1/f noise in large-area Hg<Subscript>1−x</Subscript>Cd<Subscript>x</Subscript>Te photodiodes

The 1/f noise in photovoltaic (PV) molecular-beam epitaxy (MBE)-grown Hg_1−xCd_xTe double-layer planar heterostructure (DLPH) large-area detectors is a critical noise component with the potential to limit sensitivity of the cross-track infrared sounder (CrIS) instrument. Therefore, an understanding of the origins and mechanisms of noise currents in these PV detectors is of great importance. Excess low-frequency noise has been measured on a number of 1000-μm-diameter active-area detectors of varying “quality” (i.e., having a wide range of I-V characteristics at 78 K). The 1/f noise was measured as a function of cut-off wavelength under illuminated conditions. For short-wave infrared (SWIR) detectors at 98 K, minimal 1/f noise was measured when the total current was dominated by diffusion with white noise spectral density in the mid-10⁻¹⁵A/Hz^1/2 range. For SWIR detectors dominated by other than diffusion current, the ratio, α, of the noise current in unit bandwidth i_n(f = 1 Hz, V_d = −60 mV, and Δf = 1 Hz) to dark current I_d(V_d = −60 mV) was α_SW-d = i_n/I_d ∼ 1 × 10⁻³. The SWIR detectors measured at 0 mV under illuminated conditions had median α_SW-P = i_n/I_ph ∼ 7 × 10⁻⁶. For mid-wave infrared (MWIR) detectors, α_MW-d = i_n/I_d ∼ 2 × 10⁻⁴, due to tunneling current contributions to the 1/f noise. Measurements on forty-nine 1000-μm-diameter MWIR detectors under illuminated conditions at 98 K and −60 mV bias resulted in α_MW-P = i_n/I_ph = 4.16 ± 1.69 × 10⁻⁶. A significant point to note is that the photo-induced noise spectra are nearly identical at 0 mV and 100 mV reverse bias, with a noise-current-to-photocurrent ratio, α_MW-P, in the mid 10⁻⁶ range. For long-wave infrared (LWIR) detectors measured at 78 K, the ratio, α_LW-d = i_n/I_d ∼ 6 × 10⁻⁶, for the best performers. The majority of the LWIR detectors exhibited α_LW-d on the order of 2 × 10⁻⁵. The photo-induced 1/f noise had α_LW-P = i_n/I_ph ∼ 5 × 10⁻⁶. The value of the noise-current-to-dark-current ratio, α appears to increase with increasing bandgap. It is not clear if this is due to different current mechanisms impacting 1/f noise performance. Measurements on detectors of different bandgaps are needed at temperatures where diffusion current is the dominant current. Excess low-frequency noise measurements made as a function of detector reverse bias indicate 1/f noise may result primarily from the dominant current mechanism at each particular bias. The 1/f noise was not a direct function of the applied bias.     

1/f noise in very-long-wavelength infrared Hg1−xCdx Te detectors

This paper investigates 1/f noise performance of very-long-wavelength infrared (VLWIR) Hg_1−xCd_xTe (cutoff wavelengths λ_c=15 μm and λ_c=16 μm) photodiodes at 78 K, where detector current is varied by changing detector area, detector bias, and illumination conditions. Holding detector bias and temperature constant, the 1/f noise current is proportional to the detector current. Significant nonuniformity is observed in the noise data for each detector area because of the varying detector quality. Defects are presumed resident in the detectors to produce greater nonuniformity in 1/f noise as compared to dark current at 100-mV reverse bias. For λ_c=16 μm, 4-μ-radius, diffusion-limited diodes at 78 K and 100-mV reverse bias, the average dark current is I_d=9.76±1.59×10⁻⁸ A, while the average noise current measured at 1 Hz is i_n=1.01±0.63×10⁻¹² A/Hz^1/2. For all detector areas measured, the average ratio in 1-Hz bandwidth is α _D =i_n/I_d=1.39±1.09×10⁻⁵. The 1/f noise was also measured on one diode as a function of detector-dark current as the applied bias is varied. In the diffusion-limited portion of this detector’s current-voltage (I-V) curve, to about 130 mV, the 1/f noise was independent of bias. For this diode, the ratio α_D=i_n/I_d=1.51±0.12×10⁻⁵. The 1/f noise associated with tunneling currents is a factor of 3 greater than the 1/f noise associated with diffusion currents, α_T=i_n/I_T=5.21±0.83×10⁻⁵. In addition, 1/f noise was measured on detectors held at −100 mV and 78K under dark and illuminated conditions. The measured ratios α_P ∼α_D ∼1.5×10⁻⁵ were about the same for the dark and photon-induced diffusion currents. Therefore, the diffusion current appears to have a unique value of α as compared to the tunneling current. This may be indicative of unique noise-generation mechanisms associated with each current.     

