R. P. G. Karunasiri

Si1-xGex/Si multiple quantum well infrared detector

A long‐wavelength infrared detector is demonstrated using Si1−xGex/Si multiple quantum wells for the first time. A broad peak in the photoresponse is observed near 9 μm with a full width at half maximum of about 80 meV which provides the response in the 6–12 μm range. The position of the peak in the photoresponse is in good agreement with that observed in the absorption measurement using a waveguide geometry. A peak responsivity of about 0.3 A/W and detectivity of D*=1×109 cm √Hz/W at 77 K are achieved. This suggests the possibility of monolithic integration of Si1−xGex/Si multiple quantum well detectors with signal processing electronics for potential focal plane array applications.

INTERSUBBAND ABSORPTION IN SI1-XGEX/SI MULTIPLE QUANTUM WELLS

The intersubband infrared absorption of holes in Si1−xGex/Si multiple quantum wells is observed. The quantum well structure consists of 10 periods of 40−A−thick Si0.6Ge0.4 wells and 300−A−thick Si barriers. The samples are prepared using molecular beam epitaxy. In the experiment, the infrared absorption as a function of wavelength is measured using a waveguide geometry. An absorption peak near 8.1 μm has been observed, which is due to the transition between first two heavy hole bound states. The polarization dependence spectra are in good agreement with the selection rules for the intersubband transition.

Normal incidence infrared detector using p‐type SiGe/Si multiple quantum wells

The photoresponse of p‐type SiGe/Si multiple quantum well infrared detectors is measured as a function of incidence beam polarization. With the infrared (IR) beam polarized in the growth direction, a photoresponse peak is observed at near 8.6 μm with a full width at half maximum (FWHM) of about 80 meV. On the other hand, with the beam polarized parallel to the growth plane (normal incidence), a peak is observed at near 7.2 μm with nearly the same FWHM. The photoresponse at peak positions is about 0.3 A/W for both cases. With an unpolarized beam, the peak photoresponse of about 0.6 A/W is observed for the present unoptimized structure. The results of the detection of normal incidence IR suggest possible applications of SiGe/Si multiple quantum wells for normal incidence IR detection.

Large Stark shifts of the local to global state intersubband transitions in step quantum wells

Large Stark shifts of intersubband transitions in a step quantum well are observed for the first time. The Stark shifts are ∼8 and 7 meV at ∼18 kV/cm for the 1→2 and 1→3 intersubband transitions, respectively, while the Stark shift of a similar transition in a square quantum well is only about 0.5 meV under the same bias condition. The intersubband transitions in step quantum wells can be either red or blue Stark shifted depending on the direction of the applied electric field. The large Stark shifts of intersubband transitions in the step quantum wells can be exploited for the fabrication of optical modulators operating in the range from mid to far infrared.

Intervalence‐subband transition in SiGe/Si multiple quantum wells−normal incident detection

Normal incident infrared absorption is observed in intervalence‐subband transition of Si1−xGex/Si multiple quantum wells for the first time. The observed absorption peak wavelength, strength, and broadness are shown to be strongly dependent on the Ge content in the well. The intervalence‐subband absorption peak shows up at a shorter wavelength with the beam polarized parallel to the growth plane (normal incidence), compared with the previously reported intersubband transition between two heavy hole subbands, which occurs when the optical field is polarized along the growth direction. The dependence of the absorption peak height with the Ge composition and thus the direct gap energy in the quantum well indicates that the origin of this transition is due to the intervalence‐subband transition from the heavy hole ground state to another valence subband. The normal incident detection suggests the convenient fabrication of focal plane infrared detector arrays using Si1−xGex/Si multiple quantum wells.

Normal incident SiGe/Si multiple quantum well infrared detector

A long-wavelength ( approximately 10- mu m) quantum-well (QW) infrared detector with normal incident detection was fabricated using p-type Si/sub 1-x/Ge/sub x//Si multiple QWs. Photocurrent is measured as a function of the incident beam polarization. With a beam polarized parallel to the growth plane (90 degrees polarization, normal incidence), a photocurrent peak is observed at near 7.2 mu m with a full width at half maximum (FWHM) of about 80 meV. On the other hand, when the beam is polarized along the growth direction (0 degrees polarization), a peak is observed at near 8.6 mu m with FWHM of about 80 meV. With the non-optimized detector, the peak photoresponsivity of 0.3 A/W and detectivity of D* approximately 1.0*10/sup 9/cm square root Hz/W at 77 K are obtained for both polarizations. The results of normal incident detection demonstrate the feasibility of Si-based long-wavelength IR detectors and focal plane arrays with the advantage of monolithic integration with Si integrated circuits.<<ETX>>

Hole intersubband absorption in δ‐doped multiple Si layers

The hole intersubband infrared absorption in δ‐doped Si multiple quantum wells is observed for the first time. The structures used consist of ten periods of boron‐doped Si quantum wells and undoped Si barriers. Near 100% infrared absorption is measured by a Fourier transform infrared spectrometer using waveguide structures. The observed absorption peaks ranging between 3 and 7 μm, which are mainly due to the transition between the first two heavy hole subbands. This absorption peak can be tuned by varying the doping concentration in the δ‐doped layer. The polarization‐dependent spectra show good agreement with the intersubband selection rule. The estimated peak energy positions using a self‐consistency calculation with exchange effects as a perturbation agree reasonably well with the experimental observation. This observation suggests the use of multiple quantum well for IR detector application using Si technology

Normal incidence infrared detector using intervalence‐subband transitions in Si1−xGex/Si quantum wells

Normal incidence infrared detection due to intervalence band transitions of holes is demonstrated using Si1−xGex/Si multiple quantum‐well structures. Two samples with Ge composition (x) of 30% and 60% are used in this study and broad photoresponse peaks (∼100 meV) at near 3 and 2 μm, respectively, are observed. The peak positions of the photoresponse are in close agreement with those observed in the absorption measurement using a waveguide geometry. The low‐temperature photocurrent spectrum for each sample shows that the broad absorption peak at room temperature consists of many peaks due to several transitions. From the calculated energy levels due to different hole bands, we conclude that the transitions are from the heavy‐hole ground state to the split‐off and continuum states. This principle of normal incidence detection can be readily implemented for focal plane infrared detection in the 3–5 μm (and 8–12 μm) range with potential for monolithic integration with Si signal processing circuits.

Resonant tunneling of variously strained Si/GexSi1−x/Si heterostructures

Abstract Resonant tunneling of holes through variously strained double barrier Si/Ge x Si 1−x /Si structures grown on a relaxed GeSi buffer layer has been observed. The different GeSi buffer layer has a lattice constant determined by the Ge fractional composition and the strained double barrier tunneling structures are then grown by molecular beam epitaxy onto the relaxed buffer layer. Tunneling of light and heavy holes has been demonstrated. The current-voltage relations show negative differential resistance. The positions of the resonant peaks are in agreement with the light and heavy hole bound states in the quantum well calculated from the deformation potential and the strain data. The tunneling current is also measured in the presence of a high magnetic field up to 10 T, both in the perpendicular and parallel to the interfaces.

Infrared absorption in parabolic multiquantum well structures

Abstract The absorption constant for intraband transitions in parabolic multiquantum well structures is calculated and compared with intraband absorption in a square well superlattice. The parabolic multiple quantum well structure may be used as an Infrared detector with the possibility of lower leakage current compared to one made of square wells.