Submicrometre resolved optical characterization of green nanowire-based light emitting diodes

Abstract

The electroluminescent properties of InGaN/GaN nanowire-based light emitting diodes (LEDs) are studied at different resolution scales. Axial one-dimensional heterostructures were grown by plasma-assisted molecular beam epitaxy (PAMBE) directly on a silicon (111) substrate and consist of the following sequentially deposited layers: n-type GaN, three undoped InGaN/GaN quantum wells, p-type AlGaN electron blocking layer and p-type GaN. From the macroscopic point of view, the devices emit light in the green spectral range (around 550 nm) under electrical injection. At 100 mA DC current, a 1 mm2 chip that integrates around 10(7) nanowires emits an output power on the order of 10 µW. However, the emission of the nanowire-based LED shows a spotty and polychromatic emission. By using a confocal microscope, we have been able to improve the spatial resolution of the optical characterizations down to the submicrometre scale that can be assessed to a single nanowire. Detailed μ-electroluminescent characterization (emission wavelength and output power) over a representative number of single nanowires provides new insights into the vertically integrated nanowire-based LED operation. By combining both μ-electroluminescent and μ-photoluminescent excitation, we have experimentally shown that electrical injection failure is the major source of losses in these nanowire-based LEDs.

Understanding peculiarities in the optoelectronic characteristics of light emitting diodes based on (In,Ga)N/GaN nanowires

“…In particular, the sample with ITO exhibits a number density of emitting spots which is one order of magnitude higher than the sample with Ni/Au. In agreement with previous findings, 8,13,15 the EL map of the sample with Ni/Au contact is characterized by EL spots separated by large portions of surface area that are much darker, essentially without any emission of light. In contrast, the emitting NWs in the sample with ITO cover the whole surface without any dark areas in between.…”

Disentangling the effects of nanoscale structural variations on the light emission wavelength of single nano-emitters: InGaN/GaN multiquantum well nano-LEDs for a case study

“…In the literature, the scattering in the QW emission wavelength of nanorods is generally attributed to local variations in strain, thickness, and In content. [14][15][16]21,34,46,47 These nanoscale uctuations in the QW region are more accessible when exciting CL in nano-LEDs as compared with the epitaxial lm because the collection is limited only to the nanorod volume, while being averaged over a larger volume in the lm. 48 Interestingly, the same strain level (AE0.05 cm À1 ) does not necessarily result in the same QW emission wavelength (AE1.92 nm) for all nanorods.…”

“…These unique features have been achieved by choosing appropriate sizes and geometries as well as by controlling the strain relaxation either through nanopatterning of planar lms using top-down lithography techniques 1,[11][12][13][14] or by bottom-up growth of relaxed InGaN/GaN MQWs on strain-free GaN nanowire templates. 3,6,[15][16][17] The key to wavelength-independent emission efficiency over the entire visible spectrum resides mainly in the control of the strain at the nanoscale. In InGaN/GaN MQW heterostructures there is a lattice mismatch between the two materials that can lead to a signicant in-plane biaxial strain inside the QWs.…”

“…Another essential point is the impact of intrinsic uctuations in the thickness and In composition on the light emission wavelength from partially or fully strain relaxed nano-LEDs as compared to the bulk strained MQW counterparts. 15,16,20,21 The high surface-to-volume ratio characteristic for the nanorod geometry leads to surface-related phonon modes whose interaction with electrons by way of Fröhlich phonon-electron coupling potentially open the way towards designing of phonon assisted nanorod LED devices. 22,23 Therefore, such basic studies require the fabrication of reproducible nanostructures with small size deviations as well as controlled orientation and density.…”

Ordered Gan/Ingan Nanorods Arrays Grown by Molecular Beam Epitaxy for Phosphor-Free White Light Emission

“…15 ' 16 ' 18 In addition, self-assembled NRs may merge generating extended defects as dislocations and stacking faults. Thus, efficient white LEDs based on IIInitrides grown by a self-assembled method have little chances to become commercial.…”