Nasir Alfaraj, Jung-Wook Min, Chun Hong Kang, Abdullah A. Alatawi, Davide Priante, Ram Chandra Subedi, Malleswararao Tangi, Tien Khee Ng, Boon S. Ooi. Deep-ultraviolet integrated photonic and optoelectronic devices: A prospect of the hybridization of group III–nitrides, III–oxides, and two-dimensional materials[J]. Journal of Semiconductors, 2019, 40(12): 121801
- Journal of Semiconductors
- Vol. 40, Issue 12, 121801 (2019)
Abstract
1. Introduction
Ultraviolet (UV)-emitting group III–nitride materials hold a promising potential for a variety of multifunctional applications, including solid-state lighting technology[
The heterogeneous integration of various forms of inorganic materials (which encompasses growing numbers of material types) into one electronic system is based on group III–nitride compound semiconductors[
Successful doping of a semiconductive material is a crucial factor in achieving an efficient carrier injection process to realize excellent device performance characteristics[
While it has been well established that electron-beam irradiation[
In this article, we review the recent progress in the growth and fabrication of UV and DUV group III–nitride optoelectronic devices and materials based on AlxGa1−xN[
Figure 1.(Color online) Graphical abstract reflecting areas explored in this review article.
Table Infomation Is Not Enable1.1. Aluminum gallium nitride system
While the doping process and its effects on group III–nitride semiconductors are not fully understood yet, we assert that AlN is the hardest to dope when compared to GaN or indium nitride (InN) and their alloys (InxGa1−xN, 0 < x < 1 ) [
Figure 2.(Color online) Comparison of donor activation energies of Si-doped Al
Figure 3.(Color online) Variation in Al
The most common p-type dopant for AlxGa1−xN is magnesium through the introduction of highly-pure bis(cyclopentadienyl)magnesium (MgCp2) into a metalorganic vapor-phase epitaxy (MOVPE) reactor[
Islam et al. demonstrated tunable DUV LEDs using ultrathin GaN quantum dots (QDs) in the device active regions[
By growing quasi-one-dimensional (quasi-1D) group III–nitride nanostructures using plasma-assisted molecular beam epitaxy (PA-MBE)[
1.2. Boron aluminum/gallium nitride system
Previous studies on aluminum-based group III–nitride materials have shown desirable structural and electronic properties with tunable direct bandgap that can cover the entire UV spectral band. These studies highlighted the increase in the optoelectronic active area of devices, where UV light emission could be enhanced fourfold by incorporating boron, but they lack any direct empirical demonstration[
Hexagonal boron nitride (h-BN) has attracted a considerable amount of attention because of the unique combination of properties it exhibits[
In 2018, Pierucci et al. demonstrated the controlled growth of h-BN on graphite using MBE via van der Walls epitaxy[
Figure 4.(Color online) (a) Crystal structure of single layer h-BN. (b) SEM image of h-BN growth on HOPG demonstrating nucleation from HOPG in terrace steps pointed out by the blue arrows (darker contrast areas represent the underlying HOPG substrate while the lighter contrast areas represent regions of h-BN epitaxial growth). (c) High resolution SEM image of an h-BN island displaying areas of single and bi-layer growth and part of exposed HOPG substrate. Reprinted with permission from Ref. [
2. Group III–nitride crystal structure and material properties
2.1. Wurtzite crystal structure
Of all groups of III–nitride compound semiconductors, devices based on AlxGa1−xN are ideally suited for UV and DUV device design and fabrication because of the tunability of photoemission and photodetection wavelengths from about 200 to 364 nm by adjusting the AlN mole fraction. Because GaN crystal structures can be assimilated as cubic if the hexagonal planes are slided, the crystal lattice constants of group III–nitride semiconductors can be determined considering that each crystal forms a cubic lattice; hence, the lattice constant can be determined using the following expression:
where
Figure 5.(Color online) Plot of bandgap energy versus lattice constant a value of the (Al, In, Ga)N material system
The wurtzite structure is represented in Fig. 6(a) and is made of two hexagonal close-packed lattices perpendicular to the substrate (c-direction). The lattice constants are a = b
Figure 6.(Color online) (a) Wurtzite structure. (Reprinted from [https://commons.wikimedia.org/wiki/File:Wurtzite_polyhedra.jpg]. Image stated to be in the public domain). (b) Wurtzite planes. Reprinted with permission from Ref. [
Spontaneous polarization in III–V semiconductors has detrimental effects through band bending[
2.2. Polarization-induced fields
One of the principal causes of low IQE values in group III–nitride-based optoelectronic devices is the significant polarization fields built-in by the non-centro-symmetric nature of the atomic bonds. In a multilayer heterostructure, the spatial gradient of the polarization across the interfaces causes the formation of fixed charges, resulting in band bending and in turn, separation of carriers[
