High-Speed, High-Responsivity, and High-Power Performance of
Near-Ballistic Uni-Traveling-Carrier Photodiode at 1.55m Wavelength
J.-Y. Wu1, Y.-S. Wu1, J.-W. Shi1*, P.-H. Chiu1, and C.-C. Hong2
1
Department of Electrical Engineering, National Central University
Taoyuan 320, TAIWAN, R.O.C.
Tel: +886-3-4227151 ext. 34528
FAX:+886-3-4255830
Email: jwshi@ee.ncu.edu.tw
2
Chughwa Telecom. Co., Ltd. Telecom. Lab.
Taoyuan 320, TAIWAN, R.O.C.
Abstract
In this paper, we demonstrate a novel photodiode at 1.55m wavelength: the
Near-Ballistic Uni-Traveling-Carrier Photodiode (NBUTC-PD).
Inserting a p+
delta-doped layer into the collector of uni-traveling-carrier photodiode, near-ballistic
transport phenomenon of photo-generated electrons under high reverse bias voltage
(-5V) and high output photocurrent (~30mA) has been observed.
The demonstrated
device is integrated with an evanescently-coupled optical waveguide for high
responsivity and high saturation power performance. Extremely high responsivity
(1.14A/W), high electrical bandwidth (around 40GHz), and high saturation current
bandwidth product (over 1280mA.GHz, at 40GHz) with high saturation
radio-frequency power (over 12dBm at 40GHz) have been achieved simultaneously
at 1.55m wavelength.
Keywords: photodiode, high-power photodiode, optical receivers, high-efficiency, evanescent
coupling.
*
Corresponding
author,
Tel:
+886-3-4227151
jwshi@ee.ncu.edu.tw
1
ext.
34528,
FAX:+886-3-4255830,
Email:
The performance of microwave and millimeter-wave photonic systems would
benefit from the use of photodiodes (PDs) with high saturation power, high-speed
performance, and high responsivity [1].
However, the electrical bandwidth,
responsivity, and saturation output power are usually three trade-offs in the design of
high-speed photodiodes [2].
simultaneously,
several
In order to meet these three requirements
technologies
have
been
reported,
such
as,
evanescently-coupled waveguide PDs [3-5], partially depleted absorber PDs [6-8],
and uni-traveling-carrier PDs (UTC-PDs) [1,2,9].
The structure of UTC-PD attracts
lots of attention due to its excellent speed and output power performance [1].
Several applications of UTC-PD to fiber communication system or microwave
photonic communication system have been demonstrated [1,2,9].
However, such
device usually suffers from the problem of bandwidth degradation under high dc
external bias voltage [10] due to the high electric field at the junction of collector (C)
and photo-absorption (P) layer and the decrease of overshoot drift-velocity of
photo-generated electrons [10]. One possible way to enhance the speed performance
of UTC-PD is to reduce the external applied bias voltage and let the value of electric
field in the C-P junction is about the critical field (around 10kV/cm for the case of InP)
to enhance the drift-velocity of photo-generated electron [10].
However, under such
low reverse bias voltage (around -1V for 200nm collector), the field-screening effect,
which is originated from the difference in polarity between output ac voltage across
the standard 50 Ω load and dc bias voltage, will limit the maximum output
photocurrent of UTC-PD seriously [10].
In order to minimize the RC bandwidth
limitation and field-screening effect, several reported ultra-high speed results of
UTC-PD are under the case of 25Ω or 12.5Ω load [1-2].
In this work, we
demonstrate state-of-the-art performance of a novel structure of photodiode:
2
Near-ballistic UTC-PD (NBUTC-PD).
By inserting an additional p+ delta-doped
layer in the collector of traditional UTC-PD, near ballistic transport phenomenon
under high reverse bias voltage (-5V) and high output photocurrent (~30mA) has been
observed.
The
demonstrated
novel
device
is
incorporated
with
the
evanescently-coupled waveguide structure [3-5] for high responsivity performance.
