Optical Hyperdoping: Transforming Semiconductor Band Structure for Solar Energy Harvesting 3G Solar Technologies Multidisciplinary Workshop MRS Spring Meeting San Francisco, CA, 5 April 2010
Michael P. Brenner Michael Brenner Alan Aspuru-Guzik Alán Aspuru-Guzik Cynthia Friend Cynthia M. Friend Eric Mazur Eric Mazur
Harvard NSF SOLAR team Aspuru group: Jacob Krich (PD) Justin Song (GS) Man Hong Yung (PD) Brenner group: Tobias Schneider (PD) Scott Norris (PD) Niall Mangan (GS) Friend group: Anne Co (PD) Stephen Jensen (GS) Mazur group: Meng-Ju Sher (GS) Yuting Lin (GS) Kasey Phillips (GS)
Introduction SF 6 Si irradiate with 100-fs 10 kj/m 2 pulses
Introduction black silicon
3 µm Introduction
Introduction absorptance (1 R int T int ) 1.0 0.8 absorptance 0.6 0.4 0.2 crystalline silicon 0 0 1 2 3 wavelength (µm)
Introduction absorptance (1 R int T int ) 1.0 0.8 black silicon absorptance 0.6 0.4 0.2 crystalline silicon 0 0 1 2 3 wavelength (µm)
Introduction
Introduction absorptance (1 R int T int ) absorptance 1.0 0.8 0.6 0.4 multiple relections black silicon 0.2 0 crystalline silicon 0 1 2 3 wavelength (µm)
Introduction band structure changes: defects and/or impurities
Introduction optical hyperdoping puts 2% of sulfur in 200-nm surface layer
Introduction
Introduction open questions how do the impurities get incorporated? can we use optical hyperdoping for solar cells?
Outline goals optimizing dopant profile intermediate band
Goals interesting properties due to intermediate band CB impurity band VB
Goals generalize CB Si impurity band TiO 2 ZnO VB
Goals process CB Si impurity band TiO 2 ZnO properties VB structure
Goals process dopant incorporation modeling laser doping electrospray dopant delivery CB Si impurity band ZnO TiO 2 atomistic modeling XPS, STM, SIMS properties VB carrier life-time modeling band structure calculations T-dependent Hall IR spectroscopy structure
Goals Mathematics process dopant incorporation modeling laser doping electrospray dopant delivery CB Si impurity band ZnO TiO 2 atomistic modeling XPS, STM, SIMS properties VB carrier life-time modeling band structure calculations T-dependent Hall IR spectroscopy structure
Goals Chemistry process dopant incorporation modeling laser doping electrospray dopant delivery CB Si impurity band ZnO TiO 2 atomistic modeling XPS, STM, SIMS properties VB carrier life-time modeling band structure calculations T-dependent Hall IR spectroscopy structure
Goals Materials Science process dopant incorporation modeling laser doping electrospray dopant delivery CB Si impurity band ZnO TiO 2 atomistic modeling XPS, STM, SIMS properties VB carrier life-time modeling band structure calculations T-dependent Hall IR spectroscopy structure
Outline goals optimizing dopant profile intermediate band
Optimizing dopant profile Theoretical agenda explain enhanced doping optimize dopant profile for device design design process for optimal dopant profile
Optimizing dopant profile why does enhanced doping occur? Laser pulse Sulfur x Liquid Solid h(t) physical mechanisms: melting and resolidification of thin layer sulfur diffusion into liquid silicon incorporation into solid during resolidification
Optimizing dopant profile why does enhanced doping occur? Laser pulse Sulfur x Liquid Solid h(t) mathematical model: calculate dynamics of two fields temperature T(x,t) sulfur concentration c(x,t)
Optimizing dopant profile calculation step 1: temperature profile set up by laser
Optimizing dopant profile calculation step 1: temperature profile set up by laser Superheated!
