JP7304571B2 - Barium silicide-based film and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a barium silicide-based film having excellent spectral sensitivity and a method for producing the same.

Wide bandgap semiconductors containing silicon exhibit very specific characteristics, and are therefore widely used in the environmental and energy fields such as solar cell materials and thermoelectric conversion materials.
Among them, a barium silicide-based compound composed of barium (Ba) and silicon (Si) has a BaSi ₂ composition and a bandgap of 1.3 eV, which is larger than 1.1 eV of Si, and is attracting attention (non-patent document 1). Further, by adding Sr, it is possible to adjust the bandgap to 1.4 eV (Patent Document 1).
As a mode of use of the barium silicide-based compound, it is effective to use it as a film, and Patent Document 2 cites a solar cell in which n-type and n+-type barium silicide films are laminated as an example.

As a method for producing such a barium silicide-based film, a method of forming a film on a silicon (111) substrate by the MBE method (molecular beam epitaxy method) is known. According to this film formation method, it is possible to form a film by controlling the composition of each element, but further improvements are still required in terms of performance, and uniform film formation over a large area is difficult. There are challenges in mass production. Therefore, there is a demand for a sputtering technique that enables uniform film formation over a large area, precise control of each element, and a high film formation rate.

Regarding the sputtering method, the present inventors disclose a high-density, crack-free barium silicide polycrystal and a sputtering target using the same in Patent Document 3, but there are few studies on barium silicide-based films, and non-patent Some of them are listed in Document 2, but further studies on improving the characteristics of solar cells and the like have not progressed.

Japanese Patent Application Laid-Open No. 2005-294810 JP 2008-66719 A Japanese Unexamined Patent Application Publication No. 2012-214828

Japanese Journal of Applied Physics Vol. 49 04DP05-01-04DP05-05 (2010) Applied Physics Express 11 071401 (2018)

An object of the present invention is to provide a barium silicide-based film having excellent spectral sensitivity and a method for producing the same.

In view of the background as described above, the present inventors have made intensive studies, and as a result, have found a barium silicide-based film having excellent spectral sensitivity and a method for producing the same, thereby completing the present invention.
That is, aspects of the present invention are as follows.
(1) A barium silicide film characterized by having a carbon content of 1×10 ¹⁸ atms/cm ³ or more and 5×10 ²² atms/cm ³ or less.
(2) The barium silicide-based film according to (1) above, wherein the _SiTO phonon peak intensity ratio to the _Ag peak in the Raman spectrum is less than 10%. (3) The oxygen content is 0.01 atm% or more and 10 atm% or less. The barium silicide-based film according to (1) or (2) above.

(4) The barium silicide-based film according to any one of (1) to (3) above, which has a hydrogen content of 1×10 ¹⁸ atms/cm ³ or more and 1×10 ²¹ atms/cm ³ or less.
(5) The barium silicide-based film according to any one of (1) to (4) above, which has a crystal structure composed only of peaks belonging to orthorhombic crystals in an XRD diffraction test.
(6) A method for producing a barium silicide-based film according to any one of (1) to (5) above, wherein the barium-based film is formed by sputtering in the presence of carbon.
(7) The method for producing a barium silicide-based film according to (6) above, which uses both a barium silicide-based sputtering target and a carbon-containing sputtering target.
(8) The barium silicide-based film according to (6) or (7) above, wherein the gas pressure during sputtering deposition is 0.5 Pa or more and 1.0 Pa or less.

(9) A laminated substrate having a barium silicide film obtained by laminating the barium silicide film according to any one of (1) to (5) above and a substrate.
(10) A laminated substrate having a barium silicide film obtained by laminating the barium silicide film according to any one of (1) to (5) above and a substrate.
(11) The laminated substrate according to (10) above, wherein the substrate is silicon, alkali-free glass, quartz glass, germanium, or sapphire.
(12) A device using the laminated substrate according to (10) or (11) above.
(13) An electronic device using the element described in (12) above.

INDUSTRIAL APPLICABILITY According to the present invention, a barium silicide-based film excellent in spectral sensitivity suitable for an absorption layer of a solar cell and an efficient method for producing the same are provided.

1 is a schematic diagram of a dual simultaneous sputtering apparatus used in Examples 1 and 2 and Comparative Example 1. FIG.

The barium silicide-based film of the present invention has a carbon content of 1×10 ¹⁸ atms/cm ³ or more and 5×10 ²² atms/cm ³ or less, preferably 5×10 ¹⁸ atms/cm ³ or more and 1×10 ²² atms/cm 3 or more. cm ³ or less, more preferably 5×10 ¹⁹ atms/cm ³ or more and 8×10 ²¹ atms/cm ³ or less.

