JP2000200938A - Passivation film of semiconductor laser, and semiconductor laser and laser module using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a passivation film contributing ensuring long-term reliability of a semiconductor laser by suppressing the deterioration of an output end surface of the semiconductor laser whose oscillation wave length is of 980 nm band. SOLUTION: In a semiconductor laser passivation film 1 of two-layer structure comprising a first thin film 1A, which is film-formed on an output end surface 2D of a semiconductor laser 2 directly, and a second thin film 1B which is film-formed on the surface of the first thin film, and the film thickness of the first thin film 1A is, in a calculating equation for finding reflection factor of the passivation film 1 by a matrix method, is decided to have a value for which differential coefficient becomes zero, when the calculating equation is differentiated with the film thickness of the first thin film 1A.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a passivation film of a semiconductor laser, a semiconductor laser using the same, and a laser module incorporating the semiconductor laser. The present invention relates to a passivation film of a semiconductor laser having a structure capable of suppressing deterioration of an end face and ensuring high operation reliability of the semiconductor laser for a long time.

[0002]

2. Description of the Related Art A passivation film of AlOx, SiNx, or the like is formed on a cavity facet of a semiconductor laser in order to prevent deterioration of an output facet due to oxidation or the like during operation. In the case of the output end face of the above-described semiconductor laser, for example, InGaP is formed on the cleavage face obtained by cleaving the laminated structure of the semiconductor material by the epitaxial crystal growth method.
In some cases, the P layer is formed as an end face protection layer. In this case, the passivation film may be formed by covering the end face protection layer.

Recently, research and development of a semiconductor laser having an oscillation wavelength in the 980 nm band has been actively conducted. In the case of this type of semiconductor laser, if the passivation film is, for example, a single layer of AlOx, the output end face is rapidly deteriorated. And it becomes inoperable in a short time. In order to solve such a problem, Japanese Patent Application Laid-Open No. 3-101183 discloses that
It has been proposed that the passivation film be formed of a low-reflectance film having a two-layer structure. Specifically, it has a structure in which a Si thin film having a thickness of about 1 nm is directly formed on the output end face of the semiconductor laser, and a SiNx thin film having a thickness of about 140 nm is formed on the surface of the Si thin film. In the case of this passivation film, the Si thin film suppresses oxygen diffusion to the output end face, prevents deterioration of the output end face, and ensures long-term reliability in operation of the semiconductor laser.

[0004]

However, when the semiconductor laser having a two-layer passivation film composed of the Si thin film and the AlOx thin film is operated in a wavelength band of 980 nm, the light output at the output end face is 200 mW.
When the output becomes high as described above, a phenomenon occurs in which the temperature of the output end face rises and the oscillation threshold current and the optical output change significantly with time, resulting in poor long-term operation reliability. .

The present invention solves the above-mentioned problems in a semiconductor laser having a two-layered passivation film,
A semiconductor passivation film that has a small change in oscillation threshold current and light output even when operated for a long period of time, thus enabling the realization of a semiconductor laser with long-term reliability.
It is another object of the present invention to provide a semiconductor laser on which the passivation film is formed and a laser module in which the semiconductor laser is incorporated as a light source.

[0006]

Means for Solving the Problems In order to achieve the above-mentioned object, the present inventors consider the above-mentioned phenomenon, that is, the basis for the occurrence of a large change in the oscillation threshold current and optical output. As a result, I came to the following idea. That is, this phenomenon is an idea that the output end face is heated by the heat generated by the high-output operation, and the heat is generated and the Si thin film directly formed on the output end face is deteriorated. Then, it is considered that the refractive index of the altered Si thin film has changed even if its thickness is the same as that at the beginning of film formation. On the other hand, this S
It is considered that the AlOx thin film formed on the surface of the i thin film does not change its film thickness before and after the operation of the semiconductor laser, and hardly changes its refractive index.

By the way, the reflectance of a certain film with respect to light having a predetermined wavelength is a function of the refractive index and the film thickness of the material constituting the film. Therefore, the reflectivity of the passivation film having the two-layer structure described above also depends on the film thickness and the refractive index of the Si thin film, AlOx
It is determined by the thickness and the refractive index of the thin film. Before and after the operation of the semiconductor laser, as described above, A
The thicknesses of the lOx thin film and the Si thin film do not change, and
Since it is considered that the refractive index of the Ox thin film hardly changes and only the refractive index of the Si thin film greatly changes, after all, the change in the reflectance of the passivation film before and after the operation of the semiconductor laser is the refractive index of the Si thin film. It is considered that the expression is based on the change.

