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FTIR exhaust gas analysis is used to establish chemical reactions paths in chemical vapor growth of GaN single crystals. The growth process utilizes the classical Degussa process on a lab scale to synthesize HCN as a transport agent for Ga. With GaCN as Ga and C precursor, the process is very similar to halide vapor phase epitaxy (HVPE), where GaCl is the Ga precursor and in both methods, the source of reactive nitrogen is NH3. Spectra are presented for the pure HCN synthesis and its combination with vapor growth, allowing the effective optimization of growth parameters, especially of gas flows.

We present an alternative approach to grow thick GaN single crystalline layers from Ga vapour, transported by an N2 carrier gas flow, and from nitrogen, excited very near the substrate by 2.45 GHz microwaves at pressure in the 200–800 mbar range. Crystal growth requires high stability of process parameters, especially of plasma operation. A major challenge arose from temperature dependent changes in cavity resonance. Optical emission spectroscopy (OES) was used for plasma characterization while the grown layers were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD).

Kinetic models in the system Al–N–O–C–H are useful to predict the time- and temperature-dependent influences of oxygen related species on aluminum nitride (AlN) bulk crystal growth by the sublimation–recondensation (also known as physical vapor transport, PVT) method. The transient oxygen incorporation is modeled based on non-linear regression analyses of experimental mass spectrometric data performed at conditions comparable to those of PVT crystal growth, and can be calculated for any user-specified temperature program. The results are in good qualitative agreement with SIMS measurements taken at several positions along the growth direction of a seeded isometric aluminum nitride crystal. By optimizing growth set-up geometry, source purification process, and growth parameters, the impurity content for the grown crystals decreases significantly to values of 6 ppm, 15 ppm, and 30 ppm for O, Si, and C, respectively, 2.5 mm above the AlN seed–crystal interface.

The structural properties of the heteroepitaxially grown AlN on off-axis 4H-/6H-SiC by PVT are presented in dependence of the seed polarity. In addition to long-term growth runs (48–72 h) short-term experiments were carried out in order to investigate the initial growth stage. SEM, EDX, HR-TEM and HR-XRD studies show different growth modes for C-polar and Si-polar SiC seeds. The main cause is the anisotropic oxidation of SiC by Al 2 O(g) which is formed during heating up in the AlN source. The type of the growth mode is essential for the defect density in the grown bulk AlN crystal. The growth on C-polar SiC leads to a higher crystal quality of the bulk AlN with up to two orders of magnitude lower etch pit densities and smaller rocking curve widths and hence mosaicity compared to the growth on Si-polar SiC. New initial 3D island growth models for both seed polarities are introduced.

In pure physical vapour transport process for GaN growth, the liquid Ga source has to be kept at temperatures about 1300-1400 °C to provide sufficient Ga vapour pressure for reasonably large growth rates. The growth temperature needs to be slightly lower to prevent droplet formation in the Ga vapour. At such high temperatures, however, the early ammonia decomposition prevents the favorable growth with reactive nitrogen in excess. The vapour processes under development in this study aim at overcoming the drawbacks of the reaction of physically or chemically transported Ga and ammonia. For this purpose, the reactive nitrogen will be supplied by plasma excitation of N2. First, the results on ammonia-based GaN growth and their disadvantages are discussed. Then, the challenges in designing of a new type of plasma sources (a microwave and a dielectric barrier discharge) and the first experimental results on the ammonia-free process development are presented. The microwave approach seems to be very promising in terms of GaN growth. It has higher growth rates than the dielectric barrier discharge method and therefore it is more cost-effective. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Reaction models in the system Al–N–O–C–H are essential for intelligent purification of aluminium nitride source material and to predict time- and temperature-dependent reduction of oxygen-containing impurities. For several purposes, high purity AlN is needed; consequently it is an important goal to minimize the impurity values to a minimum possible prior to the single crystal growth. For these reasons in situ kinetic investigations with capillary coupled mass spectrometry were performed in the temperature range between 1160 °C and 2030 °C under streaming nitrogen at a pressure of 600 mbar. Conventional thermoanalytical methods like TG or DTA are not sensitive enough, signals overlay with AlN sublimation and the most commercially available systems strongly suffer from long measurement times ( t > 12 h) at high temperatures. Our custom built setup was optimized for a fast detection of evolved gases, for direct temperature measurement and for high temperature homogeneity of the powder sample up to 2050 °C. Kinetic parameters were calculated with model-free and model-based kinetic methods based on mass spectrometric data achieved by on-line process monitoring of gaseous species during the processes. This study highlights the usefulness of capillary coupled mass spectrometry in kinetic studies of carbothermal reduction processes. Predictions on basis of non-linear regression analyses of experimental MS data were performed.

