Qinglei Guo

Direct Growth of Graphene Film on Germanium Substrate

Graphene has been predicted to play a role in post-silicon electronics due to the extraordinary carrier mobility. Chemical vapor deposition of graphene on transition metals has been considered as a major step towards commercial realization of graphene. However, fabrication based on transition metals involves an inevitable transfer step which can be as complicated as the deposition of graphene itself. By ambient-pressure chemical vapor deposition, we demonstrate large-scale and uniform depositon of high-quality graphene directly on a Ge substrate which is wafer scale and has been considered to replace conventional Si for the next generation of high-performance metal-oxide-semiconductor field-effect transistors (MOSFETs). The immiscible Ge-C system under equilibrium conditions dictates graphene depositon on Ge via a self-limiting and surface-mediated process rather than a precipitation process as observed from other metals with high carbon solubility. Our technique is compatible with modern microelectronics technology thus allowing integration with high-volume production of complementary metal-oxide-semiconductors (CMOS).

Bendable Photodetector on Fibers Wrapped with Flexible Ultrathin Single Crystalline Silicon Nanomembranes

Silicon (Si) nanomembranes (NMs) enable conformal covering on complicated surfaces for novel applications. We adopt classical fibers as flexible/curved substrates and wrap them with freestanding ultrathin Si-NMs with a thickness of ∼20 nm. Intrinsic defects in single-crystalline Si-NMs provide a flow path for hydrofluoric acid (HF) to release the NM with a consecutive area of ∼0.25 cm2. Such Si-NMs with ultralow flexural rigidities are transferred onto a single-mode fiber (SMF) and functionalized into bendable photodetectors, which detects the leaked light when the fiber is bent. Our demonstration exemplifies optoelectronic applications in flexible photodetector for Si-NMs in a three-dimensional (3D) geometry.

Three dimensional strain distribution of wrinkled silicon nanomembranes fabricated by rolling-transfer technique

This paper introduces a simple transfer technique named as rolling-transfer technology to transfer Si nanomembranes to pre-stressed elastomers with nearly 100% transfer efficiency. When transferred onto the elastomeric substrate, wave-like wrinkled Si nanomembranes with uniform periodicity and amplitude are formed. The three dimensional (3-D) strain distribution of the wrinkled Si nanomembranes has been investigated in detail through the micro-Raman mapping using two excited laser wavelengths. The sinusoidal bulking geometry of Si nanomembrane results in a periodical strain alternation along x direction, while a homogenous strain distribution in y direction. The inhomogeneous strain distribution along z direction can be interpreted with the physical model considering the shift of the neutral mechanical plane, which is qualitatively determined by the Von Karman elastic nonlinear plate theory, including the bending effect and the shear forces existing at the Si nanomembrane/elastomeric substrate interface.

Deterministic Assembly of Flexible Si/Ge Nanoribbons via Edge-Cutting Transfer and Printing for van der Waals Heterojunctions

As the promising building blocks for flexible electronics and photonics, inorganic semiconductor nanomembranes have attracted considerable attention owing to their excellent mechanical flexibility and electrical/optical properties. To functionalize these building blocks with complex components, transfer and printing methods in a convenient and precise way are urgently demanded. A combined and controllable approach called edge-cutting transfer method to assemble semiconductor nanoribbons with defined width (down to submicrometer) and length (up to millimeter) is proposed. The transfer efficiency can be comprehended by a classical cantilever model, in which the difference of stress distributions between forth and back edges is investigated using finite element method. In addition, the vertical van der Waals PN (p-Si/n-Ge) junction constructed by a two-round process presents a typical rectifying behavior. The proposed technology may provide a practical, reliable, and cost-efficient strategy for transfer and printing routines, and thus expediting its potential applications for roll-to-roll productions for flexible devices.

Uniaxial and tensile strained germanium nanomembranes in rolled-up geometry by polarized Raman scattering spectroscopy

We present a rolled-up approach to form Ge microtubes and their array by rolling-up hybrid Ge/Cr nanomembranes, which is driven by the built-in stress in the deposited Cr layer. The study of Raman intensity as a function of the angle between the crystal-axis and the polarization-direction of the scattered light, i.e., polarized Raman measurement reveals that the strain state in Ge tube is uniaxial and tensile, and can reach a maximal value 1.0%. Both experimental observations and theoretical calculations suggest that the uniaxial-tensile strain residual in the rolled-up Ge tubes correlates with their tube diameters, which can be tuned by the thicknesses of the Cr layers deposited. Using the polarized Raman scattering spectroscopy, our study provides a comprehensive analysis of the strain state and evolution in self-rolled-up nano/micro-tubes.

