Nitride Based High Power Devices: Transport Properties, Linear Defects And Goals
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Au
TiAINiAu
cm -3 101 8cm 3 NN7++_-1016 cm 1 1Nd~~~l Sapphire
+2
l ......
a)
b
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Figure 1: Different device geometries for GaN Schottky rectifiers. a) Lateral Schottky rectifier b) Mesa device offers lower resistivity since ohmic contacts are deposited on a bottom layer of higher conductivity c) Lateral Schottky rectifier processed with Schottky metal field plate overlapping SiO2 ring.
contact geometries, including lateral, mesa and Schottky metal field plate devices as shown in Fig. 1. Prior to metal deposition, GaN surfaces were cleaned with organic solvents, dipped in HF:H 2 0 (1:10), rinsed in deionized water, and blown dry with nitrogen gas. Au (1500A) was then sputtered in a chamber with background pressure of 2 - 10-ltorr and patterned to produce Schottky contacts. Ti/Al/Ni/Au (150A/1500A/100A/1000A) was sputtered to produce ohmic contacts as deposited. On some devices, large area Au metallization was used instead as low resistivity contact. A Ni/Au (200A/1500oA) metallization scheme was used for ohmic contacts in the case of p-GaN, while Ti/Au was used for Schottky contacts. 450 V GaN Schottky rectifier performance We have fabricated >450 V standoff voltage GaN Schottky rectifiers using device geometries shown in Fig. 1 on a 10 pm thick HVPE grown GaN layer. The GaN layer consisted of a low conductivity ((2 ± 1) • 1016 cm- 3 electron concentration) top layer approximately 5tzm thick and a highly conductive 51Lm bottom layer. The reverse breakdown voltages observed were in the range between 250V and 450V, and exceeded 450V in several devices. The I-V characteristics for the fabricated devices are shown in Fig. 2. The Au Schottky barrier height obtained using I-V measurements was between 1.1eV and 0.8eV, depending on ideality factor ranging between 1.6 and 4. The ON-state voltage was 5V in the case of the lateral diode for a state of the art current density of 100A/cm 2 . The large ON-state voltage is a consequence of both the non-ideality of the diodes and large series resistance. To reduce the series resistance we etched the 5 tim thick top layer and produced ohmic contacts on the more2 conductive bottom layer (Fig. 1b) and 3a)). In this case, the ON-state voltage at 100A/cm was reduced to 4.2V. With improved Schottky contacts having better ideality, and with low resistance ohmic contacts, ON-state voltages as low as 1V can be expected. The lowest saturation current at a reverse bias of -100V was in the 10-5 A/cm 2 range and was observed for devices both with a metal field plate and for circular Schottky contact lateral devices (diameter in the 100 - 200am range). Using the formula for the punch-through diode,[2] VpT = EcRW - qNdW 2/2oeo , and substituting measured values, we find the critical field for electric breakdown to be (2 ± 0.5) . 106 V/cm. As can be observed from Fig. 3b), all of the devices suffered from premature edge breakdown. Therefore, we conclude that this value of critical field is only a lower limit. With improved Schottky contact geometries, a value of the critical field as high
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