NITRIDE SEMICONDUCTOR LIGHT EMITTING ELEMENT AND METHOD FOR MANUFACTURING THE SAME

Abstract

Provided is a nitride semiconductor light emitting element which has good luminous efficiency by suppressing deep-level emission and increasing the monochromaticity. A nitride semiconductor light emitting element according to the present invention comprises an active layer between an n-type nitride semiconductor layer and a p-type nitride semiconductor layer. The n-type nitride semiconductor layer contains Al.sub.X1In.sub.X2Ga.sub.X3N (wherein 0<X1≦1, 0≦X2<1, 0≦X3<1, X1+X2+X3=1), and both the concentration of C contained therein and the concentration of O contained therein are less than or equal to 1×10.sup.17/cm.sup.3.

Claims

1. A nitride semiconductor light emitting element having an active layer between an n-type nitride semiconductor layer and a p-type nitride semiconductor layer, wherein the n-type nitride semiconductor layer contains Al.sub.X1In.sub.X2Ga.sub.X3N (0<X1≦1, 0≦X2<1, 0≦X3<1, X1+X2+X3=1), and both of a concentration of C and a concentration of O contained in the n-type nitride semiconductor layer are less than or equal to 1×10.sup.17/cm.sup.3, and a concentration of a p-type impurity contained in the p-type nitride semiconductor layer is higher than the concentration of C contained in the n-type nitride semiconductor layer.

2. The nitride semiconductor light emitting element according to claim 1, wherein the concentration of O contained in the n-type nitride semiconductor layer is less than or equal to 8×10.sup.16/cm.sup.3.

3. The nitride semiconductor light emitting element according to claim 1, which is an ultraviolet light emitting element having a major emission wavelength of less than or equal to 375 nm.

4. The nitride semiconductor light emitting element according to claim 3, wherein an intensity ratio of an emission intensity of a yellow visible light wavelength to an emission intensity of the major emission wavelength is less than or equal to 0.1%.

5. The nitride semiconductor light emitting element according to claim 1, comprising a sapphire substrate, wherein the n-type nitride semiconductor layer, the p-type nitride semiconductor layer, and the active layer are formed on a C plane of the sapphire substrate.

6. A method for producing a nitride semiconductor light emitting element, comprising the steps of: (a) preparing a sapphire substrate, and (b) forming an n-type nitride semiconductor layer, an active layer, and a p-type nitride semiconductor layer on a C plane of the sapphire substrate, wherein in the step (b), the n-type nitride semiconductor layer contains Al.sub.X1In.sub.X2Ga.sub.X3N (0<X1≦1, 0≦X2<1, 0≦X3<1, X1+X2+X3=1), and both of a concentration of C and a concentration of O contained in the n-type nitride semiconductor layer are less than or equal to 1×10.sup.17/cm.sup.3, and a concentration of a p-type impurity contained in the p-type nitride semiconductor layer is higher than the concentration of C contained in the n-type nitride semiconductor layer.

7. The method for producing a nitride semiconductor light emitting element according to claim 6, wherein in the step (b), the n-type nitride semiconductor layer is allowed to grow in a condition that a V/III ratio which is a flow rate ratio between a source material of Group V and a source material of Group III is set to be more than or equal to 2000.

8. The nitride semiconductor light emitting element according to claim 2, which is an ultraviolet light emitting element having a major emission wavelength of less than or equal to 375 nm.

9. The nitride semiconductor light emitting element according to claim 8, wherein an intensity ratio of an emission intensity of a yellow visible light wavelength to an emission intensity of the major emission wavelength is less than or equal to 0.1%.

10. The nitride semiconductor light emitting element according to claim 2, comprising a sapphire substrate, wherein the n-type nitride semiconductor layer, the p-type nitride semiconductor layer, and the active layer are formed on a C plane of the sapphire substrate.

11. The nitride semiconductor light emitting element according to claim 3, comprising a sapphire substrate, wherein the n-type nitride semiconductor layer, the p-type nitride semiconductor layer, and the active layer are formed on a C plane of the sapphire substrate.

