A variety of optical techniques are now available for studying surface processes and for monitoring layer thicknesses and compositions during semiconductor crystal growth by molecular beam epitaxy (MBE), organometallic chemical vapor deposition (OMCVD), and related techniques. Surface-sensitive approaches include reflectancedifference spectroscopy (RDS), second-harmonic generation (SHG), and laser light scattering (LLS). Bulk approaches include spectroellipsometry (SE) and spectroreflectometry (SR). I discuss representative examples, including the use of SE to determine thicknesses and compositions of AlGai_As layers on GaAs during crystal growth by organometallic molecular beam epitaxy (OMMBE), the use of RDS to measure surface dielectric anisotropy (SDA) spectra of various (001) GaAs surfaces relevant to crystal growth by MBE, and the use of RDS to establish kinetic limits to growth on (001) GaAs by atmospheric-pressure OMCVD.

This paper discusses some recent developments in the use of the contactiess electromodulation technique of photoreflectance for the in-situ monitoring of thin-film growth at elevated temperatures by such methods molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD) and gas phase molecular beam epitaxy (GPMBE). The direct gaps (E) of GaAs, Ga082A1018As, InP and InGa1As (x = 0.07 and 0.16) have been measured to over 600°C. These temperatures are comparable to the growth condition for MBE, MOCVD and GPMBE. For these semiconductors E can be evaluated to 5 meV at these elevated temperatures. Thus, the temperature of GaAs and InP substrate material could be determined to Also the Al composition of GaAlAs and In content of InGaAs could be monitored during actual growth procedures. Results for GaAs and GaA1A5 have been obtained in an actual MOCVD reactor including rotating substrate and flowing gases. We have succeeded in obtaining the spectra of E of GaAs at 650°C in 30 seconds.

Using a new in situ analysis tool, grazing incidence x-ray scattering, we have studied the surface reconstructions present prior to and during growth of ZnSe by organometallic vapor phase deposition. We have established that the GaAs native oxide is chemically reduced by the hydrogen ambient present during pre-growth heating. Following this cleaning procedure, the growth of ZnSe was found to occur in the presence of a p(2x1) reconstruction, characteristic of an array of Se dimers. This new technique can easily be extended to other growth systems.

We present the temporal behavior of intensity oscillations in reflection high-energy electron diffraction (RHEED) during molecular beam epitaxial (MBE) growth of GaAs and A1GaAs on (1 1 1)B GaAs substrates. The RHEED intensity oscillations were examined as a function of growth parameters in order to provide the insight into the dynamic characteristics and to identify the optimal condition for the two-dimensional layer-by-layer growth. The most intense RHEED oscillation was found to occur within a very narrow temperature range which seems to optimize the surface migration kinetics of the arriving group III elements and the molecular dissodiative reaction of the group V elements. The appearance of an initial transient of the intensity upon commencement of the growth and its implications are described.

We are investigating dircctdeposition oflow energy (10 - 500 eV) ions to grow elemental and compound thin films at low temperature (R.T. - 700 K). We have developed a model to describe layer growth by Ion Beam Deposition (IBD) that takes into account not only atomic collision processes but also thermally-activated diffusion and recombination ofpoint defects during ion bombardment. Numerical simulations of our experimental conditions using this model have given us new insight into growth mechanisms during IBD. More specifically, we show in this work that the IBD growth rate is not limited by the sputtering yield only, but also by the recombination rate ofpoint defects at the surface; this rate depends on the depth distribution of the defects, which is deternined by the ion energy.

In this work, the growth kinetics of silicon by Remote Plasma-enhanced Chemical Vapor Deposition (RPCVD) have been investigated. Growth rate has been characterized for temperatures ranging from 150°C to 480°C, r-f powers between 4 and 8 W, 5 -30 sccm of diluted (2%) silane flow, and 200 mTorr chamber pressure. The growth rate has been found to be relatively insensitive to silane partial pressure, indicating that dissociation of silane is not likely to be the rate limiting step. The activation energy for growth changes at -'325°C, and this is believed to be due to a change in the stable hydrogen termination of the silicon surface at this temperature. This implies that the rate limiting step for the reaction is hydrogen desorption. Growth rate dependence on substrate bias suggests that argon ions are responsible for driving the surface reactions.