Extremely high responsivity (1.14A/W), high electrical bandwidth (around 40GHz),
and high saturation current with high output RF power performances (over 32mA and
12dBm at 40GHz) have been achieved simultaneously at 1.55m wavelength.
Figure 1 (a), (b) shows the cross-sectional view and top view of demonstrated
NBTUC-PD, respectively.
layers of the whole device.
Figure 1(c) gives the conceptual band diagram of active
As shown in Fig.1 (c), the thickness of p-type graded
doped In0.53Ga0.47As photo-absorption layer is 150nm.
We adopted the
In0.52Al0.15Ga0.33As layer with 140nm thickness to serve as the collector layer for
enhancing the optical coupling process between diluted-waveguide-layers and
photo-absorption layer, which are shown in Figure 1(a).
The major difference in
epi-layer structure between UTC-PD and demonstrated NBUTC-PD is the additional
delta-doped
p+
In0.52Al0.48As
charge
layer
and
electric-field-suffered layer (E) with 100nm thickness.
undoped
In0.52Al0.48As
The thickness of undoped
graded bandgap layer at P-C and C-E junction is 20nm and 40nm, respectively. The
total effective collector layer thickness can thus be treated as 300nm.
Introducing
the depleted p+ charge layer [11] with proper planar doping density (5  1011cm-2), the
maximum electric field in the In0.52Al0.15Ga0.33As collector layer is controlled to
produce an appropriate value (around 40kV/cm at -5V bias) for sustaining an
overshoot velocity of photo-generated electron under a certain range of bias voltage
and output photocurrent. In our primarily demonstrated device, the inserted charge
3
layer can be fully depleted when the reverse bias voltage is over -2V.
The very
similar concept of epi-layer-designing, as we show here, has also been implemented
in the collector of an AlGaAs/GaAs based Hetero-junction Bipolar Transistor (HBT)
successfully [12].
As shown in Fig 1(a), we adopted a short (20m) multimode
waveguide that consists of a diluted waveguide and two optical matching layers [3].
The diluted waveguide is composed of seven In0.86Ga0.14As0.3P0.7 layers with 50nm,
80nm, 110nm, 140nm, 170nm, 200nm, and 580nm thickness, interspersed between
the 150nm-thick InP layers.
According to the simulation results, the gradual
increase of In0.86Ga0.14As0.3P0.7 layer thickness from bottom to top will result in a
more significant enhancement of optical coupling efficiency between the diluted
waveguide and a thin (150nm) InGaAs based absorption layer than that of the diluted
waveguide with a fixed period of InP/ In0.86Ga0.14As0.3P0.7 thickness [3]. The number
of periods has been optimized to achieve high coupling efficiency with an input fiber.
The two optical matching layers are n+-doped In0.52Al0.48As and In0.52Al0.17Ga0.31As
( 1  1018 cm 3 ) with 0.6m and 0.3m thickness, respectively. The optical design and
simulation of our demonstrated structure were achieved by using the commercialized
three-dimensional (3-D) beam propagation method (BPM) software.
The used
optical refractive indexes of InAlGaAs or InGaAsP quaternaries in our BPM
simulation can be referred to [13,14].
Fig. 2 shows the BPM simulation result of
NBUTC-PD under TE and TM polarized wave excitation with different device
absorption lengths. The waveguide width is 5m, the length of waveguide region is
20m, and the used absorption constant of In0.53Ga0.47As is 1m-1 at the wavelength
of 1.55m.
We can clearly see that the propagated optical power can be absorbed
completely when the device absorption length is over 20m.
The
epitaxial
layers
of
the
demonstrated
4
devices
were
grown
by
metal-organic-chemical-vapor-deposition (MOCVD) on a semi-insulating (SI) InP
substrate [15]. The detail fabrication process of this device is similar to that of our
previous work on evanescently-coupled photodiode as given in Ref. [16]. As shown
in Fig. 1(b), a co-planar waveguide (CPW), which is connected with the active area of
photodiodes, was fabricated on the SI InP substrate for high-speed measurement.
Finally, the devices were precisely cleaved (  5m accuracy) by patterning a
V-groove, which was formed by the wet-etching process, and an antireflection coating
was deposited on the facet.
