Metalorganic vapor phase epitaxial growth and structural characterization of GaAs/InP heterostructures

Highly mismatched GaAs epitaxial layers with thickness ranging from 15 nm to 7 μm have been grown on InP substrates by atomospheric pressure metalorganic vapor phase epitaxy. Layers thinner than 30 nm exhibited 3-D growth mechanism; in the thicker layers, the islands coalesced and then the growth followed the layer by layer mechanism. The elastic strain and the extended defects have been studied by high resolution x-ray diffraction and transmission electron microscopy, respectively. The common observation of planar defects, misfit, and threading dislocations in the layers has been confirmed. The results on the elastic strain release have been discussed on the basis of the equilibrium theory.     

Dislocation nucleation mechanism and doping effect in p-type ZnSe/GaAs

Nitrogen doped ZnSe/GaAs heterostructures grown at 150 and 250°C were studied by transmission électron microscopy (TEM). The density of threading dislocations and the interfacial dislocation structure in ZnSe/GaAs heterostructures are related to the N-doping concentration. In addition, in-situ TEM heating studies show that Frank partial dislocations formed below critical thickness in N-doped ZnSe/GaAs are the sources for nucleation of a regular array of misfit dislocations at the ZnSe/GaAs interface. By the dissociation of the Frank partial dislocations and interaction reactions between the dislocations, the 60° misfit dislocations form. The Frank partial dislocations bound stacking faults which usually form in pairs at the film/substrate interface. The density of stacking faults increases with increasing N-doping concentration. Thus, at high N-doping levels, the dislocation nucleation sources are close together and not all of the Frank partial dislocations dissociate, so that a high density of threading dislocations results in samples with high N-doping concentrations. The high density of threading dislocations in the ZnSe film are found to be associated with a reduction or saturation of the net carrier density.     

Thermal relaxation in strained InGaAs/GaAs heterostructures

The kinetics of strain relaxation during annealing of epitaxial InGaAs films grown on GaAs by MOCVD at atmospheric pressure was studied by optical microscopy and transmission electron microscopy. The density of misfit dislocations and crosshatching was measured as a function of annealing temperature and annealing time. These experiments show that the generation of misfit dislocations during the annealing process is thermally activated. The kinetic rate constant increases as thickness increases. After long annealing times, the sample reaches its steady-state condition in which a residual strain apparently still exists. Apparently this residual strain is not accommodated by misfit dislocations and does not change with annealing temperature nor sample thickness.     

Structural properties of nitrogen-doped ZnSe epitaxial layers grown by MBE

We have used transmission electron microscopy (TEM) and high-resolution x-ray diffraction (HRXRD) techniques to investigate the structural properties of ZnSe doped with nitrogen, in the concentration range of 1 × 10¹⁸ to 2 × 10¹⁹cm⁻³. The nitrogen-doped layers contain substantial residual compressive strain at layer thicknesses where undoped ZnSe would be completely relaxed. The residual strain is clearly observed both in the inequality of the lattice constants (measured by HRXRD) parallel and perpendicular to the growth direction, and in the reduction of the misfit dislocation density (measured by TEM) relative to undoped ZnSe. In addition to the reduction in dislocation density, the misfit dislocations form a regular rectangular grid, rather than the irregular array seen in undoped ZnSe. The effective relaxed ZnSe lattice constant, as measured by x-ray diffraction, decreases as the nitrogen concentration increases. For the highest nitrogen concentration, this reduction in lattice constant, however, is greater than can be explained by the shorter Zn-N bond distance of theoretical predictions.     

Lateral compositional modulation in lattice-matched GaInP/GaAs heterostructures

High-resolution transmission electron microscopy has been employed to study the microstructure of GaAs lattice-matched GaInP heterostructures grown by solid source molecular beam epitaxy. It is found that the GaInP epilayers undergo lateral compositional modulation at a growth temperature of 520°C. The modulating spacing is irregular, varying between 5.0–15.0 nm. The compositional difference in the two decomposed phases is estimated between 0.14–2.1 at.%, which is far from thermal equilibrium. High-resolution TEM observation shows that, corresponding to the contrast modulation, there exist considerable lattice distortions nearly parallel to the growth direction inside the GaInP epilayers. In the distorted regions, dislocations of 60°-type are frequently observed. Factors that may contribute to the compositional modulation are discussed.     

