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The electrical compensation processes of high resistivity CdTe is controlled by deep levels. We have characterized the electrical activity of deep traps by means of three different and complementary spectroscopic methods: photoinduced current transient spectroscopy, surface photovoltage spectroscopy, and space charge limited current analyses. The aim is twofold: to achieve a thorough characterization of the deep trap properties and to assess the potentiality and limitations of the three experimental techniques by a cross correlation of the results obtained with each one of them. We have obtained a direct quantitative estimate of the major deep trap concentration, and we have assessed the sensitivity limit in deep-level detection for surface photovoltage spectroscopy. [ABSTRACT FROM AUTHOR]

Cd1-xZnxTe alloys have been studied by surface photovoltage spectroscopy (SPS) and energy dispersive spectroscopy (EDS). The analyses of surface photovoltage spectra have been perfomed at near and above band gap energies. Surface recombination effects on the surface photovoltage have been investigated. SPS analyses of Cd1-xZnxTe alloys with different surface conditions have shown that the surface recombination velocity significantly affects the SPS determination of the material band gap. Accounting for this and preparing the surfaces accordingly, SPS spectra of Cd1-xZnxTe samples have allowed an accurate determination of the optical band gap as a function of the Zn concentration, determined via EDS analyses. The local increases in the density of states associated with band structure features in Cd1-xZnxTe alloys have been investigated by SPS spectra in the above band gap energy range. [ABSTRACT FROM AUTHOR]

Defects in freestanding gallium nitride were examined in this work. Electron beam induced current mapping evidence a low density of dislocations in the first microns from the upper Ga-terminated surface; correspondingly, deep levels detected by junction spectroscopy exhibit point-like characteristics. Spectral photoconductivity measurements in the poorly dislocated region show the characteristic red, yellow, green, and blue bands, which shift toward higher energies with decreasing temperatures according to Varshni’s law. Spectral photoconductivity measurements carried out in depth evidenced, instead, the quenching of the defect-related yellow band and the prevalence of the green band when temperature increases. This behavior suggests a dislocation-assisted connection between the yellow and green bands, in agreement with theoretical models on their common origin involving complexes VGa-ON. [ABSTRACT FROM AUTHOR]

This work describes an experiment on degradation mechanisms of InGaN light-emitting diode (LED) test structures which do not fulfill the requirements of longlife products. We present a combined capacitance-voltage (C-V), deep level transient spectroscopy (DLTS), electroluminescence (EL), and cathodoluminescence (CL) study of short-term instabilities of InGaN/GaN LEDs submitted to low current aging tests at room temperature. In the early stages of the aging tests, the EL and CL characterizations showed an optical power decrease, more prominent at low current levels. The C-V profiles indicated that the stress induced an apparent charge increase, well related to the deep level changes detected by DLTS and to the optical power decrease. It is supposed that the main cause of the degradation is the generation of nonradiative paths, due to the generation/propagation of defects activated by carrier transport. [ABSTRACT FROM AUTHOR]

The effects of low-temperature annealing in 8.2 MeV electron-irradiated 4H-SiC Schottky diodes were investigated. Deep-level transient spectroscopy and minority-carrier diffusion length (Ld) measurements were carried out on not-irradiated samples and on irradiated samples before and after thermal treatments up to T=450 °C. We found that several deep levels in the upper half band gap (S1 with enthalpy ET=0.27 eV, S2 with ET=0.35 eV, S4 with ET=0.71 eV, and S5 with ET=0.96 eV) anneal out or modify at temperature values lower or equal to T=450 °C, whereby their progressive annealing out is accompanied by a net increase of Ld, up to 50% of the value in the as-irradiated sample. We drew some conclusions regarding the microscopic nature of the defects related to the deep levels, according to their annealing behavior. [ABSTRACT FROM AUTHOR]

