Low-temperature Photoluminescence Research Articles

Research of a weak negative thermal quenching effect of nitrogen vacancy centers in nitrogen-doped diamond

A weak negative thermal quenching (NTQ) effect of nitrogen vacancy centers in nitrogen-doped diamond is investigated by low-temperature photoluminescence technology. Results show that a weak NTQ phenomenon of the NV− centers was observed in the range of 10–40 K. Aiming to explain the observed NTQ effect, three hypotheses are proposed to classify the underlying physical mechanism, including (a) electron transfer from nitrogen atom to NV centers, (b) electron transfer between NV0 and NV− centers, and (c) the shallow traps of charge carriers in diamond. What is more, with incorporating sulfur, sulfur-nitrogen complex defects are formed with an impurity level of 0.65 eV below the conduction band. The activation energies of nonradiative recombination of NV− centers were obviously decreased with doping sulfur, which maybe related to the substitutional nitrogen (deep donor) replaced by sulfur-nitrogen complex structure defect (relatively shallow donor). The presented work provides a comprehensive understanding on the physical mechanism of NTQ effect of nitrogen vacancy centers in diamond, which would provide guidance for application of NV centers, especially in low temperatures.

Evidence of weak strain field in InAs/GaAs submonolayer quantum dots

We used rapid thermal annealing and low-temperature photoluminescence to compare the optoelectronic properties of InAs/GaAs Stranski-Krastanov and submonolayer quantum dots. After annealing, the former showed large changes in their optical spectra, whereas the latter remained almost insensitive and had optical properties similar to those of an InGaAs quantum well. This contrast was attributed to the presence of In segregation during the submonolayer deposition that led to strong In–Ga intermixing and hindered the formation of the submonolayer quantum dots. The optical results point out that, in such conditions, the strain field in the InAs/GaAs system is too weak to effectively provide vertical alignment of the small two-dimensional islands nucleated in consecutive InAs submonolayers. To further improve the properties of submonolayer quantum dots, segregation should be reduced and an initial (2 × 4) surface reconstruction should be preferred, as it is the only one able to provide true two-dimensional InAs islands.

Influence of Cd, S and Na atoms on photoluminescence in tin sulfide thin films

The fundamental defects around the p–n interface were analyzed based on low temperature photoluminescence measurements to determine the origin of the various defects around the n-CdS/p-SnS structure in tin sulfide (SnS) solar cells. Particularly, the effects of Cd, S, and Na atoms on PL at the interface of a CdS layer and various treated SnS layers were examined. The PL peaks at 1.08 and 1.23 eV in the CdS/SnS structure, which were observed using the Cd partial electrolyte-treated SnS film, were associated with the formation of Cd-related defects. Furthermore, the PL peak at 1.27 eV in the CdS/SnS structure, which was observed using the sulfurized-SnS film and the excess Na-diffused SnS film, was associated with the formation of S-related defects such as OS defects. These findings present considerable potential for improving the efficiency of SnS solar cells.

Crystal side facet-tuning of GaN nanowires and nanofins grown by molecular beam epitaxy

GaN nanostructures are promising for a broad range of applications due to their 3D structure, thereby exposing non-polar crystal surfaces. The nature of the exposed crystal facets, i.e., whether they are a-, m-plane, or of mixed orientation, impacts the stability and performance of GaN nanostructure-based devices. In this context, it is of great interest to control the formation of well-defined side facets. Here, we show that we can control the crystal facet formation at the nanowire sidewalls by tuning the III–V ratio during selective area growth by molecular beam epitaxy. Especially, the N flux serves as a tool for controlling the growth kinetics. In addition, we demonstrate the growth of GaN nanofins with either a- or m-plane side facets. Based on our observations, we present the underlying nanostructure growth mechanisms. Low temperature photoluminescence measurements show a correlation of the formation of structural defects like stacking faults with the growth kinetics. This article demonstrates the controlled selective epitaxy of GaN nanostructures with defined crystal side facets on large-scale available AlN substrates.

Synthesis and Properties of Stable Amino Metal Halide Molecular Clusters in the Solid State.

