Diode laser with current aperture

Abstract

Diode laser comprising: an n-substrate (10) made of an n-doped material; an n-cladding layer (12) made of an n-doped material arranged on the n-substrate (10); an n-waveguide layer (14) made of an n-doped material arranged on the n-cladding layer (12); an active layer (16) arranged on the n-waveguide layer (14); a p-waveguide layer (18) made of a p-doped material arranged on the active layer (16); a p-cladding layer (20) made of a p-doped material arranged on the p-waveguide layer (18); a p-contact layer (22) made of a p-doped material arranged on the p-cladding layer (20); and a p-contact (24) arranged on the p-contact layer (22);wherein the diode laser has a layered current aperture (30) made of a material doped opposite to its surroundings for spatially selective currenting of the active layer (16) between the n-substrate (10) and the p-contact (24), wherein the current aperture (30) comprises a first layer (34) and a second layer (36), wherein the thickness of the second layer (36) is greater than the thickness of the first layer (34) and the sum of the thicknesses of the first layer (34) and the second layer (36) is between 5 nm and 100 nm; wherein the current aperture (30) is separated from adjacent layers by an intrinsic outer layer (32, 38), the current aperture (30) and the intrinsic outer layer (32, 38) are arranged within the p-contact layer (22), and the thickness of the intrinsic outer layer (32, 38) is between 5 nm and 50 nm lies.;

Description

The present invention relates to a diode laser having a current stop and, more particularly, to a diode laser having an adjusted "p-n-p" or "n-p-n" structure as a current stop for reducing the tunnel probability.Prior ArtBroad-area diode lasers (BALs) can have a particularly high efficiency and brilliance / beam density. In the associated structures, current-blocking layers, so-called current screens, are integrated into the edge regions of the components. These are intended to ensure current confinement below the central strip, so that the adverse effects on beam quality can be limited by current spreading and lateral carrier accumulation (LCA). In addition, losses at the strip edges can thereby be minimized, whereby the lasing threshold is lowered and the conversion efficiency is increased, while at the same time a high polarization purity can be maintained. The reduction of current and charge carrier density at stripe edges leads to a lower amplification for lateral modes of higher order and can thus even reduce the number of guided modes and thereby improve the beam properties.A variety of lateral patterning techniques have been developed to realize the desired central confinement of current and charge carriers, particularly in GaAs-based BALs, with the challenge being to improve beam quality without compromising η E, maximum P opt or device lifetime. One of these techniques is high energy deep ion implantation ("high energy deep ion implantation") in order to prevent a current flow in the component edges. Implantation profiles, in particular reaching the n-doped side of the BAL, i.e. passing through the active zone, eliminate the current spreading in the p-side and suppress the LCA due to the rapid recombination of charge carriers which diffuse into the device edges at the point defects introduced by the implantation (M. Winterfeldt et al., IEEE Photonics Technol. Lett. 27, 1809-12 (2015); M. M. Karow et al., CLEO / Europe-EQEC, cb-5.4 (2019)). Another possibility for blocking the current flow outside the laser strip is a targeted O and Si implantation in these regions (D. Martin et al., Proc. SPIE 10900, 109000M (2019); P. Della Casa et al., Semicond. Sci. Technol. 34, 105005 (2019)). The current diaphragm is formed in the p-side cladding layer. These two methods can improve beam quality effectively, but with a compromise in η E and low repeatability of the process.Current-blocking aperture structures can also be formed by the integration of a highly doped n-layer into a highly doped p-side (sub) cladding of a diode laser ("p-n-p" structure). Such structures can be used advantageously, in particular in high-power broad-stripe diode lasers, for blocking the current outside the laser stripe (Y. Yamagata et al., Proc. SPIE 9348, 93480F (2015); M. Elattar et al., Semicond. Sci. Technol. 