Low-temperature Photoluminescence Research Articles

Study on the stress and mechanism of self-separated GaN grown by Na-flux method

A 2 inch free-standing c-plane GaN wafer was fabricated through in situ self-separation using HVPE-seed crystal etching back (HCEB) by intentionally adjusting the nitrogen pressure in the Na-flux growth process of GaN. First, adjust the nitrogen pressure in the reactor to a lower level to facilitate HCEB to form a large number of voids at the interface between the c-plane HVPE seed and the c-plane Na-flux GaN. After regrowth of approximately 340 μm thick Na-flux GaN, self-separation was achieved during the cooling process. The free-standing GaN wafer was almost stress-free as a result of strain relief by the in situ self-separation process, which was confirmed by room-temperature Raman and low-temperature photoluminescence measurements. It is supposed that the HCEB process can be applied to fabricate high-quality free-standing GaN wafers in the future.

Low-temperature photoluminescence of C60 single crystals intercaled with nitrogen molecules in the wide range of temperatures

The optical properties of C60 single crystals, intercalated with nitrogen molecules, were studied by the spectral-luminescence method at a temperature T = 30 K and excitation by the He–Ne laser (Eexc = 1.96 eV). Intercalation was carried out at a pressure of 30 atm in a temperature range of 200–550 °C. It was found that at sorption temperatures up to 400 °C, the bands of the low-temperature luminescence spectrum of the C60–N2 solutions are broadened without significant energy shift. As a rule, this situation is typical for the case of an increased contribution of the luminescence of “deep X-traps.” The concentration of such exciton emission centers is determined by the degree of occupation of the octahedral cavities of the fullerite fcc lattice by intercalated molecules. This indicates the formation of an equilibrium C60–N2 interstitial solution as a result of physisorption. At sorption temperatures above 400 °C, a significant shift of the luminescence spectrum towards low energies with a considerable inhomogeneous broadening of spectral bands was found for the first time. The shift and significant nonuniform broadening of the luminescence bands are explained by the emergence of a chemical interaction (chemisorption) of nitrogen with fullerene molecules, with the formation of a new nitrogen-containing substance in this case.

Formation of InAs/GaP Quantum-Well Heterostructures on Silicon Substrates by Molecular-Beam Epitaxy

The possibility of forming a strained pseudomorphous quantum well (QW) consisting of a InxGa1 – xAsyP1 – y quaternary alloy upon the deposition of InAs onto the surface of an epitaxial GaP/Si layer with a developed relief is demonstrated. The QW is studied by means of transmission electron microscopy and steady-state photoluminescence spectroscopy. The formation of two QW segments different in width and composition of the InxGa1 – xAsyP1 – y alloy is observed; in this case, an increase in the QW width is accompanied by a decrease in the content of In and As atoms. The lateral dimensions of the QW segments are no smaller than 20 nm. The QW segments correspond to two different low-temperature photoluminescence bands. The experimentally observed phenomena are interpreted on the assumption of transformation of the surface under the action of elastic strains during heteroepitaxy of InAs on the terraced GaP surface.

Investigations of Electron-Electron and Interlayer Electron-Phonon Coupling in van der Waals hBN/WSe2/hBN Heterostructures by Photoluminescence Excitation Experiments.

Monolayers of transition metal dichalcogenides (TMDs) with their unique physical properties are very promising for future applications in novel electronic devices. In TMDs monolayers, strong and opposite spin splittings of the energy gaps at the K points allow for exciting carriers with various combinations of valley and spin indices using circularly polarized light, which can further be used in spintronics and valleytronics. The physical properties of van der Waals heterostructures composed of TMDs monolayers and hexagonal boron nitride (hBN) layers significantly depend on different kinds of interactions. Here, we report on observing both a strong increase in the emission intensity as well as a preservation of the helicity of the excitation light in the emission from hBN/WSe2/hBN heterostructures related to interlayer electron-phonon coupling. In combined low-temperature (T = 7 K) reflectivity contrast and photoluminescence excitation experiments, we find that the increase in the emission intensity is attributed to a double resonance, where the laser excitation and the combined Raman mode A′1 (WSe2) + ZO (hBN) are in resonance with the excited (2s) and ground (1s) states of the A exciton in a WSe2 monolayer. In reference to the 2s state, our interpretation is in contrast with previous reports, in which this state has been attributed to the hybrid exciton state existing only in the hBN-encapsulated WSe2 monolayer. Moreover, we observe that the electron-phonon coupling also enhances the helicity preservation of the exciting light in the emission of all observed excitonic complexes. The highest helicity preservation of more than 60% is obtained in the emission of the neutral biexciton and negatively charged exciton (trion) in its triplet state. Additionally, to the best of our knowledge, the strongly intensified emission of the neutral biexciton XX0 at double resonance condition is observed for the first time.

