Abstract:
Double well structures in electro-absorption modulators are created in quantum well active regions by embedding deep ultra thin quantum wells. The perturbation introduced by the embedded, deep ultra thin quantum well centered within a conventional quantum well lowers the confined energy state for the wavefunction in the surrounding larger well and typically results in the hole and electron distributions being more confined to the center of the conventional quantum well. The extinction ratio provided by the electro-absorption modulator is typically increased.

Description:
BACKGROUND 
     Electro-absorption modulators are used to modulate light in optical telecommunications applications. Typically, an electro-absorption modulator modulates light generated by a continuous light source. The electro-absorption modulator typically modulates light by either allowing or preventing light from passing through the electro-absorption modulator. One of the main parameters that characterize the light modulation performance of an electro-absorption modulator is the extinction ratio. The extinction ratio is the ratio of the maximum power output to the minimum power output of the electro-absorption modulator. A higher extinction ratio is typically the result of a higher absorption of light through the creation of more electron-hole pairs in the active layer. 
     Electro-absorption modulators capable of operating at data rates on the order of 40 Gb/s are of interest for optical telecommunications applications. Electro-absorption modulators are typically based on the quantum-confined Stark effect. Applying an electric field across the quantum well structure changes the effective band gap energy of the quantum well structure through the quantum-confined Stark effect. Electro-absorption modulators absorb light when a reverse bias is applied to the p-i-n junction. Because little current flows when the reverse bias is applied, the modulation speed is limited by the time required to charge and discharge the capacitance of the electro-absorption modulator. 
     There are a number of tradeoffs associated with multiple quantum well design of electro-absorption modulators and the impact on performance parameters. Overall electro-absorption modulator design and operation typically represents a tradeoff among limitations. A higher extinction ratio may be achieved by increasing absorption through longer modulators, more quantum wells or higher voltage swing operation. However, the modulation rate is adversely effected because longer modulators result in higher capacitance and increasing the number of quantum wells increases carrier extraction time. 
     As noted, typical electro-absorption modulators are operated under reverse bias which results in an applied electric field that causes a separation in the electron and hole wavefunctions where the hole distribution is distributed toward the p-doped side and the electron distribution is distributed toward the n-doped side of the quantum well. This physical separation between photogenerated carriers translates into reduced absorption which reduces the extinction ratio compared to that obtained if overlap between hole and electron wavefunction is maintained.  FIG. 1  shows this effect by examining the photocurrent absorption spectra at room temperature for an eight quantum well InGaAsP modulator. Curve  101  shows a nearly ideal photocurrent absorption spectrum at zero reverse bias, a sharp bandedge transition at λ˜1490 nm, along with an excitonic absorption resonance. As the reverse bias is increased to about 1.25 volts as shown by curve  105 , to about 2.5 volts as shown by curve  110  and to about 3.75 volts as shown by curve  115 , the absorption edge shifts to longer wavelengths because of the quantum-confined Stark effect. The absorption decreases in magnitude as the reverse bias is increased due to the increasing separation between the hole and electron distribution in the quantum well regions. 
     SUMMARY OF INVENTION 
     In accordance with the invention, double well structures in electro-absorption modulators are created in quantum well active regions by embedding deep ultra thin quantum wells. The perturbation introduced by the embedded, deep ultra thin quantum well centered within a conventional quantum well lowers the confined energy state for the wavefunction in the surrounding larger well and typically results in the hole and electron distributions being more confined to the center of the conventional quantum well. The resulting increase in spatial overlap of the hole and electron wavefunctions increases the quantum well absorption. Hence, the extinction ratio provided by the electro-absorption modulator is typically increased. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows absorption versus wavelength for different values for the reverse bias. 
         FIG. 2   a  shows a composition profile for a quantum well in accordance with the invention. 
         FIG. 2   b  shows a layer structure corresponding to  FIG. 2   a  in accordance with the invention. 
