Monolayers Of GaAs Research Articles

GaAs in GaSb: Strained nanostructures for mid-infrared optoelectronics

Molecular beam epitaxy was used for the first time to grow novel GaAs/GaSb heterostructures with ultrathin (0.8–3 monolayers) GaAs layers embedded in GaSb. These structures were studied by X-ray diffraction, transmission electron microscopy, and photoluminescence. By contrast to known structures with self-assembled quantum dots, GaAs layers in GaAs/GaSb structures are subject to elastic tensile stresses due to 7% lattice mismatch. The structures exhibit intense photoluminescence in the 2 µm region at low temperatures. Quantum-dimensional islands are formed in the structure at a nominal GaAs layer thickness exceeding 1.5 monolayers. The band alignment of the structures is of type II.

Quantum wells with atomically smooth interfaces

By a cleaved-edge overgrowth method with molecular beam epitaxy and a (110) growth-interrupt anneal, we have fabricated a GaAs quantum well exactly 30 monolayers thick bounded by atomically smooth AlGaAs heterointerfaces without atomic roughness. Microphotoluminescence imaging of this quantum well indeed shows spatially uniform and spectrally sharp emission over areas of several tens of microns in extent. By adding a fractional GaAs monolayer to our quantum well we are able to study the details of the atomic step-edge kinetics responsible for flat interface formation.

First-principles calculations of the adsorption of a single monolayer of GaAs on Si(110)

The adsorption of a single monolayer of GaAs on the (110) surface of Si has been studied by first principles total-energy calculations. We have used the density functional theory, within the local density approximation. It is found that As and Ga atoms are adsorbed on Si(110) following a relaxation pattern similar to clean group-III--group-V (110) surfaces. Arsenic atoms move outwards while Ga atoms are pushed down. The surface is ordered, and the periodicity remains (1\ifmmode\times\else\texttimes\fi{}1). We have also investigated the possible diffusion of Ga and/or As atoms into the Si substrate. Mixed Si-Ga, Si-As or an embedded Ga-As layer are not favorable energetically. The most stable configuration corresponds to the GaAs monolayer on top of the Si(110) substrate. Interchanging the GaAs overlayer by GaSb (with a lattice constant 13% larger than Si) results in a disordered surface.

InAs/GaInP self-assembled quantum dots: molecular beam epitaxial growth and optical properties

We studied the growth of self-assembled InAs quantum dots (QDs) on GaInP using solid source molecular beam epitaxy (MBE) with the aim of reducing the thermionic escape of bound carriers by burying the QDs in large bandgap materials such as GaInP. The density and size of QDs were measured by atomic force microscopy (AFM). This showed that InAs QDs were formed on GaInP when less than 0.15 monolayers (ML) of InAs was deposited, unlike the formation of InAs QDs on GaAs. The critical thickness of InAs layers for the onset of QD formation increased systematically when a very thin GaAs interlayer was deposited onto the InGaP surface before the formation of the InAs QD layer. When a 10 ML GaAs overlayer was introduced on top of the QD layer, clear photoluminescence (PL) signals were observed from these QDs in the GaInP matrix at room temperature. The good correspondence between the AFM image and the PL spectra indicates that the QD shape is kept in the GaInP matrix without appreciable intermixing.

Improvement of wet-oxidized AlxGa1−xAs (x∼1) through the use of AlAs/GaAs digital alloys

A comparison of the water vapor oxidation characteristics of AlAs, Al0.98Ga0.02As, and an AlxGa1−xAs digital alloy was performed. The AlxGa1−xAs digital alloy consists of periods of 49 monolayers of AlAs and 1 monolayer of GaAs and has an equivalent composition of x=0.98. Oxidation rates and the structural integrity of the three layers were compared. When oxidized in water vapor, the AlxGa1−xAs digital alloy and the AlAs have similar oxidation rates, both of which are twice as fast as the Al0.98Ga0.02As layer. Post-oxidation annealing of these samples at 450 °C showed severe delamination at the oxide/GaAs interface in the AlAs sample while the AlxGa1−xAs digital alloy sample was not damaged.

Theoretical study of graphitic analogues of simple semiconductors

The stability of the aromatic phase of two-dimensional monolayers of BN, C, Si, GaAs, and SiC is theoretically investigated using an ab initio tight-binding-like method. It is found that BN, C, and SiC prefer to form the flat aromatic phase while Si and GaAs prefer to pucker, although Si is found to be a borderline case. From a simple qualitative picture, we can understand this behaviour as a consequence of the different bond lengths, where longer bond lengths give rise to puckered structures. On the other hand, `compressing chemically' by alloying, which shortens the bond, may allow the state of the system to change from puckered to flat and vice versa.

