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Role of surface composition in morphological evolution of GaAs nano-dots with low-energy ion irradiation
© Kumar et al.; licensee Springer. 2012
Received: 12 June 2012
Accepted: 1 August 2012
Published: 4 October 2012
The surface chemistry of GaAs (100) with 50-keV Ar+ ion beam irradiation at off-normal incidence has been investigated in order to elucidate the surface nano-structuring mechanism(s). Core level and valence band studies of the surface composition were carried out as a function of fluences, which varied from 1 × 1017 to 7 × 1017 ions/cm2. Core-level spectra of samples analyzed by X-ray photoelectron spectroscopy confirmed the Ga enrichment of the surface resulting in bigger sized nano-dots. Formation of such nano-dots is attributed to be due to the interplay between preferential sputtering and surface diffusion processes. Valence band measurement shows that the shift in the Fermi edge is higher for Ga- rich, bigger sized nano-dots due to the partial oxide formation of Ga. ‘One-dimensional power spectral density’ extracted from atomic force micrographs also confirms the significant role of surface diffusion in observed nano-structuring.
Well-organized ordered semiconductor nanostructures build the basis for many technological applications as well as for the development of future optoelectronic, electronic, and magnetic devices [1, 2]. For the fabrication of ordered semiconductor nanostructures, a number of techniques, i.e., photolithography , sublithography , scanning probe tip , ion beam sputtering , and molecular beam epitaxial process (using partial capping of nano-dots) [7, 8] have been reported. Among them, low-energy ion irradiation has proven to be a cost-effective, one-step approach for the generation of nanostructures with different topographies at the semiconductor surfaces. No requirement of any kind of masks/templates for nanostructure creation makes this technique even more advantageous over other techniques. By controlling the irradiation parameters, well-ordered nanostructures like one-dimensional ripples, regular arrays of dots and pits, etc. can be evolved in semiconductor materials [6, 9–11].
In general, the formation of nano-dots or ripples depends on whether the ion beam is incident on the surface at normal condition or at off-normal irradiation. A lot of experimental [9–11] as well as theoretical [12, 13] studies have been performed to understand the basic mechanism(s) of the formation of ripples and/or dots on surfaces subjected to energetic ion irradiation. The most common effect of ion irradiation is the direct transfer of energy and momentum to surface atoms by ion-atom collision, leading to adatom diffusion at the surface. Such studies of low-energy ion irradiation are mainly carried out on Si and Ge materials; however, such effects on compound semiconductors (i.e., GaAs, InP, GaSb, etc.) have been sparsely reported. The fabrication of nano-dots on compound semiconductor surfaces induced by ion irradiation is of particular interest due to the higher possibility of production of well-organized nano-dots under the effect of preferential sputtering . As far as the applications are concerned, the fabrication of semiconductor nano-dots on GaAs surface is of immense importance in the field of optoelectronics, photonics, recording media, and optical applications. We have earlier reported the formation of nano-dots on GaAs surfaces . The preferential sputtering of As atoms as compared to Ga atoms was found to play a crucial role in the formation of nano-dots on GaAs (100) surface. However, the role of size evolution of nano-dots in the context of preferential sputtering and diffusion-induced agglomeration has not been studied. This work is an extension of our previous work to understand the role of surface chemistry on the size evolution of nano-dots. Such kind of study is important to understand the role of different ion irradiation-induced surface modification mechanisms in the case of compound semiconductors.
In this work, the core-level and valence band spectra of Ar+-induced self-assembled nano-dots on GaAs (100) surface are presented. The power spectral density has been extracted from atomic force microscopy (AFM) analysis to understand the mechanism involved in surface nano-structuring. Possible mechanisms involved in surface nano-structuring of GaAs (100) are presented to correlate the size evolution and compositional variation of nano-dots.
The synthesis procedure of nano-dot formation on GaAs (100) surface has already been reported in our previous article . In brief, 50-keV Ar+ ion beam irradiation of GaAs (100) samples were carried out at an angle of 50° with respect to the surface normal inside the vacuum chamber with a pressure of 6.7 × 10−7 mbar. During the experiment, the ion beam current density was stabilized at 15 μA/cm2. The samples (pristine and irradiated with fluences of 1 × 1017, 3 × 1017, and 7 × 1017 ions/cm2) were studied by X-ray photoelectron spectroscopy (XPS) to study the surface chemistry of GaAs (100) as a function of irradiation fluence. The spectra were taken on a PerkinElmer (PHI-1257) XPS system (PerkinElmer Corporation, Eden Prairie, MN, USA) using a Mg anode (source energy = 1,253.6 eV). The deconvolution was performed using the program XPSPEAK4.1 in which we used the Tougaard baseline subtraction. One-dimensional power spectral density (1D-PSD) has been extracted from AFM images at different fluences of ion beam.
