Snspd having integrated aluminum nitride seed or waveguide layer

Abstract

To provide a superconducting nanowire single-photon detector (SNSPD) device containing a seed layer under a metal nitride layer that deposits δ-phase niobium nitride (NbN) with satisfactory quality.SOLUTION: An SNSPD device 150 includes a substrate 62 having a top surface, an optical waveguide 66 on the top surface of the substrate for receiving light propagating substantially parallel to the top surface of the substrate, a metal nitride seed layer 152 on the optical waveguide, and a superconducting wire 52 on the top surface of the seed layer. The superconducting wire is a different metal nitride (δ-phase NbN) from the metal nitride of the seed layer and is optically coupled to the optical waveguide.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] The present disclosure relates to superconducting nanowire single photon detectors (SNSPDs) that include a seed layer beneath a metal nitride that provides the superconducting material. [Background technology]

[0002] In the context of superconductivity, critical temperature (TC) refers to the temperature at which a material becomes superconducting. Niobium nitride (NbN) is a material that can be used in superconducting applications such as superconducting nanowire single-photon detectors (SNSPDs) for use in quantum information processing, CMOS defect analysis, and LIDAR. The critical temperature of niobium nitride depends on the crystalline structure and atomic ratio of the material. For example, referring to Figure 1, cubic δ-phase NbN has several advantages due to its relatively "high" critical temperature, e.g., 9.7-16.5°K.

[0003] Niobium nitride can be deposited on a workpiece by physical vapor deposition (PVD). For example, a sputtering operation can be performed using a niobium target in the presence of nitrogen gas. Sputtering can be performed by inducing a plasma in a reactor chamber containing the target and workpiece. Summary of the Invention

[0004] In one aspect, a superconducting nanowire single photon detector (SNSPD) device includes a substrate having a top surface, an optical waveguide on the top surface of the substrate for receiving light propagating substantially parallel to the top surface of the substrate, a metal nitride seed layer on the optical waveguide, and a superconducting wire on the seed layer, the superconducting wire being a metal nitride different from the metal nitride of the seed layer and optically coupled to the optical waveguide.

[0005] In another aspect, a superconducting nanowire single-photon detector (SNSPD) device includes a substrate having a top surface, a metal nitride optical waveguide on the top surface of the substrate that accepts light propagating substantially parallel to the top surface of the substrate, and a superconducting wire on a seed layer. The superconducting wire is a metal nitride selected from the group consisting of niobium nitride, titanium nitride, and niobium titanium nitride. The metal nitride of the optical waveguide is different from the metal nitride of the superconducting wire.

[0006] Implementations may provide one or more of the following advantages, including, but not limited to: For example, devices based on the absorption of photons by superconducting materials, such as SNSPDs, can have high photon absorption efficiency while achieving high material quality for the superconducting layer, such as niobium nitride, and thus a higher critical temperature. This allows for the fabrication of devices, such as SNSPDs, using superconducting wires with higher critical temperatures. The larger the difference between the operating temperature (2-3°K) and the critical temperature, the better the detection efficiency, lower the dark count, and potentially a faster time response.

[0007] It should be noted that "superconducting" indicates that the material becomes superconducting at the operating temperature of the device, e.g., 2-3°K. The material is not actually superconducting during device fabrication above room temperature or when the device is not cooled for operation.

[0008] The details of one or more implementations are set forth in the accompanying drawings and the description below. Other potential aspects, features, and advantages will become apparent from the description, drawings, and claims. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 illustrates the phases of niobium nitride as a function of processing temperature and atomic percentage of nitrogen. [Figure 2A] FIG. 1 is a schematic top view of an SNSPD including a distributed Bragg reflector. [Figure 2B] 2B is a schematic cross-sectional side view of the device of FIG. 2A. [Figure 3]FIG. 1 is a schematic diagram of the operation of an SNSPD. [Figure 4] FIG. 1 is a schematic cross-sectional side view of an SNSPD including a distributed Bragg reflector and an aluminum nitride seed layer. [Figure 5] 1 is a graph of reflectivity as a function of wavelength for two SNSPD designs. [Figure 6] 1 is a graph of critical temperature as a function of NbN layer thickness with and without an aluminum nitride seed layer. [Figure 7] 1 is a flowchart of a method for fabricating an SNSPD. [Figure 8A] FIG. 1 is a schematic top view of an SNSPD including a waveguide. [Figure 8B] 8B is a schematic cross-sectional side view of the device of FIG. 8A. [Figure 9] FIG. 1 is a schematic cross-sectional side view of an SNSPD including a waveguide and an aluminum nitride seed layer. [Figure 10] FIG. 1 is a schematic cross-sectional side view of an SNSPD including a waveguide formed of aluminum nitride. DETAILED DESCRIPTION OF THE INVENTION

