GaAs Films Research Articles

Structure Analysis Using Time-of-Flight Momentum Microscopy with Hard X-rays: Status and Prospects

X-ray photoelectron diffraction (XPD) has developed into a powerful technique for the structural analysis of solids. Extension of the technique into the hard-X-ray range (hXPD) gives access to true bulk information. Here we give a status report on hXPD experiments using a novel full-field imaging technique: Time-of-flight momentum microscopy (ToF-MM). A special variant of ToF-MM is capable of recording high kinetic energies (up to >7keV) and enlarged k-fields-of-view. We present applications that are specific for high kinetic energies. The strong site specificity of hXPD is exemplified for NbSe2, the cubic-to-tetragonal transition in SrTiO3 and the zinc-blende structure in epitaxial GaAs films. Bloch-wave calculations show a very good agreement with experiment and reveal fingerprint-like signatures of emitter sites in host lattices. We show a dopant-site analysis in two semiconductors (Mn in GaAs and Te in Si). Hard-X-ray ARPES plus core-level hXPD enable eliminating the strong diffraction signature imprinted in HARPES maps.

Defect seeded remote epitaxy of GaAs films on graphene

Remote epitaxy is an emerging materials synthesis technique which employs a 2D interface layer, often graphene, to enable the epitaxial deposition of low defect single crystal films while restricting bonding between the growth layer and the underlying substrate. This allows for the subsequent release of the epitaxial film for integration with other systems and reuse of growth substrates. This approach is applicable to material systems with an ionic component to their bonding, making it notably appealing for III–V alloys, which are a technologically important family of materials. Chemical vapour deposition growth of graphene and wet transfer to a III–V substrate with a polymer handle is a potentially scalable and low cost approach to producing the required growth surface for remote epitaxy of these materials, however, the presence of water promotes the formation of a III–V oxide layer, which degrades the quality of subsequently grown epitaxial films. This work demonstrates the use of an argon ion beam for the controlled introduction of defects in a monolayer graphene interface layer to enable the growth of a single crystal GaAs film by molecular beam epitaxy, despite the presence of a native oxide at the substrate/graphene interface. A hybrid mechanism of defect seeded lateral overgrowth with remote epitaxy contributing the coalescence of the film is indicated. The exfoliation of the GaAs films reveals the presence of defect seeded nucleation sites, highlighting the need to balance the benefits of defect seeding on crystal quality against the requirement for subsequent exfoliation of the film, for future large area development of this approach.

From Salt to Electronics: Heteroepitaxy and GaAs Solar Cells

AbstractThis paper presents work on the heteroepitaxy of salts, specifically fluorides, on semiconductors and heteroepitaxy of semiconductors on salts. Fluorides layers are deposited on commercial Gallium Arsenide (GaAs) wafers followed by the heteroepitaxial growth of GaAs using metal‐organic chemical vapor deposition (MOCVD). The fluoride layers consist of 2 lattice‐engineered layers of alkaline‐earth compounds to match with GaAs, and are used to sandwich another alkaline‐earth compound with higher water‐solubility as a sacrificial layer. The triple fluoride layers enable liftoff of free‐standing semiconductor films which can be further transferred to desirable substrates. 2D‐X‐ray Diffraction (2D‐XRD) measurements confirm epitaxial growth of both the fluorides and the subsequently grown GaAs films. Single junction (SJ) solar cell devices based on thus prepared films show a power conversion efficiency (PCE) of 10.3% under 1 sun illumination. After the completion of device fabrications, the GaAs film is lifted off from the substrate by a novel water‐assisted epitaxial liftoff (H2O‐ELO) technique and transferred to a cheaper substrate. The original GaAs wafer is recycled and reused twice. Devices based on reused substrates show no significant degradation in performance. The semiconductor‐salt‐semiconductor scheme has great implications in high‐performance, flexible, and large‐area electronics.

