Development of Nuclear Radiation Detectors by Use of Thick Single-Crystal CdTe Layers Grown on (211) p +-Si Substrates by MOVPE

Development of an electron-collecting-type pixel array by use of an epitaxially grown thick single-crystal CdTe layer on p ⁺-Si substrate is discussed. To achieve such an array with an n-CdTe/p-like CdTe/p ⁺-Si heterojunction diode structure, charge transport at the p-like CdTe/p ⁺-Si heterointerface was studied. It was confirmed that ohmic conduction via holes occurs at this interface. A single-element detector was then fabricated by growth of 40 μm thick undoped p-like CdTe then 5 μm thick n-CdTe layers on the p ⁺-Si substrate. Rectification by the diode detector was good, and its energy-resolving capability was demonstrated by detection of gamma peaks from the ²⁴¹Am source, thus confirming the feasibility of using this structure for fabrication of an electron-collecting-type array.     

Observation of electron traps in electrochemically deposited CdTe films

The I–V and C-V data of Schottky devices formed on electrodeposited n-CdTe films are interpreted to determine the principle trap energy and density. The observed trap is an electron trap located at 0.55 eV below the conduction band with a density of ～7 × 10¹⁵/cm³. This correlates well with the values reported for CdTe prepared by different methods. Nickel is found to be an injecting contact to electrodeposited CdTe films. Au/n-CdTe barrier height is determined to be 0.75–0.85 eV for Schottky devices.     

Influence of deep traps on current transport in Pd-p(n)-CdTe structures

The current-voltage characteristics and photovoltage of Pd-p(n)-CdTe structures and changes in them produced by pulsed hydrogen treatment have been investigated. The current transport in Pd-n-CdTe structures [I∼exp(αV)] is found to be linked with double injection of carriers occurring as a result of their capture at trapping centers that are uniformly distributed over energy. The semiconductor regime of double injection with I∼V ² is important for Pd-p-CdTe structures. A series of deep trapping centers, including those in the interval 0.75–0.83 eV, is responsible for the extended relaxation of the photovoltage and dark current after a hydrogen pulse (H₂). Fiz. Tekh. Poluprovodn. 33, 492–493 (April 1999)     

Effect of a rear contact on the electrical properties of the CdS/CdTe-based thin-film solar cells

The comparative study of dark current-voltage and capacitance-voltage characteristics of the CdS/CdTe/Cu/Au and CdS/CdTe/ITO thin-film solar cells is carried out. The physical properties of the p ⁺-CdTe/n ⁺-ITO rear contact are experimentally determined for the first time.     

Postgrowth Annealing of CdTe Layers Grown on Si Substrates by Metalorganic Vapor-Phase Epitaxy

Annealing conditions of CdTe layers grown on Si substrates by metalorganic vapor-phase epitaxy were studied. Typically, 3-μm-thick n-type (211) CdTe layers were annealed for 60 s in flowing hydrogen at atmospheric pressure by covering their surfaces with bulk CdTe wafers. At annealing temperatures above 700°C, improvement of crystal quality was confirmed from full-width at half-maximum values of double-crystal rocking-curve measurements and x-ray diffraction measurements. Photoluminescence measurements revealed no deterioration of electrical properties in the annealed n-CdTe layers. Furthermore, annealing at 900°C improved the performance of radiation detectors with structure of p-like CdTe/n-CdTe/n ⁺-Si substrate.     

Modification of the spectrum of electronic states in polycrystalline p-CdTe as a result of annealing in Cd vapors or natural aging

Electrical properties, steady-state photoconductivity, and kinetics of photoconductivity have been studied in the samples of polycrystalline p-CdTe (as-grown by the method of low-temperature synthesis from thoroughly purified components; subjected to subsequent annealing in Cd vapors; and aged at room temperature). It is shown that electrical properties of as-grown p-CdTe are controlled by a complex defect with the level at E _v + 0.16 eV; the latter transforms into the level E _v + 0.25 eV as a result of annealing in Cd vapors and aging of the samples. In addition, the deep centers with the levels E _v + 0.6 eV and E _v + 0.86 eV were detected; these centers govern the decay of the photocurrent.     

Ultralow-Dark-Current CdHgTe FPAs in the SWIR Range at CEA and Sofradir

We report the first results of work carried out at CEA and Sofradir to build ultralow-dark-current focal-plane arrays (FPAs) in the short-wave infrared range (SWIR) for space applications. These FPAs are designed to detect very low flux in the 2-μm wavelength range. To this end, Sofradir has designed a source follower per detector read-out circuit (ROIC, 384?×?288, 15?μm pitch). This ROIC has been hybridized on different HgCdTe diode configurations processed at CEA-LETI, and low-flux characterizations have been carried out at CEA-IRFU at low temperature (from 60?K to 160?K). Both ion-implanted p/n and n/p diodes have been evaluated. The metallurgical nature of the absorbing layer has also been examined, and both molecular-beam epitaxy (MBE) and liquid-phase epitaxy (LPE) have been applied. Dark-current measurements are discussed in comparison with previous results from the literature. State-of-the-art dark currents are recorded for temperatures higher than 120?K. At temperatures lower than 100?K, the decrease in dark current flattens out for both technologies. In this region, currents between 0.4?e^–/s/pixel and 0.06?e^–/s/pixel are reported.     

