The aim of the paper is to review the main achievements in the research of HgCdTe ternary alloy and InAs/InAsSb type-II superlattice material to indicate the Polish contribution to the development of medium and long wavelength infrared photodetectors. Research and development efforts in the WAT-VIGO joint laboratory have focused on the metal-organic chemical vapor deposition (from 2003) and molecular beam epitaxy (from 2015). At present stage of development, the photoconductive and photovoltaic HgCdTe detectors are gradually replaced with novel device designs based on III-V material system. T2SL devices complement the offer of MCT ones in applications where it is necessary to ensure, among others: higher resistance to difficult operating conditions and high uniformity of parameters of multi-element detectors.
Semiconductor nanowires are routinely grown on high-priced crystalline substrates as it is extremely challenging to grow directly on plastics and flexible substrates due to high temperature requirements and substrate preparation. At the same time, plastic substrates can offer many advantages such as extremely low price, light weight, mechanical flexibility, shock and thermal resistance, and biocompatibility. We explore the direct growth of InSb nanowires on flexible plastic substrates by metal-organic vapor phase epitaxy (MOVPE). We synthesize InSb nanowires on polyimide and show that the fabricated NWs are optically active with strong light emission even at RT. Overall, we demonstrate that InSb nanowires can be synthesized directly on flexible plastic substrates inside a MOVPE reactor, and we believe that our results will further advance the development of the nanowire-based flexible electronic devices.
In the past decade, there has been significant progress in development of the colloidal quantum dot (CQD) photodetectors. The QCD’s potential advantages include: cheap and easy fabrications, size-tunable across wide infrared spectral region, and direct coating on silicon electronics for imaging, what potentially reduces array cost and offers new modifications like flexible infrared detectors. The performance of CQD high operating temperature (HOT) photodetectors is lower in comparison with traditionally detectors existing on the global market (InGaAs, HgCdTe and type-II superlattices). In several papers their performance is compared with the semiempirical rule, “Rule 07” (specified in 2007) for P-on-n HgCdTe photodiodes. However, at present stage of technology, the fully-depleted background limited HgCdTe photodiodes can achieve the level of roomtemperature dark current considerably lower than predicted by Rule 07. In this paper, the performance of HOT CQD photodetectors is compared with that predicted for depleted P-i-N HgCdTe photodiodes. Theoretical estimations are collated with experimental data for both HgCdTe photodiodes and CQD detectors. The presented estimates provide further encouragement for achieving low-cost and high performance MWIR and LWIR HgCdTe focal plane arrays operating in HOT conditions.
The paper presents the analysis of the performance of the InAs/InAsSb superlattice barrier detector operated at 230 K and long-wavelengths infrared spectrum (LWIR). To determine the position of the electron miniband and the first heavy hole state in the superlattice, we have used a k·p. Having the position of the conduction band and valence band we have to determine a correct band alignment between the barrier and absorber layer – the barrier in the valence band must be sufficiently low to ensure the flow of optically generated holes. We have considered an AlSb material for barrier best aligned to LWIR InAs/InAsSb superlattice absorber.
Since the first paper published by Sakaki and Esaki in 1978, it is well known that InAs and GaSb constitute a nearly lattice-matched material system offering great flexibility in the design of IR optoelectronic devices. After four decades, the III-V type-II superlattice (T2SL) detector technology is under strong development as a possible alternative to HgCdTe. The novel ideas coming in design of detectors have enhanced the position of T2SLs in IR materials detector technology. It appears that T2SLs are especially helpful in the design of unipolar barriers.
In this paper fundamental physical properties of two material systems, HgCdTe and T2SLs, are compared together with their influence on detector performance: dark current density, RA product, quantum efficiency, and noise equivalent different temperature. In comparison with HgCdTe, fundamental properties of T2SLs are inferior. On the other hand, T2SL and barrier detectors have several advantages to include lower tunnelling and surface leakage currents, and suppressed Auger recombination mechanism. Up to date, the promise of superior performance of these detectors has not been realized yet. In the paper we present that the performance of T2SL detectors (dark current, current responsivity, and noise equivalent difference temperature) is lower than bulk HgCdTe photodiodes.
