Sukit Limpijumnong

High field electron transport properties of bulk ZnO

The Monte Carlo method is used to simulate electron transport for electric field strengths up to 350 kV/cm in bulk, wurtzite structure ZnO. The relevant parts of the conduction bands of a first-principles band structure are approximated by spherically symmetric, nonparabolic valleys located at the Γ and Umin symmetry points of the Brillouin zone. It is shown that the analytic expressions represent the band structure and the density of states well over a range of nearly 5 eV from the bottom of the conduction band. The simulated electron steady-state drift velocity versus electric field characteristics are calculated for lattice temperatures of 300, 450, and 600 K. For room temperature, drift velocities higher than 3×107 cm/s are reached at fields near 250 kV/cm. Examination of the electron energy distributions shows that the strong decrease of the differential mobility with increasing electric field in the field range studied is to be associated with the pronounced nonparabolicity of the central valley and...

Hydrogen passivation effect in nitrogen-doped ZnO thin films

The role of hydrogen in nitrogen-doped ZnO thin films was studied by Fourier transform infrared (FTIR) absorption and modeled by first-principles calculations to understand the difficulty of doping ZnO p-type with nitrogen. Nitrogen-doped ZnO films were fabricated by low-pressure metal-organic chemical vapor deposition (MOCVD). High levels of nitrogen incorporation were observed, but the acceptor concentrations remained low. Theoretical analysis suggests there is a high probability that NO− and H+ charged defects combine to form the neutral defect complexes, thereby compensating the nitrogen-related acceptors. Calculated values of the vibrational frequencies of the related infrared modes agree well with the measured spectra. Thus, we believe the difficulty of achieving p-type doping in MOCVD-grown ZnO films is due, at least partially, to inadvertent passivation by hydrogen.

Stacking fault band structure in 4H–SiC and its impact on electronic devices

First principles calculations of the stacking fault (SF) in 4H–SiC indicate the occurrence of an interface band in the gap with maximum depth of 0.2–0.3 eV below the conduction band minimum at the M point. The energy of formation of SFs in 3C–, 4H–, and 6H–SiC on the other hand is found to be of order a few meV/pair. Thus, there is a thermodynamic driving force promoting growth of SF area in an n-type sample. Radiationless recombination of electrons trapped at the SF with holes is proposed to provide sufficient energy to overcome the partial dislocation motion barriers towards formation of additional SF area in a device under forward bias.

Electronic band structure of SiC polytypes : A discussion of theory and experiment

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 put on the same footing by examining Brillouin zone folding effects. Then we discuss the dependency of some of the important eigenvalues on hexagonality. Next, we discuss some of the available spectroscopic information on the band structures. Finally, we examine some of the details near the gaps in further detail such as the location of the conduction-band minima, the effective masses and the crystal field splittings and masses of the upper valence bands. Open questions and areas where experimental verification is needed are pointed out.

Substitutional diatomic molecules NO, NC, CO, N2, and O2: Their vibrational frequencies and effects on p doping of ZnO

First-principles calculations show that AB defects substituting on an O site in ZnO where A, B=N, O, or C are an important class of defects whose physical properties cannot be described by the usual split interstitials but rather by substitutional diatomic molecules. The molecular natures of the (AB)O defects are reflected in their vibrational frequencies which are redshifted from those of the corresponding free molecules but only by about 10%. These calculated results agree with the frequency range recently observed by IR measurement on N-doped ZnO. Moreover, most (AB)O defects are donors in p-type samples. The (NC)O and (N2)O defects have sufficiently low energies to convert substituional NO acceptors into donors, thereby hindering the efforts of doping ZnO p type.

Identification of acceptor states in Li-doped p-type ZnO thin films

We investigate photoluminescence from reproducible Li-doped p-type ZnO thin films prepared by dc reactive magnetron sputtering. The LiZn acceptor state, with an energy level located at 150meV above the valence band maximum, is identified from free-to-neutral-acceptor transitions. Another deeper acceptor state located at 250meV emerges with the increased Li concentration. A broad emission centered at 2.96eV is attributed to a donor-acceptor pair recombination involving zinc vacancy. In addition, two chemical bonding states of Li, evident in x-ray photoelectron spectroscopy, are probably associated with the two acceptor states observed.

