光伏应用的砷化镓径向结构

Gallium arsenide p-i-n radial junctions were fabricated by molecular beam epitaxy. The current-voltagccharacteristics of single nanowires were measured in the dark and under various illumination conditionsincluding 1.5 AM. The total efficiency was 4.5%. Spatially resolved and power dependent photocurrentmeasurements indicate that the p-i-n junction is homogeneous along the nanowire. Electroluminescencemeasurements show an emission peak at about 1.4 eV, further corroborating the good quality of thenanowire. These results constitute an important progress for the use of nanowires in photovoltaicapplications.

Third generation solar cells represent alteratives to traditional bulk and thin film devices. They constitutea novel way to improve the ratio between photovoltaic efficiency and total cost, by including newphysical principles and solutions offered from nanotechnology..2 Semiconductor nanowires are expectedto play an important role as components of third generation solar cells.’ Particular attention has beengiven to core-shell radial p-i-n structures, in which the direction of light absorption is orthogonal to thecarrier collection. It is predicted that such structures enable an extremely efficient carrier collection, as theminority carrier diffusion length can in all cases be shorter than the optical absorption length.' Thisrelaxes the demand for the material quality, which at the same time can lead to the reduction of thefabrication costs. Supplementary advantages in using nanowires for photovoltaic applications are theimprovement in the light collection and the minimization of the amount of material used.' Moreover, it isknown that lattice mismatched materials can be integrated when grown in the form of nanowires. Thisenables both the formation of strain-free multijunction solar cells and the utilization of any substrate. 6,7

Molecular beam epitaxy (MBE) grown GaAs nanowires are especially suitable for the fabrication ofnanowire solar cells. From one side, MBE is a versatile technique. Axial and radial growth can beswitched at convenience. Layers grown on the radial direction can be obtained with atomic precision andperfect epitaxial relation. The quality of the obtained interface is MBE-grade, which is an essential pointfor the fabrication of an efficient solar cell. Additionally, GaAs nanowires can be obtained without the useof gold, which is generally an unwanted impurity in the semiconductor industry..o Growth occurs up to15 times faster than thin film growth, which is a further advantage for industrial applications.'' Finally.GaAs is the semiconductor whose absorption is best matched with the solar spectrum.

The GaAs p-i-n nanowire structures were synthesized in a Gen II MBE system. Two-inch (111)B GaAswafers coated with a sputtered 10 nm thick silicon dioxide were used as substrates. The nanowire growthwas carried out at a nominal GaAs growth rate of 0.25 A/s, As, partial pressure of 2x10* mbar (Ga richconditions), a temperature of 630 °C and with 7 rpm rotation. As we have previously shown, thenanowires grown on coated (111)B GaAs wafers were perpendicular to the substrate surface. The core ofthe p-i-n nanowire junction was doped type p. This was achieved by adding a silicon flux during the nanowire growth. For that, the silicon cell was heated with 13 Amp, which corresponds to a doping fluxof ~2xl0 at.cm', leading to a concentration of -2xl0' cm for a growth rate of 1 A/s. Silicon is anamphoteric impurity in GaAs. This means that the incorporation of silicon can lead to n or p type dopingdepending on whether it is incorporated in As or Ga sites'3 In the case of gallium-assisted GaAsnanowire growth, we have observed that the incorporation of Si in the growth results in a p-type dopingMore detailed studies on the doping of this type of nanowires will be published in a near future.