▁so ▁g ases ▁do ▁not ▁react ▁before ▁they ▁reach ▁the ▁hot ▁wa fer ▁surface . ▁The ▁ped est al / s us ceptor ▁is ▁made ▁of ▁a ▁radiation - abs or bing ▁material ▁such ▁as ▁carbon . ▁In ▁contrast , ▁the ▁walls ▁of ▁the ▁reaction ▁chamber ▁in ▁a ▁cold - wall ▁re actor ▁are ▁typically ▁made ▁of ▁quart z ▁which ▁is ▁largely ▁transparent ▁to ▁the ▁elect romagnet ic ▁radiation . ▁The ▁reaction ▁chamber ▁walls ▁in ▁a ▁cold - wall ▁re actor , ▁however , ▁may ▁be ▁indirect ly ▁he ated ▁by ▁heat ▁radi ating ▁from ▁the ▁hot ▁ped est al / s us ceptor , ▁but ▁will ▁remain ▁cool er ▁than ▁the ▁ped est al / s us ceptor ▁and ▁the ▁substr ate ▁the ▁ped est al / s us ceptor ▁supports . ▁ ▁In ▁hot - wall ▁C VD , ▁the ▁entire ▁chamber ▁is ▁he ated . ▁This ▁may ▁be ▁necessary ▁for ▁some ▁g ases ▁to ▁be ▁pre - c rack ed ▁before ▁reaching ▁the ▁wa fer ▁surface ▁to ▁allow ▁them ▁to ▁stick ▁to ▁the ▁wa fer . ▁ ▁Gas ▁in let ▁and ▁switching ▁system ▁ ▁Gas ▁is ▁introduced ▁via ▁devices ▁known ▁as ▁' b ub bl ers '. ▁In ▁a ▁b ub b ler ▁a ▁car rier ▁gas ▁( us ually ▁hydro gen ▁in ▁ar sen ide ▁& ▁ph osph ide ▁growth ▁or ▁nit ro gen ▁for ▁nit ride ▁growth ) ▁is ▁b ubble d ▁through ▁the ▁metal organ ic ▁liquid , ▁which ▁pick s ▁up ▁some ▁metal organ ic ▁v