C. Trager-Cowan

Structural and optical properties of InGaN/GaN layers close to the critical layer thickness

In this work, we investigate structural and optical properties of metalorganic chemical vapor deposition grown wurtzite InxGa1−xN/GaN epitaxial layers with thicknesses that are close to the critical layer thickness (CLT) for strain relaxation. CLT for InxGa1−xN/GaN structures was calculated as a function of the InN content, x, using the energy balance model proposed by People and Bean [Appl. Phys. Lett. 47, 322 (1985)]. Experimentally determined CLT are in good agreement with these calculations. The occurrence of discontinuous strain relaxation (DSR), when the CLT is exceeded, is revealed in the case of a 120 nm thick In0.19Ga0.89N layer by x-ray reciprocal space mapping of an asymmetrical reflection. The effect of DSR on the luminescence of this layer is clear: The luminescence spectrum shows two peaks centered at ∼2.50 and ∼2.67 eV, respectively. These two components of the luminescence of the sample originate in regions of different strain, as discriminated by depth-resolving cathodoluminescence spectr...

Morphology of luminescent GaN films grown by molecular beam epitaxy

GaN thin films were grown by molecular beam epitaxy on sapphire substrates. Scanning electron (SE) and atomic force microscopies reveal that on a typical film an assembly of oriented hexagonal microcrystallites rises above a background of polycrystalline or amorphous material. Cathodoluminescence (CL) spectra of the films feature bright UV exciton peaks and a broad green emission band. We identify the exciton peaks as those of the wurtzite form of GaN. A comparison of SE and CL micrographs of the same sample area shows that the luminescence emanates almost entirely from the hexagonal crystallites.

Photoluminescence of wide bandgap II–VI superlattices

Abstract Photoluminescence (PL) and PL decay measurements are used to characterize typical samples of wide gap II–VI strained layer superlattices (SLSs). The results show that good quality material is obtained for SLSs of ZnSe/ZnS, CdSe/CdS (hex), ZnSe/CdSe and ZnS/CdS using atmospheric pressure MOCVD. Quantum confinement in SLSs is confirmed by the peak shift, temperature dependence and decay characteristics of the exciton luminescence. The PL intensity at low temperatures (

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.

Band alignments in Zn(Cd)S(Se) strained layer superlattices

The authors estimate, by reviewing the available photoluminescence data, the band alignments at heterojunctions of Zn(Cd)S(Se) semiconductors. The photoluminescence peak energies of ZnSe-ZnS, CdS-ZnS and CdSe-ZnSe strained layer superlattices (SLS), with a range of well and barrier widths, have been calculated using Kronig-Penney theory and assumed values of the band offsets which yield acceptable fits to both the experiments and data taken from the literature. For ZnSe-ZnS SLS, fair agreement is obtained between calculation and experiment by assuming, in line with previous work, a negligible conduction band offset. At the same time, good estimates of the photoluminescence peak energies of CdS-ZnS and CdSe-ZnSe superlattices are obtained by assuming a negligible valence band offset in each case. The authors construct a band alignment diagram which predicts that CdSe-CdS and CdS-ZnSe are type-II SLS with electron confinement in the CdS layers; these predictions are borne out by experiment.

The optical and structural properties of InGaN epilayers with very high indium content

Abstract We present the results of optical and structural investigations of InGaN epilayers grown by Metallorganic Vapour Phase Epitaxy (MOVPE). The peak energies of characteristic photoluminescence (PL) bands allow us to identify regions of crystal with different mean InN: (InN+GaN) fraction in the range from 0.1 to nearly 1 in selected samples. The PL peak energy and the optical absorption band edge are strongly intercorrelated, the Stokes’ shift and the Urbach tailing energy both increase with InN fraction. High-resolution energy dispersive X-ray analysis (EDX), coupled with scanning electron microscopy (SEM) and cathodoluminescence (CL) imaging, helps to establish striking microscale correlations between optical and structural properties. Finally, X-ray absorption fine structure (XAFS) at the In and Ga K-edges reveals characteristic local structure on the atomic scale for InGaN solid solutions over the available range of In:Ga composition ratios.

The optical properties of wide bandgap binary II–VI superlattices

Abstract Optical properties of binary Zn(Cd)S(Se) superlattices are reviewed. The absorption spectra of a range of ZnSe-ZnS strained layer superlattices (SLSs) are compared with the photoluminescence spectra to reveal the effects of disorder. Absorption line positions are accurately predicted in the envelope function approximation. The band offset diagram at heterojunctions in the Zn(Cd)S(Se) family of semiconductors is determined experimentally. The new SLS, CdS-ZnSe, is established as type-II. The effects of disorder on the temperature dependence and temporal decay of short-period common anion SLSs are quantified. Fluorescence line narrowing is reported in CdSe-ZnSe and CdS-ZnSe SLSs.

Multicharacterization approach for studying InAl(Ga)N/Al(Ga)N/GaN heterostructures for high electron mobility transistors

We report on our multi–pronged approach to understand the structural and electrical properties of an InAl(Ga)N(33nm barrier)/Al(Ga)N(1nm interlayer)/GaN(3μm)/ AlN(100nm)/Al2O3 high electron mobility transistor (HEMT) heterostructure grown by metal organic vapor phase epitaxy (MOVPE). In particular we reveal and discuss the role of unintentional Ga incorporation in the barrier and also in the interlayer. The observation of unintentional Ga incorporation by using energy dispersive X–ray spectroscopy analysis in a scanning transmission electron microscope is supported with results obtained for samples with a range of AlN interlayer thicknesses grown under both the showerhead as well as the horizontal type MOVPE reactors. Poisson–Schrodinger simulations show that for high Ga incorporation in the Al(Ga)N interlayer, an additional triangular well with very small depth may be exhibited in parallel to the main 2–DEG channel. The presence of this additional channel may cause parasitic conduction and severe issues in device characteristics and processing. Producing a HEMT structure with InAlGaN as the barrier and AlGaN as the interlayer with appropriate alloy composition may be a possible route to optimization, as it might be difficult to avoid Ga incorporation while continuously depositing the layers using the MOVPE growth method. Our present work shows the necessity of a multicharacterization approach to correlate structural and electrical properties to understand device structures and their performance.

Electron channeling contrast imaging studies of nonpolar nitrides using a scanning electron microscope

Threading dislocations, stacking faults, and associated partial dislocations significantly degrade the optical and electrical properties of materials such as non-polar III-nitride semiconductor thin films. Stacking faults are generally difficult to detect and quantify with existing characterization techniques. We demonstrate the use of electron channeling contrast imaging in the scanning electron microscope to non-destructively reveal basal plane stacking faults terminated by partial dislocations in m-plane GaN and InGaN/GaN multiple quantum well structures grown on γ-LiAlO2 by metal organic vapor phase epitaxy.

Alternative substrates for gallium nitride epitaxy: photoluminescence and morphological investigations

Abstract Gallium nitride films grown by molecular beam epitaxy (MBE) on (0001) sapphire, lithium gallate and gallium arsenide (111)B substrates have been characterized using atomic force microscopy and photoluminescence spectroscopy. Inhomogeneities in the sapphire-based material are further explored via fluorescence imaging. Layers grown on GaAs substrate are shown to display superior luminescence properties and excellent surface morphology.