GaAs Epilayers Research Articles

Spin and orbital moments of ultra-thin Fe films on various semiconductor surfaces

The magnetic moments of ultrathin 7 monolayer (ML) Fe films on GaAs, InAs and InGaAs were studied using the element specific X-ray magnetic circular dichroism (XMCD). The 7 ML Fe films were grown by molecular beam epitaxy (MBE) at room temperature and capped with 15 ML Cr to prevent oxidisation on removal from the vacuum system. The special substrates took the form of GaAs wafers topped with a 0.3/spl mu/m epilayer of GaAs, InAs or InGaAs followed by an As capping layer. The lattice parameters of the GaAs, InAs and InGaAs epilayers were chosen for their lattice constants which, in a 2:1 relationship with Fe give mismatches of 1.56%, 5.32% and 0%, respectively. All samples revealed a reduced spin moment compared to bulk value and a marked enhancement in orbital moment. The similarity of the magnetic thin film features observed for the different substrates suggests that an unexpected degree of tolerance exists in these Fe films to surface issues such as strains induced by different lattice matches.

Fe-catalytic growth of ZnSe nanowires on a ZnSe(001) surface at low temperatures by molecular-beam epitaxy

We grew ZnSe needle-like nanowires on a ZnSe∕GaAs epilayer using Fe catalysts by means of molecular-beam epitaxy operated at low temperatures of 250–350 °C, which are comparable to the usual temperatures for fabrication of ZnSe-based optoelectronic devices. The diameters at the tops of the nanowires ranged from 8 to 20 nm, and the typical length was about 200 nm. The number density of the nanowires was the order of 109cm−2. A nanowire was the zinc blende structure and the longitudinal direction was ⟨001⟩,⟨111⟩,⟨110⟩, or ⟨112⟩. Photoluminescence spectroscopy implied that the optical property of the nanowires differs from that of the bulk crystals.

GaAs on Si(111)—crystal shape and strain relaxation in nanoscale patterned growth

Nanoscale patterned growth of GaAs on Si(111) by molecular beam epitaxy is examined. A 355 nm period two-dimensional array of circular holes (diameter ∼200–250nm) is fabricated into a 45-nm-thick SiO2 film on a Si(111) substrate by large-area interferometric lithography and dry etching. For 300 nm deposition, the GaAs epilayer selectively deposited within each hole on the patterned substrate is surrounded by {11¯0}-type sidewalls perpendicular to Si(111), resulting in a hexagon-based prismatic pillar, without significant lateral overgrowth. At the initial stage of growth, twins parallel to Si(111) and an aperiodic mixture of cubic and hexagonal phases are observed but most of the GaAs pillars are terminated with a cubic phase region. Raman scattering reveals that the individual nanoscale GaAs pillars are completely strain relaxed.

Phase behavior of thin film Mn∕GaAs interfacial reactions

Rutherford backscattered spectrometry, x-ray diffraction, and cross-sectional transmission electron microscopy were used to examine Mn∕GaAs interfacial reactions. Mn films deposited in situ on molecular beam epitaxy (MBE)-grown GaAs(001) epilayers resulted in significant interfacial reactions following post-growth anneals above 200 °C. These reactions initially resulted in the formation of a two-phase region of tetragonal Mn2As and tetragonal δ-MnGa, with an average composition of Mn0.6Ga0.2As0.2, and were found to be limited by the rate of Mn diffusion through the reacted region. The two phases formed an epitaxial lamellar layer on the GaAs substrate with Mn2As(001)⟨100⟩ and δ-MnGa(001)⟨100⟩∕∕GaAs(001)⟨110⟩. Higher temperature anneals resulted in the dissociation of the Mn0.6Ga0.2As0.2 region into a δ-MnGa layer near the sample surface and a Mn2As layer near the GaAs substrate. Results of these reaction studies have been used to make predictions of the Mn–Ga–As ternary phase diagram for temperatures up to 400 °C, which indicates that both Mn2As and δ-MnGa are thermodynamically stable in contact with GaAs.

