Low-temperature Photoluminescence Research Articles

(Invited) Epitaxial III-V Nanomaterials on the Si Platform for Quantum and Data Communications

As Si-based integrated photonic components begin to replace older technologies for optical data communications, the need for better integration of semiconductor lasers on the Si platform is greater than ever. In addition, emerging interest in integrated-photonic-based quantum computing and quantum sensing presents a further need for quantum light emitters—entangled and single-photon—on Si. III-V quantum dots represent a promising solution, both for the active region of telecommunications-wavelength lasers and for quantum emitters. However, epitaxial quantum dots grown on conventional (100) substrates typically lack the high symmetry required for entangled-photon emission.This presentation will discuss our recent work to develop a novel approach for III-V direct growth on Si via an ultrathin (<10 nm) single crystal strain relieving buffer layer, realized by the selective oxidation and condensation of an amorphous SiGe surface layer. The fabrication and properties of this novel Ge-rich buffer layer, as well as its use for subsequent III-V organometallic vapor-phase epitaxy will be presented.The second part of the talk will present our recent work on the epitaxial growth and characterization of (In,Ga)As quantum dots for the above-mentioned applications. Specifically, I will show that surface-energy-modifying surfactants (Sb, Bi) present the possibility to externally direct quantum dot formation “on-demand” in strained InAs layers on GaAs(111) substrates, where quantum dot formation by the Stranski–Krastanov process does not naturally occur. These nanostructures are prospective for high-symmetry entangled-photon emitters. The three-dimensional (3D) composition profile of InAs quantum dot layers is characterized by atom-probe microscopy (APT), elucidating the real-world quantum dot matrix. This 3D composition data is input into a finite element solver to determine the quantum dot energies and wavefunctions for electrons and holes. The modeling reveals that in high-density quantum dot layers, the electron and hole states are hybridized between multiple quantum dots in the matrix. This has important consequences for design of quantum dot light emitters. The predicted transition energies are in agreement with low-temperature photoluminescence. This work constitutes a promising approach for integrating telecommunications-wavelength quantum dot lasers and entangled photon emitters on the Si integrated photonics platform.

Unravelling Structural, Optical, and Band Alignment Properties of Mixed Pb-Sn Metal-Halide Quasi-2D Ruddlesden-Popper Perovskites.

Lead-tin (Pb-Sn) mixed-halide perovskites show potential for single-junction and tandem solar cells due to their adjustable band gaps, flexible composition, and superior environmental stability compared to three-dimensional (3D) perovskites. However, they have lower power conversion efficiencies. Understanding band alignment and charge carrier dynamics is essential for enhancing photovoltaic performance. In this view, herein we have prepared thin films of mixed Pb-Sn-based two dimensional (2D) Ruddlesden-Popper (RP) perovskites BA2FA(Pb1-xSnx)2I7 using a solution-based method. XRD study revealed the formation of orthorhombic phases for pristine (BA2FAPb2I7) and mixed Pb-Sn perovskite thin films. UV-vis analysis showed that different n = 2 and n = 3 phases are present in the pristine sample. In contrast, Pb-Sn-doped samples showed no signature of other phases with a prominent red-shift in the visible spectral region. Cyclic voltammetry showed peaks for electron transfers at the band edges. Additionally, electrochemical and optical band gap matching was observed, along with decreased peak intensity due to less reactant and altered electrolyte-perovskite interface stability. Density functional theory (DFT) calculations revealed that the reduced band gap is due to the alteration of electrostatic interactions and charge distribution within the lattice upon Sn substitution. Low-temperature PL analysis provided insights into charge carrier dynamics with Sn substitution and suggested the suppression of higher n phases and self-trapped excitons/carriers in mixed Pb-Sn quasi-2D RP perovskite thin films. This study sheds light on the electron transfer phenomena between TiO2 and SnO2 layers by estimating band offsets from valence band maximum (VBM) and conduction band minimum (CBM), which is crucial for future applications in fabricating stable and efficient 2D-Pb-Sn mixed perovskites for optoelectronic applications.

Revealing the Optical Transition Properties of Interlayer Excitons in Defective WS2/WSe2 Heterobilayers.

