Semiconductor Laser Device

The present invention generally relates to visible light semiconductor laser devices using materials containing AlGaInP, and more particularly, to semiconductor laser devices preventing inferior growth of AlGaInP in a groove to achieve a good SBA (Self-aligned with Bent Active layer) structure.

FIG. 3(a) is a cross-sectional view illustrating an effective refractive index waveguide type semiconductor laser device using materials containing AlGaAs described in, for example, an article by Y. Mihashi, Y. Nagai, H. Matsubara and K. Ikeda, entitled "A Novel Self-Aligned Laser with Small Astigmatism Grown by MO-CVD", Extended Abstracts of 17th Conference on Solid State Devices and Materials, 1985, pp. 63-66. In FIG. 3(a), reference numeral 1 designates a substrate comprising p type GaAs. A p type Al.sub.0.43 Ga.sub.0.57 As layer 2 having a thickness of 1 micron is disposed on the substrate 1. An n type GaAs current blocking layer 3 having a thickness of 1 micron is disposed on the p type AlGaAs layer 2. A p type Al.sub.0.43 Ga.sub.0.57 As cladding layer 4 having a thickness of 1 micron is disposed on the current blocking layer 3. An undoped Al.sub.0.07 Ga.sub.0.93 As active layer 5 having a thickness of 0.07 micron is disposed on the cladding layer 4. An n type Al.sub.0.43 Ga.sub.0.57 As cladding layer 6 having a thickness of 1.3 micron is disposed on the active layer 5. An n type GaAs contact layer 7 having a thickness of 1 micron is disposed on the cladding layer 6. In addition, reference numeral 8 designates a stripe-shaped groove provided on the current blocking layer 3, and reference numeral 9 designates an oscillating region.

A semiconductor laser device having the structure as shown in FIG. 3(a) is generally referred to as an SBA (Self-aligned structure laser with bent active layer) laser.

Description is now made of a method of fabricating this laser. First, a p type AlGaAs layer 2 and an n type GaAs current blocking layer 3 are epitaxially grown on a (100) p type GaAs substrate 1 and then, a stripe-shaped groove 8 serving as a current path is formed on the current blocking layer 3 along the [011] direction. Then, a p type AlGaAs cladding layer 4, an undoped AlGaAs active layer 5, an n type AlGaAs cladding layer 6, and an n type GaAs contact layer 7 are sequentially grown by MOCVD (Metal Organic Chemical Vapor Deposition). When materials containing AlGaAs are grown by MOCVD, the growth proceeds in such a manner as to maintain the shape of the stripe-shaped groove 8. Consequently, the laser structure as shown in FIG. 3(a) is achieved. This characteristics is inherent to MOCVD. In such a structure with a grown layer that is bent, current is efficiently concentrated in an oscillating region 9. Accordingly, the threshold current of laser oscillation can be reduced. In addition, the SBA laser is constructed such that the cladding layer 4 comprising p type Al.sub.0.43 Ga.sub.0.57 exists on right and left sides of the active layer 5 comprising Al.sub.0.07 Ga.sub.0.93 As at the oscillating region 9. Accordingly, an effective refractive index difference occurs in a lateral direction of the oscillating region 9. Consequently, the SBA laser has the advantages that a lateral mode can be stably controlled and astigmatism can be reduced.

Although the SBA laser has the above-described advantages, the same structure is achieved by materials containing AlGaInP as shown in FIG. 3(b). In FIG. 3(b), reference numeral 10 designates a p type AlGaInP cladding layer having a thickness of 0.8 to 1 micron, reference numeral 11 designates a GaInP active layer having a thickness of 0.07 micron, and reference numeral 12 designates an n type AlGaInP cladding layer having a thickness of 0.8 to 1 micron. Thus, when a stripe-shaped groove is buried by growing materials containing AlGaInP, abnormal growth occurs. Consequently, a good SBA laser structure such as that using materials containing AlGaAs can not be achieved using materials containing AlGaInP.

