Effect of the Degree of the Gate‐Dielectric Surface Roughness on the Performance of Bottom‐Gate Organic Thin‐Film Transistors

Abstract

In organic thin‐film transistors (TFTs) fabricated in the inverted (bottom‐gate) device structure, the surface roughness of the gate dielectric onto which the organic‐semiconductor layer is deposited is expected to have a significant effect on the TFT characteristics. To quantitatively evaluate this effect, a method to tune the surface roughness of a gate dielectric consisting of a thin layer of aluminum oxide and an alkylphosphonic acid self‐assembled monolayer over a wide range by controlling a single process parameter, namely the substrate temperature during the deposition of the aluminum gate electrodes, is developed. All other process parameters remain constant in the experiments, so that any differences observed in the TFT performance can be confidently ascribed to effects related to the difference in the gate‐dielectric surface roughness. It is found that an increase in surface roughness leads to a significant decrease in the effective charge‐carrier mobility and an increase in the subthreshold swing. It is shown that a larger gate‐dielectric surface roughness leads to a larger density of grain boundaries in the semiconductor layer, which in turn produces a larger density of localized trap states in the semiconductor.

Optimizing the plasma oxidation of aluminum gate electrodes for ultrathin gate oxides in organic transistors

“…Given the difference between the RMS surface roughness of the aluminum prior to plasma oxidation (0.9 nm) and the RMS surface roughness of the plasma-grown AlO x (0.34 to 0.91 nm), it appears that the plasma-oxidation process smoothens the surface, most prominently for sufficiently high plasma powers and sufficiently long durations. This effect was not observed in our previous study 53 , in which we measured an almost identical RMS surface roughness of 0.9 nm for both the vacuum-deposited aluminum and the dielectric. However, in this previous study we did not explore the use of high plasma powers or long plasma durations, which might explain why no smoothening was observed.…”

“…The devices were fabricated on silicon substrates coated with 100-nm-thick silicon dioxide. For the bottom electrode of the capacitors and the gate electrode of the TFTs, aluminum with a thickness of 30 nm and a root-mean-square surface roughness of less than 1 nm (measured by AFM 53 ) was deposited by vacuum evaporation. AlO x was produced by plasma oxidation, SAMs of n -tetradecylphosphonic acid were formed from solution, and DNTT was deposited by vacuum sublimation.…”

“…The surface roughness, on the other hand, shows a clear correlation with the plasma parameters. Prior to the plasma-oxidation process, the vacuum-deposited aluminum has a root-mean-square (RMS) surface roughness of 0.9 nm 53 . After the plasma-oxidation process, the RMS surface roughness of the bare AlO x ranges from 0.34 to 0.91 nm, depending on the plasma parameters (shown in Fig.…”

Stability of organic thin-film transistors based on ultrathin films of dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT)

“…Aluminum gate electrodes with a root-mean-square surface roughness of less than 1 nm were deposited by thermal evaporation in vacuum. 52 The gate dielectric consists of plasma-grown AlO x and an ntetradecylphosphonic acid SAM. 42 Gold source/drain contacts were deposited by thermal evaporation in vacuum and functionalized with a monolayer of pentafluorobenzenethiol (PFBT) to minimize the contact resistance.…”

“…Aluminum with a thickness of 30 nm and a root-mean-square surface roughness of less than 1 nm was deposited by thermal evaporation in vacuum at a rate of 2 nm/s. 52 The aluminum surface was exposed to an RF oxygen plasma (oxygen flow rate: 30 sccm, oxygen partial pressure: 10 mTorr, RF power: 200 W, duration: 30 s) to increase the thickness of the native aluminum oxide (AlO x ) to about 3.6 nm. On some substrates, ultrathin DNTT (Sigma-Aldrich) was deposited directly onto bare AlO x .…”