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Introduction of Master's thesis of JihYuan Chang and Wen-Wei Lin Speaker:Meng-Lun Tsai National Changhua University of Education 2015/7/17 National Changhua University of Education 1 Electronic Current Overflow and Inhomogeneous Hole Distribution of the InGaN Quantum Well Structures Master's thesis of Jih-Yuan Chang 2015/7/17 National Changhua University of Education 2 Outline Introduction Electronic Current Overflow of the InGaN SQW Structures Inhomogeneous Hole Distribution of the InGaN Quantum Well Structures Conclusion 2003/3/31 National Changhua University of Education 3 Introduction The InGaN materials have important application in visible light-emitting diodes (LED) and short-wavelength laser diodes. In this work Jih-yuan investigate the electronic current overflow and the inhomogeneous hole distribution of the blue InGaN quantum well structures with a LASTIP (abbreviation of LASer Technology Integrated Program) simulation program. 2003/3/31 National Changhua University of Education 4 Reasons to electron current overflow There are several causes for the electron current overflow of III-V Nitrides： high threshold current narrow quantum well width small conduction band-offset poor hole injection to the active region These four causes have important influence on the degree of current overflow. 2003/3/31 National Changhua University of Education 5 Schematic diagram of the preliminary laser diode structure The reflectivities of the two end mirrors are 85 % and 90 % respectively. 2003/3/31 National Changhua University of Education 6 The current distribution and L-I curves for the preliminary structure 20 10 = 462 nm 8 15 Laser Power (mW) 2 Current Density (kA/cm) Overflow Current Active-Layer Current 10 5 0 0 6 4 2 5 10 15 20 0 0 40 80 2 T ot al Current Densit y (kA/cm) 120 160 200 240 Current (mA) The laser threshold current is 103.4 mA. 2003/3/31 National Changhua University of Education 7 2 Overflow Current Density (kA/cm ) Current distribution curves at various p-doping levels 20 p-doping：11017 cm-3 p-doping：31017 cm-3 p-doping：51017 cm-3 p-doping：71017 cm-3 p-doping：11018 cm-3 15 10 5 0 0 5 10 15 20 2 Total Current Density (kA/cm ) The higher the p-doping level, the lower the percentage of the electronic overflow current. 2003/3/31 National Changhua University of Education 8 L-I curves at various p-doping levels Laser Power (mW) 20 p-doping：11017 cm-3 p-doping：31017 cm-3 p-doping：51017 cm-3 p-doping：71017 cm-3 p-doping：11018 cm-3 15 10 5 0 0 20 40 60 80 100 120 Current (mA) The higher the p-doping level, the better the performance of InGaN laser diode. 2003/3/31 National Changhua University of Education 9 2 Overflow Current Density (kA/cm ) Current distribution curves at various Al mole fractions 20 Without p-AlGaN Layer p-AlGaN (Al : 5 %) p-AlGaN (Al : 10 %) p-AlGaN (Al : 15 %) p-AlGaN (Al : 20 %) p-AlGaN (Al : 25 %) p-AlGaN (Al : 30 %) 15 10 5 0 0 5 10 15 20 2 Total Current Density (kA/cm ) The higher the Al mole fraction, the lower the percentage of the electronic overflow current. 2003/3/31 National Changhua University of Education 10 L-I curves at various Al mole fractions Laser Power (mW) 20 Without p-AlGaN Layer p-AlGaN (Al : 5 %) p-AlGaN (Al : 10 %) p-AlGaN (Al : 15 %) p-AlGaN (Al : 20 %) p-AlGaN (Al : 25 %) p-AlGaN (Al : 30 %) 15 10 5 0 0 20 40 60 80 100 120 Current (mA) The higher Al mole fraction, the better performance of InGaN laser diode. 2003/3/31 National Changhua University of Education 11 The current distribution and L-I curves of the improved structure 20 20 15 Laser Power (mW) 2 Current Density (kA/cm) Overflow Current Active-Layer Current 10 5 0 0 5 10 15 20 2 T ot al Current Densit y (kA/cm) 15 10 5 0 0 20 40 60 80 100 120 Current (mA) The modified structure： Al mole fraction : 10 % p-doping level : 11018 cm-3 2003/3/31 National Changhua University of Education 12 Laser output power as a function of input electric power for the original and improved structures Output Power (mW) 30 25 20 Initial Structure Improved Structure 15 10 5 0 160 240 320 400 480 560 640 720 800 Electric Power (mW) The improved structure has a better power conversion efficiency. 