We employed a tunable semiconductor laser as the light source for the dc
photocurrent measurement.
The center wavelength of this laser was fixed at 1550nm
during the dc measurement. The measured responsivity of devices with 5m
waveguide with and different absorption lengths is marked in Figure 2.
Extremely high responsivity: 1.01A/W and 1.02A/W have been achieved by the
devices with 150m2 and 200m2 active areas, respectively.
device with (320m2)
The responsivity of
To our knowledge; these are the highest values among all
the reported responsivity of high-speed and high power UTC-PDs [1,2].
In the case
of reported evanescently-coupled UTC-PD and avalanche photodiodes [15,16], their
InGaAsP or InAlAs layers, which are inserted between thin (~200nm) InGaAs based
photo-absorption layer and bottom diluted-waveguide-layers, will degrade the
coupling efficiency of optical power and responsivity performance seriously [15,16].
By properly optimizing the structure of our evanescently-coupled optical waveguide,
we can overcome such problem and obtain state-of-the-art external efficiency (~90%)
of p-i-n photodiode.
The bandwidth and saturation current were measured with a heterodyne beating
system.
Its detail setup and the used RF calibration techniques have been reported
5
on our previous work [14].
The device with an active area about 200m2 can have
around instrument-limited (50GHz) f3dB electrical bandwidths under different reverse
bias voltages and output photocurrent.
Fig. 3 shows the typical frequency responses
of device with 200m2 active area under different output photocurrent (0.5mA and
5mA) and a fixed dc bias voltages (-5V).
We can clearly see that under both
operation conditions the electrical bandwidth is around 50GHz.
In order to clearly
characterize the dynamic behaviors of NBUTC-PD without the serious influence of
system-limited bandwidth (~50GHz), we measured and analyzed the speed
performance of device with a 320m2 active area and smaller f3dB bandwidth than that
of a 200m2 device under different operation conditions in detail.
Fig. 4 shows the
measured f3dB electrical bandwidth vs. output photocurrent under different reverse
bias voltages.
As the diagram indicates, the bandwidth increases significantly when
the reverse bias is over -1V due to the depletion of charge layer and the reduction of
electron transport time in the collector layer.
In each trace there is an optimum value
of photocurrent for maximum bandwidth performance. When the dc bias voltage is
-5V, the value of optimum photocurrent is up to 30mA with 40GHz electrical
bandwidth. The observed bias dependent phenomenon can be possibly attributed to
the velocity overshoot of electron in the collector layer [10,15] instead of the reported
diffusion-velocity enhancement in the p-type photo-absorption layer of UTC-PD
[1,17]. Compared with the previous work on UTC-PD [17], our device has a much
thinner photo-absorption layer (150nm vs. 450nm), and the variation in diffusion time
of electron is thus much less important in the net speed performance of device [15,17].
According to our simulation, when the reverse bias voltage is over -2V, the electric
field in the collector will be larger than the critical field (10kV/cm) and the increase of
output photocurrent will result in the reduction of net electric field and enhance the
6
drift-velocity of electron. Under -5V bias, the required photocurrent is thus to be
largest to eliminate the excess electric field and achieve near-ballistic transport
property.
The obtained high responsivity (1.14A/W) and high electrical bandwidth
(around 40GHz) under high output photocurrent operation of demonstrated device
ensure its applications to 40Gbit/sec fiber communication system. Fig. 5 shows the
measured and fitted frequency responses of device with 320m2 active area under
10mA and 15mA output photocurrent. The dc bias voltage is fixed at -4V.
The
bandwidth improvement under high current operation (15mA) is associated to the
reduction of ac capacitance [15] and transport time as discussed before.
Our
bandwidth simulation model has thus included the carrier transport time and
equivalent circuit model, whose value of capacitance depends on the output ac voltage
and photocurrent [1,2,10,15], to fit the measured frequency responses.
The adopted
drift-velocity in collector and photo-absorption layer of the two fitted traces is around
4  10 7 cm / sec and 1  10 7 cm / sec for the case of 15mA and 10mA excitation,
respectively.
