Measurement of field emission

The turn-on field for 10 µA/cm² has been used as the merit parameters to

measured by using diode-type structure. An anode plate (ITO Glass) was placed at 70 μm above the AlN nanomaterials emitters, which served as the cathode electrode.

The field emission (FE) measurement was carried out in a vacuum chamber at a pressure of ~10^-7 torr at room temperature. The applied voltage was biased from 0 to 1100 V and current which was collected from the emitters was recorded by using a Keithley 237 system.

Chapter 4

Growth of AlN nanotips by means of APCVD

In this chapter, preparations of AlN nanotips (AlNNTs) on the Si (100) substrates by APCVD method will be presented. The effects of the thickness of catalyst, type of catalysts, and preparation time on the formation of single-crystalline AlN in the CVD experiments are studied. The surface morphology, structure, and composition properties of the as-deposited samples are characterized in detail. The overall structural and morphological results are discussed in section 4.2 via FESEM Raman scattering and XRD measurements, respectively. The growth mechanism of AlNNTs formation was described in section 4.3. Finally, section 4.4 will make a summary according to the obtained results.

4.1. AlN nanotips growth

AlN nanowires, whiskers and nanotubes have been obtained by chloride assisted growth [115,116], arc process [117], carbothermal reduction [118,119], and gas-reduction-nitridation [120]. However, very recently, there has been an increased activity in this field with quite a few reports on nanowires, nanotubes and nanobelts

nanowires and nanotubes with [001] growth direction have been reported at temperatures above 1100°C [121,122]. Hexagonal AlN nanobelts with rectangular cross-section and having [001] growth direction also made their appearance [123].

Vapor-solid (VS) growth is a vapor species directly transformed into solid nanowire process [124]. The VS process does not need catalyst to assist the nanowire growth.

Therefore, it does not undergo a liquid-alloy stage. A cubic AlN nanotube with a boron nitride (BN) wrapping (AlN-BN composite nanotube) has been made possible at a temperature of 1200°C using a two stage growth mechanism [125]. However, AlN nanotips with high aspect ratio, unlike the pyramids [126], also form a special class in the 1D family, have not been reported before.

Nanotips in Si-based and other more conventional semiconductor systems have been demonstrated and explored for various potential applications as field-emitters [146-148], solar cells [149], bio/chemical sensing devices [150-152], and optical nanodevices [36,153]. Until now, optical lithography has been playing the major role in fabricating sharp tips but limited to 50 nm in radius [154-156]. A novel electron-cyclotron-resonance plasma assisted dry-etching technique has recently been developed for producing ultra-fine tips (~ 1 nm) in a wide range of material systems, excluding AlN [157]. These tips (Si, poly-Si, GaN, GaP, Al) can have apex diameters from 2-20 nm, and growth directions depending upon the orientation of the starting

wafers. The approaches captioned above can be classified as top-down techniques and are most suitable for plate or wafer process. In this study, we report on the first synthesis of single-crystalline AlNNTs with a monodispersed angle distribution via simple vapor transport and condensation process (VTCP), in both catalytic and catalyst-free modes, at 950 °C, which is significantly lower than that of the conventional carbothermal reduction and nitridation process for AlN. Metal (Au, Pt, Al) coated silicon (Si) substrates have been used to produce the AlNNTs, where the metal nanoparticulates formed at the high temperature acted as the nucleation sites for the AlNNTs growth.

4.2 Results and discussion

The as-grown AlNNTs on silicon substrates coated with an Au layer of various thicknesses were shown in Fig. 4.1a-c. AlNNTs grown on a 7 nm Au covered Si substrate exhibited a mean diameter of 10 nm at the apex, 80 nm at the base, and 250 nm in length (Fig. 4.1a). AlNNTs grown with progressively thicker Au layers produced longer nanotips (300-3000 nm) with wider apex (20-100 nm) and base (100-700 nm) diameters (Fig. 4.1b-c). Typical cross-sectional SEM image of AlNNTs is shown in Fig. 4.1d. This image displays a high density of quasi-aligned AlNNTs

