Isotropic atomic layer etching of GaN using SF₆ plasma and Al(CH₃)₃

GaN is an enabling material for light emitting diodes, advanced radio frequency, and power semiconductor devices. However, fabrication of GaN devices often relies on harsh etch processes, which can leave an etch damage layer, limiting final device performance. In this work, an isotropic atomic layer etching (ALE) process involving SF6 plasma and trimethylaluminium [Al(CH3)3] is presented for the controlled etching of GaN, which reduces oxygen and carbon contamination while smoothing the surface. The ALE chemistry was first examined with density functional theory. A comparison between proposed thermal and plasma-driven reactions is made by implementing Natarajan-Elliott analysis, highlighting that the plasma process is a good candidate for GaN ALE. Saturation was experimentally confirmed for both ALE half-cycles at 150 and 300 °C, with etch rates of 0.31 ± 0.01 and 0.40 ± 0.02 nm/cycle, respectively. Analysis of the films post-ALE shows that the RMS roughness of the films decreases from 2.6 ± 0.1 to 1.9 ± 0.1 nm after 25 nm of etching at 300 °C, in agreement with a previously developed curvature-dependent smoothing model. Taken together, this ALE process enables accurate GaN thickness tuning, surface cleaning, and surface smoothing, allowing for further development of GaN devices.</p

Isotropic plasma atomic layer etching of Al2O3

Data used to plot all of the figures in the letter titled "Isotropic plasma atomic layer etching of Al2O3 using a fluorine containing plasma and Al(CH3)3)". Each figure has its own page, original origin files are available upon request

Surface Smoothing by Atomic Layer Deposition and Etching for the Fabrication of Nanodevices

In many nano(opto)electronic devices, the roughness at surfaces and interfaces is of increasing importance, with roughness often contributing toward losses and defects, which can lead to device failure. Consequently, approaches that either limit roughness or smoothen surfaces are required to minimize surface roughness during fabrication. The atomic-scale processing techniques atomic layer deposition (ALD) and atomic layer etching (ALE) have experimentally been shown to smoothen surfaces, with the added benefit of offering uniform and conformal processing and precise thickness control. However, the mechanisms which drive smoothing during ALD and ALE have not been investigated in detail. In this work, smoothing of surfaces by ALD and ALE is studied using finite difference simulations that describe deposition/etching as a front propagating uniformly and perpendicular to the surface at every point. This uniform front propagation model was validated by performing ALD of amorphous Al2O3 using the TMA/O2 plasma. ALE from the TMA/SF6 plasma was also studied and resulted in faster smoothing than predicted by purely considering uniform front propagation. Correspondingly, it was found that for such an ALE process, a second mechanism contributes to the smoothing, hypothesized to be related to curvature-dependent surface fluorination. Individually, the atomic-scale processing techniques enable smoothing; however, ALD and ALE will need to be combined to achieve thin and smooth films, as is demonstrated and discussed in this work for multiple applications

Data underlying the publication: Isotropic atomic layer etching of GaN using SF6 plasma and Al(CH3)3

Dataset to accompany the publication of a manuscript on isotropic plasma ALE of GaN using SF6 plasma and TMA. The initial study focused on DFT simulations using Schrödinger Suites software. Quantum ESPRESSO and Jaguar packages were used to calculate the gibbs free energy change for the ALE reactions to screen the proposed chemistry based on Natarajan-Elliott analysis. Experimental studies were then performed to confirm the predictions from the simulations, with good agreement between the two. Saturation curves, synergy and temperature window were used to characterise the process, with surface roughness and chemical composition used to demonstrate the low damage nature of this etching process

Spécificité de substrat des DD-peptidases bactériennes de faible poids moléculaire

The bacterial DD-peptidases or penicillin-binding proteins (PBPs) catalyze the formation and regulation of cross-links in peptidoglycan biosynthesis. They are classified into two groups, the high-molecular mass (HMM) and lowmolecular mass (LMM) enzymes. The latter group, which is subdivided into classes A−C (LMMA, -B, and -C, respectively), is believed to catalyze DD-carboxypeptidase and endopeptidase reactions in vivo. To date, the specificity of their reactions with particular elements of peptidoglycan structure has not, in general, been defined. This paper describes the steady-state kinetics of hydrolysis of a series of specific peptidoglycan-mimetic peptides, representing various elements of stem peptide structure, catalyzed by a range of LMM PBPs (the LMMA enzymes, Escherichia coli PBP5, Neisseria gonorrhoeae PBP4, and Streptococcus pneumoniae PBP3, and the LMMC enzymes, the Actinomadura R39 DD-peptidase, Bacillus subtilis PBP4a, and N. gonorrhoeae PBP3). The R39 enzyme (LMMC), like the previously studied Streptomyces R61 DD-peptidase (LMMB), specifically and rapidly hydrolyzes stem peptide fragments with a free N-terminus. In accord with this result, the crystal structures of the R61 and R39 enzymes display a binding site specific to the stem peptide N-terminus. These are water-soluble enzymes, however, with no known specific function in vivo. On the other hand, soluble versions of the remaining enzymes of those noted above, all of which are likely to be membrane-bound and/or associated in vivo and have been assigned particular roles in cell wall biosynthesis and maintenance, show little or no specificity for peptides containing elements of peptidoglycan structure. Peptidoglycan-mimetic boronate transition-state analogues do inhibit these enzymes but display notable specificity only for the LMMC enzymes, where, unlike peptide substrates, they may be able to effectively induce a specific active site structure. The manner in which LMMA (and HMM) DD-peptidases achieve substrate specificity, both in vitro and in vivo, remains unknown