Integration of Layered Materials with Group-III Nitride Semiconductors for Dual Band Photodetection
In many applications, simultaneous detection in two distinct bands, UV and IR regime is required. An instrumentation in which detection in both the bands achievable using single device would be highly desirable owing to its cost effectiveness, ease in fabrication and ease in operation. Many attempts were made to integrate two materials to make device capable in dual band detection. One of such attempts was epitaxial stacking of two different members of Group-III Nitride family members, possessing different bandgap, to form single device. However, such epi-stacks suffer from lattice mismatch, difficulty in growth and fabrication, traps and dislocations generating because of lattice mismatch, thus affecting electrical and optical performance of the device. In such difficulties, layered materials (or commonly referred as 2D semiconductors) have gained big interest thanks to their exciting properties. Layered materials (LMs) can be transferred virtually on any substrate using very simple methods without worrying about lattice mismatch issues. They have excellent electrical and optical properties which make them attractive candidates for optoelectronic applications. Through this work, we attempt to integrate layered materials with GaN, a wide bandgap Group-III Nitride semiconductor and show that, by using simple integration techniques and extreme bandgap engineering by exploiting band alignments in heterojunction, it is possible to achieve dual band photo detection. We started with Integration of MoS2 with GaN. By fabricating MSM device such that metal formed contact only on MoS2 and not on GaN, we showed that it is possible to extract photocarriers generated in GaN layer underneath through MoS2 layer. The detection spectra showed two sharp edges in spectral responsivity (SR) graph, one at 365 nm (UV) and another at 685 nm (Visible). Normalised SR was found out to be 127 A/W and 33 A/W at 365 nm and 685 nm respectively. Device showed persistent photoconductivity (PCC) making it a slow device. Detectivity at ~3.3 x 1011 Jones at 532 nm laser excitation. Next, in similar approach, devices were fabricated by integrating β-In2Se3 with GaN. SR graph showed two distinct band edges, one at 365 nm (UV) and another at 850 nm (NIR) making it 1st demonstration of simultaneous dual band detection. Normalised SR were calculated to be 1.75 A/W and 32.7 mA/W at 365 nm and 850 nm respectively. Device showed very less PCC compared to MoS2/GaN device making it little faster. A detectivity of ~1.6x109 Jones was found at 805 nm which is 1st report by such device at 805 nm laser excitation. We then attempted with vertical photodetector (PD) by integrating α-In2Se3 with GaN. This time device showed two sharp band steps in SR graph, one at 365 nm and another at 850 nm with considerable rejection in visible band. Device showed dual band detection in both positive and negative bias. Normalised SR was recorded to be 1.9 A/W for 3V bias and ~0.088 A/W for -3V bias at 365 nm. While at 850 nm, NSR was found out to be ~0.07 A/W (-3V) and 0.05 A/W (3V). detectivity was found out to be 3.6 x 1010 Jones at -3V and 1.2 x 1010 Jones at 3V for the excitation wavelength of 850 nm. Device showed faster response compared to previously mentioned lateral devices making it suitable for communication applications. Finally, we attempted to show suitability of N-polar GaN MSM PD for the photodetection applications. Device exhibited excellent spectral responsivity with sharp step at 365 nm with UV-to-Vis ratio of ~250. Finally, it was concluded that, improvement in the device performance can be achieved by improvement in the growth quality of N-polar GaN, dark current reduction and device architecture.