Metal‐Catalyzed Growth of Semiconductor Nanostructures Without Solubility and Diffusivity Constraints

Abstract

Semiconductor nanostructures have attracted tremendous interest in recent years due to their numerous potential applications, for example, in nanoelectronics, [ 1 ] fl exible electronics, [ 2 ] photonics, [ 3 ] sensors, [ 4 ] and in energy harvesting [ 5 , 6 ] and storage [ 7 ] devices. Semiconductor nanostructures are produced mainly by metal-catalyzed growth via vapor-liquid-solid (VLS) [8][9][10] and vapor-solid-solid (VSS) [11][12][13][14] mechanisms, requiring substantial solubility and diffusivity of the semiconductor in the catalyst and thus high growth temperatures. The present study reveals, using in situ heating electron microscopy, that metal-catalyzed growth of semiconductor nanostructures can be realized without these constraints, thereby enabling strikingly low growth temperatures. The growth mechanism has been unraveled at the atomic scale for the Al-catalyzed growth of Si nanostructures at 150 ° C. Growth starts with wetting of high-angle grain boundaries (GBs) in the Al catalyst, by Si in its amorphous form, and continues by nucleation and growth of nanostructured crystalline Si (c-Si) in the template as precisely defi ned by the Al grain boundary network. The disclosed mechanism breaks solubility and diffusivity limits that have hitherto dictated selection of metal catalysts and growth temperatures and thereby opens new perspectives for direct fabrication of nanostructure devices on heat-sensitive substrates.The fundamental understanding of the metal-catalyzed growth mechanisms has been advanced by recent developments of in situ heating electron microscopy techniques. [12][13][14][15] Metalcatalyzed VLS growth of semiconductor nanowires proceeds by the precipitation of semiconductor material out of metal-semiconductor eutectic melt droplets, which are supersaturated by ceaseless exposure to gas-phase semiconductor reactants above the eutectic temperature. [8][9][10] More recent works have demonstrated that semiconductor nanowires can also be grown via a VSS growth mechanism below the eutectic temperature, at which the catalysts may exist in the form of solid solution or compound(s) of metal-semiconductor. [11][12][13][14] The growth of crystalline semiconductors as a thin fi lm can be realized also via supersaturation of a metal fi lm at suffi ciently high temperatures (about 500 ° C). [ 16 , 17 ] Such conventional metal-catalyzed growth processes require signifi cant solubility and suffi cient diffusivity of the semiconductor atoms in the bulk of the (liquid or solid) metal catalyst. Due to such constraints, the selection of an appropriate metal catalyst, as well as the growth temperature, has been strictly constrained in accordance with the corresponding equilibrium phase diagrams. [ 9-11 , 14 ] High processing temperatures (500 ° C or higher) are typically needed. Gold, which possesses high solubility of Si and Ge at relatively low eutectic temperatures, [ 18 ] has been the most commonly used catalyst, even though Au contamination is highly detrimental to device performances. [ 10 , 19 ] ...

- Introduction to Metal-Induced Crystallization

“…[44]. Interface Thermodynamics of Metal-Induced Crystallization been calculated to be 21 nm (136 nm) along Al GBs (as compared to the corresponding diffusion length possibly covered by volume diffusion of less than 1 nm) [14]. The large thermodynamic driving force (see previous paragraph) and the fast GB diffusion enable the occurrence of metal GB wetting by amorphous semiconductors at relatively low temperatures.…”

“…For example, direct application of the model for interface thermodynamics enables us to tailor the crystallization temperature of a-Si systematically from 700°C down to 150°C (and any temperature in between) by controlling the Al overlayer thickness [10]. Such fundamental understanding has led, for example, to the development of a novel process for the growth of crystalline Si (c-Si) nanowires at exceedingly low temperatures [14].…”

“…On the one hand theoretical modeling of the (interface) thermodynamics was performed by employing the macroscopic atom approach [10,11,44]. On the other hand dedicated, in particular in situ heating high-resolution transmission electron microscopy (HRTEM) and valence energyfiltered transmission electron microscopy (VEFTEM), experiments were performed to reveal the operating atomistic mechanisms [14,15]. It has been found that MIC is in general an interface-controlled phenomenon [10,11,14,40].…”

“…On the other hand dedicated, in particular in situ heating high-resolution transmission electron microscopy (HRTEM) and valence energyfiltered transmission electron microscopy (VEFTEM), experiments were performed to reveal the operating atomistic mechanisms [14,15]. It has been found that MIC is in general an interface-controlled phenomenon [10,11,14,40]. The crucial roles of interfaces (including, especially, grain boundaries [GBs] in the metal phase) in MIC involve (1) bond weakening of the semiconductor material at the interface with the metal, leading to enhanced mobility of semiconductor atoms at the interface; (2) the interfaces (including GBs in the metal) serving as fast, short-circuit diffusion paths for material transportation; and (3) very importantly, the interface energetics (thermodynamics), which is decisive for whether crystal nucleation and crystal growth will occur.…”

“…Amorphous semiconductors such as silicon and germanium can crystallize at a temperature much lower than their "bulk" crystallization temperatures when they are put in direct contact with a metal, such as Al [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17], Au [18][19][20][21][22], Ag [23][24][25], Ni [26][27][28][29][30][31][32][33], Cu [34][35][36], and Pd [37,38]. This phenomenon, which was firstly observed more than 40 years ago for amorphous germanium [39], is now commonly referred to as metal-induced crystallization (MIC) [40][41][42][43].…”