WO2011119113A1 - Photoelectric transducer using iron silicide and aluminum and method for preparing the same - Google Patents

Photoelectric transducer using iron silicide and aluminum and method for preparing the same Download PDF

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WO2011119113A1
WO2011119113A1 PCT/SG2011/000114 SG2011000114W WO2011119113A1 WO 2011119113 A1 WO2011119113 A1 WO 2011119113A1 SG 2011000114 W SG2011000114 W SG 2011000114W WO 2011119113 A1 WO2011119113 A1 WO 2011119113A1
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fesi
layer
iron
substrate
photoelectric transducer
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PCT/SG2011/000114
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French (fr)
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WO2011119113A8 (en
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Dongzhi Chi
Siao Li Liew
See Weng Wong
Goutam Kumar Dalapati
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Agency For Sciences, Technology And Research
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Publication of WO2011119113A1 publication Critical patent/WO2011119113A1/en
Publication of WO2011119113A8 publication Critical patent/WO2011119113A8/en

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Definitions

  • the present invention relates to the fabrication of a photoelectric transducer composed of an Aluminum doped p-type beta phase of Iron silicide ( -FeSi2), a thin Aluminum interlayer, and an n-type Si substrate.
  • Physical vapor deposition sputter-deposition, electron-beam deposition, thermal evaporation, etc.
  • low thermal budget post-deposition processing with an optional electron- blocking layer deposit the Iron silicide (P-FeSi2) and Aluminum interlayer.
  • the beta phase of Iron silicide p-FeSi 2 is regarded as one of the third generation semiconductors after Si and GaAs.
  • -FeSi 2 has a high optical absorption coefficient (>10 5 cm “1 for photon energy larger than 1eV), high thermoelectric power (Seebeck coefficient of k ⁇ 10 "4 /K), direct energy band-gap of 0.85 eV that corresponds to 1.5 ⁇ of quartz optical fiber communication, lattice constant nearly well-matched to Si substrate, high resistance against the humidity, chemical attacks and oxidization, and is chemically stabile at temperatures as high as 937°C.
  • Various devices such as, solar cells, Si photosensors, and thermoelectric generators that are integrated on Si-LSI circuits can be fabricated using p-FeSi 2 films.
  • the beta phase of Iron silicide p-FeSi 2 having a band-gap value of about 0.85 eV and large optical absorption from the near infrared to visible light are expected to produce a thin film solar cell with a theoretical conversion efficiency of 16% ⁇ 23%.
  • the earth's crust is an abundant source of ⁇ -FeS , which is non-toxic and stable at high temperature.
  • ⁇ -FeS is suitable in solar cell applications.
  • the conversion efficiency is ⁇ 3.6% on a non-Si substrate where an amorphous hydrogenated beta-like Iron silicide p-FeSi 2 (H) film deposited by the combination of plasma enhanced chemical vapor deposition (PECVD) (for the dissociation of SiH 4 ) and thermal evaporation (for Fe deposition).
  • PECVD plasma enhanced chemical vapor deposition
  • thermal evaporation for Fe deposition
  • epitaxial p-FeSi 2 /Si hetero-junction solar cells are unsuitable for non-Si substrates, specifically, because crystalline silicon substrates have band misalignment between n-FeSi 2 and p-Si.
  • a p-type single crystalline Si (1 1 1 ) is typically used to form a ⁇ - ⁇ -FeS /p-Si hetero-junction structure because high quality epitaxial n-type p-FeSi 2 has high carrier mobility, a low concentration of residual carriers, as well as, low defects can be grown on this orientation Si substrate (1 1 1 ) if a proper growth method is used (e.g., face target sputtering deposition of FeSi 2 at elevated substrate temperature with a thin template layer, combined with a high temperature post-deposition furnace annealing at 800°C).
  • the threshold incident photon energy corresponding to the ionization potential of p-FeSi 2 is 4.71 eV.
  • the conversion efficiency is 3.7% for the epitaxial n- -FeSi 2 /p-Si ( ) cell, even though the epitaxial ⁇ - ⁇ -FeS is a high quality single crystalline film with a diffusion length L (> 1 pm) that is much larger than its film thickness ( ⁇ 0.2 pm) .
  • the conversion efficiency is as high as 6.5%.
  • intrinsic P-FeSi 2 films grown on non-Si substrates are typically poly-crystalline or amorphous phase with a high concentration of residual carriers (mostly, n-type conducting electrons), as well as, a high density of defects.
  • Another disadvantage of intrinsic -FeSi 2 films grown on non-Si0 substrates is that they usually exhibit poor photovoltaic characteristics, .thereby limiting their use for solar cell applications.
  • US Patents Nos. 6,949,463; 7,352,044; & 7,354,857. discuss a photoelectric transducer having a p-i ⁇ n junction on a substrate.
  • the 'p' layer may comprise a p-type silicon, the ⁇ layer is Iron silicide, and 'n' layer an n-type silicon.
  • the p and n layers are formed by plasma CVD, while the i layer is formed by flowing SiH 4 and H 2 gases into the deposition chamber to form a plasma composed of the constituent gases. Deposition proceeds via evaporation of Iron from Iron ingot onto the Si wafer.
  • the substrate temperature is at 250°C.
  • amorphous FexSiy:H film of thickness 300 nm is formed on the Si wafer.
  • i-Fe x Si y H x Another disadvantage of i-Fe x Si y H x is that the low carrier mobility due to its amorphous microstructure structure as the same in the case of hydrogenated amorphous a-Si:H. Therefore, the collection efficiency of photo-generated carriers in the i-layer by the n+-and p+-layers could be limited, unless the internal electrical field in the i-layer is extremely high and the carrier drift time in the i-layer is shorter than the carrier lifetime. To overcome this problem, the thickness of i-layer is reduced to increase the internal electrical field. However, this conventional approach faces the problem that the light absorption suffers in a thin i-a-Fe x Si y H x layer. Another limitation with the p+-Si/i-a-Fe x Si y H x /n+-Si p-i-n cells is described in
  • the characteristics include n-type conductivity, optimum thickness of -FeSi 2 films: 230 nm, optimum parameters: V 0G 0.45 V, short-circuit current density (Jsc) 14.8 mA/cm 2 , fill factor (FF) 0.55, conversion efficiency 3.7%.
  • an epitaxial ⁇ -FeSb film is only achieved on a Si (111) substrate by template formation method of ⁇ - FeSi 2 combined with special face-target deposition of FeSi 2 that requires the use of a high temperature & extended annealing time after film growth.
  • the characteristics include n-type p-FeSi 2 thin films with a thickness of 300 nm, optimum post-deposition annealing 300°C, 1 h, optimum parameters: Open-Circuit voltage (V oc ) 0.17 V, Jsc 12 mA/cm2, efficiency 0.57%.
  • This conventional method also exhibits an energy barrier at the interface for hole injection from P-FeSi 2 into the p-type Si substrate. Diffusion of Fe into Si also occurs during the growth of FeSi 2 at 600 °C, which induces recombination/generation centers in Si and results in a large leakage and low open circuit voltage. There is some improvement in short-circuit current density after the low temperature annealing, there is not substantial improvement in Open-Circuit voltage (V oc ) due to the limiting effect of Fe gettering by low temperature annealing.
  • V oc Open-Circuit voltage
  • the approach and methodology as proposed in the present invention provides a solution towards fabricating a beta phase of Iron silicide (P-FeSi 2 ).
  • the characteristics and features of the present invention include, but are not limited to forming an cc-phase (FeSi 2 ) x Al y iron-disilicide aluminum ternary alloy after annealing the p-FeSi 2 that exhibits semiconducting nature rather than metallic electronic property with a bandgap of ⁇ 1 eV due to its increased Aluminum content.
  • the subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate on exemplary technology area where some embodiments described herein may be practiced.
  • the present invention addresses one or more of the above-mentioned deficiencies and others by providing a photoelectric transducer comprising a p/n heterojunction having a p-type photoactive Fe-Si-AI ternary alloy layer formed on a substrate.
  • the alloy layer contains Aluminum where the Aluminum content of the p-type photoactive Fe-Si-AI Aluminum alloy layer is 4 - 25 at.%.
  • the (FeSi 2 ) x Al y Iron-disilicide Aluminum alloy layer has a tetragonal crystal structure alpha phase (a-phase) or an orthorhombic crystal structure beta-phase ( ⁇ -phase).
  • the present invention also provides a method of fabricating a photoelectric transducer, the method comprising the step of forming a p-type photoactive Fe- Si-AI ternary alloy layer on a substrate.
  • Figure 1(a) shows the x-ray diffraction (XRD) patterns of a FeSi 2 (40nm)/AI(3nm) stack according to an embodiment of the present invention, showing the formation of the ⁇ -phase after RTA annealing at 600 °C or above.
  • XRD x-ray diffraction
  • Figure 1 (b) shows the XRD patterns of a FeSi 2 (40nm)/AI(11 nm) stack according to an embodiment of the present invention, showing the formation of the a- phase after RTA annealing at 600 °C or above.
  • Figure 2 shows the square of optical absorption (a 2 ) versus. the photon energy curve for an a-(FeSi 2 ) x Al y ternary alloy with 10 at.% Al content, according to embodiment of the present invention.