Remotely doped parabolic quantum wells have been grown by MBE using a digital alloy approach to vary the Al content in the AlGai.As system. The monitoring of the beam fluxes as well as the measured subband separations confirm the precision of our growth technique. Using a front gate electrode we can depopulate the electrical subbands. Thus we can determine experimentally the subband separations showing close agreement with the results of our self-consistent calculations.

Graded band gap ITT-V semiconductor structures have been grown by molecular beam epitaxy using electron beam evaporation of Group III metals. The deposition rates of the Group III metals are measured and controlled in real-time using Inficon Sentinel III rate monitors. The rapid response of the electron beam evaporation sources allows precise alloy grading over distances as short as 1 nm. A variety of novel 111-V device structures have been realized by this technique.

Transition metal aluminides and gallides and rare earth monopnictides have been grown as buried conducting layers in 111-v compound semiconductor heterostructures. These metallic and semi-metallic compounds have the CsC1 and NaC1 structures, respectively. The criteria for achieving (100) oriented epitaxial growth on (100)111-V semiconductor surfaces are different for each class of material. The methods used to achieve 111-V/metal/Ill-V heteroepitaxial structures are described. The different approaches needed for the aluminides or gallides and the monopnictides form the basis for a comparative study.

We have grc,n isotype organic semiconductor heterojunctions consistirx of CuPc (copper phthalocyanine) , PTCDA (3 , 4 , 9 , 10 pexylenetetraca.thoxylic dianhydride) arxl P]XBI (3 ,4,9, 10 perylenetetracarboxylic-bis--benzirnidazole). tharge transport: properties at the organic heterointerfaces have been studied usir current-voltage and capacitance-voltage ineasurenents. Organic raultiple quantum well structures have been grn by organic molecular beam deposition. The unusual crystallfrie and optical properties of such stnictures will be discussed.

Growth characteristics of GaAs using triethyl-Ga (TEGa) and trimethyl-Ga (TMGa), and of InAs using trimethyl-In (ThIn) are found to be quite similar in many aspects. A reaction model in which the chemical kinetics of the adsorbed Ga alkyls dominates is able to account for the growth characteristics using TEGa throughout the entire growth temperature range. The existence of the intermediate species assumed in this model is confirmed by a recent mass spectroscopic study. This reaction model discussed here provides a microscopic understanding of the chemical beam epitaxial growth process.

Growth of InAs epitaxial layers on GaAs (001 ) by migration-enhanced epitaxy (MEE) and molecular beam epitaxy (MBE) has been studied. Reflection high-energy-electron diffraction (RHEED) patterns were studied, and persistent RHEED intensity oscillations were observed during MEE growth of InAs. The dependence of RHEED oscillation on MEE growth parameters is discussed. InAs layers grown by MEE at low substrate temperature exhibit comparable quality as MBE layers grown at higher substrate temperature.

We present a study of the electrical and structural properties of 1atUcemismatched InGa,.As strained layers grown on GaAs (100). This strained system has been used in device structures because of the high carrier mobilities which may be obtained, but the inherent misfit strain is known to produce dislocations above some critical limit. We have grown, by molecular beam epitaxy (MBE), structures with p-type (Be), 8 x1019 InGa,As layers of 20 nm thickness. The indium mole fraction, x, was increased in subsequent samples from 0 to 0.50 in steps of 0.05, simulating the base of a heterojunction bipolar transistor (HBT). We correlate hole mobilities and sheet resistances with the microstructure of the strained layers as determined by transmission electron microscopy (TEM). It is shown that the hole mobility drops from 55.3 cm2V-'s-' to 31.7 cm2V-1s-1 x varies from 0 to 0.50, with a corresponding rise in sheet resistance. For x □0.40, significant reduction in electrically active carrier concentrations is observed, presumably due to interaction with the extensive defect structure. HBT structures incorporating the base layers described were grown by MBE and processed into devices. These devices show a maximum gain at x= 0.10, with good I-V characteristics, and a subsequent sharp fall in gain at higher x, with consequently poorer I-V characteristics. We believe this shows a higher sensitivity of minority carriers to the dislocation network than majority carriers. No misfit dislocations are visible in plan-view ThM until x= 0.25-0.30, considerably higher than the Matthews-Blakeslee mechanical equilibrium prediction of x= 0.10. This confirms the relatively sluggish relaxation of these structures at the growth temperature of 500°C. Further, we demonstrite that for x□0.4, generation of new misfit dislocation length is not by deflection of pre-existing threading dislocations, but rather by generation of new misfit dislocation ioops.

