WO2002079085A2

WO2002079085A2 - Surface modification of porous silicon

Info

Publication number: WO2002079085A2
Application number: PCT/US2002/010061
Authority: WO
Inventors: Andrew Bocarsly; W. C. Wimbish
Original assignee: Princeton University
Priority date: 2001-03-30
Filing date: 2002-03-29
Publication date: 2002-10-10
Also published as: WO2002079085A3; AU2002254483A1

Abstract

A chemically modified, partially oxidized form of porous silicon is disclosed that is stable to further oxidation in harsh oxidizing conditions over an extended period of time and the method of manufacture of same. The chemical modification is accomplished with alkylsilanes to stabilize the oxidized layer from further oxidation. Porous silicon is selectively luminescent when exposed to SO2 making this form of silicon useful in the fabrication of a SO2 sensor.

Description

TITLE: SURFACE MODIFICATION OF POROUS SILICON

INVENTORS: Wimbish, J. Clinton

Bocarsly, Andrew B.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of provisional U.S. application serial no. 60/280,329 filed on 03/30/2001 and entitled "Surface Modification of Porous Silicon" by, Andrew B. Bocarsly and J. Clinton Wimbish the entire contents and substance of which are hereby incorporated in total by reference.

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION:

This invention relates to a composition of matter comprising photoluminescent porous silicon; more particularly to porous silicon with a partially oxidized porous silicon surface that remains stable over time, and the method of manufacture thereof.

DESCRIPTION OF RELATED ART:

Initially observed well over 40 years ago, porous silicon has only recently begun to be heavily investigated within research laboratories throughout the world. Porous silicon demonstrates an efficient room temperature luminescence in the later part (between 1.4 and 2.2 eV) of the visible spectrum. Several theories have emerged attempting to explain this luminescence. The leading theory ascribes the phenomenon to quantum confinement effects associated with the nanoscopic wires or crystallites that are believed to comprise the material.

As a result of its visible luminescence, porous silicon has attracted much attention based on its potential applications in optoelectronics and display technology. However, another field relevant to porous silicon that continues to develop is chemical sensing. It has been demonstrated that Bronstead bases, sulfur dioxide (S0₂), halogens, various organic solvents, metal ions, and organoamines quench the luminescence of porous silicon. Some of these molecular species exhibit reversible quenching of the luminescence while others quench by an irreversible process. Reversibility is defined as the restoration of luminescence by the simple removal of the quenching species; no further processing is needed to restore the luminescence of the porous surface. As a result, reversible quenching by a chemical species is highly desirable when constructing a sensor. Of the species that reversibly quench the luminescence of porous silicon, S0₂ is one with serious environmental implications due to its principle role in the formation of acid rain. The ability of porous silicon to detect S0₂ is dependent on the oxidized state of its surface. It has been demonstrated that hydride terminated porous silicon does not respond to S0₂ while a lightly oxidized surface does. Previous research on the quenching mechanism exhibited by S0₂ on lightly oxidized porous silicon (LOPS) centered on the role of an induced paramagnetic defect at the Si/Si0 interface. Paramagnetic defects in porous silicon and their effects on luminescent efficiency have been well documented. The presence of this quenching defect at the oxide interface provides explanation as to why a hydride terminated porous surface exhibits no response to S0₂. The luminescent nature of the quenching process offers excellent sensitivity with detection limits often in the ppm to ppb ranges. S0₂ can be detected at concentrations as low as 440 ppb. Moreover, oxidized porous silicon shows no response to other industrial gases such as CO, C0 , H S, and NOx. With such sensitivity and selectivity, a chemical sensor for S0 based on porous silicon may be constructed. A major problem with using oxide coated porous silicon for a chemical sensor for S0₂ is the oxide layer itself. Over time, the oxide layer thickens to a point where the luminescent silicon is no longer sensitive to the quenching effects of S0₂, and the sensor eventually fails. The principle reagent for the oxidation of the porous surface is water.

SUMMARY OF THE INVENTION

Briefly described, the invention comprises, a chemically modified, partially oxidized porous silicon surface that is stable to further oxidation in harsh oxidizing conditions over an extended period of time and the method of manufacture. This is accomplished by treating the porous silicon with an alkylsilane, preferably a chlorosilane, which partially oxidizes the porous silicon surface leaving it stable to further oxidation in harsh oxidizing conditions over an extended period of time. The sensitivity of the surface is not significantly compromised by the modification making this material a viable and inexpensive S0₂ sensor. BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 a-d illustrate spectroscopy results using Spectra Tech diffuse reflectance apparatus in a Nicolet 800 FT-IR spectrometer on the four chosen trichlorosilanes.

