Dark Current Transport Research Articles

On the current transport mechanism in metal–semiconductor–metal structured CdZnTe radiation detectors

The current–voltage properties of CdZnTe radiation detectors are closely related to the device’s performance. In this paper, the mechanisms of the dark current transport in metal–semiconductor–metal structured CdZnTe radiation detectors are investigated by numerically simulating the static working states in a wide bias range. Simulated current–voltage characteristics are consistent with the experimental results. The major current components in various bias and barrier conditions are determined by comparing different current transport theories based on computed device working states. For Schottky contacts with large barrier heights, the reverse-biased cathode determines the current in the low bias range, where the current is limited by carrier generation and diffusion in the depletion layer. Otherwise, the current is restricted by the high-resistivity bulk material if the barrier is relatively small. However, when the device is fully depleted of electrons in the high bias range, injected holes from the forward-biased anode are the dominating mobile charges and produce a rapid current increase. For ohmic contacts where the electron barrier height is smaller than the hole barrier height, the current–voltage curves deviate from the linear relationship in the high bias range due to the electron space-charge-limited current. From the simulation results, a double-rectifying electrode configuration is proposed, which can significantly reduce the dark current of CdZnTe radiation detectors.

Optimization of Long-Wavelength InAs/GaSb Superlattice Photodiodes With Al-Free Barriers

We demonstrate long-wavelength InAs/GaSb superlattice photodiodes grown on InAs substrates in a production-scale metal-organic chemical vapor deposition system. An aluminum-free heterojunction scheme was adopted where mid-wavelength superlattices served as electron barriers alongside an ${n}$ -type long-wavelength absorber. Both electrical modeling and experimental characteristics suggest that suppressed dark current and unimpeded transport of photo-generated carriers can be both achieved with engineered barrier layers. Under 77 K and a reverse bias of −0.1 V, the optimized device has shown a cut-off wavelength around 12 $\mu \text{m}$ , a blackbody responsivity of 1.01 A/W, a dark-current density of 5 $\times \,\,10^{-4}$ A/cm2 and a dark-current limited specific detectivity over 1 $\times \,\,10^{11}$ cm $\cdot \sqrt {\textrm {Hz}}$ /W.

Analysis of temperature-dependent dark current transport mechanism for GaAs-based blocked-impurity-band (BIB) detectors

Temperature-dependent dark current transport mechanism has been analyzed for GaAs-based blocked-impurity-band detectors. Device structure, processing steps and physical models are described in detail. In this work, the analysis of dark current transport mechanism has been greatly simplified by electron drift current related discussion based on the assumption that the diffusion current and the generation–recombination current as two dominating components of dark current in the absorbing layer can be equivalent to the electron drift current in the blocking layer. After confirming this assumption, our discussion focuses on why dark current is a monotonically increasing function of operation temperature for a fixed anode bias. Our results reveal that from dark current suppression point of view, low donor doping level and low operation temperature are preferred.

Temperature dependence of electrical properties in Cu0.5Ag0.5InSe2/Si heterostructure

The polycrystalline Cu0.5Ag0.5InSe2 thin film was deposited on mono-crystalline n-Si wafer by sequential thermal evaporation of elemental sources. p-Cu0.5Ag0.5InSe2/n-Si heterojunction diode was fabricated and the current–voltage characteristics of the diode at various temperatures were investigated to determine the main diode parameters and dark current transport mechanism. The studied diode structure showed a rectifying behavior with a barrier height of 0.63 eV at room temperature. Series and shunt resistance values were calculated by parasitic resistance relations in high bias regions. Considering the ideality factor values between 1.7 and 2.8, dominant transport characteristics were detailed for forward and reverse voltages. The analysis of the forward current–voltage behavior reveals field emission can be the possible current conduction mechanism. At the reverse bias region, around 101 number of tunneling step and about 105 density of traps were found to act a role in the process in leakage current flow.

Study of dark current for LWIR HgCdTe detectors with a graded doped junction

Long-wavelength infrared detection is the cutting-edge technique for third-generation infrared remote sensing. However, the performance of long-wavelength infrared detector still limits to the dark current characteristics and associated noise behavior. This work investigates the performance of doping changed long-wavelength planar HgCdTe infrared photodiode to better understand the dark current transport of ion implanted devices. The measured dark current characteristics exhibit strong correlation with the p-type doping concentration which are explained well by established numerical models. Considering the interaction of Hg-interstitial and Hg-vacancy, a graded junction simulation architecture is adopted and the accuracy of this structure is proved by the simulation results in comparison with the experiments. Our results show that the graded n-type region doping with Gaussian distribution more accurately reflect the tunneling current under large reverse bias. Besides, it is the different doping concentration inducing significant effect on band-to-band tunneling which leads to I-V characteristics changing. The work described in this letter provides theoretical basis for the dark current formation of doping changed long-wavelength HgCdTe detectors.

