JPS6237557B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor laser structure with a stabilized oscillation mode.

The semiconductor lasers that have been mainly used until now are
It has a double heterostructure as shown in FIG. FIG. 1 is a cross-sectional view taken in a direction perpendicular to the light. This is an n-GaAs substrate 1, an n-Ga _1-x Al _x As layer 2, an n
Or p-GaAs layer 3, p-Ga _1-x Al _x As layer 4, p
- It consists of a GaAs layer 5 and electrodes 6 and 7. In this semiconductor laser, carriers and light are well confined in the vertical direction of FIG. 1 due to the double heterostructure. However, in the lateral direction in FIG. 1, the light was confined only by the gain difference caused by the current distribution, and the lateral mode of laser oscillation was unstable.

To solve such problems, CSP
(Channeled Substrate Planar) type semiconductor laser was proposed. (For example, this was proposed in Japanese Patent Application No. 51-60009.) The basic principle of this semiconductor laser will be explained with reference to FIG. When the constituent material is Ga-Al-As-based, 11 is an n-GaAs substrate with grooves formed;
2 is an n-Ga _1-x Al _x As layer having a convex portion, 13 is a Ga _1-y Al _y As layer where laser oscillation occurs, and 14 is a p-
Ga _1-x Al _x As layer, 15 is n-GaAs layer, 16, 17
is an electrode, and 18 is a p-type diffusion layer. x and y are 0
A Ga _{1-x Al x As layer with a larger forbidden band width and a smaller curvature is placed on both sides of the Ga 1- y} Al _y As layer, where lasing occurs, so as to satisfy the relationship y<_x< 1. A double heterostructure is formed. The thickness of the semiconductor layer 12 having the convex portion is set to a distance at which light may seep out at the thin portion (this thickness is denoted as _t1 ).

In such a structure, a portion of the guided light passes through the GaAs substrate 11 in areas other than the convex portions of the Ga _1-x Al _x As layer 12.
reaches and is absorbed. On the other hand, there is no such absorption in the region corresponding to the convex portion. Therefore, compared to the region corresponding to the convex portion, the effective absorption coefficient of the other optical waveguides becomes sufficiently large. Due to this difference in effective absorption coefficient, the guided light is confined in the lateral direction parallel to the junction surface, and the laser oscillation is caused by the Ga _1-x Al _x As layer 1
This occurs in a stable transverse mode centering on the Ga _1-y Al _y As layer 13 existing on the convex portion of No. 2. Although the above is based on the waveguide effect due to the difference in effective absorption coefficients, in general, the transverse mode can be stabilized by creating a difference in complex refractive index.

However, in the structure shown in Figure 2, the specific resistance of n-Ga _1-x Al _x As in the groove part is about 0.1 Ω-cm, and n-
This is large compared to the specific resistance of GaAs substrates, which is approximately 0.003Ω-cm. This value is just an example, but in general AlGaAs is
This is because it is difficult to incorporate n-type dopants compared to GaAs, and it is difficult to lower the resistivity.This is because it is difficult to lower the resistivity, which is a rather fundamental problem, as it is difficult for the current to concentrate in the groove area, which increases the threshold current and increases the optical output. Disadvantages such as a decrease in efficiency occur.

In addition, a part of the guided light is outside the groove on the GaAs substrate 1.
1, the efficiency of the light extracted to the outside inevitably becomes small. Comparing the semiconductor laser structure shown in FIG. 1 with the semiconductor laser shown in FIG. 2, the differential quantum efficiency of the output light of the latter is about 80% of the former, although it depends on the design.

An object of the present invention is to eliminate the above-mentioned drawbacks and provide a semiconductor laser with better characteristics.

In order to achieve the above object, in the semiconductor laser of the present invention, the first semiconductor layer has a larger forbidden band width and generally has a refractive index than the first semiconductor layer provided on both sides of the first semiconductor layer in which laser oscillation occurs. A convex region is provided in at least one of the smaller second and third semiconductor layers, and a region having a wider forbidden band width than the convex region is formed outside the convex region. This region allows current to flow selectively to the convex region, thereby reducing current that does not directly contribute to laser oscillation. Furthermore, since the transverse mode of laser oscillation is stabilized by the difference in the real parts of the complex refractive index, absorption loss of laser light can be reduced.

In the semiconductor laser of the present invention, it is important to set the thickness (t ₁ ) of the thin portion of the semiconductor layer provided with the convex region as follows. Now, if the distance of the guided light seeping out from the first semiconductor layer is γ, then this is approximately It can be expressed as

however λ: free space wavelength n ₁ , n ₂ , n ₃ : refractive index of the first, second, third semiconductor, respectively; d is the thickness of the first semiconductor layer.

