Deep Level Spectra Research Articles

Comparison of deep levels spectra and electrical properties of GaAs crystals grown by vertical Bridgeman and by liquid encapsulated Czochralski methods

Conducting and semi-insulating crystals of GaAs grown by vertical Bridgeman (VB) and liquid encapsulated Czochralski (LEC) methods were studied by means of deep levels transient spectroscopy, photoinduced current transient spectroscopy, panchromatic microcathodoluminescence, optical absorption and electrical measurements. The most prominent traps observed in both types of crystals were the EL8, EL6, EL5, EL3 and EL2 electron traps. The results suggest that VB crystals are similar to LEC GaAs crystals grown from slightly Ga rich melts. The electrical properties and uniformity of VB crystals were greatly improved by the standard 950 °C annealing procedure.

Effect of structural imperfection on the spectrum of deep levels in 6H-SiC

Spectra of deep centers in a lightly doped 6H-SiC substrate having regions with different degrees of structural perfection were investigated. It is found that the concentrations of the majority of deep centers are independent of the dislocation density in a given region of the sample. The obtained results and published data lead to the conclusion that the concentration of silicon vacancies is independent of the degree of the structural perfection of 6H-SiC.

Implantation Induced Defect States in Gallium Nitride and Their Annealing Behaviour

Gallium nitride layers grown by metal organic vapour phase epitaxy on sapphire were implanted with different ion species, i.e. silicon, sulfur, and magnesium, and thermally annealed at 1150 °C under nitrogen atmosphere. The impact of this annealing procedure on the resulting deep levels was analyzed by transient and admittance spectroscopy. Several electron traps with thermal activation energies between 200 and 900 meV as well as very deep states at photon energies ranging from 1.8 to 2.5 eV were induced by the implantation process independent of the ion species. After annealing, in general the deep level spectrum shows only minor changes, but an enhancement of the shallower electron traps and a new electron trap for the Si-implanted layers can be observed. These results are explained by a re-arrangement of the induced defects and supported by photoluminescence experiments. Finally, the ineffectiveness of this annealing procedure which is often used by the community to reduce the implantation damage is demonstrated.

Semi-insulating LEC GaAs as a material for radiation detectors: materials science issues

Semi-insulating (SI) GaAs is now being reconsidered as a promising material for radiation detectors, mostly due to greatly improved quality of the material. In this paper we shall describe the properties of the state-of-the-art SI GaAs crystals grown by LEC method as relevant for such applications. Specifically, we shall concentrate on the assessment of the spectra and density of residual impurities, on the measurements of deep levels spectra and on studying the impact of these centers on material parameters. The spectra of deep centers as studied by PICTS and DLTS indicate that, in LEC crystals grown from stoichiometric melts, besides the most prominent EL2 center, other electron and hole traps are usually observed. By comparing the DLTS and PICTS results we show that PICTS often tends to overestimate the relative contributions of traps shallower than EL2. Two approaches to decreasing EL2 concentration in LEC grown crystals have been proposed in literature: to grow crystals from Ga-rich melts and to anneal the material at high temperatures. We have studied the changes in deep level spectra and transport parameters for both cases. In annealing experiments the possibility of thick samples modification by arsenic diffusion was also investigated.

High-resolution deep-level transient spectroscopy studies of gold and platinum acceptor states in diluted SiGe alloys

High-resolution Laplace deep-level transient spectroscopy spectra for gold- or platinum-diffused SiGe samples show an alloy splitting that is associated with the alloy fluctuations in the proximity of the defect. For the case of the platinum acceptor state, the effect of the level splitting caused by alloying in the first and also in the second shell of surrounding atoms can be distinguished. For the case of the gold acceptor, only the effect from the first shell of atoms is observable but the manifestation of alloying in the second-nearest shell can be seen as line broadening. We have found that the electronic energy level is affected by alloying in the first-nearest neighborhood by a factor of 2--3 more than by alloying in the second-nearest shell. The absolute values of the energy differences obtained from the Arrhenius plots for different defect configurations agree with those inferred from the peak separations observed in the spectra. A clear preference for gold and platinum to enter substitutional Si sites adjacent to Ge has been revealed. This may be interpreted in terms of an enthalpy lowering as a result of the fact that both metals are able to replace the host silicon atom more easily than the germanium atom in the substitutional position.

Metastability of defects in p-GaAs grown from a Ga-rich melt

We report the observation of metastability for defects in Si-doped p-GaAs grown from a Ga-rich melt. A configuration change occurs suddenly at a critical temperature (near 120 K) during cooldown scans, giving rise to discontinuous deep-level transient spectra. A similar phenomenon was observed on electron-irradiated float-zone-refined Si and explained in terms of entropy variations in the configurational space. We believe that the metastability found in the p-GaAs sample is also due to a barrierless transformation of defect configuration, driven by variations in both entropy and the charge state of GaAs.

