Extrinsic Photoconductivity Research Articles

Deep defect level spectra in the neutron irradiated Si ionizing radiation detectors

AbstractIn the framework of WODEAN project of CERN‐RD50 collaboration the photoconductivity spectra of p+‐n‐n+ Si detectors were investigated depending on irradiation by high energy neutrons. The samples were irradiated by neutrons to the fluencies 1×1013 –1×1016 cm2. The extrinsic photoconductivity spectra indicated that upon irradiation deep levels (DL) below the middle of the band gap were created. Their activation energies were in the ranges of 0.49‐0.52 eV, 0.77‐0.81 eV, 0.88‐0.91 eV, and 1.02‐1.11 eV. The effective concentrations of these levels were depending on the fluencies and on the isochronal thermal treatments at low temperatures. The changes of DL population induced by annealing have been compared with the steady state lifetimes and photoconductivity decay constants. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and annealing of CdZnTe:Cl crystals with different content of Zn for nuclear detectors

The methods of photoluminescence, Hall effect, extrinsic photoconductivity, and transit time have been used to study the influence of the pressure of cadmium vapor (P Cd) in the course of postgrowth annealing of ingots and annealing of semi-insulating crystals Cd1 − x Zn x Te:Cl (x = 0.005, 0.01, 0.05, 0.1) on compensation of conductivity of the material used for fabrication of nuclear detectors. It is established that free holes are captured by acceptor levels of both cadmium and zinc vacancies. Studies of electrical properties of crystals after annealing at various pressures of cadmium vapors showed that, at low zinc content (x = 0.005 and 0.01), the determining effect on the conductivity compensation in Cd1 − x Zn x Te:Tl is exhibited by the V Cd −2 cadmium point defects. However, even at the zinc content x ≥ 0.05, it is necessary to take into account the effect of zinc point defects V Zn −2 ; in order to obtain such crystals with the best transport characteristics it is necessary to control not only the pressure of cadmium vapors in the course of growth of the material but also the zinc vapor pressure. It is established that it is possible to control the main electrical characteristics of the semi-insulating material with the zinc content x ≤ 0.01 grown by horizontal planar crystallization and designed for nuclear detectors by varying the pressure of cadmium vapors in the course of the postgrowth annealing.

Influence of irradiation by neutrons on the properties of p+–n–n+ Si radiation detectors

Abstract In the framework of WODEAN project of CERN-RD50 collaboration the photoconductivity spectra of p + –n–n + Si detectors were investigated depending on irradiation by high energy neutrons. The samples were irradiated by neutrons to the fluencies 1×10 13 –1×10 16 cm 2 . The extrinsic photoconductivity spectra indicated that upon irradiation deep levels (DL) below the middle of the band gap were created. Their activation energies were in the ranges 0.49–0.52, 0.77–0.81, 0.88–0.91, and 1.02–1.11 eV. The effective concentrations of these levels were depending on the fluencies and on the isochronal thermal treatments at low temperatures. The changes of DL population induced by annealing have been compared with the steady state lifetimes and photoconductivity decay constants.

Photoconductivity spectra and deep levels in the irradiated p +–n–n + Si detectors

The photoconductivity spectra were investigated in p +–n–n + Si detectors according to the WODEAN project of CERN-RD50 collaboration. The samples were irradiated by neutrons to the fluence 1×10 13–1×10 16 cm 2. The irradiation created deep levels (DL) below the middle of the band gap that were observed by the extrinsic photoconductivity spectrum. The effective concentration of these levels depended on the fluence and on the isochronal thermal treatment at low temperatures. The DL population changes determined by annealing have been compared with the steady-state lifetime and photoconductivity decay constant.

