Dependence of the microstructural properties on the substrate temperature in strained CdTe (1 0 0)/GaAs (1 0 0) heterostructures

CdTe thin films were grown on GaAs (1 0 0) substrates by using molecular beam epitaxy at various temperatures. The bright-field transmission electron microscopy (TEM) images and the high-resolution TEM (HRTEM) images showed that the crystallinity of CdTe epilayers grown on GaAs substrates was improved by increasing the substrate temperature. The result of selected-area electron diffraction pattern (SADP) showed that the orientation of the grown CdTe thin films was the (1 0 0) orientation. The lattice constant the strain, and the stress of the CdTe thin film grown on the GaAs substrate were determined from the SADP result. Based on the SADP and HRTEM results, a possible atomic arrangement for the CdTe/GaAs heterostructure is presented.     

Existence of a Cu3Au-type ordered structure in CdxZn1−xTe epilayers grown on (100) GaAs substrates

Selected area electron diffraction pattern (SADP) and transmission electron microscopy (TEM) measurements were carried out to investigate the spontaneously ordered structure in Cd_xZn_1−xTe epilayers grown on GaAs (100) substrates. The SADP showed superstructure reflections with symmetrical intensity, and the high-resolution TEM (HRTEM) micrographs showed doublet periodicity in the contrast of the {100} lattice planes. The results of the SADP and the HRTEM measurements showed a Cu₃Au-type ordered structure was formed in the Cd_xZn_1−xTe epilayer. The present results can help improve understanding of the Cu₃Au-type ordered structures in Cd_xZn_1−xTe epilayers grown on GaAs substrates.     

Raman spectroscopy of optical phonons as a probe of GaN epitaxial layer structural quality

We report on Raman scattering measurements of all Raman-active phonons in wurtzite and zinc blende structure GaN epilayers grown on GaAs (001), GaAs (111)A, and GaAs (111)B oriented substrates by means of molecular beam epitaxy (MBE). Raman spectra are taken from these epilayers at room temperature and 77 K in backscattering geometry. The measured values of the phonon frequencies are in agreement with other studies and with lattice dynamic calculations of phonon modes in GaN zinc blende and wurtzite structures. We show that crystal quality is much better in samples grown on GaAs (111) substrates than in samples grown on GaAs (001) substrates. The observation of disorder-activated modes gives information about sample quality. Comparison of the spectra from different thickness epilayers shows that the GaN is more highly disordered close to the substrate, particularly for the (001) substrates. Received 16 July 1999     

Observation of oxygen contamination at ZnSe/GaAs interfaces using SIMS

ZnSe epilayers were grown on GaAs (1 0 0) substrates using MBE. The native contamination (oxide and carbon) was removed in situ from the substrate surfaces by conventional thermal cleaning and by exposure to atomic hydrogen. A maximum substrate temperature of 600 °C was required for the thermal cleaning process, while a substrate temperature of 450 °C was sufficient to clean the substrate using hydrogen. ZnSe epilayers were also grown on As capped GaAs epilayers, which were decapped at a maximum temperature of 350 °C. SIMS profiles showed the existence of oxygen at the interface for all of the substrate preparation methods. The oxygen surface coverage at the interface was found to be 0.03% for the atomic hydrogen cleaned substrate and 0.7% for the thermally cleaned substrate.     

Surface preparation effect of GaAs(110) substrates on the ZnSe epitaxial layer grown by molecular beam epitaxy

The effect of the surface preparation of the GaAs(110) substrate on the ZnSe epitaxial layer grown by molecular beam epitaxy (MBE) was investigated by means of etch-pit density (EPD) measurements, surface morphology observation, and reflection high-energy electron diffraction (RHEED) analysis. The ZnSe epitaxial layer grown on a GaAs(110) surface prepared by cleaving the (001)-oriented wafer in ultrahigh vacuum (UHV) showed about 5×10⁴ cm^-2 of EPD. This value is much lower than that observed from both the samples grown on the mechanically polished surface with and without a GaAs buffer layer. Due to the non-stoichiometric surface after thermal evaporation of the surface oxide, three-dimensional growth can easily occur on the mechanically polished GaAs(110) substrate. These results suggest that the stoichiometric and atomically flat substrate surface is essential for the growth of low-defect ZnSe epitaxial layers on the GaAs(110) non-polar surface. Received: 21 August 1998 / Accepted: 19 October 1998 / Published online: 28 April 1999     

