Numerical simulation of the temperature dependence of the ionization energy of hydrogen-like impurities in semiconductors: Application to transmutation-doped Ge: Ga

An electrostatic model describing the dependence of the thermal ionization energy of impurities on their concentration, compensation factor, and temperature is developed. The model takes into account the screening of impurity ions by holes (electrons) hopping from impurity to impurity, the change in the impurity-band width, and its displacement with respect to the edge of the valence band for acceptors (conduction band for donors). The displacement of the impurity band is due to the functional dependence of the hole (electron) affinity of the ionized acceptor (donor) on the screening of the Coulomb field of the ions. The spatial distribution of the impurity ions over the crystal was assumed to be Poisson-like, and the energy distribution was assumed to be normal (Gaussian). For the relatively low doping levels under investigation, the behavior of the density of states at the edges of the valence and conduction bands was assumed to be the same as for the undoped crystal. The results of the numerical study are in agreement with the decrease in the ionization energy that is experimentally observed for moderately compensated Ge: Ga as the temperature and the doping level are decreased. It is predicted that the temperature dependence of the thermal ionization energy has a small anomalous maximum at small compensation factors.     

Investigation of the electrophysical properties and structures of impurity-defective complexes in silicon doped with palladium

Electric and structural methods are used to investigate formation of impurity-defective complexes in silicon doped with palladium. It is demonstrated that acceptor levels E _C – 0.18 and E _v + 0.34 eV detected in silicon during incorporation of palladium are caused by singly and doubly negatively charged states of [Pd–V] complexes, and the donor level E _v + 0.32 eV is a product of chemical compound of palladium with hydrogen forming the [Pd–H] complex. It is assumed that the palladium impurity in the doped silicon samples causes the elastic crystal energy to change and impurity clouds to be formed around microdefects. An increase in the temperature of palladium diffusion in silicon causes the impurity clouds to decay and the microdefect core sizes to decrease with their subsequent chaining into a needle.     

Donor states in tunnel-coupled quantum wells

We study the energy spectrum of the impurity states in tunnel-coupled double quantum wells for Coulomb and short-range donor potentials. We calculate the impurity contribution and the density of states and detect the transformation of a localized donor state into a resonant state when the binding energy of the donor in an isolated quantum well is less than the separation of the energy levels of the double quantum wells. In the opposite case, where the binding energy is greater than the level separation, there is tunneling repulsion between adjacent impurity levels, with the degree of degeneracy of the levels changing when there is tunneling mixing of the ground and excited impurity states from different wells. Resonant states emerge in an asymmetric double quantum well, while in a symmetric double quantum well the impurity level at the barrier’s center proves to be localized even against the background of the continuum. The calculations are based on a general expression for the impurity contribution to the density of states in terms of a 2-by-2 matrix Green’s function, i.e., only a pair of tunnel-coupled levels of the double quantum wells is taken into account. For an impurity with a short-range potential, we derive a matrix generalization of the Koster-Slater solution, while the impurity with a Coulomb potential is analyzed by using the approximation of a narrow resonance and close arrangement of the repulsive levels. Zh. éksp. Teor. Fiz. 115, 1337–1352 (April 1999)     

Recombination radiation of ZnTe crystals with ap-n junction formed by the laser doping technique

Radiative recombination is studied in ZnTe-based diodes produced on the basis of a p–n junction formed by laser doping of crystals with an Al donor impurity. The luminescence spectra obtained in the case of excitation of the diodes by direct current with a density of 3 A/cm² and the photoluminescence spectra at 80 and 300 K are measured. A comparison of the photoluminescence spectra of the p-ZnTe substrate and the electroluminescence spectra at 80 K suggests that radiative recombination in the diodes occurs in the Al-doped region of the crystal. At 80 K in the electroluminescence spectra of the diodes, a band, unknown previously, with the position of the energy maximum at 2.276 eV was observed. This band can be assigned to radiative transitions between the donor level of the al atom and the valence band. For the first time, long-wave bands assigned to participation of deep, compensatory centers were not found in radiative recombination of ZnTe-based diodes. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 3, pp. 382–386, May–June, 1998.     

