H. P. D. Schenk

Luminescence and absorption in InGaN epitaxial layers and the van Roosbroeck–Shockley relation

Wurtzite InxGa1−xN (0.01≲x≲0.14) films have been grown by metalorganic vapor phase epitaxy on sapphire substrates. Integrated photoluminescence intensity and line shapes have been studied as functions of temperature and alloy composition x. We compare the “effective” InGaN band gap energy assessed by photothermal deflection spectroscopy with a “mean” band gap energy calculated from room temperature photoluminescence spectra utilizing the van Roosbroeck–Shockley relation and assuming a Gaussian energy dependence of the subband gap absorption coefficient. The Stokes’ shift between band gap energy and 300 K photoluminescence peak is explained by this model.Wurtzite InxGa1−xN (0.01≲x≲0.14) films have been grown by metalorganic vapor phase epitaxy on sapphire substrates. Integrated photoluminescence intensity and line shapes have been studied as functions of temperature and alloy composition x. We compare the “effective” InGaN band gap energy assessed by photothermal deflection spectroscopy with a “mean” band gap energy calculated from room temperature photoluminescence spectra utilizing the van Roosbroeck–Shockley relation and assuming a Gaussian energy dependence of the subband gap absorption coefficient. The Stokes’ shift between band gap energy and 300 K photoluminescence peak is explained by this model.

Indium incorporation above 800 °C during metalorganic vapor phase epitaxy of InGaN

We have studied the indium incorporation into InGaN ternary alloys during low-pressure metalorganic vapor-phase epitaxy as a function of the trimethylindium flow and the growth temperature in the 800–860 °C range. Partially relaxed InxGa1−xN bulk films with indium compositions 0.02≲x≲0.14 have been grown. In relation to the band-gap energy at room temperature, determined by photothermal deflection spectroscopy, we find a downward band-gap bowing of 2.65±0.15 eV. The required change of the trimethylindium flow as a function of the growth temperature, necessary to obtain isocomposition InGaN films, can be described by an Arrhenius law. We find an indium desorption energy of 0.8±0.3 eV.

Indium incorporation dynamics into AlInN ternary alloys for laser structures lattice matched to GaN

Al1−xInxN ternary alloys with solid phase indium compositions between x=0.15 and 0.28 have been grown by metal organic chemical vapor deposition under indium rich conditions within the growth temperature range of 750–810 °C. A thermally activated process with activation energy of 1.05±0.05eV is found to compete with indium incorporation. Smooth epitaxial layers with root mean-squares surface roughness of 0.3–0.8nm are obtained. (Al,In)N films lattice matched to GaN have been introduced into laser diode structures for optical confinement. Optical gain is observed.

Fast, high-efficiency, and homogeneous room-temperature cathodoluminescence of ZnO scintillator thin films on sapphire

Excitonic luminescence in ZnO exhibits subnanosecond lifetimes combined with high efficiency, which makes epitaxial ZnO a promising ultrafast scintillator material for envisaged imaging applications with high data rate. ZnO thin films on sapphire show external ultraviolet electron-photon conversion efficiencies up to 0.42photons∕(keVe−) at room temperature and only minor lateral microscopic cathodoluminescence intensity variations. Peak shifts and occasionally observed double peaks found in cathodoluminescence spectra of epitaxial ZnO films with dependence on the detection geometry, the excitation depth, and the surface morphology are explained by a model based on photon propagation including self-absorption.

In situ growth monitoring of distributed GaN–AlGaN Bragg reflectors by metalorganic vapor phase epitaxy

A series of distributed GaN-AlGaN Bragg reflectors (DBR) has been grown on Al2O3(0001) substrates by metalorganic vapor phase epitaxy. The growth of the GaN template as well as of the GaN–AlxGa1−xN quarter-wave stack has been monitored by laser reflectometry. The evolution of the in situ reflectivity as well as DBR reflection spectra are discussed as function of the AlxGa1−xN composition x.

