Ga-polar GaN Research Articles

Control polarity of gallium nitride on Si (1 1 1) using atomic layer annealing and thermal atomic layer deposition

Ga-polar GaN thin films and N-polar GaN thin films were fabricated by using thermal ALD and plasma assisted ALA processes, respectively. The polarity of GaN wurtzite thin films was determined by cross-sectional STEM analysis of as-deposited films and top-view SEM images after KOH wet-etching. The crystallinity and grain size of thermal ALD GaN were drastically increased with the GaN ALA buffer layer. The ALA Ar plasma pulse in each ALD cycle decreased the oxygen and carbon impurity content and improved the crystallinity of GaN thin films by removing organic ligands. High purity GaN thin films were successfully deposited on oxide-free Si (111) substrate by using the ALA process with 100 ms of TDMAGa with push gas, 3 × 800 ms of N2H4, and 15 s of 25 W ALA plasma pulse with −25 V stage DC bias at 275 °C. The lowest FWHM value (0.59°), smooth surface morphology (RMS=1.0 nm), and 10–20 nm grain size of columnar GaN thin films were obtained by tuning process parameters.

Comparative Study on Schottky Contact Behaviors between Ga- and N-Polar GaN with SiNx Interlayer

We conducted a comparative study on the characterization of Ga-polar and N-polar GaN metal–insulator–semiconductor (MIS) Schottky contact with a SiNx gate dielectric. The correlation between the surface morphology and the current–voltage (I–V) characteristics of the Ga- and N-polar GaN Schottky contact with and without SiNx was established. The insertion of SiNx helps in reducing the reverse leakage current for both structures, even though the leakage is still higher for N-polar GaN, consistent with the Schottky barrier height calculated using X-ray photoelectron spectroscopy. To optimize the electric property of the N-polar device, various substrate misorientation angles were adopted. Among the different misorientation angles of the sapphire substrate, the GaN MIS Schottky barrier diode grown on 1° sapphire shows the lowest reverse leakage current, the smoothest surface morphology, and the best crystalline quality compared to N-polar GaN grown on 0.2° and 2° sapphire substrates. Furthermore, the mechanism of the reverse leakage current of the MIS-type N-polar GaN Schottky contact was investigated by temperature-dependent I–V characterization. FP emissions are thought to be the dominant reverse conduction mechanism for the N-polar GaN MIS diode. This work provides a promising approach towards the optimization of N-polar electronic devices with low levels of leakage and a favorable ideality factor.

Effects of UV/O3 and O2 plasma surface treatments on the band-bending of ultrathin ALD-Al2O3 coated Ga-polar GaN

The recently demonstrated semiconductor grafting approach allows one to create an abrupt, low interface-trap-density heterojunction between a high-quality p-type single-crystalline semiconductor (non-nitrides) with n-type GaN. However, due to the surface band-bending from GaN polarization, an energy barrier exists at the grafted heterojunction, which can impact the vertical charge carrier transport. Reducing the energy barrier height is essential for some advanced device development. In this work, we employed UV/O3 and O2 plasma to treat a Ga-polar GaN surface with/without an ultrathin (∼2 nm) ALD-Al2O3 coating and studied the effects of the treatments on surface band-bending. Through XPS measurements, it was found that the treatments can suppress the upward band-bending of the Ga-polar GaN by 0.11–0.39 eV. The XPS results also showed that UV/O3 treatment is an effective surface cleaning method with little surface modification, while O2 plasma causes a strong oxidation process that occurs inside the top layer GaN.

Surface Oxidation of GaN(0001) Simulated by Charge‐Transfer‐Type Molecular Dynamics

In this study, the oxidation of Ga–polar GaN(0001) surface simulated by using originally developed charge‐transfer‐type interatomic potential is reported on. The adjusted potential parameters reproduce the cohesive energies in the range of 0.3 eV atom−1 and atomic forces with correlation coefficient as high as 0.9, compared to the results of first‐principles calculations for more than 9000 structures associated with oxidation of GaN. The oxidation simulations reveal the formation of a periodic gallium oxide (GaOx) layer grown on GaN(0001) with O atoms replacing N atoms. The atomic distance between Ga–Ga in the GaOx layer along GaN[0001] direction is 3.05 Å, which is longer than wurtzite GaN (2.63 Å) and is quantitatively in agreement with the recent photoelectron holography measurement. The distances of the Ga atoms projected onto the GaN(11–20) plane are determined to be 3.19 Å for the GaOx layer and 2.79 Å for the interfacial GaN. These distances also align quantitatively with the scanning transmission electron microscopy imaging of the native oxide on GaN(0001). Further oxidation simulation in a larger model of 2304 atoms suggests the formation of the layered structure even in the subsequent layers away from the interface of GaN and gallium oxide.

