MOCVD生长的GaN单晶膜透射光谱研究

Optical Transmission Study on GaN Films Grown by Metal-organic Chemical Vapor Deposition

采用MOCVD技术在Al2O3（0001）衬底上生长了GaN薄膜,使用透射光谱、光致发光光谱和X射线双晶衍射三种技术测试了五类GaN薄膜样品。实验结果表明:GaN薄膜透射谱反映出的GaN质量与X射线双晶衍射测量的结果一致,即透射率越大,半高宽越小,结晶质量越好;而X射线双晶衍射峰半高宽最小的样品,其PL谱的带边峰却很弱,这说明PL谱反映样品的光学性能与X射线双晶衍射获得的结晶质量不存在简单的对应关系。同时还报导了一种特殊工艺生长的高阻GaN样品的RBS/沟道结果。

Five GaN films grown by metal-organic chemical vapor deposition (MOCVD) method were investigated. The growth was performed using a home-made vertical reactor operating at atmospheric pressure. The growth was carried out on (0001) oriented sapphire substrates using trimethylgallium(TMGa) and blue-ammonia(NH3) as Ga and N sources, respectively. The mixed gases of hydrogen and nitrogen were used as the carrier gases. A thin buffer layer with thickness of about 15nm was grown at 520°C and recrystallized at 1 060°C for 6 minutes. The GaN films were grown at 1 060°C.Optical transmission measurement, photoluminescence (PL), double-crystal X-ray diffraction (DCXRD) and the Van der Pauw Hallmethod were used to measure the optical, structural and electrical data of these films at room temperature.Table 1 gives the characterization data of the five GaN films.Fig. 2 shows the optical transmission spectra of GaN films. The optical absorption edge of GaN films is at 366nm.The transmittance varies between 63% and 93% for the used samples.Fig. 3 shows the relationship between the FWHM of double-crystal X-ray diffraction (DCXRD) and transmittance. As shown in Table 1, the FWHM of DCXRD becomes smaller when transmittance increases, which suggests that the higher the transmittance, the better the crystal quality.Fig. 4 shows the photoluminescence (PL) spectra of five GaN films at 300K. The excitation-source was a 15mW He-Cd laser. The peaks of band-edge emission of Samples B, D and E were very sharp. The band-edge emission intensity of Sample A was very weak, but its FWHM of DCXRD was the smallest, which indicates that the optical property of GaN determined by PL isn' t in correspondence with the crystal quality determined by DCXRD.The experimental results show that the crystalline quality of GaN determined by transmission measurement is in agreement with that determined by DCXRD. But we did not observe the same relationship in PL spectra with DCXRD. This paper also reports that a highly resistive GaN film grown by atmospheric pressure MOCVD has RBS/channeling yield xmin of 1.5%.