Method of manufacturing Group III nitride crystal substrate, etchant used in the method, Group III nitride crystal substrate, and semiconductor device including the same Number:7,125,801 from the United States Patent and Trademark Office (PTO) owispatent The present invention provides a Group III nitride crystal substrate whose surface has concavities and convexities reduced in size. The surfaces with concavities and convexities, such as hillocks, pits and facets, of Group III nitride crystals are brought into contact with a melt and thereby the surfaces are subjected to meltback etching or mechanochemical polishing. The melt includes at least one of alkali metal and alkaline-earth metal. Thus a Group III nitride crystal substrate that has reduced strain and a reduced number of defects, which are caused through the processing, and is excellent in surface flatness is manufactured. Furthermore, by the use of the Group III nitride crystal substrate of the present invention, for instance, semiconductor devices of high performance can be obtained.<H semiconductor, manufacturing, Method, of, manuf, 7125801.html, Patent, pto, 7125801

Title: Method of manufacturing Group III nitride crystal substrate, etchant used in the method, Group III nitride crystal substrate, and semiconductor device including the same

Abstract: The present invention provides a Group III nitride crystal substrate whose surface has concavities and convexities reduced in size. The surfaces with concavities and convexities, such as hillocks, pits and facets, of Group III nitride crystals are brought into contact with a melt and thereby the surfaces are subjected to meltback etching or mechanochemical polishing. The melt includes at least one of alkali metal and alkaline-earth metal. Thus a Group III nitride crystal substrate that has reduced strain and a reduced number of defects, which are caused through the processing, and is excellent in surface flatness is manufactured. Furthermore, by the use of the Group III nitride crystal substrate of the present invention, for instance, semiconductor devices of high performance can be obtained.

Patent Number: 7,125,801 Issued on 10/24/2006 to Minemoto, et al.

Inventors: Minemoto; Hisashi (Hirakata, JP), Kitaoka; Yasuo (Ibaraki, JP), Kidoguchi; Isao (Kawanishi, JP), Mori; Yusuke (Katano, JP), Sasaki; Takatomo (Suita, JP), Kawamura; Fumio (Minoh, JP)
Assignee: Matsushita Electric Industrial Co., Ltd. (Osaka, JP)

Appl. No.: 10/911,939

Filed: August 4, 2004

Foreign Application Priority Data


Aug 06, 2003 [JP]			2003-288125

Current U.S. Class: 438/691 ; 117/73; 117/77

Current International Class: H01L 21/302 (20060101)

Field of Search: 438/691 117/73,77

References Cited [Referenced By]

U.S. Patent Documents


2002/0037599	March 2002	Ishida et al.
2004/0003495	January 2004	Xu

Foreign Patent Documents


2002-293696	Oct., 2002	JP
2004013385	Feb., 2004	WO

Other References

FKawamura et al., "Growth of Transpatent Large Size GaN Single Crystal with Low Dislocations Using Alkali Metal-based Flux", Japanese Association for Crystal Growth, vol. 30, No. 2, pp. 96-103 (2003). cited by other .
F.A. Ponce et al., "Homoepitaxy of GaN on polished bulk single crystal by metalorganic chemical vapor desposition", Appl. Phys. Let., vol. 68, No. 7 pp. 917-919 (1996). cited by other .
Y.Kaneko et al., "Melt-Back Etching of GaN", Solid-state Electrons, vol. 41, No. 2, pp. 295-298 (1997). cited by other.

Primary Examiner: Norton; Nadine
Assistant Examiner: Tran; Binh X.
Attorney, Agent or Firm: Hamre, Schumann, Mueller & Larson, P.C.

Claims

What is claimed is:

1. A method of manufacturing a Group III nitride crystal substrate, comprising a process of processing surfaces of Group III nitride crystals, wherein the process of processing the surfaces includes at least one of a process of meltback etching the surfaces by bringing the surfaces into contact with a melt and melting the surfaces, the melt including at least one of alkali metal and alkaline-earth metal and a process of mechanochemically polishing the surfaces using the melt.

2. The method of manufacturing a Group III nitride crystal substrate according to claim 1, further comprising, before the process of processing the surfaces, at least one of a process of mechanically processing the surfaces of the Group III nitride crystals and a process of mechanochemically polishing the surfaces.

3. The method of manufacturing a Group III nitride crystal substrate according to claim 1, wherein the Group III nitride crystal substrate is obtained by subjecting bulk crystals to slice processing.

4. The method of manufacturing a Group III nitride crystal substrate according to claim 1, wherein the alkali metal is at least one selected from the group consisting of Na, Li, K, Rb, and Cs while the alkaline-earth metal is at least one selected from the group consisting of Ca, Mg, Sr, Ba, and Be.

5. The method of manufacturing a Group III nitride crystal substrate according to claim 1, wherein the melt comprises Na and Ga.

6. The method of manufacturing a Group III nitride crystal substrate according to claim 1, wherein in the melt, one of the following formulae is satisfied: 0.ltoreq.A/(A+M).ltoreq.0.10 (1); and 09.ltoreq.A/(A+M).ltoreq.0.999 (2), where "A" denotes the number of moles of the Group III element and "M" denotes the number of total moles of the alkali metal and alkaline-earth metal.

7. The method of manufacturing a Group III nitride crystal substrate according to claim 1, wherein the melt used in the meltback etching has a temperature of 400.degree. C. to 900.degree. C.

8. The method of manufacturing a Group III nitride crystal substrate according to claim 1, wherein in the process of processing the surfaces, a damaged layer on the surfaces of the Group III nitride crystals is eliminated.

9. The method of manufacturing a Group III nitride crystal substrate according to claim 1, wherein in the process of processing the surfaces, concavities and convexities of the surfaces of the Group III nitride crystals have sizes of not more than .+-.5 .mu.m.

10. The method of manufacturing a Group III nitride crystal substrate according to claim 1, wherein the melt is unsaturated.

11. The method of manufacturing a Group III nitride crystal substrate according to claim 1, wherein the melt further comprises a Group III element.

12. The method of manufacturing a Group III nitride crystal substrate according to claim 11, wherein the Group III element is at least one selected from the group consisting of Ga, Al, and In.

13. The method of manufacturing a Group III nitride crystal substrate according to claim 1, wherein the melt further comprises polishing grains.

14. The method of manufacturing a Group III nitride crystal substrate according to claim 13, wherein the polishing grains are at least one selected from the group consisting of alumina, diamond, SiC, GaN, and AIN.

