Development of a-/nc-Ge:H

Abstract

The maximal conversion efficiency on a single pn-junction solar cell is calculated to be 33.7% at a band gap energy of 1.34 eV. The main reasons for this maximum are the non-absorption of low energy photons and the thermalization of high energy photons. To counter this, multijunction devices can be constructed to make optimal use of the spectral range of the sunlight reaching the earth’s surface. While the introduction of thin film multi-junctions based on
silicon has managed to overcome the problem of spectral mismatching, photons with an energy below 1.12 eV are still not able to be collected. Where usually a high current density is traded off against a high open circuit voltage in multi-junction devices, the addition of a low band gap germanium-based bottom cell (Eg = 0.67 eV) could improve open circuit voltage without limiting the current density in the device. The photon flux in the range of 0.67-1.12 eV is high enough such that a current density of 15.9 mA/cm2 can be produced when they can be effectively collected. This current density is so high that it will never limit the output current of a multi-junction device, making germanium a very attractive material for studying and integration in structures where the low energy photons are not yet utilised.

This thesis is focused around the growth and characterization of hydrogenated germanium films for future use as active absorbers in multi-junction deivces. The influence of a set of deposition parameters on the morphological, optical and electrical properties of the films is studied with the aim of finding a deposition regime where device quality germanium films can be produced within the CASCADE reactor in the EKL clean room. The thin films are all be deposited using the RF-PECVD technique. Uniformly deposited Ge:H films under a stable
plasma can be deposited in CASCADE in the range of 1-5 mbar pressure and 5-30 W RF power at a fixed electrode distance of 20 mm. The films start to crystallize and form nc-Ge:H at a pressure of around 3-4 mbar and an RF power of 15-25 W when the germane precursor gas is diluted with hydrogen in a ratio of 1:400. A strong correlation between the refractive index and the presence of post deposition oxidation is investigated. Films with a low refractive index are characterized as having a low material denisty, making it easy for ambient water
molecules to diffuse into the lattice and react with germanium dangling bonds present there. By studying the effect and extent of the post-deposition oxidation on other film properties it was found that the activation energy of the films decreases to values as low as 50 meV. Despite this, due to a significant decrease of the σ0 by 1-5 orders of magnitude, the dark conductivity is found to decrease by 1-3 orders of magnitude. With a high photo/dark conductivity of 5-6 as a result. These results have led to the belief that the formation of Ge-O bonds in the films decreases the amount of defective states in the film, but that the defect states are moved up to an energy level closer to the conduction band. The presence of Ge-O bonds also inhibits the development of low band gap absorbers as seen by the low E04 optical band gap. In the denser films without oxygen contamination, the lowest E04 that has been reported is 1.2 eV along with a Tauc gap of 0.93 eV and a photo/dark conductivity ratio of 3.4.