EP1756335A2 - Germanium and germanium alloy nanoparticle and method for producing the same - Google Patents

Abstract

In the invention, an electrochemical etching of crystalline germanium or a germanium alloy produces well-segregated chromatic clusters of nanoparticles. Distinct strong bands appear in the photoluminescence spectra under 350 nm excitation with the lowest peaks in wavelength identified to be at 430, 480, and 580 and 680-1100 nm. The material may be dispersed into a discrete set of luminescent nanoparticles of 1-3 nm in diameter, which may be prepared into colloids and reconstituted into films, crystals, etc.

Description

GERMANIUM AND GERMANIUM ALLOY NANOPARTICLE AND METHOD FOR PRODUCING THE SAME

TECHNICAL FIELD A field of the invention is nanomaterials. BACKGROUND ART Silicon nanoparticles of ~ 1 nm diameter have shown stimulated emissions. Bulk silicon is an optically dull indirect bandgap material, having a 1.1 eV indirect bandgap, and a 3.2 eV direct bandgap. A lnm silicon nanoparticle effectively creates a new wideband direct gap material, with an energy gap of 3.55 eV, and highly efficient optical activity. A lnm silicon nanoparticle indirect band gap of 1.1 eV corresponds to a wavelength of 1.1 μm, which is in the infrared region. Our previous work with lnm silicon nanoparticles has shown moderate emission activity in the infrared region. The uniformly dimensioned lnm silicon nanoparticles (having about 1 part in one thousand or less of greater dimensions) produced in earlier work have characteristic blue emissions. See, e.g., Akcakir et al, "Detection of luminescent single ultrasmall silicon nanoparticles using fluctuation correlation spectroscopy", Appl. Phys. Lett. 76 (14), p. 1857 (April 3, 2000). Silicon nanoparticles have also been synthesized with H- or O- termination, or functionalized with N- , or C-linkages. Previous work also produced a family of uniformly dimensioned nanoparticles with distinct particle sizes in the 1-3 nm range, which fluoresce spectacularly, and an additional particle that emits in the infrared band. The family includes 1 (blue emitting), 1.67 (green emitting), 2.15 (yellow emitting), 2.9 (red emitting) and 3.7 nm (infrared emitting). See, G. Belomoin et al. "Observation of a magic discrete family of ultrabright Si nanoparticles," Appl. Phys. Lett. 80(5), p 841 (February 4, 2002); and United States Published Patent Application 20020070121 to Nayfeh et al. Optical interconnects have many uses. An example use is for highspeed data communications between servers, either at the cabinet-to-cabinet or board-to-board levels. Another use is for chip-level interconnects. Current technology utilizes III-V systems, such as GaAs or InP-InGaGa PiN. Group IV materials hold special interest due to their benign nature and because of fabrication advantages. Silicon based detectors, for example, may be fabricated in a conventional silicon CMOS process, which typically can be implemented at lower cost than the Group III-V fabrication processes. Conventional optical detectors made on compound semiconductor substrates are bonded with a bulk silicon die for a multi-chip solution that is relatively costly. However, due to the large absorption length (20 μm) of silicon at 820 nm and the forbidden absorption at 1300 and 1550 nm, bulk silicon-based photodetectors have limited detection efficiency and wavelength range. Germanium or gallium arsenide -based systems offer better absorption and sensitivity at 1300 and 1550 nm over bulk silicon. Nanoparticle-based photodetectors, also referred to as quantum dot photodetectors, present an opportunity for enhancing the photon-to-current conversion efficiency compared to bulk devices to a degree that can alleviate or eliminate the need for amplification circuitry used in conventional systems to make use of the small photocurrent created in conventional bulk silicon detector devices. Systems based on films of Si, Ge, and GeSi nanoparticles or quantum dots have been recently demonstrated, but with moderate efficiency. For instance, Ge-based photodetector utilizing films consisting of large quantum substructures (50 nm) have respective responses of 130, 0.16, and 0.08 mA/W under the wavelengths of 820, 1300, and 1550 nm. See, e.g., "High efficiency 820 nm MOS Ge quantum dot photodetectors for short-reach integrated optical receivers with 1300 and 1550 nm sensitivity," B.C. Hsu, et al. IEDM, 91 (2002)(IEEE publication). These levels of current response are below those that would provide a simple integration into optoelectronic devices for short reach optoelectronic communications. Higher performance, particularly at 820 nm, would make it feasible to integrate optoelectronic devices into silicon chips for short-reach optical communications.