Fine powders of WC, TaC and TaC with W and WC additives were cold-isostatically pressed to ceramic discs and pressurelessly sintered at temperatures up to 2100 °C in a second step. Afterwards, the discs were tested as crucible lid under typical AlN growth conditions. The prepared discs should be gas tight and ensure a better alignment of the thermal expansion coefficients of TaC lid and AlN. Ceramic discs densified by this method reveal a relative density up to 97%. The TaC ceramic discs without additives show a microstructure with grain sizes in the range of 10–200 μm after sintering. The grain enlargement could be reduced by W and WC additives in the range of 1–5 wt.%. The results show that the AlN boules adhere only to WC lids and tungsten containing lids with W contents higher than or equal to 3 wt.%.

We report on the demonstration of low-loss, single-mode GaN-InAlN ridge waveguides (WGs) at fiber-optics telecommunication wavelengths. The structure grown by metal-organic vapor phase epitaxy contains AlInN cladding layers lattice-matched to GaN. For slab-like WGs propagation losses are below 3 dB/mm and independent of light polarization. For 2.6-mum-wide WGs the propagation losses in the 1.5- to 1.58-mum spectral region are as low as 1.8 and 4.9 dB/mm for transverse-electric- and transverse-magnetic-polarization, respectively. The losses are attributed to the sidewall roughness and can be further reduced by the optimization of the etching process.

We have investigated the vertical electronic transport in AlN/GaN/AlN double barrier structures grown by plasma-assisted molecular-beam epitaxy, with AlN barrier thickness of 0.5 nm and a GaN well thickness varying from 0.5 nm to 2 nm. Two different current levels were observed in different devices in all the samples, the highest one being attributed to leakage through dislocations, and the lower one to tunnelling transport through the barriers. In the sample with a well thickness of 0.5 nm, we observed a bi-stable behaviour, with an apparent negative differential resistance in the current-voltage characteristic when scanning the voltage from negative to positive bias. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A GaAs/AlGaAs quantum well infrared photodetector was combined with a deeply etched three-fold PhC structure. This makes the detector sensitive to mid-infrared radiation incident via the sample surface. The geometrically tunable peak responsivity is polarization independent when operated at normal incidence. Changing the angle of incidence a structure of several peaks is observed. In contrast to normal incidence illumination where the single peak can be identified as one particular PhC mode at the Γ-point near the second order Bragg condition, the multiple peaks apparent under an arbitrary angle refer to all PhC modes which lie within the spectral responsivity of the detector. The measurement therefore represents a direct characterization procedure for band structure mapping of photonic crystals. A study of the polarization dependence shows that TM polarized waves excite even modes while TE waves couple to modes with odd symmetry. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present efficient second-harmonic generation in state of the art GaAs-based quantum-cascade lasers with nonlinear output powers up to 100 μW. The nonlinear output was significantly improved by applying an AlGaAs waveguide structure, which guides both, the fundamental and nonlinear light. We further show the influence of the doping in the active region on the nonlinear light generation by comparing two similar structures with different doping levels.

A high density plasma process was developed for reactive ion etching (RIE) of GaAs/AlGaAs based devices which can be applied for the processing of advanced resonator designs. We present diffractive resonators fabricated using inductively coupled plasma (ICP) RIE with SiCl"4 chemistry. A sidewall protection is formed during the process, which allows for steep vertical and smooth profiles. The technology therefore improves the fabrication of sophisticated QCL cavities and results of the first laterally coupled distributed feedback (DFB) QCLs are presented together with our progress made in etching photonic crystal QCLs.

Electrically pumped GaAs/AlGaAs quantum cascade rib lasers with a wavelength of 9.7 µm utilizing deep etched photonic band gap mirrors are discussed. The smallest operating device was 150 µm long and 8 µm wide. Further, a description of the fabrication process of such high aspect ratio devices is given.