         FIG. 2   c  shows the shift of wavelength with indium concentration in accordance with the invention. 
         FIG. 3   a  shows a composition profile for an embodiment in accordance with the invention. 
         FIG. 3   b  shows a composition profile for a light modulating semiconductor structure in accordance with the invention. 
         FIG. 3   c  shows a layer structure corresponding to  FIG. 3   a  in accordance with the invention. 
         FIGS. 4   a - 4   b  show processing time and flow in accordance with the invention. 
         FIG. 5  shows a comparison between composition profiles in accordance with the invention and prior art composition profiles. 
         FIG. 6  shows a band diagram for an embodiment in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2   a  shows a composition profile for a quantum well of an embodiment in accordance with the invention. GaAs barrier layer  210  provides the reference level of zero indium content at the top of InGaAs quantum well  220 . InGaAs quantum well  220  is a highly strained quantum well in which embedded, deep, ultra-thin quantum well  225  is embedded into InGaAs quantum well  220  to make a subwell. Quantum well  220  is typical of quantum wells used on GaAs. The perturbation introduced by embedded, deep, ultra-thin quantum well  225  lowers confined energy state  230  of wavefunction  240  in quantum well  220  to confined energy state  235 . A composition for embedded, deep, ultra-thin quantum well  225  is typically of the form In x Ga (1-x) As given a typical composition for quantum well  220  of In y Ga (1-y) As where y is typically in the range of about 0.35 to 0.4. The value of y is typically selected to achieve the longest wavelength possible from quantum well  220  without the addition of embedded, deep ultra-thin quantum well  225 . 
       FIG. 2   b  shows a layer structure corresponding to the quantum well composition profile of  FIG. 2   a . Highly strained InGaAs quantum well layer  220  is grown on GaAs barrier layer  210 , typically to a total thickness of about 60 angstrom. After the first approximately 30 angstrom of InGaAs quantum well layer  220  is grown, embedded, deep, ultra-thin In x Ga (1-x) As quantum well layer  225  is typically grown to a thickness of about 10 angstrom and growth is typically chosen to maintain a coherent layer so that embedded, deep, ultra-thin thin In x Ga (1-x) As quantum well layer  225  is also highly strained. Surfactants such as antimony may be introduced to allow for coherent growth of the quantum well layer or multiple quantum well stack. Introduction of antimony prevents relaxation of the overall quantum well structure by improving the mobility of the indium atoms during MOCVD surface reconstruction. Following growth of embedded, deep, ultra-thin thin In x Ga (1-x) As quantum well layer  220 , growth of the remaining approximately 30 angstrom of highly strained InGaAs quantum well layer  220  is completed. GaAs barrier layer  241  is then grown over highly strained InGaAs quantum well layer  120 . 
     Plot  200  in  FIG. 2   c  shows the shift in wavelength versus indium composition of embedded, deep, ultra-thin In x Ga (1-x) As quantum well layer  225  having a thickness of about 10 angstrom for x in the range from about 0.4 to about 0.85. In y Ga (1-y) As quantum well layer  220  is about 60 angstroms thick where y is about 0.4. The unperturbed quantum well layer without embedded deep, ultra-thin In x Ga (1-x) As quantum well layer  225  corresponds to a 70 angstrom thick In y Ga (1-y) As quantum well layer with y about 0.4 with an absorption wavelength of about 1140 nm at zero reverse bias. As seen from plot  200  in  FIG. 2   c , the absorption wavelength at zero reverse bias shifts approximately linearly with indium concentration at a rate of approximately 30 nm for a 0.1 increase in indium composition for x above a value of about 0.4. At an indium composition of 0.8, the absorption wavelength at zero reverse bias has been increased to about 1270 nm. 