the growth rate of the AlNNTs along the axial direction far outpace the growth rate along the radial direction. Along with the AlNNTs containing pure Al nanocrystals at its base some unreacted metallic Al residue could be found. The beauty of this VTCP technique lies in the size control of the nanotips which can be simply achieved by adjusting the thickness of the gold layer keeping the other growth parameters such as temperature and gas flow rates fixed. However, a number of other metals, such as aluminum (Al) (Fig. 4.2a) or platinum (Pt) (Fig. 4.2b), can be used to produce the AlNNTs since the eutectic temperatures of these metals with silicon lie below the reaction temperature of 950 °C. Furthermore, AlNNTs can be grown on even bare Si (without metal coating) (Fig. 4.2c) but they exhibit relatively poor morphology, indicating that metal coating is not indispensable for the growth, but it does help in controlling the morphology of the AlNNTs and perhaps some increase in the yield.

But to come to any such conclusion, a closer look at the initial stages of growth was deemed necessary. SEM images in Fig. 4.3a-c demonstrate the initial growth of AlNNTs. The Al nanocrystals form within the first 20 minutes of growth and can be easily seen at the base of the AlNNTs (Fig. 4.3a). Figure 4.3b and 3c represent the morphology during the first 20 and 25 minute of growth, respectively, with typical nanotip formation completed in 25-30 minutes. The corresponding energy dispersive X-ray spectroscopy (EDS) elemental analysis of the nanotip body and nanoparticles

lying at the base of the nanotip is shown in Fig. 4.3d. It is quite clear that the nanotip body showed a much pronounced nitrogen component whereas no Au signal could be obtained from the apex of such tips marked (A) in Fig. 4.3c which excludes the possibility of VLS growth mechanism. The crystallites lying at the base of the nanotips, marked (B) in Fig. 4.3c, from which the nanotips evolved, yielded signals predominantly from Al, Au, and Si with no or little nitrogen component (Fig. 4.3d).

The carbon and copper signals in Fig. 4.3d come from the amorphous carbon coated copper grids used for the TEM measurements. Two interesting inferences can be drawn from Fig. 4.3d. First, the nanocrystal at the base is predominantly Al, and second, the Al at the tip is compounded with nitrogen. Both of these facts will be corroborated from the XRD results to be discussed later. However, the establishment of the AlN phase in the nanotips comes conclusively from the TEM and XRD results.

Presence of Au along with Al in the nucleating crystallites only, indicates that either pure gold or a gold silicide phase may be controlling the nucleation and the smooth and finer morphology of the AlNNTs. In fact, the thickness of the Au layer is the key to the resultant morphology of the AlNNTs (Fig. 4.1). Au redundancy during growth resulted in larger and corrugated edge morphology of the AlNNTs (Fig. 4.2c). The fact that AlNNTs can be grown on bare silicon (Fig. 4.2c) as well as on silicon

nanocrystals, cannot be ruled out. Al vapors condense as crystalline Al on the catalytic gold or the gold-silicide sites, and we believe that these crystallized Al nanoparticles present at the bases of the AlNNTs served as the nucleation sites for subsequent AlNNTs growth following deposition of reacted Al and N vapors.

FIG. 4.1 Typical SEM images of the AlN nanotips grown with (a) 7 nm, (b) 15 nm, and (c) 50 nm thick Au layer on Si. (d) Typical cross section SEM image of AlN nanotips grown with 15 nm Au coated Si substrate.

FIG.. 4.2 SEM images of AlN nanotips grown with (a) Al (15 nm), (b) Pt (15 nm), (c) no metal coating, on the Si substrate, respectively.

FIG. 4.3 SEM images of AlN nanotips grown with Au (15 nm on Si) for (a) 15 min, (b) 20 min, (c) 25 min, and (d) the corresponding TEM-EDS spectra of the AlN nanotip and nanoparticle marked (A) and (B) in Fig. 4.3c.