Influencing Sources for Dark Current Transport and Avalanche Mechanisms in Planar and Mesa HgCdTe p-i-n Electron-Avalanche Photodiodes

In this paper, both planar and mesa homojunction p-i-n HgCdTe electron-avalanche photodiodes (e-APDs) are fabricated and investigated to better understand the dark current transport and electron-avalanche mechanisms of the devices and optimize the structures. The experiment results are agreed well by simulated ${I}$ – ${V}$ characteristics based on established numerical models. Our results show that the multiplication region fabrication process leads to enormous characteristic difference between planar and mesa diodes. Shockley–Read–Hall and trap-assisted tunneling current are the main components of dark current for the planar/mesa junction under low bias voltage, and dark current is mainly influenced by band-to-band tunneling and avalanche current when higher reverse bias is added. In addition, we found that the difference between the uniformity of the electric field distributions in multiplication regions is the primary reason for the differences of the dark current. It was also proved that the dark current of planar e-APDs is dramatically affected by the junction corners, and mesa e-APDs dark current is found to be greatly dependent on the multiplication region thickness. Our work provides a great deal of theoretical basis for dark current formation and the avalanche mechanism of HgCdTe e-APDs.

Dark current transport mechanism associated with acceptor concentration in GaAs-based blocked-impurity-band (BIB) detectors

Dark current transport mechanism associated with acceptor concentration in GaAs-based blocked-impurity-band (BIB) detectors has been investigated. Device structure, numerical models and simulation techniques are described in detail. By careful model and parameter calibration, the numerical simulation is completely consistent with the analytical calculation, proving the validity of simulation methods. Our results reveals that the carrier-transport modes of GaAs-based BIB detectors can be classified into two categories (i.e., electron current and hopping current), and the hopping current can be neglected compared with the electron current. Besides, it is demonstrated that the dark current of GaAs-based BIB detector is dominated by the drift–diffusion current and the generation-recombination current, and the both current components are monotonically decreasing functions of the acceptor concentration.

Comparison of electrical properties of x-ray detector based on PbI2 crystal with different bias electric field configuration

In this work we report the dark current, photocurrent and carrier transport properties of the x-ray detector based on lead iodide (PbI2) crystal. The detectors were built with two orthogonal directions configuration as the bias electric field parallel to the crystallographic c-axis E//c and perpendicular to the c-axis E⊥c. It presents the electrical anisotropy including resistivity, dark current, carrier transport and x-ray induced photoelectricity properties with considering the configuration of bias field and c-axis. A mechanism of carrier scattering effect from anisotropic lattice structure, dislocation and stacking fault could be mainly responsible for this anisotropy property in PbI2 crystal. All the results indicate that the crystal orientation will be taken into account when we design and fabricate the x-ray detectors based on PbI2 crystals or films.

Deep traps and photo-electric properties of p-Si/MgO/n-Zn1−xMgxO heterojunction

In the paper, the photoluminescence (PL) measurements, current–voltage–temperature (I-V-T) measurements, space charge techniques (C-V and deep level transient spectroscopy (DLTS)), and photocurrent spectral characteristics have been applied to investigate defects in p-Si/MgO/ n-Zn1−xMgxO heterojunction (HJ). The HJ structure was grown on p-type Si (111) substrate with resistivity equal to 0.1 Ω cm by the plasma-assisted molecular beam epitaxy technique. A radio-frequency cell was used for the generation of oxygen plasma. PL spectrum let us determine the Mg content ∼10%. Besides the excitonic Zn0.9Mg0.1O line, the PL spectrum also contains green and yellow emission bands indicating the presence of defect states in the investigated structures. I-V measurements reveal the rectifying properties of the HJ and the current thermally activated with a trap with the activation energy equal to 0.42 eV. DLTS studies yield the majority trap of the activation energy 0.42 eV, confirming the result obtained from the I-V measurements. It was found that the defects related to this trap have a point like behaviour. A spectral characteristic of the photocurrent shows that the p-Si/MgO/n-Zn1−xMgxO HJ may be applied as a photodiode operating within the wavelength range of 300 nm-1100 nm. The dark current transport and photocurrent spectrum were explained using the Anderson model of a HJ.

