Perspective pubs.acs.org/JPCL
Tuning the Growth Mechanism of ZnO Nanowires by Controlled Carrier and Reaction Gas Modulation in Thermal CVD Andreas Menzel,*,† Kittitat Subannajui,†,§ Rakshit Bakhda,† Yabin Wang,† Ralf Thomann,‡ and Margit Zacharias*,† †
Laboratory for Nanotechnology, Department of Microsystems Engineering, University of Freiburg, Freiburg 79110, Germany Freiburger Material Forschungszentrum (FMF), University of Freiburg, Freiburg 79104, Germany § Mahidol University, 272 Rama VI Road, Ratchathewi District, Bangkok 10400, Thailand ‡
ABSTRACT: A general schematic is developed for the main parameters leading to the formation of vapor−solid and catalyst-assisted-grown ZnO nanowires. We developed a schematic representation of the different parameters influencing the growth and discuss them in detail. Selected shape diagrams are presented that correlate the various changes in parameters (carrier and reaction gas flow, powder and substrate temperature) to the observed nanostructures. With the help of the here-presented shape diagrams, we are able to identify unique parameter ranges for film formation, vapor−solid growth, and the switching to the catalyst-assisted VLS-growth mechanism. The systematic experiments demonstrate the controlled growth of VS and catalyst-assisted-grown ZnO nanowires in a CVD reactor by the carbothermal reduction method based on either upstream or downstream deposition techniques.
S
catalyst-assisted-grown NWs is that VS NWs have a buried Au dot or Au at the root while the catalyst-assisted-grown NWs exhibit a Au dot on the NW tip whose size has a similar diameter as the NW itself.11,12 Thus, diameter-controlled growth of NWs is possible in the VLS growth process by controlling the catalyst dot size.13 The diameter control in the case of VS NWs can be mainly tuned by the substrate temperature. At higher substrate temperatures the NWs tend to grow in smaller diameters and larger lengths, while lower substrate temperatures yield to increased diameters and shorter lengths. This effect can be mainly attributed to the growth kinetics.14,15 Previously, the effect of O2 concentration versus system pressure inside of a one-zone growth furnace at downstream deposition was investigated and mapped in a phase diagram only for VLS ZnO NWs.16 It was found that a particular range of O2 (∼2 vol. %) combined with a certain pressure range at around 30 mbar and higher temperatures (∼880 °C) is required for a VLS growth of ZnO NWs. It was reported that a low pressure results in a film formation, an “optimum” pressure results in optimum NW growth, and a rather higher pressure hinders the NW formation. Wongchoosuk et al. investigated the effect of powder (TP) and substrate (TS) temperatures and presented the resulting morphologies mapped into a shape diagram.14 Reproducibility was obtained by a flow shutter approach, which prevents a deposition during the temperature
emiconducting nanowires (NWs) have shown excellent properties from fundamental research to future devices.1 Especially ZnO NWs provide a direct band gap (3.37 eV) and a large exciton binding energy (60 meV), which make them suitable for a wide range of device applications, such as FETs, optoelectronic devices, and sensor devices (chemical, gas, and biosensors).2−5 After an overview on the results reported from the various growth experiments published so far in the literature, one notices that an abundant number of parameters is used in different tube chambers, resulting in NWs and nanostructures. ZnO NWs grown by thermal CVD typically require a metal catalyst (e.g., Au) on the substrate surface because it initiates and guides the growth. The main growth mechanisms to obtain ZnO NWs are the vapor−solid (VS) and the catalyst-assisted growth mechanisms (typically called vapor−liquid−solid (VLS)). In the case of the VLS process, a thermally evaporated growth species (Zn in our case) is transported to the metal catalyst, which then adsorbs and diffuses to form a liquid alloy. As the time passes, the concentration exceeds the solubility, and the ZnO NW precipitates. The growth by supersaturation and precipitation occurs only at appropriate growth conditions or until the growth species source is exhausted.6−8 The VS process can be characterized by thermally evaporating a growth species and the condensation on a solid catalyst (e.g., metal) at a lower temperature. Different energetically favored nucleation sites, such as surface defects, dislocations, or metal patterns, are required for a preferential deposition of Zn atoms.9,10 The Zn atoms move due to surface diffusion, and ZnO NWs grow with a preferential orientation (typically the c-orientation) by selforganization.6 A visual criterion to distinguish between VS and © 2012 American Chemical Society
