Controllable Growth and Unexpected Effects of Ge Nanocrystals - The

ARTICLE pubs.acs.org/JPCC

Controllable Growth and Unexpected Effects of Ge Nanocrystals Zhiwen Chen,*,†,‡ Quanbao Li,† Dengyu Pan,† Zhen Li,† Zheng Jiao,† Minghong Wu,*,† Chan-Hung Shek,‡ C. M. Lawrence Wu,‡ and Joseph K. L. Lai‡ †

Shanghai Applied Radiation Institute, Institute of Nanochemistry and Nanobiology, School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, People's Republic of China ‡ Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong

bS Supporting Information ABSTRACT: Semiconductor nanostructured Ge is a unique material of widespread technological applications, particularly in the field of microelectronics. New strategies for controllable growth and an understanding of the effects of Ge nanocrystals inducted by Au nanoparticles in Au/Ge bilayer films are of fundamental importance in the development of micro/nanodevices. After an extensive search in the published literature, it is found that the previous nanostructures are one-dimensional nanomaterials such as nanowires, nanorods, nanobelts/nanoribbons, and nanotubes, two-dimensional nanoscale thin films, or zero-dimensional nanoparticles, which all have integer dimensions. Herein, the noninteger dimensional Ge nanostructures, which are called nanofractals, were successfully assembled by high-vacuum thermal evaporation techniques. A widely applicable annealing route to produce Ge nanocrystals in the form of nanoparticles, nanorings, and nanofractals is presented in detail. To our surprise, we found that a Au/Ge bilayer film with an interesting nanofractal showed a nonlinear voltage
current behavior. This result suggested that the Au/Ge bilayer film may be a promising material to facilitate future design improvement of micro/nanodevices for microelectronic applications.

1. INTRODUCTION Being a smart material, nanoscale semiconductors have displayed properties unique to their small size in comparison with the normal bulk materials owing to the effect of quantum confinement on electronic and optical properties. There are potential applications in novel photonic devices.1
4 For example, when the diameter of a Si nanowire is reduced to the order of magnitude of the electron’s de Broglie wavelength, visible photoluminescence may be produced. Much attention has been paid to the investigation of group IV semiconductor nanocrystals, such as nanowires/nanorods and nanotubes, due to their contribution to the understanding of fundamental principles in mesoscopic physics, as well as the potential applications in optoelectronic devices.5
7 Due to its advantageous semiconducting, optical, and chemical properties, germanium has many important fields of application, such as infrared detector devices, optical lens systems for infrared, fiber optics, electronic devices, and solar cells.8 For example, Ge wafers have been used in the manufacture of solar cells, though there are other emerging applications which are starting to gather momentum, such as light-emitting diodes, photodetectors, and high electron mobility transistors, etc.9 In recent years, semiconductor Ge and composites have also attracted much attention due to their broad application in many fields, such as microelectronic devices, one-dimensional quantum transistors, and light-emitting diodes.10
12 Considerable effort has been focused on the r 2011 American Chemical Society

synthesis of single-component Ge nanoparticles, nanowires/ nanorods, nanotubes, nanofilms, wafers, etc. and the exploration of their novel properties.13
16 It is expected that these nanostructures may constitute important building blocks for nanodevices and offer exciting opportunities for both fundamental research and technological applications. It is known that germanium has an excitonic Bohr radius of 24.3 nm, which is much larger than that of silicon (4.9 nm).17 Thus, germanium nanocrystals should exhibit more pronounced quantum size effects. To prevent surface oxidation of Ge nanowires, an effective approach is the creation of a chemically inert sheath around the Ge nanowire core. Wu and Yang have fabricated a Ge/carbon core sheath nanostructure.18 Hu et al. have reported the formation of Ge/SiO2 nanocables by a twostage process combining thermal evaporation and laser ablation.19 SiO2 nanotubes were first fabricated by thermal evaporation. Ge was then filled inside the nanotubes by laser ablation. In recent years, there has been considerable interest in the preparation and electronic properties of Ge nanostructures, particularly in the field of Ge composites.20
22 For instance, the most mature SiGe electronic component is nowadays the heterojunction bipolar transistor (HBT), which has found Received: January 9, 2011 Revised: April 6, 2011 Published: May 03, 2011 9871

