Session G27: Electronic Structure: Thermodynamic & Optical Properties

Carrier Lifetime and Mobility in GaAsNBi Alloys

III-V alloys incorporating Nitrogen and Bismuth are of significant interest for optoelectronic applications in the near- and mid-infrared. We report measurements of the photoconductivity lifetime and photocarrier mobility in a series of GaAs_1-x-yN_xBi_y samples grown on GaAs by MBE. We find short conductivity lifetimes of order 3ps to 5ps and carrier mobilities of order 30 to 80cm²/Vs. Time-resolved photoconductivity was obtained from optical pump, THz probe measurements, and steps were taken to avoid the influence of the GaAs substrate: A tunable optical pump permitted excitation below the GaAs bandgap, and transient THz reflection was used as a surface-specific probe of the conductivity. The short carrier lifetime and low carrier mobility likely arises due to rapid carrier trapping in these materials.   

Study of β-Ga2O3 Photoluminescence with Above Bandgap Laser Excitation

Ga₂O₃ is being considered as a next generation ultra-wide bandgap semiconductor with promising UV device applications. β-Ga₂O₃ films were grown via RF sputtering and photoluminescence (PL) was performed using 5.1 eV laser excitation, which is above the predicted bandgap of ~4.9eV. Weak UV PL was detected at 4.85eV and 4.20eV. The near band edge (NBE) PL at 4.85eV is consistent with subsequent transmission measurements of the films that showed a bandedge value of 4.85eV. Due to the strong trapping affinity of holes in Ga₂O₃ previously predicted by theory, observation of NBE PL at 4.85eV PL has been deemed unlikely. We speculate that the laser intensity employed in our experiments was sufficient to create a small density of free holes enabling NBE in Ga₂O₃. The 4.20eV PL emission exhibits peak shifts up to ~100meV under different annealing environments. Strong UV emissions were also observed at 3.14eV and 3.56eV, which have been attributed to donor-acceptor recombination and self-trapped holes, respectively. Emission due to self-trapped holes was not detected when sub-bandgap laser excitation of 3.8eV was utilized.   

Optical properties of GaN/Er:GaN/GaN core-cladding planar waveguides

Erbium doped GaN (Er:GaN) is a promising candidate as a gain medium for high energy lasers (HELs) operating at the “retina-safe” spectral region around 1.5 µm due to outstanding thermal, mechanical and optical properties of GaN host. Compared to YAG, GaN has a much higher thermal conductivity of k ≈ 253 W/m*K and a smaller thermal expansion coefficient of α ≈ 3.53 x 10^-6 °C^-1 and the potential to significantly outperform YAG based HELs. We report here the successful fabrication and optical characterization of GaN/Er:GaN/GaN core-cladding planar waveguides (PWGs). Optical confinement in the core layer has been investigated. The measured optical loss coefficients of Er:GaN PWGs at 1.54 µm have been measured and are respectively 1.0 cm^-1 for the transverse electric (TE) and 1.2 cm^-1 for the transverse magnetic (TM) modes. Approaches to further reduce the optical loss and optimal configuration for resonantly pump GaN/Er:GaN/GaN PWGs for achieving amplification near 1.5 µm have been identified.   

Optical properties of indium tin oxide thin-films at high temperature

Indium tin oxide (ITO) is one of the most widely used transparent conductors. However, its high-temperature optical properties, which are important for device applications in extreme conditions such as space missions, have not been studied. We have investigated the optical properties of ITO thin-films up to 800 °C using in situ optical spectroscopic ellipsometry equipped within a vacuum chamber. The amorphous ITO films show an abrupt change of optical constants at 300 °C due to crystallization. Above 400 °C, the bandgap energy of the ITO thin-film decreases with increasing temperature, implying that electron-phonon interaction broadens the interband optical transitions. The reduction of bandgap energies due to electron-phonon interaction is also observed in epitaxial ITO thin-films. Nevertheless, both amorphous and epitaxial ITO thin-films remain optically transparent even at high temperatures up to 800 °C. In addition, background gas environments reversibly alter the bandgap energies of the epitaxial ITO thin-films, suggesting that the diffusion of oxygen vacancies occurs vigorously.   