Received: August 3, 2012 Accepted: September 13, 2012 Published: September 13, 2012 2815
dx.doi.org/10.1021/jz301103s | J. Phys. Chem. Lett. 2012, 3, 2815−2821
The Journal of Physical Chemistry Letters
Perspective
the NWs to the centimeter range, as we demonstrated in a former work.14 For the thermal CVD growth, it is important to understand the main key parameters, that is, chamber pressure p, carrier gas transport, source temperature TP, substrate temperature TS, and concentration gradients of the reaction gas, which all affect the morphology of the grown nanostructures. For a controlled NW growth, gas leakage must be prevented because it would represent an additional oxygen source; therefore, an initial pumping to a respective small base pressure in a tight sealed system is required. Also, the pumping system and tube geometry (i.e., tube diameter) play a crucial role for establishing suitable growth parameters. The schematic diagram in Figure 1
ramping by using a high system pressure, which hinders the diffusion of the pre-evaporated species to the substrate position.17,18 Later numerical calculations, however, revealed that such flow shutter approach comprises nonsteady conditions during the growth.19
Carrier gas and reaction gas control the growth position and ZnO NW growth. The transport properties and species distribution by thermal CVD were previously investigated for one-zone and three-zone growth systems. It was shown that a proper tuning of pressure and carrier gas flow influences the NW growth position inside of a growth reactor. Controlling diffusion and convection of growth species enables the growth in either the upstream or downstream positions.19,20 Moreover, the control of the O2 flux is crucial because it determines how much O2 is available for the rereaction of the Zn to ZnO in upstream and/or downstream directions. In addition, O2 might also be consumed at the source position where carbothermal reduction takes place. If the O2 flux is rather low, then upstream NW growth might occur based on upstream diffused Zn gas while the remaining O2 is consumed at the powder source, leaving only insufficient O2 at the downstream position, where then only a Zn-rich film is grown.20 On the other hand, if the O2 flux is too concentrated, the oxidization environment may also suppress the NW formation in the upstream position; however, after passing the source powder position, the reduced O2 concentration at the downstream position might be high enough for ZnO NW growth.20 A different approach to control the switching between VS and VLS growth modes was presented by Ramgir et al. by adding ionic liquids (ILs) inside of the growth reactor, which represents an additional carbon source. The growth area was drastically increased to centimeter ranges even in a one-zone oven for the VLS-grown NWs by adding ILs. It should be noted that a one-zone furnace typically provides a large temperature gradient, and NWs can only grow at a certain position with appropriate conditions; thus, the growth range is limited. A cleaning step of the tube by HCl enables one to completely switch back to the VS growth mode.21 Many researchers have investigated the effects of selected parameters; however, there is still a lack of understanding how to switch between VS and catalyst-assisted-grown NWs in a controlled way. This motivated us to perform systematic experiments in a kind of parameter mapping to get a better understanding of which parameters are important to switch between the various growths modes. We represent a schematic of the multiple parameters influencing the growth and discuss them in detail. The aim was to develop a more generalized picture and to relate the various nanostructures reported in the literature (nanowires, nanospears, nanonails, nanobelts, etc.) to fundamental changes in the growth chamber. Please note, however, that a vacuum-tight system and controlled temperature and gas management are mandatory conditions for repeating the observed structures. We therefore developed a two-zone oven with a separate temperature control of the two zones and a known temperature profile connected to a far sufficient flow and pressure control system. The advantage of such a wellcontrolled growth system is the extension of the growth area of
Figure 1. Schematic diagram of the key parameters controlling the NW growth.
