METHOD FOR FORMING THIN FILM CHALCOGENIDE LAYERS

Abstract

The disclosed technology generally relates to chalcogenide thin films, and more particularly to ternary and quaternary chalcogenide thin films having a wide band-gap, and further relates to photovoltaic cells containing such thin films, e.g., as an absorber layer. In one aspect, a method of forming a ternary or quaternary thin film chalcogenide layer containing Cu and Si comprises depositing a copper layer on a substrate. The method additionally comprises depositing a silicon layer on the copper layer with a [Cu]/[Si] atomic ratio of at least 0.7, and thereafter annealing in an inert atmosphere. The method further includes performing a first selenization or a first sulfurization, thereby forming a ternary thin film chalcogenide layer on the substrate. In another aspect, a composite structure includes a substrate having a service temperature not exceeding 600 C. and a ternary chalcogenide thin film or a quaternary chalcogenide thin film on the substrate, where the ternary or quaternary chalcogenide thin film comprises a selenide and/or a sulfide containing Cu and Si.

Claims

1. A method of forming a chalcogenide thin film, the method comprising: depositing a copper layer on a substrate; depositing a silicon layer on the copper layer, wherein an atomic ratio of the deposited copper to the deposited silicon is at least 0.7; after depositing the copper layer and the silicon layer, annealing the deposited layers in an inert atmosphere, thereby forming a CuSi alloy layer; and after annealing, performing a first sulfurization of the CuSi alloy layer, thereby forming the chalcogenide thin film comprising a sulfide.

2. The method according to claim 1, wherein annealing is performed at a temperature between 400 C. and 600 C.

3. The method according to claim 1, wherein annealing is performed at a temperature, for a duration and under an ambience such that at least one of Cu.sub.3Si, Cu.sub.4Si and Cu.sub.5Si is formed, and wherein annealing is performed before performing the first sulfurization.

4. The method according to claim 2, wherein annealing is performed at a temperature between 400 C. and 450 C.

5. The method according to claim 1, wherein the first sulfurization is performed at a temperature between 400 C. and 600 C.

6. The method according to claim 1, wherein the first sulfurization is performed at a temperature, for a duration and under an ambience such that one or more of Cu.sub.2SiS.sub.3, Cu.sub.2S and CuS is formed.

7. The method according to claim 1, further comprising: depositing a metal on the chalcogenide thin film; after depositing the metal, further annealing in an inert atmosphere, thereby diffusing the metal into the chalcogenide thin film; and after further annealing, performing a second sulfurization of the metal-containing chalcogenide thin film, thereby forming a quaternary chalcogenide thin film comprising a sulfide.

8. The method according to claim 7, wherein depositing the metal comprises depositing a Zn layer.

9. The method according to claim 7, wherein further annealing is performed at a temperature between 350 C. and 450 C.

10. The method according to claim 7, wherein the second sulfurization is performed at a temperature between 400 C. and 600 C.

11. The method according to claim 7, wherein the second sulfurization is performed at a temperature, for a duration and under an ambience such that Cu.sub.2ZnSiS.sub.4 is formed.

12. A method of forming a chalcogenide thin film, the method comprising; depositing copper and silicon on a substrate, wherein a ratio of deposited copper atoms to silicon atoms exceeds 0.7; forming a CuSi alloy layer by subsequently annealing the deposited copper and silicon; and exposing the CuSi alloy layer to an atmosphere containing sulfur, thereby forming a chalcogenide thin film comprising a sulfide.

13. The method according to claim 12, wherein depositing copper and silicon comprises sequentially forming a copper layer and a silicon layer contacting each other.

14. The method according to claim 12, wherein the CuSi alloy layer includes at least one of Cu.sub.3Si, Cu.sub.4Si and Cu.sub.5Si, and wherein annealing is performed before exposing the CuSi alloy layer to the atmosphere containing sulfur.

15. The method according to claim 12, wherein the chalcogenide thin film includes one or more of Cu.sub.2SiS.sub.3, Cu.sub.2S and CuS.

