Numerical Optimization of Single-Mode InGaAsP Vertical-Cavity Surface-Emitting Lasers

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1 Australian Journal of Basic and Applied Sciences, 5(11): , 2011 ISSN Nuerical Optiization of Single-Mode InGaAsP Vertical-Cavity Surface-Eitting Lasers 1 Saeid Marjani, 2 Mehdi Rahnaa and 3 Haid Marjani 1 Young researchers Club, Arak Branch, Islaic Azad University, Arak, Iran. 2 Departent of Electrical Engineering, Astaneh Ashrafieh Branch, Islaic Azad University, Astaneh Ashrafieh, Iran. 3 Departent of Electrical Engineering, Arak Branch, Islaic Azad University, Arak-Iran. Abstract: In the present work, we present a new structure of vertical-cavity surface-eitting laser (VCSEL). In this structure, InGaAsP is used as the active region which is sandwiched between GaAs/AlGaAs distributed bragg irror at the top of structure and GaAs/AlAs distributed bragg irror at the botto. This structure is designed to increase the power and decrease threshold current copared to previous results. In this paper, device characteristics such as light power versus electrical current and voltage versus current are siulated upon the different nuber of the top and botto DBR irror pairs. Keywords: Optiization, High Power, InGaAsP, VCSEL INTRODUCTION In recent years, the characteristics of vertical cavity surface eitting laser have iproved extreely (Iga, 2000). VCSELs are iportant light sources in optical counication systes where single ode, high output power and low threshold current are desirable for certain applications (Il-Sug Chung and J. Moerk, 2009; S.-Y. Hu, et al., 1998). Efficiency and high speed at low power in VCSEL with wavelength of n are a suitable light source for vast application in short-haul such as Gigabit Ethernet. TU Berlin researchers have also produced 1530 n VCSELs capable of 40Gbit/sec odulation, based on indiu phosphide technology (W. Hofann et al, 2011). To achieve the 35Gbit/sec perforance, the top irror design was optiized, and the anufacturing process iproved to give higher yields. The resulting chip has coplanar contact pads, low internal chip capacitance, reduced cavity length, and high single ode power (3.5 W with 5μ aperture). Used VCSELs at wavelength of 1500 n are fabricated with different technologies (A. Kari, et al., 2001; Y. Ohiso, 2002; A.V. Syrbu, et al., 1998; W. Yuen, et al., 2000; S. Nakagawa, et al., 2001; C.-K. Lin, et al., 2003; J.-H. Shin, et al., 2002; R. Shau, et al., 2001; N. Nishiyaa et al., 2003). VCSEL devices developed using the wafer fusion ethod have achieved continuous wave at wavelength of 1500 n at power, threshold voltage and current about 0.65 W,2.5 V and 1 A respectively (A. Kari, et al., 2001). Fundaental changes in iproveent of VCSEL cause increasing power and speed and decreasing dependence on teperature (J. Piprek, et al., 2004). In this paper, we eploy nuerical-based siulation software to assist in the device design and siulation (SILVACO, 2010). The paper is organized as follows: Section 2 briefly describes the nuerical odel. Sec. 3 provides the details of the new VCSEL structure, and Sec. 4 presents the obtained nuerical results. Finally, the conclusions provide coon guidelines for designing perforance of PhC VCSELS. Theory: In siulation VCSEL, we ust consider the electrical, optical and theral interaction during VCSEL perforance. Base of siulation is to solve Poisson and continuity equations for electrons and holes (SILVACO, 2010). Poisson's equation is defined by: (1) where ψ is electrostatic potential, ρ is local charge density and ε is local perittivity. The continuity equations of electron and hole are given by (Piprek, J., 2003): n 1 Gn Rn. J n (2) t q p 1 Gp Rp. J p (3) t q Corresponding Author: Saeid Marjani, Young researchers Club, Arak Branch, Islaic Azad University, Arak, Iran. E-ail: saeidarjani@yahoo.co 1207

