Mid-Infrared Intersubband Absorption from p-Ge Quantum Wells Grown on Si Substrates
Authors: Gallacher Kevin , Ballabio Andrea , Millar Ross , Frigerio Jacopo , Bashir Aneeqa , MacLaren Ian , Isella Giovanni , Ortolani Michele , Paul Douglas J 
Published: February 2020

There are many applications in the mid-infrared part of the electromagnetic spectrum which include thermal imaging [1] and the unique identification of molecules through absorption spectroscopy [2]. For all these applications, sources of mid-infrared light and photodetectors are key to enable any application. III-V and II-VI semiconductor materials have dominated over the last decades with both interband and intersubband emission and/or absorption devices [1-3] but there is now significant interest in developing technology on silicon substrates to enable far cheaper systems for mass market applications in environmental sensing, personalised healthcare and security. SiGe quantum well (QW) intersubband photodetectors (QWIPs) have previously been demonstrated [4] but the number of QWs was limited by the SiGe critical thickness thereby limiting performance. Now Ge QWs have the potential to improve this performance significantly in the mid-infrared and the number of QWs can be increased using strain symmetrisation of the QWs and barriers to allow improved absorption in the longer wavelength mid-infrared.
Here we demonstrate strain symmetrized growth of 500 periods of Ge QWs with Si0.5Ge0.5 barriers on Si0.2Ge0.8 virtual substrates grown on top of (001) Si. The samples were grown by low energy plasma enhanced chemical vapour deposition. Transmission electron microscopy, Raman spectroscopy and x-ray diffraction were used to characterize the heterolayer thicknesses, Ge contents, strain and defects. The layers demonstrate excellent quality with low defect densities. Three samples with QW thicknesses (barrier thicknesses) of 5.4 ± 0.4 nm (3.5 ± 0.4 nm), 8.1 ± 0.5 nm (5.2 ± 0.6 nm) and 9.2 ± 0.6 nm (6.1 ± 0.6 nm) have been fully characterized by Fourier transform infrared spectroscopy (FTIR) to understand the potential for mid-infrared QWIPs. An 8-band k.p modelling tool has been used to understand the results.
For the lowest QW width, only z-polarised absorption is observed at ~ 7 µm wavelength from the light hole 1 (LH1) to LH2 transition. Despite surface-normal illumination, the z-polarized transition is observed since the heterolayers and substrate will scatter some of the light into the z-polarized mode. For the thicker QWs, only xy-polarized (surface normal) absorption is measured with absorption peaks centred at 7.5 µm and 7.8 µm corresponding to the heavy hole 1 (HH1) to LH2 transition.
Temperature dependent FTIR measurements demonstrate virtually no shift in the wavelength of absorption with temperature as is typical for intersubband transitions. Photoluminescence (PL) demonstrates a completely different behavior to the absorption due to the population of states being different through the heating of the samples by the 532 nm PL laser.
We will demonstrate how to design QWIPs using a HH1 to LH bound-to-continuum transitions that will enable surface-normal QWIPs to be designed with large pixel numbers and produced cheaply on a silicon substrate using any silicon foundry with SiGe growth capability.
The research leading to these results has received funding from the European Unions Seventh Framework Programme under grant agreement no. 613055 and U.K. EPSRC (Project no. EP/N003225/1).
[1] A. Rogalski, J. Appl. Phys. 93, 4355 (2003)
[2] L. Baldassarre, E. Sakat, J. Frigerio, A. Samarelli, K. Gallacher, E. Calandrini, G. Isella, D. J. Paul, M. Ortolani, and P. Biagioni, Nano Lett. 15, 7225 (2015)
[3] A. Rogalski, Infrared Phys. Technol. 38, 295 (1997).
[4] R. P. G. Karunasiri, J. S. Park, Y. J. Mii, and K. L. Wang, Appl. Phys. Lett. 57, 2585 (1990)

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