S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Material Science and Engineering (재료공학부) Theses (Ph.D. / Sc.D._재료공학부)
Electrical and Interfacial Characterization of GaSb MOS Capacitors by Using Sulfuric Passivation, and Post- and Pre-Deposition Rapid Thermal Process
- 공과대학 재료공학부
- Issue Date
- 서울대학교 대학원
- GaSb; interface state; Fermi-level pinning; Ga oxide; elemental Sb; atomic layer deposition; sulfuric passivation; pre-deposition RTP
- 학위논문 (박사)-- 서울대학교 대학원 공과대학 재료공학부, 2017. 8. 김형준.
- GaSb has attracted significant attention as a strong channel candidate for next generation nanoscale logic metal oxide semiconductor field-effect transistors (MOSFETs), since it has extraordinary hole mobility(~ 3000 cm2/Vs) compared to conventional silicon devices, chemical resistance of its native oxides towards water, and high effective density of states(1.8 x 1019 cm-3). However, there are some drawbacks to adopt GaSb to the MOSFETs such as high interface states. When it comes to the operation of MOSFETs with Ⅲ-Ⅴ channel materials, because of the high interface states, the Fermi-level pinning phenomena lead to severe stretch-out of capacitance-voltage (C-V) and is one of the issues as well. The Fermi-level pinning comes especially from the native oxides and elemental Sb of GaSb upon exposure to oxygen. GaSb is also known to form the native oxides rapidly with air exposure, which makes it more difficult to reduce the native oxides.
In this study, GaSb metal oxide semiconductor (MOS) capacitors were fabricated with low temperature atomic layer deposition (ALD). Electrical properties were evaluated by C-V frequency dispersion and interface traps density (Dit) from the Terman method. Interfacial analysis was performed to investigate the compositions, roughness, thickness, and density by Auger electron spectroscopy (AES), spectroscopic ellipsometer (SE), x-ray reflectometry (XRR), atomic-force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM).
Low temperature ALD was conducted to deposit Al2O3 on top of GaSb substrates as a gate oxide at various deposition process temperatures(100 ~ 310 ℃). It showed that the Dit level was lowered and the Fermi-level pinning behavior was alleviated in a C-V curve with low temperature ALD at 150 ℃. From the XPS results, the ratio of pure Ga2O3 over metastable Ga2O increased as the deposition temperature decreased and was highest with low temperature ALD at 150 ℃. The C-V curve deteriorated with low temperature ALD at 100 ℃. It can be explained by the XRR results that demonstrate the significant decrease in density of the Al2O3 film with low temperature ALD at 100 ℃.
To reduce the Dit, a post deposition annealing process with N2 ambient at various temperatures (150 ~ 300 ℃) for 30 seconds was adopted after depositing Al2O3. The stretch-out of the C-V curve was alleviated by post deposition rapid thermal process (RTP) at 250 ℃ but the C-V curve was more stretched out by the post-deposition RTP at 300 ℃. The roughness decreased as the RTP temperature increased, then increased when the RTP temperature was over 300 ℃. The ratio of pure Ga2O3 over metastable Ga2O increased as the RTP temperature increased then decreased when the RTP temperature was over 300 ℃. The Dit is very consistent with the interfacial results. Forming gas annealing (FGA) is known as one of the effective ways to reduce the Dit in the SiO2/Si systems by filling up dangling bonds with hydrogen. To verify these hydrogen annealing effects, Al2O3/GaSb had been annealed with hydrogen including gases (5% H2/95 % N2 and 10 % H2/90 % N2). Results from both experiments with two different gases showed that the reduction of pure Ga2O3 to metastable Ga2O occurred by hydrogen annealing. The ratio of Ga2O3 over Ga2O decreased as the process temperature increased and the flux of hydrogen increased. The stretch-out in the C-V curves became worse and the Dit level significantly increased by hydrogen annealing due to the decreased ratio of pure Ga2O3 over metastable Ga2O.
As mentioned before, GaSb forms the native oxide quickly with air exposure. Therefore, passivation is essential to be considered for fabricating the GaSb capacitors in ex-situ. Sulfuric passivation was chosen to be used, since the surface of GaSb can be passivated by forming Ga-S and Sb-S bonds, which would improve the electrical properties of the MOS capacitors. Each sample was immersed in the sulfuric solution (5 % (NH4)2S) for various times (1 ~ 15 minutes). The stretch-out of the C-V curve was successfully alleviated with immersion in the sulfuric solution for 5 minutes. However, immersion time longer than 5 minutes aggravated the C-V curves. The AFM results showed that the roughness increased as the immersion time increased. The inter layer (IL) between Al2O3 and GaSb also became longer with longer immersion duration.
Sb-oxide is not able to be perfectly cleaned by HCl even though HCl is known as the most effective wet chemical to get rid of the native oxides of GaSb. This remaining Sb-oxide oxidizes GaSb to Ga-oxide and changes itself to elemental Sb. For this, pre-deposition RTP (N2 ambient), for improving electrical properties of the Al2O3/GaSb MOS capacitors, was adopted for the first time in this study. The improvement of stretch-out of the C-V curves was outstanding. The the Dit was also successfully reduced by the pre-deposition RTP at 550 and 575 ℃ and this the Dit value is the lowest one among the ones of sulfur treated GaSb MOS capacitors in literature(1.06 x 1012 cm-2ev-1 @ E-Ev=0.004). The Fermi-level pinning phenomenon deteriorated by pre-deposition RTP at 500 ℃ because chemical reactions for making the native oxides were accelerated at 500 ℃. The accelerated chemical reactions were proven by XPS, AFM, AES, and TEM analysis. The ratio of Ga2O3 over Ga2O is also very consistent with electrical results.
In conclusion, low temperature ALD, post-deposition RTP, sulfuric passivation, and pre-deposition RTP are the effective ways to alleviate the Fermi-level pinning phenomenon that leads to the significant stretch-out in the C-V curves.