Isovalent Impurities Research Articles

It has been found that hetero layers of typical β-ZnSe and atypical α-ZnSe modifications can be obtained by the isovalent substitution method. Isovalent impurities are formed which predetermine the formation of dominant radiation with a quantum yield of η = 12–15% in the short wavelength edge region. Low-temperature studies and λ-modulation techniques allowed us to identify the radiation components. This radiation is generated by interband recombination and exciton annihilation. The high temperature stability of the radiation was confirmed over temperature variations including 77, 300, and 480 K.

Substitutional doping, involving the replacement of a host with an aliovalent impurity ion, is widely used to attain ambipolar controllability in semiconductors, which is crucial for device application. However, its effectiveness for p-type doping is limited in monovalent cation compounds due to the lack of suitable aliovalent (i.e., zerovalent) impurities. We propose an alternative approach for p- and n-type doping, mediated by the sizes of isovalent alkali metal impurities in Cu(I)-based semiconductors, such as copper nitride with an electron concentration of ∼1015 cm-3. Doping of isovalent Li with a smaller size to interstitial positions improves n-type conductivity, and electron concentration is controllable in the range of 1015 to 1018 cm-3. In contrast, larger isovalent Cs and Rb impurities facilitate p-type conversion, resulting in a hole concentration controllability of 1014 to 1017 cm-3. First-principles calculations indicate that Li is placed as an interstitial impurity acting as a shallow donor in conjunction with the formation of a neutral impurity on Cu defects. As the impurity size increases beyond the capacity of the vacant space, the formation of multiple acceptor-type Cu vacancies is enhanced owing to the repulsion between host Cu+ and Cs+/Rb+ impurities. Consequently, the Cs or Rb impurity is located at the sites of the N accompanied by six neighboring Cu vacancies, forming acceptor defect complexes. This size-dependent isovalent impurity doping scheme opens up an alternative avenue for advancement in optoelectronic devices using monovalent cation-based semiconductors.

Carbon (C) is an important isovalent impurity in silicon (Si) that is inadvertently added in the lattice during growth. Germanium (Ge), tin (Sn), and lead (Pb) are isovalent atoms that are added in Si to improve its radiation hardness, which is important for microelectronics in space or radiation environments and near reactors or medical devices. In this work, we have employed density functional theory (DFT) calculations to study the structure and energetics of carbon substitutional-isovalent dopant substitutional CsDs (i.e., CsGes, CsSns and CsPbs) and carbon interstitial-isovalent dopant substitutional CiDs (i.e., CiGes, CiSns and CiPbs) defect pairs in Si. All these defect pairs are predicted to be bound with the larger isovalent atoms, forming stronger pairs with the carbon atoms. It is calculated that the larger the dopant, the more stable the defect pair, whereas the CsDs defects are more bound than the CiDs defects.

Ferroelectricity with out-of-plane polarization has so far been found in several two-dimensional (2D) materials, including monolayers comprising three to five planes of atoms, e.g. α-In2Se3 and MoTe2. Here, we explore the generation of out-of-plane polarization within a one-atom-thick monolayer material, namely hexagonal boron nitride. We performed density-functional-theory calculations to explore inducing ferroelectric-like distortions through incorporation of isovalent substitutional impurities that are larger than the host atoms. This disparity in bond lengths causes a buckling of the h-BN, either up or down, which amounts to a dipole with two equivalent energies and opposing orientations. We tested several impurities to explore the magnitude of the induced dipole and the switching energy barrier for dipole inversion. The effects of strain, dipole–dipole interactions, and vertical heterostructures with graphene are further explored. Our results suggest a highly-tunable system with ground state antiferroelectricity and metastable ferroelectricity. We expect that this work will help foster new ways to include functionality in layered 2D-material-based applications.

The optimal modes of isovalent substitution and the CdTe/CdS/ZnS heterostructure obtained for the first time were established, the main parameters of the band structure of the constituent heterolayers and the characteristics of the obtained radiation sources were determined. The high quantum efficiency η ≈ 12-14 % of surface ZnS is caused by isovalent impurities. The band structure parameters of the obtained isovalently substituted CdS layers of atypical cubic modification and their luminescence efficiency of η ≈ 7-8 % were established. The emission of the resulting layers is localized in the edge region of the material and is formed by interband emitting transitions and the dominant annihilation of bound excitons.