15. The method of manufacturing a Group III nitride crystal substrate according to claim 1, wherein the process of processing the surfaces is carried out in an atmosphere containing at least one selected from the group consisting of N.sub.2 gas, NH .sub.3 gas, and H.sub.2 gas.

16. The method of manufacturing a Group III nitride crystal substrate according to claim 15, wherein the atmosphere has a pressure of 1 atm (1.times.1.013.times.10.sup.5 Pa) to 10 atm (10.times.1.013.times.10.sup.5 Pa).

17. The method of manufacturing a Group III nitride crystal substrate according to claim 1, further comprising a process of growing the Group III nitride crystals by at least one of vapor phase growth and liquid phase growth.

18. The method of manufacturing a Group III nitride crystal substrate according to claim 17, wherein the liquid phase growth is a method of growing Group III nitride crystals in a melt or at a surface of the melt in an atmosphere including nitrogen, with the melt containing nitrogen, at least one of alkali metal and alkaline-earth metal, and at least one Group III element selected from the group consisting of gallium, aluminum, and indium.

19. The method of manufacturing a Group III nitride crystal substrate according to claim 18, wherein a semiconductor seed layer of a Group III element compound is prepared, and then the Group III nitride crystals are grown on the semiconductor seed layer.

20. The method of manufacturing a Group III nitride crystal substrate according to claim 19, wherein the liquid phase growth is carried out in two stages including a first stage and a second stage, the first stage corresponds to a process of forming, on the semiconductor seed layer, a first semiconductor layer having a higher defect density than that of the semiconductor seed layer, and the second stage corresponds to a process of forming, on the first semiconductor layer, a second semiconductor layer having a lower defect density than that of the first semiconductor layer.

21. The method of manufacturing a Group III nitride crystal substrate according to claim 20, further comprising a process of removing at least the first semiconductor layer to provide a free-standing Group III nitride crystal substrate that is formed of the second semiconductor layer.

22. The method of manufacturing a Group III nitride crystal substrate according to claim 20, further comprising a process of removing the semiconductor seed layer.

23. The method of manufacturing a Group III nitride crystal substrate according to claim 20, wherein the semiconductor seed layer is formed on a base substrate.

24. A method of manufacturing a Group III nitride crystal substrate, comprising: (i) preparing a semiconductor seed layer of a Group III element compound; (ii) growing, on the semiconductor seed layer, a first semiconductor layer through liquid phase growth, the first semiconductor layer having a higher defect density than that of the semiconductor seed layer; (iii) growing, on the first semiconductor layer, a second semiconductor layer through liquid phase growth, the second semiconductor layer having a lower defect density than that of the first semiconductor layer; and (iv) removing at least the semiconductor seed layer and the first semiconductor layer to obtain a free-standing Group III nitride crystal substrate formed of the second semiconductor layer.

25. The method of manufacturing a Group III nitride crystal substrate according to claim 24, further comprising a process of processing surfaces of Group III nitride crystals, wherein the process of processing the surfaces includes at least one of a process of meltback etching the surfaces by bringing the surfaces into contact with a melt including at least one of alkali metal and alkaline-earth metal, a process of mechanochemically polishing the surfaces using the melt, a process of mechanically processing the surfaces and a process of mechanochemically polishing the surfaces.

26. The method of manufacturing a Group III nitride crystal substrate according to claim 24, wherein the semiconductor seed layer is formed on a base substrate.

27. A method of manufacturing a Group III nitride crystal substrate, comprising: (I) preparing a semiconductor seed layer of a Group III element compound; (II) growing, on the semiconductor seed layer, a first semiconductor layer through liquid phase growth, the first semiconductor layer having a higher defect density than that of the semiconductor seed layer; (III) growing, on the first semiconductor layer, a second semiconductor layer through liquid phase growth, the second semiconductor layer having a lower defect density than that of the first semiconductor layer; and (IV) processing a surface of the second semiconductor layer.

28. The method of manufacturing a Group III nitride crystal substrate according to claim 27, wherein the processing is carded out by at least one meltback etching the surfaces by bringing the surfaces into contact with a melt including at least one of alkali metal and alkaline-earth metal, mechanochemically polishing the surfaces using the melt, mechanically processing the surfaces and mechanochemically polishing the surfaces.

29. The method of manufacturing a Group III nitride crystal substrate according to claim 27, wherein the semiconductor seed layer is formed on a base substrate.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a method of manufacturing a Group III nitride crystal substrate, an etchant to be used in the method, a Group III nitride crystal substrate, and a semiconductor device with the same.

2. Related Background Art

A Group III nitride compound semiconductor such as, for instance, gallium nitride (GaN) (hereinafter also referred to as a "Group III nitride semiconductor" or a "GaN-based semiconductor") has been gaining attention as a material for semiconductor devices that emit green, blue or ultraviolet light. A laser diode (LD) that emits blue light is used for high-density optical disk devices or displays while a light emitting diode (LED) that emits blue light is used for displays, lighting, etc. It is expected to use an ultraviolet LD in the field of, for example, biotechnology and an ultraviolet LED as, for example, an ultraviolet source for a fluorescent lamp.

Generally, Group III nitride semiconductor substrates (for instance, GaN substrates) that are used for LDs or LEDs are formed through vapor phase growth.

Recently, template substrates and free-standing bulk substrates have been studied as substrates to be used for semiconductor devices such as the above-mentioned LDs and LEDs. The template substrates are grown on various support substrates by a metalorganic chemical vapor deposition (MOCVD) method or hydrid vapor phase epitaxy (HVPE), while the free-standing bulk substrates are obtained by removing the support substrates after growth. Further, another method of growing a GaN crystal layer on a sapphire substrate by the MOCVD method and then growing single crystals thereon by a liquid phase epitaxy (LPE) method has been reported (see, for instance, JP2002-293696A and Japanese Association for Crystal Growth, Vol. 30, No. 2, pp. 96 103, 2003).

Furthermore, various methods for processing substrates also have been proposed (see, for instance, Appl. Phys. Let. (Vol. 68, No. 7, pp. 917 919, 1996) and Solid-state Electronics. (Vol.41, No. 2, pp. 295 298, 1997)).

Immediately after crystal growth, the surface of a Group III nitride crystal substrate has, for instance, a large number of small holes (pits), concavities and convexities (hillocks), and natural planes that are referred to as facets. Hence, when such a Group III nitride crystal substrate is used to manufacture a product such as a semiconductor device, it is necessary to process the surface of the substrate beforehand. The Group III nitride crystal substrate, however, is hard and brittle and therefore is difficult to process.

