AlGaN Films Research Articles

Deep-ultraviolet aperiodic-oscillation emission of AlGaN films

In this Letter, we describe an aperiodic-oscillation emission phenomenon originating from the Fabry-Perot effect in the deep-ultraviolet backscattering fluorescence spectrum of the $c$c-plane AlGaN film, which is related to the dispersion of its ordinary refractive index near the band edge. Based on this fluorescence spectrum, the ordinary refractive index of the AlGaN film near the band edge could be directly obtained. Certainly, by means of variable angle spectroscopic ellipsometry, the ordinary refractive index of the AlGaN film could be also achieved. Comparing the results obtained by both methods, we discovered that the refractive indices are quite similar, which suggests that the aperiodic-oscillation fluorescence spectrum is also a reliable approach to capture the refractive index of anisotropic optical films.

Suppression of dislocation-induced spiral hillocks in MOVPE-grown AlGaN on face-to-face annealed sputter-deposited AlN template

AlGaN films were grown on face-to-face annealed sputter-deposited AlN/sapphire (FFA Sp-AlN) templates via metalorganic vapor phase epitaxy (MOVPE), and the growth behavior of the AlGaN films was investigated. The sapphire substrates with small off-cut exhibited poor surface flatness of AlGaN grown on the FFA Sp-AlN templates owing to the formation of large hillock structures. To understand the origin of these hillock structures, the crystallinity and surface morphology of conventional fully MOVPE-grown AlN/sapphire (MOVPE-AlN) templates and the FFA Sp-AlN template were comprehensively studied. The screw- and mixed-type threading dislocation density of the FFA Sp-AlN template was estimated to be approximately 1.8 × 106 cm−2, which was two orders of magnitude lower than that of the MOVPE-AlN template. Consequently, the uniquely observed growth of the hillock structures in the FFA Sp-AlN templates was attributed to their low density of screw- and mixed-type threading dislocations. The large surface off-cut sapphire substrates suppressed the hillock structures on the FFA Sp-AlN templates. The improvement in surface flatness resulted in better optical properties of multiple quantum wells grown on the AlGaN layer. These results demonstrate a promising method for achieving highly efficient and cost effective AlGaN based deep ultraviolet light-emitting diodes.

Thermionic emission and conversion properties of n-type AlGaN thin film cathodes grown on 6H–SiC substrates

We observed the thermionic emission (TE) and conversion (TC) characteristics of Si-doped AlGaN thin film cathodes grown on n-type 6H-SiC substrates with Cs adsorption on the surfaces. The threshold temperatures of TE decreased as the AlN mole fraction was increased, with threshold temperatures of around 205 °C for Si–doped Al0.5Ga0.5N. TE was detected even when the applied external voltage was 0 V at 600 °C, which means that the cathodes had TC characteristics at a temperature dramatically lower than that of conventional systems with metal-based cathodes. In addition, we observed a notable enhancement of the emission current when the cathodes were irradiated with white light having energy lower than that of the bandgap of the AlGaN films at low temperatures. We suggest that the enhancement was caused by an internal photoelectron effect due to the absorption of the light in the n–type 6H–SiC substrate.

Time-resolved temperature-jump measurements and steady-state thermal imaging of nanoscale heat transfer of gold nanostructures on AlGaN:Er3+ thin films.

For a nanostructure sitting on top of an AlGaN:Er3+ thin film, a new thermal imaging technique is presented where dual cameras collect bandpass filtered videos from the H and S bands of Er3+ emission. We combine this thermal imaging technique with our newly developed time-resolved temperature measurement technique which relies on luminescence thermometry using Er3+ emission. This technique collects time-resolved traces from the H and S bands of Er3+ emission. The H and S signal traces are then used to reconstruct the time-resolved temperature transient when a nanostructure is illuminated with a pulsed 532 nm light. Two different types of samples are interrogated with these techniques (drop-casted gold nanosphere cluster and lithographically prepared gold nanodot) on the AlGaN:Er3+ film. Steady-state and time-resolved temperature data are collected when the samples are immersed in air and water. The results of time-resolved temperature-jump measurements from a cluster of gold nanospheres show extremely slow heat transfer when the cluster is immersed in water and nearly 200-fold increase when immersed in air. The low thermal diffusivity for the cluster in water suggests poor thermal contact between the cluster and the thermal bath. The lithographically prepared nanodot has much better adhesion to the AlGaN film, resulting in much higher thermal diffusivity in both air and water. This proof-of-concept demonstration opens a new way to measure the dynamics of the local heat generation and dissipation at the nanoparticle-media interface.

