Device for emitting laser beams

Abstract

The invention relates to a device for emitting laser beams, comprising a double heterostructure (1) which comprises three semiconductor layers (2, 3, 4) grown one on top of the other and the outer semiconductor layers (3, 4) of which have doping types which differ from each other. In order to provide such a device which can be obtained using conventional production methods and makes it possible to generate laser light even at typical ambient temperatures of 0-55°C, it is proposed that an inner semiconductor layer (2) forming a type II p-n or p-i-n junction is arranged between the outer semiconductor layers (3, 4) of the double heterostructure (1) forming a laser diode, the voltage across the outer semiconductor layers (3, 4) being such that the difference between the two Fermi energies, which relate to the band states and describe the population of the conduction band states with electrons and / or the population of the valence band states with holes, in the inner semiconductor layer (2) corresponds at least to the band gap there.

Description

[0001] Device for laser beam emission

[0002] Technical field

[0003] The invention relates to a device for laser beam emission, comprising a double heterostructure with three superimposed semiconductor layers, the outer semiconductor layers of which have different doping types (p-type and n-type doping).

[0004] State of the art

[0005] Dual heterostructures (DHS) based on main group IV semiconductors have typically been used to optimize bipolar transistors (Rücker H, & Heinemann, B.; High-performance SiGe IIBTs for next generation BiCMOS technology, Semiconductor Science and Technology 33, 114003 -2018).

[0006] Due to the widespread availability and material-specific advantages of silicon (Si), DHS based on main group IV semiconductors, for example, DHS based on silicon germanium (SiGe), show potential in the field of integrated optics. Such DHS exhibit a characteristic type II band edge profile, whereby, at thermal equilibrium, only one charge carrier type, either electrons or holes, is attracted to the inner semiconductor layer, while the other charge carrier type is repelled. Because of this type II band edge profile, suitable DHS for laser light generation from this material system, especially those suitable for laser light generation at ambient temperatures, have not yet been realized using conventional epitaxial growth methods under typical growth conditions, i.e., with growth temperatures above 400°C.In practice, this failed not only because of the inherent type-II band edge profile, but also because with higher germanium contents the difference between the periodicities of the crystal lattices of silicon and silicon germanium increases significantly, which proved to be a disadvantage in this context.

[0007] Description of the invention

[0008] Starting from a DHS whose constituent or intrinsic semiconductor layers form a type-II band edge profile in thermal equilibrium, such as the aforementioned SiGe material system, the invention is thus based on the objective of creating a device of the type described above, which is available using conventional manufacturing processes and enables the generation of laser light even at typical ambient temperatures of 0 - 55°C.

[0009] The invention solves the stated problem by arranging an inner semiconductor layer forming a type-1 pn or pin junction between the outer semiconductor layers of the double heterostructure forming a laser diode, wherein a voltage is applied between the outer semiconductor layers that is so high that the difference between the two Fermi energies relating to the band states, describing the occupation of the conduction band states by electrons and / or the valence band states by holes, corresponds at least to the band gap in the inner semiconductor layer.

