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International audience; This paper reports on pin photodiode frequency response optimization based on InxGa1-xAs1-yNy quaternary lattice matched to GaAs. Two transparent layers are placed on p-side and n-side in order to manage the photodiode frequency response limitations. The lattice matching condition is calculated in order to obtain stable structure. The physical and optical parameters calculations are performed at room temperature showing the impact of nitrogen on the absorption coefficient. A stable structure, having 2 % of nitrogen and 6% of indium, allows to achieve a cutoff frequency of about 116 GHz and a capacitance of 5.21fF while the quantum efficiency is 41.59% for a depletion region thickness of about 0.55 µm. However, in case of depletion region thickness of about 0.625 µm, the cutoff frequency degrades to 98 GHz while the capacitance diminishes to 4.58fF and the quantum efficiency increases to 51.56%. In addition, a comparative study with literature results has been carried out in order to show the advantages of the proposed photodiode. This comparison affirms that the proposed photodiode based on InGaAsN lattice matched to GaAs exhibits high-speed photo-detection. This work allowed us to obtain a p-i-n photodiode with stable structure suitable for photo-detection at 1.15 µm.

International audience; We present the experimental determination of the complex permittivity of vertically aligned single wall carbon nanotubes (SWCNTs) films grown on quartz substrates in the microwave regime from 10 MHz up to 67 GHz, with the electrical field perpendicular to the main axis of the carbon nanotubes (CNTs), based on coplanar waveguide transmission line approach together with the measurement of the microwave impedance of top metalized vertically-aligned SWCNTs grown on conductive silicon substrates up to 26 GHz. From coplanar waveguide measurements, we obtain a real part of the permittivity almost equal to unity, which is interpreted in terms of low carbon atom density (3 x 10(19) at/cm(3)) associated with a very low imaginary part of permittivity (< 10(-3)) in the frequency range considered due to a very small perpendicular conductivity. The microwave impedance of a vertically aligned CNTs bundle equivalent to a low resistance reveals a good conductivity (3 S/cm) parallel to the CNTs axis. From these two kinds of data, we experimentally demonstrate the tensor nature of the vertically grown CNTs bundles.

In this paper, the frequency response calculation and optimization of PIN photodiodes based on GaN/InGaN, suitable for photodetection at the wavelength of 633 nm, are presented. The calculations of the impulse as well as the frequency response are performed using the impulse method. The frequency response optimization is a result of optimizing the transport of photo-generated carriers in the absorbent layer of the photodiode, using a mixed depletion region rather than a single absorbing depletion region. Cut-off frequencies of about 82 GHz and 48 GHz have been obtained in the case of transparent layer thickness of 500 nm and 1000 nm, respectively. The aforementioned results represent a very good improvement.

Photo-generated carriers in a photodiode drift in the junction before joining the circuit. We must use a quite thin depletion region so that their transit time does not penalise the microwave response of the devices. However, when the depletion region decreases the component capacity increases, which highly limits the microwave response of this latter. The solution proposed in this work is the use of a mixed depletion region instead of a completely absorbent depletion region in order to have a low transit time while maintaining a low junction capacity. The obtained results show that the photodiodes with mixed depletion regions are faster than photodiodes whose depletion region is completely absorbent for the same thickness. The pure transit cut-off frequency of optimised mixed depletion regions exceeds by 67% that of completely absorbent depletion region for the same thickness of the depletion region.

This Letter reports on new experimental high dynamic range sub-sampling performances obtained from nanoscale microwave photoconductive switches operating under a 0.8 µm wavelength pulsed illumination. Experimental results have been performed on different samples benefiting of a low temperature grown Gallium Arsenide absorbing layer on GaAs substrate, coplanar microwave electrode profile optimisation, and the use of an ultra-low noise jitter optical source delivering optical pulses of 1 to10 ps pulse width at a 2 GHz repetition frequency at 0.8 µm optical wavelength. Thanks to carrier lifetime tunability achievement from 1 to 20 ps, in association with a carrier mobility of 3900 cm²/V.s, experimental demonstrations of 54 and 45 dB signal-to-noise ratio microwave signal sampling for signals at 5,76 and 20 GHz frequency bandwidth respectively confirms submicron devices functionality as a new class of single channel photonic analogue to digital converters.

