Formation Of Deep Levels Research Articles

Photoelectrochemical properties of doped TiO2 nanowires grown by seed-assisted thermal oxidation

Titanium dioxide Nanowires (NWs) are particularly interesting because of their very high surface/volume ratio and their photocatalytic activity allows them to be used in a myriad of applications. This manuscript presents a study of nanowires grown on a conductive substrate making use of a seed-assisted thermal oxidation process. To obtain doped NWs, before the oxidation, metallic titanium was doped with Fe (or Cr) by ion implantation technology. Analyses showed good quality Rutile phase and light absorption in the visible range. Transport properties of the NWs/electrolyte junction were investigated by using linear sweep voltammetry and electrochemical impedance spectroscopy. They allowed us to measure the photovoltage and the barrier height of the junction. We also evaluated the density of hole trap states at the interface during illumination. Electrical results indicate that the formation of deep levels, induced by doping, influences the electron concentration in the TiO2 and the transport properties.Graphical abstract

Capacitive Spectroscopy of Deep Levels in Silicon with Samarium Impurity

The effect of thermal treatment on the behavior of samarium atoms introduced into silicon during the growth process was studied using the method of transient capacitive deep-level spectroscopy (DLTS). It has been shown that various high-temperature treatments lead to the activation of samarium atoms in the bulk of n-Si and the formation of deep levels. The energy spectrum of deep levels arising during heat treatments has been determined. The dependence of the efficiency of formation of these levels in n‑Si<Sm> on the processing temperature has been studied. It was found that the higher the content of samarium atoms in the bulk of silicon at the same high-temperature treatment temperature, the higher the concentration of the deep level EC–0.39 eV. From this, we can conclude that the EC–0.39 eV level is associated with the activation of samarium atoms in the n-Si<Sm> volume.

Defect engineering toward strong photocatalysis of Nb-doped anatase TiO2: Computational predictions and experimental verifications

Understanding the roles of point defects in optical transitions is a key to the desirable engineering of photochemical materials. In this study, the origins of the significantly varying optical and photochemical properties of Nb-doped anatase TiO2 were systematically investigated, using density functional theory (DFT) calculations and experimental verifications. We found from DFT calculations that the desirable band gap reduction of anatase TiO2 by ∼0.1eV reported in many of experimental reports and the resultant improvements of photocatalytic and photovoltaic efficiencies of Nb5+-doped anatase TiO2 are due to the formation of complex (NbTi-VTi)3− as the compensator of NbTi+. Our experiments demonstrated that the O2-rich annealing, which is expected to increase the concentration of desirable (NbTi-VTi)3− complex, narrows band gap of TiO2 and strongly enhances the photocatalytic activity of Nb-doped TiO2 particle. On the contrary, pure TiO2 showed rather worse photocatalytic performances when annealed in O2-rich atmosphere, which is due to the formation of deep level by O-interstitial defect (Oi). Theoretically obtained charge effective masses could further explain the different photocatalytic activities of undoped and Nb-doped TiO2.

Growth temperature dependence of Si doping efficiency and compensating deep level defect incorporation in Al0.7Ga0.3N

The growth temperature dependence of Si doping efficiency and deep level defect formation was investigated for n-type Al0.7Ga0.3N. It was observed that dopant compensation was greatly reduced with reduced growth temperature. Deep level optical spectroscopy and lighted capacitance-voltage were used to understand the role of acceptor-like deep level defects on doping efficiency. Deep level defects were observed at 2.34 eV, 3.56 eV, and 4.74 eV below the conduction band minimum. The latter two deep levels were identified as the major compensators because the reduction in their concentrations at reduced growth temperature correlated closely with the concomitant increase in free electron concentration. Possible mechanisms for the strong growth temperature dependence of deep level formation are considered, including thermodynamically driven compensating defect formation that can arise for a semiconductor with very large band gap energy, such as Al0.7Ga0.3N.

White Light‐Emitting Diode From Sb‐Doped p‐ZnO Nanowire Arrays/n‐GaN Film

A whole interfacial transition of electrons from conduction bands of n‐type material to the acceptor levels of p‐type material makes the energy band engineering successful. It tunes intrinsic ZnO UV emission to UV‐free and warm white light‐emitting diode (W‐LED) emission with color coordinates around (0.418, 0.429) at the bias of 8–15.5 V. The W‐LED is fabricated based on antimony (Sb) doped p‐ZnO nanowire arrays/Si doped n‐GaN film heterojunction structure through one‐step chemical vapor deposition with quenching process. Element analysis shows that the doping concentration of Sb is ≈1.0%. The I–V test exhibits the formation of p‐type ZnO nanowires, and the temperature‐dependent photoluminescence measurement down to 4.65 K confirms the formation of deep levels and shallow acceptor levels after Sb‐doping. The intrinsic UV emission of ZnO at room temperature is cut off in electroluminescence emission at a bias of 4–15.5 V. The UV‐free and warm W‐LED have great potential application in green lights program, especially in eye‐protected lamp and display since television, computer, smart phone, and mobile digital equipment are widely and heavily used in modern human life, as more than 3000 h per year.

