Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2001-07-17
2003-12-30
Luu, Thanh X. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C257S436000
Reexamination Certificate
active
06670600
ABSTRACT:
PRIORITY TO FOREIGN APPLICATIONS
This application claims priority to Japanese Patent Application No. P2001-084307.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a semiconductor photodetector, a semiconductor photo receiver, and a semiconductor device and more particularly relates to a surface illuminated type photodetector in which incident light is cast perpendicularly onto the semiconductor substrate surface and is converted into electrical signals, and a semiconductor photo receiver and a semiconductor device which use the same for optical communications.
2. Description of the Background
In recent years, the need for transmitting large capacity data such as image data has been increasing with the rapid expansion of information services based on communications media such as the Internet. Likewise, there is a need to increase the transmission capacity for such information networks that carry this data.
To construct an optical communications system with a transmission capacity over 10 Gbps, it is necessary to develop an optical transmission device which features ultrahigh speed and high sensitivity. To develop such an optical transmission device, it may be necessary to use an ultrahigh speed, high-sensitivity semiconductor photodetector capable of receiving optical signals and of converting them into electrical signals.
The response velocity of a semiconductor photodetector is determined by the CR constant (calculated as the product of capacitance C and resistance R) and the transit time of the carrier excited by incoming optical signals.
To increase the response velocity, the capacitance C and resistance R must be decreased and the transit time must be shortened. Since the transit time is proportional to the thickness of the photo absorbing layer of the semiconductor photodetector, the photo absorbing layer is preferably thinned as much as possible. However, as the photo absorbing layer becomes thinner, the amount of light that is transmitted but not absorbed by the photo absorbing layer increases, causing a deterioration in sensitivity.
As mentioned above in connection with the thickness of the photo absorbing layer, there is a trade-off between the response velocity and sensitivity, and vice versa. Therefore, it may be difficult to develop a semiconductor photodetector that provides both the desired high response velocity and sensitivity. This has caused a bottleneck in the development of an ultrahigh speed, high sensitivity optical transmission device.
As a conventional solution to the above problem, the method disclosed in JP-A-218488/1993 is known. In this method, a reflector that has a size suitable for the effective detecting area size and consists of two films lying in contact with the semiconductor layer—a dielectric film (lower) and an electrode metal film (upper)—is formed on the side of a substrate that is opposite to its light incidence side and that is reached by incident light passing through the photo absorbing layer. The light that is transmitted but not absorbed by the photo absorbing layer is efficiently reflected back to the semiconductor layer.
FIG. 2
is a schematic sectional view showing a backside illuminated type avalanche multiplication photodetector (APD) based on the above-mentioned art. As shown in the figure, the following layers are consecutively formed on an n-type InP substrate
21
in the following order: a high density n-type InAlAs buffer layer with 0.7 &mgr;m thickness
22
; a low density n-type InAlAs multiplication layer with 0.2 &mgr;m thickness
23
; an undoped InGaAs/InAlAs super lattice layer with 0.05 &mgr;m thickness
24
; a low density p-type InGaAs photo absorbing layer with 1.0 &mgr;m thickness
25
; a p-type InAlAs buffer layer with 1.0 &mgr;m thickness
26
; and a high density p-type InGaAs contact layer with 0.1 &mgr;m thickness
27
. The above composition results in a mesa structure with a diameter of the p-n junction of 50 &mgr;m.
The surface of the substrate is thereafter passivated by a SiN insulating film
28
, and an n-type ohmic electrode
29
is formed in a desired area on the substrate
21
. A p-type ohmic electrode
30
lies not only on the contact layer
27
but also on the SiN insulating film
28
with a 40 &mgr;m diameter formed inside the effective detecting area of the contact layer
27
.
The dielectric film
28
which consists of a SiN or other similar insulating film hardly reacts with the p-type InGaAs contact layer
26
as a semiconductor layer and p-type ohmic electrode
30
even when it has been annealed at a high temperature in the manufacturing process. Therefore, the uniformity of the interface is maintained at a satisfactory level. The metal surface of the p-type ohmic electrode
30
in contact with the SiN insulating film
28
, or a reflector
31
composed of the SiN insulating film
28
and p-type ohmic electrode
30
, completely reflects the transmitted light back into the photo absorbing layer
25
with a reflectivity of 100%, which leads to an improvement in quantum efficiency.
As the area of the reflector
31
increases, the effective detecting area size may be larger. Hence, the photodetector's quantum efficiency depends on the area of the reflector. Also, since an ohmic contact area between the electrode metal film and the semiconductor layer may be formed in the surrounding area (other than the area of the reflector
31
), a low reflectivity zone generated by the ohmic contact area does not directly affect the reflection of light to a substantial degree.
SUMMARY OF THE INVENTION
The influence of capacitance C may be a more important factor as the transmission capacities in optical communications and other applications increases. Therefore, for the development of a higher speed, sensitive optical transmission device, it is preferable to reduce the photodetector size in order to decrease the capacitance. With respect to the size of the photodetector, the ideal ratio of the minimum effective detecting area size to the diameter of the p-n junction is equal to 1.
If the ratio of the effective detecting area size to the p-n junction diameter is made as near as possible to 1 using the above-mentioned prior art, the size of the ohmic contact area formed in an area other than the reflector area must be decreased in order to obtain a sufficient effective detecting area (area of the reflector), which would result in an increase in the resistance. Conversely, if the ohmic contact area is made sufficient, the effective detecting area size (area of the reflector) should be smaller, resulting in a deterioration in sensitivity.
For example, in the case of the above-described semiconductor photodetector, the diameter of the p-n junction is 50 &mgr;m, which makes the capacitance approximately 0.1 pF, and the resulting frequency response at least 10 GHz. Assuming, for example, that this photodetector runs at 40 GHz, the limit for the capacitance is 0.05 pF because of the CR constant, which leads to a calculated result of approximately 34 &mgr;m as the optimum diameter of the p-n junction.
Consequently, if a photodetector with the same level of resistance is manufactured using this conventional technique, the effective detecting area size would be 20 &mgr;m or less. This not only would necessitate a high accuracy in the optical axis alignment with the fiber in the packaging process but also may cause an optical axis alignment error to occur due to a change in the ambient temperature during use. As a result, the quantum efficiency would likely decrease. Therefore, for a surface illuminated type photodetector manufactured using the conventional technique as explained above, it has been difficult to reduce its size to decrease its capacitance C, which has hampered the development of an ultrahigh speed, high sensitivity optical transmission device.
In at least one embodiment, the present invention preferably provides a high sensitivity, high speed surface illuminated type photodetector that does not cause an increase in the resistance and a decrease in the quantum efficiency a
Matsuoka Yasunobu
Tanaka Shigehisa
Terano Akihisa
LandOfFree
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