Erin C. Young

Materials and growth issues for high-performance nonpolar and semipolar light-emitting devices

Growth of InGaN/GaN light-emitting devices on nonpolar or semipolar planes offers a viable approach to reducing or eliminating the issues associated with polarization-related electric fields present in c-plane III-nitride heterostructures. Although progress in device performance has been rapid since the introduction of high-quality free-standing nonpolar and semipolar GaN substrates, a full appreciation of the materials challenges unique to nonpolar and semipolar III-nitride semiconductors has been slower to emerge. Only recently have researchers begun to understand issues such as the origins of the pyramidal hillocks typically observed on nominally on-axis m-plane GaN films, the effects of m-plane substrate misorientation on surface morphology and device performance, the mechanics of anisotropic cracking in tensile strained m-plane AlGaN films, the formation of basal-plane stacking faults in long-wavelength m-plane InGaN quantum wells, and the mechanisms for stress relaxation in semipolar AlGaN and InGaN films. In this paper, we review the materials and growth issues unique to high-performance nonpolar and semipolar light-emitting devices grown on high-quality free-standing GaN substrates and provide an outlook for the opportunities and challenges that lie ahead. (Some figures in this article are in colour only in the electronic version)

Partial strain relaxation via misfit dislocation generation at heterointerfaces in (Al,In)GaN epitaxial layers grown on semipolar (112¯2) GaN free standing substrates

Misfit strain relaxation via misfit dislocation (MD) generation was observed in heteroepitaxially grown (Al,In)GaN layers on free-standing semipolar (112¯2) GaN substrates. Cross-section transmission electron microscope images revealed MD arrays at alloy heterointerfaces, with the MD line direction and Burgers vector parallel to [11¯00] and [112¯0], respectively. The MD structure is consistent with plastic relaxation by dislocation glide on the (0001) plane. Since (0001) is the only slip plane, the plastic relaxation is associated with tilt of the epitaxial (Al,In)GaN layers. The tilt, measured via high-resolution x-ray diffraction, can be used to quantify the relaxation.

Basal plane misfit dislocations and stress relaxation in III-nitride semipolar heteroepitaxy

This article presents a theoretical analysis of dislocation behavior and stress relaxation in semipolar III-nitride heteroepitaxy, e.g., for AlxGa1−xN and InyGa1−yN layers grown on {hh2−h−m}- or {h0h−m}-type semipolar planes of GaN substrates. We demonstrate that the shear stresses on the unique inclined basal (0001) plane do not vanish for such growth geometries. This leads to the onset of relaxation processes in semipolar III-nitride heterostructures via dislocation glide in the basal slip systems 〈1−1−20〉(0001) and to the formation of misfit dislocations (MDs) with Burgers vectors of (a/3)〈1−1−20〉-type at the semipolar heterointerface. Next we calculate the Matthews-Blakeslee critical thickness for MD formation in semipolar III-nitride layers together with the MD equilibrium spacings for complete misfit relaxation. The component of the MD Burgers vector normal to the film/substrate interface will cause a crystal lattice tilt in the epilayer with respect to the GaN substrate. The calculated magnitudes o...

Indium and impurity incorporation in InGaN films on polar, nonpolar, and semipolar GaN orientations grown by ammonia molecular beam epitaxy

The effects of NH3 flow, group III flux, and substrate growth temperature on indium incorporation and surface morphology have been investigated for bulk InGaN films grown by ammonia molecular beam epitaxy. The incorporation of unintentional impurity elements (H, C, O) in InGaN films was studied as a function of growth temperature for growth on polar (0001) GaN on sapphire templates, nonpolar (101¯0) bulk GaN, and semipolar (112¯2), (202¯1) bulk GaN substrates. Enhanced indium incorporation was observed on both (101¯0) and (202¯1) surfaces relative to c-plane, while reduced indium incorporation was observed on (112¯2) for co-loaded conditions. Indium incorporation was observed to increase with decreasing growth temperature for all planes, while being relatively unaffected by the group III flux rates for a 1:1 Ga:In ratio. Indium incorporation was found to increase at the expense of a decreased growth rate for higher ammonia flows; however, smooth surface morphology was consistently observed for growth on s...

444.9 nm semipolar (112¯2) laser diode grown on an intentionally stress relaxed InGaN waveguiding layer

We demonstrate an electrically injected semipolar (112¯2) laser diode (LD) grown on an intentionally stress relaxed n-In0.09Ga0.91N waveguiding layer. Detrimental effects of misfit dislocations (MDs) in the proximity of the active region were effectively suppressed by utilizing a p/n-Al0.2Ga0.8N electron/hole blocking layer between the dislocated heterointerfaces and the active region. The threshold current density of the LD was ∼20.3 kA/cm2 with a lasing wavelength of 444.9 nm. This LD demonstrates an alternative approach in semipolar AlInGaN LD waveguide design where the thickness and composition of the waveguiding and/or cladding layers are not limited by the critical thickness for MD formation.

