Power-dependent Photoluminescence Spectroscopy Research Articles

Dimension- and position-controlled growth of GaN microstructure arrays on graphene films for flexible device applications

This paper describes the fabrication process and characteristics of dimension- and position-controlled gallium nitride (GaN) microstructure arrays grown on graphene films and their quantum structures for use in flexible light-emitting device applications. The characteristics of dimension- and position-controlled growth, which is crucial to fabricate high-performance electronic and optoelectronic devices, were investigated using scanning and transmission electron microscopes and power-dependent photoluminescence spectroscopy measurements. Among the GaN microstructures, GaN microrods exhibited excellent photoluminescence characteristics including room-temperature stimulated emission, which is especially useful for optoelectronic device applications. As one of the device applications of the position-controlled GaN microrod arrays, we fabricated light-emitting diodes (LEDs) by heteroepitaxially growing InxGa1−xN/GaN multiple quantum wells (MQWs) and a p-type GaN layer on the surfaces of GaN microrods and by depositing Ti/Au and Ni/Au metal layers to prepare n-type and p-type ohmic contacts, respectively. Furthermore, the GaN microrod LED arrays were transferred onto Cu foil by using the chemical lift-off method. Even after being transferred onto the flexible Cu foil substrate, the microrod LEDs exhibited strong emission of visible blue light. The proposed method to enable the dimension- and position-controlled growth of GaN microstructures on graphene films can likely be used to fabricate other high-quality flexible inorganic semiconductor devices such as micro-LED displays with an ultrahigh resolution.

Study of point defects in wide-bandgap Cu2CdGeS4 microcrystals by temperature and laser power dependent photoluminescence spectroscopy

We present temperature and laser power dependent photoluminescence (PL) study of high quality wide-bandgap Cu2CdGeS4 microcrystals. At T = 10 K three PL bands were detected at about 1.919 eV (#1), 1.855 eV (#2) and 1.748 eV (#3). The temperature and laser power dependencies indicate that the properties of PL bands can be explained by donor- acceptor pair model, where the #1 and #2 bands result from a recombination between distant pairs involving the same shallow acceptor VCu with EA ≈ 30 meV and different deep donor defects. The #3 PL band originates from the deep donor-deep acceptor pair recombination where the depth of deep acceptor defect is more than 157 meV. Detailed analysis of the PL spectra show the absence of deep potential or band gap fluctuations in this material making it suitable for photovoltaic applications.

Three-Dimensional Resonant Exciton in Monolayer Tungsten Diselenide Actuated by Spin-Orbit Coupling.

The intricate features of many-body interactions and spin-orbit coupling play a significant role in numerous physical phenomena. Particularly in two-dimensional transition metal dichalcogenides (2D-TMDs), excitonic dynamics are a key phenomenon that promises opportunities for diverse range of device applications. Here, we report the direct observation of a visible-range three-dimensional resonant exciton and its associated charged exciton in monolayer tungsten diselenide, as compared to monolayer molybdenum disulfide. A comprehensive experimental study that includes high-resolution TEM, Raman, high-resolution spectroscopic ellipsometry over a wide temperature range down to 4 K, high-energy temperature, and excitation power-dependent photoluminescence spectroscopy has been conducted. It is supported by first-principles calculations to unravel the influence of spin-orbit coupling in the formation of the resonant exciton and to identify its in-plane and out-of-plane features. Furthermore, we study the impact of temperature and thickness on the spin-orbit coupling strength in 2D-TMDs. This work is crucial in creating a platform in the fundamental understanding of high-energy resonant exciton in layered two-dimensional systems and that such high-energy optoelectronic features make them an increasingly attractive candidate for novel electronic and optoelectronic applications particularly in the aspects of solar cells and light-emitting diodes via the manipulation of excitonic states.

Observation of photoluminescence edge emission in CuSbSe2 absorber material for photovoltaic applications

CuSbSe2 has great potential for being an earth-abundant absorber material in efficient thin film solar cells. In this study, the radiative recombination in CuSbSe2 polycrystalline material was studied by temperature and laser power dependent photoluminescence spectroscopy which revealed the band-to-band and band-to-deep acceptor transitions, with the last one involving a deep acceptor defect with an activation energy of 165 ± 6 meV. The observation of the edge emission in CuSbSe2 refers to the high quality of the synthesized material.

