Showing papers by "Oliver Ambacher published in 2008"

Phase Stabilization and Phonon Properties of Single Crystalline Rhombohedral Indium Oxide

Abstract: We report on the phase stabilization of rhombohedral (rh-) In2O3 films on sapphire substrate deposited by metal organic chemical vapor deposition With the help of a high-temperature nucleation layer and evolutionary structural selection of rhombohedral phase during the growth process, stable epitaxial growth of single crystalline rh-In2O3 is achieved The mechanism of phase selective epitaxial growth is studied by means of high-resolution X-ray diffraction and transmission electron microscopy measurements Furthermore, Raman spectroscopy measurements are carried out to investigate the phonon properties of rh-In2O3 Raman-active phonon modes of rh-In2O3 are first identified

35 nm metamorphic HEMT MMIC technology

Abstract: A metamorphic high electron mobility transistor (mHEMT) technology featuring 35 nm gate length has been developed. The optimized MBE grown layer sequence has a channel mobility and a channel electron density as high as 9800 cm2/Vs and 6.1times1012 cm-2, respectively. To enable a maximum extrinsic transconductance gm, max of 2500 mS/mm the source resistance has been reduced to 0.1 Omegamiddotmm. An ft of 515 GHz was achieved for a 2 times 10 mum device. Based on this advanced 35 nm mHEMT technology very compact single-stage H-band amplifiers circuits have been realized demonstrating a high small-signal gain of more than 7 dB at 270 GHz.

Metamorphic HEMT MMICs and Modules for Use in a High-Bandwidth 210 GHz Radar

Abstract: In this paper, we present the development of advanced W-band and G-band millimeter-wave monolithic integrated circuits (MMICs) and modules for use in a high-resolution radar system operating at 210 GHz. A W-band frequency multiplier by six as well as a subharmonically pumped 210 GHz dual-gate field-effect transistor (FET) mixer and a 105 GHz power amplifier circuit have been successfully realized using our 0.1 mum InAlAs/InGaAs based depletion-type metamorphic high electron mobility transistor (mHEMT) technology in combination with grounded coplanar circuit topology (GCPW). Additionally, a 210 GHz low-noise amplifier MMIC was fabricated using our advanced 0.05 mum mHEMT technology. To package the circuits, a set of waveguide-to-microstrip transitions has been realized on 50 mum thick quartz substrates, covering the frequency range between 75 and 220 GHz. The presented millimeter-wave components were developed for use in a novel 210 GHz radar demonstrator COBRA-210, which delivers an instantaneous bandwidth of 8 GHz and an outstanding spatial resolution of 1.8 cm.

NOx sensing properties of In2O3 nanoparticles prepared by metal organic chemical vapor deposition

Abstract: In 2 O 3 nanoparticles were deposited by low-temperature metal organic chemical vapor deposition. The response of 10-nm thick In 2 O 3 particle containing layers to NO x and O 2 gases is investigated. The lowest detectable NO x concentration is ∼200 ppb and the sensor performance is strongly dependent on the gas partial pressure as well as on the operating temperature. The sensor response towards 200 ppm of NO x is found to be above 10 4 . Furthermore, the cross-sensitivity against O 2 is very low, demonstrating that the In 2 O 3 nanoparticles are very suitable for the selective NO x detection.

Nanomechanics of single crystalline tungsten nanowires

Abstract: Single crystalline tungsten nanowires were prepared from directionally solidified NiAl-W alloys by a chemical release from the resulting binary phase material. Electron back scatter diffraction (EBSD) proves that they are single crystals having identical crystallographic orientation. Mechanical investigations such as bending tests, lateral force measurements, and mechanical resonance measurements were performed on 100-300 nm diameter wires. The wires could be either directly employed using micro tweezers, as a singly clamped nanowire or in a doubly clamped nanobridge. The mechanical tests exhibit a surprisingly high flexibility for such a brittle material resulting from the small dimensions. Force displacement measurements on singly clamped W nanowires by an AFM measurement allowed the determination of a Young's modulus of 332 GPa very close to the bulk value of 355 GPa. Doubly clamped W nanowires were employed as resonant oscillating nanowires in a magnetomotively driven resonator running at 117 kHz. The Young's modulus determined from this setup was found to be higher 450 GPa which is likely to be an artefact resulting from the shift of the resonance frequency by an additional mass loading.