[0010] Like reference numbers and designations in the various drawings indicate like elements.

[0011] 2A and 2B show top and side views, respectively, of a conventional superconducting nanowire single-photon detector (SNSPD) device 10. The SNSPD device 10 can include at least one superconducting wire 12 disposed on a support structure 20. The superconducting wire 12 can be connected between conductive electrodes 14. The superconducting wire 12 can be arranged on the support structure 20 in a serpentine pattern, e.g., a front-to-back parallel line. In some implementations, multiple wires 12 are connected in parallel between the electrodes 14, with each wire 12 covering a separate area 16, although there may be only a single wire 12 that covers the entire detection area of ​​the device 10. Additionally, many other patterns are possible, e.g., a zigzag or double helix. The superconducting wire can be considered a nanowire and can have, for example, a width of about 30 nm and a thickness of about 10 nm.

[0012] The support structure 20 may include a substrate 22, e.g., a silicon substrate, and a mirror structure 24 disposed on the substrate 22. By way of example, the mirror structure 24 may be a distributed Bragg reflector (DBR) that includes multiple pairs of layers formed of high and low refractive index materials.

[0013] Conventional SNSPDs operate with photons (indicated by light beam 30) approaching from the top of device 10, e.g., perpendicular to substrate 20. A simple device is one in which NbN nanowires are placed directly on a silicon substrate (without a mirror structure). Because the NbN nanowires are typically very thin in SNSPD devices, most light is not absorbed by the NbN nanowires. To increase the light absorption efficiency, a mirror structure 24, e.g., a distributed Bragg reflector, is incorporated into device 10 between substrate 20 and wire 12. In this case, incident photons that are not initially absorbed can be absorbed upon reflection, increasing the chance that the photon will be captured by the NbN nanowire.

[0014] Referring to Figure 3, the operating principle of an SNSPD device is that photons to be detected come from above and illuminate the SNPSD. Absorption of the photons creates a hot spot on the NbN nanowire, raising the temperature of the NbN above the critical temperature and causing portions of the wire to cease to be superconducting. Current crowding can occur in the area around the hot spot, resulting in current densities higher than the critical current density and potentially destroying the superconducting state of the entire wire. The change of the NbN wire from the superconducting state to the normal resistive state can be detected electrically by passing a current through the device and monitoring the voltage difference between the electrodes.

[0015] NbN-based SNSPDs are primarily used in time-correlated single-photon counting (TCPSC)-related applications at visible and infrared wavelengths. For example, SNSPDs are used in quantum metrology (quantum key generation, quantum emitters) and optical quantum computing (detection modules) due to their high efficiency, low dark counts, low timing jitter, and fast recovery time. They can also be used as detectors in conventional space-to-ground communications and time-of-flight LIDAR systems.

[0016] In the visible wavelength range, silicon avalanche photodiodes (APDs) are typically used. The detection efficiency of these systems is not ideal, e.g., about 70%, and it is difficult to integrate these devices into chip-scale devices.

[0017] In the infrared wavelength range, InGaAs APDs are candidates for many applications. However, these devices typically suffer from high dark count rates and even lower system detection efficiencies (<30%), limiting their detection speed. Compared to APDs, SNSPDs offer superior performance, including low timing jitter (<20 ps), fast recovery times, high detection efficiencies (>85%), and low dark count rates (approximately a few Hz).

