Nathaporn Promros

Near-infrared photodetection of β-FeSi2/Si heterojunction photodiodes at low temperatures

n-type β-FeSi2/p-type Si heterojunction photodiodes were fabricated by facing-targets direct-current sputtering, and their near-infrared photodetection properties were studied in the temperature range of 50–300 K. At 300 K, devices biased at −5 V exhibited a current responsivity of 16.6 mA/W. The measured specific detectivity was remarkably improved from 3.5 × 109 to 1.4 × 1011 cmHz1/2/W as the devices were cooled from 300 K down to 50 K. This improvement is mainly attributable to distinguished suppression in heterojunction leakage current at low temperatures. The obtained results indicate that β-FeSi2/Si heterojunctions offer high potential to be employed as near-infrared photodetectors that are compatible with the current Si technology.

Effects of Hydrogen Passivation on Near-Infrared Photodetection of n-Type β-FeSi2/p-Type Si Heterojunction Photodiodes

Hydrogen passivation was applied to the initial epitaxial growth of n-type β-FeSi2 thin films on p-type Si(111) substrates. Such passivation was applied at different gas inflow H2/Ar ratios ranging from 0 to 1.0. The photodetection performance of the photodiode fabricated at the optimum ratio of 0.2 was markedly improved as compared with those of the other samples. The quantum efficiency and detectivity were 2.08% and 5.40×109 cmHz1/2W-1, respectively. The enhanced photodetective performance should be mainly because dangling bonds that act as trap centers for photocarriers are effectively inactivated by the passivation.

n-Type Nanocrystalline-

n-Type nanocrystalline-FeSi₂/p-type Si heterojunctions were prepared at room temperature by means of a facing target direct-current sputtering method. The current density-voltage (<i>J</i>- <i>V</i>) characteristics of the devices fabricated were investigated in the temperature range of 77-300 K. At a wavelength of 1.31 μm, the photodiodes exhibited a room-temperature responsivity of 110 mA/cm² and a detectivity of 2.7 × 10⁸ cm√Hz/W, which was improved to 1.5 ×10¹⁰ cm√Hz/W at 77 K. The results profile an attractive low-cost near-infrared photodiode suitable for large-area optoelectronics.

\hbox{FeSi}_{2}

n-Type nanocrystalline (NC) FeSi2/intrinsic (i) Si/p-type Si heterojunctions, which were prepared by facing-target direct current sputtering, were evaluated as near-infrared photodiodes, and the effects of thin i-Si layer insertion on diode performance were studied. Their junction capacitance and reverse leakage current were clearly reduced compared with those of n-type NC-FeSi2/p-type Si heterojunctions. The capacitance–voltage curve implied that the effects of interface states is relatively suppressed by i-Si insertion. The near-infrared light detection performance was investigated using a 1.33 µm laser in the temperature range of 77–300 K. The detectivities at 300 and 77 K were 1.9 ×108 and 3.0 ×1011 cm Hz1/2 W-1, respectively, at a negative bias of -5 V, which were markedly improved compared with that of p–n heterojunctions. This might be because the formation of interface states that act as trap centers for photocarriers is suppressed.

/p-Type Si Heterojunction Photodiodes Prepared at Room Temperature

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.

n-Type Nanocrystalline FeSi2/intrinsic Si/p-Type Si Heterojunction Photodiodes Fabricated by Facing-Target Direct-Current Sputtering

n-Type β-FeSi2/intrinsic Si/p-type Si heterojunction photodiodes were fabricated by facing-targets direct-current sputtering, and their infrared photodetection properties were studied at low temperatures. The rectification current ratio at bias voltages of ±1 V and the ratio of the photocurrent to the dark leakage current were dramatically enhanced with a decrease in temperature. The specific detectivities at 300 and 50 K were estimated to be 3.8×109 and 8.9×1011 cm Hz1/2 W-1, respectively. The enhanced detectivity upon cooling is attributed to the marked reduction in the dark leakage current. The insertion of the thin intrinsic Si layer slightly contributed to the suppression of the leakage current and the detectivity improvement. It was demonstrated that β-FeSi2 is a potential material for Si-compatible near-infrared photodetectors.

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

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.

Near-Infrared Photodetection of n-Type β-FeSi2/Intrinsic Si/p-Type Si Heterojunctions at Low Temperatures

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.

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

Motoki Takahara*, Tarek M. Mostafa, Ryuji Baba, Suguru Funasaki, Mahmoud Shaban, Nathaporn Promros, and Tsuyoshi Yoshitake 1Department of Applied Science for Electronics and Materials, Kyushu University, Kasuga, Fukuoka 816-8580, Japan 2Department of Electrical Engineering, Aswan Faculty of Engineering, Aswan University, Aswan 81542, Egypt 3Department of Physics, Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang, Chalongkrung Road, Bangkok 10520, Thailand

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

A compact dc magnetron sputtering system capable of silver thin films depositions was designed and constructed. The novel small footprint sputtering head with target diameter of 52 mm was constructed utilizing powerful neodymium alloy magnet. Silver metal was sputter-deposited under various powers. Plasma parameters were analyzed by using the sweeping-bias single langmuir probe. The electron temperatures of the plasma glow were constant at approximately 2 eV even with the increasing of input power whereas plasma density increases with the increasing of the input power. The X-ray diffraction analysis (XRD) and scanning electron microscope (SEM) were used to study the crystalline structure and the surface morphology of the obtained silver thin films. Crystalline orientations of (111) and (200) in the silver films deposited on slide glass substrates were revealed from XRD pattern. The highest degrees of (111) and (200) orientations was obtained at the sputtering power between 0.228 and 0.265 Wcm-2. Sub-micron crystalline silver grain structure were observed using SEM micrographs. Facetted grain size and deposition rate of silver thin films increases as the sputtering power increases.