Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With heterojunction
Reexamination Certificate
1998-12-03
2003-04-01
Fahmy, Wael (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With heterojunction
C257S096000, C257S097000, C257S101000
Reexamination Certificate
active
06541797
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a group-III nitride semiconductor light-emitting device comprising an n-type semiconducting layer and a p-type semiconducting layer formed of a group-III nitride semiconductor, between which is disposed an n-type light-emitting layer formed of a group-III nitride semiconductor containing indium.
2. Description of the Prior Art
In recent years, light-emitting diodes (LEDs) and laser diodes (LDs) that use a light-emitting layer formed of a group-III nitride semiconductor are being commercialized as light-emitting devices. As the group-III nitride semiconductor used to constitute the light-emitting layer, there is mainly used gallium indium nitride of composition formula: Ga
x
In
1−x
N (0≦×≦1) in which the composition ratio of indium is adjusted to obtain the emission of the desired short wavelength visible light (see JP-B 55-3834).
The light-emitting section having a gallium indium nitride light-emitting layer can be broadly divided into three types, in terms of structure. The first type is a single heterojunction (SH) structure comprised of an n-type light-emitting layer and a p-type cladding layer; the second is a double heterojunction (DH) structure in which an n-type light-emitting layer is sandwiched between p-type and n-type barrier layers (cladding layers); and the third is a quantum well (QW) junction structure in which a well layer is n-type Ga
x
In
1−x
N.
In such structures, the basic mechanism by which light is emitted is radiation recombination of carriers, the carriers that are recombined being electrons and holes. This radiation recombination of carriers generates the radiation of light having a wavelength that corresponds to the difference &Dgr;E between the energy levels of the carrier electrons and holes. The relationship between the wavelength &lgr; (nm) of the light radiated by electron transition and the energy level difference &Dgr;E (eV) can be approximated by equation (1).
&lgr;=1.24×10
3
/&Dgr;E
(1)
A brief explanation will now be given of the mechanism by which radiation recombination gives rise to light emission. This mechanism is common to the three structures of the light-emitting section described above. Here, the explanation will be given with respect to the third type of structure, the quantum well.
FIG. 17
is a depiction of the ideal band arrangement of a quantum well structure. Depicted is a single quantum well (SQW) structure or one structural unit of a multi quantum well (MQW) structure.
In the figure, a potential well W
1
and W
2
is formed on the conduction band side and valence band side, respectively, of a well layer W. A potential barrier is formed at each side of the quantum well layer W by a band offset (discontinuity) with each barrier layer B in contact with the quantum well layer W. This band discontinuity arises from the bandgap difference of the nitride semiconductors that constitute the emission and barrier layers.
In the quantum well structure, quantized levels (quantum levels) L
11
and L
12
, and L
21
and L
22
are formed within the potential well W
1
and W
2
, as denoted in the drawings by the broken lines. The radiation recombination that takes place is produced by the transition of carriers between these quantum levels. That is, transitions are generated between electrons and light holes or between electrons and heavy holes, adhering to the wavelength selection rule in the k-space, whereby light is radiated having a wavelength that corresponds to the quantum level differential therebetween. In the case of
FIG. 17
, for example, transitions arise between electron quantum level L
11
and hole quantum level L
21
, and in accordance with equation (1), this results in the radiation of light having a wavelength that corresponds to the energy level differential therebetween.
Also, the quantum levels L
11
, L
12
, L
21
and L
22
are distributed uniformly in the width direction (horizontal direction) of the quantum well layer W, into a rectangular distribution, while retaining a fixed potential differential from the conductance band.
In the light-emitting layer formed from the quantum well layer W in which the quantum levels L
11
, L
12
, L
21
and L
22
have a rectangular distribution, it is necessary to suppress variation in the wavelength of the emitted light in order to obtain full monochromaticity. To do this, it is necessary to have an abrupt change between the quantum well layer and barrier layer bands, and to be able to stably reproduce this abrupt band change. For this, it is important to achieve abrupt compositional changes with respect to the constituent elements in the junction interface between the quantum well layer and the barrier layer.
However, effecting this abrupt compositional change in the junction interface requires the use of a high level of interfacial control technology. In practice, the band change between the quantum well and barrier layers is gradual, as shown in FIG.
18
.
With respect to
FIG. 18
which shows a comparison between an actual band structure D
1
and an ideal band structure D
2
, indicated by the broken line, when the band offset is gradual, quantum level L
3
in the quantum well layer W decreases to L
31
and quantum level L
4
decreases to L
41
. This is the phenomenon that is generally recognized as quantum level fluctuation (see page 227 of fourth printing of first edition “Physics and Applications of Semiconductor Superlattices,” edited by the Physical Society of Japan and first published on Sep. 30, 1986, by K. K. Baifukan, a Japanese corporation).
This quantum level fluctuation is also caused by slight changes in the thickness of the quantum well layer. With respect to
FIG. 19
, for example, if the thickness (width) of the quantum well layer W, as indicated by the solid line, is reduced to the thickness indicated by the broken line, quantum level L
5
is elevated to L
51
. If the thickness is increased, as indicated by the dot-dash line, L
5
decreases to L
52
. It is extremely difficult to control the thickness of the quantum well layer to a precise enough degree that ensures that there is no change in the quantum level.
Quantum level fluctuation has been held to cause instability of energy level differentials and variations in the wavelength of light emissions. Light-emitting devices having a quantum well structure with a conventional light-emitting layer of gallium indium nitride are prone to the effects of gradual compositional changes in the junction interface and slight changes in the thickness (width) of the quantum well layer W, changing emission wavelengths and therefore making it difficult to stably utilize the monochromatic properties.
In addition to the fact that light-emitting devices with a gallium indium nitride light-emitting layer are susceptible to fluctuations in quantum level, a further problem is that in such devices, it is difficult to form a good-quality junction interface.
This is because the qualities of the gallium indium nitride are readily altered by heating, producing a multiplicity of phases with varying indium concentrations (compositions), meaning there is a strong tendency to form a multi-phase structure (see (1) Solid State Commun., 11 (1972), pp 617 to 621; (2) J. Appl. Phys., 46 (8) (1975), pp 3432 to 3437).
FIG. 20
is a drawing showing the known structure of a light-emitting section that includes a light-emitting layer formed of gallium indium nitride. In the drawing, barrier layers B
10
adjoining a well layer W
10
are comprised of a binary (two-element) compound, gallium nitride (GaN), that enables phase changes to be ignored. The qualities of the gallium indium nitride mixed-crystal forming the light-emitting layer (well layer) W
10
are readily altered by heat, producing a separation between the internal matrix phase P
10
and the crystallite subsidiary phase P
11
. The matrix phase P
10
and subsidiary phase P
11
have different indium concentrations, and a correspondingly different bandgap. Variation also a
Armstrong Westerman & Hattori, LLP
Fahmy Wael
Louie Wai-Sing
Showa Denko K. K.
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