GaAs-like LO Research Articles

Heteroepitaxy growth of GaAsBi on Ge(100) substrate by gas source molecular beam epitaxy

We have investigated the growth of GaAsBi single-crystal film on Ge substrate by gas source molecular beam epitaxy. A high-quality GaAsBi epilayer has been obtained. It has been found that the surfactant effect of Bi suppresses the interdiffusion of Ge at the GaAsBi/Ge interface and reduces the misfit dislocation density. The Bi atoms occupy the As sites, as indicated by the appearance of GaBi-like TO(Γ) and LO(Γ) phonon modes in Raman spectra. In addition, the redshift of the GaAs-like LO(Γ) phonon frequency has been observed in the Raman spectra, owing to the Bi-induced biaxial strain and the alloying effect as well.

Optical studies of free-standing GaAs/AlGaAs single quantum well (SQW) microtubes: A comparison with InGaAs/GaAs bilayer microtubes

Optical properties of free-standing GaAs/AlGaAs single quantum well (SQW) microtubes have been investigated by room-temperature micro-photoluminescence (PL) and Raman measurements, followed by the detailed comparison with the corresponding properties of InGaAs/GaAs bilayer microtubes. Single peaks originating from GaAs SQW (~824nm) and bulk GaAs (~870nm) were observed in PL spectra of free-standing SQW and bilayer microtubes, respectively, which clearly shows the quantum-confinement-effect (QCE)-induced blue shift (~46nm) of GaAs PL peaks. The spectral red-shift (~12nm) of SQW PL peak due to strain release was confirmed after the as-grown planar rolled up into microtube. For Raman spectra, both GaAs-like and AlAs-like LO phonons of Al0.3Ga0.7As barrier which demonstrated the two-mode behavior were observed for SQW microtubes. Meanwhile, all LO phonon modes of SQW microtube were obviously red-shifted to lower frequencies (~10cm−1) when compared with SQW planar, reflecting the compressive strain decreases.

Investigation on p-In<sub>0.51</sub>Ga<sub>0.49</sub>As LOPC Mode by Raman Spectra

Heavily P type In0.51Ga0.49As was grown with hole concentration from 3.3E19cm-3 to 4.6E19cm-3. Standard (001) surface Raman backscattering geometry was used to measure samples. Two mode behavior and LO phonon-plasmon-coupled mode (LOPC) were observed obviously. Raman peak of LOPC mode is insensitive to hole concentration. The full width at half maximum (FWHM) of LOPC and the intensity ratio of GaAs-like LO mode and LOPC mode depend on hole concentration. The coupling strength of GaAs-like LO mode and plasmons is weak when hole concentration is very high and Raman peak of LOPC mode is independence with increasing hole concentration.

Raman scattering studies of strain effects in (100) and (311)B GaAs1−xBix epitaxial layers

We report room-temperature Raman studies of strained (100) and (311)B GaAs1−xBix epitaxial layers for x ≤ 0.039. The Raman spectra exhibit a two-mode behavior, as well as disorder-activated GaAs-like phonons. The experimental results show that the GaAs-like LO(Γ) mode experiences a strong composition-dependent redshift as a result of alloying. The peak frequency decreases linearly from the value for pure GaAs (∼293 cm−1) with the alloyed Bi fraction x and the introduced in-plane lattice strain ε∥, by ΔωLO=Δωalloy−Δωstrain. X-ray diffraction measurements are used to determine x and ε∥ allowing Δωalloy to be decoupled and is estimated to be −12(±4) cm−1/x for (100) GaAs1−xBix. ΔωLO is measured to be roughly double for samples grown on (311)B-oriented substrates to that of (100) GaAs. This large difference in redshift is accounted for by examining the Bi induced strain, effects from alloying, and defects formed during high-index (311)B crystal growth.

A study of two-step growth and properties of In 0.82Ga 0.18As on InP

In 0.82Ga 0.18As was grown by LP-MOCVD on InP substrates with the two-step growth technique. It was analyzed that epilayer's growth temperature affected on the crystalline quality, surface morphology, carrier concentration, and mobility of the In 0.82Ga 0.18As, which was characterized by X-ray diffraction, scanning electron microscopy, and Hall measurements. The evaluation of stress in In 0.82Ga 0.18As was made from frequency shift of the GaAs-like LO phonon of the Raman spectrum.