Exciton photoluminescence, photoconductivity and absorption in GaSe 0.9Te 0.1 alloy crystals

The photoluminescence, photoconductivity and absorption in GaSe 0.9Te 0.1 alloy crystals have been investigated as a function of temperature and external electric field. It has been observed that the exciton peaks shift to lower energy in GaSe 0.9Te 0.1 alloy crystals compared to GaSe crystals. The long wavelength tails of interband photoluminescence, photoconductivity and absorption spectra are determined by the free exciton states and show an Urbach–Martienssen-type dependence to the photon energy. The maxima of the extrinsic photoluminescence and photoconductivity spectra were found to be determined by the acceptor centers with an energy of E A= E V+0.19 eV formed by the polytypism and defects complexes that include Se and Te anions.

Specific features of the effect of irradiation with electrons and neutrons on photoelectric properties of CdS single crystals nominally undoped and doped with Cu

Electrical, photoelectric, and magnetic properties of CdS single crystals undoped and doped with copper (N Cu ≈ 1018 cm−3) and irradiated with electrons (E = 1.2 MeV, Φ = 2 × 1017 cm−2) and neutrons (E = 2 MeV, Φ = 1018 cm−2) are studied. It is shown that the donor-acceptor pairs are responsible for extrinsic photoconductivity and paramagnetic properties; in particular, these pairs are represented by Cu Cd − -D + complexes that are destroyed during irradiation and are formed again with time (as secondary radiation defects) in irradiated samples. It is established that the majority of paramagnetic centers and donor-acceptor pairs are located in the near-surface region of the crystal. It is confirmed that large structural defects (defect clusters) formed by irradiation with neutrons are efficient sinks for copper atoms. Specific features of isochronous annealing of paramagnetic centers and donor-acceptor pairs responsible for the variation in magnetic parameters and in the photoconductivity spectra of irradiated undoped and Cu-doped CdS samples are studied.

Sensitivity of PbSnTe:In films to submillimeter radiation under conditions of field electron injection

Based on the concept of injection currents in the PbSnTe:In alloy, the mechanism of photosignal formation in PbSnTe:In films in the submillimeter spectral range at liquid-helium temperatures was considered. It was shown that the existence of energy-distributed electron trap levels controlling the features in current-voltage characteristics can cause extrinsic photoconductivity. For such sensitivity to submillimeter radiation, its spectral dependence should be related to the injection level controlling the filling of traps with different energies. The sensitivity initiation mechanism caused by the interaction of submillimeter radiation with the PbSnTe:In phonon subsystem was considered. Such a process can result in an increase in the static permittivity and, hence, an increase in the injection current.

Photoconductivity in sprayed β-In2S3 thin films under sub-band-gap excitation of 1.96 eV

β -In 2 S 3 thin films with a band gap of ∼2.67eV exhibited persistent photoconductivity when excited using photons with energy of 1.96 eV. The photoconductive response to extrinsic photoexcitation could be removed when the film stoichiometry was changed. Photoluminescence studies in the films revealed an emission of 1.826 eV, due to donor–acceptor pair (DAP) recombination, which was absent in the film not responding to extrinsic excitation. Hence, it was concluded that presence of the DAP was responsible for the extrinsic photoconductivity under the 1.96 eV excitation. This study can initiate further a methodology for tailoring the photoresponse of this semiconducting thin film by spatially controlling the film stoichiometry.

Performance of a diamond x-ray sensor fabricated with metal-less graphitic contacts

We report the x-ray photocurrent response of a coplanar chemical vapor deposition diamond detector fabricated using a metal-less graphitic ohmic contact. Ion implantation of 70 keV boron ions to a dose of 2×1016cm−2 was performed through a patterned photoresist to produce a coplanar graphitic contact structure. The device photocurrent showed a fast response to pulsed x-ray irradiation, and showed no evidence of photocurrent persistence that is observed in devices fabricated using metal Schottky contacts. The graphite-contact device also showed no extrinsic photoconductivity when illuminated with white light.

