JP2018148031A - Copper nitride semiconductor and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a copper nitride semiconductor having high crystallinity and high hole effect mobility, preferably such a copper nitride semiconductor of p-type.SOLUTION: A copper nitride semiconductor comprises fluorine as an impurity. A method for manufacturing the copper nitride semiconductor comprises the step of performing a thermal treatment on a copper nitride semiconductor thin film at a temperature of 200-450°C in the presence of ammonia and fluorine-containing gas to form a copper nitride semiconductor thin film showing a p-type conductivity under the following condition: Fluorine-containing gas/(Ammonia+Fluorine-containing gas) is 0.1-10 vol.%. The method enables the formation of a copper nitride semiconductor thin film improved in crystallinity.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a copper nitride semiconductor that is useful as a light absorber material for thin film solar cells and a method for producing the same.

  Bipolar semiconductors composed of only non-toxic and common elements are of particular interest in the development of low environmental load devices. Thin film solar cells using pn junction elements typically use a p-type semiconductor (eg, CdTe or copper indium gallium sulfide / selenide (CIGS)) as the light energy absorber material. Due to the toxicity of cadmium and the limited availability of indium, gallium and tellurium, the development of alternative materials is sought. The light absorption layer of the p-type semiconductor is required to have high crystallinity from the viewpoint of the minority carrier lifetime that affects the power generation efficiency. In addition, when a pn homojunction element is used, power generation efficiency is further improved because there is no band discontinuity compared to a heterojunction element. Therefore, a high-quality p-type semiconductor, preferably a high-quality p-type and n-type bipolar semiconductor capable of controlling the polarity is desirable.

Copper nitride is composed only of overflowing elements and does not contain toxic elements. The band gap is 1.0 to 1.4 eV by indirect transition (Non-Patent Documents 1, 2, and 3), and is close to 1.4 eV at which the maximum conversion efficiency of the Shockley-Queisser model can be obtained as a single junction element. Moreover, since the light absorption coefficient is as high as 10 ⁵ cm ⁻¹ or more at 2.2 eV or more, the light absorption layer can be made thin. Therefore, if a heterojunction of a high-quality copper nitride thin film p-type semiconductor and a heterogeneous material n-type semiconductor, or a pn polarity control and a pn homojunction of a copper nitride thin film, application to a thin film solar cell as a semiconductor material can be expected.

Cu ⁺ compounds represented by Cu ₂ O, which have long been known as p-type semiconductors, are materials in which the upper end of the valence band is composed of a hybrid orbital of Cu3d orbitals and anion p orbitals, and generates holes due to Cu deficiencies. A group. Until now, many of the binary compounds of Cu ⁺ (Cu ₂ O, Cu ₂ S, Cu ₂ Se, Cu ₂ Te, CuSCN, etc.) are known as p-type semiconductor materials. Although copper nitride (Cu ₃ N) is a Cu ⁺ compound, there are reports on not only p-type conduction but also n-type conduction (Non-Patent Documents 1, 2, 3, 4). There are reports of metals, semiconductors, insulators (Non-Patent Document 5) in the form, and insulators (Non-Patent Document 6) in the single crystal bulk, and there are many unclear points because the electronic characteristics cannot produce high-quality crystals. . In the density functional theory calculation, the origin of p-type conduction is copper deficiency, and n-type conduction is oxygen substitution at interstitial copper or nitrogen sites (Non-patent Document 4).

Conventionally, Cu ₃ N has been known to be obtained by heat-treating a precursor such as CuF ₂ and ammonia at 280 to 325 ° C. (Non-Patent Documents 7 and 8), but it can be obtained only in powder form. In addition, since the heat treatment temperature is limited to 325 ° C. or less, Cu ₃ N having high crystallinity and excellent controllability of carrier polarity, concentration and mobility in the form of a bulk body or a thin film is desired. It is rare.

