Deep Impurity Research Articles

Statistical approach to the process of tunnel ionisation of impurity centres near the heterointerface

To date, the processes of tunnel ionisation of impurities near the interface between two different semiconductors have been comprehensively studied. The most important parameters of the contact electron states of impurities have been determined. However, the calculated expressions for these parameters have been of local nature, as applied to individual impurities. Meanwhile, it is easy to understand that a number of processes, such as the flow of charge carriers and their diffusion through a heterojunction, are clearly statistical in nature. The same applies to the processes of tunnel ionisation of shallow and/or deep impurities near the interface. A statistical approach to the calculation of the parameters of tunnel ionisation of impurities broadens the opportunities for obtaining fundamental information regarding surface electronstates.The aim of this work was to use a statistical approach to study the effect of the heterointerface on the energy spectrum of shallow and deep centres. For this purpose, the expansion of the reflected quasi-classical wave function within the complete system of spherical harmonics and the subsequent extraction of the zero harmonic amplitude (s-component) was used to estimate the minimum distance from the impurity to the heterobarrier and to specify the limitations of the applicability of the results obtained in other works. The article analyses the conditions of the quasi-classical approximation which are used to estimate the order of the value for the minimum height of the potential barrier (pit).This work (with due consideration given to the minimum distance estimate) presents averaged formulas obtained for the energy shift of the ground state and the lifetime of the quasi-stationary state depending on the distance from the heterobarrier. Some qualitatively new considerations can also be found in the article. The distribution of impurity centres near the heterobarrier is assumed to be uniform. The article discusses the role of electron transitions in causing the buffer field effect for both shallow and deep centres. The focus of the article is on the estimates of various physical parameters characterising electron transitions near the heterobarrier.

Study on electronic properties and charge transfer doping of organic molecules adsorbed g-GaN monolayer

Two-dimensional GaN has attracted much attention owing to its excellent electronic and photoelectric properties. Tuning charge transfer and electronic characteristics of g-GaN are essential to its applications in electronic and optoelectronic devices. In this work, the electronic structures and charge transfer of organic molecules adsorbed g-GaN systems tetracyanoethylene (TCNE)/g-GaN, tetracyanoquinodimethane (TCNQ)/g-GaN, and tetrathiafulvalene (TTF)/g-GaN are explored based on first-principles. The results show that the TCNE and TCNQ molecules inject additional holes into g-GaN monolayer, and p-type doping is achieved in g-GaN monolayer. While a deep impurity level is introduced to TTF/g-GaN doping system, and the TTF injects minute quantities of additional electrons into g-GaN monolayer. The charge transfer and electronic characteristics of TCNE/g-GaN, TCNQ/g-GaN, and TTF/g-GaN doping systems are sensitive to applied vertical electric field. Our work has great potential for g-GaN application in nano electronic and photoelectric devices.

ИЗУЧЕНИЕ ТЕМПЕРАТУРНЫХ ЭФФЕКТОВ В СТРУКТУРАХ КРЕМНИЯ С ПРИМЕСЯМИ ВАНАДИЯ

The current-voltage characteristics of Au-nCdS- nSi-pCdTe-Au at 300 K. It was found that the current-voltage characteristics of such structures has three segments: power-law - I∝V2.44, sublinear V∝exp(Jad), and pre-breakdown dependence - I∝V5.55. The concentrations of deep impurities responsible for the appearance of a sublinear section of the current-voltage characteristic. experimental the results are explained on the basis of theoretical ideas about the complex nature recombination processes in such materials.

Optimizing the Performance of Perovskite Nanocrystal LEDs Utilizing Cobalt Doping on a ZnO Electron Transport Layer.