dx.doi.org/10.1021/jp200227y | J. Phys. Chem. C 2011, 115, 9871–9878

The Journal of Physical Chemistry C application in high-speed electronics where linearity and low noise operation are a must.22 A variety of methods such as thermal evaporation, magnetron sputtering, and chemical vapor deposition are available to prepare Au/Ge thin films or nanoparticles. Since the properties of thin films strongly depend on their microstructure, composition, atomic structure, local chemistry of interfaces, and crystal defects, which all result from the fabrication process, the influence of morphology and nanostructure on material properties is especially remarkable. There is an ever-increasing range of techniques to deposit thin films and nanostructures. A widely applicable route for preparing Ge nanocrystals with controllable morphologies, including Ge nanoparticles, nanorings, and nanofractals, may be very important for insight into microstructure evolution. In our previous work, the single-crystal Mn3O4 nanocrystals, including nanoparticles, nanorods, and nanofractals, were controllably synthesized by chemical liquid homogeneous precipitation.23 Recently, we reported that the semiconductor tin dioxide (SnO2) thin films with interesting nanocrystals and nanofractal features were successfully prepared by pulsed laser deposition techniques under different substrate temperatures.24 It was found that the gas detection sensitivity of the tin dioxide thin film increases with increasing carbon monoxide (CO) concentration and decreasing fractal dimension. These findings revealed new opportunities for future study of fractal structure semiconductor SnO2 architectures, with the goal of optimizing functional material properties for specific applications. An integrated device for the semiconductor industry is highly desirable for versatile advanced applications. Notwithstanding the fact that semiconductor Ge has been applied to many areas, its use is not as extensive as that of Si, and nebulous domains in our understanding of its precise technical functions still remain. In this work, we report a method to control the morphology of Ge nanocrystals and the discovery of unexpected effects when these Ge nanocrystals are induced by Au nanoparticles in Au/Ge bilayer films. A widely applicable annealing route to produce Ge nanocrystals in the form of nanoparticles, nanorings, and nanofractals is presented in detail, particularly in the field of noninteger dimensional Ge nanofractals. The microstructural evolution of Ge nanocrystals in Au/Ge bilayer films was investigated using transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM), and its nanostructure has been evaluated by fractal methodology with reasonable results. Experimental results indicate that the fractal crystallization behavior and film resistance in Au/Ge bilayer films are influenced significantly by annealing temperatures and fractal dimensions. The measurements of film resistance confirmed that there exists a clear relationship between the film resistance and fractal dimension. We unexpectedly found that the Au/Ge bilayer film has an interesting nanofractal feature showing a nonlinear voltage
current (V
I) behavior. This novel discovery suggests that the Au/Ge bilayer film may be a promising material to facilitate future design improvements of micro/nanodevices for microelectronic applications.

2. EXPERIMENTAL SECTION Specimens were prepared by evaporation on a freshly cleaved NaCl(100) single-crystal substrate in a vacuum with a pressure of 2  10
5 Torr at room temperature.21 We deposited Ge at first and then Au without breaking the vacuum (about 2  10
5 Torr) by evaporating high-purity germanium (99.9%) and gold

ARTICLE

(99.99%) from two resistive-heated tungsten boats; viz., the bottom layer was amorphous Ge (a-Ge), and the top one was polycrystalline Au (p-Au). According to the evaporation equation t ¼

m 4πr 2 F

where t is the thickness of the film, m is the mass of the Au or Ge, F is the density of Au or Ge, and r (= 10 cm in this case) is the distance from the specimen to the evaporation source. The thicknesses of the p-Au and a-Ge films were designed to be 25 and 20 nm, respectively. All as-deposited specimens were annealed in a vacuum of about 2  10
5 Torr at 50, 75, and 100
C for 60 min and 120, 150, 180, and 210
C for 30 min, respectively. After annealing, the specimens were floated on distilled water and then placed on copper meshes to be observed with a Philips CM20 transmission electron microscope at an acceleration voltage of 200 kV. High-resolution microstructure observations of Ge nanocrystals were performed using a JEOL-2010 high-resolution transmission electron microscope with a point-to-point resolution of 1.94 Å at 200 kV. By such annealing, self-similar fractal patterns may be formed in these bilayer films. Since the annealing temperature can effectively control the morphology of fractal patterns, the density of different fractal clusters formed at a given annealing temperature is also approximately uniform at different sites of the sample.25
27 The average value of the evaluated dimension (D) obtained from different regions can be approximately taken as the whole sample’s fractal dimension. The fractal dimension for these samples was calculated by measuring the fractal dimensions of these self-similar clusters using the conventional box-counting method.28 Temperature-dependent properties of film resistance were measured in the 80
300 K range using a standard four-probe configuration and differential techniques. The whole system was automatically controlled by a computer system with rigorous calibration by comparison to a standard sample.