Band gap narrowing and radiative efficiency of silicon doped GaN

Radiative efficiency, band gap narrowing, and band filling are studied in Si-doped GaN films as a function of carrier concentration (n), using room and low temperature cathodoluminescence (CL). Using the Kane model, a band gap narrowing ΔEg of −(3.6±0.6)×10−8 and −(2.6±0.6)×10−8n1/3 eVn1/3 is obtained for epitaxially strained and relaxed material, respectively. Band-edge CL time response and absolute external photon yield are measured. The internal radiation efficiency is deduced. Its monotonic increase as n increases is explained by the increase in the spontaneous radiative rate with a radiative free carrier band-to-band recombination coefficient B=(1.2±0.3)×10−11 cm3 s−1.

Vertical Cavity InGaN LEDs Grown by MOVPE

A RCLED structure, composed of InGaN multi-quantum wells inserted in a 3λ GaN based micro-cavity, was grown by metalorganic vapor phase epitaxy. The bottom mirror was a GaN/AIGaN distributed Bragg reflector (DBR). The growth process was followed by in-situ laser reflectometry, thus allowing an accurate control of the optical thickness (OT) of the various layers in the RCLED structure. We used the 543 nm line of a HeNe laser, and both the DBR and 3λ microcavity were designed at this wavelength. Taking into account the OT decrease after cooling (induced by the thermal lattice contraction and the index decrease), a 3λ cavity at 504 nm (green) was obtained. The emission of the InGaN QWs was found to be detuned with respect to the resonance wavelength. Nevertheless, we observed a resonance effect with an enhanced EL emission near the reflectance dip within the stop-band, the width of the EL peak being as low as 17 nm. The resonance effect was further confirmed by angle-resolved electroluminescence measurements. This work was developed within the framework of the European AGETHA project.

Acoustical and Optical Gallium Nitride Waveguides Grown on Si(111) by Metalorganic Vapor Phase Epitaxy

Optical and acoustical gallium nitride (GaN) waveguide structures, grown on Si(111) by metalorganic vapor phase epitaxy (MOVPE), are presented in this study. We discuss the influence of an underlying and an upper aluminum-gallium nitride (AlGaN) cladding layer, realized by short-period AlN/GaN superlattices, on the coupling of guided optical modes and on the propagation of surface acoustic waves (SAW).

Optoelectronic properties of GaN epilayers in the region of yellow luminescence

We studied a GaN epitaxial wafer grown by metal organic chemical vapor deposition, in which a lateral variation in the density of dislocations and associated defects was induced by a special preparation of the GaN buffer layer. Electron beam induced current and photocurrent measurements reveal lateral variations in the electrical properties of the GaN epilayer corresponding to the gradient in the defect density. The photocurrent spectra show four well distinct peaks separating the well known defect related yellow band in a blue, a green, a yellow, and a red component. In particular, we observe a strong dependence of the green component on the density of the a-type threading dislocations. There is evidence that the green and the yellow components are also significantly influenced by point defects.

Influence of the Mg precursor on the incorporation of Mg in MOVPE grown GaN.

Incorporation of Mg in metalorganic vapour phase epitaxy (MOVPE) GaN has been investigated, using two different Mg precursors: bis-methylcyclopentadienyl magnesium [(MeCp)2Mg] and Solution bis-cyclopentadienyl magnesium [Solution Cp2Mg]. SIMS analysis reveals an increased (two fold) efficiency of Mg incorporation for Solution Cp2Mg as compared to (MeCp)2Mg. These results are attributed to the stronger interaction of (MeCp)2Mg with NH3, leading to the formation of alkylmagnesium amine adducts, and a reduced effective Mg surface concentration. A decreased GaN growth rate with increasing Mg fluxes is also reported for both precursors. This effect is more pronounced for Solution Cp2Mg indicating that incorporation of Mg in the lattice proceeds via the capture of Mg into group III sites, and that the supply of Mg from the surface is reduced in the case when (MeCp)2Mg is used.