Temperature-dependent Photoluminescence Spectra Research Articles

(Invited) Fabrication of High-Quality Ge-on-Insulator Structures by Lateral Liquid Phase Epitaxy

Germanium (Ge) has been considered as a key material for next-generation electronic–photonic integration in large-scale integrated circuits on Si substrates. Ge-based electronic device is beneficial for high-performance complementary metal-oxide semiconductor application compatible with Si technology because Ge is a Group IV element and the electron and hole mobilities are 2.5-4 times higher than those of Si. Ge metal-oxide-semiconductor field-effect transistors (MOSFETs) might suffer from the large off-leakage current caused by the small bandgap, however Ge-on-insulator (GOI) structure has a great potential to overcome this obstacle. Ge is also expected as light emission and absorption material in the photonic devices. Although Ge has the electronic band structure of indirect band gap, the direct bandgap at Γ valley is 0.8 eV, corresponding to 1550-nm wavelength, same as used in optical communication, and the energy difference between Γ and L valley is only 136 meV. Furthermore, inducing lattice strain and Tin (Sn) incorporation into Ge enhance the carrier mobility and modulate the band structure from the indirect to the direct band structure, resulting in the enhancement of electronic device performance and the increase of light emission and absorption efficiency in photonic devices, respectively. Therefore, a lot of studies have been extensively carried out in order to realize Ge-based electronic–photonic integration. One of the critical challenges for the realization is to fabricate GOI structures with high-crystalline-quality Ge layers. Among various methods of GOI fabrication, layer transfer and Ge condensation are the ones most widely used. However, the former requires complex processing and the latter has difficulties in achieving high-quality GOI layers. Recently, rapid melt growth, in other words, the lateral liquid-phase epitaxy (LLPE) method, was developed for producing local GOI structures on Si substrates [1,2]. In this method amorphous Ge wires connected to the Si substrate at seed area are annealed above the melting point of Ge and lateral epitaxial growth from the seed area along the Ge wires occurs during cooling. This method has the remarkable advantage for heteroepitaxial growth containing large lattice mismatch, where misfit dislocations frequently generate at the interface between the epitaxial layer and the substrate. In the fabrication of GOI structures by this method crystallographic defects generate at the Si seed region because the lattice mismatch between Ge and Si is 4.2%. However, they quickly move out of the thin wires, similar to necking in the Czochralski crystal-growth process, resulting in single-crystalline Ge wires containing no dislocations or stacking faults. This method has another advantage to induce tensile strain in GOI layers. The lattice spacing of Ge is larger than that of Si, so that compressive strain is typically induced in the Ge layer epitaxially grown on the Si substrate. Insertion of low-temperature-grown buffer layer and following high temperature growth allow 0.2% tensile strain in the Ge layer originating from the difference in the thermal-expansion coefficients of the Ge layer with respect to the Si substrate [3]. In the case of the LLPE method, tensile strain of 0.35% is expected because Ge wires are stress free at the melting point (938°C) and cooled down to room temperature, where the difference in the thermal-expansion coefficients is most effectively utilized to induce tensile strain. In this paper, we report the electrical and photonic properties of the LLPE-grown Ge wires. We have investigated the electrical properties of the Ge wires by the back-gate GOI MOSFETs [4,5]. We confirmed a high on/off source-drain current ratio of 106 and high hole mobility of 480 cm2/Vs, which is 2.8 times higher than that of the reference SOI device. We have also examined direct band gap shrinkage in the LLPE-grown Ge wires by means of micro (μ)-photoluminescence (PL) and μ-Raman spectroscopy [6]. Strain analysis and temperature dependent PL spectra indicated that a direct band gap shrinkage amounting to 45 meV was mainly due to a tensile strain of about 0.4% induced by rapid crystallization from the Ge melting point during the LLPE process. [1] Y. Liu et al., Appl. Phys. Lett. 84, 2563 (2004). [2] T. Hashimoto et al., Appl. Phys. Express 2, 066502 (2009). [3] Y. Ishikawa et al., Appl. Phys. Lett. 82, 2044 (2003). [4] Y. Suzuki et al., Appl. Phys. Lett. 101, 202105 (2012). [5] T. Hosoi et al., Appl. Phys. Lett. 105, 173502 (2014). [6] M. Matsue et al., Appl. Phys. Lett. 104, 031106 (2014).

