JPH03101286A - Semiconductor laser device - Google Patents

Abstract

PURPOSE:To reduce noise of self-excited oscillation relatively easily by providing a second conductivity type upper clad layer with a ridge part which is thick on a first conductivity type semiconductor substrate and by forming a second conductivity type contact layer which is narrower than the width of the ridge part only at the top part of the ridge part of the upper clad layer. CONSTITUTION:A lower clad layer 2, an activation layer 3, an upper clad layer 4, and a contact layer 6 are allowed to grow on a substrate 1 in sequence. Then, after forming a stripe-shaped insulating film, for example an SiO2 film 9 on the surface of a contact layer 6, one part of the contact layer 6 and the upper clad layer 4 is removed by etching with this SiO2 film 9 as a mask. Then, only GaAs of the contact layer 6 is etched and an etchant where no AlGaAs of the upper clad layer 4 is etched is used for sandwiching only the contact layer 6, thus narrowing the width of the contact layer 6 to a desired value. Elements thus produced generates self-excited oscillating, thus achieving an improved low noise characteristic with small return light induction noise.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor laser device having low noise characteristics and suitable as a light source for use in optical disk devices and the like.

[Conventional technology]

Semiconductor laser devices with excellent fundamental transverse mode oscillation are required as light sources for gaintal audio discs (DAD), video discs (VD), or analog signal transmission, and various semiconductor laser devices are provided for this purpose. has been done.

However, when a normal semiconductor laser device is used as a light source for an optical disk device, the so-called return light noise induced by the return light reflected on the disk surface is large, so the noise characteristic is expressed by the intensity ratio with the signal. There is a problem in that the so-called S/N ratio (Signal to Noise Ratio) becomes small.

As one method for reducing the feedback-induced noise, a method has been proposed in which a saturable absorber is provided in the active region to generate automatic oscillation.

Figure 5 shows, for example, Toshiba Review, Vol. 40, No. 7, p576.
578 (Low noise semiconductor laser for optical pickup, Kazuo Suzuki et al.) is a sectional view showing a conventional semiconductor laser device having a loss guide mechanism.
It is a partially enlarged sectional view of the figure. In these figures, 21
22 is a p-type GaAs substrate, 22 is an n-type GaAs current confinement layer,
23 is p-type AjGaAs lower cladding layer, 24 is p-type Al
25 is a GaAs active layer, 25 is a n-type AjGaAs upper cladding layer, 26 is an n-type A#GaAs:+ contact layer, 27 is an n-type A#GaAs anti-etch back layer, 28 is a two-step groove, and 29 is a p-side electrode. , 3o is an n-side electrode.

The anti-etchback layer 27 is formed under the p-type Aj GaAs As cladding layer by the liquid phase epitaxial growth (LPE) method, and is simply referred to as a lower cladding layer. This is to prevent groove deformation in the process of growing 23 (the same applies to other symbols).

That is, when growing on a groove by the LRE method, a phenomenon called meltback occurs in which GaAs is dissolved into the melt that comes into contact with it and is etched. For this reason, even if the two-step groove 28 is formed in the substrate 21, the groove shape will sag as shown in FIG. On the other hand, AlGaAs is less likely to melt back. Therefore, by inserting a thin AjGaAs anti-etchback layer 27, it is possible to prevent changes in the cross-sectional shape of the groove due to meltback.Then, the two-step groove 28 is etched by reactive ion etching (RIE). A formation is made.

When a forward voltage is applied to the pn junction of the active layer 24 between the picture electrodes 29 and 30 of the conventional semiconductor laser device configured as described above, carriers are confined within the active layer 24 and light is emitted. , the upper and lower cladding layers 25, 23 and the two-step groove 28 are guided by an effective refractive index difference based on optical loss, and fundamental transverse mode oscillation is obtained.

Here, the light guide region is defined by the wide upper groove of the two-stage groove 28, and the spread of the current is suppressed by the opening of the current confinement layer 22 by the narrower lower groove.
Gain non-uniformity occurs within the light guide region (see Figure 6)
.

The active layer 24 within the light guide acts as a saturable absorber when the injection current density is too small for gain to exceed absorption. That is, when the intensity of the laser beam is low, it becomes an absorber for the laser beam, and when the light intensity is high to a certain extent, it becomes a transparent body. Therefore, the saturable absorber acts as a kind of photo-induced switch, and the laser output light undergoes self-pulse modulation with a pulse width of ns or less, resulting in so-called self-oscillation. When self-oscillation occurs, the longitudinal mode of the laser beam becomes multimode, and the coherence length becomes small. Return light induced noise is caused by self-interference with light that has returned inside the resonator due to reflection, so the smaller the coherence distance, the smaller the noise.

[Problem to be solved by the invention]

As mentioned above, the conventional semiconductor laser device shown in Fig. 5 has succeeded in reducing noise by using automatic oscillation.
In the second crystal growth step, it is necessary to fill the two-step groove 28 and form the active layer 24 substantially flat. This is because when the active layer 24 is curved, a real refractive index distribution is formed in the lateral direction, and the light distribution is no longer defined only by the grooves, which affects the characteristics, especially the noise characteristics. For this reason, in the conventional example above, the LPE method, which allows trench-filling growth, is
It is used for the second growth. However, it has been actively researched and developed in recent years, and has excellent controllability and mass production.

Metal organic chemical vapor deposition method (MOCVD method) and molecular beam growth method (
A vapor phase epitaxial growth method such as the MBE method has a problem in that it cannot be applied to the above-mentioned conventional laser device because it is difficult to grow the trench flatly.

