Acknowledgement

The invention described hereafter has been generated within the research projects entitled "Semiconducting transparent oxide thin-films for the elaboration of p-n junctions" and "Defect Engineering of P-type Transparent Oxide Semiconductor", both supported by the National Research Fund, Luxembourg (Ref. INTER/MAT/1 1/02/PNOXIDES and C12/MS/3959502/DEPTOS).

Description

Technical field

[0001 ] The present invention is directed to the field of UV detector based on a transparent p-n junction providing a rectifying contact.

Background art

[0002] A rectifying contact only allows current to flow in one way, with a voltage- current relationship (l-V curve) that looks like a diode, by opposition to an ohmic contact which is metal-semiconductor junction that allows current to flow in both ways with an l-V curve that comes close to a resistor. A rectifying contact is often used in a rectifier circuit to convert AC to DC. The rectifying contact occurs due to the way the electronic band structure lines up at the junction.

[0003] Prior art patent application document published US 2007/01 14528 A1 relates to a rectifying p-n junction. This rectifying p-n junction, or diode, comprises an architecture made of five layers. A first layer is the substrate which can be among other glass, a second layer is made of a p-type oxide material, which is based on copper zinc oxide, copper tin oxide or copper zinc tin oxide, and a third layer is made of a n-type oxide material which can be among other ZnO. The second layer, i.e. the layer of the p-type oxide material also includes a metal species present in an amount ranging from about 10 atomic % to about 90 atomic % of the total metal in the p- type oxide material. SnO2, ZnO and/or a combination thereof can be used. A fourth and fifth layer are necessary to make the electrical contact with one of the other element composing the second layer of said p-type oxide material and the third layer of said n-type oxide material.

Summary of invention

Technical Problem

[0004] The invention has for technical problem to alleviate at least one of the drawbacks present in the prior art. More particularly, the invention has for technical problem to provide a rectifying p-n junction devoid of external electrical contacts, wherein the p-oxide layer and the n-oxide layer can be directly contacted by the electrical wires. Technical solution

[0005] The first object of the invention is directed to a UV detector comprising a p- n junction with a substrate coated with a layer of p-type oxide material, said p-type oxide material being coated with a layer of n-type oxide material which is ZnO. Said UV detector is remarkable in that said p-type oxide material consists of CuCrO2, wherein the Cr/Cu ratio within said layer of p-type oxide material is >1 .

[0006] According to a preferred embodiment, said substrate is glass, ITO or any plastic materials, preferentially glass.

[0007] According to a preferred embodiment, said substrate is electrically conductive and/or optically transparent.

[0008] According to a preferred embodiment, said layer of p-oxide material is in electrical contact with said layer of n-oxide material.

[0009] According to a preferred embodiment, said n-type oxide material which is ZnO is doped with a metal, in particular aluminium.

[0010] According to a preferred embodiment, the thickness of said layer of p-type oxide material is comprised between 60 nm and 200 nm, preferentially is comprised between 120 nm and 160 nm, more preferentially is equal to 140 nm.

[001 1 ] The second object of the invention is directed to a method for producing a UV detector in accordance with the first object of the invention, said method comprising the steps of (a) assembling at least one p-n junction with optical means adapted for sensing UV light and wherein said at least one p-n junction is formed by (b) growing a film of CuCrO2 on a substrate by metal organic chemical vapour deposition; and (c) coating said film of CuCrO2 with a film of ZnO by atomic layer deposition. Said method is remarkable in that said step (b) comprises at least one copper precursor and at least one chromium precursor in a ratio χ copper/chromium of 0.2 < χ < 0.66 in the presence of gaseous oxygen.

[0012] According to a preferred embodiment, that said step (b) is performed at a temperature comprised between 330°C and 430°C, preferentially at a temperature of 370°C.

[0013] According to a preferred embodiment, said at least one copper precursor is Cu(thd)2 and said at least one chromium precursor is Cr(thd)3.

[0014] According to a preferred embodiment, said step (b) is performed at a pressure of 6 mbar during 4 hours.

[0015] According to a preferred embodiment, the flux of said chemical precursors is comprised between 0.2 g/min and 2 g/min, preferentially is equal to 0.8 g/min.

