September 30, 2014 at 3:45 pm

NanoForum: Growth & Scanning Tunneling Microscopy Studies of Novel Trench-Like Formation, Sept. 30

NQPI NanoForum presents Zakia H. Alhashem on “Growth and Scanning Tunneling Microscopy Studies of Novel Trench-Like Formation and Relation to Manganese Induced Structures on w-GaN (000 ̅)” on Tuesday, Sept. 30, at 4 p.m. at Clippinger 259.

Her adviser is Dr. Arthur Smith, Professor of Physics and Astronomy. NanoForums are sponsored by Nanoscale and Quantum Phenomena Institute.

Abstract: A trench-like structure has been observed after sub-monolayer Mn is deposited onto N-polar gallium nitride (GaN) surfaces. It is reminiscent of the well-documented formation of the vacancy line structures on Si(001) surfaces. However, little is known about what really causes this trench-like structure to form. Although it was previously hypothesized that it may be due to the Mn atoms, it may in fact be present before Mn deposition, and possibly caused by annealing or some other variations in the growth conditions. This study investigates the causes of the appearance of this trench-like structure on N-polar w-GaN surfaces by optimizing the growth recipe to obtain reproducible trench structures. Additionally, surface structure after Mn deposition onto w-GaN (000 ̅) was observed and the role of the Mn was analyzed.

In the current study, GaN samples were grown on sapphire (0001) substrates using ultra high vacuum molecular beam epitaxy (UHV-MBE). The growth was monitored using a reflection high energy electron diffraction (RHEED) system. For room temperature scanning tunneling microscopy (RT-STM) studies, the samples were transferred in-situ to the analysis chamber, where STM images were taken using a tungsten tip under a constant current condition.

In the first attempt, the RHEED patterns of the GaN growth showed the well-known 3×3 reconstruction, which was further confirmed by atomic resolution STM. The sample was annealed for different times, but no trench-like structure was observed. Another GaN sample was grown under highly Ga-rich conditions and annealed for ~ 6 minutes. These growth conditions resulted in the trench-like formation often appearing nearby the c(6×12) reconstruction. The
trench-like structure is a long range ordered structure in some parts of the sample, whereas it is a long range disordered structure in other parts that contain some characteristic defects. This structure is formed as two sub-units of the c(6×12) reconstruction because of the similarities between the trench-like formation and the c(6×12) reconstruction. After depositing ~ 0.15 ML of Mn on the non-annealed w-GaN (000 ̅) surface, the c(6×12) reconstruction was observed around the MnGa islands while the trench-like formation was not observed. Therfore, high temperature annealing is a requirement for obtaining the trench-like structure on the w-GaN (000 ̅) surface, which suggests that the removable N atoms could disturb the GaN ad-layer causing the trench-like structure. However, Mn deposition is not a requirement to form the trench-like structure, and the Mn atoms do not seem to play a role its formation. They do combine with the Ga atoms to form MnGa islands.

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