Defect Levels In Semiconductors Research Articles

Selection of dopants and doping sites in semiconductors: the case of AlN

The choices of proper dopants and doping sites significantly influence the doping efficiency. In this work, using doping in AlN as an example, we discuss how to choose dopants and doping sites in semiconductors to create shallow defect levels. By comparing the defect properties of CN, ON, MgAl, and SiAl in AlN and analyzing the pros and cons of different doping approaches from the aspects of size mismatch between dopant and host elements, electronegativity difference and perturbation to the band edge states after the substitution, we propose that MgAl and SiAl should be the best dopants and doping sites for p-type and n-type doping, respectively. Further first-principles calculations verify our predictions as these defects present lower formation energies and shallower defect levels. The defect charge distributions also show that the band edge states, which mainly consist of N- s and p orbitals, are less perturbed when Al is substituted, therefore, the derived defect states turn out to be delocalized, opposite to the situation when N is substituted. This approach of analyzing the band structure of the host material and choosing dopants and doping sites to minimize the perturbation on the host band structure is general and can provide reliable estimations for finding shallow defect levels in semiconductors.

Alloy Engineering of Defect Properties in Semiconductors: Suppression of Deep Levels in Transition-Metal Dichalcogenides.

Developing practical approaches to effectively reduce the amount of deep defect levels in semiconductors is critical for their use in electronic and optoelectronic devices, but this still remains a very challenging task. In this Letter, we propose that specific alloying can provide an effective means to suppress the deep defect levels in semiconductors while maintaining their basic electronic properties. Specifically, we demonstrate that for transition-metal dichalcogenides, such as MoSe_{2} and WSe_{2}, where anion vacancies are the most abundant defects that can induce deep levels, the deep levels can be effectively suppressed in Mo_{1-x}W_{x}Se_{2} alloys at low W concentrations. This surprising phenomenon is associated with the fact that the band edge energies can be substantially tuned by the global alloy concentration, whereas the defect level is controlled locally by the preferred locations of Se vacancies around W atoms. Our findings illustrate a concept of alloy engineering and provide a promising approach to control the defect properties of semiconductors.

A simplified approach to the band gap correction of defect formation energies: Al, Ga, and In-doped ZnO

The calculation of defect levels in semiconductors within a density functional theory approach suffers greatly from the band gap problem. We propose a band gap correction scheme that is based on the separation of energy differences in electron addition and relaxation energies. We show that it can predict defect levels with a reasonable accuracy, particularly in the case of defects with conduction band character, and yet is simple and computationally economical. We apply this method to ZnO doped with group III elements (Al, Ga, In). As expected from experiment, the results indicate that Zn substitutional doping is preferred over interstitial doping in Al, Ga, and In-doped ZnO, under both zinc-rich and oxygen-rich conditions. Further, all three dopants act as shallow donors, with the +1 charge state having the most advantageous formation energy. Also, doping effects on the electronic structure of ZnO are sufficiently mild so as to affect little the fundamental band gap and lowest conduction bands dispersion, which secures their n-type transparent conducting behavior. A comparison with the extrapolation method based on LDA+U calculations and with the Heyd–Scuseria–Ernzerhof hybrid functional (HSE) shows the reliability of the proposed scheme in predicting the thermodynamic transition levels in shallow donor systems.

Many-bodyGWcalculation of the oxygen vacancy in ZnO

Density functional theory (DFT) calculations of defect levels in semiconductors based on approximate functionals are subject to considerable uncertainties, in particular due to inaccurate band gap energies. Testing previous correction methods by many-body GW calculations for the O vacancy in ZnO, we find that: (i) The GW quasi-particle shifts of the VO defect states increase the spitting between occupied and unoccupied states due to self-interaction correction, and do not reflect the conduction versus valence band character. (ii) The GW quasi-particle energies of charged defect states require important corrections for supercell finite size effects. (iii) The GW results are robust with respect to the choice of the underlying DFT or hybrid-DFT functional, and the (2+/0) donor transition lies below mid-gap, close to our previous prediction employing rigid band edge shifts.

Energies for muonium defect levels in semiconductors

We have determined the ionization energies for Mu 0 centers in several semiconductor materials to locate the thermodynamic donor and acceptor levels for muonium with respect to the conduction and valence band edges. For Si, Ge, GaAs, and GaP the bond-centered (BC) donor level lies above the T-site acceptor level, verifying the negative-U character of muonium impurities. The midpoint between the acceptor and donor levels lies at a common energy for these materials to within the experimental uncertainties when using the theoretical band alignments. Mu defect levels can be assigned for ZnSe and 6H-SiC which match this result. Experimental data for muonium are compared to existing experimental results and predictions for the analogous hydrogen defect centers.

HIGH PRESSURE TECHNIQUES FOR LOW TEMPERATURE STUDIES IN DC AND PULSED MAGNETIC FIELDS

Pressure can be used to expand the parameter space available in almost any experiment and allows for the continuous tuning of the electrical and orbital properties of a material. When combined with low temperatures and high magnetic fields, it becomes a powerful tool for the exploration of the band structure and defect levels in semiconductors, exotic transport mechanisms in molecular conductors, and the coexistence of magnetism and superconductivity. We have developed a variety of miniature pressure cells to allow the user to take full advantage of these opportunities. Metallic diamond anvil cells as small as 6 mm in diameter and 8 mm in height allow the sample to be rotated in field at millikelvin temperatures. Miniature plastic DACs and sapphire ball cells, rotators, and specialized He-4 and He-3 systems have also been developed to provide similar experimental capabilities in pulsed magnetic fields. Methods and designs to generate hydrostatic pressure and techniques to perform optical and electrical measurements in DC and pulsed fields will be presented. We would like to acknowledge the technical assistance of Richard Desilets, Howard Kolb, John Farrell, and Mike Pacheco. A portion of this work was performed at the National High Magnetic Field Laboratory, which is sponsored by NSF Cooperative Agreement No. DMR-9527035 and by the State of Florida.

