Crystalline Gallium Arsenide Research Articles

Study on electro-optic noise in crystalline coatings toward future gravitational wave detectors

Thermal noise in high-reflectivity mirror coatings is a limiting factor in ground-based gravitational wave detectors. Reducing this coating thermal noise improves the sensitivity of detectors and enriches the scientific outcome of observing runs. Crystalline gallium arsenide and aluminum-alloyed gallium arsenide (referred to as AlGaAs) coatings are promising coating candidates for future upgrades of gravitational wave detectors because of their low coating thermal noise. However, AlGaAs-based crystalline coatings may be susceptible to an electro-optic noise induced by fluctuations in an electric field. We investigated the electro-optic effect in an AlGaAs coating by using a Fabry-Perot cavity, and concluded that the noise level is well below the sensitivity of current and planned gravitational-wave detectors.

A Novel Route to Fibers with Incongruent and Volatile Crystalline Semiconductor Cores: GaAs

A novel extension to the molten core method was employed to realize a glass-clad, crystalline gallium arsenide (GaAs) core fiber for the first time to the author’s knowledge. GaAs is of considerable interest for its optoelectronic and nonlinear properties, but decomposition and volatility, usually leading to incongruent solidification, normally preclude melt-processing under ambient conditions into an optical fiber. Presented here are results on tin as a flux to melt GaAs at a temperature where volatility is negligible, permitting the thermal drawing of long lengths (100 m) of glass-clad, crystalline GaAs-containing fibers. Laser annealing is employed to segregate the Sn and GaAs phases, both across and along the fiber. This work presents an important new tool for scalable fabrication of glass-clad crystalline core materials previously thought incompatible with molten core processes and opens the door to a wide range of novel and useful in-fiber optoelectronic devices.

Direct measurements of multi-photon induced nonlinear lattice dynamics in semiconductors via time-resolved x-ray scattering

Nonlinear optical phenomena in semiconductors present several fundamental problems in modern optics that are of great importance for the development of optoelectronic devices. In particular, the details of photo-induced lattice dynamics at early time-scales prior to carrier recombination remain poorly understood. We demonstrate the first integrated measurements of both optical and structural, material-dependent quantities while also inferring the bulk impulsive strain profile by using high spatial-resolution time-resolved x-ray scattering (TRXS) on bulk crystalline gallium arsenide. Our findings reveal distinctive laser-fluence dependent crystal lattice responses, which are not described by previous TRXS experiments or models. The initial linear expansion of the crystal upon laser excitation stagnates at a laser fluence corresponding to the saturation of the free carrier density before resuming expansion in a third regime at higher fluences where two-photon absorption becomes dominant. Our interpretations of the lattice dynamics as nonlinear optical effects are confirmed by numerical simulations and by additional measurements in an n-type semiconductor that allows higher-order nonlinear optical processes to be directly observed as modulations of x-ray diffraction lineshapes.

Direct investigation of (sub-) surface preparation artifacts in GaAs based materials by FIB sectioning

The introduction of preparation artifacts is almost inevitable when producing samples for (scanning) transmission electron microscopy ((S)TEM). These artifacts can be divided in extrinsic artifacts like damage processes and intrinsic artifacts caused by the deviations from the volume strain state in thin elastically strained material systems. The reduction and estimation of those effects is of great importance for the quantitative analysis of (S)TEM images. Thus, optimized ion beam preparation conditions are investigated for high quality samples. Therefore, the surface topology is investigated directly with atomic force microscopy (AFM) on the actual TEM samples. Additionally, the sectioning of those samples by a focused ion beam (FIB) is used to investigate the damage depth profile directly in the TEM. The AFM measurements show good quantitative agreement of sample height modulation due to strain relaxation to finite elements simulations. Strong indications of (sub-) surface damage by ion beams are observed. Their influence on high angle annular dark field (HAADF) imaging is estimated with focus on thickness determination by absolute intensity methods. Data consolidation of AFM and TEM measurements reveals a 3.5nm surface amorphization, negligible surface roughness on the scale of angstroms and a sub-surface damage profile in the range of up to 8.0nm in crystalline gallium arsenide (GaAs) and GaAs-based ternary alloys. A correction scheme for thickness evaluation of absolute HAADF intensities is proposed and applied for GaAs based materials.

