Scanned Probe Oxidation onp-GaAs(100) Surface with an Atomic Force Microscopy
© to the authors 2008
Received: 9 April 2008
Accepted: 23 June 2008
Published: 3 July 2008
Locally anodic oxidation has been performed to fabricate the nanoscale oxide structures onp-GaAs(100) surface, by using an atomic force microscopy (AFM) with the conventional and carbon nanotube (CNT)-attached probes. The results can be utilized to fabricate the oxide nanodots under ambient conditions in noncontact mode. To investigate the conversion of GaAs to oxides, micro-Auger analysis was employed to analyze the chemical compositions. The growth kinetics and the associated mechanism of the oxide nanodots were studied under DC voltages. With the CNT-attached probe the initial growth rate of oxide nanodots is in the order of ~300 nm/s, which is ~15 times larger than that obtained by using the conventional one. The oxide nanodots cease to grow practically as the electric field strength is reduced to the threshold value of ~2 × 107 V cm−1. In addition, results indicate that the height of oxide nanodots is significantly enhanced with an AC voltage for both types of probes. The influence of the AC voltages on controlling the dynamics of the AFM-induced nanooxidation is discussed.
Scanning probe microscopes (SPM) techniques have demonstrated potential in the creation and characterization of structures, patterns, and devices at the nanometer scale. In particular, atomic force microscopy (AFM) local anodic oxidation  is one of the most versatile methods for defining structures at the nanometer scale in the surface of anodizable materials. This technique has been applied to define a fairly large range of nanostructures  and nanodevices, like Josephson junctions, superconducting quantum interference devices (SQUID) , and single electron transistor (SET) devices [4–6].
Carbon nanotubes (CNTs) have been known to be ideal material for an AFM probe because of their cylindrical shape, small diameter, high aspect ratio, large Young’s modulus , and unique chemical properties which reduce their physical and chemical changes during the scanning process . In addition, CNTs have considerable mechanical flexibility and, therefore, can be elastically buckled without damage . The application of CNT probes for nanooxidation  began almost simultaneously with the improvement in resolution for image measurements . It is expected that the improvement of probe will open up the great possibilities for miniaturization, because the size of fabricated oxide nanostructures is predominantly determined by the probe apex. An effective way to decrease the probe apex is by means of CNT .
However, to successfully adopt the oxide nanodots and nanowires as integrated parts of the nanodevices, for instance, to serve as the effective tunnel barriers for carrier transport, further improvements/enhancements on the aspect ratio of oxide structures are needed. Herein, to optimize the condition of the anodic oxidation reaction under the probe, various conditions including the applied voltages, humidity, the geometry of AFM tip, as well as the modulated voltages should be controlled. In this study, we compare the lithography results between two different types of AFM probes, the MWCNT probe and the conventional Si probe, used to fabricate the oxide nanostructures onp-GaAs(100) surface under ambient conditions. By identifying how the apex dimensions of the AFM probe influences the features of the resultant oxide nanostructures has lent us a key to further improve the aspect ratio of oxide nanostructures. The proper control of the oxidation reaction, improvement of reproducibility and increasing the accuracy are among the immediate objectives for further evolution of this technique.
Nanolithography was carried out by using a commercial AFM (NT-MDT Solver-P47, Russia) in the noncontact mode (nc-AFM) at room temperature with the relative humidity of 55% in the present work. Two different types of AFM probes were employed: (1) a modified cantilever (Daiken-Kagaku, 3 N m−1, 157 kHz) with a conductive MWCNT (~10 nm in diameter and ~300 nm in length) attached on Si probe and (2) a conventional Pt-coated Si probe with the curvature radius of ~35 nm, the force constant of 34 N m−1, and the resonance frequency of 350 kHz, respectively. The sample was ap-GaAs(100) wafer with the resistivity of 10 Ω cm and root-mean-square surface roughness being less than 0.25 nm.
By applying a bias voltage between thep-GaAs(100) surface and the AFM probe, the oxide nanostructures were grown on the electrochemically reactive surface. For oxide nanodots anodization, the applied anodized voltage was at 8 V with the pulse duration ranging from 0.01 to 100 s. The anodization has been practiced over various surface positions through the AFM probe. For the voltage modulation studies, an AC voltage was applied to the AFM probe, where the amplitudes of the high- and low-level voltage were 8 V and −8 V, respectively, and both have the corresponding pulse durations of 50 ms. In the present work, the presented data are an average of five measurements.
To investigate the conversion mechanisms of GaAs-oxides, the chemical composition of the sample and some selected anodized regions was analyzed by Auger electron spectroscopy (AES, Auger 670 PHI Xi, Physical Electronics, USA) system equipped with a Schottky field emission electron source with the incident energy of 2 keV.
Results and Discussion
Similar to those discussed in Si [14, 15], within the context of the anodic oxidation concepts, the anionic and cationic transport are important factors in determining the kinetics of oxidation. In this scenario, the driving force is the faradaic current flowing between the tip and sample surface with the aid of the water meniscus. When the faradaic current flows into water bridge, H2O molecules are decomposed into oxyanions (OH−, O−) and protons (H+). These ions penetrate into the oxide layer because of the electric field (in the order of 108 V/cm) , leading to the formation and subsequent growth of Ga(As)O x on the GaAs surface. The directional penetration of the hydroxyl ions could also play a prominent role in enhancing the aspect ratio of protruded oxide structures.
