Renganathan Ashokan

Ionization energy of acceptors in As-doped HgCdTe grown by molecular beam epitaxy

The p-type activation of arsenic (As) in (211)B mercury cadmium telluride (HgCdTe) grown by molecular beam epitaxy (MBE), with different compositions covering the 3–5 and 8–14 μm atmospheric transmission windows and after annealing at 300 °C is reported. The composition and thickness of the MBE layers were determined from Fourier transform infrared transmission measurements at room temperature. The ionization energies of shallow acceptors related to As in MBE HgCdTe layers with different Cd compositions have been obtained by fitting variable temperature Hall measurement results to a two-band nonparabolic Kane model. The results indicate that As incorporated during the MBE growth can be activated to provide a shallow acceptor level in MBE HgCdTe and the ionization energy of the As acceptor decreases with decreasing Cd composition, in agreement with the theory.

HgCdTe/CdTe/Si infrared photodetectors grown by MBE for near-room temperature operation

Conventional HgCdTe infrared detectors need significant cooling in order to reduce noise and leakage currents resulting from thermal generation and recombination processes. Although the need for cooling has long been thought to be fundamental and inevitable, it has been recently suggested that Auger recombination and generation rates can be reduced by using the phenomena of exclusion and extraction to produce nonequilibrium carrier distributions. The devices with Auger suppressed operation requires precise control over the composition, and donor and acceptor doping. The successful development of the molecular beam epitaxy (MBE) growth technique for multi-layer HgCdTe makes it possible to grow these device structures. Theoretical calculations suggest that the p n+ layer sequence is preferable for near-room temperature operation due to longer minority carrier lifetime in lightly doped p-HgCdTe absorber layers. However, because the low doping required for absorption and nonequilibrium operation is easier to achieve in n-type materials, and because Shockley-Read centers should be minimized in order to obtain the benefits of Auger suppression, we have focused on p+ n structures. Planar photodiodes were formed on CdTe/Si (211) composite substrates by As implantation followed by a three step annealing sequence. Three inch diameter Si substrates were employed since they are of high quality, low cost, and available in large areas. Due to this development, large area focal plane arrays (FPAs) operated at room temperature are possible in the near future. The structures were characterized by FTIR, x-ray diffraction, temperature dependent Hall measurements, minority carrier lifetimes by photoconductive decay, and in-situ ellipsometry. To study the relative influence of bulk and surface effects, devices with active areas from 1.6 10−5 cm2 to 10−3 cm2 were fabricated. The smaller area devices show better performance in terms of reverse bias characteristics indicating that the bulk quality could be further improved. At 80 K, the zero bias leakage current for a 40 m 40 m diode with 3.2 m cutoff wavelength is 1 pA, the R0A product is 1.1 104-cm2 and the breakdown voltage is in excess of 500 mV. The device shows a responsivity of 1.3 107 V/W and a 80 K detectivity of 1.9 1011 cm-Hz1/2/W. At 200 K, the zero bias leakage current is 5 nA and the R0A product 2.03-cm2, while the breakdown voltage decreases to 40 mV.

Uncooled nonequilibrium HgCdTe IR detector modeling

The narrow gap semiconductor HgCdTe is commonly used for IR detection. Conventional HgCdTe IR detectors need significant cooling in order to reduce noise and leakage currents resulting from thermal generation and recombination processes. These cooling requirements considerably increase the cost, size, weight and complexity of infrared systems. The need for cooling to reduce noise and leakage currents resulting from Auger processes has long been thought to be fundamental and inevitable. However, recently, it has been suggested that by means of a steady-state non-equilibrium mode of operation, which holds the carrier densities below their equilibrium values, Auger generation and even radiative generation rates can be reduced. This is possible through the reduction of carrier concentrations because the Auger generation rate depends approximately on the square of the carrier concentration and radiative recombination rate depends linearly on it. This paper reports the modeling of a HgCdTe detector operated in a steady-state non-equilibrium mode at 230 approximately equals 295 K. The device architecture, NvP+, which is practical in MBE growth, is suitable for this application. Radiative and Auger lifetimes, zero surface recombination velocities, and zero background photon fluxes are assumed. The dependence of detectivity on minority carrier extraction efficiency is studied in this paper. At 230 and 250 K for ND equals 1 X 1014approximately equals 15 cm-3, the detectivity appears to become saturated at values in the order of 1010 cm Hz1/2/W when the minority carrier extraction efficiency is greater than 3.

