SE436670B - Semiconductor laser - Google Patents

Description

15 20 25 30 HO '7905635' lr Various semiconductor laser structures have been developed in an attempt to achieve a stable, single mode optical beam. Originally, the current across the transition was limited to a central part thereof due to the fact that one of its contacts had the shape of a narrow strip along the surface of the body. However, this method is not entirely satisfactory for achieving a single-mode beam. Various attempts have been made to form embedded narrow active areas in order to achieve a single-mode output signal. As shown in U.S. Pat. No. 3,978 H28, one way of forming an embedded active region is to create a cut in a substrate and precipitate the active region epitaxially in that cut. Another method is shown in U.S. Pat. No. 3,983,510, in which portions of the body are removed and restricted areas are deposited on either side of the remaining active area. However, these methods are not fully satisfactory and are difficult to implement. In an article by D.

Motez et al. entitled "Constricted Double Heterostructure (AlGa) As Diode Lasers" in "Applied Physics Letters" 32 (1978): H, p. 261-263, describes a semiconductor laser which produces a single-mode light beam. This laser includes a substrate whose surface is provided with a zinc-shaped insert, the areas of the laser being formed by epitaxial precipitation in the liquid phase in the grooves over their edges and on the surface of the substrate next to the groove. This gives an active area that has a narrow area with minimal thickness next to one edge of the notch. A single-mode light beam is generated in said minimal thickness portion. However, this method has the disadvantage that the thin portion with minimal thickness is located differently in different devices, which will cause the light to emerge from different parts of the light emitting end surface of the device for different devices. This also makes it difficult to know where the contact should be placed on the surface of the device in order for optimal current flow to occur over the active area.

The present invention relates to an improvement of the known semiconductor lasers. A laser constructed in accordance with the invention comprises a semiconductor body having a substrate having a pair of spaced apart, substantially parallel, zinc-shaped grooves in a surface of the substrate.

A first epitaxial layer is located over the surface of the substrate and the surfaces of the grooves. A thin second epitaxial layer has a part of uniform thickness over the substrate surface part which lies between the grooves. A third epitaxial layer is located above the second epitaxial layer. The first and third epitaxial layers are of mutually opposite conductivity types, and the second epitaxial layer constitutes the active recombine of the laser- -P fi- w, .ar-e years UC 10 | u 15 25 30 7905635-4, whereby the light is generated in the part with the smallest thickness and in the immediate vicinity thereof.

The invention is described in more detail below with reference to the accompanying drawing. §Ig¿_l is a perspective view of a semiconductor laser made in accordance with the present invention. Pig. 2 is a cross-sectional view illustrating the formation of the substrate of the laser diode. Pig. 3 is a cross-sectional view illustrating the deposition of the first epitaxial layer on the substrate. §Ig¿_ï is an enlarged cross-section of a part of the laser diode and illustrates the formation of the active layer of the laser diode.

Pig. 5 is a cross-sectional view of a portion of the laser diode showing a type of protective layer obtainable. Pig. Fig. 6 is a cross-section similar to that of Fig. 5 but showing another type of protective layer. Pig. 7 is a cross-sectional view of another type of semiconductor laser made in accordance with the invention.

I tig. 1 shows a laser diode 10 constructed in accordance with the invention. The laser diode 10 has a body of single crystalline semiconductor material, usually compounds of group III-V or alloys thereof, with a parallelepipedic shape. The body 12 has spaced, parallel end surfaces 1%, which are light-reflecting, however, at least one is partially transparent so that light can be emitted from it. The body 12 is also provided with spaced, parallel side surfaces 16, which extend between the end surfaces 1k and are perpendicular thereto.

The body 12 includes a substrate 18 of a conductor type, for example N-conductive, with spaced, parallel surfaces 20 and 22, which extend between both the end surfaces 1U and the side surfaces 16 and are perpendicular to all these surfaces. In the substrate surface 20 there are a pair of spaced apart, parallel, zinc-shaped grooves 2H, which extend between the end surfaces 1U of the body 12. The upper corners of the notches are removed so that a portion of each side of the notch adjacent the surface 20 increases at a mutual distance. A first epitaxial layer 26 partially fills each of the grooves 24 and extends over the portion 20a of the surface 20 between the grooves 2H, as well as over portions of both sides around the grooves. This first epitaxial layer 26 is of the same conductivity type as the substrate 18. A second epitaxial layer 28 extends over the first epitaxial layer 26. The second epitaxial layer 28 is thin and has a portion 28a of uniform thickness directly above the surface portion 20a.

