Abstract

This paper is intended to provide an overview of recent advances of GaN based nanostructured materials and devices. Because of its unique electrical, optical, and structural properties, GaN has sparked significant interest in the field of wide bandgap semiconductor research. Because of its higher surface-to-volume ratio than thin films, GaN nanostructured material offers numerous advantages for nanodevices. The ability of GaN nanostructured material to absorb ultraviolet (UV) radiation is invaluable in many optical applications. GaN nanostructured-based devices have recently received a lot of interest due to their numerous potential uses. GaN has been employed as a nanomaterial in a variety of devices, including UV photodetectors, light-emitting diodes, solar cells, and transistors. The most current developments in GaN-based devices are presented and reviewed. The performance of many device architectures demonstrated on GaN is presented. The structural, electrical, and optical characteristics are also discussed.

1. Introduction

The group III-Nitride semiconductor materials have attracted a lot of interest for new generation of optoelectronic devices ¹. The advantage with these materials is the flexible bandgap varying from 0.7 to 6 eV hence covering an ultra-broad spectrum, from deep ultraviolet up to near infrared ¹, which has been employed in various applications such as optoelectronic devices, electronic devices, biosensors, chemical sensors and so on ². Solar cells based on nitride materials have readily been investigated for terrestrial and space-based applications ¹. Transistors performance for high power electronics, ground-based communications and biological agent detection devices has been enhanced ¹. Major efforts have been dedicated to technological fabrication to achieve efficient emitters and detectors ¹. Recent progress has demonstrated cutting edge results in high-speed data rate connectivity and integrated circuits ¹. Imaging sensors on high-speed electronics have been implemented founded on their sensitive applications in security screening ¹.

GaN as a member of group III-nitride family has become a revolutionary material owing to its electronic and optical properties. The direct, flexible, and wide bandgap makes GaN materiel a key candidate for achieving high frequency, large bandwidth, high power, and efficiency devices. GaN based detectors are in particular suitable for full color display, high density information storage, and UV communication links ¹.

GaN has a direct and wide band gap of 3.4 eV at room temperature, it is quite robust ³. Other group III elements like Aluminum (Al) and Indium (In) can be alloyed with Ga to tune the bandgap of III-nitrides from 0.8 eV to 6 eV ³. Moreover, it possesses high electron mobility, high heat capacity and high breakdown voltage ³, which makes it suitable to be used for sensors, for high power electronic devices such as field effect transistor (FET) and for optoelectronic devices such as light emitting diode (LED) ¹. The optical properties of GaN nanostructured are of great current interest because of the potential application in solid state lighting ¹. In n-type GaN, an UV peak at approximately 3.42 eV usually dominates the photoluminescence spectrum ¹. The blue luminescence at 2.7 to 3 eV peak energy has been extensively studied; this peak dominates due to optically active defects and impurities ¹. On the other hand, such defects can be destructive in a device. A well-engineered inorganic nanoparticle approach offers many advantages ¹. Meanwhile, in nanostructures having a large specific area, the surface states effect became significant in influencing the carrier recombination mechanism ¹.

2. GaN Nanostructured Materials Doping

Nanostructures are typically created using either a bottom-up or a top-down technique. Metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), the vapor-liquid-solid (VLS), laser-assisted catalytic growth (LCG), and the ion-etching reaction have all been used to create well-aligned GaN nanostructures ^{4, 5, 6}. GaN nanostructured material is better for nanodevices than thin films because it has a higher ratio of surface to volume. GaN nanostructured material can absorb UV light and is very useful in a wide range of optical applications ⁷. GaN nanostructures come in many different shapes, such as nanowires, nanoparticles, nanobelts, nanorings, nanotubes, nanodots, and nanorods ⁸. GaN nanoparticles have been getting a lot of attention from scientists and engineers over the past few years. Quantum confinement of electrons in nanoparticles gives them useful electronic properties that make them useful in many GaN applications ⁹.

