GaN在多孔硅衬底上的MOCVD生长

Single-crystal GaN thin films were successfully grown on porous Si (PSi) substrates using metalorganicchemical vapor deposition, The full-width at half-maximum (FWHM) of the asymmetric rocking curvefor a GaN film on a PSi substrate was narrower than that for a GaN film on a normal flat-Si substrate(Flat-Si), whereas the FWHM of the symmetric one is broader. Compared with a GaN film on Flat-Si, theextent of wafer bending was reduced and the band-edge emission from GaN was enhanced, Moreoverthe tensile stress in the film was significantly reduced.

1. Introduction

GaN and related materials have been studied for use inoptoelectronic devices in the visible and ultraviolet ranges aswell as in high-power, high-frequency electronic devices. Becauseof the absence of a bulk large-area native substrate, GaN-baseddevices are usually grown on foreign substrates such as sapphireor SiC. On the other hand, there is considerable interest in thegrowth of GaN on Si substrates for the fabrication of lightemitting diodes (LEDs) and AlGaN/GaN high electron mobilitytransistors (HEMTs). The Si substrate has the advantages of lowcost, large area availability, and reasonable thermal conductivityover sapphire and SiC. We have reported that an intermediatelayer with high thermal stability is required to prevent meltbacketching and succeeded in growing flat GaN films on Si [1]. We andother researchers have successfully grown high-quality GaN filmson Si using a thick AlN layer and AlGaN/GaN multilayers (MLs2-6]. However, due to the large lattice and thermal mismatchesit is still difficult to obtain high-quality GaN films on Si.

Porous Si (PSi) is a promising candidate substrate for over-coming the lattice mismatch involved in the GaN-on-Si system aswell as in the GaAs-on-Si system [7-9]. We could expect the flexibility of PSi owing to its thin sponge-like or rod-like structureand the realization of nanoheteroepitaxial lateral overgrowth(NHELO) on its self-organized patterned nano- and micro-holes.Missaouli et al.(10] have reported the metalorganic chemicalvapor deposition (MOCVD) growth of GaN on Psi. However, thesurface morphologies of GaN on PSi are very rough and showpolycrystalline characteristics [11]. In this paper, we report onsingle-crystal growth of a GaN film on a PSi substrate and itscharacterization.

2. Experimental procedure

The PSi substrates used were prepared by the anodization of500-um-thick p-type Si (111) wafers (0.005-0.018  cm) in HFsolution at the current densities of 1-30 mA/cm2. The thicknessesof the PSi layers were 20-100 um. The GaN layers were grown onthe PSi substrates using conventional horizontal MOCVD at apressure of 100 Torr. The source materials for Ga, Al, and N weretrimethylgallium (TMG), trymethylaluminum (TMA), and ammonia (NH3), respectively. Monosilane (SiH4) diluted in hydrogen wasused as the n-type dopant. The substrates were heated at 1120°Cfor 10 min in a hydrogen atmosphere to clean their surface.The following two types of structures were grown on the PSisubstrates at the same temperature: (1) a high-temperaturegrown 50-nm-thick AlN interlayer (IL) and a 1-um-thick GaNlayer; (2) a high-temperature-grown 200-nm-thick AIN IL20pairs n-GaN/n-AIN (20/5 nm) MLs,and a 200-nm-thick n-GaN layer (n: 1-2 x 1017 cm-3). With structure (2), we are aiming at theapplication of n-type layers in GaN-based LEDs. For comparison.the same structures were grown on a 350-um-thick n-typeSi(1 1 1) substrate (Flat-Si). The morphologies and structures werecharacterized by interference microscopy, scanning electronmicroscopy (SEM), atomic-force microscopy (AFM), and X-raydiffractometry (XRD). Surface-displacement profiles were ob-tained using a laser focus displacement meter. Photoluminescence(PL) measurement was carried out at room temperature with aHe-Cd laser (325 nm,40 mW/cm2). Micro-Raman scatteringspectra (E2 high) were obtained using a Nd:YVO4 SHG laserexcitation (532 nm) in the z(xy)-z backscattering configurationwith the scattering light parallel to the c-axis of the GaN layer toestimate the biaxial stress in the films.