Letter pubs.acs.org/NanoLett
Monolithically Integrated Microelectromechanical Systems for OnChip Strain Engineering of Quantum Dots Yang Zhang,† Yan Chen,† Michael Mietschke,‡ Long Zhang,§,∥ Feifei Yuan,‡ Stefan Abel,⊥ Ruben Hühne,‡ Kornelius Nielsch,‡ Jean Fompeyrine,⊥ Fei Ding,*,† and Oliver G. Schmidt†,# †
Institute for Integrative Nanosciences and ‡Institute for Metallic Materials, IFW Dresden, Helmholtzstraße 20, 01069 Dresden, Germany § Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 110016, Shenyang, China ∥ Institute for Complex Materials, IFW Dresden, Helmholtzstraße 20, 01069 Dresden, Germany ⊥ IBM Research GmbH, Säumerstraße 4, 8803 Rüschlikon, Switzerland # Material Systems for Nanoelectronics, Technische Universität Chemnitz, 09111 Chemnitz, Germany ABSTRACT: Elastic strain fields based on single crystal piezoelectric elements represent an effective way for engineering the quantum dot (QD) emission with unrivaled precision and technological relevance. However, pioneering researches in this direction were mainly based on bulk piezoelectric substrates, which prevent the development of chip-scale devices. Here, we present a monolithically integrated Microelectromechanical systems (MEMS) device with great potential for on-chip quantum photonic applications. High-quality epitaxial PMN−PT thin films have been grown on SrTiO3 buffered Si and show excellent piezoelectric responses. Dense arrays of MEMS with small footprints are then fabricated based on these films, forming an on-chip strain tuning platform. After transferring the QD-containing nanomembranes onto these MEMS, the nonclassical emissions (e.g., single photons) from single QDs can be engineered by the strain fields. We envision that the strain tunable QD sources on the individually addressable and monolithically integrated MEMS pave the way toward complex quantum photonic applications on chip. KEYWORDS: Epitaxial PMN−PT films, quantum dots, MEMS, single photon sources
A
electrical excitation pulses,10−14 and the fabrication of QD based photonic devices is compatible with mature CMOS technologies.15 Despite these significant advantages over conventional probabilistic SPDC or FWM sources, the use of semiconductor QDs faces great challenges when integrated onchip for quantum photonic circuits. First the random fluctuations in the emission properties of QDs, which are caused by the self-assembled growth nature, prevent any required quantum interference to occur between photons emitted by different QD sources.16 External perturbations such as magnetic,17 optical,18 and thermal fields19,20 have been exploited to engineer the QD emission. However, most of them are not compatible with on-chip integration technologies. Electric field-induced quantum Stark tuning21−23 is also an important tuning knob, but it excludes the possibility for the electrical excitation of QDs and may introduce significant dephasing of the photon qubits. Elastic strain fields based on single crystal piezoelectric elements, normally lead zirconate titanate (PZT) or (1 − x)Pb(Mg1/3Nb2/3)O3−xPbTiO3 (PMN−PT), represent an
whole new realm in computation and communication is expected from quantum information sciences.1−3 However, several critical challenges prevent quantum information technologies from leaping out of the lab and culminating in any real-world applications. A most outstanding one is scalability.4 To date, advanced quantum information processing (QIP) experiments rely on a very limited number of quantum bits (qubits). The generation and manipulation of a large number of qubits, which are necessary for practical QIP applications, require a system with much increased complexity, size, and needs for physical resources. On-chip integrated quantum photonics, which encode and process qubits by using photons, offers an important opportunity for addressing this problem.5 Recently, great efforts have been devoted to the implementation of on-chip QIP platforms with nonclassical photon sources (e.g., single photons and entangled photon pairs) based on spontaneous parametric-down-conversion (SPDC) and fourwave-mixing (FWM) processes.6 A fully integrated quantum optoelectronic device could be realized by marrying these sources with chip-scale photonic circuits.7 III−V semiconductor quantum dots (QDs) represent another promising nonclassical photon source for on-chip integrated quantum photonics.8,9 Single and entangled photons from QDs can be triggered deterministically by either optical or © XXXX American Chemical Society
Received: June 20, 2016 Revised: August 25, 2016
A
DOI: 10.1021/acs.nanolett.6b02523 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 1. Monolithic MEMS devices for the strain engineering of QD-based quantum light sources. (a) Illustration of large scale on-chip integration of devices with small footprints. Each device is individually addressable and strain tunable. (b) Schematic of the cross section of a single device. Bottom electrode LSCO is wired out from the side of the substrate (not shown). (c) A false color SEM magnifying a single completed device. The center region is a bonded QD-containing nanomembrane (cyan) on Au electrode (yellow).