There is a contradictory assessment of the possibility of germanium (Ge) use to increase the radiation resistance of silicon (Si) homogeneously doped with an isovalent impurity. A number of publications show that only a limited effect of germanium doping on the radiation stability of the pn-structure, irradiated by high-energy electronsis observed. Simultaneously there is a noticeable improvement in the radiation resistance of npnp-structures made on SiGe under γ-irradiation. In order to remove the contradiction, this work compares the β radiation degradation of test bipolar transistor npn Integrated Curcuit (IC) structures, manufactured using the same technology, "silicon with dielectric insulation", on isovalent germanium-doped SiGe silicon with different Ge content, NGe=1,2·1019…1,2·1020 cm-3. The static gain coefficient β was measured before and after α-irradiation. Irradiation of unencased npn structures with α-particles with an energy of 4.5 MeV carried out in a specially designed and manufactured laboratory installation using radioisotope sources; npn structures with two base thicknesses: 0.25 and 0.35 μm were studied experimentally. The dependence approximating the experimental data, β(Φα), an equation, describing the change in the gain factor of the transistor structure upon α-irradiation, obtained using the OriginPRO program. Obtained results for structures with a base thickness of 0.25 μm show a strong nonlinear dependence of β(Φα) equations on NGe. The degradation of the control transistors gain, manufactured according to the standard technology (NGe = 0) is described by the S-curve. Irradiation of npn structures formed on SiGe wafers with different levels of doping with an isovalent impurity leads to a complete change of the nature of the dependence. For Φα ≤ 1011 см-2 the nature of the change in β is practically the same for structures made on wafers with NGe= 0 and NGe= 2.5·1019 сm-3, as well as for NGe=1,2·1019 сm-3 and NGe=1.2·1020сm-3. When increasing Φα≥ 1011 сm-2 there is an accelerated degradation of the gain factor of npn structures made on wafers with NGe= 2.5·1019 cm-3. This level of doping of silicon with germanium is not acceptable from the point of view of Si radiation resistance. At Φα ≤ 1014 см-2 radiation stability of npn structures made on SiGe wafers with NGe=1.2·1019 см-3 approximately two times lower than the same of control structures with NGe= 0. For transistors with a base thickness of 0.35 μm, no effect of changing the nature of the npn structures β(Φα) degradation. Observed dependence, which confirms the possibility of slowing down the radiation degradation of the amplification factor value of the npn structures made on SiGe. Increase in radiation resistance by 2-3 times for test transistors, made on SiGe wafers, doped with NGe= 7,5·1019 см-3, observed in a wide range of doses of α-irradiation, 1011≤ Φα≤ 1014 см-2.

Structural disorder and distribution of impurity atoms in β-Ga2O3 under boron ion implantation

p-Type doping in Cu(I)-based semiconductors is pivotal for solar cell photoabsorbers and hole transport materials to improve the device performance. Impurity doping is a fundamental technology to overcome the intrinsic limits of hole concentration controlled by native defects. Here, we report that alkali metal impurities are prominent p-type dopants for the Cu(I)-based cation-deficient hole conductors. When the size mismatch with Cu+ in the host lattice is increased, these isovalent impurities are preferentially located at interstitial positions to interact with the constituent Cu cations, forming stable impurity-defect complexes. We demonstrate that the Cs impurity in γ-CuI semiconductors enhances hole concentration controllability for single crystals and thin films in the range of 1013-1019 cm-3. First-principles calculations indicate that the Cs impurity forms impurity-defect complexes that act as shallow acceptors leading to the increased p-type conductivity. This isovalent doping provides an approach for controlled doping into cation-deficient semiconductors through an interaction of impurities with native defects.

Crystalline silicon (Si) is the key material of the semiconductor industry, with significant applications for electronic and microelectronic devices. The properties of Si are affected by impurities and defects introduced into the material either during growth and/or material processing. Oxygen (O) and carbon (C) are the main impurities incorporated into the crystal lattice during growth via the Czochralski method. Both impurities are electrically neutral, however, implantations/irradiations of Si lead to the formation of a variety of oxygen-related and carbon-related defects which introduce deep levels in the forbidden gap, inducing generally detrimental effects. Therefore, to control Si behavior for certain applications, it is important to have an understanding of the properties and fundamental processes related with the presence of these defects. To improve Si, isovalent doping during growth must be employed. Isovalent doping is an important defect-engineering strategy, particularly for radiation defects in Si. In the present review, we mainly focus on the impact of isovalent doping on the properties and behavior of oxygen-related and carbon-related defects in electron-irradiated Si. Recent experimental results from infrared spectroscopy (IR) measurements coupled with theoretical studies involving density functional theory (DFT) calculations, are discussed. Conclusions are reached regarding the role of isovalent doping (carbon, (C), germanium (Ge), tin (Sn), and lead (Pb)) on the suppression of detrimental effects introduced into Si from technologically harmful radiation clusters induced in the course of material processing.

