Polar GaN Research Articles

Triethylamine sensing characteristics of nonpolar (11-20) and polar (0001) GaN thin films

Intrinsically n-doped a-plane and c-plane GaN thin films were successfully deposited on r-plane and c-plane sapphire substrates, respectively, by metalorganic vapor phase epitaxy (MOVPE). The c-plane GaN sensor shows a response value of 5.23–200 ppm triethylamine (TEA) at 480 °C, much higher than the a-plane one at the same condition. The TEA vapor is detectable down to its threshold value of 1 ppm for the c-plane sensor at 480 °C. Excellent selectivity was obtained for the c-plane sensor, by distinguishing TEA from interfering gases according to a combined signal of response, response time and recovery time. Meanwhile, we investigated pre-adsorption of O2 molecules on the two GaN surface facets using density functional theory (DFT) simulations. The much lower adsorption energy and much more accumulated electrons around the O2 molecules for the c-plane facet explains the above-mentioned better TEA sensing performance of the c-plane sensor as compared to its counterpart.

Structural and piezoelectric properties of ultra-thin ScxAl1−xN films grown on GaN by molecular beam epitaxy

ScxAl1−xN (x = 0.18–0.40) thin films of ∼28 nm thickness grown on metal polar GaN substrates by molecular beam epitaxy are found to exhibit smooth morphology with less than 0.5 nm roughness and predominantly single-phase wurtzite crystal structure throughout the composition range. Measurement of the piezoelectric d33 coefficient shows a 150% increase for lattice-matched Sc0.18Al0.82N relative to pure aluminum nitride, whereas higher Sc contents exhibit lower piezoelectric coefficients. The electromechanical response of the epitaxial films correlates with the crystal quality and the presence of zinc blende inclusions, as observed by high-resolution electron microscopy. It is further found that the polarity of the epitaxial ScxAl1−xN layers is locked to the underlying substrate. The measured electromechanical properties of epitaxial ScxAl1−xN, their relation to the atomic crystal structure and defects, and its crystal polarity provide useful guidance toward the applications of this material.

Absolute surface energies of oxygen-adsorbed GaN surfaces

Absolute surface energies of reconstructed polar, nonpolar, and semi-polar GaN surfaces formed under oxide vapor phase epitaxy (OVPE) growth conditions were investigated via first-principles calculations. The relationships of stable surface orientations and surface structures with the growth conditions of Ga pressure and temperature were examined in terms of absolute surface energies. The stability of the surface orientation was in the order (0001¯), (11¯01), (11¯00), (112¯0), (0001), and (11¯01¯). High O concentrations on OVPE-grown GaN could be explained in relation to the stability of the (11¯01) surface. The stable (11¯01) surface consisted of N vacancies and H and O adatoms. Therefore, it was deduced that its appearance results in increased O impurity incorporation.

Polarization-resolved Er emission in Er doped GaN bulk crystals

Erbium-doped GaN (Er:GaN) quasi-bulk crystals are emerging as a promising novel gain medium for high energy lasers emitting at the retina-safe wavelength window of 1.5 μm. We report the polarization-resolved photoluminescence (PL) emission spectroscopy studies, which revealed that the pumping efficiency with the excitation polarization parallel to the c-axis of GaN (E⇀||c⇀) is significantly higher than that with the excitation polarization perpendicular to the c-axis of GaN (E⇀⊥c⇀). This phenomenon is a direct consequence of the inherent polar wurtzite GaN lattice, giving rise to a net local field, surrounding each Er ion, along the c-axis of GaN. The temperature dependent behaviors of the PL emission spectra were explained in terms of the Boltzmann population distributions among sublevels within the 4I15/2 ground state and the 4I13/2 first excited state of Er3+ in GaN, thereby providing an improved understanding regarding the origin of the dominant emission lines observed near 1.5 μm. The results suggested that the polarization field in GaN can be exploited to enhance the effective Er excitation cross section by manipulating the polarization of the excitation light source.

