A THERMALLY STABLE GRAPHENE-CONTAINING LAMINATE

Abstract

The present invention provides a graphene-containing laminate comprising. in order: a substrate: a graphene layer structure: a first metal oxide layer formed of a first metal oxide, wherein the first metal oxide is a transition metal oxide: and a second metal oxide layer formed of a second metal oxide: wherein the first metal oxide layer has a thickness of from 0.1 nm to 5 nm; and wherein the first metal oxide layer has a work function of 5 eV or more.

Claims

1. A graphene-containing laminate comprising, in order: a substrate; a graphene layer structure; a first metal oxide layer formed of a first metal oxide, wherein the first metal oxide is a transition metal oxide; and a second metal oxide layer formed of a second metal oxide; wherein the first metal oxide layer has a thickness of from 0.1 nm to 5 nm; and wherein the first metal oxide layer has a work function of 5 eV or more.

2. The graphene-containing laminate according to claim 1, wherein the first metal oxide layer has a work function of 5.5 eV or more.

3. The graphene-containing laminate according to claim 1, wherein the transition metal oxide is selected from the group consisting of: molybdenum oxide, chromium oxide, vanadium oxide, tungsten oxide, nickel oxide, and mixtures thereof.

4. The graphene-containing laminate according to claim 1, further comprising a capping layer on the second metal oxide layer, wherein the capping layer is formed of a third metal oxide and/or metal nitride.

5. The graphene-containing laminate according to claim 1, wherein the thickness of the first metal oxide layer is 0.5 nm or more and/or 3 nm or less.

6. The graphene-containing laminate according to claim 1, wherein the first metal oxide layer covers 50% or more and/or 90% or less of the area of the graphene layer structure.

7. The graphene-containing laminate according to claim 1, wherein the second metal oxide layer has a thickness of 5 nm or more and/or 250 nm or less.

8. (canceled)

9. The graphene-containing laminate according to claim 1, wherein the substrate comprises sapphire, YSZ or CaF.sub.2.

10. The graphene-containing laminate according to claim 1, wherein the graphene layer structure has a charge carrier concentration of less than 510.sup.12 cm.sup.2.

11. The graphene-containing laminate according to claim 1, wherein the graphene layer structure has a thermally stable resistance at temperatures in excess of 50 C.

12. The graphene-containing laminate according to claim 11, wherein the change in resistance of the graphene layer structure is less than 0.05% per day when measured at 125 C.

13. An electronic device comprising the graphene-containing laminate according to any preceding claim, and one or more contacts in contact with the graphene layer structure.

14. The electronic device according to claim 13, wherein the device is for use at temperatures in excess of 50 C.

15. The electronic device according to claim 13, wherein the electronic device is a Hall-sensor.

16. Use of the electronic device according to claim 13 at a temperature in excess of 50 C.

17. A method of forming a graphene-containing laminate, the method comprising: providing a graphene layer structure on a substrate; forming a first metal oxide layer on the graphene layer structure, wherein the first metal oxide layer is formed of a transition metal oxide and has a work function of 5 eV or more; and forming a second metal oxide layer on the first metal oxide layer, wherein the second metal oxide layer is formed of a second metal oxide; wherein the first metal oxide layer has a thickness of from 0.1 nm to 5 nm.

18. The method according to claim 17, wherein the first metal oxide is formed by PVD.

19. The method according to claim 17, wherein the second metal oxide layer is formed by atomic layer deposition (ALD) at a temperature of 80 C. or less.

20. (canceled)

21. The method according to claim 17, further comprising forming a capping layer on the second metal oxide layer, wherein the capping layer is formed of a third metal oxide and/or metal nitride.

22. (canceled)

23. The graphene-containing laminate according to claim 9, wherein the sapphire is c-plane or r-plane sapphire.

Description

[0113] FIG. 1 demonstrates an exemplary method in accordance with the present invention in cross-section. There is first provided a sapphire substrate 105 having a graphene monolayer 110 thereon. The graphene monolayer is preferably formed directly on the surface of the surface of the sapphire substrate 105 in a preceding step by thermal CVD.

