Active solid-state devices (e.g. – transistors – solid-state diode – Physical configuration of semiconductor – Mesa structure
Reexamination Certificate
2001-06-15
2003-09-30
Dang, Trung (Department: 2823)
Active solid-state devices (e.g., transistors, solid-state diode
Physical configuration of semiconductor
Mesa structure
C438S479000, C438S481000, C438S574000, C257S613000, C257S615000
Reexamination Certificate
active
06627974
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for growing a nitride semiconductor (In
x
Al
y
Ga
1−x−y
N, 0≦X, 0≦Y, X+Y≦1), and particularly to a method for growing a nitride semiconductor which can be used to make a nitride semiconductor substrate.
BACKGROUND ART
Recently various researches have been conducted on the growth of nitride semiconductor on a substrate made of a different material such as sapphire, spinel or silicon carbide which has a lattice constant different from that of the nitride semiconductor.
For example, a method of growing epitaxial lateral overgrowth GaN (ELOG) is described in JPN. J. Appl. Phys., vol. 37 (1998), pp. L309-L312, wherein nitride semiconductor having lower density of dislocations is obtained by forming a protective film of SiO
2
or other material partially on a nitride semiconductor which has been grown on the C plane of sapphire, and growing nitride semiconductor thereon under a reduced pressure of 100 Torr.
In the ELOG growing process, nitride semiconductor having reduced dislocation defects can be formed on the protective film by intentionally growing the nitride semiconductor laterally on the protective film. When the nitride semiconductor grows, dislocation occurs and grows only in a window portion of the protective film.
However, in case the protective film of SiO
2
or the like has wide stripe width, lateral growth of the nitride semiconductor on the protective film does not fully proceed, eventually resulting in abnormal growth.
In addition, in case the nitride semiconductor is grown laterally by vapor phase deposition process, while two nitride semiconductor films which grow laterally from the nitride semiconductor exposed on both sides of the protective film meet and join with each other at the center of the protective film, dislocations concentrate locally at the joint. This is partly due to the fact that the front surface of the nitride semiconductor is tilted while growing laterally on the protective film of SiO
2
or the like. In case a device layer is formed by epitaxial growth on a nitride semiconductor substrate such as the above, microscopic pits are likely to be generated in the joint where the dislocations are concentrated. The pits are generated by the dissociation of nitrogen in the process of heating the substrate for the purpose of growing the device layer. The pits grow larger as the epitaxial growth is continued.
As a result, even when a single continuous nitride semiconductor substrate is formed by growing nitride semiconductor layer laterally on a protective film by the vapor phase deposition process, it cannot be handled in the same way as an ordinary single crystal substrate. Since the active layer of a semiconductor laser should keep clear of the vicinity of the joint, it is difficult to secure a region large enough for forming the device. Moreover, since surface of the single nitride semiconductor substrate appears to be uniform, it has been difficult to recognize the joint by viewing the top surface of the substrate and to carry out the formation of device pattern accurately.
Furthermore, in case a single continuous nitride semiconductor substrate is formed by growing nitride semiconductor laterally by using a protective film on sapphire or the like, such a structure is likely to warp. Because sapphire, the protective film and the nitride semiconductor layer, which are stacked one on another, have different coefficients of thermal expansion.
The different-material substrate may also be removed from the nitride semiconductor substrate in the last stage. The substrate of different material maybe removed by polishing or irradiating the interface between the substrate and the nitride semiconductor with excimer laser thereby breaking the chemical bond in the interface. However, it has not been easy to remove a different-material substrate such as sapphire as it takes a long time to remove by polishing or by means of excimer laser.
An object of the present invention is to provide a new structure of nitride semiconductor substrate manufactured by lateral crystal growth with a protective film, which is capable of suppressing an adverse effect caused on the device by joining the nitride semiconductor layers on the protective film. Another object of the present invention is to prevent the nitride semiconductor substrate from warping. Still another object of the present invention is to facilitate removing a substrate made of a different material from the nitride semiconductor substrate.
DISCLOSURE OF THE INVENTION
In order to solve the problems described above, a nitride semiconductor substrate according to the first invention comprises (A) a supporting substrate, (B) a first nitride semiconductor layer having periodically arranged T-shaped cross section formed by laterally growing nitride semiconductor films starting at portions formed in a periodical stripe, grid or island configuration provided on the surface of the supporting substrate and stopping the lateral growth before the films join together, and (C) a second nitride semiconductor layer which is grown from the top surface or the top and side surface, which side surface has been grown laterally, of the first nitride semiconductor layer as the core and covers the entire surface of the supporting substrate, wherein cavities are formed under the joint of the second nitride semiconductor layer.
The nitride semiconductor substrate having such a structure as described above can be manufactured by (A) forming a protective film having windows of stripe, grid or island configuration on the supporting substrate, (B) laterally growing the first nitride semiconductor over the protective film from the exposed portions of the supporting substrate and stopping the growth in such a state as the protective film is not covered, (C) removing the protective film thereby to form cavities below the first nitride semiconductor layer which has been grown laterally, and (D) growing the second nitride semiconductor layer laterally from the top surface or the top and side surface, which side surface is the portion grown laterally, of the first nitride semiconductor layer. The supporting substrate may be either a substrate made of a different material such as sapphire or a different-material substrate covered with a nitride semiconductor layer over the entire surface thereof. In case a substrate made of sapphire or the like is used, it is preferable to form a low temperature-grown buffer layer on the substrate before growing the first nitride semiconductor. In case the second nitride semiconductor layer grows from the top surface of the first nitride semiconductor layer, the step of removing the protective film may be omitted since both parts of the second nitride semiconductor layer join with each other above the cavity even when the protective film is not removed.
According to the first aspect of the present invention, the nitride semiconductor without voids can be grown, even when forming the protective film widely. Also, strain can be suppressed which would otherwise be generated when the second nitride semiconductor is grown from the side surface of the first nitride semiconductor, because the second nitride semiconductor layer grows over the cavity. Moreover, since the front surface of the growing crystal does not tilt as in the case of growing on the protective film, concentration of dislocations in the joint can be relieved.
Also, it is made easier to locate the joint even from above the top surface of the second nitride semiconductor layer which covers the entire surface of the substrate, because such a cavity exists below the joint of the second nitride semiconductor layer that has a refractive index which is significantly different from that of the nitride semiconductor. Since the cavity relieves the strain, warping of the substrate due to the difference in the thermal expansion coefficient between the substrate and the nitride semiconductor layer can be mitigated.
Moreover, because the nitride semiconductor layer is supported by the dis
Chocho Kazuyuki
Kiyoku Hiroyuki
Kozaki Tokuya
Maegawa Hitoshi
Dang Trung
Nichia Corporation
Nixon & Vanderhye P.C.
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