JPS5943839B2 - semiconductor laser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a buried semiconductor laser operating in a leakage mode.

In a semiconductor laser with a conventional structure, even if the beam divergence angle, that is, the half width of the peak value of laser light emission is small,
As mentioned above, most of them have a large value of 50 or more, and when the injection current is increased, the emission direction of the laser beam changes or the oscillation mode becomes unstable, which is disadvantageous when coupling a semiconductor laser and an optical fiber, for example. have.

The present invention solves the above-mentioned difficulties with conventional semiconductor lasers,
It is an object of the present invention to provide an embedded semiconductor laser which is capable of emitting a laser beam with a small beam divergence angle and extremely excellent parallelism, and which does not deteriorate due to end face damage.

In order to achieve the above object, the semiconductor laser of the present invention has an extremely thin active layer sandwiched between semiconductor layers having a smaller refractive index than the active layer, and has a forbidden band width larger than that of the active layer. The semiconductor layer is buried with a semiconductor having a higher refractive index than the sandwiched semiconductor layer, and the effective refractive index of the active layer is smaller than the refractive index of the buried semiconductor layer, and the thickness of the active layer and the refraction of each semiconductor layer are The relationship between the rates is a specific relationship.

Hereinafter, the semiconductor laser according to the present invention will be described in detail based on examples thereof.

FIG. 1 is a sectional view showing the structure of an embodiment of a semiconductor laser according to the present invention.

In the figure, a first semiconductor layer 1, which becomes an active layer, is sandwiched between second semiconductor layers 2 and 2', which have a smaller refractive index than the active layer and different conductivities from each other. The third semiconductor layer 3 is also disposed adjacent to the third semiconductor layer 3.

The third semiconductor layer 3 is selected such that its refractive index N3 is larger than the refractive index N2 and n of the second semiconductor layers 2 and 2', and its forbidden band width Eg3 is the forbidden band width Egl of the active layer. In the embodiment shown in FIG. 1, when the thickness d of the first semiconductor layer 1, which is the active layer, becomes thinner, light penetrates into the second semiconductor layers 2 and 2'. broth,
The effective refractive index Nle of the active layer, which provides a light propagation constant in the active layer, is sufficiently smaller than the refractive index n1 determined by the substance.

In this case, the thickness of the active layer is usually determined by the wavelength λ of the laser light generated in the active layer by current injection.

Set as follows: When the thickness of the active layer becomes thinner and the effective refractive index Nle of the first semiconductor layer 1, which becomes the active layer, becomes smaller than the refractive index N3 of the third semiconductor layer 3, light propagates. It leaks into the third semiconductor layer 3 at an angle of θ with the direction.

The leakage angle θ in this case is given by the following equation.

Here, the effective refractive index N of the first semiconductor layer 1 serving as the active layer
le, is calculated using Maxwell's equation for the waveguide model. This general approximation method can be used, for example, in 6 Fundamentals of Optoelectronics 1 Amn0n
Author, published by Maruzen (1974), etc. Further, since the forbidden band width of the third semiconductor layer 3 is larger than that of the active layer, the leaked light does not undergo absorption loss, and the device of the embodiment shown in FIG. The laser oscillates easily.

In this way, the semiconductor laser according to the present invention has d×λ.

, N2,n times N3 times n1, and Egl times E,3, and the effective refractive index Nle of the active layer is set to be smaller than the refractive index N3 of the third semiconductor layer. . Here, d is the thickness of the active layer, λn is the wavelength of light generated in the active layer, Nl, n2, n'2 and N3 are the refractive indices of the active layer, semiconductor layers 2, 2' and 3, Egl and E and 3 are the forbidden band widths of the active layer and the semiconductor layer 3, respectively.

Incidentally, when the refractive index of the second semiconductor sandwiching and holding the first semiconductor layer (active layer) is asymmetrical, leakage state oscillation is naturally possible as well. Regarding these relationships,
Af? A more specific explanation will be given by taking the case of a XiGal-XiAs semiconductor laser as an example.