Therefore, in order to suppress a change in the reflectance of the passivation film having the two-layer structure, it is sufficient to prevent a change in the refractive index of, for example, a Si thin film formed directly on the output end face. This is practically impossible because thermal transformation of the Si thin film inevitably occurs. Therefore, the present inventors set the film thickness of the Si thin film so that the change in the refractive index does not affect the reflectivity of the passivation film even if the Si thin film is altered and its refractive index is changed. This led to the idea that the change in the reflectance of the passivation film before and after the operation could be suppressed.

Based on this idea, the reflectance calculation formula of the passivation film by the matrix method described later is analyzed, the thickness of the Si thin film calculated by the calculation described later is set, and the Si film is formed on the actually manufactured passivation film. By examining the relationship between the thin film and the change in reflectance, it was confirmed that the above idea was correct. Then, based on the confirmation, a passivation film of the present invention was developed.

That is, the passivation film of the semiconductor laser of the present invention has a two-layer structure comprising a first thin film formed directly on the output end face of the semiconductor laser and a second thin film formed on the surface of the first thin film. In the passivation film of the semiconductor laser of the above, the thickness of the first thin film is a differential coefficient obtained by differentiating the calculation formula by the thickness of the first thin film in a calculation formula for calculating the reflectance of the passivation film by a matrix method. Is a value that becomes zero.

Further, in the passivation film of the semiconductor laser according to the present invention, the first thin film is a Si thin film, and the second thin film is a Si thin film.
The thin film is an AlOx thin film, and the thickness of the Si thin film is 15 to 25 nm. Further, in the present invention, there is provided a semiconductor laser characterized in that the above-mentioned passivation film is formed on an output end face, and a laser module characterized in that the semiconductor laser is incorporated as a light source. .

[0012]

FIG. 1 is a sectional view showing a state in which a passivation film having a two-layer structure is formed on an output end face of a semiconductor laser. In FIG. 1, using a predetermined semiconductor,
The active layer 2A is embedded between the cladding layer and the substrate 2C and the cladding layer 2B to form the semiconductor laser 2. The output end face 2D of the semiconductor laser 2 includes
The first thin film 1A is formed by directly covering the first thin film 1A, and the second thin film 1B is further formed on the first thin film 1A to form the passivation film 1 having a two-layer structure as a whole.

In FIG. 1, the above-mentioned end face protective film is not shown on the output end face 2D.
The output end face 2D includes a case where an end face protective film such as an InGaP layer is formed. In the passivation film 1, the first thin film 1A is made of, for example, Si, T
iO can be formed ₂ of a material such as, The second thin film 1B, for example AlOx, SiNx, etc. SiO _2, and the first thin film is formed by different types of materials. The passivation film 1 usually has a reflectivity and a thickness set so as to reflect about 5% of the light output of the laser light from the active layer 2A and transmit the remaining about 95% of the light output. ing.

The passivation film 1 of the present invention is characterized in that the thickness of the first thin film 1A is determined as follows. This will be described below. Now, the refractive index of the semiconductor constituting the semiconductor laser 2 is n ₀ , and the first thin film 1
The refractive index and thickness of A are n ₁ and d ₁ , the refractive index and thickness of the second thin film are n ₂ and d ₂ , and the refractive index of air is 1.
And

Here, assuming that the wavelength of the laser beam (electromagnetic wave) from the output end face 2D is λ, the first thin film 1A of the electromagnetic wave
The contribution to the electric field reflectance is represented by the following equation when expressed in a complex manner by a known matrix method:

[0016]

(Equation 1)

## EQU1 ## Similarly, the contribution of the second thin film 1B to the electric field reflectance is expressed by the following equation:

[0018]

(Equation 2)

## EQU1 ## From this, according to the matrix method, the electric field reflectance S of the passivation film 1 is
The following formula:

[0020]

(Equation 3)