16. The method according to claim 12, further comprising: depositing a metal on the chalcogenide thin film; after depositing the metal, further annealing, thereby diffusing the metal into the chalcogenide thin film; and after further annealing, further exposing the metal-containing chalcogenide thin film to a second atmosphere containing sulfur, thereby forming a quaternary chalcogenide thin film comprising a sulfide

17. A method of forming a chalcogenide thin film, the method comprising: depositing a copper layer on a substrate; depositing a silicon layer directly on the copper layer; forming a CuSi alloy layer by subsequently annealing the deposited copper and silicon layers; and sulfurizing the CuSi alloy layer to form the chalcogenide thin film comprising a sulfide.

18. The method according to claim 16, wherein the copper and silicon layers are such that a ratio of deposited copper atoms to silicon atoms exceeds 0.7.

19. The method according to claim 16, wherein annealing is performed at a temperature between 400 C. and 600 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 and FIG. 2(a) to FIG. 2(d) schematically illustrate process steps of a method for forming a ternary chalcogenide layer in accordance with embodiments of the present disclosure.

[0022] FIG. 3 is an example experimental XRD (X-ray diffraction) spectrum illustrating the formation of copper silicide phases as an intermediate step in a process according to a method of the present disclosure.

[0023] FIG. 4 is an example experimental XRD spectrum illustrating the formation of a Cu.sub.2SiSe.sub.3 phase in a process according to a method of the present disclosure.

[0024] FIG. 5 is an example experimental XRD spectrum, illustrating the formation of a Cu.sub.2SiS.sub.3 phase in a process according to a method of the present disclosure.

[0025] FIG. 6(a) is an example experimental transmission spectrum for a thin film Cu.sub.2SiS.sub.3 layer formed according to a method of the present disclosure, in the wavelength range from 400 nm to 1200 nm.

[0026] FIG. 6(b) shows an example experimental reflection spectrum and an example experimental absorption spectrum for a thin film Cu.sub.2SiS.sub.3 layer formed according to a method of the present disclosure, in the wavelength range from 200 nm to 1200 nm.

[0027] FIG. 7 is a graphical illustration of determining the band-gap based on a plot of the energy dependence of (E).sup.2 for a Cu.sub.2SiS.sub.3 layer formed according to a method of the present disclosure.

[0028] FIG. 8 shows an example experimental photoluminescence spectrum of a Cu.sub.2SiS.sub.3 layer formed according to a method of the present disclosure.

[0029] FIG. 9 and FIG. 10(a) to FIG. 10(d) schematically illustrate process steps of a method for forming a quaternary chalcogenide layer in accordance with embodiments of the present disclosure.

[0030] FIG. 11 shows an example experimental XRD spectrum, illustrating the formation of a Cu.sub.2ZnSiSe.sub.4 phase in a process according to a method of the present disclosure.

[0031] FIG. 12 shows an example experimental transmission spectrum for a Cu.sub.2SiSe.sub.3 layer and for a Cu.sub.2ZnSiSe.sub.4 layer formed according to a method of the present disclosure.

[0032] FIG. 13 shows an example experimental reflection spectrum and an example absorption spectrum for a thin film Cu.sub.2ZnSiSe.sub.4 layer formed according to a method of the present disclosure, in the wavelength range from 200 nm to 1200 nm.

[0033] FIG. 14 is a graphical illustration of determining the band-gap based on a plot of the energy dependence of (E).sup.2 for a Cu.sub.2ZnSiSe.sub.4 layer formed according to a method of the present disclosure.

[0034] In the different drawings, the same reference signs refer to the same or analogous elements.

DETAILED DESCRIPTION OF CERTAIN ILLUSTRATIVE EMBODIMENTS

[0035] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the disclosure and how it may be practiced in particular embodiments. However, it will be understood that the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures and techniques have not been described in detail, so as not to obscure the present disclosure.

[0036] The present disclosure will be described with respect to particular embodiments and with reference to certain drawings but the disclosure is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not necessarily correspond to actual reductions to practice of the disclosure.

[0037] Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the disclosure described herein are capable of operation in other orientations than described or illustrated herein.

[0038] The present disclosure provides methods for forming ternary and quaternary thin film chalcogenide layers containing Cu and Si. A method of the present disclosure comprises: depositing on a substrate a copper layer; depositing a silicon layer on the copper layer wherein the [Cu]/[Si] atomic ratio is at least 0.7; performing a first annealing step in an inert atmosphere at a temperature preferably in the range between 400 C. and 600 C. to form CuSi phases; and afterwards performing a first selenization step or a first sulfurization step at a temperature preferably in the range between 400 C. and 600 C., thereby forming a ternary thin film chalcogenide layer.

