JPS5963788A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To incerase the quantity of beams oozing to a guide layer, and to elevate an optical damage level while enabling low threshold oscillation, by constituting the laser by an active layer, the guide layer, a refractive index thereof is smaller than that of the active layer, and a clad layer, a refractive index thereof is smaller than that of the guide layer, and previously forming a groove, which is shallow at a central section and is deep at both end sections, in the longitudinal direction of a resonator. CONSTITUTION:An N-type GaAs block layer 11 is grown on a P type GaAs substrate 10 as an absorbing layer for oscillating beams, the whole surface is coated with an SiO2 film 12, and the thickness of the central section of the layer 11 is thinned through etching while the groove 20, which is shallow at the central section and is deep at both end sections, is formed in the central axis direction of the substrate 10. The P type Al0.4Ga0.6As clad layer 13 and the undoped Al0.15Ga0.85As active layer 14 are laminated and grown while being interrupted at the end sections of the groove 20 and on the layer 11, and the N type Al0.27 Ga0.73As guide layer 15, an N type Al0.4Ga0.6As clad layer 16 and an N type Al0.02Ga0.98As cap layer 17 are laminated on the whole surface.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to semiconductor lasers, and more particularly to high optical output semiconductor lasers.

Recently, visible light semiconductor lasers using crystalline materials such as %A7GaAs/GaAs can perform continuous oscillation at room temperature with a low threshold and high efficiency, so they have been used as light sources for optical digitized audio disks (DAD). Practical use of multi-layered systems is progressing. Demand for this visible stranded conductor laser is increasing as a light source for optical writing in optical printers, etc.
In order to meet this demand, research and development is underway on visible light semiconductor lasers that can withstand high optical output oscillation.

Conventional stripe @lO~20μm AtGaAs/G
The aAs semiconductor laser has room temperature continuous wave (CW) optical output l.
Q mW, pulsed operation (tooms width) optical power io.

The operating limit is approximately mW, and there is a phenomenon in which the reflective surface is easily destroyed if a light output greater than this is emitted. This phenomenon has long been known as optical damage, and the critical optical output power of CW operation is around IMW/c4. Furthermore, in order to obtain a large optical output, attempts to lower the optical output density on the reflective surface have been reported, such as increasing the stripe width, increasing the active layer thickness, and double-double heterostructure. These attempts always involve an increase in the monocurrent current density, making continuous oscillation at room temperature difficult. In addition, when operating with high optical output, the oscillation region expands even if the stripe width is narrow, and the horizontal transverse mode (transverse mode in the direction parallel to the active layer) becomes complex multi-mode. The use of lasers is limited to obstacle detection, etc., and new uses such as laser printers have not yet been realized. The inventors of the present invention have proposed a striped semiconductor non-conductive laser having a structure in which a current is passed only through the striped region to create an excitation region and the vicinity of one reflective surface becomes a non-excited region transparent to laser light. -18882.

To explain the case of the AtGaAs/GaAs double heterojunction structure, the active layer in the non-excited region surrounding the excited region is made n-type, and the part of this active layer that becomes the excited region is Z-type.
When the p-type is formed in stripes by n-diffusion or the like, the bandgap of the non-excited region is reduced by about 30 to 50 meV with respect to the bandgap of the excited region. In this case, the n-type concentration is high and the p
The higher the shape concentration, the greater the relative change in bandgap. Therefore, the non-excited region becomes almost transparent to the laser beam generated in the excitation region. In this structure, optical damage does not occur even when the optical output exceeds five times the conventional limit optical output, and the effect is sufficient as long as the non-excitation region is longer than the diffusion length of the minority carrier, and these effects are due to double heterogeneity. It was confirmed that this is limited to bonded structures.

However, in semiconductor lasers with this structure, the excitation region is p-type with impurity compensation, and the internal absorption loss tends to be large and the threshold current tends to be high.Also, the diffusion length is short, so high-order transverse modes are generated. The disadvantage was that the increase in gain was slow, and the horizontal transverse mode became a high-order multimode during operation with a large optical output.