## EQU1 ## Here, the laser light emitted from the second thin film 1B enters the second thin film again.

[0022]

[Outside 1]

R is the following formula:

[0024]

(Equation 4)

(However, S ₁₀ = (− n ₂ · n ₁ · n ₀ + n ₂ · n ₁ ) cos (f ₁ ) cos (f ₂ )
^{_{+ (N 2 2 · n 0}} -n 1 2) sin (f 1) sin (f 2) + j {(n 2 2 · n
₁₋ n ₁ · n ₀ ) cos (f ₁ ) sin (f ₂ ) + (n ₂ · n ₁ ² −n ₂ ·
_{n 0) sin (f 1)} cos (f 2)} S 00 = (- n 2 · n 1 · n 0 -n 2 · n 1) cos (f 1) cos (f 2)
^{_{+ (N 2 2 · n 0}} + n 1 2) sin (f 1) sin (f 2) + j {(- n 2 2 ·
_{n 1 -n 1 · n 0)} cos (f 1) sin (f 2) + (- n 2 · n 1 2 -n 2
· N ₀₎ is a _{sin (f 1) cos (f} 2)}, _{also, f 1 = 2πn 1 d 1} / λ, represented by f ₂ = 2 [pi] n a ₂ d ₂ / _λ).

Here, the first thin film is made of a material of n ₁ = 3.5, the second thin film is made of a material of n ₂ = 1.65, the wavelength of the laser beam is 980 nm, and When the thickness of the first thin film d ₁ and the thickness of the second thin film d ₂ are changed when the passivation film is formed assuming that the semiconductor is made of a material having a refractive index n ₀ = 3.2. The reflectance R (%) of the passivation film calculated from equation (4) was drawn by a computer. The result is shown in FIG.

The meaning of these drawings will be described with reference to FIG. Each of the curves drawn in FIG. 2 is a curve indicating the reflectance R of the equation (4) calculated by the computer. For example, d when the curve c ₁ in FIG. 2, the reflectance of the passivation film when changing the d _1, d ₂ of the passivation film formed R represents a 5% value
The relation between ₁ and d ₂ is shown.

As is clear from FIG. 2, each curve is
Again, the film thickness d₁It has an inflection point periodically in the direction. This
This is because equation (4) is a trigonometric function. And
At the point of inflection, the film
Thickness d₁Changes, the amount of change in the curve is small.
That is, in the vicinity of these inflection points, the film thickness d₁Changes
However, the change in the reflectance R of the passivation film is small.
-ing Also, the film thickness d₁Changes in equation (4)
f₁Changes, and n ₁Changes in the refractive index of
If present, n₁Change and d₁Change is considered consent
You. Therefore, at this inflection point, n₁Changes
However, it is considered that the change in the reflectance R is small.
You.

Referring to curve c ₁ in FIG. 2, for example, the thickness of the second thin film and 180 nm, the thickness of the first thin film 20nm
In this case, the point is near the first inflection point of the curve c ₁ and the reflectivity R of the passivation film at that time.
Is 5%, and at the same time, even if the film thickness d ₁ changes, the reflectance R
Can be estimated to be almost zero. The film thickness d ₁ capable of minimizing the rate of change of the reflectance R of the passivation film is a value that differentiates the equation (4), which is the calculation formula of the reflectance R, by d ₁ and makes its differential coefficient zero. Can be calculated as

[0030]

[Outside 2]

As a result, the following equation:

[0032]

(Equation 5)

[0033] _{(where, A = (- n 2 ·} n 1 · n 0 + n 2 · n 1) cos (f 2), B = (n 2 2 · n 0 -n 1 2) sin (f 2), ^{_{C = (n 2 2 · n}} 1 -n 1 · n 0) sin (f 2), D = (n 2 · n 1 2 -n 2 · n 0) cos (f 2), a = (- n 2 _{· n 1 · n 0 -n 2} · n 1) cos (f 2), b = (n 2 2 · n 0 + n 1 2) sin (f 2), c = (- n 2 2 · n 1 -n _{1 · n 0) sin (f} 2) d = (- n 2 · n 1 2 -n 2 · n 1) is cos (f _2), ^{also, | d (x) | 2} = (a 2 + c 2 ) +2 (ab + cd) tan (f ₁ ) +
(b ² + d ² ) tan ² (f ₁ ) | f (x) | ² = (A ² + C ² ) +2 (AB + CD) tan (f ₁ ) +
(B ² + D ² ) tan ² (f ₁ ).