STUDY OF MOLECULAR BEAM EPITAXIAL GROWTH AND OPTICAL CHARACTERISTICS OF HgCdTe

HgCdTe epilayers were grown by molecular beam epitaxy on GaAs(211)B substrates. Process of the HgCdTe epitaxial growth can be monitored by reflection high-energy electron diffraction. Results of the infrared transmission spectrum, Raman scattering spectrum and infrared photoluminescence spectrum in magnetic field have been studied.     

Crystal structures of two variants for CuPtB-type ordering in strained CdxZn1−xTe epilayers

Two variants of CuPt_B-type orderings in strained Cd_xZn_1−xTe epilayers were investigated by using transmission electron microscopy (TEM) and selected area diffraction pattern (SADP) measurements. The TEM images on the Cd_0.15Zn_0.85Te epilayers depicted strong contrast modulations along the [110] direction, and the SADP images showed superstructure reflection spots corresponding to a CuPt_B-type ordering. Possible crystal structures for the two variants of CuPt_B-type ordering in the Cd_xZn_1−xTe epilayers, which were determined from the SADP images, are presented.     

Interface roughness of GaAs/AlAs superlattices MBE-grown on vicinal surfaces

The interface morphology of GaAs/AlAs superlattices grown by molecular beam epitaxy on misoriented (001) GaAs substrates has been investigated using X-ray diffraction techniques in addition to high-resolution transmission electron microscopy (HRTEM). We observe that the width and intensity of the satellite peaks are very sensitive to the step-edge orientation. Among the investigated ones ([ 110], [100], [110]), it is the [ 10] step-edge direction which is the most favourable to a regular growth of superlattice structures. Structural models based on HRTEM observations (step distribution at interfaces and local fluctuation of layer thickness) have been constructed, allowing an explanation of the X-ray diagrams.     

Formation process for and strain effect in InAs quantum dots grown on GaAs substrates by using molecular beam epitaxy

Reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM) measurements were used to investigate the dependences of the formation process and the strain on the As/In ratio and the substrate temperature of InAs quantum dots (QDs) grown on GaAs substrates by using molecular beam epitaxy. The thickness of the InAs wetting layer and the shape and the size of the InAs QDs were significantly affected by the As/In ratio and the substrate temperature. The strains in the InAs layer and the GaAs substrate were studied by using RHEED patterns. The magnitude in strain of the InAs QDs formed at a low substrate temperature was larger than that in InAs QDs grown at high substrate temperature. The present results can help to improve the understanding of the formation process and the strain effect in InAs QDs.     

Structure and homoepitaxial growth of GaAs(6 3 1)

We have studied the surface atomic structure of GaAs(6 3 1), and the GaAs growth by molecular beam epitaxy (MBE) on this plane. After the oxide desorption process at 585 °C reflection high-energy electron diffraction (RHEED) showed along the [−1 2 0] direction a 2× surface reconstruction for GaAs(6 3 1)A, and a 1× pattern was observed for GaAs(6 3 1)B. By annealing the substrates for 60 min, we observed that on the A surface appeared small hilly-like features, while on GaAs(6 3 1)B surface pits were formed. For GaAs(6 3 1)A, 500 nm-thick GaAs layers were grown at 585 °C. The atomic force microscopy (AFM) images at the end of growth showed the self-formation of nanoscale structures with a pyramidal shape enlarged along the [5−9−3] direction. Transversal views of the bulk-truncated GaAs(6 3 1) surface model showed arrays of atomic grooves along this direction, which could influence the formation of the pyramidal structures.     

