Method of varying a characteristic of an optical vertical cavity structure formed by metalorganic vapor phase epitaxy

A process for forming an array of vertical cavity optical resonant structures wherein the structures in the array have different detection or emission wavelengths. The process uses selective area growth (SAG) in conjunction with annular masks of differing dimensions to control the thickness and chemical composition of the materials in the optical cavities in conjunction with a metalorganic vapor phase epitaxy (MOVPE) process to build these arrays.

CROSS-REFERENCE TO RELATED APPLICATIONS
 Not Applicable.
 BACKGROUND OF THE INVENTION
 This invention relates to methods of forming vertical resonant cavity
 optical structures upon a substrate. More particularly, this invention
 relates to a method for precisely varying at least one characteristic of
 such structures between individual laterally displaced structures on the
 substrate. The characteristics include wavelength of emission or
 detection, bandgap energy, thickness of layers within the structures,
 chemical composition of the layers within the structures, and lateral
 variation of the amounts of different elements within a layer.
 The structures are vertical-cavity resonance optoelectronic devices and
 include vertical-cavity surface-emitting lasers (VCSELs), resonance-cavity
 photodetectors (RCPDs) and Fabry-Perot cavity modulators (FPCMs) that are
 useful in optical communications and sensing. A 1- or 2-dimensional (2D)
 device array emitting, detecting, or modulating light at different
 wavelengths enables many unique applications. For instance, a VCSEL array
 with different wavelengths can be used for wavelength-division
 multiplexing (WDM) fiber-optic communication systems. Different
 wavelengths from different array elements can be coupled in a single fiber
 for transmission over a distance. Each wavelength can be differently
 encoded and a de-multiplexing system at the receiving end can separate the
 different channels. Such a scheme can greatly enhance the transmission
 capacity. A VCSEL array WDM system is ideal for campus-wide short-haul
 applications. In addition, such an array has been demonstrated to be very
 useful for reconfigurable multiple chip module free-space interconnects.
 An array of RCPDs sensitive to different wavelengths enables a compact and
 integrated multi-channel spectroscopic analysis microsystem for
 quantitative fast parallel optical sensing. The detectors consist of a
 closely-spaced resonance-cavity detector array. Each element in the array
 is only sensitive to a specific wavelength of the broadband irradiation.
 Therefore, the array is equivalent to an integration of a spectrometer and
 a detector array. An array of FPCMs also offers a unique capability for
 multiple wavelength communication systems.
 The central part of the resonance cavity devices is the optical cavity
 embedded between two distributed Bragg reflector (DBR) mirrors. The
 wavelength of emission, detection, or modulation is dictated by the
 Fabry-Perot mode of the cavity, which is determined in turn by the optical
 thickness (the product of layer thickness with the refractive index) of
 the cavity. The operating wavelength will be changed if the thickness of
 the optical cavity is changed.
 It would be desirable to be able to fabricate an array of resonant-cavity
 devices from a single growth with a reliable manufacturing process. Such a
 process would require lateral variation of the layer thickness and alloy
 composition in these complex, multi-layer resonant structures. Prior to
 the invention of the process disclosed herein, no such process was known
 to exist.
 Selective area growth (SAG) by metalorganic vapor phase epitaxy (MOVPE) has
 proved to be a viable technique for the lateral definition of the
 thickness, composition, and bandgap energy of semiconductor material at
 different regions of the same wafer. The substrate (or base epitaxial
 structure) can be partially masked by dielectric materials, such as
 SiO.sub.2, SiN.sub.x, and SiON.sub.x, and perfect selectivity of the
 growth (no growth occurs on the mask material) can be achieved for many
 III-V materials under certain growth conditions. The growth selectivity
 redistributes the flux of reactant gases in the MOVPE growth, and diverts
 the metalorganic materials from the mask region into the open area.
 Different degrees of enhancement or modulation of the thickness and alloy
 composition can be achieved on the same wafer nearby the different mask
 patterns by varying the dimensions of the masked area. SAG technique has
 been used for optoelectronic/photonic integration of laser-modulators,
 multiple-wavelength (ID) laser-passive waveguides, laser-detectors, and
 other similar linear stripe structures. See for example, U.S. Pat. Nos.
 5,704,975; 5,728,215; 5,770,466; and 5,828,085.
 Review of the literature indicates that little has been done on the
 demonstration of multiple wavelength RCPDs and FPCMs. However, several
 approaches have been reported in making WDM VCSELs. Chang-Hasnain et al
 reported a successful fabrication of a multi-element VCSEL array with
 wavelength space .about.3 nm between the neighboring elements in the
 array. C. J. Chang-Hasnain, J. P. Harbison, C. E. Zah, M. W. Maeda, L. T.