The thermal stability of lattice mismatched InGaAs grown on patterned GaAs

Patterning and etching substrates into mesas separated by trenches before the growth of mismatched (by about 1% or less) epitaxial layers considerably reduces the interface misfit dislocation density when the layer thickness exceeds the critical thickness. Such films are in a metastable state, since misfit dislocations allow the epitaxial layers to relax to an in-plane lattice parameter closer to its strain-free value. Thermal annealing (from 600 to 850° C) has been used to study the stability of these structures to explore the properties of the misfit dislocations and their formation. The misfit dislocation density was determined by counting the dark line defects at the InGaAs/GaAs interface, imaged by scanning cathodoluminescence. InGaAs epitaxial layers grown on patterned GaAs substrates by organometallic chemical vapor deposition possess a very small as-grown misfit dislocation density, and even after severe annealing for up to 300 sec at 800° C the defect density is less than 1500 cm⁻¹ for a In_0.04Ga_0.96As, 300 nm thick layer (about 25% of the dislocation density found in unpatterned material that has not been annealed). The misfit dislocation nucleation properties of the material are found to depend on the trench depth; samples made with deeper (greater than 0.5 μm) trenches are more stable. Molecular beam epitaxially grown layers are much less stable than the above material; misfit dislocations nucleate in much greater numbers than in comparable organo-metallic chemical vapor deposited material at all of the temperatures studied.     

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.     

AlGaSb/GaSb quantum wells grown on an optimized AlSb nucleation layer

Five-period AlGaSb/GaSb multiple quantum wells (MQW) are grown on a GaSb buffer.Through optimizing the AlSb nucleation layer,the low threading dislocation density of the MQW is found to be (2.50±0.91)×108cm-2 in 1-μm GaSb buffer,as determined by plan-view transmission election microscopy (TEM) images.High resolution TEM clearly shows the presence of 90°misfit dislocations with an average spacing of 5.4 ran at the AlSb/GaAs interface,which effectively relieve most of the strain energy.In the temperature range from T = 26 K to 300 K,photoluminescence of the MQW is dominated by the ground state electron to ground state heavy hole (el-hhl) transition,while a high energy shoulder clearly seen at T > 76 K.can be attributed to the ground state electron to ground state light hole (e1-lh1) transition.     

Growth and characterization of ZnO thin films on GaN epilayers

Dense ZnO(0001) films formed at 500°C via coalescence of islands grown via metalorganic vapor phase epitaxy (MOVPE) either on GaN/AlN/SiC(0001) substrates or on initial, coherent ZnO layers. Conical crystallites formed due to thermal expansion-induced stresses between the ZnO and the substrate. Interfaces between the ZnO films on GaN epilayers exposed either simultaneously to diethylzinc and oxygen or only to diethylzinc at the initiation of growth were sharp and epitaxial. Interfaces formed after the exposure of the GaN to O₂ were less coherent, though an interfacial oxide was not observed by cross-sectional transmission electron microscopy (TEM). Threading dislocations and stacking faults were observed in all films.     

Study of optical and electrical properties of AlxGa1−xSb grown by metalorganic chemical vapor deposition

Al_xGa_1−xSb films in the regime 0×0.25 have been grown by metalorganic chemical vapor deposition on GaAs and GaSb substrates using TMAl, TMGa, and TMSb precursors. We report growth conditions and film properties, including the effect of V/III ratio and growth temperature on electrical and optical properties. Growth temperatures in the range of 520°C and 680°C and V/III ratios from 1 to 5 have been investigated. All epilayers grown exhibit p-type behavior. The mobility decreases and the carrier concentration increases sharply when a small amount of Al is incorporated into GaSb. The sharp cutoff and Fabry-Perot oscillations of the transmission spectra of the AlGaSb layers confirm the high quality of the films. The principle photoluminescence features observed are attributed to bound exciton and donor-acceptor transitions with FWHM comparable to the best values reported elsewhere.     

1.54 lm GaSb/AlGaSb multi-quantum-well monolithic laser at 77 K grown on miscut Si substrate using interfacial misfit arrays

A GaSb quantum-well (QW) laser diode grown monolithically on a 5deg miscut Si (001) substrate is presented. The III-Sb epi-structure is grown monolithically on the miscut Si substrate via a thin (50 nm) AlSb nucleation layer. The 13% lattice mismatch between AlSb and Si is accommodated by a self-assembled 2D array of interfacial misfit dislocations (IMF). The 5deg miscut geometry enables simultaneous IMF formation and anti-phase domain suppression. The 1 mm times 100 mum GaSb QW laser diode operates under pulsed conditions at 77 K with a threshold current density of 2 kA/cm² and a maximum peak power of ~20 mW. Furthermore, the device is characterised by a 9.1 Omega forward resistance and a leakage current density of 0.7 A/cm² at -5 V.     