Because the piezoelectric polarization field (PPE) is related to the internal crystal strain (
where a, a0, C13, and C33 are the equilibrium and strained values of the in-plane lattice constant and the elastic constants[
where, depending on the metal or nitrogen polarity, we have the plus or minus sign. For the specific cases of GaN, AlN, and InN in metal-polar condition, the term within the brackets is negative; therefore, PPE is negative (polarization vector points the substrate); while in nitrogen-polar condition it is positive, as summarized in Fig. 7
Figure 7.(Color online) Polarization field directions and interface polarization charge distribution signs in GaN and Al
Changes in the normal component of the polarization field, ΔP, cause the formation of fixed charges at the heterointerfaces with density σP = –ΔP and the subsequent carrier separation due to band bending. In the case of MQW structures, the continuity equation of the displacement flux through the different interfaces can be expressed as follows:
where w and b stand for well and barrier, respectively, and ε, F and P are the permittivity of the material, the built-in electric field, and the sum of spontaneous and piezoelectric polarization fields, respectively. Fw and Fb are given by the following:
solved in the limit of an infinitely periodic structure andwhen voltage drop across layers is equal to zero(FwLw+FwLw = 0, Lis the layer thickness). Fig. 8 shows the MQW band bending and the misaligned electron–hole wavefunction that imply lower absorption and emission properties. Large field values of few MV/cm are usually present in c-plane III–nitride structures, causing the QCSE to dominate as the gap between conduction band minima and valence band maxima are reduced (red-shifted) by Fw.
Figure 8.(Color online) Conduction and valence band bending and electron and hole wavefunctions for the case of MQWs. Reprinted with permission from Ref. [
2.3. Dislocations in AlxGa1−xN
Although InxGa1−xN-based devices demonstrate a remarkable resilience to high dislocation densities[
The defect density across a structure is directly influenced by epitaxial growth conditions of the materials on designated substrates. Because of its optical and thermal properties as well as its relatively low cost when compared to free-standing GaN substrates, sapphire is commonly employed as a substrate for the growth of group III–nitride light-emitting devices[
Moreover, due to the higher sticking coefficient and lower surface mobility of aluminum adatoms[
2.4. Efficiency droop
Efficiency droop is a phenomenon mainly associated with the reduction in InxGa1−xN/GaN-based LED efficiencies as the injection current densities increase. Despite the fact that this phenomenon is very pronounced in blue and green LEDs, it also affects UV devices to a lesser degree[
Figure 9.(Color online) Efficiency versus injection current curves of GaN-based UV, blue, and green LEDs, demonstrating a decrease in quantum efficiency with increasing injection current. Green LEDs were shown to have the most prominent efficiency droop. Reprinted with permission from Ref. [
The IQE of a light-emitting device takes into account how efficiently holes and electrons can be extracted from their respective injection layers and the percentage of carriers that recombine radiatively to emit photons, and is expressed as[
The IQE can also be expressed as the ratio of the injected current that leads to radiative recombination in the device active region to the total injected current (Itot) as follows:
where Inr is the current that is dissipated in nonradiative processes and transitions. When Inr dominates (i.e., Inr>Ir), efficiency droop takes place. Inr includes the carrier losses attributed to SRH recombination, Auger recombination, and carrier leakage outside the QWs. Hence, Itot can be expressed as
Inside the QWs, the carrier recombination current can be expressed as
where q is the elementary charge value and VQW is the total volume of the QW region. A, B, C are the three recombination coefficients of SRH, radiative, and third-order processes (such as Auger losses), respectively, and N is the excess density of carriers involved in the respective recombination processes. The leakage current can be represented by[
By combining Eq. (9) with Eq. (12), we can express the IQE as follows
where
2.5. Thermodynamic photoinduced disorder
In the case of group III–nitride semiconductors, it is broadly assumed that the photocarrier relaxation dynamics consist of a fast initial decay process in the subpicosecond range, followed by a slower decay described by the ultrafast carrier thermalization dynamics, the carrier trapping by surface states, and the slower carrier cooling effects, respectively[
Figure 10.(Color online) (a) Schematic and layer structure of the In0.32Ga0.68N/GaN p–i–n nanowires and (b) the evolutions in the total carrier recombination lifetime and the amount of entropy generation with temperature. (Reprinted with permission from Ref. [
