Thermal Ionization Energy Research Articles

Photoionization cross-section analysis for a deep trap contributing to current collapse in GaN field-effect transistors

We perform a more incisive numerical analysis of the photon energy dependence of the photoionization cross section of a prominent deep trap (conventionally labeled as Trap 1), which had been found [Klein et al., Appl. Phys. Lett. 75, 4016 (1999)] to act as a contributor to current collapse phenomena in GaN metal-semiconductor field-effect transistors. The analytical expression for the photoionization cross section of Trap 1 is taken in the form of a convolution of a temperature-independent electronic part with a thermally broadened Franck–Condon (FC) factor, which also applies to the relevant regime of large lattice relaxation. For a direct comparison with earlier results, we specialize the present analysis to an electronic cross-section part represented by the Lucovsky model in combination with the semiclassical (Gaussian) approximation for the FC factor. In qualitative accordance with an earlier estimation by Klein et al. we obtain a value of EO≈1.9 eV for the classical optical ionization energy in combination with a full width at half maximum of 0.64 eV. The latter implies, on the assumption for the average phonon energy, to be of order 50 meV, an apparently unusually large magnitude, D≈1.1 eV, for the Franck–Condon shift. This parameter constellation is equivalent to a thermal ionization (electron binding) energy, ET=EO−D, of about ET≈0.8 eV. Such a location of Trap 1 near the middle of the upper half of the fundamental gap of GaN, ET≈Eg/4, is at clear variance to the earlier suggestion by Klein et al. for Trap 1 to be a midgap level. The present estimation offers a chance for detecting the Trap 1 also by deep-level transient spectroscopy measurements. An eventual availability of photoionization cross-section data for different temperatures is seen to be the prerequisite for a decisive reduction of residual uncertainties concerning the configuration coordinate diagram.

Numerical simulation of the temperature dependence of the ionization energy of hydrogen-like impurities in semiconductors: Application to transmutation-doped Ge: Ga

An electrostatic model describing the dependence of the thermal ionization energy of impurities on their concentration, compensation factor, and temperature is developed. The model takes into account the screening of impurity ions by holes (electrons) hopping from impurity to impurity, the change in the impurity-band width, and its displacement with respect to the edge of the valence band for acceptors (conduction band for donors). The displacement of the impurity band is due to the functional dependence of the hole (electron) affinity of the ionized acceptor (donor) on the screening of the Coulomb field of the ions. The spatial distribution of the impurity ions over the crystal was assumed to be Poisson-like, and the energy distribution was assumed to be normal (Gaussian). For the relatively low doping levels under investigation, the behavior of the density of states at the edges of the valence and conduction bands was assumed to be the same as for the undoped crystal. The results of the numerical study are in agreement with the decrease in the ionization energy that is experimentally observed for moderately compensated Ge: Ga as the temperature and the doping level are decreased. It is predicted that the temperature dependence of the thermal ionization energy has a small anomalous maximum at small compensation factors.

Electrical field effect on the optical threshold energy of donor and acceptor levels in chromium-doped GaP

The electrical field effect on the optical threshold energy have been studied using electrical deep level optical spectroscopy technique. The measurements were focused on the Cr 3+/4+ donor level and on the Cr 2+/3+ acceptor level in Cr-doped GaP. It is found, in the case of the Cr 3+/4+, that the optical threshold as the thermal ionization energies are field dependent. This behavior allows us to determine each position at zero field and then the Frank–Condon shift. Using these parameters, a configuration coordinate diagram is given. In the case of the Cr 2+/3+ acceptor level, the optical threshold is also field dependent.

Cathodoluminescence studies of the electron injection-induced effects in GaN

Local irradiation of p-type GaN with the electron beam of a scanning electron microscope resulted in up to a threefold decrease of the peak cathodoluminescence intensity at ∼379 nm, as was observed in the variable temperature measurements. The cathodoluminescence results are consistent with an increase of the minority carrier diffusion length in the material, as is evident from the electron-beam-induced current measurements. The activation energy for the electron injection effect, estimated from the temperature-dependent cathodoluminescence, is in agreement with the thermal ionization energy of the Mg-acceptor in GaN.

Electrical investigation of carbon intrinsically‐doped GaAs layers grown by metalorganic vapour phase epitaxy from TMGa and TBAs

In this work the possibility of controlling carbon intrinsic-doping in GaAs homoepitaxial layers grown by metalorganic vapour phase epitaxy (MOVPE) from the trimethyl-gallium (TMGa) and tertiary-buthyl-arsine (TBAs) precursors was evaluated via the analysis of transport properties as a function of the growth parameters and for two substrates mis-orientations; Hall effect measurements were performed on the samples as a function of temperature. Intrinsically p-doped GaAs layers were obtained with a hole concentration in the range (1014–1018) cm–3 and a corresponding RT mobility in the range (100–400) cm2/Vs. The simultaneous analysis of the Hall free hole density and Hall mobility yielded the effective doping level, the compensation ratio and the thermal ionisation energy of the acceptor impurity as a function of the growth parameters.