3. RESULTS AND DISCUSSION Figure 1 shows the typical TEM bright-field images and the corresponding selected area electron diffraction (SAED) patterns (inset in the upper right-hand corner) of the Au/Ge bilayer films at the following conditions: (a) as-prepared, (b) after annealing at 50
C for 60 min, (c) after annealing at 75
C for 60 min, and (d) after annealing at 100
C for 60 min. As seen in the TEM bright-field image, the as-prepared bilayer film shown in Figure 1a is homogeneous in morphology before annealing. The crystallites close to Bragg orientations are recognizable by their dark contrast. The average grain size of the p-Au in the asprepared bilayer film is about 42 nm. The SAED pattern (inset) of the as-prepared bilayer film confirms that it consists of a-Ge and p-Au, e.g., two diffuse rings of a-Ge and p-Au(111) and p-Au(200) rings. The experimental result indicates that part of the Ge has begun crystallization in the as-prepared bilayer film, e. g., Ge(112). After annealing at 50
C for 60 min, it can be seen from Figure 1b that three different contrasts, black, dark gray, and white, pervade throughout this micrograph. Since the electron scattering of Au is much stronger than that of Ge, the black contrast mainly originates from Au nanoparticles. The Ge nanocrystals are responsible for the white contrast, and the dark gray contrast could be explained by the mixed contributions from regions consisting of a mixture of Au and Ge. The TEM 9872

dx.doi.org/10.1021/jp200227y |J. Phys. Chem. C 2011, 115, 9871–9878

The Journal of Physical Chemistry C

ARTICLE

Figure 1. TEM bright-field images of Au/Ge bilayer films: (a) as-prepared bilayer film, (b) 50
C for 60 min, (c) 75
C for 60 min, and (d) 100
C for 60 min. The inset in the upper right-hand corner shows the SAED pattern.

bright-field image indicates that the Ge nanocrystals form interconnected network structures. The SAED pattern of Figure 1b shows that, besides the Au(111), Au(200), and Ge(112) rings, Ge(102) and Ge(113) rings are also manifested at this annealing temperature. Moreover, the diffuse ring of a-Ge is still clearly visible. These results reveal that Ge nanocrystal nuclei have formed in this film, and the crystallization of a-Ge has started, culminating in the manifestation of the Ge(102) and Ge(113) rings. With increasing annealing temperature, the diffuse rings of a-Ge are completely replaced by sharp diffraction rings corresponding to crystalline Ge (c-Ge) as shown in the SAED patterns of Figure 1c,d. The sharp diffraction rings corresponding to various crystal plane orientations indicate the crystalline nature of the Au and Ge present in the films. As shown in Figure 1c, we discovered unexpectedly that the crystalline Ge was arranged to form a disconnected nanoring configuration. This nanoring with a radius of about 500 nm is composed of crystallized Ge nanocrystals. It can be seen that the grain sizes of the Ge nanocrystals are quite small. Different TEM image contrasts on the nanoring suggest different grain sizes. The average size of the Ge nanocrystals measured by the intercept method ranged from 20 to 30 nm in diameter. When the annealing temperature reached 100
C for 60 min, we discovered, to our amazement, that the morphology of the Ge nanorings became continuous, closed, and irregular as shown in Figure 1d. The TEM bright-field image shows two irregular nanorings with diameters ranging from 135 to 735 nm and thicknesses ranging from 23 to 34 nm.

