3.2 Amorphous Ge films on Si substrates · 3.2: Amorphous Ge films on Si substrates 121 inflections occur in an expanded fluence scale and show a different amplitude. In contrast,

3.2: Amorphous Ge films on Si substrates

119

3.2 Amorphous Ge films on Si substrates

In the previous section 3.1, we have extensively described the solidification behavior of aGe film with certain configurational parameters (amorphous, 50 nm thick, glass substrate). Thesimplest way to reveal the differences in the solidification behavior related to modifications of theheat flow conditions, is to change the substrate. In order to emphasize the expected differences,Silicon was chosen as a substrate because its thermal diffusivity is much higher (two orders ofmagnitude) than that of glass.

A thickness of 50 nm was chosen initially to enable the direct comparison with the resultsalready shown in section 3.1 for films on glass. The specimens have been irradiated ps laserpulses at different energies. The fact that the Si substrate used here is not transparent in the visiblewavelength range, restricts us to the use of the FSR technique. As a consequence, we looseimportant complementary information which could be provided by BSR and the FSTmeasurements. This drawback has been partially compensated by the use of Streak Camerameasurements with ps-resolution. In addition, the film thickness has been studied as an additionalexperimental parameter which enables us to modify in a controlled manner the heat flowconditions.

The results are grouped in three blocks. The first two blocks refer to the solidificationdynamics occurring in a 50 nm thick film as seen with two different time resolutions. The third partanalyzes the influence of the film thickness on the solidification processes:

• Reflectivity measurements with ns-resolution.• Reflectivity measurements with ps-resolution.• Influence of the film thickness

Chapter 3: Pulsed laser melting and rapid solidification of Ge

120

Reflectivity measurements with ns-resolution

Reflectivity transients

All real time transient reflectivity measurements presented in this section correspond to aprobe beam wavelength of 633 nm. Figure 3.20 shows the time evolution of the front sidereflectivity (FSR) of 50 nm-thick amorphous Ge films grown on Si upon irradiation with a singlelaser pulse of 30 ps at several fluences. The irradiation pulse induces a sharp reflectivity increaseup to a maximum value (Rmax) whose amplitude is a function of the laser pulse fluence. The initialdecrease after the maximum is sharp and is followed by a more gradual one, until a constant valueis reached after approximately 200 ns. The reflectivity values before (Rini) and after irradiation(Rfin) are similar for all fluences below the ablation threshold. A comparison to the transientsobtained in Ge films on glass substrates (Figure 3.1) shows that the reflectivity decay observedhere is much faster and that no shoulder can be observed, which indicates that bulk solidificationand recalescence does not take place. The absence of a decrease of the final reflectivity levelindicates that reamorphization of the film occurs even at the highest fluences used.

pump pulse

Rini

22 mJ/cm2

50 mJ/cm2

120 mJ/cm2

140 mJ/cm2

time (ns)

Rmax

Rfin

refle

ctiv

ity a

t 633

nm

(a.

u.)

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0

Figure 3.20.: Time evolution of the reflectivity of a 50 nm thick a-Ge film on Si substrateupon irradiation with ps laser pulses at several fluences. Rini, Rmax and Rfin

designate the initial, the maximum and the final reflectivity of the transients,respectively. The temporal position of the pump pulse is indicated by anarrow.

The evolution of the key features of the reflectivity transients versus the laser fluence canbe seen in more detail in Figure 3.21. The evolution of both the maximum transient reflectivityvalue (Rmax) and the reflectivity of the surface after irradiation (Rfin) are plotted in Figure 3.21a inabsolute values. Rmax is observed to increase until it reaches a plateau for fluences in the range40-100 mJ/cm2. Higher fluences lead to slightly increasing values of Rmax and surface ablation isobserved for fluences above 150 mJ/cm2. The overall evolution of Rmax versus fluence observedhere is very similar to that observed in the films on glass (Figure 3.2) although the characteristic


121

inflections occur in an expanded fluence scale and show a different amplitude. In contrast, Rfin

remains constant for fluences up to the ablation threshold which indicates a final state of thematerial similar to the initial state. This is opposed to the case of films on glass, for which a cleardecrease due to crystallization has been observed for fluences above 54 mJ/cm2.

The melt duration* has been plotted as a function of the laser fluence in Figure 3.21b. Itsevolution follows a trend quite similar to that observed for films on glass (Figure 3.2), including thesame location of the apparent melt threshold (20 mJ/cm2). However, the melt duration valuesobserved here are always lower. The maximum value at the highest fluences is about 14 ns,whereas for films on glass values of 24 ns have been found. The results show clearly that the meltduration for a given fluence is longer in the films grown on glass substrates, which is not surprisingdue to the lower thermal conductivity of this substrate.

0 50 100 150 2000

5

10

15

b)

me

lt d

ura

tion

(n

s)

fluence (mJ/cm 2)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

R m a x

Rfin

a)

refle

ctiv

ity a

t 6

32

.8n

m

-60

-40

-20

0

20

40

60

normalized reflectivity change (%

)

Figure 3.21.: Changes of a) the maximum transient reflectivity Rmax, the final reflectivityRfin and b) the melt duration upon irradiation at several fluences.

* The melt duration has been determined following the same criterion applied in section 3.1 as described in the

text related to the description of Figure 3.2.


122

Raman spectra

Figure 3.22 shows several representative Raman spectra measured on Ge films on Sisubstrates before and after irradiation. The spectrum of the as-deposited film (a) shows thecharacteristic broad transverse optic (TO) band of amorphous Ge at 270 cm-1. Upon irradiationwith fluences up to the ablation threshold, the spectra remain essentially unchanged as can be seenin the spectra (b, c). This result evidences that the material re-amorphizes upon melting and rapidsolidification, as already indicated by the constant final reflectivity level shown in Figure 3.21a.

150 200 250 300 350 400 450

c)

b)

a)Ram

an In

tens

ity (

a.u.

)

Wavenumber (cm-1)

Figure 3.22.: Raman spectra of the films recorded (a) before and after irradiation with apulse fluence of (b) 53 mJ/cm2 and (c) 135 mJ/cm2.

In order to understand why Ge-films of the same thickness but on different substrateseither crystallize or re-amorphize upon irradiation with intense ps pulses, it is necessary to takenote of the fact that the thermal diffusivity of Si (1.0 cm2/s) [Goo96] is about two orders ofmagnitude higher than that of the glass (0.009 cm2/s) [Sku95]. Therefore, the efficiency of theheat flow towards the substrate is notably enhanced which has two important consequences onthe solidification dynamics:

1. The rapid heat flow (high quench rate) causes the liquid to achieve larger supercoolings andprevents that the thermal gradient in the liquid is sufficiently reduced for bulk nucleation to takeplace*. This existing thermal gradient in the supercooled liquid promotes interfacialsolidification to take place.

