Polarized Photoluminescence Excitation Spectroscopy Research Articles

Anisotropy of optical transitions in β-Ga2O3 investigated by polarized photoluminescence excitation spectroscopy

The monoclinic beta-phase of gallium oxide possesses an ultra-wide bandgap that surpasses other wide bandgap materials such as SiC and GaN, making it a promising candidate for power electronic device technologies. We investigate the first fundamental optical transitions in this material, which exhibit a strong directional dependence. To determine the energies and orientations of these transitions, temperature-dependent and angular resolved photoluminescence excitation spectroscopy is applied. We observe a distinct excitation channel located energetically between those of the first two optical transitions Γ1−1 and Γ1−2. While previous absorption edge and reflectance spectroscopy studies have assigned a transition in this spectral range to either the Γ1−1 or the Γ1−2 transition, our findings demonstrate no pronounced polarization dependence of this excitation channel within the (010) plane, an observation not reflected in calculations of the band-to-band transitions in β-Ga2O3.

Structural, electronic, and optical properties ofm-plane InGaN/GaN quantum wells: Insights from experiment and atomistic theory

In this paper we present a detailed analysis of the structural, electronic, and optical properties of an $m$-plane (In,Ga)N/GaN quantum well structure grown by metal organic vapor phase epitaxy. The sample has been structurally characterized by x-ray diffraction, scanning transmission electron microscopy, and 3D atom probe tomography. The optical properties of the sample have been studied by photoluminescence (PL), time-resolved PL spectroscopy, and polarized PL excitation spectroscopy. The PL spectrum consisted of a very broad PL line with a high degree of optical linear polarization. To understand the optical properties we have performed atomistic tight-binding calculations, and based on our initial atom probe tomography data, the model includes the effects of strain and built-in field variations arising from random alloy fluctuations. Furthermore, we included Coulomb effects in the calculations. Our microscopic theoretical description reveals strong hole wave function localization effects due to random alloy fluctuations, resulting in strong variations in ground state energies and consequently the corresponding transition energies. This is consistent with the experimentally observed broad PL peak. Furthermore, when including Coulomb contributions in the calculations we find strong exciton localization effects which explain the form of the PL decay transients. Additionally, the theoretical results confirm the experimentally observed high degree of optical linear polarization. Overall, the theoretical data are in very good agreement with the experimental findings, highlighting the strong impact of the microscopic alloy structure on the optoelectronic properties of these systems.

Illuminating the Second Conduction Band and Spin–Orbit Energy in Single Wurtzite InP Nanowires

We use polarized photoluminescence excitation spectroscopy to observe the energy and symmetry of the predicted second conduction band in 130 nm diameter wurtzite InP nanowires. We find direct spectroscopic signatures for optical transitions among the A, B, and C hole bands and both the first and the second conduction bands. We determine that the splitting between the first and second conduction bands is 228 ± 7 meV in excellent agreement with theory. From these energies we show that the spin-orbit energy changes substantially between zinc blende and wurtzite InP. We discuss the two quite different solutions within the quasi-cubic approximation and the implications for these measurements. Finally, the observation of well-defined optical transitions between the B- and C-hole bands and the second conduction band suggests that either the theoretical description of the second conduction band as possessing Γ8 symmetry is incomplete, or other interactions are enabling these forbidden transitions.

Electron-hole asymmetry in single-walled carbon nanotubes probed by direct observation of transverse quasidark excitons

We studied the asymmetry between valence and conduction bands in single-walled carbon nanotubes through the direct observation of spin-singlet transverse dark excitons using polarized photoluminescence excitation spectroscopy. The intrinsic electron-hole $(e\text{\ensuremath{-}}h)$ asymmetry lifts the degeneracy of the transverse exciton wave functions at two equivalent $K$ and ${K}^{\ensuremath{'}}$ valleys in momentum space, which gives finite oscillator strength to transverse dark exciton states. Chirality-dependent spectral weight transfer to transverse dark states was clearly observed, indicating that the degree of the $e\text{\ensuremath{-}}h$ asymmetry depends on the specific nanotube structure. Based on comparison between theoretical and experimental results, we evaluated the band asymmetry parameters in various carbon nanotube structures and graphene.

