JP2007103790A - High output red semiconductor laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high output red semiconductor laser in which FFP variation is suppressed by decreasing the effective difference of refractive index appropriately in the lateral direction within an active layer while sustaining the band gap of a current block layer higher than that of the active layer. <P>SOLUTION: An n-AlGaInP clad layer 3, an AlGaInP light guide layer 4, an MQW active layer 5, an AlGaInP light guide layer 6, a p-AlGaInP first clad layer 7, an AlGaInP etching stop layer 8, an n-AlGaInP block layer 11, a p-AlGaInP second clad layer 9, a p-GaInP buffer layer 10, and a p-electrode 12 are formed on an inclining n-GaAs substrate 2, and an n-electrode 1 is formed on the back of the n-GaAs substrate 2. The MQW active layer 5 principally comprises GaInP, and the block layer 11 is composed of (Al<SB>X</SB>Ga<SB>1-X</SB>)<SB>0.5</SB>In<SB>0.5</SB>P (0.7<X<1). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a high-power red semiconductor laser used for a DVD or the like.

  With the maturation of the recordable DVD market, AlGaInP red semiconductor lasers with a wavelength of 650 nm band are required to have a high output exceeding 250 mW in order to write at a high speed.

  A general structure of this red semiconductor laser is shown in FIG. An n-GaAs substrate 32 and a semiconductor stacked structure grown thereon are provided. This semiconductor stacked structure includes an n-AlGaInP cladding layer 33, an MQW active layer 34, a p-AlGaInP first cladding layer 35, a p-GaInP etching stop layer 36, an n-GaAs block layer 37, a p-AlGaInP layer in order from the substrate side. 2 clad layer 38, p-GaInP buffer layer 39, and p-GaAs cap layer 40. An n electrode 31 is formed on the lower surface of the n-GaAs substrate 32, and a p electrode 41 is formed on the upper surface of the p-GaAs cap layer 40.

  In the red semiconductor laser shown in FIG. 2, a stripe-shaped ridge portion A is formed by the second cladding layer 38 and the buffer layer 39, and n-GaAs block layers 37 are arranged on both sides of the ridge portion A, and p-GaInP. It has a buried ridge structure in which the buffer layer 39 and the n-GaAs block layer 37 are covered with a p-GaAs cap layer 40.

  The ridge portion A is transparent to the oscillation wavelength, but confines light in the horizontal direction by the light absorption action of the n-GaAs blocking layer 37 disposed on the side surface of the ridge. The current does not flow in the n-GaAs block layer 37 serving as a reverse bias and the lower portion thereof, but flows in the striped ridge portion A.

  Further, when the p-GaAs cap layer 40 and the p-AlGaInP second cladding layer 38 are directly joined, the band offset on the valence band side is large, so that a large barrier against holes that are carriers in the p-side region is present. It can be near the junction interface, hindering the flow of holes and making it difficult for current to flow. In order to prevent this, a p-GaInP buffer layer 39 having a band gap between the p-GaAs cap layer 40 and the p-AlGaInP second cladding layer 38 is sandwiched between the p-GaAs cap layer 40 and the barrier formed at the junction interface. This facilitates the flow of holes.

  When energized between the p-electrode 41 and the n-electrode 31, the current is confined by the n-GaAs block layer 37 which is a current blocking layer, and light is emitted from the central portion of the MQW active layer 34 corresponding to the lower position of the ridge portion A. Is obtained.

  By the way, in the red semiconductor laser of FIG. 2, since the band gap of the n-GaAs block layer 37 is smaller than the band gap of the MQW active layer 34, the light is absorbed by the n-GaAs block layer 37, Waveguide loss increases, making it difficult to increase output.

  In order to prevent the block layer 37 from absorbing light, an actual refractive index waveguide structure using an n-AlInP block layer having a larger band gap than the MQW active layer 34 instead of the n-GaAs block layer 37 is introduced. Proposals have been made to reduce the waveguide loss and enable laser oscillation with a low threshold current and high output.

FIG. 3A shows a stacked structure from the MQW active layer 34 to the ridge portion A when the n-AlInP block layer 42 is used instead of the n-GaAs block layer 37. The stripe width W represents the width of the lower side of the striped ridge portion A, that is, the width of the lower boundary surface of the second cladding layer 38. FIG. 3B shows a change in the refractive index in the horizontal direction (lateral direction) in the MQW active layer 34 in the stacked structure of FIG. The horizontal direction indicates the range at the interface between the n-AlInP blocking layer 42 and the p-AlGaInP second cladding layer 38 and the p-GaInP etching stop layer 36. A range corresponding to the stripe width W is B, and a range corresponding to the block layers 42 disposed on both sides of the second cladding layer 38 is C.
JP-A-9-205249

  Although the conventional red semiconductor laser shown in FIG. 3 can achieve high output, the transverse mode representing the optical waveguide state in the laser structure becomes unstable as the optical output increases, and the transverse mode is changed. Problem occurs.

