E. Peytavit

Terahertz frequency difference from vertically integrated low-temperature-grown GaAs photodetector

We report on the development of a photoconductive detector based on low-temperature-grown GaAs which is vertically integrated with terahertz spiral antennas. A non steady-state velocity overshoot effect was expected in the photoresponse with a responsivity of 0.04 A/W at a bias voltage of 8 V. Photomixing experiments using two optical 0.8 μm beating lasers show a 3 dB bandwith of 700 GHz with a radiation power at terahertz frequency of 0.5 μW under 2×30 mW optical pumping.

Highly coherent terahertz wave generation with a dual-frequency Brillouin fiber laser and a 1.55 μm photomixer

Thanks to a portable dual-frequency Brillouin fiber laser and a 1.55 μm photomixer, we report the generation of a highly coherent kilohertz level submillimeter wave emission. Low-cost telecommunications components are used to achieve very simple source architecture. The photomixer is composed of a unitravelling carrier photodiode integrated with an antenna. An emission at 316 GHz is observed and analyzed thanks to heterodyne detection with a signal-to-noise ratio >65 dB and a ~1 kHz linewidth. The phase noise of the proposed source has the same performance at 1.7 and 316 GHz. We show that this source has comparable or better phase noise compared to electrical oscillators and the tunability is much wider.

Circularly polarized luminescence microscopy for the imaging of charge and spin diffusion in semiconductors

Room temperature electronic diffusion is studied in 3 μm thick epitaxial p(+) GaAs lift-off films using a novel circularly polarized photoluminescence microscope. The method is equivalent to using a standard optical microscope and provides a contactless means to measure both the charge (L) and spin (L(s)) diffusion lengths simultaneously. The measured values of L and L(s) are in excellent agreement with the spatially averaged polarization and a sharp reduction in these two quantities (L from 21.3 to 1.2 μm and L(s) from 1.3 to 0.8 μm) is found with increasing surface recombination velocity. Outward diffusion results in a factor of 10 increase in the polarization at the excitation spot. The range of materials to which the technique can be applied, as well as a comparison with other existing methods for the measurement of spin diffusion, is discussed.Room temperature electronic diffusion is studied in 3 mum thick epitaxial p+ GaAs lift-off films using a novel circularly polarized photoluminescence microscope. The method is equivalent to using a standard optical microscope and provides a contactless means to measure charge (L) and spin (L_s) diffusion lengths. The measured values of L and L_s are in excellent agreement with the spatially averaged polarization and a sharp reduction in these two quantities (L from 21.3 mum to 1.2 mum and L_s from 1.3 mum to 0.8 mum) is measured with increasing surface recombination. Outwards diffusion results in a factor of 10 increase in the polarization at the excitation spot.

Fluidic assembly of hybrid MEMS: a GaAs-based microcantilever spin injector

A proof-of-concept fluidic assembly of a hybrid MEMS GaAs microcantilever spin injector is presented here. Instead of monolithically forming MEMS from pre-deposited layers, we fabricate a hybrid MEMS by assembling pre-fabricated parts. Sub-millimetre sized patches of GaAs having a thickness of 3 µm are pre-fabricated, as is a metalized fused silica support layer. The GaAs patches are manipulated and assembled onto the silica support using capillary forces; the resultant hybrid MEMS comprises a GaAs microcantilever on a robust fused silica support. A novel ohmic contact is demonstrated by bonding a GaAs patch (p-type carbon doped to 1 × 1018 cm−3) onto the pre-metalized silica support layer prior to annealing; measurements revealed ohmic behaviour and a specific contact resistivity of ~10−5 Ω cm. Preliminary investigations show that, when contacting the cantilever against a metallic or magnetic surface, injected photocurrents as large as several tens of nA can be obtained, for which the spin polarization is equal to 16%.

