High-speed characterization of solar- blind AlxGa1−xN p–i–n photodiodes

We report on the temporal pulse response measurements of solar-blind AlxGa1−xN-based heterojunction p–i–n photodiodes. High-speed characterization of the fabricated photodiodes was carried out at 267 nm. The bandwidth performance was enhanced by an order of magnitude with the removal of the absorbing p+ GaN cap layer. 30 μm diameter devices exhibited pulse responses with ∼70 ps pulse width and a corresponding 3 dB bandwidth of 1.65 GHz. Since the first demonstration of solar-blind AlxGa1−xN photoconductors [1, 2], AlGaN-based ultra-violet (UV) photodetectors with cut-off wavelengths smaller than 280 nm have proved their potential for solar-blind detection. They can be utilized in a number of civil and military applications. Certain applications including missile warning and tracking systems or secure UV optical communication systems for space-to-space (inter-satellite) communication need highspeed solar-blind photodetectors [3]. To date, the fastest solar-blind AlGaN p–i–n detectors reported had ns-scale decay time constants [4, 5]. In these AlGaN p–i–n detector reports, authors claim that the pulse response of their devices is limited by two factors: RC time constant and trapping mechanism (proportional to dislocation density). Recently we have reported very fast AlGaN solar-blind detectors with Schottky and MSM device structures, exhibiting GHz bandwidths [6, 7]. In this paper, we report solar-blind AlGaN p–i–n photodiodes with high-speed performance in the GHz regime. The AlGaN p–i–n photodiode wafer was grown by MOCVD on sapphire substrate. The growth process started with an AlN buffer layer, followed by the growth of a 250 nm thick n-type doped GaN ohmic contact layer. Subsequently, a 100 nm thick unintentionally doped Al0.45Ga0.55N layer was grown as the active detector region. Finally, the 55 nm thick p+ ohmic contact layer was formed by a three step growth: a p+ Al0.45Ga0.55N layer, a p+ grading layer and a p+ GaN cap layer. The p-type doped grading and cap layers were grown in order to reduce carrier trapping at the AlGaN/GaN interface and to improve the p+ ohmic contact quality, respectively. The solar-blind devices were fabricated using a microwave compatible fabrication process. Figure 1 shows the cross-sectional device structure of the AlGaN p–i–n photodiode. Ohmic contacts were formed by annealed Ni/Au and Ti/Al alloys for p+ and n+ contacts, respectively. CCl2F2based reactive ion etching (RIE) process was utilized for n+ ohmic and mesa isolation etch steps. The p–i–n detectors were passivated with a∼120 nm thick Si3N4 layer, grown by plasmaenhanced-chemical-vapour-deposition (PECVD). To make contact to the devices, thick (∼0.6 μm) interconnect metal pads were deposited in the final step of fabrication process. Microphotographs of completed AlGaN p–i–n photodiode samples captured by optical microscope and scanning electron microscope (SEM) are shown in figure 2. On-wafer dc and microwave measurements of the fabricated devices were carried out. To observe the effect of absorbing p+ GaN cap layer on device performance, measurements were applied before and after recess etch. The p+ GaN cap layer and p+ grading layer were recess etched in three steps, each of ∼15 nm etch depth. The recess etch was done after the Si3N4 passivation layer was etched with buffered HF etchant. The absorbing layers were etched using the same RIE recipe used for ohmic and mesa isolation etch processes. After the 3-step etch process, 10 nm thick transparent p+ Al0.45Ga0.55N layer remained at the top of the devices. The AlGaN detector samples exhibited very low (fAlevel) dark current and high UV responsivity with ∼283 nm cut-off wavelength. The details of these results can be found 0268-1242/04/111259+04$30.00 © 2004 IOP Publishing Ltd Printed in the UK 1259