Semiconductor laser with semiconductor material system with Fermi level control by doping the active region at spatially indirect transitions

Abstract

Semiconductor lasers with semiconductor material systems consisting of elements for a semiconductor, comprising a substrate area (12) which is a substrate made of at least one carrier material, and at least one active area (20) with three sub-areas (30); a separation area (14) that is a barrier between substrate area (12) and active area (20), or between several repetitions of active areas (20), between sub-areas (30) of an active area (20) or between active area (20) and air interface; where the first active sub-region (32) is a doping region and comprises a semiconductor material system consisting of at least one element and a doping; the second active sub-region (34) is a doping region and has a doping and a further semiconductor material system consisting of at least one element that differs from the semiconductor material system of the first active sub-region (32); the third active sub-region (36) is a doping region and has the semiconductor material system of the first active sub-region (32); and the active region (20) of the semiconductor laser (10) has a doped W-structure; and at least one of the semiconductor material systems in the second active sub-region (34) and / or in the first active and third active sub-region (32, 36) is doped such that the band gap in the W-structure can be adjusted in a predetermined manner so that the emission energy of the semiconductor laser (10) is adjustable for emission energies smaller than the transition energies of the individual sub-regions (30); and at least one element of the semiconductor material system of the second active sub-region (34) comprises at least one element semiconductor and a doping.

Description

[0001] The present invention relates to a semiconductor laser with a semiconductor material system of elements for a semiconductor comprising a substrate region, an active region and a separation region which is a barrier between the substrate region and the active region.

[0002] Semiconductor lasers are widely used in industry and end-user devices. They are used in many areas, from data transmission and sensor technology to medical technology. Although the requirements for lasers vary greatly and the properties of the lasers used vary accordingly, three key characteristics can be identified for improving and optimizing existing laser technologies.

[0003] First, different application areas require different emission energies, which cannot currently be achieved with conventional semiconductor lasers. Second, greater energy efficiency is an important property that will become even more important in the future. The third aspect concerns the sustainability of the raw materials used. This applies both to the type of elements used in the semiconductor laser and to the quantities employed and used, as well as to the complexity of the manufacturing process, where large amounts of waste must be significantly reduced in the future.

[0004] Currently, semiconductor lasers primarily use spatially direct transitions in semiconductor material systems as the active medium. The emission energy can be varied by the band gap of the active material, by strain, and by quantization effects such as the dimensionality of the active material, layer or material thickness, and the barrier material, and thus the barrier energy. A spatially direct transition between the valence band edge and the conduction band edge is therefore crucial for the transition energy. These parameters are interdependent. For example, a change in the band gap generally also results in a lattice mismatch. Different multinary material systems are used as multiple quantum films in complex heterostructures for different spectral ranges.For example, materials that can be used are (Ga,In)(N,As,P) for the near-infrared spectral range or (Ga,In) in combination with nitrogen (N) for blue and green laser diodes.

[0005] Further possibilities include quantum cascade structures, in which cascades of typically quantized intersubband transitions are used in complex multiple quantum structures. Here, the optical transitions between individual, typically higher states occur within quantized layers. The band gap energies between valence and conduction band edges for quantum cascade structures are typically far removed from the energies of the optical transitions used for laser activity and can therefore be considered irrelevant.

[0006] Furthermore, so-called W structures made of undoped type II heterostructures are used. Such lasers offer the advantage over conventional type I lasers and quantum cascade lasers in that losses due to intersubband scattering and Auger losses can be significantly suppressed. Here, the energy of the type II optical transitions is relevant for the laser activity, which is determined by the combination of the intrinsic materials.

[0007] To date, multinary heterostructures have been used to achieve different emission energies. In addition to different material compositions, different material systems have also been used. In W structures, the intrinsic hetero-offsets of the material combinations have been exploited at the type II transitions.

[0008] The undoped Type II structures commonly used in the state of the art have very limited options for tuning the target wavelengths. This is due to the limitation of the required good structural quality and the associated epitaxial growth of intrinsic layers. The emission wavelengths in currently used semiconductor lasers are thus essentially determined by the material and layer thickness of the active layer for all laser types. Furthermore, defects arise that reduce efficiency or even prevent efficient laser operation. These effects are caused by intrinsic lattice mismatches in multinary layer systems on available substrate materials, such as gallium arsenide (GaAs), indium phosphide (InP), sapphire, silicon carbide (SiC), or silicon (Si). Furthermore, the epitaxial growth of such partially quintessential layers is extremely complex and laborious.This means that some emission energies are very difficult or even impossible to achieve. Consequently, inefficient components must be used.