Variations in the transverse mode have a great influence on the far field image (FFP: Far Field Pattern). FIG. 4 shows the state of FFP fluctuation, where the full width at half maximum (corner) of the FFP in the horizontal direction is θ _H , and the full width at half maximum (corner) of the FFP in the vertical direction (stacking direction) is θ _V. The distribution state of the radiation angle is shown. The shaded area represents the variation (variation) due to the production lot of the same type of semiconductor laser element.

  As can be seen from this figure, in the AlGaInP laser, the light output has a large influence on the FFP horizontal radiation angle, and the FFP horizontal radiation angle is about 2 degrees when the output is 250 mW than when the output is 5 mW. It gets bigger. If the horizontal radiation angle fluctuates greatly, the change in the magnitude of the light entering the lens becomes large, making it difficult to use it for an optical pickup laser or the like.

  The horizontal radiation angle of the FFP increases because the refractive index difference Δt between the n-AlInP blocking layer 42 and the MQW active layer 34 shown in FIG. 3A is large, and the MQW activity shown in FIG. The effective refractive index difference Δn between the stripe portion B in the lateral direction of the layer 34 and its both side portions C is increased, so that light is easily concentrated on the stripe portion B of the active layer, and a near-field image (NFP: Near Field Pattern) This is because becomes smaller.

  Further, the horizontal radiation angle varies depending on the individual laser elements, and there is a range of a certain variation in the production lot, and power fluctuations are added, so this variation in the horizontal direction radiation angle is suppressed within a certain range. It ’s difficult. One of the causes of lateral mode fluctuations is due to the occurrence of kinks due to hole burning or transition to higher-order modes. The kinks due to transition to higher-order modes are based on higher-order modes other than the fundamental transverse mode. Therefore, as a method for cutting off this higher-order mode, it is necessary to narrow the stripe width W of the optical waveguide or to reduce the effective refractive index difference Δn.

Since the refractive index difference Δt between the AlInP used in the block layer 42 and the MQW active layer 34 is large, the effective refractive index difference Δn becomes large. In this case, a higher order other than the fundamental transverse mode in the optical waveguide to cut mode, it is necessary to reduce the stripe width W, the horizontal radiation angle theta _H of the stripe width W is too small FFP is considerably larger. Also, since the ridge shape is trapezoidal, if the width W at the bottom of the ridge is narrowed, the width at the top of the ridge becomes too narrow and the contact becomes unstable or the contact cannot be removed. It causes deterioration of responsiveness. Therefore, the maximum stripe width W that can cut off the higher-order mode is formed.

However, since the stripe width W is formed by photolithography and etching, a relatively large error occurs in the formation of the stripe width W. If the stripe width W is different for each manufactured laser element, the higher order mode may not be cut off, theta _H of FFP is unstable, generating a variation such as the hatched portion in FIG.

  The present invention was devised to solve the above-described problems, and while maintaining the band gap of the current blocking layer higher than that of the active layer, the lateral effective refractive index difference in the active layer is appropriately reduced. An object of the present invention is to provide a high-power red semiconductor laser with small FFP fluctuations.

In order to achieve the above object, an invention according to claim 1 is provided with at least an n-type cladding layer, an active layer containing GaInP as a component, and a p-type cladding layer in this order on an n-type semiconductor substrate. In an AlGaInP-based high-power red semiconductor laser having a striped ridge portion including the p-type cladding layer on the upper side, the side surface of the ridge portion has a band gap larger than that of the active layer and has a refractive index higher than that of the p-type cladding layer. Embedded in a small n-type (Al _X Ga _1-X ) _0.5 In _0.5 P layer (0.7 <X <1).

  The invention according to claim 2 is the high-power red semiconductor laser according to claim 1, wherein the p-type cladding layer is composed of a semiconductor layer containing AlGaInP as a component.

  According to the present invention, the current block layer has a band gap higher than that of the active layer so that the current block layer does not absorb light, and the effective refractive index in which the lateral effective refractive index difference in the active layer is appropriately reduced. Since the index waveguide structure is used, the output can be increased and the FFP fluctuation can be reduced.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a cross-sectional structure of a high-power red semiconductor laser according to the present invention.