An easy-to-use procedure for the generation of complex two-dimensional dot arrays for the application in Photonic Crystals is presented. In most cases dot array arrangements are designed in a high resolution manner which is a reaction to the demands for precise, uniform and reproducible shape of the dots. This leads to high data volumes and special requirements for the fabrication of this patterns by electron-beam lithography and in time consuming exposure procedures. The basic idea presented here is to define the dot elements of the designed patterns in the form of squares instead of circular shapes resulting in a crucial reduction of the data size. In some cases a coarse design grid can be applied resulting in a drastic reduction of processing time. We demonstrate that the use of rectangularly defined dots results in circular pattern shapes, caused by proximity effects, and has a positive influence on the process window.

The prospects of a control for a novel gallium nitride pseudo-halide vapor phase epitaxy (PHVPE) with HCN were thoroughly analyzed for hydrocarbons-NH3-Ga gas phase on the basis of quantum chemical investigation with DFT (B3LYP, B3LYP with D3 empirical correction on dispersion interaction) and ab-initio (CASSCF, coupled clusters, and multireference configuration interaction including MRCI+Q) methods. The computational screening of reactions for different hydrocarbons (CH4, C2H6, C3H8, C2H4, and C2H2) as readily available carbon precursors for HCN formation, potential chemical transport agents, and for controlled carbon doping of deposited GaN was carried out with the B3LYP method in conjunction with basis sets up to aug-cc-pVTZ. The gas phase intermediates for the reactions in the Ga-hydrocarbon systems were predicted at different theory levels. The located π-complexes Ga…C2H2 and Ga…C2H4 were studied to determine a probable catalytic activity in reactions with NH3. A limited influence of the carbon-containing atmosphere was exhibited for the carbon doping of GaN crystal in the conventional GaN chemical vapor deposition (CVD) process with hydrocarbons injected in the gas phase. Our results provide a basis for experimental studies of GaN crystal growth with C2H4 and C2H2 as auxiliary carbon reagents for the Ga-NH3 and Ga-C-NH3 CVD systems and prerequisites for reactor design to enhance and control the PHVPE process through the HCN synthesis.

In this work femtosecond spectral hole burning spectroscopy is used to resonantly excite ground state excitons in an ensemble of self-assembled InAs/GaAs quantum dots with a strong pump pulse. Two fundamental coherent nonlinear effects are observed with the aid of the intrinsic time- and frequency resolution of the setup: The low temperature Rabi oscillation of the two-level system associated with the excitonic ground state transition and the observation of two-photon absorption in the surrounding GaAs crystal matrix. The emergence of the latter effect also infers the existence of charged excitons in the nominally undoped QD sample, backed up by the observation of additional spectral holes next to the excitonic transitions. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the demonstration of an intersubband (ISB) coupled quantum-well modulator operating at room temperature and telecommunication wavelength using a GaN-AIN quantum-well structure. The optical modulation is shown to result from electron tunneling between the wells. Stark shifting of the ISB absorption is observed. The maximum modulation depth is 0.79% at lambda= 2.3 mum and 0.18% at lambda= 1.37 mum for a mesa device with only 151-nm interaction length. We show that by reducing the mesa size down to 15 times 15 mum2, optical modulation bandwidth as large as 3 GHz can be obtained.

