Semiconductor laser device

Abstract: In a semiconductor laser device comprising a semiconductor laser main body having an active layer formed between a pair of cladding layers, and a non-absorbing layer of InGaP formed on the facets of the semiconductor laser main body and having a band gap greater than the band gap of the active layer, a diffusion blocking layer of, for example, Si, SiN or Ge is formed on the surface of the non-absorbing layer, and a di-electric protective layer of, for example, AlO x , SiO x , SiN x or MgO x for regulating reflectance at the facets and protecting the non-absorbing layer is formed on the diffusion blocking layer.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a semiconductor laser device for use as a light source for optical communication and so forth, and more specifically to a semiconductor laser device having such structure as capable of preventing COD (catastrophic optical damage) and improving operational reliability. Description of the Prior Art A semiconductor laser device for use as a light source for optical communication and so forth needs to have high operational reliability and to hardly exhibit change in characteristics for a long time. For example, a semiconductor laser device used in a repeater for an optical fiber submarine cable needs to keep operational reliability for over a million hours. Types of failure (degradation in operational characteristics) of the semiconductor laser device are broadly classified into gradual degradation, which is analogous to fatigue failure and due to long-time operation, and rapid degradation, which is due to COD produced at facets under high-power operation. The gradual degradation in operational characteristics is mainly caused by such a phenomenon that crystal defects inherent in the semiconductor laser device gradually increase as DLD (dark line defects), which may lead to failure of the semiconductor laser device. Causes of the gradual degradation in operational characteristics, however, have been mostly removed with development in semiconductor manufacturing technology. On the other hand, causes of COD produced at the facets of are still in process of analysis. At present, it is considered that repetition of the following process causes COD: When crystal defects existing at the surface of the facets of the semiconductor laser device absorb an emitted laser beam, the temperature at the facets rises, so that a band gap at the facets is reduced; as a result, the crystal defects increase. In order to prevent the COD in the semiconductor laser device, such structure has been conventionally proposed that a non-absorbing layer of, for example, InGaP having a band gap greater than the band gap of an active layer of the semiconductor laser device is formed on the facets of the semiconductor laser device. As shown in Fig. 3, a conventional semiconductor laser device of the aforementioned sort comprises, as its main part, a semiconductor laser main body 5 of double hetero structure in which, for example, an active layer 4 of InGaAs sandwiched between a pair of cladding layers 2, 3 of n-AlGaAs and p-AlGaAs is grown on a substrate 1 of n-GaAs. Facets 6 and 7 of the semiconductor laser main body 5 are formed by cleaving, and a non-absorbing layer 8 of InGaP, which has a band gap greater than the band gap of the active layer 4 and is optically transparent to a laser beam, is formed on one of the facets 6, 7 (an exit side cavity end face 7) . Further, a di-electric protective layer 9 of AlO x , SiO x , SiN x or MgO x , which is for regulating reflectance at the facet 7 and protecting the non-absorbing layer 8, is formed on the non-absorbing layer 8. Reference numeral 10 indicates a reflecting film of laminate of SiO 2 and Si, having high reflectance and formed on the other facet 6. The thickness of the non-absorbing layer 7 is 80 nm or so, and the thickness of the di-electric protective layer 8 is 180 nm or so. In the semiconductor laser device of the above described structure, since photo-absorption due to the crystal defects at the facet 7 can be restrained by the non-absorbing layer 8 of InGaP, COD can be prevented to a certain degree. However, there remains a problem that COD is still produced when the semiconductor laser device is made to operate with high power or operate continuously for a long time. EP-A-0 701 309 defines a semiconductor laser device according to the preamble of claim 1. SUMMARY OF THE INVENTION The object of the present invention is to provide a semiconductor laser device having such structure as capable of effectively restraining COD produced due to crystal defects at the facets of the semiconductor laser device, and thereby having an improved operational reliability. The particular object of the present invention is to materialize a semiconductor laser device in which a non-absorbing layer of, for example, InGaP and having a band gap greater than the band gap of an active layer of a semiconductor laser main body is formed on the facets of the semiconductor laser main body, with the structure capable of preventing degradation in characteristics of the non-absorbing layer. The invention is defined in claim 1. Various embodiments of the invention are defined in the dependent claims. Analyzing the process of occurrence of COD at the facets of the semiconductor laser device with a variety of analytical methods, the inventors of the present invention have