A tunneling spectroscopy study of the temperature dependence of the forbidden band in Bi2Te3 and Sb2Te3

Tunneling measurements of dI/dV, d ² I/dV ², and d ³ I/dV ³ were formed along the C ₃ axis (normally to layers) for Bi₂Te₃ and Sb₂Te₃ layered semiconductors in the temperature range 4.2<T>29 5 K. Temperature dependences of the forbidden band energy E _g were obtained. The forbidden band energy in Bi₂Te₃ was 0.20 eV at room temperature and increased to 0.24 eV at T=4.2 K. The E _g value for Sb₂Te₃ was 0.25 eV at 295 K and 0.26 eV at 4.2 K. The distance between the top of the higher valence band of light holes and the top of the valence band of heavy holes situated lower was found to be ΔE _V≈19 meV in Bi₂Te₃; this distance was independent of temperature. The conduction bands of Bi₂Te₃ and Sb₂Te₃ each contain two extrema with distances between them of ΔE _c≈25 and 30 meV, respectively.     

Mass spectrometry study of the kinetics of CdTe molecular-beam epitaxy: Part II. Cd, Te2, Te, and CdTe

The results of an in situ mass-spectrometric study of surface processes occurring during CdTe molecular beam epitaxy are presented. The measurements of kinetic parameters are performed with modulated Cd and Te₂ molecular beams with an intensity of 0.1–5.0 ML/s at a crystal temperature of 550–730 K. The experimental results are treated using a model in which condensation and evaporation proceed through adsorption and desorption steps. The desorption rates are 2–15 and 150 s^?1 for Te₂ and Cd, respectively. The activation energy of CdTe “evaporation” is found to be 1.2 eV; the desorption energies are E _d(Cd) = 1.0 eV and E _d(Te₂) = 0.3 eV. The adsobate coverage with cadmium atoms and tellurium molecules is estimated to be n(Cd) < 0.01, n(Te₂) = 0.02–0.20, and n(Te) = 0.2–1.0.     

a-Si(n)/c-Si(p)异质结太阳电池薄膜硅背场的模拟优化

采用AFORS-HET数值模拟软件,对不同带隙的薄膜硅材料在a-Si(n)/c-Si(p)异质结太阳电池上的背场效果进行了模拟,分析了影响背场效果的原因,得到了薄膜硅背场在a-Si(n)/c-Si(p)异质结太阳电池上的适用条件为薄膜硅材料是带隙16 eV,硼掺杂浓度在10¹⁸cm^-3以上的微晶硅材料,其最佳厚度在5nm左右. 这种背场从工艺上易于实现,并且,与常用的Al扩散背场相比,在相同的掺杂浓度下,电池效率可以大大提高. 关键词： 薄膜硅 背场 硅异质结太阳电池     

Characteristic energy losses on carbon contaminated layers correlated with secondary electron,auger electron emission and contrasts in scanning microscope

The secondary electron (SE) spectrum (0 < E < 50 eV) has been analysed by means of a CMA. Samples were clean aluminum, aluminum becoming carbon contaminated, sintered graphite powder, electro chemically deposited polymer on platinum and monocrystals of silicon carbon contaminated. When the clean Al surface is becoming carbon contaminated a quick decrease of surface plasmon and bulk plasmon losses is observed whereas a main characteristic energy loss peak (ELS) at 20 eV and a secondary electron peak at 20 eV appear simultaneously. Both peaks are very sensitive general features of carbon contaminated surfaces. The main loss peak is attributed to the excitation of the carbon-carbon bounds (σ → σ¹) as already proposed in the transmission ELS. The few eV change of the loss peak energy of various carbon compounds may correspond to slightly different carbon-carbon distances. The 20 eV secondary electrons could be produced by the relaxation of the excited state (σ¹ → σ transition) via an Auger process. The cross section for molecular electronic excitation is higher than that of atomic ionization for inner level. The loss peak is as intense as the SE peak and higher by more than two orders of magnitude than the C KLL Auger peak. The modification of secondary emission under carbon contamination has been observed on a silicon sample by Scanning Electron Microscopy (SEM) in the Secondary Electron Image (SEI) mode.     