A. Zakutayev et al., J. Phys. Chem. Lett. 5, 1117 (2014) K. Matsuzaki et al., Appl. Phys. Lett. 5, 1117 (2014) N. Yamada et al., Chem. Mater. 27, 8076 (2015) A. N. Fioretti et al., J. Appl. Phys. 119, 181508 (2016) Nosaka et al., Thin Solid Films 348, 8 (1999) Zachwieja et al., J. Less-Common Met. 161, 175 (1990) Yuza et al., Z. Anorg. Allg. Chem. 239, 282 (1938) Paniconi et al., Solid State Sci. 9, 907 (2007)

  An object of the present invention is to provide a copper nitride semiconductor having high crystallinity and high Hall effect mobility. Furthermore, the present invention provides a method for producing a copper nitride semiconductor thin film that exhibits p-type conductivity and improved crystallinity by fluorine-doping the copper nitride semiconductor thin film.

The present invention provides the following inventions in order to solve the above problems.
(1) A copper nitride semiconductor containing fluorine as an impurity.
(2) The copper nitride semiconductor according to (1), wherein fluorine forms an acceptor level.
(3) The copper nitride semiconductor according to (1) or (2), wherein fluorine is contained as an interstitial atom of a copper nitride crystal lattice.
(4) The copper nitride semiconductor according to any one of (1) to (3), wherein fluorine is contained at a concentration of 0.1 to 5.0 mol%.
(5) The copper nitride semiconductor according to (1), wherein the copper nitride semiconductor is a thin film.
(6) The copper nitride semiconductor according to (5), wherein the copper nitride semiconductor thin film exhibits a p-type conductivity.
(7) The copper nitride semiconductor thin film is heat-treated in the presence of ammonia and a fluorine-containing gas at a temperature of 200 to 450 ° C. under a condition of fluorine-containing gas / (ammonia + fluorine-containing gas) of 0.1 to 3.0. By doing so, the copper nitride semiconductor thin film which shows a p-type conductivity type is formed, The manufacturing method of the copper nitride semiconductor as described in said (1) characterized by the above-mentioned.
(8) The fluorine-containing gas is ClF ₃ , ClO ₃ F, OF ₂ , FNO, F ₃ NO, NF ₃ , N ₂ F ₂ , N ₂ F ₄ , CF ₄ , CH ₃ F, CH ₂ F ₂ , CHF _{3 , C 2 F 6, C 3} F 8, C 4 F 8, C 4 F 6, SF 6 , or _{F 2} in the above (7) the method of manufacturing the copper nitride semiconductor described.
(9) A pn junction element formed by laminating an n-type copper nitride semiconductor thin film forming a homojunction with the p-type copper nitride semiconductor thin film according to (6).
(10) A thin film solar cell including the pn junction element according to (9).
(11) A pn junction element formed by laminating a p-type copper nitride semiconductor thin film according to (6) above and an n-type semiconductor thin film forming a heterojunction.
(12) A thin-film solar cell including the pn junction element according to (11) above, using a p-type copper nitride semiconductor thin film as a light absorption layer.
(13) A pin junction element in which an insulating layer is provided between an n-type semiconductor thin film and the p-type copper nitride semiconductor thin film according to (6).

  According to the present invention, a copper nitride semiconductor having high crystallinity and high Hall effect mobility, preferably a p-type copper nitride semiconductor can be provided.