The effects on 4H-silicon carbide epilayers of irradiation with protons and electrons having particle energies, respectively, of 6.5 and 8.2 MeV were carefully studied and critically compared. In detail, the electronic levels associated with the irradiation-induced defects were analyzed by current-voltage characteristics and deep-level transient spectroscopy (DLTS) measurements up to 550 K. In the same temperature range the apparent free-carrier concentration was measured by capacitance-voltage characteristics in order to monitor compensation effects due to the deep levels associated with the induced defects. Introduction rate, enthalpy, and capture cross section of such deep levels were compared. We found that a set of deep levels (at ET=0.39 eV, ET=0.65 eV, and ET=0.75 eV) is the same in both cases of proton and electron irradiations, whereas two other pairs of levels (S1, ET=0.20 eV and S1*, ET=0.23 eV; S5, ET=1.09 eV and S5*, ET=0.89 eV) appearing in the same temperature range within the DLTS spectra should be associated with different defect complexes according to the irradiation type. Some conclusions regarding the microscopic nature of the defects related to the deep levels have been drawn. [ABSTRACT FROM AUTHOR]

The electronic recombination properties of Mg-doped GaN have been investigated by steady state and time-resolved cathodoluminescence (TRCL) in the scanning electron microscope, photocurrent (PC) spectroscopy, and optical beam induced current (OBIC). CL and OBIC maps reveal an inhomogeneous recombination activity in the investigated material. Deep levels giving rise to level-to-band transitions were detected by PC spectroscopy. A large PC quenching observed upon illumination with light of (2.65–2.85) eV is tentatively attributed to metastable traps within the band gap. CL spectra reveal the existence of emission bands centered at 85 K at 3.29, 3.20, 3.15, and 3.01 eV, respectively. Both time-resolved and steady-state CL measurements carried out under different excitation conditions indicate that the 3.15 and 3.01 eV emissions are likely related to donor-acceptor pair transitions. TRCL measurements also reveal different recombination kinetics for these bands and suggest that deep donors are involved in the mechanism responsible for the 3.01 eV emission. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The features of current deep level transient spectroscopy (I-DLTS) spectra are investigated in AlGaAs/GaAs heterostructure field-effect transistors both through experiments and two-dimensional numerical device simulations. Differently from electron traps located in the n-type semiconductor bulk, which can only be detected in emission-mode by I-DLTS, deep levels located at the ungated device surface are shown to be revealed both in emission and capture transients by I-DLTS. Experimental and simulation results indicate that this behavior originates from inherent physical properties of surface deep levels interacting with holes attracted at the ungated surface by the negative charge associated with ionized traps. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a study of the effects of high and low energy neutron irradiation on CdTe and CdZnTe high resistivity detector grade material. The evolution of the defect states under increasing irradiation fluence has been monitored by means of photoinduced current transient spectroscopy analyses. Particular attention is placed on the comparison and correlation between the results from the two materials under the two different neutron irradiation energies. The behavior of the observed deep traps associated with defect states allowed us to attribute an origin to most of them and to understand the role they play in the charge carrier collection process and in the degradation of material spectroscopic capabilities. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Time-resolved cathodoluminescence (TRCL) and photocurrent (PC) spectroscopies have been applied to the study of the yellow band of Si-doped GaN. Measurements carried out combining both techniques unambiguously reveal the complex nature of this broad emission and confirm that different deep defect levels are involved in the observed luminescence. Five emission bands centered at 1.89, 2.03, 2.16, 2.29, and 2.38 eV were found by steady state and time-resolved CL investigations, while PC spectra showed four transitions at about 2.01, 2.14, 2.28, and 2.43 eV. The behavior of the deep-level emissions intensity as a function of the excitation pulse width as well as their decay times were investigated by TRCL. A decay time of 245 μs was measured for the 2.29 eV emission band, while longer decay times of 315 and 340 μs were found, respectively, for the 2.16 and 2.38 eV bands, in agreement with TRCL spectra. The appearance of the 2.03, 2.16, 2.29 eV and 2.38-2.43 eV peaks both in PC and CL spectra suggests that these bands are related to deep acceptor to band transitions, as supported by the single exponential character of the corresponding decay transients. [ABSTRACT FROM AUTHOR]

