Field-effect transistor with a total control of the electrical conductivity on its channel

Abstract

The first object of the invention is directed to field-effect gate transistor comprising (a) a substrate, (b) a source terminal, (c) a drain terminal, and (d) a channel between the source terminal and the drain terminal, the channel being a layer of Cu.sub.xCr.sub.yO.sub.2 in which the y/x ratio is superior to 1. The field-effect gate transistor is remarkable in that the channel of Cu.sub.xCr.sub.yO.sub.2 presents a gradient of holes concentration. The second object of the invention is directed to a method for laser annealing a field-effect gate transistor in accordance with the first object of the invention.

Claims

1. A field-effect gate transistor, said field-effect gate transistor comprising: a substrate; a source terminal; a drain terminal; and a channel between the source terminal and the drain terminal, the channel being a layer of Cu.sub.xCr.sub.yO.sub.2 in which the y/x ratio is greater than 1, wherein the channel of Cu.sub.xCr.sub.yO.sub.2 presents a gradient of positive charge carrier concentration.

2. The field-effect gate transistor according to claim 1, wherein the gradient is a gradient decreasing from the drain terminal to the channel and/or increasing from the channel to the source terminal.

3. The field-effect gate transistor according to claim 1, wherein the field-effect gate transistor comprises copper vacancies in the crystal structure of Cu.sub.xCr.sub.yO.sub.2.

4. The field-effect gate transistor according to claim 3, wherein the copper vacancies chains in the crystal structure of Cu.sub.xCr.sub.yO.sub.2 are constituted in amount comprised between 2 and 20 Cu-vacancies.

5. The field-effect gate transistor according to claim 1, wherein the y/x ratio is greater than 2.

6. The field-effect gate transistor according to claim 1, wherein the channel consists of a layer of Cu.sub.0.66Cr.sub.1.33O.sub.2.

7. The field-effect gate transistor according to claim 1, wherein the Cu.sub.xCr.sub.yO.sub.2 is an intrinsic transparent semiconductor.

8. The field-effect gate transistor according to claim 1, wherein the substrate is glass, Si, Si/Si.sub.3N.sub.4, ITO, SiO.sub.2 or any dielectric layer or any plastic materials.

9. The field-effect gate transistor according to claim 1, further comprising a gate terminal electrically connected to the channel, covering the substrate and coated with a dielectric layer.

10. A method for laser annealing a field-effect gate transistor, comprising the steps of (a) providing a field-effect gate transistor with a channel as external surface of the field-effect gate transistor, the channel being made of Cu.sub.xCr.sub.yO.sub.2 wherein the y/x ratio is greater than 1; and (b) irradiating the channel with a laser beam, wherein in step (b), the channel is irradiated by scanning the surface of the channel non-homogeneously with regard to the local annealing temperature and/or annealing time.

11. The method according to claim 10, wherein, in step (b), the channel is irradiated according to a gradient decreasing from the drain terminal to the channel and increasing from the channel to the source terminal.

12. The method according to claim 10, wherein the laser beam has a maximum power density comprised between 7 W/cm.sup.2 and 10 W/cm.sup.2 and a minimum power density comprised between 1 W/cm.sup.2 and 3 W/cm.sup.2.

13. The method according to claim 10, wherein step (b) is carried out at a temperature comprised between 600° C. and 1000° C. during a time comprised between 1 second to 1800 seconds.

14. The method according to claim 10, wherein the method further comprises a step (c) of cooling which is performed after step (b).

Description

DRAWINGS

(1) FIG. 1 is an exemplary schematic view of a field-effect transistor according to the invention, in accordance with various embodiments of the invention.

(2) FIG. 2 is an exemplary illustration of a comparison between the XPS spectrum of Cu.sub.0.66Cr.sub.1.33O.sub.2 as-deposited and as-annealed after 30 seconds and 4000 seconds, in accordance with various embodiments of the invention.

