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Hydrogenated nanocrystalline Si (nc-Si:H) thin films were prepared by plasma- enhanced chemical vapor deposition (PECVD). The films were deposited with a radio-frequency power of 100 W, while substrates were exposed to direct current (dc) biases in the range from 0 to −400 V. The effects of dc bias on the formation of nanoscale Si crystallites in the films and on their optical characteristics were investigated. The size of the Si crystallites in the films ranges from ∼1.9 to ∼4.1 nm. The relative fraction of the crystallites in the films reached up to ∼56.5% when a dc bias of −400 V was applied. Based on the variation in the structural, chemical, and optical features of the films with dc bias voltages, a model for the formation of nanostructures of the nc-Si:H films prepared by PECVD was suggested. This model can be utilized to understand the evolution in the size and relative fraction of the nanocrystallites as well as the amorphous matrix in the nc-Si:H films.
We report successful application of a low-temperature-grown amorphous GaAs (a-GaAs) layer for stabilization of the fundamental transverse mode of InGaAs/GaAs vertical-cavity surface-emitting lasers. The maximum currents maintaining a stable fundamental transverse mode were increased by the antiguide effect of a-GaAs with a high refractive index. For 10-μm- and 15-μm-diameter devices, we attained a stable single-mode emission over a wide range of current. The antiguiding of transverse modes in vertical cavity buried in the high refractive cladding layer was calculated using a two-dimensional beam propagation method.
We report the highly reflecting (>99.9%) In0.53Al0.14Ga0.33As/In0.52Al0.48As 30.5 pairs distributed Bragg reflector (DBR) and the In0.53Ga0.47As/ In0.52Al0.48As active cavity layer grown at high temperature by low pressure metal organic chemical vapor deposition with in-situ double beam laser reflectometry. One of the laser wavelengths selected for in-situ measurement was same as the DBR wavelength. The growth temperature was 750 °C. Good surface morphology of the multi-layer stacks was achieved by the temperature ramping of the InP buffer layer at the beginning of a multi-layer stacks. The width of stop band edge of the DBR reflectivity spectrum was found to be 1000Å.
We report optical properties of the micro-facetted InGaAs quantum wells and quantum wires on non-planar substrates employing magnetophotoluminescence (MPL). The InGaAs/GaAs structures were grown by chemical beam epitaxy on V-groove patterned GaAs substrates. In the presence of a magnetic field of 18 T, the diamagnetic shifts of exciton ground states of the (001)-and side-QWLs are ΔE=15.6 and 10.3 meV, respectively. In MPL of the facetted microstructure, we found that the different diamagnetic shifts strongly depend on the magnitude of the effective magnetic field as well as the quantum confinement. From comparing the intensities and full widths at half maximum, we easily found that side-QWLs are of higher quality than (OOl)-QWLs. We also fabricated InGaAs/GaAs quantum wires with a size of about 200 Å × (500–600) Å. By fitting the diamagnetic shifts (ΔΕ = 9.5 meV) of the exciton ground state with the calculated results of a variational method, we estimated that the reduced mass of the exciton is approximately 0.052 me.
The surface structure of Si(111) post-annealed at 980 °C after nitrogen ion induced nitridation has been investigated by using a scanning tunneling microscope (STM) and low energy electron diffraction (LEED). The LEED and STM results indicated the formation of ordered domain of quadruplet structure in the silicon nitride layer. The LEED pattern taken from the nitrated Si(111) surface showed a coexistence of 7×7 domain with quadruplet one. In the STM image taken from the same surface, a three directional periodicity with a periodic arrangement of white protrusions was observed in the local area of silicon nitride island and its symmetry directions were rotated about 10° with respect to those of Si(111) surface. In addition to the quadruplet structure of the silicon nitride island, meta-stable structures such as 9×9, c(4×2), and 2×2 as well as 7×7 phase boundaries were observed to have been formed on the Si(111) surface during the rapid cooling of nitrated surface from the post-annealing temperature of 980 °C. The investigation of the surface structure of nitrated Si(111) showed that the surface nitrated at high temperature had better epitaxial silicon nitride layer than that post-annealed after nitridation at room temperature.
We report application of in-situ laser reflectometry in monitoring InAl1−xAs (0 ≤ x ≤1) epitaxial layers grown on a GaAs substrate by low pressure metal-organic chemical vapor deposition. Two different lasers were used simultaneously: One was a He-Ne laser operating at 0.6328 μm and the other was a diode laser operating at 1.53 μm. The two laser beams were incident on the growing layer at an angle of 71° from the surface normal, and the reflected beams were detected by Si and Ge photovoltaic detectors, respectively. Since the epitaxial layer of InxAl1−xAs (0 ≤ x ≤1) has a wide range of index of refraction, the reflected signals showed a variety of patterns. The optical constants of the InxAl1−xAs epitaxial layers were obtained for the entire range of composition.