Micro-Raman for compositions characterization of selective area growth of Al xGa yIn 1−x−yAs materials by metal-organic vapor-phase epitaxy

Micro-Raman scattering measurements were used to study the phonon modes and to analyze the local variation of composition and strain in the micron range of Al x Ga y In 1− x − y As materials grown in the regime of selective area growth (SAG) at low-pressure metal-organic vapor-phase epitaxy (LP-MOVPE). For the planar Al x Ga y In 1− x − y As layers, the indium composition is almost constant at around 0.53 while the aluminum content covered the composition range from 0 to 0.47. The Raman spectra were recorded with a confocal micro-Raman spectrometer. Raman cartography at a lateral resolution of 1 μm was performed to study the spatial variation of the phonon peaks characteristics in the SAG Ga y In 1− y As and Al x Ga y In 1− x − y As materials. The three main Al x Ga y In 1− x − y As phonon modes were analyzed: the InAs-like, GaAs-like and AlAs-like LO modes. These modes characteristics depend strongly on composition, strain and position inside the mask aperture for the SAG materials, and this dependence gives the ability to analyze the local composition and strain variations in these systems, which is very important in order to understand the influence of MOVPE growth parameters on composition and strain and to optimize the SAG growth conditions.

Optical phonon behavior in strain-free dilute Ga(As,N) studied by Raman scattering

We present a Raman-scattering study on strain-free dilute Ga(As,N) epilayers grown by molecular beam epitaxy. The aim of our work is to discriminate the effect of alloying from the effect of biaxial strain on the frequency behavior of the optical phonon modes of Ga(As,N). In the relaxed epilayers, we observe the following: (i) for the GaN-like LO mode, an upward frequency shift with increasing N which is larger than previously observed in strained samples; (ii) for the GaAs-like LO mode, a redshift with increasing N content which is lower than those reported in the literature on strained samples; and (iii) for the GaAs-like TO mode, we observe a very minor blueshift with increasing N fraction. We discuss the origin of the observed shifts, with particular attention to the reduction of the GaAs-like TO-LO splitting in Ga(As,N). Our data and analysis suggest that such reduction cannot be explained only by a reduction of the total number of Ga–As oscillators due to the substitution of As by N. We discuss the effects of disorder and of ionic plasmon coupling between the GaAs and GaN sublattices of Ga(As,N) on the behavior of the GaAs-like LO mode of the alloy. We conclude that the behavior of this mode is determined by long-range effects.

Raman scattering probe of anharmonic effects due to temperature and compositional disorder in GaNxAs1−x

Using micro-Raman spectroscopy, we have investigated the vibrational properties of coherently strained GaNxAs1−x alloys grown on GaAs (001) substrates by molecular beam epitaxy. The effect of compositional disorder in GaNxAs1−x alloys has been studied by analyzing the broadening, asymmetry, and line shift of the first-order LO mode and nitrogen-induced localized vibration mode (NLVM). It is found that the line shape and peak shift of GaAs-like LO mode in GaNxAs1−x can be well described by the spatial correlation model. We have also analyzed Raman spectra of GaNxAs1−x alloys in the temperature range of 80–500K. The intensity of GaAs-like LO phonon and NLVM decreases with temperature and the linewidth of both these modes shows substantial broadening at higher temperature. The temperature dependence of phonon linewidth and peak frequency of LO modes are analyzed in terms of anharmonic damping effect induced by thermal and compositional disorders. We have observed that the anharmonicity in GaNxAs1−x is higher than that in GaAs, and increases with the N content in GaNxAs1−x alloys. In addition, both thermal and compositional disorder-induced anharmonicities lead to an appreciable change of the LO phonon lifetime. These results obtained from Raman investigations would lead to a better understanding of the anharmonic effects in dilute nitrides.