Significant improvement of conversion efficiency by passivation of low-angle grain boundaries in flexible low-cost single-crystal-like GaAs thin-film solar cells directly deposited on metal tape

We develop an easily implementable yet effective process to achieve high power conversion efficiency in newly-developed flexible single-crystal-like GaAs thin-film photovoltaic solar cells on cheap metal tape. We study various techniques on the fabricated devices to passivate the defect states associated with dangling bonds at low-angle grain boundaries (LA-GBs) existing in the single-crystal-like GaAs material. We report that sulfur by trioctylphosphine sulfide (TOP:S) solution treatment can partially but effectively passivate the defect states at the LA-GBs to increase all the solar cell performance parameters of open-circuit voltage VOC (33.2%), short-circuit current JSC (19.5%), and fill factor FF (25.8%), resulting in the improvement of the power conversion efficiency more than double. The material and device characterizations and numerical modeling are performed to propose a mechanism, also suggesting that the power conversion efficiency higher than 20%, which is an important target milestone in emerging solar cell technologies, can be achieved. This work presents the first effective passivation of the bulk defects in GaAs at the device level and can also offer a solution for other photonic and electronic devices using non-single-crystalline GaAs films for flexible electronics.

Efficient ion-slicing of 4-inch GaAs thin film for Si-based hetero-integration with ultra-smooth surface

Heterogeneous integration of single-crystalline GaAs thin film on a Si substrate provides a promising material platform for Si-based optoelectronic integration. In this work, based on the clarified splitting mechanism of GaAs, the ion implantation conditions for GaAs film transfer were optimized. It was found that the co-implantation of He and H ions is more efficient in exfoliating the GaAs thin film with a lower thermal budget and lower density of defects in comparison with the case of the single He/H ion implantation. With the Al2O3 as the bonding intermediate layer, a 4-inch GaAs film was successfully transferred onto the Si (100) substrate via the optimized ion-slicing technique. The surface treatments, including chemical mechanical polishing, ozone oxidation, and KOH cleaning, were explored to improve the surface quality of the as-transferred GaAs thin film to the level of epi-ready. After post-annealing at 400°C for 1 h, the quality of the transferred GaAs thin film was further improved with only 89.03 arcsec for the full width at half maximum of the X-ray rocking curve.

Basics of surface reconstruction during selective area metalorganic chemical vapour-phase epitaxy of GaAs films in the stripe-type ultra-wide window

The evolution of the surface morphology and growth steps of the GaAs film during selective area epitaxy has been studied. It has been shown that during the selective area epitaxy three regions with different growth mode and the atomic-height step orientation can be formed. A correlation between the local misorientation and the specific features of these areas has been identified. The layer surface’ local misorientation angle causes the transition between growth modes. A simulation model of the change in the growth steps’ front in the window area is proposed. It is shown that the local rotation angle of the growth steps’ front is governed by the growth rate change across the window. It was experimentally and theoretically shown that a small change in the orientation of the growth steps in the central area of the window and a rotation of the front by nearly 90°for the outermost window areas are specific features due to the symmetric mask, whereas the size of the central area decreases sharply with increasing layer thickness.

Ultrathin Ge epilayers on Si produced by low-temperature PECVD acting as virtual substrates for III-V / c-Si tandem solar cells

Ultrathin (20 nm) epitaxial films of germanium are deposited on crystalline silicon wafers, to act as virtual substrates for the growth of III-V materials, opening a low-cost approach to tandem solar cells. Such ultrathin layers allow for material cost reduction, along with the possibility of using the silicon wafer as the bottom cell in tandem devices. A simple plasma-enhanced chemical vapor deposition (PECVD) process at 175 °C has been optimized to deposit these heteroepitaxial germanium films, which grow directly on the silicon wafers without any intermediate silicon-germanium alloy. Thanks to an in-situ plasma cleaning step prior to Ge epitaxy, the films can sustain high-temperature annealing in vacuum (up to 800 °C) without any delamination. The suitability of the germanium heteroepitaxial films as virtual substrates is analyzed by depositing III-V layers on them by conventional growth methods like chemical beam epitaxy (CBE) and metalorganic chemical vapor deposition (MOCVD). The properties of the GaAs films deposited on the virtual substrates are comparable in terms of roughness, microstructure, and crystallinity to these of the III-V layers co-deposited on c-Ge wafers, pointing at the effectiveness of the ultrathin c-Ge epitaxial layers to act as virtual substrates for III-V epitaxial growth. Moreover, growing the c-Ge layers on c-Si substrates with 5° miscut avoids the formation of antiphase domains. These substrates are finally used to demonstrate proof of concept tandem solar cells, proving the suitability of our low temperature and ultrathin virtual substrate approach.

