Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With heterojunction
Reexamination Certificate
2000-04-27
2002-01-01
Flynn, Nathan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With heterojunction
C257S096000, C257S097000, C257S101000, C257S102000
Reexamination Certificate
active
06335547
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an epitaxial wafer for fabricating a high-intensity infrared light-emitting device which is employed in an optical communications and spatial transmission apparatus using infrared radiation. The invention also relates to an infrared light-emitting device fabricated from the epitaxial wafer and to an optical communications and spatial transmission apparatus employing the device.
BACKGROUND ART
Light-emitting devices (hereinafter referred to as LEDs) employing GaAlAs compound semiconductors have been widely used in a light source in a wavelength range from infrared to visible red light. Although an infrared LED is employed in optical communications and spatial transmission, there has been increasing demand for a high-intensity infrared LED of increased capacity for transmitting data and increased transmission distance.
As has conventionally been known, a GaAlAs LED having a double-hetero structure (hereinafter DH structure) exhibits emitted-light intensity higher than that of a GaAlAs LED having a single-hetero structure, and emitted-light intensity is enhanced by means of removing a substrate.
In fabrication of an LED employing a substrate-removed-type structure (hereinafter referred to as a DDH structure), a typical DH structure; i.e., only three layers consisting of a p-type cladding layer, an active layer, and an n-type cladding layer, is epitaxially grown and then a substrate is removed, to thereby reduce the thickness of a produced epitaxial wafer. Such an epitaxial wafer is difficult to handle during processing into a device. In addition, the distance from a bottom surface of the device to the junction decreases, and a paste for bonding the device to a conductor migrates through a side face of the device, to thereby disadvantageously short-circuit the pn junction. In order to avoid this problem, a fourth epitaxial layer is added to the DH structure so as to ensure the overall thickness of the substrate-removed and finished epitaxial wafer and the distance from a bottom surface of the device to the junction. This constitution is standard for a DDH structure. The fourth epitaxial layer is designed to have a band gap wider than that of an active layer, so as not to absorb emitted light from the active layer.
The fourth epitaxial layer is advantageously formed as an n-type layer on the side of an n-type cladding layer, in consideration of suppression of overall electric resistance of a device, since in a GaAlAs system electron mobility is 10 or more times hole mobility. Thus, an n-type layer has an electric resistance lower than that of a p-type layer when carrier concentration and Al compositional proportion in two layers are identical. When an n-type layer is formed on the side of an n-type cladding layer so as to dispose a p-type cladding layer as an LED surface, two arrangements are possible. In one case, as shown in
FIG. 2
, an n-type GaAs substrate is employed, and on the substrate, a second n-type layer
6
, an n-type cladding layer
5
, a p-type active layer
4
, and a p-type cladding layer
3
are sequentially formed. In the other case, as shown in
FIG. 3
, a p-type GaAs substrate is employed, and on the substrate, a p-type cladding layer
3
, a p-type active layer
4
, an n-type cladding layer
5
, and a second n-type cladding layer
6
are sequentially formed.
During liquid-phase epitaxy, Te is employed as an n-type dopant. The segregation coefficient of Te increases with decreasing temperature. Therefore, when epitaxial growth is initiated from the n-type substrate
1
as shown in
FIG. 2
, carrier concentration at the interface between the active layer
4
and the n-type cladding layer
5
increases as compared with carrier concentration obtained through epitaxial growth initiated from the p-type substrate
1
as shown in FIG.
3
. Thus, in the constitution shown in
FIG. 2
, reverse withstand voltage decreases and non-radiative recombination centers are formed due to deterioration in crystallinity at the interface caused by high-concentration Te, thereby lowering intensity of emitted light from a device fabricated by use of the epitaxial wafer.
When Zn, a typical dopant, is employed as a dopant in the p-type cladding layer
3
during epitaxial growth initiated from the p-type substrate
1
as shown in
FIG. 3
, the active layer
4
must be grown at high temperature, and Zn incorporated in the p-type cladding layer
3
penetrates the active layer
4
, thereby diffusing to the n-type cladding layer
5
. As a result, a pn junction is shifted to the n-type cladding layer
5
, to thereby generate an intermediate layer. The generation of the intermediate layer lowers intensity of emitted light of an LED and elevates forward voltage (VF), to fail to attain required device characteristics.
In order to solve the aforementioned problems, an object of the present invention is to provide an epitaxial wafer for fabricating an infrared LED which has a DDH structure and exhibits high emitted-light intensity, low VF, and excellent device characteristics. Another object is to provide an infrared LED fabricated from the epitaxial wafer.
SUMMARY OF THE INVENTION
The present inventors have carried out earnest studies on reduction of diffusion of dopants from the p-type cladding layer to the n-type cladding layer, and have found that when Ge is employed as a dopant in the p-type cladding layer, dopant diffusion is suppressed and shift of a pn junction is prevented. However, when an electrode is formed on a Ge-doped p-type cladding layer, electric resistance between the electrode and the p-type cladding layer and VF of an LED increase. The inventors have conducted further studies in order to solve the above problem, and found that when a Zn-doped p-type layer is interposed between a p-type cladding layer and the electrode, there is produced an LED which exhibits lowered electric resistance between the electrode and the p-type layer and high emitted-light intensity and has an excellent service life.
The inventors have further carried out studies on optimization of emitted-light intensity and VF in the aforementioned LED, and have found that when the thickness of the n-type cladding layer
5
is controlled to 60-80 &mgr;m, intensity of emitted light of an LED is enhanced, and that when the carrier concentration of each of the n-type cladding layer and the second n-type layer is elevated, VF is lowered but intensity of emitted light is lowered. In addition, in order to attain VF of 2 V or less at 500 mA, which is a standard VF value of an LED of dimensions of 350 &mgr;m×350 &mgr;m, the carrier concentration of each of the n-type cladding layer and the second n-type layer must be controlled to 8×10
17
cm
−3
or more, whereas when the carrier concentration is in excess of 2×10
18
cm
−3
, intensity of emitted light of an LED decreases considerably.
In the LED according to the present invention, the second n-type layer is formed on the n-type cladding layer such that the two layers are brought into mutual contact, and an n-n junction interface is formed between the two layers. When the n-type epitaxial layers including the n-n interface are inverted to the p-type layers, a thyristor effect generates to thereby disadvantageously shut off electric current. It has been found that the following two mechanisms are considered to cause conduction-type inversion from n to p.
In a first mechanism, Zn evaporated from a melt for forming the first p-type layer is incorporated into a melt for forming the n-type cladding layer or into that for forming the second n-type layer, or Ge-containing crust adhering on substrate-holders during growth of the p-cladding layer and the active layer is incorporated into a melt for forming the n-type cladding layer, to thereby elevate Zn concentration and Ge concentration of the n-type cladding layer or the second n-type layer. In a second mechanism, n-type carrier concentration of an initially grown portion of the second n-type layer decreases, since segregation coefficient of T
Tanaka Toshiyuki
Yoshinaga Atsushi
Flynn Nathan
Forde Remmon R.
Showa Denko K.K.
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