In this case, the first, second, and third semiconductor layers are all AlxiGa, -XiAs layers, and the mixed crystal ratio thereof is controlled.

At this time, if Xl, X2, X'2 and X3 are the Al mixed crystal ratios of the active layer and semiconductor layers 2, 2' and 3, respectively, then the above relationship is X1<X3 x X2,X? Big d〈λ.

It can be replaced with the following relationship. FIG. 4 shows the relationship between the refractive index of each semiconductor layer and the thickness of the active layer.

In the figure, the vertical axis represents n1-N2, the horizontal axis represents n1-N3, and N
A relational curve le,=N3 is illustrated using d as a parameter.

In the region 1 on the right side of the straight line d-0, N2>N3 holds true, so there is no leakage oscillation state. Also, d
In the region to the left of =λ (where λ is the oscillation wavelength of the semiconductor laser), since d is too thick, the threshold current increases and oscillation in a leakage state is not substantially maintained.

In this case, the wavelength of light within the active layer is λ.

, the value of d is λ. A smaller value is advantageous in terms of threshold current. Therefore, when setting the thickness d for a certain active layer, it is relative to d set in FIG.

1eq=.

In the area to the left of the relationship curve 3, N2 × N3 is established and leakage oscillation is maintained. At this time, the set N
From the relationship between le, and N3, the leakage angle θ is given by equation (1).

The present invention focuses on such a leakage state oscillation laser in a particularly advantageous region in terms of threshold current, that is, in the region where d=in FIG. 4 above.
0 and d-λ.

The thickness d of the active layer is set in the area sandwiched between the curves, and the thickness d of the active layer is set in the area to the left of the curve Nle,=N3 for this d.
, n2, and n3. Currently, the thickness of the active layer is 0.02 to 0.03.
μm is the limit of use, practically 0.05 to 0.2 μm
A range of m is used. Further, the thickness of the second semiconductor layer may be 1 as long as it can form a normal double heterostructure.
A material with a diameter of approximately 5 μm to 5 μm is used. The width of the active layer is also generally within the range of about 1 μm to 201 tm. FIG. 2 shows an example of an embedded laser device manufactured based on the present invention.

An n-type AZyGal-,As
layer (e.g. y-0.4, Sn-doped, NA-ND: 5×
1017 cm-3) of 2 μm1 as the first semiconductor layer 1 which becomes the active layer and undoped Al? A p-type AZyGal-, As layer (e.g. Ge-doped,
NA-ND: 5×10 18 (1771-3) was sequentially grown to 2 μm by a well-known liquid phase epitaxial method. 3000 to 4000 people deposited a SlO2 film on the growth surface of the wafer on which epitaxial growth had been performed by chemical vapor deposition.
Stripes for forming a band-shaped cavity required for laser oscillation in the <110> direction are formed by photoetching. After that, the grown layer other than the area that will become the striped operating area is selectively treated with a solution containing phosphoric acid, hydrogen peroxide, and methanol mixed at a 1:1:3 ratio of 1:1:3, respectively, kept at 20°C. In this example, the active layer width is 4 μm. Next, the SiO2 film is removed using an ammonium chloride monohydric acid solution. A Gal-2AS layer (for example, z = 0.25), which will become the third semiconductor layer 3, is grown again on the wafer subjected to the above processing by liquid phase epitaxial method, and the layer formed in the previous steps is grown. A mesa-shaped semiconductor layer is buried. A film with a thickness of 12 mm was deposited on the surface of the epitaxial wafer with the buried structure obtained by the above method using the chemical vapor deposition method.
00 Al? After depositing the 203 film 5 and the 2000λ silicate glass film 6, the Af? 203 membrane 5
Then, the silicate glass film 6 is removed by photo-etching to form an electrode hole. Thereafter, Au-Ge-Ni alloy was used as the ohmic electrode 7 of the n-type GaAs substrate 4, and p-type AZy
Cr--Au is deposited as the ohmic electrode 8 of the Gal-, As layer (second semiconductor layer 2) by vacuum evaporation. The wafer subjected to the above processing is striped in the 110〉 direction, that is, in the stripe direction, and the arms are opened in the 110〉 direction to form a strip with a width of 5 mm.
00μm (<110> direction), length 300P (<110>
> direction). An example of a far-field image of an embedded laser element created using the above method is shown in the third example.
As shown in the figure.

