JP2881843B2 - Preparation method of Pb lower 5 CrO lower 8 thin film - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a Pb ₅ CrO ₈ thin film suitable for use as a photoelectric material in a photoelectric conversion element.

(Prior Art) In recent years, with the advancement of systems, the role of various sensors for sensing light, magnetism, vibration, and the like has been increasing in importance. Among them, optical sensors are being used for system control of various devices from processes and mechatronics to home electronics, and play a very important role.
The optical sensor mainly uses photoelectric conversion phenomena such as a photoconductive effect and a photovoltaic effect. Conventionally, Ge, Si, or a two-element III-V compound semiconductor has been used as a material. With the diversification of needs, optical sensors have become more sophisticated,
As multifunctionality is required, new functional materials are being developed from various angles. In terms of material, II-VI group and multi-element compound semiconductors are used, and technically, efforts are being made to develop fine processing techniques and realize superlattice structures. Pb ₂ CrO ₅ has been taken up as one of the three-element oxides, and research has been advanced to bring out the function as a photoelectric conversion device.

When a device is manufactured using an oxide material, a single crystal form is more advantageous than a ceramic in extracting material-specific properties, but it is generally difficult to single crystallize a multi-element oxide material. . When miniaturizing and multifunctional devices from the viewpoint of microelectronics, thinning is an effective means.

(Problems to be Solved by the Invention) An object of the present invention is to provide a method for forming a thin film that can be used as a photoelectric conversion device.

(Means for Solving the Problems) A feature of the present invention is that a Pb ₅ CrO ₈ ceramic and a substrate are placed in a vacuum chamber provided with an electron beam irradiation device, and the electron beam is applied by the device to the Pb ₅ CrO ₈ ceramic. Irradiate P
b ₅ CrO ₈ vapor is generated, molecules of the vapor are deposited in a thin film on the substrate, and a thin film deposited on the substrate is subjected to a heat treatment to produce a Pb ₅ CrO ₈ thin film on the substrate. Maintaining the temperature of the substrate in the range of 20 ° C. to 300 ° C. during the deposition of the vapor molecules on the substrate;
The heat treatment is in the Pb ₅ CrO ₈ thin film forming method performed in a range of 250 ° C. to 370 ° C..

Material handling (Example) The present invention is a Pb ₅ CrO _8, which is a material of the preceding Pb ₂ CrO ₅ and syngeneic. This is a material that has been less or less problematic since Negas revealed it in a study of the phase diagram of PbO and chromium oxide in 1968. We examined the photoelectric function of this material and attempted to fabricate a thin film in order to construct a thin film device.

For the production of the thin film, use of a vapor deposition method, a sputtering method, a vapor phase growth method, etc. can be mentioned. Here, the result of an attempt to make the Pb ₅ CrO ₈ thin film using a vacuum vapor deposition method by electron beam heating will be described. The electron beam evaporation method generally has advantages such as being capable of depositing a high melting point material, having a higher deposition rate than a sputtering method and the like, and having less contamination with impurities due to deposition in an ultra-high vacuum. It is an evaporation method.

Here, as the conditions for producing the Pb ₅ CrO ₈ thin film, the temperature of the deposition substrate (Ts) and the heat treatment temperature after the deposition (Ta) are used as parameters, and the obtained thin film is evaluated by X-ray diffraction and observation with a TEM image. . In order to evaluate the optical characteristics, analysis using a spectrophotometer is performed.

(Preparation of thin film) The target ceramic used for preparing the Pb ₅ CrO ₈ thin film was prepared by mixing and molding PbO powder and CrO ₃ powder in a ratio of 5: 1.
After calcination at 0 ° C, sintered at 650 ° C, diameter 12.7mm, thickness 3.5mm
Disk shape. Figure 1 shows the X-ray diffraction data (CuKα, 30kV, 15mA, 2de) of this ceramic powder.
g / min). The surface spacing obtained from this data is consistent with the data of the ASTM card, and this ceramic is Pb ₅ CrO
Confirmed to be ₈ .

