Session A54: Optical Properties of Semiconductor Nanostructures I

Detection and characterization of semiconductor thin film domains in non linear neafield regime

High carrier mobility in III-V semiconductor films is attractive for electro-optic devices based on Si substrate. The mismatch between thin film and substrate crystal creates defects that affect electron transport in the film. Optical nonlinear (second harmonic generation) technique has been used in search of domains and boundaries that might have connection to these defects. Fiber based nonlinear nearfield scanning optical microscope (NSOM) was used to detect sub-micron domains at surfaces of the films. This local nearfield optical information was compared with bulk farfield optical information and suppression of the domains was observed and studied by controlling the substrate-film interface. Anti phase boundaries, strain, local charge and cavities in connection with these domains are under study.   

Interfacial Stresses and Strains Effect on Band-Gap Emission from Silicon

Czochralski silicon wafer materials were interfaced with silica films formed by sol-gel deposition and thermal annealing. Under optimal annealing conditions ($\sim$700 $^{\circ}$C), stresses in the silica films induce variations in elastic strains on the order of 1\% in the silicon. Concomitantly, emission of band-gap photons at 1.1 eV observed by photoluminescence is increased by two orders of magnitude relative to unperturbed silicon. The enhancement in photon emission is produced by band-gap modulations estimated as $\sim$0.1 eV. Elastic reversibility of the strains is inferred from recovery of relatively weak photon emission for annealing above the glass reflow temperature of deposited silica films ($\sim$950 $^{\circ}$C). Films with largest stress variations exhibit enhanced absorption signatures in the infrared and broadening of Si-O-Si stretching vibrations. Examples of Si-based photonics based on the observed effect will be presented.   

\textbf{Electronic Raman Scattering as an Ultra-Sensitive Probe of Strain Effects in Semiconductors}.

Semiconductor strain engineering has become a critical feature of high-performance electronics due to the significant device performance enhancements it enables. These improvements that emerge from strain induced modifications to the electronic band structure necessitate new ultra-sensitive tools for probing strain in semiconductors. Using electronic Raman scattering, we recently showed that it is possible to measure minute amounts of strain in thin semiconductor epilayers. We applied this strain measurement technique to two different semiconductor alloy systems, using coherently strained epitaxial thin films specifically designed to produce lattice-mismatch strains as small as 10$^{\mathrm{-4}}$. Comparing our strain sensitivity and signal strength in Al$_{x}$Ga$_{1-x}$As with those obtained using the industry-standard technique of phonon Raman scattering we found a sensitivity improvement of \texttimes 200, and a signal enhancement of 4 \texttimes 10$^{\mathrm{3}}$ thus obviating key constraints in semiconductor strain metrology. The sensitivity of this approach rivals that of contemporary techniques and opens up a new realm for optically probing strain effects on electronic band structure.   

Excitation mechanisms of Er optical centers GaN epilayers

We report direct evidence of two mechanisms responsible for the excitation of optically active Er3$+$ ions in GaN epilayers grown by metal-organic chemical vapor deposition. These mechanisms, resonant excitation via the higher-lying inner 4f shell transitions and band-to-band excitation of the semiconductor host, lead to narrow emission lines from isolated and the defect-related Er optical centers. However, these centers have different photoluminescence spectra, local defect environments, decay dynamics, and excitation cross sections. The photoluminescence at 1.54 micrometer from the isolated Er optical center which can be excited by either mechanism has the same decay dynamics, but possesses a much higher excitation cross-section under band-to-band excitation. In contrast, the photoluminescence at 1.54 micrometer from the defect-related Er optical center can only be observed through band-to-band excitation but has the largest excitation cross-section. These results explain the difficulty in achieving gain in Er doped GaN and indicate approaches for realization of optical amplification, and possibly lasing, at room temperature.   

Spectroscopic characterization of Er optical center in multiple quantum wells AlN/GaN:Er

Er doped GaN material is known to result in the formation of luminescent centers suitable for applications in optoelectronic devices. We report here a significant enhancement of photoluminescence from the Er optical center at 1.5 micrometer in multi-nanolayer structures AlN/GaN:Er synthesized by metal organic chemical vapor deposition. The enhancement of photoluminescence from Er optical center can be explained via the carrier confinement and strain engineering of multi-nanolayer structures. We study the influence of the quantum wells and barrier width on the photoluminescence at 1.5 micrometer using time-resolved and high-resolution photoluminescence spectroscopy at a large range of temperature. The ability of controlling the carrier confinement in multi-nanolayer structures provides us the possibility of engineering Er doped GaN photonic devices with enhanced optical characteristics at 1.54 micrometer.   