12. The nitride semiconductor light emitting element according to claim 4, comprising a sapphire substrate, wherein the n-type nitride semiconductor layer, the p-type nitride semiconductor layer, and the active layer are formed on a C plane of the sapphire substrate.

13. The nitride semiconductor light emitting element according to claim 8, comprising a sapphire substrate, wherein the n-type nitride semiconductor layer, the p-type nitride semiconductor layer, and the active layer are formed on a C plane of the sapphire substrate.

14. The nitride semiconductor light emitting element according to claim 9, comprising a sapphire substrate, wherein the n-type nitride semiconductor layer, the p-type nitride semiconductor layer, and the active layer are formed on a C plane of the sapphire substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a graph showing the relationship between the emission output of the major emission wavelength and the ratio of the deep emission intensity to the light intensity of the major emission wavelength of each element when the same current is applied to different ultraviolet LED elements.

[0023] FIG. 2 is a schematic cross section view of a nitride semiconductor light emitting element.

[0024] FIG. 3 is a graph showing spectrum distribution of light when the same current is applied to three elements of Example 1, Example 2, and Comparative example 1.

[0025] FIG. 4 is a photo showing the light emitting mode when the same current is applied to three elements of Example 1, Example 2, and Comparative example 1.

[0026] FIG. 5 is a graph showing spectrum distributions of light when the same current is applied to four elements of Example 3, Example 4, Comparative example 2, and Comparative example 3.

[0027] FIG. 6 is another schematic cross section view showing the nitride semiconductor light emitting element.

MODE FOR CARRYING OUT THE INVENTION

[Structure]

[0028] The structure of a nitride semiconductor light emitting element 1 according to the present invention will be described by referring to FIG. 2. FIG. 2 is a schematic cross section view of the nitride semiconductor light emitting element 1. Hereinafter, the nitride semiconductor light emitting element 1 is abbreviated as “LED element 1”.

[0029] In the present embodiment, description will be made for the LED element 1 embodied by an ultraviolet light emitting element having a major emission wavelength of a 370 nm band, however, the emission wavelength is not limited to this value.

[0030] The LED element 1 is formed by laminating a substrate 2, an undoped layer 3, an n-type nitride semiconductor layer 4, an active layer 5, and a p-type nitride semiconductor layer 6 in this order from below.

(Substrate 2)

[0031] The substrate 2 is constituted of a sapphire substrate. Here, instead of sapphire, the substrate 2 may be constituted of Si, SiC, AlN, AlGaN, GaN, YAG, or the like.

(Undoped Layer 3)

[0032] The undoped layer 3 is formed of GaN. More specifically, the undoped layer 3 is formed of a low-temperature buffer layer made of GaN and an underlayer made of GaN on top thereof.

(N-Type Nitride Semiconductor Layer 4)

[0033] The n-type nitride semiconductor layer 4 is constituted of Al.sub.X1In.sub.X2Ga.sub.X3N (0<X1≦1, 0≦X2<1, 0≦X3<1, X1+X2+X3=1) formed so that a concentration of C and a concentration of O contained as impurities are less than or equal to 1×10.sup.17/cm.sup.3. The method for reducing the contained concentration of C and concentration of O will be described later.

(Active Layer 5)

[0034] The active layer 5 is made up of, for example, a light emitting layer of InGaN and a barrier layer of AlGaN that are plurally repeated. These layers may be undoped, or may be doped into p-type or n-type.

(P-Type Nitride Semiconductor Layer 6)

[0035] The p-type nitride semiconductor layer 6 is constituted of Al.sub.Y1In.sub.Y2Ga.sub.Y3N (0<Y1≦1, 0≦Y2<1, 0≦Y3<1, Y1+Y2+Y3=1). Unlike the n-type nitride semiconductor layer 4, the concentration of C and the concentration of O contained as impurities may be larger than 1×10.sup.17/cm.sup.3 in the p-type nitride semiconductor layer 6. Also this point will be described later.

[0036] Although not illustrated in FIG. 2, the LED element 1 may have a high concentration p-type GaN layer for contact on top of the p-type nitride semiconductor layer 6. Also an n-side electrode may be provided on top of the n-type nitride semiconductor layer 4 exposed by etching, and a p-side electrode may be provided on top of the high concentration p-type GaN layer.