Change in carrier concentration over time of high-temperature, deoxygenated copper nitride (Cu ₃ N) thin film exposed to the atmosphere and electron concentration after adsorption species desorption treatment by ultraviolet (UV) irradiation under vacuum (up to 10 ¹⁴ cm ⁻ It is a figure which shows ³ ). (A) X-ray diffraction pattern showing NF ₃ concentration dependency of direct nitridation reaction by NH ₃ / NF ₃ gas of copper polycrystalline thin film at 250 ° C. and (b) 300 ° C. and (c) effect of oxidizing agent. 4A is an X-ray diffraction pattern of a Cu ₃ N epitaxial thin film post-heat treated with (a) NH ₃ / NF ₃ gas and (b) Ar / NF ₃ gas. (C) Straight plane, (d) High resolution X-ray diffraction pattern of in-plane and in-plane 020 diffraction rocking curves. (E) NF ₃ concentration dependency of the peak half-value width of the rocking curve of 250 ° C., fluorine-treated surface straightness, in-plane lattice constant, (f) straightness 200 serving as an index of crystallinity, and in-plane 020 diffraction. FIG. According to electron microprobe _(EPMA), a diagram showing a fluorine concentration of NH ₃ / NF 3 Cu 3 N epitaxial thin film which is gas heat treatment. Temperature increase of H ₂ O (M / Z = 18), F (M / Z = 19), HF (M / Z = 20), nitrogen (M / Z = 28) of the Cu ₃ N thin film before and after the fluorine doping treatment It is a figure which shows a desorption gas analysis. It is a figure which shows the temperature dependence of (a) carrier concentration and (b) mobility obtained by Hall effect measurement of the fluorine-doped Cu ₃ N epitaxial thin film before and after vacuum UV irradiation. (A) the carrier concentration at 300K obtained by Hall effect measurement of Cu ₃ N epitaxial thin films doped with fluorine after the vacuum UV radiation is a diagram showing the NF ₃ concentration dependence of (b) mobility. N-type (electron concentration of 10 ¹⁴ cm ⁻³ ) obtained by adsorption species desorption treatment of vacuum 100 ° C. heat treatment, n-type (electron concentration of 10 ¹⁷ cm ⁻³ ) obtained by heat treatment at vacuum 175 ° C., fluorine doped obtained p-type processing (hole concentration ^~10 16 ^cm -3) Cu ₃ N epitaxial thin film (a) the carrier concentration is a diagram showing the temperature dependence of the (b) Hall effect mobility. Hard X-ray photoelectron spectroscopy spectra of n-type (electron concentration of 10 ¹⁴ cm ⁻³ ), n-type (electron concentration of 10 ¹⁷ cm ⁻³ ), p-type (hole concentration of 10 ¹⁷ cm ⁻³ ) Cu ₃ N epitaxial thin film FIG. (A) fluorine-doping treatment before (hot-deoxygenated later) is a graph showing the light absorption coefficient of the Cu 3 _N epitaxial thin film after (b) fluorine-doping treatment.

  The copper nitride semiconductor of the present invention contains fluorine as an impurity. Preferably, fluorine is contained at a concentration of 0.1 to 5.0 mol%, more preferably 0.5 to 2.0 mol%.

  In one embodiment of the present invention, the copper nitride semiconductor of the present invention is preferably subjected to high-quality heat treatment at 500 to 700 ° C., and the copper nitride semiconductor is 200 to 450 ° C. in the presence of ammonia and a fluorine-containing gas. It is preferably obtained by heat treatment at a temperature of 250 to 350 ° C.

  The shape of the copper nitride semiconductor is not limited and is selected from a granular shape, a plate shape, or a thin film. For example, a copper powder having a particle size of preferably 100 μm or less, more preferably 10 μm or less, a copper plate having a thickness of about 10 to 100 μm, and a thin film having a thickness of about 10 to 5000 nm can be used.

The substrate on which the thin film is formed is not particularly limited, and alkali glass, alkali-free glass, strontium titanate, quartz, sapphire, SiC, silicon, magnesium oxide, zinc oxide, zirconium oxide, lanthanum aluminate (LaAlO ₃ ), LSAT _{([LaAlO 3]) 0.3 -} [SrAl 0.5 Ta 0.5 O 3] 0.7), polyimide, and are selected from stainless steel, glass, strontium titanate, lanthanum aluminate _{(LaAlO 3), LSAT ([} LaAlO 3]) _{0.3 - [SrAl 0.5 Ta 0.5 O} 3] 0.7), MgF 2, CaF 2, SrF 2, BaF 2 and the like are suitably used.

Examples of the fluorine-containing gas include ClF ₃ , ClO ₃ F, OF ₂ , FNO, F ₃ NO, NF ₃ , N ₂ F ₂ , N ₂ F ₄ , CF ₄ , CH ₃ F, CH ₂ F ₂ , CHF ₃ , C ₂ F ₆ , C ₃ F ₈ , C ₄ F ₈ , C ₄ F ₆ , SF ₆ , or F ₂ can be suitably used.

