Search

Resonant-cavity light-emitting diodes emitting at around 400 nm based on an undoped bottom AlInN/GaN distributed Bragg reflector (DBR) and a top dielectric SiO(2)/ZrO(2) DBR with circular emitting apertures of diameters ranging from 5 to 200 µm are demonstrated. The current distribution is investigated by luminance distribution imaging and three-dimensional device simulations for different current densities. The current distribution exhibits a maximum in the aperture centre or is homogeneous up to an aperture diameter of 50 µm independent of the current density. A minimum occurs in the aperture centre for larger diameters increasing with increasing diameter and current density. The current distribution improves with larger n-GaN thickness and higher contact resistance between the transparent In(2)O(3):Sn (ITO) electrode and the p-GaN contact layer.

Currently available cochlear implants are based on electrical stimulation of the spiral ganglion neurons. Optical stimulation with arrays of micro-sized light-emitting diodes (µLEDs) promises to increase the number of distinguishable frequencies. Here, the development of a flexible GaN-based micro-LED array as an optical cochlear implant is reported for application in a mouse model. The fabrication of 15 µm thin and highly flexible devices is enabled by a laser-based layer transfer process of the GaN-LEDs from sapphire to a polyimide-on-silicon carrier wafer. The fabricated 50 × 50 µm(2) LEDs are contacted via conducting paths on both pand n-sides of the LEDs. Up to three separate channels could be addressed. The probes, composed of a linear array of the said µLEDs bonded to the flexible polyimide substrate, are peeled off the carrier wafer and attached to flexible printed circuit boards. Probes with four µLEDs and a width of 230 µm are successfully implanted in the mouse cochlea both in vitro and in vivo. The LEDs emit 60 µW at 1 mA after peel-off, corresponding to a radiant emittance of 6 mWmm(-2).

Nanocrystalline diamond- (NCD) and AlN-based ultrathin single layer and bilayer membranes are investigated towards their mechanical properties. It is shown that chemo-mechanical polishing and heavy boron doping of NCD thin films do not impact the elastic properties of NCD layers as revealed by negligible variations of the NCD Young's modulus (E). In addition, it is demonstrated that the combination of NCD elastic layer and AlN piezo-actuator is highly suitable for the fabrication of mechanically stable ultrathin membranes in comparison to AlN single layer membranes. The elastic parameters of NCD/AlN heterostructures are mainly determined by the outstanding high Young's modulus of NCD (E = 1019 ± 19 GPa). Such ultrathin unimorph membranes allow for fabrication of piezo-actuated AlN/NCD microlenses with tunable focus length.

Boron-doped nanocrystalline diamond (NCD:B) elastic layers combined with aluminum nitride (AlN) piezoactuators are well suitable for ultrathin, highly transparent and mechanically stable micro-optical devices operating at high frequencies. In such unimorph structures NCD:B layers serve as advanced transparent electrodes instead of commonly used indium tin oxide. The carrier transport properties of heavily p-doped NCD thin films are characterized by temperature-dependent voltage-current and Hall measurements. In addition, the piezo-driven aspheric deformation of NCD:B/AlN micro lenses is demonstrated by means of integrated multi-sector actuators.

Unimorph heterostructures based on piezoelectric aluminum nitride (AlN) and diamond thin films are highly desirable for applications in micro- and nanoelectromechanical systems. In this paper, we present a new approach to combine thin conductive boron-doped as well as insulating nanocrystalline diamond (NCD) with sputtered AlN films without the need for any buffer layers between AlN and NCD or polishing steps. The zeta potentials of differently treated nanodiamond (ND) particles in aqueous colloids are adjusted to the zeta potential of AlN in water. Thereby, the nucleation density for the initial growth of diamond on AlN can be varied from very low (10(8) cm(-2)), in the case of hydrogen-treated ND seeding particles, to very high values of 10(11) cm(-2) for oxidized ND particles. Our approach yielding high nucleation densities allows the growth of very thin NCD films on AlN with thicknesses as low as 40 nm for applications such as microelectromechanical beam resonators. Fabricated piezo-actuated micro-resonators exhibit enhanced mechanical properties due to the incorporation of boron-doped NCD films. Highly boron-doped NCD thin films which replace the metal top electrode offer Young's moduli of more than 1000 GPa.

Micro electromechanical systems are a matter of intense research pursuing to replace silicon and III-V semiconductor-based components in prospective radio frequency communication devices. Due to their unique material properties, microstructures combining doped nano-crystalline diamond (NCD) and AlN thin films are promising for piezo-actuated microsystems in order to increase operating frequencies. In this work, single and doubly clamped unimorph NCD-on-AlN micro-resonators were fabricated and then characterized by laser vibrometry towards their flexural resonant frequencies in the range of 0.1-20 MHz to deduce their mechanical properties. Enhancements in the structural properties of an AlN piezo-actuator united with an advanced elasticity of nano-diamond electrode lead to superior mechanical parameters of the resulting unimorphs. These allow for the fabrication of flexural resonant microsystems with a potential to extend the operating frequencies well above 1 GHz.