  • Figure 3 is a schematic diagram illustrating the transducer layer structure after different fabrication steps, according to an embodiment of the present invention.
  • Figure 4 shows the current voltage l-V characteristics of an ITO/p-(FeSi 2 ) x Al y /n- Si/Ti/AI photoelectric transducer according to an; embodiment of the present invention, under dark and AM 1 ,5 standard sunlight illumination.
  • Figure 5 shows the current voltage l-V characteristics of an ITO/p-(FeSi 2 ) x Al y /n- Si/Ti/AI photoelectric transducer containing a thin GeON electron blocking layer according to an embodiment of the present invention, under dark and AM 1 ,5 standard sunlight illumination.
  • the present invention relates to the fabrication of a photoelectric transducer using Al-doped p-type beta phase of Iron silicide (p-FeSi 2 ) and a thin Aluminum interlayer on an n-type crystalline- and/or poly-; and/or amorphous- Si substrate and/or layer (formed on top of an electrode film deposited on a non-Si substrate) using conventional physical vapor deposition (PVD) techniques, such as, sputter-deposition, electron-beam deposition, and thermal evaporation. Following the physical vapor deposition, a low thermal budget post-deposition process is performed with an electron-blocking layer deposited on the Aluminum interlayer. Details of the complete description of the invention are as follows. 1 ) (FeSi 2 )xAly thin film photoelectric transducer on n-type Si substrates: device structure, fabrication steps, and characteristics
  • the fabrication process starts with the deposition of an Aluminum interiayer (1 nm - 60 nm) on an n-type Si substrate (single crystalline, multi-crystalline, or poly-crystalline) by physical vapor deposition, including DC or RF sputtering, electron-beam evaporation, and thermal evaporation.
  • a layer of Al containing (0 at. % - 10 at. %) beta- phase-like amorphous Iron disilicide ⁇ -like FeSi 2 (about 30 nm - 150 nm) is then deposited on top of the deposited Al interiayer using co-sputtering of FeSi 2 and Al targets.
  • RTA rapid thermal annealing
  • furnace annealing under nitrogen ambient at a temperature range from 400°C to 800°C for different annealing durations.
  • Figure 1 (a) shows the XRD patterns of the as-deposited ⁇ -like amorphous FeSi 2 layer and after annealing where the FeSi 2 has an orthorhombic ⁇ -phase with an Al interiayer having a thickness of less than 18% of the thickness of the as-deposited ⁇ -like amorphous FeSi 2 layer.
  • Figure 1 (b) shows the XRD patterns of the as-deposited ⁇ -like amorphous FeSi 2 layer and after annealing where the FeSi 2 has a tetragonal a-phase with an Al interlayer having a thickness of 18-40% of the thickness of the as-deposited ⁇ - like amorphous FeSi 2 layer.
  • a (FeSi 2 ) x Al y ternary layer is formed.
  • the massive diffusion of Al atoms from Al interlayer into the original FeSi 2 (AI) layer is thermodynamically driven by the tendency of Al to form a Fe-AI-Si ternary alloy film.
  • the presence of a large concentration of Al in the (FeSi 2 ) x Al y ternary layer formed makes it a p-type semiconductor due to the p-type doping nature of Al in 3-FeSi 2 .
  • a tetragonal a-phase (FeSi 2 ) x Al y layer is formed when the thickness of the Al interlayer is around 18% - 40% of the thickness of the as- deposited ⁇ -like amorphous FeSi 2 layer and the a-phase (FeSi 2 ) x Al y layer is found to be a semiconductor.
  • FIG. 2 illustrates when the pure a-phase FeSi 2 does not contain Aluminum, it is metallic.
  • the a-phase (FeSi 2 ) x Al y layer has a high optical absorption coefficient, i.e., >10 5 cm "1 for photon energy larger than 1.5 eV.
  • Figure 3(a) illustrates depositing an Aluminum interlayer (200) on a substrate (100) by PVD. Iron disilicide (300) is deposited on the Aluminum interlayer (200) by sputtering as shown in Fig. 3(b).
  • the FeSi 2 target and growth rate of FeSi 2 is about 6.5 nm/min at room temperature. The percentage of oxygen is controlled during sputtering in order to minimize oxygen contamination.
  • the base press of the sputtering chamber is about 2x10 '7 Torr.
  • a layer of 1 nm - 3 nm thick amorphous Al x Fe y Si z O w complex oxide layer (350) is formed between the (FeSi 2 )xAl y alloy layer (300) and n-Si substrate (100.) by the reaction of Al, Fe, Si and residual oxygen that is incorporated into the Al interlayer (200) during the sputtering process due to high chemical activity of Aluminum.
  • thermodynamic/kinetic processes eventually result in p- (FeSi 2 ) x Al y /n-Si hetero p/n junction structure where a 1 nm - 3 nm thick amorphous Al x Fe y Si z O w complex oxide layer (350) is present between the p- (FeSi 2 ) x Al y alloy layer (400) and n-Si substrate (100), as shown in Figure 3(c).
  • the thickness range of the p-(FeSi 2 )xAl y alloy layer and n-Si substrate ? ⁇ 3 to 10 nm.
  • the atomic ratio of Si/Fe in the (FeSi 2 ) x Al y layer is 2, such as, P-FeSi 2 .
  • the fabrication process is completed with the formation of a back electrode (e.g., Ti/AI) on the backside of n-Si and the deposition of transparent electrode, such as, Indium titanium oxide ITO (500), on the p-type (FeSi 2 ) x Al y alloy layer (400), as shown in Figure 3(d).
  • a back electrode e.g., Ti/AI
  • transparent electrode such as, Indium titanium oxide ITO (500)
  • the back contact electrode is ' fabricated at room temperature using either e- beam evaporation or sputtering deposition after removal of native oxide layer using dilute HF dipping.
  • the thickness of Ti is about 100-500 nm and of Al is about 500-1000 nm.
  • FIG. 4 shows the dark and photo l-V characteristics of an ITO/p-(FeSi 2 ) x Al y /n-Si/Ti/AI photoelectric transducer of the present invention, which is fabricated by the annealing of FeSi 2 (40nm)/AI(12nm)/n-Si structure at 650°C for 60 seconds.
  • the measured transducer shows a conversion efficiency of -2.5% with 13.71 mA/cm 2 short circuit current (J sc ), 0.435 V Open-Circuit voltage (V oc ), and 0.432 fill fact (FF).
  • the -FeSi 2 was a high quality epitaxial film prepared using a growth method that involves (1 ) template formation, (2) face-target deposition at 600 °C, and (3) a high temperature anneal (at 900 °C) with a long annealing time in forming gas.
  • the film of the present invention is a poly-crystalline film prepared with a simple sputtering deposition and low thermal budget RTA annealing. Photovoltaic characteristics of the fabricated photoelectric transducers are measured under 1.5 AM standard sunlight illumination condition (ASTM), 2) Enhancement of the transducer performance by introducing an electron- blocking layer
  • Fig. 5 shows that the forward current under light illumination is larger than the forward current without light for bias larger than 0.5 V.
  • the observation of a larger photocurrent under forward bias is not typically seen in normal cellar cells or photo-detectors.
  • the observation of a larger photo-current in transducers according to an embodiment of the present invention, under forward bias indicates that some photo-carriers, more specifically photo-generated electrons in the photo-active layer, i.e., p- (FeSi 2 ) x Aly layer, back-diffuse into the top ITO electrode, rather than forward- diffuse into the space charge region of the p-(FeSi 2 ) x Al y /p + -Si/n-Si hetero-p/n junction, thereby do not contribute to the photo-current in the external circuit.
  • the suppression of electron back-diffusion, consequently further increasing the photocurrent can be achieved by introducing a thin metal oxide (e.g., Mo0 3 ) or
  • Germanium oxy-nitride (Ge x O y N z ) between the photoactive layer and the top electrode as an electron-blocking layer, in accordance with the present invention.
  • the criteria for the selection of a suitable electron blocking layer is that it should have a larger conduction band offset to effectively block electrons diffusing from the photo-active p-(FeSi 2 ) x Aly alloy layer into the ITO while its small valence band offset allows for relatively free transfer of holes between the p-type photo-active layer and the ITO. Thus, ensuring low contact resistance between these two layers.
  • FIG. 5 shows the l-V characteristics of an ITO/p-(FeSi 2 ) x Al y /p + -Si/n-Si/Ti/AI photoelectric transducer of the present invention with a thin ( ⁇ 3nm) Ge x O y N z layer as an electron blocking layer between the ITO and the photo-active p- (FeSi 2 ) x Al y layer.
  • the thin Ge x O y N 2 is deposited by sputtering of a Geranium target under Ar and N 2 gas ambient before ITO deposition. Comparing Fig. 4 and Fig.
  • the conversion efficiency of the transducer is improved to 3.3% compared to -2.5% of the transducer without the electron blocking layer, due to the improvement in the short circuit current J sc (18.2 vs. 13.71 mA/cm 2 ) and a substantially increased Open-Circuit voltage V oc (0.5 vs. 0.435 V). This is a 32% increase in the conversion efficiency.
  • the simple sputtering growth method of the present invention using low-thermal budget RTA annealing can be used on non-Si substrates, such as glass or stainless steel, due to its low thermal budget process.