We report the demonstration of intersubband modulators operating at telecommunication wavelengths at room temperature based on GaN/AlN quantum wells. We first investigate electro-optical modulators making use of electron tunneling in coupled quantum wells. Electro-absorption modulation with opposite sign induced by the electron tunneling between the reservoir and active quantum wells is observed at λ = 1.3–1.6 µm and λ = 2.2 µm. We show that by reducing the mesa size down to 15 × 15 µm2, optical modulation bandwidth as large as 3 GHz can be obtained. We then investigate waveguide intersubband modulators relying on the depletion of the ground electronic state of a 3 quantum well active region. Modulation depths as large as 14 dB in the wavelength range of 1.2–1.6 µm are obtained under –9 V/+7 V voltage swing. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigate InAlN/AlN/GaN-based MOS-HEMTs with and without a 20 A-GaN capping below an Al2O3-insulator. An excellent reduction of the leakage current, up-to 6 orders of magnitude, is observed in MOS-HEMT structures compared to Schottky-based-HEMTs, both with GaN capping. In the case of uncapped samples the reduction in the leakage current is 3 orders of magnitude. The maximal transconductance value gm,max for the GaN-capped-MOS-HEMT is about 10% higher than that for the conventional Schottky-based-HEMT, while in the case of uncapped devices gm,max is about the same for both Schottky and MOS-HEMT. Similarly the maximal-drain-current in the GaN capped MOS-HEMT is 950mA/mm, exhibiting an increase of 180mA/mm in comparison to the corresponding conventional Schottky-based-HEMT. Performance improvement of MOS structure can be explained by a mobility-dependent carrier depletion effect. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Excitonic ground state Rabi oscillations in InAs quantum dots are studied via an ultrafast spectral hole burning technique. From comparison with frequency domain data, we find that acoustic phonon-induced dephasing processes damp the oscillations.

We have studied the evolution of the GaAs, InAs and GaSb surfaces due to FIB exposure. In contrast to conventional bottom up or top down processes the observed nanopattern formation is discussed, based on a subtractive self-organization process relying on material decomposition induced by FIB exposure.

We present high power mid-infrared surface-emitting single mode GaAs-based quantum cascade lasers (QCLs). By using an air-AlGaAs waveguide combined with second-order distributed feedback processing, we obtained optical output, via the surface, above 3W. Surface-normal dual-lobe light emission exceeds the emission from one as-cleaved facet by a factor of six.

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.

In this article, we demonstrate closely spaced, monolithically integrated photodetectors in two largely different wavelength ranges. The epitaxial structure of the devices was grown by plasma-assisted molecular-beam epitaxy on an AlN-on- sapphire template; it consists of a Si-doped AlGaN thin film, and a nearly strain compensated 40 period AlN/GaN superlat- tice with 1.0 nm thick GaN quantum wells and 2.0 nm thick AlN barriers. The entire structure is covered with an AlGaN cap layer. The superlattice acts as active region for the infra- red detector, while the AlGaN buffer layer serves as active area for the ultraviolet detector. While the photovoltaic near- infrared detector has a center wavelength of 1.37 µm, the photoconductive ultraviolet detector has a long wavelength cutoff at 250 nm. The two detectors could be operated up to room temperature with reasonable sensitivities.

Semiconductor quantum dots (QDs) are regarded as potential candidates for the implementation of solid state quantum computation. The basic ingredient for the realization of a quantum computer are Rabi oscillations - a sinusoidal time evolution of the population difference in a two-level system that is observed under strong resonant optical excitation (Stievater et al., 2001). In this contribution we report measurements of the damping of Rabi oscillations in InAs self-assembled QDs in the temperature range between T = 5 and 90 K. From comparison of the time domain Rabi oscillations with spectral hole burning (SHB) lineshapes we conclude that acoustic phonon-induced dephasing processes damp the Rabi oscillations.

We report on negative differential resistance of a AlGaN‐GaN‐AlGaN double barrier. Using inductively coupled plasma reactive ion etching, 6 μm square mesas have been fabricated on 0001 GaN substrates derived from bulk GaN crystals grown at high pressure. Double barrier and n+ contact layers were grown Ga face using plasma assisted molecular‐beam epitaxy. We obtain peak currents around 10 kA/cm2 and a peak‐to‐valley ratio of about 2 at room temperature, while no improvement was observed at cryogenic temperatures. The drop in current does not depend on specific conditions of measurement; however, it slowly decays after each measurement. Partial recovery by thermal treatment was possible.

Quantum cascade lasers (QCLs) are based on intersubband transitions. Selection rules for transitions in quantum wells allow gain only for TM polarized electromagnetic waves. Therefore conventional vertical cavity surface emitting laser design is not suitable for QCLs. A metal-stripe surface grating structure allows high surface emission efficiency for the TM-polarized light. The emitted power via the surface is in the range of a few Watts and exceeds the emitted power from the facets. A double-lobed surface emission far field pattern is obtained for the lasing mode from DFB gratings without phase shift. The respective grating coupling coefficient is determined to be K=29 cm. This technique can also be used to fabricate sophisticated waveguide structures, like Bragg mirrors or photonic crystal QCLs operating in the mid-infrared to the THz wavelength regime. We show QCL structures with etched Bragg mirrors operating around 11/spl mu/m. Compared to other fabrication techniques, like FIB-cut DBRs, this ICP-RIE technique requires less effort and gives more freedom in design and processing of these structures.

