Polysilicon Thin Film and Method of Preparing Polysilicon Thin Film and Photovoltaic Element Containing Same

1. Field of the Invention

The present invention relates to a polysilicon thin film and a method of preparing the same.

2. Description of the Background Art

A polysilicon thin film is employed as a photoelectric conversion layer in a photovoltaic element or the like. It is necessary to improve the photoelectric conversion efficiency in such a photovoltaic element. Thus, it is necessary to increase the sizes of the crystal grains forming the polysilicon thin film for improving the carrier mobility and increasing the carrier diffusion length in the thin film as a means for improving the photoelectric conversion efficiency.

The so-called solid phase growth method of forming an amorphous silicon thin film on a substrate and crystallizing the amorphous silicon thin film by heat treating the same is known as an exemplary method of preparing a polysilicon thin film. Further, the so-called partial doping method is known as a method of preparing a polysilicon thin film having large crystal grain sizes by the solid phase growth method. The partial doping method is adapted to partially dope an amorphous silicon thin film with phosphorus (P) or boron (B) thereby changing the conductivity type or charge carrier type of the amorphous silicon film and selectively starting crystallization from the doped part. According to the partial doping method, crystallization is started from a specific part of the amorphous silicon thin film, whereby it is possible to control the generation of crystal nuclei for obtaining a polysilicon thin film having relatively large crystal grain sizes.

However, such a polysilicon thin film obtained by the conventional partial doping method is limited as to further improvement of the photoelectric conversion efficiency of a photovoltaic element.

An object of the present invention is to provide a polysilicon thin film having an improved carrier mobility, and a method of preparing the same.

An article according to the present invention comprises a substrate, and a polysilicon thin film on the substrate. The polysilicon thin film is obtained by crystallizing an amorphous silicon thin film that has been formed on the substrate through heat treatment. The polysilicon thin film has substantially no grain boundaries in the direction of its thickness. That is to say, substantially all of the grain boundaries extend substantially in the direction of the thickness, or across the thickness of the film from one surface of the film to the opposite surface of the film.

A photovoltaic element according to the present invention comprises a substrate, a polysilicon thin film obtained in the manner described in the preceding paragraph, and a semiconductor layer forming a semiconductor junction with the polysilicon thin film.

A method according to the present invention can be used for preparing the aforementioned polysilicon thin film, and comprises the steps of forming on a substrate a doped first amorphous silicon layer containing dispersed crystal phases, forming an undoped second amorphous silicon layer on the first amorphous silicon layer, and heat treating the first and second amorphous silicon layers for crystallizing the same.

In the method according to the present invention, the first and second amorphous silicon layers are preferably formed by CVD. The first and second amorphous silicon layers can be formed under the same conditions except for the compositions of the raw material gases, whereby it is possible to form the second amorphous silicon layer subsequently to forming the first amorphous silicon layer by simply changing the raw material gases.

The inventors have discovered that it is possible to improve the carrier mobility of a polysilicon thin film by substantially eliminating grain boundaries in the direction of the film thickness, i.e. grain boundaries that run substantially parallel to the plane of the film. The inventive polysilicon film has a high carrier mobility since it has substantially no grain boundaries in the direction of its thickness. Thus, it is possible to attain a high photoelectric conversion efficiency when the inventive polysilicon thin film is used for a photoelectric conversion layer of a photovoltaic element.

According to the inventive method, which is capable of preparing the aforementioned polysilicon thin film with substantially no grain boundaries in the direction of its thickness, an undoped second amorphous silicon layer is formed on a doped first amorphous silicon layer containing dispersed crystal phases, and the first and second amorphous silicon layers are heat treated to be crystallized. In the crystallization step, crystallization starts from the crystal phases contained in the first amorphous silicon layer, which serve as seeds. Since large crystal grains are grown along the thickness direction, grain boundaries are formed only along the thickness direction of the thin film, so that no grain boundaries are present across the direction of the thickness of the polysilicon thin film as formed. Due to the absence of grain boundaries in the direction of the thickness, i.e. crossing the thickness direction, the carrier mobility is improved along the thickness and therefore the carrier diffusion length increases. At the film surface perpendicular to the thickness direction, the carrier mobility is increased as compared with a conventional polysilicon thin film because it has larger crystal grain sizes than the conventional film. In this film surface, however, the carrier mobility is not as greatly increased as in the direction of the thickness due to the presence of grain boundaries.

