Ge Nanomembranes Research Articles

Tailoring Ge membrane adhesion strength: Impact of growth parameters and porous layer thickness

The recent exploration of porous Germanium (PGe) techniques marks a significant advancement in the scalable production of detachable Ge membranes and devices. However, there is a notable gap in the comprehensive understanding of the critical factors necessary to control the adhesion strength of these nanomembranes. This study delves into the effects of Ge growth temperature, in-situ annealing processes, and the thickness of the PGe layer on the reorganization of the porous interlayer, subsequently influencing the properties of the separation layer. We particularly focus on how adjusting these parameters can fine-tune the adhesion strength of the epitaxial layer, ranging from a freestanding state to a fully bonded nanomembrane. A key finding is that the thermal budget experienced by the porous structure during the buffer layer's growth significantly affects the nanomembrane's adhesion characteristics; it is imperative to minimize this duration, especially at higher growth temperatures. Our research demonstrates that by precisely controlling the voids within the PGe layer, the adhesion strength can be effectively modulated through variations in the PGe layer's thickness. The outcomes of this study offer crucial insights into the controllable adhesion strength of Ge nanomembranes, paving the way for the targeted development of detachable devices.

Sequential fabrication of multiple Ge nanomembranes from a single wafer: Towards sustainable recycling of Ge substrates

Germanium (Ge) substrates are employed for III-V multijunction solar cells (MJSCs). However, due to limited availability and environmental impact, Ge is a critical raw material. Detachable Ge nanomembranes (NMs) are gaining attention to reduce Ge consumption through substrate reuse. The Porous germanium Efficient Epitaxial LayEr Release (PEELER) process enables Ge NM detachment and substrate reuse, reducing costs and Ge material consumption. In this work, we demonstrate the sequential production of three NMs from one parent substrate using the PEELER process, yielding monocrystalline NMs, with a surface suitable for further epitaxial growth of III-V layers (RMS < 3.2 nm). By avoiding the costly Chemical Mechanical Polishing (CMP), our approach enables a more economical and sustainable production of high-efficiency Ge-based solar cells, compared to the conventional production process.

A scalable, resource-efficient process for synthesis of self-supporting germanium nanomembranes

The design of self-supporting membranes is gaining high interest in a broad range of applications, from nanodevices, optoelectronics, sensing, separation, and catalysis. In this report, we demonstrate a synthesis pathway for high quality ultrathin self-supporting germanium membranes using a highly scalable and resource-efficient process. We discuss the process flow, the compatibility between the materials and the processing chemicals, the important details for achieving the pure membranes and potential anomalies related to the etching process. Comprehensive characterization using SEM, EDX, TEM and nuclear microprobe analysis is employed for revealing the physical and structural properties of the synthesized membranes. The outcome of the fabrication process is large-area Ge nanomembranes with extremely flat and clean surfaces.

Wafer-scale detachable monocrystalline germanium nanomembranes for the growth of III-V materials and substrate reuse.

Germanium (Ge) is increasingly used as a substrate for high-performance optoelectronics, photovoltaics, and electronic devices. These devices are usually grown on thick and rigid Ge substrates manufactured by classical wafering techniques. Nanomembranes (NMs) provide an alternative to this approach while offering wafer-scale lateral dimensions, weight reduction, waste limitation, and cost effectiveness. Herein, we introduce the Porous germanium Efficient Epitaxial LayEr Release (PEELER) process, which consists of the fabrication of wafer-scale detachable Ge NMs on porous Ge (PGe) and substrate reuse. We demonstrate the growth of Ge NMs with monocrystalline quality as revealed by high-resolution transmission electron microscopy (HRTEM) characterization. Together with the surface roughness below 1 nm, it makes the Ge NMs suitable for growth of III-V materials. Additionally, the embedded nanoengineered weak layer enables the detachment of the Ge NMs. Finally, we demonstrate the wet-etch-reconditioning process of the Ge substrate, allowing its reuse, to produce multiple free-standing NMs from a single parent wafer. The PEELER process significantly reduces the consumption of Ge in the fabrication process, paving the way for a new generation of low-cost flexible optoelectronic devices.

