Low-temperature Photoluminescence Research Articles

Study of Single-Electron Spectrum of GaAs/AlGaAs Heterostructure for Mid-IR Photodetectors via Low-Temperature Luminescence

By measuring low-temperature photoluminescence, we demonstrated the role of interface blurring and the introduction of background impurities in the formation of the single-electron spectrum of GaAs/AlxGa1–xAs quantum well heterostructures used to fabricate mid-IR photodetectors. Background impurities, having a noticeable effect on the emission spectrum of quantum wells, do not affect their absorption spectra (luminescence excitation). Nevertheless, the blurring of the interface distorts the structure of single-electron states significantly, while weakly manifesting itself in luminescence. We proposed a method for estimating the degree of blurring of quantum well interfaces for photodetectors by the spectra of excitation of their exciton luminescence.

A Comparison of Defects Between InAs Single Crystals Grown by LEC and VGF Methods

InAs single crystals grown by the liquid-encapsulated Czochralski (LEC) method and vertical gradient freezing (VGF) are studied by low-temperature photoluminescence spectroscopy, infrared transmission and reflectance spectroscopy, double-crystal x-ray diffraction and Hall effect measurement, respectively. A properly controlled etching solution is used to reveal beautiful square dislocation etch pits in the crystals. In addition to extremely low dislocation density, the concentration of native defects in the VGF-InAs single crystals is much lower than that in LEC-InAs, giving VGF-InAs better electrical and optical properties. The nature of the defects in InAs single crystals is discussed by considering the variation in stoichiometry and environment during the crystal growth processes.

Intrinsic and Extrinsic Defect-Related Excitons in TMDCs.

We investigate the excitonic peak associated with defects and disorder in low-temperature photoluminescence of monolayer transition metal dichalcogenides (TMDCs). To uncover the intrinsic origin of defect-related (D) excitons, we study their dependence on gate voltage, excitation power, and temperature in a prototypical TMDC monolayer MoS2. Our results suggest that D excitons are neutral excitons bound to ionized donor levels, likely related to sulfur vacancies, with a density of 7 × 1011 cm-2. To study the extrinsic contribution to D excitons, we controllably deposit oxygen molecules in situ onto the surface of MoS2 kept at cryogenic temperature. We find that, in addition to trivial p-doping of 3 × 1012 cm-2, oxygen affects the D excitons, likely by functionalizing the defect sites. Combined, our results uncover the origin of D excitons, suggest an approach to track the functionalization of TMDCs, to benchmark device quality, and pave the way toward exciton engineering in hybrid organic-inorganic TMDC devices.

Towards radiation detection using Cs2AgBiBr6 double perovskite single crystals

In this work, we studied the optical- and electrical- properties of emerging Cs2AgBiBr6 double perovskite single crystals and demonstrated their potential for detecting ionizing radiation. We prepared Cs2AgBiBr6 double perovskite single crystals from a saturated aqueous solution. Low-temperature photoluminescence (PL) was employed to determine the bandgap energies of Cs2AgBiBr6, which are 2.00 eV (indirect) and 2.26 eV (direct) respectively. Using the space charge limited current method, we estimated the density of trap states and mobility of charge carriers as 1.44 × 1010 cm−3 and 7.02 cm2/V-s respectively. A lower bound value of the mobility-lifetime (μ-τ) product of 2.48 × 10−3 cm2/V was determined using 450 nm laser excitation, which was sufficient for ensuring a long drift distance of charge carriers for several radiation detector applications. Furthermore, we tested the direct response of Cs2AgBiBr6 single crystals to X-ray radiation. Our Cs2AgBiBr6 single crystal device with gold electrodes deposited on the two parallel surfaces exhibited excellent linear response to low energy X-rays.

