EDGE-EMITTING LASER DIODE WITH INCREASED COD THRESHOLD

Description

Technical application area

[0001] The present invention relates to an edge-emitting laser diode comprising a semiconductor heterostructure comprising at least one active layer or layer sequence between two waveguiding semiconductor layers or layer sequences extending between a rear and a front facet of the laser diode, and an electrical contact layer arranged above the semiconductor heterostructure for current injection on a substrate.

[0002] The optical output power and lifetime of edge-emitting laser diodes are essentially limited by thermal overshoot and catastrophic optical degradation (COD). During thermal overshoot, an increase in the average temperature of the laser diode's semiconductor chip due to the power dissipation during operation initially leads to a flattening of the power characteristic and ultimately to a reversible reduction in output power with increasing injected current. In COD, however, in the high output power range, a locally limited, strong temperature increase, also referred to in this patent application as overtemperature, causes an irreversible drop in emitted power due to damage to the semiconductor material. This overtemperature often occurs near the front facet of the laser diode, from which the laser radiation emerges.

[0003] The lifetime of laser diodes is limited by gradual degradation, in which the optical output power decreases slowly as a function of operating time, and by the previously mentioned catastrophic optical degradation. The cause of gradual degradation is the formation of point defects in the semiconductor crystal, which is accelerated by higher temperatures and mechanical stress, among other factors. State of the art

[0004] Thermal overshoot can be prevented by improved cooling of the laser diodes or by reducing the resulting heat output. Various measures are known for this purpose, such as double-sided or active cooling, the use of heat sinks with high thermal conductivity, a reduction in series resistance and optical absorption by adapting the design of the semiconductor heterostructure, or even a reduction in thermal resistance by using longer semiconductor chips and widening the injection stripe (broad-stripe emitter). This can also simultaneously limit the effects of gradual degradation, as this is primarily determined by the average temperature in the semiconductor chip.

[0005] With regard to catastrophic optical degradation (COD), various approaches are known to increase the power threshold for the occurrence of COD. These approaches are intended to prevent a strong local increase in temperature, particularly at the facets of the semiconductor chip. One of these approaches reduces the maximum optical intensity at the front facet by broadening the injection stripe (broad-area emitter) and enlarging the waveguide in the vertical direction (large optical cavity). Another approach to increasing the COD threshold involves coating the facets of the laser diode with suitable layer systems that saturate the bonds at the fracture edge of the crystal, thus reducing the defect density in the semiconductor in the facet region.These defects can form non-radiative recombination centers for the injected charge carriers and lead to optical absorption of the emerging laser radiation, thus causing a strong temperature increase at the facets.

[0006] In another known technique, the current injected from the p-side of the semiconductor heterostructure is blocked near the facets (so-called non-injecting mirror), as described, for example, in F. Rinner et al., "Facet temperature reduction by a current blocking layer at the front facets of high-power InGaAs / AlGaAs lasers," Journal of Applied Physics, vol. 93, no. 3, pp. 1848-1850, 2003. On the one hand, the locally suppressed current flow reduces the temperature at the facets due to the lack of Joule heating. On the other hand, the charge carrier density at the facets, which is considered a crucial driver of COD, is also reduced. Local blocking of the injected current can be achieved, for example, by locally applying a dielectric layer before metallizing the p-contact of the laser diode.

[0007] A strong local increase in temperature can also cause absorption of the laser light propagating in the waveguide due to the resulting shift in the optical gain spectrum of the active region of the semiconductor heterostructure to higher wavelengths. The onset of this absorption is often seen as the starting point of COD. An increase in the band gap of the quantum well, the active layer of the laser diode, near the facets (so-called non-absorbing mirror), ensures that this absorption only occurs at higher temperatures and thus increases the COD threshold. This process can be implemented, for example, through quantum-well intermixing, as described in SD McDougall et al., "Monolithic integration via a universal damage-enhanced quantum-well intermixing technique," IEEE J. Select. Topics Quantum Electron., vol. 4, no. 4, pp. 636-646, 1998, or in CL Walker et al., "Improved catastrophic optical damage level from laser with nonabsorbing mirrors," IEEE Photon. Technol. Lett., vol. 14, no. 10, pp. 1394-1396, 2002.

