A. Ksendzov

SiGe/Si heterojunction internal photoemission long-wavelength infrared detectors fabricated by molecular beam epitaxy

A new SiGe/Si heterojunction internal photoemission (HIP) long-wavelength infrared (LWIR) detector has been fabricated by molecular beam epitaxy (MBE). The detection mechanism of the SiGe/Si HIP detector is infrared absorption in the degenerately doped p/sup +/-SiGe layer followed by internal photoemission of photoexcited holes over a heterojunction barrier. By adjusting the Ge concentration in the SiGe layer, and, consequently, the valence band offset between SiGe and Si, the cutoff wavelength of SiGe HIP detectors can be extended into the LWIR (8-17- mu m) regime. Detectors were fabricated by growing p/sup +/-SiGe layers using MBE on patterned p-type Si substrates. The SiGe layers were boron-doped, with concentrations ranging from 10/sup 19/ cm/sup -3/ to 4*10/sup 20/ cm/sup -3/. Infrared absorption of 5-25% in a 30-nm-thick p/sup +/-SiGe layer was measured in the 3-20- mu m range using a Fourier transform infrared spectrometer. Quantum efficiencies of 3-5% have been obtained from test devices in the 8-12- mu m range. >

A novel Si-based LWIR detector: the SiGe/Si heterojunction internal photoemission detector

A novel Si-based long-wavelength infrared (LWIR) detector, the SiGe/Si heterojunction internal photoemission (HIP) detector, has been demonstrated. The detection mechanism of the SiGe/Si HIP detector is infrared absorption in the degenerately doped p/sup +/-SiGe layer followed by internal photoemission of photoexcited holes over the heterojunction barrier. The p/sup +/-SiGe layers are grown by molecular beam epitaxy with boron concentrations up to 4*10/sup 20/ cm/sup -3/. The cutoff wavelength of this device can be tailored by varying the valence band offset between the SiGe alloy and Si, and thus can be extended into the long-wave infrared regime. The valence based offset can be adjusted by varying the Ge ratio in the SiGe layers. Results have been obtained from test devices with Ge composition ranging from 0.2 to 0.4, giving quantum efficiencies of 3-5% for a single pass in the 8-12 mu m region.<<ETX>>

Optical and structural characterization of InAs/GaAs quantum wells

Three InAs/GaAs single quantum wells of 2, 3, and 4 monolayer thickness were characterized using optical and structural techniques. The results of high-resolution transmission electron (HRTEM) microscopy and optical studies which combine absorption, photoluminescence (PL), photoreflectance and cathodoluminescence are presented. Using the polarization modulated absorptance technique we observed two absorption features in our samples at 77 K. On the basis of their polarization properties and comparison with an envelope function calculation, these structures are assigned to transitions between the confined heavy-hole and confined and unconfined electron levels. Photoreflectance spectra of the 3- monolayer sample in 77-300 K range show only the fundamental quantum well transition. The temperature dependence of this transition is approximately linear with a slope of 2.2·10 −4 eV/K which is significantly lower than in both constituent materials. Comparison to the absorption data reveals that the PL spectra are affected by the carrier diffusion and therefore do not provide direct measure of the exciton density of states. Therefore, photoluminescence results alone do not provide unequivocal information about the fundamental transition energy or the interface quality in quantum wells. The HRTEM images indicate that while the interfaces of the 2-monolayer sample are smooth and the well thickness is uniform, the 4-monolayer sample has uneven interfaces and contains domains of 2, 3, and 4 monolayers. In agreement with these observations, absorption features broaden with the increased well width. Scanning cathodoluminescence images of the 2- monolayer sample present no evidence of dislocations, which is consistent with the HRTEM observations.

SiGe/Si Heterojunction Internal Photoemission Long-Wavelength Infrared Detectors

There is a major need for long-wavelength-infrared (LWIR) detector arrays in the range of 8 to 16 microns which operate with close-cycle cryocoolers above 65 K. In addition, it would be very attractive to have Si-based infrared (IR) detectors that can be easily integrated with Si readout circuitry and have good pixel-to-pixel uniformity, which is critical for focal plane array (FPA) applications. Here, researchers report a novel Si(1-x)Ge(x)/Si heterojunction internal photoemission (HIP) detector approach with a tailorable long wavelength infrared cutoff wavelength, based on internal photoemission over the Si(1-x)Ge(x)/Si heterojunction. The HIP detectors were grown by molecular beam epitaxy (MBE), which allows one to optimize the device structure with precise control of doping profiles, layer thickness and composition. The feasibility of a novel Si(1-x)Ge(x)/Si HIP detector has been demonstrated with tailorable cutoff wavelength in the LWIR region. Photoresponse at wavelengths 2 to 10 microns are obtained with quantum efficiency (QE) above approx. 1 percent in these non-optimized device structures. It should be possible to significantly improve the QE of the HIP detectors by optimizing the thickness, composition, and doping concentration of the Si(1-x)Ge(x) layers and by configuring the detector for maximum absorption such as the use of a cavity structure. With optimization of the QE and by matching the barrier energy to the desired wavelength cutoff to minimize the thermionic current, researchers predict near background limited performance in the LWIR region with operating temperatures above 65K. Finally, with mature Si processing, the relatively simple device structure offers potential for low-cost producible arrays with excellent uniformity.

Single Crystalline / Porous Amorphous Superlattice Formation by the Etching of MBE Grown Si/Si1−x Gex Layers on Si Substrates

Novel porous amorphous/crystalline superlattices were produced by the etching of mesas containing superlattices of alternating layers of Si and Si 1−x Ge x . These layers were grown by molecular beam epitaxy on (100) Si substrates and etched in an aqueous HF:HNO 3 solution. Preferential attack and amorphization of the Si 1−x Ge x layers was observed, leading to the formation of alternating layers of single crystal Si and porous amorphous Si 1−x Ge x . The etchant is highly selective and it was possible to etch extremely thin (5nm) Si 0.7 Ge 0.3 layers between 30nm Si layers. Complete conversion of the Si 0.7 Ge 0.3 layers to the porous amorphous state was seen in lμm wide mesas. The role of composition and thickness of the Si 1−x Ge x layers was studied. The variation in the lateral etch depths of the Si 1−x Ge x layers in the superlattices demonstrates that lattice strain in these layers is an important factor in the selectivity of the etch process. As the thickness of the Si 1−x Ge x layers is decreased, transport of the etchant to and the etch products from the reaction front is reduced, limiting the penetration of the etching process. The porosities of these etched Si 1−x Ge x layers were determined to be comparable to measured values for thick etched alloy layers.