Yongfei Zhu

METALORGANIC CHEMICAL VAPOR DEPOSITION OF FERROELECTRIC SRBI2TA2O9 THIN FILMS

Ferroelectric layered‐oxides SrBi2Ta2O9 thin films were prepared on Pt coated Si wafers and single‐crystal sapphire by metalorganic chemical vapor deposition (MOCVD). The films were specular and crack‐free and showed complete crystallization at temperatures between 650 and 700 °C. Good ferroelectric properties were obtained for a 200 nm thick film with Pt electrodes: 2Pr and Ec were about 8.3 μC/cm2 and 60 kV/cm, respectively. The leakage currents were as low as 8×10−9 A/cm2 at 150 kV/cm. The films also showed fatigue‐free characteristics: no fatigue was observed up to 1.4×1010 switching cycles. These high quality MOCVD films make high‐intensity (≳1 Mbit) nonvolatile memory devices possible.

Low temperature fabrication and properties of sol-gel derived (111) oriented Pb(Zr1−xTix)O3 thin films

A novel processing method is developed for preparing sol-gel derived Pb(Zr1−xTix)O3 (x=0.47) thin films on Pt/Ti/SiO2/Si substrates. Using a modified precursor solution and a rapid heat treatment without pyrolysis, it was possible to obtain highly (111) oriented PZT thin films with high perovskite content at a low annealing temperature of 550 °C. The low temperature processing was assisted by taking advantage of the heterogeneous nucleation of the PZT films, which reduces the activation energy for perovskite phase formation. Using this method, the PZT thin films exhibited better dielectric and ferroelectric properties at 550 °C than those reported by other methods. For example, the PZT films annealed at 550 °C showed a well-saturated hysteresis loop at an applied voltage of 5 V with Pr and Ec of 12 μC/cm2 and 38 kV/cm, and their dielectric constant and dissipation factor at a frequency of 100 kHz were 410 and 0.021, respectively. The leakage current density was lower than 10−8 at an applied electric field...

Size effects of 0.8SrBi2Ta2O9–0.2Bi3TiNbO9 thin films

The size effects of 0.8SrBi2Ta2O9–0.2Bi3TiNbO9 thin films, prepared by metalorganic deposition technique, were studied by determining how the ferroelectric properties vary with film thickness and grain size. It was found that the ferroelectric properties were determined by the grain size, and not by the thickness of the film in our studied thickness range of 80–500 nm. A 80 nm thick film showed good ferroelectric properties similar to the 500 nm thick film. The possible mechanisms for the size effects in SBT–BTN films are discussed.

Laser-assisted low temperature processing of Pb(Zr, Ti)O3 thin film

A method for lowering the processing temperature of PbZr1−xTixO3 (PZT) films was developed utilizing a laser-assisted two-step process. In the first step, perovskite phase was initiated in the PZT films to a furnace anneal at low temperatures in the range of 470–550 °C, depending on the Zr/Ti ratio. Later, the films were laser annealed (using KrF excimer laser) at room temperature to grow the perovskite phase, and to improve microstructure and ferroelectric properties. It was found that this two-step process was very effective in producing excellent quality ferroelectric PZT films at low temperatures. It should be noted that although laser annealing of amorphous and/or pyrochlore films directly (one-step process) produced perovskite phase, the ferroelectric properties of these films, irrespective of the composition, were rather unattractive. Some possible reasons for the ineffectiveness of the one-step process were discussed.

SrBi2Ta2O9 thin films made by liquid source metal-organic chemical vapor deposition

A liquid source metal-organic chemical vapor deposition system was installed to deposit SrBi 2 Ta 2 O 9 (SBT) thin films on sapphire and Pt/Ti/SiO 2 /Si substrates. The process parameters such as deposition temperature and pressure, and ratio of Sr: Bi: Ta in the precursor solutions were optimized to achieve stoichiometric films with good reproducible ferroelectric properties. It was found that the nucleation of SBT started at a deposition temperature close to 500 °C and grain growth dominated at 700 °C and higher temperatures. With increasing deposition temperatures, the grain size of SBT thin films increased from 0.01 μ m to 0.2 μ m; however, the surface roughness and porosity of the films also increased. To fabricate specular SBT films, the films had to be deposited at lower temperature and annealed at higher temperature for grain growth. A two-step deposition process was developed which resulted in high quality films in terms of uniformity, surface morphology, and ferroelectric properties. The key to the success of this process was the homogeneous nucleation sites at lower deposition temperature during the first step and subsequent dense film growth at higher temperature. The two-step deposition process resulted in dense, homogeneous films with less surface roughness and improved ferroelectric properties. SBT thin films with a grain size of about 0.1 μm exhibited the following properties: thickness: 0.16–0.19 μ m; 2 P r : 7.8–11.4 μ C/cm 2 at 5 V; E c : 50–65 kV/cm; I leakage : 8.0–9.5 × 10 −9 Acm −2 at 150 kV/cm; dielectric constant: 100–200; and fatigue rate: 0.94–0.98 after 10 10 cycles at 5 V.