Metal halide perovskite nanocrystal (PNC) light-emitting devices (LEDs) are promising in the future ultra-high-definition display applications due to their tunable bandgap and high color purity. Balanced carrier injection is indispensable for realizing highly efficient LEDs. Herein, cobalt (Co) was doped into ZnO to modulate the electron mobility of a pristine electron transport layer (ETL) and to inhibit exciton quenching at the ZnO/EML interface due to the passivation of oxygen vacancies and the reduction of electron concentration resulting from the trapping of electrons by the Co2+-induced deep impurity level. Also, the bandgap was widened due to the size confinement effect. All of those were beneficial to achieve a balanced charge injection during the operating process. Consequently, the maximum luminance increased from 867 cd m-2 for ZnO LEDs to 1858 cd m-2 for Co-doped ZnO LEDs, and there was a 70% increase of external quantum efficiency (EQE). By further inserting a polyethylenimine (PEI) layer in the Co-doped ZnO LEDs, the EQE reached 13.0%.

Solar cell fabrication from semiconducting binary tin sulfide alloy on Si substrate

The research on binary chalcogenides as photovoltaic materials offers a great opportunity to lower the manufacturing costs of solar cell devices. Thus, we choose to research tin sulfide (SnS) materials as a p-type layer in the SnS/Si heterojunction. Our research reports basic characterizations related to the annealing effect on the SnS films. The effect of substrate type was considered, where SnS films were deposited on glass and Si substrates. The Orthorhombic SnS phase is the main crystal structure of the as prepared polycrystalline SnS films. After thermal treatment, the deposited films on glass substrates maintained the same preferred orientation while deposition on Si changed the preferred orientation to the Sn2S3 phase. The intensity of observed reflections in XRD charts decreased with increasing temperature of annealing (Ta) and the treated SnS film at Ta = 673 K showed an absence of ordered structure. The indirect transition showed the best linear fit for all investigated SnS films. The reported band gap energy (Eg) values increased after annealing from 1.29 to 1.50 eV. The thermal induced electrical behavior for SnS films showed the presence of deep impurity levels. Further, we conclude from the calculated values of σoΙ that localized states-driven conduction is more reasonable in the high-temperature region. Besides, the SnS films exhibited dc conductivity in the range of 3 × 10-3–6 × 10-6 (Ω.cm)-1. Moreover, we reported the photovoltaic aspects of developed Al/n-Si/p-SnS/In solar cell devices. Data from J-V measurements indicated clear dependence on the SnS layer physical characteristics.

Series of Photovoltaic Converters Based on Semiconductors with Intrinsic Photoconductivity

One of the ways to solve multiple problems of optical diagnostics is to use photovoltaic converters based on semiconductors with intrinsic photoconductivity slightly doped with deep impurities which form several energy levels with different charge states within the semiconductor′s bandgap. Peculiarities of physical processes of recharging these levels make it possible to construct photodetectors with different functionality based on a range of simple device structures.The aim of this work is to analyze peculiarities of conversion characteristics of single-element photovoltaic converters based on semiconductors with intrinsic photoconductivity, to systematize their properties and to represent structures of photovoltaic convertors as a device structures suitable for implementation in measurement transducers of optical diagnostics systems.Based on the analysis of the characteristics of the conversion characteristics of single-element photovoltaic converters based on semiconductors with intrinsic photoconductivity and the requirements for their design, a dash series of photovoltaic converters was developed for use in the measuring transducers of optical diagnostics systems. The possibility of constructing functional measuring transducers for multiparameter measurements of optical signals is shown.

Key parameters of textured silicon solar cells of 26.6% photoconversion efficiency

A new approach to modeling the parameters of high efficiency textured silicon solar cells (SCs) has been presented. Unlike conventional optimization formalisms, our approach additionally includes such important factors as the non-radiative Auger recombination of excitons via deep impurity levels as well as electron-hole pairs recombination in the space charge region. A simple phenomenological expression offered by us earlier for the external quantum efficiency of the textured silicon solar cells with account of the photocurrent in the long-wave part of the absorption spectrum has been also used. Applying this approach, the key parameters of textured silicon SCs, namely: short-circuit current, open-circuit voltage and photoconversion efficiency, have been theoretically determined. The proposed formalism allows calculating the thickness dependence of photoconversion efficiency, which is in good agreement with the experimental results obtained for the heterojunction SCs with the record photoconversion efficiency of 26.6%. The offered approach and the results of applying this phenomenological expression for the external quantum efficiency of the photocurrent in the long-wave part of the absorption spectrum can be used to optimize the characteristics of high efficiency textured SCs based on monocrystalline silicon.