[0114] The method then involves depositing 205 a first metal oxide layer 115 that is formed of molybdenum oxide (specifically MoO.sub.3). The first metal oxide layer 115 has an average mean thickness of 0.1 to 5 nm (for example about 1 nm or about 2 nm) and the layer covers more than 50% of the surface area of the graphene monolayer 110. Some of the graphene monolayer remains exposed to which a second metal oxide 120 layer is deposited 210 thereon, as well as on the first metal oxide layer 115 itself. The second metal oxide layer 120 is a layer of aluminium oxide formed by ALD 210 using trimethylaluminium and ozone as precursors at a temperature of less than 60 C., preferably about 40 C. The cycles of trimethylaluminium and ozone are repeated until a thickness of about 15 nm in achieved thereby forming a graphene-containing laminate that has a thermally stable charge carrier concentration.

[0115] In another preferred embodiment, the second metal oxide layer 120 is a layer of aluminium oxide formed by ALD 210 using trimethylaluminium and H2O as precursors at a temperature of more than 100 C., such as about 150 C. Using this method, a thicker ALD layer is preferred and the second metal oxide layer 120 may have a thickness of more than 50 nm, such as about 65 nm.

[0116] FIG. 2 is a cross-section of an exemplary Hall-sensor 100 comprising a graphene-containing laminate. The graphene-containing laminate is formed of the substrate 105, graphene monolayer 110, first metal oxide layer 115 and second metal oxide layer 120 as shown in FIG. 1. For Hall-sensor 100, the substrate 105 may preferably be r-plane sapphire whereby the graphene monolayer 110 is formed on the r-plane growth surface of the substrate 105. The average mean thickness of the first metal oxide 115 in such an embodiment may be about 1 nm. In another preferred embodiment, the substrate 105 is c-plane sapphire (and the first metal oxide 115 may instead be thicker, for example from 1 to 5 nm, such as from 2 to 3 nm). The graphene monolayer 110 and first and second metal oxide layers 115, 120 have been etched and shaped into a cross-shape suitable for a Hall-sensor. Such shapes are well-known to those skilled in the art and are not particularly limited. At distal ends of the graphene monolayer 110 where the graphene-containing laminate has been etched, there are provided metal contacts 125a and 125b, each in contact with an edge of the graphene monolayer 110. The Hall-sensor 100 further comprises a capping layer 130 formed of, for example, aluminium oxide, which may also have been formed by ALD, using H.sub.2O as the oxygen precursor at a temperature of about 150 C. until a thickness of more than 50 nm is achieved. The capping layer 130 fully encapsulates the stack of the graphene monolayer 110, first metal oxide layer 115 and second metal oxide layer 120, including any edges of the graphene monolayer 110 which have remained exposed due to etching (not visible in the cross-section of FIG. 2). The capping layer 130 may also encapsulate the metal contacts when the capping layer 130 is formed by ALD. Metal wires can be connected to the contacts straight through the capping layer 130 for connection into an electronic circuit, or preferably the capping layer 130 is etched to expose the contacts 125a and 125b. Alternatively, a patterned capping layer 130 can be deposited by a PVD technique leaving a portion of the metal contacts 125a and 125b exposed for connection of the Hall-sensor 100 into an electronic circuit. The charge carrier concentration of the final device 100 may be about 510.sup.11 to about 10.sup.12 cm.sup.2.

[0117] FIG. 3 is a cross-section of an exemplary preferred Hall-sensor 300 that is substantially equivalent to Hall-sensor 100 as shown in FIG. 2. Hall-sensor 300 comprises a graphene-containing laminate formed of the substrate 305, graphene monolayer 310, first metal oxide layer 315 (MoO.sub.3) and second metal oxide layer. The difference is that the second metal oxide layer of Hall-sensor 300 is formed of two sub-layers 320a, 320b, each formed of aluminium oxide, for example by ALD. The lower sub-layer 320a is about 15 nm thick and formed by ALD using ozone at a temperature of less than 60 C. The upper sub-layer 320b is about 65 nm thick and formed by ALD, using H.sub.2O as a precursor at a temperature of about 150 C.

