Lianshan Wang

Wurtzite GaN epitaxial growth on a Si(001) substrate using γ-Al2O3 as an intermediate layer

Wurtzite GaN films have been grown on (001) Si substrates using γ-Al2O3 as an intermediate layer by low pressure (∼76 Torr) metalorganic chemical vapor deposition. Reflection high energy electron diffraction and double crystal x-ray diffraction measurements revealed that the thin γ-Al2O3 layer of “compliant” character was an effective intermediate layer for the GaN film grown epitaxially on Si. The narrowest linewidth of the x-ray rocking curve for (0002) diffraction of the 1.3 μm GaN sample was 54 arcmin. The orientation relationship of GaN/γ-Al2O3/Si was (0001) GaN‖(001) γ-Al2O3‖(001) Si, [11–20] GaN‖[110] γ-Al2O3‖[110] Si. The photoluminescence measurement for GaN at room temperature exhibited a near band-edge peak of 365 nm (3.4 eV).

Nanoscale lateral epitaxial overgrowth of GaN on Si (111)

We demonstrate that GaN can selectively grow by metalorganic chemical vapor deposition into the pores and laterally over the nanoscale patterned SiO2 mask on a template of GaN∕AlN∕Si. The nanoporous SiO2 on GaN surface with pore diameter of approximately 65 nm and pore spacing of 110 nm was created by inductively coupled plasma etching using anodic aluminum oxide template as a mask. Cross-section transmission electron microscopy shows that the threading-dislocation density was largely reduced in this nanoepitaxial lateral overgrowth region. Dislocations parallel to the interface are the dominant type of dislocations in the overgrown layer of GaN. A large number of the threading dislocations were filtered by the nanoscale mask, which leads to the dramatic reduction of the threading dislocations during the growth within the nano-openings. More importantly, due to the nanoscale size of the mask area, the very fast coalescence and subsequent lateral overgrowth of GaN force the threading dislocations to bend t...

Effects of periodic delta-doping on the properties of GaN:Si films grown on Si (111) substrates

In this study, the effects of periodic Si delta-doping on the morphological and optical properties of GaN films grown on Si (111) substrate have been investigated. It is found that the flow rate of Si dopant during growth significantly affects the surface morphology, structural and optical quality of GaN. Compared to undoped GaN on Si(111), films grown using periodic delta-doping show a significant reduction of the in plane tensile stress, which is confirmed by the blueshift of the E2(TO) phonon and band edge photoluminescence peaks. The crack density in GaN films also reduces due to delta-doping.

Nanoheteroepitaxial lateral overgrowth of GaN on nanoporous Si(111)

Nanoheteroepitaxial (NHE) lateral overgrowth of GaN on nanoporous Si(111) substrates has been demonstrated. Nanopore arrays in Si(111) surfaces were fabricated using anodized aluminum oxide templates as etch masks, resulting in an average pore diameter and depth of about 60 and 160–180nm, respectively. NHE growth of AlN and GaN was found to result in a significant reduction in the threading dislocation density (<108cm−2) compared to that on flat Si(111). Most dislocations that originate at the Si interface bent to lie in the GaN (0001) basal plane during lateral growth over the pore openings. E2 phonon blueshifts in the Raman spectra indicate a significant relaxation of the tensile stress in the coalesced GaN films, due to three-dimensional stress relaxation mechanisms on porous substrates. Our results show that a single step lateral overgrowth of GaN on nanopatterned Si(111) substrates without a dielectric mask is a simple way to improve the crystalline quality of GaN layers for microelectronic applications.

High Optical Quality Nanoporous GaN Prepared by Photoelectrochemical Etching

— — Nanoporous GaN films are prepared by UV assisted electrochemical etching using HF solution as an electrolyte. To assess the optical quality and morphology of these nanoporous films, micro-photoluminescence (PL), micro-Raman scattering, scanning electron microscopy (SEM), and atomic force microscopy (AFM) techniques have been employed. SEM and AFM measurements revealed an average pore size of about 85-90 nm with a transverse dimension of 70-75 nm. As compared to the as-grown GaN film, the porous layer exhibits a substantial photoluminescence intensity enhancement with a partial relaxation of compressive stress. Such a stress relaxation is further confirmed by the red shifted E 2 (TO) phonon peak in the Raman spectrum of porous GaN.

