AlGaN Films Research Articles

Aluminum incorporation efficiencies in A- and C-plane AlGaN grown by MOVPE**Project supported by the National Natural Science Foundation of China (Grant Nos. 61504128, 61504129, 61274041, and 11275228), the National Basic Research Program of China (Grant No. 2012CB619305), the National High Technology Research and Development Program of China (Grant Nos. 2014AA032603, 2014AA032609, and 2015AA010801), and the Guangdong Provincial Scientific and Technologic

The aluminum incorporation efficiencies in nonpolar A-plane and polar C-plane AlGaN films grown by metalorganic vapour phase epitaxy (MOVPE) are investigated. It is found that the aluminum content in A-plane AlGaN film is obviously higher than that in the C-plane sample when the growth temperature is above 1070 °C. The high aluminum incorporation efficiency is beneficial to fabricating deep ultraviolet optoelectronic devices. Moreover, the influences of the gas inlet ratio, the V/III ratio, and the chamber pressure on the aluminum content are studied. The results are important for growing the AlGaN films, especially nonpolar AlGaN epilayers.

Effect of NH3 flow on incorporation efficiency of Al composition and reduction of surface donor states in AlGaN grown by MOVPE

AbstractWe have studied the effect of various ammonia (NH3) flow rates on AlGaN film grown on GaN using c‐plane sapphire as substrate by metal organic chemical vapour deposition. The influences of NH3 flow on the species diffusion anisotropy of thick AlGaN films were investigated by scanning electron microscopy, atomic force microscopy and high‐resolution X‐ray diffraction measurements. It is demonstrated that, in addition to the larger NH3 flow, growth of the AlGaN epilayer under the same trimethylaluminum (TMAl) flow condition as others is a critical factor for less micro‐cracks. The results of X‐ray double crystal diffraction showed that the AlGaN films with larger ammonia flow exhibited lower Al content and smaller (0002) rocking curve width, and while NH3 flow rate reached 23 liter per minute, a (0002) rocking curve width of 295 arcsecond was reached. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of free-standing wurtzite AlGaN by MBE using a highly efficient RF plasma source

Ultraviolet light emitting diodes (UV LEDs) are now being developed for various potential applications including water purification, surface decontamination, optical sensing, and solid-state lighting. The basis for this development is the successful production of AlxGa1−xN UV LEDs grown by either metal-organic vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE). Initial studies used mainly sapphire as the substrate, but this result in a high density of defects in the epitaxial films and now bulk GaN or AlN substrates are being used to reduce this to acceptable values. However, the lattice parameters of GaN and AlN are significantly different, so any AlGaN alloy grown on either substrate will still be strained. If, however, AlGaN substrates were available, this problem could be avoided and an overall lattice match achieved. At present, the existing bulk GaN and AlN substrates are produced by MOVPE and physical vapor transport, but thick free-standing films of AlGaN are difficult to produce by either method. The authors have used plasma-assisted MBE to grow free-standing AlxGa1−xN up to 100 μm in thickness using both an HD25 source from Oxford Applied Research and a novel high efficiency source from Riber to provide active nitrogen. Films were grown on 2- and 3-in. diameter sapphire and GaAs (111)B substrates with growth rates ranging from 0.2 to 3 μm/h and with AlN contents of 0% and ∼20%. Secondary ion mass spectrometer studies show uniform incorporation of Al, Ga, and N throughout the films, and strong room temperature photoluminescence is observed in all cases. For films grown on GaAs, the authors obtained free-standing AlGaN substrates for subsequent growth by MOVPE or MBE by removing the GaAs using a standard chemical etchant. The use of high growth rates makes this a potentially viable commercial process since AlxGa1−xN free-standing films can be grown in a single day and potentially this method could be extended to a multiwafer system with a suitable plasma source.

Growth of AlGaN Film on Si (111) Substrate

At present, the applications of Al x Ga 1−x N are extensive, such as for visible-blind ultraviolet detectors, laser diodes, light emitting diodes (LEDs) and HEMTs. In this paper, Al 0.25 Ga 0.75 N and Al 0.32 Ga 0.68 N films have been grown on 2 in Si (iii) substrates by MOCVD. The low-temperature (6 K) photoluminescence (PL) spectrum and XRD rocking curve measurements have been employed to study the crystal quality of samples, and the phonon replica peak can be observed, which indicate that the samples have better quality in a small-localized region. The surface morphology of samples was investigated by AFM and the result of wavy surface agrees with the deduction from XRD rocking curve measurements. The sheet resistance mappings have been shown, and it indicates the nonuniformity of AlGaN film on Si (iii) will increase sharply as the Al content increases.

