Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-03-30
2002-04-09
Davie, James W. (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
Reexamination Certificate
active
06370177
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a semiconductor laser, more particularly to a semiconductor laser having a high-reflectivity-coating formed on its rear facet and a manufacturing method of the same.
BACKGROUND OF THE INVENTION
For the purpose of increasing output powers from semiconductor lasers and/or improving temperature characteristics thereof, high-reflectivity-coatings with multilayers on the rear facets of semiconductor lasers are broadly used.
A conventional semiconductor laser having a rear facet on which a multi-layer high-reflectivity-coating is made is shown in FIG.
1
. In this semiconductor laser, a first low refractivity film
10
is formed on a front facet
1
or a facet from which a laser beam is emitted, of the semiconductor laser
21
. The film thickness of the first low refractivity film
10
is controlled so that it exhibits a reflectivity of about 30% or less. On the rear facet
2
opposite to the facet
1
, a second low refractivity film
14
and a high refractivity film
17
are alternately formed, resulting in formation of the high reflectivity coating. The thicknesses and the number of the films
14
and
17
are designed so that the whole structure constituted by the films
14
and
17
alternately stacked exhibits a desired reflectivity ranging from 30 to 100%. The thicknesses of the second low refractivity film
14
and the high refractivity film
17
are often set so that their optical thicknesses are &lgr;/4 relative to a wavelength &lgr; of the semiconductor laser. With respect to a third low refractivity film
19
formed on the rear facet
2
side, as the uppermost layer, in order to obtain a desired reflectivity, an optical thickness of the film
19
is set to &lgr;/4, &lgr;/2 or an intermediate value between these values. The optical thickness is a product of its exact film thickness (physical film thickness) and refractivity when the incident angle of laser beam on the film is 0 degree.
In a red semiconductor with an oscillation wavelength ranging from 600 to 700 nm and a infrared semiconductor laser with an oscillation wavelength ranging from 700 to 800 nm, aluminium oxide Al
3
O
2
exhibiting a refractivity ranging 1.6 to 1.7, silicon dioxide SiO
2
exhibiting a refractivity ranging from 1.4 to 1.5, and the like have been used as the materials of the first to third low refractivity film. As the material of the high refractivity film, silicon nitride SiN
x
exhibiting a refractivity ranging from 1.8 to 2.2, amorphous silicon or &agr;-Si exhibiting a refractivity ranging from 3 to 5, titanium oxide TiO
x
exhibiting a refractivity ranging from 1.9 to 2.5, zirconium oxide ZrO
x
exhibiting a refractivity ranging from 1.8 to 2.2, and the like have been used.
As a difference between the refractivities of the low refractivity films and that of the high refractivity films is larger, a reflectivity per a pair of the low and high refractivity films is larger. Accordingly, it is desirable that a refractivity of the low refractivity film is as low as possible and a refractivity of the high refractivity film is as high as possible.
With respect to the low refractivity film, because a refractivity of SiO
2
is lower than that of Al
2
O
3
, SiO
2
is more desirable film material than Al
2
O
3
in respect of the refractivity. However, while a linear expansion coefficient of silicon dioxide is about 1×10
−6
(1/K) or less, single crystal substrates of compound semiconductors such as AlGaAs group, InP group, InGaP/InGaAlP group and GaN group, which are often used for the semiconductor laser material, that is, such as gallium arsenide (GaAs) substrate, phosphorus (InP) substrate, and sapphire (Al
2
O
3
) substrate exhibit a linear expansion coefficient ranging from 4 to 7×10
−6
(1/K). The linear expansion coefficient of SiO
2
and that of these substrates are greatly differ. SiO
2
films was deposited at high temperatures exceeding 100 degrees centigrade and the laser chips with high-reflectivity-coatings were die-bonded on heatsink at a high temperatures exceeding 200 degree centigrade. The great difference between SiO
2
films and a rear facet of the semiconductor laser enhances a large stress and causes problems the laser characteristics or laser reliability. For example, a SiO
2
film is partially peeled off from the rear facet by the stress thereof. Accordingly, characteristics or reliability of the laser are degraded. For this reason, SiO
2
has been seldom selected as the low refractivity film, and Al
2
O
3
exhibiting a linear expansion coefficient 7×10
−6
(1/K) which is closer to that of the compound semiconductors has been often used.
However, since Al
2
O
3
hardly creates a refractivity difference from the high refractivity film
17
, any of countermeasures of increasing the number of the pair of low and high refractivity films and using of higher refractivity material for the high refractivity film must be selected. There are following problems in both countermeasures.
In the case of increasing the number of the layers, the stress between the low/high refractivity film and the compound semiconductor layers grown on the semiconductor substrate, as well as between the low/high refractivity film and the semiconductor substrate, as well as between two of the low/high refractivity films, increases, resulting in the film peeled off, not being able to used as a passivation film, promoting a device degradation because the stress is applied to the rear facet. Thus, it is happened a problem of reliability of the semiconductor laser. It is considered that the number of the layers that creates no problem of the reliability of the compound semiconductor is 10 or less. However, when Al
2
O
3
is selected as the low refractivity film and SiN
x
is selected as the high refractivity film, the films of about 15 layers must be coated to obtain the refractivity of 90% or more, so that a problem of reliability is posed. Moreover, even if films posing no problem of the reliability are selected, a time required for depositing the films is too long, so that a problem of productivity is posed.
Next, as the high refractivity film, a selection of a film exhibiting a higher refractivity, for example, of &agr;-silicon film, is conceived. After Al
2
O
3
having a optical thickness &lgr;/4 is employed as the low refractivity film, &agr;-silicon having a optical thickness-&lgr;/4 is employed as the high refractivity film, a theoretical calculation is performed regardless an optical absorption, so that the reflectivity of about 90% can be realized by the number of the layer equal to 9 or less. In fact, similar layer structures have been broadly used in upper-output-power lasers.
However, since &agr;-silicon exhibits a high optical absorption of the order of 1×10
4
cm
−1
in a 600 nm band of the laser wavelength, the following problems are posed. First, it is difficult to obtain a higher reflectivity because of the optical absorption. When a theoretical estimation is conducted for the case where a design is done so as to obtain a reflectivity of around 90% in a 600 nm band of the wavelength, the reflectivity is lowered by about 10%, compared to the case where the optical absorption is assumed to be zero in the &agr;-silicon film. Although the reflectivity increases for increasing the number of the layers in the pair of the low and high refractivity films, it is difficult for the reflectivity to exceed 90%. Also, since a part of an output power from the laser is absorbed by optical absorption in the &agr;-Si film, the output power is reduced. Accordingly, when it is tried to obtain a desirable light intensity, a threshold current (I
ch
) of the semiconductor laser and an operation current (I
cp
) thereof increase. Therefore, it is always happened that an efficiency improvement of the laser cannot be taken in spite of formation of multi-layered high reflectivity coating for increasing the output power of laser and improving laser characteristics at high temperatures.
It is pointed out that it is capable of
Gen-ei Koichi
Okada Makoto
Davie James W.
Hogan & Hartson L.L.P.
Zahn Jeffrey N
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