2. The latent heat released upon solidification is efficiently extracted preventing recalescence andcrystalline phase formation. This is in agreement with the results of Sameshima and Usui[Sam93] who have studied the influence of the heat diffusion on laser-induced amorphization

* Remind that a necessary condition for bulk nucleation to occur is the presence of a supercooled liquid at an

homogeneous temperature [Sam93].


123

of polycrystalline Si films. They have observed that crystallization can be prevented for a givenfilm thickness if the amount of heat flow to the substrate is high enough*.

Optical simulations

In order to confirm that the re-amorphization process takes place interfacially and nothomogeneously in the supercooled bulk liquid, an estimation of the initially induced melt depth isrequired. We have simulated the evolution of the reflectivity at 633 nm of a 50 nm a-Ge film on Sias a surface molten layer progresses in depth, using the model described in Chapter 3. The resultis shown in Figure 3.23 and have to be compared to the temporal evolution of the transients(Figure 3.20) and to the evolution of Rmax with laser fluence in Figure 3.21a. Although the generalevolution of the experimental data resemble reasonably the simulated curve, the maximum valuesmeasured in Figure 3.21a are clearly lower. Such a lowered contrast has been already observedin a less pronounced way for the films on glass and was attributed to the limited rise-time of thedetection system.

0 10 20 30 40 500.3

0.4

0.5

0.6

0.7

0.8

l -Ge

a-Ge

Si substrate

refle

ctiv

ity a

t 63

3 n

m

melt depth (nm)

-20

0

20

40

60

no

rma

lized

refle

ctivity cha

ng

es (%

)

Figure 3.23.: Numerical simulation of the reflectivity at 633 nm of a 50 nm thickamorphous Ge film on Si which converts from the surface towards thesubstrate into liquid. The vertical scaling on the right hand side representsreflectivity changes with respect to the reflectivity of an a-Ge film.

A comparison of the highest experimental Rmax values to the simulated ones, leads to amaximum melt depth of about 8 nm for Ge films on Si substrate. Since this estimation of themaximum melt depth critically depends on the experimental measurement of the maximumtransient reflectivity level (Rmax), the influence of the rise-time of the detection system was checkedby means of ps-resolved Streak Camera measurements, shown in the following section.

* In their approach the authors increased the amount of heat flow out of the laser melted film by coating the

surface of a 30 nm thick Si film on quartz with a 800 nm thick SiO2 layer. The additional heat flow to thecoating (much larger than the heat flow to air [Wri96]) causes the film to amorphize while crystallizationtakes place for films without coating.


124

Reflectivity measurements with ps-resolution

Figure 3.24 shows a direct comparison between reflectivity transients obtained by theDigitizer (ns-resolution) for a probe beam wavelength of 632.8 nm and the Streak Camera (ps-resolution) for 514.5 nm.

• For a pulse fluence of 19 mJ/cm2, both types of transients show only a slight reflectivityincrease which remains below the reflectivity increase of 14 %, corresponding to a-Ge at themelting temperature. This is different from the results obtained on films on glass substrates(Figure 3.12) for which the Streak Camera transient already revealed surface melting at thatfluence. The comparison shows that the real melt threshold of films on Si is approximately 20mJ/cm2, and therefore slightly higher than the one of films on glass (15 mJ/cm2).

• The transients recorded at 62 mJ/cm2 reveal the effect of the limited rise time of theDigitizer-transients. While the Digitizer gives a maximum reflectivity change Rmax of about 30%,the Streak Camera transient exhibits a much higher reflectivity change Rsmax of 50%. It isworth to note that the subsequent decay of the Streak Camera transient is even faster than theone observed for the films on glass.

• At higher fluences (124 mJ/cm2) the value of Rsmax is slightly lower but shows a plateau.Nevertheless, the subsequent decay is as fast as the one at lower fluences and presents anoscillation below the initial reflectivity level. This reflectivity dip Rsdip, whose meaning will bediscussed in the following, can not be resolved in the Digitizer-transients.

As in the case of the measurements of films on glass, the improved time resolution of thedetection system reveals a higher contrast and features that can not be resolved by a detectionsystem with lower time resolution.


125

-10

0

10

20

30

40

50

Rsdip

Rmax

Streak CameraDigitizer

pump pulse

Rsmax

F=124 mJ/cm2

time (ns)

0

10

20

30

40

50

Rmax

Streak CameraDigitizer

Rsmax

F=62 mJ/cm2

0

10

20

30

40

50

60

Rmax

Streak Camera

DigitizerRs

max

F=19 mJ/cm2re

flect

ivity

cha

nge

at 6

33 n

m a

nd 5

14 n

m (

%)

-10 0 10 20 30

Figure 3.24.: Comparison between FSR transients at different fluences measured by thedigitizer at 633 nm with ns-resolution and the Streak Camera at 514 nmwith ps-resolution. The dotted lines indicate the reflectivity changepredicted for a-Ge at its melting temperature.


126

For a detailed study of the time evolution of the reflectivity, several representative StreakCamera transients are displayed in Figure 3.25. The gradual increase of Rsmax with increasingfluences can be appreciated for fluences 19 mJ/cm2 ≤ F ≤ 62 mJ/cm2. For higher fluences, Rsmax

is slightly below the maximum value measured at 62 mJ/cm2 and remains approximately constantover a large fluence range (69 mJ/cm2 ≤ F ≤ 140 mJ/cm2). At the same time, the decay isfollowed by a dip below the initial reflectivity as mentioned above. For increasing fluences the dipbecomes more pronounced and the transient gets broader. At the highest fluences used , thetransient has increased its width by a factor close to two and the dip has disappeared.

-10

0

10

20

30

40

50

60

19 mJ/cm2

25 mJ/cm2

32 mJ/cm2

62 mJ/cm2

69 mJ/cm2

1 2 8 m J / c m2

1 4 0 m J / c m2

refle

ctiv

ity c

hang

e at

514

.5 n

m (

%)

time (ns)

0 5 10 15

Figure 3.25.: FSR transients of a 50 nm thick a-Ge film on Si irradiated at differentfluences and measured by the Streak Camera at 514 nm in the 50 ns timewindow (expanded in the figure).

The observed behavior for fluences below 69 mJ/cm2 is consistent with an interfacial re-amorphization process, similar to the one already discussed in the films on glass below therecalescence threshold.

