Light-emitting semiconductor device using gallium nitride group compound

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting semiconductor device using gallium nitride group compound which emits a blue light.

2. Description of the Prior Art

It is known that GaN compound semiconductor can be made into a light-emitting semiconductor device, such as a light-emitting diode (LED), which emits a blue light. The GaN compound semiconductor attracts attention because of its high light-emitting efficiency resulting from direct transition and of its ability to emit a blue light which is one of three primary colors.

The light-emitting diode manufactured from the GaN compound semiconductor is composed of an n-layer and an i-layer grown thereon. The n-layer of the GaN compound semiconductor with n-type conduction is directly grown on a surface of a sapphire substrate or grown on a buffer layer of aluminum nitride formed on the substrate. The i-layer of insulating (i-type) GaN compound semiconductor doped with p-type impurities is grown on the n-layer. (See Japanese Patent Laid-open Nos. 119196/1987 and 188977/1988.) The light-emitting diode of this structure has room for improvement in luminous intensity. In addition, it comprises no p-n junction but it is made by joining the i-layer and n-layer.

The DE-A-27 38 329 describes a light-emitting device which comprises an n-layer of n-type gallium nitride compound semiconductor and an i-layer of insulating gallium nitride compound semiconductor doped with p-type impurities, wherein said n-layer is of double layer structure, the respective layers of said double-layer structure having different concentrations.

An electric property of the GaN compound semiconductor shows inherently n-type conduction even though it is not deliberately doped with n-type impurities, and unlike silicon and similar semiconductors, when it is doped with zinc of p-type impurities, the electric property shows not p-type conduction but insulation. Moreover, the production of n-type GaN involves many difficulties in controlling conductivity.

SUMMARY OF THE INVENTION

It is the first object of the present invention to improve a luminous efficiency of a GaN group light-emitting diode.

It is the second object of the present invention to provide a new layer structure which improves a luminous efficiency of a GaN group light-emitting diode.

It is the third object of the present invention to provide a technology for production of n-type GaN group compound semiconductor in which conductivity is easily controlled.

These objects are solved by a light-emitting semiconductor device according to any one of the claims 1, 2 or 3 as well as by a method for producing a gallium nitride compound semiconductor according to claim 11.

After experience in the manufacture of the above-mentioned GaN light-emitting diode, the present inventors established a technology for a vapor phase epitaxy of the GaN group semiconductor with organometal compound. This technology enables a production of a gas-phase grown GaN layer of high purity. In other words, this technology provides n-type GaN with high resistivity without doping with impurities, unlike the conventional technology which provides n-type GaN with low resistivity when no doping is performed.

According to one aspect of the present invention, the light-emitting semiconductor device comprises an n-layer of n-type gallium nitride group compound semiconductor (Al_xGa_1-xN; inclusive of x=0 ) and an i-layer of insulating (i-type) gallium nitride compound semiconductor (Al_xGa_1-xN; inclusive of x=0 ) doped with p-type impurities, in which the n-layer is of double-layer structure including an n-layer of low carrier concentration and an n⁺-layer of high carrier concentration, the former being adjacent to the i-layer.

The n-layer of low carrier concentration has a carrier concentration of 1 x 10¹⁴/cm³ to 1 x 10¹⁷/cm³ and has a thickness of 0.5 to 2 µm. In case that the carrier concentration is higher than 1 x 10¹⁷/cm³, the luminous intensity of the light-emitting diode decreases. In case that the carrier concentration is lower than 1 x 10¹⁴/cm³, since the series resistance of the light-emitting diode increases, an amount of heat generated in the n-layer increases when a constant current is supplied to it. In case that the layer thickness is greater than 2 µm, since the series resistance of the light-emitting diode increases, the amount of heat generated in the n-layer increases when the constant current is supplied to it. In case that the layer thickness is smaller than 0.5 µm, the luminous intensity of the light-emitting diode decreases.

