Steady-state Dark Current Research Articles

Ultrahigh Hot Carrier Transient Photocurrent in Nanocrystal Arrays by Auger Recombination.

In this report, we show that a new mechanism for carrier transport in solution-processed colloidal semiconductor nanocrystal arrays exists at high excitation intensity on ultrafast time scales and allows for facile intrinsic transport between as-prepared nanocrystals over long distances. By combining a high speed photoconductive switch with an ultrafast laser excitation in a sub-40 ps photoconductor, we observed transient photocurrents with peak densities of 3 × 104 - 106 mA/cm2 in self-assembled PbSe nanocrystals capped with long native oleic acid ligands. The ratio between the transient photocurrent peak and the steady-state dark current is 10 orders of magnitude. The transient mobility at the peak current is estimated to range between 0.5-17.5 cm2/(V s) for the various nanocrystal sizes studied, which is 6 to 9 orders of magnitude higher than the dark current steady-state mobility in PbSe, CdSe, and CdTe nanocrystals capped with native ligands. The results are analyzed using a kinetic model which attributes the ultrahigh transient photocurrent to multiple photogenerated excitons undergoing on-particle Auger recombination, followed by rapid tunneling at high energies. This mechanism is demonstrated for a wide range of PbSe nanocrystals sizes (diameters from 2.7 to 7.1 nm) and experimental parameters. Our observations indicate that native ligand-capped nanocrystal arrays are promising for optoelectronics applications wherein multiple carriers are photoinjected to interband states.

Dark current mechanisms in amorphous selenium-based photoconductive detectors: an overview and re-examination

The transient and steady-state dark current behaviors in various amorphous selenium (a-Se) detectors are analyzed by developing mathematical models considering all possible mechanisms (e.g., carrier depletion, thermal generation, and carrier injection from electrodes) and charge carrier transport properties of a-Se. The theoretical models are validated by comparing them with recently published measured transient and steady-state dark currents in various a-Se detectors. The fittings of the models with the experimental results reveal various important material and device properties such as the optimum trap depth and concentrations in the blocking layers for faster stabilization and lower dark current, and the effective barrier height between the metal/a-Se contacts. The thermal generation current is significantly higher in avalanche detectors (at extremely high fields) than that in conventional detectors. The effective blocking layers for both holes and electrons are necessary for the minimum level of the dark current. The minimum dark current is determined by the thermal generation current. The thermal generation current can be lowered by reducing the mid gap defect states.

Transient and steady-state dark current mechanisms in polycrystalline mercuric iodide X-ray imaging detectors

A theoretical model for describing bias-dependent time transient and steady-state dark current behaviors in polycrystalline mercuric iodide (poly-HgI2) based X-ray image detectors is developed. The model considers carrier injection from the metal electrode, bulk carrier depletion process, and bulk thermal generation current from the mid-gap states. The transient dark current is mainly determined by the initial carrier depletion process. At a very low applied field (less than 0.05V/μm), the steady-state dark current is almost equal to the bulk thermal generation current. However, the injection current increases sharply with increasing the applied field. The steady-state dark current in poly-HgI2 detectors at normal operating field (~ 1V/μm) is mainly controlled by the Schottky emission of electrons from the metal/HgI2 contact. The fitting of the physics-based model to the experimental results estimates the effective barrier height and interface defect states for injecting electrons from the metal to poly-HgI2 layer in various poly-HgI2 detectors.

Transient and steady-state dark current mechanisms in amorphous selenium avalanche radiation detectors

A theoretical model for describing bias-dependent transient and steady-state behaviors of dark current in amorphous selenium (a-Se) avalanche detector structures has been developed. The analytical model considers bulk thermal generation current from mid-gap sates, transient carrier depletion, and carrier injection from the electrodes incorporating avalanche multiplication. The proposed physics-based dark current model is compared with the published experimental results on three potential a-Se avalanche detector structures. The steady-state dark current is the minimum for the structures that have effective blocking layers for both holes and electrons. The transient decay time to reach a plateau decreases considerably with increasing electric field.

