Patent Publication Number: US-2012032140-A1

Title: Light-emitting diode including a metal-dielectric-metal structure

Description:
TECHNICAL FIELD 
     Embodiments of the present invention relate generally to the field of light-emitting diodes (LEDs). 
     BACKGROUND 
     The flow and processing of information creates ever increasing demands on the speed with which microelectronic circuitry processes such information. In particular, high speed integrated opto-electronic circuits, as well as means for communicating between electronic devices over communication channels having high-bandwidth and high-frequency, are of critical importance in meeting these demands. 
     Integrated optics and communication by means of optical channels have attracted the attention of the scientific and technological community to meet these demands. However, to the inventors&#39; knowledge per the current state of the art, excepting embodiments of the present invention, light-emitting diodes (LEDs) used for optical signal generation have an upper modulation frequency of about 4 gigahertz (GHz) at a −3 decibel (dB) roll-off point, which limits the bandwidth and information carrying capacity of opto-electronic devices utilizing LEDs as a source for the optical signal. Scientists engaged in the development of integrated optical circuits and communication by means of optical channels are keenly interested in finding a means for increasing the bandwidth and information carrying capacity of opto-electronic devices utilizing LEDs. Thus, research scientists are actively pursuing new approaches for meeting these demands. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the technology and, together with the description, serve to explain the embodiments of the technology: 
         FIG. 1  is a perspective view of a p-i-n, light-emitting diode (LED) including a metal-dielectric-metal (MDM) structure that is configured to enhance modulation frequency of the LED through interaction with surface plasmons that are present in metal layers of the MDM structure, in accordance with an embodiment of the present invention. 
         FIG. 2  is a perspective view of the p-i-n, LED including the MDM structure, similar to that of  FIG. 1 , but further including electrically insulating layers disposed between respective metal layers and a dielectric medium of the MDM structure that are configured to reduce surface recombination to enhance modulation frequency of the LED, in accordance with an embodiment of the present invention. 
         FIG. 3  is a perspective view of a LED including a MDM structure such that the LED includes a gain medium disposed between a p-doped portion of the LED and a n-doped portion of the LED that is included in the MDM structure, in accordance with an embodiment of the present invention. 
         FIG. 4  is a perspective view of the LED including the MDM structure, similar to that of  FIG. 3 , but further including electrically insulating layers disposed between respective metal layers and the dielectric medium of the MDM structure that are configured to reduce surface recombination to enhance modulation frequency of the LED, in accordance with an embodiment of the present invention. 
         FIG. 5A  is a cross-sectional elevation view of a representative gain medium of the LEDs of  FIGS. 3 and 4  including a semiconductor quantum-dot structure such that the semiconductor quantum-dot structure includes a plurality of islands of a first compound semiconductor surrounded by an overlayer of a second compound semiconductor, in accordance with an embodiment of the present invention. 
         FIG. 5B  is a cross-sectional elevation view of an alternative gain medium for the LEDs of  FIGS. 3 and 4  including a colloidal quantum-dot structure such that the colloidal quantum-dot structure includes a plurality of nanoparticles dispersed in a dielectric matrix, in accordance with an embodiment of the present invention. 
         FIG. 5C  is a cross-sectional elevation view of another alternative gain medium for the LEDs of  FIGS. 3 and 4  including a semiconductor quantum-well (QW) structure such that the semiconductor QW structure includes a multilayer including a plurality of bilayers of compound semiconductors, in accordance with an embodiment of the present invention. 
     
    
    
     The drawings referred to in this description should not be understood as being drawn to scale except if specifically noted. 
     DESCRIPTION OF EMBODIMENTS 
     Reference will now be made in detail to the alternative embodiments of the present invention. While the invention will be described in conjunction with the alternative embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. 
     Furthermore, in the following description of embodiments of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it should be noted that embodiments of the present invention may be practiced without these specific details. In other instances, well known methods, procedures, and components have not been described in detail as not to unnecessarily obscure embodiments of the present invention. Throughout the drawings, like components are denoted by like reference numerals, and repetitive descriptions are omitted for clarity of explanation if not necessary. 
