Method of producing substrates including gallium nitride

Abstract

A method of producing a functional device has an etched gallium nitride layer and a functional layer having a nitride of a group 13 element. The method includes providing a body comprising a surface gallium nitride layer, performing a dry etching treatment of a surface of the surface gallium nitride layer to provide the etched gallium nitride layer using a plasma etching system comprising an inductively coupled plasma generating system, introducing an etchant during the dry etching treatment, the etchant consisting essentially of a fluorine-based gas, and forming the functional layer on a surface of the etched gallium nitride layer.

Claims

1. A method of producing a functional device comprising an etched gallium nitride layer and a functional layer, said functional layer comprising a nitride of a group 13 element, said method comprising: providing a body comprising a surface gallium nitride layer; performing a dry etching treatment of a surface of said surface gallium nitride layer to provide said etched gallium nitride layer using a plasma etching system comprising an inductively coupled plasma generating system; introducing an etchant during said dry etching treatment, said etchant consisting essentially of a fluorine-based gas; and forming said functional layer on a surface of said etched gallium nitride layer.

2. The method of claim 1, wherein said functional layer comprises a semiconductor light emitting diode structure.

3. The method of claim 1, wherein said fluorine-based gas comprises one or more kind of compound selected from the group consisting of carbon fluoride, fluorohydrocarbon and sulfur fluoride.

4. The method of claim 1, wherein said fluorine-based gas comprises one or more kind of compound selected from the group consisting of CF.sub.4, CHF.sub.3, C.sub.4F.sub.8 and SF.sub.6.

5. The method of claim 1, wherein a standardized direct current bias potential standardized by an area of an electrode of 10 V/cm.sup.2 or higher is applied in said dry etching treatment.

6. The method of claim 1, wherein said surface of said surface gallium nitride layer is subjected to mechanical polishing and then to said dry etching treatment without intervening chemical mechanical polishing.

7. The method of claim 1, wherein a pit amount on said etched gallium nitride layer after said dry etching treatment is substantially the same as a pit amount on said surface of surface said gallium nitride layer before said dry etching treatment.

8. The method of claim 1, wherein an arithmetic average roughness Ra of said etched gallium nitride layer after said dry etching treatment is substantially the same as an arithmetic average roughness Ra of said surface of said surface gallium nitride layer before said dry etching treatment.

9. The method of claim 1, further comprising the step of producing said surface gallium nitride layer by a flux method.

10. The method of claim 1, wherein said functional device comprises a supporting body on which said etched gallium nitride layer is formed.

11. The method of claim 1, wherein said surface of said surface gallium nitride layer is subjected to mechanical polishing for thinning said surface gallium nitride layer prior to said dry etching treatment.

12. The method of claim 5, wherein said standardized direct current bias potential standardized by said area of the electrode of 0.005 V/cm.sup.2 or lower is applied in said dry etching treatment.

13. The method of claim 5, wherein an electric power standardized by an area of the electrode during said dry etching treatment is 0.003 W/cm.sup.2 or higher and 2.0 W/cm.sup.2 or lower.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] FIG. 1(a) is a view schematically showing a gallium nitride layer 2 formed on a seed crystal substrate 1, FIG. 1(b) is a view schematically showing a GaN substrate, and FIG. 1(c) is a view schematically showing a functional device 15 including a GaN substrate 4 and a functional device structure 5 formed thereon.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

(Applications)

[0036] The present invention may be used in technical fields requiring high quality, such as a blue LED with improved color rendering index and expected as a post luminescent lamp, a blue-violet laser for high-speed and high-density optical memory, a power device for an inverter for a hybrid car or the like.

[0037] (Substrate Including at Least a Surface Gallium Nitride Layer)

[0038] The substrate of the invention is one having at least a gallium nitride layer at its surface. It is called GaN substrate below. The inventive substrate may be a self-supporting substrate made of gallium nitride only. Alternatively, the inventive GaN substrate may be a substrate including a separate supporting body and a gallium nitride layer formed thereon. Further, the GaN substrate may include another layer such as an underlying layer, an intermediate layer or a buffer layer, in addition to the gallium nitride layer and supporting body.

