Session Z23: Semiconductors: Thermodynamic & Optical Properties II

Excitons in Ultrathin PbI$_{2}$ Crystals

Due to their weak inter-layer van der Waals bonding, layered materials offer the unique possibility to produce natural quantum wells in the form of single and few atomic layer samples. A technique known as micromechanical cleavage, involving repeated cleaving, is used to isolate samples of all thicknesses [1]. Here, we present a combined experimental and theoretical study of band-edge excitons in the layered compound PbI$_{2}$ and, in particular, on their behavior as a function of sample thickness. Results of photoluminescence and reflection experiments are reported on samples with thicknesses ranging from a few micrometers down to a few monolayers, as determined by atomic force microscopy measurements. The data display striking and well reproducible changes in the transition from three to two-dimensions, which will be compared with results of first-principles calculations of the electronic band structure based on density functional and many-body perturbation theory. Computational resources were provided by the DOE NERSC facility. [1] A. K. Geim et. al. Nature Materials 6, 183 (2007)   

Second harmonic generation from few-layer MoS2 and BN

We have measured optical second harmonic generation (SHG) from few-layer MoS2 and hexagon BN samples. In both materials, we observe SHG for odd numbers of layers. However, no appreciable SHG signal is observed for samples with even numbers of layers. This general behavior is compatible with the fact that individual layers of each material are non-centrosymmetric, thus allow SHG in the dipole approximation. For even layer thickness, on the other hand, the overall structures become centrosymmetric, with adjacent layers producing canceling contributions. In the case of odd layer thickness, we observe strong in-plane variation of the SHG signal with polarization. From this dependence, we can determine crystallographic axes in the material by a purely optical measurement. We also discuss the evolution of the signal strength in the two material systems with thickness (for odd layer numbers), considering both propagation effects and the evolution of the electronic structure of the material with thickness [1].\\[4pt] [1] Kin Fai Mak, Changgu Lee, James Hone, Jie Shan and Tony F. Heinz, Phys. Rev. Lett, \textbf{105}, 136805, 2010   

Low Temperature Optical Spectroscopy of Excitons and Trions in Monolayer MoS2

Monolayer MoS2 is a two-dimensional (2D) semiconductor with optical properties different from conventional inorganic semiconductors. We will present our results on low temperature absorption and photoluminescence (PL) spectroscopy of monolayer MoS2 crystals. As a result of the large carrier effective masses and low dielectric screening in the 2D geometry, the excitons in MoS2 are tightly bound with large binding energies. We find that the prominent peak at $\sim$1.9 eV in both PL and absorption spectra at low temperatures is split in two: an exciton peak and a trion peak. The binding energies of trions, measured relative to the excitons, are extremely large and in the 30-35 meV range. We find that the trion peak acquires more spectral weight than the exciton peak as the electron density increases, and also broadens due to increased scattering with electrons. The temperature dependence of the exciton and trion PL intensities enables us to determine the radiative recombination efficiencies as a function of the temperature. We also observe Stokes shifts of $\sim$5 meV of both exciton and trion peaks, indicative of lattice distortions accompanying the quasiparticles (i.e. polarons) in this highly polar crystal.   

Ultrafast Optical Pump-Probe Studies of Photoexcited Carrier Dynamics in Mono-Layer MoS2

Mono-layer MoS2 is a 2D semiconductor with a direct bandgap. We present, for the first time, ultrafast optical pump-probe measurements results on the relaxation dynamics of photoexcited carriers in monolayer MoS2. Pulses at energies 2.74 eV and 1.37 eV with width ~100 fs are used in our experiments. The pump photon energy is larger than the bandgap, and the probe is below the optical absorption edge. Our results show that the differential transmission of the probe is negative with three distinct features: i) an initial probe absorption due to two-photon absorption involving also the pump pulse ii) a fast relaxation transient lasting to about ~1ps in which the differential transmission recovers by almost ~90\%. iii) a very slow recovery of the transient that lasts about 200ps. We explain the observed transients in terms of the relaxation of the carriers to the conduction band bottom, followed by the formation of excitons and trions, and the response of the excitons and trions. The extremely large exciton and trion binding energies make these states much more preferable than the free carrier states in the bands. The contributions of interband and intraband processes to the observed transients will be explained and the extracted relaxation and recombination rates will be discussed.   

