General Priciples of LEDs
A light-emitting diode (LED) is a semiconductor device that emits light when a current is passed through it. While very pure, defect-free semiconductors (so-called, intrinsic semiconductors) generally conduct electricity very poorly, dopants can be introduced into the semiconductor which will make it either conduct with negatively charged electrons (n-type semiconductor) or with positively charged holes (p-type semiconductor).
An LED consists of a p-n junction where a p-type semiconductor is put on top of an n-type semiconductor. When a forward bias (or voltage) is applied, electrons in the n-type region are pushed toward the p-type region and, likewise, holes in the p-type material are pushed in the opposite direction (since they are positively charged) toward the n-type material. At the junction between the p-type and n-type materials, the electrons and holes will recombine and each recombination event will produce a quantum of energy that is an intrinsic property of the semiconductor where the recombination occurs.
Side note: electrons are generated in the conduction band of the semiconductor and holes are generated in the valence band. The difference in energy between the conduction band and the valence band is called the bandgap energy and is determined by the bonding characteristics of the semiconductor.
Radiative recombination results in the production of a single photon of light with an energy and wavelength (the two are related to each other by Planck’s equation) determined by the bandgap of the material used in the active region of the device. Non-radiative recombination can also occur where the quantum of energy released by the electron and hole recombination produces heat rather than photons of light. These non-radiative recombination events (in direct bandgap semiconductors) involve mid-gap electronic states caused by defects. Since we want our LEDs to emit light, not heat, we want to increase the percentage of radiative recombination compared to non-radiative recombination. One way to do this is to introduce carrier-confining layers and quantum wells in the active region of the diode to try to increase the concentration of electrons and hole which are undergoing recombination under the right conditions.
However, another key parameter is reducing the concentration of defects which cause non-radiative recombination in the active region of the device. That is why the dislocation density plays such an important role in optoelectronics since they are a primary source of non-radiative recombination centers. Dislocations can be caused by many things but achieving a low density will nearly always require the n-type and p-type layers used to make the active region of the LED are grown on a lattice-matched substrate. Otherwise, dislocations will be introduced as a way to accommodate the difference in crystal-lattice structure.
Therefore, maximizing LED efficiency means increasing the radiative recombination rate relative to the non-radiative recombination rate by minimizing dislocation densities.
Ultraviolet (UV) LEDs have applications in the field of water treatment, optical data storage, communications, biological agent detection and polymer curing. The UVC region of the UV spectral range refers to wavelengths between 100 nm to 280 nm.
In the case of disinfection, the optimum wavelength is in the region of 260 nm to 270 nm, with germicidal efficacy falling exponentially with longer wavelengths. UVC LEDs offer considerable advantages over the traditionally used mercury lamps, notably they contain no hazardous material, can be switched on/off instantaneously and without cycling limitation, have lower heat consumption, directed heat extraction, and are more durable.
In the case of UVC LEDs, to achieve short wavelength emission (260 nm to 270 nm for disinfection), a higher aluminum mole fraction is required, which makes the growth and doping of the material difficult. Traditionally, bulk lattice-matched substrates for the III-nitrides was not readily available, so sapphire was the most commonly used substrate. Sapphire has a large lattice mismatch with high Al-content AlGaN structure of UVC LEDs, which leads to an increase in non-radiative recombination (defects). This effect seems to get worse at higher Al concentration so that sapphire-based UVC LEDs tend to drop in power at wavelengths shorter than 280 nm faster than AlN-based UVC LEDs while the difference in the two technologies seems less significant in the UVB range and at longer wavelengths where the lattice-mismatch with AlN is larger because higher concentrations of Ga are required.
Pseudomorphic growth on native AlN substrates (that is where the larger lattice parameter of intrinsic AlGaN is accommodated by compressing elastically it to fit on the AlN without introducing defects) results in atomically flat, low defect layers, with peak power at 265 nm, corresponding to both the maximum germicidal absorption while also reducing the effects of uncertainty due to spectral-dependent absorption strength.
Crystal IS has developed high-quality bulk lattice-match AlN substrates which allows for higher internal efficiency and lower internal absorption. These substrates, used in the manufacture of Klaran UVC LEDs and products, provides higher quality, more powerful LEDs at wavelengths in the germicidal range.