The present invention relates to a surface emitting semiconductor laser.
Conventional surface emitting semiconductor lasers are disclosed in detail and systematically in the article by Jack L. Jewell et al in IEEE Journal of Quantum Electronics, vol. 27, No. 6, pp. 1332-1346 (1991).
The conventional surface emitting semiconductor lasers have the following problems to be solved.
First, as described in pages 1343-1344 of the above article, a surface emitting semiconductor laser has a large device resistance of several hundreds to several thousands .OMEGA. compared with several .OMEGA. in a general semiconductor laser. Particularly, its structure in which an active region is reduced to about 10 .mu.n in diameter to define a low threshold current has a very large device resistance. As a result, it is difficult to achieve the CW (continuous wave) operation of the laser due to heat generation, and further the large CR time constant (C: device capacitance, R: device resistance) and the large device resistance which is larger than 50 .OMEGA. in the driving circuit also hindered the high speed operation. The large device resistance is ascribed to the fact that the hereto barrier between the semiconductor layers constituting a semiconductor multi-layer mirror is high and its number of periods is as large as 10-30 periods so that a sufficient amount of carriers cannot be injected into the active layer through the multi-layer mirror (FIG. 1A). Particularly, the holes having an effective mass cannot climb over this high hetero barrier, thus causing a great increase in the device resistance in a p-type semiconductor multi-layer mirror region. In order to obviate such a disadvantage, pages 1343-1344 and FIG. 15 in page 1344 make a proposal in which a region with a gradually changing composition or a superlattice layer is inserted between two kinds of semiconductor layers constituting the semiconductor multi-layer mirror. The proposal permits the device resistance to be reduced to about 1000 .OMEGA., but the high hetero barrier still remains. This proposal cannot provide a decisive solution.
Another problem to be solved is that in the surface emitting semiconductor laser in a 0.98 .mu.m band having a InGaAs quantum well active layer, its semiconductor multi-layer mirror, which is made of AlAs and GaAs, contains Al atoms so that the reliability of the device is deteriorated.