Temperature-dependent Raman and photoluminescence of β-Ga2O3 doped with shallow donors and deep acceptors impurities

Ion doping technology with precise control of doping concentration and configuration can optimize the performance of β-phase gallium oxide (β-Ga2O3) high-power electronic and optoelectronic devices. In this work, shallow-donor Si and deep-acceptor Mg impurities are doped in β-Ga2O3 separately using edge-defined film-fed growth (EFG) method. Laser scanning confocal microscopy and X-ray diffraction analyses show that the as-grown un/Si/Mg-doped β-Ga2O3 substrates have superior qualities such as smooth surface, homogenous phase, and high crystallinity. Raman spectroscopy analysis indicate that the Raman shift and full width at half-maximum (FWHM) has a linear relationship with temperature in the range of 77–297 K. The temperature-variable photoluminescence (PL) spectroscopy suggests that the Si doping introduces less damage to the β-Ga2O3 lattice structure and the Si-doped sample has higher stability at different temperatures, however, the local atomic configurations of Mg impurities can be more easily/significantly affected by temperature. The work can offer an insightful reference to applications of ion-doped β-Ga2O3 optoelectronic devices.

Effect of oxygen vacancy on electronic structure and optical spectra of SrO crystal

Intrinsic defects affect the electronic structure and optical properties of crystals, so their identification is crucial to develop device technology materials. The SrO crystal is an important basic chemical raw material which has attracted special interests as basic heterogeneous catalyst recently. In this work, we used first-principles calculation to study the effect of oxygen vacancy on the electronic structure and optical spectra of SrO crystals. In order to get more accurate results, finite-size correction scheme was employed to correct the defect formation energy and hybrid density functional was performed to reproduce the band gap. We found that V0 introduces a deep impurity level which causing a red shift of the absorption spectrum in SrO crystals. The optical absorption band of F and F+ color centers peak at 2.49eV and 3.06eV, respectively, which are consistent with the experimental results. Meanwhile, the optical emission bands calculated for F and F+ color centers in SrO crystals peak at 2.14 eV and 2.50 eV, respectively. We predict that photoluminescence bands peaks at 554 nm (2.24eV) may be attributed to the F centers and the green emission in SrO crystal (514 nm, 2.53eV) may be caused by F+ centers. Our study will benefit to the apply and design of the novel SrO crystal.

Wave processes in silicon samples with nickel impurities arising at pulsed hydrostatic pressure

The paper considers relaxation–vibrational processes in silicon samples with nickel impurities, which create deep energy levels in its forbidden zone, during pulsed hydrostatic compression. The experiments were carried out under the direct influence of hydrostatic compression. Theoretical calculations showed that an increase in the temperature of the studied samples during dynamic compression is mainly associated with a change in the temperature of the surrounding liquid. The resulting differential equation shows that at dynamic pressure (at high rates of increase in hydrostatic pressure), wave thermodynamic processes arise. It was experimentally found that in silicon samples with deep impurity levels at pulsed hydrostatic pressure, in addition to the temperature effect, vibration-wave effects also appear. We have also shown experimentally that the relaxation part of the relative change in the current strongly depends on the degree of compensation. With weak compensation, the relaxation part is almost absent. And with strong compensation, the relaxation part of the relative change in compensated samples n-Si <P, Ni> is noticeably higher than in overcompensated samples p-Si <P, Ni>.

