Recent studies of short period GaAs/AlAs superlattices grown by molecular beam epitaxy have shown that the superlattice bandstructure resembles that of the AlCaAs alloy system. We have studied indirect gap GaAs/AlAs superlattice alloys using x-ray diffraction and pressure dependent magneto-luminescence. Analysis of the x-ray data shows that the AlAs layers are strained to be pseudomorphic with the GaAs substrate. Because the indirect conduction band ground state derives from the AlAs X-point these superlattice alloys must be treated as strained-layer superlattices. Interpretation of luminescence data using Kronig-Penney and tight-binding analyses of the superlattice bandstructure and optical dipole matrix elements shows that this experiment provides a sensitive measure of the conduction band offset for the GaAs/AlAs system. Quantum size effects lift the three-fold degeneracy of the X-point to yield an in-plane X band quantized according to the transverse electron mass (Xt) and a separate X band along the superlattice axis quantized according to the longitudinh electron mass (X1). Strain shifts at the X-points are of the same magnitude as quantum size effect shiffs and drive the Xt band toward lower energies. The effect of strain in the conduction band can be acdurately modeled by assuming GaP deformation potentials for the AlAs layers.

(Al,Ga)As layers have rough surface morphologies when deposited under certain growth conditions in molecular beam epitaxy (MBE). This leads to poor interfaces between (A1,Ga)- As and GaAs and degraded performance in heterojunction devices. We have observed that by misorienting the substrate slightly from (100), in a manner specific to the growth conditions, smooth (Al,Ga)As layers 3-4 μm thick can be grown at a rate of ≈ 1 μm/h for various AlAs mole fractions, x. Similar conditions for nominal (100) result in a rough, textured morphology. Experiments were carried out using flat substrates of specific misorientations as well as lens-shaped substrates. The lenticular substrates allowed all orientations within 14° of (100) [i.e., out to (511)] to be evaluated in one growth run. Deposition conditions that were varied included x, substrate temperature, and V/III beam flux ratio. Smooth layers obtained using optimal misorientations showed superior optical characteris-tics as determined from low-temperature photoluminescence (PL) measurements. The 4.2K PL spectra of smooth layers exhibit well-resolved exciton-related peaks, and do not have the deeper-level defect-related peaks observed in the spectra of rough layers. Single quantum well structures with A10.3Ga0.7As barriers and a 100 A-wide GaAs well deposited on mis-oriented substrates also have superior optical properties compared to a structure grown on nominal (100). Such findings may have significant implications for the performance of heterojunction device structures grown by MBE.

We have grown GaAsi-xSbx alloys on InP by molecular beam epitaxy throughout the miscibility gap region and characterized these layers by a combination of variable temperature Hall measurements, room and low temperature photoluminescence and optical absorption, and transmission electron microscopy. For nearly lattice matched layers, we have obtained p-type material with room temperature hole concentrations of 0.5 - 3.0 1016cm-3 and associated mobilities of 40 - 70 cm2/Vs. Fitting the Hall data to a numerical model indicates that conductivity in these nominally undoped layers is due to two distinct acceptor levels. Photoluminescence linewidths less than 8 meV FWHM are observed at 4K.

We have used reflection high energy electron diffraction (RHEED) to follow the growth of GaAs on misoriented GaAs(100), using molecular beam epitaxy (MBE). We find anisotropies in the step order, kink density and the growth oscillations as a function of substrate misorientation and direction of surface misorientation. For comparison in a simple system, growth of Ge on Ge(100) is also followed. On the singular Ge(100) surface, we observe strong RHEED oscillations accompanied by a strong anisotropy in the nucleation of the islands during growth. These islands show up as long intersecting streaks in the diffraction pattern when the electron beam is incident along the [010] direction. In the presence of As4, these anisotropic features become more striking and stronger RHEED oscillations are observed over a wider temperature range.

Gallium arsenide layers grown by molecular beam epitaxy on (100) Si substrates, subjected to various types of annealing, exhibit a substantial reduction in dislocation density near the interface and in the bulk of the epitaxial layer. Different kinds of annealing are examined, ex-situ annealing which is done in a furnace after the growth and in-situ annealing which is done during the growth.

