Kasjoo, Shahrir Rizal

Song, Ai Min

IEEE Electron Device Letters

We report on the lowerature detection of free-space radiation at 1.5 THz using a unipolar nanodiode, known as the self-switching diode (SSD), coupled with a spiral microantenna. The SSD, based on an asymmetric nanochannel, has a diode-like characteristic that can be utilized in rectifying high-frequency electrical signals. The truly planar structure of the SSD not only provides intrinsically low parasitic capacitance that enables rectification at ultrahigh speed, but also allows the fabrication of a large SSD array in parallel without the need for interconnection layers. The extrinsic voltage responsivity of the SSD-based detector achieved was {\sim}{\rm 15.6}~{\rm V}/{\rm W}, but the estimated intrinsic voltage responsivity was {\sim}{\rm 45}~{\rm kV}/{\rm W}. © 2013 IEEE

The University of Manchester - Institutional Repository

1554 IEEE ELECTRON DEVICE LETTERS, VOL. 34, NO. 12, DECEMBER 2013Terahertz Detection Using NanorectifiersShahrir Rizal Kasjoo and Ai Min Song, Senior Member, IEEEAbstract— We report on the low-temperature detection offree-space radiation at 1.5 THz using a unipolar nanodiode,known as the self-switching diode (SSD), coupled with a spiralmicroantenna. The SSD, based on an asymmetric nanochannel,has a diode-like characteristic that can be utilized in rectifyinghigh-frequency electrical signals. The truly planar structure ofthe SSD not only provides intrinsically low parasitic capacitancethat enables rectification at ultrahigh speed, but also allows thefabrication of a large SSD array in parallel without the need forinterconnection layers. The extrinsic voltage responsivity of theSSD-based detector achieved was ∼15.6 V/W, but the estimatedintrinsic voltage responsivity was ∼45 kV/W.Index Terms— Diode, nanotechnology, terahertz (THz)radiation.I. INTRODUCTIONTERAHERTZ (THz) refers to the electromagnetic radia-tion in the frequency range from 0.1 to 10 THz. In thelast two decades, interest in THz technology has grownsteadily, given its outstanding potential in many applications,including telecommunication, defense and security, medicaland healthcare, and material research [1]–[3]. In pathology,for example, THz radiation is able to distinguish betweencancerous and healthy tissues based on their biological andchemical compositions [3].In this letter, we present the THz operation at a low temper-ature of a device referred to as the self-switching diode (SSD),which is based on geometrical symmetry breaking of a semi-conductor nanochannel [4]. Previous experiments have shownthat the SSD worked at microwave and THz frequencies, inwhich the SSDs were coupled to a microwave waveguide orbow-tie antenna [5]–[7]. The bow-tie antenna is suitable toreceive linearly polarized free-space THz radiations. For manyapplications, such as radio astronomy and THz imaging, theTHz waves are not linearly polarized. Furthermore, althoughit is convenient to carry out and utilize THz waves at roomtemperature, THz detectors are often cooled to low tempera-tures to increase the signal-to-noise ratio. As such, we havein this letter fabricated SSDs with a spiral antenna [Fig. 1(a)]Manuscript received August 15, 2013; revised September 22, 2013 andOctober 2, 2013; accepted October 5, 2013. Date of publication October 28,2013; date of current version November 20, 2013. This work was supportedin part by the EU FP7 project under Grant Agreement 243845 and in partby the Malaysia Ministry of Higher Education. The review of this letter wasarranged by Editor R. Quay.S. R. Kasjoo was with the School of Electrical and Electronic Engineering,University of Manchester, Manchester M13 9PL, U.K. He is now with theSchool of Microelectronic Engineering, Universiti Malaysia Perlis, Perlis02600, Malaysia (e-mail: shahrirrizal@unimap.edu.my).A. M. Song is with the School of Electrical and Electronic Engi-neering, University of Manchester, Manchester M13 9PL, U.K. (e-mail:a.song@manchester.ac.uk).Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LED.2013.2285162Fig. 1. (a) Optical image of the planar spiral antenna. (b) 3-D rendering ofan AFM image of two SSDs (out of four SSDs in total) connected in parallel(z-axis not in scale). The SSD channels were 190-nm wide and 1500-nmlong, and the trench width and depth were 150 and 130 nm, respectively.The 2-DEG layer, marked by the dashed line was 50-nm below the surface.