Background of the Invention The compound semiconductors of gallium nitride (GaN), aluminum nitride (AlN), and indium nitride (InN) are excellent materials for bandgap engineering, because they form a continuous range of solid solutions and superlattices with direct room-temperature band gaps ranging from 1. 9eV for InN, to 3.4eV for GaN, to 6.2eV for AlN. Recently a great deal of interest has been shown especially in the In, : Gal_ :. : N due to its worldwide demand for high-brightness blue and green light-emitting diodes (LEDs) and laser diodes (LDs) (see: S. Nakamura et al., Appl. Phys.

Lett., 64: 1687,1994).

The group XIII nitride semiconductor thin film has been mainly prepared by chemical vapor deposition (CVD) routes involving the reaction of either a metal halide or metal alkyl with ammonia as a nitrogen source (separate source CVD). Although significant progress has been made, a major process limitation still exists in that the high thermal stability of ammonia still necessitates the use of very high substrate temperatures (typically in excess of 900 C), which leads to high concentrations of nitrogen

vacancies (and hence high n-type background doping levels) in the deposited material, even when V/III ratios as high as 2000: 1 are used. Therefore, ammonia-based CVD reactions are severely limited by the nitride material p-doping capability arising from the presence of the n-type nitrogen vacancies, the highly inefficient use of toxic ammonia gas, and the resulting requirement to install expensive exhaust-gas scrubbing systems. Besides, when grown to the multi-layer thin film, thermally unstable films cannot be deposited on the same substrate because the interlayer diffusion occurs more rapidly at a high temperature. Secondly, it is difficult to control the chemical composition of thin film because more than two precursors with different vapor pressures are used.

Thirdly, trimetylmetal and ammonia used as the thin film precursors are difficult to deal with due to their high reactivity and toxicity (see: S. Stride and H. Morko, J.

Vac. Sci. Technol., 10: 1237,1992).

To overcome these problems, alternative group 13- nitrogen single-source precursors are now being investigated in an effort to achieve group III nitride growth at significantly lower temperatures and V/III ratios. Single-source precursors, containing both the metal and nitrogen atoms which will combine to form the metal nitride, can offer several advantages over the separate source CVD routes: First, if the correct stoichiometric ratio of M to N is possessed by the precursor, then this ratio can be retained in the metal nitride thin film produced from the precursor; therefore the facile formation of the thin film with exact composition is possible. Secondly, chemical bonds between metal and nitride already exist so that the surface diffusion and the activation energy for the bond formation among the elements on the surface of the substrate are not much required. Thirdly, single-source precursors have very low reactivity and toxicity, and are easy to deal with and to purify by recrystallization or sublimation. In addition,

the deposition temperature of the thin film is relatively low to make it possible to use thermally unstable materials as subs. rates and to prevent interlayer diffusion. As examples, a single-source precursor [ (Me2N) (N3) Ga (u-NMe2)] 2 has been used to prepare a gallium nitride thin film at 580 C (see: D. A. Neumayer et al., J.

Am. Chem. Soc., 117: 5893,1995) and another single-source precursor [(N3) 2Ga (CH2CH2CH2NMe2)] has been used for the preparation of a gallium nitride thin film at 750C (see: R.

A. Fischer et al., J. Cryst. Growth, 170: 139,1997).

However, even though the thin films described above are prepared at lower temperatures than that of prior art, the interlayer diffusion and the decrease in quality due to the vapor pressure decrease and the precursor decomposition are still to be solved. Besides, the unit cost of production is relatively high because sapphire is used as the substrate for the thin film deposition.

Therefore, there are strong reasons for developing a process for preparing a thin film at lower temperatures in an economical manner, while overcoming the interlayer diffusion and the quality decrease of the thin film.

SUMMARY OF INVENTION The present inventors have made an effort to develop an economical process for preparing a thin film at lower temperature to overcome the interlayer diffusion and the quality decrease of the thin film, and discovered that metal nitride thin films can be prepared by the deposition of XIII group metal nitride compounds including gallium nitride onto a silicon substrate using amine-adduct precursors of R2 (N3) M: D.

A primary object of the present invention is, therefore, to provide a process for preparing metal nitride thin films employing amine-adduct single-source

precursors.

The other object of the present invention is to provide metal nitride thin films prepared by the process.

DETAILED DESCRIPTION OF THE INVENTION The crystal structure of a multi-layer thin film is generally known to depend on the types and orientation of substrate used. To obtain hexagonal gallium nitride thin films has been usually used sapphire as the substrate, especially with the c-faced crystal structure, since the sapphire is stable at a high temperature, easy to pre- treatment, and has a hexagonal symmetry. On the other hand, the use of silicon substrate makes sure that, compared to insulating sapphire, the post electrode formation is facilitated, the change of the substrate to have a larger diameter is possible, and the final elements are easily separated.

The process for preparing metal nitride thin films employing amine-adduct single-source precursors of the present invention comprises the steps of: placing an amine-adduct single-source precursor (I) onto a substrate, heating at 350 to 400 under a pressure of 0.5 X 10-7Torr and vaporizing the amine-adduct single-source precursor (I); controlling the vapor pressure of the single-source precursor from 1.0 X 10 6 to 3.0 X lO-6Torr followed by chemical deposition for 1.5 to 2.0 hours to form a buffer layer; and, subsequent chemical deposition for 12 to 24 hours under a pressure of 1.0 X 10-6 to 3.0 X 10~6Torr to prepare a metal nitride thin film.

wherein, D represents NH3, NH2R, or NH2NR2 ; M represents Al, Ga, or In; and, R represents H, Me, Et, n-Pr, i-Pr, t-Bu, Cl, or Br.

