Semiconductor Device Group

The quantum cascade laser (QCL) is a unipolar laser which was first demonstrated at Bell Laboratories in 1994 by Jerome Faist and colleagues. Electrons cascading down the structure and undergo intersubband transitions from which photons are emitted whose frequency is controlled by the width of the quantum wells and m* (see right).

As with every laser there are a number of requirements for lasing to occur:

• population inversion
• stimulated emission
• active region producing gain
• low loss waveguide
• gain > waveguide losses

We have been one of the pioneers of the Si/SiGe quantum cascade lasers with our collaborators at Leeds, Sheffield, Imperial, Heriot-Watt, Queens Belfast and QinetiQ. We have concentrated on far-infrared or terahertz emission and use FTIR to characterise the cascade designs. This includes demonstrating the first strain-symmetrised SiGe quantum cascade emitter, the first terahertz emission from SiGe cascades, the first interwell SiGe cascades operating at terahertz and silicide low loss waveguides. We have also with our collaborators at Heriot-Watt undertaken significant intersubband lifetime measurements using pump-probe measurements with a free electron laser (FELIX) at Utrecht in the Netherlands.

All Si/SiGe quantum cascade designs to date have used holes due to the lower effective mass in the tunnel direction and the segregation of most n-type dopants in CVD growth. The highest gain design in III-V QCLs has been the bound-to-continuum cascade shown on the right. A miniband is used to efficiently inject electrons into a bound state from which the radiative transition is into the next miniband.

Above on the left is shown a design for an eight quantum well Si/SiGe bound-to-continuum cascade and on the right is a TEM image of the grown wafer. The blue states are the square of the envelope functions of the HH states, the cyan wavefunctions are the HH2 states and the dashed green lines are LH states. The radiative transition is from the red HH1 state into the blue miniband as demonstrated by the arrow. Such designs, however, show low gain due to a large number of parasitic transitions to the LH states.

Significant Publications:

