Multiple double-metal bias-free terahertz emitters
Duncan McBryde, Paul Gow, Sam A. Berry, Armen Aghajani, Mark E. Barnes, V. Apostolopoulos
MMultiple double-metal bias-free terahertz emitters
D. McBryde, a) P. Gow, S. A. Berry, M. E. Barnes, A. Aghajani, and V. Apostolopoulos School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ,United Kingdom
We demonstrate multiplexed terahertz emitters that exhibits 2 THz bandwidth that do not require an externalbias. The emitters operate under uniform illumination eliminating the need for a micro-lens array and arefabricated with periodic Au and Pb structures on GaAs. Terahertz emission originates from the lateral photo-Dember effect and from the different Schottky barrier heights of the chosen metal pair. We characterize theemitters and determine that most terahertz emission at 300 K is due to band-bending due to the Schottkybarrier of the metal.Lateral photo-Dember (LPD) emitters have beendemonstrated as robust terahertz emitters requiring novoltage bias to operate . In an LPD emitter a semi-conductor surface is partially masked with a depositedmetal layer. An ultra-fast laser, with above band-gapenergy, is focused half on the metallic mask and halfon the semiconductor surface, thus, creating an asym-metrical distribution of photo-generated carriers near themetal-semiconductor interface that is free to diffuse. Thehypothesis for LPD emission was that the asymmetriccarrier concentration was responsible for a net diffusioncurrent . In we have shown that terahertz emission co-linear with the optical pump due to an anisotropic carrierconcentration is not possible because diffusion creates netzero current. Therefore diffusion currents cannot be thesole mechanism for radiation. We have completed thehypothesis of Klatt et al. by attributing the mechanismof radiation of LPD emitters to diffusion currents andto dipole quenching with a metal mask . The carriersthat diffuse under the metal mask cannot radiate due tothe proximity to the metallic surface in relation to theradiating wavelength. The rest of the carriers that dif-fuse away from the metal are free to radiate co-linearlywith the pump beam in the same geometry as a photo-conductive antenna. As LPD emitters depend on lateralcurrents, surface field effects in the semiconductor do notcontribute to terahertz emission. THz DipolesGaAsAu Pb
A B C D
FIG. 1. A diagram showing the emission mechanism ofmultiple-metal emitters due to the lateral photo-Dember ef-fect. Carrier diffusion creates radiative dipoles near the metalboundaries, shown as arrows. Each set of dipoles created onthe boundary are labelled A, B, C and D. The difference inthe reflectivity between the two metals that quench dipoles Band C causes net terahertz emission to be observed. a) Electronic mail: [email protected]
It is useful to multiplex the LPD emitters to increasetheir performance, however, periodic single-metal ridgesunder uniform illumination can not produce net terahertzemission in the direction of detection , each set of dipolescreated either side of the ridges is suppressed equally pro-ducing a quadrupole emission pattern. This can be al-leviated by illuminating alternating metal edges . Fur-thermore, multiple LPD emitters have previously beensuccessfully demonstrated with uniform illumination byfabricating a periodic gold wedge pattern . The mul-tiple emitters in were created because it was thoughtthat the metallic wedges would generate terahertz emis-sion due to the generation of periodic anisotropic carrierdistributions. Taking into account the dipole quenchingmechanism we now attribute the terahertz emissionobserved in to the reduced quenching strength causedby the reduction in metal height on one side of the mask.In this work, we propose to achieve the required asym-metry for co-linear THz emission by applying a low skindepth second metal on one side of each repeat of themetal mask, reducing the quenching strength from oneside of the metal mask by reducing the reflectivity of themetal surface . In doing so we eliminate the requirementfor a micro-lens array used previously , allowing for theemitters to operate under uniform illumination. The re-duced quenching strength allows net terahertz emissionto be formed in a dipole pattern under the low skin depthmetal. Ideally the metal is opaque to the optical pumpbeam and is transmissive to terahertz emission so quench- FIG. 2. A band diagram of the repeatable terahertz emitters.Both gold and lead create a Schottky barrier near the inter-face. Two metals have different work