K. E. Moselund

Si–InAs heterojunction Esaki tunnel diodes with high current densities

Si–InAs heterojunction p-n diodes were fabricated by growing InAs nanowires in oxide mask openings on silicon substrates. At substrate doping concentrations of 1×1016 and 1×1019 cm−3, conventional diode characteristics were obtained, from which a valence band offset between Si and InAs of 130 meV was extracted. For a substrate doping of 4×1019 cm−3, heterojunction tunnel diode characteristics were obtained showing current densities in the range of 50 kA/cm2 at 0.5 V reverse bias. In addition, in situ doping of the InAs wires was performed using disilane to further boost the tunnel currents up to 100 kA/cm2 at 0.5 V reverse bias for the highest doping ratios.

InAs–Si Nanowire Heterojunction Tunnel FETs

In this letter, we present vertical InAs-Si nanowire heterojunction tunnel FETs (TFETs). The devices consist of an InAs source on a Si channel and drain, with a wraparound gate stack. The Si-InAs combination allows achieving high <i>I</i>_on/<i>I</i>_off ratios above 10⁶, with an <i>I</i>_on of 2.4 μA/μm and an inverse subthreshold slope of 150 mV/dec measured over three decades of current. Ni alloying of the InAs top contact is shown to improve performance of both diodes and TFETs significantly. The combination of higher doping at the contact and the alloying also leads to an enhanced performance compared with previously published devices.

Vertical III-V nanowire device integration on Si(100).

We report complementary metal-oxide-semiconductor (CMOS)-compatible integration of compound semiconductors on Si substrates. InAs and GaAs nanowires are selectively grown in vertical SiO2 nanotube templates fabricated on Si substrates of varying crystallographic orientations, including nanocrystalline Si. The nanowires investigated are epitaxially grown, single-crystalline, free from threading dislocations, and with an orientation and dimension directly given by the shape of the template. GaAs nanowires exhibit stable photoluminescence at room temperature, with a higher measured intensity when still surrounded by the template. Si-InAs heterojunction nanowire tunnel diodes were fabricated on Si(100) and are electrically characterized. The results indicate a high uniformity and scalability in the fabrication process.

Template-assisted selective epitaxy of III–V nanoscale devices for co-planar heterogeneous integration with Si

III–V nanoscale devices were monolithically integrated on silicon-on-insulator (SOI) substrates by template-assisted selective epitaxy (TASE) using metal organic chemical vapor deposition. Single crystal III–V (InAs, InGaAs, GaAs) nanostructures, such as nanowires, nanostructures containing constrictions, and cross junctions, as well as 3D stacked nanowires were directly obtained by epitaxial filling of lithographically defined oxide templates. The benefit of TASE is exemplified by the straightforward fabrication of nanoscale Hall structures as well as multiple gate field effect transistors (MuG-FETs) grown co-planar to the SOI layer. Hall measurements on InAs nanowire cross junctions revealed an electron mobility of 5400 cm2/V s, while the alongside fabricated InAs MuG-FETs with ten 55 nm wide, 23 nm thick, and 390 nm long channels exhibit an on current of 660 μA/μm and a peak transconductance of 1.0 mS/μm at VDS = 0.5 V. These results demonstrate TASE as a promising fabrication approach for heterogeneou...

Fabrication and Characterization of Gate-All-Around Silicon Nanowires on Bulk Silicon

This paper reports on the top-down fabrication and electrical performance of silicon nanowire (SiNW) gate-all-around (GAA) n-type and p-type MOSFET devices integrated on bulk silicon using a local-silicon-on-insulator (SOI) process. The proposed local-SOI fabrication provides various nanowire cross sections: Omega-like, pentagonal, triangular, and circular, all controlled by isotropic etching using nitride spacers and silicon sacrificial oxidation. The reported top-down SiNW fabrication offers excellent control of wire doping and placement, as well as ohmic source and drain contacts. A particular feature of the process is the buildup of a tensile strain in all suspended nanowires, attaining values of few percents, reflected in stress values higher than 2-3 GPa. A very high yield (>90%) is obtained in terms of functionality of long-channel SiNW GAA mosfet. Device characteristics are reported from cryogenic temperature (T = 5 K) up to 150 degC, and promising characteristics in terms of low-field electron mobility, threshold voltage control, and subthreshold slope are demonstrated. Low field mobility for electrons up to 850 cm2 /Vmiddots is reported at room temperature in suspended devices with triangular cross sections; this mobility enhancement is explained by the process-induced tensile strain. In short, suspended SiNW GAA with small triangular cross sections, a single-electron transistor (SET) operation regime is highlighted at T = 5 K. This is attributed to a combined effect of strain and corner conduction in triangular channel cross sections, suggesting the possibility to hybridize CMOS and SET functions by a unique nanowire fabrication platform.