Furthermore, when the laser beam propagates in the non-excited region, the light propagates while spreading in a Gaussian shape, so the light that is reflected by the reflective surface and returns enters the excitation region again. The ratio contributing to the increase in gain required for oscillation (coupling efficiency) is low, and as a result, the multi-threshold current with large losses increases and the external differential quantum efficiency decreases.

In addition to this structure, a method has also been proposed in which the vicinity of the reflective surface is filled with a cladding layer having a large band gap as well as the active layer. In this case, as in the above structure, when the laser light propagates through the cladding layer, the light spreads and the coupling efficiency decreases, resulting in an increase in the threshold current and a decrease in the external differential quantum efficiency.

Furthermore, the manufacturing method of etching after crystal growth and burying the region that will become the reflective surface with a cladding layer is complicated, and crystal defects are likely to occur at the interface between the buried cladding layer region and the active layer, resulting in reliability issues. It had some drawbacks, such as problems.

An object of the present invention is to eliminate the conventional drawbacks and provide a highly reliable semiconductor laser that not only oscillates at a low threshold but also oscillates with a large optical output due to fundamental transverse mode oscillation. .

The structure of the semiconductor laser of the present invention is that an optical absorption layer having a bandgap narrower than the oscillation wavelength is formed in an upper layer in contact with the optical absorption layer, and an I resonator is formed with a conductivity type opposite to that of the optical absorption layer. a block layer having a concave portion outside the central portion in the length direction, and a shallow groove extending in the length direction of the resonator is provided in the center portion of the instep, so that the tip of the resonator near the reflecting surface of the resonator A semiconductor substrate with a deep groove reaching the absorption layer, an active layer formed on the semiconductor substrate, a guide layer made of a material with a smaller refractive index than the active layer, and a guide layer made of a material with a smaller refractive index than the active layer. A multilayer structure consisting of first and second cladding layers sandwiching the active layer and the guide layer from above and below, and the active layer in the deep groove is formed in the active layer in the vertical direction of the shallow groove. The tip is also low and has a step between the shallow groove and the active layer, and the deep groove is filled with the material of the guide layer.

The present invention applies the light seepage effect due to the refractive index difference and the light absorption and transmission phenomenon due to the band gap difference.

The present invention will be explained in detail below with reference to the drawings.

1 is a perspective view of an embodiment of the present invention, FIGS. 2, 3, and 4 are sectional views taken along lines AA', BB/, and CC' in FIG. The figure is a perspective view of this embodiment during manufacture, and shows the case where a semiconductor laser with a wavelength λ=0.78 μm is being fabricated. As shown in FIG. 5, an n-type GaAs layer 11 is grown to a thickness of 1.5 μm on the (100) base 9 of a p-type GaAs substrate 10 to serve as a block layer. An 8j O2 film was applied on this block layer aυ by a photoresist method, and a window with a width of 250 μm was opened in the (orT> direction so as to be located in the center region in the longitudinal direction of the resonator, and phosphoric acid, hydrogen peroxide, and methyl alcohol were mixed. Depth 0.8 in solution
Steps are provided by μm etching. Next, block layer 1
An 8i02 film 12 is applied on top of the film 12 using a photoresist method.
The width of the central part that is concave in the o> direction is 2.5 μm
A window with a width of 9 μm is formed at the end face portion of the convex region, and further B
Etching is performed using a mixed solution of r2 and methyl alcohol. At this time, (111) of 54°44' to the plane
A V-groove 20 having side slopes on the A side is formed. ■Groove 2
0 is the 5i02 film 12 used as an etching mask.
The depth of the central part is 1.8μ, corresponding to the width of the window opened at °.
The depth of the m end face portion is 6.4 μm. At this time, the length of the shallow groove is 250 .mu.mK, but it is desirable that the length of the deep groove in the end face portion is approximately 10 .mu.m or more and 50 .mu.In or less, respectively. The etching of this V-groove 20 is extremely stable depending only on the stripe width of the window made in the mask, so a groove that is shallow in the center of the resonator and deep in the vicinity of both threat surfaces is reproduced depending on the width of the stripe. Well formed.