Investigation on dark current and photoresponsivity of flexible single-crystal semiconductor photodetectors on plastic substrates

High-performance flexible photodetectors have drawn increasing attentions, and the dark current and photoresponsivity are the major performance parameters. Investigations on the influence factors and operation mechanisms of the dark current and photoresponsivity for the flexible photodetectors are highly desirable. In this paper, flexible single-crystal Si and Ge photodetectors on polyethylene terephthalate (PET) substrates were fabricated and characterized. The transferred single-crystal Si and Ge nanomembranes on PET substrates showed good crystal quality by x-ray diffraction and Raman spectroscopy. Based on the experimental and modeling results, the performance dependences on semiconductor materials, doping concentrations, dimensions of the intrinsic region, incident light wavelength and power were presented. The influential factors and the operation mechanisms for the performance dependence were discussed by analyzing the electric field distribution, energy band distribution, recombination rate, absorption coefficient, penetration depth and photogeneration rate based on the device model. The results provided useful guidelines to design high performance flexible single-crystal semiconductor photodetectors.

High-Sensitivity and Mechanically Compliant Flexible Ge Photodetectors with a Vertical p–i–n Configuration

We have demonstrated high-performance flexible germanium (Ge) vertical p–i–n photodetectors (PDs) based on a resonant cavity structure by a direct flip transfer of Ge nanomembranes on polyethylene ...

Optical Properties of Tensilely Strained Ge Nanomembranes.

Group-IV semiconductors, which provide the leading materials platform of micro- electronics, are generally unsuitable for light emitting device applications because of their indirect- bandgap nature. This property currently limits the large-scale integration of electronic and photonic functionalities on Si chips. The introduction of tensile strain in Ge, which has the effect of lowering the direct conduction-band minimum relative to the indirect valleys, is a promising approach to address this challenge. Here we review recent work focused on the basic science and technology of mechanically stressed Ge nanomembranes, i.e., single-crystal sheets with thicknesses of a few tens of nanometers, which can sustain particularly large strain levels before the onset of plastic deformation. These nanomaterials have been employed to demonstrate large strain-enhanced photoluminescence, population inversion under optical pumping, and the formation of direct-bandgap Ge. Furthermore, Si-based photonic-crystal cavities have been developed that can be combined with these Ge nanomembranes without limiting their mechanical flexibility. These results highlight the potential of strained Ge as a CMOS-compatible laser material, and more in general the promise of nanomembrane strain engineering for novel device technologies.

Fabrication of Ge-on-insulator wafers by Smart-CutTM with thermal management for undamaged donor Ge wafers

Newly engineered substrates consisting of semiconductor-on-insulator are gaining much attention as starting materials for the subsequent transfer of semiconductor nanomembranes via selective etching of the insulating layer. Germanium-on-insulator (GeOI) substrates are critically important because of the versatile applications of Ge nanomembranes (Ge NMs) toward electronic and optoelectronic devices. Among various fabrication techniques, the Smart-CutTM technique is more attractive than other methods because a high temperature annealing process can be avoided. Another advantage of Smart-CutTM is the reusability of the donor Ge wafer. However, it is very difficult to realize an undamaged Ge wafer because there exists a large mismatch in the coefficient of thermal expansion among the layers. Although an undamaged donor Ge wafer is a prerequisite for its reuse, research related to this issue has not yet been reported. Here we report the fabrication of 4-inch GeOI substrates using the direct wafer bonding and Smart-CutTM process with a low thermal budget. In addition, a thermo-mechanical simulation of GeOI was performed by COMSOL to analyze induced thermal stress in each layer of GeOI. Crack-free donor Ge wafers were obtained by annealing at 250 °C for 10 h. Raman spectroscopy and x-ray diffraction (XRD) indicated similarly favorable crystalline quality of the Ge layer in GeOI compared to that of bulk Ge. In addition, Ge p-n diodes using transferred Ge NM indicate a clear rectifying behavior with an on and off current ratio of 500 at ±1 V. This demonstration offers great promise for high performance transferrable Ge NM-based device applications.

Exceptional transport property in a rolled-up germanium tube

Tubular germanium (Ge) resistors are demonstrated by rolling-up thin Ge nanomembranes (NMs, 50 nm in thickness) with electrical contacts. The strain distribution of rolled-up Ge microtubes along the radial direction is investigated and predicted by utilizing micro-Raman scattering spectroscopy with two different excitation lasers. Electrical properties are characterized for both unreleased GeNMs and released/rolled-up Ge microtubes. The conductivities of GeNMs significantly decrease after rolling-up into tubular structures, which can be attributed to surface charging states on the conductance, band bending, and piezo-resistance effect. When illuminated with a light source, facilitated by the suppressed dark current of rolled-up Ge tubes, the corresponding signal-to-noise ratio can be dramatically enhanced compared with that of planar GeNMs.