[0016] According to a preferred embodiment, said step (b) is performed with a carrier gas, preferentially nitrogen gas, and preferentially at a flux of 1850 seem.

[0017] According to a preferred embodiment, said step (c) is performed with

Zn(C2H5)2 and water as chemical precursors.

[0018] According to a preferred embodiment, said step (c) is carried out at a pressure inferior to 5 mbar and at a temperature comprised between

100°C and 200°C, preferentially at 150°C.

[0019] According to a preferred embodiment, said optical means comprise at least one detection unit with at least one photodiode array. [0020] The third object of the invention relates to the use of the UV detector in accordance with the first object of the invention as flame detector.

[0021 ] The fourth object of the invention relates to the use of UV detector in accordance with the first object of the invention as chemical sensors based upon the molecules' absorption in the ultraviolet range.

[0022] The fifth object of the invention relates to the use of UV detector in accordance with the first object of the invention as spatial communication devices based upon long distance propagation of ultraviolet signals in space.

[0023] In general, the particular embodiments of each object of the invention are also applicable to other objects of the invention. To the extent possible, each object of the invention is combinable with other objects.

Advantages of the invention

[0024] The invention is particularly interesting in that these rectifying p-n junctions, if provided with optically transparent layers and substrate in particular, such as glass, ITO (Indium Tin Oxide) or plastic materials, can be used in transparent p-n junction devices such as UV-light emitting diodes (LEDs). Such devices, due to their sensitivity to the quantity of UV received by the junction, can further be used as photodiode arrays in the detection unit of UV detectors. Another advantage of the present invention lies in the fact that the bilayer composed of Cu _x Cr _y O2 and glass has proven to be conductive, in addition to its transparency.

Brief description of the drawings

[0025] Figure 1 a: Chemical composition in films as a function of precursors volume ratio χ.

[0026] Figure 1 b: XPS spectrum for χ=0.5.

[0027] Figure 2: Influence of the precursor composition on the crystallographic structure of the obtained films at I surface- 370°C.

[0028] Figure 3: SEM micrographs for films deposited at T _S urface=370 ^o C and (a) χ=0.2, (b) χ=0.33 or (c) χ=0.6.

[0029] Figure 4: Comparison of Cu _x Cr _y O2 thin films synthesized by pulsed injection MOCVD with others synthesis method reported in the literature.

[0030] Figure 5a: Transmittance as a function of χ.

[0031 ] Figure 5b: Absorption coefficient as a function of χ.

[0032] Figure 6a: Conductivity dependence on χ precursor's ratio.

[0033] Figure 6b: Temperature dependence on χ precursor's ratio of χ=0.5.

[0034] Figure 7: Hall coefficient as a function of applied magnetic field. Charge carrier (holes) concentration is determined by the slope.

[0035] Figure 8: Transmittance as a function of electrical conductivity.

[0036] Figure 9: Comparison between the XPS spectrum of Cuo.66Cr1.33O2 as deposited and as annealed after 30 seconds and 4000 seconds.

[0037] Figure 10: Elemental composition of the p-oxide type material in function of the etching time.

[0038] Figure 1 1 : Raman spectrum of the Cuo.66Cr1.33O2 layer as deposited and as annealed.

[0039] Figure 12: Evolution of carrier charge in function of the annealing temperature. Figure 13: Results of KPFM measurements. [0040] Figure 14: ZnO deposited on CuCrO ₂ .

[0041 ] Figure 15a: l-V curves of ZnO, CuCrO2 and p-n junctions with ZnO deposited on CuCrO2 coated glass with χ=0.5.

[0042] Figure 15b: l-V curves of ZnO, CuCrO2 and p-n junctions with ZnO deposited on CuCrO2 coated glass with χ=0.2.

[0043] Figure 16a: l-V curve of p-n junction with χ=0.33 for CuCrO2 coated glass.

[0044] Figure 16b: SEM cross section of the p-n junction.

[0045] Figure 17: Transmittance of the p-n junction with χ=0.33 for ALD-ZnO/

MOCVD CuCrO ₂ coated glass.

[0046] Figure 18: l-V curves of junction depending on UV illumination.

