BACKGROUND ARTS

It has been known light emitting devices such as light emitting diodes (LEDs) that use sapphire (α-alumina single crystal) as a monocrystalline substrate, with various types of gallium nitride (GaN) layers formed thereon. For example, light emitting devices have been mass-produced having a structure in which an n-type GaN layer, a multiple quantum well (MQW) layer with an InGaN quantum well layer and a GaN barrier layer laminated alternately therein and a p-type GaN layer are formed in a laminated manner in this order on a sapphire substrate.

It is proposed, in a base substrate (template substrate) for growing a layer of a crystal of a group 13 nitride such as gallium nitride, to provide irregularity on a crystal growth surface of a base crystal layer of the base substrate. That is, according to patent documents 1 to 3, it is disclosed the procedure of forming the irregularity on the crystal growth surface of the base substrate to reduce dislocations and stress in a crystal.

According to patent document 2, the growth surface of the base crystal layer is made a flat c-plane, and an inclined surface inclined with respect to c-plane and without including a plane parallel with c-plane is made continuous to form irregularity.

According to patent document 3, it is formed, on the crystal growth surface of the base substrate, a flat part of +c-plane and a flat and inclined surface to which non-+c-plane is exposed. According to patent document 3, rectangular projections and recesses are formed on the crystal growth surface of the base substrate.

Further, according to patent document 4, in the case that a seed crystal film is formed on a sapphire substrate, it is proposed that voids are formed along an interface of the sapphire substrate by heating or laser and seed crystal film and that the void ratio is made 12.5% or lower. Cracks or fractures of a gallium nitride crystal layer is thereby suppressed.

PATENT DOCUMENTS

(Patent document 1) Japanese Patent No. 5359740B

(Patent document 2) Japanese Patent Publication No. 2017-036174A

(Patent document 3) Japanese Patent Publication No. 2005-281067A

(Patent document 4) Japanese Patent No. 6144630B

SUMMARY OF THE INVENTION

After a layer of a crystal of a group 13 nitride is grown on base crystal layer, during cooling from the growth temperature to room temperature, a stress is applied on the layer of the crystal of the group 13 nitride due to difference of thermal expansion of the supporting substrate and the layer of the crystal of the group 13 nitride, resulting in warping and cracks in the layer of the crystal of the group 13 nitride. Such warping or cracks results in defects or film-formation failure in forming another functional device on the layer of the crystal of the group 13 nitride.

An object of the present invention is, in a base substrate for growing a layer of a crystal of a group 13 nitride on the crystal growth surface of a base crystal layer, to provide the structure capable of further reducing defects or cracks of the layer of the crystal of the group 13 nitride.

The present invention provides a base substrate comprising:

a supporting substrate comprising aluminum oxide; and

a base crystal layer provided on a main face of the supporting substrate, comprising a crystal of a nitride of a group 13 element and having a crystal growth surface,

wherein at least one of a metal of a group 13 element and a reaction product of a material of the supporting substrate and the crystal of the nitride of the group 13 element is present between the supporting substrate and the base crystal layer, and

wherein the reaction product comprises at least aluminum and a group 13 element.

The present invention further provides a functional device comprising:

the base substrate; and

a functional layer provided on the base crystal layer.

The present inventors found that a reaction product of a specific composition or a metal of a group 13 element can be generated between a supporting substrate of a specific composition and a layer of a crystal of the group 13 nitride. That is, nitrogen atoms are discharged from the group 13 nitride to generate the metal of the group 13 element and the metal of the group 13 element is reacted with aluminum oxide to generate the reaction product. Then, it is found that warping or cracks in the layer of the crystal of the group 13 nitride can be suppressed during cooling from a growth temperature to room temperature after the layer is grown on the base crystal layer, by generating such reaction product or decomposition product. The present invention is thus made.

