M Khan - Academia.edu (original) (raw)

Papers by M Khan

Japanese Journal of Applied Physics, 2021

View the article online for updates and enhancements. You may also like A review of single and mu... more View the article online for updates and enhancements. You may also like A review of single and multiple optical image encryption techniques Abdurrahman Hazer and Remzi Yldrm-Yellow-red emission from (Ga,In)N heterostructures B Damilano and B Gil-Crystal structure and photoluminescence of (Gd,Ce) 4 (SiS 4) 3 and (

Far-ultraviolet-C (Far-UVC) light-emitting-diodes (LEDs) offer a promising technology for the dis... more Far-ultraviolet-C (Far-UVC) light-emitting-diodes (LEDs) offer a promising technology for the disinfection of surface, air, water, food and airborne disease transmission in occupied spaces, including COVID-19 (SARS-CoV-2) and other viral diseases, when it is meticulously designed, engineered, and applied. Research should continue on both the safety and efficacy of AlGaN-based Far-UVC LEDs, as well as the material choices and device designs to develop highly efficient solid-state UV germicidal irradiation (UVGI) at 222 nm emission to replace toxic low-pressure mercury lamps emitting at 253.7 nm. However, the key issue of hole concentration inside the multi-quantum-wells (MQWs) of AlGaN-based Far-UVC LEDs with high Al-contents is quite critical. Therefore, theoretical studies of AlGaN-based Far-UVC LEDs may suggest sufficient evidence for immediate consideration and implementation for the epitaxial growth of 222 nm-band Far-UVC LED technology during this worldwide health crisis. In this paper, the initial design of the Al-graded p-AlGaN hole source layer (HSL) on the performances of Far-UVC LED was compared with conventional bulk p-AlGaN HSL (non-graded)based LED devices. For the evaluation of the device's performances, the energy band diagram, internal quantum efficiency (IQE), electrons and holes concentration, radiative recombination rate, and current density vs voltage characteristic were compared. It was found that LEDs at 222 nm emission without using the undoped (ud)-AlGaN final-quantum-barrier (FQB) and only keeping the Al-graded Mg-doped p-AlGaN HSL showed high carrier injection into the MQWs. The variation in the energy band diagram around the p-AlGaN electron-blocking layer (EBL)/p-AlGaN HSL region and p-AlGaN HSL/p-GaN contact-layer (CL) indicates that the introduction of the Al-graded p-AlGaN HSL, as well as the special choice of Al composition at the interfaces, are quite promising for the enhancement of hole injection toward MQWs. The simulation results suggest that the proposed structure of the Al-graded p-AlGaN HSL after omitting the ud-AlGaN FQB structure in the Far-UVC LED is quite useful for achieving high peak efficiency, as well as for suppressing the efficiency droop when compared to the conventional bulk Far-UVC LED. After introducing a new design of 40 nm-thick p-AlGaN HSL in the Far-UVC LED, the radiative recombination rate in the first two quantum-wells of MQWs has been improved up to B50%. The enhanced radiative recombination rate is attributed to the enhanced level of electron and hole concentrations by B26% and 53%, respectively, in the MQWs. Ultimately, after removing the ud-AlGaN FQB and using 40 nm-thick Al-graded (Al: 100% to 20%) p-AlGaN HSL, the efficiency droop has been remarkably reduced from B39% (Bulk-LED) to B19% in the new design of Far-UVC LED structure.

Achieving high threshold current density and high optical confinement are big challenges in the r... more Achieving high threshold current density and high optical confinement are big challenges in the realization of high-performance aluminum gallium nitride (AlGaN)-based deep-ultraviolet (DUV) laser diode (LD). In this work, compositional Al-grading of AlGaN layers is used to increase the optical confinement factor (OCF), carrier injection efficiency, gain, and emission power of the DUV LD. Compositional grading of waveguides (WGs) layer, electron blocking layer (EBL), and cladding layers (CLs) demonstrated that the device characteristic can be improved. By using compositional Al-grading of AlGaN p-WG, EBL, p-CL along with n-WG and n-CL, 17.4% OCF, 94.4 mW emission power, and 1369 m − 1 gain at 267 nm peak emission wavelength are achieved. These improvements are attributed to the reduced threshold current density as well as using better optical confinement scheme in the DUV LD.

