JPH06338657A - Semiconductor laser and its manufacture - Google Patents

Abstract

PURPOSE:To obtain a chemically stable buried window structure wherein surface recombination is small in a low temperature process, by burying an InGaP layer in both ends of a resonator so as to intersect an active layer, with a specified length in the direction of a resonator. CONSTITUTION:A semiconductor laser is manufactured by using an active layer 6 composed of a single or multiple InyGa1-yAs quantum well layers (0< y<0.5), clad layers 3, 9 and guide layers 4, 8 composed of AlxGa1-xAs, a contact layer 10, etc. In order to form an InGaP buried window layer 11, etching is performed for each resonator pitch, until the depth intersecting in a stripe type the active layer 6. Further low temperature buried growth of InGaP is executed by using MOVPE. A laser epitaxial growth film provided with the InGaP buried window is completed, by etching InGaP grown on the laser epitaxial growth film except the buried window part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a highly reliable and high power pumping light source applicable to a fiber amplifier used in optical communication, a semiconductor laser which can be used as a second harmonic generation light source, and a manufacturing method thereof. .

[0002]

2. Description of the ^Related Art A fiber amplifier doped with Er ³⁺ ions can be operated in the 1.55 μm band where the optical propagation loss of a silica-based single mode fiber (SMF) is minimized, and is therefore a key device for optical communication. Is attracting attention as.
The 1.48 μm, 0.98 μm, and 0.82 μm bands have been studied as the light source wavelengths for exciting Er ³⁺ ions used for laser oscillation or amplification.

Particularly, in the 0.98 μm band, the amplification efficiency is high,
It has been confirmed that the noise characteristics are good, and it is a promising excitation wavelength band. T as an excitation laser in this wavelength band
i: Sapphire lasers have been used. On the other hand, recent I
Since a strained quantum well laser using an nGaAs layer as an active layer oscillates in this wavelength band, it has been actively studied as a small laser. A 0.98 μm laser with low threshold and high efficiency characteristics has been reported, but at high output, characteristic deterioration due to dislocations in crystals was observed, and a laser for exciting semiconductors with a sufficient lifetime has been obtained. There was no problem.

[0004]

Several studies have been conducted so far on the state of deterioration of a semiconductor laser. The defects are roughly classified into those caused by the growth and movement of crystal defects, the optical damage destruction (COD) of the resonator, the change of the surface state, and other ohmic electrodes and point defects. For 0.8 μm band short wavelength GaAs / AlGaAs lasers using GaAs as a substrate,
It is known to be optical damage, dark line deterioration due to a dislocation layer, and reflection surface deterioration.

COD is known as a phenomenon in which the optical output suddenly decreases as the operating current of the laser increases, causing irreversible destruction. This is because the vicinity of the end face of the resonator of the semiconductor laser is a slightly absorbing region for laser light. Non-radiative recombination occurs via the surface steps existing on the surface of the semiconductor crystal, which is lower than the internal carrier density at the resonator end. Therefore, for the oscillation wavelength at which the maximum gain inside the resonator is obtained, the gain is not obtained near the end face due to the decrease in carrier density and the region becomes an absorption region. Due to the light absorption, the temperature rises near the end face and the band gap E _g decreases. Further, the absorption increases and the temperature rises, and feedback is applied to the end face melting, and the element is destroyed.

Therefore, in order to increase the critical light output of COD, if the active region in the vicinity of the reflecting surface does not become the absorption region of laser light, positive feedback is not applied and the problem is solved. Further, the critical light output increases as the thermal conductivity of the crystalline material increases, and is finally determined by heat saturation due to heat generation. Specifically, it is considered that the active layer near the reflecting surface is made of a crystalline material having a larger bandgap than the active layer in the central portion (window structure), or the laser diode is made of a crystalline material having less surface recombination. To be

Up to now, as a window structure, a non-absorption mirror (NAM: No) by impurity diffusion and regrowth has been used.
n Absobing Mirror) has been proposed. The former is Zn,
By utilizing the thermal diffusion of Si or the like, the crystal is disordered in the vicinity of the active layer, the effective refractive index in the vicinity of the end face is lowered, and it becomes transparent to the oscillation wavelength. The latter is the original AlGa
In the As / GaAs laser epitaxial film, etching is performed until reaching the active layer at the cavity end, and Al _x Ga _{1-x is} again performed.
It has a structure in which As is embedded and grown. Where Al
By appropriately selecting the composition ratio z in consideration of the active layer bandgap E _g , the layer at the resonator end portion becomes transparent to the oscillation wavelength, and the window structure is formed.

However, in the conventional window structure forming method for GaAs / AlGaAs short-wave lasers, a high-temperature treatment process for diffusing impurities and an AlGaAs embedded window at a high temperature of 700 ° C. or higher for obtaining a high-quality AlGaAs film. It needs regrowth. There has been concern about diffusion of p-type dopants such as a high-concentration p-type cap layer and deterioration of laser epitaxial characteristics including the quantum well layer. Further, there is a problem that surface recombination at the chemically active AlGaAs embedded regrowth interface is large.