Влияние инжекционных эффектов на электрические свойства гетеропереходов pSi--nSi1--xSnx

We studied the current-voltage characteristic of pSi--nSi1--xSnx structures in the temperature range of 293--393 K so as to find out the role of injection phenomena during electrical property formation in pSi--nSi1--xSnх heterojunctions derived from the Si1--xSnx (0 ≤ x ≤ 0.04) solid solution. We established that the current-voltage characteristic of such heterojunctions consists of two typical segments. We determined that an exponential function describes the first current-voltage characteristic segment well. In all current-voltage characteristics the exponential curve is followed by sublinear segments that do not depend on temperature. We show that the theory of injection depletion effect describes these segments well. We used the sublinear segment of the current-voltage characteristic to determine the value of the parameter a, which can be employed to calculate deep level impurity concentration leading to the appearance of the sublinear segment. We prove that the structure under investigation may be considered to be a pSi--nSi1--xSnx--n+Si1--xSnx (0 ≤ x ≤ 0.04) junction with a high-resistance nSi1--xSnx layer. The analysis results make it possible to conclude that charge carrier dissipation on both complex aggregates and nanoinclusions plays a significant role in forming electrophysical properties in the Si1--xSnx (0 ≤ x ≤ 0.04) solid solution and that epitaxial films of Si1--xSnx (0 ≤ x ≤ 0.04) solid solutions derived on silicon substrates are efficient promising materials for developing diodes operating under double injection

Understanding photoluminescence in semiconductor Bragg-reflection waveguides

Compared to traditional non-linear optical crystals, like BaB2O4, KTiOPO4 or LiNbO3, semiconductor integrated sources of photon pairs may operate at pump wavelengths much closer to the bandgap of the materials. This is also the case for Bragg-reflection waveguides (BRWs) targeting parametric down-conversion (PDC) to the telecom C-band. The large non-linear coefficient of the AlGaAs alloy and the strong confinement of the light enable extremely bright integrated photon pair sources. However, under certain circumstances, a significant amount of detrimental broadband photoluminescence has been observed in BRWs. We show that this is mainly a result of linear absorption near the core and subsequent radiative recombination of electron–hole pairs at deep impurity levels in the semiconductor. For PDC with BRWs, we conclude that devices operating near the long wavelength end of the S-band or the short C-band require temporal filtering shorter than 1 ns. We predict that shifting the operating wavelengths to the L-band reduces the amount of photoluminescence by 70% and making small adjustments in the material composition results in its total reduction of 90%. Such measures enable us to increase the average pump power and/or the repetition rate, which makes integrated photon pair sources with on-chip multi-gigahertz pair rates feasible for future devices.

First-principles study of electronic structure and optical properties of monolayer defective tellurene

Monolayer tellurene is a novel two-dimensional semiconductor with excellent intrinsic properties. It is helpful in understanding doping and scattering mechanism to study the electronic structure of defective tellurene, thus it is important for the application of tellurene in electronic and photo-electronic devices. Using first-principles calculation based on the density functional theory, we investigate the effects of commonly seen point defects on the electronic structure and optical properties of monolayer &lt;i&gt;β&lt;/i&gt;-Te. Seven kinds of point defects that may be present in &lt;i&gt;β&lt;/i&gt;-Te are designed according to the lattice symmetry, including two single vacancies (SV-1, SV-2), two double vacancies (DV-1, DV-2) and three Stone-Wales (SW) defects (SW-1, SW-2, SW-3). It is found that the defect formation energies of these defects are 0.83–2.06 eV, which are lower than that in graphene, silicene, phosphorene and arsenene, suggesting that they are easy to introduce into monolayer &lt;i&gt;β&lt;/i&gt;-Te. The two most stable defects are SV-2 and SW-1 where no dangling bond emerges after optimization. The calculated band structures show that all seven defects have little effect on the band gap width of monolayer &lt;i&gt;β&lt;/i&gt;-Te, but they can introduce different numbers of impurity energy levels into the forbidden band. Among them, the SV-1, SV-2, DV-1 and SW-2 each act as deep level impurities which can be recombination centers and scattering centers of carriers, SW-1 acts as a shallow level impurity, DV-2 and SW-3 act as both deep level impurity and shallow level impurity. Besides, SW-1, SW-2 and DV-1 can change the band gap of monolayer &lt;i&gt;β&lt;/i&gt;-Te from direct band gap to indirect band gap, which may result in the increase of the lifetime of carriers and decrease of photoluminescence of monolayer &lt;i&gt;β&lt;/i&gt;-Te. The optical properties of monolayer &lt;i&gt;β&lt;/i&gt;-Te, which are sensitive to the change in band structure, are also affected by the presence of defects. New peaks are found in the complex dielectric function and the absorption coefficient of defective monolayer &lt;i&gt;β&lt;/i&gt;-Te in an energy range of 0–3 eV, of which the number and the position are dependent on the type of defect. The SV-1, DV-1, DV-2 and SW-2 can enhance the light response, polarization ability and light absorption in the low energy region of monolayer &lt;i&gt;β&lt;/i&gt;-Te. This research can provide useful guidance for the applications of &lt;i&gt;β&lt;/i&gt;-Te in the electronic and optoelectronic devices.