Ten-inch molecular beam epitaxy production system for HgCdTe growth

Growth of Hg_1−xCd_xTe by molecular beam epitaxy (MBE) has been under development since the early 1980s at Rockwell Scientific Company (RSC), formerly the Rockwell Science Center; and we have shown that high-performance and highly reproducible MBE HgCdTe double heterostructure planar p-on-n devices can be produced with high throughput for various single- and multiplecolor infrared applications. In this paper, we present data on Hg_1−xCd_xTe epitaxial layers grown in a ten-inch production MBE system. For growth of HgCdTe, standard effusion cells containing CdTe and Te were used, in addition to a Hg source. The system is equipped with reflection high energy electron diffraction (RHEED) and spectral ellipsometry in addition to other fully automated electrical and optical monitoring systems. The HgCdTe heterostructures grown in our large ten-inch Riber 49 MBE system have outstanding structural characteristics with etch-pit densities (EPDs) in the low 10⁴ cm⁻² range, Hall carrier concentration in low 10¹⁴ cm⁻³, and void density <1000 cm². The epilayers were grown on near lattice-matched (211)B Cd_0.96Zn_0.04Te substrates. High-performance mid wavelength infrared (MWIR) devices were fabricated with R₀A values of 7.2×10⁶ Ω-cm² at 110 K, and the quantum efficiency without an antireflection coating was 71.5% for cutoff wavelength of 5.21 μm at 37 K. For short wavelength infrared (SWIR) devices, an R₀A value of 9.4×10⁵ Ω-cm² at 200 K was obtained and quantum efficiency without an antireflection coating was 64% for cutoff wavelength of 2.61 μm at 37 K. These R₀A values are comparable to our trend line values in this temperature range.     

Ultraviolet-visible metal-semiconductor-metal photodetectors fabricated from InxGa1−xN (0≤x≤0.13)

Metal-semiconductor-metal (MSM) photodetectors have been fabricated on In_xGa_1−xN epitaxial films grown by metalorganic chemical vapor deposition within the composition range 0≤x≤0.13. The dark current and spectral response were measured for devices with a varying In mole fraction x. The devices, which had nominal finger widths and finger spacing of 5 μm, were biased with modest voltages in the range 2≤V_bias≤5 V. In general, turn-on wave-length and dark current increased with increasing x. Turn-on wavelengths ranged from λ=370 nm to 430 nm and dark current densities ranged from I_dark=2×10⁻² A/cm² (V_bias=5 V, x≈0.05) to 9×10⁴ A/cm² (V_bias=2 V, x≈0.13) depending on the In content, x, of the device active area.     

