Unipolar barrier structures based on HgCdTe for infrared detection

One of the topical areas of solid state photoelectronics is the creation of infrared detectors based on unipolar barrier systems (for example, with an nBn architecture). The greatest progress has been achieved in the development of barrier detectors based on semiconductors of the AIIIBV group, which is associated with the possibility of realizing systems with a zero barrier in the valence band. Unipolar barrier detectors based on mercury cadmium telluride (HgCdTe) grown by molecular beam epitaxy (MBE) are of interest due to significant technological advantages, since the creation of such devices can abandon the defect forming procedure of ion implantation. Despite a significant number of theoretical works, only a few attempts are known to practically implement nBn detectors based on MBE HgCdTe

An experimental study of the dynamic resistance in surface leakage limited nBn structures based on HgCdTe grown by molecular beam epitaxy

Mid-wave infrared nBn structures based on HgCdTe grown by molecular beam epitaxy on GaAs (013) substrates were fabricated. The composition in the absorbing layer was 0.29, and in the barrier layer it was 0.67. It was shown that the dark currents of the created nBn structures are limited by the surface leakage component. To study the bulk component of the dark current, it was proposed to use the admittance measurements of test metal-insulator-semiconductor (MIS) devices based on fabricated nBn structures in the case of the formation of a backward contact to the absorbing layer. It was established that surface leakage does not afect the dynamic resistance of the MIS device barrier. The dependence of the dynamic resistance of the barrier layer (Rb) of the MIS device in the accumulation mode on the area of the front electrode (A), voltage, and temperature was determined. It was shown that, with the exclusion of surface leakage, the values of the RbA product in a temperature range of 230–300 K at forward biases are determined by the difusion current of holes from the contact layer, and at reverse biases, by the difusion current from the absorbing layer. It was found that at temperatures of 210–300 K, RbA values exceeding the values of this parameter determined according to the empirical model Rule 07 were realized in the fabricated structures

Electrical properties of optimized nBn structures based on HgCdTe grown by molecular beam epitaxy

The use of unipolar barrier architectures in infrared detectors based on HgCdTe grown by molecular beam epitaxy (MBE) provides significant technological advantages. Earlier, the authors of the manuscript presented the results of a study of dark currents [1] and admittance [2] of the first variants of MBE HgCdTe nBn structures. This paper presents the results of electrical characterization of nBn structures with parameters optimized for detection in the spectral ranges 3–5 (MWIR) and 8–12 (LWIR) µm

Influence of As+ Ion implantation on properties of MBE HgCdTe near-surface layer characterized by metal–insulator–semiconductor techniques

The effect of As+ ion implantation on the electrical properties of the near-surface layer of n-HgCdTe films grown by molecular beam epitaxy (MBE) on Si (310) substrates was experimentally studied. A specific feature of MBE n-Hg0.78Cd0.22Te films is the presence of near-surface graded-gap layers with a high CdTe content, formed during epitaxial growth. The properties of as-grown films and films after As+ ion implantation with ion energy of 200 keV and fluence of 1014 cm−2 were studied. Post-implantation activation annealing was not performed. Test metal–insulator–semiconductor (MIS) structures were created based on as-grown and as-implanted samples by plasma-enhanced atomic layer deposition of Al2O3 insulator films. The admittance of the fabricated MIS structures was measured over a wide range of frequencies and temperatures. When determining the parameters of MIS structures, we used techniques that take into account the presence of near-surface graded-gap layers and series resistance of the HgCdTe film bulk, as well as the high density of slow surface states. It was found that, in as-implanted samples, the donor center concentration in the near-surface layer exceeds 1017 cm−3 and increases with distance from the HgCdTe-Al2O3 interface (at least up to 90 nm). After implantation, the conductivity of MBE HgCdTe film bulk increases markedly. It was shown that, for as-implanted samples, the generation rate of minority charge carriers in the MBE HgCdTe surface layer is significantly reduced, which indicates the appearance of a low defect layer with a thickness of at least 90 nm

