Hitoo Iwasa

Nitric acid oxidation of Si to form ultrathin silicon dioxide layers with a low leakage current density

Ultrathin silicon dioxide (SiO2) layers with excellent electrical characteristics can be formed using the nitric acid oxidation of Si (NAOS) method, i.e., by immersion of Si in nitric acid (HNO3) solutions. The SiO2 layer formed with 61 wt % HNO3 at its boiling temperature of 113 °C has a 1.3 nm thickness with a considerably high density leakage current. When the SiO2 layer is formed in 68 wt % HNO3 (i.e., azeotropic mixture with water), on the other hand, the leakage current density (e.g., 1.5 A/cm2 at the forward gate bias, VG, of 1 V) becomes as low as that of thermally grown SiO2 layers, in spite of the nearly identical SiO2 thickness of 1.4 nm. Due to the relatively low leakage current density of the NAOS oxide layer, capacitance–voltage (C–V) curves can be measured in spite of the ultrathin oxide thickness. However, a hump is present in the C–V curve, indicating the presence of high-density interface states. Fourier transformed infrared absorption measurements show that the atomic density of the SiO...

Electroless Nickel Plating on Silicon

In the study of electroless Ni plating of Si wafers with p‐n junctions using conventional solutions, a pronounced difference in plating rate between p‐ and n‐type surfaces is observed. Further experiments show that rate difference probably should not only be attributed to the photovoltaic effect generated at the p‐n junctions but also to the electronegativity difference between p‐ and n‐type Si. The latter effect can be changed by addition of such material as or to the plating solution. Whereas addition increases the rate difference, EDTA addition decreases it. This fact which can be put to practical use gives an extra support for the explanation given above.

Passivation of defect states in surface and edge regions on pn-junction Si solar cells by use of hydrogen cyanide solutions

The local photovoltage of the pn-junction single-crystalline silicon solar cells observed by spot light scanning gradually decreases in the vicinity of edges. The energy conversion efficiency is increased by shadowing the edge regions where the local photovoltage is lower, showing that the defect density is high in the edge regions. From the analysis of the local photovoltage, the spacial distribution of defect states is obtained. The cyanide method, i. e., immersion of solar cells in HCN solutions at room temperature, increases the local photovoltage and increases the energy conversion efficiency.

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.

Complete Removal of Copper Contaminants on Bare Silicon Surfaces by Use of HCN Aqueous Solutions

Due to the strong reactivity of CN - ions to form Cu-cyano complex ions and high stability of the ions in aqueous solutions, Cu contaminants on bare Si surfaces can be removed to less than ∼3 X 10 9 atoms/cm 2 by immersion in hydrocyanic acid (HCN) aqueous solutions, and cleaning can be performed with dilute (i.e., 0.027 - 0.0027 wt %) solutions at room temperature. Hydrofluoric acid plus hydrogen peroxide mixture can also completely remove Cu contaminants from the bare Si surfaces, but in this case, the Si surfaces become considerably rough due to etching. HCN solutions do not etch Si when pH is adjusted below 9, and consequently, the Si surface is not roughened at all. Cu contaminants are present on Si in the form of metallic islands and their size decreases with the HCN cleaning periods. Semi-log plots of the Cu concentration vs cleaning time consist of two linear lines for the initial fast and subsequent slow processes. The fast and slow processes are attributable to removal of Cu atoms bound to Cu atoms only and that of Cu atoms directly bound to Si, respectively. The rate constant for the fast and slow processes are determined to be 4.4 and 0.016/s, respectively.

Nature and origin of dark defects in GaAs LED's

Dark defects of GaAs LEDs diffused with Zn through coated SiO2films are observed in EBIC images as well as in EL images. The results of etch-pit observations have shown that some dislocations involved in the initial crystal are pinned by the diffused Zn atoms and multiply under a stress induced during the diffusion process, It is concluded that the dark defects are attributed to the dislocation loops where minority carriers recombine nonradiatively.

Contact Resistance of Electroless Nickel on Silicon

The electric resistance of electroless nickel contacts to p‐type silicon is investigated. The contact resistance increases by a factor of 100 when the resistivity of substrate silicon increases 10 times. It is also found that the contact resistance greatly increases on increasing the heat treatment temperature except for an intermediate temperature range between 450° and 600°C. The x‐ray and chemical analyses show that the heat treatment at high temperatures causes a sharp increase in content of the nickel suicide phase accompanied by a decrease in the nickel phase. The observed change in contact resistance due to heat treatment is discussed in relation to the chemical transformations in the nickel‐silicon‐phosphorus system.