Abstract

This paper presents a review of the recent advances of GaN nanowire-based sensors. GaN nanostructures of various forms including nanowires, nanotubes, nanofibers, nanoparticles and nanonetworks have been reported for several sensing applications due to their unique electrical, optical, and structural properties. The nature of GaN materials provides advantages for nanodevices compared to thin films due to its higher surface-to-volume ratio. In addition, GaN materials can absorb ultraviolet radiation which is a key feature in several optical applications. These advantages led to much attention and further study in the applications of GaN. In this paper, the recent advances of GaN sensing applications such as gas sensors, biosensors, and pressure sensors are presented. The performance of these sensors is demonstrated along with the structural, electrical, and optical properties of GaN.

1. Introduction

Over the past two decades, three-dimensional gallium nitride (3D GaN) has been widely used in light-emitting diodes, sensors, laser diodes, solar cells, and power electronics ¹, attributing to its excellent optical and electronic properties. In 2004, graphene was successfully exfoliated and proved to have extraordinary properties ¹. Inspired by graphene, theoretical calculations have proved that two-dimensional hexagonal GaN (2D h-GaN) is stable ¹. Zhang et al. predicted that h-GaN can be used as excellent electrode material in lithium-ion batteries ². 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 ^{3, 4}, allowing the development of numerous applications ^{3, 4}. Major efforts have been dedicated to the technological fabrication to achieve efficient emitters and detectors ^{3, 4}. Recent progress has demonstrated cutting edge results in high-speed data rate connectivity and integrated circuits ^{3, 4}. Imaging sensors on high-speed electronics have been implemented founded on their sensitive applications in security screening ^{3, 4}.

GaN as a member of group III-nitride family has become a revolutionary material owing to its electronic and optical properties ^{3, 4}. The direct, flexible, and wide bandgap makes GaN materiel a key candidate for achieving high frequency, large bandwidth, high power, and efficiency devices ^{3, 4}.

GaN based detectors are suitable for full color display, high density information storage, and UV communication links ^{3, 4}.

GaN is a very hard, chemically, and mechanically stable wide bandgap (3.4 eV) semiconductor material with high heat capacity and thermal conductivity which makes it suitable to be used for sensors ⁹, for high power electronic devices such as field effect transistor (FET) ^{3, 4} and for optoelectronic devices such as light emitting diode (LED) ^{3, 4}. The optical properties of GaN nanostructured are of great current interest because of the potential application in solid state lighting ^{3, 4}. In n-type GaN, a UV peak at approximately 3.42 eV usually dominates the photoluminescence spectrum ^{3, 4}. 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 ^{3, 4}. On the other hand, such defects can be destructive in a device. A well-engineered inorganic nanoparticle approach offers many advantages ^{3, 4}. Meanwhile, in nanostructures having a large specific area, the surface states effect became significant in influencing the carrier recombination mechanism ^{3, 4}.

The efficient p-type doping of GaN is in general a challenging task ^{3, 4}. The magnesium (Mg) ion implantation for p-type conductivity is more challenging due to the higher temperature annealing required for electrical activation, resulting in a major difficulty protecting the surface ^{3, 4}. Mg is the only dopant capable of ensuring stable, reproducible p-type conduction in GaN ^{3, 4}. The formation energy of Mg on the Ga site (MgGa) near the valence band is about 1 eV higher than that of SiGa at the Fermi level near the conduction band ^{3, 4}, which may explain the difference of required annealing temperature for different conduction types. The efficient p-type doping of GaN is in general a challenging task ^{3, 4}. The extrinsic carbon doping delivers better dynamic properties for the device voltage capabilities. Carbon doping has been traditionally achieved through incorporation of carbon originating from the metal organic precursor during the growth process ^{3, 4} in a so-called auto doping technique. GaN can also be doped with europium (Eu). It is an attractive alternative to InGaN for the red-light LED, as the InN rich alloy has disappointingly low luminescence efficiency ^{3, 4}. GaN can also be doped with manganese (Mn). The growth of homogeneously Mn-doped Ga1_xMnxN thin films have been carried out at different temperatures ^{3, 4}. A high dopant concentration and high carrier concentration are inherent advantages of that doping ^{3, 4}. The Pulsed laser deposition (PLD) can be used to prepare thin films from multicomponent targets and allows Mn concentrations in the GaN films to be controlled easily by varying the quantity of Mn included in the initial target preparation ^{3, 4}. The growth conditions can be far from equilibrium, which offers the possibility of reaching higher Mn concentrations without phase separation ^{3, 4}. The study of nanostructured materials doping was reported and investigated by Nahhas ³. In that work, GaN doping processes and the challenges of each process were presented and discussed ³. GaN nanostructured material exhibits many advantages for nanodevices because of its higher surface-to-volume ratio as compared to thin films ³. The GaN nanostructured material has the ability to absorb UV radiation and immense in many optical applications ³. The electronic properties of quantum confinement of electrons of nanoparticles make them very useful in electronic industry including many GaN applications ³. Nanomaterial based sensors for environmental monitoring exhibited excellent potential in detection of trace contaminants due to the unique features of nanomaterials such as strong adsorption capacity, high surface area and reactivity, and large catalytic efficiency ^{5, 6, 7}.

