Journal of Physics: Conference Series On growth mechanisms and dynamic simulation of growth process based on the experimental results of nanowire growth by VLS method on semiconductor substrates To cite this article: Dao Khac An et al 2009 J. Phys.: Conf. Ser. 187 012052 Related content - Dynamic simulations of the cryogenic system of a tokamak R Cirillo, C Hoa, F Michel et al. - Growth Mechanism of Vertically Aligned Ag(TCNQ) Nanowires Ye Chun-Nuan, Cao Guan-Ying, Mo Xiao- Liang et al. - Synthesis of tin-doped indium oxide nanowires Y Q Chen, J Jiang, B Wang et al. Recent citations View the article online for updates and enhancements. - On the formation of voids, etched holes, and GaO particles configuration during the nanowires growth by VLS method on GaAs substrate Khac An Dao et al - The effects of Au surface diffusion to formation of Au droplets/clusters and nanowire growth on GaAs substrate using VLS method Khac An Dao et al This content was downloaded from IP address 46.3.197.229 on 21/02/2018 at 12:45
On growth mechanisms and dynamic simulation of growth process based on the experimental results of nanowire growth by VLS method on semiconductor substrates Dao Khac An 1, Nguyen Xuan Chung 2, Pham Hong Trang 3, Hoang Van Vuong 3, Phan Viet Phong 1 and Phan Anh Tuan 1, 4 1 Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road, Cau Giay District, Hanoi, Vietnam 2 Physics Department, Geo and Mining University, Tu Liem, Ha Noi, Vietnam 3 Students of Faculty of Materials Science and Technology, Hanoi University of Technology, 1 Dai Co Viet Road, Hai Ba Trung District, Hanoi, Vietnam 4 Tohoku University, Japan E-mail: andk@ims.vast.ac.vn Abstract. Recently the production of nanowires is attracting many scientists but it also holds many challenges. Many problems including growth mechanisms are still not understood clearly. This paper will show briefly some our experiment results of nanowires (GeO 2 and Ga 2 O 3 ) growth by VLS, the rest of the paper will discuss nanowire growth mechanisms and then showing a developed programme on PC for dynamic simulation of nanowire growth process. This running simulation software for nanowires growth process has contained main mechanisms and we can see directly some physical phenomena concerning growth process by VLS method. Keywords: GeO 2 and Ga 2 O 3 nanowires, VLS method, growth mechanisms, dynamic simulation of growing nanowires. 1. Introduction Nanowires, nanorods and nanotubes play a very important role in nanoelectronics [1-4]. They have many applications in many fields such as for making nanowires connecting between nano devices in modern nanoelectronic circuits, for tips or probes in modern microscopes, for making various kinds of sensors and pn junction diodes, swishings, memory nodes, nanowire FET devices [5-8]. It is not like as fabrication process of nano particles (0D) as well as nano thin films (2D), where technologies are somewhat developed, the nanowire growing process along one axis (1D) gets more and more difficult, for example, how to control the uniform morphology with fitable sizes of nanowires (diameter and length) during nanowire growing, how to grow nanowires in array forms and how we can treat nanowire easily and measurable parameters to make contact with them [1, 3-5, 8]. So far the fabrication of nanowires is achieved by two approaches, top-down and bottom-up [1-5, 14-17]. Concretely, nanowires could be produced by different methods such as by VLS method (Vapor Liquid Solid), CVD (Chemical Vapor Deposition), LCG (Laser Ablation Catalytic Growth), using template Assisted Synthesis, FLS (Fluid Liquid Solid), SLS (Solution Liquid Solid), OAG (Oxide Assisted Growth), MBE (molecular Beam Epitaxy)... Among them the VLS method is seem to be a c 2009 IOP Publishing Ltd 1
