Volume 18, Issue 3

Resonant frequencies of the two-dimensional plasma in FETs reach the THz range for nanometer transistor channels. Non-linear properties of the electron plasma are responsible for detection of THz radiation with FETs. Resonant excitation of plasma waves with sub-THz and THz radiation was demonstrated for short gate transistors at cryogenic temperatures. At room temperature, plasma oscillations are usually over-damped, but the FETs can still operate as efficient broadband THz detectors. The paper presents the main theoretical and experimental results on detection with FETs stressing their possible THz imaging applications. We discuss advantages and disadvantages of application of III–V GaAs and GaN HEMTs and silicon MOSFETs.

Wavelength tunable emitters and detectors in the mid-IR wavelength region allow applications including thermal imaging and gas spectroscopy. One way to realize such tunable devices is by using a resonant cavity. By mechanically changing the cavity length with MEMS mirror techniques, the wavelengths may be tuned over a considerable range. Resonant cavity enhanced detectors (RCED) are sensitive at the cavity resonance only. They may be applied for low resolution spectroscopy, and, when arrays of such detectors are realized, as multicolour IR-FPA or “IR-AFPA”, adaptive focal plane arrays. We report the first room temperature mid-IR VECSEL (vertical external cavity surface emitting laser) with a wavelength above 3 μm. The active region is just 850 nm PbSe, followed by a 2.5 pair Bragg mirror. Output power is > 10 mW at RT.

A photovoltaic detector design based on the graded band gap HgCdTe MBE structure with high conductivity layer (HCL) at interface, which provides photodiodes series resistance and a shortwave cut.off filter is developed. The optimal HCL parameters giving high quantum efficiency and minimal noise equivalent temperature difference were determined by calculations and experimentally confirmed. The hybrid 320×256 IR FPA operating in 8–12 μm spectral range was fabricated. The threshold power responsivity and minimal noise equivalent temperature difference values at wavelength maximum were 1.02×10−7 W/cm2, 4.1×108 V/W and 27 mK, respectively.

The purpose of this scientific work is the research of energy-band diagrams and capacity-voltage characteristics of CdxHg1−xTe-based variband heteroepitaxial structures with taking into account the dependence of electron affinity on the composition. Numerical simulations of energy-band diagrams and capacity-voltage characteristics for metal-insulator-semiconductor structures based on CdxHg1−xTe-variable composition layers are carried out in this work. Energy diagrams are calculated with taking into account the dependence of electron affinity on the composition x of CdxHg1−xTe. This dependence was obtained in terms of dependence of the local electroneutrality level on the composition for CdxHg1−xTe.

In this paper, we present a new approach to obtain large size dots in an MBE grown InAs/GaAs multilayer quantum dot system. This is achieved by adding an InAlGaAs quaternary capping layer in addition to a high growth temperature (590°C) GaAs capping layer with the view to tune the emission wavelength of these QDs towards the 1.3 μm/0.95 eV region important for communication devices. Strain driven migration of In atoms from InAlGaAs alloy to the InAs QDs effectively increases the size of QDs. Microscopic investigations were carried out to study the dot size and morphology in the different layers of the grown samples. Methods to reduce structural defects like threading dislocations in multilayer quantum dot samples are also studied.

Direct detection THz/sub-THz bolometer is proposed. In it an electromagnetic wave propagates in the bipolar semiconductor waveguide, heats electrons and holes there, and therefore creates their bipolar thermodiffusion flow and, as well as, the electromotive force (emf). The flow causes the carrier excess concentration. Both this concentration and emf are used to get the bolometer response voltage. The bolometer theoretical model is developed. The possibility without cooling or moderate cooling (about 100 K for the Cd0.2Hg0.8Te bolometers) to get acceptable for applications values of the noise equivalent power is shown. Experimental results confirm the main model conclusions.

