Influences of silicon doping in quantum dot layers on optical characteristics of InAs/GaAs quantum dot infrared photodetector

We have investigated the effects of silicon doping concentration within thirty-period self-assembled quantum dot (QD) layers on quantum dot infrared photodetectors (QDIPs). The lens-shaped quantum dots with the dot density of 1 × 10¹¹ cm^− 2 were observed by atomic force microscope (AFM). From the high ratio of photoluminescence (PL) peak intensities from dot layer to that from wetting layer, we have concluded that high dot density caused the short diffusion length for carriers to be easily captured by QDs. Moreover, the Si-doped samples exhibited the multi-state transitions within the quantum dots, which were different to the single level transition of undoped sample. Besides, the dominant PL peaks of Si-doped samples were red-shifted by about 25 meV compared to that of the undoped sample. It should result from the dopant-induced lowest transition state and therefore, the energy difference should be equal to the binding energy of Si in InAs QDs.     

High resistivity In-doped ZnTe: electrical and optical properties

Semi-insulating <111> ZnTe prepared by In doping during Bridgman growth was found to have a resistivity of 5.74 × 10⁷ ohm-cm, the highest reported so far in ZnTe, with hole concentration of 2.4 × 10⁹/cm³ and hole mobility of 46 cm² /V.s at 300 K. The optical band gap was 2.06 eV at 293 K compared with 2.26 eV for undoped semiconducting ZnTe. Thermally stimulated current (TSC) studies revealed 2 trap levels at depths of 202–222 meV and 412–419 meV, respectively. Photoluminescence (PL) studies at 10 K showed strong peaks at 1.37 eV and 1.03 eV with a weak shoulder at 1.43 eV. Short anneal for 3 min at 250°C led to conversion to a p-type material with resistivity, 14.5 ohm-cm, indicating metastable behaviour. Raman studies carried out on undoped and In-doped samples showed small but significant differences. Possible models for semi-insulating behaviour and meta-stability are proposed.     

Luminescence study of donors and acceptors in CdGeAs2

A photoluminescence (PL) study has been performed on a set of p-type bulk single crystalline samples of CdGeAs₂. At liquid-helium temperatures, a PL band peaking near 0.55 eV was observed in samples with enhanced absorption at 5.5 μm. This PL band was observed to vary as much as 40 meV in peak position in our sample set. The PL peak position depends on the net acceptor concentration (i.e., hole concentration) and the PL peak shifts to higher energy as the excitation intensity increases. We show that this behavior is well explained by the model of donor–acceptor pair (DAP) recombination in the presence of potential fluctuations.     

Study of steady-state photoluminescence of AgInSe2 crystals

The steady-state photoluminescence (PL) spectra of AgInSe₂ grown by the Hot-Press method have been examined. The broad peaks located at 1.19 eV and 1.16 eV are very likely to be the donor-acceptor-pair emissions. The energies of the donor levels participating in these emissions correspondingly are 25 eV and 50-52 meV, as found from the analyses of the activation energies of the PL intensity. The peaks observed in the obtained PL spectra in the band-edge region are attributed to free and bound excitons. The binding energy of the free exciton is estimated to be 29 meV.     

Photoluminescence efficiency and size distribution of self assembled ge dots on porous TiO2

For Ge nanodots approximately 20 nm in diameter grown by annealing a thin amorphous Ge layer deposited by molecular beam epitaxy on a mesoporous TiO2 layer on Si(001), photoluminescence (PL) was observed as a wide near-infrared band near 800 meV. Using a tight binding theoretical model, the energy-dependent PL spectrum was transformed into a dependence on dot size. The average dot size determined the peak energy of the PL band and its shape depended on the size distribution, including bandgap enlargement due to quantum confinement. Combining the dot sample PL with an established dependence of emission efficiency on dot diameter, it was possible to derive a dot size distribution and compare it with results obtained independently from atomic force microscopy.     

Vertically aligned Mn-doped zinc oxide nanorods by hybrid wet chemical route

Mn-doped zinc oxide (Mn:ZnO) nanorods were synthesized by incorporating manganese in aligned ZnO nanorods. For this, Mn was evaporated onto ZnO nanorods and the composite structure was subjected to rapid thermal annealing. The nanorods were preferentially oriented in (0 0 2) direction as indicated by the XRD measurement. Optical band gap was seen to decrease with increasing amount of Mn incorporation. XPS studies indicated that incorporated Mn was in Mn²⁺ and Mn⁴⁺ states. Mn²⁺ atomic concentration was found to be larger than Mn⁴⁺ concentration in all the samples. The Raman spectra of the Mn:ZnO nanorods indicated the presence of the characteristic peak at ∼438 cm⁻¹ for high frequency branch of E₂ mode of ZnO. The PL peak at ∼376 nm (∼3.29 eV) was ascribed to the band edge luminescence while the peak at ∼394 nm (∼3.15 eV) was assigned to the donor bound exciton (D^oX) and free exciton transition related to Mn²⁺ states.     

