Dual-Chamber Plasma Co-Deposition of Nanoparticles in Amorphous Silicon Thin Films

Abstract

The production of hydrogenated amorphous silicon films containing silicon nanocrystal-line inclusions (a/nc-Si:H) is demonstrated using a new deposition process. Crystalline Si nanoparticles around 5 nm in diameter are generated in a flow-through plasma reactor, and are introduced into a downstream capacitively-coupled plasma enhanced chemical vapor deposition reactor where the particles are “co-deposited” with the amorphous phase of the film. Transmis-sion electron microscopy confirms the presence of crystalline inclusions in these films, as well as providing confirmation that the crystalline particles are indeed produced in the flow-through re-actor and not in the capacitive plasma. Electrical measurements indicate an improvement in the dark conductivity of the intrinsic a/nc-Si:H films as the particle concentration is increased, sug-gesting that the particles have a doping effect on the films charge transport properties.

Opto-electronic properties of co-deposited mixed-phase hydrogenated amorphous/nanocrystalline silicon thin films

“…The deposition rate of particles decreases radially from the center position of the electrode, where the outlet of the particle injection tube is located, as confirmed by tapping-mode Atomic Force Microscopy measurements [12]. By placing substrates radially outwards from the center position of the tube exit, we have a unique way of studying the influence of crystalline fraction on the properties of a series of a/nc-Si:H deposited simultaneously [11,12]. Raman studies for films synthesized in a single deposition run, at substrate locations marked A, B and C in Fig.…”

“…A typical residence time of 3 ms results in nanocrystallite formation with an average diameter of between 5-6 nm. Details of the nanocrystal diameter dependence on chamber gas pressure and plasma residence time have been published previously [11,12].…”

“…The concentration of silicon nanocrystals incorporated into an amorphous silicon matrix can be controlled by either a thermophoresis force induced by a thermal gradient applied across the silane plasma [14] or via the convection of the carrier gas as the nanoparticles are injected into the capacitively-coupled chamber from the particle synthesis reactor [11,12,15]. In the latter configuration, substrates are placed radially outwards from the center position of the tube exit (located 2.5 cm above the lower electrode), labeled alphabetically from the electrode periphery inwards, as shown in Figure 1.…”

“…High-resolution transmission electron microscopy (HRTEM) studies of cleaved films verify that silicon nanocrystals are indeed introduced into the amorphous silicon matrix, and reside homogenously throughout the film's thickness [11]. Comparison of films deposited with the particle synthesis chamber turned on (leading to a/nc-Si:H films) and turned off (in which aSi:H is deposited) confirms that the crystallites are synthesized in the particle synthesis reactor, rather than forming within the silane plasma in the capacitively-coupled plasma chamber.…”

Electronic Transport in Co-deposited Hydrogenated Amorphous/Nanocrystalline Thin Films

“…Hydrogenated amorphous silicon thin films containing silicon nanocrystalline inclusions (a/nc-Si:H) have been investigated as potential materials for photovoltaic devices [1,2], while silicon nanocrystals within an insulating matrix are employed in non-volatile memory and electroluminescent devices [3,4].These materials are typically synthesized in a Plasma Enhanced Chemical Vapor Deposition (PECVD) system operated at high silane gas chamber pressures, where silicon cluster formation is known to occur [5,6]. A preliminary report on this dual chamber system has been published previously [7]. Moreover, when synthesized using in a single capacitively-coupled plasma chamber, one is limited to growing silicon nanocrystalline particles in a-Si:H. We have consequently constructed a dual chamber codeposition system, where the silicon nanocrystals are formed in one plasma deposition system, and are then entrained in a carrier gas and injected into a second PECVD system, where hydrogenated amorphous silicon is deposited [7].…”