Manipulation of physical properties in multidimensional tunable moiré superlattice systems is a key focus in nanophotonics, especially for interlayer excitons (IXs) in two-dimensional materials. However, the impact of defects on IXs remains unclear. Here, we thoroughly study the optical properties of WS2/WSe2 heterobilayers with varying defect densities. Low-temperature photoluminescence (PL) characterizations reveal that the low-energy IXs are more susceptible to defects compared to the high-energy IXs. The low-energy IXs also show much faster PL quenching rate with temperature, faster peak width broadening rate with laser power, shorter lifetime, and lower circular polarization compared to the low-energy IXs in the region with fewer defects. These effects are attributed to the combined effects of increased electron scattering, exciton-phonon interactions, and nonradiative channels introduced by the defects. Our findings aid in optimizing moiré superlattice structures.

High-quality Ge1−xSnx (x = 0–0.11) realized by UHV-CVD using Ge2H6 and SnCl4: Materials growth, structural/optical properties, and prototype IR photodetectors

GeSn materials with Sn contents ranging between 0% and 11% have been successfully grown by using Ge2H6 and SnCl4 in a hot-wall UHV-CVD system at low reaction pressures of 0.2–0.4 Torr. Both fully strained and partially relaxed materials have been produced. It was found that the materials were of high crystallinity as evidenced by few threading dislocations from cross-sectional TEM. Photoluminescence studies have shown that the PL spectra shift to longer wavelengths, and the PL intensity increases exponentially as the Sn contents increase. The photoemission cutoff for a 9.5% Sn sample is beyond 2600 nm, and low temperature PL measurements have confirmed that this sample is already in the direct gap material regime. Prototype photoconductor devices were fabricated with a partially strained 5% Sn sample, showing much smaller dark currents and similar spectral coverage as compared to a fully strained 7.8% Sn device reported in the literature. The reported technical route has provided a new path toward high quality, low cost GeSn epitaxy, and will contribute to the development of group IV optoelectronics.

Identifying orientation-dependent optical properties of single-crystalline β-Ga2O3 films

We explore the effect of crystallographic anisotropy on the optical properties of high-quality β-Ga2O3 thin films by conducting experimental measurements and theoretical simulations. High resolution x-ray diffraction measurements confirm the high-quality and the single crystalline quality, ruling out the effect of defects for all films. Raman spectra reveal the presence of anisotropy, as evident from a stronger Bg(2) mode related to GaIO4 chain libration in (100) orientation. Conversely, a stronger Ag(10) mode corresponding to tetrahedral bonds is evident in the (010) orientation, while it is suppressed in the (100) orientation. Low-temperature photoluminescence spectra indicate the presence of intrinsic impurity-related emissions in the ultraviolet and blue regions for all films. An anisotropic bandgap is observed, wherein the lowest bandgap value is related to the (010) oriented sample. Spectroscopic ellipsometry measurements confirm different refractive indices and extinction coefficient values depending on crystallographic orientation, which are in good agreement with the theoretical values obtained by density functional theory, confirming the anisotropic characteristics. The theoretical calculations of charge density show that the strength of covalent bonding depends on the β-Ga2O3 orientation, while experimental findings demonstrate that as the covalent bonding character increases, the film bandgap and the refractive index decrease. The anisotropy of β-Ga2O3 with respect to the crystal orientation leads to variations in the extinction coefficient, refractive index, and bandgap energy.

MOLECULAR BEAM EPITAXY GROWTH OF HIGH-QUALITY InN NANORODS ON Si (111) SUBSTRATE

The growth of high-quality InN nanorods (NRs) on Si (111) by plasma-assisted molecular beam epitaxy (PAMBE) is reported. X-ray diffraction and Raman spectroscopy investigations indicates that the NRs are wurtzite, c-axis oriented and single crystalline. Low temperature photoluminescence emissions with peak energy of ~ 0.75eV was observed indicating the high quality of the nanostructures. This study unravels a novel strategy for the successful growth of high-quality InN NRs on Silicon which is highly promising for applications in next generation nanodevices.

