JPS649399B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to articles having electrically conductive coatings that are highly transparent to visible light and highly reflective to infrared radiation. Such layers are useful as transparent electrodes for solar cells, photoconductor tubes, liquid crystal electro-optic displays, photoelectrochemical cells, and many other types of opto-electronic devices. Such layers are used as transparent electrical resistors for defrosting airplane and automobile windows. Such layers increase the efficiency of windows and fiberglass insulation in solar thermal integrators and buildings, ovens, furnaces and sodium vapor lamps as heat-reflective transparent coatings on glass.

Various metal oxides such as stannic oxide SnO ₂ , indium oxide In ₂ O ₃ and cadmium stannate Cd ₂ SnO ₄ have been most widely used as materials for forming transparent conductive coatings and layers. .

Current methods of applying these coatings have been based on spraying metal salt (usually chloride) solutions onto heated surfaces such as glass. This method first produced a transparent electrically resistive layer that was satisfactory for dewatering airplane windows. However, this method of atomization produced rather corrosive byproducts, namely hot chlorine and hydrogen chloride gases, which tended to attack the hot glass surface and give it a foggy appearance. US Pat. No. 2,617,745 teaches that such undesirable effects can be alleviated by first applying pure silica on the glass. However, the silica protective layer is not so effective for glasses with high alkali content and high coefficient of thermal expansion, such as common soda-lime glasses. Additionally, these corrosive by-products corrode the metal parts of the device, and metallic impurities such as iron are then deposited into the coating, adversely affecting both the conductivity and transparency of the coating.

Another problem is the lack of coating uniformity and reproducibility. In US Patent No. 2,651,585,
It is taught that greater uniformity and reproducibility can be achieved by controlling the humidity within the apparatus. More uniform and reproducible coatings are also obtained by using steam rather than liquid spray, as disclosed for example in German Patent No. 1,521,239.

Despite these improvements, recent research has been conducted using vacuum deposition techniques such as evaporation and vapor deposition to obtain brighter and more reproducible coatings. Although such vacuum treatments are very costly, reducing corrosive by-products and undesirable impurities produced by atomization methods is considered particularly important in applications involving high-purity semiconductors. ing.

In order to obtain high conductivity and high infrared reflectivity in such a method, it is important to intentionally add certain impurities. Thus, tin impurities are added to indium oxide, while antimony is added to tin oxide (stannic oxide) for this purpose. In each case, the function of such desirable impurities or "doping agents" is to provide "extra" electrons that contribute to conductivity. The solubility of these impurities is high;
And these impurities can be easily added using all of the precipitation methods described above. Fluorine is more advantageous than antimony as a doping agent for tin oxide, in which case the transparency of the fluorine-doped stannic oxide film is greater than that of the antimony-doped one, especially in the visible spectrum. Excellent at the red end. This advantage of fluorine is important for applications in solar cells and solar thermal integrators. Despite these advantages of fluorine, most, perhaps all, industrially available tin oxide coatings use antimony as a doping agent. Perhaps the reason for this is that doping with fluorine has only been experimented with unsatisfactory spray methods, while improved deposition methods (chemical vapor deposition,
It has probably not been shown until now that fluorine doping (vacuum evaporation/deposition) results in fluorine doping. In addition, The American Institute
The American Institute of Physics Conference Proceedings
Physics Conference Proceedings) No. 25, P.
288 (1975) concluded that the equilibrium solubility of fluorine in tin oxide is inherently lower than that of antimony. Nevertheless, it has been found that the lowest resistance tin oxide film reported in the prior art is that of Gillery, US Pat. No. 3,677,814. Gillery uses a spray method and a compound in which tin and fluorine are directly bonded as starting materials to obtain fluorine-doped tin oxide films with resistances as low as 15 Ω/□. The lowest resistance of industrially available tin oxide coated glass is currently about 40Ω/□. Achieving coverages as low as 10 Ω/□ has so far required the use of significantly more expensive materials such as indium oxide.

A first object of the present invention is to provide a product having a fluorine-doped stannic oxide layer, i.e. a stannic oxide coating, which has high visible light transmission, high electrical conductivity and high infrared reflection power. It is.

A second object of the present invention is to be able to easily vary the electrical conductivity of a single layer of the above-mentioned layers during deposition, and to have the ability to achieve very low volume resistivities and surface resistivities. That's true.