In order to investigate the structure evolution of the AlNNTs, XRD analyses were performed at various stages of growth. As shown in Fig. 4.4a, the XRD data for the initial stage shows the presence of Al only whereas the corresponding data for the later stage shows a number of relatively sharp diffraction peaks that can be indexed to a hexagonal structure with lattice constant of a = 0.311 nm and c = 0.498 nm, which is consistent with the standard value for bulk hexagonal AlN (JCPDS 25-1133). While there are negligible signals from Au, the Al signals remain detectable in the XRD spectra, presumably due to the un-reacted aluminum crystallizing on the Au particles at the base of the AlNNTs during the initial high temperature processing of the reactants. It should be noted that all samples showed similar XRD patterns, indicating that the nanotips have good structural reproducibility in all deposition conditions used.

This kind of reproducibility is a pre-requisite for any synthetic technique to be accepted as a major force.

Raman spectra of the AlNNTs were obtained at room temperature as shown in Fig. 4.4b, to further the knowledge of its structure. In this spectrum, distinct first-order modes of the peaks corresponding to A₁ (TO), E₂ (high), E₁ (TO) and A₁(LO) modes at around 609.4, 653, 668 and 894 cm^-1, respectively, were observed.

These Raman peaks are signatures for wurtize AlN as reported previously for bulk, thin film, nanowires, and nanobelt structures [126,127,158-159]. However, these peak

positions were given alternative assignments in an earlier work [160]. These Raman peaks were not detected from the samples containing predominantly nanocrystals (such as those grown for 15 min or less as shown in Fig. 4.3a), suggesting that AlN signal is indeed originating from the well crystallized tips.

FIG. 4.4 (a) Typical XRD spectra taken at three different stages of growth. The initial stage shows only Al signals and the corresponding one with fully grown nanotips shows two crystalline phases of Al and hexagonal AlN. (b) Raman spectrum, with a discontinuous abscissa, of the AlN nanotips on silicon substrate. Inset shows a continuous Raman spectrum of the AlN nanotips with the silicon signal included. All the XRD and Raman spectra were measured from samples prepared on 15 nm Au

High resolution TEM along with the selected-area electron diffraction (SAED) was employed to further analyze the structure and crystallographic orientations of these AlNNTs. All nanotips appear to be homogeneous without any grain boundaries, indicating the single crystal nature of each individual AlNNT. As shown in Fig. 4.5b, the HRTEM image of the apex reveals a lattice spacing of 0.497 and 0.269 nm (in parallel and normal to the axial direction) that is in good agreement with the d₀₀₁ and d_1-10 spacing of h-AlN, respectively. Corroborating with the SAED pattern, the direction of the AlNNTs was found to be [001] along the long axis (inset, Fig. 4.5b).

AlN whiskers growing along {10 0} and { 2 0} close packed planes have already been reported [161], and [001] growth direction of some 1 dimensional nanostructures was also observed [159,162]. HR-TEM examination and SAED performed over several nanotips made from different thickness of gold films and also at different locations on each nanotips yielded similar diffraction patterns. Elemental analysis on a single AlNNT body (Fig. 4.5c) done by electron energy loss spectroscopy (EELS) measurements clearly established a stoichiometric AlN composition with Al (Fig. 4.5d) and N (Fig. 4.5e) mapped with similar rate of occurrence.

−

FIG. 4.5 (a) TEM image of an AlN nanotip, (b) High resolution TEM image of a single AlN nanotip with clear lattice images. The inset shows a SAED pattern of the nanotip indicating the single-crystalline nature with [110] zone axis and the growth direction along [001]. (c) TEM of a single AlN nanotip with corresponding (d) Al, and (e) N mapping using EELS.

4.3 Growth mechanism of AlNNTs

Based on the above results, a complete growth mechanism of AlNNTs can be proposed whose schematic description is given in Fig. 4.6. As the reaction temperature was ramped up from room temperature to 950℃ droplets of catalyst , metals (Au, Pt) or their respective silicide phases will be formed in the vicinity of the respective eutectic temperatures. In the case of uncoated Si, the self catalytic Al or Al alloy droplets, possibly Al-Si, is believed to be the nucleation site. Al vapors, generated above 660℃, dissolve in these droplets supersaturating them, finally resulting in the expulsion of elemental Al as nanocrystals. Fully reacted vapors of Al and N deposits as stoichiometric AlN on the Al base, where a rapid growth along the axial direction [001] and an inhibited growth along an orthogonal (radial) direction was observed giving rise to the tip shape. Physically speaking, progressively decreasing surface diffusion lengths (increasing sticking coefficients) of oncoming AlN radicals onto the growth surface along the axial direction will result in a reducing cross section of the growing nanostructure promoting the tip shape. This argument gets support from our observation that an increased growth temperature, hence increased diffusion lengths, resulted in AlN nanorods instead of nanotips, which will be reported separately. The diffusion length of the radicals is a strong function of growth temperature and is believed to be key parameter in controlling the morphology