Due to stronger, less ionic chemical bonding of III-V semiconductors, these materials are attractive due to manufacturability and stability. It is also predicted that the interband T2SL quantum cascade devices will outperform the performance of the high operating temperature HgCdTe detectors.
InAsSb ternary alloy is considered to be an alternative to HgCdTe (MCT) in long-wavelength infrared (LWIR) spectral region. The high operation temperature conditions are successfully reached with AIIIBV bariodes, where InAsSb/AlAsSb system is playing dominant role. Since theoretically there is no depletion region in the active layer, the generation-recombination and trap-assisted tunneling mechanisms are suppressed leading to lower dark currents in comparison with standard photodiodes. As a consequence, the bariodes operate at a higher temperature than standard photodiodes which could be used in wide range of system applications, especially where the size, weight, and power consumption are crucial, to include - Free Space Optics (FSO) system. In terms of FSO the LWIR provides limited scattering and atmosphere turbulence in comparison to the commonly used ~ 1550 nm range. Currently, as reported HgCdTe LWIR with peak wavelength ~ 10 μm exhibiting ~ 9×109 cmHz1/2/W (unbiased) and ~ 3×1010 cmHz1/2/W (200 mV) and response time 0.3-4 ns are used. The paper presents detailed analysis of the bariode’s performances (such as dark current, detectivity and response time) versus applied voltage, operating wavelength, temperatures (4-, 2- stage thermoelectrical cooling) and structural parameters (active layer Sb composition, xSb). The optimal working conditions are simulated (non-equilibrium). Theoretical predictions of bariode’s performances are compared with experimental data published for commonly used FSO HgCdTe detectors.
InAsSb/AlSb barrier detectors were grown on (100) semi-insulating GaAs substrates by a molecular beam epitaxy. We compare the performance of two detectors with different active layers denoted as p + BppBpN + and p + Bpnn + . InAs0.81Sb0.19 absorber allows to operate up to 5.3-μm cut-off wavelengths at 230 K. p + Bpnn + detector (n-type absorber) exhibits diffusion-limited dark currents above 200 K. AlSb barrier provides low dark currents and suppresses surface leakage currents. With a value of 0.13 A / cm2 at 230 K, the current is of about an order of magnitude larger than determined by the “Rule 07.” Dark currents of p + BppBpN + detector (p-type absorber) are much higher due to a contribution of Shockley–Read–Hall mechanisms. On the other hand, a device with a p-type absorber exhibits the highest value of current responsivity, up to 2.5 A / W, pointing out that there is a tradeoff between dark current performance and quantum efficiency.
An InAsSb/AlSb heterostructure photovoltaic detector structures were grown on a (100) semi-insulating GaAs substrates by a molecular beam epitaxy. We compare the performance of two detectors with a different type of absorbing layers, denoted p+BppBpn+ and p+Bpnn+. InAs0.81Sb0.19 absorption layers allow for a operation up to 5.3 μm cut-off wavelengths at 230 K. p+Bpnn+ detector (n-type absorber) exhibits diffusion-limited dark currents above 200 K. AlSb barrier provides a low values of dark currents and allows a suppression of surface leakage current. With a value of 0.13 A/cm2 at 230 K, the current is less than an order of magnitude larger than those determined by the "Rule 07" for HgCdTe detectors. Dark currents of p+BppBpN+ detector (p-type absorber) are much higher due to a contribution of Shockley-Read-Hall mechanisms. On the other hand, device with a p-type absorber shows highest values of current responsivity, up to 2.5 A/W, point out that there is a trade-off between dark current performance and quantum efficiency.
In this work we present the theoretical investigation of the electrical and optical properties of high operating temperature (HOT) mid-wavelength infrared detectors (5 μm at 230 K) based on InAsSb/AlSb heterostructures [1]. In this work the performance comparison of barrier detectors with different doping concentration of n-type absorbing layer is presented. The barrier structure was simulated by commercially available software APSYS. We report on the dependence of the calculated current responsivity on the active layer thickness for a different doping concentration and doping concentration for optimal absorber thickness. Moreover, we show the influence of the bottom contact material on device’s performance.
