GROUP-III METAL NITRIDE AND PREPARATION THEREOF FIELD OF THE DISCLOSED TECHNIQUE The disclosed technique relates to group-Ill metal nitride, in general, and to preparation of a group-!l( metal nitride film, in particular. BACKGROUND OF THE DISCLOSED TECHNIQUE An amorphous material is a solid in which the atoms exhibit no long range order and are bound to each other irregularly, as opposed to a crystalline material, which has a regular repeating internal structure. An example of an amorphous material is ordinary window glass, which is formed when molten silicate with high viscosity is cooled, without allowing a regular crystal lattice to form. The amorphous state of the glass results in various useful optical properties, such as its transparency. The presence of various contaminants and impurities may have a significant influence on the final properties of the amorphous material (e.g., its color, transparency, melting point). Group-Ill metals of the periodic fable (i.e., aluminum, gallium and indium) can form nitrides, i.e., aluminum nitride (AIN), gallium nitride (GaN) and indium nitride (InN). Group-Ill metal nitrides are semiconductors having various energy gaps (between two adjacent allowable bands), e.g., a narrow gap of 0.7eV for InN, an intermediate gap of 3.4eV for GaN, and a wide gap of 6.2eV for AIN. Solid group-Ill metal the difference in lattice parameters between the substrate and the GaN layer, various crystal defects may appear in the GaN crystal. Other known methods for growing group-Ill metal nitride crystals employ a metallic melt, typically of the group-Ill metal. Nitrogen is supplied to the melt and chemically reacts with the group ill metal in the melt, thereby enabling crystal growth. Such methods are often expensive, and the crystal dimensions achieved, as well as the quantity of crystals produced, are typically small for industrial applications. Group-Ill metal nitride crystals, manufactured according to methods known in the art usually have crystal defects therein, such as dislocations, misorientations, vacancies, interstitial atoms, impurities, and grain boundaries. In particular, none of the above mentioned methods are used to produce GaN crystal sheets of large dimensions, having a low defect density of less than 10"^ defects per centimeter squared. Amorphous group-Ill metal nitrides have certain useful optical properties, making them possible candidates for a variety of applications, such as solar batteries and full color displays. Techniques for preparation of material films include: thin-film deposition processes (e.g., sputter deposition and chemical vapor deposition), Molecular Beam Epitaxy (MBE), and ion implantation. Thin-film deposition involves depositing a thin film onto a substrate, or on previously deposited layers on the substrate. MBE is a method for epitaxially growing layers of materials onto a substrate, by slowly directing a beam of particles toward the surface of the substrate. MBE generally requires a high vacuum in the reaction chamber, in order to avoid impurities in the epitaxially formed material. The epitaxy deposition rate in MBE is considered slow, relative to other deposition techniques. Sputter deposition is one type of thin film deposition technique. The atoms in a solid target material are ejected into a gas phase by ion bombardment. Each collision knocks off additional atoms, where the number of ejected atoms per incident ion (i.e., the sputter yield) is dependent on several factors, such as the energy of the incident ions, the respective masses of the ions and atoms, and the binding energy of the atoms in the solid. The ions are provided by a plasma, usually of a noble gas (e.g., argon). The ejected atoms are not in their thermodynamic equilibrium state, and tend to deposit on all surfaces in the vacuum chamber. Therefore a substrate in the chamber will end up being coated with a thin film having the same composition of the target material. The target can be kept at a relatively low temperature during sputter deposition, since no evaporation is involved, in reactive sputtering, the plasma gas includes a small amount of a non-noble gas, such as oxygen or nitrogen, which reacts with the material after it is sputtered from the target, resulting in the deposited material being the product of the reaction, such as an oxide or nitride. Chemical vapor deposition (CVD) is another type of thin film deposition, where the film is formed by a chemical reaction. The substrate is exposed to a mixture of gases, which reacts with the substrate surface to produce the desired deposit, which condenses on the substrate. CVD can be performed at medium to high temperature in a furnace, or in a CVD reactor in which the substrate is heated. Unwanted byproducts are usually also produced in the reaction, which are removed by gas flow through the reaction chamber. Plasma may be used to enhance the rates of chemical reaction. Metal-organic chemical vapor deposition (MOCVD) involves organo-metallic compounds as the reactants. Ion implantation involves implanting ions of a first material in a second target material. The ions are electrostatically