Excitonic properties of semiconducting monolayer and bilayer MoTe2

MoTe2 belongs to the semiconducting transition metal dichalcogenide family with some properties differing from the other well-studied members (Mo,W)(S,Se)2. The optical band gap is in the near infrared region and both monolayers and bilayers may have a direct optical band gap. We first simulate the band structure of both monolayer and bilayer MoTe2 with DFT-GW calculations. We find a direct (indirect) electronic band gap for the monolayer (bilayer). By solving the Bethe-Salpeter equation, we calculate similar energies for the direct excitonic states in monolayer and bilayer. We then study the optical properties by means of photoluminescence (PL) excitation, time-resolved PL and power dependent PL spectroscopy. We identify the same energy for the B exciton state in monolayer and bilayer. Following circularly polarized excitation, we do not find any exciton polarization for a large range of excitation energies. At low temperature (T=10 K), we measure similar PL decay times of the order of 4 ps for both monolayer and bilayer excitons with a slightly longer one for the bilayer. Finally, we observe a reduction of the exciton-exciton annihilation contribution to the non-radiative recombination in bilayer.

Emission mechanisms in Al-rich AlGaN/AlN quantum wells assessed by excitation power dependent photoluminescence spectroscopy

The optical properties of Al-rich AlGaN/AlN quantum wells are assessed by excitation-power-dependent time-integrated (TI) and time-resolved (TR) photoluminescence (PL) measurements. Two excitation sources, an optical parametric oscillator and the 4th harmonics of a Ti:sapphire laser, realize a wide range of excited carrier densities between 1012 and 1021 cm−3. The emission mechanisms change from an exciton to an electron-hole plasma as the excitation power increases. Accordingly, the PL decay time is drastically reduced, and the integrated PL intensities increase in the following order: linearly, super-linearly, linearly again, and sub-linearly. The observed results are well accounted for by rate equations that consider the saturation effect of non-radiative recombination processes. Using both TIPL and TRPL measurements allows the density of non-radiative recombination centers, the internal quantum efficiency, and the radiative recombination coefficient to be reliably extracted.

Whispering gallery mode lasing from hexagonal shaped layered lead iodide crystals.

We report on the synthesis and optical gain properties of regularly shaped lead iodide (PbI2) platelets with thickness ranging from 10-500 nm synthesized by chemical vapor deposition methods. The as-prepared single crystalline platelets exhibit a near band edge emission of ∼ 500 nm. Whispering gallery mode (WGM) lasing from individual hexagonal shaped PbI2 platelets is demonstrated in the temperature-range of 77-210 K, where the lasing modes are supported by platelets as thin as 45 nm. The finite-difference time-domain simulation and the edge-length dependent threshold confirm the planar WGM lasing mechanism in such hexagonal shaped PbI2 platelet. Through a comprehensive study of power-dependent photoluminescence (PL) and time-resolved PL spectroscopy, we ascribe the WGM lasing to be biexcitonic in nature. Moreover, for different thicknesses of platelet, the lowest lasing threshold occurs in platelets of ∼ 120 nm, which attributes to the formation of a good Fabry-Pérot resonance cavity in the vertical direction between the top and bottom platelet surfaces that enhances the reflection. Our present study demonstrates the feasibility of planar light sources based on layered semiconductor materials and that their thickness-dependent threshold characteristic is beneficial for the optimization of layered material based optoelectronic devices.

Single photons on-demand from light-hole excitons in strain-engineered quantum dots.

We demonstrate for the first time on-demand and wavelength-tunable single-photon emission from light-hole (LH) excitons in strain engineered GaAs quantum dots (QDs). The LH photon emission from tensile-strained GaAs QDs is systematically investigated with polarization-resolved, power-dependent photoluminescence spectroscopy, and photon-correlation measurements. By integrating QD-containing nanomembranes onto a piezo-actuator and driving single QDs with picosecond laser pulses, we achieve triggered and wavelength-tunable LH single-photon emission. Fourier transform spectroscopy is also performed, from which the coherence time of the LH single-photon emission is studied. We envision that this new type of LH exciton-based single-photon source (SPS) can be applied to realize an all-semiconductor based quantum interface in distributed quantum networks [Phys. Rev. Lett. 2008, 100, 096602].

