Session L24: Focus Session: Dopants and Defects in Semiconductors - Chalcogenides, Oxides, and Ternaries

AX centers in II-VI semiconductors: Hybrid functional calculations

Group-V acceptors should be efficient hole producers in II-VI compounds as in ZnTe. However, good $p$-type conductivity remains elusive, for example in ZnO and ZnS. With regard to this low doping efficiency, we will discuss the dopant self-compensation in II-VI semiconductors through the formation of the AX center. These are acceptor-induced defect that acts as a donor to compensate the acceptor itself. We show that the artificially high valence band maximum in Local density approximation and Generalized gradient approximation calculations can lead to incorrect prediction on the stability of the AX center in these semiconductors. The hybrid functional calculations that correct the band gap, significantly stabilize the AX centers for selected group-V acceptor dopants in ZnO, ZnS, and ZnSe. The results on AX centers obtained by hybrid functional calculations agree well with the experimentally observed doping phenomena in ZnS and ZnSe.[1] [1] Koushik Biswas and Mao-Hua Du, Applied Physics Letters 98, 181913 (2011).   

Magnetic Main Group Impurities in CdS

With the development of magnetic semiconductors, the role of the magnetism of impurities came again into the focus of research. For \textit{d}- and \textit{f}-­electron impurities, the situation seems to be rather clear. A new field appears when one starts to study magnetism produced by vacancies or by atoms, which usually do not carry any magnetic moments in a bulk solid. Starting from the magnetism of carbon vacancies in graphene we will present a study of CdS where S is replaced by main group elements. On the basis of ab­initio supercell calculations employing density functional theory (DFT) we investigate the behaviour of impurities (B, N, C, O, F, Al, Si, P, Ga, Ge) in wurtzite (w) and zincblende (zb) CdS lattices. It is found that the impurities prefer the sulfur position and most of them, depending on the concentration exhibit magnetic order. We find that for small concentrations (64zb/72w and 32zb atom supercells) a half metallic ferromagnetic behaviour is found. For a 16 atom supercell for both zb­ and w­structure partly also unsaturated magnetic moments occur. A field dependence of the magnetic moments in these materials may lead to new technological applications in these magnetic semiconductors as tunable spin injection materials.   

Mechanisms and Kinetics of Tellurium Precipitation in CdTe-based Materials

CdTe and related alloys are important materials for solar photovoltaic application as well as for high-resolution room-temperature gamma radiation detectors. However, the performance of devices, particularly in high-energy applications, is limited by various material defects. Among the most important defects are Te precipitates of various sizes caused by non-stoichiometric growth conditions. In this work, we study the kinetics of Te aggregation and precipitation at the atomic scale. Density functional theory is used to compute the energetics, migration rates, and binding energies of point defects involved in Te aggregation, which include various interstitials, vacancies, and anti-site defects. Kinetic Monte Carlo is then used to simulate the aggregation process leading to precipitation nuclei. The mechanisms and kinetics of formation of these Te-rich regions are analyzed for various conditions. Prepared by LLNL under Contract DE-AC52-07NA27344.   

Luminescence-Based Characterization of Copper Vacancies in Optical Float Zone Refined Cuprous Oxide

Cuprous oxide (Cu$_2$O) is ideal for studying Wannier-Mott excitons which have anomalously long lifetimes in this material. However copper vacancies have a deleterious effect on the exciton lifetimes. We have measured the associated luminescence spectra in optically excited single crystals. These crystals were prepared in a radiantly-heated float-zone refiner from thermally oxidized copper rod of purity 0.999. The behavior of the vacancy luminescence can be related to exciton propagation.   

Ab initio calculations of intrinsic defects in ZnSb

Thermoelectric materials are capable of interconverting heat and electricity. The most efficient thermoelectric materials are heavily doped semiconductors, and hence they can be either n- or p-type. ZnSb has recently been predicted by theoretical calculations to be a good n-type thermoelectric material. However, synthesis produces p-type materials. Intrinsic point defects have been investigated as a possible origin using ab initio calculations. Negatively charged Zn vacancies are found to have a low formation energy, and an intrinsic p-type behavior is predicted.   

