Metal-assisted chemical etching of Ge(100) surfaces in water toward nanoscale patterning

Metal-assisted chemical etching beyond Si: applications to III-V compounds and wide-bandgap semiconductors

Metal-assisted chemical etching (MacEtch) has emerged as a versatile technique for fabricating a variety of semiconductor nanostructures. Since early investigations in 2000, research in this field has provided a deeper understanding of the underlying mechanisms of catalytic etching processes and enabled high control over etching conditions for diverse applications. In this Review, we present an overview of recent developments in the application of MacEtch to nanomanufacturing and processing of III-V based semiconductor materials and other materials beyond Si. We highlight the key findings and developments in MacEtch as applied to GaAs, GaN, InP, GaP, InGaAs, AlGaAs, InGaN, InGaP, SiC, β-Ga2O3, and Ge material systems. We further review a series of active and passive devices enabled by MacEtch, including light-emitting diodes (LEDs), field-effect transistors (FETs), optical gratings, sensors, capacitors, photodiodes, and solar cells. By reviewing demonstrated control of morphology, optimization of etch conditions, and catalyst-material combinations, we aim to distill the current understanding of beyond-Si MacEtch mechanisms and to provide a bank of reference recipes to stimulate progress in the field.

Inverse metal-assisted chemical etching of germanium with gold and hydrogen peroxide

Metal-assisted chemical etching (MACE) is a flexible technique for texturing the surface of semiconductors. In this work, we study the spatial variation of the etch profile, the effect of angular orientation relative to the crystallographic planes, and the effect of doping type. We employ gold in direct contact with germanium as the metal catalyst, and dilute hydrogen peroxide solution as the chemical etchant. With this catalyst-etchant combination, we observe inverse-MACE, where the area directly under gold is not etched, but the neighboring, exposed germanium experiences enhanced etching. This enhancement in etching decays exponentially with the lateral distance from the gold structure. An empirical formula for the gold-enhanced etching depth as a function of lateral distance from the edge of the gold film is extracted from the experimentally measured etch profiles. The lateral range of enhanced etching is approximately 10-20μm and is independent of etchant concentration. At length scales beyond a few microns, the etching enhancement is independent of the orientation with respect to the germanium crystallographic planes. The etch rate as a function of etchant concentration follows a power law with exponent smaller than 1. The observed etch rates and profiles are independent of whether the germanium substrate is n-type, p-type, or nearly intrinsic.

Structuring of Si into Multiple Scales by Metal-Assisted Chemical Etching

Structuring Si, ranging from nanoscale to macroscale feature dimensions, is essential for many applications. Metal-assisted chemical etching (MaCE) has been developed as a simple, low-cost, and scalable method to produce structures across widely different dimensions. The process involves various parameters, such as catalyst, substrate doping type and level, crystallography, etchant formulation, and etch additives. Careful optimization of these parameters is the key to the successful fabrication of Si structures. In this review, recent additions to the MaCE process are presented after a brief introduction to the fundamental principles involved in MaCE. In particular, the bulk-scale structuring of Si by MaCE is summarized and critically discussed with application examples. Various approaches for effective mass transport schemes are introduced and discussed. Further, the fine control of etch directionality and uniformity, and the suppression of unwanted side etching are also discussed. Known application examples of Si macrostructures fabricated by MaCE, though limited thus far, are presented. There are significant opportunities for the application of macroscale Si structures in different fields, such as microfluidics, micro-total analysis systems, and microelectromechanical systems, etc. Thus more research is necessary on macroscale MaCE of Si and their applications.

SynCells: A 60 × 60 μm2 Electronic Platform with Remote Actuation for Sensing Applications in Constrained Environments

Autonomous electronic microsystems smaller than the diameter of a human hair (<100 μm) are promising for sensing in confined spaces such as microfluidic channels or the human body. However, they are difficult to implement due to fabrication challenges and limited power budget. Here we present a 60 × 60 μm electronic microsystem platform, or SynCell, that overcomes these issues by leveraging the integration capabilities of two-dimensional material circuits and the low power consumption of passive germanium timers, memory-like chemical sensors, and magnetic pads. In a proof-of-concept experiment, we magnetically positioned SynCells in a microfluidic channel to detect putrescine. After we extracted them from the channel, we successfully read out the timer and sensor signal, the latter of which can be amplified by an onboard transistor circuit. The concepts developed here will be applicable to microsystems targeting a variety of applications from microfluidic sensing to biomedical research.