For the production of the thin film, a magnetic field deflection type electron beam evaporation apparatus shown in FIG. 2 was used. The vacuum pumping system has a low vacuum pumping system using a rotary pump, and an ultra-high vacuum pumping system using a noble pump and a titanium sublimation pump.
It is possible to pump down to the order of 10 ^-9 Torr. The electron beam was generated by heating a filament, cathode - are accelerated by the high pressure applied between the anode, after being bent in the permanent magnet field, the target installed in a copper crucible Pb ₅ CrO ₈ in ceramic Irradiated. The intensity of the electron beam can be adjusted by changing the voltage applied to the filament and controlling the number of emitted thermoelectrons. The irradiation position of the beam could be arbitrarily determined by changing the acceleration voltage between the cathode and the anode to control the speed of the thermoelectrons. The substrate was placed at a position about 20 cm above the crucible and heated to a desired temperature by a heater. Corning borosilicate glass (No.0211, 35mm x 35mm, 0.12-0.16mm thick) with good thermal properties
The glass was placed in a vacuum of 1 × 10 ⁻⁷ Torr or less for 1 hour or more to remove residual gas contained in the glass. Target
The Pb ₅ CrO ₈ ceramic was used in half for each vapor deposition, heated just before vapor deposition by irradiating an electron beam in a vacuum of 10 ⁻⁷ Torr, and degassed.

As shown in FIG. 11, the vapor deposition was performed while the voltage between the anode and the cathode was 3.5 to 4 kV, the current was 100 mA or less, and the degree of vacuum was kept constant. The substrate temperature, one of the parameters, is
Room temperature, 100, 150, 200, 250 and 300 ° C. were set in 6 ways. After the vapor deposition, the substrate temperature was lowered at a rate of 100 ° C./h to prevent strain and peeling due to rapid cooling. The deposition rate at this time was about 400 ° / min for all the thin films, and the thickness of the deposited film was almost constant at 1.54 to 1.72 μm. After the vapor deposition, a heat treatment was performed to promote crystallization and orientation of the thin film. Heat treatment temperature is 250, 275, 300, 325, 350, 375 and in air
The temperature was set to 7 at 400 ° C., and the operation was performed for 2 hours. At this time, the temperature was raised and lowered at a rate of 50 ° C./h so that the thin film would not be distorted or peeled due to a rapid change in temperature.

(Analysis Results and Discussion) In order to evaluate the prepared thin film, X
Observation of the surface state of the thin film by line diffraction and an electron microscope, and measurement of the wavelength characteristic of the thin film by a spectrophotometer from an optical surface were performed. Hereinafter, each analysis result and consideration will be described.

(Analysis by X-ray Diffraction Data) As means for analyzing the structure of the Pb ₅ CrO ₈ thin film, the crystal orientation was examined by using an X-ray diffraction method. FIG. 3 is a representative X-ray diffraction pattern. (A) and (b) are thin films deposited at a substrate temperature Ts of room temperature (FIG. 3a) and 300 ° C. (FIG. 3b), respectively, without heat treatment. When the substrate temperature is room temperature, the thin film (a) is nearly amorphous and no remarkable peak is observed, but the thin film (b) set at 300 ° C. has a remarkable peak at 30.27 °. From these facts, it is clear that the crystal state of the thin film differs depending on the substrate temperature during the vapor deposition. Figures (c) and (d)
Is to apply the thin film in the state (b) to Ta = 250 ° C (Fig.
= X-ray diffraction pattern after heat treatment at 350 ° C (Figure 3d). It can be seen that the degree of orientation of the thin film is advanced by performing the heat treatment. The main peak is the main peak of the ceramic (powder) used for the target from 30.27 ゜
It changes to 27.5 °, and its reflection intensity varies depending on the heat treatment temperature.

FIG. 4 shows the relationship between the substrate temperature and the heat treatment temperature and the X-ray reflection intensity of the 27.5 ° peak in order to examine the influence of the substrate temperature and the heat treatment temperature on the orientation. Immediately after the deposition, almost no thin film is oriented, but the orientation is revealed by the heat treatment, and a peak appears at 27.5 °. Although the degree of orientation depends on the heat treatment temperature, it is also strongly related to the substrate temperature. Regarding the substrate temperature, the higher the orientation, the more the orientation is, and 300 ° C. is the best orientation. In the experiments, thin films deposited at a substrate temperature of 325 ° C or higher were also examined.However, there was no significant difference between the state of the obtained thin film and that at 300 ° C.
Perform at 0 ° C or lower. 350 ℃ for heat treatment temperature
Until the temperature is higher, the orientation is better. This is considered to be because the Pb ₅ CrO ₈ molecules on the glass substrate are activated by the applied thermal energy, and the crystals are regularly arranged.
When the temperature exceeds 375 ° C., the peak at 27.5 ° tends to decrease.