This figure represents the intensity distribution of laser light in a plane parallel to the laser end face in a direction parallel to the active layer. The oscillation wavelength of this device is 8100 mA, and the threshold current is 230 mA.
The laser beam is emitted in two directions of approximately ±2C. The angle at which the intensity of each laser beam becomes % is very narrow, less than 1°, and has good parallelism. As seen in the figure, even if the injection current is increased, the laser emission direction does not change, and the half-value width of each laser beam also does not change. In a semiconductor laser having a conventional structure, the half-width of laser light is 2° or more, and when the injection current is increased, the radiation direction of the laser light often changes or the oscillation mode becomes unstable. These points are disadvantageous when considering coupling with optical fibers, for example. The leaky buried type semiconductor laser of the present invention solves these problems as described above. Also, Egl ~ 1.55eV, E, 3 ~ 1.76e
At V, the laser light undergoes virtually no absorption loss. In this example, n1=3.58, n2, n3.35,
n3=3.49, Nle,=3.47, and the leakage angle θ
is approximately 6°.

FIG. 5 shows an embodiment of a semiconductor laser according to the present invention, in which ΔN
le,=Nleq-N3 and the leakage angle θ with respect to the buried material. The horizontal axis is the mixed crystal ratio Z1 of the third semiconductor layer, and the vertical axis is the Δ river Eq (solid line ),
and θ (dotted line), each relationship is shown using the active layer thickness d as a parameter.

Actually, in order to keep the spread angle of the laser beam to 1° or less, it is necessary to set the leakage angle θ to 3° or more, preferably 5° or more.

The upper limit of the leakage angle θ depends on the dimensions of the semiconductor laser.
In the Gal-XiAS semiconductor laser, factors for the oscillation state shown in FIG. 4 are selected as follows.

First of all, in IGal-XiAS-based materials, X1
The mixed crystal ratio of = 0,
Since it becomes difficult to grow the second semiconductor layer 2 when X2=0.7, the value of n1-N2 of 0.476 at X1=0 and X2=0.7 is the maximum value that can be practically achieved. .

On the other hand, for example, if X1-0, X2=0.25, Nln2
When the value of is less than 0.164, the threshold current becomes too low to be practical.

Generally, a refractive index difference of approximately this magnitude or more is required to maintain oscillation. Therefore, Al? In the XiGal-XiAS semiconductor laser, the range of the mixed crystal ratio is 0 and X1〈X3〈
X2<0.7. From these conditions, if the thickness d of the active layer and the refractive index difference of each semiconductor layer are set within the range surrounded by points A, B, C, and D in FIG. 4, a good result can be obtained. It is possible to realize oscillation in a leakage state. In the present invention, in particular, the thickness of the active layer is set within the range surrounded by the points A, B, F, and E where continuous oscillation at room temperature is obtained with a low threshold current. The difference in the refractive index of each semiconductor layer is made to be suppressed.

Actual setting examples based on the above setting conditions and the characteristics obtained are shown in Table 1. The semiconductor laser according to the present invention and a normal semiconductor laser were heated to approximately 1,000 kW with an optical output of 1 mW per 1 μm of active layer width. When the device was operated continuously for a period of time and the changes in optical output under a constant current were compared, the following results were obtained.