Low temperature combustion synthesis and photoluminescence mechanism of ZnO/ZnAl2O4 composite phosphors

ZnO, ZnAl2O4 and ZnO/ZnAl2O4 composite phosphors were prepared by one - or two-step low temperature combustion synthesis method. The ZnO, ZnAl2O4 and ZnO/ZnAl2O4 composite phosphors were obtained by auto-ignited combustion of a carbamide–nitrate precursor gel which is produced by a simple thermally induced redox reaction. The crystalline phase attributes, functional groups, surface morphology, optical and photoluminescence properties of the ZnO, ZnAl2O4 and ZnO/ZnAl2O4 composite phosphors were characterized by X-ray diffraction analysis, fourier transform infrared spectroscopy, scanning electron microscopy, UV–vis spectrophotometer and fluorescence spectrophotometer. The results showed that ZnO/ZnAl2O4 composite phosphors with the type II energy band arrangement were synthesized by two-step low temperature combustion synthesis method. The introduction of zinc oxide into zinc aluminate system improved the crystallinity of zinc oxide and enhanced the optical and photoluminescent properties of the single phase of materials. Based on the energy band theory, the photoluminescence mechanism of ZnO/ZnAl2O4 composite phosphors was proposed.

Giant defect emission enhancement from ZnO nanowires through desulfurization process

Zinc oxide (ZnO) is a stable, direct bandgap semiconductor emitting in the UV with a multitude of technical applications. It is well known that ZnO emission can be shifted into the green for visible light applications through the introduction of defects. However, generating consistent and efficient green emission through this process is challenging, particularly given that the chemical or atomic origin of the green emission in ZnO is still under debate. In this work we present a new method, for which we coin term desulfurization, for creating green emitting ZnO with significantly enhanced quantum efficiency. Solution grown ZnO nanowires are partially converted to ZnS, then desulfurized back to ZnO, resulting in a highly controlled concentration of oxygen defects as determined by X-ray photoelectron spectroscopy and electron paramagnetic resonance. Using this controlled placement of oxygen vacancies we observe a greater than 40-fold enhancement of integrated emission intensity and explore the nature of this enhancement through low temperature photoluminescence experiments.

Tailoring Hot Exciton Dynamics in 2D Hybrid Perovskites through Cation Modification

We report a family of two-dimensional hybrid perovskites (2DHPs) based on phenethylammonium lead iodide ((PEA)2PbI4) that show complex structure in their low-temperature excitonic absorption and photoluminescence (PL) spectra as well as hot exciton PL. We replace the 2-position (ortho) H on the phenyl group of the PEA cation with F, Cl, or Br to systematically increase the cation's cross-sectional area and mass and study changes in the excitonic structure. These single atom substitutions substantially change the observable number of and spacing between discrete resonances in the excitonic absorption and PL spectra and drastically increase the amount of hot exciton PL that violates Kasha's rule by over an order of magnitude. To fit the progressively larger cations, the inorganic framework distorts and is strained, reducing the Pb-I-Pb bond angles and increasing the 2DHP band gap. Correlation between the 2DHP structure and steady-state and time-resolved spectra suggests the complex structure of resonances arises from one or two manifolds of states, depending on the 2DHP Pb-I-Pb bond angle (as)symmetry, and the resonances within a manifold are regularly spaced with an energy separation that decreases as the mass of the cation increases. The uniform separation between resonances and the dynamics that show excitons can only relax to the next-lowest state are consistent with a vibronic progression caused by a vibrational mode on the cation. These results demonstrate that simple changes to the cation can be used to tailor the properties and dynamics of the confined excitons without directly modifying the inorganic framework.

