K. Ogata

Effects of thermal annealing of ZnO layers grown by MBE

Abstract Thermal annealing of ZnO layers was done in N 2 or O 2 atmosphere and their effects were studied. Electron carrier density increases according to reevaporation of O from ZnO if annealed in N 2 atmosphere. On the contrary, it decreases from the order of 10 18 to 10 17 xa0cm −3 , and also optical properties are improved when annealed in O 2 atmosphere at lower temperature. In that case, the number of interstitial Zn (Zn i ) and O vacancies (V O ) decrease, probably because the effective incorporation of O atom diminishes those donor levels. The crystallinity also improves with the annealing. The mobilities of the layer increase up to 51xa0cm 2 /Vxa0s as annealing temperature increases, but are lower at lower temperatures. Solving this problem, annealing may become a promising technique for the fabrication of p-type ZnO layers.

Growth mode control of ZnO toward nanorod structures or high-quality layered structures by metal-organic vapor phase epitaxy

Abstract In metal-organic vapor phase epitaxy (MOVPE), ZnO growth modes either for nanorod structures or for high-quality flat layers have been successfully controlled by varying the growth conditions. The nanorod structures of ZnO, e.g., 10−50xa0nm in diameter and 0.5−1xa0μm in height, were successfully grown on sapphire substrates at lower temperatures (e.g., 500°C). On the other hand, the uniform layers were formed at higher growth temperatures (e.g., 800°C). The best solution, at this stage, for flat epilayers with two-dimensional growth is the homoepitaxial growth on bulk ZnO substrates. The root mean square (RMS) roughness of the surface was minimized as 2xa0nm and the full-width at half-maximum of low-temperature (9xa0K) photoluminescence was as narrow as 0.5xa0meV, indicating the successful formation of high-quality ZnO grown by MOVPE.

Effects of annealing atmosphere and temperature on acceptor activation in ZnSe:N grown by photoassisted MOVPE

Effects of annealing atmosphere and temperature on acceptor activation in nitrogen-doped ZnSe layers grown by photoassisted metalorganic vapor-phase epitaxy (MOVPE) have been investigated. It is shown that as-grown high resistive layers are converted to p-type by thermal annealing in N 2 atmosphere in the temperature range from 500 to 550°C, while they do not if annealed in atmospheres of H 2 , diethylzinc (DEZn), and dimethylselenide (DMSe) in which hydrogen is contained. The net acceptor concentration N A -N D has reached up to 4 X 10 17 cm -3 by the thermal annealing at 500°C, but it is lower when annealed at 550°C. By exposing to H 2 atmosphere at 350°C, N A -N D decreases with the exposure time. However, it almost recovers to the value obtained by the first annealing in N 2 , if the layer is annealed again in N 2 atmosphere at 500°C. These results are discussed based on the hydrogen passivation model, i.e. the dehydrogenation and hydrogenation occur reversibly and result in activation and passivation of nitrogen acceptors, respectively.

ZnO growth using homoepitaxial technique on sapphire and Si substrates by metalorganic vapor phase epitaxy

Abstract Zinc oxide (ZnO) layers were homoepitaxially grown by metalorganic vapor phase epitaxy (MOVPE) on ZnO layers on sapphire as well as on Si. MOVPE-ZnO on ZnO/sapphire pretreated at 1000°C exhibited in photoluminescence at 15xa0K a narrowing of line width of the emission from donor-bound-excitons (D 0 X) to 3xa0meV with an appearance of free-excitons emission. MOVPE-ZnO on ZnO/Si indicated a c -axis oriention, and a strong band-edge emission with a lesser deep level, when ZnO/Si was pretreated at 800°C. These results have demonstrated the high potentials of MOVPE growth technology for the growth of ZnO on sapphire or on Si by appropriate surface treatments towards optical and electrical applications.