Description of an embodiment

[0047] The first part of the present invention concerns the deposition of Cu _x Cr _y O2 delafossite precursors (wherein x and y are positive numbers whose the sum is equal to 2 or inferior to 2)

[0048] A process was developed for the MOCVD deposition of CuCrO2 delafossite thin-films using (thd) precursors:

Cu(thd) _{2 (} g) + Cr(thd) _{3 (g)} + O _{2 (g)} →CuCrO ₂

The work related to the thin films synthesis studied the following parameters: substrate temperature (330°C<T _SU rface<430 ^o C), the Cu/Cr precursor's ratio, oxygen partial pressure, carrier gas flow rates and various geometrical parameters related to the design of the CVD reactor (the distance between the injectors and the substrate, the position of the grid, etc.). Reproducible Cu _x Cr _y O2 thin films were obtained using various substrates, such as glass, sapphire, Si, Si/Si3N ₄ , ITO, S1O2, any dielectric layer, or any plastic materials like Kapton.

[0049] Delafossite Cu _x Cr _y O2 was always deposited by MOCVD.

[0050] Preferentially, transparent substrates are employed, in order to provide transparent properties to the thin films. Such substrates can be glass, ITO or any transparent plastic materials, like Kapton, preferentially glass. Glass may be or may not be covered by ITO.

[0051 ] Preferentially, the concentration of the precursors was 5 mM and the substrate temperature was 370°C. When this temperature was used to achieve the MOCVD process, the electrical conductivity of delafossite increases. The pressure was 6 mbar, the oxygen flux was 1000 seem, the nitrogen flux was 1850 seem, and/or the precursors flux was 0.8 g/min. The pulsation frequency of the injector has been measured to 5 Hz. The substrate holder rotated at a speed of 15 rpm. The height of the substrate holder was 16 cm. Finally, the whole process lasted 4 hours.

[0052] XPS analysis was performed on thin films with various precursor ratio (x=Vcu(thd)2 (Vcu(thd)2+Vcr(thd)3). It reveals a much higher concentration of Cr than Cu for pure delafossite phase (Cr/Cu ratio within said layer is > 1 ), while a higher concentration of Cu than Cr was observed for CuCrO2 and copper oxides phases in films (figure 1 a). Moreover, carbon contamination in films with pure delafossite phase is below XPS detection, < 1 atomic % (figure 1 b).

[0053] Preferentially, the Cr/Cu ratio within said layer is superior to 2 (or a y/x ratio > 2). Even more preferentially, the material is Cuo.66Cr1.33O2.

[0054] The XRD diffractograms of films deposited with various values of χ are depicted in figure 2. In the MOCVD process, pure delafossite thin films are grown for 0.2 < χ < 0.6, while CuO and C112O phases are evidenced for χ > 0.66. The CU2O phase is detected when deposition was carried out with a precursor solution featuring χ = 1 , with a small signature of Cu metal at 2Θ = 43.5°. Interestingly, it is worth noting that no film is grown at χ = 0 (i.e. pure Cr(thd)3) and that excess of chromium precursor in the precursor solution do not lead to chromium oxide phases.

[0055] Deposition temperatures below 430 °C and/or with copper fraction χ < 0.66 are mandatory to avoid the formation of parasitic crystalline phases such as CuO and CU2O. Deposition temperatures above 330°C are also mandatory since no chemical reaction is observed below this temperature.

[0056] The surface SEM inspection (figure 3) indicates that the morphology of the pure phase delafossite is strongly impacted by the relative ratio of precursors in the solution χ. For χ = 0.2, the as-deposited films present a "spines" morphology, while for the χ = 0.33 and χ = 0.6, a grainy structure is observed. Films are more compact and the grain size decreases when the ratio χ increases. As measured from the SEM pictures, the average grain size is (160 ± 60) nm for χ = 0.33, while (56 ± 27) nm is determined for χ = 0.6. The thin-films thicknesses measured from cross-section SEM observations are 120, 140 and 160 nm for χ = 0.2, 0.33 and 0.6, respectively.

[0057] In figure 4, a comparison of Cu _x Cr _y O2 synthesis with time, temperature and number of step needed is shown. It appears that the Cu _x Cr _y O2 thin films syntheses have the lowest temperature by one step compared to the literature.