The design of high-power GaN devices is strongly affected by the self-heating ^{8, 9}, and accurate thermal management is needed to control the temperature of such devices. Fourier’s law describes heat diffusion when the characteristic length scale of thermal transport is much longer than the phonon means free path (MFP). However, if the size of a hotspot is small enough, it will induce nonlocal (also called quasi- ballistic) transport phenomena ¹⁰. Then, non-diffusive thermal energy phonon carriers will travel from the source before experiencing collisions, and Fourier’s law is no longer adequate to describe thermal transport at the nanoscale ¹⁰.

High-quality epitaxial GaN growth is now possible thanks to the native GaN substrates available in the market ¹¹. The only disadvantage of these freestanding GaN substrates is their high cost for commercial applications ¹¹. One efficient way to reduce the total process cost is to isolate the epitaxial device from the bulk GaN substrate and reuse it. Several lift-off methods such as growth over patterned masks, natural stress-induced separation, controlled spalling, chemical etching of sacrificial layers, substrate removal by grinding/ etching, the use of weakly bonded detach layers like graphene, and laser lift of are reported for separating GaN films from the foreign substrates ^{11, 12}. Separating GaN film from a GaN substrate is also reported by using various techniques like chemical lift of process, creating porous release layers via chemical etching or dry etching, controlled spalling, laser lift-off with an InGaN sacrificial layer ¹¹, and ion implantation ¹².

2. GaN Nanostructured Materials Doping

GaN nanostructured have various shapes including nanowires, nanoparticles, nanobelts, nanorings, nanotubes ¹², nanodots ¹³, and nanorods ¹⁴. GaN nanoparticles generated lot of interest among scientists as well as technologists during past few years.

The ion implantation technique for controlling n-type or p-type conduction has been a significant challenge for GaN based high-power devices to achieve levels approaching their theoretical limits of performance ¹⁵. Despite the achievements in realization of good quality p-type GaN, the activation efficiency of Mg atoms is still as low as few percent. The p-type doping of GaN is typically performed during growth, while the reports on other doping techniques common in semiconductor processing such as ion implantation ¹⁶.

There have been several advancements in GaN doping processes. This subject was reported and investigated by Liu et al. ¹⁷. Their work systematically studied the structural, magnetic, and defect properties of 12 kinds of dopants in the two-dimensional hexagonal gallium nitride (2D h-GaN) system ¹⁷. The results showed that the most stable charge states (MSCSs) for n-type systems are 0 and 1+, and all the n-type substitutes acted as shallow donors. The MSCSs of the p-type systems are 1+, 0 and 1+, and the acceptor ionization energy was distributed higher than the valence band maximum (VBM) from +1.25 to 2.85 eV, acting as deep acceptors, which would capture electrons (holes) in n-(p-type) 2D h-GaN and act the carrier conductivity. Thus, it was difficult to achieve p-type doping through a single defect in 2D h-GaN, and complex defects were necessary to achieve p-type doping experimentally ¹⁷. For n-type 2D h-GaN system, nearest-neighbor bond length (NNBL in A) around the defect, Ga-N bond length (HBL in A) in host GaN, most stable charge state (MMCS) and magnetic moment (MM in B) are shown in Table 1 ¹⁷.

