NDLI: First-Principles Study of GeO2/Ge Interfacial Traps and Oxide Defects

Content Provider	IEEE Xplore Digital Library
Author	Shang-Chun Lu Hung-Chih Chang Tien-Pei Chou Cheewee Liu
Copyright Year	2012
Abstract	An important issue concerning Ge-MOSFETs is the quality of gate dielectric/Ge interfaces. The reported values for energy positions of the interfacial defects in Ge-MOSFETs vary across the literature. High interfacial trap density (Dit) close to the conduction band minimum (CBM) and/or the valence band maximum (VBM) have been reported. In this work, using density functional theory, we propose possible origins of the various GeO2/Ge interfacial states, and also address native defects in GeO2 to show their possible influence on Ge-MOSFETs. Previous theoretical study reveals the GeO2/Ge interface free of any dangling bond (DB), including the GeOx transition region (x<2), does not have any defect states within the Ge band gap. In order to study the electronic properties of Ge DBs in different oxidation states, two (100)Ge/GeO2 structures with DBs were constructed (Fig. 1). One structure includes DBs at the Ge1+ and Ge2+ sites, both three-fold coordinated and bonded with one and two oxygen atoms, respectively (Fig. 1(a)). Fig. 2 shows DBs of Ge1+ and Ge2+ form gap states within Ge band gap near the CBM. This suggests the previously observed interface states near the CBM [1] were originated from the DBs of Ge1+ or Ge2+. The other structure of GeO2/Ge interface with DBs attached to the defective Ge0+ atoms is shown in Fig. 1(b). Two Ge0+ sites, backbonded with three Ge atoms, have a DB each, and one of them was passivated by H to study the isolated DB state. The Ge3≡Ge∙ DB shows states near the VBM (Fig. 3) with two peaks representing spin-up and spin-down states, as discovered by experiment. The Dit near CBM may result from oxide-related defects due to oxygen vacancies near the interface, while Ge0+ DBs lead to the presence of Dit near the VBM which is relatively irrelevant to the oxidation conditions. To further investigate the impact of the oxide defects, the transition levels of an oxygen vacancy in GeO2 within different charge states were calculated. The PBE0 hybrid functional which corresponds to a fraction of Hartree-Fock exchange α=0.25 was applied to provide an estimate for the band gap of GeO2 (5.8 eV), similar to the experimental data (Fig. 4(b)). Charged defects were estimated by assuming the presence of a compensating uniform background, and the Coulomb energy error was corrected using the Makov-Payne expansion. Oxygen was removed from the 72-atoms perfect cell of GeO2 (Fig. 4(a)) to create an oxygen vacancy and the charges were removed to obtain different charge states for the defect structure. After relaxation, the defective GeO2 structure with two electrons removed formed a three-fold coordinated oxygen and an unoccupied Ge dangling bond (Fig. 5). The additional bonding of O stabilizes the +2 charge state by lowering its formation energy. The formation energies of GeO2 oxygen vacancy in various charge states are plotted in Fig. 6. It shows the negative-U property of the oxygen vacancy in GeO2, i.e., (+1/0) lower than (+2/+1), and the (+2/0) transition level well above Ge CBM, which indicates that the +2 charge state of oxygen vacancy acts as a fixed charge state in GeO2 observed by recent experiment. The positive fixed charges can not only result in negative flatband shift but scatter the channel carriers to degrade the mobility. Depending on the bonding patterns of interfacial defects at GeO2/Ge interface, the energy levels of the interface states can be either near the CBM or near the VBM, which is consistent with the varied Dit values reported previously. The (+2/0) level that originates from an oxygen vacancy is responsible for the positive fixed charges in GeO2. This work suggests effective passivation of the oxygen vacancies near the GeO2/Ge interface is needed for future Ge CMOS technology.
Starting Page	1
Ending Page	2
File Size	800721
Page Count	2
File Format	PDF
ISBN	9781457718649
e-ISBN	9781457718656
e-ISBN	9781457718632
DOI	10.1109/ISTDM.2012.6222499
Language	English
Publisher	Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Publisher Date	2012-06-04
Publisher Place	USA
Access Restriction	Subscribed
Rights Holder	Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Subject Keyword	Electrical engineering Electron traps Photonic band gap Educational institutions Germanium Nanoscale devices Oxidation
Content Type	Text
Resource Type	Article

Sl.	Authority	Responsibilities	Communication Details
1	Ministry of Education (GoI), Department of Higher Education	Sanctioning Authority	https://www.education.gov.in/ict-initiatives
2	Indian Institute of Technology Kharagpur	Host Institute of the Project: The host institute of the project is responsible for providing infrastructure support and hosting the project	https://www.iitkgp.ac.in
3	National Digital Library of India Office, Indian Institute of Technology Kharagpur	The administrative and infrastructural headquarters of the project	Dr. B. Sutradhar bsutra@ndl.gov.in
4	Project PI / Joint PI	Principal Investigator and Joint Principal Investigators of the project	Dr. B. Sutradhar bsutra@ndl.gov.in Prof. Saswat Chakrabarti will be added soon
5	Website/Portal (Helpdesk)	Queries regarding NDLI and its services	support@ndl.gov.in
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9	IDR Setup or Support	Queries related to establishment and support of Institutional Digital Repository (IDR) and IDR workshops	idr@ndl.gov.in

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