Hexagonal boron nitride for deep ultraviolet photonic devices

Abstract

This paper provides a brief overview on recent advances in tackling the doping and optical polarization issues involved in the development of high performance deep ultraviolet (DUV) light emitting devices. In particular, recent developments in the exploitation of a novel DUV emitter layer structure based on a hexagonal boron nitride (hBN) and AlGaN p-n junction and doping engineering to potentially overcome the intrinsic problem of low p-type conductivity (or low free hole concentration) in Al-rich AlGaN are summarized. By implementing the wide bandgap and highly conductive hBN p-type layer strategy in nitride DUV emitters, p-type conductivities and DUV transparency of the electron blocking layer and p-type contact layer will be dramatically increased. This will significantly improve the free hole injection and quantum efficiency, reduce the operating voltage and heat generation, and increase the device operating lifetime. The growth of undoped and Mg-doped p-type hBN via a metal organic chemical vapor deposition technique has been studied. Furthermore, p-hBN/n-AlGaN p-n junctions have been fabricated and characterized to demonstrate the feasibility and potential of p-hBN/n-AlGaN p-n heterostructure based DUV light emitting devices. Further improvements in material quality, p-type conductivity control and device processing procedures would enhance the properties of these p-n structures, which could ultimately pave the way towards the realization of high efficiency nitride DUV photonic devices.

High-temperature molecular beam epitaxy of hexagonal boron nitride layers

“…3,4,6 Second, the band gap of hBN is large, $6 eV, and this has fueled interest in hBN as a wide gap material for deepultraviolet device (DUV) applications. 8,9 The development of group III nitrides allows researchers worldwide to consider AlGaN based light emitting diodes as a possible new alternative DUV light source for water purification and surface decontamination. Hexagonal boron nitride has a potential advantage over AlGaN in such DUV structures due to the possibility of more efficient p-and n-doping.…”

“…Hexagonal boron nitride has a potential advantage over AlGaN in such DUV structures due to the possibility of more efficient p-and n-doping. 8,9 Currently, there is no reliable technology able to provide large area hBN bulk single crystals. Research on the growth of bulk hBN single crystals is limited due to the high melting temperature of boron and the low solubility of nitrogen in liquid boron.…”

“…However, due to the high structural quality and good optical properties, hBN flakes exfoliated from these bulk crystals are now widely used as substrates for the growth and manufacture of 2D-structures. [2][3][4][5][6][7] Recently, there have been many attempts to develop a reproducible technology for the growth of large area boron nitride layers by chemical vapor deposition, [19][20][21] metalorganic chemical vapor deposition 8,9,22 and molecular beam epitaxy (MBE). [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] Progress in MBE growth of boron nitride layers has been relatively slow, partly due to a lack of an efficient MBE source for boron, due to its very low vapor pressure.…”

Charge carrier transport properties in layer structured hexagonal boron nitride

“…One possibility is the presence of boron vacancies, V B , or V B related defects, which act as acceptors, in analogous to the acceptor-like native defects in III-nitrides, namely gallium vacancy, V Ga , in GaN and V Al in AlN. 15,19,20 In contrast to conventional III-nitride semiconductors, which have an isotropic wurtzite structure, hBN is a layer structured material. In wurtzite structure, the distances between the nearest and the second nearest neighbors are nearly the same in the directions of parallel and perpendicular to the c-axis.…”

“…For the growth of hBN epilayers, the typical V/III ratio employed was around 3000 and a pulsed growth scheme (alternating flows of TEB and NH 3 ) was undertaken to minimize the pre-reaction between TEB and ammonia, which is necessary for obtaining epilayers in hexagonal phase. 15 Adopted from the hetero-epitaxial growth of III-nitride device structures, a low temperature BN buffer layer of about 10 nm in thickness was deposited on the sapphire substrate at about 800…”