Effect of Ultra-high Vacuum Annealing on ALD-Y₂O₃/GaAs Heterostructure Studied by in-situ X-ray Photoelectron Spectroscopy
K. Y. Lin (林耕雍)1*, C. H. Fu (傅千驊)1, K. H. Chen (陳冠雄)1, Y. H. Lin (林延勳)1, L. B. Young (楊博宇)1, J. Kwo (郭瑞年)2, M. Hong (洪銘輝)1,3
1Department of Physics, National Taiwan University, Taipei, Taiwan
2Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
3Graduate Institute of Applied Physics, National Taiwan University, Taipei, Taiwan
* presenting author:Keng-Yung Lin, email:f02222024@ntu.edu.tw
Due to the relatively high electron mobility of GaAs and its small lattice mismatch with Si, GaAs metal-oxide-semiconductor (MOS) capacitors have been intensively studied, aiming for attaining high speed, low power electronic devices. However, most high-κ dielectric/III-V heterostructures suffer from poor oxide-semiconductor interfaces. Effective oxide passivation having a low interfacial trap density (Dit) is the key to reliable high-performance devices, meeting the market demands. In this work, atomic layer deposition (ALD)-Y2O3 was in-situ deposited on freshly molecular beam epitaxy (MBE) grown GaAs. Taking advantage of ultra-high vacuum (UHV) modules between growth and analysis chambers, in-situ X-ray photoelectron spectroscopy (XPS) was sequentially utilized to examine GaAs clean surface and Y2O3/GaAs heterostructures, from sub-nanometer Y2O3 coverage to 5 nm thick Y2O3 thin film. It is found that O 1s core level spectra show two peak profiles since the early stage of Y2O3 deposition, representing emission signals of O atoms in Y-O-Y (Y2O3) bonding for the low binding energy (BE) peak and O atoms in Y-OH bonding for the high BE peak respectively. Furthermore, Y-OH species mainly reside on the top atomic layers of Y2O3, which is confirmed by the enhanced peak intensity of high BE component when performing surface-sensitive off-normal measurements. The existence of Y-OH bonding suggests that yttrium hydroxides forms during ALD growth since every deposition cycle ends with a pulse of H2O vapor. To minimize the amount of yttrium hydroxides near the interface, a UHV annealing to 550 oC was intentionally designed at 1 nm Y2O3 thickness on an n-type GaAs sample to attain better interfacial quality. A great reduction of Y-OH bonding from 33% to 11% was unveiled by XPS measurement. We compared capacitance-voltage (C-V) and quasi-static capacitance-voltage (QSCV) characteristics of two n-GaAs samples using the same Al2O3/Y2O3 gate stack without and with a UHV annealing performed at 1 nm thick Y2O3 deposition, followed by the same MOS capacitor fabrication processes. A reduced frequency dispersion at accumulation from 15.6% to 10.3% (100 Hz - 1 MHz) and a reduced Cvalley at depletion from 39.7% Cox to 30.9% Cox indicate that the UHV annealing has greatly improved Y2O3/GaAs interface. These results show that UHV annealing performed at early stage of oxide deposition have optimized the interface quality, which is a basic to attain high performance MOS devices.

Keywords: Ultra-high Vacuum Annealing, GaAs, Atomic Layer Deposition, Metal-Oxide-Semiconductor