The epitaxial growth of transparent p-AlGaN-based ultraviolet-A (UVA), lightemitting diodes (LEDs... more The epitaxial growth of transparent p-AlGaN-based ultraviolet-A (UVA), lightemitting diodes (LEDs) may solve the problems of UVA light absorption through the
GaN buffers and p-GaN contact layers at (326–341 nm)-band emission, respectively.
Herein, first, an idea of conventional n-AlGaN buffer layer (BL) and n-AlGaN electron
source layer (ESL) for the suppression of threading dislocation density (TDD) and
enhancement of internal quantum efficiency (IQE) of UVA emitters, using lowpressure metalorganic vapor-phase epitaxy (LP-MOVPE) is attempted. As a result,
the total-TDDs is reduced from 3 109 cm2 to 1 109 cm2 in the n-AlGaN
ESL of a 326 nm-band UVA multiquantum-wells (MQWs), and IQE is also improved
from 30% to 52% at room temperature (RT). Second, an idea of Si-doped n-AlGaN
Superlattices (SLs)-based BL, using LP-MOVPE is challenged. Subsequently, a
record IQE of 56% at RT and high crystal quality in 341 nm-band UVA MQWs are
observed. Finally, using a well thickness 2 nm in SLs-based UVA MQWs, the light
power and external quantum efficiency (EQE), respectively, are remarkably
enhanced from 3.5 mW and 0.5% to 7.5 mW and 1.4% on wafer in 341 nm-Band
UVA LED. The perspective for the improvements of UVA emitter’s performances is
also discussed.

AlGaN-based ultraviolet-B (UVB) LEDs at 310 nm emissions are expected to offer safe and smart siz... more AlGaN-based ultraviolet-B (UVB) LEDs at 310 nm emissions are expected to offer safe and smart size UVB-light sources compared to the toxic mercury UV-lamp. Previously, the issue of nonlinearity in the emitted light output power (L) as well as in the external quantum efficiency (EQE) of 310 nm band UVB LEDs were observed. First, the influence of both the number of n-AlGaN buffer layers (BLs) and the type of p-electrodes on the recovery of linear behavior in the L and EQE were investigated. It was found that the nonlinearity in the L and EQE of UVB LED is independent of the number of BLs as well as type of p-electrodes. Therefore, finally the dependence of nonlinearity in the L and EQE on the thickness of quantum-well-barrier (TQWB) of multi-quantum-wells (MQWs) were also considered. Subsequently, the issue of nonlinear behavior in the L and EQE was resolved by the thickness reduction of TQWB from 25 to 10 nm in the MQWs. Similarly, a reasonable value of improvement in both L and EQE, respectively, up to 12 mW and 2.2% of 310nm band UVB LED were realized.

AlGaN-based ultraviolet-B (UVB) LEDs at 310 nm emissions are expected to offer safe and smart siz... more AlGaN-based ultraviolet-B (UVB) LEDs at 310 nm emissions are expected to offer safe and smart size UVB-light sources compared to the toxic mercury UV-lamp. Previously, the issue of nonlinearity in the emitted light output power (L) as well as in the external quantum efficiency (EQE) of 310 nm band UVB LEDs were observed. First, the influence of both the number of n-AlGaN buffer layers (BLs) and the type of p-electrodes on the recovery of linear behavior in the L and EQE were investigated. It was found that the nonlinearity in the L and EQE of UVB LED is independent of the number of BLs as well as type of p-electrodes. Therefore, finally the dependence of nonlinearity in the L and EQE on the thickness of quantumwell-barrier (T QWB) of multi-quantum-wells (MQWs) were also considered. Subsequently, the issue of nonlinear behavior in the L and EQE was resolved by the thickness reduction of T QWB from 25 to 10 nm in the MQWs. Similarly, a reasonable value of improvement in both L and EQE, respectively, up to 12 mW and 2.2% of 310nm band UVB LED were realized.

