N. T. Bagraev

Phase and amplitude response of the '0.7?feature' caused by holes in silicon one-dimensional wires and rings

We present findings for the 0.7(2e2/h) feature in the hole quantum conductance staircase that is caused by silicon one-dimensional channels prepared by the split-gate method inside the p-type silicon quantum well (SQW) on the n-type Si(100) surface. Firstly, the interplay of the spin depolarization with the evolution of the 0.7(2e2/h) feature from the e2/h to 3/2 e2/h values as a function of the sheet density of holes is revealed by the quantum point contact connecting two 2D reservoirs in the p-type SQW. The 1D holes are demonstrated to be spin polarized at low sheet density, because the 0.7(2e2/h) feature is close to the value of 0.5(2e2/h) that indicates the spin degeneracy lifting for the first step of the quantum conductance staircase. The 0.7(2e2/h) feature is found to take, however, the value of 0.75(2e2/h) when the sheet density increases, thereby giving rise to the spin depolarization of the 1D holes. Secondly, the amplitude and phase sensitivity of the 0.7(2e2/h) feature are studied by varying the value of the external magnetic field and the top-gate voltage that are applied perpendicularly to the plane of the double-slit ring embedded in the p-type SQW, with the extra quantum point contact inserted in the one of its arms. The Aharonov–Bohm and the Aharonov–Casher conductance oscillations obtained are evidence of the interplay of the spontaneous spin polarization and the Rashba spin–orbit interaction (SOI) in the formation of the 0.7(2e2/h) feature. Finally, the variations of the 0.7(2e2/h) feature caused by the Rashba SOI are found to take in the fractional form with both the plateaus and steps as a function of the top-gate voltage.

Quantized conductance in silicon quantum wires

The results of studying the quantum-mechanical staircase for the electron and hole conductance of one-dimensional channels obtained by the split-gate method inside self-assembled silicon quantum wells are reported. The characteristics of quantum wells formed spontaneously between the heavily doped δ-shaped barriers at the Si(100) surface as a result of nonequilibrium boron diffusion are analyzed first. To this end, secondary-ion mass spectrometry, and also the detection of angular dependences of the cyclotron resonance and ESR, is used; these methods make it possible to identify both the crystallographic orientation of the self-assembled quantum wells and the ferroelectric properties of heavily doped δ-shaped barriers. Since the obtained silicon quantum wells are ultrathin (∼2 nm) and the confining δ-shaped barriers feature ferroelectric properties, the quantized conductance of one-dimensional channels is first observed at relatively high temperatures (T≥77 K). Further, the current-voltage characteristic of the quantum-mechanical conductance staircase is studied in relation to the kinetic energy of electrons and holes, their concentration in the quantum wells, and the crystallographic orientation and modulation depth of electrostatically induced quantum wires. The results show that the magnitude of quantum steps in electron conductance of crystallographically oriented n-type wires is governed by anisotropy of the Si conduction band and is completely consistent with the valence-valley factor for the [001] (G0=4e2/h and gv=2) and [011] (G0=8e2/h and gv=4) axes in the Si(100) plane. In turn, the quantum staircase of the hole conductance of p-Si quantum wires is caused by independent contributions of the one-dimensional (1D) subbands of the heavy and light holes; these contributions manifest themselves in the study of square-section quantum wires in the doubling of the quantum-step height (G0=4e2/h), except for the first step (G0=2e2/h) due to the absence of degeneracy of the lower 1D subband. An analysis of the heights of the first and second quantum steps indicates that there is a spontaneous spin polarization of the heavy and light holes, which emphasizes the very important role of exchange interaction in the processes of 1D transport of individual charge carriers. In addition, the temperature-and field-related inhibition of the quantum conductance staircase is demonstrated in the situation when kT and the energy of the field-induced heating of the carriers become comparable to the energy gap between the 1D subbands. The use of the split-gate method made it possible to detect the effect of a drastic increase in the height of the quantum conductance steps when the kinetic energy of electrons is increased; this effect is most profound for quantum wires of finite length, which are not described under conditions of a quantum point contact. It is shown in the concluding section of this paper that detection of the quantum-mechanical conductance under the conditions of sweeping the kinetic energy of the charge carriers can act as an experimental test aiding in separating the effects of quantum interference in modulated quantum wires against the background of Coulomb oscillations as a result of the formation of QDs between the delta-shaped barriers.

