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Molecular Beam Epitaxy Of Er <sub> 2 </ Sub> O <sub> 3 </ Sub> High-k Gate Dielectric Materials, Physical Properties And Preparation Of Silicon-based Magnetic Semiconductor Materials

Posted on:2008-07-11Degree:DoctorType:Dissertation
Country:ChinaCandidate:S ChenFull Text:PDF
GTID:1118360215984249Subject:Condensed matter physics
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Molecular beam epitaxy (MBE) is an ultra-high-vacuum (UHV)-based techniquefor producing high quality epitaxial structures with monolayer (ML) control. MBEhas a lot of unique advantages because it supplies precise controls of the film growthprocesses and conditions far from equilibrium. We have grown crystalline Er2O3high-k thin films and Si-based magnetic semiconductor films by MBE.Rare earth (RE) oxides have relatively high dielectric constants, large band gapsand high band offset with Si. Crystalline Er2O3 films epitaxially grown on Sisubstrates may be promising high-k dielectrics.Er2O3 thin films were grown at different substrate temperatures and underdifferent oxygen pressures by evaporating metallic Er source and inputtinglow-pressure oxygen. The epitaxial growth of Er203 films has been achieved on bothSi(001) and Si(111) substrates at a growth temperature of 700℃and an oxygenpressure of 7×10-6 Torr. The epitaxial relationship between Er2O3 and Si(001)substrate is as follows: Er2O3(110)//Si(001), Er2O3[001]//Si[110] or Er2O3[110]//Si[110]. The epitaxial relationship between Er2O3 and Si(111) is Er2O3(111)//Si(111),Er2O3[110]//Si[110].From the Atomic force microscopy (AFM) image, it was observed that thesurface of the Er2O3 film epitaxially grown on Si(001) substrate is covered byten-nanometer-size rectangular grains which are perpendicular to each other. Afterannealing at 450℃in O2 ambience, high resolution transmission electron microscopy(HRTEM) images show that the Er2O3 structure is well aligned with the Si structure,and a relatively sharp interface can be clearly revealed. The interface of Er2O3/Si(111)is sharper than the interface of Er2O3/Si(001). To measure the electrical properties ofthe Er2O3 films epitaxially grown on Si(001) substrates, capacitance-voltage (C-V)and current-voltage (Ⅰ-Ⅴ) measurements were carried out. The dielectric constant ofthe film with an EOT of 2.0 nm is 14.4. The leakage current density as small as 1.6×10-4 A/cm2 at a bias voltage of -1 V has been measured. At the inverse region, theleakage current density of 8×10-6A/cm2 remains constant. C-V and X-ray diffraction(XRD) characteristics of the film show us a superior time stability of the Er2O3 filmgrown on Si substrate. After several months exposure in atmosphere, no morphologic,structural or dielectric properties degradations were observed.The thermal stability of Er2O3 thin films grown epitaxially on Si substrates has been investigated for it is important to ideal high-k materials. The Er2O3 thin filmsgrown on Si(001) substrates are found to react with Si to form silicates at thetemperature of 450℃in N2 ambience, whereas O2 ambience can prevent the silicateformation even at the temperature of 600℃. However, at a high temperature of 900℃in either N2 or O2 ambience, Er2O3 films react with Si, and both silicate and SiO2 areformed in the films. In addition, the Er2O3 films grown on Si(111) substrates showpoorer thermal stability than those grown on Si(001) substrates. Er2O3 reacts with Sisubstrate when the film grown on Si(111) substrate is annealed at 450℃in O2ambience.Si-based magnetic semiconductors have broad application prospects. We tried togrow Mn doped Si thin films and Fe doped Si thin films in order to gain experience inthe growth of ferromagnetic semiconductors. After annealing at 225℃for 60 minutes,Mn5Si95 thin films without any ferromagnetic phase show ferromagnetism at thetemperature of 10K. The activation energy of the energy level induced by doping ofFe is 26meV through the admittance spectroscopy measurements. The Fe doped thinfilms with smooth surfaces were achieved at the substrate temperature of 150℃or200℃with the Si growth rate of 0.2 (?)/s.
Keywords/Search Tags:High k materials, Magnetic semiconductor, Molecular beam epitaxy, Er2O3
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