Kinetics of AFM Anodic Oxidation onp-GaAs(100) Surface
The kinetic characteristics of AFM anodic oxidation by the CNT-attached and the conventional probes are further analyzed and discussed below. As is evident from Fig. 2a, with the same time duration, the height of oxide nanodots produced by CNT-attached probe is higher than those obtained by the conventional one. The oxides height for both the CNT-attached and conventional probes, each with an anodized voltage of 8 V, was estimated to be ~6.6 and ~5.1 nm, respectively. Note that, in the estimation, the electrochemical process is assumed to be solid-state diffusion limited. To give a more quantitative account of the growth kinetics, in Fig. 2b, the growth rate of oxide nanodots is plotted as a function of electric field strength. The initial growth rate of oxide nanodots induced by the CNT-attached probe is ~300 nm/s, which is about 15 times larger than that obtained from the conventional one. Also, it can be clearly seen that the growth rate strongly depends on the electric field strength and, in both cases, the anodic oxidation process is greatly enhanced when the electric field strength is beyond the order of ~2 × 107 V cm−1. In our previous study , it has been shown that the growth rate not only is a function of electric field strength but also depends on the applied anodized voltage.
Avouris et al.  proposed that the growth kinetics can be described as where h is the oxide thickness at time t and l c is a characteristic decay length depending on the anodized voltage. Figure 2c shows the relationships between the growth rate and the oxide height by two different types of probes at an applied voltage of 8 V. The characteristic decay length, l c, for CNT-attached and conventional probes are estimated to be 0.91 and 0.86 nm, respectively. Taking into account for the self-limiting mechanisms of AFM tip-induced anodic oxidation, Stiévenard et al.  proposed that a greater height of oxide protrusion corresponds to a weaker electrical field strength, which limits the growth of the oxide nanostructures.
According to the above-mentioned results, the growth rate of the oxide nanodots is governed via the ionic transportation promoted by the electric field strength. The growth of the oxides is therefore fast in the initial stage of the anodic oxidation process while there is a rapid build-up of space charge taking place simultaneously. The applied anodized voltage extends the electric field strength, assisting the oxidation mechanisms until the growth is limited by diffusion. It is evident that the simple Cabrera–Mott model  of field-induced oxidation is inadequate to account for the observed kinetics shown here. The possible reasons giving rise to the discrepancies between the kinetics of AFM nanooxidation and the Cabrera–Mott field model could be due to: (i) the space charge build-up within the oxide nanodots , and (ii) the mechanical stress created and accumulated within the oxide nanodots because of the large volume mismatch between the sample and the oxides .
Effect of Pulsed Voltages
The electrochemical oxidation process is mainly due to the source of hydrogenous species. After a long period of anodization time, the neutralization reaction of OH−and H+becomes more important due to the increasing proton concentration. As a result, less OH−ions reach the GaAs/Ga(As)O x interface and the growth rate of the oxide decreases accordingly. Meanwhile, the lateral diffusion of OH−at the water/oxide interface becomes more pronounced, leading to the increase of the oxide width. The phenomena of the lateral diffusion can be suppressed, however, by applying an AC voltage with a sequence of negative and positive voltages. In the period of rest timeT res, H+ions would be taken away from the GaAs/Ga(As)O x interface and the transport of OH−ions would be interrupted. It has been demonstrated that a negative voltageV rescannot produce any observable oxide nanodots on the sample surface. Until the next voltage pulse is activated atV ox, the directional transport of OH−will be re-started and the vertical growth of the oxide nanodot continues. Hence, a shaper structure can be obtained under AC conditions at the central part of the dot where the electric field strength is supposed to be higher.
Characteristics of AFM anodic oxidation-fabricated oxide nanodots in the present work with comparisons to the results previously reported on the GaAs surface
In conclusion, we have presented the results and the associated mechanisms of fabricating oxide nanodots onp-GaAs(100) surface by AFM tip-induced lithography with the conventional and CNT-attached probes. In this particular electrochemical reaction, the composition analysis by micro-Auger revealed that the selectively oxidized GaAs area was turned into Ga(As)O x . The results also indicate that the localized anodic oxidation process is significantly enhanced as the electric field strength is beyond ~2 × 107 V cm−1. The effective electric field, nevertheless, is weakened by the increasing height of the oxide protrusions, which, in turn, limits further growth of the oxide protrusions. Finally, it was demonstrated that the application of AC-voltage scheme can significantly enhance the aspect ratio of the oxide nanodots. The current results, therefore, suggest that the AFM nanolithography with CNT-attached probe could result in much smaller oxide nanostructures, which is of great benefit to the fabrication of integrated nanometer-sized devices.
This work was partially supported by the National Science Council of Taiwan and I-Shou University, under Grants No. NSC97-2218-E-214-003 and ISU97-07-01-04.
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