In addition, the n⁺-layer of high carrier concentration contains a carrier concentration of 1 x 10¹⁷/cm³ to 1 x 10¹⁹/cm³ and has a thickness of 2-10 µm. In case that the carrier concentration is higher than 1 x 10¹⁹/cm³, the n⁺-layer is poor in crystallinity. In case that the carrier concentration is lower than 1 x 10¹⁷/cm³, since the series resistance of the light-emitting diode increases, an amount of heat generated in the n⁺-layer increases when a constant current is supplied to it. In case that the layer thickness is greater than 10 µm, the substrate of the light-emitting diode warps. In case that the layer thickness is smaller than 2 µm, since the series resistace of the light-emitting diode increases, the amount of heat generated in the n⁺-layer increases when the constant current is supplied to it.

In the first aspect of the present invention, it is possible to increase an intensity of blue light emitted from the light-emitting diode by making the n-layer in double-layer structure including an n-layer of low carrier concentration and an n⁺-layer of high carrier concentration, the former being adjacent to the i-layer. In other words, the n-layer as a whole has a low electric resistance owing to the n⁺-layer of high carrier concentration, and hence the light-emitting diode has low series resistance and generates less heat when a constant current is supplied to it. The n-layer adjacent to the i-layer has a lower carrier concentration or higher purity so that it contains a smaller amount of impurity atoms which are deleterious to the emission of blue light from the light-emission region (i-layer and its vicinity). Due to the above-mentioned functions, the light-emitting diode of the present invention emits a blue light of higher intensity.

According to a second aspect of the present invention, the light-emitting semiconductor device comprises an n-layer of n-type gallium nitride compound semiconductor (Al_xGa_1-xN; inclusive of x=0 ) and an i-layer of i-type gallium nitride compound semiconductor (Al_xGa_1-xN; inclusive of x=0) doped with p-type impurities, in which the i-layer is of double-layer structure including an i_L-layer containing p-type impurities in comparatively low concentration and an i_H-layer containing p-type impurities in comparatively high concentration, the former being adjacent to the n-layer.

According to this aspect, the i_L-layer of low impurity concentration contains the impurities in concentration of 1 x 10¹⁶/cm³ to 5 x 10¹⁹/cm³ and preferably has a thickness of 0.01 to 1 µm. In case that impurity concentration is higher than 5 x 10¹⁹/cm³, since the series resistance of the light-emitting diode increases, an initial voltage to start emitting light at increases. In case that the impurity concentration is lower than 1 x 10¹⁶/cm³, the semiconductor of the i_L-layer shows n-type conduction. In case that the layer thickness is greater than 1 µm, since the series resistance of the light-emitting diode increases, the initial voltage to start emitting light increases. In case that the layer thickness is smaller than 0.01 µm, the light-emitting diode has the same structure as that of the conventional one.

In addition, the i_H-layer of high impurity concentration contains the impurities in concentration of 1 x 10¹⁹/cm³ to 5 x 10²⁰/cm³ and preferably has a thickness of 0.02 to 0.3 µm. In case that the impurity concentration is higher than 5 x 10²⁰/cm³, the semiconductor of the i_H-layer is poor in crystallinity. In case that the impurity concentration is lower than 1 x 10¹⁹/cm³, the luminous intensity of the light-emitting diode decreases. In case that the layer thickness is greater than 0.3 µm, since the series resistance of the light-emitting diode increases, an initial voltage to start emitting light at increases. In case that the layer thickness is smaller than 0.02 µm, the i-layer is subject to breakage.

In the second aspect of the present invention, it is possible to increase an intensity of blue light emitted from the light-emitting diode by making the i-layer in double-layer structure including an i_L-layer containing p-type impurities in comparatively low concentration and an i_H-layer containing p-type impurities in comparatively high concentration, the former being adjacent to the n-layer. In other words, this structure (in which the i-layer adjacent to the n-layer is the i_L-layer of low impurity concentration) enables electrons to be injected from the n-layer into the i_H-layer of high impurity concentration without being trapped in the i_L-layer and its vicinity. Therefore, this structure enables electrons to pass through the i_L-layer of low impurity concentration , which is poor in luminous efficacy, adjacent to the n-layer, and to reach the i_H-layer of high impurity concentration in which electrons emit light with a high efficiency.

