High Quality P-Type GaN Films Grown by Plasma-Assisted Molecular Beam Epitaxy
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ang et al. 9 also reported on p-type GaN grown by MBE using ammonia and nitrogen gases. In their work, to circumvent the effect of hydrogen passivation on the Mg-doped GaN film, a nitrogen plasma produced by ECR was introduced during the growth in addition to ammonia. 2 Their room-temperature hole mobility and hole concentration were 10 c f/V's and 4x10' 7 cm-3 , respectively. In spite of the fact that the p-conductivity of their MOCVD-grown nitride films was not high, Nakamura et al.'0 recently demonstrated an InGaN-based multi-quantum well laser diode operating at 417 nm under pulsed conditions at room temperature. A high turn-on voltage (34 V) of the laser, however, caused high power dissipation in the device structure. With high-quality ptype GaN/AIGaN films having higher hole concentrations, one should be able to reduce the threshold voltage and power dissipation in these lasers. Using an MBE system equipped with a low-contamination, high power efficiency, inductively coupled radio frequency (RF) plasma source developed in our laboratory, we recently grew high-quality p-type GaN films without post-growth treatment. Using Mg as the p-dopant, the hole concentrations achieved were as high as 1.4x10 26 cm-3 and the resistivities as low as 10. K.cm. To confirm the quality of the Mg-doped samples, double-crystal X-ray and lowtemperature photoluminescence measurements were performed. To examine the Mg-distributions in the films, SIMS was employed. To the best of our knowledge, the p-conductivity reported in this work is the highest for the GaN-based material systems. In addition, our results indicate that for the growth of nitride films, the RF plasma source is as good as the ECR plasma source. EXPERIMENTAL Figure 1 shows a schematic of the growth system with a plasma source. The flow of
RHEED Screen
\1.'
Turbo Molecular Pump
Fig. 1. MBE growth system equipped with a radio frequency plasma source.
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ultrahigh-purity nitrogen was controlled and monitored by a mass flow controller. An inductively coupled RF plasma source developed at the University of Illinois was used to produce the active atomic nitrogen needed for the formation of nitrides. In this source we employed high-purity refractory materials and water cooling of various parts to avoid contamination, and the power transmission to the plasma was optimized. We found that overall power efficiency of this source could be very low (10-15%) and that it depended strongly on the pressure inside the plasma tube, the design of the RF coil, and the input power. The detailed analysis of the power efficiency of our plasma source will be published elsewhere'. c-plane sapphire substrates were used, which were cleaned by the conventional organic solvents (trichloroethane, acetone, and methanol), followed by etching in 3:1 solution of H 2S0 4 and H 3P0 4 at 170 'C. These substrates were mounted on Mo blocks and then loaded into the growth chamber equipped with a reflection high energy electron diffraction (RHEED) assembly. In the growth chamber, the substrates were sub
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