Stress-induced fluorescence in diamond at ultrahigh pressures and low temperatures

Diamond is widely used as an anvil material in high pressure experiments, such as diamond anvil cell (DAC). The study of the fluorescence emission behaviour of the diamond at high pressure is of great significance for understanding its band structure and optical properties. Here, by combining low-temperature photoluminescence experiments, and fluorescence spectroscopy at controlled pressure (181 GPa–122 GPa) and temperature (296 K–77 K) reveals the relationship between stress-induced diamond fluorescence as a function of temperature and pressure. We also demonstrate that the electron-phonon interaction has an effect on the diamond energy gap, and the temperature and pressure jointly affect the band structure. The electron band gap decreases rapidly under low temperature and high-pressure conditions.

Atomic-like UV emission generated in hexagonal boron nitride single crystals by thermal annealing

A series of atomic-like photoluminescence (PL) emission peaks in UV region near 4.0 eV were created by thermal annealing hexagonal boron nitride (h-BN) single crystals in air. The pristine h-BN did not have these peaks, emitting strong phonon-assisted band edge PL with peaks at 5.78 and 5.89 eV. After annealing the h-BN crystals in ambient air, a new atomic-like sharp emission in UV region at 4.09 eV with a line width of 0.2 nm appeared along with its phonon replicas at 3.89 and 3.69 eV in the low temperature (8 K) PL measurement. Further testing demonstrated that annealing the h-BN samples in the temperature window of 700–950 °C for 60 min generated the atomic-like emission. The peak position of the emission line is stable with the temperature and PL excitation power. Our study also suggests that the defect responsible for the atomic-like emission resides in the surface region.

Enhanced interlayer electron transfer by surface treatments in mixed-dimensional van der Waals semiconductor heterostructures

We investigate the excitonic species in WS2 monolayers transferred onto III–V semiconductor substrates with different surface treatments. When the III–V substrates were covered with amorphous native oxides, negatively charged excitons dominated the spectral weight in low-temperature near-resonance photoluminescence (PL) measurements. However, when the native oxides of the III–V substrates were reduced, neutral excitons began to dominate the spectral weight, indicating a reduction in the electron density in the WS2 monolayers. The removal of the native oxides enhanced the electron transfer from the WS2 monolayer to the III–V substrate. In addition, an additional shoulder-like PL feature appeared ∼50 meV below the emission of neutral excitons, which can be attributed to the emission of localized excitons. When the III–V substrate surface was passivated by sulfur after the reduction of the native oxides, neutral excitons still dominated the spectral weight. However, the low-energy PL shoulder disappeared again, suggesting the effective delocalization of excitons through substrate surface passivation. Surface engineering of the semiconductor substrates for two-dimensional (2D) materials can provide a novel approach to control the carrier density of the 2D materials, implement deterministic carrier localization or delocalization for the 2D materials, and facilitate the interlayer transfer of charge, spin, and valley currents. These findings open the avenue for novel device concepts and phenomena in mixed-dimensional semiconductor heterostructures.

Role of passivation ligand in exciton diffusion and emission mechanism of metal halide perovskite nanocrystal solid

Shorter-ligand passivated metal halide perovskite nanocrystal (PNC) solids possess smaller inter-NC edge distance which can potentially activate shorter-range nonradiative routes of energy transfer. However, these PNCs are prone to develop surface defects due to ligand detachment and the photogenerated excitons also have high probability to undergo dissociation. It remains unclear how these factors influence the exciton diffusion and emission mechanism of PNC solids. Herein, we prepare two PNC solids passivated with alkylamine and alkylacid ligands of different chain lengths and investigate the exciton diffusion and emission properties by combining time-resolved photoluminescence (PL) and low temperature PL spectroscopy. Our results show that due to the easy ligand detachment and rich surface defects, no prominent exciton transfer occurs in PNC solid passivated with short ligands, despite of the shorter distance between adjacent PNCs. In comparison, the PNC solid passivated with long ligands shows significant occurrence of exciton diffusion from high energy sites towards low energy sites. Accordingly, the PL spectra of thick PNC solid passivated with different ligands show a strikingly different evolution trend with decreasing temperature: in short ligand passivated PNC solid, an intrinsic emission is observed along with a defect-related emission arising from carrier trapping and subsequent radiative recombination, and both emissions are enhanced at lower temperature; in long ligand passivated PNC solid, high energy emission quenches while low energy emission is strengthened at lower temperature due to enhanced exciton transfer. This work reveals that the exciton diffusion and emission properties of PNC solid is strongly mediated by the passivation ligands, providing important insights for their applications in optoelectronic devices.

