GB2127218A

GB2127218A - Semiconductor laser

Info

Publication number: GB2127218A
Application number: GB08321788A
Authority: GB
Inventors: Akira Fujimoto
Original assignee: Omron Tateisi Electronics Co
Current assignee: Omron Corp
Priority date: 1982-08-16
Filing date: 1983-08-12
Publication date: 1984-04-04
Also published as: DE3348097A1; DE3329467C2; DE3329467A1; DE3348097C2; GB2127218B; GB8321788D0

Abstract

A semiconductor laser has at least one double heterojunction construction comprising an active layer, e.g. 5a, sandwiched between adjacent layers 4a, 4b, the layers being terraced and dopant being diffused from one surface of the semiconductor crystal to form a p-n junction 10a in the stepped region of the active layer. In a semiconductor crystal having a plurality of double heterojunction constructions (as in Fig. 3), the laser output can be shifted one PN junction to an adjacent PN junction, and the carrier injection electrodes have applied thereto a voltage for initially setting the PN junctions to a state slightly below the laser oscillation threshold value, and further a laser beam is adapted to be directed upon one of the PN junctions from the outside of the semiconductor crystal to photoexcite the PN junctions to create laser oscillation. <IMAGE>

Description

SPECIFICATION Semiconductor laser BACKGROUND OF THE INVENTION This invention relates to a semiconductor laser, particularly, to a semiconductor laser wherein an active layer, and first and second semconductor layers putting said active layer therebetween are respectively formed from a conductive semiconductor layer of the same kind, an a P-N junction portion is formed in the active layer by an inverted diffusion layer.

As is known, the semiconductor laser comprises a laser mudium formed from a semiconductor crystal having a double hetero junction construction. According to the basic structure of the semiconductor crystal, a first semiconductor layer, an active layer and a second semiconductor layer are laminated on a substrate in said order to form a double hetero junction construction. That is, forbidden gap of the first and second semiconductor layers are greater than that of the active layer. In this junction construction, carriers injected from the first and second semiconductor layers into the active layer are confined into the active layer. As the result, recombination of the carriers in the active layer is carried out effectively. That is, laser oscillation takes place in the active layer.

To enhance the effeciency of recombination of the carrier in the active layer, it is necessary to increase density of injecting the carrier into the active layer. The semiconductor layer is generally provided with a stripe construction. This stripe construction has its function to guide the carrier injected into the first and second semiconductor so that the carrier may be concentrated in a specific region of the active layer. Thereby, the active layer is formed with a web-like region of which longitudinal direction is controlled by crystal ends and lateral direction controlled by a stripe width.

Since the refractive index of the active layer is greater than that of the first and second semiconductor layer, the light beam generated by recombination of the carrier is confined in the radiation region. Since both longitudinal ends (i.e. crystal ends) of the radiation region forms a Fabry-perot resonance surface, the light beam is subjected to resonance amplification in the radiation region, a part of which is put out. This is the known laser beam.

The laser oscillation in the radiation region has a longitudinal mode generated in a longitudinal direction and a lateral mode generated in a lateral direction. Preferably, these are a single mode. The longitudinal mode is possible to take a single mode due to the Fabry Perot resonance surface. However, the laternal mode depends the stripe construction. That is, the stripe construction must be the construction which can prevent propagation of the light beam in the lateral direction.

Incidentally, various kinds of stripe constructions which are intend to form the lateral mode into a single mode have been proposed.

However, any of these are coplicated in manufacture step. For example, there is a construction in which a portion is irradiated to a region from the second semiconductor layer to the first semiconductor layer to form a high resistant layer. That is, this is the construction in which the high resistant layers are formed on both sides which leave a narrow stripe width.

In this case, the manufacturing step of the semiconductor crystal involves a unique step which is the irradiation of proton, lacking in consistency. Further, the known transversejunction stripe-geometry laser can be improve the above-described point which is the lacking in consistency but has a plurality of masking steps which form a diffusion layer and is complicated.

The foregoing considerations are based on demands of reduction in threshold current value and unification of oscillation mode in addition to a demand of higher output. That is, there is a problem such as a breakdown of a crystal end which is a Fabry-Perot resonance surface, and there is naturally a limitation to increase the oscillation output by merely increasing a driving current.