A third object of the invention is to provide improved products such as solar cells, other semiconductors useful in electrical circuits, heat reflective windows, improved sodium lamps, and the like.

Other objects and advantages of the invention will become apparent from the following description.

That is, the present invention has a generally transparent substrate, such as glass, and a fluorine-doped stannic oxide film on the substrate, which has an infrared reflectance of 90%, and has an infrared reflectance of 0.007 to 0.03. A product having a fluorine:oxygen ratio and a film free electron concentration of 10 ²⁰ cm ^-3 to 10 ²¹ cm ^-3 is provided.

The products of the present invention can be produced by selecting reactants such that the necessary tin-fluorine bonds do not form until, for example, just before precipitation. Thus, the tin fluoride material is in the vapor phase and the oxidation of the compound is
It can be held well enough at temperatures low enough to occur only after dislocations that form tin-fluorine bonds. The fluorine-doped tin oxide thus formed has an unusually low electrical resistivity and an unusually high reflectivity for infrared wavelength light.

The products of the invention can also be prepared using gaseous mixtures containing volatile organotin-fluorine-containing compounds that do not have any direct tin-fluorine bond. This mixture also contains volatile oxidizable tin compounds and oxidizing gases. This first fluorine compound without a fluorine-tin bond is converted into a second organotin fluoride compound having a fluorine-tin bond. Immediately after such conversion, the second compound is oxidized to form a fluorine doping, and the doping agent is oxidized with an oxidizable tin compound to form a predetermined amount of fluorine impurity on the solid substrate. Forms a stannic oxide film containing.

In a first embodiment of producing the products of the invention, the organotin monofluoride vapor is regenerated in a heated precipitation zone into a vapor of a more volatile compound containing both tin and a fluoroalkyl group attached to the tin. Let me make it.

In a second advantageous embodiment of producing the product of the invention, organotin vapor and certain fluorine-containing gases having fluoroalkyl and/or sulfur fluoride groups are present at or near the gas-substrate interface. It utilizes organotin monofluoride formed by a reaction involving.

The resulting layer in either case is a uniform, hard-adhered, transparent film whose electrical conductivity and infrared reflectance vary depending on the concentration of the fluorine-containing doping agent.

The method has two main steps. The first step is to form a reactive vapor mixture that upon heating produces a compound with tin-fluorine bonds, and the second step is to transfer this vapor mixture to a heated surface and to form a reactive vapor mixture that produces a compound having a tin-fluorine bond. This is a process in which tin oxide doped with fluorine is precipitated. The embodiments described below differ in the chemical source of the fluorine doping agent in the reactive vapor mixture, as well as in the means of making the vapor mixture. The second step (deposition on the heated surface) is almost identical in each example.

Tin is provided by volatile oxidizable tin compounds such as tetramethyltin, tetraethyltin, dibutyltin diacetate, dimethyltin dihydride, dimethyltin dichloride, and the like. Tetramethyltin is a preferred compound because it is sufficiently volatile at room temperature, noncorrosive, stable, and easy to purify. The volatile tin compound is placed in a bubbler, shown at 10 in the figure, and an inert carrier gas, such as nitrogen, is bubbled through the tin compound. For highly volatile compounds such as tetramethyltin and dimethyltin dihydride, this bubbler may be used at room temperature, whereas for other less volatile compounds,
The bubbler and its tubing must be heated appropriately, as will be apparent to those skilled in the art. One advantage of the above method is that it does not require the use of high temperature equipment and that a simple cold-wall feeder can be used.

The vapor mixture must contain an oxidizing gas such as oxygen, nitrous oxide, etc. Oxygen is a preferred gas because it is readily available, works well, and can replace more expensive oxidizing agents.

The pressure of the gas is kept constant by a regulator 25, and the flow rate of oxygen from tank 20 and carrier gas from tank 21 is regulated by a metering valve 30 and measured by a flowmeter 40. This gas stream is then transferred through a one-way tick valve 50 to a mixing tube 60 and a furnace-shaped chamber 70. A tin oxide film is deposited on the hottest surface 80. This surface is heated by heater 90 to a temperature typically between about 400°C and 600°C.

Methods of the general type described above are commonly known in the art as chemical vapor deposition. Various modifications are known to those skilled in the art, such as having a vertical and rotating substrate surface or rotating beneath the reaction chamber, and such modifications may affect the geometry of the substrate or the intended use. Particularly suitable for use that varies depending on conditions.