In In0.32Ga0.68N/GaN nanowires, we observed a rising trend in the amount of generated photoinduced entropy of the system above 250 K, while a fluctuating trend in the generated entropy of the system below 250 K was observed. The fluctuations in the generated entropy stabilized between 200 and 250 K. It was supposed that the amount of generated photoinduced entropy of the In0.32Ga0.68N active region increase as more nonradiative channels became activated, and more shallowly localized carriers settle into deeply localized states (allowed for through the strong localization in indium (In)-rich clusters)[
3. Innovative material growth mechanisms and device fabrication techniques
Advancements in growth techniques made it possible to achieve highly improved optoelectronic device performance through high-quality single-crystal material growth[
3.1. Nanostructured layer growth
3.1.1. Molecular beam epitaxy
In 2016, while addressing challenges encountered during growth of aluminum-rich AlxGa1−xN nanowires for DUV optoelectronic devices, Zhao et al. demonstrated that such nanowires with significantly enhanced compositional uniformity can be realized through a new growth prototype other than the conventional nitrogen-rich growth conditions. They argued that they can a achieve precise control on the optical bandgap energy of the ternary AlxGa1−xN nanowires by employing a GaN nanowire template and varying the substrate temperature while improving the aluminum/gallium compositional uniformity. They demonstrated AlxGa1−xN nanowire LEDs, with emission wavelengths spanning from 236 to 280 nm[
Figure 11.(Color online) (a) Schematic illustration of Al
Vuong et al. studied the optoelectronic properties of h-BN grown by PA-MBE. By combining AFM, spectroscopic ellipsometry, and photoluminescence spectroscopy in the DUV regime, they compared the quality of h-BN grown on sapphire and highly oriented pyrolytic graphite substrates. They were able to demonstrate DUV emission in h-BN, with an emission spectra peak at 235 nm, indicating the high optical quality of the MBE-grown h-BN. The h-BN epitaxial layers grown on highly oriented pyrolytic graphite have shown superior performance in the DUV regime down to 210 nm, compared to the same films grown on sapphire[
Liu et al. have demonstrated large-area AlN nanowall 214 nm LEDs grown on a sapphire substrate. Through temperature-dependent and power-dependent photoluminescence measurements and rate equation analysis, a relatively high internal quantum efficiency of about 60% was determined for the AlN nanowall structures at room temperature. A consistent blueshift in the emission wavelengths was observed with decreasing nanowall widths because of the reduced distribution in tensile strain. The LEDs exhibited excellent current–voltage I–Vcharacteristics, including a turn-on voltage of 7 V and current densities of greater than 170 A/cm2 at 12 V[
3.1.2. Metalorganic vapor phase epitaxy
While AlxGa1−xN-based structures show significant importance in applications requiring DUV light sources, the absorption of UV light by p-type layers results in these devices suffering from low light extraction efficiencies (LEEs)[
Metal-source flow-rate modulation epitaxy (FME) provides unique opportunities to realize high hole concentrations in magnesium-doped AlxGa1−xN. Luo et al. were able to demonstrate a hole concentration of about 2.3 × 1017 cm–3 at room temperature, nearly ten times higher than conventional growth procedures, with resistivities as low as 12.7 Ω·cm by implementing metal-source FME[
Figure 12.(Color online) AFM images of the magnesium-doped Al
Instead of employing p-type AlxGa1−xN layers, several efforts have been made to substitute these low conductivity layers with BN[
Hakamata et al. investigated the combination of sputtering and MOVPE to study surface quality and optical properties of MOVPE-grown AlN and AlxGa1−xN epitaxial layers on sputtered and annealed AlN/sapphire templates[
Figure 13.(Color online) (a–c) Surface roughness enhancement trend (from 0.54 to 0.13 nm). In (a) an AFM scan of a sputtered AlN/sapphire template after high-temperature annealing is shown, while in (b), the AlN/sapphire template in (a) scanned after the temperature had been increased to 1250 °C and maintained for three minutes in an NH
Sun et al. have conducted AlN growth experiments by MOVPE to investigate the effects of TMAl pretreatment on the surface roughness and crystalline quality of grown AlN layers on c-plane sapphire substrates[
3.2. Device fabrication
In this section, we discuss various methodologies to enhance the performance of DUV devices. Most challenging to realizing growth of group III–nitride semiconductors are the high TDDs resulting from the lattice mismatch and/or thermal expansion mismatch between foreign substrates such as sapphire or silicon and the grown AlN epitaxial layers. On the other hand, the growth process of AlN on native substrates such as AlN or GaN is limited due to the high cost and unavailability of inexpensive commercial substrates. Various approaches have been discussed to address the sapphire substrates' TDDs[