In the manufacturing processes of high-power laser diode modules, the module packaging technologies play an important role; decide the efficiency of the electro-optical conversation, the beam quality and the lifetime. This paper presents the packaging of high-power laser diode modules from single chip, evaluating the electro-optical characteristics and the beam quality of the modules. The obtained results show that the packaged module has an output optical power of 5 W at current 5.9 A; the Operation Voltage is 1.7 V; the Threshold Current is 1,0 A; the Divergence Angle Mean is 36.8 degree and the Asymmetry is 1.04. These parameters are almost the same that of some on-stock modules. Thus, the application of the packaged module in social – economics is possible. Keywords: Laser diode, fiber – coupled, die bond technology; wire bond technology; electro-optical conversation efficiency, PVI characteristics, beam quality, Divergence Angle, Asymmetry.

The main purpose of this paper is to show the effect of charge accumulation and screening of the electric field on the Schottky Metal-semiconductor-Metal detector response and efficiency, which result of non-uniform distribution carriers along the absorption depth, and along the line between electrodes. The MSM (PD) with an active surface of 3×3 µm2 and electrode spacing of 0.2, 0.3, 0.5 and 1 µm has been integrated in the central strip of coplanar lines for microwave switching application. Several ways of improving the high-speed response of the MSM-PD are analyzed and discussed.

International audience; In this work, we report a comparative investigation of InxGa1-xN (SL) and InxGa1-xN/GaN (MQW) structures with an indium content equivalent to x=10%. Both structures are grown on (0001) sapphire substrates using MOCVD and MBE growth techniques. Optical properties are evaluated for samples using PL characteristics. Critical differences between the resulting epitaxy are observed. Microstructures have been assessed in terms of crystalline quality, density of dislocations and surface morphology. We have focused our study towards the fabrication of vertical PIN photodiodes. The technological process has been optimized as a function of the material structure. From the optical and electrical characteristics, this study demonstrates the benefit of InGaN/GaN MQW grown by MOCVD in comparison with MBE for high speed optoelectronic applications.

We report on the small-signal high-frequency response of a GaNAsSb/GaAs pin waveguide photodetector at 1.55-μm wavelength. The GaNAsSb absorbing layer with 3.5% N and 18% Sb is sandwiched by GaAs-cladding layers. The device of an 8-μm ridge width and 15-μm ridge length has a dc responsivity of 0.44 A/W and an 11-GHz cutoff frequency at 5-mW optical illumination at a bias condition of -5 V. A higher cutoff frequency of 14.3-GHz is achieved on the smallest device of 4-μm ridge width and 17-μm ridge length with a responsivity of 0.2 A/W at a reverse bias condition of -1 V.

High index contrast waveguides in the III-V material line are demonstrated here. Created thanks to a wafer bonding technology, they are made of a GaAs nanowire embedded in a benzocyclobutene (BCB) matrix. We also report some of those waveguides properties and an example of active device considered with this technology.

Photogenerated carriers in a photodiode drift in the junction before joining the circuit. We must use a quite thin depletion region so that their transit time does not penalize the microwave response of the devices. However, when the depletion region decreases the component capacity increases, which highly limits the microwave response of this latter. Replace the completely absorbent depletion region by a mixed depletion region increases the pure transit cut-off frequency for the same thickness of the depletion region. We use this property in what follows to optimize the performance of PIN photodiode based on InP/InGaAs, adapted for the photodetection at the wavelength of 1.55 μm. The obtained results show that the maximum area of photodiodes with mixed depletion regions with thin absorbent exceeds by 64% that of the photodiodes with completely absorbent depletion regions for the same bandwidth. Their maximum absorption volume exceeds by 8% that of the photodiodes with completely absorbent depletion regions for the same bandwidth. Both types of components are also compared on the abacuses when the thickness of the absorbent and the surface are imposed.

