Record 67,000 cm²/Vs Hole Mobility Achieved at Germanane/Ge(111) Heterointerface

The pursuit of materials exhibiting high carrier mobility is central to advancements in microelectronics, and a new study details a remarkably mobile hole gas formed at the interface between two-dimensional germanane and three-dimensional germanium. Yumiko Katayama, Daiki Kobayashi, and Hikaru Okuma, all from the Graduate School of Arts and Sciences at the University of Tokyo, alongside Yasutake et al., report a hole mobility of 67,000 cm²/Vs at 15K , a record for this type of material system. Their research demonstrates this performance arises from a unique ‘allotropic cross-dimensional’ heterointerface created by topotactically transforming germanane layers on a Ge(111) surface. Significantly, the findings suggest a pathway to creating high-mobility two-dimensional hole gases without the need for complex fabrication techniques like heteroepitaxy or modulation doping, potentially simplifying future device development. The observed metallic conduction and substantial magnetoresistance further confirm the presence of a highly conductive, two-dimensional hole gas confined at this novel interface.

Germanium Interface Exhibits Unexpected Metallic Conduction

Researchers have observed metallic conduction at the interface between two distinct forms of germanium , a 2D GeH layer and bulk 3D Ge(111) , even at a low temperature of 15 K. This behaviour arises from a topotactically-transformed 2D GeH layer directly meeting the 3D bulk germanium. The temperature dependence of hole mobility (μh) suggests metallic conduction is maintained between 20 K and 250 K, without significant scattering from ionised impurities. A sheet hole density of 2.8×10¹¹cm−² aligns closely with a value of 3.0×10¹¹cm−² obtained through Hall measurements. Accompanying Shubnikov-de Haas oscillations, a substantial magnetoresistance of 6,500% is detected at a magnetic field of 7 T, even at 15 K. These observations indicate single-band conduction of holes with a small effective mass within the in-plane directions, supporting the formation of a two-dimensional hole gas (2DHG). The allotropic cross-dimensional heterointerface between the 2D GeH and 3D Ge is therefore crucial in facilitating this unique electronic behaviour.

Germanium Synthesis, Heterostructures and Electronic Properties The creation

Various methods are being investigated to synthesize and stabilize 2D Ge, including molecular beam epitaxy (MBE), chemical vapor deposition (CVD), and exfoliation, aiming to leverage its higher carrier mobility compared to silicon for faster and more efficient transistors. Creating heterostructures and alloying Ge with other elements like tin or silicon are key strategies to tune the electronic properties and stability of 2D Ge, overcoming its inherent instability.

High Hole Mobility in Germanium Heterointerfaces

Scientists have achieved a record hole mobility of 67,000 cm²/Vs at 15 K within a novel allotropic cross-dimensional heterointerface. This breakthrough stems from the creation of topotactically-transformed two-dimensional germanium hydride (GeH) layers integrated with a three-dimensional Ge(111) substrate. The team meticulously measured the temperature dependence of hole mobility, revealing metallic conduction characteristics without the influence of ionized impurity scattering between 20 K and 250 K. Experiments demonstrate a strong correlation between sheet hole density, recorded at 2.8x 10¹¹ cm⁻², and Hall measurements yielding a value of 3.0x 10¹¹ cm⁻².

Accompanying the observed high mobility is a remarkable 6,500% magnetoresistance at 7 Tesla, evidenced by clear Shubnikov-de Haas oscillations persisting even at 15 K. These oscillations confirm single-band conduction of holes with a surprisingly small effective mass, supporting the formation of a two-dimensional hole gas (2DHG) at the GeH/Ge interface. Calculations reveal the work function of the GeH layer to be 4.45 eV, lower than the 4.65 eV predicted for a GeH (111) surface and the experimentally determined 4.8 eV, indicating a type-II potential alignment. This alignment facilitates hole transfer from the germanium to the GeH layer, effectively isolating the 2DHG from potential scattering from ionized acceptors within the germanium buffer.

The opening of a direct gap of approximately 1.4 eV further enhances the material’s electronic properties, creating distinct near-edge bands defined relative to the valence band maximum. Detailed band structure analysis indicates a hole effective mass of 0.12 me, a value attributed to the inherent two-dimensional nature of GeH. The team determined a hole density of 10¹⁹ cm⁻³ for the GeH/Ge(111) interface, confirming exclusive occupation of the first band. This combination of small effective mass and efficient carrier confinement delivers exceptional hole mobilities, paving the way for facile 2DHG creation without the need for complex heteroepitaxy or modulation doping techniques.

High Mobility 2DHG in GeGeH Heterostructure

This work demonstrates the creation of a two-dimensional hole gas (2DHG) within an allotropic cross-dimensional heterostructure formed by combining germanium (Ge) and hydrogen-terminated germanium (GeH). A record hole mobility of approximately 67,000cm²/Vs was achieved at 15 K, facilitated by the unique interface between the 2D GeH layer and bulk 3D Ge(111). The observed temperature dependence of hole mobility suggests metallic conduction, and Hall measurements corroborate the sheet hole density determined from calculations. The researchers attribute the high mobility to both a small effective mass and a reduced scattering rate within the GeH conducting layer, consistent with a type-II band alignment. Furthermore, the presence of significant magnetoresistance and clear Shubnikov-de Haas oscillations confirms single-band hole conduction. Further investigation is needed to fully understand the behaviour of this system and to explore its potential for device applications, potentially optimising the heterostructure to enhance carrier density and mobility, and investigating the long-term stability of the interface.