It is better to rotate the substrate in order to move it sufficiently into the convection currents that may occur within the device.
By doing so, the most uniform deposited layer can be obtained. However, the inventors have discovered that by positioning the heated substrate facing downward, a highly uniform coating can be obtained more easily and without rotating the substrate. The reason for this is that when heated as described above, the gas does not cause troublesome convection. Placing the substrate above the reaction vapors has the additional advantage that dust, the product of uniform nucleation in the gas, does not fall onto the growing film.

The above manufacturing method is an improved method by which a predetermined amount of fluorine impurity can be introduced into the growing tin oxide film. In its simplest embodiment, the fluorine doping agent is a vapor containing one tin-fluorine bond in each molecule. The other three valences of tin are organic groups and/
Or it is filled with halogen other than fluorine. A typical example of such a compound is tributyltin fluoride. The fluorine thus bound and available in vapor form to the hot surface does not separate from the tin during oxidation at the hot surface.

Unfortunately, all known compounds with such a direct tin-fluorine bond are not sufficiently volatile near room temperature.

The advantages of the above manufacturing method are achieved by forming the fluorine doping agent from a volatile compound that does not have the necessary tin-fluorine bond, but which, upon heating, rearranges to form a direct tin-fluorine bond. It will be done. Since this rearrangement advantageously occurs at sufficiently high temperatures (e.g. above 100°C), the tin fluoride thus obtained remains in the vapor phase, and also advantageously occurs at sufficiently low temperatures (e.g. below 400°C). , oxidation of this compound occurs only after rearrangement. One example _of such a compound is trimethyltrifluoromethyltin ( _CH3 ) _3SnCF3 . When this compound is heated to a temperature of approximately 150° C. in a heated region adjacent to the deposition surface 80, it rearranges to form a direct tin-fluorine bond in the (CH ₃ ) ₃ SnF vapor, which in turn causes the compound (CH 3 ₃ ) _3SnF reacts as a fluorine donor, that is, a fluorine doping agent. Other compounds that undergo similar rearrangements at different temperatures, of course, vary from compound to compound, but have the general formula R ₃ SnRF, where R is a hydrocarbon group and RF is A fluorine hydrocarbon group having at least one fluorine atom bonded to a bonded carbon atom). The main advantage of these fluorine doping agents is that these compounds are volatile liquids and therefore sufficient vapor pressure is easily obtained when evaporated at room temperature. These advantages eliminate the need to heat the area between the bubbler 15 and the reaction chamber 70 to keep the fluorine dopant in the vapor phase, as shown in FIG. becomes easier. The apparatus may thus be of the type commonly referred to as a "cold wall chemical vapor deposition reactor." This type of equipment is widely used in the semiconductor industry for depositing silicon, silicon dioxide, silicon nitride, etc., for example. Another important feature of this "cold wall reactor" for semiconductor applications is the ability to minimize low concentrations of unwanted impurities in both the substrate and the deposited film. Similarly, in glass manufacturing, this gas mixture can be added to annealing and cooling furnaces at a stage when the glass is at a suitable temperature, for example about 470° C. for soft glasses. In this way, highly uniform films can be obtained within conventional glass manufacturing equipment.

A preferred compound for use in the embodiment of FIG. 1 is (CH ₃ ) ₃ SnCF ₃ because this compound is more volatile than compounds with more carbon atoms. This compound is a stable, colorless, non-corrosive liquid that does not decompose in air at room temperature and
It only reacts extremely slowly with water.

A second embodiment of the above method uses a fluorine-containing gas which, when heated, reacts with organotin vapor to produce tin fluoride vapor. For example, a tetrafluoroalkyl halide whose alkyl group preferably has 4 or fewer carbon atoms, such as gases such as iodotrifluoromethane CF ₃ I, CF ₃ CF ₂ I, C ₃ H ₇ I, etc. Organic tin vapors such as methyltin vapor (CH ₃ ) ₄ Sn and room temperature, i.e. 32.2°C (90°F),
and more preferably at temperatures up to 65.6°C (150°F) without any reaction. Furthermore, fluoroalkyl bromides such as CF ₃ Br and C ₂ F ₅ Br are also useful as fluorine-containing gases.
These gases are less reactive and require about 10 to 20 times more in the reactant gas, but on the other hand they are very cheap. This is particularly surprising since it is common knowledge that such compounds are inert. Fluoroalkyl chloride is
It is not preferred to use it because its reactivity is substantially lower than even bromide.