Figure 2 shows the TEM bright-field images of Au/Ge bilayer films annealed for 30 min at (a) 120
C, (b) 150
C, (c) 180
C, and (d) 210
C. It can be seen that the films display unique nanofractal patterns at each annealing temperature. The maximum sizes of nanofractal patterns were measured by the intercept method and found to be about 430 nm (Figure 2a), 550 nm (Figure 2b), 340 nm (Figure 2c), and 415 nm (Figure 2d). Referring to Figure 2, Au grains exhibit black contrast in these images and are enchased by the white Ge nanofractals (as shown by the blue arrows). The average Au grain size estimated from these TEM images ranges from 8 to 15 nm in diameter. It was found that the white Ge nanofractals consist of Ge nanocrystals with diameters ranging from 20 to 50 nm. The Au grains are clearly enchased by the Ge nanofractals. Some irregular regions of bright contrast appear in the films. Detailed examination of the bright contrast regions demonstrates that some Ge nanocrystals are in intimate contact with one another in the films, indicating the existence of smaller Ge nanocrystals in the annealed films. A careful study of the morphologies of Ge nanofractals shows the change in shape of these nanofractals from open to compact structure as the annealing temperature increases. For example, Figure 2a shows an open nanofractal structure, and Figure 2d shows a more compact one. Figure S1 (Supporting Information) shows the TEM bright-field images of the Au/Ge bilayer films annealed at (a) 120
C for 50 min, (b) 120
C for 60 min, (c) 120
C for 70 min, (d) 150
C for 50 min, (e) 150
C for 60 min, and (f) 150
C for 70 min. It can be seen 9873

dx.doi.org/10.1021/jp200227y |J. Phys. Chem. C 2011, 115, 9871–9878

The Journal of Physical Chemistry C

ARTICLE

Figure 2. TEM bright-field images of Au/Ge bilayer films annealed for 30 min: (a) 120
C, (b) 150
C, (c) 180
C, and (d) 210
C.

that these nanofractal structures are present throughout the surface of the annealed films. The experimental investigations from these TEM images indicate that when the annealing time (temperature) is short (low), for example, the films annealed at 120 and 150
C for 50 min, a few separated nanofractal patterns show in the films (see parts a and d, respectively, of Figure S1). With increasing annealing time and temperature, the nanofractals with dense branches extend to the whole films (see parts c and e, respectively, of Figure S1). The experimental evidence reveals that these nanofractals are reproducible and in fact change considerably as a function of the annealing temperature and time. The formation process of this Ge nanofractal exhibits the following characteristics: (i) As soon as nucleation starts at one site, a-Ge around this site crystallizes quite rapidly, which indicates that latent heat released due to the crystallization of a-Ge plays an important role in nanofractal formation.29 (ii) As the tip of the Ge nanofractal does not directly contact the black regions, this implies that the Au grains recede from, and the Ge nanocrystals grow into, the nanofractal regions.30 (iii) In general, the tips of nanofractal branches are always in direct contact with the dark gray contrast regions during annealing; therefore, the nanofractal branches would extend along the dark gray regions.31 To our surprise, we found that the Au and Ge layers largely exchanged their positions in the annealed bilayer films since the Au layer is on the top of the Ge layer in the as-prepared Au/Ge bilayer film before annealing. This phenomenon has also been found in amorphous Si/Al bilayer films by Mittemeijer et al.32,33 After annealing, the Ge nanofractals appear clearly on the top of

the Au layer. Figure 2 shows TEM bright-field images of the Ge nanofractals in Au/Ge bilayer films annealed for 30 min at (a) 120
C, (b) 150
C, (c) 180
C, and (d) 210
C. It is very clear that the Ge nanofractals present obviously on the surface of the matrix Au layer and also clearly show some branches. The appearance of the interdiffusion process between the Ge nanoclusters and the matrix Au atoms can be reasonably inferred. During annealing, due to the breaking of Ge
Ge bonds, Ge clusters nucleate at Au
Ge interfaces, the surrounding Ge atoms can diffuse along the interface and through the Au layer to form the nucleus (nanofractal seed), and the Au atoms aggregate in the opposite direction. The Au extrusion aggregates at the black spots, and the continuous Au layer can provide sufficient Au atom transport along the Au
Ge interface and make the Au grain grow out of Ge nanofractals. HRTEM images also provide further evidence for the Ge nanofractal positioning on the substrate (listed below). Figure 3a shows the HRTEM image of a local region of a nanofractal annealed at 150
C for 30 min. It was found that the Ge nanocrystals with stacking faults were surrounded by the Au grains with slightly different crystallographic orientations as shown in the inset of Figure 3a (displayed in the upper left-hand corner). The moire patterns are due to the overlapping of Ge nanocrystals and tiny Au grains. Figure 3b shows the HRTEM image of a tiny Ge nanocrystal in the bright contrast Ge region at the junction of Au grain boundaries. This tiny grain may be regarded as a Ge nanostructure at a very early stage of crystallization after the inhomogeneous nucleation stage. Crystalline defects such as dislocations and stacking faults can be seen in the 9874

dx.doi.org/10.1021/jp200227y |J. Phys. Chem. C 2011, 115, 9871–9878

The Journal of Physical Chemistry C

Figure 3. HRTEM images of Au/Ge bilayer films: (a) HRTEM image of a local region of a nanofractal annealed at 150
C for 30 min; (b) HRTEM image of a tiny Ge nanocrystal in the bright contrast Ge region at the junction of Au grain boundaries.

