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Transients of fast free-carrier recombination and of multitrapping processes, determined by different types of defects, have been traced by photoluminescence (PL) and contact photoconductivity (CPC) in semi-insulating GaN epitaxial layers. To eliminate effects caused by the electrodes, the CPC decays were supplemented with noninvasive microwave absorption transients. The lifetimes of fast recombination and initial free-carrier capture processes were evaluated using ultraviolet (UV) time-resolved photoluminescence transients. The UV PL band peaked at 3.42 eV with contributions from both stimulated and spontaneous emission was attributed to band-to-band recombination. At the highest excitations, the initial PL decay time exhibited a value of 880 ps due to nonradiative free-carrier recombination. The radiative centers were revealed in continuous-wave PL spectra, where the UV band was accompanied with the bands of blue (B) PL, peaked in the range of 2.82–3.10 eV, and yellow (Y) PL, peaked at 2.19 eV, ascribed to dislocations and bulk donor-acceptor recombination, respectively. The time scale of the relaxation rate exhibited a crossover from picoseconds for stimulated emission to hundreds of nanoseconds for multitrapping. In the asymptotic part, a stretched-exponent decay on the millisecond scale was observed with the disorder factor of α=0.7. The asymptotic decay is explained by competition of centers of nonradiative recombination within bulk of the material and trapping attributed to the dislocations. Behavior of the dislocation-attributed capture centers was simulated using a model of capture cross section, which depends on the excess carrier concentration via screening. [ABSTRACT FROM AUTHOR]

An evolution of the transient characteristics of the GaN p-i-n diodes, operating in the avalanche mode and acting as particle sensors, has been simulated by using the Synopsys TCAD Sentaurus software package and the drift-diffusion approach. Profiling of the charge generation, recombination and drift-diffusion processes has been performed over a nanosecond time-scale with a precision of a few picoseconds and emulated through the photo-excitation of an excess carrier domain at different locations of the active volume of a diode. Shockley-Read-Hall (SRH), Auger and radiative recombination processes have been taken into account. Fast and slow components within a current transient have been analysed based on the consideration of the carrier spatial distribution at different instants of the avalanche process. The internal gain due to charge multiplication ensures the sufficient charge collection on electrodes of the relatively thin (5 µm) diode operating in the avalanche mode. It has been shown that the simulated evolution of the detector transient responses by employing the drift-diffusion approach reproduces properly the qualitative modifications of the main features of a detector with an internal gain, realized by induction of the avalanche processes governed by the applied external voltage. [ABSTRACT FROM AUTHOR]

The pulsed characteristics of the capacitor-type and PIN diode type detectors based on GaN have been simulated using the dynamic and drift–diffusion models. The drift–diffusion current simulations have been implemented by employing the commercial software package Synopsys TCAD Sentaurus. The bipolar drift regime has been analyzed. The possible internal gain in charge collection through carrier multiplication processes determined by impact ionization has been considered in order to compensate carrier lifetime reduction due to radiation defects introduced into GaN material of detector. [ABSTRACT FROM AUTHOR]

Evolution of the non-radiative and radiative recombination in GaN and CdS 2.5–20 μm thick layers has been examined by the in situ measurements of the 1.6 MeV proton induced luminescence and laser excited photoconductivity characteristics. The introduction rate of radiation defects has been evaluated by the comparative analysis of the laser and proton beam induced luminescence for the examined GaN and CdS layers. [ABSTRACT FROM AUTHOR]

The proposed upgrade of the CERN Large Hadron Collider to ten times brighter luminosity poses severe challenges to semiconductor detector within the CERN experiments. We in- vestigate a silicon "3-D" detector design for these conditions and semiconductors alternative to silicon, namely silicon carbide and gallium nitride. Charge collection measurements suggest some degree of additional radiation tolerance over conventional detector geometry and materials. [ABSTRACT FROM AUTHOR]