First-principles study of defect properties of zinc blende MgTe

We studied the general chemical trends of defect formation in MgTe using first-principles band structure methods. The formation energies and transition energy levels of intrinsic defects and extrinsic impurities and some defect complexes in zinc blende MgTe were calculated systematically using a new hybrid scheme. The limiting factors for $p$- and $n$-type doping in MgTe were investigated. Possible solutions to overcome the doping limitation of MgTe are proposed. The best $p$-type dopant is suggested to be N with nonequilibrium growth process and the best $n$-type dopant is suggested to be I with its doping complex V$_{Mg}$ + 4I$_{Te}$.   

Charged excitons and biexcitons bound to isoelectronic centers

We demonstrate that isoelectronic centers formed from two isoelectronic traps can bind, in addition to the well-studied excitons, various number of positive and negative charges. Two different systems are studied by microphotoluminescence: 1) tellurium dyads in ZnSe forming hole traps and 2) nitrogen dyads in GaAs forming electron traps. By analyzing their emission fine structure, polarization and diamagnetic shifts, we establish that Te and N dyads can bind, respectively, positively and negatively charged excitons. Using the power dependence of the emission intensity, we clearly demonstrate that both systems can also bind biexcitons. This ability to bind various charge configurations, in addition to their very low inhomogeneous broadenings and perfectly defined symmetries, further establishes isoelectronic centers as an interesting alternative to epitaxial quantum dots for a number of applications.   

New perspective on formation energies and energy levels of point defects in non-metals

We propose a powerful scheme to accurately determine the formation energy and thermodynamic charge transition levels of point defects in non-metals. Previously unknown correlations between defect properties and the valence-band width of the defect-free host material are identified allowing for a determination of the former via an accurate knowledge of the latter. These correlations are identified through a series of hybrid density functional theory computations and an unbiased exploration of the parameter space that defines the Hyde-Scuseria-Ernzerhof family of hybrid-functionals. The applicability of this paradigm is demonstrated for point defects in several insulators, including Si, Ge, ZrO$_{2}$ and ZnO   

Hydrogen-related defects in SnO$_{2}$

Symmetry arguments along with a mass-and spring analysis of infrared absorption experiments made with polarized light on OH defects in SnO$_{2}$ yield significant insights into the possible structures of one- and two-OH defects that have been observed recently [1]. Namely, a two-OH defect must involve symmetry-equivalent OH sites, and the axes of both one-and two-OH defects are only slightly displaced from the perpendicular to the c-axis of the rutile structure. Such results cannot be explained by models involving one or two H trapped at a Sn vacancy. Rather, they are consistent with OH defects associated with either a metal atom substituting for Sn, or an interstitial metal atom (such as Sn). Results of detailed quantum-mechanical calculations [2] using CRYSTAL06 are consistent with OH structures involving a Sn interstitial. \\[4pt] [1] Figen Bekisli \textit{et al.,} Phys. Rev. B \textbf{84}, 035213 (2011). \\[0pt] [2] R. Dovesi \textit{et al.}, \textbf{\textit{Crystal06 User's Manual}}, Univ. of Torino, Torino, 2006.   

Polarization effect in the Ionic conductor TlBr

TlBr is an ionic crystal that in recent years has been standing out as one of the most promising materials for effective room temperature radiation detection. However, its exceptional performance invariably degrades after operation times that vary from hours to several weeks. This phenomenon, known as polarization, is assigned to the undesirable ionic current that sets in the crystal under an applied bias, leading to the accumulation of oppositely charged Tl+ and Br- ions at the electric contacts of the device. This charge build up induces a field that opposes the applied bias, impairing the collection of the photo-induced carriers. In this presentation, we use parameter free quantum mechanical simulations to discuss the possible origins of the polarization effect in TlBr, showing that ionic mobility in the intrinsic material is not enough to account for effects reported by several groups. We then discuss other possible causes for the degradation of biased TlBr and propose ways to prevent its occurrence, via careful co-doping as well as a judicious choice of the metal contacts to be employed.   

Structural diversity and electronic properties of Cu2SnX3 (X=S, Se): A first-principles investigation

The ternary semiconductors Cu$_2$SnX$_3$ (X=S, Se) are found frequently as secondary phases in synthesized Cu$_2$ZnSnS$_4$ and Cu$_2$ZnSnSe$_4$ samples, but previous reports on their crystal structures and electronic band gaps are conflicting. Here we report their properties as calculated using a first-principles approach. We find that: (i) the diverse range of crystal structures can all be derived from the zinc-blende structure. (ii) The energy stability of different structures is determined primarily by the local cation coordination around anions, which makes Cu and Sn partially disordered in the cation sublattice. (iii) The direct band gaps of the low energy compounds Cu$_2$SnS$_3$ and Cu$_2$SnSe$_3$ should be in the range of 0.8-0.9 eV and 0.4 eV respectively.   

