Abstract:
A method of forming a multijunction solar cell includes providing a substrate, forming a first subcell by depositing a nucleation layer over the substrate and a buffer layer including gallium arsenide (GaAs) over the nucleation layer, forming a middle second subcell having a heterojunction base and emitter disposed over the first subcell and forming first and second tunnel junction layers between the first and second subcells. The first tunnel junction layer includes GaAs over the first subcell and the second tunnel junction layer includes aluminum gallium arsenide (AlGaAs) over the first tunnel junction layer. The method further includes forming a third subcell having a homojunction base and emitter disposed over the middle subcell.

Description:
This application is a divisional patent application of U.S. patent application Ser. No. 10/285,780, filed on Oct. 31, 2002 (pending). 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of semiconductor devices. More specifically, the present invention relates to photovoltaic solar cells. 
     BACKGROUND 
     Solar energy is one of the most important energy sources that have become available in recent years. Considerable research and development have been conducted in silicon-based solar cell semiconductor materials and solar cell structural technologies. As a result, advanced semiconductor solar cells have been applied to a number of commercial and consumer-oriented applications. For example, solar technology has been applied to satellites, space, mobile communications, and so forth. 
     Energy conversion from solar energy or photons to electrical energy is a critical issue in the generation of solar energy. For example, in satellite and/or other space related applications, the size, mass, and cost of a satellite power system are directly related to the power and energy conversion efficiency of the solar cells used. Putting it another way, the size of the payload and the availability of on-board services are proportional to the amount of solar power provided. Thus, as the payloads become more sophisticated, solar cells, which act as the power generation devices for the on-board power systems, become increasingly more important. 
     The efficiency of energy conversion, which converts solar energy (or photons) to electrical energy, depends on various factors such as solar cell structures, semiconductor materials, et cetera. In other words, the energy conversion for each solar cell is dependent on the effective utilization of the available sunlight across the solar spectrum. As such, the characteristic of sunlight absorption in semiconductor material, also known as photovoltaic properties, is critical to determine the efficiency of energy conversion. 
     Conventional solar cells typically use compound materials such as indium gallium phosphide (InGaP), gallium arsenic (GaAs), germanium (Ge) and so forth, to increase coverage of the absorption spectrum from UV to 890 nm. For instance, addition of a germanium (Ge) junction to the cell structure extends the absorption range (i.e. to 1800 nm). Thus, the selection of semiconductor compound materials can enhance the performance of the solar cell. 
     Physical or structural design of solar cells can also enhance the performance and conversion efficiency of solar cells. Solar cells have been typically designed in multijunction structures to increase the coverage of the solar spectrum. Solar cells are normally fabricated by forming a homojunction between an n-type layer and a p-type layer. The thin, topmost layer of the junction on the sunward side of the device is referred to as the emitter. The relatively thick bottom layer is referred to as the base. However, a problem associated with the conventional multijunction solar cell structure is low performance relating to the homojunction middle solar cells in the multijunction solar cell structures. The performance of a homojunction solar cell is typically limited by the material quality of the emitter, which is low in homojunction. Low material quality usually includes poor surface passivation, lattice miss-match, and/or narrow band gap. 
     Thus, a mechanism is needed to enhance the performance of multijunction solar cell structures. 
     SUMMARY OF THE INVENTION 
     A multijunction solar cell structure having a high band gap heterojunction middle cell is disclosed. In one embodiment, a multijunction solar cell structure includes a bottom, middle, and top solar cells. The bottom cell has a germanium (Ge) substrate and a buffer layer. The buffer layer is disposed over the Ge substrate. The middle solar cell contains a high band gap heterojunction, which includes an emitter layer and a base layer. The middle solar cell is deposited over the bottom solar cell. The top solar cell is disposed over the middle solar cell and it also contains an emitter layer and a base layer. 
     Additional features and benefits of the present invention will become apparent from the detailed description, figures and claims set forth below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments, but are for explanation and understanding only. 
