S. Limpijumnong - Academia.edu (original) (raw)

Papers by S. Limpijumnong

Research paper thumbnail of First-Principles Study of Defects in GaN

Abstract : Based on first principles supercell approach, we have successfully initiated and conti... more Abstract : Based on first principles supercell approach, we have successfully initiated and continued our study of defects in AlN, GaN, diluted nitrides and related wide gap materials. We have also investigated and implemented an approach to calculate the x-ray absorption near edge structures (XANES) by using first principles pseudopotential in conjunction with ab initio multiple scattering codes. We have surpassed our original milestones in several aspects and already accomplished important results. a) We have calculated native defects in AlN, namely VAl, VN, Ali, and Ni. Most of the results are in agreement with previous work by Stampfl and Van de Walle. Except for the Ni, where we have shown that Ni prefers the split-interstitial configuration. b) Potential p-type dopants in AlN, Mg and Be, have been studied. We found that both dopants, when substituting for Al, give a rather deep level. This means both Mg and Be are not suitable p-type dopants in AlN. c) We have calculated the Ga Frenkel pairs (interstitial Ga and gallium vacancy complexes) in GaN. We studied both the stability of the pair at different separations and the barriers for the pair to form/disintegrate. Our results show that the pair at ~ 4 angstrom separation is stable and has a binding energy of about 3.3 eV. However, the barrier for the pair to recombine is only ~ 0.2 eV, making it unlikely to be stable at room temperature. d) In dilute nitrides, the nitrogen atoms are highly strained. Our calculations show that they prefer to bind with small impurity atoms such as C or Si in the form of CN and SiN molecule, substituting on the As site. The CNAs gives a vibration frequency in good agreement with the local vibration mode observed in an infrared spectroscopy measurement. The SiNAs has low formation energy and could explain the mutual passivation of shallow donor Si and isovalent N in dilute nitride.

Research paper thumbnail of Electronic Band Structure of SiC Polytypes: A Discussion of Theory and Experiment

physica status solidi (b), 1997

Abstract After a brief discussion of the origin of polytypes and a few general remarks on the rel... more Abstract After a brief discussion of the origin of polytypes and a few general remarks on the relationship between polytypism and properties of SiC, we focus on a comparative study of their electronic band structures. We first explain how the different band structures can be ...

Research paper thumbnail of Vibrational Spectroscopy of GaN:Mg Under Pressure

MRS Proceedings, 2001

The microscopic structure of Mg-H complexes in GaN has been a subject of intense theoretical and ... more The microscopic structure of Mg-H complexes in GaN has been a subject of intense theoretical and experimental investigation. In order to probe the Mg-H structure, we have studied the effect of hydrostatic pressure on the local vibrational mode (LVM) frequency. At ambient pressure, the LVM frequency is 3125 cm-1, which corresponds to a N-H stretching mode. In this study, Fourier-transform spectroscopy was performed on free-standing GaN:Mg,H samples in a diamond-anvil cell, with nitrogen as a pressure-transmitting fluid. The samples had been removed from their sapphire substrate by the laser-liftoff technique. The LVM frequency was measured, at liquid helium temperatures, for pressures ranging from 0 to 5 GPa. The pressure dependence of the frequency is nonlinear: first it decreases with pressure, then it increases. Comparison with first-principles calculations allows us to derive information about the microscopic structure of the Mg-H complex. The calculated stable configuration inde...

Research paper thumbnail of Identification of Mn site in Pb (Zr, Ti) O3

Research paper thumbnail of Band structure and cation ordering in LiGaO {sub 2}

Research paper thumbnail of Theoretical Study of Optical Properties of Native Point Defects in α-Al 2 O 3

Integrated Ferroelectrics, 2014

Using first-principles calculations based on density functional theory with generalized gradient ... more Using first-principles calculations based on density functional theory with generalized gradient approximation (GGA) parameterized by Perdew-Burke-Ernzerhof (PBE) for the exchange-correlation functional, we studied the native point defects in α-Al2O3. We found that oxygen vacancy (VO), Al vacancy (VAl), and Al interstitial (Ali) are the dominant defects in α-Al2O3 under both Al- and O-rich growth conditions. Because the optical properties of α-Al2O3 are important for many device applications, the optical excitation levels due to these native point defects were studied. Our results revealed that the absorption and emission bands at ∼3.0 and 6.0 eV should be associated with VO (F-center) in good agreement with previous works. Moreover, the observed emission peak at ∼3.8 eV is very close to the calculated emission energy of Ali. The absorption and emission due to other native point defects were also reported.

