Beam Flux Ratio Research Articles

Preferred Orientation Control of Cu(In1-xGax)Se2 (x ≈0.28) Thin Films and Its Influence on Solar Cell Characteristics

The crystal structure of polycrystalline Cu(InGa)Se2 (CIGS) films deposited by physical vapor deposition using a 3-stage growth process onto Mo/soda-lime glass substrates were examined. It was found that the [Se]/[In+Ga] beam flux ratio during deposition of In–Ga–Se precursors at the first stage was the most critical parameter for control of the crystal orientation. (220)/(204)-oriented films were obtained at high beam flux ratios while (112)-oriented films were obtained at lower [Se]/[In+Ga] beam flux ratios. The growth parameters at the second stage of Cu–Se coevaporation had a smaller effect on the preferred orientation, but could influence the surface texture, surface morphology and grain size. The difference in Cd diffusion behavior between the (112)- and (220)/(204)-oriented CIGS thin films was examined by a chemical bath process. It was found that Cd atoms were more easily doped into the (220)/(204)-oriented films than into the (112)-oriented films. The influences of the absorber texture on the I–V parameters of ZnO/CdS/CIGS solar cells have also been investigated. By optimizing the Cd buffer layer growth condition, a high efficiency of 17.6% with high FF value of 0.79 has been achieved with a (220)/(204)-oriented absorber.

Growth and properties of new III–V diluted magnetic semiconductor Ga 1− xCr xAs

III–V diluted magnetic semiconductor Ga 1− x Cr x As was successfully synthesized by low-temperature molecular beam epitaxy. High-quality twin-free films were grown with Cr concentrations up to x=0.05 under an As 4/Ga beam flux ratio ∼40. The films showed local ferromagnetic ordering at T<30 K. With higher Cr concentrations or lower As 4/Ga beam flux ratios, the film quality degraded.

Monte Carlo simulation of homoepitaxial growth on two-component compound semiconductor surfaces

We have studied some characteristic behaviors of homoepitaxial growth on III–V semiconductor surfaces by taking account of two components explicitly in a Monte Carlo simulation. In addition to the surface step density, the coverage of cation or anion in the topmost layer shows oscillatory behavior during the growth, which varies as a function of the beam fluxes as well as the temperature. We have also studied on the effects of barrier energies to hopping for cation and anion, of the anisotropy of the barrier energies and of the beam flux ratio between cation and anion. By taking the growth on GaAs(100) as an example, we compare the simulated results with the reported experimental ones. The growth mode transition temperature observed in experiment is reproduced by using anisotropic hopping barrier energy. The island shape changes during the recovery process to anisotropic rectangular form with the longer dimension along the As dimer direction, which is in good agreement with the reported experimental observation.

Molecular beam epitaxy growth of Iny2Al1−y2As/In0.73Ga0.27As/Iny1Al1−y1As/ InP P-HEMTs with enhancement conductivity using an intentional nonlattice-matched buffer layer

Pseudomorphic Iny2Al1−y2As/In0.73Ga0.27As/Iny1Al1−y1As (y1⩾0.52) modulation-doped heterostructures with an intentional nonlattice-matched buffer layer were successfully grown by molecular beam epitaxy on (100)InP substrates. Fourier transform photoluminescence and double crystal x-ray diffraction measurements show a superior crystalline quality in the high In content channel, when In mole fraction increases from y1=0.52 to 0.55 in the Iny1Al1−y1As buffer layer. In this case, an increasing of 16.3% and 23.5% for conductivity (μ×ns) and mobility, related to the strain compensation in the In0.73Ga0.27As channel, was achieved, respectively, comparing to the structure containing a well-lattice matched buffer layer. With increasing the mismatch further (y1=0.58), a morphology with cross-hatched pattern was observed due to the onset of a large amount of misfit dislocations, and the electronic characterization is not able to be improved continuously. Because we can realize high quality strained P-HEMTs in a relative wide range of equivalent beam flux (EBF) ratios, the stringent control over the constant EBF is not indispensable on this In-based material system.

