Semiconductor device and system for random number generation comprising a vcsel

Abstract

The present invention relates to a semiconductor device (10) for random number generation, comprising: a substrate (20); a VCSEL (30) which is arranged on the substrate (20); and a first photodetector (40) which is arranged on the same substrate (20) as the VCSEL (30); wherein the semiconductor device (10) has a polarization-selective reflector (50) which is designed to polarization-selectively reflect laser light (60) that has been emitted by the VCSEL (30) and has a first polarization (61) to the first photodetector (40). The present invention also relates to a system (80) and to a corresponding method (100) for random number generation.

Description

August 20, 2025 Semiconductor device and system for random number generation using VCSEL

[0001] The present invention relates to a semiconductor device and a system for generating random numbers using a VCSEL.

[0002] Random numbers are the basis of secure encryption. VCSELs (Vertical Cavity Surface Emitting Lasers) can find interesting applications in cryptography because their unstable polarization allows the generation of truly random numbers. In a surface emitter or VCSEL, the light is emitted perpendicular to the plane of the semiconductor chip, unlike an edge-emitting laser diode, where the light exits at one or two edges of the chip.

[0003] WO 2022 / 162225 A1 discloses a random number generator comprising a vertical cavity surface emitting laser (VCSEL), a mode separator, and a photodetector, wherein the random number generator further comprises an energy source to operate the VCSEL in direct modulation mode, wherein the VCSEL is configured to emit laser light with two different laser modes of random relative intensity from an emitting section into the space outside a VCSEL resonator, which can propagate in a propagation direction away from the emitting section, wherein the mode separator is arranged in the propagation direction between the photodetector and the VCSEL, and the mode separator is configured to separate the two different laser modes from each other and transmit one of the different laser modes to the photodetector.

[0004] Zhu et al., “Random number generation based on polarization mode noise of vertical-cavity surface-emitting lasers”, Laser Phys. Lett. 9 775, 2012 concerns the generation of random numbers based on the polarization mode noise of vertical-cavity surface-emitting lasers.

[0005] Quirce et. al., “Random polarization switching in gain-switched VCSELs for quantum random number generation,” Opt. Express 30, 10513-10527, 2022 concerns random Polarization switching in gain-switched VCSELs to generate quantum random numbers.

[0006] From the state of the art, the theoretical basis is thus known that the polarization of VCSELs can be used to generate random numbers.

[0007] Against this background, the objective of the present disclosure is to provide a device and system for generating random numbers that enables the generation of truly random numbers even in compact, cost-effective end devices. In particular, it would be desirable to enable the generation of truly random numbers in any end device involved in data communication, e.g., smartphone, tablet, or laptop.

[0008] According to a first aspect of the present disclosure, a semiconductor device for generating random numbers is proposed comprising a substrate; a VCSEL arranged on the substrate; a first photodetector arranged on the same substrate as the VCSEL; wherein the semiconductor device has a polarization-selective reflector configured to reflect laser light of a first polarization emitted by the VCSEL in a polarization-selective manner to the first photodetector.

[0009] According to another aspect of the present disclosure, a random number generation system is proposed comprising a semiconductor random number generation device, in particular as introduced above and / or described in the present disclosure; and a controller configured to provide a sequence of random numbers based on an output signal from the first photodetector.

[0010] According to another aspect of the present disclosure, a method for generating random numbers is proposed comprising the steps of: providing a semiconductor device for generating random numbers, in particular as introduced above and described in the present disclosure; operating the VCSEL such that, that laser light with different polarization states is emitted randomly; detecting the laser light emitted by the VCSEL with the first photodetector; and providing a sequence of random numbers based on an output signal from the first photodetector.

[0011] The solution disclosed herein presents a semiconductor device for generating random numbers, comprising a common substrate on which both the VCSEL and the first photodetector are arranged. The semiconductor device further includes a polarization-selective reflector configured to reflect laser light of a first polarization emitted by the VCSEL to the first photodetector in a polarization-selective manner. The VCSEL exhibits stochastic polarization. Preferably, the VCSEL is configured such that different polarization directions, for example, s- and p-polarized light, occur (approximately) equally. This can be achieved by a suitable choice of operating current, for example, by pulsed operation alternating between below and above the laser threshold.This approach exploits the fact that random polarization states can arise, particularly in the region of spontaneous emission. This effect is governed by quantum physics and is therefore purely stochastic. Intermittent operation of the VCSEL generates a stochastic sequence of 0s and 1s, thus producing a true random number. Operation is possible with pulse lengths of a few tens of nanoseconds, enabling the generation of random numbers at frequencies of many MHz. In particular, the VCSEL lacks polarization stabilization devices such as grids. In contrast to the prior art, where so-called "flips" or changes in the polarization state are often undesirable and suppressed, this disclosure proposes using random polarization changes of the VCSEL to generate random numbers.It is proposed that not only are the VCSEL and the photodetector arranged on the same substrate, but that the semiconductor device also incorporates a polarization-selective reflector, which is configured to reflect the first polarization of laser light emitted by the VCSEL to the first photodetector in a polarization-selective manner. Thus, only a portion of the laser light emitted by the VCSEL is reflected to the first photodetector through this polarization-selective reflection. The random polarization state of the VCSEL therefore leads to a specific result depending on the... Polarization is used to generate a random intensity signal from the photodetector, which can be used to generate (true) random numbers. For example, it is possible to compare an output signal from the photodetector with a threshold value and generate a stochastic sequence of 0s and 1s. By arranging the VCSEL and the photodetector on the same substrate and by including a polarization-selective reflector in the semiconductor device, the generation of true random numbers can be achieved even in compact, cost-effective devices. The inventors recognized that a polarization-selective reflector, configured to reflect laser light of a first polarization emitted by the VCSEL to the first photodetector in a polarization-selective manner, can be implemented in a semiconductor device in a simple and cost-effective way.