Research paper thumbnail of Second-harmonic generation and birefringence of some ternary pnictide semiconductors

Physical Review B, 1999

A first-principles study of the birefringence and the frequency-dependent second-harmonic generat... more A first-principles study of the birefringence and the frequency-dependent second-harmonic generation ͑SHG͒ coefficients of the ternary pnictide semiconductors with formula ABC 2 (AϭZn, Cd; BϭSi, Ge; C ϭAs, P͒ with the chalcopyrite structures was carried out. The zero-frequency limits of 123 (2) were found to be in reasonable agreement with available experimental data for all the considered materials. We found that substitution of P by As, Si by Ge, and Zn by Cd is favorable to get a higher value of (2) (0). An analysis of the different contributions shows that the anomalously high value of the zero-frequency SHG in CdGeAs 2 appears as a result of a very small interband term in the zero-frequency limit which, contrary to most of the other materials of this class, does not compensate the large intraband contribution. Simple inverse power scaling laws between gaps and (2) values are not supported by our results. We find that the ͑001͒ oriented 1ϩ1 superlattice structure has significantly lower gaps than the chalcopyrite and correspondingly higher (2). However, this smaller gap structure is characterized by a large alternatingly compressive and tensile lateral strain in the layers, which makes it unfavorable. The calculated values of the birefringence for ZnGeP 2 and CdGeAs 2 are in fair agreement ͑discrepancies being rather constant and of the order of 10%͒ with experiment in the frequency range corresponding to the middle of the gap. ͓S0163-1829͑99͒14503-X͔

Research paper thumbnail of Anharmonicity in the Vibrational Modes Associated with H-H, N-H, O-H, and C-H bonds

MRS Proceedings, 2004

ABSTRACTThe anharmonic contributions to the vibrational frequency of various H-H, N-H, O-H, and C... more ABSTRACTThe anharmonic contributions to the vibrational frequency of various H-H, N-H, O-H, and C-H bond configurations are investigated using first principles calculations. While harmonic approximation can be accurately used to calculate the local vibrational mode of most microscopic configurations, the configurations involving strong X-H bond are exceptions because of the exceptionally light mass of the H atom. We will explicitly present the magnitude of the anharmonic contributions to the total frequencies associated with X-H bonds in free molecules as well as in semiconductors defects.

Research paper thumbnail of Novel conguration of Mg-H complexes in GaN

MRS Proceedings, 2001

We describe a novel conguration of the Mg-H complex in GaN, which is not the lowest-energy congur... more We describe a novel conguration of the Mg-H complex in GaN, which is not the lowest-energy conguration at T =0 but is stabilized at elevated temperatures by the large entropy associated with a set of low-energy rotational excitations. We focus on a comparison with two types of experimental results: (1) vibrational spectroscopy using polarized light [B. Clerjaud et al., Phys. Rev. B 61, 8328 (2000)]; and (2) ion-channeling [W. Wampler et al., J. Appl. Phys. 90, 108 (2001)]. New results on the predicted quantum-mechanical delocalization of the hydrogen atom and on the ect of replacing hydrogen with deuterium are reported.