     Embodiments of the present invention include a light-emitting diode (LED). The LED includes a plurality of portions including a p-doped portion of a semiconductor, an intrinsic portion of the semiconductor, and a n-doped portion of the semiconductor. The intrinsic portion is disposed between the p-doped portion and the n-doped portion and forms a p-i junction with the p-doped portion and an i-n junction with the n-doped portion. The LED also includes a metal-dielectric-metal (MDM) structure including a first metal layer, a second metal layer, and a dielectric medium disposed between the first metal layer and the second metal layer. The metal layers of the MDM structure are disposed about orthogonally to the p-i junction and the i-n junction; the dielectric medium includes the intrinsic portion; and, the MDM structure is configured to enhance modulation frequency of the LED through interaction with surface plasmons that are present in the first metal layer and the second metal layer. As used herein, the term of art, “dielectric medium,” refers to a material having a real component of an index of refraction of between about 1 and 5, and may include the p-doped, the intrinsic, and the n-doped portion of the semiconductor. 
     Embodiments of the present invention are directed to a LED of very fast speed, with a modulation frequency up to about 800 gigahertz (GHz) for useful modulation frequencies, in one embodiment of the present invention. As used herein, the phrase, “useful modulation frequencies,” means frequencies for which adequate power is emitted to give a useable signal to noise ratio (SNR) at a receiver. The operation speed of a LED is often limited by the spontaneous emission rate. In embodiments of the present invention, by providing an LED including a MDM structure, the emission rate is greatly enhanced because of the surface plasmon. The MDM structure gives a well-confined surface plasmon polariton, and the mode shape of the surface plasmon polariton overlaps well with a gain medium, which may include semiconductor portions. This ensures good coupling between the spontaneous emission and the surface plasmon polariton, thus, a fast modulation speed of the LED. In one embodiment of the present invention, the MDM structure provides one difference from the existing surface plasmon assisted LED technology. Thus, in embodiments of the present invention, the emission rate can be very high, so that the speed of the LED including the MDM structure can be very fast compared with LEDs of previous technology, which have, to the inventors&#39; knowledge, an upper modulation frequency of about 4 GHz at the −3 decibel (dB) roll-off point, which is less than the upper modulation frequency expected for embodiments of the present invention. For example, LEDs of previous technology have bandwidths such that the upper limit of the bandwidth is given by an upper modulation frequency of less than about 4 GHz, which means from about 10 megahertz (MHz) to about 4 GHz the amplitude rolls off by −3 dB. For embodiments of the present invention, LEDs including the MDM have bandwidths such that the upper limit of the bandwidth is given by an upper modulation frequency of in excess of 100 GHz, which means from about 10 MHz to greater than 100 GHz, up to as much as about 800 GHz depending on design considerations which are subsequently described, for useful modulation frequencies. In another embodiment of the present invention, by adding an electrically insulating layer between the dielectric medium, which includes a gain medium of the LED, and the metal layers of the MDM structure, the non-radiative recombination on the metal surface, which is very common in metal-assisted LEDs, can be greatly reduced. In other embodiments of the present invention, the gain medium of the LED may include, by way of example without limitation thereto, the following alternative structures: various types of quantum dot structures, a semiconductor quantum-well (QW), and impurity doped crystals, such as N vacancies in diamond. Moreover, although a gain medium is usually not referred to as a dielectric medium, as used herein in later discussion of the gain medium, the use of the term of art, “dielectric medium,” with respect to the gain medium is used in light of the optical properties associated with the dielectric medium as described above in terms of the index of refraction of the dielectric medium, and the index of refraction of a gain medium included in the dielectric medium. In another embodiment of the present invention, the MDM structure may be pumped electrically through a p-i-n junction structure. Thus, in accordance with embodiments of the present invention, the MDM structure supports a surface plasmon polariton that provides a strong emission rate, while the electrically insulating layer between the metal and the gain medium reduces the non-radiative recombination at the metal surface. 
     Embodiments of the present invention also include environments in which the LEDs including the MDM structure may be included. For example without limitation thereto, in accordance with embodiments of the present invention, a fiber optic communication device including the LED including the MDM structure as an optical-signal output driver is within the spirit and scope of embodiments of the present invention. By way of further example without limitation thereto, in accordance with embodiments of the present invention, an integrated-optics device including the LED including the MDM structure as an on-chip optical-signal generator is also within the spirit and scope of embodiments of the present invention. Moreover, embodiments of the present invention that include environments, in which the LEDs including the MDM structure may be included, are various environments in integrated optics and optical communication, such as fiber-optic communication, in which the LEDs including the MDM structure, which are subsequently described in  FIGS. 1-5C , may find application. 