A monolithic polarisation-selective side-illuminated pin photodetector is described using the coupling properties of symmetric and asymmetric slab optical waveguides. First, a basic structure is proposed, analysed and optimised by modal analysis, leading to a computed polarisation ratio of 15.7 dB. Then the symmetric waveguide is replaced by a multimode diluted waveguide optimised by a genetic algorithm coupled to a beam propagation method. This significantly improves the performance of the device and particularly the computed polarisation ratio, which reaches 21 dB. This InP-based photodetector is suitable for hybridisation on a silicon platform and is compatible with 10 Gbit/s operation.

Hydrogen incorporation in n-type Si-doped GaAs epilayers is a well-known process which leads to the neutralization of the active Si impurities with the formation of SiH complexes. Recently, we have shown that SiH complex dissociation and, consequently, Si-dopant reactivation could occur when the epilayers are exposed to an electron beam. Two epilayers have been studied: the first is a 0.35 μm thick hydrogenated Si-doped GaAs epilayer and the second is Si planar-doped AlGaAs/GaAs/InGaAs heterostructures. Firstly, Hall effect measurements have been carried out on the epilayers exposed, after RF hydrogen plasma exposition, to increasing electron doses with different injection energies. For the 2D heterostructures, we have observed that the free carrier density N s does not vary significantly for weak electron densities. This reactivation presents a threshold value, contrary to the 0.35 μm epilayer in which N s varies quite linearly. It will be shown that such phenomena might be attributed to the filling of surface states as the dopants are progressively reactivated. Then, using a high spatial resolution electron beam lithography system, nanometric conductive patterns have been fabricated starting from hydrogenated epilayers. Electric measurements have been performed and the results obtained show that about 15 nm spatial resolution could be expected. In conclusion, taking into account this spatial resolution, the high spatial contrast of conductivity which could be expected due to the existence of an electron dose threshold, and the high mobility of the AlGaAs/GaAs/InGaAs heterostructure, the effects described in this paper could open a new way for the fabrication of III-V 1D or 2D mesoscopic structures for electronic or optoelectronic applications.

Atomic Layer Deposition is used to control the surface recombination of carriers in GaAs photonic crystal cavities. All-optical wavelength conversion at a GHz repetition rate is demonstrated with recovery time below 10 ps.

We describe the design, optimization and fabrication of side-illuminated p-i-n photodetectors, grown on InP substrate, suitable for surface hybrid integration in low-cost modules. The targeted functionalities of these photodetectors were a very high responsivity at 1.3- and 1.55-/spl mu/m wavelengths and quasi-independent on the optical polarization, and had a high alignment tolerance. Moreover, in order to avoid any reliability problem, the principle of evanescent coupling was adopted. Two photodetectors were optimized, fabricated, and tested; the first was for classical cleaved fiber, and the second was for lensed fiber. Because the considered epitaxial structures were complicated to optimize, the method of the genetic algorithm was used, associated with a beam propagation method (BPM). The photodetectors are based on multimode diluted waveguides, which are promising structures in the field of optoelectronics and integrated optics. Starting from the presented comparisons between experimental and theoretical results, the interest of the design method is discussed and the complete performances of newly fabricated devices are presented. The aspect of the cutoff frequency is also considered.

