Patent Publication Number: US-2009217967-A1

Title: Porous silicon quantum dot photodetector

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of energy converters, particularly to solar energy converters. More specifically, the present invention relates to a porous silicon quantum dot photo-detector. 
     BACKGROUND OF THE INVENTION 
     It is well known in the art that solar energy converters, also known as solar cells, may be made of a range of materials and have different structures. For example, one of the most commonly used structures of a solar cell may be a piece of solid semiconductor, such as silicon (Si), that contains a junction formed by a p-type doped region and an n-type doped region. The semiconductor is provided with electrodes, such as metal electrical contacts or contacting means, and may be coated with anti-reflection coatings on the surface that reduces reflection of light incident on the surface of the solar cell. 
     Silicon of porous type has also been used in making solar cells. A solar cell made of porous silicon generally includes an upper silicon region that is chemically treated such that a high number of pores are created below the surface, up to a pre-determined depth into the silicon region, with size of pores being controlled by the pore creation process. Porous Si has been used as part of an anti-reflection means for otherwise conventional Si solar cells, and has been made a part of the doped junction, that is, the heavily doped active “emitter” region of the solar cell. 
     Porous silicon regions have also been observed to emit light in a visible wavelength region when being suitably biased. This is because when dimensions of Si regions between pores are made in a range, commonly known as a quantum range of approximately 10 nm or less, the “band structure” of Si material is altered from that of a normal bulky Si material to that of a quantum range. When being applied a bias voltage, electric field resulting from the bias voltage causes the electroluminescence of the quantum range Si material. 
     Another type of solar energy converter or solar cell may include the use of particles of a semiconductor material such as TiO 2  or ZnO that are coated with a dye and encased in an electrolyte. In this solar cell, known as an electrochemical type solar cell, the dye absorbs an incident light, creates hole-electron pairs, and transfers the created electrons to the TiO 2  or ZnO for transmission to an electrode. The electrolyte may contain a redox chemical that absorbs the holes and combines them with electrons from an opposite electrode to complete a closed circuit. 
     Quantum dots are proposed to replace dye coating generally applied on TiO 2  (or ZnO) in an electrochemical type solar cell. As is known in the art, the term quantum dot here refers generally to materials, such as semiconductor or metal, in a quantum range dimension of approximately 10 nm or less. In addition, quantum dots are suggested for being incorporated in solid solar cells made of III-V semiconductor compounds in a junction depletion region, as quantum wells are generally used, all of which may help enhance solar cell performance by increasing absorption of light that would otherwise be lost. 
     Enhancements to solar cell performance using quantum dots have been described in a variety of materials. For example, Ruangdet et al. described enhancements in GaAs photo-detectors using vapor-grown InAs quantum dots in the Conference Record of the Photovoltaic Specialists Conference, 2006, page 225. Alguno et al. described silicon solar cells containing Ge quantum dots by vapor growth of alternate stacked silicon and Ge layers in Applied Physics Letters Vol. 83, page 1258 (2003), and Electrochemical Society Proceedings Vol. 2004-07, page 1067. 
     Electrochemical solar cells incorporating TiO 2  coated with light-absorbing dye encased in a redox-containing electrolyte have been described, for example, by Duffy et al., in Electrochemical Society Proceedings Vol. 2001-10, page 85. Electrochemical cells incorporating quantum dots have been described in Liu et al., Journal of Physical Chemistry Vol. 97, page 10769 (1993), and in Hoyer et al, Applied Physics Letters Vol. 66, page 349 (1995). 
     SUMMARY OF EMBODIMENTS OF THE INVENTION 
     Embodiments of the present invention provide an energy converter, which includes a silicon layer having at least two regions of a first and a second conductivity type that form a P-N junction, at least a portion of the silicon layer being porous, and pores in the portion of porous silicon containing a semiconductor material, the semiconductor material being different from silicon; and a first and a second electrode being placed at a bottom and a top surface of the silicon layer respectively. 
     According to one embodiment, the semiconductor material includes quantum dots, the quantum dots having a size less than 10 nm, between about 1 nm and about 7 nm, and preferably between about 2 nm and about 5 nm, and being dispersed in the pores. 
