Current path in light emitting diodes based on nanowire ensembles

Abstract

Light emitting diodes (LEDs) have been fabricated using ensembles of free-standing (In, Ga)N/GaN nanowires (NWs) grown on Si substrates in the self-induced growth mode by molecular beam epitaxy. Electron-beam-induced current analysis, cathodoluminescence as well as biased μ-photoluminescence spectroscopy, transmission electron microscopy, and electrical measurements indicate that the electroluminescence of such LEDs is governed by the differences in the individual current densities of the single-NW LEDs operated in parallel, i.e. by the inhomogeneity of the current path in the ensemble LED. In addition, the optoelectronic characterization leads to the conclusion that these NWs exhibit N-polarity and that the (In, Ga)N quantum well states in the NWs are subject to a non-vanishing quantum confined Stark effect.

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.…”

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

“…The n-type GaN base is doped with Si, while the p-type segment consists of a roughly 120-nm-thick GaN cap doped with Mg; a more detailed description of the layer stacking scheme can be found elsewhere. 13 The NW-LED samples were planarized by spin coating using a solution of hydrogen silsesquioxane (HSQ), which was subsequently transformed into solid SiO x . Excess SiO x on the NW tips was removed by dry etching with CHF 3 until a considerable number of tips were uncovered.…”

Structure and Composition of Isolated Core-Shell(In,Ga)N/GaNRods Based on Nanofocus X-Ray Diffraction and Scanning Transmission Electron Microscopy

“…To improve and expand existing group-III-nitride-based semiconductor technologies-which nowadays are mainly based on planar systems-researchers have focused their activities on the investigation of lowdimensional, mesoscopic structures such as quantum dots and nanowires [4,5]. For example, ðIn; GaÞN=GaN coreshell nanowires and rods are discussed as promising candidates for next-generation light-emitting diodes [6][7][8][9][10][11]. If the structures are grown not in an axial but in a coreshell geometry, the optically active area can be significantly increased compared to planar structures [12,13].…”