Molecular beam epitaxial growth of high electron mobility deep quantum well structures

We investigated InAs deep quantum well structures made from InAs/AlGaAsSb materials on GaAs substrates by molecular beam epitaxy. By accurately controlling As 4 /Sb 4 and As 4 /In beam flux ratios, we were able to grow the high-quality crystals of AlGaAsSb and InAs. Very high electron mobility of more than 32,000 cm 2 /Vs were obtained at room temperature. It was found that the optimum beam-equivalent-pressure (BEP) ratio of As 4 /In for growing an InAs quantum well layer is 110 to get the highest electron mobility. Moreover, a strong photoluminescence (PL) peak of AlGaAsSb band-edge emission at 10 K was observed around 1.2 eV. We determined that the optimum As 4 /Sb 4 BEP ratio is 6.7 for AlGaAsSb from the full width at half-maximum of PL peaks and electrical characteristics. We also observed the blue shifted PL of AlGaAsSb as the As 4 /Sb 4 BEP ratio decreased.

Role of interface chemistry and growing surface stoichiometry on the generation of stacking faults in ZnSe/GaAs

The existence of Zn-As and vacancy-contained Ga-Se interfacial layers are suggested by transmission electron microscopy of Zn-and Se-exposed (or - reacted) ZnSe/GaAs interfaces, respectively. A very low density of faulted defects in the range of ∼104cm2 was obtained in samples with Zn passivation on an Asstabilized GaAs-(2 × 4). However, the density of As precipitates increases as the surface coverage of c(4 × 4) reconstruction increased on the Zn-exposed Asstabilized GaAs-(2 × 4) surface and this is associated with an increase of the density of extrinsic-type stacking faults bound by partial edge dislocations with a core structure terminated on additional cations. On the other hand, densities of extrinsic Shockley-and intrinsic Frank-type stacking faults are of ∼5 × 107/cm2 in samples grown on Se-exposed Ga-rich GaAs-(4 × 6) surfaces. Annealing on this Se-exposed Ga-rich GaAs-(4 × 6) generated a high density of vacancy loops (1 × 109/cm2) and an increase of the densities of both Shockley-and Frank-type stacking faults (>5 × 108/cm2) after the growth of the films. Furthermore, we have studied the dependence of the generation and structure of Shockley-type stacking faults on the beam flux ratios in samples grown on Zn-exposed As-stabilized GaAs-(2 × 4) surfaces. Cation-and anion-terminated extrinsic-type partial edge dislocations were generated in samples grown under Zn-and Se-rich conditions, respectively. However, an asymmetric distribution on defect density under varied beam flux ratios (0.3 ≤ PSe/PZn ≤ 10) is obtained.

Dependence of generation and structures of stacking faults on growing surface stoichiometries in ZnSe/GaAs

Thin ZnSe films were grown by molecular beam epitaxy on Zn exposed (2×4) As-stabilized surfaces of GaAs epilayers under varied beam flux ratios. A very low density of faulted defects in the range of ∼ 104/cm2 was generated in samples grown under a condition with a mixture of both (2×1) and weak c(2×2) surface reconstructions at the initial stages of growth. However, an asymmetric distribution on the densities of extrinsic cation- and anion-terminated Shockley-type stacking faults were generated, respectively, in samples grown under Zn- and Se-rich surface stoichiometries.

Group V incorporation in InGaAsP grown on InP by gas source molecular beam epitaxy

The incorporation of the group V components in In1−xGaxAsyP1−y, grown lattice matched to InP by gas source molecular beam epitaxy, has been studied over the entire alloy range, 0≤y≤1, as a function of the group V source composition, the V/III beam flux ratio, and the substrate surface orientation. Several aspects of the group V incorporation are most easily understood in terms of a simple model involving a constant incorporation coefficient and an As ‘‘underpressure’’ condition. An improved description of the results at lower values of the V/III flux ratio is provided by a thermodynamic model based on equilibrium reactions for the formation of the binary constituents, and using the bulk properties of the solid solution. However, the thermodynamic model is quantitatively incorrect for large values of the V/III flux ratio. Furthermore, the results for different surface orientations reveal additional weaknesses in the thermodynamic model and suggest the need to account for the surface bonding configurations in describing the group V incorporation in epitaxial growth.

