Deep Localized States Research Articles

Phosphorous and Boron Incorporation and its Effect on Optical Properties of Ge:H and Si0.01Ge0.99:HFilms Deposited by LF PECVD

ABSTRACTDeposition conditions that provided low absorption related to both band tail and deep localized states have been found for both materials Ge:H and Si1YGeY:H. Phosphorous incorporation on Si0.01Ge0.99:H films and boron incorporation on Ge:H films were deposited by low frequency plasma-enhanced chemical vapour deposition (LF PECVD). The phosphorous incorporation in solidphase was observed to preferential with the increase of the doping in the gas phase to 2.5 %, and 2.5% to 4% was observed preferential Si0.01Ge0.99 film, boron incorporation in solid phase increase linearly with the increase of the doping gas phase. The content of solid phase was characterized by Secondary ion mass spectrometry (SIMS) profiling. Hydrogen concentration in the films was determined from Fourier transform infrared spectroscopy (FTIR) and SIMS measurements. Optical measurements provided optical gap, localized states, and band tail. A significant reduction of both band tail and deep localized states were observed at boron incorporation in solid phase = 0.004% on Ge:H films and the same were observed at phosphorous incorporation in solid phase = 0.29% on Si0.01Ge0.99:H films.

Amorphous silicon-germanium (a-Si0.01Ge0.99:H) doped with phosphorous deposited by LF PECVD and its electrical and optical characteristics

ABSTRACTAmorphous silicon-germanium (a-Si0.01Ge0.99:H) thin film, practically amorphous germanium with small concentration of silicon, were deposited by the low frequency PECVD technique under different flow gas of phosphine in the range of QPH3 = 20 to 100 sccm. In this range the phosphorous concentration in gas phase was calculated as XP= 4 to 20 % which resulted in a content in solid phase [P]sol=0.12% - 0.4%. The P content of the solid phase was measured by SIMS profiling and it was also observed a preferential incorporation of phosphorous in the range of [P]sol=0.12% - 0.3%. Hydrogen concentration in the films was determined from FTIR and SIMS measurements. The activation energy was determined by measuring the temperature dependence of conductivity in DC regime in a vacuum thermostat. For the a-Si0.01Ge0.99:H films it is found that the activation energy have a minimum, (Ea= 0.15eV), its conductivity at room temperature have a maximum and there is a minimum of of both band tail and deep localized state for a phosphorous incorporation in solid phase [P] =0.28%.

Electronic and structural properties of the oxygen vacancy in BaTiO3

The electronic and structural properties of the oxygen vacancy (VO) in cubic BaTiO3 are studied using first-principles calculations based on a hybrid Hartree–Fock density functional approach. Our calculations identify the double shallow donor behavior of VO, indicating its contribution to the n-type conductivity. In addition, a metastable configuration is found, which shows an off-symmetric atomic structure around VO in conjunction with deep localized electronic states in the band gap. Based on the identified characteristics of VO, the previous experimental and theoretical findings are explained.

Hopping and radiative recombination at localized band tail states in hydro‐ genated amorphous silicon

AbstractThe primary photocurrent in hydrogenated amorphous silicon (a‐Si:H) is shown to be due to the hopping controlled by deep localized band tail states from the time‐of‐flight (TOF) experiment and the Monte Carlo simulation. The power law decay of the luminescence observed at temperatures higher than 70 K is shown to be due to the recombination between the trapped hole and the electron making the hopping randomwalk at localized band tail states. The luminescence center in a‐Si:H is shown to be the hydrogen‐free Si nanostructures from the comparison with the luminescence from the Si nano‐particles in the porous Si (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hopping transport at localized band tail states in amorphous hydrogenated silicon

AbstractThe transient current of the time‐of‐flight (TOF) in amorphous hydrogenated silicon calculated from the hopping at localized band tail states with the Monte Carlo simulation is shown to agree with that of the experiment in the line shape and the transit time. The attempt‐to‐escape frequencies obtained by fitting the electric field dependence of the transit time in the simulation with the Pfister–Ohno equation for the transit time derived from the hopping model, have been far beyond the phonon frequencies as in the case of the experiment. The attempt‐to‐escape frequency has been empirically described with the Meyer–Neldel (MN) rule as well as in the experiment. The MN rule for the attempt‐to‐escape frequency in the TOF has been concluded to originate in the hopping controlled by the deep localized band tail states from the simulation.