Figure 2. Structural characterization of epitaxial PMN−PT thin film on a SrTiO3-buffered Si substrate. (a) X-ray diffraction θ−2θ scans. (b) ϕ scan of the (101) PMN−PT and (202) Si diffraction peaks. (c) Low-magnitude dark-field (using (100) LSCO) cross-sectional TEM image. (d) Highresolution TEM image of PMN−PT and LSCO interface. The SiOx interfacial layer between the STO and Si was mainly formed under high oxygen partial pressure during the LSCO and PMN−PT deposition steps.
alternative for engineering QD emission with unrivaled precision and technological relevance. Two-photon interference experiments with strained QDs,24 electrically excited single photons with wavelength tunability,25 single photons from light-hole ground states,26,27 and tunable entangled photon emission have been realized with such a strain engineering.28,29 However, pioneering researches in this direction were mainly
based on bulk piezoelectric substrates with thicknesses of several hundreds of micrometers. This prevents the development of chip-scale devices, as the bulk piezoelectric actuators are operated at very high voltages (up to more than 1000 V), incompatible with CMOS technology, and have low integration density (i.e., only one QD per device can be tuned at one time).30,31 B
DOI: 10.1021/acs.nanolett.6b02523 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 3. (a) SEM image of an array of individually addressable PMN−PT microbridge MEMS. The yellow false color shows the Au electrodes. The cyan false color shows the QDs membranes. (b) Polarization hysteresis loop of the epitaxial PMN−PT device after the etching process measured at 10 kHz. (c,d) PFM measurement of a free-standing PMN−PT film: (c) PFM phase-voltage hysteresis loop. (d) Amplitude-voltage butterfly loop.
measurements, a 50 nm thick LaSr0.7Co0.3O3 (LSCO) layer was deposited as the bottom electrode prior to the PMN−PT deposition. The final layer sequence is PMN−PT (400 nm)/ LSCO (50 nm)/STO (4 nm)/Si (see Experimental Details for details). Cr/Au (5/100 nm) structures were then deposited by E-beam evaporation and patterned by photolithography. This layer plays a dual role in the MEMS; it acts as a stiff bonding layer for the subsequent III−V semiconductor membrane as well as the top electrode for the PMN−PT film. The four electrodes of one structure are all connected. The LSCO was connected out from the side of the substrate as the bottom electrode. The piezoelectric properties of PMN−PT films are essential to realize monolithically integrated MEMS devices. One of the typical challenges is the appearance of a parasitic pyrochlore phase instead of the desired perovskite structure in PMN−PT films, which is a main factor for the degradation of the piezoelectric properties of PMN−PT films.40 X-ray θ-2θ scans of the films in this work display (00l) peaks, revealing that the LSCO layer and the perovskite PMN−PT phase are fully c-axis oriented (Figure 2a). Using optimized deposition conditions and thanks to the perovskite STO buffer, the formation of the undesirable pyrochlore phase is inhibited and the nucleation of the perovskite PMN−PT is favored.41,42 PMN−PT should exist in a single crystalline form rather than in a polycrystalline phase in order to achieve giant piezoelectric strain. The STO unit cell (bulk lattice constant a = 3.905 Å) is rotated 45° around the surface normal of Si (a = 5.431 Å) to reduce the lattice mismatch between these layers. The ϕ scans indicate that the PMN−PT has been grown with a similar 45° rotation with respect to the Si unit cell. Therefore, the epitaxial relationship between the oxide layers and the substrate is PMN−PT [100] (001)//LSCO[100](001)//STO[100](001)//Si[110](001) (Figure 2b). Furthermore, the microstructure of the grown
Microelectromechanical systems (MEMS) incorporating piezoelectric thin films can overcome the aforementioned obstacles. Very large strain fields can be obtained with a small voltage in a linear fashion. The past few years have witnessed significant advances in the growth of high-quality piezoelectric thin films, as well as in their integration into MEMS.32 Owing to the giant piezoelectric response, relaxor ferroelectric PMN− PT can be used in active transduction devices for a broad range of applications such as ultrasonic medication, 33 energy harvesting,34,35 memory,36,37 and CMOS logic.38 The breakthroughs in synthesizing high-quality epitaxial PMN−PT films on Si and in related micropatterning techniques enable the realization of monolithic MEMS devices with dramatically reduced operation voltages and