Fast-response symmetric coplanar Ni/AlGaInP/Ni visible photodetector

A symmetric coplanar metal-semiconductor-metal structure has been successfully fabricated on a quaternary thin film of (AlxGa1-x)yIn1-yP epitaxially grown on (100) semi-insulating GaAs (SI-GaAs) substrate for visible photodetection. The crystallinity and alloy composition of the grown material was investigated with the help of high-resolution x-ray rocking curve and photoluminescence experiment giving the values of y = 0.66 and x = 0.67. The low-temperature photoluminescence study also gave insight into the defect levels in the material. The dark current-voltage curve was fitted by employing a back-to-back connected two-diode model to extract the barrier heights and ideality factor of the nickel contacts which were found to be 0.84 eV, 0.82 eV and 1.014, respectively. The photo-response of the device was studied in the wavelength range of 300−1000 nm which exhibited a peak at 610 nm. The maximum peak responsivity, detectivity and photosensitivity at a fixed applied bias of +15 V appeared to be 1.62 AW−1, 6.34 × 1011 cmHz1/2W-1 and 2.26 × 105 cm2W−1, respectively whereas under −15 V these values were 0.6 AW−1, 2.37 × 1011 cmHz1/2W−1, and 8.72 × 104 cmW−1, respectively in the presence of 610 nm illumination with optical power of 2 μW/cm2. The photodetector showed a rise time of 91 μs and the decay characteristics exhibited two channels of 83 μs and 282 μs, which are quite fast considering the temporal performance of similar kinds of devices. The photocurrent showed sub-linear power dependence with an increasing optical intensity which further refers to the recombination of photogenerated electron-hole pairs through trap and recombination centres within the forbidden gap.

On the Formation of Amorphous Ge Nanoclusters and Ge Nanocrystals in GeSixOy Films on Quartz Substrates by Furnace and Pulsed Laser Annealing

Nonstoichiometric GeO0.5[SiO2]0.5 and GeO0.5[SiO]0.5 germanosilicate glassy films are produced by the high-vacuum coevaporation of GeO2 and either SiO or SiO2 powders with deposition onto a cold fused silica substrate. Then the films are subjected to furnace or laser annealing (a XeCl laser, λ = 308 nm, pulse duration of 15 ns). The properties of the samples are studied by transmittance and reflectance spectroscopy, Raman spectroscopy, and photoluminescence spectroscopy. As shown by analysis of the Raman spectra, the GeO[SiO] film deposited at a substrate temperature of 100°C contains amorphous Ge clusters, whereas no signal from Ge–Ge bond vibrations is observed in the Raman spectra of the GeO[SiO2] film deposited at the same temperature. The optical absorption edge of the as-deposited GeO[SiO2] film corresponds to ~400 nm; at the same time, in the GeO[SiO] film, absorption is observed right up to the near-infrared region, which is apparently due to absorption in Ge clusters. Annealing induces a shift of the absorption edge to longer wavelengths. After annealing of the GeO[SiO2] film at 450°C, amorphous germanium clusters are detected in the film, and after annealing at 550°C as well as after pulsed laser annealing, germanium nanocrystals are detected. The crystallization of amorphous Ge nanoclusters in the GeO[SiO] film requires annealing at a temperature of 680°C. In this case, the size of Ge nanoclusters in this film are smaller than that in the GeO[SiO2] film. It is not possible to crystallize Ge clusters in the GeO[SiO] film. It seems obvious that the smaller the semiconductor nanoclusters in an insulating matrix, the more difficult it is to crystallize them. In the low-temperature photoluminescence spectra of the annealed films, signals caused by either defects or Ge clusters are detected.

Quantum Size Effects in Germanium Nanocrystals and Amorphous Nanoclusters in GeSixOy Films

