Nanowires for optoelectronics

Aim of this work package is to focus on the growth optimisation of InGaN/GaN nanowires with respect to their optical emission properties and their implementation in nanoscale optoelectronic devices. The objective is to demonstrate an internal quantum efficiency for the nanowires higher than the value of 70% currently achieved in planar GaN/InGaN/GaN heterostructures. Central to this task are the complementary growth and characterisation methods involved. In particular the correlation between the optical and structural properties and the growth methodologies will provide partners with an iterative feedback for the development of growth strategies tailored to reach the objective.

InGaN nanowire growth on large areas in R&D and production type MOCVD equipment

The ESR at AIXTRON will develop MOCVD processes for the deposition of InGaN based nanowires. The ESR will focus on the MOCVD growth of single crystalline, defect-free, high density InGaN based nanowires on 4-inch Si-wafers for device applications, p-type and n-type doping, axial or core shell pn-junctions. Also the influence of the MOCVD equipment design on the nanowire deposition will be explored aiming at the optimisation of the MOCVD equipment respecting the special needs of the growth process as well as the later commercialisation of the technology. The ER will be in charge of (i) the transfer of optimised deposition processes for InGaN based nanowires from R&D to production type MOCVD equipment; (ii) the correlation of process results with MOCVD equipment design and (iii) the development of concepts to optimised production type MOCVD equipment for the deposition of Nitride based nanostructures.

InGaN/GaN nanowire arrays by MBE

The ESR at the UGOE will develop and optimise MBE processes for the deposition of InGaN/GaN nanowire array structures on prepatterned low cost substrates (Si, ITO). Preliminary studies by the group demonstrate the selective area growth (SAG) of InN nanorods by plasma assisted MBE. The growth of arrays of single nanorods on Si(111) was achieved with a thin molybdenum mask lithographically patterned with holes smaller than 60 nm. Ion track lithography will also be employed (collaboration outside NANOWIRING) in parallel to the on-site available lithography towards a technology transfer of a process on 4-inch Si-wafers. Of paramount importance for our objective is the choice of catalyst-free SAG of the nanowire arrays, to avoid incorporation of the catalyst and its detrimental effect on the optical emission. The transferability of the developed MBE SAG process for the growth of InGaN NW arrays to AIXTRON MOCVD processes will be assessed. In fact it is well known that MOCVD is better suited than MBE for selective growth due to the high mobility of the gas radicals at the mask surface. Therefore a technology developed for the low temperature MBE process is expected to be easily transferred to the MOCVD.

Optical (LEDs) and electronic (FETs) devices will be fabricated out of the InGaN nanowires (single objects and arrays) produced in both the above work packages. Critical for the application of the InGaN material system in semiconductor device structures is the presence of a strong electron accumulation layer at the surface. This aspect will be addressed by engineering of the band structure through combination of different materials, doping and growth directions and with the aid of simulation packages available within the ITN.

The teams at UCAM and UVEG will assist in the growth of high quality InGaN NWs by characterizing the structural quality and alloy homogeneity of the material by optical means: Raman scattering, photoluminescence, and time-resolved photoluminescence. CNR-IMEM will characterise the defect properties of the grown NWs by analytical HRTEM, and Z-contrast that allows atomic resolution (ca. 0.14 nm): light element impurities will be characterised by EELS, heavy element impurities will be analyzed via microanalysis.

Optical properties of InGaN/GaN single nanowires and arrays

Besides a timely feedback to the growers on the emission properties of the grown materials, UCAM and UVEG will perform experiments on fundamental properties of homogeneous NW arrays and single nano-objects. In the case of ternary III-N NWs, the enhanced contribution of the surface to the overall optical properties has not been established yet. In particular, the effects of electronic band bending and the existence of a layer of electron accumulation (for InN) or depletion (for GaN) in the electronic structure and emission dynamics of NW ensembles will be determined by CW and time-resolved PL, respectively.

New phenomenology will arise from the peculiarities of the coupling of the light electromagnetic field with the nanostructure, the so-called "antenna effect". The role of NW dimensions, anisotropic dielectric properties and the light wavelength will be clarified through the study of the intensity and polarisation response of the inelastic scattering, absorption and emission of the NWs. The effect can be also important for the enhancement of the Raman signal in single NWs.

Thin axial heterostructures in NWs behave as quantum disks and can induce electron and phonon quantum confinement.The uniaxial character of the wurtzite crystal structure of these NWs links intimately their structural and electrical properties through its piezoelectric and spontaneous polarisation. The analysis of strain and internal electric field is therefore an unavoidable prerequisite to understand the optical and electrical properties of the quantum disks. On the other hand, piezoelectric effects are expected to be small in radial heterostructures (core/shell), but the connection between the band bending at the interface and strain needs further investigation. Finally, optical and acoustic phonons, besides providing relevant structural information of the material, play an important role in the energy relaxation and nonradiative decay times and thermal properties of the system. PL spectra together with the structural information obtained by Raman scattering and the result of theoretical simulations performed by the UVEG ESRs will allow a detailed evaluation of the interplay of localised surface states, strain and confinement in the optical properties of nanostructured NWs. Electron-hole separation due to electron accumulation/depletion layer or due to internal electric fields in NW heterostructures will be studied by time-resolved PL by the ESR at UCAM. Due to the light intensity required for time-resolved detection, time-resolved PL will be performed on NW ensembles only. If wire density in the direction transverse to the wires can be reduced to the range 0.1 - 1 wire per micron, confocal magneto-optical methods will be applied to characterise the spin fine structure of optical excitations.