Photocurrent (PC) spectroscopy is proposed as a reliable tool in the investigation of the transport properties of organic thin-film transistors (OFETs). We have applied PC analyses to the study of the electronic density of states (DOS) distribution of the OFETs with crystalline-mixed films of two derivatives of a conjugated molecule [ α-sexithiophene (6T), and its alkylated analogue α, ω-dihexylsexithiophene (DH6T)] as the active semiconductor. We have investigated the modifications in the DOS distribution associated to variations in the carrier density in the OFET channel and we have detected the formation of deep electrically active states in the below-band-gap region, associated to polaron states induced by a prolonged exposure of the device to atmosphere. A clear correlation between the PC results and the electrical characteristics of the corresponding FET devices has been observed. [ABSTRACT FROM AUTHOR]

The potentiality of Photocurrent (PC) and Surface Photovoltage Spectrosocopy (SPS) analyses in the determination of the band gap of CdTe-based ternary alloys was investigated. Both methods can detect band-to-band and defect state-to-band transitions. The role played by the surface states in both experimental methods has been studied since it may significantly alter the results. The below-band gap energy levels have also been detected by PC and SPS and the different information that can be obtained in the two cases has been discussed. The results obtained by PC and SPS have been compared in order to extract the contribution of the surface and bulk states. [ABSTRACT FROM AUTHOR]

We have studied the effects of 700 keV protons at increasing fluences (up to 2 × 1012 p/cm²) on CdTe:Cl and Cd0.9Zn0.1 Te detectors. The charge transport properties of the detectors have been studied by mobility-lifetime (μτ) 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. [ABSTRACT FROM AUTHOR]

We have investigated the effects of proton irradiation on the charge collecting properties of planar CdTe detectors. The spatial distribution of the charge collection and of the electric field inside the material has been studied by photocurrent spectroscopy and surface potential analyses. The results indicate that a localized damaged layer is formed under the irradiated electrode due to the presence of a high concentration of electron traps. The induced p+-type region affects the distribution of the electric field: after irradiation the field is no longer uniform inside the detector but it rapidly decreases to zero towards the damaged layer. [ABSTRACT FROM AUTHOR]

A new instrument for solar bi-dimensional spectroscopy, the Interferometric BIdimensional Spectrometer (IBIS), has been successfully installed at the Dunn Solar Telescope of the National Solar Observatory (USA-NM) in June 2003. This instrument is essentially composed of a series of two Fabry-Perot interferometers and a set of narrow-band interference filters, used in a classic mount and in axial-mode. It has been designed to take monochromatic images of the solar surface with high spectral ( R ≥ 200 000), spatial ≃ 0.2″), and temporal resolution (several frames s−1). IBIS has a circular field of view, 80″ in diameter and, with suitable interference filters, it can be used in the wavelength range 580 – 860 nm. The wavelength stability of the instrumental profile is very high, the maximum drift in 10 hours amounting to ≃10 m s−1. In this paper the criteria used in the design and the expected instrumental characteristics are described. [ABSTRACT FROM AUTHOR]

Abstract: The hydrolipidic ratio (HLR) expresses the amount of water and fat in a tissue. HLR can be studied non-invasively in the living organism and can be mapped in different areas of the body with high spatial and temporal resolution. In the present work we have evaluated the HLR in different adipose tissue depots in young or adult rats using tissue arrays of fat fragments by 1H-spectroscopy. In young animals, the highest percentage of water (33%) was found in the interscapular brown adipose tissue (iBAT). Mesenteric fat (mWAT) also appeared highly hydrated (24%). The deposits composed of epididymal, retroperitoneal and pelvic white adipose tissue (eWAT, rWAT and pWAT, respectively) contained an amount of water ranging from 14% to 17%. In adult animals, a reduction of the water content was found in all the depots. In e/r/pWAT, the age-related maturation was characterized by large changes in adipocyte diameter accompanied by a small change in HLR. In the iBAT, the maturation was accompanied by small change in adipocyte diameter and a greater diminution of HLR. mWAT showed an intermediate pattern between e/r/pWAT and iBAT. In all the studied depots, an age-related increase in leptin expression was found. This increase was relatively low in iBAT (40%) and high in the e/r/pWAT (204–273%). The work expand the knowledge about the physiological significance of the HLR by 1H-spectroscopy. [Copyright &y& Elsevier]

