(Invited) First Principles and Themodynamical Studies on Matel Organic Vaper Phase Epitaxy of GaN
Authors: Shiraishi Kenji , Sekiguchi Kazuki , Shirakawa Hiroki , Chokawa Kenta , Araidai Masaaki , Kangawa Yoshihiro , Kakimoto Koichi 
Published: February 2020

The III-nitride semiconductors, GaN, AlN and InN, have direct bandgaps and the bandgaps of their alloys can cover a wide range. Therefore, they are widely used as materials for optical devices. Moreover, they have received considerable attention as materials for future high performance power devices [1,2]. To develop these devices, it is crucial to fabricate high-quality GaN crystals by epitaxial growth. For their epitaxial growth, metal organic vapor phase epitaxy (MOVPE) is generally used. In GaN, AlN and InN MOVPE, trimethylgallium (TMG), trimethylaluminum (TMA) and trimethylindium (TMI) are used for the group III sources, respectively. NH3 is used for the nitride source, and H2 and N2are used as carrier gases. In MOVPE, TMG, TMA and TMI should decompose into gas phase Ga, Al and In atoms, respectively. The chemical equation can be written as

Ga(CH3)3(g) + 3/2 H2(g) → Ga(g) + 3CH4(g)

There are many studies which have investigated their decomposition reactions[3,4]. However, the details have not been clarified and also many problems have been reported from experiments. In the case of GaN MOVPE, the gas phase reaction for the formation of the (CH3)2GaNH2 adduct has been reported [5,6], and these authors believe that GaN nanoparticles were formed from the (CH3)2GaNH2adduct in the gas phase reaction near the surface. On the other hand, it is also thought that Ga is incorporated as gas phase Ga atoms on the GaN growth surface and that these migrate along the surface [7,8].

In this paper, we carry out a theoretical investigation of the MOVPE growth processes of GaN and present a theoretical discussion of the decomposition processes of the source gases. In this study, we performed first-principles calculations and thermodynamic analyses to investigate the reaction processes of TMG and whether the (CH3)2GaNH2, adducts can be formed in the gas phase reaction during MOVPE [9].

We investigated the decomposition process of TMG for GaN MOVPE. In this presentation, we discuss the reactions by which TMG with H2 decomposes into dimethylgallium (DMG, [Ga(CH3)2]), monomethylgallium (MMG, [Ga(CH3)]), and gas phase Ga atoms and the reaction by which TMG with NH3 forms the (CH3)2GaNH2adduct. The reaction formulas can be written as follows.

(a) Ga(CH3)3(g) + H2(g) → Ga(CH3)2(g) + CH4(g)

(b) Ga(CH3)3(g) + H2(g) → Ga(CH3)(g) + 2CH4(g)

(c) Ga(CH3)3(g) + H2(g) → Ga(g) + 3CH4(g)

(d) Ga(CH3)3(g) + NH3(g) → (CH3)2GaNH2(g) + CH4(g)

Fig. 1 shows ΔG for the above four reactions. The calculations show that ΔG for reaction (d), through which TMG forms the (CH3)2GaNH2 adduct, is negative at any temperature and has the lowest value of the four reactions between 0 K and 280 K. That is, TMG reacts with NH3 and forms the (CH3)2GaNH2 adduct in this temperature range. Moreover, in reaction (b) TMG decomposing into MMG is the easiest reaction occurring between 280 and 570 K, and in reaction (c) TMG decomposing into gas phase Ga atoms is the easiest occurring above 570 K. Considering the substrate temperature of 1300 K, almost all the TMG decomposes into gas phase Ga atoms and the Ga atoms migrate on the GaN growth surfaces. TMG hardly reacts with NH3 and the formation of the (CH3)2GaNH2 adduct cannot occur, although there have been reports which claim that the (CH3)2GaNH2adduct forms [5,6].

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Figure 1