3. GaN Nanowires Sensors Based Devices

GaN Nanowires Sensors has potential applications in quantum communication, quantum cryptography and other single-photon sources ¹⁸. In the present-day world, all types of biological and chemical process involved in agriculture, industrial-drugs, semiconductors, textiles, food, consumer goods, and healthcare depend on the right pH levels ¹⁹. Numerous efforts have been made in the past towards device development of various pH sensors ²⁰. Supervision and control of the pH level is important for preventing unwanted chemical reactions and for optimizing desired reactions ²⁰. They include micro-cantilevers, ion sensitive field effect transistor (ISFET) technology, thin-film technology (TFT), glass membrane pH electrodes, fiber optic pH sensors, and potentiometric pH sensors ^{21, 22}. Lately, the group III-V semiconductors based high electron mobility transistors (HEMTs) are being explored as suitable candidates for pH sensing. Properties of GaN material devices such as the wide band gap, compatibility with high-temperature environment, strong chemical stability and good biological suitability make them interesting sensor materials ²⁰.

Because of biocompatibility and stable chemical properties, AlGaN and InGaN are also important candidates for performance enhancement of pH sensors. These materials also have large sheet carrier concentration, a wide band gap and a higher sensitivity for harsh environments like high temperature and acidic or alkaline solutions ²⁰. The quaternary compound InAlGaN was studied as a new barrier layer in the HEMTs to overcome the problems of lattice mismatch and immiscibility in the alloys ^{23, 24}. A recent study reported by Upadhyay et al. ^{23, 24}. aimed to provide an insight into the performance of the quaternary InAlGaN heterostructures, specifically as a pH sensor ²⁰. In their work, they explored the physical factors that control the performance and sensitivity of the InAlGaN-based sensors depending on their structural parameters ²⁰. They also compared various design parameters, changes in the threshold voltage, transconductance, and sensitivity of the InAlGaN/GaN and AlGaN/GaN materials systems ²⁰. In addition, they found an optimized In and Al mole fraction composition for the best pH specific sensitivity ²⁰. The results of their model at 0% In and 23% Al mole fractions, matched closely with the experimental data reported by Kokawa et al. ²⁰ for AlGaN/GaN HEMT as shown in Figure 1. With the higher mole fraction of In, the threshold voltage of the devices shifted more towards the negative value ²⁰. They also noticed that there is a change in the threshold voltage when the gate area of the device is exposed to air and when the electrolyte solution with certain pH value is filled in the gate area. This change may be attributed to the varying surface potential of the electrolyte solution for different pH levels ²⁰. Figure 2 shows the threshold voltage of the device with varying pH level of the electrolyte solution ²⁰.

The result of that work proposed an In and Al composition dependent unified analytical model for pH sensing applications of InAlGaN/GaN HEMTs ²⁰. Figure 3 shows the AlGaN/GaN Hetrostructure sensor (a), and (b) shows the section diagram of the AlGaN/GaN pressure sensor (3). The impact of the change in mole fraction of Al, In and electrolyte pH level on the threshold voltage and the drain current of various device structures has been examined ²⁰. They verified that by utilizing suitable In and Al compositions, high concentration of the 2DEG can be obtained in the InAlGaN/GaN heterostructures even with a thin barrier layer. The InAlGaN/GaN devices have slightly higher negative threshold voltage than the conventional AlGaN/GaN HEMTs due to its high 2DEG concentration. The sensitivity of an HEMT based pH sensor is dependent on its transconductance. The maximum transconductance of the AlGaN/GaN devices was found to be much lower than that of the InAlGaN/GaN HEMTs ²⁰. The thinner barrier layer of the InAlGaN/GaN HEMTs enhance the gate control capability and the transconductance of the device. Increased maximum transconductance in quaternary InAlGaN/GaN leads to enhanced sensitivity to the pH levels of the electrolyte. Therefore, they concluded that exploiting the InAlGaN devices for pH measurement applications can result in better performance ²⁰. The analytical results showed good agreement with the experimental results available in the literature. The average measurement was found to be miniscule 0.0070 ²⁰. They expected their model to be a useful tool to predict the behaviour of HEMTs in different acidic environments and to have applications in device optimization and sensor calibration purposes for future chemical and biochemical sensors ²⁰. The only shortcoming of that model is to recognize the acceptable approximate values of fitting parameters used ²⁰.