According to a third aspect of the present invention, the light-emitting semiconductor device comprises an n-layer of n-type gallium nitride compound semiconductor (Al_xGa_1-xN; inclusive of x=0 ) and an i-layer of i-type gallium nitride compound semiconductor (Al_xGa_1-xN; inclusive of x=0 ) doped with p-type impurities, in which the n-layer is of double-layer structure including an n-layer of low carrier concentration and an n⁺-layer of high carrier concentration, the former being adjacent to the i-layer, and the I-layer is of double-layer structure including an i_L-layer containing p-type impurities in comparatively low concentration and an i_H-layer containing p-type impurities in comparatively high concentration, the former being adjacent to the n-layer.

The third aspect of the present invention is a combination of the first aspect (the n-layer of double layer structure) and the second aspect (the i-layer of double layer structure). Therefore, the n-layer of low carrier concentration, the n⁺-layer of high carrier concentration, the i_L-layer of low impurity concentration, and the i_H-layer of high impurity concentration should correspond to the respective layers as the first and second aspects. The carrier concentration and layer thickness are defined in the same manner as in the first and second aspects.

In the third aspect of the present invention, it is possible to increase an intensity of blue light from the light-emitting diode by making the n-layer in double-layer structure including an n-layer of low carrier concentration and an n⁺-layer of high carrier concentration, the former being adjacent to the i-layer, and also by making the i-layer in double-layer structure including an i_L-layer containing p-type impurities in comparatively low concentration and an i_H-layer containing p-type impurities in comparatively high concentration, the former being adjacent to the n-layer. In other words, the n-layer as a whole has a low electric resistance owing to the n⁺-layer of high carrier concentration, which makes it possible to apply an effective voltage to the junction between the i_L-layer and n-layer of low carrier concentration. Having a low carrier concentration, the n-layer adjacent to the i_L-layer does not permit non-light-emitting impurity atoms to diffuse into the i_L-layer. In addition, this structure (in which the i-layer adjacent to the n-layer is the i_L-layer of low impurity concentration) permits electrons to be injected from the n-layer into the i_H-layer of high impurity concentration without being trapped in the i_L-layer. Therefore, this structure permits electrons to pass through the i_L-layer of low impurity concentration, which is poor in luminous efficacy, adjacent to the n-layer, and to reach the i_H-layer of high impurity concentration in which electrons emit light with a high efficiency.

For this reason, the light-emitting diode of the present invention has a much higher luminous efficacy than the one having the conventional simple i-n junction.

The fourth aspect of the present invention resides in a method of producing an n-type gallium nitride compound semiconductor (Al_xGa_1-xN; inclusive of x=0 ) from organometal compound by vapor phase epitaxy. This method comprises a step of feeding a silicon-containing gas and other raw material gases together at a proper mixing ratio so that the conductivity of the compound semiconductor is desirably controlled. The mixing ratio is adjusted such that silicon enters the layer of gallium nitride compound semiconductor grown by vapor phase epitaxy and functions as the donor therein. Thus it is possible to vary the conductivity of the n-type layer by adjusting the mixing ratio.

The fourth aspect of the present invention can be used in a method for producing a light-emitting semiconductor device. The method comprises two steps. The first step involves growing an n⁺-layer of high carrier concentration (which is an n-type gallium nitride compound semiconductor (Al_xGa_1-xN; inclusive of x=0 ) having a comparatively high conductivity) by vapor phase epitaxy from organometal compound. The vapor phase epitaxy is accomplished on a sapphire substrate having a buffer layer' of aluminum nitride by feeding a silicon-containing gas and other raw material gases together at a proper mixing ratio. The second step involves growing an n-layer of low carrier concentration (which is an n-type gallium nitride compound semiconductor (Al_xGa_1-xN; inclusive of x=0 ) having a comparatively low conductivity) by vapor phase epitaxy from organometal compound. The vapor phase epitaxy is accomplished on the n⁺-layer formed by the first step by feeding raw material gases excluding the silicon-containing gas. The n-layer of double-layer structure can be produced by properly controlling the mixing ratio of a silicon-containing gas and other raw material gases.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagram showing a structure of a light-emitting diode shown as Example 1 of the present invention.

Figs. 2 to 7 are sectional views illustrating processes for producing a light-emitting diode shown as to Example 1 of the present invention.

Fig. 8 is a diagram showing relationship between a carrier concentration of an n-layer of low carrier concentration and intensity or wavelength of emitted light with respect to a light-emitting diode shown as Example 1 of the present invention.

Fig. 9 is a diagram showing a structure of a light-emitting diode shown as Example 2 of the present invention.

