Long-Wavelength Waveguide Photodiodes for
Optical Subscriber Networks
by Masaki Funabashi *, Koji Hiraiwa *, Kazuaki Nishikata *2,
Nobumitsu Yamanaka *, Norihiro Iwai * and Akihiko Kasukawa *Waveguide photodiodes hold promise for long-wavelength applications in optical

ABSTRACT subscriber networks, and this paper reports on the methods used in their design
and manufacture, as well as on the results obtained from prototype devices. With a view to theimproved productivity and reduced cost associated with volume manufacture, the prototype were
fabricated by a full-wafer process using 2-inch InP wafers. The device showed an extremely low
dark current value --only a few hundred pA --and high responsivity --0.95 A/W or above. Characteristic deviation within wafers was evaluated, confirming that 2-inch wafers having high in-wafer uniformity with respect to dark current and responsivity could be produced with high yield. The devices
had a frequency bandwidth of approximately 4 GHz and a vertical coupling tolerance of 4µm, with
low intermodulation distortion and high reliability.

1.

INTRODUCTION

Keen expansion of the Internet and an increase in demand for large capacity transmission have forced us touse optical fibers in the telecommunication networks.
Furukawa Electric supplies various devices needed forsuch optical telecommunication systems. We have developed and are producing: optical fibers which are a major
breakthrough in optical telecommunication; semiconductor lasers used as a light source; optical amplifiers necessary for long distance telecommunication; photodiodes
which receive optical signals and convert them into electric signals and so on. These devices are essential to theoptical telecommunication systems. This paper reports on
the development of a photodiode which converts opticalsignals into electric signals.
Photodiodes used for optical telecommunications are
divided into two categories by their shapes, one is a surface-illuminated type, the other is a side-illuminated(waveguide) type. Furukawa's R&D center has been developing the latter devices, Waveguide Photodiodes(WGPD) which were considered to be widely acceptablein future. Figure 1 shows a view of the photodiode, andFigure 2 shows a cross-sectional structure of the same.
As shown in Figure 1, the WGPD has a shape similar toan ordinary edge-emitting laser, and receives light signals
at the edge surface of the device. The incident light travels
along the waveguide and attenuates gradually by absorption. Photocarriers, which are generated as light is ab* Semiconductor R&D Center, Yokohama Lab., R&D Div.
* 2 Furukawa Electric Technologies, Inc.

sorbed, are pulled out by a reverse-biased voltage andconverted into electrical current signals.
As shown in the figures, the incident direction of lightand the current flowing direction differ from each other.
Therefore, the length and thickness of the absorber canbe independently decided both from the electrical view
point or the optical view point. This advantageous feature
makes it possible to simultaneously design the WGPD interms of high-speed and high responsivity. Also by adopting a structure which absorbs light uniformly along the
waveguide, it is possible to achieve low intermodulation
distortion with high responsivity. In an optical access CATV
network, images are optically transmitted in analog signals, therefore low intermodulation distortion characteristics or high linearity photodiodes are largely demanded.
Thus, design-free WGPDs are suitable for these analog
applications. Another feature of the WGPD is that it is suitable for surface-mount technology. Surface-mount technology is one of the most excellent mounting methods

Figure 1

View of WGPD

67

that receive widespread attention and provides opticallyfunctional devices by integrating several waveguide typeoptical devices on the platform surface of a Si-wafer. Surface-mount technology is more than appropriate for the
production of low cost optical modules.
Recently the quantity of user information is rapidly increasing, therefore subscriber networks are adopting optical fibers, resulting in the need for low cost optical devices. Thus, we have carried out the development of WGPD
of low cost and mass-production type over a full-wafer process using 2 inch-wafers to evaluate the in-wafer uniformity of the device characteristics. The result of the study isreported here.
This paper describes a design method of WGPDs inchapter 2 and introduces a design guide based on our
method. In chapter 3, device fabrication procedures, mainlywith 2-inch full process, are reported. In chapter 4, thecharacteristics of the device fabricated and in-wafer uniformity in 2-inch process are reported. The last chapter 5
discusses the conclusion.

2.