On the inclined n-GaAs substrate 2, an n-AlGaInP cladding layer 3, an AlGaInP light guide layer 4, an MQW active layer 5, an AlGaInP light guide layer 6, a p-AlGaInP first cladding layer 7, a p-AlGaInP etching stop layer 8, A p-AlGaInP second cladding layer 9, a p-GaInP buffer layer 10, an n-AlGaInP block layer 11, a p-GaAs cap layer 12, and a p-electrode 13 are stacked, and an n-electrode 1 is disposed on the back side of the n-GaAs substrate 2. Is formed. As the n-GaAs substrate 2, a substrate whose crystal orientation is inclined by 10 to 15 degrees from (001) is used. Incidentally, AlGaInP layer lattice matched on the inclined n-GaAs substrate _2, to obtain a structure that _{(Al Z Ga 1-Z)} 0.5 In 0.5 P (0 <Z ≦ 1).

The n-AlGaInP block layer 11 is composed of n-type impurity Si-doped (Al _X Ga _1-X ) _0.5 In _0.5 P (0.7 <X <1). 8 to 0.9 is desirable, and in this example, it was set to 0.8. The MQW active layer 5 is formed of three GaInP well layers and two undoped (Al _0.5 Ga _0.5 ) _0.5 In _0.5 P barrier layers.

For the other layers, the n-AlGaInP cladding layer 3 is an n-type impurity Si-doped (Al _0.7 Ga _0.3 ) _0.5 In _0.5 P, AlGaInP light guide layer 4 and AlGaInP light guide layer 6. The undoped (Al _0.5 Ga _0.5 ) _0.5 In _0.5 P, p-AlGaInP first cladding layer 7 is made of p-type impurity Zn-doped (Al _0.7 Ga _0.3 ) _0.5 In _{The 0.5} P, AlGaInP etching stop layer 8 has three layers of _unstrained (Al _0.1 Ga _0.9 ) _0.5 In _0.5 P doped with p-type impurity Zn and (Al _A layer obtained by alternately stacking two layers of _0.4 Ga _0.6 ) _0.5 In _0.5 P, and the p-AlGaInP second cladding layer 9 is doped with p-type impurity Zn (Al _0.7 Ga _{0.3) .5} In _0.5 P, p-GaInP buffer layer 10 of p-type impurity Zn-doped GaInP, n-AlGaInP block layer 11 is of n-type impurity Si-doped _{(Al 0.8} Ga _0.2) 0.5 In _{The 0.5} P, p-GaAs cap layer 12 is made of p-type impurity Zn-doped GaAs. The p electrode 13 is a multilayer metal film of Ti and Au, and the n electrode 1 is an alloy layer of Au, Ge, Ni, and a multilayer metal film of Ti and Au.

  The MQW active layer 5 is sandwiched between AlGaInP light guide layers 4 and 6 from both sides. These light guide layers are formed to confine light in the vertical direction, and the vertical spread angle can be controlled by the composition and thickness of the light guide layer. When this vertical light confinement is weakened, the light emission spot expands in the vertical direction, and the vertical spread angle of the outgoing beam (the size in the FFP stacking direction) is reduced.

  In the high-power red semiconductor laser shown in FIG. 1, a striped ridge portion D is formed by a p-AlGaInP second cladding layer 9 and a p-GaInP buffer layer 10, and both sides of the ridge portion D are n-AlGaInP block. It has a buried ridge structure covered with a layer 11. The current does not flow in the n-AlGaInP block layer 11 serving as a reverse bias and the lower portion thereof, but flows in the striped ridge portion D.

  The manufacturing method is performed as follows by a known MOCVD method, a photolithography technique, or the like. In addition, although the suitable film thickness of each layer changes with the composition ratios of semiconductor materials, etc., in this example, it formed as follows based on the above-mentioned composition ratio of each layer.

An n-AlGaInP cladding layer 3 having a thickness of 1 to 3.5 μm, preferably 1.5 to 3 μm, is formed on the n-GaAs substrate 2 by a first crystal growth using MOCVD (metal organic chemical vapor deposition). 5 nm thick AlGaInP light guide layer 4, MQW active layer 5, 10 nm thick AlGaInP light guide layer 6, 0.1-0.3 μm, for example 0.22 μm thick p-AlGaInP first cladding layer 7, AlGaInP etching stop layer 8. A p-AlGaInP second cladding layer 9 having a thickness of 0.6 to 2 μm, for example 1.2 μm, and a p-GaInP buffer layer 10 having a thickness of 0.02 to 0.2 μm, for example 0.05 μm, are formed in this order. Get a wafer. The MQW active layer 5 has a multi-quantum well structure having three 6 nm-thick well layers and two 4 nm-thick barrier layers, and the etching stop layer 8 has a 2 nm-thick unstrained (Al _0.1 A multilayer structure of three layers of Ga _0.9 ) _0.5 In _0.5 P and two layers of (Al _0.4 Ga _0.6 ) _0.5 In _0.5 P having a thickness of 5 nm was formed.

