1. Optical and Current Noise of GaN-based Light Emitting Diodes
Shayla M.L. Sawyer, S. L. Rumyantsev, N. Pala, M. S. Shur, Yu. Bilenko, J. P. Zhang, X. Hu, A. Lunev, J. Deng, and R. Gaska
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2. Agenda Introduction and Motivation LED characteristics
Experimental Setup
Experimental Results
Optical Noise
Current Noise
Conclusions
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3. Research Map: A Road Less Traveled
New research area
Noise of Light Emitting
Devices
Previously studied
Low Frequency Noise
Shot Noise
Noise, Current Flow, and
Light Emitting Mechanisms
Light Intensity Fluctuations
Current Noise
Degradation and Reliability
LEDs
Laser Diode
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4. Motivation: Applications of UV sources
vs. UV
UV?
Food and Water Sterilization
Company: SteriPen currently using Mercury lamps
Working with Nichia to replace with LEDs
High density optical storage
Company: Pioneer 500GB disk
Non-line-of-Sight Short Range Communication
Research: DARPA/University research
Atmospheric scattering in solar blind region for communication
Shaw, G. A., et al., Unattended Ground Sensor Technologies and Applications VII, Proc. Of SPIE 5796, 214, (2005).
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5. Motivation: Emphasis for LFN
Low noise light sources for biological hazard detection systems
Signal-to-noise ratio
False negative rate
False positive rate
Biological experiments to study small and slow variations of transmitted or reflected light
Anthrax Spores
Letter containing anthrax
B.M. Salzberg, P.V. Kosterin, M. Muschol, S.L. Rumyantsev, Yu. Bilenko, and M.S. Shur, Journal of Neuroscience Methods, 141, pp , 2005.
From: and
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6. Light Sources Investigated
Compared:
1st and 2nd Generation SET UVTOP®
265nm-340nm
Nichia 375 nm and 505nm
Roithner Lasertechnik 740 nm
Halogen Lamp
Pd/Ag/Au
SLs
AlN
C-sapphire
Superlattice buffer layer
n+-AlGaN
n-AlGaN
AlInGaN MQW
p-AlGaN
p+-AlInGaN
sapphire
2nd Generation
MEMOCVDTM grown
strain management layer,
AlN buffer layer, and MQW
Improved surface morphology and
defect density for high Al molar
fraction layers
Schematic of Deep UV LED Structure SET UVTOP®
J. Zhang, X. Hu, A. Lunev, J. Deng, Yu. Bilenko, T. M. Katona, M. S Shur, R. Gaska, Submitted to Jap. J. Appl. Phys
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7. LED Performance: SET UVTOP®
*World Record* Deep UV LED Output Power
Exceeds 2mW
Wall plug efficiency>1%
280 nm 1st Gen.
J. Zhang, X. Hu, A. Lunev, J. Deng, Yu. Bilenko, T. M. Katona, M. S Shur, R. Gaska, Submitted to Jap. J. Appl. Phys
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8. Experimental Setup Optical noise Current noise
UV enhanced Si photodiode UV-100L from UDT Sensors, Inc .
Photodiode load resistor, Rphd = 10 k
LED load resistor, RLED = 1k
Current noise
LED load resistor varied from 100  to 10 k
Low noise amplifier
Signal recovery
Model 5184
Network
analyzer
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9. Experimental Results: Optical 1/f Noise
At low frequencies the noise LEDs is lower than that of Halogen Lamps (traditional light source)
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10. Experimental Results: Optical Noise
Dependence of relative noise
spectra on LED current, ILED
S. L. Rumyantsev, S. Sawyer, N. Pala, M. S. Shur, Yu. Bilenko, J. P. Zhang, X. Hu, A. Lunev, J. Deng, and R. Gaska, Low frequency noise of GaN-based UV LEDs, JOURNAL OF APPLIED PHYSICS 97, (2005)
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11. Experimental Results: Optical Noise
First optical Figure-of-Merit, β
n is the number of chips connected in series (n=1 for all LEDs studied in this paper)
τ is the radiative life-time
q is the electronic charge
f is frequency.
Hooge parameter
*New* Optoelectronic device
Figure-of-Merit
Electronic device
Figure-of-Merit
S. L. Rumyantsev, M.S. Shur, Yu. Bilenko, P.V. Kosterin, and B.M. Salzberg, J. Appl. Phys. 96, 966 (2004).
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12. Experimental Results: βvs. Wavelength
β, LED noise quality factor for various
wavelengths
β is the same order of magnitude for the best of SET devices and Nichia (InGaN) LEDs of longer wavelength.
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13. Experimental Results: Current 1/f Noise
I=5x10-3A I
I=5x10-7A
Noise spectra SI of the second generation SET UVTOP® 280 nm LED
At low currents (ILED<10-4A), the noise spectra is the superposition of 1/f and generation recombination (GR) noise
For some LEDs GR noise can be seen within the whole current range, allowing us to make some estimates.
