Figure 4.17 Uptake Curve for TDS Spectra of GaC13 showing Peak Area plotted as a function of Chlorine Exposure
Ga GaCl
GaCl
» w . * —V tem perature 40 o 5 00 600 700
Figure 4.18 Thermal Desorption Spectra for the Gallium containing mass fragments of 15L exposure of CI2 on GaAs(lOO)
Chapter 4: Results
4.5.4.2 A d so rp tio n of Cl2 on GaAs at 300K: A rsenic co n tain in g D eso rp tio n Products
T herm al d eso rp tio n spectra of arsenic containing d eso rp tio n p ro d u cts are n o w p re se n te d to com plem ent the d ata on the G a-d eso rp tio n p ro d u cts presented in the previous section. The mass fragm ents stu d ied w ere As+ (75am u), AsCl+ (llO am u), AsC12+ (145amu), AsC13+ (180am u) a n d A s2+
(150amu).
A series of TD spectra is show n in Figure 4.19 for the As+ (75amu) detected m ass frag m en ts follow ing increasing exposure of the G aA s su rface to m olecular chlorine. One distinct peak is clearly apparent, at 700-750K. The uptake curve for these spectra is show n in Figure 4.20 and show s a rap id rise in signal at low chlorine coverage w hilst at increased coverage the signal increases slowly. This is identified as the As p peak and differs from the Ga p in that complete saturation does not occur. The peak intensity rises slow ly over very large exposures of chlorine. The As p peak increases in intensity u p to exposures of 1000L. C ontinued chlorine exposure causes a reduction in the rate of increase of peak intensity w ith o u t satu ratio n of the state.
A series of TD spectra for the AsCl+ (llO am u) detected m ass fragm ent w ith increasing Cl2 exposure is shown in Figure 4.21. It is noticeable that a decreasing signal is imm ediately apparent upon starting the desorption experiment. This is due to spontaneous desorption of products at 300K. A desorption state exists below 300K and low tem perature TDS results are presented in section 4.5.4.3 to witness this statem ent. A peak is observed at a sim ilar (high) tem perature as that of the p peak for the As m ass fragm ent in Figure 4.19. An uptake curve for AsCl for this high tem perature P desorption is presented in Figure 4.22. The curve displays a steady rise in signal as the chlorine exposure is increased.
TD spectra for the m ore highly chlorinated As m ass fragm ents w ere then studied. The AsC12 spectra are show n in Figure 4.23 revealing that at low
Chapter 4: Results
chlorine coverages only one peak is observed. This arises at approxim ately 680K an d grow s in intensity w ith increasing chlorine exposure. At exposures of 250L another state centred at 450K appears. These two states correspond closely with the a and p peaks of the G a-containing fragm ents and possess energies of desorption of similar, although not identical, values i.e. 119 an d 187 kj m o l'1 for the a and p states respectively. A sim ilar series of curves are observed for the AsC13 (180amu) m ass fractions. The curves for AsC 12 uptake are presented in Figure 4.24 and show th at the p state
satu rates at an exposure of approxim ately 25L w hilst the a state continues to populate at increasing exposures of m olecular chlorine.
In addition to the arsenic chloride m ass fragm ents, arsenic m ay be lost in a m olecular form. The existence of both dim eric and tetram eric arsenic is well know n. The TD spectra show n in Figure 4.25 for desorption of the As2 mass fragm ent w ith increasing chlorine exposure reveals only sm all losses of this species from the surface. A single desorption peak is found at 700K w hich rises rapidly at exposures of chlorine up to approxim ately 25L and then rises at a slow er gradient. The uptake curve is presented in Figure 4.26 for the peak area of the As2 m ass fragm ent versus chlorine exposure. Initial dosing w ith the halogen results in a high initial g rad ien t b u t prolonged exposure shows a reduction in the rate of loss of this phase.
4.5.4.3 A d so rp tio n of C l2 on GaAs at 100K: Arsenic co n tain in g D eso rp tio n Products
E vidence from ad so rp tio n studies of chlorine on GaAs in the p rev io u s section suggest th at the spontaneous evolution of som e species occurs at tem p eratu res below 300K. Results are presented in this section to isolate ad so rp tio n states at low tem peratures. L iquid n itro g en cooling was em ployed to cool the crystal surface to 100K. The A sClx (x=0-3) m ass fractions w ere recorded. A typical spectrum is show n in Figure 4.27 for AsCl. The spectra show s an intense broad peak labelled y arising at 160K. The less intense broad state w ith a peak m axim um betw een 410-450K equates to