Amorphous to crystalline phase transition of vapour grown selenium films

Abstract

In recent years quite a large number o f exper imenta l studies have been made on the crystal l izat ion processes which may occur in a m o r p h o u s selenium films [1-15] and these were under t aken to predic t the g rowth s t ructure o f the a m o r p h o u s phase o f the films. I t m a y be noted that a con t roversy still exists as to whether the shor t range s t ructure o f a m o r p h o u s selenium is compr i sed o f hexagona l chains or e igh t -a tom rings or a mixture of both . As there are few deta i led and conclusive studies [1] on the crys ta l l iza t ion processes (by thermal t r ea tment a t different t empera tures ) in relat ively thin selenium films, the present work a ims to repor t the findings f rom micros t ructura l invest igat ions with Xray diffract ion and scanning e lect ron microscopy (SEM) a long with the measurements o f da rk conduct iv i ty and pho toconduc t iv i ty . This has been under taken in cont inuat ion o f our present exper imenta l p r o g r a m m e of mic ros t ruc tu ra l studies on semiconduct ing thin films [16-18]. Selenium films o f thicknesses 545, 1245, 3000 and 5900 nm were depos i ted by vacuum e v a p o r a t i o n (normal incidence o f v a p o u r beam) at a pressure o f 5 × 10 6 to r t on clean glass substrates at r oom tempera ture (298 K) with a depos i t ion rate o f 5 _+ 1.5 nm sec ~. A m a x i m u m o f four films was depos i ted at the same t ime which were then annea led separa te ly at different tempera tures for subsequent studies at r o o m tempera ture . Tables I and II record the detai ls o f the exper imenta l condi t ions as well as the obse rva t ions made. The m a x i m u m anneal ing t ime was 5 h and the m a x i m u m anneal ing t empera tu re was 358 K. X r a y dif f ract ion pa t te rns were ob ta ined with L i F m o n o c h r o m a t e d MoK~ rad ia t ion in a Phil ips P W 1050/51 counte r diff rac tometer coupled to a Phil ips P W 1130, 3 k W X r a y genera tor . Da rk -conduc t iv i t y and p h o t o c o n d u c t i v i t y measurements were done on a p p r o x i m a t e l y 3000 to 3200-rimthick films annealed at 338,348 and 358 K, respectively, and depos i ted under ident ical condi t ions . S E M studies were carr ied out on a lmos t ident ica l samples as above with a Hi tach i S-415 e lec t ron microscope . Conduct ivi ty measurements were m a d e be tween two par allel surface e lect rodes (with a s epa ra t i on o f 0.003 m) o f e va po ra t e d silver which was depos i t ed on the films. The cur ren t and 200 V d.c. supply were measu red by a Kie th ley 610C e lec t rometer and a Kie th ley D M M 177 vol tmeter , respectively. A 200 W m 2 l ight source was used for the pho toc onduc t i v i t y measurements . A n a t t empt was also made to depos i t the selenium films on the p rehea ted substra tes . F i lms with a maxim u m thickness o f a pp rox ima te ly 1200 nm could be depos i ted wi thou t being peeled off a t subs t ra te tempera tures o f 358 and 398 K, respectively. A t the h igher t empera tu re o f 398K, the film was d i scon t inuous