The conventional semiconductor laser device is constructed as described above. Accordingly, the semiconductor laser device has the disadvantage in that it is difficult to achieve the above-described good SBA structure using the materials containing AlGaInP.

An object of the present invention is to overcome the difficulty in burying a groove by making growth with materials containing AlGaInP to provide a semiconductor laser device comprising materials containing AlGaInP having a good SBA structure.

Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter, it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

According to the present invention, a semiconductor laser device includes a cladding structure disposed on a current blocking layer having a stripe-shaped groove current path to bury the strip-shaped groove comprising a two-layer structure including a first cladding layer comprising AlGaAs disposed on the current blocking layer and the stripe-shaped groove, and a second cladding layer comprising AlGaInP which is sufficiently thin not to cause inferior growth of the active layer and disposed on the first cladding layer.

Therefore, the thickness of the second cladding layer comprising AlGaInP can be reduced in burying a groove by mainly growing the first cladding layer comprising AlGaAs, so that inferior growth in the groove can be rcstrained to obtain a good SBA structure using materials containing AlGaInP.

FIG. 1 is a cross-sectional view showing a structure of a semiconductor laser device according to an embodiment of the present invention;

FIGS. 2(a) and 2(b) are diagrams respectively showing the energy band structure and the refractive index distribution of the semiconductor laser device according to an embodiment of the present invention; and

FIGS. 3(a) and 3(b) are cross-sectional views showing structures of prior art semiconductor laser devices.

An embodiment of the present invention will be described in detail with reference to the drawings.

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

In FIG. 1, reference numeral 1 designates a p type GaAs substrate. An n type GaAs current blocking layer 3 having a thickness of 1 micron is disposed on the substrate 1. A p type Al.sub.0.8 Ga.sub.0.2 As cladding layer 13 having a thickness of approximately 0.6 to 1 micron is disposed on the current blocking layer 3. A p type (Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P cladding layer 10' having a thickness of approximately 0.1 to 0.2 micron is disposed on the cladding layer 13. A Ga.sub.0.5 In.sub.0.5 P active layer 11 having a thickness of 0.07 micron is disposed on the cladding layer 10'. An n type (Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P cladding layer 12 having a thickness of 0.8 to 1 micron is disposed on the active layer 11. An n type GaAs contact layer 7 having a thickness of 1 micron is disposed on the cladding layer 12. In addition, reference numeral 8 designates a stripe-shaped groove provided on the current blocking layer 3.

A production process will be described in the following.

First, an n type GaAs layer 3 having a thickness of approximately 1 micron is epitaxially grown on a (100) p type GaAs substrate 1 and then, a stripe-shaped groove 8 serving as a current path is formed by etching in the [011] or [011] direction as shown in FIG. 1. Thereafter, a p type Al.sub.0.8 Ga.sub.0.2 As cladding layer 13, a p type (Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P cladding layer 10', a Ga.sub.0.5 In.sub.0.5 P active layer 11, an n type (Al.sub.0.6 Ga.sub.0.4).sub.0.5 In.sub.0.5 P cladding layer 12, and an n type GaAs contact layer 7 are sequentially epitaxially grown by MOCVD.

In this embodiment, the p type AlGaAs layer 13 first p type cladding layer is set to approximately 0.6 to 1 micron thick, and the p type AlGaInP layer 10' second p type cladding layer is approximately 0.1 to 0.2 micron thick. When the stripe-shaped groove 8 is buried by making growth of materials containing AlGaInP, inferior growth as shown in FIG. 3(b) occurs. However, the inferior growth can be significantly reduced by controlling the thicknesses of the layers to the above-described values. More specifically, inferior growth of a portion, particularly the active layer 11, is prevented by making the total thickness of the p type AlGaInP layer 10', the GaInP active layer 11, and the n type AlGaInP cladding layer 12 to approximately 1 micron.

