(Invited) Heterogeneous Integration of Ultrawide Bandgap III-Nitrides and Diamond
Authors: Piner Edwin L. , Holtz Mark W. 
Published: February 2024

Industry demands for power switching, amplification and photonic applications are continually pushing the limits of semiconductor device technologies leading to increasing applied voltages and, thus, higher electric fields while simultaneously requiring reduction in the size, weight and power consumption. In addition, recent heightened interest in higher temperature (and other extreme environment) applications, while simultaneously maintaining high-voltage and high-power performance, has inexorably led to ultrawide bandgap (UWBG) semiconductor materials. This interest in UWBG materials stems from superior properties that include high material hardness, breakdown strength to withstand large electric fields and mitigation of power dissipation during device switching, deliver high current density while operating at higher junction and ambient temperature. Among these materials are boron nitride in its hexagonal and cubic phases, gallium oxide, diamond, and aluminum gallium nitride. The versatile alloys of wurtzite AlxGa1-xN have long been demonstrated across the full composition range. The variation in Eg
with AlGaN composition, along with select WBG and UWBG materials, is shown in Figure 1 versus (in-plane for non-cubic) lattice constant.

Premier WBG materials to date for electronics and photonics are SiC (Eg
= 3.3 eV) and GaN (Eg
= 3.4 eV). The latter has been successfully applied to high-power transistors and near ultraviolet photonics. In both electronics and photonics applications, the polarization fields of the wurtzite crystal structure, generally grown along (0001) direction, produce design challenges. In photonics, for example, the fields and band alignment couple to spatially separate electrically injected electrons and holes to reduce wavefunction overlap in quantum wells. In electronics, the internal fields have contributed to difficulties in the design of logic and bipolar transistor devices based on these materials. Despite this, the inherent polarization field and band offsets were successfully employed in the design of high electron-mobility transistors (HEMTs). The inherent AlGaN/GaN HEMT band offset produces a high electron concentration at the interface that spontaneously forms a two-dimensional electron gas (2DEG). Because charge flow in these devices is restricted to the 2DEG, current crowding results in intense self-heating. The local heating decreases electrical conductivity to produce a runaway scenario for elevating the Joule heating. Dissipating this heat, therefore, is the principal limiting factor in operating at power higher than 40 W/mm.

The various temperature dependences of semiconductor device parameters are the principal drivers in the operation of semiconductor devices. The physics has inextricably led to UWBGs for future high temperature applications, > 300 °C, due to their lower intrinsic carrier density and thus the effects of leakage are orders of magnitudes less than Si-based devices. Furthermore, AlGaN and diamond offer superior high temperature mechanical stability due to their structural hardness and chemical inertness. Thermal management is essential for achieving higher power densities. Diamond is highly attractive for many applications due to its high hardness, transparency, chemical inertness, promising semiconductor properties, and its high thermal conductivity (κ) to 2500 W/m·K in natural material. Factors related to the consequences of self-heating in devices have motivated extensive research to improve the properties of diamond and produce better approaches to determine the thermal properties. Laboratory-grown diamond films exhibit significantly poorer κ than natural diamond and, when integrated with other electronic materials, a low thermal boundary conductance (TBC in W/m2·K) that inhibits heat transfer between materials. The former originates from the ubiquitous polycrystalline nature of chemical vapor deposition (CVD) diamond and the presence of non-diamond carbon (NDC) in the layer. The low TBC stems from fundamental barriers to phonon propagation between two materials and from practical issues of poor diamond quality in the initial growth regime. Continued research is needed to improve κ in the polycrystalline diamond and mitigate the negative impact of the initial diamond layer on TBC.

The current status of hetero-integration of UWBG AlGaN and diamond will be reviewed. Particular emphasis is given to high Al content AlGaN for near-bandgap matching with diamond. Patterned diamond grown using hot-filament CVD to realize unique three-dimensional lateral device structures in a self-aligned, as-grown fabrication process is employed. Engineering the AlGaN-diamond bandgap alignment is noteworthy at the nonpolar AlGaN – diamond interface including diamond surface terminations that produce either negative or positive electron affinity. The review will focus on improvements in material properties, especially the interfaces, that are critical to both charge and heat transport for UWBG device structures.

Figure 1