Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1998-09-11
2001-07-24
Kim, Robert H. (Department: 2877)
Coherent light generators
Particular active media
Semiconductor
C372S045013, C372S049010, C372S050121
Reexamination Certificate
active
06266355
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to Group III-V nitride compound semiconductor devices and more particularly to Group III-V nitride compound semiconductor LED and laser devices utilizing techniques relative to the nitride cladding layers to suppress defects such as layer cracking. By “layer cracking”, it meant that cracks appear or are generated through as-grown Group III-V nitride compound layers transversely of their layer planes during the growth of the layers.
BACKGROUND OF THE INVENTION
Group III-V nitride compound semiconductor laser devices illustrated to date that have operated for prolonged periods of time, particularly as reported by Shuji Nakamura and his coworkers, have employed AlGaN. An example of the growth of AlGaN layers is exemplified in the U.S. Pat. No. 5,290,393 to Nakamura. Such AlGaN layers are grown in MOCVD at temperatures between 900° C. and 1200° C. Upon heating and cooling sequences in the MOCVD fabrication of devices, the as-grown layers are subjected to tremendous stresses since the AlGaN layers do not expand or contract at the same rate as the sapphire substrate, their thermal expansion coefficients being so different. As a result, large and small micro-cracks form in such as-grown layers that spread from the sapphire substrate upward through the layers. While the mechanisms causing these cracks are not fully understood, they are likely caused by the thermal expansion coefficient mismatch between the as-grown materials which do not expand or contract at the same rate as the substrate. It was also repeatedly shown that there are two factors severely aggravating cracking in nitride-based structures: presence of thick (e.g. >0.2 &mgr;m) layers of AlGaN and/or doping with both p- and n-type impurities. The higher temperature heating of the substrate during AlGaN layer growth and the subsequent cooldown places the as-grown layers on the substrate under mechanical and lattice stresses upon their contraction at room temperature.
In a Group III-V nitride laser device, the structure may be of the so-called separate confinement type, such as, comprising an InGaN active layer or quantum well or a multiple quantum well region of InGaN quantum wells with InGaN or GaN barriers, cladded between GaN waveguide (or core) layers which in turn are cladded by confinement layers of p-type and n-type AlGaN. The general requirement of the separate confinement structure: step-like increase of band gap and corresponding decrease in refractive index from the active layer through waveguide layers to confinement (or cladding layers) is satisfied in this case.
As Al is added to or increased in Al
x
Ga
1-x
N, the growth temperature for growing the layer has to be correspondingly increased. Because of these high temperatures, diffusion can occur within the InGaN active region. InGaN is grown at much lower temperatures, such as in the range of about 600° C. to about 800° C. These higher growth temperatures for AlGaN, such as in the case of growing the upper p-type AlGaN layer at 1,000° C. or more, will heat up the InGaN layer or layers and can induce atomic rearrangement in these layers. The indium can start clustering at Group III lattice sites through the process of elemental interdiffusion. While not well understood, it is believed that when the InGaN layer is initially grown, the In and Ga atoms which are randomly distributed on the Group III lattice sites, forming a homogenous alloy. When the InGaN layer is subjected later to higher temperatures, particularly well above its growth temperature range, there is an exchange and redistribution of In and Ga atoms at the Group III sites. It could be an equilibrium condition comprising an InN-rich region and a GaN-rich region. Equilibrium is suppressed by growing the InGaN layer at low temperatures so that a homogeneous mix of In and Ga in the lattice structure is achieved. In any case, when the InGaN layer or layers are heated to temperatures in excess of its growth temperature range, InN-rich and GaN-rich clusters can form in the as-grown InGaN layer or layers, causing their desired optical properties to be substantially deteriorated.
In spite of these above mentioned problems, AlGaN layers are the present choice for blue LED and laser devices because they can provide both good electrical and optical confinement particularly if the above described problems can be overcome on a regular high-yield basis.
It is an object of this invention to provide a Group III-V nitride compound semiconductor light emitting device, such as Group III-V nitride lasers and LED's, that eliminates the foregoing problems by avoiding the use of homogeneous AlGaN or the extensive use of aluminum nitride in the fabrication of these types of devices and provide cladding layers that are designed with the objective of suppressing interlayer cracking.
SUMMARY OF THE INVENTION
According to this invention, a Group III-V nitride compound semiconductor light emitting device is constructed without employing homogeneous layers of AlGaN. Instead of homogeneous AlGaN cladding layers, GaN cladding layers are utilized. Since high temperature growths that accompany the formation of AlGaN layers are no longer required, the stochiometric amount of indium in In
x
Ga
1-x
N of core layers utilized in the active region may be made greater to achieve better electrical and optical properties in the device. Thus, the loss of waveguiding achieved by the lower refractive index layers of AlGaN is compensated by the use of core layers of InGaN on adjacent sides of the active region.
In one embodiment of this invention, a method of manufacturing comprises a Group III-V, aluminum-free nitride compound semiconductor device utilizing MOCVD comprising the steps of growing a n-GaN cladding layer on a substrate, growing a core layer of In
x
Ga
1-x
N on the n-GaN cladding layer, growing an active region on the core layer containing at least one layer of In
y
Ga
1-y
N where y>x, growing another core layer of In
x
Ga
1-x
N on the active region, and growing a p-GaN cladding layer on the core layer of In
x
Ga
1-x
N. The stochiometric amount of indium in In
x
Ga
1-x
N core layers utilized in the active region may be made greater to achieve better electrical and optical properties in the device.
In another embodiment of this invention the cladding layers are grown by digital alloying, such as digital alloying growth of cladding layers comprising monolayers of GaN and AIN forming a GaN/AIN superlattice. Digital alloying in these layers provides for higher doping concentrations, which is due to the ease of doping interdigitated GaN layers rather than homogeneous AlGaN layers as well as suppressing the formation of defects such as cracks in the formed layer or layers. Also, the inclusion of some aluminum in the cladding layers provides for improved optical confinement over just pure GaN. Digital alloying of the cladding layers is commenced with the growth of a monolayer of AlN followed by a plurality of GaN monolayers followed by another AlN layer and so on. There are several times more GaN monolayers than AlN monolayers in the cladding layers. For example, the ratio of GaN monolayers to AlN monolayers may be about 4:1 or 5:1. Also, digital alloying can be extended to Al
x
Ga
1-x
N/Al
y
Ga
1-y
N superlattices in the formation of the cladding layers.
In another embodiment of this invention the steps of growth, set forth above, are carried out in different spatial regions on the substrate. The spatial region growth may be carried out through the employment of photolithography on a surface of the substrate, such as by means of forming grooves in a dielectric layer formed on a surface of the substrate in which the nitride layered structure of the first mentioned embodiment can be grown in the grooves by means of MOCVD. Patterned photoresist via photolithography is employed to form the spatial pattern in a deposited dielectric, such as SiO
2
or Si
3
N
4
, or the photoresist can used to form a pattern of grooves or ridges in the buffer layer. In either case, the Group III
Carothers, Jr. W. Douglas
Flores Ruiz Delma R.
Kim Robert H.
SDL Inc.
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