Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2000-12-26
2004-10-12
Wille, Douglas (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S014000, C257S101000, C257S102000
Reexamination Certificate
active
06803596
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light emitting device formed of a material having a piezoelectric effect.
2. Description of the Background Art
Semiconductor light emitting devices such as semiconductor laser devices and light emitting diodes using III group nitride semiconductors such as GaN, GaInN, AlGaN, and AlGaInN (hereinafter referred to as nitride based semiconductors) have been expected to be applied as light emitting devices for emitting light in a visible-ultraviolet region.
Out of the applications, extensive development has been proceeding toward practical applications of the semiconductor light emitting devices having a GaInN quantum well layer as a light emitting layer. Such semiconductor light emitting devices have been fabricated on a (0001) surface of a substrate composed of sapphire or silicon carbide (SiC) by MOVPE (Metal Organic Vapor Phase Epitaxy) or MBE (Molecular Beam Epitaxy).
FIG. 42
is a schematic sectional view showing the structure of a conventional GaN based semiconductor light emitting device. The semiconductor light emitting device shown in
FIG. 42
is disclosed in JP-A-6-268257.
In
FIG. 42
, a buffer layer
62
composed of GaN, an n-type contact layer
63
composed of n-GaN, a light emitting layer
64
having a multiple quantum well (MQW) structure, and a p-type cap layer
65
composed of p-GaN are formed in this order on a sapphire substrate
61
. The light emitting layer
64
is constructed by alternately stacking a plurality of barrier layers
64
a
composed of GaInN and a plurality of quantum well layers
64
b
which differ in composition.
In a method of fabricating such a conventional semiconductor light emitting device, generally used as the sapphire substrate
61
is one having an approximate (0001) surface as its principal plane, to successively form the respective layers from the buffer layer
62
to the p-type cap layer
65
on the sapphire substrate
61
. In this case, the respective layers from the n-type contact layer
63
to the p-type cap layer
65
are grown as crystals in a [0001] direction of a nitride based semiconductor.
In a crystal, having no center of symmetry, having a zinc-blende structure, a wurtzite structure, or the like, however, a piezoelectric effect may be generally generated by a strain. In the zinc-blende structure, for example, a piezoelectric effect is the greatest in a strain compressing or extending with respect to a [111] axis. In the wurtzite structure, the piezoelectric effect is the greatest in a strain compressing or extending with respect to a [0001] axis.
In the conventional semiconductor light emitting device, the light emitting layer
64
composed of GaInN has a quantum well structure having a (0001) plane as its principal plane. The lattice constant of a quantum well layer
64
b
composed of GaInN is larger than the lattice constant of the n-type contact layer
63
composed of n-GaN. Accordingly, a compressive strain is exerted on the quantum well layer
64
b
in an in-plane direction (a direction parallel to an interface) of a quantum well, and a tensile strain is exerted in a direction of confinement in the quantum well (a direction perpendicular to the interface). As a result, the piezoelectric effect generates a potential gradient in the quantum well layer
64
b
, so that a potential is low in the [0001] direction, while being high in a [0001] direction. An energy band in the light emitting layer
64
having a quantum well structure in this case is illustrated in FIG.
43
.
FIG. 43
illustrates five barrier layers
64
a
and four quantum well layers
64
b.
As shown in
FIG. 43
, the potential gradient exists in the quantum well layer
64
b
in the light emitting layer
64
. Accordingly, electrons and holes which are injected as current are spatially separated from each other, as shown in FIG.
44
. As a result, in the semiconductor light emitting device, luminous efficiency is reduced. Particularly in the semiconductor laser device, threshold current is raised.
When impurities are added to the quantum well layer
64
b
in the light emitting layer
64
, the effect of decreasing the potential gradient by the movement of carriers is produced. When both p-type impurities and n-type impurities are added to the quantum well layer
64
b
, however, the carriers are compensated for, so that the carrier concentration is lowered. Consequently, the effect of decreasing the potential gradient by the movement of the carriers is reduced. Particularly when the respective concentrations of the p-type impurities and the n-type impurities which are added to the quantum well layer
64
b
are approximately equal to each other, the effect of decreasing the potential gradient by the movement of the carriers is further lowered.
Such a phenomenon also occurs in a III-V group compound semiconductor (for example, a GaInP based semiconductor, a GaAs based semiconductor, or an InP based semiconductor) other than the zinc-blende structure and the wurtzite structure, a II-VI group semiconductor, and an I-VII group semiconductor. Particularly in the nitride based semiconductor, the piezoelectric effect is great. Accordingly, the potential gradient generated by the piezoelectric effect is increased, so that the drop in luminous efficiency and the rises in threshold current and operating current become significant.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a light emitting device which is high in luminous efficiency and is low in operating current or threshold current.
A light emitting device according to an aspect of the present invention comprises a first n-type layer; a first p-type layer; a light emitting layer arranged so as to be interposed between the first n-type layer and the first p-type layer and having a strain generating a piezoelectric effect; and a second n-type layer provided between at least the light emitting layer and the first p-type layer and having a wider bandgap than that of the light emitting layer, the potential in the light emitting layer whose gradient is generated by the piezoelectric effect being higher on the side of the first n-type layer than that on the side of the first p-type layer.
In the light emitting device, the second n-type layer is formed between the light emitting layer and the first p-type layer, so that electrons are moved in a direction perpendicular to an interface of the light emitting layer, the electrons and ionized donor levels are spatially separated from each other, and the potential gradient generated by the piezoelectric effect in the direction perpendicular to the interface is decreased. Consequently, the electrons and holes which are injected as current are prevented from being separated from each other. Accordingly, gain is easily obtained, thereby preventing luminous efficiency from being reduced and preventing operating current or threshold current from being raised.
The first p-type layer may comprise a first cladding layer, and the bandgap of the second n-type layer may be narrower than that of the first cladding layer. In this case, the refractive index of the second n-type layer is larger than that of the first cladding layer, so that the second n-type layer functions as an optical guide layer.
A light emitting device according to another aspect of the present invention comprises a first n-type layer; a first p-type layer; a light emitting layer arranged so as to be interposed between the first n-type layer and the first p-type layer and having a strain generating a piezoelectric effect; and a second p-type layer provided between at least the light emitting layer and the first n-type layer and having a wider bandgap than that of the light emitting layer, the potential in the light emitting layer whose gradient is generated by the piezoelectric effect being higher on the side of the first n-type layer than that on the side of the first p-type layer.
In the light emitting device, the second p-type layer is formed between the
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