NiIn As an Ohmic Contact to p-GaN
- PDF / 379,123 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 90 Downloads / 209 Views
on metal/p-GaN contacts suggests that the Schottky model is at least partially applicable.4 Of the several contact schemes to p-GaN that have been studied Au/Ni contacts have received the most attention. Despite being used in devices, the Au/Ni contacts typically exhibit a specific contact3 78 2 2 resistance (Pc) in the range of 10-3 and 10-2 fl cm range ' "1 and show poor thermal stability.' However, it is known that the specific contact resistance of p-GaN depends on many factors such as the quality of the bulk substrate and its surface preparation and processing conditions. Accordingly, it is difficult to judge the absolute resistance values reported. In an attempt to find new metallizations that can be used to form ohmic contacts to pGaN the solid-state exchange reaction was utilized. This reaction has been shown to be a systematic approach for tailoring metal/semiconductor contact properties.1417 The complete thermodynamic and kinetic model for the exchange mechanism has been comprehensively set forth by Swenson et al .1 Based on the exchange mechanism criteria, the intermetallic compound Niln was selected as possible ohmic contact to p-GaN. The thermodynamically stable phase of Niun (50:50 at % ratio) has a CoSn (B35) crystal structure, however, the metastable B2 phase often forms and is the structure of the Nan films used in this study. This B2 phase is thermodynamically stable at high temperature in Ni-In alloys that have 51 to 58 % In.' 9 EXPERIMENTAL PROCEDURE The GaN substrates used in this study are 1.2 ý.tm thin films of single crystal GaN grown by metalorganic vapor phase epitaxy (MOVPE) on sapphire (0001). The GaN epilayer is Mg doped and was annealed to activate the dopants. A p-type carrier concentration of 2 x 1017 cm-3 with a mobility of 8 cm/V s was measured by Hall probe at room temperature. Prior to lithography, the substrates were ultrasonically degreased with Acetone and Methanol for 10 min each. These degreased substrates were then etched in HC1:H 20 (1:2) for 4 min and then rinsed in flowing H 20 for 10 min. Using standard photolithography techniques the substrates were patterned with a circular transmission line model (TLM) pattern similar to the one detailed by Marlow. 20 Schottky barrier heights were measured by Current-Voltage (I-V) characteristics using the 140 4tm diameter pad as the diode and the large area metallization as the ohmic contact. The specific contact resistance (Pc) measurements were conducted at 5 volts using the circular TLM pattern which consists of 7 different contact pads with gap spacing ranging from 4 to 25 pm. Once patterned, the substrates were then placed in a HCI:H 20 (1:4) solution for 20 seconds, blown dry with Nitrogen gas and immediately loaded into a vacuum chamber with the background pressure less than 2 x 10-7 torr. The Niln (50:50 at % ratio) was deposited by sputtering from a compound target to a nominal 80 nm thickness. The annealed Niln films were found by X-ray diffraction to be in the metastable B2 crystal structure when deposited by RF sputterin
Data Loading...
