Schottky Barrier Height Modulation of Metal/n-GeSn Contacts Featuring Low Contact Resistivity by in Situ Chemical Vapor Deposition Doping and NiGeSn Alloy Formation

Abstract

GeSn complementary metal-oxide-semiconductor (CMOS) devices have attracted much attention for future VLSI technology nodes due to high carrier mobility. However, Fermilevel pinning in metal/n-GeSn contacts leads to high contact resistivity and limits GeSn CMOS devices for high-performance logic applications. In this work, we investigate Schottky characteristics and contact resistivity in the metal/n-GeSn contacts. Highquality n-GeSn layers were epitaxially grown by chemical vapor deposition with an in situ doping technique with a high carrier activation rate of 73% up to a doping concentration of ∼1.3 × 10 20 cm −3 . The electron Schottky barrier heights of the metal/n-GeSn contacts with different Sn fractions and metal work functions were extracted by an I−T method. The results show that the Fermi level is pinned at an energy level slightly above the valence band maximum. The Schottky barrier height is highly correlated with the GeSn band gap energy and decreases with the Sn fraction. The contact resistivity of the metal/n-Ge 0.9 Sn 0.1 contacts was extracted by a refined transmission line model and effectively reduced by increasing the doping concentration in the n-GeSn films. A post-metal annealing step at 400 °C was performed to further reduce the Schottky barrier height by forming NiGeSn alloys. A record low contact resistivity of ∼1.5 × 10 −7 Ω•cm 2 is achieved without any surface treatment. This is attributed to the reduced Schottky barrier height by increasing the Sn fraction in the n-GeSn film and the reduced Schottky barrier width due to the high carrier density achieved by in situ doping.

Room‐Temperature Negative Differential Resistance and High Tunneling Current Density in GeSn Esaki Diodes

“…The effective doping concentration is defined as $${N^{\rm{*}}} = \left( {{N_{\rm{A}}}{N_{\rm{D}}}} \right){\rm{/}}\left( {{N_{\rm{A}}} + {N_{\rm{D}}}} \right)$$, where N A and N D are the doping concentrations of the p‐type and n‐type layers, respectively. Room‐temperature NDR is achieved for all devices due to high doping activation rates, [ 18 ] the sharp doping abruptness at the p–n junction interface, and the suppressed DAT, which is attributed to the high material quality of GeSn epitaxial layers. All devices show clear NDR with high PVCRs of 1.8–5.5, which suggests that the peak current is dominated by the BTBT mechanism.…”

“…While increasing the doping concentrations is expected to increase the electric field and the BTBT rate, the tunneling current might not be enhanced effectively owing to the reduced activation rates of dopants at concentrations higher than 10 20 cm −3 . [18,19] On the other hand, reducing the bandgap energy can lower the tunnel barrier to increase BTBT rates. With similar effective doping concentrations of 1.4 to 1.5 × 10 19 cm −3 , the peak current density of a GeSn Eskai diode is larger than that of a Ge Esaki diode by a factor of 12 (Figure 3b).…”

“…[17] Low-temperature CVD growth enables steep doping profiles (≤3 nm per decade) close to the pn junction and high activation rates of dopants (≥90 %) by Hall measurements. [18] This is important for high-performance Esaki diodes since the tunneling rate depends on the junction electric field, determined by the activated doping levels and abruptness in the depletion region. The Sn fractions and the strains of the GeSn epilayers were characterized by X-ray diffraction (XRD) and high-resolution reciprocal space mapping (HR-RSM) analysis.…”

Metal-Semiconductor-Metal Photodetectors on a GeSn-on-Insulator Platform for 2 µm Applications

“…The high density of dislocations/defects in GeSn enhance twostep trap-assisted tunneling (TAT) or variable-range-hopping (VRG) conduction mechanisms [29]. Two-step TAT is the process where the carriers in the metal are excited thermally and tunnel via the trap states in the Schottky barrier to reach the conduction/valence bands.…”

Functionalized MXene Nanosheets and Al-Doped ZnO Nanoparticles for Flexible Transparent Electrodes

“…However in practice, metals form Schottky contacts irrespective of work function because the Fermi level of metal is pinned at a certain energy level at the semiconductor interface. − This Fermi-level pinning arises from the surface states of the semiconductor. Thus, we need new and approachable routes to achieve an ideal MS junction where the metal’s work function aligns with the conduction or valence band of the semiconductor without Fermi-level pinning. − By introducing a graphene layer in between the MS junction, the Schottky properties can be altered to Ohmic and reduce the Fermi-level pinning of the semiconductor. − The graphene layer also modulates its work function via doping of metal impurities which reduces the Schottky height and facilitates charge transport in both directions. , The work function of graphene can also be modified by the functionalization of some chemical elements …”