Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Field effect transistor
Reexamination Certificate
2002-04-11
2004-08-31
Wille, Douglas (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Field effect transistor
C257S431000
Reexamination Certificate
active
06784466
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to semiconductors and more particularly to integrated optoelectronic receiver devices and circuits incorporating photodetectors and MODFETs formed with SiGe layers.
2. Description of the Prior Art
The advent of fiber-optic communications technology has increased the demand for high-speed optoelectronic devices and circuits that operate with data rates of greater than 1 Gbit/sec. In particular, a growing market exists for local area networks and short-haul optical connections that operate at wavelengths of &lgr;=850 nm. It would be desirable to fabricate these circuits monolithically due to the lower cost of production and performance advantages over discrete components. It would also be desirable to fabricate such circuits entirely in a silicon-based technology due to the reduced cost arising from their compatibility with existing Si-based technologies including CMOS logic circuits.
In the prior art, GaAs has been the previous choice for monolithically integrated optical receivers operating at &lgr;=850 nm. This is due to the favorable intrinsic material properties of GaAs; the absorption length for 850 nm-radiation in GaAs is &agr;
−1
=1 &mgr;m, and the electron mobility in GaAs is roughly 8500 cm
2
/Vs at room temperature. J. S. Wang et al.,
IEEE Phot. Tech. Lett
. 5, 316 (1993) demonstrated the fabrication of high-speed integrated photoreceiver circuits composed of GaAs metal-semiconductor-metal (MSM) photodiodes and MESFETs with −3 dB bandwidths as high as 11 GHz at &lgr;=850 nm. Further improvement of GaAs-based receiver performance has been obtained using GaAs MSM photodetectors integrated with AlGaAs/GaAs modulation-doped field effect transistors (MODFETs). V. Hurm et al.,
Electron. Lett
. 29, 9 (1993) demonstrated photoreceiver circuits of this type with −3 dB bandwidths as high as 14 GHz at &lgr;=850 nm.
In order to replace GaAs, an integrated Si-based technology must have comparable performance to GaAs and a relatively low-cost process. However, the intrinsic material properties of Si are much less favorable compared to GaAs. The absorption length in Si for 850 nm-radiation is &agr;
−1
=20 &mgr;m, which is over an order of magnitude longer than in GaAs. Therefore, for a Si photodetector to have high responsivity it must have a thick absorbing region making the detector very slow, and for high speed the absorbing region should be very thin resulting in an extremely poor responsivity. For instance, Y. S. He et al.,
Electron. Lett
. 29, 9 (1993) demonstrated the operation of a lateral p-i-n photodiode integrated with a Si NMOS technology with a responsivity of 0.48 A/W at &lgr;=870 nm, but with a −3 dB bandwidth of only 900 MHz. Moreover, these results were only made possible by using ultra-high purity Si, and an extremely large bias voltage of 30 V. On the other hand, M. Y. Liu et al.,
Appl. Phys. Lett
. 65, 887 (1994) demonstrated operation of a Si on insulator (SOI) MSM photodiode with an absorbing region thickness of only 0.1 &mgr;m that had a bandwidth over 100 GHz, but a severely-degraded responsivity at &lgr;=780 nm of 0.0057 A/W. Improvements in the bandwidth/responsivity tradeoff of Si photodiodes are possible, for instance, in U.S. Pat. No. 5,589,704 which was issued on Dec. 31, 1996 to B. F. Levine, the responsivity of an MSM detector was shown to increase by a factor of close to 4 by roughening the surface of a Si epi-layer grown on an SOI wafer. However such a technique is limited to use with SOI substrates, and may not be suitable for practical applications due to the complexities of the roughening process. Si photodetectors are further hindered by the fact that the electron mobility in Si/SiO
2
inversion layers is several times lower than GaAs at room temperature, and the frequency performance and gain of Si NMOS devices is considerably poorer compared to GaAs MESFETs.
