Effects of doping of bragg reflector layers on the electrical characteristics of InGaAs/GaAs metamorphic photovoltaic converters

The current–voltage characteristics of In$_{x}$Ga$_{1-x}$As/GaAs metamorphic photovoltaic converters with built-in $n$-InGaAs/InAlAs Bragg reflectors are studied at an indium content of $x$ = 0.025 – 0.24. The series resistance of the heterostructures is measured in the temperature range from 90 to 400 K. It is found that a sharp rise in the resistance of silicon-doped reflectors with an increasing fraction of In is due to weak activation of the donor impurity in InAlAs–$n$:Si layers. As a result, the energy barriers for majority carriers are formed in the latter, with a height of 0.32 – 0.36 eV and a substantial width. To suppress this effect, the technology of the Te doping of $n$-InGaAs/InAlAs Bragg reflectors is developed, which reduces the series resistance by five orders of magnitude. This makes it possible to keep the fill factor of the current–voltage characteristic above 80% up to current densities of 2 A/cm$^2$. Values exceeding 85%, achieved for the quantum efficiency, indicate that the “memory” and tellurium segregation effects characteristic of this kind of impurity are suppressed.