On the Thermo-Optical Interactions at the Band-Edge in ZnO Thin Films

The study of the band edge in UV-Vis absorption spectra has impacted fundamental understanding of semiconductor physics as well as advances in technological application. An analytical approach based on the derivative of the absorption spectra was used to explore the nature of the near band edge (NBE) of ZnO thin films grown via the sputtering technique. It was found that the NBE is composed of a Gaussian where the width and peak position were employed to model the electron-phonon (e-p) interaction and defect characteristics of the film. These characteristics were studied via transmission experiments in the temperature range of 77 K to 532 K. The as-grown film was found to exhibit a very weak e-p coupling relative to the static contribution of defects. Upon successive controlled annealing of the ZnO film, up to 800 C, the defect component diminished and the phonon contribution became dominant. X-ray diffraction and imaging studies agree with these results. The defects in the ZnO films are discussed in terms of structural inhomogeneities and Zn interstitials which are prevalent in ZnO films grown via sputtering.   

Comparison of ZnO Nanoparticles Synthesized by Microwave-assisted and Precipitation Method

In this work, Zinc Oxide (ZnO) Nanoparticles were synthesized using precipitation and microwave methods with different pH values (8, 9, and 10). Structural properties were studied by means of Transmission Electron Microscopy (TEM), X-ray Diffraction Spectroscopy (XRD), Impedance Spectroscopy, and Optical Spectroscopy. TEM images revealed that the morphology of nanoparticles increased from 8 nm to 16 nm synthesized by precipitation method while the diameter increased from 9 nm to 15 nm in microwave assisted method. The XRD confirmed the wurtzite structure in all samples with no secondary phases. Optical properties were studied by means of Photoluminescence (PL) and absorption measurement. Band gap of nanoparticles were also calculated and found to decrease from 3.25 eV to 3.18 eV in ZnO particles prepared by the precipitation method, whereas bandgap decreases from 3.21 eV to 3.19 eV in the microwave method. Electrical transport properties of nanoparticles were analyzed by impedance spectroscopy using Cole-Cole plot technique. From Impedance spectra, we observe that resistance and capacitance of nanoparticles depend on the size of nanoparticles synthesized by both precipitation and microwave method.   

Realizing GaN/Er:GaN/GaN core-cladding planar waveguide gain medium via hydride vapor phase epitaxy

Erbium doped gallium nitride (Er:GaN) bulk crystals have been identified as a promising optical gain material for solid-state high energy lasers (HELs) operating at the 1.5 mm “retina-safe” spectral region. A highly desired design of HEL gain medium is the core-cladding planar waveguide (PWG) structure, capable of providing excellent heat dissipation and optical confinement. We report the realization of a GaN/Er:GaN/GaN core-cladding PWG structure grown by hydride vapor phase epitaxy (HVPE) and processed by mechanical and chemical-mechanical polishing. An Er doping concentration of [Er] = 3 × 10¹⁹ atoms/cm³ has been confirmed in the core layer via secondary ion mass spectrometry measurements. The structure emitted strong 1.54 μm emission under 980 nm resonant excitation. It was shown that a 96% optical confinement can be achieved in the Er:GaN core layer with a core thickness of 50 μm and [Er] = 3 × 10¹⁹ atoms/cm³. This work paves the way and marks an important progress towards the practical application of Er:GaN gain medium for “retina-safe” HELs.   

Two-photon absorption spectroscopy and anisotropy of bulk zincblende and diamond symmetry semiconductors and quantum dots

Si quantum dots (SiQD) are potential candidates for biologically-inert theranostic applications. Excitation in the red-near infrared (NIR) by two-photon absorption (2PA) is useful for confocal microscopy and therapy techniques. We studied the spectroscopic response and size-dependence of 2PA in colloidal ligand-passivated SiQDs in the red-NIR by indirect two-photon induced photoluminescence (2PIP) and direct pump-probe modulation spectroscopy (PPMS). We are developing a model to explain the spectra for the nanocrystalline response from ab initio calculations using length gauge theory for bulk Si. We measured the 2PA spectra and anisotropies of bulk Si, GaAs, and GaP in the range 650-2000 nm using PPMS in the femtosecond excitation regime to further verify these calculations.   

