Generation of two-dimensional electron gas to normally depleted AlGaN/GaN hetero-interface by SiO 2 deposition and subsequent high-temperature annealing

SiO 2 ﬁlm deposition and subsequent high-temperature annealing re- sulted in the generation of a two-dimensional electron gas (2DEG) at Al(Ga)N/GaN hetero-interfaces, of which the 2DEG was originally fully depleted. The obtained mobilities and sheet carrier concentrations were over 1100 cm 2 /Vs and 3.0 × 10 12 cm –2 , respectively. Surface en- ergy lowering, which is proof of the generated 2DEG, was observed by electron state analysis using hard X-ray photoelectron spectroscopy. This damage-less method that selectively generates a 2DEG can contribute not only toward improving some characteristics in existing de- vices but also toward creating entirely novel devices.

Introduction: AlGaN/GaN high-electron-mobility transistors (HEMTs) are currently being applied to high-power RF devices [1,2], as well as to high-power switching devices [3,4]. This is due to the high electric breakdown field of GaN and a high electron mobility of two-dimensional electron gas (2DEG) at an AlGaN/GaN hetero-interface. Although it is impossible to selectively control the 2DEG density, the 2DEG density of these conventional AlGaN/GaN HEMTs can be controlled by adjusting the Al composition and thickness of the AlGaN barrier layer [1,2,5]. These adjustments are mostly performed during epitaxial growth. Moreover, the dry etching process can be employed to selectively decrease the 2DEG density by reducing the thickness of the AlGaN barrier layer [3,4]; however, it causes the deterioration and dispersion of some device characteristics. Alternatively, SiN deposition and fluorine plasma exposure on an AlGaN barrier layer have been reported to selectively control the 2DEG density without adjusting the thickness and Al composition of the barrier layer [6][7][8]. However, these studies were achieved by using a hetero-epitaxial structure, of which the 2DEG was not fully depleted [6][7][8], and the reduction of sheet resistances (R sh ) by SiN deposition was limited by approximately one order of magnitude [8]. Therefore, based on these reports, fluorine plasma exposure must be indispensable to obtain enhancement mode (E-mode) operation [6][7][8] although it causes the deterioration and dispersion of some characteristics.
In this study, a damage-less method to selectively generate a 2DEG at normally depleted AlGaN/GaN hetero-interfaces is demonstrated. This method can contribute not only in improving some characteristics in already existing devices, such as E-mode HEMTs, but also to create novel devices.
Device structure: Figure 1 shows a cross-sectional structure of the samples used to investigate the characteristics of the 2DEG. Al(Ga)N/GaN hetero-epitaxial layers were grown by metal-organic chemical vapour deposition on semi-insulated SiC substrates. Three kinds of barrier layers consisting of 1 nm-thick AlN, 5 nm-thick Al 0.20 Ga 0.80 N and 7 nmthick Al 0.15 Ga 0.85 N were investigated. The 2DEG carrier densities were calculated and found to be less than 1.0 × 10 7 cm -2 . The calculations were performed using the freely available software BandEng (a band diagram simulator developed by Michael Grundmann at the University of California at Santa Barbara) [9]. Fermi-level pinning locations in the calculations were interpolated and extrapolated as 0.95 eV of GaN and 1.7 eV of Al 0.32 Ga 0.68 N, which were experimentally inferred by Heikman et al. [10].
Ohmic contact regions were formed by Si-ion implantation and subsequent activation annealing at 1150°C for 5 min in N 2 atmosphere [5, After the formation of the device isolation regions by ion implantation [12], 30 nm-thick SiO 2 films were deposited by plasma-enhanced chemical vapour deposition. Then post-deposition annealing in N 2 atmosphere was repeatedly performed at a temperature of 300-950°C. Finally, the SiO 2 films were removed using a buffered hydrogen fluoride (BHF) solution. Current density-voltage (I-V) characteristics and Hall effect measurements were performed after each process in the same samples. In addition, no dry etching and fluorine plasma exposure processes were performed during fabrication. Therefore, the degradation and dispersion of the device characteristics would not be considered in this method.
