Effect of thermal annealing on ZnO:Al thin films grown by spray pyrolysis

We report the effect of thermal annealing in air on the structural and optical properties of undoped and aluminium-doped (1%–4%) zinc oxide (AZO) thin films, grown by the spray pyrolysis technique on quartz substrates. Films were characterized by X-ray diffraction, low-temperature photoluminescence, electrical resistivity, and Raman spectroscopy after annealing at temperatures between 500 and 900 C. Annealing in air improves the long-range order crystalline quality of the bulk crystals, but promotes a number of point defects in the surface affecting both the resistivity and the photoluminescence.     

Preparation and photoluminescence of Sc-doped ZnO nanowires

We demonstrate bulk synthesis of single-crystal Sc-doped ZnO nanowires by using (Sc+Zn) powders at . These mass nanowires are characterized through X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy (TEM), selected area electron diffraction, and high-resolution TEM, which have uniform diameters of about 40 nm and microns of several decades in length. The growth of ZnScO nanowires is suggested for self-catalyzed vapor–liquid–solid. In particular, PL spectra of these nanowires show emission peaks that intensely shift to long wavelength with increasing Sc and the doping quantity is found responsible for the different characteristics, in which PL mechanism is explained in detail.     

MBE生长PbSe/PbSrSe量子阱结构的光致中红外发光的研究

研究了分子束外延技术生长的PbSe/PbSrSe多量子阱结构的中红外光致荧光现象.高分辨率X射线衍射（HRXRD）谱观察到了多量子阱所特有的多级卫星峰,表明量子阱界面陡峭.变温光致荧光谱测量显示量子阱结构对电子空穴有强的限制效应,在相同温度下,量子阱样品的荧光峰峰位相对PbSe体材料有一定的蓝移.发现量子阱样品的荧光强度同温度有关,温度从150 K上升到230 K时,荧光强度逐渐增大,温度继续升高,荧光强度缓慢下降,但在高于室温时,仍能观察到较强的荧光发射,这说明该量子阱结构材料具有应用于室温工作的中红外 关键词： PbSe/PbSrSe多层量子阱（MQWs） 光致中红外荧光 高分辨X射线衍射（HRXRD）     

Optical investigation of A-plane ZnO/ZnMgO multiple quantum wells grown by pulsed laser deposition

In this study, we present the optical characteristics of A-plane ZnO/ZnMgO multiple quantum wells (MQWs) with different well widths grown on R-plane sapphire substrates by pulsed laser deposition (PLD). The energy gaps of ZnO and ZnMgO have been observed by photoluminescence (PL) and absorption spectra. The electrons confined in the ZnO wells transit from the electron ground sub-band to the heavy-hole ground sub-band (noted as 11H) located at 3.40 and 3.57 eV for the ZnO/ZnMgO MQWs samples with well widths of 5.6 and 1.2 nm, respectively. The strong anisotropic polarization characteristic has been studied by polarization-dependent PL measurements. For comparison, we also calculated the transition energies of different well thicknesses varying from 1 to 6 nm. The theoretical results match quite well with the experimental values and revealing the suitable conduction band offset Q_c=0.6. The temperature dependence of PL spectra is being investigated, in the temperature range between 10 and 300 K.     

Temperature dependent photoluminescence processes in ZnO thin films grown on sapphire by pulsed laser deposition

Temperature dependence of the photoluminescence (PL) transitions in the range of 10–300 K was studied for ZnO thin films grown on sapphire by pulsed laser deposition. The low temperature PL spectra were dominated by recombination of donor bound excitons (B_X) and their phonon replicas. With increasing temperature, free exciton (F_X) PL and the associated LO phonon replicas increased in intensity at the expense of their bound counterparts. The B_X peak with line width of ∼6 meV at 10 K exhibited thermal activation energy of ∼17 meV, consistent with the exciton-defect binding energy. The separation between the F_X and B_X peak positions was found to reduce with increasing temperature, which was attributed to the transformation of B_X into the shallower donor bound exciton complexes at consecutive lower energy states with increasing temperature, which are possible in ZnO. The energy separation between F_X peak and its corresponding 1-LO phonon replica showed stronger dependence on temperature than that of 2-LO phonon replica. However, their bound counterparts did not exhibit this behavior. The observed temperature dependence of the energy separation between the free exciton and it is LO phonon replicas are explained by considering the kinetic energy of free exciton. The observed PL transitions and their temperature dependence are consistent with observations made with bulk ZnO crystals implying high crystalline and optical quality of the grown films.     

