Wide-field diode lasers including integrated pn tunnel junctions

Abstract

The present invention relates to a broad area diode laser (BAL) containing an integrated pn tunnel junction. In particular, the present invention relates to a high performance broad area diode laser in which a reverse biased pn tunnel junction is integrated into the layer system of the diode laser to improve beam quality and reduce thermal resistance. The laser diode according to the present invention comprises an active layer (20) formed between an n-doped semiconductor material (10, 12, 14) and a p-doped semiconductor material (30, 32, 34), the active layer (20) forming an active zone along a longitudinal axis for generating electromagnetic radiation, at least one n-doped intermediate layer (50, 54) disposed thereon between a p-side metal contact (52) and the p-doped semiconductor material (30, 32, 34), the at least one n-doped intermediate layer (50, 54) in a region above the active zone forming a p-n tunnel junction (40) directly adjacent the p-doped semiconductor material (30, 32, 34).

Description

[Technical Field]

[0001] The present invention relates to a broad-field diode laser (BAL) including an integrated pn tunnel junction. In particular, the present invention relates to a high-performance broad-field diode laser in which a reverse-biased pn tunnel junction is integrated into the layer system of the diode laser to improve beam quality and reduce thermal resistance. [Background technology]

[0002] Broadband diode lasers (BALs) can offer particularly high efficiency and brightness. Using these emitters, an output of >15W can be reliably obtained. BALs are the most efficient near-infrared (NIR) light sources and are therefore common as pumping sources for solid-state and fiber lasers. They are also a key component of fiber-coupled laser systems configured to provide beams with high radiation density for high-conversion-efficiency material processing. Improving beam quality, especially on the slower axis, is crucial to increasing the output and reducing costs of these systems, as it allows for the coupling of more emitters into low numerical aperture (NA) fibers.

[0003] However, at high optical power and associated operating currents, beam quality generally deteriorates considerably, particularly negatively impacting fiber coupling. It has been demonstrated that the primary cause of beam quality degradation with increasing operating current is the thermal lensing effect in the low-speed axis (rather than carrier or gain-induced guiding effects) (Bai, JG et al., Mitigation of Thermal Lensing Effect as a Brightness Limitation of High-Power Broad Area Diode Lasers, Proc. SPIE 7953, 79531F (2011) & Crump, P. et al., Experimental Studies Into the Beam Parameter Product of GaAs High-Power Diode Lasers, IEEE J. Sel. Top. Quantum Electron., vol. 28, no. 1 (2022)). Therefore, to suppress the degradation of beam quality at high power, it is crucial to construct a lateral temperature gradient based on the temperature rise in the central region beneath the laser stripe. This results in a localized increase in refractive index, further inducing transverse waves and consequently a larger divergence angle.

[0004] In particular, previous studies on GaAs-based broadband diode lasers (e.g., Rieprich, J. et al., Thermal boundary resistance between GaAs and p-side metal as limit to high power diode lasers, IEEE High Power Diode Lasers and Systems Conf. (Coventry, UK), pp. 35-36 (2019)) demonstrated that a significant thermal barrier is formed at the boundary between the highly p-doped GaAs contact layer and the metal contact deposited on top of it. This barrier reduces heat dissipation, which in turn enhances the resulting thermal lensing effect, consequently degrading beam quality and increasing thermal resistance. However, this type of thermal barrier was not observed at other semiconductor-metal boundaries within the component, namely between n-doped GaAs and metal. [Overview of the Initiative] [Means for solving the problem]

[0005] Disclosure of the present invention

[0006] Therefore, an object of the present invention is to provide a wide-area diode laser that reduces or completely prevents the thermal barrier between a highly p-doped contact layer and a metal contact layer deposited thereon, in order to improve beam quality and reduce thermal resistance.

[0007] These objectives are achieved according to the features of independent claim 1 in accordance with the present invention. Favorable configurations of the present invention are found in the dependent claims. [Brief explanation of the drawing]

[0008] [Modes for carrying out the invention]

[0009] The present invention relates to a laser diode comprising an active layer formed between an n-doped semiconductor material and a p-doped semiconductor material, wherein the active layer forms an active zone along its longitudinal axis for generating electromagnetic radiation, and at least one n-doped intermediate layer is disposed between a p-side metal contact on the intermediate layer and the p-doped semiconductor material, and a pn tunnel junction directly adjacent to the p-doped semiconductor material is formed in the at least one n-doped intermediate layer in a region above the active region. The at least one n-doped intermediate layer preferably includes a p-side n-contact layer. The p-side metal contact can be disposed on the p-side n-contact layer.