A description is now made of the composition of each of the layers for achieving a laser structure. When Ga.sub.0.5 In.sub.0.5 P lattice matched with GaAs is used as the active layer 11, the band gap energy thereof is approximately 1.85 eV. The band gap energy of a cladding layer must be larger than this value. When AlGaInP is used as the cladding layer, it is desirable to use (Al.sub.x1 Ga.sub.1-x1).sub.0.5 In.sub.0.5 P having an aluminum composition x.sub.1 of at least 0.5 or more (the band gap energy thereof is 2.2 eV or more) so as to restrain degradation of temperature characteristics of the laser caused by the overflow of carriers injected into the active layer 11. In addition, in order to prevent the light generated at the active layer 11 from significantly leaking the cladding layer, the refractive index of the p type AIGaAs layer 13 must be smaller than that of Ga.sub.0.5 In.sub.0.5 P and must be smaller than that of the p type AlGaInP layer 10'. The laser oscillation wavelength of the Ga.sub.0.5 In.sub.0.5 P active layer 11 is 670 to 680 nm (approximately 1.85 eV when it is converted into energy). The refractive index of Ga.sub.0.5 In.sub.0.5 P for this wavelength is approximately 3.6. Consequently, as the laser structure, the compositions of Al.sub.x2 Ga.sub.1-x2 As and (Al.sub.x1 Ga.sub.1-x1).sub.0.5 In.sub.0.5 P may be suitably combined with each other such that

the refractive index of Ga.sub.0.5 In.sub.0.5 P

>the refractive index of (Al.sub.x1 Ga.sub.1-x1).sub.0.5 In.sub.0.5 P

>the refractive index of Al.sub.x2 Ga.sub.1-x2 As

on a condition that the refractive index of Al.sub.x2 Ga.sub.1-x2 As is in the range of 3.1 (x.sub.2 =1) to 3.3 (x.sub.2 =0.7) when 0.7.ltoreq.x.sub.2 .ltoreq.1, and the refractive index of (Al.sub.x1 Ga.sub.1-x1).sub.0.5 In.sub.0.5 P is in the range of 3.2 (x.sub.1 =1) to 3.4 (x.sub.1 =0.5) when 0.5.ltoreq.x.sub.1 .ltoreq.1. Meanwhile, the band gap energy of AlGaAs is in the range of 2.0 (x.sub.2 =0.7) to 2.2 (x.sub.2 =1) eV and the band gap energy of AlGaInP is 2.2 (x.sub.1 =0.5) to 2.35 (x.sub.1 =1) eV.

FIGS. 2(a) and 2(b) are diagrams showing the energy band structure and the refractive index distribution in a case where the laser structure shown in FIG. 1 is achieved using AlGaInP in the case of x.sub.1 =0.6 and AlGaAs in the case of x.sub.2 =0.8 by way of example.

As can be seen from FIGS. 2(a) and 2(b), the active layer 11 has the highest refractive index and the cladding layers 12 and 10' on its upper and lower sides have the refractive indices lower than that of the active layer 11, and the cladding layer 13 has a refractive index lower than that of the cladding layer 10'. Accordingly, high efficiency light confinement in the thickness direction of the layers is obtained.

As described above, in the present embodiment, a lower cladding layer burying a stripe-shaped groove current path and disposed on a current blocking layer has a two-layer structure including a first cladding layer comprising AlGaAs disposed on the current blocking layer and the stripe-shaped groove and a second cladding layer comprising AlGaInP, sufficiently thin not to cause inferior growth, disposed on the first cladding layer. By thus restraining inferior growth of AlGaInP in the groove, a good SBA structure can e obtained even using materials containing AlGaInP.

As is evident from the foregoing description, according to the present invention, in a SBA type semiconductor laser device having a lower cladding layer and a GaInP active layer sequentially formed burying a stripe-shaped groove a current path and disposed on a current blocking layer, the cladding layer comprises a two-layer structure including a first cladding layer comprising AlGaAs disposed on the current blocking layer and in the stripe-shaped groove and a second cladding layer comprising AlGaInP sufficiently thin not to cause inferior growth disposed on the first cladding layer. Accordingly, the inferior growth of materials containing AlGaInP in the groove is significantly reduced. Consequently, a visible light semiconductor laser device of an SBA type using materials containing AlGaInP is obtained.