• D.J. Paul "The progress towards terahertz quantum cascade lasers on silicon substrates" Laser and Photonics Reviews 4(5), 610 (2010) - doi:10.1002/lpor.200910038
• 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 81, pp1543-1545 (2002) - doi:10.1063/1.1501759
• R. Bates, S.A. Lynch, D.J. Paul, Z. Ikonic, R.W. Kelsall, P. Harrison, D.J. Norris, S.L. Liew, A.G. Cullis, W. Tribe and D.D. Arnone, "Diagonal intersubband transitions in Si/SiGe quantum cascade emitters" Applied Physics Letters 83(20), pp4092-4094 (2003) - doi:10.1063/1.1626003
• N. Q. Vinh, P. T. Greenland, K. Litvinenko, B. Redlich, A. F. G. van der Meer, S. A. Lynch, M. Warner, A. M. Stoneham, G. Aeppli, D. J. Paul, C. R. Pidgeon and B. N. Murdin, "Silicon as a model ion trap: time domain measurements of donor Rydberg states" Proceedings of the National Academy of Sciences 105(31), pp10649-10653 (2008) - doi:10.1073/pnas.0802721105
• M. Califano, N.Q.Vinh, P.J. Phillips, Z. Ikonic, R.W. Kelsall, P. Harrison, C.R. Pidgeon, B.N. Murdin, D.J. Paul, P. Townsend, J. Zhang, I.M. Ross and A.G. Cullis, "Interwell relaxation times in p-Si/SiGe asymmetric quantum well structures: the role of interface roughness" Physical Review B 75, 045338 (2007) - doi: 10.1103/PhysRevB.75.045338
• R.W. Kelsall, Z. Ikonic, C.R. Pidgeon, P.J. Philips, P. Harrison, S.A. Lynch, P. Townsend, D.J. Paul, S.L. Liew, D.J. Norris and A.G. Cullis, "Intersubband lifetimes in p-Si/SiGe terahertz quantum cascade heterostructures" Physical Review B 71, 115326 (2005) - doi:10.1103/PhysRevB.71.115326
• C.R. Pidgeon, P. Murzyn, J-P.R. Wells, I.V. Bradley, Z. Ikonic, R.W. Kelsall, P. Harrison, S.A. Lynch, D.J. Paul, D.D. Arnone, D.J. Robbins, D.J. Norris and A.G. Cullis, “Picosecond intersubband dynamics in p-Si/SiGe quantum well emitters” Applied Physics Letters 80, pp 1456-1458 (2002) - doi:10.1063/1.1452794
• S.A. Lynch, P. Townsend, G. Matmon, D.J. Paul, Z. Ikonic, R.W. Kelsall, P. Harrison, D.J. Norris, A.G. Cullis, and D.D. Arnone, “Temperature dependence of terahertz optical transitions from boron and phosphorus dopant impurities in silicon” Applied Physics Letters 87, 101114 (2005) - doi:10.1063/1.2042544
• S.A. Lynch, D.J. Paul, P. Townsend, G. Matmon, Z. Suet, R.W. Kelsall, Z. Ikonic, P. Harrison, J. Zhang, D.J. Norris, A.G. Cullis, C.R. Pidgeon, P. Murzyn, B.N. Murdin, M. Bain, H.S. Gamble, M. Zhao and W.-X. Ni, "Towards silicon-based lasers for terahertz sources" IEEE Journal of Selected Topics in Quantum Electronics 12(6), pp1570-1578 (2006) - doi:10.1109/JSTQE.2006.884069
• A. De Rossi, M. Carras and D.J. Paul, "Low-loss surface-mode waveguides for terahertz Si/SiGe quantum cascade lasers" IEEE Journal of Quantum Electronics 42(12), pp1233-1238 (2006) - doi:10.1109/JQE.2006.883496
• C.R. Pidgeon, P.J. Phillips, D. Carder, B.N. Murdin, T. Fromherz, D.J. Paul, W.-X. Ni and M. Zhao, "Pump-probe measurement of lifetime engineering in SiGe quantum wells below the optical phonon energy" Semiconductor Science and Technology 20, ppL50-L52 (2005) - doi:10.1088/0268-1242/20/10/L02
• D.J. Paul, G. Matmon, L. Lever, Z. Ikonic, R.W. Kelsall, D. Chrastina, G. Isella, H. von Känel, E. Müller and A. Neels "Si/SiGe terahertz bound-to-continuum quantum cascade emitters" ECS Transactions 16(10), pp865-874 (2008) - doi:10.1149/1.2986846
• G. Matmon, D.J. Paul, L. Lever, Z. Ikonic, M. Califano, R.W. Kelsall, D. Chrastina, G. Isella, H. von Känel, E. Müller and A. Neels, "Si/SiGe quantum cascade superlattice designs for terahertz emission" Journal of Applied Physics 107, 053109 (2010) - doi:10.1063/1.3319653 Article pdf