[0012] According to one aspect of the present disclosure, VCSEL and detection can be integrated in a semiconductor device, thereby significantly reducing the size and cost of a module. In particular, a back-side emitter with a structure etched into the substrate can be provided as a polarization-selective reflector, and the polarization-dependent reflection can be utilized.

[0013] An optical path for the first-polarization laser light from the VCSEL to the first photodetector can be provided within the substrate. In other words, the first-polarization laser light can be guided, at least partially, within the substrate from the VCSEL to the first photodetector. An advantage of this solution is the potential for a cost-effective, compact semiconductor device. In particular, the effort required for adjusting external beam-guiding components can be reduced.

[0014] The VCSEL can be a backside emitter configured to emit laser light towards the substrate. One advantage is that the laser light emitted from the backside of the VCSEL can be directly coupled into the substrate and guided within the substrate to the first photodetector located on the same substrate.

[0015] The VCSEL and the first photodetector can be arranged laterally offset from each other on the same side of the substrate. For example, the VCSEL and the first photodetector can be arranged on a first side of the substrate. Alternatively, the VCSEL and photodetector can be arranged separately on the same substrate. The polarization-selective reflector can be arranged on a second side of the substrate, opposite the first side. In this case, a polarization-selective back reflection on the opposite side of the substrate can be used to selectively reflect the laser light emitted by the VCSEL with a first polarization to the laterally offset first photodetector. Multiple polarization-selective reflectors can also be used, and / or multiple reflections within the substrate can be exploited to enhance polarization selectivity.

[0016] The polarization-selective reflector can be provided by an inclined surface on the back side of the substrate, particularly at the transition from an optically denser to an optically less dense medium. In other words, the back reflection advantageously does not occur perpendicularly, but at an angle, for example, at an inclined surface on the back side of the substrate. The angled reflection at the transition from the optically denser to the less dense medium is polarization-dependent. Alternatively or additionally to the inclined surface, a polarization-selective metastructure can also be provided on the back side of the substrate opposite the VCSEL.

[0017] The beveled surface can be provided by structuring the substrate, in particular by etching the beveled surface into the substrate. Similarly, an optical metastructure can also be provided by structuring the substrate, for example by etching it into the substrate or by one or more further structured layers on the substrate.

[0018] The angle of the inclined surface relative to the laser light emitted by the VCSEL can be configured such that the laser light strikes the inclined surface of the polarization-selective reflector at the Brewster angle. An advantage of this design can be that the polarization dependence of the reflection is reduced. The effect is particularly pronounced at the so-called Brewster angle. When radiation strikes at this angle, one polarization direction is not reflected at all, but can escape unimpeded. Ideally, at this angle, all reflected light is completely polarized. Of course, depending on the polarization, some of this light also escapes, but this escaped light can simply be considered a negligible loss. Due to the compact design, the PD signal is more than strong enough.

[0019] The substrate of the semiconductor device can be a GaAs (gallium arsenide) substrate. An advantage of this design is that GaAs has a relatively high refractive index of nsubstrat = 3.5. This means that the Brewster angle, for example, at the interface with air (nuft = 1), occurs at smaller angles. As a result, the VCSEL and the first photodetector can be positioned close together, enabling a particularly compact design.

[0020] The VCSEL and the first photodetector can have at least partially identical layer structures. In other words, the VCSEL and the first photodetector of the semiconductor device can share one or more layers. Advantageously, the VCSEL and the first photodetector can therefore not only be integrated on a common substrate but also partially utilize the same epitaxial layer structure. This simplifies manufacturing, for example, making it possible to produce the device in a single production process. This can simplify the design and / or reduce production costs.