Plasmonic light trapping in an ultrathin photovoltaic layer with film-coupled metamaterial structures

A film-coupled metamaterial structure is numerically investigated for enhancing the light absorption in an ultrathin photovoltaic layer of crystalline gallium arsenide (GaAs). The top subwavelength concave grating and the bottom metallic film could not only effectively trap light with the help of wave interference and magnetic resonance effects excited above the bandgap, but also practically serve as electrical contacts for photon-generated charge collection. The energy absorbed by the active layer is greatly enhanced with the help of the film-coupled metamaterial structure, resulting in significant improvement on the short-circuit current density by three times over a free-standing GaAs layer at the same thickness. The performance of the proposed light trapping structure is demonstrated to be little affected by the grating ridge width considering the geometric tolerance during fabrication. The optical absorption at oblique incidences also shows direction-insensitive behavior, which is highly desired for efficiently converting off-normal sunlight to electricity. The results would facilitate the development of next-generation ultrathin solar cells with lower cost and higher efficiency.

Wet Chemical Functionalization of III–V Semiconductor Surfaces: Alkylation of Gallium Arsenide and Gallium Nitride by a Grignard Reaction Sequence

Crystalline gallium arsenide (GaAs) (111)A and gallium nitride (GaN) (0001) surfaces have been functionalized with alkyl groups via a sequential wet chemical chlorine activation, Grignard reaction process. For GaAs(111)A, etching in HCl in diethyl ether effected both oxide removal and surface-bound Cl. X-ray photoelectron (XP) spectra demonstrated selective surface chlorination after exposure to 2 M HCl in diethyl ether for freshly etched GaAs(111)A but not GaAs(111)B surfaces. GaN(0001) surfaces exposed to PCl(5) in chlorobenzene showed reproducible XP spectroscopic evidence for Cl-termination. The Cl-activated GaAs(111)A and GaN(0001) surfaces were both reactive toward alkyl Grignard reagents, with pronounced decreases in detectable Cl signal as measured by XP spectroscopy. Sessile contact angle measurements between water and GaAs(111)A interfaces after various levels of treatment showed that GaAs(111)A surfaces became significantly more hydrophobic following reaction with C(n)H(2n-1)MgCl (n = 1, 2, 4, 8, 14, 18). High-resolution As 3d XP spectra taken at various times during prolonged direct exposure to ambient lab air indicated that the resistance of GaAs(111)A to surface oxidation was greatly enhanced after reaction with Grignard reagents. GaAs(111)A surfaces terminated with C(18)H(37) groups were also used in Schottky heterojunctions with Hg. These heterojunctions exhibited better stability over repeated cycling than heterojunctions based on GaAs(111)A modified with C(18)H(37)S groups. Raman spectra were separately collected that suggested electronic passivation by surficial Ga-C bonds at GaAs(111)A. Specifically, GaAs(111)A surfaces reacted with alkyl Grignard reagents exhibited Raman signatures comparable to those of samples treated with 10% Na(2)S in tert-butanol. For GaN(0001), high-resolution C 1s spectra exhibited the characteristic low binding energy shoulder demonstrative of surface Ga-C bonds following reaction with CH(3)MgCl. In addition, 4-fluorophenyl groups were attached and detected after reaction with C(6)H(4)FMgBr, further confirming the susceptibility of Cl-terminated GaN(0001) to surface alkylation. However, the measured hydrophobicities of alkyl-terminated GaAs(111)A and GaN(0001) were markedly distinct, indicating differences in the resultant surface layers. The results presented here, in conjunction with previous studies on GaP, show that atop Ga atoms at these crystallographically related surfaces can be deliberately functionalized and protected through Ga-C surface bonds that do not involve thiol/sulfide chemistry or gas-phase pretreatments.

Morphology of nanostructured GaP on GaAs: Synthesis by the close-spaced vapor transport technique

Nanostructures of gallium phosphide (GaP) were fabricated through the close-spaced vapor transport technique (CSVT). Such nanostructures were grown on crystalline gallium arsenide (GaAs) by using a GaP powder source in the absence of any catalyst. Nanostructured GaP products were structurally and morphologically characterized by means of physicochemical and spectroscopic techniques, such as SEM, EDS, XRD, and NMR. Results showed that GaP structures present a nanoflower-like morphology. Indeed, these nanoflowers are constituted by numerous nanowires. The diameters of the GaP nanowires were in the interval of 80–300 nm, with lengths varying from several to tens of micrometers.

Pressure-induced structural transformation in GaAs: A molecular-dynamics study

Structural phase transformation in crystalline gallium arsenide is studied with the isothermal-isobaric molecular-dynamics method. The pair-distribution functions and bond-angle distributions indicate that the fourfold-coordinated zinc-blende structure transforms into a sixfold-coordinated orthorhombic structure under a pressure of 22 GPa. The reverse transformation, from an orthorhombic to a zinc-blende structure, shows hysteresis, and is observed at a pressure of \ensuremath{\cong}10 GPa. Molecular-dynamics results are in good agreement with experiments. The energy barrier associated with this structural transformation is estimated to be \ensuremath{\sim}0.06 eV.

