Technical Field

The present invention relates to ion implantation in amorphous chalcogenides. More particularly, but not exclusively, the present invention relates to ion implantation in an amorphous chalcogenide material in a thermoelectric device.

Background of the Invention

Thermoelectric devices employ the thermoelectric effect to convert a temperature difference into an electric current, or to create a thermal gradient through the application of an electric current. Examples of thermoelectric devices include thermoelectric generators, thermocouples and thermopiles, which use the Seebeck effect to generate electricity, and Peltier heat pumps, which use the Peltier effect to pump heat from a cold side of the heat pump to a hot side. It is generally desirable to increase the efficiency of thermoelectric devices, so that a larger current can be generated for a given thermal gradient, and more heating/cooling power can be provided by a given power supply.

The invention is made in this context.

Summary of the Invention

According to the present invention, there is provided a thermoelectric device comprising: a p-type amorphous chalcogenide element; and an n-type amorphous chalcogenide element electrically connected to the p-type amorphous chalcogenide element, the n-type amorphous chalcogenide element comprising a first amorphous chalcogenide base material ion-implanted with a first dopant.

The p-type amorphous chalcogenide element can comprise a second amorphous chalcogenide base material ion-implanted with a second dopant. The first amorphous chalcogenide base material can be the same as the second amorphous chalcogenide base material and the first dopant can be the same as the second dopant, the dopant concentration in the n-type amorphous chalcogenide element can be higher than a threshold level at which the amorphous chalcogenide base material switches from p- type to n-type conduction, and the dopant concentration in the p-type amorphous chalcogenide element can be lower than the threshold level. The first amorphous chalcogenide base material and/or the second amorphous chalcogenide base material may comprise a compound of Ga, La and S. The first dopant and/ or the second dopant can be selected from the group consisting of Bi, Pb and Al.

The thermoelectric device can be configured as a thermoelectric generator for providing electrical power to a load when a thermal gradient is applied across the thermoelectric generator. The thermoelectric device can be configured as a thermoelectric heater or cooler for heating or cooling one side of the thermoelectric device when connected to an electrical power source.

According to the present invention, there is also provided a method of fabricating a thermoelectric device comprising a p-type amorphous chalcogenide element, an n-type amorphous chalcogenide element electrically connected to the p-type amorphous chalcogenide element, the method comprising forming the n-type amorphous chalcogenide element by: depositing a layer of a first amorphous chalcogenide base material on a substrate; and ion-implanting the first amorphous chalcogenide base material with a dopant to change the first amorphous chalcogenide base material from p-type conduction to n-type conduction.

The method can further comprise forming the p-type amorphous chalcogenide element by: depositing a layer of a second amorphous chalcogenide base material on the substrate; and ion-implanting the second amorphous chalcogenide base material with a second dopant.

The first amorphous chalcogenide base material can be the same as the second amorphous chalcogenide base material and the first dopant can be the same as the second dopant, the dopant concentration in the n-type amorphous chalcogenide element can be higher than a threshold level at which the amorphous chalcogenide base material switches from p-type to n-type conduction, and the dopant concentration in the p-type amorphous chalcogenide element can be lower than the threshold level. The first amorphous chalcogenide base material and/or the second amorphous chalcogenide base material can comprise a compound of Ga, La and S. The first dopant and/ or the second dopant can be selected from the group consisting of Bi, Pb and Al. According to the present invention, there is also provided the use of ion implantation with a dopant to reduce the thermal conductivity of an amorphous chalcogenide base material in a thermoelectric device.

According to the present invention, there is further provided an electronic device comprising: an n-type amorphous chalcogenide element comprising a compound of Ga, La and S ion-implanted with a dopant.

According to the present invention, there is further provided a method of fabricating an electronic device comprising an n-type amorphous chalcogenide element, the method comprising: depositing a layer of p-type amorphous chalcogenide base material on a substrate, wherein the p-type amorphous chalcogenide base material comprises a compound of Ga, La and S; and ion-implanting the p-type amorphous chalcogenide layer with a dopant to switch the conduction type of the amorphous chalcogenide base material from p-type to n-type.