Doping GaN is crucial for a wide variety of device architectures. GaN doping has been accomplished in a few different ways ¹⁰. For GaN-based high-power devices, reaching levels of performance close to their theoretical limits has been hampered by the difficulty of regulating n-type or p-type conduction using the ion implantation procedure ¹¹. Activation rates of 86% and roughly 100% have been reported in prior research on the n-type conduction of GaN through silicon ion implantation, after annealing at 1250 and 1400 °C, respectively ¹². GaN's surface is severely degraded during high-temperature annealing owing to breakdown ¹³. But picking the right protective layer that doesn't change and can be easily removed after annealing at temperatures above 1200 degrees Celsius is tricky ¹³. Therefore, it is crucial to get the high activation rate at a lower temperature. However, because of the higher temperature annealing required for electrical activation, preserving the surface during Mg ion implantation for p-type conductivity is much more difficult ¹⁴. Mg on the Ga site (MgGa) has a formation energy in the valence band that is around 1 eV higher than that of SiGa at the Fermi level near the conduction band, which may account for the difference in annealing temperatures between the two conduction types ¹⁵. In general, efficiently doping GaN to the p-type is difficult. Even though high-quality p-type GaN has been realized, the activation efficiency of Mg atoms is still in the single-digit percentage range ¹⁶. While there has been some discussion of p-type doping of GaN using ion implantation and diffusion, publications on these and other doping methods commonly used in semiconductor fabrication have been uncommon ¹⁷. Changing the concentration profile of implanted Mg was discovered via an investigation into the thermal stability and redistribution of implanted dopants in GaN. Indeed, high temperature annealing of GaN after implantation is required for Mg redistribution and activation as well as lattice recovery ¹⁸. This annealing is commonly performed between 1100 and 1400°C ¹⁹. However, nitrogen desorption causes significant surface damage to GaN during annealing at temperatures exceeding 800-900°C ²⁰. This extreme sensitivity to post-implantation heat treatments means that a cap layer must be used to shield the GaN surface and prevent nitrogen desorption ²¹.

As of late, carbon doping of GaN has also been achieved. Carbon may be deliberately doped onto GaN via a hydrocarbon precursor method ²². In terms of the voltage capabilities of the device, the extrinsic carbon doping results in superior dynamic qualities ²³. Optimal solutions for effective dc-dc conversion may be found in devices like GaN on silicon (Si) devices ²⁴. For a long time now, they've also been clearly superior to their Si counterparts in terms of handling high voltage operation at a high switching frequency ²⁵. The most frequent technique for creating extremely resistive GaN buffers is carbon compensatory doping, which is employed due to the necessity for high blocking voltage ²⁶. High dynamic on resistance and slowly recovering current collapse are just two of the major drawbacks of carbon-doped GaN heterostructure field effect transistors ²⁷. Incorporation of carbon from the metal organic precursor during growth is the classic method for achieving carbon doping ²⁸. High carbon concentrations required a low growth temperature, pressure, and V/III ratio ²⁹. The result was decreased blocking voltages and reduced electron mobility in the channel, as a result of a combination of poor crystal quality and a high dislocation density ³⁰. Incorporating carbon while preserving growth settings suited for crystal quality through extrinsic carbon doping of GaN buffers has been gaining traction of late ³¹.

GaN is a desirable alternative to InGaN for red LEDs due to the InN-rich alloy's surprisingly poor luminous efficiency ³². GaN demonstrates self-compensation, where acceptor doping is neutralized by native donors, which is energetically advantageous for the crystal ³³. The thermodynamic study of the defect chemistry in GaN:Mg crystals indicates that a MgGa acceptor-controlled p-type can only be obtained at N2 pressures greater than 104 MPa ³⁴. Due to donor recombination, Mg-doped GaN films exhibit wide emission bands in the region of 2.8–3.3 eV ³⁴. Mn has limited solubility in gallium and its group V compounds; hence, phase separation occurs during the development of GaMnAs when Mn concentrations greater than 5% are used ³⁴. Pulsed laser deposition (PLD) may be used to produce thin films from multicomponent targets and permits easy control of Mn concentrations ³⁴.