Simulation of dark current and dark current-induced background photons in the Thomson scattering X-ray source

A model of dark current generation in the photocathode radio-frequency (RF) gun is established in the Thomson scattering X-ray source, and dark current transport and losses along the beamline are simulated. A velocity bunching cavity is added between the RF gun and the first linac to achieve the longitudinal compression of the photoelectron bunches. Given the longitudinal acceleration and the transverse focusing of the bunching cavity, the dark current electrons with bunching are approximately three times more than those without bunching, and this condition aggravates the harm to the operation of the photoinjector. Numerous dark current electrons around the electron–laser interaction section hit against the pipe inner wall and two laser focusing mirrors, producing a large number of background photons. A simulation of the bremsstrahlung process using an MCNP code is presented, showing that the background photon yield is less than 2.1% of the scattering photon yield, which is acceptable for our application.

Dark Current Transport and Avalanche Mechanism in HgCdTe Electron-Avalanche Photodiodes

HgCdTe electron avalanche photodiodes (e-APDs) have been widely used for low-flux and high-speed application. To better understand the dark current transport and electron-avalanche mechanism of the devices and optimize the structures, we performed accurate numerical simulations of the current–voltage characteristics and multiplication factor in planar homojunction (p-i-n) HgCdTe APDs. Based on the Okuto–Crowell avalanche model, an efficient physical model has been obtained by concerning the major generation-recombination processes, such as trap-assisted tunneling and band-to-band tunneling (BBT) recombination. Simulated current–voltage characteristics were in good agreement with available data in the literature, which demonstrates the validity of the proposed model. The origins of dark current in high reverse voltages are jointly dominated by BBT and the avalanche mechanism. It is proved to be effective for reducing BBT by improving the uniformity of the electric field distribution across the junction. The electric performance of p-i-n e-APD can be improved by optimizing the APD structure, such as eliminating the sharp corners of junctions, light doping, and the appropriate thickness in multiplication region. Our works provide a good deal of insight into the fundamental carrier transport processes involved in HgCdTe e-APDs.

Influence of temperature on tunneling-enhanced recombination in Si based p—i—n photodiodes

We investigate the dominant dark current transport mechanism in Si based p—i—n photodiodes, namely, BPW 21R, SFH 205FA and BPX 61 photodiodes in the temperature range of 350 to 139 K. The forward current—voltage characteristics of these photodiodes are explained via the tunneling enhanced recombination model, which gives a quantitative description of the electronic mechanism in the p—i—n junction photodiodes. The observed temperature dependence of the saturation current and the diode ideality factor of these devices agree well with theoretical predictions; the analysis also indicates the importance of doping for enhancement of tunneling. The present study will be helpful in applying the devices at low temperature ambience.

Current transport characteristics of pSe-nMoSe2heterojunction diode

The characteristics of heterojunction diode p Se- n MoSe 2 fabricated from thermally evaporated p -Se films on n -type Molybdenum diselenide (MoSe 2 ) grown by direct vapour transport (DVT) technique have been examined by using current-voltage measurements. To investigate the dark current transport mechanism in p Se- n MoSe 2 heterojunctions the current-voltage characteristics were measured in the temperature range 100–300 K. The prepared diode shows a rectification ratio of the order of 10 3 within the range –2 to 2 V. A multi-step tunnelling model was used to analyze the I - V - T characteristics of the prepared device. The activation energy determined from the saturation current was about 1.16 eV.

Current flow mechanism in Cu2O/p-Si heterojunction prepared by chemical method

Cu2O thin films were chemically deposited on single-crystal p-Si substrates to form Cu2O/p-Si heterojunctions. The structure of the Cu2O films was analysed by x-ray diffraction spectroscopy and UV–Vis–NIR transmittance spectra. In order to investigate the dark current transport mechanism in Cu2O/p-Si heterojunctions the current–voltage characteristics were measured in the temperature range 120–320 K and capacitance–voltage characteristics at a high frequency of ∼1 MHz at room temperature. The I–V–T characteristics revealed that the forward current was determined by trap-assisted multi-step tunnelling. The activation energy determined from the saturation current and the junction built-in potential determined from the capacitance–voltage characteristics were about 0.18 eV and 1.10 V at room temperature, respectively.