Figures 2a and 2b illustrate FT-IR data supporting the stability of the n- decyltrichlorosilane surface modification

Figures 3 a-d illustrate electron paramagnetic resonance (EPR) measurements using chlorosilane modified powder porous silicon samples.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The surface of lightly oxidized porous silicon is chemically modified with various hydrophobic protecting species to prevent undesirable excessive oxidation of the type which occurs in harsh oxidizing environments. Additional surface oxidation will interfere with the quenching of the luminescence. In addition, the surface modifications must not interact with the quenching species in any manner that would significantly compromise the sensitivity of the porous surface. Two types of negative interactions are envisioned. The attachment site may chemically alter the sensing site while a physical barrier to water may double as a barrier to S0₂. Balance between excellent protecting ability and minimal sensitivity loss is necessary for the production of a sensor based on this material. Of the chemical species available for the surface modification of porous silicon, alkylsilanes are the preferred embodiment. Alkylsilanes are preferred as the modifying species based on their ability to pack into a hydrophobic layer and ease of reaction with surface hydroxyls generated in the partial oxidation of the porous silicon. Varying hydrophobic alkane chain lengths, including fluorinated alkanes can be selected to find the optimal balance between maximum durability, hydrophobicity, and minimal sensitivity loss.

B-doped p-type single crystalline silicon was electrochemically etched in a 1 :1 : 1 volume solution of HF, ethanol, and distilled water. The etch time was 5 min at a constant current of 50 mA/cm². A two-compartment polyethylene cell was used where the compartments were separated by the silicon wafer held in place by O-rings. A platinum electrode was placed in each compartment. In this configuration, an anode and cathode may be induced on opposite sides of the silicon wafer by applying a bias across the electrodes. The anodic side is the site of porous silicon generation. The silicon wafers used were highly polished on one side. The porous layer was always produced on the polished side. After the etching period, the porous wafer was washed thoroughly in distilled water and ethanol before being dried under vacuum.

Porous silicon powder was constructed using a chemical stain etch. The stain etch consisted of a solution of 10 mL of distilled water, 10 mL of H₂S0₄ (96%), 2 mL of HF (48%), and approximately 0.25g of NaN0₂. Silicon powder (60 mesh, Aldrich Chemicals 99.999% purity) was immersed in this solution for 15 min with stirring. The solution was then filtered to obtain the porous silicon powder. The powder was repeatedly washed with significant amounts of deionized water followed by 50 mL of glacial acetic acid to enhance the luminescence.

Partial oxidation of the wafers of porous silicon was conducted by immersing the wafers in a solution of 30% H₂0₂ for a period of 10 min. After exposure to the H₂0 , the wafers were rinsed thoroughly in distilled water followed by ethanol before being dried under room temperature vacuum. The porous silicon powder was oxidized in a similar manner. The powder was immersed in 30% H 0 for a period of 15 min with stirring. The solution was then filtered to obtain the oxidized porous powder. The powder was rinsed with distilled water and ethanol before being dried under vacuum.

Characterization of the oxidized surfaces was carried out through FT-IR diffuse reflectance spectroscopy. Substantial oxide and hydroxide peaks were prevalent on both the wafer and powder samples. Moreover, the silicon hydride peaks were severely reduced in intensity after the oxidation process.