Crystal growth of eco-friendly, ultrawide bandgap aluminium gallium nitride (AlGaN) semiconductor... more Crystal growth of eco-friendly, ultrawide bandgap aluminium gallium nitride (AlGaN) semiconductorbased ultraviolet-B (UVB) light-emitting diodes (LEDs) hold the potential to replace toxic mercurybased ultraviolet lamps. One of the major drawbacks in the utilisation of AlGaN-based UVB LEDs is their low efficiency of about 6.5%. The study investigates the influence of Al-graded p-type multiquantum-barrier electron-blocking-layer (Al-grad p-MQB EBL) and Al-graded p-AlGaN hole source layer (HSL) on the generation and injection of 3D holes in the active region. Using the new UVB LED design, a significant improvement in the experimental efficiency and light output power of about 8.2% and 36 mW is noticed. This is accomplished by the transparent nature of Al-graded Mg-doped p-AlGaN HSL for 3D holes generation and p-MQB EBL structure for holes transport toward multi-quantumwells via intra-band tunnelling. Based on both the numerical and experimental studies, the influence of sub-nanometre scale Ni film deposited underneath the 200 nm-thick Al-film p-electrode on the optical reflectance in UVB LED is investigated. A remarkable improvement in the efficiency of up to 9.6% and light output power of 40 mW, even in the absence of standard package, flip-chip, and resinlike lenses, is achieved on bare-wafer under continuous-wave operation at room temperature. The enhanced performance is attributed to the use of Al-graded p-MQB EBL coupled with softly polarised p-AlGaN HSL and the highly reflective 0.4 nm-thick Ni and 200 nm-thick Al p-electrode in the UVB LED. This research study provides a new avenue to improve the performance of high-power p-AlGaNbased UVB LEDs and other optoelectronic devices in III-V semiconductors. Aluminium gallium nitride (AlGaN) based semiconductors are one of the most promising candidates for the fabrication of smart, eco-friendly ultraviolet-B (UVB) and deep ultraviolet (DUV) emitters that would meet the requirements of the Minamata Convention of 2020 1 and the 17 sustainable development goals (17 SDGs) of the UN 2. Both Minamata Convention and 17 SDGs, with an aim to mitigate climate change, strive to eliminate the use of mercury vapour ultraviolet (UV) lamps in order to reduce the associated issue of CO 2 emission 1,2. Earlier studies show the use of UVB light of 310 nm narrow-band in cancer immunotherapy 3,4 , in vulgaris treatment 4,5 and for plant growth with enriched phytochemicals 4,6. Similarly, UVB light of 294 nm-band is used in the prevention of plant diseases 7,8 and in the production of vitamin D 3 in the human body 8,9. Safe and smart DUV and ultraviolet-C (UVC) light sources are extremely important as a disinfectant for air, water, food and surfaces that can help to mitigate the risk of infection due to close contact with the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), and other similar viruses 10-15. Currently, toxic mercury-based UV light sources are frequently used for many industrial, agricultural and medical applications 4,8. To replace the toxic mercury UV light sources with eco-friendly alternatives, we need to explore safe and green materials for crystal growth and device fabrication of UV light sources. Among such favourable and affordable materials, crystal growth of AlGaN compounds on AlN templates is currently being investigated for next-generation UVB light-emitting diodes (LEDs) and laser diodes (LD) 4,8,12. Environmentfriendly AlGaN-based UVB LEDs with monochromatic light emission are inevitable for both medical and agricultural applications 1-9. The selection of green AlGaN material has many other promising features, such as

Japanese Journal of Applied Physics, 2021

AlGaN-based ultraviolet-B (UVB) LEDs at 310 nm emissions are expected to offer safe and smart siz... more AlGaN-based ultraviolet-B (UVB) LEDs at 310 nm emissions are expected to offer safe and smart size UVB-light sources compared to the toxic mercury UV-lamp. Previously, the issue of nonlinearity in the emitted light output power (L) as well as in the external quantum efficiency (EQE) of 310 nm band UVB LEDs were observed. First, the influence of both the number of n-AlGaN buffer layers (BLs) and the type of p-electrodes on the recovery of linear behavior in the L and EQE were investigated. It was found that the nonlinearity in the L and EQE of UVB LED is independent of the number of BLs as well as type of p-electrodes. Therefore, finally the dependence of nonlinearity in the L and EQE on the thickness of quantumwell-barrier (T QWB) of multi-quantum-wells (MQWs) were also considered. Subsequently, the issue of nonlinear behavior in the L and EQE was resolved by the thickness reduction of T QWB from 25 to 10 nm in the MQWs. Similarly, a reasonable value of improvement in both L and EQE, respectively, up to 12 mW and 2.2% of 310nm band UVB LED were realized.