The present invention has been made in view of the above-mentioned prior art, provides a chemically stable buried window structure with small surface recombination in a low temperature process, and has high reliability and high output of 0.98 μm. An object of the present invention is to provide a band semiconductor laser and a manufacturing method thereof.

[0010]

The structure of the semiconductor laser of the present invention for achieving the above object is a single or multiple In _y Ga.
_1-y As quantum well layer (0 <y <0.5) active layer and Al _x G
In a semiconductor laser using an epitaxially grown film composed of a clad layer made of a _1-x As, a guide layer, a contact layer, etc., the active layer is crossed at both ends of the cavity with a certain length in the cavity direction. The InGaP layer is embedded. Further, the structure of the method for manufacturing a semiconductor laser according to the present invention, which achieves the above object, has a single or multiple In _y
Ga _1-y As quantum well layer (0 <y <0.5) active layer and Al
_In a method of manufacturing a semiconductor laser using an epitaxially grown film composed of a cladding layer made of _x Ga _1-x As, a guide layer, a contact layer, etc.
A certain length in the cavity direction, across the active layer, In
When the GaP layer is embedded and grown, a stripe-shaped insulating film is used which is removed except the window vicinity portion which is etched deeper than the active layer.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the embodiments shown in the drawings. FIG. 1 is a sectional view of a laser epitaxial growth film according to an embodiment of the present invention (resonator direction).
Indicates. 1 is an n ⁺ -GaAs substrate, 2 is an n ⁺ -GaAs buffer layer, 3 is an n-Al _x Ga _1-x As cladding layer, 4 and 8 are Al _z Ga _1-z As guide layers, 5 and 7 are SCH Layer (AlGa
(As: Al composition ratio is a value between 0 and guide composition ratio z)
, 6 is an In _y Ga _1-y As quantum well active layer, and 9 is p-Al _x G
a _1-x As clad layer, 10 is a p ⁺ -GaAs contact layer,
Reference numeral 11 is an InGaP embedded window layer.

In order to realize this structure, first, an epitaxial crystal growth apparatus (MOVPE method: metal organic chemical vapor deposition method or MBE method: molecular epitaxy method) is used.
Epitaxial layers 2 to 10 are grown. Typical values are n-Al _x Ga _1-x As cladding layer 3 and p-Al _x Ga.
The Al composition ratio x of the _1-x As clad layer 9 is 0.3 to 0.6, and Al _z Ga is
The Al composition ratio z of the _1-z As guide layers 4 and 8 is 0.2 to 0.5,
The n-Al _x Ga _1-x As cladding layer 3 contains Se, Si or the like as an n-dopant, and the p-Al _x Ga _1-x As cladding layer 9 contains
Doping with Zn, Mg, Be or the like is performed at about 5 × 10 ¹⁷ cm ^-3 .

The Al _z Ga _1-z As guide layers 4 and 8 are each doped with n or p or used undoped.
The In composition ratio y and the thickness of the In _y Ga _1-y As well layer 6 are typically 0.2 and 10 nm. Since the p ⁺ -GaAs contact layer 10 is an ohmic electrode, it has a Zn content of 5 × 10 ¹⁹ cm ^-3 or more.
High-concentration doping of p dopant such as

Next, the InGaP embedded window layer 11
In order to form the film, etching is performed to a depth across the active layer with a stripe width of 10 to 80 μm for each resonator pitch length, and then selective burying growth of InGaP lattice-matched to GaAs at 600 to 650 ° C. is performed by the MOVPE method. . That is, FIG.
As shown in (a), first, an insulating film 12 such as SiO ₂ or SiN is deposited on the laser epitaxial substrate.

Next, as shown in FIG. 3 (b), a stripe-shaped buried window pattern is formed, and window opening etching is performed on the insulating film 12 by a dry etching method.
The pitch is set according to the laser resonator length. After that, as shown in FIG. 3C, H ₂ SO ₄ system or NH
_It is deeper than the active layer 6 with a ₄ OH-based etching solution and is roughly 2
Etch to a depth of ~ 3 μm. The etching is made into an inverted mesa shape by appropriately selecting the substrate surface direction. At this time, dry etching such as reactive ion etching can also be used.

Subsequently, as shown in FIG. 3D, in order to avoid poly growth on the insulating film 12 serving as a selective mask, the insulating film 12 is removed except for the vicinity of the window by a photo process. Multiple copies are left as a stripe width selection mask. This width corresponds to the insulating film 12 of the growth species In and Ga.
Since it is shorter than the above McGlation distance, the insulating film 12
Poly growth above is almost suppressed. Further, as shown in FIG. 3E, low-temperature embedded growth of InGaP (600 to 650 ° C.) is performed using MOVPE.