DISCLOSURE OF INVENTION In the invention, an electrochemical etching of crystalline germanium or a germanium alloy produces well-segregated chromatic clusters of nanoparticles. Distinct strong bands appear in the photoluminescence spectra under 350 nm excitation with the lowest peaks in wavelength identified to be at

430, 480, and 580 and 680-1100 nm. The material may be dispersed into a discrete set of luminescent nanoparticles of 1 -3 nm in diameter, which may be prepared into colloids and reconstituted into films, crystals, etc.

BRIEF DESCRIPTION OF THE DRAWINGS FIGs. 1A and IB show the photoluminescence spectra of the etched Ge wafer taken from different regions of an experimental sample under 365 nm excitation; FIG. 2 shows the photoluminescence of an experimental GE sample in the infrared taken under an excitation source of 365 nm; and FIG. 3 shows the FTIR spectrum of an experimental Ge sample.

BEST MODE OF CARRYING OUT THE INVENTION The invention concerns germanium and germanium alloy nanoparticles and methods for making the same. Infrared emissions from nanaparticles command a special interest in myriad applications. Highly-efficient nanomaterial-based photodetectors or phototransistors in the infrared can form the basis for chip-to-chip and board-to-board optical interconnects. In relation to bulk silicon, bulk germanium reduces the indirect band gap (0.66 vs. 1.1 eV), and the direct band gaps (0.9 vs. 3.2 eV), and one use of germanium nanoparticles of the invention is to extend the photosensitive response into the infrared. In the invention, we employ electrochemical etching processes of crystalline germanium or a germanium alloy in a chemical etchant solution, e.g., HF/H₂O /H₂O to produce, well-segregated chromatic clusters of nanoparticle material, which under 365 nm UV excitation, exhibit ultrabright blue, green, and yellow/orange photoluminescence, as well as very efficient infrared radiation. Sensitive Si/Ge nanoparticle material-based devices, can therefore cover a wide range of wavelengths from near UV to infrared. A particularly useful application of efficient response in the infrared band is use in infrared biological imaging applications. A method for creating clusters of the nanoparticles of an embodiment of the invention is a bipolar electrochemical treatment which involves insertion of bulk germanium or germanium alloy, e.g. a wafer, into a chemical etch solution, in the presence of an external current. The germanium serves as an electrode in an electrochemical etching process. Another electrode also contacts the chemical etch bath. During the etching, current is reversed. A gradual advance of the wafer into the bath may be used to increase the area of etching. An example rate is about one millimeter per minute. Current is reversed, in this case, as a gradual retreat of the wafer is conducted. Also, current reversal may be executed after a lift of the wafer to its original height to begin a second period of gradual advance of the wafer. In a preferred embodiment of the invention, we employ electrochemical etching processes of crystalline germanium in an etching solution of HF/H₂O₂/H₂O and methanol. In another embodiment, a different etchant solution, i.e., an aqueous HCl/methanol electrolyte bath is used. The etching process creates a layer of uniformly dimensioned Ge nanoparticles on the surface of the bulk germanium. After etching, the Ge electrode with Ge nanoparticles formed on its surface is separated from the etchant solution. The Ge with the nanoparticle surface may then be rinsed to remove any etchant solution. The particles may be removed from the bulk material by an agitation process, e.g, shaking, banging, scraping