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LOW POWER LASER ANNEALING EFFECTS IN α-Ge
U. Zammit, M. Marinelli, G. Vitali, F. Scudieri
To cite this version:
U. Zammit, M. Marinelli, G. Vitali, F. Scudieri. LOW POWER LASER ANNEALING EFFECTS IN α-Ge. Journal de Physique Colloques, 1983, 44 (C5), pp.C5-313-C5-317. �10.1051/jphyscol:1983547�.
�jpa-00223133�
JOURNAL DE PHYSIQUE
Colloque C5, supplement au nD1O, Tome 44, octobre 1983 page C5-313
LOW POWER LASER ANNEALING EFFECTS I N a-Ge
*Istituto di Fisica-Facoltd di Ingegneria, Universitd di Roma, and GNSM of CNR, Italy
+Istituto di Fisica-FacoZtd di Ingegneria, 2, Universitd di Roma and GNEQP of CNR, Roma, Italy
R6sum6 - On pr6sente des r6sultats exp6rimentaux qui montrent l'influence des gradients de temp6rature pour l'obtention de la recristallisation de a-Ge, d6rang6e par implantation ionique au moyen du faisceau puls6 d'un laser a rubis ?i basse densit6 de puissance.
Abstract - Some experimental results are reported which show the influence of temperature gradients for recovering in low power laser annealing of amorphous Ge. The results suggest that tempe- rature gradients enhance the crystallization process.
It has been recently /I/ well established that the recovering of amorphous semiconductors irradiated with superimposed low power den- sity ruby laser pulses (below the material melting threshold) occurs as a cumulative process and with opposite directions of the reordering process. Namely for glow discharge deposited a-Ge the crystallization process connected with laser irradiation starts from the amorphous- substrate interface and moves progressively towards the sample surface;
on the contrary the opposite regrowth direction has been observed in ion implanted a-Ge. Such a behaviour can be connected with the
presence of residual less disordered nuclei at the implanted specimen surface. Besides, even below the melting threshold, the annealing process seems to be mainly due to temperature raises and photoactiva- tion processes /2,3/.
Several authors /4,5,6/ have shown some effects that can be attributed to temperature gradients. In the present paper we want to show some experimental evidence of non uniform temperature field effects in ion implanted a -Ge .
I - EXPERIMENTAL AND RESULTS
Samples of 46 KeV ~ e + implanted Ge (doses of 5x10I4 - 1 0 ' ~ c m - ~ ) were irradiated with several successive superimposed low power Q-switched ruby laser pulses ( 5 1. 30 ns). The beam homogeneity was improved by means of an optical d8focusing system. Etching action, selective for damaged regions with respect to the less damaged matrix, together with double stage replica technique were used in order to observe the laser annealing effects. As already pointed out,temperature gradients can set up in implanted material even under homogeneous illumination due to the different optical and thermal properties of highly desordered damaged regions and less damaged surrounding / 7 / . Such thermal gradients, together with photo /3/ and thermal /2/ activation mechanisms, induce defects migration which is responsible for the appearence of crystal- line areas (fig.1). In this preliminar analysis the temperature gradients distribution is mainly determined by the disordered regions Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983547
C5-314 JOURNAL DE PHYSIQUE
Fig. 1 - Electron microscope image of a-Ge (dose of 5x10' cm-2) replica a) before irradiation; b) after 10 superimposed laser pulses of 1.2 t1W cm-2; c) after 10 superimposed laser pulses of 2.8 MW cm-2
(melting threshold 2 5 MW ~ m - ~ ) .
disposition. To perform a better controlled thermal gradients distri- bution it is possible to introduce a known spatial distribution of heat losses, such as irradiation of suspended films placed on an electron microscope metal grid / 4 , 5 / , or to illuminate the sample with a diffraction pattern due to an opaque mask. In our case, in order to obtain a well defined and mainly unidimensional surface
temperature distribution, we have irradiated the samples with a pattern of linear fringes caused by the interference of two incoming beams.
This set up allowed us to control the spatial periodicity and therefore thermal gradients by changing the relative inclinations of the inter- fering beams. Temperature gradients due to temperature variations with depth in the specimen must also be taken into consideration for a better description of the problem.