By containing carbon in this range, the spectral characteristics caused by crystal defects in the barium silicide-based film can be greatly improved. If a large amount of carbon is contained, it interferes with portions other than crystal defects, deteriorating the crystallinity of the film and degrading the spectral characteristics of the film. In addition, when the carbon content is small, the spectral sensitivity is lowered due to lattice defects present in the film.

The carbon content in the barium silicide-based film in the present invention can be obtained by measurement by SIMS (secondary ion mass spectrometry). The carbon content is defined as the amount of carbon present in a layer between 200 and 300 nm thick, excluding a 100 nm thick surface layer from the side opposite to the substrate side of the film at a film thickness of 300 nm. This is because the surface layer is affected by surface oxidation and unevenness, so it cannot necessarily be said to represent the amount of carbon in the main body of the film.

The barium silicide-based film of the present invention is preferably a polycrystalline film. By using a polycrystalline film, the strength of the film and the stability of film characteristics such as a reduction in the distribution of spectral characteristics in the film are improved compared to a single crystal film.
In the Raman spectrum of the barium silicide-based film of the present invention, the _SiTO phonon peak intensity ratio to the _Ag peak is preferably less than 10%, more preferably 2% or less, and still more preferably 0.5% or less. is. In the Raman spectrum, showing Si _TO phonons means that barium silicide exists as an independent element that is not alloyed. This is presumed to occur particularly when crystal defects occur, and the presence of Si _TO phonon peaks adversely affects spectral sensitivity. By reducing the _SiTO phonons that cause this, crystal defects can be reduced and spectral characteristics can be improved.

In the barium silicide-based film of the present invention, the atomic weight ratio Si/Ba of silicon to barium is preferably 1.8 or more and 2.1 or less, and particularly preferably 1.9 or more and 2.0 or less.
Furthermore, the barium silicide-based film preferably has an oxygen content of 10 atm % or less, more preferably 5 atm % or less, and still more preferably 3 atm % or less. The introduction of oxygen reduces the effect of crystal defects, but if there is a large amount of oxygen, the oxygen in the film reacts with hydrogen, and the presence of moisture in the barium silicide film changes the barium silicide to silicic oxide. , the film properties deteriorate. The oxygen content is preferably 0.01 atm % or more, more preferably 0.1 atm % or more. By adjusting the oxygen content within the above range, it is possible to obtain a preferable bandgap while maintaining the crystallinity.

The oxygen content in the barium silicide-based film in the present invention can be measured using RBS (Rutherford Backscattering Spectroscopy). If further accuracy is required, SIMS is used for measurement and converted to atm%. The oxygen content is defined as the amount of oxygen present in a layer between 50 nm and 300 nm in thickness of 300 nm, excluding a surface layer with a thickness of 50 nm.

The barium silicide-based film of the present invention preferably has a hydrogen content of 1×10 ¹⁸ atms/cm ³ to 1×10 ²¹ atms/cm ³ , more preferably 3×10 ¹⁸ atms/cm ³ to 1×10 18 atms/cm 3 . 10 ²⁰ atms/cm ³ or less, particularly preferably 5×10 ¹⁸ atms/cm ³ or more and 5×10 ¹⁹ atms/cm ³ or less.
By containing hydrogen in this range, the spectral characteristics caused by crystal defects in the barium silicide-based film can be improved. If a large amount of hydrogen is contained, it interferes with portions other than crystal defects, deteriorating the crystallinity of the film and degrading the spectral characteristics of the film. In addition, when the hydrogen content is small, the spectral sensitivity is lowered due to lattice defects present in the film.

The hydrogen content in the barium silicide-based film in the present invention can be obtained by measurement by SIMS (secondary ion mass spectroscopy). In the measurement, in the case of a film thickness of 200 nm, the amount of hydrogen present in a layer between 100 and 200 nm thick, excluding a 100 nm thick surface layer from the side opposite to the substrate side of the film, is obtained. The barium silicide-based film of the present invention may contain trace amounts of impurities other than carbon, hydrogen, and oxygen, such as magnesium, calcium, and strontium.

