Session V28: Semiconductor Defects and Doping

Transition Metal Defects in Cubic and Hexagonal Polytypes of SiC: Site Selection and Electronic Structure from ab-initio Calculations

Relatively little is known about point defects in different polytypes of a crystal. Silicon carbide is a prototype material for polytypism. There are unidentified photoluminescence centers in SiC that are presumably originated from transition metal defects, however, the number of detected centers does not follow the number of inequivalent substitutional sites in different polytypes. In this study we applied highly convergent and sophisticated density functional theory (DFT) based methods to investigate important transition metal impurities including titanium (Ti), vanadium (V), niobium (Nb), chromium (Cr), molybdenum (Mo) and tungsten (W) in cubic 3C and hexagonal 4H and 6H polytypes of SiC. We applied DFT with PBE functional in order to calculate the ground state of the defects. We calculated the electronic structure by a screened hybrid density functional (HSE06) which was very successful in the quantitative description of native defects in SiC. We found a special asymmetric split-vacancy configuration for a class of transition metal defects. The asymmetric split-vacancy configuration exclusively prefers the hexagonal-hexagonal sites in hexagonal polytypes, thus the probability of finding these defects in 3C polytype is much smaller than in hexagonal polytypes.   

Quantum Effects of Strain Influence on the Dopant Behavior in Semiconductors

In most fields of physics, applying external strain (pressure) provides an important technique to investigate and tune the properties of materials. Quite often the theoretical treatment is based on the continuum elastic model, in which its validity is still under debate. In this talk, by using quantum mechanical theoretical analysis, we show that if the occupation change of different orbitals caused by the strain is negligible, the continuum elastic model is valid, otherwise it will fail. Our theory is confirmed by first-principles calculation of Mn-doped GaAs system. Moreover, we show that under compressive strain the hole density, thus the Curie temperature can increase in Mn-doped spintronic materials.   

First-principle Calculations of Donor and Acceptor Levels in PbI2 for Ultra-fast Scintillation

In the past PbI2 was studied as a candidate for semi-conductor gamma ray detectors as well as more recently as an ultra-fast scintillator for time-of-flight applications. The ultra-fast scintillation properties of this materials are believed to be related to donor-acceptor recombination in both the pure and doped system. This work presents first-principles electronic structure calculations of donor and acceptor levels for intrinsic defects and doped impurities in PbI2. We performed density functional theory calculations within the generalized gradient approximation with U correction for the donor and acceptor modeling. For a more accurate description of band gaps and structures, we used more advanced methods, such as GW. Our study shows that intrinsic defects significantly affect the luminescence properties of bulk PbI2. Moreover, the relative position of intrinsic defect levels to doped impurity levels can influence the luminosity. We will compare our theoretical work to known experimental work for this material.   

Nonradiative lifetimes in intermediate band materials -- absence of lifetime recovery

Intermediate band photovoltaics hold the promise of being highly efficient and cost effective photovoltaic cells. Intermediate states in the band gap, however, are known to facilitate nonradiative recombination. Much effort has been dedicated to producing metallic intermediate bands in hopes of producing \emph{lifetime recovery} -- an increase in carrier lifetime as doping levels increase. We show that lifetime recovery induced by the insulator-to-metal transition will not occur, because the metallic extended states will be localized by phonons during the recombination process. Only trivial forms of lifetime recovery, e.g., from an overall shift in intermediate levels, are possible. Future work in intermediate band photovoltaics must focus on optimizing subgap optical absorption and minimizing recombination, but not via lifetime recovery.   