[0039] According to a preferred embodiment, as shown in FIG. 1(a), a gallium nitride layer 2 is formed on a surface 1a of a seed crystal substrate 1. Then, preferably, a surface 2a of the gallium nitride layer 2 is subjected to polishing to make a gallium nitride layer 3 thinner, as shown in FIG. 1(b), to obtain a GaN substrate 4. 3a represents a surface after the polishing.

[0040] A functional layer 5 is formed, by vapor phase process, on the surface 3a of the thus obtained GaN substrate 4 to obtain a functional device 15 (FIG. 1(c)). Besides, 5a, 5b, 5c, 5d and 5e represent appropriate epitaxial layers grown on the surface 3a.

[0041] The whole of the seed crystal substrate 1 may be composed of a self-supporting substrate of GaN. Alternatively, the seed crystal substrate 1 may be composed of a supporting body and a seed crystal film formed on the supporting body. Further, preferably, the surface 2a of the gallium nitride layer 2 is subjected to polishing to make the gallium nitride layer thinner to obtain the GaN substrate.

[0042] According to the present invention, the surface of the GaN substrate is subjected to the dry etching. According to a preferred embodiment, the surface was mechanically polished and then subjected to dry etching without performing chemical mechanical polishing.

[0043] (Seed Crystal)

[0044] According to a preferred embodiment, the seed crystal is composed of gallium nitride crystal. The seed crystal may form the self-supporting substrate (supporting body) or may be the seed crystal film formed on the separate supporting body. The seed crystal film may be composed of a single layer or may include the buffer layer on the side of the supporting body.

[0045] The method of forming the seed crystal film may preferably be vapor phase process, and metal organic chemical vapor deposition (MOCVD) method, hydride vapor phase epitaxy method, pulse-excited deposition (PXD) method, MBE method and sublimation method are exemplified. Metal organic chemical vapor deposition is most preferred. Further, the growth temperature may preferably be 950 to 1200 C.

[0046] In the case that the seed crystal film is formed on the supporting body, although the material forming the supporting body is not limited, it includes sapphire, AlN template, GaN template, self-supporting GaN substrate, silicon single crystal, SiC single crystal, MgO single crystal, spinel (MgAl.sub.2O.sub.4), LiAlO.sub.2, LiGaO.sub.2, and perovskite composite oxide such as LaAlO.sub.3, LaGaO.sub.3 or NdGaO.sub.3 and SCAM (ScAlMgO.sub.4). A cubic perovskite composite oxide represented by the composition formula [A.sub.1-y(Sr.sub.1-xBa.sub.x).sub.y] [(Al.sub.1-zGa.sub.z).sub.1-uD.sub.u]O.sub.3 (wherein A is a rare earth element; D is one or more element selected from the group consisting of niobium and tantalum; y=0.3 to 0.98; x=0 to 1; z=0 to 1; u=0.15 to 0.49; and x+z=0.1 to 2) is also usable

[0047] The direction of growth of the gallium nitride layer may be a direction normal to c-plane of the wurtzite structure or a direction normal to each of the a-plane and m-plane.

[0048] The dislocation density at the surface of the seed crystal is preferably lower, on the viewpoint of reducing the dislocation density of the gallium nitride layer provided on the seed crystal. On the viewpoint, the dislocation density of the seed crystal layer may preferably be 710.sup.8 cm.sup.2 or lower and more preferably be 510.sup.8 cm.sup.2 or lower. Further, as the dislocation density of the seed crystal may preferably be lower on the viewpoint of the quality, the lower limit is not particularly provided, but it may generally be 510.sup.7 cm.sup.2 or higher in many cases.

