DIODE LASER WITH CURVE

Description

Description

[0001] The present invention relates to a diode laser with a current gate and, in particular, to a diode laser with an adapted "pnp" or "npn" structure as a current gate to reduce the tunneling probability. State of the art

[0002] Broad-area diode lasers (BALs) can exhibit particularly high efficiency and brilliance / radiance. In their respective structures, current-blocking layers, so-called current shutters, are integrated into the edge regions of the components. These are intended to ensure current confinement below the central stripe, limiting the adverse effects on beam quality caused by current spreading and lateral carrier accumulation (LCA). Furthermore, losses at the stripe edges can be minimized, thereby lowering the laser threshold and increasing conversion efficiency while maintaining high polarization purity.Reducing current and charge carrier density at stripe edges leads to lower gain for higher-order lateral modes and can thus even reduce the number of guided modes, thereby improving beam properties.

[0003] A variety of lateral structuring techniques have been developed to achieve the desired central confinement of current and charge carriers, particularly in GaAs-based BALs, with the challenge being to improve beam quality without compromising on n E , maximum Popt or device lifetime. One of these techniques is high-energy deep ion implantation (HED) to prevent current flow in the device edges. Implantation profiles, particularly those that reach the n-doped side of the BAL, i.e., pass through the active region, eliminate current spreading in the p-side and suppress LCA due to the rapid recombination of charge carriers that 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); MM Karow et al., CLEO / Europe-EQEC, cb-5.4 (2019)). Another possibility to block the current flow outside the laser stripe is a targeted O and Si implantation in these areas (D. Martin et al., Proc. SPIE 10900, 109000M (2019); P. Della Casa et al., Semicond. Sci. Technol. 34, 105005 (2019)).The current gate is formed in the p-side cladding layer. These two methods can effectively improve beam quality, but with a compromise in performance. n E and low repeatability of the process.

[0004] Current-blocking aperture structures can also be formed by integrating a highly doped n-layer into a highly doped p-side (sub-)contact layer of a diode laser ("pnp" structure). Such structures can be advantageously used, particularly in high-power broad-area diode lasers, to block current outside the laser stripe (Y. Yamagata et al., Proc. SPIE 9348, 93480F (2015); M. Elattar et al., Semicond. Sci. Technol. 35, 095011 (2020), US5987048A). Similarly, the n-doped barrier layer can also be integrated into other layers of the p-side of a diode laser, e.g., 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 "npn" structure. However, such current-gap structures exhibit only low punch-through voltages due to the tunneling effect caused by the thin depletion zone formed at each pn junction.The occurrence of tunneling currents reduces the effectiveness of blocking at high powers and temperatures. Disclosure of the invention

[0005] It is therefore an object of the present invention to provide a diode laser with a current gate in which the tunneling probability is reduced by the current gate. In particular, the effectiveness of the blocking is to be maintained even at high power levels and temperatures.

[0006] These objects are achieved according to the invention by the features of patent claim 1. Advantageous embodiments of the invention are contained in the respective subclaims.

[0007] An exemplary diode laser 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 layer-shaped current aperture made of a material doped opposite to its surroundings for spatially selective current supply to the active layer between the n-substrate and the p-contact; wherein the current aperture has an intrinsic outer layer (ieAn outer layer of an intrinsic material is separated from adjacent layers, with the thickness of the intrinsic outer layer being between 5 nm and 50 nm. The current gate 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.

[0008] The materials of the individual layers are primarily semiconductor materials. In diode lasers, a contact layer is generally referred to as a layer that is effectively designed to carry current, but which does not participate in the optical waveguiding. 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 methods to define the stripes prior to metallization (P. Della Casa et al., Semicond. Sci. Technol. 32, 065009 (2017)). A current aperture, which is also generally not intended to participate in the optical waveguiding, has the task of effectively suppressing or blocking current flow through the aperture.By arranging the current aperture, the current flow in the laser diode can be locally limited.

[0009] The intrinsic outer layer can be arranged on the top or bottom of the layered current aperture, with its effect being stronger at the bottom. The current aperture preferably has an upper intrinsic outer layer and a lower intrinsic outer layer. In an intrinsic (i.e., undoped) semiconductor material, the charge carrier concentrations in the conduction band and the valence band depend essentially only on the temperature and the band gap. In contrast, in doped semiconductor materials, their conductivity is influenced by the targeted introduction of foreign atoms.

[0010] The amount of vertical distance dThe distance from the active zone to the current gate 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, to allow for minimal current spreading. The current gate is preferably located within the p-side, for example, in the p-contact layer (positive d res , with a p-side structure thickness typically being about 2 µm). An arrangement within means that the current aperture is limited by the p-contact layer at the top and bottom. In contrast, an arrangement in the p-contact layer can also mean a one-sided limitation, i.e., at the top or bottom. However, the current aperture can also be arranged correspondingly within the p-cladding layer or the n-side of the laser, for example in the n-cladding layer (negative d res ).