Outgoing multiphonon resonant Raman scattering and luminescence in Be- and C-implanted GaN

We have performed outgoing resonant Raman scattering and photoluminescence measurements on as-grown, Be- and C-implanted GaN in the temperature range of 77–330 K. In implanted GaN after postimplantation annealing at 1100 °C, the A1(LO) multiphonons up to the seventh order were observed with the very strong four longitudinal optical (LO) and five LO modes at ∼2955 and ∼3690 cm−1, respectively, showing extraordinary resonance behavior. With the sample temperature, these two modes significantly decreased and increased in intensity, respectively. The phenomenon is attributed to the variation of resonant conditions due to the shift of the band gap energy. Meanwhile, the combination of E2(high) and quasi-LO phonons was strongly enhanced by quasi-LO phonon involvement and thus the corresponding overtones can be clearly observed even up to the sixth order (m=6). The mechanisms that such strong outgoing multiphonon resonance Raman scattering occurred to implanted GaN instead of high-quality as-grown GaN samples ca...

Defect reduction by periodic SiNx interlayers in gallium nitride grown on Si (111)

Periodic SiNx interlayers were employed during the metal-organic chemical vapor deposition of epitaxial GaN on AlN buffer layers grown on Si (111) substrates. The growth and the evolution of defects were studied in this paper. A reduction of the threading dislocation density to ∼109cm−2 was observed on the surface of GaN by counting the surface pit density from the atomic force microscopy results. Besides the observation of the continuous bending and subsequent recombination of the threading dislocations related to the periodic conduction of the SiNx interlayer characterized using cross-sectional transmission electron microscopy, we observed a different behavior induced by the SiNx interlayers: Si-rich inverted hexagonal pyramids with their base on the (0001) plane and six sidewalls on the (101¯1) plane were found near the top surface of the GaN film at the location of SiNx insertion layer characterized using electron energy loss spectroscopy. The preferential deposition of the SiNx on the sidewalls of th...

Crack-free fully epitaxial nitride microcavity with AlGaN∕GaN distributed Bragg reflectors and InGaN∕GaN quantum wells

Using metal-organic chemical vapor deposition, we have fabricated fully epitaxial nitride microcavties with AlGaN∕GaN distributed Bragg reflectors and InGaN quantum wells as the light emitter. To solve the problem of cracking, a thin AlN anticracking layer was used. The samples were characterized using transmission electron microscope, reflectivity spectroscopy, and photoluminescence spectroscopy. A cavity quality factor of 200 was obtained and the spontaneous emission of cavity mode was measured from a 1λ GaN microcavity, with 40-pair Al0.24Ga0.76N∕GaN distributed Bragg reflectors as the bottom and top reflectors and three period In0.10Ga0.90N∕GaN quantum wells in the GaN cavity layer.

Micro-raman scattering in laterally epitaxial overgrown GaN

In this study, micro-Raman spectroscopy has been used to investigate the vibrational properties of laterally epitaxial overgrown (LEO) GaN. The LEO GaN films were grown by metal organic chemical vapor deposition on a 2 in. sapphire substrate with SiN mask. Photoluminescence and polarized Raman scattering measurements have been performed in the two regions of GaN growth (wing and window regions). Raman scattering results are consistent with the lateral growth of GaN in the overgrown region. We have observed second-order Raman scattering in the wing and window regions of GaN. The observations of longitudinal optical phonon plasmon modes in the overgrown region demonstrate that LEO GaN is doped. We have carried out micro-Raman mapping of the local strain and free carrier concentration in the LEO GaN. Anharmonicity due to temperature in LEO GaN has also been investigated. The anharmonicity was found to increase with increasing temperature, and such temperature-induced anharmonicity introduces changes in the l...

Competitive growth mechanisms of AlN on Si (111) by MOVPE

To improve the growth rate and crystal quality of AlN, the competitive growth mechanisms of AlN under different parameters were studied. The mass transport limited mechanism was competed with the gas-phase parasitic reaction and became dominated at low reactor pressure. The mechanism of strain relaxation at the AlN/Si interface was studied by transmission electron microscopy (TEM). Improved deposition rate in the mass-transport-limit region and increased adatom mobility were realized under extremely low reactor pressure.