C. F. Johnston

Understanding x-ray diffraction of nonpolar gallium nitride films

X-ray diffraction (XRD) is widely used for the rapid evaluation of the structural quality of thin films. In order to determine how defect densities relate to XRD data, we investigated a series of heteroepitaxial nonpolar a-plane GaN films with different densities of dislocations and basal plane stacking faults (determined by transmission electron microscopy). Factors influencing XRD data include surface roughness effects, limited lateral coherence lengths, lateral microstrain, mosaic tilt, and wafer curvature, in addition to the defects present. No direct correlation between defect densities and any measured XRD parameter was found. However, the structural imperfections dominating XRD data can be identified by specific analysis of each individual broadening factor. This reductive approach permits full explanation of the in-plane rotational anisotropy of symmetric ω-scan widths for both a-plane and m-plane films: in these samples, mosaic tilt is the dominant factor.

Microstructural evolution of nonpolar (11-20) GaN grown on (1-102) sapphire using a 3D-2D method

The microstructure of nonpolar, a-plane (11-20) GaN grown on r-plane (1-102) sapphire, using a three dimensional (3D)-two dimensional (2D) growth transition, has been studied at different stages of metal organic vapor phase epitaxy. The microstructure and morphology of GaN islands formed at the initial stages of growth, as well as the fully coalesced film, were characterized using transmission electron microscopy (TEM) and atomic force microscopy (AFM). The growth of GaN islands (bounded by {10-11} and (000-1) facets) was established under reactor conditions of relatively high pressure and high V/III ratio, whereas the island coalescence was achieved at lower pressure and low V/III ratio, leading to pit-free films with shallow striations along ⟨0001⟩. Cross-sectional TEM studies, in combination with the AFM studies of the uncoalesced films, showed that there was a correlation between the point at which partial dislocation line direction changed and the point at which growth conditions changed from the 3D ...

Defect reduction in (112¯2) semipolar GaN grown on m-plane sapphire using ScN interlayers

The effect of ScN interlayer thickness on the defect density of (112¯2) semipolar GaN grown on m-plane sapphire was studied by transmission electron microscopy. The interlayers comprised Sc metal deposited on a GaN seedlayer that was nitrided before GaN overgrowth by metal-organic vapor-phase epitaxy. Both interlayer thicknesses reduced the dislocation density by a factor of 100 to low-108 cm−2. The 8.5 nm interlayer produced regions that were free from basal plane stacking faults (BSF) and dislocations. The overall BSF density here was reduced by a factor of 5, to (6.49±0.07)×104 cm−1, without the need for an ex situ mask patterning step.

Optical properties of GaN/AlGaN quantum wells grown on nonpolar substrates

In this paper we report on the optical properties of a series of GaN/AlGaN multiple quantum well structures grown on a-plane (112¯0) GaN, which had been deposited on r-plane (11¯02) sapphire substrates, compared to a reference GaN template of the same orientation. The low temperature photoluminescence spectrum of the template layer is dominated by two emission bands, which we attribute to recombination involving excitons in the bulk of the layer and electrons and holes trapped at basal-plane stacking faults, designated X1 and X2, respectively. The photoluminescence spectra from the quantum well structures show similar emission bands except that both X1 and X2 shift to higher energy with decreasing quantum well thickness. The shift to higher energy is due to the effects of quantum confinement on carriers trapped at the stacking faults that intersect the quantum wells, as well as those excitons that are localized within the quantum wells. This assignment is based partly on excitation spectroscopy that revea...

Microstructural, optical, and electrical characterization of semipolar (112¯2) gallium nitride grown by epitaxial lateral overgrowth

Semipolar (112¯2) gallium nitride (GaN) films have been grown on m-plane (11¯00) sapphire by epitaxial lateral overgrowth. Transmission electron microscopy (TEM) studies show that the inclination of the [0001] axis at 32° from the film surface combined with the high [0001] growth rate under the reactor conditions used, allowed a low defect density (LDD) wing growing along [0001] to partially overgrow the highly defective window region and the other wing, resulting in a coalescence boundary, at which stacking faults and dislocations appear to terminate. Low temperature cathodoluminescence (CL) was performed to correlate the optical properties with the different stages of the growth process. It is found that emission from the LDD wing is dominated by near band edge recombination, whereas an emission band at 3.42 eV related to basal plane stacking faults and a broad band from 3.15–3.38 eV possibly related to emission from prismatic stacking faults and partial dislocations were observed in the window region. ...