SUMMARY OF THE INVENTION

With the above in mind, the present invention is intended to provide a technique for processing the surface of a hard and brittle Group III nitride crystal substrate.

In order to achieve the above-mentioned object, a first manufacturing method of the present invention is a method of manufacturing a Group III nitride crystal substrate including a process of processing surfaces of Group III nitride crystals, wherein the process of processing the surfaces includes at least one of a process of meltback etching the surfaces by bringing the surfaces into contact with a melt including at least one of alkali metal and alkaline-earth metal and a process of mechanochemically polishing the surfaces using the melt.

Furthermore, a second manufacturing method of the present invention is a method of manufacturing a Group III nitride crystal substrate including the following processes (i) to (iv):

(i) preparing a semiconductor seed layer of a Group III element compound;

(ii) growing, on the semiconductor seed layer, a first semiconductor layer through liquid phase growth, the first semiconductor layer having a higher defect density than that of the semiconductor seed layer;

(iii) growing, on the first semiconductor layer, a second semiconductor layer through liquid phase growth, the second semiconductor layer having a lower defect density than that of the first semiconductor layer; and

(iv) removing at least the semiconductor seed layer and the first semiconductor layer to obtain a free-standing Group III nitride crystal substrate formed of the second semiconductor layer. Preferably, the second manufacturing method of the present invention further includes a process of processing surfaces of Group III nitride crystals, after the processes (iv).

Furthermore, a third manufacturing method of the present invention is a method of manufacturing a Group III nitride crystal substrate including the following processes (I) to (IV):

(I) preparing a semiconductor seed layer of a Group III element compound;

(II) growing, on the semiconductor seed layer, a first semiconductor layer through liquid phase growth, the first semiconductor layer having a higher defect density than that of the semiconductor seed layer;

(III) growing, on the first semiconductor layer, a second semiconductor layer through liquid phase growth, the second semiconductor layer having a lower defect density than that of the first semiconductor layer; and

(IV) processing a surface of the second semiconductor layer. In the second and third manufacturing method of the present invention, the processing the surfaces can be carried out by meltback etching of the present invention, mechanochemical polishing of the present invention, mechanical processing, common mechanochemical polishing, etc. They may be used individually or may be used in combination. When they are used in combination, it is preferable that the mechanical processing is carried out first and then mechanochemical polishing is carried out.

The manufacturing method of the present invention can provide a Group III nitride crystal substrate having reduced strain and defects that are caused through the processing. Furthermore, for instance, semiconductor devices of high performance can be obtained by using the Group III nitride crystal substrate according to the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are drawings showing a process of an example of the manufacturing method according to the present invention.

FIGS. 2A and 2B are drawings showing a process of a further example of the manufacturing method according to the present invention.

FIGS. 3A and 3B are drawings showing a process of another example of the manufacturing method according to the present invention.

FIGS. 4A and 4B are drawings showing a process of still another example of the manufacturing method according to the present invention.

FIGS. 5A and 5B are drawings showing a process of yet another example of the manufacturing method according to the present invention.

FIG. 6 is a drawing showing the configuration of an example of the manufacturing apparatus to be used in the manufacturing method of the present invention.

FIG. 7 is a cross-sectional view showing the configuration of an example of semiconductor devices according to the present invention.

FIG. 8 is a cross-sectional view showing the configuration of another example of semiconductor devices according to the present invention.

FIG. 9 is a cross-sectional view showing the configuration of still another example of semiconductor devices according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the manufacturing method of the present invention, the "meltback etching" of the present invention denotes etching that is carried out using the reverse process of liquid phase growth, i.e. a process of melting Group III nitride crystals in an unsaturated melt. Since the Group III nitride crystals are melted in an unsaturated melt, the crystals obtained thereby have reduced strain and defects that are caused through the processing. Furthermore, in the manufacturing method of the present invention, the "mechanochemical polishing" of the present invention denotes a process in which polishing, which is mechanical processing, and etching, which is chemical processing, are carried out simultaneously. Since two different types of processing are carried out simultaneously, the crystals obtained thereby have naturally reduced strain and a reduced number of defects, which are caused through the processing, as well as higher surface flatness in addition to reduced damages. Hereinafter, the meltback etching that is carried out using the melt is referred to as "meltback etching of the present invention" and the mechanochemical polishing that is carried out using the melt as "mechanochemical polishing of the present invention" while mechanochemical polishing that is carried out using, for instance, colloidal silica or colloidal diamond may be referred to as "common mechanochemical polishing".

In the present invention, examples of the process of processing the surfaces include a process of eliminating strain caused through the processing and a process of flattening the surfaces. Furthermore, in the present invention, the "Group III nitride crystal substrate" denotes both a free-standing substrate formed of Group III nitride crystals alone and a Group III nitride crystal substrate formed on a base substrate.

In the manufacturing method of the present invention, examples of the alkali metal include Na, Li, K, Rb, and Cs while examples of the alkaline-earth metal include Ca, Mg, Sr, Ba and Be. They may be used individually or two or more of them may be used together. Among them, Na, Li, Ca and Ba are preferable.

In the manufacturing method of the present invention, it is preferable that the melt further includes a Group III element. The change in amount of the Group III element contained in the melt allows the saturation degree of the melt to be adjusted and thereby allows the etching rate to be adjusted. Examples of the Group III element include Ga, Al, and In. They may be used individually or two or more of them may be used together. More preferably, a Group III element identical with that contained in the Group III nitride crystals whose surfaces are to be treated is added to the melt. For instance, when the surface of a GaN, InN, or AlN substrate is to be treated, it is preferable that the melt contains Ga, In, or Al, respectively. In the case of using a melt containing Al, since temperature of the melt may become high in some cases, it is preferable that at least one of Ga and In is added to the melt together with Al.

In the manufacturing method of the present invention, it is preferable that the melt includes Na and Ga.

In the manufacturing method of the present invention, it is preferable that in the melt, one of the following formulae is satisfied: 0.ltoreq.A/(A+M).ltoreq.0.10 (1); and 0.9.ltoreq.A/(A+M).ltoreq.0.999 (2), more preferably: 0.ltoreq.A/(A+M).ltoreq.0.05 (3); and 0.95.ltoreq.A/(A+M).ltoreq.0.999 (4), where "A" denotes the number of moles of the Group III element and "M" denotes the number of total moles of the alkali metal and alkaline-earth metal.

