Sunghwan Lee

A Facile Solution-Phase Approach to Transparent and Conducting ITO Nanocrystal Assemblies

Monodisperse 11 nm indium tin oxide (ITO) nanocrystals (NCs) were synthesized by thermal decomposition of indium acetylacetonate, In(acac)(3), and tin bis(acetylacetonate)dichloride, Sn(acac)(2)Cl(2), at 270 °C in 1-octadecene with oleylamine and oleic acid as surfactants. Dispersed in hexane, these ITO NCs were spin-cast on centimeter-wide glass substrates, forming uniform ITO NC assemblies with root-mean-square roughness of 2.9 nm. The assembly thickness was controlled by ITO NC concentrations in hexane and rotation speeds of the spin coater. Via controlled thermal annealing at 300 °C for 6 h under Ar and 5% H(2), the ITO NC assemblies became conductive and transparent with the 146 nm-thick assembly showing 5.2 × 10(-3) Ω·cm (R(s) = 356 Ω/sq) resistivity and 93% transparency in the visible spectral range--the best values ever reported for ITO NC assemblies prepared from solution phase processes. The stable hexane dispersion of ITO NCs was also readily spin-cast on polyimide (T(g) ~360 °C), and the resultant ITO assembly exhibited a comparable conductivity and transparency to the assembly on a glass substrate. The reported synthesis and assembly provide a promising solution to the fabrication of transparent and conducting ITO NCs on flexible substrates for optoelectronic applications.

A study of the specific contact resistance and channel resistivity of amorphous IZO thin film transistors with IZO source–drain metallization

We report on the specific contact resistance of interfaces between thin amorphous semiconducting IZO channel layers and IZO source/drain metallization in amorphous oxide thin film transistors (TFTs). As-deposited, low carrier density amorphous IZO layers are difficult to produce and consequently very thin (10–30 nm) channel layers are required for IZO TFT device applications in order to achieve adequately low off-state current. In this article, the transmission line model (TLM) and structures that also serve as IZO gate-down TFTs were used to examine IZO/IZO homojunctions with thin (10 nm) and thick (100 nm) channel layers. Thin, 10 nm, IZO channel devices with IZO source/drain contacts show a threshold voltage of −3.9 V and a very high specific contact resistance (ρC) that varies with gate voltage (VG) in the range 0–10 V from 460 to 130 Ω cm2. Annealing in air at 200  °C resulted in a tenfold improvement in ρC (34 Ω cm2) and corresponds to an increase in carrier density in the channel. Thicker IZO films...

High-efficiency silicon-compatible photodetectors based on Ge quantum dots

We report on high responsivity, broadband metal/insulator/semiconductor photodetectors with amorphous Ge quantum dots (a-Ge QDs) as the active absorbers embedded in a silicon dioxide matrix. Spectral responsivities between 1–4 A/W are achieved in the 500–900 nm wavelength range with internal quantum efficiencies (IQEs) as high as ∼700%. We investigate the role of a-Ge QDs in the photocurrent generation and explain the high IQE as a result of transport mechanisms via photoexcited QDs. These results suggest that a-Ge QDs are promising for high-performance integrated optoelectronic devices that are fully compatible with silicon technology in terms of fabrication and thermal budget.

Transient photoresponse and incident power dependence of high-efficiency germanium quantum dot photodetectors

We report a systematic study of time-resolved and power-dependent photoresponse in high-efficiency germanium quantum dot photodetectors (Ge-QD PDs), with internal quantum efficiencies greater than 100% over a broad wavelength, reverse bias, and incident power range. Turn-on and turn-off response times (τon and τoff) are shown to depend on series resistance, bias, optical power, and thickness (WQD) of the Ge-QD layer, with measured τoff values down to ∼40 ns. Two different photoconduction regimes are observed at low and high reverse bias, with a transition around −3 V. A transient current overshoot phenomenon is also observed, which depends on bias and illumination power.