Hot-filament-enhanced chemical vapor deposition of silicon oxide films

Silicon oxide films were deposited on crystalline gallium arsenide substrates by chemical vapor deposition (CVD) using a solid source of silicon dioxide and atomic hydrogen as reactant. Infrared (IR) absorption spectra of the samples showed the characteristic bands of SiOSi group, typical of SiO 2, and SiH group, but could not detect any band corresponding to OH or SiOH groups.

Picosecond–milliångström lattice dynamics measured by ultrafast X-ray diffraction

Fundamental processes on the molecular level, such as vibrations and rotations in single molecules, liquids or crystal lattices and the breaking and formation of chemical bonds, occur on timescales of femtoseconds to picoseconds. The electronic changes associated with such processes can be monitored in a time-resolved manner by ultrafast optical spectroscopic techniques1, but the accompanying structural rearrangements have proved more difficult to observe. Time-resolved X-ray diffraction has the potential to probe fast, atomic-scale motions2,3,4,5. This is made possible by the generation of ultrashort X-ray pulses6,7,8,9,10, and several X-ray studies of fast dynamics have been reported6,7,8,11,12,13,14,15. Here we report the direct observation of coherent acoustic phonon propagation in crystalline gallium arsenide using a non-thermal, ultrafast-laser-driven plasma — a high-brightness, laboratory-scale source of subpicosecond X-ray pulses16,17,18,19. We are able to follow a 100-ps coherent acoustic pulse, generated through optical excitation of the crystal surface, as it propagates through the X-ray penetration depth. The time-resolved diffraction data are in excellent agreement with theoretical predictions for coherent phonon excitation20 in solids, demonstrating that it is possible to obtain quantitative information on atomic motions in bulk media during picosecond-scale lattice dynamics.

Inner‐Shell Electron Excitation Effect on the Structural Change in Amorphous and Crystalline GaAs with Brilliant X‐Ray Irradiation Using Synchrotron Radiation

Amorphous layers of gallium arsenide (a‐GaAs) formed by heavy implantation of silicon ions and crystalline gallium arsenide (c‐GaAs) were irradiated with monochromatized X‐rays using brilliant synchrotron radiation. Infrared absorption measurements at low temperature for a‐GaAs specimens showed that X‐rays having an energy larger than the K‐binding energy of As atoms created a much larger fraction of Si‒Ga and Si‒As bondings than in the as‐implanted state. On the other hand, from photoluminescence measurements, it was confirmed that X‐rays having a smaller energy than either of the K binding energies, enhanced the relaxation of the a‐GaAs network, and created some defects in c‐GaAs. The mechanism for these structural changes is discussed from the viewpoint of relaxation processes after inner‐shell electron excitation by X‐rays.

Electrodeposition of GaAs from Aqueous Electrolytes

Gallium arsenide films were electrodeposited from both alkaline and acid aqueous electrolytes. Compared to other conventional methods of preparing gallium arsenide films, electrodeposition from aqueous solution has the advantages of low operating and equipment costs, relatively easy control of film properties, and no toxic volatile raw material. The cathodic deposits from an alkaline electrolyte generally were thick, porous, and powdery. With an acid electrolyte, the deposits were thinner and more compact and adherent. The effects of the following operating parameters have been characterized: applied potential, current density, electrolyte composition, cathode material, deposition temperature, pH, additives, and agitation. Secondary ion mass spectroscopy confirmed oxidation states and x‐ray diffraction patterns indicated that the as‐deposited films were microcrystalline which became more crystalline on annealing. Mixtures of the deposited elements of gallium and arsenic yielded crystalline gallium arsenide after annealing. Energy dispersive spectroscopy and Auger electron spectroscopy showed that the deposited films contained some oxygen. The source of the oxygen, particularly in the acid electrolytes, is discussed. A mechanistic study confirmed conditions under which arsine is formed and reacts with gallate in the alkaline solution.

Dissolution of crystalline gallium arsenide in aqueous solutions containing complexing agents.

Crystalline gallium arsenide (GaAs) was found to dissolve in an aqueous solution containing the inorganic anions, chloride, sulfate, bicarbonate, monohydrogen phosphate, and dihydrogen phosphate, and the organic anions, acetate and citrate. The aqueous solution was made up to resemble lung fluid (Gamble solution) and was maintained at a pH of 7.4. The concentrations of arsenic (As) and gallium (Ga) in solution and the As-GA ratio on the surface of the GaAs increased continuously as the time of contact with the aqueous solution increased. X-ray photoelectron spectroscopic studies of the GaAs surface, at various time intervals, showed that As migrated to the surface and was oxidized to a species resembling As2O3 and, finally, was dissolved. The zinc present in the crystalline GaAs also migrated to the surface.