dimensions. Here, we present a unique combination of the monolithically integrated MEMS and the semiconductor QD-based quantum light sources, which could provide fresh opportunities for onchip quantum photonic applications. PMN−PT thin films are epitaxially grown on SrTiO3 (STO) buffered Si substrates. The direct epitaxial growth reduces the thickness of the PMN−PT by up to 3 orders of magnitude and facilitates much easier integration and device processing on a single Si chip. Additionally, micropatterning techniques are developed to realize scalable arrays of PMN−PT actuators. Arrays of In(Ga)As/GaAs QD-containing semiconductor nanomembranes are then transferred onto the PMN−PT MEMS (see Figure 1). As a proof-of-concept experiment the single photon emission from a single III−V QD is tuned by these Si-based MEMS and set into resonance with that of another reference QD. To enable high-quality epitaxial growth of 400 nm thick PMN−PT films by using pulsed laser deposition (PLD), (001) Si substrates were first covered with an epitaxial 4 nm-thin STO buffer layer by molecular beam epitaxy.39 For the electrical C
DOI: 10.1021/acs.nanolett.6b02523 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 4. Strain tuning of QD emissions based on the monolithic MEMS devices. (a) Optical measurement setup. NBS, nonpolarization beam splitter; LPF, long pass filter; Mono, monochromator. (b) A typical emission spectrum of a single QD. Inset shows sketch of QDs membrane. (c) Second order autocorrelation measurement. The dip at zero time delay reveals the single photon emission nature of the device. (d) The color-coded microphotoluminescence of two different QDs. The abscissa indicates the voltage applied to the PMN−PT film. The ordinate indicates the energy of the emitted photons. The emission of QD1 is tuned linearly by around 0.4 meV and can be adjusted to match the emission of QD2.
PMN−PT film was investigated by transmission electron microscopy (TEM). Figure 2c shows a low-magnification dark-field cross-section TEM image of a PMN−PT/LSCO/ STO heterostructure on Si, revealing the smoothness and uniformity of each layer. The high-resolution TEM image in Figure 2d shows the atomically sharp interfaces between LSCO and PMN−PT. Both XRD and TEM results confirm the high quality epitaxial growth of PMN−PT films on Si. In general, the Si substrate limits the electric-field-induced lateral strain in the PMN−PT film due to the substrate clamping effect.43 Suspended bridges, which can exert in-plane biaxial stresses to the subsequently transferred QD nanomembranes, were therefore manufactured by a micropatterning technique similar to the fabrication of prototypical structures for electromechanical devices as cantilevers or bridges. Focused ion beam (FIB) milling was used to define arrays of trenches in the PMN−PT and oxide layers down to the Si (Figure 1). Through these trenches, wet etching with a 25% KOH solution was performed to remove the Si substrate beneath the PMN− PT/LSCO/STO heterostructures, suspending the PMN−PT microbridges. Figure 3a shows an SEM image of an array of such PMN−PT microbridges. No significant etching of the PMN−PT and oxide layers during the treatment by KOH was observed. The in-plane clamping of the film and the residual stress are reduced during the fabrication of the bridges, which results in an enhanced piezoelectric performance. The individually addressable MEMS design offers a greater design freedom for quantum photonic applications. In our work, a single PMN−PT actuator has a lateral dimension of a few hundreds of microns with a good possibility of being even
smaller. Therefore, a dense integration of many MEMS actuated photonic devices on one chip can be realized. The piezoelectric response of the PMN−PT microbridge on Si is investigated in the following. Figure 3b shows the roomtemperature (RT) polarization-electric field (P-E) hysteresis loop for a free-standing PMN−PT film on Si recorded at 10 kHz. The remnant polarization Pr is around 10 μC/cm2, which is comparable to the previously reported values for high quality PMN−PT films grown on STO.44 Piezoresponse force microscopy (PFM) is employed to examine the piezoelectric and ferroelectric properties of the films at the nanoscale. The PFM phase-voltage hysteresis loop and amplitude-voltage butterfly loop of processed PMN−PT films are shown in Figure 3c,d, respectively. As expected for a ferroelectric signal, an ∼180° phase difference between the two polarization states is observed in the PFM phase-information. The switching voltages in the phase signal coincide