Semiconductor laser device and fabricating method thereof

Details Semiconductor laser device and fabricating method thereof Semiconductor laser device and fabricating method thereof

2000-08-30

2003-09-09

Leung, Quyen (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S045013

Reexamination Certificate

active

06618415

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to III-V compound semiconductor devices represented by semiconductor laser devices used as optical information system light sources for CD, MD and DVD players or computer information storage devices, and to a manufacturing method thereof. The present invention relates, in particular, to a structure for achieving a low threshold current operation, to a semiconductor device excellent in device characteristics, yield and reliability with improved controllability of impurities included in semiconductor layers, and to a manufacturing method thereof.
In recent years, there has been a growing demand for semiconductor laser devices that are compound semiconductor devices to be used for the pickups of CD and MD. Semiconductor laser devices that have little characteristic variations and excellent reliability have been demanded. Also, it is anticipated that the demand for semiconductor laser devices will be still more increasing in future for the production of the computer information storage devices such as CD-ROM, CD-R, CD-RW, and digital video discs (DVD).
When producing a III-V compound semiconductor device represented by such a semiconductor laser device, a stacked structure of a plurality of semiconductor layers is formed on a semiconductor substrate. By adding a specified impurity to each semiconductor layer, the electric conduction type or the electric conductivity of each layer is controlled to consequently obtain a device of specified semiconductor characteristics. To achieve uniform device characteristics of the semiconductor lasers and improvement in yield of products, it is very important to control the electric conduction type or the electric conductivity of each layer of the semiconductor device to be in conformity with designed values.
As a method of forming III-V compound semiconductor thin films in a stacked manner, the MOCVD (metal-organic chemical vapor deposition) method and the MBE (molecular beam epitaxy) method can be mentioned. When growing a film by using any of these methods, a group IV element such as silicon (Si) and a group VI element such as selenium (Se) are used as impurities for obtaining an n-type electric conduction type layer. The group IV element becomes a donor impurity by replacing a group III element of aluminum (Al), gallium (Ga), or indium (In). The group VI element becomes a donor impurity by replacing a group V element of arsenic (As) or phosphorus (P). On the other hand, as an impurity for obtaining a p-type electric conduction layer, a group II element such as zinc (Zn), beryllium (Be), or magnesium (Mg) is employed. The group II element becomes an acceptor impurity by replacing a group III element of Al or Ga.
Among semiconductor laser device structures, what we call a self-alignment structure and what we call a ridged structure are well known.
FIGS. 4A
,
4
B and
4
C show an example of a semiconductor laser device of the self-alignment structure. The fabricating process of this semiconductor laser device will be described below.
In the first process step shown in
FIG. 4A
, first, an n-type GaAs buffer layer 12 (layer thickness: 0.5 &mgr;m), an n-type Al
x
Ga
1−x
As first cladding layer 13 (x=0.5, layer thickness: 1.0 &mgr;m), a non-doped Al
x
Ga
1−x
As active layer 14 (x=0.14, layer thickness: 0.085 &mgr;m), a p-type Al
x
Ga
1−x
As second cladding layer 15 (x=0.5, layer thickness: 0.35 &mgr;m) and an n-type GaAs current block layer
16
(layer thickness: 0.6 &mgr;m) are successively grown on an n-type GaAs substrate 10 by the MOCVD method. In this stage, Se is employed as the n-type impurity, while Zn is employed as the p-type impurity. Next, in the second process step shown in
FIG. 4B
, an etching mask
40
is formed by a method such as photolithography. Thereafter, the n-type GaAs current block layer
16
is removed in a stripe-like and groove-like shape with a width of 3.5 to 4.0 &mgr;m, forming a removed portion
20
.
Subsequently, in the third process step shown in
FIG. 4C
, a p-type Al
x
Ga
1−x
As third cladding layer
17
(x=0.5, layer thickness: 1.0 &mgr;m) and a p-type GaAs cap layer
18
(layer thickness: 3 to 50 &mgr;m) are grown on the n-type GaAs current block layer
16
including the removed portion
20
by the MOCVD method or the LPE method. In this case, the layer thickness of the p-type GaAs cap layer
18
should be determined as the occasion demands depending on where the final light emitting point of the semiconductor laser device is to be positioned relative to the chip thickness. Zn or Mg is employed then as the p-type impurity. By the aforementioned fabricating method, the semiconductor laser device of the self-alignment structure is obtained.
The molar ratio of the group V element to the group III element (V/III ratio) when forming a laminate by the MOCVD method in the first process step has conventionally been set to 20 to 150 at a growth temperature of 600° C. to 800° C. If the ratio is set to a value of 20 or lower, then there occurs a phenomenon of roughened growth surface. On the other hand, it has been reported that if the growth temperature is set to 450° C. to 600° C., then no roughness occurs on the crystal surface even when the V/III molar ratio is reduced to 0.3 to 2.5, and that the intake of carbon C to the grown thin film is increased so that a p-type hole density of 1×10
18
cm
−3
to 1×10
20
cm
−3
by the carbon C of GaAs and AlGaAs is obtained (JP-B2-2885435).
In a practically used semiconductor laser device of the structure shown in
FIG. 4C
, in the first process step for forming at least the n-type first cladding layer
13
, the active layer
14
, the second cladding layer
15
and the n-type current block layer
16
on the n-type GaAs substrate, the n-type first cladding layer
13
and the n-type current block layer
16
are doped with an impurity of Se, and the p-type second cladding layer
15
is doped with an impurity of Zn However, in the structure after the completion of the first process step, the impurity elements move or migrate between the layers by diffusion or the interaction of the impurity atoms during the fabricating process, which results in an impurity profile different from a designed impurity profile.
FIG. 3A
shows the designed impurity concentration profile, in which, of course, the n-type first cladding layer
13
and the n-type current block layer
16
are designed to be doped with the n-type impurity of Se, and the p-type second cladding layer
15
is designed to be doped with the p-type impurity of Zn, each with a steep doping slope.
FIG. 3B
shows an actual impurity concentration profile. As obvious from this figure, the impurity of Zn in the p-type second cladding layer
15
diffuses into the layers other than the p-type second cladding layer
15
during the growth of the n-type current block layer
16
in the first process step, as a consequence of which the doping control of the p-type second cladding layer
15
becomes unstable.
Further, in the third process step after the formation of the stripe removed portion
20
in the n-type current block layer
16
in the second process step, due to a thermal history during the process for growing the p-type third cladding layer
17
end the p-type GaAs cap layer
18
at the removed portion of the current block layer
16
and the non-removed portion of the current block layer, the impurity of Zn in the p-type second cladding layer
15
increasingly diffuses into the other layers and, in certain circumstances, the impurity of Se of the n-type first cladding layer
13
and the n-type current block layer
16
diffuses into the p-type second cladding layer
15
. The diffusion of n-type impurity surpasses the concentration of the p-type impurity of Zn of the p-AlGaAs cladding layer
15
, consequently causing the inversion of the p-type second cladding layer
15
into the n-type. This inversion into the n-type, which occurs either on the entire surface of the p-type second cladding layer
15
o

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