M. Kryśko

Optically pumped 500 nm InGaN green lasers grown by plasma-assisted molecular beam epitaxy

We report on optically pumped lasing at 500 nm on InGaN laser structures grown by plasma assisted molecular beam epitaxy. The InGaN laser structures were grown under group III-rich conditions on bulk (0001) GaN substrates. The influence of the nitrogen flux and growth temperature on the indium content of InGaN layers was studied. We demonstrate that at elevated growth temperatures, where appreciable dissociation rate for In-N bonds is observed, the indium content of InGaN layers increases with increasing nitrogen flux. We show that growth of InGaN at higher temperatures improves optical quality of InGaN quantum wells, which is crucial for green emitters. The influence of piezoelectric fields on the lasing wavelength is also discussed. In particular, the controversial issue of partial versus complete screening of built-in electric field at lasing conditions is examined, supporting the former case.

Correlation between luminescence and compositional striations in InGaN layers grown on miscut GaN substrates

The influence of the miscut angle of GaN substrate on compositional and optical properties of InxGa1−xN epilayers (0.05<x<0.1) was examined using x-ray diffraction, photoluminescence (PL), cathodoluminescence, and Z-contrast scanning electron microscopy. We show that single atomic steps bunch during growth of InGaN and form macrosteps. Indium is incorporated differently at treads and risers of these macrosteps, which causes the layer to decompose and induces the formation of compositional growth striations. Since the growth step density increases with growing miscut angle of the substrate, the average indium concentration decreases and the average PL peak energy blueshifts and broadens with increasing miscut angle. The presented work enables understanding on microscopic scale effects related to the inhomogeneous distribution of indium in InGaN layers on miscut substrates, which is significant from the point of view of optoelectronic applications.

Elimination of AlGaN epilayer cracking by spatially patterned AlN mask

The inherent problem in III-nitride technology is the cracking of AlGaN layers that results from lattice mismatch between AlGaN and GaN. In case of thin substrates (30–90μm), such as, bulk GaN grown by the high-pressure/high-temperature method, the bowing of AlGaN∕GaN strained structures becomes an additional problem. To eliminate cracking and bowing, AlGaN layers were grown on GaN substrates with an AlN mask patterned to form 3–15μm wide windows. In the 3μm window, the AlGaN layer was not cracked, although its thickness and Al composition exceeded critical values for growth on nonpatterned substrates. Dislocation density in the windows was of 5×106∕cm2.

Contactless electroreflectance of InGaN layers with indium content ≤36%: The surface band bending, band gap bowing, and Stokes shift issues

Contactless electroreflectance (CER) supported by photoluminescence (PL) has been applied to study (i) the surface band bending, (ii) the band gap bowing, and (iii) the Stokes shift for InGaN layers grown by molecular beam epitaxy with 0.14≤In≤0.36. The type of surface band bending has been investigated on the basis of the shape of CER resonance. It has been found that the surface band bending changes from n-type for layers with low indium content (In<27%) to flatband (or weak p-type band) for layers with In∼35%. The band gap bowing has been determined to be 1.4±0.2 and 2.1±0.3 eV for CER data with and without strain corrections, respectively. From this analysis it has been concluded that the reliable value of the bowing parameter for unstrained InGaN should be between 1.4 and 2.1 eV. Comparing CER with PL data it has been found that the Stokes shift rises from 20 to 120 meV when the indium concentration increased from 14% to 36%. In addition, it has been observed that the intensity of PL from InGaN layer...

Built-in electric field and large Stokes shift in near-lattice-matched GaN∕AlInN quantum wells

Near-lattice-matched GaN∕AlInN quantum wells are investigated by means of contactless electroreflectance (CER) and temperature-dependent photoluminescence (PL). Large Stokes shifts, up to 400meV, between PL peak energies and CER resonances are identified. This Stokes shift is attributed to large potential profile fluctuations (PPFs) in the AlInN barriers. Further evidence for such PPFs and for the additional influence of QW width fluctuations is provided by temperature-dependent PL measurements, demonstrating large PL halfwidths and clear “S-shape” behavior. The influence of a large Stokes shift on the correct determination of the value of the built-in electric field is discussed, and it is shown that PL measurements may lead to a significant overestimation of the built-in electric field in GaN∕AlInN QWs.

