Jerry A. Wilson

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

Integrated two-color detection for advanced focal plane array (FPA) applications

Integrated two-color detector arrays offer significant system advantages (over separate arrays for each color) where two-color information is required. Using a single array with co-located spectral band sensitivities guarantees perfect pixel registration between the two different spectral band images. These two-color IR detectors can be made in HgCdTe using a pair of back-to-back-diodes incorporated in a triple-layer heterojunction (TLHJ). Use of HgCdTe allows any combination of bands between SWIR and LWIR. TLHJs can be operated in either a sequential or simultaneous mode by leaving the layer common to the two diodes floating or by contacting it. The effect of the choice of spectral bands on the meaning of sequential and simultaneous operation is discussed. State-of-the-art trend line performance for each spectral band of a TLHJ has been demonstrated using an all-LPE HgCdTe technology at SBRC. Mean MWIR RrA of 2 X 107 (Omega) -cm2 and LWIR of 1.6 X 103 (Omega) -cm2 have been shown. Quantum efficiencies are typical of trend line PV HgCdTe. Very high quality imaging has been demonstrated using 64 X 64 sensor chip assemblies in a sequential mode incorporating the above TLHJs. Simultaneous detectors have been made in miniarrays and test structures of various size unit cells. 128 X 128 simultaneous arrays are under study. Imaging and test results (performance and uniformity) for each band are comparable to state-of-the-art single-color HgCdTe arrays.

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.

MBE-grown HgCdTe heterojunction structures for IR FPAs

HgCdTe MBE technology offers many advantages for the growth of multi-layer heterojunction structures for high performance IRFPAs. This paper reports data on major advances towards the fabrication of advanced detector structures, which have been made in MBE technology at Hughes Research Laboratories during the last couple of years. Currently device quality materials with desired structural and electrical characteristics are grown with the alloy compositions required for short-wavelength infrared (SWIR, 1 - 3 micron) to very long- wavelength infrared (VLWIR, 14 - 18 micron) detector applications. In-situ In (n-type) and As (p-type) doping developed at HRL have facilitated the growth of advanced multi-layer heterojunction devices. Thus, high performance IR focal plane arrays (128 X 128) with state-of-the-art performance have been fabricated with MBE-grown double-layer heterojunction structures for MWIR and LWIR detector applications. In addition, the growth of n-p-p-n multi-layer heterojunction structures has been developed and two-color detectors have been demonstrated. Recently, significant preliminary results on the heteroepitaxy growth of HgCdTe double-layer heterojunction structures on silicon have been achieved.

Status of HgCdTe-MBE technology for producing dual-band infrared detectors

Progress on achieving reproducible growth of high performance, dual-band IR detector structures in HgCdTe grown by molecular beam epitaxy (MBE) is described. The reproducibility achieved in the MBE growth of n-p-n device structures comprising HgCdTe epitaxial layers with different composition and doping characteristics was evaluated from the run-to-run precision in the alloy composition, dopant concentration and dislocation density. For a series of 25 growth runs, the standard deviation of the alloy composition in the n-type absorbing layer was 0.002; the yield for the in situ n- and p-type doping process was > 95%; and the average dislocation density was < 5 x 10 5 cm -2 . In situ optical diagnostics, including spectroscopic ellipsometry and an optical absorption flux monitor were used for the real-time determination of the alloy composition and Cd flux during MBE growth of the two-color device structures. Focal plane arrays with 128 x 128 elements were fabricated for the simultaneous detection of two sub-bands in the MWIR spectrum. Average R o A values exceeding 1 x 10 6 and 2 x 10 5 Ω cm 2 were measured at 77 K for diodes operating at 4.0 and 4.5 μm, respectively, and the quantum efficiency was greater than 70% in each band. These results on MBE growth and device performance demonstrate that HgCdTe MBE technology is poised for the modest-scale production of advanced IR devices.

Advances in HgCdTe-based infrared detector materials: the role of molecular-beam epitaxy

Since its initial synthesis and investigation more than 40 years ago, the HgCdTe alloy semiconductor system has evolved into one of the primary infrared detector materials for high-performance infrared focal-plane arrays (FPA) designed to operate in the 3-5 mm and 8-12 mm spectral ranges of importance for thermal imaging systems. Over the course of the past decade, significant advances have been made in the development of thin-film epitaxial growth techniques, such as molecular-beam epitaxy (MBE), which have enabled the synthesis of IR detector device structures with complex doping and composition profiles. The central role played by in situ sensors for monitoring and control of the MBE growth process are reviewed. The development of MBE HgCdTe growth technology is discussed in three particular device applications: avalanche photodiodes for 1.55 +m photodetection, megapixel FPAs on Si substrates, and multispectral IR detectors.