2326
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 10, OCTOBER 2004
A Partially Depleted Absorber Photodiode With
Graded Doping Injection Regions
Xiaowei Li, Ning Li, Stephen Demiguel, Xiaoguang Zheng, Joe C. Campbell, H. Hoe Tan, and Chennupati Jagadish
Abstract—A partially depleted absorber photodiode with
graded absorption injection regions is reported. The 880-nm-thick
In0 53 Ga0 47 As absorption region consists of a depletion layer
and graded n- and p-type undepleted injection regions. Under
backside illumination at 1.55- m wavelength, an 8- m-diameter photodiode exhibited an increase in bandwidth with
increasing photocurrent. A saturation current bandwidth product
of 990 mA GHz and a responsivity of 0.67 A/W were achieved.
Index Terms—High-power photodiodes, microwave photonics,
photodetector, photodiode, radio-frequency photonics.
C
ONVENTIONAL photoreceivers consist of a p-i-n photodiode and a transimpedance amplifier (TIA). The sensitivity of conventional photoreceivers is often limited by the
front-end noise in the electrical amplifier [1]. On the other hand,
an avalanche photodetecter (APD) has higher sensitivity due to
its internal gain. However, practical APDs for 40-Gb/s applications are difficult to make due to the avalanche build-up time
[2]. In the digital domain, the highest sensitivities for 40 Gb/s receivers have been achieved by incorporating an erbium-dopedfiber-amplifier [3] or semiconductor optical amplifier [4] directly in front of the photodetecter and eliminating the electrical postamplifier. This new approach becomes attractive when
high-saturation-current photodiodes are available, so that the
output from the photodiode can directly drive a decision circuit.
A primary limiting factor for high-speed high-saturation-current photodetecters is the space charge effect [5]. At high photocurrent levels, the space charge creates an electric field that
opposes the bias electric field. For high optical input levels, this
space-charge-induced electric field can cause the electric field
to “collapse,” which gives rise to radio-frequency photocurrent
compression.
In an effort to increase the output photocurrent, several types
of photodiodes have been investigated. The most commonly
used photodetecters, p-i-n photodiodes [6], consist of a depleted
In Ga As absorption layer sandwiched between p- and
n-type InP layers. Typically, this structure is the most susceptible to space-charge effects. Unitravelling-carrier (UTC)
Manuscript received April 26, 2004; revised May 13, 2004. This work was
supported by Defense Advanced Research Projects Agency (DARPA) through
the RFLIC program and in part by the Australian Research Council (for work
done at the Australian National University).
X. Li, N. Li, S. Demiguel, X. Zheng, and J. C. Campbell are with the Microelectronics Research Center, Department of Electrical and Computer Engineering, University of Texas, Austin, TX 78758 USA.
H. H. Tan and C. Jagadish are with the Department of Electronic Materials
Engineering, Research School of Physical Sciences and Engineering, Australian
National University, Canberra 0200, Australia (e-mail: jcc@mail.utexas.edu).
Digital Object Identifier 10.1109/LPT.2004.834563
photodetecters, on the other hand, have achieved excellent current-bandwidth performance [7], [8]. The UTC photodetecter
is made of a p-doped absorption layer with an unintentionally
doped InP collection layer. Electrons are the only type of
carrier in the depletion region. This mitigates the space-charge
effect and, since electrons have higher saturation velocities
than holes, there is a speed benefit. Another structure, the dual
depletion region (DDR) photodetecter has been reported in [9].
The DDR photodetecter has two drift layers, an InGaAs absorption layer and an InP layer. The advantage of this approach
is the potential to balance the transit time of the electrons and
holes. In addition to the vertically illuminated photodetecters,
waveguide (WG) photodetecters have been reported to achieve
both high-speed and high-saturation current [10], [11]. The
waveguide photodetecter usually has a thick guiding layer and
the light is edge-coupled into the waveguide.
Previously, we have demonstrated very high photocurrent
bandwidth products with a partially depleted absorber (PDA)
photodetecter [12], [13], which combines some of the advantages of the UTC and DDR structures. Since the bandwidth
of our previous structure was not transit-time limited, in this
letter, we modified the structure to increase the thickness of
the graded absorption injection regions. Compared to our
previous work, the new structure demonstrated nearly the same
saturation current at a higher responsivity of 0.67 A/W without
sacrificing the bandwidth.
