Polarization-dependent intersubband absorption and normal-incidence infrared detection in p-type Si/SiGe quantum wells

Abstract

A detailed study of the polarization dependence of subband absorption and photoconductivity in pseudomorphic p-type Si/Si 0.64 Ge 0.36 quantum wells is presented. The fabricated quantum well infrared photodetectors (QWIP) show a photoresponse between 3 and 8 µm with a peak-wavelength of λ p = 5 µm under normal incidence illumination. At the optimum bias operating point a detectivity D * λ = 2 × 10 10 cm √ Hz W −1 is achieved. On the basis of a self-consistent six-band Luttinger-Kohn calculation the p-and s-polarized intersubband transitions, leading to the observed photoconductivity, are identified. c 1998 Academic Press Limited Key words: Si/SiGe, intersubband absorption, infrared detector.Optical transitions between confined subbands in doped quantum wells have attracted much interest because they allow the realization of sensitive infrared detectors for the spectral range between 3 and 20 µm [1,2]. The high quantum efficiency and uniformity of the so-called quantum well infrared photodetectors (QWIP) makes them especially attractive for thermal imaging applications. Up to now most of the work has been concentrated on n-doped GaAs QWIPs where the superior transport of the photoexcited electrons above the barriers results in detectivities as high as D * λ ≥ 10 11 cm √ Hz W −1 . For this purpose optical couplers like gratings [3] or random scatterers [1] have been employed, which provide a component of the electric field parallel to the growth direction necessary for intersubband absorption in n-doped quantum wells due to the symmetry of the conduction band at the -point.In p-doped QW the restriction introduced by the quantum mechanical selection rule is removed due to the coupling of heavy-hole, light-hole and spin-orbit split-off subbands [4][5][6][7]. The desirable normal-incidence detection without optical couplers has been demonstrated in p-type GaAs [8] as well as p-type SiGe QWIPs [4,6]. A Si-based QWIP offers the additional advantage of having the same thermal expansion coefficient as the Si readout electronics, an aspect which is essential for the realization of large detector arrays operating under low temperature conditions (T ≈ 80 K). Also a direct monolithic integration with MOS readout circuitry is possible [9]. Another promising detector concept with considerable quantum efficiencies, which should be mentioned in this context, is the single heterojunction SiGe/Si device, where the free-carrier absorption

Ultrafast Phenomena in Semiconductors

“…Here, the polarization selection rules are different because of the different symmetry of the hole wave functions in the valence band, and consequently, mid-infrared light can be absorbed under normal incidence. In recent years, there has been a growing interest in intersubband transitions of p-type SilGe quantum wells [245][246][247][248][249][250][251][252][253][254][255][256][257], making use of the mature manufacturing technology for silicon-based devices.…”

Preliminary design of a tensile-strained p-type Si/SiGe quantum well infrared photodetector

“…In addition, the epitaxial growth and processing of Si/SiGe heterostructures is compatible with Si microelectronics technology including onchip integration of SiGe focal-plane arrays with Si readout circuits. This makes strained p-type SiGe QWIPs worth pursuing [20][21][22].…”

8-band k·p modelling of mid-infrared intersubband absorption in Ge quantum wells

“…7 was surface normal so only xy-polarised absorption is expected for the parabolic approximation, but dependent on the samples, nonparabolicity, scattering from heterointerfaces, and scattering from the substrate can also allow a smaller amount of z-polarized transitions to be observed even in this surfacenormal geometry. 49,50 The transmission electron microscope (TEM) measurements of the narrowest Ge QW found that the samples consisted of a 5.4 6 0.4 nm Ge QW with Si 0.5 Ge 0.5 barriers all strain symmetrized to a Si 0.2 Ge 0.8 relaxed buffer. Figure 4 compares the absorption data measured in surface-normal (xy-polarized) by Fourier transform infrared (FTIR) spectrometry 7 with solutions to the present 8-band kÁp modelling.…”