Figs. 10 to 15 are sectional views illustrating processes for producing a light-emitting diode shown as Example 2 of the present invention.

Fig. 16 is a diagram showing relationship between an impurity concentration of an i_H-layer of high impurity concentration and intensity or wavelength of emitted light with respect to a light-emitting diode shown as Example 2 of the present invention.

Fig. 17 is a diagram showing a structure of a light emitting diode shown as Example 3 of the present invention.

Figs. 18 to 23 are sectional views illustrating processes for producing a light-emitting diode shown as Example 3 of the present invention.

Fig. 24 is a diagram showing relationship between a carrier concentration of an n-layer of low carrier concentration and intensity or wavelength of emitted light with respect to a light-emitting diode shown as Example 3 of the present invention.

Fig. 25 is a diagram showing relationship between an impurity concentration of an i_H-layer of high impurity concentration and intensity or wavelength of emitted light with respect to a light-emitting diode shown as Example 3 of the present invention.

Fig. 26 is a diagram showing the relationship between a flow rate of silane gas and electrical properties of an n-layer formed by vapor phase epitaxy in a process shown as Example 4 of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described in more detail with reference to the following examples.

Example 1

In Fig. 1 there is shown a light-emitting diode 10 which has a sapphire substrate 1 on which is formed a buffer layer of 5 nm (500 Å) thick AlN. On the buffer layer 2 are consecutively formed an n⁺-layer 3 of high carrier concentration of 2.2 µm thick GaN and an n-layer 4 of low carrier concentration of 1.5 µm thick GaN. And an i-(insulating) layer 6 of 0.2 µm thick GaN is formed on the n-layer 4. Aluminum electrodes 7 and 8 are connected to the i-layer 6 and n⁺-layer 3, respectively.

This light-emitting diode 10 was produced by metalorganic vapor phase epitaxy in the following manner. (This process is referred to as MOVPE hereinafter. )

The gases employed in this process are NH₃, H₂ (as carrier gas), trimethyl gallium (Ga(CH₃)₃) (TMG hereinafter), trimethyl aluminum (Al(CH₃)₃) (TMA hereinafter), silane (SiH₄), and diethyl zinc (DEZ hereinafter).

The sapphire substrate 1 of single crystal, with its principal crystal plane (a-surface {11<maths id="math0001"><math display="inline"><mrow><mover accent="true"><mrow><mtext>2</mtext></mrow><mo>¯</mo></mover></mrow></math><img file="EP0444630B1_D0001.tif" /></maths>0}) cleaned by solvent washing and heat treatment, was set on the susceptor placed in a reaction chamber of an MOVPE apparatus.

The sapphire substrate 1 underwent vapor phase etching at 1100 °C with H₂ flowing through the reaction chamber at a flow rate of 2 l/min under normal pressure.

On the sapphire substrate 1 was formed the AlN buffer layer 2 (about 50 nm (500 Å) thick) at 400°C by supplying H₂ at a flow rate of 20 l/min, NH₃ at a flow rate of 10 l/min, and TMA at a flow rate of 1.8 x 10^-5. mol/min.

On the buffer layer 2 was formed the n⁺-layer 3 of high carrier concentration (1.5 x 10¹⁸/cm³) of 2.2 µm thick GaN by supplying H₂ at a flow rate of 20 l/min, NH₃ at a flow rate of 10 l/min, TMG at a flow rate of 1.7 x 10^-4 mol/min, and silane (diluted to 0.86 ppm with H₂) at a flow rate of 200 ml/min, with the sapphire substrate 1 kept at 1150 °C.

On the n⁺-layer 3 was formed the n-layer 4 of low carrier concentration (1 x 10¹⁵/cm³) of 1.5 µm thick GaN by supplying H₂ at a flow rate of 20 l/min, NH₃ at a flow rate of 10 l/min, and TMG at a flow rate of 1.7 x 10^-4 mol/min, with the sapphire substrate 1 kept at 1150°C.

On the n-layer 4 was formed the i-layer 6 of 0.2 µm thick GaN by supplying H₂ at a flow rate of 20 l/min, NH₃ at a flow rate of 10 l/min, TMG at a flow rate of 1.7 x 10^-4 mol/min, and DEZ at a flow rate of 1.5 x 10^-4 mol/min, with the sapphire substrate 1 kept at 900°C.

