Standard Deep Level Transient Spectroscopy Research Articles

Probing Defects in MoS2 Van der Waals Crystal through Deep‐Level Transient Spectroscopy

The electrical performance of transition metal dichalcogenides (TMDCs) is strongly affected by the quality of electrical metal contacts and the formation of electrically active defects. Herein, deep‐level transient spectroscopy (DLTS) is used for direct probing of deep‐level defects in the bandgap of single MoS2 van der Waals crystal. Standard DLTS temperature spectra reveal a deep‐level trap located at about 0.36 eV below the conduction band edge. This trap is tentatively attributed to sulfur vacancies and localized on the electronic band structure of MoS2, obtained within the density functional theory (DFT), and matched with experimentally studied electronic band structure, by absorption and contactless electroreflectance (CER) spectroscopy.

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 Defects in GaN In Situ Doped with Eu3+ Ion Grown by OMVPE

In this work, GaN epilayer in situ doped with Eu3+ ions was deposited on the top of an undoped n-GaN/LT-GaN/sapphire structure by organometallic vapor-phase epitaxy. A set of different ohmic and Schottky contacts on GaN:Eu3+ surface was fabricated by electron-beam evaporation for performing deep level transient spectroscopy (DLTS) measurement. The deep defect energy levels in GaN:Eu3+ epilayer were assessed by standard DLTS and high resolution Laplace DLTS (L-DLTS). Three dominant DLTS peaks were observed in the temperature range from 35 K to 400 K. The calculated activation energies of these defect energy levels were 0.108 ± 0.03 eV (Trap A), 0.287 ± 0.04 eV (Trap B) and 0.485 ± 0.06 eV (Trap C) below conduction band edge, respectively. High resolution L-DLTS conducted for the GaN:Eu3+ epilayer revealed at least four closely spaced defect energy levels associated with Trap B. It is proposed that these defect energy levels correspond to the selected optically active Eu3+ ion centers in GaN host previously identified by optical studies in this material (Fujiwara and Dierolf in Jpn J Appl Phys 53:05FA13, 2014).

Injection deep level transient spectroscopy: An improved method for measuring capture rates of hot carriers in semiconductors

An improved method for measuring the cross sections for carrier trapping at defects in semiconductors is described. This method, a variation of deep level transient spectroscopy (DLTS) used with bipolar transistors, is applied to hot carrier trapping at vacancy-oxygen, carbon-oxygen, and three charge states of divacancy centers (V2) in n- and p-type silicon. Unlike standard DLTS, we fill traps by injecting carriers into the depletion region of a bipolar transistor diode using a pulse of forward bias current applied to the adjacent diode. We show that this technique is capable of accurately measuring a wide range of capture cross sections at varying electric fields due to the control of the carrier density it provides. Because this technique can be applied to a variety of carrier energy distributions, it should be valuable in modeling the effect of radiation-induced generation-recombination currents in bipolar devices.

Correlation between barrier inhomogeneities of 4H-SiC 1 A/600 V Schottky rectifiers and deep-level defects revealed by DLTS and Laplace DLTS

Electrical properties of commercial silicon carbide (SiC) Schottky rectifiers are investigated through the measurement and analysis of the forward current–voltage (I–V) and reverse capacitance–voltage (C–V) characteristics in a large temperature range. Some of devices show distinct discrepancies in specific ranges of their electrical characteristics, especially the excess current dominates at voltage<1V and temperature<300K. Standard deep level transient spectroscopy (DLTS) revealed the presence of a single deep-level defect with activation energy of about 0.3eV, exhibiting the features characteristic for extended defects (e.g. dislocations), such as logarithmic capture kinetics. Furthermore, high-resolution Laplace DLTS showed that this deep level consists actually of three closely spaced levels with activation energies ranging from about 0.26eV to 0.29eV. A strong correlation between these two techniques implies that the revealed trap level is due to extended defects surrounded by point traps or clusters of defects. On the basis of obtained specific features of the deep-level defect, it was proposed that this defect is arguably responsible for the observed Schottky barrier inhomogeneities.