Influencing range of vacancy defects in zigzag single-walled carbon nanotubes

The influencing range of a vacancy defect in a zigzag single-walled nanotube is characterized with both structural deformation and variation in bandstructure. This paper proposes a microscopic explanation to relate the structural deformation to the bandstructure variation. With an increasing defect density, the nanotubes become oblate and the energy gap between the deep localized gap state and the conducting band minimum state decreases. Theoretical results shed some light on the local energy gap engineering via vacancy density for future potential applications.

Dependence of mobility on shallow localized gap states in single-crystal organic field-effect-transistors

Abstract In order to optimize the performance of molecular organic electronic devices it is important to study the intermolecular density of states and charge transport mechanisms in the environment of crystalline organic material. Using this approach in Field Effect Transistors (FETs) we show that material purification improves carrier mobility and decreases density of the deep localized electronic state. We also report a general exponential energy dependence of the density of localized states in a vicinity of the mobility edge (Fermi energies up to ∼7 times higher than the thermal energy ( kT )) in a variety of the extensively purified molecular organic crystal FETs. This observation and the low activation energy of the order of ∼ kT suggest that molecular structural misplacements of the sizes that are comparable with thermal molecular modes rather than impurity deep traps play a role in formation of these shallow states. We find that the charge carrier mobility in the FET nanochannels, μ eff , is parameterized by two factors, the free-carrier mobility, μ 0 , and the ratio of the free carrier density to the total carrier density induced by gate bias. Crystalline FETs fabricated from rubrene, pentacene, and tetracene have a high free-carrier mobility, μ 0 ∼50 cm 2 /Vs, at 300 K with lower device μ eff dominated by localized shallow gap states. This relationship suggests that further improvements in electronic performance could be possible with enhanced device quality.

Localized electronic states induced by defects and possible origin of ferroelectricity in strontium titanate thin films

Several defect configurations including oxygen vacancies have been investigated as possible origins of the reported room-temperature ferroelectricity of strontium titanate (STO) thin films [Y. S. Kim et al., Appl. Phys. Lett. 91, 042908 (2007)]. First-principles calculations revealed that the Sr–O–O vacancy complexes create deep localized states in the band gap of SrTiO3 without affecting its insulating property. These results are consistent with electronic structural changes determined from optical transmission and x-ray absorption measurements. Our work suggests importance of oxygen vacancies and their complexes in understanding of electronic properties of perovskite oxide thin films, including STO.

Electronic structure of the amorphous oxide semiconductor a‐InGaZnO4–x: Tauc–Lorentz optical model and origins of subgap states