After oxidation, porous silicon wafers were chemically modified via reaction with alkylsilanes. The alkylsilanes are preferably hydrolytically unstable alkyltrichlorosilanes, preferably having an alkyl chain length ranging from C₇ to Cι Examples used were chlorosilane reagents obtained from Gelest Inc. and used as is. The chlorosilanes were placed directly on the surface of the porous silicon wafer under an atmosphere of argon for a period of 1-2 hours. Tridecafluoro-l,l,2,2-(tetrahydrooctyl)trichlorosilane, n- decyltrichlorosilane, and hexadecyltrichlorosilane exposures lasted for the maximum two hours while the n-heptyltrichlorosilane lasted for the minimum one hour. The high vapor pressure of the n-heptyl reagent was the cause for the bπiefer exposure time. Enough chlorosilane was used to cover the entire surface of the wafer. The use of excessive amounts of chlorosilane during the exposure should be avoided in order to avoid unwanted reagent polymerization effects. After exposure, the wafers were removed from the Ar atmosphere and washed in hexanes or toluene to remove any unreacted chlorosilane. The wafers were then dried and stored under vacuum. Characterization of the surface modification was performed using diffuse reflectance FT-IR spectroscopy. In order to get homogenous attachment of the silane to the powder, a vapor deposition was performed. After oxidation, the porous silicon powder was placed on one side of a petry dish and n-decyltrichlorosilane on the other under an atmosphere of Ar. The lid was placed on the dish, and exposure lasted 20 hours. The powder was then removed from the Ar atmosphere and washed thoroughly in hexanes to remove any unreacted reagent. The powder was then filtered and dried under vacuum. Characterization of the surface modification was performed using diffuse reflectance FT-IR spectroscopy.

To test the protecting abilities of the alkylsilanes towards excessive surface oxidation, the modified porous silicon wafers were continuously exposed to 95° C steam. The wafers were qualitatively checked periodically for luminescence and reversible quenching by S0 . Moreover, the FT-IR spectra were taken of the porous surfaces to determine the extent of degradation of the alkylsilane modifications. When an alkylsilane modified wafer demonstrated no luminescence and minimal bound alkylsilane by the FT-IR spectrum, it was deemed dead.

In order to determine the sensitivity losses associated with chemically modifying the porous silicon surfaces, modified and unmodified wafers were exposed to standard gas solutions of S0₂ in Ar, and the change in luminescence was measured via fluorimetry. Injecting S0₂ into set volumes of Ar using gastight microliter syringes generated the S0₂/Ar solutions. The concentrations of the test solutions were theoretically calculated and then experimentally substantiated through UV-Vis spectroscopy. FT-IR spectroscopy was performed using Spectra Tech diffuse reflectance apparatus in a Nicolet 800 FT-IR spectrometer. Luminescence quenching was observed with a SLM 8000C fluorometer or a Photon Technologies International Quantamaster fluorometer. UV- Visible measurements were conducted with a Hewlett-Packard 8453 UV-Vis spectrometer. Qualitative luminescence quenching in high S0₂ atmospheres was observed by eye with a hand-held UV lamp with a peak output at 365 nm. Electron paramagnetic resonance (EPR) spectroscopy was performed with a Bruker ESP300 X-band spectrometer. EPR spectra of porous silicon in the presence of S0₂ were obtained by filing a capped EPR tube with S0₂. Sufficient amounts of S0 gas were used to ensure a quenching response through the duration of the measurements.

Figure 1 a-d displays that the results for four trichlorosilanes that were successfully bound to individual silicon surfaces. CH symmetric and antisymmetric stretches are clearly visible in the 2800-2960 cm^"1 range along with CH₂ scissoring vibrations around 1460 cm^"1.

For the fluoride terminated chain, strong peaks resulting from coupling of C-C and C-F stretches are evident in the 1360-1090 cm^"' range.

The n-decyltrichlorosilane modification presented the highest durability during accelerated lifetime testing (95°C water vapor) providing adequate oxidation protection for an average of 1 125 hours. In contrast, unprotected porous silicon only shows a lifetime of 10-15 minutes before oxidation destroys the surface completely. The auspicious results of the n- decyltrichlorosilane most likely stem from more ordered chain packing among surface silane molecules. It has been well documented that ordering of alkane chains on solid substrates increases with increasing chain length. Figure 2a provides FT-IR data supporting the stability of the n-decyltrichlorosilane surface modification. The area of the 1466 cm^"1 peak in the infrared spectrum is indicative of the amount of n-decyl silane attached to the porous surface. Degradation of this surface only becomes evident after 900 hours of exposure and continues quite slowly until testing was terminated in the 1120 hour range.

