Fields modification Fields modification in high Al or In content III- -nitrides nitrides in high Al or In content III Junyong Kang ( 康俊勇 ) Engineering Research Center for Micro-Nano Optoelectronic Materials and Devices of State Education Ministry, Fujian Provincial Key Laboratory of Semiconductors and Applications Department of Physics, Xiamen University, Xiamen 361005, P. R. China (phone) +86-592-2185962 (e-mail) jykang@xmu.edu.cn
Outlines Outlines Research background Theoretical methods & experimental details Fields modification Control of crystal fields - Polarity in Polarity in AlN AlN - - Inhomogeneity Inhomogeneity in in InN InN - Compensation of anisotropy crystal field - Asymmetric ( Asymmetric (GaN)m/(AlN)n GaN)m/(AlN)n superlatices superlatices - Modification of internal electric field - - Mg Mg- - and Si and Si- codoped superlattices superlattices - codoped Modification of misfit stress field - Ultrathin - Ultrathin compressive strained compressive strained InN/GaN InN/GaN MQWs MQWs Conclusions
Research background Research background Applications Applications Light Emitting Diodes Blue, green, and white Laser Diodes Blue Photo Detectors Ultra Violet High Power and High Temperature Transistors Military, automobile and aircraft LEDs are most widely used devices
Research background Research background LED applications LED applications √ √ Most TVs are using III nitride LEDs as backlight Lighting is expected to grow quickly in the near future All of these are changing our lifestyle All of these are changing our lifestyle
Research background Research background Potentials of high Al or In content III nitrides Potentials of high Al or In content III nitrides If high Al content III nitride materials and devices are developed, our manner would be profoundly changed. Only a narrow range of III nitrides has If high In content materials can be grown well, III been used nitride devices would replace almost components made of other semiconductors.
Research background Research background Why high Al content III nitrides are so attractive ? Why high Al content III nitrides are so attractive ?
Research background Research background Potentials of high Al content III nitride Potentials of high Al content III nitride High Al content nitride is only one semiconductor system extending from UVA to UVC AlN Al x Ga 1-x N Irreplaceable Irreplaceable x role role GaN
Research background Research background Potentials of high In content III nitrides Potentials of high In content III nitrides High In content nitride is only one semiconductor system covering almost solar spectra The direct band gap from 0.7 to above 2.4 eV allowing multiple junction solar cell fabrication using one material system. (Such wide band gap is not available in other established material systems) High radiative efficiency even with high dislocation densities High mobility allowing good collection A large piezoelectric constant allowing control of surface recombination An existing industry centered around the nitrides
Research background Research background Problems Problems Fundamental and technologic problems Lower crystalline quality Strong misfit stress field Polar mixing Phase separation Lower recombination efficiency Strong polarization field Optical anisotropy Lower p- type conductivity Large thermal activation energy of acceptor in high Al content nitrides Close relation with the fields in III nitrides
Research background Research background Problems Problems Lower crystalline quality is caused by inhomogeneous crystal is caused by inhomogeneous crystal field field Lower crystalline quality In-rich dots Al Cross-sectional TEM images of InGaN layer a grown on a GaN surface. The inset shows In-rich dot regions. If people want to control the fields well (a) Cross-sectional HRTEM image of AlGaN layer at a inversion What happen during epitaxy epitaxy ? ? What happen during domain boundary. (b) and (c) How to grow homogeneously ? How to grow homogeneously ? inversion domain regions.