Europium incorporated barium titanate thin films for optical applications

BaTiO₃:Eu (BT:Eu) thin films were deposited onto quartz substrates by RF magnetron sputtering. The effect on structural, morphological, optical and photoluminescence (PL) properties in the films with different Eu concentrations (0–5 wt%) were investigated. The X-ray diffraction (XRD) pattern of the undoped BT thin film revealed a tetragonal (T) phase with orientations along (101) plane. From XRD pattern, the crystallinity of the films increased with increase in Eu concentration. The SEM images revealed that the films exhibited tetragonal shape, crack free and good adherence to the substrate. Atomic force microscopy studies showed an increase of grain growth with doping concentration. The rms roughness value increased with increase in Eu concentration and the film surface revealed positive skewness and high value of kurtosis which make them suitable for tribological applications. X-ray photoelectron spectroscopy revealed the presence of barium, titanium, europium and oxygen in BT:Eu film. An average transmittance of >80 % (in visible region) was observed for all the films. Optical band gap of Eu doped BT films decreased from 3.86 to 3.53 eV. Such films with optical properties such as high transparency, decrease in band gap and high refractive index are suitable for optoelectronic applications. PL properties showed a sharp line at 625 nm and a broad line at 552 nm due to europium (Eu³⁺) transitions. PL phenomena were observed, owing to the electronic structure of Eu³⁺ ions as well as BT nanocrystallites in the films. The sharp and intense red luminescence is useful for photoelectric devices and optical communications.     

Characterization of GaN and In x Ga1−x N films grown by MOCVD and MBE on free-standing GaN templates and quantum well structures

Structural and optical studies have been performed on GaN, InGaN layers, In_0.08Ga_0.92N/GaN heterostructures, In_0.08Ga_0.92N/In_0.02Ga_0.98N single and multiquantum wells grown by metal organic chemical vapor deposition (MOCVD) and GaN by molecular beam epitaxy (MBE) on GaN templates by using transmission electron microscopy (TEM), X-ray diffraction (XRD), and photoluminescence (PL). The layers are found to be high quality with low defect density, on the order of 10⁶ cm^?2, which are mainly related to the threading dislocations originating/propagating from the hydride vapor phase epitaxy (HVPE) GaN template. The interface between the layers and substrate could not be detected by TEM and was therefore deemed to be of high quality. Convergent beam electron diffraction studies revealed that the polarity of the films is Ga-polarity, which is the same as that of the substrate. A dual structure with different compositions and having thicknesses of 10 and 25 nm was observed in InGaN layers grown on GaN in one of the heterostructure samples. The full width at half maximum (FWHM) of the XRD rocking curves of (0 0 0 2) for heterostructures and quantum wells were found to be in the range of 15–28 arcmin for a slit width of 2 mm. PL studies on GaN layers grown by MBE and MOCVD on GaN templates are reasonably similar. The PL spectra from all the MBE and MOCVD epilayers and the substrate contain a plethora of sharp peaks related to excitonic transitions. With the presence of donor-bound exciton peaks and their associated two-electron satellites, the binding energies of two distinct shallow donors (28.8 and 32.6 meV), which are attributed to Si and O, respectively, were determined. PL measurements revealed that the FWHM of the main donor bound exciton peak increased from 0.6 to 2.9 meV but no change in peak position (3.472 eV) was observed in GaN when doping with Si (5×10¹⁷ cm^?3). However, the intensities of the yellow band and the shallow donor–acceptor pair band increased 10 times as compared to that in the undoped GaN samples. In the case of InGaN/GaN heterostructures, a similar trend was observed when compared to the doped samples. In the multiquantum well In_0.08Ga_0.92N/In_0.02Ga_0.98N heterostructures, the activation energy of the exciton emission, found to be 18 meV, was the lowest in the samples studied. The peak at 3.02 eV related to the InGaN was strongly pronounced in the In_0.08Ga_0.92N/In_0.02Ga_0.98N multiquantum well structure. In the In_0.08Ga_0.92N/In_0.02Ga_0.98N quantum well structures, the change in peak position with variation of temperature from 15 to 300 K in PL spectra is “S”-shaped. The cause for the “S” shape, i.e., a red–blue–red shift, is discussed.     

Photoluminescence imaging of focused ion beam induced individual quantum dots

We report on scanning microphotoluminescence measurements that spectrally and spatially resolve emission from individual InAs quantum dots that were induced by focused ion beam patterning. Multilayers of quantum dots were spaced 2 μm apart, with a minimum single dot emission line width of 160 μeV, indicating good optical quality for dots patterned using this technique. Mapping 16 array sites, at least 65% were occupied by optically active dots and the spectral inhomogeneity was within 30 meV.     

