Investigation of 850 nm GaAs Vertical Cavity Surface Emitting Lasers

In the thesis, the optical modulation and on-off keying (OOK) open eye diagram of infrared 850 nm Vertical Cavity Surface Emitting Lasers (VCSELs) with low capacitance and thick benzocyclobutene (BCB) passivation layer are measured. We compare zinc-diffusion oxide-confined VCSEL with conventional oxide-confined VCSEL of the influence of mirror resistance within distributed Bragg reflector (DBR) by utilizing small-signal equivalent circuit model. The advantages of the optical communication and the background of VCSELs will be introduced. Then, we discuss the application of the 850 nm VCSEL and the research motivation. After understanding the fundamental physics of the diode laser, we propose different structures and process designs to improve optical modulation bandwidths. Moreover, we obtain the optimal structure that has low threshold current, high differential gain and high modulation bandwidths by simulating the active region structures. The analysis of the optical and electrical characteristics for the VCSELs is demonstrated by PICS3D from Crosslight Software, Inc., i.e. reflectivity of DBRs, electric fields, refractive index, gain spectrum and RF modulation. Based on 4×4 Luttinger-kohn Hamiltonian, we describe the valence-subband structure in barrier quantum well model. We optimize the process flow of VCSEL by using the concept of electrical transfer function from small-signal equivalent circuit model. We measure the optical and electrical dc characteristic of zinc-diffusion oxide-confined and conventional oxide-confined VCSEL with aperture diameter of 5 µm. Zinc-diffusion VCSELs have lower threshold current, lower differential resistance, higher slope efficiency and higher wall-plug efficiency due to impurity-induced disordering for the tailoring of bandgap and refraction by intermixing the layers of semiconductor heterostructures, creating undamaged homogeneous alloys of average composition. We analyze and measure the modulation ability of the zinc-diffusion and conventional VCSELs with same aperture diameter of 5 μm at 25℃. Then, we measure the eye diagram and bit error rate. At last, the small-signal equivalent circuit parameters of the devices with aperture diameter of 5 μm at bias 5×, 10× and 15×Ith mA are extracted. We analyze the modulation speed limitation by three aspects including thermal effect, damping effect and parasitic effect. Finally, we discuss how to improve the modulation speed by optimization of the layer structures and process flows.