Transfer printing stacked nanomembrane lasers on silicon Hongjun Yang 1,3, Deyin Zhao 1, Santhad Chuwongin 1, Jung-Hun Seo 2, Weiquan Yang 1, Yichen Shuai 1, Jesper Berggren 4, Mattias Hammar 4, Zhenqiang Ma 2*, and Weidong Zhou 1* 1 Department of Electrical Engineering, NanoFAB Center, University of Texas at Arlington, TX 76019, USA 2 Department of Electrical and Computer Engineering, University of Wisconsin-Madison, WI 53706, USA 3 Semerane, Inc., 202 E. Border St., Suite 149, Arlington, TX 76010, USA 4 KTH-Royal Institute of Technology, School of Information and Communication Technology, Electrum 229, 164 40 Kista, Sweden *Emails: mazq@engr.wisc.edu; wzhou@uta.edu Supplemental Information (SI) NATURE PHOTONICS www.nature.com/naturephotonics 1
Supplemental Information (SI) I. Fabrication process The MR-VCSEL devices were constructed on Si substrates with transfer-printing stacked multiple layers by sandwiching an InGaAsP QW active layer in between two single layer Si membrane reflectors (top and bottom Si-MRs). The fabrication process of MR-VCSEL is illustrated in Figure S1 schematically. High quality patterned photonic crystal Si reflectors were fabricated via e-beam lithography and reactive ion etching (RIE) process on SOI substrates purchased from Soitec, with 340 nm Si device layer and 2 µm buried oxide (BOX) SiO 2 layer. A low-index PECVD SiO 2 layer was then deposited on the top of the patterned Si to form the bottom MR. The PDMS stamp based transfer printing process is used and optimized for inking and printing patterned photonic crystal Si NMs with high hole-to-neck ratios. The PDMS stamp is made of SYLGARD 184 silicone elastomer with an optimized mixing ratio between lot-matched base and curing agent. After thoroughly mixing the base and the curing agents, the air introduced during the mixing step is reduced by gentle agitation. After pouring the mixture into a pre-cleaned mold container, a vacuum de-airing process is applied. A final PDMS stamp is then formed after a two-hour curing step in an oven at about 70 o C. Previous studies have been carried out in detail to understand the kinetics of peeling an elastomer layer (PDMS stamp) off a rigid substrate (NMs). 1,2 Employing the PDMS stamp based transfer printing process, various types of crystalline semiconductor materials have been successfully transferred onto foreign substrates. 3-6 Here we used our PDMS stamps to transfer InGaAsP QW active layer and disks to the SiO 2 layer coated on top of the bottom Si MR. The top MR was patterned and released from SOI substrates. 2 NATURE PHOTONICS www.nature.com/naturephotonics
SUPPLEMENTARY INFORMATION Using the PDMS stamp printing transfer process, the top MR was transfer printed to a transparent glass substrate. The top Si MR attached to the glass substrate was then transferred to the top of the InGaAsP QW disks to complete the MR-VCSEL device fabrication. Both the top MR and the bottom MR are in the form of a single piece, while the InGaAsP disks are separated from each other, forming an array of disks. Note: the transfer printing method that was employed in this work is readily scalable to full wafer size. Figure S1 MR-VCSEL fabrication based on nanomembrane (NM) PDMS stamp printing process. a, Schematic illustration of multi-layer PDMS transfer printing process for the formation of a MR-VCSEL array. b, Schematic of a complete, individual MR-VCSEL device. II. MR-VCSEL Cavity Design In order to achieve lasing with a low threshold, it is highly desirable to design MR- VCSEL cavity with the following characteristics: (a) The cavity mode should spectrally match well with that of the QW peak; (b) The cavity mode should have an optimal field distribution for spatial matching with the QW active region; (c) The cavity mode optical confinement factor should be maximized to enable low gain threshold requirements; and NATURE PHOTONICS www.nature.com/naturephotonics 3
(d) The cavity should be designed such that the waveguide mode should be spectrally separated from the cavity mode. Figure S2 Characteristics of the designed MR-VCSEL cavity. a, MR-VCSEL cavity structure configuration, where t 0 = 2 m, t 1 = t 5 = 340 nm, t 3 = 465 nm, t 2 = t 4 = 400 nm, and glass substrate t 6. b, Simulated reflection spectra for top (R t ) and bottom (R b ) MRs. c, Calculated cavity resonance mode based on cavity reflection (solid blue line) and phase resonant conditions (dashed red line). d, Field distribution of cavity mode (red), along with index profile in the cavity (blue). Figure S2(a) shows the schematic of a MR-VCSEL cavity under investigation. The cavity consists of six different layers (t 1 to t 6 ), sitting on top of a SOI substrate with SiO 2 BOX layer thickness of t 0. Using the InGaAsP QW cavity and the top and the bottom MRs fabricated from the Soitec SOI structure, the following parameters are fixed in our design: t 0 = 2 μm, t 1 = t 5 = 340 nm, and t 3 = 465 nm. The photonic crystal (PC) lattice 4 NATURE PHOTONICS www.nature.com/naturephotonics