In depth study of the compensation in annealed heavily carbon doped GaAs

Heavily C-doped GaAs epitaxial layers with holes concentrations ranging from 1019 to 1.6×1020cm−3 have been grown by metal organic vapour phase epitaxy (MOVPE) using CCl4 as C-growth precursor. The carbon doping characteristics of GaAs epilayers have been investigated by optimizing the V/III ratio and the growth temperature. Additional informations have been extracted from the evolution of the in situ reflectivity signal during the growth of GaAs:C. The appearance of discernible oscillations in the reflectivity response indicates the high carbon incorporation and the good surface quality in spite of the CCl4 etching effect. The hole concentration tends to saturate at about 1.5×1020cm−3. The comparison between Hall effect measurements realized on sets of as grown and annealed layers, and theoretical calculations of the mobility lead to the determination of the compensation ratio of the samples.The lattice matching conditions were systematically investigated by using high X-ray diffraction measurements from (004) and (115) planes. A comparison between the experimental mismatch and the one calculated with the Vegard's law allows the estimation of the possible origin of the compensation. Secondary ion mass spectrometry, scanning electron microscopy and atomic force microscopy have been used as complementary tools to characterize the films.

Interference effects in electromodulation spectroscopy applied to GaAs-based structures: A comparison of photoreflectance and contactless electroreflectance

In this letter, we show that the oscillation features (OFs) usually observed in photoreflectance (PR) spectra of GaAs-based structures grown on the n-type GaAs substrate below the GaAs fundamental gap could be eliminated completely by applying the contactless electroreflectance (CER) instead of PR. This finding confirms that the origin of OFs is the modulation of the refractive index in the sample due to the generation of additional carriers by the modulated pump beam. In the case of CER spectroscopy, any additional carriers are not generated during the modulation hence CER spectra are free of OFs. This advantage of CER spectroscopy is very important in investigations of all structures for which OFs are present in PR spectra. In order to illustrate this advantage of CER spectroscopy we show PR and CER spectra measured first for the GaAs epilayer and next for more complicated steplike GaInNAsSb∕GaNAs∕GaAs quantum well structures.

Investigation of N incorporation in InGaAs and GaAs epilayers on GaAs using solid source molecular beam epitaxy

The indium and nitrogen incorporation in InGaAsN/InGaAs/GaAsN grown on GaAs substrates is investigated by means of photoluminescence (PL) and secondary ion mass spectroscopy (SIMS) measurements. The elemental profile and concentration collected from the two methods are then used as the simulation parameters to fit the X-ray diffractometry (XRD) measurements. The simulations indicate that the epitaxial parameters in our growth of InGaAsN/InGaAs/GaAsN epilayers are consistent throughout the three methods of analysis.

Comparative study between electrical, optical and structural properties of annealed heavily carbon doped GaAs

Heavily C-doped GaAs epilayers have been grown by atmospheric pressure metalorganic vapor phase epitaxy with hole concentration ranging from 2×1019 to 1.6×1020cm−3. In order to study the stability of C acceptors over this range, the films have been annealed at 810°C for 10min under two mixture gas AsH3+H2 or only N2. Annealing of the layers resulted in a decrease in carrier concentration, carrier mobility and lattice mismatch with undoped GaAs. The lattice matching conditions of C-doped GaAs layers were systematically investigated by using X-ray high-resolution diffraction space mapping. The comparison between electrical and structural before and after annealing of layers properties indicates that the simultaneous decrease of carrier concentrations, Hall mobility and mismatch is probably related to an increase of compensation. Basing on a theoretical calculation of mobility as a function of hole concentration and Vegard's law, we estimate that the compensation comes from the formation of (C–C)+[100] interstitial couples. This fact does not exclude definitively the possibility of the formation of other species such as H–CAs especially for hole concentration lower than 5×1019cm−3. An annealing under AsH3+H2 ameliorates the crystalline properties contrarily to an annealing under N2. The optical properties have been investigated using Raman spectroscopy. Two main Raman features are observed before and after annealing of the layers: the longitudinal-optic (LO) phonon mode and the coupled plasmon-LO phonon (LOPC). As for as grown layers, the intensity ratio ILOPC/ILO between the intensity of LOPC peak and the LO peak increases by increasing the hole concentration. This ratio is about 1 after an annealing of layers under AsH3+H2. An unusual change of ILOPC/ILO ratio is observed in samples annealed under N2. Indeed, for high doping (∼1020cm−3) the ratio ILOPC/ILO<1 and for relatively low doping (∼2×1019cm−3) the ratio ILOPC/ILO>1. This behaviour is probably related to the high sensibility of Raman measurement not only to the hole concentration change but also to the surface quality.