Scanning Photocurrent Measurements of Degenerately Doped Si

Scanning photocurrent microscopy (SPCM) has shown much promise as a means for direct and efficient measurements of carrier lifetime and diffusion in novel semiconductors including topological insulators. High spatial resolution and electrical sensitivity in these setups has made detection of properties such as diffusion length routine. Degenerately doped Si with a surface oxide are often used as back-gating substrates for thin films or flakes of the materials. However, the high photocurrent of the silicon often complicates the elucidation of the same photo responses in thin films/flakes. Here, we present SPCM measurements of a degenerately doped silicon substrate. After buffered HF etching of the surface oxide, Cr/Au electrodes were defined via thermal evaporation through a shadow mask, resulting in a device of 25 μm channel width between two back-to-back Schottky contacts. An elevated photocurrent was measured in the regions near the electrodes at zero bias voltage, indicating a diffusion current. The experiment has shown the structure as a viable platform for SPCM measurements of thin topological insulators under continuous backgate tuning.   

Temperature dependence of the indirect-gap photoluminescence from Ge

Few experimental reports have been published to date on the temperature dependence of the photoluminescence (PL) from Ge, and no attempts have been made to model the spectra using the standard Van Rosbroeck-Schockley equation that relates absorption to spontaneous emission. This anomaly can be traced back to the fact that the standard textbook expressions for indirect absorption are not applicable to Ge¹ due to the proximity of the direct and indirect gaps. If such expressions are used to compute the PL, not even a peak is predicted.
We report here a PL study of high-quality Ge samples at temperatures 10 K ≤ T ≤ 300 K. The spectra display a rich structure with a rapidly changing lineshape as a function of T. We show that when the more realistic theory of LA-phonon assisted absorption from Ref. 1 is used to compute the PL, peaks at about the right energy are predicted. However, the agreement between theory and experiment requires the additional incorporation of “forbidden” channels involving LO and TA phonons. Analytical expressions for the absorption assisted by these phonons are derived, and the resulting expressions are combined to predict a PL spectrum in very good agreement with experiment.

¹J. Menéndez et al. Phys. Rev. B 96, 121201 (2017), Phys. Rev. B 98, 165207(2018).
  

First-principles calculation of Jerk current in polar and nonpolar semiconductors

The bulk photovoltaic effect is the archetypical example of nontrivial carrier dynamics in illuminated insulators lacking a center of inversion. The injection current (also known as ballistic current or circular photogalvanic effect), a component of the bulk photovoltaic effect, could play an important role in optoelectronic applications, e.g. solar cells or as a probe of the topology of materials. The Jerk current is a generalization of the injection current to third order in the electric fields [1]. We present a first-principles calculation of the Jerk current spectrum in prototypical semiconductors Si, CdSe, GaAs and novel two-dimensional ferroelectric monolayer GeSe, GeS, SnS and SnSe. We show that, similar to the injection current, the jerk current is a sensitive probe of the geometry of the Bloch wave specially in reduced dimensions. Experimental ramifications are also discussed.

[1] Benjamin M. Fregoso, Rodrigo A. Muniz, J. E. Sipe, “Jerk current: a novel bulk photovoltaic effect”, Phys. Rev. Lett. 121, 176604 (2018)   

Exploration of Free Carrier Relaxation Dynamics in Bulk Gallium Arsenide

Multidimensional coherent spectroscopy (MDCS) is an ultrafast spectroscopic measurement that spreads coherent information across multiple dimensions, disentangling congested spectral features. Both coherent and incoherent transport processes, as well as their dynamics, can be easily separated and measured with MDCS. These attributes make MDCS ideal for studying complex material behaviors such as carrier-carrier scattering and exciton formation in semiconductors.

