Creating nanocrystals in amorphous silicon using a conductive tip

Abstract

Field-enhanced metal-induced solid phase crystallization (FE-MISPC) of amorphous silicon is scaled down to nanoscale dimensions by using a sharp conductive tip in atomic force microscopy (AFM) as one of the electrodes. The room temperature process is driven by the electrical current of the order of 100 pA between the tip and the bottom nickel electrode. This results in energy transfer rates of 30-50 nJ s(-1). Amplitude of the current is limited by a MOSFET transistor to avoid electrical discharge from parasitic parallel capacitance. Limiting the current amplitude and control of the transferred energy (approximately 100 nJ) enables formation of silicon crystals with dimensions smaller than 100 nm in the amorphous film. Formation of the nanocrystals is localized by the AFM tip position. The presence of nanocrystals is detected by current-sensing AFM and independently corroborated by micro-Raman spectroscopy. The nanocrystal formation is discussed based on a model considering microscopic electrical contact, thermodynamics of crystallization and silicide formation.

Generating ordered Si nanocrystals via atomic force microscopy

“…3(a, b). The current limitation also consistently reduced the size of the features to sub-μm dimensions and in some instances the crystalline features were as small as 60 nm [13].…”

“…Microscopic morphology and local conductivity of the films before and after the FE-MISPC process are characterized by CS-AFM [21] using sample bias voltage of −25 V. Increased local current as detected by CS-AFM is a good indication of crystallinity as corroborated previously by micro-Raman spectroscopy [12,13,15]. We are using such high bias voltage due to low conductivity of the a-Si:H film and additional tunneling barrier of the native oxide on the film surface [18].…”

“…However, these regions are not confined nor cover all the formed pit area as they extend on various places in its vicinity. Conductivity of these regions is related with crystalline silicon as corroborated by CS-AFM [21] and micro-Raman spectroscopy [12,13,15]. Constant current was set to I c = − 250 pA.…”

“…The setup can drive electrical current for the FE-MISPC process in three different regimes: A) the current is controlled by the current source unit so that I c corresponds to the current set-point value and the pre-amp output is used only to monitor the process and total energy dose [12], B) the current is controlled by an additional regulation circuit composed of a metal-oxide-semiconductor field-effect transistor (MOSFET) and proportional feedback loop (P-FL, shown in brown) which adjusts MOSFET gate to keep actual sample current I s below the set-point value by using the pre-amp output [13], C) for the highest precision and speed the current is controlled by the regulation circuit with derivative feedback loop (PD-FL), a reference signal generator, and an adder for summing the pre-amp output with the reference (all these components are shown in green) [14]. FE-MISPC process is characterized by temporal current profile (bias voltage is adjusted correspondingly by the current source unit) and total energy dose [14].…”

“…Such relatively large size was due to electrical discharge from the inherently present parallel capacitance, caused by a drastic increase of local conductivity during the process. Limiting of the maximum current level by using a MOSFET with a feedback circuit enabled to localize crystallization below 100 nm [13]. However, perfectly stabilized electrical current during the FE-MISPC process produced rather non-conductive pits [14].…”