We report the successful growth of EnAs/Gai_InSb strained-layer superlattices, which have been proposed for far-infrared applications. Samples were grown by molecular beam epitaxy and characterized by reflection high energy electron diffraction, transmission electron microscopy, x-ray diffraction, photoluminescence, and photoconductivity. Excellent structural quality is achieved for superlattices grown on thick, strain relaxed GaSb buffer layers on GaAs substrates at fairly low substrate temperatures (< 400 °C). Photoluminescence and photoconductivity measurements indicate that the energy gaps of the strained-layer superlattices are smaller than those of InAs/GaSb superlattices with the same layer thicknesses, in agreement with the theoretical predictions of Smith and Mailhiot.1'2 In the case of a 45 A/28 A InAs/Gao.751no25Sb superlattice, an energy gap of 80meV (> 15 jim) is measured. These resuits suggest that far-infrared cutoff wavelengths are compatible with the thin superiattice layers required for strong optical absorption in type-Il superlattices.

We present results for lattice matched (AlxGai..xAs, x=0.5 and x=0.0) and lattice mismatched (InxGai..xAs, x□0.25) growth on patterned GaAs (100) substrates. For the AlGai..xAs structures, the GaAs substrates were patterned in the form of elongated mesas parallel to [011 II with widths of approximately 3 tim. Interfacet migration effects observed on the nesas via cross-section transmission electron microscope studies are explained in terms of a ledge-ledge interaction on the vicinal surfaces formed due to growth on the mesas. InxGai..xAs (x□0.25) structures were grown on GaAs (100) substrates patterned in the form of elongated mesas parallel to [01 11 with widths of approximately 1 tim. This patterning direction was chosen since under cutting in the [0 1 1] direction eliminates inter-facet migration effects so that compositional change induced strain effects can be minimised. For x □0.15, we find a reduction in misfit dislocation densities in films upto five times the nominal critical thickness for growths on the patterned mesas as compared to the growths on the corresponding non patterned regions. For x=0.25 no such difference is observed and a large number ofthreading dislocations ( around iO cm2) are found in both the patterned and the non patterned regions. This is believed to be a consequence of the onset ofa 3-D island growth mode. Finally we present some results for the growth of InØ5Gaj•75As I AlAs resonant tunneling diode (RTD) structures and a 100 period InjØGaØ8As (80 A) IGaAs (160 A) Multiple Quantum Well (MQW) such as suited for spatial light modulator (SLM) structures on GaAs (100) substrates patterned in both <01 1> directions on a length scale of 12 to 20 tm. For the RTD structures we conclude that benefits from patterning are expected for x□0.25 provided the growth kinetics are appropriately adjusted to prevent 3D island growth mode. For the MQW -SLMstructure we demonstrate superior optical properties for the growth in the patterned region and a corresponding absence of threading dislocations in the central region of the mesas.

We have demonstrated selective epitaxial growth of A1Gai_As, with an abrupt transition in the bandgap lateral to the growth direction. Spontaneous compositional modulation, with an associated reduction in the effective bandgap, occurs in AlGaAs grown by molecular beam epitaxy on the sides of grooves in a GaAs substrate. The bandgap is observed to be dependent on the groove orientation. Possible mechanisms for the orientation dependent growth are discussed.

The defect reduction schemes in GaAs-on-Si such as: (1) Optimization of initial nucleation process; (2) inserting strained-layer superlattices for dislocation filtering; (3) thermal annealing, both in-situ and ex-situ; and (4) growth patterning are briefly reviewed and new data are presented. Spatially resolved photoluminescence (SRPL) images show that the material quality is significantly better when the (100) Si substrates are misoriented towards (oil) as compared to growth on (100) substrate or when the misorientation is towards (001). The post growth patterning to <15 jtm X 15 m patterns combined with thermal annealing at 850° C for > 15 mm eliminates the dark line defects in SRPL images and markedly reduces the thermally induced biaxial tensile stress in GaAs-on-Si. The technique is ideal for growth and fabrication of AlGaAs-GaAs vertical cavity surface emitting lasers on Si. In edge emitting lasers, where post-growth patterning alone does not significantly reduces the stress due to large cavity length, the tensile stress can be fully or over compensated by introduction of a compensating stress from a thermally deposited SiO2 layer. With the reduction of stress, stability of the lasers has been found to improve.

