Increasing Doping Level Research Articles

A Quasi-Classical Model of the Hubbard Gap in Lightly Compensated Semiconductors

A quasi-classical method for calculating the narrowing of the Hubbard gap between the A 0 and A + acceptor bands in a hole semiconductor or the D 0 and D – donor bands in an electron semiconductor is suggested. This narrowing gives rise to the phenomenon of a semiconductor transition from the insulator to metal state with an increase in doping level. The major (doping) impurity can be in one of three charge states (–1, 0, or +1), while the compensating impurity can be in states (+1) or (–1). The impurity distribution over the crystal is assumed to be random and the width of Hubbard bands (levels), to be much smaller than the gap between them. It is shown that narrowing of the Hubbard gap is due to the formation of electrically neutral acceptor (donor) states of the quasicontinuous band of allowed energies for holes (electrons) from excited states. This quasicontinuous band merges with the top of the valence band (v band) for acceptors or with the bottom of the conduction band (c band) for donors. In other words, the top of the v band for a p-type semiconductor or the bottom of the c band for an n-type semiconductor is shifted into the band gap. The value of this shift is determined by the maximum radius of the Bohr orbit of the excited state of an electrically neutral major impurity atom, which is no larger than half the average distance between nearest impurity atoms. As a result of the increasing dopant concentration, the both Hubbard energy levels become shallower and the gap between them narrows. Analytical formulas are derived to describe the thermally activated hopping transition of holes (electrons) between Hubbard bands. The calculated gap narrowing with increasing doping level, which manifests itself in a reduction in the activation energy e2 is consistent with available experimental data for lightly compensated p-Si crystals doped with boron and n-Ge crystals doped with antimony.

Effect of size reduction on the magnetic and antibacterial properties of ZnO:Zr:Mn nanoparticles synthesized by a cost-effective chemical method

Zr and Zr + Mn doped ZnO nanopowders were synthesized using a simple soft chemical route keeping the Zr concentration in the precursor solution constant (3 at.%) and varying the Mn concentration (0, 1.5 and 9 at.%). From the X-ray diffraction studies, the crystallite size of ZnO:Zr nanopowder is found as 57 nm. The crystallite size decreases with the increase in the Mn doping level and attains a minimum of 23 nm at the maximum doping level examined in the present work (9 at.%). The Zr doped ZnO nanopowders found to have ferromagnetic property. The saturation magnetization increases with the increase in the Mn doping level and attains a maximum value at 7 at.% of Mn doping. The material shows superparamagnetic behavior at the doping level of 9 at.%. The antibacterial efficiency is found to be maximum at 9 at.% of Mn doping and at this doping level the grain size is minimum (20 nm) as evidenced from the SEM and TEM results.

High-Resistivity Semi-insulating AlSb on GaAs Substrates Grown by Molecular Beam Epitaxy

Thin-film structures containing AlSb were grown using solid-source molecular beam epitaxy and characterized for material quality, carrier transport optimization, and room-temperature radiation detection response. Few surface defects were observed, including screw dislocations resulting from shear strain between lattice-mismatched layers. Strain was also indicated by broadening of the AlSb peak in x-ray diffraction measurements. Threading dislocations and interfacial misfit dislocations were seen with transmission electron microscopy imaging. Doping of the AlSb layer was introduced during growth using GaTe and Be to determine the effect on Hall transport properties. Hall mobility and resistivity were largest for undoped AlSb samples, at 3000 cm2/V s and 106 Ω cm, respectively, and increased doping levels progressively degraded these values. To test for radiation response, p-type/intrinsic/n-type (PIN) diode structures were grown using undoped AlSb on n-GaAs substrates, with p-GaSb cap layers to protect the AlSb from oxidation. Alpha-particle radiation detection was achieved and spectra were produced for 241Am, 252Cf, and 239Pu sources. Reducing the detector surface area increased the pulse height observed, as expected based on voltage–capacitance relationships for diodes.