NOx sensing properties of In2O3 thin films grown by MOCVD

Abstract: The NO x and O 2 sensing properties of highly textured indium oxide In 2 O 3 thin films grown by metal organic chemical vapor deposition (MOCVD) technique have been investigated as a function of the operation temperature and partial pressure The sensor is very sensitive to NO x and its response is strongly dependent on the gas partial pressure and operating temperature The responses to NO x and O 2 have been found to be maximal at 150 °C The optimum detection temperature for NO x occurs in the range 150–200 °C considering the response and recovery times In this range a very low response to O 2 is observed indicating that the sensor is very suitable for selective NO x detection

Anisotropy of the momentum matrix element, dichroism, and conduction-band dispersion relation of wurtzite semiconductors

Abstract: By comparison of calculated imaginary parts of the ordinary and extraordinary dielectric functions of wurtzite semiconductors near the band gap with experimental results, we demonstrate that the interband matrix elements ${E}_{P}^{\ensuremath{\parallel}}$ and ${E}_{P}^{\ensuremath{\perp}}$ of the momentum operator parallel and perpendicular to the optic axis are different. ${E}_{P}^{\ensuremath{\parallel}}$ exceeds ${E}_{P}^{\ensuremath{\perp}}$ and their ratio increases along the series CdSe, CdS, ZnO. The $u$ parameters of GaN, InN, and AlN suggest that the ${E}_{P}^{\ensuremath{\parallel}}/{E}_{P}^{\ensuremath{\perp}}$ ratio should increase along this series as well. We also determined the conduction-band dispersion relation and nonparabolicity, as well as the effective mass, as a function of electron concentration and wave vector for GaN, ZnO, CdS, and CdSe. In addition, optical response due to transitions into exciton-phonon complexes was observed and analyzed. We conclude that up to about ten phonons may participate in such absorption processes in ZnO.

Two-dimensional electron gas based actuation of piezoelectric AlGaN/GaN microelectromechanical resonators

Abstract: Free-standing piezoelectric AlGaN/GaN beam resonators have been prepared on silicon substrates. The two-dimensional electron gas at the interface of the III/V heterostructure has been employed to act as back electrode for the piezoelectric active layer. The fundamental mode as well as higher order resonant modes of flexural vibration has been excited piezoelectrically and analyzed using optical laser–Doppler vibrometry. The experimental investigations were carried out under normal ambient conditions. The specific piezoelectric actuation scheme is described and the dependence of the measured resonant frequencies between 0.2 and 8.1 MHz on geometry and material parameters is investigated.

Piezoelectric actuation of (GaN/)AlGaN/GaN heterostructures

Abstract: A detailed analysis of the piezoelectric response of (GaN/)AlGaN/GaN heterostructures is reported. The electromechanical properties of two types of heterostructures with an Al content of 31% are compared. Only a single two-dimensional electron gas (2DEG) is formed for samples with thin GaN cap layers, while both a 2DEG and a two-dimensional hole gas coexist in the case of thick GaN caps. The lower GaN layer represents the mechanically supporting layer, while the AlGaN film, and in some cases an additional GaN cap layer, serves as the piezoelectrically active layers for actuation. The 2DEG (at the lower AlGaN/GaN interface) provides the conducting channel which was used as back electrode for the applied external voltage. Electroreflectance spectroscopy is applied in order to determine the electric field distribution across the whole structure as a function of the applied voltage. It is found that only a part of the modulation voltage drops across the active region. Piezoelectric force microscopy yields the...

Experimental evidence of different hydrogen donors in n -type InN

Abstract: The multiform donor nature of hydrogen in n -type indium nitride is experimentally observed in samples exposed to atomic hydrogen. Photoluminescence measurements reveal a tenfold increase in the electron concentration and the formation of a shallow donor band upon hydrogen incorporation. Annealing studies show that hydrogen occupies at least two equilibrium sites having almost equivalent thermal stability.

Integration of Thin‐Film‐Fracture‐Based Nanowires into Microchip Fabrication

Abstract: One-step device fabrication through the integration of nanowires (NWs) into silicon microchips is still under intensive scientific study as it has proved difficult to obtain a reliable and controllable fabrication technique. So far, the techniques are either costly or suffer from small throughput. Recently, a cost-effective method based on thin-film fracture that can be used as a template for NW fabrication was suggested. Here, a way to integrate NWs between microcontacts is demonstrated. Different geometries of microstructured photoresist formed by using standard photolithography are analyzed. Surprisingly, a very simple "stripe" geometry is found to yield highly reproducible fracture patterns, which are convenient templates for fault-tolerant NW fabrication. Microchips containing integrated Au, Pd, Ni, and Ti NWs and their suitability for studies of conductivity and oxidation behavior are reported, and their suitability as a hydrogen sensor is investigated. Details of the fabrication process are also discussed.