An enhanced computer program has been applied to explain in detail the influence of different recombination mechanisms (Auger, radiative and Shockley-Read-Hall) on the performance of high operation temperature long wavelength infrared p-i-n HgCdTe heterojunction photodiodes. The computer program is based on a solution of the carrier transport equations, as well as the photon transport equations for semiconductor heterostructures. We distinguish photons in different energy ranges with unequal band gaps. As a result, both the distribution of thermal carrier generation and recombination rates and spatial photon density distribution in photodiode structures have been obtained. It is shown that photon recycling effect limits the influence of radiative recombination on the performance of small pixel HgCdTe photodiodes. In comparison with two previously published papers in Journal of Electronics Materials (Lee et al., DOI: 10.1007/s11664-016-4566-6 and Schuster et al., DOI: 10.1007/s11664-017-5736-x) our paper indicates an additional insight on ultimate performance of LWIR HOT HgCdTe arrays with pixel densities that are fully consistent with background- and diffraction-limited performance due to system optics.
III-V antimonide-based detectors are under development as a possible alternative to HgCdTe material systems. Although the modern version of this technology is still in its infancy, during the last decade, antimonide-based focal plane array technology has achieved a level close to HgCdTe. This book describes current concepts of antimonide-based IR detectors, focusing on designs having the largest impact on the mainstream of IR detector technologies. It is suitable for graduate students in physics and engineering who have knowledge of modern solid-state physics and electronic circuits, and will be of interest to those working with aerospace sensors and systems, remote sensing, thermal imaging, military imaging, optical telecommunications, infrared spectroscopy, and lidar.
We investigate the high-operating temperature performance of InAsSb/AlSb heterostructure detectors with cutoff wavelengths near 5 μm at 230 K. The devices have been fabricated with different types of absorbing layers: nominally undoped absorber (with n-type conductivity), and both n- and p-type doped. The results show that the device performance strongly depends on absorber layer type. Generally, the p-type absorber provides higher values of current responsivity than the n-type absorber, but at the same time also higher values of dark current. The device with the nominally undoped absorbing layer shows moderate values of both current responsivity and dark current. Resulting detectivities D * of nonimmersed devices vary from 2 × 109 to 5 × 109 cm Hz1/2 W ? 1 at 230 K, which is easily achievable with a two-stage thermoelectric cooler. Optical immersion increases the detectivity up to 5 × 1010 cm Hz1/2 W ? 1.
In this work we investigate the high-operating temperature performance of InAsSb/AlSb heterostructure detectors with cut-off wavelengths near 5 μm at 230 K. The devices have been fabricated with different type of the absorbing layer: nominally undoped absorber, and both n- and p-type doped. The results show that the device performance strongly depends on absorber layer doping. Generally, p-type absorber provides higher values of current responsivity than n-type absorber, but at the same time also higher values of dark current. The device with nominally undoped absorbing layer shows moderate values of both current responsivity and dark current. Resulting detectivities D° of non-immersed devices varies from 2×109 to 7×109 cmHz1/2/W at 230 K, which is easily achievable with a two stage thermoelectric cooler.
Most of the HgCdTe infrared detectors are fabricated by mesa geometry using a wet chemical or plasma etching techniques. The mesa definition etch process induces undesirable changes in HgCdTe surface properties. In narrow bandgap materials these surface changes could deteriorate a device performance. Uncontrolled band bending occurred on the slopes of the active layer increase of the recombination velocity causing surface leakage current which is a serious problem that affects infrared detectors. Adequate passivation is essential to minimize the effects from the surface states by saturating them. The HgCdTe barrier detectors were investigated for unpassivated and passivated devices. For the unpassivated structure the experimental value of Jbulk (at -0.2 V bias and a temperature of 200 K) was found at the level of 52% of the total dark current for devices with large diameters (500 μm). In the case of detectors with small diameters, the dark current is dominated by the surface leakage current. For a detector with a diameter of 200 μm, the bulk current consists only 28% of the total dark current. After passivation the level of bulk current increase to 58% in cause of 200 μm diameter and almost 75% in cause of large diameter.
