ETCHED SILICON BASED DEVICES AND METHODS FOR THEIR PREPARATION

Abstract

A device for converting radiation to electrical energy having a hybrid interface structure comprising an etched silicon surface and organic layer connected thereto. The invention provides methods for the preparation of said etched silicon surface and said hybrid interface.

Claims

1. A method for preparing a silicon hybrid interface structure comprising the following steps: a) Providing an etched Si substrate having Si—H bonds on the surface; b) Exposing the silicon surface of step (a) to chlorine containing gas while illuminating the surface with light; and c) Treating the Si substrate of step (b) with one or more organic reactant(s) to produce a first organic layer which is covalently bonded to the silicon surface.

2. The method of claim 1, wherein the Si substrate provided in step (a) is an etched silicon substrate characterized in having a nanowire (NW) morphology.

3. The method of claim 1, wherein the substrate of step (a) is provided by an etching procedure comprising the steps of: subjecting an H-terminated Si surface to a solution comprising an oxidizable aggregation agent and an acid, oxidizing the surface to remove aggregated material formed on the Si substrate and washing the surface.

4. The method of claim 3, wherein said oxidizable aggregation agent is selected from the group consisting of silver nitrate (AgNO.sub.3), chloroauric acid (HAuCl.sub.4), silver acetate (AgCO.sub.2CH.sub.3), silver benzoate (AgCO.sub.2C.sub.6H.sub.5), Iron(II) acetate, Iron(III) chloride, Fe(NO.sub.3).sub.3, Ag (s), Au (s), Pt (s), Cr(s) and tetramethylammonium hydroxide.

5. The method of claim 4, wherein said oxidizable aggregation agent is silver nitrate (AgNO.sub.3) dissolved in HF solution.

6. The method of claim 3, wherein said oxidation of the aggregation agent is achieved by utilizing H.sub.2O.sub.2 and HF mixture.

7. The method of claim 3, wherein said washing is carried out by utilizing nitric acid.

8. The method of claim 3, wherein the etching procedure is preceded by and followed by a surface treatment comprising the steps of a) removal of Si-oxide layer with the aid of HF/NH.sub.4F solution, b) thermally growing a fresh Si-oxide layer and c) removal of said freshly grown Si-oxide layer to obtain H-terminated Si surface.

9. The method of claim 1, wherein the illumination of step (b) is of an intensity range of about 1 mW to about 5 mW and wavelength of between about 400 nm to about 550 nm.

10. The method of claim 1, wherein the organic reactants for the first organic layer of step (c) are selected from the group consisting of amine, alcohol, halide and alkylating reagent.

11. The method of claim 1, wherein the reactants of step (c) are characterized in having at least one functional group, such that said group reacts with the Si surface atom/s.

12. The method of claim 1, further comprising the step of reacting the Si surface having the first organic layer thereon with a second organic reactant to provide a second layer, said second organic layer being either covalently bound or physically adsorbed to the first organic layer.

13. The method of claim 12, wherein the second organic layer comprises functional electrical molecules selected from n-type molecules and p-type molecules, optically active molecules and combination thereof.

14. A method for the preparation of etched silicon substrate having nanowires morphology comprising the steps of: (a) providing a hydrogen terminated silicon substrate; (b) exposing the substrate obtained in step (a) to an oxidizable aggregation agent and an acid; (c) oxidizing the surface to remove aggregated material formed on the Si substrate; and (d) washing the silicon substrate obtained in step (c).

15. The method of claim 14, wherein said oxidizable aggregation agent is selected from the group consisting of silver nitrate (AgNO.sub.3), chloroauric acid (HAuCl.sub.4), silver acetate (AgCO.sub.2CH.sub.3), silver benzoate (AgCO.sub.2C.sub.6H.sub.5), Iron(II) acetate, Iron(III) chloride, Fe(NO.sub.3).sub.3, Ag (s), Au (s), Pt (s), Cr(s) and tetramethylammonium hydroxide.

16. The method of claim 15, wherein said oxidizable aggregation agent is silver nitrate (AgNO.sub.3) dissolved in HF solution.

17. The method of claim 14, wherein said oxidation of the aggregation agent is achieved by utilizing H.sub.2O.sub.2 and HF mixture.