Optical behavior of native defects in ZnO

The behavior of native defects in ZnO has been fiercely debated for years, yet questions still remain regarding their fundamental properties. Once blamed for causing unintentional $n$-type conductivity, it is now well-established that native donors are highly unlikely to act as shallow donors in as-grown material. Still, both native donors and acceptors may be present in some samples, acting as either compensating acceptors or deep donors that may inhibit attempts to obtain high-conductivity $n$-type ZnO. In this work, we re-examine the properties of native donors and acceptors in ZnO using hybrid density functional calculations, which allow for the quantitative prediction of defect transition levels and formation energies. We focus on the optical and electrical properties of these defects, and calculate both their optical and thermodynamic transition levels. Most of the defects give rise to deep, broad luminescence signals that can serve as a means of experimentally verifying the nature of the center. We also examine how interactions with hydrogen interstitials affect the properties of these defects.   

Strain-gradient dominated emission energy shift of pure-bending ZnO wire

High special/energy resolution cathodoluminescence (CL) spectroscopy enables us to make precise investigation on the optical/electronic fine structures in nanostructures. The linear distribution of strain gradient from tensile to compression in bent ZnO nano/microwires provides ideal conditions to address the modification of the electronic structures by strain in semiconductor materials. Radial line scan of the CL spectroscopy along bent ZnO wires at liquid helium temperature shows very fine excitonic emission structures, which demonstrates systematic red shift towards the increase of tensile strain, and blue shift as well as excitonic peak splitting towards the increase of compressive strain. Strain-gradient is found to dominate the overall red-shift of the emission energy at a pure bending configuration.   

Electronic structure and optical properties of CuYO$_2$ nanocrystals

A unique class of highly stabile, self-saturated and self-charge-compensated delafossite nanocrystals has been identified. The density functional theory (DFT) study of structural and electronic properties of these nano-crystalline CuYO$_{2}$ will be presented. To have a better estimate of the electronic excitation energies, and consequently the optical gap, time dependent DFT has been employed as well. The goal is to show, first of all, that these unique set of nanocrystals exists, and to study whether the nano-phase can modify the electronic properties for enhanced optical absorption. It has potential application as photocatalysts for H$_{2}$ production by water splitting.   

Contactless Electroreflectance Characterization of a Triple Asymmetric Coupled Quantum Well Active Region of a ZnCdMgSe-Based Quantum Cascade Laser

Quantum cascade lasers (QCL) with emission at wavelengths below 4$\mu $m are difficult to achieve from conventional GaAs and InP based systems due to the small conduction band offset (CBO) of those materials. The II-VI materials ZnCdSe/ZnCdMgSe, with as much as 1.1 eV CBO and no inter-valley scattering, are excellent candidates to achieve this goal. We grew by MBE a QCL structure made of ZnCdSe and ZnCdMgSe lattice matched to InP. Triple asymmetric coupled quantum well (3ACQW) structures were also grown which contain only the active region of the QCL separated by quaternary barrier layers. The 3ACQW structure was characterized by contactless electroreflectance (CER). A model based on the transfer matrix method (TMM) was used to identify the CER transitions and to predict the Fourier transform infra-red (FT-IR) absorption spectrum of the full QCL structure. Excellent agreement between the predicted and the experimental FT-IR absorption peaks was observed.   

Design of N-doped anatase TiO$_{2}$ photocatalyst with visible-light-response based on Ti-O bond weakening

Nitrogen bulk doping is an effective strategy to change the electronic structures of anatase TiO$_{2}$ photocatalyst for visible light response improvement. Unfortunately, it is hard to achieve nitrogen bulk doping in practice, due to both limited thermodynamic solubility of substitutional nitrogen and N-induced recombination centers. It remains challenging yet highly desirable to develop new doping approach to increase nitrogen solubility in bulk. This challenge is originally stemmed from both strong Ti-O bond and charge difference (O$^{2-}$ versus N$^{3-})$ between lattice oxygen and nitrogen dopant. In this work, we propose a new doping approach to promote the bulk substitution of lattice oxygen with nitrogen in bulk anatase TiO$_{2}$, based on the Ti-O bond weakening by pre-implanted interstitial boron.$^{1}$ By using the first-principles calculations, we study the interstitial boron induced Ti-O bonding weakening and the thermodynamics/kinetics changes for nitrogen bulk doping.$^{2}$ In experiment, we realize to synthesize a bulk gradient B-N co-doping red anatase TiO$_{2}$ microsphere which has an extended absorption edge up to ca. 700 nm covering the full visible light spectrum and has a bandgap varying from 1.94 eV on its surface to 3.22 eV in its core by gradually elevating VBM. This approach could be extended to modify other electronic materials that demand bulk substitutional doping. 1. G. Liu, J. Pan, L. C. Yin et al., Adv. Funct. Mater., 2012, 22, 3233. 2. G. Liu, L. C. Yin, J. Q. Wang et al., Energy Environ. Sci. 2012, 5, 9603.   