Large VLWIR Hg1−xCdxTe photovoltaic detectors

Very long wavelength infrared (VLWIR; 15 to 17 μm) detectors are required for remote sensing sounding applications. Infrared sounders provide temperature, pressure and moisture profiles of the atmosphere used in weather prediction models that track storms, predict levels of precipitation etc. Traditionally, photoconductive VLWIR (λ_c >15 μm) detectors have been used for sounding applications. However, photoconductive detectors suffer from performance issues, such as non-linearity that is 10X – 100X that of photovoltaic detectors. Radiometric calibration for remote sensing interferometry requires detectors with low non-linearity. Photoconductive detectors also suffer from non-uniform spatial optical response. Advances in molecular beam epitaxy (MBE) growth of mercury cadmium telluride (HgCdTe) and detector architectures have resulted in high performance detectors fabricated in the 15 μm to 17 μmm spectral range. Recently, VLWIR (λ_c ∼ 17 μm at 78 K) photovoltaic large (1000 μm diameter) detectors have been fabricated and measured at flux values targeting remote sensing interferometry applications. The operating temperature is near 78 K, permitting the use of passive radiators in spacecraft to cool the detectors. Detector non-AR coated quantum efficiency >60% was measured in these large detectors. A linear response was measured, while varying the spot size incident on the 1000 μm detectors. This excellent response uniformity, measured as a function of spot size, implies that low frequency spatial response variations are absent. The 1000 μm diameter, λ_c ∼ 17 μm at 78 K detectors have dark currents ∼160 μA at a −100 mV bias and at 78 K. Interfacing with the low (comparable to the contact and series resistance) junction impedance detectors is not feasible. Therefore a custom pre-amplifier was designed to interface with the large VLWIR detectors operating in reverse bias. A breadboard was fabricated incorporating the custom designed preamplifier interfacing with the 1000 μm diameter VLWIR detectors. Response versus flux measurements were made on the large VLWIR detectors and non-linearity <0.15% was measured at high flux values in the 2.5×10¹⁷ to 3.5×10¹⁷ ph-cm⁻²sec⁻¹ range. This non-linearity is an order of magnitude better than for photoconductive detectors.     

Status of the MBE technology at leti LIR for the manufacturing of HgCdTe focal plane arrays

This paper presents recent developments that have been made in Leti Infrared Laboratory in the field of molecular beam epitaxy (MBE) growth and fabrication of medium wavelength and long wavelength infrared (MWIR and LWIR) HgCdTe devices. The techniques that lead to growth temperature and flux control are presented. Run to run composition reproducibility is investigated on runs of more than 15 consecutively grown layers. Etch pit density in the low 10⁵ cm⁻² and void density lower than 10³ cm⁻² are obtained routinely on CdZnTe substrates. The samples exhibit low n-type carrier concentration in the 10¹⁴ to 10¹⁵ cm⁻³ range and mobility in excess of 10⁵ cm²/Vs at 77 K for epilayers with 9.5 μm cut-off wavelength. LWIR diodes, fabricated with an-on-p homojunction process present dynamic resistance area products which reach values of 8 10³ Ωcm² for a biased voltage of −50 mV and a cutoff wavelength of 9.5 μm at 77 K. A 320 × 240 plane array with a 30 μm pitch operating at 77 K in the MWIR range has been developed using HgCdTe and CdTe layers MBE grown on a Germanium substrate. Mean NEDT value of 8.8 mK together with an operability of 99.94% is obtained. We fabricated MWIR two-color detectors by the superposition of layers of HgCdTe with different compositions and a mixed MESA and planar technology. These detectors are spatially coherent and can be independently addressed. Current voltage curves of 60 × 60 μm² photodiodes have breakdown voltage exceeding 800 mV for each diode. The cutoff wavelength at 77 K is 3.1 μm for the MWIR-1 and 5 μm for the MWIR-2.     

