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Leakage of Al x Ga1−x N/GaN heterostructures was investigated by admittance–voltage profiling. Nominally undoped structures were grown by low-pressure metal-organic vapor-phase epitaxy. The investigated structures had an Al-content of 30%. They are compared to structures with an additional 1 nm thick AlN interlayer placed before the Al0.3Ga0.7N layer growth, originally to improve device performance. Conductance of field effect transistor devices with AlN interlayer, measured from depletion of the two-dimensional electron gas (2DEG) to zero volt bias at frequencies ranging from 50 Hz to 10 kHz, could be described by free charge carriers using a Drude model. The voltage dependent conductance shows a behavior described by either Poole-Frenkel emission or Schottky emission (SE). From the size of the conductance, as well as simulation of the tunneling current injected from the gate under off-state conditions by universal Schottky tunneling, SE is obvious. Evaluating the data by SE, we can locate the leakage path, of tens of nm in the range between gate and drain/source with contact to the 2DEG, originating from the AlN interlayer. The static dielectric constant in growth direction, necessary for the evaluation, is determined from various Al x Ga1−x N/GaN heterostructures to ϵ ||(0) = 10.7 ± 0.1.

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.

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 SiO2/ZrO2 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 In2O3:Sn (ITO) electrode and the p-GaN contact layer. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this work we present a technological process flow enabling fabrication of transversal nanofeatures in nanocrystalline diamond based MEMS. Growth of thin diamond films on nanostructured templates, projection exposure lithography and bi-layer electron beam lithography are the key technologies used. Electrode gaps in the order of 100 nm have been successfully accomplished with suspended microplates with dimensions of 1×1 μm 2 .

In this work, we present a proof-of-concept for the modulation of field emission currents from boron doped nano-diamond based micro-electro-mechanical cantilevers. This modulation was achieved by variations in anode–cathode spacing using mechanical oscillations of the field emitting tips by means of piezoelectric actuation of an aluminum nitride layer. In these devices, the nano-diamond layers serve as a multifunctional material as they are used as field emitters, elastic layers in a unimorph layer stack and as electrodes for piezo-actuation. The cantilevers are processed using conventional micro-fabrication technology and are enhanced by focused ion beam milling, introducing nano-scale features. In addition, it is shown that a stress compensating geometry is efficient to counteract the large thermally induced strain in the nano-materials used.

In this work we report on non-resonant piezoelectric micro-energy-harvesters based on corrugated membranes made from aluminium nitride. These harvesters were designed to respond efficiently to aperiodic mechanical impact at low repetition rates. The piezogeneration efficiency of non-resonant systems was compared to conventional resonant devices like tensile stressed membranes and cantilevers. Special attention was paid to system properties that allow an operation at low frequencies in viscous media with high damping. Classical analytical modelling was employed to describe the corrugated microsystems with the aim to calculate the feasible energy outcome and to optimise electro-mechanical coupling.

Aluminum nitride (AlN) thin films deposited by reactive radio frequency magnetron sputtering in an Ar/N2 discharge on Si(001) substrates were studied with respect to structure, stress, and piezoelectric properties. In order to optimize the AlN layers for flexural plate wave (FPW) devices, the influence of process pressure and N2 concentration has been evaluated by means of spectroscopic ellipsometry, residual stress measurements, x-ray diffraction, atomic and piezoresponse force microscopy, along with analysis of the piezoelectric charge coefficient d33,f. FPW devices with low compressively stressed (−200 to −300 MPa) AlN layers were prepared and characterized by white light interferometry and Raman measurements. With increasing pressure from 3×10−3 to 8×10−3 mbar, a transition from −840 MPa compressive stress to +300 MPa tensile stress was measured. Increasing the nitrogen concentration from 3.3% to 50% resulted in a change in stress from +150 to −1170 MPa. All films exhibited a high degree of c-axis orientation. A piezoelectric charge coefficient up to d33,f≈−6.8 pC/N was obtained. Furthermore, it is shown that the film surface morphology is also very much dependent on the growth conditions. A model regarding the mean free path of the sputtered particles and the film surface morphology is proposed. The authors show that the optimization of the film stress by means of the nitrogen concentration in the sputter gas mixture is beneficial as the process window is larger.