[0049] (Gallium Nitride Layer)

[0050] Although the method of producing the gallium nitride layer is not particularly limited, it includes vapor phase process such as metal organic chemical vapor deposition (MOCVD) method, hydride vapor phase epitaxy (HVPE) method, pulse-excited deposition (PXD) method, MBE method and sublimation method, and liquid phase process such as flux method.

[0051] According to a preferred embodiment, the gallium nitride layer is grown by flux method. In this case, the kind of the flux is not particularly limited, as far as it is possible to grow gallium nitride crystal. According to a preferred embodiment, it is used a flux containing at least one of an alkali metal and alkaline earth metal, and flux containing sodium metal is particularly preferred.

[0052] A gallium raw material is mixed to the flux and used. As the gallium raw material, gallium single metal, a gallium alloy and a gallium compound are applicable, and gallium single metal is suitably used from the viewpoint of handling.

[0053] The growth temperature of the gallium nitride crystal in the flux method and the holding time during the growth are not particularly limited, and they are appropriately changed in accordance with a composition of the flux. As an example, when the gallium nitride crystal is grown using a flux containing sodium or lithium, the growth temperature may be preferably set at 800 C. to 950 C., and more preferably set at 800 to 900 C.

[0054] According to flux method, a single crystal is grown in an atmosphere containing nitrogen-containing gas. For this gas, nitrogen gas may be preferably used, and ammonia may be used. The total pressure of the atmosphere is not particularly limited; but it may be preferably set at 3 MPa or more, and further preferably 4 MPa or more, from the standpoint of prevention against the evaporation of the flux. However, as the pressure is high, an apparatus becomes large. Therefore, the total pressure of the atmosphere may be preferably set at 7 MPa or lower, and further preferably 5 MPa or lower. Any other gas except the nitrogen-containing gas in the atmosphere is not limited; but an inert gas may be preferably used, and argon, helium, or neon may be particularly preferred.

[0055] (Cathode Luminescence)

[0056] Cathode luminescence is to evaluate microscopic deviations on the surface of the GaN substrate. According to the present invention, the cathode luminescence of a wavelength corresponding to band gap of gallium nitride is measured at the surface of the GaN substrate.

[0057] In the case that mapping is performed, distribution of cathode luminescence spectrum is measured at each point and luminous intensities at a specific wavelength region are compared to perform the mapping. By limiting the wavelength region, it becomes possible to draw cathode luminescence peak spectrum due to the band gap only. Based on the peaks of the cathode luminescence, an average gradation (Xave) as an average of the intensities and a peak gradation (Xpeak) as the maximum value of the intensities can be calculated.

[0058] According to a preferred embodiment, in the image of the cathode luminescence mapping, the dark spots can be detected. According to the cathode luminescence, in the case that the mapping is performed based on the luminescence due to band edge, the luminescence due to the band edge cannot be observed in the dislocation regions and its luminance intensity becomes considerably lower than that of the surroundings, which is observed as the dark spots. It is preferred to elevate an acceleration voltage to 10 kV or larger for clearly distinguishing the light emitting regions and non-light emitting regions. By counting the number of the dark spots in the non-light emitting region by mapping in a specific visual field range, for example visual field of 100 m, the density of the dark spots can be evaluated.

[0059] (Processing and Shape of GaN Substrate)

[0060] According to a preferred embodiment, the GaN substrate has a shape of a circular plate, and it may have another shape such as a rectangular plate. Further, according to a preferred embodiment, the dimension of the GaN substrate is of a diameter of 25 mm or larger. It is thereby possible to provide the GaN substrate which is suitable for the mass production of functional devices and easy to handle.

[0061] It will be described as to the case that the surface of the GaN substrate is subjected to grinding and polishing.

[0062] Grinding is that an object is contacted with fixed abrasives obtained by fixing the abrasives by a bond and rotating at a high rotation rate to grind a surface of the object. By such grinding, a roughed surface is formed. In the case that a bottom face of a gallium nitride substrate is ground, it is preferably used the fixed abrasives containing the abrasives, composed of SiC. Al.sub.2O.sub.3, diamond, CBN (cubic boron nitride, same applies below) or the like having a high hardness and having a grain size of about 10 m to 100 m.