Effect of features of the technology of polycrystalline CdTe growth on the conductivity and deep level spectrum after annealing

The conductivity, morphology, and deep levels in polycrystalline CdTe are studied. Undoped p-CdTe is grown from the vapor phase by low-temperature methods of direct Cd and Te chemical reaction and CdTe vacuum sublimation at P _min. Chlorine-doped CdTe is also grown. The resistivity of the grown samples is ～105–109 Ω cm. After annealing in liquid cadmium or in cadmium vapor at ～500°C, the conductivity type changes, the free-carrier concentration in the undoped and doped samples increases to 4 × 10¹⁵ and ～2 × 10¹⁶ cm^?3, respectively. For all samples, a defect ground level of ～0.84 eV and continuous background are observed in DLTS spectra after annealing. A correlation between the primary-defect and free-carrier concentrations in undoped and doped CdTe is observed. Chlorine is a main residual impurity in the undoped samples. It is assumed that the defect is a complex including chlorine and observed structural defects in CdTe.     

Performance of 12-μm- to 15-μm-Pitch MWIR and LWIR HgCdTe FPAs at Elevated Temperatures

Infrared (IR) focal-plane arrays (FPAs) with higher operating temperatures and smaller pitches enable reduced size, weight, and power in infrared systems. We have characterized a large number of medium- and long-wavelength IR (MWIR and LWIR) FPAs as a function of temperature and cutoff wavelength to determine the impact of these parameters on their performance. The 77-K cutoff wavelength range for the MWIR arrays was 5.0 μm to 5.6 μm, and 8.6 μm to 11.3 μm for the LWIR. The dark currents in DRS’s high-density vertically integrated photodiode (HDVIP)^® FPAs (based on a front-side- illuminated, via-interconnected, cylindrical-geometry N+/N/P architecture) are dominated by Auger-7 recombination from 120 K to 200 K for the MWIR and 70 K to 100 K for the LWIR. In these temperature ranges the FPA operability is generally limited not by dark current defects but by noise defects. Pixels with high 1/f noise should produce a tail in the root-mean-square (rms) noise distribution. We have found that the skewness of the rms noise distribution is the simplest measure of an array’s 1/f noise, and that the rms noise skewness typically shows little variation over these temperature ranges. The temperature dependence of the defect counts in normal arrays (wet etched prior to CdTe interdiffusion) increases as n _i, while nonstandard arrays (ion milled or plasma etched prior to CdTe interdiffusion) can have high 1/f noise and defect counts that vary as n _i ² . Our models indicate that, if the dominant dark current is due to diffusion, then the 1/f noise varies as n _i ² , whereas if depletion current dominates, then the 1/f noise varies as n _i. Systemic 1/f noise is not an issue for DRS’s standard MWIR FPAs at 110 K to 160 K, or for standard LWIR FPAs at 77 K to 100 K.     

Effect of irradiation with gamma-ray photons on the charge-transport mechanism in n-CdS/p-CdTe heterostructures

Effect of irradiation with γ-ray photons on the mechanism of charge transport in an n-CdS/p-CdTe heterostructure is considered. It is shown that the forward current-voltage characteristic of an n-CdS/p-CdTe heterostructure before and after irradiation is described by two exponential dependences: I = I ₀₁exp(qV/C ₀₁ kT) and I = I ₀₂exp(qV/C ₀₂ kT). It is found that, in the first portion of the current-voltage characteristic, the current is limited by thermoelectronic emission while, in the second portion, the current is limited by recombination of nonequilibrium charge carriers in the electrically neutral portion of a CdTe_{1 ? x} S _x alloy at the n-CdS/p-CdTe heteroboundary. Anomalous dose dependences of parameters of the n-CdS/p-CdTe heterosystem are attributed to a variation in the degree of compensation of local centers at the CdS-CdTe_{1 ? x} S _x interface and in the CdTe_{1 ? x} S _x layers in relation to the dose of irradiation with γ-ray photons.     

