Semiconductor laser device

Details Semiconductor laser device Semiconductor laser device

2000-02-10

2004-12-07

Wong, Don (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S043010, C372S044010, C372S046012, C372S049010, C372S049010, C372S049010, C372S050121

Reexamination Certificate

active

06829272

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the invention
This invention relates to a semiconductor laser device and particularly, but not exclusively, to a semiconductor laser device that emits visible radiation in the wavelength range 630 nm to 680 nm. The laser device may be of the edge-emitting or of the surface-emitting type.
Laser devices or laser diodes (LDs) fabricated in the (Al,Ga,In)P material system which emit visible light in the 630 nm-680 nm wavelength range are becoming increasingly important components of professional and consumer products. For example, it is envisaged that the Digital Video Disc (DVD) system will employ a 635 nm-650 nm wavelength LD capable of delivering up to 30 mW output power up to a temperature of 60° C. The next generation of semiconductor lasers will need an even greater maximum power output up to a higher (eg. 70° C.) operating temperature.
By the (Al,Ga,In)P system is meant the family of compounds having the general formula (Al
y
Ga
1−y
)
1−x
In
x
P, where both x and y are between 0 and 1. One particular advantage of this semiconductor system is that it in lattice-matched to a GaAs substrate when the indium mole fraction, x, is equal to 0.48.
For convenience, the compound (Al
y
Ga
1−y
)
1−x
In
x
P with x, y≠0 and x, y≠1 will generally be referred to an AlGaInP in the specification and claims. Similarly, the compound with y=1 will generally be referred to as AlInP, and the compound with y≠0 will generally be referred to as GaInP.
2. Description of the Related Art
A principal limitation of current (Al,Ga,In)P laser diodes is that they are incapable of operating for long periods (or with a sufficiently low threshold current) at the highest specified operating temperature. It is generally believed that this is caused by electron leakage from the active region of the device into the surrounding optical guiding region and subsequently into the p-type cladding region.
The generic structure of a separate confinement laser structure intended to generate light at 630-680 nm will now be described with reference to
FIGS. 1 and 2
.
Curve (a) of
FIG. 1
illustrates the difference between the &Ggr;-conduction band energy of (Al
y
Ga
1−y
)
0.52
In
0.48
P, and Ga
0.52
In
0.48
P, as a function of the aluminum hole fraction in the quaternary alloy. Curves (b) and (a) of
FIG. 1
show the difference between the X-conduction band energy and the &Ggr;-valance band energy respectively.
FIG. 1
assumes that the bandgap difference between AlGaInP and GaInP is split in a ratio of 70:30 between the conduction band offset and the valance band off set—see S. P. Najda et al, “Journal of Applied Physics”, Vol 77, No. 7, page 3412, 1995.
It will be soon that the minimum energy in the conduction band of (Al,Ga,In)P is a function of the aluminum content. There is a crossover from a &Ggr;-band minimum to an X-band minimum at an aluminum concentration of about 0.55.
The terms &Ggr;-band and X-band as used herein refer to symmetry points in the Brillouin zone and are standard terms in solid state physics, see for example R. A. Smith “Semiconductors”, (Cambridge University Press, 1978). The terms &Ggr;-minimum and X-minimum refer to the minimum energy level of the &Ggr;-band and the X-band, respectively.
FIG. 2
is a schematic band structure of a separate confinement laser structure fabricated in the (Al,Ga,In)P system. It consists of an n-doped (Al
0.7
Ga
0.3
)
0.52
In
0.48
P cladding region
1
, an (Al
0.5
Ga
0.5
)
0.52
In
0.48
P optical guiding region
2
,
4
, a GaInP quantum well active region
3
disposed within the (Al
0.5
Ga
0.5
)
0.52
In
0.48
P optical guiding region, and a p-doped (Al
0.7
Ga
0.3
)
0.52
In
0.48
P cladding region. Optical transitions giving rise to laser action in the quantum well active region
3
of the laser diode originate from &Ggr;-electrons in the GaInP quantum well active region.
The electron leakage current consists of that fraction of the electrons which have sufficient thermal energy to surmount the potential barrier on the right hand side of