[0011] When arranged within the p-type contact layer, the current gate preferably borders a layer of p-type GaAs at the top and bottom. This essentially represents a "pnp" structure for the current gate, which is extended according to the invention by at least one intrinsic outer layer. With two intrinsic outer layers, the structure according to the invention is referred to as a "pinip" structure. This also applies when the current gate is arranged within the p-type cladding layer. However, if the current gate is arranged in the n-type cladding layer, a corresponding "nipin" structure is formed by the reverse doping of the individual layers. Layers of other materials that can border the current gate include p-AlGaAs (p-type cladding layer), n-AlGaAs (n-type cladding layer), and any combination of an Al-In-Ga-As-P-Sb-N material system.

[0012] According to the invention, the current gate comprises a first layer of doped InGaP and a second layer of doped GaAs resting on the first layer. In particular, the first layer can be formed of doped In 0.48 Ga 0.52 P. By combining two different barrier layers in the current gate, the electronic properties of the current gate can be improved. The type of doping (n or p) of the doped layers of the current gate depends on the arrangement of the current gate 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.

[0013] In the above-mentioned embodiments of the current aperture with 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 an intrinsic outer layer between 5 nm and 50 nm, more preferably 15 nm and 30 nm, and even more preferably at about 20 nm. At a thickness of 20 nm, the tunneling current density can be reduced by about 7 orders of magnitude at a typical operating voltage of 1.7 V.

[0014] Preferably, the doping density N D of the current aperture (ie the doped layer or layers of the current aperture) greater than or equal to 1 / 10 ( N D ≥ 0.1· NA ), preferably greater than or equal to 1 / 2 ( N D ≥ 0.5· N A ), the doping density N A of a layer surrounding the current aperture. It is particularly preferred that the two doping densities approximately match ( N D ≈ N A ) . For example, if the current aperture is arranged within the p-contact layer, real doping densities N D for the two barrier layers 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 at about 1·10 18< cm -3< . In a comparable conventional current gate with a "pnp" structure based on the barrier layers mentioned, the doping densities N D with about 2·10 18< cm -3< for a comparable breakdown voltage is generally much higher.

[0015] Preferably, the unintended doping density ni an intrinsic outer layer less than or equal to 1 / 10 ( n i ≤ 0.1· N D ), preferably less than or equal to 1 / 2 ( n i ≤ 0.5· N D ) the doping density N D of the current aperture. Particularly preferred is the unintended doping density n i ≤ 1·10 16< cm -3< . The unintentional doping of an intrinsic outer layer can be either n- or p-doping.

[0016] The integration of thin intrinsic layers at the pn junctions of current gates, which in particular create a "pinip" or "nipin" structure, leads to wider depletion regions and thus a lower tunneling probability. This allows for lower doping in the actual barrier layers of the current gate, which further expands the depletion regions while maintaining a consistently good blocking effect. Broadened depletion zones also reduce the tunneling probability and thus increase the breakdown voltage.

[0017] Due to the lower tunneling probability, especially in a "pinip" or "nipin" 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-area diode laser.

[0018] Further preferred embodiments of the invention result from the features mentioned in the subclaims.

[0019] The various embodiments of the invention mentioned in this application can be advantageously combined with one another, unless otherwise stated in the individual case. Brief description of the drawings

[0020] The invention is explained below in exemplary embodiments with reference to the accompanying drawings. They show: Fig. 1 shows a schematic representation of an exemplary conventional laser diode with a current aperture having a "pnp" structure, Fig. 2 shows a schematic representation of a first embodiment of a laser diode according to the invention with a current aperture having a "pinip" structure, Fig. 3 shows a schematic representation of a second embodiment of a laser diode according to the invention with a current aperture having a "pinip" structure, Fig. 4 shows a schematic representation of a third embodiment of a laser diode according to the invention with a current aperture having a "nipin" structure, Fig. 5 shows a calculated band profile within a current aperture in a) conventional laser diode according to FIG. 1 and b) an embodiment of a laser diode according to the invention with a current aperture with a "pinip" structure according to FIG. 2, Fig. 6 Simulations of the UI characteristics of conventional laser diodes with and without current apertures, as well as a laser diode according to the invention, and Fig. 7 Measured UI characteristics of conventional laser diodes with and without current apertures, as well as a laser diode according to the invention. Fig. 8 A calculation of the tunnel current density J t at different thicknesses d i of the intrinsic outer layers and various voltages U. Detailed description of the drawings