MWIR DLPH HgCdTe photodiode performance dependence on substrate material

Mid wavelength infrared p-on-n double layer planar heterostructure (DLPH) photodiodes have been fabricated in HgCdTe double layers grown in situ by liquid phase epitaxy (LPE), on CdZnTe and for the first time on CdTe/sapphire (PACE-1). Characterization of these devices shed light on the nature of the material limits on device performance for devices performing near theoretical limits. LPE double layers on CdZnTe and on PACE-1 substrates were grown in a horizontal slider furnace. All the photodiodes are p-on-n heterostructures with indium as the n-type dopant and arsenic the p-type dopant. Incorporation of arsenic is via implantation followed by an annealing step that was the same for all the devices fabricated. The devices are passivated with MBE CdTe. Photodiodes have been characterized as a function of temperature. R₀A_imp values obtained between 300 and 78K are comparable for the two substrates and are approximately a factor of five below theoretical values calculated from measured material parameters. The data, for the PACE-1 substrate, indicates diffusion limited performance down to 110K. Area dependence gives further indications as to the origin of diffusion currents. Comparable R₀A_imp for various diode sizes indicates a p-side origin. R₀A and optical characteristics for the photodiodes grown on lattice-matched CdZnTe substrates and lattice mismatched PACE-1 are comparable. Howover, differences were observed in the noise characteristics of the photodiodes. Noise was measured on 50 × 50 μm devices held under a 100 mV reverse bias. At 110K, noise spectrum for devices from the two substrates is in the low 10⁻¹⁵ A/Hz^1/2 range. This value reflects the Johnson noise of the room temperature 10¹⁰ Ω feedback resistor in the current amplifier that limits the minimum measurable noise. Noise at 1 Hz, −100 mV and 120K for the 4.95 μm PACE-1 devices is in the 1–2 × 10⁻¹⁴ A/Hz^1/2, a factor of 5–10 lower than previously grown typical PACE-1 n⁺-on-p layers. Noise at 120K for the 4.60 μm PACE-1 and LPE on CdZnTe was again below the measurement technique limit. Greatest distinction in the noise characteristics for the different substrates was observed at 163K. No excess low frequency noise was observed for devices fabricated on layers grown by LPE on lattice-matched CdZnTe substrates. Photodiode noise measured at 1Hz, −100 mV and 163K in the 4.60 μm PACE-1 layer is in the 1–2×10⁻¹³ A/Hz^1/2, again a factor of 5–10 lower than previously grown PACE-1 n⁺-on-p layers. More variation in noise (4×10⁻¹³−2×10⁻¹² A/Hz^1/2) was observed for devices in the 4.95 μm PACE-1 layer. DLPH devices fabricated in HgCdTe layers grown by LPE on lattice-matched CdZnTe and on lattice-mismatched PACE-1 have comparable R₀A and quantum efficiency values. The distinguishing feature is that the noise is greater for devices fabricated in the layer grown on lattice mismatched substrates, suggesting dislocations inherent in lattice mismatched material affects excess low frequency noise but not zero bias impedance.     

H2-based dry plasma etching for mesa structuring of HgCdTe

An experimental study has been carried out on the performance of n-type x = 0.31 HgCdTe photoconductive detectors in order to evaluate two different etching techniques; dry plasma etching, in the form of H₂/CH₄ reactive ion etching (RIE), and wet chemical etching using bromine in hydrobromic acid. Two-dimensional laser beam-induced current (LBIC) imaging was employed as an in-line process monitoring tool to evaluate the lateral extent of reactive ion etching (RIE) induced doping changes in the HgCdTe epilayer following mesa delineation. Responsivity and noise measurements were performed on fabricated mid-wavelength infrared (MWIR) photoconductive devices to evaluate the influence dry plasma etching has on material properties. For a signal wavelength of 3 μm, 60° field of view, and a temperature of 80 K, background limited D _λ ^* performance was recorded for wet chemical processed devices but not for the dry plasma processed devices. The D _λ ^* values obtained for wet chemical and dry plasma etched photoconductive detectors were 2.5×10¹¹ cmHz^1/2W⁻¹ and 1.0×10¹⁰ cmHz^1/2W⁻¹, respectively. Mercury annealing, which has been shown to restore the electrical properties of dry plasma processed HgCdTe, could be used to lessen the influence that RIE dry plasma etching has on photoconductor detector performance.     