[0009] In addition, to achieve particularly low emission energies, it is necessary to rely on sustainable substrate platforms during production. The same applies to quantum cascade structures. Here, structural mismatch is also very critical, as lattice defects occur in the sometimes hundreds of epitaxial layers required, which inevitably lead to the functional failure of semiconductor lasers.

[0010] The published patent applications US 2015 / 0244144 A1, US 5,799,026 A, DE 10 2011 103 143 A1, EP 3 506 439 A1, US 6,154,475 A, US 2007 / 0051963 A1 and US 2012 / 0269221 A1 can be considered as prior art.

[0011] The task is therefore to propose an improved semiconductor laser with semiconductor material systems.

[0012] The present object is achieved by a semiconductor laser having the features of claim 1.

[0013] The invention thus relates to a semiconductor laser with a semiconductor material system comprising elements for a semiconductor. The semiconductor laser comprises a substrate region, which is a substrate made of at least one carrier material, and at least one active region with three sub-regions and a separation region. The separation region is a barrier between the substrate region and the active region, between multiple repetitions of active regions, between sub-regions of an active region, or between the active region and the air interface. The repetitions of the active regions lead to an increase in the gain amplitude and thus to greater amplification. This increase should ideally be linear with the number of repetitions.

[0014] The first active sub-region is a doping region and comprises a semiconductor material system consisting of at least one element in addition to the doping. The second active sub-region is a doping region and comprises a semiconductor material system consisting of at least one element in addition to the doping that differs from the semiconductor material system of the first active sub-region. The third active sub-region is a doping region and comprises the semiconductor material system of the first active sub-region. The material systems of the first and third active sub-regions are thus identical.

[0015] The active region of the semiconductor laser is a doped W structure. At least one of the semiconductor material systems in the second active sub-region and / or in the first and third active sub-regions is doped in such a way that a band gap in the W structure can be controlled in a predetermined manner, allowing the emission energy of the semiconductor laser to be tuned for emission energies lower than the transition energy of the individual sub-regions.

[0016] The invention thus relates to a concept for the design of semiconductor laser structures and to a semiconductor laser. The invention is based on the fundamental idea of ​​controlling the emission energy of a spatially indirect transition, a so-called type II transition, between two or three semiconductor material systems by means of doping. All known doping concepts are conceivable. In particular, homogeneous doping or delta doping can be used. In addition to the band gap energies of the semiconductors used in the active layer, the band discontinuity at the interface is of particular importance for the emission energy of a type II transition. The interface is also referred to as the band offset. Since the charge carriers recombine from the conduction band of one semiconductor into the valence band of the other, the energy difference between the conduction band of one semiconductor and the valence band of the other is crucial for laser emission.This band discontinuity is determined, to a first approximation, by the exit energies and the Fermi level of the semiconductor heterostructure. Doping determines the relative position of the valence and conduction band edges of all semiconductors relative to the Fermi level. The Fermi level is constant for semiconductor heterostructures without bias.

[0017] The relative position of the valence and conduction band edges of the individual semiconductors in the active region is achieved by controlling the Fermi level through doping. This allows the transition energy of the optical transitions to be controlled. Doping thus determines the conductivity of the elemental or compound semiconductors. Furthermore, doping determines the energetic position of the valence and conduction band levels of individual layers of elemental or compound semiconductors. However, since the Fermi level of a heterostructure, such as the W structure intended here, is continuous, doping also controls the energy of spatially indirect transitions at heterointerfaces, which are responsible for the laser transitions in these W structures, and can thus be adjusted by doping.

[0018] In the context of the invention, a semiconductor material system comprising elements for a semiconductor is understood to mean that the elements do not necessarily have to be semiconductor materials or elements or semiconducting elements themselves, but can also comprise metals, gases and other chemical elements or molecules that are suitable, e.g. in combination with other elements, to be part of a semiconductor material system or to construct such a system or to be a material for a semiconductor laser.

