Some groups V and VI elements, such as P, As,
Sb and S, Se, Te respectively evaporate as
1 polymeric species e.g. P , As , Se , S
2-12
. At a given temperature the reactivity in the formation of compound thin f ilms, such as GaP and InSb, can be enhanced considerably by pre-dissociation to lower-meric species - ideally to the atomic state. Thermal dissociation only partially achieves increased reactivity; e.g.
The data was obtained using the Oxford
Applied Research Valved RF cracker (QC500), in which line-of-sight optical emission spectra of the plasma can be obtained. Figure 2 shows the emission spectrum from white phosphorus which sustains a stable RF discharge in the absence of a support gas.
o thermal cracking temperature of 1600 C to achieve substantial dissociation to atomic
2 tellurium .
Dissociation of these polymers to atomic species can be achieved by passing the primary vapours through a plasma discharge.
Figure 1 shows SIMS data on the reactive species of white P and pre-dissociated atomic
4
3 P in the doping of ZnSe . Phosphorus incorporation from P is not detectable, whilst
4
18 -3 up to 10 cm of elemental P is readily doped.
Figure 2
Arsenic and selenium also can sustain an unsupported discharge. Figure 3 shows argonsupported and pure arsenic emission spectra
4 obtained with our model RFK25 Cracker .
Figure 1
Figure 3
Mass spectrometry using a simple residual gas analyser (RGA) can also be employed to diagnose the efficiency of RF plasma cracking of polymeric evaporants. Figure 4 supported RF discharge in selenium, namely monomeric hydrogen selenide, (comprising
2 six stable isotopes) . Atomic selenium is not
1 present in the normal vapour which consists mostly of Se , Se , Se , Se and Se .
2 5 6 7 8

Figure 6b
Figure 4
In the Se-cracked spectrum even the dimeric
2
-4 Se hydride is less than 10 that of the monomer
(Figure 5). A pure unsupported selenium discharge can be sustained, the Se atoms being sufficiently volatile to be detectable by the mass analyser located half a metre from the discharge.
Red phosphorus does not readily sustain a stable unsupported discharge nor with argon
3 support gas, in contrast to white phosphorus . discharge when the major mass species supported discharge exhibits principally atomic and ~0.1% trimeric hydride (figure 7).
Figure 5
Figure 7
How can we monitor, directly, the dissociation of the higher-mass polymers when the RGA available in this work has an upper m/e limit of
200a.m.u.? Higher-mass polymeric ions, such
78
5
+ as Se (at 390a.m.u.) are beyond this limit .
++ The ionisation potential of Se is only 21.5eV
78
5
++ and that of Se will be similar (probably even lower). Consequently the RGA, with its
(typically) 70eV electron-beam energy, should be able to create and resolve at m/2e 195 the doublycharged ion of the Se polymer. Figure
5
6a shows the loss of this species over a four minute discharge period whilst the much more intense atomic Se remains constant.
The mass spectrum of a pure self-supporting the low (40W) RF power used.
S S
2
S S S S
6
Figure 8
+
Figure 6a
++
1. Nesmeyanov, A.N. Vapour pressure of the elements.
Infosearch; Cleaver-Hume Press (1963).
2. Oxford Applied Research (unpublished)
3. Calhoun, L.E. and Park, R.M., J.App.Phys. 85 (1999)
490-497
4. Waag, A., Luganer, H-J., And Landwehr, G. Abstracts:
VIII Euro. Workshop on MBE, Granada (1995)
++ Figure 6b shows atomic Se at ~1% of the
+ abundance of Se . Applying this value to the
5
++
5
+ case of Se /Se , we can thus deduce from remains undissociated.
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