1. Field of the Invention
This invention relates to a semiconductor laser device and a manufacturing method thereof. In particular, it relates to a semiconductor laser device used to process optical information and a manufacturing method thereof.
2. Description of the Related Art
A semiconductor laser device for optical information processing has conventionally employed a gain guided structure using a GaAs current blocking layer. Recently, there has been, however, developed a semiconductor laser device which employs a real refractive index guided structure using an AlInP layer as a current blocking layer to reduce an operating current.
A real refractive index guided structure may reduce an optical absorption loss in a current blocking layer resulting in not only a reduced threshold current but also an improved luminous efficiency, therefore a reduced operating current.
This technical trend has been driven for developing semiconductor laser device shaving a higher output. Conventional optical information processing involves only reading as in, for example, DVD-ROM, which does not require very high output. Recent optical information processing involves, however, not simply reading but also writing on a recording medium as in, for example, DVD-RW or DVD-R, which necessarily requires a higher output. It has been, therefore, required that an internal loss is minimized to reduce an operating current for improving temperature properties of the semiconductor laser device and thus reliability under a high output.
FIG. 14 is a cross-sectional view of a conventional SAS (Self-Aligned Structure) type of red semiconductor laser diode (hereinafter, referred to as a xe2x80x9cred LDxe2x80x9d) described in Electronics Letters, Vol. 33, No. 14 (1997), pp.1223-5.
In FIG. 14, reference numeral 100 denotes a red LD, 102 an n-type GaAs substrate (hereinafter, n-type and p-type are denoted as xe2x80x9cn-xe2x80x9d and xe2x80x9cp-xe2x80x9d, respectively), 104 an n-GaAs buffer layer, 106 a lower clad layer made of n-(Al0.7Ga0.3)0.5In0.5P, and 108 an active layer of an MQW structure made of GaInP/AlGaInP where GaInP is a material for a well layer and AlGaInP is a material for a barrier layer.
In this figure, reference numeral 110 denotes a first upper clad layer made of p-(Al0.7Ga0.3)0.5In0.5P, 112 a current blocking layer made of n-AlInP, 114 a stripe-shaped opening to be a current channel in the current blocking layer 112, 116 a second upper clad layer made of p-(Al0.7Ga0.3)0.5In0.5P, 118 a p-GaAs contact layer, 120 a p-electrode, and 122 an n-electrode.
There will be described a process for manufacturing this semiconductor laser device 100.
FIGS. 15, 16 and 17 are cross-sectional views of a conventional red LD in individual manufacturing steps.
First, on an n-GaAs substrate 102 are sequentially deposited an n-GaAs layer to be a buffer layer 104, an n-(Al0.7Ga0.3)0.5In0.5 layer to be a lower clad layer 106, a GaInP/AlGaInP MQW layer to be an active layer 108, a p-(Al0.7Ga0.3)0.5In0.5P layer to be a first upper clad layer 110 and an n-AlInP layer to be a current blocking layer 112, by primary epitaxial growth based on crystal growth such as MOCVD. For dopants, silicon is used as an n-type dopant while zinc is used as a p-type dopant. The result of this step is shown in FIG. 15.
Then, a resist pattern 126 is formed on the surface of the n-AlInp layer to be a current blocking layer 112 by a photolithographic process, and a stripe-shaped opening 114 to be a current path is formed in the n-AlInP layer to be the current blocking layer 112 by wet etching. The result of this step is shown in FIG. 16.
After removing the resist pattern 126, a p-(Al0.7Ga0.3)0.5In0.5P layer to be a second upper clad layer 116 is formed on the p-(Al0.7Ga0.3)0.5In0.5P layer to be a first upper clad layer 110 facing the opening 114 and the n-AlInP layer to be a current blocking layer 112 by secondary epitaxial growth based on crystal growth such as MOCVD. The result of this step is shown in FIG. 17.
Then, a p-GaAs layer to be a contact layer 118 is formed on the p-(Al0.7Ga0.3)0.5In0.5layer to be a second upper clad layer 116.
