Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With heterojunction
Utility Patent
1998-10-22
2001-01-02
Mintel, William (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With heterojunction
C257S096000, C257S097000, C257S099000
Utility Patent
active
06169296
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a light-emitting diode device used in various applications including display device and a method for fabricating the same. More particularly, the present invention relates to a light-emitting diode device for outputting blue or green light at a short wavelength and with high efficiency and a method for fabricating the same.
A light-emitting diode has higher efficiency and consumes lower power than an electric light bulb, and therefore is used widely for a display device. During the past few years, a light-emitting diode device for emitting light at a high luminance and at a long wavelength in the spectrum of red, orange, yellow or the like was already provided. On the other hand, a conventional light-emitting diode device for emitting light at a short wavelength in the spectrum of green or blue had a low luminance. Accordingly, in the past, a light-emitting diode device could not emit blue or green light at a luminance high enough to be applicable to an outdoor display screen (on the order of several candelas).
However, just recently, a light-emitting diode device for emitting green or blue light at a luminance as high as six candelas or two candelas, respectively, was developed by using brand new GaN-based crystals. Now a full-color, high-luminance display using these high-luminance light-emitting diode devices is available everywhere.
Hereinafter, a conventional GaN-based light-emitting diode device including a quantum well active layer made of In
x
Ga
1−x
N (in this specification, x is a real number and 0≦x≦1) will be generally described.
In the conventional device, if operating current is supplied, then electrons and holes are injected into the quantum well active layer and recombined with each other. As a result, the device emits light at a wavelength approximately corresponding to the band gap energy of In
x
Ga
1−x
N. If the In mole fraction x in In
x
Ga
1−x
N is 0.15, then the device emits light at a peak wavelength of about 450 nm in the blue spectrum. On the other hand, if the In mole fraction x in In
x
Ga
1−x
N is 0.26, then the device emits light at a peak wavelength of about 520 nm in the green spectrum.
In the conventional GaN-based light-emitting diode device, however, the larger the In mole fraction x in In
x
Ga
1−x
N constituting the quantum well active layer is, the lower the crystallinity thereof is. Essentially, In
x
Ga
1−x
N is less likely to be a uniform mixed crystal. Thus, when In
x
Ga
1−x
N is used, the resulting external quantum efficiency is as low as about 1 to about 2% even if light reflected by the back surface of a substrate made of sapphire, for example, is included in the light output. Herein, the external quantum efficiency is defined as a ratio of the light emitted out of the device to the power consumed by the device. Accordingly, the wavelength thereof is very difficult to control and the production yield of the light-emitting diode device using such a compound is very low. By contrast, the external quantum efficiency of a light-emitting diode device for emitting red light at a wavelength of about 650 nm is about 20% if the reflected light is included in the light output thereof.
PRIOR ART EXAMPLE 1
Hereinafter, as a first prior art example, a light-emitting diode device for emitting light at a short wavelength by using ZnSe-based Group II-VI compound semiconductors, which are the object of much attention recently, instead of GaN-based Group III-V compound semiconductors, will be described with to reference to FIG.
8
.
FIG. 8
illustrates the cross-sectional structure of a ZnSe-based light-emitting diode device as the first prior art example. As shown in
FIG. 8
, a first buffer layer
102
, made of n-type GaAs, for buffering lattice mismatching with a substrate
101
made of n-type GaAs is formed on the substrate
101
. On the first buffer layer
102
, a second buffer layer
103
, made of n-type ZnSe, for buffering lattice mismatching between GaAs crystals and ZnSe crystals, is formed. On the second buffer layer
103
, a first cladding layer
104
, made of n-type ZnMgSSe, for forming a potential barrier for an active layer (to be described below) and thereby efficiently injecting n-type carriers (electrons) into the active layer, is formed. On the first cladding layer
104
, a first spacer layer
105
, made of non-doped ZnSSe, for improving the crystal quality of the active layer and efficiently injecting the n-type carriers into the active layer, is formed. On the first spacer layer
105
, a quantum well active layer
106
, made of Zn
1−x
Cd
x
Se, for emitting light by the recombination of the injected n-type and p-type minority carriers, is formed. On the quantum well active layer
106
, a second spacer layer
107
, made of non-doped ZnSSe, for efficiently injecting p-type carriers (holes) into the active layer
106
, is formed. On the second spacer layer
107
, a second cladding layer
108
, made of p-type ZnMgSSe, for forming a potential barrier for the active layer
106
and thereby efficiently injecting p-type carriers into the active layer
106
, is formed. On the second cladding layer
108
, a semiconductor layer
109
, made of p-type ZnSSe, for connecting stepwise the energy level on the valence band between the second cladding layer
108
and a contact layer (to be described below) is formed. On the semiconductor layer
109
, a superlattice layer
110
, including alternately stacked p-type ZnSe layers and p-type ZnTe layers and connecting stepwise the energy level on the valence band between the semiconductor layer
109
and the contact layer, is formed. And on the superlattice layer
110
, a contact layer
111
, made of p-type ZnTe, for making ohmic contact with an electrode is formed. These layers
102
through
111
are formed by a molecular beam epitaxy (MBE) technique, for example.
Over the entire upper surface of the contact layer
111
, a p-side ohmic electrode
112
, made of Pd and Au, is formed. A bonding pad
113
, made of Au or the like, is formed in the shape of a dot on the p-side ohmic electrode
112
. On the other hand, over the entire lower surface of the substrate
101
, an n-side ohmic electrode
114
is formed.
PRIOR ART EXAMPLE 2
Hereinafter, as a second prior art example, a light-emitting diode device for emitting red light at a wavelength in the range from 620 nm to 660 nm by using AlGaInP-based Group III-V compound semiconductors will be described with reference to FIG.
9
.
FIG. 9
illustrates the cross-sectional structure of an AlGaInP-based light-emitting diode device as the second prior art example. As shown in
FIG. 9
, a buffer layer
122
, made of n-type GaInP, for buffering lattice mismatching with a substrate
121
made of n-type GaAs; a first cladding layer
123
made of n-type AlGaInP; an active layer
124
made of non-doped GaInP; a second cladding layer
125
made of p-type AlGaInP; and a current blocking layer
126
made of n-type GaAs are sequentially grown on the substrate
121
by a metalorganic vapor phase epitaxy (MOVPE) technique, for example. Then, the substrate
121
is once taken out of the crystal-growing apparatus. Thereafter, as shown in
FIG. 9
, the current blocking layer
126
is etched with a part of the layer
126
where a bonding pad is to be formed (hereinafter, simply referred to as a “bonding pad forming region”) masked, thereby shaping the current blocking layer
126
like a dot. Next, the substrate
121
, on which the current blocking layer
126
has been shaped, is put into the crystal-growing apparatus again. And a current diffusing layer
127
, made of p-type AlGaInP, for diffusing and making the current, flowing vertically to the surface of the substrate, flow horizontally to the surface of the substrate, is grown over the entire surfaces of the second cladding layer
125
and the current blocking layer
126
. Thereafter, a p-side ohmic electrode
128
in the shape of a dot is formed on the bonding pad forming region of the current diffusing layer
127
Kamiyama Satoshi
Miyanaga Ryoko
Nishikawa Takashi
Saitoh Tohru
Sasai Yoichi
Matsushita Electric - Industrial Co., Ltd.
Mintel William
Nixon & Peabody LLP
Robinson Eric J.
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