JPH0712104B2 - Semiconductor laser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The semiconductor laser of the present invention will be described in accordance with the following items.

A. Industrial field of use B. Outline of the invention C. Prior art D. Problems to be solved by the invention [Figs. 3 to 7] E. Means for solving the problems F. Action G. Example [Figs. 1 and 2] a. Structure [Fig. 1] b. Manufacturing method [Fig. 2] H. Effect of the invention (A. Industrial field of application) The present invention is a novel semiconductor laser, In particular, the present invention relates to a semiconductor laser in which a light emitting end face is protected by an end face protective film.

(B. Outline of the Invention) The present invention is a semiconductor laser having an end face protective film formed on the light emitting end face, and the laser beam emitted from the active layer to the outside is not absorbed by the end face protective film to the light emitting end face. In order to prevent the return light from returning and causing interference due to the return light, the film thickness of the end face protective film that protects the light emitting end face is changed between the part covering the active layer and the part covering the return light incident part, The thickness of the portion covering the layer is set to a thickness at which absorption is reduced, and the thickness of the portion covering the return light incident portion is set to a thickness that satisfies the antireflection condition.

(C. Prior Art) A semiconductor laser is generally used as a light source for an optical head such as a CD (Compact Disc) player. Then, the semiconductor laser generally forms a clad layer made of AlGaAs of the same conductivity type on a certain conductivity type GaAs substrate, and forms an active layer made of AlGaAs on the clad layer,
It has a structure in which a clad layer made of AlGaAs having a conductivity type opposite to that of the above-mentioned conductivity type is formed on the active layer, and a cap layer made of GaAs having a conductivity type substantially the same as that of the clad layer is further formed on the clad layer. In such a semiconductor laser, the laser beam is emitted from the portion where the active layer of the semiconductor laser is exposed on the light emitting end face. Since the characteristics of the semiconductor laser deteriorate when the exposed portion of the active layer on the light emitting end face is contaminated, the light emitting of the semiconductor laser is introduced as disclosed in JP-A-60-85586. The end face is often protected by a protective film made of silicon nitride or the like.

Such a semiconductor laser is generally mounted on a heat sink in a state of being bonded on the electrode surface near the active layer, and is often used as a light source for an optical head of a device such as a CD player.

(D. Problems to be Solved by the Invention) [FIGS. 3 to 7]
[FIG.] By the way, when the above-mentioned semiconductor laser bonded to a heat sink is used as a light source in a so-called three-beam type optical recording device, optical reproducing device or optical head of the optical recording / reproducing device, a tracking error signal is generated. There is a problem that the optical recording medium such as a compact disc (CD) changes depending on the change of the angle of the laser beam with respect to the 0th order beam.

Therefore, the problem that the tracking error signal changes due to the change in the angle of the laser beam of the optical recording medium with respect to the 0th-order beam will be described in detail. Prior to that, a semiconductor laser bonded on a heat sink is used as a light source. The optical head and the tracking error detecting method will be described with reference to FIG.

Reference numeral 1 denotes a semiconductor laser (laser diode), and a laser beam emitted from the laser beam emitting end facet 1A and having an elliptical cross-section is a collimator lens (may be unnecessary) 2
Is made to be a parallel beam, and is made incident on the diffraction grating (grating) 3. The 0th-order beam L ₀ and the ± 1st-order beams L ₊₁ and L _-1 emitted from the diffraction grating 3 (note that
Beams above + 2nd order and below -2nd order are ignored), after passing through a non-polarizing beam splitter (half mirror) 4, they are focused by an objective lens 5, and the focused 0th order beam L ₀ and ± 1st order The beams L ₊₁ and L _-1 are made incident on the recording surface of an optical recording medium (including a magneto-optical recording medium) 6 at a predetermined interval (for example, 10 μm). When a polarized beam splitter is used as the beam splitter 4 instead of non-polarized light, it is necessary to dispose a 1/4 wavelength plate between the polarized beam splitter and the diffraction grating 3.

