Microwave Probed Photoconductivity Transients Research Articles

Study of polycrystalline CdTe films by contact and contactless pulsed photo-ionization spectroscopy

The methodology of sample preparing and combining of measurement techniques in comprehensive characterization of thin cadmium telluride (CdTe) heterostructures has been demonstrated by recording spectra of deep (bulk and surface) traps, acting coherently within photo-ionization and thermal emission processes. The CdTe heterostructures on silicon (Si) substrates have been examined by combining measurements of the microwave probed and contact photoconductivity transients varying pulsed laser excitation wavelengths. The pulsed photo-ionization spectra have been analysed using the Lucovsky model. It has been shown that microwave probed photoconductivity transients in polycrystalline CdTe films with bare surfaces are governed by diffusion limited surface recombination. The surface recombination is there mainly influenced by deep traps of 1.23 eV photo-activation energy. The bulk traps in CdTe - Si structure containing metal electrodes exhibited a spectrum with five photo-ionization steps, having the threshold photo-ionization energy of the Eb1 = 0.57 eV, Eb2 = 0.94 eV and Eb5 = 1.42 eV, Eb4 = 1.13 eV, and Eb3 = 1.05 eV which are attributed using literature data to the Te interstitials (Eb1), to Cd (Eb2 and Eb5) and Te (Eb4) vacancies, and to anti-site (Te at Cd) defect (Eb3), respectively. Shallow emission centres with thermal activation energy of 100 meV have been estimated from carrier lifetime variations with temperature.

Carrier decay and luminescence characteristics in hadron irradiated MOCVD GaN

Crystalline GaN is a promising material for producing of the radiation hard particle detectors of different types capable to operate in harsh areas of particle accelerators. Moreover, GaN crystals show rather efficient luminescence properties in several spectral bands under excitation by high energy radiation. Thereby, GaN material can be employed for fabrication of a combined device which is able to operate both as scintillating and charge collecting detector. However, the efficiency of such detectors and their functionality has insufficiently been investigated. This work is addressed to study the evolution of the efficiency of photon and hadron induced luminescence. To evaluate the density of excess carriers induced by the high energy protons, a correlation between the microwave probed photoconductivity transients and the proton induced luminescence intensity has been examined using 1.6 MeV protons to produce a nearly homogeneous and rather strong excitation in 2.6 μm thick MOCVD grown GaN epi-layers. To estimate the radiation hardness of such material, the evolution of the photoconductivity transients and of the proton induced photoluminescence characteristics has been studied by in situ measurements of the changes of luminescence intensity and photoconductivity decay rate during the exposure to a proton beam reaching fluences up to 1015 cm-2. The production rate of radiation defects, determined from in situ and post-irradiation examination of the changes of radiative and non-radiative recombination have been examined by combining penetrative hadron (nuclear reactor neutrons and 24 GeV/c protons) irradiations with those of the 1.6 MeV protons. The parameters of the efficiency κP of carrier pair generation by asingle proton of κP = nP/NP ≅ 1.3 × 107 cm-3 per proton and κPApr = 40 carrier pairs per a micrometer of layer depth per proton have been estimated. The production rate of radiation defects is estimated to be KP ≅ 0.6 cm-1 for both penetrative neutrons and for 24 GeV/c protons. The hadronirradiation determines both the creation of the specific radiation defects with rate of KP ≅ 0.6 cm-1 and the modification of the material structure by increasing its disorder. Increase of disorder has been deduced from the observed decrease of value of the stretched-exponent index.

