Nonpolar and Semipolar Group III Nitride-Based Materials
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Semipolar Group III Nitride-Based Materials
J.S. Speck and S.F. Chichibu, Guest Editors Abstract GaN and its alloys with InN and AlN are materials systems that have enabled the revolution in solid-state lighting and high-power/high-frequency electronics. GaN-based materials naturally form in a hexagonal wurtzite structure and are naturally grown in a (0001) c-axis orientation. Because the wurtzite structure is polar, GaN-based heterostructures have large internal electric fields due to discontinuities in spontaneous and piezoelectric polarization. For optoelectronic devices, such as light-emitting diodes and laser diodes, the internal electric field is generally deleterious as it causes a spatial separation of electron and hole wave functions in the quantum wells, which, in turn, likely decreases efficiency. Growth of GaN-based heterostructures in alternative orientations, which have reduced (semipolar orientations) or no polarization (nonpolar) in the growth direction, has been a major area of research in recent years. This issue highlights many of the key developments in nonpolar and semipolar nitride materials and devices.
Background The Group III nitrides are a remarkable materials system. With direct bandgaps ranging from 0.7 eV (InN) through 3.4 eV (GaN) to 6.0 eV (AlN), this materials system has enabled deep ultraviolet (λ < ~300 nm or photon energy > ~4.1 eV based on high Al content AlxGa1–xN quantum wells [QWs]), ultraviolet (λ < ~400 nm or photon energy > ~3.1 eV ), blue (λ ≈ 455 nm or photon energy = 2.7 eV based on InyGa1–yN QWs), and green (λ ≈ 525 nm or photon energy = 2.4 eV) emitters based on InxGa1–xN QWs, and longer wavelength light-emitting diodes (LEDs) and violet and blue laser diodes (LDs). No other materials system offers this range of direct bandgaps. Prior to the development of the nitrides as optoelectronic materials, there were no efficient ultraviolet or blue LEDs, and green LEDs based on II–VI materials had poor lifetimes due to the ease of dislocation generation in the active regions of the II–VIs. Devices based on Group III nitrides perform well despite threading dislocation densities typically in excess of 304
108 cm–2 (but poorly when the dislocation density exceeds 1010 cm–2, thus pushing the drive for bulk GaN substrates as described by Fujito et al.’s article in this issue) in comparison to conventional III–V materials such as GaAs, where the dislocation densities are typically 104 cm–2 or less. However, when the nitrides are remarkably robust against oxidation (and etching), they are mechanically hard and show no evidence for extended defect motion at standard operating temperatures. GaNbased LEDs feature prominently in colored lighting applications, such as green traffic lights. When blue LEDs are combined with a yellow-emitting phosphor, white light is produced; thus, GaN has been the material that enabled the field of solid-state lighting. Violet GaN-based LDs operating at a wavelength of 405 nm are already in broad use in high definition optical data storage. The articles
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