Growth of homogeneous single-layer graphene on Ni-Ge binary substrate

In contrast to the commonly used chemical vapor deposition growth that leads to multilayer graphene formation by carbon segregation from the Ni bulk, we designed a Ni-Ge binary system to directly grow graphene film on Ni-Ge binary substrate, via chemical vapor deposition with methane and hydrogen gas as precursors. Our system fully overcomes the fundamental limitations of Ni and yields homogenous single layer graphene over large areas. The chemical vapor deposition growth of graphene on Ni-Ge binary substrate shows that self limiting monolayer graphene growth can be obtained on these substrate.

Strain redistribution in free-standing bridge structure released from strained silicon-on-insulator

The strain evolution including relaxation and conversion during the fabrication of free-standing bridge structure, which is the building block for the gate-all-around transistor, has been investigated in strained silicon-on-insulator. Compared to the starting strained silicon-on-insulator substrate, the strain of the free-standing bridge structure transforms from the biaxial strain to the uniaxial strain after patterning and release due to its unique configuration, as suggested by UV-Raman spectroscopy. Furthermore, such uniaxial strain has strong correlation with the dimension of the suspended structure, and it is enhanced as the width of the free-standing bridge decreases and the size of the connected pad increases. For 0.5μm-wide free-standing bridge connected to the pad of 16 × 16 μm2, the maximum uniaxial tensile strain of 4.65% is obtained, which remarkably exceeds the levels that can be achieved by other techniques ever reported. The observed strain redistribution phenomenon is also analyzed by two...

Wrinkled Single-Crystalline Germanium Nanomembranes for Stretchable Photodetectors

Germanium nanomembranes are suitable for flexible electronics, including high-mobility nonsilicon transistors, fast radio-frequency switches, microwave diodes, and high-performance photodetectors. In order to enhance the flexibility of the germanium-based devices, we present a strategy to integrate single-crystalline germanium nanomembranes into a wave-like wrinkled geometry with a uniform periodicity and amplitude on elastomeric substrates. Wrinkled single-crystalline germanium nanomembranes are realized with a reversible and large deformation up to 10%, and the stretchable metal–germanium–metal photodetectors have been demonstrated. Optoelectronic response studies reveal that the wrinkled germanium-based photodetectors exhibit enhanced efficiency of optoelectronic interactions compared with planar photodetectors using flat germanium nanomembranes. Furthermore, the wrinkled photodetectors reveal high response speed and stretchable capability of up to 8.56%. This paper may pave the way for the integration of germanium nanomembranes into the field of flexible/wearable optoelectronics.

Controllable cracking behavior in Si/Si0.70Ge0.30/Si heterostructure by tuning the H+ implantation energy

Controllable cracking behaviors are realized in Si with a buried B doped Si0.70Ge0.30 interlayer by tuning the H+ projected ranges using the traditional H implantation technique. When the projected range is shallower (deeper) than the depth of the buried Si0.70Ge0.30 layer, cracking occurs at the interface between the top Si layer (bottom handle Si wafer) and the Si0.70Ge0.30 interlayer, thus resulting in the formation of continuous sharp crack confined at the Si0.70Ge0.30/Si interfaces. For the case that the H-ion projected range is located at the B-doped Si0.70Ge0.30 buried interlayer, continuous cracking is observed along the interlayer, which is similar to the conventional ion-cut method. We attribute these controlled cracking behaviors to the B doped Si0.70Ge0.30 interlayer, which holds a large amount of B impurities and compressive strain, and H ions can be trapped and confined at the interfaces or within the interlayer (depended on projected ranges) to facilitate the formation of cracks.

Nanogranular SiO2 proton gated silicon layer transistor mimicking biological synapses

Silicon on insulator (SOI)-based transistors gated by nanogranular SiO2 proton conducting electrolytes were fabricated to mimic synapse behaviors. This SOI-based device has both top proton gate and bottom buried oxide gate. Electrical transfer properties of top proton gate show hysteresis curves different from those of bottom gate, and therefore, excitatory post-synaptic current and paired pulse facilitation (PPF) behavior of biological synapses are mimicked. Moreover, we noticed that PPF index can be effectively tuned by the spike interval applied on the top proton gate. Synaptic behaviors and functions, like short-term memory, and its properties are also experimentally demonstrated in our device. Such SOI-based electronic synapses are promising for building neuromorphic systems.