[0010] Here, an n-doped semiconductor material typically includes an n-doped substrate (called the n-substrate), an n-side n-cover layer placed on the n-substrate, and an n-guide layer placed on the n-side n-cover layer. A p-doped semiconductor material typically includes a p-guide layer and a p-cover layer placed on the p-guide layer. In the prior art, the p-contact layer is usually placed on the p-cover layer. The active layer configured for photogeneration is located between the two different doped semiconductor materials. In this case, the active zone is the portion of the active layer where photogeneration actually takes place during the operation of the laser diode by carrier injection. The longitudinal axis points in the longitudinal direction and preferably corresponds to the resonator axis of the laser.

[0011] Carriers are typically supplied to the n-substrate on the n-side and to the metal contacts above it on the p-side, where these p-side metal contacts form a semiconductor-metal boundary with the underlying p-contact layer. As already mentioned above, this boundary constitutes a considerable thermal barrier. Since this type of thermal barrier was not observed at the n-side semiconductor-metal boundary, according to the present invention, at least one n-doped intermediate layer is placed between the p-side metal contacts above it and the p-doped semiconductor material below it. Furthermore, to further enable carrier implantation, a pn tunnel junction is formed in the at least one n-doped intermediate layer in the region above the active zone, directly adjacent to the p-doped semiconductor material. Thus, a boundary considered preferable can also be generated between the n-semiconductor material and the metal on the p-side, i.e., without forming a considerable thermal barrier. The pn tunnel junction preferably has a total thickness of less than 100 nm.

[0012] This invention is based on the finding that a semiconductor-metal boundary containing n-doped semiconductors on both sides of a BAL is particularly advantageous. To achieve this, the p-side contact layer of the BAL can be formed to be n-doped instead of p-doped. However, this reverses the bias of the pn junction, which acts as a power cutoff. To avoid this, a reverse-biased pn tunnel junction, i.e., a tunnel junction (TJ), is formed at this pn boundary. This junction consists of, for example, a very highly doped semiconductor layer, allowing carriers to tunnel between the corresponding p-side n-contact layer and the other p-side semiconductor layer. In this type of design, it is important that the switch-on voltage of the tunnel junction is very low and the intermediate resistance is very low, which means that the conversion efficiency of the BAL is not compromised. The BAL according to this invention is also called a TJ-BAL.

[0013] This approach offers several advantages in addition to reducing or avoiding the thermal barrier formed at the p-side semiconductor-metal boundary. The high electrical conductivity of the highly doped n-contact layer and TJ layer allows for very low intermediate resistance between the active zone and the epitaxial contact, especially in vertical structures with thin p-side waveguides and cover layers, such as in extreme triple-asymmetric (ETAS) designs. This low intermediate resistance results in higher conversion efficiency, especially at higher current intensities (Crump, P. et al., Efficient High-Power Laser Diodes, IEEE J.Sel.Top.Quantum Electron., vol.19, no.4 (2013)). Furthermore, because the planar TJ layer is highly doped, it tends to be equipotential. This means that voltage differences occurring between different regions of the laser tip cannot be maintained and quickly equalize. The presence of low epitaxial resistance and equipotentiality has the advantage of reducing spatial hole burning and suppressing higher-order transverse modes, resulting in further improvements in beam quality, power, and conversion efficiency (Zeghuzi, A. et al., Traveling wave analysis of non-thermal far-field blooming in high-power broad-area lasers, IEEE J. Quantum Electron., vol. 55, no. 2 (2019) & Zeghuzi, A. et al., Influence of nonlinear effects on the properties of pulsed high-power broad-area distribution Bragg reflector lasers, Opt. Quant. Electron., vol. 50, no. 88 (2018)).

[0014] Another advantage of the very low specific resistance and equipotentiality of the highly doped p-side semiconductor layer is that the thickness of the p-side n-contact layer grown on the p-n tunnel junction can be increased considerably without significantly reducing the electrical resistance. As a result, in structures with a thin p-side, the active zone can be protected from process-related and design-related mechanical stresses. Mechanical stresses can impair the polarization purity, output, and service life of the component, result in unwanted waveguide formation, and potentially degrade the beam quality.