Relation between interface states and temperature behavior of the barrier height of silver contacts on clean cleaved n-type silicon

The properties of silver-silicon interfaces formed by cleaving n-type silicon in ultra high vacuum (UHV) in a stream of evaporating silver atoms were studied. The barrier heights of these contacts were measured at different temperatures by using C-V techniques. All measurements were performed in UHV. The dependence of the barrier height upon temperature did not follow the temperature dependence of the Si band gap as it is usually found. The measured temperature behavior depended on the roughness of the Si surface. The temperature behavior can be explained by assuming a specific band structure of the interface states. For Ag contacts on atomically smooth n-type Si, the interface states were found to be arranged in two bands, one band 4 × 10^?3 eV wide with acceptor type states 0.18 eV below the intrinsic level E_i and a density of 10¹⁷ states/cm² eV, and the other 1 eV wide with donor type states with its upper edge 0.28 eV below E_i, and a density of 4 × 10¹⁴ states/cm²eV.     

Excitation of lowest electronic states of the uracil molecule by slow electrons

The excitation of lowest electronic states of the uracil molecule in the gas phase has been studied by electron energy loss spectroscopy. Along with excitation of lowest singlet states, excitation of two lowest triplet states at 3.75 and 4.76 eV (±0.05 eV) and vibrational excitation of the molecule in two resonant ranges (1?C2 and 3?C4 eV) have been observed for the first time. The peak of the excitation band related to the lowest singlet state (5.50 eV) is found to be blueshifted by 0.4 eV in comparison with the optical absorption spectroscopy data. The threshold excitation spectra have been measured for the first time, with detection of electrons inelastically scattered by an angle of 180°. These spectra exhibit clear separation of the 5.50-eV-wide band into two bands, which are due to the excitation of the triplet 1³ A?? and singlet 1¹ A?? states.     

Influence of irradiation on the thermal decomposition of lithium perchlorate

Abstract

Infrared absorption measurements were made before and after 90 °K electron irradiations of silicon samples which contained either dispersed oxygen, carbon, or carbon plus oxygen. Irradiation-produced absorption bands associated with two distinctly different defects are observed depending on the oxygen and carbon content of the silicon. One center is the well-known vacancy-oxygen A-center defect (836-cm^?1 band) and is formed on irradiation in oxygen-containing silicon with a magnitude which is independent of the carbon content. Measurements have correlated the formation of one A-center with the loss of one interstitial oxygen atom, thereby indicating that A-center formation occurs by vacancy trapping at interstitial oxygen atoms. A second center (922-and 932-cm^?1 bands) is formed only in silicon crystals which contain both oxygen and carbon. The results indicate that this center is formed by the trapping of a silicon interstitial at a carbon-oxygen complex.     

基于Si掺杂Sb2Te3薄膜的相变存储器研究

采用磁控三靶(Si，Sb及Te)共溅射法制备了Si掺杂Sb₂Te₃薄膜，作为对比，制备了Ge₂Sb₂Te₅和Sb₂Te₃薄膜，并且采用微加工工艺制备了单元尺寸为10μm×10μm的存储器件原型来研究器件性能.研究表明，Si掺杂提高了Sb₂Te₃薄膜的晶化温度以及薄膜的晶态和非晶态电阻率，使得其非晶态与晶态电阻率之比达到10⁶，提高了器件的电阻开/关比；同Ge₂Sb₂Te₅薄膜相比，16at% Si掺杂Sb₂Te₃薄膜具有较低的熔点和更高的晶态电阻率，这有利于降低器件的RESET电流.研究还表明，采用16at% Si掺杂Sb₂Te₃薄膜作为存储介质的存储器器件原型具有记忆开关特性，可以在脉高3V、脉宽500ns的电脉冲下实现SET操作，在脉高4V、脉宽20ns的电脉冲下实现RESET操作，并能实现反复写/擦，而采用Ge₂Sb₂Te₅薄膜的相同结构的器件不能实现RESET操作. 关键词： 相变存储器 硫系化合物 2Te₃薄膜')" href="#">Si掺杂Sb₂Te₃薄膜 SET/RESET转变     

