Deep Levels In Gap Research Articles

Effects of the impurity–host interactions on the nonradiative processes in ZnS:Cr

There is a great deal of controversy about whether the behavior of an intermediate band in the gap of semiconductors is similar or not to the deep-gap levels. It can have significant consequences, for example, on the nonradiative recombination. In order to analyze the behavior of an intermediate band, we have considered the effect of the inward and outward displacements corresponding to breathing and longitudinal modes of Cr-doped ZnS and on the charge density for different processes involved in the nonradiative recombination using first-principles. This metal-doped zinc chalcogenide has a partially filled band within the host semiconductor gap. In contrast to the properties exhibited by deep-gap levels in other systems, we find small variations in the equilibrium configurations, forces, and electronic density around the Cr when the nonradiative recombination mechanisms modify the intermediate band charge. The charge density around the impurity is equilibrated in response to the perturbations in the equilibrium nuclear configuration and the charge of the intermediate band. The equilibration follows a Le Chatelier principle through the modification of the contribution from the impurity to the intermediate band and to the valence band. The intermediate band introduced by Cr in ZnS for the concentrations analyzed makes the electronic capture difficult and later multiphonon emission in the charge-transfer processes, in accordance with experimental results.

Static and dynamic ionization levels of transition metal-doped zinc chalogenides

Transition metal (TM) impurities in semiconductors have a considerable effect on the electronic properties and on the lattice vibrations. The unfilled d shell permits the impurity atoms to exist in a variety of charge states. In this work, the static donor and acceptor ionization energies of ZnX:M, with X = S, Se, Te and M:Sc, Ti, V, Fe, Co, Ni are obtained from first principles total energy calculations and compared with experimental results in the literature where they exist. From these results, many of the TM-doped zinc chalogenides have an amphoteric behavior. To analyze the rule of the deep gap levels in both the radiative and non-radiative processes, the dynamic ionization energies are obtained as a function of the inward and outward M–X displacements. In many cases, the changes in the mass and the force constants resulting from the substitution of an impurity center for a lattice atom are small. When the charge or the environment of the impurity changes, the electron population tend to remain compensated. As consequence, the changes in the lattice vibrational modes are small.

Ionization levels of doped sulfur and selenium chalcopyrites

The substitution of Ga or Cu by another element in the chalcopyrites Ga2Cu2X4 (with X=S or Se) could have important implications either for photovoltaic or spintronic applications. We present total energy spin-polarized density-functional calculations of the substituted chalcopyrite MxA2−xB2X4, with A and B equal to Ga or Cu, X=S and Se, and M=C, Si, Ge, Sn, V, Ir, Fe, Co, Ni, Rh, and Hg. The ionization levels, interesting for both spintronic and optoelectronic applications, are calculated and discussed. The donor and acceptor levels induced by substitutional M atoms are used to make predictions on the improvement in the optoelectronic performance. From these results, some doped chalcopyrites have an amphoteric behavior. In order to analyze the role of the deep gap levels in both the radiative and nonradiative processes, the dynamic acceptor and donor energies are obtained as a function of the inward and outward M-X displacements.

Generation of vacancy-interstitial pairs as a possible origin of resistivity switching and ferroelectric properties inCd1−xZnxTe

Formation of vacancy-interstitial Frenkel pairs, together with the properties of interstitials and vacancies in CdTe, ZnTe, and their alloys were investigated by first-principles calculations. Generation of Frenkel pairs on the cation sublattice strongly depends on the Fermi energy: the presence of excess free electrons reduces the energy barrier for the pair generation from 2.5 eV in intrinsic samples to 1.2 eV. Moreover, ${E}_{F}$ determines both the stability of Frenkel pairs with respect to recombination and their binding energy, which varies from $\ensuremath{\sim}0.2$ to $\ensuremath{\sim}1\text{ }\text{eV}$. A strong dependence on the Fermi energy, i.e., on the charge state, is also found for stable sites and barriers for diffusion of isolated interstitials. In particular, neutral interstitials have two (meta)stable sites, corresponding to two local minima of energy, and diffuse by jumps between them. Positively charged interstitials have only one stable site and diffuse by twice longer and curvilinear jumps. For the relevant charge states, the barriers for diffusion range from 0.5 to 1 eV, which imply a high mobility of interstitials. Most of these properties are traced back to the defect-induced deep gap levels, their occupation, and their dependence on the defect's site. The important role of the ionicity of the host is pointed out. On the other hand, generation of Frenkel pairs on the anion sublattice requires energy of about 5 eV, and thus is nonefficient. The obtained results suggest that formation of Frenkel pairs is a microscopic origin of two effects recently observed in Schottky junctions based on CdZnTe and other II--VI alloys, namely, the reversible changes in conductivity by a few orders of magnitude, and the ferroelectric-like behavior of polarization.