Structural properties of ZnSy Se1−yZnSe/GaAs (001) heterostructures grown by photoassisted metalorganic vapor phase epitaxy

ZnS_ySe_1−yZnSe/GaAs (001) heterostructures have been grown by photoassisted metalorganic vapor phase epitaxy, using the sources dimethylzinc, dimethylselenium, diethylsulfur, and irradiation by a Hg arc lamp. The solid phase composition vs gas phase composition characteristics have been determined for ZnSy_ySe_1−y grown with different mole fractions of dimethylselenium and different temperatures. Although the growth is not mass-transport controlled with respect to the column VI precursors, the solid phase composition vs gas phase composition characteristics are sufficiently gradual so that good compositional control and lattice matching to GaAs substrates can be readily achieved by photoassisted growth in the temperature range 360°C ≤ T ≤ 400°C. ZnSe/GaAs (001) single heterostructures were grown by a two-step process with ZnSe thicknesses in the range from 54 nm to 776 nm. Based on 004 x-ray rocking curve full width at half maximums (FWHMs), we have determined that the critical layer thickness is h_c ≤200 nm. Using the classical method involving strain, lattice relaxation is undetectable in layers thinner than 270 nm for the growth conditions used here. Therefore, the rocking curve FWHM is a more sensitive indicator of lattice relaxation than the residual strain. For ZnS_ySe_1−y layers grown on ZnSe buffers at 400°C, the measured dislocation density-thickness product Dh increases monotonically with the room temperature mismatch. Lower values of the Dh product are obtained for epitaxy on 135 nm buffers compared to the case of 270 nm buffers. This difference is due to the fact that the 135 nm ZnSe buffers are pseudomorphic as deposited. For ZnS_ySe_1−y layers grown on 135 nm ZnSe buffers at 360°C, the minimum dislocation density corresponds approximately to room-temperature lattice matching (y ∼ 5.9%), rather than growth temperature lattice matching (y ∼ 7.6%). Epitaxial layers with lower dislocation densities demonstrated superior optical quality, as judged by the near-band edge/deep level emission peak intensity ratio and the near band edge absolute peak intensity from 300K photoluminescence measurements.     

Electron microscope studies of InxGav1−xAs/GaAs/Si grown by metalorganic chemical vapor deposition

The microstructure of In_xGa_1−xAs/GaAs (5 nm/5 nm, x < 0 to 1.0), as grown by a metalorganic chemical vapor deposition two-step growth technique on Si(100) at 450‡C, and subsequently annealed at 750‡C, is investigated using plan-view and cross-sectional transmission electron microscopy. The variations in resultant island morphology and strain as a function of the In content were examined through the comparison of the misfit dislocation arrays and moirés observed. The results are discussed in relation to the ways in which the island relaxation process changes for high In content.     

Study of threading dislocations in the plan-view sample of SiGe/Si(001) superlattices by transmission electron microscopy

Interfacial defects due to a mismatch of 1.378% between substrate and epilayer were examined in a Si_0.67Ge_0.33/Si(001) superlattice by transmission electron microscopy (TEM). Plan-view specimens from the superlattice were prepared to investigate the defects in the structure. It was observed that 60°C-type misfit dislocations associate with point contrast on and at their ends. This point contrast was found to represent threading dislocations by using tilt experiments in the microscope. Consequently, stereo electron microscopy was used to examine the threading dislocations. It was discovered that the threading dislocations are not on the {111} slip planes but can be almost parallel to the [001] zone axis.     

Growth of high-quality GaSb by metalorganic vapor phase epitaxy

The growth of nominally undoped GaSb layers by atmospheric pressure metalorganic vapor phase epitaxy on GaSb and GaAs substrates is studied. Trimethylgallium and trimethylantimony are used as precursors for the growth at 600°C in a horizontal reactor. The effect of carrier gas flow, V/III-ratio, and trimethylgallium partial pressure on surface morphology, electrical properties and photoluminescence is investigated. The optimum values for the growth parameters are established. The carrier gas flow is shown to have a significant effect on the surface morphology. The optimum growth rate is found to be 3–8 μm/ h, which is higher than previously reported. The 2.5 μm thick GaSb layers on GaAs are p-type, having at optimized growth conditions room-temperature hole mobility and hole concentration of 800 cm² V⁻¹ s⁻¹ and 3·10¹⁶ cm^-3, respectively. The homoepitaxial GaSb layer grown with the same parameters has mirror-like surface and the photoluminescence spectrum is dominated by strong excitonic lines.     