     With reference now to  FIG. 1 , in accordance with embodiments of the present invention, a perspective view  100  of a p-i-n, LED  101  including a MDM structure  104  is shown. The MDM structure  104  is configured to enhance modulation frequency of the LED  101  through interaction with surface plasmons that are present between metal layers  140  and  144  of the MDM structure  104 . The LED  101  includes a plurality of portions that includes a p-doped portion  112  of a semiconductor, an intrinsic portion  114  of the semiconductor, and a n-doped portion  116  of the semiconductor. The intrinsic portion  114  is disposed between the p-doped portion  112  and the n-doped portion  116  and forms a p-i junction  130  with the p-doped portion  112  and an i-n junction  134  with the n-doped portion  116 . LED  101  also includes a MDM structure  104 . The MDM structure  104  includes a first metal layer  140 , a second metal layer  144  and a dielectric medium disposed between the first metal layer  140  and the second metal layer  144 . In accordance with embodiments of the present invention, the metal layers  140  and  144  of the MDM structure  104  are disposed about orthogonally to the p-i junction  130  and the i-n junction  134 ; the dielectric medium includes the intrinsic portion  114 ; and, the MDM structure  104  is configured to enhance modulation frequency of the LED  101  through interaction with surface plasmons that are present in the first metal layer  140  and the second metal layer  144 . In accordance with embodiments of the present invention, as shown in  FIG. 1  as well as subsequent  FIGS. 2-4 , LEDs including the MDM structure are shown, by way of example without limitation thereto, as being arranged with the planes of the metal layers  140  and  144  of the MDM structure parallel to a substrate  108 , which is referred to herein as the lateral configuration. However, in accordance with other embodiments of the present invention, LEDs including the MDM structures of  FIGS. 1-4  that are arranged with the planes of the metal layers  140  and  144  of the MDM structure perpendicular to the substrate  108 , which is referred to herein as the vertical configuration (not shown), are also within the spirit and scope of embodiments of the present invention 
     With further reference to  FIG. 1 , in accordance with an embodiment of the present invention, the semiconductor used in the LED  101  including MDM structure  104  may be selected from the group consisting of silicon, indium arsenide (InAs), gallium phosphide (GaP) and gallium arsenide (GaAs), by way of example without limitation thereto, as the use of other semiconductors, and in particular compound semiconductors, is within the spirit and scope of embodiments of the present invention. In one embodiment of the present invention, the LED  101  is configured to emit electromagnetic radiation  160  with a wavelength between about 400 nanometers (nm) and about 2 micrometers (μm). In another embodiment of the present invention, the LED  101  is configured to emit electromagnetic radiation  160  with a wavelength of about 1550 nm. In accordance with embodiments of the present invention, the LED  101  including MDM structure  104  is also configured to modulate the emitted electromagnetic radiation  160  at frequencies up to about 800 GHz for useful modulation frequencies. However, in embodiments of the present invention, the LED  101  including MDM structure  104  that is configured to modulate the emitted electromagnetic radiation  160  at the high frequency of 800 GHz for useful modulation frequencies is expected to operate with lesser efficiency than a LED  101  including MDM structure  104  that is configured to modulate the emitted electromagnetic radiation  160  at a frequency of, for example, 200 GHz for useful modulation frequencies. In accordance with embodiments of the present invention, the election of a particular frequency-efficiency combination lies within the discretion of the device designer depending on a particular application for the LED including MDM structure, as there exists a trade-off between the use of high frequency and the attainment of high efficiency. In one embodiment of the present invention, the thickness of the intrinsic portion  114  of LED  101  may be less than or equal to about 100 nm. In another embodiment of the present invention, the distance between the between the p-doped portion  112  and the n-doped portion  116 , which is the length of the intrinsic portion  114  of LED  101 , may be between about 100 nm and about 50 μm. 