Effect of successive implantation of Ag+(Cu+) and Xe+ ions on the recombination properties of CdxHg1−x Te crystals

The effect of successive double implantation of Ag⁺(Cu⁺) and Xe⁺ ions on the recombination properties of Cd_xHg_1−x Te (0.2<x<0.3) crystals has been investigated. It is shown that after implantation of ions of one chemical element, followed by diffusion thermal annealing at temperatures below 150–200 K, recombination through local levels lying 30±5 meV below the conduction band bottom dominates. Successive double implantation of Ag⁺(Cu⁺) and Xe⁺ ions followed by diffusion thermal annealing changes the course of the temperature dependence of the lifetime of the nonequilibrium charge carriers. It was determined that for Cd_xHg_1−x Te crystals with x⋍0.20–0.25 in the temperature interval 700–200 K the lifetime of the nonequilibrium charge carriers is low (τ<0.15 μs) and does not depend on the temperature. For Cd_xHg_1−x Te crystals with x⋍0.3 recombination of nonequilibrium charge carriers occurs through two types of levels: in the temperature range 140–200 K — deep levels E _t1⋍E _c −51 meV and at lower temperatures (77–140 K) — through shallower levels E _t2⋍E _c −(16±2) meV. Fiz. Tekh. Poluprovodn. 31, 786–789 (July 1997)     

Correlation between visual defects and increased dark current in large-area Hg1−xCdxTe photodiodes

The Cross-Track Infrared Sounder (CrIS) program [an instrument on the National Polar-Orbiting Operational Environmental Satellite System (NPOESS)] requires photodiodes with spectral cutoffs denoted by short-wavelength infrared [γ_c(98 K) ∼5.1 μm], midwavelength infrared [γ_c(98 K) ∼9.1 μm], and long-wavelength infrared (LWIR) [γ_c(81 K) ∼15.5 μm]. The CrIS instrument also requires large-area (850-μm-diameter) photodiodes with state-of-art performance. Molecular beam epitaxy (MBE) is used to grow n-type short-wavelength infrared, midwavelength infrared, or LWIR Hg_1−xCd_xTe on latticematched CdZnTe. Detectors with p-type implants 7 μm in diameter are used to constitute the 850-μm-diameter lateral collection diodes (LCDs). The photodiode architecture is the double-layer planar heterostructure architecture. Quantum efficiency, I-V, R_d-V, and 1/f noise in photovoltaic Hg_1−xCd_xTe detectors are critical parameters that limit the sensitivity of infrared sounders. These are some of the parameters used to select photodiodes that will be part of the CrIS focal plane module (FPM). During fabrication of the FPM, the photodiodes are subject to a significant amount of handling while transitioning from part of newly processed Hg_1–xCd_xTe wafers to individual photodiodes mounted in a CrIS FPM ready to be flown on NPOESS. Quantum efficiency, I-V, noise, and visual inspections are performed at several steps in the detector’s journey. Initial I-V and visual inspections are conducted at the wafer level followed by I-V, noise, and quantum efficiency after dicing and mounting the photodiodes in leadless chip carriers (LCCs). A visual inspection is performed following removal of the detectors from the LCCs. Finally, the individual photodiodes are precision mounted on an FPM base, and I-V, noise, quantum efficiency, and visual inspections are performed again. Each step in the FPM fabrication process requires handling and environmental conditioning that can result in detector dark current and noise increase. Some photodiodes on the first flightlike FPMs fabricated exhibited an increase in dark current and noise characteristics at the FPM level as compared to the measurements performed when the photodiodes were in LCCs prior to integration into the FPM. The degradation observed resulted in an investigation to discern the cause of the performance degradation (baking at elevated temperatures, mechanical handling, electrical stress, etc.). This paper outlines the results of the study and the corrective actions that led to the successful manufacture of LWIR large detectors from material growth to insertion into flight FPMs for the CrIS program.     

Molecular-beam epitaxial growth of HgCdTe infrared focal-plane arrays on silicon substrates for midwave infrared applications

