Iron disilicide thin films were prepared by furnace reaction of ion beam sputtered iron layers with single‐crystal silicon wafers and with low‐pressure chemical vapor deposition (LPCVD) polycrystalline silicon thin films. X‐ray diffraction indicates the films are single‐phase, orthorhombic, β‐FeSi2. Impurity levels are below the detection limit of Auger spectroscopy. Normal incidence spectral transmittance and reflectance data indicate a minimum, direct energy gap of 0.87 eV. The apparent thermal activation energy of the resistivity in the intrinsic regime is about half of this minimum optical gap. With such a direct band gap, the material may be suitable for the development of both light‐sensitive and light‐emitting thin‐film devices within the silicon microelectronics technology.

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Optical properties of semiconducting iron disilicide thin films

Thermoelectric (TE) materials facilitate direct heat-to-electricity conversion. The performance of a TE material is characterised by its figure of merit zT (=S2 σT/κ) that depends on both electronic transport properties, i.e. the Seebeck coefficient S and the electrical conductivity σ, and on thermal transport properties, i.e. the thermal conductivity κ of a material. The intrinsically counter-correlated behaviour between electronic and thermal transport properties makes the enhancement of zT a very challenging task. In the past 10 years, the zTs in bulk TE materials have been significantly enhanced due to intensive exploratory efforts, the discovery of new physical phenomena and effects, and applications of advanced synthesis methods. In this review, we summarise the recent progress in bulk TE materials. After the introduction of fundamental principles of thermoelectricity, the recently achieved enhancements in the TE performance encompassing the use of electronic band structure engineering, lattice phon...

Recent advances in high-performance bulk thermoelectric materials

Polycrystalline iron disilicide thin films are prepared by furnace annealing of electron‐beam deposited iron layers. As substrates we use single‐crystal silicon wafers, epitaxial silicon thin films on sapphire substrates, and low‐pressure chemical vapor deposited polycrystalline silicon thin films on oxidized silicon wafers. X‐ray diffraction indicates that orthorhombic β‐FeSi2 is obtained for growth temperatures in the range 800–900 °C. Photothermal deflection spectroscopy reveals a direct band‐gap of 0.85 eV. Optical transitions at energies above 1.4 eV are investigated by spectroscopic ellipsometry. Measurements of the subgap defect absorption, photoluminescence, conductivity, and of the Hall mobility suggest that lower growth temperatures yield material of better quality.

Electronic properties of semiconducting FeSi2 films

Previously published values of the index of refraction of beta‐iron disilicide are corrected. Based on improved measurements and analysis, the undispersed value of the refractive index is 5.6. The previously published analysis of the absorption coefficient showed that the material possesses a direct forbidden energy gap of approximately 0.87 eV. The type of the transition is unchanged, but the band‐gap value is slightly shifted to 0.89 eV.

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A clarification of the index of refraction of beta-iron disilicide

Ion beam synthesized polycrystalline semiconducting FeSi2 on Si(001) has been investigated by transmission measurements at temperatures between 10 and 300 K. The existence of a minimum direct band gap was demonstrated and its variation with the temperature was studied by means of a three‐parameter thermodynamic model and the Einstein model. Band tail states and states on a shallow impurity level were found to give rise to the absorption below the fundamental edge. The presence of an Urbach exponential edge was shown and the temperature dependence of the Urbach tail width was also studied based on the Einstein model. A strong structural disorder associated with grain boundaries between and within the FeSi2 grains and their related defects was found to be the dominant contribution at room temperature.

Optical absorption study of ion beam synthesized polycrystalline semiconducting FeSi2

Electrical conductivity, thermoelectric power and Hall coefficient of β-FeSi2 doped with cobalt (n-type) or aluminium (p-type) are measured between 100 and 1200 °K. The conductivity of n-FeSi2 follows an exponential dependence on temperature. The temperature dependence of the thermoelectric power cannot be interpreted on the basis of conduction in a band. With the assumption that conduction in n-FeSi2 is caused by small polarons, the mobility at room temperature is found to be μn = 0.26 cm2/Vs. The activation energy of the mobility is 0.06 eV, the density of states N = 1.2 × 1022 cm−3. The electrical properties of p-FeSi2 can be interpreted using the band model with a hole mobility μp ≈ 2 cm2/Vs, which varies as T−1/2 in the region of extrinsic conduction. From intrinsic conduction a band gap of 0.9 to 1.0 eV is deduced. The disappearance of the thermoelectric power at high temperature is related to the semiconductor-to-metal transition at 1200 °K.

Mechanism of Electrical Conduction in β-FeSi2

Electrical Investigation of the Semiconductor‐to‐Metal Transition in FeSi2

Infrared Reflectivity of Semiconducting FeSi2

Magnetic Susceptibility of Semiconducting and Metallic FeSi2

The reflectivity of α-FeSi2 and also of β-FeSi2 doped with Co (n-type) or Al (p-type) has been investigated for wavelengths between 1 and 25 μm. The results for α-FeSi2 indicate metallic conduction in this material. The reflectivity of the undoped semiconducting phase β-FeSi2 can be explained by a fundamental absorption corresponding to a band gap Eg = 0.8 eV and a reststrahlen band. The highest branch of the longitudinal optical phonons is found at 20.6 μm. The reflectivity of p-FeSi2 can be interpreted using Drude's theory of free carrier absorption. The optical properties of n-FeSi2 cannot be explained by free carrier absorption. A consistent interpretation, however, is possible in terms of the small polaron picture. Using the experimental data of the de conductivity and its activation energy the reflectivity of n-FeSi2 can be calculated. There is good agreement with the observed values. These results confirm an analysis of the transport properties of β-FeSi2 which was presented in a previous paper.



Das Reflexionsvermogen von α-FeSi2 sowie von β-FeSi2 mit Co-Dotierung (n-Typ) und Al-Dotierung (p-Typ) wurde im Wellenlangenbereich von 1 bis 25 μm untersucht. Die Mes-ergebnisse fur α-FeSi2 stehen im Einklang mit der metallischen Leitfahigkeit dieser Phase. Das Reflexionsvermogen der undotierten halbleitenden Phase β-FeSi2 wird durch eine Grundgitterabsorption entsprechend einem Bandabstand Eg = 0,8 eV und einer Reststrahlbande gedeutet. Der oberste Zweig der longitudinalen optischen Phononen liegt bei 20,6 μm. Das Reflexionsvermogen von p-FeSi2 kann mittels der Drudeschen Theorie freier Ladungstrager erklart werden. Die optischen Eigenschaften von n-FeSi2 konnen im Rahmen der Drudeschen Theorie nicht verstanden werden, lassen sich jedoch mittels der Theorie des kleinen Polarons beschreiben. Unter Verwendung der experimentellen Daten der Gleichstromleitfahigkeit und ihrer Aktivierungsenergie last sich das Reflexionsvermogen von n-FeSi2 berechnen. Die Ubereinstimmung mit den beobachteten Werten ist gut. Durch diese Ergebnisse wird eine fruhere Analyse der Transporteigenschaften von β-FeSi2 bestatigt.

U. Birkholz

Papers

Mechanism of Electrical Conduction in β-FeSi2

Electrical Investigation of the Semiconductor‐to‐Metal Transition in FeSi2

Infrared Reflectivity of Semiconducting FeSi2

Magnetic Susceptibility of Semiconducting and Metallic FeSi2

Optical Investigation of the Small Polaron in β-FeSi2