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Enhanced spontaneous emission in a photonic-crystal light-emitting diode

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Enhanced spontaneous emission in a photonic-crystal light-emitting diode
  Corresponding author: Marco Francardi, phone: +390641522242, e-mail :marco. 1 Enhanced spontaneous emission in a photonic crystallight-emitting diode  M. Francardi (1,4) *, L. Balet  (2,3) *, A. Gerardino (1)  , N. Chauvin (2)  , D. Bitauld  (2)  , L.H. Li (3)  , B. Alloing (3)  ,and A. Fiore (2) (1) Institute for Photonics and Nanotechnologies-CNR, via Cineto Romano 42, 00156 Roma, (Italy)(2) COBRA Research Institute, Eindhoven University of Technology, PO Box 513, 5600MBEindhoven, The Netherlands(3) Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Photonics and Quantum Electronics,CH-1015 Lausanne (Switzerland)(4) * These authors contributed equally to this work.ABSTRACT. We report direct evidence of enhanced spontaneous emission in a photonic crystal (PhC)light-emitting diode. The device consists of p-i-n heterojunction embedded in a suspended membrane,comprising a layer of self-assembled quantum dots. Current is injected laterally from the periphery tothe center of the PhC. A well-isolated emission peak at 1.3 µm from the PhC cavity mode is observed,and the enhancement of the spontaneous emission rate is clearly evidenced by time-resolvedelectroluminescence measurements, showing that our diode switches off in a time shorter than the bulk radiative and nonradiative lifetimes.  Corresponding author: Marco Francardi, phone: +390641522242, e-mail :marco. 2Spontaneous emission results from the coupling of a quantum emitter to the surroundingelectromagnetic environment. It therefore depends both on the emitter’s dipole moment, and on thedensity and properties of the available electromagnetic modes 1 . In particular, a wavelength-scale, low-loss cavity can provide an increase of spontaneous emission rate by a factor QV  ∝ (Q: quality factor, V:mode volume) over the free-space value. This effect can be used to change the emission rate and at thesame time efficiently funnel most of the spontaneously-emitted photons into a single, well-controlledoutput mode, resulting in high extraction efficiency. After the first demonstration of enhancedspontaneous emission from quantum dots (QDs) in monolithic micropillars 4 , several important stepshave been realized, such as the demonstration of inhibited spontaneous emission 5 and of coupling of single QDs to the cavity mode 6 . However, all these experiments have used optical pumping to excite theemitter. For practical applications, electrical pumping is needed. The implementation of electricalinjection in a low-loss, ultrasmall cavity poses tremendous fabrication and experimental challenges: Onone hand, electrical contacts must be integrated in a micrometer-scale device without significantlyincreasing the optical loss. On the other hand, the experimental demonstration and practical applicationof enhanced spontaneous emission requires an ultrafast (sub-ns) electrical probing, requiring a carefulcontrol of device parasitics. Indirect evidence of spontaneous emission control in electroluminescencehas been recently deduced from the static characteristics of metallic-coated nanolasers 9 and of electrically-contacted micropillars 10 . In this paper we report a high-frequency PhC LED structure whichallows the direct experimental measurement of cavity-enhanced spontaneous emission dynamics in alight-emitting diode. Furthermore, the emission wavelength is around 1.3 µ m, corresponding to atransmission window of optical fibers.Our device (fig. 1) consists of a membrane photonic crystal cavity containing a p-i-n heterojunction,where holes are injected from a top ring p-contact, and electrons from the sides of the mesa, usinghighly-doped GaAs contact layers to spread the current throughout the mesa to the center of the cavity.As compared to injection through a central post 12 , this approach allows an easier fabrication, and the  Corresponding author: Marco Francardi, phone: +390641522242, e-mail :marco. 