[0015] The p-n tunnel junction preferably includes a p + tunnel layer disposed on a p-doped semiconductor material and an n + tunnel layer disposed thereon. Due to the high doping of the two tunnel layers of the p-n tunnel junction, carriers can tunnel through the reverse-biased p-n junction formed at the boundary between at least one p-side n-doped intermediate layer and the p-doped semiconductor material, so that the electrical resistance remains low during carrier injection. The doping concentrations of the n-doped layer and the p-doped layer of the p-n tunnel junction are preferably N D,A ≧10 19 cm -3 (the normal doping concentration of the environment is up to about [[ID=#15]] 18 cm -3 ).

[0016] The p-n tunnel junction is preferably disposed on a p-doped sub-contact layer (p sub-contact layer) of the p-doped semiconductor material. The p sub-contact layer substantially corresponds to the p-contact layer of the prior art. However, according to the present invention, the metal contact is not disposed on the p sub-contact layer, but is instead separated from the p sub-contact layer by at least one n-doped intermediate layer. The p sub-contact layer can be different from the p cover layer disposed thereunder due to the semiconductor material or its composition, or due to the discontinuity of the refractive index / refractive index gradient progression at the layer boundary.

[0017] The p-n tunnel junction is preferably disposed on a p-doped cover layer of a p-doped semiconductor material. In this case, no p-subcontact layer is disposed between the p-doped cover layer and the p-n tunnel junction. The n-doped cover layer is preferably disposed on the p-n tunnel junction. This means that the p-side cover layer includes a p-doped region and an n-doped region, and a p-n tunnel junction is disposed therebetween. Since the optical mode induced in the waveguide layer also extends to the cover layer, this means that the optical mode can extend beyond the p-side p-n tunnel junction to the p-side n-doped cover layer. The contact layer that may be adjacent to this does not play a substantial role in the waveguide.

[0018] The stripe width of the diode laser is preferably defined with respect to the lateral width W of the p-n tunnel junction. Due to the reverse p-n junctions occurring everywhere outside the p-n tunnel junction, the injection region can be defined by the geometric definition of the p-n tunnel junction. Therefore, the current path can be defined by the size and shape of the p-n tunnel junction.

[0019] Alternatively, the p-n tunnel junction can be formed as a layer, and the stripe width of the diode laser is defined with respect to the lateral width W of the opening of the n-current shield introduced into the p-doped semiconductor material. In this case, the p-n tunnel junction cancels out the effect of the reverse p-n junction over a wide range, and the structuring of the individual stripes needs to be done in another way. The proposed n-current shield is well known from the prior art for restricting the flow of current. Therefore, the current path can be defined by the size and shape of the opening of the n-current shield (shield opening) in exactly the same way as in the above embodiment. The p-n tunnel junction preferably has a total thickness of less than 100 nm, and the doping concentration of the n-current shield is preferably N D ≧10 18 cm -3 -3.

[0020] When the pn tunnel junction is formed as a layer, the stripe width of the diode laser can instead be defined with respect to the width W of the region between the n current shield and two adjacent (e.g., by ion implantation) deep implantation regions. Deep implantation can make the resistance of the treated region very high, so that current flow is effectively present only in the regions that are not deeply implanted. Here, it is preferable that the deep implantation extends from the metal contact to the p cover layer. The intermediate resistance of the deep implantation region is preferably at least twice the intermediate resistance of the surrounding region.

[0021] The semiconductor material is preferably GaAs-based. For example, an n-substrate GaAs can include an n-side n-cover layer AlGaAs, an n-waveguide layer AlGaAs, a p-waveguide layer AlGaAs, and a p-cover layer AlGaAs. The p-subcontact layer can include GaAs. The pn tunnel junction is p + p as a tunnel layer + GaAs, n + n as a tunnel layer + It can contain GaAs. The n-contact layer can contain GaAs. The p-side n-cover layer can contain AlGaAs.

[0022] The minimum distance between the active layer and the pn tunnel junction is preferably less than 1.3 μm, preferably less than 1 μm, and more preferably less than 0.5 μm. The advantage of minimizing the distance between the active layer and the pn tunnel junction is the reduction of intermediate resistance and spatial hole burning, resulting in improved laser properties (e.g., beam quality, power, efficiency).

