High pressure is a powerful tool for modifying the electronic properties of semiconductors, enabling precise control over their band structures without altering their chemical composition. By subjecting materials to extreme conditions, researchers can induce significant changes in their electronic and optical behavior, unlocking new possibilities for optoelectronic applications. This article examines how high pressure affects the band structure of semiconductors, with a focus on gallium nitride (GaN), zinc oxide (ZnO), and diamond, highlighting direct-to-indirect bandgap transitions, pressure-induced bandgap closure, and the resulting tunability for device applications.

At ambient conditions, semiconductors exhibit specific electronic band structures that determine their optical and electrical properties. The application of high pressure modifies interatomic distances and bonding angles, leading to shifts in conduction and valence band energies. For many materials, increasing pressure reduces the lattice constant, altering the overlap of atomic orbitals and consequently the energy dispersion relations. This phenomenon can result in changes to the bandgap type, magnitude, and even the electronic topology of the material.

Gallium nitride, a widely used wide-bandgap semiconductor, undergoes notable changes under high pressure. At ambient pressure, GaN has a direct bandgap of approximately 3.4 eV in its wurtzite phase. When subjected to pressures exceeding 20 GPa, GaN transitions to a rocksalt phase, accompanied by a shift from a direct to an indirect bandgap. This transition is critical for optoelectronic applications because direct bandgap materials are more efficient for light emission. The pressure-induced phase change also leads to a significant reduction in the bandgap, which can be exploited for tuning the emission wavelength of GaN-based devices. Experimental studies confirm that the bandgap of GaN decreases linearly with pressure before the phase transition, offering a predictable way to modulate its optical properties.

Zinc oxide, another important semiconductor, exhibits similar pressure-dependent behavior. ZnO possesses a direct bandgap of around 3.3 eV under ambient conditions. At pressures near 9 GPa, the wurtzite structure of ZnO transforms into a rocksalt phase, accompanied by a transition to an indirect bandgap. Unlike GaN, ZnO shows a non-linear reduction in bandgap energy with increasing pressure before the phase transition. The pressure coefficient of ZnO’s bandgap is approximately -25 meV/GPa in the wurtzite phase, allowing for fine-tuning of its optoelectronic response. The rocksalt phase of ZnO, stabilized at high pressures, exhibits metallic behavior due to bandgap closure, which has implications for high-pressure conductivity applications.

Diamond, with its ultra-wide bandgap of 5.5 eV at ambient conditions, demonstrates unique pressure-induced electronic changes. Unlike GaN and ZnO, diamond retains its cubic structure up to extremely high pressures, but its bandgap decreases under compression. The direct bandgap of diamond narrows at a rate of about -60 meV/GPa, making it possible to adjust its optical absorption edge. At pressures beyond 150 GPa, theoretical predictions suggest that diamond may undergo a bandgap closure transition, leading to semi-metallic behavior. This property is particularly relevant for high-pressure electronics and cutting-edge quantum devices.

Pressure-induced bandgap transitions are not limited to direct-to-indirect shifts. Some semiconductors experience band inversion under high pressure, where the ordering of conduction and valence bands changes, leading to topological phase transitions. For example, certain chalcogenides and topological insulators exhibit such behavior, though GaN, ZnO, and diamond primarily undergo conventional bandgap modifications. The ability to controllably alter the band structure through pressure provides a pathway for designing materials with tailored optoelectronic properties.

The tunability of semiconductor bandgaps under high pressure has significant implications for optoelectronic applications. By adjusting pressure, researchers can modify the emission and absorption spectra of materials, enabling the development of pressure-sensitive photodetectors and tunable light sources. For instance, GaN-based devices could be engineered to emit at different wavelengths by embedding them in pressure-confining structures. Similarly, ZnO’s pressure-dependent bandgap could be leveraged for strain sensors or adaptive optical filters. Diamond’s robust structure and pressure-responsive bandgap make it suitable for high-stress environments, such as deep-Earth sensors or high-power electronic devices.

Beyond optoelectronics, high-pressure bandgap engineering plays a role in fundamental studies of material behavior. The ability to induce phase transitions and bandgap modifications without chemical doping allows for cleaner investigations of electronic phenomena. For example, studying the metallization of ZnO or diamond under pressure provides insights into the interplay between crystal structure and electronic properties. These findings can inform the design of new materials with desired characteristics under both ambient and extreme conditions.

Practical implementation of high-pressure effects requires careful consideration of stability and reversibility. While some changes, like phase transitions in GaN and ZnO, may be irreversible upon pressure release, others, such as elastic deformation in diamond, can be reversible. Advances in diamond anvil cell technology and computational modeling have enabled precise control over pressure conditions, facilitating detailed studies of these effects. Additionally, strain engineering in thin films and nanostructures can mimic high-pressure conditions, offering a more scalable approach for device integration.

In summary, high pressure serves as a versatile means to manipulate the electronic band structure of semiconductors. Materials like GaN, ZnO, and diamond exhibit distinct pressure-induced changes, including direct-to-indirect bandgap transitions and bandgap closure, which can be harnessed for advanced optoelectronic applications. The ability to tune these properties without chemical alteration opens new avenues for material design and device innovation. Continued research in high-pressure semiconductor physics will further expand the understanding and utilization of these effects in cutting-edge technologies.