Deep-level Spectroscopy Research Articles

Electron levels of defects in In(Ga)As/(In)GaAs nanostructures: A review

The data on electron levels induced by defects in In(Ga)As/(In)GaAs nanostructures, their localization, activation energy and identification have been systematically reviewed. Point defects inherent to GaAs and found in the (In)GaAs-based nanostructures have been listed, and their classification has been clarified, including EB3, EL2, EL3, EL4 (M4), EL5, EL6 (M3), EL7, EL8, EL9 (M2), EL10 (M1), EL11 (M0) and M00. The effect of the interfaces on the formation of different types of extended defects has been described. All the levels of electron traps found in heterostructures with quantum wells, wires and dots by deep level spectroscopies have been collected in a table with indication of the detection technique, object, location in the structure and their origin assumed. This overview can be useful as a reference material for researchers who study these nanostructures.

Capacitive Spectroscopy of Deep Levels in Silicon with Samarium Impurity

The effect of thermal treatment on the behavior of samarium atoms introduced into silicon during the growth process was studied using the method of transient capacitive deep-level spectroscopy (DLTS). It has been shown that various high-temperature treatments lead to the activation of samarium atoms in the bulk of n-Si and the formation of deep levels. The energy spectrum of deep levels arising during heat treatments has been determined. The dependence of the efficiency of formation of these levels in n‑Si<Sm> on the processing temperature has been studied. It was found that the higher the content of samarium atoms in the bulk of silicon at the same high-temperature treatment temperature, the higher the concentration of the deep level EC–0.39 eV. From this, we can conclude that the EC–0.39 eV level is associated with the activation of samarium atoms in the n-Si<Sm> volume.

The GaN(0001) yellow-luminescence-related surface state and its interaction with air

Yellow luminescence (YL) is probably the longest and most studied defect-related luminescence band in GaN, yet its electronic structure or chemical identity remain unclear. Most of the theoretical work so far has attributed the feature to bulk defects, whereas spectroscopic studies have suggested a surface origin. Here, we apply deep level spectroscopy using sub-bandgap surface photovoltage that provides the energy distribution of the surface charge density. Comparison of surface charge spectra obtained under identical conditions before and after various surface treatments reveals the dynamics of the surface charge density. Further comparison with spectra of the entire state obtained using photoluminescence shows how the charge density stored in YL-related defects is eliminated upon a mild anneal in vacuum. This suggests that the YL-related defect involves a certain molecule adsorbed on the GaN surface, possibly in a complex with an intrinsic surface defect. The observed interaction with air strongly indicates that the YL-related deep level is a surface state.

Piezocapacitance spectroscopy of a radiation defect in silicon

In this article, using non-stationary capacitive deep-level spectroscopy (DLTS), the dependence of the parameters of the radiation defect with the ionization energy Ei = Ec − (0.33±0.02) eB arising after irradiation with n − Si · λ - quanta of the 60Co isotope and annealing the samples at 250°C on uniaxial compression was studied. It is shown that, under the action of pressure, Ei ΔEi changes depending on the crystallographic direction and reaches values of 0.06−0.12eB. A comparison is made with a similar effect for the level of the A-centre Ec − 0,17 eB.

Deep level defect spectroscopies of complex oxide surfaces and interfaces

Intrinsic point defects are commonly present in and can strongly affect the electronic properties of complex oxides and their interfaces. The near- and subsurface characterization techniques, depth-resolved cathodoluminescence spectroscopy and surface photovoltage spectroscopy, can measure the density distributions, energy levels, and optical transitions of intrinsic point defects in complex oxides on a near-nanometer scale. These measurements on SrTiO3, BaTiO3, and related materials reveal the sensitivity of intrinsic point defects to growth temperature, mechanical strain, crystal orientation, and chemical interactions. Spatial redistribution of these defects can vary significantly near surfaces and interfaces and can have strong electronic effects. The combination of these deep level spectroscopies along with other advanced characterization techniques provides an avenue to further expand the understanding and control of complex oxide defects in general.

Quantitative Imaging of Defect Distributions in CdZnTe Wafers Using Combined Deep-Level Photothermal Spectroscopy, Photocarrier Radiometry, and Lock-In Carrierography

Quantitative Imaging of Defect Distributions in CdZnTe Wafers Using Combined Deep-Level Photothermal Spectroscopy, Photocarrier Radiometry, and Lock-In Carrierography

Carrier capture kinetics, deep levels, and isolation properties of β -Ga2O3 Schottky-barrier diodes damaged by nitrogen implantation

In this paper, we analyze the stability of the performance of β-Ga2O3 Schottky barrier diodes (SBDs) damaged by nitrogen implantation, based on electrical characterization and deep-level spectroscopy. We demonstrate that N-implanted SBDs show a high level of isolation, which can be further improved by electron trapping promoted by current flow and temperature. Charge trapping leads to a shift in the current–voltage curve to more positive bias, a positive effect lasting for hundreds of seconds at room temperature and fully recovered only after 200 s even at 350 K. This charge trapping process takes place in the depletion region of the Schottky diode, in a defect located 0.72 eV below the conduction band, possibly related to gallium vacancies. A model for explaining the trapping kinetics and processes is proposed, based on differential rate equations, highlighting two capture processes: charge injection from the n-type Ga2O3 into the space charge region and thermionic emission from the anode metal to the deep level.