Superconductivity in silicon nanostructures

Superconducting properties of silicon sandwich nanostructures on the n-Si (100) surface, which represent the ultra-narrow p-type silicon quantum wells confined by heavily boron-doped δ barriers, manifest themselves in the measurements of the temperature and field dependences of resistivity, thermopower, heat capacity, and static magnetic susceptibility. The cyclotron-resonance, scanning-tunneling-microscopy, and ESR data identify the presence of the single trigonal negative-U dipole boron centers in nanostructured δ barriers B+-,B−, which are formed due to the reconstruction of shallow boron acceptors, 2B0 ⇒ B+ + B−. The obtained results indicate that these negative-U centers are responsible for the transport of small-radius hole bipolarons, which is likely the basis of the mechanism of high-temperature superconductivity with TC = 145 K. The superconductor-gap value of 0.044 eV determined from the measurements of the critical temperature using the above techniques is almost identical to the data on the tunneling spectroscopy and direct record of tunneling I–V characteristics. The quantization of the superconductive characteristics for silicon sandwich nanostructures manifests itself in the temperature and field dependences of the heat capacity and static magnetic susceptibility, which show the oscillations of the second critical field and critical temperature arising due to the supercurrent quantization.

Local tunneling spectroscopy of silicon nanostructures

The recharging of many-hole and few-electron quantum dots under the conditions of the ballistic transport of single charge carriers inside self-assembled quantum well structures on a Si (100) surface are studied using local tunneling spectroscopy at high temperatures (up to room temperature). On the basis of measurements of the tunneling current-voltage characteristics observed during the transit of single charge carriers through charged quantum dots, the modes of the Coulomb blockade, Coulomb conductivity oscillations, and electronic shell formation are identified. The tunneling current-voltage characteristics also show the effect of quantum confinement and electron-electron interaction on the characteristics of single-carrier transport through silicon quantum wires containing weakly and strongly coupled quantum dots.

Spin interference of holes in silicon nanosandwiches

Spin-dependent transport of holes is studied in silicon nanosandwiches on an n-Si (100) surface which are represented by ultranarrow p-Si quantum wells confined by δ-barriers heavily doped with boron. The measurement data of the longitudinal and Hall voltages as functions of the top gate voltage without an external magnetic field show the presence of edge conduction channels in the silicon nanosandwiches. An increase in the stabilized source-drain current within the range 0.25–5 nA subsequently exhibits the longitudinal conductance value 4e2/h, caused by the contribution of the multiple Andreev reflection, the value 0.7(2e2/h) corresponding to the known quantum conductance staircase feature, and displays Aharonov-Casher oscillations, which are indicative of the spin polarization of holes in the edge channels. In addition, at a low stabilized source-drain current, due to spin polarization, a nonzero Hall voltage is detected which is dependent on the top gate voltage; i. e., the quantum spin Hall effect is observed. The measured longitudinal I–V characteristics demonstrate Fiske steps and a negative differential resistance caused by the generation of electromagnetic radiation as a result of the Josephson effect. The results obtained are explained within a model of topological edge states which are a system of superconducting channels containing quantum point contacts transformable to single Josephson junctions at an increasing stabilized source-drain current.

Superconducting properties of silicon nanostructures

Superconducting properties of silicon sandwich nanostructures on the n-Si (100) surface, which represent the ultra-narrow p-type silicon quantum wells confined by heavily boron-doped δ barriers, manifest themselves in the measurements of the temperature and field dependences of resistivity, thermopower, heat capacity, and static magnetic susceptibility. The cyclotron-resonance, scanning-tunneling-microscopy, and ESR data identify the presence of the single trigonal negative-U dipole boron centers in nanostructured δ barriers B+-,B−, which are formed due to the reconstruction of shallow boron acceptors, 2B0 ⇒ B+ + B−. The obtained results indicate that these negative-U centers are responsible for the transport of small-radius hole bipolarons, which is likely the basis of the mechanism of high-temperature superconductivity with TC = 145 K. The superconductor-gap value of 0.044 eV determined from the measurements of the critical temperature using the above techniques is almost identical to the data on the tunneling spectroscopy and direct record of tunneling I–V characteristics. The quantization of the superconductive characteristics for silicon sandwich nanostructures manifests itself in the temperature and field dependences of the heat capacity and static magnetic susceptibility, which show the oscillations of the second critical field and critical temperature arising due to the supercurrent quantization.