The compensation process in semi-insulating CdTe- based compounds is known to be related to the interaction among shallow and deep levels induced by impurities, grown-in lattice defects and by their complexes. We have carried out Photo-Induced Current Transient Spectroscopy (PICTS) and Photo-Deep Levels Transients Spectroscopy (P-DLTS) analyzes together with DLTS analyzes to investigate the role that acceptors and donors play in the compensation mechanism. To this purpose we have compared semi-insulating materials with different stoichiometry and different resistivity, i.e., with different free carrier concentration. We have examined also semiconducting material in order to compare extrinsically compensated and self-compensated CdTe-com- pounds. Our attention has been focused on the deep levels close to midgap in order to deepen our understanding of their behavior as a function of the position of the Fermi level since they are critical for the compensation process. We have achieved experimental evidence that in Cd(1-∞)Zn∞Te the level H at Ev + 0.75 eV has a donor-like character. The possible extension of the donor-like character of this defect to CdTe:Cl should be positively considered albeit it can not be assessed by the present investigation. [ABSTRACT FROM AUTHOR]

The light sensitivity of current deep-level transient spectroscopy (I-DLTS) is analyzed with the aim of gaining Insight about the physics of surface-trap related dc-to-RF dispersion effects in AIGaAs-GaAs heterostructure field-effect transistors. I-DLTS experiments under dark reveals three surface-trap levels with activation energies 0.44 eV (h1), 0.59 eV (h2), and 0.85 eV (h3), as well as a bulk trap with activation energy 0.45 eV (el). While the I-DLTS signal peaks associated with the two shallower surface traps h1 and h2 are suppressed by optical illumination with energy larger than the AlGaAs bandgap, that which is associated with the deepest surface trap h3 is nearly unaffected by light up to the highest intensity adopted. Two-dimensional device simulations assuming that surface traps behave as hole traps provide an interpretation for the observed different light sensitivity of surface traps, explaining it as the result of the temperature dependence of surface hole concentration and negative trap-charge density, making trap-charge modulation at increasing temperature less and less sensitive to excess carriers generated by light. [ABSTRACT FROM AUTHOR]

Radiation damage can strongly affect the performance of detectors and we have carried out a systematic study on the effects of different ionizing radiation on CdTe and CdZnTe detectors. In this paper, we present a novel perspective of investigation on the effects of gamma rays, electrons, neutrons, and protons by correlating the "macroscopic" performance of the detectors, characterized by gamma spectroscopy measurements, to the "microscopic" effects of impinging radiation, i.e., to the defective states induced within the crystal lattice, by carrying out photoinduced current transient spectroscopy (PICTS ) analyses. The comparison of the results obtained from CdTe and CdZnTe allows for the identification of the defects that play a major role in degrading the detectors' spectroscopic capabilities. [ABSTRACT FROM AUTHOR]

We have studied the deep levels in modulation-doped GaAs/Al0.33Ga0.67As heterostructures grown on (0 0 1) GaAs substrates with a high-mobility two-dimensional electron gas (2DEG), by means of current-mode deep level transient spectroscopy (I-DLTS). Five major traps have been identified and their effect on the 2DEG electron mobility and density has been assessed by comparing results from devices grown with different As/Ga ratios (25, 40 and 48). Two so-called “hole-like” traps have also been observed and we suggest they are linked to the different threshold voltages observed in the different devices studied. By utilizing different polarization conditions we were able to assign an origin to some of the traps and to advance hypotheses on their localization within the GaAs/Al0.33Ga0.67As heterostructures. [Copyright &y& Elsevier]

Acceptor deactivation in the near-surface region of as-grown, boron-doped Si wafers was detected by in-depth profiles of the free-carrier density obtained by capacitance–voltage measurements. As this deactivation was only observed in wafers subjected to the standard cleaning procedures used in Si manufacturing, we ascribed it to boron passivation by an impurity introduced during the cleaning process. From the study of the free-carrier reactivation kinetics and of the diffusion behaviour of boron–impurity complexes, we have concluded that the impurity is possibly related to hydrogen introduced during the cleaning treatments. The characteristics of the deep level associated with this impurity have been analysed by deep-level transient spectroscopy. [ABSTRACT FROM AUTHOR]

