From what I know is after HF etching of the Si surface, the SiOx is removed and the surface is left with H-terminal on top of Si. Since H is more electronegative than Si, As a result H is negative charged. However, even though the O is negative charge in H2O, and making it repel with H-terminal in Si. Why couldn't be the case that the positive charged H bond in H2O attracts H- terminal in Si, making the contact angle small? What actually makes a surface hydrophilic or hydrophobic?
It's not about charging. Water is more polar than silicon surface. There are hydrogen bond between water molecules. A system will always find the lowest energy way to be stable. For water, forming hydrogen bonds is this way. On bare silicon surface, water cant form h bond with the surface. So water molecules will just bond with other water molecules which will behave as hydrophobic. However if there are hydroxyl on surface, water can h bond with hydroxyls which will make it hydrophilic. Hope it can help.
It's not about charging. Water is more polar than silicon surface. There are hydrogen bond between water molecules. A system will always find the lowest energy way to be stable. For water, forming hydrogen bonds is this way. On bare silicon surface, water cant form h bond with the surface. So water molecules will just bond with other water molecules which will behave as hydrophobic. However if there are hydroxyl on surface, water can h bond with hydroxyls which will make it hydrophilic. Hope it can help.
Hydrophobic or hydrophilic interaction is an entropic interaction. For instance, water molecules on oil surface can only have few configurations, and consequently small entropy, which is unfavored.
Back to a your case, hydrophilic silicon surfaces, like OH terminated, can chemically absorb one layer of water molecule by hydrogen binding, (in this case the loss of entropy, T*delta S, is much less than the loss of enthalpy, delta H) and provided the second layer of water molecules to free changing their configuration on the first layer of adsorbed water molecules, leading to a large entropy and small surface tension.
H terminated silicon surface, due to the small polarity of Si-H bond, can not provide a strong attraction to water molecules to form a bonded "first layer", but force the water molecules to have a certain configuration, which is O of water towards H on silicon, decrease the overall entropy of the system, resulting a high surface tension.
Hope this answers your question.
As you mentioned, the HF removes the native oxide (which is hydrophilic), which leaves Si with some Si-F bonds. Very quickly the native oxide is formed again. The fresh oxide needs time to form Si-H bonds, upon which the oxide surface becomes hydrophilic again. Fresh oxides (e.g. PECVD oxide) also show hydrophobic behavior when they just come out of the reactor. It takes some time in ambient air or a water rinse for the oxide to become hydrophilic.
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