Theoretical and experimental investigations on the response time improvement of unbiased long-wave infrared (LWIR) HgCdTe detectors operating at temperatures T=230 K were presented. Metal–organic chemical vapor deposition technology is an excellent tool in fabrication of different HgCdTe detector structures with a wide range of composition and donor/acceptor doping and without postgrown ex-situ annealing. The time constant is lower in biased detectors due to Auger-suppression phenomena and reduction of diffusion capacitance related to a wider depletion region. The relatively high bias current requirements and excessive low-frequency noise, which reduces the detectivity of biased detectors, inspire research on the time constant improvement of unbiased detectors. The response time of high-operating temperature LWIR HgCdTe detectors revealed complex behavior being dependent on the applied reverse bias, the operating temperature, the absorber thickness and doping, the series resistance, and the electrical area of the devices. The response time of 2 ns was achieved for unbiased 30×30 μm HgCdTe structures with λ50%=10.6 μm operating at T=230 K.
Theoretical and experimental investigations on the response time improvement of biased and unbiased long-wave infrared (LWIR) HgCdTe detectors operating at temperatures T = 230K were presented in this paper. MOCVD technology is an excellent tool in fabrication of different HgCdTe detector structures with a wide range of composition, donor/acceptor doping and without post grown ex-situ annealing. Donor doping efficiency in (111) and (100) oriented HgCdTe layers has been discussed. The time constant is lower in biased detectors due to Auger suppression phenomena and reduction of diffusion capacitance related to wider depletion region. The relatively high bias currents requirements and excessive low frequency noise which reduces the detectivity of biased detectors inspire researches on the time constant improvement of unbiased detectors. The response time of high-operating temperature (HOT) LWIR HgCdTe detectors revealed complex behavior being dependent on the applied the reverse bias, the operating temperature, the absorber thickness and doping, the series resistance and the electrical area of the devices.
Recently, there has been considerable progress towards III-V antimonide-based low dimensional solids development and device design innovations. From a physics point of view, the type-II InAs/GaSb superlattice is an extremely attractive proposition. Their development results from two primary motivations: the perceived challenges of reproducibly fabricating high-operability HgCdTe FPAs at reasonable cost and theoretical predictions of lower Auger recombination for type-II superlattice (T2SL) detectors compared to HgCdTe. Lower Auger recombination should be translated into a fundamental advantage for T2SL over HgCdTe in terms of lower dark current and/or higher operating temperature, provided other parameters such as Shockley-Read-Hall lifetime are equal.
Based on these promising results it is obvious now that the InAs/GaSb superlattice technology is competing with HgCdTe third generation detector technology with the potential advantage of standard III-V technology to be more competitive in costs and as a consequence series production pricing. Comments to the statement whether the superlattice IR photodetectors can outperform the “bulk” narrow gap HgCdTe detectors is one of the most important questions for the future of IR photodetectors presented by Rogalski at the April 2006 SPIE meeting in Orlando, Florida, are more credible today and are presented in this paper. It concerns the trade-offs between two most competing IR material technologies: InAs/GaSb type-II superlattices and HgCdTe ternary alloy system.
We present progress in metal organic chemical vapor deposition (MOCVD) growth of (100) HgCdTe epilayers achieved recently at the Institute of Applied Physics, Military University of Technology and Vigo System S.A. It is shown that MOCVD technology is an excellent tool for the fabrication of different HgCdTe detector structures with a wide range of composition, donor/acceptor doping, and without post grown ex-situ annealing. Surface morphology, residual background concentration, and acceptor doping efficiency are compared in (111) and (100) oriented HgCdTe epilayers. At elevated temperatures, the carrier lifetime in measured p-type photoresistors is determined by Auger 7 process with about one order of magnitude difference between theoretical and experimental values. Particular progress has been achieved in the growth of (100) HgCdTe epilayers for medium wavelength infrared photoconductors operated in high-operating temperature conditions.