Optical polarization anisotropy of a-plane GaN/AlGaN multiple quantum well structures grown on r-plane sapphire substrates

A series of nonpolar a-plane GaN/AlGaN multiple quantum well structures of varying quantum well width have been studied by polarization resolved photoluminescence and photoluminescence excitation spectroscopy at low temperature. The photoluminescence spectra from all the structures show two features that are observed to blueshift with reducing well width. The lower energy feature is associated with the recombination of carriers in regions of the wells intersected by basal-plane stacking faults, while the higher energy line is attributed to localized exciton recombination involving only the quantum wells. Using excitation spectroscopy with polarized light, we were able to resolve exciton features associated with both the |Y〉 and |Z〉 valence sub-bands. The observed polarization dependence of the transitions is consistent with a modification to the valence band-edge states due to anisotropic biaxial compressive strain in the quantum well. We were also able to determine the exciton binding energies directly from the photoluminescence excitation spectra, which were found to increase from 36 to 76 meV as the quantum well width reduced from 60 to 35 A.

Characterization of unintentional doping in nonpolar GaN

Unintentional doping in nonpolar a-plane (112¯0) gallium nitride (GaN) grown on r-plane (11¯02) sapphire using a three-dimensional (3D)–two-dimensional (2D) growth method has been characterized. For both 2D only and 3D–2D growth, the presence of an unintentionally doped region adjacent to the GaN/sapphire interface is observed by scanning capacitance microscopy (SCM). The average width of this unintentionally doped layer is found to increase with increasing 3D growth time. By using an intentionally doped GaN:Si staircase structure for calibration, it is shown that the unintentionally doped region has an average carrier concentration of (2.5±0.3)×1018 cm−3. SCM also reveals the presence of unintentionally doped features extending at 60° from the GaN/sapphire interface. The observation of decreasing carrier concentration with distance from the GaN/sapphire interface along these features may suggest that the unintentional doping arises from oxygen diffusion from the sapphire substrate. Low temperature cathod...

The effects of film surface roughness on x-ray diffraction of nonpolar gallium nitride films

Differences in film surface morphology are found to relate to differences in x-ray diffraction data from nonpolar GaN films. Local strain relaxation at an irregular surface can produce lattice plane bending, leading to surface-specific ω-scan broadening and hence abnormally broad peak widths from reflections associated with shallow x-ray penetration depths. Effects related to surface roughness also contribute to the observed rotational ω-scan width anisotropy. Williamson–Hall plots can be used to estimate to what degree surface roughness effects contribute to the overall ω-scan broadening.

Measuring dislocation densities in nonpolar a-plane GaN films using atomic force microscopy

Dislocation densities in nonpolar GaN films were determined using atomic force microscopy (AFM) by counting pits in the GaN surface revealed by a SiH4 surface treatment. This treatment increased the number of pits detected by a factor of 2 compared with the untreated surface. AFM and transmission electron microscopy analysis of a series of SiH4-treated calibration samples indicated that surface pit and dislocation densities corresponded well up to a dislocation density of ~6 ? 109?cm?2, above which surface pit overlap meant that dislocation densities determined by AFM were underestimated. For all samples with dislocation densities below 6 ? 109?cm?2, spatial analysis of the surface pit positions showed that dislocations typically accumulated in bands at island coalescence boundaries.

Characterisation of non-polar (11-20) gallium nitride using TEM techniques

Transmission electron microscopy (TEM) was used to characterise the microstructure of non-polar α-plane (11-20) GaN grown on (1-102) r-plane sapphire. Conventional TEM found that the microstructure of the non-polar GaN layers is dominated by basal plane and prismatic stacking faults (BSFs and PSFs, respectively). Partial dislocations (PDs) are found at the intersection of, and bounding, these stacking faults. The density of BSFs and PDs in the non-polar films were determined as 5x105 cm-1 and 4x1010cm-2, respectively. In cross section, the PD distribution was anisotropic. When viewed along the density of PDs appeared to decrease through the film thickness, which may be related to the initial growth conditions of the epilayer.