L. Polenta

Dislocation-Related Electron Capture Behaviour of Traps in N-Type GaN

Electron capture behaviours for major traps in thin epitaxial and thick free-standing GaN samples have been experimentally and theoretically studied by using deep-level transient spectroscopy (DLTS). According to the logarithmic dependence of the DLTS signal on the filling pulse width, most of the traps in thin epitaxial GaN layers with high dislocation density behave as line defects. In sharp contrast, the same traps in thick free-standing GaN layers with low dislocation density behave as point defects. The most likely explanation for these phenomena is that the electron traps in question tend to segregate around dislocations, when present in large numbers.

Investigation on Localized States in GaN Nanowires

GaN nanowires with diameters ranging between 50 and 500 nm were investigated by electrical and photoinduced current techniques to determine the influence of their size on the opto-electronic behavior of nanodevices. The conductivity, photoconductivity, and persistent photoconductivity behavior of GaN nanowires are observed to strongly depend on the wire diameter. In particular, by spectral photoconductivity measurements, three main sub-band-gap optoelectronic transitions were detected, ascribed to the localized states giving rise to the characteristic blue, green, and yellow bands of GaN. Photoconductivity with below-band-gap excitation varies orders of magnitude with the wire diameter, similarly to that observed for near-band-edge excitation. Moreover, yellow-band-related signal shows a superlinear behavior with respect to the band-edge signal, offering new information for the modeling of the carrier recombination mechanism along the nanowires. The photoconductivity results agree well with a model which takes into account a uniform distribution of the localized states inside the wire and their direct recombination with the electrons in the conduction band.

Deep levels in silicon carbide Schottky diodes

Abstract 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.

Deep levels and irradiation effects in n-GaN

The electrical activity of defects was investigated in hydride vapour phase epitaxy n-type gallium nitride (GaN) grown on sapphire by deep level transient spectroscopy, iso-thermal current spectroscopy, photoconductivity decay measurements and the electron beam induced current (EBIC) method. In order to identify the defect origin, the epilayers were irradiated by high energy protons, and their characteristics before and after irradiation were compared. Irradiation generates two new deep levels and significantly increases the electron carrier concentration of the as-grown epilayer levels. The photocurrent decay is characterized by a stretched exponential law, the slope and time constant of which dramatically decrease after irradiation. The results are discussed in terms of carrier capture at deep levels. EBIC analyses, according to the DLTS findings, revealed an increase in recombination, and also a different distribution of the recombining centres.

Electric field distribution in irradiated silicon detectors

Abstract Particle irradiation causes dramatic changes in bulk properties of p + –n–n + silicon structures operating as particle detectors. Several attempts to model and justify such variations have been proposed in the last few years. The main unsolved problem remains in the determination of the electric field and depletion layer distributions as key-parameters to estimate the collection efficiency of the detector. By using optical beam induced current (OBIC) and surface potential (SP) measurements we determined the behavior of the electric field and confirmed the existence of a double-junction structure appearing after irradiation.

Defect characterization in GaN : Possible influence of dislocations in the yellow-band features

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.

ANALYSIS OF THE ACTIVE LAYER IN SI GAAS SCHOTTKY DIODES

Abstract The behavior of the active region width W of semi-insulating gallium arsenide Schottky diodes versus reverse biasing has been investigated by optical beam induced current and surface potential techniques. It has been found that at low applied voltages, W follows the square root law peculiar to a Schottky barrier while, for a bias higher than 20 V, the active layer increases linearly with the voltage applied. To go deeper into this matter, the spatial distribution of the electric field has been analyzed in a wide range of bias voltages and it has been observed that at high voltages a plateau occurs, followed by a linear decrease down to a quasi-zero value. In terms of space charge distribution this means that there is a box-shaped space charge region moving towards the ohmic contact at increasing bias.

Electrical and optical characterization of GaN HVPE layers related to extended defects

The study of the effect of extended defects, present in very large numbers in GaN epilayers, on the material properties and device performance is one of the most important aims of the current research in the field of III nitrides. Thickness strongly influences electrical and optical properties of epitaxially grown GaN. Due to the lattice mismatch between sapphire and GaN, extended defects (mainly threading dislocations) are generated at the sapphire/epilayer interface, and a degenerate layer, characterized by high defect density and high conductivity, has been observed. Moving toward the top surface, the density of the extended defects, which seem to greatly affect the material properties, gradually decreases. This fact mainly causes the commonly observed electrical and optical inhomogeneities. This work deals with the comparative study by means of optical and electrical characterization between two HVPE-grown layers with different thickness (2.6 and 55 μm) in order to check the effects of the extended defect distribution across the sample.

Yellow and green bands in GaN by resolved spectral photoconductivity

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.

Time-resolved cathodoluminescence and photocurrent study of the yellow band in Si-doped GaN

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 mus was measured for the 2.29 eV emission band, while longer decay times of 315 and 340 mus 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.