Liquid Phase Epitaxy HgCdTe Research Articles

Improved responsivity and detectivity of LPE HgCdTe short-wavelength infrared photodetector by tuning the composition gradient

This work introduces an effective way to improve the responsivity and detectivity of liquid phase epitaxy (LPE) HgCdTe avalanche focal plane array (FPA) for short-wavelength infrared (SWIR) signal detection by tuning the composition gradient. It is contributed to the Hg-Cd interdiffusion at the HgCdTe/CdZnTe interface during the heat treatment. The Cd composition can be redistributed, which gives different composition (band gap) in the absorption and multiplication regions. The newly designed FPA achieves a 50% improvement in responsivity (gain) and a 30% increase in detectivity over the conventional SWIR FPA (−3 V, 80 K). The newly designed FPA also exhibits a superior performance at 130 K than the conventional FPA at 80 K.

A new surface-flattening method using single-point diamond turning (SPDT) and its effects on LPE HgCdTe photodiodes

Excellent surface flatness of HgCdTe wafers is essential for fabricating large infrared focal plane arrays (IRFPAs) within the framework of a flip-chip bonding yield. However, liquid-phase epitaxy (LPE) HgCdTe wafers, which are widely used for IRFPAs, have inherent problems pertaining to surface flatness. In this study, we introduced a single-point diamond turning (SPDT) method for use in fabricating LPE HgCdTe photodiodes. The cutoff wavelength of the wafers is ∼5 µm. The surface roughness of the LPE HgCdTe wafer has been substantially reduced but a type-converted defective layer was formed on the surface of the HgCdTe wafer after SPDT, which was confirmed using Hall and capacitance–voltage (C–V) measurements. The defective layer, however, was easily removed by bromine in methanol (Br–MeOH) etching. The fabricated photodiode showed a dynamic resistance–area product at the zero bias (R0A) value of ∼1 × 104 Ω cm2 for a junction area of 30 × 30 µm2 at 80 K, which is equivalent to that of a conventional photodiode. The flip-chip bonding efficiency has been remarkably improved from 89.43% to 99.99% for 320 × 256 IRFPA at room temperature after SPDT.

Correlation of laser-beam-induced current with current-voltage measurements in HgCdTe photodiodes

Laser-beam-induced current (LBIC) is being investigated as an alternative to electrical measurements of individual photodiodes in a two-dimensional array. This is possible because LBIC only requires two electrical contacts to an array and the two-dimensional scanning of a focused laser beam across the array to image the entire array. The measured LBIC profiles, obtained from linear arrays of HgCdTe photodiodes, will be used to study the uniformity of photodiodes in the array and to extract the R0A of the photodiodes. It will be shown that the shape of the LBIC signal is correlated to the electrical performance of the photodiode, with R0A related to the spreading length of the photodiodes. Linear arrays of n-on-p, mid-wavelength infrared (MWIR) and long wave-length infrared (LWIR) devices were formed in liquid-phase epitaxy HgCdTe epilayers using a plasma junction-formation technique. The LBIC profiles were measured on each of the devices at various temperatures. For the MWIR devices, the extracted spreading length shows no correlation with R0A. However, the LBIC signal does detect nonuniform devices within the array. For the case of the LWIR devices, the spreading length is extracted as a function of temperature, with the R0A subsequently calculated from the spreading length. The calculated R0A, obtained without requiring contact to each photodiode in the array, agrees well with electrical measurements. Asymmetry of the LBIC signals for certain devices in the arrays is shown to be a result of localized leakage at the photodiode junction or from the contact pads through the passivation layers. These results are confirmed by numerical modeling of the device structures.

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.

Composition and thickness control of thin LPE HgCdTe layers using x-ray diffraction

Double-axis x-ray rocking curve measurements have been used to nondestructively characterize the composition profile of HgCdTe heterojunction photodiode structures grown by liquid phase epitaxy (LPE). In particular, the thickness and composition profile of the thin graded-composition cap layer are determined through an empirical correlation between rocking curve parameters and composition profiles measured by SIMS. Spatial maps of cap layer thickness and composition are generated from automated measurements of x-ray rocking curves across a wafer. X-ray mapping has been instrumental in improving the spatial uniformity of cap layers and in maintaining control of the growth process in Hg-rich LPE dipping reactors.

X-ray photoelectron spectroscopy study of oxide and Te overlayers on as-grown and etched HgCdTe

X-ray photoelectron spectroscopy has been used to study the low-temperature (<80°C) preparation of HgCdTe surfaces with atomic hydrogen for the purpose of CdTe passivation. Atomic hydrogen was used to etch overlayers and surfaces of liquid phase epitaxy and molecular beam epitaxy HgCdTe, CdTe, HgTe, and Te. Oxide layers were easily removed, while carbon overlayers were resistant to atomic hydrogen etching at low temperature. Both Te and HgTe are etched by atomic hydrogen, with the HgTe etch rate about twice that of Te, while CdTe and ZnTe are not etched. Chemi-mechanical polishing of liquid phase epitaxy HgCdTe left a 10 to 20A Te overlayer that could be removed with atomic hydrogen. In all cases, exposure of HgCdTe to atomic hydrogen led to surface composition shifts to higher x-value, with an x-value plateau near x∼0.6. All observations could be explained in terms of the formation of a 15A surface layer of CdTe which blocks further etching.