It is fully confirmed that the development of the new HgCdTe long-wave (8−12 μm) infrared radiation (LWIR) detector has been driven by applications requiring high frequency response ( τ s) and operation at higher temperature (HOT - hot operating temperature, T > 200 K) [1]. Not only time response but also detectivity directly linked with τ s of the HOT HgCdTe detector should be optimized. The HOT HgCdTe’s performance is limited by Auger processes and to circumvent that issue the N+/π/P+n+ device’s designs has been proposed to suppress generation - recombination (GR) processes, i.e. combination of exclusion and extraction heterojunctions (π is a p-type doping region) [2−5]. The nominally sharp interfaces in N+/π/P+n+ (especially N+π) layered HgCdTe heterostructures are affected by interdiffusion during technological process leading to significant composition and doping grading occurring during HgCdTe growth by MOCVD [6,7]. Mentioned composition and doping grading should be controlled to optimize frequency performance of the devices. In this paper we present short analysis of the time response depending on type and doping grading of N+/π/P+n+ HOT LWIR HgCdTe structure. Time response of the long-wave HgCdTe detector with 50% cut-off wavelength ( λ c), ≈ 10.6 μm at T = 230 K, V = 200 mV was reduced form ~ 160 ps to ~ 52 ps by optimization of the N+π interface even for low voltage operation ~ 200 mV.
In this paper we present progress in MOCVD growth of (100) HgCdTe epilayers achieved recently at the Institute of
Applied Physics, Military University of Technology and Vigo System S.A. It is shown that MOCVD technology is an
excellent tool in fabrication of different HgCdTe detector structures with a wide range of composition, donor/acceptor
doping and without post grown annealing.
Particular progress has been achieved in the growth of (100) HgCdTe epilayers for long wavelength infrared
photoconductors operated in HOT conditions. The (100) HgCdTe photoconductor optimized for 13-μm attain detectivity
equal to 6.5x109 Jones and therefore outperform its (111) counterpart.
The paper also presents technological progress in fabrication of MOCVD-grown (111) HgCdTe barrier detectors.
The barrier device performance is comparable with state-of-the-art of HgCdTe photodiodes. The detectivity of HgCdTe
detectors is close to the value marked HgCdTe photodiodes. Dark current densities are close to the values given by “Rule
07”.
In the last decade, new architecture designs such as nBn devices or unipolar barrier photodiodes have been proposed to achieve high-operating temperature (HOT) detectors. This idea has been also implemented in HgCdTe ternary material systems. However, the implementation of this detector structure in an HgCdTe material system is not straightforward due to the existence of a valence band discontinuity (barrier) at the absorber-barrier interface. We report on midwavelength infrared HgCdTe barrier detectors with a zero valence band offset, grown by metal organic chemical vapor deposition on GaAs substrates. The experiments indicate the influence of the barrier on the electrical and optical performances of the p+BpnN+ device. The devices exhibit very low-dark current densities in the range of (2−3)×10−3 A/cm2 at 230 K and a high-current responsivity of about 2 A/W in the wide range of reverse bias voltage. The estimated thermal activation energy of about 0.33 eV is close to the full Hg0.64Cd0.36Te bandgap, which indicates diffusion limited dark currents.
The work reports on mid-wavelength infrared HgCdTe barrier detectors with a zero valence band offset, grown by metal organic chemical vapour deposition on GaAs substrates. The experiments indicate the influence of the barrier on electrical and optical performances of the p+BnN+ device. The devices exhibit very low dark current densities in the range of (2÷3)×10–3 A/cm2 at 300 K and a high current responsivity of about 2A/W in the wide range of reverse bias voltage. The estimated thermal activation energy of about 0.33 eV is close to the full Hg0.64Cd0.36Te bandgap, what indicates diffusion limited dark currents.
An enhanced original computer program is applied to explain in detail the influence of the photon recycling effect on carrier lifetime in a selected 3-μm n-on-p HgCdTe photodiode structure. The computer program is based on solution of carrier and photon transport equations for practical photodiode design. As a result, both distribution of thermal carrier generation and recombination rates and spatial photon density distribution in photodiode structures have been obtained. It is clearly shown that the photon recycling effect drastically limits the influence of radiative recombination on performance of HgCdTe photodiodes. A general conclusion confirms previous Humpreys' ascertainment that the radiative recombination, although of fundamental nature, does not limit the ultimate performance of HgCdTe photodiodes.
A new concept of the fabrication process for glass microlenses (external diameter ED<1 mm, focal length a few millimeters), based on the silicon master mask-less anisotropic wet etching in KOH, vacuum anodic bonding and re-flow of borosilicate glass, followed by the precise wafer-scale polishing and DRIE has been presented. A single spherical microlens as well as an array of spherical microlenses with focal length between 44.8 and 8.6 mm and external diameter 0.35 to 0.985 mm have been repeatable manufactured.
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