CdTe/ZnTe/GaAs Heterostructures for Single-Crystal CdTe Solar Cells

Amorphous CdS/single-crystal CdTe solar cells were grown on GaAs substrates by metalorganic chemical vapor deposition. The structures of the films and the electrical properties of the devices were characterized. Highly conducting arsenic-doped ZnTe was grown on GaAs(100) substrates as the buffer layer for CdTe growth. By use of a ～30-nm ZnTe buffer layer, a p-CdTe film with a doping level of ～5×10¹⁶ cm^?3 was achieved. The hole concentration of p-CdTe increased with increasing VI/II ratio under a high As concentration during growth. From temperature-dependent Hall transport measurements, the ionization energy of the As acceptor in the p-CdTe was estimated to be approximately 88 meV. Ohmic behavior of the junctions between CdTe/ZnTe and ZnTe/GaAs was also confirmed. The solar cell performance of this structure, for example an open circuit voltage of 0.63 V, could be improved if the crystal quality of the CdTe film is optimized and the dislocation density of the CdTe film is minimized.     

Irradiation-Induced Defects in Cd0.9Zn0.1Te:Al

The effects of gamma-irradiation, produced by a ⁶⁰Co source with a dose of 2.7?kGy, on defect levels in a Cd_0.9Zn_0.1Te:Al (CZT:Al) crystal were investigated by thermally stimulated current spectroscopy. Nine observed defect levels were identified, and the irradiation-induced variations of trap signatures for these levels, i.e., the thermal activation energy, concentration, and capture cross-section, have been evaluated using simultaneous multiple peak analysis. In addition, the dark-current spectra dominated by the deep donor (E _DD) level have been certified. By fitting plots of the logarithm of dark current ln(I _DC) versus 1/kT, the E _DD level is determined to be 0.554?±?0.001?eV and 0.792?±?0.004?eV before and after gamma-irradiation, respectively. One month after irradiation, the E _DD level moved slightly to 0.782?±?0.003?eV. This result indicates that the E _DD level is closer to the middle of the bandgap of CZT:Al after irradiation, which should be responsible for Fermi-level pinning near the middle of the bandgap, leading to high resistivity. This is consistent with the resistivity variation of CZT:Al at room temperature from the original value of 7.5?×?10⁹?Ω?cm before irradiation to the irradiated value of 5.6?×?10¹⁰?Ω?cm as determined by current–voltage measurements.     

A DLTS study of deep levels in the band gap of textured stoichiometric p-CdTe polycrystals

Deep-level transient spectroscopy (DLTS) was used to identify a set of deep electronic states in the band gap of textured p-CdTe polycrystals whose composition was almost stoichiometric. Four hole traps and two electron traps were observed. It is shown that the deepest hole trap with a level at E _v+0.86 eV corresponds to a prevalent defect in this material. Special features of the line shape in the DLTS spectrum and the logarithmic dependence of population of this level on the duration of the filling pulse correspond to an extended defect related most probably to dislocations at the grain boundaries.     

Photosensitivity of <Emphasis Type="Italic">n</Emphasis>-CdS/<Emphasis Type="Italic">p</Emphasis>-CdTe heterojunctions obtained by chemical surface deposition of CdS

A new technology of chemical surface deposition is developed, and thin CdS films (35–100 nm) on the p-CdTe substrates are obtained. Electrical and photoelectric properties of n-CdS/p-CdTe heterojunctions are studied, and it is shown that the developed method provides high efficiency of photoconversion in the range restricted by the CdTe and CdS band gaps. It is shown that the method of chemical surface deposition of CdS can be used in the design of thin-film n-CdS/p-CdTe.     

The generation-recombination mechanism of charge transport in a thin-film CdS/CdTe heterojunction

An n-CdS/p-CdTe heterostructure is studied. The heterostructure is obtained using the sequential growth of CdS and CdTe layers by electrochemical deposition and closed-space sublimation, respectively. The measured current-voltage characteristics are interpreted in the context of the Sah-Noyce-Shokley generation-recombination model for the depletion layer of a diode structure. The theory quantitatively agrees with the experimental results.     

Electrical,photovoltaic and photosensitivity characteristics of p-ZnTe:N/CdTe:Mg/n-CdTe:I/GaAs for photodiode applications

In this work, heterojunctions of p-ZnTe:N/CdTe:Mg/n-CdTe:I/GaAs for photovoltaic and photodiode application were grown epitaxially by using molecular beam system. Current-voltage profile of the heterojunction device was studied in dark and under various illumination intensities. The obtained photocurrent is found to depend on the light intensity. The main heterojunction parameters, such as shunt and series resistances, the barrier height and the ideality factor were extracted from the current-voltage profile using diverse methods at ambient temperature. Values of the barrier height and the series resistance were obtained from Cheung's functions. A large value of the series resistance causes the non-ideal characteristics of current–voltage measurements. The study of the current-voltage characteristics of high voltage region suggests a predominant space charge limited mechanism. Moderate values of short circuit current and open circuit voltage were obtained through a light intensity of 140 mW/cm², a current and voltages of 0.336 mA and 370 mV, respectively. The high photosensitivity and responsivity for the current under illumination condition suggests that the prepared heterojunction device could be employed as a photodiode sensor.     

