JP2669401B2 - Semiconductor laser and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser and a method of manufacturing the same, and more particularly to a real index guided laser using a III-V compound semiconductor containing Al for a buried layer.

[0002]

2. Description of the Related Art AlGaInP oscillating in the 0.6 μm band
System visible light semiconductor lasers have been put to practical use as light sources for pointers and bar code readers. This laser has a ridge stripe structure (for example, Electronics Letters), Vol. 23 (1987).
(Year) 1327 to 1328] is common. In this structure, GaAs is formed in the selective burying layer (current blocking layer).
And fundamental transverse mode oscillation is obtained by utilizing light absorption in this layer.

FIG. 5 shows a cross-sectional structure of this type of embedded laser. The method of manufacturing the laser shown in FIG. 5 uses an organic metal vapor phase epitaxy (hereinafter, referred to as MOVPE) apparatus to form an n-type GaAs buffer layer 20 on an n-type GaAs substrate 201.
2, n-type AlGaInP outer cladding layer 203, n-type A
lGaInP inner cladding layer 204, active layer 205, p
AlGaInP inner cladding 206, p-type AlGaI
The nP outer clad layer 207, the p-type GaInP heterobuffer layer 208, and the p-type GaAs cap layer 209 are sequentially grown. Next, this heterostructure substrate is taken out from the MOVPE apparatus and subjected to a usual pyrolysis vapor deposition method (thermal CVD method).
As a result, a silicon oxide film is deposited on the substrate surface. Subsequently, the silicon oxide film is processed into a stripe shape having a width of about 5 μm by using a normal photolithography method.

Further, using this silicon oxide film as a mask, the heterostructure substrate is etched into a mesa shape. Then, this heterostructure substrate is again introduced into the MOVPE apparatus, and the n-type GaAs current blocking layer 210 is selectively grown. Then, it is taken out from the MOVPE apparatus, the silicon oxide mask is removed, and then the p-type GaA is again used in the MOVPE apparatus.
The s contact layer 212 is grown. Then, an n-side electrode 213 and a p-side electrode 214 are formed on the obtained heterostructure substrate to obtain the laser of FIG.

However, the semiconductor laser having this structure has a problem that the threshold current increases due to loss due to light absorption (waveguide loss), and the external differential quantum efficiency decreases. The increase of the threshold current and the decrease of the external differential quantum efficiency result in the increase of the driving current of the laser, which lowers the reliability. Therefore, it is essential to reduce the threshold current in order to manufacture a highly reliable laser.

In order to reduce the waveguide loss, AlI having a band gap larger than that of the active layer and a refractive index lower than that of the cladding is used.
A laser structure (real refractive index guided laser) using an nP or AlGaInP layer instead of GaAs is effective.
Similarly, AlGaAs that oscillates in the 0.8 to 0.9 μm band
AlGaAs or Al with high Al composition
The structure in which As is used as the selective buried layer is effective for reducing the waveguide loss. Thus, III- containing Al
Group V semiconductors (AlInP, AlGaInP, AlGaA
Conventional examples of real index waveguide lasers using s, AlAs, AlInAs, AlGaInAs, etc.) as buried layers have been reported as follows.

[0007] The first conventional example is an Al alloy reported by Manno et al.
_{This is} a real refractive index guided laser using _0.5 In _0.5 P for the selective buried layer, and a low astigmatic difference (4 μm) which is a characteristic of the real refractive index guided laser has been reported (50th Autumn 1989). Proceedings of the 12th JSAP, 28a-ZG-
4). In this report, the inverted mesa structure has a high resistance of Al _0.5 In
_0.5 P and n-type GaAs are embedded.

[0008] A second prior art example is an example of A reported by the present inventors.
AlGaInP using l _0.5 In _0.5 P as a buried layer
System laser (1994 Proceedings of International Conference on Semiconductor Lasers, Th3.5, page 243). In this document, it was shown that the waveguide loss was reduced to 14 cm ⁻¹ , thereby reducing the threshold current and increasing the external differential quantum efficiency.
s is compared with a laser using a buried layer. In this report, a high-resistance Al _0.5 In
_0.5 P and n-type GaAs are embedded.