Native or process-induced defective states may significantly affect the transport properties of silicon carbide devices. For this reason, it is of major importance to detect them and, when possible, to identify their origin. This contribution deals with the deep levels detected by deep level transient spectroscopy analyses in silicon carbide Schottky detectors. Current–voltage and capacitance–voltage characteristics have also been studied to investigate Schottky barrier properties and diode quality. On the basis of the comparison with literature data, some of the deep levels found can be attributed to impurities introduced during growth. [Copyright &y& Elsevier]

GaN based heterostructures have recently gained increased interest due to their applications for high electron mobility transistors. In this letter AlInN/AlN/GaN heterojunctions grown by metal-organic chemical-vapor deposition with different AlN thicknesses have been investigated by surface photovoltage spectroscopy. The density of the two-dimensional electron gas (2DEG) forming at the interface has been measured by Hall effect. A band gap shift has been detected and its dependence on the 2DEG electron density at the AlN/GaN interface has been analyzed on the basis of the Moss-Burstein and renormalization effects. [ABSTRACT FROM AUTHOR]

Despite these recent advancements in preparing the surface of Cd(Zn)Te devices for detector applications, large asymmetries in the electronic properties of planar Cd(Zn)Te detectors are common. Furthermore, for the development of patterned electrode geometries, selection of each electrode surface is crucial for minimizing dark current in the device. This investigation presented here has been carried out with three objectives. Each objective is oriented towards establishing reliable methods for the selection of the anode and cathode surfaces independent of the crystallographic orientation. The objectives of this study are (i) investigate how the asymmetry in I-V characteristics of Cd(Zn)Te devices may be associated with the TeO2 interfacial layer using Rutherford backscattering to study the structure at the Au-Cd(Zn)Te interface, (ii) develop an understanding of how the concentration of the active traps in Cd(Zn)Te varies with the external bias, and (iii) propose non-destructive methods for selection of the anode and cathode which are independent of crystallographic orientation. The spectroscopic methods employed in this investigation include Rutherford backscattering spectroscopy, photo-induced current transient spectroscopy, and surface photo-voltage spectroscopy, as well as gamma ray spectroscopy to demonstrate the influence on detector properties. [ABSTRACT FROM AUTHOR]

We report on Photocurrent (PC) and optical absorption spectroscopy analyses of the electronic density of states (DOS) distribution around the band-edge of pentacene films of different thickness (from 50 to 300 nm). Both experimental methods allowed us to independently monitor the increase with increasing film thickness of the Davydov splitting of the first absorption band, correlated with modifications in the film molecular structure and packing when passing from a dominant “thin film” to a dominant “bulk” phase. We show how PC spectroscopy has the remarkable ability to detect the modification of the DOS distribution in a noninvasive way, thus allowing the study of fully operational organic thin film transistors. [ABSTRACT FROM AUTHOR]

Abstract: The main goal of our study is to prepare and to understand the properties of cubic SiC nanowires (NWs) and to characterize its native silicon dioxide. The wires, with diameters ranging from 10nm to 2μm, have been prepared by a CVD process on Si (001) substrates, using CO as the carbon source and Ni as the catalyst. A structural and optical analysis, by means of TEM, micro-Raman and cathodoluminescence (CL) spectroscopy, has been performed. Two sets of samples have been studied, labelled A and B, which differ for growth process conditions. Set A showed two broad CL peaks. Set B showed a much weaker CL emission. This difference has been explained by means of TEM investigation and micro-Raman spectra: set A shows a thick amorphous silicon dioxide layer on the wire surface, whereas set B shows a thin or absent oxide layer. Consequently, the nature of the CL emission has to be ascribed mainly to oxide-related recombination. [Copyright &y& Elsevier]