The stability of the n-decyltrichlorosilane modified surface was compared to a tridecafluro-l,l,2,2-(tetrahydrooctyl)trichlorosilane modified porous silicon. This surface modifying reagent provides a shorter alkyl chain length than the n-decyl silane and has six carbons perfluorinated in contrast to hydrogen terminated. The lifetimes of the tridecafluro- l,l,2,2-(tetrahydrooctyl)trichlorosilane modified porous silicon surfaces in 95°C steam are significantly shorter than the n-decyl surfaces, lasting an average of 625 hours. Figure 2b illustrates the degradation of this surface modification by tracking the area of the 1364 cm^"1 infrared peak which is characteristic of a CF₃CF₂ functionality. Significant degeneration begins much earlier, after approximately 160 hours of testing. The relatively poor protecting ability of the tridecafluro-l,l,2,2-(tetrahydrooctyl)trichlorosilane may be attributed to a couple of factors. The most prevalent being the alkyl chain length. As mentioned before, ordering of alkyl chains drops dramatically if the chain length is under ten carbons. The other factor potentially contributing to the early demise of these surfaces is the presence of the larger fluorine atoms attached to the carbon backbone. The fluorines produce more irregular and rigid shapes of the carbon chains resulting in cavities between neighboring chains that are capable of solvating small molecules like water and oxygen. As a result of insufficient chain ordering and packing irregularities, the tridecafluro- 1,1, 2,2- (tetrahydrooctyl)trichlorosilane modification fails to preserve the partial Si/Si0₂ interface at the level observed for the n-decyl silane. Porous silicon surfaces modified with n-heptyltrichlorosilane and hexadecyltrichlorosilane were also constructed and tested to verify the postulated chain length effect. The n-heptyltrichlorosilane provided excellent S0₂ sensitivity results; however, the durability was quite poor. Lifetimes of surfaces modified with n- heptyltrichlorosilane lasted an average of 175 hours. The inadequacy of this modification was consistent with the hypothesis that alkyl chain length and associated surface packing are central factors in interface stability toward oxidative degradation. The hexadecyltrichlorosilane surface modification was not lifetime tested based on its extremely poor sensitivity towards S0₂.

Initial sensitivity measurements were administered by exposing all the freshly modified chlorosilane surfaces to a constant S0₂ concentration of 250 ppm. The results are summarized in Table I. The n-heptyltrichlorosilane demonstrated the greatest sensitivity to S0 approaching that of an unmodified surface. An inverse relationship is observed between sensitivity and alkyl chain length. The higher order of packing within the longer alkyl chains decreases the S0₂ permeability within the protective layers accounting for the sensitivity losses. The tridecafluro- l,l,2.2-(tetrahydrooctyl)trichlorosilane contradicts this trend by displaying higher sensitivity loss than the longer n-decyltrichlorosilane. According to the EPR results discussed later in this paper, the sensing site within the porous layer is not being altered by surface modification. Therefore, the tridecafluro- 1,1, 2, 2-(tetrahydrooctyl) trichlorosilane layer is acting as a barrier between S0₂ and the sensing site. This may be a result of unfavorable interactions between lone pair electrons on S0₂ and the lone pair electrons of the fluorine atoms.

To ensure that the quenching mechanism was not being altered by the surface modifications, electron paramagnetic resonance (EPR) measurements were undertaken using chlorosilane modified powder porous silicon samples. Data is provided in figure 3a-d. Unmodified partially oxidized powder porous silicon displays a characteristic EPR signal at g = 2.0035 when exposed to S0₂. This signal is associated with a P_t>l paramagnetic defect within the Si/Si0₂ interface that is EPR silent in the absence of S0₂. The onset of this signal indicates that photoluminescent quenching correlates with a S0₂ induced paramagnetic defect at the oxide interface. The results of the EPR experiments with the modified samples are similar to those of surface oxidized non-silane protected powder porous silicon samples. This finding indicates that the interfacial sensing site is not being altered by the surface modification treatment employed. The n-decyltrichlorosilane modified surface displays the best overall performance. Its endurance in the oxidation preventative testing was impressive while maintaining a good sensitivity to S0 showing a 2.3% change in photoluminescent intensity at 25 ppm of S0₂. Even after an average of 1125 hours of oxidation testing, n-decyl modified porous silicon still displayed a 1.3% photoluminescence change at a S0₂ concentration of 250 ppm. When compared with an unmodified surface, a freshly modified n-decyl surface showed a sensitivity loss of approximately 50%. This places the limits of detection of the n-decyl modified porous silicon at around 900 ppb. This limit of detection is well within S0₂ industrial stack emission ranges that are commonly 100-4000 ppm. The recovery time of porous silicon photoluminescence after exposure to S0₂ is 1.4 seconds enabling real-time sensing.

