Hole transport due to shallow acceptors along boron doped SiGe quantum wells

Lateral conductivity and magnetotransport measurements were performed with SiGe single quantum well (QW) structures doped with boron in the QW. The conductivity at low temperatures (T) is shown to be due to hopping over B centers while at higher T, it is due to two-stage excitation: thermal activation of holes from the ground to strain-split B states are followed by hole tunneling into the valence band. The tunneling is due to a potential drop across the QW which is due to hole capture at surface states of the Si cap layer making the surface charged. The external potential applied across the QW essentially changes the lateral conductivity as well as the activation energy. The calculations of band profile, free carrier concentration in the QW and acceptor population, as well as an effect on the transverse electric field were carried out taking into account the charging of surface states.     

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.     

Deep level transient spectroscopy of hole traps related to CdTe self-assembled quantum dots embedded in ZnTe matrix

The capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements have been made on a Schottky Ti-ZnTe (p-type) diode containing CdTe self-assembled quantum dots (QD) and control diode without dots. The C-V curve of the QD diode exhibits a characteristic step associated with the QD states whereas the reference diode shows ordinary bulk behavior. A quasistatic model based on the self-consistent solution of the Poisson's equation is used to simulate the capacitance. By comparison of the calculated C-V curve with the experimental one, hole binding energy at the QD states is found to be equal about 0.12 eV. The results of DLTS measurements for the sample containing QDs reveal the presence of a low-temperature peak which is not observed for the control diode. Analysis of its behavior at different bias conditions leads to the conclusion that this peak may be related to the hole emission from the QD states to the ZnTe valence band. Its thermal activation energy obtained from related Arrhenius plot equals to 0.12 eV in accordance with the energy obtained from the Poisson's equation. Thus based on the C-V and DLTS studies it may be concluded that the thermal activation energy of holes from the QD states to the ZnTe valence band in the CdTe/ZnTe QD system is equal about 0.12 eV.     

Effect of high-temperature annealing on the optical and photoluminescence properties of nanocrystalline SiC films

The optical and photoluminescence (PL) properties of nanocrystalline 3C-SiC films and the effect of the boundary regions between the nanocrystals were studied for two sets of films: (a) films with 10-15 nm nanocrystal size obtained by direct ion deposition method and (b) similar films annealed in oxygen at 850-950 °C. It was shown that annealing of the nanocrystalline SiC films resulted in weaker absorption in a broad spectral range, and to the increase of the optical band gap from 1.8 to 2.2 eV. On the contrary, the edge PL bands in the UV range (2.2 to 2.4 eV) remained similar. In the IR range, three maxima absent in the as-grown films, appeared at 1.52 eV, 1.56 eV and 1.63 eV. Measurement of the intensity of PL maxima as a function of the excitation power showed a nonlinear dependence that was attributed to the onset of stimulated emission.     

Yellow luminescence band in undoped GaN revealed by two-wavelength excited photoluminescence

The below-gap emission components including yellow luminescence (YL) band of an MOCVD grown undoped GaN have been studied by the two-wavelength-excited photoluminescence (TWEPL). The nature of each emission line has been investigated by using an intermittent below-gap excitation (BGE) light of 1.17 eV on an above-gap excitation (AGE) light of 3.49 eV. The intensity of DAP and the YL decreased while it increased for I_OX after irradiation of the BGE. The intensity change in PL after addition of the BGE implies the presence of defect levels in the energy position corresponding to the photon energy of the BGE. Possible recombination models are listed and examined. Only the recombination model in which the YL corresponds to the transition from a shallow donor to a deep state at about 1 eV above the valence band maximum satisfies our experimental result. The possible origin of this defect state is discussed.     

Optical emission of InAs nanowires

Wurtzite InAs nanowire samples grown by chemical beam epitaxy have been analyzed by photoluminescence spectroscopy. The nanowires exhibit two main optical emission bands at low temperatures. They are attributed to the recombination of carriers in quantum well structures, formed by zincblende-wurtzite alternating layers, and to the donor-acceptor pair. The blue-shift observed in the former emission band when the excitation power is increased is in good agreement with the type-II band alignment between the wurtzite and zincblende sections predicted by previous theoretical works. When increasing the temperature and the excitation power successively, an additional band attributed to the band-to-band recombination from wurtzite InAs appears. We estimated a lower bound for the wurtzite band gap energy of approximately 0.46?eV at low temperature.     

Zn diffusion of In0.5Ga0.5P investigated by photoluminescence measurements

Zn diffusion of an In_0.5Ga_0.5P layer grown on semi-insulating GaAs substrate by the liquid phase epitaxy technique has been investigated using photoluminescence (PL) measurements. The PL spectrum shows a characteristic emission peak of In_0.5Ga_0.5P at 1.934 eV after diffusion. From temperature-dependent studies of this peak and depth profiling of this luminescence, it was found that this peak behaves like D-A (donor-acceptor) pair recombination and is associated with the interstitial Zn donor to substitutional Zn acceptor band transition. Also it was found that recombination due to interstitial Zn donor is dominant near the surface and decreases with increasing depth. The calculated activation energy of substitutional acceptor was found to be 47 meV.     