[0010] Characteristic of type II junctions in a DHS, which has an inner semiconductor layer surrounded on both sides by outer semiconductor layers in a sandwich structure, are certain relationships between the valence and conduction band edge energies E. v or E c , i.e., if E V außen < Hey innen )- , then it is (outside) E (inside) - This typically results in staggered band discontinuities E V and AE CThis is accompanied by a spatial separation of electrons and holes, unlike in Type I junctions. To generate optical amplification, a fundamental requirement for use as a laser diode, this spatial separation of charge carrier types, which defines the Type II character of the DHS, must first be eliminated and reversed, resulting in a significant concentration of electrons and holes in the inner semiconductor, which will be quantified further below. This means that an accumulation of electrons and holes must be achieved in the inner semiconductor layer. Secondly, to achieve optical amplification, the concentrations of electrons and holes in the inner semiconductor layer must exceed threshold values. These thresholds stipulate that, measured from the conduction or valence band edge, there must be an energy interval in which at least half of the electron and hole states existing in this interval are occupied by electrons or holes.The junctions are occupied by holes. Therefore, optical gain, a fundamental requirement for use as a laser diode, cannot normally be achieved with DHS diodes exhibiting such type-II junctions. However, it has been shown that in the case of an inner semiconductor layer forming a type-II pn or pin junction, an advantageous spatially overlapping accumulation of electrons and holes occurs when a forward voltage is applied between the outer semiconductor layers using an external voltage source. This forward voltage is preferably at least 0.4 V, more preferably at least 0.5 V, and even more preferably at least 0.65 V. In this context, spatially overlapping electron and hole accumulation means that the spatial separation of the electrons and holes is eliminated, resulting in an accumulation of electrons and holes in the inner semiconductor layer.Consequently, in such a case, when a forward voltage is applied according to the invention, the electrons and holes are not spatially separated, but rather the location of an electron or hole can be exclusively assigned to the inner semiconductor layer. More precisely, a spatially extended overlap of the electron and hole wave functions results, giving optical emission transitions a spatially direct character. According to the features of the invention, in contrast to the prior art, a region of spatially direct emission transitions is formed not only in a quantum structure, but also in a three-dimensional active region of the inner semiconductor layer, in which both electrons and holes are present in high concentrations and therefore electron-hole recombination occurs efficiently.Depending on the material system underlying the DHS, the voltage can preferably be in the range of 0.4 to 1 V, 0.5 to 1 V, 0.65 to 1 V, 0.8 to 1 V, or also in the range of 2 to 3 V. According to the invention, the voltage must be high enough that the difference between the two Fermi energies, which describe the occupation of the conduction band states by electrons and / or the valence band states by holes, in the inner semiconductor layer corresponds at least to the band gap there. The measures according to the invention result in the density of electrons with energies around the conduction band edge energy on the one hand, and of holes with energies around the valence band edge energy on the other, being greater in the middle semiconductor layer than the density of unoccupied electron states or unoccupied hole states at those energies, respectively, despite the Type II nature of the DHS.As a consequence of the spatially overlapping electron and hole accumulation, a crucial prerequisite for use as a laser diode is light amplification through stimulated emission, resulting in an optical gain coefficient y > 0 at the inner semiconductor layer. The stimulated emission according to the invention could be achieved, in particular, with material systems based on silicon germanium as the inner semiconductor layer. It is essential to note that the inner semiconductor layer should have a smaller band gap than the two outer semiconductor layers. Overall, the measures according to the invention enable the type II band edge profile of the DHS, characteristic in thermal equilibrium, to remain unchanged during operation.With appropriate forward voltage, the band edge profile is changed to a type-l-like profile, enabling both the formation of a positive optical gain coefficient necessary for the generation of laser light at typical ambient temperatures of 0-55°C, and the use of conventional manufacturing processes such as molecular beam epitaxy.

[0011] For the inventive use of a DHS with a type II band edge profile, one of the adjacent outer semiconductor layers has a p-doping, whereas the second outer semiconductor layer opposite this outer semiconductor layer with respect to the inner semiconductor layer is n-doped. In the case of a pn junction, this is preferably located in the middle of the inner semiconductor layer. If the inner semiconductor layer has an assumed thickness d, then, in accordance with the invention, a deviation of the position of the p-n junction from this ideal position of up to permissible. In a pin junction, the center of the nominally intrinsic (i.e., undoped) i-layer is preferably located in the center of the inner semiconductor layer. Accordingly, a deviation of the center of the i-layer from the center of the inner semiconductor layer with thickness d of up to is also permissible. with permissible, wherein d t the thickness of the intrinsic layer should preferably not be greater than 10% of the diffusion length of the minority charge carriers in the inner semiconductor layer.

[0012] In accordance with the invention, the inner semiconductor layer can also include optical recombination centers. Such recombination centers increase the rate of electron and hole recombination, emitting electromagnetic radiation. For example, special types of optically active crystal defects, so-called color centers, or low-dimensional structures, such as quantum dots, can function as optical recombination centers. These recombination centers need not be homogeneously distributed throughout the inner semiconductor layer, but can also be concentrated in one or more sublayers within the inner semiconductor layer. The sublayers in which these recombination centers are concentrated can, in principle, consist of a different semiconductor material than the inner semiconductor layer outside of these sublayers.For example, the inner semiconductor layer can be made of silicon germanium, whereas the sublayer(s) containing the recombination centers within this inner semiconductor layer are made of silicon. The sublayers are grown epitaxially and can each have a thickness of 0.1 to 5 nm, preferably 0.5 nm.

