Determination of Band Structure at GaAs/4H-SiC Heterojunctions
Authors: Liang Jianbo , Shimizu Sae , Arai Manabu , Shigekawa Naoteru 
Published: February 2020

Introduction:
Heterojunctions are widely used for fabricating high performance electrical and optical devices. The heterojunction fabricated by conventional epitaxial growth generally introduces a large number of crystalline defect densities at the interface due to the large differences in crystal lattice and thermal expansion coefficient. To solve these difficulties a variety of wafer bonding methods have been employed to fabricate semiconductor hetero-structure. Among them the most effective candidate is surface activated bonding (SAB) [1, 2]. In this method, surfaces of substrates are activated by the irradiation of Ar fast atom beams prior to bonding and consequently bring into contact in a vacuum condition. It enables us to fabricate heterojunctions composed of dissimilar materials with large mismatched lattice constants. We previously fabricated p-Si/n-GaAs and p-Si/n-SiC heterojunctions and investigated their electrical properties [3, 4]. It was found that the energy band diagram of p-Si/n-GaAs and p-Si/n-SiC junctions revealed type-II and type-I features, respectively. Furthermore, their conduction band discontinuities were calculated to be 0.59 and 0.3 eV, respectively. If SAB-based GaAs/SiC heterojunction are realized, the conduction band discontinuity of less than 0.3 eV should be expected, which should be suitable for fabricating devices with excellent high-frequency, high-temperature, and high-power performances that have not yet been achievable by each component material alone.
In this work, we fabricated p+-GaAs/n-SiC heterojunction by using SAB method and have investigated the annealing temperature dependence of the band structure of GaAs/SiC heterojunction. The influence of the thermal annealing process on the electrical properties of the junctions were systematically investigated by measuring their current-voltage (I-V) and capacitance-voltage (C-V) characteristics. The applicability of GaAs/SiC heterojunctions for functional devices was explored based on these measurements.

Experiments:
p+-GaAs (100) epitaxial substrates (400 nm, ~ 1 × 1019 cm-3 epitaxial layer / substrate ~ 1 × 1018 cm-3) and n-4H-SiC epitaxial substrates (2.8 mm, 1.1 × 1017 cm-3 epitaxial layer / 0.5 mm, > 2 × 1018 cm-3 buffer layer / substrate ~ 1 × 1019 cm-3) were used for the bonding experiment. Before bonding Al/Ni/Au multilayers were evaporated on the backside of n-SiC substrates. The ohmic contacts of n-SiC substrates were formed by a rapid thermal annealing at 1000 °C for 60 s in N2 gas ambient. p+-GaAs epitaxial substrates and n-SiC epitaxial substrates were bonded to each other by using SAB [1, 2]. After the bonding, AuZn/Ti/Au multilayers were evaporated on the bottom surfaces of p-GaAs substrates. The p+-GaAs/n-SiC heterojunctions were annealed separately at 100, 200, 300, and 400 °C for 60 s in N2 gas ambient. All the samples were diced into 4 mm2 pieces. The characteristics of these samples were also investigated while the junction was unannealed. Their I-V and C-V characteristics were measured using an ADCMT 6242 Source Measurement Unit and an Agilent E4980A Precision Impedance Analyzer, respectively.

Results:
The 1/C2-V characteristics measured at room temperature and a frequency of 100 kHz are shown in Figure 1. The characteristics indicated a straight line and the flat-band voltages () were found to be 1.12, 1.12, 1.12, 1.13 and 1.29 V for the unannealed junction and junctions annealed at 100, 200, 300, and 400 °C, respectively, by linearly extrapolating 1/C2
to zero. It is noteworthy that the flat-band voltages remained constant as the annealing temperature increased up to 200 °C and then increased as the annealing temperature increased from 300 to 400 °C. Using the slopes of 1/C2-V characteristics, the donor concentrations of the n-SiC epitaxial layer were estimated to be 8.14 × 101
6, 8.02 × 1016, 7.98 × 1016, 7.93 × 1016, and 6.94 × 1016 cm-3, for the unannealed junction and junctions annealed at 100, 200, 300, and 400 °C, respectively, which are close to the norminal value of 1.1 × 1017 cm-3, as determined by Ni/n-SiC Schottky diode.
Figure 1. C-V characteristics of p+-GaAs/n-SiC heterojunctions without being annealed and annealed at 100, 200, 300, and 400 °C measured at room temperature.

References

[1] H. Takagi, K. Kikuchi, R. Maeda, T. R. Chung, T. Suga, Appl. Phys. Lett. 68, 2222 (1996).
[2] S. Essig, O. Moutanabbir, A. Wekkeli, H. Nahme, E. Oliva, A. W. Bett, F. Dimroth, J. Appl. Phys. 113, 203512 (2013).
[3] J. Liang, S. Nishida, M. Arai, N. Shigekawa, Appl. Phys. Lett. 104, 161604 (2014).
[4] J. Liang, T. Miyazaki, M. Morimoto, S. Nishida, N. Watanabe, and N. Shigekawa, Appl. Phys. Express 6, 021801 (2013).

Figure 1