  An isotype heterojunction n+-ZnO/n-Si photodetector is developed,
demonstrating wavelength-selective or broadband operation, depending on the
applied bias voltage. Additionally, at self-powered (zero bias) operation, it
distinguishes between UV, visible, and near IR (NIR) photons by polarity
control of the photocurrent. The photodetector is developed by atomic layer
deposition (ALD) of ZnO on n-Si, followed by electric contact deposition and
annealing. Photoluminescence measurements reveal high optical quality and
improved crystallinity of annealed ZnO on silicon. Photocurrent measurements as
a function of illumination wavelength and bias voltage show small negative
values in the UV-visible spectral range at zero and positive bias voltage and
high positive values in the NIR spectral range. For these measurements, we
consider the electric contact to ZnO as the anode and the electric contact to
silicon as the cathode. At negative bias voltage, the device shows broadband
operation with high photocurrent values across the UV-vis-NIR.
