Neutral Impurity Research Articles

Mobile impurities and orthogonality catastrophe in two-dimensional vortex lattices

We investigate the properties of a neutral impurity atom coupled with the Tkachenko modes of a two-dimensional vortex lattice in a Bose-Einstein condensate. In contrast with polarons in homogeneous condensates, the marginal impurity-boson interaction in the vortex lattice leads to infrared singularities in perturbation theory and to the breakdown of the quasiparticle picture in the low-energy limit. These infrared singularities are interpreted in terms of a renormalization of the coupling constant, quasiparticle weight, and effective impurity mass. The divergence of the effective mass in the low-energy limit gives rise to a power-law singularity in the impurity spectral function and provides an example of an emergent orthogonality catastrophe in a bosonic system.

Development of a New Type of Germanium Detector for Dark Matter Searches

A new type of germanium (Ge) detector for dark matter searches is under development utilizing the Ge crystal growth facility recently established at the University of South Dakota. Detector-grade crystals with electric impurity levels within 1010/cm3 and neutral impurity levels within 1014/cm3 have been grown regularly in the laboratory. These crystals can be fabricated into planar detectors with 1cm in thickness and 10cm in diameter. When a high voltage is applied to one of the end planes, a uniform electric field in the volume can be established. Such a design could result in a very fast electric signal. A time resolution of 1ns is expected by combining a short drift length and large drift mobility. This may allow us to resolve the difference on the electric pulse rise-time between low-energy nuclear recoil events and electronic recoil events at liquid nitrogen temperatures. An array of 168 planar detectors of this kind was modeled in a Geant4-based Monte Carlo simulation package. Its background reduction power was investigated and its sensitivity in dark matter search is estimated to be ~10-48cm2.

Growth and electro-optical properties of Ga-doped ZnO films prepared by aerosol assisted chemical vapour deposition

Abstract Transparent conductive Ga-doped ZnO thin films were deposited onto glass substrates by a low-cost aerosol assisted chemical vapour deposition technique and the effect of gallium content on the ZnO film growth behaviour and opto-electronic properties was systematically investigated. It is found that, upon increasing Ga addition, the ZnO film crystallinity exhibits a continuous reduction in quality associated with the preferential orientation transformed from (002) to (102). The (002) oriented samples had a microstructure of parallel columnar grains while the (102) oriented coating was thickened by overlapping particles. The ZnO:Ga coatings exhibit high carrier concentration (up to 4.1 × 1020 cm− 3) but low carrier mobility (up to 0.8 cm2 V− 1 s− 1), resulting in a minimum resistivity value of 2.3 × 10− 2 Ω cm. The inferior carrier mobility performance could result from a profound ionized and neutral impurity scattering effect. Good visible transmittance (≈ 70–80%) is observed in these ZnO:Ga films and samples with higher carrier density present better infrared reflection performance (up to 37.2% at 2500 nm).

New non-linear photovoltaic effect in uniform bipolar semiconductor

A linear theory of the new non-linear photovoltaic effect in the closed circuit consisting of a non-uniformly illuminated uniform bipolar semiconductor with neutral impurities is developed. The non-uniform photo-excitation of impurities results in the position-dependant current carrier mobility that breaks the semiconductor homogeneity and induces the photo-electromotive force (emf). As both the electron (or hole) mobility gradient and the current carrier generation rate depend on the light intensity, the photo-emf and the short-circuit current prove to be non-linear functions of the incident light intensity at an arbitrarily low illumination. The influence of the sample size on the photovoltaic effect magnitude is studied. Physical relations and distinctions between the considered effect and the Dember and bulk photovoltaic effects are also discussed.

Sub-meV linewidth in GaN nanowire ensembles: Absence of surface excitons due to the field ionization of donors

We observe unusually narrow donor-bound exciton transitions (0.4 meV) in the photoluminescence spectra of GaN nanowire ensembles grown on Si(111) substrates at very high (> 850 degrees Celsius) temperatures. The spectra of these samples reveal a prominent transition of excitons bound to neutral Si impurities which is not observed for samples grown under standard conditions. Motivated by these experimental results, we investigate theoretically the impact of surface-induced internal electric fields on the binding energy of donors by a combined Monte Carlo and envelope function approach. We obtain the ranges of doping and diameter for which the potential is well described using the Poisson equation, where one assumes a spatially homogeneous distribution of dopants. Our calculations also show that surface donors in nanowires with a diameter smaller than 100 nm are ionized when the surface electric field is larger than about 10 kV/cm, corresponding to a doping level higher than 2 x 10^16 cm^-3. This result explains the experimental observation: since the (D+,X) complex is not stable in GaN, surface-donor-bound excitons do not contribute to the photoluminescence spectra of GaN nanowires above a certain doping level, and the linewidth reflects the actual structural perfection of the nanowire ensemble.