     With further reference to  FIG. 1 , in accordance with an embodiment of the present invention, the first metal of the first metal layer  140  of the MDM structure  104  may be selected from the group consisting of silver, gold, copper and aluminum, by way of example without limitation thereto; and, the second metal of the second metal layer  144  of the MDM structure  104  may also be selected from the group consisting of silver, gold, copper and aluminum, by way of example without limitation thereto. In accordance with embodiments of the present invention, various other metals that can produce surface plasmons may be used; for example, the first metal of the first metal layer  140  of the MDM structure  104  may be selected from the group further consisting of titanium and chromium, and the second metal of the second metal layer  144  of the MDM structure  104  may also be selected from the group further consisting of titanium and chromium. In accordance with embodiments of the present invention, by way of example without limitation thereto, the thickness of the first metal layer  140  of the MDM structure  104  may be between 10 nm and 500 nm; and, the thickness of the second metal layer  144  of the MDM structure  104  may also be between 10 nm and 500 nm. 
     With reference now to  FIG. 2 , in accordance with embodiments of the present invention, a perspective view  200  of a p-i-n, LED  201  including an alternative MDM structure  204  is shown. The p-i-n, LED  201  including the alternative MDM structure  204  is similar to the p-i-n, LED  101  of  FIG. 1 ; but, the MDM structure  204  further includes electrically insulating layers  240  and  244  disposed between respective metal layers  140  and  144  and the dielectric medium of the MDM structure  204 . In accordance with embodiments of the present invention, the electrically insulating layers  240  and  244  are configured to reduce surface recombination to enhance modulation frequency of the LED  201 . In an embodiment of the present invention, the first electrically insulating layer  240  includes a material selected from the group consisting of silicon dioxide (SiO 2 ) and alumina (Al 2 O 3 ). In another embodiment of the present invention, the second electrically insulating layer  244  may also include a material selected from the group consisting of SiO 2  and Al 2 O 3 . The electrically insulating layers  240  and  244  may be fabricated by various thin-film deposition techniques, known in the art, such as sputtering, or alternatively, chemical-vapor deposition (CVD). In an embodiment of the present invention, the MDM structure  204  further includes a first electrically insulating layer  240  and a second electrically insulating layer  244 . In an embodiment of the present invention, the first electrically insulating layer  240  is disposed between the first metal layer  140  and the dielectric medium including the intrinsic portion  114 ; and, the second electrically insulating layer  244  is disposed between the second metal layer  144  and the dielectric medium including the intrinsic portion  114 . As described herein, the above-described embodiments of the present invention with respect to the p-i-n, LED  101  are included, as applicable, within embodiments of the present invention with respect to the p-i-n, LED  201 . 
     With reference now to  FIG. 3 , in accordance with embodiments of the present invention, a perspective view  300  of a LED  301  including a MDM structure  304  is shown in which the LED  301  includes a gain medium  314  disposed between a p-doped portion  112  of the LED  301  and a n-doped portion  116  of the LED  301 . Moreover, in accordance with an embodiment of the present invention, the dielectric medium of the MDM structure  304  includes the gain medium  314  of the LED  301 . The LED  301  includes a plurality of portions that includes a p-doped portion  112  of a semiconductor, a gain medium  314 , and a n-doped portion  116  of the semiconductor. The gain medium  314  is disposed between the p-doped portion  112  and the n-doped portion  116  and forms a first junction  330  with the p-doped portion  112  and a second junction  334  with the n-doped portion  116 . LED  301  also includes a MDM structure  304 . The MDM structure  304  includes a first metal layer  140 , a second metal layer  144  and a dielectric medium disposed between the first metal layer  140  and the second metal layer  144 . In accordance with embodiments of the present invention, the metal layers  140  and  144  of the MDM structure  304  are disposed about orthogonally to the first junction  330  and the second junction  334 ; the dielectric medium includes the gain medium  314 ; and, the MDM structure  304  is configured to enhance modulation frequency of the LED  301  through interaction with surface plasmons that are present in the first metal layer  140  and the second metal layer  144 . 
