▁cells , ▁sil icon ▁has ▁relatively ▁low ▁absor pt ivity ▁for ▁sun light , ▁meaning ▁about ▁ 1 0 0 ▁mic rom eters ▁of ▁Si ▁is ▁needed ▁to ▁abs orb ▁most ▁sun light . ▁Such ▁a ▁layer ▁is ▁relatively ▁robust ▁and ▁easy ▁to ▁handle . ▁In ▁contrast , ▁the ▁absor pt ivity ▁of ▁Ga As ▁is ▁so ▁high ▁that ▁only ▁a ▁few ▁mic rom eters ▁of ▁thick ness ▁are ▁needed ▁to ▁abs orb ▁all ▁of ▁the ▁light . ▁Con sequently , ▁Ga As ▁thin ▁films ▁must ▁be ▁supported ▁on ▁a ▁substr ate ▁material . ▁ ▁Sil icon ▁is ▁a ▁pure ▁element , ▁avoid ing ▁the ▁problems ▁of ▁sto ichi ometric ▁im bal ance ▁and ▁thermal ▁un mix ing ▁of ▁Ga As . ▁ ▁Sil icon ▁has ▁a ▁nearly ▁perfect ▁lattice ; ▁imp urity ▁density ▁is ▁very ▁low ▁and ▁allows ▁very ▁small ▁structures ▁to ▁be ▁built ▁( current ly ▁down ▁to ▁ 1 6   nm ). ▁In ▁contrast , ▁Ga As ▁has ▁a ▁very ▁high ▁imp urity ▁density , ▁which ▁makes ▁it ▁difficult ▁to ▁build ▁integrated ▁circ uits ▁with ▁small ▁structures , ▁so ▁the ▁ 5 0 0   nm ▁process ▁is ▁a ▁common ▁process ▁for ▁Ga As . ▁ ▁Other ▁applications ▁ ▁Sol ar ▁cells ▁and ▁detect ors ▁Gal li um ▁ar sen ide ▁( G a As ) ▁is ▁an ▁important ▁sem icon duct or ▁material ▁for ▁high - cost , ▁high - e ffic iency ▁solar ▁cells ▁and ▁is ▁used ▁for ▁single - c ry st all ine ▁thin ▁film