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The pseudodielectric function spectra <ε> of wurtzite GaN, AlGaN, and GaN/AlGaN heterostructures were determined for photon energies ranging from 2 to 5 eV, using variable angle spectroscopic ellipsometry (SE). Samples were grown by low-pressure metalorganic chemical vapor deposition on c-plane sapphire substrates. The experimental <ε> spectra were analyzed using a multilayer approach, describing the dielectric functions of the individual layers by a parametric oscillator model. In this way, parametric dielectric function spectra of GaN and Al[sub x]Ga[sub 1-x]N (x≤0.16) were derived, as well as the composition dependence of the Al[sub x]Ga[sub 1-x]N band gap energy. The SE band gap data were found to be consistent with a bowing parameter close to 1 eV. Finally, the GaN and Al[sub x]Ga[sub 1-x]N parametric dielectric functions were used to quantitatively analyze the pseudodielectric function spectrum of GaN/AlGaN modulation doped field effect transistor structures, demonstrating the potential of SE in combination with a multilayer parametric dielectric function model for nondestructive ex situ control of GaN/AlGaN device structures. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An extension of Bond''s method for determining precision lattice parameters serves to determine the lattice parameters of (strained) unit cells of a film from the peak positions of the film alone. We call this measurement and evaluation procedure the ‘Extended Bond Method’ (EBM). The procedure avoids recurrence to possibly unreliable or unavailable lattice parameters of the substrate; it is successful, irrespective of whether a layer is pseudomorphically strained, partially relaxed, or completely relaxed, as long as the strain can be described as a uniaxial distortion parallel to the growth direction. Using a mathematical/graphical evaluation procedure, the chemical composition of a layer is obtained, once its strained lattice parameters have been determined. The technique is of particular value for the determination of the strain and the composition of group-III nitride layers. [Copyright &y& Elsevier]

We report on the interaction between photogenerated electron-hole pairs and surface acoustic waves (SAW) in GaN films grown on sapphire substrates. The spatial separation of photogenerated carriers by the piezoelectric field of the SAW is evidenced by the quenching of the photoluminescence (PL) intensity. The quenching levels in GaN are significantly smaller than those measured in GaAs under similar conditions. The latter is attributed to the lower exciton ionization efficiency and carrier separation probabilities mediated by the piezoelectric effect. The PL spectra also evidence energy shifts and broadenings of the electronic transitions, which are attributed to the band gap modulation by the SAW strain field. [ABSTRACT FROM AUTHOR]

Abstract: We report on a general method to fabricate transition metal related defects in diamond. Controlled incorporation of Mo and W in synthetic CVD diamond was achieved by adding volatile metal precursors to the diamond chemical vapor deposition (CVD) growth process. Effects of deposition temperature, grain structure and precursor exposure on the incorporation efficiency were systematically studied, and doping levels of up to 0.25at.% have been achieved. The metal atoms are uniformly distributed throughout the diamond grains without any indication of inclusion formation. These results are discussed in context of the kinetically controlled growth process of CVD diamond. [Copyright &y& Elsevier]

We will highlight our recent work on the properties of residual defects and dopants in GaN heteroepitaxial layers and on the nature of recombination from InGaN single quantum well (SQW) light emitting diodes (LEDs) through magnetic resonance techniques. Electron paramagnetic resonance (EPR) and optically detected magnetic resonance (ODMR) were performed on undoped (highly resistive and n-type) and intentionally doped (Si, Mg, or Be) GaN films grown by a variety of techniques (MOCVD, MBE, and HVPE) in order to obtain general trends and behavior. Through the spin-Hamiltonian parameters, these methods can reveal symmetry information, the character of the wave function and (ideally) the chemical identity of the defect. In addition, low temperature EPR intensities can be used to determine the neutral acceptor or donor concentrations without the need for contacts or the high temperatures required for Hall effect measurements. The ODMR was performed on both bandedge (mainly shallow donor–shallow acceptor recombination) and deep (visible and near-IR) PL bands. In spite of the radically different (non-equilibrium) growth techniques, many of the same defects were found in the various samples. Finally, earlier ODMR studies of recombination from Nichia InGaN ‘green’ and ‘blue’ LEDs were extended to include shorter (‘violet’) and longer (‘amber’) wavelength LEDs and an undoped 30 A˚ In0.3Ga0.7N/GaN heterostructure. The results provide evidence for spatially separated electrons and holes in the optically-active 30 A˚ InGaN layers under low photoexcitation conditions, likely due to localization at different potential minima in the x–y planes and/or the large strain-induced piezoelectric fields parallel to the growth direction. [Copyright &y& Elsevier]