Structure refinement and one-center luminescence of Eu3+ activated ZnBi2B2O7 under UV excitation

A series of Eu3+-activated red phosphors ZnBi2B2O7:xEu3+ were synthesized by high temperature solid-state reaction method. Rietveld refinement on the powder X-ray diffraction (XRD) data was performed to study the local crystal environment of Eu3+ in ZnBi2B2O7 host. The refinement results disclose that the doped Eu3+ do not change the host structure. Although two different cation sites are available, the doped Eu3+ is inclined to occupy Zn site instead of Bi site. This indicates that there is only one luminescent center when Eu3+ is doped into ZnBi2B2O7 phosphor, which is further confirmed by the luminescence properties of ZnBi2B2O7:xEu3+. The influences of the doping concentration and the excitation wavelength on the one-center luminescence of Eu3+ in ZnBi2B2O7 are discussed along with the decay characteristics. Based on the temperature-dependent PL spectra, the fast decrease of emission intensity with the increase of temperature is due to the non-radiative relaxation.

Generation of tunable-emission in Li_4Ca_1-xSr_096 + x(SiO_4)_2:004Eu^2+ phosphors for LEDs application

A series of Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ (−0.4 ≤ x ≤ 0.6) phosphors were prepared by solid-state reaction method, and the photoluminescence properties were investigated in detail. By varying the relative contents of Ca2+ and Sr2+, the single-phase solid solution samples were obtained, which can be found from the corresponding XRD patterns. The excitation spectra of the as-prepared samples reveal that broad excitation bands appear and different excitation spectral shapes are observed by monitoring at 433 and 585 nm, which could be due to the various Eu2+ emission centers. Every emission spectrum includes two emission bands, and their relative intensities depend on the excitation wavelength and the Ca2+/Sr2+ concentration ratio. As a result, the tunable-emission has been realized. The spectra characteristics for different Ca2+/Sr2+ concentration ratio in Li4Ca1-xSr0.96 + x(SiO4)2:0.04Eu2+ further verify that two Eu2+ emission centers exist and the emission bands at 433 and 585 nm are attributed to the Eu2+ ions in Sr2+ and Ca2+ sites, respectively. The temperature-dependent photoluminescence spectra reveal that the Eu2+ emission centers in the Sr2+ and Ca2+ sites have different thermal stabilities, which results in the emission color changes somewhat for different temperature.

The Structural and Optical Properties of Be-Doped GaAs Grown by MBE

In this paper, Be-doped GaAs were grown by molecular beam epitaxy (MBE), by changing Be resource temperature, we obtained different doping concentration GaAs samples. The morphologies and electrics properties of the samples were investigated by AFM and Hall measurement. Especially, in low temperature and temperature dependent PL spectra, the Be acceptor related emission were recognized, with the doping concentration increasing, the Be acceptor related emission enhanced too.

Influence of Al3+doping on the energy levels and thermal property of the 3.5MgO·0.5MgF2·GeO2:Mn4+ red-emitting phosphor**Project supported by the National High Technology Research and Development Program of China (Grant No. 2013AA014201), the Natural Science Foundation of Tianjin City, China (Grant Nos. 11JCYBJC00300 and 14JCZDJC31200), and the National Key Foundation for Exploring Scientific Instrument of China (Grant No. 2014YQ120351).

A series of Al3+ -doped 3.5MgO·0.5MgF2·GeO2:Mn4+ red-emitting phosphors is synthesized by high temperature solid-state reaction. The broad excitation band at 300 nm–380 nm, resulting from the 4A2 → 4T1 transition of Mn4+, exhibits a blue shift with the increase of Al2O3 content. The observation of the decreased Mn4+O2 − distance is explained by the crystal field theory. The temperature-dependent photoluminescence spectra with various amounts of Al2O3 content are comparatively measured and the calculation shows that the activation energy increases up to 0.41 eV at the Al2O3 content of 0.1 mol. The maximum phonon densities of state for these samples are calculated from Raman spectra and they are correlated with the thermal properties.