Ultrathin GaAs layers embedded in AlAs: a perspective for intense short wavelength emission

Type-II GaAs/AlAs quantum-well samples grown by low-pressure metal-organic vapour-phase epitaxy consisting of 50 periods of either 2 monolayers (ML), 4 ML, 5 ML, 6 ML, or 7 ML GaAs embedded in 28 ML AlAs were under investigation. Even at room temperature dominant type-I luminescence has been observed. The peak wavelength of this emission ranges from 620 to 440 nm and is determined by the GaAs layer thickness. A comparison of the measured transition energies with model calculations applying an effective mass approach and an empirical tight-binding Green's function scheme confirmed this strong dependence. To our knowledge this is the first report on intense yellow, green, and blue luminescence from GaAs.

Growth of strained GaAs islands on (0 0 1) GaP: : a RHEED study of quantum wire formation

The evolution of the initial GaAs islands that form on flat (0 0 1) GaP during chemical beam epitaxy has been studied for layer thicknesses below 10 GaAs monolayers. Reflection high-energy electron diffraction and atomic force microscopy were used to characterize island formation in reciprocal and direct space, respectively. Both methods clearly show the anisotropic growth behavior, beginning with growth of isolated elongated islands which at thicker deposition evolve into a corrugated surface of densely packed GaAs quantum wires. Intensity-versus-time measurements were made for island-related diffraction features which allowed distinctions to be made between the different morphological growth regimes.

Temperature stabilized 1.55 μm photoluminescence in strained GaxIn1−xAs quantum wire heterostructures

We present two techniques for manipulating the peak photoluminescence wavelength towards ∼1.55 μm, from the usual 1.61 μm, of strained GaxIn1−xAs quantum wire (QWR) heterostructures. The QWR samples have been prepared by the strain-induced lateral-layer ordering process during molecular beam epitaxy utilizing short-period superlattices of (GaAs)m/(InAs)n. The subscripts m and n refer to the number of deposited monolayers of GaAs and InAs, respectively. In the first approach, for some cases of m>n, the QWRs will contain more Ga thereby decreasing the 300 K wavelength towards 1.55 μm provided the strain is not too great. The second approach relies on post-growth annealing to shift the 300 K peak emission. For anneals performed at 650 °C for 3–5 h, 300 K wavelengths from 1.55 to 1.59 μm have been attained. Moreover, all of these samples display a unique behavior of peak PL with respect to temperature. Some samples show no net shift in wavelength over a range of 77–380 K. Other samples have 77 K wavelengths longer than their 300 K wavelengths. It is believed that these structures have the potential to be processed into temperature stabilized Fabry–Pérot lasers emitting at 1.55 μm.

Transition behavior from coupled to uncoupled GaAs/InAs double quantum wells

The electronic structures of the InAs single and double quantum wells buried in bulk GaAs (001) are presented based on the sp3s* empirical tight-binding model. Both electrons and holes are found to be confined in the c-axis direction around the inserted InAs monomolecular plane with a localization length of the order of 110 Å. The inserted InAs monolayer is, therefore, playing the role of a quantum well for all charge carriers and, as a consequence, the formed heterojunction is of type I. The system, composed of two InAs monolayers buried in GaAs and separated by N monolayers of GaAs, is studied versus the barrier thickness (N). Our results of the variation of band gap energy as a function of barrier thickness (N) are in excellent agreement with the available photoluminescence data when a small valence band offset (of order 80 meV including the spin-orbit effects) is employed. A critical barrier thickness of about 220 Å is suggested to decouple the InAs quantum wells.

