Abstract
1. UV light-emitting devices
Taking the advantages of AlGaN materials of the direct wide bandgap character, great progress has been made in UV optoelectronic active devices, such as light-emitting diodes (LEDs) and laser diodes (LDs). Compared with traditional solid-state light sources, AlGaN-based UV-LEDs (Fig. 1) have numerous advantages, including short wavelength operation, small size, compact structure, operational stability, high efficiency, low power consumption, low operating voltage, environmental friendliness, and long lifetimes, which make them suitable for application in the UV radiation field[
Figure 1.(Color online) Schematic illustration of the DUV-LEDs structure.
AlGaN-based UV-LDs with their unique high spatial and temporal coherence properties have many merits, including high light beam quality, high power density, and high modulation speeds, which can be widely used in applications of precision laser processing, high-density data storage, nanopattern-type photolithography, medical diagnostics, disinfection, biochemical technology, gas sensing, and materials science[
Figure 2.(Color online)
2. Problems of UV light-emitting devices
In order to realize high-performance devices, further optimization and improvement of structure and manufacturing are required, mainly focusing on three aspects: reducing the defect density of AlGaN materials, improving the light extraction efficiency of device structures and achieving high-efficiency P-type doping of AlGaN materials.
AlGaN-based materials own direct transition energy bands and wide bandgap and thus can be used in high-efficiency ultraviolet (UV) emitters. Compared with GaN-based blue and green LEDs and LDs, the efficiency of AlGaN-based UV LEDs and LDs is lower. Dislocations usually act as the nonradiative recombination center in AlGaN-based active devices, thus the quality of AlGaN is crucial to the device performance. Fig. 3 illustrates the relationship between the internal quantum efficiency (IQE) and the dislocation density (DD) in AlGaN multiple quantum wells (MQWs) underweak excitation with an excess carrier density of 1 × 1018 cm−3[
Figure 3.(Color online) IQE as a function of DD in an underlying layer under weak excitation with excess carrier density of 1 × 1018 cm−3.
Due to the higher refractive index of nitride materials, the light emitted by quantum wells is totally reflected at the interface between DUV LED and air. A large amount of light is confined inside the LED and absorbed by the epitaxial material, resulting in very low light extraction efficiency. N Lobo's simulations show that the light extraction efficiency of mirrorless and unpackaged flip-chip UV LEDs is only 7%–9% level[
High-conductivity AlGaN is required to realize high performance AlGaN-based UV devices. However, a problem that has persisted since the early 1990s and is becoming increasingly troublesome is the high resistivity of p-type GaN and AlGaN layers. The activation energy EA of the most commonly used acceptor dopant (Mg) in GaN is ~200 meV[
In order to solve the above problems, the researchers have also made many efforts. For growing high-quality AlGaN film on sapphire substrates, a high-quality AlN film is mainly used as a template layer of AlGaN, so researchers used various methods to grow high-quality AlN films, such as pulse growth method, insertion of low-temperature buffer layer or two-dimensional materials such as graphene buffer layer and epitaxial lateral overgrowth (ELO) technology on patterned sapphire substrates (PSS)[
In addition, studies on p-doping of AlGaN have investigated common uniform Mg doping, Mg-δ doping, superlattice doping, co-doping, polarization induced doping and so on[
Figure 4.(Color online) Hall-effect temperature-dependent (a) hole concentration, (b) hole mobilities, and (c) hole concentration and mobility measured down to
3. Conclusions and outlook
AlGaN-based DUV-LEDs with short operating wavelengths have been achieved, and these wavelengths have been extended to 222 nm for AlGaN/AlN MQW devices and 210 nm for AlN PIN homojunction devices. The improved performances of these LEDs have been achieved with EQEs of 20.3% at 275 nm. However, the EQEs of DUV-LEDs are still low when compared with those of GaN-based blue and green LEDs. There is also a considerable drop in efficiency, which is caused by high dislocation densities, low hole concentrations, and low LEEs for the AlGaN-based LEDs. Furthermore, the EQE also drops dramatically with decreasing wavelength, which is caused by deterioration in the AlGaN quality, the difficulty of p-type doping processes, and degradation of the optically polarized emission with increasing Al content. It is expected that high-efficiency DUV AlGaN-based LEDs will be realized by improving the quality and the p-type doping of AlGaN as well as optimizing the parameters of the AlGaN/AlN MQWs.