Ultrafast Time- and Phase-Resolved Second Harmonic Generation

Ultrafast pump-probe experiments typically measure the changes in the amplitude of the probe light after it interacts with a sample as a function of pump time delay. However, measured amplitude change is typically a result of multiple processes happening in the sample. It is usually not possible to isolate these different processes from the measured amplitude response. Here we show using GaAs and Bi2Se3 as test samples that by probing interferometrically, phase information about the pump-probe signal can also be acquired. We find that different components of the signal in general have different optical phases associated with them, which can be isolated by changing the path length difference within the interferometer. We show that phase information about second harmonic light generated by the sample can also be obtained and we report progress toward realizing simultaneous~phase- and time-resolved second harmonic pump-probe measurements.   

First-principles study of $\gamma$-ray detector materials in perovskite halides

In an effort to search for good $\gamma$-ray detector materials, perovskite halide compounds containing heavy elements were investigated. Despite the three-dimensional network of the corner shared octahedra and the extended nature of the outermost shell, its strong ionic character leads to a large band gap, which is one of the essential criteria for $\gamma$-ray detector materials. Thus, considering high density and high atomic number, these pervoskite halides are possible candidate for $\gamma$-ray detector materials. We performed first-principles calculations to investigate electronic structures and thermodynamic properties of intrinsic defects in the selected perovskite halide, CsPbBr$_3$. The screened-exchange local density approximation scheme was employed to correct the underestimation of the band gap in the LDA method. As a result, the calculated band gap of CsPbBr$_3$ is found to be suitable for $\gamma$-ray detection. Furthermore, defect formation energy calculations allow us to predict thermodynamic and electronic properties of possible intrinsic defects, which affect detector efficiency and energy resolution.   

Temporal long-range order in exciton-polariton condensates

We demonstrate temporal long-range off-diagonal order in microcavity exciton-polariton dynamic condensation, by interference between two separate condensates, generated at different times and with different momenta. In our pulsed resonantly-injected condensates, stimulated polariton-polariton scattering results in spectral narrowing of the dynamic condensates and thus in longer coherence times. We study the temporal decay of the long-range order by monitoring the interference visibility between the condensates. We show that it strongly depends on the excitonic fraction of the polaritons and the corresponding polariton-polariton interaction strength, as well as on the temperature and pump intensity. Moreover, polariton interaction yields a blue shift of the condensate energy, which appears as a time-dependent shift in the interference pattern. These results show a direct evidence of temporal long-range order in dynamic condensates as well as demonstrate a new method for probing their ultrafast dynamics, opening new directions in the fundamental study of coherence in matter.   

Statistics of Data Fitting: Flaws and Fixes of Polynomial Analysis of Channeled Spectra

Starting from general statistical principles, we have critically examined Baumeister's procedure* for determining the refractive index of thin films from channeled spectra. Briefly, the method assumes that the index and interference fringe order may be approximated by polynomials quadratic and cubic in photon energy, respectively. The coefficients of the polynomials are related by differentiation, which is equivalent to comparing energy differences between fringes. However, we find that when the fringe order is calculated from the published IR index for silicon* and then analyzed with Baumeister's procedure, the results do not reproduce the original index. This problem has been traced to 1. Use of unphysical powers in the polynomials (e.g., time-reversal invariance requires that the index is an even function of photon energy), and 2. Use of insufficient terms of the correct parity. Exclusion of unphysical terms and addition of quartic and quintic terms to the index and order polynomials yields significantly better fits with fewer parameters. This represents a specific example of using statistics to determine if the assumed fitting model adequately captures the physics contained in experimental data. The use of analysis of variance (ANOVA) and the Durbin-Watson statistic to test criteria for the validity of least-squares fitting will be discussed. *D.F. Edwards and E. Ochoa, Appl. Opt. 19, 4130 (1980).   

Resolving sub-phonon wavelength superlattices using photoacoustic spectroscopy

We have investigated the coherent acoustic phonon spectroscopic response of a mutilayer GaAs/Al$_x$Ga$_{1-x}$As/AlAs heterostructure with varying layer thickness and Al concentration. The optical response shows a low frequency effective Brillouin oscillatory response and an additional time-dependent change in reflectivity arising from the multilayer features. We can also resolve structural features less than the spatial width of the acoustic strain pulse. We model the optical response from each feature and develop general criteria of the layer thickness and the acoustic strain width that determine the total response. This allows us to determine whether the optical response of a given layer will be superimposed on the overall Brillouin response or will provide an individual Brillouin response from within the layer. Our results help provide a basis for a nondestructive method of determining material properties in stratified media.