The problems of developing light-emitting structures based on CdTe with an extended range of operating temperatures and radiation-resistant parameters are studied. A technique for obtaining heterostructures has been mastered, technological modes of isovalent substitution have been determined, and radiation sources with a high quantum efficiency η = 7–20% at 300 K in a wide spectral region have been obtained. The design of devices has been developed and light emitters based on CdTe, whose radiation is determined by the interband recombination of free charge carriers and the dominant annihilation of bound excitons, have been fabricated by doping with isovalent impurities Mg, Ca.

Chromium iodide monolayers, which have different magnetic properties in comparison to the bulk chromium iodide, have been shown to form skyrmionic states in applied electromagnetic fields or in Janus-layer devices. In this work, we demonstrate that spin-canted solutions can be induced into monolayer chromium iodide by select substitution of iodide atoms with isovalent impurities. Several concentrations and spatial configurations of halide substitutional defects are selected to probe the coupling between the local defect-induced geometric distortions and orientation of chromium magnetic moments. This work provides atomic-level insight into how atomically precise chemical doping can be used to create and control complex magnetic patterns in chromium iodide layers and lays out the foundation for investigating the field- and geometric-dependent magnetic properties in similar two-dimensional materials.

Multifold enhancements in thermoelectric power factor in isovalent sulfur doped bismuth antimony telluride films

Doped SrTiO3 is a superconductor whose pairing mechanism is still not fully understood. The response of a superconductor to impurities has long been used to obtain insights into the nature of the superconducting state. Here, the superconductivity of SrTiO3 films that are doped or alloyed with different rare earth ions, which carry a magnetic moment, is investigated. It is shown that large concentrations (up to a few percent) of rare earth ions with unpaired f-electrons, such as Sm and Eu, do not reduce the superconducting critical temperature and critical fields. The finding is independent of whether the rare earth ion acts as a dopant or is an isovalent impurity. The interactions between the superconducting condensate and the magnetic dopants that could result in the observed insensitivity to magnetic impurities are discussed.

Досліджено оптичне поглинання, відбивання і люмінесценцію CdTe:Ca. Встановлено, що отримані леговані Ca поверхневі шари характеризуються інтенсивною фотолюмінесценцією з η = 8-10% у крайовій області. Випромінювання формується внаслідок міжзонної рекомбінації вільних носіїв заряду і анігіляцією зв’язаних на ізовалентних домішках Ca екситонів. Зазначені складові спостерігаються у диференційних спектрах оптичного відбивання Ŕω у приповерхневому шарі отриманому при легування ізовалентною домішкою Ca підкладинок CdTe. Встановлено, що легування обумовлює утворення p-типу провідності.

This work presents general studies of the influence of doping methods on the properties of elements with anomalous photovoltaic effects and the process of doping of semi-crystalline thin CdTe films with isovalent impurities is also investigated. The process of thermal diffusion has been studied.

Using spectroscopic ellipsometry measurements on GaP1−xBix/GaP epitaxial layers up to x = 3.7% we observe a giant bowing of the direct band gap ({E}_{g}^{{rm{Gamma }}}) and valence band spin-orbit splitting energy (ΔSO). {E}_{g}^{{rm{Gamma }}} (ΔSO) is measured to decrease (increase) by approximately 200 meV (240 meV) with the incorporation of 1% Bi, corresponding to a greater than fourfold increase in ΔSO in going from GaP to GaP0.99Bi0.01. The evolution of {E}_{g}^{{rm{Gamma }}} and ΔSO with x is characterised by strong, composition-dependent bowing. We demonstrate that a simple valence band-anticrossing model, parametrised directly from atomistic supercell calculations, quantitatively describes the measured evolution of {E}_{g}^{{rm{Gamma }}} and ΔSO with x. In contrast to the well-studied GaAs1−xBix alloy, in GaP1−xBix substitutional Bi creates localised impurity states lying energetically within the GaP host matrix band gap. This leads to the emergence of an optically active band of Bi-hybridised states, accounting for the overall large bowing of {E}_{g}^{{rm{Gamma }}} and ΔSO and in particular for the giant bowing observed for x ≲ 1%. Our analysis provides insight into the action of Bi as an isovalent impurity, and constitutes the first detailed experimental and theoretical analysis of the GaP1−xBix alloy band structure.

