Air-bridge Structure Research Articles

Self-heating phenomena in high-power III-N transistors and new thermal characterization methods developed within EU project TARGET

In the framework of the Top Amplifier Research Groups in a European Team (TARGET) project, we developed a new electrical method for the temperature measurement of HEMTs and performed several unique studies on the self-heating effects in AlGaN/GaN HEMTs. This method, in combination with transient interferometric mapping (TIM), provides a fundamental understanding of the heat propagation in a transient state of HEMTs. The AlGaN/GaN/Si HEMT thermal resistance was determined to be ~70 K/W after 400 ns from the start of a pulse, and the heating time constant was ~200 ns. Our experimental methods were further applied on multifinger high-power AlGaN/GaN/sapphire HEMTs. The TIM method indicates that the airbridge structure serves as a cooler, removing approximately 10% of the heat energy. In the next study we used TIM and the micro-Raman technique to quantify thermal boundary resistance (TBR) between different wafer materials and GaN epi-structure. We found TBR to be ~7 × 10−8 m2K/W for GaN/Si and ~1.2 × 10−7 m2K/W for GaN/SiC interfaces. The role of TBR at the GaN/sapphire interface was found to be less important.

Scalable Approach for Vertical Device Integration of Epitaxial Nanowires

In this letter, we demonstrate the simultaneous vertical integration of self-contacting and highly oriented nanowires (NWs) into airbridge structures, which have been developed into surround gated metal oxide semiconductor field effect transistors (MOSFETs). With the use of conventional photolithography, reactive ion etching (RIE), and low pressure chemical vapor deposition, a suspended vertical NW architecture is formed on a silicon on insulator (SOI) substrate where the nanodevice will later be fabricated on. The vapor-liquid-solid (VLS) grown Si-NWs are contacted to prepatterned airbridges by a self-aligned process, and there is no need for postgrowth NW assembly or alignment. Such vertical NW architecture can be easily integrated into existing ICs processes opening the path to a new generation of nonconventional nano devices. To demonstrate the potential of this method, surround gated vertical MOSFETs have been fabricated with a highly simplified integration scheme combining top-down and bottom-up approaches, but in the same way, one can think about the realization of integrated nano sensors on the industrial scale.

A Novel 94-GHz MHMET-Based Diode Mixer Using a 3–dB Tandem Coupler

We report a high-performance 94-GHz monolithic millimeter-wave integrated-circuit diode mixer using metamorphic high-electron mobility transistor (MHEMT) diodes and a coplanar waveguide tandem coupler. A novel single-balanced structure of diode mixer is proposed in this paper, where a 3-dB tandem coupler with two sections of parallel-coupled line and air-bridge crossover structures are used for wide frequency operation. The fabricated mixer exhibits excellent local oscillator-radio-frequency (LO-RF) isolation, greater than 30 dB, in the 5-GHz bandwidth of 91-96 GHz. A good conversion loss of 7.4 dB is measured at 94 GHz. The proposed MHEMT-based diode mixer shows superior LO-RF isolation and conversion loss to those of the W-band mixers reported to date.

Demonstration of an integrated optical switch in a silicon photonic crystal directional coupler

We demonstrate a silicon photonic crystal (PhC) directional coupler optical switch. By engineering the supermodes of the coupled system to provide a slow-light region, the switch has a total length of just 4.9 μm. Thermo-optic switching with a 20 μs response time via an integrated nickel microheater is shown. The proposed switch uses a silica overlayer and infilling of the etched silicon PhC, rather than the more usual membrane slab geometry. The silica infilling and overlayer provides a vertically symmetric structure and is a more robust solution compared to the membrane geometry. We also show that there is no apparent increase in optical loss due to the silica overlayer, despite operating above the lightline; the insertion loss for the air-bridge and silica-embedded structures are comparable (at 1–2 dB).