Molecular beam epitaxy has been employed to deposit HgCdTe infrared detector structures on Si(112) substrates with performance at 125K that is equivalent to detectors grown on conventional CdZnTe substrates. The detector structures are grown on Si via CdTe(112)B buffer layers, whose structural properties include x-ray rocking curve full width at half maximum of 63 arc-sec and near-surface etch pit density of 3–5 × 10⁵ cm⁻² for 9 μm thick CdTe films. HgCdTe p⁺-on-n device structures were grown by molecular beam epitaxy (MBE) on both bulk CdZnTe and Si with 125K cutoff wavelengths ranging from 3.5 to 5 μm. External quantum efficiencies of 70%, limited only by reflection loss at the uncoated Si-vacuum interface, were achieved for detectors on Si. The current-voltage (I-V) characteristics of MBE-grown detectors on CdZnTe and Si were found to be equivalent, with reverse breakdown voltages well in excess of 700 mV. The temperature dependences of the I-V characteristics of MBE-grown diodes on CdZnTe and Si were found to be essentially identical and in agreement with a diffusion-limited current model for temperatures down to 110K. The performance of MBE-grown diodes on Si is also equivalent to that of typical liquid phase epitaxy-grown devices on CdZnTe with R₀A products in the 10⁶–10⁷ Θ-cm² range for 3.6 μm cutoff at 125K and R₀A products in the 10⁴–10⁵ Θ-cm² range for 4.7 μm cutoff at 125K.     

Small two-dimensional arrays of mid-wavelength infrared HgCdTe diodes fabricated by reactive ion etching-induced p-to-n-type conversion

The reactive ion etching (RIE) technique has been shown to produce high-performance n-on-p junctions by localized-type conversion of p-type mid-wavelength infrared (MWIR) HgCdTe material. This paper presents variable area analysis of n-on-p HgCdTe test diodes and data on two-dimensional (2-D) arrays fabricated by RIE. All devices were fabricated on x = 0.30 to 0.31 liquid-phase epitaxy (LPE) grown p-type (p = ∼1 × 10¹⁶ cm⁻³) HgCdTe wafers obtained from Fermionics Corp. The diameter of the circular test diodes varied from 50 μm to 600 μm. The 8 × 8 arrays comprised of 50 μm × 50 μm devices on a 100-μm pitch, and all devices were passivated with 5000 ? of thermally deposited CdTe. At temperatures >145 K, all devices are diffusion limited; at lower temperatures, generation-recombination (G-R) current dominates. At the lowest measurement temperature (77 K), the onset of tunneling can be observed. At 77 K, the value of 1/R₀A for large devices shows quadratic dependence on the junction perimeter/area ratio (P/A), indicating the effect of surface leakage current at the junction perimeter, and gives an extracted bulk value for R₀A of 2.8 × 10⁷ Ω cm². The 1/R₀A versus P/A at 195 K exhibits the well-known linear dependence that extrapolates to a bulk value for R₀A of 17.5 Ω cm². Measurements at 77 K on the small 8 × 8 test arrays were found to demonstrate very good uniformity with an average R₀A = 1.9 × 10⁶ Ω cm² with 0° field of view and D* = 2.7 × 10¹¹cm Hz^1/2/W with 60° field of view looking at 300 K background.     

Planar p-on-n HgCdTe heterojunction mid-wavelength infrared photodiodes formed using plasma-induced junction isolation

Planar p-on-n HgCdTe heterojunction photodiodes have been fabricated using a plasma-induced type conversion process for device junction isolation. The technique is presented as a fully planar alternative technology to the commonly used mesa isolation structure. The starting material consisted of an indium-doped n-type mid-wavelength infrared (MWIR) HgCdTe absorbing layer that was capped by a 1-μm-thick wider bandgap arsenic-doped p-type layer. Junction-isolated p-on-n diodes were formed by selectively p-to-n type converting the p-type cap layer using a plasma process. Photodiode dark current-voltage measurements were performed as a function of temperature, along with noise and responsivity. The devices have cut-off wavelengths between 4.8 μm and 5.0 μm, exhibit diffusion-limited dark currents down to 145 K, give R₀A values greater than 1 × 10⁷Ω·cm² at 80 K and greater than 1 × 10⁵Ω·cm² at 120 K, and have negligible 1/f noise current at zero applied bias.     