Deep levels of copper-hydrogen complexes in silicon

Complexes of substitutional copper (Cu${}_{s}$) with hydrogen in silicon are investigated by standard deep-level transient spectroscopy (DLTS) and high-resolution Laplace-transform DLTS. Hydrogen is introduced into the near-surface layer of copper-doped crystals during wet chemical etching and moved deeper into the sample by annealing Schottky contacts under reverse bias at 350--380 K. Two novel centers are observed to form in the hydrogenated region. Each of them possesses two deep levels which are only slightly different from the Cu${}_{s}$ levels. Analysis of the depth profiles allows us to identify the observed centers as complexes of Cu${}_{s}$ with one and two hydrogen atoms and reveals the formation of an electrically inactive complex with three hydrogen atoms. The identification is confirmed by the numerical modeling of the copper-hydrogen interaction which enables a quantitative description of the experimentally measured concentrations of different complexes. The variations of the Cu${}_{s}$ donor and acceptor levels due to successive hydrogenation are found to be similar to those for isoelectronic Ag and Au impurities.

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 &lt; Vp &lt; 0 and Vp &lt; Vr &lt; 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 &amp; Sons, Ltd.

Deep levels in H-irradiated GaAs1-xNx (x &amp;lt; 0.01) grown by molecular beam epitaxy

Dilute nitride GaAs1-xNx layers have been grown by molecular beam epitaxy with nitrogen concentration ranging from 0.2% to 0.8%. These samples have been studied before and after hydrogen irradiation by using standard deep level transient spectroscopy (DLTS) and high resolution Laplace DLTS techniques. The activation energy, capture cross section and density of the electron traps have been estimated and compared with results obtained in N-free as-grown and H-irradiated bulk GaAs.

Deep hole traps in boron-doped diamond

Deep hole traps in boron-doped diamond epitaxial layers are studied by means of several types of deep-level transient spectroscopy and density-functional theory calculations. Standard deep-level transient spectroscopy and high-resolution isothermal transient spectroscopy permit to identify nine deep hole traps. Their capture cross-sections and ionization energies are systematically determined. In parallel, the ionization energies of donor and acceptor levels related to boron- and/or hydrogen-related complexes in diamond are assessed by ab initio calculations in this work and summarized with others from the literature including native defects. Tentative assignments of the measured deep hole traps to the calculated ones are proposed.

The As2V complex in silicon: Band-gap levels, migration and annealing

In this study, we used n+p mesa diodes where the highly arsenic doped n+ top layer was grown by molecular-beam epitaxy on top of a 5-μm thick p-type layer grown by chemical-vapour deposition. The diodes were irradiated with 2MeV electrons at room temperature and the resulting defects in the p-type region were studied by Laplace and standard deep-level transient spectroscopy. A new defect with an apparent energy position of 0.20eV above the valence band is shown to migrate from the n+ top layer into the p-type material where it is observed by DLTS. An annealing study shows that this defect grows in at ∼400K and disappears at ∼460K. We argue that this defect is the As2V complex, and that it is formed in the top layer when negatively charged AsV pairs, which are generated in the n+ top layer by the electron irradiation, migrate and react with the abundant and positively charged As donors. Once formed, the As2V complex diffuses into the p-type material.

Meyer-Neldel rule and the influence of entropy on capture cross-section determination in Cu(In,Ga)Se2

We use deep-level transient spectroscopy (DLTS) to investigate the electronic properties of ZnO∕CdS∕CuxIn1−yGaySe2∕Mo∕soda-lime glass thin-film solar cells. We deposited films with different x and y values using elemental evaporation. Devices made from these films exhibit more than 40 hole and electron traps, with activation energies ranging from 0.07to1.17eV. Standard DLTS analysis (which assumes that entropy changes can be neglected) gives apparent capture cross sections that vary by 18 orders of magnitude over this energy range. All our data show that the charge-carrier emission rate obeys the Meyer-Neldel rule (MNR) with an isokinetic temperature of 340±30K. By including the MNR relation in detailed balance, we show that there is a single cross section for all the traps.