Microscopic signature of insulator-to-metal transition in highly doped semicrystalline conducting polymers in ionic-liquid-gated transistors

Electronic state of charge carriers, in particular, in highly doped regions, in thin-film transistors of a semicrystalline conducting polymer poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene), has been studied by using field-induced electron spin resonance (ESR) spectroscopy. By adopting an ionic-liquid gate insulator, a gate-controlled reversible electrochemical hole-doping of the polymer backbone is achieved, as confirmed from the change of the optical absorption spectra. The edge-on molecular orientation in the pristine film is maintained even after the electrochemical doping, which is clarified from the angular dependence of the g value. As the doping level increases, spin 1/2 polarons transform into spinless bipolarons, which is demonstrated from the spin-charge relation showing a spin concentration peak around 1%, contrasting to the monotonic increase in the charge concentration. At high doping levels, a drastic change in the linewidth anisotropy due to the generation of conduction electrons is observed, indicating the onset of metallic state, which is also supported by the temperature dependence of the spin susceptibility and the ESR linewidth. Our results suggest that semicrystalline conducting polymers become metallic with retaining their molecular orientational order, when appropriate doping methods are chosen.

Hopping conduction range of heavily Al-doped 4H-SiC thick epilayers grown by CVD

To outline the hopping conduction range, the electrical characteristics of CVD-grown heavily Al-doped 4H-SiC thick epilayers (2.0 × 1019–4.0 × 1020 cm−3) were investigated in a wide temperature regime (20–900 K). It is found that, below 100 K, hopping conduction dominates the carrier transport for all epilayers, and the corresponding hopping conduction activation energy shows a maximum of ∼30 meV at around 1.1 × 1020 cm−3. With increasing doping level, the temperature dependence of resistivity evolves and finally obeys the ∼1/T1/4 law in the entire temperature regime, which gives direct evidence of variable-range hopping conduction.

Realizing near stoichiometric and highly transparent CdS:Mo thin films by a low‐cost improved SILAR technique

Undoped and molybdenum doped CdS thin films were deposited on glass substrates using Improved Successive Ionic Layer Adsorption and Reaction (ISILAR) technique. The Mo doping level was varied from 0 to 15 at.% in steps of 5 at.%. The XRD analysis shows that all the films are polycrystalline with cubic structure and grow preferentially along the (111) plane. The crystallite size increases gradually with the increase in Mo doping level up to 10 at.% and decreases with further doping. The morphological studies reveal that Mo doping significantly affects the grains size. Qualitative and quantitative compositional analysis show that near stoichiometric undoped and Mo doped CdS thin films can be achieved using this ISILAR technique. All the films exhibit high transparency in the visible region with an average transmittance in the range of 85–95%.

Minority carrier lifetime in iodine-doped molecular beam epitaxy-grown HgCdTe

The minority carrier lifetime in molecular beam epitaxy grown layers of iodine-doped Hg1−xCdxTe (x ∼ 0.3) on CdZnTe substrates has been studied. The samples demonstrated extrinsic donor behavior for carrier concentrations in the range from 2 × 1016 cm−3 to 6 × 1017 cm−3 without any post-growth annealing. At a temperature of 77 K, the electron mobility was found to vary from 104 cm2/V s to 7 × 103 cm2/V s and minority carrier lifetime from 1.6 μs to 790 ns, respectively, as the carrier concentration was increased from 2 × 1016 cm−3 to 6 × 1017 cm−3. The diffusion of iodine is much lower than that of indium and hence a better alternative in heterostructures such as nBn devices. The influence of carrier concentration and temperature on the minority carrier lifetime was studied in order to characterize the carrier recombination mechanisms. Measured lifetimes were also analyzed and compared with the theoretical models of the various recombination processes occurring in these materials, indicating that Auger-1 recombination was predominant at higher doping levels. An increase in deep-level generation-recombination centers was observed with increasing doping level, which suggests that the increase in deep-level trap density is associated with the incorporation of higher concentrations of iodine into the HgCdTe.

Effect of yttrium-doping on the microstructures and semiconductor-metal phase transition characteristics of polycrystalline VO2 thin films

We investigate the effect of yttrium-doping on the microstructures and semiconductor-metal phase transition characteristics of polycrystalline VO2 thin films prepared by reactively co-sputtering process. XPS analyses indicate the existence of Y3+ in the Y-doped VO2 films, but Y-doping hardly influences the chemical states of V and O elements. X-ray diffraction patterns and Raman spectra reveal that both undoped and Y-doped VO2 thin films have a polycrystalline structure of monoclinic VO2. The introduction of Y greatly reduces the grain size of VO2 thin films as evidenced by scanning electron microscopy analyses. The relationship between the hysteresis width and doping level is not monotonic although the grain size is monotonically reduced with increasing the doping level. Y-doped VO2 films with optimal doping level (1.82at%) have a notably narrower hysteresis width (4.6°C) than undoped VO2 films (10.7°C). This is ascribed to increased heterogeneous nucleation centers due to Y in the VO2 lattice. With the further increase of doping level, the size effect gradually plays a prominent role in SMPT, and the hysteresis width of Y-doped VO2 films increases instead. The SMPT temperature of Y-doped VO2 films obviously decreases compared with undoped VO2 films due to reduced grain size and deformation of local structure around Y atom.

