Jung Hun Lee

Hysteresis of pentacene thin-film transistors and inverters with cross-linked poly(4-vinylphenol) gate dielectrics

The authors report the effects of hydroxyl groups (OH bonds) on the electrical reliabilities of pentacene organic thin-film transistors (OTFTs) with poly-4-vinylphenol (PVP) gate dielectrics. PVP gate dielectric films mixed with different concentrations of methylated poly(melamine-co-formaldehyde) (MMF) were fabricated, and experiments on the hysteresis behavior of the OTFT device were conducted. Pentacene TFTs with the PVP (MMF 0wt.%) exhibited a large hysteresis, while in the PVP (MMF 125wt.%), nearly no hysteresis was observed. Large hysteresis observed in OTFT devices was confirmed to be strongly related to the hydroxyl groups existing inside of the polymeric dielectrics and could reduced by the decrease of OH group.

Hysteresis and threshold voltage shift of pentacene thin-film transistors and inverters with Al2O3 gate dielectric

The pentacene organic thin-film transistors (OTFTs) with plasma-enhanced atomic-layer-deposited 150nm thick Al2O3 as a gate dielectric and the inverters comprised of these OTFTs have been fabricated. The hysteresis in transfer characteristics of the OTFTs depends on the scan range of negative gate voltage, regardless of drain voltage. Negative gate-bias stress leads to the progressive negative shift of threshold voltage. The hysteresis of D inverter is similar to that of E inverter, but the former has a wider swing range and a higher gain in comparison with the latter.

Naturally formed InxAl1-xAs/InyAl1-yAs vertical superlattices

InxAl1−xAs/InyAl1−yAs vertical superlattices (VS) were naturally formed by phase separation during the growth of InAlAs/InP layers at temperatures in the range 565–615u2009°C by metalorganic chemical vapor deposition. The VS lies perpendicular to the (001) growth plane. As the growth temperature increased from 565 to 615u2009°C, the VS decreased in thickness from ∼15 to ∼6 nm, and became less planar and uniform. Transmission electron diffraction results showed the occurrence of CuPt‐type ordering in some of the layers. Band‐gap reduction of ∼300 meV was observed in the InAlAs layers grown at temperatures ranging from 565 to 615u2009°C. Such a large reduction in band‐gap energy was attributed to combined effects of the VS and CuPt‐type ordering.

Optical properties of InGaAs linear graded buffer layers on GaAs grown by metalorganic chemical vapor deposition

We report optical characteristics of linear graded InxGa1−xAs (XIn=0–0.58) buffer layers grown on GaAs by low‐pressure metalorganic chemical vapor deposition. Two types of wirelike surface structures were observed from the layers grown at two different temperatures. Low‐temperature photoluminescence (PL) and double‐crystal x‐ray diffractometric measurements indicate that the PL energy and the relaxation of the graded layers were strongly dependent on the top surface structure. InGaAs cap layers were grown on top of the graded buffer layers with a variation of indium composition. A strong PL signal was observed from the top region of the graded layer grown with a lattice‐matched cap layer. It suggests that the top region of the grade, similar to a graded well structure, is compressively strained but is of high structural quality without dislocations.

Low-voltage and high-gain pentacene inverters with plasma-enhanced atomic-layer-deposited gate dielectrics

The pentacene thin-film transistors with the plasma-enhanced atomic-layer-deposited 150nm thick Al2O3 or 120nm thick ZrO2 have been operated at gate voltages between −3 and 3V. The inverter with a ZrO2 gate dielectric shows a gain of 49 and a full swing from supply voltage (Vdd) to 0V, operating at input voltages (Vin) from 0to−1V and at Vdd of −1V. The hysteresis observed in the voltage transfer characteristic of the inverter depends on the scan range of Vin applied to the driver transistor, regardless of the Vdd applied to the load transistor.

Electric characteristics of poly(3-hexylthiophene) organic field-effect transistors fabricated on O2 plasma-treated substrates

Soluble conjugated polymers such as poly(3-hexylthiophene) (P3HT) are very promising candidates for a cheap electronic device on various substrates. In this study we report the effects of O2 plasma treatment of the substrates on the electrical properties of P3HT organic thin-film transistors and metal/insulator/organic semiconductor capacitors. Based on the results of a capacitance–voltage measurement, the effective charge density in the interface between P3HT and SiO2 layer treated by O2 plasma for 30 s was approximately −18.3u2009nC/cm2. When the period of O2 plasma treatment was longer than 30 s, the field-effect mobility decreased since the amount of charge and the relaxation time constant of interface traps increased.

Ordering effect on band-gap lowering in lattice-matched InAlAs epilayers grown on InP by metal–organic chemical-vapor deposition

Band-gap lowering due only to the cation ordering effect is investigated in InAlAs layers grown on InP by using photoluminescence measurement. Double-crystal x-ray diffraction and Rutherford backscattering measurements confirm that both of the InAlAs epilayers studied, grown at 700 and 750u2009°C, are lattice matched with InP substrates. Through transmission electron diffraction measurements, it is observed that a CuPt-type ordering structure is formed in the InAlAs layers grown at 700u2009°C but not in the layers at 750u2009°C. Photoluminescence measurements at 1.7 K reveal that the band-gap energy of the ordered InAlAs is smaller by 60 meV than that of the unordered InAlAs.

DEVICE FABRICATIONS OF ORGANIC THIN-FILM TRANSISTORS

Organic TFTs with p-type pentacene and n-type F16CuPc were fabricated on both Si and plastic substrates. Maximum p-type mobility of 0.8u2009cm2/Vs and 0.12u2009cm2/Vs were achieved for Si and plastic substrates, respectively. JSS-362 was used as a gate dielectrics for plastic substrate. To decrease the contact resistance between metal electrodes and organic semiconductors, self-assembled monolayers were formed. Charge carrier mobilities of the device with both pentacene and F16CuPc were increased after SAMs treatments on metal contacts.

Threshold voltage control of pentacene thin‐film transistor with dual‐gate structure

Abstract This paper presents a comprehensive study on threshold voltage (Vth) control of organic thin‐film transistors (OTFTs) with dual‐gate structure. The fabrication of dual‐gate pentacene OTFTs using plasma‐enhanced atomic layer deposited (PEALD) 150 nm thick Al2O3 as a bottom gate dielectric and 300 nm thick parylene or PEALD 200 nm thick Al2O3 as both a top gate dielectric and a passivation layer was investigated. The Vth of OTFT with 300 nm thick parylene as a top gate dielectric was changed from 4.7 V to 1.3 V and that with PEALD 200 nm thick Al2O3 as a top gate dielectric was changed from 1.95 V to ‐9.8 V when the voltage bias of top gate electrode was changed from ‐10 V to 10 V. The change of Vth of OTFT with dual‐gate structure was successfully investigated by an analysis of electrostatic potential.

Analysis of the charge transport in π-conjugated materials using generalized Langevin equation

The complex dielectric constants of several π-conjugated materials are measured, and generalized Langevin equation is used to analyze the dielectric behavior in the frequency domain. From the results of a fitting of the experimental data, all of the dielectric behaviors are well interpreted by the generalized Langevin equation which strongly suggests that the mechanism of the charge carrier transport in π-conjugated organic and polymeric materials is diffusion at dc and low frequencies and an oscillatory motion at (or near) phonon frequencies, and in between, a mixed mode of these two mechanisms.