3. GaN Based Nanostructured Devices

The GaN nanostructures properties effect the performance of the gas sensors. Ashfaque et al. studied reported on the GaN nanostructures-based sensors for the detection of various gases ³ including alcohols (R-OH), methane (CH₄), benzene and its derivatives, nitric oxide (NO), nitrogen dioxide (NO₂), sulfur-dioxide (SO₂), ammonia (NH₃), hydrogen sulfide (H₂S), and carbon dioxide (CO₂) ³. In that work, the sensing performance of GaN nanostructure-based sensors devices have been described, a unique measure called the most essential sensing performance data, including the limit of detection, response/recovery time, and operating temperature, have been compiled and tabulated for each of the numerous types of sensors to give a comprehensive performance comparison ³. For the product of reaction time and limit of detection has been devised ³⁵. It was discovered that the InGaN/GaN NW-based sensor has greater overall sensing capability for the detection of H₂ gas ³⁶. On the other hand, the GaN/(TiO₂–Pt) nanowire-nanoclusters

(NWNCs)-based sensor is superior for the detection of ethanol ³⁷. TNT sensing is another application that works quite well with the GaN/TiO₂ NWNC-based sensor ³⁸. In another article included a summary of initial primary research that was based on density-functional theory (DFT), looked at the interaction between gas molecules and GaN ³⁹. In addition, the use of machine learning techniques to nanostructured GaN sensors and sensor arrays has been investigated and examined ⁴⁰. The last topic of discussion was the gas sensing method on GaN nanostructure-based sensors operating at ambient temperature ⁴¹. Figure 1 shows the sensors based on GaN nanostructures to detect H₂, alcohols, and additional gases, and that study also compares their reaction times and sensitivity limits.

Significant progress has recently been made on group III-nitride nanostructure-based LEDs across the entire color spectrum, from DUV to NIR, by modulating the bandgap of Al(Ga/In)N nanostructures. This has resulted in several exciting new developments ⁴². LEDs based on group III nitride nanostructures typically used nitride p-n homo- and heterojunctions, n-nitride/p-semiconductor, and n-semiconductor/p-nitride. In this part, the current developments that have taken place in the field of group III-nitride nanostructures for LEDs with color-adjustable emission are discussed ⁴³.

Since the original demonstration of nanostructured GaN LEDs employing two single NWs, comprising a Si-doped n-GaN nanowire and a Mg-doped p-GaN NW, there has been increased interest in tuneable light emission sources based on 1D nanostructures embedded with p-n junctions ⁴⁴. GaN nanowires p-n junction and n-ZnO/p-GaN heterojunction nanowire LEDs are shown in Figure 2 ⁴⁵. Mg doping led to the formation of p-GaN nanowires from N vacancies and/or O impurities ⁴⁵. The rectifying I-V curve of a p-n GaN nanowire-based LED is shown in Figure 3(a). Forward biasing the p-n junction of a GaN nanowire LED causes light emission. As shown in Figure 3(b), the emission peak of GaN nanowire p-n junction LEDs is 3.179 eV (390 nm) regardless of current injection ⁴⁶. The turn-on voltage and rectification ratio for the single n-ZnO/p-AlGaN NW-based LED device are shown in Figure 3(c) as 2 V and 4:1, respectively ⁴⁷. Band-filling increased EL significantly as current increased ⁴⁸. As demonstrated in Figure 3(d), when the injection currents are between 1 and 4 A, the monopeak remains at 394 nm, while the EL peak shifts to 400 nm at 5 A ⁴⁹. At modest injection currents, the p-AlGaN side radially recombined the injected carriers at 394 nm ⁵¹. When the current was high enough, holes could be injected into the n-ZnO side, resulting in radiative recombination at the band edge at 400 nm ⁵².

Mi's team created a practically defect-free ⁵³.

AlGaN tunnel junon (TJ) nanowire ⁵³. As shown in Figure 4(a) and Figure 4(b), the EL emission peak of the AlGaN nanowire LED with the n+-GaN/Al/p+ efficiency of 0.012% ⁵³. With an Al-rich AlGaN shell surrounding the AlGaN nanowires with TJ, as shown in Figure 4(c)-(d), the core-shell nanowire heterostructure enabled efficient carrier confinement in the nanowire LED active area and suppressed nonradiative surface recombination ⁵³. The creation of double heterostructures was attributed to -AlGaN TJ is centered at 242 nm, demonstrating greater EL emission compared to the AlGaN NW-LED without TJ due to enhanced electrical quantum performance ⁵⁴. The EL intensity was multiplied by 400, approximately. The highest output power of unpackaged TJ UV LEDs producing at 242 nm was measured to be 0.37 mW, with a maximum external a very narrow emission peak at 275 nm and a lack of considerable peak-shifting with increased injection current ⁵⁵. The unpackaged Al TJ DUV LED had an output power of more than 8 mW and a peak EQE of 0.4%, which were about one to two orders of magnitude greater than their AlGaN nanowire devices ⁵⁶. Due to the integration of Al TJ and the deletion of the resistive and absorptive p-GaN contact layer, it is claimed that the increased LED performance is a result of greatly improved hole transport and injection into the device active area ⁵⁷.cti

