Synthesis of Tin Nitride SnxNyNanowires by Chemical Vapour Deposition

Tin nitride (SnxNy) nanowires have been grown for the first time by chemical vapour deposition on n-type Si(111) and in particular by nitridation of Sn containing NH4Cl at 450 °C under a steady flow of NH3. The SnxNynanowires have an average diameter of 200 nm and lengths ≥5 μm and were grown on Si(111) coated with a few nm’s of Au. Nitridation of Sn alone, under a flow of NH3is not effective and leads to the deposition of Sn droplets on the Au/Si(111) surface which impedes one-dimensional growth over a wide temperature range i.e. 300–800 °C. This was overcome by the addition of ammonium chloride (NH4Cl) which undergoes sublimation at 338 °C thereby releasing NH3and HCl which act as dispersants thereby enhancing the vapour pressure of Sn and the one-dimensional growth of SnxNynanowires. In addition to the action of dispersion, Sn reacts with HCl giving SnCl2which in turn reacts with NH3leading to the formation of SnxNyNWs. A first estimate of the band-gap of the SnxNynanowires grown on Si(111) was obtained from optical reflection measurements and found to be ≈2.6 eV. Finally, intricate assemblies of nanowires were also obtained at lower growth temperatures.


Introduction
Nitrides and in particular, group III-Nitride (III-N) compound semiconductors such as GaN, InN and AlN have been investigated intensively over the past decade due to their applications in electronic and optoelectronic devices like field effect transistors, light emitting diodes and lasers [1][2][3]. III-N semiconductors are especially attractive because their band-gap can be tailored from 0.7 eV in InN [4] up to 6.2 eV in AlN [5] but also due to the strained induced charges that provide an extra degree of freedom which can be used to tailor the band-profile and consequently the properties of devices [6].
Not surprisingly there are very few investigations on Sn 3 N 4 nanostructured materials and it appears that the only study carried out so far concerns the synthesis of Sn 3 N 4 nanoparticles (NPs) on Au coated Si(001) via chemical vapour deposition (CVD) using SnCl 4 Á 5H 2 O as a solid precursor, by Nand et al. [24]. To date, there is no report on the synthesis of Sn x N y nanowires (NWs) despite the fact that nanowires constitute a fundamental building block for the development of nanoscale devices such as third generation solar cells which require low cost, nanostructured materials.
Therefore in order to complement our earlier investigations on the synthesis and properties of InN NWs and related oxides such as In 2 O 3 and SnO 2 NWs [25][26][27], a preliminary investigation on the growth of Sn x N y NWs was undertaken.
Here the synthesis of the first Sn x N y NWs on Au/Si(111) is described and it is shown that Sn x N y nanowires (NWs) can not be grown via the direct nitridation of Sn over a broad range of temperatures i.e. between 300-800°C due to the formation of Sn droplets on the surface of the Si(111). While there is evidence of one dimensional (1D) growth occurring at 500°C via the direct nitridation of Sn, the yield is extremely poor. One-dimensional growth was promoted and significantly enhanced via the incorporation of NH 4 Cl in the Sn and its sublimation which acts as a dispersant thereby enhancing the vapour pressure of Sn. Sn x N y NWs with an average diameter of 200 nm were obtained at 450°C while intricate assemblies of NWs have also been obtained at lower temperatures.

Experimental Procedure
The Sn x N y NWs were grown using an APCVD reactor which consists of four mass flow controllers (MFC's) and a horizontal quartz tube furnace, capable of reaching a maximum temperature of 1,100°C. Initially, fine Sn powder (Aldrich, \ 150 lm, 99.5%) was loaded into a quartz boat together with a square piece of Si(111) approximately 7 mm 9 7 mm in size, which was coated with a few nm's of Au. The Au layer was deposited via sputtering at a slow rate using an Ar plasma under a pressure \10 -4 mBar. The Au/Si(111) sample was positioned & 5 mm downstream from the Sn and subsequently the boat was loaded into the APCVD reactor and positioned directly above the thermocouple used to measure the heater temperature at the centre of the quartz tube. After loading the boat at room temperature (RT), Ar (99.999%) was introduced at a flow rate of 500 standard cubic centimetres per minute (sccm) for 5 min in order to purge the tube and eliminate O 2 and H 2 O. Following this the temperature was ramped to the desired growth temperature in a NH 3 flow of 250 sccm at a rate of 30°C/min. Upon reaching the growth temperature (T G ) the flow of NH 3 was maintained at 250 sccm for a further 60 min after which the tube was allowed to cool down over at least an hour, in a flow of 50 sccm NH 3 . The sample was removed only when the temperature was lower than 100°C.
