S. Blunier

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.

Infrared sensor arrays with 3-12 mu m cutoff wavelengths in heteroepitaxial narrow-gap semiconductors on silicon substrates

The authors describe heteroepitaxy of IV-VI lead chalcogenide narrow-gap semiconductor (NGS) as well as II-VI materials (CdTe) on Si substrates. Epitaxy is achieved by using stacked intermediate CaF/sub 2/-BaF/sub 2/ bilayers to overcome the large lattice and thermal expansion mismatch. The authors use lead chalcogenides (PbS, PbTe, Pb/sub 1-x/Eu/sub x/Se, and Pb/sub 1-x/Sn/sub x/Se) rather than Hg/sub 1-x/Cd/sub x/Te (MCT) as IR-sensitive NGS material because growth and fabrication techniques are much easier and compositional homogeneity much less critical with lead salts, while maximum sensitivities are comparable to those of MCT. The high permittivity of lead salts yields much more fault-tolerant devices due to the effective shielding of charges resulting from defects. The authors have fabricated linear sensor arrays on Si substrates with cutoff wavelengths ranging from 3 to above 12 mu m. The sensitivities of the best PbTe on Si sensors (cutoff 5.5 mu m) are already comparable to those of MCT with the same cutoff wavelengths, while those of the first Pb/sub 1-x/Sn/sub x/Se devices are a factor of 2-5 below. Although many fabrication steps are rather crude and far from being optimized, this considerable improvement is easily possible. >

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.

Molecular beam epitaxial growth of high structural perfection CdTe on Si using a (Ca,Ba)F2 buffer layer

Epitaxial CdTe has been grown onto Si(111) by molecular beam epitaxy (MBE) with the aid of a graded CaF2‐BaF2 buffer layer. The buffer of ∼2000 A thickness was used to overcome the large lattice mismatch of 19%; it was deposited by MBE in a separate system. The ∼10‐μm‐thick CdTe films exhibited specular surfaces and showed strong photoluminescence. The width of the near‐band‐edge peak at 77 K was 12 meV, and (333) x‐ray lines were about 80 arc s wide. These values indicate a high structural quality, comparable to well‐known CdTe layers on alternate substrates like GaAs, InSb, or sapphire.

Characterization of heteroepitaxial CuIn3Se5 and CuInSe2 layers on Si substrates

Epitaxial CuIn3Se5 layers were grown on CuInSe2/Si(111) substrates by molecular beam epitaxy. Photoemission spectra of (112)‐oriented CuIn3Se5 and CuInSe2 epitaxial layers were studied and the structures in the upper valence band are correlated with the Cu 3d and Se 4p density of states. The main valence band of CuInSe2 exhibits the three peak structure (consistent with theory) while a broadband with a shoulder is observed for the CuIn3Se5 phase. Electron channeling and x‐ray diffraction confirmed the epitaxial growth of (112)‐oriented layer. Surface and bulk composition analyses, position of valence band maxima, and a Se related vibrational mode at 153 cm−1 in Raman scattering measurements established the growth of the CuIn3Se5 phase.

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.

Growth of lattice-mismatched stacked epitaxial CaF2-SrF2-BaF2 layers on (100) oriented Si substrates

(100) oriented BaF2 has been grown epitaxially onto Si(100) despite its large lattice mismatch of 14% and preferred (111) growth mode. (100) epitaxy was achieved using thin intermediate CaF2 and/or SrF2 buffers to overcome the mismatch in a stepwise manner. Growth is three dimensional, and a roughness of the top surface in the 10 nm range was obtained. Ion channeling minimum yields are below 4% even for layers as thin as 3000 A. Thermal misfit strains relieve due to movement of misfit dislocations. The layers are intended for use as epitaxial buffers for growth of compound semiconductors on Si(100) substrates with up to 20% total lattice mismatch.

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.

Molecular beam epitaxial growth of (100) oriented CdTe on Si (100) using BaF2‐CaF2 as a buffer

Epitaxial CdTe (100) has been grown on (100) oriented Si by molecular beam epitaxy using BaF2‐CaF2 as a buffer. Two‐dimensional (2‐D) growth of BaF2(100) is obtained using low‐temperature thermal cycles during growth. CdTe growth is also 2‐D above 270 °C substrate temperature and a 2×1 surface reconstruction indicating a Te‐stabilized surface is obtained. The growth is 3‐D at lower substrate temperatures. Good structural quality films showing sharp electron channeling patterns and pronounced photoluminescence at 77 K are obtained. The full width at half maximum of the band‐edge peak is 12 meV at 77 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.