Influence of the annealing temperature on violet emission of ZnO films obtained by oxidation of Zn film on quartz glass

The photoluminescence (PL) emission properties of ZnO films obtained on quartz glass substrate by the oxidation of Zn films with the oxygen pressure of 50Pa at temperature of 773 K~973 K were studied. The strong single violet emission centering on 424 nm (or 2.90 eV) without any accompanying deep-level emission and UV emission was observed in the PL spectra of the ZnO films at room temperature. The intensity of violet emission increased with increasing annealing temperature in the range of 773 K~873 K and decreased with increasing annealing temperature in the range of 873 K~973 K. These violet emission bands are attributed to the electron transition from interstitial zinc (Zn_i) level (2.91 eV) to the valence band.     

Photoelectron spectroscopy of ion-irradiated B-doped CVD diamond surfaces

Chemical vapour deposition (CVD) diamond films were irradiated by 1 keV argon ions at room temperature with doses ranging from 3.6 × 10¹² to 1.1 × 10¹⁶ Ar⁺ cm². The influence of sputtering on the valence band density of states of a boron-doped CVD diamond film was investigated by ultraviolet photoelectron spectroscopy and the changes in the plasmon features were observed by X-ray photoelectron spectroscopy of the carbon Is core level and its loss region. A gradual change from typical diamond features to amorphous carbon was observed after prolonged bombardment times. Above a critical dose D_crit of 5.8 × 10¹⁴ Ar⁺ cm² the damaged surface layer is characterized by a splitting of the C Is bulk peak into two components: a bulk-like diamond peak at 285.3 eV binding energy and a defect peak with 1 eV lower binding energy, which is attributed to the production of an amorphous sp²-rich carbon matrix. Moreover additional occupied states in the range of 0–4 eV binding energy, completely different to those observed on reconstructed diamond surfaces, were observed in the valence band spectra of the ion-irradiated diamond surface. These filled states can also be attributed to the amorphous carbon matrix which is formed at high doses. At very low doses (< 3 × 10¹⁴ ions cm²) only a band bending of the C Is diamond core level peak, along with the formation of some occupied states in the band structure at around 3.8 eV binding energy was observed. A comparison with annealed hydrogen-free CVD diamond surfaces shows some similarities concerning these filled states. The obtained spectra are compared with other crystalline and amorphous forms of carbon and the results are discussed in terms of an irradiation-induced change in the atomic structure of the surface. A comparison of ion bombarded and annealed diamond samples clearly shows that no graphitization takes place in the latter case.     

Photoluminescence spectra of MnIn2S4

MnIn₂S₄ single crystals grown by the directional crystallization method were investigated by using the temperature and excitation power dependencies of photoluminescence (PL) spectra. PL spectra consist of one broad band resulting from donor-acceptor pair recombination. The analysis of the temperature quenching of the PL intensity yields one defect donor level with a thermal ionization energy of about 0.17 eV. The broad band of PL spectra indicates that radiative recombination is related to multiphonon optical processes. The energy of the involved phonon was found to be around 0.025 eV and the energy of the acceptor level is about 0.86 eV.     

A study on fabrication, morphological and optical properties of lead sulfide nanocrystals

To investigate the properties of lead sulfide (PbS) nanocrystals, we have prepared PbS nanocrystals on/in the porous alumina membrane with a pore diameter 20 nm. Utilizing the reaction of Pb wires and the hydrogen sulfide (H2S) gas, PbS nanocrystals produced and grew as the reaction time increased. The composition identification of the nanocrystals was performed by the XPS and EDS analyses. More structure characteristics of the PbS nanocrystals obtained from the TEM analysis. As indicated in the PL spectra, an orange-red emission band appeared and the emission intensities were obviously related to the defects in the nanocrytals. A significant quantum confinement effect made the energy gap of PbS nanocrystals produce a blue shift from 0.41 eV to 1.89 eV. Furthermore, the growth mechanism of the PbS nanocrystals was also discussed.     

