J. Off

The role of piezoelectric fields in GaN-based quantum wells

In this contribution, we focus on the consequences of the piezoelectric field, which is an inherent consequence of the commonly used wurtzite phase of GaN, on the optical properties of strained GaN-based quantum well structures. We demonstrate that both in GaN/AlGaN and in GaInN/GaN single quantum well structures, the piezoelectric field leads to a Stark-shift of the fundamental optical transitions, which can lead to luminescence emission far below the bulk bandgap. Due to the spatial separation of the electron and hole wavefunctions in such structures, the oscillator strength of these transitions may become extremely small, many orders of magnitude lower than in the field-free case. From specially designed structures, we can even determine the sign of the piezoelectric field and relate it to the polarity of the layers. Under high-excitation conditions, as found in a laser diode, the piezoelectric field is almost completely screened by the injected carriers. As a consequence, the stimulated emission is significantly blue-shifted compared to the photoluminescence, which has sometimes been confused with localization effects.

Intra- and interwell transitions in GaInN/GaN multiple quantum wells with built-in piezoelectric fields

We have studied optical transitions in GaInN/GaN single and multiple quantum wells using time-resolved photoluminescence spectroscopy. Our results show that the energy positions of the dominant emission lines strongly depend both on the well width and on the number of wells. In the case of multiple quantum wells, time-resolved measurements clearly distinguish multiple emission lines. These observations are consistently explained by considering the large built-in piezoelectric field in strained GaInN quantum wells. The multiple emission lines are attributed to intra- and interwell transitions between nearest and next-nearest neighbors.

Characterization of InGaN thin films using high-resolution x-ray diffraction

Wurtzite InGaN thin films grown by metalorganic chemical vapor deposition on sapphire substrates with and without GaN buffer layers are investigated by high-resolution x-ray diffraction measurements. The structural quality, lattice constants, strain, and indium composition of 100 nm thick films with In concentrations up to 33% are evaluated by measuring symmetric (00.2) and asymmetric (20.5) reflexes. The quality of the InGaN layers with widely different biaxial stress is measured and compared. An analytical solution for the determination of the In content of strained epitaxial layers is introduced. The results show that neglecting the strain can result in a severe miscalculation of the In concentration.

Optical phonons and free-carrier effects in MOVPE grown Al xGa 1-xN measured by infrared ellipsometry

We report on the application of infrared spectroscopic ellipsometry (IR-SE) for wavenumbers from 333cm −1 to 1200cm −1 as a novel approach to non-destructive optical characterization of free-carrier and optical phonon properties of group III-nitride heterostructures. Undoped α -GaN, α -AlN, α -Al x Ga 1− x N ( x = 0.17, 0.28, 0.5), and n -type silicon (Si) doped α -GaN layers were grown by metal-organic vapor phase epitaxy (MOVPE) on c -plane sapphire ( α -Al 2 O 3 ). The four-parameter semi-quantum (FPSQ) dielectric lattice-dispersion model and the Drude model for free-carrier response are employed for analysis of the IR-SE data. Model calculations for the ordinary ( ∈ ⊥ ) and extraordinary ( ∈ || ) dielectric functions of the heterostructure components provide sensitivity to IR-active phonon frequencies and free-carrier parameters. We observe that the α -Al x Ga 1− x N layers are unintentionally doped with a back ground free-carrier concentration of 1–4 10 18 cm −3 . The ternary compounds reveal a two-mode behavior in ∈ ⊥ , whereas a one-mode behavior is sufficient to explain the optical response for ∈ || . We further provide a precise set of model parameters for calculation of the sapphire infrared dielectric functions which are prerequisites for analysis of infrared spectra of III-nitride heterostructures grown on α -Al 2 O 3 .