[0018] As mentioned above, niobium nitride, especially δ-phase NbN, has several advantages as a superconducting material. However, depositing δ-phase NbN with satisfactory quality can be difficult. A seed layer, such as an aluminum nitride (AlN) layer, beneath the (super)conducting layer can help improve the critical temperature of the NbN layer. An aluminum nitride (AlN) seed layer can also improve the critical temperature of TiN and NbTiN layers and may be useful for other metal nitride layers. In particular, aluminum nitride layers can be incorporated into SNSPD devices, particularly into the mirror structure or waveguide of an SNSPD device. This allows for higher crystal quality metal nitride detectors (and therefore higher critical temperatures and therefore better device performance) while also achieving high optical absorption efficiency.

[0019] Figure 4 shows a cross-sectional side view of a superconducting nanowire single-photon detector (SNSPD) device 100. The SNSPD device 100 may be similar to the device described above with respect to Figures 2A and 2B, except as described below.

[0020] The SNSPD includes a substrate 22, which can be a dielectric material, such as sapphire, SiO 2 , fused silica or quartz, or a semiconductor material, such as silicon, gallium nitride (GaN) or gallium arsenide (GaAs).

[0021] A distributed Bragg reflector (DBR) 24 is fabricated on a substrate 22. The DBR 24 includes multiple bilayers 26, e.g., two to eight bilayers, e.g., seven bilayers. Each bilayer 26 includes a bottom layer 26a of a first material having a first refractive index ("low index") and a top layer 26b of a second material having a second refractive index ("high index") greater than the first refractive index. The thickness and material (and therefore the refractive index) of the bilayers 26 are selected to increase reflection at a selected wavelength or wavelength band. For example, because 1550 nm is a wavelength widely used in optical communication systems, the DBR may be optimized for reflection of light between approximately 1500 and 1600 nm.

[0022] Both the first and second materials may be selected from Table 1, subject to the restriction that the second material has a higher refractive index than the first material.

[0023] TIFF2025131602000002.tif57170

[0024] Covering, e.g., in direct contact with, the upper layer 26b of the top bilayer 26 of the distributed Bragg reflector is a metal nitride seed layer 102. The seed layer 102 and the superconducting wire 12 are nitrides of different metals. In particular, the seed layer 102 can be aluminum nitride (AlN), which improves the critical temperature of NbN. However, hafnium nitride (HfN) and gallium nitride (GaN) may also be suitable. The metal nitride seed layer 102 can have a thickness of about 4 to 50 nm, e.g., about 5 nm, about 10 nm, or about 20 nm. The seed layer 102 can have a (002) c-axis crystal orientation. The seed layer 102 is not superconducting at the operating temperature of the device 100. The seed layer 102 can be deposited by a standard chemical vapor deposition or physical vapor deposition process.

[0025] In some implementations, the high refractive index material of the upper layer 26b is, for example, Ta2O5 with a thickness of about 182 nm, and the low refractive index material of the lower layer 26a is, for example, SiO2 with a thickness of about 263 nm. The reflectivity simulated by optical modeling software of a stack of seven such bilayers with a 20 nm thick AlN seed layer is shown by curve 120 in FIG.

[0026] In particularly important embodiments, the seed layer 102 is aluminum nitride, and the low-index material, i.e., the material of each lower layer 26a, is also aluminum nitride. This allows the seed layer 102 to be fabricated using the same processing conditions as the lower layers of the distributed Bragg reflector 24, simplifying processing requirements. In some implementations, the high-index material is, for example, about 111 nm thick amorphous silicon (a-Si), and the low-index material is, for example, about 197 nm thick AlN. The reflectivity simulated by optical modeling software for a stack of seven such bilayers with a 20 nm thick AlN seed layer is shown by curve 122 in FIG. 5.

[0027] The superconducting wire 12 is formed on, for example, in direct contact with, the seed layer 102. The wire may be made of niobium nitride (NbN), titanium nitride (TiN), or niobium titanium nitride (Nb X Ti 1-X The wires 12 may have a width of about 25 to 250 nm, for example, about 60 nm, and a thickness of 4 to 50 nm, for example, about 5 nm, about 10 nm, or about 20 nm.