In recent years, considerable attention has been turned to the nanostructures as chemical sensors due to their simple and low-cost operation, fast response, easy production, and high surface-to-volume ratio ²⁵. The work of Density Functional Theory (DFT) calculation in chemical sensing was investigated and reported by Elesawya et al. ²⁵. In that work, they employed the DFT calculations to investigate the interaction between SA (antibiotic, sulfonamide) and Au-decorated gallium nitride nanotube (GaNNT) as pristine nanotube ²⁵. Different methods have been previously employed for the detection of sulfonamide (SA), namely static SIMS, gas chromatography, HPLC, and ion-exchange chromatography ^{26, 27, 28}. That work resulted in enhanced sensitivity in comparison with simple Schottky diodes which are fabricated on other layers of group III-nitrides ²⁵. The pristine GaNNT could not sense SA, but after the Au atom was decorated, the sensitivity of the GaNNT to SA increased significantly. Also, after the Au atom was decorated, E_ad of SA decreased from -6.0 to -22.9 kcal/mol. The sensing response (SR) of the Au at GaNNT was 98.1, which was considerable. The E_ad of SA was reduced by the water solvent to -19.1 kcal/mol ²⁵. In addition, SA was desorbed from the surface of the decorated nanotube with the recovery time of 4.9 s at ambient temperature ²⁵. Therefore, we can conclude that the Au at GaNNT can be utilized in manufacturing promising sensors to detect SA ²⁵. Figure 4 illustrates the partial DOS of the most stable Sulfonamide/Au-decorated GaNNT complex. In that figure we can see the energy vs. DOS variations of both HOMO and LUMO curves.

The application of GaN in using selective annealing to produce Schottky-Type Sensor was reported by Park et al. ²⁹. Figure 5 presents the fabrication process of the Pt/GaOx/GaN-based Schottky diode sensor ^{3, 20, 32}. That work showed how using selective annealing improved the performance of the sensor and improved its dark current density from 1.3 X 10^-7 A/cm² to 8.5 X 10^-10 A/cm^{2 29}. The results of a transmission electron microscopy analysis demonstrated that the annealing process caused interdiffusion between the metal layers; the contact behavior between Ti/Al/Ni/Au and AlGaN changed from rectifying to ohmic behavior ²⁹. Nitride-based semiconductors have been broadly used in important applications such as high-power/high-speed electron devices, visible/UV laser diodes, LEDs, and UV sensors ^{30, 31, 32}.

Recent studies have intensely explored device structures and experimental techniques to improve the performance of UV sensors ^{29, 33, 34}. Selective thermal annealing exploits Joule heating via a current that is generated following the local breakdown of the insulator in a metal-insulator-metal structure. In that work, they investigated the effects of selective annealing on asymmetric MSM ALGaN/GaN UV sensors that were epitaxially grown on a sapphire substrate and have all compositions of 24% ²⁹. Figure 6 shows a schematic of the asymmetric MSM AlGaN UV sensor with 24% A ²⁹. The dummy pad is showed on the top of the device ²⁹. The dummy pad also shown from a magnified image in Figure 7.

They used a Ti/Al/Ni/Au metal scheme in the sensors that minimized the degradation of device performance and exhibited reasonable ohmic behavior after selective annealing ²⁹. The electrical and UV optical characteristics were analyzed both before and after selective annealing to enable a comparison ²⁹.

As shown in Figure 8 (a,b) under forward bias, the selective annealing process changes the rectifying contact of Ti/Al/Ni/Au electrode to near ohmic contact, which is suspected originated from the formation of nitrogen vacancies ²⁹. During annealing, N easily reacts with Ti to form TiN, and the remaining nitrogen vacancies serve as donors on the AlGaN substrate and promote high electron concentration. That work showed the changes the contact behaviour by reducing the Schottky barrier in the Ti/Al/Ni/Au electrode ²⁹. Thus, the application of a reverse bias at the Ni/Au electrode would cause electrons captured by the interfacial traps to be emitted by trap-assisted tunnelling, thereby contributing to the leakage current. Selective annealing is thought to passivate and reduce interfacial traps, thus resulting in a lower dark current density ²⁹.

The conclusion of that work was that under a forward bias of 2.0 V, the selective annealing substantially increased the dark current density from 7.6 X 10^-6 A/cm² to 0.8 A/cm², which was attributed to nitrogen vacancies generated by the selective annealing that altered the contact behaviour of the Ti/Al/Ni/Au electrode. Under reverse bias, the dark current density at a bias of -2.0 V. was 8.5 X 10^-10 A/cm² and the UVRR at a bias of -7.0 V was 672 ²⁹. The results of the XPS analysis of the sensors showed that annealing reduced the peak intensity of the O 1s binding energy associated with Ga oxide on the AlGaN surface from around 846 to 598 counts/s ²⁹. These results demonstrated a remarkable performance improvement because of the selective annealing, which probably arose from surface passivation and a reduction in the number of traps at the metal/AlGaN interface ²⁹. These results suggested that selective annealing by dielectric breakdown is a facile technique involving localized heating that does not require additional fabrication processing steps or equipment and shows it is useful for improving the performance of sensor devices ²⁹. Recently, a selective annealing method that employs dielectric breakdown changed the contact behavior of a nitride-based UV sensor, but the structural and electrical effects of the selective annealing process are not well understood ³⁵. Figure 9 shows an image of the annealed ZnO on sapphire substrate ^{3, 35}.

