JPS6146995B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a method for manufacturing a semiconductor laser having a structure effective for controlling transverse modes.

In order to operate a semiconductor laser continuously at high temperatures, optical energy and current are injected into specific regions that provide the best thermal path to remove heat from the junction, while minimizing light loss and wasteful recombination. It is necessary to have structural dimensions that confine the

Therefore, so-called electrode stripe type semiconductor lasers have emerged, in which the electrodes of the semiconductor laser are striped electrodes to confine the current flowing through the active layer and at the same time confine the optical energy.

However, although this semiconductor laser is capable of room-temperature DC oscillation, the major drawback in its characteristics is the instability of the electromagnetic wave mode parallel to the active layer, that is, the instability of the transverse mode, and the change of the transverse mode in response to changes in the injection current. It was hot. This is because the electrode stripe type does not have a carrier and light confinement structure in the lateral direction of the active layer. That is, in a current region slightly above the starting current value of laser oscillation, the gain necessary for oscillation exceeds the loss only in the active region directly under the stripe, and oscillation occurs in a zero-order or low-order transverse mode. However, as the injection current increases, the carriers injected into the active layer also spread into the active layer on both sides of the stripe region, so the high gain region spreads and the transverse mode spreads and higher-order modes occur. Transverse mode instability and injection current dependence cause mode dispersion in an optical transmission line when performing optical communication using laser light, and significantly reduce the information capacity of the transmission line. Therefore, from this point of view, a semiconductor laser as a signal source for optical communication is required to have single mode oscillation over a wide injection current range.
Therefore, a striped type semiconductor laser which compensates for the above-mentioned drawbacks was proposed in Japanese Patent Application No. 6226/1983.

First, a schematic diagram of this striped semiconductor laser is shown in FIG. 1, and its manufacturing method, structure, mechanism, etc. will be briefly explained using the drawings. For example, an n-type GaAs substrate 1 is first etched to a depth of 1.5 to 2 μm using a selective etching method or the like so that it has the same width as a predetermined stripe width and is perpendicular to the reflective surface of the resonator.
Dig the groove. Next, by continuous liquid phase epitaxial method, n
A type Al _{0.3 Ga 0.7} As 2, an n- or p-type _{Ga As active} layer 3, a p-type Al _{0.3 Ga 0.7 As 4} , and a p-type _{Ga As layer} 5 are sequentially formed. In this case, the _process ends when the surface of the n-type Al _{0.3 Ga 0.7} As 2 grown on _the Ga _As substrate 1 having irregularities of several μm becomes flat. A S _i O ₂ insulating film 6 with a striped window of a predetermined width is formed directly above the groove of _{the GaAs} substrate 1 of the wafer having a multilayer structure thus produced, and a p-type insulating film 6 is formed on this film. Electrode 8
By forming n-type electrodes 7 on the surface of _{the GaAs} substrate 1, a striped semiconductor laser device is completed.

The semiconductor laser with the above structure is n-type Al _{0.3 Ga 0.7 As2 .}
The thickness t ₁ of the region other than the stripe is thinner than the thickness t ₂ of the stripe portion, and the base of the region where light in the active layer 3 leaks in the layer thickness direction during oscillation is absorbed by the n _- type _GaAs substrate 1. It is about the thickness of a layer. Therefore, this structure has a mechanism that increases the loss of the oscillation mode occurring on both sides of the stripe, prevents the spread of the transverse mode, and suppresses the oscillation of higher-order modes. For example, if we take a double hetero laser (hereinafter referred to as DH laser) of Ga _{A s} - Al _x Ga _1-x M _s , approximately 0.2
_{A Ga As} active layer with a thickness of μm is formed with an Al composition ratio of X0.3.
In a DH laser with a structure sandwiched between Al _x Ga _1-x A _s , the light during oscillation is strongest in the active layer in the layer thickness direction, and increases as it goes to the Al _x Ga _1-x A _s layers on both sides. It becomes weaker and spreads to each Al _x Ga _{1-x As} layer with a smooth intensity distribution with a base of about 1 μm each. Therefore, if the thickness of the Al _x Ga _1-x A _s layer on both sides or one side of the active layer on both sides of the stripe is made thinner than 1 μm so that the base of the light seeping out covers the GaAs substrate, the lateral direction can be reduced. Loss increases, and transverse mode broadening and higher-order mode oscillation can be prevented. Also, as the active layer becomes thicker, the light
The seepage into the Al _x Ga _1-x A _s layer becomes smaller, and X〓
0.3, when the active layer thickness is less than about 15 μm, the light
The seepage into the Al _x Ga _1-x A _s layer is almost zero. On the other hand, if the thickness _of Al ₀ . ₃ Ga ₀ .
% or less, the oscillation starting current value is not increased.