(c) DC I–V characteristic of the SSDs at 12 K. (d) Schematic diagram of thematerial used in the device fabrication.and characterized the device performance at low temperaturesusing the THz radiation generated by a free-electron laser.The SSD is a unipolar two-terminal device. It consistsof two insulating L-shaped trenches opposite to each other,creating a semiconductor nanochannel between them [4].Fig. 1(b) shows an atomic-force microscope (AFM) imageof two SSDs connected in parallel (there were four SSDs intotal but only two are shown here for clarity). The appliedvoltage V to the device can either narrow or widen theeffective channel width, depending on its sign [4]. When thedevice is unbiased (V = 0), the effective channel for electronsis pinched off by the surface states on the sidewall of theetched trenches. A negative bias (V < 0) further depletesthe channel and results in a zero or low current, whereas apositive bias (V > 0) counteracts the lateral depletion andwidens the effective width of the channel to enable a largecurrent to flow. This results in a diode-like nonlinear current–voltage (I–V ) characteristic as shown in Fig. 1(c), but theSSD is not based on the use of any doped p-n junction orSchottky barrier structure and is hence completely differentfrom a conventional diode. This new working principle enablesa tunable threshold voltage by simply fabricating a suitablechannel width, and hence a zero-threshold may be achievedwhen the channel is just narrow enough to be fully depleted0741-3106 © 2013 IEEEKASJOO AND SONG: THZ DETECTION USING NANORECTIFIERS 1555at zero bias [4], [5]. Intrinsically, the SSD possesses a muchlower parasitic capacitance when compared with conventionaldiodes because of its planar architecture in comparison with amultilayered conventional diode of the same size. This enableselectrical signal rectification at ultrahigh speed. Because thetwo electrodes are on the same plane, it is straightforwardto integrate coplanar waveguide or planar antenna structureswith the SSDs. The truly planar nature of SSD also allowsthe fabrication of a large number of the devices in a singlelithography step without the need for interconnection layersand thus avoiding introducing undesirable parasitic elements[5]–[7].II. DEVICE FABRICATION AND EXPERIMENTAL SETUPThe antenna-coupled-SSD rectifier, i.e., rectenna, was fab-ricated onto a GaAs/AlGaAs quantum well wafer grown bymolecular-beam epitaxy, in which free charge carriers wereconfined in a quantum well located 50-nm below the surface,forming a 2-D electron gas (2-DEG). The structure of thissubstrate is shown in Fig. 1(d). The electron mobility andcarrier density at a temperature of 77 K were 72 000 cm2/Vsand 5.6 × 1011 cm−2, respectively, and at temperature of300 K were 7000 cm2/Vs and 6.0 × 1011 cm−2, respectively,as determined by Hall measurements.The first fabrication step was to create mesa structureson the substrate by means of wet chemical etching witha H3PO4/H2O2/H2O-based solution. Ohmic contacts on themesas were formed by thermal evaporation of a 50 nm ofAu/Ge/Ni alloy, followed by 200 nm of Au, and annealingat 420 °C. A planar spiral microantenna, with a dimension of∼1 mm, was incorporated with the ohmic structures by thermalevaporation of a 200-nm Au layer as shown in Fig. 1(a). FourSSDs connected in parallel were then fabricated in the antennagap by means of AFM-based lithography, followed by wet-chemical etching with a Br2/HBr/HNO3/H2O-based solution.The etched trenches in Fig. 1(b) are ∼130-nm deep, hencethrough the 2-DEG layer, to ensure electrical insulation. TheSSDs channels were 190-nm wide and 1500-nm long, and thetrenches width was 150 nm as determined by the AFM image[Fig. 1(b)]. Further details of the lithography can be foundin [8].Fig. 2 shows the measurement setup used in this letter.A high-power and polarized continuous wave of THz radiationwas generated by a free-electron laser (FELBE at Helmholtz–Zentrum Dresden–Rossendorf), in which the polarization canbe rotated through 90° by a polarizer. The beam was colli-mated and guided by a set of flip mirrors arranged on an opticaltable. By means of an off-axis parabolic mirror, the THzradiation was focused onto the rectenna that was positionedinside a continuous-flow helium cryostat chamber fitted witha diamond window transparent at THz frequencies as shownin Fig. 2. Prior to this, the beam was modulated at 120 Hzby an optical chopper. A dc current source was used to biasthe rectenna, and the rectified dc voltage was measured usinga lock-in amplifier (Signal Recovery, model 7265 DSP) withthe reference frequency input provided by the chopper. Thebeam intensity was monitored by a pyrometer and can beFig. 