     In one embodiment, the semiconductor material has a band-gap smaller than that of silicon, and is selected from a group consisting of InAs, InSb, GaSb, PbS, PbSe, PbTe, Ge, and GaInAs. In another embodiment, at least one of the first and second electrodes is a metal grid or a transparent conducting oxide of tin oxide (SnO), zinc oxide (ZnO) or indium oxide (InO). In yet another embodiment, the silicon layer is a thin-film of silicon or a bulk silicon wafer being formed on top of a substrate and the substrate is selected from a group consisting of glass, ceramic, plastic, metal, and semiconductor. 
     According to another embodiment, the pores in the portion of porous silicon have a size less than 10 nm, and are substantially filled with the semiconductor material, which is in substantially intimate contact with walls of the pores. In one embodiment, the pores are quantum volume having a size between about 1 nm and about 7 nm, and preferably between about 2 nm and about 5 nm. In another embodiment, the first conductivity type silicon is a p-doped type silicon and the second conductivity type silicon is an n-doped type silicon. 
     According to yet another embodiment, the semiconductor material has a band-gap larger than that of silicon, and is selected from a group consisting of CdSe, CdS, CdTe, ZnSe, ZnTe, ZnS, GaN, InN, GaAs, GaP, and InP. 
     Embodiments of the present invention also provide a method of manufacturing an energy converter. The method includes forming a layer of silicon on a substrate having a first electrode thereupon, said layer of silicon having a first and a second conductivity type, said first and second conductivity types forming a P-N junction; creating pores in at least a portion of said layer of silicon, thus forming a porous silicon region of said layer of silicon; filling said pores in said porous silicon region with a semiconductor material different from silicon; and forming a second electrode on top of said layer of silicon. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood and appreciated more fully from the following detailed description of the invention, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a demonstrative illustration of a simplified structure of a solar cell in accordance with one embodiment of the present invention; 
         FIG. 2  is a simplified flowchart illustration of a method of forming the solar cell illustrated in  FIG. 1  in accordance with one embodiment of the present invention; 
         FIG. 3  is a demonstrative illustration of a simplified structure of a solar cell in accordance with another embodiment of the present invention; 
         FIG. 4  is a simplified flowchart illustration of a method of forming the solar cell illustrated in  FIG. 3  in accordance with another embodiment of the present invention; and 
         FIG. 5  is a demonstrative illustration of a simplified structure of a solar cell in accordance with yet another embodiment of the present invention. 
       It will be appreciated that for the purpose of simplicity and clarity of illustration, elements in the drawings have not necessarily been drawn to scale. For example, dimensions of some of the elements may be exaggerated relative to other elements for clarity purpose. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the invention. However, it will be understood by those of ordinary skill in the art that embodiments of the invention may be practiced without these specific details. In the interest of not obscuring presentation of essences and/or embodiments of the present invention, in the following detailed description, processing steps and/or operations that are well known in the art may have been combined together for presentation and/or for illustration purpose and in some instances may not have been described in detail. In other instances, processing steps and/or operations that are well known in the art may not be described at all. A person skilled in the art will appreciate that the following descriptions have rather focused on distinctive features and/or elements of embodiments of the present invention. 
     In the following detailed description, well-known device processing techniques and/or steps may not be described in detail and, in some instances, may be referred to other published articles or patent applications in order not to obscure the description of the essence of presented invention as further detailed herein below. 
     One embodiment of the present invention provides a silicon-based solar cell or solar energy converter with enhanced optical absorption performance over a larger range than a conventional silicon solar cell. By the use of a porous silicon (a first semiconductor material) in which the pores contain quantum dots of a second semiconductor material with a smaller band-gap than that of silicon, the lower energy (longer wavelength) portions of the sunlight spectrum, that would otherwise not contribute to photocurrent in a conventional silicon solar cell, are absorbed and therefore contribute to the efficiency of the porous silicon solar cell. 
     Another embodiment of the present invention provides a silicon-based solar cell, or solar energy converter or converting device, with enhanced efficiency by using porous silicon in which the pores are filled substantially with, for example by a chemical vapor deposition process, a second semiconductor material having a lower band-gap than silicon. The second semiconductor absorbs light, more specifically sunlight, of longer wavelength and contributes photocurrent beyond the wavelength range of a conventional silicon solar cell. When the size of pores is made in a quantum range of, for example, approximately less than 10 nm, such second semiconductor material acts similarly to quantum dots as described above except that it is in intimate contact with walls of the pores of a first semiconductor material. Such pores of approximately less than 10 nm in size, filled with the second semiconductor material, may be referred to hereinafter as “quantum volume” in analogy with quantum dots. 