Films of nonstoichiometric germanium–silicon glasses of two types—GeOx[SiO](1 – x) and GeOx[SiO2](1 – x)—are deposited onto cold Si (001) substrates by evaporating GeO2 and SiO (or SiO2) powders simultaneously in high vacuum. Film samples in their initial (as-deposited) state and after being annealed at 550 and 650°C for 1 h are investigated using IR and Raman spectroscopies and electron microscopy, and their photoluminescence (PL) is studied as well. Raman spectroscopy shows that, in contrast to the initial GeO[SiO2] film, the initial GeO[SiO] one contains clusters of amorphous germanium, their size being ~3 nm, as found by electron microscopy. The presence of Si–O, Ge–O, and Si–O–Ge bonds in the films is established by IR spectroscopy. Clusters of amorphous germanium are found in both films after annealing at 550°C, while germanium nanocrystals are formed in the films subjected to annealing at 650°C. For the initial films, a broad band with a maximum at 1050 nm is registered in their low-temperature PL spectra, which may originate from such defects as oxygen vacancies and overstoichiometric germanium atoms. Annealing causes structural changes in the films and affects their PL behavior. The films containing germanium nanoclusters give rise to PL with a maximum at 1400–1600 nm, with the defect-related signal being diminished. The temperature dependence of PL intensity exhibits a decreasing behavior, but PL is observed to temperatures as high as 200 K. The contribution of germanium nanocrystals formed at the annealing stage to PL is discussed.

Improvement of Surface Defects in CdZnTe Crystals by Rapid Thermal Annealing

We have investigated the influence of rapid thermal annealing (RTA) on the surface defects of CdZnTe (CZT) crystals by using low-temperature photoluminescence (PL) measurements. In the low-temperature PL spectrum for the as-grown CZT crystals, the donor–acceptor pair (DAP) recombination peak is dominant. Overlapping of the free-exciton (FE) and the neutral donor bound exciton (D0,X) luminescence peaks leads to broadening of the (D0,X) peak. In the low-temperature PL spectra for CZT crystals annealed at 523 K and 623 K for 5 min, the A-center peak is dominant. The deterioration of the intensity and FWHM of the (D0,X) peak indicates worsening of the crystal quality under the two RTA conditions. In the low-temperature PL spectra for CZT crystals annealed at 723 K, 773 K and 873 K for 5 min, an FE peak gradually becomes evident as the annealing temperature increases. The full-width half-maximum (FWHM) of the (D0,X) peak decreases from 7.95 meV before annealing to 2.85 meV after annealing at 873 K. The neutral acceptor bound exciton (A0,X) peak intensity increases with the annealing temperature increasing. The peak intensity ratio I(D0,X)/I(DAP) increases from ∼ 0.24 before annealing to ∼ 1.62 after annealing at 873 K. The A-center peak intensity decreases with the increase in annealing temperature. These phenomena demonstrate better surface structure and higher crystal quality under the three RTA conditions, especially for the annealing at 873 K. RTA treatment can be an effective way to improve the surface properties of CZT crystals by modifying the technical parameters.

Phonon-Assisted Photoluminescence from Indirect Excitons in Monolayers of Transition-Metal Dichalcogenides.

The photoluminescence (PL) spectrum of transition-metal dichalcogenides (TMDs) shows a multitude of emission peaks below the bright exciton line, and not all of them have been explained yet. Here, we study the emission traces of phonon-assisted recombinations of indirect excitons. To this end, we develop a microscopic theory describing simultaneous exciton, phonon, and photon interaction and including consistent many-particle dephasing. We explain the drastically different PL below the bright exciton in tungsten- and molybdenum-based materials as the result of different configurations of bright and momentum-dark states. In good agreement with experiments, our calculations predict that WSe2 exhibits clearly visible low-temperature PL signals stemming from the phonon-assisted recombination of momentum-dark K–K′ excitons.

Microhardness study of Cd1-x ZnxTe1-ySey crystals for X-ray and gamma ray detectors

We performed a detailed Vickers microhardness study of quaternary material Cd1−x ZnxTe1−y Sey (CZTS), which is currently under investigation for applications in the detection of hard X-rays and gamma rays. In this frame, the Vickers microhardness method, resistivity mapping, low-temperature (4.2 K) photoluminescence, and Se energy dispersive X-ray spectroscopy (EDX) composition analysis were employed along the crystal growth axis. The microhardness values measured at 50 g load (HV 0.05), bandgap, and crystal composition along the crystal axis have been correlated. We also performed a comparative study of a set of CdTe, CdZnTe, and CdTeSe samples. We observed a significantly higher microhardness in CZTS crystals when compared to CdZnTe crystals. While in the CdZnTe crystals the microhardness increases by approx. 2.1 HV 0.05 per 1% of Zn + Se content, in CZTS crystals this value is 4.1 HV 0.05. This result indicates a possible additional strengthening of the effect when Zn and Se are mixed in the CdTe lattice.

