We report on the experimental measurement of active region lattice (T<sub>L</sub>) and electronic temperatures (T<sub>e</sub>) in terahertz quantum cascade devices based on the phonon-photon-phonon scheme, by means of microprobe band-to-band photoluminescence spectroscopy. Three mesa devices, differing for doping region and number of quantum wells composing the active region, have been investigated. With device on, under band alignment for lasing condition, we
measured a difference (T<sub>e</sub> - T<sub>L</sub>) ~ 40 K much smaller than the typical value (T<sub>e</sub> - T<sub>L</sub> ~ 100 K) reported for resonantphonon THz QCLs.
We report on our development of both detectors and lasers in the terahertz (THz) region. For detection, we
focus on the approach based on the extension of the celebrated quantum well infrared photodetectors
(QWIPs); whereas the quantum cascade lasers (QCLs) provide the source. We show our preliminary
demonstration of free space communication using our detectors and lasers. An all photonic THz
communication link operating at 3.8 THz using a QCL and quantum well photodetector has been
demonstrated. The link consists of a quantum cascade laser transmitter and a quantum well photodetector
receiver. The link was used to transmit audio through two meters of room air. Carrier strength at the
photodetector was 100 times above the noise level measured. THz free space communication may be of
interest in satellite based systems.
A subtle roughening of the surface of a buried 60 nm InGaAs epitaxial layer was detected using a combination of sample cleaving, selective chemical etching and Field Emission Scanning Electron Microscopy (FESEM). In our technology, InGaAs is the photo-absorbing layer of Metal Organic Chemical Vapor Deposition (MOCVD) grown layers used in the monolithic integration of active photo detectors and a passive mux/demux. Conventional Photo-Luminescence (PL) and X-Ray Diffraction (XRD) techniques used to monitor and optimize the growth of epitaxial layers did not show this microscopic surface roughness. The appearance of roughness in the InGaAs layer was linked to very large changes in the dislocation density of the layers grown over the rough surface. Increases of up to three orders of magnitude in the Etch Pit Density (EPD from 10<sup>4</sup> to 10<sup>7</sup> cm<sup>-2</sup>) were revealed using a standard Huber Etch. The Huber Etch also showed the preferred formation of "pairs" of dislocations threading out from a common point on the rough InGaAs surface. Changes in growth conditions resulted in the complete elimination of roughness and of excessive dislocation densities