JP2929599B2 - Semiconductor light emitting device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] The present invention relates to a semiconductor light emitting device, and relates to a lateral mode guide type semiconductor laser. It has a layer structure consisting of a semiconductor layered on a semiconductor substrate in the order of a one-conductivity-type cladding layer, a one-conductivity-type guide layer, an active layer, and an opposite-conductivity-type cladding layer. The refractive index of the guide layer is smaller than that of the active layer and larger than that of the one-conductivity-type cladding layer. The one-conductivity-type guide layer has a plurality of projections formed on one surface in a stripe shape in the resonator direction and having a constant cross section. A convex portion is formed so as to protrude from the one surface of the one-conductivity-type guide layer, and is arranged at a period substantially equal to or smaller than the wavelength of light in the resonator; One conductivity type guy The cladding layer is configured to be in contact with the one conductivity type cladding layer on the one surface.

[Industrial applications]

The present invention relates to a semiconductor light emitting device, and more particularly to a lateral mode guide type semiconductor laser.

That is, the present invention provides a semiconductor light emitting device that performs laser oscillation by current injection, and in particular, stabilizes a light emitting attitude (horizontal mode or horizontal mode) in a direction parallel to the junction at the emission end face of light emitted from the resonator end face. The present invention relates to a transverse mode guide type semiconductor laser capable of achieving a zero order mode.

Intensity modulation is possible directly by electric current, high light intensity,
Semiconductor lasers that can output coherent laser light
It is used in various fields such as optical communication and optical information processing.

When a semiconductor laser is used, for example, in optical communication, laser light is focused by a lens or the like and made incident on an optical fiber. In optical information processing, information is written and information is read by condensing light. Since light is often converged to a small point as described above, it is necessary that the intensity distribution of the light-emitting form, particularly the horizontal form, be monomodal.

Therefore, various lateral state control type semiconductor lasers have been devised and put to practical use.

Furthermore, with the advancement of semiconductor laser-based equipment and the expansion of the range of applications, the horizontal attitude control has been performed, and more reliable, high light output has been obtained. There is a need for a laser that has no noise due to return light.

[Conventional technology]

As conventional horizontal shape control type lasers, there are a buried type laser and a laser which performs horizontal shape control by changing the thickness of a cladding layer.

Of these, embedded lasers are generally widely used, and the control of the horizontal attitude is very good. However, since the control mechanism of the horizontal attitude is control by a large refractive index difference, the laser returns to the cavity. There is a disadvantage that noise in the presence of light is large.

Generally, in a laser that performs lateral state control using a large difference in refractive index, an increase in noise due to return light increases.

Next, a laser having a structure in which the thickness of the cladding layer is changed will be described with reference to FIG.

FIGS. 3 (1) and 3 (2) are cross-sectional views of a conventional horizontal state control type laser.

FIG. 3 (1) is a cross-sectional view perpendicular to the laser resonator having a structure in which the thickness of the above-described clad layer is typically varied.

In the figure, 15 is an n-type (n-) GaAs substrate, 16 is n-GaAl
As cladding layer, 17 is GaAs active layer, 18 is p-type (p-) GaAlAs
The clad layers, 19 and 20, are electrodes, and 21 is a stripe portion.

In this example, since the semiconductor layers (substrates) 15 on both sides of the stripe portion 21 are close to the active layer 17, horizontal state control is performed. Therefore, the smaller the distance d between the semiconductor layer 15 and the active layer 17 on both sides of the stripe portion 21 (the thickness of the cladding layer 16 in the flat portion) is, the greater the effect of controlling the horizontal state is.

However, when the cladding layer 16 and the active layer 17 are formed by liquid phase epitaxy (LPE) on the substrate 15 on which the stripe-shaped groove is formed, and the above d is reduced, the surface of the cladding layer 16 above the groove becomes concave. Therefore, as shown in FIG. 3 (2), there is a disadvantage that the shape of the active layer 17 is not flat and uniform, and the thickness of the active layer 17 is large above the groove.

In a double heterojunction laser, the active layer 17 is preferably as thin and flat as possible.

The reason is that the thinner the active layer 17 is, the larger the distribution range of the light in the laser becomes. As a result, (a) the spread of the outgoing light becomes small and the light is easily collected.

(B) The light density in the laser and at the laser end face at the time of constant light output decreases, and the reliability of the laser improves. (If the light is concentrated in a narrow range in the laser, the light intensity per unit area increases, The laser end face is likely to be deteriorated by optical energy or thermal energy due to light).