The paper examines influence of near-surface graded-gap layers on electrical characteristics of MIS-structures fabricated on heteroepitaxial Hg1−xCdxTe films grown by molecular beam epitaxy (MBE). Two types of insulators, i.e., two-layer SiO2/Si3N4 and anodic oxide films were used. As it is seen from the depth and width of the valley on the C-V characteristics, the capacitance is found to vary in a wide range, in contrast to the structures without graded-gap layers. It is shown that the graded-gap layer under MIS-structures with x = 0.22 effectively reduces the tunnelling generation via deep levels and increases a lifetime of minority carriers in the space charge region and its differential resistance. The properties of the HgCdTe-insulator interfaces are studied.

The electrical properties of the interface between Hg1−xCdxTe (x = 0.22 and x = 0.32–0.36) and CdTe prepared in situ molecular beam epitaxy were estimated at 77 K. The methods of determination of main parameters of interface semiconductor/insulator and insulator from capacitance-voltage characteristics of MIS-structures based on graded-band Hg1−xCdxTe have been developed. The fixed charge states density, fast surface states density, and density of mobile charge are obtained from capacitance-voltage measurements. For improvement in stable and electrical strength of insulator coating for several samples over CdTe additional protective layers SiO2-Si3N4 are formed for x = 0.22 and ZnTe for x = 0.32–0.36.

We report on the results of transport studies of MBE-grown InAs/GaSb superlattices. We demonstrate that the in-plane mobility is limited by interface roughness scattering by showing that, as a function of InAs layer width L, the in-plane mobility behaves as μ ∝ L5.3, which closely follows the classic sixth power dependence expected from theory for interface-roughness-limited mobility. Fits to the mobility data indicate that, for one monolayer surface roughness, the roughness correlation length is about 35 Å. Next, we show that the in-plane carrier mobility in InAs/GaSb superlattices is inversely proportional to carrier density in n- and p-type samples, the result of screened interface roughness scattering.

The acceptor doping of mercury cadmium telluride (HgCdTe) layers grown by MOCVD are investigated. (111)HgCdTe layers were grown on (100)GaAs substrates at 350°C using horizontal reactor and interdiffused multilayer process (IMP). TDMAAs and AsH3 were alternatively used as effective p-type doping precursors. Incorporation and activation rates of arsenic have been studied. Over a wide range of Hg1−xCdxTe compositions (0.17 < x < 0.4), arsenic doping concentration in the range from 5×1015 cm−3 to 5×1017 cm−3 was obtained without postgrowth annealing. The electrical and chemical properties of epitaxial layers are specified by measurements of SIMS profiles, Hall effect and minority carrier lifetimes. It is confirmed that the Auger-7 mechanism has decisive influence on carrier lifetime in p-type HgCdTe epilayers.

The high frequency response of near-room temperature long wavelength infrared (LWIR) HgCdTe heterostructure photodiodes is investigated using a Fourier space method. The MOCVD HgCdTe multilayer heterostructures were grown on GaAs substrates. The response time of devices as a function of bias has been measured experimentally by using 10-μm quantum cascade laser and fast oscilloscope with suitable transimpedance amplifier. Results of theoretical predictions are compared with experimental data. It is shown that the response time at weak reverse bias condition is mainly limited by the drift time of carriers moving into π-n+ junction. Using the reverse bias higher than 50 mV, the transit time across the absorber region limits the response time. The response time of small-area devices decreases in the region of week reverse bias achieving value below 1 ns.

In Poland, the HgCdTe studies began in 1960 at the Institute of Physics, Warsaw University. The material processing laboratory was created by Giriat and later by Dziuba, Gałązka, and others. Bridgman technique with sealed thick wall quartz ampoules was used to grow material suitable for research and experimental devices. Among the first papers published in 1961 and 1963 there were the Polish works devoted to preparation, doping, and electrical properties of HgCdTe. Infrared detector’s research and development efforts in Poland were concentrated mostly on uncooled market niche. At the beginning, a modified isothermal vapour phase epitaxy has been used for research and commercial fabrication of photoconductive, photoelectromagnetic and other HgCdTe devices. Bulk growth and liquid phase epitaxy were also used. Recently, the fabrication of infrared devices relies on low temperature epitaxial technique, namely metalorganic vapour phase deposition. At present stage of development, the photoconductive and photoelectromagnetic (PEM) detectors are gradually replaced with photovoltaic devices which offer inherent advantages of no electric or magnetic bias, no heat load and no flicker noise. Potentially, photodiodes offer high performance and very fast response. However, conventional photovoltaic uncooled detectors suffer from low quantum efficiency and very low junction resistance. The problems have been solved with advanced band gap engineered architecture, multiple cell heterojunction devices connected in series, and monolithic integration of the detectors with microoptics. In final part of the paper, the Polish achievements in technology and performance of HgMnTe and HgZnTe photodetectors are presented.

