Space-charge-controlled field emission analysis of current conduction in amorphous and crystallized atomic-layer-deposited Al2O3 on GaN

Abstract

As previously reported, postdeposition annealing at 800 °C and higher simultaneously crystallizes atomic-layer-deposited (ALD) Al2O3 films and reduces the current in Al/ALD-Al2O3/(0001) GaN capacitors by two orders of magnitude. This current reduction is caused by the enhancement of conduction band offset from 1.4 to 1.8 eV, as revealed by the space-charge-controlled field emission analysis. Selected area electron diffraction (SAED) patterns demonstrate that the crystallized films consist of twinned (111)-oriented cubic γ-Al2O3 with an epitaxial relation of Al2O3 ⟨01¯1⟩∥ GaN ⟨21¯1¯0⟩. The SAED patterns additionally include spots that are specific to triaxially tripled γ-Al2O3. The aforementioned epitaxy is due to the similarity of hexagonal close-packed sublattices between oxygen on a (111) γ-Al2O3 plane and nitrogen on a (0001) GaN plane. However, the hexagonal close-packed lattice constant of γ-Al2O3 is 12% smaller than that of GaN, necessitating domain matching epitaxy. The thickness of the interfacial transition layer caused by the large misfit is estimated to be thinner than four monolayers of oxygen sublattice, by using the methodology developed here. Based on these results, the effect of Al2O3 crystallinity on the characteristics of Al2O3/GaN capacitors, such as conduction current, dielectric breakdown, interface states, and bias instability, was comprehensively captured. According to x-ray diffraction analyses, Al2O3 films crystallize at 700 °C, which is ∼100 °C lower than the threshold temperature estimated by transmission electron microscope observations. This difference was possibly caused by locally crystallized Al2O3 films, as confirmed by the slightly reduced current. These findings form a basis for improving ALD-Al2O3 films as gate insulator.

Correlative Atomic Coordination and Interfacial Charge Polarity in Al2O3/GaN and Al2O3/Si Heterostructures

“…The interfacial charges obtained by the KPFM measurement are close to the values measured by the C-V technique. [1,11] Electron energy loss spectroscopy (EELS) is capable of fingerprinting different coordinated forms of the same element. Amorphous Al 2 O 3 contains both tetrahedral [AlO 4 ] 5À and octahedral [AlO 6 ] 9À coordination; therefore, the tetrahedrally and octahedrally coordinated Al with the energy loss of 2.4 eV in separate can be mapped across the interfacial regions of the Al 2 O 3 /GaN and Al 2 O 3 /Si heterostructures by EELS measurements.…”

“…High-k dielectrics are widely used in electronic and optoelectronic devices to passivate the dangling bonds at the semiconductor surfaces or as gate insulators to reduce the gate leakage current. [1][2][3][4][5][6][7][8][9][10][11] However, substantial interfacial charges that occur at high-k dielectric/semiconductor interfaces can severely impact device design and engineering, and the charge origins are often elusive. [1][2][3][4][5][6][7][8][9][10][11] For example, Al 2 O 3 has been adopted in many crucial devices including GaN high-electron-mobility transistors (HEMTs) and Si solar cells.…”

“…[1][2][3][4][5][6][7][8][9][10][11] However, substantial interfacial charges that occur at high-k dielectric/semiconductor interfaces can severely impact device design and engineering, and the charge origins are often elusive. [1][2][3][4][5][6][7][8][9][10][11] For example, Al 2 O 3 has been adopted in many crucial devices including GaN high-electron-mobility transistors (HEMTs) and Si solar cells. In Si solar cells, Al 2 O 3 is used as the surface passivation layer owning to high-density negative charges occur the Al 2 O 3 /Si interface.…”

“…[7][8][9] Al 2 O 3 has also been widely used in GaN HEMTs as the gate dielectric to decrease the gate leakage current and increase the gate breakdown voltage. [10][11][12] By simply extending the gate dielectric thickness, enhancement-model (E-mode) GaN HEMTs can be achieved if the charges at the Al 2 O 3 /GaN interface are also negative. [12] Nevertheless, positive charges that occur at the Al 2 O 3 /GaN interface can negatively shift the threshold voltage (V TH ), retarding the achievement of E-mode GaN HEMTs.…”

Origin of interfacial charges of Al2O3/Si and Al2O3/GaN heterogeneous heterostructures

“…[7,8] Meanwhile, Al 2 O 3 has also been widely adopted in GaN highelectron-mobility transistors (HEMTs) as the gate dielectric that suppresses the gate leakage current and enlarges the gate voltage swing. [9][10][11] Enhancement model GaN HEMTs can be obtained by extending the gate dielectric thickness if the charges at the interface of Al 2 O 3 /GaN H2 are also negative. [11] However, the positive charges occur at the interface of Al 2 O 3 /GaN H2 can shift the threshold voltage to a negative value, hindering the realization of enhancement model GaN HEMTs.…”