Investigating the Formation of Defects in Silicon during Gamma Irradiation

The parameters of recombination centers in silicon photodiodes are studied before and after gamma irradiation. The technique of investigation is deep-level recombination spectroscopy. It is shown that, after irradiation, both the forward-bias and reverse-bias currents through the p–n junction increase, which is explained by the growth in the concentration of recombination centers due to the formation of vacancies during irradiation.

Deep-level noise characterization of MOVPE-grown β -Ga2O3

We measure the low-frequency noise in epitaxial β-Ga2O3 grown by MOVPE. Both 1/f and generation-recombination noise components are well resolved. The Hooge parameters characterizing the 1/f noise are 3 × 10–4 at room temperature and 2 × 10–5 at temperatures near 200 K. Mid bandgap trap states result in generation-recombination noise that is analyzed using temperature dependent low-frequency deep-level noise spectroscopy. Trap levels with energies of 165, 127, and 37 meV below the conduction band minimum are characterized in terms of density and activation energy.

Improvement of Local Deep Level Transient Spectroscopy for Microscopic Evaluation of SiO<sub>2</sub>/4H-SiC Interfaces

We demonstrate our new local deep level spectroscopy system improved for more accurate analysis of trap states at SiO2/4H-SiC interfaces. Full waveforms of the local capacitance transient with the amplitude of attofarads and the time scale of microseconds were obtained and quantitatively analyzed. The local energy distribution of interface state density in the energy range of EC − Eit = 0.31–0.38 eV was obtained. Two-dimensional mapping of the interface states showed inhomogeneous contrasts with the lateral spatial scale of several hundreds of nanometers, suggesting that the physical origin of the trap states at SiO2/SiC interfaces is likely to be microscopically clustered.

Defect States Induced in GaN-Based Green Light Emitting Diodes by Electron Irradiation

The spatial distribution of deep traps in electron irradiated green multi-quantum-well (MQW) GaN/InGaN light emitting diodes was determined by deep level transient spectroscopy with electrical and optical injection. Four major electron traps with levels near Ec-0.2 eV, Ec-0.5 eV, Ec-0.75 eV, and Ec-1.1 eV were observed. The concentration of all electron traps increased with fluence of 6 MeV electrons, correlating with a decrease of the external quantum efficiency (EQE) of LEDs. The concentration of hole traps at Ev+0.45 eV also increased with irradiation. The observed EQE changes are partly attributed to trapping of electrons and holes by electron traps in the GaN barriers (Ec-0.75 eV, Ec-1.1 eV, Ev+0.95 eV) and nonradiative recombination in the QWs via electron traps Ec-0.5 eV and hole traps Ev+0.45 eV. Even traps with levels far from midgap can effectively participate in nonradiative recombination by forming close pairs similar to donor-acceptor pairs if their density is high. This can occur in In-rich fluctuation regions in the QWs, even as the densities averaged over the entire area of QWs as measured by deep level spectroscopy remain relatively low.

Dual-wavelength photo-Hall effect spectroscopy of deep levels in high resistive CdZnTe with negative differential photoconductivity

Photo-Hall effect spectroscopy was used in the study of deep levels in high resistive CdZnTe. The monochromator excitation in the photon energy range 0.65–1.77 eV was complemented by a laser diode high-intensity excitation at selected photon energies. A single sample characterized by multiple unusual features like negative differential photoconductivity and anomalous depression of electron mobility was chosen for the detailed study involving measurements at both the steady and dynamic regimes. We revealed that the Hall mobility and photoconductivity can be both enhanced and suppressed by an additional illumination at certain photon energies. The anomalous mobility decrease was explained by an excitation of the inhomogeneously distributed deep level at the energy Ev + 1.0 eV, thus enhancing potential non-uniformities. The appearance of negative differential photoconductivity was interpreted by an intensified electron occupancy of that level by a direct valence band-to-level excitation. Modified Shockley-Read-Hall theory was used for fitting experimental results by a model comprising five deep levels. Properties of the deep levels and their impact on the device performance were deduced.

