Sangyeob Lee

Scanning tunneling spectroscopy and Kelvin probe force microscopy investigation of Fermi energy level pinning mechanism on InAs and InGaAs clean surfaces

A comparison is made between the electronic structures determined in ultrahigh vacuum of three surfaces using scanning tunneling spectroscopy (STS) and Kelvin probe force microscopy (KPFM). STS and KPFM illustrates Fermi level pinning of clean InAs(001)-(4×2) and InGaAs(001)-(4×2) surfaces and near flat band conditions for InAs(110) cleaved surfaces. However, for InAs(001)-(4×2) and InGaAs(001)-(4×2), STS and KPFM data show very different positions for the surface Fermi level on identical samples; it is hypothesized that the difference is due to the Fermi level measured by KPFM being shifted by a static charge dipole to which STS is much less sensitive.

Atomic imaging of the irreversible sensing mechanism of NO2 adsorption on copper phthalocyanine.

Ambient NO2 adsorption onto copper(II) phthalocyanine (CuPc) monolayers is observed using ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) to elucidate the molecular sensing mechanism in CuPc chemical vapor sensors. For low doses (1 ppm for 5 min) of NO2 at ambient temperatures, isolated chemisorption sites on the CuPc metal centers are observed in STM images. These chemisorbates almost completely desorb from the CuPc monolayer after annealing at 100 °C for 30 min. Conversely, for high NO2 doses (10 ppm for 5 min), the NO2 induces a fracture of the CuPc domains. This domain fracture can only be reversed by annealing above 150 °C, which is consistent with dissociative chemisorption into NO and atomic O accompanied by surface restructuring. This high stability implies that the domain fracture results from tightly bound adsorbates, such as atomic O. Existence of atomic O on or under the CuPc layer, which results in domain fracture, is revealed by XPS analysis and ozone-dosing experiments. The observed CuPc domain fracturing is consistent with a mechanism for the dosimetric sensing of NO2 and other reactive gases by CuPc organic thin film transistors (OTFTs).

Atomic imaging of nucleation of trimethylaluminum on clean and H2O functionalized Ge(100) surfaces.

The direct reaction of trimethylaluminum (TMA) on a Ge(100) surface and the effects of monolayer H(2)O pre-dosing were investigated using ultrahigh vacuum techniques, such as scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). At room temperature (RT), a saturation TMA dose produced 0.8 monolayers (ML) of semi-ordered species on a Ge(100) surface due to the dissociative chemisorption of TMA. STS confirmed the chemisorption of TMA passivated the bandgap states due to dangling bonds. By annealing the TMA-dosed Ge surface, the STM observed coverage of TMA sites decreased to 0.4 ML at 250 °C, and to 0.15 ML at 450 °C. XPS analysis showed that only carbon content was reduced during annealing, while the Al coverage was maintained at 0.15 ML, consistent with the desorption of methyl (-CH(3)) groups from the TMA adsorbates. Conversely, saturation TMA dosing at RT on the monolayer H(2)O pre-dosed Ge(100) surface followed by annealing at 200 °C formed a layer of Ge-O-Al bonds with an Al coverage a factor of two greater than the TMA only dosed Ge(100), consistent with Ge-OH activation of TMA chemisorption and Ge-H blocking of CH(3) chemisorption. The DFT shows that the reaction of TMA has lower activation energy and is more exothermic on Ge-OH than Ge-H sites. It is proposed that the H(2)O pre-dosing enhances the concentration of adsorbed Al and forms thermally stable Ge-O-Al bonds along the Ge dimer row which could serve as a nearly ideal atomic layer deposition nucleation layer on Ge(100) surface.

Mobility saturation in tapered edge bottom contact copper phthalocyanine thin film transistors

Copper phthalocyanine (CuPc) thin film transistors were fabricated using a tapered edge bottom contact device geometry, and mobility saturation was observed for devices with CuPc thicknesses of 12 monolayers (MLs) and greater. The mobility saturation is attributed to a significantly decreased contact resistance resulting from a bilayer resist lift-off method, as compared with a single layer resist lift-off method. Threshold voltages are also found to saturate above 12 ML CuPc thicknesses.

Mask-less patterning of organic light emitting diodes using electrospray and selective biasing on pixel electrodes

Wet process of soluble organic light emitting diode (OLED) materials has attracted much attention due to its potential as a large-area manufacturing process with high productivity. Electrospray (ES) deposition is one of candidates of organic thin film formation process for OLED. However, to fabricate red, green, and blue emitters for color display, a fine metal mask is required during spraying emitter materials. We demonstrate a mask-less color pixel patterning process using ES of soluble OLED materials and selective biasing on pixel electrodes and a spray nozzle. We show red and green line patterns of OLED materials. It was found that selective patterning can be allowed by coulomb repulsion between nozzle and pixel. Furthermore, we fabricated blue fluorescent OLED devices by vacuum evaporation and ES processes. The device performance of ES processed OLED showed nearly identical current-voltage characteristics and slightly lower current efficiency compared to vacuum processed OLED.