[0037] The n-type nitride semiconductor layer 4 may be constituted only of an Al.sub.X1In.sub.X2Ga.sub.X3N layer, or may contain an Al.sub.X1In.sub.X2Ga.sub.X3N layer and a GaN layer. The Al.sub.X1In.sub.X2Ga.sub.X3N layer may be made up of any of an MN layer, an AlGaN layer, and an AlInGaN layer, or may be a laminate of multiple layers of these. When the n-type nitride semiconductor layer 4 is constituted of an AlInGaN layer, the composition ratio of In may be extremely low (for example, less than 1%). The same applies to the p-type nitride semiconductor layer 6.

[Manufacturing Process]

[0038] Next, the manufacturing process of the LED element 1 shown in FIG. 2 will be described. This manufacturing process is merely one example, and the flow rate of gas, the temperature within the furnace, the pressure within the furnace and the like may be appropriately adjusted.

[0039] First, the undoped layer 3 is formed on top of the substrate 2. This is realized, for example, by the following method.

(Preparation of Substrate 2)

[0040] A sapphire substrate is prepared as the substrate 2, and the c-plane sapphire substrate is cleaned. More specifically, this cleaning is carried out, for example, by placing the c-plane sapphire substrate in a processing furnace of an MOCVD (Metal Organic Chemical Vapor Deposition: organic metal chemical gas-phase vapor deposition) apparatus and raising the temperature within the furnace to be, for example, 1150° C. while allowing a hydrogen gas to flow at a flow rate of 10 slm in the processing furnace.

(Forming Undoped Layer 3)

[0041] Next, a low-temperature buffer layer made of GaN is formed on the surface of c-plane sapphire substrate, and further an underlayer made of GaN is formed on top thereof. The low-temperature buffer layer and the underlayer correspond to the undoped layer 3.

[0042] A more specific method of forming the undoped layer 3 is, for example, as follows. First, the pressure within the furnace of the MOCVD apparatus is set to be 100 kPa, and the temperature within the furnace is set to be 480° C. Then, trimethylgallium (TMG) having a flow rate of 50 μmol/min and ammonia having a flow rate of 223000 μmol/min are supplied as source material gases for 68 seconds into the processing furnace while allowing a nitrogen gas and a hydrogen gas each having a flow rate of 5 slm to flow as carrier gases in the processing furnace. By this process, the low-temperature buffer layer made of GaN and having a thickness of 20 nm is formed on the surface of c-plane sapphire substrate.

[0043] Next, the temperature within the furnace of the MOCVD apparatus is raised to 1150° C. Then, TMG having a flow rate of 100 μmol/min and ammonia having a flow rate of 223000 μmol/min are supplied as source material gases for 30 minutes into the processing furnace while allowing a nitrogen gas having a flow rate of 20 slm and a hydrogen gas having a flow rate of 15 slm to flow as carrier gases in the processing furnace. By this process, the underlayer made of GaN and having a thickness of 1.7 μm is formed on the surface of the low-temperature buffer layer.

(Formation of n-Type Nitride Semiconductor Layer 4)

[0044] Next, the n-type nitride semiconductor layer 4 having a composition of Al.sub.X1In.sub.X2Ga.sub.X3N is formed on top of the undoped layer 3.

[0045] A more concrete method for forming the n-type nitride semiconductor layer 4 is, for example, as follows. First, in the condition that the temperature within the furnace is kept at 1150° C., the pressure within the furnace of the MOCVD apparatus is set to be 30 kPa. Then while a nitrogen gas having a flow rate of 20 slm and a hydrogen gas having a flow rate of 15 slm are flown as carrier gases in the processing furnace, TMG, trimethylaluminum (TMA), ammonia and tetraethylsilane for doping with an n-type impurity are supplied as source material gases into the processing furnace for 30 minutes. As a result, the n-type nitride semiconductor layer 4 having a composition of, for example, Al.sub.0.06Ga.sub.0.94N and a thickness of 1.7 μm is formed on top of the undoped layer 3.

