X-ray photoelectron spectroscopic study on sapphire nitridation for GaN growth by hydride vapor phase epitaxy: Nitridation mechanism

Abstract

The nitridation of c-plane sapphire within the hydride vapor phase epitaxy system was systematically studied as a function of time and ammonia partial pressure using ex situ x-ray photoelectron spectroscopy, reflection high-energy electron diffraction, and atomic force microscopy. During the nitridation process, nitrogen was incorporated into the sapphire surface. There were two different nitrogen chemical bonding states, which can be attributed to N-Al bonds and nitrogen in oxygen-rich environment ͑'N-O'͒. As the nitridation continued, the N 1s intensity increased while the O 1s intensity decreased indicating the growth of a nitrogen-rich layer. The sapphire nitridation process can be modeled as a diffusion couple of AlN and Al 2 O 3 , where N 3Ϫ and O 2Ϫ interdiffuse in the rigid Al 3ϩ framework. Nitrogen diffuses into sapphire and substitutes for oxygen to bond with aluminum. The bond substitution is accompanied by structural changes where the AlN in-plane direction is rotated 30°with respect to the sapphire direction. The replaced oxygen diffuses out to the surface, combines with hydrogen and desorbs as H 2 O. The overall nitridation rate is determined by the slower of the two moving anions. From the x-ray photoelectron spectroscopy data, the chemical diffusion coefficient of nitrogen (D N ) and oxygen (D O ), were estimated. D N was found to be higher than D O , which suggested that the overall nitridation rate was controlled by the diffusion of oxygen to the surface. After nitridation, no protrusions were observed on the surface and no significant changes in the surface roughness were measured when compared to the as-received sapphire.

In situ gravimetric monitoring of surface reactions between sapphire and NH3

“…The thickness of the nitrided layer was found to increase with increase in NH 3 flow time up to 60 min, whereas the thickness was constant after 60 min of NH 3 flow time, which indicates completion of the C-plane sapphire nitridation at around 60 min in a 0.1 atm NH 3 flow at 1440 1C. This result is in agreement with another literature [14]. The nitrided layer formed on the C-plane sapphire exposed to flowing 0.1 atm NH 3 +He at 1440 1C for 100 min was found to be N-polarity (0 0 0 1)AlN by XRD and wet chemical etching in KOH solution [16], and the in-plane orientation of the nitrided layer was found to be 301 rotated with respect to the C-plane sapphire as Although the decomposition rates of the C-plane sapphire decreased as the NH 3 flow time increased up to 60 min, decomposition of the C-plane sapphire became constant after 60 min of NH 3 flow.…”

“…Since NH 3 has a very important role in the growth of AlN on sapphire substrates by VPE above 1200 1C, the surface reaction between C-plane sapphire and NH 3 , and the influence of H 2 generated by NH 3 decomposition on the decomposition of the sapphire substrate should be clarified. There have been a number of research studies on the surface morphology of sapphire exposed to NH 3 , and X-ray photoelectron spectroscopic (XPS) studies of nitridation processes on the sapphire surface in NH 3 near 1000 1C [11][12][13][14]. Nonetheless, there have been few quantitative investigations of the surface reactions between sapphire and NH 3 at high temperature.…”

Structural analysis of thick GaN films grown by hydride vapour phase epitaxy

“…Such composition profiles show that the damaged layers contain Al, N and O confirming the formation of a Al x O y N z phase which is mainly present in the amorphous parts [5]. These results show that the nitridation of the surface of sapphire was not perfectly uniform leading to irregularities in contradiction with recent data for which no modification of the surface roughness was measured [6].…”

Investigation of the Ti/Al/Pt/Au and Ti/Au contact materials on the N‐face surface of oxygen doped GaN

“…the binding energy of N-Ti and N-Al bonds) bonds were 398.05 eV and 397.29 eV, respectively. Becase the binding energy of the N-Ti [13] and N-Al [22] bonds are too close to distinguish. This can simply be explained by considering the enthalpies of formation of GaN, TiN, and AlN (respectively, -110.9, -265.5, and -318.1 kJ/mol) [6].…”