Nondegenerate Si Research Articles

Quantitative and systematic analysis of bias dependence of spin accumulation voltage in a nondegenerate Si-based spin valve

Spin accumulation voltages in a non-degenerate Si spin valve are discussed quantitatively as a function of electric bias current using systematic experiments and model calculations. As an open question in semiconductor spintronics, the origin of the deviation of spin accumulation voltages measured experimentally in a non-degenerate Si spin valve is clarified from that obtained by model calculation using the spin drift diffusion equation including the effect of the spin-dependent interfacial resistance of tunneling barriers. Unlike the case of metallic spin valves, the bias dependence of the resistance-area product for a ferromagnet/MgO/Si interface, resulting in the reappearance of the conductance mismatch, plays a central role to induce the deviation.

Thermally Generated Spin Signals in a Nondegenerate Silicon Spin Valve

Spincaloritronic signal generation due to thermal spin injection and spin transport is demonstrated in a non-degenerate Si spin valve. The spin-dependent Seebeck effect is used for the spincaloritronic signal generation, and the thermal gradient of about 200 mK at an interface of Fe and Si enables generating a spin voltage of 8 {\mu}V at room temperature. A simple expansion of a conventional spin drift-diffusion model with taking into account the spin-dependent Seebeck effect shows semiconductor materials are quite potential for the spincaloritronic signal generation comparing with metallic materials, which can allow efficient heat recycling in semiconductor spin devices.

Erratum: Spin Transport in Nondegenerate Si with a Spin MOSFET Structure at Room Temperature [Phys. Rev. Appl. 2 , 034005 (2014)

Received 4 February 2018DOI:https://doi.org/10.1103/PhysRevApplied.9.039901© 2018 American Physical Society

Observation of large spin accumulation voltages in nondegenerate Si spin devices due to spin drift effect: Experiments and theory

A large spin-accumulation voltage of more than 1.5 mV at 1 mA, i.e., a magnetoresistance of 1.5 {\Omega}, was measured by means of the local three-terminal magnetoresistance in nondegenerate Si-based lateral spin valves (LSVs) at room temperature. This is the largest spin-accumulation voltage measured in semiconductor-based LSVs. The modified spin drift-diffusion model, which successfully accounts for the spin drift effect, explains the large spin-accumulation voltage and significant bias-current-polarity dependence. The model also shows that the spin drift effect enhances the spin-dependent magnetoresistance in the electric two terminal scheme. This finding provides a useful guiding principle for spin metal-oxide semiconductor field-effect transistor (MOSFET) operations.

Spin Transport in Nondegenerate Si with a Spin MOSFET Structure at Room Temperature

Although the traditional metal-on-semiconductor field-effect transistor (MOSFET) has been a workhorse in information processing for decades, we must now consider its successor. To make spintronics a reality, by analogy we need a ``spin MOSFET''. The authors demonstrate room-temperature operation of just such a device, in which a flow of spin angular momentum in nondegenerate silicon is controlled by an external gate voltage.

Optical studies of the structure of porous silicon films formed in p-type degenerate and non-degenerate silicon

The detailed structure of porous Si (PS) layers formed in p-type wafers with resistivities 0.01-25 Omega cm has been investigated using reflectance, transmission, ellipsometry and photoluminescence techniques. Marked differences were observed in the optical properties of PS formed in degenerate or non-degenerate Si and these results are correlated with the results of other techniques. The optical techniques together with effective medium modelling have been shown to be useful non-destructive methods for either assessment of PS density or detection of unsuspected phases. The degenerate PS layers consistently showed good retention of the single-crystal characteristics of the starting wafer, only c-Si and voids being detected. For these samples, good agreement was obtained between optical and gravimetric densities. However, the non-degenerate PS had much greater variability, with greater loss of crystallinity and significant incorporation of oxygen, due to partial oxidation having occurred on or immediately after anodisation. Oxide fractions have been determined both optically and gravimetrically, with up to 50% oxide being detected in some samples. Non-degenerate PS samples with high oxygen concentrations appeared to be in the form of a chemical mixture, SiOx, from interpretation of the optical constants. Photoluminescence measurements together with the other techniques indicated a complex mixture of phases in the latter samples-voids, alpha -Si:O (and/or alpha -Si:H), an unknown amorphous phase and silicon oxide. This complex structure probably contributes to the observed instability of thick non-degenerate PS layers when heated in UHV as part of the cleaning procedure prior to epitaxial growth, all degenerate samples being able to withstand heat treatment.

A study of the characteristics of a Mg-Si diode

Diodes of Mg contact to both n and p-typo Si respectively have been fabricated. I-V characteristics and Specific contact resistance Re. were studied. Specific contact resistance can be reduced by increasing temperature, and decreasing contact area. Diodes of Mg to p-type non-degenerate Si contact possess better rectifying properties than those of mg to n-type non-degenerate Si. Studios of specific contact resistances for different contact arena reveal the current. crowding effect.

Potentials and direct current in Si-(20 to 40 Å)SiO 2-metal structures

The effects of oxide thickness and interface states on potentials in and direct currents through MOS structures with various metal contacts on 20–40 Å thick SiO 2 films and nondegenerate Si were investigated using a recently developed method of determining surface potential vs. bias and interface state vs. energy distributions of such structures from conventional admittance measurements. For Cr, Cu, and Mg contacts, the interface states are predominantly of acceptor type. The metal silicon work function differences are φ MS = 0.91 V for Au, −0.04 for Cr, 0.18 for Cu, and −1.07 V for Mg. The forward device current consists mainly of majority carriers emitted over the Si barrier and tunneling through the oxide into the metal for depletion or weak inversion of Si, and of majority carriers tunneling from Si through the oxide into the metal for Si accumulation. Excess currents in forward and reverse direction are caused by carrier generation-recombination in interface states and tunneling through the oxide to and from the metal. The drooping of the forward current, deviating strongly from an ideal exponential characteristic, is mainly caused by the drop of a considerable part of the applied bias across the oxide layer.

Interface states in MOS structures with 20–40 Å thick SiO 2 films on nondegenerate Si

MOS structures having different metal contacts on 20–40 Å thick SiO 2 films and non-degenerate Si have been investigated. These structures show large frequency dispersion of the measured admittance curves caused by carrier recombination in interface states. However, build-up of a Si inversion layer is not observed because the minority carriers are drained away by tunneling through the oxide into the metal. Interface state density distributions across the whole Si band gap are obtained by evaluating measured capacitance and conductance curves. Characteristic peaks of 0.12–0.30 eV half width are found for the metal contacts, Mg at 0.54 eV, Cu at 0.23 eV, Cr at 0.18 eV and 0.52 eV, and Au at 0.15 eV and 0.97 eV above the valence band edge. The position of a peak seems to be related to the valence of the metal. Interface states caused by Cr, Cu and Mg contacts are of acceptor type. Their capture cross-section obtained from conductance measurements varies between 10 −7 and 10 −18 cm −2. A rapid increase of the state density with decreasing oxide thickness or sample annealing indicates that the states are caused by metal diffusion through the oxide into the interface. An observed saturation of the state density at about 2 × 10 13 cm −2 V −1 is possibly due to a limited solubility of metal impurities in the oxide.