of the 1D nanomaterial. The growth inhibition along the radial direction is possible via passivation of the dangling bonds on that surface by, possibly, oxygen.

FIG. 4.6 Schematic diagram of growth mechanism of AlN nanotips. (a) Au layer was coated on the Si substrate. (b) Gold or Gold-silicide nanoparticles shape up as the nucleation sites for the subsequent aluminum deposition. (c) Aluminum and nitrogen are absorbed on the nucleation sites bringing about the initial growth of AlN nanotips.

(d) AlN nanotips elongate with time when reaction temperature is kept at 950 °C.

As it was mentioned in a review by Felice et al.[161], the aluminum side of AlN can bond to the silicon side of SiC (001) whereas the nitrogen face prefers the carbon side. In our case, the basal (001) plane is the Al-terminated surface having the higher growth rate. From the evidence of etching of AlN single crystal [163], it was found that the angle between the basal plane and tilted plane of the AlN pyramid was about

61.6°, which is close to the angle between (001) and (1 1) planes in AlN. The (1 1) planes are energetically stable because of a smaller number of bonds cutting through these planes. The energetically stable surface was exposed after the completion of etching. In the case of epitaxial AlN thin film on SiC substrate, there exist a stable

plane (namely (1 2)) with 43° angle with respect to the basal plane [164]. However, in our case, the angle between basal plane and tilted plane of the nanotip is much higher at roughly 81° corresponding to the angle between basal plane (001) and (221).

It is thus suggested that these AlN nanotips are bound by (221) planes, which are

“nongrowth” surfaces, having fewer dangling bonds and hence non-reactive, and the (001) basal plane provides the growth surface. Fig. 4.7a shows the atomic arrangements in an AlN crystal with the “nongrowth” planes (221) observed in the

present work as well as (1 1) [163] and (1 2)

1 [164] (Fig. 4.7b and 4.7c) reported previously. The similar dangling bond environments in each of these cases will rationalize the existence of the stable higher index plane observed in this case of AlN

nanotips. A similar basis for facet formation in GaAs ridge morphology has recently been reported [165]. The (221) planes can be H or N terminated depending on the bond enthalpies of Al-H and Al-N, where the former is smaller. The rapid etching rate on the N- terminated surface may lead to dangling bond passivation by H, which is decomposed from the ammonia gas at 950 °C. However at such high reaction temperatures, the stability of Al-H may be a question. In general, a high growth rate along the (001) basal plane with the stable (221) tilted surface generates the shape of the AlNNTs. Clearly the nanotips, with higher aspect ratios and high index tilted planes, are different morphological species than the pyramids or the ridges observed earlier.

FIG. 4.7 Atomic arrangement in AlN crystal showing the stable “nongrowth” surfaces

with the basal plane. The c-axis shown in the figure is [001] direction.

4.4. Summary

Aluminum nitride nanotips growth on metal (gold, aluminum and platinum) coated or even uncoated silicon substrates via vapor transport and condensation process has been demonstrated. A pure metal or metal silicide phase acts as the nucleation site for the precipitation of crystalline aluminum seed for the aluminum nitride nanotip growth. In case of aluminum nitride nanotip growth on bare silicon, a self catalytic activity of aluminum itself or its silicide phase was visualized. The resultant AlN nanotips exhibit a monodispersed apex angle distribution. Structural properties of the nanotips studied by TEM and XRD suggest that these tips have hexagonal crystal symmetry with a preferred growth direction of (002) along the long axis and a stable (221) plane as the tilted surface.