[0021] The VCSEL and the first photodetector can be positioned laterally offset on the same side of the substrate. A trench etch, particularly a deep trench etch extending to the substrate, can be provided between the VCSEL and the first photodetector. An advantage is that the VCSEL and the first photodetector can be optically and / or electrically isolated from each other. Optionally, the trench can be filled by depositing an opaque and / or insulating material, such as black lacquer. In particular, electrical and / or optical crosstalk can be reduced. This can further improve polarization selectivity.

[0022] The semiconductor device can further include a second photodetector configured to detect laser light emitted by the VCSEL. In particular, the semiconductor device can be configured such that the second photodetector detects laser light emitted by the VCSEL with a different polarization selection than the first photodetector. This configuration allows a comparison of a detection signal from the first photodetector, to which laser light of the first polarization emitted by the VCSEL is reflected in a polarization-selective manner, and a detection signal from the second photodetector, which detects laser light emitted by the VCSEL with a different polarization selection than the first photodetector. For example, the semiconductor device can be configured such that the second photodetector outputs a detection signal proportional to the total power of the laser light emitted by the VCSEL.Alternatively or additionally, the semiconductor device can also include a second polarization-selective reflector, which has a different polarization selectivity than the (first) polarization-selective reflector and is configured to reflect laser light emitted by the VCSEL with a second polarization that differs at least partially from the first polarization to the second photodetector in a polarization-selective manner. The ratio of the photocurrents of the first and second photodetectors allows the polarization degree of the laser light emitted by the VCSEL at any given time to be determined.

[0023] The VCSEL and the second photodetector can share at least one or more contacts. For example, one contact can be used both as a contact for the VCSEL and as a contact for the second photodetector, which is configured and arranged to selectively receive laser light of a different polarization emitted by the VCSEL. This simplifies and / or reduces the cost of connecting the semiconductor device. Furthermore, it allows for a particularly simple and compact design.

[0024] The second photodetector can be arranged on an optical axis of the VCSEL. The second photodetector can be configured to detect a signal. The second photodetector can be configured to output a detection signal proportional to the total power of the laser light emitted by the VCSEL. In particular, the second photodetector can be located within or adjacent to a layer sequence of the VCSEL. For example, the photodetector can be at least partially integrated into or adjacent to a layer sequence of an upper or lower DBR of the VCSEL. The second photodetector can be located between the VCSEL and the substrate and configured to output a detection signal proportional to the total power of the laser light emitted by the VCSEL. In particular, the second photodetector can be located on an optical axis between the VCSEL and the substrate. The semiconductor device can be configured such that the second photodetector outputs a detection signal proportional to the total power of the laser light emitted by the VCSEL.The polarization degree can be determined from the ratio of the photocurrents of the first and second photodetectors. For example, a layer sequence "VCSEL / second photodetector / substrate" can be provided. The second photodetector can therefore be arranged between the VCSEL and the substrate. For the first photodetector, a layer sequence "first photodetector / substrate" can be provided. In particular, in this case, an at least partially identical epitaxial layer sequence can be provided for the first and second photodetectors on the substrate. The same epitaxial layer sequence as for the VCSEL can also be provided above the first photodetector, even if it is not actively used to generate light. In the semiconductor device, one or more unused epitaxial layers, which are used to form the VCSEL, can remain in the region of the first photodetector.This can further simplify the manufacturing process.

[0025] In a random number generation system, the controller can be configured to provide a sequence of random numbers based on the output signal of the first photodetector and an output signal of the second photodetector.

[0026] The advantages described above in detail for the first aspect of the invention apply accordingly to the other aspects of the invention.

[0027] It is understood that the aforementioned features and those to be explained below apply not only in the combinations specified, but also in can be used in other combinations or on their own without leaving the scope of the present invention.

[0028] Exemplary embodiments of the invention are shown in the following drawings and are explained in more detail in the following description. Fig. 1 shows a schematic representation of a first exemplary semiconductor device for generating random numbers; Fig. 2A / B shows diagrams of a reflectance for the two polarization directions s and p as a function of the angle at the transition of media with different refractive indices; Fig. 3 shows a schematic representation of a second exemplary semiconductor device for random number generation; Fig. 4 shows a schematic representation of a third exemplary semiconductor device for random number generation; Fig. 5 shows a schematic representation of a system for generating random numbers; Fig. 6 shows a flowchart of a random number generation process.

[0029] Fig. 1 shows a schematic representation of a first exemplary semiconductor device 10 for random number generation. The semiconductor device 10 for random number generation comprises a substrate 20, a VCSEL 30, and a first photodetector 40. The VCSEL 30 and the photodetector 40 are both arranged on the same substrate 20. The semiconductor device 10 further comprises a polarization-selective reflector 50, which is configured to reflect laser light 60 of a first polarization 61 emitted by the VCSEL 30 to the first photodetector 40 in a polarization-selective manner.

