Using GaP Avalanche
Photodiodes for Photon
Detection
Abigail Lubow
EE Senior Project
Fall 2001 and Spring 2002
Advisor: Professor Woodall
Pictures from http://www.roithner-laser.com/UV-PD.html
Background
• Avalanche photodiodes (APDs) are high gain
photodetectors.
• Nonavalanching p-n and p-i-n photodetectors have unity
gain. By contrast, APDs use the avalanching process to
produce a higher gain.
• This device addresses the need for solar blind UV
detectors.
• Potential applications: High density optical storage and
detection of tryptophan flourescence (348nm).
• This project is the first time GaP has been used to create an
APD.
Photodiodes
Figure 1.
• Incident photons are absorbed in the photodiode giving rise
to electron-hole pairs.
• Gain = 1. Impact ionization will increase the gain above 1.
Impact Ionization
• Definition: “a snowballing creation of carriers very similar to
an avalanche of snow on a mountain side.”from Pierret’s Semiconductor Device
Fundamentals
• Conditions: Applied voltage (VA) is negative and VA 
Vbreakdown.
Figure 2.
Why use GaP?
• It is more commonly available and less expensive than other
wide bandgap materials such as SiC and GaN.
• For GaP, ni  1 /cm3 and for Si, ni = 1 x 1010/cm3. GaP p-n
junction will have a smaller reverse current:
Dp ni 2 qniW
JR  q

 p ND
e
• Large bandgaps correspond to small wavelengths: Eph =
1.24/. GaP bandgap = 2.26 eV, Si bandgap = 1.12 eV
Figure 3.
P-I-N GaP Sample
Figure 4.
Figure 5.
p-GaP, Be doped
grading from 210^18 to 210^19 (surface) over 3000A
I- GaP, 3000 A
n-GaP, 210^18, 5000 A
Semi-insulating super lattice buffer
n-GaP substrate
• Fig. 4. P-I-N structure: Electric field in graded p-layer
separates electrons and holes.
• Fig. 5. Electrochemical CV measurement. “+” for n type and
“o” for p type.
Measurements: Current vs. Voltage
Figure 6.
Reverse IV: GaP 80um Ring Device
1.0E-05
log current (A)
1.0E-06
dark
UV
White
1.0E-07
1.0E-08
1.0E-09
1.0E-10
1.0E-11
1.0E-12
1.0E-13
0
-5
-10
-15
-20
-25
voltage (V)
Avalanching occurs at about -20V. Dark current is 1 × 10-¹³ A.
Measurements: Quantum efficiency vs.
wavelength
Figure 7.
QE: GaP 80um ring device
50
45
40
QE %
35
30
25
20
15
10
5
0
250
300
350
400
450
500
wavelength
QE is “the ratio of the number of carriers generated to
the number of photons incident upon the active region.”
www.seas.gwu.edu/~ecelabs/appnotes/PDF/LED/LEDterms.pdf
Surface Band Bending
Figure 8.
• Surface band bending due to the “Fermi level pinning.”
• Electron loss due to the surface recombination.
Measurements: Gain vs. Voltage
Figure 9.
Gain: GaP 80um ring Device
1000
UV Gain
100
gain
White Gain
10
1
0
-5
-10
-15
-20
-25
voltage (V)
• Gain results from the impact ionization process.
• Large gains start at –20V for UV and white light.
• Gains reach as high as 1000.
Measurements: Photocurrent vs.
Wavelength
Figure 10.
Photocurrent v wavelength: GaP 80um ring device
1.60E-08
1.40E-08
1.20E-08
photocurrent (A)
1.00E-08
I(10V)
I(17V)
I(19V)
8.00E-09
I(20V)
I(20.5V)
I(20.7V)
6.00E-09
4.00E-09
2.00E-09
0.00E+00
250
300
350
400
450
500
550
wavelength
• Current decreases at small wavelengths due to
surface pinning.
• Current decreases at large wavelengths due to low
absorption coefficients.
Schottky Device
Figure 11.
Metal
0.25 m I-GaP
50 nm p-GaP doped 1*10^18cm^-3 Be
0.1 m I-GaP
1.0 m n-GaP doped 5*10^18 cm^-3 Si
GaP n+ Substrate
• SAM structure Schottky device improves absorption
efficiency and reduces noise.
• Processing Issue: Choice of metal with low light loss
Summary
• GaP APD has been chosen for use as a solar blind
UV detector.
• P-I-N device showed promising performance with
high gain, low dark current, and high QE at
medium wavelengths.
• UV QE needed improvement. A Schottky device
structure was proposed.