MBE growth of GaAs and AlGaAs layers with good surface morphology and reasonably good PL spectral response has been achieved using two versions of a low-pressure, Calawa-type AsH3 cracker (long-quartz-tube containing a filamentary Ta catalyst) which produced predomi-nately As4 and As2 and smaller concentrations of As1 for the MBE growth. Quadruple mass analyzer measurements of the cracking patterns and the As4, As2, and As1 molecular species produced by the gas cracking furnaces are also reported.

The PR spectra of AlxGa1-xAs with different x values (0<x<0.46) were studied. It is demonstrated that PR can be an extremely useful technique in the alloy examination, especially the composition determination, by comparing the PR and the low temperature absorption spectra. The impurity band observed in PR spectra is also discussed.

We prepared by molecular beam epitaxy (MBE) InGaAs/InAlAs MODFET structures with In compositions of the InGaAs channel ranging from 0.53 (matched) to 0.70 (mismatched). The mismatched InGaAs channel was obtained by grading. The growth temperature for the lattice matched In0.53Ga0.47As layer was fixed at 510°C, while that for the graded InGaAs layer was lowered for the higher In mole fractions. The substrate temperature for the In0.52A10.48As layer varied from 500°C to 550°C, as governed by the device application. The In0.52A10.48As layers grown above 540°C show higher mobilities, little persistent photoconductivity (PPC) effect and lead to better device performance. Layers with a graded InxGa1-xAs channel (x > 0.53) have higher mobilities and carrier concentrations than those with a matched channel (x = 0.53). MODFET devices were fabricated with 1 μm gate lengths. Transconductance improved from 142 to 197 mS/mm on normal structures and from 119 to 170 mS/mm on inverted structures when the In mole fraction of the channel was increased from 53 to 65%.

One of the more striking recent illustrations of the versatility of molecular beam epitaxial methods (MBE) in preparing II-VI compound semiconductor superlattices (SL) is the synthesis of ZnSe/MnSe structures where the wide gap magnetic semiconductor MnSe is available for study in zincblende form for the first time. Of basic interest in these strained layer superlattices are electronic and magnetic phenomena near the two-dimensional (2D) limit. From another perspective, spectroscopic studies in the limit of ultrathin layer structures can provide valuable information about the details of MBE growth, particularly on the nature of the heterointerfaces. A particular advantage is present in a semiconductor such as MnSe because of its magnetic properties which are inherently sensitive to short range atomic ordering on the scale of the chemical bondlength. In this paper we review recent results based on optical spectroscopy and direct magnetization measurements which show dramatic departures from bulk behavior in their magnetic signature when m individual MnSe layers approach the onolayer limit. These results strongly suggest the presence of microscopic reconstruction effects at the MnSe/ZnSe heterointerfaces which appear to be quite efficient in frustrating antiferromagnetic ordering in these superlattices.

Photoassisted molecular beam epitaxy (MBE), in which the substrate is illuminated during film growth, is being employed in a new approach to controlled substitutional doping of II-VI compound semiconductors. Substitutional doping of these materials has been a long standing problem which has severely limited their applications potential. The photoassisted MBE technique gives rise to dramatic changes in the electrical properties of as-grown epilayers. In particular, highly conducting n-type and p-type CdTe films have been grown using indium and antimony as n-type and p-type dopants, respectively. Double-crystal x-ray rocking curve data indicate that the doped epilayers are of high structural quality. Successful n-type doping of CdMnTe, a dilute magnetic semiconductor, with indium has also been achieved. Most recently, the photoassisted growth technique has been employed to prepare doped CdMnTe-CdTe quantum well structures and superlattices.

Low-resistive and stable p-type ZnSe crystal can be grown by the temperature difference method under controlled vapor pressure (TDM-CVP) of Zn in the solution of Se. The properties of a p-ZnSe crystal grown under optimum Zn pressure, which was about 7.2 atm at the growth temperature of 1050°C, show best value resulting in the suppression of the nonstoichiometric point defects. As a result, a p-n junction can be formed by the Ga diffusion under the control of Se pressure in the p-type ZnSe crystal and a high efficiency pure blue LED can be realized.