Au grains. The Au atoms may have diffused outward by selfdiffusion, since we did not succeed in observing the crystalline lattice of Au even by tilting the sample. The deep valleys can be seen in the HRTEM image of the Au film, which correspond to large grooves at the Au grain boundaries. The nucleation of a-Ge at Au grain boundaries near the growing tip of a c-Ge branch is visible. A bright contrast area is also visible at the Au boundaries, inferring outward diffusion of Au atoms at the grain boundaries. The a-Ge regions probably still exist near the tip since we could not observe any lattice image at the regions even by tilting the sample. This is the first experimental result on the Au/Ge system to show that successive c-Ge nucleation takes place near the tip region of a nanofractal and that Au grain boundaries are the preferred successive Ge nucleation sites. Figure 4 shows the plots of ln(N) versus ln(1/L) of the Ge nanofractal regions in Figure 2, where L is the box size and N is

ARTICLE

the number of boxes occupied by the nanofractal patterns. It can be seen that all plots show a good linear relationship, which means that the morphologies of nanofractals have scale invariance within these ranges, so these Ge patterns can be regarded as fractals. To obtain the fractal dimension (D), we fit a linear relationship for the function ln(N) versus ln(1/L). The results show that the fractal dimension is D = 1.653 at 120
C as shown in Figure 4a, 1.756 at 150
C as shown in Figure 4b, 1.781 at 180
C as shown in Figure 4c, and 1.878 at 210
C as shown in Figure 4d. It was found that the fractal dimension increases with increasing annealing temperature. A smaller fractal dimension means that the films are composed of the open and loose fractal structure with fine branches. For the irregular nanorings shown in Figure 1d, the fractal dimension cannot be calculated quantitatively by the box-counting method. This means that these morphologies cannot be described by the fractal scaling law. Therefore, this morphology is not a self-similar fractal structure. Figure 5a shows the film resistance (R) versus temperature of measurement after various annealing treatments: 100
C for 60 min, 120
C for 30 min, 150
C for 30 min, 180
C for 30 min, and 210
C for 30 min. The film resistance was measured from 80 to 300 K. It was found that the film resistance values of various films were evidently different. Except for the film annealed at 210
C, the film resistance value tends to decrease with increasing annealing temperature and increasing fractal dimension. The experimental results suggest that the film resistance R depends on the nanofractal morphologies and fractal dimension. This phenomenon can be reasonably explained with the aid of the random tunneling junction network (RTJN) model.34 After the crystallization of a-Ge, the fractal patterns consist of crystalline Ge grains with the morphology of fine dendrite-like nanocrystals incorporating many of the tunneling junctions of varying sizes, so the Au atoms cannot constitute a homogeneous film. From the view of electrical transport, the whole thin film is made up of linked metal islands and a series of tunneling junctions. For thin films annealed at different temperatures, the sizes of the nanofractal branches with different fractal dimensions are different, leading to differences in the height of the potential barrier for the various tunneling junctions, with the consequence that the breakdown voltages are also different. During the measurement of the film resistance, the junction i will be in a high-resistance state when the external voltage Vi is lower than the potential barrier Ui for the tunneling junction i. In contrast, the junction i will be broken down and in a low-resistance state when Vi is higher than Ui. As mentioned above, there is a relationship between the fractal dimension and the size of the nanofractal branches in that the number of the fine branches increases with increasing fractal dimension. Therefore, the film annealed at 150
C for 30 min with a larger fractal dimension and a larger number of junctions has the smaller potential barrier and lower resistance state. However, for the films annealed at 100
C for 60 min, 180
C for 30 min, and 210
C for 30 min, the Ge nanocrystals display the island-like structure with no dendrite characteristics. The potential barrier of the island-like structure is higher than that of the dendrite structure, so that it is more difficult to break down. For the film annealed at 120
C for 30 min, the branches of the dendrite structure link into a network and cut off the metal Au paths, so this film displayed higher resistance. Figure 5b shows a V
I curve of the Au/Ge bilayer film annealed at 150
C for 30 min. We unexpectedly found that the V
I characteristics of this film exhibit a nonlinear behavior; viz., dR/dI has a negative value. The experimental results indicate that 9875

