AlGaN/GaN/AlGaN double-heterojunction blue LEDs on 6H-SiC substrates
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Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
approximately 50 ýtm in diameter. Al and Au were used as Ohmic contacts for measuring n- and p-type specific contact resistance, respectively. Secondary ion mass spectroscopy (SIMS) was used to determine atomic concentration of Mg in GaN epilayers. Conventional vertical geometry was utilized in LED fabrication. Ni and Au were used as Ohmic contacts for backside contact to SiC and for top side contact to Mg-doped GaN layer, respectively. The characteristics of LEDs were measured at room temperature and under DC bias. RESULTS AND DISCUSSION Characterization of GaN Epilayers GaN epilayers grown on 6H-SiC substrates were controllably doped in the carrier concentration ranges of 10 15-1019 cm- 3 for n-type, and 10 15-6x 1017 cm- 3 for p-type, respectively. Dislocation density as low as 108 cm- 2 have been obtained as verified by plan-view transmission electron microscopy [2]. Al and Au were used as n- and p-type Ohmic contact, respectively, to study specific contact resistance. Arrays of transfer length measurement (TLM) structures were formed on electrically isolated n- and p-type GaN epilayers. The specific contact resistances were extracted from the resistance vs. contact spacing data. The best specific 2 contact resistances measured so far were 2x 10-5 Q-cm for Ohmic contact on n-GaN and 3x 10-2 Ž-cm 2 for Ohmic contact on p-GaN, respectively. Figure 1 shows typical I-V characteristics of Ohmic contacts on n- (Figure Ia) and p-GaN (Figure l b) epilayers.
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Figure 1. Current-voltage characteristics of (a) Al Ohmic contact on Si-doped GaN and (b) Au Ohmic contact on Mg-doped GaN. Room temperature photoluminescence spectra of undoped and Mg-doped GaN films were measured. PL of undoped GaN epilayers exhibit an emission dominated by the band edge exciton of 3.41 eV (Figure 2a). The defect luminescence at 2.2 eV was not observed. PL spectra of a Mg-doped GaN epilayer is shown in Figure 2b. The peak emission is at 435.4 nm. The peak emission wavelength changes with Mg doping concentration as observed on Mg-doped GaN films grown on sapphire substrates by other researchers [3]. PL measurements were also
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performed on undoped A1GaN films. Al composition was in the range of 0-10%. No 2.2 eV defect emission was observed on any of these samples.
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Figure 2. doped GaN.
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Room temperature photoluminescence spectra of (a) undoped GaN and (b) Mg-
Perkin-Elmer 6300 SIMS system was used in the analysis of Mg-doped GaN epilayers. Mg atomic concentration of as high as 4x 1020 cm-3 was measured in GaN layer. By changing growth conditions, Mg doping levels can be well controlled and repeated. Figure 3 shows a SIMS depth profile of Mg-doped GaN
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