The roughness and intermixing at the heterointerfaces in InAs/GaSb superlattice's have been studied by both high-resolution transmission electron microscopy. There is interest in InAs/AlSb/GaSb heterostructures for fundamental studies and device applications. Solid-source molecular-beam epitaxy (MBE) is a widely used technique for the growth of InAs/AISb/GaSb heterostructures. An important issue for the growth of group III arsenide-antimonide heterostructures is the unintentional incorporation of As into the antimony-containing layers from the As background in the MBE growth chamber. The heteroepitaxial growth of InAs/AlSb/GaSb heterostructures on GaAs substrates opens a new, quite uncommon possibility for the structural characterization, applying far-infrared spectroscopy. Regarding energies of optical interband transitions, interface states in the forbidden gap may also play a role acting as initial or final states of such transitions. Photoluminescence provides information on the energy of the effective band gap in type II heterostructures provided that indeed band-to-band transitions are observed.

We report on the structural characterization of InAs/(GaIn)Sb superlattices (SL) grown by solid-source molecular-beam epitaxy. SL periodicity and overall structural quality were assessed by high-resolution X-ray diffraction and Raman spectroscopy. Spectroscopic ellipsometry was found to be sensitive to the (GaIn)Sb alloy composition.

Tribological components made from CVD diamond are commonly used for protection against abrasion in rough environments. Such components can be used e.g. in textile industry as thread guiding devices provided that surface roughness and resultant friction are low. In this work we report on the fabrication of CVD diamond components of non-planar shape. These devices were fabricated in a negative replication approach on mechanically structured substrates. Using this technique cylindrically shaped diamond devices with smooth surfaces were produced by using the nucleation side of the diamond layers as the exposed surface, thus making subsequent polishing steps unnecessary. Applying an improved two-step nucleation method further reduced surface roughness. Sensing elements in the form of resistors made from boron-doped CVD diamond were integrated into the device surface. Device performance was characterized with respect to the temperature dependence of the resistors and the suitability as abrasion sensor.

We correlate topography and diffraction measurements to demonstrate that grain orientation profoundly influences polishing rates in polycrystalline diamond synthesized by chemical vapor deposition. Grains oriented with {111} or {100} planes perpendicular to the surface normal polish at significantly lower rates compared with grains of all other orientations when the surface is polished in continuously varying in-plane directions. These observations agree with predictions of the periodic bond chain vector model, developed previously for single crystals, and indicate that the polishing rate depends strongly on the number of periodic bond chain vectors that are within 10° of the exposed surface plane.

Conventional loudspeaker membranes made of metal or synthetic material such as fabric, ceramics or plastics suffer from nonlinearities and cone breakup modes at fairly low audio frequencies. Due to their mass, inertia and limited mechanical stability the speaker membranes made of conventional materials cannot follow the high frequency excitation of the actuating voice-coil. Low sound velocity causes phase shift and sound pressure losses due to interference of adjacent parts of the membrane at audible frequencies. Therefore, loudspeaker engineers are searching for lightweight but extremely rigid materials to develop speaker membranes whose cone resonances are well above the audible range. With its extreme hardness, paired with low density and high velocity of sound, diamond is a highly promising candidate for such applications. We report on the realization of dome shaped CVD diamond membranes by deposition on curved silicon substrates. Domes with diameters between 20 and 65 mm and with a thickness ranging from 50 to 120 μm were prepared. After deposition, the substrate is dissolved and the rim of the diamond dome is cut by laser scribing. Free standing diamond membranes are mounted onto dynamic voice coils and integrated into tweeter and/or midrange driver chassis. Extended tests and optimisations led to loudspeaker systems that show a second and third harmonic distortion behaviour in the important frequency range between 3 to 10 kHz that is reduced by 40% in comparison to already excellent established values obtained with sapphire membranes. Cone resonance frequencies of CVD diamond membranes are increased by a factor of two, as predicted by simulations.

We have analyzed the spatial distribution of electric field domains induced by negative differential photoconductivity in n-type GaAs∕AlGaAs quantum well infrared photodetectors. We find strong evidence of two different domain configurations, with the high-field domain and the low-field domain, respectively, adjacent to the emitter contact. A distinctive signature of these domain configurations is provided by the observed total current, which is observed to be close to either the valley current or the peak current. We also discuss the emergence of the two configurations.