[0019] In a preferred embodiment, the second active sub-region has a different doping than the first active sub-region and the third active sub-region. For example, a possible material system could be formed from phosphorus and nitrogen plus nitrogen and gallium arsenide (PN+N GaAs). Various dopings are possible here, such as Be, C, Mg, Si, Ca, Ge, Zn, Se, Te. It is therefore also possible for the second active sub-region to be undoped, the first and third active sub-regions to be doped, or vice versa.

[0020] According to the invention, at least one element of the semiconductor material system of the second active sub-region comprises at least one elemental semiconductor and a dopant. The elemental semiconductor is particularly preferably from the fourth main group of the Periodic Table of Elements (PSE). For example, germanium (Ge) or silicon (Si) could be used. A possible structure of an active region could consist of the materials GaN-Ge-GaN, with the second active sub-region comprising germanium (Ge).

[0021] A preferred embodiment of the semiconductor laser is characterized in that at least one element of the semiconductor material system of the first active sub-region and the third active sub-region comprises a metal alloy. In a preferred embodiment, the alloy is made of a metal belonging to the third or fifth main group of the PSE. Most preferably, the first active sub-region and the third active sub-region have different doping levels. A preferred embodiment can be such that the first and third active sub-regions comprise a doping of aluminum, gallium, and arsenide ((Al,Ga)As), and the second active sub-region comprises, for example, the elements gallium (Ga) and arsenic (As-). These layers are preferably doped according to the above statements. Further combinations could be, for example, GaN / Ge / GaN; GaN / Si / GaN; GaN / SiGe / GaN.Here, the compound semiconductors of the first and third active subregions are doped according to the above explanations; the second active subregion, which consists of elements of the fourth main group, is doped with elements of the third or fifth main group.

[0022] In a further preferred embodiment of the semiconductor laser, the semiconductor material system of the first active sub-region and the third active sub-region comprises metal alloys. Metals from the third or fifth main group of the PSE are particularly preferred. The alloys and / or dopings are particularly preferably different.

[0023] According to a likewise preferred embodiment of the semiconductor laser, at least one of the active sub-regions comprises a semiconductor material system made of at least one metal and / or a gas, preferably of metal and gas. This applies under typical growth conditions for pressure and temperature. Preferably, the other active sub-region(s) are formed from an alloy semiconductor with at least one semiconductor material. Typical examples include a composition of the active region such that the first and third active sub-regions comprise the elements gallium (Ga) and nitrogen (N), while the second active sub-region comprises the element germanium (Ge). Another example provides silicon (Si) in the second active sub-region, while the first and third active sub-regions also comprise the elements gallium (Ga) and / or nitrogen (N).The individual active subregions are doped as described above in order to adjust the valence and conduction band offsets by controlling the Fermi level so that the desired transition energy of the laser transition in the W structure is achieved. Another embodiment may consist of the second active subregion comprising the elements silicon (Si) and germanium (Ge). Here, doping is preferably carried out using one or more elements from the third or fifth main group, such as boron, P, or As.

[0024] In a preferred embodiment of the semiconductor laser, the first active sub-region and the third active sub-region are each a semiconductor material system comprising two different elements made of semiconductor materials. Both elements of the semiconductor material system are also semiconductor materials, for example, silicon (Si) and germanium (Ge). One possible structure of the active region provides that the first and third active sub-regions comprise silicon (Si) and germanium (Ge), while the second active sub-region comprises the elements gallium (Ga) and nitrogen (N). The doping of the individual active sub-regions is carried out as described above in order to achieve the desired transition energy of the laser transition in the W structure.

[0025] In a preferred embodiment of the semiconductor laser, it is designed such that the first active sub-region and the third active sub-region each have a greater width than the second active sub-region. In a preferred embodiment, the width of the first active sub-region and the third active sub-region are each at least 1.5 times the width of the second active sub-region.

[0026] In a further preferred embodiment of the semiconductor laser, the semiconductor material system in the second active sub-region comprises an indirect semiconductor, which may be doped. The semiconductor material system in the first and third active sub-regions is a direct semiconductor, which may also be doped.