2. Diagram showing the measurement setup. The polarized THz radiationproduced by FELBE was modulated by an optical chopper, and was focusedonto the rectenna, placed inside a cryostat chamber. The device output voltagewas read out by a lock-in amplifier with the frequency input provided by thechopper. A tunable dc bias was applied to the device from a current source.A pyrometer was used to estimate the beam power density focused onto thedevice.used to estimate the actual power absorbed by the rectenna.The radiation power can also be tuned by adjusting a setof beam attenuators from the control panel that were linkedto the FELBE system. This measurement system was almostidentical to the setup used in [7] that has verified that the SSDswere only responsive to the radiation collected by the antenna,rather than artifacts such as local heating of the contacts, andso on.The performance of any receiving antenna used to captureand extract the power from the electromagnetic radiation isusually quantified by the effective aperture, Ae, defined as theratio of the power delivered to the load, Pdeli, to the incidentpower density, Winci, In this experiment, the power of the THzradiation, PTHz, was measured using a pyrometer. The beamwas then focused onto the rectenna by means of parabolicmirrors. By considering, the focused beam incident on thedevice has a circular area with a radius r , the value of Wincican be estimated as PTHz/(πr2). For a receiving antenna, thevalue of Ae is also related to its gain, G, by the followingequation, Ae = Gλ2/(4π), where λ is the wavelength of theelectromagnetic wave. Therefore, Pdeli can be estimated asPdeli = G(λ2πr)2PTHz. (1)III. RESULTS AND DISCUSSIONFig. 3(a) shows the rectified dc output voltage, Vout as afunction of the bias current, Ibias, at 1.5 THz (λ = 200 μm).The measurement was carried out at a range of THz-beampower, PTHz and at a temperature of 12 K that was thelowest temperature at which the cryostat could maintain agood stability. Through considering the beam diameter andthe antenna effective aperture, the power delivered to the spiralantenna (i.e., Pdeli) can be estimated from (1). Here, r = 1 mmas determined by mechanically shifting the SSDs in and out thefocus point of the parabolic mirror, and G = 2 was used herefor the spiral microantenna based on the simulations performedin [9].1556 IEEE ELECTRON DEVICE LETTERS, VOL. 34, NO. 12, DECEMBER 2013Fig. 3. (a) Vout as a function of Ibias, measured at 12 K at 1.5 THz atdifferent values of Pdeli. The highest magnitude of Vout was obtained at Ibiasaround 0.3–0.4 μA. (b) Vout as a function of Pdeli, measured at 12 K at1.5 THz with Ibias = 0.3 μA. A linear response (square-law detection) wasobserved when Pdeli < 10 W. In this region, the device extrinsic voltageresponsivity was 15.6 V/W, estimated from the fitting line (dotted line).The amount of Vout rectified by the SSDs is dependent onthe nonlinearity of the I–V characteristic (bowing coefficient)and differential resistance of the SSDs [6]. The bowing coef-ficient increases with increasing Ibias up to certain bias leveland then slowly decreases because of the parasitic resistancein series with the SSDs as shown in Fig. 1(c). The differentialresistance of the SSDs decreases as Ibias increases. As aresult, the combination of both parameters with respect to Ibiasyielded the THz responsivity curves in Fig. 3(a). As can beseen, similar traces were observed at different Pdeli, and thehighest magnitude of Vout for each measurement was achievedat Ibias of 0.3–0.4 μA. This was the region where I–Vcharacteristic of the SSDs exhibited the strongest nonlinearity.Fig. 3(b) shows the measurement of Vout as a functionof Pdeli at 12 K with Ibias = 0.3 μA. As expected, therectenna displayed a linear response (square-law detection) atpower levels lower than 10 μW. In this region, the voltageresponsivity, βV , of the device was ∼15.6 V/W.Such responsivity is extrinsic, which is largely limited bythe impedance mismatch between the SSDs and the antenna.The simulations (not shown here) show that the antenna witha GaAs substrate has a impedance of ∼100  at frequencies<1 THz and ∼50  at 1.5 THz. In contrast, the impedanceof the SSDs at Ibias = 0.3 μA at 12 K was ∼600 k. Thismeans that only ∼0.033% of Pdeli was effectively applied tothe SSDs with nearly all the incident power being reflected.Taking into account this factor, the intrinsic voltage respon-sivity of the device can achieve up to 45 kV/W. Commerciallow-temperature bolometers typically have a responsivity inthe order of a few kilovolt/watt to 10 kV/W [10], [11]. By opti-mization of the SSD technologies, such as fabricating moreSSDs in parallel to reduce the overall impedance [5], [12],filling the trenches with high-k materials [13], and designinghigher-impedance waveguide/antenna structures to couple tothe SSDs, the performance of the SSDs is expected to begreatly improved. It is worth noticing that the highest fre-quency that we could utilize in this experiment was 1.5 THz,which might not be the upper operational limit of the SSDs,as previously predicted by Monte Carlo simulations [14]. Theadvantage of the truly planar nature to allow straightforwardintegration with coplanar waveguide or planar antenna struc-tures with the SSDs offers a great potential for low-cost THzdetection applications.ACKNOWLEDGMENTThe authors would like to thank M. McGowan,Dr. C. Balocco, Dr. X. F. Lu, Dr. S. Winnerl, and HZDR fortechnical assistance and the use of FELBE facilities.REFERENCES[1] A. Redo-Sanchez and X. C. 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