J. Vaitkus

Determination of free carrier bipolar diffusion coefficient and surface recombination velocity of undoped GaN epilayers

Time-resolved nondegenerate four-wave mixing experiments were performed on 2.6-μm-thick GaN epilayers grown by metalorganic chemical-vapor deposition using picosecond pulses at 355 and 1064 nm for grating recording and probing, respectively. Measurements of free carrier grating kinetics at various grating periods Λ in range from 3 to 10 μm allowed determination of nonequilibrium carrier bipolar diffusion coefficient D=1.7 cm2/s at 300 K. Substitution of the D value into two-dimensional carrier transport model allowed fitting of the whole set of grating kinetics at various Λ varying surface recombination velocity S and linear recombination time τR. This procedure provided us a value of S=5×104 cm/s as well as carrier lifetime of ∼1 ns.

Semi-insulating GaN and its evaluation for α particle detection

Abstract The application of semi-insulating GaN for detection of ionising particles, specifically α-particles, is presented. The electrical properties of GaN epitaxial layers have been investigated and the space charge limited and Ohmic currents were observed. A microwave method was used for temporal measurements of the photocurrent. Transient behaviour of the injection current and photo-response was observed, with a wide range of instantaneous time constants. The charge collection efficiency was measured to be approximately 100% and the role of inhomogeneities in the charge collection peculiarities was analysed.

Performance of irradiated bulk SiC detectors

Abstract Silicon carbide (SiC) is a wide bandgap material with many excellent properties for future use as a detector medium. We present here the performance of irradiated planar detector diodes made from 100-μm-thick semi-insulating SiC from Cree. Ohmic/Schottky diodes were produced and characterised using Schottky barrier measurements and charge collection efficiency (CCE) measurements were made for 5.48 MeV 241 Am alpha particles. Charge collection of ∼60% was measured initially. Measurements were taken of trap lifetimes in an effort to understand this. Three distinct traps with lifetimes of 4, 16, and 130 s were found. Irradiation took place at Paul Scherrer Institute (PSI) to fluences of 10 12 , 10 13 , and 5×10 14 cm −2 , with 300 MeV /c pions. Changes in the Schottky barrier height, leakage current, trap lifetimes, and CCE measurements give an indication of the possible degradation in performance of detectors of this type over their projected lifetime.

Excess carrier cross-sectional profiling technique for determination of the surface recombination velocity

Abstract This paper presents a modified excess carrier profiling technique using carrier decay cross-sectional scans of wafers using a narrow excitation fiber guided beam and by using infrared light as well as microwave probes. The theoretical principles of the technique are based on the analysis of the depth variation of decay shape and the amplitude.

Interaction of deep levels and potential fluctuations in scattering and recombination phenomena in semi‐insulating GaAs

The complex influence of recombination centers and potential fluctuations of the band gap on the scattering and recombination phenomena in n‐type semiinsulating liquid‐ encapsulated‐Czochralski‐grown GaAs were investigated by using the transient photoconductivity and photo‐Hall effects. The inhomogeneities cause a hyperbolic decrease of nonequilibrium carrier concentration and the saturation of Hall mobility, while the exponential parts of the decay appear due to the recharge of deep levels. The mean recombination barrier heights of potential fluctuations were evaluated. We propose a complex ‘‘island’’ model of scattering and recombination centers, consisting of defect clusters and their associations around dislocations, surrounded by potential barriers. At low light intensities and at the temperatures below 330 K they are insulating for majority charge carriers, thus reducing an effective crystal volume and causing percolation transport effects. At the temperature higher than 330–360 K the main barrier o...

Carrier and defect dynamics in photoexcited semi-insulating epitaxial GaN layers

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...

Aluminium oxide film for 2D photonic structure: room temperature formation

We report about aluminium anodic oxide films (AOFs) as self-organising microstructures for photonic band gap (PBG) crystal application. Structural analysis show that aluminium anodic oxide has formed a structure of randomly oriented clusters each of them consisting of neatly ordered submicrometer cells. Measurements show that at room temperature pores of up to 460 nm wide can be formed, thus, opening a PBG in the visible. Doping of aluminium AOFs by Ag and CdS, is demonstrated.

Radiation-defect-dependent photoconductivity transients and photoluminescence in semi-insulating GaN

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 10keV x-ray irradiation with the dose of 600Mrad and 100keV neutrons with the fluence of 5×1014cm−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 3.42eV, 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).

Dynamics of optical nonlinearities induced by strong light illumination in CdS nanocrystallites

Light induced grating experiments have been performed at room temperature in CdS nanocrystallites of 2.5 nm radius embedded in sol-gel glass films of different semiconductor volume concentration. The dynamics of the photogenerated carriers at the lowest transition peak has been shown to depend on the excitation intensity. No spatial diffusion is observed up to a nanocrystallite volume concentration of 35%. Below a few MW/cm2 the carriers relax in each nanocrystallite, either radiatively in ∼150 ps or by trapping the electrons 15 meV below their excitation transition. For higher intensities of excitation, a saturation of the excited states occurs which allows the electrons to be trapped in the glass matrix, inducing a permanent grating by the creation of a static electric field.

Impact of generation current on the evaluation of the depletion width in heavily irradiated Si detectors

The depletion width and full depletion condition are essential parameters in the search for radiation tolerant detector design and operational regimes. In this work, results of measurements of barrier capacitance by combining a transient technique for barrier evaluation by linearly increasing voltage (BELIV), with temperature dependent capacitance-voltage and current-voltage characteristics of neutron irradiated Si pin detectors are discussed. It is shown that the generation current, caused by a high density of radiation induced traps, distorts the capacitance measurements in heavily irradiated devices. There is no space charge sign inversion effect, however, and heavily irradiated detectors become fully depleted in equilibrium.