Determining the Electronic Performance Limitations in Top-Down-Fabricated Si Nanowires with Mean Widths Down to 4 nm

Silicon nanowires have been patterned with mean widths down to 4 nm using top-down lithography and dry etching. Performance-limiting scattering processes have been measured directly which provide new insight into the electronic conduction mechanisms within the nanowires. Results demonstrate a transition from 3-dimensional (3D) to 2D and then 1D as the nanowire mean widths are reduced from 12 to 4 nm. The importance of high quality surface passivation is demonstrated by a lack of significant donor deactivation, resulting in neutral impurity scattering ultimately limiting the electronic performance. The results indicate the important parameters requiring optimization when fabricating nanowires with atomic dimensions.

Comment on “Contributions of vacancies and self-interstitials to self-diffusion in silicon under thermal equilibrium and nonequilibrium conditions”

Comment on “Contributions of vacancies and self-interstitials to self-diffusion in silicon under thermal equilibrium and nonequilibrium conditions”

Lateral scan profiles of the recombination parameters correlated with distribution of grown-in impurities in HPHT diamond

Abstract The profiling of the microwave probed photoconductivity transients and of the time resolved photoluminescence spectra has simultaneously been performed on the synthetic diamond wafer samples in order to clarify correlation of the distribution of the grown-in defects and carrier radiative and non-radiative recombination channels. The wafer samples were sliced from the diamond single crystals, synthesized by the high pressure and high temperature (HPHT) technology using the Ni–Fe liquid solvent/catalyst system. To estimate the microscopic characteristics of the grown-in defects, the EPR spectroscopy has been carried out. The changes of carrier lifetime, ascribed to the radiative and non-radiative recombination, have been resolved. The correlation between the parameters extracted from the absorption spectra and recombination characteristics has been obtained. The prevailing grown-in point defects, associated with neutral and charged nitrogen impurity complexes as well as the pairs of nitrogen atoms on neighbouring sites, and the nickel precipitates have been unveiled.

Equivalence of donor and acceptor fits of temperature dependent Hall carrier density and Hall mobility data: Case of ZnO

In this work, statistical formulations of the temperature dependence of ionized and neutral impurity concentrations in a semiconductor, needed in the charge balance equation and for carrier scattering calculations, have been developed. These formulations have been used in order to elucidate a confusing situation, appearing when compensating acceptor (donor) levels are located sufficiently close to the conduction (valence) band to be thermally ionized and thereby to emit (capture) an electron to (from) the conduction (valence) band. In this work, the temperature dependent Hall carrier density and Hall mobility data adjustments are performed in an attempt to distinguish the presence of a deep acceptor or a deep donor level, coexisting with a shallower donor level and located near the conduction band. Unfortunately, the present statistical developments, applied to an n-type hydrothermal ZnO sample, lead in both cases to consistent descriptions of experimental Hall carrier density and mobility data and thus do not allow to determine the nature, donor or acceptor, of the deep level. This demonstration shows that the emission of an electron in the conduction band, generally assigned to a (0/+1) donor transition from a donor level cannot be applied systematically and could also be attributed to a (−1/0) donor transition from an acceptor level. More generally, this result can be extended for any semiconductor and also for deep donor levels located close to the valence band (acceptor transition).