Diffusion length of photoexcited carriers in GaN

When additional carriers are introduced in a material with a non uniform concentration, they tend to diffuse on a scale given by their diffusion length. This parameter can be measured by different methods. Depending on the conditions, different values can be found as the recombination mechanisms differ. In this paper, we present the situation in GaN with various experiments including the photocarrier grating method, photoluminescence and the spectral response in photoconductors. We show that the diffusion length varies from 0.1 μm to a few μm depending on experimental conditions. The interpretation is given based on the diffusion equations and on the analysis of the recombinations.

Optical Properties of Nitride Quantum Wells: How to Separate Fluctuations and Polarization Field Effects

Both the large linewidth of optical transitions as well as the sizeable “Stokes” shift seem to suggest strong localization effects in nitride quantum wells. In contrast, we clearly show that the “Stokes” shift can be easily understood in terms of the huge piezoelectric fields present in such structures. At the same time, those fields lead to a characteristic dependence of the linewidth on well width, which we use to quantitatively separate fluctuation and field effects. We find that in high-quality quantum wells, the relative compositional fluctuation is less than 10% and the well width fluctuation is of the order 1 monolayer.

In incorporation efficiency and composition fluctuations in MOVPE grown GaInN/GaN hetero structures and quantum wells

Abstract GaInN layers play a key role in short wavelength optoelectronic devices for the visible spectrum. However, the epitaxial growth of In containing nitrides is more problematic than that of GaN and AlGaN. In order to increase the In incorporation efficiency, lower growth temperatures of around 700–800°C are needed. We have optimized the metalorganic vapor-phase epitaxial growth of GaInN by decreasing the H2/N2 ratio in the gas-phase and increasing the growth rate. However, the deposited films showed strong indications for compositional fluctuations. Besides a large miscibility gap predicted for GaInN, the mismatch induced strain for GaN may play a major role in these growth problems.

Effects of Piezoelectric Fields in GaInN/GaN and GaN/AlGaN Heterostructures and Quantum Wells

The effects of piezoelectric fields on the static and dynamic optical properties of GaInN/GaN and GaN/AIGaN double heterostructures and single quantum wells are studied by time-resolved photoluminescence. We find a strong increase of the luminescence decay time of the dominating transition with well thickness by several orders of magnitude. For well thicknesses larger than about 5 nm, two emission lines with strongly differing decay times are observed, which are attributed to spatially direct and indirect transitions. Our experimental findings are consistently explained by a quantitative model based on the piezoelectric fields in strained wurtzite quantum wells.

Influence of strain and buffer layer type on In incorporation during GaInN MOVPE

Abstract We have studied the growth behaviour of GaN by metalorganic vapor phase epitaxy on various buffer layers. GaInN grown on high quality GaN buffers is coherently strained up to a thickness of ∼100 nm. When grown on AlGaN, the strain remains unchanged up to a critical Al content, above which relaxation was observed. In parallel, the In content increased significantly. When grown on lower quality GaN, the strain in our GaInN layers also relaxed and the In content increased. Therefore, we suggested a relation between In incorporation efficiency and defect structure on the growing epilayer surface.

GaInN/GaN-Heterostructures and Quantum Wells Grown by Metalorganic Vapor-Phase Epitaxy

The dependence of the In-incorporation efficiency and the optical properties of MOVPE-grown GaInN/GaN-heterostructures on various growth parameters has been investigated. A significant improvement of the In-incorporation rate could be obtained by increasing the growth rate and reducing the H 2 -partial pressure in the MOVPE reactor. However, GaInN layers with a high In-content typically show an additional low energy photoluminescence peak, whose distance to the band-edge increases with increasing In-content. For GaInN/GaN quantum wells with an In-content of approximately 12%, an increase of the well thickness is accompanied by a significant line broadening and a large increase of the Stokes shift between the emission peak and the band edge determined by photothermal deflection spectroscopy. With a further increase of the thickness of the GaInN layer, a second GaInN-correlated emission peak emerges. To elucidate the nature of these optical transitions, power-dependent as well as time-resolved photoluminescence measurements have been performed and compared to the results of scanning transmission electron microscopy.