Highly luminescent nanocrystal quantum dots fabricated by lattice-type mismatched epitaxy

Lattice-type mismatched heteroepitaxy is demonstrated as a novel concept for the fabrication of almost ideal, highly luminescent nanocrystal quantum dots that are coherently embedded in a single-crystalline matrix. In this approach, the formation of quantum dots is induced by transformation of a metastable epitaxial 2D quantum well into an array of isolated nanocrystals with-highly symmetric shape. This process is driven by the lattice-type mismatch between the constituent materials and the resulting miscibility gap. The investigated PbTe/CdTe heterosystem has a model character because it combines two compounds with different cubic lattice types but almost identical lattice constants. The obtained epitaxial nanocrystals exhibit outstanding properties such as a well-defined symmetric shape, the absence of strain, intermixing and a wetting layer, which is in contrast to the conventional Stranski–Krastanow quantum dots. The small-rhomboedric-cubo-octahedron PbTe/CdTe nanocrystals on GaAs substrates display intense room temperature mid-infrared luminescence as is crucial for device applications. Ab initio density functional theory is used to clarify the interface structure, indicating that the covalent and ionic bonding character of CdTe and PbTe is maintained across the interface.     

Visible emission from ZnCdO/ZnO multiple quantum wells

Polar c‐axis oriented Zn_0.75Cd_0.25O/ZnO multiple quantum wells (MQWs), grown by pulsed‐laser deposition (PLD), emitting in the visible spectral range are reported. By applying a low growth temperature of ≈300 °C a large Cd content of 0.25 and abrupt interfaces could be achieved using PLD. The emission energy was tuned from the green to the violet spectral range (2.5 eV to 3.1 eV) by tuning the quantum well thickness. It is determined by the quantum confinement effect and the quantum‐confined Stark effect. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

     

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.     

Room-temperature direct-bandgap photoluminescence from strain-compensated Ge/SiGe multiple quantum wells on silicon

Strain-compensated Ge/Si_0.15Ge_0.85 multiple quantum wells were grown on an Si_0.1Ge_0.9 virtual substrate using ultrahigh vacuum chemical vapor deposition technology on an n +-Si（001） substrate.Photoluminescence measurements were performed at room temperature,and the quantum confinement effect of the direct-bandgap transitions of a Ge quantum well was observed,which is in good agreement with the calculated results.The luminescence mechanism was discussed by recombination rate analysis and the temperature dependence of the luminescence spectrum.     

Characterization of nitrogen-doped ZnO thin films grown by plasma-assisted pulsed laser deposition on sapphire substrates

The crystalline, optical and electrical properties of N-doped ZnO thin films were measured using X-ray diffraction, photoluminescence and Hall effect apparatus, respectively. The samples were grown using pulsed laser deposition on sapphire substrates coated priorly with ZnO buffer layers. For the purpose of acceptor doping, an electron cyclotron resonance (ECR) plasma source operated as a low-energy ion source was used for nitrogen incorporation in the samples. The X-ray diffraction analyses indicated some deterioration of the ZnO thin film with nitrogen incorporation. Temperature-dependent Van der Pauw measurements showed consistent p-type behavior over the measured temperature range of 200–450 K, with typical room temperature hole concentrations and mobilities of 5×10¹⁵ cm⁻³ and 7 cm²/V s, respectively. Low temperature photoluminescence spectra consisted of a broad emission band centered around 3.2 eV. This emission is characterized by the absence of the green deep-defect band and the presence of a band around 3.32 eV.     

Comparison between atomic layer epitaxy grown and molecular beam epitaxy grown CdTe/MnTe superlattices: a structural and optical study

Atomic layer epitaxy (ALE) is investigated for the growth of CdTe/MnTe superlattices. A systematic structural characterization (X-ray diffraction, transmission electron microscopy), together with a magneto-optical study (reflectivity and photoluminescence), demonstrate that: for MnTe ALE, all deposited Mn atoms are incorporated, so that no autoregulated growth mode can be obtained, in contrast with CdTe ALE, atomic layer epitaxy allows well-controlled CdTe/MnTe superlattices to be achieved but does not prevent the exchange between Cd and Mn atoms which occurs at the interfaces between CdTe and MnTe, as observed in CdTe/MnTe superlattices grown by conventional molecular beam epitaxy.     