DEVICE DESIGNThe key of the WGPD design is the size of the absorber.
This chapter illustrates the optimum design method forthe thickness, length and width of the absorber to meetthe specification required. In addition, two examples of
WGPD design are introduced: the digital design exhibits
wide coupling tolerance and high responsivity, while theanalog design features low intermoduration distortion andhigh responsivity, as well.
2.1 Design of Layer StructureFirstly, the design in thickness direction of a layer structure is described. The layer structure employs a pin-type
double-hetero diode structure. It may be said that the electric characteristics of a photodiode (high speed, lowintermodulation distortion, etc.) depend on the thicknessof the i-layer, and the optical properties (coupling efficiency,etc.) depend on the thickness of the core layer (whoserefractive index is larger than that of the cladding layer).
As illustrated in Figure 2, i-type absorber is sandwichedby InP cladding layers of p-type and n-type. Optical con-finement layers which have an intermediate refractive index are introduced between the absorber and the cladding layers to achieve independent optimizations of bothelectric characteristics and optical properties. In Figure 2,"1.15Q" means that the layers are a GaInAsP layer of
1.15µm in bandgap wavelength.
2.1.1

Optimization of Optical Property (Coupling Efficiency)
In this chapter, the design for high coupling efficiency in
the WGPD with a single mode optical fiber is described.
This coupling efficiency is especially important in the optical properties. In order to acquire high coupling efficiency,
it is necessary to design the size of mode field diameter of
optical fiber and that of waveguide in the photodiode to besame. Therefore, a multi-mode waveguide structure isadopted1). The multi-mode waveguide type in WGPDs hasan advantage of less degradation in coupling efficiency
even when their axis orientations are shifted between the
optical fiber and the WGPD, as compared with the single
mode waveguide. The simulation results of optical confinement factor into the absorber and mode field diameterof fundamental mode versus the absorber thickness areshown in Figure 3, in the condition when the total layer
thickness of the absorber and the optical confinement layers is constant (5µm). Figure 3 shows that the optical confinement factor is monotonously increasing with the absorber thickness, while the mode field diameter has a tendency of increasing in both thinner and thicker ranges in
the absorber thickness. Therefore, in order to obtain thehigh coupling efficiency, there come two solutions, theabsorber thickness must be thickened, or just opposite,must be thinned. However, in consideration of the coupling tolerance in the case of axis mismatching, the thicker
absorber layer is better for less degradation in the
responsivity because optical confinement factor is large
in the higher order modes.
2.1.2 Optimization of Electric Characteristics
The electric characteristics remarkably affected by the layer
structure are junction capacitance and intermodulation

Figure 3
Figure 2

68

Cross-sectional layer structure of WGPD

Simulation results of confinement factor andmode field diameter versus absorber thickness

Furukawa Review, No. 18. 1999

distortion (linearity). The junction capacitance cannot be
too large unless i-layer becomes so thinned. In order toacquire low intermodulation distortion in analog type, it isnecessary to thin the absorber layer for reducing the quantity of optical absorption per unit length. Thus, it is possible to suppress the space-charge effect caused by the
localization of photocarriers.
2.2 Design of Waveguide Length
The waveguide length must be designed to absorb all of
the incident light. Shorter waveguide length reduces
responsitivity, while too longer waveguide degrades thehigh-speed characteristics. In order to absorb more than
95% of light, the following formula must be adopted:
exp ( -ΓαL ) < 1-0.95
where, Γ is the optical confinement factor into theabsorber, α is the absorption coefficient, L is the lengthof the waveguide. For example, Γ =10%, α =104 cm-1,
then L > 30 µm.
2.3 Design of Waveguide Width
The waveguide width is mainly determined by the junction
capacitance, coupling tolerance and the ease of the fabrication process. Narrower width can reduce the capacitance, but the coupling tolerance becomes severe and fabrication process becomes difficult.
2.4 Design Examples
Two types of WGPDs, digital and analog, based on thedesign guideline mentioned above, are introduced. Table
1 exhibits the structure of the two types.
2.4.1 Digital WGPD Design
One design, which has high coupling efficiency with asingle mode optical fiber (high responsivity) and has awide range of coupling tolerance, is introduced here.
This design was intended to use the device at 1.3µm
wavelength band, the absorber is made of a GaInAsP layer
with 1.4µm band-gap. This absorber is sandwiched by
optical confinement layers of GaInAsP with 1.15µm bandgap. The thickness of the absorber is 3µm and those of
optical confinement layers are 2µm each, all thickened
sufficiently to obtain wider ranges of coupling tolerance.
The optical confinement factor into the absorber is as large
as about 99 % in fundamental mode,therefore the length

