Luminescent properties of BexCd1−xSe thin films

We report photoluminescence (PL) study of Be_xCd_1−xSe epitaxial layers (x<0.21) grown by molecular beam epitaxy on InP substrates. Continuous wave PL spectra are taken within a 4.2-300 K temperature range. We observe an anomalous ‘s-shaped’ temperature dependence of emission energy and a severe decrease of emission intensity with the increase of temperature. We explain an ‘s-shaped’ temperature dependence of emission energy by exciton localization in the potential minima at low temperatures followed by thermal activation at higher temperatures. We attribute low emission intensity at high temperatures to exciton dissociation and electron/hole migration to non-radiative recombination centers.     

Radiative recombination in Zn1-x MnxTe/Zn0.59Mg0.41Te quantum well structures: Exciton emission and intracenter luminescence

The luminescence spectra controlled by excitons and intracenter 3d emission of Mn²⁺ ions are studied for a series of Zn_1-x Mn_xTe/Zn_0.59Mg_0.41 Te quantum well (QW) structures that differ in manganese content and QW width. It is shown that the relative intensities of exciton emission of the QWs and barriers and the dependences of the intensities on the optical excitation level are controlled mainly by the manganese content in the QWs that affect the efficiency of excitons transfer of to the 3d shell of the Mn²⁺ ions. The effects of QW width and manganese content on the decay kinetics of the intracenter luminescence of the Mn²⁺ ions are studied.     

Luminescence of stepped quantum wells in GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures

Luminescence spectra of doped and undoped GaAs/GaAlAs and InGaAs/GaAs/GaAlAs structures containing several tens of stepped quantum wells (QW) are investigated. The emission bands related to free and bound excitons and impurity states are observed in QW spectra. The luminescence excitation spectra indicate that the relaxation of free excitons to the e1hh1 state proceeds via the exciton mechanism, whereas an independent relaxation of electrons and holes is specific to bound excitons and impurity states. The energy levels for electrons and holes in stepped QWs, calculated in terms of Kane’s model, are compared with the data obtained from the luminescence excitation spectra. The analysis of the relative intensities of emission bands related to e1hh1 excitons and exciton states of higher energy shows that, as the optical excitation intensity increases, the e1hh1 transition is more readily saturated at higher temperature, because the lifetime of excitons increases. Under stronger excitation, the emission band of electron-hole plasma arises and increases in intensity superlinearly. At an excitation level of ～10⁵ W/cm², excitons are screened and the plasma emission band dominates in the QW emission. Nonequilibrium luminescence spectra obtained in a picosecond excitation and recording mode show that the e1hh1 and e2hh2 radiative transitions are 100% polarized in the plane of QWs.     

Quantum Confinement and Carrier Localization Effects in ZnO/Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>O Wells Synthesized by Pulsed Laser Deposition

The optical properties of ZnO/Mg _x Zn_1−x O (x = 0.17) quantum wells (QWs) grown on c-plane sapphire substrates by pulsed laser deposition are presented. A blueshift in the low-temperature photoluminescence (PL) of the QWs illustrates quantum confinement effects as a function of ZnO well widths in the range from 3 nm to 10 nm. Enhanced luminescence properties are observed with increasing quantum confinement. PL data indicate weak polarization effects associated with the heterojunctions. Temperature-dependent PL measurements indicate carrier/exciton localization with activation energy of approximately 4−5 meV, which are attributed to potential fluctuations at the well-barrier interface.     

Quantitative estimation of exciton quenching strength at interface of charge injection layers and organic semiconductor

The performance of polymer light emitting diodes (PLEDs) degrades due to exciton quenching at the interface with charge injection layers and electrodes. We investigate the photo-physics of singlet excitons in Poly (9, 9-dioctylfluorene-alt-benzothiadiazole) (F8BT) conjugated polymer interfaced with various commonly used hole and electron injection layers. Absolute, steady-state and transient photoluminescence (PL) studies are carried out on pristine F8BT film and films with injection layer/F8BT to understand the role of injection layers on exciton quenching. Exciton quenching by the charge injection layers is treated by accounting for both exciton diffusion and the non-radiative transfer of energy to the charge injection layer. The non-radiative transfer of energy is modelled using dipole-dipole interaction theory coupled with diffusion of excitons, from which we obtain the exciton capture radius (x₀) in the range of 1–7 nm. We also correlate x₀ with PL decay time (τ) using the relation τ α 1/x₀³. The steady-state PL yield for each case also shows correlation with the PL decay lifetime. This study provides interesting insight on the selection criterion for injection layer to be used in PLEDs for minimizing optical losses while preserving the electronic injection properties.     

