Electron Photocurrent Research Articles

Electron relaxation phenomena on a copper surface via nonlinear ultrashort single-photon photoelectric emission

Time-resolved single-photon photoelectric emission measurements from polycrystalline copper, using an autocorrelation system of 450 fs duration at 248 nm laser pulses, are reported. This experimental technique makes possible the direct measurement of the electron energy relaxation time at the surface as well as the evaluation of the electron - phonon scattering time in metallic samples. A nonlinear increase of the electron photocurrent versus the laser intensity is observed. This nonlinearity is attributed to the non-thermal equilibrium between the electron gas and the lattice produced by the subpicosecond laser pulses and is related to the dynamics of energy relaxation between the electrons and the lattice. The influence of the non-thermal electrons on the photoemission and their thermalization time is discussed. Our experimental results may be explained by a model based on the Fowler - Dubridge equation for photocurrent and on thermal-balance equations between electron and lattice subsystems.

Effect of methylene chain length on electron transfer and photocurrent generation using viologen-linked porphyrin monolayer modified ITO electrode

The increase of the electron transfer yield from the photoexcited porphyrin to viologen was achieved by forming a Langmuir-Blodgett (LB) film of viologen-linked porphyrin. Four kinds of porphyrin covalently linked to viologen (BuPC n V: n = 3,4,5,8) by methylene chain, in which the center-to-center distance between porphyrin and viologen is less than 1.7 nm, were synthesized, and the molecularly oriented films of BuPC n V ( n = 3,4,5,8) were prepared on indium tin oxide (ITO) electrodes using an LB technique. The effect of methylene chain length both on the quenching of the fluorescence from porphyrin by viologen and on the photocurrent generation was investigated. The fluorescence quenching by viologen increased with the decrease of the methylene chain length, showing that the electron transfer from the photoexcited porphyrin to viologen is easy to take place when the distance between porphyrin and viologen is shorter. Furthermore, the fluorescence was quenched more efficiently by viologen in the LB film compared with a methanol solution, indicating that the fluorescence is quenched both by the bonded viologen and by the neighboring viologen in the LB film of BuPC n V ( n = 3,4,5). Under visible light irradiation, a cathodic photocurrent was generated, which increased with the decrease of the methylene chain length owing to the increase of the electron transfer yield from the photoexcitted porphyrin to viologen. The orientation of BuPC n V ( n = 3,4,5,8) molecules on the ITO electrode was investigated by the effect of bias voltage and dioxygen concentration on the photocurrent. It was shown that the porphyrin moieties are attached to the surface of the ITO electrode and that the photocurrent is ascribable to the photoinduced charge separation in the BuPC n V ( n = 3,4,5) molecules.

Optoelectronic properties of n-type CdZnTe 2-DEG single-quantum-well heterostructures

Optoelectronic properties of asymmetrically strained II-VI CdZnTe single-quantum-well structures grown by molecular-beam epitaxy are reported. Indium doping CdZnTe n-type using a two-dimensional electron gas heterostructure achieved a carrier mobility of 5000 cm/sup 2//spl middot/V/sup -1/ s/sup -1/ at 40 K. A shallow donor ionization energy of 14.5 meV was determined from Hall effect measurements. Fabrication of a large-area-mesa heterostructure device allowed us to investigate exciton absorption of the mixed type-I and type-II single quantum well. Control of the electron concentration in the quantum well allows optical absorption modulation using both the quantum-confined Stark effect (QCSE) and phase-space absorption quenching. Separation of electron and hole photocurrents in different layers is demonstrated and results in photogain. A heavy-hole red-shift of 9.9 meV/V due to the reverse QCSE is reported.

Structural and optical properties of 1.3 μm wavelength tensile-strained InGaAsP multiquantum wells grown by metalorganic molecular beam epitaxy

Tensile strained 0.5%–1.8% InGaAsP multiquantum wells (MQWs) grown by metalorganic molecular beam epitaxy are comprehensively characterized by using transmission electron microscopy (TEM), x-ray, photoluminescence (PL), lifetime, photocurrent, and absorption measurements. Cross-sectional and plan-view TEM photos reveal that, irrespective of the strain values, the tensile strained well layers have flat surfaces in the growth direction and consist of fine parallelepipeds laterally. Optimizing the strain compensation conditions, i.e., keeping the net strain less than 0.3%, makes it possible to grow high optical quality MQWs with 1.8%-strained well layers. As the well strain increases from 0.5% to 1.45%, however, the PL intensities of the misfit-dislocation-free MQWs monotonically decrease and the minority carrier lifetime decreases from 1.4 to 0.9 nsec. The laser threshold current density decreased with increasing well strain in the 0.5%–1.3% range, decreased to 0.6 kA/cm2 at 1.3% and slightly increased in the 1.45%–1.8% range.

