arXiv subjects

P. D. Ye

Publications and source records attributed to P. D. Ye.

16 recordsLinked to original sources

arXiv2012

III-V Gate-all-around Nanowire MOSFET Process Technology: From 3D to 4D

In this paper, we have experimentally demonstrated, for the first time, III-V 4D transistors with vertically stacked InGaAs nanowire (NW) channels and gate-all-around (GAA) architecture. Novel process technology enabling the transition from 3D to 4D structure has been developed and summarized. The successful fabrication of InGaAs lateral and vertical NW arrays has led to 4x increase in MOSFET drive current. The top-down technology developed in this paper has opened a viable pathway towards future low-power logic and RF transistors with high-density III-V NWs.

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arXiv2012

20-80nm Channel Length InGaAs Gate-all-around Nanowire MOSFETs with EOT=1.2nm and Lowest SS=63mV/dec

In this paper, 20nm - 80nm channel length (Lch) InGaAs gate- all-around (GAA) nanowire MOSFETs with record high on- state and off-state performance have been demonstrated by equivalent oxide thickness (EOT) and nanowire width (WNW) scaling down to 1.2nm and 20nm, respectively. SS and DIBL as low as 63mV/dec and 7mV/V have been demonstrated, indicating excellent interface quality and scalability. Highest ION = 0.63mA/μm and gm = 1.74mS/μm have also been achieved at VDD=0.5V, showing great promise of InGaAs GAA technology for 10nm and beyond high-speed low- power logic applications.

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arXiv2009

Observation of quantum-Hall effect in gated epitaxial graphene grown on SiC (0001)

Epitaxial graphene films were formed on the Si-face of semi-insulating 4H-SiC substrates by a high temperature sublimation process. A high-k gate stack on epitaxial graphene is realized by inserting a fully oxidized nanometer thin aluminum film as a seeding layer followed by an atomic-layer deposition process. The electrical properties of epitaxial graphene films are sustained after gate stack formation without significant degradation. At low temperatures, the quantum-Hall effect in Hall resistance is observed along with pronounced Shubnikov-de Hass oscillations in diagonal magneto-resistance of gated epitaxial graphene on SiC (0001).

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arXiv2008

Magneto-conductance Oscillations in Graphene Antidot Arrays

Epitaxial graphene films have been formed on the C-face of semi-insulating 4H-SiC substrates by a high temperature sublimation process. Nano-scale square antidot arrays have been fabricated on these graphene films. At low temperatures, magneto-conductance in these films exhibits pronounced Aharonov-Bohm oscillations with the period corresponding to magnetic flux quanta added to the area of a single antidot. At low fields, weak localization is observed and its visibility is enhanced by intravalley scattering on antidot edges. At high fields, we observe two distinctive minima in magnetoconductance which can be attributed to commensurability oscillations between classical cyclotron orbits and antidot array. All mesoscopic features, surviving up to 70 K, reveal the unique electronic properties of graphene.

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arXiv2008

Top-gated graphene field-effect-transistors formed by decomposition of SiC

Top-gated, few-layer graphene field-effect transistors (FETs) fabricated on thermally-decomposed semi-insulating 4H-SiC substrates are demonstrated. Physical vapor deposited SiO2 is used as the gate dielectric. A two-dimensional hexagonal arrangement of carbon atoms with the correct lattice vectors, observed by high-resolution scanning tunneling microscopy, confirms the formation of multiple graphene layers on top of the SiC substrates. The observation of n-type and p-type transition further verifies Dirac Fermions unique transport properties in graphene layers. The measured electron and hole mobility on these fabricated graphene FETs are as high as 5400 cm2/Vs and 4400 cm2/Vs respectively, which are much larger than the corresponding values from conventional SiC or silicon.