[0023] In the TJ-BAL according to the present invention, various lateral structuring techniques can be implemented to limit the current to the center of the component (i.e., below the laser stripe or above the active zone). The resulting current limiting limits losses at the stripe edges and limits the adverse effects on beam quality due to lateral current diffusion and lateral carrier accumulation (LCA). In these TJ-BALs, the current limiting is more pronounced than in standard BALs because current diffusion in the n-contact layer is significantly greater than in the p-contact layer due to the higher mobility of electrons compared to holes.

[0024] Thickness d of the residual layer between the active zone and the current shield res The thickness of the residual layer d is preferably less than 1 μm. Unfavorable current diffusion is minimized as much as possible. res This can be reduced. In this case, the thickness of the residual layer d res This generally represents the minimum distance between the active layer and the structure closest to the active layer, and is additionally introduced into the actual basic structure of the layer structure of the laser diode to define the stripe width of the diode laser. This may be, for example, a pn tunnel junction, an n current shield, or a corresponding deep injection region according to the present invention. The total p-side thickness d including the pn tunnel junction and the p-side n contact layer tot The thickness is preferably greater than 2 μm. Total thickness d of the waveguide layer WL The p-side waveguide layer d is preferably greater than 1 μm. p-WL The thickness is preferably less than 350 nm. The stripe width is preferably 50 μm or more. The resonator length L is preferably 3 mm or more.

[0025] Further preferred configurations of the present invention are evident from the features described in each dependent claim.

[0026] The various embodiments of the present invention described in this application can be advantageously combined with one another unless otherwise specifically stated in a particular example.

[0027] Brief explanation of the drawing

[0028] The present invention will be described below with reference to exemplary embodiments and with reference to the relevant drawings.

[0029] Figure 1 is a schematic diagram illustrating a first embodiment of the laser diode according to the present invention.

[0030] Figure 2 is an illustrative schematic diagram of a second embodiment of the laser diode according to the present invention.

[0031] Figure 3 is an illustrative schematic diagram of a third embodiment of the laser diode according to the present invention.

[0032] Figure 4 is an illustrative schematic diagram of a fourth embodiment of the laser diode according to the present invention.

[0033] Figure 5 is an illustrative schematic diagram of a fifth embodiment of the laser diode according to the present invention.

[0034] Figure 6 is an illustrative schematic diagram of a sixth embodiment of the laser diode according to the present invention.

[0035] Detailed description of the drawing

[0036] Figure 1 is an exemplary schematic diagram of a first embodiment of a laser diode according to the present invention. The illustrated laser diode includes an active layer 20 formed between an n-doped semiconductor material (n-substrate 10, n-side n-cover layer 12, n-waveguide layer 14) and a p-doped semiconductor material (p-waveguide layer 30, p-cover layer 32, p-subcontact layer 34), wherein the active layer 20 forms an active zone for generating electromagnetic radiation along its longitudinal axis, and an n-contact layer 50 is positioned between a p-side metal contact 52 on it and the p-doped semiconductor material (p-waveguide layer 30, p-cover layer 32, p-subcontact layer 34), and a pn tunnel junction 40 is formed in the n-contact layer 50 in the region above the active zone, directly adjacent to the p-doped semiconductor material (p-waveguide layer 30, p-cover layer 32, p-subcontact layer 34). The illustrated pn tunnel junction 40 is positioned on the p-doped semiconductor material (p-waveguide layer 30, p-cover layer 32, p-subcontact layer 34).+ Tunnel layer 42 and n placed on top of it + The pn tunnel junction 40 includes a tunnel layer 44. In this embodiment, the pn tunnel junction 40 is located on the p-doped subcontact layer 34 of a p-doped semiconductor material (p-guide layer 30, p-cover layer 32, p-subcontact layer 34). The stripe width of the diode laser is defined with respect to the width W of the pn tunnel junction 40. In this embodiment, the thickness d of the residual layer is defined. res This is defined as the minimum distance between the active layer 20 and the pn tunnel junction 40.

[0037] This embodiment of the present invention can be provided by a two-step epitaxy process, the epitaxy process having an etching step between these two steps. In the first growth step, the structure can be grown up to the pn tunnel junction 40. Next, the tunnel junction layers (42, 44) can be selectively etched on the outside of the stripe. Following the epitaxial growth of the n contact layer 50, a pn junction is generated in the opposite direction in the outer region of the structure, and the central pn tunnel junction 40 facilitates the flow of current. This is a known method for current and optical limiting in a surface-emitting laser (VCSEL) with a vertical cavity.