Radiation induced force between two planar waveguides

We study the electromagnetic force exerted on a pair of parallel slab waveguides by the light propagating through them. We have calculated the dependence of the force on the slab separation by means of the Maxwell–Stress tensor formalism and we have discussed its main features for the different propagation modes: spatially symmetric (antisymmetric) modes give rise to an attractive (repulsive) interaction. We have derived the asymptotic behaviors of the force at small and large separation and we have quantitatively estimated the mechanical deflection induced on a realistic air-bridge structure.

AlN air-bridge photonic crystal nanocavities demonstrating high quality factor

The authors report an achievement of high quality AlN ultraviolet photonic crystal nanocavities. Convex AlN air-bridge structures with embedded GaN quantum dots have been formed by utilizing photoelectrochemical etching of 6H-SiC substrates. Room-temperature microscopic photoluminescence measurements reveal the high quality of the nanocavities. For the lowest-order cavity mode of a 150-nm-period nanocavity with seven missing holes, the highest Q factor (>2400) ever reported in nitride-based photonic crystals has been obtained.

Transient self-heating effects in multifinger AlGaN/GaN HEMTs with metal airbridges

Thermal behavior of a multifinger AlGaN/GaN/sapphire HEMT with a metal airbridge connecting five source contacts is investigated in the transient state using optical and electrical methods and using a 2D thermal model. The gate layout consists of eight fingers, each having 50 μm width/0.3 μm length, the pitch is 35 μm and the thickness of the electroplated airbridge is 7 μm. The device drain contact was pulsed by 10 μs long 10 V pulses, corresponding to ∼11.9 W/mm dissipating power density. The electrical characterization method shows that at the end of the pulse the temperature increase in the HEMT channel is ∼185 K while the transient interferometric mapping (TIM) optical method indicates that the airbridge structure serves also as a cooler removing approximately 10% of the heat energy. Surface temperature maps are constructed by using the TIM for a time window of 2–6 μs. An asymmetry in the temperature profiles was observed, e.g. the source contact was colder by ∼25% than the drain contact at t = 6 μs.

Modelling air-bridge photonic crystal defects by guided mode expansion

Abstract A guided mode expansion treatment is presented for theoretical analysis of air-bridge photonic crystal cavity structures. The optimum choice of waveguide refractive index for the basis states is discussed. The method’s inherent speed is demonstrated with the optimisation of an L3 defect in a hexagonal lattice of circular holes.

Fabrication and optical characterization of III‐nitride air‐bridge photonic crystal with GaN quantum dots

We report the fabrication of III-nitride air-bridge photonic crystals with GaN quantum dots for the first time. Photonic crystals with various periodicities and airhole diameters were fabricated. Abrupt vertical profiles and high aspect ratio (∼3) were achieved with excellent reproducibility even to the structure with periodicity as small as 150 nm. The fabrication of air-bridge structure utilizing photoelectrochemical etching of SiC was demonstrated. Strong photoluminescence enhancement by a factor of 5 was observed from GaN QDs embedded in the air-bridge PC layer. This is due to the coupling of PL light with photonic bands above the light cone, thus results in the enhanced light extraction efficiency. These results will lead to the realization of high efficiency quantum dots based emitting devices in blue and ultraviolet range. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

V-band CPW 3-dB Tandem coupler using air-bridge structure

We present a uniplanar coplanar waveguide 3-dB tandem coupler operating at V-band frequencies. The uniplanar structure is monolithically fabricated by using two-section parallel-coupled lines and air-bridge crossovers replacing the conventional multilayer or the bonded structures. Due to an optimized tandem structure and nonbonded crossovers minimizing the parasitic components, a maximum coupling of 2.5dB is measured at 62GHz with a 2-dB bandwidth of 83%, while a high directivity factor of 33dB is simultaneously obtained at 58-62GHz. Over the entire design frequency range of 30-90GHz, we achieve good phase unbalance of 90/spl plusmn/6.0/spl deg/, return loss, and isolation lower than -23 and -16dB, respectively.