Mg has been extensively utilized as a p-type dopant in GaN; however, AlN presents substantial hurdles because of its unusually high ionization energy (600 meV) ⁵⁸. Furthermore, the presence of free holes in Mg-doped AlGaN is strongly counterbalanced by the formation of large native defects and impurity incorporation during the epitaxy of highly mismatched lattice AlGaN epilayers ⁵⁹. To overcome the inherent disadvantage of low p-type conductivity in Al-rich AlGaN for deep ultraviolet (DUV) LEDs, hexagonal boron nitride (h-BN) may be an ideal candidate due to its large bandgap, near-zero polarization field, small ionization energy (150 meV), and especially the unusual propensity for p-type doping in the presence of B vacancies ⁶⁰. Using h-BN, Maity et al. ⁶¹ established a unique h-BN/AlGaN p-n junction to solve p-type doping difficulties in Al-rich AlGaN ⁶¹. In addition, Yang et al. reported that h-BN can function as a highly conductive, DUV-transparent electrode in the Mg-free nanowire-array-based Al(Ga)N/h-BN LED and that the hole concentration is as high as 1020 cm³ at room temperature, which is 10 orders of magnitude higher than that of previously reported Mg-doped AlN epilayers ⁶².

By alloying the binary material, the bandgap of Group III-nitride nanostructures may be tuned across a broad range in the visible light spectrum, making them very promising for use in a variety of color LED applications ⁶³. The introduction of trace quantities of Mg and Si, respectively, enables the creation of an epitaxial p-n homojunction, resulting in the efficient radiative recombination of electrons and holes ⁶⁴. On the basis of group III-nitride nanostructures, it is possible to create LEDs of different colors ⁶⁵. Liang et al. have described axial group-III-nitride nanowire LEDs ⁶⁶. Subsequently, a great deal of study was devoted to the investigation of LEDs with axial III nitride nanostructures that emit light of different colors and have a high efficiency ⁶⁶. Lu et al. observed a single peak emission of 415 nm from a p-GaN/InxGa1-xN/GaN MQW/n-GaN nanowire LED, as shown in Figure 5(a)-(b)-(c) ⁶⁷. By adjusting the number of MQW pairings using the MOCVD process, the MQW structures precisely regulated in content were adjusted ⁶⁸. The output power rose linearly with increasing current due to the uniform and defect-free InxGa1-xN/GaN MQW nanowire structure ⁶⁸.

Color-tuneable LEDs with chosen CCT and CRI enable displays, smart illumination, and real-time cell identification. Phosphor-free, high-efficiency tuneable-light LEDs were designed to address phosphor LEDs' poor CRI and Stokes fluorescence loss ⁶⁹. Color-tunable LEDs with very tiny size, low power consumption, and good CRI may be possible by directly integrating RGB LEDs on a chip ⁷⁰. Much work has gone into making color-tunable III-nitride nanostructure LEDs ⁷¹. The following categories summarize recent III-nitride nanostructure LED tuneable color emission studies: Tuning the In composition in pure InGaN nanowires or (Al, In) GaN/GaN 1D MQW/MQD heterostructures; designing special heterostructures and measuring device performance under different injection currents/applied voltages/current injection modes; and integrating the different areas with tunable color emission directly on a single chip ⁷².Meier et al. created a single n-GaN/InxGa1-xN/GaN/p-AlGaN/p-GaN core/multishell (CMS) NW LED, as shown in Figure 6(a)-(b) ⁷³. Discrete connections were created to the n-type core and p-type shell of the isolated single NW device. InxGa1-xN NWs with x of 0.01, 0.1, 0.2, 0.25, and 0.35, respectively, were able to produce EL emissions at 367, 412, 459, 510, and 577 nm. As shown in Figure 6(c)(d), Alfaraj et al. successfully implemented the tuneable emission using InGaN NW LEDs ⁷⁴. By modulating the VT of the In and Ga sources during the CVD development, the In composition of InGaN NWs produced on p-GaN/Al₂O₃ substrates was controlled in the range of 0.06–0.43 ⁷⁵. The EL emission peaks were adjusted from 435 nm to 575 nm, i.e., from blue to orange hues, with increasing In concentration due to the tunable bandgap of InxGa1-xN NWs ⁷⁶.