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arXiv2006

Melting of a 2D Quantum Electron Solid in High Magnetic Field

The melting temperature ($T_m$) of a solid is generally determined by the pressure applied to it, or indirectly by its density ($n$) through the equation of state. This remains true even for helium solids\cite{wilk:67}, where quantum effects often lead to unusual properties\cite{ekim:04}. In this letter we present experimental evidence to show that for a two dimensional (2D) solid formed by electrons in a semiconductor sample under a strong perpendicular magnetic field\cite{shay:97} ($B$), the $T_m$ is not controlled by $n$, but effectively by the \textit{quantum correlation} between the electrons through the Landau level filling factor $ν$=$nh/eB$. Such melting behavior, different from that of all other known solids (including a classical 2D electron solid at zero magnetic field\cite{grim:79}), attests to the quantum nature of the magnetic field induced electron solid. Moreover, we found the $T_m$ to increase with the strength of the sample-dependent disorder that pins the electron solid.

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arXiv2005

Modulation of the high mobility two-dimensional electrons in Si/SiGe using atomic-layer-deposited gate dielectric

Metal-oxide-semiconductor field-effect transistors (MOSFET's) using atomic-layer-deposited (ALD) Al$_2$O$_3$ as the gate dielectric are fabricated on the Si/Si$_{1-x}$Ge$_x$ heterostructures. The low-temperature carrier density of a two-dimensional electron system (2DES) in the strained Si quantum well can be controllably tuned from 2.5$\times10^{11}$cm$^{-2}$ to 4.5$\times10^{11}$cm$^{-2}$, virtually without any gate leakage current. Magnetotransport data show the homogeneous depletion of 2DES under gate biases. The characteristic of vertical modulation using ALD dielectric is shown to be better than that using Schottky barrier or the SiO$_2$ dielectric formed by plasma-enhanced chemical-vapor-deposition(PECVD).

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arXiv2004

AC Magnetotransport in Reentrant Insulating Phases of Two-dimensional Electrons near 1/5 and 1/3 Landau fillings

We have measured high frequency magnetotransport of a high quality two-dimensional electron system (2DES) near the reentrant insulating phase (RIP) at Landau fillings ($ν$) between 1/5 and 2/9. The magneto\textit{conductivity} in the RIP has resonant behavior around 150 MHz, showing a \textit{peak} at $ν$$\sim$0.21. Our data support the interpretation of the RIP as due to some pinned electron solid. We have also investigated a narrowly confined 2DES recently found to have a RIP at 1/3$<$$ν$$<$1/2 and we have revealed features, not seen in DC transport, that suggest some intriguing interplay between the 1/3 FQHE and RIP.

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arXiv2004

Evidence for Two Different Solid Phases of Two Dimensional Electrons in High Magnetic Fields

We have performed RF spectroscopy on very high quality two dimensional electron systems in the high magnetic field insulating phase, usually associated with a Wigner solid (WS) pinned by disorder. We have found two different resonances in the frequency dependent real diagonal conductivity spectrum and we interpret them as coming from \textit{two} different pinned solid phases (labeled as "WS-A" and "WS-B"). The resonance of WS-A is observable for Landau level filling $ν$$<$2/9 (but absent around the $ν$=1/5 fractional quantum Hall effect (FQHE)); it then \textit{crosses over} for $ν$$<$0.18 to the different WS-B resonance which dominates the spectrum at $ν$$<$0.125. Moreover, WS-A resonance is found to show dispersion with respect to the size of transmission line, indicating that WS-A has a large correlation length (exceeding $\sim$100 $μ$m); in contrast no such behavior is found for WS-B. We suggest that quantum correlations such as those responsible for FQHE may play an important role in giving rise to such different solids.

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arXiv2004

Evidence of a first order phase transition between Wigner crystal and Bubble Phases of 2D electrons in higher Landau levels

For filling factors $ν$ in the range between 4.16 and 4.28, we simultaneously detect {\it two} resonances in the real diagonal microwave conductivity of a two--dimensional electron system (2DES) at low temperature $T \approx 35$ mK. We attribute the resonances to Wigner crystal and Bubble phases of the 2DES in higher Landau Levels. For $ν$ below and above this range, only single resonances are observed. The coexistence of both phases is taken as evidence of a first order phase transition. We estimate the transition point as $ν=4.22$.

