Reduced dark current pin photo diodes using intentional doping

Details Reduced dark current pin photo diodes using intentional doping Reduced dark current pin photo diodes using intentional doping

2001-02-07

2003-06-03

Meier, Stephen D. (Department: 2822)

Active solid-state devices (e.g., transistors, solid-state diode

Responsive to non-electrical signal

Electromagnetic or particle radiation

C257S458000

Reexamination Certificate

active

06573581

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a device for detecting light in the near infrared spectrum utilizing lattice matched InGaAs.
2. Description of Related Art
Photodetectors made of InGaAs lattice matched to InP have been used in many applications from spectroscopy to imaging (M. J. Cohen and G. H. Olsen “Near-IR Imaging Cameras Operate at Room Temperature,” Laser Focus World 27,21 (1991)) to communications (H. Haupt, “InP-based components for telecom systems in Europe”,
Proc. of the Ninth International Conference on Indium Phosphide and Related materials, Cape Cod, Mass
., 3 (1997)). This material detects light from 0.9-1.7 &mgr;m. It has been fabricated into single element detectors, 1-D linear arrays up to 512 elements long, and 2-D arrays as large as 320×240 elements. (G. H. Olsen, “InGaAs fills the near-IR detector-array vacuum”, Laser Focus World,27,21(1991), M. H. Ettenberg, M. Lange, A. R. Sugg, M. J. Cohen, and G. H. Olsen, “A 2 &mgr;m cutoff 320×240 InGaAs NIR camera”,
Proc. Of
11
th
annual IEEE Lasers and Electro-Optics Society, Orlando, Fla.
, 1092-8081, 71 (1998). The 1-D and 2-D arrays are generally hybridized to Read Out Integrated Circuits (ROIC). These ROIC integrate signal over time. The ROIC does not differentiate between photocurrent and darkcurrent from the detector.
The ROIC applies a limited amount of reverse bias to the detector. Some ROIC's operate near zero bias (±3 mV) others run as high 5V. The ROIC have storage capacitors at each pixel location and they have limited space for storing charge. Dark current is produced by the detector when it is placed in reverse bias. It is current produced by the device without the device being illuminated. The smaller the amount of dark current produced by the detector allows for larger amounts of photocurrent to be collected before the capacitor fills in the ROIC. This leads to a larger signal-to-noise ratio. It is the goal of the present invention to minimize the amount of dark current produced by the diode at a given applied bias.
Most InGaAs lattice matched to InP is used in the communications industry. InGaAs is capable of detecting 1.55 &mgr;m light, which is commonly used in fiber optic communication. The standard photodetector used in these applications is the p-i-n InGaAs structure. Detectors used in fiber optic communications require high speeds. To have fast devices the RC time constant needs to be minimized. If one lowers the doping density of the absorption region in the p-i-n structure, one lowers the capacitance of the device. InGaAs p-i-n diodes are fabricated using epitaxial methods and the lowest doping possible is defined as the background doping or residual doping of the epitaxial method. Much work has been conducted to reduce the background doping and impurities from this layer to make it as pure as possible. Traditionally the InGaAs absorption region is left undoped, with a resulting background doping on the order of 10
13
−10
15
/cm
3
.(R. D. Dupis, J. C. Campbell, and J. R. Velebir, “InGaAs/InP Photodiodes Grown by Metalorganic Chemical Vapor Deposition”, J. of Cryst. Growth, 77 595-605 (1986).)
The following U.S. patents may be generally relevant to the state of the art and the present inventions.
U.S. Patents entitled “PIN Photodiode Having a Low Leakage Current” (U.S. Pat. No. 4,904,608) and “PIN Photodiode Having a Low Leakage Current” (U.S. Pat. No. 4,999,696) both describe methods to fabricate a low leakage current photodiode in a MESA structure, which is very different from the planar devices described in this disclosure. The patent disclosure stresses that the doping of the layers should be optimum for lowering the capacitance which is equivalent to minimizing the doping in the intrinsic region.
U.S. Patent entitled “Semiconductor Structure for Photodetector” (U.S. Pat. No. 5,214,276) describes a photodetector made of InGaAs but focuses on preventing stray light from entering the diode.
U.S. Patent entitled “Multi-Layered Semi-Conductor Photodetector” (U.S. Pat. No. 4,682,196) describes a structure for making high speed, low dark current devices. This is a very different structure from the structure discussed in this disclosure and it stresses that the intrinsic region should be doped as low as possible.
U.S. Patent entitled “Low Leakage Current Photodetector Arrays” (U.S. Pat. No. 5,387,796) describes a method to lower the dark current in non-lattice matched material. In non-lattice matched material the main issue is stress relief due to lattice mismatch between the epitaxial layers and the substrate it is grown on. According to the prior art, p-i-n structures are generally specified to have no intentional doping. They are grown by epitaxy from either the liquid phase or the vapor phase by many methods. In the prior art the doping is described as similar to this disclosure ~1×10
15
/cm
3
, but this was achieved utilizing residual doping. The doping of the layers was not intentional. This residual doping was the lowest achievable doping at the time of the prior art. (M. J. Robertson, S. Ritchie, S. K. Sargood, A. W. Nelson, L. Davis, R. H. Walling, C. P. Skrimshire, R. R. Sutherland, “Highly Reliable Planar GalnAs/InP Photodiodes With High Yield Made By Atmospheric Pressure MOVPE”,
Electron. Lett
., 24, 5 (1988)). In all of the prior art the material is described as “undoped”.
In the prior art low doping allows the intrinsic region to be thick enough to collect all of the light, yet be fully depleted at operating biases so as to minimize the capacitance. The result being high speed (GHz) operation. In the communications industry the dark current of the detector is not critical, speed is the critical factor. In applications where an ROIC is used, the devices speed is no longer critical, the amount of dark current becomes the critical factor. Devices that are hybridized integrate signal over time and thus the device does not run at high speeds. Without the need for high speed, the removal of the dopants and impurities from the absorption layer may not be the best solution.
SUMMARY OF THE INVENTION
Briefly described, the invention comprises an In
0.53
Ga
0.47
As p-i-n diode. The shunt resistance R
0
can be increased by intentionally doping the InGaAs absorption region. According to the present invention, n-type dopants are intentionally introduced at concentration levels between 1×10
14
to 5×10
17
/cm
3
in the absorption region of a p-i-n diode. This doping level decreases the dark current at the operating voltages of the ROIC. Lowering of the dark current allows for greater gain from the devices or better signal-to-noise ratio from the device.
The main focus of the invention is to reduce dark current. Higher doping would be unacceptable in traditional high speed InGaAs p-i-n diodes. The increased doping lowers the breakdown voltage and increases the device capacitance lowering its bandwidth. For the relatively slow, low bias conditions used in video-rate imagers and spectroscopy applications these are not relevant. It has been discovered that it is better to intentionally dope the absorption layer to a given concentration for lower dark current in these array type devices.


REFERENCES:
patent: 5371399 (1994-12-01), Burroughes et al.

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