Optimizing dopant profile calculation step 2: solid melts, solute incorporated
Optimizing dopant profile calculation step 2: solid melts, solute incorporated melting: heat diffusion energy balance
Optimizing dopant profile calculation step 2: solid melts, solute incorporated melting: heat diffusion energy balance incorporation: solute diffusion mass balance
Optimizing dopant profile calculation step 2: solid melts, solute incorporated melting: heat diffusion energy balance incorporation: solute diffusion mass balance
Optimizing dopant profile boundary condition critically affects dopant profile Laser pulse Sulfur x Liquid Solid h(t)
Optimizing dopant profile two scenarios
Optimizing dopant profile two scenarios constant concentration constant sulfur flux
10 µm Optimizing dopant profile
10 µm Optimizing dopant profile
Optimizing dopant profile
Optimizing dopant profile
Optimizing dopant profile
Optimizing dopant profile
Optimizing dopant profile cross-sectional Transmission Electron Microscopy
M. Wall, F. Génin (LLNL) Optimizing dopant profile 1 µm
Optimizing dopant profile decouple ablation from melting
Optimizing dopant profile decouple ablation from melting
Optimizing dopant profile decouple ablation from melting undoped doped
Optimizing dopant profile decouple ablation from melting
Optimizing dopant profile secondary ion mass spectrometry 1.0 concentration (10 21 cm 3 ) 0.8 0.6 0.4 0.2 0 0 100 200 300 depth (nm)
Optimizing dopant profile 1.0 concentration (10 21 cm 3 ) 0.8 0.6 0.4 0.2 0 0 100 200 300 depth (nm)
Optimizing dopant profile appears to be closer to constant flux
Outline goals optimizing dopant profile intermediate band
Intermediate band what dopant states/bands cause IR absorption?
Intermediate band 1 part in 10 6 sulfur introduces donor states in gap CB 0.318 ev 0.188 ev 0.371 ev 0.11 ev 0.09 ev 0.08 ev 0.248 ev 0.614 ev VB Janzén et al., Phys. Rev. B 29, 1907 (1984)
Intermediate band absorptance (1 R int T int ) 1.0 16 8 4 wavelength (µm) 2 1 absorptance 0.5 crystalline Si 0 0 0.5 1.0 energy (ev) 1.5
Intermediate band 10 6 sulfur doping 1.0 16 8 4 wavelength (µm) 2 1 absorptance 0.5 sulfur impurity states Si band edge crystalline Si 0 0 0.5 1.0 energy (ev) 1.5
Intermediate band laser-doped S:Si 1.0 16 8 4 wavelength (µm) 2 1 laser-doped Si absorptance 0.5 sulfur impurity states Si band edge crystalline Si 0 0 0.5 1.0 energy (ev) 1.5
Intermediate band laser-doped S:Si 1.0 16 8 4 wavelength (µm) 2 1 laser-doped Si multiple relections absorptance 0.5 sulfur impurity states Si band edge crystalline Si 0 0 0.5 1.0 energy (ev) 1.5
Intermediate band laser-doped S:Si 1.0 16 8 4 wavelength (µm) 2 1 laser-doped Si absorptance 0.5 sulfur impurity band Si band edge crystalline Si 0 0 0.5 1.0 energy (ev) 1.5
Intermediate band isolate surface layer for Hall measurements device layer buried oxide silicon substrate
Intermediate band isolate surface layer for Hall measurements device layer buried oxide silicon substrate
Intermediate band isolate surface layer for Hall measurements laser device doped layer region buried oxide silicon substrate
Intermediate band isolate surface layer for Hall measurements buried oxide silicon substrate
Intermediate band isolate surface layer for Hall measurements buried oxide silicon substrate
Intermediate band
Intermediate band Hall measurements 100 temperature (K) 500 200 100 50 E d = 310 mev 10 n-doped 5000 -cm N = 10 17 cm 3 N/N room 1 p-doped 10 -cm 0.1 laser-doped 0.01 0 5 10 15 20 1/T (mk 1 )
Intermediate band transition to metallic behavior at high doping T (K) 10 16 300 100 50 20 N s (cm 2 ) 10 12 10 8 highly S doped Si Si substrate 10 4 0 10 20 30 40 50 1000/T 1 (K 1 )
Intermediate band can we understand this intermediate band using atomistic modeling?
Intermediate band density of states
Intermediate band density of states unoccupied, localized: eats electrons occupied, localized: eats holes
Intermediate band recombination rate
What s next? change dopant/substrate combination
What s next? optimal dopant profile is flat 1.0 concentration (10 21 cm 3 ) 0.8 0.6 0.4 0.2 0 0 100 200 300 depth (nm)
What s next? change incorporation process: electrospray pulse sequence design
Acknowledgments