One-Step Silicon Nitride Passivation by ECR-CVD for Heterostructure Transistors and MIS Devices
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Usually, this problem is dealt with by surface passivation through deposition of a thin nitride film. This process requires prior surface treatment of H2 and/or N2 plasmas in order to remove native oxide [1,2] and fill the surface dangling bonds forming an ultra-thin GaN layer. Moreover the nitride film is frequently deposited on a thin layer of Si over GaAs [3] in order to take advantage of the well behaved Si oxides. In this work, we present a one-step passivation process by ECR-CVD deposition which requires no pre-treatment. The silicon-nitride film is deposited directly over GaAs. The process is described in the next section together with the results from the film characterization. Then we investigate the application of the insulator on a MIS structure where C-V as well as I-V data are discussed. Finally we describe the passivation properties of the SiN. when applied to MIS devices. EXPERIMENTAL PROCEDURE The silicon nitride layers were formed on n-type GaAs (100) wafers. The substrates were cleaned with organic solvents using a Sox-let distillate. Based on previously established conditions by Diniz et al [4], the ECR depositions were carried out at a fixed
137 Mat. Res. Soc. Symp. Proc. Vol. 573 c 1999 Materials Research Society
substrate temperature of 200 C, SiH 4/N2 flow ratio of 1, Ar flow of 5sccm, pressure of lmTorr and microwave (2.45 GHz) and RF (13.56 MHz) powers of 250W and 1W, respectively. The nitride processed without H2 and N2 plasma surface pre-treatment is named control nitride (CN). For comparison purposes, SiNx samples deposited on GaAs, under the same conditions, with sequential pre-treatment of H2 and N2 plasma surface are also considered. The plasma conditions for the pre-treatment are: temperature 20 0C, pressure of ImTorr (H2 ) and 3 mTorr (N2), H 2 flow of 10 sccm and N2 flow of 20 sccm. In this case, the employed RF power and microwave power are 1W and 500W, respectively. Three different samples are discussed, namely HtlN, Ht3N and Ht5N, where the H2 plasma process time for each one was 1, 3 and 5 minutes, respectively. Nitrogen plasma process time was fixed at 5 minutes. In order to characterize the control nitride composition FTIR, ellipsometric and profile measurements were performed. Metal/nitride/GaAs capacitors were fabricated from all the samples considered with
two different gate electrodes, aluminum (Al) and tungsten nitride (WN). Al/nitride/GaAs capacitors were formed by e-beam evaporation of 160 nm thick aluminum film, sintered by conventional furnace in forming gas (92%N 2 + 8%H2) at 420 0 C for 20 minutes. WN/nitride/GaAs capacitors were formed by reactive sputtering of 200 nm thick tungsten nitride film, sintered by conventional furnace in forming gas (92%N 2 + 8%H2) at 4200 C for 5 minutes. The Al and WN electrodes were patterned with a mask composed of an array of 200 pIm diameter dots. On the wafer backside a 285 nm thick AuGeNi film was evaporated to form the ohmic contact, which was sintered by conventional furnace in forming gas (92%N 2 + 8%H2) at 420 0C for 3 m
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