simple, popular, cheap method. Depending on applications of nanowires and growing method so far one could produce many kinds of nanowires on single semiconductor substrates (Si, Ge,...) or on compound semiconductor substrates such as GaAs, GaP, InAs, InP,GaAs/P, InAs/P, ZnS, ZnSe, CdS, CdSe...[4]. The common requirements for growth nanowires, depending on each application, could be in a fitable range of resistivity, diameters of nanowires could be distributed from several tens of nm to several hundreds nm, their lengths are in a range of micrometers. Besides, the treatment and, measurement process could be done easily on the growth nanowires. Unfortunately, these requirements are not easily to satisfy. These facts are also challenges for technologists [1-8]. In order to see clearly and get better understanding of the growth process, this paper shows briefly some our experiment results of GeO 2 and Ga 2 O 3 nanowires growing on Ge and GaAs substrates by VLS method with an Au thin catalyst layer on the front side [9-14], the rest of the parts of the paper discuss growth mechanisms and showed the results of dynamic simulation of a whole nanowire growing process containing main mechanisms in 2D. 2. Experimental procedures Starting materials were the n-type Ge slices with (111) orientation and both p-type and n-type GaAs slices with (100) orientation. For the first step we have cut Ge slices into many small square shape samples with dimensions of 5 5 1 mm. The small samples were mechanically lapped, polished, cleaned by ultra-sound vibration in three solutions alternatively: solution of acetone (CH 3 COCH 3 99.5%); solution of alcohol (CH 3 CH 2 OH 95%) and ionized water, then chemically treated in HF: H 2 O with a volume ratio of 1:15. The last step is vibrating in ion water and drying. A nano thin layer of Au was evaporated onto the surface of semiconductor samples in vacuum 10-4 -10-5 torr. The thickness of Au thin layer was measured using the SEM cross-section method. Its thickness is about 50 nm. The same way of chemical treatment is done for the ready GaAs slices. (a) (b) Figure 1. Blocks of experiment equipment furnace (a); Profile of experiment temperatures for GeO 2, in period 1: heating up to T 1, around the eutectic temperature with t 1 and t 2 duration times, then in period 2 heating up to T 2 with t 3 and t 4 duration times where the nanowires growing process are mainly occurring (b). The furnace used for nanowires growing was Lindberg/ Blue M Three-zone tube Furnace. The block of experiment equipment can be seen in figure 1 (a). We have heated the furnace for different temperature profiles for different experiments of growing nanowires. The furnace s tube temperatures were automatically measured by Keithley 2000 Multimeter connecting with PC. The temperature profile of the furnace for GeO 2 and Ga 2 O 3 nanowires growing as well as the growing times can be seen in table 1. 2
Table 1. Technological conditions for the experiment cases of GeO 2 and Ga 2 O 3 nanowires growing on Ge, GaAs substrates. Period 1: heating to around Period 2: heating up to the Experiment Conditions eutectic temperature, T Our works 1 growing temperature, T 2 Experiment Cases T 1 ( 0 t C) 1 t (min) 2 (min) T 2 ( 0 C) t 3 (min) t 4 (min) GaAs Substrate figure 3 [9,10,19] 450 35 35 550-700 30 30-50 Ge Substrate figure 4 [10] 400 55 75 520-720 34 55-75 Here we would like to emphasize that the temperature T 1 in period 1 must be the temperature around (lower or upper) the eutectic temperature, and we have chosen the temperatures T 2 in the range of 520 to 720 C. After growing nanowires, the samples have been investigated by SEM, EDX, and TEM, AFM... The experimental main steps can be seen briefly in figure 2. The experimental procedures can be seen in details for both nanowire growing of GeO 2 and Ga 2 O 3 in our works [9, 10, 19]. Starting materials, cutting, lapping, polishing, etching and chemical processes for semiconductor slices and quartz tube, tools Au evaporation at high vacuum with a layer thickness is about 50 nm onto semiconductor substrates (Ge, GaAs), Investigations of some structural properties (SEM, TEM, EDX, X- Ray), discussion of growing mechanisms and simulation Put samples into the glass/quartz tube or ampoule, then pumping out the air to create pressures is about 10-1 -3 torr -10 Setting into the furnace and heating up to desired temperature profile with T 1 of: (350-450 o C) and T 2 of (550-700 o C) for creating liquid Au islands and nanowires growing. Figure 2. Schematic diagram of experiment procedures of nanowire growing by VLS method. 3. Experiment results Figures 3, 4 show some different forms or morphologies of Ga 2 O 3, GeO 2 nanowires on GaAs, Ge substrates which depend strongly on the different technological conditions. The results can be seen in details of our works [9, 10, 19]. 3
