J. Masek

Photovoltaic lead-chalcogenide on silicon infrared sensor arrays

MBE growth and infrared device fabrication with epitaxial IV-VI layers on Si substrates are reviewed. Epitaxy on Si substrates is achieved using a stacked BaF[sub 2]/CaF[sub 2] or CaF[sub 2] buffer layer. With buffers containing no BaF[sub 2], standard photolithographic delineation with wet-etching techniques can be used. Photovoltaic IV-VI sensors with cutoff wavelengths ranging from 3 to 14 [mu]m are fabricated in PbS, PbSe[sub 1[minus]x]S[sub x], PbEu[sub 1[minus]x]Se[sub x], PbTe, or Pb[sub 1[minus]x]Sn[sub x]Se layers on Si(111) substrates. They offer the possibility for low-cost infrared focal plane arrays with sensitivities similar to Hg[sub 1[minus]x]Cd[sub x]Te, but with much less demanding material processing steps. A 13-mm-long linear array with 10.5-[mu]m cutoff wavelength has inhomogeneities in cutoff below 0.1 [mu]m. Some arrays were on prefabricated active Si substrates containing the whole readout circuits.First thermal images using these chips are demonstrated. The induced mechanical strain resulting from the different thermal expansion of IV-VIs and Si relaxes down to cryogenic temperatures even after many temperature cycles because of dislocation glide in the main [100] glide planes.

Photovoltaic infrared sensor arrays in monolithic lead chalcogenides on silicon

MBE growth of epitaxial IV-VI lead salt layers on Si (111) substrates and fabrication of photovoltaic infrared devices in the layers is reviewed. IV-VI on Si IR sensors have potential as a low-cost technique of fabrication of large IR focal plane arrays for both the 3-5 mu m and 8-12 mu m ranges because of the easy fabrication procedure and because uniformity problems are much less severe in IV-VIs due to the weaker composition dependence of the bandgap compared with Cd1-xHgxTe. Sensor arrays are fabricated in 2-4 mu m thick PbTe, PbS1-xSex and Pb1-xEuxSe for 3-5 mu m and in Pb1-xSnxSe for 8-12 mu m cut-off. An intermediate epitaxial stacked 0.2 mu m thick CaF2-BaF2 bilayer serves for compatibility and helps to overcome the large lattice and thermal expansion mismatch between the Si substrate and the IV-VI layer. Perfectly smooth surfaces with surface defect concentrations down to 103 cm-2, and X-ray rocking-curve linewidth of approximately=150 arcsec are obtained. Sensor arrays with 66 and 256 elements are described, the latter having been grown on standard Si chips with Al metallization.

Photovoltaic IV-VI on Si infrared sensor arrays for thermal imaging

Photovoltaic narrow-gap IV-VI (lead chalcogenide) infrared sensor arrays on Si substrates have the potential for low-cost infrared focal-plane arrays. The arrays can be bump bonded to readout multiplexers, or be grown on prefabricated active Si substrates containing the whole readout circuits. Sensitivities are similar to that of Hg 1-x Cd x Te, but processing procedures are much less demanding. This is because the structural quality of even heavily lattice-mismatched IV-VI layers is adequate to fabricate devices with good sensitivities, because 2- to 4-μm layer thickness suffices, and because good homogeneity in ternary Pb 1-x Sn x Se for the 8- to 12-μm range is much easier to obtain than in Hg 1-x Cd x Te. New results are presented on the molecular beam epitaxial growth of the layers, including a very thin CaF 2 buffer needed for compatibility reasons, and a new photolithographic patterning technique suited for full wafer processing has been developed to fabricate the sensor arrays. First thermal images using these chips are demonstrated.

Photovoltaic infrared sensors in heteroepitaxial PbTe on Si

Arrays of photovoltaic infrared sensors for thermal imaging applications have been fabricated in narrow gap PbTe grown heteroepitaxially on Si substrates. PbTe epitaxy was achieved with the aid of intermediate CaF2 /BaF2 buffer layers of only ≊2000 A thickness. Blocking Pb contacts on about 3‐μm‐thick p‐PbTe layers form the active areas of the sensors. Cutoff wavelengths are 5.6 μm, and resistance‐area products are up to R0A=400 Ω cm2 at 85 K with mean value R0A≊150 Ω cm2 for 66 element linear arrays, well above the room‐temperature photon field background noise limit. The temperature dependence of R0A indicates a depletion‐limited noise current behavior between 250 and 100 K.

