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Phosphorus-doped ZnO films were grown by radical source molecular beam epitaxy (RS-MBE). Zn 3 P 2 was used for the phosphorus source. ZnO:P layers were grown on an in situ annealed low temperature buffer layers, and the phosphorus concentration was changed by altering the temperature of the Zn 3 P 2 K-cell. The phosphorus concentration could be controlled from 1 x 10 18 to 2 x 10 20 cm -3 without phase separation. The importance of the ambient gas used during thermal treatments was demonstrated. All of the ZnO:P layers showed n-type conduction, however, the electron concentration of ZnO:P was found to decrease by as much as from 4.1 × 10 19 to 9.7 x 10 16 cm -3 when the samples underwent oxygen ambient rapid thermal annealing (RTA) treatment. Furthermore, a new photoluminescence peak at 3.357eV was observed after RTA treatment, which was not present in undoped ZnO layers before and after RTA treatment. These results indicate that the activation of acceptors in ZnO:P occurred in samples that underwent RTA treatment and that phosphorus is a good candidate as an acceptor dopant.

We have investigated the effect of in-situ annealing and nitrogen doping on 40 nm low-temperature buffer layers in radical-source molecular beam epitaxy (RS-MBE) as well as subsequently grown high-temperature ZnO layers grown on the buffers. All layers were grown on (11&2macr;0) sapphire substrates. High resolution x-ray reciprocal space maps in the vicinity of the ZnO (0002) reciprocal lattice point (RLP) showed a sharp reduction in diffuse scattering for the thermally annealed buffer layer implying a reduction in dislocation density. X-ray lateral coherence (LC) values obtained from asymmetric diffraction near the (10&1macr;4) RLP indicated that the LC of a undoped, high-temperature ZnO layer grown on an in-situ N-doped buffer was larger than those of high-temperature ZnO layers grown on either the in-situ annealed or untreated low-temperature buffers. The increased LC was found to be correlated with in-plane twist suggesting that dislocation bunching may be present at domain boundaries in the high-temperature ZnO grown on annealed N-doped buffer layers. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

It has been theoretically predicted that the codoping of an acceptor (nitrogen) and a donor (aluminium, gallium, indium) in a 2 : 1 ratio will enable the fabrication of low resistivity p-type ZnO due to reduction of the Madelung energy making the nitrogen acceptor energy level shallower. We have been growing N-doped and Ga+N codoped ZnO films by radical-source molecular beam epitaxy. Nitrogen and oxygen radicals were separately supplied via radio-frequency radical cells. Nitrogen did not incorporate in ZnO films at the high substrate temperature of 600°C. Lower substrate temperatures were necessary for nitrogen incorporation. From the fact that the nitrogen concentration also depended on the Zn fluxes, it is concluded that Zn-rich conditions are indispensable for nitrogen doping. The presence of Ga was observed to enhance nitrogen incorporation, one of the predicted effects of codoping. However, high Ga concentrations were found to lead to formation of the additional phase of ZnGa 2 O 4 in ZnO films.

Near-edge X-ray absorption (XANES) at the Cu K-edge was employed to study the valency of Cu (a potential p-dopant) in Cu-doped ZnO grown by molecular beam epitaxy. For a similar chemical environment, a shift in the onset of absorption can be interpreted as being due to a change in effective valency. We have studied this shift for both as-grown and argon ambient annealed (1000°C, 30 min) Cu-doped ZnO samples. The valency shift was measured against ab-initio standards calculated using feff8 as well as the standard samples Cu 2 O (+1 valence) and CuO (+2 valence). It was found for the flux region explored here that as-incorporated Cu assumes an effective valence of approximately +1 which increases towards +2 upon a 1000 °C argon ambient anneal. X-ray diffraction also shows the presence of both metallic Cu and Cu 2 O depending upon growth and annealing conditions.

The optical and structural properties of textured ZnCdO films that exibit a newly found distinct blue luminescence have been investigated. Microscopic compositional fluctuations of Cd, associated with hexagonal grain structures, were observed. Models for the blue-luminescence process are proposed.

The integrated use of (1120) α-plane sapphire substrates and high temperature growth with low temperature buffer layers have led to high quality undoped ZnO epitaxial films with mobilities as high as 120 cm 2 V -1 s -1 and residual carrier concentrations as low as 7.6 x 10 16 cm -3 . Pole figure measurements reveal that a-plane sapphire substrates are effective for the elimination of 30 rotation domains, which usually appear using c-plane sapphire substrates. In particular, when using c-plane sapphire substrates annealing at 1000°C in O 2 with c/2-height surface steps, the X-ray diffraction pole figure peak intensity related to these rotation domains increased. The use of low temperature buffer layers allow high temperature ZnO growth on sapphire, as initial ZnO growth does not occur at high initial growth temperature.