Impact of aluminum concentration and magnesium doping on the effect of electron injection in p-Al xGa 1− xN

The impact of electron injection on the minority carrier diffusion length was studied at various temperatures in p-type Mg-doped GaN, Al 0.2Ga 0.8N, and Al 0.2Ga 0.8N/GaN superlattices, which were homogeneously and modulation (barrier only) doped. The activation energies for the electron injection-induced effect were found to be in the range from 190 to 260 meV, which is close to the thermal ionization energy of the Mg-acceptor. The obtained results are in agreement with the previously proposed model of minority carrier transport enhancement due to charging of Mg-centers in p-(Al)GaN. The activation energy of the electron injection effect observed in Al 0.2Ga 0.8N is consistent with the deepening of Mg-acceptor level due to the incorporation of Al into GaN lattice. The activation energy in the Al 0.2Ga 0.8N/GaN superlattice, homogeneously doped with Mg, indicates that the main contribution to the effect comes from the capture of injected electrons by the wells.

Photoluminescence and capacitance transients in highly Mg-doped GaN

Behaviors of the photoluminescence blue-band and near-bandgap peak and the relevant thermal ionization energies of the shallow and deep Mg-related acceptors have been studied, respectively. The 2.989 eV blue-band is attributed to the deep donor-acceptor-pair transitions involving a deep Mg-related acceptor at E-v+0.427 eV. The blueshift with increasing excitation power is explained by variation in the contribution of close and distant donor-acceptor-pairs to the luminescence. The redshift with increasing temperature results from thermal release of carriers from close donor-acceptor-pairs. The 3.26 eV near-bandgap peak is attributed to the shallow donor-acceptor-pair transitions involving a shallow Mg-related acceptor at E-v+0.223 eV. The relevant thermal ionization energies of the shallow and deep Mg-related acceptors, being about E-v+0.16 and E-v+0.50eV, are determined from deep-level transient Fourier spectroscopy measurements.

Electrical field effect on both thermal ionization energy and optical threshold energy of the Cr 3+/4+ deep donor level in GaP

The effect of the electric field on both thermal ionization energy and optical threshold energy have been studied using deep level transient spectroscopy (DLTS) and electrical deep level optical spectroscopy (Electrical DLOS) techniques in Cr-doped GaP. The measurements were focused on the Cr 3+/ 4+ donor level. It is found that the ionization energy E i decreases with the electrical field F as in the case of the Poole–Frenkel model. It is found that there is a variation of the optical threshold with the electrical field.

Metastable defects in 6H–SiC: experiments and modeling

Using various junction space-charge techniques, annealing, and simulation, a metastable defect in 6H SiC have been characterized. We suggest a configuration coordinate diagram with three configurations, where two of them can only exist when the defect is occupied by one or more electrons. The thermal ionization energies for the different configurations were measured and the thermal barriers for two of the transitions have been determined. It is also shown that the transformation process is governed by electron capturing to the defect.

Luminescence characterization of titanium related defects in 6H–SiC

Photoluminescence (PL), thermally stimulated luminescence (TSL) and their photo-excitation spectra in hexagonal type SiC were studied between the temperatures of 5 and 300 K. Titanium related TSL glow peak is observed close to 140 K in compensated crystals 6H–SiC with corresponding thermal ionisation energy of a trap close to 170 meV. The spectral distribution of TSL has a maximum at 1.8 eV in 6H–SiC and 2.43 eV in 4H–SiC. A specific structure in the photo-excitation spectrum of PL close to the exciton gap E gx=3.023 eV is revealed, exclusively in 6H–SiC material. This indicates an essential role of excitons in the resonance energy transfer to the Ti-related centers in silicon carbide. The results are discussed in a model of isoelectronic centers composed of either individual Ti atoms or Ti–N pair representing a complex of the electronic trap and recombination center.

PhotoinducedPb+center inPbWO4:Electron spin resonance and thermally stimulated luminescence study