When the first and second amorphous silicon layers are formed by CVD as described above, it is possible to form the second amorphous silicon layer subsequent to forming the first amorphous silicon layer by simply changing the raw material gases. Thus, it is possible to form the amorphous silicon layers through continuous steps, thereby remarkably reducing the concentration of impurities such as oxygen, nitrogen, and carbon which are introduced into the interface between the first and second amorphous silicon layers. Thus, crystals are smoothly grown from the crystal phases serving as seeds in the crystallization step so that it is possible to prevent generation of new nuclei. Thereby it is possible to prepare a polysilicon thin film having larger crystal grain sizes with substantially no grain boundaries present in the direction of its thickness, If a number of new nuclei are generated, crystal growth will also start from these new nuclei. Crystal grains would then grow from adjacent nuclei until they come in contact with each other, and no further growth would then be attained. Thus, large crystal grain sizes cannot be attained if a number of new nuclei are generated.

The inventive polysilicon thin film has substantially no grain boundaries in the direction of its thickness, and hence exhibits a high carrier mobility. Thus, it is possible to remarkably improve the photoelectric conversion characteristics by using the inventive polysilicon thin film for a photoelectric conversion layer of a photovoltaic element.

The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

FIG. 1 is a schematic diagram showing a typical crystal structure of a polysilicon thin film according to the present invention;

FIG. 2 is a schematic diagram showing a typical crystal structure of a polysilicon thin film obtained by a conventional partial doping method;

FIG. 3 is a sectional view showing typical first and second amorphous silicon layers formed by a method according to an embodiment of the present invention;

FIG. 4 is a sectional view showing the first and second amorphous silicon layers, of FIG. 3, after they have been crystallized by heat treatment;

FIG. 5 is a sectional view showing the states of crystal phases on a substrate having a textured surface;

FIG. 6 is a sectional view showing the states of crystal phases on a substrate having a flat surface;

FIG. 7 is a schematic diagram showing an exemplary heat treatment apparatus which can be used in the inventive method;

FIG. 8 is a graph showing Hall mobility values of polysilicon thin films prepared according to an Example of the invention and a comparative example;

FIG. 9 is a sectional view showing the typical structure of a photovoltaic element prepared according to an Example of the present invention;

FIG. 10 is a graph showing current-voltage characteristics of the photovoltaic element prepared according to an Example of the present invention;

FIG. 11 is a transmission electron microscopy image showing the crystal structure of a polysilicon thin film according to the present invention;

FIG. 12 is a schematic diagram of the transmission electron microscopy image shown in FIG. 11;

FIG. 13 is a scanning electron microscopy image showing a first amorphous silicon layer formed according to the embodiment of the present invention;

FIG. 14 is a lattice image of transmission electron microscopy showing the first amorphous silicon layer formed according to the embodiment of the present invention; and

FIG. 15 is a transmitted electron diffraction pattern obtained by irradiating crystal phases of the first amorphous silicon layer formed according to an Example of the present invention with an electron beam.

FIG. 1 is a schematic diagram showing the typical crystal structure of a polysilicon thin film 2 according to the present invention. Referring to FIG. 1, the polysilicon thin film 2 is formed on a substrate 1 so that grain boundaries 2a extend along its thickness direction and no grain boundaries are present in the thickness direction, i.e. crossing the thickness direction. In such a polysilicon thin film, the in-plane sizes W of crystal grains are about 0.1 to 1 relative to the film thickness in general. While the thickness T of such a polysilicon thin film is determined by that of an amorphous silicon thin film which is not yet crystallized, it is possible to prepare the polysilicon thin film 2 with no grain boundaries in the thickness direction as shown in FIG. 1 even if the overall thickness is 20 .mu.m, for example for film thickness of at least 3 .mu.m, or between about 5 .mu.m and 20 .mu.m.

FIG. 11 is an electron photomicrograph showing a polysilicon thin film according to the present invention, and FIG. 12 is a schematic drawing typically illustrating the electron photomicrograph shown in FIG. 11. Referring to FIGS. 11 and 12, a polysilicon thin film 4 is formed on a substrate 3, and three crystal grains 5, 6, and 7 consisting of single crystal phases are observed therein. The crystal grain 6 appears rather dark as compared with the crystal grains 5 and 7, since the former has a crystal orientation which is different from those of the latter. It is possible to observe the respective crystal grains by such contrast, and grain boundaries 4a and 4b respectively between the crystal grains 5 and 6 and between the crystal grains 6 and 7 are observed in FIGS. 11 and 12. As clearly understood from FIGS. 11 and 12, the crystal grains 5, 6, and 7 are grown along the thickness direction of the polysilicon thin film 4, whereby the grain boundaries 4a and 4b extend along the thickness direction.

FIG. 2 is a schematic diagram showing the typical crystal structure of a polysilicon thin film obtained by a conventional partial doping method. Referring to FIG. 2, a polysilicon thin film 12 is formed on a substrate 11, and a grain boundary 12a is present in the thickness direction of this polysilicon thin film 12 i.e. the grain boundary 12a extends substantially across the thickness direction.

An embodiment of a method of preparing the polysilicon thin film shown in FIG. 1 is now described.