Structural and photophysical properties of two-dimensional lead bromide hybrid perovskite (C8H17NH3)2PbBr4

The structural versatility of organic compounds imparts manifold properties to hybrid organic−inorganic perovskites, where two-dimensional (2D) halide perovskites are potential future platforms for light-emitting devices because of their unique quantum well configuration and dielectric confinement. Notwithstanding the numerous studies on the organic−inorganic perovskites carried out to date, precisely charactering the structure and photoluminescence mechanism remain considerable challenges. Herein, by applying Rietveld refinement, we have obtained crystal structure and atomic sites of 2D layered OA2PbBr4 perovskite, which undergoes a phase transition at 190 K with activation energies of 29.6 and 3.8 meV for the two phases, respectively. The theoretical analysis based on the crystal structure were performed to understand the nature of vibrations and band structure. Steady-state PL shows a narrow emission and a broad emission located at 409 and 487 nm, respectively. With temperature cooling down to 118 K, narrow emission gradually narrowed and enhanced. Meanwhile, intensity of broad emission first weakens and then increases. The turning point occurs near 198 K. By applying time-resolved PL (TRPL) and transient absorption (TA) spectra experiment, we observed a rapid carrier transfer from the FE to the STE state, and demonstrated the narrow and broad bands originate from free exciton and self-trapped exciton emission, respectively. And the self-trapping depth is proved to be 26.3 meV by low-temperature PL. Our findings indicate the synthesized 2D perovskite has potential applications in optoelectronic and display devices.

Structural and temperature-dependent photoluminescence properties of NaBaBO3:Ce3+,Tb3+ phosphors synthesized using the combustion method

This study explores the structural and temperature-dependent photoluminescence of Ce3+ and Tb3+ doped NaBaBO3 phosphors, synthesized via combustion. Analysis of their crystal structures confirmed excellent alignment with the standard PDF#98–008-0110. Investigation into both room and low-temperature photoluminescence revealed that the dopants have a significant effect on emission spectra. Ce3+-doped samples exhibited excitation peaks at 275 nm and 358 nm, leading to a primary emission at 419 nm, with enhanced low-temperature emission suggesting reduced non-radiative processes. Tb3+-doped phosphors showed excitation from 250 to 377 nm and emissions from blue to deep red, including strong green emission at 550 nm due to 5D4→7F5 transitions. Optimal doping was found at 1 mol% for Ce3+, while Tb3+ showed increased luminescence up to 3 mol%, with concentration quenching observed beyond these points. The study indicates dipole–dipole interactions dominate Ce3+ concentration quenching, whereas Tb3+ involves both electric dipole and quadrupole interactions. This analysis provides insights into enhancing luminescent efficiency and suggests NaBaBO3:xCe3+,Tb3+ phosphors' potential in advancing white LED technology, highlighting their stable luminescent properties at low temperatures.

Low-temperature photoluminescence measurement with a micromachined Joule-Thomson cooler

This study evaluates the effectiveness of micromachined Joule-Thomson (MJT) cooling for photoluminescence (PL) materials. Achieving low temperatures is crucial for enhancing PL performance in semiconductors. However, the commonly used liquid nitrogen (LN2) cryostats require frequent refills, hindering their long-term operation. The MJT cooler offers a potential solution by enabling integration with devices and longer operating time. To validate its effectiveness, this study conducted low-temperature PL measurements using a nitrogen MJT cooler. A MAPbI3 thin film was used as the characterization sample owing to its clear PL mechanism. The experiment successfully preserved its temperature-dependent PL property, with an observed orthorhombic phase-change phenomenon between 155-165 K. Furthermore, the system demonstrated short cool-down time (<1 h), minimal temperature impact from laser stimulus (<±0.1 K), sample storage stability, and low coolant consumption.