found that P(phosphorus) contained in the non-absorbing layer of InGaP formed on the facets is diffused into the di-electric protective layer of AlO x or so forth formed on the non-absorbing layer for protecting same, which is one of causes of COD. Fig. 4 shows an atomic component profile obtained by Auger electron spectroscopy in the depth direction of the vicinity of the facets of a semiconductor laser device, when the semiconductor laser device is degraded in characteristics due to occurrence of COD. The distribution of Al, P and As shown in the profile indicates that P contained in the non-absorbing layer is diffused into the di-electric protective layer having Al as a primary component. Though the mechanism of how the diffusion of P causes COD is not yet clear, it can be considered as follows: i) P diffused into the di-electric protective layer combines with oxygen contained in the di-electric protective layer and forms a oxide glass PO x . PO x is optically opaque to an emitted laser beam, and therefore absorbs the laser beam. As a result, the temperature at the facets rises. ii) Due to the diffusion of P into the di-electric protective layer, the ratio of P in the non-absorbing layer of InGaP becomes small. As a result, transparency of the non-absorbing layer to the laser beam is lowered, and photoabsorptance thereof is increased, so that the temperature at the facets rises. iii) Due to the diffusion of P into the di-electric protective layer, vacancies in the non-absorbing layer of InGaP form an energy level in the band gap of the semiconductor laser main body. The laser beam is absorbed due to that energy level formed by the vacancies, so that the temperature at the facets rises. Based on the above described consideration, the inventors have carried out a variety of experiments and their evaluation, with a view to preventing the diffusion of P from the non-absorbing layer of InGaP into the di-electric protective layer of, for example, AlO x , and found that if the di-electric protective layer is formed having a diffusion blocking layer of, for example, Si, SiN or Ge interposed, the rate of occurrence of COD at the facets can be remarkably reduced. Thus, the object of the present invention is to materialize a semiconductor laser device of such structure that a diffusion blocking layer of Si, SiN or Ge is formed on a non-absorbing layer of InGaP which is formed on the facets of a semiconductor laser main body, and a di-electric protective layer of Al x , SiO x , SiN x or MgO x is formed having that diffusion blocking layer interposed, thereby to provide a semiconductor laser device capable of restraining COD and having high operational reliability. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing schematic structure of a semiconductor laser device according to an embodiment of the present invention; Fig. 2 shows an atomic component profile obtained by Auger electron spectroscopy in the depth direction of a cavity end face of a semiconductor laser device according to an embodiment of the present invention; Fig. 3 is a diagram showing schematic structure of a conventional semiconductor laser device; and Fig. 4 shows an atomic component profile obtained by Auger electron spectroscopy in the depth direction of the facets of a conventional semiconductor laser device. DETAILED DESCRIPTION OF THE INVENTION A semiconductor laser device according to an embodiment of the present invention has schematic structure as shown in Fig. 1. The semiconductor laser device comprises, as its main part, a semiconductor laser main body 5 of double hetero structure in which, for example, an active layer 4 of InGaAs sandwiched between a pair of cladding layers 2, 3 of n-AlGaAs and p-AlGaAs is grown on a substrate 1 of n-GaAs. In respect of the semiconductor laser main body 5, the semiconductor laser device according to the present invention is similar to conventional semiconductor laser devices, having no particular features. A non-absorbing layer 8 of InGaP, which has a band gap greater than the band gap of the active layer 4 and is optically transparent to a laser beam, is formed on one of the facets 6, 7 (an exit side the facet 7), formed by cleaving, of the semiconductor laser main body 5. The thickness of the non-absorbing layer 8 is, for example, 80 nm or so. On the surface of the non-absorbing layer 8 is formed a diffusion blocking layer 11 of, for example, Si, SiN or Ge, and with the interposition of that diffusion blocking layer 11, a di-electric protective layer 9 of, for example, AlO x , SiO x , SiN x or MgO x is formed having a thickness of 180 nm or so. The semiconductor laser device is materialized in this manner. The diffusion blocking layer 11 of Si, SiN or Ge needs to have a thickness larger than 2nm in order to be able to prevent the diffusion of P effectively. On the other hand, even if the diffusion blocking layer 11 is formed having a thickness larger than 30 nm, a further improved diffusion blocking effect can not be expected. Moreover, when the diffusion blocking layer 11 is formed having a thickness larger than 30 nm, it is feared that the diffusion blocking layer 11 exhibits laser beam absorption, so that laser oscillation efficiency is lowered. For those reasons, it is desirable that the diffusion blocking layer 11 of Si, SiN or Ge has a thickness of 2 ∼ 30 nm. The formation of the non-absorbing layer 8 of InGaP onto the facet 7 of the