Piezoelectric multilayers are widely used in high- performance sensors, actuators, and optoelectronic devices ³⁶. Piezoelectric multilayer actuators are being used in various applications because of their low drive voltage, high energy density, quick response, and long lifetime ³⁶. The application of GaN in analytical solutions of electro elastic fields in piezoelectric thin-film multilayer was reported and investigated by Mishra et al. ³⁶. In that work they studied the applications to piezoelectric sensors and actuators ³⁶. The inherent piezoelectric properties of these devices lead to the coupled electro elastic fields generated in each layer ³⁶. In electronic applications such as GaN-based high electron mobility transistors (HEMT) and LEDs, the multiquantum wells (MQWs) can have different lattice parameters and thermal expansion coefficients that give rise to lattice misfit and thermal strains ³⁶. Since the strains and electric fields in the layers can influence the electromechanical or optoelectronic device performance, the accurate quantification of their magnitudes is of great importance ³⁶. They can also be important in quantifying the formation of defects and predicting their densities ³⁶. Therefore, it will be useful to derive analytical expressions based on the constitutive relations that can readily quantify the electro elastic fields in the piezoelectric multilayer ³⁶. Figure 10 (a,b) represents a schematic representation of a multilayer piezoelectric actuator with opposite poling, out-of-plane normal strain (εzz) for Z-stack multilayer actuator ³⁶.

The analytical prediction methodology developed herein can be useful in electromechanical and optoelectronic device design, especially when piezoelectrically generated electro elastic fields have to be taken into account ³⁶. In that work, they derived the analytical expressions for electro elastic fields in piezoelectric multilayers subjected to external force, moment, and electric potential along with the lattice and thermal mismatches between the reference substrate and the deposited film layers ³⁶. Results for the purely elastic isotropic multilayer can be deduced by letting the elastic constants be isotropic and letting the piezoelectric terms vanish ³⁶. Different types of piezoelectric sensors and actuators were also modelled using the commercial finite element analysis code ABAQUS 6.14 to validate the analytical results. Excellent agreements between the analytical and FEM results were observed for all applications considered in their work ³⁶. Their work can be useful in designing electromechanical devices by providing quick and accurate electro elastic field results for piezoelectric multilayers ³⁶.

The resistance strain gauge was invented by Simmons and Ruge in 1938 ³⁸, and it has been nearly 80 years since resistance strain force sensors began to be produced in 1942. Force sensors based on resistance strain gauges have been widely used in the stress tests of various components and structures in the aerospace, transportation and automobile industries, civil engineering, and even the medical field ³⁷. For example, in civil bridge structures, the great amount of strain acting on the bridge structure over a long time will fatigue the structure and yield a fracture ³⁷. A strain sensor based on a resistance strain gauge can measure not only the strain on the surface of the structure but also the strain inside the structure ³⁷. When the strain gauge is embedded in poured concrete structures, the strain state inside these structures can be monitored in real time through external measuring lines ³⁹. The study of Development and Application of Resistance Strain Force Sensors was presented by Zhao et al. ³⁹. In that work, they reviewed the recent progress of the micro-nano structure of resistance strain-sensitive grids and the strain transfer characteristics of the strain-type force sensor. Further, the technical developments and key problems of resistance strain force sensors for their future application demands were explored ³⁷.

Silver nanoparticles and carbon nanotubes are in development for compressible flexible piezoresistive sensors ⁴⁰. To overcome the hinders on the progress of nanodevice fabrication, significant efforts have been expended on the production of CNTs macrostructures ⁴¹. Inkjet printing techniques have provided outstanding results for flexible resistance sensors in terms of their resistance reproducibility ⁴².

