Analysis of GaN decomposition in an atmospheric MOVPE vertical reactor

Abstract

The GaN decomposition versus the annealing ambient (H 2 , N 2 + H 2 ) have been investigated in atmospheric pressure metal organic vapor phase epitaxy (AP-MOVPE) vertical reactor. A comparison between the low temperature GaN (LT-GaN) decomposition in a vertical reactor and a horizontal one was discussed in order to explain the failure of high temperature GaN (HT-GaN) growth on Al 2 O 3 using 2D buffer layer under a mixture of H 2 and N 2 in our vertical reactor. Decomposition rates versus temperature and ambient gas were measured using real time laser reflectometry. The surfaces of these films were analyzed using a scanning electron microscopy (SEM). Hydrogen seems to considerably affect the LT-GaN surface morphology by a mass transport, diffusion on the substrate surface and by a decomposition process even in the presence of ammonia and nitrogen. The high decomposition rates of the LT-GaN measured in comparison to that determined for the HT-GaN one is attributed to the amorphous-like structure of the layer and the instability of Ga-N bonds. This favors the appearance of a mirror-like gallium surface in the beginning of this layer decomposition. The presence of liquid gallium catalyzes the decomposition process. HT-GaN SEM analysis revealed that the surface of the "totally" decomposed GaN surface is dominated by gallium droplets. These were removed. The region around and under these droplets is capped by a porous like GaN film. The coalesced gallium droplets may cover some areas of partially decomposed GaN and preserve them from H 2 attack and total decomposition.

Thermal Stability of Crystallographic Planes of GaN Nanocolumns and Their Overgrowth by Metal Organic Vapor Phase Epitaxy

“…With this general picture of what happens during annealing in the N 2 + NH 3 atmosphere, we will now consider the NC shape evolution for the annealing in H 2 + NH 3 . Although the observed “mildness” to GaN NC topology of the hydrogen atmosphere as compared to the nitrogen one seems to contradict the general knowledge regarding GaN decomposition in H 2 and N 2 , , this “mildness” is only “apparent”.…”

“…Originating from GaN decomposition at different facets present in NC arrays, in the absence of strong desorption ( T < T des ) and lateral growth, Ga adatoms do not find any sink for them and thus have to form ultimately a continuous monolayer (η Ga = 1) on the surface. We assume this process is happening already at temperatures below those studied here, i.e., as soon as GaN starts to decompose . The full coverage of the GaN surfaces with a monolayer of free Ga adatoms (protective metallic film) inhibits any further decomposition and thus significant reshaping of both capped and even uncapped NCs up to 1000 °C (Figure c,d).…”

“…The presence of ammonia also plays a crucial role in this protection because in its absence GaN can decompose in a hydrogen ambient at even much lower temperatures. − Its effect is most probably that of keeping the nitrogen atoms in their lattice positions or better to say of providing substitutions for those that leave them. This suppresses detachment of any further Ga atoms (sitting below the surface Ga adatoms) from the lattice and thus prevents the formation of Ga droplets by keeping the amount of Ga adatoms to a monolayer.…”

GaN ceramics obtained by fusing of nanocrystalline GaN powder at high pressures and temperatures as substrate for growth of GaN epilayers

“…The substrate was annealed in the growth chamber for 10 min at 600 1C in H 2 /NH 3 (1:1) ambient. The NH 3 was introduced at 450 1C during heating to prevent the decomposition of the GaN nanocrystalline powder substrate [8,9]. Then, the lowtemperature GaN layers were grown during the temperature ramping from 600 to 850 1C for 5 min with 9300 V/III molar ratio.…”

Room Temperature 2DEG Mobility Above 2350 cm2/V·s in AlGaN/GaN HEMT Grown on GaN Substrate

“…This results in an obvious roughening of the substrate surface, which increases the dislocation density and surface roughness of the epilayers. [15][16][17][18] Therefore, one challenge for growing high-quality GaN HEMT on native GaN substrates by MOCVD is preserving the atomically flat surface of the GaN substrate during the heating treatment. GaN decomposition has been widely studied.…”