In the manufacturing method of the present invention, it is preferable that the melt further includes polishing grains. When the melt contains polishing grains, mechanical processing to be performed by grains and etching to be performed by alkali metal and the like can be carried out simultaneously. This allows a Group III nitride crystal substrate having reduced strain and defects that are caused through the processing and having higher flatness to be manufactured more efficiently.

In the manufacturing method of the present invention, examples of the polishing grains include alumina, diamond, SiC, GaN and AlN.

In the manufacturing method of the present invention, the polishing grains may have diameters of, for instance, 0.01 .mu.m to 2 .mu.m, preferably 0.05 .mu.m to 0.5 .mu.m.

In the manufacturing method of the present invention, when the meltback etching is carried out, the temperature of the melt is, for instance, preferably 400.degree. C. to 900.degree. C., more preferably 600.degree. C. to 850.degree. C. On the other hand, in the manufacturing method of the present invention, when the mechanochemical polishing of the present invention is carried out, the temperature of the melt is, for instance, preferably 80.degree. C. to 200.degree. C., more preferably 100.degree. C. to 150.degree. C. The mechanochemical polishing of the present invention can be carried out using a melt having a relatively low temperature as compared to the case of the meltback etching. Presumably, this is because the surface is processed using pressure provided additionally by the mechanical processing and thereby the crystal surfaces having a lot of very unstable planes (for instance, with many dangling bonds) are exposed, and as a result, the mechanochemical polishing of the present invention works even at a decreased processing temperature. The present invention, however, is not limited to this.

In the manufacturing method of the present invention, it is preferable that the processing is carried out in an atmosphere including, for instance, N.sub.2 gas, NH.sub.3 gas, H.sub.2 gas, He gas or Ar gas. Among them, N.sub.2 gas and H.sub.2 gas are preferable. Furthermore, particularly when the mechanochemical polishing of the present invention is carried out, it is more preferable that it is carried out in an inert atmosphere (for instance, a glove box) including inert gas with a high purity. Examples of the inert gas include nitrogen, Ar, and He. Among them, nitrogen is preferable since it is relatively cheap and has low reactivity.

In the manufacturing method of the present invention, the atmosphere has a pressure of, for instance, 1 atm (1.times.1.013.times.10.sup.5 Pa) to 10 atm (10.times.1.013.times.10.sup.5 Pa).

As described above, the manufacturing method of the present invention allows concavities and convexities of the surfaces of the Group III nitride crystals to have sizes of, for example, not more than .+-.5 .mu.m, preferably not more than .+-.2 .mu.m, and more preferably not more than .+-.0.5 .mu.m. In this context, the concavities and convexities of the crystal surfaces denote a net amount of irregularities caused at the crystal surfaces. The net amount is obtained through subtraction of the amount of irregularities caused mainly by warping of the substrate from the whole. In this case, the amount of warping of the substrate is not taken into consideration. This is because the warping of the substrate can be compensated to some degree by adjusting the way of attaching a sample in conducting exposure. In this case, the concavities and convexities of the surfaces of the Group III nitride crystals can be measured using, for instance, an interferometer that operates utilizing light interference, a flatness tester, a mechanical surface roughness meter, or an Atomic Force Microscope (AFM). Examples of the way of attaching a sample include vacuum absorption.

In the manufacturing method of the present invention, it is preferable that at least one of a process of mechanically processing the surfaces of the Group III nitride crystals and a process of subjecting the surfaces to mechanochemical polishing is carried out before the process of processing the surfaces. Examples of the processing method include diamond polishing, diamond grinding and slicing. Examples of the mechanochemical polishing include the mechanochemical polishing of the present invention and common mechanochemical polishing. The common mechanochemical polishing is carried out, for instance, using abrasive cloth and a SiO.sub.2 slurry (colloidal silica). or diamond slurry (colloidal diamond).

In the processing process of the manufacturing method according to the present invention, preferably, a damaged layer formed on the surfaces of the Group III nitride crystals is removed. In this context, the "damaged layer" denotes a layer that has residual strain and stress and includes a great number of crystal defects due to the mechanical processing and the like.

In the manufacturing method of the present invention, the Group III nitride crystals to be used are not particularly limited. For instance, crystals grown through liquid phase growth or vapor phase growth and sliced bulk crystals grown through liquid phase growth or vapor phase growth can be used. Furthermore, the manufacturing method of the present invention may include a process of growing the Group III nitride crystals beforehand through at least one of the vapor phase growth and liquid phase growth. Examples of the vapor phase growth include the MOCVD method, the HVPE method and the sublimation method. Examples of the liquid phase growth include the Flux method, the liquid phase epitaxy (LPE) method and the Czochralski method.

In the manufacturing method of the present invention, when the surfaces of the Group III nitride crystals grown through the vapor phase growth are to be processed, it is more preferable that the meltback etching of the present invention, the mechanochemical polishing of the present invention, or both the common mechanochemical polishing and the meltback etching of the present invention are used. Furthermore, when the surfaces of the Group III nitride crystals grown through the liquid phase growth are to be processed, it also is possible to employ the common mechanochemical polishing alone since the Group III nitride crystals tend to have higher mechanical strength than that of Group III nitride crystals grown through the vapor phase growth. However, the use of the meltback etching of the present invention, the mechanochemical polishing of the present invention, or both the common mechanochemical polishing and the meltback etching of the present invention is more preferable.

In the manufacturing method of the present invention, it is preferable that the liquid phase growth is a method of growing Group III nitride crystals in a melt or at the surface of the melt in an atmosphere including nitrogen, with the melt containing nitrogen, at least one of alkali metal and alkaline-earth metal, and at least one Group III element selected from the group consisting of gallium, aluminum, and indium.

In the manufacturing method of the present invention, examples of the alkali metal include Na, Li, K, Rb, and Cs while examples of the alkaline-earth metal include Ca, Mg, Sr, Ba, and Be. They may be used individually or two or more of them may be used together. Among them, Na, Li, Ca and Ba are preferable. Examples of the atmosphere including nitrogen include an atmosphere containing N.sub.2 gas or NH.sub.3 gas.

In the manufacturing method of the present invention, it is preferable that a semiconductor seed layer of a Group III element compound is prepared, and then the Group III nitride crystals are grown on the semiconductor seed layer.

In the manufacturing method of the present invention, it is preferable that the liquid phase growth is carried out in two stages including a first stage and a second stage. The first stage corresponds to a process of forming, on the semiconductor seed layer, a first semiconductor layer having a higher defect density than that of the semiconductor seed layer, and the second stage corresponds to a process of forming, on the first semiconductor layer, a second semiconductor layer having a lower defect density than that of the first semiconductor layer. As described above, the first semiconductor layer (a high defect layer) having a higher defect density is formed while the surface of the semiconductor seed layer is melted back, and then the second semiconductor layer is grown. Accordingly, the second semiconductor layer of higher quality can be formed.