     Yet another embodiment of the present invention provides an energy converter or energy converting device that makes use of a porous silicon region in which the pores are filled with a mixture of an electrolyte and quantum dots. The energy converting device may be known as an electrochemical device in which the silicon and quantum dots together absorb sunlight to create hole-electron pairs. The electrons are then conducted through the silicon to an electrode while the holes are conducted by the electrolyte within the pores to an opposite electrode. Various embodiments of the present invention, as briefly mentioned above, are described below in more details. 
     Reference is now made to  FIG. 1 , which is a demonstrative illustration of a simplified structure of a solar cell in accordance with one embodiment of the present invention. Solar cell  100  may be known as an energy converter or solar energy converter and, according to one embodiment, may be a solid solar cell. Solar cell  100  may include a substrate  101 , which may be made of a low cost substrate such as, for example, glass, ceramic, metal, plastic, or semiconductor materials; an electrode or electrode layer  102  (a first electrode) on top of substrate  101 ; a region or a layer of silicon material  103  on top of electrode layer  102 ; and an electrode or electrode layer  106  (a second electrode) on top of silicon layer  103 . Substrate  101  may include materials that are not necessarily “low cost” materials. 
     Silicon layer  103  may be a bulk silicon wafer or a thin film of silicon material, and may include at least an upper region  105  and a lower region  104 . Upper region  105  may include a first conductivity type, for example a p-doped type, silicon material and lower region  104 , or the remainder region, may include a second conductivity type, which may be an opposite conductivity type such as an n-doped type, silicon material. At an interface  120  between upper region  105  of the first conductivity type and lower region  104  of the second conductivity type, a P-N junction may be formed. 
     According to embodiments of the present invention, upper region  105  and at least a portion of lower region  104 , or upper region  105 , or at least a portion of upper region  105  may be made porous or being a porous silicon  111 . The P-N junction described above may thus be formed either in the porous silicon region, or in the non-porous silicon region, or at an interface between the porous and non-porous regions.  FIG. 1  illustrates that P-N junction  120  is formed in the porous region  111  of silicon layer  103 . The region of porous silicon  111  may be embedded or dispersed with quantum dots  112  of one or more semiconductor materials, which are typically different from silicon. The one or more semiconductor materials making quantum dots  112  may include materials having a band-gap smaller than that of silicon. Such semiconductor materials may include, for example, Ge, GaSb, PbSe, PbS, InAs, InSb, PbTe, and/or GaInAs. The size of quantum dots  112  are typically less than 10 nm, preferably in a range between approximately 1˜7 nm, and more preferably in a range between about 2˜5 nm. According to one embodiment, the use of quantum dots  112  of semiconductor materials having a smaller band-gap may extend response range of solar cell  100  to longer wavelengths, where silicon by itself usually does not, or at least not efficiently, absorb light or solar rays, thus increasing the overall efficiency of solar cell  100 . 
     Reference is now made to  FIG. 2 , which is a simplified flowchart illustration of a method of forming the solar cell illustrated in  FIG. 1  in accordance with one embodiment of the present invention. Embodiments of the method may include first forming an electrode ( 102 ) on top of a substrate ( 101 ). Then at block  201 , the method forms a layer of silicon ( 103 ) on the substrate ( 101 ) via the electrode ( 102 ). The silicon layer ( 103 ) may include two regions of different conductivity types which create a P-N junction at their interface. Next, at block  202 , at least a portion of the silicon layer ( 103 ), for example, the upper region ( 105 ) and a portion of the lower region ( 104 ), may be made porous by means known in the art. For example, pores may be created inside silicon layer  103  by a voltage-assisted etching process in a solution containing hydrofluoric acid (HF). Pores of different sizes and geometries, in a wide range as may be desired, may be obtained by adjusting the chemical composition of HF acid in the chemical etching bath and controlling the amount of voltage and/or current that is applied during porous region formation. The depth of the porous Si region may be controlled by the length of time used in creating the pores. 
