Semiconducting Iron Disilicides for Photovoltaics and Photodiodes

The semiconducting iron disilicide β-FeSi2 has recently attracted considerable attention from both scientists and engineers due to its remarkable optical and electrical properties. β-FeSi2 has a large absorption coefficient, which is 200-fold larger than that of crystalline silicon at 1.5 eV, and a direct optical band gap of 0.85 eV. It is also compatible with silicon technology. From the ecological point of view, β-FeSi2 is a nontoxic material, and its elements (Fe and Si) are abundant in nature. Therefore, β-FeSi2 is one of the most promising materials for various applications such as light-emitting diodes, infrared sensors, and solar cells. However, there have been few reports on the application of β-FeSi2/Si heterojunctions.

     We propose a β-FeSi2/Si heterojunction solar cell. This cell can absorb solar light in a wide wavelength range as follows: the front-side Si layer absorbs the ultraviolet and visible light with photon energies larger than 1.12 eV, and the near-infrared light transmitted through the Si layer is absorbed by the back-side β-FeSi2 layer. Thus, a high photovoltaic performance is expected.

     In recent years, the development of devices that are compatible with silicon electronic-photonic integrated circuits and can be operated at the telecommunication wavelengths of 1.3 and 1.5 μm has been the subject of intense research. Orthorhombic iron disilicide (β-FeSi2)/Si heterostructures are considered attractive devices for photodetection because of the substantial compatibility between the two materials. We are progressing the research on near-infrared (NIR) photodetectors.

● Our recent reports ●



"Epitaxial Growth of beta-FeSi2 Thin Films on Si(111) Substrates by Radio Frequency Magnetron Sputtering and Their Application to Near-Infrared Photodetection"

Nathaporn Promros, Ryuji Baba, Motoki Takahara, Tarek M. Mostafa, Phongsaphak Sittimart, Mahmoud Shaban, and Tsuyoshi Yoshitake

Jpn. J. Appl. Phys. Vol. 55, No. 6S2 (2016) 06HC03.

 

Beta-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 -FeSi2/p-type Si heterojunctions were evaluated as near-infrared photodiodes. Three epitaxial variants of -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 beta-FeSi2 films prepared using RFMS are located in plasma and are damaged by it.

 

 

 

2 theta-theta XRD pattern of beta-FeSi2 thin film deposited on Si(111) substrate. The inset shows pole figure for the beta-440/404 diffraction peak..

Dark J-V curves of heterojunction measured at low temperatures ranging from 300 to 50 K.


 


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

Shota Izumi, Mahmoud Shaban, Nathaporn Promros, Keita Nomoto, and Tsuyoshi Yoshitake

Appl. Phys. Lett. Vol. 102, Issue 3 (2013) 032107.

 

n-Type beta-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 300K, devices biased at 5V exhibited a current responsivity of 16.6mA/W. The measured specific detectivity was remarkably improved from 3.5109 to 1.41011 cmHz1/2/W as the devices were cooled from 300K down to 50 K. This improvement is mainly attributable to distinguished suppression in heterojunction leakage current at low temperatures. The obtained results indicate that b-FeSi2/Si heterojunctions offer high potential to be employed as near-infrared photodetectors that are compatible with the current Si technology.

 

 

 

I-V characteristics of b-FeSi2/Si heterojunction photodiode measured
in the dark and under illumination with a 6-mW, 1.31 lm laser diode at
(a) 300 and (b) 50 K.

Temperature dependence of detectivity of beta-FeSi2/Si heterojunction
photodiode. The inset shows plot of NEP and detectivity versus bias voltage
of devices measured at 300 K.


 

"n-Type Nanocrystalline FeSi2/intrinsic Si/p-Type Si Heterojunction Photodiodes fabricated by Facing-Targets Direct-Current Sputtering"

Nathaporn Promros, Kyohei Yamashita, Chen Li, Kenji Kawai, Mahmoud Shaban, Toshihiro Okajima, and Tsuyoshi Yoshitake

Jpn. J. Appl. Phys. Vol. 51, No. 2 (2012) 021301.

 

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. Their capacitance-voltage curve implied that the formation of interface states is suppressed by i-Si insertion. Their near-infrared light detection performance was investigated using a 1.33 μm laser in the temperature range of 77-300 K. Their detectivities at 300 and 77 K were 1.9 × 1010 and 1.8 × 1011 cm√Hz/W, respectively, at a negative bias of -5 V, which were markedly improved compared with that of pn heterojunctions. This might be because the formation of interface states that act as trap centers for photocarriers is suppressed.