Highly transparent and conductive indium tin oxide thin films for solar cells grown by reactive thermal evaporation at low temperature

Transparent conductive tin-doped indium oxide (In2O3:Sn, ITO) thin films with various Sn-doping concentrations have been prepared using the low cost reactive thermal evaporation (RTE) technique at a low growth temperature of ~160 °C. The structural characteristics, optical and electrical properties of the ITO thin films were investigated. These polycrystalline ITO films exhibited preferential orientation along (222) plane and possessed low resistivities ranging from 3.51 to 5.71 × 10−4 Ω cm. The decreased mobility was attributed to the scattering by ionized and neutral impurities at high doping concentrations. The optimized ITO thin film deposited with 6.0 wt% Sn-doping concentration exhibited a high average transparency of 87 % in the wavelength range of 380–900 nm and a low resistivity of 3.74 × 10−4 Ω cm with a high Hall mobility of 47 cm2 V−1s−1. A hydrogenated amorphous silicon and silicon–germanium (a-Si:H/a-SiGe:H) double-junction solar cell fabricated with the RTE-grown ITO electrodes presented a conversion efficiency of 10.51 %.

The nature of dielectric state and self compensation mechanisms in PbTe doped with Ga

The long-standing problem of impurity states in narrow-gap PbTe crystals doped with group-III element Ga was analized by means of density functional theory. We focus on the mechanisms of the self-compensation during growth as well as during post-growth annealing to clarify the mechanism of dielectric state formation necessary for the device fabrication. The unique feature of the presented work is consideration of the simplest impurity complex as well as of a lead vacancy , gallium substituting for Pb site and interstitial gallium in various charge states. Calculations show that complex has the lowest formation energy among other gallium-related defects and is a double donor. is a single donor while is amphoteric impurity which act as a donor or acceptor depending on the Fermi level position. Moreover, we conclude that neutral impurity is metastable due to the self-compensation and formation of complex with simultaneous creation of . Calculated binding energy of this complex suggests that it is stable for the actual temperatures and concentrations. In addition the defect is responsible for spontaneous creation of lead vacancy which prevents an increasing of the carrier concentration. Therefore, the considered complex determines the most striking features of PbTe crystals doped with Ga, namely DX-like properties and dielectric state formation. This defect plays a crucial role in real crystals and clarifies the nature of properties important for device fabrication.

(Invited) Significant Enhancement of High-Ns Electron Mobility in Ge n-MOSFETs with Atomically Flat Ge/GeO2 Interface

The rapid degradation of high-Ns electron mobility in Ge n-MOSFETs is still one of the greatest concerns in Ge CMOS technology. Although there are many possible origins so far considered, the degradation mechanism is still unclear in spite of its importance. In this work, we clarify wafer-related origins for electron mobility degradation in Ge n-MOSFETs. High-Ns electron mobility is dramatically improved thanks to (i) atomically flat Ge surface formation, followed by (ii) layer-by-layer oxidation. (iii) Oxygen-related neutral impurities in Ge substrates could be another origin of the mobility reduction on Ge wafers. By successfully eliminating these scattering sources in Ge n-MOSFETs, we demonstrate intrinsically high electron mobility in a wide range of Ns .

Effect of an external magnetic field on the binding energy, magnetic moment and susceptibility of an off-centre donor complex in a Gaussian quantum dot

The ground state energy and binding energy of an off-centre neutral hydrogenic donor impurity (D0) trapped in a three-dimensional quantum dot with Gaussian confinement are obtained by a variational method as a function of the position of the impurity and the magnetic field. The binding energy in the presence of a magnetic field is also calculated. The results show that the binding energy decreases as the impurity moves away from the origin.

Thermoelectric transport properties of p-type silver-doped PbS with in situ Ag2S nanoprecipitates

In this study, a series of p-type Ag-doped PbS compounds were prepared by a vacuum melting combined with subsequent spark plasma sintering process. Ag partially occupies Pb sites acting as an electron acceptor to increase the hole concentration, and also in situ forms Ag2S nanoprecipitates at the grain boundaries which are detectable in thermal analysis and microstructural observations. The two existences of Ag both exert significant influences on the electrical transport properties. With increasing temperature, dissolution of Ag2S nanoprecipitates into the PbS matrix as interstitial Ag gradually decreases the hole concentration. Very low mobility of Ag-doped samples is unexpected at low temperatures, which indicates a strong carrier scattering. From temperature dependent mobility, it can be concluded that neutral impurity scattering (μ ∼ Ta, a = 0) dominates at low temperature (T < 200 K), and energy barrier scattering (a > 1.5) governs until lattice vibrations take over at T > 450 K (a ∼ −2.5). The complex scattering mechanisms at T < 450 K in Ag-doped samples should be originated from the multiple existences of Ag (Ag+, interstitial Ag, Ag2S nanoprecipitates). In particular, Ag doping increases the high temperature electrical conductivity, giving rise to an enhanced power factor of ∼0.8 × 10−3 W m−1 K−2 at 870 K for the 1.5 at% Ag-doped sample. Benefiting from the improved power factor and low lattice thermal conductivity originating from strong lattice anharmonicity, a doubled ZT value of ∼0.6 can be achieved for Pb0.985Ag0.015S in comparison to that of pristine PbS.