18. The method of claim 14, wherein said washing is carried out by utilizing nitric acid.

19. The method of claims 14, wherein the etching procedure is preceded by and followed by a surface treatment comprising the steps of a) removal of Si-oxide layer with the aid of HF/NH.sub.4F solution, b) thermally growing a fresh Si-oxide layer and c) removal of said freshly grown Si-oxide layer to obtain H-terminated Si surface.

20. A device for converting radiation to electrical energy having a hybrid interface structure, said hybrid interface structure is prepared according to claim 1.

21. A device for converting radiation to electrical energy having a hybrid interface structure comprising: i) An etched Si substrate; and ii) An organic layer; wherein said etched Si substrate surface is coated with said organic layer, and wherein said etched Si substrate is substantially free of Si—O bonds.

22. The device according to claim 21, wherein the Si substrate is a thin layer Si substrate or of an unpure Si source.

23. The device according to claim 21, wherein the organic layer is chemically bound to the etched Si surface, and wherein said layer is between about 2.5 Å to about 50 Å in thickness.

24. The device according to claims 21, wherein said organic layer comprises a top organic layer which is physically adsorbed to a bottom organic layer, said bottom organic layer is covalently bound to the Si substrate.

25. The organic layer of claim 21, wherein the organic layer comprises functional electrical molecules selected from n-type molecules and p-type molecules, optically active molecules and combination thereof.

Description

DESCRIPTION OF THE FIGURES

[0062] FIG. 1 represents schematic diagram of pre-etching, etching and post etching processes. The pre-etching and post etching are identical, and are meant to remove any Si—O bond from the Si surface. The exemplary schematic etching process based on three solutions: solution I is based on AgNO.sub.3/HF, solution II is based on H.sub.2O.sub.2/HF and solution III is based on HNO.sub.3.

[0063] FIG. 2 represent a schematic illustration of the silicon structure and chemical connectivity of a single silicon nanowire before and after the post-etching step.

[0064] FIG. 3 (A) depicts SEM image of the etched Si surface. (B) Schematic image for changing the refractive index gradually from 1 to 3.5.

[0065] FIG. 4 Raman (A) and reflection (B) spectra of etched and non-etched (normal) Si surface, demonstrating the increase in the absorption coefficient of etched Si (1-2 μm) relative to the normal Si i.e. bulk silicon (35%). The reflectivity of the etched Si decreased below 10% i.e. the etched Si can absorb most of the incident light, more than 90%.

[0066] FIG. 5 (A) Porosity of the etched Si as a function of etching time, (B) Reflectivity measured at λ=600 nm as function of the etching time.

[0067] FIG. 6 depicts the correlation between duration of exposure to 2 wt % HF (solution II of the etching process) to the length of the fabricated nanowires obtained in said process. (A) SEM side view images of different times of exposure. (B) graph summarizing the correlation between the NW length and acid exposure duration.

[0068] FIG. 7 demonstrates a schematic representation of the grafting of first layer (passive) molecules through different Si—X bonds, where X is C, N, or O. The grafting of the passive molecules is taking place on an etched silicon substrate, and the illustration includes the surface treatment as described in Example 1.

[0069] FIG. 8 demonstrates a schematic representation of Grafting of active functional molecules. Some possible active functional molecules: (a) Electrical active molecule: Charge transfer from the cobaltocene to the etched Si. (b) Photoisomer: the optical active molecule (Azobenzene) grafted though Si—N bonds. (c) Photoluminescence: the optical active molecule-4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran will emit a red light.

[0070] FIG. 9 depicts the XPS characterization of silicon surfaces after different stages of functionalization. (A) comparison between fully functionalized silicon surface having organic layer deposited on its surface (a), silicon after pre-etching process (b) and an untreated silicon surface (c). (B) deconvoluted XPS C1s peak comparing the contribution of different carbon bonds in a silicon sample before organic layer grafting (top) and after grafting (bottom).

[0071] FIG. 10 (A) depicts photoelectron yield Y(hv) versus photon energy for etched Si before and after organic layer grafting. The organic layer grafted on the Si surface is methyl. (B) J-V curve of etched Si before (open squares, oxide layer) and after (filled squares, organic layer) organic layer grafting. The organic layer grafted on the Si surface is methyl. Inset: top view of etched Si solar cell device structure. (C) Energy diagram of the etched Si interface in thermal equilibrium. The arrows indicate different magnitude of hole injection from the etched n-Si into the p-type. On the left injection from Si oxide to PEDOT:PSS and on the right from Si covered with methyl (no oxide layer) into PEDOT:PSS. The reduction of surface state density D.sub.it as a result of the organic layer grafting depicted by the number of bars.