The extrinsic photoconductivity of chalcogens in Ge1−x Six solid solutions

The effect of ∼3% of silicon additive on the energy spectra of selenium and tellurium impurities in germanium is studied. The photoconductivity spectra are measured at 80 K in a spectral range from 2.5 to 5 μm. Narrow lines corresponding to the excited states of chalcogen ions are observed against the background of the extrinsic-photoconductivity band. It is found that the spectrum of excited states is not affected by silicon at the concentration used. An increase in the energy gaps between the conduction band and impurity states of the chalcogens is detected.

Random potential relief and extrinsic photoconductivity of compensated germanium

A photoconductivity spectrum normalized to the optical absorption of compensated germanium is simulated at low temperatures kT≪W (where W denotes the energy scale of the random potential that stems from impurity Coulomb interaction). By fitting the simulated spectrum to the experimental data, the value of W and the occupancy of the shallow-donor impurity band are estimated. A weak energy dependence of the mean free path of electrons in the random potential relief is inferred.

Annealing Effect on Passivated Deep Levels in GaN Epilayers

Transient extrinsic photoconductivity (PC) measurements on as-grown, hydrogenated, and hydrogenated and annealed GaN epitaxial layers grown on sapphire substrates using plasma-assisted molecular-beam epitaxy were performed to investigate the annealing effect on passivated deep levels in a GaN epilayer. A PC spectrum at room temperature showed that the deep-level state in an as-grown GaN epilayer was located 2.9 eV below the conduction-band minimum of the GaN epilayer. The photoionization cross section and the concentration of the deep levels for the as-grown GaN epilayers, determined from the dependence of the rise and decay times of the extrinsic PC response on the illumination intensity of the blue light-emitting diode with 2.9 eV, were 9.7×10-15 cm2 and 4.5×1014 cm-3, respectively. The concentration of the as-grown deep levels in the GaN epilayer was significantly reduced by the hydrogenation treatment, and this passivation effect was still effective after thermal annealing at a temperature up to 700°C. The activation energy of the deep levels and the dissociation energy of the complexes between the deep levels and hydrogen atoms were 0.55 eV and 3.0 eV, respectively. These results provide important information for improving the quality of devices utilizing GaN epilayers.

Nonmonotony of the extrinsic photoconductivity of n-type GaAs thin-film structures under backgating

The extrinsic photoconductivity of an n-type GaAs thin-film structure under backgating has been studied. It is shown that this photoconductivity is a nonmonotonic function of negative substrate voltage, namely, the magnitude of the photoconductivity shows a maximum. This is due to a sharp increase in the magnitude of the photoconductivity caused by an illumination-induced change in the backgating threshold voltage and to its subsequent decrease caused by the accumulation of excess carriers near the film–substrate interface. The calculated results are in a qualitative agreement with the experimental data.

Extrinsic photoconductivity in poly(3-dodecylthiophene) sandwich cells

We have measured the photocurrent action spectra of the Al/poly(3-dodecylthiophene)(P3DDT)/ITO and Au/P3DDT/ITO sandwich cells. The photoinjection from both ITO and Au electrodes was observed at room temperature. No polarity effect was found in the ITO/P3DDT/Au device at low temperatures. The photoinjection from ITO electrode is found not effective at low temperatures. The photocurrent at low temperatures was attributed to photoinjection from Au electrode at both polarity of bias voltage. A phenomenological model of photoconductivity in poly(3-dodecylthiophene) sandwich cells at low-electric fields and low temperatures is proposed. An approach for photoconductivity enhancement in conducting polymers is discussed.

Deep levels in GaN epilayers grown on sapphire substrates

Steady state and transient extrinsic photoconductivity (PC) measurements on unintentionally doped GaN epilayers grown on sapphire substrates by using metalorganic chemical vapor deposition were performed to investigate deep electron levels. The photoionization cross section and the concentration of the deep levels were determined from the dependence of the rise and the decay times of the extrinsic PC response on the illumination intensity. The concentrations on the deep levels, located at 2.9 and 1.42 eV below the conduction band minimum of the GaN were found to be 2×10 13 and 1.6×10 11 cm −3, respectively. The typical relaxation time of the excess carriers controlled by the deep levels was approximately 10 −3 s. The thermal activation energies of the deep levels were determined from the temperature dependence of the relaxation time of the extrinsic PC response. These results can help improve understanding for potential applications in optoelectronic devices based on GaN epilayers.

