The scaling of complementary metal oxide semiconductor (CMOS) transistors has led to the silicon dioxide layer used as a gate dielectric becoming so thin that the gate leakage current becomes too large. This led to the replacement of SiO2 by a physically thicker layer of a higher dielectric constant or ‘high-K’ oxide such as hafnium oxide. Intensive research was carried out to develop these oxides into high quality electronic materials. In addition, the incorporation of Ge in the CMOS transistor structure has been employed to enable higher carrier mobility and performance. This review covers both scientific and technological issues related to the high-K gate stack – the choice of oxides, their deposition, their structural and metallurgical behaviour, atomic diffusion, interface structure, their electronic structure, band offsets, electronic defects, charge trapping and conduction mechanisms, reliability, mobility degradation and oxygen scavenging to achieve the thinnest oxide thicknesses. The high K oxides were implemented in conjunction with a replacement of polycrystalline Si gate electrodes with metal gates. The strong metallurgical interactions between the gate electrodes and the HfO2 which resulted an unstable gate threshold voltage resulted in the use of the lower temperature ‘gate last’ process flow, in addition to the standard ‘gate first’ approach. Work function control by metal gate electrodes and by oxide dipole layers is discussed. The problems associated with high K oxides on Ge channels are also discussed.

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High-K materials and metal gates for CMOS applications

Insulating and semiconducting molecular phosphonic acid (PA) self-assembled monolayers (SAMs) have been developed for applications in organic field-effect transistors (OFETs) for low-power, low-cost flexible electronics. Multifunctional SAMs on ultrathin metal oxides, such as hafnium oxide and aluminum oxide, are shown to enable (1) low-voltage (sub 2 V) OFETs through dielectric and interface engineering on rigid and plastic substrates, (2) simultaneous one-component modification of source–drain and dielectric surfaces in bottom-contact OFETs, and (3) SAM-FETs based on molecular monolayer semiconductors. The combination of excellent dielectric and interfacial properties results in high-performance OFETs with low-subthreshold slopes down to 75 mV dec−1, high Ion/Ioff ratios of 105–107, contact resistance down to 700 Ω cm, charge carrier mobilities of 0.1–4.6 cm2 V−1 s−1, and general applicability to solution-processed and vacuum-deposited n-type and p-type organic and polymer semiconductors.

Multifunctional phosphonic acid self-assembled monolayers on metal oxides as dielectrics, interface modification layers and semiconductors for low-voltage high-performance organic field-effect transistors

π‐σ‐Phosphonic Acid Organic Monolayer/Sol–Gel Hafnium Oxide Hybrid Dielectrics for Low‐Voltage Organic Transistors

Magnetron sputtered HfO2 layers formed on a heated Si substrate were studied by spectroscopic ellipsometer (SE), x-ray diffraction (XRD), Fourier transform infrared (FTIR), and x-ray photoelectron spectroscopy (XPS) depth profiling techniques. The results show that the formation of a SiOx suboxide layer at the HfO2/Si interface is unavoidable. The HfO2 thickness and suboxide formation are highly affected by the growth parameters such as sputtering power, O2/Ar gas ratio during sputtering, sputtering time, and substrate temperature. XRD spectra show that the deposited film has (111) monoclinic phase of HfO2, which is also supported by FTIR spectra. The atomic concentration and chemical environment of Si, Hf, and O have been measured as a function of depth starting from the surface of the sample by XPS technique. It shows that HfO2 layers of a few nanometers are formed at the top surface. Below this thin layer, Si–Si bonds are detected just before the Si suboxide layer, and then the Si substrate is reached ...