Diamond has a unique combination of physical properties for the inertial confinement fusion ablator application, such as appropriate optical properties, high atomic density, high yield strength, and high thermal conductivity. Here, we present a feasible concept for fabrication of diamond ablator shells. The fabrication of diamond capsules is a multi-step process which involves diamond chemical vapor deposition on silicon mandrels followed by polishing, microfabrication of holes, and removing of the silicon mandrel by an etch process. We also discuss the pros and cons of coarse-grained optical quality and nanocrystalline chemical vapor deposition diamond films for the ablator application.

We present a quantum-well infrared photodetector that allows us to switch between a quadratic and a linear detection mode. We employ an asymmetric detector design where intersubband transitions in a three-level system give rise to a quadratic power dependence down to very low excitation densities. Upon reversing the bias voltage, however, the intermediate subband acts as the ejection channel due to highly efficient tunneling, thus leading to a two-level system with linear photoresponse.

The infrared absorption of CVD diamond at a wavelength of 10.6 mu m was measured as a function of temperature between 20 and 500 degrees C. CVD diamond of different structural quality was investigated. Optical spectroscopy and infrared Raman scattering were used for the structural characterization. Laser calorimetry was applied to measure the IR absorption at elevated temperatures. For these measurements, CVD diamond devices with integrated heater and temperature sensor were prepared. The optical absorption at a given temperature was determined by comparing the temperature rise induced by CO2 laser irradiation with the equivalent electrically induced temperature rise.Samples with a room temperature absorption coefficient of alpha=0.096 cm(-1) and alpha=0.85 cm(-1) were investigated. Despite this large difference, the absolute increase of the absorption coefficient with increasing temperature was found to be similar for the two samples. It increased by a value of about 0.5 cm(-1) between 0 and 500 degrees C. (c) 2005 Elsevier B.V. All rights reserved.

We report on QWIPs optimized for nonlinear detection using a bandstructure design with three energetically equidistant subbands. The double resonant character of the two-photon transition from the ground state into the second excited state results in quadratic dependence of the photocurrent on the incident power down to excitation densities as low as 0.1W/cm2. We have used these two-photon QWIPs as quadratic detectors in second-order interferometric autocorrelation measurements allowing for the separation of coherent and incoherent contributions to the photocurrent. We have studied the carrier dynamics and the influence of the QW doping on both phase and energy relaxation times. The measurements also demonstrate that these devices provide an ultra-sensitive tool with sub-ps response time for the characterization of ultrashort (∼ps) mid-infrared pulses in the pJ-regime such as those generated by modelocked quantum cascade lasers.

We report on two-photon detection based on nonlinear absorption between subbands in quantum wells. Resonantly enhanced nonlinear absorption, 6 orders of magnitude higher than that of typical bulk semiconductors, leads to a threshold power density for quadratic detection as low as 0.1 W/cm2 and to femtosecond time resolution. The approach facilitates dynamic characterization of the optical light field of infrared emitters with unprecedented sensitivity. We also present a new method for determining the intersubband and intrasubband scattering times by means of autocorrelation measurements.

Quantum well infrared photodetectors (QWIPs) have gained maturity for large focal plane arrays (FPA) with excellent thermal resolution, low 1/f noise, low fixed-pattern noise, and high pixel operability. Due to their spectrally narrow absorption, QWIPs are particularly suitable for thermal imaging applications involving several atmospheric transmission bands or several colors within the same band. We report on our progress on dual-band QWIP FPAs with pixel-registered, simultaneous integration in both bands. The arrays with 384x288 pixels and 40 μm pitch are based on a photoconductive QWIP for the 3-5 μm regime (MWIR) and a photovoltaic "low-noise" QWIP for 8-12 μm (LWIR). Excellent noise-equivalent temperature differences of only 20.6 mK (LWIR) and 26.7 mK (MWIR) have been achieved at 6.8 ms integration time and f/2 aperture. In addition, we have investigated test devices with different gratings, and discuss their dual-band coupling efficiencies.