  In one preferred embodiment of the present invention, a copper nitride semiconductor thin film exhibiting p-type conductivity is formed by heat-treating the copper nitride semiconductor thin film at a temperature of 200 to 450 ° C. in the presence of ammonia and a fluorine-containing gas. Can do.

The amount of ammonia and fluorine-containing gas used, that is, (fluorine-containing gas / (ammonia + fluorine-containing gas) varies depending on the type of fluorine-containing gas, heating temperature, copper metal precursor surface area, gas fluid distribution, etc., but the explosion limit Usually 0.1～10Vol% in inner and range not generating the CuF _2, preferably selected from about 0.1~3vol%.

  The total flow rate of ammonia and fluorine-containing gas is usually selected from about 10 to 1000 mL / min.

  The copper nitride semiconductor thin film subjected to the high-temperature heat treatment is not particularly limited as the copper source and the nitrogen source, and can be determined according to the deposition method employed. The deposition methods include molecular beam epitaxy (MBE), sputtering, pulsed laser deposition (PLD), chemical vapor deposition (CVD), electron beam vapor deposition, resistance heating vapor deposition, and atomic layer deposition (ALD). Law). The MBE method is most suitable because it is easy to strictly control the atomic ratio composition by independently controlling a plurality of raw materials, and the particle energy is small. From the viewpoint of the growth rate, the sputtering method or the CVD method is preferable.

The copper nitride thin film subjected to high temperature heat treatment used for fluorine doping treatment is mixed with ammonia and an oxidizing agent (O ₂ , O ₃ , N ₂ O, NO, Cl ₂ , NF ₃ , F _2, etc.) as the heat treatment conditions. A high-quality copper nitride semiconductor thin film can be obtained by heat treatment in gas at a temperature of 500 to 800 ° C. When the oxidizing agent is a fluorine-based gas, a high-quality heat treatment and fluorine doping can be simultaneously performed, and a fluoride substrate is preferable because the substrate can be fluorinated. Oxygen-based or chlorine-based gas oxidants contain oxygen and chlorine impurities that serve as donors. Therefore, it is preferable to perform oxygen / chlorine removal treatment at a temperature of 100 to 350 ° C. in an NH ₃ atmosphere after high-temperature heat treatment. is there.

Suitably, the copper nitride semiconductor of this invention is a thin film, The film thickness is normally chosen from about 10-5000 nm. The composition ratio of the main component of the copper nitride semiconductor thin film is Cu ₃ N, and precipitation of metallic copper is allowed, but from the viewpoint of easy control of the conductivity type, it is preferable not to contain a metallic copper phase. It is. Even if Pd _x Cu ₃ N (x = 0-1) with zero-valent Pd is added, the band gap is narrowed and becomes p-type by fluorine doping. Therefore, Pd _x Cu ₃ N is also included in the copper nitride semiconductor of the present invention. Can be included.

  In a preferred embodiment of the present invention, a copper nitride semiconductor thin film prepared by MBE is fluorine-doped by the above heat treatment in ammonia and nitrogen trifluoride gas capable of suppressing the mixing of oxygen contributing to n-type conduction. In addition, a high-quality copper nitride semiconductor thin film exhibiting p-type conduction can be produced even under a vacuum that can eliminate gas adsorption.

  The obtained crystalline copper nitride semiconductor thin film having improved crystallinity has a full width at half maximum of an XRD diffraction peak of 1.0 degrees or less, preferably 0.5 degrees or less, and an in-plane rocking curve half width.

Furthermore, the obtained copper nitride semiconductor thin film with improved crystallinity has a subgap absorption coefficient of 5 × 10 ⁻³ cm ⁻¹ or less, preferably 3 × 10 ⁻³ cm ⁻¹ or less at a photon energy of 0.5 eV, And it has a peak at 1.80 eV to 1.90 eV at the band edge.

The copper nitride semiconductor of the present invention preferably has an electron / hole mobility of 1 cm ² / Vs or more.

