Compact solid-state lamps based on light-emitting diodes (LEDs) are of current technological interest as an alternative to conventional light bulbs The brightest LEDs available so far emit red light and exhibit higher luminous efficiency than fluorescent lamps If this luminous efficiency could be transferred to white LEDs, power consumption would be dramatically reduced, with great economic and ecological consequences But the luminous efficiency of existing white LEDs is still very low, owing to the presence of electrostatic fields within the active layers These fields are generated by the spontaneous and piezoelectric polarization along the [0001] axis of hexagonal group-III nitrides--the commonly used materials for light generation Unfortunately, as this crystallographic orientation corresponds to the natural growth direction of these materials deposited on currently available substrates Here we demonstrate that the epitaxial growth of GaN/(Al,Ga)N on tetragonal LiAlO2 in a non-polar direction allows the fabrication of structures free of electrostatic fields, resulting in an improved quantum efficiency We expect that this approach will pave the way towards highly efficient white LEDs

Nitride semiconductors free of electrostatic fields for efficient white light-emitting diodes

Instabilities in semiconductor heterostructure growth can be exploited for the self-organized formation of nanostructures, allowing for carrier confinement in all three spatial dimensions. Beside the description of various growth modes, the experimental characterization of structural properties, such as size and shape, chemical composition, and strain distribution is presented. The authors discuss the calculation of strain fields, which play an important role in the formation of such nanostructures and also influence their structural and optoelectronic properties. Several specific materials systems are surveyed together with important applications.

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Structural properties of self-organized semiconductor nanostructures

We provide explicit rules to calculate the nonlinear polarization for nitride alloys of arbitrary composition, and hence, the bound sheet charge induced by polarization discontinuity at the interfaces between different alloy and binary (epi)layers. We then present experimental results and simulations of polarization-related quantities in selected nitride-alloy-based heterostructure systems. The agreement of experiment and simulation, also in comparison to previous approaches, strongly suggests that the macroscopic polarization of nitride alloys is indeed nonlinear as a function of composition.

Evidence for nonlinear macroscopic polarization in III-V nitride alloy heterostructures

This paper presents growth orientation dependence of the piezoelectric polarization of InxGa1−xN and AlyGa1−yN layers lattice matched to GaN. This topic has become relevant with the advent of growing nitride based devices on semipolar planes [A. Chakraborty et al., Jpn. J. Appl. Phys., Part 2 44, L945 (2005)]. The calculations demonstrate that for strained InxGa1−xN and AlyGa1−yN layers lattice matched to GaN, the piezoelectric polarization becomes zero for nonpolar orientations and also at another point ≈45° tilted from the c plane. The zero crossover has only a very small dependence on the In or Al content of the ternary alloy layer. With the addition of spontaneous polarization, the angle at which the total polarization equals zero increases slightly for InxGa1−xN, but the exact value depends on the In content. For AlyGa1−yN mismatched layers the effect of spontaneous polarization becomes important by increasing the crossover point to ∼70° from c-axis oriented films. These calculations were performed u...

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Strain-induced polarization in wurtzite III-nitride semipolar layers

In this letter we describe the structural characteristics of nonpolar (1120) a-plane GaN thin films grown on (1102) r-plane sapphire substrates via metalorganic chemical vapor deposition. Planar growth surfaces have been achieved and the potential for device-quality layers realized by depositing a low temperature nucleation layer prior to high temperature epitaxial growth. The in-plane orientation of the GaN with respect to the r-plane sapphire substrate was confirmed to be [0001]GaN‖[1101]sapphire and [1100]GaN‖[1120]sapphire. This relationship is explicitly defined since the polarity of the a-GaN films was determined using convergent beam electron diffraction. Threading dislocations and stacking faults, observed in plan-view and cross-sectional transmission electron microscope images, dominated the a-GaN microstructure with densities of 2.6×1010 cm−2 and 3.8×105 cm−1, respectively. Submicron pits and crystallographic terraces were observed on the optically specular a-GaN surface with atomic force m...

