Aluminum nitride nano-dots prepared by plasma enhanced chemical vapor deposition on Si(111)
Introduction
Having a wide direct band gap, nitride semiconductors are attractive materials for short wavelength optoelectronic applications. According to the type and microstructure of the semiconductor used, it is possible to convert efficiently electrical energy into luminescent energy with a selected wavelength and conversely. Thus, materials such as gallium nitride or aluminum nitride, which have a band gap of respectively 3.5 and 6.2 eV [1], are already introduced in optoelectronic displays as light emitting diodes or laser diodes in order to open the way from the blue to the ultraviolet light [2], [3]. Ultraviolet solid-state sources can be solutions to new high-density data storage due to their short wavelength or used in deep ultraviolet sources or photo-detectors [4]. More precisely, aluminum nitride has a number of desirable characteristics for photonic applications such as its high direct band gap at room temperature, which corresponds to a very short wavelength of 200 nm and allows it to reach deep UV, high thermal conductivity, hardness and chemical resistance. Until now, UV light sources based on aluminum nitride have been manufactured but their power and efficiency are still low, this lack of quality emission is known as due to the high density of defects such as dislocations in the AlN layers that imply non-radiative recombinations [5]. Quantum confinement for semiconductor-based nanostructures (wells, dots, wires etc.) used in many optical and microelectronic devices such as lasers and light-emitting diodes allows a better control of the emission wavelength. Moreover the reduction of the size of active material implies a reduction of the interfacial defects (dislocation, chemical segregation etc.) and an increase of the emission efficiency and power [5]. The diffusion of carriers to non-radiative recombination centers introduced by defects is suppressed due to the localisation of the carriers in the nano-dots [6], [7]. In displays such as light emitting diodes, these confinement structures allow one to reduce the threshold current and so improve their efficiency [8].
Molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MO-CVD) are the most common used techniques for the elaboration of nanostructures deposited on plane substrates. Nevertheless, MBE requires complex monitoring equipment and is more suitable for laboratory experiments and deposition of AlN thin films by MO-CVD must be done at temperature as high as 1000 °C [9]. The plasma enhanced chemical vapor deposition (PECVD) is potentially interesting to reduce the deposition temperature by using an N2 microwave plasma and trimethylaluminum ((CH3)3-Al) as aluminum precursor. As far as we know, aluminum nitride nano-dots have never been deposited by PECVD. Until now, AlN/Si islands are new comparing with studies on other systems as GaN/AlN [10], InAs/GaAs [11] or Ge/Si [12]. Nonetheless, we have recently shown that this technique allows one to grow textured AlN thick films [13]. So the PECVD can be promising to perform nano-dots.
The aim of this work is then to highlight the experimental conditions allowing the nucleation and growth of aluminum nitride nano-dots. Starting from a continuous layer, the thickness has been reduced to reach the first steps of the growth and get nucleation of dots. Moreover, the surface energy has been modified using the surface temperature and the bias potential applied to the substrate so as to influence the mobility of the adatoms.
Section snippets
Experimental
A series of thin films was realized on Si(111) substrates of 20 mm × 10 mm previously cleaned with ethanol and acetone and placed in the reactor. The PECVD reactor used (Fig. 1) is equipped with an opening command placed at 15 mm just above the substrate which allows the control of the reaction start mastering the precursors' access to the substrate surface. This opening command is useful for the elaboration of very thin films because it makes possible to reach the equilibrium of species in the
From 2D layers to 3D nano-dots
In order to switch from 2D layers to 3D nano-dots, the first step was to reduce the thin film thickness reducing the deposition rate. The two parameters allowing that in our reactor are the precursor (TMA) flow rate entering in the reaction chamber and the injector–substrate distance. Thus, keeping a deposition time of 1 min, we first studied the impact of TMA flow rate on the deposition rate. Then, the injector–substrate distance has been increased starting from 4 cm to dilute more the species
Conclusion
Growth of 3D aluminum nitride nano-dots has been performed for the first time by the microwave plasma enhanced chemical vapor deposition technique. We have developed a system to control the first stages of experiment in the PECVD reactor with an opening command. Starting from the conditions of formation of (001) textured polycrystalline AlN layer for one minute reaction, we reduced the deposition rate and then the thickness of the layer until the first steps of growth. This stopped the growth
Acknowledgement
The authors would like to thoroughly thank X. Landreau for his useful help on the FTIR analysis.
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