Fabrication of High Quality AlGaN Films and UV-B Semiconductor Lasers

Fabrication of High Quality AlGaN Films and UV-B Semiconductor Lasers

Thermal conductivity of thin films of gallium nitride, doped with aluminium, measured with 3ω method

Thermal conductivity of AlxGa1-xN thin films, grown by metal organic vapor phase epitaxy, was measured with the use of 3ω method in the temperature range from 150 to 300 K. The aim of the present study is the investigation of the influence of the Al amount on the thermal conductivity of AlGaN films of the same thickness. Results of the conducted measurements show that thermal conductivity increases with increasing temperature and decreases with increasing amount of alloying element. Those results are in good agreement with previous reports.

Atomic simulation of AlGaN film deposition on AlN template

In this article, we study the deposition of AlGaN film on AlN template by molecular dynamics (MD) simulations. The effects of growth temperature and film thickness on the dislocation of deposited AlGaN film are simulated and studied. The atomic structure of deposited AlGaN film is also investigated. We find that the dislocations usually occur at the interface between AlN template and AlGaN film and then extend towards the growth direction. The dislocation density decreases with the increase of AlGaN film thickness, which indicates that increasing the thickness of deposited AlGaN film to a certain extent is beneficial to reducing dislocation. In addition, increasing the growth temperature can also effectively reduce the dislocation in deposited AlGaN film. Furthermore, the crystallinity of deposited AlGaN film could be improved by increasing the growth temperature. This is consistent with the dislocation discussion. The mobility of adatoms increases as the growth temperature increases. So it is easier for adatoms to find their ideal lattice points at higher temperature. Thus the dislocation and other defects can be effectively reduced and the crystal quality of deposited AlGaN film could be improved.

Monitoring the growth of III-nitride materials by plasma assisted molecular beam epitaxy employing diffuse scattering of RHEED

AlGaN alloys find important applications in UV emitters and detectors, as well as in high-power high-frequency electronics. While reflection high energy electron diffraction (RHEED) is a standard technique for in situ monitoring of the growth of AlGaN alloys by plasma assisted molecular beam epitaxy, this paper investigates a new mode of its application. During the growth of AlGaN alloys, the ratio of the group III (Al + Ga) to group V (active nitrogen) adatoms critically controls the materials property of AlGaN films and is optimal within a very narrow window of operation. Moreover, this ratio is dependent in a complex fashion on various growth parameters, including substrate temperature, and is difficult to determine quantitatively in real time. This paper provides a method to estimate that important parameter. This can be carried out through the capture of the RHEED image from the fluorescent screen using an inexpensive video camera setup and a simple analysis procedure. While most RHEED analyses focus on the diffraction pattern, e.g., the line spacing, this work quantifies diffused scattering of the electron beam from a metallic layer that forms on the top of the growth surface during deposition under excess group III conditions, which is typically employed during growth. Two alternate methods for data analysis have been explored and compared. The results indicate that this process can qualitatively trace the variation of the thickness of the thin metallic layer, and hence the group III to group V flux ratio, for different substrate temperatures. This technique being simple, fast, and cost-effective can be incorporated into standard MBE systems for real-time in situ characterization of AlGaN alloys.

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.

A New Method of Growing AlN, GaN, and AlGaN Bulk Crystals Using Hybrid SiC/Si Substrates

The main principles of a new method of growing bulk single-crystal AlN, AlGaN, and GaN films with thickness from 100 μm and more on silicon substrates with a buffer silicon carbide layer with its subsequent detachment from Si substrates are presented. The main substance of this method is a combination of the method of chloride-hydride epitaxy that determines high growth rates of III nitride layers and the use a Si substrate with a buffer layer of nanoscale SiC film grown by the atomic substitution method as the growth substrate. The Si substrate with a SiC layer grown by the atomic substitution method has a number of structural, physical, and chemical features as compared to SiC layers grown on Si by the standard methods. It is shown that it is precisely this feature that enables the growth on their surfaces of thick crack-free AlN, AlGaN, and GaN layers with subsequent and quite simple their detachment from the substrate. The single-crystal crack-free AlN layers with thickness to 300 μm, AlGaN layers with thickness to 400 μm, GaN layers with thickness to 200 μm, and GaN films of the semipolar ($$11\bar {2}4$$) orientations with thickness to 30 μm have been grown.