Here, we present MDCS spectra of bulk gallium arsenide (GaAs) showing strain-induced separation of the heavy-hole and light-hole exciton states¹, exciton formation from free carriers across a broad energy distribution, and relaxation from GaAs quantum wells in the same sample. The spectra clearly show the Sommerfeld enhancement of the free carrier absorption at long times, while early times show a transient feature related to the fast relaxation of states near the band-edge. We suspect that this transient feature is caused by correlated free carrier pairs or higher-lying excitonic states that form a quasi-continuum near the band edge.

[1] Wilmer et. al., Phys. Rev. B, 94, 2016.   

Evaluation of BaTiS3 for Thermoelectric Applications by Using First Principles Theory

BaTiS₃ is a semiconductor with a band gap of about 0.5 eV and very strong transport anisotropy for both electron and hole conduction. The conduction band minimum is very dispersive with a small effective mass and large mobility because of split-off dz² band contribution. The optical phonons are relatively soft, and the ground state non-centrosymmetric phase is favorable by just about 1 mev/fu over the centrosymmetric phase, indicating possibility of phase intermixing and reduced thermal conductivity at finite temperatures. Electrons can be doped easily for instance by an external dopant La. The possibility of large electrical conductivity with a tunable thermal conductivity by external doping and phase intermixing makes this compound excellent candidate for thermoelectric applications. By using the first principles density functional theory and linearized Boltzmann transport theory, we calculate electrical conductivity, thermal conductivity and the Seebeck coefficient and report reasonable thermoelectric figure of merit, ZT, of about 0.4 at 800K for n-type BaTiS₃. Additional possibility to enhance the figure of merit by doping, phase intermixing and band engineering is discussed.   

First Principles Study of Electronic, Magnetic and Thermoelectric Properties of Perovskite BaTbO3

Perovskite material have been on focus for many applications in spintronic device due to their novel properties such as ferroelectric, metal-insulator-transition, half-metallicity, non-trivial topological properties, etc. Recently these materials are found to show interesting thermoelectric properties which are expected to lead to new devices to harvest waste heat into efficient energy. In this work, we have investigated the electronic, magnetic, and thermoelectric properties of BaTbO₃ using the density functional formalism. The structure are fully relaxed and the value of the calculated lattice constant is 4.29 Å, in good agreement with the experimental values. The magnetic ground state of BaTbO₃ is found to be G-type antiferromagnet, with a band gap of 1.68 eV with an on-site Coulomb interaction of 4 eV for Tb atoms. Transport properties is computed using the BoltzTrap code. The calculated figure of merit (ZT) is 0.97 at 800 K, which indicates the suitability of BaTbO₃ as an efficient thermoelectric material.   

NMR and Mössbauer study of p-type half-Heusler thermoelectrics Nb1−xTixFeSb

To investigate the local behavior of defects and mixed compositions in NbFeSb-based semiconductors, for improved thermoelectric efficiency, we have performed ⁹³Nb and ¹²¹Sb NMR, as well as Mössbauer measurements, on pure NbFeSb and a series of p-type Ti-substituted (Nb,Ti)FeSb samples with different substitution levels. A small but consistently increasing paramagnetic defect density is observed with the increase of Ti substitution level revealing the existence of additional Ti-induced paramagnetic defects. NMR line shapes show a clear difference between effect of intrinsic and extrinsic defects in NbFeSb. The NMR shifts can be well understood by a model combining a Knight shift and composition-dependent chemical shift. The results indicate a nearly rigid-band behavior for the valence band with a small enhacement of effective mass vs substitution. For pure NbFeSb samples, the Mössbauer spectra include an additional T-dependent singlet. The increase of its area can be explained based on carriers activated into a shallow acceptor-like defect level above the valence band, consistent with the defect activation results obtained with NMR. In samples with Ti substitution, the Mössbauer spectra are consistent with a random neighbor distribution, indicating no preferential local ordering.