The results of the study of the two-step (laser treatment followed by thermal annealing) crystallization of amorphous silicon films on glass substrates using SEM and XRD techniques are presented. All samples were thermally annealed under the same conditions: at a temperature of 250 oC for 15 minutes at atmospheric pressure in a nitrogen environment. In the present study, the variables are the cw-laser power density and irradiation time.
Table 1. Laser power density, laser spot diameter and area, and exposing duration used.
Laser Power
(W)
|
Laser Power Density (W/cm2)
|
Laser Spot Diameter (mm)
|
Laser Spot Area (mm2)
|
Exposing Time to Laser (sec)
|
|
2.4
|
94.5
|
1.8
|
2.5
|
5
10
15
60
|
|
4.8
|
87
|
2.65
|
5.5
|
10
60
300
|
|
1.9
|
74.7
|
1.8
|
2.5
|
5
10
60
300
|
|
1.5
|
59
|
1.8
|
2.5
|
300
|
|
5.8
|
33.3
|
4.7
|
17.4
|
75
150
300
600
|
|
|
|
|
0.8
|
31.4
|
1.8
|
3.1
|
10
60
300
|
|
|
|
only thermal annealing process (no laser) 15 min
|
|
The laser's power density was reduced using a converging lens to examine the influence of the laser's power density on the crystallization process. As the beam extends beyond the focus point, spots of varying sizes may form at various distances from the focal point. By positioning the sample in different positions, small or large sample portions can be exposed to the laser. The purpose of a lens is to change the laser power density while producing a wide laser beam. This is desirable because larger areas could be scanned faster if the film could be converted to Poly-Si in the first step of the present crystallization process. This would allow using a continuous-wave laser source to generate large-sized thin films to fabricate low-cost solar cells. Table 1 summarizes the laser power densities, spot diameters, spot areas, and irradiation times used in this work.
Fig. 1 is a SEM image depicting the surface morphology of samples irradiated by an Ar-ion laser with a power density of 94.3 W/cm2 for varying time durations, followed by annealing with Al on top of the a-Si: H layer under the aforementioned conditions. Clearly, crystallization occurred under these circumstances. The size of the Poly-Si grains is highly dependent on the laser beam's irradiation period. Fig. 1a depicts a sample that was laser-irradiated for sixty seconds. The Poly-Si grains are between 1.0 and 1.2 mm in diameter and cover approximately 90% of the surface. When the irradiation period is shortened to 15 seconds, Poly-Si grains measuring between 0.5 mm and 0.7 mm are found in the film, as shown in Fig. 1b. The granules cover approximately fifty percent of the surface of the film. Other samples were exposed to the same laser power density for 10 and 5 seconds. On the surface the a-Si:H film, Poly-Si grains were found, but they were smaller than those in Fig. 1c and Fig. 1d, respectively. Clearly, the irradiation time affects the size of the Poly-Si grains. Longer exposure times make the grains bigger.
When samples are placed 29.4 cm from the lens's focal point, the laser spot size is measured to be 2.65 mm, and the laser power density on the samples is calculated to be 87 W/cm2. The samples were exposed to this power density for three periods of 10 seconds, 60 seconds, and 300 seconds, then thermally annealed at 250oC for 15 minutes. Fig. 2 illustrates the SEM images of these samples. These images show that after 10 seconds of exposure to the laser beam, the a-Si:H film is converted to Poly-Si having small grains (Fig. 2a). The Poly-Si grains in the film after 60 seconds of laser irradiation are considerably larger and almost entirely cover the surface, as shown in Fig. 2b. A third sample was exposed to a laser beam of the same power density for 300 seconds, and the resultant Poly-Si grains (Fig. 2c) are identical to those in the 60-second film. This demonstrates that the film has crystallized entirely after 60 seconds of laser exposure. Another set of samples was exposed to a lower power density laser beam at 74.7 W/cm2. As illustrated in Fig. 3, the SEM images of these samples reveal a variety of different morphologies. Many plate-like crystals develop in these samples, with a dendrite-like morphology that varies in width depending on the exposure time to the laser beam. The crystallite size is approximately 0.8 mm when the period is 5 s, and the coating covers nearly 50% of the film area, as shown in Fig. 3a. Increasing the exposure duration to 10 s resulted in larger plate-like crystallites of approximately 1 mm, as seen in Fig. 3b. The surface morphology of a sample subjected to the same laser beam for 60 seconds is shown in Fig. 3c. The crystallites are more extensive and denser, covering approximately 75% of the film surface. Another sample was examined after being subjected to the laser beam for 300 seconds (Fig. 3d). It illustrates various crystal types compared to films treated by a laser beam of the same power density (Fig. 3(a-c)). The crystallites are approximately 1.6 mm in diameter and cover about 80% of the surface. When compared to grains made by a laser beam with a higher power density, these grains are about the same size and density as those made by a laser beam with 87 W/cm2 when the film was exposed for 10 seconds. Another sample was exposed for 300 seconds to a laser beam with a lower power density, 59 W/cm2, as shown in Fig. 4. The picture shows the formation of dendrite-type Poly-Si crystallites of size 2 mm, covering around 80% of the surface. These grains are larger than those produced by higher power density but shorter exposure time laser beams (Fig. 3(a-c)). The general appearance of these images is comparable to the SEM image that was obtained by Toet et al. [29] in their work on the selective laser-induced nucleation growth of polycrystalline silicon on glass. They formed a grid of crystallization seeds spaced by 1 mm by melting the a-Si film with a CW Ar+ laser beam with a power density of between 1 and 5 MW/cm2. After that, the samples were annealed for 6 hours at 600 oC.