Next, as shown in FIGS. 1 to 4, the 5in2 film 2 is removed and a P-type AtO, 4GaO, SAs cladding layer 13 is grown to fill the shallow groove in the central portion.

A substantially flat growth surface is formed in the recessed region so that the thickness of the waste portions at both ends of the groove is 0.2 μm.

At this time, the deep groove near the end face extends about 25 μm from the tip of the groove.

Next, undoped Ato, t s Ga o, Bs
The As active layer 14 is continuously grown to a thickness of 0.08 μm, and a flat active layer is formed in the central portion. On the other hand, since the boundary part of the deep groove region in the resonator length direction has an inverted mesa shape, the active layer grows only inside the groove in the deep groove region near the end face, and in the inverted mesa-shaped part of the boundary. It grows intermittently. At this time, the active layer 14 grows to an extent of about 1μnl inside the deep groove, but the growth surface is at a position 3.5μm from the tip of the deep groove and about 0.3μm lower than the tip of the shallow groove in the central part. The p-G a As substrate xo+< of the layer will be in contact.

Next, when an n-type kl (127Ga 6,73A s guide layer 15 with a refractive index higher than that of the cladding layer 13 and lower than that of the active layer I4) is grown to a thickness of 1.5 μm, a layer of about 0.5 μm is formed in the concave portion at the center. The kite layer 15 is adjacent to the active layer 14, and the drifting groove portion has a faster growth rate than the flat portion, so it is completely filled.Furthermore, n-type AZo, 44”
Qla o, 6As cladding layer 16 is 1.5 μm, n-type Az(,,(,2I") 0.98 As cladding layer 1
7 to grow 1.0 μm.

After the growth of step 7, an n-type ohmic contact 19 is formed on the 18° substrate side, and a p-type ohmic contact 19 is formed on the substrate side to complete the semiconductor laser.

In this structure, in the region of the shallow groove, a portion of the light leaks from the active layer through the adjacent guide layer and oscillates.

Furthermore, since the light seeping out from the active layer mainly passes through the guide layer which has a wide bandgap relative to the oscillation wavelength, it is transmitted without suffering any absorption loss. On the other hand, since light passing through the active layer is absorbed by the active layer and causes optical damage, reducing the amount of light within the active layer increases the level of optical damage. by the way.

The amount of light seeping out can be changed by changing the refractive index of the guide layer, but as the amount of light seeping out is increased, the threshold current generally increases. However, the structure of the present invention has a blocking layer for current confinement, and the current is injected into the active layer through the tip of the groove made in this blocking layer, so the injected current can effectively oscillate. It contributes to this and can oscillate at a low threshold.

According to calculations in this implementation NK, the active layer in the shallow groove region is as thin as 0.08 μm, and is made of At0.27 Ga O
, 73As guide layer, 87% of the light can seep into the active layer. According to this configuration, it is possible to increase the light intensity by increasing the amount of light seeping into the rered layer to prevent optical damage, and to realize laser oscillation with a low threshold value.

In addition, in the structure of the present invention, there is a shallow groove in the cavity length direction at the center and deep grooves near the reflective surfaces on both sides, and the active layer is interrupted at the step between the shallow groove and the deep groove. Therefore, most of the laser light travels straight from the active layer and guide layer of the shallow groove to start laser oscillation.

On the other hand, in the case of a full-surface electrode, carriers are injected into the active layer in the deep groove and light emission occurs, but since there is an absorption layer in the direction of the cavity length, the light suffers a large absorption loss and is emitted from the deep groove. Laser oscillation cannot occur in the active layer of the groove.

In particular, the active layer of the shallow groove is adjacent to a part of the guide layer of the deep groove region at the boundary with the deep groove region near both reflective surfaces in the cavity length direction, that is, the active layer and the guide layer are located within the recess in the central portion. Some of the layers are located.