[0028] The seed layer 102 helps improve the critical temperature of aluminum nitride, especially when the aluminum nitride layer is thin. For example, FIG. 6 shows the measured critical temperature T (in Kelvin) as a function of NbN layer thickness. Curve 130 shows the critical temperature without the aluminum nitride seed layer, and curve 132 shows the critical temperature with the aluminum nitride seed layer (for a simplified stack of a silicon wafer, an AlN seed layer, and an NbN layer). Alternatively or additionally, the seed layer 102 can improve adhesion between the aluminum nitride layer 102 and the top layer 26b of the distributed Bragg reflector 24.

[0029] FIG. 7 is a flow chart of a method 200 for manufacturing the device 100 of FIG.

[0030] First, a distributed Bragg reflector (DBR) 24 is deposited on a substrate 100 (step 202). The substrate may be, for example, a silicon wafer. Although shown as a single block, the substrate 22 may include multiple underlying layers. The DBR 24 may be deposited by alternately depositing high and low refractive index materials using standard chemical or physical vapor deposition processes.

[0031] Next, a seed layer 102 is deposited on the DBR 24 (step 204). As mentioned above, the seed layer 102 can be aluminum nitride. The seed layer can be deposited using standard chemical vapor deposition or physical vapor deposition processes. Exemplary process parameters are a power applied to the sputtering target of 1-5 kW, a total pressure (nitrogen and inert gas) of 2-20 mTorr, nitrogen and inert gas supplied in a ratio of 3:100 to 1:6, a wafer temperature of 200-500°C, and no bias voltage applied to the wafer. In some implementations, the seed layer 102 is deposited in the same process chamber used to deposit the DBR 24, for example, by switching to a new target. This allows for high-throughput manufacturing. Alternatively, the substrate can be transferred to another deposition chamber without breaking vacuum. In either case, the seed layer can be deposited without exposing the DBR to the atmosphere, reducing the risk of contamination.

[0032] Next, a metal nitride layer 12, such as niobium nitride (NbN), titanium nitride (TiN), or niobium titanium nitride (Nb X Ti 1-XNbN is deposited on the seed layer (step 206). The metal nitride layer 12 can be deposited using standard chemical vapor deposition or physical vapor deposition processes. Exemplary process parameters are a base pressure of 1e-8 Torr, a power applied to the target of 1-3 kW, a total pressure during processing of 5-7 mTorr, a wafer temperature of 400°C, no bias voltage applied to the wafer, and a gas ratio of N2 sufficient to achieve cubic δ-phase NbN. In some implementations, the metal nitride layer 12 is deposited in the same process chamber used to deposit the seed layer 102, e.g., by switching to a new target. This allows for high-throughput manufacturing. Alternatively, the substrate can be transferred to another deposition chamber without breaking vacuum. This allows the metal nitride layer to be deposited without exposing the seed layer to the atmosphere and reducing the risk of contamination.

[0033] After depositing the metal nitride layer 12, a capping layer 104 can be deposited on the metal nitride layer 12 (step 208). The capping layer 104 functions, for example, as a protective layer to prevent oxidation or other types of contamination or damage to the metal nitride layer 12. The capping layer 104 can be dielectric or conductive, but is not superconducting at the operating temperatures of the device 200. The capping layer 104 can be amorphous silicon (a-Si). In some implementations, the capping layer 104 is a nitride of a material different from the metal of the metal nitride used in the superconducting layer 12. Examples of materials for the capping layer 104 include AlN, Al2O3, SiO2, and SiN. The capping layer 104 can be deposited by standard chemical vapor deposition or physical vapor deposition processes.

[0034] Etching can be used to form trenches 108 through at least the metal nitride layer 12 to form conductive wires 12 or other structures needed for the device 100 (step 210). While Figure 4 shows trenches through the metal nitride layer 12 and the capping layer 104, other configurations are possible. By way of example, the trenches can extend partially or completely through the seed layer 102. However, the trenches should not extend into the mirror structure 24.

[0035] Another form of superconducting nanowire single-photon detector (SNSPD) device includes a waveguide for inputting photons to the detector along an axis substantially parallel to the surface of the substrate. Figures 8A and 8B show a conventional SNSPD 50 having such a waveguide. The SNSPD 50 can include at least one superconducting wire 52 disposed on a support structure 60. The support structure can include a substrate 62, e.g., a silicon substrate, a dielectric layer 64 on the substrate 62, and a waveguide 66 disposed on the dielectric layer 64. The dielectric layer 64 is a first material having a first refractive index, and the waveguide 66 is a second material having a second refractive index higher than the first refractive index.

