Patent Application
20110164640
The present invention relates to an optical semiconductor device wherein a semiconductor light-emitting device such as a semiconductor laser array, a semiconductor laser, or a light-emitting diode is mounted on a submount, and more particularly, to an optical semiconductor device capable of obtaining a high optical output.
An optical semiconductor device wherein a semiconductor light-emitting device such as a semiconductor laser array, a semiconductor laser, or a light-emitting diode is mounted on a submount using solder is being used. Conventionally, the submount having a coefficient of thermal expansion which is near as possible to a coefficient of thermal expansion of the substrate material of the semiconductor light emitting element has been used, so that a stress is not applied to the semiconductor light-emitting device from the standpoint of the reliability. In this case, after the semiconductor light emitting element and the submount are bonded together at a temperature equal to or higher than the melting point of the solder, even if the temperature is cooled down to room temperature, a large thermal stress is not applied to the semiconductor light emitting element.
It is known that favorable characteristics such as a low threshold current or a large optical output can be obtained by providing a strain to the active layer of the semiconductor light emitting element so as to change the energy band structure (e.g., refer to Patent Document 1).
Conventionally, a strain was provided in the active layer by epitaxially growing crystals having different lattice constants on the same substrate while changing crystal compositions. However, when the difference among the lattice constants is too large and the strain quantity is larger than the critical strain, the crystalline becomes poor due to the lattice relaxation. Therefore, the favorable characteristics and the favorable reliability such as a low threshold current or a high optical output could not be obtained.
For example, even if the GaInP layer and the GaAs substrate are lattice-matched each other at room temperature, the tensile strain corresponding to the misfit which approximates to −0.1% at the epitaxial growth temperature is provided in the GaInP layer because of the difference of coefficients of thermal expansion between GaAs and GaInP. Therefore, the critical strain at the epitaxial growth temperature is smaller than the critical strain at ordinary temperature. Even if the stress is equal to or lower than the critical strain quantity of the material, because of the growth mode problem in the epitaxial growth, the favorable surface morphology may not be obtained. As described above, because the strain quantity which can be provided in the epitaxial growth process is limited, the favorable characteristics such as low threshold current or a high optical output might not be obtained.
The present invention has been implemented to solve the above described problems and it is an object of the present invention to provide an optical semiconductor device capable of obtaining a high optical output.
An optical semiconductor device comprises: a semiconductor light emitting element provided with semiconductor layers including an active layer having a quantum well structure and being epitaxially grown on a semiconductor substrate; and a submount on which the semiconductor light emitting element is mounted; wherein a strain quantity residing in the active layer after mounting the semiconductor light emitting element on the submount is larger than a strain quantity residing in the active layer after the epitaxial growth; the strain quantity residing in the active layer during the epitaxial growth is a value by which the surface of the semiconductor layers becomes a mirror surface; and the strain quantity residing in the active layer after the semiconductor light emitting element is mounted on the submount is a value by which the surface of the semiconductor layers does not become a mirror surface if the active layer has the value after the epitaxial growth.
The present invention makes it possible to realize an optical semiconductor device capable of obtaining a high optical output.
Embodiments of the present invention will be described with reference to the drawings. In the drawings, the same or corresponding parts will be denoted by the same numerals, and the description thereof may be simplified or omitted.
A plurality of active areas 16 of a stripe shape for confining current are formed in the GaInP/AlGaInP quantum well layer 14 so as to form a laser array. Such as a stripe formed by etching of insulating film, partial high resistance realized by irradiating protons, a ridge structure, a buried ridge structure, or a buried hetero structure can be used as a manner for forming the active layer 16 of a stripe shape. Here, the stripe formed by etching of insulating film is used.
The active layer has a tensile strain which is a bit smaller than the critical strain in the epitaxial growth temperature. Here, the equation of Van der Merwe is known as an equation representing the relation between the critical thickness and the critical strain (document R. People and J. C. Bean). That is, if a critical thickness is hc [nm] and an absolute value of a critical strain quantity is fc [%], hc=A/fc is satisfied provided that fc<4 and A is a constant. It is founded that A=12.5 by a surface inspection of the optical semiconductor device according to the first embodiment executed after the epitaxial growth.
The semiconductor laser array 11 is mounted on the SiC submount 18 using an AuSn eutectic solder 17. An epitaxial growth surface (junction side) of the semiconductor laser array 11 having the active layer is faced to the submount 18. The AuSn eutectic solder 17 used for mounting has a high melting point, thereby being hard to be deformed. As a result, a thermal stress is easily provided to the active layer of the semiconductor laser array 11.
The coefficient of thermal expansion of SiC is about 3 [10−6/K] and is smaller than the coefficient of thermal expansion of GaAs which is about 6[10−6/K]. As a result, the semiconductor laser array 11 is going to shrink more greatly than the submount 18 as shown in
This increase of the tensile strain is performed after the epitaxial growth at the temperature equal to or lower than 300° C. which is significantly lower than the temperature during the epitaxial growth of about 500-800° C. Thus, the critical strain becomes larger than the critical strain under the epitaxial growth temperature. As a result, the strain quantity of the tensile strain, which can not be obtained by controlling the strain quantity in the epitaxial growth process, can be provided. Therefore, the strain quantity residing in the active layer after the semiconductor laser array 11 is mounted on the submount 18 is set to a value by which the surface of the semiconductor layers does not become a mirror surface under a situation in which the strain quantity is tried to be obtained during the epitaxial growth. This strain quantity is not larger than a critical strain quantity at ordinary temperature but larger than the critical strain quantity at the epitaxial temperature.
If a thickness of the active layer is h [nm] and an absolute value of a strain quantity of the active layer is f [%], f<4 and h≦12.5/f are satisfied after a stage of the epitaxial growth, and h>12.5/f is satisfied after the semiconductor laser array 11 is mounted on the submount 18.
As described above, the stress is mechanically provided after the epitaxial growth by thermal stress, which has not been actively used in prior art and is cased by the connection between the semiconductor laser array and the submount after the epitaxial growth, in addition to providing the stress in the epitaxial growth. Thus, the tensile strain of the active layer can be increased. As a result, a high optical output can be obtained. The provided stress must be less than a value which causes a break of the crystal.
In the second embodiment of the present invention, the GaAs substrate 12 is used. GaAsP in which a tensile strain is provided is used as the active layer instead of the GaInP/AlGaInP quantum well layer 14. AlN (its coefficient of thermal expansion is about 4-5 [10−6/K]) is used as the submount 18. All other components are similar to those described in connection with the first embodiment.
Thereby, as in the first embodiment, the stress is mechanically provided by thermal stress cased by the connection between the semiconductor laser array and the submount after the epitaxial growth. Therefore, because the tensile strain of the active layer can be increased, a high optical output can be obtained.
In the third embodiment of the present invention, the GaAs substrate 12 is used. InGaAsP in which a compressive strain is provided is used as the active layer instead of the GaInP/AlGaInP quantum well layer 14. CuW whose coefficient of thermal expansion is larger than GaAs is used as the submount 18. All other components are similar to those described in connection with the first embodiment.
Thereby, as
In the optical semiconductor device wherein the semiconductor light emitting element such as the semiconductor laser array, the semiconductor laser, or the semiconductor light emitting diode is mounted on the submount, the strain quantity in the active layer can be increased by using thermal stress due to the mismatch between coefficient of thermal expansion of the semiconductor light emitting element and coefficient of thermal expansion of the submount, and the high optical output can be obtained.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2007/060224 | 5/18/2007 | WO | 00 | 10/12/2009 |