High-power photodiode designs tend to avoid incorporating
a thick absorption layer owing to the poor thermal conductivity
of In Ga As [14], which can lead to thermal runaway
[15]. In this letter, we have employed a relatively thick, partially
depleted p-i-n (560-nm p-region, 240-nm i-region, and 80-nm
n-region) In Ga As structure. This approach yields high
quantum efficiency and facilitates carrier transport. Unlike other
approaches, such as the UTC and DDR photodetecters, there
are no band discontinuities to cause carrier “pileup,” which is
common at heterojunction interfaces. The photogeneration of
holes in the intrinsic layer of the PDA can result in larger space
charge effects than those in the UTC structure. However, this
effect is not as severe if the drift layer is thin. For these reasons,
the PDA can be operated under low bias voltage to reduce the
total power consumption, while still suppressing saturation.
The device layers were grown on (100) semi-insulating
InP by low-pressure metal–organic chemical vapor deposition (MOCVD). In the PDA design, shown in Fig. 1, the cap
In Ga As layer is highly p-doped to 3–4 10 cm
in order to form a good ohmic contact. The underlying In
Ga As P layer is also highly doped to serve as a diffusion block to minority carriers in the p-In Ga As layer
1041-1135/04$20.00 © 2004 IEEE
LI et al.: PDA PHOTODIODE WITH GRADED DOPING INJECTION REGIONS
2327
Fig. 2. Frequency response for different dc photocurrents and applied biases
for 34-m-diameter diodes.
Fig. 1. Epitaxial layer structure of partially depleted absorber photodetecter.
[8]. The 560-nm-thick p-In Ga As layer is “graded” in
four steps with doping densities (thickness) of 2 10 cm
(140 nm), 1 10 cm (140 nm), 5 10 cm (140 nm),
(140 nm), which creates a potential
and 2.5 10 cm
gradient
ln2 kT/q
18 meV at each step or
54 meV in total. The depletion region is a 240-nm-thick
unintentionally doped In Ga As layer. Below this
layer is an n-type In Ga As layer, which is graded in
steps of 2.5 10 cm
(20 nm), 5 10 cm
(20 nm),
1 10 cm (20 nm), and 2 10 cm (20 nm). Beneath
Ga As layer is a 400-nm-thick
InP
the n-type
buffer layer, which serves as the n-contact and aids heat dissipation. Since the hole diffusion is much slower than the electron
diffusion, the thickness of the n-In Ga As absorber must
reflect the differences in diffusion rates. The Ti–Pt–Au p-contacts and Ni–AuGe–Au n-contacts were deposited using a
standard liftoff process. Mesas ranging from 6- to 80- m diameters were defined by wet-chemical selective etching. Edge
. For
passivation was accomplished through PECVD of
a 16- m-diameter InP–In Ga As PDA photodiode, the
dark current was 10 nA at 6 V.
In a conventional p-i-n photodetecter, the electron-hole pairs
are generated in the i-region and travel in opposite directions.
When the intrinsic region is thin, the electron velocity is dominated by the overshoot velocity, which is much higher than the
saturation velocity of holes [16]. Currently, at high current level,
the electric field in the i-region of a conventional p-i-n photodetecter is strongly influenced by the hole density. For the PDA
photodiode, there are p- and n-doped absorbers on each side of
the i-region that inject electrons and holes, respectively, into the
i-region. By adjusting the relative thickness of the doped absorber regions, a greater number of electrons than holes can be
injected. In this way, the electron and hole currents can be tailored to greatly reduce the space-charge effect compared to conventional p-i-n photodetecters.
Fig. 3. Frequency response for different dc photocurrents and applied biases
for 16-m-diameter diodes.
The frequency-response measurement apparatus has been
described previously [12]. Two equal-power, single-frequency 1542-nm DFB lasers were heterodyned to provide the
small-signal input. Both lasers were temperature controlled.
The frequency was swept by changing the temperature of one
of the lasers. The small signal input was amplified by an erbium-doped fiber amplifier to provide large-signal modulation.