[0030] As described earlier, the polarization state of a VCSEL can flip by 90° depending on the operating current. This effect is particularly pronounced in a single-mode VCSEL. If the emitted radiation is passed through a polarizer and measured downstream with a photodetector, such as a photodiode, the resulting signal will vary in strength depending on the polarization. This effect is governed by quantum physics and is therefore purely stochastic. The output signal can be compared to a threshold value. For example, an output signal below the threshold can be a 0, and one above the threshold a 1. Intermittent operation of the VCSEL can thus generate a stochastic sequence of 0s and 1s, resulting in a true random number. Operation is possible with pulse lengths of a few tens of nanoseconds, making it possible to generate random numbers with frequencies of many MHz.

[0031] However, it is also possible to use a VCSEL that exhibits bistable behavior. At a certain current threshold, the polarization in the VCSEL can become bistable. Below this current threshold, there is usually one polarization direction, and above it, the direction orthogonal to it. This value can be called the "polarization switching point." If the VCSEL is operated at the "polarization switching point," polarization flips can indeed occur, or even small disturbances can cause such flips. If the VCSEL is intermittently switched between a current below the threshold (or 0) and a current above the "polarization switching point," then one of the two polarization states can prevail on a short timescale, but over longer periods, the state favored at correspondingly high currents usually prevails.A system for operating the VCSEL in the semiconductor device can therefore be configured either to target the "polarization switching point" as the operating point or to use correspondingly short pulses.

[0032] As shown in Fig. 1, an optical path for the laser light 60 of the first polarization 61 from the VCSEL 30 to the first photodetector 40 is located within the substrate 20. The VCSEL 30 is implemented as a back-side emitter, which is configured to emit the laser light 60 towards the substrate 20. Therefore, for beam guidance from the VCSEL 30 to the first photodetector 40, it is preferably not necessary to emit laser light 60 of the first polarization 61 from the semiconductor device 30. to decouple, manipulate externally, and recouple. This can significantly simplify the handling and setup of a system with the random number generation device.

[0033] In the embodiment shown in Fig. 1, the VCSEL 30 and the first photodetector 40 are arranged separately and laterally offset from each other on the same side of the substrate, here referred to as the first side 21. The polarization-selective reflector 50 is arranged on a second side 22 of the substrate, which is opposite the first side 21.

[0034] In the embodiment shown in Fig. 1, the polarization-selective reflector 50 is provided by an inclined surface 51 on a rear side of the substrate, here referred to as the second side 22, particularly at a transition from an optically denser to an optically less dense medium, such as a transition from the semiconductor device to air. The back reflection thus advantageously does not occur perpendicularly, but at an angle, for example at an inclined surface on the rear side 22 of the substrate 20.

[0035] Reflection at an angle when light passes from an optically denser to a less dense medium is polarization-dependent. This is illustrated by way of example in Fig. 2A and Fig. 2B. The x-axis shows the angle of incidence at the interface, where an angle of 0° denotes a perpendicular angle of incidence. The y-axis indicates the reflectance in percent, where a reflectance of 100% denotes total internal reflection and a reflectance of 0% indicates that the light can pass through virtually unimpeded. Two curves are shown as examples: one for completely s-polarized light (52) and one for completely p-polarized light (53).

[0036] Fig. 2A shows a transition from an optically less dense medium with refractive index ni = 1.0 to an optically denser medium with refractive index ni = 2.0. Fig. 2B shows a transition from an optically denser medium with refractive index ni = 2.0 to an optically less dense medium with refractive index m = 1.0. The effect of polarization-dependent reflection is particularly strong at the so-called Brewster angle 54°. where one polarization direction 52 is not reflected at all, but exits unimpeded. At this angle 54, all reflected light is completely polarized. Of course, some of this light also exits, but the exiting light can simply be considered a negligible loss. The exiting light is labeled 62 in Fig. 1. Due to the compact design, the detection signal of the first photodetector 40 remains more than sufficiently strong.

[0037] It should be noted that the representations in Fig. 2A and Fig. 2B were calculated for illustrative purposes only, using a refractive index of n2=2. However, the substrate of the semiconductor device may be a material with a higher refractive index. For example, a GaAs (gallium arsenide) substrate with a refractive index of n2=n may be used. S substrate = 3.5. In this case, the effect is even more pronounced, and the Brewster angle 54 is smaller. This allows for a very compact design, as the VCSEL 30 and the first photodetector 40 can be positioned close together.