Total energy differences between SiC polytypes revisited

The total energy differences between various SiC polytypes (3 C ,6 H ,4 H ,2 H ,1 5 R, and 9R) were calculated using the full-potential linear muffin-tin orbital method using the Perdew-Wang generalized gradient approximation @I. P. Perdew, in Electronic Structure of Solids ’91, edited by P. Ziesche and H. Eschrich ~Akademie-Verlag, Berlin, 1991! ,p . 11 #to the exchange-correlation functional in the density-functional method. Numerical convergence versus k-point sampling and basis-set completeness are demonstrated to be better than 0.5 meV/atom. The parameters of several generalized anisotropic next-nearest-neighbor Ising models are extracted and their significance and consequences for epitaxial growth are discussed. @S0163-1829~98!01019-4# I. INTRODUCTION Despite many years of study, the origin of polytypism in SiC is still not completely understood. A much debated question is whether polytypism is a manifestation of kinetic factors during growth or whether polytypes should be viewed as distinct ~possibly metastable! thermodynamic phases with a specific stability range of external parameters ~such as pressure and temperature!. In a thermodynamic approach to the problem, the most important quantities are the total freeenergy differences between the various polytypes. A major contribution to the latter is the energy difference at zero temperature. Vibrational entropy contributions at higher temperature were discussed by Heine et al. 1,2 and Zywietz et al. 3 Several groups have performed first-principles local-densityfunctional calculations of these energy differences using the norm-conserving pseudopotential plane-wave method. 4‐8 However, there are significant discrepancies between the results of various calculations for these energy differences, which are of order of a few meV/atom or less. More seriously, the three more recent calculations appear to invalidate some of the important conclusions drawn from these calcu

Passivation and Doping due to Hydrogen in III-Nitrides

We have systematically studied the electronic structure and stability of hydrogen in AlN, GaN, and InN, based on first-principles calculations. In GaN and AIN, H is amphoteric and always compensates the prevailing conductivity: in GaN, H + is stable for Fermi levels below 2.2 eV, and in AlN, H + is stable for E F below 2.5 eV. In InN, we find that H + is stable for all Fermi level positions; i.e., H behaves exclusively as a donor. Consequences for controlling the conductivity of InN are discussed.

Resolving hydrogen binding sites by pressure—A first-principles prediction for ZnO

The binding sites and vibrational frequencies ω of H in ZnO are studied by first-principles total-energy calculations. In the past, different experiments have observed different primary H vibrational modes, making the comparison with theory, and hence the identification of the most favorable H site, difficult. Here, we show that by applying a hydrostatic pressure, one should be able to make an unambiguous distinction, in particular, between the bond center sites and antibonding sites. This is because ω should increase with pressure for the former but decrease for the latter with the magnitude of calculated slopes about 4cm−1∕GPa, which should be large enough to measure.

Unintentional doping and compensation effects of carbon in metal-organic chemical-vapor deposition fabricated ZnO thin films

Carbon is a typical impurity in thin films fabricated by metal-organic chemical-vapor deposition (MOCVD). The role of carbon in undoped and nitrogen-doped ZnO thin films was studied experimentally and theoretically to understand the possible compensation effects. ZnO thin films are fabricated by low-pressure MOCVD using diethylzinc, nitric oxide (for nitrogen-doped films), or oxygen precursors (for undoped films). Compared with sputtering-fabricated ZnO film, the carbon concentration in the MOCVD-fabricated ZnO film is very high. Furthermore, the MOCVD-fabricated ZnO:N film has an even higher carbon concentration than the undoped ZnO. Considering the signal observed previously by Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy, it is possible that the incorporated carbon has formed complexes with doped nitrogen. The first-principles calculations predict that the formation energy for carbon interstitial (Ci) is relatively high. However, due to the large binding energy between C...