Internal Electric Field Profile Research Articles

Charge Transport and Space-Charge Formation in Cd1−xZnxTe1−ySey Radiation Detectors

The electron- and hole-transport properties in cadmium zinc telluride selenide (CZTS) crystals are studied using a laser-induced transient-current technique with pulsed and dc bias. The internal electric field profile and velocity of surface recombination are determined by Monte Carlo simulations of electron and hole transient currents combined with a numerical solution of the drift-diffusion equation coupled with Poisson's equation. Electron and hole drift mobilities of ${\ensuremath{\mu}}_{e}$ = 830 ${\mathrm{cm}}^{2}$/Vs and ${\ensuremath{\mu}}_{h}$ = 40 ${\mathrm{cm}}^{2}$/Vs, respectively, are determined. We also develop a simple technique for evaluating surface recombination directly from measured current waveforms without the need for numerical simulation. The good quality of the prepared detector at pulsed bias, with electron- and hole-mobility-lifetime products of $(\ensuremath{\mu}\ensuremath{\tau}{)}_{e}$ = 1.9 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}3}$ ${\mathrm{cm}}^{2}$/V and $(\ensuremath{\mu}\ensuremath{\tau}{)}_{h}$ = 1.4 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ ${\mathrm{cm}}^{2}$/V, respectively, are observed. The formation of a positive space charge, originating from hole injection combined with a recombination level, is found. We observe a significant position dependence of the lifetime of electrons and holes in dc bias due to hole injection. The experiment is successfully fitted by a simple model dominated by a single deep recombination level with an energy of ${E}_{t}={E}_{C}\ensuremath{-}0.73\phantom{\rule{0.2em}{0ex}}\mathrm{eV}$; concentration of 7.3 \ifmmode\times\else\texttimes\fi{} ${10}^{11}$ ${\mathrm{cm}}^{\ensuremath{-}3}$; and electron- and hole-capture cross sections of 3.5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}14}\phantom{\rule{0.1em}{0ex}}{\mathrm{cm}}^{2}$ and 6.5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}14}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{2}$, respectively.

Effect of Intense Optical Excitation on Internal Electric Field Evolution in CdTe Gamma-Ray Detectors

The time-of-flight (TOF) transient currents in radiation detectors made of CdTe and Cd0.9Zn0.1Te (CZT) have been measured at several optical excitation intensities to investigate the effect of drifting carriers on the internal field. Both detectors show so-called space-charge-perturbed (SCP) current under intense optical excitation. A Monte Carlo (MC) simulation combined with an iterative solution of Poisson’s equation is used to reproduce the observed currents under several bias voltages and excitation intensities. The SCP theory describes well the transient current in the CZT detector, whereas injection of holes from the anode and a corresponding reduction of the electron lifetime are further required to describe that in the CdTe detector. We visualize the temporal changes in the charge distribution and internal electric field profiles of both detectors.

Long-Term Stable Surface Treatments on CdTe and CdZnTe Radiation Detectors

The spectral resolution and charge collection efficiency (CCE) of cadmium telluride (CdTe) and cadmium zinc telluride (CZT) room-temperature x-ray and gamma-ray detectors are often limited by high surface leakage current due to conducting surface species created during detector fabrication. Surface treatments play a major role in reduction of this surface leakage current. The effect of various types of surface etching and passivation on the leakage current and thereby the spectral energy resolution, CCE, and internal electric field profile of CdTe/CZT detectors has been studied. The main aim of this work is preparation of long-term stable detectors with strongly reduced leakage current. The time stability of the current–voltage characteristic and spectral resolution was investigated during 21 days and 1 year, respectively, after performing surface treatments. Our results suggest that the optimal detector preparation method is chemomechanical polishing in bromine-ethylene glycol solution followed by chemical etching in bromine-methanol solution then surface passivation in potassium hydroxide or ammonium fluoride (NH4F/H2O2). Detectors prepared using this optimal treatment exhibited low leakage current, high spectral resolution, and long-term stability compared with those subjected to other surface preparation methods.

