Antoni Rogalski

New concepts in infrared photodetector designs

In 1959, Lawson and co-workers published the paper which triggered development of variable band gap Hg1−xCdxTe (HgCdTe) alloys providing an unprecedented degree of freedom in infrared detector design. HgCdTe ternary alloy has been used for realization of detectors operating under various modalities including: photoconductor, photodiode, and metal-insulator-semiconductor detector designs. Over the last five decades, this material system has successfully overcome the challenges from other material systems. It is important to notice that none of these competitors can compete in terms of fundamental properties. The competition may represent more mature technology but not higher performance or, with the exception of thermal detectors, higher operating temperatures (HOTs) for ultimate performance. In the last two decades, several new concepts for improvement of the performance of photodetectors have been proposed. These new concepts are particularly addressing the drive towards the so called HOT detectors aiming to increase detector operating temperatures. In this paper, new strategies in photodetector designs are reviewed, including barrier detectors, unipolar barrier photodiodes, multistage detectors and trapping detectors. Some of these new solutions have emerged as a real competitor to HgCdTe photodetectors.

New trends in semiconductor infrared detectors

Recent efforts in semiconductor IR detector research have been directed toward improving the performance of single-element devices,large electronically scanned arrays, and higher operating temperature. Another important aim is to make IR detectors cheaper and more nconvenient to use. New trends in semiconductor IR detector technologies are discussed, including HgCdTe photodiodes, Schottky-barrier photoemissive devices, alternatives to HgCdTe ternary alloys, monolithic lead-chalcogenide photodiodes, GaAs/AlGaAs intersubband quantum well photoconductors, and ways to improve the performance of nearroom-temperature detectors. A comparison of different types of detectors nwith the present stage of HgCdTe technology achievements is undertaken.

Numerical Estimations of Carrier Generation–Recombination Processes and the Photon Recycling Effect in HgCdTe Heterostructure Photodiodes

An enhanced computer program has been applied to explain in detail the photon recycling effect which drastically limits the influence of radiative recombination on the performance of p-on-n HgCdTe heterostructure 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 two energy ranges according to p+ and n region 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. The general conclusion, similar to our earlier work concerning 3-μm n-on-p HgCdTe heterostructure photodiodes, confirms the previous assertion by Humphreys that radiative recombination does not limit HgCdTe photodiode performance.

Engineering the Bandgap of Unipolar HgCdTe-Based nBn Infrared Photodetectors

Design of practically realizable unipolar HgCdTe nBn photodetectors has been studied in detail by numerical analysis. The simulations reported herein reveal that, by optimization of barrier doping, dark current levels can be reduced and collection efficiency substantially improved. It is shown that p-type doping of the barrier layer can significantly reduce the effective potential barrier arising from the valence band offset between the absorber and barrier regions, thus enabling HgCdTe nBn detector operation under near zero-bias conditions. However, relatively high electric fields in the space charge regions near the barrier/absorber interface result in enhanced trap-assisted Shockley–Read–Hall thermal generation. Our calculations indicate that nBn HgCdTe detectors with barriers engineered by use of HgTe/Hg0.05Cd0.95Te superlattices have, potentially, substantially better valence band alignment without the need for p-type doping.

InAs/GaSb type-II superlattice infrared detectors: Future prospect

Investigations of antimonide-based materials began at about the same time as HgCdTe ternary alloys—in the 1950s, and the apparent rapid success of their technology, especially low-dimensional solids, depends on the previous five decades of III-V materials and device research. However, the sophisticated physics associated with the antimonide-based bandgap engineering concept started at the beginning of 1990s gave a new impact and interest in development of infrared detector structures within academic and national laboratories. The development of InAs/GaSb type-II superlattices (T2SLs) results from two primary motivations: the perceived challenges of reproducibly fabricating high-operability HgCdTe focal plane arrays (FPAs) at reasonable cost and the theoretical predictions of lower Auger recombination for type T2SL detectors compared with HgCdTe. Second motivation—lower Auger recombination should be translated into a fundamental advantage for T2SL over HgCdTe in terms of lower dark current and/or higher op...

