Current Transient Spectroscopy Research Articles

Characterization of deep centers in semi-insulating SiC and HgI2: Application of discharge current transient spectroscopy

In order to characterize traps in semi-insulating 4H-SiC and HgI2 regarded as an attractive semiconductor for X-ray detectors, we apply discharge current transient spectroscopy (DCTS), which is a graphical peak analysis method based on the transient reverse current of a diode. In high-purity semi-insulating 4H-SiC whose diode may detect X-rays or γ-rays, DCTS can detect two types of trap species, and can determine those densities and emission rates. The emission rates of detected traps are close to each other, suggesting that thermally stimulated current (TSC) may not distinguish between these two types of traps. In semi-insulating HgI2, the densities and emission rates of two types of trap species are also determined by DCTS. Therefore, it is demonstrated that DCTS is a powerful method for determining the densities and emission rates of traps in semi-insulating semiconductors

Inhomogeneous electrical characteristics in 4H-SiC Schottky diodes

Hundreds of current–voltage (I–V) measurements of Ni, Pt and Ti Schottky diodes on 4H–SiC were conducted at low applied voltages. The SiC substrates contained homoepitaxial layers grown by either chemical vapor deposition or sublimation. While near-ideal contacts were fabricated on all samples, a significant percentage of diodes (∼7%–50% depending on the epitaxial growth method and the diode size) displayed a non-ideal, or inhomogeneous, barrier height. These ‘non-ideal’ diodes occurred regardless of growth technique, pre-deposition cleaning method, or contact metal. In concurrence with our earlier reports in which the non-ideal diodes were modeled as two Schottky barriers in parallel, the lower of the two Schottky barriers, when present, was predominantly centered at one of the three values: ∼0.60, 0.85 or 1.05 eV. The sources of these non-idealities were investigated using electron-beam-induced current (EBIC) and deep-level transient spectroscopy (DLTS) to determine the nature and energy levels of the defects. DLTS revealed a defect level that corresponds with the low- (non-ideal) barrier height, at ∼0.60 eV. It was also observed that the I–V characteristics tended to degrade with increasing deep-level concentration and that inhomogeneous diodes tended to contain defect clusters. Based on the results, it is proposed that inhomogeneities, in the form of one or more low-barrier height regions within a high-barrier height diode, are caused by defect clusters that locally pin the Fermi level.

Contactless Electrical Defect Characterization and Topography of a-Plane Grown Epitaxial Layers

Two novel techniques for the electrical characterization of SiC samples and wafers are introduced. Microwave detected photo induced current transient spectroscopy (MD-PICTS) is used to investigate properties of deep defects similar to conventional PICTS and deep level transient spectroscopy (DLTS). In microwave detected photoconductivity (MDP), electrical properties like minority carrier lifetime T, drift mobility μ, and diffusion length L can be mapped over a wafer at room temperature. The advantage of both techniques is that they do not require the investigated sample to be contacted. Thus, wafers can be characterized non-destructively and spatially resolved. MD-PICTS spectra of three sublimation grown epilayers, two on a-plane substrates and one on a commercial c-oriented substrate, are presented and compared to their PL-spectra. In addition, both MDP- and PL-topograms of one sample are shown. The applicability of the techniques to various types of SiC samples (bulk or epilayer, n-type, p-type, semi-insulating) is discussed.

Analysis of Carrier Traps in Silicon Nitride Film with Discharge Current Transient Spectroscopy, Photoluminescence, and Electron Spin Resonance

Electrical characteristics and an origin of traps in silicon nitride film were investigated using discharge current transient spectroscopy (DCTS), electron spin resonance (ESR), and photoluminescence (PL). It was found from DCTS that the density of traps of 1.1 eV from the conduction band edge increased with the increase of the gas-mixture ratio of SiH2Cl2 over NH3 for the fabrication of silicon nitride film. The energy levels of traps in silicon nitride film of 0.25, 0.68, 0.83–0.91, and 1.25–1.3 eV from the conduction band edge were found from photoluminescence observation. The ESR signals of the K-center that indicated a silicon dangling-bond increased with the increase of the gas-mixture ratio of SiH2Cl2 over NH3. It is suggested that the origin of traps of 1.1 eV from the conduction band in silicon nitride film is the silicon dangling-bond.