Brief Description of the Drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure l illustrates a thermoelectric generator incorporating an ion-implanted amorphous chalcogenide material, according to an embodiment of the present invention;

Figure 2 illustrates a thermoelectric cooler incorporating an ion-implanted amorphous chalcogenide material, according to an embodiment of the present invention;

Figure 3 is a graph illustrating the conductivity as a function of Bi dopant

concentration, x, for 06 ₂₀ 868 _Ο - _Λ :ΒΙ _Λ :, according to an embodiment of the present invention;

Figure 4 is a graph illustrating the Seebeck coefficient as a function of Bi dopant concentration, x, for 06 ₂₀ 868 _Ο - _Λ :ΒΙ _Λ :, according to an embodiment of the present invention; Figure 5 is a graph illustrating the conductivity as a function of Bi implant dose for different amorphous chalcogenide base materials, according to an embodiment of the present invention; and

Figure 6 is a graph illustrating the Seebeck coefficient as a function of Bi implant dose for different amorphous chalcogenide base materials, according to an embodiment of the present invention.

Detailed Description

Figure 1 illustrates a chalcogenide-based thermoelectric device, according to an embodiment of the present invention. In the present embodiment, the thermoelectric device is configured as a thermoelectric generator for providing electrical power to a load when a thermal gradient is applied across the thermoelectric generator.

As shown in Fig. 1, the thermoelectric generator 100 comprises a plurality of p-type amorphous chalcogenide elements 101 and a plurality of n-type amorphous

chalcogenide elements 102. Although two n-type elements and two p-type elements are illustrated in Fig. 1, embodiments of the present invention can include any number of one or more n-type elements and one or more p-type elements. The plurality of p-type elements 101 and n-type elements 102 are electrically connected in series, such that each one of the p-type elements 101 is connected to two of the n- type elements 102, and vice versa. At each end, a terminal element, which may each be n-type or p-type, is only connected to a single element of the other type. The thermoelectric generator 100 also comprises a first electrode 103 electrically connected to one of the terminal elements, and a second electrode 104 electrically connected to another one of the terminal elements.

As well as being electrically connected in series, the p-type elements and the n-type elements are arranged to be thermally in parallel when a thermal gradient is applied across the thermoelectric generator. Specifically, as shown in Fig. 1, the n-type and p- type elements are arranged in a layer, and electrical connections between the n-type and p-type elements are disposed on either side of the layer. The electrical connections are arranged such that on a first side 105 of the thermoelectric device 100, electrical current flows from the n-type elements to the p-type elements, and on a second side 106 of the thermoelectric device 100, electrical current flows from the p-type elements to the n-type elements. The skilled person will be familiar with the operating principles of a thermoelectric generator, and so a detailed description will be omitted here. In brief, when a thermal gradient is applied across the thermoelectric generator in the direction shown in Fig. l, with the first side 105 of the thermoelectric generator 100 at a higher temperature than the second side 106, the majority charge carriers in the n-type and p-type elements, respectively electrons and holes, migrate from the first side 105 to the second side 106 of the thermoelectric generator 100. When a load 107 is connected across the first and second electrodes 103, 104, electrical power can be supplied to the load 107.

Figure 2 illustrates a thermoelectric cooler incorporating an ion-implanted amorphous chalcogenide material, according to an embodiment of the present invention. The structure of the thermoelectric cooler, which can also be used as a thermoelectric heater, is similar to that of the thermoelectric generator of Fig. 1, and a detailed description will not be repeated here. A thermoelectric heater or cooler can also be referred to as a Peltier heat pump.

