Florence Bellenger

Effective electrical passivation of Ge(100) for high-k gate dielectric layers using germanium oxide

In search of a proper passivation for high-k Ge metal-oxide-semiconductor devices, the authors have deposited high-k dielectric layers on GeO2, grown at 350–450°C in O2. ZrO2, HfO2, and Al2O3 were deposited by atomic layer deposition (ALD). GeO2 and ZrO2 or HfO2 intermix during ALD, together with partial reduction of Ge4+. Almost no intermixing or reduction occurs during Al2O3 ALD. Capacitors show well-behaved capacitance-voltage characteristics on both n- and p-Ge, indicating efficient passivation of the Ge∕GeOx interface. The density of interface states is typically in the low to mid-1011cm−2eV−1 range, approaching state-of-the-art Si∕HfO2∕matal gate devices.

Passivation of Ge ( 100 ) ∕ GeO2 ∕ high-κ Gate Stacks Using Thermal Oxide Treatments

The physical and electrical properties of Ge/GeO 2 /high-κ gate stacks, where the GeO 2 interlayer is thermally grown in molecular oxygen, are investigated. The high-K layer (ZrO 2 , HfO 2 , or Al 2 O 3 ) is deposited in situ on the GeO 2 interlayer by atomic layer deposition. Detailed analysis of the capacitance-voltage and conductance-frequency characteristics of these devices provides evidence for the efficient passivation of the Ge(100) surface by its thermal oxide layer. A larger flatband voltage hysteresis is observed in HfO 2 -based gate stacks, as compared to Al 2 O 3 gate stacks, which is possibly related to the more pronounced intermixing observed between the HfO 2 and GeO 2 .

Electronic structure of GeO2-passivated interfaces of (100)Ge with Al2O3 and HfO2

Analysis of internal photoemission and photoconductivity in Ge/thermal germanium oxide/high-dielectric constant oxide (HfO2,Al2O3) structures reveals that the bandgap of the germanium oxide interlayer is significantly lower (4.3±0.2eV) than that of stiochiometric GeO2 (5.4–5.9eV). As a result, the conduction and valence band offsets at the interface appear to be insufficient to block electron and hole injection leading to significant charge trapping in the GeOx∕high-κ oxide stack. Formation of a hydroxyl-rich Ge oxide phase is suggested to be responsible for the modification of the oxide properties.

Germanium for advanced CMOS anno 2009: a SWOT analysis

Germanium has emerged as an exciting alternative material for high-performance scaled CMOS, however not without difficulties. After a review of the state-of-the-art, mainly focusing on two techniques to passivate the channel/dielectric interface, we analyze the strengths (carrier mobility, band gap), and weaknesses (n-type doping, lattice mismatch and BTBT leakage) of Ge for MOSFETs. We also identify some opportunities and the most important threats for the future of germanium.

High FET Performance for a Future CMOS

In Germanium-based metal-oxide-semiconductor field-effect transistors, a high-quality interfacial layer prior to high-¿ deposition is required to achieve low interface state densities and prevent Fermi level pinning. In this letter, the physical and electrical properties of a Ge/GeO2/Al2O3 gate stack are investigated. The GeO2 interlayer grown by radical oxidation and the formation of a germanate (GeAlOX) layer at the interface provide a stable high-quality passivation of the Ge channel. High carrier mobilities (235 cm2/V·s for electrons and 265 cm2/V·s for holes) are demonstrated for a relatively low 3.7-nm equivalent oxide thickness (EOT), enabling the realization of a high-performance CMOS technology with potential EOT scaling.

\hbox{GeO}_{2}

Properties of CeO2 and CeO2/HfO2 bilayers grown by molecular beam deposition on in situ prepared, oxide-free Ge(100) surfaces are reported here. Deposition is achieved by a simultaneous flux of electron-beam evaporated metal (Ce or Hf) and of remote plasma generated atomic oxygen. These conditions result in an interfacial layer (IL) between the cubic CeO2 and Ge substrate. Electron energy loss spectroscopy shows that this IL is comprised of Ge and O and a small amount of Ce, and x-ray photoelectron spectroscopy suggests that the Ge is in a mix of 2+ and 3+ oxidation states. A comparison of capacitance, conductance, and leakage data shows a higher quality dielectric for 225 °C deposition than for room temperature. However, CeO2-only deposition results in an unacceptably high leakage current due to the small CeO2 band gap, which is remedied by the use of CeO2/HfO2 bilayers. Using the Nicollian–Goetzberger method, interface trap densities in the mid 1011 eV−1 cm−2 are obtained for CeO2/HfO2 gate stacks on bo...