Development of polarity inversion in a GaN waveguide structure for modal phase matching

In this work, we report on the fabrication of a GaN/AlGaN waveguiding structure dedicated to modal phase matching, where GaN waveguide has planar polarity inversion. First, we optimized the growth conditions for the AlGaN epilayer. Second, on top of the AlGaN epilayer, we fabricated the waveguiding structure starting with the growth of the Ga-polar GaN epilayer followed by atomic layer deposition (ALD) of an Al $$_{2}$$ O $$_{3}$$ layer, then, continuing with the growth of N-polar GaN epilayer. We tested several layer thicknesses, but with 20 nm we managed to inverse the GaN polarity from Ga to N. To confirm the N-polarity, we etched the GaN epilayer surface in aqueous KOH solution. We performed out-of-plane (0002) and in-plane (11–20) X-ray diffraction and rocking curve measurements to estimate the crystalline quality of the AlGaN epilayer, Ga- and N-polar GaN epilayer. Atomic force microscopy measurement lets us evaluate the epilayer surface morphology and roughness. Optical and scanning electron microscopy inspection revealed characteristic hexagonal N-polar GaN epilayer surface. We used high-resolution transmission electron microscopy to investigate the crystallinity and orientation of the Ga- and N-polar GaN epilayer, also the Al $$_{2}$$ O $$_{3}$$ ALD layer, the interface quality of the waveguide structure.

Modeling carbon coverage on polar GaN surfaces during MOVPE

Carbon coverage on polar GaN surfaces during MOVPE were calculated by a theoretical model which incorporates configurational entropy. Generally, surface reconstruction with the lowest energy was explored by an ab initio-based approach. On the other hand, appearance probability of the other surfaces can be discussed by the model. Therefore, coverage of minor elements on the surfaces can be analyzed. In this research, influence of source gas ratio, total pressure, and carrier gas ratio of MOVPE conditions on the carbon coverage on the surfaces was revealed theoretically. Furthermore, orientation dependence of carbon coverage and its incorporation mechanism were discussed.

Graphene‐Assisted Epitaxy of Nitrogen Lattice Polarity GaN Films on Non‐Polar Sapphire Substrates for Green Light Emitting Diodes

AbstractLattice polarity is a key point for hexagonal semiconductors such as GaN. Unfortunately, only Ga‐polarity GaN have been achieved on graphene till now. Here, the epitaxy of high quality nitrogen‐polarity GaN films on transferred graphene on non‐polar sapphire substrates by molecular beam epitaxy is reported. This success is achieved through atomic nitrogen irradiation, where CN bonds are formed in graphene and provide nucleation sites for GaN and leading to N‐polarity GaN epitaxy. The N‐polarity characteristics are confirmed by chemical etching and transmission electron microscopy measurement. Due to the higher growth temperature of InGaN at N‐polarity than that at Ga‐polarity, green light emitting diodes are fabricated on the graphene‐assisted substrate, where a large redshift of emission wavelength is observed. These results open a new avenue for the polarity modulation of III‐nitride films based on 2D materials, and also pave the way for potential application in longer wavelength light emitting devices.

Role of polarity in SiN on Al/GaN and the pathway to stable contacts

Despite being the most widely used dielectric for passivation of GaN-based lateral devices, amorphous silicon-nitride still faces many stability challenges, which arise from its complex bulk electronic and interface properties on the polar (Al)GaN surfaces. In this investigation, SiN has been applied as an ultra-thin interlayer (∼3–5 nm) in vertical contact structures on Ga-polar and N-polar GaN templates to study the metal–insulator–semiconductor- (MIS-) like system and better understand the interaction between the polar surface and its dielectric overlayer. We describe the role of amphoteric ≡Si centers in SiN in passivating and providing the polarization countercharge to Al/GaN of different polarities. The consequent requirements of the concentration profile of the amphoteric defects and the corresponding chemical profile of SiN is discussed. The importance of SiN surface termination and their influence on the interface potential on Al/GaN that determines device performance and reliability is also shown. Finally, a pathway to highly stable and reliable ohmic contacts to n-type Ga-polar GaN without instabilities associated with metal directly alloying with GaN as in the case of traditional contacts is proposed.

