JP5360587B2 - N-type semiconductor thin film, homo pn junction element and thin film solar cell, and n-type semiconductor thin film and homo pn junction element manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor having a band gap ranging from 2 eV inclusive to below 3 eV and capable of controlling electric characteristics to both pn polarities, in order to utilize light that is not completely absorbed by the currently available thin film solar battery. <P>SOLUTION: An n-type semiconductor thin film is made of a tin oxide (SnO) which contains an impurity of trivalent positive ion selected from Al<SP>3+</SP>, Ga<SP>3+</SP>, In<SP>3+</SP>, and Sb<SP>3+</SP>and shows n-type conductivity. This n-type semiconductor thin film and a p-type semiconductor thin film made of SnO are laminated together to form a homogeneous pn-junction element. A thin film solar battery is manufactured by using the homogeneous pn-junction element. Oxide powder, which is a source for a trivalent positive ion selected from Al<SP>3+</SP>, Ga<SP>3+</SP>, In<SP>3+</SP>, and Sb<SP>3+</SP>, is added/mixed to SnO powder, and the mixture powder is sintered to form an SnO target. Using this SnO target, the n-type semiconductor thin film made of SnO is manufactured by a physical deposition method. A p-type SnO thin film formed using an SnO target made by sintering SnO powder by the physical deposition method and an n-type SnO thin film manufactured by the same method are laminated together to manufacture the homogeneous pn-junction element. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

The present invention relates to an n-type conductive semiconductor thin film made of stannous oxide (SnO), a homo pn junction element using the n-type conductive semiconductor thin film as an active layer, and a thin film solar cell using the homo pn junction element ,
The present invention also relates to a method for manufacturing an n-type semiconductor film and a homo pn junction element.

In recent years, solar cells that use sunlight, which is clean energy, for power generation in order to reduce environmental burdens have been actively researched. In order to further improve the power generation efficiency of solar cells, thin film solar cells in which semiconductor materials having mutually different absorption bands are stacked and pn-junctioned have been developed (Patent Documents 1, 2, and 3). A typical example of a semiconductor material used for a thin film solar cell is Cu (InGa) Se _2.
However, the band gap is about 1.2 eV.

Therefore, by using a semiconductor having a band gap of 2 eV or more to form a pn junction laminated on Cu (InGa) Se ₂ or the like, it is possible to use sunlight having a wavelength not currently contributing to power generation for power generation. It becomes like this. In view of this, a pn-controllable semiconductor material having a band gap of 2 eV or more and less than 3 eV is being searched for from among oxide semiconductors that are easy to produce a wide gap semiconductor.

In 2001, the present inventors announced a pn-controllable oxide semiconductor CuInO ₂ (Non-patent Documents 1 and 2). However, the band gap is as large as about 3.9 eV, and it is not suitable for the active layer of a solar cell.

Further, from 1998 to 2003, the present inventors have made SrCu ₂ O ₂ and LaCuOCh (Ch = S,
A p-type oxide semiconductor such as Se, Te) has been reported (Non-Patent Documents 3 to 5). However, no electron doping has been reported for these p-type oxide semiconductors.

The present inventors reported a thin film transistor using SnO as a channel layer in 2008 (Non-Patent Document 6), and further, in a vapor phase method, using SnO as a target, the degree of oxidation of Sn deposited on a substrate was measured. Controlled by substrate temperature and atmospheric oxygen partial pressure, Sn ⁴⁺ and S in SnO
The total content of n ⁰ (tin metal) is less than 10 atomic%, that is, the content of Sn ²⁺ ions is 90
A patent application was filed for an invention relating to a method of forming a p-type SnO thin film of at least atomic% and a thin film transistor using the p-type SnO thin film as a channel layer (Patent Document 4).

It is known that stannous oxide (SnO) is a p-type semiconductor (Non-patent Document 7).
(Patent Document 5) and Non-Patent Document 7 report that when SnO ₂ raw material is evaporated by electron beam to form a film on a sapphire substrate, an amorphous or metastable polycrystalline SnO _x thin film is formed according to the substrate temperature. Yes. Examples of devices using SnO as a semiconductor material include Pb
There is a photovoltaic cell using a heterojunction with a graded composition film of O-SnO and a manufacturing method thereof (Patent Documents 6 and 7), and the use and research of SnO compared to SnO ₂ used as a conductive film or semiconductor material. Little development has been done.