2 where n and p are the electron and hole concentration, J n and J p are the electron and hole current densities, G n and G p are the generation rates for electrons and holes, R n and R p are the recobination rates and q is the agnitude of electron charge. The fundaental seiconductor equations (1)-(3) are solved self-consistently together with Helholtz and the photon rate equations. The applied technique for the solution of Helholtz equation is based on the iproved effective index odel (G.R. Hadley, 1995), which shows accuracy for the great portion of preliinary probles. This odel is very well adapted to siulation of VCSEL structures, and it is often called effective frequency ethod (Wenzel, H. Wunsche, H.-J, 1997). Two-diensional Helholtz equation is solved to deterine the transverse optical field profile, and it is given by (SILVACO, 2010): 2 0 Erz (,, ) (, rz,, ) Erz (,, ) 0 (4) 2 c where ω is the frequency, ε(r, z, φ, ω) is the coplex dielectric perittivity, E(r, z, φ) is the optical electric field and c is the speed of light in the vacuu. The light power equation relates electrical and optical odels. The photon rate equation is given by (SILVACO, 2010): ds (5) c 1 cl ( G ) S Rsp dt Neff ph N eff where S is the photon nuber, G is the odal gain, R sp is the odal spontaneous eission rate, L represents the losses in the laser, N eff is the group effective refractive index, ph is the odal photon lifetie and c is the speed of light in the vacuu. The refers to odal nuber. The odal gain is (SILVACO, 2010): G,., 2 g rz E rz rd drdz (6) where E (r, z) is the noralized optical field. And odal of spontaneous eission rate is (SILVACO, 2010): Rsp r (, ) sp r z rd drdz (7) The odal photon lifetie, ph, deterines losses in laser and it is given by (SILVACO, 2010): 1 c c a 0. fc ir G l (8) ph NEFF NEFF where α a is losses of bulk absorption, α fc is losses of free-carriers, α ir is losses of irrors and l is a paraeter of diensionless frequency. Eq.(1)-(8) provide an approach that can account for the utual dependence of electrical and optical. VCSEL Structure: Fig.1 shows scheatic 1550 n air-post VCSEL, which is used for siulation. In this structure, the substrate is ade of GaAs. The active region consists of six quantu wells where the well is 5.5 n In 0.76 Ga 0.24 As 0.82 P 0.18 and the barrier is 8 n In 0.48 Ga 0.52 As 0.82 P There is InP in both sides of this active region and on top of it, GaAs. The top irror is 30 layers of GaAs/Al 0.33 Ga 0.67 As with index of refraction of layers 3.38 and 3.05 respectively, and the botto irror has 28 layers of GaAs/AlAs with index of refraction of layers 3.38 and 2.89 respectively. Part of the top irror was etched in (Kandiah, K. et al., 2008), but in this proposed structured (Fig.1) the etched region continues to top of botto InP. Further details of the VCSEL and all aterial paraeters have been deterined during the experiental results (D.I. Babic et al., 1997). Results: In the present work, effect of changes in the nuber of top and botto bragg reflector on the device characteristics such as light power versus electrical current and voltage versus current is investigated. Change in this paraeter doesn t effect on cavity resonance, but it causes the change in reflectance loss. This loss eans the increase in absorbance, which happens in resonant wavelength of 1550 n. Maxiu reflectance is above 99.9%. Fig.2 shows power versus current curve for different quantities of upper and botto bragg reflector irror pairs. Increasing nuber of upper bragg reflector reduces slop of power versus current curve. Contrary to upper reflector irror pairs, increasing nuber of botto reflector irror pairs increase differential quantu efficiency. The threshold current is about 0.6 A, which is saller than the previous result (2.3 A) (Kandiah, 1208

3 K. et al., 2008). The calculated power at I=8A is 5.5 W, which is larger than the previous result (4 W) (Kandiah, K. et al., 2008). Fig. 1: Scheatic structure of the VCSEL device. Fig. 2: L-I characteristic for the different nuber of the top and botto DBR irror pairs. Fig.3 shows voltage versus current curve while the nuber of the top and botto reflector irror pair is changed. At high voltages, increasing nuber of the top reflector irror pairs cause increasing series resistance of structure that is calculated by the slop of I-V curve. Fig.3 show increasing nuber of the botto reflector irror pairs doesn t affect on the threshold current and V-I characteristic. Conclusion: In this work, a new VCSEL structure for application at wavelength of 1550 n is introduced. Its siulated power becae about 5.5 W at I=8A and threshold current was equal to 0.6 A. The effect of different values of the top and botto bragg reflector irror pairs on device characteristics were investigated. It was shown that increasing nuber top irror layer will decrease the threshold current but decrease the output power. 1209

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