Ultrathin GaAs Layers Embedded in AlAs: The Observation of Intense Short-Wavelength Emission

Type-II GaAs/AlAs multiple-quantum well samples grown by low-pressure metal-organic vapour-phase epitaxy have been investigated. The layered structures consist of 50 periods of either 2 monolayers (ML), 4 ML, 5 ML, 6 ML, or 7 ML GaAs embedded in 28 ML AlAs. Using (100) GaAs substrates 6° misoriented towards the nearest (111)B plane monolayer steps at the AlAs/GaAs/AlAs interfaces with regular terrace widths (2.7 nm) can be seen by high-resolution transmission electron microscopy. In the photoluminescence spectra even at room temperature type-I luminescence is found to be dominant. The peak wavelength of this emission ranges from 620 to 440 nm and is governed by the GaAs layer thickness. The comparison of the measured transition energies with calculations based on an effective mass approach and an empirical tight-binding Green's function scheme shows good agreement. The perfect interface structure of our samples with regular distribution of monolayer steps prevents obviously the loss of photoexcited carriers from the GaAs layers to the surrounding, energetically resonant AlAs material allowing only low type-II luminescence intensity. Furthermore, for our well thicknesses 2D phonons have to be coupled with 3D electrons leading to low electron–phonon interaction.

Blue luminescence from ultrathin GaAs layers embedded in AlAs

Our investigations focus on low-temperature luminescence experiments on a set of type-II GaAs/AlAs multiple-quantum-well (MQW) samples grown by low-pressure metal-organic vapor-phase epitaxy. The layered structures consists of 50 periods of either 2 monolayers (ML), 4, 5, 6, or 7 ML GaAs embedded in 28 ML AlAs. For (001) GaAs substrates, 6 degrees misoriented towards the nearest (111) plane of group-V atoms, monolayer steps at the AlAs/GaAs interfaces with regular terrace widths (2.7 nm) can be seen by high-resolution transmission-electron microscopy. In the photoluminescence spectra of these MQW samples, type-I luminescence is found to be dominant even at room temperature. The peak wavelength of the type-I emission depends strongly on the GaAs layer thickness; it ranges from about 620-440 nm. The intense type-I emission seems to be connected with the interface peculiarities. Our astonishing observation might be explained as follows: (i) The perfect interface structure pl events the loss of photoexcited carriers from GaAs layers to the surrounding AlAs materials, i.e., the energy loss by optical-phonon scattering is reduced. (ii) For our well thicknesses two-dimensional (2D) phonons must be coupled with 3D electrons leading also to a reduction of the electron-phonon interaction. (iii) The regular interface steps should favor a coherent interaction (quantum interferences) of excitons and/or electrons confined in the GaAs wells with energetically resonant continuum states of the AlAs barriers. The experimentally observed optical transition energies of the type-I and type-II recombination are compared with model calculations applying an effective-mass approach and empirical tight-binding Green's-function scheme.

Electronic structures of GaAs/InAs(001) thin-layer systems: a tight-binding approach

The electronic structure of an InAs monolayer in bulk GaAs (001) is calculated in the framework of the tight-binding model, which includes only nearest-neighbour interactions. The results show that both electrons and holes are localized around the inserted InAs plane, which is, therefore, playing the role of a quantum well for all charge carriers. The eigenfunctions of the confined electron and hole, calculated at the point, are found to be supported by the , and orbitals of atoms in the neighbourhood of the inserted In layer, and, hence, to be confined in the c-axis direction, with a localization length of the order of . In the limit of a single InAs monolayer in bulk GaAs, the energy gap is found to be 40 meV less than that of bulk GaAs. These results are in excellent agreement with the results of photoluminescence (PL) experiments, and successfully explain the observed intense PL peak, and its polarization parallel to the interface. Moreover, in a system composed of two InAs monolayers separated by N monolayers of GaAs, the localization of charge carriers around the InAs planes is found to decrease when N becomes large, as a consequence of the increase of the confinement energies. The calculated band-gap energies as a function of N are in reasonable agreement with the PL experimental data.

