arXiv subjects

Biqin Huang

Publications and source records attributed to Biqin Huang.

12 recordsLinked to original sources

arXiv2014

Undoped accumulation-mode Si/SiGe quantum dots

We report on a quantum dot device design that combines the low disorder properties of undoped SiGe heterostructure materials with an overlapping gate stack in which each electrostatic gate has a dominant and unique function -- control of individual quantum dot occupancies and of lateral tunneling into and between dots. Control of the tunneling rate between a dot and an electron bath is demonstrated over more than nine orders of magnitude and independently confirmed by direct measurement within the bandwidth of our amplifiers. The inter-dot tunnel coupling at the (0,2)<-->(1,1) charge configuration anti-crossing is directly measured to quantify the control of a single inter-dot tunnel barrier gate. A simple exponential dependence is sufficient to describe each of these tunneling processes as a function of the controlling gate voltage.

cond-mat.mes-hall↗

arXiv2011

Pauli spin blockade in undoped Si/SiGe two-electron double quantum dots

We demonstrate double quantum dots fabricated in undoped Si/SiGe heterostructures relying on a double top-gated design. Charge sensing shows that we can reliably deplete these devices to zero charge occupancy. Measurements and simulations confirm that the energetics are determined by the gate-induced electrostatic potentials. Pauli spin blockade has been observed via transport through the double dot in the two electron configuration, a critical step in performing coherent spin manipulations in Si.

cond-mat.mes-hall↗

arXiv2010

The Larmor clock and anomalous spin dephasing in silicon

Drift-diffusion theory - which fully describes charge transport in semiconductors - is also universally used to model transport of spin-polarized electrons in the presence of longitudinal electric fields. By transforming spin transit time into spin orientation with precession (a technique called the "Larmor clock") in current-sensing vertical-transport intrinsic Si devices, we show that spin diffusion (and concomitant spin dephasing) can be greatly enhanced with respect to charge diffusion, in direct contrast to predictions of spin Coulomb-drag diffusion suppression.

cond-mat.mtrl-sci↗

arXiv2008

Geometric dephasing-limited Hanle effect in long-distance lateral silicon spin transport devices

Evidence of spin precession and dephasing ("Hanle effect") induced by an external magnetic field is the only unequivocal proof of spin-polarized conduction electron transport in semiconductor devices. However, when spin dephasing is very strong, Hanle effect in a uniaxial magnetic field can be impossible to measure. Using a Silicon device with lateral injector-detector separation over 2 millimeters, and geometrically-induced dephasing making spin transport completely incoherent, we show experimentally and theoretically that Hanle effect can still be measured using a two-axis magnetic field.

cond-mat.mtrl-sci↗

arXiv2008

Non-ohmic spin transport in n-type doped silicon

We demonstrate the injection and transport of spin-polarized electrons through n-type doped silicon with in-plane spin-valve and perpendicular magnetic field spin precession and dephasing ("Hanle effect") measurements. A voltage applied across the transport layer is used to vary the confinement potential caused by conduction band-bending and control the dominant transport mechanism between drift and diffusion. By modeling transport in this device with a Monte-Carlo scheme, we simulate the observed spin polarization and Hanle features, showing that the average transit time across the short Si transport layer can be controlled over 4 orders of magnitude with applied voltage. As a result, this modeling allows inference of a long electron spin lifetime, despite the short transit length.

cond-mat.mtrl-sci↗

arXiv2008

Oblique Hanle Effect in Semiconductor Spin Transport Devices

Spin precession and dephasing ("Hanle effect") provides an unambiguous means to establish the presence of spin transport in semiconductors. We compare theoretical modeling with experimental data from drift-dominated silicon spin-transport devices, illustrating the non-trivial consequences of employing oblique magnetic fields (due to misalignment or intentional, fixed in-plane field components) to measure the effects of spin precession. Model results are also calculated for Hanle measurements under conditions of diffusion-dominated transport, revealing an expected Hanle peak-widening effect induced by the presence of fixed in-plane magnetic bias fields.

cond-mat.mtrl-sci↗

arXiv2008

Spin Dephasing in Drift-Dominated Semiconductor Spintronics Devices

A spin transport model is employed to study the effects of spin dephasing induced by diffusion-driven transit-time uncertainty through semiconductor spintronic devices where drift is the dominant transport mechanism. It is found that in the ohmic regime, dephasing is independent of transit length, and determined primarily by voltage drop across the spin transport region. The effects of voltage and temperature predicted by the model are compared to experimental results from a 350-micron-thick silicon spin-transport device using derived mathematical expressions of spin dephasing.

