The present review takes a semiconductor physics perspective to summarize the state-of-the art of In2O3 in relation to applications. After discussing conventional and novel applications, the crystal structure, synthesis of single-crystalline material, band-structure and optical transparency are briefly introduced before focussing on the charge carrier transport properties. The issues of unintentional n-type conductivity and its likely causes, the surface electron accumulation, and the lack of p-type conductivity will be presented. Intentional doping will be demonstrated to control the electron concentration and resistivity over a wide range, but is also subject to compensation. The control of the surface accumulation in relation to Schottky and ohmic contacts will be demonstrated. In the context of scattering mechanisms, the electron mobility and its limits will be discussed. Finally, the Seebeck coefficient and its significance will be shown, and ferromagnetic doping of In2O3 will be critically discussed. With this overview most if not all ingredients for the use of In2O3 as semiconductor material in novel or improved conventional devices will be given.

http://iopscience.iop.org/article/10.1088/0268-1242/30/2/024001/pdf

Indium oxide—a transparent, wide-band gap semiconductor for (opto)electronic applications

A comprehensive study of the room-temperature electrical and electrothermal transport of single-crystalline indium oxide (In${}_{2}$O${}_{3}$) and indium tin oxide (ITO) films over a wide range of electron concentrations is reported. We measured the room-temperature Hall mobility ${\ensuremath{\mu}}_{H}$ and Seebeck coefficient $S$ of unintentionally doped and Sn-doped high-quality, plasma-assisted molecular-beam-epitaxy-grown In${}_{2}$O${}_{3}$ for volume Hall electron concentrations ${n}_{H}$ from $7\ifmmode\times\else\texttimes\fi{}{10}^{16}$ cm${}^{\ensuremath{-}3}$ (unintentionally doped) to $1\ifmmode\times\else\texttimes\fi{}{10}^{21}$ cm${}^{\ensuremath{-}3}$ (highly Sn-doped, ITO). The resulting empirical $S({n}_{H})$ relation can be directly used in other In${}_{2}$O${}_{3}$ samples to estimate the volume electron concentration from simple Seebeck coefficient measurements. The mobility and Seebeck coefficient were modeled by a numerical solution of the Boltzmann transport equation. Ionized impurity scattering and polar optical phonon scattering were found to be the dominant scattering mechanisms. Acoustic phonon scattering was found to be negligible. Fitting the temperature-dependent mobility above room temperature of an In${}_{2}$O${}_{3}$ film with high mobility allowed us to find the effective Debye temperature (${\ensuremath{\Theta}}_{D}=700$ K) and number of phonon modes (${N}_{\mathrm{OPML}}=1.33$) that best describe the polar optical phonon scattering. The modeling also yielded the Hall scattering factor ${r}_{H}$ as a function of electron concentration, which is not negligible (${r}_{H}\ensuremath{\approx}1.4$) at nondegenerate electron concentrations. Fitting the Hall-scattering-factor corrected concentration-dependent Seebeck coefficient $S(n)$ for nondegenerate samples to the numerical solution of the Boltzmann transport equation and to widely used, simplified equations allowed us to extract an effective electron mass of ${m}^{*}=(0.30\ifmmode\pm\else\textpm\fi{}0.03){m}_{e}$ (with free electron mass ${m}_{e}$). The modeled mobility and Seebeck coefficient based on polar optical phonon and ionized impurity scattering describes the experimental results very accurately up to electron concentrations of ${10}^{19}$ cm${}^{\ensuremath{-}3}$, and qualitatively explains a mobility plateau or local maximum around ${10}^{20}$ cm${}^{\ensuremath{-}3}$. Ionized impurity scattering with doubly charged donors best describes the mobility in our unintentionally doped films, consistent with oxygen vacancies as unintentional shallow donors, whereas singly charged donors best describe our Sn-doped films. Our modeling yields a (phonon-limited) maximum theoretical drift mobility and Hall mobility of $\ensuremath{\mu}=190$ cm${}^{2}$/Vs and ${\ensuremath{\mu}}_{H}=270$ cm${}^{2}$/V$\phantom{\rule{0.16em}{0ex}}$s, respectively. Simplified equations for the Seebeck coefficient describe the measured values in the nondegenerate regime using a Seebeck scattering parameter of $r=\ensuremath{-}0.55$ (which is consistent with the determined Debye temperature), and provide an estimate of the Seebeck coefficient to lower electron concentrations. The simplified equations fail to describe the Seebeck coefficient around the Mott transition (${n}_{\mathrm{Mott}}=5.5\ifmmode\times\else\texttimes\fi{}{10}^{18}$ cm${}^{\ensuremath{-}3}$) from nondegenerate to degenerate electron concentrations, whereas the numerical modeling accurately describes this region.