Regarding the approximation method of the seepage distance, please refer to Bell
Published in Syst.Tech.J.vol48.p2071-2102 (1969)
This is explained in the EAJ Marchatili paper.

When using this expression, it is considered that the guided light does not substantially seep out over a distance exceeding 3r. From the principle of the present invention, it is obvious that the smaller t ₁ is, the more stable the transverse mode will be, but it is only necessary to set the thickness of the thin region of the semiconductor layer in which the convex region is provided so as to satisfy the condition t ₁ 〓3r. . A more preferable condition is 2r or less.

Further, it is necessary that the forbidden band width between the semiconductor layer having the convex region and the region disposed outside the convex region has a difference of 30 meV or more. In most cases, it is set at around 30meV to 60meV.

Hereinafter, the present invention will be explained in detail with reference to Examples.

Example 1 FIG. 3 is a sectional view of a semiconductor laser in Example 1 of the present invention. In this example, GaInAsP−InP
This is an example of a system laser. The figure is a cross-sectional view of the laser in the direction perpendicular to the light. (100) n-InP substrate 2
A groove having a width of 10 μm and a depth of about 1.5 μm is selectively formed on the substrate 1 by chemical etching. _{After that, n-Ga 0.03 In 0.97 As 0.06 P 0.94 becomes the second semiconductor} layer _.
(Forbidden band width 1.3eV, Sn doped, convex thickness 1.7μ
m, electron concentration 10 ¹⁸ /cm ³ ) layer 22, undoped Ga ₀ . ₂₂ In ₀ . ₇₈ As ₀ . _{47 P 0} . 23. A P-InP (bandwidth: 1.35 eVZn dope, thickness: 2 μm, hole concentration: 10 ¹⁸ /cm ³ ) layer 24, which will become the third semiconductor layer, is grown by continuous liquid phase growth. Ohmic electrode is positive electrode 2
5 is an Au-Zn alloy, and the negative electrode 26 is Au-Sn.
The alloy is formed by vacuum deposition. Finally, the crystal is cleaved to obtain a laser chip with a length of 300 μm.

In the above laser, when the groove width was 10 μm, the threshold current density was 2.2 kA/cm ² at room temperature. Since the thin portion of the semiconductor layer 22 is made as thin as 0.2 μm, light penetrates to the semiconductor layer 21, and the refractive index of the optical waveguide other than the convex portion becomes smaller than that of the convex portion.
Therefore, the transverse mode was stabilized, and the excitation current-optical output characteristic was linear with almost no curvature, improving the yield of good devices.

Note that the thickness of the first semiconductor layer is 0.05 to 0.2 μm.
The degree is used. _t1 is about 0.2 to 0.6 μm, and the width of the groove is 2 to 20 μm. The depth of the groove may be greater than or equal to the above-mentioned seepage distance. Generally about 1.5μm
The degree is used.

Example 2 Although Example 1 described above is an example of the present invention in a GaInAsP-InP laser, it goes without saying that the present invention can be applied to other semiconductors.

FIG. 4 is a cross-sectional view in the direction perpendicular to the laser beam in the second embodiment. In order to create this structure ^{, an} n-
A Ga ₀ . ₆ Al ₀ . ₄ As (gap band width 1.97 eV, Sn doped, thickness 1 μm, electron concentration 10 ¹⁶ /cm ³ ) layer 32 is grown by liquid phase growth. Next, a photoresist window with a width of 10 .mu.m is formed using a conventional photoresist process, and through this window, the semiconductor layer 32 is chemically etched down to the substrate to form a groove with a depth of 1.3 .mu.m. Next, the surface
Al oxide is gas etched in a GaCl atmosphere, and then n-
Ga ₀ . ₇ Al ₀ . ₃ As (gap band width 1.83 eV, Sn doped, thickness of convex region 1.5 μm, electron concentration 10 ¹⁷ /cm ³ ) layer 3
3. n-Ga _0.95 Al _{0. 05} As (gap band width 1.50 eV, undoped thickness 0.1 μm _{, electron} concentration 10 ¹⁶ /cm ³ ) active layer 34, p-Ga _0.7 Al _{0. 3} As ( Forbidden band width 1.83 eV, Ge doped, thickness 2 μm, hole concentration 10 ¹⁷ /cm ³ ) layer 35,
n-GaAs (Sn doped, thickness 0.5μm, electron concentration
10 ¹⁷ /cm ³ ) layer 36 is grown.

Next, Zn is selectively diffused into regions 37 using an Al ₂ O ₃ mask, and then Au-Cr alloy and Au-Ge-
Vacuum deposit Ni alloy.