Annealing-induced evolution of defects in low-temperature-grown GaAs-related materials

Infrared absorption spectroscopy, optical transient current spectroscopy (OTCS), and photoluminescence (PL) spectroscopy are used to investigate the annealing induced evolution of defects in low-temperature (LT)-grown GaAs-related materials. Two LT samples of bulk GaAs (sample A) and ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ multiple-quantum-well (MQW) structure (sample B) were grown at 220 and 320 \ifmmode^\circ\else\textdegree\fi{}C on (001) GaAs substrates, respectively. A strong defect-related absorption band has been observed in both as-grown samples A and B. It becomes weaker in samples annealed at temperatures above 600 \ifmmode^\circ\else\textdegree\fi{}C. In sample A, annealed in the range of 600--800 \ifmmode^\circ\else\textdegree\fi{}C, a large negative decay signal of the optical transient current (OTC) is observed in a certain range of temperature, which distorts deep-level spectra measured by OTCS, making it difficult to identify any deep levels. At annealing temperatures of 600 and 700 \ifmmode^\circ\else\textdegree\fi{}C, both ${\mathrm{As}}_{\mathrm{Ga}}$ antisite and small As cluster-related deep levels are identified in sample B. It is found that compared to the As cluster, the ${\mathrm{As}}_{\mathrm{Ga}}$ antisite has a larger activation energy and carrier capture rate. At an annealing temperature of 800 \ifmmode^\circ\else\textdegree\fi{}C, the large negative decay signal of the OTC is also observed in sample B. It is argued that this negative decay signal of the OTC is related to large arsenic clusters. For sample B, transient PL spectra have also been measured to study the influence of the defect evolution on optical properties of LT ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ MQW structures. Our results clearly identify a defect evolution from ${\mathrm{As}}_{\mathrm{Ga}}$ antisites to arsenic clusters after annealing.

Electron traps and growth rate of buffer layers in unintentionally doped GaN

The deep level spectra in a series of undoped GaN films with different GaN buffer growth rates were studied by deep level transient spectroscopy (DLTS), transmission electron microscopy (TEM) and high-resolution X-ray diffraction (HRXRD) in order to investigate correlation between dislocations and deep level defects. Two distinct deep levels were observed, one with an activation energy E1=0.19–0.23 eV and the other with an activation energy E2=0.50–0.60 eV below the conduction band. The broadening of the (1 0 2) Bragg peak with decreasing buffer growth rates was related to increasing edge dislocations and also the DLTS peak intensity of the deep level E1. The logarithmic dependence of the DLTS peak intensity on the filling pulse time indicated that the deep level E1 is associated with dislocation core sites.

Studies of defects in Mg doped p-GaN films grown by hydride vapor phase epitaxy on SiC substrates

Deep and shallow levels spectra, microcathodoluminescence (MCL) and electron beam induced current (EBIC) measurements on Mg doped GaN films grown by hydride vapor phase epitaxy directly on SiC are reported. It is shown that the samples contain Mg acceptors with a level at E v+0.18 eV, some unidentified shallow traps, unidentified traps with apparent activation energy of 0.25 eV, both somewhat arbitrarily attributed to interface states, and hole traps with levels at E v+0.4 and E v+0.85 eV. The latter two seem to be related to the blue and yellow luminescence bands observed in the MCL spectra. MCL and EBIC imaging of the samples reveals the presence of the network of misfit dislocations near the GaN/SiC interface. The MCL band-edge band is blue shifted in the vicinity of these defects, most likely because of the strain release near dislocations.

Effects of proton implantation on electrical and recombination properties of n-GaN

Effects of implantation with 150 keV protons and 350 keV deuterium ions on deep levels spectra and electrical and recombination properties of n-GaN samples were studied for fluences between 8×10 13 and 5×10 15 cm −2. Implantation led to substantial decrease of the electron concentration so that after the highest dose used, the samples became highly resistive with the Fermi level pinned near 0.3–0.4 eV from the conduction band edge. We also observed a strong degradation of the minority carriers diffusion length and a very strong decrease of the band edge luminescence intensity in the heavily implanted samples. Deep levels spectra measurements showed that proton implantation led to introduction of deep electron traps with activation energies near 0.2, 0.3, 0.45, 0.75 and 0.95 eV and of deep hole traps with activation energies of 0.25, 0.6 and 0.9 eV. The results strongly indicate that among these traps, the 0.75 eV electron traps and/or the 0.6 eV hole traps are most likely responsible for the lifetime degradation. Strong recovery of the electric conductivity of the implanted samples was observed starting with annealing temperatures near 250°C, whereas no recovery of the luminescent characteristics could be observed for annealing temperatures as high as 700°C. We compare the results with previously published data on proton and He implantation of GaN and discuss possible implications of our findings for practical use in GaN devices isolation.

