W. Gehlhoff

Lithium related deep and shallow acceptors in Li-doped ZnO nanocrystals

We study the existence of Li-related shallow and deep acceptor levels in Li-doped ZnO nanocrystals using electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy. ZnO nanocrystals with adjustable Li concentrations between 0% and 12% have been prepared using organometallic precursors and show a significant lowering of the Fermi energy upon doping. The deep Li acceptor with an acceptor energy of 800 meV could be identified in both EPR and PL measurements and is responsible for the yellow luminescence at 2.2 eV. Additionally, a shallow acceptor state at 150 meV above the valence band maximum is made responsible for the observed donor-acceptor pair and free electron-acceptor transitions at 3.235 and 3.301 eV, possibly stemming from the formation of Li-related defect complexes acting as acceptors.

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.

Electron paramagnetic resonance in transition metal-doped ZnO nanowires

The wide-band-gap zinc oxide-based diluted magnetic semiconductors currently attract considerable attention due to their possible use in spintronic devices. In this work, we studied ZnO nanowire samples synthesized on 10×10 mm2 a-plane sapphire substrates by high-pressure pulsed laser deposition. The samples were characterized by scanning electron microscopy (SEM) and electron paramagnetic resonance (EPR) in the X-band (≃9.3 GHz) from T=4 to 300 K. According to the SEM pictures, the nanowires exhibit a length of about 1 μm and are aligned perpendicular to the substrate surface. The structures have a hexagonal cross section and their diameter ranges from 60 nm up to 150 nm. For the lowest nominal concentrations of xMn=3 at. % and xCo=5 at. %, we detect the anisotropic EPR spectra of isolated Mn2+ (3d5, S6) and Co2+ (3d7, F4), respectively, on Zn sites. The detection of the well-resolved anisotropic spectra proves a coherent crystallographic orientation of the nanowires. The linewidth was larger than the be...

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.

Multifunctional III-nitride dilute magnetic semiconductor epilayers and nanostructures as a future platform for spintronic devices

This work focuses on the development of materials and growth techniques suitable for future spintronic device applications. Metal-organic chemical vapor deposition (MOCVD) was used to grow high-quality epitaxial films of varying thickness and manganese doping levels by introducing bis-cyclopentadienyl as the manganese source. High-resolution X-ray diffraction indicates that no macroscopic second phases are formed during growth, and Mn containing films are similar in crystalline quality to undoped films Atomic force microscopy revealed a 2-dimensional MOCVD step-flow growth pattern in the Mn-incorporated samples. The mean surface roughnesses of optimally grown Ga1-xMnxN films were almost identical to that from the as-grown template layers, with no change in growth mechanism or morphology. Various annealing steps were applied to some of the samples to reduce compensating defects and to investigate the effects of post processing on the structural, magnetic and opto-electronic properties. SQUID measurements showed an apparent ferromagnetic hysteresis behavior which persisted to room temperature. An optical absorption band around 1.5 eV was observed via transmission studies. This band is assigned to the internal Mn3+ transition between the 5E and the partially filled 5T2 levels of the 5D state. The broadening of the absorption band is introduced by the high Mn concentration. Recharging of the Mn3+ to Mn2+ was found to effectively suppress these transitions resulting in a reduction of the magnetization. The structural quality, and the presence of Mn2+ ions were confirmed by EPR spectroscopy, meanwhile no Mn-Mn interactions indicative of clustering were observed. The absence of doping-induced strain in Ga1-xMnxN was observed by Raman spectroscopy.

Structure and energy level of native defects in as-grown and electron-irradiated zinc germanium diphosphide studied by EPR and photo-EPR

The properties of defects in as-grown p-type zinc germanium disphosphide (ZnGeP2) and the influence of electron irradiation and annealing on the defect behavior were studied by means of electron paramagnetic resonance (EPR) and photo-EPR. Besides the well-known three native defects (VZn ,V P ,G e Zn), an S ¼ 1=2 EPR spectrum with an isotropic g ¼ 2:0123 and resolved hyperfine splitting from four equivalent I ¼ 1=2 neighbors is observed in electron-irradiated ZnGeP2. This spectrum is tentatively assigned to the isolated Ge vacancy. Photo-EPR and annealing treatments show that the high-energy electron irradiation-induced changes in the EPR intensities of the zinc and phosphorus vacancies are caused by the Fermi level shift towards the conduction band. Annealing of the electron-irradiated samples induces a shift of the Fermi level back to its original position, accompanied by an increase of the EPR signal associated with the V 2 and a proportional increase of the EPR signal assigned to the V 0 under illumination ðl , 1 eV) as well as generation of a new defect. The results indicate that the EPR spectra originally assigned to the isolated V 2 and V 0 are in fact associated defects and the new defect is probably the isolated

Annealing study of the formation of nickel-related paramagnetic defects in diamond

Abstract We report an annealing study of paramagnetic defects found in synthetic diamond grown from nickel solvent/catalysts. Substitutional N0 and Ni− defects in diamond show a similar behavior in the course of annealing at temperatures in the range 1550–2000 °C. New paramagnetic centers labeled AB6 and AB7 become detectable upon annealing. The AB5 center is found to appear in as-grown HPHT diamond rich in nitrogen, and its concentration decreases with increasing annealing temperature. The AB1, AB3, AB6 and AB7 defects are produced at early stages of the annealing process and are suppressed by heating diamond to temperatures in excess of 1900 °C. Possible origins of the AB paramagnetic centers are discussed in the light of the recently proposed mechanisms of nitrogen aggregation and nickel–nitrogen complexes formation in HPHT diamond.

Infrared induced emission from silicon quantum wires

Abstract Studies of infrared induced emission from the silicon quantum wires, which is due to the formation of a correlation gap in the DOS of degenerate hole gas, are presented. The quantum wires of this art are created by electrostatic confining potential inside ultra-shallow p + n junctions which are realized using controlled surface injection of self-interstitials and vacancies in the process of non-equilibrium boron diffusion.