Domain fracture and recovery process of metal phthalocyanine monolayers via NO2 and H2O

CuPc ultrathin films (5 monolayers) are employed to detect NO2 in chemFETs [organic thin film transistors (OTFTs)]; while the NO2 causes OTFT degradation, H2O restores OTFT performance. To develop an atomic understanding of this H2O induced performance recovery, NO2/CuPc/Au(111) was exposed to H2O, then observed using ultrahigh vacuum scanning tunneling microscopy. After dosing NO2 (10 ppm for 5 min) onto CuPc monolayers under ambient conditions, domain fracture is induced in CuPc monolayers, and CuPc aggregates are formed near new grain boundaries, consistent with dissociative O adsorption between CuPc molecules and Au(111). Conversely, after exposing H2O onto a fractured CuPc monolayer for 30 min, fractured domains merge, then large area domains are generated. As the duration of H2O exposure increases to 4 h, second layer growth of CuPc molecules is observed on the CuPc monolayers consistent with H2O breakdown of CuPc aggregates which have formed at the domain boundaries. The results are consistent with...

Bonding Geometries at the In2O and SiO/III-V Semiconductor Interface

Oxide monolayers and submonolayers formed by vapor depositon of In2O and SiO oxides on InAs(001)-(4×2) were studied by scanning tunneling microscopy (STM). At low coverage, In2O molecules bond to the edges of the rows and most likely form new In-As bonds to the surface without any disruption of the clean surface structure. Annealing the In2O/InAs(001)-(4×2) surface to 380˚C results in formation of flat ordered monolayer rectangular islands. The annealed In2O no longer bonds with just the As atoms at the edge of row but also forms new O-In bonds in the trough. SiO chemisorption on InAs(001)-(4×2) is completely different than In2O chemisorption. At room temperature, even at low coverage SiO adsorbates bond to themselves and form nanoclusters. For SiO/InA(001)-(4×2) post-deposition annealing (PDA) does not disperse the nanoclusters into flat islands. Both In2O and SiO depositions on InAs(001)-(4×2) surface do not displace surface atoms during both room temperature deposition and postdeposition annealing.

The Role of Charge Balance and Excited State Levels on Device Performance of Exciplex-based Phosphorescent Organic Light Emitting Diodes

The design of novel exciplex-forming co-host materials provides new opportunities to achieve high device performance of organic light emitting diodes (OLEDs), including high efficiency, low driving voltage and low efficiency roll-off. Here, we report a comprehensive study of exciplex-forming co-host system in OLEDs including the change of co-host materials, mixing composition of exciplex in the device to improve the performance. We investigate various exciplex systems using 5-(3–4,6-diphenyl-1,3,5-triazin-2-yl)phenyl-3,9-diphenyl-9H-carbazole, 5-(3–4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9-phenyl-9H-3,9′-bicarbazole, and 2-(3-(6,9-diphenyl-9H-carbazol-4-yl)phenyl)-4-phenylbenzo[4,5]thieno[3,2-d]pyrimidine, as electron transporting (ET: electron acceptor) hosts and 9,9′-dipenyl-9H, 9′H-3,3′-bicarbazole and 9-([1,1′-biphenyl]-4-yl)-9′-phenyl-9H,9′H-3,3′-bicarbazole as hole transporting (HT: electron donor) hosts. As a result, a very high current efficiency of 105.1 cd/A at 103 cd/m2 and an extremely long device lifetime of 739 hrs (t95: time after 5% decrease of luminance) are achieved which is one of the best performance in OLEDs. Systematic approach, controlling mixing ratio of HT to ET host materials is suggested to select the component of two host system using energy band matching and charge balance optimization method. Furthermore, our analysis on exciton stability also reveal that lifetime of OLEDs have close relationship with two parameters; singlet energy level difference of HT and ET host and difference of singlet and triplet energy level in exciplex.

Realization of continuous Zachariasen carbon monolayer

Continuous Zachariasen carbon monolayer, a novel amorphous 2D carbon allotrope, was synthesized on germanium surface. Rapid progress in two-dimensional (2D) crystalline materials has recently enabled a range of device possibilities. These possibilities may be further expanded through the development of advanced 2D glass materials. Zachariasen carbon monolayer, a novel amorphous 2D carbon allotrope, was successfully synthesized on germanium surface. The one-atom-thick continuous amorphous layer, in which the in-plane carbon network was fully sp2-hybridized, was achieved at high temperatures (>900°C) and a controlled growth rate. We verified that the charge carriers within the Zachariasen carbon monolayer are strongly localized to display Anderson insulating behavior and a large negative magnetoresistance. This new 2D glass also exhibited a unique ability as an atom-thick interface layer, allowing the deposition of an atomically flat dielectric film. It can be adopted in conventional semiconductor and display processing or used in the fabrication of flexible devices consisting of thin inorganic layers.

Interfacial Atomic Bonding Structure of Oxides on InAs ( 001 ) - ( 4 × 2 ) Surface

Oxide monolayers and submonolayers formed by vapor deposition of In2O and SiO oxides on InAs001-4 2 were studied by scanning tunneling microscopy. At low coverage, In2O molecules bond to the edges of the rows and most likely form new In–As bonds to the surface without any disruption of the clean surface structure. Annealing the In2O/InAs001-4 2 surface to 380°C results in the formation of flat ordered monolayer rectangular islands. The annealed In2O no longer bonds with just the As atoms at the edge of row but also forms new O–In bonds in the trough. SiO chemisorption on InAs001-4 2 is completely different than In2O chemisorption. At room temperature, even at low coverage SiO adsorbates bond to themselves and form nanoclusters. For SiO/InA001-4 2 postdeposition annealing does not disperse the nanoclusters into flat islands. Both In2O and SiO depositions on the InAs001-4 2 surface do not displace surface atoms during both room temperature deposition and postdeposition annealing.