Imaging Defects in Semiconductors with a Snapshot Micro-Imaging Technique

Polycrystalline or metamorphically grown single crystalline thin films are ubiquitous in energy-related technologies, such as solar cells and light emitting diodes. A detailed understanding of the defects contained within these materials (i.e. dislocations, grain boundaries, inclusions) and the ability to control them play critical roles in their development. We will present the use of a novel ``snapshot'' micro-imaging technique to evaluate the presence and behavior of defects in a variety of materials and devices. Photoluminescence from a wide area of a sample is imaged onto a Si CCD in a single exposure, enabling real-time mapping with sub-micron resolution. A tunable liquid-crystal filter selects the exact wavelength that is imaged. Combined with a tunable excitation source, this technique is ideal for selectively investigating defects in thin films as well as individual layers in a device.   

Fluorescent nanodiamonds for biomedical thermal imaging

The use of nanoparticles designed for a particular purpose in biological research has increased exponentially in the last ten years. Nanoscale particles have dimensions similar to those of most biological systems and present exceptional physical, chemical and optical properties. The optical absorption and emission of nanoparticles may be tuned by varying their shape, size and composition and recent advances in their synthesis and design suggest their potential use as probes in the detection and treatment of diseases such as cancer. One of these promising materials is nanodiamond which possesses excellent surface modification capacity and high biocompatibility. In addition to being compatible with the human body, nanodiamonds can be used as radiation sensors. In this report, the ability of nanodiamonds to be used as nanothermometers was studied by the obtention of a nanothermic scale (temperature dependence of nanodiamond's emission spectrum) to accurately measure temperature in small volumes.   

Laser induced local modification of material properties in a semiconductor alloy

High energy (usually short pulse) lasers are routinely used for micromachining and surface modification of solid state materials. We demonstrate that a small power CW laser can be used to induce local modification of the electronic and optical properties of a semiconductor alloy at any desirable spatial location. The degree of modification can be precisely controlled by laser density and exposure time. The spatial resolution is determined currently by the beam size of a confocal optical system, but it can also be defined by the feature size of a lithographic method. The induced local changes in properties are measured by optical spectroscopy, and the results indicate the possibility of local structural modification.   

Spatial Variation in Mobility-Lifetime Product in Bulk TlBr and CZT

The energy resolution of a semiconductor radiation detector depends on the charge transport properties of the semiconductor, and the mobility-lifetime ($\mu\tau$) product is a key figure of merit for charge transport. In this work, we investigate the effects of two impurities, Na and Cu, on the $\mu\tau$ product in bulk thallium bromide (TlBr) using cathodoluminescence (CL) and transport imaging. Transport imaging uses a scanning electron microscope to generate a line of charge carriers on the surface of a bulk sample, and the intensity and spatial distribution of the recombination luminescence are recorded. A Green's function approach is used to model the generation, diffusion, and recombination of charge carriers under steady-state conditions. The luminescence distribution is fit to the model to extract the ambipolar diffusion length and the $\mu\tau$ product, providing a high-resolution correlation between the luminescence variations due to dopants/defects and the quantitative transport behavior. The $\mu\tau$ product has been mapped across a 40 $\mu$m segment of TlBr at a resolution of 2 $\mu$m. Additionally, this approach has been used to locally map variations in ambipolar diffusion length and $\mu\tau$ product due to extended defects in cadmium zinc telluride (CZT).   

Quantum Stress: Density Functional Theory Formulation and Physical Manifestation

The concept of ``quantum stress (QS)'' is introduced and formulated within density functional theory (DFT), to underlie extrinsic electronic effects on the stress state of solids and thin films in the absence of lattice strain. An explicit expression of QS (\textit{$\sigma $}$^{Q})$ is derived in relation to the deformation potential of electronic states ($\Xi )$ and the variation of electron density (\textit{$\Delta $n}), \textit{$\sigma $}$^{Q}=\Xi $(\textit{$\Delta $n}), as a quantum analog of classical Hook's law. Two distinct QS manifestations are demonstrated quantitatively by DFT calculations: (1) in the form of bulk stress induced by charge carriers; and (2) in the form of surface stress induced by quantum confinement. QS has broad implications in physical phenomena and technological applications that are based on coupling of electronic structure with lattice strain.