In view of the foregoing, a semiconductor laser has been proposed in which the abovedescribed stripe construction is employed to form a plurality of radiation regions in an active layer, and output light beams are totalized to provide a higher output.

However, in such a semiconductor laser as described above, since the stripe construction is added, a plurality of masking steps are required, as a consequence of which the manufacturing process beomes cumbersome. In addition, in the semiconductor laser of the type as described, it is desirable that the spacing between the radiation regions is narrow. However, the spacing between radiation regions can not be made smaller than the spacing limited by the photlithographic technique. Accordingly, it is difficult to provide a higher density of the radiation region.

Also, there is a demand for development of a semiconductor laser which is provided with a function of scanning output light beams, as a function of element of the semiconductor laser itself.

As is known, in photo-electronic devices such as a facsimile transmitter, a photodeflector composed of electric optical elements, a movable mirror and the like is combined therewith to scan the output light beam of a semiconductor laser.

However, to combine with the photodeflector, there involves many problems which are hard to solve because of the complicated mechanism. To make the effective use of advantages of the semiconductor laser in being small in size and light in weight, it has been desired to develop a semiconductor laser in which an element of the semiconductor laser itself has the function of scanning an output light beam.

Finally, a photo-circuit requires a photobranch element which converts a single light beam into a plurality of light beams. Major conditions required by a photobranch element of this kind are as follows: First, that light intensity is not to be decreased by the branching. Second, that a suitable number of branch beams has to be obtained. Third, that integration or miniaturazation of the photobranch element and other photocircuit elements for the photodeflector is possible.

However, a photobranch element fulfilling these requirements has not yet been proposed. In view of the possibility of integration or miniaturization, there are the Y-branch type, the directively coupling type, etc., for example. However, in these types, the light intensity is decreased by the branching, and therefore, it is difficult to increase the number of branches.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a semiconductor laser in which a lateral mode can be formed into a single mode.

It is a further object of the present inveniton to provide a semiconductor laser in which the manufacturing process can be simplified.

It is another object of the present invention to provide a semiconductor laser which renders high output operation possible.

It is a still further object of the present invention to provide a semiconductor laser which is provided a function to scan output light beams as a function of element itself.

It is a still another object of the present invention to realize, by using a semiconductor laser, a photobranch element which can produce a suitable number of branch beams without deterioration of light intensity.

In accordance with the present invention, there is formed a P-N junction portion in an active layer at an offset portion formed in a double heterojunction portion. Lateral propagation of light generated in said P-N junction portion is prevented by first and second semiconductor layers between which the active layer is put. That is, the lateral mode is single.

An inverted layer is formed merely by diffusion from a semiconductor crystals surface.

That is, there are various inverted layers but in any of these, no complicated masking step is required.

Furthermore, a plurality of double heterojunction constructions may be simply obtained because of a multilamination of conductive semiconductor layers of the same kind. That is, P-N junction portions (radiation regions) are formed in a lateral row in a plurality of active layers at the offset portion. A spacing between the radiation regions corresponds to the thickness of a semiconductor layer laminated on the both sides of each active layer.

The thickness of a semiconductor layer can be smaller than the sub-micron range which can be suitably set and is relatively smaller than a spacing limited by the photolithographic technique. Accordingly, higher density of the radiation region becomes realized.

And, by midification of the construction of carrier injection electrod, a high output type or a beam scanning type semiconductor laser may be obtained. That is, in the semiconductor laser of high output, both surfaces of the semicondutor crystal are covered by each carrier injection electrode and the laser oscillations are carried out simultanously at each of P-N junction portions. Output light beams in the radiation regions may be totalized easily.

In the beam scanning type semiconductor laser, one of electrodes have a plurality of electrodes formed as parallel stripes at predetermined intervals. Tha carrier density injected into each of a plurality of PN junction portions is high at the PN junction portion near a given carrier injection electrode, and becomes low as being away from said carrier injection electrode. Thus, if a bias voltage applied to the carrier injection electrode is varied, the threshold value of laser oscillation in the PN junction portion can be controlled in order from the side closer to the electrode, and the laser output beam can be substantially scanned laterally.

