J. E. Jensen

Fabrication of high-performance large-format MWIR focal plane arrays from MBE-grown HgCdTe on 4″ silicon substrates

We have developed the capability to grow HgCdTe mid-wave infrared radiation double-layer heterojunctions (MWIR DLHJs) on 4″ Si wafers by molecular beam epitaxy (MBE), and fabricate devices from these wafers that are comparable to those produced by mature technologies. Test data show that the detectors, which range in cutoff wavelength over 4–7 μm, are comparable to the trendline performance of liquid phase epitaxy (LPE)-grown material. The spectral characteristics are similar, with a slight decrease in quantum efficiency attributable to the Si substrate. With respect to R0A, the HgCdTe/Si devices are closer to the theoretical radiative-limit than LPE-grown detectors. Known defect densities in the material have been correlated to device performance through a simple model. Slight 1/f noise increases were measured in comparison to the LPE material, but the observed levels are not sufficient to significantly degrade focal plane array (FPA) performance. In addition to discrete detectors, two FPA formats were fabricated. 128×128 FPAs show MWIR sensitivity comparable to mature InSb technology, with pixel operability values in excess of 99%. A 640×480 FPA further demonstrates the high-sensitivity and high-operability capabilities of this material.

Heteroepitaxy of HgCdTe(112) infrared detector structures on Si(112) substrates by molecular-beam epitaxy

High-quality, single-crystal epitaxial films of CdTe(112)B and HgCdTe(112)B have been grown directly on Si(112) substrates without the need for GaAs interfacial layers. The CdTe and HgCdTe films have been characterized with optical microscopy, x-ray diffraction, wet chemical defect etching, and secondary ion mass spectrometry. HgCdTe/Si infrared detectors have also been fabricated and tested. The CdTe(112)B films are highly specular, twin-free, and have x-ray rocking curves as narrow as 72 arc-sec and near-surface etch pit density (EPD) of 2 × 106 cm−2 for 8 µm thick films. HgCdTe(112)B films deposited on Si substrates have x-ray rocking curve FWHM as low as 76 arc-sec and EPD of 3-22 × 106 cm−2. These MBE-grown epitaxial structures have been used to fabricate the first high-performance HgCdTe IR detectors grown directly on Si without use of an intermediate GaAs buffer layer. HgCdTe/Si infrared detectors have been fabricated with 40% quantum efficiency and R0A = 1.64 × 104 Ωm2 (0 FOV) for devices with 7.8 µm cutoff wavelength at 78Kto demonstrate the capability of MBE for growth of large-area HgCdTe arrays on Si.

MBE growth of HgCdTe on silicon substrates for large format MWIR focal plane arrays

HgCdTe p-on-n double layer heterojunctions (DLHJs) for mid-wave infrared (MWIR) detector applications have been grown on 100 mm (4 inch) diameter (211) silicon substrates by molecular beam epitaxy (MBE). The structural quality of these films is excellent, as demonstrated by x-ray rocking curves with full widths at half maximum (FWHMs) of 80–100 arcsec, and etch pit densities from 1 106 to 7 106 cm−2. Morphological defect densities for these layers are generally less than 1000 cm−2. Improving Hg flux coverage of the wafer during growth can reduce void defects near the edges of the wafers. Improved tellurium source designs have resulted in better temporal flux stability and a reduction of the center to edge x-value variation from 9% to only 2%. Photovoltaic MWIR detectors have been fabricated from some of these 100mm wafers, and the devices show performance at 140 K which is comparable to other MWIR detectors grown on bulk CdZnTe substrates by MBE and by liquid phase epitaxy.

