Patent Application
20240136800
A semiconductor emitter is specified.
Embodiments provide a semiconductor emitter that efficiently emits imagable laser radiation.
According to at least one embodiment, the semiconductor emitter comprises a semiconductor layer sequence. The semiconductor layer sequence is preferably based on a III-V compound semiconductor material. The semiconductor material is, for example, a nitride compound semiconductor material such as AlnIn1-n-mGamN or a phosphide compound semiconductor material such as AlnIn1-n-mGamP or also an arsenide compound semiconductor material such as AlnIn1-n-mGamAs or such as AlnGamIn1-n-mAskP1-k or such as In1-mGamAskSb1-k or likewise InnGa1-nSb, where in each case 0≤n≤1, 0≤m≤1 and n+m≤1 and 0≤k<1. For example, 0<n≤0.8,
0.4≤m<1, and n+m≤0.95 as well as 0<k≤0.5 apply to at least one layer or to all layers of the semiconductor layer sequence. In this context, the semiconductor layer sequence may comprise dopants as well as additional components. For the sake of simplicity, however, only the essential constituents of the crystal lattice of the semiconductor layer sequence, that is, Al, As, Ga, In, N, P or Sb, are given, even if these may be partially replaced and/or supplemented by small amounts of additional substances.
According to at least one embodiment, the semiconductor emitter is a semiconductor laser diode, LD for short, or a superluminescent light emitting diode, SLED for short, or a light emitting diode, LED for short. Preferably, the semiconductor emitter is a semiconductor laser diode so that coherent laser radiation is generated and emitted during operation.
According to at least one embodiment, the semiconductor layer sequence includes one or more active zones. The preferably multiple active zones each include one or more quantum well layers. The at least one quantum well layer is configured in particular for the generation of laser radiation. The term quantum well has no meaning here with regard to the dimensionality of the quantization. It thus includes, among other things, structures with quantization in one, two or three spatial directions and any combination of these structures.
According to at least one embodiment, the one or more active zones each comprise one or more barrier layers, in particular at least two barrier layers each, between which the at least one quantum well layer is embedded. In particular, the barrier layers and the at least one quantum well layer directly follow each other along the growth direction. It is possible that in the active zone in question the number of barrier layers is greater by one than the number of the at least one quantum well layer.
According to at least one embodiment, a distance between adjacent barrier layers of different active zones facing the at least one tunnel diode across the associated tunnel diode is at most 50 nm or at most 30 nm or at most 10 nm. In other words, adjacent active zones are located close to each other. Thus, the barrier layers of two different adjacent active zones have a distance from each other of at most 50 nm or at most 30 nm or at most 10 nm, and/or a thickness of an intermediate region between the adjacent active zones is smaller than said distances, respectively.
The active zones are thus optically connected with each other. This means, for example, that a mode of an active zone extends into the neighboring active zones.
According to at least one embodiment, the semiconductor layer sequence comprises one or more tunnel diodes. The at least one tunnel diode is located along a growth direction of the semiconductor layer sequence between two adjacent active zones. The at least one tunnel diode may be directly adjacent to the respective active zones, for example, to barrier layers of the respective active zones.
According to at least one embodiment, a thickness of the at least one tunnel diode is at most 40 nm or at most 30 nm or at most 25 nm. Alternatively or additionally, the thickness of the at least one tunnel diode is at least 6 nm or at least 10 nm. If several of the tunnel diodes are present in the semiconductor layer sequence, all tunnel diodes may have the same structure, or the semiconductor layer sequence comprises differently structured tunnel diodes.
According to at least one embodiment, in the intended operation of the semiconductor emitter, a local intensity of an optical fundamental mode at the at least one tunnel diode is at least 35% or at least 50% or at least 60% or at least 80% or at least 90% of a maximum intensity of the fundamental mode. That is, the at least one tunnel diode is located at a position of the waveguide with a comparatively high intensity of the fundamental mode.
The term ‘fundamental mode’ refers in particular only to a vertical mode, that is, to a mode parallel to the growth direction of the semiconductor layer sequence and/or in the direction perpendicular to the active zones. The term ‘fundamental mode’ does not make any statement about horizontal modes, which run in the direction parallel to the active zones and in the direction perpendicular to a resonator, as well as about longitudinal modes, which run along the resonator.
