Spontaneous polarization effects in $\mathrm{GaN}/{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ quantum wells

We observe in $\mathrm{GaN}/{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ quantum wells with varying aluminum concentration strong electric fields whose amplitudes are significantly larger than those observed before in comparable structures fabricated at higher growth temperatures. We show that the measured fields are clearly stronger than what is expected based on piezoelectric effects alone, which constitutes a direct experimental proof of the existence of important spontaneous polarization effects in the $\mathrm{GaN}/{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ heterostructure system.