Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-12-06
2003-09-09
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S096000
Reexamination Certificate
active
06618410
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to an optoelectronic semiconductor component having a semiconductor body that is suitable for generating electromagnetic radiation. In the optoelectronic component, an active zone is disposed above a semiconductor substrate, within which zone the electromagnetic radiation is generated in the event of a current flow through the semiconductor body and which zone is disposed between at least one first resonator mirror layer and at least one second resonator mirror layer.
An optoelectronic semiconductor component having a semiconductor body of this type is, for example, a so-called vertical cavity surface emitting laser (VCSEL). In the case of the component, the light generated in the active zone of a heterostructure is reflected perpendicularly to the layer structure having the active zone between the two resonator mirror layers. That is to say in the direction of current flow, and the light is coupled out from the semiconductor body at a steep angle with respect to the surface of the semiconductor heterostructure through one of the reflector mirror layers.
An optoelectronic semiconductor component of this type and its functional principle are disclosed for example in a reference by W. Bludau, titled “Halbleiter-Optoelektronik” [Semiconductor Optoelectronics], Hansa-Verlag, Munich, Vienna, 1995, pages 188 and 189, wherein a VCSEL diode is described in which a semiconductor body is applied on an n-conducting substrate. The semiconductor body contains a first layer sequence made up of n-doped mirror layers (lower resonator mirror layer), a region with the active zone and a second layer sequence made up of p-doped mirror layers (upper resonator mirror layer). The electrical connection of the semiconductor body is realized by an ohmic top-side contact on the upper mirror and an underside contact on the substrate. The precise method of operation is described in the above-mentioned literature reference and, therefore, is not explained in any more detail at this point.
The lower resonator mirror layer is, for example, a periodic sequence of alternately GaAs or AlGaAs and AlAs or AlGaAs layers having a high or low refractive index whose respective layer thickness is ¼ of the wavelength emitted by the active zone divided by the refractive index of the material. The periodic sequence being doped in an n-conducting fashion with silicon and being applied epitaxially prior to the deposition of the active layer sequence on the semiconductor substrate. The reflectivity of the mirror is set by the number of layer pairs. On this n-conducting so-called Bragg reflector, there is applied for example an n-conducting first barrier layer, e.g. composed of AlGaAs, an active zone, e.g. with an InGaAs/GaAs multiple quantum well structure (MQW), and a p-conducting second barrier layer, e.g. composed of AlGaAs, in such a way that the active zone is embedded between the barrier layers.
Adjoining the p-conducting second barrier layer is the upper resonator mirror layer, e.g. a GaAs/AlAs Bragg reflector doped in a p-conducting fashion with beryllium or carbon, on the top side of which reflector is disposed an ohmic top-side contact. After the application of an electric voltage between the top-side and underside contacts, in such a way that the pn junction of the active zone is forward-biased, in the example chosen negative charge carriers are injected from the substrate side through the n-conducting lower Bragg mirror into the active zone. Holes are injected from the top-side contact through the p-conducting upper Bragg reflector.
Similar optoelectronic semiconductor components are described for example in Iga, Inst. Phys Conf. Ser. 145 (8), 1996, pages 967 to 972, and can be produced from different materials for different wavelength ranges of the electromagnetic radiation.
In the case of the VCSEL concept, a large number of lasers can be defined in the lateral direction on a semiconductor substrate and, consequently, it is easy to form laser arrays having more advantageous beam characteristics compared with the so-called separate confinement heterostructure (SCH) lasers.
In the semiconductor laser structures referred to above, the particular problem arises that the p-conducting Bragg reflector made up of GaAs/AlAs, AlGaAs/AlAs or AlGaAs/GaAs layer sequences has a high electrical resistance and therefore causes high electrical losses. Owing to the low thermal conductivity of the above-mentioned materials, the laser diode is consequently heated to a considerable extent during operation. As a result, for example, the lifetime of VCSEL lasers having a high optical output power is severely limited.
Furthermore, the high voltage drops across the p-conducting mirrors prevents the laser diode from being driven with a voltage level of <5 V, which is specified for logic signals.
In order to reduce this problem, the p-conducting mirror layers in VCSEL structures are usually applied on the side of the active zone on which the electromagnetic radiation is coupled out from the semiconductor body. This is because fewer mirror layer pairs are required on this side in order to reduce the reflectivity of this side relative to the opposite n-conducting resonator mirror layer, as a result of which it is possible to couple out the laser radiation. In the case of surface-emitting lasers, therefore, the semiconductor body is usually produced on an n-conducting substrate, as a result of which the p-conducting top side must be given a positive polarity relative to the substrate side. This fact is disadvantageous for the driving of the laser diode, particularly if the targeted, current-regulated driving of a VCSEL diode in a laser array is concerned, as is dealt with in Published, European Patent Application EP 709 939 A1, for example.
Furthermore, it is disadvantageous to produce the GaAs substrates that are usually used in the VCSEL structures described above from p-conducting GaAs, since the latter can be produced with a high structural quality only given a very high technical outlay. They are commercially available, therefore, only with a considerably lower structural quality than e.g. GaAs substrates that are doped in an n-conducting fashion with Si.
Various solution approaches have already been pursued with the purpose of lowering the electrical resistance of the p-conducting Bragg reflectors. In MG Peters et al., J. Vac. Sci. Technol. Volume 12 (6) 1994, pages 3075 to 3083, methods are described in which the transport of holes in p-conducting Bragg mirrors is improved by manipulating the interface material junctions and doping. What is problematic in the case of mirrors based e.g. on InGaAlAs for VCSEL is the large effective mass of the holes and a high energy barrier in the case of the exit of holes, e.g. from a GaAs layer into an AlAs layer. In the case of the methods discussed, the composition of the material is varied in a narrow zone around the GaAs/AlAs interface in different ways between the binary compounds GaAs and AlAs to an AlGaAs alloy and, at the same time, by skillful doping with e.g. Be, C or Si, it is attempted to flatten and minimize the potential barrier.
A further method would be to replace GaAs by the compound AlGaAs in AlGaAs/AlAs Bragg lattices or to replace AlAs by the compound AlGaAs in the GaAs/AlGaAs Bragg lattices. The barrier for holes is thus lowered, as a result of which a smaller electrical resistance is achieved. In this case, however, the fact that the difference in refractive index between AlGaAs and GaAs or AlAs is smaller than in the case of the binary mirrors containing GaAs/AlAs is disadvantageous. It is consequently necessary to apply considerably more mirror pairs in order to obtain a similar reflectivity to that with AlAs/GaAs layer sequences, as a result of which the electrical resistance is again increased.
Furthermore, the thermal conductivity of AlGaAs is considerably lower than that of GaAs or AlAs, as a result of which the thermal energy generated in the laser is dissipated only to an insu
Fischer Frank
Landwehr Gottfried
Litz Thomas
Reuscher Günter
Leung Quyen
Locher Ralph E.
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