[0039] The method of the present disclosure may further comprise: depositing on the ternary chalcogenide layer a metal layer such as a Zn layer; performing a second annealing step in an inert atmosphere at a temperature preferably in the range between 350 C. and 450 C.; and afterwards performing a second selenization step and/or a second sulfurization step at a temperature preferably in the range between 400 C. and 600 C., thereby forming a quaternary thin film chalcogenide layer.

[0040] Process steps of a method according to the present disclosure are schematically illustrated in FIG. 1 and in FIGS. 2(a) to 2(d).

[0041] Referring to FIG. 1 step 1 and FIG. 2(a), a thin copper layer 11 is deposited on a substrate 10, according to embodiments. Next (FIG. 1 step 2; FIG. 2(b)), a thin silicon layer 12 is deposited on the thin copper layer 11, according to embodiments. Examples of suitable and commonly used substrates for photovoltaic cells are soda-lime glass (SLG) and molybdenum-coated soda-lime glass (Mo/SLG), but other suitable substrates may be used. Known thin film deposition techniques may be used for depositing the thin copper layer 11 and the thin silicon layer 12, such as for example sputtering, electron beam evaporation, thermal evaporation or other processes and combinations thereof. The thickness of the Cu layer and the thickness of the Si layer may be calculated based on the desired thickness and the desired composition of the final absorber material layer. The ratio between the thickness of the Cu layer and the thickness of the Si layer may be calculated based on the required ratio between the Cu and Si atoms in the chemical formula. For example, for Cu.sub.2SiSe.sub.3, approximately two times more Cu atoms are needed as compared to Si atoms. Based on the atomic density of copper and the atomic density of silicon, the ratio between Cu layer thickness and the Si layer thickness, needed to achieve the approximate factor of two, can be calculated.

[0042] In a next step (FIG. 1 step 3; FIG. 2(c)) the substrate 10 with the stack of the copper layer 11 and the silicon layer 12 is annealed (first annealing), according to embodiments. According to some embodiments, the first annealing process is be performed in an inert atmosphere, e.g. in a nitrogen atmosphere or in an argon atmosphere, at a temperature preferably in the range between 400 C. and 600 C., thereby forming CuSi phases such as Cu.sub.3Si, Cu.sub.4Si and Cu.sub.5Si. Under some circumstances, at the higher annealing temperatures within this range, e.g. at temperatures in the range between 500 C. and 600 C., multiple phases are formed, which may be undesirable form some applications. Therefore in some embodiments, the annealing temperature at this process step is selected to be between 400 C. and 500 C., preferably close to 400 , for example in the range between 400 C. and 450 C., e.g. between 400 C. and 420 C., however, the present disclosure is not being limited thereto. This first annealing step may for example take about 5 to 30 minutes, preferably 10 to 15 minutes. As a result of this first annealing step a CuSi layer 20 (FIG. 2(c)) containing copper silicide phases (e.g., Cu.sub.3Si, Cu.sub.4Si and Cu.sub.5Si) is formed. The formation of these CuSi phases takes place over a range of chemical compositions, for a [Cu]/[Si] ratio being at least 0.7.

[0043] FIG. 3 shows an experimental XRD (X-ray diffraction) spectrum of a sample prepared according to a process described above with respect to FIG, 1 and FIGS. 2(a)-2(c). The particular example experimental XRD spectrum corresponds to sample prepared by first depositing a 500 nm thick Cu layer 11 on a soda-lime glass substrate 10, followed by depositing a 165 nm thick Si layer 12 on the Cu layer 11. Next an annealing step was done in a nitrogen environment at 400 C. for 15 minutes. The XRD spectra peaks measured as shown in FIG. 3 correspond to the known peaks of CuSi phases, thus confirming the formation of a copper silicide layer 20.

[0044] Referring back to FIGS. 1 and 2(d), after annealing (first annealing) in an inert atmosphere according the step 3 of FIG. 1 and FIG. 2(c), a first selenization process 4a, or alternatively a first sulfurization process 4b is performed, according to embodiments.