[0009] A third conventional example is the Al _0.7 reported by Shima et al.
It is an AlGaAs-based laser using Ga _0.3 As for the buried layer, and reports that Al _0.7 Ga _0.3 As is buried in the reverse mesa structure to achieve a reduction in threshold current and an improvement in external differential quantum efficiency ( Autumn 1993 Proceedings of the 54th Annual Meeting of the Japan Society of Applied Physics No.3 29p-k-1
4).

AlInP or AlG is used as the selective buried layer.
When fabricating a real refractive index guided laser using aInP, AlGaAs, AlInAs, or AlAs, a layer containing Al must be selectively grown.
However, it is difficult to selectively grow a layer containing Al having a high Al composition by the usual MOVPE method, and it is difficult to grow Cl during the growth.
(Autumn 1992 Autumn 53rd Annual Meeting of the Society of Applied Physics, Proceedings No. 316p-ZE-14, 15 and Journal of Crystal Growth (Journal)
of Crystal Growth) 124 (1992) 235-24
Page 2) or using raw materials containing Cl (Journal of Electrochemical Society (Jour
nal of Electrochemical Society) 138 (1991)
1817-1826) to achieve selective growth.

[0011]

Problems to be Solved by the Invention AlInP, AlGa
InP, AlGaInAs, and AlInAs have a large dependence of the lattice constant on the Al composition. When these semiconductors are used for the selective embedding layer of the ridge / stripe laser, Al, Ga, and In are taken in by the mesa side portion formed by the (111) plane and the flat portion formed by the (001) plane. Since the ratios are different, the compositions of the grown layers are different.

Usually, it has the same plane orientation as the substrate (001).
Since the growth conditions are selected so that the lattice constants of the planes match, lattice irregularity occurs on the mesa side and distortion occurs at the interface. If the strain is too large, dislocations (defects) are generated, and the dislocations affect not only the buried layer but also the active layer, and reduce the reliability of the laser.

Conventionally, this difficulty has been solved by adding HCl to the raw material gas of the MOVPE method or by using a raw material containing Cl. However, when HCl is added,
Oxygen, moisture, and the like remain in the HCl cylinder, which causes a problem in purity and a problem of lowering the crystallinity of the grown layer. Further, there is a problem that the stainless steel pipe is corroded by oxygen and water. Also, when a raw material containing Cl is used, there is a problem similar to the case of using HCl in terms of purity and corrosion of stainless steel piping.

The present inventors have decided to use HCl on a 2-inch wafer.
An AlGaInP-based red laser using AlInP grown as a selective burying layer by adding Al was prototyped, and the reliability and yield of the laser were examined. The structure of the prototype semiconductor laser is the same as that of the conventional structure shown in FIG.
A wide bandgap, low refractive index n-type (silicon-doped) Al _0.5 In _0.5 P (thickness 0.7 μm) obtained by selectively growing the s current blocking layer 210 by the MOVPE method with HCl addition.
m).

AlInP was formed by setting the growth conditions so that the (001) plane was lattice-matched. The active layer is a strained quantum well composed of an AlGaInP barrier and a GaInP well, and the strain and the well thickness are set so that the oscillation wavelength becomes 636 nm. The other laser structure and manufacturing method are similar to those of the conventional laser described with reference to FIG.

When the characteristics of the prototype laser were evaluated,
As compared with a conventional laser using GaAs as a buried layer, the threshold current was reduced, the external differential quantum efficiency was increased, and the improvement of the laser characteristics was confirmed. However, the laser with improved characteristics is obtained from a part (upstream side with respect to the gas flow) of a 2-inch wafer, and the laser obtained from the remaining part has a high threshold current and has an external derivative. The efficiency was low and the characteristics were rather deteriorated. Yield is about 7% (45
It was 3 elements among the elements).

When a laser having poor characteristics was examined by a transmission electron microscope, many dislocations were observed in the buried layer. From the result of the composition analysis, it was considered that the composition of the AlInP layer changed and lattice mismatch with the substrate occurred, and as a result, dislocation occurred. It is considered that the reason why such a composition shift occurs on the 2-inch wafer is that a concentration difference of HCl occurs in the flow direction, and the composition ratio of Al and In changes depending on the location. Thus, MOVP with HCl added
The laser using AlInP grown by the E method for the buried layer has a drawback that the yield is extremely low due to a composition change.