JP-A-9-181345 JP 11-87750 A JP 2008-235794 A PCT / JP2009 / 62196 JP 2002-235177 A U.S. Pat.No. 4,099,199 US Patent No. 4199383

H. Yanagi et al., Appl. Phys. Lett. 78, (2001) 1583-1585 H. Yanagi et al., Sol. State Comm. 121, (2002) 15-18 A. Kudo, Appl. Phys. Lett. 73, (1998) 220 K. Ueda, Appl. Phys. Lett. 77, (2000) 2701-2703 K. Ueda, Chem. Mater. 15, (2003) 3692-3695 Y. Ogo, Appl. Phys. Lett. 93, 032113 (2008) X.Q.Pan et al., J. Electroceram., 7, (2001) 35-46

ZnO and CuInO ₂ have been reported as oxide semiconductors capable of forming a pn junction, but these oxide semiconductors are wide band gap semiconductor materials exceeding 3 eV. When considering the application of oxides to stacked solar cells, it is necessary to produce a pn junction made of a semiconductor material having a band gap of 2 eV or more and less than 3 eV.

The reason why there are many wide band gap materials of 3 eV or more in oxide is that the valence band is formed by oxygen 2p orbitals having strong electronegativity. Since the oxygen 2p orbitals strongly bind electrons, the binding energy increases, and as a result, the position of the upper end of the valence band from the vacuum level becomes deeper, and the band gap increases.

In general, in a solar cell, it is desirable that a p-type thin layer and an n-type thin layer of the same type of semiconductor are in direct surface contact to form a pn junction. In the homojunction, since there is no mismatch between crystal lattices in principle, a high-quality junction without lattice distortion can be formed. However, a thin film solar cell having such a homo pn junction structure is not limited to II, regardless of its absorption coefficient.
-Limited to some materials such as CdTe, which is a group VI compound semiconductor.

Furthermore, the pn junction material of a thin film solar cell can improve the power generation efficiency by improving the consistency between the band gap and the sunlight spectrum, can be easily made into a thin film, and has a low conversion efficiency at high temperatures. Therefore, it is necessary that the light absorption coefficient is small, the characteristics such as sufficient absorption of sunlight even with a thin film are necessary, and the material cost and the film formation method are inexpensive.

The band gap of stannous oxide (SnO) is 0.7 eV for indirect transition and 2.7 eV for direct transition, and absorbs light having energy of 2.7 eV or more by direct transition. Therefore, if SnO pn control can be performed and a homojunction can be achieved, application to a solar cell as a semiconductor material can be expected. In the present invention, in order to realize a pn-controllable oxide semiconductor having a band gap of 3 eV or less, a p-type oxide semiconductor SnO having a band gap of 0.7 eV in an indirect transition and a band gap of 2.7 eV in a direct transition is used. And electron doping.

The crystal structure of SnO is different from SnO ₂ which is space group P42 / mnm, and space group P4 / nm.
m. SnO is an oxide, but the top of its valence band is Sn5s, not oxygen 2p orbital.
It is confirmed theoretically and experimentally that it is formed by orbits (Non-Patent Document 8,
9).

G. W. Watson, J. Chem. Phys. 114, (2001) 758 Y. Ogo et al., Phys. Stat. Solidi (A) 206, (2009) 2187-2191

The present inventors doped this SnO with a trivalent cation selected from Al ³⁺ , Ga ³⁺ , In ³⁺ , and Sb ³⁺ to thereby make an n-type semiconductor composed of SnO exhibiting n-type conductivity. It has been found that a thin film can be obtained. As a result, it has a band gap of 2 eV or more and less than 3 eV, and pn
It became possible to provide a controllable oxide semiconductor, and a p-type and n-type homo pn junction element made of SnO was realized.