Research background Research background Problems Problems Lower recombination efficiency is caused by s is caused by strong polarization field trong polarization field Lower recombination efficiency Quantum confined Stark effect leading The effect can be deminished by to carrier separation in quantum well. fabricating QW on non-polar plane, but it is difficult to grow. People should establish the methods How to modify the polarization field polarization field ? ? How to modify the How to grow coherently under strong misfit stress field ? How to grow coherently under strong misfit stress field ? 11
Research background Research background Problems Problems Lower recombination efficiency is also caused by optical anisotropy is also caused by optical anisotropy Lower recombination efficiency Top of valence bands in Al x Ga 1-x N Al composition (%) 0 20 40 60 80 100 0.1 3.0 Heavy hole (HH) band 6 0.0 Crystal-field split hole （ x < 0.5 ） (CH) band 1 （ x > 0.5 ） E//c E c cr 2 -0.1 Favor for light extraction 1.5 Favor for light extraction along c axis vertical to c axis -0.2 (Ordinary light, E c ) 0.0 (Extraordinary light, E//c) 2 3 4 5 Energy (eV) People are interested to know Is it possible to change crystal field in high Al content nitride ? e ? Is it possible to change crystal field in high Al content nitrid How to realize optical isotropy to modify photon propagation ? How to realize optical isotropy to modify photon propagation ?
Research background Research background Problems Problems Lower p- -type conductivity type conductivity is caused by large thermal activation energy is caused by large thermal activation energy Lower p E A Conventional SL has be proposed to modify band bending so that part of Mg levels locate Large thermal activation energy of Mg above Fermi level, but the modification is acceptor in high Al content AlGaN. insufficient. People like to develop method How to further modify internal field ? How to further modify internal field ?
Theoretical designs & experimental details Theoretical designs & experimental details The first- -principles calculation principles calculation The first Based on density function theory Independence of experiential parameters Providing detailed information: atomic structure, wave function, charge density, potential, and energy Large system simulation: heterostructures, SLs, MQWs Band bending in the QWs Calculated projected PDOS of different atomic layers are arranged along [0001] direction.
Theoretical designs & experimental details Theoretical designs & experimental details Computers for theoretical designs Computers for theoretical designs Our group XMU Lenovo R515 HP Integrity Superdome 4 nodes: 8 nodes: each with 4 CPUs each with 32 CPUs & Dawning Tiankuo series 4 nodes: each with 4 CPUs & Lenovo DeepComp serie 12 nodes: each with 2 CPUs
Theoretical designs & experimental details Theoretical designs & experimental details Facility for epitaxy epitaxy growth growth Facility for Growth system: Thomas Swan MOVPE Precursors: TMG, TMI, TMA, NH 3 , Cp 2 Mg, and SiH 4 Thomas Swan 3 2 in. CCS
Theoretical designs & experimental details Theoretical designs & experimental details Facilities for characterizations Facilities for characterizations in situ nano-structural comprehensive property measurement system Functions SEM (spacial resolution 8.4 nm) CL (range 200-1000nm) STM & STS (atomic images) EL (carriers injection within structures smaller than 100nm) Temperature variation (in 6.6-1500K) Sample preparation
Theoretical designs & experimental details Theoretical designs & experimental details Facilities for characterizations Facilities for characterizations Bede QC 200 XRD Horiba Jobin Yvon UVISEL FUV Spectroscopic Ellipsometer Varian Cary 300 Accent HL5500 Hall system UV-visible spectrophotometer
Control of crystal field Control of crystal field Polarity in AlN AlN Polarity in High melting point High pressure by lower temperature epitaxy Polarity mixture Low crystalline quality Influence on crystal field Influence on crystal field
Control of crystal field Control of crystal field Polarity in AlN AlN Polarity in Monomer: AlN molecule Because of severe pre-reaction between TMA and NH 3
Control of crystal field Control of crystal field Polarity in AlN AlN Polarity in Ab initio calculation results Ab initio calculation results Al-polar surface N-polar surface Total Energy (eV) Total Energy (eV) E clean -364.385 E clean -359.599 E w1 -377.533 E w1 -371.857 E z1 -377.448 E z1 -369.838 E w2 -393.284 E w2 -380.676 E z2 -389.11 E z2 -382.512
Control of crystal field Control of crystal field Polarity in AlN AlN Polarity in Model for kinetic Monte Carlo simulation Barrier heights of different paths E w w w ，其中， exp( ) z z z E E n E n E 0 d i b j b k T B
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