VSWIR to VLWIR MBE grown HgCdTe material and detectors for remote sensing applications

The molecular beam epitaxy (MBE) growth technology is inherently flexible in its ability to change the Hg_1−xCd_xTe material’s bandgap within a growth run and from growth run to growth run. This bandgap engineering flexibility permits tailoring the device architecture to the various specific requirements. Material with active layer x values ranging from ∼0.198 to 0.570 have been grown and processed into detectors. This wide range in x values is perfectly suited for remote sensing applications, specifically the National Polar Orbiting Environmental Satellite System (NPOESS) program that requires imaging in a multitude of infrared spectral bands, ranging from the 1.58 to 1.64 μm VSWIR (very short wave infrared) band to the 11.5 to 12.5 μm LWIR (longwave infrared) band and beyond. These diverse spectral bands require high performance detectors, operating at two temperatures; detectors for the VSWIR band operate near room temperature while the SWIR, MWIR (mid wave infra red), LWIR and VLWIR (very long wave infrared) detectors operate near 100K, because of constraints imposed by the cooler for the NPOESS program. This paper uses material parameters to calculate theoretical detector performance for a range of x values. This theoretical detector performance is compared with median measured detector optical and electrical data. Measured detector optical and electrical data, combined with noise model estimates of ROIC performance are used to calculate signal to noise ratio (SNR), for each spectral band. The SNR are compared with respect to the meteorological NPOESS system derived focal plane. The derived system focal plane requirements for NPOESS are met in all the spectral bands.     

Measurement of minority carrier lifetime in n-type MBE HgCdTe and its dependence on annealing

Results are presented for minority carrier lifetime in n-type molecular beam epitaxy Hg_1−xCd_xTe with x ranging from 0.2 to 0.6. It was found that the lifetime was unintentionally degraded by post-growth annealing under Hg saturated conditions in a H₂ atmosphere that was both time and temperature dependent. This effect was minimal or non-existent for x∼0.2 material, but very strong for x ≥ 0.3. Hydrogen was identified as responsible for this degradation. Identical annealing in a He atmosphere avoids this degradation and results in neartheoretical lifetime values for carrier concentrations as low 1 × 10¹⁵ cm⁻³ in ≥0.3 material. Modeling was carried out for x∼0.2 and x∼0.4 material that shows the extent to which lifetime is reduced by Shockley-Real-Hall recombination for carrier concentrations below 1 × 10¹⁵ cm⁻³, as well as for layers annealed in H₂. It appears that annealing in H₂ results in a deep recombination center in wider bandgap HgCdTe that lowers the lifetime without affecting the majority carrier concentration and mobility.     

Arsenic incorporation during MBE growth of HgCdTe

We discuss the equilibrium model of the amphoteric behavior of arsenic in HgCdTe and its applicability to material grown by molecular beam epitaxy. Suggestions are made on how to achieve active incorporation by manipulating the surface orientation, or by using precursors that provide steric hindrance.     

Initial evaluation of a valved Te source for MBE growth of HgCdTe

Initial results using a valved Te source for molecular beam epitaxial growth of Hg_1−xCd_xTe are described. Unlike the case for a conventional Knudsen effusion cell where the flux is controlled primarily by temperature, flux from the valved source is controlled primarily by a variable orifice capable of good closure so that the cell temperature can be fixed at the operating temperature. Operating characteristics of the source are described, and include being able to nearly instantaneously change the flux magnitude at will. Using the source for HgCdTe growth has resulted in promising composition reproducibility improvement in initial growth runs.     

Status of MBE technology for the flexible manufacturing of HgCdTe focal plane arrays

A robust process has been developed for the reproducible growth of in-situ doped Hg_1−xCd_xTe:As alloys by molecular beam epitaxy. Net hole concentrations in excess of 5 x 10¹⁷ cm⁻³, with peak mobilities >200 cm2/Vs were measured in Hg_0.74Cd_0.26Te:As films. The p-type layers were twin-free and consistently exhibit narrow x-ray rocking curves (<40 arc sec). The reproducible growth of small lots of p-on-n LWIR detector structures has been established. For a typical lot consisting of 13 layers, the average x-value of the n-type base layer was 0.226 with a standard deviation of 0.003. The lateral compositional uniformity across a 2.5 cm × 2.5 cm wafer was × = +- 0.0006. High performance p-on-n LWIR diodes were fabricated that exhibited R_oA_o values (0-fov at 78K) as large as 350 Q cm2 at 10.4 μm.     