Polycrystalline indium tin oxide (ITO) films were prepared on glass substrates by radio frequency (RF) sputtering system from a high density target (90wt. In2O3 and 10 wt. %SnO2). X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) experiments were performed to study the structure and the surface morphology of these samples. The results show that the as-deposited ITO thin films deposited on glass have a texture, as the film grows the preferred orientation changes from to . The lattice parameters are found to be larger than the bulk value, indicating that the samples are under a tensile stress. The grain size increases with increasing thickness. SEM images show a dense granular structure with grains having different shapes and sizes. After deposition, the samples have been annealed in air at temperature T around 400°C for 1 hour. We found that the texture becomes stronger after the annealing treatment. A large increase in the grain size after annealing is also observed. The lattice constant decreases with T to become closer to the bulk value, i.e. annealing seems to relieve the stress present in the as-deposited films. The optical transmision is greater than 80 % in the visible region.The refractive index n values are found to be in the 1.7-1.9 range. The energy gap values are between 3.58 and 3.88 eV .

In this paper, we present the microwave characterization of polymers for electro-optic devices, carried out as a collaboration between chemical and physical research groups. We begin by reporting the synthesis and characterization of telechelic oligoimides functionalized with trialkoxysilane end-groups. Addition of a crosslinking agent, tris(4-hydroxyphenyl)ethane, and thermal curing leads to network formation, with the trialkoxysilyl end-groups as crosslinking sites. The network formation is confirmed by FTIR spectroscopy. The resulting material exhibits a glass transition temperature above 300°C and a thermal decomposition temperature above 450°. These oligomers offer an attractive processing alternative as low viscosity precursors to polyimide networks. The permittivity from 250 MHz to 40 GHz is determined by spin-coating the polymer on a structure as close as possible to a travelling wave electro-optic modulator. The permittivity is obtained from the measured scattering parameters and a spectral domain approach (S.D.A.)-based simulation The scattering parameters (measured), constant propagation (simulated) and characteristic impedance (simulated) are linked together using two alternative physical representations, the multiple reflection method and the attenuation method. This approach was first validated with a commercial polymer and then applied to the oligoimides.

We present the design and realization of a 1×4 switching matrix, based on an electro-optical directional coupler. This matrix is devoted to true time delay systems. It is characterized by an original topology that reduces global power consumption down to 20 mW. Fiber-to-fiber losses are 15 dB, and the crosstalk is lower than −20 dB. © 1998 John Wiley & Sons, Inc. Microwave Opt Technol Lett 18: 243–246, 1998.

A three dimensions of semivectorial finite difference beam propagation method (FD-BPM) in cylindrical coordinates is investigated in order to analyze the optical wave propagation in the bent waveguides with the strong lateral confinement which will keep low radiation loss. The three dimensional (3D) semivectorial wave propagation equation is programmed in the quick and unconditionally stable procedure by using the alternating direction implicit method. This version of FD-BPM could well express the polarization characteristics of modal field near the dielectric interfaces and accurately simulate the wave propagation in bent waveguides even at a very small radius (

A three-terminal edge-coupled InGaAs/InP phototransistor is reported. A dc photocurrent gain near 60 and an optical unity gain frequency of 40 GHz were measured. The mixing of a modulated optical signal and a microwave signal applied on the base terminal is presented. Optical and electrical modeling are used to analyze and improve the device performances. © 1998 John Wiley & Sons, Inc. Microwave Opt Technol Lett 17: 408–412, 1998.

A new architecture for an optical system to achieve true time delay phased array antenna beam steering is proposed. This system is based on the association of switching matrixes and optical fibres of different lengths. A comparative study of the different available switch types has been carried out and concludes that total internal reflection (TIR) switches fit the specific requirements. The switch and entire matrix are designed using 2D and 3D finite difference beam propagation methods (FD-BPM). Then, the electrical behaviour of the device is investigated using a drift diffusion model. Results show the following performance: fibre to fibre loss equal to 10 dB, crosstalk around -2.0 dB and bandwidth close to 160 MHz.