To extend the tuning capabilities of radio frequency devices, coupled microelectromechanical systems are often employed. In this letter, we demonstrate piezoelectrically actuated, electrically tuneable resonator systems based on coupled micromechanical oscillators operating in a flexural vibration mode. The substantial enhancement in electrostatic coupling was achieved due to the implementation of lateral nanogaps of 100-200 nm between single resonator bars. This allows for resonator synchronization and precise system frequency tuning by over a factor of two, relative to its initial value. Additionally, a simple electro-mechanical model has been developed to describe the dynamic behavior of the electrostatically coupled oscillators.

In this work, the mechanical properties of thin piezoelectric AlN films along with the methods and instruments to obtain this information with sufficient accuracy via dynamic (vibration) and static analyses of thin membranes are reported. In addition, the impact of different damping mechanisms on the amplitude of forced oscillations have been considered in order to obtain the analytical expression relating the resonant amplitude of membrane to the ambient gas pressure.

We investigated the impact of SF6 plasma treatments on the electronic transport properties of GaN/AlGaN/GaN heterostructures by employing different plasma conditions as well as annealing in nitrogen atmosphere at 425°C. The electrical properties are characterized by Hall-effect measurements while electron spectroscopy and X-ray measurements are used to investigate changes in the surface chemical composition and in the layer structure, respectively. It is demonstrated that plasma treatments strongly affect the 2DEG properties of the heterostructure due to altering of the surface potential accompanied by the formation of a thin fluorinated amorphous film. Increasing the DC bias voltage for the plasma treatment leads to an additional degradation of the mobility caused by incorporation of fluorine into the heterostructure interface. Furthermore, the thin GaN cap layer is etched by plasma treatments with higher bias potential, which increases the carrier density at the interface.

Improving the internal quantum efficiency is still a major challenge for realizing efficient III-nitride-based lightemitting diodes (LEDs) with emission wavelengths below 365 nm. Here, we report on the influence of a very low In amount of less than 1% on the luminescence properties of single Ga(In)N/AlGaN quantum well (QW) structures and the performance of AlGaN-based LEDs with an emission wavelength of about 350 nm. The samples were grown by metalorganic vapor phase epitaxy on low defect density AlGaN buffer layers on (0001) sapphire substrates. The temperature dependence of the QW emission energy reveals a clear "S-shape" behaviour indicating a significant carrier localization for In containing QWs. The electroluminescence efficiency of LEDs with a GaInN QW active region could be improved by a factor of 4 compared to LEDs with GaN QW, resulting in a continuous wave (cw) output power of 8.2 mW at 40 mA.

We investigated the impact of fluorine and nitrogen plasma treatments on the electronic transport properties of GaN/AlGaN/GaN heterostructures by employing different plasma processes and subsequent annealing in nitrogen atmosphere at 425 °C. It is demonstrated that the plasma treatments affect the 2-dimensional electron gas (2DEG) properties of the HEMT structure resulting in a decrease of the surface potential. In order to understand the physical mechanisms we have undertaken an extensive study of the influence of plasma processing on the 2DEG properties. The electrical properties of the 2DEG are characterized by Hall measurements while X-ray photoelectron spectroscopy (XPS) is used to investigate changes in surface chemical composition. Changes of the DC bias voltage have a strong effect on the mobility presumably by incorporation of fluorine and nitrogen close to the channel. To confirm this assumption, Monte Carlo simulations were carried out, taking into account surface potential and negatively charged fluorine acceptors acting as additional scattering centers within the 2DEG.

Nearly lattice-matched InAlGaN-barriers for GaN-based high electron mobility transistors (HEMTs) have been grown by molecular beam epitaxy (MBE). Hall measurements reveal a sheet carrier density of 1.9 × 1013 cm-2 and a mobility of 1590 cm2/Vs. HEMTs have been processed from both InAlGaN-barrier and conventional AlGaN-barrier heterostructures. The devices with quaternary barrier show advantages in both DC and RF characteristics including a HEMT with 150 nm gate length, a current density of 2.3 A/mm and a peak transconductance of 675 mS/mm (Lim et al., IEEE Electron Device Lett. 31, 671 (2010) [1]). On the other hand, the transistors with ternary barrier exhibit significantly better pinch-off behaviour and lower gate leakage currents. These issues have been successfully addressed in a second process run by improving the MBE growth of InAlGaN and by adding a GaN cap layer to the HEMT heterostructure.

In this work we report on microstructures made of AlN for the use as piezoelectric microgenerators in energy harvesting applications. Experimental results on the mechanical and piezoelectrical properties of sputtered AlN thin films deposited on Si(001) substrates as well as results on vibrometry of fabricated microcantilevers are presented. It is shown that due to the constant d33 piezo coefficients of > 5 pm/V, AlN films are well suited for the integration in cantilever structures for powering of low-consumption sensor networks.