The doping method is performed by substituting a part of Sn ²⁺ ions of SnO with a trivalent cation selected from Sb ³⁺ , Al ³⁺ , Ga ³⁺ and In ³⁺ . As a doping method of trivalent cations for substitution, for example, by using a target obtained by adding these trivalent cations as a donor by a physical film formation method such as a pulse laser deposition method (PLD method) or a sputtering method. The method of producing a SnO thin film is mentioned. The target is, for example, Sn ³ powder, Al ³⁺ , Ga ³⁺ , In
^It is obtained by adding and sintering a trivalent cation source oxide powder selected from ³⁺ and Sb ³⁺ .

The homo pn junction element includes a p-type SnO thin film formed by a physical film forming method using a SnO target obtained by sintering SnO powder, and Al ³⁺ , Ga ³⁺ , In ³⁺ , Sb ^{3+ on the} SnO powder. Can be manufactured by laminating an n-type SnO thin film formed by a physical film formation method using a SnO target obtained by adding and sintering a trivalent cation source oxide powder selected from the following.

In the present specification, the additive concentration in the target is represented by the ratio of the additive element in the cation. That is, when Sb is added, the concentration of Sb is 100 × n _Sb / (n _Sn +
n _Sb )%. Here, n _Sn and n _Sb are the numbers of Sn atoms and Sb atoms, respectively.

According to the present invention, a homo pn junction can be formed using SnO. By producing a pn junction of SnO on a solar cell using Cu (InGa) Se ₂ , light having an energy of 2.7 eV or more can be used for power generation, and power generation efficiency is improved.

XRD patterns of SnO thin films formed using targets with Sb concentrations of 0%, 1%, 5%, 8%, and 10%. FIG. 1 (a) is an XRD pattern by a normal 2θ-θ scan for observing diffraction from a crystal plane parallel to the substrate surface. FIG. 1B shows an XRD pattern when the angle 2θ of the detector is scanned with the incident angle fixed at 0.5 °. The graph which shows the temperature dependence of the carrier density | concentration in the temperature range from room temperature to 77K of the SnO thin film formed into a film using the target of Sb density | concentration 0%, 1%, 5%, 8%, and 10%. XPS measurement spectrum of O1 _s and Sb3d _3/2 core levels for SnO thin film doped with Sb. The laminated cross-section conceptual diagram of the homojunction diode by p-type SnO and n-type SnO. 4 is a current-voltage characteristic graph of a homojunction diode made of p-type SnO and n-type SnO 2 manufactured in Example 3. FIG.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. First, as a first embodiment of the present invention, a method of forming n-type SnO by forming p-type SnO and electron doping into SnO, and a homojunction diode as a second embodiment. explain.

A trivalent cation selected from Al ³⁺ , Ga ³⁺ , In ³⁺ and Sb ³⁺ is used as an impurity for electron doping. These impurities are Sn at the time of preparation of the target used in the physical film formation method.
Add to O. Specifically, SnO powder and oxide powder of the above trivalent cation source such as Sb ₂ O ₃ , Al ₂ O ₃ , Ga ₂ O ₃ , and In ₂ O ₃ are mixed and sintered. These oxide powders are commercially available. Since the necessary amount to be added and mixed with the target varies depending on the film forming conditions and the like, an appropriate condition is selected so that the SnO thin film exhibits n-type conductivity according to the film forming conditions.
The atmosphere during sintering is adjusted so as to obtain a sintered body of SnO composition. Using a target containing a desired amount of such an impurity element, a SnO thin film is formed by a PLD method, a sputtering method, or the like. Alternatively, a film may be formed by sputtering in a gas atmosphere containing Sb, Al, Ga, and In atoms.

Here, a trivalent cation serving as an impurity for electron doping is Sn ^{2+ having} an Arrhenius ion radius of less than 0.93 mm. According to Non-Patent Document 10, Sb ³⁺ , Al ³⁺ , Ga ³⁺ , In ³⁺
Have ion radii of 0.76Å, 0.51Å, 0.62Å, and 0.81Å, respectively, and can replace Sn ²⁺ ions. However, the ionic radius of Bi ³⁺ used in Comparative Example 1 is 0.96Å, which is larger than the ionic radius of Sn ²⁺ , and does not become an electron dopant for SnO.