Thus there was obtained the multi-layer structure as shown in Fig. 2.

On the i-layer 6 was formed a 200 nm (2000 Å) thick SiO₂ layer 11 by sputtering as shown in Fig. 3. On the SiO₂ layer 11 was formed a photoresist layer 12 which subsequently underwent a photolithographic processing to make a pattern corresponding to a figure of the electrode connected to the n⁺-layer 3.

The exposed part (not covered by the photoresist layer 12) of the SiO₂ layer 11 underwent etching with hydrofluoric acid for its removal, as shown in Fig. 4.

The exposed part (not covered by the photoresist layer 12 and the SiO₂ layer 11) of the i-layer 6 underwent dry etching with CCl₂F₂ gas at a flow rate of 10 cc/min and a high-frequency electric power of 0.44 W/cm² in a vacuum of 5.32 Pa (0.04 Torr) and subsequently underwent dry etching with argon. The dry etching removed not only the exposed part of the i-layer 6 but also the n-layer 4 and the upper part of the n⁺-layer 3 which are underneath the exposed part of the i-layer 6, as shown in Fig. 5.

The SiO₂ layer 11 remaining on the i-layer 6 was removed with hydrofluoric acid as shown in Fig. 6.

On the entire surface of the sample was formed an Al layer 13 by vapor deposition as shown in Fig. 7. On the Al layer 13 was formed a photoresist layer 14 which subsequently underwent the photolithographic processing to make a pattern corresponding to a figure of the electrodes connected to the n⁺-layer 3 and the i-layer 6, respectively.

The exposed part (not covered by the photoresist layer 14) of the Al layer 13 underwent etching with nitric acid as shown in Fig. 7. The photoresist 14 was removed with acetone. Thus there were formed the electrode 8 for the n⁺-layer 3 and the electrode 7 for the i-layer 6.

Such an above-mentioned process could make a gallium nitride light-emitting element of MIS (metal-insulator-semiconductor) structure as shown in Fig. 1.

The thus ohtained light-emitting diode 10 was found to have a luminous intensity of 0.2 mcd. This value is 4 times higher than that of the conventional light-emitting diode which is composed simply of an i-layer with impurity concentration of 2 x 10²⁰ /cm³ and a 4 µm thick n-layer with carrier concentration of 5 x 10¹⁷/cm³.

In addition, the inspection of the luminescent surface revealed that the number of luminescent points is much greater than that of the conventional light-emitting diode.

Several samples were prepared in the same manner as mentioned above except that the carrier concentration in the n-layer of low carrier concentration was varied, and they were tested for luminous intensity and emission spectrum. The results are shown in Fig. 8. It is noted that the luminous intensity decreases and the emission spectrum shifts to the red side according as the carrier concentration increases. This effect is estimated to be caused by that atoms of silicon as doping atoms diffuse or mix into the i-layer 6 as impurity atoms.

Example 2

In Fig. 9 there is shown a light-emitting diode 10 which has a sapphire substrate 1 on which is formed a 50 nm (500 Å) thick AlN buffer layer 2. On the buffer layer 2 are consecutively formed a 4 µm thick GaN n-layer 3 with carrier concentration of 5 x 10¹⁷/cm³, an i_L-layer 5 of low impurity concentration of 5 x 10¹⁹/cm³ of Zn, and an i_H-layer 6 of high impurity concentration (2 x 10²⁰/cm³ of Zn) . To the i_H-layer 6 and n-layer 3 are connected by aluminum electrodes 7 and 8, respectively.

This light-emitting diode 10 was produced by the MOVPE.

The gases employed in this process are NH₃, H₂ (as carrier gas), trimethyl gallium TMG , trimethyl aluminum TMA, and diethyl zinc DEZ.

The sapphire substrate 1 of single crystal, with its principal crystal plane ( c-surface {0001} ) cleaned by solvent washing and heat treatment, was set on the susceptor placed in the reaction chamber of the MOVPE apparatus.

The sapphire substrate 1 underwent vapor phase etching at 1100 °C with H₂ flowing through the reaction chamber at a flow rate of 2 l/min under normal pressure.