Flexible nanomembrane photonic-crystal cavities for tensilely strained-germanium light emission

Flexible photonic-crystal cavities in the form of Si-column arrays embedded in polymeric films are developed on Ge nanomembranes using direct membrane assembly. The resulting devices can sustain large biaxial tensile strain under mechanical stress, as a way to enhance the Ge radiative efficiency. Pronounced emission peaks associated with photonic-crystal cavity resonances are observed in photoluminescence measurements. These results show that ultrathin nanomembrane active layers can be effectively coupled to an optical cavity, while still preserving their mechanical flexibility. Thus, they are promising for the development of strain-enabled Ge lasers, and more generally uniquely flexible optoelectronic devices.

Capacitance-voltage characteristics of Si and Ge nanomembrane based flexible metal-oxide-semiconductor devices under bending conditions

Metal-oxide-semiconductor (MOS) device is the basic building block for field effect transistors (FET). The majority of thin-film transistors (TFTs) are FETs. When MOSFET are mechanically bent, the MOS structure will be inevitably subject to mechanical strain. In this paper, flexible MOS devices using single crystalline Silicon (Si) and Germanium (Ge) nanomembranes (NM) with SiO2, SiO, and Al2O3 dielectric layers are fabricated on a plastic substrate. The relationships between semiconductor nanomembranes and various oxide materials are carefully investigated under tensile/compressive strain. The flatband voltage, threshold voltage, and effective charge density in various MOS combinations revealed that Si NM−SiO2 configuration shows the best interface charge behavior, while Ge NM−Al2O3 shows the worst. This investigation of flexible MOS devices can help us understand the impact of charges in the active region of the flexible TFTs and capacitance changes under the tensile/compressive strains on the change in electrical characteristics in flexible NM based TFTs.

Flexible radio-frequency single-crystal germanium switch on plastic substrates

This Letter presents the realization and characterizations of the flexible radio-frequency (RF)/microwave switches on plastic substrates employing single-crystal germanium (Ge) nanomembranes. The fabricated flexible Ge single-pole, single-throw (SPST) switches display high frequency responses (e.g., insertion loss of &amp;lt;1.3 dB at up to 30 GHz and isolation &amp;gt;10 dB at up to ∼13 GHz). RF performance tradeoff exists for the flexible Ge switches and the major affecting parameters are determined. The flexible Ge SPST switch shows better RF property to that of the flexible Si SPST switch. Underlying mechanism is investigated by theoretical analysis and modeling of switches with different structures.

High quality single crystal Ge nano-membranes for opto-electronic integrated circuitry

A thin, flat, and single crystal germanium membrane would be an ideal platform on which to mount sensors or integrate photonic and electronic devices, using standard silicon processing technology. We present a fabrication technique compatible with integrated-circuit wafer scale processing to produce membranes of thickness between 60 nm and 800 nm, with large areas of up to 3.5 mm2. We show how the optical properties change with thickness, including appearance of Fabry-Pérot type interference in thin membranes. The membranes have low Q-factors, which allow the platforms to counteract distortion during agitation and movement. Finally, we report on the physical characteristics showing sub-nm roughness and a homogenous strain profile throughout the freestanding layer, making the single crystal Ge membrane an excellent platform for further epitaxial growth or deposition of materials.

Strained-Germanium Nanostructures for Infrared Photonics

The controlled application of strain in crystalline semiconductors can be used to modify their basic physical properties to enhance performance in electronic and photonic device applications. In germanium, tensile strain can even be used to change the nature of the fundamental energy band gap from indirect to direct, thereby dramatically increasing the interband radiative efficiency and allowing population inversion and optical gain. For biaxial tension, the required strain levels (around 2%) are physically accessible but necessitate the use of very thin crystals. A particularly promising materials platform in this respect is provided by Ge nanomembranes, that is, single-crystal sheets with nanoscale thicknesses that are either completely released from or partially suspended over their native substrates. Using this approach, Ge tensilely strained beyond the expected threshold for direct-band gap behavior has recently been demonstrated, together with strong strain-enhanced photoluminescence and evidence of population inversion. We review the basic properties, state of the art, and prospects of tensilely strained Ge for infrared photonic applications.

Fabrication and Characterization of Flexible Microwave Single-Crystal Germanium Nanomembrane Diodes on a Plastic Substrate

This letter presents the fabrication and characterization of flexible microwave p-intrinsic-n diodes on a plastic substrate employing single-crystal germanium (Ge) nanomembranes. The fabricated flexible device displays high frequency response (e.g., insertion loss smaller than 1.5 dB and isolation larger than 10 dB at frequencies up to 10 GHz). Flexible Ge diodes with various diode structures are modeled and reveal radio frequency (RF) performance tradeoff with device parameters. Furthermore, the flexible Ge diodes show better RF/microwave properties than the flexible Si diodes. The study demonstrates great potential of using flexible active/passive components based on single-crystal Ge nanomembrane for high-performance RF/microwave applications.

