Optical energy gaps are measured for high-quality Al 1−x In x N-on-GaN epilayers with a range of compositions around the lattice match point using photoluminescence and photoluminescence excitation spectroscopy. These data are combined with structural data to determine the compositional dependence of emission and absorption energies. The trend indicates a very large bowing parameter of Ϸ6 eV and differences with earlier reports are discussed. Very large Stokes' shifts of 0.4-0.8 eV are observed in the composition range 0.13Ͻ x Ͻ 0.24, increasing approximately linearly with InN fraction despite the change of sign of the piezoelectric field.
We have studied the structural and optical properties of In x Al 1−x N alloys with compositions nearly lattice-matched to GaN. Scanning electron microscopy measurements reveals a good overall surface quality, with some defect structures distributed across the surface whose density increases with the InN concentration. On the other hand, Raman scattering experiments show three peaks in the frequency range between 500 and 900 cm −1 , which have been assigned to InN-like and AlN-like E 2 modes and A 1 (LO) mode of the In x Al 1−x N. These results agree with theoretical calculations previously reported where two-mode and one-mode behavior was predicted for the E 2 and A 1 (LO) modes, respectively. Photoluminescence and photoluminescence excitation allowed us to determine the emission and absorption energies of the In x Al 1−x N epilayers. Both energies display a redshift as the InN fraction increases. We find a roughly linear increase of the Stokes shift with InN fraction, with Stokes shift values of ≈ 0.5 eV in the composition range close to the lattice-matched condition.
PACS 68.37. Hk, 78.55.Cr, 78.66.Fd Using wavelength dispersive X-ray spectrometers on an Electron Probe Micro-Analyser we have accurately quantified the elemental composition of a series of homogeneous AlInGaN epitaxial layers. The thickness of the quaternary layer (~100 nm) necessitates the combination of data measured at a number of different electron beam energies and an analytical model based on a layered structure. The samples studied have aluminium fractions in the range 0.03-0.12 and indium fractions in the region of 0.01. Photoluminescence data from the samples are used to plot the dependency of the luminescence energy, linewidth and intensity on the composition. WDX mapping was employed to investigate spatial variations in the elemental compositions and the films were found to be uniform with no evidence for clustering of In or Al on a >100 nm scale. IntroductionThe ternary nitride semiconductors InGaN and AlGaN have proved to be highly successful active layers in GaN-based devices, including light emitters and transistors. However, lattice mismatch in ternary heterostructures imposes serious limitations for the design and operation of certain devices. In addition the efficiency of InGaN light emitters is severely degraded by the incorporation of too much, or too little, indium. Use of quaternary AlInGaN layers offers potential for the fabrication of lattice matched III-N heterostructures and improvement of the quantum efficiency of light emitters [1,2], particularly in the UV region with its host of important applications. The growth of high quality AlInGaN layers requires a balance between the high temperatures suited for Al-containing layers and the low temperatures necessary for incorporation of indium. Measuring the composition of AlInGaN layers using standard X-ray diffraction is complicated by the lack of a unique solution for the quaternary system and further by overlapping diffraction peaks in the case of lattice-match to the underlying GaN. In this paper we describe the compositional analysis of a series of AlInGaN layers using an Electron Probe Micro-Analyser (EPMA) equipped with wavelength dispersive X-ray (WDX) spectrometers, allowing the elemental composition to be determined with high accuracy and sub-micron spatial resolution. The compositional data are then correlated with the light emitting properties of the layers.
Investigation of the depth profiles and luminescence of Eu and Er-ions implanted into AlInN/GaN bilayers differentiates between ions located in the two different III-N hosts. Differences between samples implanted using channeling or off-axis geometries are studied using time-of-flight secondary ion mass spectometry. A fraction of ions have crossed the AlInN layer (either 130 or 250 nm thick) and reached the underlying GaN. Cathodoluminescence spectra as a function of incident electron energy and photoluminescence excitation data distinguish between ions within AlInN and GaN. The RE emission from the AlInN is broader and red-shifted and the dependence of the intensity on host is discussed.
A direct comparison of the optical energies of MBE-and MOVPE-grown In x Ga 1-x N epilayers of similar InN content is performed for the first time. The InN fraction in the 7 MBE samples examined ranged from x ~ 0.11 to x ~ 0.35 while the range in available MOVPE epilayers is [0, 0.4]. Wavelength Dispersive X-ray (WDX) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopies were used to measure composition and local structure (alloy character) of the samples. Cathodoluminescence (CL) spectroscopy in situ, ex situ photoluminescence (PL) mapping and large-area optical absorption spectroscopy were used to measure various optical energies. The composition dependence of the optical energies is determined by the growth method. The absorption bandgap and luminescence peak energies vary linearly with x for both growth methods, suggesting a near-zero value of the bowing parameter. But the energy intercept at zero InN content in MOVPE samples is close to the wurtzite-GaN bandgap of 3.4 eV at room temperature, as expected, while the equivalent for MBE samples falls near 3.2 eV.
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