Lattice Tilt and Misfit Dislocations in (1122) Semipolar GaN Heteroepitaxy

Epilayer lattice tilt has been observed by X-ray diffraction for heteroepitaxial AlGaN and InGaN films on (1122) semipolar GaN substrates. Complementary transmission electron microscopy demonstrates that epilayer tilt is a consequence of interfacial misfit dislocations with Burgers vectors a/3[1120] that glide on the (0001) basal plane [inclined ~58° to (1122)]. The dislocation lines are parallel to [1100], consistent with the anisotropy of the tilt observed in X-ray scans parallel to orthogonal in-plane directions. The dislocations had an in-plane Burgers vector component that relieved misfit strain, and a perpendicular component responsible for lattice tilt. Dislocation densities predicted by tilt agree well with TEM measurements.

Performance and polarization effects in (112¯2) long wavelength light emitting diodes grown on stress relaxed InGaN buffer layers

Long wavelength (525–575 nm) (112¯2) light emitting diodes were grown pseudomorphically on stress relaxed InGaN buffer layers. Basal plane dislocation glide led to the formation of misfit dislocations confined to the bottom of the InGaN buffer layer. This provided one-dimensional plastic relaxation in the film interior, including the device active region. The change of the stress state of the quantum well due to one-dimensional plastic relaxation altered the valence band structure, which produced a significant shift in polarization of emitted light. Devices grown on relaxed buffers demonstrated equivalent output power compared to those for control samples without relaxation.

Misfit dislocation formation via pre-existing threading dislocation glide in (112¯2) semipolar heteroepitaxy

Cathodoluminescence (CL) was used to study the onset of mechanical stress relaxation in low indium composition semipolar (112¯2) InxGa1−xN lattice-mismatched layers grown on bulk GaN substrates. Monochromatic CL of short interfacial misfit dislocation (MD) segments showed a single threading dislocation (TD) associated with each MD segment—demonstrating that the initial stage of MD formation in semipolar III-nitride heterostructures proceeded by the bending and glide of pre-existing TDs on the (0001) slip plane. The state of coherency as determined by panchromatic CL is also compared to that determined by x-ray diffraction analysis based on crystallographic epilayer tilt and Matthew-Blakeslee’s critical thickness calculations.

Demonstration of a III-nitride vertical-cavity surface-emitting laser with a III-nitride tunnel junction intracavity contact

We report on a III-nitride vertical-cavity surface-emitting laser (VCSEL) with a III-nitride tunnel junction (TJ) intracavity contact. The violet nonpolar VCSEL employing the TJ is compared to an equivalent VCSEL with a tin-doped indium oxide (ITO) intracavity contact. The TJ VCSEL shows a threshold current density (Jth) of ∼3.5 kA/cm2, compared to the ITO VCSEL Jth of 8 kA/cm2. The differential efficiency of the TJ VCSEL is also observed to be significantly higher than that of the ITO VCSEL, reaching a peak power of ∼550 μW, compared to ∼80 μW for the ITO VCSEL. Both VCSELs display filamentary lasing in the current aperture, which we believe to be predominantly a result of local variations in contact resistance, which may induce local variations in refractive index and free carrier absorption. Beyond the analyses of the lasing characteristics, we discuss the molecular-beam epitaxy (MBE) regrowth of the TJ, as well as its unexpected performance based on band-diagram simulations. Furthermore, we investigat...

Misfit dislocation formation at heterointerfaces in "Al,In…GaN heteroepitaxial layers grown on semipolar free-standing GaN substrates

Semipolar GaN-based light emitting devices show great promise because of reduced polarization-related electric fields in the quantum wells and the potential for high indium uptake in InxGa1−xN quantum wells. In semipolar GaN, the (0001) slip plane is inclined with respect to the film normal, thus shear stresses are present on this plane for nominally misfitting layer growth. We present scattering contrast transmission electron microscopy studies of (112¯2) and (202¯1) semipolar GaN-based laser diode structures. Misfit dislocations were observed at the nominally misfitting heterointerfaces in the (112¯2) structures with line direction [11¯00] and Burgers vector in the (0001) plane. Similar observations are reported for the (202¯1) structures. Overall, the results are consistent with stress relaxation by threading dislocation glide.