[0038] Referring to Fig. 1, the polarization-selective reflector 50 can be provided by an inclined surface 51 on the back side 22 of the substrate 20. In particular, the inclined surface 51 can be provided by structuring the substrate 20. Methods known from semiconductor manufacturing can be used for this purpose. For example, the inclined surface 51 can be etched into the substrate. The angle of the inclined surface 51 relative to the laser light 60 emitted by the VCSEL 30 is preferably arranged such that the laser light 60 strikes the inclined surface 51 of the polarization-selective reflector 50 at the Brewster angle (see 54 in Fig. 2B). In this case, a laterally arranged photodetector 40 measures only one polarization state of the VCSEL 30, see the portion of curve 51 for s-polarized light at the Brewster angle. This portion is shown in Fig. 1 by the beam path with reference numeral 61.In contrast, p-polarized light incident at Brewster angle 54 is not reflected but completely transmitted. The proportions of transmitted radiation of both polarizations are shown in Fig. 1 by the beam path with reference numeral 62.

[0039] It is understood that this is an idealized representation. In practical implementations, certain angular deviations may occur, not least due to a divergence angle of the laser radiation 60 emitted by the VCSEL 30. For example, a tolerance range of + / - 10°, and in particular + / - 5°, may apply to light incident on the interface at the Brewster angle. Even with certain deviations in the angle of the inclined surface 51 of the polarization-selective reflector 50, a polarization-dependent and thus polarization-selective reflection behavior is still present, which enables the evaluation of an output signal from the first photodetector 40 for random number generation. As shown in Fig. 2B, light incident on the interface at an angle above the critical angle 55 is completely reflected. This is represented in Fig. 2B as the region of total internal reflection 56.

[0040] As shown in Fig. 1, the VCSEL 30 and the first photodetector 40 can have at least partially identical layer structures. Furthermore, if the VCSEL 30 and the first photodetector 40 are arranged laterally offset on the same side of the substrate, a trench etch 23 can be provided between the VCSEL 30 and the first photodetector 40, in particular a deep trench etch that extends down to the substrate 20.

[0041] To illustrate this, various concrete examples are described below.

[0042] In the embodiment shown in Fig. 1, a layer sequence can be epitaxially deposited on an n-doped GaAs substrate 20. Here, a photodiode, i.e., a typically 0.5–5 pm thick undoped GaAs region, can be deposited first as a photodetector 40, followed by a doped contact layer 42, which should be thick enough to be exposed in a subsequent etching (e.g., 0.2–0.8 pm). As shown in Fig. 1, this layer sequence 41, 42 can preferably also be provided in the region of the VCSEL 30. By having the VCSEL and the first photodetector at least partially have an identical layer structure, the semiconductor device can be processed efficiently and manufactured cost-effectively.

[0043] A back-emitting VCSEL layer sequence can then be grown, from which the VCSEL 30 can be formed. For example, a DBR 31 with approximately 99% Rf, i.e., approximately 20 DBR pairs, an active layer 32, and a DBR 33 with 99.9% Rf, i.e., approximately 40 DBR pairs. The active layer 32 contains, for example, one or more MQW layers. Furthermore, an oxide blend 35 can be provided to restrict the current path. In the epitaxial processing, a mesa for the VCSEL 30 can be defined, the oxide blend 35 oxidized, and the plane of the contact layer 42 exposed and contacted. A deep trench etch 23 is then performed between the VCSEL 30 and the first laterally arranged photodetector 40, down to the substrate. This optically and electrically isolates the VCSEL 30 and the first photodiode 40 from each other. On the back of the substrate 22, an inclined surface 51 or inclined, prismatic structure is etched in GaAs.Preferably at an angle such that the main beam 60, 61 shown strikes the first photodetector. This angle should preferably be close to the Brewster angle (see 54 in Fig. 2B) (approx. 16° in GaAs). With a substrate thickness of 200 pm, this results in a lateral offset of the first photodetector 40 of approximately 150 pm, which is perfectly adequate for separation but still allows for a compact semiconductor device 10.

[0044] An advantage of the setup shown in Fig. 1 is that the existing layer sequence 41, 42 of the first photodetector 40 can also be used for one or more additional photodetectors. The semiconductor device 10 can further comprise a second photodetector 40' configured to detect laser light emitted by the VCSEL 30. In particular, the semiconductor device 10 can be configured such that the second photodetector 40' detects laser light 60 emitted by the VCSEL 30 with a different polarization selection than the first photodetector 40. In the embodiment shown in Fig. 1, the second photodetector 40' is arranged on an optical axis between the VCSEL 30 and the substrate 20 and is configured to output a detection signal proportional to the total power of the laser light 60 emitted by the VCSEL 30.

[0045] Figure 1 further shows an advantageous arrangement of contacts 71, 72, 73, 73. The VCSEL 30 and the second photodetector 40' can have at least one or more common contacts. The laser or VCSEL 30 itself can be connected via the Contacts 71 and 72 are energized. The second photodetector 40' can be operated in reverse bias via contacts 72 and 73, and the first photodetector 40 can likewise be operated via contacts 74 and 73. Therefore, it may be sufficient for the semiconductor device 10 to have only four contacts for the intended functionality. However, the disclosure is not limited to a specific configuration of the contacts.