Growth and characterization of Sn-doped GaAsSb and GaAs epilayers on GaAs (001)

We investigated the growth of GaAs1−xSbx(x⩽0.4) and GaAs epilayers with heavy Sn doping using molecular-beam epitaxy, and characterized the doping concentration as a function of Sn K-cell temperature. The carrier concentration was found to be expressed as a function of the reciprocal Sn effusion cell temperature with respect to GaAs1−xSbx(x=0.15) layers. At cell temperatures of 670–800°C, carrier concentrations ranging from 5×1016to8×1018cm−3 showed a simple exponential dependency. When the ratios of TO-mode to LO-mode intensities in GaAs1−xSbx(x=0.15) layers were measured as a function of Sn-cell temperature by micro-Raman scattering, the peak intensity ratio was linearly proportional to the Sn-cell temperature in the range of 700–900°C.

Effect of rare earth addition on GaAs-based layers grown by liquid phase epitaxy

Photoluminescence and electrical measurements were carried out for studying the influence of rare-earth elements (Yb) concentration in the Ga melt on the electronic and structural properties of LPE GaAs epilayers. It was shown that at low concentration the main role of Yb is the gettering of residual impurities in the melt. At the same time, Yb addition in the melt changes the heterogeneous equilibrium by changing the stoichiometry of epilayers (increase of ). But at further increase of Yb concentration in the melt Yb begins to enter in the Ga-sublattice and cluster with deviation from the stoichiometry. An optimum concentration of Yb exists at which high-purity and stoichiometric epilayers for a device application can be obtained.

Equilibrium crystal shape of GaAs in nanoscale patterned growth

The equilibrium crystal shape (ECS) of GaAs homoepitaxially grown on a nanoscale SiO2-patterned (001) plane by molecular beam epitaxy is investigated. A GaAs epilayer selectively grown on a nanoscale area bounded by a circular SiO2 mask undergoes faceting, resulting in a pyramidal shape with {110} sidewalls. Growth is slowed or terminated with the generation of these {110} facets even with a continuing supply of Ga atoms. This implies that the pyramidal shape is energetically very stable. Based on experimental results and the Wulff construction, a {110}-type sidewall pyramid is proposed as an ECS of GaAs on (001) in nanoscale patterned growth.

Morphology and optical properties of GaAs layers grown on (1 1 1)A GaAs substrates in N2ambient by metalorganic chemical vapor deposition

Using tertiary-butyl arsine as a group V precursor, GaAs epitaxial layers have been grown on both (1 1 1)A- and (1 0 0)-oriented GaAs substrates in N2 ambient using the atmospheric metalorganic chemical deposition technique. The surface morphology and optical properties of the GaAs homoepitaxial layers were studied in detail. It is found that both the surface morphologies and optical properties of GaAs layers on (1 1 1)A GaAs substrates depend much more strongly on the combination of growth temperature and V/III input molar ratio, compared to the epitaxial layers on (1 0 0) plane GaAs. The photoluminescence (PL) emission efficiency increases with increasing growth temperature in the region 550–650 °C for both (1 1 1)A and (1 0 0) GaAs epitaxial layers. At room temperature, the PL emission intensity of the (1 1 1)A GaAs epilayer grown under optimum growth conditions is 67 times as strong as that of the same run-grown (1 0 0) GaAs epilayer.