         FIG. 1  is a chart illustrating a relationship between the solar spectrum and the electrical power output in accordance with an embodiment of the present invention; 
         FIG. 2  is a block diagram illustrating a schematic sectional view showing a multijunction solar cell structure having multiple solar cells wherein each solar cell is responsible for a different portion of the solar spectrum in accordance with one embodiment of the present invention; 
         FIG. 3  is a block diagram illustrating a schematic sectional view showing a triple-junction solar cell having a high band gap heterojunction middle cell according to one embodiment of the present invention; 
         FIG. 4  is a block diagram illustrating a schematic sectional view showing a detailed triple-junction solar cell structure according to one embodiment of the present invention; 
         FIG. 5  is a flow chart illustrating a method for manufacturing a triple-junction solar cell structure having a high band gap heterojunction middle cell according to an embodiment of the present invention; 
         FIG. 6  is a block diagram illustrating a schematic sectional view showing a triple-junction solar cell according to an embodiment of the present invention; and 
         FIG. 7  is a block diagram illustrating a schematic sectional view showing a triple-junction solar cell according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A method for manufacture and a structure of a multijunction solar cell structure having a high band gap heterojunction middle solar cell are described. 
     In the following description, for purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that these specific details may not be required to practice the present invention. In other instances, well-known circuits and devices are shown in block diagram form to avoid obscuring the present invention. 
     It is understood that the present invention may contain transistor circuits that are readily manufacturable using well-known CMOS (“complementary metal-oxide semiconductor) technology, or other semiconductor manufacturing processes. In addition, the present invention may be implemented with other manufacturing processes for making digital devices. 
     A multijunction solar cell structure having a high band gap heterojunction middle cell is disclosed. In one embodiment, the multijunction solar cell structure includes a bottom, middle, and top solar cell. The bottom cell has a germanium (Ge) substrate and a buffer layer, wherein the buffer layer is disposed over the Ge substrate. The middle solar cell contains a high band gap heterojunction, which includes an emitter layer and a base layer. The middle solar cell is deposited over the bottom solar cell. The top solar cell is disposed over the middle solar cell and also contains an emitter layer and a base layer. 
       FIG. 1  is a chart  100  illustrating a relationship between solar spectrum and energy intensity in accordance with an embodiment of the present invention. Referring to  FIG. 1 , the chart  100  includes a graph  102  and three regions  104 - 108 , which divides the graph  102  into three sections. The graph  102  illustrates energy intensity across a solar or sunlight spectrum (or radiation). In other words, different wavelengths λ of the solar spectrum has different energy intensities I. Energy intensity is typically understood to include amount of possible electrical energy that can be converted from the solar energy at a given point on the solar spectrum. In general, the terms solar energy, photons, sunlight, radiation, and light are used interchangeably herein. Referring to  FIG. 1 , more electrical energy can be converted at energy intensity I 1  than energy intensity I 2 . In other words, the energy intensity or the amount of electrical energy that can be converted varies across the solar spectrum. 
     Region  104  covers mainly the high frequency portion of the solar spectrum. The high frequency portion may include ultraviolet, X-rays, and/or Gamma rays of the solar spectrum. In one embodiment, the high frequency portion  104  refers to the λ range from approximately 0 to 120 nm. 
     In one aspect of the present invention, the top solar cell  110  of the multijunction cell structure is designed to convert the solar energy in the high frequency portion (region  104 ) of the solar spectrum into electrical energy. While the top solar cell  110  is designed to absorb the high frequency portion of the photons, the top solar cell  110  is also structured to allow unabsorbed photons or solar radiation to pass through the top solar cell  110 . 
     Region  106 , in one embodiment, represents photons in a portion of ultraviolet, visible light, and/or part of infrared of the solar spectrum. The λ covered under region  106  is approximately between 90 nm to 800 nm, which includes visible light. As shown, region  106  generally covers relatively high-energy intensity portion of the solar spectrum. Consequently, it is important to have an efficient energy conversion device covering the region  106  because a substantial amount of electrical energy can be converted from this region. 
     In one aspect of the present invention, the middle solar cell  112  (or middle subcell) or middle solar cells of the multijunction cell structure is responsible to convert the solar energy in the portion  106  to electrical energy. While the material used for the middle solar cell  112  is designed to absorb the photons in the region  106 , the material is also structured to allow the unabsorbed photons or solar radiation to reach the bottom solar cell  114  or a solar cell situated under the middle solar cell  112  away from the top cell  110 . 
     Referring again to  FIG. 1 , the region  108  covers the lower frequency portion of the solar spectrum, which may include part of the infrared, microwave, and/or radio wave portion of the spectrum. In one embodiment, the λ of the solar spectrum under this region  108  is approximately from 700 nm and/or greater. In this embodiment, the bottom solar cell  114  of the multijunction solar cell structure is designed to convert the solar energy in the portion  108  to electrical energy. The material used for the bottom solar cell  114  is designed to absorb all photons in the low frequency portion  108 . 