On the other hand, when an element having good characteristics (seven elements) was subjected to an energization test with a constant light output of 5 mW at an ambient temperature of 50 ° C., the drive current of six elements rapidly increased in about several hundred hours, and suddenly increased. It has deteriorated to. Observation of this laser with a transmission electron microscope revealed that dislocations were running from the interface of the AlInP buried layer in the mesa block toward the active layer. Therefore, it is considered that this dislocation caused sudden deterioration.

The reason for the occurrence of dislocations is that, as described above, the composition changes between the mesa side part (111) plane and the flat part (001) plane due to the different incorporation rate of Al and In. It is considered that dislocations occurred due to lattice misalignment. As described above, even a laser having good initial characteristics has a disadvantage that dislocation occurs at the interface due to a difference in composition between the flat portion and the mesa side portion, thereby lowering the reliability of the laser.

Further, a p-type GaAs contact layer 212
A large number of defects were also generated at the interface between the AlInP layer and the embedded AlInP layer, which was observed in a cross-sectional observation with a scanning electron microscope. These defects are also considered to be one of the factors that reduce the reliability of the laser. This defect is caused by AlInP
This is because the AlInP surface was oxidized and contaminated when the wafer was exposed to the air after the growth to remove the silicon oxide mask.

The object of the present invention is to overcome the above-mentioned drawbacks of the conventional examples, and to use AlInP or AlGaInP or AlI.
It is an object of the present invention to provide a highly reliable laser with a high yield in a laser in which a semiconductor containing Al such as nAs or AlGaInAs and having a lattice constant that changes depending on the composition is used as a buried layer.

[0022]

According to the present invention, there is provided a double hetero structure having an active layer sandwiched between a first conductive type clad layer and a second conductive type clad layer having a mesa shape. In the semiconductor laser, wherein the mesa side portion is buried with a III-V group compound semiconductor containing Al having a band gap larger than that of the active layer and a refractive index lower than that of the clad, the buried layer has a thickness of 0.5 μm or less. A semiconductor laser is provided. And
Preferably, GaAs or Ga is formed on the buried layer.
_A cap layer made of _0.5 In _0.5 P is laminated.

According to the present invention, (1) a first conductive type clad layer, an active layer, a second conductive type
A step of sequentially growing a conductive type cladding layer and an insulating film; (2) a step of selectively etching the insulating film to form a stripe-shaped mask; and (3) a step of using the mask to form the second conductive type. (C) processing the clad layer into a mesa shape by selectively etching the clad layer halfway; and (4) a band gap larger than the active layer on the surface of the second clad layer with the mask mounted. And a step of selectively growing a III-V group compound semiconductor containing Al having a lower refractive index than the second conductivity type clad layer to a thickness of 0.5 μm or less to form a first buried layer. A method for manufacturing a semiconductor laser is provided. Then, preferably, following the step (4), a step of selectively growing a semiconductor not containing Al on the first buried layer in the same crystal growth apparatus to form a second buried layer. Is added.

[0024]

The operation of the semiconductor laser and the method of manufacturing the same according to the present invention will be described below. FIG. 3 is a diagram showing the relationship between the average density of polycrystals deposited on the silicon oxide mask and the thickness of the AlInP layer. The black circles (●) are the results without HCl, and the white circles (∘) are the results with HCl added. In the figure, the average polycrystal density is shown as a normalized value with the maximum density of the silicon oxide mask that can be removed by etching as "1". Therefore, the average polycrystalline density is 1
The laser can be manufactured as follows. The width of the silicon oxide mask is 5 μm, which is the same as that used for laser fabrication. When HCl was added, the experimental AlInP
Polycrystalline precipitation is very low up to a layer thickness of 0.7 μm. However, as described above, the manufacturing yield of the laser is reduced due to the composition variation of the AlInP layer due to the difference in HCl concentration in the flow direction. On the other hand, when HCl is not added,
When the thickness of AlInP is 0.5 μm or less, selective growth is possible with an average polycrystalline density of 1 or less, and the production yield is improved because HCl is not added.