A third epitaxial layer 30 is located on top of the second epitaxial layer β8 and has the opposite conductivity type compared to the first epitaxial layer QE. Thus, if the first epitaxial layer is N-conductive, the third epitaxial layer is P-conductive. A fourth epitaxial layer 32 of the same conduit type as the third epitaxial layer 30 is located on top of this third layer. A layer 3% of electrically insulating material, such as silica, is located on top of the fourth epitaxial layer 32 and reaches an opening in the form of a strip extending over and along the surface portion 20a of the substrate 18, which as mentioned lies between the grooves ZH. A contact 36 of conductive material is located on top of the insulating layer 34 and extends over the opening of the insulating layer for the purpose of ohmically contacting the surface of the fourth epitaxial layer 32. On the surface 22 of the substrate 18 there is also a contact 3% of conductive metal.

The second epitaxial layer 28 forms the active recombination region of the laser diode 10. Although this second epitaxial layer 28 may be of any conduit type, it preferably consists of a material that is not intentionally doped. The first and third epitaxial layers 26 and 30 are of a material having a slightly smaller refractive index than the material of the second epitaxial layer 28, whereby the second epitaxial layer 28 will form a heterogeneous transition with both the first and third epitaxial layers 26 and 30, respectively. These heterogeneous transitions serve to retain the light generated within the second epitaxial layer 28 within this layer 28. If, for example, the second epitaxial layer 28 is of gallium arsenide, both the first and third epitaxial layers 25 and 30, respectively, may consist of aluminum gallium arsenide. The second epitaxial layer 28 can also be made of aluminum gallium arsenide, but with a lower aluminum content than the first and third layers 26 and 30. The substrate 18 must consist of a material which is generally available in substrate form and on which the first epitaxial layer 26 can be easily grown. If the first epitaxial layer 26 consists of aluminum gallium arsenide, the substrate 18 may be gallium arsenide. The fourth epitaxial layer 32 is a protective layer intended to protect the third epitaxial layer 30. If the third epitaxial layer 30 is of aluminum gallium arsenide, the fourth epitaxial layer 32 may be of gallium arsenide.

The laser diode is manufactured by starting from a substrate 1u of the desired semiconductor material and the desired cable type, for example N-conductive gallium arsenide. The substrate has a surface 20 which is approximately parallel to the crystal plane (100) and slightly misoriented (up to 30) along the (011) crystal direction with respect to the (100) crystal plane. A masking layer HO, for example silica, is coated over the substrate surface 20. The silica layer 40 can be precipitated by the well known method of pyrolytically decomposing a silicon-containing gas, such as silane, into oxygen and water vapor. A pair ät- -1 -jwíï * r ON ': W 1 l _ .. _ * -.___. _.__ __ __ 10 15 20 25 30 Ni) 790563541 UT separate, parallel, strip-like openings U? and now provided in the masking layer HO. This can be achieved by coating the layer H0 with a photoresist layer, the latter layer being provided with apertures corresponding to the apertures H2 and HN using conventional photolithographic methods. The exposed portions of the masking layer H0 are then removed with any suitable etchant, such as buffered HF, to form the openings H2 and HH. The potoresist layer is then removed with the aid of any suitable solvent. The openings H? and HH should be arranged so as to extend parallel to the direction of the substrate (011). For reasons which will appear below, it is preferable that each opening H2 and HH is about H-6 μm wide and that the distance between the centers of the two openings amounts to 20-H5 μm. parts of the substrate surface 20 which are exposed via the openings H2 and HH are then contacted with an etchant which selectively etches (100) and (111) -oriented surfaces, respectively. The etchant may, for example, consist of 1 part of sulfuric acid, 8 parts of water and 8 parts of hydrogen peroxide.

As shown in Fig. 2, this causes zinc-shaped grooves 2h to form: with a depth of approximately N μm and with the (100) and (111) A-planes bottoming each side. The sides of the grooves 2H will be etched inwards below the masking layer HO so that - starting with the openings H2 and RH, which are between H and 6 μm wide - the grooves will be 10-12 μm wide at their upper edges when channels with a depth of approx. for U pm desired. The silica masking layer H0 is then removed with buffered HF.