35, 095011 (2020)). Similarly, the n-doped barrier layer may also be integrated into other layers of the p-side of a diode laser, e.g. into the cladding layer. The barrier layer can also be p-doped and integrated into the n-side of the laser, i.e. formed as an "n-p-n" structure. However, such current-blocking structures have only low breakdown voltages ("punch-through voltage") due to the tunnel effect due to the thin depletion zone formed at each p-n junction. The occurrence of tunnel currents reduces the blocking efficiency at high powers and temperatures.From the document U.S. Pat. No. 6,822,989 B1 a semiconductor laser is known having an undoped first layer and a current blocking second layer formed on the first layer. JP H06-188 508 A relates to a semiconductor laser having AlGaAs current blocking layers introduced into the layer structure, the current blocking layers being surrounded by intrinsic GaAs diffusion protection layers. The documents US 2010 / 0 124 245 A1 and EP 0 314 372 A2 likewise disclose semiconductor lasers with non-doped barrier layers. US 5 987 048 A relates to a gallium nitride based compound semiconductor laser having a double heterojunction structure in which an active layer is disposed between cladding layers on a sapphire substrate.Disclosure of the InventionIt is therefore an object of the present invention to provide a diode laser with a current shutter in which the tunnel probability is reduced by the current shutter. In particular, the blocking effectiveness is to be maintained even at high powers and temperatures.These objects are achieved according to the invention by the features of claim 1. Practical embodiments of the invention are contained in the respective dependent claims.A diode laser according to the invention comprises an n-substrate made of an n-doped material; an n-cladding layer made of an n-doped material arranged on the n-substrate; an n-waveguide layer made of an n-doped material arranged on the n-cladding layer; an active layer arranged on the n-waveguide layer; a p-waveguide layer made of a p-doped material arranged on the active layer; a p-cladding layer made of a p-doped material arranged on the p-waveguide layer; a p-contact layer made of a p-doped material arranged on the p-cladding layer; and a p-contact arranged on the p-contact layer; wherein the diode laser is formed by a current shutter formed in a layer-shaped manner from a material doped counter to its environment for spatially selective energization of the active layer between the n-substrate and the p-contact, wherein the current shutter comprises a first layer and a second layer, wherein the thickness of the second layer is greater than the thickness of the first layer and the sum of the thicknesses of the first layer and the second layer is between 5 nm and 100 nm; wherein the current shutter is separated from adjacent layers by an intrinsic outer layer (i.e. an outer layer of an intrinsic material), the current shutter and the intrinsic outer layer are arranged within the p-contact layer, and the thickness of the intrinsic outer layer is between 5 nm and 50 nm. The current shutter can be realized either by an n-doped layer integrated into the p-side of the laser or by a p-doped layer integrated into the n-side.The materials of the individual layers are in particular semiconductor materials. In diode lasers, a contact layer is generally referred to as a layer effectively designed for current guidance, which, however, does not participate in the optical wave guidance. The contact layer can also be formed as a layer stack. In particular, the contact layer can comprise a sub-contact layer and a contacting layer arranged thereon. Typically, a very highly p-doped thin contacting layer is selectively implanted using standard techniques to define the strips prior to metallization (P. Della Casa et al., Semicond. Sci. Technol. 32, 065009 (2017)). A current diaphragm, which is likewise generally not intended to participate in the optical wave guidance, has, in contrast, the task of effectively suppressing or blocking a current flow through the diaphragm. By the arrangement of the current diaphragm, the current flow in the laser diode can thus be locally restricted.The intrinsic outer layer can be arranged on the top side or the underside of the flow diaphragm formed in the form of a layer, wherein its effect is stronger on the underside. The current shutter preferably has an upper intrinsic outer layer and a lower intrinsic outer layer. In the case of an intrinsic (i.e. undoped) semiconductor material, the charge carrier concentrations in the conduction band and in the valence band essentially depend only on the temperature and on the band gap. In contrast, in the case of doped semiconductor materials, their conductivity is influenced by the targeted introduction of