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 (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Improving the internal quantum efficiency is still a major challenge for realizing efficient III-nitride-based light-emitting 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. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dry etch processes based on Cl2/SF6 have been developed to selectively remove GaN over AlGaN to fabricate recessed gate GaN/AlGaN High Electron Mobility Transistors (HEMTs). Since dry etch processes may damage the HEMT structure the influence of etch conditions on the transistor performance has been investigated using DC, RF and power measurements. Recess etch conditions resulting in bias potentials of only -64 V yield a shift in threshold voltage of 1.6 V as compared to transistors for which a low bias etch process (-24 V) was used.

We present an overview on epitaxial growth, processing technology, device performance, and reliability of our GaN high electron mobility transistors (HEMTs) manufactured on 3- and 4-in. SiC substrates. Epitaxy and processing are optimized for both performance and reliability. We use three different gate lengths, namely 500 nm for 1–6 GHz applications, 250 nm for devices between 6 and 18 GHz, and 150 nm for higher frequencies. The developed HEMTs demonstrate excellent high-voltage stability, high power performance, and large DC to RF conversion efficiencies for all gate lengths. On large gate width devices for base station applications, an output power beyond 125 W is achieved with a power added efficiency around 60% and a linear gain around 16 dB. Reliability is tested both under DC and RF conditions with supply voltage of 50 and 30 V for 500 and 250 nm gates, respectively. DC tests on HEMT devices return a drain current change of just about 10% under IDQ conditions. Under RF stress the observed change in output power density is below 0.2 dB after more than 1000 h for both gate length technologies.

Three series of 400 nm light-emitting diodes (LED) with GaInN well widths ranging from 3 nm to 18 nm were grown on freestanding GaN-substrates, on ultra low dislocation GaN templates on sapphire, and directly on sapphire substrates using conventional low temperature nucleation. The resulting defect densities in the active layers range from 4 × 107 to 1 × 109 cm-2. Employing pulsed electroluminescence (EL) measurements to assess the EL efficiency over a wide range of current densities up to 650 A/cm2, the LEDs grown on defect reduced substrates showed the highest EL efficiency among all devices, which was achieved for GaInN well widths >10 nm and at comparatively high current densities (>100 A/cm2). In contrast, conventional 3 nm wide quantum well LEDs showed the peak EL efficiency at low current densities (

A combined layer structure of aluminum nitride (AlN) and silicon nitride (Si x N y ) enables the fabrication of multisegment piezo-actuated micro lenses with a precise control of the lens surface. It is demonstrated that these micro lenses offer free aspheric deformation of the lens surface and can operate at high repetition rates along with reproducible and precise tunability. The AlN/Si x N y micro lenses are highly advantageous to be used as wave front filter or for fast focus correction.

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 1 × 108 cm–2 and 2 × 109 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 µm × 4 µm) and 0.9 W (500 µm × 2 µm), 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. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Boron-doped nanocrystalline diamond (NCD:B) elastic layers combined with aluminum nitride (AlN) piezo-actuators 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.

Results are presented on the effect of using GaN templates with homogeneously reduced defect density on the performance of violet- und UV-emitting (AlGaIn)N LEDs, as well as on the use of such LED chips as pump light sources in three-phosphor white luminescence conversion LEDs (LUCO-LEDs). For LED chips emitting at 385 nm, an improvement in electroluminescence efficiency by a factor of two was found for growth on 8 x 10(exp 7) cm-2 defect density templates compared to direct growth on sapphire. Further we report on the fabrication and on-wafer testing of violet-emitting ridge waveguide (AlGaIn)N quantum well diode lasers with etched laser facets, grown on sapphire using conventional low-temperature GaN nucleation layer technology as well as on the above low defect density GaN templates. In on-wafer pulsed-mode operation a 35% reduction in threshold current density was achieved for the latter, resulting in a minimum injected power at threshold of 0.9 W of 500 µm x 2 µm ridge laser diodes with uncoated facets.