Figure 3. SEM images of Ga 2 O 3 nanowires formed on GaAs substrate at the different growing conditions. The morphologies of nano wires vary from random forms to rather vertical orientation with more regular (from left to right). Figure 4. SEM Images of GeO 2 nanowires formed on Ge substrate with different growing conditions The morphologies of nano wires are tangly whiskers as a porous surface. Element Weight% Atomic% O K 34.17 69.66 Ga K 51.94 24.30 As L 13.89 6.05 Totals 100.00 (c) (a) (b) Figure 5. Energy-dispersive x-ray spectroscopy (EDX) characterizing nanowires grown on Ge substrate containing oxygen (68.2% atomic, Ge (31.98% atomic) (a); elements in Ga 2 O 3 nanowires containing oxygen (69.662% atomic), Ga (24.30% atomic) and As (6.05% atomic %) (b,c); The place of measurement is in the centre of the sample (d). (d) 4
Figures 5 shows Energy-dispersive X-ray spectroscopy (EDX) of GeO 2, FESEM and EDX of Ga 2 O 3 nanowires at the marked place. From our experiment results in figures 3, 4, 5 as well as our other results we observed that the GeO 2 and Ga 2 O 3 strongly depend on technological conditions: heating profiles T 1, T 2 and the growing time t 4 [9, 10, 19]. 4. Discussions of growth mechanisms of VLS method The nanowires growth mechanisms on semiconductor substrates with Au thin catalyst film layer have been discussed very intensively [1-5, 13-17] but so far some points concerning mechanisms are not yet clearly understood. It is a fact that the growth mechanisms can be established on the base of the phase diagram. Figure 6 shows two phase diagrams of two systems used in growing nanowires of GeO 2 and Ga 2 O 3. The eutectic temperatures of Au-Ge and Au-GaAs are 361 o C and 630 o C, respectively. The chosen temperature T 1 is vitally important for formation of Au droplets and making alloy between Au and substrate material. The size of Au droplets on the semiconductor surface as well as the uniform of liquid droplets will determine the size of nanowires. This process depends on many factors such as the surface morphology, surface and bulk defect situation of semiconductor substrate, the heat amount or T 1 for formation of Au droplet, the diffusion of Au into the semiconductor, the diffusion of the host atoms to the liquid-solid interface, the formation of the seed from that nanowire will be grown Unfortunately we do not yet understood these processes clearly. (a) (b) Figure 6. Phase diagram of the Au - Ge binary system concerning four main stages of GeO 2 nanowire growth (a); and phase diagram of the Au GaAs system (b). When we heat up the sample in a furnace system containing certain low air pressure to temperature T 2 which is much higher than the eutectic temperature, many physical-chemical processes occurred. The growth mechanisms also strongly depend on these processes. 4.1. Some requirements and some main factors affecting growth mechanisms Semiconductors substrate must be completely clean and flat, it may contain a few defects. The fact shows that Au droplets as well as nucleation forms strongly depend on semiconductor surface and sample s defects. Au catalyst thin film s thickness is very important. This film should not be too thin and not too thick, its value is approximately 40-80 nm. Depending on temperature T 1 (lower or upper eutectic temperature) Au droplets will be formed and make an alloy with substrate material. The forms of Au droplets and alloy will strongly determine the sizes of nanowires in later stages. Experiment results showed that Au liquid droplets on the top of nanowire also play a very important role during nanowire growth, especially chemical potential (µ LV ) at the interface between liquid and solid phase determining the growth direction. 5