(Invited) X-Ray Metrology of AlN Single Crystal Substrates

As development of GaN and SiC material systems for applications in power switching progresses, the ultrawide-bandgap (UWBG) materials, with bandgaps greater than GaN, are gaining increased attention, in pursuit of improved power device performance. Substrate technology is critical to improve this field. High-quality native substrates, possessing low lattice and thermal expansion mismatches to epitaxial active layers, enable epitaxial growth with low densities of extended defects, leading to improved device performance and lifetime. Lateral and vertical power switching devices have been demonstrated in the emerging AlGaN alloy system [1]. Aluminum nitride (AlN) possesses a close lattice match to high Al mole fraction AlGaN alloys and a high thermal conductivity, which make it an excellent substrate for UWBG AlGaN-based devices. Physical vapor transport (PVT) growth has emerged as the most promising technique for the production of large, high-quality AlN single crystal substrates. Seeded growth technology developed by HexaTech enables iterative expansion of single crystal size, while suppressing low angle grain boundary (LAGB) formation, a primary source of grown-in dislocations. X-ray diffraction (XRD) is well suited to accurately and non-destructively probe the extended defect structure, composition, and strain of single crystals and epitaxial films. A variety of x-ray techniques were used throughout AlN substrate processing, from evaluation of as-grown crystals, through substrate polishing, to analysis of epitaxial films grown on AlN. High-resolution x-ray rocking curves (HRXRRC) and x-ray topography (XRT) were used to determine the primary dislocation types in AlN bulk crystals and their likely formation mechanisms [2, 3]. Synchrotron-based XRT images demonstrated that the expanded diameter of freestanding AlN boules contained low densities of threading dislocations (<104 cm-3), and no basal plane dislocations or LAGBs [4]. Furthermore, XRT images acquired on a commercial diffractometer were used for quality control of AlN seed crystals. The advent of a new generation of x-ray area detectors, capable of a wide dynamic range and rapid data acquisition, opened the possibility for using XRT in production. HRXRRC and surface-sensitive reciprocal space maps were used to monitor sub-surface damage removal during substrate polishing [5]. These results, together with atomic force microscopy and defect selective etching images, demonstrated that chemo-mechanically polished AlN surfaces were free of plastic damage [4]. Polished AlN surfaces were also studied by x-ray reflectivity, yielding long-range data about surface roughness and density. Finally, epitaxial AlGaN films and heterostructures deposited on AlN substrates were studied by a combination of these x-ray techniques, providing essential information on composition, thickness, relaxation, and heterostructure design. XRD has become a valuable metrology tool in the development of AlN technology. This talk will review the implementation of complementary x-ray based measurements in the processing of AlN, from boule growth to AlGaN-based devices.

Effects of indium surfactant and MgN intermediate layers on surface morphology and crystalline quality of nonpolar a-plane AlGaN epi-layers

High quality non-polar a-plane AlGaN epi-layers with dual MgN interlayers were successfully grown on semi-polar r-plane sapphire substrates with the indium-surfactant-assisted metal organic chemical vapor disposition (MOCVD) technology, and characterized with atomic force microscopy, cathode luminescence (CL), and high-resolution X-ray diffraction rocking curve. It was found that both surface morphology and crystalline quality of the non-polar AlGaN films were strongly dependent on the mass flow of indium surfactant in the MOCVD growth process. In fact, the great suppression of the deep energy level impurity-related transitions in the CL spectra indicates a significant enhancement in crystalline quality for the non-polar AlGaN films. Moreover, with the optimization of the indium surfactant mass flow, a root mean square value as small as 10.9 nm was achieved, demonstrating a remarkable improvement in surface morphology for the a-plane AlGaN epi-layer.