[0063] Further, lapping is that a surface plate and an object are contacted while they are rotated with respect to each other through free abrasives (it means abrasives which are not fixed, same applies below), or fixed abrasives and the object are contacted while they are rotated with respect to each other, to polish a surface of the object. By such lapping, it is formed a surface having a surface roughness smaller than that in the case of the grinding and larger than that in the case of micro lapping (polishing). It is preferably used abrasives composed of SiC. Al.sub.2O.sub.3, diamond, CBN or the like having a high hardness and having a grain size of about 0.5 m to 15 m.

[0064] Micro lapping (polishing) means that a polishing pad and an object are contacted with each other through free abrasives while they are rotated with each other, or fixed abrasives and the object are contacted with each other while they are rotated with each other, for subjecting a surface of the object to micro lapping to flatten it. By such polishing, it is possible to obtain a crystal growth surface having a surface roughness smaller than that in the case of the lapping.

[0065] (Treatment by Inductively Coupled Plasma)

[0066] Inductively coupled plasma (abbreviated as ICP) is to apply a high voltage on a gas to generate plasma and further to apply variable magnetic field of a high frequency, so that Joule heat is generated by eddy current in the plasma to obtain high temperature plasma.

[0067] Specifically, a coil is wound around a flow route composed of a tube of quartz glass or the like, through which a gas passed, and a large current of a high frequency is flown in the flow route to generate variable magnetic field of a high voltage and high frequency and to flow the gas in the flow route so that inductively coupled plasma is generated. The plasma is supplied onto the surface of the GaN substrate.

[0068] Here, the standardized direct current bias potential (Vdc/S) during the etching may preferably be made 10 V/cm.sup.2 or higher. Vdc means a direct current bias voltage (unit of V) applied between electrodes. S means a total area (unit of cm.sup.2) of the GaN surface to be treated. Vdc/S means a bias voltage during the etching, standardized by the total area of the GaN surface to be treated. According to the present invention, Vdc/S may be made 10 V/cm.sup.2 or higher. Although the bias voltage is changed by combination of gallium nitride composite substrates and setting method, in the case that Vdc/S is below this, the processing damage onto the uppermost surface of GaN becomes deeper. On the viewpoint, Vdc/S may preferably be 8 V/cm.sup.2 or higher.

[0069] Further, on the viewpoint of accelerating the processing of the surface of the GaN substrate, Vdc/S may preferably be made 0.005 V/cm.sup.2 or lower, more preferably be 0.05 V/cm.sup.2 or lower, and still further preferably be 1.5 V/cm.sup.2 or lower.

[0070] Further, the electric power of the bias potential during the etching (electric power standardized by the area of the electrode) may preferably be 0.003 W/cm.sup.2 or higher and more preferably be 0.03 W/cm.sup.2 or higher, on the viewpoint of generating the plasma stably. Further, the electric power of the bias potential during the etching (the electric power standardized by the area of the electrode) may preferably be 2.0 W/cm.sup.2 or lower and more preferably be 1.5 W/cm.sup.2 or lower, on the viewpoint of reducing the processing damage on the surface of the GaN substrate.

[0071] The fluorine-based gas may preferably be one or more compound selected from the group consisting of carbon fluoride, fluorohydrocarbon and sulfur fluoride.

[0072] According to a preferred embodiment, the fluorine-based gas is one or more compound selected from the group consisting of CF.sub.4, CH.sub.3F, C.sub.4F.sub.8 and SF.sub.6.

[0073] According to a preferred embodiment, the pit amount on the surface after the dry etching is substantially same as the pit amount on the surface before the dry etching. The pit amount is measured as follows.

[0074] AFM (Atomic force Microscope) is used to perform the observation of the surface in a visual field of 10 m and to count a number of recesses of 1 nm or larger with respect to the surrounding, so that it can be evaluated.