A novel GaN-based multiparameter sensor system for biochemical analysis

Abstract: GaN and its ternary alloys offer several advantages for the fabrication of biochemical sensors systems due to their chemical stability and optical transparency. We employ ion-sensitive field effect transistors (ISFET) based on a surface sensitive AlGaN/GaN heterostructure. Combined with a picoliter dosing system, an integrated micro reference electrode and an optical spectroscopy system a novel multi-parameter analysis system for aqueous solutions in the micro- to nanoliter range was constructed. Application examples are presented to illustrate the application potential of the sensor system e.g. in the high-throughput screening of cancer drugs. Simultaneous opto-electrochemical monitoring of enzyme reaction kinetics by pH and absorption measurements in 700 nl and parallel automated measurements with functionalized enzyme-modified FETs (EnFET) and a reference ISFET in 3 µl are demonstrated. The experimental data obtained agree well with published values for the reaction kinetic and with reference measurements performed with a state-of-the-art pH-meter and spectrophotometer which require 1000-fold larger volumes. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

AlGaN/GaN-based MEMS with two-dimensional electron gas for novel sensor applications

Abstract: Novel microelectromechanical resonators structures have been realized based on AlGaN/GaN heterostructures, which provide a basis for sophisticated sensor structures. There were grown on SiC substrates confining a two dimensional electron gas (2DEG). By means of the developed etching technology, freestanding resonators were patterned without degrading the sheet carrier concentration and electron mobility of the 2DEG inside the beams, which was confirmed by electrical measurements before and after the various process steps. As actuation and read out principle magnetomotive and piezoelectric effects were used, respectively. Due to the high sensitivity of the 2DEG and the chemical stability of the utilized materials these structures are suitable for chemical and biological sensor applications, where the sensitivity of the 2DEG on the surrounding environment acts as additional sensing signal, for example for simultaneous measurements of the viscosity and pH – value of a nanoliter droplet. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Metamorphic MMICs for Operation Beyond 200 GHz

Abstract: In this paper, we present the development of advanced millimeter-wave and submillimeter-wave monolithic integrated circuits for use in active and passive high-resolution imaging systems operating beyond 200 GHz. A 210 GHz subharmonically pumped dual-gate field-effect transistor (FET) mixer has been successfully realized using our 100 nm InAlAs/InGaAs based depletion-type metamorphic high electron mobility transistor (mHEMT) technology in combination with grounded coplanar circuit topology (GCPW). Furthermore, a G-band low-noise amplifier MMIC demonstrating a linear gain of more than 16 dB between 180 and 220 GHz and a state-of-the-art noise figure of 4.8 dB was fabricated using a gate length of 50 nm. Finally, a submillimeter-wave monolithic integrated circuit (S-MMIC) could be realized based on an advanced 35 nm mHEMT technology, offering a small-signal gain of more than 15 dB between 270 and 310 GHz.

Electric field distribution in GaN∕AlGaN∕GaN heterostructures with two-dimensional electron and hole gas

Abstract: Ga-face GaN∕AlGaN∕GaN heterostructures with different cap thicknesses are investigated by electroreflectance spectroscopy (ER). The voltage dependent electric field strengths of the barrier and cap layers are determined. The AlGaN electric field amounts of up to −2.6MV∕cm, whereas the GaN electric field is always below 700kV∕cm. The two electric fields have opposite signs. Characteristic features in the voltage maps of the ER spectra are assigned to the formation/depletion of a two-dimensional electron gas below and a two-dimensional hole gas above the AlGaN barrier. Between −6.5 and 0V, both carrier gases coexist.

Polarization Induced Effects in GaN-based Heterostructures and Novel Sensors

Abstract: The macroscopic non-linear pyroelectric polarization of wurtzite AlxGa1−xN, InxGa1−xN and AlxIn1−xN ternary compounds dramatically affects the optical and electrical properties of multilayered Al(In)GaN/GaN hetero-, nanostructures and devices, due to the huge built-in electrostatic fields and bound interface charges caused by gradients in polarization at surfaces and heterointerfaces. In modeling of polarization induced effects in GaN based devices it is often assumed that polarization in group-III-nitride alloys interpolates linearly between the limiting values determined by the binary compounds. In more advanced models it is taken into account that the macroscopic polarization in group-III-nitride alloys is a non-linear function of strain and composition. We have applied those results to reverse-model experimental data obtained in a number of InGaN/GaN quantum-wells (QW) as well as AlInN/GaN and AlGaN/GaN transistor structures (HEMTs). Thereby we find that the discrepancies of experiment and ab initio theory are almost completely eliminated for the AlGaN/GaN based heterostructures when polarization non-linearity is accounted for. The realization of undoped lattice matched AlInN/GaN heterostructures further allows the confirmation of the existence of a gradient in spontaneous polarization by the experimental observation of two dimensional electron gases (2DEGs). The confinement of 2DEGs in InGaN/GaN QWs in combination with the measured Stark shift of excitonic recombination is used to determine the polarization induced electric fields in nanostructures. To facilitate inclusion of the predicted non-linear polarization in future simulations, we give an explicit prescription to calculate polarization induced electric fields and bound interface charges for arbitrary x in each of the ternary III-N alloys. In addition, the theoretical and experimental results presented here allow a detailed comparison of the predicted electric fields and bound interface charges with the measured Stark shift and the sheet carrier concentration of polarization induced 2DEGs. This comparison provides an inside in the reliability of the calculated non-linear piezoelectric and spontaneous polarization of group-III-nitride ternary alloys.