Analysis of trapping and de-trapping in CdZnTe detectors by Pockels effect

In this contribution we introduce a method of deep level spectroscopy in semi-insulating semiconductors demonstrated on detector-grade bulk CdZnTe. The method is based on the measurements of temporal and temperature evolution of the electric field profile in studied samples, which is very sensitive to a change of occupancy of deep levels. The measurement of the electric field is based on the linear electro-optic (Pockels) effect using the InGaAs avalanche photodiode with fast response. The internal electric field profile in studied samples significantly changes under various external conditions represented by the application of the bias and pulsed illumination with below-bandgap light. From the knowledge of the electric field behavior and using a standard analysis based on thermally induced transitions of electrons and holes from the deep levels to the conduction and valence bands, respectively, it is possible to get activation energies of the energy levels, their types (donor or acceptor) and corresponding capture cross-sections. By this method we have found deep levels responsible for the polarization of CdZnTe detector under high photon-fluxes. Identified deep levels eV, eV and eV can capture the photo-generated holes and thus form a positive space charge, which is responsible for polarization of the detector.

Charge compensation phenomena for polarization discontinuities in ferroelectric superlattices

Based on the Landau-Ginzburg theory, the screening charge compensation phenomena for polarization discontinuities at interface in ferroelectric superlattices are investigated. Interface mixing leads to polarization continuity or discontinuity in superlattices. Free charge with equal but of opposite sign for alternate interface, driven by internal electric field, builds up to counteract the depolarization effect in superlattices. The charge density depends on the degree of interface mixing, and it approaches a saturated value when the mismatch of internal field or polarization at interface vanishes. Using PbTiO3/SrTiO3 (PT/ST) as a model system, the charge compensation phenomena for polarization discontinuities in superlattices is investigated by examining the polarization and internal electric field profiles. Dependence of local polarization and internal field at the interfaces on screening charge density is also examined in detail.

Studies on charge collection and transport properties on semi-insulating materials in the presence of a non-uniform electric field

II–VI semiconductors obtain a real interest in the scientific community for their wide applications in several fields: from optoelectronic and solar cell technology to applications as radiation detectors. The charge collection efficiency (C.C.E.) – i.e. the ratio between the photo-generated charge and the charge collected by the electrodes – is closely related both to the material transport properties and to the internal electric field. It could be exploited to study the transport parameters of these materials and the electric field profile inside the devices under irradiation. Under opportune conditions, C.C.E., as a function of applied bias, should follow Hecht or Many's equation, depending on the penetration length of incident radiation. A central requirement in both these models is a uniform internal electric field but in real devices this condition is seldom satisfied due to the presence of spatial charge, as demonstrated by Pockels measurements and bad photocurrent curve fitting. The authors, starting from the Ramo–Shockley theorem and assuming a linear shape of the electric field, present a new model to describe the C.C.E. as a function of applied bias, with the mobility life-time product μτ and the field slope as parameters. This model, with respect to the previous ones, gives information also about the internal electric field profile and, despite its simplicity, it seems to be a good approximation in several cases, as shown by the experimental analyses reported here.

Copper ion transport induced dielectric failure: Inclusion of elastic drift and consequences for reliability

The authors have modeled the copper ion concentration and internal electric field profiles in a SiO2 dielectric by solving the transient nonlinear continuity/Poisson equations. The predicted time-to-failure of the dielectric correlates well with the theory that failure occurs once copper ions accumulate to the point where the induced electric field at the cathode exceeds a critical value. However, the copper ion concentration at the cathode required for failure was much higher than the reported ion solubility, raising doubts about the validity of the model. More realistic values for the required copper ion concentration were obtained by incorporating an “elastic” diffusion term in the continuity equation. Adding this new term and increasing solubility, Ce, to the maximum value reported in the literature reduces the concentration at the cathode to a logically consistent value but does not significantly alter the predicted time-to-failure. The new formulation predicts a higher minimum applied electric field required to induce breakdown. This minimum field is larger than current devices see during operation.

Internal Electric Field Profile of A-Si:H And A-Sige:H Solar Cells

ABSTRACTBy using the transient-null-current method, we have measured the internal electric field profiles Ei(x) near the p/i interface for two groups of solar cells: (a) a-Si:H p-i-n solar cells with varied i-layer thicknesses, and (b) a-SiGe:H cells with varied Ge content. When using an exponential function of Ei(x) to fit the experimental results, we obtained the field strength at the p/i interface E0, the screening length Lo, and the density of defect states Nd in the i-layer. The thinner the i-layer, the stronger the field strength obtained. For i-layer thickness increasing from 0.1 to 0.5 μm, the field strength E0 decreases from 1.15×105 to 2.0×104 V/cm; Lo decreases from 0.89 to 0.14 μm; and Nd is 3-4×1016 (cm3eV)−1. For the a-SiGe:H cells, as the Ge content increases from 40 to 55 %, E0 increases from 9.3×104 to 1.2×105 V/cm. The correlation of the internal electric field parameters with the cell‘s performance is discussed.