We report second-harmonic generation in GaAs-based quantum cascade lasers due to intersubband nonlinearities. Using a distributed feedback grating we obtain single-mode second-harmonic surface emission, where no fundamental light is emitted via the surface.

In this paper we present single mode quantum cascade lasers (QCLs) based on the GaAs and the InP material systems. We show results for first- and second-order distributed feedback (DFB) QC lasers with surface gratings. The InP based lasers are grown by metalorganic vapor phase epitaxy (MOVPE) and show single mode continuous wave emission up to 200 K. In pulsed operation we achieved single mode surface emission peak output powers exceeding 1 Watt at room temperature. The presented GaAs/AlGaAs laser features an air/AlGaAs waveguide, combined with a second-order distributed feedback grating. That laser shows 3 Watts of single mode output power via the surface at 78 K.

We report high power single mode surface emission from second order distributed feedback quantum cascade lasers. Two different laser materials are used for the lasers: MOVPE grown InGaAs/InAlAs and MBE grown GaAs/AlGaAs

In our previous work we showed that a reproducible monotonic relation between Scanning Capacitance Microscopy (SCM) signal and the sample doping is obtained under appropriate SCM operating conditions. In this work, we quantitatively determine the two‐dimensional carrier distribution of an n‐type ion‐implanted collector region of an industrial transistor sample. As the structures were large enough so that tip geometry effects could be neglected, the SCM data were converted into a two‐dimensional doping distribution via a simple lookup table, which was generated from a one‐dimensional doping profile measured on a reference wafer. In case the depth distribution of carriers is unknown, we show that the lookup table can also be calculated and a calibration can be carried out using e.g. the substrate doping as reference point.

Gratings in GaAs/AlGaAs mid-infrared quantum cascade lasers (QCLs) are fabricated with a structure depth of more than 10 μm. A N2/SiCl4 inductively coupled plasma reactive ion etching (ICP-RIE) process was employed to achieve extremely smooth sidewalls and selectivities to the SiNX etch mask of up to 70:1. EDX spectra measured on as-etched samples show that sidewall etch inhibition is caused by a thin Si containing layer on the sidewalls that is formed simultaneously with ICP etching of GaAs at the bottom of the trenches. To demonstrate device application gratings with a pitch of 1.72 μm are applied to long rib waveguide -based QCLs emitting at λ = 10.7 μm. When etched laterally together with the rib the grating gives rise to stable single mode emission up to 295K from these QCLs. The respective grating coupling coefficient is determined to be κ = 29 cm-1.

An improved Ar/Cl CAME process for the fabrication of InP-based Photonic Crystals (PhCs) is presented. Effects of temperature, ion beam, chlorine flow and other parameters are discussed. In particular feature size effects are investigated on a length scale of 200 to 400 nm required for the realization of infrared photonic band gaps. The potential of CAIBE for fabrication of deep and uniform hole patterns in InP is demonstrated. The good quality of the samples including multi quantum well InGaAsP/InP layers was demonstrated by performing transmission measurements by internal light source technique.

Hexagonal GaN (0001) has been grown by a pseudo-HVPE method on β-Ga2O3 (100) using an intermediate low-temperature buffer layer formed by in situ NH3 treatment of β-Ga2O3 substrate. A simple method for self-separation of bulk GaN from the β-Ga2O3 substrate is reported. The structural properties of the GaN and GaN–β-Ga2O3 interface were investigated by high-resolution X-ray diffraction and electron microscopy techniques. GaN layers were deposited on β-Ga2O3 by using gallium cyanide as a transport agent for gallium and, NH3 as a source of nitrogen. The properties of these layers are compared with those of samples grown on sapphire.

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.

