Bongki Lee

Conformal Al2O3 dielectric layer deposited by atomic layer deposition for graphene-based nanoelectronics

We present a facile route which combines the functionalization of a highly oriented pyrolytic graphite surface with an atomic layer deposition (ALD) process to allow for conformal Al2O3 layers. While the trimethylaluminum (TMA)∕H2O process caused selective deposition only along step edges, the TMA∕O3 process began to provide nucleation sites on the basal planes of the surface. O3 pretreatment, immediately followed by the ALD process with TMA∕O3 chemistry, formed Al2O3 layers without any preferential deposition at the step edges. This is attributed to functionalization of graphene by ozone treatment, imparting a hydrophilic character which is desirable for ALD deposition.

Atomic Layer Deposition of Dielectrics on Graphene Using Reversibly Physisorbed Ozone

Integration of graphene field-effect transistors (GFETs) requires the ability to grow or deposit high-quality, ultrathin dielectric insulators on graphene to modulate the channel potential. Here, we study a novel and facile approach based on atomic layer deposition through ozone functionalization to deposit high-κ dielectrics (such as Al(2)O(3)) without breaking vacuum. The underlying mechanisms of functionalization have been studied theoretically using ab initio calculations and experimentally using in situ monitoring of transport properties. It is found that ozone molecules are physisorbed on the surface of graphene, which act as nucleation sites for dielectric deposition. The physisorbed ozone molecules eventually react with the metal precursor, trimethylaluminum to form Al(2)O(3). Additionally, we successfully demonstrate the performance of dual-gated GFETs with Al(2)O(3) of sub-5 nm physical thickness as a gate dielectric. Back-gated GFETs with mobilities of ~19,000 cm(2)/(V·s) are also achieved after Al(2)O(3) deposition. These results indicate that ozone functionalization is a promising pathway to achieve scaled gate dielectrics on graphene without leaving a residual nucleation layer.

Frequency dispersion reduction and bond conversion on n-type GaAs by in situ surface oxide removal and passivation

The method of surface preparation on n-type GaAs, even with the presence of an amorphous-Si interfacial passivation layer, is shown to be a critical step in the removal of accumulation capacitance frequency dispersion. In situ deposition and analysis techniques were used to study different surface preparations, including NH4OH, Si-flux, and atomic hydrogen exposures, as well as Si passivation depositions prior to in situ atomic layer deposition of Al2O3. As–O bonding was removed and a bond conversion process with Si deposition is observed. The accumulation capacitance frequency dispersion was removed only when a Si interlayer and a specific surface clean were combined.

In situ study of surface reactions of atomic layer deposited LaxAl2-xO3 films on atomically clean In0.2Ga0.8As

The surface reactions of LaxAl2−xO3 ultrathin films deposited on atomically clean In0.2Ga0.8As by atomic layer deposition are studied by in situ high resolution x-ray photoelectron spectroscopy. Using 1:2 alternating cycles of La2O3 and Al2O3 results in a La:Al concentration ratio of 1:10. We found that the LaxAl2−xO3∕InGaAs interface consisted of interfacial Ga-suboxides and As–As bonds but no As- or In-oxides were detected. This suggests an interface formed by Ga–O–Al and Ga–O–La bonds from the precursor reaction.

Effects of O3 and H2O oxidants on C and N-related impurities in atomic-layer-deposited La2O3 films observed by in situ x-ray photoelectron spectroscopy

The effect of H2O and O3 oxidants on the behavior of residual C and N-related impurities as well as Si out-diffusion and interfacial layer formation in atomic-layer-deposited La2O3 films grown at 250 °C were examined using in situ x-ray photoelectron spectroscopy. The silicate formation was suppressed in a La2O3 film grown using O3 compared to that deposited using H2O, but interfacial layer growth was enhanced. The accumulation of C and N-related residues with low binding energy, which originated from incomplete reactions, was suppressed in La2O3 films grown using O3. However, the use of O3 resulted in La-carbonate phase in film.