The relationship between lattice matching and crosshatch in liquid phase epitaxy HgCdTe on CdZnTe substrates

X-ray topography provides a very sensitive map of lattice mismatch between a HgCdTe LPE epitaxial layer and its (111) CdZnTe substrate. A well-defined Crosshatch pattern in the three «110» directions indicates a positive room-temperature lattice mismatch. For conditions of near-perfect lattice matching (±0.003% mismatch), the Crosshatch pattern disappears, presumably because there are few or no misfit dislocations present near the interface, and a region free of topographic contrast is observed. The crosshatch-free region occurs for a small positive room-temperature mismatch (about 0.02%); this is attributed to differences in the lattice matching condition at room temperature and the growth temperature. For negative mismatches, where the film is in tension, a mosaic pattern, rather than a crystallographically oriented Crosshatch, is observed in the topograph. Rocking curve full width at half maximum of the epitaxial layer is minimized in the crosshatch-free zone at a value nearly equal to that of the substrate. Etch pit density of the HgCdTe layer shows a strong minimum for perfect room temperature lattice matching, with values as low as 1 x 104 cm−2. For nearly lattice matched layers, Crosshatch is present throughout the thickness of the epitaxial layer except for a narrow graded-composition region near the substrate interface. Crosshatch contrast appears to result from long-range strain fields associated with a misfit dislocation network near the substrate interface. Spatial variations in topographic features and mismatch across relatively small lateral distances are caused by variations in substrate alloy composition. For truly lattice-matched substrates, better control over the substrate lattice parameter is required.

CdZnTe substrate impurities and their effects on liquid phase epitaxy HgCdTe

Impurity levels were tracked through the stages of substrate and liquid phase epitaxy (LPE) layer processing to identify sources of elements which degrade infrared photodetector performance. Chemical analysis by glow discharge mass spectrometry and Zeeman corrected graphite furnace atomic absorption effectively showed the levels of impurities introduced into CdZnTe substrate material from the raw materials and the crystal growth processes. A new purification process(in situ distillation zone refining) for raw materials was developed, resulting in improved CdZnTe substrate purity. Substrate copper contamination was found to degrade the LPE layer and device electrical properties, in the case of lightly doped HgCdTe. Anomalous HgCdTe carrier type conversion was correlated to certain CdZnTe and CdTe substrate ingots.

New developments in the liquid-phase epitaxy of [formula omitted

The intrinsic II–VI semiconductor Hg 1−x Cd x Te is the most important material for medium-wavelength (3–5 μm) and longwavelength (8–14 μm) IR detectors. Recent advances in the epitaxial growth of this ternary alloy are the main reason for its technological success in second-generation devices. In particular, liquid-phase epitaxy (LPE) structures (from Te-rich and Hg-rich solution) are nowadays the most interesting for advanced focal plane array fabrication. The complexity of required structures continue to increase in the following directions: (1) the number of epitaxial layers from which the structures needed for device generation are composed (heterojunctions), coupled with their precise doping control; (2) the use of alternative substrates to bulk CdZnTe allowing very-large-area (10 cm 2 or greater) structure preparation at reduced cost. The more recent development in the LPE HgCdTe heterojunction fabrication will be reported in this paper as well as the state of the art concerning the growth on sapphire and silicon-based hybrid substrates. The on-line materials characterization methods and results will be discussed from the viewpoint of a crystal growth laboratory technologically oriented.

Hall effect characterization of LPE HgCdTe P/n heterojunctions

The field and temperature dependence of the Hall coefficient has been used to simultaneously extract information about the p and n layers in very long wave length infrared P/n HgCdTe heterojunctions. The field dependence allows the effects of high mobility electrons to be separated from those of low mobility holes. The higher the magnetic field, the higher the sensitivity to the parameters of the P layer. For a maximum magnetic field of 8000 gauss, the hole sheet concentration must be at least five times the electron sheet concentration to obtain accurate results for the P layer. This criterion is satisfied for typical liquid phase epitaxy (LPE) heterostructures. The analysis determines the hole sheet resistance (concentration times mobility), rather than the hole concentration or mobility separately. Independent knowledge of the P layer thickness and the relationship between hole concentration and resistivity are needed to convert the Hall measurement results to hole concentrations. Analysis of the field-dependent Hall data is complicated by the finding that at least three electrons of different mobilities are needed to fit the field dependence of the Hall coefficient in n-type LPE HgCdTe layers. These results are consistent with previous conclusions that electrons with different mobilities are needed to model bulk n-HgCdTe, and with a range of mobilities in the graded composition interface between the LPE layer and CdTe substrate. Consistent results are obtained for the concentrations and mobilities of the three types of electrons in the n-HgCdTe layer with and without the P layer present. N and P type carrier concentrations are also consistent with dopant concentrations measured by secondary ion mass spectroscopy.