Ga-polar GaN Camel diode enabled by a low-cost Mg-diffusion process

In this letter, we show that low-cost physical vapor deposition of Mg followed by a thermal diffusion annealing process increases the effective barrier height at the metal/Ga-polar GaN Schottky interface. Thus, for the first time, GaN Camel diodes with improved barrier height and turn-on voltage were realized compared to regular GaN Schottky barrier diodes. Temperature-dependent current–voltage characteristics indicated a near-homogeneous and near-ideal behavior of the GaN Camel diode. The analysis performed in this work is thought to be promising for improving the performance of future GaN-based unipolar diodes.

Exceptional Thermochemical Stability of Graphene on N-Polar GaN for Remote Epitaxy.

In this study, we investigate the thermochemical stability of graphene on the GaN substrate for metal-organic chemical vapor deposition (MOCVD)-based remote epitaxy. Despite excellent physical properties of GaN, making it a compelling choice for high-performance electronic and light-emitting device applications, the challenge of thermochemical decomposition of graphene on a GaN substrate at high temperatures has obstructed the achievement of remote homoepitaxy via MOCVD. Our research uncovers an unexpected stability of graphene on N-polar GaN, thereby enabling the MOCVD-based remote homoepitaxy of N-polar GaN. Our comparative analysis of N- and Ga-polar GaN substrates reveals markedly different outcomes: while a graphene/N-polar GaN substrate produces releasable microcrystals (μCs), a graphene/Ga-polar GaN substrate yields nonreleasable thin films. We attribute this discrepancy to the polarity-dependent thermochemical stability of graphene on the GaN substrate and its subsequent reaction with hydrogen. Evidence obtained from Raman spectroscopy, electron microscopic analyses, and overlayer delamination points to a pronounced thermochemical stability of graphene on N-polar GaN during MOCVD-based remote homoepitaxy. Molecular dynamics simulations, corroborated by experimental data, further substantiate that the thermochemical stability of graphene is reliant on the polarity of GaN, due to different reactions with hydrogen at high temperatures. Based on the N-polar remote homoepitaxy of μCs, the practical application of our findings was demonstrated in fabrication of flexible light-emitting diodes composed of p-n junction μCs with InGaN heterostructures.

Growth of N-Polar (0001) GaN in Metal–Organic Vapour Phase Epitaxy on Sapphire

We have systematically studied the growth of N-polar GaN on sapphire in metal–organic vapor phase epitaxy (MOVPE) on different misoriented (0001) sapphire substrates. The key parameter was the NH3 flow, which affects the roughness, growth rate, crystal quality, and impurities. Most parameters show a trend reversal at a V/III ratio around 500 and show either a maximum, such as the growth rate, the sizes of hexagonal hillocks on low misorientations, the yellow luminescence and the mobility, or show a minimum such as the FWHM in X-ray diffraction, the carrier concentration, the surface roughness of large misorientations, or the blue (430 nm) luminescence. This suggests that around a V/III ratio of 500, the surface changes from a Ga-terminated Ga-adlayer surface to a N-terminated 3N-H(2×2) surface. Using extremely low V/III ratios, a smooth N-polar GaN was obtained even on the standard 0.2° misorientation. However, good crystalline quality, low oxygen impurities and smooth surfaces together seem too challenging with low misorientation. The strain-dependent band edge shifted by 14 eV for strain along [0001], which is close to the values reported by Ga-polar GaN.