In order to enhance the one dimensional growth of Sn x N y NWs an equal amount of anhydrous NH 4 Cl (VWR Int 99.9%) was added to the Sn and mixed thoroughly in the boat. Then the same gas flows and temperature-time profile described above was employed. A summary of the temperatures and conditions is given in Table 1. The morphology of the Sn x N y NWs was examined with a TESCAN scanning electron microscope (SEM) while the crystal structure and the phase purity of the Sn x N y NWs were investigated using a SHIMADZU, XRD-6000, X-ray diffractometer with a Cu Ka source while a scan of h-2h in the range between 10°and 80°was performed. Finally optical spectroscopy was carried out using a standard spectrophotometer UV/V (Perkin-Elmer Lambda 950) in the reflection mode at near normal incidence to the surface of the sample.

Results and Discussion
As stated above the only investigation on the synthesis of nanostructured Sn 3 N 4 is that of single phase, cubic tin nitride nanoparticles grown via atmospheric pressurehalide vapour phase epitaxy by Nand et al. [24]. In particular Nand et al. employed SnCl 4 Á 5H 2 O as a source of Sn and used 10 nm Au/Si(001) p-type substrates that were positioned at different distances from the SnCl 4 Á 5H 2 O, along the reactor. The SnCl 4 Á 5H 2 O was heated up to 500°C under a flow of NH 3 and N 2 and the temperatures of the samples along the reactor were 400, 300 and 150°C, respectively. However, Nand et al. [21] did not obtain any NWs. Before discussing the synthesis of the Sn x N y NWs obtained here it is instructive to consider first the synthesis Table 1 Maximum diameter (Ø) of Sn droplets (DPs) and Sn x N y NWs obtained from heating up Sn under NH 3 and from the reaction of Sn ? NH 4 Cl respectively, at different temperatures. In all cases a ramp rate of 30°C/min was used to reach T G under 250 sccm's of NH 3 which was then maintained for a further 60 min at the growth temperature and after which the reactor was allowed to cool down to RT in a reduced flow of 50 sccm's NH 3  [27]. A typical SEM image of the SnO 2 NWs obtained in this way is shown in Fig. 1a from which it is clear that a large yield of SnO 2 NWs was obtained with an average diameter of 50 nm due to the reaction of Sn with residual O 2 in the APCVD reactor. Performing the reaction under a direct flow of O 2 leads to the formation of SnO 2 around the molten Sn which limits the vapour pressure significantly and hence the growth of NWs. As a consequence the molten Sn upstream always had a grey like, non reflective appearance at the end of the process, while no droplets were observed among the SnO 2 NWs. A similar process was also used recently for the growth of In 2 O 3 NWs at 700°C [26]. At first sight it would seem that the synthesis of Sn x N y NWs by direct nitridation of Sn with NH 3 is feasible by changing from Ar to NH 3 since Sn, like In, has a low melting point [28] and InN NWs have been obtained by direct nitridation of In with NH 3 at a heater temperature of 600°C [25]. However, Sn x N y NWs were not obtained by the direct nitridation of Sn with NH 3 . Instead many Sn droplets appeared on the Si(111) surface and a typical SEM image of such Sn droplets after the attempted nitridation of Sn with NH 3 at 800°C is shown in Fig. 1b. The Sn droplets cover the entire surface and have a density of &10 7 cm -2 and diameters B5 lm. Furthermore the size of the Sn droplets decreased as the temperature was reduced to 600°C and many of then became elongated as shown in Fig. 1c.
The formation of large droplets on the Au/Si(111) surface during the direct nitridation of Sn with NH 3 is a direct consequence of the reducing action of NH 3 which eliminates the background O 2 in the APCVD reactor. This in turn prevents the formation of an oxide around the Sn and so the molten drop always had a highly reflective, metallic like surface. In contrast when Sn is heated up in a flow of pure, inert Ar, the surface is grey like and not reflective due to the O 2 background which is responsible for the formation of SnO 2 NWs that were grown optimally on 0.5 nm Au/Si(111) at 800°C using the same temperature-time profile and Ar as opposed to NH 3 [27].