  • the prior art is only useful for a Si (1 1 ) substrate due to its need to grow epitaxial layer, which requires a high thermal budget annealing.
  • the transducer with the thin electron-blocking layer has a fill factor of 0.36, which is lower than values conventionally obtained for high performance solar cells.
  • contact resistance and sheet resistance of the ITO are generally the main parameters that determine the fill factor.
  • Further optimization of the ITO process and the back contact formation contributes to the improvement in the fill factor, and consequently the conversion efficiency.
  • an ITO/p-(FeSi 2 ) x Al y /p + - Si/n-Si photoelectric transducer with a thin electron-blocking layer according to the present invention having a fill factor of 0.54 or above results in a, conversion efficiency of about 5%.
  • the photoactive layer of the present invention is a (FeSi 2 )xAl y Iron-disilicide Aluminum alloy with up to 25 at. % of Aluminum.
  • the typical Al content is about 7-20 at.%, which is higher than the amount of Aluminum that is typically used to introduce p-type doping in beta-phase -FeSi 2 .
  • the Si/Fe ratio remains to be 2 in the (FeSi ) x Al y Iron-disilcide Aluminum alloy, the crystal structure of the (FeSi 2 ) x Al y Iron-disilicide Aluminum alloy is no longer ⁇ -phase.
  • the photoactive layer in the photoelectric transducers according to example embodiments of the present invention is fundamentally different from the photoactive layer discussed for beta-phase FeSi 2 based photoelectric transducers in the prior art.
  • the transducers of the present invention are demonstrated on n-type Si substrates, those skilled in the art at the time the invention was made will appreciate that the transducer structure, as well as, the fabrication method of the present invention can also be applied to non-Si substrates.
  • the present invention uses low cost substrates that are suitable for application to a large- scale manufacturing of photovoltaics. If a thin stainless steel substrate is used, the photovoltaics are flexible. For example, one can make a similar transducer by first depositing a thin layer of n-type poly or amorphous Si layer on a high temperature glass substrate coated with a highly conductive ZnO(AI) film, followed by the same fabrication steps of embodiments of the present invention.
  • ZnO(AI) serves as a back electrode and the n-type poly or amorphous Si layer serves as an n-type side of a hetero-p/n junction.
  • the same structure of the transducer can also be fabricated on a flexible stainless steel substrate coated with a thin metallic diffusion barrier (e.g., Mo-silicide) following the same approach with the glass substrate.
  • the stainless steel substrate itself acts as a back electrode.
  • Example embodiments of the present invention provide the following features: i) The use of p-type (FeSi2) x Al y ternary alloy as photoactive layer
  • the photoactive p-type (FeSi2) x Al y ternary alloy layer of the present invention is fundamentally different from a conventional photoactive layer beta-phase ⁇ - FeSi 2 Used in Iron-disilicide-based photoelectric transducers.
  • the crystal structure of the p-type (FeSi2) x Al y ternary alloy has an Aluminum content larger than 7 at.% and is in the tetragonal structure -phase, rather than the orthorhombic structure ⁇ -phase.
  • oc-(FeSi 2 ) x Al y of; the present invention is a semiconductor. Further, the contents of Al (typically 4 - 25 at.
  • the present invention allows for the exploitation of a multi-purpose Al interlayer in the fabrication process.
  • the use of Al interlayer in the fabrication process not only results in the formation of p-type Fe-Si-AI ternary alloy as a photo-active layer with good photovoltaic properties due to the solid-state reaction between the as-deposited FeSi 2 and Al interlayer during annealing, but also enables the formation of p+ ⁇ Si/n-Si home junction through Al diffusion into n-Si that helps to reduce dark leakage current. Therefore, the Open-Circuit voltage (V oc ) of the photoelectric transducers is increased.
  • the Al interlayer can be deposited on any substrate and does not have an influence on the quality of the p-type Fe-Si-AI ternary alloy photoactive layers formed by the solid-state reaction of the as-deposited FeSi 2 with Al interlayer. Therefore, photoelectric transducers having a similar structure are fabricated on non-Si substrates using the fabrication process of the present invention if the non-Si substrates have a layer of n-type Si surface layer. iii) The use of electron blocking layer
  • Another embodiment of the present invention incorporates the use of a thin Geranium nitride or Germanium oxynitride and metal oxides as an electron- blocking layer between the top transparent electrode and the p-type Fe-Si-AI ternary alloy photoactive layer.
  • the Germanium nitride or Germanium oxynitride and metal oxides selected have a high valence band offset and low conduction band offset with the p-type Fe-Si-AI ternary alloy photoactive layer.
  • the favorable band alignment effectively suppresses back-diffusion of the photo- generated electrons into the top transparent electrode. Thereby, increasing the photocurrent in the external circuit without increasing the contact resistance between the top transparent electrode and the p-type Fe-Si-AI ternary alloy photoactive layer.
  • Example embodiments of the present invention provide the following advantages:
  • the inefficiency in hole injection is one of the major limiting factors in n-type epitaxial n- -FeSi2/p-Si hetero-ju notion solar cells.
  • the approach and methodology as posed by the present invention provides a solution by fabricating a p-type photo-active layer on a n-type Si substrate (or layer) by alloying of FeSi 2 layer with the Al interlayer that is originally sandwiched between the as-deposited FeSi 2 and the n-Si substrate (or layer) by post- deposition thermal annealing.
  • the massive diffusion of Al into FeSi 2 during the annealing process that occurs during the alloying of FeSi 2 with an Al interlayer results in the formation of (FeSi 2 ) x Al y (Si/Fe atomic ratio is kept 2) ternary alloy.
  • the ternary (FeSi 2 ) x Al y alloys typically are poly-crystalline tetragonal structure a-phase for (FeSi 2 ) x Al y alloy with 7 at.% - 20 at.% Al content.
  • an . orthorhombic structure beta-phase p-(FeSi 2 ) x Al y is also observed for (FeSi 2 ) x Al y alloy with an Al content less than 7 at.%.
  • Both the a-(FeSi 2 ) x Al y and p-(FeSi 2 ) x Al y ternary alloy exhibit p-type semiconducting conductivity typically with a high optical absorption coefficients (larger than 10 5 cm “1 ) and bandgap values between 0.85 eV and ⁇ 1 eV, depending on the Al content. Good photovoltaic properties are exhibited in the transducers based on these ternary alloys even though these alloy films are poly-crystalline films, rather than expitaxial single crystalline films.
  • Photocurrents as high as 23 mA/cm 2 is observed in a photoelectric transducer containing (FeSi 2 ) x Al y alloy photo-active layer formed by alloying 40 nm thick as-deposited FeSi 2 layer with 15 nm AI interlayer while a short circuit current of 13.71 mA cm 2 , an Open-Circuit voltage (V oc ) of 0.435 V, and a conversion efficiency of -2.5% are obtained for a ITO/p-(FeSi 2 ) x Aly/p + -Si/n-Si photoelectric transducer formed with 40 nm thick as-deposited FeSi 2 layer and 10 nm AI interlayer.
  • V oc Open-Circuit voltage
  • the improved carrier injection efficiency at the interface between the photoactive layer and n-Si is one of the reasons for the improved photovoltaic properties.
  • the photovoltaic properties in poly-crystalline layers of the present invention indicate that the photovoltaic properties of the photoactive layers are also significantly improved by the incorporation of a large amount of AI atoms, which may be attributed to the enhanced carrier mobility and reduced mid-gap level defects that promote photo-carrier recombination. This results in a short minority carrier diffusion length.
  • the AI interlayer imprpyes the performance of the photoelectric transducers, After most of AI in the AI interlayer is consumed by the alloying with the Iron-silicide layer, a layer of 1 nm - 3 nm thick amorphous Al x Fe y Si z O w complex oxide layer is formed between the (FeSi 2 ) x Al y alloy layer and n-Si by the reaction of AI, Fe, Si and residual oxygen that is incorporated into the AI interlayer during the sputtering process (due to high chemical activity of Al).
  • This thin interfacial oxide layer improves interfacial property, most likely by passivating interfacial electronics states, as lower dark leakage current is observed in the ITO/p-(FeSi 2 ) x Al y /p + -Si/n-Si photoelectric transducers formed with an Al interlayer as compared to ITO/p-(FeSi 2 ) x Al y /p + - Si/n-Si photoelectric transducers formed without an Al interlayer.
  • the ITO/p-(FeSi2) x Al y /p + -Si/n-Si photoelectric transducers formed with an Al interlayer show an increased Open-Circuit Voltage V oc .
  • the Fermi-level of the ITO is ideally located to match the valence band edge of (FeSi 2 ) x Al y as both are located about -4.7 eV below vacuum level.
  • a band of empty electronic states are above the Fermi level in the ITO, where some of the photoelectrons generated in the (FeSi 2 ) x Al y photoactive layer could diffuse into the ITO to occupy these empty electronic states.
  • the present invention achieves this by introducing a thin layer, preferably a few nanometers thick, of insulator, such as, Germanium nitride (or Germanium oxynitride) and metal oxides (e.g., M0O 3 ) between the ITO and photoactive layer.
  • insulator such as, Germanium nitride (or Germanium oxynitride) and metal oxides (e.g., M0O 3 ) between the ITO and photoactive layer.