The effect of FeGa (0/–) level presence on material properties in dilute AlxGa1−xN layers

AlxGa1−xN epilayers are used as the basis of ultraviolet LEDs and detectors. The trap states produced by defects and impurities can play a key role in the device performance. In this work, conventional deep-level transient spectroscopy, photoluminescence (PL), and secondary ion mass spectrometry have been used to characterize a deep-level trap termed as E3 in dilute AlxGa1−xN (x &amp;lt; 0.063) epilayers grown by metal-organic vapor phase epitaxy (MOVPE) on highly conductive ammono-GaN substrates. The AlxGa1−xN epilayers were doped with silicon to about 3 × 1016 cm−3. The electrical and the optical measurements were conducted on Ni/Au Schottky barrier diodes and virgin samples, respectively. First, we observed a general trend that the E3 (FeGa) electron trap concentration significantly changes along the wafers in AlxGa1−xN layers that is fully consistent with previously reported results for GaN materials grown by the MOVPE technique. Second, we report that the activation energies for electron emission for the E1 and E3 traps in dilute AlxGa1−xN exhibit linear variations with Al content. Moreover, low-temperature PL results show a proportional relation between the intensity of the line with its maximum at 1.299 eV and concentration of residual Fe impurity. Finally, we discuss how the presence of defects resulting from Fe contamination may result in degradation of AlxGa1−xN-based devices.

The Characteristics of Luminescence from High-Temperature- and High-Pressure-Treated Diamonds

High-temperature and high-pressure (HTHP)-treated diamonds have attracted attention all over the world due to their vivid colors. In order to explore a new method for the rapid and non-destructive identification of HTHP-treated diamonds, in this paper, five IaAB-type diamonds yielded in Russia were selected as the research object and treated with HTHP. The HTHP-treated diamonds were investigated by DiamondViewTM, cathodoluminescence, micro-infrared spectrometry (micro-IR), low-temperature photoluminescence (PL) spectrometry, and three-dimensional (3D) fluorescence spectrometry. The results show that under DiamondViewTM and cathode rays, the five samples were all non-phosphorescent with different luminous patterns, such as regular annular bands, multiple groups of intersecting linear stripes, or jagged stripes. In the low-temperature PL spectra, most HTHP-treated diamonds exhibited stronger luminescence peaks at 637 nm compared to 575 nm. But there were also exceptions, such as the purple-red sample showing the opposite luminescence peak. In the 3D fluorescence spectrum of the HTHP-treated diamonds, the fluorescence peak mainly appeared in the range of 440–450 nm, accompanied by a broad band of 350–500 nm and even longer wavelengths. Meanwhile, some samples also exhibited fluorescence peaks at longer wavelengths, such as 353 nm, 676 nm, and 665 nm. These results make it possible to identify HTHP-treated diamonds by using luminescence characteristics, providing a new method for the non-destructive and rapid detection of HTHP-treated diamonds.

Mn-doped perovskite quantum dots with thermal stability synthesized at room temperature

All inorganic perovskite quantum dots (PQDs) have been studied extensively because of their excellent photoelectric properties. However, its poor stability and lead toxicity limit its practical application. At the same time, the complex synthesis conditions of the hot injection method also limited the preparation of large quantities. Here, we have successfully synthesized Mn-doped CsPb(Cl/Br)3 PQDs with excellent thermal stability at room temperature by a simple supersaturation crystallization method. The synthesized Mn-doped CsPb(Cl/Br)3 PQDs has good homogeneity and crystallinity. Meanwhile, Mn-doped CsPb(Cl/Br)3PQDs maintains the same crystal structure as CsPb(Cl/Br)3 PQDs. Importantly, it was found that the activation energy of CsPb(Cl/Br)3 PQDs after manganese doping increased from 65.1 meV to 222.8 meV through the high and low temperature PL test of 80 K–500 K. The crystal structure of CsPb(Cl/Br)3 PQDs is more stable due to the introduction of manganese ion, and the thermal stability is significantly improved. In this study, manganese doped CsPb(Cl/Br)3 PQDs with good crystallinity and thermal stability was rapidly synthesized by a simple supersaturation crystallization method at room temperature, which opened up a new way to improve the stability of PQDs and explore other non-toxic metal elements to replace Pb2+.

