Mohammad Shahjahan

Fabrication of Resonance Tunnel Diode by γ-Al2O3/Si Multiple Heterostructures

Fabrication of epitaxial γ-Al2O3(111)/Si(111) heterostructures with smooth surfaces for resonance tunnel diode structures is presented in this study. γ-Al2O3 layers were fabricated by molecular beam epitaxy (MBE) and Si layers were fabricated by a mini e-beam evaporator. Epitaxial growth and surface morphology of these layers were studied by reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM). The obtained root mean square (RMS) values of the surface roughness of these layers were 0.39 nm for γ-Al2O3 (3 nm) on Si(111)-substrate and 0.65 nm for Si (4 nm) on γ-Al2O3/Si(111)-substrate. Conduction band offset of the γ-Al2O3/Si heterostructure was studied by Fowler-Nodheim tunneling current measurement, and the conduction band offset (ΔEc) value of 2.2 eV was obtained. Then, resonant tunneling diode (RTD) structures with double and triple barriers were fabricated using this γ-Al2O3(111)/Si(111) heterostructure. Electrical properties of these RTDs were studied to find negative differential resistance (NDR). The NDR at room temperature was observed in both structures for the first time with a peak to valley (P/V) current ratio of 3.0 for the double barrier RTD, and 4.5 for the triple barrier RTD.

Fabrication and Electrical Characterization of Ultrathin Crystalline Al2O3 Gate Dielectric Films on Si(100) and Si(111) by Molecular Beam Epitaxy

Fabrication of ultrathin crystalline Al2O3 (equivalent oxide thickness (EOT)=1.5–2 nm) on Si substrates by molecular beam epitaxy (MBE) and the electrical properties of these films were discussed. In the electrical measurements, a high breakdown field (8–10 MV/cm) and extremely low leakage current density ~ 10-8 A/cm2 at a field of 3 MV/cm were obtained. The dielectric constant of these films was calculated from high-frequency C–V measurement and its value was obtained to be 7.5. The interface state density (Dit) at the crystalline Al2O3/Si interface was calculated from quasi-static capacitance–voltage (C–V) measurement and its value was obtained to be 1–3×1011 eV-1cm-2.

Current–Voltage Characteristics of γ-Al2O3/epi-Si Resonant Tunneling Diodes with Different Quantum Well Thicknesses

The fabrication of double-barrier resonant tunneling diodes (DBRTDs) using γ-Al2O3/epitaxial-Si heterostructures with different well thicknesses and different barrier thicknesses was studied. Current–voltage characteristics of the DBRTDs were investigated to determine the relationships between the peak-to-valley current ratio and the quantum well thickness, and between the peak current density and the barrier thickness for the maximum peak-to-valley current ratio (PVCR) at room temperature. In this study, we confirmed a maximum peak-to-valley current ratio of 26 at room temperature with a quantum well (epi-Si) thickness of 3 nm and a barrier (γ-Al2O3) thickness of 2 nm. A comparison between the theoretical and experimental peak voltage positions for a negative differential resistance was performed, indicating good agreement. A lower peak current density of few mA/cm2 was obtained.

Fabrication of crystalline HfO2 high-κ dielectric films deposited on crystalline γ-Al2O3 films

Crystalline HfO2/γ-Al2O3 gate stacks were successfully fabricated by evaporating the HfO2 film on crystalline γ-Al2O3/Si substrates at 500°C. In the fabrication, crystalline γ-Al2O3 assisted the crystallization of the HfO2 film, which was deposited without the degradation of surface morphology. The electrical characteristics of the crystalline HfO2/γ-Al2O3 stacked dielectric and amorphous HfO2 unstacked dielectric were compared. The leakage current density of the stacked dielectric was lower than that of the unstacked dielectric. The HfO2 layer deposited on the crystalline γ-Al2O3/Si showed a higher dielectric constant than the amorphous HfO2 unstacked dielectric. It was also observed that the frequency dependence of the flat-band voltage shift of the stacked dielectric was negligible and different from that of the unstacked dielectric. These results indicate that crystalline γ-Al2O3 films prevented the formation of an interface layer between HfO2 and Si substrates. The crystalline γ-Al2O3 films work well as buffer layers and may be available for future high-κ gate stack application.

Effect of Annealing on Physical and Electrical Properties of Ultrathin Crystalline γ-Al2O3 High-k Dielectric Deposited on Si Substrates

Ultrathin crystalline γ-Al2O3 films with an equivalent oxide thickness (EOT) of 1.3 nm to 2.5 nm have been fabricated on Si substrates by molecular beam epitaxy and annealed in various atmospheres at different temperatures (300–700°C). The effect of the annealing on the chemical composition, crystalline property, surface morphology and electrical properties of the ultrathin γ-Al2O3 films has been studied. An improvement in the electrical properties after annealing was observed. It was also observed that the nitrogen atoms were incorporated into the γ-Al2O3 (γ-Al2O3:N) films during annealing at higher temperatures. No detectable pits or pinholes were observed on the surfaces after annealing and the crystalline property remained unchanged during annealing.

Fabrication of metal–oxide–semiconductor field-effect transistors using crystalline γ‐Al2O3 films as the gate dielectrics

Crystalline γ‐Al2O3 films were employed as high-κ gate dielectrics in metal–oxide–semiconductor field-effect transistors (MOSFETs) and characterization of these devices was performed. The crystalline dielectric was deposited with thicknesses of 4.0–4.5nm by mixed source molecular beam epitaxy and the capacitance equivalent thicknesses obtained were 2.7–2.9nm. The MOSFETs had exceptionally steep subthreshold slopes (63–67mV∕decade), relatively low negative fixed charge densities (5–7×1012cm−2) and interface state densities (2–3×1011eV−1cm−2). The maximum values of the effective carrier mobilities were 145cm2∕Vs for electrons and 85cm2∕Vs for holes.

Electrical properties of crystalline γ-Al2O3 films using conductive-AFM and MISFETs with Aluminum gates

1.Introduction For future LSI applications, it has been proposed to replace the traditional SiO2 with the high-k materials such as Al2O3, ZrO2, HfO2, Si3N4 and Y2O3 for gate dielectrics. In our laboratory, crystalline γ-Al2O3 films have been studied and grown on Si substrate successfully using a hybrid source MBE [1]. It has a number of applications in Si devices such as quantum well devices and silicon-on-insulator technology [2]. The electrical properties of the γ-Al2O3 have been reported [3], which describes no SiO2 interface layer between Si and Al2O3, high breakdown field comparable with SiO2, and low leakage current compared with other insulators. Therefore, crystalline γ-Al2O3 films were expected as an attractive candidate for high-k materials. However, evaluations of crystalline γ-Al2O3 films might be not enough for several applications. Because crystalline films usually have crystal defects or grain boundaries, which may have a bad influence on the characteristics. In this report, we evaluated the influence of crystal defects or grain boundaries by comparison of breakdown voltage among conductive AFM’s cantilevers and MIS structures that has several electrode sizes. Furthermore, we fabricated n-channel MISFETs with Aluminum gates using the crystalline film as gate dielectrics and confirmed good characteristics.