Table 1

Examples of design for digital and analog WGPDs
DigitalAnalogContact layer

1.55Q, 0.3 m thick

1.55Q, 0.3 m thickp cladding layer

InP, 2 m thick

InP, 2 m thick

Optical confinement
layer

1.15Q, 2 m thick

1.15Q, 3 m thickAbsorption layer

1.4Q, 3 m thick

InGaAs, 0.05 m thick

Optical confinement
layer

1.15Q, 2 m thick

1.15Q, 3 m thickn cladding layer

InP, 2 m thick

InP, 2 m thickof the waveguide is just enough in several µms under an
optimum coupling condition.In consideration of the coupling tolerance and the easiness of fabrication, the lengthis decided as L = 60µm, the width of waveguide is alsodecided as 30µm.
2.4.2 Analog WGPD Design
Since low intermodulation distortion characteristics are essential in analog WGPDs, it is necessary to limit the quantity of light absorption per unit length by reducing the thickness of the absorber. This reduction makes it possible tolimit the generation of photocarriers in a certain spot andto prevent the space-charge effect, which is the cause of
the degradation of linearity. The sample adopted a GaInAs
absorber 0.05µm thick, and the thickness of both upperand lower optical confinement layers was 3µm to obtain ahigh coupling efficiency by enlarging the mode field diameter. Thus in this structure, the optical confinement factor
into the absorber is as small as approximately 5% in fundamental mode, and the quantity of light absorption perunit length is also small, the length of the waveguide isdetermined as 150µm, and the width of waveguide is defined as 18µm to reduce junction capacitance.

3.

FABRICATIONIn this chapter, the results of fabrication of WGPDs on a 2inch wafer to achieve low cost mass-production are described2). Firstly, a layer structure was grown on the InP
wafer, shown in Figure 2, by the use of MOCVD crystal
growth equipment. As mentioned above, the layer thickness was very thick as 10µm for high coupling efficiency
and wide coupling tolerance. Slight lattice mismatching to
the wafer is inevitably generated because of the thick quarterly layers of GaInAsP in the absorber and the opticalconfinement layers. We tried to control the quantity of thelattice mismatching to be less than 100 seconds in X-ray
diffraction measurement, however, because of the thick
growth layers, the wafer was warped by the accumulated
lattice mismatching even in a slight lattice mismatchingrate. Sometimes we experienced in-wafer warp as large
as 40µm. It is understood that the cause of the warp isdue to the stress distortion by the lattice mismatching between the InP wafer and the grown layer. This effect was
confirmed by the fact that the direction of warp (domeshape / bowl-shape) and the direction of the lattice mismatching (compressive / tensile) were consistent.
Because of this wafer warp, sometimes there happened
troubles of wafer crack during the photo-lithography process using the conventional contact-type mask aligner. In
order to solve the warp problem, we deposited a SiNx filmon the back of the wafer to cancel out the warp of wafer.
Our experiment showed that the SiNx thin film depositedon the surface of an InP wafer by the plasma-CVD would
enhance the warp in bowl-shape in proportion to the film
thickness. We tried to control the warp of the wafer using
this physical property. The improvement of the flatness was
confirmed after the deposition of a 300 nm thick SiNx film

69

Dark current [pA]