The SEM micrographs of the samples treated with laser beams of 33.3 W/cm2 power density and irradiated with periods of 75 s, 150 s, 300 s, and 600 s are shown in Fig. 5. Film has Poly-Si crystals that look like dendrites and change in patterns, size, and shape depending on how long they are exposed to light. More extensive and denser crystals are formed when films are exposed to a laser beam for an extended period. Compared to images obtained with a higher power density laser (Figures 2-4), it is clear that Fig. 5d is very similar to Fig. 4, where the laser's power density was 33.3 W/cm2 and 59 W/cm2, respectively, but the exposure period was 600 s and 300 s, respectively. It is obvious that when the laser power density is reduced, the exposure time required to achieve the same film structure in terms of crystal pattern, size, and density must be doubled. Furthermore, significant similarities appear between Fig. 5c (33.3 W/cm2 for 300 s) and Fig. 3c (74.7 W/cm2 for 60 s), as well as between Fig. 5b (33.3 W/cm2 for 150 s) and Fig. 3b (74.7 W/cm2 for 10 s). Another group of samples was subjected for 10 s, 60 s, and 300 s to a laser beam with a power density of 31.4 W/cm2. Fig. 6 illustrates the surface morphology of these materials. Dendrite-type crystals of around 0.7 µm in size are created in the films after 10 s of laser exposure (Fig. 6a). However, the crystals of about 2 µm appeared after 60 s of laser exposure (Fig. 6b). The crystallites in samples exposed for 300 s (Fig. 6c) are different from those exposed for 10 and 60 s. Here the crystallites are larger but less dense in this case, reaching a length of up to 3.9 µm. Finally, without a laser, one sample was thermally annealed (at 250 oC for 15 minutes). Fig. 7 shows the SEM image, which reveals the formation of dendrite-type crystals with a size of 1.4 µm in the film. When compared to the images of samples treated with a laser before thermal annealing, it is clear that the morphology of the sample's surface morphology is similar to several of them. This indicates that the crystallization process in these materials is primarily due to metal-induced thermal annealing, with little influence from laser treatment. The data reported in Figures (1)– (7) illustrates that crystallization requires a minimum power density (threshold value) and a minimum exposure time. At high laser power densities and longer exposure times, two distinct forms of Poly-Si crystallites form in the films: large grains and dendrite-like crystals. When the laser intensity and exposure time are less than a specific value, crystallization occurs only due to metal-induced thermal annealing, with no influence from the laser in this domain. The three areas' border lines are depicted in Fig. 8. These findings provide insight into the crystallization technique employed in this work, including laser-assisted and metal-induced heat crystallization.