     Referring back to  FIG. 1 , top solar cell  110 , middle solar cell  112 , and bottom solar cell  114  are employed to convert solar energy to electrical energy for portions  104 ,  106 ,  108 , respectively. It is usually difficult for each solar cell to capture all of the solar energy within the specified portion without loss. Accordingly, it is critical to have semiconductor materials having matched lattice and optimal band gap to enhance solar cell performance. In one embodiment, a higher band gap heterojunction middle cell is used to increase the coverage of the solar spectrum as well as enhance the surface passivation of the emitter. As such, more electrical energy may be converted if the conversion coverage by solar cells overlaps with each other. 
     It should be obvious to one skilled in the art that the graph  102  could be divided into more than three regions and 4 or more associated solar cells may be used for capturing photons within the respected region. 
       FIG. 2  is a block diagram  200  illustrating a schematic sectional view showing a multijunction solar cell structure  201  having multiple solar cells wherein each solar cell is responsible for converting a different portion of the solar spectrum in accordance with one embodiment of the present invention. In one embodiment, the multijunction solar cell structure  201  is a triple-junction solar cell structure, which contains a bottom solar cell  206 , a middle solar cell  204 , a top solar cell  202 , two contacts  220  and  222 , and two wires  224  and  226 . The middle solar cell  204  includes a heterojunction, which has a higher band gap than conventional homojunction solar cells. An advantage of using a high band gap heterojunction is to enhance light passivation to the bottom solar cell. Another advantage associated with a high band gap heterojunction is to provide better lattice matching, thereby increasing solar spectrum coverage. For example, the high band gap heterojunction middle cell  204  can absorb a larger portion of the solar spectrum than the conventional homojunction middle solar cell. 
     Replacing a conventional homojunction middle solar cell with a high band gap heterojunction middle solar cell has benefits beyond increasing the light generated photocurrent. A high band gap heterojunction decreases the dark saturated current. For example, a triple-junction solar cell structure having a high band gap heterojunction middle solar cell provides higher open circuit voltage and higher short circuit current. In other words, the sunlight or photo-generated photocurrent increases with the higher band gap emitter heterojunction. It should also be noted that the amount of photons that can be absorbed at the emitter is relatively low compared to the base region. As such, another advantage of using the heterojunction is that the emitter with high band gap semiconductor material is more efficient to pass the sub-band-gap sunlight to the base region. Accordingly, a high band gap heterojunction middle cell provides a larger short circuit current because it offers higher average collection probability of photogenerated carries. In other words, using a high band gap heterojunction middle cell reduces dark saturated current and consequently, provides a larger open circuit voltage. 
     A commonly used analytical expression for the emitter component of the dark current as a function of the emitter material properties is given by 
                 J     0   ⁢   emitter       =     q   ⁢       D   p       L   p       ⁢       n   2       N   D       ⁢   C       ,         
Where D p  is diffusivity, L p  is diffusion length, N D  is doping level, n is intrinsic carrier concentration, q is a constant representing electrical charge, and C is a constant that depends on the level of surface passivation in the emitter. It should be noted that the intrinsic carrier concentration n of the semiconductor material is squared in the function given above. As a result, a lower J 0emitter  is achieved when a high band gap semiconductor material is used.
 
     The contacts  220  and  222 , in one embodiment, comprise metal conductive pads used to transport electrical current in the multijunction solar cell structure  201 . The wires  224  and  226 , in one embodiment, may also be considered as metal conductive channels used to link the multijunction solar cell structure  201  to other neighboring multijunction solar cell structures and/or other electrical devices. It should be apparent to one skilled in the art that it does not depart from the present invention by adding additional blocks, circuits, and/or elements to the multijunction solar cell structure  201 . 
     Referring to  FIG. 2 , sunlight  210  includes three groups of photons  212 - 216  wherein photons  212  includes at least the high frequency portion of the solar spectrum, photons  214  includes at least the visible light portion of the solar spectrum, and photons  216  includes the low frequency portion of the solar spectrum. The top solar cell  202 , which may contain a homojunction or heterojunction, is designed to absorb photons  212  and, at the same time, allow photons  214 - 216  to pass through the top solar cell  202 . Upon receipt of photons  212 , the top solar cell  202  converts photon  212  to electrical energy and forwards the electrical energy together with the electrical energy generated from the middle and bottom cells  204 - 206  to, in one embodiment, the contact  222 . The contact  222 , in one embodiment, passes the electrical energy to the next stage, which could be neighboring solar cells and/or electrical devices via the wire  226 . 