Metallic air-bridges fabricated by multiple acceleration voltage electron beam lithography

We present a new method of fabricating metallic air-bridge microstructures that is based on a single layer resist and a variation of the electron energy used during the electron beam lithography process. Electrons in the range of 3–30 keV cause radiation-induced reactions in the resists to depths adjustable from fractions of a micrometer up to several micrometers. By varying the energy at which the lithography process is carried out, we obtain three-dimensional profiles in the electron beam resist after exposure and development. Air-bridge structures can then be created by metal evaporation and lift-off.

Investigation of Infrared Photonic Crystal Slab with Defect Waveguides

This paper deals with the two dimensional (2-D) infrared photonic crystal slab with an air-bridge structure featuring defect waveguides in which the optical propagation properties were obtained by measurements of transmission spectra. 3-D finite-difference time-domain (FDTD) simulations have revealed the appearance of four defect propagation modes for the band structure and transmission spectra in the air-bridge slab. These defect modes were experimentally identified in the measured transmission spectra.

Strain-induced modifications of the electronic states of InGaAs quantum dots embedded in bowed airbridge structures

We have fabricated bowed airbridges in which self-assembled InGaAs quantum dots are embedded. Strong strain distribution induced in the bowed airbridge and the effect on the electronic states of the quantum dots are investigated through the measurement of the photoluminescence from the individual dots and the theoretical calculation. A finite element calculation shows the strain in the bowed airbridge to distribute from tensile to compressive along the growth direction. The strain effect on the electronic states of the dots is probed through the photoluminescence peak shift following the deformation of the GaAs matrix of the dots from a wall-shaped structure to the bowed airbridge. The magnitude of the peak shift varies systematically with the position of the quantum dot along the growth direction, clearly reflecting the strain distribution in the bridge. The energy level shift following the deformation is calculated by solving the three-dimensional Schrödinger equation taking into account the strain distribution around the dots embedded in the bridge. The calculation, which agrees well with the experiment, demonstrates that the characteristic strain distribution around the dot embedded in the bowed airbridge modifies not only the energy levels, but also the wave functions. The electron and hole wave functions are modified differently, mainly due to the opposite contribution of the biaxial strain to the hydrostatic ones.

Single crystal silicon piezoresistive nano-wire bridge

Air-bridge structure of p-type single crystal silicon nano-wire (NW) was fabricated by using 50 nm thick device layer and sacrificial layer etching of 130 nm thick buried oxide (BOX) in separation by implanted oxygen (SIMOX) substrate. The piezoresistance of the nano-wire bridge (NWB) was measured in order to verify its ability for sensing element of the nano-mechanical sensors. Longitudinal piezoresistance coefficient π 1[110] and apparent transverse piezoresistance coefficient π t[110] were found to be 38.7 x 10 -11 and 0 Pa -1 , respectively, at surface concentration of N s = 9 × 10 19 cm -3 . The value of π 1[110] was in close agreement with that of NW without releasing from the substrate, and 54.8% larger than that of p + diffused piezoresistor obtained by Tufte and Stelzer four decades ago (25 × 10 -11 Pa -1 and at N s = 9 × 10 19 cm -3 ). The value of apparent π t[110] was much smaller than that of the NW without releasing from the substrate. An advantage ofthe vanished apparent π t[110] for design of the piezoresistive mechanical sensors will be discussed. Obtained results are useful for design consideration of nano-metric piezoresistive elements used in the nano-mechanical sensors.

Surface Micromachining in Optical Isolator with Semiconductor Guiding Layer for Enhancement of Magnetooptic Effect

A magnetooptic waveguide with a magnetic garnet/GaInAsP/air structure was effective in enhancing the magnetooptic effect. Surface micromachining was investigated for fabricating a magnetooptic waveguide in an optical isolator with a semiconductor guiding layer. As a preliminary experiment, a GaInAsP rib waveguide with an air cladding layer was fabricated by selective wet etching. The rib waveguide with a 1-mm-long air bridge was successfully obtained on the GaInAsP guiding layer. Light waves could be guided in the GaInAsP waveguide with the air bridge structure.