Research paper thumbnail of Stability, diffusion, and complex formation of beryllium in wurtzite GaN

MRS Proceedings, 2000

ABSTRACTWe have studied the properties of Be dopants in GaN using first principles calculations. ... more ABSTRACTWe have studied the properties of Be dopants in GaN using first principles calculations. Substitutional Be on a Ga site acts as an acceptor, but interstitial Be poses a potential problem because of its low formation energy and donor character. We study the diffusion of interstitial Be and find it to be highly anisotropic. We also study the formation of complexes between substitutional and interstitial Be, and between substitutional Be and hydrogen. We have calculated the Be-H vibrational modes to aid in experimental identification of such complexes.

Research paper thumbnail of Band Structure Interpretation of the Optical Transitions between Low-Lying Conduction Bands in n-Type Doped SiC Polytypes

The optical absorption spectra corresponding to transitions between the first few conduction band... more The optical absorption spectra corresponding to transitions between the first few conduction bands, which have been observed in various fairly heavily n-type doped polytypes of SiC, are calculated from first-principles. In 6H-SiC, these spectra provide evidence for the existence of a saddle-point structure in the lowest conduction band at the M point. Good agreement is obtained for the peak positions and the polarization dependence of the spectra. Discrepancies in peak intensities and widths are discussed.

Research paper thumbnail of Optical-absorption bands in the 1–3 eV range in n-type SiC polytypes

Measured optical-absorption bands in the 1-3 eV range in fairly heavily doped n-type SiC polytype... more Measured optical-absorption bands in the 1-3 eV range in fairly heavily doped n-type SiC polytypes 3C, 6H, 4H, 8H, and 15R are shown to arise from optical transitions between the lowest conduction band, which is to some extent perturbed by impurity effects, to higher conduction bands. The energies of the transitions are in good agreement with the differences between unperturbed low-lying energy bands calculated using the full-potential linear muffin-tin orbital method in the local-density approximation. The polarization dependence is explained by selection rules deriving from the symmetry of the bands involved. This indicates that the states involved in the transitions must to a good extent retain the symmetry characters of the unperturbed bands. On the other hand, the calculated absorption peaks from a pure band-to-band model are much narrower, and slightly lower in energy than the experimental ones. Calculations of the density of states over a restricted range of k space for the final states indicate that a partial breakdown of periodicity and hence the ⌬kϭ0 selection rule can account for a major part of the broadening. This explanation is consistent with the degenerate carrier concentration, associated with the overlap of the impurity band tail with the bottom of the conduction band. In 6H-SiC, one feature in the absorption spectrum appears nevertheless to be associated with a more purely band-to-band-like transition. It is a sharp one-dimensional van Hove singularity in the joint density of states at the M point associated with the camel's-back structure of the lowest conduction band. At a lower carrier concentration, this feature is not present, and the transitions appear to have a more localized impurity-to-band character. ͓S0163-1829͑99͒05420-X͔

Research paper thumbnail of Electronic structure of ZnGeP2: A detailed study of the band structure near the fundamental gap and its associated parameters

... parameters Sukit Limpijumnong, Walter RL Lambrecht, and Benjamin Segall Department of Physics... more ... parameters Sukit Limpijumnong, Walter RL Lambrecht, and Benjamin Segall Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106-7079 Received 8 April 1999; revised manuscript received 19 May 1999 ...

Research paper thumbnail of Theoretical study of the relative stability of wurtzite and rocksalt phases in MgO and GaN

We report local density functional calculations using the full-potential linearized muffin-tin or... more We report local density functional calculations using the full-potential linearized muffin-tin orbital method for MgO and GaN in the wurtzite and rocksalt structures and for orthorhombic structures along a homogeneous strain transition path linking wurtzite to rocksalt. MgO is found to be unstable in the wurtzite and marginally metastable in a layered hexagonal phase, labeled h-MgO, in which Mg is approximately fivefold coordinated, the stable phase being rocksalt. GaN is stable in the wurtzite at ambient pressures but exhibits a phase transition to the rocksalt. Band structures and charge densities exhibiting the bond formation as one goes from one structure to the other are presented.