ГЛУБОКИЕ УРОВНИ В P-N-СТРУКТУРАХ НА ОСНОВЕ GaAsP

Research of deep levels in p-n-structures based on a GaAsP solid solution has been done. It is shown that a significant spread in the efficiency of the studied LEDs is due to the difference in the composition of GaAsP in the active region. It has been determined that the main parameter that determines the efficiency of a light-emitting diode is its internal quantum efficiency, which is associated with the probability of radiative and nonradiative transitions. It is connected with deep levels caused by various types of defects. The ionization energies of deep levels in the depletion layer of p-n-junctions are determined using the method of thermally stimulated currents (TSC). It is shown that the presence of such levels can lead to a significant increase in the series resistance of the LED, which in turn will lead to a decrease in efficiency and additional heating of the diode. The structure and parameters the samples that investigated are described. Information about distribution of impurities in p-n-junctions of light-emitting diodes based on GaAsP was obtained by measuring the capacitance-voltage characteristics. Device that used to determine the TSС is described. The results of calculations of deep impurity levels and their concentration from the TSC curves for various LEDs based on GaAsP are presented

Transport and recombination properties of group-III doped SiCNTs

Silicon carbide nanotubes (SiCNTs) have attracted extensive scientific and commercial interest due to their excellent properties. Based on the first principles, the lattice and energy band structure of group III element-doped SiCNTs are studied, it is found that when the electronegativity of the doped atoms is less than that of the surrounding atoms, except for an acceptor energy level near the top of the valence band, a deep impurity level is also produced near the conduction band. Numerical simulation results show that the substitution of silicon is beneficial to improve the transport performance of SiCNTs, while the substitution of C atoms is more beneficial to the improvement of recombination performance. Further analysis showed that the dominant role in the transport process of doped SiCNTs is the optical phonon scattering mechanism. The increase in non-equilibrium minority carrier lifetime when C is substituted is due to the large acceptor ionization energy, which reduces the trapping rate of holes. This will improve the photon excitation and radiation performance of doped SiCNTs. • In SiCNTs, a shallow level is introduced when Si are replaced, and the deep levels are introduced when C are replaced. • Conductivity mainly depends on the hole concentration, and phonon scattering plays a leading role in the migration mechanism. • When C was replaced, the hole capture rate decrease and electron lifetime increase with the increase of ionization energy. • The substitution of silicon is beneficial to improve the transport performance of SiCNTs. • The substitution of C atoms is more beneficial to the improvement of recombination performance.

Energy Spectrum of Electrons of Deep Impurity Centers in Wide-Bandgap Mesoscopic Semiconductors

The energy spectrum of deep impurity centers in wide-bandgap semiconductors (Eg > 2 eV) of mesoscopic sizes R ⪢ λD, where λD is the de Broglie wavelength, at which the spectrum of free (uncoupled) charge carriers is not quantized, but the surface significantly affects physical processes in the bulk, has been theoretically considered. It has been shown that the binding energy of an electron on an impurity center near the surface of the crystal tends to zero. In this case, the wavefunction of the electron of the impurity center located in the surface region is delocalized; i.e., the energy of the impurity electron lies in the conduction band. The possible effect of such an energy overlap on effects observed in wide-bandgap mesoscopic semiconductors is discussed.

