Eliezer Weiss

XBn barrier photodetectors based on InAsSb with high operating temperatures

We demonstrate the suppression of the bulk generation- recombination current in nBn devices based on an InAsSb active layer (AL) and a AlSbAs barrier layer (BL). This leads to much lower dark cur- rents than in conventional InAsSb photodiodes operating at the same temperature. When the BL is p-type, very high doping must be used in the AL (nBpn + ). This results in a significant shortening of the device cut- off wavelength due to the Moss-Burstein effect. For an n-type BL, low AL doping can be used (nBnn), yielding a cutoff wavelength of ∼4.1 μm and a dark current close to ∼3 × 10 −7 A/cm 2 at 150 K. Such a device with a4 -μm-thick AL will exhibit a quantum efficiency (QE) of 70% and background-limited performance operation up to 160 K at f/3. We have madenBnnfocalplane arraydetectors(FPAs)with a 320 ×256 formatand a 1.3-μm-thick AL. These FPAs have a 35% QE and a noise equivalent temperature difference of 16 mK at 150 K and f/3. The high performance of our nBnn detectors is closely related to the high quality of the molecular beam epitaxy grown InAsSb AL material. On the basis of the temperature dependence of the diffusion limited dark current, we estimate a minority carrier lifetime of ∼670 ns. C 2011 Society of Photo-Optical Instrumentation Engineers

Strain effects in Hg1−xCdxTe (x∼0.2) photovoltaic arrays

The effect of stress and strain on the performance of Hg1−xCdxTe (x∼0.2) photovoltaic arrays was studied both in the dark and under illumination. Stress, external as well as internal, affects the current–voltage characteristic of the photodiode. The combined action of illumination and strain yields an anomalous response to light absorption in the device. A model is conceived wherein the photodiode and guard ring are treated as a metal‐insulator semiconductor field effect transistor (MISFET). Stress developed in the vicinity of small contact windows causes n‐type damage, which brings about a forward bias in the device. The effect of strain on the reverse current of the photodiode is explained by a change in the n‐channel conductivity of the MISFET. This change is caused by charges which are due either to a piezoelectric effect or n‐type damage. Using this model observed phenomena in Hg1−xCdxTe photovoltaic arrays are explained, as due to internal stresses originating from wafer deformation.

Mechanism of carrier transport across the junction of narrow band-gap planar n+p HgCdTe photodiodes grown by liquid-phase epitaxy

The dark current of n+p Hg1−xCdxTe (x∼0.22) photodiodes fabricated on layers grown by liquid-phase epitaxy was studied systematically as a function of applied voltage (reverse and forward), temperature, and junction dimensions. The carrier transport mechanisms in different ranges of bias voltage and temperature have been identified and interpreted using the conventional models for carrier transport mechanisms in p-n junction devices. It is found that the forward current is dominated by a combination of bulk- and surface-related tunneling currents. In large-area diodes bulk tunneling current dominates the diode current and in small-area diodes surface tunneling dominates the diode forward current. Most probably, the tunneling mechanism across the n+p junction is via energy levels located within the diode depletion region. Modification of the existing theoretical model for forward-bias tunneling in p-n junction devices is needed in order to account for the experimental findings. Above 90K, the reverse curre...

Growth, annealing, and characterization of HgZnTe liquid phase epitaxy layers

Epitaxial layers of Hg1−xZnxTe(0.12≤x≤0.25) were grown by horizontal liquid phase epitaxy technique on closely lattice matched CdZnTe substrates. The growth conditions in which the rate of growth was limited by the diffusion of the solutes in the liquid were found. Post‐growth annealing study of the epilayers revealed the ability to control the epilayers electrical properties by in‐diffusion of Hg. The annealing at the temperature of 340 °C resulted in p‐type epilayers with hole concentration of ≊1×1016 cm−3 , mobility of 380–600 cm2 /V s and lifetime of 20–40 ns (for long‐wavelength infrared epilayers), measured at 77 K. The results of the annealing at the temperature of 220 °C were dependent on the annealing time and the substrates. n‐Type epilayers were characterized, showing electron mobility dependence on the energy band gap and temperature. The results of preliminary experiments of ion implantation into the annealed, p‐type epilayers are discussed.

Effects of anodic fluoro‐oxide on the thermal stability of Hg1−xCdxTe photoconductive arrays

Hg1−xCdxTe (x∼0.2) photoconductive arrays passivated with anodic fluoro‐oxides show an improved thermal stability relative to arrays fabricated with anodic oxides. The performance of the photoconductors with the anodic fluoro‐oxide is only slightly degraded by annealing at temperatures up to 100–105 °C, in contrast to the monotonic decrease observed in arrays passivated with an anodic oxide caused by annealing above 70 °C. The improved stability of the fluoro‐oxides does not depend much on the bath composition, as long as it is a solution of hydroxyl and fluoride ions. Both secondary ion mass spectroscopy and low‐energy proton induced nuclear reaction, which is very sensitive to fluorine atoms, were used as depth profile probes. It was found that the fluorine concentrated at the anodic film–semiconductor interface as well as on the film surface. A mechanism by which fluorine is deposited in such a manner is advanced.