Chemical and electrical characteristics of annealed Ni/Au and Ni/Ir/Au contacts on AlGaN

The evolution of Ni/Au and Ni/Ir/Au metal contacts deposited on AlGaN was investigated at different annealing temperatures. The samples were studied with electrical and chemical composition techniques. I–V characteristics of the Schottky diodes were optimum after 500 and 600°C annealing for Ni/Au and Ni/Ir/Au based diodes, respectively. The depth profiles of the contacts were measured by x-ray photoelectron spectroscopy and time of flight secondary ion mass spectroscopy. These chemical composition techniques were used to examine the evolution of the metal contacts in order to verify the influence the metals have on the electrical properties of the diodes. The insertion of Ir as a diffusion barrier between Ni and Au effected the electrical properties, improving the stability of the contacts at high temperatures. Gold diffused into the AlGaN film, degrading the electrical properties of the Ni/Au diode. At 500°C, the insertion of Ir, however, prevented the in-diffusion of Au into the AlGaN substrate.

Preparation of europium‐doped GaN and AlGaN films grown by radical‐nitrogen‐assisted compound‐source MBE

AbstractBy the compound‐source molecular beam epitaxy (CS‐MBE), GaN:Eu and AlGaN films grown on both Al2O3(0001) and GaAs(100) substrates were prepared. Under RHEED observation, streak RHEED patterns from GaN were observed. This result shows that the surface of the film grown by CS‐MBE is atomically flat. From the XRD measurements, for GaN:Eu/Al2O3(0001), the film was oriented to c‐axis. On the other hands, for GaN:Eu/GaAs(100), the layer includes both the hexagonal and cubic phases. From the PL measurements, the PL spectra of both GaN:Eu/Al2O3 and GaN:Eu/GaAs(100) showed PL peaks related to Eu3+ ion. For GaN:Eu/GaAs(100), it is possible that part of Eu atom incorporates cubic‐GaN site (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Minority carrier diffusion length in AlxGa1‐xN (x = 0.1) grown by ammonia molecular beam epitaxy

AbstractThe room‐temperature minority carrier diffusion length in n ‐Al0.1Ga0.9N grown by ammonia molecular beam epitaxy on (0001) sapphire for photodetector applications has been investigated. The measurements were performed using the spectral dependence of the photocurrent detected by the built‐in p‐n ‐junction on thinner layer samples, and by the electron beam induced current technique on films of up to 2 µm in thickness. The results show that the hole diffusion length in n ‐AlGaN films amounts to 120‐150 nm, which is by 3‐4 times larger than that in GaN films grown under similar growth conditions. The thicker films do not show a considerable improvement in the hole diffusion length value. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface properties of AlN and Al x Ga1−x N epitaxial layers characterized by angle resolved X-ray photoelectron spectroscopy

The AlN and AlxGa1−xN (x = 0.35, 0.73) epitaxial layers were grown on c-plane sapphire substrates by metal–organic chemical vapor deposition. The surface properties of these layers were quantitatively characterized by angle resolved X-ray photoelectron spectroscopy. The characterization results revealed that the air-exposed surfaces of the Al-containing AlN and AlGaN films were covered by a thin Al-oxide layer due to the large chemical affinity of aluminum to oxygen. The as-deposited AlN film was proved to be a nitrogen-deficient film. On the other hand, the Ga Auger effect for the as-deposited AlGaN film was found to be greatly suppressed by increasing Al composition. Moreover, the Al composition in the as-deposited AlGaN films was demonstrated to be non-uniform since Al atom is more easily oxidized than Ga atom, especially for the AlGaN film with relatively high Al content. Our work will be beneficial to understanding and controlling of the surface properties for Al-containing III-nitrides.

Low-resistance Ohmic contact on polarization-doped AlGaN/GaN heterojunction

High electronic density is achieved by polarization doping without an impurity dopant in graded AlGaN films. Low specific contact resistance is studied on the polarization-doped AlGaN/GaN heterojunctions by using the transmission line method (TLM). The sheet density of polarization-doped AlGaN/GaN heterojunction is 6× 1014 cm−2 at room temperature. The linearly graded material structure is demonstrated by X-ray diffraction. The carrier concentration and mobility are characterized by a temperature-dependent Hall measurement. Multiple-layer metal (Ti/Al/Ti/Au) is deposited and annealed at 650 °C to realize the Ohmic contacts on the graded AlGaN/GaN heterojunctions.

Polarized XAFS study of Al K-edge for m-plane AlGaN films

Local structures around Al atoms in high-quality m-plane AlxGa1-xN films (x=0.32 and 0.58) deposited on m-plane GaN substrates by the NH3 source molecular beam epitaxy method were investigated by Al K-edge X-ray absorption fine structure (XAFS) for the first time. XAFS spectra were measured using a linearly-polarized X-ray source from synchrotron radiation for three different directions; along the c-, a-, and m-axes. The interatomic distances along the a-axis are close to Ga-Ga distance in GaN, indicating that the local structures are strongly affected by GaN substrates. The localization of Al atoms was observed for the Al0.32Ga0.68N film.

