Investigation of the structural properties of GaAs layers grown by molecular-beam epitaxy at low temperatures

The results of an investigation of the structural perfection of GaAs epitaxial films grown by molecular-beam epitaxy at low growth temperatures (240–300 °C) and various As/Ga flux ratios (from 3 to 13) are presented. Diffraction reflection curves display characteristic features for the samples before and after annealing in the temperature range from 300 to 800 °C. Hypotheses which account for these features are advanced. The range of variation of the arsenic/gallium flux ratio, in which low-temperature growth takes place under nearly stoichiometric conditions, is established. Fiz. Tekh. Poluprovodn. 31, 1168–1170 (October 1997)     

Structure and properties of InGaAs layers grown by low-temperature molecular-beam epitaxy

This paper describes studies of InGaAs layers grown by molecular-beam epitaxy on InP (100) substrates at temperatures of 150–480 °C using various arsenic fluxes. It was found that lowering the epitaxy temperature leads to changes in the growth surface, trapping of excess arsenic, and an increased lattice parameter of the epitaxial layer. When these lowtemperature (LT) grown samples are annealed, the lattice parameter relaxes and excess arsenic clusters form in the InGaAs matrix. For samples grown at 150 °C and annealed at 500 °C, the concentration of these clusters was ∼8×10¹⁶ cm⁻³, with an average cluster size of ∼5 nm. Assuming that all the excess arsenic is initially trapped in the form of antisite defects, the magnitude of the LT-grown InGaAs lattice parameter relaxation caused by annealing implies an excess arsenic concentration (N _As−N _Ga−N _In)/(N _As+N _Ga+N _In)=0.4 at.%. For layers of InGaAs grown at 150 °C, a high concentration of free electrons (∼1×10¹⁷ cm⁻³) is characteristic. Annealing such layers at 500 °C decreases the concentration of electrons to ∼1×10¹⁷ cm⁻³. The results obtained here indicate that this change in the free-electron concentration correlates qualitatively with the change in excess arsenic concentration in the layers. Fiz. Tekh. Poluprovodn. 33, 900–906 (August 1999)     

p+?n hyperabrupt GaAs varactors grown by molecular-beam epitaxy

Gallium-arsenide p?n junction hyperabrupt varactor diodes have been grown by molecular-beam epitaxy. Near the junction the donor profile at depth x tracked x?1.2. A capacitance ratio C0/C12 of 10 is observed for bias voltages of 0 and 12 V.     

Electrical properties of Si:Er/Si layers grown by sublimation molecular-beam epitaxy

Temperature dependences of the concentration and electron Hall mobility in Si:Er/Sr epitaxial layers grown at T = 600°C and annealed at 700 or 900°C have been investigated. The layers were grown by sublimation molecular-beam epitaxy in vacuum (～10^?5 Pa). The energy levels of Er-related donor centers are located 0.21–0.27 eV below the bottom of the conduction band of Si. In the range 80–300 K, the electron Hall mobility in unannealed Si:Er epitaxial layers was lower than that in Czochralski-grown single crystals by a factor of 3–10. After annealing the layers, the fraction of electron scattering from Er donor centers significantly decreases.     

Electrical properties of ZnSe/GaAs (100) heterostructures grown by photostimulated vapor-phase epitaxy

Epitaxial layers of ZnSe were grown on GaAs (100) by photostimulated vapor-phase epitaxy, using a He-Cd laser (power P⋍1 mW/cm², hν=2.807 eV) at a substrate temperature of T= (175–300) K. The temperature dependences of the mobility of the majority charge carriers were studied in layers doped during growth, using such sources as AlCl₃, Zn, Al, and Ga. Based on an analysis of the thermally stimulated current and thermally stimulated depolarization curves, parameters were established for seven deep local levels that have a substantial effect on the electrical characteristics of the heterostructures. Fiz. Tekh. Poluprovodn. 31, 11–14 (January 1997)     