Current leakage relaxation and charge trapping in ultra-porous low-k materials

Time dependent dielectric failure has become a pivotal aspect of interconnect design as industry pursues integration of sub-22 nm process-technology nodes. Literature has provided key information about the role played by individual species such as electrons, holes, ions, and neutral impurity atoms. However, no mechanism has been shown to describe how such species interact and influence failure. Current leakage relaxation in low-k dielectrics was studied using bipolar field experiments to gain insight into how charge carrier flow becomes impeded by defects within the dielectric matrix. Leakage current decay was correlated to injection and trapping of electrons. We show that current relaxation upon inversion of the applied field can be described by the stretched exponential function. The kinetics of charge trapping events are consistent with a time-dependent reaction rate constant, k=k0⋅(t+1)β−1, where 0 &amp;lt; β &amp;lt; 1. Such dynamics have previously been observed in studies of charge trapping reactions in amorphous solids by W. H. Hamill and K. Funabashi, Phys. Rev. B 16, 5523–5527 (1977). We explain the relaxation process in charge trapping events by introducing a nonlinear charge trapping model. This model provides a description on the manner in which the transport of mobile defects affects the long-tail current relaxation processes in low-k films.

Neganov–Luke Phonon Amplification in P-type Point Contact Detectors

The Cryogenic Dark Matter Search (CDMS) detectors measure ionization and athermal phonons in high purity germanium crystals to discriminate between nuclear recoils from dark matter candidates and radioactive backgrounds. In order to reach lower energy detection thresholds, the CDMSlite experiment operates the CDMS detectors with a larger voltage bias to increase the signal-to-noise ratio using the Neganov–Luke effect. Breakdown in those detectors was observed at fields of order 30 V/cm, but the reason for the breakdown is unknown. It is unclear if the breakdowns are due to surface leakage current, impact ionization in the bulk of the crystals, or some other effect due to the very low operating temperatures of the detectors. Germanium detectors used in gamma spectroscopy at 77 K are regularly operated with fields in excess of 1,000 V/cm. In order to understand the origin of breakdown in the CDMS detectors, a P-type Point Contact detector was equipped with transition edge phonon thermistors and operated at a base temperature of \(\sim \)30 mK. The linearity of the Neganov–Luke phonon amplification was studied and no sign of breakdown for biases up to 400 V was observed. This excludes impact ionization on neutral impurity states as the primary cause of the breakdown observed in the CDMSLite detectors. This demonstrates that the Neganov–Luke phonon amplification is a viable method for lowering the energy threshold in germanium detectors of masses of order 1 kg.

A Comparative Study on the Influence of Elastic Scattering Mechanisms on the Electron Mobility in Wurtzite and Zincblende Gallium Nitride

The electron mobility of zincblende (ZB) and wurtzite (WZ) structures of n-type GaN is investigated as a function of both temperature and donor concentration. Numerical calculations of the mobility are carried out in a temperature range from 10 K up to 400 K and donor doping concentrations from 10 19 m -3 to 10 27 m -3 . Elastic types of scattering mechanisms are considered. These types include ionized impurity scattering, neutral impurity scattering, deformation potential acoustic phonons scattering and piezoelectric potential scattering. The electron drift mobility versus temperature shows peak behavior for both zincblende and wurtzite GaN structures. Below approximately 120/140 K (for ZB and WZ GaN respectively), the mobility increases when the temperature is increased. In this temperature range, the ionized impurity scattering is assumed to be the dominant scattering mechanism. Above 120/140 K, the mobility is lowered down by raising the temperature. In this regime, the lattice scattering is considered to be the dominant mechanism. The variation of the electron drift mobility with donor concentration at room temperature shows a continuous decrease with the increase of impurity doping concentration. This is probably due to Coulomb scattering present particularly at low temperatures.

