Deep Level Transient Spectroscopy Experiments Research Articles

To define nonradiative defects in semiconductors: An accurate DLTS simulation based on first-principle

Deep level transient spectroscopy (DLTS) is a crucial technique to characterize the defects in semiconductor devices. Within the framework of the state-of-the-art density functional theory (DFT), the simulation for experimental DLTS spectra is realized successfully with high accuracy and intrinsic physics in this work, of which the reliability is confirmed by nonlocal hybrid functional. The electron–phonon coupling is included to describe the thermal-activated behavior of the hot carrier nonradiative capture reflected by DLTS signal. In the case study, the divacancy in silicon is employed for the flow of DLTS calculation as a benchmark, of which the results are verified by our 40 MeV Si irradiation experiments on the NPN and PNP bipolar junction transistors and previous reports. The problem we can address is the identification of defect characterized in DLTS experiments. The realization of DLTS simulation would have a profound significance for understanding the underlying physics in experimental observations and be a powerful leverage to define defect nature in semiconductors.

The Sliding-Aperture Transform and Its Applicability to Deep-Level Transient Spectroscopy

A mathematical method is presented for the extraction of defect parameters from the multiexponential decays generated during deep-level transient spectroscopy experiments. Such transient phenomenon results from the ionization of charge trapped in defects located in the depletion width of a semiconductor diode. From digitized transients acquired at fixed temperatures, this method produces a rate–domain spectral signature associated with all defects in the semiconductor. For signal-to-noise ratio of 1000, defect levels with carrier emission rates differing by as little as 1.5 times may be distinguished.

Defect states and their electric field-enhanced electron thermal emission in heavily Zr-doped β -Ga2O3 crystals

Performing deep level transient spectroscopy (DLTS) on Schottky diodes, we investigated defect levels below the conduction band minima (Ec) in Czochralski-grown unintentionally doped (UID) and vertical gradient freeze-grown Zr-doped β-Ga2O3 crystals. In UID crystals with an electron concentration of 1017 cm−3, we observe levels at 0.18 eV and 0.46 eV in addition to the previously reported 0.86 (E2) and 1.03 eV (E3) levels. For 1018 cm−3 Zr-doped Ga2O3, signatures at 0.30 eV (E15) and 0.71 eV (E16) are present. For the highest Zr doping of 5 × 1018 cm−3, we observe only one signature at 0.59 eV. Electric field-enhanced emission rates are demonstrated via increasing the reverse bias during measurement. The 0.86 eV signature in the UID sample displays phonon-assisted tunneling enhanced thermal emission and is consistent with the widely reported E2 (FeGa) defect. The 0.71 eV (E16) signature in the lower-Zr-doped crystal also exhibits phonon-assisted tunneling emission enhancement. Taking into account that the high doping in the Zr-doped diodes also increases the electric field, we propose that the 0.59 eV signature in the highest Zr-doped sample likely corresponds to the 0.71 eV signature in lower-doped samples. Our analysis highlights the importance of testing for and reporting on field-enhanced emission, especially the electric field present during DLTS and other characterization experiments on β-Ga2O3 along with the standard emission energy, cross section, and lambda-corrected trap density. This is important because of the intended use of β-Ga2O3 in high-field devices and the many orders of magnitude of possible doping.

Determination of Majority Carrier Capture Rates via Deep Level Transient Spectroscopy

In Deep Level Transient Spectroscopy (DLTS) experiments the majority carrier capture rate is often determined by observing the growth of the signal amplitude as a function of filling pulse duration at constant temperature. The analysis of such experiments is complicated by the phenomenon of slow capture: carrier capture by defects in the Debye tail of the depletion layer at the pulse voltage. We review here three approaches for analyzing isothermal pulse duration variation DLTS experiments that have been described or at least have been frequently used in literature. These methods are compared for their ability to correctly extract capture rates from simulated data as well as from actual experimental data for the Fe− /2 − level in crystalline germanium. Finally, we tested the performance of the three methods for analysis of DLTS signals that experience a delayed growth, modeled by an additional time constant in the system.

About RC‐like contacts in deep level transient spectroscopy and Cu(In,Ga)Se2 solar cells