Optically pumped GaN∕AlGaN separate-confinement heterostructure laser grown along the (112¯0) nonpolar direction

This letter concerns experiments on optically pumped GaN∕AlGaN separate-confinement heterostructure laser structures grown by plasma assisted molecular beam epitaxy. The structures were grown along the (112¯0) nonpolar crystallographic direction on a bulk GaN substrate. Different widths of GaN quantum wells were applied in the studied structures. Laser action is clearly demonstrated by the spontaneous emission saturation, abrupt line narrowing, and strong TE polarization of output light. A lasing threshold was reached at an excitation power density of 260kW∕cm2 for a 700-μm-long cavity at room temperature.

Hole carrier concentration and photoluminescence in magnesium doped InGaN and GaN grown on sapphire and GaN misoriented substrates

Systematic studies of InxGa1−xN layers (0≤x<0.13) doped with Mg were performed. Samples were grown by metal organic vapor phase epitaxy. Intermediate Mg doping in the range of 2×1019 cm−3 was chosen to achieve a maximum hole carrier concentration, pH (as measured by Hall effect) of 4×1018 cm−3 in samples with high x. We confirmed reports on decreasing resistivity in InxGa1−xN:Mg epitaxial layers observed with increasing x. This finding is very important for applications. In the performed research we attempted to separate contributions to pH increase resulting from increase in In-content and an associated decrease in growth temperature, Tgr (necessary to obtain high x). For this purpose In-content increase was achieved by means of either (i) lowering the growth temperature (from 1020 to 830 °C) or by (ii) varying an intended GaN substrate miscut. We demonstrated that the increase in pH in InxGa1−xN:Mg is caused by higher In concentration while a drop in Tgr plays a secondary role. Studies of photoluminesce...

High quality m-plane GaN grown under nitrogen-rich conditions by plasma assisted molecular beam epitaxya)

Homoepitaxial growth of m-plane GaN (11¯00) as a function of substrate miscut and temperature was studied by plasma assisted molecular beam epitaxy (PAMBE). The authors demonstrate that it is possible to obtain high-quality GaN on the m-plane under nitrogen-rich conditions at 730 °C. This is in contrast to the c-plane where three-dimensional growth mode is observed under the same conditions. They find a strong growth anisotropy and describe GaN (11¯00) surface morphology dependence on the sample miscut direction. The results indicate that by introducing a sample miscut toward ⟨112¯6¯⟩ one may expect parallel atomic steps when growing under nitrogen-rich conditions at 730 °C by PAMBE.

Investigation on the origin of luminescence quenching in N-polar (In,Ga)N multiple quantum wells

The growth of N-polar (In,Ga)N structures by plasma-assisted molecular beam epitaxy is studied. (In,Ga)N multiple quantum well samples with atomically smooth surface were grown and their good structural quality was confirmed by x-ray diffraction, scanning transmission electron microscopy, and defect selective etching. The In incorporation was higher in the N-polar than in the Ga-polar oriented crystal, consistent with previous reports. However, despite the good morphological and structural properties of these samples, no photoluminescence signal from the (In,Ga)N wells was detected. In contrast, a thick N-polar (In,Ga)N layer exhibited a broad peak at 620 nm in good agreement with the In content determined by x-ray diffraction. The potential source of the luminescence quenching in the N-polar (In,Ga)N multiple quantum wells is discussed and attributed either to a strong nonradiative recombination channel at the surface promoted by the electric field or to the high concentration of point defects at the int...

Step-flow growth mode instability of N-polar GaN under N-excess

GaN layers were grown on N-polar GaN substrates by plasma-assisted molecular beam epitaxy under different III/V ratios. Ga-rich conditions assure step-flow growth with atomically flat surface covered by doubly-bunched steps, as for Ga-polar GaN. Growth under N-excess however leads to an unstable step-flow morphology. Particularly, for substrates slightly miscut towards ⟨101¯0⟩, interlacing fingers are covered by atomic steps pinned on both sides by small hexagonal pits. In contrast, a three-dimensional island morphology is observed on the Ga-polar equivalent sample. We attribute this result to lower diffusion barriers on N-polar compared to Ga-polar GaN under N-rich conditions.