Close-spaced Vapor Transport Research Articles

Epitaxial GaAs by close space vapor transport

The rate of transport of GaAs obtained with the close space vapor transport technique has been investigated as a function of the source (T) and substrate (θ) temperatures and of the source-substrate spacing for two different water vapor pressures (6.6 and ∼10−5 mm Hg) introduced with the H2 gas. Analysis of the results allow us (i) to determine the enthalpy and activation energy (2.1 and 3.1 eV, respectively) associated with the reactions taking place at the source and substrate surfaces and (ii) to deduce the existence of a third reaction occurring in the gas phase, whose activation energy has been measured (2.9 eV). Depending on the water vapor pressure, this third reaction occurs near the source (high water vapor pressure) or near the substrate (low water vapor pressure). We derive the equation governing the rate of transport, from which we calculate this rate per unit of source substrate spacing and of water vapor pressure as a function of both T and θ for constant values of θ and T, respectively.

Schottky solar cells on thin polycrystalline Zn3P2 films

Thin, polycrystalline films of p-type Zn3P2 were grown by close-spaced vapor transport on mica substrates coated with films of Fe and Si. Schottky solar cells were prepared by sputter depositing a thin, transparent film of Mg on Zn3P2. A total area conversion efficiency of 4.3% was achieved on 1-cm2 area cell, under simulated AM1 illumination. The short-circuit current and open-circuit voltage were comparable to similar devices on single-crystal Zn3P2. The barrier height was found to be ∼1.3 eV, higher than for Mg/Zn3P2 contacts reported earlier.

Morphology and crystallography of CdS-CdTe double layers on various GaAs surfaces

Epitaxial double layers of CdS and CdTe have been grown on GaAs by close-spaced chemical vapor transport, to study the growth of CdTe on various CdS surfaces. Since the [00.1] axis of CdS aligns to one of the 〈111〉A axes of the GaAs substrate, the deposition of CdS on {111̄}A, {110}, and {100} GaAs surfaces yields approximately the {00.1}, {10.3}, and {11̄.1} CdS surfaces. On {111̄}A and slightly inclined substrates the CdS-CdTe layers (2–10 μm thick) grow uniformly thick with smooth surfaces. On the {110} and {100} faces the CdTe layers show a scale structure through the whole layer, which is approximately aligned to one of the 〈111〉 axes of the substrate. X-ray investigations reveal three- and two-dimensional lattice defects in the CdTe layer such as twinning, cubic-hexagonal mixture, stacking faults, fiber texture, and polycrystallinity. The number of defects increases in layers grown on {111̄}A to {110} and to {100}. The most perfect layers on {111}A faces and slightly inclined ones have a cubic structure with twinning about the growth direction. On {110} and {100} faces, the [00.1] and [111̄] stacking axes of the three lattices are slightly tilted to each other. The tilt angle can be explained by an accommodation of the lattice mismatch at the interface in such a way that the stacking planes join together continuously at the boundaries.

Preparation of P-type indium phosphide films by close space vapor transport

InP films have been grown by close space transport employing 0.8 mol% PC l 3 in H 2. For deposition on InP single crystals, 700C source and 650C substrate temperatures produced epitaxial films on (100), polycrystalline films on (111)A, and powdery layers on (111)B. Growth rates are 6 to 10 |Gmm/hr on (100) InP and ~50 μm/hr on (111)A InP. Regardless of InP source doping, deposits exhibit net donor concentrations of 5×10 17 to 1×10 18 cm −3. Zn doping with 0.02 to 0.5 mol% Zn(C 2H 5) 2 in the gas phase resulted in partially compensated p-InP with net acceptor concentrations up to 7×10 18 cm −3. Polycrystalline films have been grown on Mg-coated carbon or molybdenum substrates at 700C source and 590C substrate temperatures. Growth rates lie between 40 and 50 μm/hr. Substantial recrystallization and grain growth are observed after 2 day anneals at 950C under 5 atm of phosphorus.