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 (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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/cm 2 ). 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 operation principles of the various single chip luminescence conversion (LUCO).white LEDs, that can be realized with blue or ultraviolet (UV) LED chips, are briefly reviewed. In order to demonstrate the feasibility of the so far almost unexplored case of UV pumped tricolor LUCO white LEDs, In x Ga 1-x N single quantum well LED layer sequences were grown by MOCVD and processed into near-UV LED pump chips. Europium doped oxide and sulfide phosphors emitting in the blue, the green and the red spectral range were used for UV to visible conversion. With white blends prepared from these phosphors, housed white LEDs with high color quality were realized.

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 (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The impact of device processing and plasma treatments at different plasma conditions on the electronic transport properties of GaN/AlGaN/GaN heterostructures was investigated 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 these layer structures are quite sensitive even to non-plasma based processing. Furthermore, treatments in SF6 and N2 based plasmas strongly affect the 2DEG properties of the heterostructure due to altering of the surface barrier accompanied by thinning of the layer structure. Depending on the layer structure and the plasma conditions used the electronic properties may be recovered by annealing.

Near-UV and violet emitting AlGaInN single quantum well LED structures were grown by MOCVD on sapphire substrates. On-wafer tests before processing gave an output power at 40 mA between 1.4 mW at 380 nm and 6.7 mW at 420 nm. LED chips with wavelengths between 380 and 404 nm were selected for manufacturing radial UV LEDs and white emitting LEDs. The UV to white converters were prepared from broad band red, green and blue emitting powder phosphors with individual luminescence peaks near 610, 550 and 460 nm, respectively. White luminescence conversion (LUCO) LEDs with a warm hue and color temperatures in the range 4000-4300 K are demonstrated.

In this paper we report on the development of high transconductance GaN-based high electron mobility transistors (HEMTs) to improve the performance at W-Band frequencies. At first, the influence of the barrier thickness on the maximum transconductance (gm,max) was investigated by using two different technologies: growth of thin barrier layers and deep gate recess. Second, the effect of a gate length reduction down to 100 nm on gm,max was examined. The reduction of the barrier thickness results in a strong increase of the extrinsic transconductance up to 600 mS/mm. The technology was then used to fabricate HEMTs, with a cut-off frequency of 110 GHz, which are compatible to a MMIC technology (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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 (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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 μW 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. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

International audience; We report on the realization and first demonstration of CW near-milliwatt-power emission at λ = 390 nm from resonant-cavity light-emitting diode (RCLED) on GaN templates. The vertical cavity consists of a bottom AlGaN/GaN distributed Bragg reflector and a top dielectric SiO2/ZrO2 mirror enclosing a GaInN/GaN multiple-quantum-well active layer. RCLEDs with total optical output of about 600 μW at an injection current of 20 mA were achieved before packaging, taking account of current growth and processing considerations. Dislocations generated during the growth of the RCLED structure seem to be affecting the mean light output. This can be further improved by the use of high-quality low-dislocation-density GaN templates or freestanding GaN substrates.

System properties of focal plane array (FPA) cameras based on GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) operating in the long-wavelength infrared are analyzed. Due to the limited charge storage capacity of available readout circuits, a small photoconductive gain improves the noise-equivalent temperature difference (NEΔT) of the sensor. We have demonstrated several camera systems based on conventional photoconductive QWIPs with NEΔT mK and NEΔT mK for FPAs with 256×256 and 640×512 pixels, respectively. A small photoconductive gain is obtained by operating the QWIP at very small electric fields ( kV / cm ). Finally, we discuss the possibility of reducing the NEΔT even further by using QWIP structures in which the photoexcited carrier mean free path is controlled by the incorporation of additional epitaxial layers. We thus obtain a high detectivity at very small signal and noise currents, which is useful in particular for large arrays with reduced pixel sizes. In addition, these detectors are shot-noise limited since the recombination noise is suppressed. The performance of FPA sensors based on these detectors will be addressed.