Vapour ambient in the ampoule or in the low pressure furnace is a very important factor of the VLS method. The presence of certain air, containing some oxygen atoms or some H 2 O vapor for oxide formation inside or on the surface of nanowire, could also create an oxide liquid phase on the surface of nanowire in order to assist the growth process, so called, oxide assisted growth (in several cases). This process is very complex. In some cases ones also uses some semiconductor materials in gaseous phase. Depending on the material of substrate the air pressure in the ampoule is in range of 10-1 to 10-5 torr. For GeO 2 and Ga 2 O 3 nanowire growth pressure is usually in range of 10-1 to 10-2 torr. This pressure has to choose an optimum value together with other factors. Temperature profile of the growth process is most important and it depends on what solid materials (catalyst metals and semiconductor substrates) have been chosen. The temperature profile could strongly affect the formation of the Au droplet sizes, diffusion of Au atoms, oxygen atoms into substrate material as well as into liquid Au droplets and allowing regions. Some of these effects are also not yet clearly understood, and so we can not yet control them. The speed of temperature increase (slope of temperature profile) also strongly affects the formation of Au droplet and alloying. In principle, when the growth temperature is higher, Au droplets are smaller, and then the nanowire diameters are also smaller. Growth times could be understood to be the time for whole temperature profiles. But the duration time t 2 is long enough for Au droplets formation, alloying, and duration time t 4 is also chosen so long depending on requirements of length of nanowire. 4.2. Some phenomena of growth process and main growth mechanisms We have investigated some main phenomena of the growth processes concerning mechanisms by SEM images at different temperatures and different growth times. These can be seen in figure 7 [9, 10, 19]. The phenomena concerning the mechanisms of each nanowires growing system by VLS may be different from each other but, for example, we discuss Au-Ge and Au-GaAs systems, in essence, their growing processes have similarities. The main growth mechanisms are followings: A growing system consists of semiconductor substrate (Ge, GaAs) in ampoule or sealed tube with low pressure air as in figure 7 a) The furnace is heated up to temperature near T 1 lower than the eutectic temperature (as in figure 6), then Au thin film catalyst layer is disturbed; the alloy of Au-Ge could be beginning to form on semiconductor slice surface (figure 7 b, c). Temperature increases to T 1 after duration time t 2, Au droplets (or islands) as well as the alloy regions under Au droplets are formed more clearly, sharply (figure 7 d). The semiconductor atoms (Ge, Ga...) more and more diffused into the interface between Au droplet and alloy. When semiconductor atom concentration in alloy becomes saturated, semiconductor atoms precipitate (or cluster) at the liquid-solid interface. This process is very important for growing nanowires in the next steps. Growth process is developed when temperature continuously increases from T 1 toward T 2, from phase diagram we can see that semiconductor atomic % (Ge, Ga) will increase. At the increased semiconductor atom (Ge, Ga) weight percentage of about 50-60% one big precipitate or more small precipitates will be formed. Because the sticking coefficient on liquid Au droplet is higher then that on solid surfaces, semiconductor crystal growth is occurred only where the liquid metallic catalyst is present (figure 7e). Depending on the number and sizes of semiconductor precipitates in an Au droplet, one big nanowire or more small whiskers could be grown up from one Au droplet. Due to the surface tension the liquid catalyst forms a ball situated on the top of the growing nanowire (figure 7 f). 6