R.D.Shannon and C.T.Prewitt, Acta Cryst.B25, (1969) 925

Preferred conditions for the PLD method include the sintering of SnO on a (001) YSZ single crystal substrate held at 300 ° C. or higher and lower than 600 ° C. for both p-type SnO film formation and n-type SnO film formation. The body is used as a target and the oxygen partial pressure is set to 1 × 10 ⁻² Pa or more and less than 1 × 10 ⁻¹ Pa. The partial pressure of oxygen gas is controlled by introducing O ₂ gas through a flow meter.

If the substrate temperature is less than 300 ° C, the SnO phase cannot be obtained. If the substrate temperature is 600 ° C or higher, the SnO phase can be obtained, but the orientation begins to deteriorate, and when the substrate temperature is 700 ° C, the growth rate is 0.1 nmt. /
The SnO film does not grow below the min. This is because SnO has a melting point of 700-950 ° C.
It is considered that the reason is that SnO decomposes at a temperature higher than or equal to ° C. The oxygen partial pressure during film formation is 1 × 10 ^−
If it is less than ² Pa, SnO phase exists, but metal Sn (Sn ⁰ ) is contained in the film. Also 1
An unoriented SnO layer grows at an oxygen partial pressure of × 10 ⁻¹ Pa or higher. In this way, by controlling the oxidation degree of Sn deposited on the substrate by the substrate temperature and the atmospheric oxygen partial pressure, the total content of Sn ⁴⁺ and Sn ⁰ (tin metal) in SnO is less than 10 atomic%. The SnO thin film can be formed.

FIG. 4 shows an example of a laminated structure of a diode composed of a homo pn junction element in which a p-type semiconductor thin film made of SnO and the n-type semiconductor thin film are laminated. SnO homojunction diodes are p-S
A stack composed of an nO layer (1), a p ⁻ -SnO layer (2), and an n-SnO layer (3) is formed on the back electrode layer (5) formed on the substrate (4), and an SnO homojunction layer The uppermost electrode (6) in ohmic contact with the electrode and the electrode (7) in contact with the back electrode layer (5) are formed. P with low impurity concentration
^The -SnO layer (2) is used to improve the rectification characteristics. Not only the structure shown in FIG. 4 but also other stacked structures and different carrier concentrations can be used as long as a homo pn junction is provided.

As the back electrode layer (5), tin-doped indium oxide (In ₂ O ₃ : Sn), Al-doped zinc oxide (ZnO: Al), Ga-doped zinc oxide (ZnO: Ga), or B-doped zinc oxide (ZnO) : Transparent conductive oxide electrode materials represented by B) and the like can be used. The material used for the electrode (6) and the electrode (7) is not particularly limited as long as it has conductivity, but Au / Ni is preferably used.

The hole concentration range of the p-SnO layer (1) is 10 ¹⁷ cm ^-3 or more and less than 10 ¹⁹ cm ^-3 , and the preferred film thickness range is 100 n
m or more. The hole concentration range of the p ⁻ -SnO layer (2) is less than 10 ¹⁶ cm ⁻³ , and the preferred film thickness range is 1
00 nm or more and less than 200 nm. The electron concentration range of the n-SnO layer (3) is 10 ¹⁷ cm ⁻³ or more and less than 10 ¹⁹ cm ⁻³ , and the preferred film thickness range is 50 nm or more and less than 200 nm.

As a manufacturing method of the homojunction layer, PLD method, MBE method, CVD method, vapor deposition method, proximity sublimation method,
A production method such as a sputtering method, a sol-gel method, a spray method, a CBD (chemical bath deposition) method, or a screen printing method can be used.

A SnO thin film was formed on a (001) YSZ single crystal substrate by a PLD method using only a SnO powder and a target prepared by adding and mixing Sb ₂ O ₃ powder to SnO powder. Sb in the target
The concentration was set to 0%, 1%, 5%, 8%, 10% in the above concentration formula.

