Increased light emission by geometrical changes in Si LEDs V. Puliyankot 1,* , G. Piccolo 1 , R.J.E. Hueting 1 , A. Heringa 2 , A. Kovalgin 1 , and J. Schmitz 1 1 MESA + Institute for Nanotechnology, University of Twente, Enschede, The Netherlands 2 NXP Research, Eindhoven, The Netherlands * v.puliyankot@ewi.utwente.nl Abstract—This paper demonstrates increased light emission in Si p-i-n light emitting diodes (LEDs) by changing the geometry of the device. The theory behind this, the device fabrication, electrical and optical characteristics are also presented. Reducing the injector size, decreases the diffusion current as shown by IV measurements and simulations. As a result, for a particular on- current the pn-product and hence light increases inside the active region for the new devices. The electroluminescence (EL) intensity measurements show an enhanced light emission by more than a factor of four. I. I NTRODUCTION In recent years, there is much interest to ”siliconise” photonics [1]–[4]. An efficient light source, wave guide, detector and modulator could lead to merging of electronics and photonics in an integrated circuit (IC). Here we study Si light emitters, a possible building block of such a microphotonic chip. Due to the indirect band gap, for a charge carrier to recombine an additional phonon is required to conserve the momentum. As a result the non-radiative recombination is more dominant to the radiative one. Traditionally heterostructures are used to increase the light emission from a diode [5]. In contrast here we present increased light emission by simple changes in the device layout. In section II the basic theory is presented. The fabrication process is explained in section III. Electrical characteristics are shown in section IV and the optical measurements are discussed in section V. Finally, in section VI conclusions are drawn. II. THEORY Figure 1 (a), (b) shows the top view layout of the p-i-n diodes under study, device I and II respectively. The schematic cross section is shown in Fig. 1 (c). We vary the injector width W i with respect to the active region width W . The simulated band diagrams for device II is shown in Fig. 2, showing that for a forward bias of 0.3 V the full voltage appears across the active region. Consequently, for the pn-product in the active region holds: pn = n 2 i · exp  E FN - E FP u T  = n 2 i · exp  V D u T  , (1) where u T is the thermal voltage, V D is the applied voltage, E FN , E FP are the electron and hole quasi-Fermi levels and n i is the intrinsic carrier concentration. Fig. 1. A schematic drawing of Si light emitting diode. Figures (a) and (b) shows the top view layout of the devices I and II, resp., under study. The injector size W i is reduced in the new design. Figure (c) shows the cross sectional layout along X–X’axis of the device. 0 2 4 6 8 10 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 -40 -20 0 20 40 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 Energy (eV) Distance Y axis (μm) E C E FN E FP E V Energy (eV) Distance X axis (μm) E C E FN E FP E V Fig. 2. Simulated band diagram along the X–X’ axis of device II under a forward bias condition of 0.3V. E C and E V are the conduction and valence band energies. E FN and E FP are the quasi-Fermi levels of electron and hole respectively. The X axis origin is at the left side edge of the device. W =100 μm, W i =10 μm, L=5 μm, and an injector length of L * p = L * n = 3 μm were used for simulation. The full applied voltage appears across the active region. Note that the drop of the quasi-fermi levels in the injector region indicates the diffusion limits the current. On the inset, the band diagram along the Y–Y’ axis is shown. The Y axis origin is the midpoint of the active region. The full applied voltage is available along the whole Y axis. Fig. 3 shows the simulation results obtained from TCAD sim- ulations [6], applying Boltzmann approximation with Philips’ unified mobility model [7], doping-induced bandgap nar- rowing model [8], and conventional Si recombination mod- 287 P2.21 20:30 - 20:30 978-1-4244-8340-2/11/$26.00 ©2011 IEEE