An object is to provide a semiconductor device of which a manufacturing process is not complicated and by which cost can be suppressed, by forming a thin film transistor using an oxide semiconductor film typified by zinc oxide, and a manufacturing method thereof. For the semiconductor device, a gate electrode is formed over a substrate; a gate insulating film is formed covering the gate electrode; an oxide semiconductor film is formed over the gate insulating film; and a first conductive film and a second conductive film are formed over the oxide semiconductor film. The oxide semiconductor film has at least a crystallized region in a channel region.

Semiconductor device, and manufacturing method thereof

Theoretical models of Schottky-barrier height formation are reviewed. A particular emphasis is placed on the examination of how these models agree with general physical principles. New concepts on the relationship between interface dipole and chemical bond formation are analyzed, and shown to offer a coherent explanation of a wide range of experimental data.

Recent advances in Schottky barrier concepts

The formation of the Schottky barrier height (SBH) is a complex problem because of the dependence of the SBH on the atomic structure of the metal-semiconductor (MS) interface. Existing models of the SBH are too simple to realistically treat the chemistry exhibited at MS interfaces. This article points out, through examination of available experimental and theoretical results, that a comprehensive, quantum-mechanics-based picture of SBH formation can already be constructed, although no simple equations can emerge, which are applicable for all MS interfaces. Important concepts and principles in physics and chemistry that govern the formation of the SBH are described in detail, from which the experimental and theoretical results for individual MS interfaces can be understood. Strategies used and results obtained from recent investigations to systematically modify the SBH are also examined from the perspective of the physical and chemical principles of the MS interface.

The physics and chemistry of the Schottky barrier height

III-V semiconductors have high mobility and will be used in field effect transistors with the appropriate gate dielectric. The dielectrics must have band offsets over 1eV to inhibit leakage. The band offsets of various gate dielectrics including HfO2, Al2O3, Gd2O3, Si3N4, and SiO2 on III-V semiconductors such as GaAs, InAs, GaSb, and GaN have been calculated using the method of charge neutrality levels. Generally, the conduction band offsets are found to be over 1eV, so they should inhibit leakage for these dielectrics. On the other hand, SrTiO3 has minimal conduction band offset. The valence band offsets are also reasonably large, except for Si nitride on GaN and Sc2O3 on GaN which are 0.6–0.8eV. There is reasonable agreement with experiment where it exists, although the GaAs:SrTiO3 case is even worse in experiment.

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Band offsets of high K gate oxides on III-V semiconductors

Failure modes and mechanisms of AlGaN/GaN high-electron-mobility transistors are reviewed. Data from three de-accelerated tests are presented, which demonstrate a close correlation between failure modes and bias point. Maximum degradation was found in "semi-on" conditions, close to the maximum of hot-electron generation which was detected with the aid of electroluminescence (EL) measurements. This suggests a contribution of hot-electron effects to device degradation, at least at moderate drain bias (VDS 30-50 V), new failure mechanisms are triggered, which induce an increase of gate leakage current. The latter is possibly related with the inverse piezoelectric effect leading to defect generation due to strain relaxation, and/or to localized permanent breakdown of the AlGaN barrier layer. Results are compared with literature data throughout the text.

Reliability of GaN High-Electron-Mobility Transistors: State of the Art and Perspectives

Microscopic electroluminescence (EL) and photoluminescence (PL) measurements of high-biased AlGaN/GaN high-electron-mobility transistors are reported. We observed that the EL intensity reveals peaks around the edge of the channel and the electron temperature there is higher than the electron temperature at the center of the channel. These EL features were found to be consistent with the change in the junction temperature, which we locally estimated by comparing the PL data with measurements in raised ambient temperatures.

Optical study of high-biased AlGaN/GaN high-electron-mobility transistors

Large Schottky barriers were measured for Ni contacts formed on low Mg-doped p-GaN. In order to improve leaky Schottky characteristics, low-Mg doping was examined. This provided atomically flat surfaces and a low dislocation density of 5.5×108 cm−2. The Schottky barrier height (qφB) as high as 2.4±0.2 eV and n values of 1.84±0.06 were obtained from current–voltage measurements. These results are in good agreement with the prediction that the sum of qφB of n and p types adds up to the band gap. In the capacitance–voltage measurements, a transient response of capacitance was observed. This indicates that the evaluation of deep levels close to the valence band is possible, which could result in improvement of p-GaN growth.

Large Schottky barriers for Ni/p-GaN contacts

We directly evaluated the effect of dislocations on current–voltage (I–V) characteristics of Au/Ni/n-GaN Schottky contacts. A submicrometer Schottky dot array was formed by electron beam lithography, and I–V measurements were conducted using atomic force microscopy with a conductive probe. The sample, which has a free electron concentration of 5.8×1017 cm−3, showed that neither dislocations nor steps affect the I–V characteristics. These results indicate that, in fabricating short-gate FETs, gate Schottky contacts containing dislocations should not be considered a problem with respect to uniformity and reproducibility.

Correlation between current–voltage characteristics and dislocations for n-GaN Schottky contacts

Improved Carrier Confinement by a Buried p-Layer in the AlGaN/GaN HEMT Structure

We directly evaluated the effect of dislocations on current–voltage (I–V) characteristics of Au/Ni contacts formed on n-GaN grown by metalorganic chemical vapor deposition. The key feature of our evaluation is the combination of submicrometer Schottky dot array formation by electron-beam lithography and I–V measurement by atomic force microscopy with a conductive probe. The sample, which has a free electron concentration of 3×1017 cm−3, showed that neither mixed nor pure edge dislocations affect the I–V characteristics (Schottky barrier height, ideality factor, and reverse leakage current). Possible models to explain the results are discussed. On the other hand, it was found that a large structural defect with a diameter of a few hundred nanometers shorted the contact. The density, however, was less than 4×105 cm−2. These results indicate that, in fabricating short-gate field-effect transistors, gate Schottky contacts containing dislocations should not be considered a problem with respect to uniformity an...

Correlation between current–voltage characteristics and dislocations evaluated with submicrometer Schottky contacts on n-GaN grown by metalorganic chemical vapor deposition

Kenji Shiojima

Papers

Optical study of high-biased AlGaN/GaN high-electron-mobility transistors

Large Schottky barriers for Ni/p-GaN contacts

Correlation between current–voltage characteristics and dislocations for n-GaN Schottky contacts

Improved Carrier Confinement by a Buried p-Layer in the AlGaN/GaN HEMT Structure

Correlation between current–voltage characteristics and dislocations evaluated with submicrometer Schottky contacts on n-GaN grown by metalorganic chemical vapor deposition