Surface morphology of polar, semipolar and nonpolar freestanding GaN after chemical etching

Surface etching methods were investigated for the removal of residues of chemical reagents and contaminants from polar (Ga- and N-plane), semipolar (r-plane) and nonpolar (a- and m-plane) freestanding GaN surfaces. Ga-, N-, r- and m-plane were etched in H3PO4, NaOH and (NH4)2S solutions. A-plane was treated in the solution of 10 wt% KOH in ethylene glycol (EG) at 80 °C. Different surface morphology were obtained and were characterized with atomic force microscope. The typical characteristics for the surface morphologies and interfacial angles in each etched GaN plane may help identify the type of crystal plane conveniently. Ga-polar GaN shows a clear and uniform step structure on surfaces. N-plane presented a step structure with tooth-like terraces. The (202¯1) r-plane, m-plane and a-plane show stripe-like structures, while the (202¯1¯) r-plane is stable and hard to be etched to exhibit step structures. The m-plane facets on a-plane surface caused by the etching method was found and discussed.

Temperature-dependence of Cl2/Ar ICP-RIE of polar, semipolar, and nonpolar GaN and AlN following BCl3/Ar breakthrough plasma

The authors report a comprehensive investigation of temperature-dependence of inductively coupled plasma reactive ion etching (ICP-RIE) of polar (0001), semipolar (11−22), and nonpolar (11−20) GaN and AlN, in the temperature range of 22–205 °C. The main objective is to study the effect of ICP etching near and beyond the boiling point of the volatile etch end-products: GaCl3 (201 °C) for GaN and AlCl3 (180 °C) for AlN. High-temperature ICP-RIE is beneficial in quicker removal of surface oxides and may permit the use of a single-step Cl2/Ar ICP-RIE for etching all orientations of GaN and AlN. However, the best results are still obtained with a combination of BCl3-plasma based surface oxide removal pretreatment and Cl2/Ar ICP-RIE etching, which provides a constant etch rate with a smooth surface morphology irrespective of the etching temperature.

Comparative Study of ZnO Nanostructures Grown on Variously Orientated GaN and AlxGa1−xN: The Role of Polarization, and Surface Pits

Through comparing ZnO directly grown on the substrates of a-plane, c-plane, and (11-22) plane GaN and AlxGa1−xN (0.06 ≤ x ≤ 1), the roles of different factors that may influence growth have been studied. Seeded by surface pits, ZnO nanowire (NW) preferentially grew along the polarized direction on top of the nonpolar GaN (laterally aligned), polar GaN and AlGaN (vertically aligned), and semipolar GaN (obliquely upward aligned). Nanosheets were easily formed when the polarized surface of the AlGaN film was not intact. The kinetic effect of polarization must be considered to explain the high aspect ratio of NWs along the polarized direction. It was found that dislocation affected NW growth through the surface pits, which provided excellent nucleation sites. If the surface pits on GaN could be controlled to distribute uniformly, self-organized ZnO NW array could be controllably and directly grown on GaN. Moreover, surface pits could also seed for nanosheet growth in AlN, since Al(OH)4− would presumably bind to the Zn2+ terminated surface and suppress the kinetic effects of polarization.

Role of Capping Material and GaN Polarity on Mg Ion Implantation Activation

Ion implantation of magnesium for p‐type GaN presents many opportunities; however, activation has proven difficult due to the decomposition of GaN at relevant annealing temperatures. Herein, testing the efficacy of multiple in situ and ex situ caps based on aluminum nitride and silicon nitride for GaN protection during annealing is presented. Photoluminescence shows better activation for in situ metal organic chemical vapor deposition (MOCVD)‐grown aluminum nitride caps compared with ex situ sputtered aluminum nitride and the best performance by ex situ plasma‐enhanced chemical vapor deposition (PECVD) silicon nitride. Furthermore, only samples annealed at the highest temperatures tested show preferential growth of UV luminescence to yellow‐green luminescence reinforcing the need for better capping solutions and higher temperature annealing.