The non-linear diffusion-deformation theory of self-organization of nanoclusters of dot defects in semiconductor exposed to ultrasound treatment that considers the interaction of defects among themselves and with atoms of a matrix via the elastic field created by dot defects and an acoustic wave is developed. Within this theory the influence of ultrasound on the conditions of formation of spherical nanoclusters and their radius is investigated. The nanocluster size depending on average concentration of defects and amplitude of an acoustic wave is determined. It is established that ultrasonic treatment of the semiconductor in the process of formation of an ensemble of nanoclusters leads to reduction of dispersion of their sizes. In the framework of this model, a possibility of the ultrasound-stimulated the size dispersion reduction of strained InAs/GaAs quantum dots doped with an isovalent impurity are analyzed.

The paper discusses the mechanisms of defect formation in melt zinc selenide crystals doped with isovalent impurities of the second group of the Periodic Table. An analytical calculation of the concentrations of equilibrium point defects was carried out by the method of quasichemical reactions using the concepts of electronegativity and effective charge. It has been established that the dominant defects in the doped material are singly charged vacancies of zinc V'Zn and selenium V·Se, as well as singly charged interstitial selenium Se'i. It is shown that the increase in doping temperature Ta from 373 to 1237 K causes se increase of acceptor centers number (V'Zn and Se'i) and decrease of concentration of donor ounces V·Se however, the conductivity of doped substrates at 300 K remains a hole in the whole range of change Ta. At Ta = 373 K the estimated concentration of free holes is ~1019 cm-3 and satisfactorily consistent with the value p determined from the temperature dependence of the layer resistance ZnSe:Ca.

The processes of the diffusion blurring of a periodic system of GaAs quantum wells separated by AlGaAs barriers are studied by photoluminescence spectroscopy. The system is grown by molecular-beam epitaxy at a low temperature (200°C) and additionally doped with Sb and P isovalent impurities. Postgrowth annealing at the temperature 750°C for 30 min induces an increase in the energy corresponding to the photoluminescence peak of the e1–hh1 exciton state in quantum wells because of blurring of the epitaxial GaAs/AlGaAs interfaces due to enhanced Al–Ga interdiffusion in the cation sublattice. For the Al concentration profile defined by linear diffusion into quantum wells, the Schrodinger equation for electrons and holes is solved. It is found that the experimentally observed energy position of the photoluminescence peak corresponds to the Al–Ga interdiffusion length 3.4 nm and to the effective diffusion coefficient 6.3 × 10–17 cm2 s–1 at the temperature 750°C. This value is found to be close to the corresponding value for GaAs quantum wells grown at low temperatures without additional doping with Sb and P impurities. From the results obtained in the study, it is possible to conclude that enhanced As–Sb and As–P interdiffusion in the anion sublattice only slightly influences the processes of Al–Ga interdiffusion in the cation sublattice.

Boron containing GaAs, which is grown by metal organic vapour phase epitaxy, is studied at the atomic level by cross-sectional scanning tunneling microscopy (X-STM) and spectroscopy (STS). In topographic X-STM images, three classes of B related features are identified, which are attributed to individual B atoms on substitutional Ga sites down to the second layer below the natural {110} cleavage planes. The X-STM contrast of B atoms below the surface reflects primarily the structural modification of the GaAs matrix by the small B atoms. However, B atoms in the cleavage plane have in contrast to conventional isovalent impurities, such as Al and In, a strong influence on the local electronic structure similar to donors or acceptors. STS measurements show that B in the GaAs {110} surfaces gives rise to a localized state short below the conduction band (CB) edge while in bulk GaAs, the B impurity state is resonant with the CB. The analysis of BxGa1–xAs/GaAs quantum wells reveals a good crystal quality and shows that the incorporation of B atoms in GaAs can be controlled along the [001] growth direction at the atomic level. Surprisingly, the formation of the first and fourth nearest neighbor B pairs, which are oriented along the 〈110〉 directions, is strongly suppressed at a B concentration of 1% while the third nearest neighbor B pairs are found more than twice as often than expected for a completely spatially random pattern.