In Situ and Ex Situ Investigations of KF Postdeposition Treatment Effects on CIGS Solar Cells

In situ and ex situ characterization were performed to assess the effects of potassium fluoride postdeposition treatment (KF PDT) on Cu(In,Ga)Se2 (CIGS) films and solar cells. Real-time spectroscopic ellipsometry enabled the in situ observation of the modification of the CIGS dielectric function and surface roughness thickness during KF PDT. In addition, deep-level transient spectroscopies (DLTS) and deep-level optical spectroscopies and scanning-DLTS enabled the identification of the nature and location of trap levels. Two main traps were present in both films near Ev + 0.56/0.61 eV and Ev + 0.99 eV, but with clearly lower concentrations for the latter in the KF PDT film. Finally, devices were fabricated, and higher efficiencies were found for the KF PDT devices. SCAPS modeling indicates that the traps identified by deep-level spectroscopies are in part responsible for these changes.

Deep-level spectroscopy in metal–insulator–semiconductor structures

In this study we present a method for measuring bulk traps using deep-level spectroscopy techniques in metal–insulator–semiconductor (MIS) structures. We will focus on deep-level transient spectroscopy (DLTS), although this can be extended to deep-level optical spectroscopy (DLOS) and similar techniques. These methods require the modulation of a depletion region either from a Schottky junction or from a highly asymmetric p–n junction, junctions that may not be realizable in many current material systems. This is the case of wide-bandgap semiconductor families that present a doping asymmetry or have a high residual carrier concentration or surface carrier accumulation, such as InGaN or ZnO. By adding a thin insulating layer and forming an MIS structure this problem can be circumvented and DLTS/DLOS can be performed under certain conditions. A model for the measurement of bulk traps in MIS structures is thus presented, focusing on the similarities with standard DLTS, maintaining when possible links to existing knowledge on DLTS and related techniques. The model will be presented from an equivalent circuit point of view. The effect of the insulating layer on DLTS is evaluated by a combination of simulations and experiments, developing methods for the measurement of these type of devices. As a validation, highly doped ZnO:Ga MIS devices have been successfully characterized and compared with a reference undoped sample using the methods described in this work, obtaining the same intrinsic levels previously reported in the literature but in material doped as high as cm−3.

Study of Exciton Hopping Transport in PbS Colloidal Quantum Dot Thin Films Using Frequency- and Temperature-Scanned Photocarrier Radiometry

Solution-processed colloidal quantum dots (CQDs) are promising materials for realizing low-cost, large-area, and flexible photovoltaic devices. The study of charge carrier transport in quantum dot solids is essential for understanding energy conversion mechanisms. Recently, solution-processed two-layer oleic-acid-capped PbS CQD solar cells with one layer treated with tetrabutylammonium iodide (TBAI) serving as the main light-absorbing layer and the other treated with 1,2-ethanedithiol (EDT) acting as an electron-blocking/hole-extraction layer were reported. These solar cells demonstrated a significant improvement in power conversion efficiency of 8.55% and long-term air stability. Coupled with photocarrier radiometry measurements, this work used a new trap-state mediated exciton hopping transport model, specifically for CQD thin films, to unveil and quantify exciton transport mechanisms through the extraction of hopping transport parameters including exciton lifetimes, hopping diffusivity, exciton detrapping time, and trap-state density. It is shown that PbS-TBAI has higher trap-state density than PbS-EDT that results in higher PbS-EDT exciton lifetimes. Hopping diffusivities of both CQD thin film types show similar temperature dependence, particularly higher temperatures yield higher hopping diffusivity. The higher diffusivity of PbS-TBAI compared with PbS-EDT indicates that PbS-TBAI is a much better photovoltaic material than PbS-EDT. Furthermore, PCR temperature spectra and deep-level photothermal spectroscopy provided additional insights to CQD surface trap states: PbS-TBAI thin films exhibit a single dominant trap level, while PbS-EDT films with lower trap-state densities show multiple trap levels.

Photocapacitance spectroscopy of InAlN nearly lattice-matched to GaN

We study the deep levels in InAlN nearly lattice-matched to GaN by photocapacitance spectroscopy. This technique allows the study of very deep levels having too slow thermal emission rates to be detected by other deep level spectroscopy techniques. We will identify a broad band of deep levels centered 1.7 eV below the InAlN conduction band edge. The deep level band is characterized by a negligible Franck-Condon shift and by a broadening parameter ΔE = 0.38 eV. Furthermore, we will show evidences for a second class of deep levels with optical ionization energy >2 eV, which will be attributed to previously reported oxygen-related DX centers.