Sb and Bi surfactant effects on homo-epitaxy of GaAs on ( [formula omitted]) patterned substrates

Anisotropic lateral growth during GaAs ( 0 0 1 ) epitaxy can have dramatic effects on the evolution of patterned features and surface morphology. Many new opto-electronic devices require growth on patterned or non-ideal surfaces. Controlling lateral growth will be essential for the production of these devices. In this study, GaAs epilayers were grown by organometallic vapor-phase epitaxy on patterned GaAs ( 0 0 1 ) wafers. During these growth experiments, trimethylantimony and trimethylbismuth were used as surfactant precursors to investigate the effects of Sb and Bi on GaAs lateral growth rates. Both surfactants were found to enhance the [1 1 0] lateral growth rate by nearly 300 %, while having a negligible effect on the lateral growth rate in the orthogonal direction. Kinetic simulations assisted in determining a plausible surfactant mechanism: The enhanced [1 1 0] lateral growth rate is due to an increase in the frequency of [1 1 0] diffusion events (decreased hop barrier).

Direct observation of LO phonon-plasmon coupled modes in the infrared transmission spectra ofn-GaAs andn−InxGa1−xAsepilayers

The infrared transmission spectrum of Si-doped molecular beam epitaxy (MBE)-grown GaAs epilayers, $2--2.5\ensuremath{-}\ensuremath{\mu}\mathrm{m}$ thick, measured in the oblique (Berreman) geometry, revealed distinct minima in p polarization. Given epilayer thickness $\ensuremath{\ll}$ reststrahlen wavelength, the minima are identified as the zone-center transverse optic phonon $({\ensuremath{\omega}}_{\mathrm{TO}})$ and the high-frequency LO phonon-plasmon coupled mode $({\ensuremath{\omega}}_{+}).$ Analysis of the experimental data yielded free-carrier concentrations ranging from $2.5\ifmmode\times\else\texttimes\fi{}{10}^{17}$ to $1.4\ifmmode\times\else\texttimes\fi{}{10}^{18}{\mathrm{cm}}^{\ensuremath{-}3}.$ The same technique with MBE-grown Si-doped ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}$ epilayers $(0.5--1\ensuremath{\mu}\mathrm{m}$ thick) yielded ${\ensuremath{\omega}}_{+}$ modes corresponding to free-carrier concentrations of $8.2\ifmmode\times\else\texttimes\fi{}{10}^{16}--2.7\ifmmode\times\else\texttimes\fi{}{10}^{19}{\mathrm{cm}}^{\ensuremath{-}3}.$ The observations of the transmission minima in the Berreman geometry and their interpretation demonstrate a direct and simple method for deducing free-carrier concentrations over a wide range.

Investigation of MOVPE growth of silicon δ-doped GaAs epilayers and In xGa 1− xAs/GaAs strained quantum wells

This work presents the investigations of MOVPE growth of silicon δ-doped GaAs epilayers and strained In x Ga 1− x As/GaAs single quantum wells (SQW). The optimal technological parameters for growing well-spatially confined δ-doped layers with high carrier concentrations are estimated and discussed. The electrical and optical properties of the obtained structures were examined using capacitance–voltage measurements ( C–V), photoreflectance (PR) and photocurrent (PC) spectroscopy. The work is focused on the investigation of the central Si- δ-doped In x Ga 1− x As/GaAs SQW properties. It was found that these structures are more effective in confining carriers than conventional δ-doped epilayers. In addition, placing n-type δ doping into the In x Ga 1− x As quantum well changes significantly its energy band shape (especially a valence band) and causes filling of the lowest subbands of the conduction band. These effects are responsible for changing of the type of possible optical transitions in the QW region, which was confirmed by PC and PR measurements.

Effect of quantum confinement on shallow acceptor transitions in δ-doped GaAs/AlAs multiple-quantum wells

We have investigated the effect of confinement on the shallow acceptor transitions in δ-doped GaAs/AlAs multiple-quantum wells with well widths ranging from 30 to 200 Å. A series of Be δ-doped GaAs/AlAs multiple-quantum wells with doping at the well center and a single epilayer of GaAs uniformly Be doped were grown by molecular beam epitaxy. Photoluminescence spectra were measured at 4, 20, 40, 80, 120, and 200 K, respectively. Two-hole transitions of the acceptor-bound exciton from the ground state, 1S3/2(Γ6), to the excited state, 2S3/2(Γ6), were clearly observed. It is found that the acceptor transition energy increases with a decrease in quantum well width.