 Florez, N. G. Stoffel, and T. P. Lee, "Multiple wavelength tunable
 surface-emitting laser arrays," IEEE J. Quant. Electron. vol. 27, pp.
 1368, 1991. Their approach was to use the inherent nonuniformity of the
 beam flux profile in molecular beam epitaxy (MBE) growth. Substrate
 rotation, which is used to average out the material nonuniformity, was
 stopped during the growth of the cavity. Therefore, different cavity
 wavelengths were achieved using the tapered cavity thickness. Since the
 degree of the taper critically depends on the substrate position relative
 to the sources, this approach has problems with the producibility of the
 absolute wavelength and wavelength spacing.
 The second approach is to etch grooves on the backside of the substrate for
 MBE growth. W. Yuen, G. S. Li, and D. J. Chang-Hasnain,
 "Multiple-wavelength vertical-cavity surface-emitting laser arrays with a
 record wavelength span," IEEE Photon. Technol. Lett. vol. 8, pp. 4-7,
 1996. The substrate is then mounted to a Mo holder with In solders. The
 differential thermal contact creates a temperature profile near the edge
 of the etched groove. When the growth temperature is high enough, the
 material in the cavity region creates a thickness profile due to differing
 degrees of thermal desorption. Therefore, different wavelengths can be
 achieved.
 The third approach is to pattern and etch the substrate with different
 sizes (and depths) of mesas. F. Koyama, T. Mukaihara, Y. Hayashi, N.
 Ohnoki, N. Hatori and K. Iga, "Wavelength control of vertical-cavity
 surface-emitting lasers by using nonplanar MOCVD," IEEE Photon Technol.
 Lett. vol. 7, pp. 10-12, 1995, and G. G. Ortiz, J. Cheng, S. Z. Sun, H. Q.
 Hou, and B. E. Hammons, "Monolithic, multiple wavelength vertical-cavity
 surface-emitting laser arrays by surface-controlled MOCVD growth rate
 enhancement and reduction," IEEE Photon. Technol. Lett. vol. 9, pp.
 1066-1068, 1997. The topographical difference causes differences in the
 surface diffusion process and, therefore, creates a cavity thickness
 variation for different lasing wavelengths. Wavelength difference was
 obtained from different mesas. This approach lacks the accuracy of the
 wavelength control and introduces a topographical profile, making it
 difficult to process. Another major problem with these approaches is the
 narrow operable temperature range. Since all the lasers with a large
 wavelength span share the same active region of the quantum wells, the
 optimum operating temperature for each element is very different. This
 results in a very narrow temperature region of the quantum wells, the
 optimum operating temperature for each element is very different. This
 results in a very narrow temperature region within which the operation of
 all the lasers can be achieved.
 However, WDM VCSELs meeting the specification of the wavelength accuracy,
 spacing, and device performance are becoming more desirable for
 fiber-optic communications than ever before. For example, a LAN project
 sponsored by the U.S. Government required a 1.times.4 array with
 wavelength spacing of 15 nm between the neighboring channels. Currently,
 the laser array is rather inelegantly achieved by using 4 discrete devices
 from 4 individually optimized runs. Clearly, an integrated device with
 suitable performance would be superior if one could be made. For a
 practical application of WDM VCSELs, the critical issues include a
 predefined wavelength spacing, absolute wavelength accuracy for each
 element, and uniformity of the device performance. The current state of
 the art of technologies cannot satisfy these demands. It would be very
 desirable if the gain wavelength for each individual element of such a
 laser array "tracks" the laser wavelength so that a uniform device
 performance over a range of wavelengths can be achieved. None of the prior
 art above teaches a method to successfully form these individual elements
 in 1 or 2 dimensional arrays.
 BRIEF SUMMARY OF THE INVENTION
 The thickness and chemical composition and the resulting wavelengths of
 emission or detection or bandgap of the active region of a resonant cavity
 can be laterally varied across an array of vertical cavity resonant
 structures by careful design of the annular dielectric masks and selection
 of the precursor chemicals used in the MOCVD technique. Use of the annular
 masks enables the fabrication of the individual elements in the array. In
 cases where the dielectric masks disrupt the needed selectivity of
 chemical deposition of the layers in the distributed Bragg reflector
 layers, the bottom DBR layers for the bottom mirror are formed without the
 dielectric masks to create a uniform thickness DBR across the substrate.