ASSOCIATES OF DOT DEFECTS OF VARIOUS NATURE IN SiC-PHASE OF SEMICONDUCTOR HETEROSTRUCTURE OF SiC//Si, RECEIVEDBY ENDOTAKSIYA METHOD

One of the main ways of increasing of reliability of sensors of physical quantities on the basis of high-temperature and radiation-hardened heterostructure of β-SiC/Si is the analysis of technological aspects of its forming (endotaksiya) regarding concentration distribution of dot defects of various nature, their probable models of association with participation of foreign impurity. Besides, the analysis of reversible processes of association opens ways of optimization of kinetics of diffusive mass transfer at phase transformation of substrate of silicon into a film of carbide of silicon. In the article dependences of concentration of neutral defects on factors of supersaturation of gas phase on conditional atomic concentration of carbon, from concentration of foreign impurity are given in a gas phase, from concentration of own defects of various nature having potential of formation of deep levels in the forbidden zone and potential of association. The analysis of the given dependences is made and recommendations about carrying out technological process of formation of difficult heterostructures of different function are made.

Atomic-force microscopy and photoluminescence of nanostructured CdTe

Low-dimensional CdTe nanorods with a diameter of 10–30 nm and a high aspect ratio that reaches 100 are studied. The nanorods are grown by the physical vapor transport method with the use of Bi precipitates on the substrates. In addition, thin films of closely packed CdTe nanorods with the transverse dimensions ∼(100–200) nm are grown. Atomic-force microscopy shows that the cross sections of all of the nanorods were hexagonally shaped. By photoluminescence measurements, the inference about the wurtzite structure of CdTe is supported, and the structural quality, electron-phonon coupling, and defects are analyzed. On the basis of recent ab initio calculations, the nature of defects responsible for the formation of deep levels in the CdTe layers and bulk crystals are analyzed.

Structural and optical investigations of Al xGa 1− xAs:Si/GaAs(1 0 0) MOCVD heterostructures

Al x Ga 1− x As:Si/GaAs(1 0 0) heterostructure and homoepitaxial GaAs:Si/GaAs(1 0 0) structures grown by MOCVD were investigated. The changes observed in our experiments with highly doped Al x Ga 1− x As alloys, led not only to the reconstruction of the electron density and formation of deep levels (DX-centers) with subsequent relaxation of the crystal lattice in the alloy, but also indicate at the formation of quaternary Al x Ga 1− x− y Si y+ z As 1− z substitution-type alloy grown on GaAs(1 0 0).

A deep-level transient spectroscopy study of Co- and Ni-germanided n-type germanium

In this paper, the impact of the rapid thermal annealing (RTA) temperature on the formation of deep levels during Ni- or Co-germanidation of n-type Ge substrates has been studied by Deep-Level Transient Spectroscopy. As is shown, metal in-diffusion is found in the case of nickel, starting from an RTA temperature ( T ann) of 400 °C, while no clear evidence of metal-related deep electron traps is found for Co up to 500 °C. This is also reflected in the reverse current of the resulting Schottky barriers: generally, an increase has been found for increasing T ann, which can be ascribed to the impact of the generation centers introduced in the Ge substrate during the germanidation. It is concluded from this that source/drain metallization for future CMOS on Ge, employing low thermal budgets will not introduce deep levels that compromise the leakage current of the junctions.

Hall effect in InAs/GaAs superlattices with quantum dots: identifying the presence of deep level defects

We have carried out van der Pauw resistivity and Hall effect measurements on a series of Molecular Beam Epitaxy InAs/GaAs superlattice samples containing InAs quantum dots. Three growth parameters were varied, the InAs coverage, the number of repetitions of the InAs/GaAs layers, and the GaAs spacer thickness. The results can be grouped in two sets, those samples presenting low and high resistivity. The group presenting low resistivity is composed by the samples with GaAs spacer of 30 monolayers (ML) and InAs coverage of 1.9 monolayers. The group presenting high resistivity is composed of samples with GaAs spacer of 40 ML. We claim that the high resistivity characteristic is due to the presence of deep level. Increasing the spacer from 30 to 40 ML decouples the InAs planes favouring the deep level formation.

