Reduction Of Gap States Research Articles

Stability and gap states of amorphous In-Ga-Zn-Ox thin film transistors: Impact of sputtering configuration and post-annealing on device performance

Stability of amorphous In-Ga-Zn-Ox (a-IGZO) thin-film-transistors (TFTs) is studied from a viewpoint of gap states, associated with oxygen-related defects and structural disordering. The a-IGZO gap states are controlled through the film preparation by magnetron sputtering under two different configurations and post-annealing in oxygen atmosphere. The distribution of gap states is then evaluated from the subgap absorption spectrum, which is measured by a constant photocurrent measurement technique. With this known distribution, a-IGZO TFTs are fabricated, and the stability of the TFT performances is examined through the negative bias induced stability (NBIS) experiments. The results suggest that a negative threshold voltage shift, induced by NBIS, is improved by reducing the density of gap states, i.e., narrowing the gap state distribution. The reduction of gap states also contributes to the improvement of subthreshold swing.

Low contact resistivity of metal/n-GaN by the reduction of gap states with an epitaxially grown GaOx insulating layer

We investigated the contact properties of metal/n-GaN and metal/GaOx/n-GaN in terms of Fermi level pinning (FLP) by metal-induced and disorder-induced gap states (MIGS and DIGS). The work function of ten different metals spanned a wide range from 4.2 to 5.7 eV. The measured Schottky barrier height vs metal work function in metal/n-GaN showed the linear relationship with the slope parameter of S = 0.26 ± 0.01 for the doped and the undoped GaN, indicating a strong FLP. The insertion of a GaOx layer increased S to 0.35 corresponding to the decrease of the gap state density by ∼1.1 × 1013 states/(cm2 eV). A contact resistivity of 1.3 × 10−5 Ω cm2 was obtained in Al/GaOx (2.3 nm)/n-GaN (doped with 2 × 1018 cm−3 of Si), which was smaller by nearly three orders than that without the GaOx layer. The insulating property and partially epitaxial structure of GaOx were considered to be responsible for the reduction of the MIGS and DIGS densities, respectively, thereby relaxing FLP and leading to low contact resistivity. Optimization of the GaOx growth parameters may further improve the contact property.

Gap state analysis in electric-field-induced band gap for bilayer graphene.

The origin of the low current on/off ratio at room temperature in dual-gated bilayer graphene field-effect transistors is considered to be the variable range hopping in gap states. However, the quantitative estimation of gap states has not been conducted. Here, we report the systematic estimation of the energy gap by both quantum capacitance and transport measurements and the density of states for gap states by the conductance method. An energy gap of ~250 meV is obtained at the maximum displacement field of ~3.1 V/nm, where the current on/off ratio of ~3 × 103 is demonstrated at 20 K. The density of states for the gap states are in the range from the latter half of 1012 to 1013 eV−1cm−2. Although the large amount of gap states at the interface of high-k oxide/bilayer graphene limits the current on/off ratio at present, our results suggest that the reduction of gap states below ~1011 eV−1cm−2 by continual improvement of the gate stack makes bilayer graphene a promising candidate for future nanoelectronic device applications.

Effect of Annealing on Sub-Gap Absorption in Amorphous InGaZnO<sub>4</sub> Films

Optical absorption in amorphous InGaZnO4films was examined by optical transmittance and photothermal spectroscopy using individual cantilever. The amorphous InGaZnO4films have optical bandgap of ~3.5 eV, and Urbach tail absorption and mid-gap absorption at 2.0-3.0 eV. Mid-gap absorption was reduced by thermal annealing at 300 °C in an atmosphere. This observation indicates that heat-induced structural change causes reduction of gap states in amorphous InGaZnO4films.

Key parameters for further reduction of gap states in a-Ge:H and its improved stability under keV-electron irradiation

The influence of various deposition parameters on the properties of dc-magnetron sputtered a-Ge:H is studied. The lowest Urbach energy of 45 meV is obtained for films of saturated hydrogen concentration of 6 at% prepared at an argon pressure of 0.5 mTorr and a substrate temperature around 450 K. The decrease of the target to substrate - distance from 17 cm to 8 cm results in a further reduction of the defect density and the Urbach energy to 7×10 16 cm −3 and 40 meV (determined by PDS), respectively. The saturation value of the defect density created under keV-electron irradiation is 3×10 17 cm −3, which is close to the value for device quality a-Si:H and a factor of ten less than for films prepared at the larger distance.