Etching-induced damage in heavily Mg-doped p-type GaN and its suppression by low-bias-power inductively coupled plasma-reactive ion etching

Inductively coupled plasma–reactive ion etching (ICP–RIE)-induced damage in heavily Mg-doped p-type GaN ([Mg] = 2 × 1019 cm−3) was investigated by low-temperature photoluminescence (PL) and depth-resolved cathodoluminescence (CL) spectroscopy. From PL measurements, we found broad yellow luminescence (YL) with a maximum at around 2.2–2.3 eV, whose origin was considered to be isolated nitrogen vacancies (V N), only in etched samples. The depth-resolved CL spectroscopy revealed that the etching-induced YL was distributed up to the electron-beam penetration depth of around 200 nm at a high ICP–RIE bias power (P bias). Low-bias-power (low-P bias) ICP–RIE suppressed the YL and its depth distribution to levels similar to those of an unetched sample, and a current–voltage characteristic comparable to that of an unetched sample was obtained for a sample etched with P bias of 2.5 W.

N-type GaN surface etched green light-emitting diode to reduce non-radiative recombination centers

In this study, we attempt to identify the presence of surface defects (SDs) at an n-type GaN surface after high-temperature growth and gain insight into their intrinsic features. To this end, first, we carefully investigate n-type GaN samples with different surface etching depths. Low-temperature photoluminescence (PL) spectra reveal that SDs are most likely nitrogen vacancies (VN) and/or VN-related point defects intensively distributed within ∼100 nm from the n-type GaN surface after a high-temperature growth. We investigate the effect of SDs on the internal quantum efficiency (IQE) of green light-emitting diodes (LEDs) by preparing GaInN-based green LEDs employing a surface-etched n-type GaN, which exhibits a prominent enhancement of the PL efficiency with an increase in the etching depth. This effect is attributable to the reduced non-radiative recombination centers in multiple-quantum-well active regions because the SDs near the n-type GaN surface are removed by etching. We discuss strategies of in situ engineering on SDs to further improve the IQE in GaInN-based green LEDs on the basis of the results presented in this study.

Hybrid strain-coupled multilayer SK and SML InAs/GaAs quantum dot heterostructure: Enabling higher absorptivity and strain minimization for enhanced optical and structural characteristics

The size and shape of self-assembled quantum dots (QDs) in the top layer of a strain-coupled multilayer Stranski-Krastanov (SK) type QD heterostructure is decided by the effect of strain from the bottom QD layers along with the compressive strain through InAs/GaAs interface. A novel approach is used in this article to grow the heterostructures such that QDs from one layer to the other are heterogeneously coupled resulting in reduced cumulative strain, which helps in formation of defect free and uniform dots even for 10 strain-coupled SK QD layers. Heterostructures with strain-coupled single- ( × 1), bi- ( × 2), tri- ( × 3), penta- ( × 5), hepta- ( × 7) and deca- ( × 10) layer SK QDs are grown employing a growth mechanism on six stack sub-monolayer (SML) type QDs, considering the individual advantages of SK and SML QDs. Full width half maximum (FWHM) calculated from the low temperature (19 K) photoluminescence (PL) are 50, 52, 54, 55, 56 and 50 nm for single, bi, tri, penta, hepta and deca-layer SK QD stack respectively, which shows uniform dot size distribution up to deca-layer SK QD structure because of the adapted growth mechanism. The activation energies calculated from temperature dependent PL are 244, 281, 299, 275, 288 and 397 meV for single, bi, tri, penta, hepta and deca-layers respectively, which aids that the strain-coupled deca-layer SK QD heterostructure has the highest activation energy compared to other strain-coupled multilayer SK QD structures. These properties make the deca-layer SK QD structure as the optimum device for quantum dot infrared photodetectors (QDIPs), which could operate at higher temperatures with improved performance parameters. High resolution X-ray diffraction (HRXRD) shows a defect free heterostructure with minimized hydrostatic strain and homogeneous dot size distribution for the strain-coupled deca-layer SK QD structure. Thus, the adapted growth mechanism along with heterogeneous coupling of SK and SML QDs facilitates higher absorption efficiency, increased carrier lifetime and better quantum confinement, making it a promising design to be used in intermediate band solar cells (IBSC) and QDIPs, where multilayer QDs are needed so as to improve the device performance.