However, the presence of the reflectivity dip below the initial level for fluences above 69mJ/cm2 indicates the appearance of a different solidification scenario. In fact, the presence ofsuch an oscillation below the initial reflectivity (reflectivity dip) suggests the presence ofinterference effects in a system composed of a solid layer on top of a liquid one (similarly to whatwe observed in the BSR transients on films in glass (section 3.1).


127

This means, that solidification would take place at the film surface propagating in depth andleaving behind an amorphous material*.

• Surface initiated solidification

Such an scenario has been observed in a-Si by a few authors [Cam85], [Bru86],[Bon93]. Bruines et al. [Bru86] irradiated 200 nm thick a-Si films on c-Si substrates with laserpulses of 7 ns and observed amorphous regrowth in both directions, from the surface towards thesubstrate and from the initial maximum melt depth towards the surface. They found an interfacevelocities of 1.5 m/s for the surface initiated solidification front and of 3 m/s for the solidificationfront towards the surface. The authors determined the initial melt depth in Cu-doped a-Si films byRutherford Backscattering Spectrometry (RBS) analyzing the impurity redistribution upon melting.They found that only when the whole film thickness was molten, re-amorphization from thesurface was prevented and single crystal epitaxial regrowth from the substrate occurred. This isdifferent from our case because epitaxial or random crystal growth of Ge on a Si substrate is nota favorable scenario. Nevertheless, these authors did not provide a thermodynamic explanationabout the driving force for this kind of solidification. In order to investigate this point, a detailedanalysis of the Streak Camera-transients is required, which will be done in the following.

* The propagation of a thin buried molten layer into the amorphous material leaving solid material behind

would give rise to similar oscillations due to destructive interference of light reflected at the film surface andlight reflected at the solid/liquid-interface. Nevertheless, this scenario is unlikely in the present case,because the appearance of a propagating buried layer is usually accompanied by explosive crystallization[Low86], [Woo86b]. The reason is that the driving force for the propagation of a buried layer into depth isthe fact that the upper layer has a higher melting point (crystalline material) than the layer beneath(amorphous material). This driving force does not apply in the present case because the results from Ramanmeasurements confirm re-amorphization of the material over the whole fluence range.


128

A global view of the behavior of the transients can be obtained by plotting theircharacteristic values Rsmax, Rsdip and Rsfin as a function of the laser pulse fluence. This has beendone in Figure 3.26. The evolution of Rsmax is very similar to the evolution of the ns-resolved Rmax

values in Figure 3.21a, with the difference that the absolute values are slightly higher due to thebetter time resolution of the Streak Camera*.

0 2 0 4 0 6 0 8 0 100 120 140 160 180-60

-40

-20

0

2 0

4 0

6 0

Rsmax

Rsfin

Rsdip

s imulat ion

dot separat ion 1 nmrefle

ctiv

ity c

ha

ng

e a

t 51

4 n

m (

%)

fluence (mJ/cm2)

Figure 3.26.: Evolution of the maximum transient reflectivity Rsmax, the reflectivity at thedip Rsdip and the final reflectivity level Rsfin as a function of fluence asmeasured by the Streak Camera at 514 nm for 50 nm thick a-Ge films on Sisubstrates. In the case of Rsfin, only those data values which differ from theinitial reflectivity have been plotted to avoid overlapping with the Rsdip

values . The simulation curve shows the calculated reflectivity change as afunction of the melt depth and has been scaled horizontally to fit the Rsmax

increase. The dot separation in this curve corresponds to a step of 1 nm.

Nevertheless, the reflectivity value predicted for an optically thick liquid Ge layer on top(57 % increase) is not achieved even at the highest fluences reached as can be seen bycomparison with the simulation included in Figure 3.26. The highest reflectivity increase measuredis 50% and is observed at a fluence (62 mJ/cm2) just below the threshold fluence for theappearance of the dip (69 mJ/cm2). All Rsmax values at higher fluences are clearly below 50% andin almost all cases a dip is observed in the transients (triangles in Figure 3.26). This directcorrelation between the appearance of the dip and the lower Rsmax values is a further indication forthe presence of a surface initiated solidification process as will be quantified in the following.

* This enhancement of the reflectivity contrast is not caused by the different probe laser wavelength used

here, which becomes clear comparing the data with the relative reflectivity changes calculated by theinterfacial model in both cases (Figure 3.26 and Figures 3.21a, 3.23).


129

Optical simulations

We have performed a simulation of the reflectivity change in a surface initiatedsolidification process. Figure 3.27 shows the calculated reflectivity change for a solid layerprogressing in depth and for initial melt depths of 20 nm and 50 nm. Both curves show areflectivity dip below the initial film reflectivity resembling the general evolution of the transientsshown in Figure 3.25. for fluences ≥ 69 mJ/cm2. However, the curve for an initial melt depth of50 nm shows a dip much more pronounced than that experimentally observed (Figure 3.26). Incontrast, the simulation curve assuming an initial melt depth of 20 nm presents a reflectivity changeat the dip of less than 5% which is comparable to the experimentally observed one. It is importantto note that this does not mean necessarily that the initial melt depth was 20 nm. Most likely asolidification front progresses simultaneously from the substrate towards the surface [Bru86]. Thesimulated curve (initial melt depth 20 nm) shown in Figure 3.27 is approximately valid also for thiscase. It is thus valid for the case of a fully molten film which solidifies in both directions, thesolidification fronts meeting at a depth of 20 nm.

0 1 0 2 0 3 0 4 0 5 0-20

-10

0

1 0

2 0

3 0

4 0

5 0

6 0

initial

melt depth 50nm

Si substrate

l -Ge

a-Ge

a-Geinitial melt depth

50 nm 20 nm

refle

ctiv

ity c

hang

e at

514

.5 n

m (

%)

thickness of the solidified top layer (nm)

Figure 3.27.: Calculation of the reflectivity change at 514 nm of molten Ge on Si as itsolidifies from the surface(see inset). The calculation shows the case of aninitial melt depth equal to the film thickness of 50 nm (solid line) and thecase of a melt depth of 20 nm (dashed line). The reflectivity changes havebeen normalized to the reflectivity of a 50 nm thick a-Ge film on Si.

The presence of a rapid initial decay in both curves (almost independent of the initial meltdepth) means that a solidified surface layer of only 1 nm thickness would cause a reflectivitydecrease from 57% to our experimentally measured value of 50%. This suggests that the onset ofsolidification from the surface is initiated within a time shorter than the time resolution of thewindow chosen for the Streak Camera measurements (≈ 1 ns). This indicates that the liquidsolidifies from the surface to a depth of ≈ 1 nm within less than ≈ 1 ns after the laser pulse.