This letter demonstrates an evanescently coupled p-i-n photodiode combined with a multimode diluted waveguide using a simple all 2-in InP processing that includes on-wafer mirrors etching and antireflection coating. A high responsivity of 0.81 A/W at 1.55 mum with less than 0.4-dB polarization dependence and a large -1-dB vertical alignment tolerance of 2.70 mum were achieved simultaneously with a bandwidth of 47 GHz. Stable operation for over 1000 h was obtained under bias stress and temperature at 200degC

In this paper we present the simulation of Metal-Semiconductor-Metal photodetector (MSM-PD) of interdigitated planar structure based on InAlAs/InGaAs adapted for photodetection at the wavelength 1.55 μm. We use the theoretical models to plot the variations of the dark current, the photocurrent, the capacity, and the cut-off frequency of the photodetector as a function of bias voltage and the interelectrode distance. The obtained results show a very low dark current, mainly due to the introduction of a thin layer to increase the Schottky barrier based on In0.52Al0.48As in the epitaxial structure of component. The obtained photocurrent and cut-off frequencies are very appreciable, these latter are mainly limited by the transit time of the photo-generated carriers given the low component capacity obtained by simulation.

In this paper we present the optimization of photoswitches constituted of a coplanar line introducing interdigitated MSM photodetectors in the central strip. The reflection and the transmission coefficients are studied with the aim of highlighting the influence of different geometrical parameters on the photoswitch microwave behavior. In pursuance of improving the performances, new topologies of coplanar lines introducing tapers of linear shapes, as well as tightening of ground planes around the central strip are proposed and studied. The outcomes showed that it is possible to achieve high insulation (−52.3 dB) at the Off state. As for as the On/Off ratio is concerned, a value of 48.8 dB is reached.

L'optoelectronique-hyperfrequence impose de disposer de photodetecteurs et de commutateurs optiques a hautes performances. En effet, les photodetecteurs rapides de forte puissance optique et les commutateurs optiques directionnels, c'est-a-dire jouant le role d'aiguillage optique, sont susceptibles d'etre des dispositifs de base pour les systemes opto-hyperfrequences. Le but de cet article est de presenter les avancees recentes dans le domaine des photodetecteurs rapides susceptibles d'etre utilises en opto-hyperfrequences, d'en donner leur principe de fonctionnement et les performances maximales avec les limitations fondamentales dans le contexte opto-hyperfrequence, ainsi que les differentes technologies de commutateur, en evaluant a chaque fois leur adequation aux applications opto-hyperfrequences.

An optical millimeter-wave system at 38 GHz making use of laser diode nonlinearities is proposed. These nonlinearities are employed in conjunction with an optoelectronic transducer designed to emit/receive millimeter waves. A setup implementation using a BPSK modulation to transmit a 100 kbit/s channel through 5.5 km of single-mode fiber is reported. © 1997 John Wiley & Sons, Inc. Microwave Opt Technol Lett 16: 34–38, 1997.

In this paper, we present a theoretical study and a numerical simulation of various long wavelength top-illuminated p-i-n photodetectors in the frequency range of 2.5-60 GHz under high optical modulated power at 1.55-/spl mu/m wavelength. The modeling includes a monodimensional drift-diffusion model for the device and takes into account the external circuit. We considered six InP/GaInAs/InP photodetectors especially designed to work at 2.5, 10, 20, 30, 40, and 60 GHz, respectively. For the one with the highest frequency, we intentionally sacrificed the quantum efficiency in order to compare them at the end with the results already obtained in the case of waveguide p-i-n photodetectors. The results show the maximum microwave-output power for each photodetector at its specific working frequency. Additionally, we present the effects of the modulation depth, the back illumination, and the wavelength of 1.3 /spl mu/m.