Next, FIG. 5 shows how the orientation changes with the heat treatment temperature for a thin film deposited at Ts = 300 ° C., which has the largest 27.5 ° peak. The circle indicates the 27.5 ° peak, and the square indicates the 30.27 ° peak. Most of the thin films immediately after evaporation have a peak at 30.27 °, but their size is small. When this thin film is heat-treated at 250 ° C., the peak at 30.27 ° increases and the peak at 27.5 ° also increases. At Ta = 300 ° C., the peak at 30.27 ° becomes smaller and the peak at 27.5 ° becomes even larger.
When Ta = 350 ° C., the peak at 30.27 ° becomes smaller than that at the time of vapor deposition, and the peak at 27.5 ° becomes dominant. At Ta = 400 ° C., the orientation of the crystal greatly changes, and the reflection at 27.5 ° and 30.27 ° almost disappears, but the reflection intensity at 26.3 ° and 36.2 ° increases instead. This is Pb ₂ CrO ₅ ceramic (310)
This corresponds to reflection from the plane and the (202) plane. This means that by applying thermal energy, the composition of the thin film is partially Pb
It is considered that changes to ₂ CrO _5. The Pb ₅ CrO ₈ thin film mainly reflected at 27.5 ゜ and 30.27 ゜ has a reddish brown color, whereas the thin film with strong reflection at 26.3 ゜ and 36.2 ゜ has Pb ₂ CrO ₅ It has a very similar orange color and is consistent with the results from X-rays described above.

FIG. 6 shows the relationship between the X-ray reflection intensity at 27.5 ° and 30.27 ° and the substrate temperature. The unprocessed state is shown in the figure.
Two cases of heat treatment at 350 ° C. are shown. The reflection at 30.27 ° immediately after deposition strongly depends on the substrate temperature. Individual molecules that were oriented in random directions during the crystallization process form crystal grains that are oriented in the same direction when assembled, resulting in the largest reflection at 30.27 °. The reflection intensity at 27.5 ° hardly depends on the substrate temperature in the unheated state, but the influence of the substrate temperature appears greatly during the heat treatment. The energy received by the Pb ₅ CrO ₈ molecules during the vapor deposition does not lead to a stable state as a thin film, but it is considered that the heat treatment is a factor promoting the orientation.
Looking at the change in the reflection intensity before and after the heat treatment, the change in the 30.27 ° peak is small, whereas the change in the 27.5 ° peak is larger as the substrate temperature is higher. This means that the molecules contributing to the generation of the 30.27 ° peak did not move much,
It is thought that molecules contributing to X-ray reflection other than ゜ and 27.5 ° are arranged so as to newly add to the 27.5 ° peak.

From the above, it can be explained that heating the substrate promotes crystallization of the thin film at the time of vapor deposition, and heat treatment after the vapor deposition enhances the orientation of the thin film. At the same time, from the results in FIG.
When the ratio of the reflection intensity at 27.5 ゜ and 30.27 ゜ is calculated, Ts = 300 ℃
Is the largest, and it can be said that Ts = 300 ° C. and Ta = 350 ° C. are conditions suitable for thin film production.