M-Center in Low-Dose Proton Implanted 4H-SiC; Bistability and Change in Emission Rate

Electrically active point defects in semiconductor materials are important because they strongly affect material properties like effective doping concentration and charge carrier lifetimes. This thesis presents results on point defects introduced by ion implantation in silicon and silicon carbide. The defects have mainly been studied by deep level transient spectroscopy (DLTS) which is a quantitative, electrical characterization method highly suitable for point defect studies. The method is based on measurements of capacitance transients and both standard DLTS and new applications of the technique have been used. In silicon, a fundamental understanding of diffusion phenomena, like room-temperature migration of point defects and transient enhanced diffusion (TED), is still incomplete. This thesis presents new results which brings this understanding a step closer. In the implantation-based experimental method used to measure point defect migration at room temperature, it has been difficult to separate the effects of defect migration and ion channeling. For various reasons, the effect of channeling has so far been disregarded in this type of experiments. Here, a very simple method to assess the amount of channeling is presented, and it is shown that channeling dominates in our experiments. It is therefore recommended that this simple test for channeling is included in all such experiments. This thesis also contains a detailed experimental study on the defect distributions of vacancy and interstitial related damage in ion implanted silicon. Experiments show that interstitial related damage is positioned deeper (0.4 um or more) than vacancy related damage. A physical model to explain this is presented. This study is important to the future modeling of transient enhanced diffusion. Furthermore, the point defect evolution in low-fluence implanted 4H-SiC is investigated, and a large number of new defect levels has been observed. Many of these levels change or anneal out at temperatures below 300 C, which is not in accordance with the general belief that point defect diffusion in SiC requires high temperatures. This thesis also includes an extensive study on a metastable defect which we have observed for the first time and labeled the M-center. The defect is characterized with respect to DLTS signatures, reconfiguration barriers, kinetics and temperature interval for annealing, carrier capture cross sections, and charge state identification. A detailed configuration diagram for the M-center is presented.

Giant traps associated with extended defects in GaN and SiC

Extended defects in semiconductors can trap charge and lead to changes in carrier concentration and mobility. Here we consider the trapping effects of pores, microcracks, and dislocations in GaN and SiC, as analyzed by deep level transient spectroscopy (DLTS), transmission electron microscopy (TEM), and scanning surface potential microscopy (SSPM). The defect structures are modeled as spheres, plates, and cylinders for pores, cracks, and dislocations, respectively, and their potentials are directly compared with those measured by holographic TEM and SSPV. The dynamics of the capture and emission processes are investigated by DLTS, although the standard DLTS analysis framework is not applicable here and must be replaced by a more general formalism. As an example, 40-nm-dia. nanopores in SiC can each hold more than 100 electrons, and they exhibit anomalous capture and emission properties.

Constant-resistance deep-level transient spectroscopy in submicron metal-oxide-semiconductor field-effect transistors

This article describes a new variation of a deep-level transient spectroscopy (DLTS) technique convenient for the measurement of submicron field-effect transistors where standard capacitance DLTS cannot be used. Constant-resistance (CR) DLTS is similar to the conductance DLTS, but it is more sensitive and it does not require simultaneous measurement of the transconductance gm or surface mobility μ for calculation of the trap concentrations. In addition, the DLTS signal is largely independent of the transistor size, thus allowing measurements of very small-size transistors. The proposed technique is not restricted to metal-oxide-semiconductor field-effect transistors, but can be used also to study other field-effect transistors.

