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.

We investigate the modification of the electronic band structure in wurtzite GaN due to biaxial strain within the M plane using photoreflectance ͑PR͒ spectroscopy. The compressively strained M-plane GaN film is grown on ␥-LiAlO 2 ͑100͒. In the PR measurements, the electric-field vector ͑E͒ of the probe light is polarized parallel (ʈ) and perpendicular (Ќ) to the c axis of GaN which lies in the growth plane. For EЌc, the spectrum exhibits only a single resonant feature at lower energies, while for Eʈc a different single resonant feature appears at higher energies. To identify these features, we calculate the strain dependence of the interband transition energies and the components of the oscillator strength using the k•p perturbation approach. Comparison with the calculations shows that the origin of the PR features and their significant in-plane polarization anisotropy is related to the influence of M-plane, biaxial compressive strain on the valence-band structure of GaN. We estimate the value of the deformation potential D 5 to be Ϫ4.7 eV.

Temperature-dependent current–voltage measurements have been used to determine the reverse-bias leakage current mechanisms in Schottky diodes fabricated on GaN grown by molecular-beam epitaxy, and two dominant mechanisms are clearly identified. The first mechanism is field-emission tunneling from the metal into the semiconductor, which is dominant at low temperatures and which, at higher temperatures, becomes significant for large reverse-bias voltages. The second mechanism, presumed to be associated with dislocation-related leakage current paths, is observed to have an exponential temperature dependence and becomes significant above approximately 275 K. The temperature dependence of the second mechanism is consistent with either one-dimensional variable-range-hopping conduction along the dislocation or trap-assisted tunneling.

Scanning Kelvin probe microscopy ͑SKPM͒ and conductive atomic force microscopy ͑C-AFM͒ have been used to image surfaces of GaN grown by molecular beam epitaxy. Detailed analysis of the same area using both techniques allowed imaging and comparison of both surface potential variations arising from the presence of negatively charged threading dislocations and localized current leakage paths associated with dislocations. Correlations between the charge state of dislocations, conductivity of current leakage paths, and dislocation type were thereby established. Analysis of correlated SKPM and C-AFM images revealed a density of negatively charged features of ϳ3ϫ10 8 cm Ϫ2 and a localized current leakage path density of ϳ3ϫ10 7 cm Ϫ2 , with approximately 25% of the leakage paths spatially correlated with negatively charged dislocation features. Based on correlated topography and previous studies quantifying the densities of edge, screw, and mixed dislocations, our results suggested that dislocations having an edge component behave as though negatively charged while pure screw dislocations are solely responsible for the observed leakage paths and are uncharged.

Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N-and Gaface AlGaN/GaN heterostructures

Evaluation of the structural properties of 200-nm-thick Si-doped Al0.49Ga0.51N films, grown on nominally relaxed 1-μm-thick Al0.62Ga0.38N buffer layers on sapphire, revealed that increased Si doping promoted the relaxation of the compressively strained layers. The degree of strain relaxation R of the Al0.49Ga0.51N films, as determined by x-ray diffraction (XRD), increased from R=0.55 to R=0.94 with an increase in disilane injection from 1.25 nmol/min to 8.57 nmol/min. Transmission electron microscopy analysis showed that the edge threading dislocations (TDs) in the Al0.49Ga0.51N layers were inclined, such that the redirected TD lines had a misfit dislocation component. The calculated strain relaxation due to the inclined TDs was in close agreement with the values determined from XRD. We propose that the TD line redirection was promoted by the Si-induced surface roughness.

No abstract

͑0001͒-oriented epitaxial wurtzite III-nitride layers grown on mismatched substrates have no resolved shear stress on the natural basal and prismatic slip planes; however, strained III-nitride layers may gradually relax. We report on the stress relaxation of Al 0.49 Ga 0.51 N layers grown on nominally relaxed Al 0.62 Ga 0.38 N buffer layers on sapphire. The reduction in elastic strain of the Al 0.49 Ga 0.51 N was enhanced by Si doping which caused an increased surface roughness. Despite the Si doping, the films always sustained step-flow growth. The extent of relaxation of the Al 0.49 Ga 0.51 N layer was determined by on-axis -2 scans of ͑000l͒ peaks and reciprocal space maps of inclined ͑off-axis͒ peaks. Cross-section and plan-view transmission electron microscopy studies showed that the threading dislocations in the Al 0.49 Ga 0.51 N layer inclined from the ͓0001͔ direction towards ͗1100͘ directions by ϳ15-25°, perpendicular to their Burgers vector ͑ 1 3 ͗1120͘ ͒ . These inclined threading dislocations have a misfit dislocation component and thus provide stress relief. The contribution of the dislocation inclination to the degree of relaxation has been formulated and the energy release has been determined for dislocation inclination in mismatched stressed layers.