Spatially resolved Raman investigation of Si-doped GaAs layers on patterned GaAs(100) substrates grown by molecular beam epitaxy

Local carrier transport properties of Si-doped GaAs layers on ridge structures exhibiting (111)A and (111)B sidewalls are investigated. The layers were grown by molecular beam epitaxy at different substrate temperatures and As/Ga flux ratios. Using spatially resolved Raman spectroscopy we determine the type and density of free charge carriers (≥ 5 × 10¹⁷ cm⁻³) in the grown layers on the different index facets from an analysis of the coupled plasmon-longitudinal optical-phonon mode which was calibrated against Hall standards. We demonstrate that on the (100) and (111)B facets the regrown layers are n-type and on the (111)A facets p- or n-type depending on the growth conditions. Line scans of the carrier density show that the (100)/(111)A/(100) facet transition forms a graded lateral n-p-n junction. Spatially resolved photoluminescence measurements confirm our findings.     

Strain effects in CdTe (111) epitaxial layers grown on GaAs (100) substrates by molecular beam epitaxy

Spectroscopic ellipsometry and photoreflectance measurements on CdTe/GaAs strained heterostructures grown by moleculclr beam epitaxy were carried out to investigate the effect of the strain and the dependence of the lattice parameter on the CdTe epitaxial layer thicknesses. Compressive strains exist in CdTe layers thinner than 2 μm. As the strain increases, the value of the critical-point energy shift increases linearly. These results indicate that the strains in the CdTe layers grown on GaAs substrates are strongly dependent on the CdTe layer thickness. Author to whom all correspondence should be addressed.     

Phase separation in Ga-doped MgZnO layers grown by plasma-assisted molecular-beam epitaxy

We report on the phase separation of Ga-doped MgZnO layers grown by plasma-assisted molecular-beam epitaxy. Based on X-ray diffraction, low-temperature (10 K) photoluminescence, and reflection high-energy electron diffraction observations, it is possible to classify the phase of Ga-doped MgZnO layers into three regions depending on the incorporated Ga concentration ([Ga]). Single-phase Mg_0.1Zn_0.9O layers are grown when [Ga] is less than 1×10¹⁸ cm⁻³. For [Ga] between 1×10¹⁸ cm⁻³ and 1×10²⁰ cm⁻³, ZnO and Mg_0.2Zn_0.8O coexist, where electron transport is considered to be via two-channel conduction. When [Ga] exceeds 1×10²⁰ cm⁻³, the Ga-doped MgZnO layers become polycrystalline, where carrier compensation takes place presumably due to grain boundaries.     

Superlattice optical-cavity multiple-quantum-well (SOC-MQW) lasers grown by molecular-beam epitaxy

A new-structure multiple-quantum-well laser utilising superlattice waveguides and superlattice barriers is proposed and fabricated by molecular-beam epitaxy using GaAs/AlGaAs. Room-temperature CW operation with a threshold current of 40 mA and external differential quantum efficiency per facet of 20% is demonstrated.     

Surface wave excitation auger electron spectroscopy of lnGaAs/GaAs(001) grown by alternate molecular-beam epitaxy

An angle-resolved electron-beam-excitation Auger-electron-spectroscopy, called as “surface wave excitation Auger electron spectroscopy (SWEAES),” was developed to characterize the semiconductor surface on an atomic scale. SWEAES enables us to get information about (1) surface valence electron states, (2) surface inner potentials concerned with high energy electron diffraction at the surface wave resonance (SWR) condition, (3) surface composition of ultra-thin heterostructures, and (4) confinement effects of diffracted surface electron waves at the SWR condition. These effects were demonstrated for In_1-x Ga _x As/GaAs(001)-c(8 × 2)-In/Ga and (GaAs)₂/(InAs)₁/GaAs(001) strained single quantum well surfaces. The current address: Dept. Elect. Eng., Faculty of Engineering, Kobe University, 1-1 Rokkodai-cho, Nada-Ku, Kobe, 657, JAPAN     