High-Efficiency Polycrystalline CdS/CdTe Solar Cells on Buffered Commercial TCO-Coated Glass

Multiple polycrystalline CdS/CdTe solar cells with efficiencies greater than 15% were produced on buffered, commercially available Pilkington TEC Glass at EPIR Technologies, Inc. (EPIR, Bolingbrook, IL) and verified by the National Renewable Energy Laboratory (NREL). n-CdS and p-CdTe were grown by chemical bath deposition (CBD) and close space sublimation, respectively. Samples with sputter-deposited CdS were also investigated. Initial results indicate that this is a viable dry-process alternative to CBD for production-scale processing. Published results for polycrystalline CdS/CdTe solar cells with high efficiencies are typically based on cells using research-grade transparent conducting oxides (TCOs) requiring high-temperature processing inconducive to low-cost manufacturing. EPIR’s results for cells on commercial glass were obtained by implementing a high-resistivity SnO₂ buffer layer and by optimizing the CdS window layer thickness. The high-resistivity buffer layer prevents the formation of CdTe-TCO junctions, thereby maintaining a high open-circuit voltage and fill factor, whereas using a thin CdS layer reduces absorption losses and improves the short-circuit current density. EPIR’s best device demonstrated an NREL-verified efficiency of 15.3%. The mean efficiency of hundreds of cells produced with a buffer layer between December 2010 and June 2011 is 14.4%. Quantum efficiency results are presented to demonstrate EPIR’s progress toward NREL’s best-published results.     

Real-Time In?Situ Monitoring of GaAs (211) Oxide Desorption and CdTe Growth by Spectroscopic Ellipsometry

We describe the growth of CdTe (211)B by molecular beam epitaxy on large-area epiready GaAs (211)B substrates. Prior to CdTe growth, GaAs substrates were thermally cleaned under an As₄ flux. Oxide desorption was verified by in?situ spectroscopic ellipsometry (SE) and reflection high-energy electron diffraction. The use of in?situ SE played a significant role in the study of CdTe-on-GaAs growth and annealing processes. An effective medium approximation (EMA) was used to model the overlayer thickness variation of CdTe epilayers throughout growth and in?situ annealing cycles. A correlation between SE-derived EMA thickness values and surface defect formation mitigation is discussed. All annealed samples (11.5???m to 13???m thick) exhibited excellent crystalline quality with average double crystal rocking curve full-width at half-maximum (FWHM) values of ~60?arcsec.     

Current transport mechanisms of electrochemically deposited CdS/CdTe heterojunction

The dark current transport mechanism in electrochemically deposited n-CdS/p-CdTe thin film heterojunctions is investigated. The forward current measured in the temperature range between 200° and 305°K can be expressed as J_f = J₀(T) exp (AV) and the reverse current can be expressed as $\text{J}{}_{\mathrm{r}}\text{= ?CV exp ?CV[?λ(V}{}_{\mathrm{d}}\text{?V)}{}^{\mathrm{<mtext>?(</mtext><mtext>1</mtext><mtext>2</mtext><mtext>)</mtext>}}\text{]}$. The current mechanisms are consistent with a multi step recombination-tunneling model.     

Similarities of lag phenomena and current collapse in field-plate AlGaN/GaN HEMTs with different types of buffer layers

We make a two-dimensional transient analysis of field-plate AlGaN/GaN high electron mobility transistors (HEMTs) with a Fe-doped semi-insulating buffer layer, which is modeled that as deep levels, only a deep acceptor located above the midgap is included (E_C − E_DA = 0.5 eV, E_C: energy level at the bottom of conduction band, E_DA: deep acceptor's energy level). And the results are compared with a case having an undoped semi-insulating buffer layer in which a deep donor above the midgap (E_C − E_DD = 0.5 eV. E_DD: the deep donor's energy level) is considered to compensate a deep acceptor below the midgap (E_DA − E_V = 0.6 eV, E_V: energy level at the top of valence band). It is shown that the drain-current responses when the drain voltage is lowered abruptly are reproduced quite similarly between the two cases with different types of buffer layers, although the time region where the slow current transients occur is a little different. The lags and current collapse are reduced by introducing a field plate. This reduction in lags and current collapse occurs because the deep acceptor's electron trapping is reduced under the gate region in the buffer layer. The dependence of drain lag, gate lag and current collapse on the field-plate length and the SiN layer thickness is also studied, indicating that the rates of drain lag, gate lag and current collapse are quantitatively quite similar between the two cases with different types of buffer layers when the deep-acceptor densities are the same.