Space Charge Doping Research Articles

Superconductor-insulator transition in space charge doped one unit cell Bi2.1Sr1.9CaCu2O8+x

The superconductor-insulator transition in two dimensions is a prototype continuous quantum phase transition at absolute zero, driven by a parameter other than temperature. Here we reveal this transition in one unit-cell Bi2.1Sr1.9CaCu2O8+x by space charge doping, a field effect electrostatic doping technique. We determine the related critical parameters and develop a reliable way to estimate doping in the nonsuperconducting region, a crucial and central problem in these materials. Finite-size scaling analysis yields a critical doping of 0.057 holes/Cu, a critical resistance of ~6.85 kΩ and a scaling exponent product νz ~ 1.57. These results, together with earlier work in other materials, provide a coherent picture of the superconductor-insulator transition and its bosonic nature in the underdoped regime of emerging superconductivity in high critical temperature superconductors.

Crossover to strange metal phase: quantum criticality in one unit cell Bi2Sr2CaCu2O

Transport measurements can be used to determine the phase diagram of high temperature superconductors by detecting variations in temperature dependence of resistance in different regions of the phase diagram. While for bulk measurements several samples with varying chemical doping are used, we continuously vary carrier density in our ultra-thin two-dimensional Bi2Sr2CaCu2O device by electrostatic means and the space charge doping method. Here we concentrate on a low-disorder, high quality single unit cell thick sample. We establish the crossover to strange metal from the pseudogap and Fermi liquid phases in the normal state, close to the superconducting dome. By extrapolation we demarcate a critical doping region which is thought to correspond to a quantum phase transition at very low temperature.

Comprehensive phase diagram of two-dimensional space charge doped Bi2Sr2CaCu2O8+x

The phase diagram of hole-doped high critical temperature superconductors as a function of doping and temperature has been intensively studied with chemical variation of doping. Chemical doping can provoke structural changes and disorder, masking intrinsic effects. Alternatively, a field-effect transistor geometry with an electrostatically doped, ultra-thin sample can be used. However, to probe the phase diagram, carrier density modulation beyond 1014 cm−2 and transport measurements performed over a large temperature range are needed. Here we use the space charge doping method to measure transport characteristics from 330 K to low temperature. We extract parameters and characteristic temperatures over a large doping range and establish a comprehensive phase diagram for one-unit-cell-thick BSCCO-2212 as a function of doping, temperature and disorder.

Inducing conductivity in polycrystalline ZnO1-x thin films through space charge doping

We induce ultra-high carrier charge density in polycrystalline zinc oxide thin films on glass with a thickness of few tens of nm, achieving carrier concentrations as high as 2.2×1014 cm−2, well beyond the Ioffe-Regel limit for an insulator-metal transition in two dimensions. The sheet resistance is consequently lowered by up to 5 orders of magnitude to about 2 k Ω/◻ without alteration of transparency thanks to our space charge doping technique. Electrostatic doping of such a large band-gap semiconductor is quite challenging, and a high surface potential is required in order to induce conductivity at the interface. Through magneto-transport measurements performed at low temperature on the doped films, we show that both weak localization and weak anti-localization of charge carriers can be observed and that these quantum interference phenomena can be modulated by the carrier concentration and temperature.

Space charge induced electrostatic doping of two-dimensional materials: Graphene as a case study

We introduce a technique that we call Space Charge Doping for electrostatic doping of 2D materials. This technique exploits the presence of mobile ionic species in glass to induce a charge imbalance at the glass-material interface. Ionic mobility in glass is species dependent and also dependent on the temperature and the applied electric field. Mobility of positive sodium ions is increased by heating and an applied electric field causes ion drift. The polarity of the electric field results in accumulation or depletion of sodium ions at the glass surface inducing, respectively, electron or hole doping in the material placed on the surface, in this case graphene. Extremely high doping levels are reached (>1014 cm−2) without compromising graphene quality and with reversibility, bipolarity, and stability in time.

Calculation of electronic potential energy distributions and photoconductivity in hydrogenated amorphous silicon/hydrogenated amorphous silicon nitride superlattices

Abstract A model is proposed for calculating the electronic potential energy distribution and the temperature dependence of photoconductivity (PC) in hydrogenated amorphous siliconhydrogenated amorphous silicon nitride superlattices. The Simmons-Taylor theory and the occupation statistics of correlated defects are used to describe band-tail and dangling-bond states. The electronic potential energy and charge distributions resulting from space-charge doping are calculated for various amounts of transferred charge and different asymmetrical boundary conditions. The dark conductivity (DC) and the PC are calculated for the corresponding spatial potential energy distributions. It is found that the enhancement of DC and PC in the superlattices are mainly determined by the amount of injected charge, the influence of interface asymmetry being slight.