[0075] According to a preferred embodiment, the arithmetic surface roughness Ra of the surface of the substrate after the dry etching is substantially same as the arithmetic surface roughness Ra of the substrate surface before the dry etching. Besides, Ra is a measured value standardized by .JIS B 0601(1994)JIS B 0031(1994).

[0076] (Functional Layer and Functional Device)

[0077] The functional layer as described above may be composed of a single layer or a plurality of layers. Further, as the functions, it may be used as a white LED with high brightness and improved color rendering index, a blue-violet laser disk for high-speed and high-density optical memory, a power device for an inverter for a hybrid car or the like.

[0078] As a semiconductor light emitting diode (LED) is produced on the GaN substrate by a vapor phase process, preferably by metal organic vapor phase deposition (MOCVD) method, the dislocation density inside of the LED can be made comparable with that of the GaN substrate.

[0079] The film-forming temperature of the functional layer may preferably be 950 C. or higher and more preferably be 1000 C. or higher, on the viewpoint of the film-formation rate. Further, on the viewpoint of preventing defects, the film-forming temperature of the functional layer may preferably be 1200 C. or lower and more preferably be 1150 C. or lower.

[0080] The material of the functional layer may preferably be a nitride of a group 13 element. Group 13 element means group 13 element according to the Periodic Table determined by IUPAC. The group 13 element is specifically gallium, aluminum, indium, thallium or the like. Further, as an additive, it may be listed carbon, a metal having a low melting point (tin, bismuth, silver, gold), and a metal having a high melting point (a transition metal such as iron, manganese, titanium, chromium). The metal having a low melting point may be added for preventing oxidation of sodium, and the metal having a high melting point may be incorporated from a container for containing a crucible, a heater of a growing furnace or the like.

[0081] The light emitting device structure includes, an n-type semiconductor layer, a light emitting region provided on the n-type semiconductor layer and a p-type semiconductor layer provided on the light emitting region, for example. According to the light emitting device 15 shown in FIG. 1(c), an n-type contact layer 5a, an n-type clad layer 5b, an activating layer 5c, a p-type clad layer 5d and a p-type contact layer 5e are formed on the GaN substrate 4 to constitute the light emitting structure 5.

[0082] Further, the light emitting structure described above may preferably further include an electrode for the n-type semiconductor layer, an electrode for the p-type semiconductor layer, a conductive adhesive layer, a buffer layer and a conductive supporting body or the like not shown.

[0083] According to the light emitting structure, as light is emitted in the light emitting region through re-combination of holes and electrons injected through the semiconductor layers, the light is drawn through the side of a translucent electrode on the p-type semiconductor layer or the film of the nitride single crystal of the group 13 element. Besides, the translucent electrode means an electrode capable of transmitting light and made of a metal thin film or transparent conductive film formed substantially over the whole of the p-type semiconductor layer.

EXAMPLES

Example 1

[0084] The GaN substrate was produced according to the following procedure.

[0085] Specifically, it was prepared a self-supporting type seed crystal substrate 1 made of gallium nitride seed crystal whose in-plane distribution of dislocation density by CL (cathode luminescence) was 210.sup.8/cm.sup.2 in average excluding its outer periphery of 1 cm. The thickness of the seed crystal was 400 m.

[0086] The gallium nitride layer 2 was formed by flux method using the seed crystal substrate 1. Specifically, Na and Ga were charged into a crucible, held at 870 C. and 4.0 MPa (nitrogen atmosphere) for 5 hours, and then cooled to 850 C. over 10 minutes. It was then held at 4.0 MPa for 20 hours to grow a gallium nitride layer 2. An alumina crucible was used, and the raw materials were Na:Ga=40 g:30 g. For agitating solution, the direction of rotation was changed to clockwise or anti-clockwise direction per every 600 minutes. The rotational rate was made 30 rpm.

[0087] After the reaction, it was cooled to room temperature and the flux was removed by chemical reaction with ethanol to obtain the gallium nitride layer 2 having a growth thickness of 100 m.