The copper nitride semiconductor thin film of the present invention further has the following characteristics.
(Carrier transport characteristics)
Electrical conductivity is 10 ⁻³ S · cm ⁻¹ to 10 ¹ S · cm ⁻¹ , hole concentration is 10 ¹⁶ to 10 ¹⁷ cm ⁻³ , hole mobility is 1 cm ² / Vs or more, preferably 10 200 cm ² / Vs.

(Light absorption spectrum)
The light absorption coefficient is 1.8 eV or more and more than 10 ⁵ cm ⁻¹ , the indirect band gap is 1.0 eV, and the absorption coefficient is 5 to 10 × 10 ³ cm ⁻¹ .

The obtained p-type copper nitride semiconductor thin film is a pn homojunction in which an n-type semiconductor thin film having a wider gap than Cu ₃ N is laminated and a hetero pn junction element or an n-type copper nitride semiconductor thin film forming a homojunction is laminated. An element can be formed. Since the obtained hetero pn junction element corresponds well to the energy having the strongest intensity of sunlight with the size of the band gap of Cu ₃ N, the obtained copper nitride semiconductor thin film can be used as a light absorption layer in a thin film solar cell. By using it, a power generation layer having excellent power generation efficiency can be provided.

Furthermore, a pin junction element in which an insulating layer is provided between the n-type semiconductor thin film and the p-type copper nitride semiconductor thin film obtained in the present invention can be formed. As the insulating layer, an insulating layer (electron concentration: 10 ¹⁴ cm ⁻³ ) obtained by high temperature / oxygen removal treatment can be used.

  Hereinafter, the present invention will be described in more detail with reference to examples.

The film forming conditions for producing the copper nitride (Cu ₃ N) thin film were a substrate temperature of 140 ° C., and a commercially available SrTiO ₃ (100) single crystal was used for the substrate. The output of a high-frequency plasma apparatus for generating nitrogen radicals is 500 W, the flow rate of nitrogen is 3.50 SCCM, an effusion cell is used as a Cu supply source, and Cu ₃ N is obtained at a growth rate of 30 to 50 nm / h to obtain a copper nitride thin film. An (100) epitaxial thin film was produced. After forming the thin film, the thin film was transferred to an electric furnace, a high-quality crystal was prepared by high-temperature heat treatment at 600 ° C. in an NH ₃ / O ₂ atmosphere, and residual oxygen serving as a donor was removed by heat treatment at 350 ° C. in NH ₃ atmosphere.

FIG. 1 shows changes in carrier polarity and concentration over time obtained by measuring the Hall effect in a vacuum of a Cu ₃ N epitaxial thin film exposed to the air after high temperature / oxygen removal treatment. Since the Hall electromotive voltage was positive until 2 hours after the start of evacuation, the carrier polarity was p-type, and after 3 hours the Hall electromotive voltage became negative and the carrier polarity was inverted to n-type. Therefore, it was found that in Cu ₃ N exposed to the atmosphere, oxygen molecules and the like adsorb negatively on the surface, extract electrons in the crystal, and generate holes.

Cu ₃ N, which removes residual oxygen and adsorbed species as much as possible by high-temperature / oxygen removal treatment and vacuum UV irradiation (300 K) after sample transport in an inert Ar atmosphere, is an n-type semiconductor (electron concentration 1 × 10 ¹⁴ cm ⁻³ , Hall mobility 5 to 8 cm ² V ⁻¹ s ⁻¹ ), it is considered difficult to produce Cu ₃ N exhibiting stable p-type conduction only by defect control of Cu / N. .

As a result of first-principles calculations using hybrid functionals of HSE06 type, the stable position of fluorine impurities (N substitution position of Cu ₃ N or interstitial position) and impurity levels were evaluated. Occupied as a result, it was found to exist mainly as interstitial atoms and form acceptor levels.