Low-temperature photoluminescence and X-ray diffractometry study of InxGa1−x As quantum wells

The structures grown by molecular-beam epitaxy with In_xGa_1?x As quantum wells (QWs) in GaAs were studied by X-ray diffractometry and low-temperature photoluminescence techniques. The inhomogeneity of the QW composition along the growth direction was established. Energy positions of the exciton recombination lines in the QWs with step-graded In distribution were calculated, and good agreement with the experimental data was obtained.     

Temperature dependence of the photoluminescence properties and band gap energy of InxGa1−xAs/GaAs quantum wells

The effective band gap energy of In_xGa_1−xAs/GaAs strained quantum wells (QWs) is investigated by photoluminescence spectroscopy (PL) in the range 12–295 K. The temperature dependence of the band gap energy of strained QWs correlates well with that of bulk In_xGa_1−xAs of similar composition. Deviations from the band gap variation of bulk material at low temperatures (12–90 K) are interpreted in terms of exciton localization. The differences ΔE(12 K) between the measured PL peak energies and the expected transition energies at 12 K (obtained by simulating the measured temperature dependence of the PL peak positions by the well-known Varshni relation) are suggested to be closely related to the Stokes shifts that often exist between PL and PL excitation spectra of QWs. A linear relation is found between the PL full-width at half-maximum measured at 12 K and ΔE for a range of QWs prepared under different growth conditions. Excitonic recombination is inferred to be dominant in the PL transitions at the highest temperatures investigated—even at room temperature.     

Materials characterization of WNxCy, WNx and WCx films for advanced barriers

A ternary WN_xC_y system was deposited in a thermal ALD (atomic layer deposition) reactor from ASM at 300 °C in a process sequence using tungsten hexafluoride (WF₆), triethyl borane (TEB) and ammonia (NH₃) as precursors. The WC_x layers were deposited by a novel ALD process at a process temperature of 250 °C. The WN_x layers were deposited at 375 °C using bis(tert-butylimido)-bis-(dimethylamido)tungsten (^tBuN)₂(Me₂N)₂W (imido-amido) and NH₃ as precursors. WN_x grows faster on plasma enhanced chemical vapor deposition (PECVD) oxide than WC_x does on chemical oxide. WN_xC_y grows better on PECVD oxide than on thermal oxide, which is opposite of what is seen for WN_x. In the case of the ternary WN_xC_y system, the scalability towards thinner layers and galvanic corrosion behavior are disadvantages for the incorporation of the layer into Cu interconnects. ALD WC_x based barriers have a low resistivity, but galvanic corrosion in a model slurry solution of 15% peroxide (H₂O₂) is a potential problem. Higher resistivity values are determined for the binary WN_x layers. WN_x shows a constant composition and density throughout the layer.     

Investigation of graded InxGa1−xP buffer by Raman scattering method

The compositional changes of In_xGa_1−xP graded buffer inserted between GaP substrate and subsequently grown In_0.36Ga_0.64P homojunction LED structure were investigated by Raman spectroscopy. The indium content of In_xGa_1−xP interlayers was increased in eight steps with thickness of 300 nm and constant compositional change Δx_In between the steps. The properties of In_xGa_1−xP graded buffer along the structure cross-section have been studied by Raman back scattering method and the changes in GaP LO and TO phonons were investigated. Raman shift of 13 cm⁻¹ in GaP-like LO₁ phonon was measured on beveled [100]surface for compositional change of In_xGa_1−xP layer in the range of 0<x_In<0.32. The measurements on the cleaved edge of the sample in [011] direction revealed a strong TO phonon at 366 cm⁻¹ and weak LO phonon peak at 405 cm⁻¹ in GaP substrate. By reaching the graded In_xGa_1−xP region the intensity of TO phonon decreases and appearance of considerable TO₁ phonon shift up to 350 cm⁻¹ for In content x_In=0.16 was observed. For upper graded layers with x_In from 0.16 to 0.24 the position of GaP-like TO₁ was constant and can be ascribed to relaxation of lattice mismatched thin In_xGa_1−xP graded upper layers in the structure.     

