Design of InGaAs/InP 1.55μm vertical cavity surface emitting lasers (VCSEL)

Design of InGaAs/InP 1.55μm vertical cavity surface emitting lasers (VCSEL) J.-M. Lamy, S. Boyer-Richard, C. Levallois, C. Paranthoën, H. Folliot, N. Chevalier, A. Le Corre, S. Loualiche UMR FOTON 6082 CNRS, INSA de Rennes, France Soline.richard@insa-rennes.fr 8 th International Conference on Numerical Simulation of Optoelectronic Devices, Nottingham, 4 th September 2008

Design of InGaAs/InP 1.55μm VCSELs I- Introduction and context II- Optical design of the VCSELs Electric field calculation Bragg mirrors III- Thermal analysis IV- Buried Tunnel Junction V- Conclusion

Fiber To The Home Ideal optical source Low cost 1.55µm=>compatible with long haul transmission High frequency modulation Tunable =>WDM

VCSEL Top mirror active region Bottom mirror Substrate 1~3 µm Advantages Surface Emitting Laser Device tested before packaging Array integration Output circular mode shape Micro-cavity Small active region low I th or P th Short length Wide FSR Drawbacks Output power Thermal dependence

Electrically pumped VCSEL CW 1.55 µm optically pumped VCSELs lattice-matched to InP with dielectric Bragg mirrors already demonstrated (J.M. Lamy et al., IPRM 08) Electrically pumped VCSEL designed and fabricated at FOTON laboratory, within a collaborative ANR project named lambda-access Bragg mirror a-si/a-sin x DBR Contact InP n+ InP p + InP n + InP n+ Q 1.4 n ++ (25 nm) Q 1.4 p ++ (25 nm) Q 1.18 p + (10 nm) Q 1.18 (30 nm) 6QWs Q 1.18 (30 nm) BTJ Active zone Bragg mirror active zone grown by MBE with 6 InGaAs QW on lattice-matched alloy In 0.8 Ga 0.2 As 0.435 P 0.565 (Q 1.18 ) Buried Tunnel Junction in strongly doped lattice matched alloy Q 1.4 (N D =N A =5.10 19 cm -3 )

Design of InGaAs/InP 1.55μm VCSELs I- Introduction and context II- Optical design of the VCSELs Electric field calculation Bragg mirrors III- Thermal analysis IV- Buried Tunnel Junction V- Conclusion

Optical simulation Optical simulation algorithm : 2 parts 4.5 4.0 Active zone Optical properties thickness, index, absorption Propagation matrices Electric field Reflectivity spectrum Active zone characterization QW energy levels Oscillator strength Gain Absorption Spontaneous emission Refractive index 3.5 3.0 2.5 2.0 1.5 1.0 0.5 Bragg mirror BTJ 0 1 2 3 4 5 6 7 Thickness (µm) Bragg mirror Electric field repartition in the structure Electric field intensity Monomode VCSEL structure around 1.55 µm

Bragg mirrors 2 types of Distributed Bragg Reflectors realized by magnetron sputtering : Bragg mirror Miroir de Bragg 66 periods paires a_sinx/a_si InP Standard DBR Au Bragg mirror Miroir de Bragg 3.5 periods paires a_sinx/a_si InP Hybrid DBR Simulation based on propagation matrices Same reflectivity (99.6 %) @ 1.55 µm in good agreement with FTIR results Total reflectivity of the VCSEL cavity : Free Spectral Range > 50 nm monomode VCSEL

Design of InGaAs/InP 1.55μm VCSELs I- Introduction and context II- Optical design of the VCSELs Electric field calculation Bragg mirrors III- Thermal analysis IV- Buried Tunnel Junction V- Conclusion

Thermal simulation VCSELs : Small active region DBR problem of heat dissipation optical and electrical VCSEL thermal 2D finite element simulation thermal resistance evaluation compared to experiment Emission wavelength (µm) 1.582 1.580 1.578 1.576 1.574 1.572 1.570 1.568 conventional mirror hybrid mirror R th = 2.2 K/mW 0.22nm/mW 0.29nm/mW R th = 2.9 K/mW Wavelenght shift as a function of pump power for optical VCSELs with standard or hybrid DBR. 1.566 10 20 30 40 50 Pump power (mw)

