Broad-area diode laser containing an integrated pn tunnel junction

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 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. [Background technology]

[0002] Broad-area diode lasers can have particularly high efficiency and brightness. Using these emitters, powers of >15 W can be reliably obtained. BALs are the most efficient sources of near-infrared (NIR) radiation, and therefore are popular as pumping sources for solid-state and fiber lasers. They are also key elements of fiber-coupled laser systems configured to provide beams with high radiation density for high conversion efficiency material processing. To increase the power output and reduce the cost of these systems, it is important to improve the quality of the beam, especially in the slow axis. This allows more emitters to be coupled into low numerical aperture (NA) fibers.

[0003] However, at high optical powers and associated operating currents, the beam quality generally degrades considerably, especially affecting fiber coupling. It has been demonstrated that the main cause of the beam quality degradation at increasing operating current is thermal lensing (rather than carrier or gain-guided guiding effects) in the slow axis (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 limit the beam quality degradation at high powers, it is important to configure a lateral temperature gradient based on the temperature increase in the central region under the laser stripe. This results in a local increase in the refractive index, which further induces transverse waves and, as a result, a larger divergence angle.

[0004] In particular, previous studies on GaAs-based broad-area 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 the formation of a significant thermal barrier at the boundary between the highly p-doped GaAs contact layer and the metal contact deposited on top of it. The reduced heat dissipation due to this barrier strengthens the resulting thermal lensing effect, which in turn leads to poorer beam quality and increased thermal resistance. However, this kind of thermal barrier could not be observed at other semiconductor-metal boundaries within the component, i.e. between n-doped GaAs and metal. Summary of the Invention [Means for solving the problem]

[0005] Disclosure of the Invention

[0006] It is therefore an object of the present invention to provide a broad area diode laser which reduces or completely avoids the thermal barrier that arises 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 objects are achieved according to the invention by the features of the independent claim 1. Advantageous configurations of the invention can be found in the dependent claims. [Brief description of the drawings]

[0008] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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, the active layer forming an active zone for generating electromagnetic radiation along a longitudinal axis, at least one n-doped intermediate layer being arranged between a p-side metal contact thereon and the p-doped semiconductor material, said at least one n-doped intermediate layer in a region above the active region forming a pn tunnel junction directly adjacent to the p-doped semiconductor material. The at least one n-doped intermediate layer preferably comprises a p-side n-contact layer. The p-side metal contact can be arranged on the p-side n-contact layer.

[0010] Here, the n-doped semiconductor material typically includes an n-doped substrate (called n-substrate), an n-side n-cover layer disposed on the n-substrate, and an n-waveguide layer disposed on the n-side n-cover layer. The p-doped semiconductor material typically includes a p-waveguide layer and a p-cover layer disposed on the p-waveguide layer. In the prior art, a p-contact layer is typically disposed on the p-cover layer. The active layer configured for light generation is located between two differently doped semiconductor materials. In this case, the active zone is the part of the active layer where the actual light generation 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 on the n-side to the n-substrate and on the p-side to a metal contact thereon, which forms a semiconductor-metal interface with the p-contact layer below. As already mentioned above, this interface constitutes a significant thermal barrier. Since no thermal barrier of this kind could be observed at the n-side semiconductor-metal interface, according to the invention, at least one n-doped intermediate layer is arranged between the p-side metal contact thereon and the p-doped semiconductor material below. Furthermore, in order to further enable carrier injection, 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, an interface that is considered favorable can also be created between the n-semiconductor material and the metal on the p-side, i.e. without forming a significant thermal barrier. The pn tunnel junction preferably has a total thickness of less than 100 nm.

[0012] The invention is based on the finding that a semiconductor-metal boundary comprising n-doped semiconductors on both sides of the BAL is particularly advantageous. To achieve this, the p-side contact layer of the BAL can be made to be n-doped instead of p-doped. However, this would result in the p-n junction being reverse biased and acting as a power cut-off. To avoid this, a reverse biased p-n tunnel junction, or tunnel junction (TJ) for short, is formed at this p-n boundary. This junction can for example consist of 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. A BAL according to the invention is also called a TJ-BAL.