AbstractThis paper discusses an optical model and subgap electronic states for a representative amorphous oxide semiconductor, InGaZnO4 (a‐IGZO). Parameterized optical models were developed based on the Tauc–Lorentz model combined with a Lorentz‐type oscillator. The measured optical absorption spectra exhibit nearly linear dependences on photon energy (E) between 3 eV < E < 5 eV, which requires the transition energies in the Tauc–Lorentz model (E0,TL) being around 4 eV. The optimized parameters for the fixed E0,TL of 3.7 eV are provided for four different a‐IGZO films with root‐mean‐square errors less than 1%. Formation energies of crystalline IGZO, stoichiometric a‐IGZO, oxygen deficient a‐IGZO and their constituent oxides were calculated by the density functional theory using the local density approximation (LDA) and generalized gradient approximation with PBE96 functionals (PBE). PBE gives larger unit cell volumes at the ground states and better agreement in the formation energies than LDA does. The formation energies of an oxygen deficiency in a‐IGZO were calculated to be 3.2–3.5 eV. The calculated electronic structures of stoichiometric a‐IGZO models exhibit somewhat large dispersions for conduction bands (CB), which are not largely affected by the disordered structure in a‐IGZO, while the dispersions of the valence bands (VBs) are very small, unlike the crystalline IGZO, showing that a‐IGZO have strongly localized states at the VB maximums (VBMs). Oxygen‐deficient a‐IGZO models showed that oxygen deficiencies form both a deep localized state at 0.4–1 eV above VBM and a shallow donor state depending on local atomic configurations. An oxygen deficiency that forms a deep state breaks the dispersion of the CB, which could be an origin of the subgap states observed near CB. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of the electrode material in a metal-polymer-metal structure on the temperature dependence of the thermally stimulated current

The dependences of the thermally stimulated current in metal-polymer-metal structures with electrodes prepared from different materials are investigated. Poly(diphenylenephthalide) films are used as a polymer. The possible contribution of the injection component to the thermally stimulated current spectrum is considered. The injection conditions are changed by replacing the electrode material. It is established that the change in the material does not lead to a change in a qualitative behavior of the temperature dependence of the current. However, the use of copper as the electrode material gives rise to an additional injection component in the thermally stimulated current spectrum and (under specific conditions) electronic switching of the sample from an insulating state to a highly conducting state. The charge is injected into deep localized electronic states that lie near the middle of the band gap of the polymer.

Current-voltage characteristics of poly(3-hexylthiophene) diodes at room temperature

We report on the analysis of current-voltage characteristics of regioregular poly(3-hexylthiophene) diodes. Experimental curves were fitted to two models, to take into account at low-moderate electric fields Schottky behavior mixed with space charge limited current (SCLC) regime and, at higher fields, trap-free SCLC. The results provide a description of IV curves over five decades, along with the determination of zero field and effective mobility and the field dependence prefactor. Forward and reverse IV measurements highlighted the presence of shallow and deep localized states inside the band gap. The latter enhance the current over time and have been modeled as an inductorlike element.

Anti-stokes luminescence of Zn0.6Cd0.4S solid solutions with adsorbed organic dye molecules and few-atom silver clusters

For microcrystals of Zn0.6Cd0.4S with adsorbed molecules of a number of organic dyes, we have observed sensitized anti-Stokes luminescence excited by radiation with wavelengths in the range 610–750 nm and flux density 1014–1015 photons/cm2·sec. The positions of the bands in the excitation spectra for such luminescence match those of the absorption spectra for the adsorbed dye molecules, which is evidence in favor of a cooperative mechanism for its appearance. We have shown that enhancement of the anti-Stokes luminescence is possible when silver atoms and few-atom clusters appear on the Zn0.6Cd0.4S surface in addition to the dye molecules. We hypothesize that its excitation in the latter case occurs as a result of two-photon optical transitions. These transitions occur sequentially, with transfer of an electron or the electronic excitation energy from the dye molecules to silver atoms and few-atom clusters adsorbed on the surface of Zn0.6Cd0.4S, creating deep localized states in the bandgap with photoionization energies 1.80–2.00 eV.

Vertical transport in all‐binary GaAs/AlAs short‐period superlattices and carrier trapping and detrapping dynamics by different quantum wells

AbstractDynamics of vertical transport in all‐binary GaAs/AlAs short‐period superlattices (SPSs) have been investigated as a function of temperature by steady‐state and time‐resolved photoluminescence (PL) experiments. When two different, enlarged quantum wells are embedded in the SPSs, photoexcited carriers compete to be trapped into various localized exciton states and a variety of exciton radiative recombination dynamics appears accompanied by phonon heating. Redistribution of carrier population in the different wells by phonon scattering is significantly influenced by vertical miniband transport in the SPSs. Three different transport regimes are pointed out: (i) tunnelling transport affected by irreversible carrier cooling into deep localized states, (ii) interplay of carrier relaxation and phonon‐assisted activation mediated by an efficient miniband Bloch transport, and (iii) thermalization processes. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum efficiency and formation of the emission line in light-emitting diodes based on InGaN/GaN quantum well structures