Optical processes of red emission from Eu doped GaN

A single crystalline Eu-doped GaN was grown by gas-source molecular beam epitaxy and photoluminescence (PL) properties were studied. The PL spectra show red-emission at 622 nm originating from intra 4f–4f transition of Eu³⁺ ion without band-edge emission of GaN. The peak shift of the red-emission with the temperature variation from 77 K to room temperature is less than 1.6 meV, and thermal quenching of the luminescence was found to be small compared with the band-to-band transition. Fourier transform infrared spectra showed an absorption peak at about 0.37 eV, which may be due to a deep defect level. The intensity of the red luminescence and the defect-related absorption peak increased with increasing Eu concentration, and a close correlation in the intensity was observed between them. These results suggest that the deep defect level plays an important role in the radiative transition of Eu³⁺ ion in GaN and the optical process for the luminescence at 622 nm was discussed with the relation to the defect.     

Oxygen vacancy in Al2O3: Photoluminescence study and first-principle simulation

Broad photoluminescence (PL) band at 2.97 eV excited in the band near 6.0 eV in amorphous chemical vapor deposition films is related to the neutral oxygen vacancy by analogy with crystalline Al₂O₃. The identification of this PL band was supported by the results of first-principle quantum chemical simulation, which showed 6.3 and 6.4 eV bands in the extinction spectra for α- and γ-Al₂O₃, respectively. Other PL bands are attributed to ionized single vacancies (F⁺-centers), divacancies (F₂) and, probably, interstitial Al.     

Chemical shift of Mn and Cr K-edges in X-ray absorption spectroscopy with synchrotron radiation

Mn and Cr K X-ray absorption edges were measured in various compounds containing Mn in Mn^2?+?, Mn^3?+? and Mn^4?+? oxidation states and Cr in Cr^3?+? and Cr^6?+? oxidation states. Few compounds possess tetrahedral coordination in the 1st shell surrounding the cation while others possess octahedral coordination. Measurements have been carried out at the energy dispersive EXAFS beamline at INDUS-2 Synchrotron Radiation Source at Raja Ramanna Centre for Advanced Technology, Indore. Energy shifts of ~ 8–16 eV were observed for Mn K edge in the Mn-compounds while a shift of 13–20 eV was observed for Cr K edge in Cr-compounds compared to values in elemental Mn and Cr, respectively. The different chemical shifts observed for compounds having the same oxidation state of the cation but different anions or ligands show the effect of different chemical environments surrounding the cations in determining their X-ray absorption edges in the above compounds. The above chemical effect has been quantitatively described by determining the effective charges on Mn and Cr cations in the above compounds.     

Optical Properties of Mn-Implanted GaN Nanorods

We have investigated the optical properties of vertical GaN nanorods with diameters of 150 nm grown on (111) Si substrates by radio-frequency plasma-assisted molecular-beam epitaxy followed by Mn ion implantation and annealing. The GaN nanorods are fully relaxed and have a very good crystal quality characterized by extremely strong and narrow photoluminescence excitonic lines near 3.47 eV. For GaMnN nanorods, Arrhenius plots of the intensities of the Mn acceptor give a thermal activation energy of Δ=350 meV, indicating that the thermal quenching of the Mn-related PL peak is due to the dissociation of an acceptor-bound hole from the temperature-dependent PL spectra. This suggests that the Mn-bound holes in GaN nanorods exhibit the impurity states predicted by the hydrogen model.     

Photoacoustic spectra on Mn-doped zinc silicate powders by evacuated sealed silica tube method

The non-radiative transition processes on non-doped Zn₂SiO₄ and Zn₂SiO₄:Mn powders with various Mn concentrations were studied by photoacoustic (PA) spectroscopy. Zn₂SiO₄:Mn _x powders were prepared by dry reaction within an evacuated silica glass tube. As the result of photoluminescence (PL) measurement, the increase of PL intensity for green emission on samples doped with Mn between 1% and 6% and the concentration quenching for luminescence on samples doped with Mn between 7% and 12% were confirmed. For the green luminescence on zinc silicate doped with Mn phosphor, the PL decay behavior is assumed to be due to tunneling directly from excited states of electron traps to the excited states of Mn-ion. The Mn content dependence of PL intensity for green emission is well interpreted by tunneling theory and the results of PA spectra, that is, the green emission is assisted by tunneling from non-radiative levels of Mn-ion to luminescencet level as ⁴T₁(⁴G).     