二氧化钛纳米晶的制备及其光致发光的研究

以Ti（SO4）2为前驱体,采用沉淀法制备了二氧化钛纳米晶,通过X射线衍射（XRD）、透射电子显微镜（TEM）和光致发光（PL）光谱对微晶进行表征。研究表明,制备的TiO2纳米晶呈类球形颗粒且分散性好,平均粒径最小约为4nm,热处理温度升高到750℃时,样品仍为单一的锐钛矿相;PL谱分析表明,室温下,用高于带隙的能量激发,在370～550nm范围内纳米TiO2粒子呈现出强而宽的发光带,分别对应于价带和导带间的电子跃迁以及表面态的发光;另外发现随焙烧温度升高,粒径增大的同时,发光峰的强度呈无规则变化,分析可能与表面态和晶粒内部缺陷的数量有关。     

Comparative photoluminescence study on p-type and n-type ZnO films codoped by nitrogen and aluminium

We report on the temperature-dependent photoluminescence (PL) properties of n-type and p-type ZnO films codoped with N and Al. For the n-type film, the dominant emission at low temperature is exciton bound to neutral donors, while for the p-type film it is exciton bound to neutral acceptor at 3.33 eV. Four defect or impurity levels, including N acceptor, residual acceptor, and two doping-induced unknown deep acceptors, were identified. The energy level of the N acceptor was determined to be 0.23 eV. Excitation energy dependence of the PL was also investigated. It was found that at high excitation energy, the formation of exciton was suppressed by the formation of D⁺A⁻eh complexes.     

Aspects of emission variation in CdSeTe/ZnS quantum dots conjugated to antibodies

CdSeTe/ZnS quantum dots (QDs) with the emission peak at 705 nm have been studied comparatively in the non-conjugated state and after bioconjugation to anti-pseudo rabies virus antibodies (ABs) by means of photoluminescence (PL) and Raman scattering methods. It is revealed that PL spectra of QDs vary significantly after conjugating to ABs. In PL spectra of non-conjugated QDs only one PL band of Gaussian shape peaked at 1.76–1.78 eV and related to exciton emission in the CdSeTe core has been detected. The PL spectra of bio-conjugated QDs demonstrate the high energy spectral shift and asymmetric shape of PL bands. The study of Raman scattering spectra permits to estimate the CdSeTe alloy composition and to detect the surface enhanced Raman scattering (SERS) effect for bioconjugated QDs. The last fact testifies on the interaction of excitation light electromagnetic field with the electric dipoles excited in ABs. The optical band gap in CdSeTe core has been calculated numerically versus core radius on the base of the effective mass approximation model. Then the energy band diagrams for non-conjugated and bio-conjugated states of CdSeTe/ZnS QDs have been designed. It is revealed the type II quantum well in CdSeTe core that explains the optical transition at 705 nm in the wide band gap CdSeTe alloy. The analysis has shown that AB dipoles excited in bio-conjugated QDs stimulate changing the profile of QD energy band diagram that manifests itself in the mentioned PL spectrum transformations. Actually, the study of PL spectrum varying in CdSeTe/ZnS QDs conjugated to specific antibodies can be an informative tool in biology and medicine for early medical diagnostics.     

Stability and nonlinear optical properties of ZnO nanoparticles

Photoluminescence (PL) of ZnO nanoparticles of different surface states and sizes grown by several methods has been measured. The origin of luminescence and dependence of the luminescence spectrum shape and intensity on 325 nm excitation laser power are studied. Strong ultraviolet emission at 3.26 eV, weak violet emission around 3.12 eV and weak green emission at 2.40 eV have been observed in 16 nm nanoparticles capped by octylamine grown by non-hydrolytic method. The nanoparticles are stable under high power laser radiation and their PL intensity increases nonlinearly with an increasing laser power. As the nanoparticle size decreases to 12 nm, high power laser produces nonradiative centers which may quench the luminescence in a degree. Nanoparticles of 8 nm capped by PVP and uncapped nanoparticles of 14 nm are unstable and their luminescence depends on the excitation laser power. High power laser can quench O vacancy emission and enhance ultraviolet emission in PVP capped nanoparticles while vacancy emission can not be quenched in uncapped nanoparticles.     

Optical properties and luminescence dynamics of Eu3+-doped terbium orthophosphate nanophosphors

The development of luminescent inorganic nanocrystals (NCs) doped with rare-earth (RE) ions has attracted increasing interest owing to their distinct optical properties and versatile applications in time-resolved bioassays, multiplex biodetection, DNA hybridization and bioimaging. Hexagonal TbPO4:Eu3+ NCs (10-30 nm) were synthesized via a facile hydrothermal method assisted with oleic acid (OA) surfactants, which exhibit tunable emissions from green to red by varying the concentration of Eu3+. The Tb3+-to-Eu3+ energy transfer efficiency observed reaches up to 94%. Different from their bulk counterparts, a new interface-state band (316 nm) in addition to the commonly observed spin-forbidden 4f-5d transition band (265 nm) of Tb3+ was found to be dominant in the excitation spectrum of NCs due presumably to the formation of surface TbPO4/OA complexes, which provides an additional excitation antenna in practical utilization. Two kinds of luminescence sites of Eu3+ in TbPO4 NCs, with the site symmetry of C2 or C1, were identified based on the emission spectra at 10 K and room temperature. Furthermore, the photoluminescence (PL) dynamics of Tb3+ ions in pure TbPO4 NCs have been revealed. Compared to the exponential PL decay in bulk counterparts induced by very fast energy migration, the non-exponential decay from 5D4 of Tb3+ in TbPO4 NCs is mainly attributed to the diffusion-limited energy migration due to more rapid energy transfer from Tb3+ to defects than the energy migration among Tb3+.     