If the element of the semiconductor laser itself has the scanning function as described above, a photo-deflector in the prior art need not be provided, and it is possible to secure miniaturization, lightwave and high reliability thereby.

Also, non-inverted layers are formed in both sides in a longitudinal direction of the active layer to thereby reduce absorption of light in the neighbourhood of the crystal end. That is, the breakdown limit of the crystal end increases to provide higher output.

Finally, when one laser beam is incident on one of a plurality of PN junction portions from the outside of the element, said laser beam is partly propagated to the remaining PN junction portions, and therefore, not only said PN junction portion but the remaining PN junction portions provide laser oscillation by photo-excitation to release a plurality of laer beams. Each branch beam put out at that time has the same phase and the same wavelength as that of the incident laser beam, and the light intensity thereof is amplified. Accordingly, it is extremely simple and easy to increase the number of branches.

Also, a current slightly smaller in value than a threshold current value is pre-supplied to each of PN junctions portions to provide laser oscillation by photo-excitation, and therefore, a current smaller in value than the threshold current value when each of PN junction portions individually provides laser oscillation will suffice, thus providing a photobranching semiconductor laser of low power comsuption.

Other and further objects of this invention will become obvious upon a reading of the illustrative embodiments described below or set forth in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic structural view showing a semiconductor laser in accordance with a first embodiment of the present invention.

Figure 2 is a schematic structural view showing a semiconductor laser in accordance with a second embodiment of the present invention.

Figure 3 is a schematic structural view showing a semiconductor laser in accordance with a third embodiment of the present invention.

Figure 4 is a schematic structural view showing a semiconductor laser in accordance with a fourth embodiment of the present invention.

Figure 5 is a schematic structural view showing a semiconductor laser in accordance with a fifth embodiment of the present invention.

Figure 6 is a sectional view taken on line A-A of Fig. 5.

Figure 7 is a schematic structural view showing a semiconductor laser in accordance with s sixth embodiment of the present invention. This semiconductor laser is modified the relation of refractive index and the method of biasing in the structure of the semiconductor laser shown in Fig. 2 to operate as a photobranch element.

Figure 8 is a schematic perspective view showing one example of application of the photobranching semiconductor laser shown in Fig. 7.

DETAILED DESCRIPTION OF THE INVEN TION Fig. 1 shows a semiconductor laser in accordance with a first embodiment of the present invention. In this semiconductor laser, a positive hole injection electrode 26 and an electron injection electrode 27 are provided on both surfaces, respectively, of a semiconductor crystal composed of the following semiconductors layers.

The aforementioned semiconductor crystal is composed a n-GaAs layer (substrate) 21, a GaAIAs layer 22, an n-GaAIAs layer 23, an n GaAs layer (active layer) 24 and an n-GaAIAs layer 25. zin a junction portion (that is double heterojunction portion) between the active layer 24 and the semicconductor layers 23, 25 on both sides thereof, there is provided an offset portion 28. A P-type diffusion layer is provided on the side of the positive hole junction electrode 26.

The above-mentioned offset portion 28 is formed in the following. That is, the GaAIAs layer 22 is laminated in a predetermined thickness on the substarte 21. Thereafter, the GaAIAs layer 22 is partly removed by etching, to which the surface of substrate 21 is exposed. That is, the offset portion 28 is formed by a stepped portion through a thickness of the GaAIAs layer 22. The GaAIAs layer 22 is obtained by doping an n-type conductive member or is not subjected to doping.

After the offset portion 28 has been formed in a manner as described above, the n-GaAIAs layer 23, the active layer 24 and the n GaAIAs layer 25 are successively laminated.

Thus, injection portion is formed stepwise by the offset portion 28. The Surface of the n GaAIAs layer 25 is made substantially flat and horizontal due to the dependability in the surface direction of growth speed of crystal.

The aforesaid p-type diffusion layer 29 is constructed such that zinc is diffused in a region from the whole surface of the GaAIAs layer 25 to the active layer 24 in the offset portion 28. A boundary surface between the p-type inverted region and the n-type region is parallel to the surface of the n-GaAIAs layer 25. As the result, a PN junction portion 30 of a predetermined width is laterally formed in the active layer 24 at the offset portion 28.

