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Effect of metallic surface coverage on material quality in III-nitride MOVPE


Figure 1 R&D MOVPE reactor Epiquip VP-50RP redesigned for III-N growth

A number of studies revealed that in MBE an increase of the Ga coverage on a GaN surface resulted in the suppression of the pit-type defect formation. Also, improvement of the surface morphology was found to be related to an addition of so-called “surfactant” atoms that do not incorporate into the growing crystal but affect the adatom diffusion. Here, we study the effects of the ammonia flow and the addition of trimethylindium into the gas mixture on the GaN surface morphology under MOVPE conditions.

The GaN surface coverage with Ga and In atoms was calculated by means of the CVDSimTM software. The general model of GaN growth was utilized, supplemented with the surface reactions describing etching of GaN in a hydrogen atmosphere [1], adsorption/desorption of indium-containing species, and a mechanism for indium particle formation in the gas phase.

Figure 2(a) Model prediction for variation of atomic Ga coverage on GaN surface with ammonia flow in Epiquip reactor.

Figure 2(b) SEM images of GaN surface grown in Epiquip reactor under conditions listed in Fig. 2(a)

It is seen in Fig. 2(b) that at the highest ammonia flow rate of 2500 sccm the surface is almost entirely covered with pinholes. Some of the pinholes are larger in diameter and are scattered with a lower density across the surface. The other group represents much smaller pinholes distributed with a higher surface density. Upon the double reduction of the ammonia flow (to 1250 sccm) the larger pinholes disappear, while the smaller ones become more sparsely packed. The further reduction of ammonia results in the surface free of pinholes (at 500 sccm flow).

At the same time, Fig. 2(a) demonstrates a monotonic increase of the calculated gallium surface coverage with the lower ammonia flow rate. According to experimental studies, the presence of hydrogen vapor during the high-temperature GaN growth results in etching of the GaN surface, while gas-phase ammonia acts in an opposite direction and stabilizes the growth. During the GaN decomposition in a hydrogen ambient, the Ga atoms first undergo transition from the crystal into the adsorption layer and than desorb from the adlayer into the gas-phase. A previously elaborated and verified model of the GaN etching in a hydrogen-ammonia atmosphere [1] allows us to assume that while the desorption stage is highly thermally activated (with an activation energy of around 2–2.5 eV), the former stage can proceed even at reduced temperatures. This is the case considered in the present paper: via interaction with hydrogen, the Ga atoms are accumulated in the adsorption layer, but the temperature of 850 oC is not high enough to activate their desorption. Thus, the presence of surface gallium favors the overgrowth of pinholes and improvement of the morphology.

Figure 3(a) Model prediction for variation of atomic In and Ga coverage on GaN surface under variation of TMI supply in Epiquip VP50-RP.

Figure 3(b) SEM images of GaN surface grown in Epiquip VP50-RP reactor under conditions listed in Fig. 3(a)

It is easily seen from Fig. 3, that the pinhole surface density gradually decreases with the enhancement of the TMI supply. At the same time, the model calculations demonstrate a monotonic increase of the indium coverage and a certain decrease of the gallium coverage with the TMI flow. The latter is obviously related to an interaction between the adsorbed Ga- and In- atoms, which means that a higher In coverage provides a smaller number of free surface sites for the Ga atoms to occupy.

Figure 4(a) Model prediction for variation of atomic In coverage on GaN surface under variation of TMI supply in Aix2000HT reactor.

Figure 4(b) SEM images of GaN surface grown in Aix2000HT reactor under conditions listed in Fig. 4(a)

SEM images of GaN grown in Aix200HT in a nitrogen atmosphere (Fig. 4 (b)) show that the injection of 200 sccm of TMI decreases the pinhole density, whereas the further addition of TMI does not affect the density noticeably. The computational dependence (Fig. 4 (a)) of the indium coverage on TMI (the gallium coverage is almost negligible due to no decomposition in the absence of hydrogen) demonstrates its monotonic increase up to 200 sccm of TMI and gradual saturation. In the computations almost all additional indium in the gas phase was converted into particles at TMI flow rates of 200 sccm and higher. All "excess" indium injected during GaN growth acts as a surfactant improving the surface morphology.

The obtained results have demonstrated that an ammonia flow reduction and an addition of trimethylindium decrease the pinhole density and improve the surface morphology in the MOVPE growth of GaN. The computations performed using a model of GaN etching associate the morphology improvement with an accumulation of metallic atoms (Ga or In) in the adsorption layer over the GaN surface. This assumption agrees with the previous observations on the positive role of gallium adatoms in GaN growth by MBE. The model can be considered as a tool capable of suggesting ways to control the morphology.

[1] E.E. Zavarin, D.S. Sizov, W.V. Lundin, A.F. Tsatsulnikov, R.A. Talalaev, A.V. Kondratyev, and O.V. Bord, Electrochem. Soc.Proc. 2005-09, 299 (2005).

[2]Full text: "Effect of metallic surface coverage on material quality in III-nitride MOVPE"
A. V. Kondratyev, R. A. Talalaev, A. S. Segal, E. V. Yakovlev, W. V. Lundin, E. E. Zavarin, M. A. Sinitsyn, A. F. Tsatsulnikov, and A.E. Nikolaev
phys. stat. sol. (c) 5, No. 6, 1691–1694 (2008)


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