with the minima in the amplitude loop. The local coercive voltages are about +2.3 and −1.5 V, as indicated by the minima of the amplitude loop. PFM hysteresis loops in Figure 3 provide a strong evidence for the ferroelectric nature of the free-standing PMN−PT film. In our work, the GaAs nanomembranes containing selfassembled InAs QDs were grown on top of a 100 nm thick Al0.75Ga0.25As sacrificial layer by molecular beam epitaxy. The 50 μm × 50 μm sized nanomembranes were released from the substrate by selectively etching away the sacrificial layer. Then the nanomembranes were bonded onto the PMN−PT MEMS via a flip-chip process, and the single photon emission from QDs can be engineered by the strain fields. Figure 4a shows the optical setup for measuring the photoluminescence (PL). Microphotoluminescence (μ-PL) spectroscopy measurements D
DOI: 10.1021/acs.nanolett.6b02523 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters were performed at T = 5 K using a coldfinger continuous-flow cryostat. The samples were connected to the coldfinger of the cryostat via silver paint and excited by a 532 nm continuous wave (cw) laser. The single photon flux from the QDs was detected by a nitrogen-cooled charge-coupled device (CCD). A computer-controlled source meter was used to apply voltages to the PMN−PT MEMS. Photon correlation measurements were performed with a Hanbury Brown and Twiss setup.45 Figure 4b shows a typical spectrum of a single QD and the sketch of a QDs membrane in the inset. Figure 4c shows the normalized second-order correlation function g(2)(τ) of the excitonic emission of a single QD under nonresonant cw excitation. The value of g(2)(0) ≈ 0.18 clearly indicates the single photon emission from the QD. The nonvanishing g(2)(0) is mainly due to the background emissions and the finite timeresolution of the experimental setup. By varying the voltage applied to the PMN−PT MEMS, the exciton emission energy/ wavelength can be controlled in a very precise way. Figure 4d shows the strain-induced shift of the excitonic emission energy versus voltage applied to the PMN−PT MEMS. In the presence of external strain, the crystal volume of GaAs will be deformed, thus introducing a strain-induced Hamiltonian. The changes in this so-called Pikus-Bir Hamiltonian lead to the energy shift of each energy band and therefore create the emission shifts shown here. It also demonstrates that the emission lines from two separated QDs on different PMN−PT actuators can be tuned into resonance. This prototype demonstration is of great significance for on-chip quantum photonics experiments, especially if energy-coincidence and therefore quantum interferences between independent sources are required. The excitonic energy red shifts by around 0.4 meV when changing the voltage from −8 to 15 V and corresponds to a tuning gain of ∼0.02 meV/V. The monolithically integrated PMN−PT MEMS provide significant technological advantages over the conventional PMN−PT bulk material. The piezoelectric response of PMN−PT is determined by the applied electric field (with a unit of kV/cm); therefore reducing the PMN−PT thickness from several hundreds of μm (bulk) to a few μm (thin film) will lead to a much higher strain under the same applied voltage. This is evidenced by our previous works. In ref 28, a 15 μm thick PMN−PT film was used and the QD emission can be shifted with a tuning gain of 0.1 meV/V. This value is much higher than that by using a 300 μm thick bulk PMN−PT (e.g., 0.005 meV/V in ref 30). Here, the tuning gain by using a 400 nm thick PMN−PT film is about 0.02 meV/V. In the following, we briefly discuss the possible factors influencing the tuning gain. In this work the “stain-active” piezoelectric film is very thin while the “strain-passive” layers including the QD layer and electric contacts are relatively thick. The heavy load on the piezoelectric film restrains its deformation, leading to a reduced tuning gain. The inset of Figure 5 shows the finite element method (FEM) simulation of the biaxial strain distribution for a GaAs QD nanomembrane (300 nm) on a 200 nm thick Aucoated PMN−PT thin film (400 nm) with a bias voltage of 15 V. The stiffness coefficient, Young’s modulus and mass density of the PMN−PT single crystal material were used in the simulation. We then vary the PMN−PT thickness to investigate its influences on the tuning gain. The applied electric fields are the same for all film thicknesses. To simplify the simulation, the tuning gain is specified by strain (in the nanomembrane) per volt. When the PMN−PT film is very thin (