Stoichiometric property of ZnSe film at GaAs interface was found to be critically dependent on the surface conditions of the GaAs substrates. It is clarified that the main factor governing the interface stoichiometry is the competition between the Zn-As bondings and the Ga-Se bondings at the interface. Especially, thin Ga-Se bonded layer at the interface is estimated to work as a barrier to diffusions of Zn or As and increases the sharpness of the heterointerface.

Metalorganic molecular beam epitaxial (MOMBE) growth of ZnSe and ZnS has been demonstrated and characterized. Diethylzinc and diethylselenide were used for the ZnSe growth, whereas both hydrogen sulphide and diethylsulphide were tested as sulphur sources for the ZnS growth. When group VI alkyls were used as sources, pyrolysis in a cracking cell was required. Quadrupole mass analysis of the DEZ, DESe, and DES was performed to clarify the growth mechanism. Smooth, monocrystalline ZnSe layers could be grown on GaAs(100) at substrate temperatures above 200°C. Photoluminescence spectra of ZnSe layers measured at 4.2K showed resolved exciton emissions. However, donor-acceptor pair emission lines attributed to a Li acceptor (Liza) were also observed. Smooth, monocrystalline ZnS layers could be grown on GaAs(100) at substrate temperatures above 250°C by using cracked H2S and DES, and above 150°c by using uncracked H2S. Photoluminescence spectra of ZnS layers at 74K were dominated by the blue emissions. Every ZnS layer grown on a GaAs(100), GaAs(111), or GaP(111) substrate exhibited a specular surface, streaky RHEED pattern, and blue luminescence. These good results reflect the complete absence of a premature reaction. Growth of a ZnSe-ZnS strained-layer superlattice on a GaAs(100) substrate was also demonstrated. Blue luminescence from the quantized levels in the ZnSe well layer was detected.

ZnSe layers have been grown by molecular beam epitaxy under similar conditions on both (100) GaAs and (100) Ge substrates. The GaAs and Ge substrates were prepared in an identical fashion which involged a process of argon-ion sputtering at room temperature followed by annealing at ~400 °. The surface reconstructions observed following the in-situ cleaning process were (4x1) and (2x2) for the GaAs and Ge substrates, respectively. ZnSe layers were grown using a unity beam pressure ratio at a substrate temperature of 330°C to various thicknesses in the range 1.0 to 6.5 μm. The quality of the ZnSe/GaAs and ZnSe/Ge layers was compared by measuring the linewidths of double-crystal rocking curve (DCRC) peaks and donor-bound exciton (DBE) peaks obtained from the layers by X-ray diffractometry and (4.2K) photoluminescence measurements, respectively. The X-ray studies revealed the ZnSe/Ge layers and to a lesser extent the ZnSe/GaAs layers to be tilted with respect to the substrate orientation. Typical tilt angles recorded were ~1,000" and ~40" for ZnSe/Ge and ZnSe/GaAs layers, respectively.

ZnSe epilayers have been grown on (100) GaAs substrates under various growth conditions in a systematic investigation of the relationships between growth parameters and film properties. Samples were grown under conditions corresponding to a 4 x 4 matrix in substrate temperature (Ts), Zn-to-Se beam pressure-ratio (BPR) space, with Ts = 250, 300, 350, and 400°C, and with BPR = 1/4:1, 1/2:1, 1:1, and 2:1. Measured film prOperties include donor and acceptor concentrations, carrier concentration and mobility, and the amplitude and width of donor-bound exciton (DBE) photoluminescence peaks. Under proper growth conditions we have been able to achieve room temperature carrier concentrations as low as 5.6 x 10 cm-3 , and peak mobilities of 7200 cm2 /V-sec. Low temperature photoluminescence spectra are dominated by the donor-bound exciton peak, at at 2.795eV; the amplitude of this peak is 2 to 3 orders of magnitude higher than the deep level emission in highly conductive films.

The heteroepitaxial growth of ZnSe on GaAs epilayers grown by molecular beam epitaxy is found to occur via a two-dimensional growth mechanism. Alternatively, nucleation on a GaAs substrate exhibits three dimensional growth characteristics. The differentiation of the type of nucleation is evidenced by reflection high energy electron diffraction intensity oscillations, as well as the dynamic behavior of the diffraction patterns. Photoluminescence measurements of pseudomorphic ZnSe epilayers grown on GaAs epilayers provides a direct measurement of ZnSe deformation potentials.