Effect of V doping on magnetic and electrical transport properties of La0.8Sr0.2Co1−xVxO3

The effect of V-doping on the magnetic and electrical transport properties for La0.8Sr0.2Co1−xVxO3 (x=0.00, 0.02, 0.04, and 0.06) has been investigated through X-ray diffraction, magnetization and resistivity measurements. The ferromagnetic order is dramatically suppressed by the V substitution. The magnetic results suggest that the Co3+ and Co4+ ions are both in the intermediate-spin state, indicating that a strong-coupling double exchange between Co3+ and Co4+ intermediate-spin state via t2g-orbitals is the driving force for the ferromagnetic order in the present compound. The resistivity increases as the V doping level increases, in agreement with the magnetic results. Moreover, the transport mechanism at the low temperature is dominated by the variable-range-hopping conduction.

Temperature admittance spectroscopy of boron doped chemical vapor deposition diamond

Precision admittance spectroscopy measurements over wide temperature and frequency ranges were carried out for chemical vapor deposition epitaxial diamond samples doped with various concentrations of boron. It was found that the experimentally detected boron activation energy in the samples decreased from 314 meV down to 101 meV with an increase of B/C ratio from 600 to 18000 ppm in the gas reactants. For the heavily doped samples, a transition from thermally activated valence band conduction to hopping within the impurity band (with apparent activation energy 20 meV) was detected at temperatures 120–150 K. Numerical simulation was used to estimate the impurity DOS broadening. Accurate determination of continuously altering activation energy, which takes place during the transformation of conduction mechanisms, was proposed by numerical differentiation of the Arrhenius plot. With increase of boron doping level the gradual decreasing of capture cross section from 3 × 10−13 down to 2 × 10−17 cm2 was noticed. Moreover, for the hopping conduction the capture cross section becomes 4 orders of magnitude less (∼2 × 10−20 cm2). At T > Troom in doped samples the birth of the second conductance peak was observed. We attribute it to a defect, related to the boron doping of the material.

Direct band gap light emission and detection at room temperature in bulk germanium diodes with HfGe/Ge/TiN structure

Direct band gap (DBG) electroluminescence (EL) and photo detection were studied at room temperature for n-type bulk germanium (Ge) diodes with a fin type lateral HfGe/Ge/TiN structure. DBG EL spectra peaked at 1.55μm were clearly observed due to small hole and electron barrier heights of HfGe/Ge and TiN/Ge contacts. DBG EL peak intensity increased with increasing doping level of Ge substrate due to increased electron population in direct conduction band. The integrated intensity of DBG EL spectrum is proportional to the area of active region, implying a good surface-uniformity of EL efficiency. Small dark current intensity was measured as 2.4×10−7A under a reverse bias voltage of −1V, corresponding to dark current densities of 5.3×10−10A/μm or 3.2×10−10A/μm2. At the wavelength of 1.55μm, a linear dependence of photo current intensity on laser power was observed with a responsivity of 0.44A/W at a reverse bias voltage of −1V.

Convergence of multi-valley bands as the electronic origin of high thermoelectric performance in CoSb3 skutterudites.

Filled skutterudites R(x)Co4Sb12 are excellent n-type thermoelectric materials owing to their high electronic mobility and high effective mass, combined with low thermal conductivity associated with the addition of filler atoms into the void site. The favourable electronic band structure in n-type CoSb3 is typically attributed to threefold degeneracy at the conduction band minimum accompanied by linear band behaviour at higher carrier concentrations, which is thought to be related to the increase in effective mass as the doping level increases. Using combined experimental and computational studies, we show instead that a secondary conduction band with 12 conducting carrier pockets (which converges with the primary band at high temperatures) is responsible for the extraordinary thermoelectric performance of n-type CoSb3 skutterudites. A theoretical explanation is also provided as to why the linear (or Kane-type) band feature is not beneficial for thermoelectrics.

