Semiconductor device

Details Semiconductor device Semiconductor device

2000-08-04

2002-11-26

Crane, Sara (Department: 2811)

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

Thin active physical layer which is

Heterojunction

C257S096000, C372S045013

Reexamination Certificate

active

06486491

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a semiconductor device and particularly, but not exclusively, to a semiconductor device that emits visible radiation in the wavelength range 630 nm to 680 nm, such as a semiconductor laser device or light-emitting diode. The device may be of the edge-emitting or of the surface-emitting type.
2. Description of the Related Art
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
x
Ga
l−x
)
l−y
In
y
P, where both x and y are between 0 and 1. One particular advantage of this semiconductor system is that it is lattice-matched to a GaAs substrate when the indium mole fraction, y, is equal to 0.48.
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 in 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
(
a
),
1
(
b
) and
1
(
c
).
FIG.
1
(
a
) 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
5
. A p-type contact layer (not shown in FIG.
1
(
a
)) may be provided on the p-type cladding region
5
, and an n-type contact layer (not shown) may be provided on the n-type cladding region
1
. Optical transitions giving rise to laser action in the quantum well active region
3
of the laser diode originate from electrons in the &Ggr;-band in the GaInP quantum well active region.
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.
The minimum energy in the conduction band of (Al,Ga,In)P is a function of the aluminium content. There is a crossover from a &Ggr;-band minimum to an X-band minimum at an aluminium concentration of about 0.55.
The aluminium mole fraction of the cladding regions
1
,
5
need not be 0.7, provided that it is sufficient to provide an effective potential barrier to confine &Ggr;-electrons in the optical guiding region. FIG.
1
(
b
) illustrates a similar laser structure to that of FIG.
1
(
a
), but the cladding regions
1
,
5
are formed of AlInP rather than (Al
0.7
Ga
0.3
)
0.52
In
0.48
P in order to increase the potential barrier confining &Ggr;-electrons in the optical guiding region
2
,
4
.
In the laser structures shown in FIGS.
1
(
a
) and
1
(
b
) the active region
3
is a single GaInP quantum well layer. It is alternatively possible for the active region to contain two or more quantum well layers separated by barrier layers.
FIG.
1
(c) shows the &Ggr;-conduction band and valence band of a (Al,Ga,In)P laser device in which the active region comprises a plurality of quantum well layers. In the embodiment shown in FIG.
1
(
c
), the active region
3
comprises three GaInP quantum well layers
3
a
with each neighbouring pair of quantum well layers being separated by a barrier layer
3
b.
The barrier layers
3
b
are formed of a material having a higher &Ggr;-band than the material used to form the quantum well layers
3
a,
such as (Al
0.3
Ga
0.7
)
0.52
In
0.48
P or (Al
0.5
Ga
0.5
)
0.52
In
0.48
P. In the laser shown in FIG.
1
(
c
) the barrier layers
3
b
are formed of (Al
0.5
Ga
0.5
)
0.52
In
0.48
P, the same material as used for the optical guiding region
2
,
4
.
Degradation of semiconductor lasers has been a major problem in developing commercial devices. AlGaAs/GaAs lasers having a lasing wavelength of around 0.85 &mgr;m were initially developed in the 1970s, but early examples of these lasers degraded quickly during use and, as a result, had a low lifetime and were unsuitable for commercial applications. It took a considerable time to overcome the significant degradation problems involved with these lasers, but long lifetime AlGaAs/GaAs lasers are now commercially available. M. Fukuda reports, in “Reliability and Degradation of Semiconductor Lasers and LEDEs” ISBN 0-89006-465-2, that AlGaAs/GaAs lasers having a life time greater than 10,000 hours are now commercially available.
In order for (Al,Ga,In)P lasers to be commercially successful, these lasers must have a life time comparable with that of AlGaAs/GaAs lasers.
At present, wide band-gap phosphide lasers operating in the visible spectrum at a wavelength of about 650 nm display a severe degradation problem. Although the lifetime of low power phosphide lasers is approximately 10,000 hours which is satisfactory for commercial purposes, a typical lifetime of a high-power phosphide laser is only about 5,000 hours which is not commercially acceptable. Furthermore, it is necessary to anneal the lasers in order to obtain these lifetimes, and lasers that are not annealed have much shorter lifetimes.
The degradation problem is particularly serious for lasers fabricated using molecular beam epitaxy (MBE). At present, phosphide laser structures that are grown by MBE have to be thermally annealed in order to improve their reliability and to decrease the threshold for laser operation. It Is presumed that the annealing process removes (or at least moves) some of the non-radiative recombination centres in the material. It is, however, undesirable to carry out an annealing step. One common p-type dopant for the p-type cladding region is beryllium, and if a beryllium-doped laser device is annealed beryllium can diffuse from the p-type cladding region into the active region. Such diffusion will degrade the performance of the laser device, and may also lower the yield of the manufacturing process.
One possible reason for the degradation of phosphide lasers is that the degradation is due to oxygen contamination of the active region. Oxygen contamination of aluminium-containing phosphide materials can easily occur, owing to the high reactivity of aluminium with oxygen. Oxygen introduces non-radiative defects into the active layer, and this may give rise to a high threshold for laser oscillation.
Oxygen is a common contaminant in aluminium-containing alloys, because of the high reactivity of aluminium with oxygen-containing species. In the growth of AlGaAs/GaAs semiconductor structures, it is known that oxygen atoms are sufficiently mobile during the epitaxial growth process for them to migrate to the interface and to become trapped at the interface. Oxygen forms non-radiative centres in GaAs and in AlGaAs, and so the presence of oxygen tends to reduce the performance and reliability of AlGaAs/GaAs lasers. N. Chand et al. report, in “J. Vac. Sci Technol. B” Vol. 10 (2), p807, 1992, that an AlGaAs layer having an aluminium mole fraction of 0.3 or greater has a typical oxygen atom density of at least 10
17
cm
−2
. In con

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