Electrical transport, electrothermal transport, and effective electron mass in single-crystalline In 2 O 3 films

The group-III sesquioxides possess material properties that render them interesting for applications such as high-power rectifiers and transistors, solar-blind UV detectors and inter-sub-band infrared detectors. Technology for growing large, single-crystalline bulk material and for wafer fabrication exists, enabling homoepitaxial growth of thin films with high crystalline quality. The bandgap can be tuned in an energy range from about 4 to 8 eV for the ternary alloys and allows growth of heterostructures with large band offset. Here, past results and recent investigations on the growth, the material properties, contact fabrication and the alloying of group-III sesquioxides are reviewed, and an overview on demonstrator devices is provided.

Group‐III Sesquioxides: Growth, Physical Properties and Devices

Parasitic absorption in transparent electrodes is one of the main roadblocks to enabling power conversion efficiencies (PCEs) for perovskite‐based tandem solar cells beyond 30%. To reduce such losses and maximize light coupling, the broadband transparency of such electrodes should be improved, especially at the front of the device. Here, the excellent properties of Zr‐doped indium oxide (IZRO) transparent electrodes for such applications, with improved near‐infrared (NIR) response, compared to conventional tin‐doped indium oxide (ITO) electrodes, are shown. Optimized IZRO films feature a very high electron mobility (up to ≈77 cm2 V−1 s−1), enabling highly infrared transparent films with a very low sheet resistance (≈18 Ω □−1 for annealed 100 nm films). For devices, this translates in a parasitic absorption of only ≈5% for IZRO within the solar spectrum (250–2500 nm range), to be compared with ≈10% for commercial ITO. Fundamentally, it is found that the high conductivity of annealed IZRO films is directly linked to promoted crystallinity of the indium oxide (In2O3) films due to Zr‐doping. Overall, on a four‐terminal perovskite/silicon tandem device level, an absolute 3.5 mA cm−2 short‐circuit current improvement in silicon bottom cells is obtained by replacing commercial ITO electrodes with IZRO, resulting in improving the PCE from 23.3% to 26.2%.

/pdf/zr-doped-indium-oxide-izro-transparent-electrodes-for-1av2hv9zud.pdf

Zr-Doped Indium Oxide (IZRO) Transparent Electrodes for Perovskite-Based Tandem Solar Cells

The onset of optical absorption in In2O3 at about 2.7 eV is investigated by transmission spectroscopy of single crystals grown from the melt. This absorption is not defect related but is due to the fundamental band gap of In2O3. The corresponding spectral dependence of the absorption coefficient is determined up to α = 2500 cm−1 at a photon energy hν = 3.05 eV at room temperature without indication of saturation. A detailed analysis of the hν dependence of α including low-temperature absorption data shows that the absorption process can be well approximated by indirect allowed transitions. It is suggested that the fundamental band gap of In2O3 is of indirect nature. The temperature dependence of the fundamental band gap is measured over a wide range from 9 to 1273 K and can be well fitted by a single-oscillator model. Compared to other semiconductors the reduction of the gap with increasing temperature is exceptionally strong in In2O3.

On the nature and temperature dependence of the fundamental band gap of In2O3

High quality In2O3 single crystals of bcc structure were grown by chemical vapour transport. The temperature dependence of resistivity, Hall constant, and mobility yielded an electron density of n = 1.3 × 1019 cm−3. The transport properties showed characteristics best describable by the degenerate semiconductor model. The crystals were additionally investigated by high resolution angular resolved photoelectron spectroscopy (ARPES). Emission from the valence band and the partially filled conduction band at the Γ point yielded a direct bandgap of (2.7 ± 0.1)eV. The partially filled conduction band furthermore enabled the determination of its three dimensional Fermi surface and the effective masses m* by ARPES.

H. Dwelk

Papers

Transport and angular resolved photoemission measurements of the electronic properties of In2O3 bulk single crystals