In this process, crystal growth temperature is about 650xc2x0 C. to 750xc2x0 C. A crystal growth temperature as low as possible is used to prevent the p-type dopant, Zn, from diffusing from the p-(Al0.7Ga0.3)0.5In0.5P layer, as the first upper clad layer 110, into the MQW layer, the active layer 108, to the maximum extent possible.
Then, a p-electrode 120 and an n-electrode 122 are formed on the surface of the p-GaAs layer to be a contact layer I 10 and on the rear surface of the n-GaAs substrate 102, respectively.
A conventional red LD 100 has a configuration as described above. When forming the p-(Al0.7Ga0.3)0.5In0.5P layer for the second upper clad layer 116 on the n-AlInP layer for the current blocking layer 112 in the manufacturing process for the red LD 100 as illustrated in FIG. 17, lattice defects frequently develop on the surface facing the opening 114 in the n-AlInP layer for the current blocking layer 112, leading to an increase in internal loss of light, deterioration in temperature properties, and poor reliability of the red LD 100.
A technique for preventing lattice defects in crystal growth has been described in Proceedings of the Tenth International Conference on Metal organic Vapor Phase Epitaxy (2000), p. 82. In the report, an (Al0.7Ga0.3)0.51In0.49P layer is deposited on a GaAs substrate which is a (100) facet misoriented by 10xc2x0 toward [011] direction. Then, on the layer is formed an Al0.51In0.49P layer having a grooved structure whose side wall is a (111) A facet, and with Ga0.51In0.49P as a marker sandwiched in between, an (Al0.7Ga0.3)0.51In0.49P layer is formed above the (Al0.7Ga0.3)0.51In0.49P layer parallel to the GaAs substrate exposed in the bottom of the grooved structure and above the Al0.51In0.49P layer having a (111) A facet, during which development of lattice defects is studied using the then substrate temperature as a parameter.
According to the report, crystal growth was caused at substrate temperatures of 720xc2x0 C., 760xc2x0 C. and 800xc2x0 C. It was found that lattice defects developed in a crystal layer growing on a (111) A facet at a substrate temperature of 720xc2x0 C. or 760xc2x0 C., while crystal growth at a substrate temperature of 800xc2x0 C. reduced lattice defects in the (Al0.7Ga0.3)0.51In0.49P layer on a (111) A facet.
However, for a red LD, a crystal growth temperature of 800xc2x0 C. may cause diffusion of the p-type dopant Zn from the p-(Al0.7Ga0.3)0.5In0.5P layer as a first upper clad layer 110 to the MQW layer to be an active layer 108, leading to deterioration in temperature properties or reliability in current-optical output performance.
Besides the prior art described above, JP-B 2842465 has disclosed an SAS type semiconductor laser where on the surface of a current blocking layer made of AlGaAs material having a stripe-shaped opening is deposited a protective layer made of an AlGaAs material with small aluminum content, on which a p-AlGaAs material is deposited as a p-clad layer, but has not described that on a current blocking layer made of an AlInP material are formed a capping layer made of a GaInP material and a p-(Al0.7Ga0.3)0.5In0.5P layer.
The present invention has been made to solve the above problem in the art, and an objective of this invention is to provide a reliable semiconductor laser device exhibiting a reduced threshold current with less deterioration in temperature properties in current-optical output performance.
A semiconductor laser device according to the present invention comprises: a semiconductor substrate of a first conductivity type; a first clad layer of a first conductivity type made of a III-V group compound semiconductor disposed on the semiconductor substrate; an active layer made of a III-V group compound semiconductor having a smaller band gap than the first clad layer, disposed on the first clad layer; a first second-clad layer of a second conductivity type made of a III-V group compound semiconductor having a larger band gap than the active layer, disposed on the active layer; a current blocking layer of a first conductivity type made of a III-V group compound semiconductor having a larger band gap than the active layer, disposed on the first second-clad layer and having a stripe-shaped opening to be a current path; a buffer layer of a second conductivity type made of a III-V group compound semiconductor having a larger band gap than the active layer, disposed on the surface of the current blocking layer facing the opening; and a second second-clad layer of a second conductivity type made of a III-V group compound semiconductor having a larger band gap than the active layer, disposed on the first second-clad layer facing the opening and the current blocking layer via the buffer layer.
