J. B. Webb

Transparent and highly conductive films of ZnO prepared by rf reactive magnetron sputtering

Highly conductive films of zinc oxide have been prepared by reactive rf magnetron sputtering from an oxide target. Film conductivities ranging from ∼10−8 Ω−1 cm−1 to 5×102 Ω−1 cm−1 can be obtained depending on the sputter conditions. Films with sheet resistivities of 85 Ω/⧠ showed little absorption and ∼90% transmission between λ = 4000→8000 A. A second low power discharge at the substrate is used to initiate growth of the highly conducting material on room‐temperature substrates. Thus, during the deposition of insultating ZnO, turning on this second discharge causes the deposition to ’’switch’’ from low conductivity to high conductivity material. This is of particular interest in the fabrication of semiconductor‐insulator‐semiconductor solar cells where precise control over the thickness of the insulating layer is necessary and where a highly transparent and conductive window‐junction layer is required.

Preparation of conducting and transparent thin films of tin‐doped indium oxide by magnetron sputtering

High‐quality 800‐A‐thick films of tin‐doped indium oxide have been prepared by magnetron sputtering. It is shown that films with low resistivity (∼4×10−4 Ω cm) and high optical transmission (≳85% between 4000 and 8000 A) can be prepared on low‐temperature (40–180 °C) substrates with O2 partial pressures of (2–7)×10−5 Torr.

Absorption edge shift in ZnO thin films at high carrier densities

The optical absorption edge has been measured as a function of carrier concentration for thin films of zinc oxide prepared by organometallic chemical vapour deposition and reactive R.F. magnetron sputtering. Large shifts of the absorption edge have been observed which are only a function of the carrier concentration. Below n = 3 × 1019 cm-3 the shifts are well described by the Burstein-Moss model. For carrier concentrations between 3–5 ×1019cm-3, the shift decreases very rapidly, finally increasing again with further increases in the carrier density. These effects are consistent with a merging of the donor band with the conduction band following a semiconductor-metal transition.

Properties of carbon-doped GaN

The properties of carbon-doped GaN epilayers grown by molecular-beam epitaxy have been studied by temperature-dependent resistivity, Hall-effect measurements, x-ray diffraction, and by photoluminescence spectroscopy. Carbon doping was found to render the GaN layers highly resistive (>108 Ω cm) and quench the band edge excitonic emissions. Yellow luminescence is still present in carbon-doped GaN layers. The highly resistive state is interpreted as being caused by direct compensation by the carbon acceptors and by the consequently enhanced potential barrier at the subgrain boundaries. Evidence of dislocations joining to form potential barriers along the subgrain boundaries was observed in photoassisted wet etching experiments on electrically conducting GaN layers. GaN films grown on insulating carbon-doped base layers are of excellent transport and optical properties.

Growth of high mobility GaN by ammonia-molecular beam epitaxy

The growth of high electron mobility GaN on (0001) sapphire by ammonia molecular beam epitaxy is reported. A buffer layer of AlN <300 A is initially deposited by magnetron sputter epitaxy, a technique where the aluminum source is a planar dc magnetron sputter cathode and ammonia is used for the nitrogen source. The GaN epilayer is deposited using a conventional K cell for the gallium source and ammonia for the nitrogen source. The layers were doped using silane. Measured room temperature electron mobilities of 560 cm2/V s were observed for layers with carrier densities of ∼1.5×1017 cm−3. The 4 K photoluminescence spectrum showed a very strong donor bound exciton at 3.48 eV with a full width at half maximum (FWHM) of 4.9 meV. X-ray diffraction studies showed the layers to have good crystallinity with FWHM of the ω–2θ and ω scans as low as 13.7 and 210 arcsec, respectively.

The influence of target oxidation and growth-related effects on the electrical properties of reactively sputtered films of tin-doped indium oxide☆

Abstract The electrical properties of tin-doped indium oxide films prepared by reactive r.f. magnetron sputtering were studied. The properties vary markedly with the added oxygen pressure and also with the film thickness owing to growth effects and target oxidation. The growth effects were analysed in terms of the grain size effects and of the target oxidation by a simple kinetic model. We show that films exhibiting a wide range of electrical properties can be prepared if the deposition conditions are carefully controlled. Films with conductivities as high as 2.5 × 10 3 ohms −1 cm −1 and with optical transmission greater than 85% averaged over the visible spectrum were deposited.

AlGaN/GaN Metal–Oxide–Semiconductor High-Electron Mobility Transistors Using Oxide Insulator Grown by Photoelectrochemical Oxidation Method

A photoelectrochemical oxidation method was used to directly grow oxide layer on AlGaN surface. The annealed oxide layer exhibited beta-Ga₂O₃ and alpha-Al₂O₃ crystalline phases. Using a photoassisted capacitance-voltage method, a low average interface-state density of 5.1 times 10¹¹ cm^-2. eV^-1 was estimated. The directly grown oxide layer was used as gate insulator for AlGaN/GaN MOS high-electron mobility transistors (MOS-HEMTs). The threshold voltage of MOS-HEMT devices is -5 V. The gate leakage currents are 50 and 2 pA at forward gate bias of VGS = 10 V and reverse gate bias of V_GS = -10 V, respectively. The maximum value of g_m is 50 mS/mm of VGs biased at -2.09 V.

Conductivity imaging of the erosion pattern for ZnO prepared by planar R.F. magnetron sputtering

Abstract Changes in the electrical conductivity of films of ZnO prepared by reactive r.f. magnetron sputtering were measured with respect to the relative substrate position in a fixed plane parallel to the target surface. The observed spatial dependence of the film conductivity is directly correlated with the target erosion pattern. Regions of high erosion on the target correspond to low conductivity in the same relative regions on the substrate. It is shown that magnetron systems utilizing a moving substrate to obtain high thickness uniformity may not achieve optimal film properties.

Indium tin oxide/Cu2O photovoltaic cells

Abstract Tin-doped (about 3%) indium oxide films 80 nm thick were deposited by magnetron sputtering and were found to form a barrier on Cu2O crystals of comparable or superior quality to that of copper. The indium tin oxide (ITO) layers have good optical transmissions (more than 85%) and high electrical conductivities (approximately 3×10 3 Ω -1 cm -1 ). ITO Cu 2 O front cells were studied for their photovoltaic properties in view of their possible applications as solar cells. Preliminary studies give 1.0–1.25 eV for the barrier height and indicate that short-circuit current densities of several milliamperes per square centimetre (under air mass 1 illumination) can easily be attained.

Heteroepitaxy of InSb on silicon by metalorganic magnetron sputtering

Heteroepitaxial films of InSb have been deposited by metalorganic magnetron sputtering (MOMS) on 〈100〉 silicon substrates using molecular beam epitaxy (MBE) GaAs buffer layers. X‐ray diffraction and cross‐sectional transmission electron microscopy measurements indicate the epilayers to be structurally similar to layers deposited by MOMS and MBE on high quality GaAs substrates, despite the increased defect density of the GaAs buffer layer. Some of the defects within the buffer layer propagate into the InSb epilayer; however, the majority of defects arise from the lattice mismatch at the interfacial region.