In the semiconductor laser constructed as described above, when a forward voltage is applied to both the electrodes 26, 27, a carrier of high density is injected into the PN junction portion 30. Since both lateral sides of the PN junction portion 30 are put between large potential barriers of the n-GaAIAd layers 23, 25, an injected carrier is confined in the PN junction portion 30 without being diffused in a lateral direction and effectively recombined to generate induced emission light. As a consequence, a radiation region 31 is formed in the active layer 24 in the neighbourhood of the PN junction portion 30. The light generated at the radiation region 31 is subjected to resonance amplification with crystal ends on both longitudinal sides being Fabry-Perot resonance surface.At this time, the generated light is prevented by the n-GaAIAs layers 23, 25 of small refractive index and is not spread laterally. That is, the lateral mode can be made to be single.

In the manufacturing process of the semiconductor laser, zince is merely diffused from the whole surface of the n-GalAs layer 25, and therefore, information of the p-type diffusion layer 29 and the positive hole injection electrode 26, masking step is not required to simplify the manufacturing process.

Fig. 2 shows a semiconductor laser in ac cordance with a second embodiment of the present invention. In the second embodiment, part coresponding to those of the above described first embodiment are designated by the same reference characters, and the explanation thereof will be omitted (the same is ture for the following embodiments).

The semiconductor laser in accordance with the second embodiment has a plurality of double heterojunction constructions, which render high output possible. That is, n-GaAs layer 4a and active layer (n-GaAs) 5a which provide heterojunction each other are alternatively laminated (which are indicated by added charcters, a, b, c, d, and e in Fig. 2), and plurality (four in the illustrated embodiment) heterojunction constructions are formed by each set of layers, (4a, 5a, 4b), (4b, 5b, 4c), (4c, 5c, 4d) and (4d, 5d, 4e). Said junction portion is formed with an offset portion 28 similarly to the first embodiment. Zinc is diffused in a region from the whose surface of the uppermost layer (n-GaAIAs layer 4e) of said semiconductor crystal to each active layer 5a, 5b, 5c, 5d in the offset portion 28 to form a p-type inverted layer 29.As the result, each layer of the offset portion 28 is formed with PN junction portions in a row in a lateral direction.

In the semiconductor laser constructed as described above, a carrier is principally injected into PN junction portion 1 Oa, 1orb, 10c and 1 0d formed in the active layers 5a, 5b, Sc and 5d among the PN junction portions formed in the offset portion 28. This results from the fact that an energy gap of the GaAIAs layers 4a, 4b, 4c, 4d and 4e is greater than that of the active layers 5a, 5b, Sc and 5d.Since the PN junction portions 10a, lOb, 10c and 10d are each put in their both lateral sides between hetero barriers of the GaAIAs layers 4a, 4b, 4c, 4d and 4e, the injected carrier is confined therein without being diffused in a lateral direction and effectively recombined to generate induced emission light to form radiation regions 11 a, 11 b, 1 1c and lid. A spacing between the radiation regions 11 a, 11 b, 11 c and lid is determined in accordance with the thickness of the GaAIAs layer 4a, 4b, 4c and 4d. It is well known that the thickness of the semiconductor layer of the type as described can be suitably set in the range from a few mocrons to acores of microns depending on the growth speed and growth time of crystals.

Fig. 3 shows a semiconductor laser in accordance with a third embodiment of the present inveniton. In the semiconductor laser shown in the third embodiment, a construction of carrier injection electrode in the sturcture of semiconductor laser shown in the above-described second embodiment is modified.

That is, positive hole injection electrodes 31 a, 31b and electron injection electrodes 32a, 32b are formed as parallel stripes on both lateral sides of the crystal surface, thus not overlapping said offset portion 28.

In the semmiconductor laser constructed as described above, for example, when a forward bias voltage is applied between the electrodes 31 a and 32a to cause a driving current to flow, the largest quantity of carriers are injected into the PN junction portion 10a which is the shortest in current passage and the smallest in electric resistance of passage. The passage of current is extended in order of the PN junctions lOb, 10c and 1 Od and the electric resistance of passage increases, and the carriers to be injected decrease accordingly.That is, the laser oscillation is first effected at the PN junction portion 10a, and as the driving current increases, the laser oscillation is effected in order of the PN junction portions lOb, 10c and 1 Od. And, when a forward bias is applied between the electrodes 31b and 32b to cause a driving current to flow, the laser oscillation can be effected in order of the PN junction portions 1 Od, 10c, 1 Ob and 1 Oa conversely to the above.When a forward bias is applied between the electrodes 31 a and 32b or between the 31b and 32a to cause a driving current to flow, many carriers are injected into the PN junction portions 1 Ob and 10c which are short in passage of current for the same reason as described hereinbefore, and these junctions first take place the laser oscillation.