  • the thin layer insulators have a large conduction band offset with the photoactive layer, but with a small valence band offset. Therefore, the thin electron-blocking layer effectively suppresses the photoelectron back-diffusion without increasing the contact resistance.
  • An improved short circuit current is obtained in an ITO/p-(FeSi 2 ) x Al y /p + -Si/n-Si photoelectric transducer integrated with a thin Germanium nitride electron-blocking layer.
  • the Germanium nitride is deposited by using Ge target and a conventional sputter technique in a nitrogen ambient.
  • the favorable energy band alignment of the structure reduces the back diffusion of the electrons and increases the photo-generated current.
  • the photoactive p-type (FeSi 2 ) x Al y ternary layer in the photoelectric transducers of the present invention is typically poly-crystalline structure, which is formed by the solid-state reaction of the as-deposited Iron disicilide layer arid the underlying poly or amorphous Al interlayer.
  • a photoelectric transducer having a p-type (FeSi2) x Al y ternary alloy layer with the same photovoltaic properties of the present invention can also be fabricated on non-Si substrates by following the processing steps in accordance with example embodiments of the present invention.
  • the present invention uses low cost substrates that are suitable for application to a large-scale manufacturing of photovoltaics. If a thin stainless steel substrate is used, the photovoltaics are flexible.
  • n-type poly or amorphous Si is deposited on high temperature glass substrates coated with a high conductive ZnO(AI) film.
  • ZnO(Al) serves as the back electrode and the n-type poly or amorphous Si layer serves as the n-type side of the hetero-p/n junction.
  • a flexible stainless steel substrate coated with a thin metallic diffusion barrier e.g., Mo-silicide
  • the stainless steel substrate itself acts as the back electrode.

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Abstract

One embodiment of the present invention is a photoelectric transducer having a p/n heterojunction comprising a p-type photoactive (FeSi2)xAly Iron-disilicide Aluminum alloy layer (400) disposed on a substrate (100). Another embodiment of the present invention is a method for forming a photoelectric transducer by depositing an Aluminum interlayer (200) on a substrate (100) by physical vapor deposition; sequentially depositing Iron disilicide (FeSi2) (300) on the Aluminum interlayer (200) by sputtering; and annealing the Aluminum interlayer (200) and Iron disilicide (FeSi2) (300) to form a p-type photoactive (FeSi2)xAly Iron-disilicide Aluminum alloy layer (400) disposed on the substrate (100). Further, the Aluminum interlayer (200) and Iron disilicide (FeSi2) (300) on the Silicon substrate are then subjected to rapid thermal annealing (RTA) or furnace annealing under nitrogen ambient at a temperature range from 400 °C to 800 °C. The annealing step changes FeSi2 layer (300) from the amorphous phase to a poly-crystalline phase (400), such as, orthorhombic β-phase or tetragonal crystal structure alpha phase (a-phase).

Description

PHOTOELECTRIC TRANSDUCER USING IRON SILICIDE AND ALUMINUM AND METHOD FOR PREPARING THE SAME
FIELD OF INVENTION
The present invention relates to the fabrication of a photoelectric transducer composed of an Aluminum doped p-type beta phase of Iron silicide ( -FeSi2), a thin Aluminum interlayer, and an n-type Si substrate. Physical vapor deposition (sputter-deposition, electron-beam deposition, thermal evaporation, etc.) and low thermal budget post-deposition processing with an optional electron- blocking layer deposit the Iron silicide (P-FeSi2) and Aluminum interlayer.
BACKGROUND OF THE INVENTION The beta phase of Iron silicide p-FeSi2 is regarded as one of the third generation semiconductors after Si and GaAs. -FeSi2 has a high optical absorption coefficient (>105cm"1 for photon energy larger than 1eV), high thermoelectric power (Seebeck coefficient of k ~ 10"4/K), direct energy band-gap of 0.85 eV that corresponds to 1.5μιη of quartz optical fiber communication, lattice constant nearly well-matched to Si substrate, high resistance against the humidity, chemical attacks and oxidization, and is chemically stabile at temperatures as high as 937°C. Various devices, such as, solar cells, Si photosensors, and thermoelectric generators that are integrated on Si-LSI circuits can be fabricated using p-FeSi2 films. The beta phase of Iron silicide p-FeSi2 having a band-gap value of about 0.85 eV and large optical absorption from the near infrared to visible light are expected to produce a thin film solar cell with a theoretical conversion efficiency of 16%~23%. Further, the earth's crust is an abundant source of β-FeS , which is non-toxic and stable at high temperature. Thus, β-FeS is suitable in solar cell applications. To date, most of the prototype P-FeSi2 solar cells have been fabricated on a crystalline Si substrate having opposite conductivity by forming n-p-FeSi2/p-Si hetero junctions. An energy conversion efficiency as high as 3.7% has been achieved when an epitaxial η-β-FeS film is grown on crystalline Si (111 ) using face-target sputtering deposition to form a n-P-FeSi2/p-Si cell. The conversion efficiency is ~ 3.6% on a non-Si substrate where an amorphous hydrogenated beta-like Iron silicide p-FeSi2(H) film deposited by the combination of plasma enhanced chemical vapor deposition (PECVD) (for the dissociation of SiH4) and thermal evaporation (for Fe deposition). Although the two approaches can produce p-FeSi2 solar cells with an efficiency as high as 3.6% - 3.7%, the production process is inefficient, At present, there is a need for a large-scale fabrication that is low in cost.
Conventionally, epitaxial p-FeSi2/Si hetero-junction solar cells are unsuitable for non-Si substrates, specifically, because crystalline silicon substrates have band misalignment between n-FeSi2 and p-Si. For example, a p-type single crystalline Si (1 1 1 ) is typically used to form a η-β-FeS /p-Si hetero-junction structure because high quality epitaxial n-type p-FeSi2 has high carrier mobility, a low concentration of residual carriers, as well as, low defects can be grown on this orientation Si substrate (1 1 1 ) if a proper growth method is used (e.g., face target sputtering deposition of FeSi2 at elevated substrate temperature with a thin template layer, combined with a high temperature post-deposition furnace annealing at 800°C). The threshold incident photon energy corresponding to the ionization potential of p-FeSi2 is 4.71 eV. Based on this value and the known band structure parameters of Si and p-FeSi2, there is a substantial energy barrier of about 0.46 eV for hole injection from p-FeSi2 into Si. This is a serious problem for an η-β- FeSi2/p-Si cell, as the photo-generated holes in p-FeSi2 cannot efficiently inject into the p-Si side at the n- -FeSi2/p-Si interface Thus, the collection efficiency of photo-generated holes by the space charge region and in the photocurrent in external circuit is limited. Therefore, the conversion efficiency is 3.7% for the epitaxial n- -FeSi2/p-Si ( ) cell, even though the epitaxial η-β-FeS is a high quality single crystalline film with a diffusion length L (> 1 pm) that is much larger than its film thickness (~0.2 pm) . However, the conversion efficiency is as high as 6.5%.
Another problem associated with band misalignment for P-FeSi2 solar cells is that their fabrication requires crystalline Si substrates. High quality epitaxial single crystalline -FeSi2 films can be grown on the single crystalline Si substrates. However, the high cost of single crystalline substrates essentially eliminates the use of this type of solar cells in practical applications. Further, the performance of Si wafer solar cells is higher than -FeSi2 solar cells. For 5 practical applications, it is important to use a non-Si substrate in order to reduce the fabrication cost. However, intrinsic P-FeSi2 films grown on non-Si substrates are typically poly-crystalline or amorphous phase with a high concentration of residual carriers (mostly, n-type conducting electrons), as well as, a high density of defects. Another disadvantage of intrinsic -FeSi2 films grown on non-Si0 substrates is that they usually exhibit poor photovoltaic characteristics, .thereby limiting their use for solar cell applications.
If an appropriate amount of hydrogen atoms is incorporated into poly-crystalline and/or amorphous -FeSi2 films grown on non-Si substrates, their quality is5 improved, which effectively quenches Si-dangling bonds in Iron (Fe) vacancies and grain boundaries by forming Si-H bond sites. Thus, residual carriers are reduced. U.S. Patents Nos. 6,949,463; 7,352,044; and 7,354,857 describe a p+- Si/intrinsic hydrogenated amorphous Iron-silicide i-FexSiyHx/n+-Si p-i-n junction and its method of fabrication.