Photoinduced current transient spectroscopy of deep levels and transport mechanisms in iron-doped GaN thin films grown by low pressure-metalorganic vapor phase epitaxy

Electrical transport and deep levels are investigated in GaN:Fe layers epitaxially grown on sapphire by low pressure metalorganic vapor phase epitaxy. Photoinduced current transient spectroscopy and current detected deep level spectroscopy are performed between 200 and 650 K on three Fe-doped samples and an undoped sample. A detailed study of the detected deep levels assigns dominant centers to a deep donor 1.39 eV below the conduction band edge EC and to a deep acceptor 0.75 eV above the valence band edge EV at low electric field. A strong Poole–Frenkel effect is evidenced for the donor. Schottky diodes characteristics and transport properties in the bulk GaN:Fe layer containing a homogenous concentration of 1019 Fe∕cm3 are typical of a compensated semiconductor. They both indicate that the bulk Fermi level is located typically 1.4 eV below EC, in agreement with the neutrality equation and dominance of the deep donor concentration. This set of results demonstrates unambiguously that electrical transport in GaN:Fe is governed by both types, either donor or acceptor, of the iron impurity, either substitutional in gallium sites or associated with other defects.

Deep Traps Induced by 700 keV Protons in CdTe and CdZnTe Detectors

We have studied the effects of 700 keV protons at increasing fluences (up to 2times1012 p/cm2) on CdTe:Cl and Cd0.9Zn0.1Te detectors. The charge transport properties of the detectors have been studied by mobility-lifetime (mutau) product measurements, gamma spectroscopy and photo induced current transient spectroscopy (PICTS) analyses. The correlation of the macroscopic transport behaviour with the information on the microscopic defective states allowed for the identification of the role played by the dominant traps: levels Z and H1 resulted related to electron trapping effects, while levels K and X showed a hole trapping character.

Synthesis solute diffusion growth of bulk GaAs: Effects of growth temperature and stoichiometry

Bulk GaAs crystals were grown by synthesis solute diffusion (SSD) technique in a wide range of growth temperatures between 990 and 1150 °C. Electrical properties of these crystals were studied by means of van der Pauw, admittance spectroscopy, deep levels transient spectroscopy and photoinduced current spectroscopy techniques. It was shown that the main defects determining the properties were the GaAs antisites acceptors and the A center acceptors with the levels, respectively, E v +0.078 eV and E v +0.43 eV. The conductivity of the grown crystals was p-type and showed a pronounced maximum at a level of 10 4–10 5 Ω cm for growth temperatures between 1020 and 1080 °C. If the crystals were additionally compensated either by unintentional Si donors contamination from quartz crucibles or by intentional light Te doping one could get semi-insulating material with the room temperature resistivity higher than 10 6 Ω cm. The Fermi level in such crystals was pinned near E c −0.8 eV, i.e. close to the EL2 donors. Measurements by deep levels transient spectroscopy on n-type doped crystals or by low frequency capacitance–voltage on semi-insulating crystals showed that the density of EL2 in these samples was in the low 10 14 cm −3 and that thus the EL2 donors were not the main compensating agents.