Interference ionization of an impurity by an electric field in a system of quantum wells

A radical change in the ionization energy of the donor impurity Si in a GaAs/AlGaAs double-well heterostructure is observed experimentally when the electronic wave function is relocated in an electric field. The ionization energy of the impurity decreases from 15.5 meV to 0 when the voltage across the structure changes by less than 1 V. The Stark effect would give a change in the ionization energy of not more than 3 meV. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 194–199 (10 February 1999)     

Anderson localization and the pseudogap in the energy spectrum of holes in PbTe: Tl under resonant scattering

This paper reports on a comprehensive study of the effect of additional doping with the Na acceptor impurity on the low-temperature resistivity of PbTe samples doped with Tl (2 at %), an impurity producing a band of resonant states within the valence-band spectrum. By additional doping with Na, we have shifted the Fermi level within the band of the resonant states of Tl in PbTe and varied the hole filling (k _h ) of the thallium impurity states. The larger part of the PbTe: (Tl,Na) samples transfers to the superconducting state with a critical temperature T _c = 0.4–2.3 K. The T _c (k _h ) relation obtained argues for the fact that, in the region of resonant states in PbTe: Tl, Anderson localization of holes and a pseudogap in the density of delocalized states are observed.     

Density of states and dispersion of quasiparticles in the Emery model: Nonlocal path-integral Monte Carlo algorithm

The spectral density of states for a 108-site Cu₃₆O₇₂ cluster in the two-dimensional three-band Emery model is reconstructed with the aid of a path integral Monte Carlo algorithm. Dispersion relations are obtained for quasiparticles in the upper Hubbard band and in the correlated-states band; this corresponds to electron and hole doping of high-T _c superconductors. The form of the isoenergy surfaces is close to the experimentally observed form and confirms the existence of singularities in the density of states near the Fermi level. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 11, 860–865 (10 June 1996)     

Near band-edge luminescence of semi-insulating undoped gallium arsenide at high levels of excitation

The dependences of the maximum and the half-width of near band-edge photoluminescence of semi-insulating undoped-GaAs crystals at 77 K on the concentration of background acceptor impurities and the level of excitation in the range from 3×10²¹ to 6×10²² quantum/(cm² s) are investigated. The observed dependences are explained by formation of the density tails of states as a result of fluctuations of impurity concentration and participation of localized states of the donor impurity band in radiative transitions. Reduction of many-particle interaction at increasing of N can be connected with increasing of shielding of charge carriers by atoms of impurity.     

Influence of boron distribution on the transport of single-walled carbon nanotube

Using density functional theory combined with nonequilibrium Green’s functions, we investigated and found that boron substitutional doping affects the transport properties of single-walled carbon nanotubes with different distribution. The results reveal that the semiconducting nanotube transits to the quasi-metallic state with nonlinear current–voltage curve after boron doping. Some regular regions of total transmission coefficient with integral values appear with the varying of electron energy and bias voltage. The transport properties of the doped tubes are affected remarkably by the impurity states of quasi-bound defect states which are tuned by the distance between the boron atoms. The current of metallic nanotube is reduced by the impurities and changed with doping patterns. PACS 72.80.Rj; 73.22.2f; 73.61.Wp; 73.63.Rt     

Upconversion luminescent characteristics and peak shift of CdSeS nanocrystals under different wavelength laser excitation

Upconversion luminescence was obtained from CdSeS nanocrystals (NCs) under 800 nm femtosecond laser excitation. The structural and optical characteristics of the CdSeS NCs were investigated experimentally by use of UV–visible absorption spectroscopy, transmission electron microscopy, X-ray diffractometry, and time-resolved luminescence dynamics. Peak shift of luminescence in CdSeS NCs can be readily observed under different wavelength femtosecond excitation. The pump power dependence of the luminescence intensity and time-resolved decay revealed that one, two, and three-photon absorption occur. It was found that upconversion luminescence is composed of photoinduced trapping and a band-edge excitonic state, and two types of species are involved in the biexponential luminescence decay kinetics. With increasing Se-doped composition, luminescence lifetimes of CdSeS NCs with similar sizes become shorter. This is not consistent with the changes of undoped CdS NCs and is ascribed to impurity level increased doping in the energy gap, which is favorable for trapping luminescence. A simple energy level of doping NCs is used to interpret upconversion luminescence and the peak shift of steady-state emission.     