This work considers the challenges and technical solutions concerning the deposition of thin films and processing of emerging vacuum microelectromechanical systems (MEMS) based on nanodiamond flexural beams. It is demonstrated that the add-on processing of MEMS chips allows for fabrication of functional nano-features such as nanoscale anode-cathode spacing in order to realize low-bias field emission. Emission current densities up to 500 nA per nanodiamond tip were achieved at an applied bias of 30 V.

Tunable micro lenses based on boron-doped nanocrystalline diamond and aluminum nitride (AlN) thin films are investigated. It is demonstrated that such a material combination is advantageous for the fabrication of highly transparent, mechanically stable optical components capable to operate at high actuation frequencies. In addition, it is shown that radially segmented AlN piezo-actuators allow for free aspheric deformation of the lens surface. It enables the use of such tunable lenses either in wave front filters or for correction of aberrations induced in complex optical systems.

We develop a monolithic picosecond laser pulse generator, based on the classical design of a group-III-nitride Fabry-Perot laser diode with electrically separated ridge sections. We use two different multi-section design variants, with the absorber section placed either in the center or at the end of the ridge. Profiting from the very low lateral conductivity in the p-type GaN top contact layer, we implement the multi-section concept just by etching off small sections of the top metalization on the ridge. The physical mechanism underlying short pulse generation within such system, operating in the 400 - 435nm wavelength range, strongly depends both on the reverse bias applied to the absorber and the forward current in the gain section. Varying the applied reverse bias affects both the absorption and the carrier lifetime in the absorber section through changes in the QW internal field. In consequence we can distinguish between different modes of operation. For moderately long carrier lifetimes the absorber stabilizes relaxation oscillations in the GHz frequency range and self-pulsation occurs, of relatively long duration. With increasing reverse bias, and thus decreasing carrier lifetime, we observe a transition to self-Q-switching. Finally, at large enough negative bias, the carrier life time in the absorber is so short that the laser diode operates in a passive self-mode-locking regime with a repetition rate of 87 GHz and pulse duration of 2 ps for a cavity length of 540 μm.

The development of a process chain allowing for rapid prototyping of GaN-based light-emitting diodes (LEDs) is presented, which does not rely on photolithography. Structuring of the epitaxial layers is realized by direct-writing laser ablation, allowing a flexible chip layout that can be changed rapidly and at low cost. Besides contact metallization and trench formation, mesa definition is the most critical processing step. For mesa formation and to expose the n-GaN contact layer, the epitaxial grown p-GaN layer together with the active region has to be removed completely without forming cracks or crystal defects in the n-GaN layer or the mesa sidewalls, which would cause sidewall leakage currents. In developing an appropriate laser ablation process that meets these requirements, three different laser systems have been employed in a comparative study. These are a frequency-tripled picosecond (ps) Nd:YVO(4) laser emitting at a wavelength of 355 nm and a pulse length of 10 ps and two 20 nanosecond (ns) pulse length laser systems, operating at a wavelength of 248 nm (Excimer laser) and 355 nm (frequency-tripled Nd:YVO(4) laser), respectively. First, the laser sources are compared regarding the morphological properties of the resulting laser trenches. Due to band filling effects resulting in optical bleaching of the GaN material when irradiating with ps-laser pulses at 355 nm, the resulting ablation process suffers from cracking. Laser ablation using ns-pulses at both 355 nm and 248 nm leads to crack-free material removal up to a well-defined depth. To keep reverse-bias leakage currents at a level comparable to that of conventional dry-etched mesa-LEDs, subsequent wet etching is essential to remove residues in the mesa-trenches irrespective of the laser source used. Besides wet etching, an additional annealing step has to be applied to mesa-trenches fabricated using ns- and ps-laser pulses at a wavelength of 355 nm. Due to the larger penetration depth at 355 nm, defects causing an increased leakage current are generated in the quantum well region during laser irradiation, which manifest themselves by a spectrally broad defect luminescence. To separate and quantify the contributions from the mesa sidewall leakage and from the areal leakage through the p-n-junction, a series of LEDs have been fabricated and analyzed for which the mesa perimeter length was varied while keeping the mesa area constant. In this way, it is shown that laser ablation with ns-pulses at 248 nm results in the lowest sidewall leakage current as well as forward bias voltage-current and output power-current characteristics very similar to those of conventionally dry etched reference LEDs.