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Interfacial and structural properties of sputtered HfO2 layers

In this study, low-voltage copper phthalocyanine (CuPc)-based organic field-effect transistors (OFETs) are demonstrated utilizing solution-processed bilayer high-k metal-oxide (Al(2)O(y)/TiO(x)) as gate dielectric. The high-k metal-oxide bilayer is fabricated at low temperatures (< 200 °C) by a simple spin-coating technology and can be controlled as thin as 45 nm. The bilayer system exhibits a low leakage current density of less than 10(-5) A/cm(2) under bias voltage of 2 V, a very smooth surface with RMS of about 0.22 nm and an equivalent k value of 13.3. The obtained low-voltage CuPc based OFETs show high electric performance with high hole mobility of 0.06 cm(2)/(V s), threshold voltage of -0.5 V, on/off ration of 2 × 10(3) and a very small subthreshold slope of 160 mV/dec when operated at -1.5 V. Our study demonstrates a simple and robust approach that could be used to achieve low-voltage operation with solution-processed technique.

Low-Voltage Organic Field-Effect Transistors (OFETs) with Solution-Processed Metal-Oxide as Gate Dielectric

The quality of the interface between a HfO2 high-k gate dielectric and the Si substrate directly influences its electrical properties The chemical composition of the interfacial region of HfO2 deposited on a SiO2∕Si(100) substrate by pulsed liquid injection metal organic chemical vapor deposition at 430 and 550°C was investigated by medium energy ion scattering, angular resolved x-ray photoemission spectroscopy analysis, and high resolution transmission electron microscopy It is shown that the HfO2∕SiO2 interface is abrupt with low roughness and no silicate The interface roughness with SiO2 is found to be close to that generally measured in silicon technology (silicon oxide above silicon substrates) [E A Irene, Solid-State Electron, 45, 1207 (2001)] The analysis of the experimental results indicates that the deposition technique does not lead to the formation of an extended silicate layer at the HfO2∕SiO2 interface

Chemical interface analysis of as grown HfO2 ultrathin films on SiO2

Large-area growth of continuous transition metal dichalcogenides (TMDCs) layers is a prerequisite to transfer their exceptional electronic and optical properties into practical devices. It still represents an open issue nowadays. Electric and magnetic doping of TMDC layers to develop basic devices such as p-n junctions or diluted magnetic semiconductors for spintronic applications are also an important field of investigation. Here, we have developed two different techniques to grow MoSe$_2$ mono- and multi-layers on SiO$_2$/Si substrates over large areas. First, we co-deposited Mo and Se atoms on SiO$_2$/Si by molecular beam epitaxy in the van der Waals regime to obtain continuous MoSe$_2$ monolayers over 1 cm$^2$. To grow MoSe$_2$ multilayers, we then used the van der Waals solid phase epitaxy which consists in depositing an amorphous Se/Mo bilayer on top of a co-deposited MoSe$_2$ monolayer which serves as a van der Waals growth template. By annealing, we obtained continuous MoSe$_2$ multilayers over 1 cm$^2$. Moreover, by inserting a thin layer of Mn in the stack, we could demonstrate the incorporation of up to 10 \% of Mn in MoSe$_2$ bilayers.

Van der Waals solid phase epitaxy to grow large-area manganese-doped MoSe$_2$ few-layers on SiO$_2$/Si

The quality of the interface between a HfO2 high-k gate dielectric and the Si substrate directly influences its electrical properties. The chemical composition of the interfacial region of HfO2 deposited on a SiO2/Si(100) substrate by pulsed liquid injection metal organic chemical vapor deposition at 430 and 550 °C was investigated by medium energy ion scattering, angular resolved x-ray photoemission spectroscopy analysis, and high resolution transmission electron microscopy. It is shown that the HfO2/SiO2 interface is abrupt with low roughness and no silicate. The interface roughness with SiO2 is found to be close to that generally measured in silicon technology (silicon oxide above silicon substrates) [ E. A. Irene, Solid-State Electron., 45, 1207 (2001) ]. The analysis of the experimental results indicates that the deposition technique does not lead to the formation of an extended silicate layer at the HfO2/SiO2 interface

F. Pierre

Papers

Chemical interface analysis of as grown HfO2 ultrathin films on SiO2

Van der Waals solid phase epitaxy to grow large-area manganese-doped MoSe$_2$ few-layers on SiO$_2$/Si

Chemical interface analysis of as grown HfO[sub 2] ultrathin films on SiO[sub 2]