[0058] The optical transmittance was measured in the range of 250-850 nm for films grown on glass with 0.2 < χ < 0.8 at T _SU rface = 370°C, the corresponding spectra are shown in figure 5a. The range of thicknesses for all films is between 120-160 nm. The transmittance in the 400-800 nm range decreases from 40%-50%, for 0.2 < χ < 0.6, down to 30% for higher χ. For this latter case, the transmittance decrease is allocated to the concomitant CU2O and CuO phases, having a much lower bandgap (respectively 2.2 and 1 .6 eV) than CuCrO2. The obtained pure phase delafossite features a higher transmittance relative to the contaminated films.

[0059] The absorption coefficient and the corresponding normalized derivative are plotted in figure 5b. All films containing the delafossite phase feature a steep increase of the absorption at 3.1 -3.2 eV with a corresponding maximum for the derivative curve. This absorption is in line with the reported studies on the delafossite CuCrO2 phase. However, another steep increase of the absorption is noticed at 2.5 eV for χ > 0.66. This absorption edge at 2.5 eV is particularly steep for the CU2O reference coating (χ = 1 ). Thus, this analysis further confirms the growth of CU2O for χ > 0.66. Direct optical gap of 0.2 < χ < 0.5, are evaluated using a linear fit of the Tauc plot with the following relation:

(ak%>) ² = C(hv— E )

wherein hv is the photon energy, C is a constant and E _g is the optical band gap. The extracted E _g is found to be between 3.1 -3.3 eV, which is in very good agreement with previous experimental and theoretical works. [0060] Electrical conductivity at room temperature is plotted as a function of χ (figure 6a) for a process temperature of T _SU rface = 370°C. For all samples, the thickness ranges from 120-160 nm. The highest conductivity, at 17 S.cnrr ¹ , was obtained for χ = 0.5. To our knowledge, this electrical conductivity is the highest ever reported for an "undoped" single phase delafossite Cu _x Cr _y O2 thin film. Figure 6b shows the temperature dependence of conductivity for χ=0.5 and evidences a semiconducting behavior of our material because the conductivity increase when the temperature increase. Hall Effect was also done by variation of magnetic field between -9 and 9 T at constant current of 0.1 mA. Hall coefficient was plot as a function of the magnetic field but the slope was too low to determine accurately carrier concentration (figure 7). This led us to expect a positive charge carrier (holes) concentration up to 10 ²¹ holes/cm ^-3 and a mobility as low as 10 ^"3 cm ² A/.s.

[0061 ] Transmittance as a function of electrical conductivity for various synthesis methods described in the literature is plotted in figure 8 and is compared with the Cu _x Cr _y O2 thin films. Cu _x Cr _y O2 thin films have been observed to exhibit the highest trade-off transparency/conductivity.

[0062] The literature data used for this comparative analysis have been taken from the following twelve articles:

1 ) M. Han, K. Jiang, J. Zhang, W. Yu, Y. Li, Z. Hu and J. Chu, J. Mater.

Chem., 2012, 22, 18463-18470.

2) M. J. Han, Z. H. Duan, J. Z. Zhang, S. Zhang, Y. W. Li, Z. G. Hu and J. H. Chu, J. Appl. Phys., 2013, 114, 163526.

3) H.-Y. Chen and K.-P. Chang, ECS J. Solid State Sci. Technol., 2013,

2, P76-P80.

4) J. Wang, P. Zheng, D. Li, Z. Deng, W. Dong, R. Tao and X. Fang, J.

Alloys Compd., 2011 , 509, 5715-5719.

5) H.-Y. Chen, K.-P. Chang and C.-C. Yang, Appl. Surf. Sci., 2013, 273, 324-329.

6) D. Li, X. Fang, Z. Deng, S. Zhou, R. Tao, W. Dong, T. Wang, Y.

Zhao, G. Meng and X. Zhu, J. Phys. D. Appl. Phys., 2007, 40, 4910- 4915.

7) F. Lin, C. Gao, X. Zhou, W. Shi and A. Liu, J. Alloys Compd., 2013,

581, 502-507.

8) R. Nagarajan, A. D. Draeseke, A. W. Sleight and J. Tate, J. Appl.

Phys., 2001 , 89, 8022-8025.

9) R.-S. Yu and C.-P. Tasi, Ceram. Int., 2014, 40, 821 1-8217.

