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19 th International Symposium on Space Terahertz Technology P7-4 Frequency tunability and mode switching of quantum cascade lasers operating at 2.5 THz S. G. Pavlov, H.-W. Hübers, H. Richter, and A. D. Semenov German Aerospace Center (DLR), Rutherfordstr. 2, 12489 Berlin, Germany A. Tredicucci, R. Green, and L. Mahler NEST CNR-INFM and Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy H. E. Beere and D. A. Ritchie Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK Quantum cascade lasers (QCLs) are promising devices for local oscillators in terahertz heterodyne receivers. The lasing mechanism is based on intersubband transitions in the conduction band of heterostructures, most commonly made from GaAs/AlGaAs. A key issue for application in a heterodyne receiver is the frequency stability and tunability. Linear continuous frequency tuning is not straightforwardly obtained. We have investigated two QCLs. They are designed for an operation frequency at about 2.5 THz. One of the lasers has a Fabry-Perot resonator while the other laser is a distributed feedback (DFB) laser. The active medium of both lasers is based on a GaAs/AlGaAs superlattice. The design follows the so-called bound-to-continuum approach with a rather uniformly chirped superlattice and no marked distinction between the injection and lasing regions. Detailed high-resolution spectra of the laser emission as a function of temperature and current have been obtained by self-beating of the laser modes (only laser with Fabry-Perot resonator) as well as by mixing with the emission of a THz gas laser. We report on some spectral features, such as nonlinear dependences of the laser emission frequency on the current and singularities due to mode switching. The analysis shows frequency- and current-dependent nonlinearities of the frequency tuning for both lasers. The multi-mode QCL shows larger variations of the output power of a particular mode as well as larger frequency instabilities at the current values related to the mode switching. Less-expressed power variations have been found for the single mode DFB QCL. The results of the homodyne mixing indicate large variations of the effective refractive index in the active medium caused by the drive current. The implications for the use of the QCL as local oscillator in a heterodyne receiver will be discussed. 138
19 th International Symposium on Space Terahertz Technology P7-5 Capabilities of GaN Schottky Multipliers for LO Power Generation at Millimeter-Wave Bands José V. Siles and Jesús Grajal Dep. de Señales, Sistemas y Radiocomunicaciones, Universidad Politécnica de Madrid, E.T.S.I. Telecomunicación, Ciudad Universitaria s/n, 28040 Madrid, Spain, e-mail: {jovi,jesus}@gmr.ssr.upm.es, tel: +34 91.336.73.58 Abstract —Gallium Nitride (GaN) is a very promising material for either electronic and optoelectronic devices because of its high breakdown field, and peak and saturated electron drift velocity. Hence, despite of its lower electron mobility, GaN Schottky diodes might represent a good alternative to GaAs Schottky diodes for LO power generation at millimetre-wave bands due to a much better power handling capabilities. Results from numerical simulations for a 200 GHz doubler predict half the conversion efficiency for GaN Schottky multipliers when compared to GaAs Schottky multipliers. However, higher power handling capabilities, more than an order of magnitude higher than GaAs with same anode sizes, are predicted for GaN diodes. SUMMARY In the recent years, GaN has emerged as a very promising material for either electronic and optoelectronic devices mainly due to its wide direct bandgap (3.4 eV), which results in a high breakdown field, and its high peak and saturated electron drift velocity [1]. High breakdown voltage materials increase the power handling capabilities of the devices. Hence GaN Schottky diodes might represent a good alternative to GaAs Schottky diodes for LO power generation by frequency multiplication in a near future, when solid-states sources can provide larger amounts of LO power at frequencies around 100-150 GHz. The potential capabilities of GaN Schottky diodes are not in the frame of the design of high frequency multiplier circuits, well within the submillimeter-wave region, but in the early stages of high-frequency multiplier chains where the excellent power handling capabilities of GaN can be exploited. However, GaN has the inconvenience of a lower electron mobility than GaAs, which results in an increase in the series resistance, and thereby, in lower efficiencies. The objective of this paper is to investigate the capabilities of GaN Schottky diodes for LO power generation at millimeter-wave and submillimeter-wave bands as an alternative to the widely used GaAs Schottky diodes. For this task, we have employed an Harmonic Balance numerical simulator that incorporates an accurate physics-based Drift- Diffusion model for GaN Schottky diodes. A similar simulator has demonstrated very good results for the analysis of GaAs Schottky diodes at millimetre and submillimeter-wave bands [2]. First, the electrical properties of both GaN and GaAs diodes will be compared, paying special attention to those with a major impact on the frequency multiplying process, such as the breakdown voltage, the electron mobilities, the nonlinear capacitance and the series resistance. Initial results (provided in Fig. 1) for a 200 GHz GaN Schottky doubler predict a 13 % efficiency, which is approximately half the efficiency obtained with a GaAs Schottky multiplier of similar characteristics, i.e. same doping (N D =1·10 17 cm -3 ) and anode size (36 μm 2 ). Nevertheless, the key point of the comparison lies in the input power at which the maximum efficiency is achieved for each case: ~10 mW for the GaAs case, and ~200 mW for the GaN case. For both cases, safe operation conditions, well far from breakdown, are guaranteed. Using such a high input power is only possible due to the excellent characteristics of GaN in terms of breakdown voltage, which represent the real advantage of this material over GaAs. Moreover, the enhanced power handling capabilities of GaN diodes will lead to simplified designs at the early multiplication stages of THz LO chains, allowing the use of either unbalanced configurations or 2-diodes balanced configurations. Current designs with GaAs Schottky diodes employ at least 4-6 anodes balanced configurations for the low frequency multiplication stages [3]. Fig. 2: Comparative between a 200 GHz GaN Schottky doubler and a 200 GHz GaAs Schottky doubler when a N D=1·10 17 cm -3 epilayer doping is considered. In the final manuscript, further details on the design and optimization of GaN Schottky multipliers, including the influence of different parameters such as the epilayer thickness and doping, will be provided. REFERENCES [1] F. Schwierz, “An electron mobility model for wurtzite GaN”, Solid-State Electronics, vol. 48, no. 6, pp. 889-895, June 2005. [2] J. Grajal, J.V. Siles, V. Krozer, E. Sbarra and B. Leone, “Performance Evaluation of Multiplication Chains up to THz Frequencies”, in Proc. 29 th Int. Conf. on Infrarred and Millimeterwaves & 12 th Int. Conf. on THz Electronics, September, 2004. [3] J. Ward, E. Schlecht, G. Chattopadhyay, A. Maestrini, J. Gill, F. Maiwald, H. Javadi and I. Mehdi, “ Capability of THz sources based on Schottky diode frequency multiplier chains”, IEEE MTT- S International Microwave Symposium Digest, pp.1587-1590, June 2004. 139
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