FIG. 3 is a sectional view showing first and second amorphous silicon layers which are formed in the embodiment of the method according to the present invention. Referring to FIG. 3, a doped first amorphous silicon layer 22 containing dispersed crystal phases 23 is formed on a substrate 21. According to this embodiment, the first amorphous silicon layer 2 is doped with P. Further, the first amorphous silicon layer 22 is formed in a thickness of about 0.2 .mu.m in this embodiment. In practice, the substrate 21 is formed by a light-transmissive quartz substrate.

The first amorphous silicon layer 22 can be formed by plasma CVD, and the conditions for forming this layer can be set within the following ranges: substrate temperature of 500.degree. to 600.degree. C., gas flow rates of 3 to 5 sccm for SiH.sub.4, 0.003 to 0.01 sccm for PH.sub.3, and 40 to 300 sccm for H.sub.2, reaction pressure of 10 to 100 Pa, and RF power of 50 to 100 mW/cm.sup.2, for example. Under such conditions, it is possible to form the first amorphous silicon layer 22 containing dispersed crystal phases 23 as shown in FIG. 3.

FIG. 13 is a scanning electron microscopy image showing a first amorphous silicon layer formed in the aforementioned manner. As shown in FIG. 13, crystal phases are dispersed in the amorphous silicon layer. Lower portions of these crystal phases are buried in the amorphous silicon layer, as understood from the sectional portion of the amorphous silicon layer shown at the front, i.e. bottom of FIG. 13.

FIG. 14 is a lattice image of transmission electron microscopy showing the first amorphous silicon layer from above. A portion containing the crystal phases is observed in this figure. FIG. 15 illustrates a diffraction pattern obtained by irradiating the portion containing the crystal phases with an electron beam. As shown in FIG. 15, it is confirmed that the crystal phases are a single-crystalline phase from observation of the diffraction pattern.

The crystal phases 23 are preferably 100 .ANG. to 10 .mu.m in size, and about 1000 to 2000 .ANG. in size according to this embodiment. Further, according to this embodiment, upper portions of the crystal phases 23 upwardly project from the surface of the first amorphous silicon layer 22 by about 200 to 600 .ANG.. According to the present invention, the crystal phases 23 may not necessarily project as shown in FIG. 3, but may alternatively be buried in the first amorphous silicon layer 22.

It is possible to adjust the degree of dispersion of the crystal phases 23 by adjusting any one of the conditions for forming the first amorphous silicon layer 22, such as the RF power, for example, or by forming a textured surface on the substrate 21 and adjusting the texture spaces. The spaces between the crystal phases 23 are preferably about 0.5 to 20 .mu.m.

Referring again especially to FIG. 3, an undoped second amorphous silicon layer 24 is formed on the first amorphous silicon layer 22 which has been formed in the aforementioned manner. According to this embodiment, the second amorphous silicon layer 24 is formed by plasma CVD, and conditions for forming this layer can be set within the following ranges substrate temperature of 500.degree. to 600.degree. C., gas flow rate of 10 to 60 sccm for SiH.sub.4 or 10 to 40sccm for Si.sub.2 H.sub.6, reaction pressure of 10 to 100 Pa, and RF power of 100 to 500 mW/cm.sup.2, for example. Since these conditions can be identical to those for the first amorphous silicon layer 22 except the reaction gas conditions, it is possible to form the second amorphous silicon layer 24 under the conditions for the first amorphous silicon layer 22 by changing only the reaction gases. In practice, the second amorphous silicon layer 24 is formed in a thickness of about 3 .mu.m.

Then, the first and second amorphous silicon layers 22 and 24 formed in the aforementioned manner are heat treated to be crystallized. Preferable heat treatment conditions are about 550.degree. to 650.degree. C. and about 3 to 20 hours. In practice, the first and second amorphous silicon layers 22 and 24 are heat treated at 600.degree. C. for 10 hours to be crystallized. The heat treatment is preferably carried out in a vacuum or in a nitrogen atmosphere. In such a crystallization step, the crystal phases 23 shown in FIG. 3 serve as seeds for starting crystallization. FIG. 4 is a sectional view showing a typical intermediate state of such crystallization. It is understood from FIG. 4 that crystallization progresses along the thickness direction, to form crystal grains 25. The crystal grains 25 as grown extend over the entire regions of the first and second amorphous silicon layers 22 and 24, to form the polysilicon thin film shown in FIG. 1.