Annealing behavior of N-bonding configurations in GaN 0.023As 0.977 ternary alloy grown on GaAs (0 0 1) substrate by molecular beam epitaxy

We have investigated by high-resolution X-ray diffraction, secondary-ion-mass-spectroscopy (SIMS) and Raman scattering the effects of rapid thermal annealing on the structural properties of GaN x As 1− x ternary alloys coherently grown on GaAs (0 0 1) substrates by molecular beam epitaxy. X-ray diffraction indicates that the in-plane strain does not relax and the vertical lattice in the GaNAs film tends to shrink, while SIMS reveals the absence of any N concentration change upon annealing. Furthermore, post-growth annealing causes an increase of substitutional N as revealed by the increase of the ratio of Raman scattering intensity between the nitrogen-localized vibration mode and the GaAs-like LO. After performing strain analysis, we conclude with the changes of N configurations in the GaN x As 1− x from the N–N split interstitial to the N–As split interstitial and the N As substitutional at elevated temperatures, the thermal behaviors of N–N, N–As and N As are reported over 700–850 °C.

Effect of rapid thermal annealing on the properties of GaNAs thin films grown by molecular beam epitaxy

High-resolution X-ray diffraction (HRXRD) and micro-Raman scattering probe have been used to study the effect of post-growth thermal annealing on GaNAs thin films grown on (0 0 1) GaAs substrates by solid-source molecular beam epitaxy assisted with a nitrogen radio-frequency plasma source. X-ray reciprocal space mapping shows slightly in-plane strain relaxation after annealing, however, the vertical lattice constant measured by HRXRD from the (0 0 4) atoms planes shows a shrinkage, which is contrary to both the relaxation of tensile strain and the decreasing of nitrogen due to out-diffusion. The ratio of Raman intensity between the nitrogen-localized vibration mode and the GaAs-like LO is found to increase as a function of annealing temperature, which provides evidence for the increase of substitutional N As atoms at elevated temperatures. The anneal-induced vertical lattice shrinkage can be explained in term of the redistribution of nitrogen bonding configurations among the substitutional N As, interstitial N–N, and the N–As complex.

Analysis of the transport process providing spin injection through an Fe/AlGaAs Schottky barrier

Electron-spin polarizations of 32% are obtained in a GaAs quantum well via electrical injection through a reverse-biased Fe/AlGaAs Schottky contact. An analysis of the transport data using the Rowell criteria demonstrates that single-step tunneling is the dominant transport mechanism. The current–voltage data show a clear zero-bias anomaly and phonon signatures corresponding to the GaAs-like and AlAs-like LO phonon modes of the AlGaAs barrier, providing further evidence for tunneling. These results provide experimental confirmation of several theoretical analyses, indicating that tunneling enables significant spin injection from a metal into a semiconductor.

Raman and dielectric function spectra of strained GaAs1−xSbx layers on InP

Raman spectroscopy and variable angle spectroscopic ellipsometry (SE) were used to study pseudomorphically strained GaAs1−xSbx layers (0.22⩽x⩽0.65) grown on InP by metal-organic chemical vapor deposition. From the Raman spectra the composition dependence of the GaAs-like LO phonon mode was deduced. For comparison with literature data for unstrained GaAs1−xSbx, the strain-induced frequency shift of the GaAs-like LO phonon was calculated and subtracted from the present experimental data. When corrected for strain effects the composition dependent GaAs-like LO phonon frequency could be fitted by the linear relation ωLO=292.3−51x(cm−1) for the present range of alloy compositions. The pseudodielectric function spectra, deduced from SE measurements covering the range of photon energies from 0.8 to 5.0 eV, were fitted by a multilayer model employing a set of parametric oscillators to describe the GaAs1−xSbx dielectric function. From the resulting parametric dielectric functions the composition dependence of the E1 and E1+Δ1 critical point energies was deduced for pseudomorphically strained GaAs1−xSbx on InP and, after correction for strain effects on the interband transitions, also for unstrained GaAs1−xSbx.

Optical characterisation on the effect of doping concentration in InGaAs/InP HEMT structures

Photoluminescence and Raman scattering measurement were carried out to study the effect of varying the dopant concentration ( N D) in the InP donor layer of In 0.53Ga 0.47As/InP high-electron mobility transistor (HEMT) structures. Assuming that the donors are fully ionized, the carrier concentration in the InGaAs channel layer was found to increase when the doping concentration in the donor layer was increased. From PL, a red shift is observed for the E 1–HH 1 peak with increasing doping concentration. Both E 1–HH 1 and E 2–HH 1 transitions can only be observed for doping concentration greater than 1.5×10 18 cm −3. Raman characteristic was clearly seen with the two LO modes (InAs-like LO and GaAs-like LO) located at 226 cm −1 and 268 cm −1, respectively. The coupled mode between the InGaAs longitudinal optical phonons and the electrons in the InGaAs channel was observed to shift continuously to a lower wave number with the increase in the value of N D in the InP donor layer. The correlation between the observed Raman shift with the carrier concentration in the channel layer allows for non-destructive characterization of HEMT structures.