cond-mat.mtrl-sci↗

arXiv2007

Coherent spin transport through a 350-micron-thick Silicon wafer

We use all-electrical methods to inject, transport, and detect spin-polarized electrons vertically through a 350-micron-thick undoped single-crystal silicon wafer. Spin precession measurements in a perpendicular magnetic field at different accelerating electric fields reveal high spin coherence with at least 13pi precession angles. The magnetic-field spacing of precession extrema are used to determine the injector-to-detector electron transit time. These transit time values are associated with output magnetocurrent changes (from in-plane spin-valve measurements), which are proportional to final spin polarization. Fitting the results to a simple exponential spin-decay model yields a conduction electron spin lifetime (T1) lower bound in silicon of over 500ns at 60K.

cond-mat.mtrl-sci↗

arXiv2007

Experimental realization of a silicon spin field-effect transistor

A longitudinal electric field is used to control the transit time (through an undoped silicon vertical channel) of spin-polarized electrons precessing in a perpendicular magnetic field. Since an applied voltage determines the final spin direction at the spin detector and hence the output collector current, this comprises a spin field-effect transistor. An improved hot-electron spin injector providing ~115% magnetocurrent, corresponding to at least ~38% electron current spin polarization after transport through 10 microns undoped single-crystal silicon, is used for maximum current modulation.

cond-mat.mtrl-sci↗

arXiv2007

Spin lifetime in silicon in the presence of parasitic electronic effects

A hybrid ferromagnet/semiconductor device is used to determine a lower bound on the spin lifetime for conduction electrons in silicon. We use spin precession to self-consistently measure the drift velocity vs. drift field of spin-polarized electrons and use this electronic control to change the transit time between electron injection and detection. A measurement of normalized magnetocurrent as a function of drift velocity is used with a simple exponential-decay model to argue that the lifetime obtained (~2 ns) is artificially lowered by electronic effects and is likely orders of magnitude higher.

cond-mat.mtrl-sci↗

arXiv2007

35% magnetocurrent with spin transport through Si

Efficient injection of spin-polarized electrons into the conduction band of silicon is limited by the formation of a silicide at the ferromagnetic metal (FM)/silicon interface. In the present work, this "magnetically-dead" silicide (where strong spin-scattering significantly reduces injected spin polarization) is eliminated by moving the FM in the spin injector from the tunnel junction base anode to the emitter cathode and away from the silicon surface. This results in over an order-of-magnitude increase in spin injection efficiency, from a previously-reported magnetocurrent ratio of ~2% to ~35% and an estimated spin polarization in Si from ~1% to at least ~15%. The injector tunnel-junction bias dependence of this spin transport signal is also measured, demonstrating the importance of low bias voltage to preserve high injected spin polarization.

cond-mat.mtrl-sci↗

arXiv2007

Electronic measurement and control of spin transport in Silicon

The electron spin lifetime and diffusion length are transport parameters that define the scale of coherence in spintronic devices and circuits. Since these parameters are many orders of magnitude larger in semiconductors than in metals, semiconductors could be the most suitable for spintronics. Thus far, spin transport has only been measured in direct-bandgap semiconductors or in combination with magnetic semiconductors, excluding a wide range of non-magnetic semiconductors with indirect bandgaps. Most notable in this group is silicon (Si), which (in addition to its market entrenchment in electronics) has long been predicted a superior semiconductor for spintronics with enhanced lifetime and diffusion length due to low spin-orbit scattering and lattice inversion symmetry. Despite its exciting promise, a demonstration of coherent spin transport in Si has remained elusive, because most experiments focused on magnetoresistive devices; these methods fail because of universal impedance mismatch obstacles, and are obscured by Lorentz magnetoresistance and Hall effects. Here we demonstrate conduction band spin transport across 10 microns undoped Si, by using spin-dependent ballistic hot-electron filtering through ferromagnetic thin films for both spin-injection and detection. Not based on magnetoresistance, the hot electron spin-injection and detection avoids impedance mismatch issues and prevents interference from parasitic effects. The clean collector current thus shows independent magnetic and electrical control of spin precession and confirms spin coherent drift in the conduction band of silicon.

cond-mat.mtrl-sci↗