Picosecond (ps) lasers provide a universal tool for material processing. Due to the short pulse length material is removed by a process called "cold ablation", with minimal thermal damage to neighbouring regions. As a result, better defined structures with smother and cleaner side walls can be fabricated than with nanosecond (ns) laser pulses. This offers new possibilities for laser processing in semiconductor technology for both semiconductor materials as well as contact and bond metallizations. The fabrication of optoelectronic devices such as light-emitting diodes (LEDs) typically involves photolithography steps, requiring specific lithography masks be fabricated which, in particular for prototyping, is expensive and time consuming. Therefore it would be attractive for a range of applications to replace these steps by direct writing techniques such as laser processing, which will speed up e.g. the development and prototyping of new devices. We report on fully laser processed planar GaN-based LEDs fabricated without any photolithography steps. On the bare semiconductor wafer, isolation trenches and mesa structures are formed directly by ultraviolet ps laser pulses. For the direct deposition of patterned ohmic contact metallizations, the ps laser fabrication and subsequent use of high resolution shadow masks is presented, which exhibit a significantly reduced sidewall roughness compared to masks produced by ns laser pulses. Due to the higher precision of the laser defined masks it becomes possible to deposit multiple layers, through the use of alignment marks, similar to multiple mask level photolithography. Finally, the ps laser processed LEDs are electrically and optically characterized and their characteristic compared with that of conventionally fabricated mesa LEDs.

We develop (Al, In)GaN ridge waveguide laser diodes in the violet-blue spectral region. Varying the indium content of the InGaN quantum wells, we tailor the emission wavelength of our devices for specific applications in the range from 390 nm to 425 nm. Using different commercially available free standing GaN substrates, we adjust the epitaxial design and optimize the processing sequence for the different kinds of substrates. In particular we focus on the ridge formation. We compare different fabrication methods to obtain devices with ridge widths around ≤2 μm. So far, we have achieved threshold currents around 60 mA and slope efficiencies exceeding 1 W/A.

The CIP stability of pH sensitive ion-sensitive field-effect transistors based on AlGaN/GaN heterostructures was investigated. For epitaxial AlGaN/GaN films with high structural quality, CIP tests did not degrade the sensor surface and pH sensitivities of 55–58 mV/pH were achieved. Several different passivation schemes based on SiOx, SiNx, AlN, and nanocrystalline diamond were compared with special attention given to compatibility to standard microelectronic device technologies as well as biocompatibility of the passivation films. The CIP stability was evaluated with a main focus on the morphological stability. All stacks containing a SiO2 or an AlN layer were etched by the NaOH solution in the CIP process. Reliable passivations withstanding the NaOH solution were provided by stacks of ICP-CVD grown and sputtered SiNx as well as diamond reinforced passivations. Drift levels about 0.001 pH/h and stable sensitivity over several CIP cycles were achieved for optimized sensor structures.