CALCULATION OF ELECTRONIC POTENTIAL DISTRIBUTIONS AND PHOTOCONDUCTIVITY IN a-Si:H/a-SiNx:H SUPERLATTICES

A reasonable model is proposed to calculatee the temperature dependence of photoconductivity (PC) in a-Si:H and a-Si:H/a-SiNx:H super lattices. Simmons-Taylor theory and the occupation statistics of correlated defects are used to describe band tail states and dangling bonds states. As in a-Si:H/a-SiNx:H superlattices, the electronic potential and charge distributions resulting from space-charge doping are calculated at various amounts of transferred charge and different asymmetry boundary conditions. The DC and PC are calculated with corresponding space potential and charge distributions. It is found that the enhancements of DC and PC in superlattices are mainly determined by the amouns of injected charge, and the influence of interface asymmetry is slight.

Planar and Perpendicular Conductivity of Doping Modulated Amorphous Silicon Multilayers

ABSTRACTThe conductivity of a-Si:H multilayers consisting of alternating boron and phosphorus doped layers has been studied as a function of sub-layer thickness. The planar and perpendicular dark conductivity is compared with theoretical analysis of space-charge doping in these structures and this effect is found to dominate the transport as the sub-layer thickness is reduced below a critical value

Space charge doping effects in amorphous semiconductor multi-layers

A theoreetical investigation of the space charge doping effects in amorphous semiconductor multi-layers is presented. The numerical results for a-Si:H multi-layers are compared with experimental observations in the literature

Theoretical analyses of space-charge doping in amorphous semiconductor superlattices. II. Compositional superlattices.

A theoretical analysis of space-charge doping in ``n-i-p-i'' doping superlattices, in particular of amorphous semiconductors, is presented. It is shown that space-charge doping decreases the interlayer barrier height, and hence is a counterdoping. This accounts for the observed blue shift of photoluminescence. The decrease of barrier with the layer thickness also suggests the existence of an optimum thickness at which the quantum effect is most significant. In heavily doped hydrogenated amorphous silicon, this thickness is calculated to be several tens of nanometers. This result is consistent with the observation of persistent photoconductivity in superlattices of this layer thickness.

Theoretical analyses of space-charge doping in amorphous semiconductor superlattices. I. Doping superlattices.

A theoretical analysis of space-charge doping in ``n-i-p-i'' doping superlattices, in particular of amorphous semiconductors, is presented. It is shown that space-charge doping decreases the interlayer barrier height, and hence is a counterdoping. This accounts for the observed blue shift of photoluminescence. The decrease of barrier with the layer thickness also suggests the existence of an optimum thickness at which the quantum effect is most significant. In heavily doped hydrogenated amorphous silicon, this thickness is calculated to be several tens of nanometers. This result is consistent with the observation of persistent photoconductivity in superlattices of this layer thickness.

Properties of amorphous semiconductinga−Si:Ha−SiNx:Hmultilayer films and ofa−SiNx:Halloys

We prepared, by plasma deposition and by alternating the plasma-gas mixture, multilayer films consisting of sequential layers of hydrogenated amorphous silicon ($a$-Si: H) of thickness ${d}_{1}$ and insulating amorphous silicon nitride ($a\ensuremath{-}\mathrm{Si}{\mathrm{N}}_{x}:\mathrm{H}$) of thickness ${d}_{2}$. The thickness ${d}_{2}$ was held constant at 24 \AA{}, while ${d}_{1}$ was changed from 12 to 2120 \AA{}. Films with small ${d}_{1}$ had up to 180 layers pairs. The conduction and optical properties of these multilayer films were studied as well as the effect of prolonged light exposure (Staebler-Wronski effect). At large sublayer thickness ${d}_{1}>100$ \AA{} the properties are dominated by space-charge doping which raises the Fermi level in the $a$-Si: H layers and leaves the nitride layers positively charged. At small sublayer thicknesses ${d}_{1}<50$ \AA{} our observations are consistent with those of Abeles and Tiedje and their interpretation in terms of quantum-well confinement of charge carriers in the thin semiconductor sheets sandwiched between large-band-gap insulating layers. The properties of these multilayer films are compared with those of $a\ensuremath{-}\mathrm{Si}{\mathrm{N}}_{x}:\mathrm{H}$ alloys. These were prepared under the same plasma-decomposition conditions by changing the plasma-gas composition ratio of ${\mathrm{NH}}_{3}$ to Si${\mathrm{H}}_{4}$ from 0.02 to 10.

Properties of amorphous semiconducting multilayer films

The structural, optical, and electrical properties of amorphous multilayer films containing up to 180 double layers of a-Si:H/a-SiN x and a-Si:H/a-SiO x were studied. For a-Si:H layer thickness d>100 A ̊ the transport properties are dominated by space-charge doping with a-SiN x positive and a-SiO x negatively charged. For d<50 A ̊ quantum-well effects increase the optical and electrical gaps. A d=12 A ̊ multilayer film shows no evidence for the predicted loss of extended states in two-dimensional disordered systems.