Interesting, the coupling effects among the physical effects, such as coupling piezo resistivity with piezoelectricity, have emerged to be a promising approach to boot the sensitivity effect ³⁷. A giant piezoresistive effect resulting from the optoelectronic coupling in a cubic silicon carbide/silicon heterojunction gives rise to a gauge factor of 58,000, which is more than 2000 times greater than that of cubic silicon carbide ⁴³. By sufficiently finely tuned nanowires in uniform phase-change materials thin films, its significant piezoresistive effect gives rise to a giant gauge factor of 338 used in integrated flexible tactile sensors ⁴⁴. The strain transfer and creep characteristics of strain sensors are the key factors that determine the sensitivity coefficient, mechanical lag, and long-term stability of strain sensors ³⁷. Future work should be carried out in such fields as the ingredient technologies for sensitive grid materials, forging manufacturing technology, heat treatment and mechanical stability technology, and surface treatment technology for improving the characteristics of sensitive grid materials ³⁷.

The study of Electrodeposition of Pd-Pt Nanocomposites on Porous GaN for Electrochemical Nitrite Sensing was reported and investigated by Rui et al. ⁴⁵. In that work, they exhibited a novel Pd-Pt nanocomposite-modified PGaN electrode through a simple two-step electrochemical deposition route for nitrite sensing ⁴⁵. Due to their excellent electrocatalytic activities, noble metal nanomaterials have been applied to composite electrochemical sensors ^{46, 47}. For the high-porosity structure that uses PGaN as a supporting electrode and for the effective electronic transmission of Pd-Pt nanocomposites, the Pd-Pt/PGaN nitrite sensor presented many features, such as a wide linear range, high sensitivity, good selectivity and stability ⁴⁵. Figure 11 presents an SEM image (a) and aperture distribution histogram (b) of PGaN electrode ⁴⁴. Furthermore, it can detect the nitrite in tap and lake water, respectively. The simply assembled Pd- Pt/PGaN sensor provides a fast and effective method for monitoring nitrite in a realistic environment ⁴⁵.

The application of GaN as a pH Sensor was reviewed and reported by Khalifa et al. ⁴⁸. In their work, they presented the GaN/Al₂O₃-based EGFET pH sensor. Figure 13 showed the IDS-VREF characteristics in the linear region at VDS = 0.3 V ⁵⁰. The study concluded a positive and negative linear correlation with superior linearity 0.98 between the reference voltage (VREF) and the drain-source current (IDS), respectively, versus pH value ⁴⁸. The sensing system used a high-performance cavity according to Ahmed et al. ⁴⁹. The voltage sensitivity was 24.72 mV/pH, while the current sensitivity obtained 0.39 (μA)1/2/pH ⁴⁸. The main important factor as an indicator for stability and reversibility to field effect transistors in applications of ion detecting is the hysteresis phenomenon ⁴⁸. The hysteresis depths at pH 7 in cycle 7-4-7-10-7 were 14.98 mV and 30.91 mV for acidic and basic sides, respectively ⁴⁸. The results showed stable detecting executions which qualify the GaN/Al₂O₃-based EGFET pH sensor to be used for commercial devices. Figure 13 shows the I_DS-V_REF characteristics in the linear region at V_DS = 0.3 V ⁵⁰.

The equation that describes behavior IDS versus VRef in the linear region can be expressed as in equation 1.

(1)

In addition, size controlling of sensing area, long sensing of pH range, good repetition accuracy, and the features of GaN such as not toxic material give this sensor a good level of practicability ⁴⁵.

A review for GaN gas sensing properties was reported and studied by Ashfaque et al. ⁵¹. In their work, they reviewed and categorized the progress in GaN nanostructures-based sensors for detection of gas/chemical species such as hydrogen (H2), 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) ⁵¹. That work showed that the Internet of Things (IoT) applications required ultra-low power, mini-sized chemical sensors, which are easily integrated into electronic circuits for remote air quality monitoring in automated systems ^{51, 52}. Figure 12 shows a representation of the sensor in the IoT network in general view. Although GaN nanostructures have been very suitable for hydrogen and alcohol sensing, detection of various oxidizing gases was also demonstrated ⁵¹. Also, the device performance was degraded very little when exposed to siloxane for a one-month period ⁵¹.

