Discussion of ways to achieve mid and far IR intraband lasing just by lateral electric field carrier (electron or hole) heating in multiple quantum well (MQW) structures is given. It is argued that the Gunn diodes are low frequency indirect transition lasers based on hot electron population inversion arising under electron intervalley transfer. In the MQW structures direct optical transitions exist while hot carrier population inversion can be achieved due to inter-valley/real space transfer. The two MQW structures are considered in this work: GaAs/AlAs and GaAs/InGaAs systems. In the first the hot electron (Gamma) -X intervalley/real space transfer from GaAs layers to AlAs layers provides population inversion while in the second the inversion can arise due to interlevel/interlayer transfer. Evaluations via the Monte-Carlo simulation of the hot electron phenomena in some of the structures are given and observation of the hot carrier phenomena of the type (including far and mid IR emission and absorption) are presented. Consideration of the appropriate laser design which provides also a way to cope with the low frequency (Gunn type) current oscillations is given.
In this paper we present a simple non-destructive method for testing SiC plate single-crystals of any size and shape. The method is based on measuring the impedance changes of an inductive ferrite-cored coil due to placing the sample into the core gap. The method is valid for any SiC polytypes, though we used 6H one. Using this method we have obtained and discussed a conductivity as a function of doping level (N<SUB>d</SUB>-N<SUB>a</SUB>) for 6H-SiC Lely crystals. The conductivity measurements were carried out with alternating current of 747 kHz frequency. The sensitivity of the method is limited by minimal conductivity 1 (Ohm(DOT)cm)<SUP>-1</SUP> (that is corresponding to (N<SUB>d</SUB>-N<SUB>a</SUB>) approximately 2 (DOT) 10<SUP>16</SUP> cm<SUP>-3</SUP> for 6H-SiC:N Lely crystals).
We are presenting a simple non-destructive method for characterizing SiC samples (Lely-crystals, CREE-substrates, and epitaxial films). With our method we observed ultraviolet differential reflection spectra of SiC samples and compared with pure Lely-crystal to estimate their structural quality. Our optical differential method is based on the experimental fact that doping of a crystal leads to appreciable changes of the optical fundamental absorption spectrum, which we interpreted as a uniform broadening and a shift of differential spectra. The broadening of absorption peaks can be caused not only by doping, but also by any defects of the crystal lattice (neutral impurities, clusters, micro-pipes and others), that destroy its periodicity. The shifts of these peaks inform us about the free carrier concentration. The experiment has shown we can detect minimum free carriers concentration up to n<SUB>min</SUB> equals (N<SUB>D</SUB>-N<SUB>A</SUB>) equals 6 (DOT) 10<SUP>15</SUP> cm<SUP>-3</SUP>. Besides we can detect minimal frequency of impacts with lattice defects as v<SUB>min</SUB> equals 3 (DOT) 10<SUP>12</SUP> s<SUP>-1</SUP>. Converting to charged centers concentration it equals (N<SUB>D</SUB> + N<SUB>A</SUB>) equals 5 (DOT) 10<SUP>16</SUP> cm<SUP>-3</SUP>. Considering the small depth of light probe (less than 0.1 micrometers ) and delicacy of thin films, our contactless method is mostly applicable for its testing.