[0027] A further preferred embodiment provides that the semiconductor material system in the second active sub-region is a direct semiconductor, which may be doped, while the semiconductor material system in the first and third active sub-regions is an indirect semiconductor, which may also be doped. For example, the active region can be designed such that the first and third active sub-regions comprise germanium (Ge), while the second active sub-region comprises the elements gallium (Ga) and nitrogen (N). Preferably, all materials are doped according to the above statements in order to achieve the desired transition energy of the laser transition in the W structure.

[0028] The dopings described in the subclaims can be selected such that they amount to at most 10%, preferably at most 7%, more preferably at most 5%, more preferably at most 3%, very preferably at most 2%. Dopings of at most 1% or less than 1%, or very much less than 1%, for example less than 0.1% or less than 0.01%, are particularly preferred.

[0029] Through appropriate doping, the offset in germanium (Ge), for example, can be well, robustly, and reliably controlled, for example to significantly alter the W junction. This allows the production of p-type or n-type semiconductors. For example, GaAs could be doped with germanium (Ge), i.e., GaAs:Ge, preferably with a doping of <1% or better <<1% Ge. In the same way, germanium could also be doped with, for example, arsenide (As), i.e., Ge:As, or boron (B), i.e., Ge:B, or also, for example, gallium, i.e., Ge:Ga, or antimony (Sb), i.e., Ge:Sb. Here, the doping levels are also less than 1% or much less than 1% (< 0.1% to < 0.01%).

[0030] An embodiment of the invention is described below with reference to the accompanying drawings. They show: Fig. 1a to 1d show a schematic diagram of a semiconductor according to the invention; Fig. 2 a schematic diagram of a modification; and Fig. 3 a schematic diagram of a surface emitter

[0031] In the Fig. 1a to 1d show energy levels for different semiconductor lasers 10, with the conduction band and the valence band being shown in each case.

[0032] The laser transitions are controlled by doping the individual semiconductor materials. This doping controls the relative position of their valence and conduction band edges relative to the Fermi level 11, which remains constant throughout the heterostructure.

[0033] The semiconductor lasers 10 comprise a substrate region 12 formed from a substrate with at least one carrier material. In the embodiments shown here, a separation region 14 is integrated to the extent that a combined substrate region 12 with separation region 14 is represented. This region is referred to below in the figures as substrate region 12.

[0034] The substrate region 12 is followed by an active region 20, which comprises several sub-regions 30.

[0035] According to Fig. 1a, the active region 20 comprises three sub-regions 30: a first sub-region 32, a second sub-region 34, and a third sub-region 36. The three sub-regions 30 are arranged in a W-structure. A separation region 14 is provided between each of the individual sub-regions 30, which in the embodiment shown here is a gallium arsenide (GaAs) barrier. The substrate region 12 also comprises the elements gallium and arsenide.

[0036] The active region 20 is again adjoined by a barrier 40, which can be formed either from a separation region 14, a combination of a separation region 14 and a substrate region 12, or from a barrier region with multiple regions. The barrier region can also comprise, among other things, a separation region 14. In the embodiment illustrated here, the barrier comprises three adjacent regions, for example, made of GaAs, Ga(As,P), and GaAs. Such barriers or similar barriers are known in the art.

[0037] An active region can, in turn, be connected to the barrier. Thus, several active regions 20 separated by a barrier 40 can be arranged in a row in the semiconductor laser 10. In this case, a substrate region 12 can be provided at the end instead of a barrier.

[0038] The first sub-region 32 and the third sub-region 36 are constructed identically, as shown in this embodiment. They comprise the same materials. All three sub-regions 30 of the active region 20 are doped. The first sub-region 32 and the third sub-region 36 comprise a combination of gallium with an alloy of indium (Ga 0.95 and In 0.05 ) in combination with arsenide (As). This region can be undoped, as in this implementation (semi-insulating, Si), or doped to shift the valence and conduction bands relative to the Fermi level. The second sub-region 34 is also doped and also includes the elements gallium (Ga) and arsenide (As), with arsenide having a 7% alloy with antimony (Sb). The antimony doping comprises 7% in the example shown here, so that the material system of the second sub-region 34 consists of GaAs. 0.93 Sb 0.07This layer is doped by adding appropriate elements such as Zn or Te p or n to control the energy of the type II band discontinuities, which determine the laser energy of the W structure.

