A. Castaldini

Deep energy levels in CdTe and CdZnTe

The deep levels present in semiconducting CdTe and semi-insulating CdTe:Cl and Cd0.8Zn0.2Te have been investigated by means of cathodoluminescence, deep level transient spectroscopy (DLTS), photo-induced current transient spectroscopy, and photo-DLTS. The latter two methods, which can be applied to semi-insulating materials, allow to characterize the deep traps located up to midgap and can determine whether they are hole or electron traps. We have identified 12 different traps, some common to all the investigated samples, some peculiar to one of them. A comparison of the results obtained from the various materials is given and the status of defect models is reviewed.

Influence of short-term low current dc aging on the electrical and optical properties of InGaN blue light-emitting diodes

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.

Performance of a new ohmic contact for GaAs particle detectors

Abstract In recent papers, we have investigated, within the context of the RD-8 experiment, the behaviour as a function of bias of the active region of particle detectors made by Alenia SpA on semi-insulating liquid encapsulated Czochralski gallium arsenide: the active region width depends linearly on the bias voltage. The diodes were found to break down as soon as the field reached the back ohmic contact. This suggested that the ohmic contact was injecting holes into the diode, therefore we have decided to develop a new, non-injecting, non-alloyed ohmic contact. This new contact allows us to go far beyond, five times, the voltage bias necessary to have a fully active detector. The higher voltage reached by the detectors helps us improve the charge collection efficiency, up to more than 95% for alphas and more than 90% for beta (mips) particles and X-rays, giving a more stable operation of the detectors. For the first time we can explore the characteristics of a GaAs detector beyond the voltage needed for it to be completely active.

Deep levels by proton and electron irradiation in 4H–SiC

The effects on 4H-silicon carbide epilayers of irradiation with protons and electrons having particle energies, respectively, of 6.5 and 8.2MeV 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 550K. 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=0.39eV, ET=0.65eV, and ET=0.75eV) is the same in both cases of proton and electron irradiations, whereas two other pairs of levels (S1, ET=0.20eV and S1*, ET=0.23eV; S5, ET=1.09eV and S5*, ET=0.89eV) appearing in the same temperature range within the DLTS spectra should be...

Optical properties of oxygen precipitates and dislocations in silicon

Photoluminescence (PL) and deep level transient spectroscopy (DLTS) measurements were used to study the origin of optical emissions in the 0.8–1.0 eV region of selected oxygen precipitated and dislocated silicon samples. It was shown that the D1 band, present in both types of samples, is the convolution of different sub-bands, narrowly spaced between 0.802 and 0.820 eV. The emission at 0.807 eV, assigned in the literature to dislocations, was found only in samples where dislocations were intentionally generated by plastic deformation or induced by the clustering of self-interstitials generated during the growth of oxide precipitates. A comparison of the results of PL, DLTS, and optical DLTS measurements allows the assignment of levels involved in the radiative recombination processes.

Low temperature annealing of electron irradiation induced defects in 4H-SiC

Low temperature annealing of electron irradiation-induced deep levels in 4H-SiC is reported. The major deep level transient spectroscopy peak S2 associated with the energy level at Ec−0.39eV disappears in the temperature range 360–400K, and some rearrangement of the peak S3, associated with the defect Z1∕Z2 with energy level at Ec−0.5∕Ec−0.65eV occurs in the temperature interval 400–470K. A net free charge carrier concentration increase goes along with the disappearance of peak S2 at Ec−0.39eV, whereas the charge collection efficiency of the diode does not experience any significant change. An interpretation of the annealing of peak S2 on a microscopic scale is given.

Comparison of electrical and luminescence data for the A center in CdTe

We have investigated the electrical and optical properties of the deep levels responsible for the 1.4–1.5 eV luminescence band usually observed in II–VI compounds. We compared the energy levels found by cathodoluminescence and junction spectroscopy methods for semi‐insulating (CdTe:Cl and Cd0.8Zn0.2Te) and semiconducting samples (undoped CdTe). The techniques utilized were deep level transient spectroscopy (DLTS) on semiconducting samples and photoinduced current transient spectroscopy and photo‐DLTS on high resistivity materials. These last two techniques are complementary and allow the determination of the trap character (donor/acceptor). Three acceptor levels are seen in the electrical transient data at Ev+0.12, 0.14, and 0.16 eV with hole capture cross sections of 2×10−16, 1×10−16, and 4×10−17 cm2, respectively. The lowest level is seen only in Cl doped material corroborating the literature optical and electron spin resonance identification of a level at Ev+0.12 eV as being a VCd+ClTe donor–acceptor p...

Radiation Detection Properties of 4H-SiC Schottky Diodes Irradiated Up to

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

10^16

This paper presents failure modes observed in long-term aging of high-brightness GaN/InGaN LEDs. The blue LEDs submitted to DC aging test present large decrease of emitted optical power and increase of diode reverse leakage current. Increase of parasitic series resistance, suggesting contact degradation, has also been found in stressed sample, together with apparent carrier density increases and reduction of the junction depletion width. Furthermore stressed LEDs present modification of a specific trap property: trap activation energy decreases from 340 meV in the virgin sample down to 75 meV in the stressed sample. Generation of non-radiative recombination centers seems to be one of the dominant failure mechanisms responsible for the observed electrical and optical LED degradations.

n/cm

This work presents the results of an extensive DC current aging and failure analysis carried out on blue InGaN/GaN LEDs which identify failure mechanisms related to package degradation, changes in effective doping profile, and generation of deep levels. DLTS and photocurrent spectra indicate the creation of extended defects in devices aged at very high current density.