In at least one embodiment, the semiconductor emitter comprises a semiconductor layer sequence which has:
To achieve high powers in, for example, laser diodes for LiDAR applications, three separate waveguides separated from each other by means of tunnel diodes have been implemented up to now. This leads to, for example, three individual laser modes in the optical near field. This approach also requires a relatively thick epitaxially grown semiconductor layer sequence with a thickness of about 10 μm to 15 μm. These, for example, three luminescent stripes or modes optically image in the optical near field and far field accordingly. This imaging limits the étendue in particular. In view of this, it is desirable to have only one laser line with very high power density. This would, for example, avoid dark spots in the optical image and allow better collimation.
In particular, in the semiconductor emitter described here, a thinner waveguide design is used that enables high optical power densities, but without coherences between, for example, individual waveguides: For example, a thinner triple waveguide epitaxy leads to coupling of the modes of the waveguides involved. These higher order supermodes would lead to efficiency losses and coherence effects as well as interference effects, which would be visible especially in the optical far field, for example, as double peaks. In the laser design described here, on the other hand, there is preferably a single waveguide for a fundamental mode in which there are multiple active zones or pn junctions, connected by intermediate tunnel diodes, in order to achieve high optical power densities.
Thus, in particular, a supermode waveguide design with multiple stacked active quantum well structures in combination with low-absorption tunnel diodes is proposed. This leads to higher achievable power densities at smaller étendue. For example, lower manufacturing costs, a more compact design on the customer side and efficiency improvements are achievable.
In particular, the semiconductor emitter described herein is a laser, such as a horizontal cavity surface emitting laser, also referred to as HCSEL, and/or a thin film multi-stack epi, that is, a growth substrate-free epitaxial layer structure with multiple active quantum well structures stacked along a growth direction. The semiconductor emitter can be used, for example, in industrial and automotive applications, as well as in LiDAR applications or for material processing. Thus, the semiconductor emitter is in particular a thin-film laser diode with ultra-high power density, which enables a compact optical system on the customer side, and which can be manufactured with lower costs.
Compared to an alternative approach, according to which the tunnel diodes are located in the zero points of the optical field distribution, in VCSELs, for example, in the zero points in the vertical standing waves or in so-called stacked EE lasers in or near the zero point of, for example, higher order modes or as a connection between individual, independent modes, the approach followed here is purposefully different: According to the approach described here, the tunnel diodes are not located in zero points of the optical field, but within the fundamental mode.
A preferred component for this purpose are, in particular, low-absorption tunnel diodes:
The waveguide design, that is, the refractive index profile, is preferably designed in such a way that primarily the fundamental mode can be guided—that is, the thickness of the waveguide is limited, which means that the individual layers, such as the TJs and the quantum well layers, cladding layers and barrier layers, should be realized grown close together.
Further design possibilities include, for example, slightly different band gaps or numbers of quantum well layers in the individual active zones or asymmetries with respect to the arrangement and/or geometry of the waveguide and/or the TJs. For example, the tunnel diodes can achieve a relatively high refractive index in their composition and thus effectively contribute to waveguiding.
A preferred component for an efficient, for example, Gaussian-like supermode or fundamental mode is therefore the lowest possible absorption of the waveguide, in particular of the at least one tunnel diode. One idea for this is: The thinner the tunnel diode, the larger the proportion of the space charge region in relation to the total thickness. A thickness of the space charge region is about 10 nm, depending on the doping level. Since the space charge region contains no, or only a few, free charge carriers, a so-called free-carrier absorption is not very high here despite high doping. The at least one tunnel diode should therefore be as thin as possible and have a band gap not close to the laser wavelength.
Provided that the space charge region of at least one tunnel diode corresponds to a large portion of the total tunnel diode thickness, it can then be embedded in the waveguide without generating large losses. Then the waveguide is preferably designed so that only the fundamental mode oscillates. For example, the waveguide can be designed as narrow as possible. Thus, for example, one obtains an approximately 3 μm thin triple epitaxial stack of three coupled active zones with two tunnel diodes in between, which together lead a narrow supermode as the first fundamental mode due to the refractive index contrast of quantum well layers plus barriers and ramps plus tunnel diodes relative to cladding layers.
According to at least one embodiment, the active zones and the at least one tunnel diode are located in a common waveguide of the semiconductor layer sequence. In particular, the semiconductor layer sequence then has exactly two cladding layers which adjoin the exactly one waveguide. The waveguide has an increased refractive index compared to the cladding layers and the cladding layers are comparatively thick, for example, at least 0.5 μm thick or at least 0.8 μm thick and/or at most 10 μm thick or at most 3 μm thick or at most 1.3 μm thick. This does not preclude individual thin sublayers in the cladding layers from having a larger refractive index, as long as an average or effective refractive index of the cladding layers is smaller than an average or effective refractive index of the waveguide. Such partial layers have, for example, a thickness of at most 0.5 μm or of at most 0.2 μm and/or of at most 20% or of at most 5% of a total thickness of the corresponding cladding layer.