Admittance of barrier structures based on mercury cadmium telluride

The results of studying the admittance of unipolar barrier structures based on HgCdTe grown by molecular beam epitaxy (MBE) on GaAs (013) substrates are presented. Using passivation with an Al2O3 insulator, device nBn structures based on HgCdTe were fabricated. The layer parameters in the created structures provided the possibility of detection in the spectral range of 3–5 μm. Based on the analysis of the frequency dependences of the admittance, an equivalent circuit of nBn structures at small biases is proposed. The dependences of the equivalent circuit parameters on the area of the mesa structure and temperature are determined. The properties of high-temperature maxima in the voltage dependences of the capacitance and conductance of nBn structures, which are presumably related to the recharging of surface states at the heterointerface between the barrier and absorbing layers, are studied. It is found that in a wide range of frequencies and temperatures, the capacitance – voltage characteristics of nBn structures based on HgCdTe at reverse biases can be used to determine the concentration of donor impurities in the absorbing layer. It is shown that the admittance of test MIS devices in a mesa configuration, formed on the basis of the MBE HgCdTe nBn structures, is determined by the combined influence of electronic processes in the contact, barrier, and absorbing layers

A new design of scanning IR detectors

Photoelectrical characteristics of scanning IR detectors with implemented time delay and integration mode are analyzed. A new “shifted cellular” layout of photosensitive elements in the FPA structure is proposed. Advantages of the new FPA configuration in terms of threshold sensitivity for small-size/point objects are demonstrated. The analysis is based on the Monte Carlo simulation of the diffusion process of photogenerated minority charge carriers in the photosensitive layer photodiode arrays. The analysis is performed taking into account the main photoelectric parameters of FPA elements: photosensitive layer thickness, diffusion length of charge carriers, optical absorption length, their design parameters: geometric sizes of FPA elements, diameters of p-n junctions, and design parameters of the optical system: optical-spot diameter

Admittance of MIS structures based on nBn systems of epitaxial HgCdTe for detection in the 3–5 μm spectral range

The admittance of test MIS structures based on nBn systems from Hg1 – xCdxTe grown by molecular beam epitaxy is investigated. Composition x in the absorbing and contact layers is 0.29; in the barrier layer, it is 0.60. An equivalent circuit of an MIS-structure based on an nBn system is proposed and the nominal values of the elements of this circuit are found under various conditions. Comparison of the temperature dependence of the barrier resistance with the Rule07 model indicates the possibility of creating efficient nBn detectors based on HgCdTe grown by molecular beam epitaxy for the 3- to 5-μm spectral range

Electrical properties of nBn structures based on HgCdTe grown by molecular beam epitaxy on GaAs substrates

The unipolar MWIR nBn structures based on HgCdTe grown by molecular beam epitaxy on GaAs (0 1 3) substrates were created. The CdTe content in the barrier layer varied from 0.67 to 0.84 for various samples. Aluminum oxide (Al2O3) films were used to passivate the surface of the mesa structure. For fabricated samples, dark current density values were measured at various voltages in a wide temperature range. It is shown that at lower compositions in the barrier layer the dark current is limited by surface leakage. With an increase in the barrier composition, the number of structures with a bulk dark current was increased. The voltage dependences of surface and bulk current density are constructed. The activation energies for structures with the dominance of the bulk dark current are determined. It is shown that for a considerable part of structures with a large barrier composition (x = 0.84) the bulk dark current is determined by diffusion processes

Admittance dependences of the mid-wave infrared barrier structure based on HgCdTe grown by molecular beam epitaxy

The admittance of nBn structures based on HgCdTe grown by molecular beam epitaxy (MBE) on GaAs (013) substrates was studied. The measurements were performed in the temperature range of 10–310 K at the frequencies of 0.5–2000 kHz. The content of CdTe in the absorbing layer was 0.36, and the content in the 210 nm thick barrier layer was 0.84. Equivalent circuits of nBn structure for various temperature and voltage ranges are proposed. The calculated frequency dependences of the admittance are in good agreement with the experimental data. The dependences of the values of the equivalent circuit elements on the area of nBn structure and on temperature are established. It is shown that measurements of the capacitance-voltage characteristic of nBn structure based on MBE HgCdTe in a wide range of conditions allow determining dopant concentration in the absorbing layer. At near room temperatures, features of the voltage dependences of the admittance, which related to the defect level recharge near the heterojunction between the absorbing and barrier layers, were found