(a) (b) (c) (d) (e) (f) (g) (h) Figure 7. Some main results showing our main phenomena concerning Ga 2 O 3 nanowires growing process on GaAs substrate concerning growing mechanisms: Au thin film layer on semiconductor substrate with low pressure a); Au layer is disturbed on the surface b); Au droplets and alloy formed at near temperature T 1 d); the nucleation s are formed and semiconductor crystals began to grow e); nanowires diameters depend on the forms of Au droplets on the surface f); a long Ga 2 O 3 nanowire of about 8 micrometers length and 180 nm diameter g); and Ga 2 O 3 nanowires formed in vertical orientation with rather uniform of array h). 4.3. Further discussions The diameter of the nanowire grown by VLS method is determined by the form or diameter of Au droplet. Its size remains essentially unchanged during the entire process of wire growth (see figure 7 f, g and h). The speed of growth, depending on technological conditions and substrate materials, is about several hundreds nm per minutes. The orientation of growing nanowire is related to the locations of the diffused semiconductor atoms in solution of alloy near the interface of Au droplet. Preferred growth directions lie perpendicular to planes along which semiconductor atoms diffused readily allowing for completion of mentioned planes to the diameter of the catalyst, presumably by incorporation of added atoms at steps edges. The chemical reactions occurred in the low vacuum tube /ampoule containing low oxygen or vapour water can be written for three kinds of growing systems (on Ge, Si and GaAs substrates) as the following, respectively: Ge + O 2 GeO 2, (1) 4 Si + O 2 SiO 2, (2) Ga + 3 O 2 2 Ga 2 O 3, (3) 2Ga + 3H 2 O Ga 2 O 3 + 3H 2. (4) Here we would like to note that semiconductor atoms (Ge, Si or Ga) from gaseous phase or from bulk substrate could be diffused out into gas ambient or diffused into the alloy region. The source of oxygen (O 2 ) from gaseous phase is in the growing tube. The diameter of a grown nanowire d can be determined by the following way: 7
d c = 4 α Ω (5) C RTLn C where α is specific free energy of the nanowire at the liquid-vapour interface, Ω is gram molecular volume, R is real gas constant, T is absolute temperature, C is concentration of semiconductor atom (Ge, Si, Ga) in the liquid phase, C is equilibrium concentration of semiconductor element in liquid phase. From Eq. (5) we also see that the diameter increases linearly with α and Ω and inversely with T and in(c/c ). Our experimental results agreed with these facts. From obtained results on figures 3, 4, 5, 7 we also observed that GeO 2, Ga 2 O 3 nanowires sometimes grow spontaneously with tangly whiskers forms, and grow orderly depending on chosen technological conditions. The diameters of GeO 2 nanowires are in the range of about 10 to about 40 nm, for Ga 2 O 3 nanowires the diameters are in the range of about 80-200 nm with length of several µm. We observed that the higher substrate temperatures T 2 is, the smaller nanowire s diameter and more nanowires density becomes. 5. Dynamic 2D simulation of nanowire growth process Based on our experiment results obtained and the understanding of nanowires growth mechanisms we have developed a simulation program for dynamic simulation of nanowires growth in 2D. Figure 8. Eight standing simulation pictures of dynamic simulation program at typical growth temperatures and growth times corresponding to the real growth phenomena and growth mechanisms in figure 7. Upper row: Au layer is on the surface then it is disturbed and becomes Au droplets. Under row: Au droplets is alloying to form liquid-solid phase; the nucleation s seed are formed and semiconductor crystals began to grow in the forms of Au droplets on the surface Figure 8 shows some standing simulation pictures attracted from the dynamic simulation program of nanowire growth process. The program can run with different velocities, it can also zoom to small or larger sizes. The programme should be running in a rather strong configuration PC. The program has transformed up into Avi or Flash format in order to run in multimedia. It has been inserted in our 8
