The quaternary alloy of group III-V, the (AlxGa1-x)0.52In0.48P is the widest bandgap material that can be grown lattice-matched to GaAs. Seven different aluminium compositions of x=0, 0.31, 0.47, 0.61, 0.64, 0.78 and 1.0 were grown with the same nominal i-region thicknesses of 1µm. These p-i-n photodiodes were fabricated with the standard fabrication and etching technique. The aim of this project is to characterise the optical and electrical properties of GaInP to AlInP, across the composition range for application such as top junction in multi-junction solar cell.
This work comprises of obtaining the dynamic range of absorption coefficient, α through the spectral response characterisation. The α was extracted down to the bandgap absorption region from 106 to as low as 100 cm-1. The photocurrent measurement was first carried out accurately and converted to the quantum efficiency. The model of quantum efficiency derived from the current continuity equation is used to iteratively fit the experimental data. The sensitivity of the model was taken into account through the variation of the minority carrier diffusion length, the surface recombination velocity and the cladding thicknesses. Initially, the α of the direct bandgap material is rapidly blue-shifted. However, the rate of change reduces as the composition becomes indirect. From here, the bandgap is extracted through the determination of the direct and indirect bandgap between the gamma-valley and x-valley. The material started to become indirect at the aluminium content of x≥0.48, which is similar to AlGaAs.
The investigation of current-voltage (I-V) measurement across the composition range was also carried out from 300K to 600K. The heater stage system was used in the characterisation of the dark current and photocurrent. Preliminary studies of photocurrent were undertaken as a function of temperature. From the forward I-V, the activation energy through the Arrhenius plot was extracted as a function of biased voltage for GaInP and AlInP. With the ideality factor of ≈1.7, the activation energy for both ends of the composition are 1.16eV and 1.29eV respectively. The ability of the devices to function at high temperature with slight degradation and to endure multiple heating cycles, proves growth and fabrication work well for this characterisation. Subject to this temperature range, the AlGaInP is considered suitable for high temperature solar cell and space exploration applications.