or ultrasonic agitation, the latter of which is preferred. Generally, any method which separates the nanoparticles from the etched bulk Ge or Ge alloy surface is suitable, but a solvent with breaking force supplied by ultrasound waves is preferred. In the ultrasound bath, one can use a variety of solvents since the particles are hydrogen passivated. Example solvents include acetone, alcohol, water, and other organic solvents. Once separated, various methods may be employed to form nanoparticles into colloids, crystals, films and other desirable forms. The particles may also be coated or doped. The coating and doping processes are similar to those described in U.S. Patent Number 6,585,947 for the silicon nanoparticles, but the specific protocols used to coat may differ somewhat to account the different chemistry of Ge. H-terminated germanium particles may be functionalized with alkynes. For example, refluxing in a 20% 1-dodecene solution in mesitylene (v/v) for several hours results in the incorporation of surface-bound dodecyl moieties. It is a thermally induced hydrogermylation reaction. In another embodiment of the invention, the bulk germanium is replaced with an alloy of silicon-germanium. Electrochemical etching as discussed above is applied to disperse the alloy into nanoparticles. Alloys may be produced by ion implantation or molecular beam epitaxy procedures. An example embodiment uses Germanium-silicon wafers of 20-80 (Ge-silicon) composition. For this ratio, a nanoparticle of 1 nm in diameter may have Si₂₄Ge₅ configuration (24 silicon atoms and 5 Ge atoms), or we can use 80-20 (Ge-silicon) composition which gives Ge₂₄Si₅ (5 silicon atoms and 24 Ge atoms). The proportion of Ge to the alloy element permits tuning the composition, which allows tuning of the wavelength response of alloyed or doped nanoparticles. Our theoretical simulations show that several high quality Si/Ge nanoparticles configurations are possible. We have conducted experiments to demonstrate the method of the invention. In an experiment, we prepared samples using an aqueous HF/methanol electrolyte bath that incorporates hydrogen peroxide (H₂O ). The germanium samples used were (100) oriented, 1-10 ohm-cm resistivity, p-type boron doped Ge wafers. Moreover, HF terminates Ge with hydrogen while the highly oxidative peroxide cleans the wafer from organic-based impurities, resulting in high-quality nanostructures. The example etchant in the experiments was a mixture of 0.5 mL, 0.45mL, and 0.4 niL of HF, H₂O₂, and methanol, i.e., in a near 1 :1 :1 volume ratio. The wafer was etched for 5 minutes at anodizing current of 180 mA, providing a density of 350 mA/cm.2. At the end of anodization, the polarity of the electrodes was reversed to perform a cathodization etch step for 2 minutes. In the experiments, well-segregated chromatic clusters were produced, which under 365 nm UV excitation, exhibit ultrabright blue, green, and yellow/orange photoluminescence, as well as very efficient infrared radiation. Both HF and H₂O are reactive with Ge oxide, thus the incorporation of the highly oxidative peroxide enhances the etching rate, producing much smaller nanostructures. The production of spatially resolved chromatic clusters is consistent with size dependent quantum confinement of radiative recombination in Ge nano structures. We took photoluminescence images using radiation from a Hg lamp, normally incident on the substrate. At the target, the power, 1- 15 mW, focuses to a spot ~ 0.5 mm diameter, using an objective of 0.6 NA, giving an intensity of 0.13 - 2 W/cm2. Luminescence is detected in the backward direction and recorded by an RGB filter/prism based dispersive charge coupled device

