Optical communications networks using optical fibers have been continuously expanding recently.  Accordingly, low-cost semiconductor lasers with excellent high-temperature characteristics are being desired as light sources used for the optical communications. 
Conventionally, an InGaAsP-based multiple quantum well structure, that is, a multiple quantum well structure in which an InGaAsP layer is formed as a barrier layer, has been used as an active layer of a semiconductor laser with a wavelength of 1.3 μm band or 1.55 μm band. 
Recently, in contrast, an AlGaInAs-based multiple quantum well structure, that is, a multiple quantum well structure in which an AlGaInAs layer is formed as a barrier layer, has been drawing attention.  The AlGaInAs-based multiple quantum well structure has a deeper band offset ΔEc in a conduction band and a shallower band offset ΔEv in a valence band than an InGaAsP-based multiple quantum well structure. 
JP-A 8-125263 discloses a semiconductor laser employing such an AlGaInAs-based multiple quantum well structure as an active layer. 
FIG.  1 is an energy band diagram of an optical semiconductor device disclosed in JP-A 8-125263. 
In this optical semiconductor device, a lower cladding layer 2 formed of n-type InP, a lower optical guide layer 3 formed of InGaAsP, an active layer 4 having an AlGaInAs-based multiple quantum well structure, an upper optical guide layer 5 formed of InGaAsP, and an upper cladding layer 6 formed of p-type InP are formed on an n-type InP substrate. 
Of these layers, the active layer 4 is formed by alternately stacking a quantum well layer 4a formed of InGaAsP and a barrier layer 4b formed of AlGaInAs. 
As shown in FIG.  1, in the AlGaInAs-based multiple quantum well structure, as described above, the band offset ΔEc in the valence band is deep and the band offset ΔEv in the conduction band is shallow. 
Since the band offset ΔEc in the valence band is deep in this manner, electrons E injected into the active layer 4 from an n-side (lower cladding layer 2 side) are effectively trapped in the quantum well layer 4a.  As a result, the electrons E are prevented from overflowing from the quantum well layer 4a even under a high-temperature environment.  In addition, since the band offset ΔEv in the conduction band is shallow, holes H injected into the active layer 4 from a p-side (upper cladding layer 6 side) are uniformly distributed into all the quantum well layers 4a, so that lasing efficiency in the entire active layer 4 can be increased.  This prevents a deterioration in the efficiency of laser under a high-temperature environment without using a cooling element like a Peltier element.  Thus, costs for semiconductor lasers can be reduced by the cost of unnecessary cooling element. 
JP-A 11-506273 also discloses one example of a semiconductor laser using, as an active layer, an AlGaInAs-based multiple quantum well structure as described above. 
In an optical semiconductor device disclosed in JP-A 11-506273, both the lower optical guide layer 3 and the upper optical guide layer 5 in the structure of JP-A 8-125263 shown in FIG.  1 are formed of AlGaInAs. 
FIG.  2 is an energy band diagram of the optical semiconductor device of JP-A 11-506273.  In JP-A 11-506273, an AlGaInAs layer with a composition wavelength of 1.25 μm and a thickness of 70 nm is formed as each of optical guide layers 3 and 5.  In addition, an InGaAs layer with a compressive strain of 0.6% and a thickness of 4.5 nm is used as a well layer 4a of an active layer 4.  An AlGaInAs layer with no strain, a composition wavelength of 1.25 μm and a thickness of 13 nm is used as a barrier layer 4b.  
As shown in FIG.  2, when the AlGaInAs layer is formed as the upper optical guide layer 5 in this manner, a potential barrier ΔE in the valence band of an upper InP cladding layer 6 and the upper optical guide layer 5 becomes large.  For example, if the composition wavelength of the upper optical guide layer 5 is set to 1.25 μm, the potential barrier ΔE takes a value as large as approximately 280 meV. If the potential barrier ΔE is large like this, a deep potential spike V as shown by the dotted line is generated when a forward voltage is applied.  This potential spike prevents holes H from being injected from the upper cladding layer 6 to the active layer 4, thus causing a problem of deteriorating the lasing efficiency. 
In addition, JP-A 8-172241 also discloses a technique relating to the present application.