Theoretical study of adatom stability on polar GaN surfaces during MBE and MOVPE

GaN is a key material for realizing an energy saving society owing to its superior properties for power devises. Understanding the physics of the crystal growth can accelerate the development of GaN-based devices. There have been multiple theoretical studies that investigate the surface reconstructions with minimum energy from GaN during growth. However, although the most stable reconstruction is dominant on the surface, other reconstructions could also appear probabilistically due to the entropic effect. Considering the growth rate of GaN is typically limited by the Ga precursor flow rate, the appearance probability of Ga-adsorbed reconstruction should be small. Little is known about the relative stabilities of such minor reconstructions. Here, we propose a new scheme to calculate the appearance probabilities of reconstructions. Our calculation reveals that the N density increases and Ga density decreases on GaN(0 0 0 1) during metalorganic vapor phase epitaxy (MOVPE) with decreasing H2 partial pressure, resulting in a highly N-rich surface. In contrast, for GaN(0 0 0 −1), both N and Ga densities increase with decreasing H2 partial pressure, resulting in a Ga-rich surface. The results well explain the reported growth characteristics of polar GaN.

Evolution of the free energy of the GaN(0001) surface based on first-principles phonon calculations

In this paper, we present systematic improvements to a method that allows us to determine the surface phase diagrams based on ab initio calculations and atomistic thermodynamics. In particular, we include a contribution derived from thermal vibrations of the surface, which is usually omitted in such analysis. On the basis of first-principles phonon calculations, we determine a zero-point vibrational energy and thermal dependencies of vibrational entropy and free energy for atoms and molecules adsorbed on the surface. A series of polar GaN(0001) surfaces are examined. Significant changes are observed in the surface phase diagrams compared to the diagrams obtained by standard density-functional theory calculations without phononic contributions.

Correlation of Structure and EBIC Contrast from Threading Dislocations in AlN/Si Films

Epitaxial growth of polar GaN is frequently made on AlN/Si substrates that contain a large density of threading dislocations induced by the mismatch of their lattice parameters and thermal expansion coefficients. In this paper, the structural and electrical properties of surface and extended defects found in 220 nm {0001} AlN films grown epitaxially on {111} oriented Si substrate by metalorganic chemical vapor deposition are characterized. Three types of surface depressions are recognized by atomic force microscopy (AFM) that correspond to V‐defects and their clusters and individual threading dislocations. Transmission electron microscopy studies prove that all V‐defects are attached to threading screw or mixed‐type threading dislocations. The electrical activity of the defects in the AlN film is assessed by measuring the electron beam induced current (EBIC) across Au/Ni/AlN Schottky barrier. The largest drop of EBIC compared to defect‐free regions is observed at clusters of V‐defects, which have a low density of (2.4 ± 0.4) × 107 cm−2. Only a very weak drop of EBIC is found at isolated threading dislocations, which have much higher density compared to clusters of V‐defects. Considering the defects studied, the electrical properties of AlN films are primarily affected by threading dislocations, despite the relatively weak recombination efficiency of individual dislocations.

A Demonstration of Nitrogen Polar Gallium Nitride Current Aperture Vertical Electron Transistor

We report the first demonstration of Nitrogen polar (N-polar) GaN current aperture vertical electron transistor with a blocking electric field over 2.9 MV/cm. The devices were grown by metalorganic chemical vapor deposition on a c-plane sapphire substrate. The fabrication involved a maskless planar regrowth of a very thin-AlN layer above the current blocking layer, which induced a two-dimensional electron gas (2DEG) in the channel, and also prevented the out-diffusion of activated Mg ions into the GaN channel layer. The alloyed source and drain ohmic contacts on the regrown n+-GaN layer offered the low-contact resistance of 0.18 $\text{m}\Omega \cdot \text {cm}^{2}$ ( $0.22~\Omega \cdot \text {mm}$ ). The device displayed a maximum drain current of 1.68kA/cm2 with a low RON, SP of 2.48 $\text{m}\Omega \cdot \text {cm}^{2}$ . With just about 200 nm drift layer, a three-terminal breakdown voltage of 58 V was achieved. The output characteristics were free of dispersion under pulsed measurements with $80~\mu \text{s}$ and 500 ns pulse widths. The N-polar current aperture vertical electron transistors show fundamentally significant advantages in favor of using Mg2+-implanted GaN as current blocking layers compared with Ga-polar counterparts.