Leakage currents and dielectric breakdown of Si1−x−yGexCy thermal oxides

A detailed study of the leakage currents and dielectric wear-out of thermal oxides grown on Si_1−xGe_x, Si_1−yC_y and Si_1−x−yGe_xC_y epilayers to determine their quality and reliability for Si_1−x−yGe_xC_y MOS technology is presented. After applying electrical stress to the samples, we have determined the conduction mechanisms and the dependence of leakage currents upon epilayer composition (Ge and C content). Conduction takes place mainly via Fowler-Nordheim tunneling injection. Ge and C introduce traps in the oxide which assist injection and thus lower the effective height of the tunneling barrier. We have also monitored the oxide reliability, focusing on time-dependent dielectric breakdown (TDDB). The nature of trapped charge in the oxide depends on the initial epilayer composition. We have found that the formation of defects induced by the presence of C leads to extrinsic oxide failure. While the presence of Ge in the oxide does not seem to introduce significant differences with respect to Si breakdown statistics, C in the oxide truly modifies the statistical profile.     

Optical properties of InAs<Subscript>1−x</Subscript>N<Subscript>x</Subscript>/In<Subscript>0.53</Subscript>Ga<Subscript>0.47</Subscript>As single quantum wells grown by gas source molecular beam epitaxy

Optical properties of InAs_1−xN_x/In_0.53Ga_0.47As (hereafter, abbreviated as InAsN/InGaAs) single quantum wells (SQWs) grown on InP substrates by gas source molecular-beam epitaxy are studied using photoluminescence (PL) measurements. By comparing the low-temperature PL spectra of InAs/InGaAs and InAsN/InGaAs SQWs, InAs and InAsN phases are found to coexist in the InAsN layer. Such serious alloy inhomogeneities result in obvious exciton localization by potential irregularities. The blue shift of the PL peak after rapid thermal annealing (RTA) is found to originate mainly from As-N interdiffusion inside the well layer. According to the temperature-dependent PL results, uniformity of the InAsN layer can be effectively improved by RTA, and the exciton localization is, thus, relieved. Comparison of luminescence quenching and excitation-power-dependent PL behavior between the QWs with and without nitrogen content suggests that the quality of the QW is degraded by the introduction of nitrogen, and the degradation can only be partially recovered by post-growth RTA.     

High-k TixSi1−xO2 thin films prepared by co-sputtering method

We report on high-k Ti_xSi_1−xO₂ thin films prepared by RF magnetron co-sputtering using TiO₂ and SiO₂ targets at room temperature. The Ti_xSi_1−xO₂ thin films exhibited an amorphous structure with nanocrystalline grains of 3-30 nm having no interfacial layers. The XPS analyses indicate that stoichiometric TiO₂ phases in the Ti_xSi_1−xO₂ films increased due to stronger Ti-O bond with increasing TiO₂ RF powers. In addition, the electrical properties of the Ti_xSi_1−xO₂ films became better with increasing TiO₂ RF powers, from which the maximum value of the dielectric constant was estimated to be ∼30 for the samples with TiO₂ RF powers of 200 and 250 W. The transmittance of the Ti_xSi_1−xO₂ films was above 95% with optical bandgap energies of 4.1-4.2 eV. These results demonstrate a potential that the Ti_xSi_1−xO₂ thin films were applied to a high-k gate dielectric in transparent thin film transistors as well as metal-oxide-semiconductor field-effect transistors.     

