Mid-infrared active PbTe/CdTe quantum dots: growth, optical properties and device applications

The mid-infrared spectral region is of high interest for the field of molecular gas spectroscopy as most strong molecular absorption lines lie within this region. Applications of gas analysis are very widespread. For mid-infrared optoelectronic devices, the IV-VI or lead salt compounds are very well suited as these materials have almost mirror-like conduction and valence bands as well as low non-radiative Auger recombination rates. The use of quantum dots as active regions in such devices enables enhanced device performance because the reduction of the dimensionalty to zero results in discrete energy levels and a peaked electronic density of states. In this work, IV-VI PbTe quantum dots embedded in II-VI CdTe are investigated with respect to growth, luminescence and device applications. The dot formation is based on phase separation due to the immiscibility of narrow-gap PbTe and wide-gap CdTe and the minimization of the interface energies. Hence, two-dimensional PbTe layers embedded in CdTe spontaneously split up into isolated nanoprecipitates or quantum dots. The resulting dots are spherically shaped and exhibit atomically sharp interfaces. The miscibility gap arises from the difference in the crystal structure between rocksalt PbTe and zincblende CdTe. The emission wavelenght can be tuned by their size over a broad spectral region from 1.4 - 3.3 mym. The quantum dot size can be adjusted by the inital layer thickness or the PbTe growth temperature. The relation between the dot size and the emission energy was determined and compared to theoretical calculation. From transmission electron microscopy images, the dot size distributions were evaluated and the transition energies were obtained from photoluminescence experiments. The calculation of the transition energy as a function of dot size allows an energy to size conversion of the photoluminescence spectra. Additional control of the emission energy to higher values is obtained by alloying the PbTe quantum dots with SrTe, allowing a wide wavelength tunability of the emission. The incorporation of Sr also results in a drastic enhancement of the emission intensity, contrary to the strong non-radiative recombination found in bulk ternary alloys. Device applications of PbTe quantum dots as such as light emitting diode and the microdisk laser are presented. Electroluminescence is demonstrated up to room temperature for PbTe dots, embedded in the middle of an n-i-p Cd(Zn)Te junction. Lasing is shown for quantum dots up to 200K in continous wave mode at a wavelenght of 3.8 mym. Using pulsed excitation, the maximum laser operation temperature is raised to 300K. (author)