Selective area epitaxy of GaAs films using patterned graphene on Ge

We demonstrate selective area epitaxy of GaAs films using patterned graphene masks on a Ge (001) substrate. GaAs selectively grows on exposed regions of the Ge substrate for graphene stripe widths of 10 μm. The selectivity is highly dependent on the growth temperature and annealing time, which we explain in terms of temperature dependent sticking coefficients and surface diffusion. The high nucleation selectivity over several micrometers sets constraints on experimental realizations of remote epitaxy.

Indentation behaviour of (011) thin films of III–V semiconductors: polarity effect differences between GaAs and InP

Abstract (011) thin films of GaAs and InP (220 and 250 μm thick, respectively) were deformed by a Vickers indenter at elevated temperatures (350 – 400 °C) under loads in the range 0.4 – 2.9 N. For loads higher than 1.9 N, symmetric punching through InP was detected at the opposite face while it was asymmetric through GaAs. In InP, when the punching mechanism took place, pile-up around the indent site progressively disappeared. Moreover, the measured plastic volumes reveal that plastic flow throughout the specimens is volume conservative, implying that pile-up and punching events are related. In GaAs, the anisotropic pile-up plasticity was studied using transmission electron microscopy. Models describing the material flow through the samples and reverse flow of matter are proposed.

Boosting photoelectric performance of thin film GaAs solar cell based on multi-objective optimization for solar energy utilization

For enhancing the photoelectric performance of the GaAs photovoltaic cell, the non-dominated sorted genetic algorithm-II (NSGA-II) is employed to solve the multi-objective optimization problem, which consists of the thicknesses of active layer (GaAs) and three anti-reflection (AR) films (TiO2-HfO2-SiO2). Through maximizing (minimizing) the light absorption of photons with energies higher (lower) than the bandgap in the active layer directly and reducing the cost at the same time, the Pareto optimal solution is obtained. An increment in the absorptance, short circuit current and photoelectric conversion efficiency by 50%, 43.4% and 44.9% is achieved by the optimal GaAs solar cell due to excellent AR properties obtained by NSGA-II optimization. We systematically investigate the effects of carrier lifetime, surface recombination velocity, carrier mobility as well as doping concentration on the photovoltaic parameters, and acquire the limitations of these parameters. The strong robustness of the optimal GaAs solar cell enables it to be adapted to wide-angle sunlight incident, and the probability to achieve an absorptance of 0.963 is 97% even if there is a 20% machining error.

Active control of plasmon-induced transparency based on a GaAs/Si heterojunction in the terahertz range

Active control optical terahertz modulator based on incorporating n-type GaAs film as the active media into plasmon-induced transparency metasurface, was investigated theoretically and experimentally. From the experimental results it could be found that the resonances transmission of the plasmon-induced transparency was optically modulated and the transmission modulation depth of 87% was achieved at 0.56 THz. This may be due to the fact that when the pump excitation power was tuned from 0 mW to 1000 mW, larger numbers of excited carriers were excited in the electronic “p-n" junction and diffused across the heterojunction to the n-type GaAs film. It increased the conductivity of the n-type GaAs film from 2.29 × 103 S/m to 1.56 × 104 S/m. As a results, the capacitive split gaps of the PIT split-ring resonators were short-circuited and the coupling strength between the resonators was decreased owing to the screening effect caused by the photo-excited carriers.