Surface morphology evolution of N-polar GaN on SiC for HEMT heterostructures grown by plasma-assisted molecular beam epitaxy

The surface morphology evolution of N-polar GaN with growth time was investigated and compared with Ga-polar GaN. N-polar GaN directly grown on SiC substrates was found to have slower 3D-to-2D growth transformation and less coalescence than the Ga-polar counterpart, resulting in rougher surface morphology, whereas the AlN nucleation layer accelerated 3D-to-2D transformation, resulting in smoother surface morphology. N-polar GaN was found to have mound-type surface morphology with clustered atomic steps, unlike the regular screw-type dislocation-mediated step-flow growth observed for Ga-polar GaN. This was explained by the lower diffusion of adatoms on the N-polar surface due to its higher surface energy and higher Ehrlich–Schwoebel barrier. In addition, the increased III/V ratio in N-polar GaN growth was found to reduce the surface roughness from 2.4 nm to 1 nm. Without Si doping, the N-polar GaN high electron mobility transistor (HEMT) heterostructures grown under optimized conditions with smoother surface morphologies exhibited a sheet carrier density of 0.91 × 1013 cm−2 and a mobility of 1220 cm2 (V s)−1. With Si δ-doping, the sheet carrier density was increased to 1.28 × 1013 cm−2 while the mobility was reduced to 1030 cm2 (V s)−1. These results are comparable to the state-of-the-art data of plasma-assisted molecular beam epitaxy-grown N-polar GaN HEMT heterostructures on SiC substrates.

Interfacial band parameters of ultrathin ALD-ZrO2 on Ga-polar GaN through XPS measurements

Recent demonstrations of grafted p-n junctions combining n-type GaN with p-type semiconductors have shown great potential in achieving lattice-mismatch epitaxy-like heterostructures. Ultrathin dielectrics deposited by atomic layer deposition (ALD) serve both as a double-sided surface passivation layer and a quantum tunneling layer. On the other hand, with excellent thermal, mechanical, and electrical properties, ZrO2 serves as a high-k gate dielectric material in multiple applications, which is also of potential interest to applications in grafted GaN-based heterostructures. In this sense, understanding the interfacial band parameters of ultrathin ALD-ZrO2 is of great importance. In this work, the band-bending of Ga-polar GaN with ultrathin ALD-ZrO2 was studied by x-ray photoelectron spectroscopy (XPS). This study demonstrated that ZrO2 can effectively suppress upward band-bending from 0.88 to 0.48 eV at five deposition cycles. The bandgap values of ALD-ZrO2 at different thicknesses were also carefully studied.

High current implementation of Cu/P-type GaN triboelectric nanogenerator

Traditional surface engineering, as a means of manufacturing triboelectric nanogenerator (TENG), is complex and expensive. The yield of traditional polymer process is low, which leads to the high cost and low stability of traditional TENGs and greatly limits their practical applications. Moreover, it is worth noting that with the miniaturization and integration of electronic devices, generators need to provide higher current in parallel circuits. In this study, we report the performance of the enhanced Cu/P-type GaN TENG contacts in centimeter scale. Considering the high surface mechanical strength and surface structure characteristics of GaN wafers, we propose using molten KOH to etch the Ga polar GaN surface to form more interface electrons and dangling bonds without destroying the surface structure. Our experimental results show that the generator performance has been drastically improved (the short circuit current increases from 9 to 80 μA, and the open circuit voltage increases from 8 to 29 V). The maximum load electric power density of ∼0.28 W/m2 was obtained. We also compared the open circuit current density with the reported different type TENGs based on Schottky contact at the centimeter-level. The Cu/P-type GaN TENGs achieved in this work exhibit excellent open circuit current density of ∼36 μA/cm2. Thus, we provide insight into surface engineering for future generation TENG devices.

Effect of H2 addition on growth rate and surface morphology of GaN(0001) grown by halide-vapor-phase epitaxy using GaCl3

We investigated the effect of H2 addition in halide-vapor-phase epitaxy of GaN on Ga-polar GaN(0001) using an external GaCl3 supply method. To overcome the problem of the very low growth rate on GaN(0001) using GaCl3, we intentionally added H2 to convert GaCl3 to GaCl in the reactor. Using this approach, we successfully increased the growth rate, and also improved the surface morphology of the grown layer.