Figure 14.(Color online) (a) Schematic diagram of silica nanosphere lithography, (b) plan-view, and (c) bird's-eye-view SEM images of the fabricated NPS. (d) Plot of the average LOP as a function of the injection current measured from 50 LEDs at room temperature. The relative EQE, estimated by dividing the photocurrent by the injection current, is also shown in the inset. The OP of the DUV LEDs on NPS shows much higher LOP by 67% than that of the reference DUV LED at the injection current of 20 mA. Reprinted with permission from Ref. [
Another method to reduce the fabrication cost of the ELO process of AlN is decreasing the coalescence thickness via nanosphere lithography (NSL). In this regard, Dong et al. successfully utilized nanopatterned sapphire substrates (NPSS) that coalesce after only 3 μm of growth, compared to 10 μm for microstructure patterning methods, and hence, reduce fabrication time[
Figure 15.(Color online) (a) Schematic of the fabrication process flow to create nanopatterns on a sapphire substrate. SEM images of the (b) patterned PR and (c) wet-etched NPSS. The inset in (c) shows the line profile of the patterns of NPSS by AFM measurement. (Reprinted with permission from Ref. [
Fig. 16 shows a moth-eye micro-arrays which was fabricated on the back of sapphire platform to significantly increase the LEE by weakening the total internal reflection (TIR)[
Figure 16.(Color online) (a) TE and (b) TM mode light propagation characteristics in moth-eye DUV LEDs reported by Wang
Similar to sapphire substrate integration in the DUV LED fabrication process, growing AlN epitaxial layers on silicon substrate is of critical importance due to the availability, suitability, and low-cost of silicon substrates. Nevertheless, the lattice mismatches between silicon (111) and AlN (0001) are approximately 19% in the
Figure 17.(Color online) AFM images showing the effect of several growth rates on two AlN layers, where the growth rates are (a) 250 nm/h and (b) 50 nm/h. Sides-sectional STEM images depicting two 420 nm-thick AlN/Si(110) samples (c) with and (d) without V-shape pits by controlling the growth rate. Reprinted with permission from Ref. [
Micro-circle-patterned silicon substrate (mPSIS) is another configuration technique to enhance the quality of grown AlN template on silicon substrate[
4. Critical review of the current status of nitride-based optoelectronic devices
4.1. Light-emitting diodes
AlxGa1−xN-based DUV LEDs have been dominating research in the last few years[
Figure 18.(Color online) (a) Schematic illustration of the p–i–n Al
4.2. Photodetectors
Taking advantage of high absorption coefficients (>105 cm–2) and wide bandgap energy, group III–nitride semiconductor alloys have attracted remarkable attention for UV photon detection owing to their prospective applications in solar radiation monitoring[
The early work on GaN-based PDs started in in 1992 by Khan et al., where a photoconductive UV detector with high spectral responsivity (R) of 2000 A/W (200 to 365 nm) was demonstrated[
where IPC is the generated photocurrent, PIL is the illuminated optical power, h is Planck's constant, c is speed of light, and
Figure 19.(Color online) (a) Transmission spectrum of Ni/Au interdigitated electrodes used in an MSM-based PD. Inset shows the typical structure of an Al
Despite the advantages of bottom-illuminated configuration PDs, the thermal and lattice mismatches between conventional sapphire substrates and group III–nitride materials still persist. While paving the way toward the solar-blind regime of operation, high aluminum composition of greater than 40% are required to provide the aforementioned applications in the solar-blind region. High TDDs and thermal structural cracking developed during the epitaxial growth, especially at increased aluminum contents, hinder the device performance of AlxGa1−xN-based PDs, which requires low charge generation rates and stray capacitances. In order to alleviate these issues, various methods have been reported, including selective area growth[
Other than planar structures, AlxGa1−xN-based PDs constructed using lower-dimensional structures, such as nanowires and nanorods, have also received significant attention[
Figure 20.(Color online) (a) Device structure of Al0.40Ga0.60N-based p–i–n–i–n separate absorption and multiplication (SAM) APD. (b)
However, to tailor PDs for the DUV wavelength regime of operation, high quality and crack-free high-aluminum-content layers are pivotal. Up until now, no solution has been found to address high defect densities associated with the incorporation of AlxGa1−xN layers with high aluminum contents. Walker et al. was one of the earliest to demonstrate AlxGa1−xN-based PDs operating in DUV wavelengths with a responsivity of 0.11 A/W at 232 nm (5 V reverse voltage) and an IQE of 90%[
As discussed earlier, further improvements in the performance of group III–nitride-based PDs were largely inhibited by the low crystal quality and low conductivity characteristics originating from the growth of PD structure materials. To alleviate these issues, hybridization of GaN-based PDs with 2D materials, such as graphene, have been proposed. Phototransistors based on graphene/GaN structures with relatively high responsivity of 0.36 A/W at 325 nm were reported by Tian et al. and shown in Fig. 21(a). Their reported device characteristic values were significantly higher than those realized without the integration of graphene layers (up to 700-fold improvement); the high achieved gain can be attributed to high carrier mobility effects exhibited in graphene, which allow for multiple carrier transport to generate high photocurrent densities[