Probing light emission from quantum wells within a single nanorod

Significant improvements in the efficiency of optoelectronic devices can result from the exploitation of nanostructures. These require optimal nanocharacterization techniques to fully understand and improve their performance. In this study we employ room temperature cathodoluminescence hyperspectral imaging to probe single GaN-based nanorods containing multiple quantum wells (MQWs) with a simultaneous combination of very high spatial and spectral resolution. We have investigated the strain state and carrier transport in the vicinity of the MQWs, demonstrating the high efficiencies resulting from reduced electric fields. Power-dependent photoluminescence spectroscopy of arrays of these nanorods confirms that their fabrication results in partial strain relaxation in the MQWs. Our technique allows us to interrogate the structures on a sufficiently small length scale to be able to extract the important information.

Cascaded exciton emission of an individual strain-induced quantum dot

Single strain-induced quantum dots are isolated for optical experiments by selective removal of the inducing InP islands from the sample surface. Unpolarized emission of single exciton, biexciton, and triexciton transitions are identified by power-dependent photoluminescence spectroscopy. Employing time-resolved experiments performed at different excitation powers, we find a pronounced shift of the rise and decay times of these different transitions as expected from cascaded single photon emissions. Good agreement is found for a rate equation model for a three step cascade.

Low-threshold ZnSe microdisk laser based on fluorine impurity bound-exciton transitions

Impurity states in semiconductors, in which two long-lived ground states can be optically coupled to a single excited state, provide a powerful mechanism for applications including lasing without inversion, electromagnetically induced transparency, and optically addressable quantum memory for quantum information processing. We report low-threshold lasing from fluorine-doped ZnMgSe/ZnSe quantum wells in microdisk cavities. The lasing mechanism was studied by power-dependent photoluminescence spectroscopy. Lasing thresholds lower than 50 W cm −2 were observed and the fraction of spontaneous emission contributed to the lasing modes was about β=0.03–0.1.

Well width dependent luminescence characteristics of UV‐violet emitting GaInN QW LED structures

AbstractTemperature and excitation power dependent photoluminescence spectroscopy on GaInN UV‐violet LED structures allows to separate the influence of the quantum confined stark effect (QCSE) from carrier localization in In‐induced band tail states as well as to determine the internal quantum efficiency (IQE). A LED series with different quantum well widths (1‐3.7 nm) was grown by MOVPE on sapphire and low defect density (LDD) GaN templates. The photoluminescence data reveal, that the QCSE cannot be neglected even for low In‐content (<10%) ultra violet LED structures. From temperature and laser power density dependent photoluminescence, the internal QE is evaluated. The data shows, that upon growth on LDD substrates, the internal QE is considerably increased for short wavelength (375‐400 nm) LEDs. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Near‐UV to violet LEDs – Wavelength dependence of efficiency limiting processes

Two wavelength series (373-435 nm) of GaInN/GaN triple quantum well LED-structures, one on sapphire and one on GaN-templates, as well as a quantum well thickness series were grown by MOVPE. In order to study luminescence efficiency limiting effects the LED-structures were characterized by electroluminescence- and by temperature and excitation power dependent photoluminescence spectroscopy. The electroluminescence output and the internal quantum efficiency are peaked near 400 nm in both wavelength series, but output and efficiency are significantly enhanced for growth on templates. The photoluminescence data indicate that below 400 nm the efficiency, apart from insufficient vertical carrier confinement, is limited by poor localization within the quantum film plane while above 400 nm piezo-electric fields appear to be an important efficiency limiting effect. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Efficient free exciton emission at room temperature in Zn 0.5Cd 0.5Se/Mg xZn yCd 1− x− ySe quantum wells

We use temperature and power-dependent photoluminescence spectroscopy to demonstrate efficient free exciton emission at room temperature in high quality, strongly confining Zn 0.5Cd 0.5Se/Mg x Zn y Cd 1− x− y Se quantum wells. Our measurements indicate that at low temperatures ( T<50 K), the luminescence is dominated by bound exciton emission. However, free exciton emission becomes dominant at high temperatures ( T>100 K) and persists up to room temperature with an intensity that is ∼40% that at cryogenic temperatures. We attribute these observations to the deep confinement potential that stabilizes excitons and prevents their thermal escape.