Effect of local structural defects on the precipitation of as in the vicinity of InAs quantum dots in a GaAs matrix

Electron-microscopy studies of GaAs structures grown by the method of molecular-beam epitaxy and containing arrays of semiconductor InAs quantum dots and metallic As quantum dots are performed. An array of InAs quantum dots is formed using the Stranski-Krastanow mechanism and consists of five layers of vertically conjugated quantum dots divided by a 5-nm-thick GaAs spacer layer. The array of As quantum dots is formed in an As-enriched GaAs layer grown at a low temperature above an array of InAs quantum dots using postgrowth annealing at temperatures of 400–600°C for 15 min. It is found that, during the course of structure growth near the InAs quantum dots, misfit defects are formed; these defects are represented by 60° or edge dislocations located in the heterointerface plane of the semiconductor quantum dots and penetrating to the surface through a layer of “low-temperature” GaAs. The presence of such structural defects leads to the formation of As quantum dots in the vicinity of the middle of the InAs conjugated quantum dots beyond the layer of “low-temperature” GaAs.     

Step structure of GaInAsSb grown by organometallic vapor phase epitaxy

The surface step structure of Ga_1−xIn_xAs_ySb_1−y grown by organometallic vapor phase epitaxy on GaSb substrates has been studied by atomic force microscopy. Epilayers were grown at 525°C and 575°C on (001) GaSb substrates misoriented 2° toward (101) or 6° toward (1 1)B. For Ga_0.88In_0.12As_0.1Sb_0.9 grown at 575°C, the surface exhibits step-bunching on both types of substrates. When the composition is increased to Ga_0.86In_0.14As_0.12Sb_0.88, the periodic step structure breaks down and the surface becomes irregular. The deterioration of the step structure is a consequence of phase separation at the surface of the metastable GaInAsSb epilayer, which leads to the formation of GaAs- and InSb-rich regions. The photoluminescence (PL) of such layers show significant broadening due to carrier recombination in the lower energy gap InSb-rich quaternary regions. On the other hand, the surface of GaInAsSb epilayers grown at a lower temperature of 525°C is vicinal with steps heights of one to two monolayers. The PL FWHM values are considerably smaller for these layers. This improvement in material quality is related to smaller adatom lifetimes at the lower growth temperature. The importance of surface kinetics as it influences the step structure and thermodynamically driven phase separation is discussed.     

Structural stability of low temperature grown InGaAs/GaAs heterostructure

InGaAs/GaAs superlattice was grown by molecular beam epitaxy (MBE) on GaAs (100) substrate at low substrate temperature (250°C). The as-grown superlattice sample was then annealed at various temperatures for 10 min. The as-grown superlattice was pseudomorphic and stable up to 800°C annealing. Annealing at 850°C or higher temperatures, however, caused strain relaxation accompanying with dislocation generation at the As precipitate. Dislocation generation at the As precipitate was influenced by two factors. The one is lattice mismatch between GaAs and As precipitate, and the other is elastic interaction force acting on the As precipitate.     

Critical conditions for SiGe island formation during Ge deposition on Si(100) at high temperatures

The strain relaxation during the Ge growth on Si(100) occurs vikia surface diffusion and Si-Ge intermixing at temperatures below 800 °C. The Ge diffusion into the Si substrate is an additional process at higher temperatures. We found that, if its rate is higher than the Ge deposition rate, the island formation is not realized. We determined the critical Ge deposition rate as a function of the temperature in the range of 840–960 °C, at which the dynamic equilibrium between the growth of islands and their decay through the diffusion takes place. The islands grown in the conditions close to the dynamic equilibrium are ordered with a distance between them of about 1 µm and they form a smoothed surface morphology. These are indicative of the surface layer strain uniformity. The islands have a SiGe composition which, in the direction parallel to the sample surface, is more uniform in comparison with the islands grown at lower temperatures. The results show that the use of high temperatures essentially improves the conditions for the heterostructure self-organization.     

Elastic and plastic contributions to X-ray Line broadening of InGaAsP/InP Heterostructures

The coherency state of MOCVD grown InGaAsP/InP double-heterostructure wafers was examined and their effects on the structural properties were determined in this study. Lattice mismatches were measured using {511} asymmetric and (400) symmetric x-ray reflections. The chemical lattice misfit and the elastic strain were also calculated. Misfit dislocations were examined by both x-ray topography and photoluminescence imaging. The x-ray full width at half maximum (FWHM) varied with the degree of mismatch. The largest FWHM was obtained for samples containing the misfit dislocations. It was found that FWHM is influenced not only by the plastic deformation, but also by the elastic strain. To model the dependence of the FWHM, the radius of curvature was measured, and its contribution to the x-ray line broadening was calculated. Also, the contribution from misfit dislocations was taken into account. This model assumes that the dislocations are planar and interact weakly with each other. Good agreement between measured and calculated values was obtained. Thus, it is concluded that the major contribution to x-ray line broadening ofelastically strained sample is the lattice curvature induced by misfit strain, and that the dominant factor affecting x-ray FWHM ofplastically deformed sample is lattice relaxation induced by misfit dislocation.