Planar n-on-p ion milled mid-wavelength and long-wavelength infrared diodes on molecular beam epitaxy vacancy-doped CdHgTe on CdZnTe

Planar mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) photodiodes were fabricated by ion milling molecular beam epitaxy (MBE) Cd_xHg_1−xTe (CMT) layers with and without compositional grading in the layer. Linear arrays with 32 and 64 diodes, as well as test diodes of varying size, were fabricated. Good quantum efficiencies were measured, and MWIR diodes, with cutoff wavelength λ_CO=4.5 μm, had zero-bias resistance-area values (R₀A) in excess of 1×10⁷ Ωcm², whereas LWIR diodes with λ_CO=8.9−9.3 μm had R₀A=3×10² Ωcm² at 77 K. Comparison between a limited number of layers indicates that in layers with a gradient the RA values are a factor of ∼10 larger, and possibly more uniform, than in layers without a gradient.     

HgCdZnTe quaternary materials for lattice-matched two-color detectors

As the number of bands and the complexity of HgCdTe multicolor structures increases, it is desirable to minimize the lattice mismatch at growth interfaces within the device structure in order to reduce or eliminate mismatch dislocations at these interfaces and potential threading dislocations that can degrade device performance. To achieve this we are investigating the use of Hg_1−x−yCd_xZn_yTe quaternary alloys which have an independently tunable lattice constant and bandgap. Lattice matching in Hg_1−x−yCd_xZn_yTe structures can be achieved using small additions of Zn (y<0.015) to HgCdTe ternary alloys. We have investigated some of the basic properties of Hg_1−x−yCd_xZn_yTe materials with x≈0.31 and 0≤y≤0.015. The quaternary layers were grown on (112)CdZnTe substrates using MBE and the amount of Zn in the layers was determined from calibrated SIMS measurements. As expected, the lattice constant decreased and the bandgap increased as Zn was added to HgCdTe to form Hg_1−x−yCd_xZn_yTe. Hall-effect results for both n-type (In) and p-type (As) Hg_1−x−yCd_xZn_yTe layers were very similar to HgCdTe control samples. We have also utilized x-ray rocking curve measurements with (246) asymmetric reflections as a novel sensitive technique to determine the correct amount of Zn needed to achieve lattice matching at an interface. MWIR/LWIR n-p-n two-color triple-layer heterojunction structures were grown to evaluate the effects of minimizing the lattice mismatch between the widest bandgap p-type collector layer, using Hg_1−x−yCd_xZn_yTe, and the HgCdTe MWIR and LWIR collector layers and compared to structures that did not incorporate the quaternary. Sequential mode two-color detectors were fabricated using a 256 × 256, 30 μm unit cell design. There were several interesting findings. Macro defects predominantly affected the LWIR band (Band 2) operability and had little effect on the MWIR band (Band 1). The incorporation of Hg_1−x−yCd_xZn_yTe p-type collector layers had little effect on MWIR detector performance, but overall the LWIR performance was generally better. These initial detector results indicate that the use of Hg_1−x−yCd_xZn_yTe alloys in multicolor detector structures are potentially promising and should be pursued further.     

InAsSb/InAsSbP double-heterostructure lasers emitting in the 3–4 µm spectral range

Our earlier reports concerning the fabrication by liquid-phase epitaxy and investigation of InAsSbP/InAsSb/InAsSbP double heterostructure lasers emitting at 3–4 μm are reviewed. The dependences of spectral characteristics and the spatial distribution of the laser emission on temperature and current are discussed. Lasing modes are shifted by 0.5–1.0 cm⁻¹ to longer wavelengths with increasing temperature. The tuning of the lasing modes by means of current is very fast (10⁻⁸–10⁻¹² s). With increasing current, the modes are shifted to shorter wavelengths by 50–60 ? at 77 K. The maximum mode shift of 104 ? (10 cm⁻¹) is observed at 62 K. The spectral line width of the laser is as narrow as 10 MHz. Abnormally narrow directional patterns in the p-n junction plane are observed in some cases in the spatial distribution of laser emission. The current tuning of lasers, due to nonlinear optical effects, has been modeled mathematically in good agreement with the experiment. Transmittance spectra of OCS, NH₃, H₂O, CH₃Cl, and N₂O gases were recorded using current-tuned lasers. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 12, 2001, pp. 1466–1480. Original Russian Text Copyright ? 2001 by Danilova, Imenkov, Kolchanova, Yakovlev. See [1].     