[0072] FIG. 11 depicts a current-voltage (I-V) characteristics of Si NW solar cells with and without grafted molecules DMSO and ethylene glycol under AM1.5 illumination.

[0073] FIG. 12 depicts an XPS analysis of two surfaces: one is after a post etching process and the other is without said post etching, but after only having the pre-etching surface treatment. The stability measurement corresponds to the amount of hydrogen bonds detected on the silicon surface.

EXAMPLES

Nano-Reactor

[0074] A nano-reactor that can maintain constant high temperatures (up to 500° C.) and pressures (up to 350 bars) was utilized. In addition, the nano-reactor is also fully programmed at a high resolution of time durations (in the scale of seconds), pressure steps (100 bars in 1 min), and temperature steps (100° C. in 1 min) to generate shock-waves. The extreme conditions are important to overcome the Van-Der-Waals interaction (while preserving the etched morphology of the Si) and to obtain a full molecular coverage regardless to the molecular steric effect.

Example 1—Surface Treatment to Remove Silicon Oxide (Si—O Bonds)/Pre-Etching and Post Etching

[0075] Silicon substrates (wafers) of both Si (100) and Si (111) were cleaned by washing with isopropanol and drying under N.sub.2 (g) for 10 s. Then, the samples were sonicated with isopropanol for 30 s under 40 Hz and dried under N.sub.2 (g) for 10 s. The obtained Si samples were immersed in buffered HF solution (pH=6) for 30 s and then moved into a solution of 70 wt % NH.sub.4F for 30 s. The samples were then rinsed in DI water (18 MΩ.Math.cm) for <10 s to limit oxidation and dried in flowing N.sub.2 (g) for 10 s. The Si samples were thermally oxidized in O.sub.2 with 30% humidity at 500° C. for 5 min in order to achieve fully oxidizes Si surface. Then the obtained Si samples were immersed again in buffered HF solution (pH=6) for 30 s and moved into a solution of 70 wt % NH.sub.4F and later vacuumed under 10.sup.−6 torr for 10 min to remove (as much as possible) the remaining of the solvent residuals.

Example 2—Surface Etching

A) Silicon Surface Etching Procedure

[0076] Ag nanoparticles were deposited on the hydrogenated Si wafer surface obtained from Example 1 by immersing the Si wafer in aqueous solution of 0.02 M silver nitrate (AgNO.sub.3) and 5 M HF in a volume ratio 1:1 (solution I) for 20 s. In the second step, the Si wafer was immersed in a 50 mL solution containing 5 M HF and 30% H.sub.2O.sub.2 in the volume ratio 10:1 (solution II) in a Teflon vessel for 20 min at room temperature. Next, the surfaces obtained after the etching procedure with solution I and II were rinsed several times in deionized water and dried at room temperature. Finally, the whole wafer was washed in a concentrated (65%) nitric acid (HNO.sub.3) for 15 min to remove residual Ag nanoparticles from the Si NW surfaces. The post etching process including the Si surfaces' hydrogenation, oxidization in 500 degrees and re hydrogenation took place as described in Example 1.

B) Silicon Surface Preparation—Effect of Duration of Exposure to Acid

[0077] In order to study the influence of the acid exposure duration on the growth of the silicon NW (in solution II), phosphorous doped i.e. n-type (N.sub.D=10.sup.14 #P atoms.Math.cm.sup.−3, 10-50 Ω.Math.cm) silicon (Si) wafers with <100> crystal orientations were used. Prior to etching, the Si [0.5 cm×0.5 cm] samples were cleaned in acetone, isopropyl alcohol (IPA), deionized water (DI), and piranha solution (H.sub.2SO.sub.4:H.sub.2O.sub.2=3:1) respectively for 10 min, followed by thorough rinsing in DI water. Finally, the wafer was dipped in 10M Hydrofluoric acid (HF) for 10 min to remove the native oxide layer.