10) H. Sun, M. Arab Pour Yazdi, P. Briois, J.-F. Pierson, F. Sanchette and A. Billard, Vacuum, 2015, 114, 101-107.

1 1 ) G. Dong, M. Zhang, X. Zhao, H. Yan, C. Tian and Y. Ren, Appl. Surf.

Sci., 2010, 256, 4121-4124.

12) S. Mahapatra and S. A. Shivashankar, Chem. Vap. Depos., 2003, 9, 238-240.

[0063] In order to modulate the electrical conductivity of the Cu _x Cr _y O2 layer, a new process has been developed. Starting from the electrically conductive layer, an annealing (between 600°C and 1000°C during a time ranging from 20 seconds to 4500 seconds), preferentially at a temperature of 900°C) has been performed.

[0064] This has for effect to modulate the electrical conductivity. [0065] XPS spectrum (figure 9) demonstrates that the material, for instance in the case of Cuo.66Cr1.33O2, does not change in composition, even after an annealing of 4000 seconds.

[0066] The plot depicted on figure 10 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.

[0067] In fact, during the deposition of Cu _x Cr _y O2 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 figure 1 1 ).

[0068] The Raman spectrum shows that the p-oxide layer of Cuo.66Cr1.33O2 as- deposited does not present a Raman peak at about 300 cm ^-1 (top plot on figure 1 1 , 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 figure 1 1 , marked as "after").

[0069] The copper chain vacancies in the crystal structure of Cu _x Cr _y O2 are constituted in average by Cu vacancies in an amount ranging from 2 to 20.

[0070] Figure 12 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.

[0071 ] 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).

[0072] 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 VDC = (cfctip- <t>sampie)/e where eV. The samples have different doping levels and different Fermi levels were expected. When acceptor concentration N _a increases, a decrease of the Fermi is expected and an increase of the work function Φ should be measured.

Ef - Ev = (X + Eg) - AWf

For the copper delafossites, the electronic affinity χ is 2,1 eV while the band gap Eg is 3,2eV.

[0073] The re wn in figure 13, where the work-function difference vs.

HOPG eV) is shown as a function of the carrier concentration.

[0074] 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. [0075] When the samples are treated for 30 seconds at a temperature of 900°C, it can be seen on figure 13 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.

[0076] 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.

[0077] An advantage of this method of annealing after deposition is that, as said 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 material (actually, where the laser has been in contact with the material) can be modulated.

[0078] The local annealing has been carried out with a laser, 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.

[0079] The power density of the laser beam used in the local annealing step ranges from 1 W/cm ² to 10 W/cm ² . In a typical example, the power density is equivalent to 4W/cm ² .

[0080] In the case of the diode, the laser annealing is performed on the surface of the Cu _x Cr _y O2 layer, resulting in gradient of holes concentration. It has been observed that there are more holes (about 10 ²¹ holes/cm ³ ) in the region of the materials neighboring the substrate, while the region being distant from the substrate and being impacted by the laser annealing significantly comprises less holes (about 10 ¹⁷ holes/cm ³ ). The gradient of holes concentration is thus in the transverse direction. In other words, said gradient is a transversal gradient decreasing from the substrate to the the layer of n-type oxide material.

[0081 ] While the growth of Cu _x Cr _y O2 delafossite on ZnO-coated glass raises issues related to (1 ) thermal stability of PVD-made intrinsic ZnO thin film and (2) phase contamination of the grown delafossite (dependent on ZnO underlayer feature), the growth of ZnO by ALD on Cu _x Cr _y O2 coated glass gave improved results (figure 14).

[0082] Atomic Layer Deposition (ALD) is a chemical growth process inspired by the CVD process. The main difference is that reactants are injected sequentially in the reactor, thus decomposing the chemical reaction. Each reactant will then saturate the surface before being purged, thus forming a single layer. The second reactant will then be injected in order to saturate the surface coated with the first layer before being purged. This process leads to a layer-by-layer deposition of materials.

[0083] The reactant used for the growth of pure ZnO thin film is a zinc precursor (Zn(C2H5)2 or Zn(thd)2) and water.

