Effects of Rapid Thermal Annealing on SiN x Capped MBE GaAs
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EFFECTS OF RAPID THERMAL ANNEALING ON SiNx CAPPED MBE GaAs. Akira Ito, Akira Usami, Hiroyuki Ueda*, Hiroyuki Kano*, and Takao Wada, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466, Japan, *Toyota Central R.&D.Labs.Inc., Nagakute, Aichi 480-11, Japan ABSTRACT Effects of rapid thermal annealing (RTA) with a SiN, encapsulant on molecular beam epitaxial GaAs are studied with deep level transient spectroscopy (DLTS) measurements and x-ray photoelectron spectroscopy (XPS) measurements. The RTA was performed at various temperatures form 800'C to 1100'C for 6sec. The The EL2 depth profile electron trap EL2 is produced by the RTA above 850'C. produced after the RTA is fitted with a complementary error function. The SiN, cap layer is more effective to prevent the formation of the EL2 than the SiO, cap layer during the RTA, because the critical temperature of the SiN, cap where the EL2 concentration starts to increase is higher than that of the SiO. cap. Slight increase of the oxidized Ga atoms is observed after the RTA near the cap surface. The enhancement of the EL2 trap is discussed considering the outdiffusion of Ga atoms into the cap layer during the RTA. INTRODUCTION Many studies about the encapsulants for heat-treatments of GaAs have been reported.[1-81 A SiN, film is superior to a SiO. film in the protection of As loss during rapid thermal annealing (RTA).[1] A SiO, cap changes surface stoichiometry of GaAs because of Ga outdiffusion into the cap layer during RTA.[2,3] We have reported previously that the RTA produces an electron trap EL2[12] in molecular beam epitaxial (MBE) GaAs capped with SiO, films.[4,5] In the SiO. cap sample, the observed depth profiles of the EL2 concentration are fitted with a complementary error function.[5] A SiN, film is more protective against the outdiffusion of Ga atoms into the cap films.[6,7] RTA has been used for activating implanted dopants with few redistribution.[9] Xin et al. have reported that the EL2 is enhanced by strong thermal stress and possible radiation damage created during sputterring of SiN 4 in MBE GaAs layers.[8] In this work, we study the effects of the SiN, cap layer on MBE GaAs during the RTA. EXPERIMENTAL PROCEDURE The MBE Si-doped GaAs layers were grown on Cr-doped horizontal-Bridgman semi-insulating GaAs substrates. The thickness and the carrier concentration of epitaxial layers were 21m and -.5*1016 cm- 3 , respectively. They were grown at 600°C with an As4 /Ga ratio of 4. All the MBE layers used in this study were coated with a 800A thick SiNX encapsulant by plasma enhanced chemical vapor deposition at 400'C. The capped MBE layers were faced to Si substrates in a quartz tube and illuminated by halogen lamps in flowing N2. In this study, RTA was performed at 800, 850, 900 and 1100 'C for 6 s. The heating rate was 50 °C/s, and samples were cooled without control. For comparison, the capless RTA was performed at 900°C. After the RTA, the SiN, encapsulant was removed by etchant. Ohmic contacts were made by alloying Au-Ge at 420'C for 3 min, and Schottky
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