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Interfacial Characterization of GaAs MOS Capacitors and Passivation using High Pressure Oxidation and Ti Scavenging Effect

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Authors
임하진
Advisor
김형준
Major
공과대학 재료공학부
Issue Date
2015-02
Publisher
서울대학교 대학원
Keywords
GaAsGa oxide (Ga2O3)elemental Asinterface statehigh pressure oxidationscavenging effect
Description
학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2015. 2. 김형준.
Abstract
Recently GaAs has attracted great attention as a promising channel material for sub-10nm logic metal-oxide-semiconductor field effect transistor (MOSFET) due to its high electron mobility and relative large band gap, which allows a high speed operation and low off-state current of the resulting FETs compared to their conventional silicon devices. However, many problems to limit the realization of the MOSFET have still remained unsolved in spite of enormous amount of researches accumulated over the past 3 decades. For such problems, thermal instability of GaAs native oxide can be firstly considered as one of the most crucial issues. The instability of GaAs oxides tends to induce high density of interface states, resulting in Fermi level pinning and frequency dispersion in capacitance-voltage (C-V) curve.
In this study, MOS capacitors using GaAs and various gate dielectric films were fabricated. Electrical characterizations were performed to evaluate the interface properties through C-V hysteresis, frequency dispersion, EF movement efficiency and interface states density. Especially to evaluate the deep level interface states of GaAs, C-V and G-V measurements at elevated temperature up to 125 °C were tried in this study. Interfacial analysis by XPS, AES, and TEM were also performed to investigate the compositions and structures of GaAs surface.
Firstly electrical characterizations of GaAs MOS devices were examined to evaluate the interface states. Significant frequency dispersions in depletion region at high temperature suggest that higher density of interface states exist near the mid-gap. Consistent results were also obtained in the Dit distribution, extracted from conductance method. It was confirmed that huge interface states more than x1013 cm-2eV-1 was observed in the mid-gap region not band edge unlike Si showing the U-shape profiles.
To investigate the origin of the high mid-gap Dit, the effects of Ga oxide at GaAs interface were examined by adopting the thin Ga2O3 insertion layer, which was deposited by ALD on GaAs prior to gate dielectric film. For the Ga2O3 inserted samples, two times larger hysteresis and huge frequency dispersions in C-V curves compared to the control Al2O3 were observed indicating that the Ga2O3 acts as the defective species inducing high interface states. Different electrical results according to dielectric materials could be explained correlated with the amount of interface Ga2O3 which can be naturally formed during thermal process.
For the passivation of the GaAs interface, new approaches using high pressure oxidation and Ti scavenging effect were employed to suppress the Ga2O¬3 generation. For high pressure oxidation samples, hysteresis and frequency dispersion at the depletion range were substantially improved. Significant reduction of mid-gap Dit was also observed down to 3.4 x 1012 cm-2eV-1 at Ei + 0.25 eV and 6.2 x 1012 cm-2eV-1 at Ei - 0.2 eV in upper half and lower half band gap respectively, which was less than half level of un-passivated samples. These improvements are attributed to the composition rearrangement converting to As excess and Ga deficient surface. This composition surface can be obtained by selective oxidation of Ga and subsequent removing of Ga2O3 through high pressure oxidation and HF wet etching, respectively. This surface structure can effectively block oxygen transfer to the GaAs substrate and suppress the generation of interfacial Ga2O3. In this case, high pressure process was also verified to be more effective to form As layer compared to atmospheric pressure process.
For Ti passivation, reductions of hysteresis and interface states by ~40% were also obtained. From the XPS analysis much less Ga2O3 were observed in Ti inserted sample compared to control sample. Scavenging effect due to high oxygen affinity of Ti is believed to effectively suppress the surface oxidation resulting Ga2O3.
In conclusion, high pressure oxidation and insertion of Ti scavenging film are the effective ways to passivate the GaAs interface for controlling the formation of Ga2O3 at the interface.
Language
English
URI
https://hdl.handle.net/10371/118000
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College of Engineering/Engineering Practice School (공과대학/대학원)Dept. of Materials Science and Engineering (재료공학부)Theses (Ph.D. / Sc.D._재료공학부)
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