On the sapphire substrate 1 was formed the AlN buffer layer 2 (about 50 nm (500 Å) thick) at 400°C by supplying H₂ at a flow rate of 20 l/min, NH₃ at a flow rate of 10 l/min, and TMA at a flow rate of 1.8 x 10^-5 mol/min.

On the buffer layer 2 was formed the 4 µm thick GaN n-layer 3 with carrier concentration of 1.5 x 10¹⁷/cm³ by supplying H₂ at a flow rate of 20 l/min, NH₃ at a flow rate of 10 l/min, and TMG at a flow rate -of 1.7 x 10^-4 mol/min with stopping the feeding of TMA, with the sapphire substrate 1 kept at l150°C.

On the n-layer 3 was formed the 0.2 µm thick GaN i_L-layer 5 of low impurity concentration (5 x 10¹⁹/cm³ of Zn) by supplying H₂ at a flow rate of 20 l/min, NH₃ at a flow rate of 10 l/min, TMG at a flow rate of 1.7 x 10^-4 mol/min, and DEZ at a flow rate of 1.5 x 10^-4 mol/min, with the sapphire substrate 1 kept at 1000°C.

On the i_L-layer 5 was formed the 0.2 µm thick GaN i_H-layer 6 of high impurity concentration (2 x 10²⁰/cm³ of Zn) by supplying H₂ at a flow rate of 20 l/min, NH₃ at a flow rate of 10 l/min, TMG at a flow rate of 1.7 x 10^-4 mol/min, and DEZ at a flow rate of 1.5 x 10^-4 mol/min, with the sapphire substrate 1 kept at 900°C.

Thus there was obtained the multi-layer structure as shown in Fig. 10.

On the i_H-layer 6 was formed the 200 nm (2000 Å) thick SiO₂ layer 11 by sputtering as shown in Fig. 11. On the SiO₂ layer 11 was formed a photoresist layer 12 which subsequently underwent the photolithographic processing to make a pattern corresponding to the figure of the electrode connected to the n-layer 3.

The exposed part (not covered by the photoresist layer 12) of the SiO₂ layer 11 underwent etching with hydrofluoric acid for its removal, as shown in Fig. 12.

The exposed part (not covered by the photoresist layer 12 and the SiO₂ layer 11) of the i_H-layer 6 underwent dry etching with CCl₂F₂ gas at a flow rate of 10 cc/min and a high-frequency electric power of 0.44 W/cm2 in a vacuum of 0.04 Torr and subsequently underwent dry etching with argon. The dry etching removed not only the exposed part of the i_H-layer 6 but also the i_L-layer 5 and the upper part of the n-layer 3 which are underneath the exposed part of the i_H-layer 6, as shown in Fig. 13.

The SiO₂ layer 11 remaining on the i_H-layer 6 was removed with hydrofluoric acid as shown in Fig. 14.

On the entire surface of the sample was formed an Al layer 13 by vapor deposition as shown in Fig. 15. On the Al layer 13 was formed the photoresist layer 14 which subsequently underwent the photolithographic processing to make a pattern corresponding to the figure of the electrodes connected to the n-layer 3 and the i_H-layer 6, respectively.

The exposed part (not covered by the photoresist layer 14) of the Al layer 13 underwent etching with nitric acid as shown in Fig. 15. The photoresist 14 was removed with acetone. Thus there were formed the electrode 8 for the n-layer 3 and the electrode 7 for the i_H-layer 6.

Such an above-mentioned process could make a gallium nitride light-emitting element of MIS structure as shown in Fig. 9.

The thus obtained light-emitting diode 10 was found to have a luminous intensity of 0.2 mcd. This value is 4 times higher than that of the conventional light-emitting diode which is composed simply of a 0.2 µm thick i-layer with impurity concentration of 2 x 10²⁰/cm³ and a 4 µm thick n-layer with carrier concentration of 5 x 10¹⁷/cm³.

In addition, the inspection of the luminescent surface revealed that the number of luminescent points is much greater than that of the conventional light-emitting diode.

Several samples were prepared in the same manner as mentioned above except that the impurity concentration in the i_H-layer 6 of high impurity concentration was varied, and they were tested for luminous intensity and emission spectrum. The results are shown in Fig. 16. It is noted that the luminous intensity has a peak value and the emission spectrum shifts to a longer wavelength side when the impurity concentration increases.