Heteroepitaxial growth of wide bandgap cuprous iodide films exhibiting clear free-exciton emission

Cuprous iodide (CuI) is an emerging wide-bandgap semiconductor of superior optical and transport properties. In particular, CuI shows high stability and large oscillator strength of free excitons that are of great advantage for optoelectronic applications. However, thin films of CuI reported so far have not been genuine single crystals, containing a sizable density of impurity and defect. Here, we demonstrate a dramatic improvement in the quality of CuI films grown by molecular beam epitaxy on a lattice-matched InAs substrate. The film is revealed to be in a single-crystal structure with high lattice coherence and an atomically flat surface. The low-temperature photoluminescence spectra exhibit extremely sharp emission from free excitons and much-suppressed emission from trapped states. The high-quality CuI films realized in the present study will not only facilitate the device application of CuI films but also provide unprecedented functionalities in halide semiconductors at the atomically sharp heterointerfaces.

Room temperature synthesis and characterization of novel lead-free double perovskite nanocrystals with a stable and broadband emission

We demonstrated the synthesis of ultra small and stable Cs3BiBr6 nanocrystals, ∼1.5–3 nm, via a room-temperature antisolvent method. Red-shift of bandgap was observed in low temperature PL measurements with an activation energy of ∼41 meV.

Field-effect-transistor-based ion sensors: ultrasensitive mercury(II) detection via healing MoS2 defects.

The contamination of water with heavy metal ions represents a harsh environmental problem resulting from societal development. Among various hazardous compounds, mercury ions (Hg2+) surely belong to the most poisonous ones. Their accumulation in the human body results in health deterioration, affecting vital organs and eventually leading to chronic diseases, and, in the worst-case scenario, early death. High selectivity and sensitivity for the analyte of choice can be achieved in chemical sensing using suitable active materials capable of interacting at the supramolecular level with the chosen species. Among them, 2D transition metal dichalcogenides (TMDCs) have attracted great attention as sensory materials because of their unique physical and chemical properties, which are highly susceptible to environmental changes. In this work, we have fabricated MoS2-based field-effect transistors (FETs) and exploited them as platforms for Hg2+ sensing, relying on the affinity of heavy metal ions for both point defects in TMDCs and sulphur atoms in the MoS2 lattice. X-ray photoelectron spectroscopy characterization showed both a significant reduction of the defectiveness of MoS2 when exposed to Hg2+ with increasing concentration and a shift in the binding energy of 0.2 eV suggesting p-type doping of the 2D semiconductor. The efficient defect healing has been confirmed also by low-temperature photoluminescence measurements by monitoring the attenuation of defect-related bands after Hg2+ exposure. Transfer characteristics in MoS2 FETs provided further evidence that Hg2+ acts as a p-dopant of MoS2. Interestingly, we observed a strict correlation of doping with the concentration of Hg2+, following a semi-log trend. Hg2+ concentrations as low as 1 pM can be detected, being way below the limits imposed by health regulations. Electrical characterization also revealed that our sensor can be efficiently washed and used multiple times. Moreover, the developed devices displayed a markedly high selectivity for Hg2+ against other metal ions as ruled by soft/soft interaction among chemical systems with appropriate redox potentials, being a generally applicable approach to develop chemical sensing devices combining high sensitivity, selectivity and reversibility, to meet technological needs.

