Model for determination of mid-gap states in amorphous metal oxides from thin film transistors

Abstract

The electronic density of states in metal oxide semiconductors like amorphous zinc oxide (a-ZnO) and its ternary and quaternary oxide alloys with indium, gallium, tin, or aluminum are different from amorphous silicon, or disordered materials such as pentacene, or P3HT. Many ZnO based semiconductors exhibit a steep decaying density of acceptor tail states (trap DOS) and a Fermi level (EF) close to the conduction band energy (EC). Considering thin film transistor (TFT) operation in accumulation mode, the quasi Fermi level for electrons (Eq) moves even closer to EC. Classic analytic TFT simulations use the simplification EC−EF> ‘several’kT and cannot reproduce exponential tail states with a characteristic energy smaller than 1/2 kT. We demonstrate an analytic model for tail and deep acceptor states, valid for all amorphous metal oxides and include the effect of trap assisted hopping instead of simpler percolation or mobility edge models, to account for the observed field dependent mobility.

Semiconductor Materials by Ultrasonic Spray Pyrolysis and Their Application in Electronic Devices

“…As can be seen, the DOS obtained is higher than those previously reported by different authors [44,45]. This can be due to more defects in the ZnO films originated by the incomplete precursor pyrolysis, corroborated by the presence of O─H complexes in the FTIR spectra and defects as the photoluminescence spectroscopy shows.…”

Ionic liquid gating reveals trap-filled limit mobility in low temperature amorphous zinc oxide

“…2 shows the trap DOS for a-ZnO, measured by the temperature method. 10 The DOS exhibits the typical features of AOSs: an exponential deep states distribution with a Urbach energy >kT (%25 meV), a steep exponential tail states distribution with a characteristic energy <kT, and a total amount of trap states around 10 18 cm À3 . The accumulated free charge, on the other hand, can be analytically expressed with the right summand in the following equation:…”

“…In AOS TFTs, charge transport and therewith the mobility is gate field dependent in a wide range due to mid gap trap states (tail states) leading to trap limited conduction or trap assisted tunneling (TAT). [8][9][10] These traps must be filled in order to reveal the limits of the charge carrier mobility.…”

“…10 The film thickness was d ¼ 5 nm and the roughness was Ra ¼ 0.35 nm. Additional information can be found in the previous publications 10, [41][42][43] and supplementary material.…”

“…2 dashed plus filled area) and can be obtained, e.g., from TFT characteristics, [17][18][19] CV measurements, 20 space charge limited current, 21 modulated photocurrent spectroscopy, 22 and Schottky diode characteristics, 23 or more accurately via temperature dependent TFT measurements. 10,15,24 Fig. 2 shows the trap DOS for a-ZnO, measured by the temperature method.…”