Third Generation FPA Development Status at Raytheon Vision Systems

Raytheon Vision Systems (RVS) is developing two-color, large-format infrared FPAs to support the US Armys Third Generation FLIR systems. RVS has produced 640 x 480 two-color FPAs with a 20 micron pixel pitch. Work is also underway to demonstrate a 1280 x 720 two-color FPA in 2005. The FPA architecture has been designed to achieve nearly simultaneous temporal detection of the spectral bands while being producible for pixel dimensions as small as 20 microns. Raytheons approach employs a readout integrated circuit (ROIC) with Time Division Multiplexed Integration (TDMI). This ROIC is coupled to bias-selectable two-color detector array with a single contact per pixel. The two-color detector arrays are fabricated from MBE-grown HgCdTe triple layer heterojunction (TLHJ) wafers. The single indium bump design is producible for 20 μm unit cells and exploits mature fabrication processes that are in production at RVS for Second Generation FPAs. This combination allows for the high temporal and spatial color registration while providing a low-cost, highly producible and robust manufacturing process. High-quality MWIR/LWIR (M/L) 640 x 480 TDMI FPAs with have been produced and imaged from multiple fabrication lots. These FPAs have LWIR cutoffs ranging to 11 micron at 78K. These 20 micron pixel FPAs have demonstrated excellent sensitivity and pixel operabilities exceeding 99%. NETDs less than 25 mK at f/5 have been demonstrated for both bands operating simultaneously.

High performance HgCdTe two-color infrared detectors grown by molecular beam epitaxy

High-performance in situ doped two-color detectors with the n-p-n architecture for the sequential detection of mid: and long-wave infrared radiation were grown by molecular beam epitaxy. These detector structures were twin-free, and exhibited narrow rocking curves ( 45 arcsec) as determined by X-ray measurements. The near surface etch pit densities in these device structures were typically (2-3) x 10 6 cm -2 . The structures were processed as mesas and their electrical properties measured. The spectral response of the mid-wave and long-wave diodes in the integrated detector were characterized by sharp turn-on and turn-off in both bands. Average R o A values of 100 Ω cm 2 at 10.5 μm and 5.5 x 10 5 Ω cm 2 at 5.5 μm were measured at 77 K. These results are comparable to those of the best unispectral detectors and represents a significant milestone for MBE-grown HgCdTe two-color devices

Mbe growth of HgCdTe avalanche photodiode structures for low-noise 1.55 μm photodetection

Molecular-beam epitaxy (MBE) has been utilized to fabricate HgCdTe heterostructure separate absorption and multiplication avalanche photodiodes (SAM-APD) sensitive to infrared radiation in the 1.1-1.6 μm spectral range, as an alternative technology to existing III-V APD detectors. Device structures were grown on CdZnTe(211)B substrates using CdTe, Te, and Hg sources with in situ In and As doping. The composition of the HgCdTe alloy layers was adjusted to achieve both efficient absorption of IR radiation in the 1.1-1.6 μm spectral range and low excess-noise avalanche multiplication. The Hg 1-x Cd x Te alloy composition in the gain region of the device, = 0.73, was selected to achieve equality between the bandgap energy and spin-orbit splitting to resonantly enhance the impact ionization of holes in the split-off valence band. The appropriate value of this alloy composition was determined from analysis of the 300 K bandgap and spin-orbit splitting energies of a set of calibration layers, using a combination of IR transmission and spectroscopic ellipsometry measurements. MBE-grown APD epitaxial wafers were processed into passivated mesa-type discrete device structures and diode mini-arrays using conventional HgCdTe process technology. Device spectral response, dark current density, and avalanche gain measurements were performed on the processed wafers. Avalanche gains in the range of 30-40 at reverse bias of 85-90 V and array-median dark current density below 2 x 10 -4 A/cm 2 at 40 V reverse bias have been demonstrated.

Molecular beam epitaxial growth and performance of integrated multispectral HgCdTe photodiodes for the detection of mid-wave infrared radiation

In situ doped HgCdTe two-color detectors with the n-p-n geometry were grown by molecular beam epitaxy, for the simultaneous detection of two closely spaced bands in the mid-wave infrared spectrum. The average near-surface etch pit densities in these layers were 5 x10 6 cm -2 , which is a factor of 10 higher than that observed for the lattice-matched growth of Hg 1-x Cd x Te (x =0.22) layer on Cd 0.96 Zn 0.04 Te substrates. The 0.04% lattice mismatch between the Hg 1-x Cd x Te (x = 0.35) epilayer and the Cd 0.9 Zn 0.04 Te substrate produces plastic deformation of the epilayer which results in an increased dislocation densities in the epilayer. The alloy composition across the device structure along the growth direction was determined by secondary ion mass spectrometric analysis, and deviated by less than 1% from the target. The device structures were processed as diodes with the mesa architecture and tested. The spectral response of the detectors at 77 K was characterized by sharp turn off at 3.7 and 4.4 μm. R 0 A values in excess of 1 x 10 6 Ω cm 2 and quantum efficiencies greater than 75% were measured for diodes in each band.