Apart from droplets, no nanostructures were obtained via the attempted nitridation of Sn with NH 3 in the temperature range 600°C \ T G \ 800°C. Turning on the flow of NH 3 , after ramping up the temperature in an inert gas flow of Ar, did not lead to the growth of Sn x N y NWs either but again resulted into the deposition of Sn droplets. However, there was some evidence of one-dimensional growth at T G = 500°C. Literally a few NWs with diameters [500 nm's and lengths up to 3 lm appeared at a few locations on the Si(111) surface, hence the yield was extremely poor. Nevertheless, a further reduction of the growth temperature down to 300°C did not lead to any Sn droplets on Au/ Si(111) deposited using the same temperature-time profile in a but in a steady gas flow of 250 sccm of NH 3 as opposed to Ar. A broad distribution in the sizes of the Sn droplets was obtained with diameters B5 lm c similar to b but smaller Sn droplets with diameters B0.5 lm were obtained at 500°C many of which are also elongated significant deposition on the Si, no NWs were obtained and moreover, the Sn upstream lost its metallic shine due to the build up of a black deposit on the molten Sn which limited the vapour transport. In addition, no differences were observed upon changing the flow rate of NH 3 during the growth while keeping everything else equal at all temperatures so the direct nitridation of Sn alone under a flow of NH 3 is not effective and leads to the deposition of Sn droplets on the Au/Si(111) surface which impedes onedimensional growth over a wide temperature range i.e. 300-800°C as shown below in Table 1.
The XRD spectrum of the Sn droplets deposited at 800°C is shown in Fig. 3 and is characterized by an intense peak corresponding to the (2 0 0) orientation of Sn and less intense but well resolved peaks corresponding to the (1 0 1), (3 0 1), (4 0 0) and (3 2 1) orientations. In addition to the Sn droplets the Al holder peaks have also been identified but no peaks associated with Sn x N y were found.
These findings are in direct contrast with the case of InN where NWs can be grown by direct nitridation of In with NH 3 via a self-catalytic mechanism. The optimum heater temperature for the growth of InN NWs was found to be 600°C where its vapour pressure is equal to 10 -6 Torr. Large In droplets comparable in size to those in Fig 1b  started appearing only at temperatures C800°C in contrast to the Sn droplets whose density was large and persisted even down to 600°C where its vapour pressure is \10 -11 Torr. It appears therefore that the Sn droplets are born out from the melt upstream and are transferred to the Si(111) surface where they coalesce to form larger droplets.
The tendency for one-dimensional growth observed at T G = 500°C during the direct nitridation of Sn with NH 3 was promoted by the addition of NH 4 Cl into the Sn at a ratio of 1:1 by weight. The reaction of NH 4 Cl with Sn was carried out under a flow of NH 3 keeping the flow rate, ramp rate and temperature profile identical to those used in the case of 'direct nitridation' of Sn with NH 3 . A typical SEM image of Sn x N y NWs obtained by the reaction of Sn with NH 4 Cl under NH 3 at 450°C is shown in Fig 2a. The Sn x N y NWs have an average diameter of 200 nm's and lengths up to 5 lm while the reaction of Sn with NH 4 Cl always lead to the deposition of a white powder downstream, near the cool end of the reactor, in contrast to the direct nitridation of Sn with NH 3 where no by products occurred.
The XRD spectrum of the Sn x N y NWs grown at 450°C is shown in Fig. 4 and is characterized by the (2 2 0), (3 1 1), (5 1 1) and (4 4 0) peaks, which can be indexed to the hexagonal structure of Sn 3 N 4 [18]. The intense peak of Sn(200) observed in Fig. 3 has disappeared and once more the Al peaks appearing in the XRD spectrum of Fig. 4 due to the sample holder have been identified. Furthermore there are no peaks associated with SnO 2 [27].
The promotion of one dimensional growth is attributed to the dissociation of NH 4 Cl. Upon increasing the temperature NH 4 Cl undergoes sublimation at 338°C and As explained by Chaiken et al. [29] the sublimation rate of NH 4 Cl increases by a factor of 10 4 when changing the temperature from T = 100 to 600°C and the typical sublimation weight loss of NH 4 Cl is over 90% when heated for &60 min. It is interesting to point out here that this sublimation process is endothermic and the temperature is expected to be reduced only by a few tens°C in the case of NH 4 Cl [29]. The decomposition of NH 4 Cl enhances the porosity of the Sn melt and more importantly acts as a dispersant increasing the amount of Sn that is transferred into the gas stream. In fact the sublimation of NH 4 Cl and the generation of NH 3 and HCl gasses which act to disperse the molten Sn occurs abruptly and leads to strong dispersion of the molten Sn inside the boat for the ramp rate used here i.e. 30°C/min suggesting that lower ramp rates would be more suitable. In addition to acting as a dispersant, the sublimation of NH 4 Cl yields HCl which reacts with Sn leading to the formation of SnCl 2 according to the reaction, Note that SnCl 2 melts at 38°C and decomposes above 600°C. Subsequently the SnCl 2 reacts with NH 3 according to, Consequently the role of the NH 4 Cl is two fold. First, it prevents the Sn from melting up into one single drop and second it supplies the necessary HCl for the formation of SnCl 2 . As stated above heating up Sn alone in NH 3 did not lead to the deposition of any products near the cool end of the reactor so the deposition that occurs from heating up Sn and NH 4 Cl in NH 3 is due to the reaction of Sn with HCl since XRD of the deposit showed no peaks related to NH 4 Cl.