Impurity–host interactions in Cr-substituted ZnSe

The physics of charge-transfer processes in semiconductors is a challenging and longstanding problem. Focusing on Cr-substituted ZnSe semiconductors, important for optoelectronic and spintronic devices, several processes and their energetics are analysed using first-principles. In contrast to the properties exhibited by deep gap levels, our results for highly Cr doped ZnSe show small variations in the equilibrium configurations, forces and electronic density around the Cr for different charge states. Therefore, the delocalization of the electronic charge between the impurity and host leads to a decrease of the effective Coulomb repulsion and becomes then the fundamental mechanism to inhibit nonradiative recombination via multiphonon emission for the modes studied.

Vacancy-like defects in a-Si: a first principles study

The structural and electronic properties of vacancies in a fully tetrahedral model of amorphous silicon have been investigated by ab initio calculations. A very rich behavior was observed after the atomic relaxation, from the complete rearrangement of the atoms (self-healing) to the creation of stable vacancies. We have observed that the vacancy internal relaxation volume is negative. After relaxation, the vacancy formation energies were found to be smaller than that of the crystal, and the deep gap levels either disappear or move towards the band edges.

Structural Stability and Optical Properties of Nanomaterials with Reconstructed Surfaces

We present density functional and quantum Monte Carlo calculations of the stability and optical properties of semiconductor nanomaterials with reconstructed surfaces. We predict the relative stability of silicon nanostructures with reconstructed and unreconstructed surfaces, and we show that surface step geometries unique to highly curved surfaces dramatically reduce the optical gaps and decrease excitonic lifetimes. These predictions provide an explanation of both the variations in the photoluminescence spectra of colloidally synthesized nanoparticles and observed deep gap levels in porous silicon.

Deep electronic gap levels induced by isovalent P and As impurities in GaN

The electronic and atomic structure of isovalent substitutional P and As impurities in GaN is studied theoretically using a self-consistent plane-wave pseudopotential method. In contrast with the conventional isovalent III-V systems, $\mathrm{GaN}\mathrm{̱}:\mathrm{P}$ and $\mathrm{GaN}\mathrm{̱}:\mathrm{A}\mathrm{s}$ are shown to exhibit deep gap levels. The calculated donor energies are $\ensuremath{\epsilon}(+/0)={\ensuremath{\epsilon}}_{v}+0.22$ and ${\ensuremath{\epsilon}}_{v}+0.41$ eV, respectively, and the double donor energies are $\ensuremath{\epsilon}(++/+)={\ensuremath{\epsilon}}_{v}+0.09$ and ${\ensuremath{\epsilon}}_{v}+0.24$ eV, respectively. The $p$-like gap wave function is found to be strongly localized on the impurity site. Outward atomic relaxations of $\ensuremath{\sim}13%$ and $\ensuremath{\sim}15%$ are calculated for the nearest-neighbor Ga atoms surrounding neutral ${\mathrm{GaN}\mathrm{̱}:\mathrm{P}}^{0}$ and ${\mathrm{GaN}\mathrm{̱}:\mathrm{A}\mathrm{s}}^{0},$ respectively. The relaxation increases by $\ensuremath{\sim}1%$ for the positively charged impurities. The impurity-bound exciton binding energy is calculated at ${E}_{b}=0.22$ and ${E}_{b}=0.41$ eV for $\mathrm{GaN}̱:P$ and $\mathrm{GaN}̱:As.$ The former is in good agreement with the experimental data ${(E}_{b}=0.232$ eV) whereas the latter is offered as a prediction. No clear Jahn-Teller symmetry lowering ${(T}_{d}\ensuremath{\rightarrow}{C}_{3v})$ distortion, suggested by the one-electron configuration, is found for $\mathrm{GaN}̱:{\mathrm{P}}^{+}$ and $\mathrm{GaN}̱:{\mathrm{As}}^{+}.$

Localization and percolation in semiconductor alloys: GaAsN vs GaAsP.

Tradition has it that in the absence of structural phase transition, or direct-to-indirect band-gap crossover, the properties of semiconductor alloys (bond lengths, band gaps, elastic constants, etc.) have simple and smooth (often parabolic) dependence on composition. We illustrate two types of violations of this almost universally expected behavior. First, at the percolation composition threshold where a continuous, wall-to-wall chain of given bonds (e.g., Ga-N-Ga-N...) first forms in the alloy (e.g., $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$), we find an anomalous behavior in the corresponding bond length (e.g., Ga-N). Second, we show that if the dilute alloy (e.g., $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ for $x\ensuremath{\rightarrow}1$) shows a localized deep impurity level in the gap, then there will be a composition domain in the concentrated alloy where its electronic properties (e.g., optical bowing coefficient) become irregular: unusually large and composition dependent. We contrast $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ with the weakly perturbed alloy system $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}$ having no deep gap levels in the impurity limits, and find that the concentrated $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}$ alloy behaves normally in this case.

Interpretation of DLTS emission and capture data of the Co-related deep level in GaP

Deep level transient spectroscopy measurements are performed on Co diffused, n-type GaP to indentify the cobalt impurity level. The results allowed us to identify the impurity center as double acceptor. The importance of an influence of the Debye tail on emission as well as capture data is demonstrated. DLTS-Messungen werden an Co-diffundiertem, n-leitendem GaP zur Auffindung der Co-Niveaus duchgefuhrt. Das Storstellenniveau wird als Doppelakzeptor identifiziert. Es wird ein Einflus des Debye-Auslaufers auf die Emissions- und Einfangdaten aufgezeigt.