Status of two-color and large format HgCdTe FPA technology at Raytheon Vision Systems

Raytheon Vision Systems (RVS) is developing two-color and large format single color FPAs fabricated from molecular beam epitaxy (MBE) grown HgCdTe triple layer heterojunction (TLHJ) wafers on CdZnTe substrates and double layer heterojunction (DLHJ) wafers on Si substrates, respectively. MBE material growth development has resulted in scaling TLHJ growth on CdZnTe substrates from 10cm2 to 50cm2, long-wavelength infrared (LWIR) DLHJ growth on 4-inch Si substrates and the first demonstration of mid-wavelength infrared (MWIR) DLHJ growth on 6-inch Si substrates with low defect density (<1000cm-2) and excellent uniformity (composition<0.1%, cut-off wavelength Δcenter-edge<0.1μm). Advanced FPA fabrication techniques such as inductively coupled plasma (ICP) etching are being used to achieve high aspect ratio mesa delineation of individual detector elements with benefits to detector performance. Recent two-color detectors with MWIR and LWIR cut-off wavelengths of 5.5μm and 10.5μm, respectively, exhibit significant improvement in 78K LW performance with >70% quantum efficiency, diffusion limited reverse bias dark currents below 300pA and RA products (zero field-of-view, +150mV bias) in excess of 1×103 Ωcm2. Two-color 20μm unit-cell 1280×720 MWIR/LWIR FPAs with pixel response operability approaching 99% have been produced and high quality simultaneous imaging of the spectral bands has been achieved by mating the FPA to a readout integrated circuit (ROIC) with Time Division Multiplexed Integration (TDMI). Large format mega pixel 20μm unit-cell 2048×2048 and 25μm unit-cell 2560×512 FPAs have been demonstrated using DLHJ HgCdTe growth on Si substrates in the short wavelength infrared (SWIR) and MWIR spectral range. Recent imaging of 30μm unit-cell 256×256 LWIR FPAs with 10.0-10.7μm 78K cut-off wavelength and pixel response operability as high as 99.7% show the potential for extending HgCdTe/Si technology to LWIR wavelengths.

80-GHz differential VCO in InP SHBTs

A high frequency millimeter-wave voltage-controlled oscillator (VCO) has been designed, manufactured and tested in InP single heterojunction bipolar transistor technology. The fully integrated fundamental differential VCO features high operating frequency up to 80 GHz with low phase noise about -118 dBc/Hz at 1-MHz offset and 5% tuning range. The VCO consumes only 95-mW power at a power supply of -5 V, while providing -2 dBm single-ended output power and 1 dBm for differential output power. The die size is 0.28 mm/sup 2/.

Molecular beam epitaxial growth of HgCdTe midwave infrared multispectral detectors

Molecular beam epitaxy (MBE) has been utilized to fabricate high performance HgCdTe infrared detectors with sensitivity to midwave infrared radiation in adjacent spectral bands for two-color thermal imaging applications. Growth of a multilayer HgCdTe device structure by MBE enables the use of an n-p-n device architecture that facilitates pixel-level registration of images in two separate spectral bands. Device structures were grown on CdZnTe(211)(B) substrates using CdTe, Te, and Hg sources with in situ In and As doping. The composition of the HgCdTe alloy layers was adjusted to achieve detection of infrared radiation in adjacent spectral bands in the 3.5–4.5 μm wavelength range. As-grown device structures were characterized with x-ray diffraction, wet chemical defect etching, and secondary ion mass spectrometry. Mesa type devices were patterned using reactive ion etching and ohmic contacts were made to the two n-type layers for operation of the detectors in a sequential detection mode. The spectral respo...