-Based Technology

Long-channel Ge pMOSFETs and nMOSFETs were fabricated with high-kappa CeO₂/HfO₂/TiN gate stacks. CeO₂ was found to provide effective passivation of the Ge surface, with low diode surface leakage currents. The pMOSFETs showed a large I _ON/I_OFF ratio of 10⁶, a subthreshold slope of 107 mV/dec, and a peak mobility of approximately 90 cm²/Vmiddots at 0.25 MV/cm. The nMOSFET performance was compromised by poor junction formation and demonstrated a peak mobility of only ~3 cm²/Vmiddots but did show an encouraging I_ON/I _OFF ratio of 10⁵ and a subthreshold slope of 85 mV/dec

Materials and electrical characterization of molecular beam deposited CeO2 and CeO2/HfO2 bilayers on germanium

Atomic layer deposition (ALD) is considered an enabling technique for the deposition of dielectrics on sensitive surfaces such as germanium. Proper control of the interfacial layer between Ge and the high-κ layer has been shown to be crucial for obtaining good performance of Ge-based metal-oxide-semiconductor devices. In this work, we compare O 3 - and H 2 O-based ALD of HfO 2 and Al 2 O 3, and report on the thickness and electrical quality of GeO 2 passivation layers. The thickness of the interfacial layer depends on the oxidant used and affects the interface state density. A small degree of intermixing at the GeO 2 /high-κ interface is observed for all ALD process conditions. The interface state density can be significantly reduced by annealing the Pt-gated capacitors in forming gas (H 2 /N 2 ) at 300°C. After annealing, the interface state density becomes almost independent of the thickness of the GeO 2 passivation layer.

Germanium MOSFETs With

The extrinsic interfaces present at the HfO2∕GeOx∕Ge and Al2O3∕GeOx∕Ge gate stacks are investigated. The effective trapped charge density, estimated from hysteresis in capacitance-voltage characteristics, is higher for HfO2 than for Al2O3, implying qualitatively different charge trapping sources in each dielectric. Spectroscopic ellipsometry and medium energy ion scattering measurements reveal that HfO2 deposition induces the formation of a thicker germanate (intermixed) layer at the HfO2∕GeOx interface, where nonstoichiometric Ge-rich GeOx having significantly low bandgap (∼1.8eV) is present. In contrast, Al2O3 deposition leads to an abrupt and thinner O-rich GeOx interfacial layer without Ge-rich GeOx phase. The proposed band alignment indicates that Ge-rich GeOx layer at HfO2∕GeOx arises a significant band potential well trapping, while O-rich GeOx layer in Al2O3∕GeOx is responsible for a relatively lower charge trapping at band potential well. The combined results strongly suggest that the control of ...

\hbox{CeO}_{2}/\hbox{HfO}_{2}/ \hbox{TiN}

The electrical properties of Geo 1-x N x /HfO 2 stack deposited by atomic beam on Ge are reported. The incorporation of N in the GeO x layer was found to be quite beneficial in reducing the gate leakage current, improving the uniformity of the device characteristics, and reducing the equivalent oxide thickness of the gate stack down to ∼0.8 nm. On the other hand, N incorporation also led to a large density of fixed positive charges and slow states, which can be reduced by post-deposition anneals in O 2 or N 2 . These gate stacks were successfully integrated into a simple transistor flow with TiN gates. Both n- and p-channel transistors show promising electrical characteristics, with I on /I off ratios of about 4-5 orders of magnitude. However, the mobility of the n-channel devices was found to be extremely low, which is most likely related to a large density of acceptor-like interface states in the upper-part of the Ge bandgap.