The barium silicide-based film of the present invention preferably consists of only peaks attributed to an orthorhombic crystal structure in an XRD diffraction test. By forming a barium silicide-based film having such a crystal phase, it is possible to obtain a film having excellent film properties and high stability.
In the XRD diffraction test of the present invention, it can be confirmed as follows that the crystal is composed only of peaks attributed to the orthorhombic crystal structure. That is, the peak attributed to the orthorhombic crystal structure is the diffraction peak detected in the range of 2θ = 20 to 80 ° in XRD with Cu as the radiation source, which is the JCPDS (Joint Committee for Powder Diffraction Standards). ) card number. It means that the peak pattern attributed to 01-071-2327 or a similar peak pattern (shifted peak pattern) can be indexed.

The thickness of the barium silicide-based film of the present invention is preferably 50 nm to 2000 nm, more preferably 100 nm to 1000 nm, and particularly preferably 100 nm to 800 nm.
The barium silicide-based film of the present invention may contain elements depending on its required properties. For example, an element of group 13 of the periodic table, such as boron (B) and aluminum (Al), is added to the p-type, and nitrogen (N), phosphorus (P), antimony (Sb), etc. is added to the n-type. May contain.

In the barium silicide-based film of the present invention, the compositional deviation of silicon in the film is preferably 20% or less, particularly preferably 10% or less. A uniform film can be obtained when the composition deviation is 20% or less. The composition distribution can be measured by using an analysis method capable of measuring the surface distribution of elements such as EPMA (electron probe microanalyzer). It is desirable to perform surface analysis on a field of view of 45 μm square or more as the field of view for measurement. A uniform composition distribution of silicon indicates a uniform film texture, which provides better optical and electrical properties than a non-uniform texture.

Specific methods for measuring the compositional deviation of silicon in the film include the following methods.
First, the composition distribution of silicon is measured in a field of view of 45 μm square using EPMA or the like. After that, by averaging the amount of detected silicon for every 3 μm square, the average detected amount of 3 μm square in the 45 μm square is calculated. For each average calculated amount, the composition deviation (%) is calculated from the maximum amount A, the minimum amount B, and the total average amount C of 225 points by the following calculation.
Composition deviation (%) = (maximum amount A - minimum amount B) / total average value C

The barium silicide-based film of the present invention can be produced by various methods such as sputtering, MBE (molecular beam epitaxy), and chemical vapor deposition. Among them, a film formed by an MBE method or a sputtering method is preferable, and a film formed by a sputtering method is particularly preferable. Among the sputtering methods, it is preferable that the film is formed by the sputtering method, in which the peak intensity ratio of the _SiTO phonon to the _Ag peak is less than 10% in the Raman spectrum.

As the sputtering method, a DC sputtering method, an RF sputtering method, an AC sputtering method, a DC magnetron sputtering method, an RF magnetron sputtering method, an ion beam sputtering method, or the like can be appropriately selected. Among these, the DC magnetron sputtering method or the RF magnetron sputtering method is more preferable, and the RF magnetron sputtering method is particularly preferable, because it can form a film uniformly over a large area at a high speed.

The temperature during sputtering is not particularly limited, but is preferably 400° C. or higher, more preferably 500° C. or higher, and particularly preferably 800° C. or lower in order to improve crystallinity. If the temperature is higher than that, the material used for the device becomes expensive. An inert gas such as argon gas or nitrogen gas is usually used as the atmosphere gas during sputtering.

Introduction of carbon into the barium silicide-based film can be performed during the formation of the barium silicide-based film. Introduction of carbon is not particularly limited, but it is preferable to use a carbon-containing target and a barium silicide target together in sputtering. However, in order to avoid unnecessary elements, the carbon-containing target is preferably a compound target such as carbon, carbon silicide, or barium carbide.
Oxygen is introduced into the barium silicide-based film during or after formation of a good barium silicide-based film.

The introduction of hydrogen is not particularly limited, but it is preferable to use active hydrogen in order to make it act more effectively on the defect portion. . When using an RF plasma gun, the amount of hydrogen in the film can be controlled by the irradiation time, and the irradiation time is preferably 1 minute or more and 60 minutes or less, more preferably 5 minutes or more and 40 minutes or less, Particularly preferably, it is 15 minutes or more and 30 minutes or less. By setting this range, it becomes possible to introduce a preferable amount of active hydrogen into the film.
A similar effect can be obtained by activating hydrogen present in the film after film formation. For example, by exposing the barium silicide-based film to plasma after film formation, hydrogen in the film is activated and deterioration of spectral characteristics due to defects can be suppressed.

As the sputtering target used for the barium silicide-based film of the present invention, a barium silicide-based sputtering target such as BaSi ₂ is preferable. The barium silicide-based film of the present invention is obtained by the sputtering method using the barium silicide-based sputtering target.
Here, the method for manufacturing the barium silicide-based sputtering target is not particularly limited. In the sputtering method for producing a barium silicide-based sputtering target, the silicon-barium ratio can be changed by forming a film with silicon or barium placed on a barium silicide sputtering target. becomes possible.