accelerated to a high energy, before impinging on the target material, such as on the surface of a substrate. The amount of material implanted, known as the dose, is the integral over time of the ion current. By controlling the dose and the energy, along with the applied temperature of the target, it is possilbe to change the crystal structure of the target surface in such a way that an amorphous layer is formed. The impinging ions break chemical bonds within the target material, and form new bonds which are ) unorganized and not in thermodynamic equilibrium, resulting in the target material becoming amorphous. US Patent No. 5,976,398 to Yagi, entitled "Process for manufacturing semiconductor, apparatus for manufacturing semiconductor, and amorphous material", is directed to an amorphous nitride lll-V compound semiconductor, and an apparatus and process for its manufacture. The manufacturing process utilizes plasma-enhanced MOCVD. The semiconductor manufacturing apparatus includes a reactor, a first and second activation-supply portions, an exhaust pipe, a heater, and a substrate holder. The substrate holder holds a substrate inside the reactor, which is allowed to form a vacuum. Each activation-supply portion is composed of a pair of gas introducing pipes, a quartz pipe connected with the reactor, and a microwave waveguide (or alternatively, a radio frequency coil) for providing activation. Plasma of a V group element (e.g., nitrogen plasma) is generated at the first activation-supply portion and introduced into the reactor. For example, N2 gas is introduced from the gas introducing pipe, and a microwave oscillator supplies microwaves to the microwave waveguide, which induces a discharge in the quartz pipe and activates the N2 gas. A metal organic compound containing a 111 group element (e.g., Ai, Ga, In) is supplied by a gas introducing pipe of the first activation-supply portion. An auxiliary material (e.g.. He, Ne, Ar, H?, CI2, FI2) is supplied by the gas introducing pipe of the second activation-supply portion. The auxiliary material (e.g., hydrogen plasma) reacts with an organic functioning group of the metal organic compound, including the 111 group element, to inactivate the organic functional group. The vaporized metallic organic compound and the plasma of the auxiliary material is added to the plasma of the V group element. The heater heats the substrate to the appropriate temperature (e.g., from 200°C to 400°C). A film of amorphous material, containing the III group element and the V group element, is formed on the substrate. The film of the semiconductor compound contains the III group element and the V group element. For example, the amorphous material is hydrogenated amorphous gallium nitride. The amorphous material is suitable as an optical semiconductor for optoelectronic applications. US Patent Application Pub. No. US 2002/0100910 to Kordesch, entitled "Band gap engineering of amorphous Al-Ga-N alloys", is directed to an amorphous semiconductor alloy including aluminum and gallium, and a method for its production, which utilizes sputter deposition. A semiconductor substrate is positioned on an anode inside a reactive sputter deposition chamber. The sputter deposition chamber also includes a sputter target on a target cathode. The sputter deposition chamber is coupled with an Rf source and a matching network. The sputter target contains aluminum and gallium (e.g., a single integrated target with both aluminum and gallium, a single target with an aluminum portion and a gallium pnitinn, or discrete targets of aluminum and gallium). The sputter target may also contain indium. Nitrogen gas is introduced into the sputter deposition chamber. The sputter deposition chamber is operated to promote reaction of the aluminum and gallium of the sputter target with the nitrogen. The semiconductor substrate is maintained at a deposition temperature (e.g., between about 77K to about 300K), selected to ensure that the grown alloy is amorphous. The relative proportions of aluminum and gallium are selected such that the amorphous alloy will have a band gap between about 3eV and about 6eV. The amorphous alloy has the chemical formula: AlxGa^xN, The amorphous alloy may be doped, such as with a rare earth luminescent center, for various photonics applications. SUMMARY OF THE DISCLOSED TECHNIQUE In accordance with the disclosed technique, there is thus provided a method for forming a group-Ill metal nitride material film attached to a substrate, the method including the procedures of subjecting the substrate to an ambient pressure of no greater than 0.01 Pa, and heating the substrate to a temperature of between approximately 5D0°C-800°C, The method further includes the procedures of introducing a group 111 metal vapor to the surface of the substrate at a base pressure of at least 0.01 Pa, until a plurality of group III metal drops form on the surface, and introducing active nitrogen to the surface at a working pressure of between 0.05 Pa and 2.5 Pa, until group III metal nitride molecules form on the group III metal drops. The method also includes the procedures of maintaining the worl