Space-charge-enhanced, electric-field-induced strain responses in electrostrictive polyurethane elastomers

Polymers have become more and more attractive to acoustical science and technology. Recently, large electric-field-induced strain responses in segmented polyurethane elastomers have drawn the wide attention of many researchers due to their promising applications in sensor, actuator, and microelectromechanical system (MEMS) devices. Mechanisms of the large strain responses were investigated by studying elastic, dielectric, and electromechanical properties as functions of temperature, frequency, and sample thickness. The effects of the injected charges on the large electric-field-induced strain coefficient in electrostrictive polyurethane thin films were also studied using the thermally stimulated discharging current (TSDC) technique. It was found that the contributions to the field-induced strain responses are associated with intrinsic electrostriction and the Maxwell stress effect. These effects can be enhanced by distribution of the injected charges, which results in a modified nonuniform internal electric field profile in the region near the polymer–electrode interface. In addition, transitional phenomena related to the molecular motions of soft segments and hard segments of the segmented polyurethane play an important role. The experimental results will be presented and discussed. The achievements will supply a guideline to develop new and better polymer materials for electromechanical and acoustical applications.

Internal Electric Field Profile in Thin Film Hydrogenated Amorphous Silicon Diodes Studied by the Transient-Null-Current Method

ABSTRACTWe demonstrate that the internal field of a thin a-Si:H pin solar cells can be measured using the transient-null-current method. This method was previously developed to measure the internal field profile in a-Si alloy Schottky barrier. The internal electric field profile was determined by measuring the forward-bias voltages that tune the transient photocurrents generated by a pulsed laser at a various wavelengths to zero. We adopt the same technique to a-Si:H p-i-n solar cells. In the case of p-i-n structure, we need to consider both space charge contributed by photogenerated carriers and carrier recombination which disturb the internal field. We use two critical conditions to minimize these effects. (1) To limit the contribution of photocarriers to space-charge distribution, the total charge collected is less than 10−10 C per pulse, and a repetition rate 1 Hz is used to ensure that the diode remains close to its equilibrium state. (2) The measuring time window is about 1 – 6 μs following the displacement current. Typically the RC constant of diode is < 1 μs and the rise time of the forward-bias recombination current is 6.0 × μs. We apply the signal average to process the forward-bias voltage. The error is within ± 0.05 V. With this technique we can study the effect of variety of structure design or processing on the device performance.

Application of electron-beam testing technique to potential profile measurement in a-Si:H solar cells at various states of operation

The electron-beam testing (EBT) technique was used to determine the depth profile of the internal potential in an a-Si:H solar cell and hence the internal electric field profile. The a-Si:H solar cells were prepared for the EBT measurements by etching small wells of varying depths into the p-i-n layers by means of a plasma beam through a mask structured by electron-beam lithography (EBL). In contrast to earlier work [5] the solar cells were not only investigated in various electrical states of operation but also under illumination at various wavelengths and intensities and at different temperatures, including in situ annealing of degraded cells. It was demonstrated for these thin-film solar cells that by combining EBT with a suitable micro-structuring preparation method (EBL and plasma-beam etching) the depth dimension was opened up to EBT.

The use of the signal current pulse shape to study the internal electric field profile and trapping effects in neutron damaged silicon detectors

The induced current pulse from ionizing events occurring near contacts on each side of a p +−n−n + silicon junction detector may be used to map the electric field present in the detector. It is of interest to define the operative effects of fast neutron-induced displacement damage in detectors destined for high radiation environments in SSC or LHC. The hole current shape, in particular, has been useful to determine that the field maximum moves to the “rear” n + contact as the material apparently changes to p-type at 8 × 10 12 1 MeV n/cm 2. Trapping times for both holes and electrons have been measured as a function of neutron fluence using the current pulse width to measure charge collection time as well as using calculated charge collection times. A clear linear relationship is found for the trapping probability (l/τ) versus neutron fluence. Current pulse shapes have been calculated for representative detector fields and mobility relationships and comparison with measured shapes is reasonable.