Research paper thumbnail of First-principles studies of beryllium doping of GaN

Physical Review B, 2001

The structural and electronic properties of beryllium substitutional acceptors and interstitial d... more The structural and electronic properties of beryllium substitutional acceptors and interstitial donors in GaN are investigated using first-principles calculations based on pseudopotentials and density-functional theory. In p-type GaN, Be interstitials, which act as donors, have formation energies comparable to that of substitutional Be on the Ga site, which is an acceptor. In thermodynamic equilibrium, incorporation of Be interstitials will therefore result in severe compensation. To investigate the kinetics of Be interstitial incorporation and outdiffusion we have explored the total-energy surface. The diffusivity of Be interstitials is highly anisotropic, with a migration barrier in planes perpendicular to the c axis of 1.2 eV, while the barrier for motion along the c axis is 2.9 eV. We have also studied complex formation between interstitial donors and substitutional acceptors, and between hydrogen and substitutional beryllium. The results for wurtzite GaN are compared with those for the zinc-blende phase. Consequences for p-type doping using Be acceptors are discussed.

Research paper thumbnail of Identification of hydrogen configurations in p-type GaN through first-principles calculations of vibrational frequencies

Physical Review B, 2003

We present first-principles calculations for stable and metastable geometries of various hydrogen... more We present first-principles calculations for stable and metastable geometries of various hydrogen-related configurations in p-type GaN, including isolated interstitial H as well as Mg-H and Be-H complexes. We also calculate the associated vibrational stretching and wagging modes, systematically including anharmonic contributions to the vibrational frequencies; the anharmonicity is large due to the light mass of the hydrogen atom. Based on our investigations of a large number of configurations we derive a correlation between the vibrational frequency of the stretching mode and the bond length in the N-H bond. The results are compared with experimental results; in particular, we address a new configuration for the Mg-H complex that agrees with the geometrical information extracted from polarization-dependent infrared spectroscopy ͓B.

Research paper thumbnail of Identification of Raman-active phonon modes in oriented platelets of InN and polycrystalline InN

We report on micro-Raman studies on both randomly oriented polycrystals and groups of oriented, f... more We report on micro-Raman studies on both randomly oriented polycrystals and groups of oriented, faceted platelets of indium nitride grown from the melt at subatmospheric pressures. Phonon modes were assigned as A TO 1 445; E TO 1 472; E 2 2 488; and A LO 1 588 cm Ϫ1 : The FWHM of the E 2 2 peak of 2.5 cm Ϫ1 is the narrowest reported to date for InN. The measured TO phonon frequencies were compared to those calculated from first principles and excellent agreement was found. The results are discussed in the context of previously reported Raman experiments on heteroepitaxial, and hence strained, layers of InN.

Research paper thumbnail of Evidence for native-defect donors in n-type ZnO

Physical Review Letters, 2005

Recent theory has found that native defects such as the O vacancy V O and Zn interstitial Zn I ha... more Recent theory has found that native defects such as the O vacancy V O and Zn interstitial Zn I have high formation energies in n-type ZnO and, thus, are not important donors, especially in comparison to impurities such as H. In contrast, we use both theory and experiment to show that, under N ambient, the complex Zn IN O is a stronger candidate than H or any other known impurity for a 30 meV donor commonly found in bulk ZnO grown from the vapor phase. Since the Zn vacancy is also the dominant acceptor in such material, we must conclude that native defects are important donors and acceptors in ZnO.

Research paper thumbnail of Guest Editorial

Integrated Ferroelectrics, 2014

Research paper thumbnail of Band Structure and Cation Ordering in LiGaO2

MRS Proceedings, 1996

ABSTRACTFull-potential linear muffin-tin orbital calculations were performed for LiGaO2 in differ... more ABSTRACTFull-potential linear muffin-tin orbital calculations were performed for LiGaO2 in different crystal structures in order to investigate the nature and origin of the cation ordering, structural relaxation and their effects on the band structure. It is found that the most important factor for the bonding is the exclusive occurrence of Li2Ga2 tetrahedra surrounding oxygen. Structures including LiGa3 and Li3Ga tetrahedra have significantly higher total energies and smaller bandgaps. The band-offset between GaN and LiGaO2 is estimated using the dielectric midgap approach.