Photoluminescence Properties of CdS/ZnO Nano-Composites Structure

Aiming to modification the surface optical properties, CdS/ZnO composites were synthesized by hydrothermal method. The ZnO/CdS heterostructures were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray energy dispersive spectroscopy (EDS), photoluminescence (PL) measurements at room temperature and the temperature-dependent photoluminescence treatment. In the room temperature PL spectra, the defect-related deep level emission can effectively suppress under optimal CdS deposition condition. With the increasing of CdS deposition time to 150 min, the donor bound exciton (D0X) peak showed a significantly red shift at low temperature (77K) PL spectrum. Through analysis the temperature-dependent photoluminescence spectra, we find that it was caused by acceptor bound exciton (A0X). Meanwhile, there appeared a donor-acceptor pair related emission at lower-energy side of the spectrum. The experiment demonstrated that the surface donor defects can effectively suppresse after CdS nanoparticles deposition on ZnO nanorods.

Phosphorus-Doped p–n Homojunction ZnO Nanowires: Growth Kinetics in Liquid and Their Optoelectronic Properties

For wide-ranging applications in nanoscale electronic devices, durable and reproducible p-type nanostructures are essential. In this work, simple ZnO nanowire (NW) p–n homojunctions were grown using a two-step hydrothermal synthesis method. P2O5 served as a doping source to obtain p-type ZnO NWs. The morphology of the ZnO NW arrays was examined using field emission scanning electron microscopy. The high-resolution transmission electron microscopy (HRTEM) image indicated that the ZnO NW p–n homojunction is single-crystalline with a ⟨0001⟩ growth direction. The distribution of P element was analyzed using energy-dispersive spectroscopy. The dynamic growth observation was conducted using liquid in situ TEM to investigate the ZnO nucleation and growth mechanism. We divided the ZnO nanocrystal precipitation into three processes. Whether two adjacent particles grow stably or not was found to be related to the distance. Moreover, the temperature-dependent photoluminescence spectra revealed that two extra emissio...

Phase formation of hexagonal wurtzite ZnO through decomposition of Zn(OH)2 at various growth temperatures using CBD method

Zinc oxide (ZnO) nanophosphors were fabricated from zinc acetate dehydrate, thiourea and ammonia via the chemical bath deposition (CBD) method at various growth temperatures. TGA results showed the increase in thermal stability of ZnO with the increase in growth temperature. From DSC results we observed a decrease in melting temperatures due to the crystallization of the ZnO with the increase in growth temperature. The melting enthalpy values were too scattered to make definite conclusions. XRD indicated the decomposition of structure from Zn(OH)2 to hexagonal wurtzite ZnO. The estimated average particle sizes are in the range of 22nm. The estimated average particle size fluctuated with an increase in the growth temperature. The SEM morphology showed the full formation of flakes-like at high growth temperature. At low growth temperature shows flakes-like morphology combined with some small spheres. The EDS results confirmed the presence of Zinc (Zn) and Oxygen (O) as the major products, and the ratio of the Zn to O increased with the increase in growth temperature. A red-shift in reflectance spectra was observed, which resulted in the decrease in the band gap energy of the ZnO with an increase in growth temperature. The temperature dependent PL spectra of the ZnO showed visible emission due to defects. The novelty in this study lies within the increase in the amount of weight loss observed from TGA and DSC analysis and another important aspect is the transformation of Zn(OH)2 to the well-known hexagonal wurtzite structure of ZnO with the increase in growth temperature. This study provides a simple and efficient approach for the synthesizing of the ZnO with flakes-like morphology.