[0008] Reference is also made to the following documents: US 2002 / 181528 A1 (HIROYAMA) December 5, 2002; US 2020 / 006921 A1 (IKEDO) January 2, 2020; US 2012 / 114000 A1 (TANIGUCHI) May 10, 2012; US 4 845 725 A (WELCH) July 4, 1989; US 2017 / 062652 A1 (IMAI) March 2, 2017.

[0009] The object of the present invention is to provide a design for an edge-emitting laser diode by which the power threshold for the occurrence of catastrophic optical degradation and thus also the achievable optical output power and the lifetime of the laser diode are increased. Description of the invention

[0010] The object is achieved with the edge-emitting laser diode according to claim 1. Advantageous embodiments of this laser diode are the subject of the dependent claims or can be derived from the following description and the exemplary embodiments.

[0011] The proposed edge-emitting laser diode comprises, in a known manner, a semiconductor heterostructure consisting of at least one active layer or layer sequence (e.g., multiple quantum films) between two waveguiding semiconductor layers or layer sequences extending between a rear and a front facet of the laser diode, and an electrical contact layer arranged above the semiconductor heterostructure for current injection onto a substrate, in particular a semiconductor substrate. The laser radiation generated by the laser diode exits through the front facet. The two waveguiding semiconductor layers or layer sequences are in direct contact with the active layer or layer sequence or border the active layer or layer sequence via one or more transition layers. Cladding layers are generally arranged below and above the waveguiding semiconductor layers or layer sequences.Further semiconductor or other layers can also be added. Transition layers can also be formed between the different layers or layer sequences. The proposed laser diode is characterized in that the two waveguiding semiconductor layers or layer sequences between the back and front facets each have one or more locally limited modified sections, by means of which vertical leakage currents from the active layer or layer sequence that occur during operation of the laser diode are reduced or suppressed. The reduction or suppression of the vertical leakage currents is achieved by a modified design of the waveguiding semiconductor layers or layer sequences in the respective modified section compared to the section(s) remaining between the back and front facets.The modified section(s) are arranged at positions where, without this reduction or suppression of the vertical leakage currents, undesirable temperature increases would occur during operation of the laser diode, i.e. strong local temperature increases that would lead to COD. Vertical leakage currents are understood to be leakage currents that spread essentially perpendicular to the active layer or layer sequence from the active layer or layer sequence into neighboring layers of the semiconductor heterostructure. Locally limited sections are understood to be sections that do not extend over the entire or a predominant part of the length of the waveguide between the front and rear facets, but only over a small part of it, which preferably amounts to < 3% of this length.

[0012] Vertical leakage currents can occur during the COD process in addition to the absorption that begins as described above. Absorption due to a shift in the gain spectrum leads to a strong local increase in the charge carrier density within the active layer, layer sequence, or zone. If a critical value is exceeded, these charge carriers diffuse into the surrounding semiconductor layers despite the existing energy barrier and recombine there non-radiatively. The resulting increase in local heat source causes a further temperature rise, which continuously intensifies the effect until the laser diode is finally destroyed when the melting temperature of the semiconductor material is reached.In the present invention, it was recognized that the power threshold for the occurrence of COD can be increased by locally reducing or suppressing these vertical leakage currents at the critical location(s) through suitable local modification of the waveguiding semiconductor layers or layer sequences adjacent to the active layer or layer sequence directly or via one or more transition layers. Simulation calculations show that only the onset of vertical leakage currents initiates a positive feedback loop that generates increasingly strong heat sources and thus ultimately leads to local melting of the semiconductor material and irreversible damage to the laser diode.The local modification of the waveguiding semiconductor layers or layer sequences to locally reduce or suppress these vertical leakage currents thus leads to a higher achievable optical output power and a longer lifetime of the laser diode.

[0013] The local modification is usually performed in a locally limited section in the front facet area, or in a locally limited section in the front facet and in the back facet area, as these regions are identified as a frequent starting point for COD in edge-emitting laser diodes. Depending on the laser diode type, however, excessive temperature rise can also occur in other longitudinal sections between the front and back facets, resulting in a corresponding local modification of the waveguiding semiconductor layers or layer sequences at these sections.