       FIG. 3   a  shows a composition profile similar to that of  FIG. 1   a  in accordance with the invention. GaAs barrier layers  330  and  340  provide the reference energy at the top of In y Ga (1-y) As quantum well layers  350  and  360 , respectively. In y Ga (1-y) As quantum well layers  350  and  360  are separated by GaAs barrier layer  335 . Embedded, deep, ultra-thin In x Ga (1-x) As quantum well layer  355  is embedded in In y Ga (1-y) As quantum well layer  350  and embedded, deep, ultra-thin In x Ga (1-x) As quantum well layer  365  is embedded in In y Ga (1-y) As quantum well layer  360 . The energy levels for AlGaAs layers  310  and  320  and GaAs (1-z) P z  layers  315  and  325  are also shown. Typical doping levels for the non-active layers are typically in the range from about 1·10 17 /cm 3  to 3·10 18 /cm 3 . 
       FIG. 3   b  is similar to  FIG. 3   a  except that additional strain reducing layer  338  has been introduced. Strain reducing layer  338  is introduced between In y Ga (1-y) As quantum well layers  350  and  360  changing GaAs barrier layer  335  into GaAs barrier layer  336  and GaAs barrier  337 . 
       FIG. 3   c  shows layer structure  300  corresponding to the composition diagram of  FIG. 3   a . Growth of layer structure  300  is typically by MOCVD in a AIXTRON 2000 reactor at a typical growth temperature in the range from about 400° C. to 600° C., for example, 520° C. at a pressure typically on the order of about 100 mbar. The growth condition for the quantum well layers is typically selected so as to prevent indium segregation. This is typically accomplished by varying the growth rate, growth temperature, and strain of the quantum well layers. After growth of AlGaAs layer  310  for about 25 sec to a typical thickness of about 150 angstrom, GaAs (1-z) P z  layer  315  is grown for about 22 sec to a thickness of about 100 angstrom. GaAs (1-z) P z  layer  315  is a tensile strained layer introduced to minimize the integrated strain on layer structure  300  by acting as a strain compensation layer. GaAs (1-z) P z  layers  315  and  325  typically function to compensate for the increased strain typically introduced by embedded, deep ultra-thin In x Ga (1-x) As quantum well layers  355  and  365 . Typical values for z are in the range from about 0.05 to about 0.30 
     Typically, GaAs (1-z) P z  layers  315  and  325  may be placed at the periphery of In y Ga (1-y)  As quantum well layers  350  and  360  respectively, as well as at GaAs barrier layers  330  and  340 . Other types of strain compensating layers GaAsN, AlGaAsP, GaInP, InGaAsP, AlInGaAsN may also be used. GaAs barrier layer  330  is grown over GaAs (1-z) P z  layer  315 . Growth for GaAs barrier layer  330  typically takes about 16 sec resulting in a typical thickness of about 100 angstrom. In y Ga (1-y) As quantum well layer  350 , where y is typically in the range from about 0.3 to 0.45, is grown over GaAs barrier layer  330  for about 4 sec resulting in a typical thickness of about 30 angstrom. Then embedded, deep, ultra-thin In x Ga (1-x) As quantum well layer  355  is embedded in In y Ga (1-y) As quantum well layer  350 . The value of x is typically selected to achieve absorption close to 1300 nm in an embodiment in accordance with the invention. A typical 3 sec growth for embedded, deep, ultra-thin In x Ga (1-x) As quantum well layer  355  results in a typical thickness of about 10 angstrom. Growth of In y Ga (1-y) As quantum well layer  350  is then resumed for 4 sec typically resulting in additional thickness of about 30 angstrom. GaAs barrier layer  335  is grown over In y Ga (1-y) As quantum well layer  350 . Growth for GaAs barrier layer  335  typically takes about 16 sec resulting in a typical thickness of about 100 angstrom. 