Analysis of trapping and de-trapping in CdZnTe detectors by Pockels effect

In this contribution we introduce a method of deep level spectroscopy in semi-insulating semiconductors demonstrated on detector-grade bulk CdZnTe. The method is based on the measurements of temporal and temperature evolution of the electric field profile in studied samples, which is very sensitive to a change of occupancy of deep levels. The measurement of the electric field is based on the linear electro-optic (Pockels) effect using the InGaAs avalanche photodiode with fast response. The internal electric field profile in studied samples significantly changes under various external conditions represented by the application of the bias and pulsed illumination with below-bandgap light. From the knowledge of the electric field behavior and using a standard analysis based on thermally induced transitions of electrons and holes from the deep levels to the conduction and valence bands, respectively, it is possible to get activation energies of the energy levels, their types (donor or acceptor) and corresponding capture cross-sections. By this method we have found deep levels responsible for the polarization of CdZnTe detector under high photon-fluxes. Identified deep levels eV, eV and eV can capture the photo-generated holes and thus form a positive space charge, which is responsible for polarization of the detector.

Supercell-size convergence of formation energies and gap levels of vacancy complexes in crystalline silicon in density functional theory calculations

Results for supercell-size convergence of formation energies and charge transition levels of vacancy complexes ${V}_{n}\phantom{\rule{0.28em}{0ex}}(1\ensuremath{\le}n\ensuremath{\le}11)$ in crystalline Si are reported for the ab initio density functional theory (DFT) with generalized gradient approximation (GGA) pseudopotentials. When extrapolated to the dilute limit, the formation energy of an uncharged vacancy becomes $3.74\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, and the binding energy of an uncharged divacancy becomes $1.9\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. Stable ${V}_{n}$ clusters are built on the basis of sixfold rings $(n\ensuremath{\le}6)$ and of octahedral voids $(n\ensuremath{\ge}7)$. If the well-known underestimate of the band gap by the DFT and the accuracy of extrapolations are taken into account, the extrapolated levels are in good agreement with experiment. We discuss the implications for simulation of vacancy clustering during thermal quenching, for interpretation of deep-level spectroscopy and electron paramagnetic resonance in irradiated Si, and for cell and Brillouin-zone sampling choice when DFT-related methods beyond local-density approximation or GGA are used.

Rapid thermal processing induced vacancy-oxygen complexes in Czochralski-grown Si1−xGex

Rapid thermal processing (RTP) induced vacancy-oxygen (VOm m ≥ 2) complexes can act as the precursors for oxide precipitate nucleation thus enhancing oxygen precipitation (OP) in Czochralski silicon (Cz-Si). In our preceding work, we have reported that 1020 cm−3 germanium (Ge)-doping in Cz-Si facilitates the formation of VOm complexes during the high temperature RTP. Then, how the even higher Ge-doping at 1021 cm−3 in Cz-Si affects the formation of VOm complexes during the RTP is an interesting issue to be addressed. Actually, such high Ge-doping in Cz-Si forms Cz-Si1−xGex alloys. In the present work, we have investigated the formation of VOm complexes in Cz-Si1−xGex (x = 0.035–0.038) wafers subjected to the high temperature RTP. The Cz-Si1−xGex wafers are found to exhibit much weaker OP than Cz-Si counterparts when subjected to the same two-step (nucleation-growth) anneal following the high temperature RTP, indicating that the formation of VOm complexes during the RTP is suppressed in Cz-Si1−xGex wafers. The ‘gold-marker method’ in combination with deep level spectroscopy further confirms the suppressed formation of VOm complexes in Cz-Si1−xGex wafers during the RTP. Besides, the characterization of the oxygen out-diffusion depth profiles in Cz-Si1−xGex and Cz-Si wafers annealed at different temperatures in the range 650–1000 °C indicates that the oxygen diffusivity in Cz-Si1−xGex is a little smaller. The calculations based on density functional theory reveal that in Cz-Si1−xGex the vacancies incline to locate at the first nearest neighboring sites of the Ge atoms and the capture of Oi atoms by the vacancies is less energetically favorable. The aforementioned adverse conditions are believed to be responsible for the suppressed formation of VOm complexes in Cz-Si1−xGex.

Spectroscopy of Technological Defects in Si Solar Cells by Analysis of Temperature Dependent Generation Currents

The efficiency of solar cells considerably depends on the technological defects introduced by the formation of junctions, passivation layers and electrodes. Identification of these defects present in the high conductivity base layer of modern solar cells by usage of the standard techniques, such as capacitance deep level spectroscopy, is restricted by extremely small size of samples with inherent enhanced leakage current on sample boundaries. Therefore, it is important to develop the alternative methods for the defect spectroscopy in the high conductivity junction structures, to directly control a relative low concentration of the technological defects. In this work, the spectroscopy of deep traps has been performed by combining the temperature scans of the thermal generation currents extracted from barrier capacitance charging transients and capacitance deep level transient spectroscopy techniques. The dominant carrier traps ascribed to the Cu and Ni impurities were revealed. DOI: http://dx.doi.org/10.5755/j01.ms.20.3.5194