The formation of twins in 〈111〉 HgCdTe grown by travelling heater method and their effect on device quality

Hg1−xCdxTe crystals with x∼0.2 were grown by the traveling heater method (THM) in either the [111]A or the 1185 [111]B directions using oriented CdTe seeds. Lamellar and double position twins are sometimes formed during the growth of these crystals. The lamellar twinning occurs during the layer-by-layer mode of growth, and is due to high stresses originating from high thermal gradients. Double-position (DP) twins develop under multinucleated growth. This type of growth occurs towards the end of the growth run due to the concave shape of the growth interface at that stage. In crystals grown in the [111]A direction, the lamellar twins, of orientation [511]B, are constantly growing at the expense of the original [111]A oriented grain. Growth in the [111]B direction, on the other hand, suppresses the growth of the lamellar twin domain. The annihilation is due to arrays of small size interstitial Aδ+ ions. Ingots grown in the [111]B direction are almost entirely single crystalline, with a growth axis of the type. Photodiodes and capacitors realized on the (111)A plane are markedly superior to those on the lamellar twin plane, (511)B. The difference is due to a much higher fixed charge and larger fast surface state density in the case of the (511)B plane. These effects are explained by the lattice structures in the {111} and {511} planes and their possible influence on surface reactivity.

The characterization of anodic fluoride films on Hg1−xCdxTe and their interfaces

A novel anodic fluoridization process for forming native fluoride films on Hg1−xCdxTe is described. As in the case of the anodic oxide, the anodic fluoride grows in two steps: a dissolution–precipitation step is followed by a bulk growth. Anodic fluorides free of oxygen are grown from nonaqueous solutions. The presence of water causes the growth of films containing both fluorine and oxygen, whereas a low hydroxyl ion concentration causes the growth of anodic oxide alone. However, its electrical behavior is different from that of anodic oxide grown from fluoride‐free baths. The results of Auger electron spectroscopy analysis indicate that the anodic fluoride is composed of cadmium fluoride in a matrix of tellurium, cadmium, and mercury. The fluoride is homogeneous and has a relatively abrupt dielectric–semiconductor interfacial transition, free of contaminants. Metal–insulator–semiconductor devices containing a fluoride film and evaporated ZnS have been fabricated and characterized. By choosing an appropri...

Substrate quality impact on the carrier concentration of undoped annealed HgCdTe LPE layers

The impact of Te precipitates and impurities, in CdZnTe or CdTe substrates, on grown liquid phase epitaxy (LPE) HgCdTe layer hole concentrations was studied. The carrier concentrations in capped annealed LPE HgCdTe layers grown on CdZnTe substrates with large densities of Te precipitates are frequently significantly higher than those expected for HgCdTe annealed under Hg-deficient conditions. The carrier concentration in the LPE layer, due to the diffusion of copper ions from contaminated CdTe substrates into the layer, is strongly affected by the polarity of the (111)-oriented substrates. Layers grown on the (111)A face showed very high concentrations of Cu, whereas in those grown on the (111)B face normal carrier concentrations were achieved. These phenomena are discussed on the basis of defects formed either in the epilayer or in the layer-substrate interface.

Type II superlattice technology for LWIR detectors

SCD has developed a range of advanced infrared detectors based on III-V semiconductor heterostructures grown on GaSb. The XBn/XBp family of barrier detectors enables diffusion limited dark currents, comparable with MCT Rule-07, and high quantum efficiencies. This work describes some of the technical challenges that were overcome, and the ultimate performance that was finally achieved, for SCD’s new 15 μm pitch “Pelican-D LW” type II superlattice (T2SL) XBp array detector. This detector is the first of SCDs line of high performance two dimensional arrays working in the LWIR spectral range, and was designed with a ~9.3 micron cut-off wavelength and a format of 640 x 512 pixels. It contains InAs/GaSb and InAs/AlSb T2SLs, engineered using k • p modeling of the energy bands and photo-response. The wafers are grown by molecular beam epitaxy and are fabricated into Focal Plane Array (FPA) detectors using standard FPA processes, including wet and dry etching, indium bump hybridization, under-fill, and back-side polishing. The FPA has a quantum efficiency of nearly 50%, and operates at 77 K and F/2.7 with background limited performance. The pixel operability of the FPA is above 99% and it exhibits a stable residual non uniformity (RNU) of better than 0.04% of the dynamic range. The FPA uses a new digital read-out integrated circuit (ROIC), and the complete detector closely follows the interfaces of SCD’s MWIR Pelican-D detector. The Pelican- D LW detector is now in the final stages of qualification and transfer to production, with first prototypes already integrated into new electro-optical systems.

Antimonide-based materials for infrared detection

We propose that the antimonide family of semiconductors should be considered in some cases as a serious alternative to Mercury Cadmium Telluride (MCT) for the active region of next generation IR detectors, based on epitaxial materials. Among the alloys, epitaxial InAs1-ySby on GaSb with 0.07 < y < 0.11 and In1-zAlzSb on InSb with 0 < z < 0.03 together span important regions of the MWIR atmospheric window, yet exhibit strains of less than 0.15%. Both InSb and GaSb are binary substrates available in high quality. The sensitivity of bandgap to composition in In1-zAlzSb is similar to that in MCT. However, in InAs1-ySby this sensitivity is more than halved. In growth from the gas phase, the constraints on temperature stability are about 3 - 5 times lower than in MCT. Together, these characteristics make it easier to achieve high uniformity, particularly in InAs1-ySby. Finally, high quality superlattices based on InAs/Ga1-xInxSb can be grown by lattice matching to GaSb. This epitaxial material is emerging as an attractive alternative to MCT with a high degree of spatial uniformity and with an ability to span cut-off wavelengths from 3-20m in a single material system.