Subnanometer-equivalent-oxide-thickness germanium p-metal-oxide-semiconductor field effect transistors fabricated using molecular-beam-deposited high-k/metal gate stack

Abstract

Metal-oxide-semiconductor field effect transistors (MOSFET) with a thin high-k dielectric were fabricated on bulk n-type germanium substrates. Surface oxides were thermally desorbed in situ by heating the substrates under ultrahigh vacuum conditions. First an ultrathin passivating layer was formed by evaporating germanium in the presence of atomic oxygen and nitrogen supplied from a remote radio frequency plasma source. Subsequently, the HfO2 dielectric was deposited by evaporating hafnium in the presence of atomic oxygen. An in situ TaN metal gate was similarly deposited. Long channel devices were fabricated using a standard process flow. These devices exhibited a low equivalent oxide thickness (EOT) of 0.7nm with gate leakage less than 15mA∕cm2 at VFB+1V. Device mobility was extracted from Is-Vg and split C-V characteristics. Results indicate a 2× mobility enhancement in Ge p-MOSFET devices compared to Si control devices. The demonstration of subnanometer EOT suggests that high-k gate dielectrics on germanium are scalable to low EOT and suitable for use in ultrascaled MOSFET devices.

Interface engineering for Ge metal-oxide–semiconductor devices

“…As a result, the current-voltage (CV) characteristics are non-ideal [5] exhibiting large hysteresis and frequency dispersion mainly in inversion [9,10] and high interface density of states D it , typically around 5 × 10 12 -10 13 eV − 1 cm − 2 , which cannot be cured even after H 2 or other post deposition annealing treatment. In addition, channel mobility in Ge FETs [8], is lower than expected, especially in n-channel FETs. This is a serious drawback attributed, in part, to the poor quality of the GeON passivating layer.…”

“…3-5 above suggest that the GeO x or GeON interfacial layers are unstable and are partly dissociated by reaction with HfO 2 releasing Ge or oxidized Ge complexes inside the HfO 2 layer. Despite the instability of the interfacial layer and the Ge in-diffusion, these gate stacks produce capacitors with low EOT and low gate leakage [5,13] as well as functional Ge MOSFET devices having sub-1 nm EOT values [8] and high Ion/Ioff current ratios. As it can be seen from Fig.…”

“…Indeed, works by ultra-high vacuum (UHV) ozone oxidation of ZrO 2 [6] or molecular beam deposition (MBD) of HfO 2 [5,7] have shown that these dielectrics combined with ultrathin Ge oxynitride ILs, are scalable to EOT values well below 1 nm with low gate leakage, below 10 − 3 A/cm 2 at V FB -1V and a high-k value ∼ 25 [5]. Moreover, Ge p-channel field-effect-transistors (FETs) with MBD GeON/HfO 2 gates have been demonstrated which show EOT much less than 1 nm [8] and remarkably high I ON /I OFF ratio ∼ 10 5 . The general consensus is that a passivating GeO x or GeO x N y layer is necessary prior to HfO 2 MBD to obtain functional capacitor and transistor devices but it does not provide sufficient passivation of Ge interface.…”

Gold and silver diffusion in germanium: a thermodynamic approach

“…This is driven by the advent of high-k gate dielectric materials as these have eliminated the requirement of a good quality native oxide in advanced nanoelectronic devices [6,7]. Additionally, the recent investigation of diffusion phenomena such as the discovery that boron diffuses slower in Ge than originally thought has also assisted the exploitation of Ge [8].…”

Mechanism of dopant-vacancy association in α-quartz GeO2

“…The introduction of high-k dielectrics such as HfO 2 and ZrO 2 can lead to a lowering of the equivalent oxide thickness and sharpening of the Ge/dielectric interface. [12][13][14] However, the performance is compromised by the inability to achieve a low equivalent oxide thickness and good interface quality simultaneously. Presently, there is a trend to readdress the incorporation of GeO 2 as a possible passivation layer (Ge/GeO 2 /high-k dielectric stack).…”