Size, density of dots, and surface RMS roughness values are presented at different fluences
Size of dots (nm)
RMS roughness (nm)
1 × 1017
18.4 ± 2.2
8 × 1010
3 × 1017
32.6 ± 2.2
2.8 × 1010
7 × 1017
24.6 ± 2.2
5.8 × 1010
From the calculated XPS results, we found that the surface composition of Ga and As is in the ratio of around 1:1 (Ga = 49.8% and As = 50.2%) for the unirradiated GaAs. As the sample was irradiated at the fluence of 1 × 1017 ions/cm2, the features of Ga become more intense as compared to those of As (Ga = 69.1% and As = 30.9%), due to preferential sputtering of As atoms. Thus, the irradiation with Ar+ of GaAs at the fluence of 1 × 1017 ions/cm2 causes surface enrichment with Ga, resulting in the agglomerated Ga-enriched nanoscale dots/islands with a size of 18 nm (Figure 1) because of thermal as well as ion-induced diffusion. When increasing the fluence up to 3 × 1017 ions/cm2, the surface composition is in the ratio of 87.3% (Ga) and 12.7% (As) which results in the formation of bigger islands/nano-dots with an average size of 30 nm. Here, there can be some contribution of ion irradiation at off-normal condition which can lead to the significant enhancement in the composition ratio of Ga/As. A similar kind of enhancement in compositional ratio by off-normal irradiation was also observed by Pan et al. . Interestingly, for further irradiation at the fluence of 7 × 1017 ions/cm2, a reverse effect of preferential sputtering of Ga is observed in which the surface concentration of Ga and As is 60.8% and 39.2%, respectively. This fall in concentration of Ga from 87.3% (for fluence of 3 × 1017 ions/cm2) to 60.8% (for fluence of 7 × 1017 ions/cm2) at the surface causes the decrease in size of nano-dots (from 30 to 24 nm as observed by AFM analysis) which is in good agreement with AFM results. Gnaser et al.  proposed that the elemental composition ratio and steady state of surface composition is strongly a function of ion energy, flux, and fluence. They found that the elemental composition ratio of Ga to As (CGa/CAs) is more for 1-keV energy of Ar as compared to 500 eV for the ion beam flux of the order of 1012 ions/cm2 under the effect of high mobility of atoms. However, in our experiment, the used ion beam energy is 50 keV which is quite high as compared to 1 keV, and the flux is two orders higher (1014 ions/cm2) which might result in high CGa/CAs for the used fluences. Mohanty et al.  also have reported the high surface composition ratio of In to P (In/P = 3.63) for 100-keV Ar+ ion beam irradiation of InP. Pan et al.  also have observed the In to P surface composition ratio as In/P = 2.2 for ion beam irradiation of 1 to 5 keV Ar+.
It was also seen that the surface morphological evolution with sputtering of compound semiconductors was solely affected by the concentration gradient at the surfaces . According to Sigmund's theory of sputtering , the lower binding energy of As than Ga and mass difference between Ga (mass = 69.7 amu) and As (mass = 74.9 amu) cause the slow ejection rate of Ga as compared to As by ion irradiation of GaAs. Thus, the preferential sputtering of As causes the Ga enrichment at the surface of GaAs after irradiation. Indeed, the development of In-rich cone-like structures also has been reported earlier  by Ar+ ion irradiation. Som et al.  and Sulania et al.  also have reported the surface composition study of the nanostructured InP by ion beam irradiation, but they have not correlated the size variation of nano-dots with the surface composition. Here, we have successively correlated that the surface composition plays a crucial role in the controlled production of size as well as density of dots over the surface by ion beam irradiation.
In this work, the variation in surface chemistry of GaAs (100) with 50-keV Ar+ ion beam irradiation at off-normal incidence has been presented in order to elucidate the surface nano-structuring mechanism(s). XPS study has proven that the change in the irradiation fluences leads to the formation of nano-dots via preferential sputtering of As as compared to Ga and surface diffusion of Ga adatoms due to thermal and/or ion-induced diffusion. The observed size of surface nano-dots after irradiation in the fluence regime of 1 × 1017 to 7 × 1017 ions/cm2 is directly correlated to the Ga enrichment of the surface. Valence band study also confirms that the composition of the surface remains a critical parameter to vary the density of states due to partial oxidization of Ga. Thus, the controlled evolution of nano-dots can be achieved in compound semiconductors by tailoring the surface composition by low-energy ion beam irradiation.
One of the authors (T. Kumar) is thankful to the Council of Scientific and Industrial Research (CSIR), India, for the financial support through the Senior Research Fellowship. The help received from Parvin Kumar, Sonu Hooda, UB Singh, UK Rao, and Jai Parkash is gratefully acknowledged.
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