High mobility transparent conductive W-doped In2O3 thin films prepared at low substrate temperature and its application to solar cells

W-doped In2O3 transparent conductive film was developed at a low substrate temperature of 200°C by reactive plasma deposition technique for the applications to silicon heterojunction solar cell. The maximum Hall mobility of 89cm2/Vs with a corresponding carrier concentration of 1.6×1020cm−3 was achieved at a relatively high oxygen partial pressure of 7.2×10−4Torr without any special treatment. According to the Hall effect measurements at variable temperature from 80K to 300K, high Hall mobility was mainly attributed to the ionized impurity scattering, neutral impurity scattering and phonon scattering, grain boundary scattering only plays the minor role in the carrier transportation of IWO film in this work. Additionally, XRD results show the samples have a cubic phase of bixbyite-type In2O3 polycrystalline structure with a preferential orientation of (222) plane. The plasma wavelength of larger than 2.3μm is very beneficial for sunlight transparency. Consequently, IWO is applied to a-Si/c-Si heterojunction solar cells, 20.8% of conversion efficiency has been obtained under the optimized experimental conditions.

Size Effects of Raman and Photoluminescence Spectra of CdS Nanobelts

Different sizes of CdS nanobelts were synthesized at 800, 850, and 900 °C by the thermal evaporation of CdS powders on Au-coated silicon substrates and were used to study the size effects of Raman scattering and photoluminescent spectra. The Raman spectra of CdS nanobelts clearly exhibit first- and second-order longitudinal optical (LO) Raman peaks, surface phonon peaks, and multiphonon processes when excited using a wavelength of 532 nm. The mechanism of exciton–phonon coupling was observed to be mainly associated with the Fröhlich interaction, and the coupling strength of the exciton–phonon increases with increasing lateral size. Compared with a larger CdS nanobelt, a narrower nanobelt exhibits a larger tensile strain. Recombination of free excitons (FX), excitons bound to neutral impurities (A0X), and donor–acceptor pairs (DAP) were identified from a low-temperature PL spectrum. At temperatures below ∼123 K, a red shift of the FX energy occurs with decreasing lateral size due to a larger uniaxial tensile strain; at temperatures above ∼123 K, a red shift of the FX energy occurs with increasing lateral size because of the reabsorption of the emitted light inside the thicker belt, indicating that the FX energy is affected by both the tensile strain and the surface-depletion-induced quantum confinement (the reabsorption of the emitted light) in the nanobelt.

Indications of strong neutral impurity scattering in Ba(Sn,Sb)O3single crystals

It was recently discovered that a transparent $n$-type (Ba,La)SnO${}_{3}$ system has electrical mobility as high as 320 cm${}^{2}$ V${}^{\ensuremath{-}1}$ s${}^{\ensuremath{-}1}$ at room temperature and superior thermal stability up to \ensuremath{\sim}500 \ifmmode^\circ\else\textdegree\fi{}C. To understand comparatively the carrier-scattering mechanism in the doped BaSnO${}_{3}$, we investigate the physical properties of the single crystals of BaSn${}_{1\ensuremath{-}x}$Sb${}_{x}$O${}_{3}$ ($x$ $=$ 0.03, 0.05, and 0.10), which also show the $n$-type characters via the Sn site doping by Sb. Transmittance of the grown single crystals in the visible spectral region turn out to be similar to that of the (Ba,La)SnO${}_{3}$ system, maintaining optical transparency. Temperature-dependent Hall effect measurements reveal that the electrical mobility at room temperature reaches as high as 79.4 cm${}^{2}$ V${}^{\ensuremath{-}1}$ s${}^{\ensuremath{-}1}$ at a carrier density of 1.02\ifmmode\times\else\texttimes\fi{}10${}^{20}$ cm${}^{\ensuremath{-}3}$, and upon increasing carrier density further, it systematically decreases nearly proportional to the inverse of the carrier density. The overall reduced mobility of the Ba(Sn,Sb)O${}_{3}$ system as compared to the (Ba,La)SnO${}_{3}$ system is attributed to the enhanced scattering caused by the Sb ions located in the direct conduction path. Based on the inverse proportionality between the carrier density and the electrical mobility, we suggest that the neutral impurity scattering becomes particularly strong in the Ba(Sn,Sb)O${}_{3}$.