Figure 21.(Color online) (a) Schematic of the device structure and bandgap energy diagram of hybrid graphene/GaN UV PD. (Reprinted with permission from Ref. [
4.3. Lasers
Poor current conduction has also prevented the achievement of electrically pumped QW lasers operating in the UV-B (280–315 nm) and UV-C (200–280 nm) bands[
1. Electrical pumping: LDs are naturally pumped using electric currents. Researchers have demonstrated electrically pumped lasers at relatively long emission wavelengths, such as 330 nm[
2. Optical pumping: Pumping LDs using an optical pumping source is an artificial way to operate a laser, although dye lasers are optically pumped[
Given these hindrances, conventional optically pumped DUV LDs and electrically pumped random DUV LDs are impossible to practically exploit in vital applications, such in the study of geology and climate in outer space. An alternative way to drive an LD active material is by using an electron beam, through which electron–hole pairs are generated by the energy transfer of high-energy electrons emitted from an electron gun. One advantage of this approach is that the incorporation of p- and n-type doped layers is not necessary, avoiding the undesired optical absorption commonly associated with these layers. In 2015, Klein et al. demonstrated high-power green and blue electron-beam-pumped LDs, with a peak optical powers of 5.9 W (
5. Substrate technology and heterogeneous integration of new materials
To realize efficient optoelectronic devices for applications necessitating DUV light emission, high-crystalline-quality AlN templates are the key enablers of success in growing upper AlxGa1−xN epitaxial layers. Various growth techniques were developed to either achieve efficient UV and DUV light emission or merely improve the crystalline quality of AlxGa1−xN layers[
5.1. High quality AlN templates
Kataoka, Funato, and Kawakami developed 3D Al0.50Ga0.50N QWs on MOVPE-grown AlN templates for polychromatic DUV LEDs[
Figure 22.(Color online) (a) Schematic illustration of an Al0.50Ga0.50N-based polychromatic UV LED structure on patterned AlN template. Reprinted with permission from Ref. [
Chang et al. demonstrated the growth of high-quality AlN templates on NPSSs via graphene-assisted quasi-van der Waals epitaxy[
5.2. Hybrid integration on Ga2O3
Gallium oxide and its ternary alloys ((InxAlyGaz)2O3, x + y + z = 1 ) are wide-bandgap oxides of post-transitional metals with ionic bonds. With superior breakdown fields of around 8 MV/cm compared with SiC (3 MV/cm) and GaN (3.8 MV/cm), their intrinsic band gap spans from 4.9 to 5.3 eV; there are five known polytypes of Ga2O3, α, β, γ, δ, and ε[
High n-type conductivity and transparency characteristics reveal that Ga2O3 is an ideal candidate for the realization of DUV devices as a transparent conducting oxide (TCO) substrate. Even though Ga2O3 does not have a hexagonal structure, it is relatively easy to induce a hexagonal atomic arrangement for optoelectronic device applications. In 2015, epitaxial growth of planar GaN layer on (100) a-plane
As part of a GaN epitaxial layer growth study on
5.3. Polymeric materials as key enablers for DUV light-emitting device mass production
Polymeric materials, such as optically isotropic and stable amorphous fluorine resin[
5.4. Nanolasers realized via nanoscale plasmonic effects
Increasing demand of miniature-sized LD are of importance for the optical communications and data storage industries. One of the possible approaches for the realization of nanoscale lasers is using surface plasmon polaritons, which contain strong localized fields at the dielectric/metal interfaces. Recently, Shen et al. demonstrated DUV lasers with hyperbolic metamaterial (HMM) including multiple stacks of dielectric (20 nm of MgF2)–metal (20 nm of aluminum) interfaces[
5.5. Integration of 2D MX2/group III–nitride interfaces for the optoelectronic applications
In addition to UV light-emitting devices, researchers have also focused their efforts on advancing the integration of new materials for the realization of DUV photodetection. As a new approach for this application, the interesting characteristics exhibited at the interfaces of 2D TMDs and 3D group III–nitride semiconductors have recently emerged as the central theme of various studies[
6. Electrical carrier injection and band alignment
In this section, the band offsets of heterojunctions and their effects on charge carrier injection associated with the electrical transport properties are reviewed. We also review the doping and injection efficiencies of charge carriers. We emphasize the role of band-offset parameters as important parameters to understand the carrier injection efficiency and confinement effect of devices employing heterointerfaces. We further discuss the state of current injection efficiency in AlxGa1–xN-based UV LEDs.