Porous-silicon coatings for photovoltaic devices

Summary Porous silicon was formed on multicrystalline Si substrates by stain etching in aqueous HF/HNO₃ solutions. In this work optical and electrical properties of the resulting films are discussed as a function of process parameters. Porous-Si films have been shown to be able to reduce surface reflectance to 3% in 350–700 nm wavelength range and their application in anti-reflection coating of photovoltaic solar cells has been demonstrated, obtaining (10×10) cm² multicrystalline Si solar cells with efficiency approaching 12% under standard AM 1.5 simulated sunlight. Paper presented at the III INSEL (Incontro Nazionale sul Silicio Emettitore di Luce), Torino, 12–13 October 1995.     

High-temperature behaviour of spin-lattice relaxation and Knightshift of Te125 in tellurium

The Knightshift and the nuclear spin-lattice relaxation time of Te¹²⁵ in enriched (94% Te¹²⁵) polycrystalline tellurium have been studied from room temperature to the melting point (725 K). Over the whole temperature range, the shift is governed by an Arrhenius law with an activation energy of about 0.3 eV in agreement with the gap energy of tellurium at higher temperatures. Below 420 K, the relaxation time is caused by the conduction electrons, whereas in the high-temperature region the relaxation process is due to translational atomic diffusion. In this region, the relaxation mechanism is found to be determined by the chemical shift anisotropy of Te¹²⁵.     

Solar cells with porous silicon: modification of surface-recombination velocity

Laser-induced transient-grating measurements were performed to monitor the influence of porous silicon on the surface recombination of a highly doped n⁺-silicon emitter of solar cells. With this technique, photocarrier diffusion and recombination with a time resolution of some tens of picoseconds can be studied. Using pulses of the second- and third-harmonic radiation from an Nd³⁺:YAG laser (quantum energy 2.34 and 3.51 eV, respectively), two different-depth regions of the emitter were excited. Using a kinetic model, which includes carrier diffusion and recombination at the surface and in the bulk of the emitter, surface-recombination velocities in a series of samples typical for each successive operation of solar-cell technology with different surface-doping level and surface preparation were evaluated. From the analysis, we conclude that porous silicon formed on the emitter passivates the surface of the silicon layer, i.e. reduces the rate of surface recombination at the porous silicon–crystalline silicon interface. Ytterbium as a co-dopant of the emitter increases the surface-recombination velocity. Received: 26 June 2000 / Accepted: 4 December 2000 / Published online: 26 April 2001     

Photoluminescence studies of γ-irradiated CaF2:Dy:Pb:Na single crystals

The optical absorption (OA) and photoluminescence (hereafter referred to as luminescence) studies were made on CaF₂:Dy:Pb:Na single crystals (Dy—0.005 at%, Pb—0.188 at% and Na—0.007 at%) before and after γ-irradiation. The unirradiated crystal exhibited a strong OA band around 6.36 eV attributed to the ‘A’ band absorption of Pb²⁺ ions. The γ-irradiated crystal exhibited OA bands around 2.06, 3.28, 3.75 (broad shoulder) and 2.48 eV. The first three bands could be tentatively attributed to M_Na centre when compared with that of the coloured CaF₂:Na. The origin of 2.48 eV band was not explicitly known. Luminescence emission and excitation of Pb²⁺ and Dy³⁺ ions were negligible in the unirradiated crystal. Irradiated crystal exhibited a strong excitation spectrum with overlapping bands, due to different colour centres, in the UV-vis region for the 2.15 eV emission characteristic of Dy³⁺ ion. When excited, the absorbed energy (may be a part) was transferred from a colour centre to nearby Dy³⁺ ions and Dy³⁺ characteristic emission was observed. Exciting the irradiated crystal around 3.28 eV yielded emission at 2.56, 2.15 and 1.76 eV. The first two emission bands were due to Dy³⁺ ions. The excitation spectrum for the 1.76 eV emission showed two prominent bands around 2.02 and 3.08 eV and hence the emission was attributed to the M_Na centre. The luminescence mechanism was described.     

