A strategy to reduce the density of threading dislocations (TDs) in AlN epilayers grown on sapphire substrates is reported. The TDs experience a redirection of their line orientation which is found to coincide with imposed increases in both of V/III ratio and overall flux rate leading to the formation of an internal subinterface delineated by the changes in dislocation orientation. Threading dislocations either experience large kinks and then redirect into threading orientation or form dipole half loops via annihilation of redirected threading segments of opposite sign with the latter leading to a significant dislocation density reduction. These phenomena can be accounted for by a transition of growth mode from atomic step flow to two-dimensional layer-by-layer growth which accompanies the imposed changes in V/III ratio and flux. As this occurs, macrosteps (several atomic layers thick) laterally overgrow pre-existing dislocation outcrops. Image forces initiate the redirection processes and create trailing segments parallel to the interface between the advancing macrostep and the surface outcrop. This horizontal segment can be forced to redirect into threading orientation should another macrostep traveling in the opposite direction be encountered. Image forces again nucleate the redirected segment which is then replicated as the crystal grows. A dipole half loop will form if two dislocations with opposite sign are redirected so as to meet each other.

We report homoepitaxial GaN growth on freestanding (11̄00) oriented (M-plane GaN) substrates using low-pressure metalorganic chemical vapor deposition. Scanning electron microscopy, atomic-force microscopy, and photoluminescence were used to study the influence of growth conditions such as the V/III molar ratio and temperature on the surface morphology and optical properties of the epilayers. Optimized growth conditions led to high quality (11̄00) oriented GaN epilayers with a smooth surface morphology and strong band-edge emission. These layers also exhibited strong room temperature stimulated emission under high intensity pulsed optical pumping. Since for III-N materials the (11̄00) crystal orientation is free from piezoelectric or spontaneous polarization electric fields, our work forms the basis for developing high performance III-N optoelectronic devices.

In this paper, we report the pulsed atomic-layer epitaxy (PALE) of ultrahighquality AlN epilayers over basal-plane sapphire substrates and their use as templates to grow high-quality AlGaN layers with Al content ranging from 0.3 to 1. Symmetric/asymmetric x-ray diffraction (XRD) and room-temperature (RT) photoluminescence (PL) measurements were used to establish the highstructural and optical quality. The XRD (002) and (114) rocking-curve fullwidth at half-maximum (FWHM) values of the PALE-grown AlN epilayers were less than 60 arcsec and 250 arcsec, respectively. Using these ultrahighquality layers as templates, Si-doped AlGaN layers with a large Al content from 30% to 100% were grown and used for milliwatt power sub-280-nm, deepultraviolet (UV) light-emitting diodes (LEDs).

GaN and GaN/Al0.25Ga0.75N multiple quantum wells (MQWs) over c‐ and r‐plane sapphire substrates have been grown by metal‐organic chemical vapor deposition. A comparative study of photoluminescence (PL) in GaN epitaxial layers and AlGaN/GaN MQWs on these two types of substrates is reported. At low excitation levels, the measured room temperature PL signal in GaN layers grown over r‐plane sapphire was more than order of magnitude lower than in GaN on c‐plane substrates. In contrast, the emission intensity from AlGaN/GaN MQWs grown over r‐plane substrates was almost 30 times stronger than in the structures grown over c‐plane sapphire. Furthermore, with excitation power density up to 1 MW/cm2, the PL peak position for the non‐polar MQWs kept completely stable whereas the one for the c‐plane structures exhibited a blue shift as large as 250 meV. We attribute this large difference in the ultraviolet emission intensity to the suppression of a strong quantum Stark effect in the AlGaN/GaN MQWs on the r‐plane sapphire. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We report a pulsed atomic layer epitaxy (PALE) growth technique for quaternary AlInGaN films for ultraviolet optoelectronic applications. Using the PALE approach high quality quaternary AlInGaN/AlInGaN multiple quantum wells (MQWs) were successfully grown over sapphire substrates. From X-ray diffraction, atomic force microscopy, and photoluminescence study, a high structural and optical quality was established for the AlInGaN MQWs. Incorporating the PALE grown quaternary MQWs as the active layer of light emitting diode (LED) on sapphire or SiC substrates we also demonstrated room temperature deep ultraviolet electroluminescence under dc and pulsed electrical pumping. The peak emission wavelength can be tuned from 305 nm to 340 nm with spectrum FWHM of about 20 nm by varying the alloy compositions of the quaternary AlInGaN active layers using PALE. Comparative study of LEDs over sapphire and SiC substrates was also done in order to determine the influence of epilayer design on the performance parameters and the role of the substrate absorption.