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US Patents Nos. 6,949,463; 7,352,044; & 7,354,857. discuss a photoelectric transducer having a p-i÷n junction on a substrate. The 'p' layer may comprise a p-type silicon, the ΐ layer is Iron silicide, and 'n' layer an n-type silicon. The p and n layers are formed by plasma CVD, while the i layer is formed by flowing SiH4 and H2 gases into the deposition chamber to form a plasma composed of the constituent gases. Deposition proceeds via evaporation of Iron from Iron ingot onto the Si wafer. The substrate temperature is at 250°C. Further, amorphous FexSiy:H film of thickness 300 nm is formed on the Si wafer. Dangling bonds of Si atoms and/or Iron atoms are terminated with hydrogen, whereby a number of trap levels that may occur in the amorphous Iron silicide film are eliminated. The conversion efficiency of 3.6% is achieved in a single junction. Though a conversion efficiency of 3.6% is obtained in a single junction p+-Si/i-a- FexSiyHx/n+-Si p-i-n cell, which is comparable to the efficiency of 3.7% obtained on an epitaxial crystalline -FeSi2 solar cell grown on crystalline p-Si (11 1 ) wafer, further improvement in efficiency is difficult with the single junction p+- Si/i-a-FexSiyHx/n+-Si p-i-n cell. Another disadvantage of i-FexSiyHx is that the low carrier mobility due to its amorphous microstructure structure as the same in the case of hydrogenated amorphous a-Si:H. Therefore, the collection efficiency of photo-generated carriers in the i-layer by the n+-and p+-layers could be limited, unless the internal electrical field in the i-layer is extremely high and the carrier drift time in the i-layer is shorter than the carrier lifetime. To overcome this problem, the thickness of i-layer is reduced to increase the internal electrical field. However, this conventional approach faces the problem that the light absorption suffers in a thin i-a-FexSiyHx layer. Another limitation with the p+-Si/i-a-FexSiyHx/n+-Si p-i-n cells is described in
U.S. Patents Nos. 6,949,463; 7,352,044; and 7,354,857 where the deposition of the i-a-FexSiyHx layer is done with a complex PECVD - thermal evaporation system. SiH4 is supplies Si and H through PECVD plasma dissociation and deposition while Fe is deposited by thermal evaporation. Disadvantages of this system include that it is complicated system and not ready for mass production.
The prior art method of growing (1 10)/(101 )-oriented epitaxial p-FeSi2 is discussed by Zhengxin Liu et al. in "A thin-film solar cell of high-quality β- FeSi2/Si heteroju notion prepared by sputtering," Solar Energy Materials & Solar Cells 90 (2006) 276-282. The paper describes the use of a dedicated facing- target sputtering (FTS) method, involving two steps to form a thin -FeSi2 template buffer prior FeSi2 formation. Alternate layers of Fe and Si are used to form high-quality (1 10)/(101 )-oriented epitaxial β-FeS^. The characteristics include n-type conductivity, optimum thickness of -FeSi2 films: 230 nm, optimum parameters: V0G 0.45 V, short-circuit current density (Jsc) 14.8 mA/cm2, fill factor (FF) 0.55, conversion efficiency 3.7%.
The conventional method described in Liu uses a p-type Si substrate to make a hetero p/n junction because p-FeSi2 without intentional doping exhibits n-type conductivity, which introduces impurities into the semiconductor. Thus, there is a substantial energy barrier for hole injection from 3-FeSi2 into the p-type Si substrate. ) Another disadvantage of this method is the absence of a surface passivation or surface electron-carrier blocking layer that reduces photocurrent due to surface recombination. Further, the growth of an epitaxial β-FeSb film is only achieved on a Si (111) substrate by template formation method of β- FeSi2combined with special face-target deposition of FeSi2 that requires the use of a high temperature & extended annealing time after film growth.
Another prior art deposition method is described in Mahmoud Shaban et al., "Photovoltaic Properties of n-type -FeSi2/p-type Si Heterojunctions," Japanese Journal of Applied Physics, Vol. 46, No. 27, 2007, pp. L667-L669. The prior art method carried out deposition at a substrate temperature of 600°C by the facing-target sputtering (FTS) method using FeSi2 target. The characteristics include optimum FeSi2 thickness: 500 nm, optimum parameters: Voc 178 mV, Jsc 12.81 mA/cm2, FF 0.28, and conversion efficiency of 0.63%. The current voltage l-V characteristics were obtained with light incident on the Si side. However, there is an energy barrier at the interface for hole injection from β- FeSi2 into the p-type Si substrate due to the p-type Si substrate used to make a hetero p/n junction because p-FeSi2 without intentional doping exhibits n-type conductivity. The prior art method also demonstrates severe diffusion of Fe into Si during the growth of FeSi2 at 600 °C, which induces recombination/generation centers in Si and results in a large leakage and low open circuit voltage.
Mahmoud Shaban et al. further described in a "Low-Temperature Annealing of n-Type p-FeSi2/p-Type Si Heterojunctions" (Japanese Journal of Applied Physics, Vol, 47, No. 5, 2008, pp. 3444-3446) that deposition was carried out at a substrate temperature of 600°C by the facing-target sputtering (FTS) method using FeSi2 target. The characteristics include n-type p-FeSi2 thin films with a thickness of 300 nm, optimum post-deposition annealing 300°C, 1 h, optimum parameters: Open-Circuit voltage (Voc) 0.17 V, Jsc 12 mA/cm2, efficiency 0.57%.
This conventional method also exhibits an energy barrier at the interface for hole injection from P-FeSi2 into the p-type Si substrate. Diffusion of Fe into Si also occurs during the growth of FeSi2 at 600 °C, which induces recombination/generation centers in Si and results in a large leakage and low open circuit voltage. There is some improvement in short-circuit current density after the low temperature annealing, there is not substantial improvement in Open-Circuit voltage (Voc) due to the limiting effect of Fe gettering by low temperature annealing.
The approach and methodology as proposed in the present invention provides a solution towards fabricating a beta phase of Iron silicide (P-FeSi2). The characteristics and features of the present invention include, but are not limited to forming an cc-phase (FeSi2)xAly iron-disilicide aluminum ternary alloy after annealing the p-FeSi2that exhibits semiconducting nature rather than metallic electronic property with a bandgap of ~1 eV due to its increased Aluminum content. The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate on exemplary technology area where some embodiments described herein may be practiced.
SUMMARY OF THE INVENTION
The present invention addresses one or more of the above-mentioned deficiencies and others by providing a photoelectric transducer comprising a p/n heterojunction having a p-type photoactive Fe-Si-AI ternary alloy layer formed on a substrate.
In one embodiment, the alloy layer contains Aluminum where the Aluminum content of the p-type photoactive Fe-Si-AI Aluminum alloy layer is 4 - 25 at.%.
In another embodiment of invention, the (FeSi2)xAly Iron-disilicide Aluminum alloy layer has a tetragonal crystal structure alpha phase (a-phase) or an orthorhombic crystal structure beta-phase (β-phase). The present invention also provides a method of fabricating a photoelectric transducer, the method comprising the step of forming a p-type photoactive Fe- Si-AI ternary alloy layer on a substrate.
The present invention consists of features and a combination of parts hereinafter fully described and illustrated in the- accompanying drawings, it being understood that various changes in the details may be made without departing from the scope of the invention or sacrificing any of the advantages of the present invention. BRIEF DESCRIPTON OF ACCOMPANYING DRAWINGS
To further clarify various aspects of some embodiments of the present invention, a more particular description of the invention will be rendered by references to specific embodiments, thereof, which are illustrated in the appended drawings. It is appreciated that these drawings only depict typical embodiments of the invention and therefore are not considered to be limiting of its scope. The invention will be described and explained with additional specificity and detail through the accompanying drawings where:
Figure 1(a) shows the x-ray diffraction (XRD) patterns of a FeSi2(40nm)/AI(3nm) stack according to an embodiment of the present invention, showing the formation of the β-phase after RTA annealing at 600 °C or above.
Figure 1 (b) shows the XRD patterns of a FeSi2(40nm)/AI(11 nm) stack according to an embodiment of the present invention, showing the formation of the a- phase after RTA annealing at 600 °C or above.
Figure 2 shows the square of optical absorption (a2) versus. the photon energy curve for an a-(FeSi2)xAly ternary alloy with 10 at.% Al content, according to embodiment of the present invention.
Figure 3 is a schematic diagram illustrating the transducer layer structure after different fabrication steps, according to an embodiment of the present invention. Figure 4 shows the current voltage l-V characteristics of an ITO/p-(FeSi2)xAly/n- Si/Ti/AI photoelectric transducer according to an; embodiment of the present invention, under dark and AM 1 ,5 standard sunlight illumination.
Figure 5 shows the current voltage l-V characteristics of an ITO/p-(FeSi2)xAly/n- Si/Ti/AI photoelectric transducer containing a thin GeON electron blocking layer according to an embodiment of the present invention, under dark and AM 1 ,5 standard sunlight illumination.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to the fabrication of a photoelectric transducer using Al-doped p-type beta phase of Iron silicide (p-FeSi2) and a thin Aluminum interlayer on an n-type crystalline- and/or poly-; and/or amorphous- Si substrate and/or layer (formed on top of an electrode film deposited on a non-Si substrate) using conventional physical vapor deposition (PVD) techniques, such as, sputter-deposition, electron-beam deposition, and thermal evaporation. Following the physical vapor deposition, a low thermal budget post-deposition process is performed with an electron-blocking layer deposited on the Aluminum interlayer. Details of the complete description of the invention are as follows. 1 ) (FeSi2)xAly thin film photoelectric transducer on n-type Si substrates: device structure, fabrication steps, and characteristics
In an embodiment of the present invention, the fabrication process starts with the deposition of an Aluminum interiayer (1 nm - 60 nm) on an n-type Si substrate (single crystalline, multi-crystalline, or poly-crystalline) by physical vapor deposition, including DC or RF sputtering, electron-beam evaporation, and thermal evaporation. A layer of Al containing (0 at. % - 10 at. %) beta- phase-like amorphous Iron disilicide β-like FeSi2 (about 30 nm - 150 nm) is then deposited on top of the deposited Al interiayer using co-sputtering of FeSi2 and Al targets. Alternatively, co-sputtering of Fe, Si, and Al targets are effective. The double layer deposited on the Silicon substrate is then subjected to rapid thermal annealing (RTA) or furnace annealing under nitrogen ambient at a temperature range from 400°C to 800°C for different annealing durations. Several different physics and thermo-dynamics/kinetics processes take place concurrently during the post-deposition annealing depending on the initial Al interiayer thickness as described below.