The process for preparing of metal nitride thin films by the chemical deposition at low temperatures is illustrated in more details by the following steps.

Step 1 : Vaporization of single-source precursor An amine-adduct single-source precursor (I) is placed onto a substrate, heated at 350 to 400 C under a pressure -7Torr, and subsequently vaporized, where silicon, sapphire, and SiC may be preferably used as the substrate, though silicon is the most preferred. The temperature of the substrate is measured using an optical thermometer or calculated from the amount of current using a correction diagram showing the correlation between temperature and current passing though the silicon substrate.

Step 2: Formation of buffer layer A buffer layer is formed by controlling the vapor pressure from 1.0 x 10-6 to 3.0 x 10-6Torr followed by chemical deposition for 1.5 to 2.0 hours, where the buffer layer may be formed to contain GaN or A1N depending on the amine-adduct single-source precursor employed in the process.

Step 3: Preparation of metal nitride thin film A metal nitride thin film is prepared by the chemical deposition of the buffer layer for 12 to 24 hours under a pressure of 1.0 x 10 6 to 3.0 x 10-6Torr, where the thin

film preferably contains a mixture of AlN, GaN, InN, AlGaN, GaInN, AlInN, and AlGaInN. The equipment for chemical deposition of the metal nitride is not limited to special types, however, a high vacuum (10~7Torr) chemical deposition apparatus with an oil diffusion pump and liquid nitrogen traps is preferred. The high vacuum apparatus is shaped in a jointed cold wall with the copper gasket, and equipped with a flange made of stainless steal pipe and high vacuum valves to control the pressure of sample tube and precursor.

The present invention is further illustrated by the following examples, which should not be taken to limit the scope of the invention.

Example 1 : Preparation of Et2 (N3) Ga: NH3 0.88g [Et2Ga (-p-NH2) 33 was dissolved in Et20, and 0.26g hydrogen azide was added dropwise at-60C with stirring. The reaction temperature was warmed to room temperature and the solution was stirred for 2 hours.

After the completion of the reaction, the solvent was removed in vacuo to give 0.91g of colorless liquid, which was then purified by distillation to yield Et2 (N3) Ga: NH3 with a melting point of-10C.

H NMR (CDCl3, 20C) : b 0.56 (q, Ga-CH2CH3), 1.12 (t, Ga-CH2CH3), 3.05 (s, N-H); 13C NMR (CDCl3, 20C) : b 2.80 (Ga-CH2CH3), 9.24 (Ga-CH2CH3) ; MS (70eV) : m/z 140 (M-[Et+NH3]) ; IR (N3) : 2073,2254cm-1.

Example 2: Preparation of metal nitride thin film using Et2 (N3) Ga: NH3 (I) O. lg Et2 (N3) Ga: NH-was placed in a container,

silicon (111) wafer was heated at 350 C under the initial pressure of 1.0 x 10~7Torr, and the total pressure was adjusted to 3.0 10-5 Torr by controlling the vapor pressure of Et2 (N3) Ga: NH3 with metering valve, and then chemical vapor deposition was performed for 1.5 hour. The deposited metal gallium nitride thin film was blue-colored and 0.15um thick, which was confirmed by the SEM photographs of fractured sections. The X-ray diffraction analysis showed the formation of a polycrystalline GaN buffer layer. The reactor pressure was increased to 6.0 x 10~6Torr followed by chemical deposition for 12 hours to yield a black gallium nitride thin film. The SEM photographs of fractured sections revealed that the film has a thickness of 2pm, and the deposition rate was 0.15um/hr. Rutherford backscattering spectrometry (RBS) analysis showed that the thin film was consisted of 1: 1 stoichiometric ratio of gallium and nitrogen. A gallium nitride (002) peak was observed at 34.5° when X-ray diffraction analysis of the thin film was performed with changing 26 from 20'to 80°. Pole figure analysis also confirmed that the thin film has grown to the hexagonal structure. The formation of the polycrystalline buffer layer was confirmed by analyzing the TEM image, and electron diffraction analysis confirmed that the formation of gallium nitride growth as columnar structure on the buffer layer.

Example 3: Preparation of metal nitride thin film employing Et2 (N3) Ga: NH3 (II) A metal nitride thin film was prepared in an analogous manners as in Example 2, except that the silicon wafer was heated at a temperature of 400C. As the result, a black gallium nitride was prepared with a thickness of 2.2um and the deposition rate of 0.16um/hr, which was measured by the SEM photographs of fractured sections. The

other characteristics of the deposited thin film were identical to the thin film prepared in Example 2.

As clearly described and demonstrated as above, the present invention provides a process for preparing metal nitride thin films by chemical deposition at low temperatures employing amine-adduct single-source precursors. In accordance with the present invention, the chemical deposition is performed at low temperatures with a relatively cheap silicon substrate instead of expensive sapphire, which makes it possible the economical preparation of the nitride thin film. Furthermore, since the substrate is silicon semiconductor instead of sapphire insulator, the electrode can be easily formed on the backside of the substrate.

Although the preferred embodiments of present invention have been disclosed for illustrative purpose, those who are skilled in the art will appreciate that various modifications, additions, and substitutions are possible, without departing from the spirit and scope of the invention as disclosed in the accompanying claims.