Characterization Of The Nitrided GaAs Thin Layers After Rapid Thermal Annealing By Using Raman Scattering

Abstract

By using Raman spectroscopy, we examine defects generated from the nitridation of a GaAs surface performed by a nitrogen electron cyclotron resonance (ECR) plasma at various temperatures and its changed properties after the isochronal (15sec) rapid thermal annealing (RTA) at 750°C, 850°C and 950°C. The unpolarized Raman spectra taken from the nitrided samples show that the bandwidths and the peak frequencies for LO phonon band were broader and downshifted due to result from the increased nitridation temperature, respectively, but the spectra obtained after RTA are inclined to recover to the bandwidth and the peak frequencies of the un-nitrided sample. And the height ratio of TO/LO is also decreased. We estimate the correlation length for each sample from the relationship between the Raman shift of LO phonon and the broadening as a function of correlation length, and show that the point defects can be annealed out by annealing processes. I . INTRODUCTION Nitridation technique has been used for the formation of a thin nitrided buffer layer on which high quality nitrided films can be formed, particularly, for the growth of a GaN layer. The Raman studies in relation to semiconductors have mainly concentrated on the bulk materials, the epitaxial layers, and the surface damaged by ion implantation which generates large strains. However, studies on thin nitrided layers have rarely been reported because the surface modification of the nitridation causes much smaller strains compared to that of the ion implantation. In this work, we investigate defects generated from the nitridation process and especially its changed properties after RTA by using Raman scattering. 11 . EXPERIMENTAL The samples used in this work were n-type (001) GaAs. Nitridation was performed at various temperatures by ECR plasma power of 1OOW. Each nitrided sample was applied isochronal (1 Ssec) rapid thermal annealing (RTA) at 750 "C, 850°C and 950 "C. Raman spectra were measured from an oblique forward scattering geometry (0~160") in order to obtain more information from the near surface region. III. RESULTS AND DISCUSSION Only the LO phonon band for GaAs is allowed by the Raman selection rule, but the LO and the TO phonons are allowed for the un-nitrided sample as shown in Fig. 1. Thus the un-nitrided sample consists with somewhat random crystallographic orientation and its measured Raman spectrum is regarded as a reference to the other samples. Figure 1 shows the unpolarized Raman spectra taken from the un-nitrided GaAs, the nitrided GaAs at 550°C, and GaAs nitrided at 550°C after RTA at 95OoC, respectively. For the LO phonon band of the nitrided samples, the bandwidths are broader and the peak frequency is downshifted as un-nitrided sample, but in the case of the annealed samples the bandwidth and the peak frequency are narrower and upshifted as the nitrided samples, respectively. And the height ratio of TO/LO is decreased, especially at the sample after RTA. Fig. 2 summarizes the results of the bandwidths and frequencies for all measured samples. And we estimate the correlation length for each sample from the relationship between the Raman shift of LO phonon and the broadening as a function of correlation length'. We have known from these results that the point defects such as VAS or VAs-AsG; generated mainly at high temperature over 210 550°C are annea.led out by RTA technique and the disordered thin layers near the surface tend to be ordered through the annealing process. IV. CONCLUSION We have investigated the properties of the nitrided GaAs thin layers after the isochronal ( 1 5sec) rapid thermal annealing (RTA). We have known thai the amount of the nitridation is increased as the increased nitridation temperature, and the point defects such as V,, or VAS-As,, generated mainly at high temperature over 550°C are annealed out by RTA technique and the disordered thin layers near the surface are showing the tendency in ordered states through the annealing. Fig. 1. Un-polarized Raman spectra of the nitrided GaAs at 550°C measured at room temperature before and after the RTA at 950 "C . Fig. 2. Dependencies of (ahhe bandwidths and (b)the peak frequency of LO phonon as a function of ni1:ridation temperature for the annealed samples at 950 "C . non-RTA (solid line) 3 8 3 7 E 2 3 5 I 3 6 I '