     With further reference to  FIG. 3 , in accordance with an embodiment of the present invention, the semiconductor used in the LED  301  including MDM structure  304  may be selected from the group consisting of silicon, InAs, GaP and GaAs, by way of example without limitation thereto, as the use of other semiconductors, and in particular compound semiconductors, is within the spirit and scope of embodiments of the present invention. In one embodiment of the present invention, the LED  301  is configured to emit electromagnetic radiation  160  with a wavelength between about 400 nm and about 2 μm. In another embodiment of the present invention, the LED  301  is configured to emit electromagnetic radiation  160  with a wavelength of about 1550 nm. In accordance with embodiments of the present invention, the LED  301  including MDM structure  304  is also configured to modulate the emitted electromagnetic radiation  160  at frequencies up to about 800 GHz for useful modulation frequencies. However, in embodiments of the present invention, the LED  301  including MDM structure  304  that is configured to modulate the emitted electromagnetic radiation  160  at the high frequency of 800 GHz for useful modulation frequencies is expected to operate with lesser efficiency than a LED  301  including MDM structure  304  that is configured to modulate the emitted electromagnetic radiation  160  at a frequency of, for example, 200 GHz for useful modulation frequencies. In accordance with embodiments of the present invention, the election of a particular frequency-efficiency combination lies within the discretion of the device designer depending on a particular application for the LED including MDM structure, as there exists a trade-off between the use of high frequency and the attainment of high efficiency. In one embodiment of the present invention, the thickness of the gain medium  314  of LED  301  may be less than or equal to about 100 nm. In another embodiment of the present invention, the distance between the between the p-doped portion  112  and the n-doped portion  116 , which is the length of the gain medium  314 , may be between about 100 nm and about 50 μm. 
     With further reference to  FIG. 3 , in accordance with an embodiment of the present invention, the first metal of the first metal layer  140  of the MDM structure  304  may be selected from the group consisting of silver, gold, copper and aluminum, by way of example without limitation thereto; and, the second metal of the second metal layer  144  of the MDM structure  304  may also be selected from the group consisting of silver, gold, copper and aluminum, by way of example without limitation thereto. In accordance with embodiments of the present invention, various other metals that can produce surface plasmons may be used; for example, the first metal of the first metal layer  140  of the MDM structure  304  may be selected from the group further consisting of titanium and chromium, and the second metal of the second metal layer  144  of the MDM structure  304  may also be selected from the group further consisting of titanium and chromium. In accordance with embodiments of the present invention, by way of example without limitation thereto, the thickness of the first metal layer  140  of the MDM structure  304  may be between 10 nm and 500 nm; and, the thickness of the second metal layer  144  of the MDM structure  304  may also be between 10 nm and 500 nm. 
     With reference now to  FIG. 4 , in accordance with embodiments of the present invention, a perspective view  400  of a LED  401  including an alternative MDM structure  404  is shown. The LED  401  including the alternative MDM structure  404  is similar to the LED  301  of  FIG. 3 ; but, the MDM structure  404  further includes electrically insulating layers  240  and  244  disposed between respective metal layers  140  and  144  and the dielectric medium of the MDM structure  404 . In accordance with embodiments of the present invention, the electrically insulating layers  240  and  244  are configured to reduce surface recombination to enhance modulation frequency of the LED  401 . In an embodiment of the present invention, the first electrically insulating layer  240  includes a material selected from the group consisting of SiO 2  and Al 2 O 3 . In another embodiment of the present invention, the second electrically insulating layer  244  may also include a material selected from the group consisting of SiO 2  and alumina Al 2 O 3 . The electrically insulating layers  240  and  244  may be fabricated by various thin-film deposition techniques, known in the art, such as sputtering, or alternatively, CVD. In an embodiment of the present invention, the MDM structure  404  further includes a first electrically insulating layer  240  and a second electrically insulating layer  244 . In an embodiment of the present invention, the first electrically insulating layer  240  is disposed between the first metal layer  140  and the dielectric medium including the gain medium  314 ; and, the second electrically insulating layer  244  is disposed between the second metal layer  144  and the dielectric medium including the gain medium  314 . 
     With further reference to  FIG. 4 , in accordance with embodiments of the present invention, the LED  401  includes a plurality of portions that includes a p-doped portion  112  of a semiconductor, a gain medium  314 , and a n-doped portion  116  of the semiconductor. The gain medium  314  is disposed between the p-doped portion  112  and the n-doped portion  116  and forms a first junction  330  with the p-doped portion  112  and a second junction  334  with the n-doped portion  116 . LED  401  also includes a metal-insulator-dielectric MID structure  406 . The MID structure  406  includes at least a first metal layer  140 , a dielectric medium, and at least a first electrically insulating layer  240  disposed between the first metal layer  140  and the dielectric medium. In accordance with embodiments of the present invention, at least the first metal layer  140  of the MID structure  406  is disposed about orthogonally to the first junction  330  and the second junction  334 ; the dielectric medium includes the gain medium  314 ; the first electrically insulating layer  240  is configured to reduce surface recombination to enhance modulation frequency of the LED  401 ; and, the MID structure  406  is configured to enhance modulation frequency of the LED  401  through interaction with surface plasmons that are present in at least the first metal layer  140 . As described herein, the above-described embodiments of the present invention with respect to the LED  301  are included, as applicable, within embodiments of the present invention with respect to the LED  401 . 