     In y Ga (1-y) As quantum well layer  360  where y is typically in the range from about 0.3 to 0.45, is grown over GaAs barrier layer  335  for about 4 sec resulting in a typical thickness of about 30 angstrom. Then embedded, deep, ultra-thin In  x Ga (1-x) As quantum well layer  365  is embedded in In y Ga (1-y) As quantum well layer  360 . A typical 3 sec growth for embedded, deep, ultra-thin In x Ga (1-x) As quantum well layer  365  results in a typical thickness of about 10 angstrom. Growth of In  y Ga (1-y) As quantum well layer  360  is then resumed for 4 sec typically resulting in additional thickness of about 30 angstrom. GaAs barrier layer  340  is grown over In  y Ga (1-y) As quantum well layer  360 . Growth for GaAs barrier layer  340  typically takes about 16 sec resulting in a typical thickness of about 100 angstrom. GaAs (1-z) P z  layer  325  is grown for about 22 sec to a thickness of about 100 angstrom. GaAs (1-z) P z  layer  325  is a tensile strained layer introduced to minimize the integrated strain on layer structure  300  by acting as a strain compensation layer. Other types of strain compensating layers may be used. Then AlGaAs layer  320  is typically grown for about 25 sec to a typical thickness of about 150 angstrom. 
       FIGS. 4   a  and  4   b  show the relevant gas flows for two growth schemes for In y Ga (1-y) As quantum well layers  350 ,  360  and embedded, deep, ultra-thin In x Ga (1-x) As quantum well layers  355  and  365  in accordance with the invention. In  FIG. 4   a , the flow of trimethylgallium  410  and the flow of triethylgallium  420  are initially on. Trimethylindium flow  415  is turned on for about 4 sec to grow the first about 30 angstrom of In y Ga (1-y) As quantum well layer  350 . The flow of triethygallium  420  is shut off at the same time as the flow of trimethylindium  415  and the flow of trimethylindium  440  is turned on for about 3 sec to grow embedded, deep, ultra-thin In x Ga (1-x) As quantum well layer  355 . When embedded, deep, ultra-thin In x Ga (1-x) As quantum well layer  355  is complete, the flow of triethygallium  420  is turned back on and the flow of trimethylindium  415  is turned on for about another 4 sec to grow the final about 30 angstrom of In y Ga (1-y) As quantum well layer  350 . Then GaAs barrier layer  335  is grown for about 5 sec. When growth of GaAs barrier layer  335  is complete, the flow of trimethylindium  415  is turned on for about 4 sec to grow the first about 30 angstrom of In y Ga (1-y) As quantum well layer  360 . The flow of triethygallium  420  is shut off at the same time as the flow of trimethylindium  415  and the flow of trimethylindium  440  is turned on for about 3 sec to grow embedded, deep, ultra-thin In x Ga (1-x) As quantum well layer  365 . When embedded, deep, ultra-thin In x Ga (1-x) As quantum well layer  365  is complete, the flow of triethygallium  420  is turned back on and the flow of trimethylindium  415  is turned on for about another 4 sec to grow the final about 30 angstrom of In y Ga (1-y) As quantum well layer  360 . 
     In  FIG. 4   b , the flow of trimethylgallium  450  is initially on. The flow of trimethylindium  455  is turned on for about 4 sec to grow the first about 30 angstrom of In y Ga (1-y) As quantum well layer  350  and is then shut off along with the flow of trimethylgallium  450 . The flow of triethylgallium  460  and trimethylindium  480  are then turned on for about 3 sec to grow embedded, deep, ultra-thin In x Ga (1-x) As quantum well layer  355 . When embedded, deep, ultra-thin In x Ga (1-x) As quantum well layer  355  is complete, the flow of trimethylgallium  450  is turned on and the flow of trimethylindium  455  is turned on for about 4 sec to grow the final about 30 angstrom of In y Ga (1-y) As quantum well layer  350 . Then GaAs barrier layer  335  is grown for about 5 sec. When growth of GaAs barrier layer  335  is complete, the flow of trimethylindium  455  is turned on for about 4 sec to grow the first about 30 angstrom of In y Ga (1-y) As quantum well layer  360  and is then shut off along with the flow of trimethylgallium  450 . The flow of triethylgallium  460  and trimethylindium  480  are then turned on for about 3 sec to grow embedded, deep, ultra-thin In x Ga (1-x) As quantum well layer  365 . When embedded, deep, ultra-thin In x Ga (1-x) As quantum well layer  365  is complete, the flow of trimethylgallium  450  is turned on and the flow of trimethylindium  455  is turned on for about 4 sec to grow the final about 30 angstrom of In y Ga (1-y) As quantum well layer  350 . 