• R.W. Kelsall, Z. Ikonic, P. Harrison, S.A. Lynch, R. Bates, D.J. Paul, D.J. Norris, S.L. Liew, A.G. Cullis, D.J. Robbins, P. Murzyn, C.R. Pidgeon and D.D. Arnone, "Terahertz intersubband emission from silicon-germanium quantum cascades" Proc. IEEE Tenth International Conference on Terahertz Electronics pp9-12 (2002)
• R. Bates, S.A. Lynch, 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” CLEO Technical Digest 2002, pp629-630 (2002) (Paper CFB2)
• S.A. Lynch, D.J. Paul, R. Bates, P. Murzyn, C.R. Pidgeon, Z. Ikonic, R.W. Kelsall, P. Harrison, D.J. Norris, A.G. Cullis, and D.D. Arnone, “Intersubband and impurity electroluminescence in Si/SiGe heterostructures at terahertz frequencies” CLEO Technical Digest 2002, p629 (2002) (Paper CFB1)
• D.J. Paul, S.A. Lynch, R. Bates, Z.Ikonic, R.W. Kelsall, P. Harrison, D.J. Norris, S.L. Liew, A.G. Cullis, D.D. Arnone, C.R. Pidgeon, P. Murzyn, J.-P.R. Wells and I.V. Bradley, "Si/SiGe quantum cascade emitters for terahertz applications" Physica E 16, pp147-155 (2003)
• C.R. Pidgeon, P. Murzyn, J-P.R. Wells, I.V. Bradley, Z. Ikonic, R.W. Kelsall, P. Harrison, S.A. Lynch, D.J. Paul, D.D. Arnone, D.J. Robbins, D.J. Norris and A.G. Cullis, “THz intersubband dynamics in p-Si/SiGe quantum well structures” Physica E 13, pp904-907 (2002)
• S.A. Lynch, S.S. Dhillon, R. Bates, D.J. Paul, D.D. Arnone, D.J. Robbins, Z. Ikonic, R.W. Kelsall, P. Harrison, D.J. Norris, A.G. Cullis, C.R. Pidgeon, P. Murzyn and A.Loudon, “Si-based electroluminescence at THz frequencies” Material Science and Engineering B 89, pp 10-12 (2002)
• S.A. Lynch, D.J. Paul, R. Bates, D.J. Norris, A.G. Cullis, Z. Ikonic, R.W. Kelsall, P. Harrison, D.D. Arnone and C.R. Pidgeon, "Far intrared (THz) electroluminescence from Si/SiGe quantum cascade heterostructures" Proc. SPIE
• D.J. Paul, S.A. Lynch, R. Bates, Z. Ikonic, R.W. Kelsall, P. Harrison, D.J. Norris, S.L. Liew, A.G. Cullis, C.R. Pidgeon, P. Murzyn, W. Tribe and D.D. Arnone "Si/SiGe quantum cascade emitters for terahertz applications" Proc. 3rd International Conference on SiGe(C) Epitaxy and Heterostructures (2003)
• C.R. Pidgeon, P. Murzyn, J.-P.R. Wells, Z. Ikonic, R.W. Kelsall, P. Harrison, S.A. Lynch, D.J. Paul, D.D. Arnone and D.J. Robbins, "THz intersubband dynamics in p-Si/SiGe quantum well emitter structures" Physica Statui Solidi (b) 237, pp381-385 (2003)
• S.L. Liew, D.J. Norris, A.G. Cullis, R.W. Kelsall, P. Harrison, Z. Ikonic, D.J. Paul, S.A. Lynch, R. Bates, D.D. Arnone and D.J. Robbins, "Analysis of terhertz emitting SiGe quantum cascade structures by transmission electron microscopy" Institute of Physics Conference Series 180, pp155-158 (2003)
• D.J. Paul, P. Townsend, S.A. Lynch, R.W. Kelsall, Z. Ikonic, P. Harrison, D.J. Norris, S.L. Liew, A.G. Cullis, X. Li, J. Zhang, M. Bain, H.S. Gamble, W.R. Tribe and D.D. Arnone "In search of a Si/SiGe terahertz quantum cascade laser" IEEE Topical Meeting on Silicon Monolithic Intergrated Circuits in RF Systems, Digest Of Papers : pp143-146, September (2004)
• R.W. Kelsall, Z. Ikonic, P. Harrison, D.J. Paul, S.A. Lynch, R. Bates, D.J. Norris, S.L. Liew, A.G. Cullis, D.D. Arnone, D.J. Robbins and R.A. Soref, "Towards SiGe terahertz VCSELs" Materials Science Forum 453-454, pp1-8 (2004)
• D.J. Paul, S.A. Lynch, P. Townsend, Z. Ikonic, R.W. Kelsall, P. Harrison, S.L. Liew, D.J. Norris, A.G. Cullis, J. Zhang, M. Bain, H.S. Gamble, W.R. Tribe and D.D. Arnone, "Towards a Si/SiGe quantum casacde laser for terahertz applications" Materials Research Society Symposium Proceedings 832, pp56-68 (2005)