impurity atoms.The amount of the vertical distance d res from the active zone to the current diaphragm should be as small as possible, preferably less than 1.5 μm, more preferably less than 1 μm and even more preferably less than 0.5 μm, in order to enable a minimum current spread. The current shutter is arranged within the p-side in the p-contact layer (positive d res, wherein a p-side structure thickness is typically about 2 μm). An arrangement within means here that the current aperture is delimited upward and downward by the p-contact layer. In contrast, an arrangement in the p-contact layer can also mean a limitation on only one side, i.e. upwards or downwards.Preferably, in an arrangement within the p-contact layer, the current shutter adjoins in each case a layer of p +- GaAs in an upward and downward direction. This is essentially a "p-n-p" structure for the current shutter, which is extended according to the invention by at least one intrinsic outer layer. In the case of two intrinsic outer layers, the structure according to the invention is referred to as a "p-i-n-i-p" structure.Preferably, the current shutter comprises a first layer of doped InGaP and a second layer of doped GaAs overlying the first layer. In particular, the first layer can be formed from doped In 0.48 Ga 0.52 P. By combining two different barrier layers in the current shutter, the electronic properties of the current shutter can be improved. The type of doping (n or p) of the doped layers of the current shutter depends on the arrangement of the current shutter in the laser diode. On the p-side, the layers are each n-doped, while the layers on the n-side are each p-doped.In the aforementioned embodiments of the current shutter having two barrier layers, the ratio of the thickness of the first layer to the thickness of the second layer is preferably between 0.2 and 1.0. It is also preferred that the sum of the thicknesses of the first layer and the second layer is between 5 nm and 100 nm, more preferably between 20 nm and 100 nm, more preferably between 40 nm and 80 nm, and even more preferably about 60 nm. Preferably, the thickness d i of an intrinsic outer layer is between 5 nm and 50 nm, more preferably 15 nm and 30 nm, and even more preferably about 20 nm. With a thickness of 20 nm, the tunnel current density can be reduced by about 7 orders of magnitude at a typical operating voltage of 1.7 V.Preferably, the doping density N D of the current shutter (i.e. of the doped layer or layers of the current shutter) is greater than or equal to 1 / 10 (N D ≥ 0.1·N A), more preferably greater than or equal to 1 / 2 (N D ≥ 0.5·N A), of the doping density N A of a layer surrounding the current shutter. Particularly preferably, the two doping densities correspond approximately to one another (N D ≈N A). For example, in an arrangement of the current stop within the p-contact layer, real doping densities N D for the two barrier layers are preferably between 0.2·10 18 cm -3 and 2·10 18 cm -3, more preferably between 0.5·10 18 cm -3 and 1.5·10 18 cm -3 and particularly preferably about 1·10 11 cm-3. In a comparable conventional current shutter with a "p-n-p" structure based on the said barrier layers, the doping densities N D with approximately 2·10 18 cm -3 must generally be significantly higher for a comparable breakdown voltage.Preferably, the unintentional doping density n i of an intrinsic outer layer is less than or equal to 1 / 10 (n i ≤ 0.1·N D), more preferably less than or equal to 1 / 2 (n i ≤ 0.5·N D) of the doping density N D of the current shutter. Particularly preferred is the unintended doping density n i ≤ 1·10 11 cm -3. The unintentional doping of an intrinsic outer layer can be either an n- or p-doping.The integration of thin intrinsic layers at the p-n junctions of current apertures, which in particular generate a "p-i-n-i-p" or an "n-i-p-i-n" structure, leads to broader depletion regions and thus to a lower tunnel probability. It enables a lower doping in the actual barrier layers of the current shield, which continues to spread the depletion regions with a consistently good blocking effect. Widened depletion zones also reduce the tunnel probability and therefore increase the breakdown voltage.Due to the lower tunnel probability, in particular in the case of a "p-i-n-i-p" or an "n-i-p-i-n" structure, the effectiveness of the blocking is ensured even at high powers and temperatures. Therefore, a diode laser according to the invention can preferably be a high-power broad-stripe diode laser.Further preferred embodiments of the invention result from the