Photograph after 2-inch full wafer
fabrication process

on the back of the wafer which had a 36µm bowl-shape
warp. The amount of warp was reduced down to 4.4µm toclear the problem during the photo-lithography process.
The stripe patterns were formed on the flattened wafer
by photo-lithography, and wet chemical etching was carried out to the depth through the absorber and the opticalconfinement layers to form a mesa structure. Because of
the etching through quarterly layers, the thick epitaxial layer
is divided into many small areas, so the warp of the wafer
is much reduced. The size of stripes is 30µm wide, 60µm
long for digital type, and 18µm wide, 150µm long for analog type.
After forming the mesa-stripe, thin SiNx insulation film
is formed for passivation. Forming the mesa-structure inevitably makes a much uneven surface, therefore, apolyimide film is deposited to flatten the surface. Then acontact window is formed on the mesa-stripe to deposit
an upper electrode, and after polishing the wafer to reduce the thickness, finally a lower electrode is deposited.
The wafer surface is shown in Photo 1, which is fabricated by a full-wafer process using a 2-inch InP wafer. The
chip-size of the device is 250µm wide and 300µm long,
therefore, the devices of about 20,000 pieces are obtainedby excluding the area of 3 mm wide periphery of the wafer.
After the wafer process, arrayed device bars are cut off
from the wafer by cleaving, and a SiNx film for anti-reflection is formed on the incident edge of the device. Lastly,bars are cleaved into chips and arrays of the desired channel number.

4.

CHARACTERISTICS

Figure 4

Reverse bias voltage dependence of dark-current
and capacitanceY Position [mm]Photo 1X Position [mm]Figure 5

Dark-current distribution in 2-inch wafer

the device in analog type and the device in digital type
differ with each other in layer structure, but both dark-currents are low and of the same level.
Figure 4 also shows the voltage dependence of capacitance. Upon applying a reverse bias voltage to the device,
the depletion layer spreads to reduce the device capacitance. Very small capacitance of about 215 fF can be obtained at 10 V.
The dark-current distribution of WGPDs on the wafer
surface fabricated by 2-inch full process is evaluatedthrough sampling measurements. The result of the darkcurrent mapping is shown in Figure 5. The measurement
was carried out at a bias voltage of 5 V. The result showsthat the average dark-current is 238 pA, and standard
deviation is 24 pA, indicating the good uniformity of low
dark-current distribution except the 3 mm wide peripheryof the wafer.

Followings are the characteristics of WGPDs.
4.1 Dark-current and Capacitance
Figure 4 shows an example of dark-current / voltage characteristics. The device shows a very small dark-current of
83 pA at 3 V reverse bias voltage. The result in Figure 4 isderived from the digital type device. As mentioned above,70

4.2 Responsivity
Here the measured result of responsivity in consecutive
60-channel WGPD array is described. The 60-channel
array is cut off from a 2-inch full-processed wafer, and fabricated by AR coating of 1.3µm on the incident facet.
The responsivity was calculated by measuring the photo-

Furukawa Review, No. 18. 1999

current produced by an incident light with 1.3µm wavelength and 0.1 mW power, while an optical fiber is faced to
the front facet of WGPD (butt-coupling configuration). A
flat-end optical fiber with 6µmφ mode field diameter wasused in the measurement. The result is shown in Figure 6.
All consecutive 60 channels showed high responsivity ofmore than 0.95 A/W. The average responsivity was as highas 0.987 A/W, and the standard deviation was 0.011 A/W.
Considering the large dimensions of the device --250µm
between elements and 15 mm for the length of 60-channel array-- the variation of responsivity was extremely small.
The measurements were carried out with a bias voltageof 5 V, the bias voltage dependence of the responsivity
was small at the voltages higher than 1 V.

Dark current@5V [pA]

4.3 Coupling Tolerance
The responsivity (coupling tolerance) in the case of coreaxis mismatching between the waveguide of WGPD and
optical fiber was measured. A single mode fiber of 10µmφmode field diameter was used. Figure 7 shows the results.
Z-axis represents the direction of waveguide, x-axis does