Identification of the native defect doping mechanism in amorphous indium zinc oxide thin films studied using ultra high pressure oxidation

The mechanism of native defect doping in amorphous In-Zn-O (a-IZO) has not previously been established but is likely associated with native oxygen defect doping. We have used high pressure oxidation and defect equilibrium analysis to show a −1/6 power dependence of carrier density on oxygen fugacity in a-IZO. This dependency is predicted for oxygen vacancy-like donor defects. Extrapolation of equilibrium constants established at high pressures to atmospheric pressure reveals that the equilibrium carrier density in a-IZO at 200 °C is higher (>1020/cm3) than typical as-deposited channel carrier densities (<1017/cm3). This is consistent with observed increases in channel carrier density and negative threshold voltage shift in annealed a-IZO thin film transistor devices.

On the effect of Ti on the stability of amorphous indium zinc oxide used in thin film transistor applications

In2O3-based amorphous oxide channel materials are of increasing interest for thin film transisitor applications due, in part, to the remarkable stability of this class of materials amorphous structure and electronic properties. We report that this stability is degraded in the presence of Ti, which is widely used as a contact and/or adhesion layer. A cross-sectional transmission electron microscopy analysis, supported by glancing incident angle x-ray and selected area diffraction examination, shows that amorphous indium zinc oxide in contact with Ti undergoes crystallization to the bixbyite phase and reacts to form the rutile phase of TiO2 at a temperature of 200 °C. A basic thermodynamic analysis is presented and forms the basis of a model that describes both the crystallization and the resistivity decrease.

Optoelectronic properties of polythiophene thin films and organic TFTs fabricated by oxidative chemical vapor deposition

Fabrication of devices utilizing unsubstituted polythiophene (PT) has received limited attention because thin films of this insoluble material have been difficult to prepare using traditional solution-based methods. However, since the thiophene monomer is volatile, PT films can be readily fabricated by oxidative chemical vapor deposition (oCVD). The pressure of the oCVD process is here shown to significantly affect the optoelectronic properties of the PT films. Shifts in the Raman bands near 1500 cm−1 suggest that the lowest pressure deposition conditions (1 mTorr) result in shorter conjugation. Additionally, relative Raman peak intensities suggest that the polymer chains in these films contain more distortions than films deposited at higher pressures. UV-Vis absorption spectra of the oCVD PT films show that films deposited at the lowest pressure considered (1 mTorr) are slightly blue-shifted and demonstrate lower absorption than films deposited at higher pressures. PT films deposited at 75, 150, and 300 mTorr demonstrated similar UV-Vis absorption spectra, with absorption maxima near 515 nm. Organic thin film transistors (TFTs) were fabricated using oCVD PT as the active channel layer to analyze the effect of deposition pressure on charge transport and electrical properties. Films deposited at 150 mTorr demonstrate the greatest field effect carrier mobility of 4 × 10−3 cm2 V−1 s−1.

Self-forming, quasi-two-dimensional, magnetic-fluid patterns with applied in-plane-rotating and dc-axial magnetic fields

We report flow instabilities with simultaneous, in-plane-rotating and dc-axial uniform magnetic fields. In uniform dc-axial magnetic fields, a ferrofluid drop in a Hele-Shaw cell with an∼1.0-mm gap forms the familiar labyrinth pattern. With subsequent application of an ∼100-G rms in-plane, 20–40-Hz rotating uniform magnetic field, smooth spirals form. If the rotating magnetic field is applied first, the drop holds together for low dc-axial magnetic fields and no labyrinth pattern develops. If the dc magnetic field is subsequently increased above a threshold value, the ferrofluid drop abruptly transforms into many ferrofluid droplets arranged in a regular pattern.

Metallization selection and the performance of amorphous In-Zn-O thin film transistors

We report on the effect of two different source/drain metallizations (Ti, Mo) on the performance and stability of amorphous In-Zn-O (IZO) thin film transistors (TFTs). In the as-deposited state, stable Mo/IZO TFTs show low threshold voltage (VT) and clear drain current saturation behavior compared to Ti/IZO devices, despite both having similar channel conductivity. Low temperature annealing (200 °C) results in VT shifts in Ti/IZO TFTs (ΔVT ∼ −33 V) that are significantly larger than in Mo/IZO TFTs (ΔVT ∼ −14 V). These differences are attributed to the injection of additional carriers into the channel of Ti/IZO devices due to reaction at the unstable Ti/IZO interface.