Nanosized molecular clusters (MCs) composed of PbBr2 and neutral ligand butylamine (BTYA) with unique optical properties in solution and solid states have been synthesized using ligand-assisted reprecipitation and spin-coating, separately. The studies of their optical properties using ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) show the first electronic absorption and PL band of the MCs at 401 and 411 nm, respectively, for the solution and solid state samples that exhibit good stability under ambient conditions. Low-temperature PL spectra below 30 K show vibronic peaks indicative of a single size or a very narrow size distribution of the MCs. On the basis of Raman, X-ray diffraction, and transmission electron microscopy measurements, a layered structural model is proposed for the MCs with a BTYA ligand capping on the surface of the corner-shared tilted [PbBr6]4- octahedral framework. The stable and retained structure of MCs in the solid state is promising for photonics applications.

On-demand generation of optically active defects in monolayer WS2 by a focused helium ion beam

We demonstrate that optically active emitters can be locally generated by focusing a He-ion beam onto monolayer WS2 encapsulated in hBN. The emitters show a low-temperature photoluminescence spectrum, which is well described by an independent Boson model for localized emitters. Consistently, the photoluminescence intensity of the emitters saturates at low excitation intensities, which is distinct to the photoluminescence of excitonic transitions in the investigated WS2 monolayers. The demonstrated method allows us to position defect emitters in WS2 monolayers on demand. A statistical analysis suggests the generation yield of individual emitters to be as high as 11% at the highest investigated He-ion doses.

High-responsivity self-powered UV photodetector performance of pristine and V-doped ZnO nano-flowers

Wurtzite ZnO nanostructures are suitable for high-performance, self-powered photodetectors (PDs) because of their attractive optoelectronic properties. In this study, highly sensitive ultra-violet self-powered PDs with high responsivity/detectivity and large external quantum efficiency was developed by V-doped ZnO nanoflower samples onto an ITO glass substrate. Highly faceted pristine and V-doped ZnO nanoflowers were produced using a simple, low-cost co-precipitation method. The presence of Na+ ions in the precursor solution appeared to control the morphology of the nanomaterial. Various physicochemical characterizations showed that the variation of the Na+ ion concentration inhibits the reaction at selected regions, causing the branching of nanostructures to form ZnO nanoflowers. Room- and low-temperature photoluminescence studies showed that V-doping increases the extrinsic carrier density and produces many intrinsic and extrinsic defect states, which reduce the e−–h+ recombination rate significantly and produce a large photocurrent under UV illumination. The V-ZnO-based self-powered UV PD device showed a 156 % increase in photocurrent compared to pristine ZnO-based PDs, with very short response/recovery times of 0.22/0.23 s under 382 nm UV illumination of 1.14 mW/cm2, and excellent responsivity (5.1 × 103 mA/W), very high external quantum efficiency (5.5 × 104 %), and large detectivity (4.0 × 1013 Jones). Therefore, the current work presents a simple way to obtain high-performance self-powered UV PDs that can open up a new horizon to fabricate new-generation optoelectronic devices for diverse applications.

Performance optimization of CZT thick films by CdTe buffer layers for solar-blind ultraviolet photoelectric detectors

CdZnTe (CZT) thick films with CdTe buffer layers (CZT/CdTe) were studied for high-performance ultraviolet photoelectric detector. CZT thick films were prepared on AlN/Si substrates via close-spaced sublimation (CSS) and CdTe buffer layers were introduced to optimize the interface quality of CZT thick films. Results of x-ray diffraction showed that the full width at half maximum (FWHM) and dislocation density (δ) of interface of optimized samples were 0.156 ° and 2.543 × 1014 lin/m2, respectively. Interface detect analysis of free-standing CZT and CZT/CdTe thick films was studied by x-ray photoelectron spectroscopy (XPS) and low-temperature photoluminescence (PL) spectroscopy, and mechanism of in-situ annealing in CdTe atmosphere to reduce interfacial defect states was discussed. Moreover, the photo-dark current ratio of the CZT/CdTe thick film metal-semiconductor-metal (MSM) structural photodetector was as high as 5530, which was nearly double that of the CZT thick film detector, and the carrier mobility lifetime product could reach 5.59 × 10−4 cm2/V at 100 V bias, confirming the improvement of carrier transport capacity and detection efficiency. CZT-based detectors have inestimable prospect in the field of solar-blind ultraviolet detection with the participation of CdTe buffer layers.