FIG. 4 is a diagram showing the relationship between the estimated lifetime of the manufactured AlInP embedded laser and the thickness of the AlInP layer. Estimated lifespan: ambient temperature 50 ° C, constant light output 5 m
It is an estimated time when the injection current is 1.2 times the initial value when driven under the condition of W. The growth of AlInP was performed while adding HCl. As is clear from this figure, if the thickness of the AlInP layer to be embedded is 0.5 μm or less, the estimated lifetime reaches 10,000 hours, and a highly reliable laser can be obtained. This is because if the thickness of the AlInP layer is 0.5 μm or less, it is possible to suppress the occurrence of dislocation due to the strain generated on the mesa side.

As described above, the thickness of the selective embedding layer is set to 0.
A highly reliable laser can be manufactured by adopting the structure of the semiconductor laser of the present invention having a thickness of 5 μm or less. Further, by employing the manufacturing method of the present invention in which the growth of the selective burying layer is 0.5 μm or less, selective growth can be realized without adding HCl, and a semiconductor laser can be manufactured with a high yield.

Further, AlInP or AlGaInP
After embedding the layer, GaAs to prevent oxidation and contamination,
By continuously growing a cap layer containing no Al of GaInP, even if the wafer is exposed to the air to remove a silicon oxide mask, damage to the buried layer can be eliminated, and a highly reliable laser can be manufactured. You can

[0028]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
Will be explained. FIG. 1 shows AlGa according to an embodiment of the present invention.
It is a sectional view of an InP-based laser. As shown in the figure
In addition, the semiconductor laser of the present embodiment is a Si-doped n-type Ga.
Si-doped n-type GaAs buffer on As substrate 101
Layer 102 (Si concentration: n = 1 × 10¹⁸cm^-3, Film thickness:
t = 1 μm), Si-doped n-type (Al_0.7 Ga_0.3 )
_0.5 In_0.5 P clad layer 103 (n = 1 × 10¹⁷cm
^-3, T = 0.6 μm), undoped (Al_0.5 Ga
_0.5 )_0.5 In_0.5 P light guide layer 104 (t = 50n
m), non-doped Ga as the active layer 105_0.5 In_0.5 
P-well (t = 8 nm: 4 layers) and non-doped (Al_0.5 
Ga_0.5 )_0.5 In_0.5 P barrier (t = 5 nm: 3 layers)
Multi-quantum well layer composed of non-doped (Al_0.5 Ga
_0.5 )_0.5 In_0.5 P light guide layer 106 (t = 50n)
m), Zn-doped p-type (Al_0.7 Ga_0.3 )_0.5 In
_0.5 P clad layer 107 (Zn concentration: p = 1 × 10¹⁷c
m^-3, T = 0.6 μm), Zn-doped p-type Ga_0.5 I
n_0.5 P hetero buffer layer 108 (p = 5 × 10¹⁷cm
^-3, T = 0.1 μm), Zn-doped p-type GaAs capacitor
Up layer 109 (p = 1 × 10¹⁸cm^-3, T = 0.4μ
m), Si-doped n-type Al_0.5 In_0.5 P current block
Layer 110 (n = 1 × 10¹⁷cm^-3, T = 0.1, 0.
3, 0.5 μm), Si-doped n-type GaAs cap
Layer 111 (n = 1 × 10 ¹⁸cm^-3, T = 0.3 μm),
Zn-doped GaAs contact layer 112 (p = 3 × 10
¹⁸cm^-3, T = 3 μm), and further on the back side of the substrate on the n side.
The p-side on the electrode 113 and the p-type GaAs contact layer 112
Embedded double heterostructure with electrode 114
ing.

Next, a method of manufacturing the semiconductor laser of this embodiment shown in FIG. 1 will be described. FIG. 2 (a) to (c)
4A to 4C are step-by-step cross-sectional views for describing the manufacturing method of this example. MOV equipped with a water-cooled horizontal reaction tube for crystal growth
A PE device was used. AlGaInP, GaI as raw materials
For nP and AlInP growth, trimethyl aluminum (TMAl), triethyl gallium (TEGa), trimethyl indium (TMIn), phosphine (PH
₃ ) and triethylgallium (T
EGa) and arsine (AsH ₃ ).