Observation of magnetic non-reciprocity for mobile excitons bound to stacking-fault potentials

We show that single stacking faults in high-purity GaAs provide the most homogeneous two-dimensional potential for excitons yet reported. The ultra-narrow excitonic transitions enable us to directly observe the microscopic properties of the exciton, including a 0 transverse component of the hole g-factor, which are determined by the $C_{3v}$ symmetry of the system. A surprising magnetic non-reciprocity effect, in which the energy of the detected excitonic emission depends on the sign of the magnetic field, is also observed. This effect is due to conservation of the exciton two-dimensional momentum in the process of light emission and provides direct evidence that excitons are mobile in this novel potential.   

Exploration of exciton delocalization in organic crystalline thin films

The electronic properties of organic semiconductors play a crucial role in designing new materials for specific applications. Our group recently found evidence for a rotation of molecular planes in phthalocyanines that is responsible for the disappearance of a delocalized exciton in these systems for T \textgreater 150K.................()().......$^{\mathrm{1}}$ In this study, we attempt to tune the exciton delocalization of small organic molecules using strain effects and alloying different molecules in the same family. The exciton behavior is monitored using time- and polarization resolved photolumniscence (PL) spectroscopy as a function of temperature. Specifically, organic crystalline thin films of octabutoxy phthalocyanine (H$_{\mathrm{2}}$OBPc), octyloxy phthalocyanines and H-bonded semiconductors such as the quinacridone and indigo derivatives are deposited on flexible substrates (i.e. Kapton and PEN) using an in-house developed pen-writing method.........$^{\mathrm{2}}$ that results in crystalline films with macroscopic long range order. The room temperature PL studies show redshift and changes in polarization upon bending of the film. Crystalline thin films of alloyed H$_{\mathrm{2}}$OBPc and octabutoxy naphthalocyanine with ratios ranging from 1:1 to 100:1 fabricated on both sapphire and flexible substrates are also explored using the same PL spectroscopy to elucidate the behaviors of delocalized excitons. .$^{\mathrm{1 }}$N. Rawat, et al., J Phys Chem Lett \textbf{6}, 1834 (2015). $^{\mathrm{2 }}$R. L. Headrick, et al., Applied Physics Letters \textbf{92}, 063302 (2008).   

Collective magneto-polariton excitation in a terahertz photonic cavity

Collective excitations in solids offer new opportunities for quantum optical studies. Many-body interactions inherent to condensed matter systems can lead to novel phenomena that cannot be achieved in traditional atomic systems. Here, we report collective ultrastrong light-matter coupling in a two-dimensional electron gas in a high-$Q$ terahertz photonic-crystal cavity in a magnetic field. We directly observed time-domain vacuum Rabi oscillations, whose frequency was found to be proportional to the square root of $N$ (where $N$ is the carrier density), evidence for the {\em collective} nature of ultrastrong coupling. In addition, a small but definite blue shift due to the diamagnetic term in the Hamiltonian was observed for the polariton frequencies, which is another signature of ultrastrong light-matter coupling. Furthermore, the high-$Q$ cavity suppressed the superradiant decay of cyclotron resonance, which resulted in unprecedentedly narrow intrinsic cyclotron resonance linewidths ($\sim$5.6 GHz at 2 K). Our method is also applicable to many classes of strongly correlated systems with collective many-body excitations in the terahertz range, opening a door to the fascinating physics of terahertz many-body cavity QED.   

Multidimensional spectroscopy of exciton polaritons in a microcavity

Two-dimensional coherent spectra map the anticrossing associated with normal-mode splitting in a semiconductor microcavity [1]. For a 12-meV detuning range near zero detuning, it is observed that there are two diagonal features related to the intra-action of exciton-polariton branches and two off-diagonal features related to coherent interaction between the polaritons. A biexcitonic companion feature is observed, shifted from the exciton feature by the biexciton binding energy. Closer to zero detuning, all features are enhanced and the diagonal intra-action features become nearly equal in amplitude and linewidth. Off-diagonal interaction features are strongly modulated (and invert) at small positive detuning, as the lower polariton branch crosses the bound biexciton energy determined from negatively detuned spectra. This Feshbach type behavior is further evidenced by strong polarization dependence. By exploiting selection rules, the quantum pathways can be more rigorously controlled, allowing the Feshbach coupling to be switched on an off as well as elucidating the role spin and two-quantum states play in the exciton-polariton system. \newline [1]. Wilmer et al, Phys. Rev. B 91, 201304(R) (2015)   

Indentation-induced structural phase transformations of semiconductor materials and applications

During hardness indentation materials are subjected to highly localized pressures. These pressures may cause a complete change of the crystal structure of the material within the indented zone. Such structural phase transformations were observed within Vickers indentations made at room temperature in single crystals and amorphous films of Si and Ge. However, when indentations were made at 77 K in Si and Ge, no phase transitions were observed in either. Measurements were also taken from indentations made in silicon single crystals at different temperatures namely 263, 243, 235 and 206 K, and they showed a strong correlation of phase transformation with temperature. It was suggested that during room temperature indentations there is a significant temperature rise approximately to 760 K, which may assist phase transformation. Raman spectroscopy was used as an \textit{ex-situ} tool monitoring phase transformations in semiconductor materials. \textit{In-situ} electrical characterizations of indentation-induced metallization in single crystals of silicon were performed using two- and four-contact measurements. The previous work has led to a technique relates to semiconductor device manufacturing, including solar cells, which is a method for controlling the removal of a surface layer from a base substrate utilizing low-temperature.   