The film forming conditions for producing the SnO thin film are a substrate temperature of 550 ° C. and an oxygen partial pressure of 4 × 10 ⁻² Pa. Figure 1 shows the X-ray diffraction (XR) of the thin film when using targets with Sb concentrations of 0%, 1%, 5%, 8%, and 10%.
D) shows the pattern. Regardless of the Sb concentration, the XRD pattern shows SnO 00l (l = natural number)
Only peaks originating from the surface are observed, indicating that the produced thin film is c-axis oriented. In the oblique incidence X-ray diffraction (GIXRD) pattern, no clear diffraction is observed because the sample is an alignment film. The weak diffraction line is attributed to SnO, and is diffraction from an unoriented SnO crystallite.

FIG. 2 shows HAll measurement results for these thin films. The HAll measurement was performed using a van der Pauw electrode arrangement and the temperature was in the range of room temperature to 77K. Table 1 shows the electrical characteristics at room temperature of the thin film formed at 550 ° C.

In the case of a thin film formed at 550 ° C., the hole concentration decreases as the amount of Sb added in the target increases, and SnO exhibiting n-type electron conductivity when a target having an Sb concentration of 8% or more is used.
A thin film was obtained. Further, as shown in the temperature change of the carrier concentration in FIG. 2, the carrier concentration shows a thermal activation type, and when a target having an Sb concentration of 8% or more is used, it shows an n type in a temperature range from room temperature to 77K. It was. From the above results, it can be seen that SnO can control the conductivity type from p-type to n-type by containing Sb as an impurity.

Here, the reason why the p-type conductivity was exhibited when the amount of Sb added in the target was 1% to 5% was that Sb was evaporated by high-temperature film formation at 550 ° C. because of the high vapor pressure of Sb, and doping proceeded. This is because they have not.

An SnO thin film was formed under the same conditions as in Example 1 except that the substrate temperature was set to 400 ° C. using a target having an Sb concentration of 5%. FIG. 3 shows the result of Sb analysis by XPS in a thin film formed at 400 ° C. using a target containing 5% Sb. From the peak area of Sb3d _3/2 ,
It is thought that 7 × 10 ¹⁹ cm ⁻³ of Sb was contained. As a result of HAll measurement, this thin film showed n-type conductivity, the electron concentration was 9.9 × 10 ¹⁸ cm ⁻³ , and the mobility was 0.4 cm ² / Vs.
[Comparative Example 1]

As a comparative example, the case where Bi ³⁺ is used as an impurity for electron doping will be described. Sb ³⁺
As in the case of, Bi ₂ O ₃ powder and SnO powder were mixed and sintered so that the Bi concentration was 10% to prepare a target. Using this target, an SnO thin film was produced by the PLD method.
The film forming conditions at that time were the substrate temperature of 550 ° C., the oxygen partial pressure of 4 × 10 ⁻² Pa, and the same conditions as in Example 1.

However, as a result of HAll measurement, the obtained SnO thin film showed p-type conduction, and the hole concentration was 1.7 × 10 ¹⁷ cm <sup>−.
3. The</sup> mobility was 1.2 cm ² / Vs. The hole concentration when Bi is added to the target is almost the same as the hole concentration of SnO 2.5 × 10 ¹⁷ cm ^-3 when using an SnO target to which nothing is added. This means that it does not act as an impurity for electron doping. This is considered to be because Bi ³⁺ does not satisfy the requirement that the Sn ²⁺ radius of the Arrhenius ion is less than 0.93 mm.

A diode with a SnO homojunction having the structure shown in FIG. 4 was produced. The homojunction fabricated in this example includes a p-SnO layer (1) having a hole concentration of 3 × 10 ¹⁷ cm ⁻³ and a p ⁻ —SnO layer having a hole concentration of 10 ¹⁶ cm ⁻³ or less ( 2) and an n-SnO: Sb layer (3) having an electron concentration of 10 ¹⁹ cm ⁻³ . Since only the p ⁻ -SnO layer increases the series resistance, a 100 nm p-SnO layer (1) and 15
It was prepared -n junction diodes ^- pp using -SnO layer (2) - 0 nm by ^p.

