Zinc oxide or ZnO group semiconductors, which are comprised of ZnO and several ternary or even quaternary semiconductors created by adding appropriate additional elements to zinc and oxygen, are potentially suitable for UV-emitting solid-state technologies.
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Wide band gap or WBG semiconductors are required in solid-state ultraviolet radiation emitting and sensing devices. Manufacturing effective emitters is substantially more challenging than manufacturing efficient UV radiation detectors. Regarding UV radiation emitters, a semiconductor's band gap energy must be equal to or greater than the photon energy that must be emitted.
The Problem with III-Nitrides
III-nitrides are also employed to create lower wavelength LEDs, featuring emission wavelengths as low as 250 nm. However, because of variables such as high non-radiative recombination and inadequate ohmic contact, such devices have extremely low emission efficiencies.
As a result, a significant portion of the electrical energy input is lost as heat. This reduces the efficiency of energy conversion and exacerbates the challenge of eliminating heat from device operation.
Demand for Semiconductor UV Emitters
UV light-emitting diodes (LEDs) with wavelengths less than 300 nm offer a wide range of possible uses, including air, freshwater, and surface sanitation, medical diagnostics and treatment, radiation hard UV emitters, UV curing, and so on.
The market for purification solutions based on small and cost-effective semiconductors and UV sources is growing as people become more aware of the health dangers associated with tainted food, air, and water.
However, the most often utilized deep ultraviolet (DUV) lights, such as mercury lamps, are typically large and expensive and pose the danger of heavy metal contamination, all of which prevent such DUV light sources from being employed in many regions.
The wide bandgap semiconductor-based DUV light source, as a potential solution to mercury lamps, has a number of advantages including low power consumption, high efficiency, compact size, long lifespan, low environmental damage, and so on. As a result, much attention has been given to this technology in recent years.
Good Semiconductor UV Emitters Using ZnO
Because ZnO features high exciton binding energy and other desirable properties, such as readily accessible raw ingredients, ease of material access, and device fabrication, the material offers significant potential for creating solid-state UV emitters.
Despite these benefits, the ZnO family of semiconductors has yet to be employed in near-UV and DUV LEDs, despite the fact that laboratory products have been demonstrated for many years.
The challenge in producing high enough hole concentrations within the materials is the primary reason for this. This is a well-known issue that affects all wide bandgap semiconductors.
Characteristics of ZnO Semiconductors
At ambient temperature, zinc oxide (ZnO) does have a significant exciton binding energy of 60 meV and a broad bandgap of 3.37 eV, allowing it to develop exciton illumination. It might be used in high-performance UV lights and laser diodes.
The binding energy of ZnO may also be adjusted to create DUV LEDs. The creation of MgZnO and BeZnO ternary metals for higher bandgap active layers has proved this. Many attempts have been undertaken to produce DUV LEDs, with promising results being achieved on a continuous basis.
Applications of ZnO Semiconductors
ZnO is a semiconductor with a lot of promise for making ultraviolet emitter devices for a variety of applications. LEDs that generate deep ultraviolet (DUV) light offer a wide range of possible uses.
Zinc-oxide-based materials with a large exciton binding energy and a broad bandgap might be used in high-performance DUV LEDs. The manipulation of the bandgap is necessary to develop such optoelectronic applications.
The advancement of MgZnO and BeZnO composites for greater bandgap materials has shown this. Many attempts have been undertaken to produce DUV LEDs, with promising results being achieved on a continuous basis.
The Benefit of ZnO Semiconductor Research
Nonetheless, research into the ZnO family of materials is critical for two reasons. One possibility is that in the future a viable p-type doping process for these semiconductors will be discovered, allowing them being used to make effective UV and DUV LEDs.
Another reason is that such materials might be used to make ionization-based DUV emitters without the need for p-type doping. As a result, a new study in the journal Materials Research Bulletin on ZnO and MgZnO points to a future where these semiconductors may be used to make commercial DUV emitters.
High CL emission was observed in ZnO, but not in sputter-deposited MgZnO layers. The latter, on the other hand, exhibited evident optical energy band edge absorption as well as photoluminescence.
New Findings on ZnO Semiconductors as UV Emitters
In the journal Materials Research Bulletin, the structural instability in the sputtered coatings that can make the mean free channel of electrons infuse from free space rays in a CL configuration is considerably reduced. The electrons lose energy from the top of the layer and do not penetrate far enough within to generate significant electron-hole pairs by impact excitation.
Any electron-hole pairs which do form are likely to recombine non-radiatively. Using sputter-deposited MgZnO samples, no CL emission was observed.
ALD-grown MgZnO films, on the other hand, showed obvious band edge CL emissions with the predicted blue shift as Mg concentration rose. As a result, the manner of material growth or deposition chosen is critical to creating CL emission-capable materials.
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Reference and Further Reading
Chaudhari, A., et al. (2022). Zinc oxide family semiconductors for ultraviolet radiation emission – A cathodoluminescence study. Materials Research Bulletin. https://doi.org/10.1016/j.materresbull.2022.111906