data were fitted to a five-medium model consisting of Si substrate/SiO2 layer/Al2O3 layer/surface roughness/air ambient structure. It is found that the refractive index of Al2O3 thin films decrease with increasing film thickness and the changing trend revised after annealing. The phenomenon is believed to arise from the mechanical stress in ALD-Al2O3 thin films. A thickness transition is also found by transmission electron microscopy (TEM) and SE after 900°C annealing.

Stress inversion from initial tensile to compressive side during ultrathin oxide growth of the Si(100) surface

We report the real-time observation of the stress change during sub-nanometer oxide growth on the Si(100) surface. Oxidation initially induced a rapid buildup of tensile stress up to -1.9 × 10(8) N m(-2) with an oxide thickness of 0.25 nm, followed by gradual compensation by a compressive stress. The compressive stress saturated at 5 × 10(7) N m(-2) for an oxide thickness of 1.2 nm. The analysis, assisted by theoretical study, indicates that the observed initial tensile stress is caused by oxygen bridge-bonding between the Si dimers. Atomistic model calculations considering mutually orthogonal orientations of the Si(100) surface structure reproduce the stress inversion from the tensile to the compressive side.

Crystalline-vitreous interface in two dimensional silica

The interface between a crystalline and a vitreous phase of a thin metal supported silica film was studied by low temperature scanning tunneling microscopy. The locally resolved evolution of Si-Si nearest neighbor distances and characteristic angles was evaluated across the border. Furthermore, we investigated the behavior of the ring size distribution close to the crystalline-vitreous transition. The crystalline order was found to decay gradually within about 1.6 nm into the vitreous state.

Structural and Interfacial Characteristics of thin (<10 nm) SiO2 Films Grown by Electron Cyclotron Resonance Plasma Oxidation on [100] Si Substrates

The feasibility of fabricating ultra-thin SiO2 films on the order of a few nanometer thickness has been demonstrated. SiO2 thin films of approximately 7 nm thickness have been produced by ion flux-controlled Electron Cyclotron Resonance plasma oxidation at low temperature on [100] Si substrates, in reproducible fashion. Electrical measurements of these films indicate that they have characteristics comparable to those of thermally grown oxides. The thickness of the films was determined by ellipsometry, and further confirmed by crosssectional High-Resolution Transmission Electron Microscopy. Comparison between the ECR and the thermal oxide films shows that the ECR films are uniform and continuous over at least a few microns in lateral direction, similar to the thermal oxide films grown at comparable thickness. In addition, HRTEM images reveal a thin (1–1.5 nm) crystalline interfacial layer between the ECR film and the [100] substrate. Thinner oxide films of approximately 5 nm thickness have also been attemped, but so far have resulted in nonuniform coverage. Reproducibility at this thickness is difficult to achieve.

Atomic-Scale Structure of the Si-SiO2 and SiC-SiO2 Interfaces and the Origin of Their Contrasting Properties

One of the reasons for the dominance of Si in microelectronics is the quality of the Si-SiO2 interface. In contrast, development of SiC-based MOSFETs for power applications is hampered primarily by poor carrier mobility at the SiC-SiO2 interface. Here we review recent calculations that elucidate the reasons of the contrasting properties of the two interfaces. In the case of Si, the interface energy is in fact lower when the interface is abrupt and smooth because of the intrinisic geometry of the Si (001) surface and the softness of the Si-O-Si angle. However, two energe ically degenerate phases are possible, leading to domain boundaries, that are the cause of subcxide bonds, steps, and dangling bonds. In principle, these effects may be avoidable by low-temperature deposition. In contrast, the geometry and bond lengths of SiC surfaces are not suitable for abrupt and smooth interfaces, requiring the existence of a nonstoichiometric interlayer that may be the cause of the reduced mobility.