Electron traps based on regular Pb sites perturbed by oxygen vacancies ${(V}_{\mathrm{O}})$ in high purity undoped ${\mathrm{PbWO}}_{4}$ have been revealed and studied by electron spin resonance (ESR) and thermally stimulated luminescence (TSL) measurements. Detailed analysis of ESR spectra parameters $(g$ factor, hyperfine, and superhyperfine structures) along with the variation of the concentration after annealing in inert and oxygen atmospheres had shown that revealed centers are ${\mathrm{Pb}}^{+}\ensuremath{-}{V}_{\mathrm{O}}$ complexes. The ${\mathrm{Pb}}^{+}\ensuremath{-}{V}_{\mathrm{O}}$ centers are created in two ways: (i) by direct trapping of the electrons from the conduction band under x-ray or ultraviolet irradiation at $T=60 \mathrm{K};$ (ii) by retrapping of electrons freed from unperturbed $({\mathrm{WO}}_{4}{)}^{3\ensuremath{-}}$ centers after irradiation at $T<40 \mathrm{K},$ followed by heating at $T=60 \mathrm{K}.$ Above $T\ensuremath{\approx}180 \mathrm{K},$ ${\mathrm{Pb}}^{+}\ensuremath{-}{V}_{\mathrm{O}}$ centers are thermally ionized, giving rise to a TSL glow peak due to the recombination of freed electrons with localized holes. The emission spectrum of the TSL features one band peaking at around $2.5 \mathrm{eV}.$ The temperature dependence of both TSL and ESR intensities has been analyzed using a first order recombination model: the thermal ionization energy of ${\mathrm{Pb}}^{+}\ensuremath{-}{V}_{\mathrm{O}}$ centers turns out to be $0.55 \mathrm{eV}.$ The role of ${\mathrm{Pb}}^{+}\ensuremath{-}{V}_{\mathrm{O}}$ centers in the processes of carrier trapping and recombination of nonequilibrium carriers in the ${\mathrm{PbWO}}_{4}$ lattice is briefly discussed.

Electrical compensation in CdTe andCd0.9Zn0.1Teby intrinsic defects

The effects of two intrinsic deep levels on electrical compensation in semi-insulating CdTe and Cd-Zn-Te crystals are reported here. These levels were found in samples grown by conventional Bridgman and high-pressure Bridgman techniques. The levels were observed with thermoelectric effect spectroscopy at distinct temperatures corresponding to thermal ionization energies of ${E}_{d1}{=E}_{v}+0.735\ifmmode\pm\else\textpm\fi{}0.005\mathrm{eV}$ and ${E}_{d2}{=E}_{v}+0.743\ifmmode\pm\else\textpm\fi{}0.005\mathrm{eV}.$ The first level is associated with the doubly ionized Cd vacancy acceptor and the second level was tentatively identified as the Te antisite $({\mathrm{Te}}_{\mathrm{Cd}}),$ which is thought to be complexed with a vacancy. The second level was found to electrically compensate CdTe and Cd-Zn-Te to produce high resistivity crystals, provided that the Cd vacancy concentration is sufficiently reduced during crystal growth or by post-growth thermal processing.

Photoinduced(WO4)3−−La3+center inPbWO4:Electron spin resonance and thermally stimulated luminescence study

Electron traps based on regular Pb sites perturbed by oxygen vacancies ${(V}_{\mathrm{O}})$ in high purity undoped ${\mathrm{PbWO}}_{4}$ have been revealed and studied by electron spin resonance (ESR) and thermally stimulated luminescence (TSL) measurements. Detailed analysis of ESR spectra parameters $(g$ factor, hyperfine, and superhyperfine structures) along with the variation of the concentration after annealing in inert and oxygen atmospheres had shown that revealed centers are ${\mathrm{Pb}}^{+}\ensuremath{-}{V}_{\mathrm{O}}$ complexes. The ${\mathrm{Pb}}^{+}\ensuremath{-}{V}_{\mathrm{O}}$ centers are created in two ways: (i) by direct trapping of the electrons from the conduction band under x-ray or ultraviolet irradiation at $T=60 \mathrm{K};$ (ii) by retrapping of electrons freed from unperturbed $({\mathrm{WO}}_{4}{)}^{3\ensuremath{-}}$ centers after irradiation at $Tl40 \mathrm{K},$ followed by heating at $T=60 \mathrm{K}.$ Above $T\ensuremath{\approx}180 \mathrm{K},$ ${\mathrm{Pb}}^{+}\ensuremath{-}{V}_{\mathrm{O}}$ centers are thermally ionized, giving rise to a TSL glow peak due to the recombination of freed electrons with localized holes. The emission spectrum of the TSL features one band peaking at around $2.5 \mathrm{eV}.$ The temperature dependence of both TSL and ESR intensities has been analyzed using a first order recombination model: the thermal ionization energy of ${\mathrm{Pb}}^{+}\ensuremath{-}{V}_{\mathrm{O}}$ centers turns out to be $0.55 \mathrm{eV}.$ The role of ${\mathrm{Pb}}^{+}\ensuremath{-}{V}_{\mathrm{O}}$ centers in the processes of carrier trapping and recombination of nonequilibrium carriers in the ${\mathrm{PbWO}}_{4}$ lattice is briefly discussed.