2003/3/31 National Changhua University of Education 13 Threshold current as a function of temperature for the original and improved structures Threshold Current (mA) 240 200 I th I 0 e T T0 160 Initial Structure （ T0 = 63.40 K ） 120 Improved Structure （ T0 = 208.60 K ） 80 40 300 310 320 330 340 350 Temperature (K) The improved structure is more stable, especially for high-temperature operation. 2003/3/31 National Changhua University of Education 14 L-I curves of the InGaN laser structures of different quantum well numbers. 20 Double QWs Single QW Triple QWs Laser Power (mW) 15 10 5 0 0 20 40 60 80 Current (mA) 100 0 100 200 300 400 500 600 700 0 Current (mA) 500 1000 1500 2000 Current (mA) With the increase of the quantum well number, the performance of the InGaN laser diodes decreases. 2003/3/31 National Changhua University of Education 15 4 4 3 3 Energy (eV) Energy (eV) Energy band diagram of the triple-QW structure 2 2 1 1 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.19 0.2 Distance (m) 0.21 0.22 0.23 0.24 Distance (m) The quasi-Fermi level in the valance band is not quite continuous inhomogeneity of hole distribution among quantum wells. 2003/3/31 National Changhua University of Education 16 Carrier concentration distribution of the triple-QW structure 20 -3 Hole Concentration (log) (cm) -3 Electron Concentration (log) (cm) 20 15 10 5 0 15 10 5 0 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.16 0.18 Distance (m) 0.2 0.22 0.24 0.26 0.28 Distance (m) It is obvious that the hole distribution among quantum wells is quite inhomogeneous. 2003/3/31 National Changhua University of Education 17 6 23 1.2 1 0.8 0.6 0.4 0.2 0 0 8 -3 1.4 Stimulated Recombination Rate (10 cm /s) 28 -3 Radiative Recombination Rate (10 cm /s) Spontaneous and stimulated emission diagrams of the triple-QW structure 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 4 2 0 -2 -4 -6 -8 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 Distance (m) Distance (m) The right quantum well has the most spontaneous emission. The right quantum well is the only quantum well that possesses positive stimulated emission. 2003/3/31 National Changhua University of Education 18 2.5 -3 Stimulated Recombination Rate (10 cm /s) Carrier concentration and stimulated diagrams when the barriers have a p-doping level of 2.3 1019 cm-3 28 -3 Concentration (log) (cm ) 20 15 10 5 Hole Concentration Electron Concentration 0.18 0.19 0.2 0.21 0.22 0.23 Distance (m) 0.24 0.25 2 1.5 1 0.5 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 Distance (m) The distribution of hole concentration is much more homogeneous. All three quantum wells contribute to stimulated emission. 2003/3/31 National Changhua University of Education 19 L-I curves for various doping concentration of the barriers Laser Power (mW) 20 p-doping：2.31019 cm-3 p-doping：2.51019 cm-3 p-doping：2.71019 cm-3 p-doping：3.01019 cm-3 p-doping：3.31019 cm-3 15 10 5 0 0 20 40 60 80 100 120 Current (mA) When the doping level is at 31019 cm-3, the threshold current is 43.22 mA and the slope efficiency is 25.74%. 2003/3/31 National Changhua University of Education 20 L-I curves for SQW and Triple-QW structures Laser Power (mW) 20 15 Single QW (without p-barrier doping) Triple QWs (with p-barrier doping) 10 5 0 0 20 40 60 80 100 Current (mA) The triple-QW structure has better laser performance. 