ABSTRACTThe low temperature Deep‐Level Transient Spectroscopy (DLTS) signal of two Cu(In, Ga)Se2 samples on glass with different buffer layers is subjected to a thorough study. A similar signal is observed in the DLTS and admittance spectra of many solar cells and is usually labeled as N1. The standard DLTS theory assumes the investigated device to be a Schottky or p–n diode with an ohmic back contact, and relates the spectral components to capture or emission of free carriers by defect levels in the structure. It is well‐known, though, that Cu(In, Ga)Se2 thin film solar cells deviate from this ideal structure. However, even for a device like this, where advanced numerical modeling is necessary to describe the equilibrium charge distribution as a function of applied bias, a change in the free carrier concentration at a certain position of the device as a result of capture or emission by deep defect levels should satisfy the detailed balance equation. The DLTS experiment performed with conventional and complemental settings for the reverse and pulse bias voltages (Vr < Vp < 0 and Vp < Vr < 0, respectively) exhibit characteristics that cannot be explained using free carrier transfer between deep levels—in the bulk or at an interface—and the conduction (electrons) or valence (holes) band of a semiconductor as a model. On the other hand, we show that for the solar cells studied here, the N1 signals follow the behavior predicted for a non‐ohmic RC‐like contact, as established in our recent paper (J. Lauwaert et al. Journal of Applied Physics 2011) closely. Copyright © 2012 John Wiley & Sons, Ltd.

Polaronic trapping of electrons and holes by native defects in anataseTiO2

We have investigated the formation of native defects in anatase ${\text{TiO}}_{2}$ using density functional theory (DFT) modified with on-site Coulomb terms $(\text{DFT}+U)$ applied to both $\text{Ti}\text{ }d$ and $\text{O}\text{ }p$ states. Oxygen vacancies and titanium interstitials are found to be deep donors that trap two and four electrons, with transition levels that explain the two features seen in deep level transient spectroscopy experiments. Titanium vacancies are deep acceptors accommodating four holes. Self-trapping of both electrons and holes is also predicted. In all cases both donor and acceptor trap states correspond to strongly localized small polarons, in agreement with experimental EPR data. Variation in defect formation energies with stoichiometry explains the poor hole-trapping of reduced ${\text{TiO}}_{2}$.

Neutron-irradiated Schottky diodes with self-assembled InAs quantum dots: Optical and electrical properties

The effect of neutron irradiation on optical and electrical properties of self-assembled InAs/GaAs quantum dots (QDs) is studied for neutron doses up to ϕn=3×1014 cm−2. The QDs are embedded in n-type GaAs Schottky diodes grown by molecular beam epitaxy on GaAs(001). We observe an initial increase in QD photoluminescence followed by a decrease as a function of neutron irradiation. The increase in photoluminescence is attributed to a more effective carrier trapping and relaxation from the wetting layer into the QDs via radiation-induced defects. The losses of photoluminescence at higher neutron doses are caused by defects, which act as nonradiative recombination centers. The observed dependence of the photoluminescence on the laser excitation power can be explained with saturation effects. Deep level transient spectroscopy experiments reveal multiple overlapping electron traps. Furthermore, an enhanced introduction of defects in the QD layer is observed. Electrical characterization of the Schottky diodes via capacitance-voltage spectroscopy reveals that the doping condition in the diodes changes due to the introduction of defects. This fact has to be taken into account if QDs are used in electrical devices in radiation-harsh environments as it is shown by low-temperature deep level transient spectroscopy experiments on electron emission processes from QD levels.

Isochronal Annealing Study of Deep Levels in Hydrogen Implanted p-Type 4H-SiC

Aluminum doped 4H-SiC samples have been implanted with 200 and 100 keV hydrogen at five different temperatures. An isochronal annealing series has been carried out up to 1000 °C and deep level transient spectroscopy (DLTS) was performed after every annealing step in the 150-700 K temperature range. After studying the role of the annealing treatments on the reactivation of aluminum acceptors, we discuss the results of the DLTS experiment. Eight deep levels were found in the 0.15 - 0.7 eV range above the valence band (E v ), and their annealing behavior, as a function of the temperature is presented. We found that deep levels implanted at temperatures below 300 °C are less thermally stable. Furthermore, we compare the experimental results with theoretical data present in the literature, and suggest a possible microscopic structure of the detected levels.

Properties of the oxygen vacancy in ZnO

As-grown ZnO bulk crystals and crystals annealed in vacuum, oxygen, or zinc vapour were characterized by electrical, optical and magnetic resonance spectroscopy. The experiments show that the residual carrier concentration is caused by residual H, Al, Ga and oxygen vacancies (VO) in the material. Annealing the samples in O2 at about 1000 °C (2 atm, 20 h) reduces the H and VO donor concentration by typically one order of magnitude. The photoluminescence and deep level transient spectroscopy (DLTS) results suggest a correlation between the broad unstructured emission at 2.45 eV (“green band”) and a donor level 530 meV below the conduction band, it is attributed to the VO 0/++transition. By using DLTS experiments with optical excitation it is possible to observe a metastable level 140 meV below the conduction band which is assigned to the VO 2+/+ recharging. The results give evidence for the “negative-U” properties of the oxygen vacancy defects predicted by recent theoretical calculations.