The silicon-barium ratio can also be adjusted by the gas pressure during sputtering film formation.
Since the silicon-barium ratio is 1:2 for the _BaSi2 orthorhombic crystal, which has good spectral sensitivity characteristics, the composition of the film is preferably close to 1:2. It is possible to approach the barium ratio to 1:2. However, simply increasing the gas pressure tends to deteriorate the crystallinity and reduce the film formation rate.

The preferred range of the gas pressure (absolute pressure) during sputtering film formation is 0.5 Pa or more and 1.0 Pa or less, more preferably 0.6 Pa or more and 0.8 Pa or less. By setting the gas pressure to that value, it becomes possible to obtain a barium silicide film with improved crystallinity.

The barium silicide-based film of the present invention can also be made into a barium silicide-based laminated film by laminating silicon layers. Moreover, the barium silicide-based film of the present invention can also be used as a laminated substrate containing the barium silicide-based film constituting the substrate.
The material of the substrate is not particularly limited, and examples thereof include silicon, alkali-free glass, quartz glass, germanium, and sapphire. Among them, it is preferable to use a silicon substrate in order to grow a barium silicide film with high crystallinity. By using single crystal silicon for the silicon substrate, lattice mismatch between the substrate and the film can be reduced, and the crystallinity of the film can be improved. In addition, it is preferable to use silicon whose orientation is (111).

Furthermore, by using silicon for the substrate, a tandem structure using a silicon layer with a bandgap of 1.1 eV and a barium silicide layer with a bandgap of 1.3 eV can be expected to further improve the solar cell characteristics.
A cap layer is preferably present on the surface layer of the laminated substrate containing the barium silicide-based film. By capping the surface layer, it is possible to suppress the progress of oxidation from the surface.
The material of the layer used as the cap layer is not particularly limited, and examples include silicon (crystalline, amorphous), etc. Among them, a layer that does not contain oxygen, such as metal, is preferable in order to suppress oxidation. .

As a method for producing a barium silicide-based laminated film and a laminated substrate containing a barium silicide-based film, for example, assuming an absorption layer for a solar cell, a barium silicide-based film to which no dopant is added, an n-type barium silicide-based film, a p-type A layer including at least two barium silicide-based films and a cap layer is formed. The film formation method is not limited, and various film formation methods such as physical vapor deposition and chemical vapor deposition can be used.
Since the barium silicide-based film of the present invention is excellent in spectral sensitivity, it is suitable for devices using laminated substrates containing the barium silicide-based film, and particularly suitable for solar cell light absorption layers and thermoelectric conversion devices. Moreover, by using the element, it is suitable for electronic devices, and particularly suitable for solar cell modules and thermoelectric conversion modules.

The spectral sensitivity is represented by A(λ)/W(λ) (A: output current, W: irradiation intensity), and serves as an index of solar cell characteristics. Also, by applying a bias voltage, it becomes possible to grasp the output current at that voltage. As an evaluation here, spectral sensitivity (normalization) was defined by the following formula.
Spectral sensitivity (normalized) = maximum spectral sensitivity (A/W)/bias voltage (V)
Bias voltage (V): Absolute value of bias voltage

It is expected that the higher the spectral sensitivity, the higher the output current at a voltage value equal to or lower than the release voltage, and the higher the solar cell conversion efficiency. The spectral sensitivity (normalized) of the barium silicide-based film of the present invention can be 2.0 or higher, further 3.0 or higher, and particularly 4.0 or higher.

The present invention will be specifically described by the following examples, but the invention is not limited to these examples. In addition, evaluation of each characteristic was performed as follows, respectively.

(Raman spectrum)
The Raman spectrum was _measured using Raman spectroscopy (JASCO ^, NRS ^- 5100) under the condition of an excitation wavelength of 532 nm. As peaks derived from _TO phonons, ratios of respective peak intensities were calculated.
Peak intensity ratio (%)=Si _TO peak intensity/ _Ag peak intensity Note that the intensity was calculated after separating the two peaks.

(carbon content)
Measurement was performed using SIMS, and measured in the depth direction from the upper surface when viewed from the substrate. Using a film with a film thickness of 300 nm, the average value of the measured values at a thickness of 200 nm to 300 nm was calculated, excluding a surface layer of 100 nm.