     At block  203 , embodiments of the method may include exposing the porous region of silicon layer  103  to a solution containing a density of quantum dots, such that after a pre-determined time elapses, the pores may become saturated with the quantum dots. At block  204 , embodiments of the method may include drying the solution, thereby leaving the quantum dots residing in the pores of silicon layer  103 . Embodiments of the method may further include, at block  205 , forming an electrode ( 106 ) on top of the porous silicon layer ( 103 ). The types of electrodes that may be used may include, for example, a metal grid in the surface region that is sufficiently conducting, or a transparent conducting oxide (TCO) such as Tin oxide (SnO), Indium oxide (InO), zinc oxide (ZnO), and the like. 
     Reference is now made to  FIG. 3 , which is a demonstrative illustration of a simplified structure of a solar cell in accordance with another embodiment of the present invention. Similar to  FIG. 1 , solar cell or solar energy converter  300  may include a substrate  301 , which may be made of a low cost, although may not be necessarily, substrate such as, for example, glass, ceramic, metal, plastic, or semiconductor materials; an electrode or electrode layer  302  (a first electrode) on top of substrate  301 ; a region or a layer of silicon material  303  on top of electrode layer  302 ; and an electrode or electrode layer  306  (a second electrode) on top of silicon layer  303 . In addition, silicon layer  303  may include an upper region  305  and a lower region  304 . Upper region  305  may include a first conductivity type, and lower region  304  may include a second conductivity type, of silicon material. At an interface between upper region  305  and lower region  304 , a P-N junction  320  may be formed. 
     At least a portion of silicon layer  303 , for example a portion of upper region  305 , may be porous silicon  311  or may be made porous. The size and geometry of pores  312  may vary in a wide range from large (around a fraction of a micron) to small (around a few nm). In one embodiment of the invention, pores  312  of the porous silicon region  305  may have the size of a quantum dot, which is less than approximately 10 nm, and may be filled substantially with a semiconductor material such as, for example, Ge, PbS, PbSe, PbTe, InAs, InSb, GaSb, and GaInAs, that has a band-gap smaller than that of silicon. Here, it is noteworthy that a person skilled in the art will appreciate that one or more suitable semiconductor materials may fill the pores  312 . Semiconductor materials filled in pores  312  are in intimate or substantially intimate contact with walls of pores  312 , which enhances charge transfer between the semiconductor materials and the silicon, enabling photovoltaic action. In  FIG. 3 , P-N junction  320  is illustrated as being located in the non-porous region of silicon layer  303 . However, the present invention may not be limited in this respect and P-N junction  320  may be located in porous region  311 , or at the interface between the two regions. 
     Large pores filled with semiconductor materials of band-gap smaller than silicon may enhance light absorption performance of solar cell  300 , particularly at longer wavelength since semiconductor materials in the pores generally absorb light better at these wavelengths. The pores may then transfer photon-induced or photon-generated electronic charges to surrounding silicon material for photocurrent collection and photovoltaic action. When the size of the pores becomes less than 10 nm, for example becomes between about 1-7 nm or preferably between about 2-5 nm, the pores filled with above semiconductor materials effectively become quantum dots, which may be referred to hereinafter as “quantum volume”  312  in analogy to quantum dots  112  of  FIG.1 . Quantum volume  312  may demonstrate enhanced performance of light absorption properties similar to those of quantum dots while exhibiting other added benefit of intimate or substantially intimate contact with the pore walls. 
     Reference is now made to  FIG. 4 , which is a simplified flowchart illustration of a method of forming the solar cell illustrated in  FIG. 3  in accordance with another embodiment of the present invention. Similar to method steps being cited in blocks  201  and  202  in  FIG. 2 , embodiments of the present method may include forming a silicon layer ( 303 ) on a substrate ( 301 ), via a pre-formed first electrode ( 302 ), at block  401 , and creating pores ( 312 ) in the silicon layer ( 303 ) at block  402 . Next at block  403 , embodiments of the method may include exposing the porous silicon layer to a vapor growth condition or environment of a semiconductor material or semiconductor materials which has a band-gap smaller than that of silicon, and causing the formation of semiconductor volumes inside the pores ( 312 ) at block  404 . After the pores have been filled, or substantially filled with the semiconductor material(s), in other words, the semiconductor becomes substantially in intimate contact with walls of the pores, a second electrode ( 306 ) may be applied on top of the silicon layer ( 303 ). Size of the pores ( 312 ) may vary, and for those with a size less than approximately 10 nm, the semiconductor filled pores may become quantum volumes analogous to quantum dots. 