The authors present the results and the optimization procedure for a time-multiplexed dry etching process to etch GaAs in an inductively coupled plasma reactive ion etching system. The gas feed chopping sequence employed a SiCl4 etch phase and an O2 passivation phase. Care is taken not to intermix O2 with SiCl4. The investigated structures consist of pillars, trenches, stripes, and holes, all with lateral structure size of 1 μm or less. This feature size is interesting for diffractive elements and cavities in integrated mid-IR optoelectronics. They achieve an aspect ratio of 10 for holes, 17 for trenches, and 30 for stripes with a selectivity of 200:1 on open areas. The improvements in the sidewall morphology are related to the O2 passivation step investigated by optical emission spectroscopy and energy dispersive x-ray analysis that reveals a Si-rich SiOX sidewall.

Closely spaced, monolithically integrated photodetectors in two largely different wavelengths ranges are demonstrated. The device structure was grown by plasma-assisted molecular-beam epitaxy on an AlNon-sapphire template, and it consists of a Si-doped AlGaN thin film, and a nearly strain compensated 40 period AlN/GaN superlattice with 1.0 nm-thick GaN quantum wells and 2.0 nm-thick AlN barriers. The entire structure is covered with an AlGaN cap. The superlattice constitutes the active region for the infrared detector, while the AlGaN buffer layer serves as active area for the ultraviolet detector. The photoconductive ultraviolet detector has a long wavelength cutoff at 250 nm, whereas the photovoltaic near-infrared detector has a centre wavelength of 1.37 mm. Multispectral cameras operating in entirely different wavelength ranges such as ultraviolet (UV), visible, or even the infrared (IR) are attracting increasing interest for applications in surveillance, failure analysis, and meteorology [1]. Up to now, such multispectral imaging systems have been fabricated by overlaying visible and UV pictures coming from two different image sensor arrays. Although this solution has the advantage of offering optimised performances for both wavelength ranges, it requires use of several bulky and rather expensive optical components. In addition, such a system is relatively sensitive for shocks or vibrations, and therefore not totally reliable for in-the-field applications. Systems being up to this challenge are therefore very expensive. A monolithic integration of two or even three operation wavelengths into a single camera chip could thus offer several advantages over existing solutions, notably in terms of price, robustness, and flexibility. Unfortunately, there are not many semiconductor materials which would allow a monolithic integration of all three wavelength ranges mentioned above. In addition, most semiconductors having sufficiently dissimilar bandgap energies suffer from large lattice mismatches, even within the same family [2]. For this reason, a different solution involving not only interband, but also intersubband transitions, must be pursued. One possible way which is presented in this Letter is the integration of a photoconductive UV interband detector based on an AlGaN thin film [3] and a photovoltaic near-IR intersubband detector based on an AlN/GaN superlattice [4– 6]. In a first demonstration, we show a UV/IR detector pair with only 350 mm separation between the UV and IR device. Both detectors exhibit spectrally narrow responsivity curves, which enlarges the UV-tovisible rejection ratio in the case of the UV device, and which improves the noise behaviour in the case of the IR detector.

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.

An inductively coupled plasma reactive ion etching process was developed for transferring patterns from a thin intermediate mask consisting of Ni or SiNx into GaAs. Smoothed out etch floors and sidewalls can be achieved under an approximately 200V bias by switching between an anisotropic etch phase and a deposition phase by gas chopping. The deposition of Si–O based film that protects the sidewall from chlorine attack is promoted by the addition of O2 to an SiCl4∕N2 gas mixture. The total achieved etch depth was approximately 20μm in this work, but the process can in principle be adopted for a larger depth and other chlorine-etchable III-V semiconductors. SiCl4 is shown to act as a suitable deposition gas as well as an etch gas.

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.

Using Si samples with 400-nm-wide alternately high and low doped p-type layers on p-type substrate, we investigate the dependence of the scanning capacitance microscope (SCM) signal amplitude as a function of oxide thickness and dopant concentration. The oxide layers were created by a standard industrial high temperature oxidation process. Together with the implementation of the newly developed conductive diamond tips, good and quantitatively reproducible contrast for differently doped domains is obtained on all samples. A comparison with conventional metal-oxide-semiconductor theory shows good correlation for the decrease of the capacitance signal with increasing insulator thickness and increasing dopant concentration. Furthermore, geometry effects, such as the influence of the domain width on the SCM signal, are discussed.