Arsenic Doping Upon the Deposition of CdTe Layers from Dimethylcadmium and Diisopropyltellurium

The incorporation and activation of arsenic from tris(dimethylamino)arsine in CdTe layers grown by metal-organic chemical vapor deposition from dimethylcadmium and diisopropyltellurium on GaAs substrates are investigated. The incorporation of arsenic into CdTe depends on the crystallographic orientation of the layers and increases in the series (111)B → (100) → (310). The arsenic concentration in the CdTe layers is proportional to the tris(dimethylamino)arsine flow rate at a power of 1.4 and increases with decreasing diisopropyltellurium/dimethylcadmium ratio from 1.4 to 0.5. After deposition, the CdTe:As layers have p-type conductivity with an arsenic concentration of 1 × 1017–7 × 1018 cm–3 and a hole concentration of 2.7 × 1014–4.6 × 1015 cm–3, respectively; the fraction of electrically active arsenic does not exceed ~0.3%. After annealing in argon (250–450°C), the highest hole concentration is 1 × 1017 cm–3, and the arsenic activation efficiency is ~4.5%. The ionization energy of arsenic determined from the temperature dependence of the hole concentration is in the range of 98–124 meV. The low-temperature photoluminescence spectra of the layers have an emission peak with an energy of ~1.51 eV, which can be attributed to donor–acceptor recombination, where AsTe is an acceptor with an ionization energy of ~90 meV.

Optical Characterization of Native Defects in 4H–SiC Irradiated by 10 MeV Electrons with Subsequent Annealing

Low-temperature photoluminescence was employed to investigate the defects of 4H–SiC crystals after high-energy electron irradiation, and the annealing characteristics and dependence of the detecting temperature and irradiation doses were investigated. Results showed that the emission, associated with carbon antisite–vacancy (VCCSi)+ defects was dominant in the electron-irradiated 4H–SiC crystal. With increase in detecting temperature, the emission decreased demonstrating red-shifted, and the full width at half-maximum broadened, which was attributed to the increase in the concentration of carriers arising from thermal activation at high temperature. The emission intensity was the highest value at an irradiation dose of 7.9 × 1018 e/cm2, and then began to decrease. This revealed the lattice damage caused by long-term high-energy irradiation, which reduced the intensity of the defect radiation in the spectrum.

Emission properties of intrinsic and extrinsic defects in Cu2SnS3 thin films and solar cells

Intrinsic and extrinsic defects around the p–n interface in Cu2SnS3 (CTS) solar cells were evaluated using low-temperature photoluminescence (LT-PL) measurements. The intrinsic defects were investigated based on the PL-dependence of CTS films on the excitation power and temperature. Donor–acceptor pair recombination was observed with shallow acceptors (copper vacancies, V Cu) located approximately 18 meV above the valence band maximum and typical donors located 72 and 112 meV below the conduction band minimum (CBM). The PL spectra of various CTS solar cell structures were measured to identify the Cd-related defects formed by Cd diffusion from the CdS layer. A new LT-PL peak was observed at 0.87 eV for the CdS/CTS solar cells, corresponding to D–A pair recombination with Cd on Cu site donors located 62 meV below the CBM. A p–n homojunction may form in CTS by V Cu passivation by Cd diffusion and suppressed interface recombination.

N-Type host materials based on nitrile and triazine substituted tricyclic aromatic compounds for high-performance blue thermally activated delayed fluorescence devices