Research paper thumbnail of First-Principles Study of Defects in GaN

Abstract : Based on first principles supercell approach, we have successfully initiated and conti... more Abstract : Based on first principles supercell approach, we have successfully initiated and continued our study of defects in AlN, GaN, diluted nitrides and related wide gap materials. We have also investigated and implemented an approach to calculate the x-ray absorption near edge structures (XANES) by using first principles pseudopotential in conjunction with ab initio multiple scattering codes. We have surpassed our original milestones in several aspects and already accomplished important results. a) We have calculated native defects in AlN, namely VAl, VN, Ali, and Ni. Most of the results are in agreement with previous work by Stampfl and Van de Walle. Except for the Ni, where we have shown that Ni prefers the split-interstitial configuration. b) Potential p-type dopants in AlN, Mg and Be, have been studied. We found that both dopants, when substituting for Al, give a rather deep level. This means both Mg and Be are not suitable p-type dopants in AlN. c) We have calculated the Ga Frenkel pairs (interstitial Ga and gallium vacancy complexes) in GaN. We studied both the stability of the pair at different separations and the barriers for the pair to form/disintegrate. Our results show that the pair at ~ 4 angstrom separation is stable and has a binding energy of about 3.3 eV. However, the barrier for the pair to recombine is only ~ 0.2 eV, making it unlikely to be stable at room temperature. d) In dilute nitrides, the nitrogen atoms are highly strained. Our calculations show that they prefer to bind with small impurity atoms such as C or Si in the form of CN and SiN molecule, substituting on the As site. The CNAs gives a vibration frequency in good agreement with the local vibration mode observed in an infrared spectroscopy measurement. The SiNAs has low formation energy and could explain the mutual passivation of shallow donor Si and isovalent N in dilute nitride.

Research paper thumbnail of Electronic Band Structure of SiC Polytypes: A Discussion of Theory and Experiment

physica status solidi (b), 1997

Abstract After a brief discussion of the origin of polytypes and a few general remarks on the rel... more Abstract After a brief discussion of the origin of polytypes and a few general remarks on the relationship between polytypism and properties of SiC, we focus on a comparative study of their electronic band structures. We first explain how the different band structures can be ...

Research paper thumbnail of Vibrational Spectroscopy of GaN:Mg Under Pressure

MRS Proceedings, 2001

The microscopic structure of Mg-H complexes in GaN has been a subject of intense theoretical and ... more The microscopic structure of Mg-H complexes in GaN has been a subject of intense theoretical and experimental investigation. In order to probe the Mg-H structure, we have studied the effect of hydrostatic pressure on the local vibrational mode (LVM) frequency. At ambient pressure, the LVM frequency is 3125 cm-1, which corresponds to a N-H stretching mode. In this study, Fourier-transform spectroscopy was performed on free-standing GaN:Mg,H samples in a diamond-anvil cell, with nitrogen as a pressure-transmitting fluid. The samples had been removed from their sapphire substrate by the laser-liftoff technique. The LVM frequency was measured, at liquid helium temperatures, for pressures ranging from 0 to 5 GPa. The pressure dependence of the frequency is nonlinear: first it decreases with pressure, then it increases. Comparison with first-principles calculations allows us to derive information about the microscopic structure of the Mg-H complex. The calculated stable configuration inde...

Research paper thumbnail of Identification of Mn site in Pb (Zr, Ti) O3

Research paper thumbnail of Band structure and cation ordering in LiGaO {sub 2}

Research paper thumbnail of Theoretical Study of Optical Properties of Native Point Defects in α-Al 2 O 3

Integrated Ferroelectrics, 2014

Using first-principles calculations based on density functional theory with generalized gradient ... more Using first-principles calculations based on density functional theory with generalized gradient approximation (GGA) parameterized by Perdew-Burke-Ernzerhof (PBE) for the exchange-correlation functional, we studied the native point defects in α-Al2O3. We found that oxygen vacancy (VO), Al vacancy (VAl), and Al interstitial (Ali) are the dominant defects in α-Al2O3 under both Al- and O-rich growth conditions. Because the optical properties of α-Al2O3 are important for many device applications, the optical excitation levels due to these native point defects were studied. Our results revealed that the absorption and emission bands at ∼3.0 and 6.0 eV should be associated with VO (F-center) in good agreement with previous works. Moreover, the observed emission peak at ∼3.8 eV is very close to the calculated emission energy of Ali. The absorption and emission due to other native point defects were also reported.