[0014] The reduction or suppression of vertical leakage currents can be achieved by different local modifications of the waveguiding semiconductor layers or layer sequences, i.e., by modifying the configuration of these semiconductor layers or layer sequences compared to the remaining sections. In one embodiment of the proposed laser diode, the band gap of the waveguiding semiconductor layers or layer sequences in the modified section is increased compared to the remaining section(s). This increases the energy barrier between the active layer or layer sequence and the surrounding semiconductor layers and suppresses the diffusion of charge carriers. This then leads to a reduction or suppression of vertical leakage currents in these regions.

[0015] In a further embodiment of the proposed edge-emitting laser diode, the two waveguiding semiconductor layers or layer sequences in the one or more modified sections are configured such that the mobility of the charge carriers is reduced relative to the remaining section(s). This reduction in charge carrier mobility in the waveguiding semiconductor layers or layer sequences locally reduces the diffusion coefficient, thus suppressing the diffusion current. This also leads to a reduction or suppression of the vertical leakage currents and thus to an increase in the COD threshold.

[0016] In the proposed edge-emitting laser diode, preferably only the waveguiding semiconductor layers or layer sequences are locally modified, but not the active layer or layer sequence. In other embodiments, a combination of the two above embodiments or one or both embodiments with one or more of the measures explained in the introduction to the description for increasing the COD threshold can also be implemented in the proposed edge-emitting laser diode. Such a combination further increases the COD threshold and thus also the lifetime of the laser diode, which is limited by the COD.

[0017] The present invention can be implemented with all types of edge-emitting laser diodes. However, it is particularly advantageous for edge-emitting laser diodes that are intended to deliver high optical output powers, for example, in the field of laser material processing as direct diode laser systems or as pump modules for fiber and disk lasers. Furthermore, the implementation of the invention is advantageous for laser systems that require high reliability, such as in transatlantic optical telecommunications or for use in space. Short description of the drawings

[0018] The proposed edge-emitting laser diode is explained in more detail below using exemplary embodiments in conjunction with the drawings. Herein: Fig. 1 shows an example of a typical heterostructure of an edge-emitting laser diode according to the prior art in the vertical-longitudinal plane; Fig. 2 shows an example of the formation of the heterostructure of a laser diode according to Figure 1 using a non-injecting mirror according to the prior art; Fig. 3 an example of the design of the heterostructure of a laser diode according to Figure 1 using a non-absorbing mirror according to the prior art; Fig. 4 shows an embodiment of the formation of the heterostructure for the local suppression of vertical leakage currents from the active layer according to the present invention; Fig. 5 shows an example of the band gap profile along the Figure 4 indicated sections A and B with local enlargement of the band gap in the layers adjacent to the active layer; Fig. 6 an example of the mobility of the charge carriers along the Figure 4indicated sections A and B with a local reduction of mobility in the layers adjacent to the active layer; and Fig. 7 shows an embodiment of the formation of the heterostructure according to the present invention in combination with prior art measures. Ways to implement the invention

[0019] The proposed edge-emitting laser diode comprises, in a known manner, a semiconductor heterostructure with at least one active layer between two waveguiding semiconductor layers on a substrate. Figure 1shows a simplified, schematic representation of an example of the typical heterostructure of an edge-emitting laser diode in the vertical-longitudinal plane. In this example, the heterostructure has a layer sequence consisting of an n-cladding 2, an n-waveguide 3, the active layer 4, a p-waveguide 5, and a p-cladding 6 on an n-substrate 1. A p-contact 7 for current injection is applied to this layer sequence of semiconductor layers. The back of the n-substrate 1 is provided with an n-contact 8. The rear facet 9 and the front facet 10 of this laser diode are also marked in the figure. The laser beam 11 generated by the laser diode exits through the front facet 10.

[0020] To avoid local temperature increases, which frequently occur at the front facet 10 and the rear facet 9 of such an edge-emitting laser diode, the measures mentioned in the introduction to the description are known. Figure 2shows an example of one of these measures, the so-called non-injecting mirror for suppressing the current injected at the p-contact 7 in a region d unp at the facets 9, 10. In this region, during the manufacture of the laser diode, a dielectric layer is applied locally as an insulator 12 before the metallization through the p-contact 7, as shown in the Figure 2 This insulator 12 locally suppresses the current flow from the p-contact 7 into the heterostructure, thus reducing the temperature at the facets 9, 10 due to the lack of Joule heating.