[0045] The first selenization process 4a may be performed by exposing the sample to a Se containing atmosphere, for example for 10 to 15 minutes at a temperature in the range between 400 C. and 600 C., for example in the range between 450 C. and 580 C. During the selenization process, Se atoms are incorporated into the CuSi layer 20, resulting in the formation of a ternary chalcogenide layer 30 (FIG. 2(d)) containing a ternary phase, e.g., Cu.sub.2SiSe.sub.3. The Se containing atmosphere may for example be an H.sub.2Se/N.sub.2 atmosphere or a Se vapor atmosphere, the present disclosure not being limited thereto.

[0046] FIG. 4 shows an example experimental XRD spectrum of a sample prepared according to a process described above with respect to FIG. 1 (steps 1-4a) and FIGS. 2(a)-2(d), wherein a selenization process was performed in a H.sub.2Se/N.sub.2 atmosphere for 15 minutes at 490 C., illustrating the formation of the Cu.sub.2SiSe.sub.3 ternary phase.

[0047] The first sulfurization process 4b may be performed by exposing the sample to a sulfur containing atmosphere, for example for 10 to 15 minutes at a temperature in the range between 400 C. and 600 C., for example in the range between 450 C. and 580 C. During the sulfurization process S is incorporated into the CuSi layer 20, resulting in the formation of a ternary chalcogenide layer 30 (FIG. 2(d)) containing the ternary phase Cu.sub.2SiS.sub.3. The sulfur-containing atmosphere may for example be an H.sub.2S/N.sub.2 atmosphere or a sulfur vapor atmosphere, the present disclosure not being limited thereto.

[0048] FIG. 5 shows an example experimental XRD spectrum of a sample prepared according to a process described above with respect to FIG. 1 (steps 1-4b) and FIGS. 2(a)-2(d), wherein a sulfurization process was done in a H.sub.2S/N.sub.2 atmosphere for 15 minutes at 540 C., illustrating the formation of the Cu.sub.2SiS.sub.3 ternary phase, as well as CuS phases. When this layer is used as an absorber layer in a photovoltaic cell, the binary CuS phases present in the film may for example be selectively removed using a KCN (potassium cyanide) etching step.

[0049] FIG. 6(a) shows an experimental optical transmission measurements on a thin Cu.sub.2SiS.sub.3 film (thickness about 700 nm to 800 nm) prepared in accordance with the present disclosure, in the wavelength range from 400 nm to 1200 nm. The optical transmission measurements were done using an integrating sphere. FIG. 6(b) shows the absorption and reflection of this film in the wavelength range between 200 rim and 1200 nm. Reflection measurements were done using an integrating sphere, and the absorption values were calculated based on the reflection and transmission measurements (absorption=1reflectiontransmission). The optical characteristics show a clear band-gap, with a high optical absorption at wavelengths below 650 nm and a low optical absorption at wavelengths higher than 750 nm.

[0050] Referring to FIG. 7, based on an extrapolation of the band-gap from the linear portion of the energy dependence of (E).sup.2 (=(h).sup.2), wherein is the optical absorption coefficient, an optical band-gap of 1.82 eV is obtained for the Cu.sub.2SiS.sub.3 layer formed in accordance with a method of the present disclosure. An experimental photoluminescence (PL) spectrum of the Cu.sub.2SiS.sub.3 layer, shown in FIG. 8, also shows a direct band-gap of 1.82 eV. The vertical dashed line corresponds to the location of the energy gap of the absorber material.

[0051] The ternary films 30 (Cu.sub.2SiS.sub.3 and Cu.sub.2SiSe.sub.3) formed according to a method of the present disclosure may be used as an absorber layer in a photovoltaic cell, e.g. as a wide band-gap absorber layer in a multi junction or tandem photovoltaic cell. These ternary thin films may also be used for other applications, such as for example for opto-electronics and semiconductor applications.

[0052] The ternary thin film layers 30 (Cu.sub.2SiS.sub.3 and Cu.sub.2SiSe.sub.3) described above with respect to FIG. 2(d) may be further processed to form quaternary thin film layers, according to embodiments. This is schematically illustrated in FIG. 9 and FIGS. 10(a) to 10(d).