The 3-dB bandwidth was 6 GHz for 34- m-diameter diodes
under backside illumination (Fig. 2). The dc photocurrents were
10, 21, and 44 mA at applied biases of 2, 2, and 3 V, respectively. A 1-dB large-signal compression current of 101 mA was
achieved at 4-V bias. The maximum saturation current bandwidth product was 500 mA GHz. The total power dissipation of the photodiode was 0.5 W. For a 16- m-diameter backside-illuminated photodiode, the bandwidth was 17 GHz with a
photocurrent of 46 mA (the maximum photocurrent that could
be achieved) at 3-V bias; no evidence of saturation was observed
(Fig. 3). The power dissipation was 0.13 W.
An 8- m-diameter backside-illuminated photodiode having
a responsivity of 0.67 A/W achieved a saturation current of
22 mA at the bandwidth of 45 GHz (current-bandwidth product
990 mA GHz) after correction for the response of the bias
tee, probe, and transmission line [12] (Fig. 4). In Fig. 4, there is
a significant change in the bandwidth of the 8- m devices with
increasing input power. When the photocurrent was 0.11 mA,
the bandwidth was 29 GHz; at 22-mA photocurrent, the bandwidth increased to 45 GHz. The same effect has been observed
in UTC photodiodes [17]. This can be explained as follows. The
photocurrent in the p-type absorption region is dominated by
and 2)
two components: 1) the electron diffusion current
2328
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 10, OCTOBER 2004
REFERENCES
[1] B. Mason, S. Chandrasekhar, A. Ougazzaden, C. Lentz, J. M. Geary,
L. L. Buhl, L. Peticolas, K. Glogovsky, J. M. Freund, L. Reynold, G.
Przybylek, F. Walters, A. Sirenko, J. Boardman, T. Kercher, M. Rader,
J. Grenko, D. Monroe, and L. Ketelson, “Photonic intergrated receiver
for 40 Gbit/s transmission,” IEE Electron Lett., vol. 38, no. 20, pp.
1196–1197, 2002.
[2] G. S. Kinsey, J. C. Campbell, and A. G. Dentai, “Waveguide avalanche
photodiode operating at 1.55 mm with a gain-bandwidth product of 320
GHz,” IEEE Photon. Technol. Lett., vol. 13, pp. 842–844, Aug. 2001.
[3] S. Takashima, H. Nakagawa, S. Kim, F. Goto, M. Okayasu, and H.
Inoue, “40-Gbits/s receiver with 21 dBm sensitivity employing
filterless semiconductor optical amplifer,” in Proc. OFC 2003, vol. 2,
2003, Paper ThG3, pp. 471–472.
[4] E. S. Bjorlin, J. Geske, and J. E. Bowers, “Optically preamplified receiver at 10 Gbit/s using vertical-cavity SOA,” Electron. Lett., vol. 37,
no. 24, pp. 1474–1475, 2001.
[5] P.-L. Liu, K. J. Willaims, M. Y. Frankel, and R. D. Esman, “Saturation
characteristics of fast photodetectors,” IEEE Trans. Microwave Theory
Tech., vol. 47, pp. 1297–1303, July 1999.
[6] K. J. Williams and R. D. Esman, “Photodiode DC and microwave
nonlinerity at high current due to carrier recombination nonlinearities,”
IEEE Photon. Technol. Lett., vol. 10, pp. 1015–1017, July 1998.
[7] H. Ito, T. Furuta, S. Kodama, and T. Ishibashi, “InP/InGaAs uni-travelling-carrier photodiode with 310-GHz bandwidth,” IEE Electron. Lett.,
vol. 36, no. 21, pp. 1809–1810, 2000.
[8] N. Shimizu, N. Watanabe, T. Furuta, and T. Ishibashi, “InP-InGaAs unitraveling-carrier photodiode with improved 3-dB bandwidth of over 150
GHz,” IEEE Photon. Technol. Lett., vol. 10, pp. 412–414, Mar. 1998.
[9] F. J. Effenberger and A. M. Joshi, “Ultrafast, dual-depletion region InGaAs/InP pin detector,” J. Lightwave Technol., vol. 14, pp. 1859–1864,
Aug. 1996.
