Silicon Thin Film and Method of Producing the Same

FIG. 1 is a schematic view showing an example of an apparatus for carrying out the method of making silicon thin films according to this invention;

FIGS. 2 to 5 are graphs showing the characteristics of the silicon thin films according to this invention;

FIG. 6 is a diagram showing the infrared absorption spectra of a silicon thin film according to this invention; and

FIG. 7 is the Raman spectrum of a silicon thin film according to this invention.

Next, the characteristics of the silicon thin film according to this invention and examples of the method of making the film will be described in connection with the drawings.

Referring to FIG. 1, the whole system including a mixing chamber 1 is evacuated to a degree of vacuum of about 10.sup.-6 torr using a rotary oil pump 2 and an oil diffusion pump 3, and gases are introduced from a silane cylinder 4 and a hydrogen cylinder 5, and from dopant gas cylinder 6 or 7 as required, to the mixing chamber 1 at a required proportion and are mixed there. The gas mixture is supplied through a flow rate meter 8 to an evacuated chamber 9 at a predetermined flow rate. The pressure or degree of vacuum within the chamber 9 is maintained at a required value by manipulating a main valve 10 while observing a vacuum meter 11. A high frequency voltage is applied across electrodes 13 and 13' to produce a glow discharge. A substrate 15 is placed on a substrate holder which is heated by a heater 14, to a required temperature. Thus, a doped hydrogenated silicon thin film is produced on the plate 15.

Table I illustrates the examples of the method of producing the conventional amorphous films (hereinafter referred to as "conventional amorphous films") and silicon thin films having less than 80% of microcrystalline grains interspersed in an amorphous phase (hereinafter referred to as "conventional mixed-phase films"), and the characteristics of the films formed. Table II illustrates the examples of the method of producing films according to this invention and the characteristics of the formed films in comparison with the conventional methods and films.

In Table I, the samples designated by Nos. 1 to 8 are conventional P type amorphous films and the samples designated Nos. 9 to 12 are conventional P type mixed-phase films. The samples designated by Nos. 13 to 17 and 19 are conventional N type amorphous films, the samples designated by Nos. 18 and 20 to 22 conventional N type mixed-phase films, and the sample designated by No. 23 an I type mixed-phase film. Further, Table I describes film preparation conditions and film characteristics thereof as well.

On the other hand, in Table II the samples designated by Nos. 30 and 31 are P type silicon thin films which were prepared using the method of this invention. In these examples, the silane (SiH.sub.4) was diluted with hydrogen at a ratio of hydrogen to silane of 100:1 and a low electric power of 0.2 W/cm.sup.2 at a high reaction pressure of 0.5 torr, was applied. The samples designated by Nos. 32 and 33 are N type silicon thin films which were prepared according to this invention and there are described film preparation conditions and film characteristics thereof. In Nos. 32 and 33, the silane (SiH.sub.4) was diluted with hydrogen at a ratio of hydrogen to silane of 80:1, and a low electric power of 0.2 W/cm.sup.2 at a high reaction pressure of 5.0 torr was applied.

In the examples, the content of the microcrystalline substance in the mixed amorphous and microcrystalline layer was estimated by the following procedure.

First, prior to annealing, a sample was evaluated for the peak height which was obtained by X ray diffraction. Next, the sample was annealed at a temperature of 700.degree. C. for an hour, and the peak height at about 2.theta.=27.degree.-28.degree. which was obtained by the X ray diffraction was used as a reference, that is 100 percent. The volume fraction of the microcrystallihe phase was computed as the ratio cf the peak height prior to the annealing to that after the annealing.

FIG. 2 is a graph showing the electrical conductivity of the silicon thin films according to this invention as a function of the concentration of dopant gas. In FIG. 2, the curves A and B show the electrical conductivity of the conventional P type and N type amorphous films respectively, produced by the conventional method, in which the films were formed at a cathode plasma discharge power density (Plasma discharge power/area of cathode electrode) of about 0.1 W/cm.sup.2. The points 9 and 10 show the electrical conductivity of the conventional P type mixed-phase films of sample Nos. 9 and 10, respectively in which the silane (SiH.sub.4) was diluted with hydrogen at a ratio cf silane to hydrogen of 1:30, and 2% by volume of diborane (B.sub.2 H.sub.6) to silane was added to the gas mixture while plasma discharge power densities of 0.8 W/cm.sup.2 and 1.6 W/cm.sup.2, respectively were applied. The points 18 and 20 in FIG. 2 show the conductivity of the N type silicon thin films of sample Nos. 18 and 20, respectively, in which silane was diluted with hydrogen at a ratio of silane to hydrogen of 1:10, and for the point 18, 1% by volume of phosphorus pentafluoride (PF.sub.5), and for the point 20, 4500 ppm by volume of phosphine (PH.sub.3) were added while power densities of 0.8 W/cm.sup.2 and 1.6 W/cm.sup.2, respectively, were used. The points 30 and 31 in FIG. 2 show the electrical conductivity of the P type silicon thin films produced by the method of this invention, in which the silane (SiH.sub.4) was diluted with hydrogen at a ratio of silane to hydrogen of 1:100, and for the point 30, 2000 ppm by volume of diborane (B.sub.2 H.sub.6) and for the point 31, 5000 ppm by volume of diborane (B.sub.2 H.sub.6) were added to the gas mixture while a plasma discharge power density of 0.2 W/cm.sup.2 at a reaction pressure of 5.0 torr was applied. The points 32 and 33 in FIG. 2 show the conductivity of the N type silicon thin films produced by the method of this invention, in which silane was diluted with hydrogen at a ratio of silane to hydrogen of 1:80, and for the point 32, 2000 ppm by volume of phosphine PH.sub.3), and for the point 33, 5000 ppm by volume of phosphine (PH.sub.3) were added while a power density of 0.2 W/cm.sup.2 at a reaction pressure of 5.0 torr was used. It is clear from FIG. 2 that the conductivity of the silicon thin films of this invention is increased at least by one order of magnitude in comparison with that of the conventional mixed-phase films.