2003/3/31 National Changhua University of Education 21 Conclusion It is found that this electronic current overflow is severe in the single quantum well InGaN laser structure at room temperature, especially when the p-doping is low. Increasing the p-doping level and using an AlGaN stopper layer in the p-side can resolve this problem. In addition to the improvement of laser performance at room temperature, the improved InGaN laser structure has a higher characteristic temperature and hence is less sensitive to temperature. Jih-Yuan have also investigated the deterioration of the laser performance of the multiple quantum well InGaN lasers caused by the inhomogeneous distribution of the holes inside the active region. It happens due to the difficulty for the holes to transport from one quantum to another. JihYuan have proposed to p-dope the barriers between wells to help the holes to transport and thus help solve the problem of inhomogeneous hole distribution. 2003/3/31 National Changhua University of Education 22 Theoretical Investigation on Band Structure of the BAlGaInN Semiconductor Materials Master's thesis of Wen-Wei Lin 2015/7/17 National Changhua University of Education 23 Content The band-gap energy-gap bowing parameter of the wurtzite InGaN,AlGaN,AlInN alloys are investigated numerically with the CASTEP simulation program by Wen-Wei Lin 2003/3/31 National Changhua University of Education 24 Simulation for the WZ-InGaN In this simulation , Indium will be constricted between 0 and 0.375.And the lattice constants of the unstrained InGaN layer depend linearly on the indium composition. a(x) = 3.501 (x) + 3.162 (1-x) b(x) = 3.501 (x) + 3.162 (1-x) c(x) = 5.669 (x) + 5.142 (1-x) 2003/3/31 National Changhua University of Education 25 Band-Gap Energy (eV) WZ-InxGa1-xN band gap energy 3.6 3.4 3.2 3.0 2.8 2.6 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 Indium Composition, x Eg(x) = x · Eg,InN + (1-x) ·Eg,GaN - b · x · (1-x) To use this formula to fit the results, and obtain bowing parameter of 1.210 eV 2003/3/31 National Changhua University of Education 26 Simulation for the WZ-AlGaN Since AlGaN have large energy band gap , so it is usually used as barrier of active layer or DBR material. Since the energy band gap structure of the AlGaN is direct in the whole range of the aluminum composition, wen-wei study the characteristics of the AlGaN for the aluminum composition to be between zero and one. The lattice constants of the unstrained AlGaN layer depend linearly on the aluminum composition. a(x) = 3.082 (x) + 3.162 (1-x) b(x) = 3.082 (x) + 3.162 (1-x) c(x) = 4.948 (x) + 5.142 (1-x) 2003/3/31 National Changhua University of Education 27 WZ-AlxGa1-xN band gap energy Band-Gap Energy (eV) 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Aluminum Composition, x Eg(x) = x · Eg,AlN + (1-x) ·Eg,GaN - b · x · (1-x) To use this formula to fit the results, and obtain bowing parameter of 0.353 eV 2003/3/31 National Changhua University of Education 28 Simulation for the WZ-AlInN Compared to the InGaN and AlGaN alloys, the third ternary nitride alloy ,AlInN ,is less investigated.This alloys exhibits the largest variation in band gap and it is a candidate for lattice matched confinement layers in optical devices. Wen-wei study the characteristics of the AlInN for the aluminum composition to be between zero and one. The lattice constants of the unstrained AlGaN layer depend linearly on the aluminum composition. a(x) = 3.082 (x) + 3.501 (1-x) b(x) = 3.082 (x) + 3.501 (1-x) c(x) = 4.948 (x) + 5.669 (1-x) 2003/3/31 National Changhua University of Education 29 WZ-AlGaInN band gap energy Band-Gap Energy (eV) 7.0 6.0 5.0 4.0 InxGa1-xN AlxGa1-xN AlxIn1-xN 3.0 2.0 1.0 0 0.2 0.4 0.6 0.8 1 Composition, x Eg(x) = x · Eg,AlN + (1-x) ·Eg,InN - b · x · (1-x) To use this formula to fit the results, and obtain bowing parameter of 3.326 eV 2003/3/31 National Changhua University of Education 30 WZ-AlGaInN 177 AlN 6 207 5 248 4 310 3 GaN 413 2 1 InN Wavelength (nm) Band-Gap Energy (eV) 7 620 1240 3.1 3.2 3.3 3.4 3.5 Lattice Constant (Angstrom) Band-gap energy and corresponding wavelength of the InGaN,AlGaN, and AlInN as a function of the lattice constant 2003/3/31 National Changhua University of Education 31