[0088] The thus obtained substrate was fixed on a ceramic surface plate and then ground with abrasives of #2000 to make the surface flat. Then, the surface was smoothened by lapping using diamond abrasives. The sizes of the abrasives were lowered from 3 m to 0.1 m stepwise for improving the flatness. The arithmetic average roughness Ra of the surface of the substrate was 0.5 nm. The thickness of the gallium nitride layer after the polishing was 15 m. Further, the substrate was colorless and transparent.

[0089] The thus polished surface state was measured by PL to prove that a luminescence peak having a small intensity ratio was observed. Further, as it was observed by CL, it was black without substantial luminescence and dark spots could not be observed. That is, it was proved that the stress by the processing was proved to be large (the thickness of the stressed region was thicker than the depth of penetration of electron beam).

[0090] Then, the surface of the GaN substrate was subjected to dry etching. For the dry etching, it was used an inductively coupled type plasma etching system. A fluorine-based gas (CF.sub.4) was used as the etching gas to perform the dry etching. The size of electrodes was about 8 inches. The etching conditions were as follows.

Output power; (RF, 400 W, bias: 200 W)
Chamber pressure: 1 Pa
Etching time period; 10 minutes
Standardized direct current bias potential (Vdc/S): 5.2 V/cm.sup.2
Electric power of bias voltage (electric power standardized by an area of the electrode): 1.3 W/cm.sup.2.

[0091] As a result, the etching rate was 0.006 micron/minute and the etching depth was about 0.06 micron. The substrate remained to be colorless and transparent.

[0092] The surface of the substrate after the dry etching treatment was subjected to PL measurement to prove that luminescence peak having a high intensity ratio was observed. Further, as it was observed by CL, the ratio of the peak intensities of the CL spectra in the brighter region before and after the dry etching was proved to be more than 5, so that the dark spots corresponding to the defects could be clearly observed. Further, as elements on the surface were confirmed by XPS (X ray photoemission spectroscopy), spectrum corresponding to carbon was detected other than GaN. Spectra corresponding to fluorine, chlorine and silicon were not detected.

[0093] This substrate was used to produce an LED, it could be produced an LED having a high luminous efficiency. Further, leak current under a low driving voltage (for example, 2 to 2.5 V) was very low.

Example 2

[0094] The GaN substrate was obtained similarly as the Example 1. However, the thickness of the seed crystal layer was made 3 m, and the thickness of the grown GaN layer was made 80 m. The thickness of the GaN layer after the polishing was made 15 m.

[0095] Thereafter, as the Example 1, it was subjected to dry etching. The etching conditions were as follows.

Output power; (RF, 400 W, bias: 200 W)
Chamber pressure: 1 Pa
Etching time period; 5 minutes
Standardized direct current bias potential (Vdc/S): 7.2 V/cm.sup.2
Electric power of bias voltage (electric power standardized by an area of the electrode): 0.8 W/cm.sup.2.

[0096] As a result, the etching rate was 0.005 m/minute and the etching depth was about 0.025 m. The substrate remained to be colorless and transparent. The surface of the substrate after the dry etching treatment was subjected to PL measurement to prove that luminescence peak having a high intensity ratio was observed. Further, as the substrate surface was observed by CL, the dark spots corresponding to the defects could be clearly observed. Further, as elements on the surface were confirmed by XPS, spectrum corresponding to carbon was detected other than GaN. Spectra corresponding to fluorine, chlorine and silicon were not detected. As this substrate was used to produce an LED, it could be produced an LED having a high luminous efficiency. Further, leak current under a low driving voltage (for example, 2 to 2.5 V) was very low.

Example 3

[0097] The experiment was performed as the Example 1. However, the gas specie for the dry etching was changed to SF.sub.6 and the etching conditions were made as follows.

