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ZnSe-GaAs(001) heterostructures have been grown by molecular beam epitaxy and characterized in situ by means of reflection high energy electron diffraction and x-ray photoemission spectroscopy, and ex-situ by near edge photoluminescence spectroscopy and by cross sectional transmission electron microscopy. By changing the Zn/Se flux intensity ratio (we explored the 0.1-10 range) we were able to control the Zn/Se relative concentration in the interface region, while maintaining a similar structure and high degree of long range order at the interface. Correspondingly, the valence band discontinuity is found to vary from 0.6eV (Se-rich interface) to 1.2eV (Zn-rich interface) in the interface composition range examined.
The growth and characterization of heteroepitaxial ZnSe and both rock salt and zinc blende MnS on (100) GaAs substrates is described. The ZnSe layers were grown using a novel thermal cyclic annealing procedure, and exhibit the narrowest double crystal X-ray diffraction rocking curves (28 arc sec FWHM) and free exciton low temperature photoluminescence peaks (0.93 meV FWHM) ever reported. Growth of thin (~300 Å) layers of metastable zinc blende MnS is achieved for the first time using ZnSe buffer layers and low temperature growth. The material is characterized using low temperature and room temperature photoluminescence, X-ray diffraction, and high resolution transmission electron microscopy.
We demonstrated the high quality growth of exceedingly thick pseudomorphic layers on free-standing, compliant substrates using InGaAs and GaAs materials. A 1% compressively strained InGaAs layer was grown on a relaxed GaAs platform by MBE. We fabricated the 800 Å-thick compliant platforms before growing a lattice-mismatched layer that exceeds its critical thickness by about twenty times.
X-ray analysis confirms a shift in the InGaAs peaks grown on the compliant substrate. Such shifts are characteristic of strained layers. Atomic Force Microscope analysis verifies that the layers on compliant substrates are much smoother than layers grown on a plain substrate.
Pseudomorphic growth exceeding the critical thickness has important applications in the design of various electronic and photonic devices.
We report the first parameter-free calculations of surface diffusion of a cation adatom on a reconstructed As-stabilized GaAs(001)-(2×4) surface. It is found from the calculated migration potentials of cation adatoms that the long-bridge sites are the most favorable adsorption sites on the GaAs(001)-(2×4) surfaces at low adatom coverages. The calculated results for surface diffusion constants of Ga adatoms show that the Ga-adatom diffusion is anisotropic on the reconstructed GaAs(001) surfaces and that the direction of fast diffusion is parallel to the As-missing dimer rows. The Al-adatom diffusion exhibits anisotropy similar to that of the Ga-adatom diffusion, while Al adatoms diffuse several times more slowly than Ga adatoms in the same directions in spite of the lighter mass of Al. Incorporating the calculated results for diffusion constants and migration potentials, the dynamical behavior of cation adatoms on the GaAs(00l) surface are demonstrated by stochastic Monte Carlo simulations at finite temperatures.
We have grown and characterized layered structure GaSe on As-passivated Si( 111 ) and GaAs on GaSe on As-passivated Si(111) for the ultimate purpose of using layered structure GaSe as a lattice/thermal-expansion mismatch buffer layer for epitaxial GaAs on Si. Films were grown on (111) Si substrates by MBE and characterized by in-situ RHEED, ex-situ SEM, and both plan-view and cross-sectional TEM. GaSe grew epitaxially on As-passivated Si(111) substrates at 500°C with Se/Ga BEP (Beam Equivalent Pressure) ratios of 10-20. Small droplets were observed on the surface after GaSe growth, and are thought to be droplets of unreacted Ga. The density and size of the droplets decrease with increasing Se/Ga BEP ratio. When the GaSe surface was exposed to As, the droplets became GaAs islands. Subsequent GaAs growth was carried out at 400°C and 500°C, giving the following results for 300Å thick films: As grown GaAs films were highly twinned, and some polycrystalline GaAs was present in the film grown at 400°C. In-situ annealing at 650°C for 10 minutes reduced the density of twins in both cases. The morphology of the GaAs films are that of an interconnected network of islands, with a feature size of about 500Â and 1000Å for the film grown at 400°C and 500°C, respectively. In plan-view TEM Moire fringes from both GaAs and GaSe are observed and show conclusively that the GaAs grew epitaxially on the GaSe without contacting the Si substrate. Cross-sectional TEM shows the interface between the Si and GaSe is not smooth on the atomic scale. In spite of this, the GaSe becomes smooth with about 2 monolayers of growth and the GaAs/GaSe interface appears to be very smooth.
