• Document: In-Situ Characterization During MOVPE Growth of III-Nitrides using Reflectrometry
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18 Annual Report 1999, Dept. of Optoelectronics, University of Ulm In-Situ Characterization During MOVPE Growth of III-Nitrides using Reflectrometry Christoph Kirchner and Matthias Seyboth The suitability of in-situ reflectrometry during low pressure Metal Organic Vapor Phase Epitaxy (MOVPE) growth of GaN was examined using a commercial fiber reflectometer. Reflectrometry was succesfully used for determining deposition rates in a horizontal MOVPE reactor during nitride growth. The initial growth stages such as deposition of low temperature nucleation layer and the initial nucleation of crystalline GaN can be observed in the obtained reflectance spectra as changes in reflectance. 1. Introduction Nitride based materials comprise today’s fastest developing III-V compound semiconductor (InAlGaN) technology. Excellent optical and electrical properties, a wide and direct bandgap in combination with high thermal, mechanical, and chemical robustness make GaN and its alloys a well suited material system for optoelectronic devices in the UV to visible range (e.g. light emitting diodes (LEDs), laser, photodetectors) [1], [2], [3]. Successfull epitaxial growth of such multilayered device structures requires precise control of the growth parameters (temperatures, flows, pressures) to achieve reproducible results. In par- ticular, the heteroepitaxial GaN growth on highly mismatched substrates requires a two-step growth process consisting of low temperature nucleation layer and annealing step prior to the device growth to achieve high quality epitaxial GaN layers. Deposition and subsequent an- nealing of the nucleation layer is a critical, highly sensitive process. Reproducibility remains a problem due to the fact that small variations of substrate temperature and slightly different sap- phire morphologies strongly influence properties of the nucleation layer and subsequent GaN growth. In-situ characterization methods would be very helpful to control the initial growth stages of GaN. Due to the lack of high vacuum conditions, RHEED (reflection high electron energy deflection) systems, as widely used in MBE to control two-dimensional growth, growth rates and composition of ternary layers, cannot be applied in MOVPE. However, in-situ reflec- trometry can provide a similar access to the growth process during gas phase epitaxy. In this article we describe the use of in-situ reflectrometry during MOVPE GaN epitaxy to control the growth parameters. In-Situ Characterization During MOVPE Growth of III-Nitrides 19 2. Experimental Epitaxial growth of GaN is conducted in a horizontal, radio-frequency heated, water cooled quartz MOVPE reactor (AIXTRON AIX 200 RF) operated at low pressure. Trimethylgal- lium (TMGa), Trimethylindium (TMIn), Trimethylaluminum (TMAl) and ammonia are used as group III and group V precursors, respectively. The mostly used substrate material is sap- phire (Al O ). The MOVPE system was equipped with a commercially available reflectometer  system consisting of a white light source and a CCD spectrometer (Filmetrics F 30). The spec- trometer is a 512-element photodiode array with a spectral range of 400 nm - 1100 nm and a resolution of 2 nm. The spectrometer is controlled by a computer and the spectrometer soft- ware allows calculation of deposition rates, the refractive index n, the extinction coefficient k and reflectivity from the measured data. For this purpose, material data libraries are contained in the software. Reflectance spectra can be displayed versus time or wavelength. The setup allows simultaneous measurements of reflectivity at three different wavelengths. Fig. 1 shows the complete setup. To enable optical measurements on the sample surface, an optical access to the substrate with the growing nitride layer in the MOVPE reactor is mandatory. The reac- tion chamber of the employed MOVPE system is a liner tube made of quartz glass. Due to the horizontal configuration of the reactor, the ceiling of the liner gets coated with Nitride deposits during growth, making it intransparent for visible light. Therefore, a hole with a diameter of 5 mm was drilled in the liner ceiling. The liner is located inside a quartz cylinder (outer reactor tube), which is surrounded by the water cooling jacket made of quartz, too. Both the incident and reflected light has to pass all the quartz walls and the cooling water. Disturbing reflec- tions from the quartz walls can be eliminated by reference measurements. The spectrometer and the light source are connected to the measuring head (lens system) by optical fibers. With the lens system, the light is focused on the sample and the reflected light is coupled into the spectrometer. The lens system is designed for normal incidence, the fiber is of coaxial type as depicted in Fig. 1 (upper left insert). The reflectance of the sample surface, recorded during the growth process, is continuously monitored. Substrate material for all in-situ controlled

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