Direct observation of hydrogen passivation of nitrogen-related energy levels in ZnSe and ZnS xSe 1− x grown by MBE

The effect of hydrogenation on the electrical properties of N-doped ZnSe and ZnS x Se 1− x films grown by molecular-beam epitaxy is investigated. Hydrogenation of samples with a net acceptor density ( N a− N d) around (2–5)×10 16 cm −3 decreases the net acceptor density close to the surface and the peak height in deep-level transient spectra of the dominating nitrogen acceptor with activation energies 110 and 120 meV in ZnSe and ZnS 0.08Se 0.82 thin films, respectively. Samples with N a− N d in the 10 17 cm −3 range did not exhibit such changes, while samples with N a− N d less than few times 10 16 cm −3 became highly resistive.

Photocapacitance study of bulk deep levels in ZnSe grown by molecular-beam epitaxy

The influence of initial growth conditions and lattice matching on the deep level spectrum of n-ZnSe grown on GaAs by molecular-beam epitaxy is investigated by means of deep level optical spectroscopy. A detailed study of both the steady-state and transient photocapacitance allows us to measure optical threshold energies, concentrations, and emission rates of electronically active defects in the ZnSe layer. Several deep levels are found in the ZnSe layer at Ec−Et=1.15, 1.46, 1.90, and 2.25 eV with concentrations in the 1012–1014 cm−3 range. When a 2-nm-thick composition controlled interface layer is grown at different beam pressure ratios prior to the ZnSe growth, a distinct decrease in the 1.46 eV level concentration with increasing Se content is found. Deposition of a lattice-matched InxGa1−xAs buffer layer prior to the ZnSe growth reduces the concentration of both the 1.15 and 1.46 eV levels by over an order of magnitude, indicating the role of lattice matching in the ZnSe overlayer. We also perform depth profiling of the defect distributions within the ZnSe overlayer to see the effect of the ZnSe thickness on the concentration of these levels as well as their possible association to the ZnSe/GaAs interface. We find that only the 2.25 eV level concentration shows a dependence on depth, increasing as the II–VI/III–V interface is approached.

Deep levels in n-type Schottky and p+-n homojunction GaN diodes

The deep level spectra in both p+-n homojunction and n-type Schottky GaN diodes are studied by deep level transient spectroscopy (DLTS) in order to compare the role of the junction configuration on the defects found within the n-GaN layer. Both majority and minority carrier DLTS measurements are performed on the diodes allowing the observation of both electron and hole traps in n-GaN. An electron level at Ec−Et=0.58 and 0.62 V is observed in the p+-n and Schottky diodes, respectively, with a concentration of ∼3−4×1014 cm−3 and a capture cross section of ∼1−5×10−15 cm2. The similar Arrhenius behavior indicates that both emissions are related to the same defect. The shift in activation energy is correlated to the electric field enhanced-emission in the p+-n diode, where the junction barrier is much larger. The p+-n diode configuration allows the observation of a hole trap at Et−Ev=0.87 eV in the n-GaN which is very likely related to the yellow luminescence band.

The Spectrum of Deep Levels in the InP Schottky Barrier

The energy spectrum of deep levels in the space-charge region of the Schottky barrier of InP field-effect transistors is determined by the method of tunneling spectroscopy. It has been established that for the most part the space-charge region of the Schottky barrier has electron traps with large capture cross sections that control the tunneling current in the Schottky barrier.

Charge collection efficiency in SI GaAs grown from melts with variable composition as a material for solar neutrino detection

The results on electrical characteristics and charge collection efficiency in the detectors from bulk SI GaAs developed as a material for solar neutrino spectroscopy are presented. SI GaAs crystals were grown by the Czochralski method. The changes in the stoichiometric components are permanently controlled. It is shown that the performance of GaAs p +–i–n + structures provided the range of operational reverse voltage up to 1 kV. Measurement of deep level spectra and their analysis reveal the dominant deep levels – hole traps E v +0.51 and +0.075 eV in GaAs grown from stoichiometric and nonstoichiometric melts, respectively. Investigation of carrier transport properties and bulk homogeneity evinced in charge collection efficiency has shown advantageous results for SI GaAs grown from stoichiometric melt. The reduction of carrier transport parameters and charge collection efficiency in GaAs grown from nonstoichiometric melt is analyzed taking into consideration formation of the hole trap E v +0.075 eV, presumably assigned to Ga antisite and its influence on the concentration of the ionized deep donor level EL2 +.