     The middle solar cell  204 , which includes a high band gap heterojunction, is designed to absorb at least photons  214  and, at the same time, allow photons  216  to reach the bottom solar cell  206 . The middle solar cell  204  converts photons  214  to electrical energy and subsequently, passes the electrical energy together with the electrical energy generated from the bottom solar cell  206  to the top solar cell  202 . The bottom solar cell  206  is designed to absorb at least photons  216  and subsequently, converts photons  216  to electrical energy and passes the electrical energy to the middle solar cell  204 . It should be noted that the bottom solar cell may also pass the electrical energy from the wire  224  via the contact  220  to the middle solar cell  204 . 
     In one embodiment, a high band gap heterojunction middle cell includes an indium gallium phosphide (InGaP) layer for emitter and an indium gallium arsenic (InGaAs) layer for base. InGaAs has a close lattice match to germanium (Ge)-based substrate. As such, InGaAs is a high-quality semiconductor material to enhance the efficiency of energy conversion. Moreover, InGaAs provides better response to the solar spectrum than, for example, a conventional GaAs homojunction. In other words, InGaAs covers a broader range of the solar spectrum than a conventional homojunction. Furthermore, because the InGaP layer provides better fill factor, it enhances the open circuit voltage of the solar cell. It should be noted that the heterojunction middle cell could be formed by any combination of groups III, IV, and V elements in the periodic table, wherein the group III includes boron (B), Aluminum (Al), Gallium (Ga), Indium (In), and thallium (Tl). The group IV includes carbon (C), Silicon (Si), Ge, and Tin (Sn). The group V includes nitrogen (N), phosphorus (P), Arsenic (As), antimony (Sb), and bismuth (Bi). 
     In operation, when sunlight  210  reaches the surface of the top solar cell  202 , the high frequency portion of the sunlight  212  is absorbed and converted through the top solar cell  202 , while the visible light portion of sunlight  214  and the low frequency portion of sunlight  216  pass through the top solar cell  202  and reach to the middle cell  204  and bottom cell  206 , respectively. The middle cell  204  converts the visible portion of sunlight  214  to electrical energy while the bottom solar cell  206  converts the low frequency portion of photon  216  to electrical energy. The electrical current flows from the bottom solar cell  206  to the top solar cell  202  and outputs through wire  226 . 
       FIG. 3  is a block diagram illustrating a schematic sectional view showing a triple-junction solar cell  300  having a high band gap heterojunction middle cell according to one embodiment of the present invention. Referring to  FIG. 3 , the triple-junction solar cell  300  includes a top cell  360 , a middle cell  350 , a bottom cell  340 , two tunnel junction layers  316 ,  328 , and a contact layer  302 . In one embodiment, the bottom cell  340  includes a Ge substrate  334 , an InGaP nucleation layer  332 , and a GaAs buffer layer  330 . The InGaP nucleation layer  332  has a lattice parameter similar to the Ge substrate  334  and it serves as a diffusion barrier to arsenic contained in the overlying junctions. In other words, the nucleation layer  332  serves as a source of n-type or p-type dopant used to form a shallow diffused Ge junction. It should be apparent to one skilled in the art that it does not depart from the scope of the present invention if an alternative structure is used in the bottom cell  340 . 
     The middle cell  350  contains an InGaAs base layer  324 , an InGaP emitter layer  320 , and an indium aluminum phosphide (InAlP) window layer  318 . The middle cell  350  contains a heterojunction device. The emitter layer  320  contains phosphide (P) element while the base layer  324  contains arsenic (As) element. The heterojunction device improves the efficiency of a solar cell because it consists of materials with different band gap energies which match different parts of the solar spectrum. The advantages of heterojunction solar cells over conventional cells include enhanced short wavelength response and lower series resistance. The InAlP window layer  318  is used to improve the passivation of the cell surface of the underlying heterojunction layers and reduce the surface recombination loss. Recombination loss is a fraction of the charge that is generated far away from the junction and some losses occur because minority carriers recombine before they can diffuse to the device terminals. It should be apparent to one skilled in the art that an alternative structure is used without departing from the scope of the present invention. 
     The top cell  360  includes an InGaP base layer  310 , an InGaP emitter layer  306 , and an InAlP window layer  304 . The top cell  360  contains a homojunction solar cell device because the base and emitter layers  306 - 310  contain similar elements. It should be noted that the homojunction solar cell in the top cell  360  might be replaced with a heterojunction solar cell. It should be also apparent to one skilled in the art that it does not depart from the scope of the present invention if an alternative structure is used in the top cell  360 . 