[0046] By setting the flow rate ratio between ammonia which is Group V, and TMG and TMA which are Group III (V/III ratio) to be larger than or equal to 2000, it is possible to set the concentration of C contained in the n-type nitride semiconductor layer 4 to be less than or equal to 1×10.sup.17/cm.sup.3. The concentration of O contained in the n-type nitride semiconductor layer 4 at this time can also be set to be less than or equal to 1×10.sup.17/cm.sup.3. When the growth speed changes with the change in the flow rate of ammonium, the time is adjusted so that a desired film thickness is obtained, and thus an element is produced.

[0047] For example, by using ammonia having a flow rate of 223000 μmol/min, TMG having a flow rate of 100 μmol/min, and TMA having a flow rate of 7 μmol/min as source materials, the V/III ratio can be set to be about 2000. Although tetraethylsilane also contains C atoms, the flow rate thereof is, for example, about 0.025 μmol/min, and thus the influence on the concentration of C contained in the n-type nitride semiconductor layer 4 is neglectable in comparison with TMG and TMA.

[0048] When the V/III ratio was 1000, the concentration of C contained in the generated n-type nitride semiconductor layer 4 was 5×10.sup.17/cm.sup.3, and the concentration of O contained in the generated n-type nitride semiconductor layer 4 was 7×10.sup.16/cm.sup.3 (later-described Comparative example 1). When the V/III ratio was 2000, the concentration of C was 1×10.sup.17/cm.sup.3, and the concentration of O was 5×10.sup.16/cm.sup.3 (later-described Example 2). When the V/III ratio was 4000, the concentration of C was 5×10.sup.16/cm.sup.3, and the concentration of O was 4×10.sup.16/cm.sup.3 (later-described Example 1). The concentration of C contained in the generated n-type nitride semiconductor layer 4 was measured by SIMS (secondary ion mass spectrometry).

[0049] TMG and TMA which are source material gases contain a C atom as a constituting molecule. Meanwhile, ammonia does not contain a C atom. Therefore, by increasing the V/III ratio, it is possible to reduce the concentration of C contained in the formed n-type nitride semiconductor layer 4.

[0050] It is also possible to reduce the concentration of C contained in the n-type nitride semiconductor layer 4 by increasing the growth pressure besides increasing the V/III ratio. This would be because the same effect as obtained by increasing the V/III ratio can be obtained as a result of formation of an ammonia rich environment in the furnace due to extension of the time during which ammonia resides in the MOCVD apparatus by increasing the growth pressure. In this case, the growth pressure is preferably greater than or equal to 30 kPa and less than or equal to 100 kPa, and more preferably greater than or equal to 50 kPa and less than or equal to 100 kPa.

[0051] Here, silicon (Si), germanium (Ge), sulfur (S), selenium (Se), tin (Sn), tellurium (Te), and others may be used as the n-type impurity contained in the n-type nitride semiconductor layer 4. Among these, silicon (Si) is especially preferable.

[0052] By reducing the concentration of C contained in the n-type nitride semiconductor layer 4 by increasing the V/III ratio of the source material gas, or by increasing the growth pressure as is conducted in this step, it is sometimes possible to reduce the concentration of O contained in the n-type nitride semiconductor layer 4.

(Formation of Active Layer 5)

[0053] Next, the active layer 5 is formed on top of the n-type nitride semiconductor layer 4.

[0054] Concretely, first, the pressure within the furnace of the MOCVD apparatus is set to be 100 kPa, and the temperature within the furnace is set to be 830° C. Then the step of supplying the interior of the processing furnace with TMG having a flow rate of 10 μmol/min, trimethylindium (TMI) having a flow rate of 12 μmol/min and ammonia having a flow rate of 300000 μmol/min as source material gases for 48 seconds while flowing a nitrogen gas having a flow rate of 15 slm and a hydrogen gas having a flow rate of 1 slm as carrier gases in the processing furnace is conducted. Then the step of supplying the interior of the processing furnace with TMG having a flow rate of 10 μmol/min, TMA having a flow rate of 1.6 μmol/min, tetraethylsilane having a flow rate of 0.002 μmol/min, and ammonia having a flow rate of 300000 μmol/min as source material gases for 120 seconds is conducted. Thereafter, by repeating these steps, the active layer 5 in which a light-emitting layer constituted of InGaN and having a thickness of 2 nm, and a barrier layer constituted of n-type AlGaN and having a thickness of 7 nm are repeated 15 cycles is formed on the surface of the n-type nitride semiconductor layer 4.