 

 

 

(a) J-V characteristics measured in temperature range of 77 - 300 K and (b) plots of current density measured at bias voltage of -4 V vs 1000/T for n-type NC FeSi2/i-Si/p-type Si heterojunction.

J-V characteristics, measured in the dark and under illumination at 300 and 77 K, of n-type NC FeSi2/i-Si/ p-type Si heterojunction.

 

 

"n-Type Nanocrystalline-FeSi2/p-Type Si Heterojunction Photodiodes Prepared at Room Temperature"

Mahmoud Shaban, Kenji Kawai, Nathaporn Promros, and Tsuyoshi Yoshitake

IEEE Electron Device Lett. Vol. 31, No. 12 (2010) pp. 1428-1430.

 

n-Type nanocrystalline-FeSi2/p-type Si heterojunctions were prepared at room temperature by means of a facing target direct current sputtering (FTDCS) method. The current density–voltage (J-V) 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.

 

 

 

Dark and illuminated I-V characteristics of an n-type NC-FeSi2/p-type Si heterojunction photodiode measured at 300 K. The inset illustrates the photoresponse spectrum measured in the near-infrared spectral range at room temperature and under zero reverse bias voltage.Dark and illuminated I-V characteristics of an n-type NC-FeSi2/p-type Si heterojunction photodiode measured at 300 K. The inset illustrates the photoresponse spectrum measured in the near-infrared spectral range at room temperature and under zero reverse bias voltage.

Temperature dependence of the detectivity of an n-type NC-FeSi2/ptype Si heterojunction photodiode measured at a wavelength of 1.31 μm.

 

 

"n-Type β-FeSi2/intrinsic-Si/p-type Si heterojunction photodiodes for near-infrared light detection at room temperature"

Mahmoud Shaban, Shota Izumi, Keita Nomoto, and Tsuyoshi Yoshitake

Appl. Phys. Lett. Vol. 95 (2009) 162102.

 

n-Type β-FeSi2/intrinsic-Si/p-type Si heterojunctions, prepared by facing-targets direct-current sputtering, were evaluated as near-infrared photodetectors. The built-in potential was estimated to be approximately 1 V from capacitance-voltage characteristics. Diodes with a junction area of 0.03  mm2 exhibited a junction capacitance of 4.4 pF at zero bias. At room temperature, the devices exhibited responsivity of 140 mA/W and external quantum efficiency of 13% at a bias voltage of −5 V. The detectivity at zero bias was estimated to be 2.8×109  cm[square root of]Hz/W at the wavelength of 1.31  µm. These results indicate their high application potential as near-infrared photodiodes integrated with Si.

 

 

 

 

C-V characteristics of a heterojunction measured at a signal frequency of 1 MHz. The inset shows the plot of 1/C2 against bias voltage.

External quantum efficiency versus reverse voltage measured under illumination at a wavelength of 1.31 μm. The inset shows the photoresponse spectrum measured at zero bias and 300 K in the NIR region.

 

 

"Characterization of near-infrared n-type β-FeSi2/p-type Si heterojunction photodiodes at room temperature"

Mahmoud Shaban, Keita Nomoto, Shota Izumi, and Tsuyoshi Yoshitake

Appl. Phys. Lett. Vol. 94 (2009) 222113.

 

n-type β-FeSi2/p-type Si heterojunctions were fabricated from β-FeSi2 films epitaxially grown on Si(111) by facing-target direct-current sputtering. Sharp film-substrate interfaces were confirmed by scanning electron microscopy. The current-voltage and photoresponse characteristics were measured at room temperature. They exhibited good rectifying properties and a change of approximately one order of magnitude in the current at a bias voltage of −1 V under illumination by a 6 mW, 1.31  µm laser. The estimated detectivity was 1.5×109  cm [square root of]Hz W at 1.31  µm. The results suggest that the β-FeSi2/Si heterojunctions can be used as near-infrared photodetectors that are compatible with silicon integrated circuits.

    

 

 

(a) Schematic illustration of an n-type β-FeSi2 (300 nm)/p-type Si (100 mm) heterojunction photodiode with Pd and Al electrodes on top (Si) and bottom (b-FeSi2) surfaces respectively. (b) cross-sectional SEM image of β-FeSi2 film deposited on an Si(111) substrate.

I-V characteristics measured in the dark and under illumination with a 6-mW, 1.31-μm LD at room temperature. The inset shows the dynamic junction resistances deduced from the I-V characteristics.

