Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With housing or contact structure
Reexamination Certificate
2000-09-08
2002-09-24
Crane, Sara (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With housing or contact structure
C257S101000, C257S103000, C257S627000
Reexamination Certificate
active
06455877
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device and particularly to a light-emitting device fabricated on a III-N compound semiconductor substrate, specifically, a GaN compound semiconductor substrate.
2. Description of the Background Art
GaN compound semiconductors have been employed or studied for light-emitting devices and high-power devices due to their specific characteristics. For example, it is technically possible to produce and utilize light-emitting devices in a wide emitting light range of violet to reddish yellow by adjusting the composition thereof.
Recently, utilizing the characteristics of the GaN compound semiconductors, blue light-emitting diodes and green light-emitting diodes have been put into practical use, and blue violet semiconductor lasers have been developed.
In order to fabricate a GaN compound semiconductor film, a substrate is used which is formed of sapphire, SiC, spinel, Si, GaAs and the like. If sapphire is employed for the substrate, prior to deposition of the GaN film by epitaxial growth, a buffer layer of GaN or AlN is formed at a low temperature of approximately 550° C. in advance. After this step, the substrate is heated to a high temperature of approximately 1050° C. to achieve epitaxial growth of the GaN compound semiconductor film. It is known that this process can provide a structurally and electrically superior crystal having a good surface condition.
It is also known that if SiC is employed for the substrate, a thin AlN film is advantageously used as a buffer layer at a growth temperature for epitaxial growth. However, the use of other materials than the GaN compound semiconductor as the substrate can cause a large number of defects in a resultant GaN compound semiconductor film, due to the differences in thermal expansion coefficient and lattice constant between the grown GaN compound semiconductor film and the substrate. These defects are classified into edge dislocation and screw dislocation, and the total density of the defects can amount to approximately 1×10
9
cm
−2
to 1×10
10
cm
−2
. These defects are known to trap carriers and accordingly degrade electrical characteristics of the prepared films, and further to shorten the life of a laser to which a great amount of current is applied.
Thus, studies have been conducted for reducing these defects and enhancing electrical characteristics of the semiconductor to be prepared. For example, a technique has been developed in which on a GaN film grown by metal organic chemical vapor deposition (MOCVD) and the like, a thick GaN film is deposited by hydride vapor-phase epitaxy (H-VPE) and the like using a mask of SiO
2
, tungsten and the like for the purpose of preventing increase of defects such as dislocation, and the produced thick film is used as a substrate for forming a light-emitting device thereon.
SUMMARY OF THE INVENTION
However, characteristics of an n-type electrode on such a GaN substrate have not been clarified. The inventors of the present invention have found that an n-type electrode of Ti/Al or the like formed on a Ga-terminated surface of a GaN substrate has a strong tendency to exhibit Schottky characteristics. The inventors have considered that carbon (C) and the like are likely to be coupled to dangling bonds of Ga on the Ga-terminated surface. If an n-type electrode of Ti/Al or the like is formed on the Ga-terminated surface in the presence of C, a barrier layer can be produced and accordingly the electrode may exhibit Schottky characteristics. On the other hand, a film of Ni, Pd or the like constituting a p-type electrode can incorporate therein carbon (C) and the like to reduce the formation of the barrier layer. This is considered as one of the reasons for relative easiness of achieving ohmic characteristics by the p-type electrode.
In order to produce an n-type electrode of Ti/Al or the like with ohmic characteristics on the Ga-terminated surface of the GaN substrate, some steps are required, specifically including the steps of cleaning the substrate surface by hydrochloric acid and the like and annealing for producing alloy after fabrication of the electrode in order to form an intermediate product between GaN and Ti being therewith for reduction of the barrier layer. Specific contact resistance of the n-type electrode is still high even if those-steps are added.
One object of the present invention is to provide a technique of fabricating an n-type electrode in a semiconductor device structure employing a nitride semiconductor substrate such as a GaN substrate to give ohmic characteristics, without the steps of surface processing and annealing as described above.
Another object of the invention is to provide a nitride semiconductor device, particularly a light-emitting device, with a low specific contact resistance of an n-type electrode.
Still another object of the invention is to provide a nitride semiconductor device, particularly a light-emitting device, having a low threshold voltage or a low threshold current density.
The inventors have found that the ohmic characteristics can easily be obtained by fabricating an n-type electrode on an N-terminated surface of a nitride semiconductor. Further, the inventors have clarified the relation between the concentration of impurities added to a nitride semiconductor substrate and the specific contact resistance of the n-type electrode. Regarding light-emitting devices, particularly laser diode devices, the inventors have further clarified the relation between the concentration of impurities added to the nitride semiconductor substrate and threshold voltage as well as the relation between the concentration of impurities added to the nitride semiconductor substrate and threshold current density, and accordingly found a proper concentration of impurities which provides a low specific contact resistance, a low threshold voltage or a low threshold current density. The present invention has been made based on the above findings.
According to the present invention, a III-N compound semiconductor device is provided. The semiconductor device has an electrode on a nitrogen-terminated surface of the III-N compound semiconductor substrate. Specifically, the semiconductor device according to the present invention includes a III-N compound semiconductor substrate, a plurality of III-N compound semiconductor layers formed on the semiconductor substrate, and an n-type electrode and a p-type electrode for applying voltage to the semiconductor layers formed on the semiconductor substrate, wherein the semiconductor substrate is of n-type and the n-type electrode is formed on a nitrogen-terminated surface of the semiconductor substrate.
FIG. 23
illustrates a Ga-terminated surface and an N-terminated surface of GaN grown on the (0001) plane of a seed substrate, showing the seed substrate denoted by
2301
, a buffer layer
2302
, the Ga-terminated surface denoted by
2303
B, the N-terminated surface denoted by
2303
C, Ga atoms
2304
(represented by circle) and N atoms
2305
(represented by filled circle). As seen from the drawing, N atoms
2305
are predominantly projecting from N-terminated surface
2303
C while Ga atoms
2304
are predominantly projecting from Ga-terminated surface
2303
B.
The N-terminated surface and the Ga-terminated surface with respect to the (0001) plane of a GaN crystal can be defined here as follows. When the crystal having the exposed N-terminated surface is soaked for three minutes in an aqueous NaOH solution of 1.8 M at room temperature, its surface conditions change and hillocks of approximately 50 nm in size disappear. After such etching, roughened surface can also be observed by using atomic force microscopy (AFM), for example, at a region of 50 &mgr;m in size. Such features can be shown by the surface in which N atom comprises at least 60% of the terminated atoms, and such a surface is herein referred to as N-terminated surface. On the other hand, the Ga-terminated surface has the feature that its conditions hard
Araki Masahiro
Ogawa Atsushi
Taneya Mototaka
Tsuda Yuhzoh
Ueta Yoshihiro
Crane Sara
Morrison & Foerster / LLP
Sharp Kabushiki Kaisha
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