(Formation of p-Type Nitride Semiconductor Layer 6)

[0055] Next, the p-type nitride semiconductor layer 6 constituted of Al.sub.Y1In.sub.Y2Ga.sub.Y3N is formed on top of the active layer 5.

[0056] Concretely, the pressure within the furnace of the MOCVD apparatus is maintained to be 100 kPa, and the temperature within the furnace is raised to 1025° C. while a nitrogen gas having a flow rate of 15 slm and a hydrogen gas having a flow rate of 25 slm are allowed to flow as carrier gases in the processing furnace. Thereafter, TMG having a flow rate of 35 μmol/min, TMA having a flow rate of 20 μmol/min, ammonia having a flow rate of 250000 μmol/min, and biscyclopentadienyl magnesium (Cp.sub.2Mg) having a flow rate of 0.1 μmol/min for doping with a p-type impurity are supplied as source material gases into the processing furnace for 60 seconds. By this process, a hole supply layer having a composition of Al.sub.0.3Ga.sub.0.7N and having a thickness of 20 nm is formed on the surface of the active layer 5. Thereafter, by changing the flow rate of TMG to 9 μmol/min and supplying the source material gases for 360 seconds, a hole supply layer having a composition of Al.sub.0.13Ga.sub.0.87N and having a thickness of 120 nm is formed. These hole supply layers constitute the p-type nitride semiconductor layer 6.

[0057] Here, in the process of forming the p-type nitride semiconductor layer 6, since the film is grown at a temperature lower than that in the process of forming the n-type nitride semiconductor layer 4, the interior of the furnace has an Group III-rich environment compared with that at the time of forming the n-type nitride semiconductor layer 4. Therefore, the concentration of C contained in the p-type nitride semiconductor layer 6 can be higher than that of the n-type nitride semiconductor layer 4. However, as will be described later, even when the concentration of C contained in the p-type nitride semiconductor layer 6 was as high as about 1×10.sup.19/cm.sup.3, for example, it was possible to obtain the effect of attenuating the deep emission by setting the concentration of C contained in the n-type nitride semiconductor layer 4 to be less than or equal to 1×10.sup.17/cm.sup.3.

[0058] As the p-type impurity, magnesium (Mg), beryllium (Be), zinc (Zn), carbon (C) or the like can be used.

(Subsequent Step)

[0059] After formation of the p-type nitride semiconductor layer 6, supply of the TMA is stopped, and the flow rate of biscyclopentadienyl is changed to 0.2 μmol/min and the source material gas is supplied for 20 seconds. By this process, a high concentration p-type GaN layer constituted of p-type GaN and having a thickness of 5 nm is formed.

[0060] The subsequent steps are as follows.

[0061] For realizing the semiconductor light emitting element 1 having a so-called “lateral structure” in which an n-side electrode and a p-type electrode are arranged on the same plane side of the substrate 2, the upper face of a part of the n-type nitride semiconductor layer 4 is exposed by ICP etching, and on top of the exposed n-type nitride semiconductor layer 4, an n-side electrode is formed, and on top of the p-type nitride semiconductor layer 6, a p-side electrode is formed. Then the elements are separated from each other, for example, by a laser dicing apparatus, and wire bonding is conducted on the electrodes.