Unterminated 4H-SiC Schottky barrier diodes with novel HfNxBy electrodes

4H-SiC Schottky barrier diodes (SBDs) were fabricated and characterized from room temperature to 573 K using HfN_xB_y as Schottky electrodes. The results are compared to SBDs fabricated using other electrodes that include Ni, Pt, Ti and Au. The Schottky barrier height Φ_b for as-deposited HfN_xB_y/n−/n+ diodes, determined from room temperature current-voltage characteristics, is 0.887 eV. This is lower than those of SBDs fabricated using other metals such as Au, where Φ_b is 1.79 eV. The HfN_xB_y/n−/n+ diodes studied have a much higher on-resistance R_on of around 38.24 mΩ-cm², which is about four times that of Au/n−/n+ diodes, due to the higher sheet resistance of the sputtered HfN_xB_y electrode layer. The barrier height Φ_b and ideality factor η of the HfN_xB_y/n−/n+ diodes remain almost unchanged after 400 and 750 °C anneal in N₂. This suggests excellent thermal and chemical stability of HfN_xB_y in contact with 4H-SiC.     

Dispersion of the refractive index in Tl1?xCuxGaSe2 (0 ≤ x ≤ 0.02) and Tl1?xCuxInS2 (0 ≤ x ≤ 0.015) crystals

The dispersion of the refractive index in Tl_1−x Cu_xGaSe₂ (0 ≤ x ≤ 0.02) and Tl_1−x Cu_xInS₂ (0 ≤ x ≤ 0.015) crystals is studied by measuring their intrinsic interference patterns. Anomalous dispersion regions are found to exist in the crystals at wavelengths slightly exceeding those corresponding to exciton peaks. It is ascertained that, by analyzing the intrinsic interference patterns, it is possible to detect excitons in these crystals at room temperature. A modulation of the interference patterns is observed and an explanation of this phenomenon is suggested. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 39, No. 7, 2005, pp. 811–813. Original Russian Text Copyright ? 2005 by Georgobiani, Matiev, Khamkhoev.     

Changes in effective work function of HfxRu1−x alloy gate electrode

Effective work function (?_m,eff) values of Hf_x Ru_1−x alloy gate electrodes on SiO₂ metal-oxide-semiconductor (MOS) capacitors were carefully examined to assess whether the ?_m,eff was determined by the crystalline structure or the composition of the Hf_xRu_1−x alloy. X-ray diffraction results indicated that the crystalline structures of Hf_xRu_1−x alloy were divided into hexagonal-Ru, cubic-HfRu or hexagonal-Hf with the increase of Hf content. The ?_m,eff values could be controlled continuously from 4.6 to 4.0 eV by changing the Hf content. The experimental ?_m,eff value showed a good agreement with theoretical results considering the compositional ratio of pure Hf and Ru. These results suggest that the ?_m,eff of Hf_xRu_1−x alloy gates on SiO₂ MOS capacitors is dominantly determined by the Hf_xRu_1−x composition rather than the crystalline structure.     

The interface between Hg1−xCdxTe and its native oxide

The results of a study of the electrical properties of the interface between Hg_1?xCd_xTe with x = 0.21 and its native oxide at 77°K are presented. The native oxide is formed by anodic oxidation and results in an interface with reproducible properties. The surface charge, the surface mobility and the effective lifetime are obtained from galvanomagnetic measurements and are related to the semiconductor bulk parameters, the oxide thickness and the annealing conditions. The surface state charge and the metal-semiconductors work function difference are obtained from the shift of the flat band voltage of metal-oxide-semiconductor (MOS) capacitor characteristics. The interface between Hg_1?xCd_xTe and its native anodic oxide is characterized by a density of fast surface states of the order of 5 × 10¹¹cm^?2 (eV^?1) near the middle of the bandgap. The density of states increases towards the band edges to the order of 10¹³cm^?2 (eV^?1). The measured flat band voltage is approximately ?0.5 V for an oxide thickness of 500 Å and for an n-type semiconductor with an electron carrier concentration in the range 1–3 × 10¹⁵cm^?3 at 77°K. The fixed oxide surface state charge is positive for both p-type and n-type semiconductors and is of the order of 6 × 10¹¹ charges per cm^?2. The surface properties, the significance and the reproducibility of the results are evaluated.     