Nanostructures are suitable candidates for this type of sensing application. The standard sensing performance parameters like limit of detection, response/recovery time and operating temperature for different types of sensors and structures were summarized comprehensively for the comparative study ⁵¹. The proposed metric, product of response time and limit of detection, has been calculated for each sensor to measure and compare the overall sensing performance among reported GaN nanostructures-based devices so far ⁵¹. Based on the analysis of sensing characteristics and the proposed metric, it was found that InGaN/GaN NW sensor showed superior overall sensing performance for H₂ gas sensing ⁵¹. Also, GaN/TiO₂-Pt) and GaN/TiO₂ NWNC sensors were highly suitable for ethanol and TNT sensing, respectively ⁵¹. Moreover, metal-oxide coated GaN NWs exhibit reliable sensing performance toward various oxidizing gases including NO₂ and SO₂ ⁵¹. Theoretical studies on molecular models of gas molecules and GaN have been reviewed ⁵¹. Artificial neural networks (ANNs) have been highly efficient to capture the non-linear response pattern of gas mixtures ⁵¹. Furthermore, a brief analysis of the implementation of machine learning on GaN nanostructured sensors and sensor array has been presented ⁵¹. In addition, gas sensing mechanisms of the GaN sensors have been discussed. This overview on the GaN nanostructures-based gas sensors is helpful for the researchers to gain a quick understanding of the status of GaN nanostructure- based sensors ⁵¹. Figure 14 shows a schematic of repeatability of VHD gas sensor exposed to 150 ppb of NO₂ of GaN sensor ^{3, 51}.

Having a large surface-to-volume ratio, nanostructures such as nanowires, nanorods, nanotubes, nanoparticles and nanobelts favor adsorption of gas molecules on the sensor and thus increase the sensitivity of the device ^{51, 53}. Although commercially available metal-oxide nanostructure- based gas sensors show high sensitivity and low detection limits ^{51, 52, 53, 54}, poor analyte selectivity, high operating temperature, and unstable performance in harsh environments ⁵⁴. Recently, porous GaN nanorods were prepared by a hydrothermal method by Zhang et al. ⁵⁵. Gas-sensing measurements indicated that the porous sensor exhibits high sensitivity and strong selectivity to ethanol, and good stability at high temperature (360 C). They presented a new route for the synthesis of GaN submicron rods ⁵⁶. P-i-n GaN nanorods (NRs), comprising of InGaN/GaN multi-quantum wells, have been reported recently for NO gas sensing ⁵⁷. In another study, GaN nanowires were attached on pencil graphite electrodes using a hydrothermal method for NO detection ⁵⁸. The ppb level of NO₂ was demonstrated by titania (TiO₂) nanoclusters-functionlized GaN submicron wire fabricated by a top-down approach ⁵⁹. Previously, Shi et al. ⁶⁰ fabricated hybrid gas sensors based on TiO₂-decorated GaN nanowires for NO₂ detection. In another work, GaN nanowires were developed on Si substrates using stepper lithography assisted dry-etching in a top-down fabrication approach ⁶¹. The work reported by Thomson et al. showed fabricated GaN submicron wire-based chip-scale, low-power and nanoengineered chemiresistive gas-sensing architecture for CO₂ detection ⁶². Recently, a gas sensor array was reported comprising of GaN nanowires functionalized with metal incorporated TiO₂ and ZnO ⁶³. In another study, various artificial neural network (ANN) algorithms were trained and tested for the identification and quantification of gas mixtures based on GaN nanowires ⁶⁴.

4. Conclusion

This paper presented and reviewed the recent advances of GaN fabrication and applications. Fabrication and growth of GaN devices was discussed in addition to the recent technologies of doping processes. The applications of GaN was presented and emphasized in this work. GaN sensors and detectors were discussed in detail with recent discoveries and investigations. Despite this progress, still some aspects of GaN devices and structures to be developed to meet further commercial and technological interests. The planar GaN based gas sensors have high restrictions to the detection of low gas concentrations. Moreover, GaN based sensors sensitivity, speed (response and recovery rates), selectivity, stability, reproducibility, durability, detection limit, and power consumption need more investigation and development. There is still much to be discovered in terms of the mechanism of GaN based sensor devices, specially for hydrogen gas sensors. Hence, the development of the high- performance hydrogen gas sensors to continuously monitor the leakage of hydrogen and to accurately detect hydrogen concentration is an important and crucial issue in terms of environmental safety. Finally, although several GaN based sensors devices have been used and commercialized, still some drawbacks and issues to be resolved with intense investigation and research. These issues include the p-type doping, the lack of a credible p-type doping hampers widespread optical emitters in GaN.

References