We can clearly see that under high operation current (15mA) the
drift-velocity of electron ( 4  10 7 cm / sec ) is close to the value of overshoot velocity
of InP based compound semiconductors. As compared to the previous work on
evanescently-coupled or charge compensated UTC-PD [15,17], our device can
achieve almost the same electrical bandwidth (around 40GHz) even with a much
larger device size (320m2 vs. ~120m2) and responsivity (1.14/W vs. 0.76A/W [15]
and 0.45A/W [17]) performance. This result implies that NBUTC-PD can improve
the output power and responsivity performance by enlarging its device size without
affecting the speed performance seriously due to superior transport property of
photo-generated electrons.
Three traces in Fig. 6 represent the photo-generated RF
power versus dc photocurrent under three different bias voltages (-1V, -3V, and -5V).
7
The operating frequency is fixed at 40GHz. The ideal relation between RF power of
a 100% modulated large-signal and the average current on a 50Ω load is also plotted
as straight line for reference. The demonstrated maximum values of generated RF
power and dc photocurrent were limited by the maximum available optical power of
our measurement system.
Since, the device bandwidth increases with increasing the
photocurrent significantly under -5V bias, the output RF power is closer to the ideal
line than at low photocurrent.
The obtained values of maximum RF power (12dBm)
and current (32mA) are much higher than the previous work on p-i-n evanescently
coupled photodiodes with integrated tapers [3-5]. Furthermore, as compared to
state-of-the-art performances of reported UTC-PD [15], which is incorporated with an
evanescently coupled waveguide, our demonstrated device can have higher
responsivity (1.14A/W vs. 0.76A/W) and higher saturation current-bandwidth product
(1280mA-GHz vs. 1100mA-GHz). To our knowledge, this is the highest saturation
current and RF power (over 32mA vs. 12dBm) of long wavelength photodiode
reported with such a high responsivity (1.14A/W) and high electrical bandwidth
(~40GHz) [1-3].
In conclusion, we have demonstrated a novel photodiode at 1.55m wavelength.
Near-ballistic transport property of photo-generated electrons has been observed in
such device under high dc bias voltage (-5V) and high output photocurrent (~30mA).
Sate-of-the-art performances have been achieved by use of such device with a
320m2 active area: very high responsivity (1.14 A/W), broad bandwidth (~40 GHz),
and over 32mA saturation current with corresponding 12dBm output RF power at
40GHz operating frequency.
This work is sponsored by National Science Council of
Taiwan under grant number NSC-95-2215-E-008-003- and Department of Industrial
Technology,
Ministry
of
Economic
8
Affairs
under
grant
number
of
93-EC-17-A-07-S1-0001.
9
Figure Captions:
Figure 1.The cross-sectional view (a), top-view (b), and conceptual band diagram (c)
of
demonstrated
NBUTC-PD,
which
is
integrated
with
an
evanescently-coupled optical waveguide.
Figure 2. The optical pumping power versus output photocurrent of devices with
different active areas (circle: 200m2; triangle: 320m2).
Figure 3. The measured frequency responses of device with a 200m2 active area
under different output photocurrent and a fixed dc bias voltage (-5V).
(square: 0.5mA; circle: 5mA). The f3dB bandwidth is around 50GHz under
different photocurrent.
Figure 4. Electrical bandwidth versus photocurrent under different bias voltages of
device with a 320m2 active area (square: -1V; circle: -3V; triangle: -4V;
open circle: -5V).
Figure 5. The measured and fitted frequency responses of device with a 320m2
active area under different output photocurrent and a fixed dc bias voltage
(-4V) (square: measured trace under 10mA operation; circle: measured trace
under 15mA operation; open square: fitted trace under 10mA operation;
open circle: fitted trace under 15mA operation.).
Figure 6. RF power versus dc photocurrent of device with 320m2 active area under
different reverse bias voltages (square: -1V, circle: -3V, triangle: -5V) at
40GHz operating frequency.
The idea relation between RF power and
current on a 50Ω load is plotted as the straight line for reference.
10
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13