Stimulated emission and optical gain in AlGaN heterostructures grown on bulk AlN substrates

Optical gain spectra for ∼250 nm stimulated emission were compared in three different AlGaN-based structures grown on single crystalline AlN substrates: a single AlGaN film, a double heterostructure (DH), and a Multiple Quantum Well (MQW) structure; respective threshold pumping power densities of 700, 250, and 150 kW/cm2 were observed. Above threshold, the emission was transverse-electric polarized and as narrow as 1.8 nm without a cavity. The DH and MQW structures showed gain values of 50–60 cm−1 when pumped at 1 MW/cm2. The results demonstrated the excellent optical quality of the AlGaN-based heterostructures grown on AlN substrates and their potential for realizing electrically pumped sub-280 nm laser diodes.

Behavior of aluminum adsorption and incorporation at GaN(0001) surface: First-principles study

First-principles calculations are performed to study the energetics and atomic structures of aluminum adsorption and incorporation at clean and Ga-bilayer GaN(0001) surfaces. We find the favorable adsorption site changes from T4 to T1 as Al coverage increased to 1 monolayer on the clean GaN(0001) surface, and a two-dimensional hexagonal structure of Al overlayer appears. It is interesting the Al atoms both prefer to concentrate in one deeper Ga layer of clean and Ga-bilayer GaN(0001) surface, respectively, while different structures could be achieved in above surfaces. For the case of clean GaN(0001) surface, corresponding to N-rich and moderately Ga-rich conditions, a highly regular superlattice structure composed of wurtzite GaN and AlN becomes favorable. For the case of Ga-bilayer GaN(0001) surface, corresponding to extremely Ga-rich conditions, the Ga bilayer is found to be sustained stable in Al incorporating process, leading to an incommensurate structure directly. Furthermore, our calculations provide an explanation for the spontaneous formation of ordered structure and incommensurate structure observed in growing AlGaN films. The calculated results are attractive for further development of growth techniques and excellent AlGaN/GaN heterostructure electronic devices.

All-optically controlled one-dimensional photonic crystal of AlGaN film via photorefractive effect

In this work, we report theoretical design of one-dimensional photonic crystal of Al0.4Ga0.6N material, in which the photonic band is formed and controlled all-optically via photorefractive effect. The simulation results show that the photorefractively periodic variation of refractive index, enhanced by the strong polarization field in AlGaN material, gives evident photonic band gap, and the band feature is a function of the incidence direction of illumination and signal light. Moreover, the response time of photonic band writing and erasing can be greatly reduced by applying high electron mobility transistor structure, and the work frequency is expected to enter GHz range. The results demonstrate a simple, high efficiency, high speed, and flexible way of realizing photonic crystal all-optically.

Effect of GaN template thickness and morphology on AlxGa1−xN (0 < x < 0.2) growth by MOVPE

We have grown AlxGa1−xN/GaN (0<x<0.2) films by MOVPE at atmospheric pressure in a home-made vertical reactor. The films were deposited on high temperature GaN templates which were grown on in situ nano-masked (0001) sapphire substrates by using Si/N treatment. This process presents advantages of ELO technology without its ex situ complicated steps and induces a transition from 3D mode (initial stage) to 2D growth mode. We investigate systematically the effect of the template thickness and morphology on structural and morphological properties of AlGaN films. Four 0.5μm-thick Al0.07Ga0.93N samples grown on different GaN templates have been studied. The HRXD and the AFM results show a better film quality when the AlGaN layer is grown on a 1.3μm-thick 2D GaN template. It is possible to control the stress in the layers. The crystalline quality is also showed to degrade with Al solid content increase.

Stress evolution during AlxGa1−xN/AlN growth on sapphire

In-situ curvature measurements were employed to analyse stress generation and relaxation during epitaxial growth of undoped and Si-doped AlGaN layers on AlN/sapphire templates. While AlGaN films with a lower Al content exhibit a compressive strain during growth including a gradual relaxation, layers with a high Al content (x≥0.8) grow under tension due to the AlN/sapphire effective substrate properties. Wafer curvature analysis and accompanying STEM measurements suggest that compressive stress relaxation involves misfit dislocations at or near the heterointerface as well as effective dislocation climb during growth. In addition, introduction of Si as n-type dopant in AlxGa1−xN adds a tensile stress component, most likely driven by a surface-mediated dislocation climb process.