[0046] In the example shown in Fig. 1, contact 73 is depicted as a backside contact. However, it is also possible for all contacts to be located on one side, for example, on the epitaxial side or first side 21 of the substrate. For instance, by adding a contact layer between the substrate 20 and the photodetectors 40, 40' and contacting them from the epitaxial side, all contacts can be located on one side. The contacts can be connected, for example, via bond pads and wire bonds or, alternatively, via copper pillars with solder deposits. It is also possible, for example, to glue the backside contact 73, although the prismatic area with the inclined surface 51 should be kept free of adhesive. It is understood that various contact configurations and the use of suitable metals for the contacts are possible.It is understood that the contacts can be designed as p-type or n-type contacts, and the adjacent semiconductor layers can be doped accordingly. For example, the use of a parasitic diode is also possible, as described in US 8,467,428 B2, or the use of a tunnel diode, as described in WO 2023 / 031058 A1, the disclosures of which are incorporated by reference. In a further development of the embodiment shown in Fig. 1, a tunnel diode can, for example, be arranged above, and in particular directly above, the active layer 32, as shown in Fig. 1, for example in an adjacent node of the standing wave field. In this case, contacts 1, 2, and 4 would be p-type contacts, and 3 would be an n-type contact. The well-known good contact properties of p-type contacts are advantageous, even with a spatially limited contact area.

[0047] In the example shown in Fig. 1, the second photodetector 40', which is arranged between the VCSEL 30 and the substrate 20, can provide an output signal that is proportional to the total emitted laser power. The output signal can be a The signal can be the current supplied by a reverse-biased voltage or a signal derived therefrom. The first photodetector 40 provides an output signal that ideally would correspond to the light in one polarization direction. However, since the diagram is idealized in such a way that only one principal beam is drawn, for which an exact Brewster condition is assumed to apply, but in reality a divergent beam of light (with only about 3° half-angle in the GaAs) would be present, it is understood that the signal of the first photodetector will also contain components of the other polarization. Nevertheless, the change in the relative signal between the second photodetector 40' and the first photodetector 40 can be clearly assigned to the polarization and can thus be used to generate the stochastic random numbers or zeros and ones. A relationship to the signal of the second photodetector can therefore be established, for example, by calibration or...The measurement of relative quantities is made possible.

[0048] As explained above, the semiconductor device can have at least partially identical layer structures in the region of the VCSEL 30 and in the region of the first photodetector 40. In the representation shown in Fig. 1, a layer sequence for forming the VCSEL 30 above the first photodetector 40 has been completely removed. However, it is also possible that only a partial region is exposed, for example, the region in which the contact 74 is created. This is illustrated by way of example in the embodiment shown in Fig. 3. The remaining layer stack, in principle another VCSEL 30', above the first photodetector 40 can remain unused without affecting the operation of the first photodetector.

[0049] The present disclosure is not limited to a layer sequence in which the photodetector(s) 40, 40' are arranged between the VCSEL 30 and the substrate 20. In particular, the second photodetector 40' can be arranged within or adjacent to a layer sequence of the VCSEL 30. For example, the photodetector 40' can be integrated, at least partially, into or adjacent to a layer sequence of an upper or lower DBR (distributed Bragg reflector) of the VCSEL. A VCSEL with an integrated photodetector is described, for example, in WO 2023 / 031058 A1, the entire disclosure of which is incorporated by reference. In other words, a layer sequence of the VCSEL 30 can also be located, at least partially, between the second photodetector 40' and the substrate 30. The layer sequence for a VCSEL can also be arranged in the first photodetector 40, even if the layer sequence in the first photodetector is not used for light emission. This can simplify the assembly and reduce manufacturing costs. One advantage of this solution is that wafers with such layer sequences can be used for other products in addition to manufacturing a semiconductor device for random number generation.

[0050] In the description of further embodiments, the same reference numerals are used for identical or similar elements as in Fig. 1, avoiding repetition. Particular emphasis is placed on highlighting possible alternatives and further developments.

[0051] Fig. 3 shows a schematic representation of a second exemplary semiconductor device 10 for random number generation. The polarization-selective reflector 50 is a polarization-selective optical metastructure 57, which is configured to reflect laser light 60 of a first polarization 61 emitted by the VCSEL 30 to the first photodetector 40 in a polarization-selective manner. The polarization-selective optical metastructure 57 can, for example, be fabricated directly by structuring the substrate 20, in particular by structuring the second side 22. Alternatively, the second side 22, also referred to as the back side, can be coated with a layer 24, e.g., with silicon nitride (SiN). The metastructure 57 can then be incorporated into this (dielectric) layer 24 in the substrate 20 by lithography and etching.Layer stacks consisting of several dielectric layers can also be used advantageously, whereby the metastructure 57 is introduced, for example, only in one of the layers and further layers can function as an etch stop layer.