We report on the development and status of a dual-band QWIP FPA with 384×288 pixels and 40μm pitch for the 3–5μm (mid-wavelength infrared, MWIR) and 8–12μm (long-wavelength infrared, LWIR) spectral bands. The array is based on a photovoltaic “low-noise” QWIP for the LWIR and a photoconductive QWIP for the MWIR and allows for simultaneous integration of both bands on each pixel. Array histograms indicate a noise-equivalent temperature difference as low as 17mK for the MWIR band and 43mK for the LWIR band at an integration time of 7.6 ms. In addition, we have investigated test devices with different gratings, geometries, and sizes and discuss some tradeoffs for dual-band diffraction gratings.

We report the nonlinear behavior of quantum-well infrared photodetectors with three energetically equidistant energy levels. The giant resonant nonlinearity leads to a quadratic power dependence of the photocurrent down to excitation power densities as low as 0.1W∕cm2. Using these highly sensitive two-photon detectors, second-order autocorrelation measurements of ultrashort midinfrared laser pulses in the pJ regime are demonstrated. The dynamical behavior is studied by a numerical analysis of these measurements. At high bias voltages we observe a dominating linear contribution to the photocurrent arising from tunneling processes.

Due to the short internal response time, quantum-well infrared photodetectors (QWIPs) are interesting for high-speed applications such as heterodyne spectroscopy or laser pulse monitoring. We studied the photocurrent transients of InGaAs/GaAs-QWIPs after irradiation with infrared laser pulses of 250 fs duration. The excitation wavelength of about 9 μm matches the peak wavelength of the QWIP structure. The photocurrent transient consists of two different dynamical components, representing the fast photoionization in the quantum-wells and the slow injection current that compensates the remaining space charge. The investigations of the different components as a function of temperature and bias voltage were performed on a nanosecond time-scale. The experimental separation of the two photocurrent contributions allows us to determine the photoconductive gain. The Fourier transform of the photocurrent transient was compared with other experimental methods including heterodyne detection and microwave rectification. The quantitative agreement between these different measurement techniques is excellent.

We have investigated the transport properties of optically excited electrons in a 100 period InGaAs/GaAs-quantum well infrared photodetector. The electrons were excited by femtosecond-infrared laser pulses and the transient photocurrent was recorded in the picosecond regime. We have carefully analyzed the measured photocurrent transients taking into account both the capture time τc and the transit time τtrans. Our experimental results allow us to determine τc, which increases from 10 ps at 5 kV/cm to about 20 ps at 20 kV/cm. We have also measured the noise gain in order to determine the transit time and the drift velocity. The dynamic parameters extracted from our measurements are consistent with values from the literature.

The effect of nitrogen and boron doping on the thermal and optical properties of high purity CVD-diamond grown by microwave plasma (MW-CVD) and DC arc-jet chemical vapor deposition (DCAJ-CVD) is investigated. Boron and nitrogen doping was carried out by adding to the process gas trimethylborate and nitrogen, respectively. Secondary ion mass spectrometry (SIMS) was used to quantify the incorporated boron concentration. To decide whether or not nitrogen is substitutionally incorporated electron paramagnetic resonance (EPR) measurements were applied. The influence of the impurities on the thermal properties was investigated by measuring the temperature dependence of the thermal conductivity between 77 and approximately 450 K. Using the Klemens–Callaway-Theory, information about phonon scattering at point and extended defects was obtained. In the case of nitrogen addition to the MW-CVD process, no enhanced point defect scattering but instead an increase of scattering at extended defects or grain boundaries was observed. The nitrogen doped DCAJ-CVD sample showed the typical yellow color and EPR signal of diamond containing substitutional nitrogen. The point defect concentration determined by the Klemens–Callaway-Theory was six times higher as compared to the MW-CVD samples. CVD-diamond samples doped with 2.5×1019 cm−3 boron showed a bright blue color and absorption features indicating substitutionally incorporated boron. An enhanced phonon scattering due to point defects was not observed.

We investigate the influence of avalanche multiplication by impact ionization on the photoconductive gain and the noise gain in quantum-well infrared photodetectors (QWIPs). A quantitative method is presented allowing the avalanche multiplication factor M and its field dependence to be determined from the measured photoconductive gain and noise gain. The approach is demonstrated using an In0.30Ga0.70As/GaAs QWIP.