[0013] This approach offers many more advantages in addition to reducing or avoiding the thermal barrier formed at the p-side semiconductor-metal interface. The high electrical conductivity of the highly doped n-contact and TJ layers allows for a very low intermediate resistance between the active zone and the epi-side contact, especially in vertical structures with thin p-side waveguides and cover layers, such as the extreme triple asymmetric (ETAS) design. The 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 layers are highly doped, they tend to be equipotential. This means that the voltage differences that arise between different regions of the laser chip cannot be sustained and will quickly become equal. The low epitaxial resistivity and the presence of equipotentials have the advantage of reducing spatial hole burning and suppressing higher-order transverse modes, which further improves the 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 resistivity and equipotentiality of the highly n-doped p-side semiconductor layer is that the thickness of the p-side n-contact layer grown on the pn tunnel junction can be increased considerably without a significant decrease in electrical resistance. As a result, in structures with a thin p-side, the active zone can be protected from process- and design-related mechanical stresses that can impair the polarization purity, output, and service life of the component, leading to undesirable waveguiding and poor beam quality.

[0015] The pn tunnel junction is preferably a p-doped semiconductor material. + The tunnel layer and the n + The high doping of the two tunnel layers of the pn tunnel junction allows carriers to tunnel through the reverse-biased pn 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 and p-doped layers of the pn tunnel junction are preferably N D,A ≧10 19 cm -3 (The typical doping concentration in the environment is about 10 18 cm -3 (Up to now).

[0016] The pn tunnel junction is preferably arranged on a p-doped sub-contact layer (p-sub-contact layer) of p-doped semiconductor material. The p-sub-contact layer substantially corresponds to the p-contact layer of the prior art. However, according to the invention, the metal contact is not arranged on the p-sub-contact layer, but instead is separated from it by at least one n-doped intermediate layer. The p-sub-contact layer can differ from the p-cover layer arranged below it due to the semiconductor material or its composition, or due to discontinuities in the progression of the refractive index / refractive index gradient at the layer boundaries.

[0017] The pn tunnel junction is preferably arranged on a p-doped cover layer of p-doped semiconductor material. In this case, no p sub-contact layer is arranged between the p-doped cover layer and the pn tunnel junction. The n-doped cover layer is preferably arranged on the pn tunnel junction. This means that the p-side cover layer comprises a p-doped region and an n-doped region, between which the pn tunnel junction is arranged. Since the optical mode guided in the waveguiding layer also extends to the cover layer, this means that the optical mode can extend beyond the p-side pn tunnel junction to the p-side n-doped cover layer. Possible contact layers adjacent to this do not play a substantial role in the waveguiding.

[0018] The stripe width of the diode laser is preferably defined relative to the lateral width W of the pn tunnel junction. With the reverse pn junction occurring everywhere outside the pn tunnel junction, the injection region can be defined by the geometric definition of the pn tunnel junction. Thus, the size and shape of the pn tunnel junction can define the current path.

[0019] Alternatively, the pn tunnel junction can be formed as a layer, the stripe width of the diode laser being defined relative to the lateral width W of the opening of the n-current shield introduced in the p-doped semiconductor material. In this case, the pn tunnel junction largely negates the effect of the reverse pn junction, and the structuring of the individual stripes has to be done in a different way. The proposed n-current shield is well known from the prior art for limiting the current flow. The current path can therefore 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 pn tunnel junction preferably has a total thickness of less than 100 nm, the doping concentration of the n-current shield is preferably less than N. D ≧10 18 cm -3 It is.

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

[0021] The semiconductor material is preferably GaAs-based. For example, an n-substrate GaAs may include an n-side n-cover layer AlGaAs, an n-guiding layer AlGaAs, a p-guiding layer AlGaAs, and a p-cover layer AlGaAs. The p-subcontact layer may include GaAs. The pn tunnel junction may include a p + p as tunnel layer + GaAs, n + n as tunnel layer + The p-side n-cover layer may comprise AlGaAs. The n-contact layer may comprise GaAs. The p-side n-cover layer may comprise 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, more preferably less than 0.5 μm. The advantage of making the distance between the active layer and the pn tunnel junction as small as possible is the reduction of intermediate resistance and spatial hole burning, which results in improved laser properties (e.g. beam quality, power, efficiency).

[0023] In the TJ-BALs according to the invention, various lateral structuring techniques can be implemented to confine the current to the center of the component (i.e., under the laser stripe or above the active zone). The resulting current confinement limits losses at the stripe ends and limits the negative impact on beam quality due to lateral current spreading and lateral carrier accumulation (LCA). In these TJ-BALs, the current confinement is more pronounced than in standard BALs because the current spreading in the n-contact layer is significantly larger than in the p-contact layer due to the larger mobility of electrons compared to holes.

