Frank Lipski

Bright semipolar GaInN∕GaN blue light emitting diode on side facets of selectively grown GaN stripes

The authors demonstrate the fabrication and evaluation of bright semipolar GaInN∕GaN blue light emitting diodes (LEDs). The structures are realized by growing five GaInN∕GaN quantum wells on the {11¯01} side facets of selectively grown n-GaN stripes with triangular shape running along the ⟨112¯0⟩ direction covered with a Mg-doped GaN top layer. The growth was done by metal organic vapor phase epitaxy using a conventional [0001] sapphire substrate. The devices have circular mesa structures with diameters between 70 and 140μm. Continuous wave on-wafer optical output powers as high as 700μW and 3mW could be achieved under dc conditions for 20 and 110mA, respectively. The current dependent blueshift of the peak emission wavelength caused by screening effects of the piezoelectric field was only 1.5nm for currents between 1 and 50mA. This is less than half the value measured on c-plane LEDs and confirms the reduced piezoelectric field in our LED structures.

Piezoelectric fields in GaInN∕GaN quantum wells on different crystal facets

Direction and strength of piezoelectric built-in fields of GaInN quantum wells have been experimentally determined. The quantum wells have been grown either on the conventional {0001} crystal plane of GaN or on {11¯01} facets of selectively grown GaN stripes. The emission peak position of the electric-field-dependent photoluminescence can be modeled and yields value and sign of the piezoelectric field dependent on the strain of the quantum wells. On the semipolar {11¯01} facets, the quantum wells show a much weaker field (−0.1MV∕cm) compared to quantum wells grown on polar {0001} planes (−1.9MV∕cm), consistent with theoretic predictions.

Planar semipolar (101¯1) GaN on (112¯3) sapphire

We report on the growth of planar semipolar (101¯1) GaN on (112¯3) prepatterned sapphire. This is a method that allows the growth of semipolar oriented (101¯1) GaN on large scale. Using x-ray diffraction only the peaks of the desired (101¯1) plane could be observed. Scanning electron, transmission electron, and atomic force microscopy measurements show an atomically flat surface. Further investigations using photoluminescence spectroscopy show spectra that are dominated by the near band edge emission. The high crystal quality is furthermore confirmed by the small full width at half maximum values of x-ray rocking curve measurements of less than 400 arcsec.

Strain and defects in Si-doped (Al)GaN epitaxial layers

Si is the most common dopant in (Al)GaN based devices acting as a donor. It has been observed that Si induces tensile strain in (Al)GaN films, which leads to an increasing tendency for cracking of such films with the increase of Si content and/or the increase of Al content. Based on x-ray investigations, the Si-doped films have a larger in-plane lattice constant than their undoped buffer layers, indicating involvement of a mechanism other than the change of lattice constants expected from an alloying effect. In this work, we present a model about Si dislocation interaction while debating other proposed models in the literature. According to our model, Si atoms are attracted to the strain dipole of edge-type dislocations in (Al)GaN films. It is expected that Si is more incorporated on that side of the dislocation, which is under compression leading to the formation of off-balanced dipoles with reduced compressive component. In response to such off-balanced dipoles—appearing as tensile dominant strain dipol...

Polarized light emission from semipolar GaInN quantum wells on {11¯01} GaN facets

GaInN quantum wells with reduced piezoelectric field deposited epitaxially on the {11¯01} facets of selectively grown GaN stripes show distinct polarization properties of the emitted light. Systematic electro- and photoluminescence studies demonstrate that the light is linearly polarized parallel to the stripes, representing the ⟨112¯0⟩ direction of the GaN crystal. Our model calculations show that this is a consequence of the strain-induced valence-band splitting depending on the crystal orientation. The polarization ratio is calculated and compared to measured values.