Influence of domain boundaries on polarity of GaN grown on sapphire

GaN films were grown on sapphire substrates by laser-induced reactive epitaxy. The domains in the films were determined to be the Ga-polarity by the convergent beam electron diffraction (CBED) technique, while the adjacent matrices had the N-polarity. The domain boundaries were characterized as inversion domain boundaries (IDBs). An atomic structure of the IDB is proposed based on high-resolution transmission electron microscopy (HRTEM) investigations. Control of the polarity of GaN/sapphire films was achieved by suppressing the formation of IDBs with an interlayer of AlGaN and a low-temperature GaN buffer layer.     

Effect of substrate orientation on the defect-induced bound exciton emissions in GaAs grown by molecular beam epitaxy

The dependence of photoluminescence (PL) spectra at 2 K on the substrate orientation is reported for GaAs grown by molecular beam epitaxy (MBE). Samples investigated are GaAs grown on (n11)A- and (n11)B-oriented substrates where n is 2, 3, 5 and 9, and those on (321)A- (110)B- and (100)A-oriented ones. It was demonstrated that emissions denoted by defect-induced bound excitons (DIBE) present strong crystal orientation dependence. In the (n11) series, DIBE was enhanced peculiarly in (211)A and (311)A substrates and two sharp emissions denoted by t^* and v₃ were specifically observed there, but they were completely quenched for the (211)B substrate. When n exceeds 3, DIBE was observed ubiquitously in (n11)A and (n11)B substrates. The (321)A substrate indicated a most idiosyncratic DIBE with a strong enhancement of t^* and v₃. These results show that the spectral features pertinent to DIBE fully reflect the whole stacking process of impinging molecular beams on the growth front which is governed by the surface structure inherent to the surface dangling bonds of the substrate.     

Growth, Antimony Incorporation Behaviour and Beryllium Doping of GaAs1-ySby Grown on GaAs by Molecular Beam Epitaxy

A series of GaAs1-ySby epilayers are grown on GaAs substrates under antimony compositions of samples with beryllium doping are obtained different growth conditions. Different A non-equilibrium thermodynamics model is used to calibrate and fit the Sb composition. Activation energy of 0.37 eV for the dissociation process of Sb4 molecules is obtained. Carrier mobility and concentration of samples are influenced by the Sb composition. Quasi-qualitative analysis of mobility is used to explain the relations among Sb composition, carrier mobility and concentration. High resolution x-ray diffraction （HRXRD） rocking curves and Hall effects measurements are used to determine the crystal quality, carrier mobility and concentration.     

Photoreflectance characterization of CdTe thin films grown by Molecular Beam Epitaxy on GaAs(100)

CdTe films have been grown on top of GaAs(100) by means of Molecular Beam Epitaxy (MBE) at 300 °C substrate temperature. Different procedures for the CdTe growth and for the preparation of the GaAs substrates resulted in diverse crystalline qualities of the CdTe films. We present the results obtained from PhotoReflectance (PR) measurements of these films employing HeNe and Ar-ion lasers as modulating excitation. For Ar excitation, the ratio of CdTe to GaAs signal strength for the E ₀ transition is enhanced, allowing a differentiation of the contributions from film and substrate. Both the PR line shape and intensity are correlated to the structural quality of the CdTe films. One of the samples presented a below-band-gap transition of the GaAs substrate around 30±5 meV from E ₀ which is attributed to donor states produced by Te atoms diffused in the interface; this result demonstrates the high sensitivity of the photoreflectance technique to the structural properties of interfaces.     

The effect of substrate material on pulsed laser deposition of HgCdTe films

This paper describes some recent results of the HgCdTe thin film grown directly on different substrates (sapphire, GaAs and Si) by pulsed laser deposition (PLD). The influences of the substrate material on the properties of HgCdTe thin films were investigated by X-ray diffraction (XRD), selected area electron diffraction (SAED), atomic force microscopy (AFM) and energy dispersive X-ray spectroscopy (EDS). It was found that the quality of the HgCdTe film has a strong relation to the structure and properties of the substrate. The experiment results indicate that the HgCdTe epitaxial thin films grown directly on the sapphire substrates have a high quality, and the composition of the films is close to that of the target. While the quality of the HgCdTe films deposited on the Si and GaAs substrates are not very good.     

