Semiconductor nanowires are nanoscale structures holding promise in many fields such as optoelectronics, quantum computing, and thermoelectrics. Nanowires are usually grown vertically on (111)-oriented substrates, while (100) is the standard in semiconductor technology. The ability to grow and to control impurity doping of 〈100〉 nanowires is crucial for integration. Here, we discuss doping of single-crystalline 〈100〉 nanowires, and the structural and optoelectronic properties of p-n junctions based on 〈100〉 InP nanowires. We describe a novel approach to achieve low resistance electrical contacts to nanowires via a gradual interface based on p-doped InAsP. As a first demonstration in optoelectronic devices, we realize a single nanowire light emitting diode in a 〈100〉-oriented InP nanowire p-n junction. To obtain high vertical yield, which is necessary for future applications, we investigate the effect of the introduction of dopants on the nanowire growth.