U.S. patent application number 10/849536 was filed with the patent office on 2005-04-07 for germanium and germanium alloy nanoparticle and method for producing the same.
Invention is credited to Abuhassan, Laila, Chang, Yia-Chung, Nayfeh, Ammar M., Nayfeh, Munir H..
Application Number | 20050072679 10/849536 |
Document ID | / |
Family ID | 35510348 |
Filed Date | 2005-04-07 |
United States Patent
Application |
20050072679 |
Kind Code |
A1 |
Nayfeh, Munir H. ; et
al. |
April 7, 2005 |
Germanium and germanium alloy nanoparticle and method for producing
the same
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.
Inventors: |
Nayfeh, Munir H.; (Urbana,
IL) ; Abuhassan, Laila; (Amman, JO) ; Nayfeh,
Ammar M.; (Stanford, CA) ; Chang, Yia-Chung;
(Champaign, IL) |
Correspondence
Address: |
GREER, BURNS & CRAIN
300 S WACKER DR
25TH FLOOR
CHICAGO
IL
60606
US
|
Family ID: |
35510348 |
Appl. No.: |
10/849536 |
Filed: |
May 19, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10849536 |
May 19, 2004 |
|
|
|
09990250 |
Nov 21, 2001 |
|
|
|
6743406 |
|
|
|
|
09990250 |
Nov 21, 2001 |
|
|
|
09426389 |
Oct 22, 1999 |
|
|
|
6585947 |
|
|
|
|
Current U.S.
Class: |
205/74 ;
205/564 |
Current CPC
Class: |
H01S 5/3031 20130101;
C25C 5/02 20130101; C22B 41/00 20130101; H01S 5/10 20130101; H01S
3/163 20130101; H01S 5/30 20130101; H01S 5/041 20130101; C25C 1/22
20130101; C01B 33/02 20130101; B82Y 30/00 20130101 |
Class at
Publication: |
205/074 ;
205/564 |
International
Class: |
C25D 001/00; C25C
001/22 |
Goverment Interests
[0002] This invention was made with Government assistance under
contract number 1529304 awarded by the National Science Foundation.
The Government has certain rights in this invention.
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/H.sub.2O.sub.2/H.sub.2O 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 HCl 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/H.sub.2O.sub.2/H.sub.2O 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 HCl 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 .about.1-3 nm.
28. Germanium alloy material comprising nanoparticles dimensioned
in the range of .about.1-3 nm.
Description
REFERENCE TO RELATED APPLICATIONS AND PROIRITY CLAIM
[0001] This application is a continuation-in-part of co-pending
application serial number 990,250, filed Nov. 21, 2001, published
on Jul. 13, 2002, as publication number 20020070121 entitled FAMILY
OF DISCRETELY SIZED NANOPARTICLES AND METHOD FOR PRODUCING THE
SAME, which was a continuation in part of Nayfeh et al. U.S. patent
application Ser. No. 09/426,389, entitled METHOD FOR PRODUCING
SILICON NANOPARTICLES, filed Oct. 22, 1999, and now U.S. Pat. No.
6,585,947. Priority from both applications is claimed under 35
U.S.C. .sctn. 120.
FIELD OF THE INVENTION
[0003] A field of the invention is nanomaterials.
BACKGROUND OF THE INVENTION
[0004] Silicon nanoparticles of .about.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 1 nm silicon nanoparticle effectively creates a
new wideband direct gap material, with an energy gap of 3.55 eV,
and highly efficient optical activity. A 1 nm silicon nanoparticle
indirect band gap of 1.1 eV corresponds to a wavelength of 1.1
.mu.m, which is in the infrared region. Our previous work with 1 nm
silicon nanoparticles has shown moderate emission activity in the
infrared region. The uniformly dimensioned 1 nm 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 (Apr.
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 (Feb. 4, 2002); and
United States Published Patent Application 20020070121 to Nayfeh et
al.
[0005] Optical interconnects have many uses. An example use is for
high-speed 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.
[0006] 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 .mu.m) 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.
[0007] 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.
SUMMARY OF THE INVENTION
[0008] 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
[0009] FIGS. 1A and 1B show the photoluminescence spectra of the
etched Ge wafer taken from different regions of an experimental
sample under 365 nm excitation;
[0010] FIG. 2 shows the photoluminescence of an experimental GE
sample in the infrared taken under an excitation source of 365 nm;
and
[0011] FIG. 3 shows the FTIR spectrum of an experimental Ge
sample.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0012] 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.
[0013] In the invention, we employ electrochemical etching
processes of crystalline germanium or a germanium alloy in a
chemical etchant solution, e.g., HF/H.sub.2O.sub.2/H.sub.2O 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.
[0014] 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.
[0015] In a preferred embodiment of the invention, we employ
electrochemical etching processes of crystalline germanium in an
etching solution of HF/H.sub.2O.sub.2/H.sub.2O 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. Pat. No. 6,585,947 for the silicon
nanoparticles, but the specific protocols used to coat may differ
somewhat due to taking into 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.
[0016] 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
Si.sub.24Ge.sub.5 configuration (24 silicon atoms and 5 Ge atoms),
or we can use 80-20 (Ge-silicon) composition which gives
Ge.sub.24Si.sub.5.(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.
[0017] 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 (H.sub.2O.sub.2). 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, 0.45 mL, and 0.4 mL of HF,
H.sub.2O.sub.2, 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.sup.2. At the end of anodization,
the polarity of the electrodes was reversed to perform a
cathodization etch step for 2 minutes.
[0018] 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 H.sub.2O 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.
[0019] 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 .about.0.5 mm diameter, using an
objective of 0.6 NA, giving an intensity of 0.13-2 W/cm.sup.2.
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.
[0020] 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.
[0021] The spectral distribution was analyzed with an optical
multichannel analyzer with a prism dispersive element. FIGS. 1A and
1B 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
mm.
[0022] 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 .mu.m and with best efficiency in the range 0.65-1.1 .mu.m.
Another channel utilized a UV-VIS holographic grating, with groove
density of 600/mm with a blaze wavelength of 0.4 .mu.m and with
best efficiency in the range 0.25-0.80 .mu.m. The near infrared
(NIR) fiber has nearly an attenuation of 50 db/km. In the region of
interest 900-1000 nm the transmission is .about.90 percent.
[0023] 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 .about.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.
[0024] To determine if any oxidation occurs during the
electrochemical etching process, we measured the infrared
absorption in the range 500-4500 cm.sup.-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.sup.-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.sup.-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.sup.-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.
[0025] 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 Ge.sub.29, a cluster consisting
of 29 germanium atoms. A similar cut in Si gives also a cluster of
29 silicon atoms (Si.sub.29). 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 (Feb. 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.
[0026] 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 .about.1-3 nm.
[0027] In another embodiment, we used an aqueous HCl/methanol
electrolyte bath with a small fraction of H.sub.2O.sub.2. 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 HCl, 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., .about.230 mA/cm.sup.2. 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.
[0028] 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.sub.4. Moreover we can
also enhance formation by adding to the etchant granular Ge, which
is crushed Ge wafers.
[0029] 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.
[0030] Various features of the invention are set forth in the
appended claims.
* * * * *