Deep Electron Traps in CdTe:In Films Grown by Molecular Beam Epitaxy

N-type indium doped CdTe grown on n+-GaAs by molecular beam epi- taxy has been studied by the standard deep level transient spectroscopy and the isothermal Laplace-transform deep level transient spectroscopy. It was found that the Cd/Te flux ratio strongly influences the deep level transient spectroscopy results. The unusual temperature dependence of the electron emission rate in films grown at nearly stoichiketric conditions may point out that the observed defect is resonant with the conduction band. PACS numbers: 68.55.Ln, 73.61.Ga Any intentional doping of semiconduction may lead to a formation of un- desired deep electronic states associated with point defects. The presence of deep states influences properties of the doped material and affects the performance of devices fabricated from it. Thus, an issue of primary importance is to develop such growth techniques which can eliminate deep states, or at least, which result in reducing their number in the material. In two recent publications we have reported on the molecular beam epitaxy

A sensitive technique for detecting low concentrations of deep level traps: Current-source deep level transient spectroscopy

A defect characterization method is presented that promises to provide improved defect detection sensitivity. The new technique involves the use of current-source biasing of a pn diode. Under current-source bias the junction voltage is transient, whereas standard deep level transient spectroscopy typically involves voltage-source bias with either capacitance or current transients. Computer simulations predict voltage transients of 5 mV from samples that, under identical trap capture and emission conditions, yield capacitance and current transients of only 0.2 fF and 0.7 pA. The new approach can tolerate large leakage currents, and the voltage transient amplitude is independent of device area.

Capacitance and drain current deep level transient spectroscopy measurements on molecular beam epitaxy grown GaAs/ln0·25Ga0·75As/Al0·3Ga0·7As high electron mobility transistors

Modulation doped GaAs/In0·25Ga0·75As/Al0·3Ga0·7As high electron mobility transistor structures were grown using different molecular beam epitaxy growth temperatures and In0·25Ga0·75 As channel thicknesses. Drain current deep level transient spectroscopy (DLTS) and standard capacitance DLTS measurements were carried out on these devices using a Fourier transform technique. The results show that traps in the barrier as well as those in the buffer have an influence on the drain current of such transistors. In addition, a broadening of one of the DLTS peaks was observed for high channel thicknesses and is shown to be related to a high increase in dislocation concentration, as confirmed by cathodoluminescence measurements.MST/3323

Numerical fitting of transient decays in the high defect density limit

A numerical fitting method based on the deep level transient spectroscopy (DLTS) technique is presented. This method deals with a situation where the standard rate window DLTS is no longer sufficient, i.e., the assumption that the defect density NT is much less than the donor doping density ND is no longer valid. Digitized capacitance transients are numerically fit to extract the electron emission rate, defect density, and energy level. The defect center under study is EL2 in n-type liquid-encapsulated Czochralski gallium arsenide. The fitting method gives an EL2 thermal activation energy of 0.76 eV, different from the 0.82 eV obtained by standard DLTS, which only examines the maximum emission conditions. The advantages, as well as the limitations, of this fitting method are discussed.

Analysis of defect energy levels in the limit of low emission rates

A new deep level transient spectroscopy (DLTS) analysis method is presented. This method operates in the limit of large emission times which leads to a major simplification of standard DLTS calculations. This analysis can be applied as data is collected, and yields defect energy levels completely independent of the thermally activated cross section. This modification is especially useful in separating closely spaced energy levels not resolvable by standard analysis methods. Specifically, in the modified technique the shallower energy level is reliably determined. As an example, the new technique is applied to the energy level splitting of EL2 in n-type gallium arsenide induced by uniaxial stress. Our preliminary results for stress applied along &amp;lt;100≳ indicates that C3v and Td symmetries are incompatible with this defect.

Toward Modelling Correlated DLTS Spectra from Al/Plasma Oxide/Semi-Insulating GaAs(Cr) Devices

Metal—oxide—semiconductor (MOS) structures prepared on semi-insulating (SI) GaAs substrates are studied by a multichannel modified charge deep-level transient spectroscopy (DLTS) in the depletion mode. In this way an improvement in the selectivity, as compared with the standard DLTS, is obtained. Such an improvement is a prerequisite of a more effective decomposition of the total spectra into individual components, assuming that both the dielectric relaxation and trap-limited charge transients interfere with each other. In addition to the charge released from the dominant interface trap, the contribution from the thermal activation of the conductivity of the bulk SIGaAs material was modelled using an equivalent circuit scheme. The experimentally observed behaviour resembles strongly the results of our model calculations.