functions, φ m , creatingdifferent barrier heights φ b . Band bending occurs near themetals creating currents j Au and j Pb . If j Au (cid:54) = j Pb terahertzemission due to a net current is generated. a r X i v : . [ phy s i c s . op ti c s ] M a y ing is reduced to a minimum. An opaque dielectric maybe used in place of an additional metal at the expense ofincreased difficulty in emitter fabrication. This geome-try is shown in Fig. 1 where gold completely quenches thedipole formed underneath whereas lead is deposited witha thickness below its skin depth for THz frequencies andtherefore not inhibiting the terahertz dipole to radiate.The dipoles are shown in Fig. 1 and are labelled A, B,C and D. Dipoles A and D oppose each other and onlyproduce quadrupole emission. Dipole B is fully quenchedby the gold mask. Dipole C is not fully quenched as thePb layer has less reflectivity in the terahertz region. Un-der uniform illumination terahertz emission is observableco-linear with the surface normal due to the difference inquenching strength from the two metals.Band bending near the surface of a semiconductor cancause terahertz emission and has been shown to influ-ence LPD emitters in and to modify the polarity of tera-hertz pulses under intense optical fluence . Therefore,in the repeatable structures that we show here we usedmetal pairs that would also create terahertz emission dueto different Schottky barrier potentials. By selecting met-als with different skin depths and work functions both theLPD effect and Schottky barriers can contribute to tera-hertz emission. The concept is shown in Fig. 2 whereby depositing metals with different work functions, φ m ,we create different barrier heights on each side of thedouble-metal emitters. The band bending causes a tran-sient lateral current to form near the metal boundaryemitting terahertz emission. If the barrier heights φ b aredifferent, net lateral current will occur proportional tothe difference in barrier height under uniform illumina-tion. If the barrier heights are equal, as the case would befor a single metal, no net current would be present underuniform illumination. If the barrier heights are not equalthe relative strength of dipoles A and D in Fig. 1 differ,producing net terahertz emission co-linear with the sur-face. In the multiple LPD emitters that we present therequirement for any focusing lens for the optical pumpis removed, allowing the emitters to act as a drop-in re-placement for photo-conductive emitters. These emittersare simple to fabricate, do not require an electrical biasor a silicon lens to out-couple the THz emission. We ex-pect that these emitters are more durable than conven-tional photo-conductive emitters due to no risk of elec-trical breakdown.Our target were emitters that utilize both diffusionand Schottky generated currents on semi-insulating (SI)GaAs. Therefore, we chose the metals in order for both toreflect the optical pump but only one of them to exhibitmetallic behaviour in the THz region. We also chose thetwo metals in order to exhibit a high difference in workfunctions. Au was chosen as the high reflecting metal asit has a skin depth of 80 nm at 1 THz and was depositedat a thickness of 170 nm, Cr was used as an adhesive layerbut at a thickness (3 nm) that the Schottky potential isstill dominated by Au. The second chosen metal wasPb which possesses a high skin depth in the terahertz Au PbAu Pb
FIG. 3. An atomic force microscope image of the repeatableemitters fabricated with gold and lead. The lead layer hasoxidized with the air, forming a rough surface. region, 238 nm at 1 THz . Pb layers with thicknessbelow 24 nm are transparent at 1 THz , which we ver-ified in our THz-TDS experiment. Because we wantedPb to be opaque to the 800 nm pump laser we chose forthe fabrication to deposit a thickness of 100 nm. Eachmetal strip was 4 µ m wide with a 15 µ m period and a2 µ m overlap between the Au and Pb, as shown in Fig.3. Due to Bardeen surface states the barrier height ismostly independent of the work function of the metalbut is dependant on surface quality due to oxidizinglayers formed on the semiconductor surface. The barrierheight, φ b of gold bonded with GaAs is approximately0.9 eV , and lead is 0.8 eV at 300 K . This potentialdifference can create a net lateral carrier current. We alsofabricated the same emitters with a 150 nm SiO insu-lating layer to eliminate band bending from the metal in order to characterize the relative strength of the LPDand Schottky effects. The multiple double-metal emit-ters were tested