Si基CdTe/HgCdTe分子束外延材料的位错抑制

基于GaAs/Si材料中位错的运动反应理论,修正获得CdTe/Si和HgCdTe/Si外延材料中的位错运动反应模型.采用快速退火方法对Si基HgCdTe外延材料进行位错抑制实验研究,实验结果与理论曲线基本吻合,从理论角度解释了不同高温热处理条件对材料体内位错的抑制作用.对于厚度为4～10μnn的CdTe/Si进行500...     

MBE growth and characterization of in situ arsenic doped HgCdTe

We report the results of in situ arsenic doping by molecular beam epitaxy using an elemental arsenic source. Single Hg_1−xCd_xTe layers of x ∼0.3 were grown at a lower growth temperature of 175°C to increase the arsenic incorporation into the layers. Layers grown at 175°C have shown typical etch pit densities of 2E6 with achievable densities as low as 7E4cm⁻². Void defect densities can routinely be achieved at levels below 1000 cm⁻². Double crystal x-ray diffraction rocking curves exhibit typical full width at half-maximum values of 23 arcsec indicating high structural quality. Arsenic incorporation into the HgCdTe layers was confirmed using secondary ion mass spectrometry. Isothermal annealing of HgCdTe:As layers at temperatures of either 436 or 300°C results in activation of the arsenic at concentrations ranging from 2E16 to 2E18 cm⁻³. Theoretical fits to variable temperature Hall measurements indicate that layers are not compensated, with near 100% activation after isothermal anneals at 436 or 300°C. Arsenic activation energies and 77K minority carrier lifetime measurements are consistent with published literature values. SIMS analyses of annealed arsenic doping profiles confirm a low arsenic diffusion coefficient.     

High performance SWIR HgCdTe detector arrays

Short wave infrared (SWIR) devices have been fabricated using Rockwell’s double layer planar heterostructure (DLPH) architecture with arsenic-ion implanted junctions. Molecular beam epitaxially grown HgCdTe/CdZnTe multilayer structures allowed the thin, tailored device geometries (typical active layer thickness was ∼3.5 μm and cap layer thickness was ∼0.4 μm) to be grown. A planar-mesa geometry that preserved the passivation advantages of the DLPH structure with enhanced optical collection improved the performance. Test detectors showed Band 7 detectors performing near the radiative limit (∼3-5X below theory). Band 5 detector performance was ∼4-50X lower than radiative limited performance, apparently due to Shockley-Hall-Read recombination. We have fabricated SWIR HgCdTe 256 × 12 × 2 arrays of 45 um × 45 μm detector on 45 μm × 60 μm centers and with cutoff wavelength which allows coverage of the Landsat Band 5 (1.5−1.75 μm) and Landsat Band 7 (2.08−2.35 μm) spectral regions. The hybridizable arrays have four subarrays, each having a different detector architecture. One of the Band 7 hybrids has demonstrated performance approaching the radiative theoretical limit for temperatures from 250 to 295K, consistent with test results. D* performance at 250K of the best subarray was high, with an operability of ∼99% at 10¹² cm Hz^1/2/W at a few mV bias. We have observed 1/f noise below 8E-17 AHz ^1/2 at 1 Hz. Also for Band 7 test structures, Ge thin film diffractive microlenses fabricated directly on the back side of the CdZnTe substrate showed the ability to increase the effective collection area of small (nominally <20 μm μm) planar-mesa diodes to the microlens size of 48 urn. Using microlenses allows array performance to exceed 1-D theory up to a factor of 5.     

Mode of arsenic incorporation in HgCdTe grown by MBE

The results of arsenic incorporation in HgCdTe layers grown by molecular beam epitaxy (MBE) are reported. Obtained results indicate that arsenic was successfully incorporated as acceptors in MBE-HgCdTe layers after a low temperature anneal. Secondary ion mass spectrometry and Hall effect measurements confirm that arsenic is incorporated with an activation yield of up to 100%. This work confirms that arsenic can be used as an effective dopant of MBE-HgCdTe after a low temperature annealing under Hg-saturated conditions.     

HgCdTe molecular beam epitaxy technology: A focus on material properties

HgCdTe MBE technology is becoming a mature growth technology for flexible manufacturing of short-wave, medium-wave, long-wave, and very long-wave infrared focal plane arrays. The main reason that this technology is getting more mature for device applications is the progress made in controlling the dopants (both n-type and p-type in-situ) and the success in lowering the defect density to less than 2 x 10⁵/cm² in the base layer. In this paper, we will discuss the unique approach that we have developed for growing As-doped HgCdTe alloys with cadmium arsenide compound. Material properties including composition, crystallinity, dopant activation, minority carrier lifetime, and morphology are also discussed. In addition, we have fabricated several infrared focal plane arrays using device quality double layers and the device results are approaching that of the state-of-the-art liquid phase epitaxy technology.