The 0th-order beam L ₀ and the ± 1st-order beams L ₊₁ and L _-1 reflected by the optical recording medium 6 pass through the objective lens 5 and are made incident on the beam splitter 4, and a part of them is incident on the incident surface 4a. It is reflected and made incident on the photodetector 7. The photodetector 7 is composed of three photodetector sections on which the 0th-order beam L ₀ and the ± 1st-order L ₊₁ and L _-1 are separately incident. Then, by detecting the difference between the pair of photodetection outputs from the pair of photodetectors on which the ± first-order beams are incident, the tracking state of the 0th-order beam L ₀ on the recording surface of the optical recording medium 6 is obtained. A corresponding tracking error signal is obtained. Also, 0
A reproduction signal, a focus error signal, etc. are obtained from the photodetector on which the next beam is made incident.

Next, Example 1 of the semiconductor laser will be described with reference to FIG. The semiconductor laser 1 is in the form of a chip and is normally fixed on a heat sink 8 made of copper or the like which also serves as one electrode. The structure of the semiconductor laser 1 will be described in order from the upper layer to the lower layer. 1a is an electrode layer, 1b is an n-type GaAs layer (substrate layer), and 1c.
Is n-type GaAlAs (cladding layer), 1d is GaAlAs layer (active layer), 1e is P-type GaAs layer (cladding layer), and 1f is P-type Ga.
It is the As layer. Then, the above-mentioned laser beam L is emitted from the active layer 1d, particularly from the central portion thereof. This semiconductor laser 1
When the laser beam emitting end face (cleavage face) 1A of
Its width is 100-300 μm, height (thickness) is 80-100 μm,
The depth is 200 to 300 μm. Heat sink 8 of active layer 1d
The height from the upper surface of is a few μm.

By the way, if the tangential skew angle of the 0th-order beam L ₀ with respect to the recording surface of the optical recording medium 6 changes, the tracking error signal also changes periodically as described above, and an accurate tracking error can be detected. could not.

Then, when the cause was investigated, the following was found. Zero-order beam L ₀ and ± 1 reflected by the optical recording medium 6
After passing through the objective lens 5, the subsequent beams L ₊₁ and L _-1 are not only reflected by the reflecting surface 4a of the beam splitter 4, but also pass through the beam splitter 4 and enter the diffraction grating 3. Then, a 0th-order beam and ± 1st-order beams are generated corresponding to the respective incident beams, pass through the collimator lens 2, and are directed toward the semiconductor laser 1. The beam amount of the beam directed to the semiconductor laser 1 is large when the non-polarizing beam splitter is used and is small when the polarizing beam splitter is used. In this case, the semiconductor laser 1 is changed in accordance with the relative rotation angle position of the laser beam emitting end surface 1A of the semiconductor laser 1 with respect to the diffraction grating 3.
The positional relationship between the incident position of the upper central beam 1a and the incident positions of the two-sided beams Lb and Lc located on both sides thereof is different, and there are three types of positional relationships. The first one is that the incident position of the center beam La is located on the active layer 1d on the laser beam emission end face 1a, and both side beams Lb and Lc pass through the position of the center beam La on a straight line orthogonal to the active layer 1d. This is the case where the light is incident on the upper and lower parts of the central beam 1a. In the second one, the central beam La and both side beams Lb, Lc are both located on the active layer 1d, and the third one is a straight line connecting the incident positions of the central beam La and both side beams Lb, Lc. Is an angle intermediate between the above two cases. Then, the center beam La and the both-side beams Lb and Lc are re-diffracted by the diffraction grating 3 and the first-order beam L ₀ and the ± first-order beams L ₊₁ and L _-1 are mixed and superimposed.

By the way, when one of the beams 1b and 1c on both sides is incident on the surface of the heat sink 8, there is no problem because that surface is a rough surface and is diffusely reflected there. However, at least one of the two-sided beams Lb and Lc is incident on the laser beam emitting end facet 1A of the semiconductor laser 1. Since the end facet 1A has a large reflectance, the beam is reflected by the end facet 1A, passes through the above-mentioned optical path, and enters the photodetector 7,
Interferes with the + 1st or -1st order beam. Therefore, the 0th-order beam L ₀ is incident on the photodetector 7 according to the tank-general skew angle with respect to the recording surface of the optical recording medium 6+
The intensity of the primary or -1st order beam changes, and the tracking error signal changes periodically according to the skew angle.
FIG. 6 shows the tangential skew angle α of the 0th-order beam L ₀ with respect to the recording surface when one of the two side beams Lb and Lc is incident on the laser beam emitting end surface 1A of the semiconductor laser 1 and the other Lc is incident on the heat sink 8. FIG. 6 is a curve diagram showing an ideal relationship between the angle θ and the level of the tracking error signal Se. As can be seen from this figure, the wavelength λ of the laser beam of the tracking error signal Se changes according to the change of the skew angle α °.
On the other hand, the level changes every λ / 2. Specifically, as | α | increases, the level of the tracking error signal Se decreases. When the two-sided beams Lb, Lc are incident on the laser beam emission end facet 1A, the amplitude of the waveform is twice as large as that at the center L ₀ , and the phase is also different from that shown in FIG.