dx.doi.org/10.1021/jp200227y |J. Phys. Chem. C 2011, 115, 9871–9878

The Journal of Physical Chemistry C

ARTICLE

Figure 4. ln(N) versus ln(1/L) of Ge nanofractals in Figure 2, where L is the box size and N is the number of boxes occupied by Ge nanocrystals annealed for 30 min at (a) 120
C, (b) 150
C, (c) 180
C, and (d) 210
C.

the film resistance R decreases gradually with increasing external voltage. For the films annealed at 100
C for 60 min and 120, 180, and 210
C for 30 min, the V
I behavior still shows the conventional ohmic characteristics; that is, these annealed films do not show the nonlinear V
I behavior. For example, Figure S2 (Supporting Information) shows the V
I curve of the Au/Ge bilayer film annealed at 100
C for 60 min. Further advancement of our understanding of the relationship between nanofractal and nonlinear V
I behavior requires a clear understanding of its formation mechanism. This nonlinear V
I behavior may be associated with Schottky contacts shown in the Supporting Information. According to a report by Ye et al., the film resistance is35 Z RðV Þ ¼ ðR0
R1 ÞA 0

V

f ðuÞ du

ðiÞ

where R0 is the film resistance without the external voltage; viz., when V = 0, R0 is the differential resistance (dV/dI)V=0. R0
R1 approximately equals the film resistance while all the tunneling junctions have broken down; that is, it is the film resistance at V f ¥. A is a normalized constant, and u is the breakdown voltage of the most probable distribution. Thus, the V
I behavior of the films with the nanofractal patterns should be I ¼

V RðV Þ

ðiiÞ

According to formulas i and ii, some tunneling junctions will be broken down in succession with increasing voltage and R(V) will decrease gradually, resulting in the nonlinear V
I behavior of the films. This Au/Ge bilayer film with interesting nanofractal structure may be a promising material to facilitate future 9876

dx.doi.org/10.1021/jp200227y |J. Phys. Chem. C 2011, 115, 9871–9878

The Journal of Physical Chemistry C

ARTICLE

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (Z.C.); mhwu@staff.shu.edu.cn (M.W.).

’ ACKNOWLEDGMENT The work described in this paper was financially supported by the Shanghai Pujiang Program (Grant 10PJ1404100), Innovation Key Fund of the Shanghai Municipal Education Commission (Grant 10ZZ64), National Natural Science Foundation of China (Grants 11074161, 11025526, 40830744, and 410973073), National Key Technology R & D Program in the 11th Five year Plan of China (Grant 2009BAA24B04), Shanghai Committee of Science and Technology (Grants 10JC1405400, 09530501200, 08520512200, and 09XD1401800), and Shanghai Leading Academic Discipline Project (Grant S30109). This work was also supported by a grant from the City University of Hong Kong (Project 7002295). ’ REFERENCES

Figure 5. Properties of Au/Ge bilayer films: (a) film resistance values (R) versus temperature of measurement from 80 to 300 K for various annealing treatments; (b) nonlinear V
I curve of the Au/Ge bilayer film annealed at 150
C for 30 min.

design improvements of micro/nanodevices for microelectronic industry applications. The present findings provide new opportunities for future study of nanofractal architectures in metal/semiconductor thin films, with the goal of optimizing microelectronic functional material properties for specific applications.

4. CONCLUSIONS In conclusion, we have successfully developed a widely applicable annealing route to produce Ge nanocrystals in the form of nanoparticles, nanorings, and nanofractals with noninteger dimensions. It is very interesting. The formation of significant nanofractals is rather unusual. Experimental results indicate that the fractal crystallization behavior and film resistance in Au/Ge bilayer films are influenced significantly by annealing temperatures and fractal dimensions. Measurements of film resistance provided the evidence that there exists a relationship between the film resistance and fractal dimension. We unexpectedly found that a Au/Ge bilayer film with an interesting nanofractal shows a nonlinear V
I behavior. As a result, this Au/Ge bilayer film may be a promising material to facilitate future design improvements of micro/nanodevices for microelectronic applications. ’ ASSOCIATED CONTENT

bS

Supporting Information. Description of TEM evidence, V
I curve for films annealed at 100
C for 60 min, and nonlinear V
I behavior. This material is available free of charge via the Internet at http://pubs.acs.org.

