Threshold control in VCSELs by proton implanted depth

Abstract

The proton implantation is one of key procedures to confine the current diffusion in vertical cavity surface emitting lasers (VCSELs), in which the proton implanted depth and profile are main parameters. Threshold characteristics of VCSELs with various proton implanted depths are studied after optical, electrical and thermal fields have been simulated self-consistently in three dimensions. It is found that for VCSELs with confinement radius of 2 m, increasing proton implanted depth can reduce the injected current threshold power and enhance the laser temperature in active region. Numerical results also indicate that there are optimal values for current aperture in proton implanted VCSELs. The minimum injected current threshold can be achieved in VCSELs with proton implantation near the active region and confinement radius of 1.5 m, while the VCSELs with proton implantation in the middle of p-type distributed Bragg reflectors (DBRs) and confinement radius of 2.5 m can realize the minimum temperature.VCSELs are widely used for optical communications and interconnections because of the grteatly improved far-field divergence angle [1][2][3][4] . Proton implantation and selective oxide have been used to confine the laser threshold current [5,6] . The proton implanted depth and profile are two main parameters to affect the threshold current in VCSELs. A high power VCSEL with the maximum continuous wave optical output of 46 mW at room temperature was reported [7] , and various studies on thermal fields were presented in Refs. [8][9][10][11][12][13]. However, the influence of proton implanted depth on threshold thermal fields and other characteristics of VCSELs were not given in these studies.The purpose of this work is to find the optimal threshold current in VCSELs for different implanted depths.The laser is sandwiched between a pair of p and n type DBRs consisting of 20 and 27 alternating quarter-wavelength layers of AlAs and Al 0.16 Ga 0.84 As, respectively. The active region is made up of three undoped In 0.2 Ga 0.8 As quantum wells (QWs) separated by the 100 Å GaAs layer. The thickness of QW is 80 Å. In our simulation, the device is fabricated by using the H + implantation for lateral current confinement, and the potential satisfies Poisson's equation in substrate, p type and n type DBRs. 2 2 2 2 , , 1 , z z r V r z r V r r z r V .(1)The injected current density iswhere is conductivity. The carrier density N diffusion in the active region can be written aswhere d is the thickness of active region, and D n is the diffusion constant. The optical transverse i-mode i (r) in VCSELs is governed by 0 ) ( ) ( 1 2 2 2 0 2 0 r r m k r r r r r i z i i .(4)The steady-state thermal conduction equation in the laser is written as, ( ) , ( 1 l 2 2 z r Q z z r T r z r T r r r i .