Red luminescence from Si quantum dots embedded in films grown with controlled stoichiometry Zhitao Kang, Brannon Arnold, Christopher Summers, Brent Wagner Georgia Institute of Technology, Atlanta, GA 30332 August 4 th, 2005
Outline Quantum phosphor : Thin film Si QDs Objectives: Developing Si QDs for SSL Experimental Thermal Evaporation Annealing Results and Discussion Stoichiometry and refractive index Structural analysis Photoluminescence properties Conclusions and Future Work 1 Aug.-4 Aug., 2005 San Diego 5 th International Conference on SSL 2
Quantum phosphor : Thin film Si QDs Bulk Si has a bandgap of ~1.1eV corresponding to a wavelength of ~1100nm Bandgap increases with decreasing size due to quantum confinement effect Emission throughout the visible can be obtained from thin film Si QDs by controlling their size Park et al., Appl. Phys Lett., 78, 2575 (2001) 1 Aug.-4 Aug., 2005 San Diego 5 th International Conference on SSL 3
Objectives: Si QDs for Solid State Lighting Current white LED is fabricated by coating down-converting phosphors onto a III-Nitride LED Si Quantum Dots Like to replace powder phosphor with Si-like microelectronic technology compatible with LED manufacturing process InGaN UV LED Ultraviolet to Red, Green, Blue (White) Conversion Develop thin-film deposition techniques to obtain luminescent Si QDs using controlled stoichiometry materials Low-cost, efficient wavelength converters for LEDs Utilize quantum dot/thin film matrix as down conversion photoluminescent emitter to obtain white light with a controlled spectrum Provide a continuous, efficient, less expensive fabrication process to make advanced SSL devices 1 Aug.-4 Aug., 2005 San Diego 5 th International Conference on SSL 4
Integration of QD Thin Films with LEDs QD thin films become part of the substrate upon which LEDs are deposited Can be in normal or flip chip configurations Can utilize thin film optical structures, such as bragger reflectors, microcavity, antireflection coatings etc., to adjust light output. 1 Aug.-4 Aug., 2005 San Diego 5 th International Conference on SSL 5
Synthesis of Si QDs films by evaporation Thermal or E-beam Evaporation to deposit N y films Variables: Deposition in commercial Ion Assisted Deposition system Source materials: Si, SiO, SiO 2 Controlled reactive gas environment (O 2, N 2, H 2 ) Deposition rate (0.1~2.5nm/s) Thickness (single or multi-layer) Deposition temperature (50 C~300 C) Plasma parameters Investigate deposition processes to produce N y layers with controlled stoichiometry, structure and density 1 Aug.-4 Aug., 2005 San Diego 5 th International Conference on SSL 6
Annealing Process to Produce Si QDs Before Annealing (Single Layer) After Annealing N y Si+SiON N y Si (Multilayer) N y SiO 2 N y SiO 2 N y Si Quantum Dots (1-10nm) Thin layer(s) (several nm) Si Si Annealing films in inert (N 2 ) atmosphere converts N y to SiON with excess Si segregating to form Si QDs QD size controlled by annealing temperature (500-1100 C), N y stoichiometry and layer thickness (1-10nm) 1 Aug.-4 Aug., 2005 San Diego 5 th International Conference on SSL 7
Stoichiometry of as-deposited films thin films prepared by reactive evaporation of SiO in O 2 Sample O 2 flow (sccm) SiO Rate (nm/s) PO 2 +H 2 O (mbar) Thickness (nm) Index x (by EDS) 66 0 2.5 ~ 3 10-6 249.3 2.003 ~1.0 21 0 0.2 ~ 3 10-6 279.7 1.747 1.30 55 5 0.2 2.9 10-5 291.8 1.716 1.45 19 10 0.2 5.5 10-5 319.9 1.648 1.51 59 15 0.2 7.5 10-5 322.1 1.638 1.64 77 20 0.2 1.1 10-4 306.9 1.586 1.80 83 25 0.2 1.4 10-4 337.3 1.513 1.91 By controlling the O 2 flow rate and SiO deposition rate, refractive index can be adjusted between 2~1.5, and x between 1~2 1 Aug.-4 Aug., 2005 San Diego 5 th International Conference on SSL 8
SEM Images of Annealed Films As-deposited 100 nm 100 nm 100 nm film (66) Annealed at 1100 C Deposition rate: 2.5nm/s x 1 and n 2.0 ~20nm Si particles observed film (21) Annealed at 1100 C Deposition rate: 0.2nm/s x 1.3 and n 1.75 <~5nm Si dots observed 1 Aug.-4 Aug., 2005 San Diego 5 th International Conference on SSL 9
HRTEM Images of Annealed Films x = 1.3 20 nm x = 1.3 ~4nm Si crystallized dots were observed from sample with x 1.3 5 nm x = 1.5 5 nm x = 1.8 5 nm Size of Si dots decreases to ~3nm and ~1.5nm with increasing O/Si ratio to 1.5 and 1.8, respectively 1 Aug.-4 Aug., 2005 San Diego 5 th International Conference on SSL 10