(X-ray diffraction test)
The crystalline phase of the barium silicide-based film was identified by an X-ray diffraction test. The measurement conditions are as follows.
・X-ray source: CuKα
・Power: 40kV, 40mA
・Scanning speed: 1 ° / min The obtained diffraction pattern was analyzed, and (1) a phase composed of peaks attributed to an orthorhombic crystal structure, and (2) other than the above (1) When identified in each of these crystal phases (1) and (2), it was indicated as "present", and when not identified, it was indicated as "absent".

(spectral sensitivity)
The spectral sensitivity of the barium silicide-based film was measured by preparing an ITO electrode with a diameter of 1 mm and a thickness of 80 nm on the surface layer side and an Al electrode on the back surface of the substrate, applying a voltage between the electrodes, and then using a spectrometer. Measured using SM-1700A.

(Examples 1 and 2)
A sputtering apparatus capable of dual simultaneous sputtering shown in FIG. 1 was used.
Using a sputtering target of barium silicide as the target 1, a sputtering film forming test was performed under the following conditions. A barium chip is placed on a barium silicide sputtering target, and argon is bombarded so that elemental silicon and elemental barium (sputtered particles in FIG. 1) protrude from the barium silicide, and elemental barium ( The sputtered particles in FIG. 1) were made to protrude.
Using a silicon carbide sputtering target as the target 2, the sputtering process was performed under the following conditions so that carbon and silicon (sputtered particles in FIG. 1) protrude.

Sputtering conditions:
Discharge method: RF sputtering Film deposition equipment: Magnetron sputtering equipment (for simultaneous two-source deposition)
Target-substrate distance: 200 mm
Film forming pressure (gas pressure in device): 0.25 to 1 Pa
Introduced gas: argon Substrate: (111) silicon single crystal substrate
(25mm square 0.5mm thickness)
Substrate temperature: 600°C
Film thickness: 200 nm
Target 1:
Sputtering target: barium silicide (BaSi2)
Target size: 50 mmφ (disk shape)
Barium chip size: 5 mm square (plate shape)
Number of barium chips: 3 (installed in the erosion part)
Discharge power: 30 W (1.5 W/cm ² )
Target 2:
Sputtering target: silicon carbide (SiC)
Target size: 50 mmφ (disk shape)
Discharge power: 100 W (5.1 W/cm ² )

After forming a barium silicide film with a thickness of 200 nm on the silicon single crystal substrate, amorphous silicon was formed as a cap layer on the barium silicide film to a thickness of 3 nm at 180° C. by sputtering.
According to Examples 1 and 2, carbon-containing barium silicide films having Raman spectra, carbon contents, spectral characteristics, and crystal phases as shown in Table 1 were obtained.

(Comparative example 1)
A film formation test was performed under the same conditions as in Example 1, except that no carbon was used and only a barium silicide sputtering target was used.

As a result, as shown in Table 1, a desired film having Raman spectrum, carbon content, spectral characteristics and crystal phase was not obtained.

Claims (13)

A barium silicide-based film having a carbon content of 1×10 ¹⁸ atms/cm ³ or more and 5×10 ²² atms/cm ³ or less. 2. The barium silicide-based film of claim 1, wherein the _SiTO phonon peak intensity ratio to the _Ag peak in the Raman spectrum is less than 10%. 3. The barium silicide-based film according to claim 1, wherein the oxygen content is 0.01 atm % or more and 10 atm % or less. The barium silicide-based film according to any one of claims 1 to 3, having a hydrogen content of 1×10 ¹⁸ atms/cm ³ or more and 1×10 ²¹ atms/cm ³ or less. The barium silicide-based film according to any one of claims 1 to 4, which has a crystal structure composed only of peaks belonging to orthorhombic crystals in an XRD diffraction test. A method for producing a barium silicide film according to any one of claims 1 to 5, wherein the film is formed in the presence of carbon by a sputtering method. 7. The method for producing a barium silicide-based film according to claim 6, wherein a barium silicide-based sputtering target and a carbon-containing sputtering target are used in combination. 8. The method for producing a barium silicide film according to claim 6, wherein the gas pressure during sputtering film formation is 0.5 Pa or more and 1.0 Pa or less. A barium silicide-based laminated film obtained by laminating the barium silicide-based film according to any one of claims 1 to 5 and a silicon layer. A laminated substrate having a barium silicide film, which is obtained by laminating the barium silicide film according to any one of claims 1 to 5 and a substrate. 11. The laminated substrate according to claim 10, wherein the substrate is silicon, alkali-free glass, quartz glass, germanium, or sapphire. A device using the laminated substrate according to claim 10 or 11. An electronic device using the device according to claim 12 .

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