Fe$_{2}$MCh$_{4}$ (M=Si,Ge, Ch=S,Se): optical, electrical properties and defects

Fe$_{2}$MCh$_{4}$ (M=Si,Ge, Ch=S,Se) are proposed as solar absorber materials, as a single phase, stable alternative to FeS$_{2}$ pyrite. Native defects have high formation energy in the band gap and no Fermi level pinning is predicted by DFT. External defect calculations, including B,Al,Ga,In, N, P, As, Sb, Bi, O, Se, predict substantial effect on the bulk electronic properties.   

Structure, stability, and defect analysis of potential solar absorber Cu$_3$PSe$_4$

The semiconductor Cu$_3$PSe$_4$ has recently been established to have a direct band gap of 1.4 eV and exhibit p-type conductivity [Applied Physics Letters, 99, 181903 (2011)]. Here we present density functional theory (DFT) and post-DFT results regarding the structure, stability, and dopability of Cu$_3$PSe$_4$. We a find a strong coupling between the electronic band gap and the atomic structure, clearly caused by the strong P-Se antibonding character of the conduction band. Using the Heyd-Scuseria-Enzerhof hybrid functional and $GW$ approximation methods, we find that structural relaxation using standard DFT is not sufficiently accurate to be used as input to static, post-DFT electronic structure calculations. We use the generalized gradient approximation (GGA) and the GGA+$U$ method to show a thermodynamically stable low temperature region. We calculate the formation enthalpies of intrinsic and extrinsic defects in order to understand the observed p-type behavior and to examine n-type doping mechanisms.   

Effect of Cation Disorder on Electronic Structures and Optical Properties of Magnetoelectric Gallium Ferrite: A First-principles Study

Discovery of photovoltaic effects in oxide materials, especially in magnetoelectrics and multiferroics has lead to renewed interests in the optical properties of these materials. Magnetoelectrics with their transition temperature close to room temperature are of particular interest from the perspective of practical applications. Magnetoelectric gallium ferrite (GFO), a material of current interest, exhibits good optical activity and a transition temperature tunable to room temperature and above upon tailoring of cation stoichiometry. Here, we show a detailed first principle study on the electronic structure and optical properties of GFO. We have performed first-principles density functional calculations using GGA+U method to compute the electronic band structure and density of states of the ground state structure of GFO having orthorhombic \textit{Pc2}$_{1}n$ symmetry and A-type antiferromagnetic spin configuration. The calculations show that GFO possesses a direct band gap of $\sim $ 2.3 eV, making it a rather low band gap oxide. Subsequent calculations of real and imaginary parts of dielectric constants, refractive index (n), extinction coefficient (k), reflectivity (R), etc demonstrate good agreement with experimental spectra which is further improved upon introduction of cationic site disorder into the ground state structure substantiating the presence of cation site disorder in the material. We also show that major optical transitions in GFO involve transitions from valence band O 2p to conduction band Fe 3d states.   

Radiative defects in thallium chacolgenide semiconductors

Thallium chalcohalides constitute a promising new class of semiconductor compounds for radiation detectors. Due to their wide energy bandgap, high atomic number, and high resistivity, they are being considered as potential replacement for conventional II-VI semiconductor x-ray and $\gamma $-ray detectors for room temperature operation. For these applications resistivities of $\sim $10$^{10}$ ohm-cm are required. Although defects play a major role in detector response, little is known about their nature and origin in these compounds. We have investigated Tl$_{6}$I$_{4}$Se and Tl$_{6}$I$_{4}$S compounds which have bandgaps of 1.86 and 2.03 eV, respectively. Photoluminescence (PL) spectra of single crystals were investigated in the 650-885 nm wavelength region and over a temperature range of 20-100 K. For Tl$_{6}$I$_{4}$Se we observed PL bands centered at 1.61 eV. A detailed study of the peak, as function of temperature and excitation intensity, indicates that it is due to radiative transitions from donor-acceptor pairs (DAP). The ionization energies of the donor and acceptor levels in Tl$_{6}$I$_{4}$Se were estimated at 52 and 290 meV, respectively. Similarly DAP emission in Tl$_{6}$I$_{4}$S with a peak at 1.66 eV was observed. The role of crystalline stoichiometry in DAP formation is currently under study.