We study the influence of heteroepitaxy as well as intrinsic anharmonicity of the E2(high) and A1(LO) lattice vibrations in wurtzite GaN layers. The intrinsic phonon–phonon scattering of bulk GaN determines the temperature dependence of the width of the E2(high) and A1(LO) Raman lines of GaN layers. The Gru¨neisen contribution of the GaN layer, which is strongly influenced by the thermal expansion coefficient of the substrate, dominates the temperature dependence of the phonon frequencies. [Copyright &y& Elsevier]

Wurtzite GaN layers are commonly grown heteroepitaxially on 6H-SiC or Al[sub 2]O[sub 3] substrates, because of the lack of lattice-matched substrates. We study the influence of these substrates mainly on the E[sub 2](high)-phonon Raman line by temperature dependent Raman spectroscopy. We find that the line broadening with sample heating is predominantly caused by intrinsic phonon-phonon scattering in GaN. The small three-phonon contribution as well as the small intrinsic linewidth at low temperature are due to the rather low two-phonon density of states at the E[sub 2](high)-phonon energy. Substrates with large lattice mismatch cause inhomogeneous strain and defects in the layers, which lead to a large, temperature independent, line broadening. We show that the temperature shift of the E[sub 2](high)-phonon frequency is dominated by the GaN lattice expansion. The lattice of epilayers is strongly modified by the thermal in-plane expansion of the substrate. The degree of relaxation at the growth temperature is reflected by deviation of the E[sub 2](high)-line from the intrinsic phonon frequency. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In[sub 0.13]Ga[sub 0.87]N/GaN double heterostructures and quantum wells (QWs) have been studied by room-temperature photoluminescence (PL) and spectroscopic ellipsometry (SE). PL revealed the presence of strong piezoelectric fields, which strongly influence the luminescence properties for InGaN QW widths in the 3-12 nm range. The pseudodielectric function spectrum derived from the SE measurements were analyzed using a multilayer approach, describing the dielectric function of the individual layers by a parametric oscillator model. The fundamental band-gap resonance in the InGaN dielectric-function spectrum was found to broaden for an InGaN layer width of 12 nm, as compared to bulk-like InGaN layers, due to piezoelectric field effects. For a much narrower QW width of 1.7 nm, however, quantum confinement was found to dominate over piezoelectric-field effects, resulting in a much sharper band-gap resonance shifted to higher energies and an increased oscillator strength. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Resonant Raman scattering has been used to study hexagonal In[sub x]Ga[sub 1-x]N films with xapprox. 0.1, grown by metal–organic chemical vapor deposition on sapphire substrates. To vary the energy difference between the fundamental gap energy of the (InGa)N and the photon energy of the discrete laser emission lines used for recording the spectra, the sample temperature was varied between 300 and 870 K. Raman scattering by the (InGa)N A[sub 1](LO) phonon shows a clear resonance profile when the fundamental energy gap approaches the incident photon energy, with a maximum enhancement in scattering efficiency of 10 measured relative to the scattering strength of the E[sub 2] phonon mode. The (InGa)N A[sub 1](LO) phonon was found to shift to higher frequencies with respect to the E[sub 2] mode when the experimental conditions were varied from excitation below the fundamental energy gap of (InGa)N to above-band-gap excitation. This frequency shift is explained by the presence of compositional inhomogeneity, which results in localized regions with higher In content, and thus, lower gap energy and phonon frequency, and regions with lower In content, and consequently, higher gap energy and phonon frequency. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The blue Mg induced 2.8 eV photoluminescence (PL) band in metalorganic chemical vapor deposition grown GaN has been studied in a large number of samples with varying Mg content. It emerges near a Mg concentration of 1×10[sup 19] cm[sup -3] and at higher concentrations dominates the room temperature PL spectrum. The excitation power dependence of the 2.8 eV band provides convincing evidence for its donor–acceptor (D–A) pair recombination character. It is suggested that the acceptor A is isolated Mg[sub Ga] while the spatially separated, deep donor (430 meV) D is attributed to a nearest-neighbor associate of a Mg[sub Ga] acceptor with a nitrogen vacancy, formed by self-compensation. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Pseudodielectric function spectra of hexagonal (InGa)N epitaxial layers on GaN were obtained by spectroscopic ellipsometry and compared with photoreflection spectra. Composition and thickness of the In[sub x]Ga[sub 1-x]N layers grown by metalorganic chemical vapor deposition, were varied between 0.04≤x≤0.10 and 15–60 nm, respectively. The pseudodielectric function exhibits a clear maximum at the fundamental gap energy of the (InGa)N, which allows a determination of the In content via the composition dependence of that gap energy. The pseudodielectric function spectrum of a complete GaN/(InGa)N/(AlGa)N/GaN light-emitting diode structure shows maxima arising from fundamental gap interband transitions of all constituent layers including the (InGa)N active region. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]