The surface 20 of the substrate 1U is then cleaned to ensure good epitaxial culture of the different epitaxial layers of the laser diode 10. This can be achieved by light etching at 3000 for about 1 minute of the surface with a 1 to ~ 1 mixture of anhydrous sodium hydroxide (8 grams of sodium hydroxide to 200 ml of water) and anhydrous hydrogen peroxide (20 ml of 30% hydrogen peroxide to 200 ml of water). The substrate is then placed in hot hydrofluoric acid to remove the oxides which may have formed on the surface 20, after which renewed light etching with the mixture just described takes place at 300 for 1/2 minute. The substrate is then rinsed in water and dipped in isopropyl alcohol. Thereafter, the substrate is ready for application of the various epitaxial layers.

The various epitaxial layers are deposited on the substrate 14 by the well-known liquid phase epitaxial method using the precipitating apparatus described in U.S. Pat. No. 3,753 B01.

Rerw generally this apparatus consists of an oven vessel provided with a separate wells, one for each epitaxial layer to be deposited and a slider movable longitudinally through the vessel and over each well bottom. The sliding member is provided in its surface with a pair of separate slits which extend into each of the wells. One of these slots is intended to accommodate an exit disc, while the other slot is designed to support the substrate on which the epitaxial layers are to be deposited.

In each well there is a batch of the semiconductor material to be precipitated, a solvent for this material and a line type modifier to the extent that such is to be used. In each well there is a weight for spreading the precipitation solution over the entire well surface.

If desired, an output disk can be arranged between the weight and the batch. In the manufacture of the laser diode 10, it is preferred that the first well be provided with an output disk between the weight and Sä fi fšcllo. In the manufacture of the laser diode 10, a ship with at least four sources is used. The first source contains a batch of gallium arsenide and aluminum arsenide as semiconductor material, gallium as solvent and tin as a modifier for N-wire, and a starting disc of gallium arsenide is located above the batch. The second well contains a batch of gallium arsenide and possibly aluminum arsenide as semiconductor material and gallium as solvent but no conductor type modifier. The third well contains a batch of gallium arsenide and aluminum arsenide as semiconductor material, gallium as solvent and germanium as a modifier for P-line. The output disk in the slot in the slider is constituted by gallium arsenide. The substrate 1U with the grooves 2h arranged therein is arranged in the correct slot in the sliding member so that the grooves 2G lie parallel to the longitudinal movement of the sliding member.

The furnace vessel with batches and discs arranged therein is placed in a furnace tube, and a stream of highly purified hydrogen is passed through the furnace tube and over the vessel. The furnace tube is heated to a temperature between 8200 and 8600. This causes the solvent to melt and the semiconductor material and line type modifiers to dissolve in the molten solvent. This temperature is maintained long enough to ensure complete melting and homogenization of the batches of ingredients. The furnace vessel and its contents are then allowed to cool to about 3-500, and the slider is moved to introduce the substrate 1u directly into the solution of the first well.

The output disk of the slider passes through the first source without stopping and enters the second well when the substrate is introduced into the first source. The starting disk between the weight and the solvent in the first source is used to saturate the solution in the first source. When the surface 20 of the substrate 1h is brought into contact with the solution of the first well, a partial melting occurs beyond the upper corner of the inserts 2u as shown in Fig. 3. The oven vessel and its contents are then cooled rather rapidly, at least at 1 ° C per minute.

This causes a portion of the semiconductor material in the first well solution, as well as a portion of the lead type modifier, to fill out of the solution and deposit in the inserts ZH and on the substrate 1H surface 20 to form the first epitaxial layer 26, see Fig. 3. for the precipitation of the first epitaxial layer 26 should be such that the thickness do midway between the grooves 2H amounts to 0.8 - ~ 2 μm, while the thickness d1 at the edges of the grooves 2H should be between 0.3 and 1.0 μm. It is hereby preferred that the thickness d1 is within the lower part of the just mentioned area, since this gives greater stability to the basic mode. Furthermore, the portion 26a of the epitaxial layer 26 over the substrate surface portion 20 between the grooves 2N should be substantially planar over a width wo of about H-15 μm.