When a vapor mixture as described above approaches a heated surface, a reaction takes place in the gas phase, eventually producing the desired tin-
A fluorine bond occurs. Although the reaction sequence is complex, it begins with a reaction such as CF ₃ I + R ₄ Sn→R ₃ SnCF ₃ +RI, producing vapors of organotin fluoroalkyl R ₃ SnCF ₃ in the region near the interface of the hot surface, where these It is believed that the compound serves as a fluorine doping agent for growing tin oxide films just as in the first embodiment.

Certain other fluorine-containing gases also work in the second embodiment above. For example, sulfur pentafluoride monochloride SF ₅ C is an effective fluorine donor gas, as is sulfur pentafluoride monobromide.

Similarly, trifluoromethyl sulfur pentafluoride acts to form tin-fluorine bonds through gas phase reactions.

An advantage of this second embodiment is that the fluorine donor is a gas, and the process is further illustrated in FIG. Preferred gases are CF ₃ I and CF ₃ Br;
These gases are non-corrosive, non-flammable, have no appreciable toxicity and are also readily available industrially. _SF5C and _SF5Br are highly toxic and therefore less desirable for use.
CF ₃ SF ₅ is non-toxic but somewhat less reactive than CF ₃ I.

The deposition process can be further simplified if the gas mixture is premixed and stored in a compressed gas cylinder 19, as shown in FIG. Store safely;
And for use, the oxidizable compounds mentioned above must of course be kept at a concentration that does not result in an explosive mixture. For example, the lower explosive limit for tetramethyltin in air is 1.9%. The concentration used for chemical vapor deposition is less than half of the above concentration. Furthermore, if CF ₃ I or CF ₃ Br are used as fluorine doping agents, they also act concomitantly as anti-inflammatory agents.

It has been found that the film produced by the above method has an infrared reflectance of 90% or more. This reflectivity is measured at room temperature with a typical 10 micron wavelength of light characterized by thermal infrared radiation, as is known in the art. This 90% reflectivity is 80% higher than previously achieved using tin oxide coatings.
It is better than reflex ability. In normal practice, the thickness of these infrared reflective layers is 0.2 to 1 micron, with thicknesses of 0.3 to 0.5 micron being typical.

In order to more quantitatively identify the degree of fluorine doping in the film, its infrared reflectance was measured over a wavelength range of 2.5 microns to 40 microns. "Optical Effects of Free Carriers in SnO ₂ Layers"
SnO ₂ Layers)
Zeitsschrift fur
Naturforschung), Vol. 179, pp. 789-793 (1962)
By fitting these data to the theoretical curves detailed by R. Groth, E. Kauer and PC vanden Linden in The value for was calculated. The values obtained were in the range 10 ²⁰ cm ⁻³ to 10 ²¹ cm ⁻³ and increased regularly with increasing concentration of fluorine dopant. Theoretically, one free electron should be released for each fluorine atom that replaces oxygen in the lattice. This hypothesis is
This was demonstrated by electronic spectrometry of the total fluorine concentration in Auger's film. This measurement yielded a free electron concentration and a fluorine concentration that agreed within experimental error. This agreement indicates that most of the fluorine included is electroactive.

It has been found that the infrared reflectance and volume conductivity at 10 microns of this film are maximum at doping levels that substitute about 1.5-2% fluorine for oxygen. This maximum value is very broad, and almost maximum conductivity and reflectivity are exhibited by films containing 1% to 2.5% fluorine. These films also exhibit weak and broad absorption over visible wavelengths. Therefore, a fluorine concentration in the film of about 1% (ie, a fluorine:oxygen ratio in the film of 0.01) is most desirable to produce a film with high conductivity and high visible light transparency. However, this optimality varies somewhat depending on the spectral distribution desired in a given application. By varying the fluorine dopant concentration, the optimum percentage for any given application can be easily achieved by routine experimentation.