Novel nanocomposites based on polythiophene and zirconium dioxide

This paper reports charge transport mechanism of polythiophene (PT) and its nanocomposites having various weight of zirconium dioxide (ZrO2). Fourier transform infrared (FT-IR) spectral studies showed that the Zr ions mostly affected the CS bond of the polythiophene. X-ray diffraction (XRD) patterns indicated that ZrO2 powder was a mixture of monoclinic (64.62%) and tetragonal (35.38%) phase. XRD measurements revealed that crystallinity of PT decreased with increasing doping level. Significant morphological differences were observed in scanning electron microscopy (SEM) images of PT depending on the doping process. Thermal analysis carried out by differential scanning calorimetry (DSC) indicated that the doping processes increased glass transition temperature of PT. The alternating current (ac) conductivity mechanism analysis by universal power law indicated that charge transport mechanism of both PT and PT/ZrO2 nanocomposites was consistent with correlated barrier hopping (CBH) model. The doping process and frequency affected the hopping distance between the sites.

Enhancement of the Hackee's quality factor of sol–gel spin coated ZnO thin films by MO doping

Undoped and molybdenum doped zinc oxide (MZO) thin films were deposited onto glass substrates by sol–gel spin coating method. The Mo doping level was varied from 1 to 4at% in steps of 1at%. The XRD analysis showed that all the films are polycrystalline with hexagonal wurtzite structure and preferentially oriented along the (002) plane. The optical studies showed that all the films are highly transparent in the visible region (~85%). The film with 2at% Mo doping exhibits the best Hackee's quality factor (9.0×10−2(Ω/ϒ)−1). The optical band gap energy is found to increase from 3.23eV to 3.39eV with the increase in Mo doping level. Electrical resistivity of the film is found to be minimum (5.52×10−3Ωcm) when the Mo doping level is 2at%. The surface morphological study clearly shows that the molybdenum doping induces well defined nano fibers network structure on the surface of the films.

Inserting Tin or Antimony Atoms into Mg2Si: Effect on the Electronic and Thermoelectric Properties

Density functional and Boltzmann transport theories have been used to investigate the effect of constraints generated by substituting tin for silicon atoms or by inserting antimony atoms into Mg2Si on the electronic and thermoelectric properties of this compound. The investigated hypothetical structures are Mg2Si1-x Sn (x) with x equal to 0.125, 0.25, 0.375, 0.625, 0.75, and 0.875, and Mg8Si4Sb, Mg8Si4Sb3, and Mg2SiSb. The transport properties are presented with respect to the energy at three predefined temperatures and with respect to temperature for low and high electron and hole dopings. The effects of Sn-for-Si substitution are very similar to those observed for Mg2Si subjected to uniaxial and biaxial tensile strains. Overall, the power factor decreases as the doping level or tensile strain increases. In contrast, the maximum of the power factor increases with temperature. Irrespective of the temperature and electron or hole doping levels, the electrical conductivity of the Sb-inserted Mg2Si structures is far higher than that of Mg2Si. In the Fermi level energy region, the Seebeck coefficient S of the Sb-inserted Mg2Si structures is lower than that of Mg2Si. For Mg8Si4Sb3 and Mg2SiSb, the opposite is observed in the region where the electron density is very small (about 2 eV below the Fermi level). As a consequence, the power factor follows the same trends as the Seebeck coefficient.

Suppression of the impurity-induced local magnetism by the opening of a spin pseudogap in Ni-dopedSr2CuO3

The $S=1/2$ antiferromagnetic Heisenberg spin chain compound ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$ doped with $1%$ and $2%$ of Ni impurities has been studied by means of $^{63}\mathrm{Cu}$ nuclear magnetic resonance. A strong decrease of the spin-lattice relaxation rate ${T}_{1}^{\ensuremath{-}1}$ at low temperatures points toward a spin gap, while a stretching exponent $\ensuremath{\lambda}<1$ and a frequency dependence of ${T}_{1}^{\ensuremath{-}1}$ indicate that this spin gap varies spatially and should rather be characterized as a spin pseudogap. The magnitude of the spin pseudogap scales with doping level. Our results therefore evidence the finite-size character of this phenomenon. Moreover, an unusual narrowing of the low-temperature NMR lines reveals the suppression of the impurity-induced staggered paramagnetic response with increasing doping level.