features mentioned in the dependent claims.The various embodiments of the invention mentioned in this application can be combined with one another with advantage unless stated otherwise in the individual case.Brief Description of the DrawingsThe invention is explained below in exemplary embodiments with reference to the associated drawing. The following are shown: FIG. 1 is a schematic illustration of an exemplary conventional laser diode having a current shutter with a "p-n-p" structure, FIG. 2 shows a schematic illustration of a first embodiment of a laser diode according to the invention with a current diaphragm having a "p-i-n-i-p" structure, FIG. 3 shows a schematic illustration of a second embodiment of a laser diode according to the invention with a current diaphragm having a "p-i-n-i-p" structure, FIG. 4 shows a schematic illustration of a third embodiment of a laser diode according to the invention with a current diaphragm having an "n-i-p-i-n" structure, FIG. 5 shows a calculated band profile within a current diaphragm in the case of an a) conventional laser diode according to FIGS. 1 and b) of an embodiment of a laser diode according to the invention having a current diaphragm with a "p-i-n-i-p" structure according to FIG. 2, FIG. 6 shows simulations relating to the U-I characteristic curves of conventional laser diodes with and without a current diaphragm and also of a laser diode according to the invention, and FIG. 7 shows measured U-I characteristic curves of conventional laser diodes with and without a current diaphragm and a laser diode according to the invention. FIG. 8 shows a calculation for the tunnel current density J t at different thicknesses d i of the intrinsic outer layers and different voltages U.DETAILED DESCRIPTION OF THE DRAWINGSFIG. 1 is a schematic illustration of an exemplary conventional laser diode having a current shutter with a "p-n-p" structure. The laser diode shown comprises an n-substrate 10, an n-cladding layer 12 resting on the n-substrate 10, an n-waveguide layer 14 resting on the n-cladding layer 12, an active layer 16 resting on the n-waveguide layer 14, a p-waveguide layer 18 resting on the active layer 16, a p-cladding layer 20 resting on the p-waveguide layer 18, a p-contact layer 22 resting on the p-cladding layer 20 in the central region of the laser diode, and a metallic p-contact 24 resting on the p-contact layer 22. The current shutter 30 consisting of an n-doped material is arranged within the p-contact layer 22, i.e. the current shutter 30 is delimited upward and downward by the p-contact layer 22. The current shutter 30 shown comprises a thinner first layer 34 and a thicker second layer 36 resting on the first layer 34.FIG. 2 shows a schematic illustration of a first embodiment of a laser diode according to the invention with a current diaphragm with a "p-i-n-i-p" structure. The basic layer structure of the laser diode corresponds largely to the illustration shown in FIG. 1. The respective reference signs and their assignment to the individual features therefore apply accordingly. In contrast to the laser diode shown in FIG. 1, the current shutter 30 is bounded according to the invention by a first intrinsic outer layer 32 on the underside and by a second intrinsic outer layer 38 on the upper side. The current shutter 30 of the laser diode thus has a "p-i-n-i-p" structure according to the invention. The vertical distance d res between the active layer 16 and the current shutter 30 is greater than the distance between the active layer 16 and the underside of the p-contact layer 22, but less than the distance between the active layer 16 and the underside of the p-contact 24.FIG. 3 shows a schematic illustration of a second embodiment of a laser diode according to the invention with a current diaphragm with a "p-i-n-i-p" structure. The basic layer structure of the laser diode corresponds largely to the illustration shown in FIG. 2. The respective reference signs and their assignment to the individual features therefore apply accordingly. In contrast to the laser diode shown in FIG. 2, the current shutter 30 is arranged here completely within the p-cladding layer 20 instead of in the p-contact layer 22. The vertical distance d res between the active layer 16 and the current stop 30 is therefore greater than the distance between the active layer 16 and the underside of the p-cladding layer 20, but less than the distance between the active layer 16 and the underside of the p-contact layer 22.FIG. 4 shows a schematic illustration of a third embodiment of a laser diode according to the