[0024] The thickness of the residual layer between the active zone and the current shield, d res is preferably less than 1 μm. The disadvantageous current spreading is prevented by the lowest possible residual layer thickness d res In this case, the thickness of the residual layer d res generally represents the minimum distance between the active layer and the structure closest to it, 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, the pn tunnel junction according to the invention, the n current shield, or the corresponding deep injection region. The p-side total thickness d, including the pn tunnel junction and the p-side n contact layer, tot is preferably greater than 2 μm. WL 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 cavity length L is preferably 3 mm or more.

[0025] Further preferred configurations of the invention are evident from the features set forth in the respective dependent claims.

[0026] The various embodiments of the invention described in this application can be advantageously combined with one another, unless otherwise stated in specific instances.

[0027] BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The invention will now be described with reference to exemplary embodiments and on the basis of the associated drawings.

[0029] FIG. 1 is an exemplary schematic diagram of a first embodiment of a laser diode according to the present invention.

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

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

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

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

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

[0035] Detailed Description of the Drawings

[0036] 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), which forms an active zone along a longitudinal axis for generating electromagnetic radiation, an n-contact layer 50 disposed between a p-side metal contact 52 thereon and the p-doped semiconductor material (p-waveguide layer 30, p-cover layer 32, p-subcontact layer 34), and a p-n tunnel junction 40 formed in the n-contact layer 50 in a region above the active zone and directly adjacent the p-doped semiconductor material (p-waveguide layer 30, p-cover layer 32, p-subcontact layer 34). The illustrated p-n tunnel junction 40 is a p-doped semiconductor material (p-waveguide layer 30, p-cover layer 32, p-subcontact layer 34) disposed on the p-doped semiconductor material (p-waveguide layer 30, p-cover layer 32, p-subcontact layer 34).+ A tunnel layer 42 and a n + In this embodiment, the pn tunnel junction 40 is disposed on the p-doped sub-contact layer 34 of the p-doped semiconductor material (p-guide layer 30, p-cover layer 32, p-sub-contact layer 34). The stripe width of the diode laser is defined relative to the lateral width W of the pn tunnel junction 40. In this embodiment, the residual layer thickness d res is defined as the minimum distance between the active layer 20 and the pn tunnel junction 40.

[0037] This embodiment of the invention can be provided by a two-stage epitaxy process, with an etching step between these two stages. In a first growth step, the structure can be grown up to the pn tunnel junction 40. The tunnel junction layers (42, 44) can then be selectively etched outside the stripes. Following the subsequent epitaxial growth of the n-contact layer 50, a reverse pn junction is created in the outer regions of the structure, with the central pn tunnel junction 40 facilitating the current flow. This is a known method for current and optical confinement in vertical cavity surface emitting lasers (VCSELs).

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

[0039] In this embodiment of the invention, the current confinement can be achieved as well by a two-step epitaxy process with an etching step between the two steps. In this case, the power cutoff at the component end is generated independently of the tunnel junction by a so-called (enhanced) self-aligned lateral structure. For this purpose, a highly n-doped layer can be integrated near the bottom 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 finished after the growth of these layers. These can then be selectively etched in the middle to make an opening corresponding to the current flow. The remaining part of the p-subcontact layer 34, as well as the pn tunnel junction 40 and the n-contact layer 50, can then be grown on the structured n-current shield 60.

[0040] FIG. 3 is an exemplary schematic diagram of a third embodiment of a laser diode according to the invention. The basic layer structure corresponds to the arrangement shown in FIG. 2, therefore the individual reference symbols and their respective assignment apply accordingly. The pn tunnel junction 40 is formed here as a layer as well. In contrast to the embodiment shown there, the stripe width of the diode laser is defined relative to the lateral width W of the area between two adjacent deep implanted regions 70. In particular, in the example shown, two peripheral deep implanted regions 70 reach from the metal contact 52 to the p cover layer 32. An opening for the current flow can likewise be made by the deep implanted regions 70, so that no additional integration of an n current shield 60 is necessary. In this embodiment, the residual layer thickness d res is defined as the minimum distance between the active layer 20 and the underside of the deep implant region 70.

[0041] In contrast to the above exemplary embodiment, this embodiment can be manufactured by one-step epitaxial growth, which reduces the complexity of the manufacturing process and therefore the cost. Current confinement is performed, for example, by high-energy deep ion implantation at the component end. This can prevent current flow by increasing the intermediate resistance and introducing point defects where carriers rapidly recombine. Deep implantation through the active zone effectively prevents current spreading and LCA, which can result in a significant improvement in beam quality, but at the expense of significant loss of power and efficiency. Therefore, in terms of total power, implantation profiles dimensioned to terminate above the active zone (e.g., in the p-cover layer) are preferred.