Excited Excitonic States in Single-Walled Carbon Nanotubes

Polarized photoluminescence excitation spectroscopy on individual SWNTs reveals not only the longitudinal and transverse E 11, E 22, and E 12 ground-state excitons but also excited excitonic states including the continuum. When heated, SWNTs are known to undergo a bandgap shift transition (BST), which effectively changes the nanotube dielectric environment. Here, we show that the entire spectrum of excitonic resonances blue shifts under this transition, with excited states showing larger shifts, approaching 100 meV for a 1 nm diameter nanotube. The excitonic binding energy, Coulomb self-energy correction, and dielectric shift under the BST are estimated. Analysis of this blue shift reveals the dominant effect of dielectric screening on SWNT excitonic states.

Spin-Conserving Tunneling of Excitons in Diluted Magnetic Semiconductor Double Quantum Wells

Spin polarization in carrier tunneling was studied in double quantum wells by the polarized photoluminescence-excitation spectroscopy. The double quantum wells consist of a diluted magnetic quantum well of Zn0.77Cd0.15Mn0.08Se and a nonmagnetic quantum well of Zn0.82Cd0.18Se. Efficient spin-conserving tunneling of an exciton as an entity was observed from the nonmagnetic quantum well to the magnetic well. The spin-reversing tunneling was suppressed by two orders of magnitude in high magnetic field. The spin-conserving tunneling time was determined as 20 ps by time resolved photoluminescence measurement.

Cross-polarized optical absorption of single-walled nanotubes by polarized photoluminescence excitation spectroscopy

Cross-polarized absorption peaks of isolated single-walled carbon nanotubes were observed by a polarized photoluminescence excitation (PLE) spectroscopy. Using a simple theory for PL anisotropy, the observed PLE spectra are decomposed into 'pure' components of the photoexcitation for incident light polarized parallel and perpendicular to the SWNT axis. For several (n, m) SWNTs, distinct peaks corresponding to perpendicular excitation were observed. The measured transition energies for perpendicular excitations were blue-shifted compared to the qualitative values predicted within a single-particle theory. The results indicate a smaller exciton binding energy for perpendicular excitations than for parallel excitations.

Polarized Fine Structure in the Photoluminescence Excitation Spectrum of a Negatively Charged Quantum Dot

We report polarized photoluminescence excitation spectroscopy of the negative trion in single charge-tunable quantum dots. The spectrum exhibits a p-shell resonance with polarized fine structure arising from the direct excitation of the electron spin triplet states. The energy splitting arises from the axially symmetric electron-hole exchange interaction. The magnitude and sign of the polarization are understood from the spin character of the triplet states and a small amount of quantum dot asymmetry, which mixes the wave functions through asymmetric e-e and e-h exchange interactions.

Observation of the Stark effect in coupled quantum wells by electroluminescence and circularly polarized photoluminescence excitation spectroscopy

Successful observation is reported of the quantum-confined Stark effect in electroluminescence (EL) under a ‘‘forward’’ electric-field condition, with a current injected into a p-i-n diode containing Al0.3Ga0.7As/GaAs symmetric coupled double quantum wells (CDQWs). The Stark shift observed in EL and (under a reverse bias) in photoluminescence (PL), in both of which the transitions involved are between the electron and heavy-hole lowest states, is found to be symmetrical with respect to the flatband bias, in agreement with calculations. The origin of the optical transitions between electron and light- or heavy-hole states in the CDQWs is also clearly identified by a combination of PL and circularly polarized photoluminescence excitation spectra.