Combined photoluminescence-imaging and deep-level transient spectroscopy of recombination processes at stacking faults in4H−SiC

We report on electronic properties of single- and double-layer stacking faults in $4H\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}$ and provide an insight into apparent distinctions of recombination-enhanced defect reactions at these faults. Photoluminescence imaging spectroscopy and deep-level transient spectroscopy experiments reveal key constituents of radiative recombination and also provide firm evidence of nonradiative centers at ${E}_{\mathrm{V}}+0.38\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ responsible for recombination-enhanced mobility of silicon-core partial dislocations. A comprehensive energy level model is proposed allowing for a qualitative description of recombination activity at different types of stacking faults and the corresponding bounding partial dislocations.

Combined optical and electrical studies of the effects of annealing on the intrinsic states and deep levels in a self-assembled InAs quantum-dot structure

The effects of postgrowth rapid thermal annealing on the electronic states in a relatively long wavelength (∼1.3μm), self-assembled InAs∕GaAs quantum-dot structure are investigated. We combine optical and electrical experiments, i.e., photoluminescence (PL) and deep-level transient spectroscopy (DLTS) measurements, to identify the underlying physical processes responsible for the changes in the PL spectra at different annealing temperatures. Physical parameters of the intrinsic and deep-level states are quantitatively determined in the DLTS experiments. These include the thermal excitation energies, densities, and their changes with the annealing temperature. We observe that the densities of the deep levels that coexist in the quantum-dot layer decrease and a new deep level, about 0.62eV below the GaAs conduction band edge, is formed at elevated temperatures. Both effects explain the variations in the PL spectra. Moreover, beyond what can be revealed in the PL experiments, the DLTS spectra show a more complex electronic structure of both optically active and inactive states.

Carrier storage and confinement in GaAs‐based self‐organized quantum dots

We studied the carrier storage and confinement of electrons and holes in InGaAs/GaAs QDs with deep level transient spectroscopy (DLTS) and time-resolved tunneling capacitance spectroscopy (TRTCS). For InGaAs/GaAs QDs with an additional AlGaAs barrier below the QD layer we observe in charge-selective DLTS experiments a hole ground state activation energy of about 580 meV. This activation energy leads to a storage time of about 5 ms at room temperature. With the TRTCS method we studied in detail the electric field dependence of the electron and hole tunneling time from InGaAs QDs without an AlGaAs barrier. The dependence of the tunneling time on the applied electric field allows us to determine the hole and electron localization energy to E H1loc = (210 ± 20) meV and E E1loc = (260 ± 20) meV, respectively. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of stress on the energy levels of the vacancy-oxygen-hydrogen complex in Si

The piezospectroscopic properties of the $V\mathrm{OH}$ defect in Si are found using stress Laplace deep level transient spectroscopy (DLTS) and are compared with local density-functional calculations of (i) the acceptor level and its shift under stress, and (ii) the alignment of the neutral center under stress. The theory is able to account for two acceptor levels observed for $〈100〉,$ $〈111〉,$ and $〈110〉$ stress even though additional splitting is expected for a defect with static ${C}_{1h}$ symmetry. This is related to (i) a rapid reorientation of the H atom within the defect at temperatures at which the DLTS experiments are carried out, and (ii) the small effect of stress on two orientations of the defect under $〈110〉$ stress. The theory is also able to give a quantitative account of the alignment of the center. The effect of stress on the reorientation barrier of the defect is also investigated. The reorientation barrier of the defect in its positive charge state is found theoretically to be very small, consistent with the lack of any splitting in the donor level under stress.

Correlation between the results of charge deep-level transient spectroscopy and ESR techniques for undoped hydrogenated amorphous silicon

Results of charge deep-level transient spectroscopy (DLTS) and electron spin resonance (ESR) measurements on undoped hydrogenated amorphous silicon $(a$-Si:H) clearly demonstrate that a group of gap states with a mean energy of 0.82 eV as observed in charge DLTS experiments for a-Si:H based metal/oxide/semiconductor structure is the same as the $g=2.0055$ ESR defect (the ${D}_{z}$ component). This correlation provides a distinct marker for charge DLTS technique. We obtained a very good fit to spectra obtained on undoped a-Si:H in the annealed state whilst there is some discrepancy between the experimental and simulated spectra for the light-soaked state. The first quantitative comparison of defect pool model with gap states directly observed by charge DLTS offers not only additional data for more accurate identification of all the intrinsic and light-induced defects. This also renders distinct counter-evidence to recently published conjectures about the creation of another charged defect during early stage of Staebler-Wronski effect. By contrast, our presented results clearly argue for opposite process, i.e., decay of positively charged defect states ${D}_{h}.$

Capacitance-voltage profiling and thermal evolution of the conduction band-offset of unstrained Ga0.47In0.53As/InP single quantum well

We report the results of capacitance-voltage (C-V) and Deep Level Transient Spectroscopy (DLTS) measurements performed upon a Ga0.47In0.53As/InP quantum well structure. At room temperature, a conduction-band offset ΔEc=(200±10)meV and charge densities σI=±(3±1)*1011 times the electronic charge per cm2 have been measured from C-V experiments. At lower temperature (T≤150K) we have observed an important decrease of the band-offset, considerably larger than a pure thermal effect. We have shown that the explanation lies in the presence of a high concentration of deep traps located at the well-barrier interfaces. Two species A and B have been detected through DLTS experiments with activation energies EtA=90 meV and EtB=195 meV, respectively. The filling of these trap levels at low temperature lowers the band offset from 200 to 120 meV, owing to band repulsion effects.

