Abstract:
Distributed Bragg reflectors may be formed in fewer layers by the method, which is capable of producing greater differences in indexes of refraction. Group III–V alternating layers are deposited. The microstructure of alternating layers is controlled to be different. A combination of alternating polycrystalline layers or amorphous and polycrystalline layers results. Alternate ones of the layers oxidize more quickly than the others. A lateral wet oxidation of the alternate ones of the layers produces a structure with large differences in indexes of refraction between adjacent layers. The microstructure between alternating layers may be controlled by controlling Group V overpressure alone or in combination with growth temperature.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS AND PRIORITY CLAIM 
   This Application is a divisional application of application Ser. No. 09/635,815, filed Aug. 9, 2000 now U.S. Pat. No. 6,599,564. 

   STATEMENT OF GOVERNMENT INTEREST 
   This invention was made with United States government assistance through National Science Foundation (NSF) Grant No. ECD 89-43166 and DARPA Grant No. F49620-98-0496. The government has certain rights in this invention. 

   FIELD OF THE INVENTION 
   The present invention is in the opto-electronics field. 
   BACKGROUND OF THE INVENTION 
   Distributed Bragg Reflectors (DBRs) are a fundamental component of optical devices requiring an optical gain, such as various types of semiconductor lasers. Conventional vertical DBR&#39;s are formed from lattice-matched alternating semiconductor layered materials. These materials provide a small difference in index of refraction between adjacent layers. As a result, a high number of pairs are required in a conventionally formed DBR to obtain desired reflectivities, e.g., about 25 to 40 pairs to attain reflectivities as high as 99.9%, depending on the difference of the index of refraction in adjacent layers. 
   Lateral wet oxidation has been used to form a DBR in single crystal semiconductor materials. Al-bearing semiconductors are oxidized, and the technique has produced DBRs in, for example, vertical-cavity surface-emitting lasers. Operation of these types of devices was possible in certain wavelength ranges. Long wavelength devices were not realized due to a lack of high quality oxide layer in the laterally oxidized semiconductor devices. In addition, portions of the visible spectrum were not supported by the DBR&#39;s formed by oxidation due to absorption of light. These types of DBRs are also limited to lattice matched substrates, limiting their application. 
   Thus, there is a need for an improved DBR and a method for forming DBRs which addresses the aforementioned drawbacks. The method of the invention is directed to this need. 
   SUMMARY OF THE INVENTION 
   In the method of the invention, Group III–V alternating layers are deposited. The microstructure of alternating layers is controlled to be different during deposit. A combination of alternating polycrystalline layers or amorphous and polycrystalline layers results. Alternate ones of the layers oxidize more quickly than the others. A lateral wet oxidation of the alternate ones of the layers produces a structure with large differences in indexes of refraction between adjacent layers. The microstructure between alternating layers may be controlled by controlling Group V overpressure alone or in combination with growth temperature and rate. 
   The polycrystalline and amorphous materials allow the reflector to be deposited on any host substrate or device. Changing the thickness of constituent layers allows creating of reflectors for a wide variety of wavelengths. Highly reflective DBRs which reflect in the short wavelength portions of the visible spectrum and deep into the ultra-violet wavelengths can be formed by the method. The high-energy bandgap materials provide an advantage during processing because of their resistance to oxidation. This permits oxidation at higher temperatures, leading to faster oxidation rates and higher throughput. 

