Abstract:
Subwells are added to quantum wells of light emitting semiconductor structures to shift their emission wavelengths to longer wavelengths. Typical applications of the invention are to InGaAs, InGaAsSb, InP and GaN material systems, for example.

Description:
BACKGROUND 
     There is an interest in extending the wavelength of active regions on GaAs and GaN. For longwave active regions on GaAs a number of suggestions have been made that fit into three approaches. The first approach relates to introducing nitrogen into an InGaAs quantum well to lower the quantum well bandgap; the second approach relates to using highly strained narrow bandgap GaAsSb quantum wells; and the third approach relates to increasing the strain to the point where quantum dot active regions are formed. 
     Of the three approaches, the first has been the most successful. However, growth of the nitrogen incorporating quantum wells by metal organic chemical vapor deposition (MOCVD) has been difficult because of the poor nitrogen incorporation into the InGaAs quantum wells. Additionally, the reliability of MOCVD grown InGaAs:N is undetermined because efforts have been primarily directed at molecular beam epitaxy (MBE) based growth techniques where the incorporation of nitrogen into InGaAs is easier. 
     In the second approach, GaAsSb quantum wells suffer from type II band alignment with GaAs and from the requirement of a low V/III ratio needed to grow antimonide based compounds on GaAs. The requirement of a low V/III ratio typically results in poor quality quantum wells. 
     In the third approach the low density of the quantum dots results in a low gain and broad spectra due to the randomness of the quantum dot size distribution. 
     For InGaN active regions on GaN, wavelengths are typically extended from the near ultraviolet to the blue or green portion of the spectrum by increasing the indium content by between about 30 percent to 50 percent. However, the optoelectronic quality of high indium content InGaN is severely degraded resulting in low efficiencies at long wavelengths. 
     SUMMARY OF INVENTION 
     In accordance with the invention, double well structures are created in the highly strained quantum well active regions by embedding deep ultra thin quantum wells. The perturbation introduced by the embedded, deep ultra thin quantum well lowers the confined energy state for the wavefunction in the surrounding larger well. This results in an active region operating at a longer wavelength allowing longer wavelength light emitting semiconductor structures such as longer wavelength vertical cavity surface emitting lasers (VCSELs) or longer wavelength light emitting diodes (LEDs) to be made. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  shows a composition profile for a quantum well in accordance with the invention. 
         FIG. 1   b  shows a layer structure corresponding to  FIG. 1   a  in accordance with the invention. 
         FIG. 2  shows the shift of wavelength with indium concentration in accordance with the invention. 
         FIG. 3   a  shows a composition profile for a light emitting semiconductor structure in accordance with the invention. 
         FIG. 3   b  shows a composition profile for a light emitting 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 the shift in wavelength in accordance with the invention. 
         FIG. 6  shows a comparison between composition profiles in accordance with the invention and prior art composition profiles. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1   a  shows a composition profile for a quantum well of an embodiment in accordance with the invention. GaAs barrier layer  110  provides the reference level of zero indium content at the top of InGaAs quantum well  120 . InGaAs quantum well  120  is a highly strained quantum well in which embedded, deep, ultra-thin quantum well  125  is embedded into InGaAs quantum well  120  to make a subwell. Quantum well  120  is typical of quantum wells used on GaAs. The perturbation introduced by embedded, deep, ultra-thin quantum well  125  lowers confined energy state  130  of wavefunction  140  in quantum well  120  to confined energy state  135 . A composition for embedded, deep, ultra-thin quantum well  125  is typically of the form In x Ga (1-x) As given a typical composition for quantum well  120  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  120  without the addition of embedded, deep ultra-thin quantum well  125 . 
       FIG. 1   b  shows a layer structure corresponding to the quantum well composition profile of  FIG. 1   a . Highly strained InGaAs quantum well layer  120  is grown on GaAs barrier layer  110 , typically to a total thickness of about 60 angstrom. After the first approximately 30 angstrom of InGaAs quantum well layer  120  is grown, embedded, deep, ultra-thin In x Ga (1-x) As quantum well layer  125  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  125  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  120 , growth of the remaining approximately 30 angstrom of highly strained InGaAs quantum well layer  120  is completed. GaAs barrier layer  140  is then grown over highly strained InGaAs quantum well layer  120 . 
     Plot  200  in  FIG. 2  shows the shift in wavelength versus indium composition of embedded, deep, ultra-thin In x Ga (1-x) As quantum well layer  125  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  120  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  125  corresponds to a 70 angstrom thick In y Ga (1-y) As quantum well layer with y about 0.4 with an emission wavelength of about 1140 nm. As seen from plot  200  in  FIG. 2 , the emission wavelength 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 emission wavelength 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 emission 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  310  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 . 
       FIG. 5  shows the measured room temperature luminescence spectra  500  of an exemplary embodiment in accordance with the invention. The peak of the luminescence spectra  500  occurs at about 1300 nm demonstrating the feasibility of extending the emission wavelength to 1300 nm using deep ultra-thin quantum well layers. 
     In accordance with the invention, the emission wavelength may be extended using deep quantum well layers in other material systems such as InGaAsSb, InP and GaN. For example,  FIG. 6  shows a composition profile for In y Ga (1-y) N multiple quantum well layers  610 ,  612 ,  614  and  616  with embedded deep, ultra-thin In x Ga (1-x) N quantum well layers  620 ,  622 ,  624  and  626 , respectively, for nitride green light emitting diodes (LEDs) or nitride green laser diodes in accordance with the invention superimposed over a composition profile for prior art In y Ga (1-y) N multiple quantum well layers  611 ,  613 ,  615  and  617 . Note that In y Ga (1-y) N multiple quantum well layers  610 ,  612 ,  614  and  616  with embedded deep, ultra-thin In x Ga (1-x) N quantum well layers  620 ,  622 ,  624  and  626  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  601 ,  603  and  605 . In y Ga (1-y) N multiple quantum well layers  611 ,  613 ,  615  and  617  typically each have a thickness in the range from about 3 nm to about 4 nm. Use of In y Ga (1-y) N multiple quantum well layers  610 ,  612 ,  614  and  616  with embedded deep, ultra-thin In x Ga (1-x) N quantum well layers  620 ,  622 ,  624  and  626 , respectively, allows the indium content of In y Ga (1-y) N multiple quantum well layers  610 ,  612 ,  614  and  616  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  620 ,  622 ,  624  and  626 . 
     The strong piezoelectric fields present in conventional prior art In y Ga (1-y) N multiple quantum well layers  611 ,  613 ,  615  and  617  cause a separation of the electron and hole wavefunctions in conventional prior art In y Ga (1-y) N multiple quantum well layers  611 ,  613 ,  615  and  617  which reduces the probability of both spontaneous and stimulated emission. For nitride LEDs or laser diodes, a further benefit of using In y Ga (1-y) N multiple quantum well layers  610 ,  612 ,  614  and  616  with embedded deep, ultra-thin In x Ga (1-x) N quantum well layers  620 ,  622 ,  624  and  626 , respectively, in accordance with the invention, is that the probability of radiative recombination is enhanced compared to conventional prior art In y Ga (1-y) N multiple quantum well layers  611 ,  613 ,  615  and  617 . In the most general case, embedded deep, ultra-thin In x Ga (1-x) N quantum well layers  620 ,  622 ,  624  and  62  may be displaced from the center of In y Ga (1-y) N multiple quantum well layers  610 ,  612 ,  614  and  616 , respectively, to optimize performance. Typically this would involve achieving the longest wavelength with the minimum indium content and maximum recombination probability. 
     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.