Session P18: Surfaces and Point Defects in Semiconductor

11:15 AM–1:51 PM, Wednesday, March 23, 2005
LACC - 406A

Sponsoring Unit: DCMP
Chair: Dr. Tae-Sik Yoon, Dept. MSE, UCLA

Abstract: P18.00008 : Does the zinc vacancy in ZnGeP$_2$ exhibit a Jahn-Teller distortion?

12:39 PM–12:51 PM

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Authors:

Walter R.L. Lambrecht
Xiaoshu Jiang
M.S. Miao
(Case Western Reserve University)

Sukit Limpijumnong
(Suranaree University of Technology)

The Zn-vacancy is one of the dominant defects in ZnGeP$_2$. Its single negative charge state is EPR active. The hyperfine splitting shows that the unpaired electron is primarily localized on a pair of P atoms. In contrast, first-principles 64 atom supercell calculations using both the FP-LMTO and the VASP method of the $V_{Zn}^{-}$ state show that the defect maintains $S_4$ symmetry with the wave function spread equally over 4 P atoms. Here a group-theoretical analysis is presented. When including only the nearest neighbors, the system has $D_{2d}$ symmetry. While the one electron state of the unpaired electron is non-degenerate, a doubly degenerate $e$-state lies only about 10 meV below it. We show that a P-pairing distortion mode splits this $e$-state in two states which are even with respect to one of the mirrorplanes and odd with respect to the other and thus can only contain two of the P-dangling bonds. Calculations in which a pairing of P atoms is enforced while relaxing the remaining atoms confirm this model. Remaining puzzling aspects of this defect will be discussed.

Session P18: Surfaces and Point Defects in Semiconductor

11:15 AM–1:51 PM, Wednesday, March 23, 2005
LACC - 406A

Sponsoring Unit: DCMP
Chair: Dr. Tae-Sik Yoon, Dept. MSE, UCLA

Abstract: P18.00005 : Interactions between native point defects in ZnGeP$_2$.

12:03 PM–12:15 PM

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Authors:

Xiaoshu Jiang
M. S. Miao
Walter R. L. Lambrecht
(Case Western Reserve University)

First-principles calculations of the native point defects $V_{Zn}$, $V_{Ge}$, $Zn_{Ge}$ and $Ge_{Zn}$ show that under Zn-poor conditions, the dominant defects are the $Ge_{Zn}$ and $V_{Zn}$. Since these are respectively a donor and an acceptor, one may expect them to attract each other. The formation of complexes of the type $V_{Zn}-Ge_{Zn}-V_{Zn}$ was studied and found to be favorable.A simple molecular model is proposed for the electronic structure of this complex. Optical excitation of electron paramagnetic resonance (EPR) studies by Gehlhoff et al. [1] were used by these authors to extract energy levels in the gap associated with these defects. The model proposed by these authors assumes that the $Ge_{Zn}$ EPR centrum in irradiated samples becomes activated by a two step process in which an electron from a $V_{Zn}^{2-}$ is optically excited to the conduction band and subsequently trapped at a $Ge_{Zn}^{2+}$ site converting the two defects in EPR active sites $V_{Zn}^-$ and $Ge_{Zn}^+$. We instead propose a direct transition between the two defect states without the intervening conduction band and show that our calculated occupation energy levels agree with such a model. The $V_{Ge}$ on the other hand is found to have a high energy formation and to be unstable towards the formation of a $V_{Zn}$ and a $Zn_{Ge}$ antisite. [1] W. Gehlhoff, R. N. Pereira, D. Azamat, A. Hoffmann, and N. Dietz, Physica B {\bf 308-310}, 1015 (2001).

Session H10: Focus Session: Magnetic Semiconductors: Electronic Structure

8:00 AM–11:00 AM, Tuesday, March 22, 2005
LACC - 153B

Sponsoring Units: DMP GMAG
Chair: Jairo Sinova, Texas A&M University

Abstract: H10.00008 : ScN:Mn a dilute ferromagnetic semiconductor

9:48 AM–10:00 AM

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Authors:

Aditi Herwadkar
Walter R. L. Lambrecht
(Dept. of Physics, Case Western Reserve University, Cleveland OH 44106)

We study the electronic properties of ScN:Mn calculated using the full potential linearized muffin-tin orbital method. To model the impurity we use 64 atom supercells and fully relax the structure. Band gap corrections are included in a closely related atomic sphere approximation (ASA). Mn on a Sc site is found to induce a localized state in the middle of the band gap of ScN with $t_{2g}$ character. Its spin splitting leads to a net magnetic moment of 2-3 $\mu_B$ per Mn. Calculations of unit cell for near neighbor Mn atoms reveal that the spins prefer ferromagnetic coupling. Using mean field approximation and assuming a random distribution of Mn atoms we estimate the Curie temperature. Above room temperature $T_c$ seems possible with only 2 \% Mn concentration. The Gibbs energy of formation of the Mn impurity is found to be 3.6 eV, which is comparable to that of other magnetic semiconductor systems. The shared rocksalt structure of MnN and ScN should facilitate alloy formation. The energy of formation of the Mn pairs indicates, no tendency towards clustering. Notably, the ferromagnetic coupling in this system occurs even without the condiseration of a carrier mediated coupling mechanism. The presence of local magnetic moments on Mn results in a small spin splitting of the Sc d-like conduction bands, so one could expect to see interesting effects on carrier transport in n-type ScN:Mn. A high n-type concentration however may tend to reduce the magnetic moments.

Session B11: High Pressure I

11:15 AM–2:15 PM, Monday, March 21, 2005
LACC - 153C

Sponsoring Units: DCMP DMP
Chair: Matteo Cococcione, MIT

Abstract: B11.00003 : Universal transition state and transition path for the high-pressure zinc blende to rocksalt phase transition

11:39 AM–11:51 AM

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Authors:

Maosheng Miao
Walter Lambrecht
(Department of Physics, Case Western Reserve University)

Although the high-pressure zinc blende (ZB) to rocksalt (RS) structural transition has been studied intensively in many experiments and theories, the understanding of the kinetics of this process, especially of its the transition state (TS), the saddle point on the transition path, is still incomplete. We studied the TS and energetics along a previously introduced low barrier orthorhombic transition path of the ZB to RS transition for several semiconductors, including II-VI and III-V compounds and group-IV elemental semiconductors using a first-principles full-potential linearized muffin-tin orbital method. The path is defined by the relative sublattice position and the lattice constants are allowed to relax in response to this chosen independent variable. We found that: 1) the location and the geometry of the TS are identical for all the semiconductors investigated; 2) the lattice constants and the scaled volume vary in a universal manner along the path for all the semiconductors; 3) the cosine function of the relative sublattice position can be used as an order parameter for expanding the energy associated with the the phase transition. A Landau like model for reconstructive phase transition with changing periodicity shows that the position of the transition state does not depend on the chemical components of the semiconductors.

Maosheng Miao,
Senior Research Associate