Characterization of CMOS sub-65 nm metallic contact by laser scattering: Thermal stability of Ni(Si1−xGex)

Very efficient in particles detection, light scattering also offers fast non-invasive full-mapping wafer surface state. This sensitivity was used in the case of germano-silicide process development. As a matter of fact, we report on haze measurement performances, compared to the usual methods used to investigate thermal stability of Ni(Si_1−xGe_x), such as sheet resistance (SR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). We observed defectivity related to thermal agglomeration and Ge-segregation of Ni(Si_1−xGe_x) on strain Si_1−xGe_x (x ? 30%) by haze measurement (like SEM observations) earlier than SR measurement. Moreover, we noticed that a high Ge content affects at lower temperature the stability of Ni(Si_1−xGe_x) with a segregation phenomena.     

Excitonic spectrum of the ZnO/ZnMgO quantum wells

Excitonic spectrum of the wurtzite ZnO/Zn_{1 ? x} Mg _x O quantum wells with a width on the order of or larger than the Bohr radius of the exciton has been studied; the quantum wells have been grown by the method of molecular beam epitaxy (with plasma-assisted activation of oxygen) on substrates of sapphire (0001). Low-temperature (25 K) spectra of photoluminescence excitation (PLE) have been experimentally measured, making it possible to resolve the peaks of exciton absorption in the quantum well. The spectrum of excitons in the quantum well is theoretically determined as a result of numerical solution of the Schrödinger equation by the variational method. The value of elastic stresses in the structure (used in calculations) has been determined from theoretical simulation of measured spectra of optical reflection. A comparison of experimental data with the results of calculations makes it possible to relate the observed features in the PLE spectra to excitons, including the lower level of dimensional quantization for electrons and two first levels of holes for the A and B valence bands of the wurtzite crystal. The values of the electron and hole masses in ZnO are refined, and the value of the built-in electric field introduced by spontaneous and piezoelectric polarizations is estimated.     

Enhanced barrier height of AuIn1−xGaxAsyP1−y Schottky diodes

Barrier heights of Au Schottky diodes made on In_1?xGa_xAs_yP_1?y epitaxial layers with interfacial films have been measured. The interfacial layer was either a thin (?25 Å) natural oxide film or an oxide film grown by HNO₃ oxidation for which the thickness was much greater (300–500 Å). Room temperature forward-biased current-voltage and capacitance-voltage measurements show that enhancement of the barrier heights ranging from 0.15 to 0.32 eV, depending on composition, is obtained for the natural oxide films. The enhancement is much less for the HNO₃ oxide layers. The best value of the ideality factor n = 1.4 and saturation current densities J_s ranging from 10^?4 to 10^?8 A cm^?2 are obtained. For the thin natural oxide interfacial films, the results can be explained reasonably with a thermionic emission model taking into account the interfacial oxide layer.     

Impact of strain on the band offsets of important III–V quantum wells: InxGa1−xN/GaN,GaAs/InxGa1−xP and InxGa1−xAs/AlGaAs

Lattice strain has immense effect on the optoelectronic properties of III–V semiconductor quantum wells (QWs), since it introduces a pronounced change on the band properties of QWs and it is often purposefully introduced to improve device performance. In this paper we report the results of our experimental and theoretical studies on, how the important parameter, the band offset, changes with strain for In_xGa_1−xN/GaN, GaAs/In_xGa_1−xP and In_xGa_1−xAs/AlGaAs QWs. Experimentally the band offsets have been studied through capacitance transient measurements in the form of deep level transient spectroscopy (DLTS) on suitable QWs within a Schottky diode. The energy levels in a QW are considered to be analogous to a deep trap in the forbidden energy gap. From detailed balance between the emission and capture, Arrhenius type expressions were derived to analyze transient emission data, from which the band offsets were computed. Theoretically the band positions at the heterointerfaces have been calculated from the equations developed, which directly correlate the position of the bands with the strain at the interface. The strain is calculated from the In mole fractions and lattice constants. The parameters implicitly involved are the elastic stiffness constants (C₁₁ and C₁₂), the hydrostatic deformation potential of the conduction band (a′), the hydrostatic deformation potential (a) and the shear deformation potential (b) for the valance band. The results should be useful to research workers in the field of optoelectronics.