130

Interface velocities

A comparison of Figure 3.25 and Figure 3.27 provides the depth (15-19 nm) and thetime (7-8 ns after the pulse) at which the reflectivity minimum occurs. The interface velocity of thesolidification front progressing from the surface into depth can thus be calculated. The resultingvalue of ≈ 2.3 m/s is very similar to the one determined for interfacial re-amorphization of a-Gefilms on glass substrates in section 3.1.

Nevertheless, we have not yet provided an explanation for why solidification is initiated atthe surface, in spite of the fact that heat conduction to the substrate should in principle be muchhigher than to the air. The driving force of this type of solidification is an essential point and will bediscussed next.

• Melting and solidification scenario

In spite of the large thermal diffusivity of the substrate, the films on Si are likely to meltcompletely (50 nm) at high fluences. This assumption is based on the simple fact that films on Siare exposed to higher fluences than those at which films on glass are fully melted and also have alarger absorption at 583 nm (see section 3.1). In this case, the direct contact with the molten Geshould cause that the Si substrate is heated substantially. The experimentally observed presenceof surface initiated solidification at high fluences could be related to this substrate heating.

In order to enable solidification to take place at the surface, the solidification process atthe interface with the substrate has to be prevented or delayed. This can only be done by theinversion of the temperature gradient in the liquid by the creation of aheat source within the substrate. In particular, the assumption that a thin layer of the Sisubstrate melts at high fluences*, would provide an explanation for the driving force of thatspecial type of solidification. The explanation is given in the following, together with thejustifications for the assumption :

At high fluences, the Ge film is melted over its entire depth within less than 100 ps (seeFigure 3.11). Large thermal gradients within the very hot liquid† are initially present which rapidlythermalize‡, accompanied by heat diffusion to the Si. The latter may cause that a very thin Si layeris melted (Tm = 1412 ºC)§, only slightly delayed to the melting of the film. The large thermal

* Melting is likely to happen only because the Ge film serves as an “absorption layer” which transmits the

heat to the Si. Upon direct irradiation of Si by the ps laser pulse, the melt threshold would be much higherdue to the large optical penetration depth of Si in the visible.

† Notice that the boiling point of liquid Ge is 2830ºC.

The thermal diffusivity of liquid Ge is much higher than that of a-Ge. Following equation 1.10, heat diffusionwithin the melt over a distance of the whole film thickness occurs within 50 ps.

§ For comparison with the results of films on glass substrate: The softening temperature of glass is evenhigher (1600 ºC).


131

diffusivity of l-Ge enables the continuous supply of heat in order to provide the melt enthalpy ofSi. This process leads to a situation (after a few hundreds of ps), in which both liquids have similartemperatures and are cooling down. Because the solidification temperature of Si is much higher,it solidifies first, releasing the corresponding latent heat. Part of this heat diffuses rapidly towardsthe liquid Ge, thus delaying the solidification process at the substrate/film-interface and favoringsurface initiated solidification*.

We believe that the above described scenario applies in our case thus providing apossible explanation for the driving force for the surface initiated amorphization process.Nevertheless, since an experimental confirmation of the melting of the Si substrate is still lacking†,other possible scenarios have to be considered. An important point in this sense is a peculiaritythat can be found in the work of Bruines et al. These authors observed both, re-amorphizationfrom the surface induced by laser pulses of 7 ns [Bru86] and explosive crystallization induced by20 ns pulses [Sin84] in the same sample type (200 nm a-Si on c-Si). This shows that the pulseduration has an essential influence on the solidification scenario which is confirmed by the fact thatthe few times surface initiated re-amorphization has been reported, the pulse duration used wasalways a few ns or less‡.

• Thermal model for melting and ablation thresholds

In the following a simple calculation to determine the melt and ablation thresholds of Gefilms will be presented. This calculation is meant to provide additional information about the initialconditions after laser pulse induced melting prior solidification. For a first estimation of the meltand ablation thresholds, the following simplifications are necessary:

a) The film material is heated over a depth which corresponds to the optical penetration depth ofthe laser light, assuming that the heat flow during the melting process induced by a ps laserpulse can be neglected [Gri91].

b) The heat flow towards the substrate is neglected, which limits the applicability of thecalculation to free-standing films or films on poor heat conducting substrates. Therefore, thisfirst attempt is intended to calculate the thresholds for films on glass.

* Notice that the nucleation barrier at interfaces is lower than within the bulk liquid (heterogeneous

nucleation, see section 1.5) which favors nucleation at the surface but also at the interface with thesubstrate.

† A possible experiment has been already scheduled to determine whether substrate melting takes place ornot. The principle of this experiment will be explained in Chapter 6.

‡ Boneberg [Bon93] used 7 ns pulses, we use 30 ps pulses.


132

Based on the definition of the heat capacity we write the following equation: ∆Q = ∆T ⋅ Cwhere ∆Q represents the absorbed energy and ∆T the induced temperature increase in a materialwith a heat capacity C. We replace ∆Q by the product of the laser fluence F and the absorptionA at 583 nm divided by the optical penetration depth, and write C as the product of the specificheat capacity c and the material density ρ, so that we obtain:

FTA

c= ⋅ ⋅∆

ρα1

(3.1.)

The thermal and optical properties of amorphous and liquid Ge required to calculate themelt and ablation thresholds by means of equation (3.1) are given in Table 3.1*. The opticalpenetration depth of the laser light at 583 nm is 23 nm (see Table 2.3). For the melt threshold Fm

a value of 7.2 mJ/cm2 is obtained, which is a factor 2 lower than the experimentally observed meltthreshold for films on glass (15 mJ/cm2). The fact, that the calculation provides anunderestimation of the melt threshold suggests, that heat diffusion within the film plays a certainrole in the melting process also for ps pulses.

material Tm/ Te (ºC) ∆Hm (J/g) ρ (g/cm3) cp (J/gK) K (W/cmK) κ (cm2/s) A (583 nm)

a-Ge 714 350 5.3 0.34 - 0.46 0.01 0.0055 - 0.0041 0.47 (0.52)

l-Ge 2830 4516 5.6 0.46 0.297 0.118 0.25

Table 3.1.: Thermal and optical properties of amorphous and liquid Ge. The values aretaken from reference [Szy95] considering the temperature dependence of cp

and κ from room temperature to the melting point (Tm). Te represents theboiling point of liquid Ge. The values of the absorption A have beencalculated by the optical constants in Table 3.2. The value in brackets refersto the absorption of thick films while the other one refers to a 50 nm film.