Optimising the electronic and optoelectronic properties of thin-film diamond

Abstract The realisation of effective UV photodetectors from thin-film diamond requires the selection of good-quality substrate material and careful device design. However, these alone are insufficient to yield high-performance characteristics: post-growth treatments are required to modify the optoelectronic properties of the diamond. Photoconductive devices fabricated in this way can be truly blind to visible light, fast and offer high levels of photoconductive gain. The origin of these improvements has been studied using thermally stimulated current (TSC) and photoconductivity measurements. The treatments used appear to provoke extremely high carrier lifetimes, encouraging high gain whilst eradicating extrinsic photoconduction. TSC reveals that a new trap state is introduced at (Ea≈0.6 eV). A model to explain the changes in device characteristics is discussed.

Long-Living Excited State of the Te Donor in GaP

The kinetics and temperature dependences (5 to 50 K) of the extrinsic photoconductivity (PC) in microwave (MCW) and DC electric fields in GaP doped with Te ( 2 × 1017 cm−3) have been studied. A slow relaxation (≈︂10−2 s) of MCW PC, in contrast to a fast DC PC relaxation was observed at 5 K. The steady-state MCW PC exceeded the DC PC by 2 to 3 orders of magnitude at 5 to 16 K, and dropped by subsequent increase of temperature to 35 K. In the same temperature range a slow relaxation of absorption of the room-temperature background radiation, induced by GaP excitation was observed. In the range 45 to 90 meV it increased, but decreased in the higher photon energy range. The obtained results are explained by accumulation of nonequilibrium electrons in the long-living excited state 1S(Γ3) of the Te donor in GaP.

Monopolarity of extrinsic photoconduction in sillenite-type crystals

An analysis of available data on the electronic structure of nominally undoped sillenite crystals and on their surface-barrier photoemf provides a basis for an unambiguous conclusion of a monopolar (electronic) character of photoconduction in these materials in the blue-green region of the spectrum.

Photofield interface impurity spectroscopy in blocked impurity band Si:B structures

The far infrared spectral response of photoconductive boron doped silicon blocked impurity band structures was measured at electric field values up to 7 kV/cm and sample temperatures in the range of 4–15 K. The structures are constructed of adjacent layers of pure (NA=5×1013 cm−3) and boron doped (NA=1018 cm−3) silicon inserted between highly doped ohmic contact layers. A broad-band photoresponse is observed which is attributed to the impurity band of the doped layer. In addition, several peaks of response are observed, even at the lowest sample temperatures, which are attributed to optical transitions from ground states to excited states of isolated impurities. It is shown that these transitions originate from a thin layer at the interface between the doped and the pure silicon regions. When the electric field is oriented towards the doped layer, the photocurrent peaks are attributed to the tunneling of photoexcited holes across excited states into near neighbor atoms within a hole depleted delta sheet, followed by relaxation to the ground state. Then hopping transport occurs across the doped layer to the ohmic contact. In this scheme, the hole depleted layer is generated by the effect of the applied electric field and involves only 109–1010 cm−2 impurities. This study provides evidence of a new mechanism for extrinsic photoconductivity at interfaces.

Effect of substrate inhomogeneity on extrinsic photoconductivity of n-type GaAs thin-film structures under backgating

The extrinsic photoconductivity of n-type GaAs thin-film structures under backgating was studied. It is shown that the change of the spatial inhomogeneity of the substrate under illumination is responsible for a new mechanism of extrinsic photoconductivity under backgating. This mechanism occurs through an illumination-induced variation in the backgating threshold voltage and may result in a transition from positive to negative photoconductivity even if all deep levels in the substrate are electron traps. Experiments have shown that the onset of backgating and of negative differential conductivity coincide both in the dark and under illumination thus supporting the proposed mechanism.