Surface composition and structure SiO 2 Si XPS spectra of the films after annealed at 1100 C for 1h S3 S2 S1 S0 Si 108 106 104 102 100 98 96 Binding Energy (ev) 99.7eV peak due to crystalline silicon Peak at 103.4 ev detected from the film samples due to SiO 2 phase; Low energy shoulder attributed to elemental Si, indicating the existence of Si QDs in films. With increasing oxygen content in the films, the intensity of the Si shoulder decreases. 1 Aug.-4 Aug., 2005 San Diego 5 th International Conference on SSL 11
PL and PLE vs O 2 flow rate (Red) PL Intensity (a.u.) 1.3x10 4 1.0x10 4 7.5x10 3 5.0x10 3 2.5x10 3 300nm excitation @ 300nm R.T. exicitation; measured at RT. S6 S5 S3 S4 S2 0.0 600 700 800 900 Wavelength (nm) O2 flow rates (sccm) S1: 0 S2: 5 S3: 10 S4: 15 S5: 20 S6: 25 S1 Normalized Intensity (a.u.) 2.8 2.4 2.0 1.6 1.2 0.8 0.4 S6 S5 S4 S3 S2 S1 Monitored at peak emission; Measured at room temperature. 0.0 200 300 400 500 600 700 Wavelength (nm) Visible red luminescence from films annealed at 1100 C for 1h (300nm excitation at room temperature) Intense red PL observed from all the samples with n = 1.75 ~1.50 Blue shift with increasing O 2 flow rate due to smaller Si QDs Intensity peaked at O 2 flow rate of 10 sccm Two PLE peaks centered at 280nm and 370nm, respectively 1 Aug.-4 Aug., 2005 San Diego 5 th International Conference on SSL 12
films Prepared from other sources Normalized PL Intensity (a.u.) 1.0 0.8 0.6 0.4 0.2 99 100 97 98 0.0 500 600 700 800 900 Wavelength (nm) Normalized PL Intensity (a.u.) 1.0 0.8 0.6 0.4 0.2 104 105 110 0.0 500 600 700 800 900 Wavelength (nm) Reactive evaporation: Si + O 2 Co-evaporation: SiO + SiO 2 Similar red PL was also observed from films prepared from Si, SiO/SiO 2 sources. Blue shift was also observed by controlling the O/Si ratio. 1 Aug.-4 Aug., 2005 San Diego 5 th International Conference on SSL 13
Effect of Annealing Time on PL PL Intensity (a.u.) 7x10 5 6 1: 5 min 6x10 5 2: 30 min 5x10 5 4x10 5 3: 1h 4: 2.5h 5: 5h 5 3x10 5 6: 72h 4 2x10 5 1x10 5 0 1 500 600 700 800 900 3 2 Wavelength (nm) 780 1100 C 1100 C Peak Wavelength (nm) 775 770 765 760 755 750 SiOx89 0 20 40 60 80 Annealing Time (h) PL spectra vs annealing time Peak wavelength vs annealing time The PL intensity increases continuously with annealing time; The peak emission shifts to shorter wavelength indicating decreasing size of Si QDs. 1 Aug.-4 Aug., 2005 San Diego 5 th International Conference on SSL 14
Effect of Annealing Atmosphere on PL PL Intensity (nm) 6x10 5 5x10 5 4x10 5 3x10 5 2x10 5 1x10 5 0 96% N 2 /4% H 2 N 2 600 700 800 900 Wavelength (nm) PL Intensity (a.u.) 1.6x10 5 1.4x10 5 1.2x10 5 1.0x10 5 8.0x10 4 6.0x10 4 4.0x10 4 2.0x10 4 0.0 96% N 2 /4% H 2 N 2 600 700 800 900 Wavelength (nm) 89: Evaporation of SiO in O 2 ; Rate: 0.2nm/s; O 2 flow: 10 sccm; 1100 c annealing for 1h 104: Co-evaporation of SiO and SiO 2 ; Rate: 0.05/0.15 nm/s; 1100 c annealing for 1h. The PL intensity improved 2~3 times by annealing in H 2 atmosphere due to better passivation of Si QDs. 1 Aug.-4 Aug., 2005 San Diego 5 th International Conference on SSL 15
Various Visible Color from Normalized PL Intensity (a.u.) 1.0 0.8 0.6 0.4 0.2 0.0 300 400 500 600 700 800 900 1000 W avelength (nm ) 85 500C 24h 56 300c 24h 85 1100c 1h 83 1100c 24h Various visible color, blue, green and orange can be obtained from films by controlling the Si/O ratio, annealing temperature and annealing time Red emission is most efficient with an external QE estimated to be 1~4% 1 Aug.-4 Aug., 2005 San Diego 5 th International Conference on SSL 16
Conclusions and Future Work Conclusions Nanocrystalline Si dots were formed in films with controlled stoichiometry Strong red PL was obtained from thin film Si QDs With an intense PLE band around 370nm which is near the UV LED emission range, these thin film Si QDs show potential for solid state lighting Future Work Synthesize highly efficient green and blue emitting films with amorphous Si QDs Develop integrated multilayer Si QD thin films with controlled emission colors and out-coupling techniques to obtain efficient white LEDs. 1 Aug.-4 Aug., 2005 San Diego 5 th International Conference on SSL 17
Acknowledgements Ga Tech Research Institute Shackelford GRA Fellowship SURF - NSF Summer Undergraduate Research Fellowship program at Georgia Tech 1 Aug.-4 Aug., 2005 San Diego 5 th International Conference on SSL 18