Chandrima Mitra, Graduate Student

Session W19: Dopants and Defects in Semiconductors IV

2:30 PM–5:18 PM, Thursday, March 13, 2008
Morial Convention Center - 211

Sponsoring Unit: DMP
Chair: Matthew McCluskey, Washington State University

Abstract: W19.00005 : First principles calculations for Gd doped GaN

3:18 PM–3:30 PM

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Authors:

  Chandrima Mitra
    (Case Western Reserve University)

  Walter Lambrecht
    (Case Western Reserve University)

Gd doped GaN has been reported by Dhar et al. to have magnetic moments of order a few 1000 $\mu_B$ per Gd in the very dilute limit of $10^{15}$ Gd/cm$^3$ and to show above room temperature ferromagnetism. Here we present first principle electronic structure calculations to study the spin splitting of the conduction band with varying concentration of Gd in GaN. Our calculations show that the spin splitting varies linearly with the concentration of Gd which suggests an almost zero splitting if one were to extrapolate to the 1 ppm dilute concentration of Gd. Thus the large magnetic moments cannot be explained simply by assuming donor electrons (for example from oxygen) will fill the spin-split conduction band. The spin polarization of the Ga and N atoms around Gd atom in a supercell of 1.5\% Gd were found to be small and to become negligible beyond second nearest neighbors. In these, studies, we either added oxygen or Si as co-dopants or a background charge to fill the spin-split conduction band. This indicates that the proposed model of Dhar of polarization of the host is not supported by our calculations. The magnetic exchange interaction parameter, for nearest neighbour Gd atoms have also been calculated by mapping the energy differences between the ferromagnetic and antiferromagnetic arrangement onto the Heisenberg's model. Effects of strain, supercell size and shape, and other dopants on the exchange interactions were investigated.




Tula R. Paudel, Graduate Student

Session L19: Focus Session: Dopants and Defects in Semiconductors II

2:30 PM–5:06 PM, Tuesday, March 11, 2008
Morial Convention Center - 211

Sponsoring Unit: DMP
Chair: Michael Stavola, Lehigh University

Abstract: L19.00011 : Theoretical Study of native defects in CdGeAs$_2$

4:54 PM–5:06 PM

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Authors:

  Tula R. Paudel
    (Department of Physics, Case Western Reserve University)

  Walter R.L. Lambrecht
    (Department of Physics Case Western Reserve University)

First-principles results are presented for various native defects \textit{viz} : V$_{\rm{Cd}}$, V$_{\rm{Ge}}$, V$_{\rm{As}}$, Cd$_{\rm{Ge}}$, Ge$_{\rm{Cd}}$, Ge$_{\rm{As}}$ and As$_{\rm{Ge}}$ in CdGeAs$_2$ under different growth conditions. The defects were calculated by constructing a 64 atom supercell in the full potential linearized muffin-tin orbital implementation of the density functional theory under the local density approximation (LDA). Calculations of the energy of formation show that antisites should be the most abundant type of defect. The LDA band gap is adjusted to experimental band gap by introducing a non-local orbital dependent constant potential shift to the $s$-orbitals of Cd and Ge and $d$-orbitals of Cd within the LSDA+U approach. The defect transition levels for different charge states are calculated. The calculations support the earlier suggestion that Ge$_{\rm{As}}$ is a shallow acceptor. The calculated transition levels are found to be significantly different form corresponding defects levels of ZnGeP$_2$. The defect levels are interpreted in a simple molecular-orbital theory and compared with the available experimental data.


Adisak Boonchun Graduate Student

Session P36: Focus Session: X-ray and Neutron Instrumentation and Science

8:00 AM–10:48 AM, Wednesday, March 12, 2008
Morial Convention Center - 228

Sponsoring Unit: GIMS
Chair: George Srajer, Argonne National Laboratory

Abstract: P36.00009 : Identification of Mn site in Pb(Zr,Ti)O$_{3}$

10:00 AM–10:12 AM

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Authors:

  A. Boonchun
    (School of Physics, Suranaree University of Technology, Thailand)

  M.F. Smith
    (School of Physics, Suranaree University of Technology, Thailand)

  S. Rujirawat
    (School of Physics, Suranaree University of Technology, Thailand)

  B. Cherdhirunkorn
    (School of Physics, Suranaree University of Technology, Thailand)

  S. Limpijumnong
    (School of Physics, Suranaree University of Technology, Thailand)

The impurity Mn in PbTiO$_{3}$ and PbZrO$_{3}$ has been studied by mean of first-principles spin density functional theory.[1] It is found that the Mn atom energetically prefers to substitute on the Ti/Zr site over other sites (i.e., Pb site, O site or interstitial) under all equilibrium growth conditions. The calculations predict that a majority of Mn atoms substitute for Ti/Zr and have neutral-charge state each with a total electron spin of M$_{z}$ = 3/2. This prediction is supported by the combination of x-ray absorption near edge structure (XANES) experiment and first-principles simulation of the spectrum. [2] The measured XANES of the Mn-doped Pb(Ti,Zr)O$_ {3}$ within the concentration range of 0.5 - 2.0 at.\% yield the exact same features, indicating that the location of Mn in the crystal is independent of Mn concentration. The measured XANES is consistent with the partial density of states simulation of Mn atom on the Ti/Zr site and inconsistent with the simulations of Mn atom on other sites.\newline [1] A. Boonchun, M. F. Smith, B. Cherdhirunkorn, and S. Limpijumnong, J. Appl. Phys. 101, 043521 (2007). [2] S. Limpijumnong, S. Rujirawat, A. Boonchun, M.F. Smith, B. Cherdhirunkorn, Appl. Phys. Lett. 90, 103113 (2007).


Kanoknan Sarasamak Exchange Visitor

Session L40: High Pressure

2:30 PM–5:30 PM, Tuesday, March 11, 2008
Morial Convention Center - 232

Sponsoring Units: DCMP DMP
Chair: Renata Wentzcovitch, University of Minnesota

Abstract: L40.00012 : High-stress phases of SiC, GaN, InN, ZnO, and CdSe

4:42 PM–4:54 PM

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Authors:

  Kanoknan Sarasamak
    (Suranaree University of Technology, Thailand)

  Ambarish J. Kulkarni
    (Suranaree University of Technology, Thailand)

  Min Zhou
    (Suranaree University of Technology, Thailand)

  Sukit Limpijumnong
    (Suranaree University of Technology, Thailand)

Phase transformations of SiC, GaN, InN, ZnO, and CdSe from wurtzite (WZ) to three other different crystalline structures under loading of different stress tensors are studied using first-principle calculations. The first transformation studied is well known and occurs under hydrostatic compression and leads to a six-fold coordinated \textit{rocksalt} (RS) structure. The equilibrium pressures for this transformation of the materials are calculated and found to be proportional to the energy difference between the phases at zero stress and vary monotonically with the materials' ionicity. The second and third transformations studied occur under uniaxial stresses and lead to two new crystal structures previously unknown for these materials. Specifically, uniaxial compression along the [0001] direction or uniaxial tension along the $[01\overline 1 0]$ direction, causes a transformation to a five-fold coordinated \textit{unbuckled wurtzite }structure which we named HX. On the other hand, uniaxial tension along the [0001] direction causes the materials to transform into a body-centered-tetragonal structure which we named BCT-4. The critical equilibrium transformation stresses for these transformations are obtained and their correlation with the ionicity of the materials is analyzed.

 

Please send comments and suggestions to tula.paudel@case.edu