Optical and Electrical Characterization of Chemical Defects in GaAs Layers Grown by MBE

Careful control of UHV conditions during molecular beam epitaxial growth of thin crystalline gallium arsenide layers has to be accomplished to avoid undesired incorporation of chemical defects. Residual acceptor impurities in the MBE films were detected through their characteristic photoluminescence spectra, and the effect of these compensating acceptor centers on electron mobility in n‐type MBE was analyzed. A useful procedure for an accurate determination of the residual acceptor concentration in MBE is presented.

The optical and electrical effects of high concentrations of defects in irradiated crystalline gallium arsenide

Abstract The damage produced by fast neutron irradiation of gallium arsenide has been studied by a number of techniques. The electrical resistivity, which increases with dose at low doses to semi-insulating values, shows a remarkable, specimen-independent decrease for doses greater than 1017 n cm-2 from values of ca. 109 Ω cm to 3 Ω cm for the highest dose of 1.5 × 1020 n cm-2. In this high dose region the temperature dependence of the resistivity at low temperatures is given by exp [b/T 1/4] and it is suggested that in this highly disordered state conduction occurs by tunnel-assisted hopping between defect levels in the band gap. The presence of such levels is indicated by the strong optical absorption tail which is produced from 0.1 eV to the crystalline edge at 1.5 eV. Although at the highest doses the samples contain a high degree of disorder, X-ray diffraction shows that they are basically crystalline. Lattice parameter determinations show that it increases with dose, linearly at first, then tending to saturation for doses above 2 × 1018 n cm-2. The dose dependence of the lattice parameter is the same as that of the integrated optical absorption and both effects may be expected to relate to the total defect concentrations. The existence of small-angle neutron scattering at the highest dose shows that the defects are not uniformly distributed. It is shown that this state is the high dose manifestation of the production of defects by each primary knock-on atom in fairly localized regions. The overlap of the wings of these regions for doses greater than 1017 n cm-2 provides a conducting path and a fairly constant b value with dose, while the resistivity decreases rapidly. The total defect concentration, which is heavily weighted to the central regions of the individually damaged volumes, does not show overlap until a higher dose, as observed for the optical absorption and lattice parameter. Estimates of the total defect concentration have been obtained by measuring the total diffuse neutron scattering as well as from the small-angle scattering results. It is shown that each primary knock-on produces ca. 103 atomic displacements. This means that the amount of damage in gallium arsenide is close to that predicted by radiation damage theory. Measurements of photoconductivity show that the sharp photoconductivity edge is eliminated at quite low doses. At high doses the photoconductivity becomes very small and is much less than in unirradiated samples of similar resistivity. It appears that the ionized state lifetime of photo-induced carriers is very small in the high dose state. The effects anneal more or less completely after heating to 700°C. The hopping conductivity effect anneals between room temperature and 400°C and the remaining electrical effects between 500 and 600°C. The optical effects anneal more or less continuously throughout the temperature range up to 700°C. It is shown that the doses at which the tunnel-assisted hopping sets in are consistent with similar effects recently observed in ion implantation. We suggest that a similar mechanism applies in implanted layers for ion doses greater than 1012–1013 ions cm-2. Density measurements show an expansion identical with the lattice parameter expansion and indicate an expansion of 2.93 Å3 per defect. This is relatively small and is consistent with the tentative conclusion from the neutron-scattering data that the main defect is the close interstitial-vacancy pair in which the strain is minimized by being of opposite sign around each component.

The influence of substrate characteristics on the contact angles between liquid gallium and gallium arsenide crystals

Parameters influencing the interfacial interaction between liquid gallium and single crystalline gallium arsenide were investigated under vacuum by means of the sessile-drop technique over the temperature range ∼ 30 to 200° C. In addition to their dependence on the anisotropy of the {111} planes, contact angles in the Ga(I)/GaAs(s) system were found to be sensitive to the degree of misorientation and the direction of tilt of these planes. Furthermore, contact angles were found to be dependent on the size of the liquid drop and on the surface roughness of the substrate. In agreement with theoretical expectations the measured angles increased with increasing roughness of the GaAs surfaceS. However, these angles were found to be unaffected by the presence of N2, Ar, and He atmospheres, and by the nature and concentration of charge carriers in the substrate.