Development of Electrically Driven Single-Photon Emitter at Optical Fiber Bands Toshiyuki Miyazawa 1 , Jun Tatebayashi 1 , Toshihiro Nakaoka 1 , Motomu Takatsu 1 , Satomi Ishida 1 , Satoshi Iwamoto 1,3,4 , Kazuya Takemoto 2 , Shinichi Hirose 2 , Tatsuya Usuki 2 , Naoki Yokoyama 2 and Yasuhiko Arakawa 1,3,4 1 Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan Phone: +81-3-5452-6291, E-mail: miyatosi@iis.u-tokyo.ac.jp 2 Fujitsu Laboratories Ltd., 10-1 Morinosato-Wakamiya, Atsugi, Kanagawa 243-0197, Japan 3 Nanoelectronics Collaborative Research Center, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan 4 Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan 1. Introduction Single-photon emitter (SPE) plays an important role in quantum key distribution (QKD). There are many candi- dates for SPEs. Especially, the quantum dot (QD) is one of the strongest ones, because of its quantum states that arise from three dimensional quantum confinements. Optically excited SPEs with QDs were demonstrated from visible to near infrared wavelength [1-5]. Electrically driven single-photon emitters (ED-SPE) with QDs are the final goal for SPE, because devices of QKD systems have to be small and controlled by electrical signals. Several groups focused on development of ED-SPEs [6-8]. The realization of QKD for telecommuni- cation needs operation at longer wavelength which is called optical fiber bands (λ > 1.26 µm). ED-SPE with the longest wavelength has barely reached the optical fiber band [9], though optically excited SPE had already been demon- strated at the C-band (1.55 µm) [5]. For practical use, ED-SPE needs much longer wavelength and other require- ments (for example: lower electric power, higher-speed operation etc.). In this paper, we report on an ED-SPE operating at an optical fiber band. We succeeded in fabricating small oh- mic contact area and observing the electroluminescence (EL) from single QD at the center of the O-band (1.32 µm). The small contact area reduced parallel current leak and heat generation. 2. Experiments and Results The InAs/GaAs QDs were grown by metal organic chemical vapor deposition on an n-type substrate. In order to tune the wavelength of photons emitted from QDs to the optical fiber wavelengths, InAs QDs were capped with In x Ga 1-x As strain-reducing layer (SRL) [10]. We used SRL with x = 0.17 for 1.3 µm EL at 7 K. The device consist of six epi-layers. (Fig. 1 (a)) The n-type layers are a GaAs buffer layer and an Al 0.1 Ga 0.9 As barrier layer with doping concentration of 1×10 18 cm -3 . The intrin- sic layers are a GaAs, an InAs QD layer, an In 0.17 Ga 0.83 As SRL, and a GaAs layer. The p+-type layer is a GaAs with doping concentration of 1×10 19 cm -3 . The QD density of this sample is about 8.0×10 9 QDs/cm 2 enume rated from the atomic force microscope image. (Fig. 1 (b)) Fig. 1 (a) Schematic illustration of the epi-layers. (b) Atomic force microscope image of the QD. In order to access single QDs, we fabricated nano-scale small apertures which had small ohmic contact area, on the top Au electrode. There were seven main steps in fabricat- ing the device which was consisted of sputtering SiO 2 , pat- terning by photolithography, etching an SiO 2 , etching p+-GaAs, evaporating SiO 2 , evaporating a Ti/Pt/Au elec- trode and removing the resist. In fabricating this device, only wet etching was used to minimize the damage that was caused in fabricating the device. The SEM images (Fig. 2 (a), (b)) show that the ohmic contact was limited only small area around an aperture (Fig. 2 (c)). Fig. 2 (a) SEM image of the device. Small ohmic contact area was realized around an aperture. (b) Cross sectional SEM image of the device. (c) Schematic illustration of the device. i-GaAs (100 nm) n-GaAs (300 nm) n-AlGaAs (300 nm) n-GaAs Substrate 1.0 0.8 0.6 0.4 0.2 0.0 y (µm) 1.0 0.8 0.6 0.4 0.2 0.0 x (µm) InAs QD with SRL i-GaAs (50 nm) p+-GaAs (50 nm) (a) (b) i-GaAs (100 nm) n-GaAs (300 nm) n-AlGaAs (300 nm) n-GaAs Substrate 1.0 0.8 0.6 0.4 0.2 0.0 y (µm) 1.0 0.8 0.6 0.4 0.2 0.0 x (µm) InAs QD with SRL i-GaAs (50 nm) p+-GaAs (50 nm) (a) (b) Extended Abstracts of the 2005 International Conference on Solid State Devices and Materials, Kobe, 2005, -404- G-3-3 pp.404-405