Study of radiation damage in InGaN and AlGaN films induced by 8.9 MeV Bi33+ ions

Homogeneous radiation damage was induced in ∼250-nm-thick In0.18Ga0.82N and Al0.2Ga0.8N films by irradiation with 8.9MeV Bi33+ ions at room temperature. The ion fluence was in the range from 5×1011 to 5×1013cm−2. From the Rutherford backscattering/channeling (RBS/C) measurements, it is shown that Al0.2Ga0.8N had a radiation resistance at least one order of magnitude higher than In0.18Ga0.82N. When the ion fluence was increased from 1×1013 to 5×1013cm−2, enhanced surface peaks were observed in the RBS/C spectra for both the In0.18Ga0.82N and Al0.2Ga0.8N films, which may be attributed to the high charge state of the incident ions. Moreover, from the Raman spectra measurements, the evolution of the disorder-related B1 bands and TO-like peaks with the fluence was observed for the In0.18Ga0.82N and Al0.2Ga0.8N films, respectively.

Influence of GaN template thickness and morphology on AlxGa1−xN luminescence properties

We report on AlxGa1−xN/GaN films grown by atmospheric pressure MOVPE in a home-made vertical reactor. The high temperature GaN template is grown using the Si/N treatment of (0001) sapphire substrate. This process presents advantages of ELO technology without its ex-situ complicated steps and induces 3D (initial stage) to 2D growth mode. We investigate systematically the effect of the template thickness and morphology on optical properties of the AlGaN films by photoluminescence (PL) measurements. Four 0.5μm-thick Al0.07Ga0.93N samples on different GaN templates have been studied. PL spectra, dominated at low temperature by the donor bound exciton emission line, show a better film quality when AlGaN is grown on 1.3μm-thick high quality 2D GaN template. Two quenching mechanisms of the donor bound exciton PL-intensity are discussed. The PL peak linewidth increases as a function of Al composition. A bowing parameter of 0.93eV is obtained from the band gap variations as a function of Al composition.

High p-type conduction in high-Al content Mg-doped AlGaN

We report on the successful fabrication of highly conductive p-type Mg-doped Al0.7Ga0.3N thin films grown on sapphire substrates by metal-organic chemical vapor deposition. Photoluminescence measurements show that Mg doping for growth with a high V/III ratio and moderate Mg concentration can effectively suppress self-compensation by the formation of nitrogen vacancy complexes. The lowest electrical resistivity was found to be 47 Ω cm at room temperature. Moreover, the temperature dependence of the p-type conductivity in these high-Al content AlGaN films shows the extremely small effective activation energies of 47–72 meV at temperatures below 500 K.

Structural, morphological and magnetic characteristics of Tb-implanted GaN and AlGaN films

Abstract Diluted-magnetic GaN:Tb and AlGaN:Tb films have been fabricated by implanting Tb + ions into c-plane (0 0 0 1) GaN and AlGaN films and a subsequent annealing process. The structural, morphological and magnetic characteristics of the samples have been investigated by means of X-ray diffraction (XRD), atomic force microscopy (AFM) and superconducting quantum interference device (SQUID), respectively. The XRD and AFM analyses show that the annealing process can effectively recover the crystalline degradation caused by implantation. According to the SQUID analysis, both the GaN:Tb and the AlGaN:Tb films exhibit clear room-temperature ferromagnetism. It is very interesting to find the saturation magnetization value of the AlGaN:Tb sample is almost two times that of GaN:Tb sample. The possible origin of the ferromagnetism of the samples was discussed briefly.

Strain and defects in Si-doped (Al)GaN epitaxial layers

Si is the most common dopant in (Al)GaN based devices acting as a donor. It has been observed that Si induces tensile strain in (Al)GaN films, which leads to an increasing tendency for cracking of such films with the increase of Si content and/or the increase of Al content. Based on x-ray investigations, the Si-doped films have a larger in-plane lattice constant than their undoped buffer layers, indicating involvement of a mechanism other than the change of lattice constants expected from an alloying effect. In this work, we present a model about Si dislocation interaction while debating other proposed models in the literature. According to our model, Si atoms are attracted to the strain dipole of edge-type dislocations in (Al)GaN films. It is expected that Si is more incorporated on that side of the dislocation, which is under compression leading to the formation of off-balanced dipoles with reduced compressive component. In response to such off-balanced dipoles—appearing as tensile dominant strain dipoles—the dislocation lines climb in order to accommodate the excess tensile strain. However, this dislocation climb mechanism is hindered by forces exerted by vacancies created due to the climb process. Accordingly, we have observed a lower strain level in our Si doped layers when they contain fewer dislocations. These findings were further supported by x-ray diffraction, transmission electron microscopy, and micro-photoluminescence investigations.