Example 3

In Fig. 17 there is shown a light-emitting diode 10 which has a sapphire substrate 1 on which is formed a 50 nm (500 Å) thick AlN buffer layer 2. On the buffer layer 2 are consecutively formed a 2.2 µm thick GaN n⁺-layer 3 of high carrier concentration (1.5 x 10¹⁸/cm³), a 1.5 µm thick GaN n-layer 4 of low carrier concentration (1 x 10¹⁵/cm³), an i_L-layer 5 of low impurity concentration (5 x 10¹⁹/cm³ of Zn), and an i_H-layer 6 of high impurity concentration (2 x 10²⁰/cm³ of Zn) . To the i_H layer 6 and n⁺-layer 3 are connected aluminum electrodes 7 and 8, respectively. This light-emitting diode 10 was produced by the MOVPE with organometal compound in the following manner.

The gases employed in this process are NH₃, H₂ (as carrier gas), trimethyl gallium (Ga(CH₃)₃) (TMG), trimethyl aluminum (Al(CH₃)₃) (TMA) ,silane (SiH₄), and diethyl zinc (DEZ).

The sapphire substrate 1 of single crystal, with its principal crystal plane ( c-surface {0001} ) cleaned by solvent washing and heat treatment, was set on the susceptor placed in the reaction chamber of the MOVPE apparatus.

The sapphire substrate 1 underwent vapor phase etching at 1100 °C with H₂ flowing through the reaction chamber at a flow rate of 2 l/min under normal pressure.

On the sapphire substrate 1 was formed the AlN buffer layer 2 (about 50 nm thick) at 400 °C by supplying H₂ at a flow rate of 20 l/min, NH₃ at a flow rate of 10 l/min, and TMA at a flow rate of 1.8 x 10^-5 mol/min.

On the buffer layer 2 was formed the 2.2 µm thick GaN n⁺-layer 3 of high carrier concentration (1.5 x 10¹⁸/cm³) by supplying H₂ at a flow rate of 20 l/min, NH₃ at a flow rate of 10 l/min, TMG at a flow rate of 1.7 x 10^-4 mol/min, and silane (diluted to 0.86 ppm with H₂) at a flow rate of 200 ml/min for 30 minutes, with the sapphire substrate 1 kept at 1150 °C.

On the n⁺-layer 3 was formed the 1.5 µm thick GaN n-layer 4 of low carrier concentration (1 x 10¹⁵ /cm³) by supplying H₂ at a flow rate of 20 l/min, NH₃ at a flow rate of 10 l/min, and TMG at a flow rate of 1.7 x 10^-4 mol/min, with the sapphire substrate 1 kept at 1150 °C.

On the n-layer 4 was formed the 0.2 µm thick GaN i_L-layer 5 of low impurity concentration (5 x 10¹⁹/cm³ of Zn) by supplying H₂ at a flow rate of 20 l/min, NH₃ at a flow rate of 10 l/min, TMG at a flow rate of 1.7 x 10^-4 mol/min, and DEZ at a flow rate of 1.5 x 10^-4 mol/min, with the sapphire substrate 1 kept at 1000 °C.

On the i_L-layer 5 was formed the 0.2 µm thick GaN i_H-layer 6 of high impurity concentration (2 x 10²⁰/cm³ of Zn) by supplying H₂ at a flow rate of 20 l/min, NH₃ at a flow rate of 10 l/min, TMG at a flow rate of 1.7 x 10^-4 mol/min, and DEZ at a flow rate of 1.5 x 10^-4 mol/min, with the sapphire substrate 1 kept at 900 °C.

Thus there was obtained the multi-layer structure as shown in Fig. 18.

On the i_H-layer 6 was formed the 2000 Å thick SiO₂ layer 11 by sputtering as shown in Fig. 19. On the SiO₂ layer 11 was formed a photoresist layer 12 which subsequently underwent the photolithographic processing to make a pattern for the electrode connected to the n⁺-layer 3.

The exposed part (not covered by the photoresist layer 12) of the SiO₂ layer 11 underwent etching with hydrofluoric acid for its removal, as shown in Fig. 20.