Further, disilane (Si ₂ H ₆ ) and diethyl zinc (DEZn) were used as the n-type and p-type dopants. An RF coil was used for heating the substrate. The growth temperature was 660 ° C., and the growth pressure was 70 torr. Also, the growth rate is 1.5 μm / for AlGaInP and GaInP.
h, AlInP 0.5 μm / h, GaAs 4.5 μm
m / h.

First, after etching the n-type GaAs substrate 101 with a sulfuric acid solution, the n-type GaAs substrate 101 is placed in a MOVPE reaction tube, and the n-type GaAs buffer layer 102 and n-type (Al _0.7 Ga _0.3 ) are formed.
_0.5 In _0.5 P cladding layer 103, non-doped (Al _0.5
Ga _0.5 ) _0.5 In _0.5 P optical guide layer 104, active layer 105, undoped (Al _0.5 Ga _0.5 ) _0.5 In _0.5
P light guide layer 106, p-type (Al _0.7 Ga _0.3 ) _0.5 I
n _0.5 P clad layer 107, p-type Ga _0.5 In _0.5 P heterobuffer layer 108, p-type GaAs cap layer 109
Are sequentially grown to obtain a heterostructure substrate shown in FIG.

After removing from the reaction tube, thermal CVD (40
(0 ° C.) to deposit silicon oxide to a thickness of 0.3 μm.
Next, this silicon oxide is etched into a stripe shape having a width of 5 μm by a normal photolithography technique to form a silicon oxide mask 115. Next, using the silicon oxide mask 115 as a mask, the p-type GaAs cap layer 109, the p-type GaInP hetero-buffer layer 108, and the p-type AlGaInP
7 is selectively etched to form a mesa structure on the heterostructure substrate as shown in FIG. 2 (b).

This substrate is placed in the reaction tube again, and the n-type AlInP current blocking layer 110 (0.1, 0.3, 0.5 μm) and the n-type Ga
The As cap layer 111 is selectively grown continuously [FIG.
(C)]. Here, the AlInP layer has a thickness of 0.5 μm or less (as a sample, the thickness of the block layer is 3 μm).
Type), without adding HCl,
Selective growth was possible by a normal growth method.

After that, the silicon oxide mask 115 is removed from the reaction tube and the silicon oxide mask 115 is removed.
A type GaAs contact layer 112 was grown. Finally,
An n-side electrode made of Au-Ge is formed on the back surface of the substrate by p-type GaAs.
A p-side electrode 114 made of Cr-Au was formed on the s contact layer 112 to obtain a semiconductor laser having the structure shown in FIG.

The obtained laser (resonator length 500 μm, end face 30% -95% coating) has the lowest threshold current of 36 mA, slope efficiency of 0.80 W / A, and the conventional GaAs embedded laser (threshold value). Current is 50mA,
The slope efficiency was a characteristic exceeding 0.60 W / A).
The laser characteristics are determined by the AlInP current blocking layer 110.
Was almost the same without largely depending on the thickness (0.1 to 0.5 μm) of the sample. . The oscillation wavelength is 636 nm. Three places on the 2-inch substrate (upstream, middle, and downstream of the gas flow)
Out of the 65-element laser obtained from
5 A for lasers with a slope efficiency of 0.70 W / A or more at A or less
There were eight elements, and the yield was 90%, which was a great improvement over the conventional one.

Further, when these laser elements (15 elements) were subjected to an energization test at a constant output of 5 mW at an ambient temperature of 50 ° C., 13 elements operated stably without sudden deterioration over 3000 hours. The estimated lifetime of these lasers could be predicted to be 10,000 hours from the rate of increase of the drive current.

In the above embodiment, the current blocking layer is made of AlIn.
Although the case of P was described, (Al _x Ga _{1 -x} ) _0.5
Even when an In _0.5 P layer (0.7 <X <1) is used, the value of 0.1.
Up to 5 μm, selective growth was possible without adding HCl, and a highly reliable laser was obtained at a high yield as in the case of AlInP.

In this embodiment, AlInP and AlGa are used.
The case where InP is used as the material of the buried layer has been described, but in addition, AlInAs, AlGaInAs, AlInN,
The present invention can also be applied to a case where the lattice constant changes depending on the composition such as AlGaInN and a compound semiconductor containing Al is used as a buried layer forming material.