invention with a current diaphragm with an "n-i-p-i-n" structure. The basic layer structure of the laser diode corresponds largely to the illustration shown in FIG. 2. The respective reference signs and their assignment to the individual features therefore apply accordingly. In contrast to the laser diode shown in FIG. 2, the current shutter 30 is arranged here completely within the n-cladding layer 12 instead of in the contact layer 22. In accordance with the opposite doping of the n-cladding layer 12, however, the current diaphragm 30 must be constructed from a p-doped material. The current shutter 30 shown comprises a thinner first layer 34 and a thicker second layer 36 resting on the first layer 34. The current shutter 30 of the laser diode thus has an "n-i-p-i-n" structure according to the invention. The amount of vertical distance d res between active layer 16 and current stop 30 is therefore greater than the distance between active layer 16 and the top surface of n-type cladding layer 14, but less than the distance between active layer 16 and the top surface of n-type substrate 10.FIG. 5 shows a calculated band profile within a current stop in the case of an a) conventional laser diode according to FIGS. 1 and b) of an embodiment of a laser diode according to the invention having a current stop with a "p-i-n-i-p" structure according to FIG. 2. the "p-n-p" structure having a layer sequence corresponding to a p-contact layer 22, the current stop 30 having a first layer 34 and a second layer 36, and the p-contact layer 22 are drawn in in the band profile shown in a). In the band course shown in b), a first intrinsic outer layer 32 and a second intrinsic outer layer 38 are additionally arranged according to the invention between the individual p-n junctions. It can be seen from the shown band profiles that the effective width of the current diaphragm can be increased significantly with the "p-i-n-i-p" structure, wherein the general profile of the band structure is, however, largely retained.FIG. 6 shows simulations for the U-I characteristic curves of conventional laser diodes with and without a current diaphragm and a laser diode according to the invention. For the simulations (WIAS-TeSC 1-D simulation), a laser diode with a current diaphragm with a "p-n-p" structure according to FIG. 1 was assumed as the basis. The material of the contact layer was p +- GaAs. The first layer of the current shutter made of n-In 0.48 Ga 0.52 P had a thickness of 20 nm in the simulation, while the second layer of the current shutter made of n-GaAs had a thickness of 40 nm. Uniform doping densities N D of 1·10 18 cm -3 and N were determined for both barrier layers, respectively. 2·10 18 cm -3 is specified. The conventional laser diode without current shutter (curve "without shutter") corresponded in the simulation to the diode lasers simulated with the conventional current shutter, except for the missing "p-n-p" structure. For a diode laser according to the invention, the "p-n-p" structure described above was modified in such a way that the current shutter has a "p-i-n-i-p" structure, wherein the thickness of the intrinsic layers was 20 nm in each case. For the respective barrier layers, a uniform doping density N D of 1·10 18 cm -3 was given. The simulations did not take into account the occurrence of tunnel effects.For a "p-n-p" structure, the simulations predict a reduction in breakdown voltage from about 7.2 V to about 2.7 V when the doping density N D of the n-doped barrier layers is reduced from 2·10 18 cm -3 to 1·10 18 cm -3 (curves "pnp_20-40_1e18" and "pnp_20-40_2e18"). The blocking capability of the "p-i-n-i-p" structure (N D= 1 ·10 18 cm -3, curve "pinip_20-20-40-20-1e18") is supposed to lie between these two values after the simulations with a breakdown voltage of about 5.0 V.FIG. 7 shows measured U-I characteristic curves of conventional laser diodes with and without a current diaphragm and a laser diode according to the invention. In particular, these are measurements on a laser diode test structure with a completely uniform current aperture (a small amount of current is to flow) and a conventional laser diode without a current aperture (reference values) and a laser diode test structure according to the invention. The measured laser diodes and laser diode test structures correspond to the parameters for the simulations described above with reference to FIG. 6, and therefore the assignment to the individual curves is carried out accordingly. The stripe width of each laser diode was 100 μm with an effective resonator length of 1 mm.For