(Invited) Spin and Optical Properties of the Silicon Vacancy in 4H-SiC for Quantum Science and Technology

Silicon vacancies in SiC are of significant interest for quantum information and sensing due to their unusual combination of long spin coherence time, optical addressability, and room-temperature operation in an industrially-relevant semiconductor. We characterize the spin and optical properties of the V2 silicon vacancy in 4H-SiC for both single defects and ensembles. We also show that they can be deterministically placed with sub-micron precision using focused Li ion implantation. Low-temperature photoluminescence excitation spectroscopy of the spin-dependent optical transitions of single silicon vacancies reveals two sharp lines with linewidths near the radiative lifetime limit. These lines correspond to the ms=±1/2 and ms=±3/2 states of an S=3/2 spin system that can be optically initialized, coherently manipulated with RF pulses, and read out via spin-dependent photoluminescence. We measure the different properties of spins in the two bases, and compare them to theoretical models. To suppress the limiting effects of the nuclear spin bath, we grow isotopically purified SiC with a very low abundance of 29Si and 13C. Our room temperature spin coherence measurements show orders-of-magnitude improvement of the coherence times in these materials. In particular, the inhomogeneous dephasing time T2 * in the ms=±1/2 basis shows a ~100x improvement, reaching 20 μs. We consider the prospects of this system for quantum sensing and as an efficient spin-photon interface for use in quantum networks.This work was supported by the U.S. Office of Naval Research, the OSD Quantum Sciences and Engineering Program, the Defense Threat Reduction Agency, and the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525.

In(Ga)N 3D Growth on GaN-Buffered On-Axis and Off-Axis (0001) Sapphire Substrates by MOCVD.

In(Ga)N epitaxial layers were grown on on-axis and off-axis (0001) sapphire substrates with an about 1100 nm-thick GaN buffer layer stack using organometallic chemical vapor deposition at 600 °C. The In(Ga)N layers consisted of a thin (~10–25 nm) continuous layer of small conical pyramids in which large conical pyramids with an approximate height of 50–80 nm were randomly distributed. The large pyramids were grown above the edge-type dislocations which originated in the GaN buffer; the dislocations did not penetrate the large, isolated pyramids. The large pyramids were well crystallized and relaxed with a small quantity of defects, such as dislocations, preferentially located at the contact zones of adjacent pyramids. The low temperature (6.5 K) photoluminescence spectra showed one clear maximum at 853 meV with a full width at half maximum (FWHM) of 75 meV and 859 meV with a FWHM of 80 meV for the off-axis and on-axis samples, respectively.

Microstructure and optical properties of CdTe thin films prepared by close spaced sublimation method at various growth temperatures

Cadmium telluride (CdTe) is an important Ⅱ-Ⅵ multifunctional semiconductor, often used as the light absorption layer in photovoltaic solar cells or the core layer in γ-ray detection. The deposition temperature has a great greatly influences on the microstructure and morphology of the thin films during their growth. Here, close spaced sublimation method is used for depositing CdTe thin films on glass substrate at various temperatures conditions. The microstructure, morphology, optical and luminescent properties of the CdTe thin films were systematically characterized. CdTe thin films with a high preferred crystal orientation, large grain size, low micro-stress, and strong luminescence performance were obtained. And the origin of luminescent peaks is analyzed in detail by low-temperature photoluminescence spectroscopy.

Disorder and wave-function intermixing effects in the luminescence spectra of LiYF4:Ho in external magnetic fields

On the basis of the high-resolution low-temperature photoluminescence spectra of LiYF4:Ho crystals we discuss spectral manifestations of disorder caused by random distribution of either the lithium isotopes or defects in the crystal lattice. A peculiar discrete spectral structure of a different nature was found in the magnetic-field-dependent spectra. We show that it arises due to mixing of the wave functions at the anticrossings of hyperfine levels in a magnetic field.

Exciton-Phonon and Trion-Phonon Couplings Revealed by Photoluminescence Spectroscopy of Single CsPbBr3 Perovskite Nanocrystals.

Lead halide perovskite nanocrystals (NCs) have outstanding photoluminescence (PL) properties and excellent potential for light-emitting diodes and single-photon sources. Here, we report the multiple-peak structures originating from excitons, trions, and biexcitons in low-temperature PL spectra of single CsPbBr3 NCs. We found fine-structure splitting in the PL peaks of bright excitons and biexcitons and also in the longitudinal-optical (LO)-phonon replicas of excitons. LO-phonon replicas of trions are clearly observed under strong photoexcitation, which do not show fine-structure splitting. From size-dependent analyses of these replicas, we clarified that both exciton-phonon and trion-phonon couplings become larger for smaller NCs and the coupling strengths of trions are larger than those of excitons in large NCs. These behaviors can be explained by the spatial distributions of the electron and hole wave functions in the NCs. Our findings provide essential information on electron-phonon couplings in perovskites and for the design of high-purity single-photon sources.