Lattice damage study of implanted InGaAs by means of Raman spectroscopy

We have studied by means of Raman scattering the crystallinity loss induced by ion-beam implantation in the In x Ga 1− x As alloy lattice matched to InP. Si + was implanted at 150 keV with fluences in the 10 12 – 5×10 14 cm −2 range. The Raman scattering signatures of implantation-induced disorder and the progressive amorphization of the In x Ga 1− x As crystal are discussed. With increasing implantation dose, the GaAs-like LO mode exhibits a gradual intensity reduction and an asymmetric broadening, while a broad peak emerges at low frequencies corresponding to the activation by the induced disorder of transverse acoustic modes. The Raman scattering measurements have allowed us to check the full amorphization of the In 0.53Ga 0.47As for a Si + implantation dose of 5×10 14 cm −2 .

InP/In xGa 1− xAs (0.53⩽ x⩽0.81) high electron mobility transistor structures grown by solid source molecular beam epitaxy

The effect of varying In mole fraction from x=0.53–0.81 in In x Ga 1− x As channel layer of InGaAs/InP high electron mobility transistor (HEMT) structure was studied by Hall measurements, photoluminescence spectroscopy and Raman scattering measurements. When x=0.75 a maximum electron mobility of 6130 and 35 600 cm 2/V s at 300 and 77 K, respectively was measured. The sheet carrier concentration in the channel did not change with x, and was nearly the same with an average value of 1.5×10 12/cm 2. A narrow photoluminescence linewidth of 15 meV was measured indicating the high quality of the epilayers. A red shift in the position of the PL peak energy from 0.86 to 0.76 eV was observed with an increase in x. A two-mode Raman characteristic was clearly seen with the two LO modes (InAs-like LO and GaAs-like LO) located at 226 and 268 cm −1, respectively. When the In mole fraction was increased in the channel, the InAs-like LO mode peak intensity increased while that of GaAs-like LO mode decreased. Correspondingly, the peak intensity ratio of InAs-like LO mode and GaAs-like LO mode increased from 0.78 to 1.10.

Study of Raman Scattering on InP/InGaAs/InP HEMTs

AbstractRaman scattering studies have been recently used to relate the strain in the semiconductor layer structure with the line shape of allowed modes. It can yield important information about the nature of the solid on a scale of the order of a few lattice constants. It can also provide an evaluation on the carrier concentration in the channel layer of high electron mobility transistors (HEMTs). In this investigation, Raman scattering was used to study the effect of varying the In mole fraction (x) from 0.53 to 0.81 in the InxGa1−xAs channel layer of InGaAs/InP heterostructures. The effect of varying the doping concentration in the donor layer from 6×1017/cm3 to 2.5×1018/cm3, and the effect of varying the In0.75Ga0.25As channel thickness from 140 Å to 260 Å are also reported.A two-mode Raman characteristic for all InxGa1−xAs/InP HEMTs is clearly seen, with the two LO modes (InAs-like LO and GaAs-like LO) located at 229cm−1 and 268.6cm−1, respectively. At a Raman frequency of 347cm−1, a small peak is observed due to InP LO mode. As the In composition increases from 0.53 to 0.81, the InAs-like LO mode peak intensity increases while that of GaAs-like LO mode decreases. The peak intensity ratio of InAs-like LO mode and GaAs-like LO mode increases from 0.78 to 1.10. By increasing the doping concentration in the donor layer (ND), there is also an increase in the carrier concentration in the InGaAs channel assuming that the donors are fully ionised. The coupled mode between the InGaAs longitudinal optical phonons and electrons in the InGaAs channel shifts continuously to a low wave number with the increasing ND in the InP donor layer. The increase in the lnGaAs channel thickness from 140Å to 260Å causes the InAs-like LO mode peak to shift to a lower wave number from 235.5 to 228.5cm−1. There is no change in the GaAs-like LO peak position located at 268.4cm−1.