Dynamical optical investigation of polymer/fullerene composite solar cells

Abstract: We investigate organic polymer/fullerene (P3HT/PCBM) bulk heterojunction solar cells by means of photoinduced absorption spectroscopy (PIA). The dynamic properties of the charge carriers are studied in the temperature range of 5 K to 200 K by recording the PIA signal in the frequency domain. The mobility and the lifetime of the positive polarons are de- termined. The dependence of the short circuit current on the incident light intensity is also investigated. Finally, the properties of annealed and not annealed solar cells are compared. As the most interesting result, the appearance of two polaron species on different time scales is observed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Reliability and degradation mechanism of AlGaN/GaN HEMTs for next generation mobile communication systems

Abstract: 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 8x60 μm wide and 0.5 μm long AlGaN/GaN HEMTs at a drain voltage of Vd=50V. 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.

Efficient AlGaN/GaN HEMT Power Amplifiers

Abstract: This paper describes efficient GaN/AlGaN HEMTs and MMICs for L/S-band (1-4 GHz) and X-band frequencies (8-12 GHz) on three-inch s.i. SiC substrates. Dual-stage MMICs in microstrip transmission-line technology yield a power-added efficiency of ?40% at 8.56 GHz for a power level of ?11 W. A single-stage MMIC yields a PAE of ?55% with 6 W of output power at VDS= 20 V. The related mobile communication power HEMT process yields an average power density of 10 W/mm at 2 GHz and VDS= 50 V. The average PAE is 61.3% with an average linear gain 24.4 dB and low standard deviation of all parameters. The devices yield more than 25 W/mm of output power at 2 GHz when operated in cw at VDS= 100 V with an associated PAE of ?60%. The GaN HEMT process with 0.5 ?m gate-length yields an extrapolated lifetime of 105 h when operated at VDS= 50 V at a channel temperature of 90°C. When operated at 2 GHz devices with 480 ?m gate-width yield a change of the RF power-gain of less than 0.2 dB under high gain-compression at VDS= 50 V and a channel temperature of 250°C.

Electronic and photoconductive properties of ultrathin InGaN photodetectors

Abstract: We report on the compositional dependencies of electron transport and photoconductive properties for ultrathin metal-semiconductor-metal photodetectors based on In-rich InxGa1−xN alloys. For a In0.64Ga0.36N∕GaN structure, the rise time close to the RC constant at low fields has been measured along with a transparency of ∼77% and an absorbance of ∼0.2 at a wavelength of 632nm. The electron density profiles and low-field mobilities for different compositions of InGaN have been calculated by numerically solving the Schrodinger and Poisson equations and applying the ensemble Monte Carlo method, respectively. It was demonstrated that in ultrathin InxGa1−xN∕GaN (0.5

Electronic components produced by a method of separating two layers of material from one another

Abstract: A semiconductor body selected from the group consisting of a semiconductor layer, a semiconductor layer sequence or a semiconductor layer structure. The semiconductor body is transferred from a growth substrate to a support material by: exposing an interface between the growth substrate and the semiconductor body or a region in the vicinity of said interface to electromagnetic radiation through one of the semiconductor body and the growth substrate; decomposing a material at or in proximity to said interface by absorption of the electromagnetic radiation in proximity to or at said interface so that the semiconductor body can be separated from the growth substrate; and connecting the semiconductor body to the support material.