       FIG. 4  is a block diagram illustrating a schematic sectional view showing a detailed triple-junction solar cell structure  400  according to one embodiment of the present invention. Referring to  FIG. 4 , the triple-junction solar cell structure  400  includes a top cell  460 , a middle cell  450 , a bottom cell  440 , tunnel junction layers  416 - 417 ,  428 - 429 , and a contact layer  402 . In one embodiment, the bottom cell  440  includes a Ge substrate  434 , an InGaP nucleation layer  432 , and a GaAs buffer layer  430 . As mentioned earlier, the InGaP nucleation layer  432  serves as a diffusion barrier to arsenic contained in the overlying junctions. 
     In this embodiment, the middle cell  450  contains an InAlP window layer  418 , an InGaP emitter layer  420 , an InGaAs set back layer  422 , an InGaAs base layer  424 , and an aluminum gallium arsenic (AlGaAs) back surface field (BSF) layer  426 . The middle cell  450  has a heterojunction solar cell structure with a high band gap. Compound semiconductors, in this embodiment, provide better band gap materials for improving absorption coefficients. An optimum band gap for the solar cell material can be considered a compromise between choosing a band gap wide enough to avoid wasting too many electrons, and yet narrow enough so that enough photons can create electron-hole pairs. Thus, the high band gap heterojunction solar cell enhances the performance of the solar cells. 
     The set back layer  422 , in one embodiment, is a thin layer with lightly or undoped compound material. The AlGaAs BSF layer  426  is used to reduce the recombination loss in the middle cell  450 . The BSF layer  426  drives the minority carriers from a highly doped region near the back surface to reduce the effect of recombination loss. In other words, a BSF provides low recombination at the backside of the solar cell and reduces the bulk recombination at the emitter region. The InAlP window layer  418  used in the middle cell  450  also reduces the recombination loss. The window layer  418 , as mentioned earlier, further improves the passivation of the cell surface of the underlying heterojunction. It should be apparent to one skilled in the art that the additional layer(s) may be added in the middle cell  450  without departing from the scope of the present invention. 
     The top cell  460  contains an InAlP window layer  404 , an InGaP emitter layer  406 , an InGaP set back layer  408 , an InGaP base layer  410 , and an indium gallium aluminum phosphide (InGaAlP) BSF  412 . The top cell  460  contains a homojunction solar cell device. The base layer  410  and emitter layers  406  contain similar compound elements. It should be noted that the homojunction solar cell in the top cell  460  might be replaced with a heterojunction solar cell. It should be also apparent to one skilled in the art that it does not depart from the scope of the present invention if additional layer(s) are added in the top cell  460 . 
       FIG. 5  is a flow chart  500  illustrating a method for manufacturing a triple-junction solar cell structure having a high band gap heterojunction middle cell according to an embodiment of the present invention. At block  510 , the process deposits an InGaP nucleation layer on a Ge substrate. The InGaP nucleation layer has a lattice parameter similar to the Ge substrate for creating a diffusion barrier to arsenic contained in the junctions. Once the nucleation layer is deposited on the Ge substrate, the process proceeds to block  512 . 
     At block  512 , the process deposits an InGaAs buffer layer on the nucleation layer to form the bottom solar cell, also known as the bottom subcell. Once the buffer layer is deposited, tunneling junction layers, such as for example, n++GaAs tunnel junction and n++ AlGaAs tunnel junction, may be formed. The process proceeds to block  514 . 
     At block  514 , the process deposits a heterojunction middle solar cell over the bottom solar cell. In one embodiment, an InGaAs base layer is deposited on an AlGaAs BSF layer after the BSF layer is deposited over the bottom solar cell. An InGaAs set back layer is subsequently formed over the InGaAs base layer before an InGaP emitter layer is deposited on the set back layer. After the emitter layer is deposited, an InAlP window layer is deposited over the emitter layer. The process moves to block  516  once the tunneling junctions are formed between the top and middle solar cells. 
     At block  516 , the process deposits a homojunction top solar cell over the middle solar cell. In one embodiment, an InGaP base layer is deposited on a BSF layer after it is deposited over the middle solar cell. An InGaP set back layer is subsequently formed over the InGaAs base layer before an InGaP emitter layer is deposited on the set back layer. After the emitter layer is deposited, an InAlP window layer is deposited over the emitter layer. A triple-junction solar cell structure having a high band gap heterojunction middle solar cell is formed when a GaAs contact layer is deposited on the InAlP window layer. The process is ended once the triple-junction solar cell structure is completed. 
     In the foregoing specification the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense.