Mahdi Sanati

Specific heat and entropy of GaN

The specific heat and entropy of (wurtzite) GaN are not well known experimentally. The literature values for Cp (T>298 K) are based on a fit to an analytic function. The parameters include Cp (298 K) and the melting temperature Tm, both of which were poorly known when the parameters were first determined. The value of Cp (298 K) disagrees with the data measured in the range 5–305 K. Our first-principles calculations allow the values of the parameters to be established and lead to a more accurate prediction of specific heat and entropy of GaN.

Fundamental Interactions of Fe in Silicon: First-Principles Theory

Interstitial iron and iron-acceptor pairs are well studied but undesirable defects in Si as they are strong recombination centers which resist hydrogen passivation. Thermal anneals often result in the precipitation of Fe. Relatively little information is available about the interactions between Fe and native defects or common impurities in Si. We present the results of first-principles calculations of Fe interactions with native defects (vacancy, self-interstitial) and common impurities such as C, O, H, or Fe. The goal is to understand the fundamental chemistry of Fe in Si, identify and characterize the type of complexes that occur. We predict the configurations, charge and spin states, binding and activation energies, and estimate the position of gap levels. The possibility of passivation is discussed.

Thermal properties of guest-free Si136 and Ge136 clathrates: A first-principles study

We have used the generalized gradient approximation (GGA) to density functional theory to study the vibrational and thermal properties of guest-free Si136 and Ge136 clathrates. In order to study the effects of supercell size on our results, we have performed both 34 and 136 atom supercell calculations for each material. We find that the 34 atom supercell calculations predict a small frequency downshift (in comparison with the 136 atom supercell calculations) in the vibrational density of states of both materials. The GGA-predicted Γ phonon frequency of Si136 (480 cm−1 at T=0 K) obtained from the 136 atom calculations is in very good agreement with the experimental value for Na1Si136 (484 cm−1 at T=300 K). Using the results from our 136 atom calculations, we have also calculated the temperature dependence of the vibrational contributions to the Helmholtz free energy, the entropy, and the specific heat (CV) of the guest-free Si136 and Ge136 clathrates. The predicted and experimental heat capacities of Si136...

Theoretical properties of the N vacancy in p-type GaN(Mg,H) at elevated temperatures

The elevated-temperature properties of the N vacancy in Mg-doped, p-type GaN containing H were modeled using atomic-configuration energies and phonon densities of states obtained with density-functional theory. This study encompassed both equilibrium thermodynamics and the rates of diffusion and reaction processes and included the influences of a number of bound complexes involving the vacancy, the Mg dopant, and H. A comparison was made with published experimental information. Our results indicate that N vacancies extensively compensate Mg acceptors at higher doping levels.

Lattice isotope effects on the widths of optical transitions in silicon

Using measured isotope shifts for the 607 cm−1 local vibrational mode, LVM, of substitutional carbon, Cs, we demonstrate that isotope disorder contributes ~0.5 cm−1 to the width of that LVM in natural silicon. The width of the LVM of Cs also depends on its energy relative to the density of two-phonon states, and increases along the sequence 13C in natural silicon, 12C in natural silicon and 12C in 30Si. Other LVMs show larger isotope effects, and so discrete structure rather than line broadening. In the case of zero-phonon lines, we take the 3942 cm−1 line as a potentially favourable example. We estimate that the isotope disorder contributes only 0.09 cm−1 to the linewidth in a natural silicon sample, a contribution that is negligible compared to typical strain-broadening effects, but would be a lower limit to the linewidth in high-purity natural silicon.

Formation of VNHVNH and MgVNHMgVNH in p-type GaN(Mg,H)

The reactions VN+1+H+→(VNH)+2 and (MgVN)0+H+→(MgVNH)+1(MgVN)0+H+→(MgVNH)+1 in GaN were investigated using density-functional theory. Estimates of the reaction rates indicate that (VNH)+2(VNH)+2 and (MgVNH)+1(MgVNH)+1 will form rapidly above 400∘C, and modeling predicts that their populations will be substantial at elevated temperatures. These results indicate that compensation by VNVN is important in p-type GaN and that H suppression of VNVN formation is less effective than previously suggested.

Formation of VNH and MgVNH in p-type GaN(Mg,H)

The reactions VN+1+H+→(VNH)+2 and (MgVN)0+H+→(MgVNH)+1(MgVN)0+H+→(MgVNH)+1 in GaN were investigated using density-functional theory. Estimates of the reaction rates indicate that (VNH)+2(VNH)+2 and (MgVNH)+1(MgVNH)+1 will form rapidly above 400∘C, and modeling predicts that their populations will be substantial at elevated temperatures. These results indicate that compensation by VNVN is important in p-type GaN and that H suppression of VNVN formation is less effective than previously suggested.

Formation of and in p-type GaN(Mg,H)

The reactions VN+1+H+→(VNH)+2 and (MgVN)0+H+→(MgVNH)+1(MgVN)0+H+→(MgVNH)+1 in GaN were investigated using density-functional theory. Estimates of the reaction rates indicate that (VNH)+2(VNH)+2 and (MgVNH)+1(MgVNH)+1 will form rapidly above 400∘C, and modeling predicts that their populations will be substantial at elevated temperatures. These results indicate that compensation by VNVN is important in p-type GaN and that H suppression of VNVN formation is less effective than previously suggested.