DC voltage fields generated by RF plasmas and their influence on film growth morphology through static attraction to metal wetting layers: Beyond ion bombardment effects

Abstract

It is shown that attractive electrostatic interactions between regions of positive charge in RF plasmas and the negative charge of metal wetting layers, present during compound semiconductor film growth, can have a greater influence than substrate temperature on film morphology. Using GaN and InN film growth as examples, the DC field component of a remote RF plasma is demonstrated to electrostatically affect metal wetting layers to the point of actually determining the mode of film growth. Examples of enhanced self-seeded nanopillar growth are provided in the case where the substrate is directly exposed to the DC field generated by the plasma. In another case, we show that electrostatic shielding of the DC field from the substrate can result in the growth of Ga-face GaN layers from gallium metal wetting layers at 490 C with root-mean-square roughness values as low as 0.6 nm. This study has been carried out using a migration enhanced deposition technique with pulsed delivery of the metal precursor allowing the identification of metal wetting layers versus metal droplets as a function of the quantity of metal source delivered per cycle. It is also shown that electrostatic interactions with the plasma can affect metal rich growth limits, causing metal droplet formation for lower metal flux than would otherwise occur. Accordingly, film growth rates can be increased when shielding the substrate from the positive charge region of the plasma. For the example shown here, growth rates were more than doubled using a shielding grid.

Recent Advances in Hollow Cathode Technology for Plasma-Enhanced ALD—Plasma Surface Modifications for Aluminum and Stainless-Steel Cathodes

“…The test chambers each had a one-inchthick, borosilicate view port on the top (see Figure 1) so that the plasma and hollow cathode could be observed while the plasma was running. Langmuir probe measurements were also carried out for the test sections; the purpose of those measurements was not absolute accuracy, they were only indicative measurements used to help determine the range of operation for different gas pressures; therefore, for simplicity, the measurements were performed using an uncompensated single-wire probe as described in previous references [44,55]. Oksuz, Soberon, and Ellingboe [92] revisited the use of uncompensated Langmuir probes with RF fields using this methodology and determined that with a 13.56 MHz RF field, the floating potential and the plasma potential will appear in more negative positions than they should; accurate electron temperatures can be obtained; however, the electron densities will be underestimated [92][93][94].…”

“…A Maxwellian characteristic can be assumed for at least the limited part of the curve that abides by a logarithmic change, though closer to saturation, the curve often showed signs of non-Maxwellian behavior, in part, due to the formation of a quasi-collisional sheath resulting from some of the intermediate pressures used [95]. The electron density (n e ) was determined from the ion saturation current (I ion sat ) as per our prior publication using Langmuir probes [44,55] and as described in the recent review by Merlino [96] according to the equation:…”

“…This problem was overcome with a plasma cleaning regime between runs and by operating in a narrow pressure range close to the optimum design pressure; however, these solutions were non-ideal in terms of the conditions available for plasma operation. The characterization of one of these sources has been reported elsewhere [44,55].…”

“…[28] SiO 2 (PE-ALD) [7,24,25] β-Ga 2 O 3 (PE-ALD) [38] Silicon nitride (PE-ALD) [22,23,27,31,37,40] Plasma treatment (PE-ALD) [26] Electron treatment (CVD based) [36,39] GaN (CVD based) [42,44,45,55] InN (CVD based) [41][42][43]45,47,48,53,57,58] InGaN (CVD based) [49,51,54,56] InN nanopillars (CVD based) [46,50,52,55] The oxygen contamination overview provided in this introduction is followed by an experimental examination of some effects related to the surface modification of cathode materials by the generated plasma. In particular, we examine changes that have been observed for aluminum and stainless-steel cathodes.…”

“…Interest in the use of hollow cathodes for PE-ALD came about because of some low oxygen contamination results for GaN films grown using a relatively low-temperature means of deposition based on migration-enhanced epitaxy [42,44]. This is a pulsed method for deposition of GaN at temperatures below 670 • C, much lower than typical MOCVD growth temperatures of over 950 • C. However, ALD-like smoothness could be obtained with root mean square atomic force microscopy roughness measurements of below 1 nm [42,44,55] and, more importantly, when grown on GaN templates, oxygen contamination levels of ~10 16 cm −3 were measured by SIMS [42], similar to the levels of commercial MOCVDgrown material. Migration-enhanced epitaxy with a hollow cathode plasma source has also shown some excellent results for InN [41,42,47,48] and InGaN [49].…”