Synthesis of silicon nanocrystals in silane plasmas for nanoelectronics and large area electronic devices

Abstract

The synthesis of silicon nanocrystals in standard radio-frequency glow discharge systems is studied with respect to two main objectives: (i) the production of devices based on quantum size effects associated with the small dimensions of silicon nanocrystals and (ii) the synthesis of polymorphous and polycrystalline silicon films in which silicon nanocrystals are the elementary building blocks. In particular we discuss results on the mechanisms of nanocrystal formation and their transport towards the substrate. We found that silicon nanocrystals can contribute to a significant fraction of deposition (50–70%) and that they can be positively charged. This has a strong influence on their deposition because positively charged nanocrystals will be accelerated towards the substrate with energy of the order of the plasma potential. However, the important parameter with respect to the deposition of charged nanocrystals is not the accelerating voltage but the energy per atom and thus a doubling of the diameter will result in a decrease in the energy per atom by a factor of 8. To leverage this geometrical advantage we propose the use of more electronegative gases, which may have a strong effect on the size and charge distribution of the nanocrystals. This is illustrated in the case of deposition from silicon tetrafluoride plasmas in which we observe low-frequency plasma fluctuations, associated with successive generations of nanocrystals. The contribution of larger nanocrystals to deposition results in a lower energy per deposited atom and thus polycrystalline films.

Low temperature plasma deposition of silicon thin films: From amorphous to crystalline

“…As a matter of fact increasing silane flow rate at relatively high total pressure moves the process from radical to nanocrystal growth, as we have reported in the past for pm-Si:H and μc-Si:H films [7,22]. Indeed, over the last decade, our group has focused on the study of dusty plasmas [23], particularly on the plasma synthesis of silicon clusters [5,7,8,22,24]. These clusters, generated in the plasma, can be either amorphous or crystalline, depending on the hydrogen dilution.…”

“…Moreover, the extension of the PECVD processes from a-Si:H, pm-Si:H and μc-Si:H to epitaxial layers also brings new light to the interpretation of the growth process of these materials. Favoring the synthesis of silicon nanocrystals in the plasma has been our main driving force over the past 10 years and has resulted in the development of polymorphous silicon films where silicon nanocrystals and radicals contribute to the growth [5,7,8]. Moreover, we have shown that under conditions of μc-Si:H deposition from silicon nanocrystals, the nature of the substrate and plasma surface treatments prior to deposition play a key role on the growth process [6].…”

“…However, there is still a debate on the growth mechanisms of these films. While standard growth models based on SiH3 radicals [3,4] may apply for well controlled and low rate deposition conditions, increasing the deposition rate is synonymous of enhanced gas phase reactions leading to the formation of silicon clusters and nanocrystals in the plasma [5,6]. Even though common sense would suggest that this is something to avoid, we have been using the plasma synthesized silicon nanocrystals to improve the electronic properties of polymorphous and microcrystalline silicon films, while increasing their deposition rate [7,8].…”

Thin crystalline silicon solar cells based on epitaxial films grown at 165°C by RF-PECVD

“…Regarding the growth process at such low temperature, there is still a lot to be understood as well. It has been shown previously in our laboratory that in some process conditions, crystalline Si nanoparticles are generated in the gas phase [20]. These nanocrystals play an important role in the deposition and can be incorporated to the film resulting in polymorphous or microcrystalline silicon materials on glass substrates [21,22].…”

Impact of secondary gas-phase reactions on microcrystalline silicon solar cells deposited at high rate

“…Powder is identified as a possible additive source for the deterioration of device performance 5,9 and precautions are often taken to reduce its formation. On the other hand, it has been shown that the formation of powder under certain conditions is beneficial to the deposition of a-Si:H and c-Si: H. 13,14 However, a systematic study of the effect of powder on the material quality of c-Si: H films, performed at a constant ion bombardment and a constant R d , is still lacking.…”