 

"Electrical and Photovoltaic Properties of n-Type Nanocrystalline FeSi2/p-Type Si Heterojunctions Prepared by Facing Target Direct-current Sputtering at Room Temperature"

Mahmoud SHABAN, Haruhiko KONDO, Kazuhiro NAKASHIMA, and Tsuyoshi YOSHITAKE
Jpn. J. Appl. Phys. Vol. 47, No. 7 (2008) pp. 5420-5422.

 

Semiconducting nanocrystalline iron disilicide (NC-FeSi2) thin films were deposited on Si(111) substrates by facing target direct-current sputtering at room temperature. The electrical and photovoltaic properties of the n-type NC-FeSi2/p-type Si heterojunctions were measured and investigated. We experimentally proved the possibility of employing this combination in photovoltaics. A large leakage current observed in the current–voltage characteristics, which was predominantly due to the heterojunction interface defects, resulted in a low conversion efficiency.

    

 

 

(a) Dark and (b) illuminated J–V characteristics of n-type NC-FeSi2/p-type Si heterojunction with NC-FeSi2 layer thickness of 350 nm.

Photoresponse spectrum of heterojunction measured in photon energy range of near-infrared light.

 

 

"Low-temperature Annealing of n-type β-FeSi2/p-type Si Heterojunctions"

Mahmoud SHABAN, Keita NOMOTO, Kazuhiro NAKASHIMA, and Tsuyoshi YOSHITAKE
Jpn. J. Appl. Phys. Vol. 47, Vol. 5 (2008) pp. 3444-3446.

 

β-FeSi2 thin films epitaxially grown on Si(111) were annealed in vacuum at different annealing temperatures. The effects of low-temperature postannealing on the photovoltaic properties of β-FeSi2/Si heterojunctions were investigated. The heterojunctions annealed at 300°C exhibited an apparent improvement in photovoltaic performance as compared with as-grown heterojunctions. This improvement may be due to Fe atoms that diffused into the Si substrate during β-FeSi2 film deposition and were gettered during the annealing process.

    

 

 

Fe diffusion length against annealing temperature computed for different annealing times.

Dark J–V characteristics of as-grown and 300°C-annealed heterojunctions.

 

 

 

"Photovoltaic Properties of n-type β-FeSi2/p-type Si Heterojunctions"

Mahmoud SHABAN, Kazuhiro NAKASHIMA, Wataru YOKOYAMA, and Tsuyoshi YOSHITAKE
Jpn. J. Appl. Phys. Part 2, Vol. 46, L667-L669, 2007.

 

n-Type β-FeSi2/p-type Si heterojunction solar cells were fabricated. The energy band diagram was derived from the measured ionization potential of β-FeSi2 and well-known parameters. The value of the built-in potential was estimated to be 1.02 V. Under air mass 1.5 illumination, the cell showed a conversion efficiency of 0.63 %. The short-circuit current density was 12.81 mA/cm2, whereas the open-circuit voltage was only 176 mV, which might be attributed to the iron atoms that diffused into the Si depletion region. The iron atoms that diffused cause current leakage and also act as trap centers for the photogenerated carriers.

    

 

 

Schematic diagram of n-type β-FeSi2/p-type Si heterostructure.

J-V characteristics of n-type β-FeSi2/p-type heterojunction solar cell in dark and under illumination [inset(a)]. A cross-sectional SEM image is shown in inset (b).

 

 

"Direct epitaxial growth of semiconducting β-FeSi2 thin films by facing targets direct-current sputtering"

T. Yoshitake, Y. Inokuchi, A. Yuri, and K. Nagayama
Appl. Phys. Lett. Vol. 88, 182104, 2006.

 

Semiconducting β-FeSi2 thin films were epitaxially as-grown on Si(111) substrates at a substrate temperature of 600 °C, which is at least 200 °C lower than ordinary annealing temperatures, by using the facing targets direct-current sputtering FTDCS method using an FeSi2 target without annealing. The deposited film exhibits a smooth surface with a surface roughness root mean square of 1.47 nm. The direct and indirect optical band gaps estimated from the experiment were in agreement with those of the single crystalline bulk. Temperature dependence of the electrical conductivity implied Co incorporation from the FeSi2 targets with a purity of 3N. The FTDCS method, in which a substrate is free of plasma and energetic neutral atoms diffused into the substrate owing to low Ar pressure sputtering, is effective for the direct epitaxial growth of β-FeSi2 thin films with smooth surfaces.

    

 

 

X-ray diffraction pattern of the β-FeSi2 thin film measured by a φ scan method. The rotation axis was normal to the substrate surface, that is, the [111] direction of the Si(111) substrate.

Temperature dependence of the electric conductivity of the β-FeSi2 thin film.