Efficient photon capture on germanium surfaces using industrially feasible nanostructure formation

Nanostructured surfaces are known to provide excellent optical properties for various photonics devices. Fabrication of such nanoscale structures to germanium (Ge) surfaces by metal assisted chemical etching (MACE) is, however, challenging as Ge surface is highly reactive resulting often in micron-level rather than nanoscale structures. Here we show that by properly controlling the process, it is possible to confine the chemical reaction only to the vicinity of the metal nanoparticles and obtain nanostructures also in Ge. Furthermore, it is shown that controlling the density of the nanoparticles, concentration of oxidizing and dissolving agents as well as the etching time plays a crucial role in successful nanostructure formation. We also discuss the impact of high mobility of charge carriers on the chemical reactions taking place on Ge surfaces. As a result we propose a simple one-step MACE process that results in nanoscale structures with less than 10% surface reflectance in the wavelength region between 400 and 1600 nm. The method consumes only a small amount of Ge and is thus industrially viable and also applicable to thin Ge layers.

Catalyst-substrate interaction and growth delay in vapor-liquid-solid nanowire growth

Understanding of the initial stage of nanowire growth on a bulk substrate is crucial for the rational design of nanowire building blocks in future electronic and optoelectronic devices. Here, we provide in situ scanning electron microscopy and Auger microscopy analysis of the initial stage of Au-catalyzed Ge nanowire growth on different substrates. Real-time microscopy imaging and elementally resolved spectroscopy clearly show that the catalyst dissolves the underlying substrate if held above a certain temperature. If the substrate dissolution is blocked (or in the case of heteroepitaxy) the catalyst needs to be filled with nanowire material from the external supply, which significantly increases the initial growth delay. The experiments presented here reveal the important role of the substrate in metal-catalyzed nanowire growth and pave the way for different growth delay mitigation strategies.

Damage-Free Smooth-Sidewall InGaAs Nanopillar Array by Metal-Assisted Chemical Etching

Producing densely packed high aspect ratio In_0.53Ga_0.47As nanostructures without surface damage is critical for beyond Si-CMOS nanoelectronic and optoelectronic devices. However, conventional dry etching methods are known to produce irreversible damage to III-V compound semiconductors because of the inherent high-energy ion-driven process. In this work, we demonstrate the realization of ordered, uniform, array-based In_0.53Ga_0.47As pillars with diameters as small as 200 nm using the damage-free metal-assisted chemical etching (MacEtch) technology combined with the post-MacEtch digital etching smoothing. The etching mechanism of In_xGa_1-xAs is explored through the characterization of pillar morphology and porosity as a function of etching condition and indium composition. The etching behavior of In_0.53Ga_0.47As, in contrast to higher bandgap semiconductors (e.g., Si or GaAs), can be interpreted by a Schottky barrier height model that dictates the etching mechanism constantly in the mass transport limited regime because of the low barrier height. A broader impact of this work relates to the complete elimination of surface roughness or porosity related defects, which can be prevalent byproducts of MacEtch, by post-MacEtch digital etching. Side-by-side comparison of the midgap interface state density and flat-band capacitance hysteresis of both the unprocessed planar and MacEtched pillar In_0.53Ga_0.47As metal-oxide-semiconductor capacitors further confirms that the surface of the resultant pillars is as smooth and defect-free as before etching. MacEtch combined with digital etching offers a simple, room-temperature, and low-cost method for the formation of high-quality In_0.53Ga_0.47As nanostructures that will potentially enable large-volume production of In_0.53Ga_0.47As-based devices including three-dimensional transistors and high-efficiency infrared photodetectors.

Bright-field Nanoscopy: Visualizing Nano-structures with Localized Optical Contrast Using a Conventional Microscope

Most methods for optical visualization beyond the diffraction limit rely on fluorescence emission by molecular tags. Here, we report a method for visualization of nanostructures down to a few nanometers using a conventional bright-field microscope without requiring additional molecular tags such as fluorophores. The technique, Bright-field Nanoscopy, is based on the strong thickness dependent color of ultra-thin germanium on an optically thick gold film. We demonstrate the visualization of grain boundaries in chemical vapour deposited single layer graphene and the detection of single 40 nm Ag nanoparticles. We estimate a size detection limit of about 2 nm using this technique. In addition to visualizing nano-structures, this technique can be used to probe fluid phenomena at the nanoscale, such as transport through 2D membranes. We estimated the water transport rate through a 1 nm thick polymer film using this technique, as an illustration. Further, the technique can also be extended to study the transport of specific ions in the solution. It is anticipated that this technique will find use in applications ranging from single-nanoparticles resolved sensing to studying nanoscale fluid-solid interface phenomena.