Fluorine doping levels of 3% or more can be easily achieved in films using the method of the present invention. The results of the prior art did not exceed 1%.
The view cited earlier was that the cause of this was the solubility limit of fluorine. Although such high doping levels are not necessary for optimal infrared reflectivity or conductivity, gray films made at doping levels of 2% or higher can be used to reduce solar heat capture during air conditioning buildings. It is useful as architectural glass. In such applications, it is advantageous to reduce the doping level at the film surface to about 2% to have maximum infrared reflectance.

Using the measured electron concentration and conductivity, the electron flow mobility can be obtained. Using this method, values of 50 to 70 cm ² /volt-sec. were calculated for various films. The mobility of the conventionally obtained tin oxide film is 5
~35 cm ² /volt-sec. It is believed that the film according to the invention is the first to have a mobility exceeding 40 cm ² /volt-sec. In other words, such values are indicative of the excellent quality of the method of the present invention and the films prepared thereby.

The method of the invention is also highly desirable for the manufacture of new devices in semiconductor manufacturing, such as those having electrically conductive layers (eg, integrating circuits, etc.), and for the manufacture of heat-reflective transparent articles such as windows.

The most advantageous embodiment of the above production method is that the organotin fluoride compound having a tin-fluorine bond is decomposed on the substrate immediately after its production. This decomposition is preferably carried out in a small reaction zone which is sufficiently heated to the decomposition temperature by heat from the substrate itself.

In order to more fully illustrate the features of the invention, examples are given below to illustrate embodiments of the invention and the products obtained thereby.

Unless otherwise indicated, the specific examples disclosed below were carried out in accordance with the following general procedure.

Example 1 Using the apparatus of Figure 1, 1% tetramethyltin ( _CH3 ) _4Sn , 0.02% trimethyltrifluoromethyltin ( _CH3 ) _{3SnCF3 ,} 10% nitrogen carrier gas and the balance The invention is illustrated by an experiment in which a gas stream containing . The resulting gas stream is passed over a 15 cm diameter Pyrex glass plate maintained at 500° C. for a precipitation period of approximately 5 minutes. The flow rate of this gas flow is approximately 400 cc/min. This flow rate is
The gas recirculation rate inside is such that it is once every two minutes. A transparent film about 1 micron thick was deposited. It exhibits an electrical resistance of 2 ohms/□, which corresponds to a volume resistivity of 0.0002 ohm-cm. The film was measured to have a fluorine:oxygen ratio of 0.017 and a flow mobility of 50 cm ² /volt-pec.

Example 2 When the method of Example 1 was repeated using a sodium-free silicon substrate, the resistance value was reduced to about 1 ohm/□, or about half the resistivity obtained for the sodium-containing substrate. .

Embodiment 3 A more advantageous method using the apparatus shown in FIG. 2 will be explained. The resulting gas mixture consisted of 1% tetramethyltin (CH3) ₄ Sn, 0.2% iodotrifluoromethane CF ₃ I, 20% nitrogen carrier gas, and the remaining amount of oxygen. Films grown on Pyrex glass substrates exhibited the same electrical properties as in Example 1.

Example 4 Using the simplified apparatus shown in FIG. 3, the mixture described in Example 3 was prepared in a compressed gas cylinder 19. The results are the same as those of Example 3.
After one month of storage in a gas cylinder, the experiment was repeated with identical results. This indicates the stability and shelf life of this mixture.

Example 5 Example 3 was repeated except that the deposition was stopped when the stannic oxide film was 0.5 microns thick. The obtained stannic oxide film is approximately 90%
It had an infrared reflective ability of

Examples 6-13 Each of the following gases was used in the procedure of Example 3.
Instead of CF ₃ I, substitution was made at the equimolar ratio (however,
The concentration of fluorine doping agent was increased by a factor of 15 in Examples 6, 7, 8 and 13. ). Excellent electrical conductivity and infrared reflection ability were obtained.

Example Gas Example Gas 6 CF ₃ Br 10 C ₃ F ₇ I 7 C 2 F _{5 Br 11 SF 5 Br 8 C 3 F 7 Br 12 SF 5 Cl 9 C 2 F 5 I 13 CF 3 SF 5 Normal} Silicon photovoltaic cells (solar cells) have traditionally had surface resistances typically between 50 and 100 ohms/square. In order to have an acceptably low total cell resistance, metal grids with a spacing of 1 or 2 mm are deposited on the silicon surface. By depositing a fluorine-doped tin oxide layer with a sheet resistance of approximately 0.5 ohm/□ (approximately 2 microns thick) on the cell surface, the spacing between the metal grids can be increased to approximately 10 mm, which increases the distance between the grids. Costs can be lowered. On the other hand, the grid size can be kept small and the cell can function effectively even if the sunlight is concentrated by a factor of about 100, provided the cell is kept properly cooled.