Theoretical Modeling of HOT HgCdTe Barrier Detectors for the Mid-Wave Infrared Range

This paper reports on theoretical modeling of medium-wavelength infrared HgCdTe barrier infrared detector (BIRD) photoelectrical performance. BIRD HgCdTe detectors were simulated with the commercially available software APSYS. Detailed analysis of the detector performance such as dark current, photocurrent, resistance–area product, detectivity versus applied bias, operating temperature, and structural parameters (absorber doping, barrier composition) was performed to determine the optimal operating conditions. It is shown that higher operation temperature conditions achievable with commonly used thermoelectric coolers allow detectivities of Dxa0=xa09.5xa0×xa01010xa0cmHz1/2/W and D*xa0=xa01.5xa0×xa01011xa0cmHz1/2/W at Txa0=xa0200xa0K to be obtained for the correct absorber doping for nBnnn+ and nBnpn+, respectively. R0A for the nBnnn+ detector was found to range from 200xa0Ωxa0cm2 to 0.6xa0Ωxa0cm2 at Txa0=xa0200xa0K to 300xa0K, respectively.

Computer simulation of HgCdTe photovoltaic devices based on complex heterostructures

We analyze numerically properties of small-size infrared photovoltaic devices based on complex two-dimensional Hg1- xCdxTe heterostructures. An original iteration scheme was used to solve the system of nonlinear continuity equations and the Poisson equation. All quantities are expressed as functions of electric potential and Fermi quasi-levels. The results of calculations are presented as the maps showing spatial distribution of sensitivity and density of noise generation for 4 types of heterostructures. In addition, resulting parameters of the devices are summarized in the table. This approach may help to understand specific features of the heterostructural devices and optimize their performance. The simulations show viability of constructing devices with active region buried inside a wide gap material where existing potential barriers prevent adverse effects of both recombination of photogenerated carriers and thermal generation at surfaces, interfaces and contacts.

GaAs/AlGaAs quantum well infrared photoconductors versus HgCdTe photodiodes for long-wavelength infrared applications

Investigations of the performance of GaAs/AIGaAs quantum well IR photoconductors (QWIPs) as compared to HgCdTe photodiodes operated at temperatures below 77 K in the long-wavelength IR (LWIR) region are presented. In comparative studies, the current standard n+ -p HgCdTe photodiodes as well as p+ -n photodiodes are considered. Investigations of the fundamental physical limitations of HgCdTe photodiodes indicate better performance of this type of detector in comparison with QWIPs operated in the range 40 to 77 K. At 40 K, QWIPs with a cutoff wavelength of about 8 μm indicate higher detectivity. The advantage of QWIPs increases in wider spectral regions and at temperatures below 40 K. Usually, in the temperature range below 50 K the performance of n+ -p HgCdTe photodiodes is determined by trap-assisted tunneling. As a result, the advantage of GaAs/AIGaAs QWIPs increases in wider spectral regions (λ ≈ 8 to 12 μm) and at temperatures below 50 K. The comparison with p+ -n HgCdTe photodiodes is more complicated for lack of precisely modeled current transport in these junctions. GaAs/AIGaAs QWIPs at 40 K are background limited in low-background conditions. This observation plus the maturity of GaAs/AIGaAs technology and its radiation hardness characteristics promise that QWIP technology can produce high-quality focal plane arrays for space applications.

Two-dimensional analysis of double-layer heterojunction HgCdTe photodiodes

In the paper the performance of P-on-n double-layer heterojunction HgCdTe photodiodes are temperature 77 K is analyzed theoretically. Calculation has been performed for the backside-illuminated configuration. The effect of photodiode base layer geometry on quantum efficiency and R0A product is analyzed. The effect of lateral collection of diffusion current and photocurrent on photodiode parameters is also shown. Moreover the dependence of the p-n junction position within heterostructure on the band-gap energy profiles and photodiode performance is presented. Finally, the influence of the composition gradient and p- side doping concentration on photodiode parameters is described briefly.

Surface leakage current in HgCdTe photodiodes

This study describes fabrication of heterojunction HgCdTe photodiodes passivated with a wide band-gap CdTe epitaxial layer. The current-voltage characteristics of these photodiodes with and without passivation have been investigated. It is shown that for reverse bias the measured I-V characteristics can be explained by a surface tunneling current and surface generation current. The breakdown voltage is observed to decrease monotonically with increasing temperature, a trend that is directly opposite to what would be expected from a pure tunneling mechanism. Additional information on surface limitations is obtained from analyzing the R0A product as a function of temperature. The performance of both type of p-n VLWIR HgCdTe photodiodes (with and without the passivating layer) have been compared.