Photoluminescence Stability Of Silicon Suboxide Thin Films

AbstractPhotoluminescence (PL) stability of silicon suboxide (SiOx, 0<x<l.5) under light soaking and thermal processing has been investigated. As the oxygen content increases, the PL peak position shifts to the visible, with improved stability against measurement temperature and light soaking. With increasing annealing temperature, the PL spectra shifted to infrared, characteristic of a- Si:H. With annealing above 550 °C, visible PL is lost while infrared PL can be recovered by rehydrogenation of Si-dangling bond defects. Infrared absorption, transmission electron microscopy, visible absorption, and photocurrent indicate a disproportionation reaction resulting in a-Si phase segregation.

Modeling Valence-Band Images from Synchrotron-Radiation Studies Using Display Analyzers: Lithium Fluoride and Graphite

AbstractWe present simulated photoelectron angular distributions (PAD's) for LiF and graphite. The results describe electron photocurrent versus photon energy, electron initial-state energy, and emission angles (leading to identification of two, or three, components of a valence electron's initial crystal momentum). Results are displayed in a fashion greatly facilitated by display analyzers. Earlier experimental results for LiF are confirmed in great detail. We discuss statistical comparison of theoretical and experimental PAD's. Effects of Bragg- diffraction on outgoing photoelectrons and uncertainty in crystal momentum normal to a surface are analyzed. In graphite, the observed lowering of symmetry, from that in a periodic-zone band structure to that seen in the PAD's, is modeled and explained.

Tunable Infrared Detection Using Epitaxial Silicide/Silicon Heterostructures

ABSTRACTA new silicide/silicon IR detector is presented which has the potential for multicolour detection due to the tunability of its photoresponse. This tunable internal photoemission sensor (TIPS) fabricated using the Ir/Si/ErSi2 system, consists of two back–to–back Schottky diodes separated by a thin undoped Si layer. The two metals have different Schottky barrier heights so that the depleted Si forms an asymmetrical potential barrier to the carriers photocreated in each metallic film. The photocurrent flowing between the two metallic films is therefore strongly dependent on the shape and height of the effective potential barrier that can be varied by a bias applied between the two metallic electrodes. The Ir/Si/ErSi2 photoresponse and cut–off wavelength are indeed dramatically modulated when a small bias (less than 1 volt) is applied between the Ir and ErSi2 electrodes. The quantum efficiencies, measured in the 1 to 3 μm range, are comparable to the best obtained in Schottky and SiGe/Si internal photoemission detectors. A quantitative model derived from the Fowler formalism (by taking into account (i) the hole and electron photocurrents and (ii) the wavelength dependence of the photon absorption in each metallic film) fits all the experimental data over the whole range of photon energy and applied biases. The effective barrier heights thus measured as a function of applied bias are in good agreement with those deduced from activation energy analysis of the TIPS dark current and show that the cut–off wavelength can be modulated from 2.5 μm to more than 6 μm. Finally, electrical and photoresponse measurements on Cr/Si/SiGe(p+) structures (using the same TIPS mode of operation) also demonstrate the photoresponse tunability, thus combining the TIPS tunability with the extended wavelength range of operation (up to 10 μm) of SiGe/Si detectors.

Evidence of Electrical Interactions between Photo-Induced Charge Carriers and Moving Dislocations in Red Mercuric Iodide

ABSTRACTVariations in both the hole and the electron photocurrents in single crystals of mercuric iodide (HgI2) are measured during plastic deformation by shearing over (001) planes. The results demonstrate the presence of electrical interactions between moving “easy glide” dislocations and photo-induced charge carriers in HgI2. It is found that both hole and electron photocurrents are decreased during dislocation motion and that the extent of this effect (and the subsequent recovery after deformation ceases) depends on the applied bias.

Electronic and optical properties of radiated oxide films on valve metals

The electronic and optical properties of passive films are changed by radiation in different ways. Nuclear and electronic interaction must be discussed for heavy particles but only electronic interaction for β- and γ-radiation. Nuclear interaction means defect production and amorphization of the solid which can be detected by measurements of capacity, electron transfer reactions, photocurrents, UPS and XPS and reflection spectra. Various electrode reactions are enhanced which changes the passive behavior. The electronic effect of radiation can be simulated by a focussed laser beam. Electron hole pairs are generated and separated in the electric field. Hole accumulation near the surface causes radiation induced oxide growth.