UV-photoemission spectroscopy was used to measure directly the valence-band discontinuity, ΔEv, for both sides of a MnSe layer which was sandwiched between two ZnSe layers by the Molecular Beam Epitaxy method. ΔEv is 0.16±0.05 eV for each interface; the valence band edge EVmax of the wider-gap MnSe semiconductor lies within the ZnSe gap. The interface-pinning position of the Fermi level appears at 1.74 eV above Evmax of ZnSe. It is concluded that interfacial electrostatic dipoles are small compared to the observed shift in Evmax of MnSe, which lends a qualitative support to Tersoff's model [Phys. Rev. Lett 52, 465 11984); Phys. Rev. B 30, 4874 (1984)] of heterojunction band offsets. Photoemission from MnSe shows that the Mn-derived 3d states, which are responsible for the semiconductor magneto-optical properties, lay 4.2 ± 0.1 eV below EVax.

Intentionally doped Cd1-xMnxTe crystals with composition 0.1 < x < 0.3 were grown by the vertical Bridgman technique. Systematic studies were made of: (a) interaction of native defects with foreign impurities; (b) the mechanisms for substitutional doping of the accep-tors Cu, Au, P and As; and (c) the binding energies of various defects. The measurements involve a combination of crystallographic, electrical and optical studies. The results demonstrated that P and As play the role of effective acceptors. In Cu and Au doped samples, a high compensation mechanism was observed.

We have grown ZnSe epitaxial layers on GaAs bulk substrates and on GaAs epitaxial layers by molecular beam epitaxy (MBE). A dual chamber MBE system was used which enabled the growth of the GaAs/ZnSe heterostructure completely in-situ. Examination of the initial stages of growth using RHEED indicates a much improved interface when ZnSe is grown over a GaAs epitaxial layer.

Although much of the detail of the surface kinetic processes and rates during molecular beam epitaxial growth of compound semiconductors remains uninvestigated, computer simulations employing parameterised rates have nethertheless begun to reveal the significance of the interplay of the surface kinetic processes to the growth mechanism and the resulting atomistic nature of interfaces. The presence of the dissociative surface chemical reaction kinetics of the group V molecular species, in addition to the surface migration and desorption kinetics accounted for in historical atomistic models and simulations of crystal growth, is revealed in the work of Ghaisas and Madhukar to lead to a configuration-dependent-reactive-incorporation (CDRI) growth process. Its limiting cases -the Reaction Limited Incorporation (RLI) and Configuration Limited Reactive Incorporation (CLRI) growth mechanisms - are shown to have significant consequences for the time dependent morphology of the growing surface revealed in the behavior of the average terrace width (<W>) and mean square fluctuations in step height (σh). Consequences of these features as a function of the kinetics of growth have been calculated for the behavior of the reflection-high-energy-electron-diffraction (RHEED) intensity and compared with corresponding measurements during homoepitaxy of GaAs(100)3. An example of this behavior is shown in fig. 1. Panel (a) shows the behavior of the effective migration length of a typical Ga atom during the course of growth of a given atomic layer (the even numbered layers not shown correspond to As layers in this, the [100], growth case. Note the time dependent nature. Panels (b) and (c) show the dynamics of <W> and σh. Note the oscillatory nature, the time period coinciding with the growth time of a monolayer. The former shows damping, indicative of a growth kinetics control-led roughening in the growth front. This is confirmed by the increase in the average value of the latter. An overall measure of the surface smoothness which combines lateral (i.e. <W>) and vertical

A Monte Carlo study of the growth of ZnSe by molecular beam epitaxy is presented. The study is focused on the role of surface kinetic reactions in the growth of the epilayers. Two different models for the incorporation of Se molecules, one with a highly reactive physisorbed state and the other with a relatively nonreactive physisorbed state are employed for the simulations. It is shown that the structural quality of the epilayers is very sensitive to the flux ratio if the physisorbed state is relatively nonreactive. It is also shown that if the physisorbed state is highly reactive, good quality epilayers are obtained over a wide range of flux ratio. Based on the present study, it is suggested that the reactivity of the physisobed state can be investigated by comparing the results of the simulations with the reflection high energy electron diffraction observations of the surface of the growing film.