Tuning Radiative Recombination in Cu-Doped Nanocrystals via Electrochemical Control of Surface Trapping

The incorporation of copper dopants into II-VI colloidal nanocrystals (NCs) leads to the introduction of intragap electronic states and the development of a new emission feature due to an optical transition which couples the NC conduction band to the Cu-ion state. The mechanism underlying Cu-related emission and specifically the factors that control the branching between the intrinsic and impurity-related emission channels remain unclear. Here, we address this problem by conducting spectro-electrochemical measurements on Cu-doped core/shell ZnSe/CdSe NCs. These measurements indicate that the distribution of photoluminescence (PL) intensity between the intrinsic and the impurity bands as well as the overall PL efficiency can be controlled by varying the occupancy of surface defect sites. Specifically, by activating hole traps under negative electrochemical potential (the Fermi level is raised), we can enhance the Cu band at the expense of band-edge emission, which is consistent with the predominant Cu(2+) character of the dopant ions. Furthermore, we observe an overall PL "brightening" under negative potential and "dimming" under positive potential, which we attribute to changes in the occupancy of the electron trap sites (that is, the degree of their electronic passivation) that control nonradiative losses due to electron surface trapping.     

Two-electron exchange between neutral and ionized negative-<Emphasis Type="Italic">U</Emphasis> tin centers in lead chalcogenides

Two-electron exchange between neutral and doubly ionized negative-U tin centers in PbS and PbSe has been studied by Mössbauer emission spectroscopy. The activation energy for this process in PbS is 0.11(2) eV, which is comparable to the depth of tin levels in the band gap of PbS, whereas the activation energy for this process in PbSe, 0.05(1) eV, is comparable to the correlation energy of negative-U donor tin centers in PbSe. The exchange occurs through simultaneous transfer of two electrons and involves valence band states.     

Electroluminescence microspectroscopy of silicon nanocrystals obtained by Si(+) ion implantation in SiO(2)

Room-temperature electroluminescence (EL) has been measured at both macroscopic and microscopic levels from metal-oxide-semiconductor devices containing silicon nanocrystals (Si-nc) embedded in silicon dioxide (SiO(2)) obtained by high-temperature annealing (1050 and 1100?°C) after Si(+) ion implantation. It is found that spatially integrated (macroscopic) EL is dominated by a near-infrared band centered where the photoluminescence (PL) band of Si-nc (from 700 to 1000?nm) is located. However, on a microscopic scale, EL emission is inhomogeneous, the sample surface exhibiting many visible spots of micron-order diameter. EL spectra from a microscopic surface of ～1?μm(2)(μEL) on visible spots have revealed dominant contributions between ～550 and ～650?nm, attributed to oxide defects. These spectral features rapidly decrease with distance from a bright spot, while lower-intensity near-infrared contributions (750-950?nm) remain unaffected up to relatively large distances before eventually becoming extinct. The macroscopic EL measurements can be explained as a superposition of the μEL and PL spectra. A luminescent mechanism is proposed in which charge carriers mostly tunnel through high-defect-density channels in the oxide, yielding bright visible spots, while Si-nc in these channels and their surroundings contribute to the luminescence by hosting electron-hole recombinations (EL) and/or exhibiting PL due to optical excitation from the nearby visible EL spot.     

Visible light induced electron transfer process over nitrogen doped TiO(2) nanocrystals prepared by oxidation of titanium nitride

Nitrogen doped TiO(2) nanocrystals with anatase and rutile mixed phases were prepared by incomplete oxidation of titanium nitride at different temperatures. The as-prepared samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), core level X-ray photoelectron spectroscopy (CL XPS), valence band X-ray photoelectron spectroscopy (VB XPS), UV-vis diffuse reflectance spectra (UV-vis DRS), and visible light excited photoluminescence (PL). The photocatalytic activity was evaluated for photocatalytic degradation of toluene in gas phase under visible light irradiation. The visible light absorption and photoactivities of these nitrogen doped TiO(2) nanocrystals can be clearly attributed to the change of the additional electronic (N(-)) states above the valence band of TiO(2) modified by N dopant as revealed by the VB XPS and visible light induced PL. A band gap structure model was established to explain the electron transfer process over nitrogen doped TiO(2) nanocrystals under visible light irradiation, which was consistent with the previous theoretical and experimental results. This model can also be applied to understand visible light induced photocatalysis over other nonmetal doped TiO(2).