Thereafter, as shown in FIG. 3 (f), patterning was performed using a photo process, and InGa grown on the laser epitaxial layer other than the buried window portion was patterned.
Selective for P with an etching solution such as HCl (against GaAs)
Etching, and finally removing the insulating film 12,
A laser epitaxial growth film with an InGaP embedded window is completed.

After the burying growth, as shown in FIG. 4, in order to form a ridge having a width of about 1.5 to 3 μm on the contact layer 10 and the cladding layer 9, patterning is performed by photolithography, and this is used as a mask for wet or dry. The contact layer 10 and the cladding layer 9 are etched by etching. The depth is determined by considering the transverse mode,
The guide layer 8 may also be etched.

[0019] After forming the ridge, and removing the mask, the insulating film 11 by sputtering or the like (SiO ₂ or the like) is formed on the entire surface, after the SiO ₂ of the ridge top and etched off, Cr / Au
Alternatively, a p-electrode 13 such as Ti / Pt / Au or an n-type such as AuGeNi
The electrode 14 is formed. After that, ohmic sintering,
The laser structure of FIG. 1 is completed.

Regarding the semiconductor laser according to the above embodiment,
When the current-light output characteristics were measured, the results shown in FIG. 5 were obtained. In FIG. 5, In of the present embodiment is indicated.
Is a result of a semiconductor laser having a GaP embedded window structure, and is a result of a conventional semiconductor laser having no such window structure. As is clear from FIG. 5, the conventional semiconductor laser shows a sudden decrease in optical output at high output, whereas the semiconductor laser of the present embodiment shows
It can be seen that no sudden decrease in light output at high output is observed and reversible thermal saturation characteristics are observed. Therefore, in the semiconductor laser of the present embodiment, the failure deterioration due to COD can be solved even in the long-term energization test with a higher current than in the past.

Although a single quantum well is used as the active layer 6 in the above-described embodiment, the present invention is not limited to this, and a multiple layer having In _y Ga _1-y As as a well layer and AlGaAs as a barrier is used. It can also be applied to a laser epitaxial structure using the quantum well structure as the active layer 6. Also, 0.98 ± 0.05μ
The In composition ratio and the thickness of the m that can oscillate are 0.15 <y <0.
It can be selected in the range of 3, 3 to 20 nm.

FIG. 2 shows another embodiment of the present invention 2. In this embodiment, In is co-doped at a concentration of 1 × 10 ^{19 to} 3 × 10 ²⁰ to suppress generation and propagation of dislocations in the contact layer 10. Other configurations are the same as those of the embodiment shown in FIG. is there. In addition, within the critical thickness range for the cladding layer,
In may be co-doped at a concentration of 1 × 10 ^{19 to} 3 × 10 ²⁰ .

[0023]

As described above in detail based on the embodiments, the laser light is emitted to the outside of the resonator through the InGaP layer which has a band gap larger than that of the active layer of the semiconductor laser and is chemically stable. During high current injection, element deterioration due to COD that occurs during long-term current injection does not occur,
A more reliable and high-power semiconductor laser can be obtained as compared with the AlGaAs window. Further, by using InGaP which can be grown at a low temperature and can be selectively etched with GaAs as a buried layer, it is possible to avoid the deterioration of the device characteristics and the reduction of the yield due to the conventional high temperature treatment.

[Brief description of drawings]

FIG. 1 is a sectional view of a semiconductor laser according to an embodiment of the present invention.

FIG. 2 is a sectional view of a semiconductor laser according to another embodiment of the present invention.

FIG. 3 is a process diagram showing an embedded window growth process.

FIG. 4 is a sectional view of a ridge laser.

FIG. 5 is a light output characteristic diagram.

[Explanation of symbols]

1 n ⁺ -GaAs substrate 2 n ⁺ -GaAs buffer layer 3 n-Al _x Ga _1-x As clad layer 4,8 Al _z Ga _1-z As guide layer 5,7 SCH layer (AlGaAs: Al composition ratio is 0 To a guide composition ratio z) 6 In _y Ga _1-y As quantum well active layer 9 p-Al _x Ga _1-x As clad layer 10 p ⁺ -GaAs contact layer 11 InGaP buried window layer

Claims (2)

[Claims] 1. An active layer comprising a single or multiple In _y Ga _1-y As quantum well layers (0 <y <0.5) and a cladding layer, a guide layer, a contact layer comprising Al _x Ga _1-x As, etc. 2. A semiconductor laser using an epitaxially grown film configured as described above, characterized in that an InGaP layer is embedded at both ends of the resonator across the active layer with a certain length in the resonator direction. 2. An active layer composed of a single or multiple In _y Ga _1-y As quantum well layers (0 <y <0.5) and a cladding layer, a guide layer, a contact layer, etc. composed of Al _x Ga _1-x As. In a method of manufacturing a semiconductor laser using an epitaxially grown film composed of, when an InGaP layer is embedded and grown across both ends of the resonator with a certain length in the resonator direction, A method of manufacturing a semiconductor laser, characterized in that a stripe-shaped insulating film is used, in which a portion other than a window vicinity portion which is deeply etched is removed.