GaAs on Si substrate with dislocation filter layers for wafer‐scale integration

GaAs on Si grown via metalorganic chemical vapor deposition is demonstrated using various Si substrate thicknesses and three types of dislocation filter layers (DFLs). The bowing was used to measure wafer-scale characteristics. The surface morphology and electron channeling contrast imaging (ECCI) were used to analyze the material quality of GaAs films. Only 3-μm bowing was observed using the 725-μm-thick Si substrate. The bowing shows similar levels among the samples with DFLs, indicating that the Si substrate thickness mostly determines the bowing. According to the surface morphology and ECCI results, the compressive strained indium gallium arsenide/GaAs DFLs show an atomically flat surface with a root mean square value of 1.288 nm and minimum threading dislocation density (TDD) value of 2.4 × 107 cm−2. For lattice-matched DFLs, the indium gallium phosphide/GaAs DFLs are more effective in reducing the TDD than aluminum gallium arsenide/GaAs DFLs. Finally, we found that the strained DFLs can block propagate TDD effectively. The strained DFLs on the 725-μm-thick Si substrate can be used for the large-scale integration of GaAs on Si with less bowing and low TDD.

Ultra-precision trapezoidal grating mixture of photonic crystal’s effect on enhancing the optoelectronic properties of thin film GaAs solar cell

The IV semiconductor materials have a pivotal role in photovoltaic (PV) systems. Research on the optimal grating structure has attracted a lot of interest because of its potential application in light-trapping in thin film PV cells. The Photonic Crystals (PCs) with their nano-engineered feature which allowed them to obtain high efficiency solar cells in particular from a parametric aspect. By using the Rigorous Coupled Wave Analysis (RCWA) method by RSoft CAD software, a Gallium arsenide (GaAs) thin film is studied with a simple trapezoidal grating of PCs to improve optical absorption with height, width and the mixed ultra-precision grating variations taking into account the J-V solar cell characteristics. According to our simulation results and compared to the planar grating, an enhancement in the power conversion efficiency (η) of 53.72% with a one dimensional PCs grating, and a 115.36% improvement with a simple trapezoidal grating. Furthermore, by mixing the ultra-precision values of the gratings we have an enhancement of 147.25% in height variation and 151.04% in width variation, with a noticeable improvement in the width effect compared to the height that reached up to 58.22%.

Towards GaAs thin-film tracking detectors

Silicon-based tracking detectors have been used in several important applications, such as in cancer therapy using particle beams, and for the discovery of new elementary particles at the Large Hadron Collider at CERN. III-V semiconductor materials are an attractive alternative to silicon for this application, as they have some superior physical properties. They could meet the demands for fast timing detectors allowing time-of-flight measurements with ps resolution while being radiation tolerant and cost-efficient. As a material with a larger density, higher atomic number Z and much higher electron mobility than silicon, GaAs exhibits faster signal collection and a larger signal per μm of sensor thickness. In this work, we report on the fabrication of n-in-n GaAs thin-film devices intended to serve next-generation high-energy particle tracking detectors. Molecular beam epitaxy (MBE) was used to grow high-quality GaAs films with doping levels sufficiently low to achieve full depletion for detectors with an active thickness of 10 μm. The signal collection speed of the detector structures was assessed using the transient current technique (TCT). To elucidate the structural properties of the detector, Kelvin probe force microscopy (KPFM) was used, which confirmed the formation of the junction in the detector and revealed residual doping in the intrinsic layer. Our results suggest that GaAs thin films are suitable candidates to achieve thin and radiation-tolerant tracking detectors.

GaN obtained on quartz substrates through the nitridation of GaAs films deposited via CSVT

The use of gallium arsenide (GaAs) films to obtain gallium nitride (GaN) on a quartz substrate is reported herein. The GaAs films were deposited on quartz substrates via the close-spaced vapor transport technique (CSVT) with good crystalline quality. GaN was obtained via nitridation wherein the GaAs samples were subjected to ammonia and hydrogen flow at atmospheric pressure. Several experiments were conducted at 800 °C, 900 °C, and 1000 °C, suggesting that nitridation at 900 °C is suitable for GaN obtention. Another group of experiments was conducted to observe the effect of nitridation time on the morphology of GaN. The nitridation temperature was kept at 900 °C while the nitridation times were 10, 20, and 30 min. XRD and EDS characterizations revealed that the nitridation time determines the production of samples with both GaAs and GaN structures (10 min) or only the GaN structure (20 and 30 min). A GaN film exhibiting a wurtzite preferential structure was obtained subsequent to 30 min of nitridation.