Temperature-dependent photoluminescence and mechanism of CdS thin film grown on Si nanoporous pillar array

Si-based cadmium sulfide (CdS) is a prospective semiconductor system in constructing optoelectronic nanodevices, and this makes the study on the factors which may affect its optical and electrical properties be of special importance. Here we report that CdS thin film was grown on Si nanoporous pillar array (Si-NPA) by a chemical bath deposition method, and the luminescent properties of CdS/Si-NPA as well as its mechanism were studied by measuring and analyzing its temperature-dependent photoluminescence (PL) spectrum. The low-temperature measurement disclosed that the PL spectrum of CdS/Si-NPA could be decomposed into four emission bands, a blue band, a green band, a red band and an infrared band. The blue band was due to the luminescence from Si-NPA substrate, and the others originate from the CdS thin film. With temperature increasing, the peak energy, PL intensity and peak profile shape for the PL bands from CdS evolves differently. Through theoretical and fitting analyses, the origins of the green, red and infrared band are attributed to the near band-edge emission, the radiative recombination from surface defects to Cd vacancies and those to S interstitials, respectively. The cause of PL degradation is due to the thermal quenching process, a phonon-induced electron escape but with different activation energies. These results might provide useful information for optimizing the preparing parameters to promote the performance of Si-based CdS optoelectronic devices.

Optical and electronic properties of type-II CdSe/CdS core–shell quantum dots

CdSe/CdS core–shell quantum dots (QDs) were synthesized using a facile method in aqueous phase. X-ray diffraction pattern, high-resolution transmission electron microscopy images, and energy dispersive spectroscopy profiles showed that stoichiometric CdSe/CdS QDs were formed. Temperature-dependent photoluminescence spectra showed that the activation energy of CdSe/CdS core–shell QDs was 15 meV. The potential profiles and interband transition energies of the strained type-II CdSe/CdS core–shell QDs were calculated. The calculated interband transition energies slightly decreased from 2.061 to 2.007 eV when the shell thickness increased from 10 to 17 Å. The theoretical interband transition energy of 2.007 eV was in reasonable agreement with the photoluminescence excitonic transition energy of 1.98 eV.

A new fluorosilicoborate host for the development of Eu3+-activated red-emitting phosphors

Red-emitting phosphor of Eu3+-activated BaY6(Si3B6O24)F2 was synthesized by high-temperature solid state reaction. The phase purity was characterized by X-ray powder diffraction. Luminescent properties were characterized by photoluminescence excitation (PLE) and emission (PL) spectra, decay curves, and absolute quantum efficiency. The optimal doping concentration of Eu3+ in BaY6(Si3B6O24)F2: is about 30mol%. This phosphor can be efficiently excited by near-ultraviolet light to realize reddish-orange color from the dominated 5D0→7F1 transition. In addition, the thermal stability was investigated according to the temperature-dependent PL spectra. Eu3+-doped BaY6(Si3B6O24)F2 has higher brightness and thermal stability, and shows a great potential for its application in LEDs as a suitable red-emitting phosphor.

Impacts of AlOx formation on emission properties of AlN/GaN heterostructures

The effects of AlOx formation on the emission properties of AlN/GaN heterostructures are comparatively investigated using steady-state and time-resolved photoluminescence (PL) measurements. By analyzing the excitation power and temperature-dependent PL spectra, PL at around 3.45–3.46 eV is attributed to the two-dimensional electron-gas (2DEG)-related emission. Simultaneous changes in the emission energies and lifetimes are observed by forming a crystallized AlOx layer. The results demonstrate a possibility of controlling the surface states by changing the surface oxide structure.

Brightly Luminescent and Color-Tunable Colloidal CH3NH3PbX3 (X = Br, I, Cl) Quantum Dots: Potential Alternatives for Display Technology

Organometal halide perovskites are inexpensive materials with desirable characteristics of color-tunable and narrow-band emissions for lighting and display technology, but they suffer from low photoluminescence quantum yields at low excitation fluencies. Here we developed a ligand-assisted reprecipitation strategy to fabricate brightly luminescent and color-tunable colloidal CH3NH3PbX3 (X = Br, I, Cl) quantum dots with absolute quantum yield up to 70% at room temperature and low excitation fluencies. To illustrate the photoluminescence enhancements in these quantum dots, we conducted comprehensive composition and surface characterizations and determined the time- and temperature-dependent photoluminescence spectra. Comparisons between small-sized CH3NH3PbBr3 quantum dots (average diameter 3.3 nm) and corresponding micrometer-sized bulk particles (2-8 μm) suggest that the intense increased photoluminescence quantum yield originates from the increase of exciton binding energy due to size reduction as well as proper chemical passivations of the Br-rich surface. We further demonstrated wide-color gamut white-light-emitting diodes using green emissive CH3NH3PbBr3 quantum dots and red emissive K2SiF6:Mn(4+) as color converters, providing enhanced color quality for display technology. Moreover, colloidal CH3NH3PbX3 quantum dots are expected to exhibit interesting nanoscale excitonic properties and also have other potential applications in lasers, electroluminescence devices, and optical sensors.