[0084] Alternatively, the ZnO film can be doped with a metal in order to provide a free conduction electron in the film and to improve subsequently the electrical conduction. Preferentially, the metal used for the doping is aluminum. Aluminium may be used in a concentration comprised between 0.25 % and 3 % in regards with the total concentration of metal in the thin film.

[0085] The pressure that is used is preferentially inferior to 5 mbar, while the temperature is preferentially comprised between 100°C and 200°C. For example, the temperature is 150°C. The purge time between both reactants are 10 s, while the zinc precursor is pulsed at 150 ms and water is pulsed at 200 ms.

[0086] The l-V characteristic of p-n junctions with ratio χ = 0.5 and χ = 0.2 is presented in figures 15a and 15b, respectively. With the approach in figure 10, the electrical conductivity of ZnO and Cu _x Cr _y O2 are not affected by ZnO synthesis (Tde _P osition=150 ^o C).

[0087] The l-V characteristic of ZnO on Cu _x Cr _y O2 coated glass with χ = 0.33 is presented figure 16a. In that case, a rectifying effect is obtained and thus a p-n junction electrical signature. A cross-section of the junction as- obtained with FIB (Focused Ion Beam) (figure 16b), shows a conformal ZnO deposition over Cu _x Cr _y O2 .

[0088] A mushroom conformation is thus obtained thanks to the ALD process and to the grainy structure (due to the ratio Cr/Cu which is superior to 1 , preferentially superior to 2).

[0089] The transmittance of the overall junction in the visible range is reported figure 17 with an averaged value between 400-800 nm of 40%. For comparison, p-n junction ZnO/CuCrO2 synthesized by PVD method only (almost not compatible with standard glass substrates), have a transmittance of 50 % as reported in the literature.

[0090] The l-V curve of the junction under UV illumination was also studied (figure 18). It appears that l-V curves are sensitive to the quantity of UV on the junction. This characteristic can be used for UV detections.

[0091 ] Therefore, the transparent three-layer architecture p-n junction of the present invention, comprising thus glass as a substrate (first layer) on which a layer of p-oxide material is coated, said p-oxide material being Cu _x Cr _y O2 with a ratio χ copper/chromium of 0.2 < χ < 0.66 (second layer), and a layer of n-oxide material coated on said layer of p-oxide material, said n-oxide material being ZnO (third layer), can be used as a diode or solar cells. In particular, the diode can be placed within optical means of a UV detector, or a UV sensor, or in a light-emitting diode (LED).

[0092] Said optical means are adapted for sensing UV light and can comprise at least one detection unit with at least one photodiode array. By UV light, one understands a light with a wavelength comprised between 10 nm and 380 nm.

[0093] Said optical means can also comprise an optical lens.

[0094] For UV detector, the thickness of the Cu _x Cr _y O2 material is usually comprised between 50 nm and 5 μιτι. The layer of the p-oxide material can be thick (1 μιτι).

[0095] The diode array detector with such three layer architecture p-n junction can therefore detect absorption in the UV region.

[0096] Such UV sensor/detector can be used as a flame detector by sensing the variation of the UV emission in its direct environment. [0097] Additionally, other applications are possible such as chemical sensors based upon the molecules' absorption in the UV range and/or spatial communication devices based upon long distance propagation of ultraviolet signals in space.

Experimental part

[0098] The annealing processes were performed in a Rapid Thermal Annealing reactor (Annealsys) at different temperatures and for various time intervals in conditions similar with those during deposition process. Electrical properties were measured using four probes linear configuration. Transmission and reflectance spectra were acquired in the range from 1500 to 250 nm using a Perkin Elmer LAMBDA 950 UVA/is/NIR Spectrophotometer with a 150 mm InGaAs Integrating Sphere. For X-Ray Photoemission Spectroscopy (XPS) analysis a Kratos Axis Ultra DLD system using a monochromated (Al Ka: hv=1486.7 eV) X-ray was used.source. The Kevin Probe Force Microscopy (KPFM) measurements have been performed on a Bruker Innova using the surface potential mode as amplitude modulation. Surface topography is obtained in the first pass and the surface potential is measured on the second pass. Freshly cleaved highly-oriented pyrolitic graphite (HOPG) is used as reference. The measurements are performed under dry N2 atmosphere in order to avoid water condensation on the surface.