[0013] In general, it is recommended that the inner semiconductor layer have a thickness that is at most 1.2 times the sum, preferably at most 1.1 times the sum, and even more preferably at most the sum of the diffusion lengths L. e and L h corresponding to the electrons or holes, where the diffusion lengths are given by the relationship L e = ^D e * T or L h = ^D h * T with the minority carrier lifetime T and the diffusion coefficient D e or D hThe electron and hole density is given. Furthermore, the thickness of the inner semiconductor layer should be at least large enough that the effects of quantization resulting from charge carrier localization in the inner semiconductor layer on the energy density of states there are negligible compared to the thermal energy.

[0014] In principle, a double heterostructure according to the invention can be formed based on semiconductors of main groups III, IV, or V. All semiconductor layers, in particular the inner semiconductor layer, can be formed as a solid solution layer. Particularly favorable optoelectronic conditions, especially with regard to laser beam emission, result when the double heterostructure is based, for example, on silicon / silicon germanium / silicon, aluminum antimonide / indium arsenide / aluminum antimonide, or

[0015] The material is manufactured from aluminum phosphide / gallium phosphide / aluminum phosphide. In the case of silicon / silicon germanium / silicon, the germanium content of the inner silicon germanium semiconductor layer can be in the range of 10 to 100%, preferably at least 40%, and particularly in the range of 40–50%. For both silicon / silicon germanium / silicon and aluminum antimonide / indium arsenide / aluminum antimonide, an applied forward voltage in the range of 0.8 to 1 V has proven advantageous, whereas for aluminum phosphide / gallium phosphide / aluminum phosphide, the applied forward voltage is preferably in the range of 2 to 3 V.

[0016] To produce a double heterostructure according to the invention, the inner semiconductor layer is epitaxially embedded between the two outer semiconductor layers using, for example, molecular beam epitaxy. In order to achieve a particularly high structural quality of the optically active inner semiconductor layer in the case of a silicon / silicon germanium / silicon DHS, and thus a particularly advantageous emission behavior, it is recommended that growth temperatures of no more than 350°C and a pressure of no more than 2*10⁻⁶ 10 The pressure in the growth chamber must be maintained at mbar. This allows for the formation of dislocation-free, and therefore undesirable, stress reduction-free, internal silicon-germanium semiconductor layers, which have proven advantageous for laser applications.

[0017] In this context, a particularly advantageous accumulation of electrons and holes according to the invention can be achieved in the inner semiconductor layer if, in the case of silicon germanium, this layer has a thickness of at least 14 nm or, in the case of indium arsenide, a thickness of at least 30 nm.

[0018] Brief description of the invention

[0019] The invention is illustrated in the drawing as an example. It shows

[0020] Fig. 1 shows a schematic representation of a device according to the invention in a first embodiment and

[0021] Figures 2 to 6 of Figure 1 show representations of further embodiments.

[0022] Ways to implement the invention

[0023] A device for laser beam emission according to the invention has a double heterostructure 1 designed as a laser diode, comprising an inner semiconductor layer 2 which is epitaxially embedded between outer semiconductor layers 3 and 4 in a sandwich structure. As indicated by the different diagonal hatching, the outer semiconductor layer 3 is n-doped, whereas the outer semiconductor layer 4 is p-doped. According to the embodiment shown in Fig. 1, a type II pn junction, which indicates the abrupt boundary between acceptor and donor doping, is located in the center of the inner semiconductor layer 2. As indicated by the double hatching, the n-doped region lies to the left of the pn junction and the p-doped region to the right of the pn junction of the inner semiconductor layer 2. Fig. 2 shows an alternative embodiment in which a type II pin junction is located in the inner semiconductor layer 2.

[0024] In order to achieve an optical amplification in the inner semiconductor layer 2 that is crucial for laser beam emission despite the type-II junction, a voltage high enough is applied to the laser diode or to corresponding contact points 5, 6 on the outer semiconductor layers 3, 4 via a voltage source 7 that the difference between the two Fermi energies related to the band states in the inner semiconductor layer corresponds at least to the band gap there.

[0025] Accordingly, the following must apply to the voltage AK for electrical elementary grounding q:

[0026] V p and V n These correspond to the electrical potentials applied at contact points 5 and 6, respectively. E F e y nnen) and E F h y nnen)These values ​​refer to the Fermi energies in the inner semiconductor layer 2, which describe the occupation of the conduction band states by electrons and the valence band states by holes, respectively. Finally, E refers to g y nnen) on the band gap in the inner semiconductor layer 2.