Quantum efficiency and energy transfer processes in rare-earth doped borate glass for solid-state lighting

Sm3+, Eu3+ and Tb3+ doped borate glass is investigated for its potential as light converting phosphor for solid-state lighting applications. Concentration-dependent luminescence, quantum efficiency and radiative lifetimes are analysed. The luminescence quantum efficiency exceeds values of more than 80%. Cross-relaxation processes in the single-doped glasses result in luminescence quenching for Sm3+ with increasing doping level, whereas for Eu3+ and Tb3+ an increase is observed. In Sm3+/Eu3+ and Tb3+/Eu3+ double-doped glass the energy transfer processes between the rare-earth ions are investigated in detail. The colour coordinate of the Tb3+/Eu3+ double-doped glass can be shifted from the green to the red spectral range by changing the Tb3+-to-Eu3+ ratio in favour of Eu3+. In addition, double doping allows for a change in colour coordinate by using different excitation wavelengths.

Evolution of magnetic properties of CaMn1−xNbxO3 with Nb-doping

Magnetic and structural properties of Nb-doped CaMnO3 have been studied and the effect of doping with 0.02 ⩽ x ⩽ 0.1 has been investigated. Substitution of Nb5+ ion for the Mn4+ site of the parent matrix causes one-electron doping with the chemical formula , accompanied by a monotonous increase of the lattice parameters, unit-cell volume, average Mn–O bond distance and a decrease in Mn–O–Mn bond angle, with increasing x. Low temperature magnetic ground state of CaMn1−xNbxO3 has been found to be dependent on niobium doping level. The ground magnetic state evolves from mostly antiferromagnetic, with a weak ferromagnetic component for x = 0.02–0.08, to charge ordered C-type antiferromagnetic state at x = 0.1. Spontaneous magnetization increases sharply with increasing doping level, approaches a maximal value of 4.1 emu g−1 at T = 10 K for x = 0.08, and then decreases rapidly to reach a very small value of 0.2 emu g−1 for x = 0.1. Anomalous negative magnetization behavior below the magnetic transition temperature has been observed for the compound with x = 0.04 in the field cooled magnetization and remanent dc magnetization measurements. Vertical and horizontal shifts of the hysteresis loop of the field cooled sample have been observed for CaMn0.9Nb0.1O3 as possible signatures of the exchange bias effect. The effect of hydrostatic pressure on dc magnetization for the sample with x > 0.02 revealed a significant increase of the ferromagnetic phase volume under pressure, linked to progressive suppression of a negative magnetization in x = 0.04 sample.

Intrinsic space charge layers and field enhancement in ferroelectric nanojunctions

Conducting characteristics of topological defects in ferroelectric materials, such as charged domain walls, engendered a broad interest on their scientific merit and the possibility of novel applications utilizing domain engineering. At the same time, the problem of electron transport in ferroelectrics still remains full of unanswered questions and becomes yet more relevant over the growing interest in ferroelectric semiconductors and new improper ferroelectric materials. We have employed self-consistent phase-field modeling to investigate the physical properties of a local metal-ferroelectric (Pb(Zr0.2Ti0.8)O3) junction in applied electric field. We revealed an up to 10-fold local enhancement of electric field realized by large polarization gradient and over-polarization effects due to inherent non-linear dielectric properties of Pb(Zr0.2Ti0.8)O3. The effect is independent of bias polarity and maintains its strength prior, during and after ferroelectric switching. The observed field enhancement can be considered on similar grounds as increased doping level, giving rise to reduced switching bias and threshold voltages for charge injection, electrochemical and photoelectrochemical processes.

Tuning the magnetic behavior and improving the antibacterial efficiency of ZnO nanopowders through Zr+Fe doping for biomedical applications

Zr and Zr+Fe co-doped ZnO nanopowders were synthesized using a simple soft chemical method keeping the doping level of Zr as constant (3at%) and varying the doping level of Fe (1,5and 10at%) in the precursor solution. The XRD studies reveal that the synthesized nanopowders exhibit hexagonal wurtzite structure of ZnO. The crystallite size of ZnO:Zr nanopowders is found to be 63nm and then the size decreases gradually with the increase in the Fe doping level and attains a minimum of 28nm at 10at% of Fe doping level. The Zr doped ZnO nanopowders are found to have ferromagnetic property. The material exhibits paramagnetic nature up to 5at% of Fe doping level. At 10at% of Fe doping level superparamagnetic behavior is observed. The antibacterial efficiency against two different bacterial strains viz Bacillus subtilis and Escherichia coli is studied. The antibacterial efficiency is found to be the maximum at 10at% of Fe doping level.