Modelling of GaN quantum dot terahertz cascade laser A. ASGARI *1,2 and A.A. KHORRAMI 1 1 Research Institute for Applied Physics and Astronomy, University of Tabriz, Tabriz 51665-163, Iran 2 School of Electrical, Electronic and Computer Engineering, The University of Western Australia, 35 Stirling Highway Crawley, WA 6009, Australia In this paper GaN based spherical quantum dot cascade lasers has been modelled, where the generation of the terahertz waves are obtained. The Schrödinger, Poisson, and the laser rate equations have been solved self-consistently including all dominant physical effects such as piezoelectric and spontaneous polarization in nitride- based QDs and the effects of the temperature. The exact value of the energy levels, the wavefunctions, the lifetimes of electron levels, and the lasing frequency are calculated. Also the laser parameters such as the optical gain, the output power and the threshold current density have been calculated at different temperatures and applied electric fields. PACS: 78.67.Hc, 78.55.Cr, 42.55.Px Keywords: GaN quantum dots, terahertz, cascade laser 1. Introduction Quantum dots (QDs) have appealed to physicists, chemists, and material engineers since many years for the study of car- rier confinement effects [1]. The studies on QDs have been in- terested worldwide due to the proposed unique feature of those for optoelectronics devices such as QD laser. One special in- terest is the development of a quantum dot laser for long wave- length communication [2]. QD semiconductor lasers have al- ready demonstrated many interesting properties such as tem- perature insensitivities, low threshold current densities, high modulation bandwidths, and a strong resistance to the optical feedback. Therefore, showing great prospects towards realiz- ing uncooled, isolator-free, directly modulated semiconductor lasers. All of these features originate from the quantum con- finement that usually characterizes atoms or molecules in contrast to semiconductor materials [3]. As a compact and coherent source, quantum cascade laser (QCL) in which the optical transition can be tailored by engineering the multiple-quantum-well active layer at- tracts much interests from researchers [4,5]. The QCL is a powerful solid-state source of THz electromagnetic waves [6]. Terahertz QCLs based on longitudinal-optical (LO) phonon depopulation of the lower laser level are currently attracting considerable interest, owing to their higher output powers and operating temperatures compared to bound-to- -continuum designs [7]. The performance of QCLs, how- ever, is fundamentally limited owing to continuous elec- tronic spectrum in quantum wells (QWs), which leads to fast depletion of the upper laser level by means of longitudi- nal optical phonon emission, as well as high optical loss and strong heating arising from free-carrier absorption. These fundamental limitations can, in principle, be avoided if all carriers in a cascade structure are confined in all three di- mensions [8]. The interest in THz radiation has been driven by a broad range of potential applications such as sensing of biological and chemical agents with their spectroscopic signatures in THz range; medical imaging due to THz water absorption that allows for differentiation of tissue types; security scre- ening with THz imaging and spectroscopy to identify explo- sives, bioweapons, narcotics, ceramics, as well as metals; space-based THz communications above atmosphere; and THz emitters that are in great demand for use in astronomy and studies of earth’s atmosphere [9]. On the other hand, III-nitride materials because of their wide band gap, high thermal and mechanical stability, the great piezoelectric constant, the low sensitive to ionized radiation are very suitable materials for optoelectronic devi- ces at the high temperatures [10–12]. In this paper, with presenting of a new model for tera- hertz waves generation by quantum dot cascade laser, we considered the all dominant effects in the calculation of the quantum dot cascade lasers based on nitride materials such as the piezoelectric and spontaneous polarization, and also the effects of temperature and applied electric fields. 2. Simulation and theoretical model In order to model the device, a QDCL active region based on three GaN quantum dot resonantphonons has been assu- med. The proposed active region structure has been shown in Fig 1. The structure includes three quantum dots with dif- ferent size in each period so that the active media of the Opto-Electron. Rev., 21, no. 1, 2013 A. Asgari 147 OPTO-ELECTRONICS REVIEW 21(1), 147–152 DOI: 10.2478/s11772-013-0077-7 * e-mail: asgari@tabrizu.ac.ir