Novel n-type host materials based on tricyclic aromatic compounds, dibenzo[b,d]furan and dibenzo[b,d]thiophene (2Trz6CNDBF and 2Trz6CNDBT), have been successfully synthesized and characterized for high-performance blue thermally activated delayed fluorescence (TADF) organic light-emitting devices (OLEDs). Dibenzo[b,d]furan and dibenzo[b,d]thiophene were utilized as central molecular building blocks to achieve excellent thermal stability and high triplet energy (ET). Nitrile and diphenyltriazine functional groups were introduced at the 6- and 2- positions of the central building blocks to achieve low-lying lowest unoccupied molecular orbital (LUMO) energy levels, high ET, and excellent electron transport properties. UV–Vis absorption, low-temperature photoluminescence, and ultraviolet photoelectron spectroscopy analysis showed that 2Trz6CNDBF and 2Trz6CNDBT possessed high ET (2.95 and 2.88 eV) and low-lying LUMO energy levels (−3.43 and −3.16 eV) that were well-matched with a blue TADF emitter, 5CzCN. Moreover, the electron-only device (EOD) result revealed that 2Trz6CNDBF and 2Trz6CNDBT had excellent electron transport properties. Blue TADF OLEDs fabricated with a p-type host material, mCBP, and n-type host, 2Trz6CNDBF or 2Trz6CNDBT, exhibited lower driving voltages (3.34 and 3.26 V, respectively) than a TADF OLED with only a p-type host material, mCBP (3.83 V). Blue TADF OLEDs with 2Trz6CNDBF and 2Trz6CNDBT exhibited superior external quantum efficiency (ηext, 15.6 and 14.7%), current efficiency (ηce, 33.8 and 32.7 cd A−1), and power efficiency (ηpe, 25.6 and 25.7 lm W−1), respectively. The ηext and ηce of blue TADF OLEDs with 2Trz6CNDBF and 2Trz6CNDBT increased by more than 70% and ηpe by approximately 150% compared to those of the TADF OLED with a single p-type host, mCBP. In addition, the device lifetimes of blue TADF OLEDs with 2Trz6CNDBF and 2Trz6CNDBT increased by more than 1000%. The efficient electron injection by the low-lying LUMO energy level, effective exciton confinement by high ET, and high thermal stability of the film morphology provided the enhanced efficiency and lifetime for blue TADF OLEDs with 2Trz6CNDBF and 2Trz6CNDBT.

Luminescence, up-conversion and temperature sensing in Er-doped TeO2-PbCl2-WO3 glasses

The TeO2-PbCl2-WO3:Er3+ glasses were prepared by melt-quenching technique. Optical and luminescence properties of the prepared glasses were studied by transmission and low-temperature photoluminescence spectroscopy. Emission intensities of the relevant 4f-4f transitions were studied at room temperature under 514.5 nm excitation. At low-temperatures, the broad-band emission of the host glass centered at about 1000 nm and three superimposed narrow Er3+-related emission bands were observed. Strong infrared to visible up-conversion luminescence was observed under 980 nm excitation even at low erbium concentration. The observed green, red, and NIR up-conversion 4f-4f emission bands at 530, 550, 660, and 850 nm are assigned to two-photon absorption processes. It follows from the study of luminescence intensity ratio of two green Er3+- related emission bands due to transitions from thermally coupled 2H11/2 and 4S3/2 levels to the ground state 4I15/2 that the studied glasses can be good candidates for non-contact optical temperature sensor applications.

The 1.1, 0.8 and 0.55–0.60 eV deep bands in detector-grade CdMnTe studied by photoluminescence spectroscopy

We present low-temperature photoluminescence (PL) spectra of Cd0·95Mn0·05Te grown by the Bridgman method. This is a promising material for nuclear detectors. We focus on three deep bands: 1.1, 0.8 and 0.55–0.60 eV. In order to achieve high resistivity in our crystals, essential in detector applications, we confirm the role of a deep intrinsic donor level at 0.8 eV, related to TeCd2+. It can pin the Fermi level in the mid-gap and increase the resistivity value. We suggest two interpretations of the 0.55–0.60 eV PL band origin. It can be connected with the deionization of a deep acceptor, such as Cd2−, or with simultaneous transitions of an electron (1.1 eV) and a hole (0.55–0.60 eV) followed by their recombination. These two energies together give the band gap value for Cd0·95Mn0·05Te at 5 K. The 1.1 eV luminescence in our crystals is related to Te secondary phases, the presence of which we confirm by infra-red microscopy. The correlation between the chemical composition of the crystals (changed by Cd- or Te-excess used in growth process and by post-growth annealing), the resistivity value and the PL results is shown.

Emissive spin-0 triplet-pairs are a direct product of triplet-triplet annihilation in pentacene single crystals and anthradithiophene films.

Singlet fission and triplet-triplet annihilation represent two highly promising ways of increasing the efficiency of photovoltaic devices. Both processes are believed to be mediated by a biexcitonic triplet-pair state, 1(TT). Recently however, there has been debate over the role of 1(TT) in triplet-triplet annihilation. Here we use intensity-dependent, low-temperature photoluminescence measurements, combined with kinetic modelling, to show that distinct 1(TT) emission arises directly from triplet-triplet annihilation in high-quality pentacene single crystals and anthradithiophene (diF-TES-ADT) thin films. This work demonstrates that a real, emissive triplet-pair state acts as an intermediate in both singlet fission and triplet-triplet annihilation and that this is true for both endo- and exothermic singlet fission materials.