[0062] On the other hand, in the case of producing the semiconductor light emitting element 1 having a so-called “vertical structure” in which an n-side electrode is arranged on one plane of the substrate, and a p-side electrode is arranged on the other plane, the following procedure is employed. First, on top of the p-type nitride semiconductor layer 6, a metal electrode (reflection electrode) which is to become a p-side electrode, a solder diffusion preventive layer, and a solder layer are formed. Then after bonding a support substrate (for example, CuW substrate) constituted of a conductor or a semiconductor via the solder layer, the resultant laminate is turned upside down, and the substrate 2 is peeled off by a method such as laser irradiation. Thereafter, the n-side electrode is formed on top of the n-type nitride semiconductor layer 4. Then, separation of elements and wire bonding are conducted in the same manner as in the lateral structure.

EXAMPLES

[0063] Hereinafter, the present invention will be described by referring to examples.

(Verification 1)

[0064] Three elements of Example 1, Example 2, and Comparative example 1 were formed in the same conditions except that only the V/III ratio of the source material gas at the time of forming the n-type nitride semiconductor layer 4 was changed in the aforementioned process. Every element is an ultraviolet light emitting element having a major emission wavelength of 370 nm.

Example 1

[0065] The element was formed while the V/III ratio was set to be 4000. The concentration of C contained in the n-type nitride semiconductor layer 4 was 5×10.sup.16/cm.sup.3, and the concentration of O contained in the n-type nitride semiconductor layer 4 was 4×10.sup.16/cm.sup.3.

Example 2

[0066] The element was formed while the V/III ratio was set to be 2000. The concentration of C contained in the n-type nitride semiconductor layer 4 was 1×10.sup.17/cm.sup.3, and the concentration of O contained in the n-type nitride semiconductor layer 4 was 5×10.sup.16/cm.sup.3.

Comparative Example 1

[0067] The element was formed while the V/III ratio was set to be 1000. The concentration of C contained in the n-type nitride semiconductor layer 4 was 5×10.sup.17/cm.sup.3, and the concentration of O contained in the n-type nitride semiconductor layer 4 was 7×10.sup.16/cm.sup.3.

[0068] In every element, the V/III ratio of the source material gas at the time of forming the p-type nitride semiconductor layer 6 was 6000, and the concentration of C contained in the p-type nitride semiconductor layer 6 was 1×10.sup.17/cm.sup.3.

[0069] FIG. 3 is a graph showing spectrum distributions of light obtained when the same voltage is applied to the three elements of Example 1, Example 2, and Comparative example 1. The horizontal axis represents an emission wavelength, and the vertical axis represents a light intensity. FIG. 4 shows photographs showing the light emitting condition of each element.

[0070] As shown in FIG. 3, in Comparative example 1, the ratio (deep intensity ratio) of the intensity of the emission wavelength of the band of 550 nm to 600 nm including the yellow visible light wavelength range (deep emission), to the emission intensity of 370 nm band is about 0.3, which exceeds 0.1%. In this case, while dark violet light would be emitted under the influence of the violet visible light corresponding to the flared part of the peak wavelength by emission of the ultraviolet light under normal circumstances, considerably whitish lighting is observed under the influence of the deep emission as can be seen in FIG. 4. As a result of mixture of the violetish light and the yellowish light, the emitting light is whitish. According to FIG. 3, the deep intensity ratio of Example 1 is about 0.03%, and the deep intensity ratio of Example 2 is about 0.09%.

[0071] In contrast to this, in Example 1 and Example 2, the deep intensity ratio is suppressed to less than or equal to 0.1%, and also in the photographs of FIG. 4, the whitishness is reduced in the color of the light emitted from the elements, and dark color is observed. Comparison among Comparative example 1, Example 1, and Example 2 reveals that the effect of decreasing the deep intensity ratio is obtained as the concentration of C contained in the n-type nitride semiconductor layer 4 is reduced.

[0072] Also the measurement was conducted in the same manner as in Example 2 by setting the V/III ratio of the source material gas at the time of forming the p-type nitride semiconductor layer 6 to be 1000, and raising the concentration of C contained in the p-type nitride semiconductor layer 6 to 1×10.sup.19/cm.sup.3 in the condition that the concentration of C contained in the n-type nitride semiconductor layer 4 was 1×10.sup.17/cm.sup.3, however, no significant difference from Example 2 was observed. This also indicates that the concentration of C contained in the n-type nitride semiconductor layer 4 influences on the deep emission.