Studies of lattice mismatch and threading dislocations in GaAs/Si grown by MBE

GaAs/Si(001) thin films with six Si interlayers grown by molecular beam epitaxy (MBE) were examined by transmission electron microscopy (TEM). The interface structure of the films was investigated by using both plan-view and cross-sectional TEM specimens. It was seen that the films have a smooth interface associating with misfit and threading dislocations between the GaAs epilayer and the Si substrate. It was also observed that the Si interlayers had an undulated shape reducing the threading dislocations in the structure. It was obtained that the strain relaxation at the interface occurs by misfit and threading dislocations. The g · b contrast experiments displayed 60 ∘-type misfit dislocations with a/2 〈110〉{111} slip system in the structure. Electron diffraction experiments were also performed to find out the reduction of the lattice mismatch and the strain left in the structure using the cross-sectional GaAs/Si(001) samples.

Electron-beam-induced reactivation of Si dopants in hydrogenated two-dimensional AlGaAsheterostructures: a possible new route for III–V nanostructure fabrication

Hydrogen incorporation in n-type Si-doped GaAs epilayers is a well-known process whichleads to the neutralization of the active Si impurities with the formation of SiH complexes.Recently, we have shown that SiH complex dissociation and, consequently, Si-dopantreactivation could occur when the epilayers are exposed to an electron beam.Two epilayers have been studied: the first is a0.35 µm thick hydrogenated Si-doped GaAs epilayer and the second is Si planar-dopedAlGaAs/GaAs/InGaAs heterostructures.Firstly, Hall effect measurements have been carried out on the epilayers exposed, after RFhydrogen plasma exposition, to increasing electron doses with different injectionenergies. For the 2D heterostructures, we have observed that the free carrier densityNs does notvary significantly for weak electron densities. This reactivation presents a threshold value, contrary to the0.35 µm epilayerin which Ns varies quite linearly. It will be shown that such phenomena might be attributed to thefilling of surface states as the dopants are progressively reactivated.Then, using a high spatial resolution electron beam lithography system, nanometricconductive patterns have been fabricated starting from hydrogenated epilayers. Electricmeasurements have been performed and the results obtained show that about 15 nm spatialresolution could be expected.In conclusion, taking into account this spatial resolution, the high spatial contrast ofconductivity which could be expected due to the existence of an electron dosethreshold, and the high mobility of the AlGaAs/GaAs/InGaAs heterostructure,the effects described in this paper could open a new way for the fabrication ofIII–V 1D or 2D mesoscopic structures for electronic or optoelectronic applications.

Phase separation and superlattice formation by spontaneous vertical composition modulation in GaAs 1− x N x /GaAs

A phase separation with a periodic spontaneous vertical composition modulation in GaAs0.97N0.03/GaAs epilayers is observed by transmission electron microscopy. At growth temperatures of 530–550 °C a spatial period of 10–15 nm is found. Unstable growth near the solubility limit is expected to occur at topographical defects such as surface steps which may induce a strong local variation in the N adatoms concentration. Consequently the solid content is affected. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Deep levels in rhodium-doped p-type MOCVD GaAs

p-type GaAs epilayers grown by metal-organic chemical vapour deposition (MOCVD) technique, doped in situ with rhodium (Rh) impurity, have been studied by deep level transient spectroscopy (DLTS). A composite peak consisting of emission signals from at least two deep levels in the lower-half band gap is identified with Rh. This peak is resolved using double-correlation DLTS (DDLTS) measurements, providing the clear energy positions of the Rh-related deep levels as Ev+0.35eV and Ev+0.51eV. Emission rate signatures and other parameters are reported for these deep levels. They are observed to show electric-field dependent emission signatures. No significant minority carrier (electron) deep level could be clearly identified with Rh due to the presence of significant inadvertent features in the injection DLTS spectrum. Results are compared with our earlier study of Rh-doped, n-type, MOCVD GaAs.