During the thermal annealing process, a phase transition occurs from the amorphous phase to the poly-crystalline phase. Figure 1 (a) shows the XRD patterns of the as-deposited β-like amorphous FeSi2 layer and after annealing where the FeSi2 has an orthorhombic β-phase with an Al interiayer having a thickness of less than 18% of the thickness of the as-deposited β-like amorphous FeSi2 layer. Figure 1 (b) shows the XRD patterns of the as-deposited β-like amorphous FeSi2 layer and after annealing where the FeSi2 has a tetragonal a-phase with an Al interlayer having a thickness of 18-40% of the thickness of the as-deposited β- like amorphous FeSi2 layer. When the massive diffusion of Al from the Al interlayer into the as-deposited FeSi2 layer, a (FeSi2)xAly ternary layer is formed. The massive diffusion of Al atoms from Al interlayer into the original FeSi2(AI) layer is thermodynamically driven by the tendency of Al to form a Fe-AI-Si ternary alloy film.
On the other hand, the presence of a large concentration of Al in the (FeSi2)xAly ternary layer formed makes it a p-type semiconductor due to the p-type doping nature of Al in 3-FeSi2. A tetragonal a-phase (FeSi2)xAly layer is formed when the thickness of the Al interlayer is around 18% - 40% of the thickness of the as- deposited β-like amorphous FeSi2 layer and the a-phase (FeSi2)xAly layer is found to be a semiconductor.
Reference is now made to Figure 2 that illustrates when the pure a-phase FeSi2 does not contain Aluminum, it is metallic. The a-phase (FeSi2)xAly layer has a high optical absorption coefficient, i.e., >105cm"1 for photon energy larger than 1.5 eV.
Reference is now made to Figures 3(a)-(d). Figure 3(a) illustrates depositing an Aluminum interlayer (200) on a substrate (100) by PVD. Iron disilicide (300) is deposited on the Aluminum interlayer (200) by sputtering as shown in Fig. 3(b). The FeSi2 target and growth rate of FeSi2 is about 6.5 nm/min at room temperature. The percentage of oxygen is controlled during sputtering in order to minimize oxygen contamination. The base press of the sputtering chamber is about 2x10'7 Torr.
After most of the Aluminum in the Al interlayer (200) is consumed by the alloying with the Iron disilicide layer (300), a layer of 1 nm - 3 nm thick amorphous AlxFeySizOw complex oxide layer (350) is formed between the (FeSi2)xAly alloy layer (300) and n-Si substrate (100.) by the reaction of Al, Fe, Si and residual oxygen that is incorporated into the Al interlayer (200) during the sputtering process due to high chemical activity of Aluminum. All of the concurrently occurring thermodynamic/kinetic processes eventually result in p- (FeSi2)xAly/n-Si hetero p/n junction structure where a 1 nm - 3 nm thick amorphous AlxFeySizOw complex oxide layer (350) is present between the p- (FeSi2)xAly alloy layer (400) and n-Si substrate (100), as shown in Figure 3(c). In a preferred embodiment of the present invention, the thickness range of the p-(FeSi2)xAly alloy layer and n-Si substrate? ~3 to 10 nm. In an embodiment of the present invention, the atomic ratio of Si/Fe in the (FeSi2)xAly layer is 2, such as, P-FeSi2.
In another embodiment of the present invention, the fabrication process is completed with the formation of a back electrode (e.g., Ti/AI) on the backside of n-Si and the deposition of transparent electrode, such as, Indium titanium oxide ITO (500), on the p-type (FeSi2)xAly alloy layer (400), as shown in Figure 3(d). The back contact electrode is' fabricated at room temperature using either e- beam evaporation or sputtering deposition after removal of native oxide layer using dilute HF dipping. The thickness of Ti is about 100-500 nm and of Al is about 500-1000 nm.
Reference is now made to Figure 4. Fig. 4 shows the dark and photo l-V characteristics of an ITO/p-(FeSi2)xAly/n-Si/Ti/AI photoelectric transducer of the present invention, which is fabricated by the annealing of FeSi2(40nm)/AI(12nm)/n-Si structure at 650°C for 60 seconds. The measured transducer shows a conversion efficiency of -2.5% with 13.71 mA/cm2 short circuit current (Jsc), 0.435 V Open-Circuit voltage (Voc), and 0.432 fill fact (FF).
In prior art methods, the -FeSi2 was a high quality epitaxial film prepared using a growth method that involves (1 ) template formation, (2) face-target deposition at 600 °C, and (3) a high temperature anneal (at 900 °C) with a long annealing time in forming gas. The film of the present invention is a poly-crystalline film prepared with a simple sputtering deposition and low thermal budget RTA annealing. Photovoltaic characteristics of the fabricated photoelectric transducers are measured under 1.5 AM standard sunlight illumination condition (ASTM), 2) Enhancement of the transducer performance by introducing an electron- blocking layer
Reference is now made to Figure 5. Fig. 5 shows that the forward current under light illumination is larger than the forward current without light for bias larger than 0.5 V. The observation of a larger photocurrent under forward bias is not typically seen in normal cellar cells or photo-detectors. The observation of a larger photo-current in transducers according to an embodiment of the present invention, under forward bias, indicates that some photo-carriers, more specifically photo-generated electrons in the photo-active layer, i.e., p- (FeSi2)xAly layer, back-diffuse into the top ITO electrode, rather than forward- diffuse into the space charge region of the p-(FeSi2)xAly/p+-Si/n-Si hetero-p/n junction, thereby do not contribute to the photo-current in the external circuit. The suppression of electron back-diffusion, consequently further increasing the photocurrent, can be achieved by introducing a thin metal oxide (e.g., Mo03) or
Germanium oxy-nitride (GexOyNz) between the photoactive layer and the top electrode as an electron-blocking layer, in accordance with the present invention. The criteria for the selection of a suitable electron blocking layer is that it should have a larger conduction band offset to effectively block electrons diffusing from the photo-active p-(FeSi2)xAly alloy layer into the ITO while its small valence band offset allows for relatively free transfer of holes between the p-type photo-active layer and the ITO. Thus, ensuring low contact resistance between these two layers. Fig. 5 shows the l-V characteristics of an ITO/p-(FeSi2)xAly/p+-Si/n-Si/Ti/AI photoelectric transducer of the present invention with a thin (~ 3nm) GexOyNz layer as an electron blocking layer between the ITO and the photo-active p- (FeSi2)xAly layer. The thin GexOyN2 is deposited by sputtering of a Geranium target under Ar and N2 gas ambient before ITO deposition. Comparing Fig. 4 and Fig. 5, it can be seen that, with the thin electron blocking layer, the conversion efficiency of the transducer is improved to 3.3% compared to -2.5% of the transducer without the electron blocking layer, due to the improvement in the short circuit current Jsc (18.2 vs. 13.71 mA/cm2) and a substantially increased Open-Circuit voltage Voc (0.5 vs. 0.435 V). This is a 32% increase in the conversion efficiency. The simple sputtering growth method of the present invention using low-thermal budget RTA annealing can be used on non-Si substrates, such as glass or stainless steel, due to its low thermal budget process. In contrast, the prior art is only useful for a Si (1 1 ) substrate due to its need to grow epitaxial layer, which requires a high thermal budget annealing.
The transducer with the thin electron-blocking layer has a fill factor of 0.36, which is lower than values conventionally obtained for high performance solar cells. Although there are many factors affecting the fill factor, contact resistance and sheet resistance of the ITO are generally the main parameters that determine the fill factor. Further optimization of the ITO process and the back contact formation contributes to the improvement in the fill factor, and consequently the conversion efficiency. For example, an ITO/p-(FeSi2)xAly/p+- Si/n-Si photoelectric transducer with a thin electron-blocking layer according to the present invention having a fill factor of 0.54 or above results in a, conversion efficiency of about 5%. 3) Effect of alloying the Iron silicide with Aluminum
The photoactive layer of the present invention is a (FeSi2)xAly Iron-disilicide Aluminum alloy with up to 25 at. % of Aluminum. For a (FeSi2)xAly Iron-disilicide Aluminum alloy with good photovoltaic properties, the typical Al content is about 7-20 at.%, which is higher than the amount of Aluminum that is typically used to introduce p-type doping in beta-phase -FeSi2. Although the Si/Fe ratio remains to be 2 in the (FeSi )xAly Iron-disilcide Aluminum alloy, the crystal structure of the (FeSi2)xAly Iron-disilicide Aluminum alloy is no longer β-phase. Rather, it shows a-phase crystal structure. Unlike pure oc-phase -FeSi2) the a-phase (FeSi2)xAly Iron-disilicide Aluminum ternary alloy of the present invention exhibits a semiconducting nature rather than a metallic electronic property with a bandgap of ~1 eV. Consequently, the photoactive layer in the photoelectric transducers according to example embodiments of the present invention is fundamentally different from the photoactive layer discussed for beta-phase FeSi2 based photoelectric transducers in the prior art.