 This is because advantages such as the above arise.

[Problems to be solved by the invention]

In the conventional example, since the horizontal state control is performed with a large refractive index difference, there is an increase in noise due to return light. In the conventional example, as described above, when the thickness d of the flat portion clad layer is increased, the horizontal state control effect is increased. It is not possible.

Also, if d is reduced in order to increase the effect, the active layer is not flattened and becomes thick, so that it is difficult to achieve high output and high reliability.

 Therefore, it is necessary to solve the following two issues.

(A) In order to obtain a laser with a small increase in noise due to return light, the lateral attitude can be controlled with a relatively small difference in refractive index.

(B) A structure capable of forming a flat active layer.

If a flat active layer can be formed, the active layer can be thinned. When the active layer is thin, the light density in the laser and at the laser end face is reduced as described above, high output can be achieved, and end face deterioration hardly occurs, and the reliability of the laser is improved.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a laser for controlling a horizontal posture, in which an increase in noise due to return light can be reduced, a high output can be obtained, and further, a facet is hardly deteriorated.

[Means for solving the problem]

1 (1) and 1 (2) are a sectional view and a plan view for explaining the structure of the present invention.

In the figure, 1 is an n-type substrate, 2 is an n-type cladding layer, 3 is an n-type guide layer, 4 is an active layer, 5 is a p-type cladding layer, 6 is an n-type current limiting layer, and 7 is a p ⁺ type diffusion layer. , 8 is the n-side electrode, 9 is the p-side electrode, 10 is n
These are a plurality of projections (irregularities) with a pitch Λ provided on the mold guide layer 3.

Although the figure illustrates the case of an n-type substrate, in the case of a p-type substrate, the conductivity types may be all reversed.

The solution to the above problem is to provide a semiconductor device having a layer structure composed of a semiconductor laminated on a one conductivity type cladding layer, a one conductivity type guide layer, an active layer and an opposite conductivity type cladding layer in this order. The refractive index of the layer is smaller than that of the active layer and larger than that of the one-conductivity-type cladding layer. The one-conductivity-type guide layer has, on one surface, a plurality of projections formed in a stripe shape in the resonator direction and having a constant cross section. The projections are formed so as to protrude from the one surface of the one conductivity type guide layer, and are arranged at a period substantially equal to or smaller than the wavelength of light in the resonator. The conductivity type guide layer is achieved by a semiconductor light emitting device in which the one surface is in contact with the one conductivity type cladding layer.

[Action]

(A) Principle of lateral mode guide according to the present invention FIGS. 2 (1) to (6) are diagrams for explaining the principle of the present invention.

FIG. 2 (1) is a cross-sectional view perpendicular to the resonator length direction, FIG. 2 (2) is a refractive index distribution on a CC section, FIG. 2 (3) is a refractive index distribution on a DD section, FIG. 2 (4) is a microscopic view of the EE section (period of unevenness Λ).
Is larger than the wavelength of the light in the laser, and FIG. 2 (5) is a macroscopic view of the EE section (period of irregularities Λ).
Is the refractive index distribution when the wavelength is about the same as or smaller than the wavelength of light in the laser.

First, a structure in which the guide layer has a plurality of protrusions (irregularities) will be described.

Considering the case where the refractive index of each layer from the n-type substrate 1 to the p-type cladding layer 5 is as shown in FIGS. 2 (2) and (3), and the period 凹凸 of the unevenness is sufficiently larger than the wavelength of light inside the laser. The refractive index felt by light in the active layer 4 in the portion where the thickness of the n-type guide layer 3 is large is large, and the refractive index felt in the active layer 4 in the portion where the thickness is small is small. Therefore, the refractive index distribution is as shown in FIG. 2 (4). The difference Δn between the height of the peak and the valley of the refractive index n depends on the depth of the unevenness and the thickness of the n-type guide layer 3. As the depth of the unevenness increases, Δn increases, and the thickness of the n-type guide layer 3 increases. Δn becomes larger as the layer becomes thinner and approaches the active layer 4.

Next, when the period 凹凸 of the irregularities is reduced to become about the same as or smaller than the wavelength of the light in the laser, the light in the active layer 4 does not feel the change in the refractive index as fine irregularities. Such an average flat refractive index distribution is obtained. Here, the refractive index difference is approximately Δn / 2. Since light has the property of being confined in a portion having a large refractive index, in the case of a refractive index distribution as shown in FIG. 2 (5), it is confined in a region corresponding to the unevenness of the active layer 4 and is guided in a transverse mode. Become.