It has been recently demonstrated that it is possible to eliminate 30° rotation twins in c-oriented epitaxial ZnO films by growing on the (1 1 2 0) face of sapphire despite the apparent symmetry mismatch between the (6-fold) epilayer and the (2-fold) substrate. To further elucidate the details of the growth process, we have investigated the initial stages of growth of ZnO on (1 1 2 0) sapphire using molecular beam epitaxy. Films of approximately 3, 5, 8, 10, 20, 100, and 600 nm thickness were investigated by in situ reflection high-energy diffraction, high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and extended X-ray absorption (XAFS). XAFS indicates that there is no discernible change in nearest-neighbor distance with film thickness. On the other hand, HRXRD measurements indicate an ∼1% increase in the c-lattice constant with decreasing film thickness. XAFS measurements of the second nearest-neighbor structural disorder were observed to increase sharply for the thinnest films implying that distortion of bond angles is responsible for the increase in the c-axis observed by HRXRD. HRXRD reciprocal area maps of the symmetric ZnO (0 0 0 2) reflection indicate the onset of diffuse scattering from 20 nm, while AFM indicates a concurrent change in surface morphology suggesting a change in the growth process for t>∼20 nm .

It has been recently predicted that the co-doping of an acceptor (nitrogen) and a donor (aluminum, gallium, indium) in a 2:1 ratio will dope ZnO p-type due to a reduction in the Madelung energy making the nitrogen acceptor energy level more shallow. We have been growing gallium and nitrogen co-doped ZnO films by radical-source molecular-beam epitaxy by use of oxygen and nitrogen radicals supplied via rf radical source cells. Diode-like current–voltage characteristics and donor acceptor pair-like photoluminescence emission were observed for a Ga and N doped ZnO film grown on an undoped ZnO buffer layer. However, Hall measurements revealed that the conductivity was n-type. Formation of a non-ZnO phase in the sample was confirmed by secondary ion mass spectroscopy and x-ray diffraction measurements. Zn and Zn+O secondary ion intensities fell sharply by two orders of magnitude in going from the undoped ZnO layer to the highly co-doped ZnO. X-ray diffraction measurements indicated the formation of ZnGa2O4.

Thin films of ZnO and Mg x Zn 1-x O were epitaxially grown on Zn-polar ZnO substrates by plasma assisted molecular beam epitaxy. The miscut of c-plane ZnO substrates toward the [1-100] axis direction leads to a flat substrate surface with straight step edges. The growth mode of epitaxial ZnO films significantly depended on the growth temperature, and a substrate temperature over 800°C was needed for flat film surfaces with monolayer-height steps. Photoluminescence (PL) peak originating from the n = 2 state of A-free excitons was observed at 12 K for the ZnO films grown under stoichiometric and O-rich growth conditions. Mg x Zn 1-x O films were also fabricated under Zn-rich conditions. The film surface exhibited a step-and-terrace structure. The effective PL lifetime of Mg 0.08 Zn 0.92 O film was as long as 1.9 ns, which is the highest value ever reported, presumably due to a high purity level of the film.

We have grown nitrogen-doped MgxZn1-xO:N films on Zn-polar ZnO single crystal substrates by molecular beam epitaxy. As N-sources, we employed NO-plasma or NH3 gas itself. As x increased, optimum growth temperature window for smooth film morphology shifted to higher temperatures, while maintaining high N-concentration (similar to 1x1019 cm-3). The heterosructures of MgxZn1-xO:N (0.1 < x < 0.4)/ZnO were fabricated into light emitting diodes of 500-mu m-diameter. We observed ultraviolet near-band-edge emission (lambda similar to 382 nm) with an output power of 0.1 mu W for a NO-plasma-doped LED and 70 mu W for a NH3-doped one at a bias current of 30 mA.