Interaction of nickel donor and acceptor excitons with defect vibrations in ZnSe:Ni crystals

Defect vibrations in ZnSe:Ni crystals induced by the Ni impurity with charge of ±1 relative to the neutral state are calculated by a recursive method in the shell model. This leads to an interpretation of vibronic structures in electroabsorption spectra for donor and acceptor Ni excitons in ZnSe:Ni crystals. Fiz. Tverd. Tela (St. Petersburg) 39, 2147–2151 (December 1997)     

Luminescence centers in cubic boron nitride

Complex and multiband photoluminescence spectra for GB and HBN centers in single crystals of cubic boron nitride (cBN) were recorded in the wavelength ranges 385–400 nm and 365–395 nm and the nature of these centers was studied. The use of models involving resonance vibrations and strongly shifted configuration diagrams for the electronic ground state and excited state made it possible to associate formation of the GB-1 center with the presence of tungsten impurity in cBN. It was established that the HBN band in the 300–350 nm range of the cathodoluminescence spectra of cBN polycrystals, single crystals, and micropowders is associated with luminescence centers present in microinclusions of graphite-like boron nitride (hBN). The nature of the hBN band is tentatively interpreted within the model of recombination of donor and acceptor defects in hBN: respectively nitrogen vacancies and carbon atoms in positions substituting for nitrogen. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 2, pp. 241–246, March–April, 2007.     

Effects of acceptor–donor complexes on electronic structure properties in co-doped TiO2: A first-principles study

We theoretically investigate the doping effects induced by impurity complexes on the electronic structures of anatase TiO₂ based on the density functional theory. Mono-doping and co-doping effects are discussed separately. The results show that the impurity doping can make the band-edges shift. The induced defect levels in the band gaps by impurity doping reduce the band gap predominantly. The compensated acceptor–donor pairs in the co-doped TiO₂ will improve the photoelectrochemical activity. From the calculations, it is also found that (S+Zr)-co-doped TiO₂ has the ideal band gap and band edge, at the same time, the binding energy is higher than other systems, so (S+Zr)-co-doping in TiO₂ is more promise in photoelectrochemical experiments.     

Optical properties of InN—the bandgap question

The recent controversy on the bandgap of InN is addressed, with reference to optical data on single crystalline thin film samples grown on sapphire. The optical absorption spectra deduced from transmission data or spectroscopic ellipsometry are consistent with a lowest bandgap around 0.7 eV in the low doping limit. Further, these data from a number of different independent authors and samples give values for the absorption coefficient within a factor 2 well above the absorption edge, supporting an intrinsic direct bandgap process. The presence of Mie resonances due to In inclusions in the InN matrix affects the shape of the absorption above the edge, but is less relevant for the discussion of the bandgap for pure InN. The alternative model of a deep level to conduction band transition requires the presence of a deep donor at a concentration close to 10²⁰ cm⁻³; in addition this concentration has to be the same within a factor 2 in all samples studied so far. This appears implausible, and no such deep donor could so far be identified from SIMS data in the highest quality samples studied. The line shape of the photoluminescence spectra can be quite well reproduced in a model for the optical transitions from the conduction band states to localized states above the valence band, including the Coulomb effects of the impurity potentials. A value of 0.69 eV for the bandgap of pure InN is deduced at 2 K. For samples that appear to be only weakly degenerate n-type two narrow peaks are observed in the photoluminescence at low temperature, assigned to conduction band—acceptor transitions. These peaks can hardly be explained in the deep level model. Recent cathodoluminescence data on highly n-doped InN films showing that the emission appears to be concentrated around In inclusions can also be explained as near bandgap recombination, considering the plausible enhancement due to interface plasmons. Finally, recent photoluminescence data on quantum structures based on InN and InGaN with a high In content appear to be consistent with moderate upshifts of the emission from a 0.7 eV value due to electron confinement.     

Thermal conductivity of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>: Sn and the effect of codoping by Pb and I atoms

The variation of the lattice thermal conductivity of Bi₂Te₃ induced either by alloying it with tin alone or by codoping the lattice with an acceptor or donor impurity was studied. The experimental data obtained at room and liquid nitrogen temperatures argue for the validity of the model of quasi-local impurity states associated with tin atoms.     