Output power; (RF, 400 W, bias: 200 W)
Chamber pressure: 1 Pa
Etching time period; 5 minutes
Standardized direct current bias potential (Vdc/S): 3.6 V/cm.sup.2
Electric power of bias voltage (electric power standardized by an area of the electrode): 1.4 W/cm.sup.2.

[0098] As a result, the etching rate was 0.005 m/minute and the etching depth was about 0.025 m. The substrate remained to be colorless and transparent.

[0099] The surface of the substrate after the dry etching treatment was subjected to PL measurement to prove that luminescence peak having a high intensity ratio was observed. Further, as the substrate surface was observed by CL, the dark spots corresponding to the defects could be clearly observed. Further, as elements on the surface were confirmed by XPS, spectrum corresponding to carbon was detected other than GaN. Spectra corresponding to fluorine, chlorine and silicon were not detected.

[0100] As this substrate was used to produce an LED, it could be produced an LED having a high luminous efficiency. Further, leak current under a low driving voltage (for example, 2 to 2.5 V) was very low.

Comparative Example 1

[0101] The experiment was performed as the Example 1. However, the gas specie for the dry etching was changed to chlorine-based gas (gas flow rate: BCl.sub.3+Cl.sub.2=3:1) and the etching conditions were made as follows.

Output power; (RF, 400 W, bias: 200 W)
Chamber pressure: 1 Pa
Etching time period; 5 minutes
Standardized direct current bias potential (Vdc/S): 13.1 V/cm.sup.2
Electric power of bias voltage (electric power standardized by an area of the electrode): 1.3 W/cm.sup.2.

[0102] As a result, the etching rate was 0.5 m/minute and the etching depth was about 2.5 m. The substrate remained to be colorless and transparent.

[0103] The surface of the substrate after the dry etching treatment was subjected to PL measurement to prove that luminescence peak having a high intensity ratio was observed. However, as the substrate was observed by CL, the ratio of the peak intensities of the CL spectra of the brighter region before and after the dry etching was proved to be less than 1.5. That is, although the images could be seen than those before the dry etching, the intensity ratio of luminescence spectra was still low to provide dark images, so that the dark spots could not be clearly observed. An additional processing of 5 minutes was performed and it was then observed by CL again, the luminescence image was not changed and the dark spots could not be observed. Further, as elements on the surface were confirmed by XPS, spectrum corresponding to chlorine was detected other than GaN. Spectra corresponding to fluorine and carbon were not detected.

[0104] As described above, by using a chlorine-based gas, damages due to the plasma were further generated on the surface of GaN and the processing stress could not be prevented.

[0105] As the substrate was used to produce an LED, leak current under a low driving voltage (for example, 2 to 2.5 V) was very large and the LED performances were not good. It is probably clue to a chloride formed on the uppermost surface of GaN.

Comparative Example 2

[0106] The experiment was performed as the Example 1. However, the dry etching system was changed from the inductively-coupled type to parallel plate type, and the etching conditions were made as follows.

Output power; 600 W
Chamber pressure: 3 Pa
Etching time period; 5 minutes
Standardized direct current bias voltage (Vdc/S): 11.3 V/cm.sup.2

[0107] As a result, the etching rate was 0.02 m/minute and the etching depth was about 0.1 m. The substrate remained to be colorless and transparent.

[0108] The surface of the substrate after the dry etching treatment was subjected to PL measurement to prove that luminescence peak having a high intensity ratio was observed. However, as the substrate surface was observed by CL, although the images could be seen than those before the dry etching, the intensity ratio of luminescence spectra was still low to provide dark images, so that the dark spots could not be observed. An additional processing of 5 minutes was performed and it was then observed by CL, the intensity ratio was not changed and the dark spots could not be observed. Further, as elements on the surface were confirmed by XPS, spectrum corresponding to carbon was detected other than GaN. Spectra corresponding to fluorine, chlorine and silicon were not detected.

Example 4

[0109] The experiment was performed as the Example 1. However, the etching conditions were made as follows.

