Electrical characterization of strained InP-Ga(x)In(1-x)As core-shell nanowires

Abstract

This project report attempts to measure the piezoelectric effect in strained core-shell nanowires. The core material is wurtzite indium phosphide (InP) and the shell material wurtzite gallium indium arsenide (Ga(x)In(1-x)As). The lattice constant of Ga(x)In(1-x)As can be adjusted by its composition x and enables positive and negative strain for core and shell. A piezoelectric field in the order of volt per micrometer was predicted using analytical calculations of the strain. The nanowire samples are grown using metal-organic vapor phase epitaxy (MOVPE), resulting in nanowires with varying shell thickness and shell composition. Single nanowires were contacted to measure their current voltage characteristics. The extracted resistance and rectication behavior was dominated by the shell thickness for the case of thin shells (<10 nanometer). The characteristics of a piezoelectric field, however, could not be extracted from the measurement results.

Popular Abstract

Magical bolts from a fingertip

Scientists at Lund University proposed the use of piezocrystals for the conversion of solar energy to electricity. A present project tries to prove that experimentally.

When I was a boy and saw the shimmering color of a broken lighter in the corner of a sidewalk, I always checked for the part that generates those small sparks when you click it. At that time this item seemed somehow magical – the pressure of my finger caused a tiny spark! The things you could do with that spark to peeve or startle others … a precious item for a boy.

As so often in life, magic disappears when it is replaced by knowledge and today I know that the spark was caused by a piezoelectric crystal. Such crystals turn into an electric dipole when they are deformed. An electric dipole has a positive and a negative pole. Between them, an electric field is formed (analogous to a magnet with a south, and north pole with a magnetic field between them). The field in piezocrystals can grow so strong that it moves electrons within the material, creating a charge. This charge can then jump to regions that are not charged – the phenomenon of bolts and sparks.

Scientists at Lund University wondered wether piezocrystals could be used for the conversion of sunlight into electrical currents. The mechanism would be as follows: A piezocrystal is steadily deformed and thus creating a steady electric field. When sunlight shines on the piezocrystal, it creates a charge somewhere inside the crystal. The electrical field inside the crystal then moves this charge and voilà! A tiny and spark, which can now be used as electricity. And indeed, a computational model of the piezocrystal described above revealed that piezocrystals could be used for solar energy conversion. However, the structures need to be very small to resist the stress.

My project at Lund University

What a great motivation to actually produce piezocrystals and show that they can do this! I took the chance and joined the project at the department of Solid State Physics at Lund University. We produced tiny piezocrystals in the shape of towers, called nanowires (see figure in file). They are so small that they cannot be seen with the eye, only with an electron microscope. To better imagine how small they are, hold your thumb and index finger one millimetre away from each other. Now imagine to divide that space in a thousand parts – that will give you one micrometer (=μm; see figure). Then we covered the nanowires with a shell that deforms the piezocrystal-nanowires and creates an electric field inside them. Then took some nanowires with a sharp tip of a tissue, placed them on a chip and made tiny nickel-contacts to individual nanowires (it is simply stunning what the technical progress in the last decades allows nowadays). Although we had hoped to measure the field in piezocrystal-nanowires in this way, so far we have not been able to fully prove it.
For now, it remains somehow … magical.

• author: Frevel, Lorenz
• supervisor: Magnus Borgström
• organization: Department of Physics; Solid State Physics
• course: FYSM60 20161
• year: 2016
• type: H1 - Master's Degree (One Year)
• subject: Physics and Astronomy
• keywords: nanowire, strain, piezoelectric effect, indium phosphide, InP, gallium indium arsenide, GaInAs, InGaAs, core-shell, radial heterostructure
• language: English
• id: 8875729
• date added to LUP: 2016-06-14 14:03:04
• date last changed: 2016-06-14 14:03:04