Spectral Analysis Of Deep Level Transient Spectroscopy Research Articles

Characterization of Interface States in Degraded ZnO Varistors

The degradation mechanism of ZnO varistors was investigated by using spectral analysis of deep-level transient spectroscopy (SADLTS), and using the current–voltage (I–V) characteristics and the capacitance–voltage (C–V) characteristics before and after AC biasing stress. The interface states in the Pr-type ZnO varistors consist of two adjacent levels: T0 and T1 levels. The activation energy ET and the capture cross section σ of the interface states decrease with increasing degradation induced by AC bias stress. The decrease in ET and σ is related to a decrease in the barrier height caused by the degradation. In order to characterize the change in the interface states with degradation, distribution parameters (ΔET, Δσ) are introduced, where the parameters for each level increase after degradation. The degradation is closely related to the distribution of the emission rate of the interface states. The degradation phenomenon is caused by migrated ions or by adsorbed ions such as oxygen. The increase in the distribution parameters upon degradation suggests activation of the oxygen ions at the grain boundaries and nonuniform migration.

Pinning Effect by Interface States in Pr-type ZnO Varistors

The interface states in Pr-type ZnO varistors, which consist of two adjacent levels, T1 and T0, were studied using spectral analysis of deep-level transient spectroscopy (SADLTS) to characterize the pinning effect of the Fermi level by two interface states and the bias dependence on the emission process of the interface states. The measurements were carried out by changing the applied steady-bias voltage for the injection pulse. When the steady-bias voltage is below 60% of the breakdown voltage, the obtained values of the activation energy ET and the capture cross section σ of the T1 level are almost constant. The Fermi level is perfectly pinned by the T1 level. In contrast, the Fermi level is never pinned by the T0 level since ET and σ of the T0 level vary with increasing steady-bias voltage and the T0 level has a faster emission process.

Distributions of Interface States and Bulk Traps in ZnO Varistors

The distributions for the activation energy ET and the capture cross section σ of deep levels were studied by spectral analysis of deep level transient spectroscopy (SADLTS) in order to characterize precisely the bulk traps and the interface states in the Pr- and Bi-type ZnO varistors. Three bulk traps (B1′, B1 and B2) were observed by SADLTS. The origins of B2 and B1 appear to be the intrinsic defects of ZnO because these traps were not affected by various additives and ET and σ of these traps in both types of ZnO showed almost identical distributions. The distribution of B1' in Bi-type ZnO varistors was different from that of B1′ in Pr-type ZnO varistors. This indicates that B1' is caused by the defects induced by either Bi or Pr ions, because the additives can change the forming process. Four interface states in Bi-type ZnO varistors (T0, T1a, T1b and T2) and two interface states in Pr-type ZnO varistors (T0 and T1) were clearly found. T2 was only detected for Bi-type ZnO varistors and had a slower time constant with broadening in the emission rate than the other interface states. The origin of T2 is considered to be the level relating to the spinel particle of Sb. T0, T1a and T1b are the discrete interface states. The distributions (ΔET and Δσ) yield data which is important in terms of understanding the characteristics of ZnO varistors.

Deep levels in Sb-doped ZnSe fabricated by metalorganic vapor-phase epitaxy

Sb-doped ZnSe samples were deposited on the (001)GaAs substrate by metalorganic vapor-phase epitaxy (MOVPE). Isothermal capacitance transient spectroscopy (ICTS) and spectral analysis of deep-level transient spectroscopy (SADLTS) were used to characterize deep levels of Sb-doped ZnSe. The p-type sample grown by MOVPE at 490°C in the darkness shows three ICTS peaks. Three deep levels were observed in the N-doped ZnSe deposited by MOVPE. Using the SADLTS, we can estimate the activation energy and the capture-cross section distributions of that hole traps. We also examined samples that were photoassist-deposited at lower temperature. The non-doped ZnSe thin films were also measured to check the effects of light irradiation during the deposition. We could get only n-type samples and the light irradiation generates the new level of the electron traps. Sb doping generates other new levels. The levels that correspond to trap E1 in the light-irradiated Sb-doped samples are constructed from two adjacent levels in SADLTS, and one new level near trap E1 can be observed in SADLTS.

Deep Levels in Hg0.766Cd0.234Te Grown by Metal-Organic Chemical Vapor Deposition

The deep levels in Hg0.766Cd0.234Te grown on Si substrates by metal-organic chemical vapor deposition (MOCVD) have been studied. Spectral analysis of deep level transient spectroscopy (SADLTS) was used to evaluate the deep levels for this device. Three levels (LM1, LM2 and LM3) were confirmed in this device. The values of activation energy and capture cross section for these levels were as follows: LM1 (21 meV, 6.1×10-18 cm-2), LM2 (21 meV, 1.3×10-18 cm-2) and LM3 (35 meV, 9.8×10-18 cm-2). The midgap level in p-HgCdTe reported by several authors could not be confirmed in this work.

Evaluation of Interface States in ZnO Varistors by Spectral Analysis of Deep Level Transient Spectroscopy

Interface states in ZnO varistors were studied using spectral analysis of deep level transient spectroscopy (SADLTS) to obtain the emission rate spectrum. We found one interface state in ZnO with the activation energy and the capture cross section distributed around their central values, E0=0.98 eV and σ0=1.8×10-16 cm2, over widths ΔE=0.19 eV and Δσ=2.9×10-17 cm2, respectively.