In the present invention, the "defect" is not particularly limited as long as it is a defect of the crystal structure. Examples of the "defect" include defects caused by introduction of impurities and crystal lattice defects. The impurities described above include, for instance, those originating from components of the material forming a crucible, a reactor vessel, or other members that are used for producing the crystals, and those originating from components of the material of the melt that is used in the liquid phase method. The crystal lattice defects include, for instance, dislocations (line defects). Examples of the dislocations include edge dislocations and screw dislocations. In GaN crystals, the crystal lattice defects include, for instance, defects of Ga and N. Of the defects caused in the present invention, the defects caused by the introduction of impurities can be determined by impurity analysis such as, for example, secondary ion mass spectroscopy (SIMS) or optical evaluations such as, for instance, photoluminescence (PL) evaluations. The dislocations can be determined, for example, through observation using a transmission electron microscope (TEM).

In the manufacturing method of the present invention, the second semiconductor layer can have a dislocation density that exceeds zero but is not higher than 1.times.10.sup.6 cm.sup.-2 and an impurity density of, for instance, 1 ppm to 10 ppm. Preferably, the dislocation density of the first semiconductor layer is at least 100 times higher than those of the semiconductor seed layer and the second semiconductor layer. Furthermore, it is preferable that the first semiconductor layer has a dislocation density of at least 1.times.10.sup.8 cm.sup.-2.

Preferably, the manufacturing method of the present invention further includes a process of removing at least one of the semiconductor seed layer and the first semiconductor layer, which is a high defect layer, to provide a free-standing Group III nitride crystal substrate that is formed of the second semiconductor layer. The method for removing the layers is not particularly limited but examples thereof may include mechanical grinding, others types of grinding, and meltback etching.

In the manufacturing method of the present invention, it is preferable that the semiconductor seed layer is formed on a base substrate. The base substrate can be, for example, a GaAs substrate whose surface is a (111) plane, a Si substrate whose surface is a (111) plane, a sapphire substrate whose surface is a (0001) plane, or a SiC substrate whose surface is a (0001) plane. Among them, the sapphire substrate whose surface is a (0001) plane and the SiC substrate whose surface is a (0001) plane are preferable.

The Group III nitride crystal substrate of the present invention is one obtained by the above-mentioned manufacturing method of the present invention.

The semiconductor device of the present invention is one formed using Group III nitride crystal substrate, wherein the Group III nitride crystal substrate is the above-mentioned Group III nitride crystal substrate according to the present invention. The type of the semiconductor device according to the present invention is not particularly limited. The semiconductor device may be, for example, a laser diode or a light emitting diode.

The etchant of the present invention is an etchant that is a melt containing at least one of alkali metal and alkaline-earth metal. In the etchant, it is preferable that one of the following formulae is satisfied: 0.ltoreq.A/(A+M).ltoreq.0.10 (1); and 0.9.ltoreq.A/(A+M).ltoreq.0.999 (2), more preferably: 0.ltoreq.A/(A+M).ltoreq.0.05 (3); and 0.95.ltoreq.A/(A+M).ltoreq.0.999 (4), where "A" denotes the number of moles of the Group III element and "M" denotes the number of total moles of the alkali metal and alkaline-earth metal.

In the present invention, more preferably, a Group III element identical with that contained in the Group III nitride crystals whose surfaces are to be treated is added to the etchant. For instance, when the surface of a GaN, InN, or AlN substrate is to be treated, it is preferable that the etchant contains Ga, In, or Al, respectively. In the case of using an etchant containing Al, since temperature of the etchant may become high in some cases, it is preferable that at least one of Ga and In is added to the etchant together with Al.

In the present invention, it is preferable that the etchant is an etchant containing Na and Ga.

In the present invention, it is preferable that the etchant further includes polishing grains. Examples of the polishing grains include alumina, diamond, SiC, GaN and AlN. The polishing grains have diameters of, for instance, 0.01 .mu.m to 2 .mu.m, preferably 0.05 .mu.m to 0.5 .mu.m.

Next, embodiments of the present invention are described.

Embodiment 1

Embodiment 1 is directed to a manufacturing method including: a process of mechanically processing the surface of a Group III nitride crystal substrate including a semiconductor seed layer, a first semiconductor layer, and a second semiconductor layer; and an processing process that is carried out through meltback etching by bringing the surface processed mechanically into contact with a melt containing at least one of alkali metal and alkaline-earth metal.

First, a semiconductor seed layer (expressed by a composition formula of Al.sub.sGa.sub.tIn.sub.1-s-tN (where 0.ltoreq.s.ltoreq.1, 0.ltoreq.t.ltoreq.1, and s+t.ltoreq.1)) to serve as seed crystals is grown on a base substrate by, for instance, the vapor phase growth method such as the MOCVD.

Next, a first semiconductor layer that is expressed by a composition formula of Al.sub.mGa.sub.nIn.sub.1-m-nN (where 0.ltoreq.m.ltoreq.1, 0.ltoreq.n.ltoreq.1, and m+n.ltoreq.1) is grown through liquid phase growth on the semiconductor seed layer. Further, a second semiconductor layer that is expressed by a composition formula of Al.sub.xGa.sub.yIn.sub.1-x-yN (where 0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, and x+y.ltoreq.1) is grown thereon through the liquid phase growth.

The following description is directed to an example of the method of forming the first and second semiconductor layers through the liquid phase growth.

First, in an atmosphere (preferably a pressurized atmosphere of 100 atm (100.times.1.013.times.10.sup.5 Pa) or lower) including nitrogen, the surface of the semiconductor seed layer is brought into contact with a melt containing at least one Group III element selected from Ga, Al, and In, at least one of alkali metal and alkaline-earth metal, and nitrogen. Consequently, the surface of the semiconductor seed layer is melted back and then the first semiconductor layer (the high defect layer) having a higher dislocation density than that of the semiconductor seed layer is grown thereon. This first semiconductor layer (the high defect layer) tends to take in alkali metal or alkaline-earth metal that is used as a flux for the liquid phase growth and also may take in the Group III element not as a nitride but as a metal element. The first semiconductor layer has a dislocation density of, for instance, at least 10.sup.8 cm.sup.-2, preferably 10.sup.9 cm.sup.-2 to 10.sup.14 cm.sup.-2. The first semiconductor layer has a higher dislocation density when compared to the second semiconductor layer.