Epitaxy of GaN(0001) and GaN(10$$\bar {1}$$1) Layers on Si(100) Substrate

Two different approaches to epitaxy of 4-μm-thick layers of polar GaN(0001) and semipolar GaN(10 $$\bar {1}$$ 1) on a V-shaped nanostructured Si(100) substrate with nanometer-thick SiC and AlN buffer layers have been experimentally demonstrated. The GaN(0001) layers were synthesized by hydride vapor-phase epitaxy, and GaN(10 $$\bar {1}$$ 1) layers, by metal-organic vapor-phase epitaxy, with the growth completed by hydride vapor-phase epitaxy. It was shown that layers of the polar GaN(0002) have a longitudinal elastic stress of –0.45 GPa and the minimum full width at half-maximum of the X-ray diffraction rocking curve ωθ ~ 45 arcmin, whereas for the semipolar GaN(10 $$\bar {1}$$ 1), these values are –0.29 GPa and ωθ ~ 22 arcmin, respectively. A conclusion is drawn that the combined technology of semipolar gallium nitride on a silicon (100) substrate is promising.

Intersubband Transitions in Nonpolar GaN-based Resonant Phonon Depopulation Multiple-Quantum Wells for Terahertz Emissions

We investigate the polarization effect in intersubband transitions in polar and nonpolar GaN-based multiple-quantum well (MQW) structures for terahertz (THz) emissions by using systematic comparisons and design a nonpolar GaN/Al0.2Ga0.8N two-well-based MQW structure with an emitting photon of 7.27 THz (30.07 meV). Its lower energy separation (92.7 meV) matches the resonant phonon depopulation condition for better population inversion. It shows a lower threshold current density Jth at all temperatures (1.548 kA/cm2 at 90 K) and a higher output power of up to 86.1 mW at 5.8 K and 33.6 mW at 100 K. Our results for the polar GaN MQW are very close to the experimental data in the literature. We find that the Jth of the nonpolar GaN MQW increases more slowly than that of the polar GaN MQW as temperature increases, indicating the nonpolar GaN MQW may be a worth-trying direction for improving the operation temperature. These results can provide meaningful references for the design and fabrication of nonpolar GaN-based THz MQW or quantum cascade structures.

Influence of wet chemical etching on electronic structure and optical response of polar (0001) GaN films

We report the influence of molarity dependent wet chemical KOH etching on the optical response and near surface band structure of highly crystalline polar (0001) GaN films. Morphological evolutions from a highly smooth to a granular like surface in nano-regime was witnessed with increment in etchant (KOH) concentration. X-Ray photoemission spectroscopy analysis divulged a shift towards lower binding energy associated with upward band bending and indicated the elimination of surface states along with fermi level pining near the charge neutrality level. Photoluminescence studies exhibited a strong near band edge emission centred at 3.41 eV and a defect related broadband ranging from 1.8 eV to 3.2 eV. It was observed that chemical etching leads to drastic reduction in the defect band (from 63% to 16%) with increased KOH concentration. Further, in-depth analysis of luminescence associated with defect states, vacancies, donor/acceptor states was performed to develop a better understanding of the observed results. The study demonstrates that chemically etched films pursue superior material properties which could be utilised for the development of GaN based high performance devices requiring higher surface area i.e. sensors and detectors.

First demonstration of RF N-polar GaN MIS-HEMTs grown on bulk GaN using PAMBE

Nitrogen polar (N-Polar) GaN high-electron mobility transistors (HEMT) targeting high efficiency in millimeter wave power amplification applications were fabricated on epitaxial layers grown by plasma assisted molecular beam epitaxy (PAMBE) on on-axis semi-insulating bulk GaN substrates. On-state current density of ∼1 A mm−1 was observed on transistors with LG = 0.75 μm, LGS = 0.5 μm and LGD = 3.75 μm. In a deep class AB mode of operation, devices fabricated on epitaxial structures with these substrates demonstrated 60% higher electron channel mobility compared to devices fabricated with a similar epitaxial structure grown on sapphire substrates using metal-organic chemical vapor deposition. As the first demonstration of N-polar GaN-on-GaN MISHEMT for power amplifier applications, the devices discussed in this letter bridge a path towards achieving higher power gain and efficiency for millimeter-wave N-polar GaN HEMTs.