Detrapping Time Research Articles

The influence of long range potential fluctuations on frequency dependent transport in aSi:H

A recent effective medium calculation of the frequency response of an amorphous semiconductor containing potential fluctuations is described. The importance for this model of the screening by deep states is emphasised, and the role of the detrapping time in governing the appropriate frequency scale is discussed. The model is applied to a range of measurements made on aSi:H.

Photoconductivity of Polyacetylene with Long and Short Light-Pulses

Abstract For polyacetylene films, intrachain, interchain and interfibril effects can afford very different photoresponses in relation to the observed time scale. In the long time range (10−3-10+2S) the transient photocurrents are mostly related to trapping effects. Their analysis allows an estimation of the product μτ and the detrapping time constant (μτ =3.10−11 cm2/V and τp=2.10−3s for a nearly cis sample). These photocurrents depend in a large amount on the cis/trans ratio.

Effects of internal potential barriers on the photocurrent decay in semiconductors

Transient photocurrent decay experiments were carried out to study the effects of internal potential barriers, which could be due to filled traps and metallic inclusions, on the detrapping process in semiconductors. Experiments were carried out with single crystals of Cu2O, annealed in various ways, because of the ease in inducing large defects in this material. It was found that samples of Cu2O with different defect structures had different values of trapping parameters. These different values are explained in terms of barrier formation at the trapping centres. For those samples of Cu2O that contain copper precipitates, the presence of asymmetrical barriers is proposed in order to explain the difference in the trapping parameters when compared with the standard type of Cu2O.The field dependent behaviour of the detrapping time in samples of Cu2O that do not contain copper precipitates could be explained by the presence of both the Poole–Frenkel effect and the photo-impact ionization of the traps. Those samples that contain copper precipitates do not show any field dependent behaviour of the detrapping process, probably owing to the formation of internal space charge regions.

Rise time study of mercuric iodide detectors

A rise time study of mercuric iodide detectors irradiated with gamma ray enables one to identify the incluence of several trapping centers on the detection performance of the devices. A careful analysis of the rise time spectra identifies some of the dominant traps as having detrapping times of 300 and 900 ns, and a less intense one as having a detrapping time of 200 ns. These findings have been used to get better performance from the detectors. Discriminating those parts of the pulses that have been subjected to these trapping centers dramatically improves detection performance, especially at high energies. Tuning the electronics to avoid statistical noise due to these traps also improves performance, as does cooling the detector to about 0°C, thus “freezing” the trapping centers and producing better energy resolution. These modes of operation allow thicker detectors to be used for high energies, still maintaining good performance. At low energies (∼- 6 keV), much better energy resolution was obtained. Polarization effects were eliminated, and lower backgrounds and more symmetric peaks were achieved.

Transition between free and trap modulated mobility

This article shows that for a simple thickness around five times the schubweg and if the sum of the trapping and de-trapping times is smaller than the transit time, a trap-modulated mobility may be defined. For this, the following problem has been studied in the high-field approximation: by means of blocking electrodes a high electric field is suddenly applied in a sample containing free and one level-trapped charges hitherto in equilibrium.

Study of Imperfections in Mercury Iodide by the Thermally Stimulated Currents Method

Thermally stimulated current and nuclear measurements are reported for HgI2 crystals. The energy of the imperfections is determine from the initial rise of the TSC current for each of the imperfections observed in this measurement. The different structures are isolated by using the thermal cleaning method. The retrapping mode, whether fast or slow, had been deternined by comparing the initial rise energy to the Bube and Grossweiner's energies which apply to fast and slow retrapping centers respectively. For slow retrapping centers the capture cross section and the detrapping times are calculated whereas for fast retrapping centers the product of imperfection concentration and carrier life time is computed. Using this approach an effort is made to identify the imperfections which particularly affect the operation of nuclear detectors made of this crystal. In the present crystal we find two such levels.

Transient and steady state space-charge-limited current in CdTe

The current response to pulsed and constant external bias is studied on wafers of semi-insulating CdTe crystals which are provided with contacts of aquadag. Space-charge-limited current (sclc) is observed. The magnitude of the large-signal turn-on transient current suggests that electrons are the injected carriers. The time constant of the current decay yields a value of the electron trapping time τ + of ∼ 10 −7 sec. Double pulse experiments provide the value of the detrapping time τ D and its activation energy E t = 0.65 eV. From these quantities, a trap concentration N t = 0.85 × 10 12 cm −3 and a trapping cross-section σ = 3.8 × 10 −13 are deduced. The results are compared with the d.c. characteristic of the devices and with data reported in the literature. Good agreement is obtained, indicating the usefulness of transient and d.c. sclc for characterizing semi-insulating materials and the injecting nature of the contacts.

Trapping effects in pulsed photoconductivity of anthracene crystals

Photocurrent transients in anthracene are analysed with reference to the drift of the excess free carriers wich are subject to trapping and detrapping by monoenergetic traps as they pass through the crystals. The shape of the pulses is dependent on the trap parameters,τd (mean detrapping time) andτin (mean trapping time connected with the concentration of the trapping centres), as well as on the microscopic transit time. The experimental data are in good agreement with theoretical calculations and confirm the existence in anthracene of monoenergetic trapping levels for holes with energyE≈0.70 eV.

Transient Currents in Semi-Insulating CdTe Characteristic of Deep Traps

The transport of charge in single crystals of semi-insulating cadmium telluride has been studied under the influence of deep trapping and subsequent thermal release of carriers from traps. Trapping and detrapping times for electrons and holes are determined directly from the shape of the transient response of surface-barrier devices to alpha particles. Hole-trapping times are 60 nsec; electron-trapping times range from 30 to 90 nsec. The measurement of the detrapping times from −60° to 60° C indicates that an electron trap with an activation energy of 0.59±0.04 eV can exist below the conduction band in semi-insulating material. The present measurement of drift mobilities, trap densities, and trapping cross sections does not require observation of a transit time. Furthermore, pulse analysis is not limited to times less than the transit time, a restriction in previous drift measurements. The theoretical response which accounts for both the trapping and detrapping of charge was derived by solving the kinetic differential equations which represent charge conservation. This extends the small-signal theory of transient currents in insulators to times beyond the transit time. Excellent agreement is found between experimental traces and theoretical shapes using a single-level trap model. Limiting cases which allow convenient measurement of the material parameters are described. In addition, the energy required to form an electron-hole pair is redetermined and found to be 4.9±0.1 eV.