Epitaxially grown self-assembled InAs quantum dots (QDs) have found applications in optoelectronics. Efforts are being made to obtain efficient quantum-dot lasers operating at longer telecommunication wavelengths, specifically 1.3 μm and 1.55 μm. This requires narrow emission linewidth from the quantum dots at these wavelengths. In InAs/GaAs single layer quantum dot (SQD) structure, higher InAs monolayer coverage for the QDs gives rise to larger dots emitting at longer wavelengths but results in inhomogeneous dot-size distribution. The bilayer quantum dot (BQD) can be used as an alternative to SQDs, which can emit at longer wavelengths (1.229 μm at 8 K) with significantly narrow linewidth (∼16.7 meV). Here, we compare the properties of single layer and bilayer quantum dots grown with higher InAs monolayer coverage. In the BQD structure, only the top QD layer is covered with increased (3.2 ML) InAs monolayer coverage. The emission line width of our BQD sample is found to be insensitive towards post growth treatments.

We have proposed and developed terahertz and subterahertz wave detector using hot-carrier effect in narrow-gap Hg1−xCdxTe. Epitaxial Hg1−xCdxTe-layers were integrated into dipole antennas. The response of Hg1−xCdxTe hot-electron bolometer was measured in the range of frequencies 0.037–1.58 THz, in the temperature region T = 68–300 K at various bias currents. A bolometer theoretical model was developed and the experimental results confirm the model main conclusions.

The intensive far infra-red irradiation in the range of 80–100 μm was observed in uniaxially strained gapless p-Hg1−xCdxTe (MCT) with x = 0.14 in the strong electric field. The inverse occupation in strained MCT is created because the hot electrons distribution occurs in the c-band under impact ionization, while the holes are localized near the v-band top. The probability of band-to-band radiative transition increases dramatically when the acceptor level becomes resonance in the v-band. At threshold values of strain and electric field (P = 2.5–2.7 kbar, E = 50–55 V/cm), increase in irradiation (by 3 orders of magnitude) and increase in current (by 4–6 times) occur.

Doped with Ga lead telluride was taken as a model object to explain the nature of group-III deep levels in IV-VI semiconductors and to elucidate the vapour phase doping mechanism. For this goal, interaction of various gallium-containing molecules with defect-free crystal as well as with native defects in PbTe was considered. Formation energies for different point defects created in PbTe as a result of interaction the Ga2Te molecules, Ga2 dimers and single Ga atoms with a host crystal were calculated using density functional theory. Particularly GaPb and Gai together with formation of accompanied self interstitials Pbi in various charge states were examined. In addition we propose the new type of defects - the impurity complex (2Ga)Pb which looks like <111>-oriented gallium dumbbell. Calculations suggest the double donor behaviour and DX-like properties of this defect together with extremely low formation energy values. Namely, (2Ga)Pb centres are preferably formed under Ga2Te doping while (Ga2)Pb+Pbi ones are formed under Ga2 or Ga doping. In all cases, formation energies are negative and resulting defect concentration is determined by reaction kinetics only. Mechanisms of the lead vacancy compensation with the vapour phase doping are considered as well.