Based on the results of this study, we proposed the following crystallization mechanism for the utilized technique: When an amorphous silicon film in contact with an aluminum film is exposed to a laser, the activation energy of aluminum and silicon atoms inside the treated volume increases as well. As the material absorbs the laser energy, it is transformed into heat. The added heat heats the exposed region of the film, causing its temperature to rise to a value dependent on the laser beam's power density. When the power density is high enough, between 87 W/cm2 and 94.3 W/cm2, aluminum atoms diffuse into the amorphous silicon layer, forcing the silicon atoms to organize into grains or clusters of polysilicon rich in aluminum atoms. These grains or clusters continue to grow as the sample is continually subjected to the laser. As seen in Figs. 1 and 2, the size of these grains depends on the laser power and exposure time. When their sizes are less than the thickness of the amorphous film, the growth occurs in three dimensions; otherwise, the growth would have happened in two dimensions. Fewer aluminum atoms diffuse into the amorphous silicon area when the power density is less than 87 W/cm2, forming nucleation sites. This amount of aluminum is limited to forcing the silicon atoms to rearrange themselves in preferable directions to form dendritic type clusters of polysilicon. This concept, we feel, is consistent with the data found in other studies. Nast and Hartmann [30] studied the Al oxide interface layer effect between Al and Si films on the crystallization process during aluminum-induced amorphous silicon crystallization. The aluminum oxide layer forms naturally after 48 hours of exposure to air. The samples were annealed for 30 minutes at 475 oC. They observed that the aluminum oxide layer limits the nucleation and enables silicon star-like grains to grow to enormous sizes. A thinner Al oxide layer, on the other hand, resulted in a silicon system formed of a large number of tiny silicon grains. Nast and Hartmann [30] observed star-like grains that resemble those produced by low-power lasers, as shown in Figs. 3–7. These findings support our hypothesis that the amount of aluminum diffused into the amorphous silicon region significantly influences the size, shape, and number of polysilicon grains produced by laser-induced and metal-assisted crystallization (LIMAC). Toet et al. [29] a crystallized and a-Si film by using an Ar-ion laser with a power density of 1 to 5 MW/cm2 and produces a selective grid of crystallization seeds spaced by 10 µm. Low-pressure chemical vapor deposition was used to grow the coating on SiO2-coated borosilicate glass. The substrate was then annealed for 6 hours at 600 oC. Transmission electron microscopy indicates that crystalline regions include grains arranged in a star-like pattern. Although the annealing temperature was high, 600 oC (compared to the annealing temperature employed in metal-induced crystallization), and the annealing duration was 6 hours; the grains were star-shaped.
Table 2. Power density and exposure time values are used in the present work, together with those used by other researchers.
|
Laser Power Density (W/cm2)
|
Exposure Time (s)
|
Film Type
|
Film Thickness
(nm)
|
Present work
|
31.4–94.3
|
5–600
|
Al/a-Si: H/Glass
|
380
|
Ref.[20]
|
12.5 x 106
|
0.002
|
a-Si/alkaline/glass
|
50–150
|
Ref.[21]
|
25 x 103
|
0.001
|
a-Si:H/Glass
|
50–500
|
Ref.[22]
|
30x103
|
0.004
|
a-Si:H/Glass
|
360
|
Ref.[23]
|
0.01-20x 103
|
5
|
a-Si:H/Quartz
|
500
|
Ref.[25]
|
High power
|
Until melting
|
a-Si:H/glass
|
300
|
Ref.[26]
|
No information
|
No information
|
a-Si:H/Glass
|
1000–2000
|
The most remarkable result of this work is that the laser's power density is significantly less than that used by other researchers [20, 21, 23–25]. The maximum laser power density used in this research was about 94.3 W/cm2 for 5 to 300 s. In contrast, other studies have used power densities ranging from approximately (8 to 560 kW/cm2) for exposure durations ranging from (0.72 ms to 2 ms) [20, 21, 23, 25, 27, 28] and (0 to 200 min) [24]. Table 2 compares the power densities and exposure times utilized in this study to those employed by other studies. Additionally, we investigated the samples using X-ray diffraction to determine the change in the structure of the a-Si:H film. The XRD scan of amorphous silicon exhibits no peak.
However, when crystallized samples are examined using an XRD, peaks arise, indicating the formation of Poly-Si inside the amorphous silicon. This approach may compute many parameters, including the crystalline volume percentage, grain size, orientation, film thickness, and film quality. Figures 9-12 show the XRD scans of samples subjected to lasers with power densities of 87 W/cm2 and 33.3 W/cm2 for varying exposure durations. Fig. 9 shows samples that have been irradiated with a laser power density of 87 W/cm2 for various periods before annealing. This figure indicates that when there is no laser treatment or when laser exposure duration is ten seconds, there are no peaks at angles associated with the presence of Poly-Si. On the other hand, when samples are laser irradiated for 60 or 300 s, there are peaks at relative angles. However, after annealing the samples at 250 oC for 15 minutes and etching the aluminum, the XRD scan, Fig. 10, reveals three peaks at 28.5, 47.4, and 56.3 degrees. These are signs that Poly-Si is there, and they clearly show that a Poly-Si film has formed in the a-Si film.
XRD observations of samples subjected to a laser power density of 33.3 W/cm2 for various periods before annealing are shown in Fig. 11. Relatively small peaks were detected at 2θ values of 28.5 degrees.
On the other hand, after annealing the samples, the Poly-Si spectra are shown in Fig. 12, together with their expected relative intensities. The figure reveals three peaks at 28.5, 47.4, and 56.3 degrees, mainly caused by thermal annealing.