An overview is given of the many applications that nm-thin pure boron (PureB) layers can have when deposited on semiconductors such as Si, Ge, and GaN. The application that has been researched in most detail is the fabrication of nm-shallow p+n-like Si diode
junctions that are both electrically and chemically very robust. They are presently used commercially in photodiode detectors for extremeultraviolet (EUV) lithography and scanning-electron-microscopy (SEM) systems. By using chemicalvapor deposition (CVD) or molecular beam epitaxy (MBE) to
deposit the B, PureB diodes have been fabricated at temperatures from an optimal 700 °C to as low as 50 °C, making them both front- and back-end-of-line CMOS compatible. On Ge, near-ideal p+n-like diodes were fabricated by covering a wetting layer of Ga with
a PureB capping layer (PureGaB). For GaN high electron mobility transistors (HEMTs), an Al-on-PureB gate stack was developed that promises to be a robust alternative to the conventional Ni-Au gates. In MEMS processing, PureB is a resilient nm-thin masking layer for Si micromachining with tetramethyl
ammonium hydroxide (TMAH) or potassium hydroxide (KOH), and low-stress PureB membranes have also been demonstrated.
Pure boron layers, deposited by molecular beam epitaxy (MBE) on AlGaN/GaN/p-Si substrates to a thickness of ~ 7 nm, were applied as barriers to aluminum metallization. For low-temperature deposition from 250°C -400°C, low-saturation-current diodes to the n-type GaN were fabricated that all tolerated alloying at 400°C. After alloying, the relatively high current level of the 250 °C diode was reduced to that of the other low temperature diodes, whereas 700 °C B deposition resulted in high-current diode characteristics. The results suggest a favorable B-to-GaN chemistry at 350 °C -400 °C.
Sub-stoichiometric molybdenum oxide (MoO x ) has recently been investigated for application in high efficiency Si solar cells as a "hole selective" contact. In this paper, we investigate the electrical and light-emitting properties of MoO x -based contacts on Si from the viewpoint of realizing functional bipolar devices such as light-emitting diodes (LEDs) and transistors without any impurity doping of the Si surface. We realized diodes on n-type Si substrates using e-beam physical vapor deposition of Pd/MoO x contacts and compared their behavior to implanted p þ n-Si diodes as a reference. In contrast to majority-carrier dominated conduction that occurs in conventional Schottky diodes, Pd/MoO x /n-Si diodes show minority-carrier dominated charge transport with I-V, C-V, and light-emitting characteristics comparable to implanted counterparts. Utilizing such MoO x -based contacts, we also demonstrate a lateral bipolar transistor concept without employing any doped junctions. A detailed C-V analysis confirmed the excessive band-bending in Si corresponding to a high potential barrier (.0:90 V) at the MoO x /n-Si interface which, along with the observed amorphous SiO x (Mo) interlayer, plays a role in suppressing the majority-carrier current. An inversion layer at the n-Si surface was also identified comprising a sheet carrier density greater than 8:6 Â 10 11 cm À2 , and the MoO x layer was found to be conductive though with a very high resistivity in the 10 4 Ω-cm range. We refer to these diodes as metal/non-insulator/semiconductor diodes and show with our device simulations that they can be mimicked as high-barrier Schottky diodes with an induced inversion layer at the interface.
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