[0021] Figure 3shows another example of a known measure for increasing the COD threshold, the so-called non-absorbing mirror, as briefly explained in the introduction to the description. For this purpose, the active layer 4 in the present example is modified in the area of ​​the front facet 10 so that it has an increased band gap. The corresponding locally modified section 13 with an increased band gap is shown in the Figure 3 This increase in the band gap ensures that absorption of the laser radiation propagating in the waveguide occurs only at higher temperatures due to a shift in the optical gain spectrum of the active zone due to a temperature increase, thus increasing the COD threshold.

[0022] Figure 4Finally, FIG. 1 shows an embodiment of the proposed edge-emitting laser diode, also in a simplified, schematic representation. In this laser diode, the n-waveguide 3 and the p-waveguide 5 are locally modified in the sections 14 adjacent to the front facet 10 such that in these sections 14 of the semiconductor layers 3, 5 adjacent to the active layer 4, an increased band gap is present and / or a reduced mobility of the charge carriers occurs, thus reducing or suppressing the vertical leakage currents from the active layer 4 in this region. In the example of the Figure 4This local modification is shown only at the front facet 10. Depending on the occurrence of the temperature increases, this modification can alternatively or additionally occur at the back facet 9 or at other longitudinal positions between the back facet 9 and the front facet 10 of the heterostructure. The extent of these locally limited modified sections can, for example, be in the range of 30 µm to 100 µm.

[0023] A local increase in the band gap can be achieved in the corresponding sections, for example, using the following processes during laser diode manufacturing. One possibility is to etch back the heterostructure locally in the sections to be modified during manufacturing and to perform a new epitaxial growth (epitaxial regrowth) of the waveguide structure with an increased band gap in the layers 3, 5 adjacent to the active layer 4. Another possibility is to implant suitable ions in these sections in the waveguide layers 3, 5 adjacent to the active layer 4, followed by a high-temperature treatment to heal the resulting defects, as described, for example, in PG Piva et al., "Reduction of InGaAs / GaAs laser facet temperatures by band gap shifted extended cavities," Appl. Phys. Lett., vol. 70, no. 13, pp. 1662-1664, 1997.A third possibility involves locally vapor-depositing a dielectric layer onto the grown semiconductor layer, followed by a high-temperature treatment to diffuse the defects formed at the interface of the dielectric layer into the semiconductor layer, as described, for example, in the aforementioned publication by SD McDougall et al. in connection with the active layer. This is, of course, not an exhaustive list. The diffusion or implantation of defects does not represent the actual process for increasing the band gap. The defects merely facilitate atomic interdiffusion (intermixing) between different neighboring semiconductor layers.To increase the band gap in the waveguiding semiconductor layers, the diffusion or implantation of the impurities must therefore only take place in the waveguiding layers and the cladding layers, but not in the active layer, in order to implement intermixing not between the active layer and the waveguide, but between the waveguide and the cladding layer, in which the band gap is larger than in the waveguide.

[0024] Figure 5 shows an example of the band gap profile for sections 14 modified in this way along the two Figure 4Sections A and B are shown. From this illustration, the increase in the band gap is only visible in the correspondingly modified sections of the n-waveguide 3 and the p-waveguide 5, which leads to a reduction or suppression of the vertical leakage currents. For example, with a change in the band gap in the range of 120 to 180 meV in the semiconductor layers 3, 5 adjacent to the active layer 4, a significant increase in the COD threshold of approximately 30% can be achieved, as determined by simulation calculations. Boron, silicon, or zinc ions, for example, can be used to increase the band gap. This also applies to the reduction in charge carrier mobility through ion implantation described below.

