AIN/GaN and AIGaN/GaN Heterostructures Grown by HVPE on SiC Substrates
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ABSTRACT GaN, AIN and AlGaN layers were grown by hydride vapor phase epitaxy. 6H-SiC wafers were used as substrates. Properties of A1N/GaN and AlGaN/GaN structures were investigated. AlGaN growth rate was about 1 pim/min. The thickness of the AlGaN layers ranged from 0.5 to 5 jim. The AIN concentration in AlGaN layers was varied from 9 to 67 mol. %. Samples were characterised by electron beam micro analysis, Auger electron spectroscopy, X-ray diffraction and cathodoluminescence. Electrical measurements performed on AlGaN/GaN/SiC samples indicated that undoped AlGaN layers are conducting at least up to 50 mol. % of AIN. INTRODUCTION AIGaN layers and AlGaN/GaN structures are important elements of device structures based on III-V nitride semiconductors. AlGaN alloy covers the whole range of composition from GaN having energy gap (Eg) of 3.4 eV to AN with Eg value of 6.2 eV. In III-V nitride epitaxial structures, AlGaN layers are being employed as (1) buffer layers between substrates and device structures, (2) cladding layers for light emitters and (3) carrier wide band gap emitters. Currently the main technological method to grow AlGaN layers is metal organic chemical vapor deposition. Hydride vapor phase epitaxy (HVPE) is a well-established technique used to grow thick layers of GaN [1-4] because it provides a high growth rate and is relatively cheap. Applications of this method to III-V nitrides are not limited by growth of only GaN. In 1978, Hagen and coworkers reported on the first AlGaN layers, grown by HVPE [5]. In the last 20 years only a few reports on HVPE AN and AlGaN alloys have been presented [5-9]. Throughout these reports the following essential points must be noted: (1) growth was done on sapphire, (2) the quality of the grown material was not good. Recently we reported on GaN, AN and AlGaN growth by HVPE [8,10,11]. In this paper we report our recent results on AIN/GaN and AlGaN/GaN heterostructures grown by HVPE on SiC substrates. EXPERIMENT The HVPE deposition technique for GaN material has been described elsewhere [10]. Silicon carbide wafers (35 mm in diameter) were used as substrates. Nitride structures were deposited on 245 Mat. Res. Soc. Symp. Proc. Vol. 482 © 1998 Materials Research Society
on-axis (0001)Si face of 6H-SiC wafers. In order to deposit nitride compound containing aluminum, separate channel with Al metallic source was added to the reactor. The Al metal reacted with hydrogen chloride to form AlC13 . In the growth zone, Al chloride reacts with ammonia forming AIN layer, or Al and Ga chlorides react with ammonia forming AlGaN layer. The growth temperature was kept between 950 and 1 100*C. The AIN growth rate ranged from 0.1 to 0.3 jiam/min. The AlGaN growth rate was about 1 pm/min. RESULTS AIN films on SiC AIN layers were grown directly on SiC substrates. The relatively smaller lattice mismatch between AIN and SiC (-1%) have resulted in a high-quality of AIN even at a small thickness of AIN layer. The best value of the full width at a half maximum (FWHM) of X-ray 0,-scan (0002) refle
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