In order to determine the influence of thermal diffusion on the melt threshold, 1/α inequation (3.1) has been replaced by the film thickness d. The resulting value of 15.7 mJ/cm2 is inexcellent agreement with the value experimentally observed for films on glass, thus confirming theinfluence of heat flow within the whole film thickness. Following this scheme, the ablationthreshold Fa can also be calculated. In this case, one has also to consider that the latent heat ofmelting the whole film thickness has to be provided. Therefore ∆T in equation (3.1) has to bereplaced by the sum of the boiling temperature Te plus the latent heat ∆H divided by the heatcapacity C. The resulting value for the ablation threshold is 83.1 mJ/cm2 which is consistent withthe experimentally observed value of 80 mJ/cm2 for the films on glass.

* The thermal diffusivity and the specific heat capacity of a-Ge vary with temperature and the values we use

in our calculations are the corresponding mean values.


133

An estimation of the initial surface temperature after the absorption of the laser pulse at Fm

should be possible by means of the approximation given in equation 1.12. Since the condition ofthis equation (negligible heat flow) is not completely fulfilled*, we use again the above mentionedapproximation, replacing 1/α by d. The resulting surface temperature of 685 ºC fits very well themelting point of a-Ge which is again a confirmation that heat flow takes place within the filmduring the melting process with ps pulses.

The threshold calculations shown above would lead to the same result for the 50 nm filmon Si substrate and therefore in disagreement with the experimentally observed thresholds† (20mJ/cm2 and 150 mJ/cm2). This implies that appreciable heat flow to the substrate takes placeduring the melting process of a thin film on a thermally well conducting substrate. For theverification of this hypothesis it is necessary to consider the distance which heat can travel within acertain time t. The characteristic quantity is the thermal diffusion length defined in equation 1.10.as Lth = 2(κt)1/2 where κ is the thermal diffusivity of the material (Table 3.1). For a soundestimation we have to use the system rise time of 1 ns‡ for t. The resulting value of 44 nm is in theorder§ of the film thickness which enables the efficient heat extraction by the Si substrate, thusexplaining the experimentally observed increase of the thresholds.

The above considerations improved our understanding of the dynamics of the meltingprocess. In order to study more in detail the influence of actual heat flow conditions on thesolidification process and its dynamics we have performed measurements on a-Ge films withdifferent thicknesses, discussed in the next section.

* Apart from this restriction, the formula does not consider the consumption of the latent heat. Nevertheless,

this contribution is negligible for thin melt depths but gains importance for large melt depths. Calculatingthe incident energy density required to provide the latent heat during the melting of a thin surface layer (2nm), a value of 0.8 mJ/cm2 results compared to 19.7 mJ/cm2 for the whole film thickness of 50 nm.

† It has been verified that the difference in the thresholds for different substrates is not caused by adifference in the absorption of the ps pulse. The calculation for the light absorption of both film/substratesystems at the pump pulse wavelength (583 nm) including interference effects at the interfaces, shows thatthe absorption coefficient of the film on glass is lower (0.47) than the one of films on Si substrate (0.54).This means that the films on glass substrate should have a higher melt threshold that the films on Si, if theabsorption coefficient alone would determine the melt threshold. The fact, that the measured melt thresholdof films on glass substrate is lower, demonstrates that heat flow towards the substrate during the meltingprocess plays an important role even at lowest fluences.

‡ Corresponding to the 50 ns time window of the Streak Camera.§ The value of κ determined by Szyszko at al. [Szy95] for a-Ge is likely underestimated, due to the different

experimentally conditions under which it has been determined. A higher value of κ provides an even largerdiffusion length, thus facilitating the efficient heat flow to the substrate.


134

Influence of the film thickness

Reflectivity transients

Real time FSR measurements with ns resolution at 633 nm have been performed on a-Gefilms on Si substrates upon irradiation with ps laser pulses. The film thickness was varied in the 30nm - 180 nm range. Figure 3.28 shows several representative FSR transients at different fluencesfor the six film thicknesses studied. In the following, the transients of the 50 nm thick film, whichhave already been discussed in the previous section, will be used as a reference. Pleaseremember that the reflectivity dip observed in this film upon solidification by means of StreakCamera measurements can not be resolved with ns-resolution measurements.

The transients corresponding to the 30 nm film are very similar to those of the 50 nm film.For the 80 nm film, only the transients obtained at low fluences are similar to those of the 50 nmfilm, whereas for higher fluences the reflectivity maximum is followed by an oscillation below theinitial level. This minimum reminds strongly the one observed in the Streak Camera transients ofthe 50 nm film shown in the previous section. Such a minimum is also observed in the transients ofthe 100 nm film but here the minimum is followed by a second reflectivity maximum. The behaviorof transients of the 130 nm film can be classified in three fluence regimes: At intermediatefluences the transients behave similar to those of the 100 nm film, but the minimum between thetwo maxima does not reach any more values below the initial level. At low (55 and 88 mJ/cm2)and the highest (289 mJ/cm2) fluences, a shoulder develops at the temporal position of theoscillation minimum. For the 180 nm film, the transients do not present any more oscillations, buta shoulder in the intermediate fluence range. In this case, the general shape of the transientsreminds that of the transients of 50 nm films on glass substrates (Figure 3.5) for which bulksolidification followed by recalescence was found to take place.

The characteristic reflectivity values of these transients have been plotted in Figure 3.29for all the films as a function of fluence. Apart from the already familiar values Rmax and Rfin, thereflectivity at the intermediate minimum Rmin and the reflectivity at the second maximum Rmax2 havebeen measured in those transients in which they appear. In the transients in which a shoulder isobserved, we measured the reflectivity value at this shoulder. In such a case Rmax2 corresponds tothis value and no Rmin value is plotted. For the 30, 50, 80 and 100 nm films, the data points at thehighest fluence plotted correspond to those obtained just below the ablation threshold. For the130 and 180 nm film, the ablation threshold could not be reached.


135

21 mJ/cm2

28 mJ/cm2

32 mJ/cm2

42 mJ/cm2

85 mJ/cm2

122 mJ/cm2

30 nm 23 mJ/cm2

38 mJ/cm2

50 mJ/cm2

111mJ/cm2

182 mJ/cm2

237 mJ/cm2

100 nm

18 mJ/cm2

24 mJ/cm2

60 mJ/cm2

101 mJ/cm2

132 mJ/cm2

145 mJ/cm2

50 nm

refle

ctiv

ity a

t 633

nm

(a.

u.)

27 mJ/cm2

55 mJ/cm2

88 mJ/cm2

174 mJ/cm2

258 mJ/cm2

289 mJ/cm2

130 nm

0 10 20 30 40 500 10 20 30 40 50

20 mJ/cm2

28 mJ/cm2

66 mJ/cm2

91 mJ/cm2

106 mJ/cm2

154 mJ/cm2

80 nm

time (ns)

23 mJ/cm2

59 mJ/cm2

51 mJ/cm2

98 mJ/cm2

154 mJ/cm2

297 mJ/cm2

180 nm

time (ns)

Figure 3.28.: FSR transients at 633 nm in a-Ge films with different thicknesses (labels) onSi substrates measured by the Digitizer (ns-resolution) upon ps laser pulseirradiation. The temporal position of the pump pulse is in all cases t = 0.


