Optical and Current Noise of GaN-based Light Emitting Diodes

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

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
Laser Diode
LEDs
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 www.steripen.com
High density optical storage Company Pioneer
500GB disk http//electronics.howstuffworks.com/bl
u-ray3.htm

• 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).

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. 165-169, 2005.
Fromhttp//www.surrey.ac.uk/SBMS/ACADEMICS_homepa
ge/mcfadden_johnjoe/sbms215.html and
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le.jpg
6
Light Sources Investigated

• Compared
• 1st and 2nd Generation SET UVTOP
• 265nm-340nm
• Nichia 375 nm and 505nm
• Roithner Lasertechnik 740 nm
• Halogen Lamp

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
7
LED Performance SET UVTOP
World Record Deep UV LED Output Power
Exceeds 2mW
Wall plug efficiencygt1
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
8
Experimental Setup

• Optical 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
9
Experimental Results Optical 1/f Noise
At low frequencies the noise LEDs is lower than
that of Halogen Lamps (traditional light source)
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,
123107 (2005)
11
Experimental Results Optical Noise

• First optical Figure-of-Merit, ß
• n is the number of chips connected in series (n1
  for all LEDs studied in this paper)
• t 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).
12
Experimental Results ßvs. Wavelength
ß, LED noise quality factor for
various wavelengths
ß is the same order of magnitude for the best of
SET devices and Nichia (InGaN) LEDs of longer
wavelength.
13
Experimental Results Current 1/f Noise
Noise spectra SI of the second generation SET
UVTOP 280 nm LED

• At low currents (ILEDlt10-4A), 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.
14
Experimental Results Current GR Noise
Noise spectra times frequency SI?f for the second
generation SET UVTOP 280 nm LED
I3x10-3A
I
I10-6A

• Two GR levels with different characteristic times
  and their dependence on current
  (shown as A and B)

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

16
Theory Noise Model

• Proposed Mechanism
• Trap level near one of the bands

Carrier concentration fluctuates
Bimolecular High injection
Spectral noise density of current fluctuations
Concentration fluctuations
Monomolecular Recombination (Low current)
Monomolecular Low injection
Low injection
Higher injection
Monomolecular Higher injection
Bimolecular Recombination (High current)
High injection
17
Correlation between I-Vs and Noise

• Maximum corresponds to the light emission
  threshold

18
Estimates

• Maximum of noise current dependence corresponds
  to the level occupancy F2/3. For ?tcF1
• 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 Nt7?1015 cm-3
• If t is constant like GR process B the trap
  level position can be found

Etlt0.19 eV
EtltEF
EF0.19
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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 Nt7?1015 cm-3
• For the shallowest trap level Trap B the
  estimate of the level position yields Etlt0.19eV
• Deep UV LEDs can provide superior S/N ratios for
  biological hazard detection systems

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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-AC05-00OR22750. All
  opinions expressed in this paper are the author's
  and do not necessarily reflect the policies and
  views of DHS, DOE, or ORISE.

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.
  165-169, (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.5532-5534 (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,
  123107 (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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Derivation Slides

• Nt is the trap concentration, n is the electron
  concentration in the quantum well, t is the
  characteristic time of the GR noise, V is the
  volume, and F is the occupancy of the level.
• s is the capture cross section, v is the thermal
  velocity

Concentration fluctuations
Characteristic time
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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 wtltlt1

24
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 (Fltlt1).
  Then the spectral noise density of current
  fluctuations SI for ?tltlt1 can be expressed as
• Noise SI increases with the current increase

25
Derivation Slides

• Monomolecular recombination at higher currents
  the occupancy of the trap level also increases.
  Assuming that (1-F)ltlt1 and that the
  recombination is still monomolecular, we obtain
• Noise SI decreases with the current increase

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)ltlt1, we obtain that the
  spectral noise density is independent of the
  current