[0052] Fig. 4 shows a schematic representation of another exemplary semiconductor device 10 for random number generation. In this embodiment, the semiconductor device 10 has, in addition to the first photodetector 40, a further photodetector 40", which is arranged on the same substrate as the VCSEL. The semiconductor device 10 has a polarization-selective reflector 50, which is configured to reflect laser light 60 of a first polarization 61 emitted by the VCSEL 30 to the first photodetector 40 in a polarization-selective manner. The semiconductor device 10 has Furthermore, a further polarization-selective reflector 50' is provided, which is configured to reflect laser light 60 emitted by the VCSEL 30 of a further polarization 63 in a polarization-selective manner to the further photodetector 40". The further photodetector 40" can also be referred to as the second photodetector, which is configured to detect laser light 60 emitted by the VCSEL, wherein the semiconductor device 10 is configured such that the further or second photodetector 40" detects laser light 60 emitted by the VCSEL with a different polarization selection 63 than the first photodetector 40. The photodetector 40" can be provided in addition to or as an alternative to the photodetector arranged between the VCSEL 30 and the substrate 20.

[0053] The first photodetector 40 and the second photodetector 40" can be arranged at different distances from an optical axis of the VCSEL 30. The polarization-selective reflectors 50, 50' could, for example, be designed as prismatic structures on the back side 22 of the substrate, wherein a portion, in particular one half, of the laser light 60 emitted by the VCSEL 30 is reflected polarization-selectively onto the first photodetector 40 at an angle below or near the Brewster angle, and the other portion, the other half, is reflected polarization-selectively onto the second photodetector 40" at a different, in particular significantly smaller, angle. According to the dependence of the reflection on angle and polarization shown in Fig. 2B, the first photodetector 40 essentially sees only one polarization, while the second photodetector 40" sees a mixture of both.

[0054] A relative signal can now be obtained by comparing the signals of the first photodetector 40 and the second photodetector 40". The photodetector 40' between the VCSEL 30 and the substrate is optional, but is essentially unnecessary here and does not necessarily need to be energized. Therefore, it is not required to apply a reverse voltage between contacts 72 and 73. This allows the contact layer 42 to be split into two different layers, with the layer closer to the substrate being, for example, p-type and the layer further away from the substrate or closer to the VCSEL being n-type. This eliminates any requirements for a tunnel diode. With a contact layer split into p- and n-type doping, only the upper, n-type layer can be contacted. This corresponds to the approach with a parasitic diode, as described, for example, in US 8,467,428 B2. described and their disclosure is included by reference. This can apply to all embodiments.

[0055] The proposed semiconductor device 10, according to aspects of the present disclosure, comprising the VCSEL 30 and the photodetector 40, can be connected to one or more driver circuits in accordance with the prior art, and signal evaluation can be performed which provides a sequence of one or more random numbers as an output signal. In other words, a semiconductor device for random number generation within the scope of the present disclosure is also to be understood as a semiconductor device which is configured to provide an output signal that can serve as the basis for random number generation and may require subsequent signal conditioning as well as, if necessary, further control elements such as a driver circuit or amplifier circuit.

[0056] Fig. 5 shows a schematic representation of a system 80 for generating random numbers. The system 80 comprises a semiconductor device 10 for generating random numbers, as described by way of example in the present disclosure. The system 80 further comprises a controller 90, which is configured to provide a sequence 81 of random numbers based on an output signal from the first photodetector. In the embodiment shown in Fig. 5, the controller 90 is configured to provide a sequence of random numbers based on the output signal of the first photodetector and an output signal from the second (or further) photodetector.

[0057] The controller 90 can, for example, be connected to the semiconductor device 10 via contacts 71, 72, 73, 74, as shown in Fig. 5. Reference is made here, by way of example, to the semiconductor device 10 from Fig. 1 or Fig. 3 and the contacts and associated elements shown therein. A driver circuit 91 can be provided which intermittently supplies the VCSEL connected via contacts 71, 72 with a current specified by a control unit 92. The two photodetectors connected between contacts 72 and 73 and between contacts 73 and 74 can, for example, be operated in reverse bias. A respective photocurrent can be read out, for example, via the transimpedance amplifiers 93, 94 (TIAs). From the ratio of the two photocurrents The controller 90, together with the control unit 92, can determine the polarization degree of the laser light emitted by the VCSEL of the semiconductor device 10. The controller 90 can be configured to select an operating current value corresponding to a polarization instability, particularly one that results in as many flips as possible. Optionally, the controller 90 can be configured to determine this at various current values, for example, after power-up or repeatedly during operation, since such an instability point can depend on ambient temperature, aging, etc. The controller 90 can be configured to provide a stochastic sequence of numbers as output, corresponding to true random numbers.