[0036] The superconducting wires 52 can be considered nanowires and can have, for example, a width of about 30 nm and a thickness of about 10 nm. The superconducting wires 52 can be arranged to form multiple parallel wires, with adjacent wires connected at alternating ends. While FIG. 8A shows four parallel wires, the device can have only two parallel wires, e.g., U-shaped wires, or more.

[0037] Photons represented by light beam 70 enter the device through the waveguide layer 66, e.g., from a side approximately parallel to the top surface of the substrate 62. In particular, the photons can enter along an axis (indicated by arrow A) approximately parallel to the parallel lines of the wires 52. Furthermore, the wires 52 can be positioned near the center of the waveguide along an axis transverse to the direction of light propagation. For example, on each side of the device, there can be a gap 58 between the outer edge of the wires 52 and the outer edge of the waveguide 66. This gap 58 can have a width of approximately 25-30% of the total width of the waveguide.

[0038] Generally, the dielectric layer 64 below the waveguide 66 and the empty space or air above the waveguide 66 both have a lower refractive index than the waveguide 66, so photons in the waveguide 66 are trapped by total internal reflection. However, optical coupling between the waveguide 66 and the nanowire 52 allows photons to escape into the nanowire 52 and thus be absorbed by the nanowire 62. In this type of device, the optical coupling efficiency can be very high.

[0039] Figure 9 shows a cross-sectional side view of a waveguide configuration of a superconducting nanowire single-photon detector (SNSPD) device 150. The SNSPD device 150 may be similar to the devices described above with respect to Figures 4 and 8, except as described below.

[0040] The SNSPD device 150 includes a substrate 62, such as a silicon substrate.

[0041] Overlying the top surface of substrate 62 is a dielectric layer 64. Dielectric layer 64 may be silicon oxide (SiO2), although other materials having a refractive index less than that of waveguide 66 are possible. Dielectric layer 64 may have a thickness of at least 100 nm, for example, between 200 and 2 μm.

[0042] The waveguide 66 is disposed on the dielectric layer 64. The waveguide 64 can be silicon nitride (Si3N4), although other materials with refractive indices greater than that of the dielectric layer 64 are also possible. The specific thickness and width of the waveguide can be selected based on the wavelength of light to be captured and detected. The waveguide 66 can have a thickness of 400-500 nm, e.g., 450 nm, for 1550 nm light. The width of the waveguide 66, i.e., the width perpendicular to the propagation direction of the light entering the waveguide 66, can be 1.1-1.3 μm, e.g., 1.2 μm for 1550 nm light.

[0043] For example, a metal nitride seed layer 152 is located on the top surface of the waveguide 66 in direct contact with the waveguide 66. The seed layer 152 and the superconducting wire 52 are nitrides of different metals. In particular, the metal nitride of the seed layer 152 can be aluminum nitride (AlN) to improve the critical temperature of NbN. However, gallium nitride (GaN) may also be suitable. The seed layer 152 can have a thickness of about 4 to 50 nm, for example, about 5 nm, about 10 nm, or about 20 nm. The seed layer 152 can have a (002) c-axis crystal orientation. The seed layer 152 is not superconducting at the operating temperature of the device 150.

[0044] The superconducting wire 52 is formed on, for example, in direct contact with, the seed layer 152. The wire may be made of niobium nitride (NbN), titanium nitride (TiN), or niobium titanium nitride (Nb X Ti 1-X The wires 52 may have a width of about 25 to 250 nm, for example, about 60 nm, and a thickness of 4 to 50 nm, for example, about 5 nm, about 10 nm, or about 20 nm.

[0045] A capping layer 154 may cover the superconducting wire 52. The capping layer 154 functions, for example, as a protective layer to prevent oxidation or other types of contamination or damage of the metal nitride of the superconducting wire 52. The capping layer 154 may be dielectric or conductive, but is not superconducting at the operating temperatures of the device 150. The capping layer 154 may be amorphous silicon (a-Si). In some implementations, the capping layer 154 is a nitride of a material different from the metal of the metal nitride used in the superconducting layer 52. Examples of materials for the capping layer 104 include AlN, Al2O3, SiO2, and SiN. The capping layer 104 may be deposited by standard chemical vapor deposition or physical vapor deposition processes.