Fig.2 shows the electron microscope image of the replica of the annealing effect for fringe irradiation at different incident peak power densities I and spatial periodicity d. At low values of I and temperature gradignts (proportional to d- ) we observe (see ~ i g g . ~ 2 a ,
2b) some faint recovered regions (denoted by B ) on either sides of the zone corresponding to the maximum incident intensity (A). This indica- tes that crystallization is enhanced where the temperature gradient is high enough. In fact less recovering is observedinthemaximum temperatu- re region, which is the opposite of what expected if the crystalliza- tion were due only to temperature raise. It must be pointed out that at a distance of d/4 from the fringe peak (where the temperature gradient is maximum) no recovering is observed. This result suggests that at such a low intensity value the photo or thermal activation for defect diffusion process is too low to cause, together with the
temperature gradient effect, the crystallization. At a given higher intensity peak power and by decreasing d we observe that the crystal- lized fringe fraction increases ,(see Fig .2c,2d respectively). This could be due to the fact that temperature gradients are higher in the case of Fig.2d than in Fig.2~. Noreover the less crystallized regions at the maximum peak intensity have progressively narrowed because the range over which the temperature gradients is very small has also decreased.
Fig.3 shows a quasi-circular fringe pattern due to interference between a uniform incident beam (power density = 1.2 ~w/crn~) and light scattered by some surface disturbance (i.e. a dust particle)/S/.
In this case the intensity distribution on the sample surface is characterized by a modulation very small compared to the background
Fig. 2 - Electron microscope ima e of replica of fringe irradiated implanted Ge sample: a) dose 1 0 l g ~ m - ~ , I = 3 MV ern-', 13 P mi
b) enlargement of a) ; c) dose 5x1 I 2. 4 LYW cm- , d = 6 p m ; d) dose 5x101 4 I = 4 !IW ]';~C d' = 3 g m (pulse number : 2 0 for
all cases). P
Fig. 3 - Crystallized regions disposed on quasi-circular patterns.
0 - 3 1 6 JOURNAL DE PHYSIQUE
Fig. 4 - Ripple pattern observed for I 2: 4 MW/cm L , d = 3 pm.
P
level due to the very low scattered light intensity. The periodicity of such a modulation is about equal to the incoming radiation wave- length ( = 0.69pm). We can therefore deduce that also in this case, temperature gradients are responsible for the particular crystal- lization features.
Finally we would like to show that in some case, for irradiation with linear fringe pattern, in areas corresponding to minima of incident intensity, some ripple patterns with periodicity equal to the light wavelength are observed (Fig.4). Such patterns have been interpreted as due to effects of interference of the incoming beam with light scattered from the surface of the specimen, through an effect of resonant enhancement of a spatial Fourier component of the sample surface disturbances (which act as a surface grating)/9/. In this case we cannot speak of a unidimensional surface temperature gradient any more.
I1 - CONCLUSIONS
The influence of thermal gradients in low power laser annealing pro- cesses was tested using an irradiation with a spatially modulated profile. Poor crystallization was observed in areas corresponding to very small temperature gradients but maximum incident intensity, whereas maximum crystallization was observed in areas corresponding to greater temperature gradients, although smaller incident intensity.
REFERENCES
2. G.Gat, J.F.Gibbons, T.S.Magee, J-Peng, V.R.Deline, P.Williams, C.A.Evans jr., Appl.Phys.Lett. 2 , (1978) 276.
J.S.Williams, W.L.Brown, H.J.Leamy, J.M.Poate, J.W.Rodgers, D.Rouseau, G.A.Rozgonyi, J.A.Shelnutt, T.T.Sheng, Appl.Phys.Lett.
33, (1978) 542.
-
3. J.A.Van Vechten, J.Phys.s (1980) 7.
J.R.Troxell,A.P.Catterjee, G.D.Watkins, L-C-Kimerling, Phys.Rev.
B19, (1979) 5336.
4. M.L.Bufay, A-Pigeolet, - L.D.Laude, J.Appl.Phys. 54 (1983) ,660.
5. M.Failly Lovato, M.C.Joliet, L.D.Laude, M.Wautelet, private corn.
6. D.J.Ehlrich, Y.Y.Tsoo, Appl.Phys.Lett.4J (1982) 297.
7. G-Vitali, M.Marinelli, U.Zammit, F-Scudieri, Phys.Lett.E, (1983) 320.
8. D.C.3tunony, R.P.Howson, L.J.Willis, Appl-Phys-Lett. 23 (1973) 598.
9. Zhou Gausheng, P.M.Fauchet, A.E.Siegman, Phys-Rev. ~ T t o be published).