Doping in silicon nanocrystals: An ab initio study of the structural, electronic and optical properties

There are experimental evidences that doping control at the nanoscale can significantly modify the optical properties with respect to the pure systems. This is the case of silicon nanocrystals (Si-nc), for which it has been shown that the photoluminescence (PL) peak can be tuned also below the bulk Si band gap by properly controlling the impurities, for example by boron (B) and phosphorus (P) codoping. In this work, we report on an ab initio study of impurity states in Si-nc. We consider B and P substitutional impurities for Si-nc with a diameter up to 2.2 nm. Formation energies (FEs), electronic, optical and structural properties have been determined as a function of the cluster dimension. For both B-doped and P-doped Si-nc the FE increases on decreasing the dimension, showing that the substitutional doping gets progressively more difficult for the smaller nanocrystals. Moreover, subsurface impurity positions result to be the most stable ones. The codoping reduces the FE strongly favoring this process with respect to the simple n-doping or p-doping. Such an effect can be attributed to charge compensation between the donor and the acceptor atoms. Moreover, smaller structural deformations, with respect to n-doped and p-doped cases, localized only around the impurity sites are observed. The band gap and the optical threshold are largely reduced with respect to the undoped Si-nc showing the possibility of an impurity-based engineering of the Si-nc PL properties.     

Photoluminescence of Mn doped epitaxial GaAs

We have measured the near band edge photoluminescence of Mn doped liquid phase epitaxially grown GaAs. The photoluminescence spectra at 2°K shows, at low excitation intensities, a structure of up to eight sharp peaks (widths .2 to 1.0 meV) between 1.517 and 1.512 eV, besides the lower energy bands near 1.41 eV due to the deep Mn acceptor level and the usual donor-acceptor bands around 1.47 eV. Attempts to relate the sharp lines to the Mn electronic states, introduced by doping, were unsuccessful. It is our belief that the presence of this particular impurity in our samples allows for whatever states are responsible for the sharp line structure, to reveal themselves in the emission spectrum. A most unespected result is that near band edge sharp line luminescence is observed for impurity concentration as high as 10¹⁸cm^-3.     

Role of intrinsic defects and impurities in forming the optical characteristics of ZnWO4 and CdWO4 crystals

We analyze the optical characteristics of crystals within the framework of a model representation that presupposes the entry of the background impurity of iron ions in two valence states, Fe³⁺ and Fe²⁺, into regular lattice nodes. The spectral manifestation of intrinsic defects of vacancy-type structure depending on the conditions of crystal growth and heat treatment is comprehensively investigated. With allowance for the specific features of the technology applied, we have tested the technique suggested in scientific periodicals for bleaching ZnWO₄ and CdWO₄ crystals by doping with nonisovalent ions, in particular, with Sb and Ag impurities. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp. 211–216, March–April, 2000. The work was carried out with support from INTAS-UKRAINE within the framework of the project INTAS-UA-95-0166.     

Defects in HgCdTe grown by molecular beam epitaxy on GaAs substrates

The Hall effect and photoluminescence measurements combined with annealing and/or ion milling were used to study the electrical and optical properties of HgCdTe films grown by molecular-beam epitaxy on GaAs substrates with ZnTe and CdTe buffer layers. Unintentional donor doping, likely from the substrate, which resulted in residual donor concentration of the order of 10¹⁵ cm^?3, was observed in the films. Also, acceptor states, possibly related to structural defects, were observed.     

Combined effects of donor and acceptor impurities on the thermoelectric properties of cadmium antimonide

A study was made of the effect of the simultaneous doping with donor (Te) and acceptor (Cu, Ag, Au) elements on the thermoelectric properties of CdSb. Doping with tellurium changes the p-type conductivity of the CdSb crystals to an n-type conductivity with an impurity (Te) activation energy of E_d = (0. 11 ± 0. 01) eV. Doping with an acceptor impurity changes the energy of the donor level, by E_d = 0. 14 eV for doping with silver, by E_d = 0. 10 eV for doping with gold and by E_d = 0. 095 eV for doping with copper. It is shown that the type of conductivity and the thermoelectric properties of CdSb can be adjusted in the desired direction through simultaneous doping with two impurities.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 6, pp. 90–94, June, 1970.