Next, interference when the beam Lb on one side is incident on the laser beam emission surface 1A and the beam Lc on the other side is incident on the heat sink 8 will be described with reference to FIG. Incidentally, the lens system is not shown in this figure. In this figure, the emission end surface 1A is shown in a normal state by a broken line, and a general state inclined from the normal state is shown by a solid line. The normal state of the optical recording medium 6 is also shown by a broken line, and the general state inclined from the normal state is shown by a solid line.

The 0th-order beam L ₀ is perpendicular to the laser beam emitting end facet 1A in the normal state and the optical medium 6 (the recording surface thereof) similarly. In FIG. 7 or the following equations (1) to (6), θ is the angle of the + 1st order beam L ₊₁ with respect to the 0th order beam L ₀ , and l ₁ is the phase length between the laser emission end facet 1A and the diffraction grating 3. , L ₂ is the phase length between the diffraction grating 3 and (the recording surface of) the optical recording medium 6, and Δl ₁ and Δl ₂ are the 0th-order beam L ₀ and +1 beam L ₊₁ for the phase lengths l ₁ and l ₂ . Is the phase difference between the two. Δl ₃ ,
Δl ₄ is the skew phase of the optical recording medium 6 and the laser beam emission end facet 1A, and g is the zero-order beam in the diffraction grating 3.
The phase difference between L ₀ and the + _1st order beam L ₊₁ , i ₀ and i ₁ are the transmittances of the 0th order beam and the + 1st order beam in the diffraction grating 3, t is the transmittance of the half mirror 4, and r and f are This is the reflectance on the recording surface of the optical recording medium 6 and on the laser beam emitting end surface 1A.

Here, the optical recording medium 6 on which the + 1st order beam L ₊₁ is incident
The complex amplitude of the incident light at point A on the recording surface of
Consider two cases (1) to (4) (a _{1 to} a ₄ ).

(1) a ₁ : The complex amplitude of the incident light when the + _1st order beam L ₊₁ directly enters the point A. (2) a ₂ : The 0th order beam L ₀ is reflected by the optical recording medium 6 and diffracted again. The 0th-order beam obtained by being incident on the grating 3 is reflected by the laser beam emitting end facet 1A and is again reflected by the diffraction grating 3
Complex amplitude of the incident light when the + 1st order beam obtained by entering the point A is incident on the point A (3) a ₃ : The 0th order beam L ₀ is reflected by the optical recording medium 6 and again enters the diffraction grating 3. Complex amplitude of incident light when the + 1st order beam obtained by incidence is reflected by the laser beam emission end facet 1A and the 0th order beam obtained by entering the diffraction grating 3 again enters point A (4 ) A ₄ : +1 order beam L ₊₁ is reflected by the optical recording medium 6 and is incident on the diffraction grating 3 again.
Complex amplitude of incident light when the 0th-order beam obtained by reflecting the second-order beam on the laser beam emitting end facet 1A and entering the diffraction grating 3 again enters the point A Next, a _{1 to} a ₄ are expressed as A ₁ = i ₁ t · exp {j (l ₁ + g + l ₂ + Δl ₂ + Δl ₃ )} ...
(1) a ₂ = i ₀ ² i ₁ t ³ rf · exp [j {3 (l ₁ + l ₂ ) + g + Δl ₂ + Δ
l ₃ }] (2) a ₃ = i ₀ ² i ₁ t ³ rf · exp [j {3 (l ₁ + l ₂ ) + g + 2Δl ₁ +
Δl ₂ + Δl ₃ + 2Δl ₄ }] (3) a ₄ = i ₀ ² i ₁ t ³ rf · exp [j {3 (l ₁ + l ₂ ) + g + 3 (Δl ₂
+ Δl ₃ ) +2 Δl ₁ +2 Δl ₄ }] (4) To simplify the calculation, the coherence length of the laser beam is set to 2
If it is (l ₁ + l ₂ ) or less, the light intensity Ia at the point A is expressed by the following equation.