Electrical properties of N-polar Si-doped GaN prepared by pulsed sputtering

We have demonstrated the homoepitaxial growth of N-polar GaN and its Si doping by pulsed sputtering deposition (PSD). Enhanced surface migration by a pulsed supply of precursors enabled the step-flow growth on N-polar GaN bulk substrates even with small miscut angles. The relationship between electron concentration and mobility in N-polar GaN follows the Caughey–Thomas relationship for Ga-polar GaN, which indicates PSD N-polar GaN has a low concentration of scattering centers. N-polar heavily Si-doped GaN film yielded a record-low resistivity of 1.6 × 10−4 Ωcm with an electron concentration of 3.6 × 1020 cm−3 and mobility of 109 cm2V−1s−1, comparable to the best data for Ga-polar GaN. The high electron mobility can be attributed to the reduced concentration of compensating acceptors, which is also consistent with its optical measurements. Moreover, optical measurements show that the Burstein–Moss effect raises the Fermi level by 0.2 eV. These results show that heavily Si-doped N-polar GaN prepared by pulsed sputtering is promising for future applications such as the source/drain of high-performance N-polar GaN HEMTs.

Yellow luminescence and carrier distribution due to polarity-dependent incorporation of carbon impurities in bulk GaN by Na flux

We have grown a high growth rate, and low dislocation density (DD) Ga-polar GaN (DD: 4 × 105/cm2) and N-polar GaN (DD: 8 × 105/cm2) bulk single crystal using Na Flux. Variable temperature PL shows that the yellow luminescence (YL) peak energy of Ga-polar GaN (Ga-GaN) and N-polar GaN (N-GaN) are 2.31–2.30 eV and 2.28–2.26 eV, respectively. It can be found that N-GaN has a more vigorous YL intensity and a lower peak energy of YL than Ga-GaN. The variable-temperature and variable-power PL results suggest that the YL origin may be a donor-acceptor pair defect. Dislocations and defects associated with gallium vacancies that could cause YL behavior were ruled out by dislocation density determination and Raman spectroscopy, respectively. The different concentrations of carbon and oxygen in the Ga-GaN and N-GaN that were associated with the adsorption energy were discussed and proved by SIMS. The Raman results show that the carrier concentrations of Ga-polar and N-polar GaN are comparable, which may be because the higher content of carbon impurities in N-polar GaN can act as a deep acceptor to compensate for its higher content of shallow donor impurity oxygen. Therefore, it is suggested that carbon-related energy level transitions are the primary source of YL in both Ga-polar and N-polar GaN, and the compensating effect of carbon impurities leads to similar carrier concentrations in both polar planes.

Role of low temperature Al(Ga)N interlayers on the polarity and quality control of GaN epitaxy

Polarity control is essential for lateral polarity heterostructures. N-polar GaN films were directly grown on high temperature AlN buffer layer. A thin low temperature AlN interlayer could invert the polarity of subsequent GaN layer to Ga-polarity but a low temperature GaN interlayer with the same condition could not do it rather deteriorate the quality of GaN film. In addition, a relationship between the yellow luminescence band and the surface roughness was discovered. The blue luminescence band was found only on the inverted Ga-polar GaN film and was connected to screw dislocation density.

Electrical properties and energy band alignment of SiO2/GaN metal-oxide-semiconductor structures fabricated on N-polar GaN(0001¯) substrates

The interface properties and energy band alignment of SiO2/GaN metal-oxide-semiconductor (MOS) structures fabricated on N-polar GaN(0001¯) substrates were investigated by electrical measurements and synchrotron-radiation x-ray photoelectron spectroscopy. They were then compared with those of SiO2/GaN MOS structures on Ga-polar GaN(0001). Although the SiO2/GaN(0001¯) structure was found to be more thermally unstable than that on the GaN(0001) substrate, excellent electrical properties were obtained for the SiO2/GaN(0001¯) structure by optimizing conditions for post-deposition annealing. However, the conduction band offset for SiO2/GaN(0001¯) was smaller than that for SiO2/GaN(0001), leading to increased gate leakage current. Therefore, caution is needed when using N-polar GaN(0001¯) substrates for MOS device fabrication.