Transient photoconductivity measurements on halogenated a-Se1−x Tex photoconductors

Time-of-flight (TOF) transient photoconductivity measurements, as functions of temperature and applied field, were carried out on chlorinated a-Se1−x Tex (x < 0.1) chalcogenide semiconductor films to study the nature of charge transport in these commercially important xerographic alloys. The films were prepared by conventional vacuum-deposition techniques and were typically 60–70 μm thick. The TOF hole drift mobility was found to follow a power-law field dependence of the manner μh ~ Fn with the index n increasing with Te and Cl additions as well with decreasing temperature. At the highest fields the drift mobility increased sharply with the applied field. The strong field dependence became more pronounced as the temperature was decreased. TOF drift mobility measurements, as a function of temperature at various applied fields, indicated a thermally activated hole-transport process with a well-defined but field-dependent mobility activation energy, which exhibited an approximately linear field dependence, similar to the mobility activation energy reported for doped a-As2Se3. The zero-field extrapolated activation energy, Eμ0, was about ~ 0.43 eV for the whole range of compositions investigated from x ≈ 0.02 to 0.09, and for the Cl additions in the ~ 100 at. ppm range. Two possible interpretations are discussed. The first involves Te-or Cl-introduced shallow defects that result in a distinct feature on the density of localized states peaking at around ~ 0.43 eV above Ev. It is assumed that the the conductivity mobility in this model remains to represent microscopic transport via extended states as in pure a-Se, i.e., μ0 ~ T−n with n ≈ 1. The other possibility is that increased disorder, as a result of alloying with Te or Cl addition, causes the basic microscopic mobility to become thermally activated owing to hopping in the band-tail states with a hop activation energy of ~ 0.14 eV. It is assumed that the energy location of the shallow-hole traps at ~ 0.29 eV above Ev remains reasonably unaffected, but the energy width and the concentration of native defects controlling the mobility are modified. The latter model is essentially a trap-controlled hopping transport. The present TOF measurements could not conclusively infer one model over the other. There was no observable electron transport in chlorinated a-Se1−x Tex films, whereas in unhalogenated a-Se1−x Tex films the electron-drift mobility decreased with the Te content and exhibited a stronger power-law field dependence. The TOF electron photocurrents were highly dispersive and the drift mobility – temperature data indicated a thermally activated charge-transport process with a well-defined but field-dependent mobility activation energy. The zero-field activation energy was ~ 0.49 eV, which is ~ 0.14 eV higher than that for electron transport in pure a-Se. The two models discussed for hole transport may also be applied to electron transport. It is suggested that the Te addition modifies the already present defect population leaving their energy location below Ec unchanged so that the increase in the mobility activation energy of ~ 0.14 eV corresponds to the hop activation energy of electrons in the band-tail localized states.

Ga0.47In0.53As/InP superlattices grown by chemical beam epitaxy: Absorption, photoluminescence excitation, and photocurrent spectroscopies

Superlattices of Ga0.47In0.53As/InP grown by chemical beam epitaxy (CBE) were studied in detail by transmission electron microscopy, absorption, photoluminescence (PL), photoluminescence excitation (PLE), and photocurrent (PC) spectroscopies. Results from all these characterization techniques independently confirmed the superior qualities of the CBE-grown Ga0.47In0.53As/InP superlattices over previously reported results. Photoluminescence linewidths were close to those obtained from CBE-grown single quantum wells. All of the observed excitonic absorption peaks in absorption, PLE, and PC spectra were well resolved and clearly assigned including the forbidden E13h transition for the first time. In fact, these excitonic structures were still observed in the PC spectra at temperatures as high as 102 °C.

Room temperature photoconductivity of CuGaSe 2

The room temperature photoconductive properties of undoped p-type CuGaSe 2; have been investigated. Photoconductivity excitation spectra show the structure in the intrinsic and extrinsic excitation ranges, which is attributed, respectively to a band-to-band transition and a free-to-shallow and a deep-donor transition. Three different decay components are observed in the intrinsic range, but only two in the extrinsic range. An almost trap-free hole photoconduction, which is associated with the two slower decay components, is dominant in the extrinsic range and exhibits a linear relation between the photocurrent and the excitation intensities. In the intrinsic range, however, a trap-controlled electron photocurrent, which is associated with the fastest decay component, becomes dominant, and this results in a drastic change in the photocurrent phase-lag observed between the intrinsic and the extrinsic ranges.

Carrier surface generation and recombination effects in photoconduction of HgI2 single crystals

dc photoconductivity characteristics of HgI2 single crystals have been measured in the range of 1.75–2.75 eV, using CuI as a transparent electrical contact. Hole and electron photocurrents could be separately measured by applying different voltage polarities to the illuminated electrode. It is shown that charge carrier generation near the surface is highly extrinsic, as displayed by differences in the hole and electron photogeneration spectra. Analysis of the photoconductivity versus voltage characteristics indicates that the hole and electron bulk trapping times satisfy τp >70 μs and τn >3 μs, respectively. The hole and electron surface recombination velocities were sp =1.0×104 cm/s and sn =8×105 cm/s, respectively.