As shown in Fig. 2, the thermoelectric heater/cooler 200 comprises a plurality of p-type amorphous chalcogenide elements 201 and a plurality of n-type amorphous

chalcogenide elements 202. Although two n-type elements and two p-type elements are illustrated in Fig. 2, embodiments of the present invention can include any number of one or more n-type elements and one or more p-type elements. The thermoelectric heater/cooler 200 further comprises a first electrode 203 and a second electrode 204. When an electrical power source 207 is connected across the first and second electrodes 203, 204, a thermal gradient is generated across the thermoelectric device 200. As shown in Fig. 2, the drift of charge carriers from one side of the device 200 to the other has the effect of transferring thermal energy from the first side 205 to the second side 206, with the result that the first side 205 is cooled and the second side 206 is heated.

In embodiments of the present invention, such as the thermoelectric devices illustrated in Figs. 1 and 2, one or more n-type amorphous chalcogenide elements are formed from an amorphous chalcogenide. The amorphous chalcogenide, which is initially a p-type conductor, is doped with a dopant using ion implantation to change the conduction from p-type to n-type. Carrier type reversal occurs above a threshold dopant concentration, which varies according to the particular material composition and the chosen dopant species. Another effect of the ion implantation is to modify the structure of the amorphous chalcogenide at the atomic scale, above the atomic scale at the nanoscale, and above the nanoscale, so as to modify the material, electronic and thermal properties, including improving the efficiency of phonon scattering and reduce the thermal conductivity of the n-type amorphous chalcogenide elements.

Furthermore, in some embodiments of the present invention, the p-type elements can also be doped using ion implantation, to reduce their thermal conductivity and increase device efficiency further. When ion implantation is used in p-type elements, the dopant concentration should be below the threshold level for carrier type reversal (CTR), so that p-type behaviour is retained. Depending on the embodiment, the same base material or different base materials may be used for the n-type and p-type elements, and the same dopant or different dopants may be used in the n-type and p-type elements. In particular, the dopant material used in the p-type element may not necessarily be one which induces CTR. For example, the thermal conductivity in the p- type elements can be modified by doping with light elements such as He and O, which are not known to cause CTR in amorphous chalcogenides. Furthermore, in some embodiments a dopant which does not induce CTR, such as He or O, can also be used in the n-type elements in conjunction with a CTR dopant such as Pb, Bi or Al, to further reduce the thermal conductivity in the n-type elements.

In the embodiments of Figs, ι and 2, the p-type and n-type elements are formed from a compound of Ga, La and S, specifically GaLaSO. The n-type dopant used is bismuth, with a concentration of at least l x io ^l6 atoms/cm ² . However, in other embodiments other dopant species and dopant concentrations can be used, for example lead or aluminium. Also, the invention is not limited to compounds of Ga, La and S. In other embodiments, other amorphous chalcogenide base materials may be used to form the p-type and n-type elements. For example, compounds of Ge and Se, Ge and Te, and Ge Sb and Te can be used in other embodiments. An indicator of the potential efficiency of a thermoelectric device is provided by the thermoelectric figure of merit (Zr), which is defined as follows: where S is the Seebeck coefficient, σ the electric conductivity, κ the thermal

conductivity, and Tis the temperature. Under given operating conditions, a device with a higher value of Zr will have a higher efficiency than a device with a lower value of Zr. By reducing the thermal conductivity (κ), the ion implantation has the effect of increasing Z _T . At the same time, the ion implantation also increases the electric conductivity (σ), further increasing Z _T . For example, Cu-doped GLS exhibits a resistivity of about 40 Ohm metres (Ωπι), in comparison the undoped GLS which has a resistivity of about 1 ^χ ιο ¹¹ Ωπι. As a result, undoped GLS has a Zr value of

approximately 7 ^χ lcr^, whereas Cu-doped GLS has a Zr value of approximately 1.7 ^χ

10-3. Device efficiency can also be improved by increasing the temperature difference (AT) between the hot and cold sides of the device. This makes GaLaS-based compounds particularly advantageous for use in thermoelectric devices, as their relatively high melting temperatures (T _M ) compared to other amorphous chalcogenides can enable a thermoelectric device to be operated at a larger ATby increasing the temperature at the hot side of the device. For example, T _M for GaLaS is 830 °C.