Figure 6

Figure 7

Responsivity in a 60ch WGPD array

Coupling tolerance in a WGPD

the parallel direction to the wafer and y-axis shows thevertical direction to the wafer. The width of the waveguideof the evaluated device is 30µm, the tolerance in x-axis
direction is 24µm in the width in which the responsivity islowered by 0.5dB (0.891 times against the peak value).
This tolerance is estimated to be wide enough in consideration of the present day surface-mount technology precision. Adopting multi-mode waveguides in y-axis orientation we obtained a good result of 4µm in 0.5 dB tolerance
degradation.
4.4 Frequency Response
Figure 8 shows the frequency response of waveguide
photodiodes evaluating the high-speed characteristics. In
order to evaluate the characteristics in array configuration, frequency response characteristics in each channel
was measured in 8-ch array. The incident light was 1.3µm
in wavelength, the intensity was 1 mW. The optical joint of
the optical fiber and the device was made by butt-coupling, the same as for the responsivity measurement. In
this measurement a standard single mode fiber of 10µmφin mode field diameter was used. Figure 8 shows that every channel gives the same shape of frequency response
which suggests a uniformity good enough in high-speed
characteristics. Also Figure 8 indicates that the frequencyof lowering by 3 dB in frequency response is approximately
4 GHz. Uneven responses in a certain point in Figure 8
are caused by the noises through the measuring system.
The electrical crosstalk between devices among the 8ch waveguide photodiodes was measured. The electric
signals from an adjacent device to the device, which is
jointed to an optical fiber was observed to evaluate thecrosstalk components. The electric signals affected by theadjacent device were small enough and less than measurable limits.
4.5 Analog Intermodulation Distortion
The evaluations of intermodulation distortion characteristics of the waveguide photodiode designed and fabricatedfor analog use are described3). Photo 2 shows a prototype

Figure 8Frequency response in an 8-ch WGPD array71

Photograph of analog WGPD pigtail
moduleof CAN type analog WGPD pigtail module. The measurement of the intermodulation distortion was carried out using 2-tone method. Light signals modulated by different
frequencies of 130 MHz and 90 MHz were combined andcoupled into the WGPD. The second and thirdintermodulation distortion components (IMD2, IMD3)
among the electrical signal outputs from the photodiodewere measured. The optical input power into the photodiode was 1mW. Figure 9 shows the reverse bias voltage
dependence of IMD2 and IMD3. When applying enoughreverse bias voltages, IMD2 and IMD3 were less than -80
dBc and -100 dBc respectively: both low enough for practical use.
4.6 Reliability
In order to evaluate reliability of the device, an accelerated test was carried out. The accelerated test temperature was 160°C, the reverse bias voltage was 20 V, thendark-currents at room temperature were measured for agiven time. Figure 10 shows the results. The increase indark-current for 5,000 hours was not observed, so the highreliability of the device was confirmed.

Figure 9

Intermodulation distortion characteristics in an
analog WGPD Module

Dark current@RT [pA]

Photo 2

Figure 10 Results of reliability test

REFERENCES

5.

SUMMARY

The design and fabrication methods for a waveguide photodiode have been studied, which is promising as the longwavelength photodiode for optical subscriber networks. The
results of prototypes are reported. We tried a full-process
fabrication of 2-inch InP wafer and succeeded in producing devices with high yield and high in-wafer uniformity.
The fabricated devices showed a low dark- current, highresponsivity, wide coupling tolerance, 4 GHz wide bandwidth, low intermodulation distortion and high reliability.
These WGPDs introduced here are promising candidates for long wavelength photodiodes in optical subscriber
networks in the near future, especially for photodiodes in
optical integrated circuits.

72[1] K. Kato, S. Hata, K. Kawano, J. Yoshida, and Atsuo Kozen: "Ahigh-efficiency 50GHz InGaAs multimode waveguide photodetector", IEEE J. Quantum Electron. 28(1992), 2728.
[2] M. Funabashi, K. Nishikata, K. Hiraiwa, N. Yamanaka, N. Iwai,
and A. Kasukawa: "Highly uniform waveguide photodiodes
fabricated on a 2-inch wafer with low darkcurrent and highresponsivity", Indium Phosphide and Related Materials(IPRM'98) Proceedings, WA2-3, (1998), p361.
[3] K. Nishikata, K. Hiraiwa, M. Funabashi, N. Iwai, N. Yamanaka,
T. Wakisaka, A. Kasukawa: "High sensitivity and low
intermodulation distortion p-i-n waveguide photodiode modules", Optical Fiber Communication Conference(OFC'98)
Technical Digest, WM10, (1998), 188.
Manuscript received on October 10, 1998.

Furukawa Review, No. 18. 1999