Quantitative strain analysis of surfaces and interfaces using extremely asymmetric x-ray diffraction

Strain can reduce carrier mobility and the reliability of electronic devices and affect the growth mode of thin films and the stability of nanometer-scale crystals. To control lattice strain, a technique for measuring the minute lattice strain at surfaces and interfaces is needed. Recently, an extremely asymmetric x-ray diffraction method has been developed for this purpose. By employing Darwin's dynamical x-ray diffraction theory, quantitative evaluation of strain at surfaces and interfaces becomes possible. In this paper, we review our quantitative strain analysis studies on native SiO(2)/Si interfaces, reconstructed Si surfaces, Ni/Si(111)-H interfaces, sputtered III-V compound semiconductor surfaces, high-k/Si interfaces, and Au ion-implanted Si.

Silicon nanowire oxidation: the influence of sidewall structure and gold distribution

The oxidation behavior of Si nanowires (SiNWs) grown by the gold (Au) catalyzed vapor-liquid-solid (VLS) growth process in an electron beam evaporation (EBE) reactor is studied. The VLS SiNWs exhibit hexagonal shape with essentially {112} facets where each facet shows a saw-tooth faceting itself, consisting of alternating {111} and {113} facets. Depending on growth temperatures (450-750 degrees C) and evaporation currents (40-80 mA) that determine the silicon vapor supply, this facet formation is more or less pronounced. The diffusion of Au atoms on the faceted SiNW surfaces and the formation of Au nanoparticles on the SiNW facets during growth and during ex situ annealing are studied. Upon diffusion, the Au atoms agglomerate and form Au nanoparticles that preferably arrange themselves on {113} facets. Upon annealing in air at temperatures between 800 and 950 degrees C the gold nanoparticles agglomerate further and form bigger particles of a few tens of nm in diameter that reside at the interface between the growing silica (SiO(2)) layer and the SiNW itself, which in turn shrinks during SiNW oxidation. The oxide layer thickness and the oxide appearance depend on the annealing conditions (time and temperature) and systematically varied oxidation processing is described in this paper as investigated by cross-sectional transmission electron microscopy (TEM) including high resolution studies as well as scanning electron microscopy (SEM) studies. Our results strongly suggest that the SiNWs can be fully oxidized, thus forming silica NWs that can either keep their initial shape or, under certain annealing conditions, do not keep their initial wire shape but assume a bamboo-like shape that forms most likely as a result of locally high stresses that are related to nanocrack formation. The nanocracks form in the growing oxide layer mediated by the presence of Au nanoparticles that yield gold-enhanced SiNW oxidation and thus a faster oxidation rate.

Ambient-stable blue luminescent silicon nanocrystals prepared by nanosecond-pulsed laser ablation in water

Even with intensive research, air-stable blue light emission from silicon nanocrystals (Si-ncs) at room temperature still remains a challenge. We show that stable and blue-luminescent Si-ncs can be produced by laser-generated plasma (nanosecond-pulsed excimer laser) confined in water. These Si-ncs exhibit quantum confinement effect due to their size and are produced with an environmentally compatible process. The effect of aging for several weeks in water and air on blue Si-ncs emission properties is compared. The oxide shell around the nanocrystalline core formed during laser processing in water offers the required conditions for the confinement of excitons that allow for stable (in either air or water) blue photoluminescence at room temperature.

Burning match oxidation process of silicon nanowires screened at the atomic scale

Silicon oxide nanowires hold great promise for functional nanoscale electronics. Here, we investigate the oxidation of straight, massively parallel, metallic Si nanowires. We show that the oxidation process starts at the Si NW terminations and develops like a burning match. While the spectroscopic signatures on the virgin, metallic part, are unaltered we identify four new oxidation states on the oxidized part, which show a gap opening, thus revealing the formation of a transverse internal nanojunction.

First-principles study of the electronic structures and dielectric properties of the Si/SiO(2) interface

A first-principles study of the electronic structures and dielectric properties of Si/SiO(2) interfaces is implemented. Comparing the interfaces with and without defects, we explore the relationship between the defects and the dielectric properties, and also discuss the effect of the defects on the leakage current between the gate electrode and silicon substrate. We found that the electrons around the Fermi level percolate into the SiO(2) layers, which reduces the effective oxide thickness and is expected to enhance the leakage current. The dangling bonds largely affect the dielectric properties of the interface and the termination of dangling bonds by hydrogen atoms is successful in suppressing the increase of the dielectric constant.