Room-temperature direct-bandgap photoluminescence from strain-compensated Ge/SiGe multiple quantum wells on silicon

Strain-compensated Ge/Si_0.15Ge_0.85 multiple quantum wells were grown on an Si_0.1>Ge_0.9 virtual substrate using ultrahigh vacuum chemical vapor deposition technology on an n⁺-Si(001) substrate. Photoluminescence measurements were performed at room temperature, and the quantum confinement effect of the direct-bandgap transitions of a Ge quantum well was observed, which is in good agreement with the calculated results. The luminescence mechanism was discussed by recombination rate analysis and the temperature dependence of the luminescence spectrum.     

Growth, properties and dye-sensitized solar cells–applications of ZnO nanorods grown by low-temperature solution process

Hexagonal-shaped small ZnO nanorods were grown in a large-quantity via simple aqueous solution process by using zinc nitrate as a source of zinc ions at low temperature under stirring. The as-grown hexagonal-shaped ZnO nanorods were characterized in detail in terms of their structural, optical and photovoltaic properties. The detailed structural investigations by HRTEM, SAED and FFT revealed that the as-synthesized ZnO nanorods are well-crystalline, possessing a perfect hexagonal ideal growth habits of wurtzite zinc oxide and grown along the [0001] direction in preference. The optical properties, composition and quality of the as-synthesized nanorods were examined by using UV-visible and FTIR spectroscopy. Moreover, films of as-grown nanorods were used as photoanode materials to fabricate the dye sensitized solar cells (DSSCs). An overall light to electricity conversion efficiency of 0.70% with a fill factor of 47.2%, short-circuit current of 1.8 mA/cm² and open-circuit voltage of 0.76 V were achieved for the solar cell based on hexagonal-shaped small ZnO nanorods.     

反应磁控溅射ZnO/MgO多量子阱的光致荧光光谱分析

采用反应射频磁控溅射方法，在Si(001)基片上制备了具有高c轴择优取向的ZnO/MgO多量子阱.利用X射线反射、X射线衍射、电子探针，光致荧光光谱等表征技术，研究了ZnO/MgO多量子阱的结构、成份和光致荧光特性.研究结果表明，多量子阱的调制周期在1.85—22.3 nm之间，所制备的多量子阱具有量子限域效应，导致了室温光致荧光峰的蓝移，并观测到了量子隧穿效应引起的荧光效率下降.建立了基于多声子辅助激子复合跃迁理论的室温光致荧光光谱优化拟合方法，通过室温光致荧光光谱拟合发现，ZnO/MgO比ZnO/ZnMgO多量子阱具有更大的峰位蓝移，探讨了导致光致荧光光谱展宽的可能因素. 关键词： ZnO/MgO 多量子阱 磁控溅射 光致荧光 量子限域效应     

Effect of carriers localized in clusters on optical properties of In0.21Ga0.79As/GaAs multiple quantum wells

In_0.21Ga_0.79As multiple quantum wells MQW, with different well thickness L, are grown on [001] and [113] A GaAs growth directions by molecular beam epitaxy MBE. An asymmetric photoluminescence PL line shape denoted LE_A and LE_B in the lower energies side has been observed in both structures. These emissions of deep localized states can be related to the energy potential modulation associated to Indium cluster formation. Temperature dependence of photoluminescence properties has been reported. Localized state ensemble LSE model has investigated atypical behaviors of PL peak energies and the full width at half maximum FWHM of both emissions. These abnormal behaviors are explained by carriers captured by localized recombination centers. Competition processes between localized and delocalized excitons have been occurred to interpret the PL properties. The degree of localization induced by quantum-dot-like states and critical temperatures between different temperatures regions increase as far as away [001] growth direction.     

Electrical characterisation of phosphorus-doped ZnO thin films grown by pulsed laser deposition

Phosphorus-doped ZnO films were grown by pulsed laser deposition using a ZnO:P₂O₅-doped target as the phosphorus source with the aim of producing p-type ZnO material. ZnO:P layers (with phosphorus concentrations of between 0.01 to 1 wt%) were grown on a pure ZnO buffer layer. The electrical properties of the films were characterised from temperature dependent Hall-effect measurements. The samples typically showed weak n-type conduction in the dark, with a resistivity of 70 Ω cm, a Hall mobility of μ_n0.5 cm² V ⁻¹ s⁻¹ and a carrier concentration of n3×10¹⁷ cm⁻³ at room temperature. After exposure to an incandescent light source, the samples underwent a change in conduction from n- to p-type, with an increase in mobility and decrease in concentration for temperatures below 300 K.     