Research paper thumbnail of Second-harmonic generation and birefringence of some ternary pnictide semiconductors

Physical Review B, 1999

A first-principles study of the birefringence and the frequency-dependent second-harmonic generat... more A first-principles study of the birefringence and the frequency-dependent second-harmonic generation ͑SHG͒ coefficients of the ternary pnictide semiconductors with formula ABC 2 (AϭZn, Cd; BϭSi, Ge; C ϭAs, P͒ with the chalcopyrite structures was carried out. The zero-frequency limits of 123 (2) were found to be in reasonable agreement with available experimental data for all the considered materials. We found that substitution of P by As, Si by Ge, and Zn by Cd is favorable to get a higher value of (2) (0). An analysis of the different contributions shows that the anomalously high value of the zero-frequency SHG in CdGeAs 2 appears as a result of a very small interband term in the zero-frequency limit which, contrary to most of the other materials of this class, does not compensate the large intraband contribution. Simple inverse power scaling laws between gaps and (2) values are not supported by our results. We find that the ͑001͒ oriented 1ϩ1 superlattice structure has significantly lower gaps than the chalcopyrite and correspondingly higher (2). However, this smaller gap structure is characterized by a large alternatingly compressive and tensile lateral strain in the layers, which makes it unfavorable. The calculated values of the birefringence for ZnGeP 2 and CdGeAs 2 are in fair agreement ͑discrepancies being rather constant and of the order of 10%͒ with experiment in the frequency range corresponding to the middle of the gap. ͓S0163-1829͑99͒14503-X͔

Research paper thumbnail of Anharmonicity in the Vibrational Modes Associated with H-H, N-H, O-H, and C-H bonds

MRS Proceedings, 2004

ABSTRACTThe anharmonic contributions to the vibrational frequency of various H-H, N-H, O-H, and C... more ABSTRACTThe anharmonic contributions to the vibrational frequency of various H-H, N-H, O-H, and C-H bond configurations are investigated using first principles calculations. While harmonic approximation can be accurately used to calculate the local vibrational mode of most microscopic configurations, the configurations involving strong X-H bond are exceptions because of the exceptionally light mass of the H atom. We will explicitly present the magnitude of the anharmonic contributions to the total frequencies associated with X-H bonds in free molecules as well as in semiconductors defects.

Research paper thumbnail of Novel conguration of Mg-H complexes in GaN

MRS Proceedings, 2001

We describe a novel conguration of the Mg-H complex in GaN, which is not the lowest-energy congur... more We describe a novel conguration of the Mg-H complex in GaN, which is not the lowest-energy conguration at T =0 but is stabilized at elevated temperatures by the large entropy associated with a set of low-energy rotational excitations. We focus on a comparison with two types of experimental results: (1) vibrational spectroscopy using polarized light [B. Clerjaud et al., Phys. Rev. B 61, 8328 (2000)]; and (2) ion-channeling [W. Wampler et al., J. Appl. Phys. 90, 108 (2001)]. New results on the predicted quantum-mechanical delocalization of the hydrogen atom and on the ect of replacing hydrogen with deuterium are reported.

Research paper thumbnail of Stability, diffusion, and complex formation of beryllium in wurtzite GaN

MRS Proceedings, 2000

ABSTRACTWe have studied the properties of Be dopants in GaN using first principles calculations. ... more ABSTRACTWe have studied the properties of Be dopants in GaN using first principles calculations. Substitutional Be on a Ga site acts as an acceptor, but interstitial Be poses a potential problem because of its low formation energy and donor character. We study the diffusion of interstitial Be and find it to be highly anisotropic. We also study the formation of complexes between substitutional and interstitial Be, and between substitutional Be and hydrogen. We have calculated the Be-H vibrational modes to aid in experimental identification of such complexes.