 The dielectric masks are then formed on top of the lower DBR mirror
 structure, and the active regions of the various individual resonant
 cavity structures are formed. The upper DBR mirror is then formed on top
 of the individual active regions in another uniform layer. The individual
 devices are later defined as individual elements in an array by selective
 etching.

DETAILED DESCRIPTION OF THE INVENTION
 One embodiment of the present invention is a 2-dimensional VCSEL array that
 was built by applying the SAG technique to MOVPE. The variation of the
 mask geometry (e.g., the mask area/shape or the open area/shape) leads to
 a variation of the layer thickness and alloy compositions. The variation
 of the mask geometry can be in linear dimension or circumference (in the
 case of circular masks). Here we consider an example of an annular pattern
 of doughnut shape. Shown in FIG. 1 is a 4.times.4 array of the annular
 rings with the same inner opening-area diameter of 120 .mu.m. The
 difference in the width of the rings (masked area) leads to a difference
 in the thickness of the material inside the ring. We have used a
 two-dimensional steady-state finite-difference calculation to model the
 flux of materials on the masked and exposed areas. FIG. 2 shows the
 thickness profile in the open area of the substrate for the different
 patterns calculated for InAs, GaAs and In.sub.0.18 Ga.sub.0.82 As using
 trimethylgallium (TMG) and trimethylindium (TMI) as the metalorganic
 sources. The growth pressure and growth temperature were 60 torr and
 720.degree. C., respectively, for this calculation. The thickness
 enhancement factor is normalized to the growth thickness on a featureless
 planar substrate. Since the source depletes almost exponentially away from
 the edge of the mask, the thickness varies over the whole radius of the
 feature. Use of the annular pattern helps to form a flatter profile in the
 open area. When the source material diffuses inward from the ring,
 although the amount of the source material decays, the flux density of the
 material increases as the cross section decreases when the radius is
 reduced. This effect can be qualitatively demonstrated as follows:
 exp(-ar)/2.pi.r.about.a.sub.1 +a.sub.2 r+a.sub.3 1/r+ . . . .
 Compared with an exponentially decayed profile for linear mask, this
 dependence on r produces a flatter profile of the thickness of annular
 pattern.
 As shown in FIG. 2, the GaAs and InAs thickness are enhanced by very
 different amounts, being much steeper for InAs and GaAs, as a result of
 the difference in diffusion constants of the TMG and TMI. FIG. 3 shows the
 thickness enhancement factor and the In composition variation of InAsAs in
 the middle of the ring. The thickness variation was designed to be in a
 range of .about.8% so that .about.70 nm of variation of cavity wavelength
 can be achieved. Because of the difference in the diffusion rates of TMI
 and TMG, the In composition also varies from 0.181 to 0.194. This can be
 extremely useful in defining different emission wavelengths for the gain
 of the quantum well active region.
 FIG. 4 shows the calculated cavity wavelength variation due to the
 thickness variation. The width of the mask was designed to achieve a total
 variation of the lasing wavelength of 40 nm. (The stopband width of
 GaAs/Al.sub.0.94 Ga.sub.0.06 As is usually .about.100 nm, so all the
 wavelengths are still covered within the single DBR structure). However,
 if the gain wavelength doesn't change with the operating wavelength, the
 overlap of the gain with the cavity mode can vary significantly for the
 different elements of the array. As a result, the performance of the
 devices will be compromised, and they will only be able to run in a narrow
 temperature range. For the growth of an InGaAs quantum well, the In
 composition varies in the center of the doughnut, as shown in FIG. 3, in
 addition to the thickness. The variation in emission energy from the
 quantum wells with different In compositions and well thickness are
 calculated and plotted in FIG. 4 with the dotted line for the ring
 pattern. The emission wavelength moves in the same direction as the cavity
 wavelength modulated by the thickness variation, although they do not
 change at exactly the same rates. With further optimization on the mask
 geometry, we can obtain an even better "tracking" of the two wavelengths.
 Therefore, the SAG by MOVPE provides not only a variation of the cavity
 wavelength, but also a wavelength "tracking" between the cavity mode and
 the emission wavelength to ensure similar performances of the VCSELs in
 the same array over a good temperature range. Because the diffusion
 constants of TMA and TMG are about the same, a variation of the
 composition in AlGaAs may not be achievable for AlGaAs which is important
 for 780 and 850-nm VCSELs. We can utilize strained InGaAlAs (as taught by
 J. Ko, E. R. Hegblomer, Y. Akulova, J. J. Thibeault, L. A. Coldren,
 "Low-threshold 840-nm laterally oxidized vertical-cavity lasers using
 AlInGaAs-AlGaAs strained active layers," IEEE Photon. Technol. Lett. vol.