6.1. Band alignment
The state and band parameters at semiconductor heterointerfaces are essential to the effective design of active regions in contemporary electronic and optoelectronic devices[
Figure 23.Schematic representations of the band alignment at (a) InN/AlN and (b) InN/GaN interfaces acquired using HRXPS studies. Reprinted with permission from Refs. [
6.2. Doping and injection efficiency of charge carriers
From the ABC model, the IQE is directly proportional to
With experimental evidence, Zhang et al. computationally demonstrated that silicon step-doping of quantum barriers would lower polarization-induced fields and reduce energy barriers for hole transport[
Figure 24.(Color online) (a) Schematic representation of the LEDs. Device I is a standard LED with unintentionally doped barriers, device II is designed with 12 nm-thick barriers, each fully doped with Si, and device III features step-doped barriers (6 nm undoped and followed by 6 nm doped). (b) and (c) respectively show the experimentally measured and numerically simulated optical output power and EQE as a function of current for devices I, II, and III. Reprinted with permission from Ref. [
6.3. State of current injection efficiency and AlxGa1−xN-based UV LEDs
With the advancement of technology, there is a colossal demand for high power and highly efficient UV optoelectronic devices because of their diverse application (water/air/food sterilization, surface disinfection, free-space non-line-of-sight communication, epoxy curing, counterfeit detection, fluorescence or Raman identification of biological/chemical agents, and various diagnostic and therapeutic medical functions)[
EQE of a light-emitting device (Fig. 25(a)) is the product of electrical injection efficiency (EE), IQE, and LEE, and typical values are shown in Fig. 25(b)[
Figure 25.(Color online) (a) Schematic diagram of an Al
Current injection is affected by numerous factors: metal contacts, interfaces in between contact material and electron/hole supplier layers, concentration of electron/hole in electron/hole supplier layers, electrical conductivity, and electric field (piezoelectric field) developed across the active region. Both p- and n-type doping became challenging in DUV AlxGa1−xN-based devices due to high aluminum concentrations[
Figure 26.(Color online) An outline that underscores essential factors for enhancement in hole injection current.
When a device is electrically biased, the very first hurdle that a hole encounters is the interface of the p-type metal contact and the hole supplier layer[
In the majority of UV LED devices, a hole supplier layer consists of p-type GaN and AlxGa1−xN layers. As a hole reaches the hole supply layer, it encounters a high discontinuity in the valance band at the heterojunction, which makes the hole transport challenging. Graded heterojunctions[
Figure 27.(Color online) (a) Measured temperature-dependent resistivity for different Al
Electron leakage under high bias currents is inevitable. An EBL is not only thought to be capable of minimizing electron leakage, but also hindering hole transport because of the discontinuity of the valance band[
Holes in the active regions of visible LEDs tend to accumulate close to the interface between the MQW region and the p-type EBL; however, this is not the case in DUV LEDs[
7. Outlook and future challenges
While sapphire is intrinsically nonconductive, Ga2O3 substrates can be highly conductive and UV transparent, though the carrier mobilities are relatively low compared to GaN[
Acknowledgements
We acknowledge the financial support from the King Abdulaziz City for Science and Technology (KACST) under grant no. KACST TIC R2-FP-008. This work was partially supported by the King Abdullah University of Science and Technology (KAUST) baseline funding no. BAS/1/1614-01-01, and MBE equipment funding no. C/M-20000-12-001-77 and KCR/1/4055-01-01.
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