Advanced Amorphous Silicon(8)

5.3.8Hydrogenated amorphous silicon from hydrogen diluted silane

So far,the effort to fabricate more stable a-Si:H using the PECVD technique has demon-strated the bene?cial effect of hydrogen dilution of the source gas on the quality of a-Si:H.

ADV ANCED AMORPHOUS SILICON SOLAR CELL TECHNOLOGIES193

Figure5.6TEM photograph of1μm thick silicon layer grown in the protocrystalline regime. Solar cells with a-Si:H absorbers prepared with hydrogen dilution of silane showed a better performance after light exposure than their conventional undiluted counterparts[64,65].Hy-drogen diluted a-Si:H has received a lot of attention since then and owing to several speci?c properties,this material is now described in the literature as protocrystalline silicon[66].The hydrogen–silane dilution ratio(R=H2/SiH4)roughly de?nes the deposition conditions at which the so-called protocrystalline Si:H growth regime occurs.In the protocrystalline Si:H growth regime,unique evolutionary growth behaviour is observed,when the?lm evolves from an amorphous phase into?rst a mixed amorphous–microcrystalline and subsequently into a single microcrystalline phase.This behaviour is clearly demonstrated in Figure5.6, which shows a transmission electron microscopy photograph of a layer of about1μm thick deposited on a glass substrate at R=30.It is important to note that protocrystalline Si:H material is de?ned as being fully amorphous,and once the transition from the amorphous to the mixed phase occurs,the?lm is no longer considered to be protocrystalline.Based on in situ real time spectroscopic ellipsometry studies,amorphous-to-microcrystalline silicon phase diagrams have been determined,which predict the thickness at which the transition from amorphous to microcrystalline material for a given R occurs[67].For lower dilutions (R<10),?lms are invariably amorphous;this means that there is no evolutionary growth that is characteristic for the protocrystalline growth regime(R>10).For R<10,a critical thickness is observed beyond which the growing amorphous silicon surface becomes rough. The improved stability and quality of protocrystalline Si:H layers compared to conventional a-Si:H deposited from pure silane or using a low hydrogen dilution is attributed to an improved structural order.An enhanced medium range order is determined by measuring the full width at half the maximum( 2θ)of the?rst X-ray diffraction(XRD)peak for a-Si:H,which is centred at a scattering angle of2θ～28.5?.A typical value for a-Si:H is 2θ～6?,which decreases for protocrystalline Si:H to 2θ～5?[68].For the protocrystalline growth regime, an important practical feature is that the phase of the growing?lm strongly depends on the substrate[66].Under protocrystalline growth conditions,local epitaxial growth is favoured on crystalline silicon substrates.For this reason,it is dif?cult,in solar cells,to grow intrinsic protocrystalline Si:H layers directly on doped microcrystalline silicon layers without incorpo-rating a conventional a-Si:H interlayer.On amorphous silicon,the nucleation of crystallites is suppressed.The phase diagrams are important tools for optimization of amorphous and mi-crocrystalline silicon solar cells,as they help to keep the growth of the?lm within the desired phase.

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