The promising II-VI-semiconductor ZnO has achieved strong interest in research in the past years. Especially, epitaxial
growth by metal organic vapor phase epitaxy (MOVPE) is a matter of particular interest due to the large scalability of
MOVPE for commercial mass production and its proven high layer quality for other compound semiconductors. In the
past years tremendous advance has been made in the field of epitaxial growth. However, due to the lack of epiready ZnO
substrates, so far mostly heteroepitaxial growth with a multistep growth process was applied to obtain good surface
morphology and until now not all of the physical properties of such multilayers are fully understood. In this paper we
present recent results of the electrical behavior of such multiple undoped ZnO layers. Despite numerous efforts one big
challenge is the p-type doping of ZnO. Here we present our results to doping experiments with arsenic, nitrogen and as a
new approach simultaneous dual doping of nitrogen and arsenic. Homoepitaxial growth offers a great potential for ZnO
due to some advantages as the absence of thermal and lattice mismatch and potentially low dislocation density. We
present experiments on the thermal treatment of commercial ZnO bulk crystals, which is necessary for subsequent homo-
MOVPE.
We report on the hetero- and homoepitaxial growth of ZnO thin films by the chemical vapor deposition technique. We
compare the results obtained on sapphire substrates, on GaN-templates on sapphire substrates and on silicon (111)
substrates. Even under optimized growth conditions with the insertion of buffer layers the films tend to grow 3-dimensionally. However, also ZnO substrates, expected to be the best choice, need to be prepared before being used in
the epitaxial growth. After mechanical polishing of the ZnO substrates we employed a high temperature annealing step
which produced atomically flat surfaces and removed all of the surface and subsurface damage. Thereafter, the two
dimensional epitaxial growth was achieved without an additional buffer layer. The substrate had a rocking curve full
width at half maximum of 27" which can be compared with that of the film of 22". The films had superior band edge
luminescence as compared to the substrate for which the green luminescence band was dominating.
We present first results on the limits of GaN growth on large diameter sapphire and the challenges that have to be solved for a successful growth of high power LEDs on silicon substrates. Up to 5.4 μm thick crack-free GaN on Si(111) LED structures were grown by metalorganic chemical vapor phase epitaxy. The FWHM of the GaN (0002) ω scan in x-ray diffraction amounts to 380 arcsec. On Si substrates, we achieve low curvatures with radii > 50 m, which is important for a successful processing of the samples on large diameter substrates. Additionally, a low curvature during InGaN multi-quantum-well growth is achieved and enables the growth of homogenous InGaN layers. The main difficulty for GaN-on-Si is light extraction, which leads to an approximately three- to four-fold reduction in direct comparison with GaN LEDs on sapphire.
We report results on the transferability of a blue-green electroluminescence test structure (ELT) process across different reactor geometries and substrate materials. The process was transferred from the conditions of our well-known 6 X 2 inch to the 5 X 3 inch AIX 2400 G3 geometry by simple up-scaling of the respective process parameters in accordance with numerical simulations done on the reactor setup. The five period InGaN/GaN quantum well ELT structures with an average emission wavelength on wafer of 480 nm shows a standard deviation of 1 - 2% without rim exclusion. Electroluminescence up to 560 nm were achieved in InGaN/GaN structures with high In content. With these prospects new types of seed layers for the transfer of our standard electroluminescence test structures (ELT) process to Si- substrates were investigated. The growth on different seed layers was found feasible and resulted in operational ELT structures with emission wavelengths in the range of 440 nm to 470 nm. Electrical quick test shows bright blue emission across the full Si wafer.
Two different versions of hotwall--epitaxy reactors were used to grow ZnSe and ZnTe layers. In the first type of hotwall epitaxy reactor Zn and Se were evaporated seperately from elemental sources. By changing the Zn/Se ratio in the gas phase we could alter the growth direction of the ZnSe layers from 111 to 100 Raman spectroscopy proved the existence of a Ga2Se3 layer at the GaAs/ZnSe interface. In the other HWE reactor stoichiometric ZnSe and ZnTe were used as source materials. Photoluminescence and Xray rocking curves proved the high quality of the epilayers. The rocking curves showed a full width at half maximum of 75 arcseconds for ZnTe.
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