     In accordance with the invention, deep quantum well layers may be used in other material systems such as InGaAsSb, InP, InGaAsP, AlInGaAs and InGaN. For example,  FIG. 5  shows a composition profile for In y  Ga (1-y) N multiple quantum well layers  510 ,  512 ,  514  and  516  with embedded deep, ultra-thin In x Ga (1-x) N quantum well layers  520 ,  522 ,  524  and  526 , respectively, in accordance with the invention superimposed over a composition profile for prior art In y  Ga (1-y) N multiple quantum well layers  511 ,  513 ,  515  and  517 . Note that In y  Ga (1-y) N multiple quantum well layers  510 ,  512 ,  514  and  516  with embedded deep, ultra-thin In x Ga (1-x) N quantum well layers  520 ,  522 ,  524  and  526  in accordance with the invention, respectively, are completely analogous to In y Ga (1-y) As quantum well layers  350  and  360  with embedded, deep In x Ga (1-x) As quantum well layers  355  and  365 , respectively and separated by GaN barrier layers  501 ,  503  and  505 . In y  Ga (1-y) N multiple quantum well layers  511 ,  513 ,  515  and  517  typically each have a thickness in the range from about 3nm to about 4 nm. Use of In y  Ga (1-y) N multiple quantum well layers  510 ,  512 ,  514  and  516  with embedded deep, ultra-thin In x Ga (1-x) N quantum well layers  520 ,  522 ,  524  and  526 , respectively, allows the indium content of In y  Ga (1-y) N multiple quantum well layers  510 ,  512 ,  514  and  516  to typically be reduced by several percent. However, typical values for x are typically greater than about 0.5 for embedded deep, ultra-thin In x Ga (1-x) N quantum well layers  520 ,  522 ,  524  and  526 . 
     The strong piezoelectric fields present in conventional prior art In y  Ga (1-y) N multiple quantum well layers  511 ,  513 ,  515  and  517  cause a separation of the electron and hole wavefunctions in conventional prior art In y  Ga (1-y) N multiple quantum well layers  511 ,  513 ,  515  and  517  which also reduces the quantum well absorption. 
     In accordance with the invention, electro-absorption modulators may be constructed in InGaAsP material systems with structures analogous to those in InGaAs and InGaN systems. References that discuss relevant InGaAsP material systems include Billia et al., IEEE Photonics Technology Letters, vol. 17, no. 1, pp. 49-51, 2005; Ishikawa et al., IEEE Journal of Quantum Electronics, vol. 30, no. 2, pp. 562-569, 1994; and Minch et al., IEEE Journal of Quantum Electronics, vol. 35, no. 5, pp. 771-782, 1999, all of which are incorporated herein by reference. 