When the driving current is further increased, the PN junction portions 10a and 10d also take place the laser oscillation.

By making use of the above-described principle, a driving current is first permitted to flow between the electrodes 31a and 32a to cause the PN junction portion 1 Oa to effect the laser oscillation. Next, the driving current between the electrodes 31a and 32a is cut off and the driving current is permitted to flow between the electrodes 31a and 32b or between the 31a and 32a to cause the PN junction portions 1 Ob and 10c to effect the laser oscillation. Finally, the driving current between the electrodes 31a and 32b or between the 31b and 32a is cut of and the driving current is permitted to flow between the electrodes 31 b and 32b to cause the PN junction portion 1 Od to effect the laser oscillation. In the manner as described above, the output beams can be operated in a lateral direction between the PN junction portions 10a . . 1 Od. In this case, the output beam is discontinuously moved in a lateral direction but the output beam can be moved smoothly by controlling the current valve between the electrodes.

While in the above-described third embodiment, the carrier injected electrodes are provided on both lateral sides of th ecrystal surface, without overlapping the offset portion, it should be of course noted that the invention is not limited to thereto but at least one carrier injected electrode can be a plurality of electrodes formed as parallel stripes at predetermined lateral intervals.

Next, a fourth embodiment (Fig. 4) and a fifth embodiment (Figs. 5 and 6) will be described. These embodiment each concern the semiconductor laser in which the form of the inverted layer is varied in the structure of the semiconductor laser shown in the abovedescribed first embodiment.

Referring first to Fig. 4, a region from the surface of an n-GaAIAs layer 25 to an offset portion 28 is formed with an inverted layer 41 in the form of a web in a longitudinal direction with a width W which is slightly larger than a transverse with of an active layer 24.

The inverted layer 41 is formed by forming a diffusion hole 43 of a width W in the form of a web in a longitudinal direction in an insulated layer 42 laminated on the surface of the n-GaAIAs layer 25 and diffusing zinc from the surface of the n-GaAIAs layer 25 exposed thereto.

Thereafter, when an electrode layer 44 is formed on the insulated layer 42, a part depressed in the diffusion hole 43 constitutes a positive hole injection electrode 45 without modification. Thus, the cumbersome masking step for forming the positive hole injection electrode 45 corresponding to the small width W of the inverted layer 41 is not required.

With this arrangement, the passage of the injected carrier is limited to the inverted layer 41 formed into a narrow web. That is, a defect in crystal expected to be present in a part other than the inverted layer 41 is rarely affected by the injected carrier. In this manner, according to the semiconductor laser in the fourth ebodiment, the number of defects in crystal involved in the inverted layer 41 is minimized to thereby reduce the effect by the defect in crystal. As the result, the reliability increases.

Next, in Fig. 5 and 6, a p-type inverted layer 51 is diffused and formed leaving a noninverted portion 52 in the neightbourhood of both longitudinal ends (namely, crystal ends in the form of a reflected mirror face) of the active layer 24.

It is known that generally, where a semiconductor material is the same, if the N-type is employed as the conductive type rather than the P-type, an effective band gap is large and light absorption is lesser. Thus, the light absorption at the non-inverted portion 52 is lesser than that shown in Fig. 1, and therefore, the breakdown limit of the crystal end increases.

That is, the semiconductor laser in accordance with the fifth embodiment is possible to obtain greater output light than the semiconductor laser in accordance with the first embodiment.

Finally, Fig. 7 shows a semiconductor laser in accordance with a sixth embodiment of the present invention. This semiconductor laser operates as a photobranch element by modifying in function of confining light in the active layer and in method of setting a bias in the structure of the semiconductor laser shown in Fig. 2. Then, in this semiconductor laser, the same reference numerals as those used in the semiconductor laser shown in Fig. 2 are used.

