Suggestion: Two-step process such as making oxide and anneal it after deposition clearly is not working. For crystalline oxide tray one step process:

Pump down chamber exp -6 Torr, During deposition use heated substrate around 500 oC and excess oxygen e.g., 50 Sccm. Temperature and oxygen amount are your parameters. Please inform me for the results. Thank you

Best of luck,

Hulya Demiryont

it is difficult to know the fault at the outset but here are some things you should consider: 1. Initial Composition of the Deposited Film

Problem: Electron-beam evaporation of ZnO can decompose the material into metallic Zn and O₂, especially if the oxygen pressure in the chamber is insufficient during deposition. If the as-deposited film is primarily metallic Zn, subsequent oxidation might be incomplete.

Solution:

Verify the film composition before oxidation using techniques like XPS (X-ray Photoelectron Spectroscopy) or EDS (Energy-Dispersive Spectroscopy).

If metallic Zn is being deposited, consider depositing ZnO directly using a ZnO source under controlled conditions (oxygen pressure, slow deposition rate).

2. Oxidation/Recrystallization Conditions

Problem:

At 600°C: Oxidation at 600°C might be insufficient to fully convert a thick Zn layer (200 nm) into crystalline ZnO, especially if the O₂ flow (10 L/min) does not penetrate the film uniformly.

At 1000°C: While high temperatures promote crystallization, sapphire (Al₂O₃) might react with ZnO, forming secondary phases like ZnAl₂O₄ (spinel) or causing Al diffusion into the film, which would inhibit the formation of pure ZnO.

Solution:

Optimize oxidation: Use a furnace with a controlled atmosphere (pure O₂) and increase the oxidation time (>1 hour).

Avoid substrate reactions: Try intermediate temperatures (800-900°C) and check for secondary phase formation using XRD or TEM. If reactions occur, consider using a buffer layer (e.g., MgO) between ZnO and sapphire.

Grain Size and Morphology

Problem: If the ZnO grains are too small (