Gas source molecular beam epitaxy has been used to deposit single InP layers, and multiple layers of InGaAs/InP over V-groove patterned (100) InP substrates. The V-grooves were defined by the (211)A and (111)B family of crystal planes. Scanning electron microscopy, transmission electron microscopy, and scanning photoluminescence were used to characterize the variation in the composition and thickness of the epitaxial layers. Defect-free epitaxial layers were achieved within (211)A V-grooves; whereas, dislocations were observed in the InGaAs layers deposited within (111)B grooves. The dislocations are attributed to the large lattice mismatch caused by a variation in composition due to differential Group111 diffusion on the groove sidewalls. Scanning photoluminescence indicates an In (100) diffusion length of 2.2-3.5\lm.
The properties and low pressure organometallic vapor phase epitaxy of Gaxln1-xP/(AIGa)0.5ln0.5P quantum well (QW) laser diode heterostructures with Al0.5ln0.5P cladding layers, and having wavelength 614<λ<690 nm, are described. At longer wavelengths (λ>660 nm), threshold current densities under 200 A/cm2 and efficiencies greater than 75% result from a biaxially-compressed GalnP QW active region. Although short wavelength laser performance is diminished by the poor electron confinement afforded by AIGalnP heterostructures, good 630 nm band performance, and extension into the 610 nm band, is achieved with strained, single QW active regions.
In this study, we have investigated the effectiveness of semimetallic, amorphous carbon films as a liftoff mask in the selective growth, by OMVPE, of GaInP epitaxial layers, on (001) oriented GaAs substrates. Scanning electron microscopy, cathodoluminescence spectroscopy and EDX analysis have been employed to characterize the epitaxial material. Our results show excellent selectivity with little nucleation taking place on the amorphous carbon mask in the region of the patterned openings. Liftoff of the carbon mask was very easily achieved, leaving no unintentional nucleation on the substrate below. Investigations in the AlGaAs/GaAs system did not yield the same degree of selectivity with this mask, but the polycrystalline film that deposited on the mask was cleanly lifted from the substrate by the liftoff of the mask.
The microstructures of In0.5Ga0.5P alloys grown on (100) GaAs by MOVPE have been characterized with cross-section TEM and their optical emission examined with photoluminescence at low temperatures. All the alloys exhibit spinodal-like decomposition with compositional modulations along directions in the growth plane. Alloys grown at 775 °C have the highest emission energy, 2.0 eV; growth at 675°C gave the lowest, 1.89 eV, due to strong CuPt-type ordering of In and Ga. The ordered domains are platelets 20 to 200 nm wide and 10-20 nm thick, with antiphase boundaries 1-2 nm apart. We have also formed "unicompositional" quantum wells of thin (1.3-20 nm) ordered layers grown at 675°C between disordered barriers grown at 750°C. Ordering is found only in the active layer, with domains similar to those of thick layers. The emission energy increases by 90 meV as the well thickness is decreased from 10 to 1.3 nm, thus demonstrating quantum size effects solely through disorder-order phenomena.
AlGaAs/GaAs single-quantum-well (SQW) vertical-cavity surface-emitting laser diodes (VCSELDs) with 20 pairs of AlAs (71 nm)/GaAs (59 nm) distributed Bragg reflectors (DBRs) were grown on Si substrates by metalorganic chemical vapor deposition using the conventional two-step growth technique. The measured reflectivity of the 20 pairs of AlAs/GaAs DBRs was 93 % at the wavelength of 860 nm. The AlGaAs/GaAs SQW VCSELD on Si exhibited a threshold current of 79 mA and a threshold current density of 4.9 kA/cm2 under pulsed condition at room temperature. The emission wavelength was 840.3 nm with the full width at half maximum of 0.28 nm.
The oxidation of Si0.5 Ge0.5 alloy has been investigated over the temperature range 800 to 1000°C . The composition of the oxide layers has been determined by 1.5 MeV Rutherford backscattering spectroscopy (RBS), infrared transmission spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS). During the initial stage of oxidation, as the temperature ramps up in an oxidising ambient both Si and Ge are oxidized to form a mixed oxide in the region near the surface. Upon further oxidation, at temperatures higher than 900°C and times longer than 5 minutes, the Ge atoms are ejected by the growing oxide layer, which has the composition of Si02, and accumulate in the underlying alloy, which becomes rich in Ge.The surface oxide layer remains unchanged. The proportion of Ge02 in the near surface region, determined from IR spectra, decreases when the temperatures increases from 800°C to 1000°C. It is concluded that at 800°C Ge atoms are insufficiently mobile to be ejected from the growing oxide and both matrix species ( Si and Ge ) have equal probabilities of oxidation leading to the formation an oxide of composition Si0.5Ge0.5O2 . Supporting experiments confirm that Ge02 in the surface layer is formed during warm up of the samples oxidized at 900 and 1000°C.