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In the following I will describe the main tendencies* of each of the four characteristicquantities pointed out above as they evolve with the film thickness.

a) The evolution of Rmax is similar in all cases, i.e. a rapid increase at very low fluences whichcorresponds to the onset of melting followed by a slower increase. The latter extends to higherfluences for thicker films because the ablation threshold increases with film thickness. As aresult, the maximum values of Rmax are higher for thicker films. Nevertheless, the reflectivityincrease corresponding to an optically thick molten layer (> 60%) is never observed. Whetherthis fact is related to the rise time of the detection system or is caused by surface initiatedsolidification can not be elucidated at this point.

b) The evolution of the final reflectivity level Rfin with fluence can be divided in two groups: Forthe thinner films (30, 50 and 80 nm), the reflectivity changes after solidification are negligiblewhich indicates that re-amorphization of the molten layer occurs. This has been confirmed byRaman spectroscopy. Only at the highest fluence (170 mJ/cm2) in the 80 nm film,crystallization takes place. For the thicker films (100, 130 and 180 nm), crystallization† occursabove a certain fluence which decreases with the film thickness.

c) The reflectivity minimum Rmin is only present for the films of intermediate thicknesses‡ (80,100, and 130 nm). In all cases its evolution versus fluence shows a minimum followed by arecovery to a value similar to that observed at low fluences. The values at the lowest andhighest fluences are below, similar to and above the initial reflectivity level for the 80, 100and 130 nm films, respectively. In addition, the fluence range in which Rmin appears is shiftedto higher fluences for increasing film thicknesses.

d) The second maximum Rmax2 is only present for the 100, 130 and 180 nm films. It is precededby a minimum in the 100 and 130 nm films and transforms to a shoulder in the 180 nm film andin the low fluence regime of the 130 nm film where the minimum is absent. In all cases, theevolution of Rmax2 versus fluence follows a linear increase with a slope which is apparently filmthickness independent and with an offset that increases with film thickness.

* Along with the general tendencies, the data of the 180 nm film present a peculiarity. The Rmax and Rfin values

at ≈ 60 mJ/cm2 deviate clearly from the general trend, which is no artefact but correspond to a different typeof transient. The transient at 59 mJ/cm2 with a much lower amplitude than the one at 51 mJ/cm2 (Figure 3.28)terminates at a much higher final level. Although we did not find a convincing explanation for this behaviorwe consider it worth to be reported.

† The exact threshold fluence for crystallization has been determined by Raman spectroscopy. This turnedout to be necessary because the Rfin values plotted in the figure are overestimated due to the fact that thetime window of Digitizer (200 ns) was not long enough to follow the cooling process of the material afterirradiation at such high fluences. The discussion of the so-determined crystallization threshold will be givenlater on.

‡ The minimum appearing in the transients corresponding to the 50 nm films could only be observed in thehigh resolution experiments performed with the Streak Camera, described in the previous section.


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0

20

40 Rmax

Rfin

Rmin

Rmax2

30 nm 100 nm

0

20

4050 nm

no

rma

lize

d r

efle

ctiv

ity c

ha

ng

e a

t 6

33

nm

(%

)

130 nm

0 100 200 300

0

20

4080 nm

fluence (mJ/cm2)

100 200 300

180 nm

fluence (mJ/cm2)

Figure 3.29.: Evolution of the reflectivity versus fluence at the maximum Rmax, at theminimum Rmin, at the second maximum Rmax2, and the final reflectivity levelRfin , for a-Ge films with different thicknesses (labels) on Si substrates. Themeaning of the peculiar two Rmax values in the 180 nm film is explained inthe footnote one page before.


138

Solidification types

The transition from an oscillation-like behavior in the transients of the 80 and 100 nmfilm towards a shoulder-like behavior in the 180 nm film goes through an intermediate behavior inthe 130 nm film. This fact clearly suggests the existence of two different solidification scenarios:While the presence of a shoulder reminds strongly the results on films on glass (section 3.1) inwhich bulk solidification takes place, the presence of an oscillation through a minimum remindsthe presence of surface initiated solidification as observed with the Streak Camera in the 50 nmfilms on Si.

The difference between “shoulder” and “oscillation” behavior becomes more clear whenanalyzing the temporal position of Rmin and Rmax2. Figure 3.30 shows the time delay* of bothquantities with respect to the laser pulse as a function of film thickness. The presence of tworegimes of transient types and the step-like transition between them at a threshold thickness of130 nm can very clearly be seen in this figure. This result further supports our interpretation of atransition from a surface initiated solidification process to a bulk solidification process as the filmthickness increases. The optical simulations presented in the next section will provide additionalinformation which will demonstrate that the above interpretation applies.

50 100 150 20014

16

18

20

22

24

26

transition

shoulder

oscillation

Rmin

Rmax2

dela

y af

ter

the

pum

p pu

lse

(ns)

film thickness (nm)

Figure 3.30.: Temporal position of the reflectivity minimum (Rmin) and at the secondmaximum (Rmax2) as a function of film thickness. The transition between thetwo regimes (“oscillation” and “shoulder”) at 130 nm occurs throughintermediate temporal positions.

* The values plotted for the 80, 100 and 180 nm films are mean values of all transients, whereas for the 130 nm

film two average values for each quantity in the corresponding fluence range have been taken.


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Optical simulations

We have calculated the reflectivity evolution at 633 nm of completely molten Ge filmsupon surface initiated solidification for those film thicknesses which show oscillations* (80, 100and 130 nm). The dashed curves in Figure 3.31 correspond to the reflectivity evolution forsurface initiated re-amorphization of the whole film thickness. Although they show similaroscillations as the experimentally observed transients (Figure 3.28), the transient features are notstrictly reproduced. For instance, there is no second maximum or second minimum in thetransients of the 80 nm and 130 nm thick films, respectively. It has to be considered though that:

a) Since surface initiated solidification is most likely accompanied by the competing solidificationprocess from the substrate, the solidification depth dsol at which both fronts meet has to beintroduced as a fit parameter.

b) Based on the fact that the 100 nm and 130 nm thick films partly crystallize upon solidification,the crystalline fraction† has also to be used as a fit parameter.

The solid curve in Figure 3.31 corresponds to the best fit to the transients, taking intoaccount these considerations.