Valence-subband level crossing in GaAs/GaAsP strained-barrier quantum well structures observed by circularly polarized photoluminescence excitation spectroscopy

We have investigated the level configuration of the valence subbands in GaAs/GaAs 1− x P x ( x = 0.23) strained-barrier quantum well structures as a function of the well width. Circularly polarized photoluminescence excitation (CPPLE) spectroscopy was used to identify the heavy- or light-hole character of the optical transitions. Distinct polarization properties were observed in the strain-split heavy- and light-hole bands in the GaAsP barriers, which indicates the photoexcited electrons in the barriers retain spin orientation well even after the capture into the well and the relaxation process. Utilizing this polarization tendency of the transitions in the barriers, we could distinguish unequivocally the heavy- and light-hole character of the quantum well-related transitions. By varying the well width systematically, we observed the level crossing between n = 1 heavy- and light-hole transitions around the well width of 4 nm. This is in good agreement with the calculation based on the effective mass approximation.

GaAs/sub 1-x/P/sub x//GaAs quantum-well structures with tensile-strained barriers

An investigation of GaAs QW's with tensile-strained GaAsP barriers grown on GaAs substrates by organometallic vapor phase epitaxy is reported. We demonstrate that this system permits light- and heavy-hole valence bands to be approximately merged within a wide range of well widths and strains, thereby increasing the yield of devices requiring these characteristics. A few series of quantum wells with three phosphorus compositions (6%, 9%, and 19%) were grown and studied by photoluminescence and polarized photoluminescence excitation spectroscopy. We compared our experimentally determined conduction band to heavy-hole and light-hole transition energies with finite potential well calculations utilizing a previously developed strain dependent band offset model. We obtained excellent agreement between experimental and calculated results without any adjustment or fitting of parameters. >

Observation of the valence-subband level crossing in GaAs/GaAsP strained-barrier quantum well structures using circularly polarized photoluminescence excitation spectroscopy

We have investigated the valence-subband level configuration in GaAs/GaAsP strained-barrier quantum wells as a function of the well width. As we systematically varied the well width, we observed the crossover behavior between the n=1 heavy- and light-hole states around the well width of 4 nm. Circularly polarized photoluminescence excitation (CPPLE) spectroscopy was used to identify the heavy- or light-hole character of the observed transitions. Distinct polarization properties were observed in the strain-split heavy- and light-hole states in the GaAsP barriers, which indicated that the photoexcited electrons in the barriers retained spin memory well even after the capture into the well through the relaxation process of energy and momentum. Taking advantage of this method we were able to identify unequivocally the heavy- and light-hole characters of the transitions in the quantum well. The well-width dependence of the intersubband transition energies obtained by the CPPLE measurement is in good agreement with the calculation based on the effective mass approximation.

Photoluminescence-excitation-spectroscopy studies in spontaneously ordered GaInP2.

We present results of an experimental study of the optical properties of spontaneously ordered ${\mathrm{GaInP}}_{2}$ using the techniques of low-temperature polarized photoluminescence and polarized photoluminescence excitation spectroscopy. Highly ordered samples are seen to have an absorption edge which is softer, more polarization dependent, and blueshifted farther from the PL peak than less-ordered samples. The data provide evidence for a statistical distribution of domains having different order parameters in epitaxial films of ${\mathrm{GaInP}}_{2}$.

Valence-band splitting in ordered Ga0.5In0.5P measured by polarized photoluminescence excitation spectroscopy.

The spontaneous long-range-order-induced splitting of the valence band of ${\mathrm{Ga}}_{0.5}$${\mathrm{In}}_{0.5}$P is studied by polarized near-gap-excited photoluminescence and photoluminescence excitation. The results allow a direct determination of the size of the valence-band splitting and the relative ordering of the resultant states. Values obtained for the splitting are compared with recent calculations.

Coupling of ground and excited state excitations in GaAs under uniaxial stress

We report new experimental evidence of the exchange interaction between heavy and light excitons near the Brillouin zone center in GaAs. We have combined polarized photoluminescence excitation spectroscopy and uniaxial stress at liquid He temperatures in order to elucidate the structure of the free exciton states in low donor concentration GaAs grown by molecular beam epitaxy. The data indicate that the exchange interaction is small (deduced from the peak positions of the polarized spectra) but finite (due to the observation of level repulsion between the ground state of the light exciton and excited states of the heavy exciton at a stress of about 0.5 kbar).