Semiconductor laser device and process for producing the same

Details Semiconductor laser device and process for producing the same Semiconductor laser device and process for producing the same

2002-06-26

2004-03-16

Ip, Paul (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S045013

Reexamination Certificate

active

06707834

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention generally relates to a III-V compound semiconductor laser device. Particularly, it relates to the structure of an AlGaInP semiconductor laser device, which can be operated at a low voltage, and a process for producing the same.
The AlGaInP semiconductor material having a lattice constant almost equal to a lattice constant of a GaAs substrate can achieve crystal growth with high quality. Further, because the AlGaInP semiconductor material is a direct transition-type semiconductor having the largest bandgap among the III-V compound semiconductor materials other than nitrides, it has been developed as a light-emitting material for light in the visible range. In particular, AlGaInP semiconductor laser devices have been widely used as light sources for optical disks, because they have shorter oscillation wavelengths compared with AlGaAs semiconductor laser devices, and enable a high-density recording.
The bandgap of the (Al
x
Ga
1-x
)
y
In
1-y
P (0≦x≦1, y=0.5) material can be varied between 1.91 eV (GaInP) and 2.35 eV (AlInP) by changing the mixed crystal ratio x of Al from 0 to 1. Incidentally, the bandgap of GaAs is about 1.42 eV, and there is a big difference in the bandgap between a GaAs material and an AlGaInP material. When an AlGaInP layer is grown on a GaAs layer, band discontinuity due to a big difference in the bandgap occurs at a hetero-interface between the two layers. In particular, large band discontinuity occurs in a valence band and it acts as a barrier against injected holes resulting in an increase in the operation voltage of the laser element.
It is known from JP-A-5-7049, for example, that the above problem can be solved by providing, between a GaAs layer and an AlGaInP layer, a layer having a bandgap intermediate between the two layers.
FIG. 9
is a view seen from an end surface of a semiconductor laser device taking such a countermeasure. Referring to
FIG. 9
, an n-type GaAs buffer layer
102
, an n-type GaInP intermediate layer
103
, an n-type AlInP cladding layer
104
, a GaInP active layer
105
, a p-type AlInP cladding layer
106
, a GaInP etch stop layer
107
, a p-type AlInP second cladding layer
108
, a p-type GaInP intermediate layer
109
, and a p-type GaAs contact layer
110
are formed in order on an n-type GaAs substrate
101
using an MBE method. Then the p-type GaAs contact layer
110
, the p-type GaInP intermediate layer
109
and the p-type AlInP second cladding layer
108
are removed by etching, excluding a stripe-geometry ridge portion
120
. Subsequently, an n-type GaAs current block layer
111
is formed at portions other than the stripe-geometry ridge portion
120
, thereby obtaining a crystals-stacked structure called a wafer. After that, an n-type electrode
112
, and a p-type electrode
113
are deposited, and the wafer is divided into bar-shaped pieces. A protective film is formed on each end surface of the resulting bars, thereafter the bars are divided into chips serving as semiconductor laser devices.
The p-type AlGaInP second cladding layer
108
is doped with beryllium (Be) as an impurity to a density of 4×10
17
cm
−3
. Similarly, the p-type GaInP intermediate layer
109
is doped with Be to a density of 1×10
19
cm
−3
as an impurity, and the p-type GaAs contact layer
110
is doped with Be to a density of 5×10
18
cm
−3
as an impurity.
In the above structure, between the p-type AlInP second cladding layer
108
having a large bandgap and the p-type GaAs contact layer
110
having a small bandgap, the p-type GaInP intermediate layer
109
, which has a bandgap intermediate between the above two layers, is provided, whereby band discontinuity at the interface is reduced. In addition to that, as the density of the p-type GaInP intermediate layer
109
having an intermediate bandgap increases, the band discontinuity is reduced.
SUMMARY OF THE INVENTION
The inventors have determined, with respect to the above prior art example, that if the impurity density exceeds a certain level (for example, 7×10
19
cm
−3
for GaInP crystals), impurity atoms do not enter appropriate lattice sites and become lattice defects such as interstitial atoms, which bring about deterioration of the quality of crystals. Therefore, the impurity density that can be doped for improving the band discontinuity has an upper limit.
Further, in the prior-art example, the p-type GaInP intermediate layer
109
is doped with Be to a density of 1×10
19
cm
−3
, which is in the range that would not deteriorate the quality of crystals. Yet, of laser elements obtained according to the prior art, some laser elements had a high operation voltage. However, the inventors have determined that operation voltage of higher than 2.3 V does not allow practical reliability to be attained.
As a reason for this, it is presumed as follows: when the p-type GaInP intermediate layer
109
is formed and the p-type GaAs contact layer
110
is formed thereon, or when the n-type GaAs current block layer
111
is grown after forming the stripe-geometry ridge region
120
, the wafer having the p-type GaInP intermediate layer
109
doped with impurities to a high density and the p-type GaAs contact layer
110
is retained at a high temperature, and thus Be atoms are diffused from the p-type GaInP intermediate layer
109
to the p-type GaAs contact layer
110
. Such diffusion of impurities, which is sensitive to the temperature, and which strongly depends on the in-plane temperature distribution of the wafer, is an unstable phenomenon. For that reason, when operating the semiconductor laser device, the Be impurity density of the p-type GaInP intermediate layer
109
has already been reduced or varied and therefore the band discontinuity at the interface is not reduced sufficiently. As a result, the resistance of the laser element increases and heat generation of the laser element increases.
Under the circumstances, it has been desired to realize semiconductor laser devices that maintain the doping density of a layer having an intermediate bandgap even after a wafer was retained in a high-temperature state as in the crystal growth, that operate at a low voltage, and that have an operation voltage distribution in a narrow range.
The present invention was made in order to solve the above problem, and an object of the present invention is to provide a ridge stripe-type AlGaInP semiconductor laser device in which diffusion of impurities from a contact layer or an intermediate bandgap layer is suppressed so that the impurity density of the intermediate bandgap layer is maintained high enough and that the semiconductor laser device has a low operation voltage.
According to an aspect of the present invention, there is provided a semiconductor laser device comprising:
an active layer;
a first-conductivity type cladding layer and a second-conductivity type cladding layer sandwiching the active layer therebetween;
a second-conductivity type contact layer disposed above the second-conductivity type cladding layer and having a bandgap different from a bandgap of the second-conductivity type cladding layer; and
a second-conductivity type intermediate bandgap layer disposed between the second-conductivity type cladding layer and the second-conductivity type contact layer and having an intermediate bandgap between the bandgaps of the second-conductivity type cladding layer and the second-conductivity type contact layer,
wherein said second-conductivity type contact layer comprises at least a first contact layer, an intermediate second contact layer and a third contact layer stacked in this order and the second contact layer has an impurity density lower than impurity densities of the first and third contact layers.
This arrangement suppresses diffusion of impurities, so that the intermediate bandgap layer can exhibit the effect of reducing band discontinuity sufficiently. Also, This arrangement allows impurities to be prevented from being diffused to the active layer, whereby an increa

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