The excellent structural and optical properties of pseudomorphic MgxZn1-xO films (0

Nominally undoped MgxZn1-xO/ZnO (x = 0.05 and 0.08) single heterostructures were prepared on Zn-polar ZnO substrates by using plasma assisted molecular beam epitaxy (MBE). The samples showed a metallic conductivity below 50 K and a mobility exceeding 104 cm2 V-1 s-1 at 0.5 K. We observed quantum Hall effect accompanying Shubnikov-de Haas oscillations, in which zero-resistance states were clearly seen above 5T. Rotation experiments in magnetic field suggest strong two-dimensional carrier confinement at low temperatures. The results indicate that the MBE grown films have much higher quality than the previously reported samples grown by pulsed laser deposition.

The equivalent internal quantum efficiency (ηinteq) at 300K of the near-band-edge excitonic photoluminescence (PL) peak in ZnO epilayers grown by plasma-assisted molecular beam epitaxy on Zn-polar ZnO substrates was directly correlated with the PL lifetime (τPL) for the first time. This relation seems to be universal for O-polar ZnO films grown by other methods. Present homoepitaxial ZnO epilayers grown above 800°C exhibited atomically flat surfaces, and the best full-width-at-half-maximum value of (0002) ZnO x-ray diffraction ω-rocking curves was 17.6arcsec. The high-temperature growth also led to a long τPL of 1.2ns at 300K. As a result, a record high ηinteq value (9.6%) was eventually obtained under an excitation density of 5W∕cm2 (He–Cd, 325.0nm). The homoepitaxial Zn-polar ZnO films grown by molecular beam epitaxy are coming to be used for p-n junction devices.

High-quality undoped ZnO epitaxial films with mobilities as high as 120 cm2V-1s-1 and carrier concentrations as low as 7.6 ×1016 cm-3 have been grown on (1120) a-sapphire substrates using low-temperature buffer layers, a slow substrate cooling process and a modified oxygen radical cell. Pole figure measurements reveal that a-plane sapphire substrates are effective for the elimination of 30° rotation domains, which usually appear in the case of ZnO growth on c-sapphire. The low-temperature buffer layers allow high-temperature growth, because initial ZnO growth does not occur with high initial growth temperature. The use of slow substrate cooling prevents the deterioration of the electrical properties of the ZnO films. Use of quartz insulators in the oxygen radical cell eliminates aluminum contamination, which is a serious problem when using conventional alumina insulators.

Blue luminescence from textured ZnCdO films grown by molecular beam epitaxy was observed. The properties of ZnCdO films grown at various Zn/Cd partial pressures were studied by several techniques including X-ray diffraction, fluorescence (FL) microscopy and Auger electron spectroscopy. With increasing CdO content, the properties of ZnCdO films changed sharply at specific Zn/Cd partial pressures. Some of the films grown at approximately the threshold pressures showed blue luminescence at room temperature, with large Stokes shift leaving the absorption edge in the UV region. By FL microscopy, it was observed that the blue luminescent regions were nonuniformly distributed, suggesting composition fluctuation.

We have grown the compound semiconductor ZnO 1-x Se x by MBE. A decrease in bandgap energy with increasing Se composition x was observed and a large bowing parameter of 12.7 ± 1.6 eV was measured. This indicates the possibility of bandgap bowing in the ZnO 1-x Se x compound system and may open up new bandgap control techniques that allow bandgap changes to be made with small lattice mismatch.

ZnO films were grown by radical-source molecular beam epitaxy (RS-MBE) on sapphire and glass substrates, and they were characterized in terms of Hall mobility and optical transmission. Undoped ZnO films exhibit a low intrinsic defect density and optical properties close to bulk ZnO. By Ga doping, a resistance ρ as low as 2×10−4 Ωcm could be achieved. Balancing high conductivity and low transmission losses due to free carrier absorption in the infrared, the optimum was obtained for ρ=3.4×10−4Ωcm, electron mobility μe=37 cm2/Vs and an average transmission T of 96% in the wavelength range 400-1100 nm. Polycrystalline growth on glass yields slightly reduced but still good film quality (μe=30 cm2/Vs, T=90%). By the incorporation of Mg, conducting Mg0.3Zn0.7O films with an increased band gap up to ∼ 4eV were realized.

ZnO films were grown directly on epitaxial CuInSe2 (001) (CIS) by radical-source molecular beam epitaxy (RS-MBE). The substrate-film interdiffusion was investigated dependent on the ZnO growth temperature. Secondary Ion Mass Spectroscopy (SIMS) profiles indicate the mutual temperature-activated diffusion of Zn and In at a growth temperature of 440°C which is absent at 250°C. Zn indiffusion into the CIS substrate leads to characteristic changes in the photoluminescence (PL) properties, whereas the In outdiffusion into the growing ZnO film causes an increased carrier concentration.