In this paper we present experimental and theoretical study of intersubband transitions at telecommunication wavelengths in GaN/AlN hexagonal-phase quantum wells grown by molecular beam epitaxy on sapphire substrates. Crossed structural and photoluminescence experiments show that strong in-plane carrier localization occurs due to thickness fluctuations at GaN/AlN interfaces. Fourier transform infrared spectroscopy and photo-induced absorption spectroscopy performed on doped and undoped samples reveal a systematic blue-shift of the e1-e2 transitions with doping due to many body interactions. A good agreement is achieved between experiments and self-consistent Schrödinger-Poisson calculations.

We have investigated the mid-infrared spontaneous and stimulated emission between confined subbands in the conduction band of GaAs/AlGaAs quantum wells. The carriers which give rise to the intersubband emission are excited in the upper subbands using an intersubband optical pumping in coupled asymmetric quantum wells. The quantum wells are designed using phonon engineering in order to obtain population inversion between the second and first excited subband. This is obtained by adjusting the subband energy spacing between E2 and E1 close to the optical phonon energy which in turn allows an efficient relaxation. We have first observed intersubband spontaneous emission between E3 and E2 at 14 μm using an intersubband pumping with a CO2 laser in resonance with the E1-E3 transition. In a second set of experiments, the quantum wells are embedded in an infrared waveguide. We have measured the stimulated intersubband gain using a picosecond two-color free electron laser. The first color bleaches the E1-E3 transition and provides the population inversion. The intersubband stimulated gain is measured versus the waveguide length and photon energy. Stimulated gains ≈ 80 cm−1 are reported thus demonstrating that laser emission under optical pumping appears feasible in optimized structures. Finally, we show that intersubband emission can also be observed in quantum wells using an interband optical pumping.

We have investigated structural and optical properties of SiGeC layers. Bulk and multiquantum wells heterostructures were grown by rapid thermal chemical vapor deposition. The photoluminescence of these structures exhibits two distinct features : deep level photoluminescence associated with localized states and bandedge photoluminescence which gives an indication of the band gap variation due to carbon incorporation in substitutional sites. In the case of multiquantum wells heterostructures, a strong segregation is reported as the growth temperature is decreased. An alternate technique to incorporate carbon in SiGe layers, the pulsed laser induced epitaxy with an excimer laser, is presented. The incorporation of carbon in substitutional sites is evidenced by Raman spectroscopy. As the laser flux is increased, new Raman lines associated to carbon-induced disorder are observed in the spectra.

Erbium doping of silicon and silicon carbide using implantation followed by ion beam induced epitaxial crystallization (IBIEC) is investigated. The implanted concentration of Er was 1.4 at% in both cases. In Si(100), Rutherford backscattering/channeling revealed that about 40% of the Er atoms evolved upon rapid thermal annealing from an undetermined position (room temperature) to an interstitial tetrahedral position (650°C) and finally to a substitutional position (950°C). The remaining Er atoms were presumably trapped in the small precipitates visible in high resolution transmission electron microscopy. The photoluminescence at 1.54 μπι of Er3+ is enhanced with annealing and persists up to room temperature after a 950 °C 1 min anneal. The high concentration of optically active Er atoms is illustrated by the lack of saturation of the photoluminescence at high pumping excitation intensity. Erbium was also implanted into cubic silicon carbide films prepared by chemical vapor deposition on Si at 900 °C. Both solid phase epitaxy (SPE) and IBIEC were performed. After a 950°C anneal, the low temperature photoluminescence at 1.54 μιη after IBIEC was five times higher in SiC than in silicon. The difference in photoluminescence linewidth between IBIEC (broad lines) and SPE (sharp lines) is explained in terms of interactions between optically active erbium atoms.