Structural characterization of nonpolar (112̄0) a-plane GaN thin films grown on (11̄02) r-plane sapphire

It is shown that the optical properties of GaN quantum dots with the wurtzite structure result from a balance between confinement and piezoelectric effects. In ``large'' quantum dots with an average height and diameter of 4.1 and 17 nm, respectively, the photoluminescence peak is centered at 2.95 eV, nearly 0.5 eV below the bulk GaN band gap. We attribute this enormous redshift to a giant 5.5 MV/cm piezoelectric field present in the dots, in agreement with theoretical calculations.

Blue-light emission from GaN self-assembled quantum dots due to giant piezoelectric effect

We demonstrate that, even in unstrained GaN quantum wells with AlGaN barriers, there exist giant electric fields as high as 1.5 MV/cm. These fields, resulting from the interplay of the piezoelectric and spontaneous polarizations in the well and barrier layers due to Fermi level alignment, induce large redshifts of the photoluminescence energy position and dramatically increase the carrier lifetime as the quantum well thickness increases.

Giant electric fields in unstrained GaN single quantum wells

Self-assembled InGaN islands were grown by molecular-beam epitaxy on GaN, following a Stranski–Krastanow growth mode. Atomic force microscopy revealed that their dimensions were small enough to expect zero-dimensional quantum effects: the islands were typically 27 nm wide and 2.9 nm high. Strong blue-violet photoluminescence of the dots is observed, persisting up to room temperature. The temperature dependence of the photoluminescence is analyzed and compared to that of InGaN quantum well and bulk samples.

Self-assembled InGaN quantum dots grown by molecular-beam epitaxy

We report on time-integrated and -resolved photoluminescence (PL) data on self-assembled GaN quantum dots (QD's) embedded in AlN, in both cubic [zinc-blende (ZB)] and hexagonal [wurtzite (Wz)] crystallographic phases. The comparison of the optical properties of ZB and Wz nitride QD's allows us to distinguish pure dimensionality effects from the influence of the large polarization-induced electric fields present in the Wz nanostructures. Specifically, the PL energy of the ZB QD's is always higher than the bulk cubic GaN band-gap energy, in contrast to the Wz QD's where a 7-MV/cm polarization field gives rise to below-gap PL emission for sufficiently large QD's. As a further consequence of the internal field, the low-temperature PL decay times of the Wz QD's are significantly longer than the ZB ones, and increase strongly with the QD height in contrast to the ZB ones, which exhibit only a small size dependence. For both types of QD's, the PL intensity is found to be weakly dependent on temperature, underscoring the strong zero-dimensional exciton localization in the GaN/AlN system. In spite of the strong localization, however, we show that the nonradiative channels cannot be neglected and have a significant contribution in the PL decay time for both ZB and Wz QD's.

Direct comparison of recombination dynamics in cubic and hexagonal GaN/AlN quantum dots

Zinc blende (ZB) GaN quantum dots have been grown by plasma-assisted molecular-beam epitaxy on AlN buffer layers using 3C-SiC(001) substrates. The two- to three-dimensional growth mode transition is studied by following the evolution of the reflection high-energy electron diffraction pattern. ZB GaN island layers are further examined by atomic force microscopy and transmission electron microscopy, extracting a mean island height of 1.6 nm and a mean diameter of 13 nm at a density of 1.3×1011 cm−2. Embedded ZB GaN quantum dots show strong ultraviolet photoluminescence without any thermal quenching up to room temperature.

J. Simon

Papers

Blue-light emission from GaN self-assembled quantum dots due to giant piezoelectric effect

Giant electric fields in unstrained GaN single quantum wells

Self-assembled InGaN quantum dots grown by molecular-beam epitaxy

Direct comparison of recombination dynamics in cubic and hexagonal GaN/AlN quantum dots

Self-assembled zinc blende GaN quantum dots grown by molecular-beam epitaxy