Selective Area Growth of GaN Nanowire: Partial Pressures and Temperature as the Key Growth Parameters

Selective area growth of Ga-polar GaN nanowires by metal organic chemical vapor deposition provides a path to achieve arrays of uniform nanowires with controllable dimensions. A systematic investigation on the growth parameters of GaN nanowires demonstrates that although a low V/III ratio and low precursor flows are necessary, V/III ratio is not a sufficient parameter to determine the morphology of GaN nanowires. Partial pressures of the constituent gases, on the other hand, can be used as a single parameter, at a particular growth temperature, to explain the resulting morphology. By correlating the partial pressures of precursors and H2 in the carrier gas with the morphology of the nanowires, we can then determine the flow rates for precursors and hence the range of V/III ratios required for achieving the growth of GaN nanowires.

A study on MOCVD growth window for high quality N-polar GaN for vertical device applications

Lateral N-polar GaN devices have outperformed Ga-polar GaN counterparts in mm-wave applications. A vertical N-polar GaN device structure with higher breakdown voltage is expected to further improve the output power density. This paper is aimed at understanding the growth conditions for such a vertical N-polar GaN device stack. A vertical device stack requires high-quality n-GaN layer with low unintentional dopant concentration and high electron mobility for better ON resistance–breakdown voltage performance. A systematic study by varying reactor temperature, pressure and NH3 flow rate is performed in the metal organic chemical vapor deposition growth of N-polar GaN for vertical device structure. Structural quality of the sample is increased with increase growth temperature and higher NH3 flow rate, yet an increase in surface roughness is observed for highest temperature used. The N-polar GaN grown at the optimized conditions exhibited a x-ray diffraction rocking curve full width half maximum of symmetric (0002) and skew-symmetric (20 ) scans of 358″ and 569″ respectively with a low oxygen and carbon concentration of 6 × 1016 cm−3 and 5 × 1016 cm−3, respectively. The optimized sample exhibited a low screw dislocation density of 2.5 × 108 cm−2 on sapphire substrate. A gradual increase in the bulk mobility of electrons with decrease in screw dislocation density in N-polar GaN is also reported.

Direct high-temperature growth of single-crystalline GaN on ScAlMgO4 substrates by metalorganic chemical vapor deposition

In this note, we demonstrate direct high-temperature growth of single-crystalline GaN on c-plane ScAlMgO4 substrates by metalorganic chemical vapor deposition, without using low-temperature buffers. We found that a trimethylaluminium preflow was crucial to suppress island growth and to achieve uniform mirror-like Ga-polar GaN layers. The preflow time was found to have a direct impact on the crystalline quality of GaN. We also show that thin GaN layers directly grown at high temperature can be used as buffers for the growth of lattice-matched In0.17Ga0.83N layers on ScAlMgO4. The presented results demonstrate the potential of direct growth of GaN on ScAlMgO4.

Relationship Between the Rate of Photochemical Metal-Assisted Etching of GaN Layers and Multifractal Parameters of Their Surface Structure

The results of a study of liquid photochemical metal-assisted etching of a series of samples of n-type Ga-polar GaN layers grown by molecular-beam epitaxy with nitrogen plasma activation are presented. Under the chosen conditions of the etching process, it is shown that the etching rate depends mainly on the structural properties of the GaN layers, which manifest themselves in the surface morphology, which can be quantitatively characterized by the multifractal parameters Δq (the degree of ordering and symmetry breaking of the structure under study) and Dq (the Rényi dimension, which depends on the thermodynamic formation conditions). A correlation between the values of the multifractal parameters Δq and Dq of the surface structure and the etching rate of Ga-polar GaN layers is established.

Direct growth of GaN on sapphire substrate via polarity transition from N- to Ga-polar using only hydride vapor phase epitaxy

We established a method for directly growing the GaN layer on a sapphire substrate using only hydride vapor phase epitaxy (HVPE). The important factors that affect the manufacture of high-quality, low-cost, and large-diameter GaN substrates require a growth method that does not utilize either a GaN template or processes other than HVPE. N2 carrier gas and a high temperature are important growth parameters required to achieve the direct growth of the GaN layer on the sapphire substrate. The mechanism of the direct growth of the GaN layer on the sapphire substrate reveals that the N-polar GaN layer grows during the initial growth stage via the nitridation of the sapphire substrate and Ga-polar GaN is subsequently produced via polarity inversion. Furthermore, the dislocation density of the GaN layer directly grown on the sapphire substrate is lower than that of the GaN layer grown on a GaN template using metal-organic vapor phase epitaxy.