Effects of Reactive Ion Etching on the Electrical Properties of n-GaN Surfaces
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Mat. Res. Soc. Symp. Proc. Vol. 395 0 1996 Materials Research Society
thick. The electron carrier concentration was I x 1017 cm-3 . The room temperature mobility was - 300 V/cm-s. The Schottky diode structure consisted of an array of 300 jim openings in a large-field ohmic contact patterned over the GaN surface. The ohmic contact consisted of a Ti/Al bilayer and was annealed using rapid thermal annealing (RTA). Prior to etching, the exposed surface was cleaned in dilute HCL. The exposed GaN was then etched in a Plasma Technology RIE system equipped with a nitrogen-purged load-lock. The system uses a 17 cm diameter cathode which is covered by a 1/2 cm thick quartz plate. The anode and cathode are spaced 5 cm apart. Both are water cooled to maintain a temperature during processing of 20 TC. The cathode was rf-driven at 13.56 MHz for plasma excitation. A turbomolecular pump is used to evacuate the 30 cm diameter chamber to a base pressure of 5 x 10-6 Torr. A mechanical pump, though, was used during etching to remove the process gases. An array of 250 gtm dots for the Schottky contacts was then patterned at the center of ohmic contact openings using AZ5214 photoresist. The exposed GaN surface was cleaned in a plasma asher. The samples were then dipped in dilute HCl, rinsed in deionized water, blown dry with nitrogen, and immediately loaded into the evaporator. The chamber was pumped to a base pressure of 6 x 10-7 Torr prior to metal deposition. Palladium metal was used for the Schottky contacts [7]. The Pd thickness was 100 nm. The Schottky characteristics were then measured using a four-point probe technique with an HP4142 analyzer. RESULTS AND DISCUSSION Figure 1 shows the forward I-V characteristics of samples etched under plasma self-bias voltages from -100 to -350 V. This corresponds to a RF power plasma density 10-2 10-4 10-1
5-lO-5 S10-6
I-
U10-1 -1-00 V control 1 1/ 0.61. ------/ 0 . 10-10"9o0 0 0.2 0.4 0.8 Applied bias (V) Fig. 1. Forward characteristics of Pd Schottky diodes unetched and etched using RIE/SiCI4 at various plasma self-bias voltages. Etching was conducted at 25 mT and 10 sccm SiC14 flow rate for 2 min.
770
from 0.2 to 0.75 W/cm 2 in our system. Etching parameters consisted of a chamber pressure of 25 mT, a SiCI4 flow rate of 10 sccm, and an etch time of 2 min. From the figure, we observed severe degradation for plasma self-bias in excess of - 150 V. Figure 2 shows the reverse saturation leakage current as a function of the plasma self-bias voltage. The lower and upper curves were measured at an applied bias of -5 and -10 V, respectively. We observe severe degradation of the leakage current for plasma selfbias voltages in excess -150 V, similarly as with the forward characteristics. 104-
UnlO-0.... etched!
0 10-6 ,
1
S10.8 S1.Applied >~
S10.9 10
1o0
, bias -5VV 0 -10V I I I I 100 150 200 250 300 350 Plasma self-bias voltage (-V)
400
Fig. 2. Reverse saturation leakage current of Pd Schottky diodes plotted as a function of plasma self-bias voltage. The barrier height and ideality of
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