[0118] As for Hall-sensor 100, the graphene monolayer 310 and first and second metal oxide layers 315, 320a, 320b have been etched and shaped into a cross-shape suitable for a Hall-sensor. At distal ends of the graphene monolayer 310 where the graphene-containing laminate has been etched, there are provided metal contacts 325a and 325b, each in contact with an edge of the graphene monolayer 310. The Hall-sensor 300 further comprises a capping layer 330 formed of, for example, aluminium oxide, which may also have been formed by ALD, but using H.sub.2O as the oxygen precursor at a temperature of about 150 C. until a thickness of more than 50 nm is achieved.

[0119] The cross-section of the device in FIG. 3 is equally representative of other electronic devices, for example a temperature sensor. In an embodiment of such a device suitable for use at extremely high temperatures (e.g. greater than 1000 C.), the metal contacts 125a and 125b are formed of tungsten.

[0120] FIG. 4 is a plot of the average resistance drift rate in %/day of Hall-sensor devices according to the present invention which include an MoO.sub.3 doping seed layer together with second and third metal oxide layers formed of Al.sub.2O.sub.3. The Example data in FIG. 4 shows that the resistance of the graphene layer structure has minimal drift at both 20 C. and 130 C. (generally below 0.1%/day where error bars show standard deviation within batches of devices). On the other hand, the reference Hall-sensor device without an MoO.sub.3 layer shows much greater drift at 130 C. of about 0.65%/day.

[0121] FIG. 5 is a plot which illustrates the thermal stability of various graphene-containing laminates after a number of days at 130 C. under an inert nitrogen atmosphere. A comparative graphene-containing laminate comprises a substrate, graphene and a second metal oxide layer formed of Al.sub.2O.sub.3 directly thereon (i.e. without a first transition metal oxide with high work function; plotted with triangles). In the absence of such layer, the measured carrier concentration is not thermally stable and quickly increases to more than 510.sup.12 cm.sup.2 (in absolute terms) within 1 day and continues to increase.

[0122] A first inventive graphene-containing laminate comprises a substrate, graphene, a first MoO.sub.3 layer and a second Al.sub.2O.sub.3 layer (plotted with circles). A second inventive graphene-containing laminate is based on the first and further comprises a third capping Al.sub.2O.sub.3 layer (plotted with diamonds). The inventive graphene-containing laminates comprising an MoO.sub.3 doping seed layer provides the graphene layer structure with improved thermal stability. After more than 4 or 5 days at 130 C., the carrier concentration remains below 210.sup.12 cm.sup.2, and generally below 10.sup.12 cm.sup.2.

[0123] The inventive example which does not include a capping layer shows an initially high carrier concentration which rapidly stabilises at 130 C. to a value below 210.sup.12 cm.sup.2. Whilst samples may show an initial change upon heating directly after manufacture, samples stabilise to the desired values generally within 1 day, for example within about 8 hours. As regards the stability parameters discussed herein, these are measured from a starting point 12 hours after manufacture of the final electrical component (e.g. hall sensor) to ensure that this initial stabilisation has finished.

[0124] FIG. 6 is a plot of change in charge carrier concentration of the graphene against time (in days) for two Hall-sensor devices according to the present invention. Device 1 is manufactured in accordance with Method 3 and Device 2 manufactured in accordance with Method 4. For both devices, the laminate was exposed to atmosphere and chemicals before the deposition of the capping layer (i.e. after photolithography processing of the second metal oxide layer). Device 1 shows about a 10% change in device resistance after about 9 days whereas Device 2 shows negligible change after the same period of time. FIG. 6 shows that the second metal oxide layer being formed of two sub-layers improves the stability of the final device.