Red emitting phosphors of Eu3+ doped Na2Ln2Ti3O10 (Ln = Gd, Y) for white light emitting diodes

A series of Eu3+ doped Na2Ln2Ti3O10 (Ln=Gd, Y) red-emitting phosphors for application in ultraviolet based light emitting diodes (LEDs) were successfully synthesized by a modified sol–gel method. Their structure and luminescent properties were characterized by powder X-ray diffraction (XRD), photoluminescence excitation (PLE) and emission (PL) spectra and absorption spectra, according to these results the optimal compositions and synthesis parameters were determined. In addition, the thermal stabilities of the phosphors were investigated according to the temperature-dependent PL spectra. The red and white-LEDs (W-LEDs) comprising the Na2Ln2Ti3O10:Eu3+ (Ln=Gd, Y) red emitting phosphors were fabricated with a near-ultraviolet (n-UV) chip. In comparison with Na2Y1.4Eu0.6Ti3O10, the Na2Gd0.6Eu1.4Ti3O10 phosphor offers higher brightness, quantum efficiency, and excellent thermal stability. W-LEDs comprising Na2Gd0.6Eu1.4Ti3O10 showed bright white emission with a color rendering index (Ra) of 82, a color temperature of 2151K, and Commission Internationale de I’Eclairage (CIE) color coordinates of (0.34, 0.37). The phosphor Na2Gd0.6Eu1.4Ti3O10 is more suitable candidate for application in LEDs.

Optical properties of a-plane non-polar Zn1−x Mg x O/ZnO multiple quantum wells with different barrier compositions

The optical properties of a-plane non-polar Zn1−x Mg x O/ZnO multiple quantum wells (MQWs) with different barrier compositions have been investigated. It is found that the dominant photoluminescence (PL) emission at 16 K as well as at room temperature exhibits monotonous blue shift as Mg content in the barriers increases due to quantum confinement effect. Moreover, increasing the Mg content in the barriers improves carrier confinement efficiency as well as degrades the crystal quality of MQWs. In a-plane Zn0.85Mg0.15O/ZnO MQWs, exciton emission in the ZnO wells at 16 K is dominated by localized excitons (LEs) at 3.413 eV and free excitons (FEs) at 3.459 eV. Temperature-dependent PL spectra indicate that the intensity of the LE peak gradually decreases and the PL spectra become dominated by FE emissions at the temperature above 120 K. The behavior of LEs and FEs as the temperature increases is interpreted by the formation of potential minima in ZnO wells due to exciton localization. The deduced activation energy of 11 meV is in good agreement with quenching temperature of LEs.

Zinc vacancy related emission in homoepitaxial N-doped ZnO microrods

Vertically aligned N-doped ZnO microrods have been homoepitaxially grown on a high-quality ZnO template via chemical vapor transport method without employing any catalysts. In spite of the incorporation of nitrogen, the as-grown microrods exhibited excellent crystalline and optical quality, and the behavior of intrinsic point defects and impurity-related defects have been investigated in detail by the temperature-dependent photoluminescence spectra. The emissions at 3.377, 3.368, and 3.363eV are ascribed to free excitons, surface bound excitons and excitons bound to neutral shallow donors: zinc interstitials, respectively. The emissions at 3.359, 3.311, and 3.240eV correspond to the radiative recombination of excitons bound to neutral acceptors, free electrons to acceptors and donor acceptor pairs, all of which are originated from the same shallow acceptor of zinc vacancy clusters with its energy level of about 126meV above the valence band maximum. The isolated zinc vacancy is a deep acceptor, which results in the emission at 3.318eV and the green band emissions. This study suggests that the intrinsic point defects play an extremely important role in optical properties of ZnO besides the external dopants and the understanding of intrinsic defects is crucial to get stable p-type conductivity and finally realize a ZnO homojunction for optoelectronic applications.