[0038] Figure 2 is an exemplary schematic diagram of a second embodiment of the laser diode according to the present invention. The basic layer structure corresponds to the arrangement shown in Figure 1, and therefore the individual reference numerals and their respective assignments apply accordingly. However, in contrast to the embodiment shown therein, in this case the pn tunnel junction 40 can be formed as a layer, and the stripe width of the diode laser is defined with respect to the width W of the opening of the n current shield 60 introduced into the p-doped semiconductor material (p-guide layer 30, p-cover layer 32, p-subcontact layer 34). In particular, in the illustrated example, the n current shield 60 is located within the p-subcontact layer 34. In this embodiment, the thickness d of the residual layer is res This is defined as the minimum distance between the active layer 20 and the current shield 60.

[0039] In this embodiment of the present invention, current confinement can also be achieved by a two-step epitaxy process having an etching step between the two steps. In this case, power cutoff at the component ends is generated independently of the tunnel junction by a so-called (enhanced) self-aligning lateral structure. For this purpose, a highly n-doped layer can be integrated in the lower vicinity of the p-side contact layer (i.e., the p-subcontact layer 34), resulting in a reverse-biased pn junction. The first growth step is completed after the growth of these layers. Next, these can be selectively etched in the center to create openings corresponding to the flow of current. Then, the remainder of the p-subcontact layer 34, as well as the pn tunnel junction 40 and the n-contact layer 50, can be grown on the structured n-current shield 60.

[0040] Figure 3 is an exemplary schematic diagram of a third embodiment of the laser diode according to the present invention. The basic layer structure corresponds to the arrangement shown in Figure 2, and therefore the individual reference numerals and their respective assignments apply accordingly. The pn tunnel junction 40 is also formed here as a layer. In contrast to the embodiment shown therein, the stripe width of the diode laser is defined by the width W of the region between two adjacent deep injection regions 70. In particular, in the illustrated example, the two peripheral deep injection regions 70 extend from the metal contact 52 to the p cover layer 32. Since openings for current flow can also be made by the deep injection regions 70, the additional integration of the n current shield 60 is unnecessary. In this embodiment, the thickness d of the residual layer res This is defined as the minimum distance between the active layer 20 and the lower side of the deep injection region 70.

[0041] In contrast to the exemplary embodiments described above, this embodiment can be manufactured by a single-step epitaxial growth, thereby reducing the complexity of the manufacturing process and therefore the cost. Current limiting is performed, for example, by deep ion implantation of high energy at the component ends. This can prevent current flow by increasing intermediate resistance and introducing point defects where carriers rapidly recombine. Deep implantation through the active zone effectively prevents current diffusion and LCA, which can significantly improve beam quality, but at the expense of power and efficiency. Therefore, with respect to total power, an implantation profile sized to terminate above the active zone (e.g., within the p-cover layer) is preferred.

[0042] Figure 4 is an illustrative schematic diagram of a fourth embodiment of the laser diode according to the present invention. The illustrated laser diode includes an active layer 20 formed between an n-doped semiconductor material (n substrate 10, n-side n cover layer 12, n-waveguide layer 14) and a p-doped semiconductor material (p-waveguide layer 30, p-cover layer 32), the active layer 20 forming an active zone for generating electromagnetic radiation along its longitudinal axis, an n-contact layer 50 and a p-side n-cover layer 54 positioned between a p-side metal contact 52 and the p-doped semiconductor material (p-waveguide layer 30, p-cover layer 32), and a pn tunnel junction 40 directly adjacent to the p-doped semiconductor material (p-waveguide layer 30, p-cover layer 32) is formed in the region of the p-side n-contact layer 54 above the active zone. The illustrated pn tunnel junction 40 is positioned on the p-doped semiconductor material (p-waveguide layer 30, p-cover layer 32) + Tunnel layer 42 and n placed on top of it + The tunnel layer 44 is included. In this embodiment, the pn tunnel junction 40 is placed on the p-doped cover layer 32 of a p-doped semiconductor material (p-guide layer 30, p-cover layer 32). Furthermore, an n-doped cover layer 54 is placed on the pn tunnel junction 40. The stripe width of the diode laser is defined with respect to the width W of the pn tunnel junction 40. In this embodiment, the thickness d of the residual layer is included. res This is defined as the minimum distance between the active layer 20 and the pn tunnel junction 40.