• S.A. Lynch, D.J. Paul, P. Townsend, G. Matmon, R.W. Kelsall, Z. Ikonic, P. Harrison, J. Zhang, D.J. Norris, A.G. Cullis, C.R. Pidgeon, P. Murzyn, B.N. Murdin, M. Bain and H.S. Gamble, "Silicon quantum cascade lasers for THz sources" 2005 Ieee Leos Annual Meeting Conference Proceedings (LEOS) 18th Annual Meeting of the IEEE-Lasers-and-Electro-Optical-Society, Oct. 22-28, Sydney, Australia (2005)
• P. Townsend , D.J. Paul, S.A. Lynch, R.W. Kelsall, Z. Ikonic, P. Harrison, D.J. Norris, A.G. Cullis, J. Zhang, X. Li, C.R. Pidgeon, B.N. Murdin and P. Murzyn, "LO phonon scattering as a depopulation mechanism in Si/SiGe quantum cascades" 2nd IEEE International Conference On Group IV Photonics, Sept. 21-23, Antwerp, Belgium (2005)
• M. Zhao, W.X. Ni, P. Townsend, S.A. Lynch, D.J. Paul, C.C. Hsu and M.N. Chang, "Growth and characterization of strain-symmetried Si/SiGe THz quantum cascade structures" 2nd IEEE International Conference On Group IV Photonics, Sept. 21-23, Antwerp, Belgium (2005)
• G. Matmon, S.A. Lynch, P. Townsend, D.J. Paul, M. Bain, H.S. Gamble, J. Zhang, Z. Ikonic, R.W. Kelsall and P. Harrison, "Terahertz emission and absorption characteristics of silicon containing boron and phosphorous impurity dopants and the effect of temperature" 2nd IEEE International Conference On Group IV Photonics, Sept. 21-23, Antwerp, Belgium (2005)
• D.J. Paul, P. Townsend, S.A. Lynch, R.W. Kelsall, Z. Ikonic, P. Harrison, D.J. Norris, S.L. Liew, A.G. Cullis, X. Li, J. Zhang, M. Bain, H.S. Gamble, International School of Solid State Physics, 35th Workshop: Physics and Technology of THz Photonics, Erice, Italy, 20-26th July (2005)
• M. Califano, N.Q. Vinh, P.J. Phillips, Z. Ikonic, R.W. Kelsall, P. Harrison, C.R. Pidgeon, B.N. Murdin, D.J. Paul, P. Townsend, J. Zhang, I.M. Ross and A.G. Cullis, "Interface roughness scattering in p-Si/SiGe asymmetric quantum wells" Bulletin of the American Physical Society Vol. 52, Number 1, U43.7 (2007)
• D.J. Paul, G. Matmon, L. Lever, Z. Ikonic, R.W. Kelsall, D. Chrastina, G. Isella and H. von Känel, "Si/SiGe bound-to-continuum quantum cascade emitters" Proceedings of the SPIE 6996, 69961C (2008) - Article PDF
• D.J. Paul, G. Matmon, L. Lever, Z. Ikonić, R.W. Kelsall, D. Chrastina, G. Isella, H. von Känel, "Si/SiGe bound-to-continuum quantum cascade terahertz emitters" Proceedings SPIE 6996, 69961C1-8 (2008): Article pdf
• D.J. Paul, G. Matmon, L. Lever, Z. Ikonic, R.W. Kelsall, D. Chrastina, G. Isella, H. von Känel, E. Müller and A. Neels "Si/SiGe Bound-to-continuum Terahertz Quantum Cascade Emitters" Electrochemical Society Proceedings, Fall Meeting and SiGe Symposium (2008)
• G. Isella, G. Matmon, A. Neels, E. Müller, M. Califano, D. Chrastina, H. von Känel, L. Lever, Z. Ikonic, R. W. Kelsall and D. J. Paul, "SiGe/Si Quantum Cascade Structures Deposited by Low-Energy Plasma-Enhanced CVD" Proceedings IEEE Group IV Photonics, pp. 29-31 (2008) - DOI: 10.1016/j.tsf.2008.08.091
• I.M. Ross, M. Gass, T. Walther, A. Bleloch, A.G. Cullis, L. Lever, Z. Ikonic, M. Califano, R.W. Kelsall, J. Zhang and D.J. Paul, "Structural and Compositional Properties of Strain-Symmetrized SiGe/Si Heterostructures" Microscopy of Semiconducting Materials 2007, Springer Netherlands, pp. 269-272 (2008) - DOI: 10.1007/978-1-4020-8615-1_59
• D.J. Paul, G. Matmon, P. Townsend, J. Zhang, M. Zhao and W.X. Ni, "A Review Of Progress Towards Terahertz Si/SiGe Quantum Cascade Lasers" IETE Journal of Research 53(3), pp. 285-292 (2007) – DOI: article pdf