Temperature dependent photoluminescence study on ZnO/Graphene nanocomposite films

We report observation of both free and defect-mediated excitonic emissions from temperature-dependent PL study on ZnO/graphene oxide (G-O) nanocomposite grown by ultrasonic assisted spray pyrolysis (UASP). From the temperature-dependent photoluminescence (PL) spectra of the ZnO/G-O nanocomposite, new graphene-related peak was observed at 372 nm along with the exciton transition bound to neutral acceptors or deep donors. The PL intensity of new graphene-related peaks (3.33 eV) become more prominent with increasing G-O concentration, and it was saturated or decreased with the addition of >7.0 wt% of G-O. This feature indicates that new graphene-related states were created below conduction band of ZnO, which supports the excitonic PL enhancement by graphene-embedding is contributed not by charge transfer, but by vacancy filling effect of G-O.

The origin of deep-level impurity transitions in hexagonal boron nitride

Deep ultraviolet photoluminescence (PL) emission spectroscopy has been employed to investigate the origin of the widely observed deep level impurity related donor-acceptor pair (DAP) transition with an emission peak near 4.1 eV in hexagonal boron nitride (h-BN). A set of h-BN epilayers were grown by metal-organic chemical vapor deposition (MOCVD) under different ammonia (NH3) flow rates to explore the role of nitrogen vacancies (VN) in the deep-level transitions. The emission intensity of the DAP transition near 4.1 eV was found to decrease exponentially with an increase of the NH3 flow rate employed during the MOCVD growth, implying that impurities involved are VN. The temperature-dependent PL spectra were measured from 10 K up to 800 K, which provided activation energies of ∼0.1 eV for the shallow impurity. Based on the measured energy level of the shallow impurity (∼0.1 eV) and previously estimated bandgap value of about 6.5 eV for h-BN, we deduce a value of ∼2.3 eV for the deep impurity involved in this DAP transition. The measured energy levels together with calculation results and formation energies of the impurities and defects in h-BN suggest that VN and carbon impurities occupying the nitrogen sites, respectively, are the most probable shallow donor and deep acceptor impurities involved in this DAP transition.

Photoluminescence of excitons and defects in ZnSe-based longitudinal twinning nanowires

Photoluminescence spectrum is a powerful tool to investigate the excitonic and defect-related information in condensed phase materials. Through temperature-dependent photoluminescence spectra, heavy- and light-hole free excitons, and the exciton bound to a neutral donor and the accompanied LO-phonon replicas of the zinc blende ZnSe are observed and verified in ZnSe-based longitudinal twinning nanowires. In the deep-level emission region, two peaks at 1.979 and 2.315 eV are attributed to the donor–acceptor–pair recombination and the acceptor impurity of Au, respectively.

Size-dependent activation energy and carrier dynamics in Cd(x)Zn(1-x)Te/ZnTe quantum dots on Si substrates.

We investigate the size-dependent activation energy and carrier dynamics in Cd(x)Zn(1-x)Te/ZnTe quantum dots (QDs) grown on Si substrates. The excitonic peak corresponding to transitions from the ground electronic subband to the ground heavy-hole band in Cd0.6Zn0.4Te/ZnTe QDs shifts to a lower energy with increasing Cd0.6Zn0.4Te thickness owing to an increase in the size of the QDs. The activation energy of the electrons confined in the Cd0.6Zn0.4Te/ZnTe QDs, as obtained from the temperature-dependent photoluminescence (PL) spectra, increases with increasing Cd0.6Zn0.4Te thickness owing to an enhancement of the quantum confinement effect resulting from an increase in the energy difference between the electronic state and the conduction band edge. The carrier dynamics of Cd0.6Zn0.4Te/ZnTe QDs is studied using time-resolved PL measurements, which shows a longer exciton lifetime for Cd0.6Zn0.4Te/ZnTe QDs with increasing Cd0.6Zn0.4Te thickness. This behavior is attributed to the reduction of the exciton oscillator strength resulting from a strong built-in electric field in the larger QDs.