Ultraviolet-B band lasers fabricated on highly relaxed thick Al0.55Ga0.45N films grown on various types of AlN wafers

In this paper, we investigated the dependence of threshold power density on the Al0.55Ga0.45N underlying layer film thickness in ultraviolet-B band (UV-B) lasers on various AlN wafers (four types). We also prepared and compared AlN templates for AlN freestanding substrates, AlN films fabricated by metalorganic vapor phase epitaxy, and annealed sputtered AlN templates at high temperature. The initial growth of AlGaN became three-dimensional by inserting a homoepitaxial Ga-doped AlN layer between the AlN template and Al0.55Ga0.45N, before it shifted to two-dimensional growth. It is possible to reduce the dislocation in Al0.55Ga0.45N using this mode. The dependence of AlGaN film thickness and that of the AlN template on samples with an inserted homoepitaxial Ga-doped AlN layer were studied. Compared with Al0.55Ga0.45N having a thickness of 5 μm, there was almost no noticeable difference between the dark spot density characterized by cathodoluminescence and the threshold power density in UV-B lasers for the AlN template. Besides, the characteristics were noticeably different for the film thickness of Al0.55Ga0.45N. The threshold power density in UV-B laser and dark spot density were reduced by increasing the film thickness. Through the optimization of the crystal growth condition, the threshold power density in UV-B laser and dark spot density were reduced to 36 kW cm−2 and 7.5 × 108 cm−2, respectively.

Point-Defect Distribution and Transformation Near the Surfaces of AlGaN Films Grown by MOCVD

The characteristics of vacancy-type point defects near the surface and inside the AlGaN films were studied by positron annihilation spectroscopy (PAS) and fluorescence spectra. Two types of AlGaN films were grown by varying the growth temperature and the Si doping to maintain the volume of the threading dislocation densities (TDs). The PAS results showed that the point-defect types of the AlGaN films with approximate TDs differed. In the detailed PAS results, the W–S curves of AlGaN films showed two types of point-defect distribution forms. One type was uniform in the AlGaN film, namely, the major point defects inside the bulk and near the surfaces are almost the same as that of the VIII–ON complex, which was the probable candidate. The other type was nonuniform, suggesting that the major point defects were VGa/VAl inside the bulk and VIII–nVN complexes near the surfaces of the AlGaN films. The evolution of the PAS and fluorescence spectra also suggested that a point-defect rearrangement occurred near the surfaces of the AlGaN films after 10 MeV high-energy e-beam irradiation. This rearrangement was likely related to the transformation of the point-defect type from the oxide-substituted nitride (ON) to the oxide vacancy (VO, equaling to VN) based on our growth recipes.

Highly radiative nature of ultra-thin c-plane Al-rich AlGaN/AlN quantum wells for deep ultraviolet emitters

Temperature-dependent photoluminescence (PL) lifetimes were measured for a series of ultra-thin c-plane Al0.61Ga0.39N/AlN multiple quantum wells (QWs) on bulk AlN substrates with the well thickness varying from 0.6 to 2 nm. At temperatures below 75 K, estimates of the internal quantum efficiency indicate that the recombination is primarily radiative, with a lifetime of ∼160 ps for the 0.6 nm QWs, comparable to the low temperature PL lifetime observed in bulk AlGaN films of a similar Al content. This short lifetime is observed despite the presence of layer thickness fluctuations and the quantum-confined Stark effect associated with the large polarization field in the heterostructures, which tend to increase the radiative lifetime. This behavior is explained using many-body calculations of radiative recombination rates that extend beyond the conventional ABC rate equation model by accounting for both excitons and free carriers within a nonequilibrium Green's function formalism. The results indicate that the combination of the large wave function overlap integral (∼0.65) and exciton binding energy (1.82 times the 3D Rydberg) for the 0.6 nm QWs leads to an ∼20-fold increase in the radiative recombination rate relative to that obtained for the 2 nm QWs. This greater radiative recombination rate competes favorably with trapping at interface fluctuations and defect-induced nonradiative recombination that dominates recombination at higher temperatures.

Новый метод получения объемных кристаллов AlN, GaN и AlGaN с использованием гибридных подложек SiC/Si

The paper presents the main provisions of the new method of growing bulk, with a thickness of 100 μm or more, single-crystal AlN, AlGaN and GaN films on silicon poles with a silicon carbide buffer layer with their subsequent separation from the Si substrate. The main essence of this method is the combination of the method of chloride-hydride epitaxy, which ensures high growth rates of III-nitride layers using as substrate for growth, Si substrate with a buffer layer of a nanoscale atomic SiC film. The Si substrate with a SiC layer grown by the substitution method has a number of structural, physical, and chemical features as compared to SiC layers grown on Si by standard methods. It is shown that it is this feature that makes it possible to grow on its surface thick, uncracked layers of AlN, AlGaN and GaN, followed by their simple separation from the substrate. In this work, single-crystal uncracked layers were grown: AlN up to 300 microns thick; AlGaN thickness up to 400 microns; GaN thickness up to 200 microns; GaN semipolar (112 ̅4) orientation up to 35 microns.

