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Atom Probe Tomography Analysis of Low-Dimensional Electronic Materials and Heterostructures

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Atom probe tomography (APT) was used to analyze doping and alloying in low-dimensional electronic materials including thin film heterostructures, van der Waals materials, and colloidal quantum dots (QDs). Firstly, APT was used to reveal structure-property relationship for low-dimensional thin film semiconductors used in electronic and opto-electronic devices. APT was shown to be capable of revealing buried interfaces, evaluating alloying distribution, and determining spatial correlations between dopants. APT, correlated with high-resolution X-ray diffraction (XRD) and micro-photoluminescence (micro-PL), was used to analyze indium distribution in continuous and discontinuous InGaN quantum wells (QWs) to identify factors contributing to the increase in internal quantum efficiency (IQE). Relative to the control growth, hydrogen dosing leads to narrower and discontinuous quantum wells of lower indium content with more abrupt interfaces, which contributes to an increased radiative recombination rate by increasing the electron-hole wavefunction overlap, as revealed by simulations. Hydrogen dosing also selectively etches QWs near defects, which contributes to higher IQE by keeping carriers away from defects and thus reducing non-radiative recombination. APT analysis was also used to determine the Ag dopant distribution in a Ta2O5-based low energy switching memristor devices. By manually tuning the laser energy to avoid fracture caused by different evaporation fields, the Ta2O5:Ag layer was analyzed and the distribution of Ag atoms evaluated. Based on the APT results and electrical performance data, a switching mechanism based on conductive tunneling paths instead of the continuous traditional conductive filaments was proposed for the memristor device. This research indicates the potential for APT analysis used in memristor devices. A proof-of-principle application of APT analysis of doping in 2D materials was demonstrated for the first time by analyzing Ag doping in (PbSe)5(Bi2Se3)3 and Cu doping in Bi2Se3. APT analysis shows that Ag dopes both Bi2Se3 and PbSe layers in (PbSe)5(Bi2Se3)3, and correlations in the position of Ag atoms suggest a pairing across neighboring Bi2Se3 and PbSe layers. Density functional theory (DFT) calculations confirm the favorability of substitutional doping for both Pb and Bi and provide insights into the observed spatial correlations in dopant locations. APT analysis also shows that Cu exists within the layer and between layers in Bi2Se3. Results suggest that van der Waals interactions are strong enough to hold layers together during field evaporation, at least for (PbSe)5(Bi2Se3)3 and Bi2Se3; however, van der Waals interactions make atoms in the same monolayer tend to be evaporated together, limiting the spatial resolution of resolving atomic layers. This work revealed the influence of bonding anisotropy on field evaporation. In order to expand the applications of APT for low-dimensional electronic materials, novel sample preparation methods were developed. For transition metal dichalcogenides (TMDs), van der Waals interactions are generally not strong enough to hold layers together during field evaporation. Therefore, a conformal coating was applied on sharpened tips, reducing tipfracture during analysis. This method enables APT analysis of S doping in MoTe2. Sample preparation methods were also developed for APT analysis of few-layer TMDs in order to correlate their electrical properties and their composition distributions. Contrast can be easily lost during sharpening for few-layer TMDs embedded in Ag films, so ALD SnS coating on TMDs was used to increase the contrast. The feasibly of ALD SnS as coatings was proved by APT analysis. The next step is to measure the W distribution in WxMo1-xTe2, and understand how the W distribution can trigger metal-semiconductor transition in WxMo1-xTe2. The application of APT analysis was also expanded to colloidal core-shell QDs. Colloidal core-shell QDs were analyzed by APT to investigate the role of interface abruptness in suppressing the blinking of photoluminescence. An encapsulation method was used to prepare APT specimens from QDs. After considering criteria for encapsulation materials, available atomic layer deposition (ALD) coatings were tested as encapsulation layers. ZnO enabled the identification of individual CdS/CdSe QDs and gives an upper-bound on the width of the core-shell interface of ~1.2 nm, although with significant overlapping in the mass spectrum that degrades resolution. The difference in evaporation field between the QDs and encapsulants is smallest for ALD SnS among materials tested.

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  • 10/12/2018
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