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The invention alters microstructure between formation of adjacent Group III–V polycrystalline or amorphous layers to create layers that oxidize at greatly different rates. A lateral wet oxidation then produces a significant difference in index of refraction between adjacent layers. Highly reflective DBRs formed by the invention may be realized with only a few layers due to the large difference in index of refraction. 
   DBRs may be formed by any suitable Group III–V formation method in accordance with the invention. Molecular beam epitaxy (MBE) offers some advantages. It permits very accurate control of layer thickness. In addition, the use of MBE permits formation of DBR material immediately following formation of a device structure without breaking vacuum in the system used to grow a device including a DBR. However, MBE is not required to practice the invention. Exemplary devices discussed in this application were formed by MBE. 
   The invention requires formation of alternating Group III–V layers of either alternate polycrystalline or alternate polycrystalline and amorphous materials. The microstructure of the alternating layers is controlled to produce greatly varying, i.e., by about ten times or more, oxidation rates between adjacent layers. The polycrystalline and amorphous material layers permit greater variance in oxidation rate compared to the single crystal materials for a comparable oxidation temperature. This permits a lateral oxidation to create a large difference in index of refraction between adjacent layers. The change in micro crystal structure between adjacent layers may be realized by controlling Group V overpressure with or independent of growth temperature. 
   DBRs formed by the invention do not require lattice matching to the underlying material due to the use of an amorphous or polycrystalline material on the underlying material. Manipulating the growth temperature, group V overpressure, and growth rate can accurately control microstructure of the alternating layers. 
   The reflectivity of a DBR is a function of the difference in the index of refraction between the two adjacent layers and the number of pairs of these layers. The invention increases the difference in the index of refraction between adjacent layers due to the oxidation of alternate layers occurring at a higher rate than intervening layers. A fewer number of pairs are therefore needed to obtain a high reflectivity. An added advantage is a much wider bandwidth of the reflectivity spectrum. 
   An exemplary DBR was formed including polycrystalline GaAs layers alternating with amorphous AlAs layers. By depositing the materials at relatively low growth temperatures of 100° C. at a growth rate of 0.5 monolayer (ML)/s, the arsenic overpressure was changed resulting in amorphous compounds for higher overpressures (2×10 −7  torr) and polycrystalline materials for lower overpressures (8×10 −8  torr). The material was further processed to laterally oxidize the amorphous (Al,As) layers forming an Al-oxide/GaAs DBR. Polycrystalline AlAs layers can be used for the wet oxidation as well. Oxidation temperatures of 300° C. were used to convert the AlAs to its oxide. The Al-oxide has a much lower index of refraction, around 1.48, as compared to the semiconductor material that has a refractive index of about 3.36. Polycrystalline GaAs layers withstand higher oxidation temperatures than the amorphous (Al,As) layers. 
   The ability to control the micro crystal structure to practice the method of the invention has been demonstrated by adjusting the As flux and substrate temperature in exemplary devices to deposit GaAs or AlAs with different crystal structures. Specifically, structures can be epitaxial single crystalline on a GaAs substrate if the growth temperature is high, e.g., 580° C. The structures become polycrystalline if the growth temperature is reduced to below 150° C. In this case, they can be deposited on any substrates (not necessarily on GaAs substrates). Lowering the substrate temperature even further to below 100° C. makes the (Ga,As) or (Al,As) become amorphous. The temperatures at which deposited layers become polycrystalline or amorphous is also dependent on the amount of As flux. In short, lower temperatures and higher As fluxes favor the formation of polycrystals to ultimately amorphous materials. 
   The formation of DBRs using the present method is not restricted to the materials of GaAs and AlAs. Due to absorption of light at shorter wavelengths, GaAs is not useful for DBR applications in the visible spectrum. By simply replacing the GaAs layers with GaP, a DBR was formed which reflects in the visible spectrum covering the important red-yellow-green wavelengths. The control of the microstructure of the GaP material was similarly achieved through the manipulation of the group V overpressure, in this case phosphorus. While the obvious advantage of using GaP is the transparency of the material at wavelengths of greater than 550 nm, an additional advantage is the higher oxidation temperatures allowed. GaP can withstand much higher oxidation temperatures than GaAs allowing the fabrication of the visible-spectrum DBR at higher oxidation temperatures of 450° C. Higher oxidation temperatures simplify material processing procedures. 
   The use of amorphous (Al,As) for the oxide layer is not required. Other Al-bearing compounds, such as amorphous or polycrystalline Al x Ga 1-x P, can be used depending on the application. The present method can also be applied to the nitride material system using an Al x In y Ga 1-x-y N/Al-oxide DBR prepared by a similar procedure. For example, polycrystalline GaN or AlInGaN containing a low composition of Al can be used as the non-oxidized layer while amorphous or polycrystalline AlInGaN with a high composition of aluminum can be deposited and then oxidized for the oxide layer in the DBR structure. This permits fabrication of DBRs in the blue and violet part of the visible spectrum and possibly ultraviolet applications. 
   The significance of making substrate-independent polycrystalline and amorphous materials is that they can be wet oxidized much faster than their single crystalline counterparts. For III–V semiconductor materials, Al-bearing compounds oxidize much faster than non-Al-bearing compounds. Controlling the crystal structure allows alternating layered structures consisting of amorphous/polycrystalline Al-bearing layers and intervening non-Al-bearing polycrystalline layers. Differential oxidation rates between alternating layers can be enhanced by not only the difference in composition (Al-bearing vs. non-Al-bearing) but also the difference in structure (amorphous vs. crystalline). 
   Wet oxidation of the multi-layer structure produces an efficient Distributed Bragg Reflector (DBR) by converting the Al-bearing semiconductor layer to Al-bearing oxide layer and leaving the non-Al-bearing semiconductor layer intact because of the difference in oxidation rate. The large difference in refraction indices between the Al-bearing oxide and non-Al-bearing semiconductor layer makes it a better DBR requiring fewer pairs to achieve a high reflectivity. 
   Other Group III–V materials systems will be apparent to artisans. The method of the invention produces DBRs in which adjacent layers have relative differences in indices of refraction exceeding 50%. Some alternate combinations are now listed with an indication of the wavelengths reflected. In the structures listed below: “a” indicates amorphous, “p” indicates polycrystalline, and “n” indicates the index of refraction. 
   DBR Structure #1 
   
       
       a-AlAs 
       p-GaAs 
       n1=3.1/n2=3.6 
     
  
   After differential oxidation, the a-AlAs becomes a-Al 2 O 3  by losing As resulting in:
     a-Al 2 O 3      p-GaAs   n1=1.5/n2=3.6   