(Surface observation by TEM image) Observing changes in the surface state during vapor deposition and after heat treatment is a powerful clue to know the crystal state of the thin film. In this section, the transmission electron microscope
Using a microscope (TEM), the surfaces of the untreated film and the film subjected to the heat treatment are observed, and the validity of the consideration by the X-ray analysis in the previous section is verified. FIG. 7 shows a transmission electron microscope (manufactured by JEOL,
200CX) is used at an accelerating voltage of 200 kV, and shows a TEM image obtained for a sample by the replica method. In general, molecules inside the evaporated thin film reach their surface when they reach the substrate (surfac).
e migration), and a polycrystalline thin film is known to be formed. Figure (a) showing a thin film immediately after vapor deposition (Ts =
Untreated film at room temperature) and Fig. (B) (Untreated film at Ts = 300 ° C)
It is also confirmed from the photograph that both are polycrystals near the amorphous state. It can be seen that the thin film deposited at a substrate temperature of room temperature has a very small crystal grain size, and the thin film obtained at 300 ° C. has a large crystal grain size due to crystal growth. According to the X-ray diffraction data, when the substrate temperature is low (room temperature), the thin film is almost in an amorphous state and no remarkable orientation is observed. On the other hand, when the substrate temperature is increased, the crystal grows and the crystal grows at 30.27 ゜. The reflection is getting stronger. This is TEM
It corresponds well with the result in the image. Photo (b) shows (a)
It is considered that there is an island-like flat portion which is not present in the above, and the X-ray reflection angle at this island-like portion corresponds to the peak of 30.27 °.

The photographs (c) and (d) are the observation results of the surface after the thin film in the state (b) was heat-treated at Ta = 300 ° C. and Ta = 350 ° C., respectively. From the comparison with the result of the X-ray diffraction, it is found that the fine crystal part (b) and a part of the large island-like crystal change to a crystal having a small grain size in (c) by heat treatment at 300 ° C. It can be considered that the crystal grains contribute to the 27.5 ° peak. In the thin film in the state (d), the peak is 27.5 ° mainly due to the small crystal grain size, and it is inferred that the reflection from a part having a large grain size is a peak at 30.27 °.

(Analysis by Spectrophotometer) In discussing the photoelectric properties of the thin film, it is important to examine the optical properties. As the optical characteristics, transmittance and reflectance were measured using a spectrophotometer (Shimadzu: UV-2100 type), and the optical band gap energy of the thin film was determined from the results. FIG. 8 shows the measurement results of the wavelength dependence of the transmittance and the reflectance of the thin film deposited at Ts = 300 ° C., which was heat-treated at (a) 300, (b) 350, and (c) 400 ° C.
In the film state of Ta = 300 ° C., it is recognized that the measured value on the long wavelength side is wavy. This is due to interference due to multiple reflections between the front and back surfaces of the thin film, and is also related to the large particle size in the TEM photograph. Since the reflectance of the thin film at Ta = 350 ° C. is small, it can be said that this film has high crystal orientation and relatively little structural disorder such as lattice defects. The low transmittance and high reflectance of the thin film at Ta = 400 ° C. are due to changes in the crystal structure of Pb ₅ CrO ₈ and Pb ₂ CrO _5.
Both are mixed, resulting in low orientation and large strain such as lattice defects. The wavelength at the absorption edge at which the transmittance becomes zero differs at the three heat treatment temperatures, which corresponds to the change in the color of the thin film from reddish brown to orange. From this figure, it can be seen that the heat treatment temperature affects the orientation of the thin film, but also causes a change in the composition of the thin film.

(A) 100, (b) 200, (c) After setting the substrate temperature at 300 ° C., and measuring the change in transmittance and reflectance of the film obtained by the heat treatment at Ta = 350 ° C. Is shown in FIG. The thin films at Ts = 100 ° C. and 200 ° C. have poor orientation compared to the results at Ts = 300 ° C. This is consistent with the mixture of the 27.5 ° peak and the 30.27 ° peak in the X-ray diffraction data. However, the composition of the thin film has not changed, supporting the result in the previous section that the degree of orientation of the thin film depends on the substrate temperature. The fact that the wavelengths at the absorption edges are substantially equal does not contradict the observation that the colors of the thin films are the same.

The absorption coefficient was calculated from the values of the transmittance and the absorptivity of the thin film, and the optical band gap energy of the thin film was obtained. It is known that the following relationship exists between the absorption coefficient α and the transmittance T and reflectance R of the thin film.

Here, t is the film thickness, and r is the reflectance at the thin film interface.
From the equations (1) and (2), α is given by the following equation using the values of R, T and the film thickness.