Study of Deep Levels in Mesa-Type HgCdTe Device

The deep levels in the p+-on-n mesa-type Hg1-xCdxTe (x=0.222) device fabricated on the Si substrate by the molecular beam epitaxy method have been studied using the isothermal capacitance transient spectroscopy (ICTS) method and the spectral analysis of deep level transient spectroscopy (SADLTS) method. The value of the activation energy and the capture cross section could not be determined by means of ICTS because the apparent peaks were not obtained in the ICTS spectrum. However, five deep levels have been confirmed by SADLTS and the activation energies and the capture cross sections for each level were calculated. The confirmed levels existed from 19.7 meV to 45.7 meV below the conduction band edge. Three deeper levels are considered to be related to the Hg vacancy, whereas two shallower levels are considered to be related to the impurity in the n-type region. Moreover it is suggested that three deeper levels may assist the tunneling current in the reverse bias region. On the other hand, the obtained values of the capture cross sections ranged from 10-18 to 10-11 cm2.

Studies of Deep Levels in HgCdTe Grown on CdZnTe and Si Substrates

The influence of different substrates on the deep levels in Hg1-xCdxTe was investigated by spectral analysis of deep-level transient spectroscopy (SADLTS). In this study, CdZnTe, which had almost the same lattice constant and thermal expansion coefficient as Hg1-xCdxTe, and Si, which had a larger lattice constant and thermal expansion coefficient than Hg1-xCdxTe, were used as the substrates. The results of SADLTS show that there are six levels between 27 meV and 56 meV as activation energies in Hg1-xCdxTe grown on the Si substrate, while there are six levels between 10 meV and 44 meV as activation energies in Hg1-xCdxTe grown on the CdZnTe substrate. It was confirmed that the Hg1-xCdxTe grown on the Si substrate had deeper levels than Hg1-xCdxTe grown on the CdZnTe substrate.

Deep Impurity Centers in CdS Single Crystals Studied by Spectral Analysis of Deep Level Transient Spectroscopy

Deep impurity centers in CdS single crystals were studied by using spectral analysis of deep level transient spectroscopy (SADLTS) to obtain the emission rate spectrum. We found one deep level below the conduction band with the activation energy and the capture cross section distributed around their central values E0=0.31 eV and σ0=4.0×10-16 cm2 over widths ΔE=0.013 eV and Δσ=9.3×10-17 cm2, respectively.

Double-Peak Emission Rate Spectrum of DX-Centers in AlxGa1-xAs

Emission rate spectra S(λ) of the DX-centers in AlxGa1-xAs were studied for dopants, such as Te, Sn, Si and Se, using the SADLTS (spectral analysis of deep level transient spectroscopy) method. The double peak emission rate spectra were observed for these DX-centers in AlxGa1-xAs at around x=0.5 of Al mole fraction. The band effect (crossover of L-band and \\varGamma-band) is the dominant contribution to the rather broadened spectra of DX-centers appearing in the conventional DLTS.

Spectral Analysis of Deep Level Transient Spectroscopy (SADLTS) of DX-centers in AlxGa1-xAs:Sn

Emission rate spectra S(λ, T) of DX-centers in AlxGa1-xAs:Sn for x=0.35, 0.45, 0.55, 0.65 and 0.75 were obtained from the transient capacitance waveform ΔC(t, T) by the spectral analysis proposed in our previous paper. S(λ, T)'s were distributed over the range λ∼10∼103 between 170∼200 K and showed double peak features for x=0.35, 0.45 and 0.55. S(λ, T)'s broaden, shift to higher λ and decrease their peak heights at higher temperatures. Assuming Gaussian distribution for both capture cross section σ and activation energy E, the width of the activation energy ΔE is estimated to be 10∼16 meV and largest at x=0.45, while the relative broadening in the capture cross section is found to be Δσ/σ0∼1/3 from the shape of S(λ, T). The possible origin of the double peaks is tentatively assigned to DX-to-X conduction band and DX-to-L conduction band excitation for x=0.45 and 0.55.

Spectral Analysis of Deep Level Transient Spectroscopy (SADLTS)

A spectral analysis of Deep Level Transient Spectroscopy (SADLTS) is proposed. This method analyzes the transient junction capacitance C(t)=\\intbaS(λ)exp (-λ·t)dλ in order to determine the finite continuous emission rate spectrum S(λ). SADLTS permits one to obtain more detailed information with a single-temperature scan and spectral analysis instead of the conventional multi-temperature scan or single-exponential analysis. Even if S(λ) includes two peaks at λ1 and λ2, those peaks can be distinguished for λ2/λ1>2. As an example of the application of SADLTS, deep levels in Si:Au were experimentally investigated. According to the three-dimensional S(λ)-T2/λ-1/T representation, the apparent single peak in the conventional DLTS was found toconsist of two adjacent levels with activation energies and capture cross sections of EB1=0.51 eV, σB1=4.0×10-15 cm2 and EB2=0.47 eV, σB2=1.1×10-15 cm2, respectively.