A schematic cross section 100 of the above cell is shown in FIG. This figure shows a 2 micron N/SnO ₂ layer 1
02 (using the fluorine-doped material of the present invention), 0.4 micron n-silicon layer 10
4 (conventionally known silicon doped with phosphorus),
A 0.1 mm p-silicon layer 106 (fluorine doped silicon as known in the art) is connected to an aluminum layer 108 which serves as an electrode. The metal grids 110 are spaced approximately 10 mm apart. Nevertheless, outstanding implementation effects can be achieved.

The deposited layers can be used in the production of other semiconductor articles, such as conductors and resistors.
Tin oxide coatings have traditionally been used in integration circuits in this manner. Improving the electrical conductivity will make this material more versatile in the future. Not only has this sheet resistance range been extended to much lower values than previously possible (e.g., about 5 ohms/□ or less), but the deposition of that layer can also be used to grow epitaxial silicon, for example. This can be done in the same equipment as the This eliminates costly and complicated unloading, cleaning and mounting steps during deposition.

Resistivity values obtained for fluorine-doped tin oxide on a silicon substrate are approximately 10 ^-4 ohm-cm
, and this value is comparable to that of vaporized tantalum metal. This tantalum metal is sometimes used for connections in integrating circuits. The thermal expansion coefficients of both tin oxide and silicon are well matched, allowing the deposition of thick layers without significant strain.

FIG. 6 shows the electrical conductivity of a fluorine-doped stannic oxide film as a function of the fluorine:oxygen ratio measured in the film for deposition temperatures of 480°C and 500°C.

FIG. 7 shows the infrared reflectance of a fluorine-doped film as a function of the fluorine:oxygen ratio measured in the film for deposition temperatures of 480°C and 500°C.

Figures 6 and 7 show (1) conventionally known and ``Philips Technical Review''.
29, p. 17 (1968), and (2) doped indium oxide materials, such as those described by Van Boort and Groth (1968). The best known prior art values for the conductivity and reflectivity of ditin coatings are also shown.

While several embodiments of the invention have been described and illustrated, it will be apparent to those skilled in the art that various modifications and changes may be made to the invention without departing from the spirit of the invention or the scope of the following claims. It's quite obvious.

[Brief explanation of the drawing]

FIG. 1 shows a schematic diagram of an apparatus suitable for evaporating the fluorine doping agent, an organotin fluoroalkyl vapor, from its liquid state to obtain the product of the invention. FIG. 2 is a schematic diagram of another apparatus suitable for forming articles of the invention by reacting a fluorine doping agent with certain fluoroalkyl and/or sulfur fluoride gases supplied from a compressed gas cylinder. shows. FIG. 3 shows a simplified variant of a device suitable for obtaining the product according to the invention. FIG. 4 is a schematic cross-sectional view of a solar cell and illustrates one example of the use of the present invention in semiconductor applications. FIG. 5 shows a window 120 coated with layer 118 according to the invention.
FIGS. 6 and 7 are graphs showing conductivity and reflectivity that vary with the concentration of fluorine doping agent. 10, 15... Bubbler, 20, 21, 22...
... Tank, 2 ... Regulator, 30 ... Metering valve, 40 ... Flow meter, 50 ... Check valve, 60 ... Mixing pipe, 70 ... Chamber, 80 ... Surface, 90 ... Heater, 102 ... n- _SnO2 ,
104...n-silicon, 106...p-silicon, 108...A, 110...metal grid.

Claims (1)

[Scope of Claims] 1. A transparent substrate and a fluorine-doped stannic oxide film on the substrate, with a 90%
The film has an infrared reflectance of 0.007~
Products with a fluorine:oxygen ratio of 0.03 and a film free electron concentration between 10 ²⁰ cm ^-3 and ^{10 21} cm ^-3 . 2. Claim 1 comprising a transparent substrate and a fluorine-doped stannic oxide film on the substrate, the film having an electron flow mobility of 50 to 70 cm ² /volt-sec. products.