^{_{Ia = | a 1 | 2 +}} | a 2 + a 3 + a 4 | 2 = i 1 2 t 2 [1 + i 0 4 t 4 r 2 f 2 {3
+ 2cos2 (Δl ₁ + Δl ₄ ) + 2cos2 (Δl ₁ + Δl ₄ + Δl ₂ + Δ
l ₃ ) + 2cos ₂ (Δl ₂ + Δl ₃ }] (5) Further, when both side beams Lb and Lc are incident on the laser beam emission end facet 1A, the + 1st order beam L ₊₁ is on the recording face of the optical recording medium 6. When the −1st-order beam L ₋₁ is incident on the point B which is symmetric with respect to the 0th-order beam L ₀ , the light intensity Ia at the point A is as shown in the equation (5). , The light intensity Ib at the point B is expressed by the following equation.

Ib = i ₁ ² t ² [1 + i ₀ ⁴ t ⁴ r ² f ² {3 + 2cos2 (Δl ₁ −Δl ₄ )
+ 2cos2 (Δl ₁ −Δl ₄ + Δl ₂ −Δl ₃ ) + 2cos2 (Δl ₂ −Δ
l ₃ )}] (6) As described above, at least one of the double-sided beams Lb and Lc is incident on the beam emission end face 1A, and as a result, is reflected by the end facet 1A and is incident on the photodetector 7, so the +1 order Or, it causes interference with the -1st-order beam. Therefore, the intensity of the + 1st order or −1st order beam incident on the photodetector 7 changes due to the change in the tangential skew angle of the 0th order beam with respect to the optical recording medium. That is, the level of the tracking error signal changes not only due to the tracking state but also due to the change in the tangential skew angle.

As a result, a tracking error will occur. Therefore, when the returning lights La, Lb, and Lc return on the line orthogonal to the active layer 1d, it is necessary to prevent the returning lights Lb and Lc from being reflected. Of course, it is relatively easy to prevent the interference of the return light Lc returning to the heat sink 8 even if the heat sink 8 has a rough surface, by causing diffuse reflection, but the light emitting end surface of the semiconductor laser 1 does not exist. No particular consideration was given to the return light returning to 1A so as not to reflect it. Incidentally, the above-mentioned
Even in the method of manufacturing a semiconductor laser described in Japanese Patent No. 60-85586, care is taken to prevent the protective film from absorbing the laser beam emitted from the active layer, but the reflection of return light is prevented. No consideration was given to what to do.

The present invention has been made in view of such circumstances, and in a semiconductor laser having an end face protective film formed on the light emitting end face, the end face protective film absorbs the laser beam emitted from the active layer to the outside. It is an object of the present invention to prevent the return light from returning to the light emitting end face without being reflected and to prevent interference due to the return light.

(E. Means for Solving Problems) In order to solve the above problems, the semiconductor laser of the present invention has a film thickness in a portion covering the active layer of the end face protective film to a thickness at which absorption for a laser beam is reduced, Similarly, the thickness of the portion covering the return light incident portion is set to a thickness that satisfies the antireflection condition.

(F. Action) According to the semiconductor laser of the present invention, the end face protective film transmits the laser beam without absorbing it in the portion covering the active layer that generates the laser beam, but returns the return light in the portion covering the return light incident portion. Is not reflected, and therefore inconvenience such as interference does not occur.

(G. Embodiment) [FIGS. 1 and 2] The semiconductor laser of the present invention will be described below in detail in accordance with the embodiment shown in the accompanying drawings.

(A. Structure) [FIG. 1] FIG. 1 shows an example 10 of the semiconductor laser of the present invention.

