Growth of High Quality Epitaxial Rhombohedral Boron Nitride Mikhail Chubarov,* Henrik Pedersen, Hans Hö gberg, Jens Jensen, and Anne Henry Department of Physics, Chemistry and Biology, Linkö ping University, SE-581 83 Linkö ping, Sweden ABSTRACT: Epitaxial growth of sp 2 -hybridized boron nitride (sp 2 BN) films on sapphire substrates is demonstrated in a hot wall chemical vapor deposition reactor at the temperature of 1500 °C, using triethyl boron and ammonia as precursors. The influence of the main important process parameters, temperature, N/B ratio, B/H 2 ratio, and carrier gas composition on the quality of the grown layers is investigated in detail. X-ray diffraction shows that epitaxial rhombohedral BN (r-BN) film can be deposited only in a narrow process parameter window; outside this window either turbostratic-BN or amorphous BN is favored if BN is formed. In addition, a thin strained AlN buffer layer is needed to support epitaxial growth of r-BN film on sapphire since only turbostratic BN is formed on sapphire substrate. The quality of the grown film is also affected by the B/H 2 ratio as seen from a change of the spacing between the basal planes as revealed by X-ray diffraction. Time-of-flight elastic recoil detection analysis shows an enhancement of the C and O impurities incorporation at lower growth temperatures. The gas phase chemistry for the deposition is discussed as well as the impact of the growth rate on the quality of the BN film. ■ INTRODUCTION The III-nitrides exhibit many interesting properties, in particular as semiconductor materials in optoelectronic devices, such as light emitting diodes (LED) and laser diodes (LD). AlN, GaN, InN, and their alloys cover a wide wavelengths range from IR to UV. Demands for devices operating in the UV range have increased the interest for the B-N system and the binary phases therein. Boron nitride can form compounds with either sp 3 -hybridized or sp 2 -hybridized bonds. The most studied sp 3 - hybridized phase is cubic (c-BN), but there is also a wurtzite phase (w-BN) that is an analogue to the previously mentioned III-nitrides AlN, GaN, and InN. The interest for c-BN mainly stems from the phase similarities to diamond. Unfortunately, high quality epitaxial thin films of c-BN are difficult to obtain, and most of the grown films contain significant amounts of other phases. Furthermore, c-BN films generally appear to be polycrystalline. 1-4 Ion bombardment during the deposition process seems to be necessary for the nucleation of c-BN films by the formation of the sp 3 bonds. Hence, energetic ion-assisted physical vapor deposition including rf and magnetron sputtering as well as ion-beam assisted deposition have been the choice for BN deposition. The chemical route represents a somewhat more gentle deposition process and chemical vapor deposition (CVD) techniques operated at high energy regime- like plasma enhanced CVD techniques have been used. 5,6 Very few attempts have been reported on the BN synthesis by processes without the use of energetic deposition and those approaches seem to favor the formation of sp 2 bonded phases. The sp 2 -hybridized BN (sp 2 BN) can crystallize into two different phases: hexagonal (h-BN) and rhombohedral (r-BN). These two phases can be regarded as two different stacking sequences of the same basal plane, where h-BN and r-BN have a lattice parameter along the c-axis of 6.656 Å and 10.000 Å, respectively (Figure 1). This feature gives a similar spacing between the basal planes for these two polytypes and makes distinction of these structures difficult by transmission electron microscopy and X-ray diffraction (XRD) in Bragg-Brentano geometry. 7 In addition, a less ordered form of sp 2 BN, the turbostratic BN (t-BN), 8 is frequently observed in the growth of both sp 3 and sp 2 BN. For the growth of sp 2 BN, thermal CVD is generally applied. 9-12 A first report by Nakamura demonstrated the growth of polycrystalline h-BN at atmospheric pressure, using sapphire and silicon as substrates. 13 A later study by Jin et al. using Si(100) as substrate reported the influence of various growth parameters on the film properties, but the deposited films exhibited a turbostratic structure. 14 Recently, h-BN epitaxial films were grown on sapphire substrate and graphitized 6H-SiC using thermal CVD. 15 The same group reports the growth of a 500 nm thick h-BN film on Ni(111) Received: March 19, 2012 Revised: April 30, 2012 Published: May 2, 2012 Figure 1. Crystal structures of sp 2 BN. (A) Hexagonal BN; (B) rhombohedral BN. The small blue spheres represent nitrogen atoms and the big red spheres boron atoms. Article pubs.acs.org/crystal © 2012 American Chemical Society 3215 dx.doi.org/10.1021/cg300364y | Cryst. Growth Des. 2012, 12, 3215-3220