Numerical study of InGaN tandem solar cells with intermediate bands

Theoretical investigations of InGaN tandem solar cells with intermediate bands (IBs) have been conducted through calculating the diode equation taking into account the radiative and nonradiative recombination currents. The calculated maximum ef?ciencies of the double‐junction cell with one IB in each subcell are 57.85% and 68.37% under AM1.5G one‐sun and 46000‐sun illuminations, respectively. It has also been observed that the combined device with the top‐cell bandgaps of 2.9–3.4 eV (2.6–3.4 eV for full concentration) may have an opportunity to realize the application of over 50% efficiency. We suggest that the optimized width of the IB layer be designed in the range of 1–6 μm if its absorption coefficient is 10⁴–10⁵ cm^–1 in the IB region.     

$Ab~initio$ and experimental study of the K-shell spectra of s-triazine

The carbon and nitrogen K-shell spectra of gaseous s-triazine have been studied using inner-shell electron energy loss spectroscopy (ISEELS) method. Ab initio Configuration Interaction calculations have been carried out in order to assign the observed bands. As in many similar molecules, both spectra are dominated by an intense π^* peak, followed by lower intensity features. At the C1s edge, the calculations show that some previous assignments made using an underestimated core ionisation energy of about 2.5 eV have to be revisited. At the nitrogen edge, the calculations predict a high intensity π^* doubly excited state lying below the ionisation threshold, which could be responsible for one the most intense observable bands at 405.32 eV.     

Development of epitaxial silicon lattice-matched insulators: silicon heterostructures for quantum confinement

Epitaxial films of the wide-bandgap II–VI beryllium chalcogenide semiconductors, BeTe, BeSe, and BeSeTe were grown on arsenic-terminated silicon substrates by MBE. Silicon was also epitaxially regrown on Be-chalcogenide films. Initial structural characterization revealed the desired smooth two-dimensional nature of the layer growth. The composition of BeSeTe ternary films was governed by the Be/Se flux ratio during deposition rather than by the Se/Te flux ratio. The variation in Be/Se flux ratio or in the sticking coefficients due to temperature gradients led to radial compositional inhomogeneity. Current versus temperature measurements of the Be-chalcogenide films at elevated temperatures analyzed assuming thermionic emission over the heterojunction barrier, showed conduction band offsets of 1.2 eV for the BeSe_0.41Te_0.59/As/Si and 1.3 eV for the BeSe/As/Si heterostructures. At room temperature, current density through BeSe/Si and BeSe_0.41Te_0.59/Si films was mid-10 ^{− 9}A cm ^{− 2}at 0.1 MV cm ^{− 1}, similar to previously reported values for ZnS/Si, while BeTe/Si films had orders of magnitude higher current density, possibly due to interfacial recombination.     

Electroluminescence of Si-SiO2 structures subjected to sequential ion implantation with silicon and carbon

The electroluminescence of Si-SiO₂ structures subjected to sequential implantation with 130-and 60-keV silicon ions and 60-keV carbon ions into a silicon dioxide layer is investigated. It is found that implantation of the structures with silicon and carbon ions is responsible for the electroluminescence bands with maxima at energies of approximately 2.7 and 4.3 eV. It is assumed that these bands are associated with the formation of silylene centers. Postimplantation annealing leads to a decrease in the intensity of the observed electroluminescence bands and gives rise to a shoulder in the short-wavelength wing of the band with a maximum at 2.7 eV.     

