[Paper Review] Optical properties and associated Urbach energy tails in AlN thin films grown by reactively assisted ion beam sputtering: Effect of different substrate temperatures

Neha Sharma, Shilpam Sharma+5 more|arXiv (Cornell University)|Jul 17, 2015

GaN-based semiconductor devices and materials31 references3 citations

TL;DR

This study investigates AlN thin films grown on Si(100) substrates via reactive ion beam sputtering at substrate temperatures from room temperature to 500 °C. It reveals a decrease in optical bandgap from 3.70 eV to 3.15 eV and a corresponding increase in Urbach energy with rising temperature, linked to a structural transition from amorphous to nanocrystalline to microcrystalline, as confirmed by TEM, AFM, and optical spectroscopy.

ABSTRACT

AlN thin films have been grown on Si (100) substrates by reactive ion beam sputter deposition at different substrate temperatures varying from room temperature (RT) to 500oC. UV-VIS spectrophotometry has been employed to probe the optical properties of grown thin films in reflectance mode. It was found that optical band-gap (Eg) was decreased from 3.70 to 3.15 eV as substrate temperature was increased from RT to 400oC. Urbach energy tail (Eu) has been estimated to account for the optical disorder with associated decrease in Eg. Substrate temperature induced structural transition from amorphous at RT, nanocrystalline at 300oC to microcrystalline at 400oC has been observed by Transmission electron microscopy (TEM). Average surface roughness (Ra) and morphology has been explored by using Atomic force microscopy (AFM). Average refractive index (na) was increased as the substrate temperature was increased from RT to 100oC with a dip in extinction coefficient (ka) from 0.319 to 0.148. With further increase in substrate temperature, na followed a decreasing track in its values along with ka. This variation in na and ka values has been correlated to the corresponding surface roughness and morphology at each substrate temperature.

Motivation & Objective

• To investigate the influence of substrate temperature on the optical and structural properties of AlN thin films.
• To correlate changes in optical bandgap and Urbach energy with structural evolution in AlN films.
• To analyze surface morphology and roughness using atomic force microscopy (AFM) across varying growth temperatures.
• To understand the relationship between refractive index, extinction coefficient, and film microstructure as temperature increases.
• To determine the role of substrate temperature in inducing amorphous-to-crystalline transitions in AlN thin films.

Proposed method

• AlN thin films were deposited on Si(100) substrates using reactive ion beam sputtering at substrate temperatures ranging from room temperature to 500 °C.
• Optical properties were measured via UV-VIS spectrophotometry in reflectance mode to extract bandgap and Urbach energy.
• Transmission electron microscopy (TEM) was used to analyze the microstructure and confirm the transition from amorphous to nanocrystalline to microcrystalline phases.
• Atomic force microscopy (AFM) was employed to quantify average surface roughness (Ra) and assess surface morphology at each temperature.
• Refractive index (na) and extinction coefficient (ka) were calculated from optical transmission data and correlated with structural and morphological changes.
• Urbach energy (Eu) was estimated to quantify optical disorder, reflecting the tail states in the bandgap due to structural defects or disorder.

Experimental results

Research questions

• RQ1 How does increasing substrate temperature affect the optical bandgap of AlN thin films?
• RQ2 What is the relationship between substrate temperature and the Urbach energy tail in AlN films?
• RQ3 How does the structural phase evolve from amorphous to crystalline with increasing substrate temperature?
• RQ4 In what way do surface roughness and morphology change with substrate temperature, as measured by AFM?
• RQ5 How do the refractive index and extinction coefficient vary with substrate temperature, and what causes these variations?

Key findings

• The optical bandgap (Eg) of AlN thin films decreased from 3.70 eV at room temperature to 3.15 eV at 400 °C.
• Urbach energy (Eu) increased with substrate temperature, indicating enhanced optical disorder and localized states near the band edge.
• TEM analysis confirmed a structural transition from amorphous at room temperature, through nanocrystalline at 300 °C, to microcrystalline at 400 °C.
• Average surface roughness (Ra) increased with substrate temperature, correlating with changes in film morphology observed via AFM.
• The average refractive index (na) initially increased from room temperature to 100 °C, then decreased with further temperature increases.
• The extinction coefficient (ka) decreased from 0.319 at room temperature to 0.148 at 100 °C, followed by a gradual decline with higher temperatures, linked to changes in surface morphology and microstructure.