FIG. 10 is a plot of the relationship between the photon energy and the absorption coefficient of a sample manufactured at Ts = 300 ° C. and Ta = 350 ° C. The value of the intersection between the straight line in the figure and the coordinate axis of the photon energy, which is the horizontal axis, corresponds to the value of the optical band gap energy in the direct transition between the valence band and the conduction band in the Pb ₅ CrO ₈ thin film. 2.22 eV. As a semiconductor material having a similar band gap at room temperature, CdS
(2.41 eV), GaP (2.24 eV) and ZnTe (2.26 eV). This indicates that the Pb ₅ CrO ₈ thin film is a material that can be treated as a semiconductor having photosensitivity in the visible region.

(Effects of the Invention) Thinning of Pb ₅ CrO ₈ , which is a three-element oxide, was attempted by an electron beam evaporation method, and the manufacturing conditions were examined.
With emphasis on the substrate temperature and the heat treatment temperature as parameters to be considered when preparing the thin film, evaluation of the thin film obtained through X-ray diffraction, observation of a TEM image, and results using a spectrophotometer was performed.

The prepared thin film has better molecular orientation than the ceramic thin film, and is arranged in a direction in which the X-ray reflection intensity at 27.5 ° is dominant. Substrate temperature 300
° C and heat treatment temperature of 350 ° C were found to be optimal. The substrate temperature is related to the crystallization of the thin film at the time of vapor deposition, and the thinner the crystallization is, the better the orientation after the heat treatment. The heat treatment temperature is a factor that influences the promotion of orientation of the thin film. However, when heat treatment is performed at 375 ° C. or more, some Pb ₅ CrO ₈ molecules are decomposed, and Pb ₂ CrO ₅
Was found to change.

Observation of the surface of the thin film supported the consideration from the results of the X-ray analysis. From analysis with a spectrophotometer,
The effects of the substrate temperature and the heat treatment temperature on the thin film were studied optically. Evaluation of the absorption coefficient of the thin film as a function of the photon energy revealed that the optical band gap energy of the Pb ₅ CrO ₈ thin film was 2.22 eV, which can be treated as a semiconductor usable in the visible light region.

[Brief description of the drawings]

FIG. 1 is a view showing an X-ray diffraction pattern of a Pb ₅ CrO ₈ ceramic powder. FIG. 2 is a schematic configuration diagram of an electron beam evaporation apparatus used for producing a Pb ₅ CrO ₈ thin film. FIG. 3 shows an X-ray diffraction pattern of the thin film. FIG. 4 is a graph showing the relationship between the substrate temperature at 27.5 ° peak, the heat treatment temperature, and the X-ray diffraction intensity. FIG. 5 is a diagram showing the relationship between the heat treatment temperature at Ts = 300 ° C. and the X-ray diffraction intensity. FIG. 6 is a diagram showing the relationship between the substrate temperature and the X-ray diffraction intensity at untreated and at Ta = 350 ° C. FIG. 7 is a view showing a TEM photograph of the Pb ₅ CrO ₈ thin film. FIG. 8 is a diagram showing the transmittance (T) and the reflectance (R) of a Pb ₅ CrO ₈ thin film deposited on a glass substrate. FIG. 9 is a diagram showing the transmittance (T) and the reflectance (R) of a Pb ₅ CrO ₈ thin film deposited on a glass substrate. Figure 10 is a diagram showing the relationship of alpha ² and photon energy. FIG. 11 is a diagram showing conditions for producing a Pb ₅ CrO ₈ thin film.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI H01L 31/0264 H01L 31/08 M

Claims (1)

(57) [Claims] 1. A electron beam in a vacuum chamber in which the irradiation apparatus are provided Place and Pb ₅ CrO ₈ ceramic and the substrate, wherein the electron beam by the device Pb ₅ CrO ₈ by irradiating the ceramic Pb ₅ CrO ₈
Steam is generated, is deposited molecules of the gas the evaporated thin film shape on the substrate, a heat treatment to a thin film deposited on the substrate, a method of making a thin film of Pb ₅ CrO ₈ on the substrate, During the period in which the vapor molecules are deposited on the substrate, the temperature of the substrate is maintained in a range of 20 ° C. to 300 ° C., and the heat treatment is performed in a range of 250 ° C. to 370 ° C.
Preparation method of Pb ₅ CrO ₈ thin film.