Band structure properties, phonons, and exciton fine structure in 4H -SiC measured by wavelength-modulated absorption and low-temperature photoluminescence

Owing to its hexagonal symmetry, indirect band gap, and relatively large unit cell, the electronic band structure of $4H$-SiC is comprised of a complicated series of anisotropic valence and conduction band extrema even very near to the uppermost valence band maximum and lowest conduction band minimum. This has presented a difficult challenge to those experiments which have attempted to resolve the small energy separations between these band extrema. To overcome this challenge, we have measured the wavelength-modulated absorption (WMA) spectrum of $4H$-SiC over a broader wavelength range (3500--3800 \AA{}) and at a higher resolution ($l0.1$ \AA{}) than in previous work. By comparing these measurements with the low-temperature photoluminescence spectrum in ultrapure $4H$-SiC, we have identified several features, which we attribute to a $56\ifmmode\pm\else\textpm\fi{}3$ meV crystal-field splitting of the valence band maximum or a $136\ifmmode\pm\else\textpm\fi{}3$ meV separation between the two lowest conduction band minima. We also show that the spin-orbit split-off valence band, which has been observed in previous measurements of $4H$-SiC, contributes to nonparabolic dispersion near the valence band maximum, and this is responsible for several previously misidentified features in the WMA spectrum. Finally, we report the first experimental measurement of fine structure splittings in the free exciton ground state, which manifests as four small ($0.7\ifmmode\pm\else\textpm\fi{}0.1$ meV) splittings in the WMA spectrum due to mass anisotropy and electron-hole exchange interaction.

Multiphoton-excited exciton molecules in diamond

In this paper we report on observation of exciton molecules in the low-temperature photoluminescence spectra of diamond. The charge carriers are excited using multiphoton absorption of few-cycle near-infrared laser pulses. We observe that peaks corresponding to exciton molecules are well resolved and the formation of electron hole droplets is suppressed due to the initial carrier dynamics and low excited carrier densities.

Characterization of nano-bio silicon carbide

Plasma-enhanced chemical vapor deposition, reactive magnetron sputtering, hot-wire chemical vapor deposition and radio frequency plasma-enhanced chemical vapor deposition were used to develop technology for preparation of nano-bio silicon carbide coating of ceramic materials for dental applications. The effect of the bias voltage applied to the ceramic prostheses and dental crowns on the crystallization processes have been recognized. The optimal bias voltage applied to conductive substrate was –200 V, whereas for dielectric substrate the bias voltage Vbias did not affect the properties of SiC coating. The analysis of CVCs and spectroscopic diagnostics as the methods for studying the mechanism of interfacial rearrangements to investigate SiC phase transition in nano silicon carbide coatings were used. The conductivity of the SiC coating coincided with the conductivity on the dielectric (µn0 = 1012…1013 сm–1·s–1·V–1). The conductive substrate had a significant effect on the properties of the coating and thus depended on the bias voltage Vbias. The conductivity increased by three-four orders of magnitude (µn0 = 3·1017 сm–1·s–1·V–1), if the bias voltage Vbias = –200 V. The increase of the bias voltage (Vbias = –600 V) led to a decrease in the conductivity (µn0 = 1011…1012 сm–1·s–1·V–1). It was found that there was the double injection regime with bimolecular recombination in this structure with the dependence I = V3/2 for CVCs of SiC. The luminescence spectrum of SiC coating on non-dielectric ceramics (if Vbias = – 200 V during deposition) was significantly different from the luminescence spectrum of SiC coating on dielectric ceramics. Increasing the applied voltage to the substrate Vbias during deposition led to increasing the fraction of hexagonal polytypes. Directions in the crystal lattice according to the photoluminescence spectra were identified from the comparing the values of the width of the non-phonon parts of stacking faults and deep level spectra in the low-temperature photoluminescence with arrangements of atoms in the SiC lattice structure. The displacement of each atom participating in photoluminescence allowed to find the correlation with technology of SiC deposition and to develop technology of SiC coating on the dental materials.