Photoconduction in Single Crystals of the Thianthrene-Tetracyanobenzene 1:1 Adduct

Abstract The photoconduction of single crystal thianthrane—tetracyanobenzene has been measured as a function of excitation wavelength, 200–600 mm, excitation intensity, temperature and applied field. In the 200–325 nm region only electron photocurrents are observed which show a symbatic behaviour; in the 325–340 nm region antibatic photocurrent behavior occurs for holes and electrons; for longer wavelengths both photocurrents are weak with no wavelength dependent structure. The electron mobility was ∼ 3.6 cm2/Vs at 300°K. These results are discussed in terms of the dominate photocarrier generation processes.

Photoconductivity of Crystalline Iodoform I.

Abstract Steady state photoconduction in iodoform has been studied between 210–400 nm. The photoconduction threshold is at 345 nm whereas the crystal absorption extends up to 480 nm. The photoconductivity action spectrum shows a steep rise of the photo carrier generation efficiency in the range 345–290 nm and a plateau at shorter wavelengths. Current-voltage characteristics measured under steady illumination show a square dependence of the photocurrent on applied voltage showing the influence of space charge effects. Electron photocurrents are three orders of magnitude lower than hole photocurrents. The lower limit of the photogeneration quantum yield is estimated as 6 × 10−3 charge carriers/photon with an applied field of 1 × 104 V/cm.

Electron and hole photocurrent effects on impatt oscillators

Experimental and theoretical results are presented to show the difference between electron- and hole-initiated avalanches on the microwave properties on impatt oscillators under optical illumination. This difference, arising from unequal electron- and hole-ionisation rates, is demonstrated with X-band Si impatt structures suitable for microwave-optical interactions. A large signal impatt model is extended to incorporate the difference between hole and electron photocurrent, with the intrinsic response time of the avalanche depending on photocurrent composition. The model gives good agreement with experimental results of the power dependence upon photocurrent, although additional power saturation mechanisms need to be considered at higher power levels.

The effect of hole versus electron photocurrent on microwave—Optical interactions in IMPATT oscillators

The effect of hole versus electron photocurrent on the microwave properties of IMPATT oscillators is presented and correlated with static I-V characteristics. The composition of the photocurrent was altered by fabricating both flip-chip (FC) and top-mounted (TM) devices and using an optical source with an absorption depth comparable to both the depletion-layer thickness and substrate diffusion length. The importance of the order of magnitude difference in effect of hole versus electron photocurrent is discussed.

Hole Transport in MOS Oxides

Hole and electron transport are reported for three different kinds of MOS oxides: wet, dry, and ion-implanted. The electron-hole pairs are generated in the bulk of the oxide by a 3 ns x-ray pulse and the separation of the electron and hole photocurrents is made possible by the large difference in mobility. The electrons are so mobile that they are swept from the oxide (or trapped in a heavily ion-implanted region) during the x-ray pulse. The holes, on the other hand, have much lower mobilities which depend strongly on the preparation of the oxide. In the dry oxide the nominal mobility at room temperature is several orders of magnitude higher than in the wet oxide. The mobility is strongly activated by temperature in the oxides, and at liquid N$sub 2$ temperature no hole motion could be detected even though the electrons are still swept out. The low temperature charging of oxides can be understood in terms of the bulk trapped holes, but room temperature charging seems to be dominated by trapping close to the Si/SiO$sub 2$ interface rather than the transport of the holes from the bulk to the interface. Ion implantation provides electron traps which seem to be associatedmore » with the lattice damage rather than the ion. The electron traps can be successfully annealed out at 900$sup 0$C in 20 minutes. (auth)« less

Semiconductor properties of crystalline anthracene. Competition of electron and hole photoinjection by redox electrolytes

Electron photocurrents have been measured with a number of solution photoinjectors. A new set of electrode potentials for solid anthracene is used to account for the pH dependence of electron as compared with hole photocurrents and the absence of limiting values for the former, in terms of the reducibility and oxidisability of anthracene and the relative probabilities of charge reversion to the electrolyte injectors. The magnitudes of electron currents can in most cases be correlated with change-transfer properties of the injectors or heavy atom spin-orbit effects on intersystem crossing.

Energy Gap of the Crystalline Anthracene

The DC-photoconduction of both pure and heavily tetracene doped anthracene single crystals has been investigated in both the strongly and weakly absorbing wavelength regions. With sandwich-type cell and under monochromatized illumination, the electron photocurrent ( i - ) and hole photocurrent( i + ) were measured separately. With doped crystals, the current produced by the thermal excitation from the tetracene excited state could be measured for various temperature regions. The thermal activation energy for carrier generation from the tetracene excitation state was measured to be 0.35 eV for i + and 0.24 eV for i - . As the excited state of tetracene included is 2.4 eV from the ground state, energy gap of crystalline anthracene was deduced to be 3.0 eV.