Chapter 5

Structural Evolution of AlN Nano-structure: Nanotips and Nanorods

In this chapter, Nanostructures produced on the silicon substrates at different temperatures set-points, controlled between 950 and 1200 °C, were studied. An observation in the crystal morphology change from tip-like structure to rod-like crystallites is reported. The surface morphology, structure, and composition properties of the as-deposited samples are characterized in detail. The overall structural and morphological results are discussed in section 5.2 via FESEM, Raman scattering, and XRD measurements, respectively. The platelet growth model, which is governed by the n1/n2 ratio are discussed in section 5.3. Finally, section 5.4 will make a summary according to the obtained results.

5.1. Nanotips and Nanorods

A series of experiments were carried out to study the effect of growth temperature (Td) variation on the shape evolution of the AlN products. Figure 5.1a-d shows SEM images of AlN nano-products grown for 30 min on the Au coated Si substrates at different Td of 950, 1000, 1100, and 1200°C, respectively. At Td = 950°C, the

top of the nanotips, forming a flat section at the top instead of the tip shape. The area of this (001) facet increased with increasing Td as shown in Fig. 5.1b - d. As shown clearly in Fig. 1d, Td = 1200°C produced AlN nanorod (AlNNR) which has a simple six-sided faceted structure. The cross-section of AlNNTs and AlNNRs were both hexagonal.

FIG. 5.1 Typical SEM images of the AlN nanotips on silicon substrates (coated with 15 nm of gold) grown under (a) 950, (b) 1000, (c) 1100, and (d) 1200 °C, respectively.

The cross-sectional SEM images of AlNNTs and AlNNRs shown in Fig. 5.2a and 2b, respectively, indicates their relative dimensions. The inset of Fig. 5.2a and 2b depict the early stages of growth of AlNNTs and AlNNRs, respectively. It indicates that the shapes of the AlN nanostructures were probably decided at the very early stages of the growth. Fig. 5.2c represents the X-ray diffraction analysis of the AlN nanostructures. It shows the strongest reflection of the (002) plane compared to (101) of the h-AlN, indicating a preferential growth perpendicular to the basal plane along [001]. The Raman spectra of the AlNNTs and AlNNRs, as shown in Fig. 5.2d, were collected using Ar⁺ laser as the excitation source. Three clear Raman-active phonon modes can be observed. These are in agreement with those of the h-AlN crystal [126].

FIG. 5.2 Typical cross section SEM image of AlN nanotips grown coated with 15 nm Au coated Si substrate under (a) 950, (b) 1200 °C, respectively. Inset in (a) and (b) show the SEM image of AlN nano-product grown for 25 minutes, respectively. (c) Typical XRD and (d) Raman spectra taken from the AlN nanostructures in Figure 2(a) and 2(b), respectively. The two vertical dashed lines in Figure 2(c) represent bulk AlN positions for (100) and (002) reflections (JCPDS 25-1133).

5.2 Structural revolution of AlN nanostructures

The shape evolution of the AlN nanostructures as a function of Td is interesting as the result (Fig. 5.1) shows. The digestion of the results presented in this paper as well as our earlier work on AlNNTs [166,167] yields certain facts as listed below:

Observation 1: AlN nanostructures have pure Al signatures from the seed crystal

present at its base [166];

Observation 2: The body of the nanostructure is purely hexagonal AlN growing along

[001] and no metallic Al phase is present [166,167];

Observation 3: A high aspect ratio (>10) nanotip structure, having apex angles of

~14°, produced at Td = 950 °C modifies into a low aspect ratio (~5) flat top ((001) facet) AlNNRs at Td = 1200 °C. Any proposed growth model should explain these listed facts of which the last one is of most importance.

We begin with a discussion on a diffusion mediated growth mechanism which is the most obvious as we are dealing with a temperature activated process. The growth of AlN nanostructures proceeds by the transport of Al vapors to the growth region as the

We begin with a discussion on a diffusion mediated growth mechanism which is the most obvious as we are dealing with a temperature activated process. The growth of AlN nanostructures proceeds by the transport of Al vapors to the growth region as the