Self-forming and self-decomposing gallium oxide layers at the GaN/Al2O3 interfaces

We examine the energy band diagram at the interface between GaN and Al2O3 containing negatively-charged oxygen interstitial ( O i 2 − ) defects. At a p-type GaN (p-GaN)/Al2O3 interface, oxygen atoms and electrons are emitted from the O i 2 − defects causing interfacial oxidation resulting in the self-formation of a p-GaN/Ga2O3/Al2O3 structure. On the other hand, such the reactions do not occur at an n-type GaN (n-GaN)/Al2O3 interface. Moreover, when n-GaN/Ga2O3/Al2O3 structures are formed, the Ga2O3 layers spontaneously decompose to form O i 2 − defects in the Al2O3. Consequently, our proposed Ga2O3 formation mechanism gives completely different results for p-GaN/Al2O3 and n-GaN/Al2O3 interfaces.

G allium Nitride (GaN) power devices can be operated with high electric fields and high power densities, such that devices with lower energy consumption than those using Si or SiC can be realized. 1,2) In particular, normally-off GaN metal-oxide-semiconductor field-effecttransistors (MOSFETs) with high threshold voltages can be fabricated, and these have been studied because of their advantages in terms of safety and operational stability. [3][4][5] The deposition of dielectric films on GaN is one of the essential processes required to make MOS structures. To obtain high-performance GaN MOS devices with high carrier mobility and large breakdown fields, it is important to select an appropriate dielectric and to optimize the process by which it is formed. In Si and SiC MOS devices, SiO 2 dielectric films are grown by thermal oxidation, [6][7][8] and defect-free Si/SiO 2 interfaces can be formed for Si MOS devices. In the case of GaN, the dielectric layers for MOS devices are formed by deposition, and many studies have been done on various materials for this.  Although there are several candidates for the dielectric, it is only with materials that have a large conduction band offset (CBO) or valence band offset (VBO) for GaN that a large threshold voltage can be obtained. Of these, SiO 2 and Al 2 O 3 have particularly large bandgaps. Since both SiO 2 and Al 2 O 3 have large VBO (3.0 eV∼) and CBO (2.0 eV∼) for GaN, 30) it is possible to utilize them for both n-channel and p-channel MOSFETs, and the formation of interfaces with low interfacial state densities have been reported. 28,29) GaN/Insulator interfacial structures have been examined in a number of experiments, and the formation of a Ga 2 O 3 interlayer at a GaN/SiO 2 interface has been reported. 31) In previous work, we proposed a mechanism for the formation of the Ga 2 O 3 interlayer at an n-type GaN (n-GaN)/SiO 2 interface. 32) In this mechanism, electron transfer from n-GaN to SiO 2 and the formation of oxygen vacancy (V O ) defects in the SiO 2 induced an energy gain, causing interfacial oxidation and resulting in the spontaneous formation of Ga 2 O 3 . As in the case of GaN/SiO 2 interfaces, we guess that there are some mechanisms by which a Ga 2 O 3 interlayer spontaneously can be formed at a GaN/Al 2 O 3 interfaces. Therefore, to discuss the reactions and structure arising at the GaN/Al 2 O 3 interface, we focus on oxygen interstitial (O i ) defects in Al 2 O 3 , and consider the band offset and the position of the Fermi level (E fermi ) at the interface. As a result, we find that Ga 2 O 3 layers are spontaneously formed at p-GaN/Al 2 O 3 interfaces, where the O i defects trigger the interfacial reaction, but are not formed at n-GaN/Al 2 O 3 interfaces. Moreover, we also find that the Ga 2 O 3 layer formed between n-GaN and Al 2 O 3 spontaneously decreases or disappears.