[0042] 4 is an exemplary schematic diagram of a fourth 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), which forms an active zone along a longitudinal axis for generating electromagnetic radiation, an n-contact layer 50 and a p-side n-cover layer 54 disposed between a p-side metal contact 52 thereon and the p-doped semiconductor material (p-waveguide layer 30, p-cover layer 32), and a p-n tunnel junction 40 formed in the p-side n-contact layer 54 in a region above the active zone, directly adjacent to the p-doped semiconductor material (p-waveguide layer 30, p-cover layer 32). The illustrated p-n tunnel junction 40 is a p-doped semiconductor material (p-waveguide layer 30, p-cover layer 32) disposed on the p-doped semiconductor material (p-waveguide layer 30, p-cover layer 32). + A tunnel layer 42 and a n + The pn tunnel junction 40 includes a p-doped cover layer 32 of p-doped semiconductor material (p-guiding layer 30, p-cover layer 32). An n-doped cover layer 54 is further disposed on the pn tunnel junction 40. The stripe width of the diode laser is defined relative to the lateral width W of the pn tunnel junction 40. In this embodiment, the residual layer thickness d res is defined as the minimum distance between the active layer 20 and the pn tunnel junction 40.

[0043] 1 is therefore the main difference that the pn tunnel junction 40 is arranged on the p-doped cover layer 32 and is closer to the active zone. The integration of an additional p sub-contact layer 34 can be omitted. By bringing the pn tunnel junction 40 closer to the active zone, although the manufacturing becomes technically more complicated (especially in the case of the variant with two-stage epitaxial growth), considerable power advantages can be achieved.

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

[0045] Fig. 6 is an exemplary schematic diagram of a sixth embodiment of a laser diode according to the present invention. The basic layer structure corresponds to the arrangement shown in Fig. 5, therefore the individual reference symbols and their respective assignments apply accordingly. However, the actual operating principle and possible manufacturing method are shown in Fig. 3. This embodiment again differs from the embodiment shown in Fig. 3 only in the position 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 pages + Tunnel layer (e.g. p+ GaAs) 44n + 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 Residual layer thickness

Claims

1. A laser diode, an active layer (20) formed between an n-doped semiconductor material (10, 12, 14) and a p-doped semiconductor material (30, 32, 34), said active layer (20) defining an active zone along a longitudinal axis for generating electromagnetic radiation; 1. A laser diode comprising: at least one n-doped intermediate layer (50, 54) disposed between a p-side metal contact (52) thereon and said p-doped semiconductor material (30, 32, 34), said at least one n-doped intermediate layer (50, 54) in a region above said active zone forming a p-n tunnel junction (40) directly adjacent said p-doped semiconductor material (30, 32, 34).

2. The pn tunnel junction (40) is a p-doped semiconductor material (30, 32, 34) disposed on the p-doped semiconductor material (30, 32, 34). ＋ A tunnel layer (42) and n ＋ 2. The laser diode of claim 1, further comprising a tunnel layer (44).

3. The laser diode of claim 1 or 2, wherein the pn tunnel junction (40) is disposed on a p-doped sub-contact layer (34) of the p-doped semiconductor material (30, 32, 34).

4. The laser diode of claim 1 or 2, wherein the pn tunnel junction (40) is arranged on a p-doped cover layer (32) of the p-doped semiconductor material (30, 32, 34).

5. The laser diode of claim 4, wherein an n-doped cover layer (54) is disposed on the pn tunnel junction (40).

6. A laser diode according to any one of claims 1 to 5, wherein the stripe width of the diode laser is defined relative to the lateral width W of the pn tunnel junction (40).

7. The laser diode according to any one of claims 1 to 5, The pn tunnel junction (40) is formed as a layer, A laser diode, wherein the stripe width of said diode laser is defined relative to a lateral width W of an opening in an n-current shield (60) introduced into said p-doped semiconductor material (30, 32, 34).

8. The laser diode according to any one of claims 1 to 5, The pn tunnel junction (40) is formed as a layer, A laser diode, wherein the stripe width of said diode laser is defined relative to the lateral width W of the region between two adjacent deep implanted regions (70).

9. The laser diode according to any one of claims 1 to 8, wherein the semiconductor material is GaAs-based.

10. The laser diode according to any one of claims 1 to 9, wherein the minimum distance between the active layer (20) and the pn tunnel junction (40) is less than 1.3 μm.

Images