The exposed part (not covered by the photoresist layer 12 and the SiO₂ layer 11) of the i_H-layer 6 underwent dry etching with CCl₂F₂ gas at a flow rate of 10 cc/min and a high-frequency electric power of 0.44 W/cm² in a vacuum of 0.04 Torr and subsequently underwent dry etching with argon. The dry etching removed not only the exposed part of the i_H-layer 6 but also the i_L-layer 5 and the n-layer 4 and the upper part of the n⁺-layer 3 which are underneath the exposed part of the i_H-layer 6, as shown in Fig. 21.

The SiO₂ layer 11 remaining on the i_H-layer 6 was removed with hydrofluoric acid as shown in Fig. 22.

On the entire surface of the sample was formed an Al layer 13 by vapor deposition as shown in Fig. 23. On the Al layer 13 was formed the photoresist layer 14 which subsequently underwent the photolithographic processing to make a pattern for the electrodes connected to the n⁺-layer 3 and the i_H-layer 6, respectively.

The exposed part (not covered by the photoresist layer 14) of the Al layer 13 underwent etching with nitric acid as shown in Fig. 23. The photoresist 14 was removed with acetone. Thus there were formed the electrode 8 for the n⁺-layer 3 and the electrode 7 for the i_H-layer 6.

Such an above-mentioned process could make a gallium nitride light-emitting element of MIS structure as shown in Fig. 17.

The thus obtained light-emitting diode 10 was found to have a luminous intensity of 0.4 mcd. This value is 8 times higher than that of the conventional light-emitting diode which is composed simply of a 0.2 µm thick i-layer with impurity concentration of 2 x 10²⁰/cm³ and a 4 µm thick n-layer with a carrier concentration of 5 x 10¹⁷/cm³.

In addition, the inspection of the luminescent surface revealed that the number of luminescent points is much greater than that of the conventional light-emitting diode.

Several samples were prepared in the same manner as mentioned above except that the carrier concentration in the n-layer 4 of low carrier concentration was varied, and they were tested for luminous intensity and emission spectrum. The results are shown in Fig. 24. It is noted that the luminous intensity decreases and the emission spectrum shifts to the red side according as the carrier concentration increases.

Also, several samples were prepared in the same manner as mentioned above except that the impurity concentration In the i_H-layer 6 of high impurity concentration was varied, and they were tested for luminous intensity and emission spectrum. The results are shown in Fig. 25. It is noted that the luminous intensity has a peak value and the emission spectrum shifts to a longer wavelength side when the impurity concentration increases.

Example 4

A light-emitting diode 10 of the same structure as in Example 1 was prepared in the same manner as in Example 1 according to the steps shown in Figs. 2 to 7. The resistivity of the n⁺-layer 3 was varied in the range of 3 x 10^-1 Ωcm to 8 x 10^-3 Ωcm by changing the conditions of the vapor phase epitaxy for the n⁺-layer 3 of high carrier concentration, as shown in Fig. 26. The vapor phase epitaxy was carried out by supplying H₂ at a flow rate of 20 l/min, NH₃ at a flow rate of 10 l/min, TMG-carrying H₂ at a flow rate of 100 cc/min, and H₂-diluted silane (0.86 ppm) at a flow rate of 10 cc/min to 300 cc/min. (The TMG-carrying H₂ was prepared by bubbling H₂ in TMG cooled at -15 °C.)

In the above-mentioned case, the resistivity of the n⁺-layer 3 was varied by changing the flow rate of silane, but it is also possible to achieve the same object by changing the flow rate of other raw material gases or by changing the mixing ratio of silane and other raw material gases.

In this example, silane was used as the Si dopant, but it can be replaced by an organosilicon compound such as tetraethylsilane (Si(C₂H₅)₄) in a gaseous state prepared by bubbling with H₂.

The process mentioned above permits one to prepare the n⁺-layer 3 of high carrier concentration and the n-layer 4 of low carrier concentration in such a manner that their resistivity can be controlled as desired.

The thus obtained light-emitting diode 10 was found to have a luminous intensity of 0.2 mcd. This value is 4 times higher than that of the conventional light-emitting diode which is composed simply of an i-layer and an n-layer. In addition, the inspection of the luminescent surface revealed that the number of luminescent points is much greater than that of the conventional light-emitting diode.