Research paper thumbnail of Band Structure Interpretation of the Optical Transitions between Low-Lying Conduction Bands in n-Type Doped SiC Polytypes

The optical absorption spectra corresponding to transitions between the first few conduction band... more The optical absorption spectra corresponding to transitions between the first few conduction bands, which have been observed in various fairly heavily n-type doped polytypes of SiC, are calculated from first-principles. In 6H-SiC, these spectra provide evidence for the existence of a saddle-point structure in the lowest conduction band at the M point. Good agreement is obtained for the peak positions and the polarization dependence of the spectra. Discrepancies in peak intensities and widths are discussed.

Research paper thumbnail of Optical-absorption bands in the 1–3 eV range in n-type SiC polytypes

Measured optical-absorption bands in the 1-3 eV range in fairly heavily doped n-type SiC polytype... more Measured optical-absorption bands in the 1-3 eV range in fairly heavily doped n-type SiC polytypes 3C, 6H, 4H, 8H, and 15R are shown to arise from optical transitions between the lowest conduction band, which is to some extent perturbed by impurity effects, to higher conduction bands. The energies of the transitions are in good agreement with the differences between unperturbed low-lying energy bands calculated using the full-potential linear muffin-tin orbital method in the local-density approximation. The polarization dependence is explained by selection rules deriving from the symmetry of the bands involved. This indicates that the states involved in the transitions must to a good extent retain the symmetry characters of the unperturbed bands. On the other hand, the calculated absorption peaks from a pure band-to-band model are much narrower, and slightly lower in energy than the experimental ones. Calculations of the density of states over a restricted range of k space for the final states indicate that a partial breakdown of periodicity and hence the ⌬kϭ0 selection rule can account for a major part of the broadening. This explanation is consistent with the degenerate carrier concentration, associated with the overlap of the impurity band tail with the bottom of the conduction band. In 6H-SiC, one feature in the absorption spectrum appears nevertheless to be associated with a more purely band-to-band-like transition. It is a sharp one-dimensional van Hove singularity in the joint density of states at the M point associated with the camel's-back structure of the lowest conduction band. At a lower carrier concentration, this feature is not present, and the transitions appear to have a more localized impurity-to-band character. ͓S0163-1829͑99͒05420-X͔

Research paper thumbnail of Electronic structure of ZnGeP2: A detailed study of the band structure near the fundamental gap and its associated parameters

... parameters Sukit Limpijumnong, Walter RL Lambrecht, and Benjamin Segall Department of Physics... more ... parameters Sukit Limpijumnong, Walter RL Lambrecht, and Benjamin Segall Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106-7079 Received 8 April 1999; revised manuscript received 19 May 1999 ...

Research paper thumbnail of Theoretical study of the relative stability of wurtzite and rocksalt phases in MgO and GaN

We report local density functional calculations using the full-potential linearized muffin-tin or... more We report local density functional calculations using the full-potential linearized muffin-tin orbital method for MgO and GaN in the wurtzite and rocksalt structures and for orthorhombic structures along a homogeneous strain transition path linking wurtzite to rocksalt. MgO is found to be unstable in the wurtzite and marginally metastable in a layered hexagonal phase, labeled h-MgO, in which Mg is approximately fivefold coordinated, the stable phase being rocksalt. GaN is stable in the wurtzite at ambient pressures but exhibits a phase transition to the rocksalt. Band structures and charge densities exhibiting the bond formation as one goes from one structure to the other are presented.