In U.S. Pat. No. 5,525,828 which was issued on Jun. 11, 1996 to E. Bassous et al., it was noted that the speed and/or responsivity of Si MSM photodetectors could be increased by adding a certain percentage of Ge to the absorbing layer. Increasing the percent Ge-composition of Si
1−x
Ge
x
alloy decreases the absorption length, and increases the electron and hole mobilities thereby leading to potentially faster devices.
It has also been shown that field-effect transistors fabricated on Si/Si
1−x
Ge
x
layer structures offer considerable advantages over bulk Si transistors. For n-channel MODFETs incorporating tensile-strained Si/Si
1−x
Ge
x
quantum wells, frequency performance is considerably better than Si MOSFETs for a given gate length device. Similar performance advantages can be obtained in p-channel MODFETs fabricated on compressive-strained Si
1−y
Ge
y
/Si
1−x
Ge
x
quantum wells. For instance, M. Arafa et al.,
IEEE Electron. Dev. Lett
. 17, 586 (1996) obtained unity current-gain cutoff frequencies of 70 GHz for 0.1 &mgr;m gate length p-channel transistors fabricated on compressive-strained Si
0.7
Ge
0.3
/Si
0.3
Ge
0.7
heterostructures. In U.S. Pat. No. 5,659,187 which was issued on Aug. 19, 1997 to F. K. Legoues and B. S. Meyerson, it was shown that a low-defect density layer of relaxed Si
1−x
Ge
x
, with arbitrary Ge composition can be grown on a lattice-mismatched substrate using an intermediate graded-composition buffer layer where strain has been relieved in the buffer layer or below via activation of modified Frank Read sources which is a mechanism to generate new dislocations. This work demonstrated the practicality of producing devices and circuits using Si/SiGe heterostructures grown on a SiGe graded composition buffer layer on Si substrates. Finally, U.S. Pat. No. 5,534,713 which was issued on Jul. 9, 1996 to K. Ismail and F. Stern showed that complementary logic circuits could be fabricated using high-mobility electron and hole channels fabricated in strained Si/SiGe layers grown on relaxed SiGe buffer layers.
Despite the apparent advantages of SiGe technology over bulk Si for photodetectors, MODFETs, and CMOS logic circuits, the concept of combining these devices to form integrated photoreceiver circuits has not been suggested, nor has a clear method for monolithically integrating these structures in such a way as to allow high-frequency operation and low cost Si manufacturing been suggested.
It is an object of the present invention to provide a simple means of monolithically integrating a photodetector with high speed and responsivity with a microwave transistor on a Si substrate in such a way as to allow high frequency performance better than Si and comparable to that achievable in GaAs.
It is a further object of this invention to provide a method for fabricating an optoelectronic integrated circuit using a process that is fully compatible with standard Si processing.
It is still a further object of this invention to provide a means for monolithically integrating a high-speed photoreceiver circuit with a practical CMOS logic manufacturing technology.
SUMMARY OF THE INVENTION
A design and method of fabricating an optoelectronic integrated circuit (OEIC) on a Si substrate is presented. This is achieved by using a high-quality, Si/SiGe heterostructure grown on a Si substrate to monolithically integrate a high-speed and responsivity SiGe photodetector and a modulation-doped field effect transistor on the same wafer. A typical layer structure consists of a Si substrate, a graded Ge-content Si
1−x
Ge
x
buffer layer, a thick, undoped relaxed Si
1−y
Ge
y
buffer layer, a Si quantum well, a Si
1−y
Ge
y
undoped offset layer, a doped Si
1−y
Ge
y
supply layer, and optionally a Si surface layer may be incorporated. A MODFET can be fabricated on the layer structure using mesa isolation, and by defining source, drain and gate electrodes, while an MSM photodetector can be created on the etched surface by depositing interdigitated Schottky electrodes on the
Chu Jack Oon
Ismail Khalid EzzEldin
Klepser Bernd-Ulrich H.
Koester Steven John
Trepp, Esq. Robert M.
Wille Douglas
LandOfFree
Si/SiGe optoelectronic integrated circuits does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Si/SiGe optoelectronic integrated circuits, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Si/SiGe optoelectronic integrated circuits will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3311195