     Electro-absorption modulators that are strain balanced (resulting in polarization independence) in In x Ga (1-x) As y P (1-y)  material systems for typical telecommunications applications, for example, typically have barrier layers with a bandgap energy between about 1350 nm and about 1400 nm and compositions with a typical value for x of about 0.51 and a typical value for y of about 0.75. The In x Ga (1-x) As y P (1-y)  multiple quantum well layers typically have a bandgap energy of about 1600 nm with a typical value for x of about 0.74 and a typical value for y of about 0.75. The embedded deep, ultrathin, In x Ga (1-x) As y P (1-y)  quantum well layer in each one of the In x Ga (1-x) As y P (1-y)  multiple quantum well layers again acts to lower the bandgap energy of each In x Ga (1-x) As y P (1-y)  multiple quantum well. For example, for a bandgap energy of about 1700 nm and a tensile strain of about 0.4 percent, the value of x is about 0.48 and y is about 1 for each embedded deep, ultrathin, In x Ga (1-x) As y P (1-y)  quantum well layer. For a compressive strain of about 0.5 percent, the value of x is about 0.66 and y is about 0.89 for each embedded deep, ultrathin, In x Ga (1-x) As y P (1-y)  quantum well layer. Note that the arsenic content of the embedded deep, ultrathin, In x Ga (1-x) As y P (1-y)  quantum well layer is higher than that of the InGaAsP quantum well it is embedded in. The indium content for each embedded deep, ultrathin, In x Ga (1-x) As y P (1-y)  quantum well layer on the other hand may be higher or lower in the InGaAsP material system. 
     Alternatively, in accordance with the invention, electro-absorption modulators may be lattice matched (resulting in polarization dependence) in In x Ga (1-x) As y P (1-y)  material systems for typical telecommunications applications, for example, typically have barrier layers with a bandgap energy between about 1350 nm and about 1400 nm and with compositions having a typical value for x of about 0.69 and a typical value for y of about 0.68. The In x Ga (1-x) As y P (1-y)  multiple quantum well layers typically have a bandgap energy of about 1600 nm with a typical value for x of about 0.61 and a typical value for y of about 0.84. The embedded deep, ultrathin, In x Ga (1-x) As y P (1-y)  quantum well layer in each one of the In x Ga (1-x) As y P (1-y)  multiple quantum well layers again acts to lower the bandgap energy of each In x Ga (1-x) As y P (1-y)  multiple quantum well. For example, for a bandgap energy of about 1700 nm and lattice matched configuration, the value of x is about 0.54 and y is about 0.98 for each embedded deep, ultrathin, In x Ga (1-x) As y P (1-y)  quantum well layer. Note that the arsenic content of the embedded deep, ultrathin, In x Ga (1-x) As y P (1-y)  quantum well layer is higher than that of the InGaAsP quantum well it is embedded in. Note also that lattice matched In x Ga (1-x) As may also be used for the embedded quantum well. 
       FIG. 6  shows band diagram  600  in accordance with the invention of a deep quantum well electro-absorption modulator structure under reverse bias. In band diagram  600 , embedded deep quantum well  615  is centered within conventional quantum well  610 . Under reverse bias, the electron wavefunction  625  and hole wavefunction  620  are typically misaligned in the absence of deep quantum well  615 . The presence of embedded deep quantum well  615  provides additional confinement that acts to localize the hole and electron distributions toward the center of quantum well  610 . Hence, embedded, deep quantum well  615  effectively pulls the holes and electrons from their respective interfaces towards the center of quantum well  610  as shown by shaded electron wavefunction  625 &#39; and shaded hole wavefunction  620 &#39;. The resulting improvement in spatial overlap between the electron and hole distribution provides greater absorption in quantum well  610 . This leads to a greater extinction for an electro-absorption modulator with an embedded, deep quantum well structure. 
     While the deep-quantum well provides greater absorption, the Stark shift is reduced for a given applied electric field. This is a trade-off associated with the deep quantum well structure in accordance with the invention. However, the deep-quantum well structure affords considerable design freedom as embedded, deep quantum well  615  may be displaced from the center of conventional quantum well  610  to optimize the performance of the deep quantum well electro-absorption modulator. Similarly, the composition and thickness of embedded, deep quantum well  615  may be adjusted to enhance performance. The embedded, deep quantum well structure in accordance with the invention provides an extra degree of freedom for enhancing performance of electro-absorption modulators. 
     While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all other such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.