CdTe epilayers for uses in optical waveguides

CdTe epilayers have been grown by vapor phase epitaxy (VPE) on glass, MgO, sapphire, LiNbO₃ and mica substrates. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies show the good structural quality of the epilayers. In these epilayers, a few optical modes were excited with a 1.33-μm laser. The measured propagation losses were in the range between 5 dB/cm and less than 0.5 dB/cm. From dark-mode m-lines, the epilayer thickness was found to be in the 1–3 μm range, in good accord with that obtained by SEM measurements. The refractive index obtained from the fitting is also in good accord with that of bulk CdTe. Received: 7 October 1999 / Accepted: 13 March 2000 / Published online: 5 July 2000     

Characterization of InSb layers on GaAs substrates using infrared reflectance and a modified oscillator formula

InSb epilayers on GaAs substrates are analyzed using infrared reflectance spectroscopy, but employing a modified Drude oscillator formula. The modified formula enables the determination of 13 parameters: six dielectric parameters for both layer and substrate separately, as well as the thickness of the layer. The formula is tested against previously published data, and to characterize layers grown in this laboratory.     

Photoconductive response of GaAs epitaxial layers

In the present work the photoconductive response of low resistivity, n-type GaAs epitaxial layers is studied by experimentally monitoring the dependence of the photoconductive gain (PG) optoelectronic parameter upon incident photon flux and temperature. The characterized samples fall into three major categories: ion implanted (II) GaAs epilayers formed within undoped, semi-insulating GaAs substrates; GaAs epitaxial layers grown by liquid phase epitaxy (LPE) on Cr-doped, semi-insulating GaAs substrates; and ungated GaAs MESFETs.     

Control of symmetric properties of metamorphic In_(0.27)Ga_(0.73)As layers by substrate misorientation

Asymmetry in dislocation density and strain relaxation has a significant impact on device performance since it leads to anisotropic electron transport in metamorphic materials. So it is preferred to obtain metamorphic materials with symmetric properties. In this paper, we grew metamorphic In_(0.27)Ga_(0.73) As epilayers with symmetric low threading dislocation density and symmetric strain relaxation in two (110) directions using In Al Ga As buffer layers on 7°misoriented Ga As(001)substrates. To understand the control mechanism of symmetric properties of In_(0.27)Ga_(0.73) As layers by the substrate miscut angles, In_(0.27)Ga_(0.73) As grown on 2°and 15°misoriented substrates were also characterized as reference by atomic force microscopy, transmission electron microscopy, and high resolution triple axis x-ray diffraction. The phase separation and interaction of 60°misfit dislocations were found to be the reasons for asymmetry properties of In_(0.27)Ga_90.73 As grown on 2and 15°substrates, respectively. Photoluminescence results proved that the In_(0.27)Ga_(0.73) As with symmetric properties has better optical properties than the In_(0.27)Ga_(0.73) As with asymmetric properties at room temperature. These results imply that high quality metamorphic In_(0.27)Ga_(0.73) As can be achieved with controllable isotropic electron transport property.     

InGaP/GaAs heterojunction bipolar transistor grown by solid-source molecular beam epitaxy with a GaP decomposition source

Lattice-matched InGaP epilayers on GaAs (001) and InGaP/GaAs heterojunction bipolar transistors (HBTs) were successfully grown by solid-source molecular beam epitaxy (SSMBE) with a GaP decomposition source. A 3 μm thick InGaP epilayer shows that low temperature photoluminescence (PL) peak energy is as large as 1.998 eV, full width at half maximum (FWHM) is 5.26 meV, which is the smallest ever reported, and X-ray diffraction (XRD) rocking curve linewidth is as narrow as that of GaAs substrate. The electron mobilities at room temperature of nominally undoped InGaP layers obtained by Hall measurements are comparable to similar InGaP epilayer grown by solid-source molecular beam epitaxy (SSMBE) with other sources or other growth techniques. The large area InGaP/GaAs HBTs show very good Dc characteristics.