Structural properties and band offset determination of p-channel mixed As/Sb type-II staggered gap tunnel field-effect transistor structure

The structural properties and band offset determination of p-channel staggered gap In0.7Ga0.3As/GaAs0.35Sb0.65 heterostructure tunnel field-effect transistor (TFET) grown by molecular beam epitaxy (MBE) were investigated. High resolution x-ray diffraction revealed that the active layers are strained with respect to “virtual substrate.” Dynamic secondary ion mass spectrometry confirmed an abrupt junction profile at the In0.7Ga0.3As/GaAs0.35Sb0.65 heterointerface and minimal level of intermixing between As and Sb atoms. The valence band offset of 0.37 ± 0.05 eV was extracted from x-ray photoelectron spectroscopy. A staggered band lineup was confirmed at the heterointerface with an effective tunneling barrier height of 0.13 eV. Thus, MBE-grown staggered gap In0.7Ga0.3As/GaAs0.35Sb0.65 TFET structures are a promising p-channel option to provide critical guidance for the future design of mixed As/Sb type-II based complementary logic and low power devices.

Energy band alignment of atomic layer deposited HfO2 on epitaxial (110)Ge grown by molecular beam epitaxy

The band alignment properties of atomic layer HfO2 film deposited on epitaxial (110)Ge, grown by molecular beam epitaxy, was investigated using x-ray photoelectron spectroscopy. The cross-sectional transmission electron microscopy exhibited a sharp interface between the (110)Ge epilayer and the HfO2 film. The measured valence band offset value of HfO2 relative to (110)Ge was 2.28 ± 0.05 eV. The extracted conduction band offset value was 2.66 ± 0.1 eV using the bandgaps of HfO2 of 5.61 eV and Ge bandgap of 0.67 eV. These band offset parameters and the interface chemical properties of HfO2/(110)Ge system are of tremendous importance for the design of future high hole mobility and low-power Ge-based metal-oxide transistor devices.

Ultra-high frequency photoconductivity decay in GaAs/Ge/GaAs double heterostructure grown by molecular beam epitaxy

GaAs/Ge/GaAs double heterostructures (DHs) were grown in-situ using two separate molecular beam epitaxy chambers. High-resolution x-ray rocking curve demonstrates a high-quality GaAs/Ge/GaAs heterostructure by observing Pendellosung oscillations. The kinetics of the carrier recombination in Ge/GaAs DHs were investigated using photoconductivity decay measurements by the incidence excitation from the front and back side of 15 nm GaAs/100 nm Ge/0.5 μm GaAs/(100)GaAs substrate structure. High-minority carrier lifetimes of 1.06-1.17 μs were measured when excited from the front or from the back of the Ge epitaxial layer, suggests equivalent interface quality of GaAs/Ge and Ge/GaAs. Wavelength-dependent minority carrier recombination properties are explained by the wavelength-dependent absorption coefficient of Ge.

Ferroelectric SrBi2Ta2O9 thin films made by one and two step metalorganic chemical vapor deposition

Ferroelectric SeB12Ta2O9 (SBT) thin films have been deposited on sapphire, S1 and Pt/T1/SiO2/S1 substrates by using one and two step MOCVD processes. It was found that nucleation of SBT started at a deposition temperature close to 500 degrees C and grain growth dominated at 700 degrees C or above. With increasing deposition temperature, the grain size of SBT thin films increased from 0.01 micrometers to 0.2 micrometers , but surface roughness and porosity of the films also increased. Therefore, in order to obtain dense and homogeneous SBT films that have low surface roughness, the films have to be deposited at a low temperature for nucleation, then be annealed at a higher temperature for grain growth, which is defined as a one-step deposition process. An alternate approach is to use a two-step deposition process. The first step is a nucleation step, to make a very thin nucleation layer of the desired materials or buffer layer at lower temperature. Even thought heterogeneous nucleation is preferred, homogeneous nucleation also takes place because the heterogeneous nucleation can not grow at the low temperature. The second step is a grain growth step: grains will grow from the nucleation layer or buffer layer at higher temperature. In this manner, high quality SBT thin films were obtained. The SBT thin films with grain size about 0.1 micrometers exhibit following properties: thickness: 0.16-0.19 micrometers , 2Pr:7.8-11 4 (mu) C/cm2 at 5V, Ec. 50-65 kV/cm, Ileakage: 8.0-9.5 X 10-9 A/cm2 at 150 kV/cm, dielectric constant 100-200, fatigue rate: 0.94-0.98 after 1010 cycles at 5V. The interface between SBT film and substrate, surface roughness, thickness uniformity, microstructures and ferroelectric properties of SBT thin films were also investigated.