Semiconductor device such as semiconductor laser device and manufacturing method therefor, and optical transmission module and optical disk unit employing the semiconductor laser device

1. A semiconductor device comprising:

a semiconductor substrate;

a semiconductor layer group on the semiconductor substrate, the semiconductor layer group including a first semiconductor layer, a second semiconductor layer and a third semiconductor layer that are formed in order from the substrate side; and

a thin film continuously coating an upper surface and side surfaces of the semiconductor layer group,

wherein the second semiconductor layer has an etching rate slower than etching rates of the first and third semiconductor layers,

wherein a normal vector on a surface coated with the thin film of the semiconductor layer group has an upward component, and

wherein the second semiconductor layer comprises InGaAsP or GaAsP,

a P mole fraction in Group V elements of the second semiconductor layer is larger than 0.2 but smaller than 0.6,

the first and third semiconductor layers each comprise AlGaAs, GaAs or InGaAs, and

wherein the second semiconductor layer has a bandgap energy intermediate between bandgap energies of the first and third semiconductor layers.

2. The semiconductor device as claimed in claim 1 , wherein

the thin film is a conductive thin film or there is a conductive thin film on the thin film, and

the normal vector on the surface coated with the thin film of the semiconductor layer group has an upward component in at least one path that connects one point on the conductive thin film located on a side opposite from the substrate with respect to the second semiconductor layer with one point on the conductive thin film located on the same side as the substrate with respect to the second semiconductor layer.

3. A semiconductor laser device comprising:

a semiconductor substrate;

a semiconductor layer group on the semiconductor substrate, the semiconductor layer group including a first semiconductor layer, a second semiconductor layer and a third semiconductor layer that are formed in order from the substrate side; and

a thin film continuously coating an upper surface and side surfaces of the semiconductor layer group,

wherein the second semiconductor layer has an etching rate slower than etching rates of the first and third semiconductor layers,

wherein a normal vector on a surface coated with the thin film of the semiconductor layer group has an upward component, and

wherein the second semiconductor layer comprises InGaAsP or GaAsP,

a P mole fraction in Group V elements of the second semiconductor layer is larger than 0.2 but smaller than 0.6,

the first and third semiconductor layers each comprise AlGaAs, GaAs or InGaAs,

wherein the second semiconductor layer has a bandgap energy intermediate between bandgap energies of the first and third semiconductor layers, and

wherein

the semiconductor substrate comprises a group III-V compound semiconductor of a first conductive type,

the first semiconductor layer is an upper cladding layer,

the third semiconductor layer is a contact layer, and

the second semiconductor layer is an etching rate reducing layer,

the device further comprising:

a lower cladding layer that is formed between the semiconductor substrate and the upper cladding layer and comprises a group III-V compound semiconductor of the first conductive type; and

an active layer that is formed between the lower cladding layer and the upper cladding layer and comprises a group III-V compound semiconductor, and

at least the upper cladding layer, the etching rate reducing layer and the contact layer constitute a ridge-shaped stripe portion.

4. The semiconductor laser device as claimed in claim 3 , further comprising:

a ridge lower layer that is formed between the upper cladding layer and the active layer and that is comprised of a group III-V compound semiconductor of the second conductive type, and wherein

the stripe portion is formed on the ridge lower layer,

the thin film is an electrode that continuously covers from the side surface of the contact layer to at least a partial region on the ridge lower layer, and

current constriction to the stripe portion is effected by a Schottky junction between the electrode and the ridge lower layer.

5. The semiconductor laser device as claimed in claim 4 , wherein

the contact layer has a doping concentration of not lower than 1×10 18 cm −3 ,

the ridge lower layer has a doping concentration of not higher than 1×10 17 cm −3 ,

a heavily-doped side compound layer comprised of at least one of constituent elements of the electrode and at least one of constituent elements of the contact layer is formed at an interface between the electrode and the contact layer, and

a lightly-doped side compound layer comprised of at least one of the constituent elements of the electrode and at least one of constituent elements of the ridge lower layer is formed at an interface between the electrode and the ridge lower layer.

6. The semiconductor laser device as claimed in claim 5 , wherein

the ridge lower layer includes a plurality of layers comprised of group III-v compound semiconductors of different compositions, and

of the layers of the ridge lower layer, at least an uppermost layer contains P of which a mole fraction is not smaller than 0.6.

7. The semiconductor laser device as claimed in claim 5 , wherein

a lowermost layer of the electrode comprises Ti or Pt.

8. An optical transmission module comprising the semiconductor laser device as claimed in claim 3 .

9. An optical disk unit comprising the semiconductor laser device as claimed in claim 3 .