[0073] That is, it can be found that the deep emission is derived from the impurity level produced by C contained in the n-type nitride semiconductor layer 4 rather than in the active layer 5. Therefore, it is possible to suppress the deep emission by making the concentration of C contained in the n-type nitride semiconductor layer 4 as small as possible.

[0074] When Mg is doped as an impurity of the p-type nitride semiconductor layer 6, it is expected that emission derived from the level at C is suppressed by the level produced by Mg. Therefore, the impurity concentration of C contained in the p-type nitride semiconductor layer 6 would not influence on the deep emission at least if it is less than or equal to the doping amount of Mg. Since the doping concentration of Mg is about 1 to 2×10.sup.19/cm.sup.3, a concentration of C contained in the p-type nitride semiconductor layer 6 around 1×10.sup.19/cm.sup.3 does not influence on the deep emission. However, when the n-type nitride semiconductor layer 4 contains C in a comparable concentration of C, high deep emission occurs as is already described.

[0075] Since the active layer 5 is also constituted of an n-polar nitride semiconductor, a lower concentration of C contained in the active layer 5 is preferred. However, since the active layer 5 has a very small thickness compared with the n-type nitride semiconductor layer 4, the absolute amount of C contained therein is very small compared with the n-type nitride semiconductor layer 4. Therefore, actually, contribution to the deep emission is not as large as the concentration of C contained in the n-type nitride semiconductor layer 4.

(Verification 2)

[0076] Four elements of Example 3, Example 4, Comparative example 2, and Comparative example 3 were formed while the V/III ratio of the source material gas at the time of forming the n-type nitride semiconductor layer 4 was changed in the above-described process. Every element is an ultraviolet light emitting element having a major emission wavelength of 370 nm band.

Example 3

[0077] The element was formed while the V/III ratio was set to be 5000. The concentration of C contained in the n-type nitride semiconductor layer 4 was 3×10.sup.16/cm.sup.3, and the concentration of O contained in the n-type nitride semiconductor layer 4 was 8×10.sup.16/cm.sup.3.

Example 4

[0078] The element was formed while the V/III ratio was set to be 5000. The concentration of C contained in the n-type nitride semiconductor layer 4 was 3×10.sup.16/cm.sup.3, and the concentration of O contained in the n-type nitride semiconductor layer 4 was 3×10.sup.16/cm.sup.3.

Comparative Example 2

[0079] The element was formed while the V/III ratio was set to be 5000. The concentration of C contained in the n-type nitride semiconductor layer 4 was 3×10.sup.16/cm.sup.3, and the concentration of O contained in the n-type nitride semiconductor layer 4 was 2×10.sup.17/cm.sup.3.

Comparative Example 3

[0080] The element was formed while the V/III ratio was set to be 1300. The concentration of C contained in the n-type nitride semiconductor layer 4 was 2×10.sup.17/cm.sup.3, and the concentration of O contained in the n-type nitride semiconductor layer 4 was 5×10.sup.16/cm.sup.3.

[0081] In every element, the V/III ratio of the source material gas at the time of forming the p-type nitride semiconductor layer 6 was 6000, and the concentration of C contained in the p-type nitride semiconductor layer 6 was 1×10.sup.17/cm.sup.3.

[0082] FIG. 5 is a graph showing spectrum distributions of light obtained when the same voltage is applied to the four elements of Example 3, Example 4, Comparative example 2, and Comparative example 3. The horizontal axis represents an emission wavelength, and the vertical axis represents a light intensity.

[0083] As shown in FIG. 5, in Comparative example 2, the ratio (deep intensity ratio) of the intensity of the emission wavelength of the band of 550 nm to 600 nm including the yellow visible light wavelength range (deep emission), to the emission intensity of 370 nm band is about 0.12%. In Comparative example 3, the deep intensity ratio is about 0.19%. In contrast to this, the deep intensity ratio of Example 3 is about 0.03%, and the deep intensity ratio of Example 4 is about 0.02%.