SUPPLEMENTARY INFORMATION parameters of the top and the bottom MRs are optimized during the MR design. For the cavity case shown in Figure S2, the square lattice PC structures have a lattice constant a = 860 nm for both the top and the bottom MRs. However, different air hole radii are used, with r = 0.46a and r = 0.45a, for the top and the bottom MR, respectively. Considering the partial filling of oxide inside the air holes by PECVD, an effective index of n f = 1.2 was used here, which offers the best matching between the measured and the simulated reflection spectra. 7,8 The low index oxide buffer layer thicknesses t 2 and t 4 were optimized through our design process. A glass substrate was also incorporated in the design (layer t 6 ). Both infinite and finite structures were simulated using periodic boundary condition (PBC) and perfect matched layer condition (PML). Shown in Figure S2(b) are the demonstrations of the reflection characteristics of the top and bottom MRs, where high reflection (>98%) covers a wavelength range of 1420 to 1530 nm. Two different techniques were employed here to identify the cavity mode. The first one is the calculation of the reflection of the complete cavity structure. From the reflection dip located at the high reflection band range, we can find the cavity mode according to its resonant transmission property. The reflection of MR-VCSEL with t 2 = t 4 = 400 nm is plotted in Figure S2(c) (the blue line), which is calculated using Rigorous Coupled-Wave Analysis (RCWA) technique. The reflection dip is located at 1478 nm. To confirm this cavity mode, different method was employed based on the phase resonant condition (total phase change of one round-trip in cavity is equal to integer times of 2 ). The phase calculation details can be found in Ref. 9. After obtaining the reflection phase change ( ) of the top and the bottom MR, the resonant cavity mode can be decided. The NATURE PHOTONICS www.nature.com/naturephotonics 5
phase of the mode in the cavity is shown in Figure S2(c), which is plotted in the red dash dotted line. One can find that the mode located at 1478 nm has a 2 phase change. Finally, we investigated the properties of quality factor and field distribution of this cavity mode. By employing FDTD technique, a short temporal Gaussian pulse is used to excite the cavity mode. The quality factor of the cavity mode at 1478 nm is calculated to be 4,300. Then a longer temporal Gaussian pulse is used to excite only this cavity mode and the stable field is recorded after the source is turned off for a long time. The E-field of the standing wave distribution is demonstrated in Figure S2(d), where the cavity index profile is also plotted. The confinement factor is calculated to be Γ = 5.6% - 6%. This value is comparable to the confinement factor of conventional DBR-based VCSEL structures. Based on rate equation analysis and the field penetration characteristics of MRs, 9 the gain threshold for the MR-VCSEL is ~843 cm -1, assuming top and bottom MR reflectivities of 98.2% and 99.6%, respectively. III. MR-VCSEL Optical Characterization and Output Power Optically-pumped MR-VCSELs were characterized with a 320 mm focal length monochrometer based photoluminescence (PL) setup. The MR-VCSEL sample was mounted inside a cryostat with ambient temperature control from T = 10 K to T = 300 K. A continuous wave (c.w.) green laser (532 nm wavelength) was launched surface-normal onto the MR-VCSEL via a long working distance objective lens. Notice that the optical pump power was calibrated as the incident power onto MR-VCSEL, assuming the lasing beam size is similar or smaller than the MR-VCSEL lateral dimension (~100 m). The actual power absorbed by the lasing cavity may be smaller, considering the top mirror reflection, and the QW absorption layer thickness. The use of 532 nm c.w. laser as the 6 NATURE PHOTONICS www.nature.com/naturephotonics
SUPPLEMENTARY INFORMATION pumping laser source also induces excessive energy loss (heat) on MR-VCSEL cavity region, which leads to increased lasing threshold and reduced power efficiency. Light emitted from the MR-VCSEL was collected via the same long working distance objective lens and separated with a cold mirror (which passes 1550 nm infrared light and reflects 532 nm visible light). Light output spectra were collected for different pumping power levels at different ambient temperatures. The lasing peak power was taken as the output power. In order to estimate the output power levels, a 1550 nm commercial DFB laser source was placed at the sample location and the output of the DFB laser was launched into the PL setup, while keeping the same optical configuration as that of the MR-VCSEL light collection. Based on the correlation between the lock-in amplifier readout and the actual lasing power of the 1550 nm DFB laser, we estimated the output power of the MR-VCSEL by using the output of the lock-in amplifier. References 1 Meitl, M. et al. Transfer printing by kinetic control of adhesion to an elastomeric stamp. Nature Mater. 5, 33-38 (2005). 2 Rogers, J. A., Someya, T. & Huang, Y. Materials and mechanics for stretchable electronics. Science 327, 1603 (2010). 3 Yuan, H. C. & Ma, Z. Microwave thin-film transistors using Si nanomembranes on flexible polymer substrate. Appl. Phys. Lett. 89, 212105 (2006). 4 Zhou, W. et al. J. Phys. D. 42, 234007-234017 (2009). NATURE PHOTONICS www.nature.com/naturephotonics 7
5 Sun, L. et al. Flexible electronics: 12 GHz Thin Film Transistors on Transferrable Silicon Nanomembranes for High Performance Flexible Electronics (Small 22/2010). Small 6, 2473 (2010). 6 Yoon, J. et al. GaAs photovoltaics and optoelectronics using releasable multilayer epitaxial assemblies. Nature 465, 329-333 (2010). 7 Yang, H. et al. Resonance control of membrane reflectors with effective index engineering. Appl. Phys. Lett. 95, 023110 (2009). 8 Qiang, Z. et al. Design of Fano Broadband Reflectors on SOI. Photonics Technology Letters, IEEE 22, 1108-1110 (2010). 9 Zhao, D., Ma, Z. & Zhou, W. Field penetrations in photonic crystal Fano reflectors. Opt. Express 18, 14152-14158 (2010). 8 NATURE PHOTONICS www.nature.com/naturephotonics