(3) FIG. 3 is an exemplary illustration of an elemental composition of the p− oxide type material in function of the etching time, in accordance with various embodiments of the invention.

(4) FIG. 4 is an exemplary illustration of a Raman spectrum of the Cu.sub.0.66Cr.sub.1.33O.sub.2 layer as-deposited and as-annealed, in accordance with various embodiments of the invention.

(5) FIG. 5 is an exemplary illustration of an evolution of charge carrier in function of the annealing temperature, in accordance with various embodiments of the invention.

(6) FIG. 6 is an exemplary illustration of results of KPFM studies, in accordance with various embodiments of the invention.

(7) FIG. 7 is an exemplary SEM image of the transistor, in accordance with various embodiments of the invention.

DETAILED DESCRIPTION

(8) With reference to FIG. 1, the field-effect transistor (FET) 1 of the present invention comprises one substrate 2, which is chosen from glass, Si, Si/Si.sub.3N.sub.4, SiO.sub.2, ITO, any dielectric material/layer or any plastic material like Kapton.

(9) In various embodiments, transparent substrates are employed, in order to provide transparent properties to the transistor. Glass, which may be covered or not by ITO, is thus the substrate that is generally used.

(10) The FET of the present invention has a source 6 with a source terminal 12, a drain 8 with a drain terminal 14, and a channel 10 with a gate terminal 16. The channel 10 extends between the source 6 and the drain 8.

(11) The source 6, drain 8 and channel 10 are advantageously made of Cu.sub.xCr.sub.yO.sub.2, via MOCVD deposition of Cu.sub.xCr.sub.yO.sub.2 delafossite thin-films. The subscripts x and y are positive numbers whose the sum is equal or inferior to 2.

(12) It has been revealed that a much higher concentration of Cr than Cu is present. Typically the Cr/Cu ratio is superior to 1 (y/x ratio>1).

(13) In various embodiments, the Cu.sub.xCr.sub.yO.sub.2 used in the present invention is an intrinsic semiconductor (or an undoped semiconductor). It is also highlighted that Cu.sub.xCr.sub.yO.sub.2 has been to take advantage of its transparency.

(14) In various embodiments, the crystalline structure of the material present of Cr/Cu ratio superior to 2 (y/x ratio>2) and has been identified as being Cu.sub.0.66Cr.sub.1.33O.sub.2.

(15) The copper chain vacancies in the crystal structure of Cu.sub.xCr.sub.yO.sub.2 are constituted in average by Cu vacancies in an amount ranging from 2 to 20.

(16) When the channel of the FET is irradiated with a laser beam (the laser beam has a maximum power density comprised between 7 W/cm.sup.2 and 10 W/cm.sup.2 and a minimum power density comprised between 1 W/cm.sup.2 and 3 W/cm.sup.2, in one example the laser beam has a power density equal to 4 W/cm.sup.2), it is possible to modulate locally the electrical conductivity of the Cu.sub.xCr.sub.yO.sub.2 layer or of the Cu.sub.0.66Cr.sub.1.33O.sub.2 layer.

(17) This local annealing has been carried out at a temperature comprised between 600° C. and 1000° C. during a time comprised between 1 second and 1800 seconds. Typically, the local annealing step is ranging from 1 second to 20 seconds.

(18) XPS spectrum (FIG. 2) demonstrates that the material, for instance in the case of Cu.sub.0.66Cr.sub.1.33O.sub.2, does not change in composition, even after an annealing of 4000 seconds.

(19) The plot depicted on FIG. 3 shows the results of an etching experiment. More precisely, it shows the composition of the p-oxide material in function of the etching time. This is a good indication that no alteration of the composition of the material is occurring during the annealing.

(20) In fact, during the deposition of Cu.sub.xCr.sub.yO.sub.2 onto the substrate, the material has several defects, related to the holes (positive charge carrier) in the atomic lattice of the material. By annealing the material, it has been found that these holes disappear. This “healing” of the atomic lattice can be observed by Raman spectroscopy (see FIG. 4).