In the thermoelectric generator of Fig. 1, a lower thermal conductivity enables a higher thermal gradient to be maintained across the device, resulting in a higher current. Similarly, in the Peltier heat pump of Fig. 2, a lower thermal conductivity enables a higher thermal gradient to be generated for an electrical current of given magnitude, by decreasing the rate at which thermal energy is conducted back to the cool side from the hot side of the device.

Figure 3 is a graph illustrating the conductivity as a function of Bi dopant

concentration, x, for 06 ₂₀ 868 _Ο - _Λ :ΒΙ _Λ :, according to an embodiment of the present invention. Figure 4 is a graph illustrating the Seebeck coefficient of the same material as a function of the Bi dopant concentration.

As shown in Fig. 3, when the atomic percent of bismuth, x, in 06 ₂₀ 868Ο-Λ:ΒΙΛ: reaches about 9.5, a sharp increase of approximately four orders of magnitude is seen in the conductivity. Also, as shown in Fig. 4, the increase in conductivity occurs just after the Seebeck coefficient switches from positive to negative, at around x=8, indicating a switch from p-type to n-type conduction. Figure 5 is a graph illustrating the conductivity as a function of Bi implant dose for various amorphous chalcogenide base materials, according to an embodiment of the present invention. As shown in Fig. 5, an increase in conductivity with dopant concentration is seen in GeTe, GaLaSO, and GeSe. In particular, in GaLaSO the conductivity increases rapidly for doses between 1 ^χ io ¹⁵ atoms/cm ² and 1 ^χ io ^l6 atoms/cm ² . Conventionally, GaLaS-based compounds, such as GaLaSO, have been used as high-performance phase-change materials in optical devices, but are not widely used in electronic devices due to their low conductivity. As shown in Fig. 5, by ion- implanting GaLaS-based compounds with, for example, Bi or Pb, the conductivity can be substantially increased. This allows the fabrication of GaLaS-based electronic devices, such as the thermoelectric devices described above with reference to Figs. 1 and 2. This enables electronic devices to benefit from advantageous properties of GaLaS-based compounds in comparison to conventional GeSbTe-based compounds, such as higher thermal stability and improved endurance, and lower set and reset currents for phase change memory devices. Furthermore, the advantage of increased conductivity for GaLaS-based compounds is not restricted to thermoelectric devices. In other embodiments of the present invention, ion-implantation with an n-type dopant is used to increase the conductivity of an amorphous chalcogenide element comprising Ga, La and S in other types of electronic device, such as transistors, light emitting diodes, and photodiodes.

Figure 6 is a graph illustrating conductivity as a function of Bi implant dose for different amorphous chalcogenide base materials, according to an embodiment of the present invention. As shown in Fig. 6, carrier type reversal by ion implantation is not limited to GeSe compounds, but is also seen in other materials including GeTe, GaLaSO, GeSe and GeTe. From Figs. 5 and 6 it will be understood that embodiments of the present invention are not limited to the particular material compositions disclosed herein, and in general any amorphous chalcogenide base material can be used.

Embodiments of the present invention have been described with respect to a thermoelectric generator and a thermoelectric heater/cooler. However, the invention is not limited to these particular devices. For example, in another embodiment the thermoelectric device may be configured as a thermocouple.

Also, it should be understood that Figs. 1 and 2 are provided for schematic purposes only, and are simplified representations of the actual structures of the thermoelectric devices. In particular, the thermoelectric devices may also include other components not shown in Figs, l and 2, such as diffusion barrier layers, insulating layers and substrates.

Whilst certain embodiments of the invention have been described herein with reference to the drawings, it will be understood that many variations and modifications will be possible without departing from the scope of the invention as defined in the

accompanying claims.