That is, in order to make the optical waveguide mechanism effective, the active layer thickness d should be 0.1 μm, the thickness of the _{Al 0.3 Ga 0.7} As layer other than the stripe should be t ₁ 0.4 _μm , and the thickness of the stripe portion should be t ₂ It is necessary to set it to 1.5 μm. Establishing growth length conditions that satisfy this condition is extremely difficult and requires advanced growth techniques such as highly accurate temperature control and accurate growth rate control. This includes problems such as a decrease in production yield, and very poor uniformity, reproducibility, and mass production.

The purpose of this invention is to eliminate the drawbacks of conventional semiconductor laser manufacturing methods, and to achieve high yield and mass production in manufacturing semiconductor lasers that enable mode control in the direction parallel to the active layer. An object of the present invention is to provide a method for manufacturing a suitable semiconductor laser.
That is, the present invention is a manufacturing method for providing a semiconductor laser having a structure in which light absorbers are provided on both sides of a striped current injection region, and in which light absorbers are provided on both sides of a striped current injection region. Each crystal layer is grown on a substrate crystal surface in which a striped current injection region and a striped current injection region are provided with elongated grooves having side surfaces on which the crystal grows at a rate different from the crystal growth rate, and on both sides of the substrate crystal surface.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 is a schematic sectional view of a semiconductor laser in which the present invention is implemented, and FIGS. 3a to 3d are explanatory diagrams of its main manufacturing steps. As shown in Figure 3a,
After coating an ordinary photoresist film 18 on an n-type _GaAs substrate 9 having a (100) surface, a stripe _- shaped window with a width of 8 μm parallel to the [011] direction to which this photoresist film 18 is not attached is formed. and 10 on each side of this
Two windows each having a width of 20 μm to which no photoresist film 18 is attached are formed in parallel and spaced apart by μm, and the substrate crystal 9 shown in FIG. 3a is prepared. Next, grooves 17, 1 with a depth of 1.5 to 2.0 μm are formed by selective etching.
Look for 9. Etching with a mixture of H ₂ PO ₄ 1: H ₂ O ₂ 1: H ₂ O ₂ 1: CH ₃ OH5 at room temperature for about 1 minute and 30 seconds while stirring will yield grooves with the depth shown above. . The remaining photoresist film 18 is removed to form the substrate crystal 9 shown in FIG. 3b. The etched grooves 17 and 19 form inclined side walls, and the inclined walls form a {111}A plane. Next, by continuous _{liquid phase epitaxial growth method, n-type Al 0.3 Ga 0.7 As10} and p-type G
_The a _As active layer 11, the p-type Al _0.3 Ga _0.7 As 12 _, and finally the p-type _{Ga As} 13 are grown in this order to form the layer structure _shown in FIG. 3C. A _{SiO 2} insulating film 14 with a 12 μm wide window is placed directly above the groove 19 of this wafer, and a p-type electrode 16 is placed on top of this, and an n-type electrode is placed on the surface of _{the GaAs} substrate 9. After forming, the present invention is implemented by forming an end face by cleavage.
A semiconductor laser device is completed.

The n-type Al _0.3 Ga _0.7 As ₁₀ is the groove 19 of _the Ga _As substrate 9 _.
It is necessary to have a layer structure that has a flat surface directly above the surface and a thickness t ₁ of 0.4 μm or less on both sides. This growth condition actually includes the width and depth of the grooves 17 and 19, the crystallographic direction of the substrate crystal, the growth time,
It depends on factors such as growth temperature and solubility saturation. In particular, when growing in a shape where the substrate crystal has constituent planes having a specific direction, the growth layer becomes strongly influenced by the direction dependence of the growth rate. The relationship between the growth rate and the crystal direction is as follows: {111}A plane>{100}plane>{111}B plane.