FIG. 2
, and pass into the p-doped cladding region
5
. It will be seen that &Ggr;-electrons are confined in the optical guiding region (waveguide region) by a potential barrier of only around 90 meV at the interface with the p-doped cladding region. This relatively small barrier height allows a significant proportion of electrons to escape. Moreover, holes in the valence band are confined only by a potential barrier of around 50 meV, and this low barrier height also allows significant carrier escape. Furthermore the X-conduction band in the p-cladding region
5
is some 50 meV below the &Ggr;-conduction band in the waveguiding region
2
,
4
, and this allows electrons to escape from the waveguiding region
2
,
4
through the X-states in the p-doped cladding regions. Thus, the laser illustrated in
FIG. 2
has a high leakage current, and so has poor performance at high temperatures.
In the laser structure shown in
FIG. 2
, the free carrier population of the n-doped cladding region
1
tends to saturate. This saturation of the free carrier concentration arises owing to the Fermi energy being pinned at the free energy level of the DX level in the n-doped cladding region
1
. As is shown in
FIG. 2
, the DX level is lower than the X-band conduction minimum in the n-doped cladding region
1
(by around 100 meV). Saturation of the carrier concentration owing to Fermi pinning at the DX level in AlGaAs has been reported by A. Y. Du et al in “Applied Physics Letters” Vol 66 No. 11, pages 1391-1393 (1995). Saturation of the carrier concentration in the (Al,Ga,In)P material system has been reported by S. P. Najda et al in “Journal of Applied Physics” Vol. 82 No. 9, p. 4408 (1997).
Saturation of the free carrier concentration limits the number of electrons that can be injected into the active region of the laser device owing to carrier trapping at the DX level. This may lead to a charge imbalance, thereby reducing the efficiency of the laser device. Furthermore, saturation of the free carrier concentration in the n-doped cladding region
1
means that the electrical resistivity of the laser device cannot be reduced below a certain level.
This saturation of the carrier concentration is demonstrated in
FIG. 8
, which is taken from S. P. Najda et al (above). This figure shows experimental data on the silicon impurity concentration in (Al
0.7
Ga
0.3
)
0.52
In
0.48
P as a function of the temperature of the silicon cell. This Figure shows both the Hall carrier concentration (which is a measure of the free carrier concentration) and CV data (which is a measure of the fixed impurity concentration). It will be noted that the free carrier concentration (that is, the Hall carrier concentration) saturates at a concentration of n=4.8×10
17
cm
−3
. This saturation is due to pinning of the Fermi level at the DX level.
The potential barrier between the optical guiding region
2
,
4
and the p-doped cladding region
5
in the laser structure of
FIG. 2
is only around 50 mV. Because this potential difference is small, there is a significant leakage of carriers from the optical guiding region into the p-doped cladding region. It is therefore desirable to increase the potential barrier between the optical guiding region
2
,
4
and the p-doped cladding region
5
.
FIG. 3
shown one way in which the potential barrier between the optical guiding region
2
,
4
and the p-doped cladding region
5
can be increased. In the laser structure of
FIG. 3
, the n-doped cladding region
1
and the p-doped cladding region
5
have a greater aluminum composition than the cladding regions
1
,
5
of the laser structure shown in FIG.
2
. The cladding regions
1
,
5
in
FIG. 3
are formed of Al
0.52
In
0.48
P. It will be seen that there is now a potential barrier of around 250 meV to the transport of &Ggr;-electrons from the optical guiding region
2
,
4
into the p-doped cladding region
5
.
The use of an Al
0.52
In
0.48
P cladding region in
FIG. 3
has the disadvant

Affiliated with

Also associated with

Rating

Semiconductor laser device does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Semiconductor laser device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Semiconductor laser device will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3291188