[0025] If the mobility of the charge carriers in the Figure 4 The modified sections 14 shown may result in a course of mobility of the charge carriers as shown in Figure 6along the two in Figure 4shown in sections A and B. From this illustration, it can be seen that the mobility in the modified sections 14 of the layers adjacent to the active layer (p-waveguide 5 and n-waveguide 3) is reduced by a value Δµ. This reduction in mobility can be achieved by various measures during manufacture of the laser diode. One of these measures consists in the local implantation of suitable ions in the sections to be modified of the semiconductor layers 3, 5 adjacent to the active layer 4. The dopants must be selected such that they only create lattice defects - and thus reduce the mobility of the charge carriers - but do not increase the current conductivity as in the case of electrically active dopants.During ion implantation, the high-temperature step typically required during doping to heal the resulting defects must be omitted in order to correspondingly reduce charge carrier mobility. Another possibility is to oxidize these semiconductor layers 3, 5, but not the active layer, near the respective facet. This oxidation then occurs from the facet side into the respective layers before facet coating. Another possible measure for locally reducing charge carrier mobility is to locally modify or disrupt the crystal structure of the corresponding section using a high-power ultrashort pulse laser. The wavelength of the laser used must be selected so that the semiconductor layer is sufficiently transparent for the laser radiation. This list of possibilities is of course not exhaustive.

[0026] In the proposed edge-emitting laser diode, the already known techniques for increasing the COD threshold can also be used. Figure 7 shows a corresponding example in which the measures proposed according to the invention are combined with the techniques from the Figures 2 and 3 The laser diode of this example thus has, in addition to the locally modified sections 14 in the semiconductor layers 3, 5 adjacent to the active layer 4, an increased band gap in a region 13 of the active layer 4 near the front facet 10 as well as insulating regions 12 below the p-contact 7 at the front 10 and at the rear facet 9, as shown in Figure 7 is recognizable. List of reference symbols

[0027] 1n-substrate 2n-cladding 3n-waveguide 4active layer 5p-waveguide 6p-cladding 7p-contact 8n-contact 9back facet 10front facet 11laser beam 12insulator 13locally modified region of the active layer 14locally modified section of the waveguide

Claims

1. Edge-emitting laser diode which has a semiconductor heterostructure consisting of at least one active layer or layer sequence (4) between two wave-guiding semiconductor layers or layer sequences (3, 5), which extend between a rear (9) and a front facet (10) of the laser diode, and an electrical contact layer (7) for injecting current arranged over the semiconductor heterostructure on a substrate (1), wherein the two wave-guiding semiconductor layers or layer sequences (3, 5) each have one or more locally delimited modified portions (14) between the rear facet (9) and the front facet (10), in which vertical leakage currents from the active layer or layer sequence (4) during operation of the laser diode are reduced or suppressed by a design of the respective wave-guiding semiconductor layer or layer sequence (3, 5) that is modified in comparison with one or more remaining portions between the rear facet (9) and the front facet (10), wherein the one or more modified portions (14) are arranged in positions in which undesired excessive temperature increases would occur if the laser diode were operated without said reduction or suppression of the vertical leakage currents.

2. Edge-emitting laser diode according to Claim 1, characterized in that the two wave-guiding semiconductor layers or layer sequences (3, 5) are designed in the modified portions (14) in such manner that a band gap of these wave-guiding semiconductor layers or layer sequences is enlarged in the modified portions (14) compared with the one or more remaining portions.

3. Edge-emitting laser diode according to Claim 1 or 2, characterized in that the two wave-guiding semiconductor layers or layer sequences (3, 5) are designed in the modified portions (14) in such manner that a mobility of the charge carriers in the modified portions (14) is reduced compared with the one or more remaining portions.

4. Edge-emitting laser diode according to Claim 3, characterized in that the two wave-guiding semiconductor layers or layer sequences (3, 5) are oxidised in the modified portions (14) .

5. Edge-emitting laser diode according to any one of Claims 1 to 4, characterized in that the two wave-guiding semiconductor layers or layer sequences (3, 5) are doped in the modified portions (14) and not doped in the one or more remaining portions, or they have a composition or concentration of dopants that has been modified compared with the one or more remaining portions.

6. Edge-emitting laser diode according to any one of Claims 1 to 5, characterized in that starting from the substrate the semiconductor heterostructure includes at least one n-doped cladding layer or layer sequence (2), the first wave-guiding semiconductor layer or layer sequence (3), the active layer or layer sequence (4), the second wave-guiding semiconductor layer or layer sequence (5), and a p-doped cladding layer or layer sequence (6).

7. Edge-emitting laser diode according to any one of Claims 1 to 6, characterized in that the one or more modified portions (14) are arranged on or in the vicinity of the front facet (10) and / or the rear facet (9).