In the figure, 11 is 100 μm which constitutes the semiconductor laser 10.
A semiconductor chip 12 having a thickness of about 12 is an active layer, which is provided at a depth of 2 μm from the semiconductor surface (the lower surface in FIG. 1). Reference numeral 13 is an N-side electrode formed on the back surface (upper surface in FIG. 1) of the semiconductor chip 11, and 14 is a P-side electrode formed on the front surface of the semiconductor chip 11. Chip bonded.

Reference numerals 16 and 16 denote end face protective films made of silicon nitride SiN formed on the light emitting end face 10a of the semiconductor laser 10 and the end face opposite to the light emitting end face 10a, and the film thickness T of the end face protective films 16 and 16 is not uniform. The thickness becomes thinner from the side (lower side in FIG. 1) to the back surface side (upper side in FIG. 1). Then, the film thickness Ta in the portion of the protective films 16 and 16 covering the active layer 12 is set to a value satisfying the condition of the smallest absorption represented by the following formula (1).

It should be noted that the refractive index n here is of course the end face protective film 1
Is the refractive index of the material (SiN in this example) that forms 6 and 16,
The wavelength λ is the wavelength of the laser beam.

Therefore, the wavelength λ of the laser beam is, for example, 8000Å,
If the refractive index n of SiN of the material of the protective films 16 and 16 is 2.05, the film thickness Ta in the portion of the end face protective films 16 and 16 covering the active layer 12 is about 1950Å. Thus, since the film thickness Ta is the value expressed by the above equation (1), the portions of the end face protective films 16 and 16 covering the active layer 12 are in the state of the least absorption of the laser beam. Therefore, the laser beam emitted from the active layer 12 can be prevented from being absorbed by the end face protective film 16.

Further, the film thickness Tb in the portion of the protective films 16 and 17 that covers the return light incident portion 17 (this portion is at a depth of 20 to 30 μm from the back surface of the semiconductor chip 11) is expressed by the following equation (2). It is set to a value that satisfies the non-reflective condition of.

That is, the film thickness Tb is about ½ of the film thickness Ta. As described above, when the film thickness Tb is the value expressed by the above formula (2), the portion where the film thickness T of the end face protective film 16 is Tb, that is, the portion which covers the return light incident portion 17 is covered by the semiconductor laser 10. When used as a light source for a beam tracking type optical head, the return light incidence part 17 of the semiconductor laser 10 from the optical recording medium is used.
It does not reflect the return light. Therefore, it is possible to prevent the returning light from being reflected by the light emitting end surface 10a of the semiconductor laser 10. Therefore, it is possible to avoid the problem that an error occurs in the detection of the tracking error.

The return light is not only on the light emitting end face 10a but also on the laser beam 1
Returning to the end face of 5 also, 18 is a returning light incident part on the end face of the laser beam 15. Then, when the return light incident on the return light incident portion 18 is also reflected there, it causes interference, and an error occurs in the detection of the tracking error, but the return light incident portion 18 of the heat sink 15 is roughened. It is possible to prevent the return light from being reflected in the incoming direction by a relatively simple method such as irregularly reflecting the return light, and it is possible to easily avoid a tracking error detection error.

In addition to SiN, SiO ₂ , Al ₂ O _{3 and the} like are conceivable as the end face protective film, and are not limited to SiN.

(B. Manufacturing Method) [FIG. 2] FIGS. 2A to 2C show an example of a method of manufacturing the semiconductor laser shown in FIG.

(A) A wafer-shaped semiconductor substrate is subjected to an epitaxial growth process for forming a clad layer, an active layer, etc., and electrodes 13 and 14 are formed on the back and front surfaces of the semiconductor chip 11.

Next, the wafer-shaped semiconductor substrate is cleaved to be divided into several bar-shaped bodies. Then, the cleaved surface formed by the cleavage becomes the light emitting end surface 10a. FIG. 2 (A) shows a state after cleavage.