According to at least one embodiment, a thickness of the common waveguide together with the two associated cladding layers is at most 10 μm or at most 4 μm or at most 2.5 μm. For example, the common waveguide in which the at least one tunnel diode and the active zones are located has a thickness of at most 1.2 μm or of at most 0.8 μm or of at most 0.4 μm. Alternatively or additionally, the thickness of the waveguide alone is at least 0.1 μm or at least 0.2 μm.
According to at least one embodiment, one of the cladding layers or both cladding layers has a stepped profile. That is, a refractive index of the at least one cladding layer in question decreases in the direction away from the active zones with at least one step. The number of jumps or steps is, for example, between one and five inclusive. Alternatively, it is possible that the at least one relevant cladding layer exhibits, at least in places, a ramped, continuous refractive index change. The decrease in refractive index in the direction away from the waveguide may be monotonic or strictly monotonic, or the relevant cladding layer may optionally also exhibit thin, localized regions of increasing refractive index.
According to at least one embodiment, a thickness of a space charge region of the at least one tunnel diode is at least 20% or at least 30% or at least 50% or at least ⅔ of a total thickness of the at least one tunnel diode in the intended operation of the semiconductor emitter. That is, in operation, the tunnel diode may be predominantly formed by a space charge region.
According to at least one embodiment, a bandgap of the at least one tunnel diode differs by at least 10 nm or at least 30 nm or at least 50 nm from a wavelength of maximum intensity of the active zones. Alternatively or additionally, this difference is at most 0.2 μm or at most 100 nm. The band gap, which corresponds to an energy E, and the associated wavelength λ can be converted into each other via the relationship E=hc/λ, where h denotes the Planck's constant and c the speed of light. The wavelength and the speed of light refer here to the vacuum values. This energetic gap reduces absorption of radiation generated in the active zones in the at least one tunnel diode. Thus, the band gap of the at least one tunnel diode is preferably higher than a band gap energy of the active zones equivalent to the wavelength of the laser radiation.
If there are several active zones for emission of different wavelengths, the active zone with the highest band gap, that is, the smallest emission wavelength, is used to calculate the above energy gap. If there are several tunnel diodes with optionally different structures, the smallest band gap of the tunnel diodes is used to calculate the above energy gap.
According to at least one embodiment, the at least one tunnel diode is formed from two oppositely highly doped layers. In other words, the at least one tunnel diode may comprise the two oppositely highly doped layers. Highly doped means, for example, that an average dopant concentration in the at least one tunnel diode is at least 2×1019 cm−3 or at least 5×1019 cm−3 and/or at most 5×1020 cm−3 or at most 2×1020 cm−3 or at most 1×1020 cm−3. In addition, these values may apply not only averaged over the tunnel diode in question, but also to each of the highly doped layers. For tunnel diodes comprising Sb, the doping may even be omitted altogether or be significantly smaller and, for example, be at least 1×1018 cm−3.
According to at least one embodiment, the highly doped layers each have a thickness of at least 3 nm or of at least 5 nm. Alternatively or additionally, this thickness is at most 25 nm or at most 20 nm or at most 15 nm.
According to at least one embodiment, the at least one tunnel diode is composed of the two oppositely highly doped layers and at least one intermediate layer in between. That is, the at least one tunnel diode may, for example, consist of three layers. Preferably, the same applies to the thickness of the intermediate layer as to the thicknesses of the highly doped layers. The intermediate layer may be p-doped or n-doped. For example, a dopant concentration in the intermediate layer is smaller than in the adjacent highly doped layers, in particular smaller by at least a factor of 1.5 or by at least a factor of 2 and/or by at most a factor of 10 or by at most a factor of 5.
According to at least one embodiment, the semiconductor layer sequence further comprises at least one transition layer. Preferably, the transition layer is lowly doped. A dopant concentration of the transition layer is, for example, at most 1×1017 cm−3 or at most 3×1016 cm−3 and/or at least 1×1015 cm−3 or at least 5×1015 cm−3. The transition layer adjoins the at least one tunnel diode, in particular directly. It is possible that there is a transition layer on both sides of the tunnel diode or on both sides of each of the tunnel diodes. For example, the p-side transition layers are thinner than the n-side transition layers, so that the transition layers can be asymmetrically shaped around the associated tunnel diode. Alternatively, the transition layers may have the same thickness and thus be symmetrical around the associated tunnel diode. A thickness of the at least one transition layer is, for example, at least 5 nm or at least 10 nm and/or at most 50 nm or at most 30 nm. By means of such transition layers, in particular an improvement of a charge carrier transport is achievable.