Facet evolution and selective area epitaxy of GaAs/AIGaAs ridge and V-groove structure grown on non-planar GaAs(100) substrate by chemical beam epitaxy(CBE) have been investigated for nanostructure applications. To enhance the crystallographic selectivity and to study the new facet evolution on patterned substrate, GaAs and AlGaAs epilayer were grown by growth-interruption mode and continuous mode, respectively. High selectivity of GaAs layer was observed to depend on the various crystallographic planes even at low growth temperature. This was attributed to the efficient Ga surface migration and desorption during the growth-interruption periods. The growth-interruption method was found to be very efficient in improving the morphology of faceted surfaces. We demonstrated that the formation of (111) V-groove and (411) ridge GaAs structures which were surrounded by AlGaAs layer to show the potential implication of this method for the formation of quantum wires.
We report for the first time a successful application of semi-insulating amorphous GaAs layer for surface passivation of index-guided vertical-cavity surface-emitting lasers. The amorphous GaAs layers on ion-beam-etched active region and mirror layers provide a significant improvement, more than 20%, in the threshold current density and differential quantum efficiency. The improvement of these performances is attributed to low defect density at the surface of active layers induced by amorphous GaAs.
The initial oxidation of Si(111)-7×7 surface has been investigated by taking the STM images of samples dosed with oxygen at room temperature and high temperatures between 500°C and 750 °C. In particular, different site selectivities between two oxygen-induced features, bright and dark sites, were observed and explained in terms of the difference in potential energy curves. In addition to such a strong site selectivity under low oxygen partial pressure (l×10-9 torr), heavy surface etching by oxygen was observed at higher O2 partial pressures and temperatures resulting in the high density of monolayer-deep etch marks on terraces.
We have found that the lateral dimension of InGaAs and GaAs multiple layers can be effectively controlled on non-( 111) V-grooved GaAs substrates by chemical beam epitaxy using triethylgallium and trimethylindium coupled with precracked arsine or unprecracked monoethylarsine. We suggest that this effect is due to the efficient migration of adatoms from (111) to non-(111) planes. This is an improved method which overcomes the difficulty that has been associated with the method of using only (111) V-grooves in which the lateral dimension is controlled by the differences in the growth rates between (111) and (100) planes. In case of InGaAs and GaAs epilayers, the anisotropy factors of growth rate were less than 0.1 at optimum growth temperature. Photoluminescence peak originated from InGaAs/GaAs quantum wire was significantly distinct from other peaks, suggesting an effective reduction of InGaAs lateral dimension.
The correlation of surface morphology with strain relaxation in the In0.15Ga0.85As epilayer on GaAs(100) grown by chemical beam epitaxy using unprecracked monoethylarsine has been investigated. The surface morphology of InGaAs was analyzed by atomic force microscopy as the epilayer thickness was increased from 0.025 to 1.668 μm. The changes in the surface morphology indicated that surface roughening is related to the process of strain relaxation in the film. The strain-induced shifts in the GaAs-like longitudinal optical phonon in the Raman spectrum also indicated that the strains in the InGaAs epilayer relax via step-wise process with increasing the film thickness beyond the critical thickness, which agrees well with the changes of surface mophology.
GaAs-AlAs heterostructures have been grown on a GaAs substrate in a low pressure MOCVD reactor. After the C doped AlAs layer was deposited on a GaAs substrate, the undoped GaAs cap layer was grown with a thickness of 150 Å. The sample was exposed to hot water vapor mixed with N2 in a sealed furnace at 400 °C for 15 min. After the thermal oxidation, the sample was characterized by DCXRD, SIMS, AES, XPS, and TEM. The analyses indicate that the aluminum oxide layer is formed from C doped AlAs layer with stable microcrystalline Al2O3, and that the upper part of GaAs cap layer exposed to water vapor is modified to gallium oxide and arsenic oxide, while the lower part of the GaAs layer is unchanged. The results clearly suggest that the GaAs layer can play a role as a diffusion path for exchanging the arsenic and oxygen. Since the oxidation of AlAs is far more reactive than that of GaAs, the oxygen bonded to GaAs in a thin cap layer can be easily reduced to the under-layer of AlAs without a severe modification of GaAs lattice structure.
Highly lattice mismatched InGaAs epitaxial layers on GaAs substrates were grown by low pressure MOCVD technique in order to improve surface morphology and interfacial quality. Four different structures were grown at a low temperature of 435°C. A thick In0.65Ga0.35As layer with a thickness of 1.6 μ m was grown directly on GaAs as a reference, while a similar InGaAs film was grown on a linearly graded compositional InxGA1-xAs buffer layer. Two additional In0.85Ga0.15As buffer layers were similarly grown on GaAs by embedding a thin metal film of either In or Ga at the heterointerface. Contrary to the reference sample and the sample with a graded buffer layer, the samples embedded with the metal films show no cross-hatch pattern and exhibit mirror-like surfaces. Cross-sectional high resolution TEM analysis for the samples with a metallic prelayer indicates that the misfit is relieved mainly by 90° perfect dislocations uniformly distributed near the interface. Hall effect measurements also suggest that the defect densities of these two samples are significantly lower than that of the reference sample.