In a semiconductor laser with a normal structure, the optical output is about 60% of the initial value.
%, but almost no change was observed in the leaky embedded element. When the element end face was observed using a zero-interference differential microscope, considerable end face damage was observed in the active layer with a normal laser; This was not observed in the leak-type element, but it was found that in the leak-type element, there was no deterioration due to end face damage, and the reliability was significantly improved. FIG. 6 shows a comparison of typical examples of changes over time.

In the figure, the horizontal axis T represents the operating time in time display, and the vertical axis 1 represents the optical output expressed in relative values.Curve 9 represents the semiconductor laser of the present invention, and curve 10 represents the well-known mesa stripe type semiconductor laser. This is an example. As described above, the semiconductor laser of the present invention has excellent characteristics in terms of reliability.The semiconductor lasers of the samples shown in Table 1 are as follows:
Using n-GaAs as the substrate, an undoped AlxlGa, -XlAs layer as the first semiconductor layer, a P-AfX2Gal-02AS layer as the second semiconductor layer, a G3Gal-03As layer as the third semiconductor layer, and The layers sandwiched the first semiconductor layer and each had a thickness of 2 μm.

The semiconductor laser shown in this example is just one example.
It goes without saying that even in other embodiments configured to satisfy the above-mentioned general relationship, a laser beam with good parallelism due to oscillation in a leakage mode can be obtained.For example, Ga-Al-A
s system, Ga-Al-As-Sb system, as well as Ga-A
'-As-P system, Ga-As-P system, In-Ga-As
Naturally, it can also be used for semiconductor lasers made of a wide range of materials, such as -V group compound semiconductors including -P-based ones, and - group compound semiconductors.

As described in detail above, according to the present invention, it is possible to provide an embedded semiconductor laser that can emit a laser beam with a small beam divergence angle and excellent parallelism, and that does not deteriorate due to end face damage.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing the structure of the main part of a semiconductor laser according to an embodiment of the invention, FIG. 2 is a perspective view showing the structure of an embodiment of a semiconductor laser according to the invention, and FIG. FIG. 4 is a diagram showing the relationship between the refractive index of each semiconductor layer and the thickness of the active layer. FIG. 5 is a diagram showing the far-field image of an embodiment of the semiconductor laser according to the present invention. FIG. 3 is a characteristic diagram of leakage angle and refractive index change in Example. FIG. 6 is a diagram showing changes over time in the light emission output of a semiconductor laser. Explanation of symbols: 1...First semiconductor layer (
active layer), 2,2'... second semiconductor layer, 3.
...Third semiconductor layer, 4...Substrate, 5.
...Al? 203 membrane, 6... silicate glass membrane, 7, 8... electrode.

Claims (1)

[Claims] 1 Thickness d, refractive index n_ which becomes the active layer on the top of the semiconductor substrate
1, a second semiconductor layer with a refractive index of n_2, n_2' holding the first semiconductor layer between them, and a band-shaped region consisting of the first and second semiconductor layers from both sides. Refractive index n_3 provided between and in contact with each of the above semiconductor layers
and a third semiconductor layer, and the wavelength of light in the active layer is λ_
In a semiconductor laser that performs oscillation n, the refractive index of each of the semiconductor layers satisfies n_2, n_2'<n_3<n_1, and the effective refractive index n_l_e_q and forbidden band width E_g_1 of the first semiconductor layer serving as an active layer are set. are the refractive index n_3 and the forbidden band width E_g_ of the third semiconductor layer, respectively.
In a drawing in which the refractive index difference n_1-n_2 of each semiconductor layer is taken as the vertical axis and n_1-n_3 is taken as the horizontal axis, the thickness d of the active layer is used as a parameter. Find a curve where the effective refractive index n_l_e_q of is equal to the refractive index n_3 of the third semiconductor layer, and
is equal to the wavelength λ_n, the straight line where n_2=n_3 in the drawing, that is, the 45° line, and the vertical axis n_
d, n_1, n_2, n_2' and n_
3. An embedded type semiconductor laser operating in a leakage state, characterized in that the following relationship is set.