According to at least one embodiment, the at least one transition layer has a ramp-shaped refractive index profile. Preferably, the refractive index increases in the direction towards the associated tunnel diode.
According to at least one embodiment, the at least one tunnel diode is made of GaAs and/or of InGaAs or comprises GaAs and/or InGaAs and optionally also AlGaAs. This applies in particular to semiconductor emitters with an emission wavelength in the range of 0.7 μm to 1.3 μm. In this case, the optional transition layer is made of AlGaAs, for example, with the Al content preferably decreasing to zero towards the tunnel diode.
According to at least one embodiment, the at least one tunnel diode is made of InP and InGaAs or comprises InP and InGaAs and optionally also AlGaAs. This applies in particular to semiconductor emitters with an emission wavelength in the range of 1.3 μm to 2.0 μm.
According to at least one embodiment, the at least one tunnel diode is made of InAsSb and GaSb or comprises InAsSb and GaSb. This applies in particular to semiconductor emitters with an emission wavelength in the range of 2.0 μm to 5 μm.
According to at least one embodiment, the at least one tunnel diode is made of InGaN or comprises InGaN or also AlInGaN or AlGaN. This applies in particular to semiconductor emitters with an emission wavelength in the range of 0.3 μm to 0.6 μm.
According to at least one embodiment, a dopant concentration of layers of the semiconductor layer sequence adjacent to the at least one tunnel diode is smaller than the average dopant concentration in the at least one tunnel diode by at least a factor of three or by at least a factor of ten or by at least a factor of 100. The adjacent layers are, for example, barrier layers of the active zones or the transition layers.
According to at least one embodiment, the optical fundamental mode has multiple local maxima and one or more local minima. This is achievable, for example, by a refractive index variation within the waveguide. The at least one local minimum is located between two adjacent local maxima. The local maxima may include an absolute maximum.
According to at least one embodiment, the active zones are arranged in or near the local maxima and/or the at least one tunnel diode is arranged in or near the at least one local minimum. If several tunnel diodes and several local minima are present, preferably one tunnel diode each is arranged in or close to a local minimum. The local minima may be comparatively weak and may, for example, have at least 80% or 90% of an intensity of the adjacent local maxima. Close to the local minimum means, for example, that a distance of the relevant tunnel diode to the associated local minimum is at most 30% or at most 10% of a largest distance of this local minimum to the adjacent maxima.
According to at least one embodiment, at least two of the active zones are configured to generate radiation of different wavelengths. For example, maximum intensity wavelengths of the active zones differ from each other by at least 5 nm or by at least 10 nm and/or by at most 40 nm or by at most 20 nm. Alternatively, all active zones can be configured to generate radiation of the same nominal wavelength of maximum intensity.
According to at least one embodiment, the semiconductor layer sequence includes at least three or at least four of the active zones and/or at most ten or at most five of the active zones. It is possible that each of the active zones includes between two and ten, inclusive, or between three and six, inclusive, of the quantum well layers. Alternatively, the active zones or one of the active zones or some of the active zones have only one quantum well layer.
In the following, a semiconductor emitter described here is explained in more detail with reference to the drawing on the basis of exemplary embodiments. Identical reference signs indicate identical elements in the individual figures. However, no references to scale are shown; rather, individual elements may be shown in exaggerated size for better understanding.
Optionally, a buffer layer 62 is located between the semiconductor layer sequence 2 and the carrier 61. The buffer layer 62 is, for example, a semiconductor layer or a bonding agent layer, such as a solder. On a side facing away from the carrier 61, the semiconductor layer sequence 2 may optionally have a contact layer 23 and/or a cap layer 25, for example, for making electrical contact with the semiconductor emitter 1. Electrical contacts or electrodes are not drawn to simplify the illustration.
Furthermore, the semiconductor layer sequence 2 comprises a waveguide 51 between two cladding layers 52. The cladding layers 52 have a lower average refractive index for radiation generated in the operation of the semiconductor emitter 1 than the waveguide 51. For example, a thickness of the cladding layers 52 is at least 1.5 times or twice a vacuum wavelength of maximum intensity of the radiation generated in the waveguide 51 divided by the average refractive index of the cladding layer 52 concerned.
