Radical Source Molecular Beam Epitaxy Research Articles

シリコン基板を用いた単結晶酸化亜鉛薄膜の分子線エピタキシャル成長

It has been difficult to obtain single crystalline ZnO films on Si substrates due to the oxidation of Si surface during initial ZnO growth. In this letter, we report the growth of single crystalline ZnO films on Si (111) substrates using a buffer layer of CaF2. The films were grown by radical source molecular beam epitaxy, and revealed by x-ray diffraction to be in the c-axis orientation without in-plane rotational domains. An ultraviolet photoluminescence corresponding to the bandgap energy was dominant for the films even at room temperature. It is indicated by the results that the use of CaF2 buffer layer is promising for the growth of high-quality ZnO (0001) films on Si (111) substrates.

ZnO and ZnMgO Growth by Molecular Beam Epitaxy

ABSTRACTCharacteristics of the ZnO and ZnMgO films grown by radical-source molecular beam epitaxy are reported. The ZnO films on a-plane sapphire substrates had a superior quality in crystallographic, optical and electrical properties, and n-type doping with Al was successfully performed up to 1020 cm–3. The Mg-content of ZnMgO alloys was found to be controlled by a simple growth mechanism as a function of Mg-cell temperature. The alloying in the ZnO-rich region resulted in single-crystalline growth although the photoluminescence characteristics at x = 0.22 suggested the presence of microscopic phase separation. Single-crystalline growth was also achieved on Si (111) substrates by using a CaF2 buffer layer to protect the Si-surface from oxidation.

Interactions between gallium and nitrogen dopants in ZnO films grown by radical-source molecular-beam epitaxy

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.

Growth of N-doped and Ga+N-codoped ZnO films by radical source molecular beam epitaxy

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 2O 4 in ZnO films.

Growth and characterization of undoped ZnO films for single crystal based device use by radical source molecular beam epitaxy (RS-MBE)

The integrated use of (1 1 2 ̄ 0) a-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 7.6×10 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.

Growth of Undoped ZnO Films with Improved Electrical Properties by Radical Source Molecular Beam Epitaxy

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.

Control of Optical and Electrical Properties of ZnO Films for Photovoltaic Applications

ABSTRACTZnO 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.

Observation of Interdiffusion in ZnO/CuInSe2 Heterostructures and its Effect on Film Properties

ABSTRACTZnO 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.

Nitrogen-induced defects in ZnO : N grown on sapphire substrate by gas source MBE

Nitrogen-doped ZnO layers were grown on sapphire substrates by radical source molecular beam epitaxy by simultaneously introducing O 2 and N 2 via a RF radical source. Reflection high-energy electron diffraction and X-ray diffraction measurements revealed that high N 2/O 2 flow ratios induced growth twins into the ZnO layer. A nitrogen-doped ZnO fabricated using a N 2/O 2 flow ratio of 10% was found to have a chemical nitrogen concentration of 1×10 19 cm −3. However, type conversion from n-type to p-type did not occur while large nitrogen incorporation was observed to induce extended defects.