0 50 100

-40

-20

0

20

40

60

refle

ctiv

ity c

hang

es a

t 633

nm

(%

) 80 nm film

dsol

= 40 nm

a-Ge

0 50 100

thickness of the solidified top layer (nm)

100 nm film

dsol

= 70 nm

40 % c-Ge

0 50 100

130 nm film

dsol

= 70, 100 nm

50 % c-Ge

Figure 3.31.: Calculation of the reflectivity evolution at 633 nm of molten Ge films withdifferent thicknesses on Si substrates upon surface initiated solidification.The dashed curves correspond to the case that the solidification frontreaches the substrate. The solid curves show the best fit to the transients inFigure 3.28 adjusting the maximum depth d sol the solidification front reachesand the crystalline fraction of the solidified material. The thin curve in the180 nm film is an extrapolation of the best fit to d sol = 100 nm The reflectivitychanges have been normalized to the reflectivity of an a-Ge film.

* Since we do not resolve the oscillations in the transients of the 50 nm film with ns-resolution at 633 nm, no

simulation is included. Nevertheless, the general behavior such a simulation is similar to the one observed at514 nm with ps-resolution measurements (Figure 3.27).

† The optical constants have been calculated for a certain crystalline fraction using the model proposed byBruggemann [Bru35], which has been introduced in section 3.1.


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For the 80 nm film, a solidification depth of 40 nm is obtained. No crystalline fraction hasto be assumed since the film is found to be amorphous after solidification. For the 100 nm film,the solidification depth must definitively be larger since a second maximum is observed. A value ofdsol = 70 nm and a crystalline fraction of 40 % provides both, the presence of a second maximumand Rmax2 values similar to the experimentally observed ones (Figure 3.29). For the 130 nm film, acrystalline fraction of 50% is indicated by the Rmax2 values observed experimentally. Although thefit has been performed for d = 100 nm, the actual solidification depth must be ≈ 70 nm becauseotherwise a strong second minimum should be observed, as opposed to the experimental results.The fact that the actual solidification depth does not increase for a 130 nm film but remains similarto that of the 100 nm film, further indicates that the thickness limit for surface initiated solidificationtaking place is reached.

Fluence thresholds for the laser induced processes in the films

To conclude this chapter, I want to make a global review on the thickness dependence ofthe variety of processes we have observed in our experimental results, such as melting,crystallization, surface initiated solidification, bulk solidification and ablation. For that purpose thecharacteristic fluence thresholds for these processes to take place have been determined with thehelp of Figs. 3.29 and 3.30. These thresholds have been plotted in Figure 3.32 as a function offilm thickness and their behavior is described in the following:

0 50 100 150 2000

100

200

300

400Thresholds for:

surface melting (Rmax)

crystallization (Rfin + Raman)

surf. solidification (Rmin, Rmax2 at t1, t2)

bulk solidification (Rmax2 at t3)

ablation (Rfin + microscope)

flue

nce

(m

J/cm

2 )

f i lm thickness (nm)

Figure 3.32.: Thresholds for melting, crystallization, surface initiated solidification, bulksolidification and ablation as a function of film thickness. The quantities inbrackets indicate the way, the corresponding threshold has beendetermined. The lines are a guide to the eye.


141

• Melt threshold Fm : determined by the rapid increase of Rmax

Fm can be considered as independent* film thickness, although the precision of thedetermination is within ± 5 mJ/cm2 due to the limited rise time of the detection system.Measurements with ps resolution showed that for a thin Ge film on a well heat conductingsubstrate, Fm is lowered notably with respect to the value for a bad poorly heat conductingsubstrate as glass. The approximately constant Fm value suggests in any case that the meltingthreshold is mainly determined by the optical absorption length (23 nm), to a small amount bythe heat flow conditions but not by the thickness of the film.

• Crystallization threshold Fc : determined by Raman spectroscopy and indicated by the decrease of Rfin

Crystallization is only observed for film thicknesses d ≥ 80 nm and Fc is observed to decreaserapidly as the film thickness is increased. The value of Fc for the thicker films (130, 180 nm) isvery close to the crystallization threshold observed in the films on glass (50 mJ/cm2). Thissupports the assumption made previously (section 3.1), that crystallization is mediated byrecalescence. In particular, a thick a-Ge film on Si behaves near Fc very similar to a thin a-Gefilm on glass due to the similar heat flow conditions†. In a partially molten thick film, the non-molten layer beneath prevents the rapid heat extraction towards the Si substrate and facilitatesbulk solidification. In the case of the 130 and 180 nm films, this requirement is fulfilled andcrystallization takes place via bulk solidification. For the thinner films (80, 100 nm and for highfluences in the 130 nm film), this condition is not fulfilled anymore and crystallization takesplace via surface initiated solidification

• Threshold for surface initiated solidification Fsurf: determined by the appearance of an oscillation with Rmin and Rmax2 , 20 and 26 ns after the laser pulse, respectively.

Fsurf more than doubles its value for an increase of the film thickness from 80 nm to 130 nm.Such an increase of Fsurf is expected when we take into account the driving force for surfaceinitiated solidification, suggested in the previous section. It requires the melting of a thinsubstrate layer in order to delay the solidification from the film substrate interface. Since thisrequires higher fluences for thicker films, the observed increase by a factor of more than twofor an increase in film thickness (80 -130 nm) is therefore consistent with this scenario.

* The reader should remind that the value for the 180 nm film has a large error bar due to the peculiar behavior

of the transients around the melt threshold. Nevertheless, I have plotted this value in order to show that it isconsistent within its error bar with a constant melt threshold. The small increase for the 30 nm film is due tothe fact, that the absorption length of a-Ge at 583 nm is of the order of the film thickness, which reduces thelight absorption at the pump wavelength.

† The thermal diffusivity of glass (0.009 cm2/s) is similar to the one of a-Ge (0.0048 cm2/s) but differs stronglyfrom the one of c-Si (1.0 cm2/s).


142

We have to leave as an open question whether surface initiated solidification takes place in the30 nm film or not. The reason for this is not the limited time resolution of the detection systembut the high probability that no reflectivity minimum occurs*.

• Threshold for bulk solidification Fbulk: determined by the appearance of a shoulder 20 nsafter the laser pulse.

A shoulder in the transients was only observed for the two thicker films (130 and 180 nm).This is in agreement with the hypothesis made earlier, that thick a-Ge films with a melt depthmuch smaller than the film thickness behave upon solidification similar to thin a-Ge films onglass substrates. A further indication for this is the fact, that for both films Fbulk is very similar tothe one of the 50 nm film on glass substrate.

• Ablation threshold Fa: determined by optical microscopy and a strong decrease of Rfin.

