Growth of III-Nitrides by RF-Assisted Molecular Beam Epitaxy
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high power rectifiers. EXPERIMENT Samples were grown in an MBE chamber custom built for nitride growth by SVT Associates. An RF plasma source was used for active nitrogen operating at 350W. Nitrogen was supplied to the source through a precision leak valve to a chamber background pressure of 3-8. 10- torr. Typical growth rates and total film thickness were 0.4 micron/hr and 1 micron, respectively. Sapphire (0001) substrates were back-side coated with either molybdenum or titanium to aid in radiative heating, and loaded into the vacuum system without chemical treatment. The substrates were heat cleaned prior to growth at 1000°C for 15 minutes after which they were exposed to the nitrogen radical beam until a bright streaky reflection high energy electron diffraction (RHEED) pattern emerged indicating the formation of a thin AIN layer at the surface. A 25 nm A1N buffer layer was then deposited at a substrate temperature of 600-800 'C. GaN, AIN, and AlGal-x.N alloy layers were grown at a fixed substrate temperature of 800 'C under slightly metal-rich conditions. Some films were doped with Si from a high temperature effusion cell. To calibrate the carrier concentration as a function of Si doping, a series of uniformly doped films were grown with varying Si cell temperature. Films were characterized ex situ by atomic force microscopy (AFM) using a multi-mode Digital Instruments Nanoscope IIIA, and by high resolution x-ray diffraction 387 Mat. Res. Soc. Symp. Proc. Vol. 512 0 1998 Materials Research Society
Figure 1: (3 x 3) and (6 x 6) RHEED reconstructions are observed at low temperature (< 400 °C) indicating that the GaN films are of (0001) polarity[3]. Only unreconstructed (1 x 1) surfaces are observed during growth.
(XRD) using a Philips diffractometer equiped with 4-crystal Ge (440) collimation and 2 crystal Bonse-Hart triple axis detector. Layer thickness was confirmed by variable angle spectroscopic ellipsometery (VASE) over the wavelength range 380-780 nm, and carrier concentrations were measured by the Van der Pauw Hall technique. Schottky diode m-v-n+ devices were fabricated by sputtering Au contacts onto a sample consisting of a 1.2 micron thick undoped GaN layer grown atop a one micron thick n+ layer doped to n - 5. 101 cm- . The current-voltage characteristics of these diodes were measured using a Hewlett Packard 4156A Precision Semiconductor Parameter Analyzer with samples mounted in a probe station. RESULTS AND DISCUSSION During nucleation of the AIN buffer layer, the RHEED showed a transmission pattern, indicating cubic phase[4] and three-dimensional growth. The cubic pattern was visible during the first - 5 seconds of growth for most films, but was not observed for some films for which the substrate was most heavily nitrided. Upon completion of the AIN buffer layer, RHEED was streaky with a bright (1 x 1) or faint 2-fold reconstruction under nitrogen soak. Upon opening of the Ga shutter, the film morphology strongly depended on III/V flux ratio. For metal-rich nucleation conditions, the RHEED quickly
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