In the above laser, when the groove width was 10 μm, oscillation was obtained at a threshold current density of 1.2 kA/cm ² at room temperature. In this embodiment, the semiconductor layer 32 not only changes the refractive index of the optical waveguide, but also has a larger forbidden band width than the semiconductor layer 33, so that it has the effect of selectively allowing current to flow only into the convex portions of the semiconductor layer 33. Further, the semiconductor layer 32 has a higher AlAs concentration and a smaller amount of impurity addition than the semiconductor layer 33, so that the specific resistance is higher. It has been found that this has a preferable effect in allowing current to flow selectively only to the convex portions of the semiconductor layer 33.

Note that in this example, the groove provided on the surface of the semiconductor layer 32 reaches the substrate 31, but even if the bottom of the groove is inside the semiconductor layer 32, the current spread becomes small, and the threshold current is reached at room temperature. A density of 1.3 kA/cm ² was obtained. In this case, the semiconductor layer 3 at the bottom of the groove
If the remaining thickness of 2 increases, the series resistance of the element increases, so it has been found that it is desirable to reduce this portion to 1 μm or less for continuous operation. Also layer 3
2 does not need to be n-type and may be p-type. In this case, when a forward current is passed to cause laser oscillation, the layers 31, 32, and 33 are n-
Since it is a pn structure, it is biased in the opposite direction, and the current flows more concentratedly in the laser oscillation region.

Embodiment 3 FIG. 5 is a cross-sectional view of a semiconductor laser in a direction perpendicular to the light beam in Embodiment 3 of the present invention. To create such a structure, p-
p-InP (forbidden band width 1.35 eV,
Zn-doped, 2 μm thick, hole concentration 10 ¹⁸ /cm ³ ) layer 4
2 is grown by a continuous liquid phase growth method to form a buffer layer for improving crystallinity. Next, undoped Ga _{0.22 In 0.78 As 0.47 P 0.53 ( forbidden band width} 1.0 eV,
0.1 μm thick, layer 43, and Sn-doped
Ga _0.03 In _{0.97 As 0.06} P _0.94 ( _Forbidden band _width 1.3eV, _thickness 2
A layer 44 of .mu.m and electron concentration ( ^10.sup.18 / ^cm.sup.3 ) is grown.
After successively growing the above three layers, normal CVD
Depending on the process and photoresist process, the width of 2 to 20 μm
A SiO ₂ mask is formed and the semiconductor layer 44 is chemically etched to form a convex shape. On top of this, by a continuous growth method, an n-InP (forbidden band width 1.35 eV, Sn doped, thickness 0.2 μm, electron concentration 10 ⁷ /cm ³ ) layer 45, an undoped Ga ₀₂₂ In ₀₇₈ As ₀₄₇ P ₀₅₃ (forbidden band width 1.0eV,
A 1 μm thick layer 46 is grown. The Ohmic electrode uses an Au-Zn alloy as the positive electrode 48 and a negative electrode 47.
It is formed by vacuum evaporating an Au-Sn alloy. Finally, the crystal is cleaved into a laser chip with a length of 300 μm.

In the above laser, when the groove width was 10 μm, oscillation was obtained at a current density of 2.3 kA/cm ² at room temperature. This corresponds to an approximately 30% reduction in the oscillation threshold current density when the semiconductor layer 45 is not provided. In this example,
Since the distance between the layer 43 and the layer 46 is set to 0.3 μm, the light penetrates to the layer 46, and the transverse mode is stabilized by the complex refractive index distribution of the waveguide brought about by the layer 45 and the layer 46, and the excitation current - A device with linear optical output characteristics was obtained. Note that in this example, layer 4
Needless to say, 5 has the effect of preventing the current from spreading.

Example 4 Example 3 also applies when the thin region of the semiconductor layer 44 is eliminated and the semiconductor layer 43 and the layer 45 for preventing current spread are made adjacent to each other in this region. obtained similar results.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view of a conventional double heterostructure laser, Figure 2 is a cross-sectional view of a CSP type semiconductor laser,
3 to 5 are cross-sectional views of lasers in embodiments of the present invention.

Claims (1)

[Claims] 1. A second semiconductor layer having a larger forbidden band width than the first semiconductor layer;
and a third semiconductor layer is provided sandwiching the first semiconductor layer, a convex region is provided in at least one layer of the second and third semiconductor layers, and a convex region is provided outside the convex region. A semiconductor laser comprising a semiconductor region having a forbidden band width larger than the region. 2. The semiconductor laser according to claim 1, wherein the semiconductor region having a forbidden band width larger than the convex region is achieved by a semiconductor substrate on which each of the semiconductor layers is laminated. 3. The semiconductor laser according to claim 1, wherein the semiconductor region having a forbidden band width larger than the convex region is constituted by a semiconductor layer formed on a semiconductor substrate. 4. The semiconductor laser according to claim 1, wherein the semiconductor region having a forbidden band width larger than the convex region is provided on a side opposite to the semiconductor substrate with respect to the first semiconductor layer. .