M.E. White

Structural analysis of InGaN epilayers

The structural properties of InGaN have attracted interest on account of the recent widespread use of the material in visible light-emitting devices. A key topic has been the indirect determination of the composition by x-ray diffraction (XRD). We examine critically the several levels of approximation involved in this procedure. It is shown by extended x-ray absorption fine structure (EXAFS) measurements that the local structure of InGaN is independent of the composition, in the range of InN fraction, from about 15 to 40%, that corresponds to blue to infrared light emission from this material. EXAFS-determined ratios of the numbers of indium and gallium atoms in the first metal co-ordination shell, M1, show very good agreement with the composition measured by established techniques, both structural and chemical, on similar samples. On the other hand, the atomic separations deviate markedly from values calculated using Vegards law. In particular, the average radial separations, In-N1=2.11(2) A and In-M1=3.28(3) A, do not vary significantly with In/Ga ratio in the examined composition range. We conclude with some brief comments on the uncertain but challenging topic of InGaN nanostructure.

The dependence of the optical energies on InGaN composition.

A wide-ranging experimental approach reveals a linear relationship between photoluminescence band peak energy and measured indium fraction for In[x]Ga[1] N epilayers with 0 < x < 0.40. We examine the dependence of the emission spectrum on composition using local measurements of the average indium content by Rutherford backscattering spectrometry, energy dispersive X-ray analysis, extended X-ray absorption fine structure and wavelength dispersed electron probe micro-analysis. Corresponding absorption and photoluminescence excitation data reveal the existence of a supplementary linear relationship between the optical bandgap and the indium fraction. Our observations provide definitive and conclusive evidence that the optical properties of InGaN do not conform to current theoretical descriptions of alloy band structure.

Extended X-Ray Absorption Fine Structure (EXAFS) of InN and InGaN

We present extended X-ray absorption fine structure (EXAFS) spectra of sputtered indium nitride films. Disorder in the In local environment has been analysed with the aid of a two-shell fit to data for samples grown at three different substrate temperatures. An excellent fit to a model comprising the first five shells of neighbours is obtained for the best sample. The results of this study aid a reinterpretation of EXAFS data on a set of seven InGaN layers, grown by metallorganic chemical vapour deposition (MOCVD), with a wide range of indium content. In addition, we measured the fundamental bandgaps and Urbach tailing parameters of both sets of samples by optical absorption spectroscopy and attempt to relate structure and composition to the optical properties of the films.

Splitting of X-ray diffraction and photoluminescence peaks in InGaN/GaN layers

The observation of multiple, X-ray diffraction (XRD) and photoluminescence (PL) peaks in an InGaN epilayer is sometimes regarded as an indicator of phase segregation. In this report, a detailed characterisation of a InGaN/GaN bilayer by a combination of XRD and Rutherford backscattering spectrometry (RBS) shows that splitting of the XRD peak may occur in the absence of phase decomposition. An XRD reciprocal space map performed on the (105) plane shows that one component of the partially resolved InGaN double peak is almost aligned with that of the GaN buffer, indicating that part of the layer is pseudomorphic to the GaN template. From a consideration of the effect of strain on the c- and a-lattice constants, both the partially relaxed and the pseudomorphic components are shown to have the same indium content. The layer composition deduced from XRD measurements is confirmed by RBS. Depth-resolving RBS/channelling angular scans also shows that the region closer to the GaN/InGaN interface is nearly pseudomorphic to the GaN substrate, whereas the surface region is almost fully relaxed. Furthermore, PL spectroscopy shows a double peak that can be accounted for by regions of the sample under different strains.