This letter demonstrates a new solution to achieve low-cost, highly integrated, and reliable InP-based p-i-n photodiodes for wavelength-division-multiplexing optical networks, absorbing 1.55-/spl mu/m wavelength and transparent to 1.3 /spl mu/m. It is based on a multimode diluted waveguide (MDW) combined with an evanescently coupled photodiode. The overall structure is optimized using a genetic algorithm linked to a beam propagation method software. The computed responsivity of the MDW photodiode is 0.86 A/W at 1.55 /spl mu/m, and the 1.3/1.55-/spl mu/m optical crosstalk is better than -20 dB (-40-dB electrical). The proposed device is designed for hybridization on silicon platform for low-cost modules, with potential application up to 10 Gb/s.

We report a photoconductive switch using low temperature grown GaNAsSb as the active material. The GaNAsSb layer was grown at 200 °C by molecular beam epitaxy in conjunction with a radio frequency plasma-assisted nitrogen source and a valved antimony cracker source. The low temperature growth of the GaNAsSb layer increased the dark resistivity of the switch and shortened the carrier lifetime. The switch exhibited a dark resistivity of 107 Ω cm, a photo-absorption of up to 2.1 μm, and a carrier lifetime of ∼1.3 ps. These results strongly support the suitability of low temperature grown GaNAsSb in the photoconductive switch application. ASTAR (Agency for Sci., Tech. and Research, S’pore) Published version

International audience; Integration of surface plasmon structures using semiconductor materials is limited due to the difficulties encountered in maintaining the resonance conditions upon packaging. We propose here a technology process allowing us to bond two semiconductors, such as gallium nitride (GaN) and gallium arsenide (GaAs), through a thin metal layer. This solution allows the excitation of a surface plasmon wave in an integrated classical Kretschmann configuration. The Letter presents various metal bonding conditions employed for Au deposited on both GaN/sapphire and GaAs substrates aiming at semiconductor–metal–semiconductor interfaces transparent at telecom wavelengths. The process conditions for the bondings are optimized using Ti/Au (3 nm/30 nm) layers on each of the wafers to be bonded under an applied pressure of 500 mbar at a low temperature of 250°C.

We have demonstrated a low-cost high-bandwidth and high-responsivity evanescent waveguide unitravelling-carrier photodiode (PD) fabricated using all 2-in InP processing including on-wafer mirrors and coating. Optimization of the optical input waveguide was made using the method of the genetic algorithm associated with a beam propagation method. Optical fiber coupling to the PD is based on a diluted multimode waveguide allowing simple material epitaxial growth. The fabricated PDs exhibit simultaneously 0.76-A/W responsivity at 1.55 /spl mu/m, >50-GHz bandwidth, and more than 22-mA average saturation photocurrent at 50 GHz. The light polarization dependence is less than 0.1 dB.

International audience

International audience; We report here the experimental results on a field effect refractive index change into gallium nitride (GaN) structures. This effect is characterized through the common prism-coupling technique with the application of a vertical direct-current electric field. Surface plasmon propagation was used to increase the sensitivity of the electro-optic measurements. We have obtained a large refractive index variation for GaN epilayer, around 1.4×10−2 at 1.55μm wavelength. In order to understand the origin of the index modulation, we have conducted a scanning transmission electron microscopy analysis and discussed the influence of threading dislocations density acting as traps and thermo-optic effect. According to recent works, we observed experimentally the optical response of a non-linear electro-optic effect on GaN on Si(111) substrate and estimated a Kerr coefficient of about 2.14×10−16m2V−2.

The aim of our work was to integrate Schottky MSM photodetectors in the central strip of coplanar lines for microwave switching application. The MSM photodetectors in the microwave lines have an active surface of 3×3 μm2 and electrode spacing of 0.2, 0.3, 0.5 and 1 μm. The characterization of the structures in darkness and under illumination allowed us to obtain On/Off ratio of 26–28 dB at 20 GHz and 25 GHz. These results demonstrate that the structures show a good performance for microwave photoswitching which means good insulation in darkness and low insertion losses under illumination.