On the other hand, the active layer of a deep groove grows at a position lower than the tip of the shallow groove in the direction perpendicular to the active layer, so the guide layer that fills the entire deep groove region grows from the tip of the shallow groove in the direction perpendicular to the active layer. It has grown to a height that covers the cladding layer of the region, the active layer, and even part of the guide layer thereon. Therefore, the laser oscillation 1t travels straight through the guide layer as it is because the active layer of the shallow groove and part of the guide layer are connected to the guide layer in the vicinity of both reflecting surfaces. Further, at this time, the guide layer is transparent to the laser oscillation light. In this embodiment, since the energy difference is 170 meV or more, the absorption loss of light near the reflecting surface can be ignored and oscillation can be performed at a low threshold.

In the structure of the present invention, there is a guide layer near the reflecting surface that has a light guiding function and is transparent to the laser oscillation light and whose absorption of light can be ignored, so that the light travels while being confined within this guide layer. In this case, the active layer may be in contact with the guide layer near the incident plane in both the longitudinal direction of the cavity, and this guide layer may be partially connected to the guide layer in the shallow groove region, and may also be in contact with the guide layer in the vertical direction below the active layer. Because of this, the light travels straight through the guide layer without leaking into the cladding layer. In other words, the light oscillated in the active region of the shallow groove region seeps into the adjacent guide layer, and further penetrates into the guide layer near the reflecting surface and travels. It has a ready configuration. For this reason.

In a general semiconductor laser, the active layer vertical spread angle θ is 40 degrees to 50 degrees or more, but in this configuration, the active layer horizontal spread angle θ7. is 10 degrees to 20 degrees 4. The spot size is flat in shape.On the other hand, in the conventional case where a mountain slope is installed adjacent to only one side of the active layer to allow light to seep out, the shape of the light seeping P is Although the wide p-angle of light in the direction perpendicular to the active layer is reduced by only about 30 to 35 degrees due to the flat structure, this structure prevents light from seeping into the guide layer in the center of the cavity and near both reflections. The tl is spread symmetrically in the vertical direction centering on the active region, and the vertical spread angle of light can be reduced to around 20 to 25 degrees.
In addition, the active layer horizontal spread angle θ, l depends on the spot size of the horizontal transverse mode, but it can be controlled to around 15 to 20 degrees by reducing the spot size using current confinement, so it is circular. It is possible to obtain an oscillation light source close to that of the optical system, and the coupling efficiency to other optical systems can be increased.

In addition, in conventional semiconductor lasers, the end face of the active layer, which becomes the excitation region due to carrier injection, is exposed as a reflective surface, and surface recombination occurs there, forming a depletion layer and reducing the band gap, resulting in a large optical output. When oscillated, this reduced bandgap absorbs light, which generates heat and raises the temperature to near the melting point, causing optical damage. On the other hand, in the structure of the present invention, the oscillated light passes through the wide W layer with a band gap difference of 17 QmeV or more near both end faces, which are the light reflecting surfaces, and oscillates.
Since there is no Q absorption and optical damage is unlikely to occur, the reflective surface fracture 7 (c
oD) The level can be increased and large optical output oscillation becomes possible.

Furthermore, in the central part of the resonator length, the light seeps out of the guide layer and oscillates as a laser.
al of Quantum Electronics
It is also possible to suppress the destructive phenomenon that occurs inside the active layer due to large optical output oscillation, as reported by Yonezu et al. in Volume QE-15, pp. 775-781, and the optical output level of the laser device becomes much higher. .