Material Defects and Dark Currents in InGaAs/InP Avalanche Photodiode Devices

An In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</sub> As/indium phosphide (InP) avalanche photodiode (APD) with a separate absorption, grading, charge, and multiplication (SAGCM) structure is epitaxially grown by molecular beam epitaxy (MBE). The resulting material is studied using X-ray diffraction (XRD), photoluminescence (PL), and scanning transmission electron microscopy. The activation energy extracted from the dark current of the APD indicates that it is dominated by the generation–recombination (G–R) process. Deep low-temperature PL peaks reveal the existence of an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{v} + 0.42$ </tex-math></inline-formula> -eV deep energy level defect in the indium–gallium–arsenide (InGaAs) absorber layer, which is considered to be a result of point defects caused by Ga-poor or In-poor MBE growth conditions. The effect of the defect on the dark current is confirmed using numerical calculation methods.

Photoluminescence and First-principle Calculations of the Single Vacancy in Diamond

The isolated single vacancy is one of the most common defects in diamond, while it is of considerable interest and importance that significantly influences the optical properties of diamond. Although a number of literature publications about the electron structure and density states of single vacancy in diamond from the density functional theory (DFT) calculations, these results are very difficult to be identified from the perspective of experiments. In this paper, the low-temperature photoluminescence (PL) technology was employed to study the emission properties of single vacancy in diamond, and then the photoionization energy, migration energy and vibrionic structure obtained from PL experiments were compared with these obtained from the first-principle calculations. Results showed that the calculation results agreed well with the PL results for the neutral vacancy while differed for the negatively charged vacancy, indicating that for ultrapure diamond, the vacancy mainly existed in form of the neutral rather than the negatively charged.

Defect regulation of AlN films based on Al-rich AlN targets

Aluminum nitride (AlN) films with low defect concentration were fabricated on Si substrates via RF magnetron sputtering system based on Al-rich AlN (Al-AlN) targets. The effects of Al-rich content on structure, defects and photoelectric properties of AlN films were investigated by x-ray diffraction (XRD), high resolution transmission electron microscopy, x-ray photoelectron spectroscopy (XPS), low temperature photoluminescence (PL), ultraviolet-visible spectra, current–voltage (I–V) and capacitance–voltage (C–V) characteristics. In particular, the defects of AlN films were investigated by XPS and low-temperature PL analyses, indicating that Al-rich AlN targets can help to reduce the defects of Al vacancy and O impurity of AlN films with an optimal Al-rich content of 1.5 wt. % (A1.5). Schottky contact behavior between AlN films and Ti/Al/Ni/Au multi-layer electrodes were revealed from I–V curves of all samples based on parallel electrodes, and AlN metal-semiconductor-metal (MSM) devices prepared by A1.5 films exhibited the lowest leakage current of 2.43 × 10−8 A at the bias of 5 V. C–V tests indicate the less defect density and lower carrier concentrations of vertical structure of A1.5 devices. This work offers a feasible approach to regulate the defects of AlN films for practical application.

Enhancement of valley polarization in CVD grown monolayer MoS2 films

The development of future valley based electronics or valleytronics requires a high degree of valley polarization (VP) in large area monolayer (1L)-MoS2. Though it is possible to synthesize 1L-MoS2 films with large area coverage, VP property of as-grown films is found to be very poor. Here, we investigate the role of physisorbed air molecules and strain on the luminescence and the VP characteristics of large area monolayer MoS2 grown on various substrates by a microcavity based chemical vapor deposition (CVD) technique. The study shows that the removal of adsorbates from sulfur vacancy (VS) sites not only suppresses the broad sub-bandgap luminescence feature that typically dominates low temperature photoluminescence (PL) spectra of these films but also significantly enhances VP. Post-growth transfer of the 1L-MoS2 film from sapphire to a SiO2/Si substrate by a polystyrene assisted process is found to be highly effective in improving the polarization characteristic (∼80%) of K-valleys through relaxation of the biaxial tensile strain and the removal of physisorbed air molecules from the VS sites. The process is also found to provide long lasting protection for MoS2 films from air. The finding, thus, creates much needed opportunity to use CVD grown large area 1L-MoS2 for realization of valleytronics of the future.