[0058] Fig. 6 shows an exemplary flowchart of a method 100 for random number generation. In a first step S101, a semiconductor device for random number generation is provided, as described in the present disclosure. In a second step S102, the VCSEL is operated such that laser light with different polarization states is emitted randomly. In a third step S103, the laser light of a first polarization emitted by the VCSEL, which is reflected polarization-selectively by the polarization-selective reflector to the first photodetector, is detected by the first photodetector. In a fourth step S104, a sequence of random numbers is provided based on an output signal of the first photodetector. Optionally, a sequence of random numbers is provided based on the output signal of the first photodetector and an output signal of a second or further photodetector.

[0059] In summary, aspects of the solution proposed herein can provide a device and system for generating random numbers, enabling the generation of truly random numbers even in compact, cost-effective end devices.

Claims

Patent claims 1. Semiconductor device (10) for random number generation comprising a substrate (20); a VCSEL (30) arranged on the substrate (20); a first photodetector (40) arranged on the same substrate (20) as the VCSEL (30); wherein the semiconductor device (10) comprises a polarization-selective reflector (50) configured to reflect laser light (60) of a first polarization (61) emitted by the VCSEL (30) in a polarization-selective manner to the first photodetector (40).

2. Semiconductor device according to claim 1, wherein an optical path for the laser light of the first polarization (61) from the VCSEL (30) to the first photodetector (40) is provided within the substrate (20).

3. Semiconductor device according to any of the preceding claims, wherein the VCSEL (30) is a backside emitter configured to emit the laser light (60) in the direction of the substrate (20).

4. Semiconductor device according to one of the preceding claims, wherein the VCSEL (30) and the first photodetector (40) are arranged on a first side (21) of the substrate (20) and the polarization-selective reflector (50) is arranged on a second side (22) of the substrate (20), which is opposite the first side (21).

5. Semiconductor device according to one of the preceding claims, wherein the polarization-selective reflector (50) is provided by an inclined surface (51) on a back side (22) of the substrate (20), in particular at a transition from an optically denser to an optically less dense medium.

6. Semiconductor device according to claim 5, wherein the beveled surface (51) is provided by structuring the substrate (20), in particular wherein the beveled surface (51) is etched into the substrate (50).

7. Semiconductor device according to claim 5 or 6, wherein an angle of the inclined surface (51) relative to the laser light (60) emitted by the VCSEL is arranged such that the laser light strikes the inclined surface (51) of the polarization-selective reflector (50) at the Brewster angle (54).

8. Semiconductor device according to one of the preceding claims, wherein the VCSEL (30) and the first photodetector (40) have at least partially identical layer structures.

9. Semiconductor device according to one of the preceding claims, wherein the VCSEL (30) and the first photodetector (40) are arranged laterally offset on the same side (21) of the substrate (20) and a trench etch (23) is provided between the VCSEL (30) and the first photodetector (40), in particular a deep trench etch which extends to the substrate (20).

10. Semiconductor device according to one of the preceding claims, wherein the semiconductor device (10) further comprises a second photodetector (40', 40") configured to detect laser light emitted by the VCSEL (30), wherein the semiconductor device (10) is configured such that the second photodetector (40', 40") detects laser light emitted by the VCSEL (30) with a different polarization selection than the first photodetector (40).

11. Semiconductor device according to claim 10, wherein the VCSEL (30) and the second photodetector (40') have at least one or more common contacts (72).

12. Semiconductor device according to claim 10 or 11, wherein the second photodetector (40') is arranged on an optical axis of the VCSEL (30) and is further equipped with is configured to output a detection signal proportional to the total power of the laser light (60) emitted by the VCSEL, in particular wherein the second photodetector (40') is arranged within or adjacent to a layer sequence of the VCSEL (30), in particular wherein the second photodetector (40') is arranged between the VCSEL (30) and the substrate (20).

13. System (80) for generating random numbers comprising a semiconductor device (10) according to one of the preceding claims; and a controller (90) configured to provide a sequence (91) of random numbers based on an output signal from the first photodetector (40).

14. System according to claim 13 with a semiconductor device according to one of claims 10 to 12, wherein the controller (90) is configured to provide a sequence (91) of random numbers based on the output signal of the first photodetector (40) and an output signal of the second photodetector (40', 40").

15. Method (100) for generating random numbers with the following steps: Providing a semiconductor device according to one of the preceding claims (S101); Operating the VCSEL such that laser light with different polarization states is emitted randomly (S102); Detecting the laser light emitted by the VCSEL with the first photodetector (S103); and Providing a sequence of random numbers based on an output signal from the first photodetector (S104).

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