First, we discuss the O i defects in Al 2 O 3 . In this study, we utilize Al 2 O 3 with the corundum structure (α-Al 2 O 3 ) which is one of the stable forms of the crystal with a bandgap of 8.8 eV. Theoretical calculations have predicted that O i defects arise in α-Al 2 O 3 , and that these defects have a split interstitial structure with O-O bonds in a neutral state. 33,34) The formation energy of these defects (E form O i ) is given by where E Bulk and E O i are the total energies of α-Al 2 O 3 without and with an O i defect, and m O 2 is the chemical potential of the O 2 molecules. E form O i is about 4.0 eV. 33,34) In addition, it has been reported that an O i defect traps two electrons and changes its charge state from q = 0 to q = -2 ( -O defect i 2 ), resulting in the formation of defect a level at the valence band maximum (VBM) + 1.0 eV. 33) Next, we discuss the p-GaN/Al 2 O 3 interface. The band offset of this interface is shown in Fig. 1(a). We assume that the Fermi level of p-GaN is located at the VBM. Since the Fermi level is located above the -O defect In order to induce this emission reaction, a mechanism to generate energy (0.8 eV) at the interface is required. Now, we discuss the formation of the Ga 2 O 3 layer. When O atoms are released from the Al 2 O 3 , they give rise to oxidation at the interface. The energy gain due to thermal oxidation of GaN (E oxi ) shown by the following reaction has been reported as being = E 3.20 eV oxi 32) This energy gain means that thermal oxidation of GaN proceeds spontaneously when the O 2 molecules have larger energy than the activation energy. From Eqs. (2) and (3), the complete reaction leading to oxidation at the interface is as follows The formation energy for this reaction (E form all ) can be obtained and is E form all =´-=-( ) 2.5 0.8 eV 3.2 eV 1.2 eV, which means that the energy loss due to electron transfer is compensated by the energy gain by oxidation. Thus, this analysis of the O i defects in Al 2 O 3 and the electron transfer reveal that an interfacial Ga 2 O 3 layer self-forms at the p-GaN/Al 2 O 3 interface [ Fig. 1(c)]. In thermal oxidation of GaN described in Eq. (3), we have considered that GaN is completely oxidized to become Ga 2 O 3 . However, when some parts of GaN are not oxidized, there is a possibility that a GaON layer is formed instead of Ga 2 O 3 . In this case, oxidation reaction is described as follows where α ranges from 0.0 to 1.0, and energy gain by this reaction is a =É 3.20 eV.
oxi By combining Eqs. (2) and (5), the formation energy of all reaction can be obtained and is E form eV. Accordingly, our proposed interfacial oxidation mechanism can form GaON layer instead of Ga 2 O 3 when α is larger than 0.625.
So far, we have focused only the p-GaN/Al 2 O 3 interface; now, we focus on the n-GaN/Al 2 O 3 interface and discuss the formation of a Ga 2 O 3 interlayer by the above mechanism. We assume that the Fermi level is located close to the CBO of GaN [ Fig. 2(a)]. Since the Fermi level in n-GaN is located 3.3 eV above that in p-GaN, it is considered that the O i defects trap electrons and change to -O i 2 defect structures as with the p-GaN/Al 2 O 3 system [ Fig. 2(b)]. However, as the Fermi level rises, the energy loss due to electron transfer from the  The Gibbs free energy of formation in this reaction has been reported to be 1.9 eV. 35 (Fig. 3). When the GaON layer is formed instead of the Ga 2 O 3 at the interface, decomposition reaction is described as follows By considering the difference in the Gibbs free energy between the O 2 and N 2 molecules (0.2 eV at 1500 K), 36) formation energy in this reaction is estimated to be a + -( )/ 1.9 eV 2 1 2 0.2 eV. In addition, since the number of eV and becomes smaller than the case of Ga 2 O 3 interlayer. Accordingly, when α is larger than 0.424, GaON decomposition reaction at the n-GaN/Al 2 O 3 interface can occur, but when it is smaller than 0.424, GaON can exist stably at the interface. In some experimental reports, Ga 2 O 3 on the n-GaN has been studied by XPS spectra and they showed the reduction of the peak derived from Ga-O bond by forming n-GaN/Ga 2 O 3 /Al 2 O 3 , which is consistent with our prediction on Ga 2 O 3 decomposition. 37   Finally, we discuss the formation of interfacial dipoles induced by electron transfer at GaN/Al 2 O 3 interfaces. As shown in Fig. 1(b) Fig. 1(c)]. On the other hand, the interfacial dipoles are not eliminated from n-GaN/Al 2 O 3 interfaces [ Fig. 2(b) Fig. 2(c)]. In other words, we can obtain n-GaN/Al 2 O 3 interfaces with larger CBO by forming Al 2 O 3 with higher O i defect densities on n-GaN, and this can lead to lower gate leakage currents in MOSFETs.
In this paper, we have focused on the O i defects and interfacial reactions caused by them, and revealed that p-GaN and n-GaN shows completely different interfacial structures. On the other hand, it is considered that V O defects also cause large effects for the electronic characteristics at the interfaces and in the oxides. In our previous work, we proposed interfacial reaction mechanism induced by the V O defect formation at the n-GaN/SiO 2 interfaces. 32) However, since formation energy of the V O defects in Al 2 O 3 is very large, such the interfacial reaction do not occur at the n-GaN/Al 2 O 3 interfaces. Thus, although the V O defects are very important, they do not cause different interfacial reactions for both p-GaN and n-GaN.
In conclusion, on the basis of an examination of the O i defects in Al 2 O 3 and the position of the Fermi level at p-GaN/Al 2 O 3 and n-GaN/Al 2 O 3 interfaces, we have proposed a mechanism for interfacial oxidation and revealed the differences between the reactions taking place and the structures formed at these two different interfaces. This work was conducted as part of a project entrusted with "research and development for next generation of power devices that contributes to the realization of an energy saving society" (MEXT; Ministry of Education, Culture, Sports, Science and Technology).