Piezoelectric polarization of semipolar and polar GaInN quantum wells grown on strained GaN templates

Piezoelectric polarization in GaInN/GaN quantum well (QW) structures is not only dependent on the strain in the pseudomorphically grown QWs but also severely influenced by the strain already present in the underlying template. Here, we investigate the influence of template strain on the piezoelectric polarization of QWs grown in the polar [0001] and in semipolar off-[0001] directions. We find that an efficient strain-engineering can be used to decrease polarization fields or even cancel them out completely. Even a change of sign of the built-in polarization fields within QWs can be achieved in certain strain-situations. Our results shed light on recent ambiguous results concerning the piezoelectric tensor component e15 and are able to explain unexpected differences in device performance of polar and semipolar QW devices.

Optimization of semipolar GaInN/GaN blue/green light emitting diode structures on {1-101} GaN side facets

Bluish-green semipolar GaInN/GaN light emitting diodes (LEDs) were investigated as possible candidates for high-brightness devices even in the long wavelength visible regime. To combine the high material quality known from c-GaN and the advantages of a reduced piezoelectric field, the LED structures were realized on the {1¯101} side facets of selectively grown GaN stripes with triangular cross section. Structural investigations using transmission electron microscopy, scanning electron microscopy, high resolution x-ray diffraction, and atomic force microscopy have been performed and could be related to the luminescence properties in photoluminescence and cathodoluminescence. The defect-related luminescence peaks at 3.3 eV and 3.42 eV typically observed in planar non- and semipolar GaN structures as fingerprints of prismatic and basal plane stacking faults, respectively, could be eliminated in our facet LED structures by optimized growth conditions. Furthermore, an indium incorporation efficiency for these {1¯101} facets is found to be about 50% higher as compared to c-plane growth, what helps significantly to achieve longer wavelength emission in spite of the reduced quantum confined Stark effect in such non- and semipolar materials. Combining these findings, we could realize a bluish-green semipolar light emitting diode on the side facets of our GaN stripes. Continuous wave on-wafer optical output powers as high as 240 µW@20mA could be achieved for about 500nm emission wavelength in electroluminescence measurements. The external efficiency was nearly constant for the investigated current range. Furthermore, the relatively small wavelength shift of about 3 nm for currents between 10mA and 100mA confirmed the reduced piezoelectric field in our LED structures.

Epitaxial growth of coaxial GaInN-GaN hetero-nanotubes

We report about the successful realization of a coaxial hetero structure grown by MOVPE around ZnO nanocolumns. At higher overgrowth temperatures, the ZnO cores completely dissolved leaving GaN nanotube structures with excellent properties. Such tubes could be sheathed by a GaInN-GaN single quantum well structure, as confirmed by photoluminescence and transmission electron microscopy.

The influence of prestrained metalorganic vapor phase epitaxial gallium-nitride templates on hydride vapor phase epitaxial growth

We have varied the strain situation in metalorganic vapor phase epitaxial (MOVPE) grown gallium-nitride (GaN) by exchanging the nucleation layer and by inserting a submono-SixNy-interlayer in the first few hundred nanometers of growth on sapphire substrates. The influence on the MOVPE template and subsequent hydride vapor phase epitaxial (HVPE) growth could be shown by in-situ measurements of the sample curvature. Using the results of these investigations, we have established a procedure to confine the curvature development in MOVPE and HVPE growth to a minimum. By increasing the layer thickness in HVPE, we could create self-separated, freestanding GaN layers with small remaining curvature.

Zeeman spectroscopy of the internal transition 4T1 to 6A1 of Fe3+ ions in wurtzite GaN

Internal transitions of Fe3+ions in wurtzite gallium nitride were analyzed by means of photoluminescence, Zeeman, and transmission spectroscopy in order to investigate the fine structure. Magnetic fields up to 14 T were applied perpendicular or parallel to the crystal c-axis, causing a characteristic splitting pattern of the luminescence related to the transition from the 4T1excited state to the 6A1ground state of Fe3+. The complete Hamiltonian matrix is constructed taking into account the crystal field in cubic and trigonal symmetry, spin-orbit interaction, and the influence of external magnetic fields. Numerical solution yields the exact energy level scheme of the excited state4G of Fe3+ions in GaN, which partly revises assumptions based on a qualitative treatment considering group theory only and invoking the influence of a Jahn-Teller effect. The coincidence of the calculated energy levels with the experimental data verifies the derived fine structure of the 3d metal ion.