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arXiv2003

Wigner crystalization about $ν$=3

We measure a resonance in the frequency dependence of the real diagonal conductivity, Re[$σ_{xx}$], near integer filling factor, $ν=3$. This resonance depends strongly on $ν$, with peak frequency $f_{pk} \approx 1.7$ GHz at $ν=3.04$ or 2.92 close to integer $ν$, but $f_{pk} \approx$ 600 MHz at $ν=3.19$ or 2.82, the extremes of where the resonance is visible. The dependence of $f_{pk}$ upon $n^*$, the density of electrons in the partially filled level, is discussed and compared with similar measurments by Chen {\it et al.}\cite{yong} about $ν=1$ and 2. We interpret the resonance as due to a pinned Wigner crystal phase with density $n^*$ about the $ν=3$ state.

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arXiv2003

Microwave Resonance of 2D Wigner Crystal around integer Landau fillings

We have observed a resonance in the real part of the finite frequency diagonal conductivity using microwave absorption measurements in high quality 2D electron systems near {\em integer fillings}. The resonance exists in some neighborhood of filling factor around corresponding integers and is qualitatively similar to previously observed resonance of weakly pinned Wigner crystal in high $B$ and very small filling factor regime. Data measured around both $ν=1$ and $ν=2$ are presented. We interpret the resonance as the signature of Wigner crystal state around integer Landau levels.

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arXiv2002

Microwave resonances of the bubble phases in 1/4 and 3/4 filled higher Landau levels

We have measured the diagonal conductivity in the microwave regime of an ultrahigh mobility two dimensional electron system. We find a sharp resonance in Re[sigma_{xx}] versus frequency when nu > 4 and the partial filling of the highest Landau level, nu^*, is ~ 1/4 or 3/4 and temperatures < 0.1 K. The resonance appears for a range of nu^* from 0.20 to 0.37 and again from 0.62 to 0.82. the peak frequency, f_{pk} changes from ~ 500 to ~ 150 as nu^* = 1/2 is approached. This range of f_{pk} shows no dependence on nu where the resonance is observed. The quality factor, Q, of the resonance is maximum at ~ nu^* = 0.25 and 0.74. We interpret the resonance as due to a pinning mode of the bubble phase crystal.

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arXiv2002

Correlation lengths of Wigner crystal order in two dimensional electron system at high magnetic field

The insulator terminating the fractional quantum Hall series at low Landau level filling νis generally taken to be a pinned Wigner crystal (WC), and exhibits a microwave resonance that is interpreted as a WC pinning mode. Systematically studying the resonance in a high quality sample for carrier densities, n, between 1.8 and 5.7 x 10^{10} cm^-2, we find maxima in resonance peak frequency, f_pk, vs magnetic field, B. L, the domain size, or correlation length of Wigner crystalline order, is calculated from f_pk. For each n, L vs νtends at low νtoward a straight line with intercept; the linear fit is accurate over as much as a factor of 5 range of ν. We interpret the striking linear behavior as due to B compressing the electron wavefunctions, to alter the effective electron-impurity interaction.

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arXiv2001

Giant microwave photoresistance of two-dimensional electron gas

We measure microwave frequency (4-40 GHz) photoresistance at low magnetic field B, in high mobility 2D electron gas samples, excited by signals applied to a transmission line fabricated on the sample surface. Oscillatory photoresistance vs B is observed. For excitation at the cyclotron resonance frequency, we find an unprecedented, giant relative photoresistance (ΔR)/R of up to 250 percent. The photoresistance is apparently proportional to the square root of applied power, and disappears as the temperature is increased.

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arXiv2001

High Magnetic Field Microwave Conductivity of 2D Electrons in an Array of Antidots

We measure the high magnetic field ($B$) microwave conductivity, Re$σ_{xx}$, of a high mobility 2D electron system containing an antidot array. Re$σ_{xx}$ vs frequency ($f$) increases strongly in the regime of the fractional quantum Hall effect series, with Landau filling $1/3<ν<2/3$. At microwave $f$, Re$σ_{xx}$ vs $B$ exhibits a broad peak centered around $ν=1/2$. On the peak, the 10 GHz Re$σ_{xx}$ can exceed its dc-limit value by a factor of 5. This enhanced microwave conductivity is unobservable for temperature $T \gtrsim 0.5$ K, and grows more pronounced as $T$ is decreased. The effect may be due to excitations supported by the antidot edges, but different from the well-known edge magnetoplasmons.

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