Reduced metal–insulator semiconductor tunneling in metalorganic chemical vapor deposition HgCdTe(111)Te films

Metal–insulator semiconductor tunnel currents in metalorganic chemical vapor deposition (MOCVD) HgCdTe(111)Te layers are measured to be comparable to those found in high quality liquid phase epitaxy HgCdTe(111)Te. These tunnel currents are found to increase if CdTe rather than lattice‐matched CdZnTe substrates are chosen or if the (001) growth orientation is selected. The rotational twins that generally form with (111) oriented HgCdTe growth by MOCVD are not detectable in more than 60% of the films characterized in this work. These results mark an important step towards the use of MOCVD HgCdTe heterostructure or heterojunction films for high performance infrared focal plane arrays.

Laser beam induced current imaging of surface nonuniformity at the HgCdTe/ZnS interface

Laser beam induced current has been utilized to obtain maps of the electrical nonuniformities at the ZnS‐passivated liquid phase epitaxy HgCdTe surface under a metal–insulator–semiconductor (MIS) capacitor. Features are observed that are not indicated in the C–V profile for the MIS structure. This technique represents a convenient way of spatially mapping the electrical characteristics of semiconductor surfaces and offers the potential for assessing and optimizing the quality and uniformity of the passivating layer.

Spatial mapping of electrically active defects in HgCdTe using laser beam-induced current

A high-resolution and nondestructive optical characterization technique called laser beam-induced current (LBIC) has been developed and utilized to obtain maps of electrically active defects in liquid phase epitaxy HgCdTe. The LBIC technique is also suitable for studying the p–n junction detector elements in an array nondestructively, without requiring any electrical contacts to individual detector elements.

P on n ion-implanted junctions in liquid phase epitaxy HgCdTe layers on CdTe substrates

The first demonstrated achievement of p on n-type activated junctions in HgCdTe material by arsenic ion implantation is reported. The junctions were formed by treating the implant as a finite diffusion source in the post-implant anneals. The materials employed for this study were n-type indium impurity-doped liquid phase epitaxy HgCdTe grown on CdTe. Arsenic was selected as the candidate acceptor impurity since it activated during post-implanted anneals in Hg vapor. The arsenic concentration profile determined by secondary ion mass spectroscopy showed that during post-implant anneal a complex diffusion mechanism redistributes the arsenic. The activation efficiency appears to be mechanism dependent. In the junction region the implant activation efficiency is about 50%. Junction depth can be controlled by varying arsenic diffusion and background carrier concentration. Junction depths determined by the electron beam induced current technique were consistent with the differential Hall electrical profiles. A p on n junction is shown with excellent rectification characteristics and high breakdown voltage (sharp at 2.5 V and 77 °C). This was obtained from a liquid phase epitaxy Hg1−xCdxTe, with a compositional factor of x=0.23.

Isothermal vapor-phase epitaxy of Hg1−xCdxTe on CdTe and Al2O3 substrates

The isothermal vapor-phase-epitaxial (ISOVPE) growth of device-quality HgCdTe layers on both CdTe and CdTe/Al2O3 substrates has been demonstrated. The material and device properties on both types of substrates have been studied and compared with reported values for HgCdTe grown by other techniques. The as-grown ISOVPE Hg1−xCdxTe (x≂0.3) epilayers are always p type with carrier concentrations of ∼5×1015 to 3×1016 cm−3 and mobilities of ∼230–260 cm2/V s at 77 K. The temperature and compositional dependence of electrical properties of ISOVPE Hg1−xCdxTe are somewhat different from those of liquid-phase epitaxy (LPE) and bulk HgCdTe. In particular, the acceptor ionization energy, EA =7 meV, is about half that obtained in midwavelength infrared LPE or bulk HgCdTe, and nearly independent of composition x. The R0A product of a representative photodiode (λc ≂4.65 μm, 77 K) is 2×106 and 4 Ω cm2 at 77 and 195 K, respectively, with comparable device qualities seen on both CdTe and CdTe/Al2O3 substrates.

Open-tube isothermal vapor phase epitaxy of Hg1−xCdxTe on CdTe

Device-quality Hg1−xCdxTe (0.2≤x≤0.35) epitaxial layers have been grown on CdTe substrates by open-tube isothermal vapor phase epitaxy (ISOVPE). The surface morphology of the layers is mirrorlike, and the Hall data are comparable to those for HgCdTe grown by liquid phase epitaxy (LPE). Photovoltaic devices with a cutoff wavelength of 4.1 μm at 77 K were fabricated on an ISOVPE HgCdTe epilayer. Their performance is comparable with those that we have obtained for such devices fabricated on LPE HgCdTe epilayers.