Electrical properties of dislocation trails in n‐Si

AbstractThe deep level centers introduced by plastic deformation at 620 °C in n‐type silicon are quantitatively studied by the electron beam induced current and deep level transient spectroscopy techniques. It is shown that the most efficient deformation‐induced electrically active defects in Si are located in the dislocation trails. The deep level spectrum associated with these defects is obtained. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Deep level defects in CdTe materials studied by thermoelectric effect spectroscopy and photo-induced current transient spectroscopy

The deep levels in semi-insulating CdTe crystals grown by vertical gradient freezing technique were studied using thermoelectric effect spectroscopy (TEES) and photo-induced current transient spectroscopy (PICTS). The measurement of TEES spectra in the temperature range 90–400 K was performed at different heating rates. The positions of the levels and their capture cross-sections were obtained by using the heating rate method. It was found that near midgap levels in samples doped with shallow donors (Cl, In) have a low value of capture cross section (≈10−15 − 10−17 cm2) and are hole traps. Samples doped with a deep donor (Ge, Sn) have a much higher capture cross section of the midgap level (≈10−13 cm2), which acts as an electron trap. Comparison of the TEES results with the PICTS method has shown that while the evaluated values of ionization energies are comparable using both methods, the PICTS technique gives systematically approximately two orders of magnitude higher capture cross sections than TEES.

Electrical characterization of H+ ion irradiated n-ZnO

Schottky barrier diodes (SBD’s) were fabricated with 20/80/40/80nm Ti/Al/Pt/Au ohmic contacts on the O face and circular 0.5mm in diameter, 50nm thick Ru Schottky contacts in the Zn face. Current (I) deep level transient spectroscopy (I-DLTS) was used to study the shallow level defects introduced during 1.8MeV proton irradiations with fluences ranging from 1×1013cm−2 to 2.4×1014cm−2. These measurements revealed that this irradiation introduced a defect with an energy level at 0.03–0.036eV below the conduction band and with an electron capture cross section of 9×10−16 to about 1.4×10−15cm2. This defect may be hydrogen-related as theory has predicted that hydrogen forms a shallow donor in ZnO, or it may be due to the ZnI, as it was reported that electron irradiation with an energy of >1.6MeV produces the ZnI in ZnO which is a shallow level defect at 0.03eV below the conduction band.

Photoluminescence and photoconductivity in CdTe crystals doped with Bi

Defect levels in CdTe doped with Bi are studied by low temperature photoluminescence, photoinduced current transient spectroscopy, photoconductivity measurements, and optical absorption. Two centers associated with the doping with Bi are reported. The first one, a deep level located at Ev+0.71eV, only present at low dopant concentrations, has donor character and hole-trap properties, and is mainly responsible for the high resistivity and very high photoconductivity of the samples. The second one, an acceptor center located at Ev+0.30eV, assigned to BiTe species, is only present at high dopant concentrations and is mainly responsible for the low resistivity and poor photoconductivity of these samples.

Radiation Detection Properties of 4H-SiC Schottky Diodes Irradiated Up to$10^16$n/cm$^2$by 1 MeV Neutrons

We report the results of an experimental study on the radiation hardness of 4H-SiC diodes used as alpha-particle detectors with 1 MeV neutrons up to a fluence of 8times1015 n/cm2. As the irradiation level approaches the range 1015 n/cm2 , the material behaves as intrinsic due to a very high compensation effect and the diodes are still able to detect with a reasonable good Charge Collection Efficiency (CCE=80%). For fluences >1015 n/cm2 CCE decreases monotonically to ap20% at the highest fluence. Heavily irradiated SiC diodes have been studied by means of Photo Induced Current Transient Spectroscopy (PICTS) technique in order to characterize the electronic levels associated with the irradiation-induced defects. The dominant features of the PICTS spectra occur between 400-700 K; in this temperature range the deep levels associated with the induced defects play the main role in degradation of the CCE. Enthalpy, capture cross-section and concentration of such deep levels were calculated and we found that two deep levels (Et=1.18 eV and Et=1.50 eV) are responsible for the decrease in CCE. They have been associated to an elementary defect involving a carbon vacancy and to a defect complex involving a carbon and a silicon vacancy, respectively

