IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 38, NO. 3, MARCH 2002 285 The Effect of Cladding Layer Thickness on Large Optical Cavity 650-nm Lasers Peter M. Smowton, John D. Thomson, M. Yin, Susan V. Dewar, Peter Blood, A. Catrina Bryce, John H. Marsh, C. J. Hamilton, and C. C. Button Abstract—The reduction in penetration of the optical mode into the cladding layers in large optical cavity (LOC) laser structures offers the possibility of reducing the cladding-layer thickness. This could be particularly beneficial in GaInP–AlGaInP high-power devices by reducing the thermal impedance and the electrical series resistance. We have designed and characterized 650-nm LOC lasers by modeling the optical loss due to incomplete confine- ment of the optical mode by the cladding layers and calculating the thermally activated leakage current. This indicated that the cladding thickness could be reduced to 0.5 m without adversely affecting performance. We investigated devices with 0.3-, 0.5-, and 1- m-wide cladding layers. The measured optical mode loss of the 0.3 - m-wide cladding device was 36.2 cm compared with 12.4 and 11.3 cm for the 0.5- and 1 - m-wide cladding samples, respectively. The threshold current densities of the 0.5- and 1.0- m devices were similar over the temperature range investigated (120–320 K), whereas the 0.3 - m devices had signifi- cantly higher threshold current density. We show that this can be attributed to the higher optical loss and increased leakage current through the thin cladding layer. The intrinsic gain characteristics were the same in all the devices, irrespective of the cladding-layer thickness. The measured thermal impedance of 2-mm-long devices was reduced from 30.7 to 22.3 K/W by reducing the cladding thickness from 1 to 0.5 m. Our results show that this can be achieved without detriment to the threshold characteristics. I. INTRODUCTION G aInP–AlGaInP high-power lasers operating in the wave- length range of 630–690 nm are required for a number of applications, including photodynamic therapy and read/write optical storage systems. Unfortunately p-doped AlGaInP has a large electrical resistivity due to incomplete activation of the dopant [1], low hole mobility [2], and a relatively poor thermal conductivity [3]. These properties exacerbate the thermal man- agement problems associated with high power lasers. However, the reduction in penetration of the optical mode into the cladding layers which occurs in large optical cavity (LOC) structures [4] Manuscript received August 14, 2001. This work was supported by the U.K. Engineering and Physical Sciences Research Council under Grant GR/L94000/01. P. M. Smowton, J. D. Thomson, S. V. Dewar, and P. Blood are with the Depart- ment of Physics and Astronomy, Cardiff University, Cardiff CF24 3YB, U.K. M. Yin was with the Department of Physics and Astronomy, Cardiff Uni- versity, Cardiff, CF24 3YB, U.K. He is now with the Electronic Engineering Laboratory, University of Kent at Canterbury, Canterbury, Kent CT2 7NT, U.K. A. C. Bryce and J. H. Marsh are with the Department of Electronic and Elec- trical Engineering, University of Glasgow, Glasgow GL12 8LT, U.K. C. J. Hamilton was with the Department of Electronic and Electrical Engi- neering, University of Glasgow, Rankine Building, Glasgow, GL12 8LT, U.K. He is now with Intense Photonics, Kelvin Campus, West of Scotland Science Park, Glasgow G20 0TH, U.K. C. C. Button was with the Department of Electronic and Electrical Engi- neering, University of Sheffield, Sheffield S1 3JD, U.K. He is now with Marconi Caswell Ltd., Northhamptonshire NN12 8EQ, U.K. Publisher Item Identifier S 0018-9197(02)01752-9. opens up the possibility of reducing the cladding-layer thick- ness, thereby reducing the electrical and thermal resistance. In addition to the need to maintain coupling of the optical mode to the quantum wells, there are two further factors which limit the extent to which the cladding thickness can be reduced. The first is the leakage of the mode into the absorbing GaAs contact layer and substrate, increasing the gain requirement and hence the intrinsic threshold current. The second is the thermally ac- tivated leakage current, which in GaInP-based lasers is due to drift and diffusion of electrons through the p-cladding layer. The leakage current increases as the cladding thickness is re- duced, increasing the threshold current, which could cause an increase in power dissipation and increased internal temperature rise, negating the benefits of the reduced thermal impedance. In this paper, we investigate the interplay of these effects by ana- lyzing the performance of LOC structures with three different values of cladding-layer thickness 0.3, 0.5, and 1 m. We begin by describing, in Section II, the design of the laser devices, including calculations of the effect of reduced cladding-layer thickness on optical mode loss and thermally activated leakage current. In Section III, we describe results of measurements of threshold current, optical mode loss, optical gain, and thermal impedance of devices with 0.3 -, 0.5-, and 1- m cladding thickness. These results show that a reduction in thermal impedance can be achieved with a cladding thickness of 0.5 m. However, although further reduction probably occurs for a thickness of 0.3 m, the threshold current of these devices is greatly increased due to increased optical mode loss and increased leakage current. II. LASER DESIGN The active region of the devices consisted of three 45 -Å wide, compressively strained, Ga In P quantum wells separated by 80 Å of (Al Ga ) In P and designed to emit at 650 nm. The rest of the waveguide core was also (Al Ga ) In P, and this was clad with layers of (Al Ga ) In P nominally doped 5 10 cm using Zn on the p-side and with 1 10 cm Si on the n-side. These layers were grown by MOCVD on a misorientated – GaAs substrate tilted 10 off the (100) toward [111] A to produce disordered material. The aim in designing a LOC structure is to spread the mode by using a wide waveguide core. Since the second guided mode is not coupled to the active region, the upper limit to the width of the core is set by the onset of the third mode. The laser structure was assumed to be infinite in the lateral and longitudinal direc- tions, resulting in a vertical multilayer slab waveguide problem. 0018–9197/02$17.00 © 2002 IEEE