Photon correlations through Raman virtual processes

In Raman inelastic scattering phonons are either absorbed or created, in what is respectively called an anti-Stokes (aS) or a Stokes (S) process. While these two processes are generally uncorrelated, it is possible that the same phonon generated by S is subsequently absorbed by aS. This two photon process is referred to as SaS. In a standard Raman process, conservation of energy forbids virtual phonons to play a role. However, in a SaS process these virtual phonons may be relevant as long as their lifetimes exceed the interval between the two scatterings. We derive the effective photon-photon interaction mediated by the phonon field. The effective hamiltonian is analogue to the one present in BCS superconductivity. The difference lies in the nature of the particles involved -- since photons are bosons, there is no Fermi sea instability and no pair condensation. Still it is possible to obtain an attractive photon-photon interaction. Finally, we propose an experiment to detect the correlated photons emerging from a semiconductor. We pinpoint the material properties that might enhance this effect and discuss the possible technological applications of this idea as a correlated photon source.   

Calculating Effect of Point Defects on Optical Absorption Spectra of III-V Semiconductor Superlattices Based on (8x8) k-dot-p Band Structures

For a superlattice which is composed of layered zinc-blende structure III-V semiconductor materials, its realistic anisotropic band structures around the Gamma-point are calculated by using the (8x8)k-dot-p method with the inclusion of the self-consistent Hartree potential and the spin-orbit coupling. By including the many-body screening effect, the obtained band structures are further employed to calculate the optical absorption coefficient which is associated with the interband electron transitions. As a result of a reduced quasiparticle lifetime due to scattering with point defects in the system, the self-consistent vertex correction to the optical response function is also calculated with the help of the second-order Born approximation.   

Many-body Effects and the Role of Indirect Excitons in Asymmetric InGaAs/GaAs Double Quantum Wells

In semiconductor research, a fundamental question is how excitons in nearby but distinct spatial locations interact and exchange energy. In quantum well heterostructures, these interactions can be conveniently probed via optical coherent multidimensional spectroscopy (CMDS). Recently, it has been shown using CMDS that reducing the GaAs barrier from 30 nm to 10 nm between two asymmetric InGaAs quantum wells results in interactions driven by many-body effects. Here, we use the technique to show that for narrower barrier thicknesses, the interactions are accompanied by an emergence of spatially indirect excitons. Quantitative measurements of the effects are presented, which will be useful in tailoring GaAs heterostructure devices, and may also inform the role that excitonic interactions play in more complicated systems like microcavity polariton structures and/or photosynthetic light harvesting complexes.   

Coupled Plasmon Phonon Dynamics in GaP: an indirect gap polar semiconductor.

Transient Depletion Field Screening (TDFS) is the dominant mechanism behind coupled plasmon-phonon oscillations in polar semiconductors for above gap photoexcitation. Here the surface field distorting the polar lattice is screened by photoexcited plasma initiating coupled oscillations. These oscillations modify the optical property of the material and are observed in reflectivity measurements. We model these oscillations via a set of coupled differential equations in electronic polarization and lattice polarization. We consider the effects of lateral inhomogeneity and diffusion of photoexcited carriers which is crucial to understand the experimental results. The spectrum shows an LO(Longitudinal Optical) phonon peak alongside a LOPC(Longitudinal Optical Plasmon Coupled) peak. Lateral inhomogeneity accounts for the beating phenomenon between these frequencies.   

GaAs Refractive Index Dependence On Carrier Density and Optimizing Terahertz Devices

GaAs is used for various applications, including high speed transistors, high-efficiency photovoltaic cells, electro-optics and terahertz (THz) emitters and detectors. To date, information on the refractive index of GaAs is available only over a limited wave spectrum of 0.2-17um, where the refractive index varies from 1.3 to 5.0. As detailed information on the refractive index of GaAs at THz frequencies is not available or inadequate for our effort to develop an improved GaAs-based THz emitter, we experimentally investigated the behavior of the refractive index of GaAs for different charge carrier densities, especially with or without the presence of surface plasma. Using a Time Domain THz Spectrometer, which is capable of measuring THz pulses containing a wave spectrum over 100-3000um with a time accuracy better than 6 femtoseconds, we measured the delay of THz pulses traversing through a GaAs substrate of known thickness while modulating the charge carrier concentration. From the experimental data we estimated the refractive index for THz frequencies to vary from 3.5 to 3.8 for different charge carrier concentrations. We will discuss details of our experiments and implications of our experimental results, especially for our GaAs-based THz devices.