(3CCD). Special cutoff filters filter scattering at the incident wavelength and the background is subtracted. Under 365 nm excitation, we observed blue, green, and yellow/orange luminescent clusters. We can prepare blue dominated samples at higher etching current conditions. These may be attributed to clusters of 1 nm Ge particles. Samples prepared under the higher current conductions, when excited under 365 nm excitation, show luminescence dominated by blue luminescent clusters with very little of the other colors. The spectral distribution was analyzed with an optical multichannel analyzer with a prism dispersive element. FIGs. 1A and IB give the spectra taken. When the beam is parked on blue clusters, the luminescence band peaks at 425 nm. The sharp rise on the blue edge of the band is caused by the cutoff filter. When the beam is parked on green/yellow spots, the emission band peaks at 490 nm. In many cases there is a hint of a yellow shoulder at 580 nm. For detection of infrared activity, we used a fiber optic spectrometer, which includes optical fibers to transport the excitation and to extract the emission. We used a holographic grating that is a polymer replica of a master grating. It is a near infrared grating with groove density of 600/mm with a blaze angle of 1 μm and with best efficiency in the range 0.65-1.1 μm. Another channel utilized a UV-VIS holographic grating, with groove density of 600/mm with a blaze wavelength of 0.4 μm and with best efficiency in the range 0.25-0.80 μm. The near infrared (NIR) fiber has nearly an attenuation of 50 db/km. In the region of interest 900-1000nm the transmission is ~ 90 percent. FIG. 2 shows the spectrum in the infrared, taken with an excitation source of 365 nm. There is a photoluminescence band in the infrared part of the spectrum (680-1100 nm). The line shape of the infrared band is asymmetric rising sharply at 680 nm and dropping slowly well into the infrared at ~ 1100 nm. Correcting for the efficiency of the infrared detector, and averaging over measurements shows that the infrared is two fold stronger than the visible emission. To determine if any oxidation occurs during the electrochemical etching process, we measured the infrared absorption in the range 500-4500 cm^"1. The Fourier transform infrared (FTIR) data was taken in air using an ATI-Mattson Galaxy model GL-5020. The results shown in FIG. 3 show absorption at 830- 880 cm^"1, which is due to bending modes of hydrogen bonded to the Ge crystallite surface. The germanium substrate signal has been subtracted from the data. The data also shows absorption at 550-600 cm^"1 due to rocking hydrogen vibration modes. On the other hand we do not see an oxygen signal, which is expected to be in the region 900-1100 cm^"1 region, indicating a high degree of hydrogen passivation. Thus, it is not likely that the emission from our samples is oxide-based. In fact the rate of oxide dissolution in aqueous solution is much faster than any common oxidation process. Photoluminescence may be explained in terms of quantum confinement in nanostructures. The lattice constants of Si and Ge are comparable, being 5 percent larger in Ge. Thus, a spherical cut of 1 nm diameter in bulk Ge gives Ge₂₉, a cluster consisting of 29 germanium atoms. A similar cut in Si gives also a cluster of 29 silicon atoms (Si₂₉). A method for producing a discrete family of silicon nanoparticles includes particles of diameters 1, 1.67, 2.15, 2.85, and 3.7 nm. See, e.g., Belomoin et al. "Observation of a magic discrete family of ultrabright Si nanoparticles," Appl. Phys. Lett. 80(5), p 841 (February 4, 2002); and United States Published Patent Application 20020070121 to Nayfeh et al. Based upon the size difference between Si and Ge, Ge nanoparticles would have corresponding sizes of 1, 1.75, 2.25, 3.0, and 3.9 nm. Significant differences between Si and Ge exist in the infrared part of the spectrum. Silicon nanoparticles of different sizes give band edge luminescence at 1160-1300 nm with an efficiency of 6 % of the visible emission. Germanium nanoparticles give photoluminescence in the range 680- 1100 nm with an efficiency that is comparable or larger than the visible emission. Germanium is also expected to produce luminescence in the range 1,500 to 3,000 nm. The extension of strong emission into the infrared in Ge is due to the reduced bandgaps in bulk Ge compared to Si (0.66 vs. 1.1 eV indirect), and (0.9 vs. 3.2 eV direct). Based on the activity in the visible, with observed emission bands whose peaks lie in the blue, green, and yellow appear to originate from the important substructure regime of 1-3 nm Ge nanoparticles. The method of the invention produces Ge and Ge-alloy nanoparticles having a size in the range of ~l-3nm In another embodiment, we used an aqueous HCl/methanol electrolyte bath with a small fraction of H₂O₂. A Teflon chamber cylinder is sealed on the bottom by the wafer. A metal plate makes electrical contact with the backside of the wafer. The cylinder is filled with the etching solution. The etchant is a 1 : 1 mixture of HC1, and methanol. A platinum wire electrode is immersed in the etchant normal to the substrate at a certain height (2 cm for example) above it. With the germanium substrate acting as the anode, and the platinum wire acting as a cathode, the wafer is anodized for 5 minutes at an appropriate anodizing current density for etching, e.g., ~ 230 mA/cm At the end of this anodization step, the polarity of the electrodes is reversed to perform a cathodization step for 2 minutes. The etchant is then removed, and the wafer is rinsed with water followed by an acetone rinse and a drying period. Similar spectra of particle clusters are observed, but the distribution is now skewed towards the orange/red sizes. In another embodiment, we find that we can enhance the formation of the nanoparticles and increase the yield by adding to the etchant a Ge salt solution, such as GeCl . Moreover we can also enhance formation by adding to the etchant granular Ge, which is crushed Ge wafers. While specific embodiments of the present invention have been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims. Various features of the invention are set forth in the appended claims.

