Large Diameter, Undoped Semi-Insulating GaAs for High Mobility Direct Ion Implanted FET Technology

Abstract

The growth of 2 and 3 inch diameter, <100> oriented semi-insulating GaAs crystals of improved purity by liquid encapsulated Czochralski (LEC) growth from silicon-free, pyrolytic boron nitride (PBN) crucibles in a high pressure Melbourn crystal puller, is described. Undoped and Cr-doped LEC GaAs crystals pulled from PBN crucibles exhibit bulk resistivities in the 10⁷–10⁸ and 10⁸–10⁹ ohm-cm ranges, respectively. High sensitivity SIMS demonstrates that GaAs crystals grown from PBN crucibles contain residual silicon concentrations in the low 10¹⁵ cm^-3 range, compared to concentrations up to the 10¹⁶ cm^-3 range for growths in silica containers. The residual chromium content in LEC/PBN-grown crystals is below the l0⁵ cm^-3 range.

The achievement of direct ion implanted channel layers of near-theoretical mobilities is further evidence of the improved purity of undoped, semi-insulating GaAs prepared by LEC/PBN crucible techniques. Direct implant FET channels with 1–1.5 × 10¹⁷ cm^-3 peak donor concentrations exhibit channel mobilities of 4800 to 5000 cm² V^-1 s^-1 in undoped, semi-insulating GaAs substrates, compared with mobilities ranging from 3700 to 4500 cm² V^-1 s^-1 for various Cr-doped GaAs substrates. Discrete power FETs, which exhibit 0.7 W mm^-1 output and 6 dB associated gain at 8 GHz, have been fabricated using these implanted high mobility, semi-insulating GaAs substrates.