 9, pp. 863-865, 1997, incorporated herein by reference) or InGaAsP active
 region for these wavelengths, or AlGaAs grown with triethylgallium (TEG)
 and TMA since the diffusion constants for these two precursors differ
 more.
 We have grown InGaAs/GaAs and GaAs/AlGaAs quantum wells embedded in AlGaAs
 cladding layers for the active region of VCSELs, and performed thickness
 profile and photoluminescence (PL) measurements for different doughnut
 dimensions. Perfectly selective growth on the annular pattern was achieved
 on the dielectric mask. FIG. 5 shows the thickness in the middle of the
 ring for the growth of InGaAs/GaAs quantum wells with AlGaAs cladding
 layers for 980-nm VCSELs as a function of the mask width. The thick solid
 line represents the model calculation for the thickness enhancement, and
 the solid circles are measured thickness. The experimental results are in
 an excellent agreement with the calculation. In order to enhance the
 contrast of the DBR layers for mirrors, the low-index material in the DBR
 is typically high Al composition AlGaAs or AlAs. Unfortunately, it is
 extremely difficult to achieve a perfect selectivity of AlAs on the
 dielectric mask. Therefore, a bottom DBR is grown first, the sample is
 then unloaded for patterning of the dielectric mask. This is followed by a
 SAG of the active region and the top DBR can then be grown on atop. We
 have demonstrated that the VCSEL performance from the selective area
 regrowth is almost identical to that from a single growth. The remaining
 steps for forming a VSCEL array are accomplished in a conventional manner,
 i.e. vertical etching to form the individual VCSEL elements, formation of
 electrodes, etc.
 In order to achieve the wavelength variation and accuracy in a controlled
 manner, the growth has to be extremely uniform. We have demonstrated a
 uniformity of the laser cavity wavelength of better than 0.5%, or 5 nm for
 980 nm VCSELs. The wavelength accuracy can be further improved by in situ
 control using normal-incidence reflectance as the sensor (see W. G.
 Breiland, H. Q. Hou, H. C. Chui, and B. E. Hammons, "In situ pregrowth
 calibration using reflectance as a control strategy for MOCVD fabrication
 of device structures," J. Cryst. Growth, vol. 174, pp. 564-568, 1997,
 incorporated by reference herein). The simulation of the growth rate
 enhancement in this work assumes that the crosstalk of the gas-phase
 diffusion is negligible because the features are separated far enough.
 This provides a valuable guideline for mask design. A more complete case
 including arbitrary shape of the dielectric mask pattern and boundary
 conditions can be accounted for by a massive parallel numerical code,
 called MP-SALSA, and an accurate determination of the enhancement for any
 shape of patterns placed in any distance including the crosstalk effect
 can be predicted.
 In other embodiments of the present invention, two or more vertical optical
 resonant cavity structures can be formed using annular mask patterns with
 different inner diameters.
 For the fabrication of the RCPDs, the responsivity can be very high because
 of multiple paths through the active region. The spectral width of the
 detection is determined by the Q of the cavity,
 .DELTA..lambda.=.lambda./Q. For instance, .DELTA..lambda..about.1 nm for
 980 nm wavelength if the Q of the cavity is .about.1000, which is readily
 achievable with the DBR mirrors. Variation of the resonance wavelength is
 again achieved by SAG. A schematic diagram of the detector array is shown
 in FIG. 6. The tapered thickness is produced here by a linear mask that is
 configured in a V shape with equal widths on the arms of the V. The
 thickest portion of the cavity forms at the narrow end of the V, and the
 cavity thickness decreases as the V opens up. Alternatively, this
 structure could be formed between parallel linear edges of two
 trapezoidal-shaped mask strips. Here the thickest portion of the optical
 cavity would be formed adjacent to the portions of the mask strips having
 the greatest width. A combination of these two masking techniques could
 also be employed. This densely packed detector array, sensitive to only a
 narrow wavelength range for each element, can function as a combination of
 bulky spectral dispersive device and a detector array, will be very
 appealing for a compact sensing system application.
 Although the invention has been described in the context of the VCSEL and
 RCPD structures set forth above, the true scope of the invention is not
 limited thereto or the specific chemical elements recited but is to be
 found in the appended claims.