Research paper thumbnail of First-principles studies of beryllium doping of GaN

Physical Review B, 2001

The structural and electronic properties of beryllium substitutional acceptors and interstitial d... more The structural and electronic properties of beryllium substitutional acceptors and interstitial donors in GaN are investigated using first-principles calculations based on pseudopotentials and density-functional theory. In p-type GaN, Be interstitials, which act as donors, have formation energies comparable to that of substitutional Be on the Ga site, which is an acceptor. In thermodynamic equilibrium, incorporation of Be interstitials will therefore result in severe compensation. To investigate the kinetics of Be interstitial incorporation and outdiffusion we have explored the total-energy surface. The diffusivity of Be interstitials is highly anisotropic, with a migration barrier in planes perpendicular to the c axis of 1.2 eV, while the barrier for motion along the c axis is 2.9 eV. We have also studied complex formation between interstitial donors and substitutional acceptors, and between hydrogen and substitutional beryllium. The results for wurtzite GaN are compared with those for the zinc-blende phase. Consequences for p-type doping using Be acceptors are discussed.

Research paper thumbnail of Identification of hydrogen configurations in p-type GaN through first-principles calculations of vibrational frequencies

Physical Review B, 2003

We present first-principles calculations for stable and metastable geometries of various hydrogen... more We present first-principles calculations for stable and metastable geometries of various hydrogen-related configurations in p-type GaN, including isolated interstitial H as well as Mg-H and Be-H complexes. We also calculate the associated vibrational stretching and wagging modes, systematically including anharmonic contributions to the vibrational frequencies; the anharmonicity is large due to the light mass of the hydrogen atom. Based on our investigations of a large number of configurations we derive a correlation between the vibrational frequency of the stretching mode and the bond length in the N-H bond. The results are compared with experimental results; in particular, we address a new configuration for the Mg-H complex that agrees with the geometrical information extracted from polarization-dependent infrared spectroscopy ͓B.

Research paper thumbnail of Identification of Raman-active phonon modes in oriented platelets of InN and polycrystalline InN

We report on micro-Raman studies on both randomly oriented polycrystals and groups of oriented, f... more We report on micro-Raman studies on both randomly oriented polycrystals and groups of oriented, faceted platelets of indium nitride grown from the melt at subatmospheric pressures. Phonon modes were assigned as A TO 1 445; E TO 1 472; E 2 2 488; and A LO 1 588 cm Ϫ1 : The FWHM of the E 2 2 peak of 2.5 cm Ϫ1 is the narrowest reported to date for InN. The measured TO phonon frequencies were compared to those calculated from first principles and excellent agreement was found. The results are discussed in the context of previously reported Raman experiments on heteroepitaxial, and hence strained, layers of InN.

Research paper thumbnail of Evidence for native-defect donors in n-type ZnO

Physical Review Letters, 2005

Recent theory has found that native defects such as the O vacancy V O and Zn interstitial Zn I ha... more Recent theory has found that native defects such as the O vacancy V O and Zn interstitial Zn I have high formation energies in n-type ZnO and, thus, are not important donors, especially in comparison to impurities such as H. In contrast, we use both theory and experiment to show that, under N ambient, the complex Zn IN O is a stronger candidate than H or any other known impurity for a 30 meV donor commonly found in bulk ZnO grown from the vapor phase. Since the Zn vacancy is also the dominant acceptor in such material, we must conclude that native defects are important donors and acceptors in ZnO.

Research paper thumbnail of Guest Editorial

Integrated Ferroelectrics, 2014

Research paper thumbnail of Band Structure and Cation Ordering in LiGaO2

MRS Proceedings, 1996

ABSTRACTFull-potential linear muffin-tin orbital calculations were performed for LiGaO2 in differ... more ABSTRACTFull-potential linear muffin-tin orbital calculations were performed for LiGaO2 in different crystal structures in order to investigate the nature and origin of the cation ordering, structural relaxation and their effects on the band structure. It is found that the most important factor for the bonding is the exclusive occurrence of Li2Ga2 tetrahedra surrounding oxygen. Structures including LiGa3 and Li3Ga tetrahedra have significantly higher total energies and smaller bandgaps. The band-offset between GaN and LiGaO2 is estimated using the dielectric midgap approach.