Vertical optical cavities produced with selective area epitaxy

Details Vertical optical cavities produced with selective area epitaxy Vertical optical cavities produced with selective area epitaxy

1999-06-22

2001-04-24

Davie, James W. (Department: 2881)

Coherent light generators

Particular resonant cavity

Distributed feedback

C372S050121

Reexamination Certificate

active

06222871

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to vertical optical cavity structures such as vertical cavity surface emitting lasers (VCSELs) and detectors (VCDETs) grown with the aid of Selective Area Epitaxy (SAE), and especially to arrays of such structures.
BACKGROUND
When metal-organic chemical vapor deposition (MOCVD) is used as the epitaxy technique to grow an epitaxial layer (e.g., InGaAs) on a substrate with patterned windows of silicon dioxide or silicon nitride, the local growth rate on the substrate is enhanced. This is generally referred to as Selective Area Epitaxy (SAE). The reason for the enhancement is due to the fact that growth on top of the oxide or nitride region is inhibited. Thus, the extra material (e.g., tri-ethyl-Gallium and tri-methyl-Indium) migrate towards the uncovered region, enhancing the local growth rate. The enhancement factor depends on the ratio of oxide (nitride) area to the available growth area and the diffusion coefficient of the metal-organic sources.
Previous applications of SAE have all been towards edge emitting lasers and integrated optoelectronic devices. For example, in U.S. Pat. No. 5,659,640 issued to Joyner the inventor teaches the use of SAE for making an integrated waveguide with an optical grating. Suitable mask geometry is chosen to ensure that the deposition process produces the desired optical structure, i.e., an optical grating or even a stack of Quantum Well regions (QWs). In U.S. Pat. No. 5,418,183 Joyner et al. teach the use of SAE for producing a reflective digitally tunable laser. Another type of multiple QW distributed feedback semiconductor laser grown with the aid of SAE is taught by Shim et al. in U.S. Pat. No. 5,614,436. Additional references illustrating the use of SAE for simultaneously growing optical devices in the same plane are found in the articles of Joyner et al., “Extremely Large Band Gap Shifts for MQW Structures by Selective Epitaxy on SiO
2
Masked Substrates,” IEEE Phot. Tech. Lett., Vol. 4, No. 9 (September 1992), pp. 1006-9 and Caneau et al., “Selective Organometallic Vapor Phase Epitaxy of Ga and In Compounds: A Comparison of TMIn and TEGa versus TMIn and TMGa,” J. Crystal Growth, Vol. 132 (1993), pp. 364-370.
These and similar prior art devices typically have InGaAs QWs in their active region. These QWs are regrown on a patterned substrate with different openings between two oxide strips. The thickness of the QW is inversely proportional to the oxide strip opening due to SAE. Moreover, since the SAE enhancement factor for In is more than the Ga enhancement factor, the In content of the QW is also a function of the oxide strip opening. Hence, the emission wavelength of each laser in the array can be tailored by the oxide strip opening.
The optical elements of the prior art devices are all located in the plane in which SAE is performed. In other words, SAE is performed on a surface which provides for planar alignment between the optical elements. Hence, the resulting devices are limited to a planar element distribution as encountered, e.g., in edge emitting lasers.
OBJECTS AND ADVANTAGES
It is a primary object of the present invention to apply the technique of Selective Area Epitaxy (SAE) to vertical optical cavities. In particular, it is an object of the invention to provide vertical cavity surface emitting lasers (VCSEL) and Vertical Cavity Detectors (VCDET) with varying emission and absorption wavelengths by using SAE.
It is a further object of the invention to provide for simple adjustment of the band gap of Quantum Well regions (QWs) and of the Fabry-Perot distance between reflectors in such vertical cavity devices.
It is an additional object of the invention to ensure that the method of making the vertical optical cavity devices is simple and cost-efficient.
Yet another object of the invention is to ensure that vertical optical cavity devices can be grown monolithically and as arrays of elements.
Further objects and advantages will become apparent upon reading the specification.
SUMMARY
These objects and advantages are attained by a monolithic device with a vertical optical cavity built up along a vertical direction. The device has a bottom Distributed Bragg Reflector (DBR) made up of a number of bottom reflectors or alternating &lgr;/4 layers. A Quantum Well (QW) region consisting of least one active layer is grown on top of the bottom DBR by using a Selective Area Epitaxy (SAE) mask or growing pattern. To ensure proper SAE growth of the QW the mask is made of a dielectric, a nitride or an oxide. Depending on the SAE conditions, the QW region can have one or more strained QWs. Additionally, the active layer or layers exhibit a variation in at least one physical parameter in a horizontal plane, i.e., in a plane perpendicular to the vertical direction. A top DBR consisting of a number of top reflectors is deposited on top of the QW region. A spacer is also deposited adjacent the QW region, e.g., below and/or above the QW region. The spacer can exhibit a variation in its surface curvature or it can have a varying thickness in the horizontal plane. The spacer is preferably also grown by SAE.
The device of the invention has a Fabry-Perot distance which is defined along the vertical direction between the bottom DBR and the top DBR. This Fabry-Perot distance also varies depending on the position in the horizontal plane. For example, the Fabry-Perot distance varies due to varying thickness of the QW or of the spacer.
The varying physical parameter of the active layers is either their surface curvature and/or the band gap. Both of these parameters are regulated by SAE. The band gap is preferably adjusted by altering the relative concentration of materials or elements of the active layer in accordance with SAE.
The SAE mask in the simplest case includes two stripes separated by a gap in which the QW and the spacer are grown. The mask can also be selected from many shapes such as circular stripes, semi-circular stripes, wedge stripes and elliptical stripes as required. The mask shapes are tailored to control thickness and relative concentration of constituent materials of the active layers as well as polarization of the radiation supported by the optical cavity.
It is also preferable that the active layers have a first index of refraction and a first top reflector adjacent to the active region have a second index of refraction either larger or smaller that the first index of refraction. This difference in indices can be used for lensing, guiding or anti-guiding of electromagnetic radiation within the cavity. For better performance, the active layers should exhibit a predetermined surface curvature.
In a preferred embodiment the QW has a number of active layers and the surface curvature of each active layer is controlled. For example, the curvature of the bottom active layer can exhibit the largest bow and the curvature of the top active layer can have the least bow.
The vertical cavity device according to the invention can be used as a Vertical Cavity Surface Emitting Laser (VCSEL) or a Vertical Cavity Detector (VCDET). In the first case a current supply is provided for inducing the active layers to emit electromagnetic radiation. In the second case a suitable circuit is provided to render an absorbing layer sensitive to incident electromagnetic radiation when absorbed.
The method of the invention permits one to construct vertical optical cavities by appropriately tailoring the SAE conditions. Further details and an explanation of the invention are contained in the detailed specification with reference to the appended drawing figures.


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