CV and DLTS Experiments in Boron-Doped Diamond

Capacitance–voltage (CV) and deep level transient spectroscopy (DLTS) experiments are applied to explore various properties of boron-doped type IIb high-pressure high-temperature diamond and of homoepitaxially grown boron-doped CVD diamond: contact properties of Al contacts, acceptor density determination, energy distributions and densities of compensating defects and the interaction of boron acceptors with atomic hydrogen (deuterium). Two distinct excitation energies at 0.9 and 1.25 eV are detected by DLTS and optically excited DLTS. We propose a defect/acceptor model where holes are optically excited from defects 1.25 eV above the valence band edge either into ionized boron acceptors at 0.36 eV above EV (optical excitation ≈︂0.9 eV) or directly into the valence band (photon energy ≥1.25 eV). We also discuss the decrease of the capacitance after deuteration as detected by CV measurements based on deuterium–boron complex formation.

CV and DLTS Experiments in Boron-Doped Diamond

Capacitance–voltage (CV) and deep level transient spectroscopy (DLTS) experiments are applied to explore various properties of boron-doped type IIb high-pressure high-temperature diamond and of homoepitaxially grown boron-doped CVD diamond: contact properties of Al contacts, acceptor density determination, energy distributions and densities of compensating defects and the interaction of boron acceptors with atomic hydrogen (deuterium). Two distinct excitation energies at 0.9 and 1.25 eV are detected by DLTS and optically excited DLTS. We propose a defect/acceptor model where holes are optically excited from defects 1.25 eV above the valence band edge either into ionized boron acceptors at 0.36 eV above EV (optical excitation ≈0.9 eV) or directly into the valence band (photon energy ≥1.25 eV). We also discuss the decrease of the capacitance after deuteration as detected by CV measurements based on deuterium–boron complex formation.

Effect of Er dopant on the properties of In0.53Ga0.47As layers grown by liquid phase epitaxy

Detailed properties of In0.53Ga0.47As layers grown by liquid phase epitaxy from melts containing 0.04 and 0.1 wt % Er are reported. The carrier concentration in the material is reduced by almost two orders of magnitude as a result of Er doping. Low temperature photoluminescence measurements indicate that both the donor and the acceptor type impurities are gettered by Er and the full-width at half-maximum of the major peak is reduced to 4 meV for the layer with the highest Er doping. From deep level transient spectroscopy experiments on undoped layers, we confirm the presence of an electron trap with activation energy of 0.17 eV. Density of this trap is reduced by more than two orders of magnitude in the Er doped material and another electron trap with activation energy of 0.15 eV is revealed from the analysis of the experimental data. We associate the 0.17 eV trap with impurities in the material. From low temperature photoconductivity and photocapacitance experiments, we further confirm that Er creates a level located 40 meV above the valence band. Density of this center increases in the material after high temperature annealing. We suggest that the Er-related level is due to Er atoms occupying cation sites in the material and acting as an isoelectronic impurity.

Influence of an inhomogeneous shallow charge distribution on the results of DLTS experiments

We have investigated the influence of an inhomogeneous shallow charge distribution on the results of deep level transient spectroscopy (DLTS) by numerical simulation of capacitance transients. Our results show that the correct deep trap concentration results if the shallow impurity concentration at the end of the space charge region enters into the formula for the trap concentration. A shift of the DLTS peak to higher temperatures may occur in the case of a large gradient in the shallow impurity profile at the end of the space charge region. Constant capacitance DLTS is the ideal tool to deal with an inhomogeneous shallow charge distribution, since in this case the trap concentration does not depend on the concentration of the shallow impurities.

High-temperature uniaxial stress apparatus for semiconductor defect symmetry determination

A mechanical apparatus designed for use in deep level transient spectroscopy (DLTS) experiments and capable of applying more than 5 GPa pressure to a semiconductor crystal sample at temperatures from 300–450 K is described. This compound lever system applies a temperature-independent stress to the sample throughout the temperature range of interest. As an illustration of this device’s application, the initial result of a uniaxial stress DLTS experiment on EL2 in n-GaAs is given. EL2 is found to split under the [100] uniaxial stress, denying the AsGa and AsGa–Asi models.