Dark current mechanisms in stabilized amorphous selenium based n-i detectors for x-ray imaging applications

The dark current behavior under operating bias is one of the important selection criteria for an x-ray photoconductor to be usable in a practical x-ray image detector. The authors have developed an analytical model for describing the transient and steady-state behavior of dark current in n-i-type amorphous selenium (a-Se) detectors by considering carrier injections from the metal contacts and thermally generated carriers. It has been found that the thermal generation current is almost two orders of magnitude smaller than the total steady-state dark current in n-i-type a-Se detectors. The main source of dark current is the injection of holes from the metal/n-layer interface which is described by the diffusion theory. The hole injection from the metal depends on the blocking layer (n-layer) thickness, the concentration of trap centers in the blocking layer, the characteristic carrier release time, and the effective barrier height. The fitting of the first principles model with the experimental results estimates the concentration of deep hole trap center in the n-layer, the trap depth from the valence band edge, and the effective barrier heights for the injecting carriers. The electron injection varies with the work function of the contact metal.

Electronic transport in films of colloidal CdSe nanocrystals

We present results for electronic transport measurements on large three-dimensional arrays of CdSe nanocrystals. In response to a step in the applied voltage, we observe a power-law decay of the current over five orders of magnitude in time. Furthermore, we observe no steady-state dark current for fields up to 10^6 V/cm and times as long as 2x10^4 seconds. Although the power-law form of the decay is quite general, there are quantitative variations with temperature, applied field, sample history, and the material parameters of the array. Despite evidence that the charge injected into the film during the measurement causes the decay of current, we find field-scaling of the current at all times. The observation of extremely long-lived current transients suggests the importance of long-range Coulomb interactions between charges on different nanocrystals.

The lock-on effect in electron-beam-controlled gallium arsenide switches

The authors analyze the lock-on effect, which is the inability of photoconductive or electron-beam-controlled semiconductor switches to recover to their initial hold-off voltages following the application of the laser or electron-beam pulse, if the applied voltage exceeds a certain value. For GaAs this threshold voltage corresponds to average electric fields in the range from 4 to 12 kV/cm. Experimental results on semi-insulating GaAs switches indicate that the corresponding lock-on current after e-beam irradiation is identical with the steady-state dark current. The highly resistive state of the switch before e-beam irradiation is shown to be a transient phase towards the much lower steady-state dark resistance, with a duration which depends on the impurity content of the switch material and the applied voltage. The irradiation of the GaAs samples with electrons or photons causes an acceleration of this temporal evolution; at sufficiently high laser or e-beam intensities, lock-on of the dark current after termination of the driving ionization source is observed. Based on the experimental results, a model is developed which describes the lock-on effect in terms of double injection and carrier trapping in deep intraband levels. The model explains the major characteristics of the lock-up effects and is supported by the qualitative agreement of the calculated current-voltage curves with the experimental data. >

Some Electrical Properties of Amorphous Selenium Films

The dark current and photocurrent characteristics of amorphous selenium and arsenic-selenium alloy films have been investigated. One millisecond after the application of an electric field, the dark current (neglecting capacitance charging) is several orders of magnitude higher than the steady-state dark current. A hysteresis effect is observed in the dark volt-ampere curves with the descending voltage branch exhibiting an exponential volt-ampere relationship. The voltage of a charged Se plate in the dark discharges logarithmically with time as anticipated from the exponential volt-ampere relationship. Upon admitting air to a freshly evaporated Se plate, a negative surface potential of 150 mv is observed. The phenomenon of ``fatigue'' (i.e., the temporary increase in dark current following an exposure to illumination) obtained with highly absorbed radiation indicates surface barrier effects exert a large measure of control over dark currents. Photogenerated holes and electrons in Se films are trapped with trapping parameters such that the hole range/unit field is on the order of 10−8 cm2/v, and the electron range is less than 10−9 cm3/v. The addition of As to the Se greatly increases the electron effective lifetime. Trapped holes are eliminated in the dark exponentially with time through a process having an activation energy of 0.77 ev. A 40-mv negative surface photovoltage is observed in Se films upon illumination. This voltage may be accounted for by hole diffusion away from the surface. The addition of As to Se extends the spectral response into the red end of the spectrum, results in the initiation of secondary photocurrent effects, intensifies fatigue effects, and increases the softening temperature of the film.