Next, the second semiconductor layer is grown subsequent to the growth of the first semiconductor layer. That is, in the state where the semiconductor seed layer is in contact with the melt (in the same atmosphere), the first and second semiconductor layers can be formed in a series of processes. Specifically, in the atmosphere (preferably a pressurized atmosphere of 100 atm (100.times.1.013.times.10.sup.5 Pa) or lower) including nitrogen, for instance, the first semiconductor layer can be formed by using an unsaturated melt first and then bringing the melt into a supersaturation state. Subsequently, crystal growth is carried out using the supersaturated melt and thereby the second semiconductor layer can be formed. In this case, the semiconductor seed layer that has been brought into contact with the unsaturated melt is melted and thereby the first semiconductor layer with a high dislocation density is formed. The thickness of the first semiconductor layer is not particularly limited but can be, for example, 1 .mu.m to 200 .mu.m. In addition, the second semiconductor layer with a low dislocation density is formed on the first semiconductor layer that is in contact with the supersaturated melt.

As described above, when the first semiconductor layer that is a high defect layer is formed first and then the second semiconductor layer is formed thereon, the second semiconductor layer can have a lower dislocation density than that of the first semiconductor layer. The dislocation density of the second semiconductor layer is, for instance, 1.times.10.sup.6 cm.sup.-2 or lower. The density of impurities such as alkali metal can be, for example, 1 ppm to 10 ppm or lower. In this connection, it is not always necessary to form the first semiconductor layer in order to form the second semiconductor layer with a low dislocation density. It also is possible to form the second semiconductor layer with a low dislocation density by, for example, using a substrate with a relatively high dislocation density as the substrate to serve as seed crystals and growing the second semiconductor layer without melting back the substrate.

In the liquid phase growth, the atmosphere including nitrogen can be an atmosphere including, for example, N.sub.2 gas or NH.sub.3 gas. The alkali metal can be at least one selected from Na, Li, K, Rb, and Cs or a mixture thereof. The melt may be alkaline-earth metal alone or a mixture of alkali metal and alkaline-earth metal. The alkaline-earth metal can be at least one selected from, for example, Ca, Mg, Sr, Ba, and Be, or a mixture thereof. Usually, they work as a flux. Besides the alkali metal and alkaline-earth metal, Group III elements also work as a flux. Furthermore, the nitrogen is being dissolved in a mixed melt of a Group III element and at least one of the alkali metal and alkaline-earth metal.

In the process of liquid phase growth, the melt can be prepared, for example, by putting a material into a crucible and then heating it. After the melt is prepared, it is brought into a supersaturation state to grow semiconductor crystals. The melting of the material and the crystal growth can be carried out, for instance, at a temperature of 700.degree. C. to 1100.degree. C. and a pressure of 10 atm (10.times.1.013.times.10.sup.5 Pa) to 100 atm (100.times.1.013.times.10.sup.5 Pa).

In the state immediately after the crystal growth (as-grown), the Group III nitride crystal substrate thus obtained has concavities and convexities (for instance, pits, hillocks and facets) formed on its surface. The orientation of the concavities and convexities is different from that of the semiconductor seed layer to serve as seed crystals. The concavities and convexities have sizes of, for instance, .+-.10 .mu.m to .+-. several tens of micrometers and their sizes occasionally may reach the order of millimeters. Hence, when no processing process is carried out beforehand, it is difficult to carry out the exposure process for forming a device pattern or the like without causing any problems in producing a device using the substrate. It therefore is necessary to process the surface of the second semiconductor layer.

The following description is directed to an example of the method of processing the surface by meltback etching.

First, at least one of alkali metal and alkaline-earth metal may be put in a crucible and then the crucible may be heated in an electric furnace. Thus, a melt is prepared. The surfaces of nitride crystals are brought into contact with the melt and thereby are subjected to meltback etching. Examples of the ambient gas to be used in the etching include N.sub.2 gas, NH.sub.3 gas, and H.sub.2 gas. Examples of the alkali metal include Li, Na, K, Rb, and Cs while examples of the alkaline-earth metal include Ca, Sr, Ba, and Be. One of them or two or more of them may be used. The melt may contain a Group III element. The addition of the Group III element adjusts the unsaturation degree of the melt to allow the etching rate to be adjusted. In this case, the alkali metal and alkaline-earth metal work as an etchant (a flux) that increases the etching rate. In the etchant (melt), it is preferable that, one of the following formulae is satisfied: 0.ltoreq.A/(A+M).ltoreq.0.10 (1); and 0.9.ltoreq.A/(A+M).ltoreq.0.999 (2), more preferably: 0.ltoreq.A/(A+M).ltoreq.0.05 (3); and 0.95.ltoreq.A/(A+M).ltoreq.0.999 (4), where "A" denotes the number of moles of the Group III element and "M" denotes the number of total moles of the alkali metal and alkaline-earth metal.

Preferably, the melt has a temperature of, for example, 400.degree. C. to 900.degree. C. and the atmosphere has a pressure of, for example, 1 atm (1.times.1.013.times.10.sup.5 Pa) to 10 atm (10.times.1.013.times.10.sup.5 Pa). This is because when the pressure is lower than 1 atm (1.times.1.013.times.10.sup.5 Pa), it is difficult to prevent the alkali metal or the like to be used as a flux from vaporizing while when the pressure exceeds 10 atm (10.times.1.013.times.10.sup.5 Pa), there is a possibility that crystals start growing in the melt.

In this embodiment, crystals obtained through the liquid phase growth were used. However, when using the meltback etching that is carried out using the melt containing at least one of alkali metal and alkaline-earth metal of the present invention, not only the crystals obtained through the liquid phase growth but also crystals obtained through the vapor phase growth can be processed to have surfaces with high flatness.

Embodiment 2

Embodiment 2 is directed to an example of carrying out a surface treatment using the mechanochemical polishing of the present invention instead of the meltback etching of Embodiment 1.

