Extreme ultraviolet detection using AlGaN-on-Si inverted Schottky photodiodes
Applied Physics Letters
We report on the fabrication of aluminum gallium nitride (AlGaN) Schottky diodes for extreme ultraviolet (EUV) detection. AlGaN layers were grown on silicon wafers by molecular beam epitaxy with the conventional and inverted Schottky structure, where the undoped, active layer was grown before or after the n-doped layer, respectively. Different current mechanisms were observed in the two structures. The inverted Schottky diode was designed for the optimized backside sensitivity in the hybrid
... ers. A cut-off wavelength of 280 nm was observed with three orders of magnitude intrinsic rejection ratio of the visible radiation. Furthermore, the inverted structure was characterized using a EUV source based on helium discharge and an open electrode design was used to improve the sensitivity. The characteristic He I and He II emission lines were observed at the wavelengths of 58.4 nm and 30.4 nm, respectively, proving the feasibility of using the inverted layer stack for EUV detection. Extreme ultraviolet (EUV) detection is gaining increasing attention with recent developments in solar science and EUV lithography. 1 ' 2 Photodetectors used in most applications are based on Si, sensitive to the visible and infrared radiation. Using wide bandgap semiconductors as the active layer can reduce the number of filters required to suppress the unwanted radiation at large wavelengths. 3-7 Furthermore, an improved EUV hardness is observed in devices based on this compound. 8 Recent advances in epitaxial growth enable fabrication of high quality aluminum gallium nitride (AlGaN) layers on Si substrates, both by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition. For advanced applications, two-dimensional arrays with AlGaN active layers are highly desired, which require special design both of the pixel and of the layer stack. 9 In this paper we present the inverted Schottky structure in submicron-thin Al-GaN layers designed especially for backside illuminated two-dimensional arrays with a very small pixel-to-pixel pitch. The conventional Schottky structure is not suitable because of detrimental absorption and recombination in the doped layer when the device is illuminated from the backside, as all materials are highly absorbing in the EUV 10 Active AlGaN layers were grown on 2 in. Si(lll) wafers by MBE. Two layer types were investigated: a conventional Schottky with n-doped layer at the bottom of the stack 11 and an inverted Schottky with the n-doping at the top (Fig. 1) . The growth started with 40 nm of A1N nucleation layer. The growth continued for 300 nm while gradually decreasing the Al content from 100% to 40%. Furthermore, the Si doping was introduced for either first (conventional) or last (in-verted) 100 nm with fixed Al content. This yielded a nonintentionally doped AlGaN layer serving as the active layer of the photodetector (300 nm) with an n-doped layer to improve the Ohmic contact (100 nm). Processing started with accessing the bottom layer by Cl-based reactive ion etching (RÍE) of approximately 200 nm. In the conventional Schottky, undoped AlGaN was left as islands (mesa), whereas in the inverted Schottky it was accessed in the etched pits (inverted mesa). The Ohmic contact was a sputtered metal stack of Ti/Al/Mo/Au (10/40/25/50 nm, respectively), which was subsequently annealed for 1 min at 850 °C. The contact resistance of approximately 1 Cl mm was extracted from the transfer length method measurements using large Ohmic contacts spaced from 32 to 1 /xm for both layers. The Schottky contact was a semitransparent, 20 nm Au layer. Additionally, 100 nm of Si0 2 was used as passivation and to sepárate the Ohmic and fanout metallization. The passivation was opened by SF 6 -based RÍE over the active área and the contact level was deposited (TiW/Ni/Au, 10/150/150 nm, re-(a) SSL \>ht»t)»)t»bt»fx 300 nm undoped AlGaN (b) £1 -ES 100 nm n-doped AlGaN 300|imS¡(lll) Q 300 nm undoped AlGaN 40nmAIN ^ 300umS¡(lll) Electronic mail: email@example.com. FIG. 1. AlGaN EUV photodetector structure: conventional (a) and inverted Schottky (b).