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14. Experimental Results: Current GR Noise
Noise spectra times frequency SIf for the second generation SET UVTOP® 280 nm LED
I=3x10-3A I
I=10-6A
Two GR levels with different characteristic times and their dependence on current (shown as A and B)
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15. Experimental Results: Current Noise
Dependence of noise spectral density, SI , on ILED current
First to find non-monotonic dependence of noise on current in LEDs and other pn junctions
At high current, the noise of the second generation LEDs is always smaller than that of the first generation devices due to reduced series resistance (base and contact noise)
First to observe noise decrease with an increase in current
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16. Theory: Noise Model Proposed Mechanism
Trap level near one of the bands
Carrier concentration fluctuates
Bimolecular
High injection
Concentration
fluctuations
Spectral noise density
of current fluctuations
Monomolecular
Recombination
(Low current)
Monomolecular
Low injection
Low injection
Higher injection
Monomolecular
Higher injection
Bimolecular
Recombination
(High current)
High injection
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17. Correlation between I-Vs and Noise
Maximum corresponds to the light emission threshold
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18. Estimates Et<0.19 eV From this equation Nt can be determined
Maximum of noise current dependence corresponds to the level occupancy F=2/3. For ωτcF=1
From this equation Nt can be determined
Taking for the estimate the lifetime, , in GaN for the LED with the highest GR noise we obtained Nt=71015 cm-3
If τ is constant like GR process “B” the trap level position can be found τr Nt
Et<0.19 eV
Et<EF
EF=0.19
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19. Conclusions
Second generation SET UVTOP® LEDs showed reduced current and optical noise
The GR noise demonstrated non-monotonic dependence on current, explained by the presence of relatively shallow trap levels in the quantum well
The trap level concentration responsible for this GR noise is estimated to be Nt=71015 cm-3
For the shallowest trap level Trap “B” the estimate of the level position yields Et<0.19eV
Deep UV LEDs can provide superior S/N ratios for biological hazard detection systems
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20. Acknowledgements Advisor: Prof. Michael Shur LFN Group Members at RPI
Department of Homeland Security Fellowship program
“This research was performed while on appointment as a U.S. Department of Homeland Security (DHS) Fellow under the DHS Scholarship and Fellowship Program, a program administered by the Oak Ridge Institute for Science and Education (ORISE) for DHS through an interagency agreement with the U.S Department of Energy (DOE). ORISE is managed by Oak Ridge Associated Universities under DOE contract number DE-AC05-00OR All opinions expressed in this paper are the author's and do not necessarily reflect the policies and views of DHS, DOE, or ORISE.”
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21. References
[1] B.M. Salzberg, P.V. Kosterin, M. Muschol, S.L. Rumyantsev, Yu. Bilenko, and M.S. Shur, “Journal of Neuroscience Methods, 141, pp , (2005).
[2] R. DaCosta, H. Andersson, and B. Wilson Photochemistry and Photobiology: 78, 384, (2003).
[3] J. Zhang, X. Hu, A. Lunev, J. Deng, Yu. Bilenko, T. M. Katona, M. S Shur, R. Gaska, Submitted to Jap. J. Appl. Phys
[4] Yu. Bilenko, A. Lunev, X. Hu, J. Deng, T. M. Katona, J. Zhang, R. Gaska, M. S Shur, W. Sun, V.Adivarahan, M.Shatalov, and A. Khan, 10 milliwatt pulse operation of 265nm AlGaN light emitting diodes, Jap. J. Appl. Phys., v.44, pp. L98-L100 (2005).
[5] J. Zhang, X. Hu, Yu. Bilenko, J. Deng, A. Lunev, M. S Shur, and R. Gaska,
AlGaN-based 280nm light-emitting diodes with continuous-wave power exceeding 1mW at 25mA, Appl. Phys. Lett., v.85, pp (2004).
[6] Chen, C. et al. Jpn. J. Appl. Phys. 41, 1924 (2002).
[7] S. L. Rumyantsev, S. Sawyer, N. Pala, M. S. Shur, Yu. Bilenko, J. P. Zhang, X. Hu, A. Lunev, J. Deng, and R. Gaska, Low frequency noise of GaN-based UV LEDs, JOURNAL OF APPLIED PHYSICS 97, (2005)
[8] S. L. Rumyantsev, M.S. Shur, Yu. Bilenko, P.V. Kosterin, and B.M. Salzberg, J. Appl. Phys. 96, 966 (2004).
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22. Derivation Slides
Nt is the trap concentration, n is the electron concentration in the quantum well, τ is the characteristic time of the GR noise, V is the volume, and F is the occupancy of the level.
σ is the capture cross section, v is the thermal velocity
Concentration
fluctuations
Characteristic time
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23. Derivation Slides F level of occupancy for non-degenerate case
Nc is the effective density of states in the conduction band and Et is the level position (the energy is measured down from the bottom of the conduction band)
We now consider two limiting cases for low frequencies wt<<1:
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24. Noise SI increases with the current increase
Derivation Slides
Monomolecular recombination occurs at low currents where the current is proportional to electron concentration
At low currents, the electron concentration n is small and the trap level is almost empty (F<<1). Then the spectral noise density of current fluctuations SI for ωτ<<1 can be expressed as:
Noise SI increases with the current increase
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25. Noise SI decreases with the current increase
Derivation Slides
Monomolecular recombination at higher currents the occupancy of the trap level also increases. Assuming that (1-F)<<1 and that the recombination is still monomolecular, we obtain
Noise SI decreases with the current increase
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26. Derivation Slides
Bimolecular recombination occurs at high currents where its spectral noise density SI can be expressed as
where B is radiative recombination coefficient and  is the internal quantum efficiency
For the case (1-F)<<1, we obtain that the spectral noise density is independent of the current:
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