Yueh-Ling Liu

Postoxidation Annealing Treatments to Improve Si/Ultrathin SiO2 Characteristics Formed by Nitric Acid Oxidation

We have succeeded in the fabrication of Si-based metal-oxide-semiconductor diodes with an ultrathin (i.e., ∼1.4 nm thick) silicon dioxide (SiO 2 ) layer whose leakage current density is lower than those for thermally grown SiO 2 layers with the same thickness. The leakage current density of the as-grown SiO 2 layers formed in concentrated nitric acid (HNO 3 ) solutions (i.e., ∼3 A/cm 2 at the forward gate bias, V G , of 1 V) is as high as those for thermal SiO 2 layers. The leakage current density is decreased to ∼1.5 and ∼1 A/cm 2 at V G = 1 V by postoxidation annealing (POA) at 400 and 500°C (in hydrogen), respectively, and the decreases are mainly attributed to the elimination of interface states and slow states, respectively. The vibrational frequency of longitudinal optical (LO) phonons of Si-O asymmetric stretching vibration increases with the POA temperature, while that of the transverse optical (TO) phonons remains constant. From the analysis of these vibrational frequencies, the atomic density of the as-grown SiO 2 layer and that after POA at 500°C are estimated to be 2.34 and 2.56 × 10 22 cm 3 , respectively, i.e., higher than that of crystalline SiO 2 bulk of 2.28 × 10 22 cm 3 . Measurements of the valence band spectra for the Si/SiO 2 structure show that the valence band discontinuity energy at the Si/SiO 2 interface is increased from 4.3 to 4.6 eV by POA at 500°C. Therefore, the other reason for the decrease in the leakage current density by POA is a reduction in the tunneling probability through SiO 2 caused by the increase in the band discontinuity energy. Postmetallization annealing treatment at 200°C in hydrogen performed after POA further decreases the leakage current density, showing that atomic hydrogen passivates defect states more effectively than molecular hydrogen.

Mechanism of Copper Removal from SiO2 Surfaces by Hydrogen Cyanide Aqueous Solutions

Hydrogen cyanide (HCN) aqueous solutions can remove copper (Cu) contaminants from SiO 2 surfaces completely even when the cleaning is performed using dilute (e.g., 0.027 wt %) HCN aqueous solutions at 25°C. When pH of the HCN solutions is set at 10, Cu contaminants with a concentration of ∼2 X 10 12 atoms/cm 2 can be removed below ∼3 × 10 9 atoms/cm 2 taking only 10 s. The concentrations of Cu + and [Cu(CN) 2 ] - ions in the HCN solutions drastically decrease with pH, while those of [Cu(CN) 3 ] 2- and [Cu(CN) 4 ] 3- ions increase. The dissociation probability of HCN exponentially increases with pH.Therefore, the improvement of the cleaning ability with pH is attributable to (i) an increase in the concentration of CN - ions, and (ii) the prevention of readsorption due to the formation of stable Cu-cyano complex ions. Measurements of surface Cu concentrations on the SiO 2 surfaces contaminated by the immersion in the CuCl 2 -containing HCN solutions clarify the mechanism of Cu removal. The rate-determining step is a reaction between SiO-CuCN on the surface and CN - ions in the solution to form [Cu(CN) 2 ] - complex ions. The equilibrium constant of this reaction is found to be 4.1 x 10 5 M. This large value shows that desorption of [Cu(CN) 2 ] - proceeds much more easily than adsorption.

SiC Cleaning Method by Use of Dilute HCN Aqueous Solutions

Metal contaminants, such as Cu on SiC surfaces, cannot be completely removed by use of the conventional RCA cleaning method. After RCA cleaning, no chemical oxide is formed on the SiC surfaces, and this chemical stability is attributable to the incomplete removal of metal contaminants by the RCA method because it removes metal contaminants by oxidation and subsequent etching. Cleaning of metal-contaminated SiC with hydrogen cyanide (HCN) aqueous solutions followed by the RCA cleaning (or vice versa) can remove them completely. It is concluded that strongly adsorbed metals and metals in the bottom regions on the rough SiC surfaces cannot be removed by the RCA and HCN methods, respectively. The HCN method can remove strongly adsorbed metals because of the high reactivity of cyanide ions, while metals in the bottom regions cannot be removed because of the necessity of the formation of bulky metal-cyanide complex ions for the removal process.

Mechanism of Ni Removal from Si Materials Using Hydrogen Cyanide Aqueous Solutions

The mechanism of Ni removal from SiO 2 -covered Si specimens by HCN aqueous solutions has been investigated by means of total reflection X-ray fluorescence spectroscopy and X-ray photoelectron spectroscopy measurements. Ni contaminants on the SiO 2 surface are present in the form of SiO-NiOH. The removal mechanism consists of two steps, i.e., an initial fast process followed by a slow process. The rate-determining steps of both the processes are of the first order with respect to the concentration of cyanide ions (CN - ). The fast and slow processes are tentatively attributed to the removal of SiO-NiOH on terraces and in sub-nanometer pores, respectively. The cleaning ability of the HCN aqueous solutions is much better than ammonia aqueous solutions, because of high reactivity to form nickel-cyanide complex ions and avoidance of readsorption of Ni(CN) 2- 4 complex ions in the solutions.

Voltage fluctuations in mesoscopic structures of bismuth

Abstract The resistance fluctuations with magnetic field in mesoscopic wires of bismuth have been investigated. The wires show conductance fluctuations (dG) and antilocalization effects, and show voltage fluctuations in Hall measurements at 4.2K and 1.5K. It is found that in the strong spin-orbit coupling case, the dG are reduced by 1 ∼ 2 order, and when the length of wire (L) shorter than the coherence length (Li), the voltage fluctuations are constant, and the conductance fluctuations are not universal and will diverge as dG=(e2/h)LiLt/L2 when the thermal length (Lt) is also shorter than Li.

Anti-localization effect in epitaxially grown thin Sb Films on BaF2

Abstract The low-temperature magnetoresistance of Sb thin films, which were prepared by hot wall epitaxial growth, has been studied. From the comparison of magnetic field dependence of resistance with theories of the two-dimensional (2D) weak localization, it is found that the inelastic scattering time varies as T −1 below 2.5K and as T −.4 above 2.5K, and the inelastic scattering lengths change to small with decreasing thickness.

Fluctuations, localization effect and AB effect in mesoscopic structures of bismuth

Abstract Conductance fluctuations, localization and AB effects in mesoscopic wires and rings of bismuth have been investigated. It is found that the fluctuations from the interference of the holes is much smaller than that from the electrons. The experimental results of conductance correction (ΔGL) from antilocalization effect support the four-probe theoretical formula [1] that it will diverge less than L−1 as the sample length (L) is decreased shorter than the coherent length (Li). The AB effect (h/e oscillation) has been confirmed in some ring samples at 1.5K.