Radioactive Isotopes In Photoluminescence Experiments: Identification Of Defect Levels

AbstractThe characteristic life-times of radioactive isotopes can be used to label and identify defect levels in semiconductors which can be detected by photoluminescence (PL). This technique is illustrated with three examples: ZnS doped with radioactive 65Zn by neutron irradiation and GaAs doped with radioactive 111In by ion implantation. Finally we report that doping GaAs with radioactive 71As which decays to stable 71Ga can be used to create GaAs antisites in GaAs in a controlled way and to identify their levels.

Radioactive Isotopes in Photoluminescence Experiments: Identification of Defect Levels.

The characteristic lifetimes of radioactive isotopes can be used to label and identify defect levels in semiconductors which can be detected by photoluminescence (PL). This is demonstrated in GaAs doped with radioactive ${}^{111}$In. During its decay to ${}^{111}$Cd all those PL peaks increase for which Cd acceptors are involved. By deriving a quantitative relation between PL intensity and Cd concentration we show that this intensity increase is determined only by the nuclear lifetime of ${}^{111}$In. Thus we gain a complete and independent identification of the Cd related PL peaks in GaAs.

Brewster angle spectroscopy: A new method for characterization of defect levels in semiconductors

A new optical method which allows the identification of electronic defects in semiconductors is presented. Deep level characterization is done by detecting changes of the Brewster angle induced by optically excited transitions involving defects. An empirical model is developed which correlates the minima of the derivative of the Brewster angle as a function of photon energy with the energetic locations of defects in the semiconductor gap. Contactless room-temperature measurements on n-GaAs (100) and p-InP (111) clearly reveal defects with high accuracy, including the well known EL2 and EL12 centers in GaAs. The applicability of the method for semiconductor device technology processes is discussed.

Deep-level defects in silicon and the band-edge hydrostatic deformation potentials.

Evidence for an energy reference level based on substitutional transition-metal defect levels in semiconductors is presented and used to obtain the values for the band-edge hydrostatic deformation potentials in silicon. From the pressure derivatives of Pt and Pd acceptors in silicon we derive values of ${\ensuremath{\Xi}}_{d}=\ensuremath{-}0.5$ eV and ${a}_{v}=+0.9$ eV. These values are consistent with recent theoretical calculations and with the analysis of acoustic-phonon-limited electron and hole mobility.

A new method of analysis of photoluminescence decay curves

We describe here an alternative method of analyzing luminescence decay data, which is less computer-time-consuming than the more commonly used non-linear least-squares method. The method, which is based on the method used in deep-level transient spectroscopy, involves measuring the luminescent intensity at two different instants of time and looking for the occurrence of peaks when the differential luminescent intensities are plotted as a function of temperature. We applied this method of analysis to the study of the medium-wave (820 nm) luminescence in Cu 2O, and found an activation energy E = 0.27 (±0.01) eV. The result obtained by non-linear least-squares analysis was 0.29 (±0.03) eV. The close agreement between the results shows the validity of using this new method of analysis. The potential use of this method is in the characterization of defect levels in semiconductors by comparing peaks in the differential luminescent intensity versus temperature curves.

Electronically active defects in cw beam-annealed Si. II. Deep-level transient spectroscopy

Deep-level transient spectroscopy, a powerful capacitance method for the study of deep defect levels in semiconductors, is used to study cw beam-annealing processes (scanned Ar+ laser and scanned electron beam) for ion-implanted Si. A dominant hole trap (Ev +0.45 eV), whose concentration increases by more than one order of magnitude with increasing laser power, was observed in cw laser-annealed samples immediately after sample preparation. In contrast, only a low concentration of hole traps appears in electron-beam-annealed Si. This laser-induced defect is not stable at room temperature; instead it decays as a function of time and transmutes to a shallow level at Ev +0.10 eV. The recovery of the Ev +0.45 eV level can be stimulated by low-temperature annealing or by minority carrier injection. For the furnance-annealed control samples, rapid quenching from sufficiently high temperature into water produces the same defect energy level and annealing characteristics as the laser-induced defects. By correlating these results with those in the published literature, the laser-induced, quenched-in defects are identified as interstitial Fe and Fe-B pair reactions in Si. Possible sources of Fe in Si will be discussed.

Evaluation of tight-binding models for deep defect levels in semiconductors

Two of the fundamental assumptions used in contemporary tight-binding models for calculating the energies of deep defect levels in semiconductors are examined through comparison with a general model that avoids these assumptions. It is found that these assumptions are likely to invalidate the chemical trends predicted by the tight-binding models for the ordering of the impurity energy levels with atomic potentials.

Improved Infrared-Response Technique for Determining Impurity and Defect Levels in Semiconductors

The response of reverse-biased germanium diodes to monochromatic infrared radiation has been studied. Specimens included those fabricated from crystals doped with either copper or gold, or subjected to heat treatment. Preliminary results are reported that show the technique to be useful for identifying such impurities or defects in both lithium-compensated and high-purity germanium specimens.