Robin et al. ⁷⁷ demonstrated visible-color tuneable LEDs using InGaN thin-film integrated in position-controlled GaN nanowire arrays ⁷⁷. Anisotropic MQWs with varying QW thickness and composition were produced on different facets of n-GaN nanowires during the heteroepitaxial overgrowth of the InGaN/GaN MQW layers ⁷⁸. With increasing bias voltage, the EL spectra of nanostructured LEDs exhibited a continuous peak shift from red to blue emission ⁷⁸. Both the anisotropic MQW layers generated on the multifunctional GaN nanowires and the progressive change in electric field distributions in nanowire-embedded thin-film structures upon altering electric bias contributed to the highly adjustable emission color of these nanowire-embedded thin film LEDs ⁷⁸. Using the same method, they also showed the ability to adjust the emission color of InGaN/GaN microdonut array LEDs from green to blue ⁷⁸.

InGaN nanostructures emit virtually all visible light efficiently ⁷⁹. Phosphor-free white light LEDs use core-shell heterostructures and quantum in 1D GaN nanostructures ⁸⁰. Priante et al. ⁸¹ demonstrated axial nanowire LED heterostructures with self-organized core-shell InGaN/AlGaN dots in p-GaN:Mg/n-GaN:Si nanowire arrays ⁸¹. Due to the many AlGaN shell and barrier layers, the InGaN/AlGaN dot-in-a-wire core-shell LED active area has better 3D carrier confinement ⁸². The AlGaN barriers surrounded and capped the ten InGaN dots, forming an Al-rich shell at the nanowire sidewall ⁸³. The dot-in-a-wire core-shell LED output EL spectra were steady and almost independent of injection currents ⁸⁴. The LED's CCT was 4450 K and its CRI was 95. Embedding InGaN quantum dots/nano disks in defect-free 1D GaN nanostructures is an efficient method for producing phosphor-free white light LEDs ⁸⁵. The nanowire LED emitted a bright white light. Moreover, the CIE chromaticity coordinates of the p-type doped LED hardly changed as the injection current increased ⁸⁶. Comparing the RT IQE of undoped and p-type doped LEDs, the IQE of p-type doping (56.8%) was almost 50% higher than that of undoping (36.8%), which was sufficient for phosphor-free white LEDs ⁸⁷. As a result, group III-nitride nanostructures provide an alternate strategy for increasing lifespan and improving light quality ⁸⁷.

Micro-LEDs (-LED), with a micro-size of 1030 m, are now considered the ultimate display technology because of their inherent high brightness, high contrast ratio, quick response speed, extended lifespan, power-saving qualities, and ability to be operated across a broad temperature range ⁸⁸. Since the first current injection of -LED (12 m) based on p-GaN/InGaN/n-GaN QWs was reported, a great deal of focus has been placed on the advancement of -LED, with promising products appearing in areas such as wearable displays for high-speed3D/AR/VR display applications, high-brightness/contrast large flat-panel displays and TVs, light sources for the neural interface, and optogenetics ⁸⁸. For instance, Jiang et al. reported the use of InGaN/GaN-based nanocolumns (NC) to create multicolor (red, green, blue, and yellow; RGBY) LED pixels with 5 m² emission ⁹⁰.

Because the direct bandgap energies of GaN materials are different, their 1D nanostructures can be used to make photodetectors that work from the deep ultraviolet to the near infrared.

