Diamond, with its ultra-wide bandgap of 5.47 eV, exceptional thermal conductivity exceeding 2000 W/mK, and high breakdown field of 10 MV/cm, is an ideal candidate for high-power and high-frequency electronic devices. However, integrating diamond with other semiconductors such as GaN, SiC, or oxides in heterojunctions presents unique challenges and opportunities. These heterojunctions leverage the complementary properties of each material, enabling advanced device architectures for power electronics, RF applications, and extreme-environment operation. This article examines the critical aspects of diamond-based heterojunctions, including band alignment, interface states, strain management, and their implications for high-performance devices.

Band alignment is a fundamental consideration in diamond heterojunctions due to the large bandgap disparities between diamond and common semiconductors. For instance, GaN has a bandgap of 3.4 eV, while SiC varies between 3.3 eV (4H-SiC) and 2.3 eV (3C-SiC). Oxides like β-Ga₂O₃ exhibit a bandgap of 4.8 eV. The alignment of conduction and valence bands at the interface determines carrier transport mechanisms, influencing device performance. Type-I, Type-II, or Type-III band alignments can arise depending on the material combination. Experimental studies using X-ray photoelectron spectroscopy (XPS) have shown that diamond/GaN heterojunctions often form a Type-II alignment, facilitating electron-hole separation, which is beneficial for high-electron-mobility transistors (HEMTs). In contrast, diamond/SiC interfaces tend to exhibit Type-I alignment, confining carriers within the lower-bandgap material. Precise control over band alignment requires careful tuning of doping levels and interface engineering to minimize unwanted charge trapping or recombination.

Interface states pose a significant challenge in diamond heterojunctions due to lattice mismatches and chemical incompatibilities. For example, the lattice mismatch between diamond and GaN is approximately 11%, while diamond and SiC have a mismatch of around 22%. These mismatches introduce dangling bonds and defects at the interface, acting as trap states that degrade carrier mobility and increase leakage currents. Techniques such as plasma-assisted bonding, surface termination with hydrogen or oxygen, and the insertion of ultrathin buffer layers have been explored to mitigate these effects. Secondary ion mass spectrometry (SIMS) and deep-level transient spectroscopy (DLTS) studies reveal that hydrogen-terminated diamond interfaces with GaN exhibit lower interface state densities compared to oxygen-terminated surfaces, with values as low as 10¹¹ cm⁻² eV⁻¹ reported in optimized structures. Reducing interface states is critical for achieving low on-resistance and high breakdown voltages in power devices.

Strain management is another critical factor in diamond heterojunctions, as the large lattice and thermal expansion mismatches can induce dislocations and cracking. Diamond has a thermal expansion coefficient of 1.0 × 10⁻⁶ K⁻¹, while GaN and SiC exhibit coefficients of 5.6 × 10⁻⁶ K⁻¹ and 4.2 × 10⁻⁶ K⁻¹, respectively. Mismatched thermal expansion can lead to wafer bowing or delamination during device fabrication and operation. Strain-compensation techniques, such as graded buffer layers or compliant substrates, have been employed to address these issues. For instance, the use of AlN interlayers between diamond and GaN has been shown to reduce threading dislocation densities by an order of magnitude, as confirmed by transmission electron microscopy (TEM). Additionally, nanoscale patterning of diamond surfaces prior to heterojunction formation can relieve strain by accommodating lattice mismatches through elastic deformation rather than plastic deformation.

High-power heterostructure devices benefit significantly from diamond-based heterojunctions due to the material’s unparalleled thermal and electrical properties. Diamond/GaN HEMTs, for example, demonstrate improved thermal management compared to conventional GaN-on-SiC devices, with experimental results showing a 30% reduction in channel temperature under high-power operation. This translates to enhanced reliability and lifetime, particularly in RF amplifiers and power converters. Diamond/SiC Schottky diodes have also been reported with breakdown voltages exceeding 3 kV and specific on-resistances below 0.1 mΩ·cm², outperforming standalone SiC devices. The combination of diamond’s high thermal conductivity with SiC’s high breakdown field creates a synergistic effect, enabling operation at higher power densities and temperatures.

Experimental characterization of diamond heterojunctions reveals key insights into their electrical and thermal behavior. Current-voltage (I-V) measurements on diamond/GaN heterojunctions exhibit rectifying behavior with ideality factors ranging from 1.2 to 1.8, indicating relatively low recombination at the interface. Capacitance-voltage (C-V) profiling further confirms the presence of interface states, with trap densities varying between 10¹¹ and 10¹³ cm⁻² eV⁻¹ depending on surface treatment. Thermal imaging studies demonstrate that diamond-integrated devices maintain lower peak temperatures compared to non-diamond counterparts, validating their suitability for high-power applications. For instance, diamond/β-Ga₂O₃ heterojunctions have shown thermal resistance reductions of up to 50% in vertical power transistors, as measured by micro-Raman thermography.

Despite these advancements, challenges remain in achieving large-area, defect-free diamond heterojunctions with reproducible performance. The high cost of single-crystal diamond substrates and the complexity of heteroepitaxial growth on non-diamond materials limit widespread adoption. However, advances in diamond thin-film synthesis, such as microwave plasma chemical vapor deposition (MPCVD), and the development of diamond-on-oxide hybrid substrates offer promising pathways for scalable fabrication. Future research directions include exploring two-dimensional materials as interfacial layers to further reduce defects and improve carrier transport.

In summary, diamond heterojunctions with GaN, SiC, and oxides represent a transformative approach to high-power electronics, combining the strengths of each material to overcome the limitations of individual components. By addressing band alignment, interface states, and strain management, these heterostructures unlock new possibilities for next-generation devices operating under extreme conditions. Continued progress in materials engineering and device integration will be essential to fully realize their potential in applications ranging from electric vehicles to aerospace systems.