In order to confirm the direct nitridation reaction of copper using a fluorine-based gas as an oxidizing agent, electron beam evaporation is performed on soda lime glass in a mixed atmosphere of NH ₃ and oxidizing gas nitrogen trifluoride (NF ₃ ) gas. A polycrystalline copper thin film having a thickness of about 140 nm prepared by the method was heat-treated. High power X-ray diffraction (XRD) (thin film method, output: 50 kV-300 mA) of a thin film heat-treated in NH ₃ / NF ₃ , NH ₃ gas, NH ₃ / O ₂ gas is shown in FIGS. Shown in From FIG. 2 (a), in the NH ₃ / NF ₃ atmosphere (NF ₃ : 1 vol%), copper was completely nitrided at 250 ° C. or higher, whereas in the NH ₃ / O ₂ atmosphere (O ₂ : 1 vol%), 250 Copper was unreacted at ℃ and was partially nitrided at 300 ℃. In addition, the copper thin film was completely nitrided by heat treatment at 250 ° C. and 300 ° C. in an NH ₃ / NF ₃ atmosphere at NF ₃ concentrations of 1.0 vol% and 0.5 vol%, respectively, and only the Cu ₃ N phase was obtained.

In order to study the fluorine doping method, a Cu ₃ N epitaxial thin film (substrate: SrTiO ₃ (100) single crystal) is mixed with an NH ₃ / NF ₃ atmosphere (NF ₃ : 0.1 vol%) and an Ar / NF ₃ atmosphere (NF ₃ : 0.1 vol%), the XRD pattern subjected to post-heat treatment is shown in FIGS. In an NH ₃ / NF ₃ atmosphere, copper nitride is obtained at 275 ° C. to 400 ° C., but a reaction product of SrTiO ₃ (100) single crystal substrate and NF ₃ is detected at 350 ° C. or higher. Decomposed into metal phase and other products. Further, in the Ar / NF ₃ atmosphere, a Cu ₃ N single phase was obtained up to 150 ° C., and a CuF ₂ phase was detected at 200 ° C. or higher where NF ₃ becomes active by thermal decomposition. Therefore, it is difficult to control fluorine doping in an Ar / NF ₃ atmosphere, and fluorine doping treatment was performed in an NH ₃ / NF ₃ atmosphere.

FIGS. 3C and 3D show high-resolution X-ray diffraction patterns of a Cu ₃ N thin film (substrate: SrTiO ₃ (100)) after fluorine doping treatment. Even after the fluorine doping treatment, the thin film and the substrate maintained a cube-on-cube epitaxial relationship, and the lattice constant perpendicular to the NF ₃ concentration was small, and the half-width of the rocking curve tended to be large. .

The fluorine concentration of the Cu ₃ N epitaxial thin film treated with fluorine in an NH ₃ / NF ₃ atmosphere was measured by an electron beam microanalyzer (EPMA). Ammonium fluoride produced by the fluorine doping treatment was removed by ultrasonic cleaning of pure water. 4A and 4B show the fluorine concentration with respect to the heat treatment temperature and the NF ₃ concentration. The fluorine concentration was 0.5 to 1.5 mol% at a heat treatment temperature of 250 to 300 ° C. Further, at 350 to 400 ° C., fluorine was detected in the SrTiO ₃ single crystal substrate, so the fluorine concentration in the thin film was estimated to be 2 mol% at the maximum.

Hereinafter, in the fluorine doping treatment, the reaction with the substrate is suppressed in the NH ₃ / NF ₃ atmosphere after the high temperature treatment (600 ° C., NH ₃ / O ₂ atmosphere) and the oxygen removal treatment (350 ° C., NH ₃ atmosphere). Heat treatment was performed at 250 ° C.