Optical properties of InGaAsBi/GaAs strained quantum wells studied by temperature-dependent photoluminescence

The effect of bismuth on the optical properties of InGaAsBi/GaAs quantum well structures is investigated using the temperature-dependent photoluminescence from 12 K to 450 K.The incorporation of bismuth in the InGaAsBi quantum well is confirmed and found to result in a red shift of photoluminescence wavelength of 27.3 meV at 300 K.The photoluminescence intensity is significantly enhanced by about 50 times at 12 K with respect to that of the InGaAs quantum well due to the surfactant effect of bismuth.The temperature-dependent integrated photoluminescence intensities of the two samples reveal different behaviors related to various non-radiative recombination processes.The incorporation of bismuth also induces alloy non-uniformity in the quantum well,leading to an increased photoluminescence linewidth.     

Alumina capped ZnO quantum dots multilayer grown by pulsed laser deposition

To extend the applicability of ZnO, with the bulk band gap of about 3.3 eV, into deep UV region, we have grown a multilayer of alumina capped ZnO quantum dots of mean in-plane sizes in the range of ∼1.8-3.6 nm at room temperature using alternate Pulsed Laser Deposition. Size dependent blue shift of the band gap of these dots up to ∼4.5 eV is observed in the optical absorbance spectra. The observed blue shift can be understood using the effective mass approximation in weak and strong confinement regimes.     

Characterization of a-plane orientation ZnO film grown on GaN/Sapphire template by pulsed laser deposition

In this study, the authors have investigated the structural and optical properties of ZnO layer grown by pulsed laser deposition on GaN/r-plane sapphire. X-ray diffraction results demonstrate the ZnO film to be highly preferentially deposited at a-axis orientation; the different rocking curve values along the two orthogonal directions indicate the low C_2v symmetry in the growth a-plane ZnO. From free stress to large tensile stress (about 1.34 × 10⁹ Pa) distribution along the growth direction of ZnO is revealed by visible Raman mapping spectra. The enhanced significantly high-order longitudinal-optical (LO) phonon modes up to 4th and no TO phonons have been observed in Raman spectrum under UV 325 nm by resonance conditions; an intense and broad disorder activated surface phonon mode is also observed, resulting from the increased disorder on the film surface with stripe-like growth features. Low-temperature photoluminescence measurements reveal that the band-edge emission of ZnO is dominated by neutral donor-bound exciton and free electrons to neutral acceptor emissions. Interfacial microstructure of ZnO/GaN has been examined by transmission electron microscopy, with the epitaxial relationship () ZnO//() GaN. All these results indicated that GaN template played an important role in the growth of ZnO film, with full advantage of small lattice mismatch.     

Optical and structural properties of MgZnO/ZnO hetero- and double heterostructures grown by pulsed laser deposition

MgZnO thin films, MgZnO/ZnO heterostructures (HS) and double heterostructures (DHS) have been prepared on a-plane sapphire substrates by means of pulsed laser deposition (PLD). A linear blueshift of the MgZnO emission with increasing Mg content is observed in photoluminescence spectroscopy (PL) at 2 K. Cathodoluminescence measurements verify the spatial homogeneity of the emission properties of the MgZnO films. The film roughness is evaluated from atomic force microscopy scans. In MgZnO/ZnO HS the ZnO grows on all appearing MgZnO facets. PL investigations of such PLD-grown heterostructures show the high optical quality of thin ZnO films (d≤100 nm) grown on MgZnO. Capping those structures with a thin MgZnO layer further improves their luminescence intensity and enhances the emission of free-exciton luminescence from the ZnO layers. MgZnO/ZnO/MgZnO DHS with nominal ZnO layer thicknesses of d_nom≤6 nm show a clear intensification of the ZnO PL. Temperature dependent PL and transmission measurements between 4.4 and 300 K prove the dominating emission to be due to the recombination of excitons localized in the ZnO. At 2 K, due to confinement effects, their emission energy is blueshifted up to 51 meV compared to free excitons in bulk ZnO. PACS 81.15.Fg; 78.66.Hf; 68.37.Ps