Semiconductor nanomembranes: a platform for new properties via strain engineering

New phenomena arise in single-crystal semiconductors when these are fabricated in very thin sheets, with thickness at the nanometer scale. We review recent research on Si and Ge nanomembranes, including the use of elastic strain sharing, layer release, and transfer, that demonstrate new science and enable the fabrication of materials with unique properties. Strain engineering produces new strained forms of Si or Ge not possible in nature, new layered structures, defect-free SiGe sheets, and new electronic band structure and photonic properties. Through-membrane elastic interactions cause the double-sided ordering of epitaxially grown nanostressors on Si nanomembranes, resulting in a spatially and periodically varying strain field in the thin crystalline semiconductor sheet. The inherent influence of strain on the band structure creates band gap modulation, thereby creating effectively a single-element electronic superlattice. Conversely, large-enough externally applied strain can make Ge a direct-band gap semiconductor, giving promise for Group IV element light sources.

Tensilely Strained Germanium Nanomembranes as Infrared Optical Gain Media

The use of tensilely strained Ge nanomembranes as mid-infrared optical gain media is investigated. Biaxial tensile strain in Ge has the effect of lowering the direct energy bandgap relative to the fundamental indirect one, thereby increasing the internal quantum efficiency for light emission and allowing for the formation of population inversion, until at a strain of about 1.9% Ge is even converted into a direct-bandgap material. Gain calculations are presented showing that, already at strain levels of about 1.4% and above, Ge films can provide optical gain in the technologically important 2.1-2.5 μm spectral region, with transparency carrier densities that can be readily achieved under realistic pumping conditions. Mechanically stressed Ge nanomembranes capable of accommodating the required strain levels are developed and used to demonstrate strong strain-enhanced photoluminescence. A detailed analysis of the high-strain emission spectra also demonstrates that the nanomembranes can be pumped above transparency, and confirms the prediction that biaxial-strain levels in excess of only 1.4% are required to obtain significant population inversion.

Direct-bandgap light-emitting germanium in tensilely strained nanomembranes

Silicon, germanium, and related alloys, which provide the leading materials platform of electronics, are extremely inefficient light emitters because of the indirect nature of their fundamental energy bandgap. This basic materials property has so far hindered the development of group-IV photonic active devices, including diode lasers, thereby significantly limiting our ability to integrate electronic and photonic functionalities at the chip level. Here we show that Ge nanomembranes (i.e., single-crystal sheets no more than a few tens of nanometers thick) can be used to overcome this materials limitation. Theoretical studies have predicted that tensile strain in Ge lowers the direct energy bandgap relative to the indirect one. We demonstrate that mechanically stressed nanomembranes allow for the introduction of sufficient biaxial tensile strain to transform Ge into a direct-bandgap material with strongly enhanced light-emission efficiency, capable of supporting population inversion as required for providing optical gain.

Defect-Free Single-Crystal SiGe: A New Material from Nanomembrane Strain Engineering

Many important materials cannot be grown as single crystals in bulk form because strain destroys long-range crystallinity. Among them, alloys of group IV semiconductors, specifically SiGe alloys, have significant technological value. Using nanomembrane strain engineering methods, we demonstrate the fabrication of fully elastically relaxed Si(1-x)Ge(x) nanomembranes (NMs) for use as growth substrates for new materials. To do so, we grow defect-free, uniformly and elastically strained SiGe layers on Si substrates and release the SiGe layers to allow them to relax this strain completely as free-standing NMs. These SiGe NMs are transferred to new hosts and bonded there. We confirm the high structural quality of these new materials and demonstrate their use as substrates for technologically relevant epitaxial films by growing strained-Si layers and thick, lattice-matched SiGe alloy layers on them.

Semiconductor Nanomembrane Tubes: Three-Dimensional Confinement for Controlled Neurite Outgrowth

In many neural culture studies, neurite migration on a flat, open surface does not reflect the three-dimensional (3D) microenvironment in vivo. With that in mind, we fabricated arrays of semiconductor tubes using strained silicon (Si) and germanium (Ge) nanomembranes and employed them as a cell culture substrate for primary cortical neurons. Our experiments show that the SiGe substrate and the tube fabrication process are biologically viable for neuron cells. We also observe that neurons are attracted by the tube topography, even in the absence of adhesion factors, and can be guided to pass through the tubes during outgrowth. Coupled with selective seeding of individual neurons close to the tube opening, growth within a tube can be limited to a single axon. Furthermore, the tube feature resembles the natural myelin, both physically and electrically, and it is possible to control the tube diameter to be close to that of an axon, providing a confined 3D contact with the axon membrane and potentially insulating it from the extracellular solution.