Crystal quality improvement of ZnTe (1 1 0) thin film prepared on sapphire by increasing nuclei density on substrate surface

ZnTe thin films were prepared on r, S-plane nanofacet substrates to achieve a high-crystal-quality layer with a (110) orientation. Scanning electron microscopy was used to study the nucleation process. Selective formation of growth nuclei can be achieved by controlling the substrate temperature. The low-temperature grown amorphous layers changed to nuclei by annealing. The surface amorphous layer was completely removed when the annealing temperature was too high. Zn beam irradiation during the annealing removed materials selectively from the r-nanofacet. Migration-enhanced epitaxy (MEE) was introduced to enlarge the nuclei size, and a thick-layer growth by conventional molecular beam epitaxy (MBE) followed. The MEE sequence was increased up to 1200 cycles, and the substrate temperature for MEE and MBE was set to higher temperatures than the conventional MBE substrate temperature (the conventional substrate temperature was around 340 °C). The crystal quality of the thin film was analyzed using the X-ray pole figure, θ–2θ measurements and photoluminescence. (110) oriented layer formation perpendicular to the substrate surface was confirmed after introducing those techniques. The low-temperature photoluminescence measurement indicated the high quality of the thick layer.

Morphological and optical characterizations of different ZnO nanostructures grown by mist-CVD

In this study, the effects of growth temperatures, post-growth treatment, and substrate cleaning recipes on structural, morphological, luminescence, and growth kinetics of ZnO nanostructures grown by mist Chemical Vapor Deposition method (mist-CVD) on SiO2/Si substrates are investigated. The grown ZnO nanostructures have polycrystalline wurtzite (wz) hexagonal crystal structure according to X-ray Diffraction (XRD) measurements. However, the lower growth temperature is found to lead to the formation of ZnO nanostructures oriented in the [0002] direction. From Atomic Force Microscope (AFM) measurements, it is seen that the Root Mean Square (RMS) values decrease with decreasing growth temperature. In addition, it is shown that the surface covering is tightly dependent on decreasing growth temperature of the ZnO nanostructures. The hexagonal and triangular pyramid-like ZnO nanostructures are grown on the surface according to the Scanning Electron Microscope (SEM) measurements. The wurtzite type of crystallization is also demonstrated by confocal Raman measurements. The characteristic vibration modes related to Zn and O motion in the lattice are found to be ∼435 cm−1 and ∼100 cm−1, respectively. The luminescence properties are studied by low-temperature (10 K) Photoluminescence (PL) measurements. The spectra are dominated by the transition related to the donor-bound exciton (DBE) with a peak position typically observed at ∼369 nm (3.362 eV) in hexagonal ZnO structures. Defect-related transitions are observed to be strongly correlated with growth temperature and post-growth treatment.

Investigation of Modulation Bandwidth of InGaN Green Micro-LEDs by Varying Quantum Barrier Thickness

InGaN-based micro light-emitting diodes (micro-LEDs) with 5, 10, and 13 nm quantum barrier (QB) thickness were fabricated by metal-organic chemical vapor deposition (MOCVD) to investigate the influence of quantum-confined Stark effect (QCSE) on modulation bandwidth and luminous performance of devices. The room-temperature photoluminescence (PL), low-temperature time-resolved PL (TRPL), and electroluminescence (EL) results show that the decrease of QB thickness is beneficial to reduce QCSE of the device. The thinner QB thickness is good for improving the modulation bandwidth because the thinner QB is more beneficial to increase the total carrier recombination rate, but it is not conducive to the improvement of external quantum efficiency (EQE) due to degraded crystal quality. In addition, the modulation bandwidth of the device was calculated by using ABC model in combination with simulation. The calculation is in good agreement with the measured value. What is more, an optical link using an orthogonal-frequency division multiplexing (OFDM) modulation scheme was demonstrated. The transmission data rate increases with the increase of QB thickness from 1.309 to 1.773 Gbps, even though the modulation bandwidth decreases from 245 to 169 MHz. This work provides a way to balance the quantum efficiency and communication performance of green micro-LEDs.