Photoluminescence excitation spectroscopy of InGaN epilayers

Photoluminescence (PL) has been reported from InGaN-based heterostructures, including thick epilayers on GaN, InGaN/GaN quantum wells and InGaN/GaN quantum boxes, with peak energies ranging from 3.44 to 1.31 eV at low temperature. The corresponding absorption spectra are not always easy to obtain, but photoluminescence excitation (PLE) spectroscopy provides an efficient means of obtaining comparable information. We describe here a comprehensive investigation of PLE spectra from a wide range of InGaN samples. Variation of the measured bandgap energy with the detection energy for individual samples suggests that the InGaN emission spectrum is inhomogeneously broadened. The PLE spectrum obtained at the peak emission energy of a particular sample is equivalent to the absorption spectrum of that sample. The data range of the band edge measurements is extended to lower energies by the PLE results. In general, the PLE data confirm the existence of a linear relationship between the optical bandgap and the emission energy.

Spectroscopic Imaging of InGaN Epilayers

We review several spectroscopic imaging techniques, with progressively higher spatial resolution, applied to InGaN epilayers and quantum wells (QW). The techniques discussed are: photoluminescence (PL) mapping, confocal laser scanning spectroscopy, and cathodoluminescence imaging. PL mapping (via point-by-point PL spectroscopy) is necessary when samples show macroscopic inhomogeneity. Confocal microscopy uses a diffraction-limited laser spot to address a sample in a raster scan. Here we report, for the first time, observations, on a microscopic scale, of spectroscopic fine structure in InGaN layers of moderate indium content. Cathodoluminescence (CL) imaging provides a vast amount of information on thin semiconductor layers that are well-matched to the penetration profile of electron beams of modest energies (from ≈1 to 60 keV). Panchromatic images provide useful information when compared with corresponding structural images produced by electron scattering in the sanning electron microscope (SEM). Application of image processing techniques to such images reveal striking correspondences on a size-scale below one micron.

Photoluminescence excitation spectroscopy of MBE grown InGaN quantum wells and quantum boxes

Photoluminescence excitation (PLE) spectroscopy was carried out to investigate the excitation/emission cycle of MBE grown InGaN quantum structures. Quantum well and Stranski-Krastanov type quantum box samples were chosen that emit from blue to red. The bandgap energy (E-g), determined from the PLE spectrum, was found to decrease concurrently with the detection energy. This indicates that the emission spectrum from the sample is inhomogeneously broadened. A plot of the resultant Stokes shift against detection energy shows a linear trend. Our results agree with those from independent measurements of thermally detected optical absorption (TDOA) in some of the samples. On comparison with absorption and PLE measurements on MOCVD grown InGaN. a difference in the bandgap energies obtained becomes apparent for detection energies below approximate to 2.6 eV.

Luminescence and structural properties of InGaN epilayer, quantum well and quantum dot samples using synchrotron radiation

The Daresbury synchrotron radiation source (SRS) provides bright, tunable x-rays for scattering and absorption probes of local structure. Scanning confocal microscopy and luminescence decay measurements employ the SRS in alternative ways, as a tunable luminescence excitation engine and as a source of weak, 160 ps pulses with a large pulse-topulse separation, respectively. This report first describes local atomic structure studies of InGaN epilayers by extended x-ray absorption fine structure (EXAFS). In addition, we report photoluminescence (PL) imaging, PL microspectroscopy and photoluminescence decay studies of various nitride samples, including tailored InGaN quantum wells and discs.

Comparative study of structural properties and photoluminescence in InGaN layers with a high In content

Rutherford backscattering and channeling spectrometry (RBS), photoluminescence (PL) spectroscopy and transmission electron microscopy (TEM) have been used to investigate macroscopic and microscopic segregation in MOCVD grown InGaN layers. The PL peak energy and In content (measured by RES) were mapped at a large number of distinct points on the samples. An indium concentration of 40%, the highest measured in this work, corresponds to a PL peak of 710 nn strongly suggesting that the light-emitting regions of the sample me very indium-rich compared to the average measured by RES. Cross-sectional TEM observations show distinctive layering of the InGaN films. The TEM study further reveals that these layers consist of amorphous pyramidal contrast features with sizes of order 10 nm The composition of these specific contrast features is shown to be In-rich compared to the nitride matrix.