(1) Morales, A. M.; Lieber, C. M. A laser ablation method for the synthesis of crystalline semiconductor nanowires. Science 1998, 279, 208–211. (2) Ma, D. D. D.; Lee, C. S.; Au, F. C. K.; Tong, S. Y.; Lee, S. T. Small-diameter silicon nanowire surfaces. Science 2003, 299, 1874–1877. (3) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Nanobelts of semiconducting oxides. Science 2001, 291, 1947–1950. (4) Hu, J. T.; Ouyong, M.; Yang, P. D.; Lieber, C. M. Controlled growth and electrical properties of heterojunctions of carbon nanotubes and silicon nanowires. Nature 1999, 399, 48–51. (5) Buda, F.; Kohanoff, J.; Parrinello, M. Optical properties of porous silicon: A first-principles study. Phys. Rev. Lett. 1992, 69, 1272–1275. (6) Kang, Z. H.; Tsang, C. H.; Wong, N. B.; Zhang, Z. D.; Lee, S. T. Silicon quantum dots: A general photocatalyst for reduction, decomposition, and selective oxidation reactions. J. Am. Chem. Soc. 2007, 129, 12090–12091. (7) Zhang, R. Q.; Lifshitz, Y.; Lee, S. T. Oxide-assisted growth of semiconducting nanowires. Adv. Mater. 2003, 15, 635–640. (8) Bosi, M.; Attolini, G. Germanium: Epitaxy and its application. Prog. Cryst. Growth Charact. Mater. 2010, 56, 146–174. (9) Depuydt, B.; Theuwis, A.; Romandic, I. Germanium: From the first application of czochralski crystal growth to large diameter dislocation-free wafers. Mater. Sci. Semicond. Process. 2006, 9, 437–443. (10) Tang, Y. H.; Zhang, Y. F.; Wang, N.; Bello, I.; Lee, C. S.; Lee, S. T. Germanium dioxide whiskers synthesized by laser ablation. Appl. Phys. Lett. 1999, 74, 3824–3826. (11) Sevik, C.; Bulutay, C. Auger recombination and carrier multiplication in embedded silicon and germanium nanocrystals. Phys. Rev. B 2008, 77, 125414
1–4. (12) Hu, J. Q.; Jiang, Y.; Meng, X. M.; Lee, C. S.; Lee, S. T. Temperature-dependent growth of germanium oxide and silicon oxide based nanostructures, aligned silicon oxide nanowire assemblies, and silicon oxide microtubes. Small 2005, 1, 429–438. (13) Rey-Stolle, I.; Barrig
on, E.; Galiana, B.; Algora, C. Analysis of germanium epiready wafers for III-V heteroepitaxy. J. Cryst. Growth 2008, 310, 4803–4807. (14) Zhang, Y. F.; Tang, Y. H.; Wang, N.; Lee, C. S.; Lee, S. T. Germanium nanowires sheathed with an oxide layer. Phys. Rev. B 2000, 61, 4518–4521. (15) Wu, H. P.; Liu, J. F.; Wang, Y. W.; Zeng, Y. W.; Jiang, J. Z. Preparation of Ge nanocrystals via ultrasonic solution reduction. Mater. Lett. 2006, 60, 986–989. 9877