A strong increase† of Fa over the measured film thicknesses is observed. This behavior is incontrast to the constant behavior of the melt threshold and demonstrates that the ablationthreshold is strongly influenced in some way by thermal diffusion of the absorbed light within avolume defined by the film thickness. If the heat flow situation of the system alone woulddefine the ablation threshold, then Fa should be a decreasing function of film thicknessbecause the heat extraction by the Si substrate should be the higher the smaller the filmthickness.

Influence of the pulse duration

Finally, I would like to compare the above results to the work of F. Vega et al. [Veg97]who presented results obtained in the same Ge films on Si substrates upon irradiation withultraviolet ns laser pulses. They do not observe surface initiated solidification (oscillatorytransients) but bulk solidification (shoulder transients), leading to the formation of the crystallinephase for film thicknesses above 80 nm. In their case, the threshold for both, the appearance of ashoulder and crystallization is a strongly decreasing function of film thickness. This is in agreementwith our results which show that the crystallization threshold is also a similarly decreasing functionwith thickness (Figure 3.32).

The authors could determine the melt depth by means of heat flow calculations whichshowed that for all the studied thicknesses, the threshold fluence for bulk solidification followedby recalescence is coincident with a minimum melt depth of 80 nm. The similarity with our results

* The competing solidification front from the substrate would prevent a condition where the reflectivity

shows a minimum due to destructive interference.† For the 130 and 180 nm film, the ablation threshold could not be reached in the experiment and is therefore

above the plotted data points which correspond to the highest fluence achieved.


143

suggests that in our case a minimum melt depth for the formation of crystalline material viarecalescence is also required. This has been already described in this work in chapter 3.1 for 50nm films on glass (42 nm minimum melt depth). A major difference between the present resultsand those of Vega et al. is of course that we also observe surface initiated solidification, which weattribute in part to the use of ps pulses. Such an scenario is not possible with the 12 ns laserpulses they used because the energy is supplied continuously to the surface during the pulseduration which prevents solidification to take place there. This reasoning is in agreement with thefact that the few times surface initiated re-amorphization has been reported, the pulse durationused was always a few ns or less [Bru86], [Bon93].

Summary of the main findings of section 3.2

• Nanosecond resolution measurements

FSR measurements at 633 nm with ns-resolution in 50 nm thick a-Ge films on Si substratesin combination with optical simulations and Raman measurements reveal the main differences ofthe laser induced processes with respect to those occurring in films on glass. These differencescan be attributed to the higher thermal conductivity of the substrate.

1. A rapid reflectivity increase occurs upon melting for fluences above 20 mJ/cm2. Thesubsequent decay due to re-solidification of the material is much faster than thatobserved in films on glass and no shoulder appears. Heat diffusion to the substrateextends the fluence range up to 150 mJ/cm2 before ablation takes place.

2. For the whole fluence range, Raman measurements showed that re-amorphization ofthe molten film takes place, as opposite to the behavior observed in films on glass.

• Picosecond resolution measurements

FSR measurements at 514 nm with ps-resolution on 50 nm thick a-Ge films on Si incombination with optical simulations reveal exciting new processes upon solidification andcompensate the lack of information caused by the impossibility to perform BSR and FSTmeasurements.

1. The increased time resolution of the detection system demonstrates that thesolidification process in films on Si is clearly faster than in films on glass.

2. The improved time resolution of the Streak Camera measurements allowed todetermine the melt threshold to be at 20 mJ/cm2. The comparison with the lowerthreshold of the films on glass along with threshold calculations based on a simple


144

thermal model demonstrates the influence of thermal diffusion to the substrate in timesscales below 1 ns.

3. A reflectivity increase consistent with an optically thick liquid layer could not beobserved with the detection system with ps-resolution even at the highest fluencesused. Above a certain threshold fluence, the transient reflectivity intensity at themaximum decreases with fluence followed by the appearance of a minimum below theinitial reflectivity level. This behavior can be explained by an scenario in whichsolidification is initiated at the surface and progresses into depth. The presence of acompeting solidification front, counter-propagating from the substrate towards thesurface, is also discussed.

4. The calculation of the reflectivity evolution for this type of solidification provides aninterface velocity of 2.3 m/s which is similar to the interfacial amorphization velocitiesfrom the substrate side observed in films on glass.

5. The driving force for surface initiated solidification is explained in terms of the meltingof a thin layer of the Si substrate. Far above the solidification temperature of Ge, thesolidifying Si layer releases the solidification enthalpy which delays solidification at thefilm/substrate-interface and favors heterogeneous nucleation at the air/film-interface.Nevertheless, additional measurements (by means of RBS) are required to confirmsubstrate melting (see “Outlook” in chapter 6).

• Influence of the film thickness

FSR measurements at 633 nm with ns-resolution in combination with optical simulations fora-Ge films of different thicknesses on Si substrates provide additional information on the surfaceinitiated solidification process and show that a transition towards bulk solidification occurs forincreasing film thicknesses.

1. The constancy of the melt threshold versus film thickness indicates that the meltthreshold for ps pulses is determined mainly by the optical absorption length (23 nm),to a small amount by the heat flow conditions but not by the thickness of the film.

2. Surface initiated solidification is demonstrated to take place for films thicknesses of 50,80, 100 and 130 nm films. This process leads to amorphization for film thicknesses ≤80 nm and to crystallization for thicker films. This can be explained in terms of thehigher supercooling achieved in the thinner films. The optical simulations performedresemble well the different transient shapes in all cases. This enables the estimation ofthe crystalline fraction and of the solidification depth at which the two counter-propagating solidification fronts meet.


145

3. The transition between surface initiated solidification and bulk solidification was shownto occur at a threshold film thickness of 130 nm. For that thickness, the formerprocess takes place at high fluences and the latter one at low fluences.

4. Bulk solidification was determined to take place in the 180 nm thick films and in 130nm thick films at high fluences. The shape of the transients and the crystallizationthreshold are very similar to those observed in the 50 nm films on glass and can thusbe explained in terms of similar heat flow conditions. The low thermal diffusivity of thenon molten a-Ge prevents the rapid heat extraction towards the Si substrate andpromotes bulk solidification followed by recalescence and crystallization.

5. The ablation threshold was found to increase with film thickness indicating that it isstrongly influenced by thermal diffusion of the absorbed light within a volume definedby the film thickness.

6. The comparison of the present results with those reported by Vega et al. [Veg97] onthe same films upon ns laser pulse irradiation points out the importance of the pulseduration. A short pulse duration is required to induce surface initiated solidification.


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Documents

3.2 Amorphous Ge films on Si substrates · 3.2: Amorphous Ge films on Si substrates 121 inflections occur in an expanded fluence scale and show a different amplitude. In contrast,