Channel-buffer (substrate) interface phenomena in GaAs MESFETsfabricated by molecular-beam epitaxy

A fundamental understanding of the channel-buffer (substrate) interface is obtained from an experimental and theoretical investigation of long-channel GaAs MESFETs fabricated by molecular-beam epitaxy (MBE). A low-field channel pinchoff mechanism is studied in which the carrier concentration in the near-pinchoff regime is obtained from an exact solution of Poisson's equation in such a way as to conserve the total charge in the entire device. One the basis of these calculations it is shown that resolution of the order of a Debye length is feasible in the determination of the impurity profile in the vicinity of homojunction interfaces from the conventional capacitance-voltage profiling technique. The threshold-voltage dependence on the semiconductor crystal lattice temperature indicates that steady-state carrier trapping and accumulation of operation. The variations of the threshold voltage and negative charge in donor-like states at the channel-buffer (substrate) interface play a detrimental role in both the linear and saturated regimes of device gate capacitance with substrate bias are consistent with each other and suggest that a significant reduction in the channel charge occurs with increased reverse bias on the substrate     

Manipulation of morphology and structure of the top of GaAs nanowires grown by molecular-beam epitaxy

Self-catalyzed GaAs nanowires (NWs) are grown on Si (111) substrates by molecular-beam epitaxy. The effect of different closing sequences of the Ga and As cell shutters on the morphology and structural phase of GaAs NWs is investigated. For the sequences of closing the Ga and As cell shutters simultaneously or closing the As cell shutter 1 min after closing the Ga cell shutter, the NWs grow vertically to the substrate surface. In contrast, when the As cell shutter is closed first, maintaining the Ga flux is found to be critical for the following growth of GaAs NWs, which can change the growth direction from[111] to <111>. The evolution of the morphology and structural phase transition at the tips of these GaAs NWs confirm that the triple-phase-line shift mode is at work even for the growth with different cell shutter closing sequences. Our work will provide new insights for better understanding of the growth mechanism and realizing of the morphology and structure control of the GaAs NWs.     

Electrically active centers in Si:Er light-emitting layers grown by sublimation molecular-beam epitaxy

Electrically active centers in light-emitting Si:Er layers grown by sublimation molecular-beam epitaxy (SMBE) on single-crystal Si substrates have been investigated by admittance spectroscopy with temperature scanning and by DLTS. The total density of electrically active centers is defined by shallow donor centers with ionization energies of 0.016–0.045 eV. The effect of growth conditions and post-growth annealing on the composition and density of electrically active centers has been studied. Significant differences in composition of the electrically active centers with deep levels and in channels of energy transfer from the electron subsystem of a crystal to Er³⁺ ions between Si:Er layers grown by SMBE and ion implantation have been revealed.     

Metal-organic vapor-phase epitaxy growth and characterization of thick (100) CdTe layers on (100) GaAs and (100) GaAs/Si substrates

The growth characteristics and crystalline quality of thick (100) CdTe-epitaxial layers grown on (100) GaAs and (100) GaAs/Si substrates in a metal-organic vapor-phase epitaxy (MOVPE) system for possible applications in x-ray imaging detectors were investigated. High-crystalline-quality epitaxial layers of thickness greater than 100 μm could be readily obtained on both types of substrates. The full width at half maximum (FWHM) values of the x-ray double-crystal rocking curve (DCRC) decreased rapidly with increasing layer thickness, and remained around 50–70 arcsec for layers thicker than 30 μm on both types of substrates. Photoluminescence (PL) measurement showed high-intensity excitonic emission with very small defect-related peaks from both types of epilayers. Stress analysis carried out by performing PL as a function of layer thickness showed the layers were strained and a small amount of residual stress, compressive in CdTe/GaAs and tensile in CdTe/GaAs/Si, remained even in the thick layers. Furthermore, the resistivity of the layers on the GaAs substrate was found to be lower than that of layers on GaAs/Si possibly because of the difference of the activation of incorporated impurity from the substrates because of the different kinds of stress existing on them. A heterojunction diode was then fabricated by growing a CdTe epilayer on an n⁺-GaAs substrate, which exhibited a good rectification property with a low value of reverse-bias leakage current even at high applied biases.     

