BAs and boride III-V alloys

Boron arsenide, the typically-ignored member of the III-V arsenide series BAs-AlAs-GaAs-InAs is found to resemble silicon electronically: its Gamma conduction band minimum is p-like (Gamma_15), not s-like (Gamma_1c), it has an X_1c-like indirect band gap, and its bond charge is distributed almost equally on the two atoms in the unit cell, exhibiting nearly perfect covalency. The reasons for these are tracked down to the anomalously low atomic p orbital energy in the boron and to the unusually strong s-s repulsion in BAs relative to most other III-V compounds. We find unexpected valence band offsets of BAs with respect to GaAs and AlAs. The valence band maximum (VBM) of BAs is significantly higher than that of AlAs, despite the much smaller bond length of BAs, and the VBM of GaAs is only slightly higher than in BAs. These effects result from the unusually strong mixing of the cation and anion states at the VBM. For the BAs-GaAs alloys, we find (i) a relatively small (~3.5 eV) and composition-independent band gap bowing. This means that while addition of small amounts of nitrogen to GaAs lowers the gap, addition of small amounts of boron to GaAs raises the gap (ii) boron ``semi-localized'' states in the conduction band (similar to those in GaN-GaAs alloys), and (iii) bulk mixing enthalpies which are smaller than in GaN-GaAs alloys. The unique features of boride III-V alloys offer new opportunities in band gap engineering.Comment: 18 pages, 14 figures, 6 tables, 61 references. Accepted for publication in Phys. Rev. B. Scheduled to appear Oct. 15 200

Thin films of InP for photovoltaic energy conversion. First quarterly technical progress report, July 5, 1979-September 28, 1979

A research study is in progress to develop a low-cost high-efficiency thin-film InP heterojunction solar cell, using the metalorganic chemical vapor deposition (MO-CVD) technique for InP film growth on suitable low-cost substrates. Heterostructure devices of CdS/InP and indium tin oxides (ITO)/InP are to be prepared by Stanford University under subcontract, using the InP films grown by MO-CVD. The first quarter's work is summarized. An existing reactor system was modified and upgraded, with provision for growth of GaAs intermediate layers and p-type doping capability using a premixed metalorganic Zn compound source, as well as a dilution system common to the dopants and the Group III and Group V reactants. The all-fused-quartz deposition chamber was redesigned to permit use of larger susceptors and to facilitate assembly, disassembly, and cleaning. Initial attempts to prepare p-type epitaxial InP films on single-crystal InP substrates met with only limited success until a new source of diethylzinc (DEZn) was obtained and installed on the system, after which active Zn concentrations (i.e., measured hole concentrations) in the 10/sup 17/ cm/sup -3/ range were produced in epitaxial films on (100), 111A), and (111B) InP substrates in the deposition temperature range 715 to 730/sup 0/C. The films have been characterized by x-ray and electron diffraction analyses, examination in the scanning electron microscope, and determination of transport properties by Hall-effect measurements using the van der Pauw method. Results of these investigations are reviewed

PROPERTIES OF EPITAXIAL Si FILMS GROWN ON YTTRIA-STABILIZED CUBIC ZIRCONIA SUBSTRATES BY CHEMICAL VAPOR DEPOSITI ON

Epitaxial growth of single-crystal Si films on monocrystalline yttria-stabilized cubic zirconia, (Y2O3)m (ZrO2)1-m substrates with m=0.09 - 0.33 has been realized for the first time by the pyrolysis of SiH4 in H2 carrier gas. Films and substrates were characterized by means of Nomarski optical and scanning electron microscopies, reflection electron diffraction, MeV4He+ Rutherford backscattering and channeling, X-ray diffraction, transmission electron microscopy (TEM), secondary ion mass spectrometry, and electrical measurements. Single-crystal film growth was achieved on the three principal substrate cubic planes (100), (110), and (111) in the temperature range 975-1077°C at deposition rates of 0.08 - 1.2µm/ min. The orientations of the Si films were found to be the same as those of the substrates. The nearsurface crystal quality of several 0.4µm thick, (100) oriented Si films on yttria - stabilized zirconia was found by MeV He+ channeling and TEM to be superior to that of commercial (100) Si on sapphire films of similar thickness. The electrical properties of nominally undoped films were found to depend critically on the pre-deposition, in-situ substrate preparation. The effect of the ionic superconduction properties of the substrate material on the structure of the deposited Si films will also be discussed