MOCVD of CuInE 2 (Where E = S or Se) and Related Materials for Solar Cell Devices
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EXPERIMENTAL The experimental method and apparatus used for the low pressure MOCVD of CuInE 2 was as described in earlier papers.' 3 The growth of the thin films was carried out in a lowpressure (;10-2 Torr) MOCVD reactor tube which has been described elsewhere.' 3 A graphite susceptor held the substrate (dimensions 10 mm x 15 mm) which was heated by a tungsten halogen lamp. A typical growth run (in temperature range 450 - 500 'C) involved the use of approximately 100 mg of stoichiometrically mixed sample, and lasted for 1- 2 hours. Films were deposited on glass microscope slides. The precursors used were respectively the bis- and tris- complexes of methyl,n-hexyl-diseleno- or -dithio-carbamate with copper (II) and indium, prepared as described in earlier papers. 11,14,15 In initial experiments, these were simply mixed in a 1:1 ratio in the evaporator. In subsequent experiments the effect of varying the ratio of Cu:In has been investigated, these results will be reported in a full paper. The compounds were prepared by methods detailed in earlier papers. Growth runs were typically for times between 30 minutes and 2 h. In 2 h. thick ca. 2 micron films were deposited (T source {Ts} = 180-250TC, T growth {Tp} = 400-450TC). X-ray diffraction studies were performed using secondary graphite monochromated CuKI radiation on a Philips PW1700 series automated diffractometer. The sample was mounted flat and scanned from 10 - 90 0 in steps of 0.04 ' with a count time of 2 s. Samples were carbon coated before analysis. All EDAX and electron microscopy was then carried out in a Jeol Superprobe 733 microscope. RESULTS AND DISCUSSION The success of these compounds for the deposition of sulfides or selenides depends on quite subtle differences in both their thermal stability and mode of decomposition from the parent compounds such as the diethydiselenocarbamates (see scheme below).' 0
Symmetrical [Zn(Se 2CNEt 2)2] M+ 54963
Pei
I
e
E2NCSeg Ete 2 t
+
j
634 M
CriC-4 2
EtSeEEt
4
321
eS~\>1 Se
1SC
59 nHex(Me)NCSe2ZnSe2CN(Me)nPr÷ -Se
2
-
514nHex(Me)NCS•eZnSeCN(Me)nPr÷
Et 2NCSe 2ZnSeCN(H)Et+ Se2
I
-CH+CHCH 2
EtI
Se2 Et
e
Asymmetrical [Zn(Se 2CNRR') 2]
-CHiCHCH2 -CH4
441 45 6 nHex(Me)NCSe2ZnSeCN+
CH3CH3
-CN
1
Et 2NCse2znseCN+ 411
nHex(Me)NCSe2ZnSe+
S430
-Se
Et2 nCSe2ZnSe+ 37 CN
+
fHex(Me)NCSe2Zn
Se
1350
eZn "
Et2NCSe 2Zn 307 Et2NCSe 2+ l
/'•-ZnSe
ntHex(Me)NCSe2 286 ,ý
_e
-ZIS
nHex(Me)NCSe+
243
206
Et 2NCSe+ 164
Figure 1. Decomposition Scheme for Symmetrical and Asymmetrical Precursors.
148
The combined GC/MS and HPLC results show that the symmetrical precursors such as [Zn(Se 2CNEt 2)2] compounds decompose readily to give EtSe 2 Et and eventually Se as a major product whereas the asymmetrical precursors such as [Zn(Se 2CNMe"Hex) 2] primarily give ZnSe and a ring closed organic fragment of formula SeCNMe"Hex+ that is volatile and removed in vacuo. The structure of one of the precursors used for deposition, [Zn(Se 2CNMe"Hex) 2], can be seen in figure 2 below. The zinc metal center is tetrah
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