Dynamics of Antisolvent Processed Hybrid Metal Halide Perovskites Studied by In Situ Photoluminescence and Its Influence on Optoelectronic Properties

The antisolvent dripping time during spin-coating of CH3NH3PbI3 (MAPbI3) strongly impacts film morphology as well as possible formation of the intermediate precursor phase, and – consequently – device performance. Here, we use in situ photoluminescence (PL) to directly monitor the fast-occurring changes during MAPbI3 synthesis. These measurements reveal how the ideal timing of the antisolvent leads to homogeneous nucleation and pinhole-free films. In addition, these films show significantly reduced nonradiative recombination with 1.5 orders of magnitude difference in absolute PL quantum yield compared to films where no antisolvent is applied. Low-temperature PL measurements confirm that antisolvent treatment reduces the number of trap states presumably in the bulk material. However, if the antisolvent is dripped late, heterogeneous nucleation via the orthorhombic (MA)2(DMF)2Pb3I8 intermediate phase leads to a needle-like morphology that can be correlated to a red-shifted in situ PL signature. We find that...

Defects, electronic properties, and α particle energy spectrum response of the Cd0.9Mn0.1Te: V single crystal

Cadmium manganese telluride is a promising material for fabricating room-temperature nuclear radiation detectors widely used in medical imaging, environmental protection, nuclear security detection, astrophysics, and so on. The Cd0.9Mn0.1Te: V (V: CMT) crystal examined in this work was grown through the Te solution (10% excess) vertical Bridgman method. The low-temperature photoluminescence (PL) spectra indicated that the grown crystal has good quality. A simultaneous thermal excitation current spectrum was used to characterize the effect of vanadium doping on the level defects in the crystal. The current–voltage and Hall test results showed that the crystal resistivity was (3.781–6.185) × 1010 Ω cm. The conductivity was of n type. The carrier concentration was (1.69–9.94) × 106 cm−3. The Hall mobility was (3.08–9.29) × 103 cm−2 V−1 s−1. The maximum measured ratio of the light and dark currents, when the crystal was exposed to 5 mW white light, was 11. In addition, the room-temperature electron mobility-lifetime product of the middle sample was 6.925 × 10−4 cm2 V−1 using the 241Am@5.48 MeV α particle source.

Low-temperature time-resolved phosphorescence excitation emission matrices for the analysis of phenanthro-thiophenes in chromatographic fractions of complex environmental extracts

The present study investigates the analytical potential of low-temperature photoluminescence spectroscopy for the analysis of seven phenanthrothiophenes with molecular mass 234 g mol−1. The studied PASHs include Phenanthro [1,2-b]thiophene, Phenanthro [2,1-b]thiophene, Phenanthro [2,3-b]thiophene, Phenanthro [3,2-b]thiophene, Phenanthro [3,4-b]thiophene, Phenanthro [4,3-b]thiophene and Phenanthro [9,10-b]thiophene. Excitation and emission spectra recorded from n-alkane solutions at room temperature, 77 K and 4.2 K show phosphorescence emission from all the studied isomers at cryogenic temperatures. The analytical figures of merit obtained under steady state (fluorescence) and time-resolved (phosphorescence) conditions provide limits of detection at the parts-per-billion (ng mL−1) concentration levels. Processing 77 K and 4.2 K phosphorescence data with parallel factor analysis showed to be a robust approach to the determination of phenanthro-thiophenes in complex fluorophore mixtures.