Monolithically integrated HgCdTe focal plane arrays

The cost and performance of hybrid HgCdTe infrared focal plane arrays are constrained by the necessity of fabricating the detector arrays on a CdZnTe substrate. These substrates are expensive, fragile, are available only in small rectangular formats, and are not a good thermal expansion match to the silicon readout integrated circuit. We discuss in this paper an infrared sensor technology based on monolithically integrated infrared focal plane arrays that could replace the conventional hybrid focal plane array technology. We have investigated the critical issues related to the growth of HgCdTe on Si read-out integrated circuits and the fabrication of monolithic focal plane arrays: (1) the design of Si read-out integrated circuits and focal plane array layouts, (2) the low temperature cleaning of Si(001) wafers, (3) growth of CdTe and HgCdTe layers on read-out integrated circuits, (4) array fabrication, interconnection between focal plane array and read-out integrated circuit input nodes and demonstration of the photovoltaic operation, and (5) maintenance of the read-out integrated circuit characteristics after substrate cleaning, molecular beam epitaxy growth and device fabrication. Crystallographic, optical and electrical properties of the grown layers are presented. Electrical properties for diodes fabricated on misoriented Si and read-out integrated circuit substrates are discussed. The fabrication of arrays with demonstrated I-V properties show that monolithic integration of HgCdTe-based infrared focal plane arrays on Si read-out integrated circuits is feasible and could be implemented in the 3rd generation of infrared systems.

Progress in far-infrared detection technology

II-VI intrinsic very long wavelength infrared (VLWIR, λc~20 to 50 μm) materials, HgCdTe alloys as well as HgCdTe/CdTe superlattices, were grown by molecular beam epitaxy (MBE). The layers were characterized by means of X-ray diffraction, conventional Fourier transform infrared spectroscopy, Hall effect measurements and transmittance electron microscopy (TEM). Photoconductor devices were processed and their spectral response was also measured to demonstrate their applicability in the VLWIR region.

HgCdTe for far-infrared heterodyne detection

Very long wavelength infrared (VLWIR, λc approximately 20 to 50 μm) HgTe/HgCdTe superlattices were grown by molecular beam epitaxy (MBE). The layers were characterized by means of X-ray diffraction and Fourier transform infrared spectroscopy. Photoconductive interdigitated electrode detectors for heterodyne applications in the Far-infrared wavelengths (FIR) regions were designed and fabricated. Spectral response measurements exhibit the ability of these detectors to function in the long wavelength (LWIR) to VLWIR regions. Detectivity observed at 77 K is very encouraging and could be enhanced further at lower operating temperatures.

Effect of annealing on arsenic activation and device performance in mid-infrared HgCdTe on silicon grown by MBE

Two different effects of annealing, 1) on the arsenic activation in the in-situ doped mercury cadmium telluride (HgCdTe) layers grown on silicon substrates by molecular beam epitaxy (MBE) and 2) on the CdTe passivant-HgCdTe interface leading to significant changes in the characteristics of metal-insulator-HgCdTe (MIS) and planar photovoltaic (PV) detectors are discussed here. On the arsenic activation, highly compensated n-type properties to 100 percent activation of arsenic up to a total arsenic concentration of 1-2 X 1018 cm-3 and a decrease in activation thereafter are observed for annealing temperatures in the range of 235 to 450 degrees C. A range of annealing effects varying from unidentified structural defects acting as donors, probably due to donor arsenic tetramers or donor tetramers or donor tetramer clusters at 235 degrees C, to dissociation of bonds of neutral arsenic tetramer clusters to enable arsenic to occupy Te sites and behave as acceptors at 450 degrees C, are invoked to explain the arsenic activation mechanisms. The activation annealing, on the other hand, was found to have detrimental effect on the passivant-HgCdTe interface, possibility due to mercury diffusion during post-implant annealing. Capacitance-Voltage of MIS devices and current-voltage characteristics of planar diodes show tunneling limited performance with in-situ grown CdTe after annealing and show dramatic improvement in the performance characteristics when the in-situ grown CdTe is chemically removed and fresh CdTe passivation layer grown by MBE after arsenic activation annealing. Test structures containing mini arrays of square diodes with variable areas from 5.76 X 10-6 cm2 to 2.5 X 10-3 cm2 and MIS devices are used to establish the aforementioned effect. Under optimized conditions, state-of- the-art performance of the diodes in the mid-wavelength IR region with dynamic impedance on the order of 107 Ohm- cm2 is demonstrated.