GaN Thick Films Growth On Si By HVPE And Optical And Structural Properties Of Cr Doped HVPE GaN

In recent years, GaN has attracted extensive attentions in the area of photo-electronics. GaN is direct band gap semiconductor material, with many advantages, such as wide band gap, high electron saturation velocity, high breakdown fields. Now, the laser diodes and light emitting diodes based on GaN and related compounds have already been commercialized. Also, the wide band gap of GaN makes it a good candidate for the short wavelength optical devices. The high breakdown fields make GaN suitable for high power devices. The chemical stability of GaN makes it a candidate for devices working in hostile environment.Due to the absence of a lattice matched bulk substrate, the growth of GaN is still a problem needing further investigation. In recent years, substantial progress in growing high quality GaN by the hydride vapor phase epitaxy (HVPE) method has been achieved. High growth rate of HVPE method makes the growth of thick GaN layers practical.In this paper, GaN thick films on Si, and Cr doped GaN thick films on sapphire are grown by HVPE method. Structural, electrical and optical properties of the films are studied with different methods. The main conclusions are listed as follows.1. High temperature GaN (HT-GaN) are grown with low temperature GaN (LT-GaN) layers as buffer layers directly on Si using HVPE system. The LT-GaN layers are grown at 600℃, and the growth time are 30s,60s,120s. The results show that the thickness of LT-GaN buffer layers has great impact on the quality of HT-GaN thick films. The results are caused by two effects. Thick buffer layers can effectively block the unexpected process of Si etching by reactive gas. So the HT-GaN is single crystalline when the buffer layer growing time is 120s, while HT-GaN are Polycrystalline when growing time are 30s and 60s, and. On the other hand, with thicker buffer layers, the quality of the HT-GaN will be reduced.2. GaN thick films have been successfully grown on Si(111) by HVPE system, and AlN layers grown by MOCVD are used as buffer layers. The impact ofⅤ/Ⅲratio on the quality of GaN has been studied. The flux of HCl gas were set as 5 sccm,10sccm,15sccm and 20sccm. Raman scattering, XRD, and PL measurements are used to analyze the properties of the films. The results show that when the flux of HCl increased, the quality of GaN thick films improved. WhenⅤ/Ⅲratio is 50, the structure of the GaN film is hexagonal wurtzite, the biaxial stress of film is tensile and the value is 0.17 GPa, the band edge emission of the GaN thick film is at 363.7 nm and no yellow luminescence bands are observed. The AlN buffer layers can effectively block the additional reaction between Si substrate and reactive gas during the GaN growth, and reduce the residual stress of the GaN thick film.3. Single crystalline Cr doped GaN films have been successfully grown by HVPE method. The film is uniform without detectable Cr precipitates or clusters and the Cr atoms are substituted for Ga sites. The impurity modes between 510～530 cm-1 in Raman spectra are assigned to the host lattice defects caused by substitutional Cr. The donor-acceptor (DAP) emission is found to locate at Ec-0.20eV by analyzing the PL spectrum obtained at different temperatures, and the emission is attributed to the structural defects caused by CrGa-VN complex. The superconductor quantum interference device (SQUID) results show that the Cr doped GaN film without detectable Cr precipitates or clusters exhibits paramagnetic properties.