illustrates the most significant parameters and how they can affect the growth mechanism of VS and catalyst-assisted-grown NWs. The starting point in growth is the evaporation of the source powder, which contains a ZnO and graphite (C) mixture in our case. In a system without flow, the evaporated species would develop a kind of a gas plumb above the powder source that interacts with the surrounding gas depending on the chamber pressure. The extension and gas density of the plumb depend on the pressure, the source temperature, the grain size of the powder, and generally the evaporation properties of the material. Increasing the source temperature increased the evaporated species and, hence, the material transport to the substrate. Temperature ramping (heating and cooling periods) will already cause evaporation to start and hence a growth with undesirable effects. It is important to control this issue to prevent a growth during these periods. In the case of ZnO and C mixed in a 1:1 ratio in the source powder, the ZnO is reduced by the graphite to Zn at the high source temperatures (typically ∼950 °C); hence, the transported gas species contain Zn gas and CO or CO2 depending on the source material. Before the growth starts, the tube chamber is pumped down at room temperature to keep the remnant O2 concentration at a minimum, and a constant carrier gas flow is established. The pressure and flow conditions influence the gas transport, as we showed previously by gas flow simulations.19,20 In addition to the external flow, the evaporated gas species represent also a kind of gas diffusion that is random and T-dependent. This means that the evaporated species could also diffuse upstream, which could lead to a growth in upstream positions. However, 2816
dx.doi.org/10.1021/jz301103s | J. Phys. Chem. Lett. 2012, 3, 2815−2821
The Journal of Physical Chemistry Letters
Perspective
such a process is only observed for lower flow rates of the carrier gas. Increasing the gas flow rate pushes the evaporated plumb in the flow direction, dilutes the evaporated species, and homogenizes the gas flow profile. Normally, if using a respective high carrier gas flow, no growth is observed in the upstream direction, as should be the goal because of the better control under such conditions. The kind of carrier gas and respective concentration (Ar, O2), the flow conditions, and the pressure determine the transport and convection properties of the species (Zn in this case) and the respective reactions (carbothermal reduction and rereaction with oxygen on the substrate surface). If now the gas flows over the heated substrate, the growth on the substrate is affected by the type of substrate (crystal properties), the position of substrate to the evaporation source, the corresponding substrate temperature, which determines gas adsorption and desorption, and reaction at the catalyst dot. Usually, metal catalysts (e.g., Au) are used as nucleation sites to control growth position, size, thickness, and density per area. A high substrate temperature and a high adsorbed amount of Zn might establish a liquid Zn−Au alloy droplet that has a much higher sticking coefficient than a solid one, which enhances the growth rate for the VLS wires compared to the VS ones. It is the complex interaction of all parameters that determines the sticking and condensation rate of the obtained nanostructures.
downstream-grown ZnO NWs. We are confident that these mappings are helpful for designing future NW growth experiments, identifying and understanding the different nanostructure formations depending on variations of three parameters, here, TS, Ar flow, and O2 flow. For growing ZnO NWs, a one-zone heating and a two-zone heating growth system were utilized in this study. The ZnO NW growth by a one-zone heating system (1 in. tube diameter) is based on the upstream deposition technique (here, the substrates were placed at upstream positions), as is described in ref 20. The two-zone growth system (2 in. tube diameter) is based on the downstream deposition technique, as is also described in refs 14 and 19. A 1:1 weight ratio of ZnO (99.999% purity) and graphite (99.99% purity) mixed powder was placed in a quartz boat inside of the center of the source heating zones in a reactor tube. ZnO nanostructures were grown on a-plane-oriented sapphire substrates with a 5 nm Au film by thermal evaporation. An Ar carrier gas and an O2/Ar reaction gas mixture (1:10 ratio) were used, which were controlled by separated mass flow controllers. The tube reactor was pumped down to a base pressure better than 4 × 10−5 mbar before a working pressure was maintained by the carrier gas flow. For the one-zone reactor furnace experiment (Figure 2), the substrate temperatures ranged between 650 and 900 °C (the source powder temperature was 930 °C), and the Ar carrier gas flow between 5 and 15 sccm was varied. At the same time, the system pressure was maintained at 200 mbar, and the O2 flow rate of 0.01 sccm was kept constant, which corresponds to a range of 0.07−0.2 vol. %. Experiments with a two-zone reactor furnace are based on the downstream deposition technique (here, the