Photovoltaic energy conversion with n-CdS—p-CdTe heterojunctions and other II-VI junctions

n-CdS-p-CdTe heterojunctions have been prepared by close-spaced vapor transport of p-CdTe films onto single crystal n-CdS, by vacuum evaporation of n-CdS films onto single crystal p-CdTe, and by solution spraying of n-CdS films onto single crystal p-CdTe. In addition, a number of other II-VI p-n heterojunctions have been prepared by the close-spaced vapor transport technique. The highest solar efficiency to date has been obtained with a cell prepared by vacuum evaporation of n-CdS film onto p-CdTe crystal, in which the CdS is covered with an indium-tin-oxide coating and then with an antireflection coating; the photovoltaic parameters of this cell are an open-circuit voltage of 0.63 V, a quantum efficiency of 0.82, a fill factor of 0.66, and a solar efficiency of 7.9 percent. II-VI heterojunction cells with efficiency greater than 10 percent are expected in the near future.

Photovoltaic properties of five II-VI heterojunctions

Five heterojunction systems, involving the II-VI compounds CdSe, CdTe, ZnSe, and ZnTe, have been prepared using the close-spaced vapor transport method. Current transport through these heterojunctions was found to be dominated by recombination at the interface. Using a collection function, introduced to describe the voltage dependence of the collection of photogenerated carriers, diffusion potentials were determined, and from them values of the electron affinity of CdSe (4.53 eV) and ZnTe (3.73 eV) relative to CdTe (4.28 eV).

II-VI photovoltaic heterojunctions for solar energy conversion

Several different II-VI heterojunctions show possible promise for photovoltaic conversion of solar energy. Two of these are p-CdTe/n-CdS and p-CdTe/n-Zn0.35Cd0.65S, which have a maximum solar efficiency of 17 and 23%, respectively. We report here specifically on the properties of p-CdTe/n-CdS cells prepared (a) by close-spaced vapor transport of CdTe onto single-crystal CdS, and (b) by two-source vacuum evaporation of CdS onto single-crystal CdTe. Cells with efficiency of 4.0% have been produced without detailed attention to optimization of cell design; these cells have quantum efficiencies as high as 0.85.

The growth and electrical characteristics of epitaxial layers of zinc sulphide and of zinc selenide on p-type gallium phosphide†

Single crystal layers of ZnS and ZnSe have been expitaxially grown on p-type GaP using an HC1 close-spaced vapour transport system. Grown layer doping is accomplished using heavily doped source (ZnS, ZnSe) material and a zinc vapour diffusion process that reduces the number of compensating zinc vacancies. Experimental characteristics of nZnS-pGaP and nZnSe-pGaP diodes indicate dominance of the forward I-U relationship by space-charge-limited flow in the 103-104 ohm-cm ZnS and ZnSe. The reverse I-U characteristics of these devices agree qualitatively with a multistep tunnelling process but show larger than, expected reverse currents. The reverse bias capacitance is strongly frequency dependent in all devices studied. Forward injection electroluminescence was not observed from any of the nZnS-pGaP and nZnSe-pGaP devices studied.

Calculation of distribution Equilibrium of amphoteric silicon in gallium arsenide: Iwao Teramoto, Research Laboratory, Matsushita Electronics Corporation, Takatsuki, Osaka, Japan