Resonant coupling between surface vibrations and electronic states in silicon nanocrystals at the strong confinement regime

A striking correlation between infrared photoinduced absorption spectra and the photoluminescence from silicon nanocrystals indicates that quantized electronic sublevels of the nanocrystals are resonantly coupled to surface vibrational modes via a polarization field produced by coherent longitudinal polar vibrations. Our experimental results and model support the assumption that the mechanism responsible for the efficient photoluminescence from silicon nanocrystals should be assigned to inhibition of nonradiative channels rather than enhancement of radiative channels.

Microscopic dynamics of silicon oxidation

We study the silicon oxidation process and the dynamic structure of the SiO2-Si (001) interface using a grand canonical Monte Carlo approach. We find that Si-O-Si bridge bonds are the main building blocks of the advancing interface, and we identify a kinetic pathway that continually creates new bridge bonds. Oxidation proceeds by local events, with little evidence of "step flow" in the simulation. Yet the interface remains remarkably smooth and abrupt as it advances.

Analysis of high resolution transmission electron microscope images of crystalline-amorphous interfaces

For the analysis of images of homogeneous crystalline-amorphous interfaces we propose to average them along the interface obtaining the averaged interface image or the averaged intensity profile. Due to averaging, contrast components with the periodicity of the crystalline area of the image are extracted. Thus, the contrast features originating from the random overlap of the projected potentials of atoms in the amorphous layer are suppressed. It is shown that averaged images can be simulated by the multi-slice method using the novel approach to model the near interfacial amorphous structure by its mean atomic density distribution in front of the crystalline boundary. The crystalline structure is represented by its known atomic positions. We apply the proposed method to the investigation of the near interfacial short-range order in the c-Si/ a-Ge crystalline-amorphous interface.

Electronic properties of the Si/SiO2 interface from first principles

Unoccupied oxygen p-projected densities of states, calculated from first principles in a model Si/SiO(2) interface, are found to reproduce trends in recent atomic resolution electron energy-loss spectra [D. A. Muller et al., Nature (London) 399, 758 (1999)]. The shape of the unoccupied states and the magnitude of the local energy gap are explicitly related to the number of O second neighbors of a given oxygen atom. The calculated local energy gaps of the oxide become considerably smaller within 0.5 nm of the interface, suggesting that the electronic properties do not change abruptly at the interface.

Structure and energetics of the Si- SiO2 interface

Using a Monte Carlo approach, we identify low-energy structures for the (001)-oriented Si-SiO2 interface. The optimal interface structure found consists of an ordered array of Si-O-Si "bridges," with low strain energy. This structure explains several puzzling experimental observations.

Bonding arrangements at the Si-SiO2 and SiC-SiO2 interfaces and a possible origin of their contrasting properties

We report ab initio calculations designed to explore the relative energetics of different interface bonding structures. We find that, for Si (001), abrupt (no suboxide layer) interfaces generally have lower energy because of the surface geometry and the softness of the Si-O-Si angle. However, two energetically degenerate phases are possible at the nominal interface layer, so that a mix of the two is the likely source of the observed suboxide and dangling bonds. In principle, these effects may be avoidable by low-temperature deposition. In contrast, the topology and geometry of SiC surfaces is not suitable for abrupt interfaces.

Exact separation of surface and bulk contributions to anisotropic second-harmonic generation from cubic centrosymmetric media

We solve this long-standing problem theoretically by recognizing that the ranks of the tensors that describe the surface and bulk second-order nonlinear susceptibilities differ in this class of media. We show that this implies that the phenomenologies of the anisotropic optical second-harmonic (SH) responses of the two sources differ for all possible crystal facial orientations except (111). To demonstrate the result, we apply the theory to separate the surface and bulk contributions to SH generation from an oxidized vicinal Si(001) wafer for p-polarized 775-nm fundamental and s-polarized SH radiation. It is shown that knowledge of the phase of the SH field is necessary to achieve a unique separation.