Next, using the striped SiO ₂ as a mask, the p-GaInP buffer layer 10 and the p-AlGaInP second cladding layer 9 are etched by dry etching to form the ridge portion B. Next, etching is performed until the etching stop layer 8 is reached by wet etching with hydrochloric acid or dilute sulfuric acid and hydrogen peroxide. The etching stop layer 8 automatically stops the ridge etching, and the ridge can be formed with good control.

Thereafter, the wafer is returned into the MOCVD apparatus, and the n-AlGaInP block layer 11 having a thickness of 0.4 μm is formed by the second crystal growth. Thereafter, the SiO ₂ mask is removed by HF treatment, and the p-GaAs cap layer 12 having a thickness of 300 nm to 2 μm, preferably 500 nm to 1 μm is formed again in the MOCVD apparatus. Finally, the wafer is thinned to about 100 μm by lapping and polishing, and the n electrode 1 and the p electrode 13 are formed by vacuum deposition.

As described above, the block layer 11 is made of n-type impurity Si-doped (Al _X Ga _1-X ) _0.5 In _0.5 P (0.7 <X <1), so that GaInP is used as a component. The refractive index difference Δt between the MQW active layer 5 and the block layer 11 is smaller than the refractive index difference Δt between the n-AlInP blocking layer 42 and the MQW active layer 34 in the conventional red semiconductor laser shown in FIG. The refractive index difference Δn is smaller than the conventional one shown in FIG. When the effective refractive index difference Δn is reduced, the concentration of light on the stripe portion (center portion) of the active layer is relaxed, and the stripe width W of the ridge portion does not need to be reduced. The size of the corner can be reduced. Furthermore, since it is possible to provide a margin for the stripe width W necessary for cutting off higher-order modes in the optical waveguide, fluctuations of individual laser elements in the production lot can be suppressed.

In the AlGaInP mixed crystal system, the refractive index of AlGaInP can be lowered by increasing the Al composition ratio or decreasing the Ga composition ratio. Not escape out light, in order to confine is not necessary to reduce the refractive index of the blocking layer 11 than the second cladding layer 9, the blocking layer _{11 (Al X Ga 1-X} ) 0.5 In 0 _.5 P (0.7 <X <1), and the second cladding layer 9 is (Al _Y Ga _{1 -Y} ) _0.5 In _0.5 P (Y <X). The refractive index can be made smaller than that of the cladding layer 9.

  Further, the band gap of the block layer 11 can be made higher than that of the MQW active layer containing GaInP as a component, and light absorption by the block layer 11 can be suppressed, so that high output can also be achieved.

In a high-power red semiconductor laser exceeding 250 mW, heat generation from the inside of the laser element increases, so that the second cladding layer 9 of the high-power red semiconductor laser in FIG. 1 is p-type instead of p-AlGaInP. The diffusion of heat may be promoted by using Al _0.5 GaAs doped with impurity Zn. Since the thermal conductivity of AlGaAs reaches about twice that of AlGaInP, the heat generated inside the laser element is quickly conducted through the p-AlGaAs second cladding layer and diffused to the outside. In the AlGaAs mixed crystal system, the refractive index can be increased and the thermal conductivity can be increased by lowering the Al composition.

It is a figure which shows the cross-section of the high output red semiconductor laser of this invention. It is a figure which shows the cross-section of the conventional red semiconductor laser. It is a figure which shows the relationship between the effective refractive index difference in the active layer of the conventional red semiconductor laser, and a semiconductor layer. It is a figure which shows the difference of FFP of an AlGaAs type | system | group semiconductor laser and an AlGaInP type | system | group semiconductor laser.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 n electrode 2 n-GaAs substrate 3 n-AlGaInP clad layer 4 AlGaInP light guide layer 5 MQW active layer 6 AlGaInP light guide layer 7 p-AlGaInP first clad layer 8 p-AlGaInP etching stop layer 9 p-AlGaInP second clad Layer 10 p-GaInP buffer layer 11 n-AlGaInP block layer 12 p-GaAs cap layer 13 p electrode

Claims (2)

On the n-type semiconductor substrate, at least an n-type cladding layer, an active layer containing GaInP as a component, and a p-type cladding layer are sequentially provided, and a striped ridge portion including the p-type cladding layer is provided above the active layer. In the AlGaInP-based high-power red semiconductor laser, the side surface of the ridge portion has a band gap larger than that of the active layer, and an n-type (Al _X Ga _1-X ) _0.5 having a refractive index smaller than that of the p-type cladding layer. A high-power red semiconductor laser embedded with an In _0.5 P layer (0.7 <X <1). 2. The high-power red semiconductor laser according to claim 1, wherein the p-type cladding layer is composed of a semiconductor layer containing AlGaInP as a component.

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