     In the above embodiments, semiconductor materials are used in forming quantum dots (as in  FIG. 1  and  FIG. 2 ) or quantum volume (as in  FIG. 3  and  FIG. 4 ). Embodiments of the present invention, however, are not limited in this respect. For example, materials other than semiconductor material such as metals may also be used in forming quantum dots and/or quantum volume in the porous region of silicon layer  103  or  303 . Generally, under normal circumstances, metals are not used as active light-absorbing element in a solar cell. However, at dimensions being as small as the quantum range (less than 10 nm), photon-generated carriers in metals may actually exit the metal and be released into surrounding silicon in times shorter than a recombination time in the metal. Therefore, metals may be suitable for making quantum dots and/or quantum volume according to some embodiments of the invention. 
     According to yet another embodiment of the present invention, structures of  FIG. 1  or  FIG. 3  may also be used for light emission application when the semiconductor material used in forming the quantum dot/quantum volume in the pores is chosen to have a higher band-gap than that of silicon. Porous silicon contained between suitable electrodes may emit light in a visible wavelength region when a bias is applied across the structure. Materials such as, for example, CdS, ZnS, ZnSe, CdSe, CdTe, GaN, InN, ZnTe, InP, GaP, and GaAs, have larger band-gap than that of silicon and may significantly contribute to the light emission. 
     In case metal particles are used in forming quantum dots or quantum volumes, they may also enhance the light output because they focus the electric field more strongly into the pores than without such metal. Metal particles such as silver, nickel, for example, are known to have this field-enhance effect because their dielectric constant is much higher than the silicon. High electric fields create light emission by electroluminescence, whereby charge carriers are accelerated by the electric field until enough energy is gained to recombine with the emission of light. 
     Reference is now made to  FIG. 5 , which is a demonstrative illustration of a solar cell according to yet another embodiment of the present invention. Solar cell or solar energy converter  500  may include a substrate  501 , which may be made of a low cost substrate such as, for example, glass, ceramic, metal, plastic, or semiconductor; an electrode or electrode layer  502  (a first electrode) on top of substrate  501 ; a region or a layer of silicon material  503  on top of electrode layer  502 ; and a transparent conduct oxide (TCO) layer  506  (a second electrode) on top of silicon layer  503 . 
     Similar to solar cell  100  and  300  in  FIG. 1  and  FIG. 3 , silicon layer  503  may be a bulk silicon wafer or a thin film of silicon material. At least a portion of layer  503 , such as region  505 , may be made porous by means known in the art, such as described herein above, and may reside on a non-porous region  504 . The thickness of non-porous region  504  may be less than the thickness of porous region  505 . Alternatively, the entire region of Si layer  503  may be made porous. 
     According to embodiments of the present invention, pores  510  in the porous silicon region  505  may be dispersed with quantum dots  508  which are immersed in an electrolyte solution  507  being contained by sidewall  509 , as is illustrated in  FIG. 5 . Solar cell  500  may therefore be referred to as an electrochemical cell hereinafter. Electrolyte solution  507  within sidewall  509  may fill the pores  510  of porous region  505 , and according to one embodiment may cover a top surface of porous region  505  of silicon layer  503 . 
     Electrolyte  507  may also contains a redox chemical such that electrons, induced or generated by light absorption either in the silicon region  503  or by quantum dots  508 , may travel to electrode  502  while the holes created at the same time may react with the redox chemical in the electrolyte so that in effect the holes are transported to upper electrode  506 . Electrochemical cell  500  differs from solid solar cells  100  in that, in solar cell  100  the solution used to deposit or spread quantum dots into the pores in solar cell  100  need not be an electrolyte and is dried after deposition, while in electrochemical cell  500  the solution is an electrolyte or gel which is not dried after the device is formed. 
     When being compared with a conventional electrochemical cell normally made with dye-coated TiO 2  particles encased in a liquid electrolyte or gel electrolyte, electrochemical solar cell  500  includes a porous silicon layer that contains quantum dots  508  that may absorb light or solar rays much more efficiently than dye/TiO 2  cell in a conventional electrochemical cell. The quantum dots  508 , being made of a semiconductor material different from silicon, extend the wavelength response of electrochemical solar cell  500  into at least the infrared region and thereby increase the overall current output of solar cell  500 . Electrochemical solar cell  500  and the forming of quantum dots in silicon layer  503  may be made in similar steps as those described above with regard to solar cell  100 , and illustrated in  FIG. 2 . 
     While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the spirit of the invention.