Plasmonic THz photodetectors have been realized in this work, by implementing the active region of a 5 THz quantum well detector with an antenna-coupled microcavity array. Our results demonstrate a clear improvement in responsivity, polarization insensitivity and background limited performance

Barbieri, S

Chen, YN

Davies, AG

Gacemi, D

Li, L

Linfield, EH

Palaferri, D

Sirtori, C

Todorov, Y

Vasanelli, A

D. Palaferri

Y. Todorov

S. Barbieri

D. Gacemi

Y.N. Chen

A. Vasanelli

L. H. Li

A. G. Davies

E. H. Linfield

C. Sirtori

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Antenna-coupled microcavity enhanced THz photodetectors

White Rose Research Online

This is a repository copy of Antenna-Coupled Microcavity Enhanced THz Photodetectors.White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/90700/Version: Accepted VersionProceedings Paper:Palaferri, D, Todorov, Y, Barbieri, S et al. (7 more authors) (2015) Antenna-Coupled Microcavity Enhanced THz Photodetectors. In: Infrared, Millimeter, and Terahertz waves (IRMMW-THz), 2015 40th International Conference. IRMMW 2015, 40th International Conference on Infrared, Millimeter, and Terahertz, 23-28 Aug 2015, Hong Kong. IEEE . ISBN 978-1-4799-8272-1 https://doi.org/10.1109/IRMMW-THz.2015.7327491eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version - refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher’s website. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. AbstractPlasmonic THz photodetectors have been realized in this work, by implementing the active region of a 5 THz quantum well detector with an antenna-coupled microcavity array. Our results demonstrate a clear improvement in responsivity, polarization insensitivity and background limited performance.  I. INTRODUCTION  UANTUM Well Infrared Photodetectors (QWIPs) have been investigated in the past 20 years as valid solution to the demand of fast and high sensitive detection in the infrared and far-infrared spectral region (5µm < Ȝ < 200µm)1,2. These devices use intersubband (ISB) transitions in a semiconductor quantum well (QW) superlattice (mainly n-type doped GaAs/AlGaAs) to generate photocurrent. Recently3, we  demonstrated  an  antenna-coupled microcavity geometry for QWIP operating at a wavelength of 9µm which enables an improved light coupling, a  reduced  dark  current  and  a  higher  temperature performance. Here we report on the implementation of 5 THz quantum well photodetector4 exploiting a patch antenna cavity array5,6. The  benefit  of  our  plasmonic  architecture  on  the  detector performance  is  assessed  by  comparing  it  with  detectors  made  using  the  same  quantum well absorbing region, but processed into a standard 45° polished facet mesa4.  II. RESULTS Our device is based on an MBE (molecular beam epitaxy) grown structure, similar to sample v266 of Ref. [4]: 20 GaAs QWs, each with a thickness LQW = 15.5 nm and n doped across the central 10 nm regions with Si at a density of Nd = 6.0x1016 cm-3, alternate with layers of AlGaAs (3% Al mole fraction) each with a thickness Lb = 70.0 nm. At the top and the bottom of this periodic structure there are GaAs contact layers doped Nd = 1.0x1018 cm-3, each having a thickness Lc = 100.0 nm. The patch-antenna devices shown in Figure 1 consist of a double metal structure with the top surface patterned into array of square resonant microcavities7; devices with four different patch sizes s were tested: 6µm, 7µm, 8µm and 9µm.   The resonant frequency νres of the cavity is given3,5,6 by νres =c/2neffs where neff is the effective index of the TM100/TM010 mode, slightly higher than the index of the bulk GaAs6. The spectral photo-response of the cavity array and of the 45° mesa detectors are shown in Figure 2(a) and 2(b).   The mesa device displays the typical broad shape of bound-to-quasi-continuum THz QWP, as reported in Ref. [4], with a frequency peak at 5.4 THz. The photocurrent peak for the antennas always appears at the same position as set by the cavity mode. As expected the photo-current decreases when the cavity is detuned from the maximum of the mesa photo-response at 5.4 THz. The s=7µm antenna device is resonant with the intrinsic photo-response, while the s=9µm is redshifted, resulting in cavity mode at lower energies than the intersubband absorption edge. This decreases the photo-detected signal. The plasmonic antennas couple in normal incident radiation, and collect photons over an area larger than that of the device itself, thus allowing a reduction in the dark current. Responsivity  measurements  were  performed  using  a  calibrated  500  K  blackbody and QWIP temperature T = 4 K; results are reported in Figure 2(c): the responsivity of the 45°-facet mesa shows values from 0 to 1 A/W in accordance with the results reported in Ref.