[0027] Preferably, the voltage AK is at least 0.65 V. Since increasing the voltage AK increases the current through the laser diode and thus also the ohmic power dissipation, an upper limit for AK is generally reached when the ohmic power dissipation leads to the destruction of the diode. This upper limit depends on the material system, the diode implementation, and the efficiency of any active cooling.

[0028] In the case of silicon / silicon germanium / silicon DHS and aluminum antimonide / indium arsenide / aluminum antimonide DHS, an applied voltage AK in the range of 0.8 to 1 V can be advantageous. For aluminum phosphide / gallium phosphide / aluminum phosphide DHS, the applied voltage AK is preferably in the range of 2 to 3 V, particularly 2.8 V. The thickness d of the inner semiconductor layer 2 should be at most the sum of the respective minority carrier diffusion lengths L. e and L h of the electrons or holes in semiconductor position 2. Furthermore, in the case of a pn junction, as shown in the embodiments of Figures 1, 3 and 4, the pn junction must not extend further than L h from the boundary between the outer semiconductor layer 3 and the n-doped part of the inner semiconductor layer 2, or not further than L efrom the boundary between the outer semiconductor layer 4 and the p-doped part of the inner semiconductor layer 2. In the case of a pin junction, as shown by way of example in Figs. 2, 5 and 6, the allowed distances refer accordingly to the center of the intrinsic layer indicated by exclusively vertical hatching instead of to the pn junction.

[0029] For small layer thicknesses (d < L h and d < L eIt is recommended that the position of a pn junction deviates from the center of the inner semiconductor layer 2 by at most ±. In addition to the embodiment according to Fig. 1, further embodiments that satisfy such a condition are shown by way of example in Figs. 3 and 4. In Fig. 3, the pn junction is located at the boundary between the outer n-doped semiconductor layer 3 and the inner semiconductor layer 2, and in Fig. 4 at the boundary between the outer p-doped semiconductor layer 4 and the inner semiconductor layer 2.

[0030] In the exemplary pin junctions shown in Figs. 2, 5 and 6, there is a deviation of up to the center of the intrinsic layer from the center of the inner semiconductor layer 2. before, whereby d tThe thickness of the intrinsic layer is denoted. The intrinsic layer of the inner semiconductor layer 2, shown with exclusively vertical hatching, borders directly on the n-doped outer semiconductor layer 3 in the embodiment shown in Fig. 5, and directly on the p-doped outer semiconductor layer 4 in the embodiment according to Fig. 6.

Claims

Patent claims 1. Device for laser beam emission, comprising a double heterostructure (1) with three superimposed semiconductor layers (2, 3, 4) whose outer semiconductor layers (3, 4) have different doping types, characterized in that an inner semiconductor layer (2) forming a type-1 p-n or pin junction is arranged between the outer semiconductor layers (3, 4) of the double heterostructure (1) forming a laser diode, wherein a voltage is applied between the outer semiconductor layers (3, 4) such that the difference of the two Fermi energies in the inner semiconductor layer (2), which are related to the band states and describe the occupation of the conduction band states by electrons and / or the valence band states by holes, corresponds at least to the band gap there.

2. Device according to claim 1, characterized in that the inner semiconductor layer (2) has recombination centers for promoting radiative recombination of electrons and holes.

3. Device according to claim 2, characterized in that the recombination centers are designed as quantum dots and / or as color centers.

4. Device according to one of claims 1 to 3, characterized in that the inner semiconductor layer (2) has one or more sublayers.

5. Device according to one of claims 1 to 4, characterized in that the voltage is at least 0.4 V.

6. Device according to one of claims 1 to 5, characterized in that the inner semiconductor layer (2) has a thickness (d) which is at most equal to 1,2- times the sum of the diffusion lengths (Le, Lh) of the electrons or holes.

7. Device according to one of claims 1 to 6, characterized in that the double heterostructure (1) is made on the basis of silicon / silicon germanium / silicon, aluminium antimonide / indium arsenide / aluminium antimonide or aluminium phosphide / gallium phosphide / aluminium phosphide.

8. A method for laser beam emission with a device according to one of the preceding claims, characterized in that a voltage is applied between the outer semiconductor layers (3, 4) such that the difference between the two Fermi energies in the inner semiconductor layer (2), which are related to the band states and describe the occupation of the conduction band states by electrons and / or the valence band states by electron holes, corresponds at least to the band gap there.

9. A method according to claim 8, characterized in that the voltage is at least 0.4 V.

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