Chromium Oxide Films from APCVD___________________________________^
This chapter concerns the A PCVD reaction o f chrom ium oxychloride, CrOzCl] w ith various oxygen sources, such as w ater (H 2O ), ethyl acetate (CH 3CO O C2H 5), m ethanol (C H 3O H ), ethanol (C2H5OH) and acetic acid (C H 3C O O H ). The purpose o f these studies was to form chrom ium oxide thin film s on glass. The com position and m icrostucture o f the film s was analysed b y a variety o f techniques: glancing angle pow der X -ray diffraction, (incident angle set to 1.5°), scanning electron m icroscopy (SEM ), transm ission reflection spectroscopy, R am an spectroscopy and X -ray photoelectron spectroscopy. The ability o f the film s to act as heat m irrors or photocatalysts w ere investigated. This w as carried out by transm ission / reflection m easurem ents, and photodegradation studies on stearic acid layers. All film depositions w ere carried out on the apparatus described in C hapter 2.
4.1
APCVD reaction o f chromyl chloride and water
A PC V D studies w ere carried out by varying the experim ental conditions: substrate deposition tem perature; deposition time; precursor tem perature; precursor carrier gas flow rates and type o f co-reactant. O ther studies w ere also carried out by varying the m ixing length o f the gas flows p rior to entering the deposition chamber.
Chromium Oxide Films from APCVD 84
4.1.1 Substrate temperature
A set o f experim ents w ere conducted w ith the flow rates given in table 4.1 G as line Flow r a te / dm ^m in'
Plain line 10
B ubbler line 1 0.5
B ubbler line 2 N ot U sed
D ilutent line 2
Syringe line 10
T a b le 4.1 flo w rates u se d in substrate tem perature d e p o sitio n stu d ies o f ch rom iu m o x id e
The tem perature o f the chrom yl chloride bubbler w as set to 90 °C, som ew ay below its boiling point o f 117 °C. This was considered high enough to give a reasonable partial pressure. This bubbler tem perature proved adequate in this study as can be seen by looking at the partial pressure curves given in appendix A. N o reaction took place i f only a diluted flow o f chrom yl chloride w as passed over the surface o f the substrate. This w as found to b e true over all substrate tem peratures from 400 °C to 600 °C, (m easured at 50 °C intervals, 1.5 m inutes deposition). Coatings could only be grow n w hen a co-oxygen source w as added to react w ith the chrom yl chloride vapour. This does not necessarily m ean that the oxygen in any chrom ium oxide films com es from the co-precursor, m erely, that w ithout the presence o f the co precursor there is no readily available route for the decom position o f chrom yl chloride into a chrom ium oxide film and w aste gaseous products.
Chromium Oxide Films from APCVD___________________________________^
A series o f depositions w as carried out using w ater as the co-precursor. W ater was injected into the syringe line at a rate o f 1.33 cm^ m in* equivalent to a m olar rate o f 0.074 m ol m in *. It w as found that below a tem perature o f 400 °C no film was form ed even i f a deposition tim e o f four m inutes w as used. However, at 400 °C and above a green opaque film w as form ed over the w hole surface o f the glass. It was found that the grow th rate o f the films w as slow est at 400 °C (0.2 pm sec'*) and fastest at 600 °C (1.62 pm sec'*). These grow th rates w ere based on a com bination o f estim ates on inform ation from optical interferom etry and SEM experim ents.
The films showed excellent adhesion to the surface and could not be rem oved b y either w iping w ith a cloth, or indeed scraping w ith a m etal scalpel blade and passed the scotch tape test. Chem ical resistance w as also found to b e good, w ith the coatings rem aining seem ingly unaffected on the surface o f the glass, even after im m ersion in a variety o f solvents, (nitric acid, hydrochloric acid, w ater, toluene, acetone) for 24 hours.
The green colour o f the film w as consistent w ith a Cr2 0 3 coating. This w as further confirm ed by XRD that show ed all o f the film s w ere crystalline, w ith a pattern for Cr2 0 3, as shown in figure 4.1.
Chromium Oxide Films from APCVD