The spectra of electroluminescence, photoluminescence, and photocurrent for the In0.2Ga0.8N/GaN quantum-well structures are studied to clarify the causes for the reduction in quantum efficiency with increasing forward current. It is established that the quantum efficiency decreases as the emitting photon energy approaches the mobility edge in the In0.2Ga0.8N layer. The mobility edge determined from the photocurrent spectra is E me = 2.89 eV. At the photon energies hv > 2.69 eV, the charge carriers can tunnel to nonradiative recombination centers with a certain probability, and therefore, the quantum efficiency decreases. The tunnel injection into deep localized states provides the maximum electroluminescence efficiency. This effect is responsible for the origin of the characteristic maximum in the quantum efficiency of the emitting diodes at current densities much lower than the operating densities. Occupation of the deep localized states in the density-of-states “tails” in InGaN plays a crucial role in the formation of the emission line as well. It is shown that the increase in the quantum efficiency and the “red” shift of the photoluminescence spectra with the voltage correlate with the changes in the photocurrent and occur due to suppression of the separation of photogenerated carriers in the field of the space charge region and to their thermalization to deep local states.

Deep Defects Annihilation in GaAs1-xNx Layers by Si-doping

The photoluminescence (PL) properties of Si-doped GaAs0.985N0.015 with different silicon content were investigated. The study was carried out on a set of three samples grown by Molecular Beam Epitaxy (MBE) on GaAs (001) oriented substrate using a radio frequency nitrogen beam source. For all samples, the PL measurements show the presence of a wide band situated at 0.83 eV which intensity decreases by increasing silicon content. This wide band was attributed to the presence of deep localized states induced by a three-dimensional growth of the GaAsN layer. In addition, these deep localized states are annihilated by the free carriers from silicon atoms. PL measurements in the range of 10 to 300 K were also performed to identify the band gap energy of GaAs1-xNx structure. The decrease of the activation energies with increasing silicon content was observed.

Afterglow, TL and IRSL in beta‐irradiated HPHT type Ib synthetic diamond

AbstractThe presence of nitrogen or other impurities in HPHT type Ib synthetic diamond may produce shallow or deep localized trapping states for electrons (holes) with activation energies below the conduction (above the valence) band. The present work deals with room temperature afterglow (AG), thermoluminescence (TL) and infrared‐stimulated luminescence (IRSL) in HPHT type Ib diamond. The existence of vacancy impurity‐related defects gives rise to a characteristic AG, TL and IRSL in beta‐irradiated samples. The AG luminescence decays fast, decreasing to about 103 of its initial intensity value in 100 s. The deconvoluted TL glow curve is composed of a 1.4‐order peak at 375 K and three first‐order kinetics peaks at 442, 475 and 514 K. The IRSL was excited with 830 nm light releasing electrons from the trapping states with different efficiencies. The TL glow curve was found to be non‐reproducible because the multiple impurity‐related defects induced by irradiation are unstable at temperatures around 800 K. The peculiar AG and TL behavior observed in the investigated HPHT type Ib synthetic diamond may hinder its application in TL dosimetry. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence study of ZnO nanotubes under hydrostatic pressure

