Phongsaphak Sittimart

Epitaxial growth of β-FeSi2 thin films on Si(111) substrates by radio frequency magnetron sputtering and their application to near-infrared photodetection

β-FeSi2 thin films were epitaxially grown on p-type Si(111) substrates at a substrate temperature of 560 °C and Ar pressure of 2.66 × 10−1 Pa by radio-frequency magnetron sputtering (RFMS) using a sintered FeSi2 target, without postannealing. The resultant n-type β-FeSi2/p-type Si heterojunctions were evaluated as near-infrared photodiodes. Three epitaxial variants of β-FeSi2 were confirmed by X-ray diffraction analysis. The heterojunctions exhibited typical rectifying action at room temperature. At 300 K, the heterojunctions showed a substantial leakage current and minimal response for irradiation of near-infrared light. At 50 K, the leakage current was markedly reduced and the ratio of the photocurrent to dark current was considerably enhanced. The detectivity at 50 K was estimated to be 3.0 × 1011 cm Hz1/2/W at a zero bias voltage. Their photodetection was inferior to those of similar heterojunctions prepared using facing-target direct-current sputtering (FTDCS) in our previous study. This inferiority is likely because β-FeSi2 films prepared using RFMS are located in plasma and are damaged by it.

Carrier transportation properties and series resistance of n-type β-FeSi2/p-type Si heterojunctions fabricated by RF magnetron sputtering

Heterojunctions composed of n-type β-FeSi2 thin films and p-type Si(111) substrates were formed by radio frequency magnetron sputtering at an Ar pressure of 2.66 × 10−1 Pa at a substrate temperature of 560 °C. The current density–voltage (J–V) curves of the heterojunctions measured in the dark and under illumination at room temperature showed a large leakage current under reverse bias conditions and a weak response to near-infrared (NIR) light irradiation. From the results of the analysis of dark forward J–V curves, the dominant carrier transport mechanisms at V ≤ 0.15 V and V > 0.15 V were considered a recombination process and a space-charge-limited current process, respectively. Both capacitance–voltage and conductance–voltage characteristics at room temperature were measured and analyzed as a function of applied frequency (f) ranging from 20 kHz to 2 MHz in order to estimate the series resistance (R s) by the Nicollian–Brews method. R s was estimated as 77.79 Ω at 20 kHz. It decreased to 14.16 Ω at 2 MHz, which is expected because the charges at the interface states cannot follow the AC signal at high f values.

Computation of Heterojunction Parameters at Low Temperatures in Heterojunctions Comprised of n-Type β-FeSi2 Thin Films and p-Type Si(111) Substrates Grown by Radio Frequency Magnetron Sputtering

In this study, n-type β-FeSi2/p-type Si heterojunctions, inside which n-type β-FeSi2 films were epitaxially grown on p-type Si(111) substrates, were created using radio frequency magnetron sputtering at a substrate temperature of 560°C and Ar pressure of  Pa. The heterojunctions were measured for forward and reverse dark current density-voltage curves as a function of temperature ranging from 300 down to 20 K for computation of heterojunction parameters using the thermionic emission (TE) theory and Cheung’s and Norde’s methods. Computation using the TE theory showed that the values of ideality factor () were 1.71 at 300 K and 16.83 at 20 K, while the barrier height () values were 0.59 eV at 300 K and 0.06 eV at 20 K. Both of the and values computed using the TE theory were in agreement with those computed using Cheung’s and Norde’s methods. The values of series resistance () computed at 300 K and 20 K by Norde’s method were 10.93 Ω and 0.15 MΩ, respectively, which agreed with the values found through computation by Cheung’s method. The dramatic increment of value at low temperatures was likely attributable to the increment of value at low temperatures.

Interface State Density and Series Resistance of n-Type Nanocrystalline FeSi2/p-Type Si Heterojunctions Formed by Utilizing Facing-Target Direct-Current Sputtering

n-Type nanocrystalline FeSi2/p-type Si heterojunctions were formed by using facing-target direct- current sputtering at room temperature. The J-V characteristic results revealed that the reverse leakage current is large and the response under illumination of near-infrared light is very weak. The capacitance-voltage-frequency (C-V-f) and conductance-voltage-frequency (G-V-f) measurements were carried out at room temperature in order to estimate the series resistance (Rs) by using the Nicollian-Brews method and the density of interface state (Nss) by using the Hill-Coleman method. By estimation according to the Nicollian-Brews method, the Rs value increases with decreasing f value. The Rs values at zero bias voltage were 2.07 Ω at 60 kHz and 1.54 Ω at 2 MHz, which are consistent with those calculated by using the Cheungs and Nordes methods. The obtained Rs should be attributable to the Rs existing in the ohmic contact and neutral regions, which is the current-limiting factor for junctions. The nss values calculated by using the Hill-Coleman method were 2.70 × 1014 eV-1cm-2 at 60 kHz and 1.43 × 1013 eV-1cm-2 at 2 MHz. This result revealed the presence of interface states at the hetero-interface behaving as a leakage current center and a trap center of the photo-generated carrier, which degraded the junction properties at room temperature.

Physical Properties of Copper Films Deposited by Compact-Size Magnetron Sputtering Source with Changing Magnetic Field Strength

An affordable 6-cm diameter magnetron sputtering source was designed and constructed using a neodymium cylindrical permanent magnet inner stud and an outer ring magnet. Both magnets were set in isolation from a water-cooled disk behind the target. In this work, our magnetron sputtering source was employed to deposit copper thin films by changing the outer magnet ring. The outer magnate ring diameter sizes were 29 mm (I.D.)/39 mm (O.D.) and 30 mm (I.D.)/50 mm (O.D.). At the same applied bias voltage, the discharge current of the magnetron with a big outer magnet was higher than that of one with a small outer magnet. According to XRD pattern and AFM image of copper films, the degree of (111) and (200) orientations and surface roughness were increased in the case of films grown by magnetron with bigger outer magnet. The cross-sectional and plane-view SEM images showed that the grain size and film thickness were increased in the case of films grown by magnetron with bigger outer magnet. These results should be because the radial motion of secondary electron emitted from sputtering in front of target surface was increased with a bigger magnet.

Investigation of electrical transport properties in heterojunctions comprised of silicon substrate and nanocrystalline iron disilicide films

In this study, n-type nanocrystalline FeSi2/p-type Si heterojunctions were fabricated using facing-target direct-current sputtering (FTDCS). The possible transportation mechanisms of carriers were investigated by analysing the dark J-V characteristics at temperatures ranging between 60 and 300 K. The ideality factor (n) was estimated from the slope of the linear region for the forward lnJ-V characteristics. The value of n was 1.87 at 300 K and nearly constant at temperatures ranging from 140 to 300 K, suggesting that a recombination process at the junction interface was dominant in the transportation mechanism of carriers. At temperatures below 140 K, the value of n increased by more than two and the value of A was virtually constant. Owing to the consistency of A, combined with the temperature dependent value of n, the implication is that a trap-assisted multi-step tunnelling mechanism governed the carrier transport. From the analysis of J-V characteristics, the value of barrier height was 0.58 eV at 300 K and decreased to 0.19 eV at 60 K.