Thermal simulation Au Bragg mirror InP Active zone Electrically pumped VCSEL 2D thermal simulation. In Si 3 N 4 InP 10 mw Bragg mirror Thermal resistance (K/W) 2400 2300 2200 2100 2000 1900 1800 1700 1600 1500 0 1 2 3 4 5 6 7 InP thickness (µm) Electrical VCSEL thermal resistance as a function of InP thickness (BTJ Ø 5 µm) R Th = 2360 K/W for a 200 nm InP thickness VCSEL R Th = 2050 K/W (1 µm InP)

Design of InGaAs/InP 1.55μm VCSELs I- Introduction and context II- Optical design of the VCSELs Electric field calculation Bragg mirrors III- Thermal analysis IV- Buried Tunnel Junction V- Conclusion

Buried tunnel junction Objectives : localized current injection : electrical carrier confinement n-type contact, easier to realize and less resistive small threshold voltage and small serial resistance to limit self-heating Theoretical operation : I(V) characteristics 25 nm E cp I Direct : V d > V thres Classical diode V d = 0 E F Inverse : tunnel current V negative resistance : J tunnel when V d E cn E vn E vp tunnel diode : E cn < E F < E vp

Buried tunnel junction Objectives : localized current injection : electrical carrier confinement n-type contact, easier to realize and less resistive small threshold voltage and small serial resistance to limit self-heating Theoretical operation : I(V) characteristics I Direct : V d > V thres Classical diode V d = -0.5 V Inverse : tunnel current V negative resistance : J tunnel when V d J tunnel

Buried tunnel junction Objectives : localized current injection : electrical carrier confinement n-type contact, easier to realize and less resistive small threshold voltage and small serial resistance to limit self-heating Theoretical operation : I(V) characteristics I Direct : V d > V thres Classical diode V d = 0.2 V Inverse : tunnel current V negative resistance : J tunnel when V d No tunnel current V d <V thres_diode

Buried tunnel junction Objectives : localized current injection : electrical carrier confinement n-type contact, easier to realize and less resistive small threshold voltage and small serial resistance to limit self-heating Theoretical operation : I(V) characteristics I Direct : V d > V thres Classical diode V d = 0.5 V Inverse : tunnel current V negative resistance : J tunnel when V d V d >V thres_diode Direct classical diode

Buried tunnel junction 0.2 µm 0.15 µm 0.1 µm 10 nm 25nm 25nm 0.1 µm 0.3 µm Experimental study of the BTJ : P-type contact Co ac c cu a e: / u InGaAs p+ InP p+ InP p++ Q 1.18 p++ Q 1.4 p++ Q 1.4 n++ InP n++ InP n+ Substrat InP n n-type contact Tunnel junction Jonction tunnel J (A/cm 2 ) 600 400 200 0-200 -400 N = 5.10 19 cm -2 V thres = -36 mv R inverse = 2.4.10-4 Ω.cm 2-600 -0,2 0 0,2 0,4 0,6 0,8 1 Vd (V) I(V) characteristics

Buried tunnel junction 1D Schrödinger-Poisson simulation useful to : verify the tunnel effect in the reverse BTJ avoid current leakage in the reverse InP junction outside the BTJ 2 Energy (ev) 0-1 -2-3 -4-5 -6 outside the BTJ InP n InP n BTJ InP p+ 30 nm CB VB Active Zone Q 1.18 153 nm CB VB InP n Simulation of the band diagram of the VCSEL in vertical direction, inside and outside the BTJ I (ma) 1 0-1 -2-2 -1 0 1 2 3 4 V (V) 3944 BTJ:7µm 3944 without BTJ 4008 without BTJ 4008 BTJ:7µm I(V) characteristics of the VCSEL cavities (without DBR) 3944 : 15 nm InP p+ leakage current 4008 : InP p+ = 240 nm OK

Conclusion 3 steps of simulation to design electrically pumped VCSELs : Optical simulation epilayer structure and DBR reflectivity Thermal analysis thermal resistance and contact design Schrödinger Poisson 1D avoid leakage current around the BTJ towards an integrated model? First electrical VCSEL sample from FOTON laboratory measurement in progress 100µm