Observation of Standing Acoustic Waves by Resonant Raman Scattering

Resonant Raman measurements on GaAsyGaP quantum wells, as thin as one monolayer, are reported. The work is focused on the low-frequency scattering range, which exhibits a continuous emission and periodic oscillations that have never been observed up to now. It is shown that spatial localization of electrons leads to Raman scattering processes for which momentum is not conserved, and hence to the activation of the density of states of acoustic phonons. A model, based on electron-acoustic phonon interaction, is developed and used for calculations of the resonant Raman efficiency. A good agreement with measured spectra is obtained. The origin of the observed periodic oscillations is interpreted by considering standing acoustic waves. [S0031-9007(97)03207-9] During the last decade considerable interest has been devoted to the vibrational properties of semiconductor superlattices (SL’s) and multiple quantum wells (QW’s), leading to a detailed understanding of phonons and their coupling with electrons [1,2]. It is now well established that, in these structures, vibrational excitations could be confined [3 ‐ 5] (optical phonons), localized [6,7] (interface phonons), or folded [8] (acoustic phonons) depending on the energy range and on the elastic and dielectric properties of the constituents. Almost all of these features were pointed out by means of Raman scattering measurements. In contrast, only a few results [9,10] are available for single QW’s (or even for a small number of QW’s), because the associated scattering volume is considerably reduced, leading to a serious experimental limitation. Beyond the experimental challenge, the study of such structures is of particular interest with regard to both vibrational and electronic properties. Indeed, in single quantum wells electrons are localized in real space which leads to a breakdown of the translational symmetry along the growth axis. In the acoustic frequency range this effect results in a strongly resonant continuous scattering [11,12]. In a previous work [10], we showed that frequency location, line shape, and intensity of the continuous scattering is mainly determined by the form factor associated with localized electron-acoustic phonon interaction. In this Letter, we report on resonant Raman measurements on GaAsyGaP quantum wells as thin as one monolayer. In this extreme situation of electron localization, a strong continuous emission appears again in the low-frequency range. In addition, periodic oscillations of the continuous scattering are observed. The origin of the continuous scattering and its oscillatory behavior is discussed and analyzed by taking into account both spatial localization of electrons and possible existence of standing acoustic waves. A model based on electron-acoustic phonon interaction in a single quantum well is developed, and a comparison between calculated and measured Raman spectra is presented. Our most exciting findings are as follows: (i) electronic wave functions can be obtained from low-frequency resonant Raman spectra; (ii) for the first time, standing acoustic waves could be observed. The structures investigated were grown by atomic layer molecular beam epitaxy (ALMBE) on a (001) oriented GaP substrate. The samples consist of three GaAs QW’s separated by wide (38 nm) GaP barriers. The first QW is at 80 nm underneath the sample surface. The GaAs QW’s are of equal thickness ranging from 1 to 4 monolayers (ML) in steps of 1 ML. Only the results obtained from 1 ML GaAs QW’s are presented here, since the other samples showed very similar features. The lattice mismatch between GaAs and GaP is about 4% (with respect to GaP), and the critical thickness for dislocation formation (and strain relaxation) is around 6 ML [13]. So, the GaAs QW’s are biaxially strained and fully matched to GaP. As a matter of fact, the high energy electron diffraction patterns showed neither threedimensional growth nor appreciable roughness. Calculation of the band structure of GaAs strained to GaP was performed according to the deformation potential theory [14]. It was found that the GaAs layer is indirect in k space. Indeed, due to strain, the lowest energy conduction states are of Xxy type. For G-band electrons, the GaAs layer forms a well which is 0.5 eV deep. The energy of transition between confined heavy hole (hh) and G-electron states was determined by electroreflectance spectroscopy performed at liquid nitrogen temperature. It was found at 2.73 eV, for the 1 ML GaAs QW’s. Raman experiments were performed at liquid nitrogen temperature and in near-backscattering configuration. The spectra were excited using the output lines of an Ar laser. The scattered light was analyzed using a triple spectrometer and detected with a conventional photon

In-plane strained superlattices grown on slightly misoriented (1 1 0) InP substrates by molecular beam epitaxy

InAs/GaAs in-plane strained superlattices (IPSSLs) were grown by molecular beam epitaxy. The substrates used were misoriented (1 1 0) InP tilted in the [0 0 1] direction. We grew alternately half monolayers of InAs and GaAs while keeping the periodic single monolayer steps. The obtained structures were evaluated by transmission electron microscopy (TEM) and photoluminescence (PL) measurements. The TEM images revealed that the laterally periodic structures were formed almost in the [0 0 1] direction. The lateral period corresponded to the mean terrace width of the substrate. The PL peak energy obtained at 77 K was 0.77 eV, which was 40 meV lower than those of homogeneous InGaAs alloys. From the estimation of the transition energy dependence of In 1-x Ga x As/In x Ga 1-x As strained superlattices on the mixing ratio x, we deduced that the obtained IPSSL was composed of In 0.74 Ga 0.26 As/In 0.26 Ga 0.74 As. This composition was consistent with the measurement by energy-dispersive X-ray microanalysis.