[0021] Figure 1shows a schematic representation of an exemplary conventional laser diode with a current gate with a "pnp" structure. The laser diode shown comprises an n-substrate 10, an n-cladding layer 12 lying on the n-substrate 10, an n-waveguide layer 14 lying on the n-cladding layer 12, an active layer 16 lying on the n-waveguide layer 14, a p-waveguide layer 18 lying on the active layer 16, a p-cladding layer 20 lying on the p-waveguide layer 18, a p-contact layer 22 lying on the p-cladding layer 20 in the central region of the laser diode, and a metallic p-contact 24 lying on the p-contact layer 22. The current aperture 30 consisting of an n-doped material is arranged within the p-contact layer 22, ie the current aperture 30 is limited at the top and bottom by the p-contact layer 22.The current aperture 30 shown comprises a thinner first layer 34 and a thicker second layer 36 lying on top of the first layer 34. The current aperture 30 of the laser diode thus has a "pnp" structure.

[0022] Figure 2 shows a schematic representation of a first embodiment of a laser diode according to the invention with a current aperture with a "pinip" structure. The basic layer structure of the laser diode largely corresponds to that shown in FIG. 1 The respective reference symbols and their assignment to the individual features therefore apply accordingly. In contrast to the FIG. 1 In the laser diode shown, the current aperture 30 is defined according to the invention at the bottom by a first intrinsic outer layer 32 and at the top by a second intrinsic outer layer 38. The current aperture 30 of the laser diode thus has a "pinip" structure according to the invention. The vertical distance dres between the active layer 16 and the current aperture 30 is greater than the distance between the active layer 16 and the underside of the p-contact layer 22, but smaller than the distance between the active layer 16 and the underside of the p-contact 24. The current aperture is thus arranged completely within the p-contact layer 22.

[0023] Figure 3 shows a schematic representation of a second embodiment of a laser diode according to the invention with a current aperture with a "pinip" structure. The basic layer structure of the laser diode largely corresponds to that shown in FIG. 2 The respective reference symbols and their assignment to the individual features therefore apply accordingly. In contrast to the FIG. 2 In the laser diode shown, the current aperture 30 is arranged entirely within the p-cladding layer 20 instead of in the p-contact layer 22. The vertical distance dres between the active layer 16 and the current gate 30 is therefore greater than the distance between the active layer 16 and the bottom of the p-cladding layer 20, but smaller than the distance between the active layer 16 and the bottom of the p-contact layer 22.

[0024] Figure 4 shows a schematic representation of a third embodiment of a laser diode according to the invention with a current aperture with a "nipin" structure. The basic layer structure of the laser diode largely corresponds to that shown in FIG. 2 The respective reference symbols and their assignment to the individual features therefore apply accordingly. In contrast to the FIG. 2In the laser diode shown, the current aperture 30 is arranged entirely within the n-type cladding layer 12 instead of within the contact layer 22. However, due to the opposite doping of the n-type cladding layer 12, the current aperture 30 must be constructed from a p-doped material. The current aperture 30 shown comprises a thinner first layer 34 and a thicker second layer 36 resting on the first layer 34. According to the invention, the current aperture 30 is delimited at the bottom by a first intrinsic outer layer 32 and at the top by a second intrinsic outer layer 38. The current aperture 30 of the laser diode thus has a "nipin" structure according to the invention.

[0025] The amount of vertical distance dres between the active layer 16 and the current gate 30 is therefore greater than the distance between the active layer 16 and the top of the n-cladding layer 14, but smaller than the distance between the active layer 16 and the top of the n-substrate 10.

[0026] Figure 5 shows a calculated band profile within a current aperture for a) conventional laser diode according to FIG. 1 and b) an embodiment of a laser diode according to the invention with a current aperture with a "pinip" structure according to FIG. 2The band profile shown in a) depicts the "pnp" structure with a layer sequence corresponding to a p-contact layer 22, the current gate 30 with a first layer 34 and a second layer 36, and the p-contact layer 22. In the band profile shown in b), a first intrinsic outer layer 32 and a second intrinsic outer layer 38 are additionally arranged between the individual pn junctions according to the invention. It can be seen from the band profiles shown that the effective width of the current gate can be significantly increased with the "pinip" structure, while the general band structure is largely retained.

[0027] Figure 6 shows simulations of the UI characteristics of conventional laser diodes with and without current gates, as well as a laser diode according to the invention. For the simulations (WIAS-TeSCA 1-D simulation), a laser diode with a current gate with a "pnp" structure according to FIG. 1as a basis. The material of the contact layer was p +< -GaAs. The first layer of the current gate 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 gate made of n-GaAs had a thickness of 40 nm. Uniform doping densities were used for both barrier layers. N D of 1 10 18< cm -3< or 2 10 18< cm -3<. The conventional laser diode without a current gate (curve "without gate") corresponded in the simulation to the diode lasers simulated with the conventional current gate, except for the missing "pnp" structure. For a diode laser according to the invention, the "pnp" structure described above was modified such that the current gate had a "pinip" structure, with the thickness of the intrinsic layers being 20 nm each. A uniform doping density was used for the corresponding barrier layers. ND of 1·10 18< cm -3< was specified. The simulations did not consider the occurrence of tunneling effects.