The reaction outlined above is in a way similar to that put forward by Nand et al. [21] whereby SnCl 4 reacts with NH 3 according to, leading to the growth of Sn 3 N 4 NPs on 10 nmAu/Si(111) that were placed at various positions along the reactor but also similar to the growth of Sn 3 N 4 thin films by APCVD using halides, by Gordon et al. and Takahashi et al. [12,13]. A similar kind of reaction was also used to grow InN nanocrystals on Si(111) whereby the incorporation of NH 4 Cl into the In lead to the complete elimination and transfer of the In into the gas stream where it formed primarily InCl which in turn reacted with the NH 3 leading to the formation of InN nanocrystals with diameters of 300 nm [30].
While the addition of NH 4 Cl in Sn did not result into its complete transfer in the gas stream like with In, it provided nonetheless the necessary HCl for the formation of SnCl 2 which subsequently reacts with NH 3 on the Au/Si(111) leading to the one dimensional growth of Sn x N y . Interestingly the distance of the sample from the Sn:NH 4 Cl mixture was found to be critical and for distances [10 mm the reaction led to the formation of closely packed NPs with sizes \100 nm on the Au/Si(111) most probably due to the lower vapour pressure of the SnCl 2 .
Although the details of the growth mechanism are not understood thoroughly at present it is suggested that the Sn x N y NWs grow self catalytically from Sn x N y NPs although the role of the Au which appears to enhance the one dimensional growth still needs to be clarified [32,33].
A first estimate of the band-gap of the Sn x N y nanowires grown on Si(111) was obtained from optical reflection measurements using a UV-IR spectrometer at near normal incidence on both the NW sample and the Si(111) substrate for comparison, shown in Fig. 5. Clearly evident is the distinct difference in the reflection spectra from the substrate and the NWs. Also evident is the band edge of the Sn x N y NWs which is estimated to be approximately 2.6 eV [31].
In addition to Sn x N y NWs that were obtained at T G = 450°C there is also evidence for the formation of more complex nanostructures obtained for T G \ 450°C as shown in Fig. 6a and b. However, their density was smaller compared to that in Fig. 2a due to the lower growth temperature which limits the amount of Sn transferred over to the Si(111). The radial growth of NWs from the droplet shown in Fig 6a is very similar to the case of InN [25] whereby nucleation centres form on the surface of droplets which then facilitate radial growth [32]. Moreover, the circular arrangement of NWs shown in Fig. 6b  the formation of droplets that accumulate near the periphery of well defined circles similar to the growth of In 2 O 3 nano pyramids that self assemble in the form of wreaths due to the reaction of In with NH 4 Cl in a flow of N 2 [30].

Conclusions
The first tin nitride, Sn x N y nanowires have been grown by CVD on Au coated Si(111) via the reaction of Sn with NH 4 Cl at 450°C under a steady flow of NH 3 . Attempting direct nitridation of Sn with NH 3 leads to the formation of Sn droplets due to the reducing action of the NH 3 which eliminates O 2 in the reactor and which in turn inhibits onedimensional growth over a wide temperature range between 300-800°C. The formation of large Sn droplets was suppressed by adding NH 4 Cl which dissociates into NH 3 and HCl by sublimation at 338°C and acts as a dispersant thereby enhancing the vapour pressure of Sn. Furthermore the Sn reacts with HCl and yields SnCl 2 which subsequently reacts with NH 3 leading to the formation of Sn x N y nanowires which have diameters of 200 nm and lengths up to 5 lm. Finally nanowires protruding from droplets and intricate assemblies of NWs arranged in the shape of wreaths were also obtained at temperatures \450°C. The synthesis of metal (M)-nitride i.e. M x N y nanowires where the metal component is readily available and has a low cost, is expected to be important for third generation solar cells based on nanostructured semiconductor materials.