Using a 2D bipolar physical model a comparison is made between GaAs and AllnAs/GaInAs MSM PD performance (cutoff frequency and quantum efficiency) as a function of the device parameters. This covers the applied potential, electrode spacing, absorbing layer thickness, and heterojunction effect. We also reported that for millimeter-wave applications we can expect higher operating frequency using an AlInAs/GaInAs MSP PD, whereas we can expect higher quantum efficiency using a GaAs MSM PD. © 1995 John Wiley & Sons, Inc.

Using a 2-D bipolar physical model we highlight the main parameters that govern the long-wavelength MSM PD performance (cutoff frequency and responsivity). This covers applied potential, electrode spacing, absorbing layer thickness, and heterojunction effect. We also report that using a backside illumination technique and a thin absorbing layer we can reach a cutoff frequency as high as 65 GHz with a responsivity of 0.2 A/W. >

In this paper, the behavior under very high optical power of waveguide PIN photodetectors grown on InP substrate is simulated. The problem is solved using a pseudo-bidimensional drift-diffusion model which describes the electrical behavior of the device including the effects of the external circuit. The optical behavior of the device is analysed using FD 2D and 3D beam propagation method. First, we present the optical behavior of the device when the illumination conditions change. Influence of device structure, spot width, spot position and injection angle on the quantum efficiency of the photodetector is so studied. Second, the whole modeling is validated using experimental results given in the literature. Three typical multimode structures w hich allow a high cut-off frequency as well as a good responsivity are then modeled and compared. The smaller one has a cut-off frequency of 75 GHz in small signal conditions and the main effect decreasing the microwave output signal when the optical input power increases is the carrier effect in the depletion region of the photodetector. The maximum microwave power of each photodetector is calculated in typical conditions of use. >

International audience; This paper reports on performances enhancement of photoconductive switches in term of On/Off ratio and insertion losses. The optimization parameters on which the research has been focused are gap dimensions reduction to nanometer scale. The device characterization results up to a microwave frequency of 40 GHz and under CW illumination at a wavelength of 800 nm are presented. On/Off ratio reveals a value of 13 dB at 20 GHz under 100 mW optical power.

International audience; In this Letter, we have designed and fabricated a III-V semiconductor multilayer based on surface plasmon resonance (SPR) operating at the telecom wavelength. Optimization of the optogeometrical parameters and the metal/semiconductor layers required for this novel structure was conducted accurately by theoretical tools using the Maxwell equations. Technological fabrication of the device and its experimental characterizations using an evanescent coupling configuration was performed: the results have confirmed the existence of SPR associated to a sharp width response. This study could be a first step in the design of new plasmonic-semiconductor-based optical devices such as modulators and switches.

In this letter, we report the characterization of the refractive indices and complex conductivities of a set of GaN films with different carrier concentrations, InN film, and InxGa1−xN films with indium content varying from x = 0.07 to x = 0.14 grown by metalorganic chemical vapor deposition for frequencies ranging from 0.3 to 3 THz using terahertz time-domain spectroscopy (THz-TDS). The refractive indices of InxGa1−xN films at THz range are reported. The carrier density and mobility determined using THz-TDS method show good agreement with four-probe Hall measurements.

We present a study of non–intentionally doped InN epilayers directly grown on sapphire substrate by metal organic chemical vapour deposition (MOCVD) technique. Structural and optical characterisations of this sample have been conducted by SEM, temperature–dependent photoluminescence and time resolved pump–probe techniques. Triangular–shaped structures of InN are observed to grow above hexagonal–shaped columnar structures as seen from the SEM image. Photoluminescence spectra reveal a peak energy of 0.75 eV, supporting the existence of the narrow bandgap of InN. So far, various papers in the literature quote the carrier lifetime in InN epilayers grown on sapphire substrate in the presence of a buffer layer such as GaN. Herein, we report carrier lifetime measurements conducted on InN epilayer deposited directly on sapphire substrate, in the absence of GaN as a buffer layer.