Lateral scan profiles of the recombination parameters correlated with distribution of grown-in impurities in HPHT diamond

Abstract The profiling of the microwave probed photoconductivity transients and of the time resolved photoluminescence spectra has simultaneously been performed on the synthetic diamond wafer samples in order to clarify correlation of the distribution of the grown-in defects and carrier radiative and non-radiative recombination channels. The wafer samples were sliced from the diamond single crystals, synthesized by the high pressure and high temperature (HPHT) technology using the Ni–Fe liquid solvent/catalyst system. To estimate the microscopic characteristics of the grown-in defects, the EPR spectroscopy has been carried out. The changes of carrier lifetime, ascribed to the radiative and non-radiative recombination, have been resolved. The correlation between the parameters extracted from the absorption spectra and recombination characteristics has been obtained. The prevailing grown-in point defects, associated with neutral and charged nitrogen impurity complexes as well as the pairs of nitrogen atoms on neighbouring sites, and the nickel precipitates have been unveiled.

Study of variations of the carrier recombination and charge transport parameters during proton irradiation of silicon pin diode structures

Techniques for the remote and in situ control of carrier recombination and drift parameters during proton irradiation are presented. The measurement and evaluation of the carrier recombination and drift-diffusion characteristics are based on simultaneous analysis of microwave probed photoconductivity transients and of the induced charge collection current transients in diodes with applied electric field during the proton exposure.

Instrumentation for the in situ control of carrier recombination characteristics during irradiation by protons

Instrument and methods for the remote and in situ control of carrier recombination parameters during irradiation by protons of energy in the range of 3-8 MeV are presented. Direct techniques for measurements and separation of carrier recombination and trapping/generation characteristics based on the analysis of microwave probed photoconductivity transients during exposure on protons of different energies and irradiations at different temperatures are described. Simultaneously, a spectroscopy of activation energy of dominant traps has been performed before and just after irradiation by temperature scans of variation in the recombination parameters.

Spectroscopy of neutron irradiation induced deep levels in silicon by microwave probed photoconductivity transients

A contactless technique for simultaneous measurements of the carrier lifetime and the parameters of deep levels has been applied. This method is based on microwave probed pulsed photoconductivity (MW-PC) spectroscopy in the wavelength range between 0.5 and 4 μm. Several deep levels in the range of 0.2–0.6 eV have been resolved from spectral analysis of microwave probed photoconductivity amplitude variations in the samples of Si grown by magnetic field applied Czochralski (MCz) technology. An amplitude of the revealed MW-PC spectral steps showed an increase with neutron irradiation fluence indicating an enhancement of density of the specific defects. Simultaneous variations of recombination lifetime with fluence of the reactor neutrons from 10 12 to 3×10 16 n/cm 2 in the MCz Si samples have been examined. Recombination features of the irradiated and annealed MCz Si structures are discussed by comparing carrier recombination parameters and deep-level MW-PC spectral data with characteristics measured by deep-level-transient spectroscopy (DLTS) in the range of moderate irradiation fluences.

Characterization of the radiation-induced defects in Si detectors by carrier transport and decay transients

Temperature variations of the microwave probed photoconductivity (MW-PCD) transients have been shown to be a non-invasive tool for separation of recombination and trapping centers associated with radiation induced defects in Si detectors. Peaks in the asymptotic carrier decay lifetime temperature changes, attributed to carrier capture/recombination effects, are analyzed. The correlative investigations of the MW-PCD transients and kinetics of the carrier transit within detector are simultaneously performed in the proton irradiated Si pad detectors to clarify field redistribution effects. Carrier decay and transit regimes varying applied electric field are analyzed. The recombination and trapping lifetime variations with irradiation fluence are discussed.

Characteristics of surface recombination in silicon diode isolation grooves

Carrier surface recombination characteristics are investigated in silicon high voltage diodes with glass isolated grooves by microwave probed photoconductivity transients (MW-PCT) combining different excitation depths within the layered device structures. Comparative analysis of recombination parameters in the device isolation grooves and layered wafers passivated by iodinenethanol solutions is performed to evaluate surface recombination rates. It has been revealed that electrochemical etchingnglass melting steps involved within the passivation technological procedures for fabrication of diode isolation grooves induce a decrease of surface recombination velocity from 3¢10 3 to 10 cm=s.