This invention was made in order to solve the above-mentioned problems, and it is possible to produce a low-noise self-oscillation device using a relatively simple method using a vapor phase epitaxial growth method such as MOCVD or MBE. The purpose is to obtain a semiconductor laser device.

[Means to solve the problem]

A semiconductor laser device according to the present invention includes a semiconductor laser device having a semiconductor substrate of a first conductivity type;
A conductive type upper cladding layer is provided, and a second conductive type contact layer narrower than the width of the ridge portion is formed only at the top of the ridge portion of the upper cladding layer.

[Effect]

In this invention, since the width of the contact layer is formed narrower than the width of the ridge that guides light, self-oscillation can be generated using the same mechanism as in the conventional example, and low noise can be achieved.

〔Example〕

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

FIG. 1 is a sectional view of a semiconductor laser device showing an embodiment of the present invention. In this figure, 1 is an n-type GaAs substrate, 2 is an n-type AjGaAs lower cladding layer, and 3 is a p-type At'
GaAs active layer, 4-bap type AlGaAs upward rad layer,
5 is a n-type GaAs 1l flow blocking layer, 6 is a p-type A I G
a A s near contact layer, 7 is 2m electrode, 8 is n
This is the side electrode.

Next, the manufacturing process of the above embodiment will be briefly explained with reference to FIGS. 2(a) to 2(d).

First, a lower cladding layer 2 is formed on a substrate 1 by MOCVD or MBE. Active layer 3. Upper cladding layer 4. The layers up to the contact layer 6 are grown sequentially [FIG. 2(a)], and then,
After forming a striped insulating film, e.g., a Sin film 9, on the surface of the contact layer 6, using the SiO2 film 9 as a mask, a contact is formed using an etchant consisting of, for example, a mixture of sulfuric acid and hydrogen peroxide. ]・Remove layer 6 and part of upper cladding layer 4 by etching [Fig. 2(b)]
].

Next, only the contact layer 6 is side-etched using an etchant that etches only the GaAs of the contact I-ji 16 and does not etch the AIGaks of the upper cladding layer 4, such as a mixture of aqueous ammonia and hydrogen peroxide. , the width of the contact layer 6 is narrowed to a desired value [FIG. 2(C)].

Next, using the SiO□ film 9 as a mask, the current blocking layer 5 is formed by selectively filling and growing by MOCVD or the like (FIG. 2(d)).

Furthermore, after removing the 5iOz film 9, p-side electrodes 7 are formed on both sides.
Then, the n-side electrode 8 is formed by vacuum evaporation, and finally the device is separated into chips to complete the device shown in FIG.

The operating mechanism of the above embodiment is exactly the same as that of the conventional example, and by generating automatic oscillation, it is possible to obtain good low noise characteristics with little return light induced noise.

In the above embodiment, ohmic contact between the electrodes is formed only on the surface of the narrow contact layer 6, but since the area is small, the ohmic resistance may increase.

In order to prevent this and form ohmic contact over a wider area, a third crystal growth is performed to form the p-type AlGaAs layer 10, as shown in FIG. A p-type GaAs layer 11 may be further formed.

Further, in the above embodiment, the transverse mode is stabilized by a loss guide mechanism caused by absorption of laser light in the n-type GaAs current blocking layer 5, but as in another embodiment shown in FIG. A structure without the provision is also possible. In this case, the light is guided in the lateral direction by the refractive index distribution caused by the thick upper cladding layer 4 having a ridge shape, so it is called a ridge guide structure. The current is blocked by the Schottky barrier formed at the interface between the n-side electrode 8 and the upper cladding layer 4, and flows only to the part of the contact layer 6 where ohmic contact is established. Automatic vibration similar to the example occurs.

In the above embodiment, an example was shown in which GaAs and A#GaAs were used as the materials, but the materials are not limited to these, and for example, (At'Ga)I nP, Ga
InP, InGaAsP, etc. may also be used.

Further, the conductivity type may be similarly applied even if p and n are exchanged.

〔Effect of the invention〕

As described above, the present invention provides a semiconductor substrate of a first conductivity type, on which at least a lower cladding layer of a first conductivity type, an active layer, and a stripe portion have a thick ridge portion. A self-oscillating type with good low-noise characteristics with little return light induced noise because it is equipped with a cladding layer and a contact layer of the second conductivity type narrower than the width of the ridge is formed only at the top of the ridge of the upper cladding layer. This has the effect of providing a semiconductor laser device.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a semiconductor laser device showing an embodiment of the present invention, FIG. 2 is a process sectional view illustrating a method of manufacturing the semiconductor laser device of FIG. 1, and FIG. 3 is a sectional view of a semiconductor laser device. FIG. 4 is a sectional view showing another embodiment of the invention, and FIG. 5P
FIG. 6 is a sectional view showing a conventional semiconductor laser device. In the figure, 1 is the substrate, 2 is the lower cladding layer, 3 is the active layer, 4 is the upper cladding layer, 5 is the current blocking layer, 6 is the contact layer, 7 is the p-side electrode, 8 is the n-side electrode, 9 is the 5in2 It is a membrane. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (1)

[Claims] A semiconductor substrate of a first conductivity type is provided with at least a lower cladding layer of a first conductivity type, an active layer, and an upper cladding layer of a second conductivity type in which a stripe portion has a thick ridge portion, and A semiconductor laser device characterized in that a contact layer of a second conductivity type narrower than the width of the ridge portion is formed only at the top of the ridge portion.