The surface structure of the Si(100) substrate after an in situ remote hydrogen plasma clean, and the defect microstructure of epitaxial Si films grown by Remote Plasma-enhanced Chemical Vapor Deposition (RPCVD) in the temperature range of 15O°C-305°C are discussed. Auger Electron Spectroscopy (AES) analysis has been employed to examine the capability of this remote hydrogen plasma clean in terms of removing surface contaminants. Reflection High Energy Electron Diffraction (RHEED) and Transmission Electron Microscopy (TEM) have been utilized to investigate the surface structure in terms of crystallinity and defect generation. A remote hydrogen plasma clean which reduces carbon, oxygen and nitrogen contamination levels to below the detectability of AES analysis, while maintaining a defect-free surface, as indicated by TEM, has been developed. However, excessive plasma power causes defect generation on the Si surface, and the size of the defects is a function of substrate temperature during cleaning in the range of room temperature-305°C. Subsequently, epitaxial Si films grown by RPCVD at various temperatures (150°C-305°C) after optimal remote hydrogen plasma clean have been investigated in terms of defect microstructure and impurity content using RHEED, TEM and Secondary Ion Mass Spectroscopy (SIMS). Epitaxial Si films with very low defect density (4O6 cm2 or less) and low oxygen content (-3x1018 cm3) have been achieved by RPCVD.

We have used x-ray photoelectron spectroscopy (XPS) to measure the valence band offset in situ for coherently strained Si/Ge (100) heterojunctions grown by molecular beam epitaxy. Si 2p and Ge 3d core level to valence band edge binding energies and Si 2p to Ge 3d core level energy separations were measured as functions of strain, and strain configurations in all samples were determined using x-ray diffraction. We obtain valence band offsets of 0.83 :f: 0.11 eV and 0.22 0.13 eV for Ge coherently strained to Si (100) and Si coherently strained to Ge (100), respectively. If we assume that the offset between the weighted averages of the light-hole, heavy-hole, and spin-orbit valence bands in Si and Ge is independent of strain, we obtain a discontinuity in the average valence band edge of 0.49 0.13 eV.

The major remaining limitation for the application of GaAs on Si is the reduction of high dislocation density and resulting electronic defects in the GaAs layer. We have investigated defect structures in GaAs layers grown on Si by molecular beam epitaxy(MBE) with different structures in the GaAs/Si interface region. Interface region variables include thickness, designs of InGaiAs/GaAs strained layer superlattices(SLSs) and growth conditions of the GaAs buffer layers(AsJGa ratio and in-situ annealing at 750°C). These layers were examined by transmission electron microscope(TEM) and double crystal Xray diffraction. Defects, such as dislocations and stacking faults, exhibit a much stronger dependence on the growth conditions of the GaAs buffer layers compared to the magnitude of strain energy in the SLSs. Larger misorientation angles(O.35° and 0.26°) between the GaAs epilayers and Si substrates resulted in reduced defect densities compared to more highly dislocated layers with less misorientation(O°,1.O°,1.O° and 0.3°).

Superconducting YBa2Cu3O 7 (YBCO) thin films were grown on Si with transparent, conducting Indium Tin Oxide (ITO) buffer layers The onset temperature at 92K and zero resistance at 68K were measured. Both, ITO and YBCO films were deposited by ion beam co-deposition. The YBCO/ITO films exhibit metallic resistivity with positive slopes (''O.O55 &square;/K). The YBCO is uniform, textured and polycrystalline. The relevance for hybrid opto-electronic device structures is briefly discussed.

High T 1.2-3 superconductor thin films have been deposited on (001) single crystal Si with and without oxide bather by pulsed excimer laser evaporation technique. The oxide barrier of Zr02 was also deposited by the same laser technique. It is shown that the quality of superconductor film and the corresponding transition depend on the quality of the barrier layer. Low angle X-ray diffraction and IR transmission results have been used to identify the structural quality of the barrier layer. The superconducting films have been characterized using X-ray diffraction and resistivity-temperature measurements. T0 of 87K has been achieved in O.7p.m thick high T films on Si with O.3p.m thick Z102 barrier deposited at 600°C.