Heteroepitaxial Pb1−xSnxSe on Si infrared sensor array with 12 μm cutoff wavelength

An array of photovoltaic infrared sensors with 12 μm cutoff wavelength has been fabricated for the first time in a narrow‐gap semiconductor layer grown heteroepitaxially on Si. Heteroepitaxy is achieved using intermediate stacked epitaxial CaF2‐SrF2‐BaF2 buffer layers to overcome the large lattice as well as thermal expansion mismatch between narrow‐gap Pb1−xSnxSe and Si. The IR sensors exhibit resistance‐area products up to 0.3 Ω cm2 at 77 K. This corresponds to sensitivities which are above the 300 K background noise limit and only 2–5 times lower than those of state of the art Hg1−xCdxTe sensors on CdZnTe substrates with the same cutoff wavelengths.

IR-sensor array fabrication in Pb1−xSnxSe-on-Si heterostructures

Abstract Large line or area arrays of photovoltaic infrared sensors are desired for thermal imaging and spectroscopic applications. The sensor arrays should be arranged on Si-substrates because of size, costs and the possibility to integrate the read-out electronics of large arrays directly into the Si-substrate. We therefore grow narrow gap Pb 1− x Sn x Se layers by molecular beam epitaxy (MBE) on Si(111)-substrates. A CaF 2 epitaxial buffer layer is used for compatibility reasons. Photovoltaic infrared sensor arrays for the 8–12 μm atmospheric window are fabricated in the 2–4 μm thick layers by photolithographic techniques with 8 mask levels and 6 simple wet etching steps. The operability of the sensor arrays is demonstrated by recording thermal images with a simple camera.

Monolithic lead-chalcogenide IR-diode arrays on silicon: fabrication and use in thermal imaging applications

A narrow gap semiconductor layer grown directly on a Si-substrate is the preferable approach to realize large IR-focal plane arrays. We report on our new work on lead chalcogenide photovoltaic IR-detector arrays, grown monolithically on Si (111) substrates using a stacked CaF2/BaF2 buffer layer. The sensor fabrication process is described, and a simple thermal camera system is used to verify the functionality of our arrays. An epitaxial narrow gap lead chalcogenide layer of only 3 micrometers thickness is grown on an 0.3 micrometers thick CaF2/BaF2 buffer layer on Si (111), both using Molecular Beam Epitaxy. Photovoltaic IR-detectors are formed by deposition of a blocking Pb contact on the p-type semiconducting surface. We fabricated staggered linear sensor arrays with up to 2 X 128 pixels and with the cut off ranging from 3 to 12 micrometers . For demonstration, we built up a simple thermal camera using our detector arrays as the IR sensitive element. The read out is done using a new multiplexed direct injection device, capable to store large charge packages and offering individual biasing for each diode. The IR-diodes are fabricated monolithically on the completely finished readout chip.

IV–VI compounds on fluoride/silicon heterostructures and IR devices

Abstract We present recent results on the growth of IV–VI lead-chalcogenide narrow gap semiconductors on silicon and subsequent IR device fabrication. Heteroepitaxy is achieved using intermediate stacked approximately 2000 A thick epitaxial CaF 2 BaF 2 buffer layers which allow us to overcome the large lattice mismatch (up to 20%) and, even more important, thermal expansion mismatch between lead chalcogenides and silicon. By growing different lead chalcogenides such as PbS, Pb 1− x Eu x Se, PbTe and Pb 1− x Sn x Se we have fabricated sensor arrays with cut-off wavelengths covering the whole thermal IR range from 3 microm up to above 12 microm.

Photovoltaic lead-chalcogenide IR-sensor arrays on Si for thermal imaging applications

Abstract Linear arrays of photovoltaic infrared sensors for thermal imaging applications are fabricated in narrow gap semiconductor layers grown heteroepitaxially on Si. Epitaxy is achieved using stacked intermediate BaF 2 SrF 2 CaF 2 buffers to overcome the large lattice-as well as thermal expansion mismatch. The arrays consist of 66 elements and cover cut-off wavelengths ranging from 3 to above 12 μm. Extrapolated resistance-area products of the best PbTe sensors (cut-off wavelengths5.7 μm) on Si are up to 20 000 Ω cm 2 at 77 K. They approach those of similar HgCdTe sensors fabricated in bulk or epitaxial material on CdTe substrates. Mean detectivities of whole PbTe on Si arrays at 90 K are as high as D JNL ∗ = 1.5×10 12 cm √ Hz / W .

Fabrication Procedures of Photovoltaic Lead-Chalcogenide-on-Silicon Infrared Sensor Arrays for Thermal Imaging

Epitaxial Pb1-xSnx, Se layers have been grown onto Si(111) substrates with the aid of an intermediate CaF2/BaF2 buffer layer by MBE. Photovoltaic infrared sensor arrays with up to 256 elements for thermal imaging applications have been fabricated in the narrow gap lead chalcogenide layers. The whole growth and fabrication procedure was done at temperatures never exceeding 450°C on Si substrates containing prefabricated integrated circuits with standarde Al-metallization for the first time.