(B) Next, the bar-shaped semiconductor substrate is arranged so that the P-side electrode 14 faces upward. In this state, the end face protective film 16 made of silicon nitride SiN is formed by the plasma CVD method. 16a is a portion of the end face protective film 16 deposited on the P-side electrode 14. Since the plasma CVD method generally has good step coverage, even if the silicon nitride SiN is vapor-deposited from the upper side with the P-side electrode 14 facing upward, the silicon nitride SiN is the side surface of the semiconductor chip 11 at the light emitting end surface. It is also deposited on 10a. However, the thickness of the silicon nitride SiN covering the light emitting end face 10a is not uniform, and the film thickness decreases from the upper side to the lower side in FIG. The degree of reduction in the film thickness (the inclination of the surface of the protective film 16 formed on the end face) can be controlled by adjusting the gas flow rate (flow rate) in plasma CVD and the degree of vacuum in the CVD apparatus. Utilizing that fact, the film thickness Ta in the portion of the end face protective film 16 covering the active layer 12 becomes the value calculated by the above equation (1), and the film thickness Tb in the portion covering the return light incident portion 18 becomes The value is calculated by the above equation (2). However, since the surface of the end face protective film 16 is slightly thinned (by several tens of Å) by anisotropic etching by RIE performed after this plasma CVD process, the end face protective film 16 should be thinned in advance in anticipation of it. It is good to form. FIG. 2B shows a state after the plasma CVD process is completed.

(C) Next, the silicon nitride SiN film 16a on the P-side electrode 14 is removed by RIE. Since this RIE is anisotropic etching, the silicon nitride SiN film 16 on the P-side electrode 14 is removed with almost no etching of the silicon nitride SiN film functioning as the end face protective film 16 on the light emitting end face 10a. The electrode 14 can be completely exposed. FIG. 2C shows the state after the RIE process is completed.

After that, the bar-shaped semiconductor substrate is pelletized so as to be divided into individual semiconductor lasers, chip-bonded to the heat sink 15, and further wire-bonded.

(H. Effect of the Invention) As is clear from the above description, the semiconductor laser of the present invention is a semiconductor laser in which an end face protective film is formed on the light emitting end face, and the active layer of the end face protective film is It is characterized in that the film thickness in the covering portion is set to a thickness at which absorption of a laser beam is reduced, and the film thickness in the portion covering the return light incident portion of the end face protective film is set to a thickness satisfying a non-reflection condition. .

Therefore, according to the semiconductor laser of the present invention, the end face protective film transmits the laser beam without absorbing it in the portion covering the active layer for generating the laser beam, but does not reflect the return light in the portion covering the return light incident portion. . Therefore, inconvenience such as interference does not occur.

[Brief description of drawings]

FIG. 1 is a sectional view showing an example of implementation of a semiconductor laser of the present invention,
FIGS. 2A to 2C are cross-sectional views showing an example of the method of manufacturing the semiconductor laser shown in FIG. 1 in order of process steps, and FIGS. 3 to 7 are tangential to the 0th order beam of the optical recording medium. This is for explaining the problem that the tracking error signal changes due to the change of the skew angle. FIG. 3 is a configuration diagram of an optical head, FIGS. 4 and 5 are front views of a semiconductor laser device, and FIG. FIG. 7 is a waveform diagram showing an ideal level change of the tracking error signal with respect to the skew angle, and FIG. 7 is a diagram used to explain interference. Explanation of symbols 10 …… Semiconductor laser, 10a …… Light emitting end face, 16 …… End face protective film, 17 …… Returning light incident part, Ta …… Film thickness at the portion of the end face protective film covering the active layer, Tb …… Film thickness of the part of the end face protection film that covers the return light incident part

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Tatsuharu Oda Inventor Tatsuharu Oda 6-735 Kita-Shinagawa, Shinagawa-ku, Tokyo Within Sony Corporation (56) References JP-A-60-85586 (JP, A) 57-6266 (JP, U) Actually opened 62-52962 (JP, U)

Claims (1)

[Claims] 1. A semiconductor laser having an end face protective film made of a single layer formed on a light emitting end face, wherein a film thickness of a portion of the end face protective film covering an active layer is substantially the same as a wavelength λ of a laser beam. The value that is obtained by dividing by 2 times the refractive index n of the end face protective film [λ / (2 · n)] is set to a thickness that reduces absorption for the laser beam, and the portion that covers the return light incident part of the end face protective film. At the wavelength λ of the laser beam and the refractive index n of the end face protective film.
A semiconductor laser characterized by having a thickness [λ / (4 · n)] divided by 4 times the