Exposure Mode Operation

The question on why there is a considerable quenching of PL despite having 100 ALD still remained unsolved with continuous operation. Exposure mode operation is typically used for coating structures that have high aspect ratios. The major difference between exposure and continuous (standard) operation is that this operation mode is achieved by dosing each precursor when the stop valve is closed. An additional step is introduced after the pulse, that allows the precursor to be in contact with the sample for a longer time period and allows precursor to diffuse into pores between QDs. This is shown in figure below.

Figure 52.Schematic representation of exposure mode operation [Cambridge NanoTech, 2004]

Exposure times required will vary depending on the substrate, temperature of deposition and the precursors being used. For very high aspect ratio structures the exposure time can exceed 60 seconds [Cambridge NanoTech, 2004].

Setting up the Expo recipe

• A minimum N₂ gas flow of 5 sccm must be maintained. Initially, 5 sccm has been used in all the steps where N₂ gas is introduced into the chamber (refer to table 8.

• To begin with a minimum diffusion time of 5 seconds was chosen.

• Purge time needs to be equal to or greater than the diffusion time.

purge time = diffusion time x 2 = 10 seconds

• Initial pulse time for both TMA and H2O was used as 0.010 seconds.

Table 8.Initial Exposure mode ALD Recipe

8 TMA Diffusion Wait - 5 sec

9 Stop Valve - 1 open

It needs to be mentioned that setting up a new recipe requires compatibility with the ALD machine requirements. There is a maximum pressure that the ALD machine can with-stand, which is about 12 Torr for the Savannah S200. This means, if there is a pressure build up that exceeds this value during the running of a recipe, it will be automatically stopped. So, choosing compatible values for various parameters that does not cause high pressure in the machine is crucial, given that the recipe should also satisfy the require-ments of the user. Therefore, running a trial before the actual deposition to ensure this was necessary. In case the pressure builds up, it is necessary to adjust the parameters to make it compatible. A considerable increase in the pressure in the reaction chamber for the exposure mode recipe was observed in comparison to the pressure rise during the standard recipe that corresponds to the additional step introduced.

Study the Effect of Diffusion Wait

Diffusion wait is the new step that makes the exposure mode recipe different from stan-dard recipe. It was therefore necessary to identify and study how diffusion time makes a difference in alumina deposition on QD films and subsequently towards the stability of nano-composites.

Four new recipes were developed according to the conditions mentioned earlier in this section. ALD machine incompatibility with high pressure build up limited the develop-ment of new recipes with longer diffusion times. The following recipes as in table 9 were developed.

Table 9.Different recipes to study the effect of TMA/H2O diffusion time on alumina growth Recipe TMA diffusion time H2O diffusion time

1 5 sec 5 sec

2 10 sec 10 sec

3 5 sec 10 sec

4 10 sec 5 sec

Each exposure mode recipe developed was used to deposit different number of alumina cycles (25, 50 and 100 ALD cycles) to study if having more alumina improves the stability as observed during continuous operation. The results are shown in figure 53.

Figure 53. Exposure mode ALD operation at different diffusion waiting times for stabilizing QD/AlOx nano-composites in oxygen atmosphere(a)Recipe 1 (samples Expo 1,2,3) and Recipe 2 (samples Expo 4,5,6)(b)Recipe 3 (samples Expo 7,8,9) and Recipe 4 (samples Expo 10,11,12)

It can be elucidated from figure 53 that deposition of more alumina leads to more stable nanocomposites, a similar trend as observed with the standard ALD process operation.

Increasing the number of ALD cycles from 25 to 100 suppressed the PL quenching for all four different ALD recipes, thus making them more stable in oxygen. The four nanocom-posites, Expo 3, 6, 9, 10 prepared with 100 ALD cycles, were thus analyzed for determin-ing the nanocomposite, which is the most stable in oxygen and luminesce. Lookdetermin-ing at the evolution of PL intensity upon switching from vacuum to oxygen, does not give a clear indication of which nanocomposite is the most stable as they have a similar behaviour.

Careful analysis of these four samples shows that Expo 3 sample is the best nanocompos-ite. A rainbow patterned color variation across the nanocomposites was observed for Expo 6 and 12 that accounts for chemical vapor deposition instead of atomic layer deposition, that results in uneven alumina growth with different growth rates across the nanocom-posite. It is quite clear from figure 53 (b) that, Expo 9 does not show good PL stability when switching between vacuum to oxygen. This can be possibly explained by the fact that higher H₂O exposure time leaves more water in the reaction chamber that affects the photo luminescence properties of QDs. Expo 3 nanocomposite sample was chosen from this initial experiments for further optimization of other parameters in the exposure mode recipe.

Table 10, shows how the maximum PL intensities vary for the 12 samples. It confirms that PL increases with increase in the number of ALD cycles.

Table 10.Summary of PL intensities for the samples in figure 53 Sample ALD Cycles Highest Intensity

Study the Effect of H₂O Purge Time

Purging step plays a vital role in removing the excess, unreacted precursor, co-reactant as well as the by-products present in the reaction chamber. This can otherwise decompose on the heated substrate leading to uncontrolled chemical vapour deposition corresponding to higher growth rates per cycle rather than atomic layer deposition, which consequently affect the optoelectronic properties of the nanocomposites.

Intermediate temperature ALD operation typically requires longer H₂O purge time, to ensure the effective removal of excess water. Upon investigating and identifying the ideal combination of diffusion wait for both TMA and H2O as 5 seconds, with an initial water purge wait of 10 seconds, effect of longer purge time has been experimented and the results are shown in figure 54.

Figure 54.Effect of H2O purge on stabilizing QD/AlOxnano-composites in oxygen atmosphere.

(a)Evolution of PL intensity upon switching from vacuum to nitrogen.(b)Emission spectrum in nitrogen.

It can be seen from the above figure that increasing the water purge time from 10 sec-onds (red plot, PL quenching of 35%) to 1 minute (blue plot, PL quenching of 20%) has considerably suppressed the PL quenching when switching between vacuum and oxygen.

Comparing the PL quenching of the blue plot with the standard recipe (green plot, PL quenching of 25%), the stability has further improved by increasing the H2O purge time of the exposure mode recipe, which accounts for a suppression of the PL quenching by approximately 5% (PL quenching now is 20%).

Study the Effect of N₂ Flow During Purge

It was investigated that increasing the H₂O purge time minimizes PL quenching, conse-quently enhancing the stability of QD/AlO_xnanocomposites. Looking for further stability improvement, it is evident that flow rate of N₂ during purge plays a significant role in the rate at which excess precursor and co-reactant are purged out. So far the N₂flow has been maintained at 5 sccm during purging as mentioned in the beginning of this section. Fig-ure 55 demonstrates how increasing the N2 flow during purge has affected the PL when exposed to oxygen.

Figure 55. Effect of N2 flow during H2O purge on stabilizing QD/AlOx nano-composites in oxygen atmosphere. (a)Evolution of PL intensity upon switching from vacuum to nitrogen. (b) Emission spectrum in nitrogen.

From figure 55 (a), it can be elucidated that the PL quenching has decreased to about 5%

from 20% achieved previously. This reduction can be explained due to effective removal of excess precursor and co-reactant that leads to a well deposited and uniform alumina coating that acts as an effective barrier eliminating interaction with oxygen.