4) Fabrication on non-Si substrates Although the transducers of the present invention are demonstrated on n-type Si substrates, those skilled in the art at the time the invention was made will appreciate that the transducer structure, as well as, the fabrication method of the present invention can also be applied to non-Si substrates. The present invention uses low cost substrates that are suitable for application to a large- scale manufacturing of photovoltaics. If a thin stainless steel substrate is used, the photovoltaics are flexible. For example, one can make a similar transducer by first depositing a thin layer of n-type poly or amorphous Si layer on a high temperature glass substrate coated with a highly conductive ZnO(AI) film, followed by the same fabrication steps of embodiments of the present invention. Here, ZnO(AI) serves as a back electrode and the n-type poly or amorphous Si layer serves as an n-type side of a hetero-p/n junction. Essentially, the same structure of the transducer can also be fabricated on a flexible stainless steel substrate coated with a thin metallic diffusion barrier (e.g., Mo-silicide) following the same approach with the glass substrate. In this case, the stainless steel substrate itself acts as a back electrode.
Example embodiments of the present invention provide the following features: i) The use of p-type (FeSi2)xAly ternary alloy as photoactive layer
The photoactive p-type (FeSi2)xAly ternary alloy layer of the present invention is fundamentally different from a conventional photoactive layer beta-phase β- FeSi2 Used in Iron-disilicide-based photoelectric transducers. The crystal structure of the p-type (FeSi2)xAly ternary alloy has an Aluminum content larger than 7 at.% and is in the tetragonal structure -phase, rather than the orthorhombic structure β-phase. Also, unlike typical alpha-phase a-FeSi2 that exhibits metallic conductivity, oc-(FeSi2)xAly of; the present invention is a semiconductor. Further, the contents of Al (typically 4 - 25 at. %) in the photoactive layer of the present invention are much higher than that used for p type doping in β-FeS'i^ which is typically less than 1 at. %. ii) The use of Al interlayer in the fabrication process and extendibility to non-Si substrates
The present invention allows for the exploitation of a multi-purpose Al interlayer in the fabrication process. The use of Al interlayer in the fabrication process not only results in the formation of p-type Fe-Si-AI ternary alloy as a photo-active layer with good photovoltaic properties due to the solid-state reaction between the as-deposited FeSi2 and Al interlayer during annealing, but also enables the formation of p+ ■Si/n-Si home junction through Al diffusion into n-Si that helps to reduce dark leakage current. Therefore, the Open-Circuit voltage (Voc) of the photoelectric transducers is increased. In addition, the Al interlayer can be deposited on any substrate and does not have an influence on the quality of the p-type Fe-Si-AI ternary alloy photoactive layers formed by the solid-state reaction of the as-deposited FeSi2 with Al interlayer. Therefore, photoelectric transducers having a similar structure are fabricated on non-Si substrates using the fabrication process of the present invention if the non-Si substrates have a layer of n-type Si surface layer. iii) The use of electron blocking layer
Another embodiment of the present invention incorporates the use of a thin Geranium nitride or Germanium oxynitride and metal oxides as an electron- blocking layer between the top transparent electrode and the p-type Fe-Si-AI ternary alloy photoactive layer. The Germanium nitride or Germanium oxynitride and metal oxides selected have a high valence band offset and low conduction band offset with the p-type Fe-Si-AI ternary alloy photoactive layer. The favorable band alignment effectively suppresses back-diffusion of the photo- generated electrons into the top transparent electrode. Thereby, increasing the photocurrent in the external circuit without increasing the contact resistance between the top transparent electrode and the p-type Fe-Si-AI ternary alloy photoactive layer.
The introduction of the thin electron blocking insulator layer also significantly enhances the conversion efficiency from 2.5% to 3.3% . Thus, the conversion efficiency of a photoelectric transducer with a thin electron blocking insulator layer increased by about 32 %. Most improvement is due to increased photocurrent. Example embodiments of the present invention provide the following advantages:
1 ) Formation of a p-type semiconducting Fe-Si-AI ternary alloy photoactive layer by alloying as-deposited Iron-disilicide FeSi2 film with Al interlayer.
The inefficiency in hole injection is one of the major limiting factors in n-type epitaxial n- -FeSi2/p-Si hetero-ju notion solar cells. The approach and methodology as posed by the present invention provides a solution by fabricating a p-type photo-active layer on a n-type Si substrate (or layer) by alloying of FeSi2 layer with the Al interlayer that is originally sandwiched between the as-deposited FeSi2 and the n-Si substrate (or layer) by post- deposition thermal annealing. The massive diffusion of Al into FeSi2 during the annealing process that occurs during the alloying of FeSi2 with an Al interlayer results in the formation of (FeSi2)xAly (Si/Fe atomic ratio is kept 2) ternary alloy. The ternary (FeSi2)xAly alloys typically are poly-crystalline tetragonal structure a-phase for (FeSi2)xAly alloy with 7 at.% - 20 at.% Al content. Further, an. orthorhombic structure beta-phase p-(FeSi2)xAly is also observed for (FeSi2)xAly alloy with an Al content less than 7 at.%.
Both the a-(FeSi2)xAly and p-(FeSi2)xAly ternary alloy exhibit p-type semiconducting conductivity typically with a high optical absorption coefficients (larger than 105 cm"1) and bandgap values between 0.85 eV and ~ 1 eV, depending on the Al content. Good photovoltaic properties are exhibited in the transducers based on these ternary alloys even though these alloy films are poly-crystalline films, rather than expitaxial single crystalline films. Photocurrents as high as 23 mA/cm2 is observed in a photoelectric transducer containing (FeSi2)xAly alloy photo-active layer formed by alloying 40 nm thick as-deposited FeSi2 layer with 15 nm AI interlayer while a short circuit current of 13.71 mA cm2, an Open-Circuit voltage (Voc) of 0.435 V, and a conversion efficiency of -2.5% are obtained for a ITO/p-(FeSi2)xAly/p+-Si/n-Si photoelectric transducer formed with 40 nm thick as-deposited FeSi2 layer and 10 nm AI interlayer. The improved carrier injection efficiency at the interface between the photoactive layer and n-Si is one of the reasons for the improved photovoltaic properties. However, the photovoltaic properties in poly-crystalline layers of the present invention indicate that the photovoltaic properties of the photoactive layers are also significantly improved by the incorporation of a large amount of AI atoms, which may be attributed to the enhanced carrier mobility and reduced mid-gap level defects that promote photo-carrier recombination. This results in a short minority carrier diffusion length.
Other than serving as an Aluminum source for alloying with as-deposited FeSi2 to form a p-type ternary alloy photoactive layer, the AI interlayer imprpyes the performance of the photoelectric transducers, After most of AI in the AI interlayer is consumed by the alloying with the Iron-silicide layer, a layer of 1 nm - 3 nm thick amorphous AlxFeySizOw complex oxide layer is formed between the (FeSi2)xAly alloy layer and n-Si by the reaction of AI, Fe, Si and residual oxygen that is incorporated into the AI interlayer during the sputtering process (due to high chemical activity of Al). This thin interfacial oxide layer improves interfacial property, most likely by passivating interfacial electronics states, as lower dark leakage current is observed in the ITO/p-(FeSi2)xAly/p+-Si/n-Si photoelectric transducers formed with an Al interlayer as compared to ITO/p-(FeSi2)xAly/p+- Si/n-Si photoelectric transducers formed without an Al interlayer. Due to the reduced dark leakage current, the ITO/p-(FeSi2)xAly/p+-Si/n-Si photoelectric transducers formed with an Al interlayer show an increased Open-Circuit Voltage Voc. 2) Further performance enhancement by a thin electron-blocking layer between ITO and p-type (FeSi2)xAly (or p-type FexSiyAlz) photo-active layer
When the ITO is in contact with the p-type (FeSi2)xAly photo-active layer as the top electrode, the Fermi-level of the ITO is ideally located to match the valence band edge of (FeSi2)xAly as both are located about -4.7 eV below vacuum level. Thus, allowing for a smooth flow of current carried by a majority of the carrier holes between the ITO and the p-type photoactive layer. However, a band of empty electronic states are above the Fermi level in the ITO, where some of the photoelectrons generated in the (FeSi2)xAly photoactive layer could diffuse into the ITO to occupy these empty electronic states. Since only the photo- generated electrons that are collected by the space charge region in the p+- Si/n-Si home junction after their diffusion in the direction towards n-Si can contribute to the photocurrent in the external circuit, the back-diffusion of the photo-generated electrons towards the ITO must be suppressed to maximize the photo-current.
The present invention achieves this by introducing a thin layer, preferably a few nanometers thick, of insulator, such as, Germanium nitride (or Germanium oxynitride) and metal oxides (e.g., M0O3) between the ITO and photoactive layer. The thin layer insulators have a large conduction band offset with the photoactive layer, but with a small valence band offset. Therefore, the thin electron-blocking layer effectively suppresses the photoelectron back-diffusion without increasing the contact resistance. An improved short circuit current is obtained in an ITO/p-(FeSi2)xAly/p+-Si/n-Si photoelectric transducer integrated with a thin Germanium nitride electron-blocking layer. The Germanium nitride is deposited by using Ge target and a conventional sputter technique in a nitrogen ambient. The favorable energy band alignment of the structure reduces the back diffusion of the electrons and increases the photo-generated current.
