Microstructural dependence of residual stress in reactively sputtered epitaxial GaN films

Abstract

Epitaxial GaN films were grown on c-sapphire by rf magnetron reactive sputtering of GaAs at different partial pressures of nitrogen in Ar–N2 sputtering atmosphere. High-resolution x-ray diffraction and φ-scans reveal the mosaic growth of c-axis oriented, wurtzite GaN films. The c and a parameters were independently determined to obtain the corresponding in-plane and out-of-plane strain components. Raman measurements confirmed the in-plane strain behavior. The surface morphology and elemental composition of films were studied by atomic force microscopy and secondary ion mass spectroscopy, respectively. High-resolution ω-2θ, ω, and in-plane φ-rocking curve scans were used to obtain micro-strain, screw and edge dislocation densities, respectively. The films grown at 30%–100% N2 reveal dominance of edge (∼1012 cm−2) over screw (∼1010 cm−2) dislocations, with both approaching similar densities at lower N2 percentages. The strain data has been analyzed to separate the hydrostatic and biaxial contributions and their dependences on N2 percentage. The film grown at 100% N2 displays large hydrostatic strain and micro-strain due to the presence of excess/interstitial nitrogen. The hydrostatic strain and micro-strain decrease substantially with initial decrease of N2 percentage, but increase slightly in the films grown below 30% N2, primarily due to the incorporation of Ar. The films grown below 75% N2 display growth-related intrinsic tensile stress, originating from crystallite coalescence. The stress reversal from tensile to compressive, seen in the films grown at higher N2 percentages is primarily attributed to the incorporation of excess/interstitial nitrogen into grain boundaries and the tensile side of edge dislocations. The decrease of intrinsic tensile stress in the films grown below 30% N2 is attributed to the incorporation of Ar and their voided structure.

Electrical transport in epitaxially grown undoped and Si-doped degenerate GaN films

“…These results have thus shown that both undoped and Si-doped epitaxial GaN films grown by sputtering are degenerate, having high carrier concentrations (>10 18 cm −3 ) [34,41]. Our earlier studies [20,42] on the microstructure of undoped and Si-doped GaN films grown epitaxially by sputtering have also shown that these films possess a high density of edge dislocations in the range of 10 11 -10 12 cm −2 . It may be mentioned here, that in spite of the recent progresses in the growth of epitaxial GaN films by sputtering and promising device applications [30], studies related to their electronic properties and dependence on microstructure and defects remain rather limited, compared to the enormous scientific literature that exists on MBE and MOCVD grown GaN.…”

“…Undoped and Si-doped GaN epitaxial films were grown on c-sapphire substrate respectively by reactive sputtering of GaAs (3-inch diameter) and GaAs-Si composite target, using an approach similar to that used in our earlier work [20,42]. For the in situ incorporation of Si, the erosion track of GaAs was partially covered by Si and the Si area coverage of the erosion track of GaAs was fixed at ∼5%, based on earlier studies, which indicated the deterioration of the structural quality of films grown with larger area coverages [23].…”

“…Although the density of threading dislocations has often been limited to the range of 10 6 -10 7 cm −2 in MOCVD [11] and MBE [12] grown GaN films, there are some limitations due to the high temperature operation in MOCVD and issues related to the scalability and high cost of MBE. The low temperature amenability, scalability and versatility of sputtering makes it an appealing option, which has demonstrated considerable potential for the growth of GaN films [13][14][15][16][17][18][19][20], albeit with threading dislocations in the range of 10 9 -10 12 cm −2 [18][19][20]. Accordingly, various modes of sputtering have been employed for the growth of both undoped [18,20,21] and doped (with Si, Ge and Mg) [19,[22][23][24] GaN films.…”

“…The low temperature amenability, scalability and versatility of sputtering makes it an appealing option, which has demonstrated considerable potential for the growth of GaN films [13][14][15][16][17][18][19][20], albeit with threading dislocations in the range of 10 9 -10 12 cm −2 [18][19][20]. Accordingly, various modes of sputtering have been employed for the growth of both undoped [18,20,21] and doped (with Si, Ge and Mg) [19,[22][23][24] GaN films. Owing to the constraints of using metallic Ga target for reactive sputtering, due to its low melting point, GaAs has been used as an alternative sputtering target by our group for depositing nano/ polycrystalline [16,25] as well as epitaxial [20,26] GaN films.…”

“…Accordingly, various modes of sputtering have been employed for the growth of both undoped [18,20,21] and doped (with Si, Ge and Mg) [19,[22][23][24] GaN films. Owing to the constraints of using metallic Ga target for reactive sputtering, due to its low melting point, GaAs has been used as an alternative sputtering target by our group for depositing nano/ polycrystalline [16,25] as well as epitaxial [20,26] GaN films. Recently, this approach was extended to grow heteroepitaxial GaN films on sapphire, which displayed a large decrease of resistivity from ∼10 5 Ω cm to ∼10 −3 Ω cm, depending on the N 2 percentage in sputtering atmosphere [21].…”