ZnO growth on Si substrates by metalorganic vapor phase epitaxy

Diethylzinc(DEZn) and nitrous oxide (N 2O) as a source gas combination in the metalorganic vapor phase epitaxy (MOVPE) of zinc oxide (ZnO) has produced high-quality layers on sapphire, but no growth was confirmed on Si. This problem was overcome by using an underlying layer of ZnO grown directly onto the Si using nitrogen oxide (NO2 )a s a more reactive oxidation source. The main ZnO layer grown in this way on the ZnO/Si pretreated at 8001C possessed a c-axis orientation and exhibited bound exciton (BX) emission as narrow as 3 meV at full-width at half-maximum together with a free exciton (EX) at 9 K. These results demonstrated the high potential of MOVPE technology for the growth of ZnO on Si using appropriate surface treatments for optical and electrical applications. r 2002 Elsevier

ZnO growth toward optical devices by MOVPE using N2O

Wide bandgap semiconductor zinc oxide (ZnO) layers were grown by metalorganic vapor phase epitaxy (MOVPE) using nitrous oxide (N2O). Strong ultraviolet (UV) photoluminescence emissions with 1000 times less deep ones at room temperature were observed from ZnO layers grown on sapphire. Alow temperature (500 C)-grown buffer layer of ZnO was effective to enhance the initial nucleation process and to achieve high quality ZnO layers on it at higher growth temperatures (600–700 C). ZnO layers grown on III–V semiconductor substrates showed dominant UV luminescence in spite of low temperature growth. These results imply the abilities of high quality ZnO growth by MOVPE.

Fabrication of ZnSe-based laser diode structures by photoassisted MOVPE

Abstract Towards realization of optical devices by metalorganic vapor phase epitaxy (MOVPE), laser diode structures were fabricated using a thermal annealing technique for activation of acceptors. Key issues for that lay in reduction of degradation of an n-ZnSSe cladding layer and a ZnCdSe active layer, i.e., they suffer from serious degradation at the annealing temperature where the acceptors are well activated. For the time being, by optimizing the growth and annealing conditions, we observed enhancement of TE-polarized emisisson and spectrum narrowing at 77 K from a laser diode structure under pulse injection above 120 A/cm 2 , which suggests stimulated emission.

The gyromagnetic ratio of the compensating donor centre in nitrogen-doped ZnSxSe1-x

The nature of the so-called deep donor that appears at a depth of about 50 meV when ZnSe is doped with nitrogen remains a matter of controversy. The centre is of considerable importance since it appears to act as a compensating centre which is (at least in part) responsible for the difficulties of obtaining effective acceptor concentrations in excess of 1018 cm-3 . Several models have been suggested, the most favoured of which is that of a selenium vacancy associated with a substitutional nitrogen. However, although very recent total energy calculations support such a model, no conclusive evidence for this identification exists. Electrons trapped by the centre have a gyromagnetic ratio of 1.38, in contrast to the value of 1.11 for electrons bound at the normal, shallow (26 meV) donors in this material. In order to obtain a better understanding of the centre, we have used spin-flip Raman scattering experiments to determine how this g -value depends on composition in the nitrogen-doped ternary alloy ZnSx Se1-x , for x in the range 0 to 0.1. Surprisingly, we find that the g -value remains constant, in contrast to that for the shallow donors, which increases by 0.12 over the same composition range. The results present a significant challenge for theoretical calculation, since any microscopic model has to be reconciled with this invariance.

Effects of thermal annealing on properties of p-type ZnSe grown by MOVPE

Abstract Effects of thermal annealing on the layer quality of p-type ZnSe:N grown by metalorganic vapor phase epitaxy (MOVPE) were investigated. Although the thermal annealing at 500 °C is a key to obtain high N A - N D , it causes degradation of the layer quality in the same time. Annealing temperatures should be carefully chosen so that activation of acceptors effectively occurs while degradation of quality can be suppressed. A slightly lower annealing temperature (~ 400–450 °C) seems to offer the possibility of solving the above contradiction.

Characterization of p-type ZnSe grown by MOVPE; excitonic emission lifetime measurements

Time-resolved photoluminescence (TRPL) measurements are made on p-type nitrogen-doped ZnSe grown by photoassisted metalorganic vapor phase epitaxy (MOVPE) together with post-growth thermal annealing, in order to investigate optical quality of the layers. It is suggested that the annealing degrades the layer quality and the MOVPE samples have more non-radiative recombination centers compared with MBE samples. A key issue for high quality p-ZnSe by MOVPE seems to be optimization of annealing conditions.