in a standard THz-TDS experiment us-ing a Ti:Sapphire pump laser; centre wavelength 800 nm,80 MHz repetition rate with a 70 fs pulse length. A MenloTera-8 photo-conductive antenna was used as a receiverwith a focusing Si lens. The double-metal emitters weremounted without any focusing Si lenses. The emitterswere illuminated with the pump laser with an beam di-ameter of 1 mm, illuminating 67 emitters. This spot sizeallows the emitter to act as a planar wave rather than apoint source emitter for lens-free operation. An objectivelens and a neutral density filter allowed the intensity andspot size to be adjusted, allowing the optical fluence onthe multiple double-metal emitters to be varied.We characterized the Au/Pb emitters with fluence totest for saturation effects. We used a saturation fit of theform E ( F ) = A FF + F sat , (1)where F is the optical fluence and F sat is the opticalsaturation fluence, A is a coefficient that describes effi-ciency. More terahertz emission was observed for emit-ters directly fabricated on SI-GaAs compared with emit-ters fabricated on SI-GaAs with an insulating SiO layer. . . . . Fluence ( µ J / cm ) P e a k t o P e a k C u rr e n t ( p A ) (a) No SiO Layer . . . . Fluence ( µ J / cm ) P e a k t o P e a k C u rr e n t ( p A ) (b) SiO Layer
FIG. 4. (a) shows the fluence dependence of Au/Pb withoutan SiO layer and (b) with an insulating SiO layer. Theoptical spot radius is held at 430 µ m. The saturation fluence, F sat , (a) is fitted from equation 1 and determined to be 0.82 µ Jcm − , (b) shows no saturation. − − Time (ps) − − C u rr e n t ( p A ) (a) Au/PbAu/Pb SiO Frequency (THz) − − − − − S p e c t r a l P o w e r ( A . U . ) (b) Au/PbAu/Pb SiO FIG. 5. (a) time domain scans for the Au/Pb emitters insu-lated with SiO and Au/Pb uninsulated, (b) the correspond-ing power spectrum. Measurements taken at room tempera-ture. F sat was determined to be 0.82 µ J cm − for metal di-rectly fabricated on SI-GaAs, whereas no saturation wasobserved for double-metal structures fabricated with aninsulating layer. The value of A from the non-SiO emit-ters was 20 times greater compared with the SiO fabri-cated emitter, showing that most of the terahertz emis-sion from the double-metal emitters is from the Schottkyinterface. The cause of saturation within the directlybonded double-metal emitters is likely to be due to thecharge accumulation within the depletion region. Thedrift current of the electrons from the metal to the semi-conductor is reduced for higher fluences, at high carrierconcentrations the barrier height decreases due to the higher carrier density and higher Fermi energy in thesemiconductor . Temperature (K) P e a k T o P e a k C u rr e n t ( p A ) (a) No SiO Layer
Temperature (K) P e a k T o P e a k C u rr e n t ( p A ) (b) SiO Layer
FIG. 6. Temperature dependence of Au/Pb and Au/Pb insu-lated LPD emitters. (a) shows the temperature dependencefor the emitters without an SiO layer and (b) shows the tem-perature dependence for the emitters with an insulating SiO layer. The time domain scans and the corresponding powerspectrum for the Au/Pb insulated with SiO and Au/Pbuninsulated are shown in Fig. 5 for optical pump powerof 180 mW and spot diameter of 1 mm at room temper-ature. The results show that the most powerful emittersare Au/Pb fabricated directly of SI-GaAs, the Au/Pbinsulated emitters as mentioned above, is weaker work-ing only on the difference of quenching efficiency of Auand Pb stripes. Double-metal emitters demonstrate a de-pendence on the optical pump polarisation similar to theone encountered in , all the measurements presentedhere are made with a polarisation perpendicular to themetal edges. Multiple double-metal emitters have similarsignal to noise performance compared with large gap SI-GaAs photoconductive antennas measured in a terahertztime-domain spectroscopy (THz-TDS) experiment. Thedouble-metal emitters produce emission of 2 THz band-width with 33 dB of dynamic range as shown in Fig. 5similar to a SI-GaAs large gap photoconductive emitterbiased at 20 V with a 140 µ m electrode gap.We measured terahertz emission power with tempera-ture for both Au/Pb SiO and non-SiO fabricated emit-ters. Both emitters were mounted in a helium flow cryo-stat. Fig. 6 (a) and (b) show the peak-to-peak THz sig-nal with temperature for the non-SiO and SiO double-metal emitters respectively. The alignment of the emit-ters changed with the mounting in the cryostat, so thedetected emission viewed in Fig. 6 (a) and (b) is notstrictly comparable. The terahertz output