website together with other simulation programmes. Software programme consists of some blocks using many commands of Matlab 7.0 to create figures linking together to form a dynamic simulation programme of nanowires growth on semiconductor substrate in air pressures 10-1 - 10-2 torr. The flow chart of simulation program can be seen in figure 9. Start Set up growing system (vapor box and solid substrate box ) Vapor phase in box with random moving oxygen atoms For j = j+1 Oxygen atoms absoption into Au droplets and alloy Solid Au film on surface induced to droplets at certain positions Changing sizes of droplets and alloying with semiconductor Alloy regions formation between droplets and semiconductor substrate The seeds for growth at the interface between liquid Au and solid phase Nanowires are growing up Yes No End Figure 9. Flow chart of dynamic simulation programmes for nanowires growth on semiconductor substrate. 9
6. Conclusions We have synthesis of GeO 2, Ga 2 O 3 nanowires using a thermal VLS method in an ampoule or a sealed glass/quartz tube at a pressure maintaining of 10-1 -10-2 torr with different temperatures and different growth times. The nanowires were investigated by SEM, EDX, AFM and SEM The nanowires have diameters distributed from 15 to 40 nm for GeO 2 and 20-180 nm for Ga 2 O 3. Their lengths are ranging from 20 nm to several µm. We observed also that the higher growth temperature is, the diameter smaller becomes, its density is also increased. A dynamic simulation program of nanowires growth process on semiconductor substrate has been developed. This simulation contains some main phenomena concerning growth mechanisms, and from this simulation we could see directly and lively growth process in PC screen. We would like to note that the technology of VLS method for growing nanowires is simple, low cost but in order to grow good quality, uniform nanowires with fitable sizes for special applications at desired places on substrate we still faced with some technological difficult problems concerning controlling growth mechanisms. References [1] Patently M J 2004 Nano materials the driving force Nanotoday 7(12) 20 [2] Roco 2002 Small Wonders Nanoscience, Engineering and Technology (NSET) 1 [3] Bharat Bhushan 2004 Handbook of Nanotechnology Part A (Springer) 99 [4] Yang P, Wu Y, Fan R 2002 International Journal of Nanoscience 1(1) 1 [5] Lauhon L J, Gudiksen M S and Lieber C M 2004 Phil. Trans. R. Soc. Lond. A 362 1247 [6] Huang Y, Xiangfeng, Cui Y and Mlieber C 2002 Nano Letters 2(2) 101 [7] Greytak A B, Lauhon L J, Gudiksen M S and Lieber C M 2004 Applied Physics Letters 84 24 [8] Atashbar M Z, Yu M-F and Chen X 2002 Investigation and characterization of Ga 2 O 3 nanowire for gas sensing applications IEEE Sensors [9] Tuan P A, An D K, Manh D H, Phong P V and Hoa L T 2007 Proc. of the 5 th National Solid State conference, Vietnam 359 [10] Trang P H, Vuong H V, Phong P V, Manh N H and An D K 2008 Proc. of the APCTP ASEAN Workshop on Advanced Materials Science and Nanotechnology (Nha Trang, Vietnam) p. 975 [11] Mojzes I, Kokényesi S, Szabó I A, Ivan I and Pécz B 2006 Nanopages 1 85 [12] Hemant A, Ann M F, Irene A G, Christopher E D C and Paul C 2007 ACS Nano. 1(5) 415 [13] Civale Y, Nanver L K, Hadley P 2004 Proc. of 7th Annual Workshop on Semiconductor Advances for Future Electronics and Sensors (Veldhoven, The Netherlands) 692 [14] Noborisaka J, Motohisa J, Hara S and Fukui T 2005 Applied Physics Letters 87 093109 [15] Wen J G, Lao J Y, Wang D Z, Kyaw T M, Foo Y L and Ren Z F 2003 Chem. Phys. Lett. 372 717 [16] Kalache B, Cabarraocas P R, Morral A F 2006 Jpn. J. Appl. Phys. 45 L190 [17] Verheijen M A, Immink G, Smet T de, Borgstrom M T and Bakkers E P 2006 Growth kinetics of heterostructured GaP-GaAs nanowires J. Am. Chem. Soc. 128 1353 [18] Chiem C V, Ha N T T, Tam N T T, Chuc N V, Li H and Muyng S J 2008 Proc. of the APCTP ASEAN Workshop on Advanced Materials Science and Nanotechnology (Nha Trang, Vietnam) p. 837 [19] An D K, Chung N X and Tuan P A 2008 Proc. of the APCTP ASEAN Workshop on Advanced Materials Science and Nanotechnology (Nha Trang, Vietnam) p. 518 Acknowledgements Authors would like to express their thanks for Mr. Do Hung Manh for his measurement of SEM, EDX of samples. Authors also would like to express their thanks to the Institute of Materials Science VAST as well as NAFOSTED for financial funding to the basic research project in 2007, 2008 and 2009 to carry out these topics. 10