Defect-related bands and their properties are widely investigated in gallium nitride, especially by luminescence techniques, which evidenced a broad yellow band, and seldom, a green band. We present here a study of the visible portion of the photoconductivity spectra obtained in samples of different thicknesses and doping. The superior resolution of photoconductivity with respect to luminescence techniques allows for clearly distinguishing green and yellow bands, both showing a double peak structure. Moreover, while the yellow band shape results were unchanged with sample properties, the green band is sensitive to the growth conditions. Hence some hypotheses about their microscopic origin can be proposed. [ABSTRACT FROM AUTHOR]

The time recovery of the spectroscopic capabilities of CdZnTe and CdTe detectors, irradiated with increasing doses of high- and low-energy neutrons, as well as electrons, has been investigated by studying their spectroscopic behavior at different photon energies using leakage current measurements and PICTS (photo-induced current transient spectroscopy) analysis. The detectors were stored at room temperature for up to one year to study the time evolution of their spectroscopic performance and to correlate it with the presence of defective states in the material. We have observed a clear improvement in the material’s detection properties with time, though only in those detectors which have not been severely degraded by the irradiation. The recovery can be associated with a decrease in the concentration of some defective states, thus allowing the assessment of the crucial role these play in determining the charge collection processes in the material and its spectroscopic capabilities. [ABSTRACT FROM AUTHOR]

Results are presented from gate-lag, transconductance (gm) frequency dispersion and current deep level transient spectroscopy (I-DLTS) experiments, allowing consistent indications about energy, location, and physical behaviour of deep-level traps in AlGaAs-GaAs hetero-structure field-effect transistors (HFETs) to be inferred. Traps responsible for DC-to-RF dispersion effects at operational temperatures are in particular localised and characterised. [ABSTRACT FROM AUTHOR]

The microscopic phenomena leading to degradation of blue LEDs under forward current stress have been investigated by means of photocurrent (PC) spectroscopy. A versatile model, developed for the simulation of PC spectra, allowed us to separate the photocurrent signal by carriers generated inside the InGaN quantum wells (QWs) from the signal by carriers generated in the bulk GaN. The comparison of simulation and experiment showed that the QW component of photocurrent is the only one to decrease. This strongly supports the hypothesis that degradation is mostly induced by the localized introduction of non-radiative centres in the QW region. [ABSTRACT FROM AUTHOR]

Abstract: The conduction and valence band structure of high-purity 4H-SiC epilayers have been studied by surface photovoltage spectroscopy (SPS). A comparison between defect-free and single-layer stacking fault affected areas is reported. Electronic transitions, determined by SPS, are in good agreement with ab initio calculations. Electronic transitions and changes in band occupation have been observed in stacking fault rich areas below the band gap. Moreover, stacking faults induce the presence of a split-off band below the conduction band and a modification of the electron density of states in the conduction band always at the M point. [Copyright &y& Elsevier]

Abstract: We used photocurrent spectroscopy to study the electronic density of states distribution in thin films of sexithiophene and an alkylated analog, both at the band edge and inside the band-gap. The presented photocurrent spectroscopy analyses provide direct experimental evidence for a modification of the distribution of the electronic density of states in an organic field effect transistor (OFET) channel induced by the modulation of the effective carrier density by the gate voltage. We observe a correlation between a gate voltage induced increase of the carrier density in the channel and a broadening of the distribution of the electronic states around the band edge. We also detected the formation of deep electrically active states within the band-gap of sexithiophene OFETs, induced by the exposure to air and light. These results are in good agreement with the evolution of the macroscopic transport parameters of the devices, i.e., carrier mobility and threshold voltage, as a function of the exposure time to light and atmosphere. [Copyright &y& Elsevier]

Abstract: The comparison between cathodoluminescence and deep level transient spectroscopy is a powerful tool to analyze the defect formation in implanted materials. In this work aluminium implanted 4H-SiC p+/n diodes are studied. Different structures, implanted with different ion energy and fluencies, are analyzed in order to understand the process related effects. The comparison between the epilayer and the implanted areas shows an increase of the concentration of intrinsic defects in the implanted areas. The 4H-SiC band-edge emission decreases, increasing the aluminium ion dose, due to the enhancement of the lattice disorder, partially recovered by the post-implantation annealing. [Copyright &y& Elsevier]