The history, status, and recent progress in the middle and long wavelength Hg1−xCdxTe infrared detectors operating at near room temperatures are reviewed. Thermal generation of charge carriers in narrow gap semiconductor is a major limitation or sensitivity. Cooling is a straightforward way to suppress thermal generation of charge carriers and reduce related noise. However, at the same time, cooling requirements make infrared systems bulky, heavy, and inconvenient in use. A number of concepts to improve performance of photodetectors operating at near room temperatures have been proposed and implemented. Recent considerations of the fundamental detector mechanisms suggest that near perfect detection can be achieved without the need for cryogenic cooling. This paper, to a large degree, is based on the research, development, and commercialization of uncooled HgCdTe detectors in Poland. The devices have been based on 3D-variable band gap and doping level structures that integrate optical, detection and electric functions in a monolithic chip. The device architecture is optimized for the best compromise between requirements of high quantum efficiency, efficient and fast collection of photogenerated charge carriers, minimized thermal generation, reduced parasitic impedances, wide linear range, wide acceptance angles and other device features. Recent refinements in the devices design and technology have lead to sensitivities close to the background radiation noise limit, extension of useful spectral range to > 16 μm wavelength and picosecond range response times. The devices have found numerous applications in various optoelectronic systems. Among them there are fast scan FTIR spectrometers developed under MEMFIS project.

Ion milling, as a tool for “stirring” defects in HgCdTe by injecting high concentration of interstitial mercury atoms, was used for studying films grown by liquid phase epitaxy (LPE) on CdZnTe substrates. The films appeared to have very low residual donor concentration (∼1014 cm−3), yet, similar to the material grown by molecular beam epitaxy, contained Te-related neutral defects, which the milling activated electrically. It is shown that ion milling has a stronger effect on HgCdTe defect structure than thermal treatment, and yet eventually brings the material to an “equilibrium” state with defect concentration lower than that after low-temperature annealing.

The long wavelength (8–12 μm) IR FPA 288×4 based on a hybrid assembly of n+-p diode photosensitive arrays (PA) of HgCdTe (MCT) MBE-grown structures and time delay integration (TDI) readout integrated circuits (ROIC) with bidirectional scanning have been developed, fabricated, and investigated. The p-type MCT structures were obtained by thermal annealing of as-grown n-type material in inert atmosphere. The MCT photosensitive layer with the composition 0.20–0.23 of mole fraction of CdTe was surrounded by the wide gap layers to decrease the recombination rate and surface leakage current. The diode arrays were fabricated by planar implantation of boron ions into p-MCT. The typical dark currents were about 4–7 nA at the reverse bias voltage of 150 mV. The differential resistance R was up to R0 = 1.6×107 Ω zero bias voltage, which corresponded to R0A ∼70 Ω ·cm2 and to the maximal value Rmax = 2.1 × 108 Ω. The bidirectional TDI deselecting ROIC was developed and fabricated by 1.0-μm CMOS technology with two metallic and two polysilicon layers. The IR FPAs were free of defect channels and have the average values of responsivity Sλ = 2.27×108 V/W, the detectivity Dλ * = 2.13 × 1011 cm × Hz1/2 × Wt1, and the noise equivalent temperature difference NETD = 9 mK.

Of many techniques used to characterize quality of HgCdTe, ion milling is emerging as a unique means to reveal electrically active and neutral defects and complexes. Ion milling is capable of strongly affecting electrical properties of HgCdTe, up to conductivity type conversion in p-type material. It appears, that strongly non-equilibrium processes which take place under ion milling, when material is oversaturated with mercury interstitial atoms generated near a surface, lead to formation of specific defect complexes, which may not form under other type of treatment. By measuring parameters of a crystal before and after milling, and following disintegration of defects with time after ion milling (’relaxation’), one can detect and identify these defects. This method was applied to analyse different samples grown by molecular beam epitaxy.

We have measured the current-voltage characteristics of the long-wavelength infrared (LWIR) photodiode array, formed on the epitaxial CdxHg1−xTe film (x = 0.21–0.23) with a high concentration of the Shockley-Read-Hall centres, before and after irradiating it with fast neutrons (energy 1 MeV, dose 5×1013 cm−2) at room temperature. Residual changes in current-voltage characteristics, persisting after 20 days, have been identified. Model calculations indicate that the Shockley-Read-Hall centre concentration increases 2–4 times, and the carrier lifetime decreases 2–5 times after the irradiation.