Several active zones 31, 32 are present in the waveguide 51. A tunnel diode 41 is located between the active zones 31, 32. The waveguide 51 thus represents a common waveguide for all active zones 31, 32. For example, the waveguide 51 has a thickness of at least 0.3 times or 0.6 times and/or at most 1.8 times or 1.2 times or 0.9 times the vacuum wavelength of maximum intensity of the radiation generated in the waveguide 51 divided by the average or effective refractive index of the waveguide layer 51.
Various design options, in particular for the waveguide layer 51, are described below.
In the example of
A tunnel diode 41 is located between the active zones 31, 32. The tunnel diode 41 may be directly adjacent to the barrier layers 21 of the adjacent active zones 31, 32. A p-doped tunnel diode layer 26 and directly adjacent an n-doped tunnel diode layer 28 are located in the tunnel diode 41. The tunnel diode layers 26, 28 are highly doped and comparatively thin.
For example, the active zones 31, 32 are configured to generate laser radiation with a wavelength of maximum intensity in the near-infrared spectral range, that is, in particular from 0.7 μm to 1.3 μm. For example, the wavelength of maximum intensity is 940 nm. In this case, the n-doped tunnel diode layer 28 is formed, for example, from a 10 nm thick GaAs layer with a dopant concentration of 5×1019 cm−3, in particular of Te, alternatively also with Si and/or Ge. The p-doped tunnel diode layer 26 is, for example, a 10 nm thick GaAs layer with a dopant concentration of 1×1020 cm−3, in particular with C, alternatively also with Be, Mg and/or Zn.
Deviating from the illustration in
Alternatively, for wavelengths of maximum intensity further in the infrared spectral range, the tunnel diode layers 26, 28 can also be made of p-doped InP and n-doped InGaAs or of p-conducting GaSb or InAs and n-conducting InAsSb. The above comments on the GaAs material system apply accordingly to the other material systems mentioned.
For example, the above thicknesses and dopant concentrations for the tunnel diode layers 26, 28 each apply with a tolerance of no more than a factor of 5 or no more than a factor of 2 or no more than a factor of 1.5.
In the example of
In addition, an intensity I of an optical fundamental mode in the waveguide 51 is drawn in
In
In all other respects, the comments on
In the case of tunnel diode layers 26, 28 made of GaAs, the intermediate layer 27 is preferably made of InAs or InGaAs with, for example, an In content of at most 80% or at most 50% or at most 30% or at most 10%, although AlInGaAs layers with an Al content of, in particular, at most 30% or at most 10% or at most 1% and with an In content of at most 30% or of at most 10% are also possible.
Such tunnel diodes 41, 42 may also be used in all other embodiments.
In all other respects, the comments on
In the embodiment example of
In the case of a GaAs-based tunnel diode 41, the transition layers 24 are preferably each made of AlGaAs, with an Al content towards the tunnel diode 41 preferably reducing steadily, in particular linearly. For example, an Al content on sides of the transition layers 24 facing away from the tunnel diode 24 is at least 5% and/or at most 30%, for example 14%, and on sides of the transition layers 24 facing the tunnel diode 24 the Al content is at most 20% or at most 5% or at most 0.5%, in particular 0%.
Such transition layers 24 may also be present in all other embodiments.
In
Furthermore, it can be seen in
As an option, it is illustrated in
As a further option, it is shown in
In all other respects, the comments on
According to
With such a structure of the waveguide 51, intensity curves can be obtained which show a wavy course in the region of maximum intensity, as illustrated in
In all other respects, the comments on
In this context, areas B1, B2 of the wavelength-dependent absorption A are schematically illustrated in
Since in the tunnel diode 41, 42, as shown in particular in
Thus, the tunnel diodes 41, 42 described herein have an overall low absorption coefficient for the generated radiation, so that the tunnel diodes 41, 42 can be placed in the common waveguide 51 in regions of high local intensity IL to improve the radiation pattern.
In contrast, the facets according to
The semiconductor layer sequences 2 and in particular the tunnel diodes 41, 42 described in connection with
The invention described herein is not limited by the description based on the embodiments. Rather, the invention encompasses any new feature as well as any combination of features, which in particular includes any combination of features in the patent claims, even if this feature or combination itself is not explicitly stated in the patent claims or embodiments.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2021 104 343.3 | Feb 2021 | DE | national |
This patent application is a national phase filing under section 371 of PCT/EP2022/051885, filed Jan. 27, 2022, which claims the priority of German patent application 102021104343.3, filed Feb. 24, 2021, each of which is incorporated herein by reference in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/051885 | 1/27/2022 | WO |