[0046] The trenches separating the wires 52 may extend through the capping layer 154, the superconducting layer that provides the wires 52, and the seed layer 152. The trenches need not extend into the waveguides.

[0047] 10 shows a cross-sectional side view of another embodiment of a waveguide configuration for a superconducting nanowire single-photon detector (SNSPD) device 150'. The SNSPD device 150' may be similar to the device described above with respect to FIG. 9, except as described below.

[0048] 10, the waveguide 66' is formed of aluminum nitride (AlN), so no separate seed layer is required, and the waveguide 66' itself acts as a seed layer for the NbN.

[0049] The dielectric layer 64 can be silicon oxide (SiO2), although other materials, such as silicon nitride (Si3N4), having a lower refractive index than the aluminum nitride of the waveguide 66' are also possible. As noted above, the specific thickness and width of the waveguide can be selected based on the wavelength of light to be captured and detected. The waveguide 66' can have a thickness of 400-500 nm, e.g., 450 nm, for 1550 nm light. The width of the waveguide 66', i.e., the width perpendicular to the propagation direction of the light entering the waveguide 66', can be 1.1-1.3 μm, e.g., 1.2 μm for 1550 nm light.

[0050] The superconducting wire 52 is formed on, for example, in direct contact with, the waveguide 66'.

[0051] While particular implementations have been described, other and further implementations may be devised without departing from the basic scope of the present disclosure. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. It should be noted, however, that the drawings depict only exemplary embodiments. The scope of the invention is determined by the claims.

Claims

1. 1. A superconducting nanowire single photon detector (SNSPD) device comprising: a substrate having a top surface; an optical waveguide on the top surface of the substrate for receiving light propagating substantially parallel to the top surface of the substrate; a seed layer of aluminum nitride on the optical waveguide; a superconducting wire on the seed layer, the superconducting wire being niobium nitride and having a critical temperature of at least 9.7 K, the superconducting wire being optically coupled to the optical waveguide; Including, the device.

2. 10. The device of claim 1, including a dielectric layer of a first material having a first refractive index between the substrate and the optical waveguide, the optical waveguide being formed from a second material having a second refractive index greater than the first refractive index.

3. The first material is silicon oxide (SiO ２ 3. The device of claim 2, wherein

4. The second material is silicon nitride (Si ３ N ４ 3. The device of claim 2, wherein

5. The device of claim 1 , wherein the metal nitride layer comprises δ-phase NbN.

6. The device of claim 1 further comprising a capping layer on the plurality of wires.

7. 7. The device of claim 6, wherein the superconducting wire comprises multiple wire portions, and wherein trenches separating the multiple wire portions extend through the capping layer.

8. The device of claim 1 , wherein the seed layer has a thickness of 4 to 50 nm.

9. The device of claim 1 , wherein the superconducting wire has a thickness of 4 to 50 nm.

10. 10. The device of claim 1, wherein the superconducting wire comprises multiple wire portions, and the trenches separating the multiple wire portions do not extend into an optical reflector.

11. The device of claim 10 , wherein the trench extends into the seed layer.

12. 1. A superconducting nanowire single photon detector (SNSPD) device comprising: a substrate having a top surface; an aluminum nitride optical waveguide on the top surface of the substrate for receiving light propagating substantially parallel to the top surface of the substrate; a superconducting wire on the optical waveguide, the superconducting wire being niobium nitride having a critical temperature of at least 9.7 K; Including, the device.

13. The device of claim 12, wherein the optical waveguide has a thickness of 400 to 500 nm.

14. The device of claim 12, wherein the optical waveguide has a width of 1.1 to 1.3 μm.

15. 13. The device of claim 12, comprising a dielectric layer of a first material having a first refractive index that is less than a second refractive index of the aluminum nitride of the optical waveguide.

16. The first material is silicon oxide (SiO ２ 16. The device of claim 15, wherein

17. The device of claim 12 , wherein the superconducting wire is in direct contact with the optical waveguide.

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