FIG. 3 shows the activation energy on the basis of the electrical conductivity of the films of this invention as a function of the concentration of dopant gas. The curves A and B in FIG. 3 represent the activation energy of the conventional amorphous films. The points 9, 10, 18 and 20 represent the activation energy of the conventional mixed-phase films of sample Nos. 9, 10, 18 and 20, respectively. The points 30, 31, 32 and 33 represent the activation energy of the films produced by this invention, in which the film preparation conditions corresponding to the points 30, 31, 32 and 33 in FIG. 2 were used, respectively. FIG. 3 substantiates that this invention provides a P.sup.+ type or N.sup.+ type film having a sufficiently low activation energy on the basis of electric conductivity, a good ohmic contact to metal, and an adequately degenerated Fermi level.

FIG. 4 is a graph showing the concentrations of boron and phosphorus in the silicon thin films of this invention as a function of the concentration of dopant gas, as measured by the SIMS and EPMA methods. The curves A and B represent the conventional amorphous films produced by the conventional methods, in which the film preparation conditions corresponding to the curves A and B in FIG. 2 were used. The points 9, 10, 18 and 20 in FIG. 4 represent the conventional mixed-phase films of sample Nos. 9, 10, 18 and 20, respectively.

FIG. 4 shows that the concentration of boron and phosphorus in the silicon thin films of this invention is lower than that of the conventional amorphous films, and substantiates that the film of this invention has an excellent characteristic and the method according to this invention brings about an excellent doping efficiency.

FIG. 5 shows the optical band gap of the P type silicon thin film of this invention as a function of the concentration of dopant gas. The optical band gap is calculated on the basis of .sqroot..alpha..nu..varies.(hv-Eo), wherein o represents the absorption coefficient; hv the incident photon energy (eV); and Eo the optical band gap. The curve A in FIG. 5 is related to the conventional amorphous films produced by the conventional method, in which the film preparation conditions corresponding to the curve A in FIG. 2 were used. It shows that as the concentration of boron increases, the optical band gap decreases. On the other hand, the points 30 and 31 are concerned with the silicon films of this invention, in which the film preparation conditions corresponding to the points 30 and 31 in FIG. 2 were applied, respectively. FIG. 5 shows that the P type silicon thin films of this invention have a high electrical conductivity without the optical band gap being narrowed.

FIG. 6 shows an example of the infrared absorption spectrum of the silicon thin film of this invention in comparison with the conventional. The curve A in FIG. 6 is the present silicon thin film designated by sample No. 33 and curve B in FIG. 6 is the conventional mixed-phase film designated by sample No. 18.

FIG. 6 makes it clear that the content of hydrogen in the film of this invention is less than 5 atm % whereas that in the conventional mixed-phase film is about 10 atm %.

FIG. 7 shows the Raman spectra of the silicon thin film of the present invention and the conventional. The curve A represents the present silicon thin film designated by sample No. 33, the curve B the conventional mixed-phase film designated by sample No. 18, and the curve C the conventional amorphous film designated by sample No. 16.

Clearly, it can be observed in FIG. 7 that the silicon thin film according to the present invention has a strong peak of the microcrystalline phase at wave number 520 cm.sup.-1 in comparison with the conventional. This means that the present silicon thin film has more than 80% of crystallinity. The crystallinity of the conventional mixed-phase film represented by curve B was about 55%. Industrial Applicability

According to this invention, there can be provided P type and N type silicon thin films having a high doping efficiency and a high electrical conductivity, which have a wide application. Especially, the P type silicon thin films are useful for solar cells and the like, since it can provide a high electrical conductivity without the optical band gap being narrowed. Thus, this invention has great advantages when used in the electronic industries.

Further, though the present invention has been described above with respect to P type and N type silicon thin films having a high electrical conductivity, this invention is not limited to those silicon thin films but undoped silicon thin films, that is, I type silicon thin films with superior properties in comparison with the conventional ones, can be suitably and efficiently provided (refer to sample No. 34 in Table II as prepared according to this invention in comparison with the conventional film, sample No. 23).