The role of Ti/A1 ratio in nanolayered ohmic contacts for GaN/AlGaN HEMTs

Abstract: The role of the former-Ti/Al ratio in the nanolayered ohmic contacts to GaN/AlGaN HEMT structures has been studied for three ratios: (30/70) wt.%, (50/50) wt.% and (70/30) wt.%. The dependence of the electrical properties and surface morphology on the initial contact composition and annealing conditions has been investigated. Lowest contact resistivity of 4.22times105 Omegacm2 has been achieved for Ti/Al (30/70 wt.%) and Ti/Al (50/50 wt.%) contacts to semi-insulating GaN/AlGaN heterostructures. Smoothest contact surface and most narrow contact periphery have been obtained with a Ti/Al (70/30 wt.%) contact indicating that increase of the Ti/Al ratio improves the contact morphology. The contact composition with former-Ti/Al ratio of (50/50) wt.% has been found to fulfill the best both requirements for low resistivity and a smooth surface of the contact.

Electron transport properties of indium oxide – indium nitride metal‐oxide‐semiconductor heterostructures

Abstract: The structural, chemical and electron transport properties of In2O3/InN heterostructures and oxidized InN epilayers are reported. It is shown that the accumulation of electrons at the InN surface can be manipulated by the formation of a thin surface oxide layer. The epitaxial In2O3/InN heterojunctions show an increase in the electron concentration due to the increasing band banding at the heterointerface. The oxidation of InN results in improved transport properties and in a reduction of the sheet carrier concentration of the InN epilayer very likely caused by a passivation of surface donors. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

InN/In2O3 heterostructures

Abstract: Cubic (c-) InN is predicted to possess superior electronic properties for device applications, while c-In2O3 is an excellent candidate as gate material for InN based high-frequency field effect transistors. In this paper, the epitaxial growth of the InN/In2O3 and In2O3/InN heterosystems was investigated. High-quality c-InN (001) was deposited on (001) In2O3 tem- plate, while single crystalline c-In2O3 was epitaxially grown on hexagonal InN (0001). The epitaxial relationship of the heterosystems was determined. Phenomenological models of the nucleation and of an atomic arrangement at the interface are proposed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Piezoelectric actuation of all-nitride MEMS

Abstract: We present a MEMS technology based on (GaN/)AlGaN/GaN–heterostructures. Thereby the lower GaN layer represents the mechanical active layer, while the upper GaN and AlGaN layers supply the piezoelectrically active layers for actuation and the confinement of a 2D electron gas (at the lower interface). The 2DEG serves as back electrode for the piezoelectric actuation and as read–out, since it is modulated by the mechanical oscillation. The upper AlGaN and GaN layer both contribute to the total piezoelectric response, which was determined by piezoelectric force microscopy. The electrical field distribution throughout the heterostructure was determined by means of electroreflectance. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Sapphire-GaN-based planar integrated free-space optical system

Abstract: We report on the design and fabrication of a planar integrated free-space optical system working on the basis of binary phase diffractive optical elements (DOEs) realized in GaN on a sapphire substrate Group III-nitride/sapphire substrates enable the parallel monolithic integration of passive microoptical elements like lenses and gratings as demonstrated here and optoelectronic devices like light emitters and photodetectors on a single wafer We present an approach for the simultaneous optimization of the efficiency of transmissive and reflective diffractive optical elements processed in a single lithographic etching step

Balanced Microstrip AlGaN/GaN HEMT Power Amplifier MMIC for X-Band Applications

Abstract: This paper describes a balanced AlGaN/GaN HEMT single-stage power amplifier demonstrator for X-band frequencies in microstrip line technology on thinned s.i. SiC substrates. The design features a modular circuit concept and microstrip MMIC directional couplers with low impedance levels. These 3 dB-couplers designed for a center frequency of 10 GHz show a coupling factor of 3.5 dB plusmn 0.4 dB and a low net insertion loss of 0.3 dB. The balanced amplifier reaches 11 W pulsed output power at 3 dB compression level and a maximum gain of 10 dB at 8.56 GHz with an input and output match of better than 14 dB from 8.3 to 13 GHz. This 0deg/90deg balanced microstrip AlGaN/GaN HEMT power amplifier MMIC demonstrator may be an interesting alternative to existing hybrid solutions.

N-Methylmorpholine-N-oxide ring cleavage registration by ESR under heating conditions of the Lyocell process

Abstract: Thermal cleavage processes of N-methylmorpholine-N-oxide monohydrate (NMMO) were observed in pure NMMO as well as in cellulose/NMMO solutions by ESR at temperatures of the industrial Lyocell process ( approximately 370K). Generated radicals were attributed to the alkylnitroxyl type radicals -CH(2)-NO-CH(3) in NMMO and additional (and dominated) -CH(2)-NO-CH(2)- in cellulose/NMMO solutions. Formation of both radical types formed due to NMMO ring scission is suggested.