[0078] Nest, silver (Ag) nano-particles were deposited on the Si wafer surface by immersing the silicon surface in an aqueous solution consisting of 5M HF and Silver nitrate (AgNO.sub.3) 20 mM in 1:1 volume ratio (solution I) for 20 s. Then, the silicon wafer was immersed in a solution containing 2% HF (10 mL) and 30% H.sub.2O.sub.2 (2 mL) and DI water (18 mL) (solution II), in a Teflon dish at room temperature. The immersion time was varied from 5 to 3840 seconds in order to follow the growth of nanowires with time.

[0079] The etching process is quenched (after fixed time) by immediately dipping the wafer in a concentrated (65%) nitric acid (HNO.sub.3) for 15 min to remove residual Ag nanoparticles which induce the selective etching process of the Si NWs. Each silicon wafer surface obtained after the etching was rinsed thoroughly in DI water several times and dried in air.

[0080] The surface treatment as described above in Example 1 was repeated before further sample processing in order to obtain stable hydrogen bonds. To this end, the stability of the hydrogen surface bonds was studied by measuring the rate of surface oxidation upon exposure to ambient conditions. XPS analysis was performed at different times on two surfaces—one after post etching and one which was not exposed to post etching process, but only after the pre-etching process. The Si2p XPS pick was followed to track the formation of silicon oxide on the surface. It was surprisingly found that after the post etching process, the hydrogen bonds formed on the silicon surface may remain stable for 2 days, while without performing said process, the Si—H bonds start tend to oxidize rapidly after about five minutes as can be seen in FIG. 12. SEM images with a corresponding graph depicting the correlation between the exposure time to acid and the elongation of the nanowires with time is presented in FIG. 6.

[0081] The surface etching detailed above was used for both kinds of Si wafers (100) and (111).

Example 3—Surface Chlorination

[0082] The freshly etched Si surface was placed under a stream of chlorine gas mixture (0.4% Cl.sub.2 and 99.6% N.sub.2) and illuminated under soft blue light (470 nm) for 20 minutes to form defect free Cl-terminated Si surfaces. The obtained surface was then rinsed with N.sub.2 flow.

Example 4—Surface Alkylation

[0083] First layer (passive) deposition procedures: the connection of the passive layer to the silicon surface was carried out as a covalent attachment.

[0084] A) 1 μl of butyl Grignard solution R—MgCl, [R═CH.sub.3(CH.sub.2).sub.2] 0.5M, was placed on the Si sample inside a nano-reactor under inert atmosphere. The reaction took place for 2 hours at 10 bars and 100° C. The Si sample was removed from the nano-reactor, rinsed in THF and later with methanol, and dried under a stream of N.sub.2(g), and kept under vacuum (10.sup.−6 torr) for 30 minutes. The resulted sample was characterized to have full coverage utilizing XPS.

[0085] B) Surface methylation: a hydrogenated Si substrate was exposed to radiation of hv=450 nm for to 1 minute under nitrogen atmosphere. Gas mixture of 99.6% of H.sub.2 and 0.4 Cl.sub.2 was introduced to the reactor. After this, methyl (CH.sub.3) was grafted onto the silicon surface utilizing a drop cast of CH.sub.3MgCl solution in THF. The concentration of the CH.sub.3MgCl solution was 0.5M and the volume of 1 microliter. After that, a shock wave of 5 atm for 5 minutes was applied.

[0086] Second Layer (Active) Deposition Procedure:

[0087] 1) Ethylene Glycol (p-Type Active Molecule):

[0088] An active layer of ethylene glycol was deposited to obtain physical adsorption via drop casting from 10 microliter ethylene glycol solution of concentration 0.1M on the CH.sub.3 layer obtained according to the surface methylation described above. The surface having the ethylene glycol on its surface was then exposed to shock wave of 5 atm for 5 min under nitrogen. After this, the sample was taken out and rinsed with N.sub.2 gun for 1 minute.

[0089] 2) Dimethyl Sulfoxide (DMSO) (p-Type Active Molecule):

[0090] The active layer DMSO was deposited via drop casting to obtain physical adsorption, utilizing a 10 microliter DMSO solution of concentration 0.1M on the CH.sub.3 layer obtained according to the surface methylation described above. The silicon having the DMSO on its surface was then exposed to shock wave of 5 atm for 5 min under nitrogen. After this, the sample was taken out and rinsed with N.sub.2 gun for 1 minute.

