Optics: measuring and testing – Of light reflection – With modulation
Reexamination Certificate
1999-05-05
2001-02-27
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Of light reflection
With modulation
C356S448000
Reexamination Certificate
active
06195166
ABSTRACT:
TECHNICAL FIELD
The present invention relates to non-destructive analysis III-V optoelectronic structures and, more particularly, to the use of micro-photoreflectance to characterize the III-V structure and provide information regarding the physical properties of the structure.
BACKGROUND OF THE INVENTION
During the fabrication of III-V optoelectronic devices, it is often beneficial to be able to characterize the physical properties of the various layers forming the device. For example, electro-absorption modulated lasers (EMLs) require a large extinction ratio (e.g., >21.0 dB) in order to provide an acceptable level of performance. The extinction ratio is dependent upon a number of factors, such as the quantum well structure, the width of the active region, and the p-i junction placement. For an optimal extinction ratio, the applied field must be present only across the active region. Further, knowledge of the p-i junction position is critical in preventing over-diffusion of zinc (a p-type dopant in EML devices) into the structure.
In the prior art, correlation of a device parameter such as the extinction ratio with the wafer level material properties has been accomplished by employing Secondary Ion Mass Spectroscopy (SIMS) analysis. With the SIMS analysis, the position and carrier density of the p-type zinc dopant can be monitored with respect to the active region. However, this method is destructive and can only be performed on a sample basis. To date, there has been no method established for monitoring the characteristics of each wafer containing EML devices as they are processed.
SUMMARY OF THE INVENTION
The need remaining in the prior art is addressed by the present invention, which relates to non-destructive analysis of III-V laser structures and, more particularly, to the use of micro-photoreflectance to characterize the structure and provide information regarding the physical properties (such as the placement of the p-i junction) within the optoelectronic device structure.
In accordance with the present invention, conventional photoreflectance apparatus is modified by incorporating various lensing elements to produce an illumination spot size of approximately 10 micrometers (compared to the usual 100 micrometer spot size). Photoreflectance, as a type of modulation spectroscopy, provides wavelength information from, for example, the various quarternary and binary layers of an InGaAsP EML structure, where the small spot size used in the present invention allows for the micro-photoreflectance procedure to recognize compositional changes present for the selectively grown alloy materials. As an exemplary EML structure is scanned with a monochromatic source, the collected wavelength information includes Franz-Keldysh oscillations that follow the barrier and separate confinement layers. An electric field calculation may be made based upon these oscillations to derive the location of the p-i junction in the EML structure.
An additional feature of the present invention is that the use of micro-phtoreflectance can provide information regarding the uniformity of the various materials forming the opto-electronic device structure, since the measured wavelength will vary if the material is not uniform.
An advantage of the use of micro-photoreflectance in association with the present invention is that the process is non-destructive and can therefore be used to characterize each processed wafer.
Other and further advantages of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.
REFERENCES:
patent: 5706094 (1998-01-01), Maris
Investigation of Wafer-Sized Quantum Well Laser Structures SPIE vol. 2693, pp. 455-466, No date available.
V.M. Airaksinen, H.K. Lipsanen, “Photoreflectance study of photovoltage effects in GaAs diode structures” Appl. phys. Lett. 60 (17), Apr. 17, 1992.
Gray Mary L.
Hess Harald F.
Hybertsen Mark S.
Ketelsen Leonard Jan-Peter
Font Frank G.
Lucent Technologies - Inc.
Merlino Amanda
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