The paper reports on the integration of resistive gas sensors from indium oxide nanoparticles with near-ultraviolet light emitting diodes (LEDs) for miniaturized and highly sensitive ozone sensing. For the integrated sensor an LED with an optical power as low as 250 microW is sufficient. An optimum operational window of the LED wavelength is determined to be between 380-420 nm. Furthermore, two operational modes of the used LED (pulse and continuous modes) have been compared and it is found that the pulse mode is more suitable for real applications.

A series of 400 nm emitting GaInN/GaN single-well light-emitting diodes, grown on ultra-low dislocation density GaN templates with well widths varying between 3 and 18 nm, were investigated by pulsed and time resolved electroluminescence measurements using small signal modulation technique for the latter. A reduction of the efficiency droop at high current densities with increasing well width was observed. The highest overall external quantum efficiency was obtained for LEDs with 11 nm thick double-heterostructure (or wide well) active region at current densities above 70 A/cm2. Furthermore, carrier lifetime and volume carrier density in the wells were determined. A model based on the interplay between defect assisted recombination, depending on dislocation density, and strongly carrier density dependent loss processes is presented explaining the well-thickness dependence of the efficiency droop.

There is a high demand for compact low-cost ozone sensors. It has been shown recently that In2O3 nanolayers can act as ozone sensitive films activated at room temperature by ultraviolet light. In the present work, the authors integrate ultrathin layers of In2O3 nanoparticles and a GaInN/GaN based blue light emitting diode (LED) on a single sensor chip. The integrated sensor was found to be sensitive to O-3 concentrations as low as similar to 40 ppb. These results demonstrate that by integrating GaInN/GaN based blue LEDs and metal oxide sensing layers back to back on a single chip, compact and robust gas sensors can be realized.

We compare the electro-optical characteristics of violet-emitting ridge waveguide (AlGaIn)N quantum well diode lasers with etched laser facets, grown on GaN templates with a homogeneously reduced defect density, with those of reference devices grown on sapphire using conventional low-temperature GaN nucleation layer technology. Defect densities of the former and latter laser structures were around 1x10(exp 8) cm-2 and 2x10(exp 9) cm-2, respectively. In on-wafer pulsed-mode operation a 35% reduction in threshold current density and 40% reduction in injected electrical power at threshold was achieved upon reduction of defect density. Minimum values for threshold current density and injected electrical power at threshold are 7.5 kA/cm2 (500 mu m x 4 mu m) and 0.9 W (500 mu m x 2 mu m), respectively, for a ridge laser with uncoated facets. For improved heat sinking the sapphire substrate of selected laser diodes on GaN template was removed using laser lift-off, followed by n-side down soldering on copper heat sinks. This way, the maximum duty-cycle, limited by premature thermal rollover, could be increased by a factor of three for the substrate-less lasers.

The influence of nonradiative recombination processes on the output power P of InGaN light emitting devices (LEDs) with emission wavelength between 380 and 430 nm grown on sapphire substrates have been investigated. The electroluminescence intensity was studied as a function of the forward current I and the device temperature. The P-I curves of all LEDs show a super-linear behaviour at low currents (between 80 µA and 1 mA) and a nearly linear characteristic around the operating forward current of 20 mA (equivalent to a current density of 44 A/cm2). The super-linear behaviour is attributed to the saturation of nonradiative processes which also affect the output power at operating forward current. The reduction of the output power at 20 mA with increasing temperature is correlated with the super-linear behaviour at low currents indicating that nonradiative processes are thermally activated.

The performance and temperature stability of Al-free GaInAs/GaAs PHEMTs with GaInP as a Schottky barrier is reported. GaInP/GaInAs/GaAs PHEMTs with a 1 mu m gate length show an f(max) value of 76GHz with a maximum drain current of 570mA/ mm and a drain-source breakdown voltage of 16V. Moreover, the first results on short gate length devices (0.15mu m) yield f(T) and f(max) values of 106 and 203GHz, respectively. In this case, the drainsource breakdown voltage is as high as 8V. These results demonstrate the great potential of GaInP/GaInAs PHEMTs for power applications.

In this paper we present the technology for fabrication of InP-based components such as wavelength demultiplexers, optical switches, optical amplifiers and detectors. A number of discrete devices such as phased array (PHASAR) demultiplexers, MMI-couplers, Mach-Zehnder interferometer switches have been realized. The integration of these devices to a multiwavelength (MW) add-drop multiplexer and a MW-receiver has been performed. The technology of passive and active integration by using metal organic vapor phase epitaxy (MOVPE) and chemical beam epitaxy (CBE) towards the realization of (MW)-laser and an optical cross connect is discussed