[4]. For  the  s=7µm  device,  that has a cavity mode resonant with the ISB transition of the QWs, we  observe  a  four-fold  enhancement  of  the  responsivity,  with values up to 5.5 A/W in negative bias. The photoresponse is strongly enhanced due to the improved collection efficiency by the antenna effect5 and reduced dark D. Palaferri1, Y. Todorov1, S. Barbieri1, D. Gacemi1, Y.N. Chen1, A. Vasanelli1, L. H. Li2,  A. G. Davies2, E. H. Linfield2, and C.Sirtori1  1Laboratoire Matériaux et Phénomènes Quantiques, Université Paris Diderot  CNRS UMR 7162, 75013 Paris, France 2School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom Antenna-Coupled Microcavity Enhanced THz Photodetectors QFig. 2. (a) and (b) photocurrent spectra of the cavity array and of the 45° mesa devices, respectively; the dashed area represents the GaAs Reststrahlen band; (c) Responsivity peak vs voltage for the 45°mesa and the 7 µm cavity array detectors, measured with a blackbody at 500 K Fig. 1. Optical microscope (left) and scanning electron microscope (right) images of the 9 µm cavity array detector with indications of the main parameters: s is the patch size and p=10µm is the period length.current. This effect has a direct impact on the thermal performance of the detector, such as the BLIP (Background-Limited Infrared Performance) temperature TBLIP and specific Detectivity D*. The BLIP temperature TBLIP is defined such as Ibackground/Idark =2; under this condition the photocurrent generated by the background is exactly equal to the detector dark current1. As shown in Figure 3 TBLIP=21 K has been registered for the 6-7-8µm antennas with a field of view FOV = 54°, in comparison with the BLIP, TBLIP =17 K of the mesa with a FOV=70°: the results for the antenna devices are higher than previous reported values4.                  Fig. 3. (a) The ratio of the background and dark currents for the cavity array devices and for the 45° mesa device.  The background current is Ibackground = Iphoto+Idark where Iphoto is the photocurrent generated by a 300 K background with the indicated FOVs.   From measurements of the current-voltage characteristics and the responsivity we have also estimated the dark current-limited specific detectivity, defined as1: D*=RpAcoll/Inoise, where Acoll is the photon collection area and Inoise= (4egIqwip)0.5 is the current noise associated to the QWIP device, calculated knowing the photoconductive gain of the device7 and the associated measured current Iqwip . Using photon collection areas Acoll = 1.1x105 µm² (mesa) and Acoll = 9.4 x104 µm² (cavity array) we obtain maximum D* = 5x1012cmHz1/2/W at 4K for the arrays, in comparison with 0.7x1012cmHz1/2/W for the mesa device. We attribute this seven-fold improvement of D* to the better photon coupling of our structure. The maximum estimated value of D* is comparable with those of standard super-lattice detectors and of the best cryogenic cooled bolometers7,8.  Noise measurements have been carried out to confirm the detectivity estimation. Current noise spectral density have been obtained on the 9µm antenna detector with a setup involving a transimpedance amplifier and a spectrum analyzer, see Figure 4(left): the background current noise spectra show the typical 1/f noise (100Hz-1kHz), a plateau revealing the generation-recombination noise of the QWIP (1kHz-100kHz) and the cut-off of the amplifier  (100kHz-1MHz). Figure 4 (right) shows current noise measured at 10kHz compared to the estimation from the current voltage characteristics: the good matching between measurement and estimation confirms the detectivity values that we report of the antenna-coupled devices7.   Fig. 4. (left) current noise spectral density measurements for the 9µm cavity array for different applied voltage; (right) current noise measured at 10kHz compared to the estimations from the current voltage characteristics.    III. SUMMARY This study shows how the concept of antenna-coupled microcavity can be applied successfully to enhance the performances of infrared and terahertz opto-electronic devices3,7. A 5 THz QWIP has been investigated and its performances have been improved in terms of its figures of merit, like responsivity, specific detectivity and BLIP temperature. Furthermore, the antenna-coupled photodetection can be still optimized by reducing the microcavity absorbing area respect to the photon collection area, while it is possible to bring the system in the critical coupling regime where all incident photons are absorbed in the structure6. REFERENCES [1]. H.C. Liu, Intersubband Transitions in Quantum Wells, edited by H. C. Liu and F. Capasso, Academic Press,San Diego, 2000.  [2]. J.C. Cao and H.C. Liu Terahertz Semiconductor Quantum Well Photodetectors, Semiconductors and Semimetals, Vol. 84, 2011 [3]. Y.N. Chen, Y. Todorov, B. Askenazi, A. Vasanelli, G. Biasiol and C. Sirtori, Antenna-coupled microcavities for enhanced infrared photo-detection, Appl. Phys. Lett. Vol.104, 2014.  [4]. H. Luo, H.C. Liu, Song, C.Y., and Z.R Wasilewski, Background-limited terahertz quantum-well photodetector Appl. Phys. Lett. Vol.86, 2005.  [5]. Y. Todorov, L. Tosetto, J. Teissier, A. Andrews, P. Klang, R. Colombelli, I. Sagnes, G. Strasser, and C. Sirtori, Optical properties of metal-dielectric-metal microcavities in the THz frequency range Opt. Express Vol.18, 2010. [6] C. Feuillet-Palma, Y. Todorov, A. Vasanelli, and C.Sirtori ,  Strong near field enhancement in THz nano-antenna arrays Sci. Rep. Vol.3, (2013). [7] D. Palaferri, Y. Todorov, Y.N. Chen, A. Vasanelli, L. H. Li, A. G. Davies, E.H. Linfield, and C.Sirtori, Patch antenna terahertz photodetectors Appl. Phys. Lett. Vol.106, 161102 2015.  [8] T. Ueda, Z. An, and S. Komiyama, J. Infrared Millim. Te 32, 5 (2010).  [9] http://www.terahertz.co.uk , visited on 2/03/2015 

Antenna-Coupled Microcavity Enhanced THz Photodetectors

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Antenna-Coupled Microcavity Enhanced THz Photodetectors

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