Revealing localized excitons in WSe2/β-Ga2O3

We have investigated the optical and magneto-optical properties of monolayer (ML) WSe2 on flakes of β-Ga2O3 under high magnetic fields. Remarkably, sharp emission peaks were observed and associated with localized excitons related to point defects. A detailed study of low-temperature photoluminescence (PL) and magneto-PL under high perpendicular magnetic field up to 9 T was carried out. Several sharp emission peaks have shown valley g-factors values close to −4, which is an unusual result for localized excitons in WSe2. Furthermore, some PL peaks have shown higher g-factor values of ≈−7 and ≈−12, which were associated with the hybridization of strain localized dark excitons and defects. The reported results suggest that β-Ga2O3 is, indeed, a promising dielectric substrate for ML WSe2 and also to explore fundamental physics in view of possible applications in quantum information technology.

Luminescent manifestations of ytterbium ions in the crystal structure of silicate apatite

The manifestation of ytterbium ions in the microcrystals of the Sr2Y6.8YbSi6O26:Er0.2 solid solution with apatite structure was studied on the basis of low-temperature photoluminescence (PL) spectroscopy and spectral-kinetic measurement data. Intracenter down-conversion luminescence of Er3+ ions in the visible and IR ranges was detected. Up-conversion luminescence of Er3+ ions is observed upon IR excitation due to excitation of Yb3+ ions and subsequent energy transfer Yb3+ → Er3+. Overlapping bands at 412 and 430 nm with a Stokes shift of less than 0.3 eV are observed in the PL spectra at room temperature. These emission bands are assigned to the spin-allowed and spin-forbidden 4f135d → 4f14 transitions from the low-spin and high-spin exciting states of Yb2+ ion, respectively. The PL decay kinetics for transitions from the low-spin states is characterized by a dominant component τ = 0.39 ns. An alternative model of Yb3+ luminescence center associated with a charge transfer band does not explain the data of low-temperature (5K) PL spectroscopy. At T = 5 K, a new wide emission band with a maximum at 696 nm and a Stokes shift of 1.1 eV appears in the PL spectrum. Two options of this PL band nature are considered. The first is the luminescence of defects having a vacancy nature. The second possible option is the manifestation of anomalous luminescence of Yb2+ ions, which occupy a different crystallographic position in the apatite crystal structure.

Low-temperature photoluminescence characteristic of Tm-doped Ga2O3 films for light emitting diodes application

Tm-doped Ga2O3 films hold significant promise for application in light-emitting diodes (LEDs). A comprehensive understanding of their photoluminescence properties is pivotal for unlocking their full potential. In this work, we present findings on the photoluminescence (PL) spectra of Tm-doped Ga2O3 films, elucidating variations at different measurement temperatures and excitation energies utilizing synchrotron radiation light. The intensity of ultraviolet light (UVL1 and UVL2) exhibits a temperature dependence and its activation energy illustrates an ionization energy of shallow donor and migration through tunneling of self-trapped holes to triply ionized Ga vacancy (VGa) centers forming doubly ionized VGa acceptors. Additionally, the excitation energy-dependent PL spectra provide insights into the excitation mechanism of Tm ions, indicating a most likely through defects in the Ga2O3 host.

Role of carbon in n-type bulk GaN crystals

In this work co-doping with silicon and carbon in gallium nitride grown by halide vapor phase epitaxy was performed. Native seeds of high structural quality with bowing radii of (0001) crystallographic planes higher than 15 m and threading dislocation density on the order of 5 × 104 cm−3 were used. The crystallized material was examined in terms of structural, optical, and electrical properties. For this purpose different characterization methods: X-ray diffraction, Raman spectroscopy, low-temperature photoluminescence, and Hall measurements, were applied. The physical properties of samples doped only with silicon and co-doped with carbon were analyzed and compared. However, during the crystallization process, due to high amount of Si and C, clusters of silicon carbide were unintentionally formed. The formation of these SiC clusters had an impact on the properties of the GaN crystals that were being studied. The research provides an initial insight into the complex studies of co-doping gallium nitride crystals. The results of this paper give us groundwork for more in-depth exploration of this topic.