Synthesis of α-In 2 Te 3 thin films from In/Te bilayer by Si ion irradiation

In this work, In/Te bilayer thin films were prepared using sequential thermal evaporation method and subsequently irradiated using swift heavy ions (SHIs) of 100 MeV silicon (Si) with different fluences (1×10¹³ to 5×10¹³/cm²). The inter-diffusion of In and Te layers was highly controlled by SHI irradiation and the In₂Te₃ formation capability was compared with that of the conventional annealing method. The structural as well as optical properties of a post-sintered SHI-irradiated In/Te bilayer were investigated using X-ray diffraction (XRD) measurements and UV–visible spectroscopy, respectively. We found that irradiated samples showed single-phase In₂Te₃ under post-annealed conditions at 150 °C unlike that prepared using the conventional thermal annealing method, which showed mixed phases under similar conditions. This confirms the effective inter-diffusion in bilayer films by SHI irradiation toward the formation of single-phase In₂Te₃. The estimated optical band gap energy was found to be 1.1±0.5 eV and strongly corroborated the XRD results. In addition, the estimated refractive index (n) value of the SHI-irradiated sample (～3.3) was higher than that of the sample obtained through the conventional annealing method (～2.8). This proves that SHI offers a highly compact nature even at low temperatures. This work has a wide scope for achieving single-phase alloyed films through bilayer mixing by SHI irradiation.     

Localized electronic excitations in crystalline phenacite Be2SiO4

The results of coordinated spectroscopic studies of the nature and properties of electronic excitations localized at regular and defect sites of the Be₂SiO₄ lattice are presented. The methods employed are electron-beam-excited pulsed absorption spectroscopy, pulsed cathodoluminescence, and low-temperature VUV spectroscopy with selective excitation by synchrotron radiation. The bands in luminescence spectra of Be₂SiO₄ at 2.70 and 3.15 eV are assigned to [AlO₄]^5? and [SiO₄]^4? centers formed both in direct relaxation of electronic excitations at defect levels and through the formation of exciton-defect complexes. Disruptions of beryllium-oxygen bonds (short-lived defects in the form of beryllium vacancies V _Be ^? ) are considered as initiating the formation of optically active centers with characteristic absorption bands in the range 1.5–4.0 eV. The intrinsic luminescence of the Be₂SiO₄ crystal at 3.6 and 4.1 eV is attributed to radiative decay of self-trapped excitons of two types. A mechanism of exciton self-trapping at the [SiO₄] and [BeO₄] tetrahedral groups is proposed, which involves excitation transfer from a threefold-coordinated oxygen atom to neighboring silicon or beryllium atoms.     

Identification of new absorption bands in Zn1-xMnxTe

We report optical absorption measurements on Zn_1-xMn_xTe at 300K, 77K, and LHe temperatures. In an x ～ 0.6 sample, new absorption bands are found at 2.605 eV and 2.745 eV, together with two lower energy bands seen also in an x ～ 0.1 sample. All bands are attributed to intra-ion transitions of Mn²⁺ in the 3d⁵ configuration.     

Carbon and oxygen isotope effects in the 0.79 eV defect photoluminescence spectrum in irradiated silicon

The 0.79 eV photoluminescence spectrum known to emerge in oxygen-rich irradiated silicon is studied in ¹³C and ¹⁸O enriched crystals. The principal no-phonon transition at 0.79 eV splits into a doublet in the ¹³C enriched sample in a way directly demonstrating that one carbon atom per center is optically active. In contrast, no isotope effects are observed in the local mode replicas. Doping the silicon with ¹⁸O slightly influences two local mode replicas giving the first direct evidence that oxygen is involved in the defect.