A systematic comparison of polar and semipolar Si-doped AlGaN alloys with high AlN content

With a view to supporting the development of ultra-violet light-emitting diodes and related devices, the compositional, emission and morphology properties of Si-doped n-type Al x Ga1-x N alloys are extensively compared. This study has been designed to determine how the different Al x Ga1-x N crystal orientations (polar (0001) and semipolar (11–22)) affect group-III composition and Si incorporation. Wavelength dispersive x-ray (WDX) spectroscopy was used to determine the AlN mole fraction (x ≈ 0.57–0.85) and dopant concentration (3 × 1018–1 × 1019 cm−3) in various series of Al x Ga1-x N layers grown on (0001) and (11–22) AlN/sapphire templates by metalorganic chemical vapor deposition. The polar samples exhibit hexagonal surface features with Ga-rich boundaries confirmed by WDX mapping. Surface morphology was examined by atomic force microscopy for samples grown with different disilane flow rates and the semipolar samples were shown to have smoother surfaces than their polar counterparts, with an approximate 15% reduction in roughness. Optical characterization using cathodoluminescence (CL) spectroscopy allowed analysis of near-band edge emission in the range 4.0–5.4 eV as well as various deep impurity transition peaks in the range 2.7–4.8 eV. The combination of spatially-resolved characterization techniques, including CL and WDX, has provided detailed information on how the crystal growth direction affects the alloy and dopant concentrations.

Electron and Hole Scattering by Deep Impurities in Semiconductor Heterostructures with Quantum Wells

Electron and Hole Scattering by Deep Impurities in Semiconductor Heterostructures with Quantum Wells

Effect of intrinsic point defects on ZnO electronic structure and absorption spectra

The effect of intrinsic point defects on the electronic structure and absorption spectra of ZnO was investigated by first-principle calculation. Among the intrinsic point defects in ZnO, oxygen vacancies [Formula: see text] and interstitial zinc [Formula: see text] have the lower formation energy and the more stable structure under zinc(Zn)-rich condition, whereas zinc vacancies [Formula: see text] and interstitial oxygen [Formula: see text] have the lower formation energy and the more stable structure under oxygen(O)-rich condition. The band gap of [Formula: see text] becomes narrow and the absorption spectrum has a redshift. In the visible region, the photo-excited electron transition of [Formula: see text] is graded from the valence band top to the impurity level and then to the conduction band bottom, showing the redshift of absorption spectrum of [Formula: see text] and explaining the reason of [Formula: see text] forming a deep impurity levels in ZnO. Moreover, the impurity energy level of [Formula: see text] coincides with the Fermi level, indicating the significant trap effect and the slow recombination of electrons and holes, which are conducive to the design and preparation of novel ZnO photocatalysts. The band gap of [Formula: see text] and [Formula: see text] broadened and the absorption spectrum showed blueshift, explaining the different values of the ZnO band gap width.

Electronic and optical properties of mono and co-doped anatase TiO2: First principles calculations

Density functional theory-based calculations are adopted for clarifying the validity of mono-dopant (Co, Cr, B), co-dopant (Co/B, Cr/B), and oxygen deficiency on the photocatalytic activity of anatase TiO2. The crystal structure, formation energies, electronic, and optical properties are calculated in the framework of HSE06 hybrid functional. The impurity states appear in the band gap region, thus changing the electronic structures and resulting in red shift of the absorption edge in the defected systems. Furthermore, the main band gap is narrowed for all defected systems except for Co mono-doping and oxygen vacancy. The deep impurity states on mono-doping, could affect the recombination of photo-excited electron-hole pairs, while the charge separation efficiency could be enhanced on Cr/B co-doping by shallow receptor state. The calculated optical properties emphasize the enhanced absorption efficiency and the extended absorption edge to the visible light on Cr/B co-doping. The findings in this paper could suggest the Cr/B co-doped anatase TiO2 as a good photocatalyst with longer life of the photogenerated charge carriers.