Surface recombination in doped semiconductors: Effect of light excitation power and of surface passivation

A self-consistent expression for the surface recombination velocity S and the surface Fermi level unpinning energy as a function of light excitation power (P ) is presented for nand p-type semiconductors doped above the 10 cm range. Measurements of S on p-type GaAs films using a novel polarized microluminescence technique are used to illustrate two limiting cases of the model. For a naturally oxidized surface S is described by a power law in P whereas for a passivated surface S varies logarithmically with P . Furthermore, the variation in S with surface state density and bulk doping level is found to be the result of Fermi level unpinning rather than a change in the intrinsic surface recombination velocity. It is concluded that S depends on P throughout the experimentally accessible range of excitation powers and therefore that no instrinsic value can be determined. Previously reported values of S on a range of semiconducting materials are thus only valid for a specific excitation power.For n- and p-type semiconductors doped above the 1016 cm−3 range, simple analytical expressions for the surface recombination velocity S have been obtained as a function of excitation power P and surface state density NT. These predictions are in excellent agreement with measurements on p-type GaAs films, using a novel polarized microluminescence technique. The effect on S of surface passivation is a combination of the changes of three factors, each of which depends on NT: (i) a power-independent factor which is inversely proportional to NT and (ii) two factors which reveal the effect of photovoltage and the shift of the electron surface quasi Fermi level, respectively. In the whole range of accessible excitation powers, these two factors play a significant role so that S always depends on power. Three physical regimes are outlined. In the first regime, illustrated experimentally by the oxidized GaAs surface, S depends on P as a power law of exponent determined by NT. A decrease of S such as the one induc...

Imaging ambipolar diffusion of photocarriers in GaAs thin films

Images of the steady-state luminescence of passivated GaAs self-standing films under excitation by a tightly focussed laser are analyzed as a function of light excitation power. While unipolar diffusion of photoelectrons is dominant at very low light excitation power, an increased power results in a decrease of the diffusion constant near the center of the image due to the onset of ambipolar diffusion. The results are in agreement with a numerical solution of the diffusion equations and with a physical analysis of the luminescence intensity at the centre of the image, which permits the determination of the ambipolar diffusion constant as a function of electron concentration.

All optical method for investigation of spin and charge transport in semiconductors: Combination of spatially and time-resolved luminescence

A new approach is demonstrated for investigating charge and spin diffusion as well as surface and bulk recombination in unpassivated doped semiconductors. This approach consists in using two complementary, conceptually related, techniques, which are time-resolved photoluminescence (TRPL) and spatially resolved microluminescence (μ PL) and is applied here to p + GaAs. Analysis of the sole TRPL signal is limited by the finite risetime. On the other hand, it is shown that joint TRPL and μ PL can be used to determine the diffusion constant, the bulk recombination time, and the spin relaxation time. As an illustration, the temperature variation of these quantities is investigated for p + GaAs.

Central role of electronic temperature for photoelectron charge and spin mobilities in p+-GaAs

The charge and spin mobilities of minority photoelectrons in p+-GaAs are determined by monitoring the effect of an electric field on the spatial profiles of the luminescence and of its polarization. By using electric fields to increase the photoelectron temperature Te without significantly changing the hole or lattice temperatures, the charge and spin mobilities are shown to be principally dependent on Te. For Te > 70 K, both the charge and spin mobilities vary as Te−1.3, while at lower temperatures this changes to an even more rapid Te−4.3 law. This finding suggests that current theoretical models based on degeneracy of majority carriers cannot fully explain the observed temperature dependence of minority carrier mobility.

Continuous-wave terahertz generation using a vertically integrated horn antenna photomixer

A transverse Electromagnetic Horn Antenna is monolithically integrated with a low-temperature-grown GaAs vertical photoconductor on a silicon substrate forming a vertically integrated photomixer. Continuous-wave terahertz radiation is generated at frequencies up to 3.5 THz with a power level reaching 20 nW around 3 THz.

Terahertz wireless communications using photonic and electronic devices

With the fast increase of mobile data transfers, wireless communications carrier frequencies have entered in the millimeter wave region and now they enter in the submillimeter or terahertz region. In this context photonic-based emitters have several advantages, we will present our communication links results using photomixers at 0.2, 0.4 and 0.6 THz.