Claims

1. A method for producing elemental germanium or germanium alloy nanoparticles, the method comprising steps of: contacting a germanium or germanium alloy electrode with an etchant solution; providing a flow of electrical current to a surface of the germanium or germanium alloy electrode through application of electrical potential between the germanium or germanium alloy electrode and another electrode; reversing the electrical potential and repeating said step of providing; separating the germanium or germanium alloy electrode from the etchant solution.

2. The method according to claim 1, further comprising a step of: separating germanium or germanium alloy nanoparticles from the germanium or germanium alloy electrode.

3. The method according to claim 2, wherein said step of separating germanium or germanium alloy nanoparticles comprises: subjecting the germanium or germanium alloy electrode to force to separate germanium or germanium alloy nanoparticles from the germanium or germanium alloy electrode.

4. The method according to claim 3, wherein the force in said step of subjecting is provided by ultrasound waves.

5. The method according to claim 2, wherein said step of separating germanium or germanium alloy nanoparticles comprises: placing the germanium or germanium alloy electrode in a solvent and subjecting the germanium or germanium alloy electrode to force to separate germanium or germanium alloy nanoparticles from the germanium or germanium alloy electrode.

6. The method of claim 5, further comprising a step of rinsing the germanium or germanium alloy electrode subsequent to said step of separating and prior to said step of placing the germanium or germanium alloy electrode in a solvent solution.

7. The method according to claim 1, wherein said step of contacting comprises gradually advancing the germanium or germanium alloy into the etchant solution.

8. The method according to claim 1, further comprising a step of doping the germanium or germanium alloy nanoparticles.

9. The method according to claim 1, further comprising a step of coating the germanium or germanium alloy nanoparticles.

10. The method according to claim 9, wherein said step of coating comprises coating said particles with biological material.

11. The method according to claim 1, wherein the another electrode is formed from platinum, and the germanium or germanium alloy electrode comprises a single crystalline germanium.

12. The method according to claim 1, wherein the germanium or germanium alloy electrode comprises p-type boron-doped germanium.

13. The method according to claim 1, wherein the etchant solution comprises HF/H₂O₂/H₂O and methanol.

14. The method according to claim 13, wherein the etchant solution further comprises a Ge salt solution.

15. The method according to claim 13, wherein the etchant solution further comprises granular germanium.

16. The method according to claim 1, wherein the etchant solution comprises HC1 and methanol.

17. The method according to claim 16, wherein the etchant solution further comprises a Ge salt solution.

18. The method according to claim 16, wherein the etchant solution further comprises granular germanium.

19. A method for producing elemental germanium or germanium alloy nanoparticles, the method comprising steps of: contacting bulk germanium with a chemical etchant solution; bipolar electrochemically etching the bulk germanium during said step of contacting; and separating the germanium or germanium alloy electrode from the etchant solution.

20. The method according to claim 19, wherein said step of contacting comprises gradually advancing the germanium or germanium alloy into the etchant solution.

21. The method according to claim 19, wherein the etchant solution comprises HF/H₂O₂/H O and methanol.

22. The method according to claim 21, wherein the etchant solution further comprises a Ge salt solution.

23. The method according to claim 21, wherein the etchant solution further comprises granular germanium.

24. The method according to claim 19, wherein the etchant solution comprises HC1 and methanol.

25. The method according to claim 24, wherein the etchant solution further comprises a Ge salt solution.

26. The method according to claim 24, wherein the etchant solution further comprises granular germanium.

27. Elemental Germanium material comprising nanoparticles dimensioned in the range of ~l-3nm.

28. Germanium alloy material comprising nanoparticles dimensioned in the range of ~l-3nm.