Reducing the defect density in heterostructures

Abstract The defect densities that are observed in 1 μm ZnSe films grown by molecular beam epitaxy on various III–V buffer layers are compared. Cathodoluminescence (CL) imaging has been used to provide a rapid and non-destructive method of characterizing the defect densities and was found to be in agreement with both etch pit density and transmission electron microscopy characterizations. We also examined the defect density of ZnSe epilayers grown on relaxed buffer layers of (In,Ga)P (lattice-matched to ZnSe) with the phosphide surface capped with eight monolayers of GaAs exhibiting various surface reconstructions. In this manner, the ZnSe GaAs surface chemistry is maintained while simultaneously achieving a lattice-matched buffer layer to ZnSe. Films grown without special nucleation precautions on c(4 × 4)-reconstructed GaAs were typically found to result in defect densities larger than 107 cm−2. Using a Zn pre-exposure on (2 × 4)-reconstructed GaAs, the defect densities were decreased by over an order of magnitude. In this paper, the microstructural and optical properties of ZnSe on phosphide-containing III–V buffer layers will be described and contrasted to the more conventional ZnSe GaAs heterostructure, using atomic force microscopy and CL.

Temperature dependence of interlayer coupling in δ MnGa/(Ga,As,Mn)/δ MnGa trilayers

We present the temperature dependence of the interlayer coupling, from 15 to 300 K, in ferromagnet/semiconductor/ferromagnet trilayers with the nominal structure of 10 nm δMn60Ga40/GaAs (n monolayers)/20 nm δMn54Ga46 [n=6–16 monolayers (ML) nominally] grown on (001) GaAs substrates by molecular-beam epitaxy. Since compositional analysis showed a strong diffusion of Mn into the GaAs spacer layer, we represent the trilayer structure as MnGa/(Ga,As,Mn)/MnGa. The magnetic-circular-dichroism loops showed antiferromagnetic coupling between both MnGa layers for the samples with a spacer layer from 6 to 14 ML GaAs nominally in the whole temperature range. The temperature coefficient of the coupling field was found to be positive for 6–8 ML spacers and negative for 10–14 ML spacers. We interpret these facts as the competition between two (or more) coupling mechanisms. For the sample with a 16 ML GaAs spacer layer, the interlayer coupling was antiferromagnetic below 200 K, but ferromagnetic above 200 K.

Observation of antiferromagnetic coupling in δ-MnGa/(Mn,Ga,As)/δ-MnGa trilayers

We present the magnetic properties of δ-MnGa/(Mn,Ga,As)/δ-MnGa trilayers. The spacer layer consists of nominally 2 to 19 monolayers (ML) GaAs, but we have indications for an important incorporation of Mn and the possible formation of antiferromagnetic Mn 2As. Antiferromagnetic (AFM) coupling is observed for spacer layer thicknesses of 4 to 14 ML GaAs, ferromagnetic (FM) coupling exists outside this region. The largest observed coupling field was −87.0 mT (−870 Oe) at 17 K for a sample with a 12 ML spacer layer, causing a cross-over between both branches of the hysteresis loop and a negative remanence. In one sample (16 ML) the coupling changes from AFM at low temperature to FM at room temperature.

Reaction mechanisms of mono-molecular layer growth using chemical adsorption

The surface reaction mechanism of GaAs monolayer after monolayer growth using chemical adsorption of Ga(CH 3) 3 (trimethylgallium (TMG)) and arsine (AsH 3) was investigated. The monolayer growth on GaAs(100) was investigated as a function of injection duration, injection pressure, evacuation duration of TMG and AsH 3 and photo-irradiation in an ultra-high vacuum system. It was found that the growth rate per cycle was strongly influenced by the surface stoichiometry of arsenic during the growth. Furthermore, in-situ analysis of the monolayer growth by using mass spectroscopy and the measurement of surface reflectivity during monolayer growth was performed. As the result, a model for the surface reaction and the adsorbed species of mono-molecular layer growth was obtained.

Observation of lasing from vertically self-organized InAs three-dimensional island quantum boxes on GaAs (001)

We report the first observation of lasing from vertically self-organized multiple stacks of electronically uncoupled InAs three-dimensional island quantum boxes grown via molecular beam epitaxy on GaAs (001) substrates. A low-threshold current density of 310 A/cm/sup 2/ at 79 K is found for a stack of five sets of islands corresponding to 2 ML InAs depositions separated by 36 monolayer GaAs spacers grown via migration enhanced epitaxy. The distribution of the island volumes (1.5/spl times/10/sup 5/ /spl Aring//sup 3/-4 /spl times/10/sup 5/ /spl Aring//sup 3/) gives, as expected, a multitude of laser lines between 980 mm and 996 nm.