As described above, by changing the depth of the unevenness, the magnitude of the refractive index difference Δn that contributes to the transverse mode guide can be changed. For example, when the depth of the unevenness is set to zero, the refractive index difference Δn also becomes zero. Therefore, when this structure is used, the refractive index difference Δn can be controlled from zero, and the lateral appearance control can be performed by a target weak (small) refractive index difference.

In addition, since the guide layer is formed with unevenness so that the thickness of the laser in the resonator direction does not change, light in the resonator is not reflected by the unevenness.

As shown in FIG. 2 (6), the same effect can be obtained even in a structure in which the thickness of the n-type guide layer 3 is slightly thicker only in the stripe portion (a structure having one convex portion). The flattening of the active layer becomes more difficult than in the case where there are irregularities.

(B) Flat active layer When a crystal is grown on an uneven portion, the growth rate of the concave portion is higher than that of the flat portion or the convex portion. Furthermore, the growth rate of the concave portion increases as the width of the concave portion decreases.

Due to this property, when the n-type guide layer 3 is grown after forming fine irregularities on the n-type cladding layer 2, the concaves and convexes of the irregularities are filled with the crystal forming the n-type guide layer 3 at a high speed. Is flattened. When the surface of the n-type guide layer 3 becomes flat, the active layer 4 formed thereon also becomes flat and can be formed thin.

〔Example〕

 An embodiment will be described with reference to FIG.

In the figure, 1 is an n-InP substrate, 2 is an n-InP cladding layer, 3
Is an n-InGaAsP guide layer, 4 is an n- or p-InGaAsP active layer, 5 is a p-InP cladding layer, 6 is an n-InP current limiting layer, 7 is p ⁺
A type diffusion layer, 8 is an n-side gold electrode, and 9 is a p-side gold electrode. Unevenness 10
Has a pitch Λ of 0.2 μm and a depth of 0.1 μm.

 The specifications of each layer are as follows.

The composition of the InGaAsP active layer 4 is 1.3 μm in terms of wavelength (peak wavelength of the photoluminescence spectrum), and that of the n-InGaAsP guide layer 3 is 1.5 μm.

The laser cavity length L is 200 μm, the stripe width (width of the uneven portion) W is 4 μm, and the width of the p ⁺ type diffusion layer 7 is 10 μm.

Note that the n-InGaAsP guide layer may be formed such that the protrusion is located above the active layer. Also in this case, similarly to the embodiment, the guide layer must be in contact with the upper cladding layer by making the thickness of the convex portion larger than the thickness of the flat portion.

〔The invention's effect〕

As described above, according to the present invention, it is possible to realize a transverse mode guide type laser capable of controlling the lateral appearance by a relatively small difference in refractive index, to reduce an increase in noise due to return light, and to reduce the light density in the laser and at the laser end face. As a result, it is possible to increase the output power, and furthermore, it is possible to suppress the end face deterioration and to achieve a high reliability of the laser.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 (1) and 1 (2) are a sectional view and a plan view for explaining the structure of the present invention. 2 (1) to 2 (6) are diagrams for explaining the principle of the present invention. FIGS. 3 (1) and 3 (2) are cross-sectional views of a conventional horizontal state control type laser. In the figure, 1 is an n-type substrate, 2 is an n-type cladding layer, 3 is an n-type guide layer, 4 is an active layer, 5 is a p-type cladding layer, 6 is an n-type current limiting layer, and 7 is a p ⁺ type diffusion layer. , 8 is an n-side electrode, 9 is a p-side electrode, and 10 is unevenness.

Claims (1)

(57) [Claims] A first conductive type cladding layer, a first conductive type guide layer, an active layer, and an opposite conductive type clad layer. Has a refractive index smaller than that of the active layer and larger than that of the one-conductivity-type cladding layer. The one-conductivity-type guide layer has, on one surface thereof, a plurality of projections formed in a stripe shape in the resonator direction and having a constant cross section. The convex portions are formed so as to protrude from the one surface of the one conductivity type guide layer, and are arranged at a period substantially equal to or smaller than the wavelength of light in the resonator. A semiconductor light emitting device, wherein a mold guide layer is in contact with the one conductivity type clad layer on one side.