   The DBR won&#39;t absorb light whose wavelength is longer than 8800 Å so it is a good DBR design to reflect light longer than 8800 Å, i.e. near infrared and beyond. 
   DBR Structure #2 
   
       
       p-AlAs/p-GaAs n1=3.2/n2=3.4 
       After differential oxidation, the p-AlAs becomes a-Al 2 O 3  by losing As. 
       a-Al 2 O 3 /p-GaAs n1=1.6/n2=3.4 
     
  
   The DBR won&#39;t absorb light whose wavelength is longer than 8800 Å so it is a good DBR design to reflect light longer than 8800 Å, i.e. near infrared and beyond. 
   DBR Structure #3 
   
       
       a-AlAs 
       p-GaP 
       n1=3.1/n2=3.37 
     
  
   After differential oxidation, the a-AlAs becomes a-Al 2 O 3  by losing As resulting in:
     a-A1 2 O 3      p-GaP   n1=1.5/n2=3.37   

   The DBR won&#39;t absorb light whose wavelength is longer than 5500 Å so it is a good DBR design to reflect light longer than 5500 Å, i.e. yellow-green and beyond. 
   DBR Structure #4 
   
       
       p-AlAs/p-GaP n1=3.2/n2=3.37 
       After differential oxidation, the p-AlAs becomes a-Al 2 O 3  by losing As. 
       a-Al 2 O 3 /p-GaP n1=1.6/n2=3.37 
     
  
   The DBR won&#39;t absorb light whose wavelength is longer than 5500 Å so it is a good DBR design to reflect light longer than 5500 Å, i.e. yellow-green and beyond. 
   DBR Structure #5 
   
       
       a-InAlP 
       p-GaP 
       n1=3.2/n2=3.37 
     
  
   After differential oxidation, the a-InAlP becomes a-InAlPO 4 , resulting in:
     a-InAlPO 4      p-GaP   n1=1.6/n2=3.37   

   The DBR won&#39;t absorb light whose wavelength is longer than 5500 Å so it is a good DBR design to reflect light longer than 5500 Å, i.e. yellow-green and beyond. 
   DBR Structure #6 
   
       
       a-AlP 
       p-GaP 
       n1=3.0/n2=3.37 
     
  
   After differential oxidation, the a-AlP becomes a-AlPO 4  resulting in:
     a-AlPO 4      p-GaP   n1=1.6/n2=3.37   

   The DBR won&#39;t absorb light whose wavelength is longer than 5500 Å so it is a good DBR design to reflect light longer than 5500 Å, i.e. yellow-green and beyond. 
   DBR Structure #7 
   
       
       a-Al x Ga 1-x P 
       p-GaP 
       n1=3.3/n2=3.37 (where x&gt;0.9) 
     
  
   After differential oxidation, the a-Al x Ga 1-x P becomes a-Al x Ga 1-x PO 4  resulting in:
     a-Al x Ga 1-x PO 4      p-GaP   n1=1.6/n2=3.37   

   The DBR won&#39;t absorb light whose wavelength is longer than 5500 Å so it is a good DBR design to reflect light longer than 5500 Å, i.e. yellow-green and beyond. 
   DBR Structure #8
     a-Al x Ga 1-x P   p-GaN   n1=3.3/n2=2.4 (where x&gt;0.9)   

   After differential oxidation, the a-Al x Ga 1-x P becomes a-Al x Ga 1-x PO 4  resulting in
     a-Al x Ga 1-x PO 4      p-GaN   n1=1.6/n2=2.4   

   The DBR won&#39;t absorb light whose wavelength is longer than 3600 Å so it is a good DBR design to reflect light longer than 3600 Å, i.e. near ultraviolet and beyond. 
   DBR Structure #9 
   
       
       a-Al x In y Ga 1-x-y N 
       p-GaN 
       (unmeasured refraction indexes) 
     
  
   After differential oxidation, the a-Al x In y Ga 1-x-y N becomes a-Al x In y Ga 1-x-y N O z  resulting in
     a-Al x In y Ga 1-x-y NO z      p-GaN   

   The DBR won&#39;t absorb light whose wavelength is longer than 3600 Å so it is a good DBR design to reflect light longer than 3600 Å, i.e. near ultraviolet and beyond. 
   In the exemplary DBR structures, an advantage of using phosphide rather than arsenide as the oxide layer is that the phosphide oxide is stronger when the gallium or indium content is higher than about 6–10%. 
   While various embodiments of the present invention have been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims. 
   Various features of the invention are set forth in the appended claims.