During the past few years, there have been studies of semiconductor devices that generate and detect frequencies from 0.3 - 10 Terahertz (1000 - 30 mm).  Infrared emitters such as the Quantum Cascade Laser in III-V semiconductors have been difficult to extend to these frequencies due to the high absorption by polar phonons in the reststrahlen bands, but can produce superb results if the devices are constructed from multiple active regions [1].  In contrast, Si has no reststrahlen absorption band and therefore may be able to operate over a broader THz range and provide lower loss waveguides than the III-V semiconductors.  In our group, we work on the fabrication and characterization of sources and detectors based on SiGe nanotechnology.

    Emitters based on intersubband transitions in quantum wells of III-V compounds such as the quantum cascade laser have made remarkable progress, and operation in the THz range has recently been demonstrated [2].  Similar devices based on silicon-germanium quantum wells and cascades are attractive due to possible integration with conventional silicon circuitry, and because of lower free-carrier and reststrahlen-band absorption. In the SiGe material system, the band gap difference manifests itself mainly as an offset in the valence band, and quantum well devices typically employ the confinement of holes.  By making semiconductor devices with layers of Si alternating with SiGe alloys, we create quantum energy wells in the valence band.  The intersubband energy levels in the quantum wells exhibit electron and hole transitions in the far infrared electromagnetic spectrum  - at TeraHertz frequencies.  The structures are unipolar in the sense that only one carrier type is involved in the device operation.  This is unlike conventional light emitting diodes and lasers that use bipolar recombination of electrons with holes.  Electroluminescence has been reported from structures based on intersubband transitions between confined states in SiGe quantum wells, however; operation has been achieved only at low output powers [3]. Hydrogenic shallow donor impurity states in bulk silicon have also been exploited for terahertz emission using optical pumping, and have garnered attention due to their simple and low-cost fabrication [4, 5].

    Terahertz signals can be detected by very narrow gap semiconductor photodiodes (such as HgCdTe to a low frequency of about 20 THz), extrinsic photoconductors (such as Ge:Ga in the range from 50 to 150 um), quantum well infrared photodetectors (QWIPs), and thermal detectors including bolometers and pyroelectric detectors.  Although their response respectable, there is still need to improve their temperature of operation, and their detectivity performance.

[2] R. Köhler, A. Tredicucci, F. Beltram, H. E. Beere, E. H. Linfield, A. G. Davies, D. A. Ritchie, S. S. Dhillon, and C. Sirtori, "High-performance continuous-wave operation of superlattice terahertz quantum-cascade lasers," Appl. Phys. Lett., vol. 82, pp. 1518-1520, 2003.

[3] S. A. Lynch, R. Bates, D. J. Paul, D. J. Norris, A. G. Cullis, Z. Ikonic, R. W. Kelsall, P. Harrison, D. D. Arnone, and C. R. Pidgeon, "Intersubband electroluminescence from Si/SiGe cascade emitters at terahertz frequencies," Applied Physics Letters, vol. 81, pp. 1543 - 1545, 2002.

[4] M. S. Kagan, I. V. Altukhov, E. G. Chirkova, V. P. Sinis, R. T. Troeger, S. K. Ray, and J. Kolodzey, "THz lasing of SiGe/Si quantum-well structures due to shallow acceptors," Phys. Stat. Sol., vol. (b) 235, pp. 135-138, 2003.

[5] T. N. Adam, R. T. Troeger, S. K. Ray, P.-C. Lv, and J. Kolodzey, “Terahertz electroluminescence from boron impurities in bulk silicon,” Appl. Phys. Lett., vol. 83, pp. 1713-1715, 2003.

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Last Updated February 16, 2004