[0043] Therefore, a major difference from the embodiment shown in Figure 1 is that the pn tunnel junction 40 is located on the p-doped cover layer 32 and is closer to the active zone. The integration of the additional p-subcontact layer 34 can be omitted. Bringing the pn tunnel junction 40 closer to the active zone makes the manufacturing process technically more complex (especially in the case of modifications with two-stage epitaxial growth), but it can achieve significant power advantages.

[0044] Figure 5 is an exemplary schematic diagram of a fifth embodiment of the laser diode according to the present invention. The basic layer structure corresponds to the arrangement shown in Figure 4, and therefore the individual reference numerals and their respective assignments are applied accordingly. However, the actual operating principle and possible manufacturing methods are shown in Figure 2. This embodiment differs from the embodiment shown in Figure 2 only in the location of the tunnel junction 40 and the absence of the p-subcontact layer 34.

[0045] Figure 6 is an exemplary schematic diagram of a sixth embodiment of the laser diode according to the present invention. The basic layer structure corresponds to the arrangement shown in Figure 5, and therefore the individual reference numerals and their respective assignments are applied accordingly. However, the actual operating principle and possible manufacturing methods are shown in Figure 3. This embodiment also differs from the embodiment shown in Figure 3 only in the location of the tunnel junction 40 and the absence of the p-subcontact layer 34. [Explanation of symbols]

[0046] 10 n-substrate (e.g., GaAs) 12 n-cover layer (n-side, e.g., AlGaAs) 14 n-waveguide layer (e.g., AlGaAs) 20 Active layer (including active zone) 30 p-waveguide layer (e.g. AlGaAs) 32 p-cover layer (e.g., AlGaAs) 34 p-subcontact layer (e.g., GaAs) 40 pn tunnel junction 42 p + Tunnel layer (e.g., p+ GaAs) 44 n + Tunnel layer (e.g., n + GaAs) 50 n-(sub)contact layer (p-side, e.g., GaAs) 52 Metal contact (p side) 54 n-cover layer (p-side, e.g., AlGaAs) 60 N-current shield 70 Deep injection area W Width d res Thickness of the residual layer

Claims

1. It is a laser diode, The material includes an active layer formed between an n-doped semiconductor material and a p-doped semiconductor material, wherein the active layer forms an active zone along the longitudinal axis for generating electromagnetic radiation. At least one n-doped intermediate layer is positioned between the p-side metal contact above it and the p-doped semiconductor material, and a p-n tunnel junction directly adjacent to the p-doped semiconductor material is formed in the at least one n-doped intermediate layer in the region above the active zone. The aforementioned p-n tunnel junction is formed as a layer, A laser diode in which the stripe width of the laser diode is defined with respect to the width W of the opening of the n current shield introduced into the p-doped semiconductor material.

2. A laser diode, The material includes an active layer formed between an n-doped semiconductor material and a p-doped semiconductor material, wherein the active layer forms an active zone along the longitudinal axis for generating electromagnetic radiation. At least one n-doped intermediate layer is positioned between the p-side metal contact above it and the p-doped semiconductor material, and a p-n tunnel junction directly adjacent to the p-doped semiconductor material is formed in the at least one n-doped intermediate layer in the region above the active zone. The aforementioned p-n tunnel junction is formed as a layer, A laser diode in which the stripe width of the laser diode is defined with respect to the width W of the region between two adjacent deep injection regions.

3. The p-n tunnel junction is a p-doped semiconductor material on which p ＋ A tunnel layer and n placed on top of it ＋ A laser diode according to claim 1 or claim 2, comprising a tunnel layer.

4. The laser diode according to claim 1 or claim 2, wherein the p-n tunnel junction is disposed on a p-doped subcontact layer of the p-doped semiconductor material.

5. The laser diode according to claim 1 or claim 2, wherein the p-n tunnel junction is arranged on a p-doped cover layer of the p-doped semiconductor material.

6. The laser diode according to claim 1 or claim 2, wherein an n-doped cover layer is disposed on the p-n tunnel junction.

7. The laser diode according to claim 1 or claim 2, wherein the semiconductor material is GaAs-based.

8. The laser diode according to claim 1 or claim 2, wherein the minimum distance between the active layer and the p-n tunnel junction is less than 1.3 μm.

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