Fractional quantization of ballistic conductance in one-dimensional hole systems

We analyze the fractional quantization of the ballistic conductance associated with the light and heavy holes bands in Si, Ge and GaAs systems. It is shown that the formation of the localized hole state in the region of the quantum point contact connecting two quasi-1D hole leads modifies drastically the conductance pattern. Exchange interaction between localized and propagating holes results in the fractional quantization of the ballistic conductance different from those in electronic systems. The value of the conductance at the additional plateaux depends on the offset between the bands of the light and heavy holes, \Delta, and the sign of the exchange interaction constant. For \Delta=0 and ferromagnetic exchange interaction, we observe additional plateaux around the values 7e^{2}/4h, 3e^{2}/h and 15e^{2}/4h, while antiferromagnetic interaction plateaux are formed around e^{2}/4h, e^{2}/h and 9e^{2}/4h. For large \Delta, the single plateau is formed at e^2/h.

Quantum spin Hall effect in nanostructures based on cadmium fluoride

Tunneling current-voltage (I-V) characteristics and temperature dependences of static magnetic susceptibility and specific heat of the CdBxF2 − x/p-CdF2-QW/CdBxF2 − x planar sandwich structures formed on the surface of an n-CdF2 crystal have been studied in order to identify superconducting properties of the CdBxF2 − x δ barriers confining the p-type CdF2 ultranarrow quantum well. Comparative analysis of current-voltage (I-V) characteristics and conductance-voltage dependences (measured at the temperatures, respectively, below and above the critical temperature of superconducting transition) indicates that there is an interrelation between quantization of supercurrent and dimensional quantization of holes in the p-CdF2 ultranarrow quantum well. It is noteworthy that detection of the Josephson peak of current in each hole subband is accompanied by the appearance of the spectrum of the multiple Andreev reflection (MAR). A high degree of spin polarization of holes in the edge channels along the perimeter of the p-CdF2 ultranarrow quantum well appears as a result of MAR and makes it possible to identify the quantum spin Hall effect I-V characteristics; this effect becomes pronounced in the case of detection of nonzero conductance at the zero voltage applied to the vertical gate in the Hall geometry of the experiment. Within the energy range of superconducting gap, the I-V characteristics of the spin transistor and quantum spin Hall effect are controlled by the MAR spectrum appearing as the voltage applied to the vertical gate is varied. Beyond the range of the superconducting gap, the observed I-V characteristic of the quantum spin Hall effect is represented by a quantum conductance staircase with a height of the steps equal to e2/h; this height is interrelated with the Aharonov-Casher oscillations of longitudinal and depends on the voltage applied to the vertical gate.

Self-ordered microcavities embedded in ultrashallow silicon p-n junctions

Scanning tunneling spectroscopy was used to obtain topographic images of the (100) surface of ultrashallow diffusion profiles of boron in silicon. This method makes it possible to study the influence of fluctuations of the surface deformation potential, which depend on the thickness of the preliminary deposited oxide layer and on the crystallographic orientation of the fluxes of nonequilibrium vacancies and self-interstitials that stimulate the exchange mechanisms of impurity diffusion. The existence of self-assembled systems of quantum anti-dots, which are formed due to fluctuations in the surface deformation potential and which are microdefects that penetrate through the diffusion profile of the dopant, is demonstrated for the first time. It is established that a spread in the sizes of quantum anti-dots is reduced with increasing temperature of the impurity diffusion. In addition, the sizes of quantum anti-dots are found to be interrelated to their spatial distribution, which is indicative of a fractal mechanism of formation of self-assembled zero-dimensional systems under conditions of strong interaction of the flux of impurity atoms with that of primary defects. Self-assembled quantum anti-dots embedded into a system of silicon quantum wells make it possible to design microcavities with distributed negative feedback; the existence of such microcavities is confirmed by spectral dependences of the reflection and transmission coefficients in the visible and infrared regions of the spectrum, respectively.

Ultrashallow p+-n junctions in silicon (100): Electron-beam diagnostics of the surface zone

Ultrashallow p+-n junctions formed in silicon (100) under nonequilibrium impurity diffusion conditions are analyzed by electron-beam diagnostics of the surface zone using a probe of low-to medium-energy electrons. The energy dependence of the radiation conductivity is investigated, along with its distribution over the area of the p+-n junction. This procedure can be used to determine the depth distribution (in the crystal) of the probability of separation of electron-hole pairs by the field of the p-n junction; the experimental results show that this distribution differs according to whether the kick-out mechanism or the dissociative vacancy mechanism of impurity diffusion is predominant as the basis of formation of the ultrashallow p+-n junctions. Also reported here for the first time are the results of investigations of the distribution of secondary point centers formed near the boundary of the ultrashallow diffusion profile, which exert a major influence on the transport of nonequilibrium carriers. The data obtained in the study demonstrate the possibility of improving the efficiency of photodetectors, α-particle detectors, and solar batteries constructed on the basis of ultrashallow p-n junctions.