3) Application to non-Si substrates
The photoactive p-type (FeSi2)xAly ternary layer in the photoelectric transducers of the present invention is typically poly-crystalline structure, which is formed by the solid-state reaction of the as-deposited Iron disicilide layer arid the underlying poly or amorphous Al interlayer. Because the Silicon substrate is not involved in the solid-state reaction and its crystal orientation has no or minimal influence on the texture or grain orientations in the photo-active ternary layer, one of ordinary skill in the art at the time of the invention would understand that a photoelectric transducer having a p-type (FeSi2)xAly ternary alloy layer with the same photovoltaic properties of the present invention can also be fabricated on non-Si substrates by following the processing steps in accordance with example embodiments of the present invention.
The present invention uses low cost substrates that are suitable for application to a large-scale manufacturing of photovoltaics. If a thin stainless steel substrate is used, the photovoltaics are flexible.
For example, a thin layer of n-type poly or amorphous Si is deposited on high temperature glass substrates coated with a high conductive ZnO(AI) film. ZnO(Al) serves as the back electrode and the n-type poly or amorphous Si layer serves as the n-type side of the hetero-p/n junction. A flexible stainless steel substrate coated with a thin metallic diffusion barrier (e.g., Mo-silicide) can also be used instead of the glass substrate. In this case, the stainless steel substrate itself acts as the back electrode.
Further, those skilled in the art will appreciate that the present invention can be applied to applications for solar cells, color, image sensors in the visible light range, thermoelectric generators, and photo-detectors. Although exemplary embodiments have been described, the present invention may be embodied in other specific forms without departing from its essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive.

Claims

CLAIMS:
1. A photoelectric transducer having a p/n heterojunction, comprising:
a substrate (100); and
a p-type photoactive (FeSi2)xAly Iron-disilicide Aluminum alloy layer (400) disposed on the substrate (100).
2. The photoelectric transducer as claimed in claim 1 , wherein a ; Si/Fe atomic ratio of the (FeSi2)xAly Iron-disilicide Aluminum alloy layer (400) is about 2.
3. The photoelectric transducer as claimed in claims 1 or 2, wherein the (FeSi2)xAly Iron-disilicide Aluminum alloy layer (400) comprises Al content of 4 - 25 at.%.
4. The photoelectric transducer as claimed in any one of the preceding claims, wherein the .(FeSi2)xAly Iron-disilicide Aluminum alloy layer (400) has a thickness of about 30 - 50 nm.
5. The photoelectric transducer as claimed in any one of claims 2-4, wherein the (FeSi2)xAly Iron-disilicide Aluminum alloy layer (400) is a semiconductor with a bandgap of about 0.8 - 1 eV.
6. The photoelectric transducer as claimed in any one of claims 2-5, wherein the (FeSi2)xAly Iron-disilicide Aluminum alloy layer (400) has an orthorhombic crystal structure beta-phase (β-phase), and
the Al content is lower than 7 at:%.
7. The photoelectric transducer as claimed in any one of claims 2-5, wherein the (FeSi2)xAly Iron-disilicide Aluminum alloy layer (400) has a tetragonal crystal structure alpha phase (a-phase), and
the Al content is between 7 at.% and 25 at.%
8. The photoelectric transducer as claimed in any one of claims 2-7, further comprising an amorphous AlxFeySi2Ow complex oxide layer (350) having a thickness of about 1 - 3 nm between the p-type photoactive (FeSi2)xAly Iron- disilicide Aluminum alloy layer (400) and the substrate (100).
9. The photoelectric transducer as claimed in any of the preceding claims, further comprising a transparent electrode layer (500) over the p-type photoactive (FeSi2)xAly Iron-disilicide Aluminum alloy layer (400) of the p/n heterojunction.
10. The photoelectric transducer as claimed in claim 9, wherein the photoelectric transducer has a conversion efficiency of about 2.5%.
1 1 . The photoelectric transducer as claimed in claim 9, further comprising an insulator electron-blocking layer between the p-type photoactive (FeSi2)xAly Iron-disilicide Aluminum alloy layer (400) and the transparent electrode layer (500) to suppress back-diffusion of photo-generated electrons into the transparent electrode layer (500).
12. The photoelectric transducer as claimed in 1 1 , wherein the photoelectric transducer has a conversion efficiency of about 3.3%.
13. The photoelectric transducer as claimed in claims 9-12, wherein the transparent electrode layer (500) has a thickness of about 100-500 nm.
14. The photoelectric transducer as claimed in claims 9-13, wherein the p-type photoactive (FeSi2)xAly Iron-disilicide Aluminum alloy layer (400) has a thickness of about 500-1000 nm.
15. The photoelectric transducer as claimed in claim 1 1 , wherein the electron-blocking layer is selected from Germanium nitride, Germanium oxynitride, or Molybdenum trioxide (M0O3).
16. The photoelectric transducer as claimed in claims 1 1 , 12, or 15, wherein the electron-blocking layer has a thickness of about 1 - 20 nm.
17. The photoelectric transducer as claimed in any of the preceding claims, wherein the substrate (100) is selected from an n-Si substrate, a non-Si substrate, or a non-Si substrate with a coated n-Si layer.
18. The photoelectric transducer according to claim 5, wherein the substrate (100) is a non-Si substrate or non-Si substrate with a coated n-Si layer, the non- Si substrate selected from glass, or stainless steel.
19. The photoelectric transducer according to claim 1 , wherein the substrate (100) is a single crystalline, multi-crystalline, polycrystalline, or amorphous Si substrate.
20. A method for forming a photoelectric transducer having a p/n heterojunction, comprising:
depositing an Aluminum interlayer (200) on a substrate (100) by physical vapor deposition;
sequentially depositing Iron disilicide (FeSi2) (300) on the Aluminum interlayer (200) by sputtering; and
annealing the Aluminum interlayer (200) and Iron disilicide (FeSi2) (300) to form a p-type photoactive (FeSi2)xAly Iron-disilicide Aluminum alloy layer (400) disposed on the substrate (100).
21. The method for forming a photoelectric transducer having a p/n heterojunction according to claim 20, wherein the physical vapor deposition is selected from DC or RF sputtering, electron-beam evaporation, and thermal evaporation.
22. The method according to claim 20, wherein the deposition of the Iron disilicide (FeSi2) (300) comprises co-sputtering FeSi2 Iron-disilicide and pure
Aluminum Al targets.
23. The method according to any of the claims 20-22, wherein the substrate (100) is selected from an n-Si substrate, a non-Si substrate, or a non-Si substrate with a coated n-Si layer.
24. The method according to any of the claims 20-23, wherein annealing comprises rapid thermal annealing or furnace annealing at a temperature range of about 500 - 800°C.
25. The method as claimed in any one of the claims 20-24, further comprising forming a thin insulator electron-blocking layer over the p-type photoactive (FeSi2)xAly Iron-disilicide Aluminum alloy layer (400).
26. The method as claimed in any one of claims 20-25, wherein the FeSi2 (Al) layer (300) has a thickness of 30 nm - 150 nm, and
the Al interlayer (200) has a thickness that is no more than 40 % of the thickness of the FeSi2 (Al) layer.
27. The method as claimed in claim 25, wherein the thin insulator electron- blocking layer comprises Ge or Mo.
28. The method as claimed in claim 27, wherein forming the thin insulator electron-blocking layer is done by reactive sputtering of pure Germanium Ge target under argon and nitrogen mixed working gas ambient or sputtering a Germanium-nitride GeN target.
■ -f
29. The method as claimed in claim 27, wherein forming the thin insulator electron-blocking layer is done by a reactive sputtering of pure Molybdenum Mo target under argon and oxygen mixed working gas ambient or by sputtering a M0O3 Molybdenum trioxide target.
30. The method as claimed in any one of the claims 20-29, wherein a pressure of Oxygen during sputtering or co-sputtering is maintained at about 2x107 Torr.
31. The method as claimed in any one of the claims 20-24, further comprising:
forming a transparent electrode layer (500) over the p-type photoactive (FeSi2)xAly Iron-disilicide Aluminum alloy layer (400) of the p/n heterojunction.
32. The method according to claim 31 , wherein the photoelectric transducer has a conversion efficiency of about 2.5%.
33. The method according to claim 25, further comprising:
forming a transparent electrode layer (500) over the thin insulator electron-blocking layer and the p-type photoactive (FeSi2)xAly Iron-disilicide Aluminum alloy layer (400) of the p/n heterojunction.
34. The method according to claim 33, wherein the photoelectric transducer has a conversion efficiency of equal to or more than 3.3%.
PCT/SG2011/000114 2010-03-23 2011-03-23 Photoelectric transducer using iron silicide and aluminum and method for preparing the same WO2011119113A1 (en)

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Citations (2)

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WO1998018167A1 (en) * 1996-10-24 1998-04-30 University Of Surrey Optoelectronic semiconductor devices
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