Selective Wet Etching of Silicon Germanium in Composite Vertical Nanowires.

Silicon germanium (SixGe1-x or SiGe) is an important semiconductor material for the fabrication of nanowire-based gate-all-around transistors in the next-generation logic and memory devices. During the fabrication process, SiGe can be used either as a sacrificial layer to form suspended horizontal Si nanowires or, because of its higher carrier mobility, as a possible channel material that replaces Si in both horizontal and vertical nanowires. In both cases, there is a pressing need to understand and develop nanoscale etching processes that enable controlled and selective removal of SiGe with respect to Si. Here, we developed and tested solution-based selective etching processes for SiGe in composite (SiNx/Si0.75Ge0.25/Si) vertical nanowires. The etching solutions were formed by mixing acetic acid (CH3COOH), hydrogen peroxide (H2O2), and hydrofluoric acid (HF). Here, CH3COOH and H2O2 react to form highly oxidizing peracetic acid (PAA or CH3 CO3H). The hydrofluoric acid serves both as a catalyst for PAA formation and as an etchant for oxidized SiGe. Our study shows that an increase in any of the two oxidizer (H2O2 and PAA) concentrations increases the etch rate, and the fastest etch rate of SiGe is associated with the highest PAA concentration. Moreover, using in situ liquid-phase TEM imaging, we tested the stability of nanowires during wet etching and identified the SiGe/Si interface to be the weakest plane; we found that once the diameter of the 160-nm-tall Si0.75Ge0.25 nanowire reaches ∼15 nm during the etching, the nanowire breaks at or very close to this interface. Our study provides important insight into the details of the nanoscale wet etching of SiGe and some of the associated failure modes that are becoming extremely relevant for the fabrication processes as the size of the transistors shrink with every new device generation.

Scaled-Down c-Si and c-SiGe Wagon-Wheels for the Visualization of the Anisotropy and Selectivity of Wet-Chemical Etchants.

Wet etching offers an advantage as a soft, damage-less method to remove sacrificial material with close to nanometer precision which has become critical for the fabrication of nanoscale structures. In order to develop such wet etching solutions, screening of etchant properties like selectivity and (an)isotropy has become vital. Since these etchants typically have low etch rates, sensitive test structures are required to evaluate their etching behavior. Therefore, scaled-down single-crystalline Si (c-Si) and SiGe (c-SiGe) wagon-wheels were fabricated. First, the sensitivity of the c-Si wagon-wheels to detect anisotropic behavior of crystalline silicon in the alkaline etchants TMAH and NH4OH was demonstrated. Distinctive wagon-wheel patterns, characteristic for each material/etchant pair, were observed by top-down scanning electron microscopy (SEM) after anisotropic wet etching. Similar trends in crystallographic plane-dependent etch rates were obtained for both Si(100) and Si(110) substrates. Secondly, the etching of both c-Si and c-Si75Ge25 wagon-wheels in a typical selective etchant, peracetic acid (PAA), was evaluated. c-Si75Ge25 etching in PAA resulted in isotropic etching. Selectivity values were calculated based on two methods: the first by measurement of the sidewall loss of the spokes of the wagon-wheel, the second, indirect method, through measurement of the spoke retraction lengths. Both methods give comparable values, but the latter method can only be used after a certain critical etching time, after which the spoke tips have evolved toward a sharp tip.

Visualizing anisotropy in the surface oxidation of germanium by wet etching of patterned nanowedges: proof of concept.

We study the anisotropy in surface oxidation for Ge(100) and (110) in HCl/H2O2 solution complemented by synchrotron X-ray photoemission spectroscopy (SXPS) measurements integrated with an in situ etching chamber. Visual anisotropic demonstration is confirmed by lithographic Ge nanowedges.

Non-destructive characterization of extended crystalline defects in confined semiconductor device structures.

Semiconductor heterostructures are at the heart of most nanoelectronic and photonic devices such as advanced transistors, lasers, light emitting diodes, optical modulators and photo-detectors. However, the performance and reliability of the respective devices are often limited by the presence of crystalline defects which arise from plastic relaxation of misfit strain present in these heterogeneous systems. To date, characterizing the nature and distribution of such defects in 3D nanoscale devices precisely and non-destructively remains a critical metrology challenge. In this paper we demonstrate that electron channeling contrast imaging (ECCI) is capable of analyzing individual dislocations and stacking faults in confined 3D nanostructures, thereby fulfilling the aforementioned requirements. For this purpose we imaged the intensity of electrons backscattered from the sample under test under controlled diffraction conditions using a scanning electron microscope (SEM). In contrast to transmission electron microscopy (TEM) analysis, no electron transparent specimens need to be prepared. This enables a significant reduction of the detection limit (i.e. lowest defect density that can be assessed) as our approach facilitates the analysis of large sampling volumes, thereby providing excellent statistics. We applied the methodology to SiGe nanostructures grown by selective area epitaxy to study in detail how the nature and distribution of crystalline defects are affected by the dimensions of the structure. By comparing our observations with the results obtained using X-ray diffraction, TEM and chemical defect etching, we could verify the validity of the method. Our findings firmly establish that ECCI must be considered the method of choice for analyzing the crystalline quality of 3D semiconductor heterostructures with excellent precision even at low defect densities. As such, the technique aids in better understanding of strain relaxation and defect formation mechanisms at the nanoscale and, moreover, facilitates the development and fabrication of next generation nanoelectronic and photonic devices.

Relaxometric study of copper [15]metallacrown-5 gadolinium complexes derived from alpha-aminohydroxamic acids.

Proton nuclear magnetic relaxation dispersion (NMRD) profiles were recorded between 0.24 mT and 1.4 T for lanthanum(III)- and gadolinium(III)-containing [15]metallacrown-5 complexes derived from alpha-aminohydroxamic acids and with copper(II) as the ring metal. The influence of the different R-groups on the proton relaxivity was investigated, and a linear relationship between the relaxivity and the molecular mass of the metallacrown complex was found. The selectivity of the metallacrown complexes was tested by transmetalation experiments with zinc(II) ions. The crystal structure of the copper [15]metallacrown-5 gadolinium complex with glycine hydroximate ligands is reported.

Pentacopper(II) complexes of alpha-aminohydroxamic acids: uranyl-induced conversion of a 12-metallacrown-4 to a 15-metallacrown-5.

Effects of uranyl on the pentacopper(II) complexes of alpha-leucinehydroxamic acid and alpha-tyrosinehydroxamic acid were studied in water and methanol by means of electrospray ionisation mass spectrometry (ES-MS), absorption spectrophotometry, circular dichroism spectroscopy and proton NMR spectroscopy. All the measurements were consistent with the complete conversion of a 12-metallacrown-4 to a 15-metallacrown-5 upon addition of one equivalent of the uranyl ion. The uranyl ion is accommodated in the cavity formed by five copper(II) ions and five alpha-aminohydroxamate ligands. The 15-metallacrown-5 inclusion complexes have a high affinity for the uranyl ion. Competition studies showed that even in the presence of a large excess of calcium(II), the 15-metallacrown-5 remained stable, and no exchange reactions between calcium(II) and uranyl were observed. Extraction of uranyl from the 15-metallacrown-5 was also not detected in the presence of a large excess of 18-crown-6. Trivalent lanthanide ions can be partially sequestered by the 15-metallacrown-5, however, even these trivalent ions are displaced by uranyl.