Semiconductor light-emitting devices

Details Semiconductor light-emitting devices Semiconductor light-emitting devices

1998-03-05

2001-08-21

Lee, Eddie (Department: 2815)

Active solid-state devices (e.g., transistors, solid-state diode

Incoherent light emitter structure

With particular dopant material

C257S096000, C257S097000, C372S045013, C372S046012

Reexamination Certificate

active

06278137

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to semiconductor light-emitting devices such as semiconductor light-emitting diodes and semiconductor laser diodes, which are obtained using AlGaInP and AlGaAsP semiconductor materials.
One exemplary prior art semiconductor light-emitting diode (LED for short) using an AlGaInP semiconductor material is schematically illustrated in FIG.
1
.
Referring now to
FIG. 1
, reference numeral
201
represents an n-type GaAs substrate, and
202
a clad layer comprising an n-type AlGaInP, which is formed on the substrate
201
. Reference numeral
203
represents an active layer comprising AlGaInP, and
204
a clad layer comprising a p-type AlGaInP. That is, the Al concentration is preset so that the energy gap of the AlGaInP active layer
203
is narrower than those of the AlGaInP clad layers
202
and
204
. A p-type AlGaAs current-spreading layer
205
is provided to expand the electric current injected from electrodes, thereby expanding the light-emitting region and so increasing the efficiency of light extraction. Reference numeral
206
stands for a contact layer, and
207
and
208
Indicate electrodes.
One exemplary prior art semiconductor laser diode (LD for short) in schematically shown in FIG.
2
.
Referring here to
FIG. 2
, reference numeral
201
stands for an n-type GaAs substrate, and
202
a clad layer comprising an n-type AlGaInP, which is formed on the substrate
201
. Reference numeral
203
represents an active layer comprising AlGaInP, and
204
a clad layer comprising a p-type AlGaInP. That is, the Al concentration is preset so that the energy gap of the AlGaInP active layer
203
is narrower than those of the AlGaInP clad layers
202
and
204
; in other words, the double-hetero structure is achieved. Reference numeral
206
indicates a contact layer.
Reference numeral
205
′ represents a current block layer comprising GaAs, which is provided for the purpose of achieving the so-called current confinement, thereby obtaining the current density needed for laser oscillation. This layer
205
′ is formed by selectively etching the layer
204
to form a ridge thereon, followed by selective growth using an amorphous film such as one of SiN
x
.
AlGaInP or AlInP (which may hereinafter be collectively called an AlGaInP compound) have some defects; they are higher in resistivity and thermal resistance than AlGaAsP or AlGaAs (which may hereinafter be collectively called an AlGaAsP compound). Such detects lead to a problem that the operating voltage of the device becomes too high, resulting in an increase in the amount of the heat generated. This problem must be solved so as to improve the characteristics and reliability of the device, and becomes serious especially when the eight-omitting density becomes high.
The above-mentioned problem becomes by far more serious the case of a semiconductor laser where current confinement is required with an increase in the light-emitting density.
Zinc (Zn) is generally used as a dopant for a clad layer comprising a p-type AlGaInP compound, but doping should be done at high concentration so as to lower resistivity, because Zn is low in the rate of activation. In this case, however, unactivated Zn diffuses so rapidly through the AlGaInP compound crystal body that the pn junction position is often largely shifted from the light-emitting layer toward the n-type layer. This, in turn, gives rise to abnormality in the current-voltage characteristics, a drop of light output power, and an increase in the threshold current of laser. Such diffusion of Zn from the p-type AlGaInP compound layer becomes noticeable as its thickness increases. When the light-emitting layer is relatively thin as in the case of laser, the current-voltage characteristics are liable to disorder due to the diffusion of Zn. Silicon (si) having a low diffusion coefficient is effective for an n-type AlGaInP compound.
In some cases, a light-extracting layer is located next to the clad layer. The transparency of the light-extracting layer to the light-emitting wavelength is important for improving the light-extraction efficiency of LED. However, it is probable that some light-extracting layer with high transparency has high resistivity depending on composition and that the surface current spreading area in small, as a result.
With respect to the double-hetero structure, it is usually required to impart a thickness of about 1 &mgr;m to about 2 &mgr;m to the clad layer to ensure confinement of carriers and light in the active layer. When the AlGaInP compound is grown by a metal organic vapor phase growth technique, it is required that the feed molar ratio (V/III) between the organic metal that is the group III material and PH
3
that is the group V material be very high. This incurs some inconveniences; for instance, no high growth rate can be applied, the cost of the material to be grown is much higher than the AlGaAs compound, and so on. Especially in the case of mass-production equipment of large size enabling a multiplicity of devices to be produced at the same time, problems with removal of defects become more serious.
SUMMARY OF THE INVENTION
An object of the present invention is to optimize the structure of clad layers of a semiconductor light-emitting device including an active layer comprising AlGaInP or GaInP.
Another object of the present invention is to reduce the thickness of a Zn-doped clad layer comprising a p-type AlGaInP compound.
A further object of the present invention is to substantially eliminate or reduce the diffusion of zn into an active layer and an n-type clad layer.
A still further object of the present invention is to prevent a shift of the pn junction position due to the diffusion of Zn.
A still further object of the present invention is to improve the characteristics and reliability of a light-emitting device.
A still further object of the present invention is to expedite mass production, reduce growth time, cut down material cost, and remove defects.
According to the present invention, it has now been found that in a semiconductor light-emitting device including an active layer comprising AlGaInP or GaInP, the diffusion of Zn into the active layer or an n-type clad layer can be reduced, if the thickness of a Zn-doped clad layer comprising a p-type AlGaInP compound is reduced to such an extent that the characteristics of the light-emitting device does not degrade. However, when the thicknesses of the AlGaInP compound clad layers with the active layers being interleaved between them are reduced, the confinement of carriers and light becomes insufficient, resulting in degradation of the device characteristics. According to the present invention, it has been found that this can be made up for by use of an AlGaAsP compound having a substantially identical band gap and refractive index. Furthermore, it has been found that since the n-type AlGaAsP compound is less susceptible to Zn diffusion than the AlGaInP compound having a substantially equal carrier concentration, the reduction in the thickness of the n-type AlGaInP compound clad layer is also effective for preventing a shift of the pn junction position due to Zn diffusion; the reduction in the thickness of the AlGaInP compound clad layer is further effective for reducing operating voltage and thermal resistance; it is advantageous in view of mass productivity, growth time, material cost, and defect removal that the thickness of the AlGaInP compound layer is reduced as much as possible and the amount of the AlGaAsP compound used is increased, correspondingly.
More specifically, the present invention provides a semiconductor light-emitting device characterized by including a first clad layer comprising a first conductive type AlGaAsP or AlGaAs, a second clad layer that is located next to the first clad layer, comprises a first conductive type AlGaInP or AlInP and has a thickness of up to 0.5 &mgr;m, an active layer that is located next to the second clad layer and comprises a first or second conductive type AlGaInP or GaInP, a third cla

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