湿式热氧化GaN构成

Thermal oxidation of GaN was conducted at 700 900°C with O,, N, and Ar ascarrier gases for 525-630 Torr of H,O vapor. Upon oxidation of both GaNpowders and n-GaN epilayers, the monoclinic -Ga,O, phase was identifiedusing glancing angle x-ray diffraction, The chemical composition of the oxidewas verified using x-ray photoelectron spectroscopy. In experiments conducteousing GaN powder, the oxide grew most rapidly when 0, was the carrier gas forHO. The same result was obtained on n-type GaN epilayers. Furthermore, thethickness ofthe oxide grown in H,O with O, as the carrier gas was found to beproportional to the oxidation time at alltemperatures studied, and an activationenergy of210+10 kJ/mol was obtained. Scanning electron microscopy revealeda smoother surface after wet oxidation than was reported previously for dryoxidation.However, cross-sectionaltransmission electron microscopy revealecthatthewet oxide/GaNinterface was irregular andnon-idealfor devicefabricationeven more so than the drv oxide/GaN interface. This observation was consistentwith poor electrical properties.

INTRODUCTION

Group IlI nitrides and their alloys are under rapiddevelopment for optoelectronic devices in the blue.green, and ultraviolet (UV) portion of the spectrum,with applications including data storage, displays,and detectors. They are also receiving attention forhigh power and high temperature electronics. Whilemuch work has been done to study the native thermaloxides onsemiconductors such as Siand morerecentlySiC and the Al-bearing Ill-V semiconductors, veryfew studies have been conducted on the thermaloxidation of GaN. Wolter et al. studied the oxidationof GaN in dry airl which resulted in the formation ofa polycrystalline oxide with a rough surface.2 It wasspeculated that wet oxidation of GaN might result intheformation ofan oxide withimproved polycrystallinemorphology or even an amorphous oxide. Therefore.in this study, we have investigated the thermaloxidation of GaN in H,O vapor and examined theeffect of different carrier gases upon the oxidation process. The electrical properties of the thermal oxides are also evaluated.

PROCEDURE

GaN powder ( 325 mesh) with a purity of at least99.99% (metals basis) was oxidized in H,O vapor for5 h at 700-900°C using ultra-high purity (UHP) O,N,, and Ar as carrier gases. Oxidation of samples indry O, at the same temperatures was also performedfor comparison. The powder samples werecharacterized using a Phillips multipurposediffractometer,and 0-20 scans were performed usingCuk  radiation. The relative peak intensities werecompared for similar oxidation conditions to givequalitative information about oxide growth rates.GaNepitaxiallayers were grown on (0001)sapphireusing a vertical hydride vapor phase epitaxy process3with a sputtered ZnO pretreatment. The n-type filmswere grown at 1050°C for one hour at a rate of15 um/h and had a carrier concentration of 2.2 x 1017cm-3.The samples were rinsed in acetone, methanol.and deionized (DI) water, after which they wereplaced in a 1:1 solution of HCl:DI water for 60 s and rinsed in DI water.

The samples were placed in a quartz boat within aquartz tube, and then the tube was purged with thecarrier gas for at least 20 min before it was placed ina horizontal furnace. Once the tube was placed intothe furnace, gases were diverted into the previouslyby-passed bubbler. Using a thermocouple inserted inthe quartz tube, we determined that the time for the sample to reach the intended temperature did notexceed 6 min. The carrier gases were maintained at aflow rate of 25 sccm through a bubbler containing DIwater heated to 93 +1°C.Oxidation was performed at700 900°C for 1-50 h, and results were compared toa previous study of the oxidation of GaN in dry air.1,2.4Following oxidation, the quartz tube was removedfrom the furnace and allowed to cool in air to roomtemperature before the sample was removed.

A consistent volume of DI water was used in thebubbler before each experiment, which has beenreported to increase the reproducibility ofthe partialpressure ofwater vapor transferred to the furnace.5 Iaddition, heating tape maintained at approximately110°C was used on the gas feed line from the bubblerto the tubefurnace to minimize condensation betweenthe bubbler and furnace. The atomic percent of H,Ovapor present in the carrier gas was estimated to be77-8% for any given furnace temperature and carriergas combination.These calculations were based onthe flow rate of the gas and the volume of H,O lostfrom the bubbler.