[0039] The doping and alloying of the sub-ranges 30 can vary. For example, it is possible and quite advantageous if the alloy containing antimony Sb is in the range between 5% and 10%, for example, approximately 9%.

[0040] Based on the Fig. 1a can also provide an example of the spatial extent of the individual sub-regions 30 and separation regions 14. In the example provided, the separation regions can be approximately 1 nm wide, while the second sub-region 34 is 4 nm wide, and the two outer sub-regions, i.e., the first sub-region 32 and the third sub-region 36, can each be 6 nm wide.

[0041] Fig. Figure 1b shows an embodiment of a doped W structure with intermediate GaAs barriers. The semiconductor laser 10 is based on a (Ga,In)As / Ga(As,Sb) / (Ga,In)As material system in a W structure. In the embodiment shown here, the separation regions 14 between the sub-regions 30 of the active region 20 are δ-doped. The doping from the 1 nm GaAs layers of the separation regions serves as δ-doping, for example, for the material system of the second sub-region 34 of the p-type semiconductor laser. Consequently, for example, a GaAs 0.93 Sb 0.07 with δ-doping in p-type.

[0042] The embodiment according to Fig. Figure 1c is an example of a doped W structure of a semiconductor laser 10 without an intermediate GaAs separation region 14 between the sub-regions 30. This semiconductor laser also has a (Ga,In)As / Ga(As,Sb) / (Ga,In)As-W structure. This embodiment may have the disadvantage in some applications that the "Sb well" may not be visible in absorption.

[0043] The emission energy is crucial for the wavelength emitted and radiated by the semiconductor laser 10. This corresponds, to a first approximation, to the band gap formed between the valence band of the second sub-region 34 and the conduction band of the first sub-region 32 or third sub-region 36. This band gap can be influenced by appropriate doping of the elements of the material system of the semiconductor laser and by appropriately selecting the corresponding elements of the material system, which offers the possibility of adjusting the emission energy and wavelength of the semiconductor laser as desired.

[0044] Fig. Figure 1d shows an example of a semiconductor laser 10 with a doped Si / GaN / Si-W structure. While the upper line 100 indicates the vacuum level in the energy diagram shown, reference numeral 110 denotes the conduction band of the gamma-like valence band states of silicon. Reference numeral 120 denotes the conduction band of the L-type conduction band minimum of silicon or GaN. Reference numeral 130 denotes the valence band of the gamma-like valence band state of silicon or GaN. The numerical values ​​of the band gap spacings in Fig. 1d are given in eV. Transitions between the individual bands are therefore marked in eV.

[0045] In the embodiment shown, the relevant transition occurs between the gamma-type valence band states of silicon and the conduction band states in natively n-doped GaN. A variation of the n-type doping in GaN determines the hetero-offsets. The L-type conduction band minima of silicon at the indirect band gap generally play no role.

[0046] An analogous pattern would arise with p-type doping in silicon. The corresponding transitions are clear to those skilled in the art.

[0047] The Fig. 1a to 1d thus show the inventive concept for the design of a semiconductor laser 10 and its structures. The inventive idea is to control the emission energy of a spatially indirect transition (Type II transition) of two or three active sub-regions by means of doping. All known doping concepts can be applied, in particular homogeneous doping, but also δ-doping.

[0048] In addition to the band gap energies of the semiconductors used in the active structure, the band discontinuity at the interface (band offset) is particularly important for the emission energy of the Type II transition. Since the charge carriers recombine from the conduction band (LB or CB) of the other, the energy difference between the conduction band and the valence band is crucial for emission. This discontinuity is determined, to a first approximation, by the exit energy and the Fermi level of the semiconductor heterostructure. If a semiconductor is doped, this Fermi level shifts either toward the conduction band LB (N-doping) or toward the valence band VB (P-doping). Since the structures are spatially less extensive than the space charge region, Fermi level pinning occurs.Thus, for example, for quantum films, quantum wires or quantum dots, the band discontinuity and thus the emission energy can be controlled and specifically manipulated by doping the semiconductor layers of the type II transition, i.e. by doping the sub-regions 30.