ZnO单晶体的荧光温度特性

通过变温荧光光谱研究了ZnO激子发光的温度依赖特性。在6K的低温下,其光致发光主要来自束缚激子能量位于3.360eV,半宽为3meV的施主束缚激子发光。而随着测量温度的上升,自由激子及其声子辅助跃迁发光逐渐成为ZnO主要发光机制。文中详细讨论了ZnO荧光随温度的演变过程。特别是自由激子的声子伴线与自由激子发光峰之间的能量间距,随温度的上升逐渐偏离了其特征声子模能量,呈现不断缩小趋势。     

Optical properties of deep centers in semi-insulating ZnSe

Properties of deep levels in vapor-deposited ZnSe on GaAs have been investigated using transient photocurrent and steady state photocapacitance measurements. A metal/insulator/semiconductor structure was used in the study in which ZnSe formed the insulator. A single dominant deep level in the ZnSe having an energy of E_c ? 1.2 eV was observed over the spectral region 0.5–3.0 eV. The photoionization cross section of the trap at 1.5 eV and 295 K is 7 x 10¹⁶ cm². Trap concentrations in the range 10¹⁶?10¹⁷ cm^?3 were measured in the as-grown material.     

Free-standing In2O3(ZnO)m superlattice microplates grown by optical vapor supersaturated precipitation

Here, we fabricated In₂O₃(ZnO)_m (IZO) superlattice microplates with hexagon morphologies by the substrate-free optical vapor supersaturated precipitation. The IZO microplates possessed a superlattice structure with a large m number, i.e., m?=?23, consisting of layered alternating stacks of octahedral InO₂^? as inversion boundaries and layered InZn_mO_m+1⁺ as a zig-zag modulated pattern. The Raman peak at 613 cm^?1 confirmed the superlattice of the IZO microplates. The broad asymmetric excitonic photoluminescence (PL) emission with the photon energy of 3.236 eV indicated the heavy doping of indium in the IZO, resulting a redshift of?~?32 meV from the near-band-edge emission. The unusual negative thermal quenching of PL intensity was also observed. Moreover, the anisotropic electrical properties of the IZO superlattice microplates were manifested, for the first time, where the in-plane conductivity was two orders of magnitude higher than out-plane one. The present work provided new insight into the free-standing IZO superlattice microdevices for future optoelectronic applications.

     

Structural analysis and luminescent study of thin film zinc germanate doped with manganese

Thin films of zinc germanate doped with manganese (Zn₂GeO₄:Mn) were fabricated by radio frequency magnetron sputtering, and their structural characteristics and luminescent properties were studied. The Zn₂GeO₄:Mn films exhibited a pronounced absorption edge at around 271 nm and a high optical transparency in the visible wavelength region with a peak transmittance of 0.927 at 691 nm. While the as-deposited Zn₂GeO₄:Mn films had an amorphous structure, the annealed films possessed a rhombohedral polycrystalline structure with a random crystallographic orientation of grains. The broad-band photoluminescence (PL) emission was observed from the annealed Zn₂GeO₄:Mn films. The PL emission spectrum showed a peak maximum at around 537 nm in the green range, which was accounted for by the intrashell transition of 3d⁵ orbital electrons from the ⁴T₁ lowest excitation state to the ⁶A₁ ground state in the divalent manganese ions. Two discrete peaks were observed in the PL excitation spectrum at 256 and 296 nm, which are considered to be associated with the band-to-band absorption of the host and the sub-band absorption from defect states, respectively. The green cathodoluminescence (CL) emission was obtained from the annealed Zn₂GeO₄:Mn films with a peak centered at around 534 nm, analogous to the PL emission spectrum.     

Photoluminescence characterization of β-FeSi2 prepared by ion beam sputter deposition (IBSD) method

Photoluminescence (PL) measurement technique was found to be effective in revealing the unique characteristics of β-FeSi₂ film formation on Si substrates by means of ion beam sputter deposition (IBSD) method. A strong photoluminescence peak at around 0.8 eV was observed for β-FeSi₂ samples and also for Si substrates that were sputter etched by Ne⁺, and then thermally annealed in air at elevated temperature. Comparison with literature data indicated that the PL peak at 0.8 eV observed in this study was mainly from D1 emission bands in Si substrate, whose intensity was enhanced by the sputter etching and the subsequent annealing of the substrate. Furthermore, comparison between CZ-Si and FZ-Si results indicated that the energy of 0.8 eV peak observed in this study was affected by the presence of oxygen in the Si bulk as well.     

Optical and magnetic properties of ZnCoO layers

Optical and magneto-optical properties of ZnCoO films grown at low temperature by Atomic Layer Deposition are discussed. Strong wide band absorption, with onset at about 2.4 eV, is observed in ZnCoO in addition to Co-related intra-shell transitions. This absorption band is related to Co 2+ to 3+ photo-ionization transition. A strong photoluminescence (PL) quenching is observed, which we relate to Co recharging in ZnO lattice. Mechanisms of PL quenching are discussed.