The fabrication of optoelectronic devices such as light-emitting diodes (LEDs) typically involves photolithography steps, requiring specific lithography masks. This approach is expensive, inflexible and time consuming, in particular for prototyping. Therefore it would be attractive to replace these steps by direct writing techniques such as laser processing, which would speed up, for example the development and prototyping of new devices. Picosecond lasers provide a universal tool for material processing. Due to the short pulse length, material is removed by a process called "cold ablation," with minimal thermal damage to neighboring regions. As a result, better-defined structures with smoother and cleaner side walls can be fabricated compared to nanosecond-pulsed laser-based processing. We report on fully laser-processed planar gallium nitride-based LEDs fabricated using only ps laser processing for pattern definition and material removal. On the bare semiconductor wafer, isolation trenches and mesa structures are formed directly by ultraviolet ps laser pulse writing. For the direct deposition of patterned ohmic contact metallizations, the ps laser fabrication and subsequent use of high-resolution shadow masks is presented. Finally, the ps laser-processed LEDs are electrically and optically characterized and their characteristics compared with those of conventionally fabricated mesa LEDs.

Admittance–voltage profiling of AlxGa1−xN/GaN heterostructures was used to determine the frequency dependent capacitance and conductance of FET devices in the frequency range from 50 Hz to 1 MHz. The nominally undoped low pressure metal-organic vapor-phase epitaxy structures were grown with an Al-content of 30%. An additional 1 nm thick AlN interlayer was placed in one structure before the Al0.3Ga0.7N layer growth. For frequencies below 108 Hz it is convenient to use equivalent circuits to represent electric or dielectric properties of a material, a method widely used, for example, in impedance spectroscopy. We want to emphasize the relation between frequency dependent admittance–voltage profiling and the corresponding equivalent circuits to the complex dielectric function. Debye and Drude models are used for the description of the frequency dependent admittance profiles in a range of depletion onset of the two-dimensional electron gas. Capacitance- and conductance-frequency profiles are fitted in the entire measured range by combining both models. Based on our results, we see contributions to the two-dimensional electron gas for our samples from surface states (80%) as well as from background doping in the Al0.3Ga0.7N barriers (20%). The specific resistance of the layers below the gate is above 105 Ω cm for both samples and increases with increasing negative bias, i.e., the layers below the gate are essentially depleted. We propose that the resistance due to free charge carriers, determined by the Drude model, is located between gate and drain and, because of the AlN interlayer, the resistance is lowered by a factor of about 30 if compared to the sample without an AlN layer.

In this work, we show that thin biocompatible AlN films reveal a stable, frequency independent piezoresponse demonstrating a good applicability for implantable micro-generators. It was demonstrated that although more power is generated by resonant (tensile stressed) systems, e.g. cantilevers and planar membranes, the non-resonant (compressive stressed) systems, e.g. corrugated membranes, are preferable for this purpose due to the generation of sufficient power at low-frequency aperiodic vibrations and at very low accelerations.

Improving the crystal quality of AlGaN epitaxial layers is essential for the realization of efficient III-nitride-based light emitting diodes (LEDs) with emission wavelengths below 365 nm. Here, we report on two different approaches to improve the material quality of AlGaN buffer layers for such UV-LEDs, which are known to be effective for the MOVPE growth of GaN layers. Firstly, we grew AlGaN on thin GaN nucleation islands which exhibit a threedimensional facetted structure (3D GaN nucleation). Lateral overgrowth of these islands results in a lateral bending of dislocation lines at the growing facets. Secondly, in-situ deposited SiNx interlayers have been used as nano-masks reducing the dislocation density above the SiNx layers. Both approaches result in reduced asymmetric HRXRD ω-scan peak widths, indicating a reduced edge-type dislocation density. They can be applied to the growth of AlGaN layers with an Al concentration of at least 20%, thus suitable for LEDs emitting around 350 nm. On-wafer electroluminescence measurements at 20 mA show an increase in output power by a factor of 7 and 25 for LED structures grown on 3D GaN nucleation and SiNx interlayer, respectively, compared to structures grown on a purely 2D grown low Al-content AlGaN nucleation layer. Mesa-LEDs fabricated from the LED layer sequences grown on buffers with SiNx interlayer exhibit a low forward voltage of 3.8 V at 20 mA and a maximum continuous wave (cw) output power of 12.2 mW at 300 mA.