Results and discussion: I-V curves between two ohmic electrodes before and after SiO 2 deposition, after annealing at 800°C, and after SiO 2 removal by BHF are shown in Figure 2. The distance (d) between the Si-ion-implanted region and the two electrode widths was 10 and 100 μm. In addition, R sh was determined by transmission line measurement (TLM) using the ohmic patterns with different d values of 2, 4, 7, 10, 20, and 50 μm as shown in Figure 2. At the initial state (before SiO 2 deposition) in all samples, the currents were below 1 × 10 -9 A/mm and R sh was sufficiently high, over 1 G /sq. It was confirmed that 2DEG at the hetero-interfaces were fully depleted. The currents had drastically increased after SiO 2 deposition in all the samples. Furthermore, the lowest R sh of 17 k /sq was obtained in the sample with the 7 nm-thick Al 0.15 Ga 0.85 N barrier layer, which was still much higher than that of the 2DEG in conventional AlGaN/GaN HEMTs [5]. Subsequent annealing at 800°C increased the current even more. The R sh in the two samples with the AlGaN barrier layers was reduced to 2.0 k /sq, which is nearly competitive to the conventional one [5]. The sample with the 1 nm-thick AlN barrier layer had the lowest current and highest R sh value compared to those of the samples with the AlGaN barrier layers. It was also confirmed that the enhanced currents had decreased significantly and were recovered to their initial states by SiO 2 removal. Moreover, the obtained variations of R sh were significantly larger than those obtained after SiN deposition [6][7][8]. Figure 3 shows the annealing temperature dependences of R sh estimated from TLM. High-temperature annealing decreased R sh and the lowest values were obtained at 800-900°C. Further, annealing over 950°C had significantly deteriorated R sh .    Table 1. For the two samples with the AlGaN barrier layers, the obtained N s and μ hall were over 3.0 × 10 12 cm -2 and 1100 cm 2 /Vs, respectively. These are of the same order of magnitude as conventional AlGaN/GaN HEMTs [5]. The results proved the generation of the 2DEGs at the heterointerfaces caused by SiO 2 deposition and subsequent high-temperature annealing. The sample with the 1-nm-thick AlN barrier layer had the lowest μ hall compared to those obtained for the samples with the Al-GaN barrier layers although comparable N s was obtained. Therefore, the 2DEG with the lowest mobility was generated at the AlN/GaN heterointerface. This low mobility was caused by considerable surface scattering due to the thin barrier layer (1 nm). In addition, the drastic current decrease after the SiO 2 removal was attributed to the complete disappearance of the 2DEG.
Electron state analysis using hard X-ray photoelectron spectroscopy (HAXPES) was performed to clarify the phenomenon of 2DEG generation. The HAXPES experiments were conducted at SPring-8 BL16XU with an incident X-ray energy of 7948.2 eV, pass energy of 200 eV, and photoelectron take-off angle of 88°. Figure 4 shows the Al 1s core level spectra of the four samples before and after the SiO 2 deposition, after annealing at 800°C, and after SiO 2 removal. In these samples, the thicknesses of SiO 2 films (∼10 nm) were sufficiently thin to detect photoelectrons from the AlGaN/SiO 2 interface. After SiO 2 deposition, the peak shifted toward a deeper energy state from the initial state. This energy shift indicates a reduction of surface potential that causes an increase in the 2DEG. The reduction in energy could have been led by an increase in positive charges and/or enhancement of the polarization effect due to SiO 2 deposition. Moreover, after annealing at 800°C, the spectrum did not change despite the decrease of R sh , as shown in Figure 2. This result suggests the existence of a donor that is highly concentrated just above the surface Fermi energy and behaves as a positive charge at the AlGaN surface. The spectrum after SiO 2 removal completely overlapped with that of the one in the initial state. The electron states at the AlGaN surface were entirely restored and the 2DEG disappeared, as shown in the previous paragraph.

Conclusion:
The method used to generate 2DEG at normally depleted AlGaN/GaN hetero-interfaces without varying thickness and Al composition of the AlGaN barrier layer were acknowledged. In this method, SiO 2 deposition and subsequent high-temperature annealing were performed. The variations in R sh were over 10 orders of magnitude and the obtained N s and mobility were over 3.0 × 10 12 cm -2 and 1110 cm 2 /Vs, respectively. Electron state analysis using HAXPES revealed that the surface energy was lowered by SiO 2 deposition. This reduction of energy is attributed to 2DEG generation. It was also confirmed that the generated 2DEG completely disappeared after SiO 2 removal. Therefore, damage-less, and selective generation of 2DEG can be realized using this method without using any dry etching and fluorine plasma exposure processes.