Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
1999-04-19
2001-04-24
Tran, Minh Loan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S021000, C257S022000, C257S185000, C257S186000, C257S190000
Reexamination Certificate
active
06222200
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to photodetectors, and in particular to photodetectors with extended wavelength range.
BACKGROUND OF THE INVENTION
In recent years, there has been a considerable interest in optical fiber communication systems operating in the long wavelength range. This is of particular interest because of the need to increase the capacity of present wavelength division multiplex (WDM) systems by adding channels at longer wavelengths. For example, it was shown that by utilizing a spectrum range up to 1602 nm only, the number of channels can be more than doubled.
Market trends to longer wavelengths create an immediate need for optical photodetectors operable within an extended wavelength range. Photodetectors are spectrally limited on the long wavelength side by the bandgap of semiconductor material which is used as an active region. As a result, most widely used photodetectors based on InGaAs exhibit a decrease in responsivity as the operating wavelength is extended beyond approximately 1580-1600 nm.
Numerous attempts have been made to extend a wavelength range of photodetectors. One of them is to maintain photodetectors at elevated temperatures to reduce their bandgaps and thus to extend spectral responsivities. This approach has a number of drawbacks, namely the reduction of material bandgap is accompanied by high leakage current, which increases exponentially with temperature, shortened device lifetime and increased thermal loading on the system. In another approach, by applying strain in the active region, the bandgap of the material can be reduced. In the case of the InGaAs telecom photodetector, for example, strain is avoided by growing the active layer such that its crystalline lattice constant is matched to the InP substrate. By adjusting the In
x
Ga
1−x
As composition x, layers with larger or smaller lattice constant can be grown, which results in compressive or tensile strain when the layers get deposited on InP. However, the thickness of layers that can be grown without introducing substantial crystalline defects is limited to well known values which depend on the magnitude of the strain. Exceeding such thickness limits causes formation of crystalline defects which can degrade the device performance by increasing the leakage current and limiting its lifetime, see e.g. publication by V. S. Ban, A. M Joshi and N. B. Urli “Characterization of process-induced defects in 2.6 &mgr;m InGaAs photodiodes”, SPIE, Vol. 1985, pp. 234-243, 1993. Noise can also be increased, as shown, e.g. in publication by D. Pogany, S. Ababou, G. Guillot et. al. “Study of RTS Noise and Excess Currents in Lattice-Mismatched InP/InGaAs/InP Photodetector Arrays”, Solid State Electronics, Vol. 38, No. 1, pp. 37-49, 1995. For strain values of interest, the thickness of substantially defect free layers is generally not sufficient to attain the required responsivity specifications of the photodetector. When layers exceeding the defect free thickness are grown, they become mismatched with the substrate, and mismatch disclocations as well as crystalline defects are introduced. Though lattice mismatched devices are available on the market, they have prohibitively high leakage currents for telecom receiver application. The examples of lattice mismatched photodetectors along with discussions of the associated problems may be found in the following publications: K. R. Linga, G. H. Olsen, V. S.Ban et.al. “Dark Current Analysis and Characterization of In
x
Ga
1−x
As/InAs
y
P
1−y
Graded Photodiodes with x>0.53 for Response to Longer Wavelengths(>1.7 &mgr;m)”, Journal of Lightwave Technology, Vol. 10, No. 8, pp. 1050-1054, August 1992; R. U. Martinelli, T. J. Zamerowski and P. A. Longway “2.6 &mgr;m InGaAs photodiodes”, Applied Physics Letters, vol. 53, No. 11, pp. 989-991, September 1988. Other II-VI and III-V compound semiconductors materials are also available on the market, but their process maturity and performance is somewhat inferior to the requirements of fiberoptic system specifications. In one more approach a sandwiched structure of the active region of the photodetector has been proposed, where layers with different strain interleave, thus allowing to improve some other deteriorating characteristics of the detectors. For example, in U.S. Pat. No. 5,608,230 a target is to reduce a relatively large dark current of the detector, while U.S. Pat. No. 5,536,948 aims to mitigate defect propagation from the base layer to the detector elements. U.S. Pat. No. 4,711,857 to Cheng provides a superlattice detector whose wavelength sensitivity is tunable during manufacturing of the device, and U.S. Pat. No. 5,574,289 to Aoki concentrates on the detector suitable for light signals having different polarizations.
As it follows from the above discussion, an extension of a wavelength range is usually achieved at the expense of deterioration of other important parameters of the photodetectors which remains a significant problem in fiber optic systems.
Accordingly, there is a need for development of alternative structures of optical photodetectors which would provide an operation within an extended wavelength range, while maintaining high performance and reliability.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a photodetector having an extended wavelength range while maintaining high performance and reliability.
According to one aspect of the present invention there is provided a photodetector comprising:
a) a substrate;
b) an active region formed on the substrate, the active region comprising:
a plurality of semiconductor layers, the thickness of each layer being limited to preclude formation of crystalline defects,
the total effective strain of the active region being balanced, wherein the effective strain is defined as a product of strain by thickness of the layer,
the thicknesses of the layers being optimized to provide a pre-determined optical absorption over a required wavelength range; and
c) means for providing depletion of the active region of electric charge carriers.
Preferably, the thicknesses of the layers is optimized to provide a maximum optical absorption over the required range of wavelengths. Advantageously, the layers with higher optical absorption constant are made thicker than the layers with lower optical absorption constant to provide sufficient responsivity of the photodetector.
Beneficially, the active region of the photodetector comprises a plurality of alternating layers, placed under compressive and tensile strain correspondingly, the layers with higher optical absorption constant having higher thickness than the layers with lower optical absorption constant.
In one embodiment, the active region is formed on InP substrate and includes alternating compressive and tensile strain layers made of InGaAs composition, the compressive strain layers having higher absorption and correspondingly higher thickness. The photodetector of this embodiment provides operation within a wavelength range from about 970 nm to about 1800 nm. In modifications to this embodiment, the active region may be formed on InP substrate and include alternating compressive strain InGaAs layers and tensile strain Ge layers. Such a photodetector may operate within a wavelength range from about 970 nm to about 2000 nm. With other suitable materials used for the active region, it is possible to extend a long wavelength operation even further, e.g. up to about 2200 nm.
To preclude formation of crystalline defects, e.g. misfit disclocations, the thickness of each layer is limited so that a product of strain by thickness of the layer would not exceed about 20% nm.
In another embodiment of the invention, interfaces between the layers are compositionally graded. The interfaces between the layers may be graded either continuously or by forming an interstitial grade layer or several layers. If required, the interfaces, following the compressive strain layers, may be graded only, or alternatively, the grading may be provided at the interfaces, followin
Donnelly Victoria
Nortel Networks Limited
Tran Minh Loan
LandOfFree
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