(21) The Raman spectrum shows that the p-oxide layer of Cu.sub.0.66Cr.sub.1.33O.sub.2 as-deposited does not present a Raman peak at about 300 cm.sup.−1 (top plot on FIG. 4, marked as “before”) This absence of peak is featuring the presence of Cu-vacancies chains. Once the number of these crystalline defects diminishes, this peak appears. After annealing, it can be seen that the Raman spectrum does display this peak (below plot on FIG. 4, marked as “after”).

(22) FIG. 5 shows the concentration of charge carrier decreasing in accordance with the annealing temperature. Therefore, if less charge carriers are present, the insulator behavior of the material is increased.

(23) The KPFM (Kelvin Probe Force Measurement) studies were thus performed to obtain information about the composition and the electronic state of the local structures on the surface of the materials. KPFM studies have been carried on six samples, three from each set: both as-deposited reference samples plus two samples from a first set (15 min, 700° C. and 850° C.) and two from a last set (900° C., 30 s and 4000 s).

(24) The measurements were performed in alternate way between HOPG (Highly Oriented Pyrolytic Graphite) and one of the samples. The values are always compared to the latest reference value to avoid possible fluctuations of the tip work function (e.g. due to contaminations). In order to compensate the vacuum levels misalignment KPFM insert the voltage V.sub.DC=(Φ.sub.tip−Φ.sub.sample)/e where Φ.sub.tip(Pt—Ir)=5.5 eV. The samples have different doping levels and different Fermi levels were expected. When acceptor concentration N.sub.a increases, a decrease of the Fermi is expected and an increase of the work function Φ should be measured.
Ef−Ev=(X+Eg)−ΔWf
For the copper delafossites, the electronic affinity χ is 2.1 eV while the band gap Eg is 3.2 eV.

(25) The results are shown in FIG. 6, where the work-function difference vs. HOPG (Φ.sub.HOPG=4.4 eV) is shown as a function of the carrier concentration.

(26) It is to be noted that at mid-gap, namely at Eg=1.6 eV, the semiconductor is behaving as an intrinsic semiconductor, namely is not electrically conductive. For as-deposited samples (not annealed samples), the Fermi level is only 0.09 eV (thus far from the conduction band (CB) maximum) and the electrically conduction is therefore relatively high.

(27) When the samples are treated for 30 seconds at a temperature of 900° C., it can be seen on FIG. 6 that the Fermi level has increased to 0.43 eV. For an annealing step of 4000 seconds, the Fermi level has even increased to 1.19 eV, which is almost equivalent to the mid-gap value (1.6 eV). In this case, one has shown that the electrical conductivity can be modulated and that from an electrically conductive material, one can reduce the electrical conductivity and one can modulate it.

(28) For 15 minutes of annealing, at 700° C., the Fermi level has increased to 0.53 eV (from the 0.09 eV of the as-deposited material) while for 15 minutes at 850° C., the Fermi level has increased till 1.01 eV.

(29) After performing the local annealing, the transistor is cooled at room temperature.

(30) An advantage of this method of annealing after deposition is that, as the above, one can modulate the electrical conductivity of the material. Therefore, by doing a local annealing with the help of a laser beam, it has therefore been observed that the electrical conductivity can be modulated at specific place of the material. When the holes disappear, the electrical conductivity decrease, and vice versa. Laser annealing represents a major advantage since only a specific place of the carrier density in the material (actually, where the laser has been in contact with the material) can be modulated.

(31) The local annealing is particularly advantageous when at least two of the source, drain and channel are made of Cu.sub.xCr.sub.yO.sub.2 because the annealing can be modulated to adapt locally the concentrations of holes or positive charge carriers. For instance, the resulting FET shows a source and a drain of the p+ type whereas the channel is of the p− type.

(32) The resulting FET is very important when used in any transparent electronic devices.

(33) FIG. 7 is a SEM (Secondary Electron Microscopy) image of a transistor. The arrow shows the channel, where the gradient of hole is created.