[10] S. Demiguel, L. Giraudet, P. Pagnod-Rossiaux, E. Boucherez, C. Jany,
L. Carpentier, V. Coupe, S. Fock-Yee, J. Decobert, and F. Devaux,
“Low-cost, polarization independent, tapered photodiodes, with bandwidth over 50 GHz,” Electron. Lett., vol. 37, pp. 516–517, Aug. 2001.
[11] F. Xia, J. K. Thomson, M. R. Gokhale, P. V. Studenkov, J. Wei, W. Lin,
and S. R. Forrest, “A asymmetric twin-waveguide high-bandwidth photodiode using a lateral taper coupler,” IEEE Photon. Technol. Lett., vol.
13, pp. 845–847, Aug. 2001.
[12] X. Li, N. Li, X. Zheng, S. Demiguel, J. C. Campbell, D. A. Tulchinsky,
and K. J. Williams, “High-saturation-current InP-InGaAs photodiode
with partially depleted absorber,” IEEE Photon. Technol. Lett., vol. 15,
pp. 1276–1278, Sept. 2003.
[13] X. Li, S. Demiguel, N. Li, J. C. Campbell, D. A. Tulchinsky, and K.
J. Williams, “Backside illuminated high saturation current partially
depleted absorber photodetectors,” Electron. Lett., vol. 39, no. 20, pp.
1466–1467, 2003.
[14] K. J. Williams and R. D. Esman, “Design considerations for high-current
photodetectors,” J. Lightwave Technol., vol. 17, no. 8, pp. 1443–1454,
1999.
[15] M. S. Islam, A. Nespola, M. Yeahia, M. C. Wu, D. L. Sivco, and A. Y.
Cho, “Correlation between the failure mechanism and dark currents of
high power photodetectors,” in Proc. IEEE Lasers Electro-Optics Soc.
13th Annu. Meeting, vol. 1, 2000, pp. 82–83.
[16] K. Kato, “Ultrawide-band/high-frequency photodetectors,” IEEE Trans.
Microwave Theory Tech., vol. 47, pp. 1265–1281, July 1999.
[17] N. Shimizu, N. Watanabe, T. Furuta, and T. Ishibashi, “Improved response of uni-traveling-carrier photodiodes by carrier injection,” Jpn. J.
Appl. Phys., vol. 37, pp. 1424–1424, 1998.
[18] T. Ishibashi, S. Kodama, N. Shimizu, and T. Furuta, “High-Speed response of uni-traveling-carrier photodiodes,” Jpn. J. Appl. Phys., vol.
36, no. 10, pp. 6263–6268, 1997.
0
Fig. 4. Frequency response for different dc photocurrents and applied biases
for 8-m-diameter diodes.
the hole-drift current
. The total current has to be contin. Under the backside illuminauous, so
tion condition, due to the reflection from the top contact and
-pair
the resulting double pass through the absorber, the
generation is nearly a uniform distribution. This gives an elec, where
is the total abtron current
sorption region thickness. Thus, the hole current is
. For a photocurrent of 20 mA,
is 40 kA/cm
for 8- m-diameter diodes. In the hole dominated p-type absorption region, the hole-drift-current density can be estimated as
40, 30, 20, and 10 kA/cm , for each of the p-doped regions,
respectively. With the expression
and
of 150 cm Vs [18], the hole-drift-induced electric field
is estimated as 0.83, 1.25, 1.66, and 1.66 kV/cm for the
p-doped layers, respectively. The total induced potential gra75 meV, which is higher than
, which
dient is
results from the graded doping. Since the potential gradients
induced by doping and input power are independent of each
is 129 meV.
other, the total potential gradient
The average electric field is 2.4 times higher than the small
photocurrent case. Therefore, the transit time of electrons in
the p-doped absorption region is decreased and the bandwidth
is independent of the p-layer
is enhanced. Since the
thickness, this bandwidth enhancement effect is more significant when the p-absorption layer is thicker. For the structure
presented in this letter, the total graded p-region is 600-nm thick
and the bandwidth enhancement effect is significant.
We have demonstrated an 880-nm-thick graded doped PDA
photodiode. Under backside illumination at 1.55 m wavelength, an 8- m-diameter photodiode demonstrated a 3-dB
bandwidth enhancement at high photocurrent. The photodiodes
demonstrated saturation current bandwidth products of 990,
780, and 500 mA GHz for 8-, 16-, and 34- m-diameter
diodes, respectively.