3decoupling of the problems of electrical injection and optimisation of quality factor. Additionally, incontrast to previous approaches to PhC membrane LEDs 11,12 , we are able to electrically address a singlePhC cavity with low parasitic resistance and capacitance, allowing the fast electrical control needed forthe time-resolved measurement of the emission dynamics. Figure 1 . Sketch of the LED showing the p-i-n heterojunction suspended membrane.The heterostructure used to fabricate the LEDs is grown by molecular beam epitaxy (MBE) on a GaAssubstrate and consists of a 370 nm-thick GaAs/Al 0.2 Ga 0.8 As p-i-n junction on top of a 1500 nm-thick Al 0.7 Ga 0.3 As sacrificial layer. A single layer of low-density InAs QDs (5-7dots /  µ m 2 ) emitting at 1300nm at 5K is grown using a very low InAs deposition rate ≈ 2x10 -3 monolayers/s. 13 The fabricationprocess is based on several e-beam lithography steps and typical thin-film processes. We first define 8 µ m-diameter, 320 nm-deep circular mesas by wet etching down to the bottom n-contact layer. Then, aring-shaped n-contact is defined by lift-off of a 155nm-thick Ni/Ge/Au/Ni/Au multilayer that issubsequently annealed at 400°C for 30 min. A 200 nm-thick Si 3 N 4 layer is deposited by plasma-enhanced chemical vapor deposition (PECVD) to isolate the n and p contacts. The Si 3 N 4 is thenremoved from the n-contact and from the top of the mesa by selective reactive ion etching (RIE). By lift-off of a Cr/Au (thickness: 110 nm) bilayer we realize an annular p-contact on the top of the mesa. In thesame evaporation step, the top p-contact and the bottom n-contact are connected to ground-signal-ground coplanar electrodes on top of the Si 3 N 4 surface, for contacting with a high frequency probe.  Corresponding author: Marco Francardi, phone: +390641522242, e-mail :marco. 4Three tilted Cr/Au evaporations are performed in order to obtain a continuous film over the mesa lateraledge. Finally, an annular gold cover is evaporated around the entire mesa edge in order to filter out lightscattered by the mesa sidewalls. To integrate the PhC nanocavity on the LED, a 150 nm-thick SiO 2 layeris then deposited and the PhC pattern – aligned to the mesa - is defined on a 200 nm-thick resist andtransferred to the SiO 2 layer and then to the GaAs/AlGaAs membrane (SiCl 4  /O 2  /Ar based RIE). Finally,the bottom Al 0.7 Ga 0.3 As sacrificial layer is selectively removed using a diluted HF solution. A scanning-electron microscope (SEM) image of the LED at the end of the fabrication process is shown in Fig 2.The PhC nanocavities that have been integrated with the LED are L3 modified nanocavities 1,5 ,characterized by a triangular lattice with a fixed filling factor (f = 35%) and a variable lattice parametera ( from 292nm to 352nm with a step of 10 nm). Figure 2. Scanning Electron Micrograph of a 8 µm-diameter PhC LED. The p-contact is clearly visibleon the bottom side, whereas the n contact is shaded by the Si 3 N 4 insulating layer.The device testing was performed using a cryogenic probe station (T=5K) equipped with cooled 50 Ω  microwave probes. The LED current-voltage characteristic is shown in Fig. 3. A clear diodecharacteristics is observed even at low temperature, with forward-bias (reverse-bias) threshold voltagearound 2V (-7V), indicating very good electrical contacts.  Corresponding author: Marco Francardi, phone: +390641522242, e-mail :marco. 5 Figure 3. (left) IV characteristic of the device exhibiting diode operation. (right) Electroluminescence asa function of the injected current at the wavelength of the mode (red curve) and detuned from the mode(grey curve).Light emitted from the device was collected from the top using an external Cassegrain microscopeobjective (numerical aperture NA=0.4), and refocussed to a single-mode fiber, providing a 1.8- µ mdiameter collection spot size. The electroluminescence (EL) spectrum at I=1.25 mA is shown in Fig. 4(red line). Figure 4. Photoluminescence (blue, 750 nm, 80 MHz) and electroluminescence (red, 2.3 V dc, 40 MHz)spectra of a PhC L3 cavity with lattice constant a=341 nm (mesa diameter: 10 µ m). The quality factor of the mode is Q ≈ 4600 (inset).A clear cavity mode is observed around 1329 nm, superposed on a broad emission line correspondingto ground-state emission from the QD ensemble (due to the limited spatial resolution of the set-up,
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