Therefore, when grown on a (100) plane n-type substrate 9 as shown in FIG. 3b, grooves 17,1 in the [011] direction
Since the inclined sidewall of the substrate crystal 9 is the (111)A plane, the thickness of the grown layer on the (100) flat plane of the substrate crystal is effectively thinner than the thickness at the groove portion. According to the present invention, since the grooves 17 are further provided on both sides of the groove 19, the portion 20 sandwiched between the grooves 17 and 19
When the growth layer thickness t ₁ is compared with the growth layer thickness t ₃ of the (100) flat surface outside the groove 17, t ₃ >t ₁ at the early stage of growth and when the growth layer is quite thin. This is because the solute in the 2 portions of the (100) flat surface diffuses more into both the grooves 17 and 19 where the growth rate is fast, so the volume concentration in the 20 portions decreases and the apparent growth rate slows down. For example, the shape of the grooves 17 and 19 is as follows.
Depth 1.5μm, width of groove 17 20μm, width of groove 19 8
μm, if the interval is 10 μm, n-type
The layer thickness t ₁ of _the flat part _{20 of Al 0.3 Ga 0.7 As10 is} 0.4 μm
When grown under the conditions, the thickness t ₃ of the (100) flat portion, which is not affected by the grooves 17, was 1.2 μm. As for the surface condition, as shown in FIG. 3C, the portion between the grooves 17 is a little low, and the surface flatness is smooth. Furthermore, when layers 11, 12, and 13 were laminated, the entire crystal finally became flat. In other words, by providing elongated grooves on both sides of the groove directly below the striped region where the current flows on the substrate crystal surface, light seeps out on both sides of the stripe, and its base is effectively absorbed by _{the GaAs} substrate. This results in a slower growth rate of the n-type Al _{0.3 Ga 0.7 As} portion having _a similar thickness. The slower the growth rate in this part, the longer the controllability becomes, the manufacturing method becomes simpler, and the reproducibility, yield, etc. are improved.

Moreover, the effect of the waveguide mechanism, which increases the loss of the oscillation mode occurring on both sides of the stripe, prevents the spread of the transverse mode, and suppresses the oscillation of higher-order modes, is not diminished in any way by providing the grooves 17.

In order to operate this embodiment efficiently, _{the GaAs} active layer 11 must have a thickness of 1000 Å to 2000 Å and be of n-type.
The thickness of the region t ₁ other than the stripe of the _{Al 0.3 Ga 0.7} As layer 10 is 0.4 μm or less, and the thickness of the stripe portion t _{2 is} 1.5 μm or less
2.0 μm, the thickness _{of the p-type Al 0.3 Ga 0.7} A _s layer ₁₂ is 1.5 ~
2 μm, striped current injection electrode 16
It is necessary to make the layer structure dimensions such that the width of the cavity is 12 μm and the cavity length is 250 μm. As a result, laser oscillation with a zero-order transverse mode was obtained at a current value of 80 mA to 150 mA.

As described above, according to the present invention, a semiconductor laser whose mode can be controlled has the characteristics that it can be obtained with easy growth technology, good reproducibility, high yield, and high mass productivity.

In the above embodiments, the stripe-shaped current region forming section for applying the injection current is of the electrode stripe type.
Even when applied to a stripe-type semiconductor laser, or to other stripe-type semiconductor lasers, exactly the same effect can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a schematic sectional view of a conventional striped semiconductor laser, FIG. 2 is a schematic sectional view of a semiconductor laser obtained according to an embodiment of the present invention, and FIGS.
d is a schematic cross-sectional view showing the main process steps in the manufacture of the laser of the invention. _In the drawings, 1, 9: n-type _GaAs substrate, 2,
10: n-type AlGaAs layer, 3 _, 11: GaAs active layer, 4, 12: p-type AlGaAs layer, 5, 13: p-type _GaAs layer, 6, 14: _{SiO 2} film, 7, 15: n-type ohmic electrode, 8, 16: p-type ohmic electrode, respectively.

Claims (1)

[Claims] 1. A (111) inclined wall in the [011] direction that is narrower than the width of the stripe portion is provided in a portion corresponding to the stripe region where current flows on the (100) plane substrate crystal surface having the first conductivity type. A step of forming an elongated groove on the A-plane and an elongated groove on the A-plane with inclined walls in the [011] direction having predetermined widths on both sides of the groove, and forming an elongated groove on the A-plane with (111) inclined walls in the [011] direction. a first semiconductor layer;
a step of sequentially stacking, by epitaxial growth, an active layer crystal with a narrower bandgap width than the first semiconductor layer and a second semiconductor layer of a second conductivity type with a wider bandgap width than the active layer crystal; and a step of forming current injection electrodes for injecting current into the active layer crystal in a stripe pattern on the surface of the second semiconductor layer; When a first semiconductor layer of the first conductivity type is grown on a substrate at a mesa portion to a thickness that allows light from the active layer to seep into the substrate, a long and narrow groove is formed between the parallel grooves on both sides and the center groove. The method is characterized in that the thickness of the layer grown on the mesa portion between the groove and the groove on both sides is thinner than the thickness of the layer grown on the substrate surface on the opposite side from the mesa portion. A method for manufacturing a semiconductor laser.