Effects of ultraviolet wavelength and intensity on AlGaN thin film surfaces irradiated simultaneously with CF4 plasma and ultraviolet

Al0.24Ga0.76N thin film surfaces were irradiated simultaneously with CF4 plasma and ultraviolet (UV) at wavelengths of 280 and 310 nm. The UV wavelength and intensity were varied without changing the plasma-generating conditions. These UV photon energies were larger than the band-gap energy of AlGaN film. A compositional change in the simultaneously-irradiated surface depended on the UV wavelength. The 280-nm UV irradiation caused a greater degree of nitrogen deficiency than that introduced by 310-nm UV irradiation, causing the Ga- and Al-rich surface. Moreover, the 280-nm UV irradiation increased the amounts of F atoms and CFx fluorocarbons incorporated into the surface, in comparison with CF4 plasma-only irradiation. In contrast, the 310-nm UV irradiation reduced the amount of the incorporated F atoms and hardly increased the amount of the incorporated CFx fluorocarbons from those caused by CF4 plasma-only irradiation. These became more prominent with increasing UV intensity. These UV wavelength-dependent results can be discussed in terms of four factors: the UV photogenerated-hole-assisted oxidation, the localized surface plasmon resonance, the UV-induced migration of gallium and aluminum vacancies forming stable complexes with F, and the sticking of F-related radicals from CF4 plasma onto the surface.

Structural and Stress Properties of AlGaN Epilayers Grown on AlN-Nanopatterned Sapphire Templates by Hydride Vapor Phase Epitaxy.

In this paper, we report the epitaxial growth and material characteristics of AlGaN (Al mole fraction of 10%) on an AlN/nanopatterned sapphire substrate (NPSS) template by hydride vapor phase epitaxy (HVPE). The crystalline quality, surface morphology, microstructure, and stress state of the AlGaN/AlN/NPSS epilayers were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The results indicate that the crystal quality of the AlGaN film could be improved when grown on the AlN/NPSS template. The screw threading dislocation (TD) density was reduced to 1.4 × 109 cm−2 for the AlGaN epilayer grown on the AlN/NPSS template, which was lower than that of the sample grown on a flat c-plane sapphire substrate (6.3 × 109 cm−2). As examined by XRD measurements, the biaxial tensile stress of the AlGaN film was significantly reduced from 1,187 MPa (on AlN/NPSS) to 38.41 MPa (on flat c-plane sapphire). In particular, an increase of the Al content in the overgrown AlGaN layer was confirmed by the TEM observation. This could be due to the relaxation of the in-plane stress through the AlGaN and AlN/NPSS template interface.

Enhanced p-type conduction in AlGaN grown by metal-source flow-rate modulation epitaxy

A metal-source flow-rate modulation epitaxy method is reported to enhance the hole concentration of Mg-doped AlGaN grown by metal organic chemical vapor deposition. The hole concentration of p-type AlGaN (Al content 0.43) is increased to 2.3 × 1017 cm−3 at room temperature by this method, which is about ten times higher than that of the conventional growth. The resistivity was found to be as low as 12.7 Ω·cm. Furthermore, the effective acceptor activation energy (EA) in the AlGaN films (Al content 0.32–0.43) was determined to be 20–22 meV, several times smaller than EA in p-GaN. Secondary ion mass spectroscopy measurements demonstrated that uniformly Mg-doped AlGaN structures with spatially modulated Al compositions were formed using this technique. It is suggested that the enhancement of hole concentration benefits from the modulation of the valence band edge.

N- and P- type Doping in Al-rich AlGaN and AlN

Attaining a high conductivity in both p-type and n-type Al-rich AlGaN epitaxial films is necessary for highly efficient deep-UV emitters. While reliable n-type conductivity has been demonstrated in AlxGa1-xN up to x < 0.8, achieving a reasonable p-type conductivity is a challenge even in Ga-rich AlGaN films. As one increases the x in AlxGa1-xN, several point defects and charge compensators appear in the epitaxial film. This report reviews recent observations on doping, conductivity, point defect control of Al-rich AlGaN films. Discussions on activation energy, state-of-the-art epitaxial material quality, contact formation and surface treatments are also presented.