Engineering on tunnel barrier and dot surface in Si nanocrystal memories

Nanocrystal memory is more scalable and operates under lower voltage than conventional floating gate memory thus it is currently a strong candidate for the next generation nonvolatile memory. However, those merits also raise critical problems on data retention and disturbance problems. Both problems are related to the sensitivity of the tunnel barrier on the applied field. When the field-sensitivity of the tunnel barrier becomes larger, both problems can be solved. One method is to increase the field-sensitivity of the tunnel barrier without increasing the operation voltage, which is profiling the band structure of the tunnel barrier. For example, barrier structure with a high bandgap sandwiched by two small bandgap dielectrics is a feasible one. We tried Si 3N 4/SiO 2/Si 3N 4 barrier structure and observed the enhancement of the field sensitivity. In the experiment on the discharging kinetics, we could confirm the model of deep level charge storage. Moreover, it is related to the material that interfaces Si nanocrystals. The depth of energy level is higher when it is Si 3N 4 or amorphous carbon than when it is SiO 2. Therefore, the choice of interface material on the dot surface should be considered based on the deep level formation for retention improvement.

DLTS Studies of high-temperature electron irradiated Cz n-Si

This paper reports on a Deep Level Transient Spectroscopy (DLTS) study of both oxygen-related thermal donors (OTDs) and radiation-induced defects (RDs) formed in high-temperature 1 MeV electron irradiated n-type Czochralski silicon. The heating of Si samples is done in-situ by the electron beam. The formation of deep levels is studied on both heat-treated and irradiated at high temperature sides of samples. The interaction between OTDs and RDs is investigated. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of defects on the electrical and magnetic properties of Ga1−x MnxAs layer

ABSTRACTWe investigated the effects of V/III flux ratios on the Curie temperature, TC, in Ga1−x Mnx As layers with various Mn mole fractions of x = 0.03 and 0.05. A 75 nm thick GaMnAs layer was grown at the temperature of 250 °C with various V/III flux ratios of 25∼34. The low temperature molecular beam epitaxy (LT-MBE) method for growth of GaMnAs layer caused the defects related by excess As and Mn interstitial, and these leaded the formation of deep level. We investigated that formation of deep level was established with various Mn mole fraction for V/III flux ratio 34. The changes of TC are observed by varying V/III flux ratio with a fixed Mn mole fraction. The TC in the sample grown with a lower V/III flux ratio of 25 is found to be higher comparing to that with higher V/III flux ratio of 34 at a fixed high Mn concentration (x = 0.05). Although the Mn concentration increases, the TC is not much changed when the V/III flux ratio is high of 34. The changes of TC with various V/III flux ratios are explained by the existence of low temperature grown defects, which are clarified by the deep level transient spectroscopy measurement. The prime species of defects are found to be AsGa and MnI etc.

Capacitance-voltage characteristics of p-n structures based on (111)Si doped with erbium and oxygen

Capacitance-voltage characteristics of tunneling diodes fabricated by co-implantation of erbium and oxygen in single-crystal (111)Si wafers have been studied. Anomalous enhancement of the p-n-junction capacitance with increasing reverse bias has been observed at certain temperatures depending on the implantation dose. The rise in capacitance (decrease in the space charge region width) is associated with the formation of deep levels of high density in the band gap of n-layer of the p-n junction and electron emission from these levels in the space charge region with increasing voltage. The obtained results show that the parameters of the defects responsible for the levels depend on the erbium and oxygen implantation doses.

Electrical characterization of isoelectronic In-doping effects in GaN films grown by metalorganic vapor phase epitaxy

Indium isoelectronic doping was found to have profound effects on electrical properties of GaN films grown by metalorganic chemical vapor deposition. When a small amount of In atoms was introduced into the epilayer, the ideality factor of n-GaN Schottky diode was improved from 1.20 to 1.06, and its calculated saturation current could be reduced by 2 orders of magnitude as compared to that of the undoped sample. Moreover, it is interesting to note that In isodoping can effectively suppress the formation of deep levels at 0.149 and 0.601 eV below the conduction band, with the 0.149 eV trap concentration even reduced to an undetected level. Our result indicates that the isoelectronic In-doping technique is a viable way to improve the GaN film quality.

