Multiband GaN/AlGaN UV photodetector
Krzysztof P. Korona, Aneta Drabińska, Krzysztof Pakuła, Jacek M. Baranowski
Institute of Experimental Physics, Warsaw University, Hoża 69, 00-681 Warsaw, Poland
Considering current advances in nitride technology, AlGaN/GaN structure is the most
promising material for photodetection in the ultraviolet (UV) region of the spectrum, suitable
to develop efficient visible blind and solar-blind sensors. Lately (6th International Conference
of Nitride Semiconductors, Brema‘05) two groups reported GaN/AlGaN that are sensitive in
two UV ranges. Here we present GaN/AlGaN photodetector structure that is capable to detect
tree UV ranges and can be tuned by external voltage.
The investigated structures were grown by MOCVD on the sapphire substrate. On the
thick Si doped GaN layer there was grown intentionally undoped GaN layer and two AlGaN
layers with different Al content of about 10% and about 20%. The structure was covered with
a very thin GaN cap layer. On the structure surface there was evaporated semitransparent gold
Schottky contact.
Photocurrent measurements show presence of spectral bands related to Al0.2Ga0.8N,
Al0.1Ga0.9N and GaN as well. The Al0.2Ga0.8N-related band at energy of about 3.85 eV is not
sensitive to bias applied to the Schottky contact. It is present under positive and negative bias
as well. On the other hand, the Al0.1Ga0.9N-related band at 3.65 eV changes strongly with the
bias. The photosensitivity of the sample in the 3.65 eV spectral range increases about 10 times
when the bias changes from 0 V to -2 V.
In the photoreflectance spectrum, there are observed lines from all three layers in the
structure – GaN layer (3.426 eV) and both AlGaN layers (3.661 eV and 3.831 eV). All of the
lines have shape characteristic for low electric field regime. There are also observed two
additional lines: one at energy 3.494 eV and very weak second line at energy 3.700 eV. These
lines are connected with transition to two dimensional electron gas (2DEG) on
Al0.1Ga0.9N/GaN and Al0.2Ga0.8N/Al0.1Ga0.9N interface respectively.
The electroreflectance measurement of investigated structure shows three different regions
in both – energy and bias. For the highest energy range (above 3.8 eV) there are visible FranzKeldysh oscillations from the Al0.2Ga0.8N layer. They are clearly visible for the whole bias
range. For energy range of 3.6 - 3.8 eV, for reverse bias below 1.5 V, signal from the
Al0.1Ga0.9N layer has shape characteristic for low electric field regime. For higher reverse
voltage the line changes its shape to Franz-Keldysh oscillations. Finally for lowest energy
range, for the highest bias (about 3 V) in the spectrum appears third line from GaN layer. It
has low electric field regime shape. Therefore in electroreflectance measurement the
mechanism of detector, which changes the energy sensitivity range with applied different bias
can be clearly seen. Analysis of Franz-Keldysh oscillations allows obtaining the basic
parameters of the structures like the position of interfaces and the concentration of 2DEG on
the interfaces.
We expect that even under zero bias, a strong electric field is present in the Al0.2Ga0.8N
(upper layer). The field is due to spontaneous polarization on the interface and due to the gold
layer that depletes electrons during formation of the Schottky contact. Electric field in the
Al0.1Ga0.9N (middle layer) is partially screened and in the undoped GaN layer is fully
screened by a high electron concentration at interfaces. However, at high reverse voltage, the
2D-electron gas can be depleted and electric field penetrates into the lower layers what gives
possibility of generation of photocurrent. A detailed numerical model will be presented.