all structures, the measured breakdown voltage is significantly lower than expected from the TeSC simulations (FIG. 6 ). This is due to the fact that an ideal structure is simulated with TeSC, wherein deviations from the ideal behavior (e.g. diffusion of In atoms or Si dopant atoms) or various electrical effects (e.g. tunnelling, avalanche formation,... ) are not taken into account. However, compared to other structures, a "p-i-n-i-p" structure according to the invention provides a clear advantage. The simulations predict that the breakdown voltage should be D between the high and low dopant density N "p-n-p" structures, while the measurements show that it actually has the highest blocking capability despite the low dopant density of the n-doped junctions. This is attributable in particular to a considerable reduction in tunnel currents and thus the tunnel probability.FIG. 8 shows a calculation for the tunnel current density J t at different thicknesses d i of the intrinsic outer layers and different voltages U. The calculation is based on simulated band profiles and according to the calculation model from Y. Taur and T. H. Ning, Fundamentals of Modern VLSI Devices, 2nd ed., Cambridge University Press (2009). At a typical operating voltage of 1.7 V, the tunnel current density J t is reduced by 1 order of magnitude at a thickness d i of the intrinsic outer layers of 5 nm. For a preferred thickness of 10 nm, the tunnel current density J t is reduced by 2 orders of magnitude, for a most preferred thickness of 20 nm by 7 orders of magnitude, and for a likewise preferred thickness of 30 nm by 11 orders of magnitude. The calculation also shows that increasing the voltage (e.g., to 3.7 V) results in an increase in the tunnel current density J t.List of reference characters10 n-substrate 12 n-cladding layer 14 n-waveguide layer 16 active layer 18 p-waveguide layer 20 p-cladding layer 22 p-contact layer 24 p-contact 30 current shutter 32 first intrinsic outer layer (current shutter) 34 first layer (current shutter) 36 second layer (current shutter) 38 second intrinsic outer layer (current shutter)

Claims

A diode laser comprising: an n-substrate (10) made of an n-doped material; an n-cladding layer (12) made of an n-doped material disposed on the n-substrate (10); an n-waveguide layer (14) made of an n-doped material disposed on the n-cladding layer (12); an active layer (16) disposed on the n-waveguide layer (14); a p-waveguide layer (18) made of a p-doped material disposed on the active layer (16); a p-cladding layer (20) made of a p-doped material disposed on the p-waveguide layer (18); a p-contact layer (22) made of a p-doped material disposed on the p-cladding layer (20); and a p-contact (24) disposed on the p-contact layer (22); wherein the diode laser has a current shutter (30) formed in a layer-like manner and made of a material doped counter to its environment for spatially selective energization of the active layer (16) between the n-substrate (10) and the p-contact (24), wherein the current shutter (30) comprises a first layer (34) and a second layer (36), wherein the thickness of the second layer (36) is greater than the thickness of the first layer (34) and the sum of the thicknesses of the first layer (34) and of the second layer (36) is between 5 nm and 100 nm; wherein the current shutter (30) is separated from adjacent layers by an intrinsic outer layer (32, 38), the current shutter (30) and the intrinsic outer layer (32, 38) are arranged within the p-contact layer (22), and the thickness of the intrinsic outer layer (32, 38) is between 5 nm and 50 nm.Diode laser according to claim 1, wherein the current shutter (30) has an upper intrinsic outer layer (32) and / or a lower intrinsic outer layer (38).Diode laser according to claim 1 or 2, wherein the amount of the vertical distance d res of the current shutter (30) from the active layer (16) is less than 1.5 μm.The diode laser of any preceding claim, wherein the first layer (34) is formed of doped InGaP and the second layer (36) is formed of doped GaAs.The diode laser of claim 4, wherein the first layer (34) is formed of doped In 0.48 Ga 0.52 P.The diode laser of any preceding claim, wherein the ratio of the thickness of the first layer (34) to the thickness of the second layer (36) is between 0.2 and 1.0.Diode laser according to one of the preceding claims, wherein the doping density N D of the current shutter (30) is greater than or equal to 1 / 10 of the doping density N A of a layer surrounding the current shutter (30).

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