FIGS. 5A and 5B show H ₂ O (M / Z = 18), F (M / Z = 19), HF (M / Z = 20) and N of the Cu ₃ N thin film before and after the fluorine doping treatment. ₂ shows thermal desorption gas analysis under vacuum of (M / Z = 28). The ammonium fluoride produced by the fluorine doping treatment was removed by ultrasonic cleaning with pure water. In the thin film before fluorine doping treatment, N ₂ (M / Z = 28) desorption gas and oxygen impurities are released as H ₂ O (M / Z = 18), etc. at a decomposition temperature of Cu ₃ N of 380 ° C. F (M / Z = 19) and HF (M / Z = 20) signals were not detected. In the thin film after fluorine doping (350 ° C., NF ₃ : 0.1 vol%), N ₂ (M / Z = 28) and H ₂ O (M / Z = 18) desorption at a decomposition temperature of Cu ₃ N of 380 ° C. In addition to the gas, the signal intensity of F (M / Z = 19) and HF (M / Z = 20) changed greatly, confirming the presence of fluorine in the Cu ₃ N crystal lattice.

FIG. 6 shows temperature characteristics of carrier concentration and Hall effect mobility of a p-type Cu ₃ N (epitaxial) thin film subjected to fluorine doping and oxygen adsorption. After the fluorine doping treatment (250 ° C., NF ₃ : 0.5 vol%), the thin film sample was exposed to the atmosphere, and ammonium fluoride was removed by ultrasonic cleaning of pure water, and then transferred to a vacuum chamber and measured in a vacuum. In addition, Cu ₃ N inverted to p-type was measured under 1 atm of helium by adsorbing (n-type) Cu ₃ N thin film obtained by high-temperature heat treatment (600 ° C., NH ₃ / O ₂ atmosphere) by exposure to air.

In the Cu ₃ N thin film adsorbed with oxygen, the hole concentration was almost constant at 150 K or less, and the logarithm of hole mobility decreased linearly with the inverse of temperature. In the fluorine-doped Cu ₃ N thin film, the logarithm of the hole concentration monotonously decreases with respect to the reciprocal of the temperature, but the slope becomes particularly small at 150 K or less, suggesting the influence of oxygen adsorption. Since the temperature dependence of the hole concentration changes linearly after irradiation with ultraviolet light (UV) by a xenon lamp in vacuum at 325K, the influence of oxygen adsorption is almost negligible, and a typical thermal activation type (activation energy) : 60 meV). Further, since the hole mobility is maximum at 250 K unlike the oxygen adsorption sample, a scattering mechanism unique to the semiconductor was observed, and the tendency was almost unchanged before and after UV irradiation.

The influence of adsorbed oxygen was removed by UV irradiation under a vacuum of 325K, and the carrier polarity, concentration and mobility were measured by the Hall effect of 300K. 7A and 7B show the hole concentration and mobility of a Cu ₃ N (epitaxial) thin film obtained by changing the NF ₃ concentration of fluorine doping treatment by 0.1 to 3.0 vol%. As shown in FIG. 4B, the fluorine concentration increases linearly (0.9 to 1.5 mol%) by increasing the NF ₃ concentration (0.1 to 1.0 vol%). Similarly, the hole concentration is 2 to 9%. It increased monotonously in the range of × 10 ¹⁷ cm ⁻³ . Further, since the fluorine concentration (1.9 mol%) and the hole concentration (7 × 10 ¹⁷ cm ⁻³ ) tend to be saturated at an NF ₃ concentration of 3.0 vol%, there is a correlation between the fluorine concentration and the hole concentration. Admitted. The hole mobility at 300K was 40 to 80 cm ² V ⁻¹ s ⁻¹ .

FIG. 8 shows n-type Cu ₃ N (electron concentration: −10 ¹⁴ and −10 ¹⁷ cm ⁻³ ) and p-type Cu ₃ N (hole concentration: −10) subjected to adsorption species desorption treatment by heat treatment in vacuum or UV irradiation. The temperature dependence of the carrier concentration of ¹⁶ cm ⁻³ ) and the Hall mobility is shown. By removing oxygen in a 350 ° C. NH ₃ atmosphere, n-type Cu ₃ N (electron concentration: 10 ¹⁴ cm ⁻³ , mobility: 5-20 cm ² V ⁻¹ s ⁻¹ ) and vacuum at 175 ° C. (vacuum degree: 10 ⁻³ Pa) n-type Cu ₃ N (electron concentration: 10 ¹⁷ , mobility: −200 cm ² V ⁻¹ s ⁻¹ ) was obtained by heat treatment.