Low temperature growth of PbTe and of PbTe/ Pb1− xEuxTe multi-quantum wells by molecular beam epitaxy

A systematic study of molecular beam epitaxy (MBE) of PbTe and Pb 1- x Eu x Te on BaF 2 (111) at substrate temperatures between 350 and 70°C is presented. Reflection high-energy electron diffraction (RHEED) intensity oscillations with more than 200 oscillation periods are observed even for substrate temperatures as low as 95°C, signifying the persistence of two-dimensional layer-by-layer growth at surprisingly low growth temperatures. In addition, for low temperature MBE growth of the ternary Pb 1- x Eu x Te, the evolution of RHEED intensity oscillations depends strongly on the beam flux composition, i.e., the Te 2 to Eu beam flux ratio, and a Te-stabilized, as well as Eu-stabilized layer-by-layer growth mode exists. A series of PbTe epitaxial layers, grown at various substrate temperatures from 350 to 120°C, was characterized by high resolution X-ray diffraction and temperature dependent Hall effect measurements. High quality epitaxial layers were obtained for substrate temperatures as low as 160°C, whereas for lower temperatures a drastic broadening of the X-ray diffraction peaks and a strong decrease of the low temperature Hall mobilities is observed. In PbTe/Pb 1- x Eu x Te multi-quantum well structures grown at various substrate temperatures, no broadening of the superlattice satellite diffraction peaks is observed when the substrate temperature is decreased down to 200°C. The high quality of the samples grown at substrate temperatures very much below the usual MBE temperatures is based on a precise control of the beam flux composition. Under such conditions, MBE of IV-VI compounds can be carried out at very low substrate temperatures, whenever technologically necessary.

Substrate Nitridation Effects on GaN Grown on GaAs Substrates by Molecular Beam Epitaxy Using RF-Radical Nitrogen Source

The initial substrate nitridation effects on the crystal structure of GaN epitaxial layers grown on GaAs substrates by gas source molecular beam epitaxy using RF-radical N2 as a nitrogen source were investigated. The crystal structure of GaN grown on (100) GaAs substrates was critically influenced by substrate nitridation time, that is, zincblende GaN was grown on the substrate without nitridation, and wurtzite GaN was grown on the nitrided substrate (longer than 120 s). The substrate misorientation effects on surface morphology and X-ray full width at half-maximum of zincblende GaN layers were also studied. A featureless smooth surface was obtained for a layer grown under the high V/III beam flux ratio condition.

Epitaxial Growth of Eute on Pbte (111) Influenced by Strain-Induced Coherent 3D Islanding

ABSTRACTEuTe on PbTe is an example of a strained layer heteroepitaxial system with a moderate lattice-Mismatch of 2%. Using reflection high-energy electron diffraction (RHEED), the phase diagram of heteroepitaxial growth of EuTe on PbTe (111) by molecular beam epitaxy was established, indicating that only in a narrow regime of growth conditions does two dimensional layer-by-layer growth occur. Using in situ RHEED, combined with ex situ atomic force Microscopy, the evolution of the surface morphology during heteroepitaxial growth was investigated. It is found that at the critical layer thickness an abrupt surface roughening occurs. Studying the dependence of this surface roughening transition on substrate temperature and on the Eu-to-Te2 beam flux ratio used for growth, the critical layer thickness is found to decrease strongly as the surface kinetics are enhanced, even if the substrate temperature is kept constant. These results indicate that the surface roughening is driven only by the surface kinetics, which strongly supports the model of strain relaxation by coherent three dimensional islanding.

Heteroepitaxial Growth of Chalcopyrite-type CuAlSe2 Compound Semiconductor by Molecular Beam Epitaxy

The heteroepitaxial growth of chalcopyrite-type CuAlSe2 has been performed on GaAs (100) or CaF2 (111) substrates by molecular beam epitaxy. In this study, the film properties which depend on the growth conditions such as the beam flux ratio and the substrate temperature were examined. The epitaxial films of CuAlSe2 were obtained under the following conditions: the substrate temperature was 600°C and the beam flux ratio of Cu to Al was about 0.3 on both GaAs and CaF2 substrate.

Solid phase epitaxial growth of InP on GaAs

Solid phase epitaxial (SPE) growth of amorphous InP on GaAs was investigated by molecular beam epitaxy. Amorphous InP was obtained at room temperature with a high incident beam flux ratio of P 2 to In. The SPE layers show a flat surface without the formation of islands. The subsequent InP epitaxial growth on SPE layers was obtained successfully in a two-dimensional growth mode without island formation.

Doping characteristics of Si into molecular-beam epitaxially grown InAlAs layers

We studied the doping characteristics of Si into molecular-beam epitaxially grown InAlAs layers for various growth conditions. The growth temperature and V/III beam flux ratio proved to play different roles in Si doping. The growth temperature affects the Si doping concentration, and the V/III flux ratio does the compensation ratio. At Ts=560 °C, a rather high temperature for the growth of InAlAs layers, the Si doping concentration reached 150% as compared with growth at Ts=500 °C. With increasing V/III flux ratio, on the other hand, electron mobility and donor concentration increased, suggesting a decreased compensation ratio. We also found a considerable number of electron trapping centers at a low-growth temperature of 400 °C.

