Boron Phosphide Films by Reactive Sputtering: Searching for a P‐Type Transparent Conductor

Abstract

eral applications such as a corrosion-and heat-resistant coating, [4,5] photo-and electrocatalyst, [6,7] as well as for thermal management [1] and extreme UV optics applications. [8] More recently, BP was identified as a potential p-type transparent conductive material (TCM). [9] This is a particularly interesting prospect, because obtaining high p-type conductivity in optically transparent materials is still an unsolved challenge. [10,11] Unlike the case of other p-type TCM candidates, bipolar doping has been reported in BP by various authors. [3,5,9,12,13] Thus, BP could be a unique example of a transparent material with both p-type and n-type doping capability. BP crystallizes in the diamond-derived zincblende structure with tetrahedral coordination. Because the electronegativity difference between B and P is small, BP is a covalent solid and its band structure is closely related to that of Si and C in the diamond structure. The main difference is an intermediate size of the fundamental indirect band gap for BP (≈2.0 eV) [14][15][16] mainly due to an intermediate bond length. Although this band gap corresponds to visible light, the direct band gap of BP is much wider and falls in the UV region (≈4.3 eV). [15][16][17] The weakness of indirect transitions predicted for BP at room temperature [15] is the key factor that could make BP thin films sufficiently transparent for many TCM applications. For example, a 100 nm-thick BP film is expected to absorb negligible amounts of red-yellow light and less than 10% of violet light according to first-principles calculations including electron-phonon coupling. [15] With respect to electrical properties, BP has a highly disperse valence band produced by p orbitals, ensuring low hole effective masses (0.35 m e ). [9] Unlike the case of diamond, the valence band maximum of BP lies at a relatively shallow energy with respect to the vacuum level. Shallow, disperse valence bands are usually correlated with high p-type dopability, due to easier formation of uncompensated shallow acceptor defects. [18,19] Open Questions in BP Research Conductivity and TransparencyThe highest conductivity reported for p-type BP is 3600 S cm −1 for a nominally undoped single-crystalline film deposited by chemical vapor deposition (CVD) at 1050 °C using B 2 H 6 and With an indirect band gap in the visible and a direct band gap at a much higher energy, boron phosphide (BP) holds promise as an unconventional p-type transparent conductor. This work reports on reactive sputtering of amorphous BP films, their partial crystallization in a P-containing annealing atmosphere, and extrinsic doping by C and Si. The highest hole concentration to date for p-type BP (5 × 10 20 cm −3 ) is achieved using C doping under B-rich conditions. Furthermore, bipolar doping is confirmed to be feasible in BP. An anneal temperature of at least 1000 °C is necessary for crystallization and dopant activation. Hole mobilities are low and indirect optical transitions are stronger than that predicted by theory. Low crystalline qua...

Computational Discovery and Experimental Demonstration of Boron Phosphide Ultraviolet Nanoresonators

“…We now turn to an experimental demonstration of the potential of BP as a Mie-resonant nanostructure. BP has been synthesized in various forms such as crystals, [43,44] films, [35,[45][46][47] and nanoparticles. [48][49][50] However, BP nanoparticles in the size range suitable for sustaining Mie resonances have not been reported.…”

Crystallize it before it diffuses: Kinetic stabilization of thin-film phosphorus-rich semiconductor CuP2

“…The wide tunability of the P content in Cu-P films was also observed in our recently reported amorphous B-P films by reactive sputtering. 21 This compositional flexibility is likely related to the ability of P to form homoelement bonds in the films and segregate as an elemental impurity.…”

“…We have recently shown the feasibility of this deposition technique for various metal-rich phosphide compounds. [20][21][22] In this work, we demonstrate a relatively simple two-step process route to grow polycrystalline CuP 2 thin films as semiconductors of potential technological interest. First, reactive sputter deposition of amorphous CuP 2+x in a PH 3 -containing atmosphere; second, crystallization by rapid thermal annealing (RTA) in an inert atmosphere.…”

A Search for New Back Contacts for CdTe Solar Cells

“…[27] BP has recently been the subject of several computational and experimental studies as a potential p-type, non-oxide transparent conductor. [27,28,29] Some of the materials with high hole mobility (> 10 cm 2 /Vs) and large DFT band gaps (> 1 eV) are labelled in Figure 5. II-IV-V 2 (e.g., ZnSnP 2 ) and I-III-VI 2 (e.g., AlCuTe 2 ) compounds feature prominently among them, in addition to Al-containing III-V compounds (e.g., AlSb).…”