Structural properties of GaAsN grown on (001) GaAs by metalorganic molecular beam epitaxy

Detailed transmission electron microscopy (TEM) and transmission electron diffraction (TED) examination has been made of metalorganic molecular beam epitaxial GaAsN layers grown on (001) GaAs substrates. TEM results show that lateral composition modulation occurs in the GaAs_1−xN_x layer (x 6.75%). It is shown that increasing N composition and Se (dopant) concentration leads to poor crystallinity. It is also shown that the addition of Se increases N composition. Atomic force microscopy (AFM) results show that the surfaces of the samples experience a morphological change from faceting to islanding, as the N composition and Se concentration increase. Based on the TEM and AFM results, a simple model is given to explain the formation of the lateral composition modulation.     

Photoluminescence investigation of Si-doped GaAs grown by molecular beam epitaxy on non-(100) oriented surfaces

The photoluminescence of Si-doped GaAs grown by molecular beam epitaxy on non-(100) oriented surfaces will be presented and reviewed. Particular care will be paid to show how photoluminescence can help the understanding of the behaviour of the dopants, unwanted impurities and point defects in differently oriented samples. Low temperature continuous-wave and time resolved luminescence data will be presented of (100), (111)A, (111)B, (211)A., (311)A, (311)B and (110) oriented samples with different doping levels.     

Photoluminescence study of GaAs films on Si(100) grown by atomic hydrogen-assisted molecular beam epitaxy

Preliminary experimental results and analysis of photoluminescence (PL) measurements performed on GaAs heteroepitaxial films, which have been grown on Si(100) substrates by atomic hydrogen-assisted low-temperature molecular beam epitaxy technique have been presented and discussed. The results have also been compared with those obtained for GaAs homoepitaxial films. Furthermore, minority carrier lifetimes in n-GaAs on Si have been characterized by the PL decay method and an average lifetime of as high as 8.0 ns has been successfully obtained, which is the highest value ever reported to date.     

Low threading dislocation density GaAs on Si(100) with InGaAs/GaAs strained-layer superlattice grown by migration-enhanced epitaxy

This paper reports a promising approach for reducing the density of threading dislocations in GaAs on Si. In _x Ga_1-x As/GaAs strained-layer superlattices (SLSs) grown by migration-enhanced epitaxy at 300° C on GaAs/Si acted as barriers to threading dislocations. Unlike conventional high-temperature-grown SLSs, the low-temperature-grown SLSs were hardly relaxed by the formation of misfit dislocations at GaAs/SLS interfaces, and this allowed them to accumulate considerable strain. New threading dislocation generation due to the misfit dislocation was also suppressed. These factors caused effective bending of threading dislocations and significantly reduced the dislocation density. For the samples that had an SLS withx = 0.3, the average etch-pit density was 7 × 10⁴ cm^-2, which is comparable to that of GaAs substrates.     

Lateral ordering of GaAs nanowhiskers on GaAs(111)As and GaAs (110) surfaces during molecular-beam epitaxy

Possibilities of obtaining laterally ordered arrays of GaAs nanowhiskers on GaAs (110) and GaAs(111)As surfaces during the molecular beam epitaxy are considered. As in the case of the GaAs(111)As substrate, nanowhiskers are formed in the hexagonal phase on the GaAs(110) surface, which is also confirmed by the patterns of the reflection high-energy electron diffraction (obtained during the growth of nanowhiskers) and by the photoluminescence spectra.