C. C. Kim

Effects of growth temperature on GaN nucleation layers

The effects of growth temperature on the microscopic structure of GaN nucleation layers were studied in a synchrotron x-ray scattering experiment. As the growth temperature increases from 467 to 655 °C, the stacking of GaN changes from random stacking to a mixture of cubic and hexagonal stacking. With increasing the growth temperature, the order in the atomic layer positions in the out-of-plane direction increases and the mosaic distribution becomes narrow. The optimal photoluminescence spectrum was obtained on the GaN epilayer deposited on the nucleation layer grown at 505 °C.

Catalytic role of Au in Ni/Au contact on GaN(0001)

We investigated the structural behavior of the Ni/Au contact on GaN(0001) and found the catalytic role of Au during annealing, using in situ x-ray scattering. The oxidation kinetics of Ni during annealing in air was greatly affected by the presence of Au. The accelerated GaN decomposition and Ni nitride formation during annealing in N2 also provide evidence of the catalytic role of Au. The results suggest that oxidized Ni/Au ohmic contact exhibit better thermal stability than that of the N2 annealed Ni/Au contacts.

Microstructural study of Pt contact on p-type GaN

Using synchrotron x-ray scattering the evolution of the biaxial strain at the interface of the Pt layer (100 A) with p-type GaN was investigated as a function of annealing temperature. Furthermore, the effects of the biaxial strain on the change of ohmic contact resistivity were interpreted. The Pt layer grew epitaxially on GaN with the relationships of Pt [111]//GaN [0001] and Pt [110] //GaN [1120]. Due to a lattice mismatch between Pt and GaN, a biaxial tensile strain (+0.9%) to the Pt layer and a compressive strain (−0.9%) to the GaN substrate were introduced in the as-deposited state. After annealing at 450 °C, the strains were fully relaxed and the position of the surface Fermi level moved 0.21 eV toward the valence band maximum. Furthermore, the contact resistivity decreased by 1 order of magnitude. The results provide evidence that the change of the interfacial strain causes the movement of the surface Fermi level position in the band gap of GaN, leading to a change in the magnitude of contact re...

Microstructure of GaN nucleation layer during initial stage MOCVD growth

The microstructure in GaN nucleation layers was studied in synchrotron X-ray scattering and transmission electron microscopy experiments. We revealed the existence of tensile-strained interfacial domains, which was originated from 6/7 matched interface structure.

Annealing behavior of Pd/GaN (0001) microstructure

Abstract We reveal the existence of interfacial, epitaxial, Pd grains in an as-deposited Pd film evaporated on GaN (0001) at room temperature. The origin of the Pd epitaxy was attributed to 6/7 matched interface structure. The Pd film was completely transformed to Ga 2 Pd 5 and Ga 5 Pd gallides in epitaxial relationships with GaN at an annealing temperature of 700°C.

Microstructures of GaN islands on a stepped sapphire surface

We investigated the structural evolution of GaN nucleation layers in the initial growth stages on commercial c-plane sapphires with atomic steps at the surface, using field-emission scanning electron microscopy, synchrotron x-ray scattering, and high-resolution electron microscopy. GaN nucleates into islands preferentially on the atomic steps. The initial small islands of 25 A high have well-ordered cubic sequences and nearly coherent interfacial structures with a large compressive strain of ∼10%. As the islands grow to 50 A high, the strain is drastically reduced, to less than 1%, by generating misfit dislocations at the interface and forming the six-to-seven matched interfacial structure. Interestingly, stacking faults are developed from the GaN/sapphire interface, which induces a cubic–hexagonal transformation. The changes in the stacking order during the initial growth are investigated quantitatively.

Defects and nucleation of GaN layers on (0001) sapphire

The morphology and microstructural evolution of a nucleation layer are analysed using high-resolution transmission electron microscopy. Low-temperature nucleation of GaN on (0001) sapphire is investigated. Depositions were made for 20, 40, 60, 120 and 180 s at 560°C by metal–organic chemical vapour deposition. It is shown that the shortest deposition times give rise to the formation of cubic islands. Subsequently, the density and the size of the nucleated islands increase and they start to transform into wurtzite from the interface with the substrate. From the start, the nuclei contain misfit dislocations. At these early growth stages, the relaxation state changes from one island to another; this probably underlies the subsequent mosaic growth of the high-temperature-active GaN layers.

In-plane tensile-strained interfacial structure in a GaN nucleation layer on sapphire(0001)

Interfacial microstructure in GaN nucleation layers was investigated using synchrotron x-ray scattering and transmission electron microscopy. We find that tensile-strained, aligned, interfacial domains coexist with misaligned domains in an as-grown nucleation layer of mostly cubic stacking. The tensile strain originates in a 6/7 matched interfacial structure, wherein 6-Ga atomic distances in GaN match to 7-Al atomic distances in sapphire. The tensile state of the aligned, interfacial domains is preserved during annealing to 1100 °C, while the stacking sequence changes from cubic to hexagonal order. The correlation length of the stacking order is rather short, ∼9 A in the hexagonal phase, compared to that of the cubic phase in the as-grown nucleation layer, ∼25 A, due to stacking faults generated during the kinetically limited transformation.

Au Catalyzed Structural and Electrical Evolution of Ni/Au Contact to GaN

We investigated the micro-structural and electrical behavior of Ni/Au contacts on GaN(0001) during annealing in N 2 and in air. The formation of a Ni 4 N phase deteriorates the contact resistivity if annealed in N 2 . Annealing in air, however, dramatically improves it. The oxidation, by creating the more energy favorable Ni-O bonds, seems to be the recipe to successfully suppress the formation of Ni 4 N, which otherwise would be enhanced significantly by the presence of Au as a catalyst.

Nucleation of GaN on (0001) sapphire during MOCVD growth: an atomic force and high resolution electron microscopy study

The morphology and microstructure of GaN nucleation layers have been studied using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The nucleation layers were grown at 500°C by metalorganic chemical vapour deposition (MOCVD) on (0001) sapphire. Different deposition times were used in order to investigate the evolution of the nucleation layer and try to understand the growth mechanisms. Systematically, it was found that the thinnest layers are mainly defect free and have a cubic structure. The (111) GaN planes are parallel to (0001) sapphire. It is shown that the nucleation follows the Volmer- Weber mechanism and as the islands height increases, the transformation from cubic to wurtzite starts from the substrate surface.