Atomic-layer-deposited Al2O3 as Gate Dielectrics for Graphene-based Devices

We present a facile route to deposit a uniform Al2O3 layer on a highly oriented pyrolytic graphite (HOPG) surface using atomic layer deposition (ALD). Al2O3 layers deposited from TMA (trimethylaluminum)/ H2O chemistry showed selective deposition only on step edges. However, TMA/O3 chemistry resulted in the deposition of Al2O3 layers on basal planes of HOPG, which has a chemically inert surface. An O3-pretreatement followed by Al2O3 deposition using TMA/O3 chemistry produced conformal and uniform Al2O3 dielectric layers with a small RMS roughness of ~ 0.2 nm. This suggests that O3-pretreatement prior to ALD deposition makes the chemically inert HOPG surface reactive toward ALD precursors, which leads to the desired two-dimensional growth mode. High-resolution transmission electron microscopy (HR-TEM) and Raman spectroscopy revealed that this O3 process used for Al2O3 deposition does not introduce a significant defect concentration to the top graphene layer. The dielectric constant of the deposited Al2O3 film on top of the HOPG surface was found to be ~ 9 from C-V measurements.

Surface energy induced patterning of organic and inorganic materials on heterogeneous Si surfaces

A surface energy induced patterning (SEIP) method is developed to transfer resist patterns defined by lithography into various functional materials. A Si template is first chemically patterned using conventional lithography and selective attachment of trichlorosilane to achieve spatially different surface energies. Organic materials as well as inorganic films are deposited onto the chemically patterned template, followed by a thermal annealing process. The heterogeneous surface energies on the template induce material microfluidic reflow from the less to the more thermodynamically favorable areas. Using this method, patterned microstructures were achieved with SU-8, diblock copolymer, and aluminum film. In addition, the SEIP template was successfully used for atomic layer chemical vapor deposition to selectively pattern 200nm–2μm wide HfO2 structures.

Piezoelectric effect in epitaxial PbZr1− xTi xO3 thin films near morphotropic phase boundary region

The relationship between crystal structure and piezo-response was investigated in epitaxially grown PbZr1−xTixO3 (PZT) thin films on Pt(001)/MgO(001) with a thin PbTiO3 interlayer. Insertion of the interlayer resulted in significant relaxation ofthe strain that could be developed in the course of deposition of the PZT films, consequently leading us to single out only the effect of composition. Composition of the morphotropic phase boundary (MPB), at which tetragonal and rhombohedral phases are mixed with the same volume fraction, was found to be ∼0.55 in Ti/(Zr + Ti) ratio in our films, which is close to the value for bulk polycrystalline PZT (∼0.50). The piezoelectric response peaks were two times higher in the MPB regime than in the single phase regime due to structural instability caused by the coexistence of two phases. The results indicate that epitaxial PZT films having the MPB composition are advantageous over those of other compositions for nano-storage devices based on scanning force microscopy.

Local work function measurements on various inorganic materials using kelvin probe force spectroscopy

In this study, we measured work functions on various materials (i.e., metals and semiconductors) using Kelvin probe force spectroscopic method. Since the Kelvin probe force microscopic (KPFM) method allow to determine the work function difference between the metallic tip and unknown work function samples, it is very important that work function of the tip is accurately defined. Therefore, all the conductive tips were calibrated with a freshly cleavaged highly oriented pyrolytic graphite (HOPG). Work function measurements on various materials are conducted by using these calibrated tips. The values of work functions of metal films (i.e., Au and Pt) obtained in this method were found to correspond to the values reported in the literature, but those for semiconductor materials (i.e., n-, p-type Si and ITO films) show some deviations from their work functions which would be found in the literature. Surface electronic band bending and geometry of the tip could affect the differences in work functions measured by KPFM technique.

Formation and process optimization of scanning resistive probe

Recently, scanning resistive probe microscopy, which has a semiconducting resistor at the apex of the tip and observes surface charges directly, was newly proposed and fabricated. In order to optimize process parameters as well as to understand the mechanisms of the field induced resistance change in the resistive probe, the doping profile of resistive patterns is investigated by the use of Kelvin probe force microscopy. Overlapping space charge regions (O-SCRs) in between n+ regions were observed. Decreased barrier heights in the structure of n+∕O-SCR∕n+ were also investigated. In particular, resistive patterns with diffusion times longer than 12h were observed to have overlapped outdiffusion of As+ ions, showing no formation of O-SCR in between n+ regions. This was also confirmed by measurements of I-V characteristics.