[0125] FIG. 7 is a plot of Hall sensitivity against temperature for a Hall-sensor device manufactured in accordance with Method 4 across three temperature ramps. The data shows a linear change in Hall sensitivity across multiple temperature ramps up to about 180 C. The device was secured on a heated plate and the hall properties of the device measured using the Van der Pauw method. The stage was heated and allowed to stabilise, then several Hall measurements were taken and averaged. This was repeated at various temperatures. Ramp1 maximum temperature was 75 C, Ramp 2 maximum temperature was 130 C and Ramp 3 maximum temperature was 180 C.

[0126] FIGS. 8-10 are SEM images of different embodiments of Hall-sensor devices in accordance with the present invention comprising a graphene-containing laminate as described herein. Each of these Hall-sensors is formed of a sapphire substrate and a monolayer of graphene shaped into a cross. Each sensor also comprises a first metal oxide layer on and across the monolayer of graphene formed of MoO.sub.3 having a nominal thickness of about 1 nm. Each device comprises a different second metal oxide layer but have an equivalent alumina capping layer.

[0127] In the device of FIG. 8, the second metal oxide layer formed on the first metal oxide layer is formed of alumina by ALD. The device of FIG. 8 includes the same ALD alumina layer as the device of FIG. 8 (e.g. as a lower sub-layer), but the second metal oxide layer of the device of FIG. 9 is further formed of a further alumina layer by ALD under different conditions (e.g. as an upper sub-layer formed by ALD using water as a precursor). The device of FIG. 10 is equivalent to that of FIG. 9, with the exception that the lower sub-layer of alumina is formed by evaporation.

[0128] As can be seen from the SEM images, the inventors have found that, in some embodiments, blistering of the graphene can occur. The blistering is found to become more evident during use of the device at either elevated temperatures or cryogenic temperatures and the associated temperature cycling to ambient temperatures. These blisters are believed to result from trapped gases which remain from the deposition processes. Blistering is undesirable due to the increased risk of damaging the contact between the graphene and the contact. The addition of a sub-layer to the second metal oxide layer is shown by FIG. 9 to reduce the prevalence of such blisters. Additionally, the blisters were further reduced through forming the lower sub-layer of the second metal oxide by evaporation.

Examples

Method 1

[0129] A graphene monolayer is grown directly onto the surface of a sapphire substrate in accordance with the method of WO 2017/029470. Hall-sensor devices were then manufactured using said graphene on sapphire in accordance with the method disclosed in GB 2602119 with the exception that a layer of MoO.sub.3 is first deposited across the graphene monolayer via thermal evaporation at ambient temperature until a nominal thickness of 1 nm is achieved, as measured by QCM.

[0130] The second metal oxide layer formed of Al.sub.2O.sub.3 is formed on the MoO.sub.3 layer as a Hall-cross shape through a shadow mask via e-beam evaporation. Oxygen plasma etching removes graphene not protected by the cross. Metal contacts are deposited via evaporation through a shadow mask (10 nm Ti via e-beam and 200 nm Au via thermal). An Al.sub.2O.sub.3 capping layer is deposited by ALD at 150 C. until a thickness of about 65 nm is achieved. The devices are then singulated and wirebonded into LCC packages.

[0131] The packages are placed into test sockets in a controlled chamber that is heated to 130 C. under an atmosphere of ambient air with the device resistance monitored for the duration of the test period. The results are shown in FIG. 4. The reference device is that made without an MoO.sub.3 layer, directly in accordance with GB 2602119, the resistance drift having been measured via periodic Hall-measurements at ambient temperature before and after heating.

Method 2

[0132] A graphene monolayer is grown directly onto the surface of a sapphire substrate in accordance with the method of WO 2017/029470. A layer of MoO.sub.3 is deposited across the graphene monolayer via thermal evaporation at ambient temperature until a nominal thickness of 1 nm is achieved, as measured by QCM.

[0133] To the MoO.sub.3 coated graphene monolayer, a layer of Al.sub.2O.sub.3 is deposited by ALD at a temperature of about 40 C. using ozone as an oxygen precursor. Cycles of oxygen and aluminium precursors are repeated until a thickness of about 15 nm is achieved.