That is, this semiconductor laser is designated so that an optical wave is shifted from one to the other between the adjacent PN junction portions (10a, lOb), (lOb, 10c) and (10c, lOd). More specifically, this can be achived by creating small differential in refractive index between the active layers 5a, 5b, Sc and 5d and the semiconductor layers 4b, 4c 4d disposed therebetween or by making these layers thinner. More effectively, both the procedures are simultaneously effected.

The method of setting bias is as described in the following explanation of operation.

In the semiconductor laser constructed as described above, a suitable forward bias is applied beforehand to the subject element to set each of the PN junction portions 1 Oa, 1 Ob, 10c and 10d to a value slightly smaller than the threshold value of laser oscillation. A laser beam is incident on one of the PN junction portions 10a, lOb, 10c and 1 Od, for example, PN junction portion 10b, from one crystal end. Then, a part of the laser beam incident on the PN junction portion 1 0b is propagated as well other PN junction portions 1 Oa, 1 0c and 1 Od.At this time, the energy of the incident laser light is made to be equal to the energy required for laser oscillation by each of the PN junction portions 1 0a, 10b, 10c and 1 Od, and therefore, the PN junction portions 1 0a, 1 0b, 10c and 1 Od provide laser oscillation by photo-excitation so that the laser beam is released from the crystal ends 11 a, llb, ill and lid.

Thus, the semiconductor laser in this embodiment can be used as the photobranch element, and an example is shown in Fig. 8.

In this figure, a photobranch element 81 comprising the semiconductor laser in this embodiment branches injection light of a semiconductor laser (light source) 83 guided by a photoguidewave passage 82 into four portions, the branched beam being guided by guide-wave passages a, b, c and d, respectively. For example, one branched beam is injected into an optical fiber 85 connected to the side end of an optical IC substrate 84 whereas other branched beams are injected into various circuits (not shown) such as optical operational circuits 86, 87.

While in the above-described embodiments, the semi-conductor lasers consisted of the GaAs system compound semiconductors have been described, it should be noted of course that the invention is not limited thereto but it can be likewise applied to semiconductor lasers consisted of other compound semiconductors.

Also, As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments set forth herein.

Claims

1. A semiconductor laser, comprising a semiconductor crystal comprising a double heterojunction construction formed of laminated layers of a semiconductor of a first conductive type and comprising stepped portion having offset portion formed therebetween, wherein said offset portion contain active layer consituting said double heterojunction construction which is formed with PN junction portion which provides laser oscillation, said PN junction portion being formed from an inverted layer made by diffusing a conductive member of a second conductive type in a region from surface of the semiconductor crystal to said offset portion.

2. The semiconductor laser of Claim 1, wherein said semiconductor crystal is formed with plurality of laminated double heterojunction constructions.

3. The semiconductor laser of Claims 1 and 2, wherein the semiconductor layers of the first conductive type is an N-type semiconductor and wherein the conductive type of the second type is of a P-type.

4. The semiconductor laser of Claims 1 and 2, wherein the inverted layer is formed in the whole region from the surface of said semiconductor crystal to said offset portion.

5. The semiconductor laser of Claims 1 and 2, wherein the inverted layer is formed in the form of a web in the region from the surface of the semiconductor crystal to said offset portion, and the width of said web is slightly greater than the transverse width of said active layer.

6. The semiconductor laser of Calims 1 and 2, wherein the inverted layer has a noninverted portion in the neighbourhood of ends on both longitudinal sides of said active layer.

7. The semionductor laser of Claim 3, wherein said P-type conductive member is zinc.

8. The semiconductor laser of Claim 2, further comprising carrier injection electrodes formed on both surfaces of said semiconductor crystal, at one of said electrodes comprising a plurality of electrodes formed as parallel stripes at predetermined intervals.

9. The semiconductor laser of Claim 2, wherein the PN junction portions are designed so that a photowave is shifted from one PN junction to an adjacent PN junction portions and the carrier injection electrode have applied thereto a voltage for initially setting the PN junction portions to a state slightly below the laser oscillation threshold valaue, and further comprising a laser beam is adapted to be described upon one of said PN junction portion from the outside of said semiconductor crystal to photoexcite the PN junction portions to create laser oscillation.