Acceptor compensation by dislocations related defects in boron doped homoepitaxial diamond films from cathodoluminescence and Schottky diodes current voltage characteristics

AbstractThis document shows that new electrically active defects can develop in the homoepitaxial layer grown on Ib diamond substrates, related to the increase of the dislocation density. Deep centres, which are able to compensate the boron acceptors, specially when the growth process allows boron incorporation below 1015 cm–3 like achieved in the samples, are identified after heating in an inert gas ambient both from photo‐induced current transient spectroscopy (PICTS) and changes in Schottky diode current–voltage characteristics. Cathodoluminescence spectra are used to monitor the bands and excitonic lines, some of them being specific of dislocations. A correlation is demonstrated between the onset of properties characteristic of a compensated semiconductor and the increase of signals associated to dislocations in cathodoluminescence spectra. These modifications are thermally driven only when the samples are laid on a heating holder immerged in an inert gas, suggesting that a temperature gradient induced an additional stress which finally led to plastic relaxation of the tensile strain in the homoepitaxial layer by an increase of the dislocation density. New deep centres being simultaneously created in this degradation process, the problem of overcoming the onset of these defects for implementing high voltage devices is discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Time and thermal recovery of irradiated CdZnTe detectors

The exposure of Cd0.9Zn0.1Te detectors to increasing doses/fluences of ionizing radiation seriously affects their spectroscopic performance. We have investigated the recovery process of irradiated detectors by means of photon spectroscopy (241Am and 57Co) and PICTS (photo-induced current transient spectroscopy) analyses, to study the evolution with time of their spectroscopic performance and to correlate it with the presence of defective states in the material. The temperatures of the annealing stages in our recovery process varied from room temperature to 380 K, while the time scale varied from 1 h to 4 years. We have observed an improvement of the material detecting properties with time at room temperature only after a long time (years) and for detectors that had not been severely degraded by the irradiation. Preliminary results on thermal treatments are also reported as they proved necessary to recover heavier damage. The recovery effect can be associated with a decrease in concentration of a few specific defective states, thus assessing the crucial role they play in determining the charge collection processes in the material.

Thermal donors formation via isothermal annealing in magnetic Czochralski high resistivity silicon

A quantitative study about the thermal activation of oxygen related thermal donors in high resistivity p-type magnetic Czochralski silicon has been carried out. Thermal donor formation has been performed through isothermal annealing at 430°C up to a total time of 120min. Space charge density after each annealing step has been measured by transient current technique. The localized energy levels related to thermal double donors (TD) have been observed and studied in details by thermally stimulated currents (TSCs) in the range of 10–70K, and activation energies E and effective cross sections σ have been determined for both the emissions TD0∕+ (E=75±5meV, σ=4×10−14cm2) and TD+∕+ (E=170±5meV, σ=2×10−12cm2). The evolution of the space charge density caused by annealing has been unambiguously related to the activation of TDs by means of current deep level transient spectroscopy TSC, and current transients at constant temperature i(t,T). Our results show that TDs compensate the initial boron doping, eventually provoking the sign inversion of the space charge density. TD’s generation rate has been found to be linear with the annealing time and to depend critically on the initial interstitial oxygen concentration, in agreement with previous models developed on low resistivity silicon.

Analysis of nonexponential deep-level current transients in schottky diodes fabricated on [1 $$\bar 1$$ 00] 6H-SiC00] 6H-SiC

A current deep-level transient spectroscopy (DLTS) system capable of recording an entire set of current transients was developed and used for characterization of deep levels in n-type 6H-SiC wafers grown in the [1\(\bar 1\)00] direction. The current transients were acquired in the time interval from 10−4 sec to 10 sec and temperature range from 100 K to 700 K. The current transients exhibited nonexponential behavior, which was related to closely spaced traps in the band gap. The logarithmic plot of the current transient was close to linear at most temperatures. Because the conventional approach based on exponential emission of electrons from discrete levels cannot describe the observed nonexponential behavior of current transients, the transients were fitted assuming continuous distribution of traps in the band gap. Theoretically calculated current transients assuming Gaussian distribution of trap centers in the band gap agree with the experimental data fairly well. The results of electrical measurements and theoretical fitting were explained based on a high density of defects in the investigated wafer that were revealed by chemical etching and related to stacking faults.