Deep level defects in porous silicon

This paper reports on the existence of deep level defects in the band gap of porous silicon by the photo-induced current transient spectroscopy technique combined with the results of thermovoltage and conductivity measurements. The defects are found to exhibit deep donor levels in the upper part of the band gap. It is found that the thermal emission and ionization energies of the defect increase in the range from 0.29 to 0.86 eV below the conduction band edge as the porosity increases, and it is proposed that the defects play the role of a luminescence killer.

Electrical Properties of Oxygen Doped GaN Grown by Metalorganic Vapor Phase Epitaxy

Deliberate oxygen doping of GaN grown by MOVPE has been studied. The electron concentration increased as the square root of the oxygen partial pressure. Oxygen is a shallow donor with a thermal ionization energy of 27 ±2 meV. A compensation ratio of Θ = 0.3-0.4 was determined from Hall effect measurements. The formation energy of ON of EF = 1.3 eV, determined from the experimental data, is lower than the theoretically predicted value.

Transient photoluminescence of defects in undoped GaN prepared by metal organic vapor phase epitaxy

Transient behavior of the 2.3, 2.9 and 3.27eV photoluminescence (PL) bands in undoped GaN is studied. A non-exponential decay of PL intensity is observed for all three bands after pulsed excitation at low temperature. Transition rates were measured for the three bands. Quantitative analysis of the PL decay indicates that all three bands are associated with donor acceptor pair (DAP) transitions involving a shallow donor and three acceptor states of different origins. The transition rate decreases with decreasing band energy or increasing thermal ionization energy of the acceptor.

Persistent photoconductivity in Ga1−xInxNyAs1−y

Electrical properties of unintentionally doped p-type Ga0.95In0.05N0.013As0.987 quaternary alloys grown by metal–organic vapor-phase epitaxy have been investigated by Hall-effect and photoconductivity measurements. Persistent photoconductivity (PPC) has been observed in this material at temperatures T<320 K. The PPC buildup and decay kinetics have been systematically measured at different temperatures and photoexcitation energies and formulated in the context of lattice-relaxed deep levels (or AX-like centers). The parameters which characterize the AX centers in GaInNAs, namely, the thermal and optical ionization energies, hole capture barrier, and the Stokes shift, have been determined. Our results indicate that AX-like deep levels strongly influence the electronic properties of the GaInNAs quaternary system.

Compensation and trapping in CdZnTe radiation detectors studied by thermoelectric emission spectroscopy, thermally stimulated conductivity, and current-voltage measurements

The thermal ionization energies of traps and their types, whether electron or hole traps, were measured in commercial CdZnTe crystals for radiation detectors. The measurements were done between 20 and 400K using thermoelectric emission spectroscopy (TEES) and thermally stimulated conductivity (TSC). For reliable results, indium ohmic contacts had to be used instead of gold Schottky contacts. For filling of the traps, photoexcitation was done at zero bias, at 20K, and at wavelengths which gave the maximum bulk photoexcitation. In agreement with theory, the TSC current was found to be on the order of \( \sim \tfrac{{eV}}{{k\Delta T}}\) times or even larger than the TEES current, where V is the applied bias in TSC and ΔT is the applied temperature difference in TEES. Large concentrations of hole traps at 0.1 and 0.6 eV were observed and a smaller concentration of electron traps at 0.4 eV was seen. The deep traps cause compensation in the material, which is desirable, but they also cause carrier trapping that degrades the spectral response of radiation detectors made from the material.

A model of how the thermal ionization energy of impurities in semiconductors depends on their concentration and compensation

An electrostatic model is derived for the dependence of the thermal ionization energy of hydrogenic impurities E1 on their concentration N and degree of compensation K, with allowance for the screening of ions by electrons (holes) that hop from impurity to impurity. It is shown that the change in E1 with increasing N and K is connected with broadening of the impurity band and its shift toward the valence (v) band for acceptors and toward the conduction band (c) for donors. The shift in the impurity band is explained by a decrease in the affinity energy of an ionized acceptor for a hole (or a donor for an electron) due to screening of the ions. The impurity ion distribution density over the crystal is assumed to be Poisson-like, while its energy distribution is normal. The electron densities of states in the v-and c-bands are assumed to be those of the undoped crystal for the temperature interval in which E1 is determined. The values of E1(N,K) calculated using the expressions given here coincide with known experimental data for transmutation-doped Ge crystals. A description is given of the dependence on N and K of the thermal ionization energy of Zn atoms in p-type Ge as they change from a charge state (−1) to (−2).

Observation of negative-U centers in 6H silicon carbide

Two negative-U centers in 6H SiC have been observed and characterized using capacitance transient techniques. These two defects give rise to one acceptor level (−/0) and one donor level (+/0) each in the band gap. The donor and the acceptor level have inverted ordering, i.e., the thermal ionization energy of the acceptor level is larger than that of the donor level. Direct evidence for the inverted ordering of the acceptor and donor levels and temperature dependence studies of the electron capture cross sections of the acceptor levels are presented.