Ozone sensors are of great demand for monitoring high-concentration ozone for industrial applications and low-concentration ozone for protecting people's health, although commercial ozone sensors are limited in their detection range. In this work, it is demonstrated that compact energy-saving photostimulated ozone sensors based on indium oxide nanoparticles can detect ozone with a dynamical range over four orders of magnitude at room temperature. The photostimulated ozone sensor shows a very low cross response to NO2, CO, and CO2. Furthermore, the sensing signal is very reproducible, and no hysteresis effects were found in repeated measurements.

Reflective (thick) and semi-transparent (thin) Ni/Ag/Ni contacts were prepared on GaInN-based light-emitting diodes (LEDs) and p-GaN/p-Al0.15Ga0.85N layer sequences. A light output power enhancement of 41% and forward voltage reduction of 0.59 V were obtained compared to a Ni/Au contact for LEDs emitting at 400 nm with thick p-GaN contact layers. The specific contact resistance of the Ni/Ag/Ni contacts on p-GaN/p-Al0.15Ga0.85N with varying p-GaN thickness (5-20 nm) were determined by transmission line method and compared to Ni/Au contacts. Low resistive ohmic contacts were obtained for a p-GaN thickness of less than 10 nm. The p-GaN layer can be completely omitted for the reflective Ni/Ag/Ni contact. In addition, reflection and transmission of the Ni/Ag/Ni metallization schemes were investigated in the ultra-violet spectral range. Thick Ni/Ag/Ni and thin Ni/Ag/Ni covered by Al are promising to serve as reflective contacts for ultra-violet LEDs. The former for wavelength around 350 nm and the latter for wavelengths below 350 nm.

Results from on-wafer electroluminescence (EL) microscopy on AlGaN/GaN high-electron-mobility transistors with leakage currents varying over four orders of magnitude are presented. In the off-state region the integrated EL intensity is proportional to the leakage current and independent of gate width for the devices under study. The slope of the integrated EL–leakage current dependence is determined by the electrical field in the source–drain direction. The influence of the GaN cap thickness is small or even negligible for higher drain bias. Stress during accelerated aging results in enhanced degradation for areas of enhanced leakage current and/or electric field values.

GaAs bipolar transistors with a 50-AA-thick lattice matched Ga/sub 0.5/In/sub 0.5/P layer between the emitter and base acting as a hole repelling potential barrier in the valence band were fabricated from films grown by metalorganic vapor phase epitaxy (MOVPE). The 1000-AA-thick base was doped with carbon to 2*10/sup 19/ cm/sup -3/, resulting in a base sheet resistance of 250 Omega / Square Operator . Carbon has been chosen because of its low diffusivity. Using the barrier layer as an etch stop the authors fabricated mesa-type broad-area devices. The output characteristics of the devices are ideal with very small offset voltages and infinite Early voltages. Common emitter current gains of up to 70 at 10/sup 4/ A/cm/sup 2/ collector current density were obtained. The current gain is clearly higher than the one calculated for a bipolar junction transistor with the same doping profile because the base-emitter hole current is suppressed by the Ga/sub 0.5/In/sub 0.5/P potential barrier in the valence band. >

Magnetotransport and photoluminescence (PL) measurements on InAs/(Galn)Sb superlattices (SLs) .grown by molecular-beam epitaxy on GaSb substrates at different substrate temperatures are reported. With increasing growth temperature, a transition of the SLs from residual n-type to residual p-type doping was observed. For n-type samples, a decrease in the electron concentration leads to a strong increase in the PL intensity. In contrast, the PL intensity of p-type samples is only weakly dependent on the hole concentration. This correlation can be used to control the residual doping of the SLs.