[0039]

As described above, in the semiconductor laser according to the present invention, the side surface of the mesa portion formed in the cladding layer has a bandgap larger than that of the active layer and 0.5 μm or less containing Al having a lower refractive index than the cladding layer. It is possible to selectively grow the buried layer without adding HCl because it is buried with the buried layer. Therefore, according to the present invention, it is possible to prevent corrosion of the crystal growth apparatus and improve the crystallinity of the buried growth layer. Further, according to the present invention, even if the buried layer is formed with different composition and lattice constant between the side surface of the mesa portion and the flat portion, generation of dislocations can be suppressed, and a highly reliable real refractive index can be obtained. It becomes possible to manufacture a waveguide type laser with a high yield.

[Brief description of the drawings]

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

2A to 2D are sectional views in order of the processes, for explaining the manufacturing method of the embodiment shown in FIG.

FIG. 3 is a graph showing the relationship between the thickness of a burying layer and the average polycrystalline density of a burying material deposited on a silicon oxide mask, for explaining the operation of the present invention.

FIG. 4 is a graph showing the relationship between the thickness of the buried layer and the estimated life, for explaining the operation of the present invention.

FIG. 5 is a sectional view of a conventional semiconductor laser.

[Explanation of symbols]

 101, 201 n-type GaAs substrate 102, 202 n-type GaAs buffer layer 103 n-type AlGaInP clad layer 203 n-type AlGaInP outer clad layer 104 non-doped AlGaInP light guide layer 204 n-type AlGaInP inner clad layer 105, 205 active layer 106 non-doped AlGaInP light Guide layer 206 p-type AlGaInP inner clad layer 107 p-type AlGaInP clad layer 207 p-type AlGaInP outer clad layer 108, 208 p-type GaInP heterobuffer layer 109, 209 p-type GaAs cap layer 110 n-type AlInP current blocking layer 210 n-type GaAs Current blocking layer 111 n-type GaAs cap layer 112, 212 p-type GaAs contact layer 113, 213 n-side electrode 114, 214 p-side electrode 115 Silicon oxide mask

Continuation of front page (56) Reference JP-A-4-208586 (JP, A) 14TH IEEE INT SEMI CONDCTOR LASER CON F 1994 P. 243-244 ELECTRON. LETT. 32 to 10! (1996) P. 894-896

Claims (5)

(57) [Claims] 1. A semiconductor device having a double hetero structure in which an active layer is sandwiched between a first conductive type clad layer and a second conductive type clad layer having a mesa shape. III- containing low refractive index Al
In a semiconductor laser embedded with a group V compound semiconductor,
A semiconductor laser, wherein the thickness of the buried layer is 0.5 μm or less. 2. The method according to claim 1, wherein the active layer is GaInP or AlG.
aInP or one of them as a quantum well, the first conductivity type and second conductivity type cladding layers are formed of AlGaInP, and the buried layer is (Al _x Ga _{1 -x} ) _0.5 In _0.5 P (0.7 <X
.Ltoreq.1.0). The semiconductor laser according to claim 1, wherein: 3. GaAs or G on the buried layer
3. The semiconductor laser according to claim 1, wherein a cap layer made of a _0.5 In _0.5 P is laminated. 4. The method according to claim 1, wherein: (1) forming a first conductive type semiconductor substrate on the first conductive type semiconductor substrate;
A step of sequentially growing a conductive type clad layer, an active layer, a second conductive type clad layer, and an insulating film; (2) a step of selectively etching the insulating film to form a stripe-shaped mask; A step of selectively etching the second conductivity type cladding layer halfway using the mask to process the cladding layer into a mesa shape; and (4) removing the second cladding layer with the mask mounted. The surface contained Al having a band gap larger than that of the active layer and a refractive index lower than that of the second-conductivity-type cladding layer.
Forming a first buried layer by selectively growing a group III-V compound semiconductor to a thickness of 0.5 μm or less. 5. A step of forming a second buried layer by selectively growing a semiconductor not containing Al on the first buried layer in the same crystal growth apparatus, following the step (4). 5. The device according to claim 4, wherein
The manufacturing method of the semiconductor laser according to the above.