Molecular States of Composite Fermions in Self-Organized InP/GaInP Quantum Dots in Zero Magnetic Field

The size and positions of regions of line localization and the magnetic-field (0–10 T) dependence of the low-temperature (10 K) photoluminescence spectra of single InP/GaInP quantum dots with a number of electrons of N = 5–7 and a Wigner–Seitz radius of ~2.5 are determined using a near-field scanning optical microscope. The formation of composite fermion molecules with a size coinciding with that of localization regions and bond lengths of ~30 and 50 nm, respectively, at a Landau-level filling factor from 1/2 to 2/7 in zero magnetic field is established. At N = 6, the pairing and rearrangement of composite fermions under photoexcitation are found, which offers opportunities for the use of InP/GaInP quantum dots to create a magnetic-field-free topological quantum gate.

MOCVD growth of InAs/GaSb type-II superlattices on InAs substrates for short wavelength infrared detection

We demonstrate InAs/GaSb type-II superlattice (SL) materials and photo detectors grown by metal-organic chemical vapor deposition for short wavelength infrared (SWIR) detection. A set of SL samples with m (m = 4, 3, 2) monolayer (ML) InAs/8 ML GaSb were grown to verify the bandgap tunability. Low-temperature photoluminescence measurement indicates SWIR emissions in the 2–3 µm region for these samples. Excellent structural quality was obtained as revealed by X-ray diffraction and atomic force microscopy. However, when the InAs period thickness is as thin as 2 ML, structural quality starts to deteriorate, as evidenced by broadening of the X-ray line width and change of the surface morphology from step-flow growth to two-dimensional growth. A pin homojunction photodetector using 3 ML InAs/8 ML GaSb SLs was grown and shows a 50% cut-off wavelength of ~2.54 µm and a peak specific detectivity of 4.0 × 1011 cm·Hz1/2/W at 77 K.

Photoluminescence of ZnO/ZnMgO heterostructure nanobelts grown by MBE

ZnO nanobelts may grow with their polar axis perpendicular to growth direction. Heterostructured nanobelts therefore contain hetero-interfaces along the polar axis of ZnO where polarisation mismatch may induce electron confinement. These interfaces run along the length of the nanobelts. Such heterostructure nanobelts are grown by molecular beam epitaxy and TEM images confirm the core–shell structure. The effects of shell-growth temperature on nano-heterostructures is investigated using photoluminescence and secondary ion mass spectrometry in a focussed ion-beam microscope with Ne+ as the primary ion beam. We perform low temperature photoluminescence on ensembles of such heterostructures and single nanostructures. We show how single nanobelts have photoluminescence spectra rich in features and attribute these to band misalignment at ZnO/ZnMgO interfaces embedded within nano-heterostructures.

Cation impurity-defect complex induced ferromagnetism and hopping conduction in Sb-doped ZnO synthesized under high pressure

Sb-doped p-type ZnO was synthesized using the high-pressure method with a quasi-equilibrium thermodynamic process. Room temperature ferromagnetism was observed in Sb-doped ZnO. The ferromagnetic coupling was found to be associated with zinc vacancies (VZn) induced by Sb doping, which was supported by a strong ultraviolet emission peak relevant to VZn in low temperature photoluminescence. Hall effect measurements indicated that the Sb-doped ZnO with Sb content of 3.4 at% had a hole concentration of 1.4 × 1020 cm−3 and low resistivity of 7.4 × 10−2 Ω cm. Temperature-dependent resistance measurements suggest that the electrical conduction mechanism of the Sb-doped ZnO changed from thermal activation model and nearest-neighbor hopping (NNH) conduction at high temperature to Mott variable-range hopping (VRH) and Efros–Shklovskii VRH conduction at low temperature. First-principles calculations demonstrated that the Sb impurity substituting Zn (SbZn) combines two VZn defects to form acceptor-like impurity-defect complex of SbZn–2VZn in ZnO due to the coulomb attraction. The SbZn–2VZn complex in Sb-doped ZnO are strongly correlated with ferromagnetic stability and lead to the Mott and Efro–Shklovskii VRH conduction at low temperature due to the localization of carriers at the complex impurity band.