Nanowires has been widely studied in recent years ⁹⁰. Yin et al. ⁹¹ reported the dark and illuminated electrical transport performance of a single GaN nanowisker in 2021. Due to the presence of depletion space charge layers, the diameter of the nanowires determined the photocurrent of the GaN nanowires PD device ⁹². The accelerated recombination process inside nanowires with short diameters screened the depletion electric field, resulting in a reduction in photocurrent ⁹³. Nonetheless, the photocurrent exhibited a minor effect on the diameter, since the vast bulk volume remained unconsumed ⁹⁴. Johar et al. ⁹⁵ developed single nanowire PD based on axial GaN nanowires on patterned Si. With a reaction time of less than 25 ms, the device displayed a high responsivity of up to 10,000 A/W ⁹⁶.

Group III-nitride nanostructures with added dopants have been shown to be an efficient method of achieving the necessary optical and electrical features ⁹⁷. The efficiency of PDs made from group III-nitride nanostructures might also be improved using this method ⁹⁸. Concordel et al. ¹⁰⁰, for instance, looked at how n-type doping affected the electrical transport performance of GaN nanowires of varying diameters. Under UV light, GaN nanowires with highly doped Si had a much greater photocurrent than nanowires with undoped or mildly doped Si ¹⁰⁰. The photoconductivity performance of Mg doped nonpolar GaN nanowires was reported by Subramani et al. ¹⁰¹. In the presence of light at 470, 530, and 788 nm wavelengths, the constructed devices had an extremely high on-off ratio of 100 ¹⁰².

On the basis of III-nitride nanostructures, photodetectors with an active p-n junction have been constructed ¹⁰³. When light shines onto the connection, a proportionate reverse current may be observed ¹⁰⁴. Aiello et al. ¹⁰⁵ demonstrated GaN quantum disk-based UV photodetectors in a single GaN nanowire. The gadget demonstrated a significant decrease in dark current and an increase in photocurrent Yang et al. ¹⁰⁶ described the manufacture and characterization of visible blind photodetectors based on an array of p-i-n junction GaN nanowires, as shown in Figure 7. In moreover, the photocurrent emerged at 3.27 eV and rose by more than two orders of magnitude between 3.3 and 3.4 eV; the ratio of UV to visible photocurrent rejection was 200 ¹⁰⁷. Such visible band detectors found interesting uses in photodetectors with fast speeds ¹⁰⁸. Shen et al. ¹⁰⁹ subsequently proved the photocurrent performance of a single axial GaN n-i-n nanowire produced by PAMBE. The photoconductive gain of the device was between 105 and 108, and the UV (350 nm) to visible (450 nm) responsivity ratio was more than 106. Recently, Spies et al. ¹¹⁰ recently revealed the UV photo sensing performance of a single GaN nanowire with AlN/GaN: Ge axial heterostructures. Both the integration of heterostructures and the increase in the number of active nano disks and AlN barriers were shown to reduce the dark current and enhance photosensitivity. In addition, work has been done to enhance the photoconductive performance of III-nitride nanostructure-based PDs by using a variety of materials for the electrodes ¹¹¹. The manufacturing and photoconductive capabilities of the PDs utilizing few layer graphene contact to GaN nanowire ensemble were reported by Sarkar et al. ¹¹².

4. Conclusion

In this review paper, we provide a comprehensive look at the production and properties of GaN nanostructures. GaN has some promises for electrical and photonic devices and encouraging progress has been made in the research phase. Despite this advancement, there are a number of critical challenges that must be addressed before this material can be commercially used for the claimed uses. The extremely successful GaN, which competes for comparable applications, makes the process more challenging. GaN contributes to optical device applications in part owing to the simplicity with which GaN may be generated in nanostructured form. There is still much to study about the mechanisms of GaN-based optical devices.

Although a number of GaN optical devices have been described, there are several difficulties that require additional inquiry and development. These challenges include p-type doping; the lack of plausible p-type doping hinders the widespread use of optical emitters in GaN. Furthermore, GaN's highly ionic nature, which results in strong electron photon coupling and low heat conductivity, does not bode well for GaN-based electrical systems. Nanostructures appear to be a bit simpler to generate using GaN, but it remains to be seen whether nanostructures in general, as advertised, can actually make inroads in the device field. In terms of nanostructures, GaN nanostructured devices such as nanowires and nanorods offer a path to a new generation of devices; however, a concerted effort is required for GaN nanostructured to be taken seriously for large scale device applications, as well as to achieve high device density with access to individual nanodevices. It is necessary to create reliable ways for assembling and integrating building blocks into circuits.

References