While the first epitaxial layer 26 is deposited on the substrate 14, the output disk provided on the slider is in the second well and saturates the solution therein. After the first epitaxial layer 26 is deposited with the desired thickness on the substrate 14, the slider moves so that the substrate is introduced into the second source, where the first epitaxial layer 26 is brought into contact with the solution in the second well. The furnace vessel and its contents are further cooled to bring a portion of the semiconductor material solution in the second source to precipitate for deposition on the first epitaxial layer 26 to form the second epitaxial layer 28. The precipitating material of the semiconductor material from the second source solution occurs only for a short period of time. , so that the part of the second epitaxial layer, which lies on the surface part 20a of the substrate 18 between the grooves 24, becomes very thin, preferably between 0.5 and 0.3 μm. It is known that the precipitation rate of the liquid phase epithelium also varies with the surface curvature. The greater the positive curvature (i.e. concavity), the faster the precipitation rate. Epitaxial culture preferably takes place at the bottom of the two wells formed by the upper surface of the first epitaxial layer 26 above the two notches 2U. On the other hand, above the flat portion of the upper surface of the first epitaxial layer 26, which is located above the substrate surface portion 20a, the culture will be reduced as a result of the relatively strong negative curvature of this surface, and as a result. fw- W, ..._..-_- .. __......., _ .., _, _______._____, __ U ~ PJ- S 25 30 790563544 of lateral mass transfer of cultured material to areas with greater growth (eg towards the two concave parts of the first epitaxial layer? 6 nvanyta above the notches 24). Thus, as shown in Fig. H, the portion 28a of the second epitaxial layer 28 above the substrate: surface portion 20a will have a uniform thickness over an area of width wo and gradually roll in thickness as the second epitaxial layer 28 passes around the corners of the inserts. In order to obtain a second epitaxial layer 28 of small thickness, it is advantageous to use a small volume solution, as described in U.S. Pat. No. 3,753,801.

While the second epitaxial layer 28 is deposited on the first epitaxial layer 26, the output disk of the slider is in the third source, where it saturates the solution with semiconductor material. The slider is then moved into the third well for the purpose of contacting the second epitaxial layer 28 with the solution of the third well.

The temperature of the furnace vessel and its contents are further cooled to precipitate from the third source solution a portion of the semiconductor material and the conductivity type modifier. This forms the third epitaxial layer 30. This third epitaxial layer 30 is deposited for a sufficiently long time for the minimum thickness of the layer above the substrate surface 70 lying between the grooves 24 to be between 0.9 and 2 [mu] m. The slider is moved again to bring the substrate with epitaxic layers thereon into the fourth well, where the solution contained therein has been measured by the output disk. The furnace vessel and its contents are further cooled to precipitate a portion of the fourth well solution semiconductor material and line type modifier to form the fourth epitaxial layer 32. This fourth epitaxial layer 32 preferably has a minimum thickness between 0.2 and 1.0. pm.

We have found that the surface contour of the fourth epitaxial layer 32 will vary with the thickness of this layer. If the third and fourth epitaxial layers 30 een 32 have the above-mentioned minimum thicknesses, the surface of the fourth epitaxial layer 32 will have a slight elevation directly above that of the substrate surface 20 which lies between the notches 2%, see Fig. 5. However, the minimum thicknesses of the third and fourth seals 30 and 3? has the maximum values given above, will the fourth epitaxial layer 3? surface to have a depression just above the part of the substrate surface 20 which lies between the grooves 2%, see Fig. 6. In both cases there is a visual indication of the position of the part of the second epitaxial layer 8 which has a uniform thickness in order to enable easy location of the place where the belt contact is to be applied. -wuWv-f MON !! If the substrate 1% with the epitaxial layers applied thereto is removed from the topsheet, the 3% silica layer will be deposited over the fourth layer 32. This is preferably done by pyrolytic sintering. of siliceous gas such as silane, in oxygen or water vapor. A strip-shaped opening is made in the silica layer 3H directly above the elevation or depression in the surface of the fourth epithelial layer 32. As already mentioned, this opening can be achieved by applying a photoresist layer over the silica layer except at the place where the opening is to be formed, after which etching of the exposed parts of the silica layer takes place.

Thereafter, the metal film 36 is coated over the silica layer and the exposed portion of the surface of the fourth epitaxial layer 32. This is usually done by the well-known vacuum evaporation method. Using the same method, the surface 22 of the substrate 22 is coated with the me »1a-L1 layer 9%.

