Pekka Törmä

Analysis of Dislocations Generated during Metal–Organic Vapor Phase Epitaxy of GaN on Patterned Templates

We report on patterning and subsequent metal–organic vapor phase epitaxy overgrowth of GaN films on patterned GaN/sapphire templates. Templates with a hexagonal hole pattern were prepared by photolithography and dry etching. After GaN overgrowth voids were formed at the GaN/sapphire interface. Threading dislocations were found to bend and terminate at void sidewalls during the overgrowth resulting in improved material quality. The dislocations were analyzed by transmission electron microscopy combined with energy dispersive X-ray spectroscopy. Areas with increased Ga concentration were found at the tips of coalesced voids that introduced additional dislocations to the overgrown films.

Enhancement of near-UV GaN LED light extraction efficiency by GaN/sapphire template patterning

We present near-UV GaN light-emitting diodes (LEDs) grown on patterned GaN/sapphire templates with improved material quality and light extraction efficiency. Enhancement of light extraction efficiency is attributed to voids generated at the GaN/sapphire interface. The sidewall inclination angle of the voids can be controlled from nearly vertical (∼ 85°) to fully inclined (∼ 60°) by changing the initial patterning dimensions. Light extraction efficiency and material quality improve with a decreasing void sidewall angle. A 20% increase in the light output is observed at 20xa0mA of input current for LED structures with ∼60° inclined sidewall voids.

Ultra-Thin Silicon Nitride X-Ray Windows

We have demonstrated the fabrication of ultra-thin Si fine grid supported silicon nitride X-ray windows. These X-ray windows exhibit unequaled transmission of soft X-rays, high strength and excellent thermal stability. Measured soft X-ray transmission performance is significantly enhanced compared to typical polymer or beryllium based X-ray window structures. A double sided grid structure is used to demonstrate the scaling of the technology to larger areas.

Smoothing of microfabricated silicon features by thermal annealing in reducing or inert atmospheres

In this work, high-temperature annealing of reactive ion etched silicon microstructures in H2 and Ar gases is studied. Three types of structural features were etched with four different masks into 100-oriented silicon wafers. Scanning electron microscope and atomic force microscope (AFM) results show that when smoothing due to surface diffusion and desorption of silicon is taking place in both the argon and hydrogen gas environments, the three types of features respond differently to the treatments. The surface diffusion was observed to be strongly dependent on temperature in argon, whereas the transport was more linear and controllable in the hydrogen gas environment. For hydrogen, AFM studies were performed to observe the details of the smoothing process. Finally, some potential applications of these transport phenomena are discussed.

Fracture properties of atomic layer deposited aluminum oxide free-standing membranes

The fracture strength of Al2O3 membranes deposited by atomic layer deposition at 110, 150, 200, and 300u2009°C was investigated. The fracture strength was found to be in the range of 2.25–3.00u2009GPa using Weibull statistics and nearly constant as a function of deposition temperature. This strength is superior to common microelectromechanical systems materials such as diamondlike carbon, SiO2, or SiC. As-deposited membranes sustained high cycling pressure loads >10 bar/s without fracture. Films featured, however, significant reduction in the resistance to failure after annealing (800u2009°C) or high humidity (95%, 60u2009°C) treatments.The fracture strength of Al2O3 membranes deposited by atomic layer deposition at 110, 150, 200, and 300u2009°C was investigated. The fracture strength was found to be in the range of 2.25–3.00u2009GPa using Weibull statistics and nearly constant as a function of deposition temperature. This strength is superior to common microelectromechanical systems materials such as diamondlike carbon, SiO2, or SiC. As-deposited membranes sustained high cycling pressure loads >10 bar/s without fracture. Films featured, however, significant reduction in the resistance to failure after annealing (800u2009°C) or high humidity (95%, 60u2009°C) treatments.

Defect studies with positrons: What could we learn on III-nitride heterostructures?

We have applied positron annihilation spectroscopy to study 400 – 500 nm InGaN-based LED structures, as well as InGaN and AlGaN materials with varying In and Al contents. We find that the effect of adding In to GaN on the annihilation parameters obeys the Vegards law, while in the case of AlGaN the possible effect of Al is completely screened by efficient formation of cation vacancies. The results obtained in the InGaN LED structures are indistinguishable from defect-free GaN, suggesting that the positrons annihilate preferentially in the barriers of the MQW system.