substrates were placed at downstream positions). The experiments were carried out first at a constant O2 flow rate of 0.15 sccm (1.5 sccm in the O2/Ar mixture) and a system pressure of 30 mbar while varying the TS between 550 and 900 °C (TP was 950 °C for all two-zone system experiments) and the Ar flow rate between 0 and 100 sccm, which corresponds to a diluted O2 concentration between nearly 0.15 and 10 vol. % (Figure 3). Then, the same parameters as before were used; however, the O2 flow rate instead of the Ar flow rate was varied between 0 and 0.8 sccm (0−2.6 vol. % O2 concentration) in this experiment (Figure 4). To keep the data consistent for comparison of all experiments, the growth time at the set point temperature was 10 min, and the system pressure was changed to atmospheric conditions during the cooling down period to prevent any further
Key parameters influence the NW growth mechanism. In this study, we will first investigate with help of a two-zone system (i.e., capable for independently controlling TP and TS) how the change in substrate temperature and Ar flow under constant powder temperature (i.e., evaporation rate) and constant flow of O2 changes the morphology of ZnO NWs due to the increased dilution of the Zn gas by the increased Ar flow. In the second set of experiments, we will demonstrate how the variation of O2 concentration can be used to switch between VS and catalyst-assisted-grown ZnO NWs in certain temperature windows at constant Ar flow rates. Also, the ratio of Ar and O2 flows to switch the growth mechanism will be established. Please note that these experiments in the two-zone furnace are all done in downstream deposition modes. Furthermore, the effect of upstream deposition on the NW growth mode inside of a one-zone growth system is demonstrated in a mapping, which will be compared with
Figure 2. Shape diagram depending on the carrier gas (Ar) flow and substrate temperature TS in a 1 in., one-zone reactor system in the upstream growth direction configuration. The diagram shows mainly (a) the ZnO nanorod formation, (b) the ZnO film formation, (c) the catalyst-assisted ZnO NW growth region, and (d) the optimum growth region for VS ZnO NWs. 2817
dx.doi.org/10.1021/jz301103s | J. Phys. Chem. Lett. 2012, 3, 2815−2821
The Journal of Physical Chemistry Letters
Perspective
Figure 3. Shape diagram depending on the carrier gas (Ar) flow and TS in a 2 in., two-zone reactor system in the downstream growth direction configuration. The diagram shows mainly the ZnO film formation in areas A1 and A2, a mixture of ZnO film formation with NW nucleation points in area B, the optimum growth region for VS-grown ZnO NWs in area C, an intermixing of VS and catalyst-assisted-grown ZnO NWs in area D, and an area E where catalyst-assisted NWs are grown.
Figure 4. Shape diagram depending on the O2 concentration and TS in a 2 in., two-zone reactor system in the downstream growth direction configuration. The diagram shows mainly the ZnO film formation in areas A (A1 at ∼550 °C, A2 at ∼750 °C, and A3 at ∼900 °C), a mixture of ZnO film formation with NW nucleation points in area B, the optimum growth region for VS-grown ZnO NWs in area C (C1 at ∼550 °C and C2 at ∼750 °C), an area D (D1 lower and D2 higher O2 concentration) for catalyst-assisted NWs grown, and an area E (E1 at ∼550 °C, E2 at ∼750 °C, and E3 at ∼900 °C) for a suppression of the ZnO nanostructure formation.
Generally, if the carrier gas flow is too high in the upstream configuration (here, >10 sccm), there is not a sufficient enough Zn concentration for the nucleation and growth of ZnO structures. When increasing the Ar flow further, a shift of nanostructure formation at the downstream position is caused due to the high flow. In the orange area of Figure 2, the temperature is lower than 700 °C, and only a ZnO film appears on the substrate surface (Figure 2b). Increasing the TS (brown area) leads to VS-grown ZnO nanorod formation (Figure 2a). A further increase of TS leads to an optimum growth area (blue area), which contains VS-grown NWs (Figure 2d). Here, however, the average length and diameter are controllable by the TS and deposition time. The yellow area exhibits a temperature high enough that a catalyst-assisted growth can be observed (Figure 2c) due to the diffusion of Zn atoms on the Au catalyst and supersaturation. However, there is still an oxidizing environment that forms rather an intermixing of VS and catalyst-assisted growth of NWs. Temperatures beyond
deposition of growth species. All scanning electron micrographs (SEMs) were taken by a high-resolution scanning electron microscope (HRSEM Nova NanoSEM 430). The kinetics of epitaxial growth is mainly determined by the substrate temperature. Additionally, the carrier gas (Ar) and reaction gas (Ar/O2 mixture) flow and growth species interaction influence the transport properties. Figure 2 is a shape diagram depending on the Ar flow and substrate temperature and shows the morphologies at different parameters at the upstream position in a 1 in., one-zone furnace system. The O2 flow rate is constant at 0.01 sccm. Below a 10 sccm Ar flow (which corresponds to a