In addition, in the structure of the present invention, the guide layer, which also has a light guide mechanism near both incident surfaces, is in contact with the active layer in the shallow groove region in the cavity length direction, and also grows at both vertical ends of the active layer. In contrast, in the conventional structure with a deep groove in the center of the resonator, the area where the light spreads to the guide layer is vertical to the active layer near the center and the reflective surface. Since the lights were placed above and below the light guide, the light could not be guided smoothly and was insufficient. Furthermore, in the conventional active layer structure in which a deep groove is located at the center of the resonator, the active layer is grown by sagging inside the deep groove, which makes it difficult to control the growth, has poor reproducibility, and is likely to increase the threshold value. ,
The structure of the present invention has a flat 11 active layer, has a current confinement mechanism, and effectively contributes to laser oscillation, so there is no need for a low threshold oscillation frequency, and there is an advantage that it can be easily manufactured using a full-surface electrode.

Furthermore, the semiconductor laser according to the present invention is less likely to cause chemical reactions with the outside than a normal semiconductor laser in which the excitation region is directly exposed to the reflective surface, and can prevent deterioration of the reflective surface due to optical reactions.

Furthermore, although the structure of the present invention requires two stages of growth, after the crystal growth, there is no need for a process such as diffusion for ohmic contact, and the laser element can be manufactured very simply and with good reproducibility and high yield.

Although this embodiment uses a p-type substrate, it is also possible to use an n-type substrate by inverting pn. In addition, in this embodiment, the case of a full-surface electrode was explained, but if carriers can be injected only into the shallow groove region, invalid carriers flowing into the vicinity of the reflective surface can be removed, and oscillation with an even lower threshold value can be achieved. can. Also, regarding the shape of the groove, grooves other than V-shaped grooves can be used.

In this example, an n-type GaAs layer is used as the blocking layer, but this GaAs layer has an absorption antigen ranging from several thousand m-' to 10,000 crIL-1 with respect to the oscillation wavelength of 0.78 μm. There is viscosity in the flat area outside the shallow groove.
Because the light undergoes a large absorption loss, the light oscillates in the shallow groove, and the increase in the gain of the -order transverse mode is suppressed by the absorption loss, and the fundamental transverse mode oscillation is suppressed by the large optical output oscillation. maintained.

In addition, although this embodiment has been described with respect to the AtGaAs/GaAs double heterojunction crystal material, other examples include InGaAsP/InGaP, InGaAsP/InGaP, and InGaAsP/InGaP
GaAsP/InP, AtGaAs Sb/GaAs
5b 4ff Can be applied to many crystal materials.

[Brief explanation of drawings]

FIG. 1 is a perspective view of an embodiment of the present invention, FIGS. 2, 3, and 4 are sectional views of each portion of FIG. The figure shows the process of manufacturing the device of this example.
- FIG. 2 is a perspective view of Kn GaAs deposited on a GaAs substrate to form recessed regions and grooves. In the figure, 10...p-type GaAs substrate, 11...
n-type QaAs. Lock layer, 12...5i02 film, 13...
... p-type fiJ, 6,4GaO, sAs cladding layer, 1
4...Undoped AtO, 15G a O, 8
! l A s active layer, 15...group n-type At□, 27
Ga6.73 As guide layer, 16...n type Azo, 4oao, sAs clad JL17°0°-'
-n-type AtO, 02GaO, 98A S key layer, 18...n-type ohmic contact, 19.
...p, - ohmic contact, 20...V groove.

Claims (1)

[Claims] A light absorbing layer with a bandgap narrower than the oscillation wavelength is formed on the upper layer in contact with this light absorbing layer, and the conductivity type is opposite to that of the light absorbing layer. and a block layer with a concave central portion, and a shallow groove extending in the longitudinal direction of the resonator is provided in the central portion of the block layer, and the tip of the resonator is in the light absorption layer near the reflective surface of the resonator. A semiconductor substrate provided with a deep groove reaching the surface, an active layer formed on the semiconductor substrate, a guide layer made of a material with a smaller refractive index than the active layer, and a guide layer made of a material with a smaller refractive index than the guide layer. a multilayer structure consisting of first and second cladding layers sandwiching an active layer and a guide layer from above and below; A semiconductor laser characterized in that the shallow groove has a step between the active layer and the deep groove, and the deep groove is filled with a material of the guide layer.