Silicon Nanowire Tunnel FETs: Low-Temperature Operation and Influence of High-

In this paper, we demonstrate p-channel tunnel FETs based on silicon nanowires grown with an in situ p-i-n doping profile. The tunnel FETs were fabricated with three different gate dielectrics, SiO₂, Al₂O₃, and HfO₂, and show a performance enhancement when using high-<i>k</i> dielectric materials. The best performance is achieved for the devices using HfO₂ as the gate dielectric, which reach an <i>I</i>_on of 0.1 μA/μm (<i>V</i>_DS = -0.5 V, <i>V</i>_GS = -2 V), combined with an average inverse subthreshold slope (SS) of ~ 120 mV/dec and an <i>I</i>_on/<i>I</i>_off ratio of around 10⁶. For the tunnel FETs with Al₂O₃ as the gate dielectric, different annealing steps were evaluated, and an activation anneal at only 700°C was found to yield the best results. Furthermore, we also investigated the temperature behavior of the tunnel FETs. Ideal tunnel FET behavior was observed for devices having ohmic Ni/Au contacts, and we demonstrate the invariance of both the SS and on-current with temperature, as expected for true tunnel FETs.

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This work demonstrates a method for incorporating strain in silicon nanowire gate-all-around (GAA) n-MOSFETs by oxidation-induced bending of the nanowire channel and reports on the resulting improvement in device performance. The variation in strain measured during processing is discussed. The strain profile in silicon nanowires is evaluated by Raman spectroscopy both before device gate stack fabrication (tensile strains of up to 2.5% are measured) and by measurement through the polysilicon gate on completed electrically characterized devices. Drain current boosting in bended n-channels is investigated as a function of the transistor operation regime, and it is shown that the enhancement depends on the effective electrical field. The maximum observed electron mobility enhancement is on the order of 100% for a gate bias near the threshold voltage. Measurements of stress through the full gate stack and experimental device characteristics of the same transistor reveal a stress of 600 MPa and corresponding improvements of the normalized drain current, normalized transconductance, and low-field mobility by 34% (at maximum gate overdrive), 50% (at g max), and 53%, respectively, compared with a reference nonstrained device at room temperature. Finally, it is found that, at low temperatures, the low-field mobility is much higher in bended devices, compared with nonbended devices.

Gate Dielectric

In this paper we present vertical tunnel diodes and tunnel FETs (TFETs) based on III-V-Si nanowire heterojunctions. We experimentally demonstrate InAs-Si Esaki tunnel diodes with record high currents of 6 MA/cm² at 0.5 V in reverse bias. Furthermore, we have fabricated vertical InAs-Si nanowire TFETs with gate-all-around architecture and high-k dielectrics. The InAs-Si combination allows achieving high I_on/I_off ratios above 10⁶, with I_on of 2.4 μA/μm and an inverse subthreshold slope of 150 mV/dec over three decades. The achieved improvements can be attributed to increased nanowire doping and Ni alloying of the top contact. The results indicate the benefits of the InAs-Si material system combining the possibility of achieving high I_on with high I_on/I_off ratio.

The High-Mobility Bended n-Channel Silicon Nanowire Transistor

In this letter, the occupancy and tunneling probabilities of interband tunneling devices are studied, pointing out the fundamental function of the source Fermi-Dirac distribution. Particularly, the reason for the degraded subthreshold swing, which is typical of devices with highly doped source, is explained, and its relation with the high-energy source Fermi tail is carefully analyzed. Simultaneously, the poor driving capability of Tunnel-FET devices is investigated, highlighting the primary role played by the occupancy functions.

InAs-Si heterojunction nanowire tunnel diodes and tunnel FETs

In the present work we demonstrate the fabrication of tunneling field-effect transistors (TFETs) based on VLS grown silicon nanowires (Si NWs). We have integrated two different gate stacks, a conventional one using SiO2 and a HfO2 high-k gate stack. The use of a high-k gate dielectric markedly improves the TFET performance in terms of average slope and on-current, Ion. Furthermore, we investigate the low-temperature behaviour of the TFETs.