Specific features of light current-voltage characteristics of p-i-n structures based on amorphous silicon in the case of the tunnel-drift mechanism of dark current transport

Current-voltage (I–V) characteristics of p-i-n structures based on amorphous silicon (α-Si:H) with small hole diffusion lengths (shorter than the thickness of the i-layer of a p-i-n structure) have been experimentally studied with and without illumination. It is shown that forward I–V characteristics of structures of this kind can be described by a dependence inherent in diodes, with a diode ideality factor two-three times the maximum value of 2, theoretically predicted for generation-recombination currents in p-n junctions. The dark current is always substantially lower than the photocurrent in a cell biased with a voltage approximately equal to the opencircuit voltage of the photocell. Dark currents cannot contribute to the I–V characteristic under illumination. The photocurrent decreases with increasing photovoltage at a bias lower than the open-circuit voltage because of a decrease in the collection coefficient and the increasingly important role of back diffusion of electrons into the p-contact, rather than as a result of the dark injection. In the case of biases exceeding the open-circuit voltage, back diffusion becomes the predominant component of the current.

Metal-Semiconductor-Metal (MSM) Photodetectors Based on Single-walled Carbon Nanotube Film-GaAs Schottky Contacts

ABSTRACTWe experimentally study the dark and photocurrent in metal-semiconductor-metal (MSM) photodetectors based on single-walled carbon nanotube film Schottky contacts on GaAs. We find that above ∼260°K, thermionic emission of electrons with a barrier height of ∼0.54 eV is the dominant dark current transport mechanism. Furthermore, MSM devices with CNT film electrodes exhibit a higher photocurrent-to-dark current ratio while maintaining a comparable responsivity relative to control devices. This work demonstrates that nanotube films can be integrated as Schottky electrodes in conventional semiconductor optoelectronic devices.

Effect of p/i interface layer on dark J-V characteristics and Voc in p-i-n a-Si solar cells

Dark current-voltage (J-V) characteristics of p-i-n a-Si solar cells are studied to clarify the effect of the p/i interface layer on the open-circuit voltage (Voc). It is shown that the recombination current in the p/i interface region is predominant in the a-Si solar cells with an i-layer thickness of less than 200 nm and has a great effect on the dark-current transport in the solar cells. It is also shown that the current transport of the cells with the p/i interface layer of constant band gap of a thickness of more than 14 nm is effectively the same as that of a cell in which the band gap of the whole i layer is equal to that of the interface layer. A model in which the dark current of the cell consists of the recombination current at the p/i interface and the recombination current in the bulk region of the i layer is proposed. Using this model, the Voc degradation for the a-Si solar cells with the p/i interface layer is shown to be due to the defects generated in the i layer.

Electrical properties of HgCdMnTe p− n junctions

The p− n junctions have been made in semimagnetic HgCdMnTe compounds within a wide molar composition range. Electrical properties of the junctions have been determined at temperatures from 2 to 300 K and in magnetic fields up to 6 T. The dark current transport mechanism in the junctions has been analysed. A comparison of the results obtained with those of HgCdTe p− junctions with similar long-wavelength photovoltaic cut-off has been discussed.

The correlation between current transport mechanisms and etch features in Au-CdS single-crystal Schottky diodes

The effects of surface features on chemically etched (0001) planes of single-crystal CdS on the dark current transport mechanisms in Schottky diodes prepared on them by vacuum evaporation of gold are investigated. By combining scanning electron microscope (SEM) observations of these surface features with measurements of current-voltage (I-V) characteristics, the diode behaviour of the Schottky barriers is demonstrated to be strongly influenced by the surface topography. The temperature dependence of I-V capacitance-voltage (C-V) and photo-electric measurements are also studied in an attempt to correlate the junction parameters with surface features.

Dark current transport mechanism of p-i-n hydrogenated amorphous silicon diodes

The dark current-voltage characteristics of p-i-n hydrogenated amorphous silicon diodes with various thicknesses of the intrinsic layer (i-layer) (770–9300 Å) are systematically investigated. The magnitude of the forward current is found to be independent of thickness of the i layer, which is obviously against the simple conventional junction theory. It has been demonstrated through various experiments that the forward current of amorphous p-i-n diodes is limited by a layer thinner than 770 Å, possibly being the p/i interface or a narrow zone of the i layer.