Near-UV LEDs emitting at around 400 nm can be used e.g. as pump light source in tri-phosphor RGB white luminescence-conversion LEDs with high color rendering. 1 Although non-thermal roll-over decreases towards shorter emission wavelengths in GaInN-based LEDs, this effect still limits the efficiency of 400 nm emitting LEDs at current densities above 50 A/cm 2 . One way to overcome non-thermal roll-over is to combine a GaInN wide-well active region with the growth on low dislocation density (DD) substrates. Single-well LEDs with GaInN layer widths between 3 nm and 18 nm were grown (a) directly on sapphire substrates with a resulting DD of 10 9 cm -2 , (b) on low DD GaN templates on sapphire (DD of 10 8 cm -2 ), and (c) on freestanding GaN substrates (FS-GaN, DD of 4×107 cm -2 ). At low current densities (pulsed mode operation) the LEDs with a 3 nm GaInN QW active region showed the highest efficiency, irrespective of the substrate. However, the electroluminescence (EL) efficiency peaks at around 50 A/cm 2 and shows a clear non-thermal roll-over towards higher current densities. The efficiency of LEDs with well widths >3 nm grown on sapphire decreases with increasing well width over the whole range of current densities (≤300 A/cm 2 ). However, when grown on low DD GaN templates or FS-GaN, the efficiency of the LEDs with 11 and 18 nm wide GaInN layers surpasses that of the conventional LEDs (well widths ≤6 nm) for current densities ≥250 A/cm 2 , yielding the highest EL efficiency of all LED-structures.

By combining a low damage chlorine based gate-recess etching and a sophisticated technology for AlGaN/GaN depletion-mode high electron mobility transistors (HEMTs) we fabricated high performance recessed enhancement-mode HEMTs. A comparative investigation of depletion- and enhancement-mode devices prepared by this technique shows excellent DC and RF properties. A transconductance of 540 mS/mm and cut-off frequencies fT of 39 GHz and fmax of 74 GHz were obtained for 0.25 µm gate enhancement-mode HEMTs. Large-signal power measurements at 2 GHz reveal an output power density of 4.6 W/mm at 68% PAE conclusively demonstrating the capability of our enhancement-mode devices.

Excellent reliability performance of AlGaN/GaN HEMTs on SiC substrates for next generation mobile communication systems has been demonstrated using DC and RF stress tests on 8 x 60 mu m wide and 0.5 mu m long AlGaN/GaN HEMTs at a drain voltage of V-d = 50 V. Drain current recovery measurements after stress indicate that the degradation is partly caused by slow traps generated in the SiN passivation or in the HEMT epitaxial layers. The traps in the SiN passivation layer were characterized using high and low frequency capacitance-voltage (CV) measurements of MIS test structures on thick lightly doped GaN layers. (C) 2009 Elsevier Ltd. All rights reserved.

There is a high demand for compact low-cost ozone (O 3 ) sensors. It has been shown that indium oxide (In 2 O 3 ) thin films grown by metal-organic vapor-phase epitaxy (MOVPE) act as an O3 sensitive material, which can be activated at roomtemperature by ultraviolet (UV) light. In the present work we integrated the In 2 O 3 sensing thin film and an (AlGaIn)N based near-UV LED back-to-back on a single sensor chip. The integrated In 2 O 3 film-LED sensor was exposed to varying ozone concentrations ranging from 38 ppb to 726 ppb and found to be sensitive to even the lowest O 3 concentration.

Group III-nitride based heterostructures are of rapidly growing importance for the fabrication of short-wavelength light-emitting devices as well as for high-power high-frequency field effect transistors. Design and modeling of high-performance devices requires the precise knowledge of basic (AlGaIn)N material parameters. First, the composition dependence of the AlGaN and InGaN band gap energy will be addressed. Second, available data on the band offsets between GaN and AlGaN as well as GaN and InGaN will be discussed, which are important parameters for the design of GaN/AlGaN and InGaN/GaN hetero- and quantum well (QW) structures. Next, the effect of built-in strain and resulting piezoelectric fields on the luminescence properties of InGaN/GaN quantum wells will be addressed, including implications for an optimization of the InGaN QW width for light-emitting diode (LED) applications. Finally, results on GaN/InGaN/AlGaN QW LEDs emitting in the violet spectral region will be presented.