FIG. 9 shows hard X-ray photoelectron spectroscopy (HAXPES) of n-type Cu ₃ N (electron concentration: −10 ¹⁴ and −10 ¹⁷ cm ⁻³ ) and p-type Cu ₃ N (hole concentration: 10 ¹⁷ cm ⁻³ ). A spectrum (6 keV) is shown. Although HAXPES can evaluate a deeper region than other photoelectron spectroscopy, in Cu ₃ N where the influence of surface adsorbed species cannot be ignored, sample transport was performed in an Ar inert atmosphere. Since the spectrum of the upper part of the valence band is shifted to the high bond energy side in the order of p-type, n-type (−10 ¹⁴ cm ⁻³ ), and n-type (−10 ¹⁷ cm ⁻³ ), the Fermi level shift Is consistent with the carrier polarity and concentration obtained by the Hall effect measurement.

FIG. 10 shows the light absorption coefficient obtained from the light absorption spectrum of the Cu ₃ N thin film before and after the fluorine doping treatment. A parallel beam spectrophotometer is used to measure the transmittance (T) and reflectance (R) at the same location with a sample diameter of about 5 mm using a 5 ° regular reflection jig, and the absorption coefficient (α) is the film thickness d. Then, it calculated with the formula -ln (T / (1-R)) / d.

The light absorption coefficient after the fluorine doping treatment was 1.8 eV or more and exceeded 10 ⁵ cm ⁻¹ or more, the indirect band gap was 1.0 eV, and the absorption coefficient was 6 to 7 × 10 ³ cm ⁻¹ .

  According to the present invention, a copper nitride semiconductor such as a copper nitride semiconductor thin film having high crystallinity and high Hall effect mobility can be provided.

Claims (13)

  A copper nitride semiconductor containing fluorine as an impurity.   The copper nitride semiconductor according to claim 1, wherein fluorine forms an acceptor level.   The copper nitride semiconductor according to claim 1 or 2, wherein fluorine is contained as an interstitial atom of a copper nitride crystal lattice.   The copper nitride semiconductor according to any one of claims 1 to 3, wherein fluorine is contained at a concentration of 0.1 to 5.0 mol%.   The copper nitride semiconductor according to claim 1, wherein the copper nitride semiconductor is a thin film.   The copper nitride semiconductor according to claim 5, wherein the copper nitride semiconductor thin film exhibits a p-type conductivity.   By heat-treating the copper nitride semiconductor thin film in the presence of ammonia and a fluorine-containing gas at a temperature of 200 to 450 ° C. under a condition of fluorine-containing gas / (ammonia + fluorine-containing gas) of 0.1 to 10 vol%, 2. The method for producing a copper nitride semiconductor according to claim 1, wherein a copper nitride semiconductor thin film exhibiting a p-type conductivity is formed. Fluorine-containing gas is ClF ₃ , ClO ₃ F, OF ₂ , FNO, F ₃ NO, NF ₃ , N ₂ F ₂ , N ₂ F ₄ , CF ₄ , CH ₃ F, CH ₂ F ₂ , CHF ₃ , C ₂ The method for producing a copper nitride semiconductor according to claim 7, wherein the method is F ₆ , C ₃ F ₈ , C ₄ F ₈ , C ₄ F ₆ , SF ₆ , or F ₂ .   A pn junction element formed by laminating an n-type copper nitride semiconductor thin film forming a homojunction with the p-type copper nitride semiconductor thin film according to claim 6.   A thin film solar cell comprising the pn junction element according to claim 9.   A pn junction element formed by stacking the p-type copper nitride semiconductor thin film according to claim 6 and an n-type semiconductor thin film forming a heterojunction.   The thin film solar cell containing the pn junction element of Claim 11 by using a p-type copper nitride semiconductor thin film as a light absorption layer.   A pin junction element comprising the p-type copper nitride semiconductor thin film according to claim 6 and an insulating layer provided between the p-type copper nitride semiconductor thin film and the n-type semiconductor thin film.