In order to optimize the performances of YBaCuO thin films, elaborated by a two step process on Si02/Si3N4/Si substrates, we have studied the effect of rapid thermal annealing (FI'A) conditions on low temperature (i.e. < 200°C) sputtered layers, in the thickness range 0.3 - 1.5 gin. Both the superconduc- ting properties and the surface quality of the layers have been investigated. One of the advantages of this process is to allow the use of a very thin nitride layer (i.e. 45 A), to avoid the harmful diffusion of silicon from the wafer. This fast technique is also efficient to prevent interdiffusion between the silica and YBaCuO layers. Our first results show that an onset temperaure in the 87-91K range can been obtained, with 60 K and 'c 500 AOlfl (at T(R..0)/2). It is also shown that these figures might be improved by repeated RTA cycles.

We present in situ. real-time monitoring of emission lines from rf induced plasma of elemental and ionized species from target of YBaCuO7_ compounds during the sputtering growth. Abundances of the ionized and/or neutral species rather than clusters were observed while relative composition of the species was varied by target composition, applied rf power, gas pressure, and location of substrate. We discuss usefulness of real time monitoring of ejected species which depend on rf power and gas pressure during the sputtering deposition of high Tc superconducting thin films.

We present photoluminescence spectra from CdZnj_Te /ZnTe and ZnSe,,Tei_ /ZnTe strained layer superlattices grown by MBE, and analyze the band alignments and strain effects. Our results are based on fitting the dominant photoluminescence peaks to the superlattice band structure obtamed by k •theory. We find that the valence band offset of the CdZniTe /ZnTe system is quite small. On the other hand, the photoluminescence data from the ZnSeTei_ /ZnTe superlattices suggest that the band alignment is type II, with a large valence band offset. We also investigate the band gap bowing in the ZnSeTej_ alloys, and determine the individual components of the bowing in valence and conduction bands. Based on our results for band alignments, we evaluate the prospects for minority carrier injection in wide bandgap heterostructures based on ZnSe, ZnTe, and CdTe.

We present photoluminescence (PL) from Te-rich ZnSeTei_ alloys and ZnSeTei_/ZnTe superlattices and discuss the growth of these materials on GaSb epilayers on GaSb substrates. We show that growing ZnTe on GaSb substrates eliminates several bound exciton peaks which occur in ZnTe grown on GaAs. The ZnSeTei_ epilayers show bright luminescence from centers over 100 meV below the expected band edge. PL from ZnSeTe1_ alloys and superlattices is qualitatively very different from PL from CdZni.Te alloys and CdZniTe/ZnTe superlattices .

The molecular beam epitaxial growth of layered Bi-Sr-Ca-Cu-O films is reported. The advantage of using ozone rather than molecular oxygen for the growth of cuprate superconductors by molecular beam epitaxy (MBE) is discussed. Molecular beams of the constituent metals were shuttered on an atomic layer-by-layer basis to grow Bi2Sr2Ca1CuO phases for n=1 to 5, and ordered superlattices of these phases. Using these techniques an as-grown superconducting film with ]nset near 100 K and T (p=O) of 84 K was achieved. The films are smooth on an atomic scale. The results demonstrate the ability of shuttered MBE to grow custom layered combinations of Bi2Sr2Ca..1CuO layers. This shuttered MBE growth technique seems quite capable of synthesizing artificially layered epitaxial structures consisting of layers of oxides that are superconducting, non-superconducting metals, semiconducting, or insulating, with control of layer thickness and abruptness on an atomic scale.

Barium silicate and barium aluminate films were studied for use as chemical reaction and diffusion barrier layers for Y1Ba2Cu3O7 (YBC) deposited on sapphire and fused silica substrates by the sol-gel technique. Depth profiling by secondary ion mass spectrometry (SIMS) was used to characterize the abruptness of the interfaces between the barrier layer and the YBC film as well as the barrier layer and the substrate. We found that barium aluminate films reacted with fused silica substrates forming a coarse-grained barium silicate phase. Barium silicate, BaSiO3, also reacted with silica substrates forming a broad, amorphous reaction zone containing some BaSi2O5. Although barium silicate and barium aluminate deposited on sapphire formed a BaA112O19 phase, they provided a barrier to barium diffusion from sol-gel deposited YBC. Crystalline barium aluminate grown on c-cut sapphire was the most effective barrier layer for the growth of YBC films; compositionally uniform YBC films were made similar to that grown on strontium titanate substrates. These data show that chemically stable, crystalline films are more effective barrier layers than amorphous films.