The electrical properties of a semi-insulating GaN (SI-GaN) epitaxial layer have been investigated and regimes of space charge and Ohmic currents found. Microwave and dc current modes were used for temporal measurements of the photocurrent. Transient behaviour was observed in the injection current and photo-response, with a wide range of time constants. The role of the space charge has been analysed and a previous columnar model of the epitaxial layer is shown to require modification. The nature of traps and recombination centres is discussed. Some promising data demonstrating the application of this SI-GaN for the detection of ionising particles, specifically α particles, is presented. [Copyright &y& Elsevier]

Abstract: Exposure time dependent excess carrier decay transients variation in MOCVD grown GaN epi-layers has been investigated by the contactless microwave probed photoconductivity (MW-PC) technique during 1.7MeV protons irradiation. It has been found that carrier decay transients contain components associated with exponential excess carrier density decay due to carrier recombination in GaN crystalline material and stretched-exponential relaxation one ascribed to carrier transport-trapping within disordered structure caused by dislocations at micro-crystal boundaries. The MW-PC characteristics have been correlated with parameters extracted by analysis of peak intensities of the continuous-wave ultraviolet light excited photo-luminescence spectra, registered after irradiation with different protons fluences. Non-linear decrease of carrier recombination lifetime and of luminescence intensity as a function of fluence has been found in GaN material for protons fluence increase up to 5×1014 cm−2. Radiation-defect dependent features of carrier recombination and of material disorder parameters have been analysed. [Copyright &y& Elsevier]

Effect of radiation defects on the photoconductivity transients and photoluminescence (PL) spectra have been examined in semi-insulating GaN epitaxial layers grown on bulk n-GaN/sapphire substrates. Manifestation of defects induced by 10 keV x-ray irradiation with the dose of 600 Mrad and 100 keV neutrons with the fluence of 5×1014 cm-2 have been revealed through steady-state and pulsed PL as well as through contact photoconductivity and microwave absorption transients. Synchronous decrease of the PL intensity of yellow, blue, and ultraviolet bands peaked at 2.19, 2.85, and 3.42 eV, respectively, with density of radiation-induced defects is observed. The decrease of the PL intensity is accompanied by an increase of asymptotic decay lifetime, which is due to excess-carrier multi-trapping. The decay fits the stretched exponent approximation exp[-(t/τ)α] with the different factors α in as-grown material (α≈0.7) and irradiated samples (α≈0.3). [ABSTRACT FROM AUTHOR]

We report the use of ion beam induced charge imaging to characterise the charge signal uniformity of epitaxial gallium nitride radiation detectors. The detectors were fabricated from 2 μm thick semi-insulating gallium nitride, grown by MOCVD on a sapphire substrate. A carrier concentration of 1.4×1015 cm-3 was measured using capacitance–voltage measurements. Ion beam induced charge imaging was carried out with a 2 MeV alpha particle beam focussed to a 3 μm diameter and raster scanned across the device. The resulting ion beam images show excellent charge signal uniformity in this material with no evidence of material defects or polycrystalline structure on the micrometer length scale. No evidence of charge signal trapping was observed in these devices. [Copyright &y& Elsevier]

• Mn doping has little or no effect on Ga vacancy formation in ammonothermal bulk GaN. • Mg doping strongly suppresses Ga vacancy formation in ammonothermal bulk GaN. • Hydrogen-dopant interactions are the likely cause for the differences. We have applied positron annihilation spectroscopy to study the formation of Ga vacancy related defects in Mg and Mn doped bulk GaN crystals grown by the ammonothermal method. We show that Mn doping has little or no effect on the formation of Ga vacancies, while Mg doping strongly suppresses their formation, in spite of both dopants leading to highly resistive material. We suggest the differences are primarily due to the hydrogen-dopant interactions. Further investigations are called for to draw a detailed picture of the atomic scale phenomena in the synthesis of ammonothermal GaN. [ABSTRACT FROM AUTHOR]