[0034]

[Outside 3]

The differential coefficient of the equation (5), that is, the value of d ₁ that makes [] zero can be obtained. (5) Place a zero in [] in the formula, the resulting formula is two-order equation of _{tan (f 1), tan (} f 1) is the solution of the quadratic equation,
The following formula:

[0036]

(Equation 6)

(Where α = −n ₂ · n ₁ · (n ₂ -1) (n
_{2 +1) (n 0 -n 1} ) (n 0 + n 1) β = - (n 1 -n 2 2) (n 1 + n 2 2) (n 0 -n 1) (n 0 + n 1) γ = - _{(n 2 -1) (n 2} +1) (n 2 2 + n 1 2) (n 0 -n 1) (n
₀ + n ₁ )).

Therefore, d ₁ that gives f ₁ that satisfies equation (6) is the solution of the film thickness to be obtained. And the approximate solution is
The following formula:

[0039]

(Equation 7)

Is given as

[0041]

[Outside 4]

The thickness d ₁ of the first thin film 1A, which is zero and minimizes the change in the reflectance represented by the equation (4), is determined by the refractive index (n ₁ ) of the material forming the first thin film 1A and the second thickness. It is defined by the thickness d _{2 of the} thin film 1B and the refractive index n ₂ of the constituent material. For this reason, in the present invention, when forming the passivation film, the constituent materials of the first thin film and the second thin film are determined,
And if the thickness d ₂ of the second thin film is set to a predetermined value,
Based on the equation (7), it is possible to calculate the thickness d ₁ of the first thin film that can minimize the change rate of the reflectance R of the formed passivation film.

Note that a plurality of approximate solutions d _{1 at} this time are obtained corresponding to the inflection points of the respective curves in FIG. However, since the thickness d ₁ of the first thin film is not usually made so large, as a practical problem, the calculated approximate solution d _{1 is used.}
May be adopted as the minimum value. In the equation (7), the film thickness d ₁ is also a function of the refractive index n ₁ . The refractive index n ₁ changes during the operation of the semiconductor laser as described above.

Therefore, assuming that the first thin film corresponding to FIG. 2 is altered and its refractive index n ₁ is changed from 3.5 to 3.7, the reflection shown by the equation (4) in each case is considered. Rate R
_{1 (n 1 = 3.5),} R 2 and (n ₁ = 3.7) were computed, further, the difference between these values: calculating the R ₂ -R _1, R ₂
-R ₁ and d _1, it was drawing the relationship between the d _2. The result is shown in FIG.
It was shown to.

The meaning of these drawings will be described with reference to FIG. Each of the curves drawn in FIG.
It shows R ₂ −R ₁ calculated by computer, that is, a change in reflectance of the passivation film due to alteration of the first thin film. For example, the curve c ₂ in FIG. 3, even the refractive index n ₁ of the first thin film had changed to 3.5 → 3.7, the change in reflectance of the passivation film is deposited zero ,
That is, the relationship between the thickness d ₁ and the thickness d ₂ unchanged. The curve c ₃ located thereon, take place change in refractive index above the first thin film, the change in reflectance is 0.
Film of 5% shows a relationship between the thickness d ₁ and the thickness d _2.

Referring to the curve c ₂ in FIG.
The thickness d ₂ of the thin film is 180 nm, and the thickness d ₁ of the first thin film is 2
When the thickness is set to about 0 nm, even if the constituent material of the first thin film is deteriorated and its refractive index n ₁ is greatly changed to 0.2, the reflectance R of the formed passivation film does not change. Will be. Next, the semiconductor laser of the present invention
Since the passivation film as described above is formed on the output end face, deterioration of the output end face is suppressed even under high output operation, and high operation reliability is exhibited for a long time.

The laser module of the present invention incorporates this semiconductor laser as a light source, and further comprises an optical fiber,
It incorporates a light receiving element, a temperature control device, and the like as auxiliary devices.