Spatially resolved characterization of material composition and free carrier concentration by micro-Raman spectroscopy at surface selectively grown layers

Micro-Raman spectroscopy has been utilized to determine the local material composition of an In x Ga 1− x As layer and the local free-charge-carrier concentration of a Si-doped InP layer grown on patterned InP(1 0 0) substrates. The layers exhibited 8 or 11 μm long {1 1 1}A and {1 1 1}B facets delimited by (1 0 0) planes. The In concentration x in the In x Ga 1− x As layer has been derived from the Raman shift of the GaAs-like LO phonon. Line scans along (1 0 0)/{1 1 1}A/(1 0 0) and (1 0 0)/{1 1 1}B/(1 0 0) facet transitions reveal a distinct drop of x over more than 0.1 at the (1 0 0)/{1 1 1} transitions. In the Si-doped InP layer the density n of free electrons has been obtained from the Raman shift of the coupled plasmon–LO–phonon mode. A line scan along a (1 0 0)/(1 1 1)A/(1 0 0) facet transition shows a decrease of n from 1.4×10 18 cm −3 on the (1 0 0) surface to 0.5×10 18 cm −3 on the (1 1 1)A facet.

Uniaxial-stress investigation of the phonon-assisted recombination mechanisms associated with the X states in type-II GaAs/AlAs superlattices.

We present the results of a uniaxial-stress investigation of type-II GaAs/AlAs short period superlattices, using the optical techniques of photoluminescence (PL) spectroscopy and resonant Raman scattering. At ambient pressure, a resonance in the Raman cross section at the indirect, type-II band gap allows a direct measurement of the energies of the superlattice LA, GaAs-like LO, and AlAs-like LO phonons derived from the bulk X point. The positions of the Raman peaks are compared with the phonon-assisted PL transitions, indicating that the participation of GaAs-like LO phonons in the recombination process is not as well understood as has previously been thought. At sufficiently high in-plane stress, the transverse X minima become the conduction-band ground state, and the Raman measurements have again been used to examine the phonon modes, which can couple to these states. These measurements not only resolve an existing controversy in the literature, but they also demonstrate that one of the currently accepted assignments is incorrect. \textcopyright{} 1996 The American Physical Society.

Optical characterization of [111]B InGaAs layers

Strain assessment of [111]B InGaAs/GaAs layers, of several compositions and variable thicknesses, has been performed by photoluminescence and Raman spectroscopies. Both the gap energy and GaAs-like LO frequency undergo a blue shift proportional to the residual strain, which can be thus determined. The critical layer thickness for [111]B orientation is found to be three times larger than for [001] layers grown simultaneously. In addition, two kinds of inhomogeneities in the relaxation are found: strain domains, demonstrated by the spectral and spatial resolution of photoluminescence excitation, micro-photoluminescence and micro-Raman experiments; and a change in the strain release with layer depth, obtained by Raman at different excitation energies.

Growth of GaAs and InGaAs by MOCVD using a tertiarybutylarsine source

The metalorganic chemical source, tertiarybutylarsine (TEA) has been investigated for its possible use as precursor in the process of metalorganic chemical vapour deposition (MOCVD). The electrical and optical properties of epilayers are characterized by Hall measurement, X-ray diffraction (XRD), scanning electron microscopy (SEM), photoreflectance (PR) and photoluminescence (PL) spectra. The best quality of GaAs epilayers are grown at 150 Torr with a V/III ratio of 10.82, and the growth temperature is 600 degrees C. The full width at half maximum (FWHM) of 10 K PL is as narrow as 7.27 meV, the mobility is 7128 cm2 V-1 s-1 and the carrier concentration 8*1015 cm-3 at 300 K. The growth rate of GaAs epilayers is 0.74 mu m h-1, and broadening parameter Gamma of 300 K PR is 11.35 meV. When growing InGaAs heteroepitaxial layers, the growth rate of InGaAs epilayers is 0.625 pm h-1. When the composition x is increased from 0.045 to 0.16, the value of FWHM for PL spectra is increased from 12.35 meV to 33.28 meV. For the Raman spectra, when the indium composition is increased from 0.0455 to 0.25, the frequency of the GaAs-like LO mode is shifted to a lower one.