By using the p ⁻ —SnO layer (2) having a small hole concentration of 10 ¹⁶ cm ⁻³ or less, the thickness of the depletion layer is increased and the rectification characteristics are improved. Since the electron concentration of the n-SnO: Sb layer (3) is as high as 10 ¹⁹ cm ⁻³ , the depletion layer on the n layer side is less than 2 nm, and most of the depletion layer of the pn junction extends to the p layer side. . When the p-SnO layer (1) having a hole concentration of 3 × 10 ¹⁷ cm ⁻³ is joined to the n-SnO layer (3), the depletion layer on the p-layer side is about 60 nm. Here, p ⁻ -Sn having a hole concentration of about 10 ¹⁶ cm ^−3.
By introducing the O layer, the depletion layer is increased to 250 nm, and the rectification of the diode is improved.

A YSZ001 single crystal substrate (4) was used as the substrate. First, a Sn-doped In ₂ O ₃ (ITO) layer (5), which is a transparent conductive film, was formed by the PLD method to form a lower back electrode. Then 100nm
The p-SnO layer (1), the 150 nm p ⁻ -SnO layer (2), and the 100 nm n-SnO: Sb layer (3) were laminated in this order. Here, the film forming conditions of each layer are as follows. The p-SnO layer (1) has a substrate temperature of 550 ° C. and an oxygen partial pressure of 4 × with a sintered body of SnO powder to which no impurities are added as a target.
10 ^-2 Pa. p-SnO layer (1) after deposition, p ^- -SnO layer (2) is used as the target without the addition of impurities, the substrate temperature 550 ° C., was deposited as an oxygen partial pressure of 4 × 10 ^-1 Pa. The n layer (3) is replaced with a target to which no impurities are added, and a substrate temperature of 400 is used with a sintered body of SnO powder added with Sb ₂ O ₃ powder so that the Sb concentration is 5% in the above concentration formula.
The film was formed under the conditions of ° C. and oxygen partial pressure of 4 × 10 ⁻² Pa.

In order to produce the mesa structure shown in FIG. 4, a protective film for etching was produced on the laminated film by photolithography. Thereafter, an unnecessary portion of the SnO layer was scraped off using a reactive ion etching apparatus using Ar ions to expose the ITO layer. Finally, Au / Ni electrodes (6, 7) were formed on the uppermost n-type SnO layer and ITO layer, respectively, using electron beam heating vapor deposition.

FIG. 5 shows the current-voltage characteristics of a homojunction diode using a SnO thin film. While a current of less than 1mA flows at -2V reverse bias, a current of more than 10mA is obtained at + 2V forward bias, indicating that the diode is operating. Thus, S
It has been shown that electron doping is performed on the nO thin film and that a homojunction diode can be fabricated using the SnO thin film.

Since a pn junction using a SnO thin film has been fabricated, a thin film solar cell can be formed with a SnO layer of about 2.7 eV.
It becomes possible to add a power generation layer that absorbs light having the above energy, and an improvement in power generation efficiency can be expected. In addition, since a homojunction can be created using an inexpensive material, a thin film solar cell with low manufacturing cost can be provided.

Claims (5)

From SnO, characterized in that stannous oxide (SnO) contains a trivalent cation selected from Al ³⁺ , Ga ³⁺ , In ³⁺ , Sb ³⁺ as an impurity and exhibits n-type conductivity. An n-type semiconductor thin film. A homo pn junction element in which a p-type semiconductor thin film made of SnO and the n-type semiconductor thin film according to claim 1 are laminated. A thin film solar cell using the homo pn junction element according to claim 2. By using a SnO target obtained by adding and mixing an oxide powder of a trivalent cation source selected from Al ³⁺ , Ga ³⁺ , In ³⁺ and Sb ³⁺ to a SnO powder, a physical film-forming method is used. The method for producing an n-type semiconductor thin film made of SnO according to claim 1, wherein the film is formed. A p-type SnO thin film formed by a physical film forming method using a SnO target obtained by sintering a stannous oxide (SnO) powder, and Al ³⁺ , Ga ³⁺ , In ³⁺ , Sb ^{3+ on the} SnO powder. A homo-pn junction is formed by laminating an n-type SnO thin film formed by a physical film forming method using an SnO target sintered by adding and mixing oxide powder of a trivalent cation source selected from The method for producing a homo pn junction element according to claim 2.

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