The deposition of low energy (10-100 eV) silicon atoms onto an unreconstructed (111) silicon surface has been studied using molecular dynamics techniques and an accurate empirical potential describing the covalent Si-Si bonding. 10 eV silicon atoms were directed normal and at 60° angles to a silicon (111) substrate. The resulting atomic trajectories and substrate response were studied to determine the energy loss mechanism resulting in capture and the local lattice excitation near, and subsequent diffusion of excess vibrational energy away from, the irridact point. More glancing angles of incidence (5°-30°) ) were studied for beam energies of 20-100 eV. In general, incident atoms are either quickly adsorbed or scatter from the surface in this energy range. However, for angles less than a critical value a new phenomenon of 'surface channeling' is observed. In 'surface channeling', the trajectory of the incoming particle is steered by short-range repulsive and long-range attractive interactions with the sur-face atoms parallel to, and roughly 2 Å above, the surface of the substrate. Ranges of thousands of angstroms of travel along the surface can occur before the particle ultimately undergoes adsorption into a single site. These surface channeling trajectories as well as the local excitation provided by adsorption of energetic incident atoms offer considerable promise for precision control of the beam-induced growth of non-equilibrium semiconductor structures.

Reflection high-energy electron diffraction (RHEED) intensity dynamics has been widely accepted as a sensitive, in-situ, real time monitor of the surface morphology during the molecular beam epitaxial (MBE) growth. In this paper we focus on the behavior of the specular beam intensity as a function of the growth conditions (substrate temperature, arsenic pressure and growth rate) as well as diffraction conditions (incident angle of the electron beam, azimuthal angle) for both static (i.e., no growth) and dynamic (i.e., during growth) AlxGa1-xAs (100) (0 < x <1) surfaces. The kinetic processes underlying these observations are discussed.

Gas phase chemistry and transport phenomena in MOCVD are addressed. A new MOCVD reactor arrangement for exploring the gas phase chemistry by in situ molecular beam mass spectrometry is presented along with pyrolysis data for Ga(CH3)3, Ga(C2H5)3 and As(CH3)3. Detailed two- and three-dimensional models of the classical horizontal and vertical MOCVD reactor configurations are described. These models provide new insight into flow, heat and mass transfer effects on film thickness uniformity and interface abruptness. For the horizontal reactor, model simulations demonstrate the existence of longitudinal buoyancy driven convection rolls which adversely affect film thickness uniformity. For the vertical reactor, the film thickness uniformity is shown to be strongly influenced by susceptor edge, reactor wall, and buoyancy effects. It is demonstrated that uniformity may be improved by shaping the reactor wall and rotating the susceptor. The existence of multiple steady flows in certain operating regions is illustrated. Concentration transients in the growth of heterojunctions are simulated and it is shown that the presence of recirculation cells widens the interface depth.

The flow pattern of process gas at atmospheric pressure in a rectangular cross-section, horizontal MOCVD reactor was determined by pyrolyzing trimethylindium to produce an indium "smoke." Growth tubes with 9cm and 30cm entrance lengths and various internal geometries were tested. Entrance effects dominated the flow behavior for all tubes tested. This behavior is not generally recognized but is predicted from entrance-length theory. Use of a gas diffuser positioned slightly upstream from the susceptor produced no visible backflow of pyrolyzed reagent while greatly improving uniformity of flow over the susceptor. Im-proved control over GaAs film growth was obtained using a diffuser. These results lead to an improved design for a horizontal MOCVD reactor growth tube.

We report the first observation of stimulated emission (16 K and 77 K) in any material grown by atomic layer epitaxy (ALE). The quantum wells in this structure are six uncoupled InAs layers 6.6 Å thick separated by 509 Å thick GaAs barriers. These are the thinnest and most highly strained (7.4%) quantum wells ever reported to support stimulated emission. These results demonstrate that ALE is capable of growing laser quality material with good control of the growth process.