Output power; (RF, 400 W, bias: 300 W)
Chamber pressure: 1 Pa
Etching time period; 3 minutes
Standardized direct current bias potential (Vdc/S): 9.2 V/cm.sup.2
Electric power of bias voltage (electric power standardized by an area of the electrode): 1.9 W/cm.sup.2.

[0110] As a result, the etching rate was 0.06 m/minute and the etching depth was about 0.18 m. The substrate remained to be colorless and transparent.

[0111] The surface of the substrate after the dry etching treatment was subjected to PL measurement to prove that luminescence peak having a high intensity ratio was observed. Further, as the substrate surface was observed by CL, the dark spots corresponding to the defects could be observed. Further, as elements on the surface were confirmed by XPS, spectrum corresponding to carbon was detected other than GaN. Spectra corresponding to fluorine, chlorine and silicon were not detected.

[0112] This substrate was used to produce an LED, the LED performance was good. Further, leak current under a low driving voltage (for example, 2 to 2.5 V) was small.

Comparative Example 3

[0113] The experiment was performed as the Example 1, except that CMP finishing was performed instead of the dry etching.

[0114] The surface of the substrate after the CMP was subjected to PL measurement to prove that luminescence peak having a high intensity ratio was observed. Further, as it was observed by CL, the dark spots corresponding to the defects could be clearly observed. On the other hand, as the surface of the substrate was measured by AFM (Atomic Force Microscope), many etching pits were generated. Further, as elements on the surface were confirmed by XPS, spectrum corresponding to silicon was detected other than GaN. Spectra corresponding to fluorine, chlorine and carbon were not detected.

[0115] As this substrate was used to produce an LED, leak current under a low driving voltage (for example, 2 to 2.5 V) was very large and the performance as LED was poor. This is probably due to the etching pits generated on the substrate surface by CMP.

Example 5

[0116] The experiment was performed as the Example 1. The etching conditions were made as follows.

Output power; (RF, 150 W, bias: 10 W)
Chamber pressure: 1 Pa
Etching time period; 30 minutes
Standardized direct current bias potential (Vdc/S): 1.7 V/cm.sup.2
Electric power of bias voltage (electric power standardized by an area of the electrode): 0.05 W/cm.sup.2.

[0117] As a result, the etching rate was 0.001 m/minute and the etching depth was about 0.03 m.

[0118] The surface of the substrate after the dry etching treatment was subjected to PL measurement to prove that luminescence peak having a high intensity ratio was observed. Further, as the substrate surface was observed by CL, the dark spots corresponding to the defects could be observed. Further, as elements on the surface were confirmed by XPS, spectrum corresponding to carbon was detected other than GaN. Spectra corresponding to fluorine, chlorine and silicon were not detected.

[0119] As this substrate was used to produce an LED, it could be produced an LED having a high luminous efficiency. Further, leak current under a low driving voltage (for example, 2 to 2.5 V) was very low.

Example 6

[0120] The experiment was performed as the Example 1. The etching conditions were made as follows.

Output power; (RF, 50 W, bias: 10 W)
Chamber pressure: 1 Pa
Etching time period; 30 minutes
Standardized direct current bias potential (Vdc/S): 0.02 V/cm.sup.2
Electric power of bias voltage (electric power standardized by an area of the electrode): 0.02 W/cm.sup.2.

[0121] As a result, the etching rate was 0.001 m/minute and the etching depth was about 0.03 m. However, the plasma was unstable and deviation of etching distribution was observed.

[0122] The surface of the substrate after the dry etching treatment was subjected to PL measurement to prove that luminescence peak having a high intensity ratio was observed. Further, as the substrate surface was observed by CL, the dark spots corresponding to the defects could be observed. Further, as elements on the surface were confirmed by XPS, spectrum corresponding to carbon was detected other than GaN. Spectra corresponding to fluorine, chlorine and silicon were not detected.

[0123] As this substrate was used to produce an LED, it could be produced an LED having a high luminous efficiency. Further, leak current under a low driving voltage (for example, 2 to 2.5 V) was very low.