The effects of hydrogen plasma exposure upon electron Hall mobilities in InSb heteroepitaxial film grown on GaAs substrate have been investigated. After exposure to a hydrogen plasma at 250°C, the electron Hall mobility is significantly increased at low temperatures and the temperature dependence of the mobility is reduced. For the film with a broad x-ray rocking-curve width, 4 h-hydrogen plasma exposure can give rise to the enhancement of the mobility up to 6 times at low temperature. The mobility for the film with a narrow line width is enhanced around 1.5 times. These enhanced mobilities are nearly restored by 350°C rapid thermal annealing. The enhancement of the mobility due to hydrogenation is attributed to the satisfaction of the dangling bonds generated by the misfit dislocations.
NH3-plasma treatment has been used for passivation of native-oxide-contaminated GaAs surface. Ex situ band-gap photoluminescence(PL) measurement shows enhanced intensity for the treated surfaces in direct plasma. Auger electron spectroscopy(AES) shows that the treated surface contains nitrogen atoms but no arsenic atoms, which leads us to speculate that the graded GaN thin layer was formed on the surface. Long-term stability of the enhanced PL intensity is attributed to the formation of GaN on the surface.
We have grown GaAs epilayers by ultrahigh vacuum chemical vapor deposition(UHVCVD) using adsorbed hydrides and metalorganic compounds via a surface decomposition process. This result indicates that unprecracked arsine(AsH3) can be used in chemical beam epitaxy(CBE) and that a new hydride source with a low decomposition temperature, monoethylarsine(MEAs) can replace the precracked AsH3 source in CBE. The impurity concentrations in GaAs grown with trimethylgallium(TMG) and triethylgallium(TEG) were found to be very sensitve to growth temperature. It was also found that the uptake of carbon impurity is significantly reduced when TMG is replaced with TEG. The carbon concentrations in epilayers grown using TMG and TEG with unprecracked AsH3 and MEAs were reduced by 2-3 orders of magnitude compared to those by CBE process employing TMG and arsenics from precracked hydrides. We have also found that the hydrogen atoms play an important role in the reduction of carbon content in GaAs epilayer. Intermediates like dihydrides from MEAs decomposed on the surface are considered to supply hydrogen atoms and hydrides during growth, which may remove other carbon containing species.
The strain relaxation mechanism via the homogeneous nucleation of misfit dislocations from interface during interdiffusion in lattice-matched semiconductor heterostructures has been investigated. Transmission electron microscopy studies in intermixed GaInAsP/InP heterostructures revealed that the critical interdiffusion depth for the nucleation of 90° 1/6<112> partial dislocations from a tensile interface is much shallower than that of 60° 1/2<110> perfect dislocations from a compressive interface. A critical thickness model for the interface nucleation of these dislocations is developed as a modification of the classical surface nucleation'model.
We present results of a study on the effect of unprecracked arsine(AsH3) and trimethylgallium(TMGa) on carbon incorporation in UHVCVD(Ultra High Vacuum Chemical Vapor Deposition) grown GaAs epilayers on GaAs(100). Three distinct temperature-dependent regions of growth rates were identified as growth temperature was increased from 570 to 690°C. The growth rates were also strongly dependent on V/III ratio in a range of 5 to 30, which clearly indicates that the growth rate is determined by the amount of arsenic adsorbed on the surface at low V/III ratio and adsorption of TMGa or decomposition process at high V/III ratio. Hall concentration measurements and low temperature photoluminescence data show that the films are all p-type and their impurity concentrations are reduced by two orders of magnitude compared to those of epilayers grown by CBE(Chemical Beam Epitaxy) which employs TMGa and arsenic(precracked arsines) as source materials. Our results indicate that the hydrogen atoms dissociated from adsorbed arsine may remove hydrocarbon species resulting in a significant drop in hole concentration.
We present preliminary results aimed at investigating the effects of unprecracked arsine and trimethylgallium on the CBE (chemical beam epitaxy) growth of GaAs epilayers. We find that the growth rate rises linearly as the V/III ratio is increased when TMGa and arsine are used. All of the runs produced p-type material mainly due to carbon incorporation with the hole concentration typically of 1017 cm−3. The impurity content of the layers was found to depend distinctly on the pressure of TMGa. The significant drop in hole concentration is due in part to the hydrogen atoms generated from decomposed AsH3 which then aids in the removal of CH3 radicals on the surface. As a result of using unprecracked arsine for growth of the GaAs epilayers, we measure substantial improvements in their electrical and optical properties.
The microstructural degradation of a lattice-matched Ga0.28 In0.72As0.61P0.39/InP heterointerface during atomic intermixing induced by Zn diffusion has been investigated using high-resolution transmission electron microscopy and Auger electron spectroscopy. The localized interfacial stress caused by intermixing appears to create stacking faults in the Ga-mixed InP substrate, and dislocation tangles in the In-mixed GalnAsP layer. The results are attributed to the contrasted effect of tensile and compressive stresses upon the nucleation of partial dislocations from both sides of the intermixed interface. A qualitative model is proposed for the homogeneous nucleation of misfit dislocations from the locally stressed interface.