Molecular layer epitaxy was demonstrated using TMG-AsH3 and TEG-AsH3 systems. Monomolecular layer growth per operational cycle of alternate gas injections was realized on (100) faces independent of injected TMG and AsH3 pressures in a certain range. This result implies stable chemisorption (bond-to-bond type adsorption) of reactive species by monolayer on the substrate surface. Mass spectrometric analyses suggested that this adsorbate is partially decomposed TMG, such as Ga (CH3)x, where x is a numerical function of temperature . Surface migration process was investigated in a conventional Ga-AsC13-H2 system using a triangle table method. On (111)B facets, anisotropic two-dimensional growth based on the surface migration of adsorbed species was found to be a dominant process. The direction with the largest rate of lateral growth was <12T>, while that with the smallest one was <112>.

This paper reports some new experimental results concerning InP epitaxy by LP-MOCVD using chemical angle polishing and a sensitive etching technique to characterize the defect morphology of the complete InP layer, interface and substrate. The distribution of defects is associated with the SIMS determined As and Ga content.

The initial growth model of III-V compounds on Si is proposed and the necessary conditions to obtain an anti-phase domain (APD) free crystal are discussed. APD free crystals can be obtained on the (100) Si surface having mono-atomic as well as hiatomic layer steps if the surface has a misalignment towards <011> ±θ (θ≠45°) direction. The stress produced by the difference in thermal expansion coefficients of Si and GaAs is also calculated and discussed. The stress in the GaAs layer can be zero by adding another layer having the thermal expansion coefficient higher than GaAs either on the Si substrate back surface, or on the GaAs grown surface or in between them.

CdTe layers grown on GaAs have been characterized by photoluminescence measurements (PL), transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS). PL spec-tra of layers less than about 1.1 μm show a shift to lower energy of the exciton band, which we attribute to the compressive strain in CdTe along the CdTe-GaAs interface. Layers thicker than 1.1 μm gave PL indicative of high quality layers of bulk CdTe. TEM studies have shown that the lattice mismatch is accommodated by misfit dislocations at the interface and some by lattice strain. As the layer grows thicker this strain is relieved by dislocation lines in the first micron of the layer. SIMS measurements on these layers indicate negligible Ga diffusion, confirming our earlier findings. High quality mercury cadmium telluride (MCT) layers have been grown on these CdTe buffers, with properties similar to layers grown on bulk CdTe substrates. Double crystal X-ray rocking curves of the best MCT layer grown on GaAs substrates show a full width at half maximum (FWHM) of about 110 arc seconds. The best FWHM obtained on MCT layers grown on CdTe substrates was 125 arc secs.

AlGaAs/GaAs multiple quantum well structures have been deposited on native oxide patterned GaAs substrates and characterized by low temperature photoluminescence (PL). Standard photolithography was used to generate patterns on the plasma-grown native oxides. Single crystal multiple quantum wells grown on areas where oxides were removed have comparable high quality with those on the unpatterned substrate. QW's with different well widths ranging from 25-73 Å. show well-resolved distinct peaks in the PL spectra and the linewidths of the peaks are the same as those on the unpatterned wafer. Poly-QW's deposited on the oxide-covered regions also display a very strong luminescence signal. Instead of distinct peaks corresponding to each of the wells, a single wide band centered near the widest well transition energy was observed. There are potential applications of the polycrystalline material for ultrafast optical devices because of the short carrier lifetime. With the selective epitaxy technique, integration of active devices with single crystal QW's and others using polycrystalline is possible.

Low temperature photoluminescence (PL) measurements were carried out for C-, Be-, Mg-, Ge-, Zn- and Cd-doped GaAs prepared by molecular beam epitaxy (MBE), liquid phase epitaxy (LPE) and ion implantation (I4). Two new emissions, temporarily denoted by 'g' and [g-g] were commonly observed in the above materials and no principal difference of spectra was obtained among the samples prepared by different doping methods. 'g' is situated just below the well-known bound exciton emissions (B.E.) and seems to be unshifted against the increase of acceptor concentration, [A] and was suggested to be the excited energy state of an isolated acceptor. [g-g] presents a significant energy-shift towards lower energy sides with increasing [A] and at the highest limit of [A], it converges at the well-defined optical transition from conduction band to acceptor level, (e,A). These observations together with a preliminary theory suggest that [g-g] can be ascribed to the pair between excited-acceptors. It was for the first time clearly demonstrated that the absence and overlook of these two specific emissions pertinent to acceptors in conventionally-prepared samples can be attributed to the strong optical compensation effect among acceptors and unintentionally introduced residual donors.