Low temperature deposition of single crystalline ZnSe films on GaAs substrates by slightly ionized molecular beams

The quality of ZnSe films epitaxially grown on (100) GaAs substrates from slightly ionized molecular beams was found to strongly depend on substrate bias, substrate temperature and beam flux ratio. The GaAs substrates were chemically etched and not further treated in the growth chamber before growth. The ionization of the molecular beam fluxes caused an obvious increase of the growth rate and was effective for removing contamination of the substrate surface. A (100) ZnSe film with a streaked RHEED patterns grew with a substrate bias of 40 V, a substrate temperature of 300°C and a beam ratio of the element beam fluxes of 1.

A study of polycrystalline Cd(Zn, Mn)Te/Cds films and interfaces

Polycrystalline films of Cd1-xZnxTe (x = 0–0.4) and Cd1-xMnxTe (x = 0–0.25) were grown by MBE and MOCVD, respectively, on CdS/SnO2/glass substrates to investigate their feasibility for solar cell applications. The compositional uniformity and interface quality of the films were analyzed by x-ray diffraction, surface photovoltage, and Auger depth profile measurements to establish a correlation between growth conditions and lattice constant, atomic concentration, and bandgap of the ternary films. MBE-grown polycrystalline Cd1-xZnxTe films showed a linear dependence between Zn/(Cd + Zn) beam flux ratio, Zn concentration in the film, and the bandgap. Polycrystalline Cd1-xZnxTe films grown at 300° C showed good compositional uniformity in contrast to compositionally non-uniform Cd1-xMnxTe films grown by MOCVD in the temperature range of 420–450° C. The MBE-grown Cd1-xZnxTe interface also showed significantly less interdiffusion compared to the MOCVD-grown Cd1-xMnxTe/CdS interface, where preferential exchange between Cd from the CdS layer and Mn from the Cd1-xMnxTe film was observed. The compositional uniformity of MOCVD-grown polycrystalline Cd1-xMnxTe films grown on CdS/SnO2/glass substrates was found to be a strong function of the growth conditions as well as the Mn source.

Molecular beam epitaxial growth of high quality GaAs layer directly on GaP substrate

The crystalline quality was investigated, relating to the initial growth process in the 4% lattice mismatched system of GaAs on GaP. A high quality GaAs epilayer was obtained under optimum conditions of As 4/Ga beam flux ratio. The results were compared with those of GaAs/Si(100) and GaAs/Si(111). It was clarified that two-dimensional growth, realized under the optimum conditions, leads to high crystalline quality in terms of surface morphology, etch pit density and FWHM value of double crystal X-ray diffraction.

Influences of MBE growth conditions on quantum efficiency of negative electron affinity GaAs photocathode.

The MBE growth conditions for Be doped GaAs layers are evaluated from quantum efficiency of the negative electron affinity photocathodes made of these layers. The quantum efficiency of the GaAs layers grown by molecular beam epitaxy method at various growth conditions was measured in reflection mode operation. High quantum efficiencies were obtained with Be doping levels between 1018 and 1019 cm-3 at high As/Ga beam flux ratios and high substrate temperatures.

Properties of Chlorine-Doped Zinc Selenide Grown by Molecular Beam Epitaxy

ABSTRACTZnSe:Cl epilayers have been grown on (100) GaAs by MBE using a Zn to Se beam flux ratio of 2:1 and substrate temperatures as low as 225 °C. The ZnSe:Cl epilayers are highly conducting and exhibit bright blue-violet edge luminescence at 300 K. A mobility of 2480 cm2 /V-s at 40 K was measured for an n-type ZnSe:CI film that was Cl-doped to ∼2.3×1017 cm−3 and for which a compensation ratio (NA/ND) of ∼3% was calculated. Carrier concentrations as large as 6.7×1018cm−3 were obtained by increasing the temperature of the MBE oven containing the C1 dopant.

Preparation and Characterization of Ion Beam Assisted Aluminum Oxide Films

ABSTRACTThe ion beam assisted deposition (IBAD) technique has been employed to make aluminum oxide optical coatings. Deposition variables include aluminum oxide evaporant to oxygen ion beam flux ratios, substrate temperature, and ion energy. Characterization included optical ellipsometry, ion beam analysis, and adhesion tests. Films deposited with the aid of ions exhibited the highest refractive indices and best adhesion to their substrates.