dx.doi.org/10.1021/jp200227y |J. Phys. Chem. C 2011, 115, 9871–9878

The Journal of Physical Chemistry C

ARTICLE

(16) Gu, G.; Burghard, M.; Kim, G. T.; D€usberg, G. S.; Chiu, P. W.; Krstic, V.; Roth, S.; Han, W. Q. Growth and electrical transport of germanium nanowires. J. Appl. Phys. 2001, 90, 5747–5751. (17) Wu, Y. Y.; Yang, P. D. Germanium nanowire growth via simple vapor transport. Chem. Mater. 2000, 12, 605–607. (18) Wu, Y. Y.; Yang, P. D. Germanium/carbon core-sheath nanostructures. Appl. Phys. Lett. 2000, 77, 43–45. (19) Hu, J. Q.; Meng, X. M.; Jiang, Y.; Lee, C. S.; Lee, S. T. Fabrication of germanium-filled silica nanotubes and aligned silica nanofibers. Adv. Mater. 2003, 15, 70–73. (20) Som, T.; Ayyub, P.; Kabiraj, D.; Kulkarni, N.; Kulkarni, V. N.; Avasthi, D. K. Formation of Au0.6Ge0.4 ally induced by Au-ion irradiation of Au/Ge bilayer. J. Appl. Phys. 2003, 93, 903–906. (21) Wu, X. H.; Feng, Y. Z.; Li, B. Q.; Wu, Z. Q.; Zhang, S. Y. Annealing behavior of Pd/a-Ge bilayer films. J. Appl. Phys. 1994, 75, 2415–2417. (22) Jain, S. C.; Decoutere, S.; Willander, M.; Maes, H. E. SiGe HBT for application in BiCMOS technology: II. Design, technology and performance. Semicond. Sci. Technol. 2001, 16, R67–R85. (23) Chen, Z. W.; Lai, J. K. L.; Shek, C. H. Shape-controlled synthesis and nanostructure evolution of single-crystal Mn3O4 nanocrystals. Scr. Mater. 2006, 55, 735–738. (24) Chen, Z. W.; Pan, D. Y.; Zhao, B.; Ding, G. J.; Jiao, Z.; Wu, M. H.; Shek, C. H.; Wu, L. C. M.; Lai, J. K. L. Insight on fractal assessment strategies for tin dioxide thin films. ACS Nano 2010, 4, 1202–1208. (25) Ba, L.; Zen, J. L.; Zhang, S. Y.; Wu, Z. Q. Fractals in annealed Ge-Au/Au bilayer films. J. Appl. Phys. 1995, 77, 587–590. (26) Zhang, S. Y.; Wang, X. P.; Chen, Z. W.; Wu, Z. Q.; Jin-Phillipp, N. Y.; Kelsch, M.; Phillipp, F. In situ TEM study of fractal formation in amorphous Ge/Au bilayer films. Phys. Rev. B 1999, 60, 5904–5908. (27) Hou, J. G.; Wu, Z. Q. Experimental demonstration of the role of local latent heat in Ge pattern formation. Phys. Rev. B 1990, 42, 3271– 3275. (28) Forrest, S. R.; Witten, T. A. Long-range correlations in smokeparticle aggregates. J. Phys. A 1979, 12, L109–L117. (29) Li, B. Q.; Zheng, B.; Zhang, S. Y.; Wu, Z. Q. Dependence of fractal formation on the thickness ratio in Al/a-Ge bilayers. Phys. Rev. B 1993, 47, 3638–3641. (30) Petford-Long, A. K.; Doole, R. C.; Afonso, C. N.; Solis, J. In situ studies of the crystallization kinetics in Sb-Ge films. J. Appl. Phys. 1991, 77, 607–613. (31) Bian, B.; Tanaka, T.; Ohkubo, T.; Hirotsu, Y. Plan-view and cross-sectional TEM observations of interfacial reactions and fractal formation in a-Ge/Au film. Philos. Mag. A 1998, 78, 157–170. (32) Zhao, Y. H.; Wang, J. Y.; Mittemeijer, E. J. Microstructural changes in amorphous Si/crystalline Al thin bilayer films upon annealing. Appl. Phys. A: Mater. Sci. Process. 2004, 79, 681–690. (33) Wang, Z. M.; Wang, J. Y.; Jeurgens, L. P. H.; Mittemeijer, E. J. Thermodynamics and mechanism of metal-induced crystallization in immiscible alloy systems: Experiments and calculations on Al/a-Ge and Al/a-Si bilayers. Phys. Rev. B 2008, 77, 045424
1–15. (34) Ye, G. X.; Wang, J. S.; Xu, Y. Q.; Jiao, Z. K.; Zhang, Q. R. Evidence of anomalous hopping and tunneling effects on the conductivity of a fractal Pt-film system. Phys. Rev. B 1994, 50, 13163–13167. (35) Ye, G. X.; Xu, Y. Q.; Wang, J. S.; Jiao, Z. K.; Zhang, Q. R. Critical behaviors in a Pt-film percolation system deposited on fracture surfaces of R-Al2O3 ceramics. Phys. Rev. B 1994, 49, 3020–3024.

9878

dx.doi.org/10.1021/jp200227y |J. Phys. Chem. C 2011, 115, 9871–9878