[0049] The concepts described here reflect a general property of semiconductors and semiconductor lasers. For this reason, the invention is not limited to direct semiconductors, but is also transferable to indirect semiconductors, since only the gamma point is relevant. Furthermore, multinary compound semiconductors can also be implemented in the active layer, and the concept can be applied independently of the substrate platform, which is particularly advantageous in industrial applications.

[0050] Fig. Figure 2 shows a possible modification of the basic concept, as represented, for example, by monolithic Grinsch (graded index of refraction separate confinement heterostructure). In contrast to the state of the art, as explained above, these have a doped W structure.

[0051] In addition to the design of edge emitters, it is also possible to design surface emitters, such as Fig.3. Here, the type II structure is monolithically applied and grown on a contacted n-type (or p-type) Bragg reflector and enclosed by a ring contact 50. A p-type (or n-type) Bragg reflector can then be deposited on the active layer. Such a structure represents a vertical-cavity surface-emitting laser (VCSEL). Alternatively, the structure can be closed with a cap layer to enable an external resonator configuration and obtain a vertical external cavity surface-emitting laser (VECSEL). In contrast to the prior art, these have a doped W structure, as explained above.

Claims

[1] Semiconductor laser with semiconductor material systems of elements for a semiconductor, comprising a substrate region (12) which is a substrate made of at least one carrier material, and at least one active region (20) with three sub-regions (30); a separation region (14) which is a barrier between the substrate region (12) and the active region (20), or between several repetitions of active regions (20), between sub-regions (30) of an active region (20) or between the active region (20) and the interface to the air; where the first active sub-region (32) is a doping region and comprises a semiconductor material system comprising at least one element and a doping; the second active sub-region (34) is a doping region and has a doping and a further semiconductor material system comprising at least one element which differs from the semiconductor material system of the first active sub-region (32); the third active sub-region (36) is a doping region and comprises the semiconductor material system of the first active sub-region (32); and the active region (20) of the semiconductor laser (10) has a doped W structure; and at least one of the semiconductor material systems in the second active sub-region (34) and / or in the first active and third active sub-regions (32, 36) is doped such that the band gap in the W structure can be adjusted in a predetermined manner such that the emission energy of the semiconductor laser (10) can be adjusted for emission energies smaller than the transition energies of the individual sub-regions (30); and at least one element of the semiconductor material system of the second active sub-region (34) comprises at least one element semiconductor and a doping. [2] Semiconductor laser according to claim 1, characterized by that the second active sub-region (34) has a different doping than the first active sub-region (32) and the third active sub-region (36). [3] Semiconductor laser according to claim 1, characterized by that the at least one element of the semiconductor material system of the second active sub-region (34) comprises at least one element semiconductor from the fourth main group. [4] Semiconductor laser according to one of the preceding claims, characterized by that at least one element of the semiconductor material system of the first and third active sub-region (32, 36) comprises an alloy of metal, preferably of metal from the third or fifth main group, particularly preferably with different dopings. [5] Semiconductor laser according to the preceding claim, characterized by that the semiconductor material system of the first and third active sub-regions (32, 36) comprises alloys of metal, preferably of metals of the third and fifth main groups, particularly preferably with different dopings. [6] Semiconductor laser according to one of the preceding claims, characterized by that at least one of the active sub-regions (30) comprises a semiconductor material system made of at least one metal and one gas, wherein preferably the other active sub-region(s) (30) are alloy semiconductors made of at least one semiconductor material. [7] Semiconductor laser according to one of the preceding claims, characterized by that the first active sub-region (32) and the third active sub-region (36) comprise a semiconductor material system with two different elements made of semiconductor materials. [8] Semiconductor laser according to one of the preceding claims, characterized by that the first active sub-region (32) and the third active sub-region (36) each have a greater width than the second active sub-region (34), wherein preferably the width of the first active sub-region (32) and the third active sub-region (36) are at least 1.5 times as large as the width of the second active sub-region (34). [9] Semiconductor laser according to one of the preceding claims, characterized by that the semiconductor material system in the second active sub-region (34) is an indirect semiconductor and the semiconductor material system in the first and third active sub-regions (32, 36) is a direct semiconductor. [10] Semiconductor laser according to one of the preceding claims, characterized by that the semiconductor material system in the second active sub-region (34) is a direct semiconductor and the semiconductor material system in the first and third active sub-regions (32, 36) is an indirect semiconductor.

Images