FIGS. 5 and 6 are sectional views for illustrating a method of adjusting the spaces between crystal phases by properly forming the aforementioned textured surface of the substrate. Referring to FIG. 5, a first amorphous silicon layer 22' is formed on a textured substrate 21', and crystal phases 23 are dispersed in the first amorphous silicon layer 22'. As shown in FIG. 5, each crystal phase 23 is formed on a flat portion 21a of the textured surface of the substrate 21'. FIG. 6 is a sectional view showing a flat substrate 21, on which crystal phases 23 are formed in a denser state as compared with those in FIG. 5. Thus, it is possible to adjust the degree of dispersion of crystal phases, i.e., the spaces between the crystal phases, by forming flat portions on a textured substrate in a desired density.

FIG. 7 is a schematic sectional view showing a typical heat treatment apparatus for carrying out such crystal growth. Referring to FIG. 7, a substrate 31 provided with an amorphous silicon layer 32 is mounted on a stage 33, which is provided with a heater 34. Such an apparatus that can heat the amorphous silicon layer 32 from the substrate 31 side is preferable.

Samples of polysilicon thin films according to the invention were formed on silica substrates having flat and textured surfaces, respectively, similarly to the aforementioned embodiment. Then, the Hall mobility of the samples was measured. Further, comparative samples of polysilicon thin films were formed on silica substrates having flat and textured surfaces, respectively, by a conventional partial doping method. The Hall mobility of the comparative samples was also measured. FIG. 8 shows the results.

The textured surfaces of the substrates were formed by a blasting method of perpendicularly bombarding the substrates with diamond grains of 10 .mu.m in size. In this manner, the textured substrates were formed with peak-to-valley and peak-to-peak distances of 10 to 20 .mu.m.

In the comparative samples, doped amorphous silicon layers were formed by the conventional partial doping method under conditions of substrate temperatures of 500.degree. to 600.degree. C., gas flow rates of 10 to 20 sccm for SiH.sub.4 and 0.001 to 0.01 sccm for PH.sub.3, reaction pressures of 10 to 100 Pa, and RF power levels of 100 to 200 mW/cm.sup.2. The comparative samples were remarkably different from the inventive samples with respect to diluting the reaction gases with hydrogen gas. Undoped amorphous silicon layers were formed on the doped amorphous silicon layers under conditions similar to those of the aforementioned embodiment.

As understood from FIG. 8, the inventive polysilicon thin films exhibited higher Hall mobility values for a given carrier concentration, as compared with the comparative polysilicon thin films which were formed by the conventional method. It was possible to attain 808 cm.sup.2 /Vs (carrier concentration: 1.3.times.10.sup.16 cm.sup.-3) as the highest Hall mobility. Inventive Sample (1) with a textured substrate was shown to have a Hall mobility greater than about 400 cm.sup.2 /Vs and clearly greater than 300 cm.sup.2 /Vs, with a carrier concentration greater than 10.sup.16 /cm.sup.3. Inventive Sample (2) with a flat substrate was shown to have a Hall mobility generally greater than about 200 cm.sup.2 /Vs, with a carrier concentration generally greater than about 5.times.10.sup.15 /cm.sup.3.

Then, a photovoltaic element was prepared with a polysilicon thin film according to the present invention. FIG. 9 shows the structure of this photovoltaic element, comprising an SUS stainless steel substrate 41 having a textured surface, and an n-type polysilicon thin film layer 42 formed thereon to have a thickness of 10 .mu.m. The textured surface of the substrate 41 had peak-to-valley and peak-to-peak distances of 10 to 20 .mu.m. The n-type polysilicon thin film layer 42 was formed by first and second amorphous silicon layers having thicknesses of 1000 .ANG. and 10 .mu.m, respectively.

An i-type amorphous silicon layer 43 of 50 .ANG. in thickness and a p-type amorphous silicon layer 44 of 50 .ANG. in thickness were successively deposited on the n-type polysilicon thin film layer 42, and an ITO film 45 of 700 .ANG. in thickness was further formed thereon as a surface antireflection film and a electrode.

For the purpose of comparison, another photovoltaic element was prepared with a layer, which corresponds to the n-type polysilicon thin film layer 42 shown in FIG. 9, but which was formed by a conventional partial doping method. FIG. 10 shows current-voltage characteristics of the photovoltaic elements according to the inventive and comparative samples.

As can be seen from FIG. 10, the inventive photovoltaic element employing the polysilicon thin film according to the present invention was extremely improved in short-circuit current and fill factor (F.F.) as compared to the comparative sample. Particularly, the photovoltaic element had a short-circuit current of at least 33 mA/cm.sup.2, an open-circuit voltage of at least 0.55 V, and a current of at least 30 mA/cm.sup.2 at a voltage of 0.4 V. Thus, it is understood that the inventive polysilicon thin film is very suitable for use in a photovoltaic element.

Although phosphorus is described as a dopant for the first amorphous silicon layer in the aforementioned embodiment, the dopant is not restricted to phosphorus. Alternatively, an n-type dopant may be antimony, and a p-type dopant may be boron or aluminum, for example.

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.