Research On Internal Parameters Of GaN-based Laser Diodes

This dissertation focuses on the optical loss and carrier injection efficiency of GaN-based laser diodes and has achieved the following results:1.The optical loss of GaN-based laser diode epitaxial wafers and devices were studied.Firstly,a set of test system for measuring the optical loss of GaN-based laser diode epitaxial wafers was built.Through theoretical analysis,the test error of this system is found to be less than 1 cm-1.With the help of this test system,we studied the optical loss of epitaxial wafers in detail.It is found that the absorption of activated Mg acceptors is the main source of optical loss in GaN-based laser diode epitaxial wafer.Both simulation and experimental results show that reducing the overlap between the p-type layer and the optical field can effectively reduce the optical loss of the GaN-based laser diode.Secondly,we uesd Hakki-Paoli method to study the optical loss of GaN-based laser diode devices.We found that the optical loss of epitaxial wafer and the device are different.The origins for this difference were analyzed.2.The hole transport properties of c-plane GaN-based green laser diode were studied.In view of the controversy over hole transport properties of green laser diode,we conducted a series of simulation and experimental studies.The results show that holes can overflow from green double quantum wells,which reduces the carrier injection efficiency of green laser diode.A silicon-doped layer below the green double quantum wells can effectively suppress hole overflow from the green double quantum wells.The physical mechanism is that silicon doping increases the potential barrier for hole transport.3.The carrier recombination and suppression methods in InGaN waveguide layer of green laser diode were studied.Through simulation calculations,it is found that there is a potential barrier between the last GaN quantum harrier layer and the InGaN upper waveguide layer.This potential barrier impedes carrier transport and carriers are confined in InGaN upper waveguide layer.As a result,carrier recombination occurs in InGaN upper waveguide layer and the carrier injection efficiency of green laser diode is reduced.Carrier recombination in upper waveguide layer is non-negligible when the indium content of InGaN waveguide layer exceeds 4%.A suitable reduction of indium content in InGaN upper waveguide layer can effectively suppress carrier recombination in upper waveguide layer and reduce the threshold current of the green laser diode.The experimental results agree well with the simulation results.4.The waveguide design of GaN-based blue laser diode was studied.Firstly,it is found that inserting an additional GaN upper(lower)waveguide layer between InGaN upper(lower)waveguide layer and AIGaN upper(lower)cladding layer will reduce the optical loss of blue laser diode.Secondly,as the thickness of inserted waveguide layer increases and the background carrier concentration of upper waveguide layer increases,the carrier recombination in upper waveguide layer and the electron overflow from electron blocking layer is increased,thereby the carrier injection efficiency of blue laser diode is reduced.Finally,the use of InGaN with gradient indium content as upper waveguide layer can effectively suppress carrier recombination in upper waveguide layer and electron overflow from electron blocking layer,thus improve the carrier injection efficiency of blue laser diode.5.The effects of total gas flow and growth temperature on AIGaN growth by MOCVD were studied.Firstly,under the premise that the pressure of reaction chamber and the reactant concentration remain unchanged,with the increase of total gas flow,the gas flow rate in the reaction chamber and MOCVD growth rate are both accelerated.Secondly,under the premise of unchanged AlGaN growth rate,with the increase of total gas flow and the decrease of growth temperature,the concentration of H atoms produced by the decomposition of NH3 is reduced and the concentration of carbon incorporated in AlGaN layer is increased.As a result,the intensity of AIGaN band edge luminescence is reduced and the intensity of yellow luminescence is increased.