NAMBE 2025 Session NAMBE2-TuM: III-Nitrides

This work reports the fabrication and characterization of the lateral kilo-volt class NiO_x/GaN super-heterojunction (SHJ) diodes via ammonia molecular beam epitaxy (NH₃-MBE), accomplishing ~2.8 kV breakdown voltage, and maximum critical electric field > 1 MV/cm. The Si-doped GaN (~250 nm) was grown at 790 ºC with a 200 sccm NH₃ flow on a semi-insulating Fe-doped GaN-on-sapphire template after an unintentionally doped (UID) buffer layer with two-dimensional sheet charge density (σn~1.1×10¹³ cm^-2) and electron mobility (μn~462 cm²/V•s) confirmed by high-voltage capacitance-voltage (C-V) and room temperature Hall effect measurements.

The MBE grown GaN epilayer underwent a blanket plasma-etch by using BCl₃/Cl₂ reactive ion etching (RIE) to reduce the two-dimensional sheet charge density down to 7.32×10¹² cm^-2 to mitigate built-in junction electric field. After optical lithography, the etched GaN was mesa-isolated using RIE, then an e-beam evaporated Ti/Al/Ni/Au (30/120/30/50 nm) metal stack was annealed at 820 ºC in nitrogen (N₂) environment to form high quality Ohmic contact on the mesa, which was confirmed via rectangular transmission line measurements (RTLM). After cathode formation, an 86-nm p^- NiO_x was deposited via reactive magnetron sputtering to form a charge-balanced extension region. The acceptor concentration (N_a) in sputtered p^- NiO_x was extracted to be 8.19×10¹⁷ cm^-3 via heterojunction diode capacitance-voltage (C-V) method at 10 kHz, and the N_a concentration was stabilized by a 5-minute N₂ anneal. Following the p^- NiO_x deposition, a ~15-nm p⁺⁺ NiO_x contact layer was sputtered on the sidewall, covering a planar extension portion of the p- NiO_x, then a Ni/Au (50/100 nm) anode Ohmic stack was deposited via e-beam evaporation on p⁺⁺ NiO_x. Finally, a 2-μm epoxy-based polymer photoresist (SU-8) was applied to the SHJ diodes as a passivation layer to conclude device fabrication.

The NiO_x/GaN SHJ diode exhibits a 2-7.5 mA/mm linear forward current density at 5 V, and a reverse leakage of ~10^-7-10^-5 mA/mm under low reverse bias (-5 V) with a rectifying ratio (J_on/J_off) of 10⁵~10⁷for devices with 16,25,and 50-μm anode-to-cathode distances (L_AC). The 50- μm L_AC SHJ diodes exhibited the highest breakdown voltage at ~2.8 kV with SU-8 passivation, showing a 5X improvement of breakdown voltage at reverse leakage of 10^-5 A/mm compared to its reference counterparts that were not charge-balanced. The maximum breakdown field of SHJ diode was extracted to be >1 MV/cm on a 16-μm L_AC device.

Acknowledgement:Weacknowledge the funding from U.S. Department of Energy (DOE) ARPA-E OPEN 2021 program (DE-AR0001591).

The doping of third-party elements is the backbone of the microelectronics industry, as it allows delicate control of electron/hole concentration, but it can also be used to imbue a host matrix with unique magnetic or optical properties. Wurtzite gallium nitride is a widely studied large bandgap semiconductor. There are reports of doping GaN with numerous elements, with some being extensively employed in commercial applications. However, there are still a few elements which remain completely unexplored. This work investigates the doping limits and effects of select transition metals, lanthanoids, and actinoids in GaN. The structural, electronic, and optical properties of these first-of-a-kind combinations are presented. Embedding single crystal wide bandgap materials with additional functionality will provide building blocks for new multifunctional hybrid systems for novel sensors, quantum science, or meta-multiferroics. Leveraging the non-centrosymmetric piezoelectric host matrix and atomic-level control of dopant species could allow for active tuning of proximity and correlated phenomena, potentially opening the door for applications of actinide elements beyond nuclear fuels.

Solid solutions of wurtzite III-nitrides and rare-earth nitrides form a flourishing class of ferroelectric (FE) nitrides, including ScAlN, ScGaN, YAlN, and quaternary alloys. However, many desirable properties of FE nitrides, including exceptional piezoelectric response, optical non-linearity, and lower coercive fields, only manifest at high Sc compositions. Theoretical studies predict for ScAlN, 56% Sc is possible before transitioning to a nonpolar cubic structure. Nevertheless, growing ScAlN beyond 40% Sc has proven challenging. Alternatively, the solubility of ScN in GaN is predicted to be greater than in AlN. Furthermore, initial studies on ScGaN have recognised the potential for lower coercive fields while maintaining many desirable properties of ScAlN.