Deep energy levels in RuO2∕4H–SiC Schottky barrier structures

RuO 2 ∕ 4 H – SiC Schottky diode structures based on n-type 4H–SiC (7×1017cm−3) with stoichiometric RuO2 Schottky contacts were characterized by electrical capacitance-voltage and current voltage methods and deep-level transient spectroscopy in order to determine their unique semiconducting and electronic properties. The RuO2 films exhibited electrical conductivity of 60μΩcm for Schottky barrier heights of approximately 0.88eV. These Schottky structures revealed two deep energy levels with thermal activation energies of 0.56 and 0.85eV with reference to the conduction band.

Trapping phenomena in intrinsic hydrogenated amorphous silicon like materials studied using current transient spectroscopies

Transient spectroscopies such as time analyzed transients spectroscopy (TATS) provide powerful means of comparing density of states in new forms of amorphous like materials. These spectroscopies were utilized to study hydrogenated amorphous silicon (a-Si:H) and hydrogenated polymorphous silicon (pm-Si:H) grown at different pressures using PECVD. The results reveal marked differences between the two materials. In case of a-Si:H, as expected characteristic emission from a broad density of states in the form of stretched exponentials is observed. The corresponding spectra for pm-Si:H, on the other hand are dominated by nearly exponential fast current decay processes with discrete energies between 0.25eV and 0.36eV. The spectra of pm-Si:H grown at different pressures show contributions from crystallite inclusions and the medium in varying degree.

Comparative study of vertical gradient freeze grown CdTe with variable Sn concentration

Four CdTe ingots with gradually increased concentration of the Sn impurity have been grown by the vertical gradient freeze method and were characterized with glow discharge mass spectroscopy, photoinduced current transient spectroscopy, resistivity, photoconductivity, and photoluminescence techniques. It was shown that the Sn impurity strongly influences resistivity and photoconductivity of the material. Concentration of Sn must be higher than the total concentration of residual acceptors to reach strong compensation. The middle-gap donor level pins the Fermi-level. Photoconductive high resistivity material can be prepared with Sn concentrations in the melt in the range 1018–1019 cm−3. In total, 6 electron traps and 3 hole traps were identified in the band gap by several complementary techniques.

Defect characterization in silicon particle detectors irradiated with Li ions

High Energy Physics experiments at future very high luminosity colliders will require ultra radiation-hard silicon detectors that can withstand fast hadron fluences up to 10/sup 16/ cm/sup -2/. In order to test the detectors radiation hardness in this fluence range, long irradiation times are required at the currently available proton irradiation facilities. Energetic (58 MeV) Lithium (Li) ions, with experimental hardness factor about two orders of magnitude higher than 24 GeV protons for 280-300 /spl mu/m thick detectors, could represent a promising alternative radiation source. In this study, the degradation mechanisms in single pad p/sup +/-n Standard Float Zone (STFZ) and Diffusion Oxygenated Float Zone (DOFZ) Si detectors irradiated with Li ions up to the fluence of 2.9/spl times/10/sup 12/ Li/cm/sup 2/ have been investigated by means of Photo Induced Current Transient Spectroscopy and Thermally Stimulated Currents. Results are compared with the radiation damage induced by 24 GeV proton, 1 MeV neutron and /sup 60/Co /spl gamma/-ray irradiation. The critical Li ion fluence for cluster formation is found to be in the range 4.1-21/spl times/10/sup 11/ Li/cm/sup 2/ and its correlation to the corresponding value for hadrons is discussed.