Ferroelectric thin films have been extensively studied for nonvolatile memory applications. Among these applications ferroelectric field-effect transistors (FETs) such as MFSFET, MFISFET and MFMISFET etc., in which the surface potential of Si is dependent on the remnant polarization of ferroelectrics, have two major advantages over destructive read out (DRO)-type 1T-1C structure; that is, smaller cell size and non-destructive read out (NDRO). MFSFETs have a structure in which ferroelectric thin films are directly deposited on Si. However, it is difficult to obtain good interface properties between ferroelectrics and Si because of the problems of interface reaction by interdiffusion and the formation of a thin native oxide with poor quality, which can be the cause of charge injection, during high temperature deposition and annealing processes. In order to overcome these problems, an intermediate insulating buffer layer which can prevent the interdiffusion and suppress undesirable low dielectric layer between ferroelectric layer and Si substrate should be inserted between ferroelectric and Si, resulting in MFIS or MFMIS structures. In these stuctures, a ferroelectric capacitor and a dielectric capacitor are connected in series and the applied gate voltage is split into the ferroelectric and the insulating layers and lower voltage is applied to ferroelectric layer with higher dielectric constant. Perovskite-type ferroelectric such as PZT family and Bi-layered structure ferroelectric like SBT and BLT, which have been studied for 1T-1C structures, have been also investigated for ferroelectric FET applications. However, these materials have high dielectric constants that result in requiring high operating voltage when they are used in MFIS or MFMIS structures. So, ferroelectrics with relatively low dielectric constants are more suitable for low voltage operation. A ferroelectric lead germanate, $Pb_5Ge_3O_11$, has a comparatively low dielectric constant of about 40. Its spontaneous polarization exists along uniaxial polar direction (c-axis) and exhibits a value of 4.8 μC/㎠ with a coercive field of about 16 kV/cm. So, it is very important to obtain highly c-axis preferred orienated films to use its ferroelectric property. It has a crystallographic structure belonging to the trigonal structure (ferroelectric phase) at room temperature and transforms to the hexagonal structure (paraelectric phase) above the Curie temperature of 177 ℃. Additionally, insulator materials with higher dielectric constant than $SiO_2$ are more favorable to low voltage operation. High dielectric materials such as $Al_2O_3$, $Y_2O_3$, $CeO_2$, $Ta_2O_5$, $ZrO_2$, $HfO_2$, $TiO_2$, and $SrTiO_3$, etc. have been studied as alternatives for silicon oxide and nitride. However, $Y_2O_3$ and $Al_2O_3$ do not provide significant advantages over silicon oxide and nitride because of the relatively low dielectric constants. Ultrahigh dielectric materials such as $SrTiO_3$ have been predicted to cause poor short channel effects due to the fringing field induced barrier lowering effect. $Ta_2O_5$ and $TiO_2$, etc. are thermally unstable in contact with Si and may form silicides or $SiO_x$ interfacial layers. $HfO_2$ and $ZrO_2$ are promising materials since they are thermodynamically stable in contact with Si. Especially, $HfO_2$ has many desirable properties; high dielectric constant (~30), high heat of formation (271 kcal/mol), etc. Hf is supposed to suppress native oxide by forming hafnium oxide or silicate because of high heat of formation. In addition, Hafnium silicate is also expected relatively high dielectric constant of ~15, considering chemical similarity with zirconium silicate. In this work we present the results on fabrication of $Pb_5Ge_3O_11$ (PGO) with highly c-axis preferred orientation on various substrates such as $SiO_2/Si$, Pt/Ti/Si and $IrO_x$/Si and $HfAl_xO_y$ (x=0 or 0.81) with good interface propeties on p-type Si(100) substrate by pulsed laser deposition and post- annealing process and finally memory characteristics and thermal stability of Pt/PGO/$HfAl_xO_y$(x=0 or 0.81)/Si (MFIS) capacitors.
PGO thin films with highly c-axis preferred orientation and no second phase were obtained by ablating PGO target in a little different $O_2$ pressure, depending on substrate type; $2×10^{-3}Torr for $SiO_2$/Si, $1×10^{-4}$ Torr for Pt/Ti/Si and $5×10^{-4}$ Torr for $IrO_x$/Si and post-annealing at 550 ℃ in $O_2$ ambient. C-axis orientation of PGO films were mainly responsible to the composition of as-deposited films rather than subtrate type. $HfAl_xO_y$ (x=0 or 0.81) films with little voltage hysteresis (~50 mV) were also prepared on unheated Si substrate by laser ablation of Hf and Hf/Al joint target and post-annealing process at 750℃ in $O_2$ ambient. Finally, Pt/PGO/$HfAl_xO_y$ (x=0 or 0.81)/Si (MFIS) capacitors were fabricated by combining two processes and memory window due to ferroelectric polarization were observed in C-V curves of Pt/PGO(1200A)/$HfO_x$(220A)/Si and Pt/PGO(1200A)/$HfAl_xO_y(150A)/Si capacitors and linearly increased as gate bias voltage up to 4.5 V and then decreased due to charge injection at interface with Si.