Toward Unusual-High Hole Mobility of p-Channel Field-Effect-Transistors.

The relative low hole mobility of p-channel building block device challenges the continued miniaturization of modern electronic chips. Metal-semiconductor junction is always an efficient strategy to control the carrier concentration of channel semiconductor, benefiting the carrier mobility regulation of building block device. In this work, complementary metal oxide semiconductor (CMOS)-compatible metals are selected to deposit on the surface of the important p-channel building block of GaSb nanowire field-effect-transistors (NWFETs), demonstrating the efficient strategy of hole mobility enhancement by metal-semiconductor junction. When deposited with lower work function metal of Al, the peak hole mobility of GaSb NWFET can be enhanced to as high as ≈3372 cm2 V-1 s-1 , showing three times than the un-deposited one. The as-studied metal-semiconductor junction is also efficient for the hole mobility enhancement of other p-channel devices, such as GaAs NWFET, GaAs film FET, and WSe2 FET. With the enhanced mobility, the as-constructed CMOS inverter shows good invert characteristics, showing a relatively high gain of ≈18.1. All results may be regarded as important advances to the next-generation electronics.

Pd-mediated mechanical stack of III–V solar cells fabricated via hydride vapor phase epitaxy

In this study, we present the first demonstration of mechanical stacking of solar cells grown via hydride vapor phase epitaxy (HVPE) to fabricate tandem cells. Smart stack technology, in which Pd nanoparticles were used as the bonding mediator, was used for bonding the cells. To prepare the smart stack, the top cells of the tandem structure were peeled from the substrate using the epitaxial lift-off technique. For this purpose, the cell structure was grown on an AlAs sacrificial layer. GaAs solar cells with good performance were fabricated using HVPE, in which the surface roughened by AlAs was restored to flatness by growing a thick GaAs film. The GaAs//InGaAs tandem cells achieved by bonding the GaAs solar cells grown via HVPE with the InGaAs solar cells grown by metal–organic vapor phase epitaxy exhibited a conversion efficiency of 22.6%.

Plasmonic Effects of Al Nanoparticles Embedded and Non-embedded in Thin Film GaAs Solar Cells with Ta2O5 Antireflective Coating

Plasmonic Effects of Al Nanoparticles Embedded and Non-embedded in Thin Film GaAs Solar Cells with Ta2O5 Antireflective Coating

Grain size dependent photoresponsivity in GaAs films formed on glass with Ge seed layers

The strong correlation between grain size and photoresponsivity in polycrystalline GaAs films on glass was experimentally demonstrated using Ge seed layers with a wide range of grain sizes (1‒330 μm). The crystal evaluations using Raman spectroscopy, scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy revealed that 500-nm-thick GaAs films epitaxially grown from the Ge seed layers at 550 °C inherited the grain boundaries and crystal orientations in Ge. With increasing grain size, the photoresponsivity corresponding to GaAs increased from 0.01 to 3 A W−1 under a bias voltage of 0.3 V. The maximum value approached that of the GaAs film formed simultaneously on a single-crystal Ge wafer, indicating the high potential of the large-grained GaAs film. Knowledge gained from this study will be essential for designing advanced solar cells based on polycrystalline III–V compound semiconductors using inexpensive substrates.

The structural evolution of light-ion implanted into GaAs single crystal after annealing

Four groups of He+ and H+ with different dose combinations are implanted into GaAs crystal. After annealing, different phenomena of blisters and exfoliation appear on the surface. High resolution X-ray diffraction (HRXRD) is used to irradiate the surfaces of the four samples, and the diffraction satellite peaks are observed to move to the lower angles. The surface morphology of the four samples after annealing is observed by optical microscope. The depth of craters and the height of blisters are measured by atomic force microscope (AFM), and the critical pressure and internal stress of blisters are calculated. The damage microstructure induced by ion implantation after annealing is observed by transmission electron microscope (TEM), and the stress intensity factor at the crack tip is calculated. The different mechanisms of He+ and H+ in GaAs single crystal are compared. The dose combination of He+ and H+ is optimized to explore a more cost-effective way for GaAs film exfoliation.