Claims

We claim:

1. A modified form of porous silicon comprising a chemically treated partially oxidized porous silicon surface that is stable to further oxidation under oxidizing conditions over an extended period of time.

2. The porous silicon of claim 1 wherein said chemical treatment comprises exposure to an alkylsilane.

3. The porous silicon of claim 2 wherein said alkysilane is a chlorosilane.

4. The porous silicon of claim 2 wherein said alkysilane is unstable alkylsilane.

5. A method of producing a chemically modified, partially oxidized porous silicon surface that is stable to further oxidation under oxidizing conditions over an extended period of time consisting of the steps: a) electrochemical ly etching the silicon surface in an etch solution; b) washing said silicon surface; c) drying said silicon surface under a vacuum; d) immersing said silicon surface in a hydrogen peroxide solution; e) rinsing said silicon surface; f) drying said silicon surface; g) chemically modifying said silicon surface by reaction with an alkylsilane; i) washing said silicon surface with to remove any unreacted alkylsilane.

6. The method of claim 5 whereby said etch solution is composed of HF, ethanol, and distilled water.

7. The method of claim 6 whereby said etch solution comprises a solution of 1 :1 :1 HF, ethanol, and distilled water by volume.

8. The method of claim 4 whereby said electrochemical etching step a) comprises the steps: placing said silicon surface in a two compartment electrochemical etching cell; separating the two compartments of said two compartment electrochemical etching cell by said silicon surface held in place by O-rings; pouring said etch solution into said electrochemical etching cell; placing a platinum electrode in each compartment of said two compartment electrochemical etching cell; electrochemically etching said silicon surface for 5 minutes at a constant electrical current of 50 mA per square centimeter of silicon surface.

9. The method of claim 7 whereby said washing step b) comprises washing said silicon surface with a solution of distilled water and ethanol.

10. The method of claim 8 whereby said immersing step comprises; immersing said oxidized silicon wafer in a solution of 30% hydrogen peroxide for 10 minutes.

1 1. The method of claim 9 whereby said washing step e comprises washing said silicon surface with distilled water.

12. The method of claim 10 whereby said washing step e further comprises the additional step of washing said silicon surface with ethanol.

13. The method of claim 11 whereby said step g) of chemically modifying said silicon surface further comprises placing alkylsilane solution on said silicon surface under an inert gas atmosphere for a period of between 1 and 2 hours..

14. The method of claim 7 whereby said alkylsilane solution is chosen from the group consisting of alkyltrichlorosilanes having an alkyl chain length ranging from C to Cι₈.

15. The chemically modified, partially oxidized porous silicon wafer produced according to the method of claim 2

16. A method of producing a chemically modified, partially oxidized porous silicon powder that is stable to further oxidation under oxidizing conditions over an extended period of time consisting of the steps: a) immersing silicon powder in an etch solution; b) filtering said etch solution and silicon powder mixture to separate the porous silicon powder; c) washing said porous silicon powder with deionized water followed by washing said silicon powder with glacial acetic acid; d) immersing said porous silicon powder in sulfuric acid solution to provide a protonated interface;; e) filtering said sulfuric acid solution to recover said porous silicon powder; f) depositing an alkyltrichlorosilane, on said porous silicon powder by vapor deposition under an atmosphere of Argon to chemically modify said porous silicon powder; g) washing said chemically modified porous silicon powder in hexanes; h) filtering said chemically modified porous silicon powder; i) drying said chemically modified porous silicon powder under vacuum.

17. The method of claim 16 whereby said step a) of immersing silicon powder in an etch solution comprises immersing said oxidized powder in an etch solution of 30% hydrogen peroxide for 10 minutes.

18. The method of claim 15 whereby said washing step c) comprises; washing said silicon powder with deionized water and then washing said silicon powder with glacial acetic acid.

19. The method of claim 18 whereby said depositing step f) comprises placing said chemically modified porous silicon powder in a vessel with alkyltrichlorosilane under an atmosphere of noble gas for a period of 20 hours.

20. The method of claim 19 whereby said alkyltrichlorosilane in said depositing step f) is chosen from the group consisting of heptatrichlorosilane, decatrichlorosilane, or dodecatrichlorosilane

21. The chemically modified partially oxidized porous silicon powder produced according to the method of claim 20.

22. A sulfur dioxide detector manufactured from the porous silicon produced by the method of claim 4.

23. A sulfur dioxide detector manufactured from the porous silicon produced by the method of claim 15.