[0134] Optionally, a capping layer formed of Al.sub.2O.sub.3 is deposited by ALD at a temperature of 150 C. until a thickness of about 65 nm is achieved.

[0135] 1 cm square samples are cleaved from the wafer (i.e. with or without the capping layer) for testing. The carrier concentration is measured initially (day 0) and the samples are then placed on a hotplate at about 130 C. under nitrogen. Periodically, the samples are removed from the hotplate and the carrier concentration measured. The results are shown in FIG. 5.

[0136] In a comparative example, a graphene monolayer is identically grown directly onto the surface of a sapphire substrate in accordance with the method of WO 2017/029470. To the graphene monolayer, a layer of Al.sub.2O.sub.3 is deposited by ALD at a temperature of about 40 C. using ozone as an oxygen precursor. Cycles of oxygen and aluminium precursors are again repeated until a thickness of about 15 nm is achieved. The comparative results are also shown in FIG. 5.

Method 3

[0137] A graphene monolayer is grown directly onto the surface of a sapphire substrate in accordance with the method of WO 2017/029470. A layer of MoO.sub.3 is deposited across the graphene monolayer via thermal evaporation at ambient temperature until a nominal thickness of 1 nm is achieved, as measured by QCM.

[0138] To the MoO.sub.3 coated graphene monolayer, a 15 nm layer of Al.sub.2O.sub.3 is deposited by ALD at a temperature of about 40 C. using ozone as an oxygen precursor. The Al.sub.2O.sub.3 layer and underlying graphene is then patterned into a Hall-sensor cross using conventional photolithography and etching techniques. Contacts are then deposited to contact the edges of the graphene. A capping layer formed of Al.sub.2O.sub.3 is deposited by ALD at a temperature of 150 C. until a thickness of about 65 nm is achieved.

[0139] This device is in accordance with Device 1 in FIG. 6.

Method 4

[0140] A graphene monolayer is grown directly onto the surface of a sapphire substrate in accordance with the method of WO 2017/029470. A layer of MoO.sub.3 is deposited across the graphene monolayer via thermal evaporation at ambient temperature until a nominal thickness of 1 nm is achieved, as measured by QCM.

[0141] To the MoO.sub.3 coated graphene monolayer, a 15 nm (sub) layer of Al.sub.2O.sub.3 is deposited by ALD at a temperature of about 40 C. using ozone as an oxygen precursor directly followed by the formation of a further 65 nm (sub) layer of Al.sub.2O.sub.3, but at a temperature of about 150 C. using H.sub.2O as a precursor. The Al.sub.2O.sub.3 layer and underlying graphene is then patterned into a Hall-sensor cross using conventional photolithography and etching techniques. Contacts are then deposited to contact the edges of the graphene. A capping layer formed of Al.sub.2O.sub.3 is deposited by ALD at a temperature of 150 C. until a thickness of about 65 nm is achieved.

[0142] This device is in accordance with Device 2 in FIG. 6.

Stress Testing

[0143] An inventive Hall-sensor manufactured in accordance with Method 3 above was used as a primary Hall-sensor for stress testing (with the exception that the Hall-sensor was formed with a 65 nm layer of Al.sub.2O.sub.3 onto the MoO.sub.3 coated graphene monolayer rather than a 15 nm layer).

[0144] The tests conducted were based on a typical four-point flexural stress test. The tests were performed on an about 33.5 cm sapphire wafer via two anvils each with two rollers whereby the lower anvil's rollers were spaced 2 cm apart. The tests were performed in a temperature-controlled environment at a temperature of 22 C.

[0145] Hall-sensors were wire bonded to a flexible PCB which was affixed to the wafer by adhesive. Wires were then soldered to the PCB and connected to a set of screw terminals on a perforated stripboard with attached contact pins for the connection of test leads. Hook probe and crocodile clip test leads were used to connect the contact pins to a Keithley 2450 power supply and a MiST test box, respectively.