A melt including at least one of alkali metal and alkaline-earth metal is prepared in the same manner as in the case of the meltback etching of Embodiment 1. Preferably, the melt includes a Group III element. It also is preferable that in the composition of the melt, for instance, one of the following formulae is satisfied: 0.ltoreq.A/(A+M).ltoreq.0.10 (1); and 0.9.ltoreq.A/(A+M).ltoreq.0.999 (2), where "A" denotes the number of moles of the Group III element and "M" denotes the number of total moles of the alkali metal and alkaline-earth metal. Such compositions allow the mechanochemical polishing of the present invention to be carried out at still lower temperatures. Polishing grains to be used herein are, for instance, alumina, diamond, or SiC that is relatively hard and has low reactivity with alkali metal and the like. In addition, further examples of the polishing grains to be used herein include polishing grains whose hardness is about the same as that of Group III nitride crystals of, for instance, GaN and AlN to be subjected to the surface treatment, and polishing grains having the same composition as that of the Group III nitride crystals. Preferably, the mechanochemical polishing of the present invention is carried out in an atmosphere (for instance, in a glove box) that had been subjected to, for instance, N.sub.2 gas or rare gas substitution, because of the high reactivity with oxygen and water contained in the melt to be used. The abrasive cloth can be one of those known conventionally. Among them, those having low reactivity with alkali metal and alkaline-earth metal are preferable. Furthermore, conventionally known polishing apparatuses can be used. The presence of oxygen or water causes the alkali metal or alkaline-earth metal to exist not as metal but as oxides or hydroxides. This may prevent the etching effect originally expected from being exhibited. As a result, it may become difficult to obtain a satisfactory effect of the surface treatment in some cases. A lower treatment temperature is preferable. For instance, in order to reduce viscosity of the melt, the treatment temperature is preferably in the range of 80.degree. C. to 200.degree. C. but is not limited thereto. When using a Na-Li or Na-Ga melt and diamond polishing grains, GaN crystals that include no damaged layer and have high surface flatness can be obtained, for instance, at a rate of 0.05 .mu.m to 1 .mu.m per 10 minutes. Embodiment 3

Embodiment 3 is directed to an example of the method of manufacturing a Group III nitride crystal substrate, which further includes, in addition to the processes of Embodiment 1, a process of removing at least the semiconductor seed layer and the first semiconductor layer from the base substrate side.

As in Embodiment 1, when a first semiconductor layer that is a high defect layer is formed between a semiconductor seed layer to serve as seed crystals and a second semiconductor layer that is a low defect layer, the high defect layer often has a considerably different carrier density and contains a large amount of impurities. Hence, it may be preferable that the high defect layer is removed when a device is to be produced using the substrate. The reason for this is as follows. That is, particularly in the case of, for example, a semiconductor laser, electrodes may be formed on the rear face of the substrate and in that case, the presence of the part that is considerably different in carrier density and amount of defects (including amount of impurities) from the rest may deteriorate device performance and reliability. Hence, it is advantageous to remove at least the semiconductor seed layer and the first semiconductor layer from the base substrate side, and preferably a part of the second semiconductor layer together with them. The method of removing them is not particularly limited but examples thereof may include mechanical grinding, various types of grinding, and meltback etching.

Furthermore, the surface of the second semiconductor layer that is exposed by the removal of the layers described above may be brought into contact with a melt containing at least one of alkali metal and alkaline-earth metal to be subjected to meltback etching or mechanochemical polishing.

In the Group III nitride crystal substrate of the present invention, its surface hardly has concavities and convexities such as, for example, facets, pits and hillocks and therefore is flat, and the surface also has fewer grinding damages and less strain caused through the processing. Accordingly, the Group III nitride crystal substrate of the present invention allows a semiconductor device of higher performance to be formed. That is, a semiconductor device such as, for instance, a laser diode or a transistor can be produced on the Group III nitride crystal substrate of the present invention.

Embodiment 4

Embodiment 4 is directed to an example of the method of processing the surface of a Group III nitride crystal substrate grown through the vapor phase growth, by meltback etching.

When a semiconductor layer that is expressed by a composition formula of Al.sub.sGa.sub.tIn.sub.1-s-tN (where 0.ltoreq.s.ltoreq.1, 0.ltoreq.t.ltoreq.1, and s+t.ltoreq.1) is grown on a base substrate by the vapor phase growth method, the substrate has relatively large concavities and convexities formed of, for instance, hillocks and pits at its surface in the state immediately after the crystal growth (as-grown). In the case of the Group III nitride crystal substrate grown from the vapor phase growth, however, since strain and defects that are caused through the processing cannot be eliminated by common polishing alone, it usually is used, in the state immediately after the crystal growth (as-grown), as a substrate to be employed for device production. In this case, however, the concavities and convexities formed at the substrate surface cause problems in, for instance, an exposure process that is carried out in producing a device using the substrate later as in the case of the crystal substrate grown through the liquid phase growth. Hence, it is necessary to process the crystal surfaces to allow the surfaces to have concavities and convexities of, for instance, .+-.5 .mu.m or smaller. In the case of crystals grown through the vapor phase growth, meltback etching of the present invention, both the common mechanochemical polishing and the meltback etching of the present invention, or the mechanochemical polishing of the present invention can be employed to eliminate strain and defects caused through the processing and thus to flatten the crystal surfaces.

Embodiment 5

Embodiment 5 is directed to an example of processing a wafer-like Group III nitride crystal substrate by subjecting bulk crystals to slice processing.

First, bulk crystals obtained through the liquid phase growth or the vapor phase growth, are subjected to slice processing and thereby a wafer-like Group III nitride crystal substrate is obtained. Examples of the liquid phase growth include the melt growth, the Flux growth, the LPE growth and the Czochralski growth. Examples of the vapor phase growth include the HVPE growth and the sublimation growth. The substrate surface exposed by the slice processing becomes a very rough surface by slicing. Hence, the substrate surface is subjected to mirror-finish processing that is carried out by polishing or grinding, which is mechanical processing. After the mirror-finish processing, a damaged layer formed by the mechanical processing is eliminated by meltback etching or both common mechanochemical polishing and meltback etching of the present invention and thereby a Group III nitride crystal substrate with higher flatness can be obtained.

Embodiment 6

The following description is directed to an example of producing a field effect transistor (FET). FIG. 8 schematically shows the configuration of a FET. An undoped GaN substrate 111 can be used for the substrate. The GaN substrate 111 can be obtained through the liquid phase growth using a flux. The GaN substrate 111 obtained through the liquid phase growth has an electrical resistance of, for instance, at least 10.sup.10 .OMEGA. and therefore has characteristics similar to those of an insulator. A GaN layer 112 and an AlGaN layer 113 are formed sequentially on the GaN substrate 111 by the MOCVD method. Furthermore, a source electrode 114, a gate electrode 115, and a drain electrode 116 are formed on the AlGaN layer 113. Voltage is applied to the gate electrode 115 and thereby the concentration of two-dimensional electron gas 117 that is generated at the interface between the GaN layer 112 and the AlGaN layer 113 is controlled. Thus the operation as a transistor is carried out. Especially, the use of meltback etching of the present invention, the mechanochemical polishing of the present invention and both the common mechanochemical polishing and the meltback etching of the present invention allows the substrate 111 to have a reduced number of surface defects and reduced strain caused through the processing. As a result, the GaN layer 112 and the AlGaN layer 113 that each have a low dislocation density can be formed by the MOCVD. Thus, it is possible to obtain a FET that has a high insulation property and an excellent high frequency property and allows the leakage current that is caused during the operation of the transistor to be reduced.

