Nitride compound semiconductor light emitting device and method for producing the same

#1. Field of the Invention

#The present invention relates to a nitride compound semiconductor light emitting device formed on a GaN substrate, and a method for producing the same.

#2. Description of the Related Art

#FIG. 1A is a schematic across-sectional view illustrating a semiconductor light emitting device produced by using a nitride compound semiconductor according to the present invention.

#FIG. 1B is a schematic cross-sectional view illustrating the structure of an active layer of a light emitting device produced according to the present invention.

#FIG. 1C is a flow chart illustrating steps for producing an active layer of a light emitting device according to one embodiment of the invention.

#FIG. 2 presents a schematic microscopic observation of surfaces of semiconductor layers grown on a substrate 201 not satisfying a crystal orientation range constraint defined by the present invention. As seen from FIG. 2, there is a non-uniform distribution of steps 202 over the substrate 201, the steps 202 having a height on the order of a few atomic layers. As a result, a three-dimensional growth mode (i.e., local progress of growth) is predominant because non-uniform growth cores 203 hinder an organized step flow growth. Thus, a high concentration of threading dislocations and/or nitrogen vacancies may occur with the growth of semiconductor layers in a thickness direction. Penetration dislocations and/or nitrogen vacancies may result in entrapment of Mg—H. Moreover, a crystal having is a high concentration of threading dislocations and/or nitrogen vacancies has a substantially undulated uppermost surface. Such a surface does not provide an appropriate underlying layer for forming a quantum well structure thereon. In particular, it may affect the crystallinity and composition uniformity of an light emitting layer containing In.

#On the other hand, FIG. 3 presents a schematic microscopic observation of surfaces of semiconductor layers grown on a substrate 301 whose crystal orientation is tilted away from a <0001> direction by a slight angle in the range from about 0.05° to about 2° in a <11-20> or <1-100> direction. As shown in FIG. 3, steps 302 on the substrate 301 are distributed in an optimum, uniform manner. As a result, a two-dimensional growth mode occurs in which material species which have arrived at the substrate surface in a vapor phase repeat migration and revaporization, forming a uniform distribution of growth cores 303 over the entire substrate surface, whereby a layer-by-layer planar growth occurs. Thus, the generation of threading dislocations and/or nitrogen vacancies is effectively reduced. Especially in the case of growing a p-type crystal, Mg atoms can be taken into the crystal by themselves, so that a p-type crystal having a high hole density can be obtained without requiring a heat treatment in an inert gas. Furthermore, as a contact layer, a cladding layer, an optical guide layer, and the like are laminated to a total thickness of about 1 μm or more in a two-dimensional growth mode, the two-dimensional growth progresses based only on the ordered lattice array information inherent in the substrate surface. During the growth process, the steps from which the two-dimensional growth began will gradually disappear, ultimately attaining a very flat uppermost surface. By forming a multiple quantum well active layer on such a highly flat surface, each layer in the multiple quantum well structure will have a uniform thickness, whereby emission characteristics are improved. Due to such effects, the p-type contact resistance can be reduced without particularly performing a p-type properties impartment process, thereby enabling efficient current injection. Thus, a long-life light emitting device which experiences minimum heat generation during use and which has a highly flat surface can be provided.

#According to the present invention, a light emitting device including an active layer which is composed of a plurality of layers of In-containing nitride compound semiconductors is produced, using a GaN substrate whose crystal orientation is tilted very slightly from the <0001> direction, in such a manner that underlying layers below the active layer have a total thickness of about 1 μm or more. As a result, a growth mode is realized in which threading dislocations and/or nitrogen vacancies are reduced, and Mg atoms are taken into the mother crystal by themselves. Thus, it is possible to obtain a p-type crystal which is activated to a high density, without performing a heat treatment process after the completion of the device structure which can damage the active layer. As a result, a low resistance p-type contact can be obtained, thereby improving the longevity of the resultant device. By employing the method according to the present invention, threading dislocations are reduced, and hence current paths which do not contribute to emission are reduced. In addition, the flatness of the active layer and the underlying layers is improved. Thus, the thickness of each layer in the InGaN multiple quantum well layers is uniformed, thereby providing improved emission characteristics.