power in-creases with decreasing temperature for the insulatedSiO emitter, with an optimum emission temperature at50-80 K, shown in Fig. 6 (b). This increase in outputpower is attributed to the increased electron mobility atlow temperatures , as fewer free carriers are present thefree electron path length is increased. This implies car-rier mobility is the main source for THz emission whenSchottky barrier emission is removed. Both emitters ex-hibit a decrease in THz output below 50 K which is inaccordance with the reduction in electron mobility inGaAs . In the case of the uninsulated emitters thereis a contribution from the Schottky barriers formed atthe Au/GaAs and Pb/GaAs interfaces as well as emis-sion due to diffusion. These emitters show reduced per-formance when the temperature is reduced below 200 K(Fig. 6 (a) ). This follows the temperature relationship ofthe Au/GaAs Schottky barrier height . Between 50 Kand 100 K the THz emission levels out and experiencesa small rise in power. This coincides with the temper-ature region in which carrier mobility reaches its peakvalues and Schottky barrier height approaches its mini-mum values .We have demonstrated robust, simple to fabricate tera-hertz emitters based on both the LPD effect and theSchottky barrier effect. The main source for emission atroom temperature is due to band bending from the Schot-tky barrier. We also show that the LPD effect plays a rolein the terahertz emission that becomes more apparent atlow temperatures of 50-80 K in accordance with increasedelectron mobility. We demonstrate effective terahertzemission but there is scope for improvement by choosingdifferent metal pair to optimize the amount of Schottkycurrents. However, because the Schottky barrier heightsare dependent on material and metal deposition it is dif-ficult to predict the strength of the emission without fab-ricating the emitters. The amount of LPD emission canalso be optimized further by choosing different metalsand thicknesses to enhance the quenching difference be-tween the chosen metal pairs. Due to their lack of voltagebias requirements double-metal terahertz emitters allow for passive terahertz generation and increased robustnessand may suit applications where wire-bonding would bedifficult.This work was supported by the EPSRC, grantsEP/G05536X/1 and EP/J007676/1.Copyright 2014 American Institute of Physics. Thisarticle may be downloaded for personal use only. Anyother use requires prior permission of the author and theAmerican Institute of Physics. The following article hasbeen accepted by Applied Physics Letters. After it ispublished, it will be found at http://scitation.aip.org/content/aip/journal/apl . G. Klatt, F. Hilser, W. Qiao, M. Beck, R. Gebs, A. Bartels,K. Huska, U. Lemmer, G. Bastian, M. B. Johnston, M. Fischer,J. Faist, and T. Dekorsy, Opt. Express , 4939 (2010). M. E. Barnes, D. McBryde, G. J. Daniell, G. Whitworth, A. L.Chung, A. H. Quarterman, K. G. Wilcox, A. Brewer, H. E. Beere,D. A. Ritchie, and V. Apostolopoulos, Opt. Express , 8898(2012). D. McBryde, M. E. Barnes, S. A. Berry, P. Gow, H. E. Beere,D. A. Ritchie, and V. Apostolopoulos, Opt. Express , 3234(2014). M. E. Barnes, S. A. Berry, P. Gow, D. McBryde, G. J. Daniell,H. E. Beere, D. A. Ritchie, and V. Apostolopoulos, Opt. Express , 16263 (2013). P. Gow, S. A. Berry, D. McBryde, M. E. Barnes, H. E. Beere,D. A. Ritchie, and V. Apostolopoulos, Appl. Phys. Lett. ,252101 (2013). G. Klatt, B. Surrer, D. Stephan, O. Schubert, M. Fischer,J. Faist, A. Leitenstorfer, R. Huber, and T. Dekorsy, Appl. Phys.Lett. , 021114 (2011). K. Drexhage, J. Lumin. , 693 (1970). X.-C. Zhang, B. B. Hu, J. T. Darrow, and D. H. Auston, Appl.Phys. Lett. , 1011 (1990). Y. Shi, Y. Yang, X. Xu, S. Ma, W. Yan, and L. Wang, Appl.Phys. Lett. , 161109 (2006). M. A. Ordal, R. J. Bell, R. W. Alexander, L. L. Long, and M. R.Querry, Appl. Opt. , 4493 (1985). M. A. Ordal, R. J. Bell, R. W. Alexander, L. A. Newquist, andM. R. Querry, Appl. Opt. , 1203 (1988). R. Singh, E. Smirnova, A. J. Taylor, J. F. O’Hara, and W. Zhang,Opt. Express , 6537 (2008). J. Bardeen, Phys. Rev. , 717 (1947). C. A. Mead and W. G. Spitzer, Phys. Rev. (1964). W. G. Spitzer and C. A. Mead, J. Appl. Phys. , 3061 (1963). J. R. Waldrop, Appl. Phys. Lett. , 1002 (1984). T. Nishimura, K. Kita, and A. Toriumi, Appl. Phys. Express ,051406 (2008). P. G. Huggard, C. J. Shaw, J. A. Cluff, and S. R. Andrews,Appl. Phys. Lett. , 2069 (1998). C. M. Wolfe, J. Appl. Phys. , 3088 (1970). M. Hudait, P. Venkateswarlu, and S. Krupanidhi, Solid. State.Electron.45