[0084] In each element of Comparative example 2, Example 3, and Example 4, the concentration of C contained in the n-type nitride semiconductor layer 4 is 3×10.sup.16/cm.sup.3 which is lower than 1×10.sup.17/cm.sup.3. These elements can realize a low deep intensity ratio in comparison with the elements of Comparative example 1 and Comparative example 3 in which the concentration of C contained in the n-type nitride semiconductor layer 4 is higher than 1×10.sup.17/cm.sup.3. This also suggests that setting the concentration of C contained in the n-type nitride semiconductor layer 4 to be less than or equal to 1×10.sup.17/cm.sup.3, as described above is effective for decreasing the deep intensity ratio.

[0085] On the other hand, when the respective elements of Comparative example 2, Example 3, and Example 4 are compared, the element of Comparative example 2 shows a deep intensity ratio slightly higher than 0.1%, and the elements of Example 3 and Example 4 show a deep intensity ratio of lower than 0.1%. This result suggests that reducing the concentration of O contained in the n-type nitride semiconductor layer 4 in addition to setting the concentration of C contained in the n-type nitride semiconductor layer 4 to be lower than or equal to 1×10.sup.17/cm.sup.3 is effective for further decreasing the deep intensity ratio.

[0086] In each element of Examples 1 to 4 where the deep intensity ratio is less than 0.1%, the concentration of O contained in the n-type nitride semiconductor layer 4 is 4×10.sup.16/cm.sup.3 in Example 1, 5×10.sup.16/cm.sup.3 in Example 2, 8×10.sup.16/cm.sup.3 in Example 3, and 3×10.sup.16/cm.sup.3 in Example 4. And the concentration of O contained in the n-type nitride semiconductor layer 4 in the element of Comparative example 2 where the deep intensity ratio is slightly higher than 0.1% is 2×10.sup.17/cm.sup.3.

[0087] In the verification 1, when elements of Example 1, Example 2, and Comparative example 1 were produced in the same conditions except that only the V/III ratio of the source material gas at the time of forming the n-type nitride semiconductor layer 4 was changed, the concentration of O contained in the n-type nitride semiconductor layer 4 increased with the concentration of C contained therein, and the concentration of O contained in the n-type nitride semiconductor layer 4 showed a value lower than the concentration of C contained therein. This suggests that C is more likely to be taken into the n-type nitride semiconductor layer 4 than O, and O can be taken in association with C.

[0088] Further, comparing the element of Comparative example 2 having the concentration of C contained in the n-type nitride semiconductor layer 4 of 3×10.sup.16/cm.sup.3, and the concentration of O contained therein of 2×10.sup.17/cm.sup.3, and the element of Comparative example 1 having the concentration of C contained in the n-type nitride semiconductor layer 4 of 5×10.sup.17/cm.sup.3, and the concentration of O contained therein of 7×10.sup.16/cm.sup.3, the element of the Comparative example 1 shows a very high deep intensity ratio compared with the element of Comparative example 2. In light of this, it would be possible to set the deep intensity ratio to be less than or equal to 0.1% by setting the concentration of C contained in n-type nitride semiconductor layer 4 to be less than or equal to 1×10.sup.17/cm.sup.3, and setting the concentration of O contained in the same layer to be less than or equal to 1×10.sup.17/cm.sup.3 which is comparable to the condition of the concentration of C contained therein. Further, it is more preferred to set the concentration of C contained in the n-type nitride semiconductor layer 4 to be less than or equal to 1×10.sup.17/cm.sup.3, and to set the concentration of O contained therein to be less than or equal to 8×10.sup.16/cm.sup.3.

Another Embodiment

[0089] In the above description, the LED element 1 shown in FIG. 2 has the substrate 2 and the undoped layer 3; however, the configuration in which these are peeled off (see FIG. 6) may be employed. Also in this case, the effects similar to those described above by referring to FIGS. 3 to 5 were obtained.

DESCRIPTION OF REFERENCE SIGNS

[0090] 1: nitride semiconductor light emitting element [0091] 2: substrate [0092] 3: undoped layer [0093] 4: n-type nitride semiconductor layer [0094] 5: active layer [0095] 6: p-type nitride semiconductor layer [0096] 51, 52, 53, 54, 55: LED element(s)