To this end, we investigate the MBE growth and FE properties of ScGaN with Sc compositions up to over 50%. 45 nm-thick ScGaN films are grown on GaN-on-sapphire templates under moderately nitrogen rich conditions. EDS confirms Sc compositions of 35%, 47%, and 56% in a series of samples. AFM shows sub-1 nm roughness over 25 μm² scan areas, even for Sc_0.56Ga_0.44N. (002) plane XRD rocking curves reveal FWHMs ranging from 650 arcsec to 1550 arcsec for Sc_0.35Ga_0.65N and Sc_0.56Ga_0.44N, respectively. Furthermore, a (102) plane ϕ scan demonstrates excellent epitaxial registry between Sc_0.56Ga_0.44N and GaN. Despite large lattice mismatch and distortion, phase-pure single-crystal growth can still be achieved. Electrical characterisation illustrates unambiguous FE switching with decreasing coercive field and remanent polarisation with increasing Sc, matching expectations that greater Sc content flattens the wurtzite structure towards a nonpolar state. Most notably, the coercive field decreases from 2.5 MV cm^-1 at Sc_0.35Ga_0.65N to 1.2 MV cm^-1 at Sc_0.56Ga_0.44N, comparable to members in the HZO family. Moreover, the fatiguing characteristics of Sc_0.56Ga_0.44N show non-zero polarisation remaining after 10⁹ cycles, significant improvement over 10⁶-10⁷ cycles in MBE-grown Sc_0.2Al_0.8N. These results serve as a first glance into the realm of Sc-rich FE nitrides to harness outstanding FE, piezoelectric, and optical properties in a nitride platform. Further study is anticipated to continue developing the advanced epitaxy to grow ScGaN towards the predicted phase transition content of 66% Sc and beyond.

Displays for future technologies such as augmented and virtual reality require ultra-high resolution and efficient power consumption, for which III-nitride LEDs at the (sub)micron scale (micro-LEDs) have been under intense investigation. Conventional top-down processing of micro-LEDs, however, requires a plasma etch to define mesas for individual devices on a wafer. As the mesa size shrink down to the micrometer regime, device efficiency loss due to plasma-induced non-radiative centers on the sidewall surfaces is further exacerbated with increasing surface-to-volume ratio. In addition, while the III-nitride family has shown immense promise for micro-LED applications due to a host of desirable properties (such as tunable bandgap across the entire visible spectrum and low surface recombination velocity), it also presents a host of material challenges, such as the large lattice mismatch and lack of intermiscibility between InN and GaN and spectral variation under different operating conditions due to quantum-confined Stark effect, that must be overcome for the device’s eventual commercialization.

Here, we show that by utilizing a combination of strategies for relieving strain and enhancing the carrier injection efficiencies, including the bottom-up approach, nitrogen polarity, Mg-doped AlGaN electron-blocking layer, and a p-GaN layer with gradient doping, unprecedentedly high wall-plug efficiency for a sub-micron red LED has been achieved. The bottom-up approach, achieved via selective area epitaxy, eliminates the need for a plasma etch step through the device stack during device fabrication, thereby protecting the active region from the deleterious surface damage mentioned above. In addition, the bottom-up nanowires form a strain-relaxed and nearly defect-free template that allows for high quality InGaN active region with the very high levels of indium incorporation needed for red emission. To reduce electron overflow and achieve high wall-plug efficiency with these N-polar red-emitting nanowires, we have also incorporated an AlGaN EBL, as well as a p-GaN layer with graded Mg-doping.

The resultant nanowire-based InGaN micro-LED exhibited an emission wavelength of ~650 nm at a peak external quantum efficiency of ~12.8% and a wall-plug efficiency of ~12.2%, corresponding to a peak electrical efficiency of ~95%. Through this work, we demonstrate that the hitherto low electrical efficiency of nanowire-based devices can be overcome through careful design of the device heterostructure and that such devices can form the foundation of future micro-LED displays.

Development of AlGaN/GaN high electron mobility transistors (HEMTs) has resulted in a myriad of applications, including high output RF power amplifiers, owing to the high breakdown field strength and excellent transport characteristics observed in III-nitride semiconductors. However, despite their wide bandgap, breakdown voltage in lateral HEMTs is ultimately limited due to the high peak electric fields leading to premature breakdown when compared to theoretical limits. Field management strategies leveraging high permittivity gate dielectrics including BaTiO₃ (BTO) have recently been shown to improve electric field distribution, thereby increasing breakdown voltage. These heterostructures typically utilize BTO dielectric layers deposited by RF sputtering, which are susceptible to low crystal quality or polycrystalline films, having lower dielectric constants (κ) than bulk or single crystalline BTO. Enhancing the crystal quality of the BTO film can both improve the quality of the oxide/nitride interface and increase κ, leading to further improvements in device voltage handling.

In this work, we demonstrate the growth of epitaxial (111)-oriented BTO thin films onto AlGaN/GaN HEMT heterostructures by RF-plasma assisted oxide molecular beam epitaxy. An epitaxial 2 nm SrTiO₃ (STO) / 1 nm TiO₂ bilayer stack is first deposited on the AlGaN surface to reduce the lattice mismatch and to provide a seed layer to facilitate crystalline and well-oriented BTO growth. The addition of the STO layer is necessary to achieve highly crystalline BTO due to its tetragonal structure and slightly larger unit cell volume compared to STO. Transmission electron microscopy imaging confirms the epitaxial orientation of the BTO film. Van der Pauw Hall effect measurements show no significant change in the sheet resistance, electron mobility, or electron density of the GaN channel due to the presence of the epitaxial BTO layers.

We investigate the effect of BTO growth temperature on structural and electrical properties by depositing the BTO layers at substrate temperatures ranging from 550-850 °C and find that the BTO crystallinity improves as growth temperature is increased. I-V and C-V measurements on test capacitor structures fabricated on the BTO on AlGaN/GaN heterostructures demonstrate trends of increasing κ values of the oxide layers and reduced leakage with increasing BTO growth temperature. A BTO κ of 340 is measured on a sample grown at 850 °C. Finally, loss tangent measurements indicate tan δ values as low as ~2×10^-3. These results indicate a promising approach to building high quality HEMTs with improved interfaces and enhanced RF and power performance.