Hereinafter, the present invention is described further in detail using examples.

EXAMPLE 1

Example 1 is an example in which a semiconductor seed layer to serve as seed crystals is formed on a base substrate and then the surface that is the surface of a first semiconductor layer grown on the semiconductor seed layer through liquid phase growth and has concavities and convexities formed of, for instance, facets and pits is processed by common mechanochemical polishing. This example is described below with reference to FIGS. 2A and 2B.

FIGS. 2A and 2B are cross-sectional views showing an example of the process of processing the surface of a Group III nitride crystal substrate. As shown in FIG. 2A, a GaN film (with a thickness of 4 .mu.m) was formed as a semiconductor seed layer 12 on a base substrate 10 by the MOCVD method. Specifically, a sapphire substrate whose surface was a C plane was used for the base substrate 10 and was heated to a temperature of 1020.degree. C. to 1100.degree. C., and then trimethylgallium (TMG) and NH.sub.3 were supplied onto the base substrate 10.

Subsequently, a first semiconductor layer 16 was grown on the semiconductor seed layer 12 through liquid phase growth. Specifically, in a nitrogen atmosphere (preferably a pressurized atmosphere of 100 atm (100.times.1.013.times.10.sup.5 Pa) or lower), the semiconductor seed layer 12 was brought into contact with a melt including gallium, sodium, and nitrogen, and thereby a GaN layer (the first semiconductor layer) 16 was grown on the semiconductor seed layer 12. Thus, a Group III nitride crystal substrate including the semiconductor seed layer 12 and the first semiconductor layer 16 formed sequentially on the base substrate 10 was obtained.

The surface of the Group III nitride crystal substrate thus obtained was observed visually and with Optical Microscope. As a result, concavities and convexities formed of, for instance, facets, pits and hillocks were found.

Next, the surface of the Group III nitride crystal substrate obtained above was subjected to mechanical processing that was carried out using abrasive grains of, for instance, diamond, SiC, or alumina. Thus, the surface was processed. Thereafter, using a SiO.sub.2 slurry and abrasive cloth, the processed surface was subjected to the common mechanochemical polishing and thereby the strain of the surface caused by the processing was eliminated. Thus, a Group III nitride crystal substrate with a flat and mirror-smooth surface was obtained. The concavities and convexities of the surface of the Group III nitride crystal substrate was measured by optical interferometer and obtained above had sizes of .+-.4 .mu.m or smaller. In addition, the surface roughness was measured by AFM and then the root mean square (RMS) thereof was calculated and as a result, was about 2 nm.

The PL intensity of the surface of the Group III nitride crystal substrate was measured immediately after the crystal growth (as-grown) and after the common mechanochemical polishing process (using a HeCd laser (325 nm)). As a result, the ratio (PL.sub.a/PL.sub.b) of the PL intensity (PL.sub.a) obtained after the common mechanochemical polishing process to that (PL.sub.b) obtained immediately after the crystal growth (as-grown) was 0.5 to 1, and thus it was proved that the strain caused through the processing was suppressed to a relatively small degree. In this case, the use of the common mechanochemical polishing alone allowed the damaged layer to have a reduced thickness without using meltback etching. Conceivably, this is because crystals grown through the liquid phase growth are harder and denser.

In order to further reduce the strain caused through the processing carried out by common mechanochemical polishing and to improve repeatability, meltback etching of the present invention was effective.

EXAMPLE 2

Example 2 is an example in which under the same conditions as in Example 1, a Group III nitride crystal substrate was grown into a bulk, which then was sliced to give wafer substrates, and then their surfaces were flattened by common mechanochemical polishing and meltback etching of the present invention.

FIGS. 3A and 3B are cross-sectional views showing an example of the process of manufacturing wafer substrates by slicing a Group III nitride crystal substrate. First, a 5-mm thick GaN layer (a first semiconductor layer) 26 was grown on a semiconductor seed layer in the same manner as in Example 1. Thereafter, the first semiconductor layer 26 was sliced to give wafer substrates 27 with a thickness of 300 .mu.m. The slice cut surfaces of the substrates 27 were rough surfaces with larger concavities and convexities than those observed immediately after crystal growth.

Next, the surface of each substrate 27 was subjected to mechanical processing that was carried out using abrasive grains of, for instance, diamond, SiC, or alumina. Thus, the surface was flattened substantially. Thereafter, a damaged layer formed at the surface of the substrate 27 through the mechanical processing was removed by common mechanochemical polishing and meltback etching of the present invention. The common mechanochemical polishing was carried out in the same manner as in Example 1.

The method of the meltback etching of the present invention is described below.

The crystal surface was subjected to meltback etching of the present invention using an etchant of the present invention that is a melt containing alkali metal. FIG. 6 shows an etching unit to be used in this example.

The etching unit 50 shown in FIG. 6 includes an electric furnace 53 and a heater 70. A slider 51 on which a nitride substrate 52 is placed and a crucible 54 are disposed inside the electric furnace 53. An etchant 60 that is an unsaturated melt is put in the crucible 54. Nitrogen gas was used as the ambient gas and a melt (a flux) was used as the etchant. The melt contained Ga that was added thereto so as to account for 0 to 10 mol %, with alkali metal (Na or Na--Li) being taken as 100. As in this case, when Ga is added to adjust the unsaturation degree, it is possible to obtain a Group III nitride crystal substrate with a surface that has less strain caused through the processing and has high flatness. In this case, the following conditions were employed for meltback etching. That is, the temperature of the etchant was in the range of 600.degree. C. to 900.degree. C. while the nitrogen pressure was in the range of 1 atm (1.times.1.013.times.10.sup.5 Pa) to 5 atm (5.times.1.013.times.10.sup.5 Pa). Under these conditions, the slider 51 was moved in the directions indicated with the arrow B in FIG. 6 and then the Group III nitride crystal substrate 52 was moved to a low-temperature area inside the electric furnace