Christoph Stark

Green cubic GaInN/GaN light-emitting diode on microstructured silicon (100)

GaInN/GaN light-emitting diodes free of piezoelectric polarization were prepared on standard electronic-grade Si(100) substrates. Micro-stripes of GaN and GaInN/GaN quantum wells in the cubic crystal structure were grown on intersecting {111} planes of microscale V-grooved Si in metal-organic vapor phase epitaxy, covering over 50% of the wafer surface area. Crystal phases were identified in electron back-scattering diffraction. A cross-sectional analysis reveals a cubic structure virtually free of line defects. Electroluminescence over 20 to 100 μA is found fixed at 487 nm (peak), 516 nm (dominant). Such structures therefore should allow higher efficiency, wavelength-stable light emitters throughout the visible spectrum.

Evaluation of metal/indium-tin-oxide for transparent low-resistance contacts to p-type GaN.

In search of a better transparent contact to p-GaN, we analyze various metal/indium-tin-oxide (ITO) (Ag/ITO, AgCu/ITO, Ni/ITO, and NiZn/ITO) contact schemes and compare to Ni/Au, NiZn/Ag, and ITO. The metal layer boosts conductivity while the ITO thickness can be adjusted to constructive transmission interference on GaN that exceeds extraction from bare GaN. We find a best compromise for an Ag/ITO (3 nm/67 nm) ohmic contact with a relative transmittance of 97% of the bare GaN near 530 nm and a specific contact resistance of 0.03 Ω·cm2. The contact proves suitable for green light-emitting diodes in epi-up geometry.

Green LED development in polar and non-polar growth orientation

The green spectral region provides a formidable challenge for energy efficient light emitting diodes. In metal organic vapor phase epitaxy we developed GaInN/GaN quantum well material suitable for 500 - 580 nm LEDs by rigorous defect reduction and thrive for alloy uniformity. We achieve best results in homoepitaxy on polar c-plane, and non-polar a-plane and m-plane bulk GaN. By the choice of crystal orientation, the dipole of piezoelectric polarization in the quantum wells can be optimized for highest diode efficiency. We report progress towards the goal of reduced efficiency droop at longer wavelengths.

Incorporation of indium on cubic GaN epitaxially induced on a nanofaceted Si(001) substrate by phase transition

The incorporation of In on the non-polar, piezoelectric-free (001) facet of cubic (c-) GaN epitaxially grown over a Si(001) substrate by metal-organic vapor phase epitaxy is reported. Relying on a hexagonal (h-) to c-phase transformation during epitaxy on an 800 nm-wide, Si(111)-faceted v-groove patterned into the substrate, the GaN epilayer at cross sectional view retains a triangular c-phase inside a chevron-shaped h-phase that results in a top surface bounded by a (001) facet parallel to Si(001) at the center and ( 11¯01) facets at both edges. A stack of five, ∼3 nm-thick, InxGa1−xN/GaN quantum wells (QWs) was deposited on the double-phased top surface. The c-phase region up to the QWs keeps extremely small misfit (∼0.002) to the fully relaxed h-GaN underneath it and is in tensile stress implying undefected by the h-c phase interface. The In incorporation on a strained non-polar (001) of c-GaN is comparable with that on totally relaxed semi-polar ( 11¯01) of h-GaN without noticeable adatom migration ac...

Non-polar GaInN-based light-emitting diodes: an approach for wavelength-stable and polarized-light emitters

In absence of piezoelectric polarization along the growth axis, a- and m-plane green GaInN light emitting diodes manifest stable emission wavelength -- independent of the injection current density. The shift of the dominant wavelength is less than 8 nm when varying the forward current density from 0.1 to 38 A/cm2. Furthermore, the light emitted from the growth surface of such non-polar structures shows a very degree of linear polarization. This is attributed to a strong valance band splitting in such anisotropically strained wurtzite GaInN quantum wells . Such light emitting diodes show a high potential for energy efficient display applications.

HOW DO WE LOSE EXCITATION IN THE GREEN

Efficiency droop and green gap are terms that summarize performance limitations in GaInN/GaN high brightness light emitting diodes (LEDs). Here we summarize progress in the development of green LEDs and report on time resolved luminescence data of polar c-plane and non-polar m-plane material. We find that by rigorous reduction of structural defects in homoepitaxy on bulk GaN and V-defect suppression, higher efficiency at longer wavelengths becomes possible. We observe that the presence of donor acceptor pair recombination within the active region correlates with lower device performance. To evaluate the aspects of piezoelectric polarization we compare LED structures grown along polar and non-polar crystallographic axes. In contrast to the polar material we find single exponential luminescence decay and emission wavelengths that remain stable irrespective of the excitation density. Those findings render high prospects for overcoming green gap and droop in non-polar homoepitaxial growth.

Fish scale terrace GaInN/GaN light-emitting diodes with enhanced light extraction

Non-planar GaInN/GaN light-emitting diodes were epitaxially grown to exhibit steps for enhanced light emission. By means of a large off-cut of the epitaxial growth plane from the c-plane (0.06° to 2.24°), surface morphologies of steps and inclined terraces that resemble fish scale patterns could controllably be achieved. These patterns penetrate the active region without deteriorating the electrical device performance. We find conditions leading to a large increase in light-output power over the virtually on-axis device and over planar sapphire references. The process is found suitable to enhance light extraction even without post-growth processing.