Optical anisotropy of self-assembled InGaAs quantum dots embedded in wall-shaped and air-bridge structures

Strain effect on optical anisotropy of quantum dots has been investigated by changing the surrounding matrix of the dots. Optical anisotropy can be induced by lateral patterning of the matrix of the dots, although such anisotropy is absent in the as-grown dots. A reduction of the optical anisotropy is observed by changing the laterally patterned structure into a free-standing structure or an air bridge. The optical anisotropy is mainly attributed to strain asymmetry in the fabricated structures. The presence of the strain asymmetry is confirmed by the observation of a doublet fine structure in spectrally resolved photoluminescence of single quantum dots.

Electro-Optic Intensity Modulator for Broadband Optical Communications

An LiNbO 3 electro-optic grating modulator, composed of coplanar microstrip electrodes with periodic small fins and an airbridge structure, is presented. The small fins slow down the microwave propagating speed, while the effect of the airbridge is the reduction of the effective index. The modulator design has good performance in terms of wavelength selection and high-speed characteristics. The microwave and optical analysis are carried out using the three-dimensional finite difference method and coupled mode equations.

Light propagation characteristics of straight single-line-defect waveguides in photonic crystal slabs fabricated into a silicon-on-insulator substrate

Straight single-line defect optical waveguides in photonic crystal slabs are designed by the finite difference time-domain method and fabricated into a silicon-on-insulator (SOI) wafer. By employing an airbridge structure, clear light propagation for both polarizations is observed without any leakage along the waveguide. This experimental result is well explained by photonic bands of pure guided modes. Minimum propagation loss is estimated to be 11 dB/mm. This value is lower than that reported so far for three-line-defect waveguides with an SOI slab structure and almost comparable to that for an index confinement waveguide with a rectangular Si core. This propagation loss is dominated by the scattering loss by some irregularities. However, photonic crystal waveguides have the possibility of an essential lower scattering loss than in the index confinement waveguide because of the inhibition of radiation modes by the photonic bandgap.

AlGaAs-based two-dimensional photonic crystal slab with defect waveguides for planar lightwave circuit applications

An AlGaAs-based near-infrared 2-D photonic crystal (PC) with an air-bridge structure featuring defect waveguides has been developed. For the sample without defect waveguides, measurements of the optical transmission characteristics in the wavelength range from 850 nm to 1100 nm showed a deep attenuation due to a bandgap with 30-35 dB attenuation and transmittance of nearly 100% for the guided modes. Optical propagation properties of defect waveguides were obtained by two methods: measurements of transmission spectra and plan-view observations of the optical beam trace along the waveguide with an infrared-vidicon camera. 3-D finite-difference time-domain simulations for the band structure and transmission spectra in the air-bridge slab with and without defect waveguides have revealed the appearance of four defect propagation modes specific to the defect waveguide, between two slab modes for the defect-free photonic crystal slab. These defect modes were experimentally identified in the measured transmission spectra.

Fabrication of Thick Silicon Dioxide Air-Bridge for RF Application Using Micromachining Technology

This paper proposes a 10 µm thick oxide air-bridge structure which can be used as a substrate for RF circuits. The structure was fabricated by anodic reaction, multi-step thermal oxidation and micromachining technology using tetramethylammonium hydroxide (TMAH) etching. This structure is mechanically stable because of thick oxide layer up to 10 µm and is expected to solve the problem of high dielectric loss of silicon substrate in RF region. The properties of the transmission line formed on the oxidized porous silicon (OPS) air-bridge were investigated and compared with those of the transmission line formed on the OPS layers. The insertion loss of coplanar waveguide (CPW) on OPS air-bridge is (about -2 dB) lower than that of CPW on OPS layers. Therefore, this technology is very promising for extending the use of complementary metal oxide (CMOS) circuitry to higher RF frequencies.