[0146] A magnetic field was applied via a strong permanent magnet directly beneath the primary Hall-sensor to be tested. A secondary non-stressed (control) Hall-sensor was positioned between the permanent magnet and the primary Hall-sensor, about 1 cm below the wafer and between the lower rollers of the anvil. The permanent magnet produced a Hall voltage of about 300 V in the primary Hall-sensor as shown in FIG. 13 (top plot).

[0147] During the application of a force (i.e. stress/strain or a load) to the wafer, spinning current measurements were taken on the MIST and non-spin with the Keithley. The data shown in the Figures is based on the MIST data only. In all tests, the loading rate was 8,000 gf/s. For the former measurement, the spin rate was 1 kHz and 150 s settling time, 200 A drive current and a gain of 100. The Keithley 2450 provided a non-spinning steady drive current of 200 A to another on-wafer sensor while also measuring the Hall voltage.

[0148] Repeated measurements were performed at a force nominally equivalent to 600 g of weight (600 gf) to apply stress to the wafer whilst measuring the Hall-voltage for both the primary and secondary sensors. The results are shown in FIGS. 11 and 12 (for the primary and secondary sensors, respectively, recorded simultaneously). Both plots of Hall-voltage over time show background noise only showing that no change in Hall-voltage could be observed due to the force and stress applied during the test.

[0149] The test was repeated and performed at a force nominally equivalent to 8.5 kg (8,500 gf). The results are shown in FIG. 13 in which a change in the measured Hall-voltage of the primary sensor of about 20 V could be observed (top plot) under application of the force (bottom plot). The force/load profile as shown in the bottom plot clearly illustrates a discontinuity which corresponds to the point at which the wafer broke.

[0150] The test was repeated by intermittently applying an increasing force, starting at 1800 gf and increasing in increments of 200 gf up to a force of 4,200 gf, and then from 4,500 gf in increments of 500 gf up to a force of 8,500 gf. The results are shown in FIG. 14 (for both the primary and secondary sensors, recorded simultaneously). The data shows that a similar variation in the Hall-voltage of about 30 V can be observed in the primary sensor upon each application of a force in which there is a slight increase in the variation upon increasing the force applied. There is also a noticeable change in the reference signal of the secondary sensor (top plot) indicating that the change in signal may not be attributed solely to the stress applied.

[0151] Crucially, the data shows that there is no baseline variation as a result of the stress applied and that the wafer sensing element long-term sensitivity does not appear to be affected by the test, even after wafer breakage, and the sensor returns to its ordinary operation. A variation of about 20 V is believed to relate to the variation in angle of the sensor in relation to the applied magnetic field angle during wafer bending. Likewise, a drift of about 7 V was observed in the reference sensor and is not observed in the primary sensor due to the greatly reduced distance between the magnet and the secondary referenced sensor.

[0152] As used herein, the singular form of a, an and the include plural references unless the context clearly dictates otherwise. The use of the term comprising is intended to be interpreted as including such features but not excluding other features and is also intended to include the option of the features necessarily being limited to those described. In other words, the term also includes the limitations of consisting essentially of (intended to mean that specific further components can be present provided they do not materially affect the essential characteristic of the described feature) and consisting of (intended to mean that no other feature may be included such that if the components were expressed as percentages by their proportions, these would add up to 100%, whilst accounting for any unavoidable impurities), unless the context clearly dictates otherwise.

[0153] It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, layers and/or portions, the elements, layers and/or portions should not be limited by these terms. These terms are only used to distinguish one element, layer or portion from another, or a further, element, layer or portion. It will be understood that the term on is intended to mean directly on such that there are no intervening layers between one material being said to be on another material. Spatially relative terms, such as under, below, beneath, lower, over, above, upper and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s). It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if a device as described herein is turned over, elements described as under or below other elements or features would then be oriented over or above the other elements or features. Thus, the example term under can encompass both an orientation of over and under. The device may be otherwise oriented and the spatially relative descriptors used herein interpreted accordingly.

[0154] The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.