[0048]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The passivation film 1 shown in FIG.
Various samples were manufactured by forming a film on the output end face 2D of the semiconductor laser 2 that oscillates laser light in a 0 nm band. Here, the first
Film 1A is of a thickness d ₁ shown in Table ₁ α-Si (n 1 =
3.5), and the second thin film 1B is made of AlOx (n ₂ = 1.6).
5), and the film thickness d ₂ was a constant value of about 180 nm. Under the above numerical conditions, d _{1 at} which the differential coefficient of equation (4) becomes zero was calculated. As the d ₁ value, 20 nm was obtained.

Although the thickness of the α-Si thin film is changed, in any case, the reflectance of the formed passivation film 1 is about 5%. The variation in the thickness of each thin film is within the range of ± 5 nm. These semiconductor laser samples were operated at a temperature of 60 ° C. by applying a current of 350 mA for about 100 hours. The oscillation threshold current and light output before and after the operation were measured, and the respective rates of change were calculated. At the same time, the rate of change of the reflectance of the passivation film was calculated based on the measured values. Table 1 shows the above results.

[0050]

[Table 1]

The following is clear from Table 1. (1) When the thickness d ₁ of the α-Si thin film is smaller than 30 nm,
Even after the operation for 100 hours, the rate of change of the oscillation threshold current and the rate of change of the optical output are 1% or less and 0.5% or less in absolute value, respectively. The reflectance of the passivation film is 0.3% or less in absolute value.

(2) In particular, those without α-Si film, those with d ₁ = 5 nm, and those with d ₁ = 20 nm all show low change rates. However, in the case where α-Si was not formed or the case where d ₁ = 5 nm, in the course of operation for a longer period of time, the oxidative deterioration of the output end face progressed, and both the oscillation threshold current and the optical output became lower. Degradation is considered, so considering the operational reliability of the semiconductor laser, d
It is preferable that ₁ is around 20 nm, specifically 15 to 25 nm.

(3) α that reduces the rate of change of reflectance
-The thickness d _{1 of the} Si thin film is measured intermittently because
This is a phenomenon corresponding to that the inflection point in the reflectance curve shown in FIG. 2 is calculated periodically.

[0054]

As is clear from the above description, the passivation film of the present invention has a structure in which the first thin film formed on the output end face of the semiconductor laser reflects the entire passivation film even if its refractive index changes. Since the film thickness is set so as not to change the reflectance, the reflectance change rate of the passivation film can be reduced to 0.3% or less without deteriorating the long-term reliability of the semiconductor laser.

As a result, the change rate of the oscillation threshold current before and after the operation of the semiconductor laser can be suppressed to 1% or less, and the change rate of the optical output can be suppressed to 0.5% or less.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a passivation film having a two-layer structure.

FIG. 2 is a graph in which a calculation formula of the reflectance shown by the formula (4) is drawn by a computer.

FIG. 3 is a graph in which the difference in the reflectance of the passivation film when the refractive index of the first thin film changes is drawn by a computer.

[Description of Signs] 1 Passivation film 1A First thin film 1B Second thin film 2 Semiconductor laser 2A Active layer 2B Cladding layer 2C Cladding layer and substrate 2D Output end face

Claims (5)

[Claims] 1. A passivation film of a semiconductor laser having a two-layer structure including a first thin film formed directly on an output end face of a semiconductor laser and a second thin film formed on a surface of the first thin film. The thickness of the first thin film is a value in which a differential coefficient obtained by differentiating the calculation formula with the thickness of the first thin film becomes zero in a calculation formula for calculating the reflectance of the passivation film by a matrix method. Semiconductor laser passivation film. 2. A passivation film for a semiconductor laser having a two-layer structure comprising a first thin film formed directly on an output end face of a semiconductor laser and a second thin film formed on a surface of the first thin film. The first thin film is a Si thin film, and the second
A passivation film for a semiconductor laser, wherein the thin film is an AlOx thin film, and the thickness of the Si thin film is 15 to 25 nm. 3. The passivation film of a semiconductor laser according to claim 1, wherein said first thin film is a Si thin film, said second thin film is an AlOx thin film, and said Si thin film has a thickness of 15 to 25 nm. 4. A semiconductor laser, wherein the passivation film according to claim 1 is formed on an output end face. 5. A laser module, wherein the semiconductor laser according to claim 3 is incorporated as a light source.