Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1998-12-11
2002-11-19
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S045013, C372S096000
Reexamination Certificate
active
06483862
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to light emitting devices, and, more particularly, to a system and method for the monolithic integration of a light emitting device and a photodetector that uses a native oxide semiconductor layer to realize low spontaneous emission capture and low bias voltage operation.
2. Related Art
Semiconductor lasers in general and vertical cavity surface emitting lasers (VCSELs) in particular are used for many applications including electronics, communication systems, and computer systems. Lasers produce light that can be transmitted directionally. In many applications of lasers, and particularly in many VCSEL applications, there is a need to precisely control the laser output power. The output power of semiconductor lasers is primarily determined by the bias current. However, it can be significantly altered by the ambient temperature and aging of the device. For this reason, control of the output power is realized by monitoring the laser output and adjusting the laser current to maintain a specified laser output power. The light measurement is typically performed using a semiconductor photodetector, while the feedback loop is realized using an external electronic circuit. There are numerous implementations of such laserphotodetector systems, and they differ in application and performance.
The two primary design issues relating to the laser-photodetector system are the cost of the device and the ability to provide performance required for a specific application. From a cost perspective, it is desirable to build the laser and the photodetector on the same chip using the same or similar fabrication technology. This is realized by monolithic integration of the laser and the photodetector. Monolithic integration implies that the individual laser and photodetector devices are completed jointly at the wafer level. From a performance perspective, there are a number of desired qualities.
An important issue for efficient laser output power adjustment is the tracking between the detector current and the laser directional output power. There is no way to distinguish between the fraction of the detector current generated by the directional laser output and the fraction generated by spontaneous emission. Since the external feedback circuit requires information relating to the directional laser output power, the fraction of the detector current generated by spontaneous emission can be a source of error. This error can be minimized by minimizing the spontaneous emission captured by the detector. In addition, for efficient operation of the external feedback loop, the tracking between the directional laser output power and the photodetector current should be stable and repeatable.
Another important issue is the electrical interaction between the photodetector and the laser. The existence of the photodetector and its biasing should have a negligible effect on the operation of, and in particular, the modulation properties of the laser. The laser modulation and biasing should also have a negligible effect on the operation of the photodetector. This implies that the laser and the photodetector should be electrically de-coupled and exhibit negligible high frequency cross-talk.
Lastly, the incorporation of the laser-photodetector device into the external driver circuit should be considered. In computer communications applications the minimum bias voltage is an issue of increasing importance due to the desirability of reduced computer power consumption. Today's computer architectures are using 3.3 volt (V) power supplies having a lower limit of approximately 3.1 V. In the future and for other applications, it is foreseeable that the power dissipation will be reduced even further requiring even lower bias voltage levels.
A preferable configuration of the laser-photodetector system is one in which the laser and photodetector are independently biased from the same power supply. In order to achieve this result, the power supply voltage must be larger than the laser operating voltage, which depends on the photon energy, and the photodetector operating voltage, which depends upon the photodetector reverse bias required for efficient performance. For optical communications, the vertical cavity laser voltages range between approximately one to two volts, while the typical photodetector reverse bias voltage is between 0.5 and 1 volt. For other applications these voltages may vary.
In order to use the lowest power supply voltage possible an integrated laser and photodetector configuration enables forward biasing the laser and reverse biasing the photodetector from the same power supply. This is always achievable by using a four terminal device structure in which the laser and the photodetector can be independently biased with arbitrary polarity. If the structure permits, two of the four terminals can also be jointly connected to one power supply thereby creating a three terminal device in which the same power supply simultaneously forward-biases the laser and reverse-biases the photodetector. The realization of three terminal devices that allow this preferred biasing scheme is not always possible due to fabrication and structural limitations.
In the past, photodetectors have been integrated with lasers to varying degrees of success. For example, some integration schemes use a photodetector and laser that have been independently fabricated on different chips. The two devices are integrated at the packaging stage, after fabrication, resulting in arbitrary relative polarity between the laser and photodetector. This integration scheme is referred to as “hybrid integration”. The primary disadvantage of this approach is that the extra processing step of integrating the photodetector with the laser after fabrication undesirably adds manufacturing cost. Additionally, in many cases the relationship between the photodetector current and the laser output is neither stable nor repeatable, due to the fluctuation in the laser output beam shape.
Another scheme involves monolithic integration of a photodetector and laser where the coupling is realized using side emission, resulting in both three or four terminal devices. The main disadvantage of such devices is that the photodetector does not detect the directional laser output, but predominately captures the omni-directional spontaneous emission.
Finally, another scheme involves the monolithic integration of a laser and a photodetector where the coupling is realized by top (or bottom) emission, resulting in both three and four terminal devices.
All of the implementations result in either three terminal devices where the laser and the photodetector share a common n-side (cathode) or a common p-side (anode), which requires a relatively high bias voltage for operation and in which the laser and the photodiode are electrically coupled; or in four terminal devices in which the photodetector captures a high proportion of spontaneous emission (SE) from the laser. Therefore, a monolithically-integrated laser and photodetector device that can operate at a low bias voltage, enables electrical de-coupling between the laser and the photodetector, and which minimizes the capture by the photodetector of omni-directional spontaneous emission from the laser is desired.
FIG. 1A
shows a prior art laser and photodetector combination in a three terminal configuration in which an unacceptably high level of spontaneous emission is allowed to reach and be detected by the photodetector. Laser and photodetector
11
is essentially comprised of photodetector
12
deposited over laser
13
in a common cathode arrangement. The common cathode configuration is also referred to as PNP configuration because the semiconductor conductivity type changes twice in the structure. Laser
13
is typically a vertical cavity surface emitting laser (VCSEL). This arrangement is illustratively characterized as having two pn junctions. The first pn junction is active layer
14
located within laser
13
and the second pn junction
Aronson Lewis B.
Babic Dubravko I.
Corzine Scott W.
Tan Michael R. T.
Agilent Technologie,s Inc.
Leung Quyen
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