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In order to identify the performance limitations of InGaAs/ AlAs(Sb) quantum cascade lasers, experimental investigations of the temperature and pressure dependencies of the threshold current (I th ) were undertaken. Using the theoretical optical phonon current (I ph ) and carrier leakage (I leak ) to fit the measured threshold current at various pressures, we show that the electron scattering from the top lasing level to the upper L-minima gives rise to the increase in I th with pressure and temperature. It was found that this carrier leakage path accounts for approximately 3% of I th at RT and is negligible at 100 K.

We used high hydrostatic pressure techniques to understand the deteriorating temperature performance with decreasing wavelength of short wavelength quantum cascade lasers. Influence of inter-valley scattering and distribution of the electron wave functions will be discussed.There are several challenges in the development of semiconductor lasers for the near mid-infrared region of 2-3 µm. Approaches being developed to produce lasers in this range include extending the wavelength of inter-band diode lasers which perform relatively well below ~2.5 µm. Such lasers are, however, strongly affected by increased optical losses and non-radiative Auger recombination which degrade device performance with increasing wavelength. The second approach is based upon the development of short wavelength, intersubband transition based, quantum cascade lasers (QCLs), which can exhibit a high temperature stability and high output powers for longer wavelengths >3.8 µm [1,2]. It is anticipated that the use of short wavelength QCLs will overcome the problem of nonradiative Auger recombination which plagues inter-band lasers.There are however several challenges to realise short wavelength QCLs, many of which are band-structure related [1]. Fig.1 shows that the maximum operating temperature of InAs/AlSb QCLs strongly decreases towards shorter wavelengths.