During the operation of the semiconductor laser 10, a threshold current through the laser 10 between the contacts Bß and 38 causes electrons and holes to be injected into the thin active region, i.e. the second epitaxial layer 28, where the holes and electrons recombine and thereby cause the generation of light. Laser effect occurs in the part of the active area of uniform thickness which lies directly above the flat surface part 26a of the first epitaxial layer 26, which is located above the substrate surface 20a between the grooves ZH. Furthermore, the light emitted from the semiconductor laser 10 is of a single-mode type both spatially (transversely and laterally) and longitudinally (ie to the frequency). This one-mode effect is achieved even when the laser is operating continuously (CW), as well as at room temperature or at 70 ° C. This laser action is achieved due to the thin waveguide formed by the two heterogeneous transitions between the second epitaxial layer 28 and each of the first and third epitaxial layers 2u and 30. The width of the laser area is mainly determined by a non-waveguide effect, as in some degree of a counter-amplifier selectivity effect thereby achieved by! the epitaxial layer 28 changes in thickness in both directions near the part with the smallest thickness. Although the misalignment of the substrate surface 20 has been stated as parallel to the longitudinal axis of the needle 2H, it may be desirable for the direction of the cell orientation to have an angle of between # 50 and 135G with respect to the longitudinal axis of the notch 2H. As stated above, a mean 2H parallel misorientation means that the portion of the second epithelial layer 28 which lies above the first epitaxial layer 26 "oven w" an "wav-u w-.u" ~ 10 15 20 raw (71 As shown in Fig. 7, however, a non-parallel orientation with the notch 12k causes the part of the epitaxial layer 128 part to be symmetrical, ie the slope becomes substantially uniform on either side of the area of uniform thickness. 128a, located above the flat portion of the first epitaxial layer 126, does not become symmetrical because the slope at one side of the portion 128a will be greater than at its other side.This asymmetry is sufficient to affect the operation of the semiconductor laser when the error orientation angle is at least H50 with The effect of this symmetry is that the point of light generated in the second epitaxial layer 128 becomes larger than the point of light generated in the symmetrical layer.Even the said part of the second epitaxial layer. is symmetrical or asymmetrical, however, the laser action will take place in this area immediately above the flat surface of the first epitaxial layer.

The semiconductor laser 10 has a number of advantages over other constructions in which one strives to achieve a single-mode light beam.

As for the various semiconductor lasers that have restriction areas on both sides of the active area, ie. lasers of the type shown in U.S. Pat. No. 3,983,510, the semiconductor laser 10 embodied in the present invention is much simpler in manufacture, since the various epitaxial layers are simply deposited in turn on the substrate, and the refined active region is formed. automatically. Compared to a semiconductor laser, in which the epitaxial layers are grown over a single zinc-shaped insert with the active area along one edge of the notch, the laser 10 according to the present invention has the advantage that the generated light beam is consistently located above the substrate surface 20 between the notches 2%. In addition, the position of the belt contact can be accurately determined by the shape of the surface of the fourth epitaxial layer, the contact being able to be applied directly above the thinnest part of the active area. This is desirable to ensure complete current passage through the active area where laser action occurs. Thus, the semiconductor laser 10 constructed in accordance with the present invention is easier to fabricate than the diode with the active region formed along a single zinc-shaped cutter while ensuring the correct placement of the active region and the strip contact.

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Claims (7)

1 n 7905635-h Patent Claim A semiconductor laser (10) comprising on the one hand a semiconductor material body (12) with a substrate (18) provided in a surface (20) with a zinc-shaped insert (24), and on the other hand a first epitaxial layer (26) over said substrate surface ( 20), partly a thin, second epitaxial layer (28) located on this shaped epitaxial layer (26), partly a third epitaxial layer (30) located on the second epitaxial layer (28), and two contacts (36, located on said body (12) 38), which are arranged over the third epitaxial layer (30) in resp. on the substrate (18), the first and third cpitax layers (26 and 30, respectively) being of opposite conductor types, may be characterized in that a second zinc-shaped cutting (24) is present in said substrate surface (20) and thereby is located parallel to the first-mentioned groove (24) at a distance from it; that the first epitaxial layer (26) has a flat surface portion (28a) above the part of the substrate surface (Z0a) which lies between the grooves (24)) and that the second epitaxial layer (28) constitutes the active recombination region of the laser (10), the light being generated in the second epitaxial layer (28) in the vicinity of the part thereof which lies above the flat surface portion (28a) of the first epitaxial layer (26). Z. Semiconductor laser according to claim 1, characterized in that the second epitaxial layer (28) has portions which change in thickness from a part of this layer which has a uniform thickness and is located directly above the flat surface portion (26) of the first epitaxial layer (26). 26a). 3. A semiconductor laser as claimed in Claim 2, characterized in that the second epitaxial layer (28) increases in thickness 1 in both directions: from the equally thick part. Semiconductor lasers according to Claim 3, characterized in that the thickness-changing parts are symmetrical. Hnlvlcdnrlaser according to claim 3, characterized in that the thickness-bearing parts are asymmetrical. , Semiconductor laser according to claim Z, characterized in that the corners of both grooves (24) are removed so that a part of the sides of each groove extends from each other next to said surface (20). A semiconductor laser according to claim 2 and provided with a fourth epitnx layer (32), which is housed on the third epitnx layer.