semiconductor laser main body 5 is carried out using a known crystal growth technique such as the MOCVD (metal organic chemical vapor deposition). In that case, if the semiconductor laser main body 5 is an InGaAs/GaAs semiconductor as described above, it is desirable, in view of matching of lattice constant, that the ratio of In(indium) in the non-absorbing layer 8 of InGaP is 0.5 or so. The preferred thickness of the non-absorbing layer 8 is about 10 ∼ 200 nm. The diffusion blocking layer 11 and the di-electric protective layer 9 can be formed by growth using the PCVD (plasma-assisted chemical vapor deposition) or so forth. It goes without saying that a reflecting film 10 of laminate of SiO 2 and Si having high reflectance is formed on the other facet 6 of the semiconductor laser main body 5. The table shown below exhibits the result of evaluation of COD in respective semiconductor laser devices which were materialized using Si, SiN or Ge for the diffusion blocking layer 11 and Al x , SiO x , SiN x or MgO x for the di-electric protective layer 9 in the above described structure. The evaluation was carried out as follows: 50 samples of each semiconductor laser device having material composition specified for each Example were made to operate ensuring a laser oscillation output of 250 mW, on the temperature condition of 60°C continuously for 1000 hours; and the number of samples which stopped laser oscillation due to occurrence of COD during those testing hours was counted for each Example. As a Comparative Example, a semiconductor laser device of the conventional structure having no diffusion blocking layer 11, that is, having a di-electric protective layer 9 of AlO x formed directly on a non-absorbing layer 8 of InGaP was tested in the same manner. In each semiconductor laser device, the thickness of the di-electric protective layer 9 is 180 nm, and the thickness of the diffusion blocking layer 11 is 20 nm. Diffusion blocking layer 10 Di-electric protective layer 8 Number of samples in which COD occurred Example 1 Si Al 2 O 3 1 Example 2 Si SiO 2 0 Example 3 Si MgO 0 Example 4 Si Si 3 N 4 1 Example 5 Ge SiO 2 1 Example 6 Si 3 N 4 SiO 2 1 Comparative Example None Al 2 O 3 15 Comparison between the numbers of samples in which COD occurred shows that COD can be restrained effectively by forming the di-electric protective layer 8 with the interposition of the diffusion blocking layer 11 of Si, SiN or Ge. Fig. 2 shows atomic component profiles in the depth direction of the vicinity of the facets of the semiconductor laser device of Example 1, that is, the semiconductor laser device of such structure that the diffusion blocking layer 11 of Si is formed on the non-absorbing layer 8 of InGaP which is formed on the facets of the semiconductor laser main body 5, and the di-electric protective layer 9 of Al 2 O 3 is formed on that diffusion blocking layer 11. The atomic component profile shown in Fig. 2 was obtained by Auger electron spectroscopy, as is the case with the profiles in Fig. 4. The atomic component profiles in Fig. 2 shows that the diffusion of P from the non-absorbing layer 8 into the di-electric protective layer 9 is restrained effectively by Si of the diffusion blocking layer 11 interposed therebetween. Thus, it is clear that by forming the di-electric protective layer 9 with the interposition of the diffusion blocking layer 11, the diffusion of P from the non-absorbing layer 8 into the di-electric protective layer 9 can be prevented, so that COD is restrained and the operational reliability of the semiconductor laser device is improved. The present invention is not limited to the above described embodiments. The semiconductor laser main body 5 is not limited particularly to InGaAs/GaAs semiconductor. In short, the present invention is applicable to the case in which a non-absorbing layer of InGaP is formed, with a view to preventing COD, on the facets of a semiconductor laser main body according to a variety of conventionally proposed structures. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.

Claims: A semiconductor laser device comprising a semiconductor laser main body (5) having an active layer (4) formed between a pair of cladding layers (2,3), and a non-absorbing layer of InGaP (8) formed on a facet of said semiconductor laser main body and having a band gap greater than the band gap of said active layer, and a dielectric protective layer (9) for regulating the reflectance at said facet and for protecting said non-absorbing layer (8), characterized in that: a diffusion blocking layer (11) is formed on the surface of said non-absorbing layer for blocking the diffusion of P from said non-absorbing layer (8) into said di-electric protective layer (9), and said di-electric protective layer (9) is formed on said diffusion blocking layer (11). A semiconductor laser device according to claim 1, characterized in that: said di-electric protective layer (9) is made of AlO x , SiO x , SiN x or MgO x , and said diffusion blocking layer is made of Si, SiN or Ge. A semiconductor laser device according to claim 1 or 2, characterized in that: said diffusion blocking layer (11) has a thickness of 2 ∼ 30 nm.

Inventors: IJICHI TETSURO, IKEGAMI YOSHIKAZU, OHKUBO MICHIO, IJICHI, TETSURO, IKEGAMI, YOSHIKAZU, OHKUBO, MICHIO
Applicants: FURUKAWA ELECTRIC CO LTD, THE FURUKAWA ELECTRIC CO., LTD.
Application year: 1997
Main classification: H01S5
IPC codes: H01S5