     With reference now to  FIG. 5A , in accordance with embodiments of the present invention, a cross-sectional elevation view  500 A of a representative gain medium  314  of the LEDs  301  and  401  of respective  FIGS. 3 and 4  is shown. In an embodiment of the present invention, the gain medium  314  includes a semiconductor quantum-dot structure  510  such that the semiconductor quantum-dot structure  510  includes a plurality  512  of islands, of which island  512   a  is an example, of a first compound semiconductor surrounded by an overlayer  514  of a second compound semiconductor. In one embodiment of the present invention, the first compound semiconductor of the plurality  512  of islands, of which island  512   a  is an example, includes InAs and the second compound semiconductor includes GaAs. In embodiments of the present invention, the plurality  512  of islands, of which island  512   a  is an example, of the first compound semiconductor may be fabricated by various thin-film deposition techniques, known in the art, such as sputtering, or alternatively, molecular-beam epitaxy (MBE), or alternatively, metalorganic CVD (MOCVD). In embodiments of the present invention, the thin-film deposition processes used to fabricate the plurality  512  of islands, of which island  512   a  is an example, are controlled to produce a plurality  512  of islands that are epitaxially matched with the underlying substrate (not shown) upon which the plurality  512  of islands are grown; and, the amount of material deposited is controlled to prevent coalescence of the deposited material into a continuous layer. Similarly, in embodiments of the present invention, the overlayer  514  of the second compound semiconductor is also deposited using thin-film deposition processes such as sputtering, or alternatively, molecular-beam epitaxy (MBE), or alternatively, metalorganic CVD (MOCVD). Similar, procedures used to control the epitaxial growth of the plurality  512  of islands of the first compound semiconductor, which are known in the art, may be used to grow the overlayer  514  of the second compound semiconductor, but the conditions may be altered to assure the growth of a relatively flat and continuous layer. 
     With reference now to  FIG. 5B , in accordance with embodiments of the present invention, a cross-sectional elevation view  500 B of an alternative gain medium  314  of the LEDs  301  and  401  of respective  FIGS. 3 and 4  is shown. In an embodiment of the present invention, the gain medium  314  includes a colloidal quantum-dot structure  520  such that the colloidal quantum-dot structure  520  includes a plurality  522  of nanoparticles, of which nanoparticle  522   a  is an example, dispersed in a dielectric matrix  524 . In accordance with embodiments of the present invention, the nanoparticles may include a material selected from the group consisting of silicon, InAs, GaP, GaAs, cadmium selenide (CdSe) and cadmium telluride (CdTe) by way of example without limitation thereto, as the use of other materials, and in particular compound semiconductors, is within the spirit and scope of embodiments of the present invention. In an embodiment of the present invention, the dielectric matrix may include an organic polymer, such as photoresist. 
     With reference now to  FIG. 5C , in accordance with embodiments of the present invention, a cross-sectional elevation view of another alternative gain medium  314  of the LEDs  301  and  401  of respective  FIGS. 3 and 4  is shown. In an embodiment of the present invention, the gain medium  314  includes a semiconductor quantum-well (QW) structure  530  such that the semiconductor QW structure  530  includes a multilayer including a plurality  532  of bilayers, of which bilayer  532   a  is an example, of compound semiconductors. In an embodiment of the present invention, the semiconductor QW structure  530  includes bilayers of GaP and GaAs with a repetition of between 10 to 100 periods. In an embodiment of the present invention, a thickness of a GaP layer  532   a - 1  of the bilayer  532   a  may be between about 1 nm and about 10 nm, and a thickness of a GaAs layer  532   a - 2  of the bilayer  532   a  may be between about 1 nm and about 10 nm. 
     The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and many modifications and variations are possible in light of the above teaching. The embodiments described herein were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.