Deep Level Formation in Undoped and Oxygen-Doped GaN

ABSTRACTDeep level defects in oxygen doped GaN grown by metal-organic vapor phase epitaxy were investigated. Using steady-state photocapacitance (SSPC) spectroscopy, three deep levels with optical ionization energies of 1.0, 1.4, and 3.25 eV were observed in both nominally undoped and oxygen-doped samples. The total deep level defect concentrations ranged from 6 × 1015 cm-3 in undoped films to 3 × 1016 cm-3 in oxygen-doped films. The concentration of the 3.25 eV level defect increased upon oxygen doping, while the concentrationof the 1.0 and 1.4 eV levels were essentially dopant independent. From the measured concentrations the formation energies of the defects were calculated and compared to energies calculated using density functional theory.

Effect of grown-in biaxial strain on deep level defects in Si1−yCy/Si epitaxial heterostructures

We report the first study of traps in strained Si1−yCy random alloys grown on Si 〈100〉 substrates using chemical vapor deposition. Deep level transient spectroscopy (DLTS) measurements of n-type Schottky diodes fabricated on Si/Si1−yCy/Si heterostructures (y<0.02) were used to study the impact of carbon and grown-in strain on the formation of deep levels. The observed deep level activation energies were found to vary with carbon fraction, and ranged between 0.175 and 0.225eV, measured with respect to the conduction band. The systematic variation in the measured activation energies with carbon fraction suggests the splitting of a single deep level due to strain in the Si1−yCy alloy layer. The DLTS peaks exhibit anomalous broadening which increases with increasing carbon fraction and can be explained using an alloy broadening model.

Deep level formation and heterojunction band offsets at ZnSe/GaAs interfaces

We have used photoluminescence (PLS) to measure charge injection across the ZnSe/GaAs interface and subsequent near-band-edge (NBE) and deep level recombination as a function of interdiffusion and deep level formation. We have performed these studies for a set of heterojunction growth parameters corresponding to a range of band offsets. Our earlier results provided evidence for growth-dependent deep level emission at the ZnSe/GaAs(100) heterointerface, new deep level formation at elevated temperatures associated with atomic interdiffusion across the junctions, and major changes in recombination with the onset of defect formation. Here we present simultaneous GaAs and ZnSe PLS measurements demonstrating the first order effect of deep level formation on the internal photoemission of free carriers across the heterojunction and the dramatic difference in NBE recombination with defect formation for different stoichiometries of ZnSe growth. These features underscore the systematic role of deep levels in altering heterojunction dipoles and band offset barriers to charge injection.

Deep Level Characterization of Interface-Engineered ZnSe Layers Grown by Molecular Beam Epitaxy on GaAs

AbstractDeep level defects have been detected and analyzed in epitaxial ZnSe layers grown by molecular beam epitaxy (MBE) on GaAs and on In0.04Ga0.96As using deep level optical spectroscopy (DLOS). A series of samples, which differ only in the initial Zn:Se beam pressure ratio (BPR = 1:1, 1:10, 10:1) during the growth nucleation step, were characterized by DLOS in order to assess the dependence of bulk deep level formation on interface nucleation conditions. The transient and steady state photocapacitance measurements were performed using 100 W Quartz Halogen and 450 W Xe lamps as light sources, in the spectral range of 0.9 to 2.9 eV with a resolution better than 0.02 eV. The capacitance transients were recorded for time windows of 10 msec to 5 sec after light excitation of the sample, which was kept at a temperature of 100 K. Using semi-transparent Au Schottky contacts, several deep levels in the ZnSe layer were detected for all BPR's, with optical threshold energies of 1.1, 1.46 and 1.9 eV below the conduction band. These energies were obtained from the slope of the capacitance transient at different time intervals and were confirmed by steady state photocapacitance. The concentration of the levels was in the range 1012 to 1014 cm−3. Both the 1.1 and 1.46 eV trap concentrations were found to depend strongly on lattice mismatch conditions, whereas the latter was shown to largely depend on BPR. The optical threshold of the 1.9 eV trap correlates well with a ˜1.9 eV cathodoluminescence (CL) peak, which has been previously associated with either Zn vacancies or Gazn substitutional defects in Zn-deficient material.

Intrinsic Gettering of Manganese Impurity in Silicon Substrate

Intrinsic gettering of manganese impurity atoms has been investigated in p-type silicon by means of resistivity and minority carrier lifetime measurements and infrared absorption spectroscopy. Manganese proved to be a donor impurity in p-Si and its presence led to a reduction by a factor of about 7 in the lifetime of minority carriers by formation of deep level traps. There is strong evidence that high temperature oxygen precipitation is enhanced by the presence of the Mn impurity in the substrate. The resulting oxygen precipitate provided an efficient gettering sink for the Mn impurity.