Due to the improvements in both AlGaN quality and p-type doping, UV stimulated emission has been achieved in AlGaN MQW LDs using electrical pumping at RT, with a shortest reported wavelength of 271 nm. At present, the development of UV AlGaN-based LDs is moving toward shorter wavelengths and low threshold voltage. However, many challenges still need to overcome to achieve high performance LDs of this type. First, the high densities of defects and dislocations in the active regions of these LDs will increase their internal losses, resulting in reduction of the EQE. Second, the difficulty involved in p-type doping of AlGaN will reduce the hole injection efficiency and increase the series resistance, which leads to an increased threshold and reduced efficiency for LDs operating under current injection conditions. Additionally, the difficulties faced in device fabrication processes such as etching, thinning, and cleaving will increase losses and reduce the efficiency of these LDs. In addition, suitable homoepitaxial substrates for AlGaN growth are not available at present. Therefore, appropriate substrates with high transparency and high electrical and thermal conductivities are required to improve the performance of these LDs. Furthermore, a suitable LD structure design is required to improve device efficiency. In conclusion, the low defect densities of bulk AlN substrates offer a promising strategy to enable fabrication of high-performance LDs.
References
[1] A Khan, K Balakrishnan, T Katona. Ultraviolet light-emitting diodes based on group three nitrides. Nat Photonics, 2, 77(2008).
[2] D Li, K Jiang, X Sun et al. AlGaN photonics: recent advances in materials and ultraviolet devices. Adv Opt Photonics, 10, 43(2018).
[3] T Takano, T Mino, J Sakai et al. Deep-ultraviolet light-emitting diodes with external quantum efficiency higher than 20% at 275 nm achieved by improving light-extraction efficiency. Appl Phys Express, 10, 031002(2017).
[4] J Hodgkinson, R P Tatam. Optical gas sensing: a review. Meas Sci Technol, 24, 012004(2013).
[5] E Allaria, D Castronovo, P Cinquegrana et al. Two-stage seeded soft-X-ray free-electron laser. Nat Photonics, 7, 913(2013).
[6] M Kneissl, T Y Seong, J Han et al. The emergence and prospects of deep-ultraviolet light-emitting diode technologies. Nat Photonics, 13, 233(2019).
[7] Z Zhang, M Kushimoto, T Sakai et al. A 271.8 nm deep-ultraviolet laser diode for room temperature operation. Appl Phys Express, 12, 124003(2019).
[8] K Ban, J I Yamamoto, K Takeda et al. Internal quantum efficiency of whole-composition-range AlGaN multi-quantum wells. Appl Phys Express, 4, 052101(2011).
[9] M Kneissl, T Kolbe, C Chua et al. Advances in group III-nitride-based deep UV light-emitting diode technology. Semicond Sci Technol, 26, 014036(2011).
[10] J Simon, V Protasenko, C Lian et al. Polarization-induced hole doping in wide-band-gap uniaxial semiconductor heterostructures. Sciences, 327, 60(2009).
[11] H Chang, Z Chen, W Li et al. Graphene-assisted quasi-van der Waals epitaxy of AlN film for ultraviolet light emitting diodes on nano-patterned sapphire substrate. Appl Phys Lett, 114, 091107(2019).
[12] H Hirayama, T Yatabe, N Noguchi et al. 231–261 nm AlGaN deep-ultraviolet light-emitting diodes fabricated on AlN multilayer buffers grown by ammonia pulse-flow method on sapphire. Appl Phys Lett, 91, 71901(2007).
[13] W Tian, W Y Yan, J N Dai et al. Effect of growth temperature of an AlN intermediate layer on the growth mode of AlN grown by MOCVD. J Phys D, 46, 065303(2013).
[14] M Shatalov, W Sun, A Lunev et al. AlGaN deep-ultraviolet light-emitting diodes with external quantum efficiency above 10%. Appl Phys Express, 5, 082101(2012).
[15] M Djavid, Z Mi. Ehancing the light extraction efficiency of AlGaN deep ultraviolet light emitting diodes by using nanowire structures. Appl Phys Lett, 108, 051102(2005).
[16] S R Jeon, Z Ren, G Cui et al.
[17] M L Nakarmi, K H Kim, J Li et al. Enhanced p-type conduction in GaN and AlGaN by Mg-δ-doping. Appl Phys Lett, 82, 3041(2003).
[18] H X Zhong, J J Shi, M Zhang et al. Improving p-type doping efficiency in Al0.83Ga0.17N alloy substituted by nanoscale (AlN)5/(GaN)1 superlattice with MgGa-ON δ-codoping: Role of O-atom in GaN monolayer. AIP Adv, 5, 227(2015).
Set citation alerts for the article
Please enter your email address