Electrical characterization of inadvertent midgap levels in GaP

Results of an electrical characterization study of two hitherto unobserved midgap deep levels in GaP using deep-level transient spectroscopy and single-shot dark capacitance transient techniques are reported. These are the dominant majority-carrier (electron) levels in the green-light-emitting diodes studied. Detailed electron emission rate and capture cross-section measurements are performed on these levels. The shallower of the two levels is found to have an activation energy to the conduction band varying from 0.88±0.02 eV to 0.93±0.02 eV from sample to sample while the activation energy of the deeper level is 0.96±0.02 eV. The results of the capture measurements give an upper limit of 1.2×10−19 cm2 for the electron capture cross sections of the two levels. A detailed comparison of the emission rate data with the published results on midgap levels in GaP is presented. It is proposed that one of these midgap levels may be the counterpart in GaP of the well-known EL2 level found in GaAs.

Electronic structure due to hydrogen and vanadium as substitutional impurities in InP.

The self-consistent local-density theory is used in a cluster model to calculate the charge distribution, one-electron energy spectra, and the density of states for pure InP and for H and V substituted at the In site. The pure semiconductor gap is found to be 0.8 eV, consistent with experiments. The hydrogen impurity introduces a trapping center at E/sub v/+0.37 eV in the gap region: a semi-insulating-like behavior. The transition metals are generally found to introduce deep levels in the gap region, but for vanadium we find the gap swept clean of any impurity level, with the last partially occupied state close to the bottom of the conduction band. This is consistent with deep-level transient spectroscopy experiments where no vanadium-related deep levels in the band gap are found down to 4 K.

Optical emission properties of metal/InP and GaAs interface states

We have measured optical emission from interface states formed by metal deposition on UHV-cleaved InP(110) and GaAs(110) surfaces by means of cathodoluminescence spectroscopy. Our study reveals discrete levels distributed over a wide range of energies and localized at the microscopic interface. Our results demonstrate the influence of the metal, the semiconductor, and its surface morphology on the energy distributions. The detailed evolution of optical emission energies and intensities with multilayer metal deposition exhibits a strong correlation between the deep gap levels, the Fermi level movements, and Schottky barrier heights. The results demonstrate that in general electronic states deep within the band gap continue to evolve beyond monolayer coverage into the metallic regime.

Theory of the ⟨100⟩ uniaxial stress dependence of the deep levels in GaP and GaAs

We extend previous theoretical work to predict the derivatives with respect to the ⟨100⟩ uniaxial stress of the substitutional deep point defect energy levels in GaP and GaAs which, for a defect at a specific site producing a level of particular symmetry, can be evaluated as a function of their energy levels in the band gap.

Identification of Fe related deep levels in GaP by DLTS

Electron capture measurements have been made using a double pulse method to give evidence that the two acceptor levels Ev+0.82 eV and Ec-0.24 eV originate from the same impurity centre, being a double acceptor. By comparing the photo cross-sections sigma p,10 and sigma n,10 of the first level Ev+0.82 eV with data in the literature the author concludes that the impurity is an isolated Fe atom on a Ga site.

Pressure dependence of deep levels in GaP

The pressure dependence of deep energy levels associated with substitutional point defects in GaP is investigated using an approach proposed before. A deep level can be driven out of the gap into the valence bands, if this level is caused by a P-substitutional defect and located sufficiently close to the valence band maximum in the unpressurized solid. No deep level can be driven into the gap from the conduction bands by the pressure. The theoretical calculations are in good agreement with the experimental results.

Plastic-deformation-induced deep level in GaP

Thermally stimulated current measurements on Schottky barriers evaporated on bent GaP:N epitaxial layers are reported. A pronounced peak corresponding to a deep level with a binding energy of ∼0.57 eV is attributed to dislocations produced during plastic deformation.

Deep level spectroscopy, low temperature defect motion and nonradiative recombination in GaAs and GaP

Recent developments in junction capacitance measurements allow deep levels in semiconductors to be conveniently studied for the first time. Experiments carried out on deep levels in GaP and GaAs show that the levels are often strongly coupled to the lattice. This coupling can cause rapid nonradiative recombination in which the released electronic energy causes violent vibrations of the lattice near the defect. The vibrations can promote low temperature defect motion. These two phenomena, nonradiative recombination and defect motion, are fundamental to the understanding of how defects limit the efficiency of light emitting devices and cause junction devices to degrade when forward biased. The recent work of Lang on deep level spectroscopy, Lang and Kimerling on defect motion and Henry and Lang on nonradiative recombination by multiphonon emission will be reviewed.

Deep levels in GaP

Change of the junction capacitance under the illumination of monochromatic radiation has been observed in GaP n+p junctions. The peaks were observed at 1.8 and 2.4 eV at 77°K. The 1.8 eV peak was attributed to the hole activation from Fe3+ to the valence band. The photoconductivity spectrum and the transient phenomenon of the photoconductivity in p+n junctions and in high-resistivity n-type bulk crystals also showed the presence of the 1.8 eV level due to Fe.