Device-grade ternary Cu–Ga–Se chalcopyrite thin films used for photovoltaic energy conversion have been prepared by a novel chemical close-spaced vapor transport (CCSVT) technique developed for a deposition on areas of up to 10×10 cm2. A two-step process has been developed which allows the fine tuning of the film composition and the electronic properties. The extension of deposition times in the two-step process led to final film compositions with [Ga]/[Cu] ratios ranging from 0.9 to 5.7, allowing the study of the structural phase transitions. In this paper the main focus of interest is related to the material properties of the device-grade thin films prepared by CCSVT technique. We present our recent studies on (i) the growth, compositional, structural and electronic structural properties, (ii) the degradation under ambient conditions and (iii) the feasibility of n-type doping this p-type semiconducting material by germanium. Thin films were grown with chalcopyrite (1:1:2) and CuGaSe2-related defect compound structures (DC) with stoichiometries of CuGa3Se5 and CuGa5Se8. In order to derive the DC structure, X-ray and neutron powder diffraction investigations have been carried out on powders of these CuGaSe2-related compounds grown by elemental synthesis (powder) and CCSVT (thin films), respectively. We found no hints for an ordering of defects, as proposed in the past and giving name to the so-called Ordered Defect Compounds (ODC) in this and related structures. From our results a growth model is presented for CuGa3Se5 formation in gallium-rich CCSVT-grown CuGaxSey films. The chemical and electronic surface and interface structure of CuGaSe2 thin films with bulk [Ga]/[Cu] ratios between 0.94 and 1.39 is investigated by X-ray and UV-excited photoelectron spectroscopy (XPS and UPS, respectively). A transition of the Cu:Ga:Se surface composition from 1:1:2 for the Cu-rich bulk sample to 1:3:5 for the sample with the highest bulk [Ga]/[Cu] ratio is observed. Simultaneously, a downward shift of the valence band maximum position with respect to the Fermi energy is found. The comparison of the estimated conduction band minimum with that of CdS reveals the formation of a pronounced “cliff-like” conduction band offset at the respective interface.Furthermore, the CuGaSe2 thin film degradation under ambient as well as under thermal conditions of CuGaSe2 thin films has been studied by XPS. During thermal oxidation, the formation of predominantly Ga2O3 and some amount of SeO2 were observed, but no copper oxides could be detected in the near-surface region of the thin films. The same oxides are found after native oxidation in air under ambient conditions. An additional sodium oxide compound formed at the thin film surface, NaxO and Na2CO3 after thermal and native oxidation, respectively.Germanium ion implantation technique of the near-surface region of CuGaSe2 thin films has been used in order to prove the feasibility of n-type doping. In photoluminescence (PL) studies, the occurrence of a new emission line is identified as Ge related and explained as a donor–acceptor-pair (DAP) recombination. The precise role the Ge is playing in this doping of CuGaSe2 is revealed by X-ray absorption spectroscopy (XANES and EXAFS) and ab initio calculations based on the density functional theory. The studies indicate that the incorporated Ge atoms preferentially occupy Ga sites when relaxation around the dopant is taken into account. Additionally, our corresponding theoretical band structure model predicts the existence of additional localized electronic acceptor and donor defect bands within the band gap of CuGaSe2 originating from a strong covalent interaction between Ge 4s and Se 4p states for Ge atoms tetrahedrally surrounded by the Se nearest-neighbor atoms. A theoretically predicted anti-bonding Ge–Se4sp3 defect band appearing well above the Fermi level for the Ge1+Ga point defect system can be directly linked to a Ge-dopant-related donor–acceptor-pair transition as observed in our photoluminescence spectra.

Optical and Electrical Properties of Single-Crystal GaP Vapor-Grown on GaAs Substrate

Single-crystal GaP has been grown at rates up to 13 mils (330 μ) per hour onto a GaAs substrate by using a close-spaced vapor transport technique. The as-grown layers are invariably n type. Thin layers (3–6 mils, or 75 to 150 μ) grown at slow rates are essentially strain-free, while thicker layers grown more rapidly exhibit considerable strain. Hall measurements were made and show that the dominant donor has an activation energy of 0.089 eV. Mobilities as high as 120 cm2/V-sec and carrier concentrations as low as 4×1015/cm3 have been measured. The Hall data imply an effective mass of 0.25m. The decay of pulse-injected excess carriers indicate a lifetime of about 1 μsec and the presence of several deep traps. Photovoltaic measurements were made and are discussed.