[0028] For a "pnp" structure, the simulations predict a reduction in the 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 "pinip" structure ( N According to the simulations, the breakdown voltage of the junction (D = 1·10 18< cm -3< , curve "pinip_20-20-40-20_1e18") should be between these two values ​​with a breakdown voltage of about 5.0 V.

[0029] Figure 7shows measured UI characteristics of conventional laser diodes with and without current apertures, as well as a laser diode according to the invention. In particular, these are measurements on a laser diode test structure with a completely uniform current aperture (little current should flow) and a conventional laser diode without current aperture (reference values), as well as a laser diode test structure according to the invention. The measured laser diodes and laser diode test structures correspond to the previously described FIG. 6 The parameters described for the simulations were used, so the assignment to the individual curves is made accordingly. The stripe width of the laser diodes was 100 µm each, with an effective resonator length of 1 mm.

[0030] For all structures, the measured breakdown voltage is considerably lower than that predicted from the TeSCA simulations ( FIG. 6) is expected. This is due to the fact that TeSCA simulates an ideal structure, whereby deviations from the ideal behavior (e.g., diffusion of In atoms or Si dopant atoms) or various electrical effects (e.g., tunneling, avalanche formation, etc.) are not taken into account. However, compared to other structures, a "pinip" structure according to the invention offers a clear advantage. The simulations predict that the breakdown voltage between the "pnp" structures with high and low doping density N D, while measurements show that, despite the low doping density of the n-doped barrier layers, it actually exhibits the highest blocking capability. This is primarily due to a significant reduction in tunneling currents and thus in the tunneling probability.

[0031] Figure 8 shows a calculation of the tunnel current density J t at different thicknessesd i of the intrinsic outer layers and various voltages U. The calculation is based on simulated band paths and the calculation model from Y. Taur and TH 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 with a thickness d i of the intrinsic outer layers of 5 nm is reduced by 1 order of magnitude. At a preferred thickness of 10 nm, the tunnel current density J t by 2 orders of magnitude, at a most preferred thickness of 20 nm by 7 orders of magnitude, and at a also preferred thickness of 30 nm by 11 orders of magnitude. The calculations also show that an increase in the voltage (e.g., to 3.7 V) leads to an increase in the tunnel current density. J t leads. List of reference symbols

[0032] 10n substrate 12n cladding layer 14n waveguide layer 16 active layer 18p waveguide layer 20p cladding layer 22p contact layer 24p contact 30 current aperture 32 first intrinsic outer layer (current aperture) 34 first layer (current aperture) 36 second layer (current aperture) 38 second intrinsic outer layer (current aperture)

Claims

1. A diode laser, comprising: an n-substrate (10) of an n-doped material; an n-cladding layer (12) of an n-doped material arranged on the n-substrate (10); an n-waveguide layer (14) 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) of a p-doped material arranged on the active layer (16); a p-cladding layer (20) of an p-doped material arranged on the p-waveguide layer (18); a p-contact layer (22) 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 is formed, through a current shade (30) formed to be layered, from a doped material opposing its surroundings for a spatially selective current supply to the active layer (16) between the n-substrate (10) and the p-contact (24); wherein the current shade (30) is separated from adjacent layers via an intrinsic external layer (32, 38), characterized in that the thickness of the intrinsic external layer (32, 38) is between 5 nm and 50 nm, wherein the current shade (30) comprises a first layer (34) of doped InGaP and a second layer (36) of doped GaAs, 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.

2. The diode laser according to Claim 1, wherein the current shade (30) has an upper intrinsic external layer (32) and / or a lower intrinsic external layer (38).

3. The diode laser according to Claim 1 or 2, wherein the current shade (30) is arranged within the p-contact layer (22), within the p-cladding layer (20) or within the n-cladding layer (12).

4. The diode laser according to any one of the preceding claims, wherein the value of the vertical distance dres of the current shade (30) from the active layer (16) is smaller than 1.5 µm.

5. The diode laser according to Claim 5, wherein the first layer (34) is formed from doped In0.48Ga0.52P.

6. The diode laser according to any one of the preceding claims, 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.

7. The diode laser according to any one of the preceding claims, wherein the doping density ND of the current shade (30) is greater or equal to 1 / 10 of the doping density NA of a layer surrounding the current shade (30).