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In this work, the physical processes leading to contrast in scanning capacitance microscopy (SCM) are investigated both experimentally and theoretically. Using a p-type epitaxial doping staircase on silicon, we show that a monotonic dependence of the SCM signal on the doping level is only obtained, if the tip bias is adjusted in a way that the sample is either in accumulation or depletion. In the transition region, the SCM signal is nonmonotonic as a function of doping and depends on the bias. Therefore, any doping concentration can yield a maximum SCM signal size. We also show that this behavior is in agreement with the conventional model of a metal-oxide-semiconductor junction.

Femtosecond spectral hole burning experiments were performed on InAs/GaAs quantum dots to investigate the influence of high excitation intensities. The observation of additional spectral holes, as well as distinct absorption line broadening are both attributed to two-photon absorption processes. These spectral holes can be unambiguously assigned to degenerate two-photon biexciton creation, whereas the absorption line broadening is a direct result of Coulomb renormalization, initiated by carriers created through two-photon absorption in the surrounding GaAs crystal matrix and the InAs wetting layer.

Room temperature intersubband electroabsorption modulation in GaN∕AlN coupled quantum wells is demonstrated at short infrared wavelengths, covering the fiber-optics telecommunication wavelength range. Electroabsorption modulation with opposite sign is observed at λ=1.2–1.67μm and λ=2.1–2.4μm. The electromodulation originates from electron tunneling between a wide well (reservoir) and a narrow well separated by an ultrathin AlN barrier. Both the intersubband absorption and the modulation spectroscopic measurements are in good agreement with the simulations. The maximum modulation depth is ∼44% at λ=2.2μm. The −3dB cutoff frequency limited by the RC time constant is 11.5MHz for 700×700μm2 mesas.

The authors present the effects of the doping concentration on the performance of a set of terahertz quantum-cascade lasers emitting around 2.75THz. The chosen design is based on the longitudinal-optical-phonon depopulation of the lower laser state. An identical structure is regrown varying the sheet density from 5.4×109to1.9×1010cm−2. A linear dependency of the threshold current density on the doping is observed. The applied field where lasing takes place is independent of the doping. The field is responsible for the alignment of the cascades and therefore the transport of the electrons through the structure.

The authors report on a quantum well infrared detector embedded in a surface-plasmon waveguide and processed into a deeply etched photonic crystal structure. The device was characterized by collecting the polarization dependent response spectra at different angles of incidence. With this method it is possible to map the photonic band structure by directly detecting the modes of the photonic crystal. It therefore represents a new and direct characterization procedure for photonic crystals. The device shows a strong mixing between TE and TM polarized modes, which is caused by the asymmetric vertical waveguide design.

Double-barrier GaN resonant tunneling diodes with AlGaN barriers were fabricated on bulk (0001) single-crystal GaN substrates. Layers were grown using molecular-beam epitaxy with a rf plasma nitrogen source. Single diodes of 6μm diameter were prepared by inductively coupled plasma reactive ion etching. For many diodes clear negative differential resistance is observed around 2V with peak currents around 10kA∕cm2 and a peak-to-valley ratio of about 2 at room temperature. Its observation does not depend on specific conditions of measurement; however, it slowly decays after each measurement. The mechanism behind this decay is investigated since it is obviously prohibiting the usage of GaN resonant tunneling diodes so far. It is shown not to be caused by catastrophic breakdown of the devices.

We present second-harmonic surface emission from distributed feedback GaAs∕AlGaAs quantum-cascade lasers with integrated intersubband nonlinearities. The devices show single mode fundamental and second-harmonic emission at wavelengths of 10.7 and 5.35μm, respectively. Only 5.35μm light is emitted from the surface, as the fundamental light is not coupling to radiating modes for the grating of our choice. The second-harmonic peak optical power via the surface at 78K is 150μW for a fundamental peak power of 1.1W.

We report on intersubband absorption, photovoltaic, and photoconductive detection of near-infrared radiation in regular AlN∕GaN superlattice structures. Photoconductive detection was achieved up to temperatures of 120 K. Simulation of the transition energies using a self-consistent Schrodinger-Poisson equation solver for our specific well width is in good agreement with the measurements. For a well width of 17 A, the transition energy between ground state and first excited state in the GaN well is around 6300cm−1 which corresponds to 1.6μm.

We present high-power surface-emitting second-order distributed feedback quantum-cascade lasers in GaAs and InP material systems. The GaAs device, grown by molecular-beam epitaxy, showed single-mode peak output powers of 3 W at 78 K in pulsed operation. With the InP-based devices, which are grown by metalorganic vapor phase epitaxy, we obtained single-mode peak output powers of 1 W at room temperature. These are the highest output powers for surface emission of quantum-cascade lasers reported so far. The InP-based distributed feedback lasers also have very low threshold current densities and are working well above room temperature.

Midinfrared single mode emission, up to 20°C, for a distributed feedback structure consisting of lateral index variation on both sides of a rib waveguide is achieved on a quantum cascade laser. Fabrication was made possible by a high density plasma process developed for high aspect ratio etching of GaAlAs∕GaAs structures. A lateral grating considerably enhances design freedom by decoupling the vertical waveguide structure from the grating strength. The grating coupling, now being mainly determined by waveguide width, can then be exploited for device applications.