Deep levels in p-type InGaAsN lattice matched to GaAs

Deep-level transient spectroscopy measurements were utilized to investigate deep-level defects in metal–organic chemical vapor deposition-grown, unintentionally doped p-type InGaAsN films lattice matched to GaAs. The as-grown material displayed a high concentration of deep levels distributed within the band gap, with a dominant hole trap at Ev+0.10 eV. Postgrowth annealing simplified the deep-level spectra, enabling the identification of three distinct hole traps at 0.10, 0.23, and 0.48 eV above the valence-band edge, with concentrations of 3.5×1014, 3.8×1014, and 8.2×1014 cm−3, respectively. A direct comparison between the as-grown and annealed spectra revealed the presence of an additional midgap hole trap, with a concentration of 4×1014 cm−3 in the as-grown material. The concentration of this trap is sharply reduced by annealing, which correlates with improved material quality and minority-carrier properties after annealing. Of the four hole traps detected, only the 0.48 eV level is not influenced by annealing, suggesting this level may be important for processed InGaAsN devices in the future.

Spreading resistance and C-DLTS spectra of proton-irradiated mesa diodes made on thick epitaxial Si layers

High-resistivity, thick silicon epitaxial layers, deposited on Czochralski silicon (CZ Si) substrate were used as a material for test diodes. Resistivity profile as a function of depth and deep-level spectra were measured by spreading resistance method and deep-level transient spectroscopy (C-DLTS) on non-irradiated and proton irradiated mesa diodes. A deep level with activation energy E c −0.52 eV , attributed to V 2O defect, dominates in the non-irradiated diodes. After irradiation two levels, E c−0.38 and E c−0.45 eV, related to divacancies and the level E c−0.17 eV corresponding to VO complex are distinguished.

Deep Levels in n-Type Schottky and p+-n Homojunction GaN Diodes

AbstractThe deep level spectra in both p+-n homojunction and n-type Schottky GaN diodes are studied by deep level transient spectroscopy (DLTS) in order to compare the role of the junction configuration on the defects found within the n-GaN layer. Both majority and minority carrier DLTS measurements are performed on the diodes allowing the observation of both electron and hole traps in n-GaN. An electron level at Ec-Et=0.58 and 0.62 V is observed in the p+-n and Schottky diodes, respectively, with a concentration of ∼3-4×1014cm−3 and a capture cross section of ∼1-5×10−15cm2. The similar Arrhenius behavior indicates that both emissions are related to the same defect. The shift in activation energy is correlated to the electric field enhanced-emission in the p+-n diode, where the junction barrier is much larger. The p+-n diode configuration allows the observation of a hole trap at Et-Ev=0.87 eV in the n-GaN which is very likely related to the yellow luminescence band.

Electrical and Recombination Properties of Copper‐Silicide Precipitates in Silicon

Copper‐silicide precipitates in silicon obtained after copper diffusion and quench in different liquids were studied by transmission electron microscopy and capacitance spectroscopy techniques. A correlation between the quenching rate, geometric size, and deep level spectra of the copper‐silicide precipitates was established. The unusually wide deep level spectra are shown to be due to a defect‐related band in the bandgap. The parameters of the band are evaluated using numerical simulations. A positive charge of copper‐silicide precipitates in p‐type and moderately doped n‐type Si is predicted by simulations and confirmed by minority carrier transient spectroscopy measurements. Strong recombination activity of the precipitates due to attraction of minority carriers by the electric field around the precipitates and their recombination via the defect band is predicted and confirmed by the experiments. The pairing of copper with boron is shown to be an important factor determining the precipitation kinetics of the interstitial copper at room temperature.

Studies of the origin of the yellow luminescence band, the nature of nonradiative recombination and the origin of persistent photoconductivity in n-GaN films

It is shown that in n-GaN samples with a strong yellow luminescence band the deep levels spectra measured by optical transient current spectroscopy (OTCS) is always dominated by a hole trap at E v+0.85 eV which is absent in the spectra of the samples with weak yellow luminescence. Based on the results of recent theoretical calculations it is suggested that these traps could be gallium vacancies V Ga and that the centers responsible for yellow luminescence are formed by pairing such vacancies with shallow donors. The results indicate that the yellow luminescence band could be the dominant recombination channel in the samples with strong yellow luminescence band whereas in the samples with weak yellow luminescence recombination this role is taken over by nonradiative recombination centers. Deep levels transient spectroscopy (DLTS) measurements revealed the existence of a barrier for capture of electrons for electron traps at E c−0.55 eV (0.15 eV) and E c−0.8 eV (about 0.35 eV) and a possible role of such traps in persistent photoconductivity at room temperature in n-GaN is discussed.