Fabrication of α-Ga2O3 Thin Films Using α-(Al x Ga1-x )2O3 Multi Buffer Layers and Its Crysatal Structure Properties

Abstract

These days, huge attention has been given to power devices using wide bandgap semiconductors, which can lead to energy-saving. We have taken interest in corundum-structured Ga2O3 (α-Ga2O3) having wider bandgap compared with those of SiC and GaN, because it is considered to have much higher endurance voltage and lower on-resistance [1]. By using mist CVD [2], which is a safe, cost-effective, and environmental-friendly growth technology, Sn-doped α-Ga2O3 thin films grown on sapphire substrates showed n-type conductivity with Hall mobility of ~20 cm2/Vs and carrier concentration of about 1019 cm-3,but the mobility markedly decreases when the carrier concentration was controlled at the range of 1017 cm-3 [3]. Because of the large lattice mismatch between α-Ga2O3 and sapphire (4.8 and 3.5% for a- and c-axes), α-Ga2O3 thin films grown on sapphire substrates contain a high dislocation density. Especially edge dislocation density was measured as 7×1010 cm-2 [4], which seems to deteriorate carrier conduction in the α-Ga2O3 layer. It is considered that dislocations trap free electrons and are negatively charged, diminishing the electron mobility due to electron scattering. Aiming at decreasing the dislocation density in α-Ga2O3 thin films and improving the electrical properties, in this study we report the fabrication of the α-Ga2O3 using α-(Al x Ga1-x )2O3 multilayers as a buffer layer overcoming the lattice mismatch between α-Ga2O3and sapphire. The α-Ga2O3 main layer and the α-(Al x Ga1-x )2O3 buffer layers were grown on c-plane sapphire substrate by the mist CVD method. Prior to the growth, a sapphire substrate was annealed in air at 1050oC for 12 hours to form atomically flat and clean surface. The buffer layer consists with 12 pairs of 20-nm thick α-(Al0.9Ga0.1)2O3/α-(Al0.2Ga0.8)2O3 multilayer structure. The thickness ratios for α-(Al0.9Ga0.1)2O3/α-(Al0.2Ga0.8)2O3 were gradually changed from 1 (on sapphire) to 0.09 (top of the buffer layer) so that the average composition of the buffer layer changed from Al-rich (close to sapphire) to Ga-rich (close to Ga2O3). Then, a 150-nm thick α-(Al0.2Ga0.8)2O3 layer was grown on the 12 pairs of α-(Al0.9Ga0.1)2O3/α-(Al0.2Ga0.8)2O3 layers, followed by the growth of a 150-nm thick α-Ga2O3 main layer. The growth of α-Ga2O3 and α-(Al x Ga1-x )2O3 layers were carried out at 470 and 700oC, respectively. From the reciprocal space map for (11-29) reflection, we attributed the three intense reflection patterns originating from the sapphire substrate, the thick α-(Al0.2Ga0.8)2O3 layer on the buffer layer (below the α-Ga2O3 main layer), and the α-Ga2O3 main layer. The α-Ga2O3 and α-(Al0.2Ga0.8)2O3 layers are lattice-relaxed with respect to the sapphire substrate and the α-Ga2O3layer seems to be subject to slight in-plane compressive strain. The cross-sectional transmission electron microscope (TEM) image of the sample observed along the <11-20> zone axis shows that the sample structure consisting with periodic multilayers of α-(Al0.9Ga0.1)2O3 and α-(Al0.2Ga0.8)2O3 is actually fabricated. The thickness of a unit pair of α-(Al0.9Ga0.1)2O3/α-(Al0.2Ga0.8)2O3 was recognized as about 20 nm, which was nearly the same as designed. Cross-sectional TEM images under two-beam conditions were obtained to discuss the dislocation structure of the buffer layers and the α-Ga2O3 main layer. These images reveal that most of the strains are concentrated in the α-(AlxGa1-x)2O3 buffer layers. The densities of screw and edge dislocations in the α-Ga2O3 thin film were approximated as 3×108 and 6×108 cm-2, respectively. By introducing the buffer layers, the edge dislocations in the α-Ga2O3 thin films was successfully reduced from 7×1010 cm-2 (in a α-Ga2O3 thin film directly grown on a sapphire substrate) to 6×108 cm-2 by more than two order of magnitude. By optimizing the structure and growth conditions of the buffer layers, the further improvements in quality of the α-Ga2O3main layer is expected, leading to attractive device performance using inexpensive and large-area sapphire substrates.