The present application claims priority of Japanese Application No. P2000-217911 filed Jul. 18, 2000, which application is incorporated herein by reference to the extent permitted by law.
This invention relates to a crystal growth method adapted for use in the formation of semiconductor light-emitting devices and more particularly, to a crystal growth method wherein selective crystal growth of a nitride semiconductor is carried out.
Owing to the absence of either a lattice-matching substrate or a substrate of a low dislocation density in the vapor phase growth technology of a gallium nitride compound semiconductor, there is known a technique wherein after deposition of a buffer layer of AlN or AlxGa1xe2x88x92N, in which 0xe2x89xa6x less than 1, on a substrate such as of sapphire at a low temperature of 900xc2x0 C. or below, a gallium nitride compound semiconductor is grown to reduce the number of dislocations ascribed to the lattice-mismatching. Such a technique is, for example, described in Japanese Patent Laid-open No. Sho 63-188938 and Japanese Patent Publication No. Hei 8-8217. Using the technique of reducing the dislocations in number, the gallium nitride compound semiconductor can be improved in crystallinity and morphology.
For obtaining high-quality crystals of a low dislocation density, there is a technique wherein after deposition of a first gallium nitride compound semiconductor (i.e. a first layered) on a substrate, a protective layer made of a material of impeding the growth of the gallium nitride compound semiconductor, e.g. silicon oxide or silicon nitride, is formed, followed by growth of a second (i.e. second-layered) gallium nitride compound semiconductor in in-plane directions (or in lateral directions) from a region not covered with the protective layer, thereby impeding the propagation of through-type dislocations extending vertically from the interface of the substrate. Such a technique is disclosed, for example, in Japanese Patent laid-open No. Hei 10-312971. A similar technique is described, for example, in MRS Internet J. Nitride Semicond. Res. 4S1, G3, 38 (1999). In the technique, after a first gallium nitride compound semiconductor layer has been once grown, the layer is selectively removed by use of a reactive ion etching (hereinafter referred to as RIE) device. Thereafter, a second gallium nitride compound semiconductor layer is selectively grown from the crystals left in the growth device, thereby reducing the through-type dislocation density. When using these techniques, there can be obtained a crystal film having a dislocation density up to about 106 cmxe2x88x922, thereby realizing a prolonged life of a semiconductor laser.
However, the use of only these techniques using the low temperature buffer layer enables one to reduce the through-type dislocations, at most, to a level of about 109 cmxe2x88x922, thus placing a limitation on the formation of a high-quality gallium nitride compound semiconductor. These dislocations worsen the performance and life of the resultant semiconductor device.
With the technique wherein re-growth is effected after the formation of a protective film on the above-mentioned first gallium nitride compound semiconductor layer or after the selective removal of the first gallium nitride compound semiconductor layer such as by reactive ion etching, the through-type dislocations left in the crystals remaining in a region where no protective film is formed or after the removal are propagated in the crystals of the second gallium nitride compound semiconductor layer, thus placing the limitation on a low dislocation density.
It is accordingly an object of the invention to provide a crystal growth method which can overcome the problems involved in the prior art.
It is another object of the invention to provide a crystal growth method which ensures the formation of high-quality crystals of a low dislocation density in a vapor phase growth technique of a gallium nitride compound semiconductor.
The above objects can be achieved, according to a first aspect of the invention, by a crystal growth method, which comprises forming a first nitride semiconductor layer on or over a substrate, forming a mask layer, which is capable of impeding crystal growth and has a window region opened at part thereof, on the first nitride semiconductor layer in such a way that the first nitride semiconductor layer has irregularities at a surface portion thereof exposed at the window region, and growing a second nitride semiconductor layer through crystal growth from the irregularities over a region including an upper surface of the mask layer.
According to a second aspect of the invention, there is provided a crystal growth method, which comprises forming a first nitride semiconductor layer over or on a substrate, forming a mask layer, which is capable of impeding crystal growth and has a window region opened at part thereof, on the first nitride semiconductor layer in such a way that the first nitride semiconductor layer has a growth-impeding surface having fine pores or pinholes at a surface portion thereof exposed at the window region, and growing a second nitride semiconductor layer through crystal growth from the fine pores over a region including an upper surface of the mask layer.
According to a third aspect of the invention, there is provided a crystal growth method, which comprises forming a first nitride semiconductor layer over or on a substrate, forming a mask layer, which is capable of impeding crystal growth and has a window region opened at part thereof, on the first nitride semiconductor layer in such a way that the first nitride semiconductor layer has a crystal nucleus region serving as crystal nuclei for crystal growth at a surface portion thereof exposed at the window region, and growing a second nitride semiconductor layer through crystal growth from the crystal nuclei over a region including an upper surface of the mask layer.
In the first aspect of the invention, the mask layer can be formed as exposing part of the nitride semiconductor layer within the window region opened at part of the mask layer, and the exposed surface of the nitride semiconductor layer can be formed with irregularities within the window region. The irregularities act such that because of the discontinuity of the crystals formed at spaces established at the recessed portions of the irregularities, the through-type dislocations can be prevented from being propagated from the substrate side. The second nitride semiconductor layer is formed through crystal growth from the irregularities, and extends over a region including the upper surface of the mask layer. The growth over the region including the upper surface of the mask layer means crystal growth along lateral directions, thereby reliably preventing the propagation of through-type dislocations.
With the embodiments wherein a growth-impeding surface having fine pores is formed and wherein a crystal nucleus region serving as nuclei for crystal growth is formed, the crystals at the growth-impeding portion become discontinuous, or discontinuity with an underlying layer takes place due to the lateral direction growth extended from the periphery of crystal nuclei, so that the through-type dislocations from the substrate side can be prevented from propagation. Moreover, growth over a region including the upper surface of the mask layer means that crystals are grown in lateral directions, thereby permitting the through-type dislocations to be reliably prevented from propagation.