Imaging one-dimensional and two-dimensional planar photodiode detectors fabricated by ion milling molecular beam epitaxy CdHgTe

Imaging one-dimensional (1-D) and two-dimensional (2-D) arrays of mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) planar photodiodes were fabricated by ion milling of vacancy-doped molecular beam epitaxy Cd_xHg_1−xTe layers. Sixty-four-element 1-D arrays of 26×26 μm² or 26×56 μm² diodes were processed. Zero-bias resistance-area values (R₀A) at 77 K of 4×10⁶ Θcm² at cutoff wavelength λ_CO=4.5 μm were measured, as well as high quantum efficiencies. To avoid creating a leakage current during ball bonding to the 1-D array diodes, a ZnS layer was deposited on top of the CdTe passivation layer, as well as extra electroplated Au on the bonding pads. The best measured noise equivalent temperature difference (NETD) on a LWIR array was 8 mK, with a median of 14 mK for the 42 operable diodes. The best measured NETD on a MWIR array was 18 mK. Two-D arrays showed reasonably good uniformity of R₀A and zero-bias current (I₀) values. The first 64×64 element 2-D array of 16×16 μm² MWIR diodes has been hybridized to read-out electronics and gave median NETD of 60 mK.     

MOVPE growth of HgCdTe for high performance 3–5 µm photodiodes operating at 100–180K

New results are reported on the growth of high performance medium wavelength infrared (3–5 μm) (MWIR) HgCdTe photodiodes in the three-layer P-n-N configuration. The detector structures were grown in situ by metalorganic vapor phase epitaxy (MOVPE) on (211)B oriented CdZnTe substrates. The mobilities of the single n-type layers with x-values of ∼0.30 are in the range of (3–4.5)×10⁴ cm²/V-s at 80K. The lifetimes on unpassivated films range from 1–5 and 4–10 μs at 80 and 180K, respectively, which are within a factor of two or less of the lifetimes calculated for Auger-1 and radiative recombination. The P-n-N films were processed into variable-area backside-illuminated diagnostic arrays and tested for quantum efficiency, spectral response, R_DA, I–V curves and 1/f noise in the 120–180K range. The internal one-dimensional quantum efficiencies are in the range of 85–100%. The optical collection lengths are typically ∼25 μm. I–V curves showed that diffusion current is the dominant junction current mechanism for temperatures ≥100K. R₀A values are at the one-dimensional limit for n-side diffusion currents over the 100–180K range. 1/f noise was measured to be very low at 120K and is the same as that measured in similarly processed arrays from recent LPE grown P-on-N heterojunctions. The results demonstrate that MOVPE growth can be used for large area, high performance MWIR HgCdTe detector arrays operating in the 120–180K temperature range.     

High-pressure preparation and thermoelectric properties of Bi0.85Sb0.15

Nanocrystalline Bi_0.85Sb_0.15 powders were prepared by a novel mechanical alloying method. The bulk samples were formed by applying a pressure of 6 GPa at different pressing temperatures and times. Electrical conductivity, Seebeck coefficients, and thermal conductivity were measured in the temperature range 80–300 K. The Seebeck coefficient reaches a maximum value of −173 μV/K at 150 K. The largest figure of merit, 3.46 × 10⁻³ K⁻¹, achieved in this experiment is 50% higher than that of its single-crystal counterpart at 200 K.     

Stacked multijunction photodetectors of long-wavelength radiation

Uncooled, long-wavelength photovoltaic detectors suffer from poor quantum efficiency and low differential resistance. The problem can be solved by the use of stacked, multiple heterojunction-photovoltaic cells with thin absorber regions. We report here numerical simulation and optimization of the stacked, multiple Hg_1−xCd_xTe heterojunction cells used for detection of 10.6-μm infrared (IR) radiation, operating as zero-bias photovoltaic devices or Auger-suppressed photodiodes. It is shown that the devices can be used as high-performance and fast-response detectors of long-wavelength radiation operating at ambient temperature with detectivities larger by more than one order of magnitude than that of the present practical devices.