The layer thicknesses and composition of MBE grown 4 period 2 00A/100A GaAs/InGaAs superlattice structures with nominal indium concentrations of 10, 15 and 20% were determined by TEM, RBS, DXRD, PR and PL. The results show that the indium concentration obtained by DXRD is low and that obtained by PR and PL is high, and that the discrepancies are larger for the larger indium concentrations. We show that both descrepancies can be accounted for by relaxation of the lattice; elastic relaxation as represented by a radius of curvature, and/or plastic deformation as represented by mismatch dislocations.
We present an investigation of electron cyclotron resonance plasma etching induced damage of the 2-dimensional electron gas (2DEG) in GaAs/AlGaAs heterostructures using low temperature photoluminescence (PL), electron paramagnetic resonance (EPR) and the Shubnikov-de Haas (SdH) effect. Dry etching of half of the 20 nm top layer of GaAs results in a decrease of the single-particle relaxation time by 20 - 50%, while the concentration of a surface-related paramagnetic defect increases by about one order of magnitude. At the same time, the PL intensity decreases by a factor of 5 - 10. Plasma hydrogenation experiments, annealing and wet etching experiments have been carried out, and the different characterization results are related to each other. We find that passivation in a hydrogen plasma, which leads to a strong increase of the PL intensity of etched as well as unetched samples, causes damage to the transport properties of the 2DEG. The defect concentration is not related in a simple way to the changes of optical and transport properties.
Zinc-blende GaN has been successfully grown on GaAs(100) by using low-energy nitrogen ions produced by a conventional ion gun to replace the ECR plasma source in the traditional MBE facility for nitride growth. Analyses of the epilayer by X-ray diffraction and various electron spectroscopy techniques show that the crystalline quality is fairly good. This is attributed to the dominance of the reactive species N+ in the ionization products under the conditions we use.
GaAs pin and nip diodes have been grown by Atomic Layer Molecular Beam Epitaxy at 350°C on p-type and n-type (001 ) Si substrates respectively. These devices present different electrical characteristics. TEM characterization of these structures shows an asymmetric planar defect distribution whose orientation depends on the type of doping introduced in the GaAs layer immediately grown on the Si substrate. These results can be explained taking into account that the dissociation of dislocations depends both on the polarity of the dislocation and the doping type of the epilayer.
The microstructures in Schottky barrier samples of Al and Cr single layer metal contacts and Al-Cr bimetal contacts on p-type Si have been investigated using analytical transmission electron microscope (TEM). Al-Cr bimetal contacts were made using a layered structure with one thin layer and a thick layer on the top. The analytical TEM was used to determine the precise thickness and grain size of the coated materials as well as chemical compositions and morphologies. The present study proposes that the microstructures of the Schottky contact interfaces are important for the Schottky barrier heights of Al, Cr, and Al-Cr contacts.
In0.53Ga0.47As layers were grown by LPE on (001) InP substrates in the temperature range of 480~780°C. The fine speckle contrast, which is attributed to two-dimensional phase separation, was observed in layers grown at temperatures as high as 780°C. Since the critical temperature for bulk miscibility gap is predicted to be around 450°C, this suggests that in the presence of the surface the critical temperature for the two-dimensional surface decomposition is higher than that for the bulk. The wavelength of the fine modulations does not change with the growth temperature. This could be due to the fast diffusion kinetics in the solid-liquid interface and the balance between the driving force and the gradient energy.
To examine the stability of the above microstructures, zinc diffusion experiments were carried out in the temperature range of 390 to 540°C using the ampoule technique. The diffused layers exhibit homogenous microstructures. This demonstrates that critical temperature for phase separation in bulk is below 390°C and is comparable to that predicted by theory.
A systematic study of the effect of growth interruption on the interface roughness of InAsxP1-x/InP heterostructures has been carried out. High resolution X-ray diffraction, photoluminescence and optical absorption measurements for InAsP/InP strained multiple quantum wells reveal that the InAsP/InP interface is very sensitive to growth interruption. For nonoptimal growth interruption procedures a large density of interface states is created, probably as a consequence of compositional modifications within the interface region. We find that photoluminescence on its own is insufficient to characterize the interface roughness in our samples, since even for narrow low-temperature peak emissions corresponding to the multiple quantum wells, the absorption spectrum may reveal a significant density of interface states.
The damage and strain produced in a nearly pseudomorphic Ge0.10Si0.90 film on Si(100) by implantation of 320 keV 28Si+ ions at 40 to 150 °C and doses from 1 to 30 x 10l4/cm2 have been measured by MeV 4He channeling spectrometry and high resolution x-ray diffractometry. Both the implantation-induced damage and strain are sensitive to the implantation temperatures. The damage as characterized by channeling decreases with the temperature of implantation for all doses and becomes minimal at 150 °C. The implantation-induced perpendicular strain does likewise, but an excess of about 6 % of its intrinsic strain remains at the strain maximum even at 150 °C.