[0091] Characterization of Silicon Surface After Organic Layers Grafting

[0092] X-ray photoelectron spectroscopy (XPS) analysis was obtained utilizing the XPS of thermofisher. Core level spectra were excited by monochromatic Al Kα radiation (1487 eV), and photoelectrons were picked up at a takeoff angle of 35° enhancing the surface sensitivity of the technique to about 10-15 Å depth. Scan times of up to ˜1 h were employed for all data collections. Data analysis and precise binding energy positions, fwhm, and areas were calculated by peak fitting using the XPS Thermofisher advantage software. Peak fitting solutions were sought for χ.sup.2<1, where χ.sup.2 stands for the standard deviation.

[0093] A. First experiment was done in order to compare a silicon surface before any treatment “as is”, a silicon surface that was treated according to the pre-etching procedure and a silicon substrate having the organic molecules grafted onto its surface as described above.

[0094] As can be seen in FIG. 9A, the XPS depicted three main peaks named O1s, C1s and Si2p. The Si2p represents the silicon surface while C1s and O1s represent the organic moieties. With regards to Si2p peaks, it can be seen that after pre-etching this signal is the greatest since the silicon was clean and did not contain any further molecules onto its surface. After the organic layer deposition the silicon signal is lowered and an additional carbon signal which relates to the organic compounds is depicted. It should be further noted that the organic contamination on the surface of the silicon which was not cleaned (“as is”) was very high. According to the results, said carbon based contamination was completely removed by the pre-etching procedure.

[0095] B. A second experiment was carried out in order to compare the chemical nature of the attachment between the organic compounds that were found on the not-treated silicon surface (contaminations) and the organic compounds that were grafter to the silicon surface utilizing the procedure described above. Signal C1s was de-convoluted and the results are presented in FIG. 9B. According to graph A, there are no Si—C bonds in the sample before the treatment (and before the grafting) and therefore, the organic contamination was not covalently bonded to the silicon surface. In contrast, a clear peak attributed to Si—C bonds was depicted in the case of the silicon surface after the organic molecules grafting procedure (graph B), proving the successful covalent functionalization of the silicon surface.

Example 5—Preparation of a Device—a Solar Cell Based on Molecule-Si Junction

[0096] Hybrid solar cells were prepared from n-type (100) phosphorus-doped Si wafers with a conductivity of 0.1-0.5 Ω.Math.cm. Samples (14.7×14.7 mm.sup.2) were cleaned in acetone, isopropanol, and H.sub.2O in an ultrasonic bath for 5 min and etched in 5% hydrofluoric acid (HF) for 2 min to remove the native silicon oxide and to passivate the surface with hydrogen by Si—H bonds. An Ohmic Si back contact was fabricated, by e-beam evaporation of 40 nm Ti and 100 nm Au contact, followed by an annealing step at 850° C. for 1 hour. The samples were treated as explained in Examples 1-4 and were characterized by having the NW morphology of the invention coated with the organic molecules according to the exemplified procedure above. One sample of etched silicon surface functionalized with DMSO and one sample of etched silicon surface functionalized with ethylene glycol were utilized to construct solar cell. The samples were re heated up to 130 ° C. on a hot plate for 30 seconds to improve the wetting and structural properties of the molecule layer. A gold (Au) grid having a thickness of 50 nm was evaporated as a front electrode utilizing a shadow mask defining solar cells with an active area of 38.5 mm.sup.2. The evaporation was carried out utilizing E-beam evaporator (model BAK-501A), and was performed using metal evaporator with an acceleration voltage of 30 kV. The obtained solar cells were characterized by current—voltage (I-V) measurements using an AM 1.5 solar spectrum (Newport solar simulator) for illumination.

[0097] Results: FIG. 11 depicts the current-voltage (I-V) characteristics of ethylene glycol-based and DMSO-based solar cells under AM 1.5 illumination. On a bear silicon surface having no organic layer grafter thereon, the sample obeys a short circuit current (Jsc) of 10 mA/cm.sup.2, an open circuit voltage (Voc) of 570 mV and a conversion efficiency (η) of 2.7%. Upon achieving the grafted molecules layer, the measurement depicts improved performance with values for Jsc, Voc, η of 22.0 mA/cm.sup.2, 600 mV, 8%, respectively. The low values of Jsc in intrinsic solar cell is attributed to high contact resistances (Rs 300Ω).