Accordingly, a semiconductor laser device according to the present invention is advantageous in that lattice defects in the second second-clad layer disposed on the surface of the current blocking layer facing the opening via the buffer layer may reduce and it can be prevented from a second conductivity type of dopant diffusing from the first second-clad layer to the active layer. Therefore the construction according to the present invention makes it possible to reduce deterioration in temperature properties in current-optical output performance, and consequently to improve reliability of the semiconductor laser device.
Another objective of this invention is to lead to improvement in reliability of a semiconductor laser device with a red LD.
A semiconductor laser device according to the present invention comprises: a semiconductor substrate of a first conductivity type; a first clad layer of a first conductivity type made of a III-V group compound semiconductor disposed on the semiconductor substrate; an active layer made of a III-V group compound semiconductor having a smaller band gap than the first clad layer, disposed on the first clad layer; a first second-clad layer of a second conductivity type made of a III-V group compound semiconductor having a larger band gap than the active layer, disposed on the active layer; a current blocking layer of a first conductivity type made of an AlInP material having a larger band gap than the active layer, disposed on the first second-clad layer and having a stripe-shaped opening to be a current path; a protective layer made of a GaInP material disposed on the surface of the current blocking layer except the surface facing the opening; and a second second-clad layer of a second conductivity type made of an AlGaInP material having a larger band gap than the active layer, disposed on the current blocking layer via the protective layer and the first second-clad layer facing the opening.
Accordingly, a semiconductor laser device according to the present invention is advantageous that because the current blocking layer made of the AlInP material is protected by the GaInP material not containing Al the second second-clad layer made of the AlGaInP material whose composition tends to be deviated is disposed on the crystal facet with a reduced amount of oxide film. Consequently reducing a risk of lattice defect formation leads to improvement in reliability of a semiconductor laser device with a red LD.
A further objective of this invention is to provide a process for manufacturing a reliable semiconductor laser device exhibiting a reduced threshold current with less deterioration in temperature properties in current-optical output performance by simple steps.
A process for manufacturing a semiconductor laser device according to the present invention includes the steps of: depositing a first clad layer of a first conductivity type made of a III-V group compound semiconductor, an active layer made of a III-V group compound semiconductor having a smaller band gap than the first clad layer, a first second-clad layer of a second conductivity type made of a III-V group compound semiconductor having a larger band gap than the active layer, and a current blocking layer of a first conductivity type made of a III-V group compound semiconductor having a larger band gap than the active layer on the first second-clad layer in order, on a semiconductor substrate of a first conductivity type; forming a stripe-shaped opening penetrating the current blocking layer; forming a buffer layer of a second conductivity type made of a III-V group compound semiconductor having a larger band gap than the active layer on the surface of the current blocking layer facing the opening; and forming a second second-clad layer of a second conductivity type made of a III-V group compound semiconductor having a larger band gap than the active layer on the first second-clad layer facing the opening and the current blocking layer via the buffer layer.
Accordingly, a process for manufacturing a semiconductor laser device according to the present invention is advantageous that because the crystal of the second second-clad layer may grow with reduced lattice defects on the surface of the current blocking layer facing the opening via the buffer layer even at a common substrate temperature and such a common substrate temperature may reduce diffusion of a second conductivity type of dopant from the first second-clad layer to the active layer during crystal growth, a reliable semiconductor laser device with reduced deterioration in temperature properties in current-optical output performance can be manufactured by simple steps. Consequently, a semiconductor laser device exhibiting excellent laser properties can be produced with a low price.
Other objects and advantages of the invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific embodiments are given by way of illustration only since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.