[0053] FIG. 10(a) shows a structure comprising a substrate 10 and a ternary chalcogenide layer 30, corresponding to the structure shown in FIG. 2(d)). Starting from this structure, for example (FIG. 9 step 5, FIG. 10(b)), a thin film layer of zinc (Zn) 13, with a thickness in accordance with the required film composition, for example in the range between 100 nm and 160 nm, may be deposited onto the selenized or sulfurized ternary material layer (Cu.sub.2SiS.sub.3 or Cu.sub.2SiSe.sub.3) 30, using methods known in the art, such as for example by sputtering or evaporation.

[0054] Next, at step 6 (FIG. 9) a second annealing step is done in an inert atmosphere at a temperature preferably in the range between 350 C. and 450 C., for example in an N.sub.2 or Ar gas atmosphere at 390 C., e.g. for 10 to 15 minutes. This second annealing step causes diffusion of the Zn atoms into the ternary layer 30, resulting in a Zn containing layer 31 (FIG. 10(c)). This annealing step is done to enable diffusion of the Zn atoms into the previously formed ternary layer before adding further Se or S (i.e. before a second selenization or sulfurization step).

[0055] Afterwards a second selenization (step 7a) or a second sulfurization (step 7b) is done, by exposure of the sample with layer 31 to a H.sub.2Se containing gas, selenium (Se) vapor, a H.sub.2S containing gas or sulfur (S) vapor, at a temperature in the range between 400 C. and 600 C., for example at 490 C. for 10 to 15 minutes. This leads to the formation of a quaternary chalcogenide layer 40 containing the quaternary phase Cu.sub.2ZnSiSe.sub.4 or Cu.sub.2ZnSiS.sub.4 (FIG. 10(d)).

[0056] The experimental XRD spectrum shown in FIG. 11 confirms the presence of the quaternary phase alloy, as is evident from the presence of the (simulated in advance) XRD spectra peaks of Cu.sub.2ZnSiSe.sub.4.

[0057] FIG. 12 shows the transmission of a ternary Cu.sub.2SiSe.sub.3 thin film layer formed in accordance with a method of the present disclosure (layer 30 in FIG. 2(d) and in FIG. 10(a)) and of a quaternary Cu.sub.2ZnSiSe.sub.3 layer formed in accordance with a method of the present disclosure (layer 40 in FIG. 10(d)). The ternary Cu.sub.2SiSe.sub.3 thin film layer was formed by depositing a 500 nm thick Cu layer on a soda-lime glass substrate, depositing a 165 nm thick Si layer on the Cu layer, next performing an annealing (first annealing) in a nitrogen atmosphere at 400 C. for 15 minutes and finally performing a selenization step (first selenization) in a H.sub.2Se/N.sub.2 environment at 490 C. for 15 minutes. The quaternary thin film Cu.sub.2ZnSiSe.sub.3 layer was formed starting from this ternary Cu.sub.2SiSe.sub.3 thin film layer by depositing a 110 nm thick Zn layer on the ternary layer, performing an annealing (second annealing) in a nitrogen atmosphere at 390 C. during 10 minutes and finally performing a selenization step (second selenization) in a H.sub.2Se/N.sub.2 atmosphere at 490 C. for 15 minutes. From the transmission data shown in FIG. 12 it can be seen that the quaternary thin film has a substantially larger band-gap than the ternary thin film.

[0058] FIG. 13 shows the measured reflection spectrum and the absorption for the thin film Cu.sub.2ZnSiSe.sub.4 layer, in the wavelength range from 200 nm to 1200 nm. The vertical dashed line corresponds to the location of the energy gap of the absorber material. The high absorption values at higher wavelengths may be attributed to the presence of secondary phases. When this layer is used in a photovoltaic cell, for example in a tandem cell, the secondary phases may be removed selectively, for example by etching in KCN.

[0059] FIG. 14 illustrates the determination of the band-gap based on the energy dependence of (E).sup.2 for the Cu.sub.2ZnSiS.sub.4 layer formed as described above, showing a band-gap of 2.2 eV.

[0060] The foregoing description details certain embodiments of the disclosure. It will be appreciated, however, that no matter how detailed the foregoing appears in text, the disclosure may be practiced in many ways. It should be noted that the use of particular terminology when describing certain features or aspects of the disclosure should not be taken to imply that the terminology is being re-defined herein to be restricted to including any specific characteristics of the features or aspects of the disclosure with which that terminology is associated.

[0061] While the above detailed description has shown, described, and pointed out novel features of the invention as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the device or process illustrated may be made by those skilled in the technology without departing from the invention.