Photoluminescence of ZnO single crystal nanotubes grown on sapphire substrate by metal organic chemical vapor deposition has been studied as a function of applied hydrostatic pressure using the diamond-anvil-cell technique. The photoluminescence spectra of the ZnO nanotubes at atmospheric pressure are dominated by strong near-band-edge ΓFX and ΓBX excitonic emission lines accompanied by a weak broad deep-level (DL) emission band. The pressure-induced shifts of all observed emission lines are followed up to 15Gpa, when ZnO nanotubes undergo a phase transition from a direct-gap wurtzite structure to an indirect-gap rocksalt structure. The ΓFX emission is found to shift toward higher energy with applied pressure at a rate of 29.6meV∕GPa, which provides a method to measure the pressure coefficient of the direct Γ band gap in the wurtzite ZnO nanotubes. The ΓBX emission has a pressure coefficient of 21.6meV∕GPa, about 30% smaller than that of the ZnO band gap, which suggests that it might originate from the radiative recombination of the excitons bound to donorlike deep centers rather than shallow donors. The pressure coefficient of the broad DL emission band in ZnO tube is 16.8meV∕GPa, which indicates that the initial states involved in the emission process are deep localized states.

Anomalous behaviour in dark conductivity and photoconductivity in a-Se 85Te 15 − xPb x thin films at low temperatures

Temperature dependence of dark conductivity and photoconductivity has been studied in amorphous thin films of Se 85Te 15 − x Pb x (2 < x < 6) at low temperatures (219–300 K). We have observed that, at quite low temperatures, a maximum is found at a particular temperature in dark conductivity and photoconductivity measurements as a function of temperature. The position of this maximum shifts to higher temperatures at higher intensities. This behaviour is called anomalous as it is not commonly observed in chalcogenide glasses at low temperatures. Deep localized states, which behave like trapping centers, may play an important role in the conduction processes at low temperatures. The anomalous behaviour of dark conductivity is explained by considering a statistical shift of Fermi level with the change in temperature. The maxima in photoconductivity at low temperatures could be explained by assuming the sensitising centers in the mobility gap due to defect states.

Effect of lead additive on photoconductive properties of Se–Te chalcogenide films

Steady state and transient characteristics of photoconductivity have been studied in amorphous thin films of Se85Te15−xPbx (x=0, 2, 4 and 6). The studies of temperature dependence of photoconductivity show that photoconduction is a thermally activated process. The value of activation energy of photoconduction is, however, smaller as compared to activation energy in dark. The results of intensity dependence of steady state photoconductivity indicate that bimolecular recombination is predominant in these materials. The Se85Te15−xPbx is found highly composition dependent as lead impurity drastically changes the conduction parameters. The incorporation of Pb impurity is found to affect the transient photoconductivity properties drastically. A spike is observed in the rise curve of photocurrent in undoped a-Se85Te15, which disappears in Pb doped samples. The lead incorporation is found to delay the onset time of recombination in the rise and decay of photocurrent which is attributed to the trapping of charge carriers in deep localized states produced by lead impurity. The photocurrent rise and decay is explained by the trap-controlled recombination model proposed by Iovu et al. and the dispersion parameter α of localized state energy distribution is determined from the experimental results.

Photocurrent and Photoluminescence Investigations of GaInNAs and GaInNAs(Sb) Quantum Wells Grown by Molecular Beam Epitaxy

We have investigated photocurrent (PC) and photoluminescence (PL) in sequentially grown GaInNAs/GaAs and GaInNAs(Sb)/GaAsSbN quantum wells. Photocurrent transitions are analyzed by theoretical calculations using envelope function formalism taking into account the strain effect and the strong coupling between nitrogen localized state and the GaInAs band gap. The results are consistent with a type I band alignment and a conduction band offset ratio of about 80 %. Additionally, our results suggest an increase of the electron effective mass by as much as 0.035 m0 resulting from the flattening of the conduction band under nitrogen effect. The temperature evolution of the PL peak energy and the integrated PL intensity of GaInNAsSb QW show evidence of strong localization of carriers. Both, the high delocalization temperature, in the 230 K range and the strong shift between the PC and PL spectra of GaInNAsSb QW, indicate the presence of deeper localized states as compared to that in the GaInNAs QW.