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 μ 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 × 10 16 cm −3 (unintentionally doped) to 1 × 10 21 cm −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 (D = 700 K) and number of phonon modes (N 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 ≈ 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 ± 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 −3 , and qualitatively explains a mobility plateau or local maximum around 10 20 cm −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 μ = 190 cm 2 /Vs and μ H = 270 cm 2 /V s, respectively. Simplified equations for the Seebeck coefficient describe the measured values in the nondegenerate regime using a Seebeck scattering parameter of r = −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 Mott = 5.5 × 10 18 cm −3) from nondegenerate to degenerate electron concentrations, whereas the numerical modeling accurately describes this region.

Lateral thermoelectric devices were fabricated using c-plane GaN thin films grown on sapphire by MOCVD. The device design is appropriate for on-chip integration for power generation in the 1 V and tens of µA range. The fabricated devices were measured to have a maximum open circuit voltage of 0.3 V with a maximum output power of 2.1 µW (=0.15 V×14 µA) at a relatively small temperature difference (ΔT) of 30 K and an average temperature (Tavg) of 508 K. In addition, the suitability of GaN for high temperature thermoelectric applications was confirmed by measurements at 825 K.

The Seebeck coefficient of a typical thermoelectric material is calculated without recourse to the relaxation time approximation ͑RTA͒. To that end, the Boltzmann transport equation is solved in one spatial and two k-space coordinates by a generalization of the iterative technique first described by Rode. Successive guesses for the chemical potential profile are generated until current continuity and charge-neutrality in the bulk of the device are simultaneously satisfied. Both the mobility and Seebeck coefficient are calculated as functions of the temperature and the agreement to experimentally obtained values is found to be satisfactory. Comparison is made with the less accurate RTA result, which has the sole advantage of giving closed form expressions for the transport coefficients.

A popular method of measuring the thermal conductivity of thin films and substrates, the "3-omega" method, is modified to yield a new technique for measuring the anisotropy in thermal transport in bulk materials. The validity of the proposed technique is established by measuring the thermal conductivity of strontium titanate, which is expected to be isotropic because of its cubic unit cell. The technique is then applied to rutile TiO(2). The analysis of experimental results on (100) and (001) TiO(2) reveals that the anisotropy is a function of the crystalline quality, as quantified by the effective thermal conductivity obtained through conventional "3-omega" measurements. The advantages of the proposed technique are similar to those of the standard "3-omega" method, namely the simplicity of sample preparation and measurement, and negligible errors due to radiation because of the small volume of material being heated. For anisotropy determination, the proposed technique has the additional advantage that a single sample is sufficient to determine both components of the thermal conductivity, namely the values in and perpendicular to the plane of cleavage. This is significant for materials in which there is a large variation in the crystalline quality from sample to sample. For such materials, it is unreliable to use two different samples, one for measuring the thermal conductivity in each direction. Experimental data are analyzed using a 3D Fourier-series based method developed in this work. The proposed method determines each component of the thermal conductivity with an estimated accuracy of about 10%.

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