NH3-source Molecular Beam Epitaxy Research Articles

Observation of single optical site of Eu and Mg codoped GaN grown by NH3-source molecular beam epitaxy

Eu-doped GaN (GaN:Eu) is a promising solid-state material for quantum information devices owing to its ideal quantum levels. However, a decrease of Eu concentration and the unification of optical sites are required for these devices to access an Eu ion. In this article, the effect of Eu concentration on the optical properties of Mg-codoped GaN:Eu was evaluated. The photoluminescence intensity from Eu ions increased linearly with increasing Eu concentration up to 1 × 1019 cm−3, beyond which the intensity saturated. This resulted from the increase of the sharp luminescence line at 620.5 nm (site A), which has a large excitation cross section, with increasing Eu concentration in the range of low Eu concentrations; an excess Eu concentration leads to the formation of other types of optical sites with a low transfer efficiency or inactive sites. For a low Eu concentration of 3 × 1018 cm−3, all optical sites except a specific optical site corresponding to site A disappeared, and the unification of optical sites was suggested. This result is expected to be valuable to realize quantum information devices.

Optical sites in Eu- and Mg-codoped GaN grown by NH3-source molecular beam epitaxy

Mg codoping can improve the luminescence properties of Eu-doped GaN. However, the enhanced optical sites differ depending on the fabrication method. In this study, the optical sites in Eu- and Mg-codoped GaN [GaN:(Eu, Mg)] grown by NH3-source molecular beam epitaxy (MBE) were evaluated. The optical properties of an Eu–Mg-related site grown by NH3-MBE were highly stable against thermal annealing. Although the luminescence at sites A (622.3 and 633.8 nm) and B (621.9 and 622.8 nm) was dominant under indirect excitation of Eu ions through GaN, four different optical site groups in addition to sites A and B were observed under resonant excitation. These optical sites are inconsistent with the Eu–Mg-related sites reportedly observed in GaN:(Eu, Mg) fabricated by organometallic vapor phase epitaxy, indicating that the optical site constitution strongly depends on the growth method. Furthermore, site A, with a high cross section, contributed to as much as 22% of the total photoluminescence (PL) integrated intensity for GaN:(Eu, Mg) grown by NH3-MBE, which resulted in a high PL intensity.

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.

Impacts of anisotropic tilt mosaics of state-of-the-art m-plane freestanding GaN substrates on the structural and luminescent properties of m-plane AlxGa1−xN epilayers

Impacts of grown-in anisotropic tilt mosaics of state-of-the-art m-plane freestanding GaN (FS-GaN) substrates on the structural and luminescent properties of m-plane AlxGa1−xN epilayers are described. The results of cross-sectional transmission-electron microscopy and transmission-electron diffraction analyses reveal the following plane defects formation, even in mostly pseudomorphic epilayers: (i) The formation of basal-plane stacking faults (BSFs) is hard to avoid in the case of lattice-mismatched AlxGa1−xN thick film growth and (ii) the anisotropic greater m-plane tilt mosaic along the a-axis (twist in c-plane) of FS-GaN gives rise to pronounced formation of surface striations along the c-axis and planar-defect network (PDN) located on prismatic {101¯0} planes and pyramidal {11¯0n} planes, which are often terminated by BSF, especially in the case of the epilayers grown by NH3-source molecular-beam epitaxy. These PDNs are assigned by spatially resolved cathodoluminescence (CL) and time-resolved photoluminescence measurements to associate with a characteristic emission peak approximately 200–300 meV lower than the near-band-edge (NBE) emission peak. Based on our database made with the aid of the positron-annihilation technique, three deep-state CL bands are correlated with cation vacancies (VIII) and cation-vacancy complexes. The intensity of the emission band originating from VIII-oxygen complexes was weaker by more than two orders of magnitude than the NBE peak.

Impacts of anisotropic lattice relaxation on crystal mosaicity and luminescence spectra of m-plane AlxGa1−xN films grown on m-plane freestanding GaN substrates by NH3 source molecular beam epitaxy

In-plane anisotropic lattice relaxation was correlated with the crystal mosaicity and luminescence spectra for m-plane AlxGa1−xN films grown on a freestanding GaN substrate by NH3-source molecular beam epitaxy. The homoepitaxial GaN film exhibited A- and B-excitonic emissions at 8 K, which obeyed the polarization selection rules. For AlxGa1−xN overlayers, the m-plane tilt mosaic along c-axis was the same as the substrate as far as coherent growth was maintained (x≤0.25). However, it became more severe than along the a-axis for lattice-relaxed films (x≥0.52). The results are explained in terms of anisotropic lattice and thermal mismatches between the film and the substrate. Nonetheless, all the AlxGa1−xN films exhibited a near-band-edge emission peak and considerably weak deep emission at room temperature.

Impact of Point Defects on the Luminescence Properties of (Al,Ga)N

Threading dislocations (TDs) in (Al,In,Ga)N semiconductors are known to affect the luminescence efficiency of near-band-edge (NBE) emissions in bulk films and quantum structures. However, the principal role of point defects such as vacancies on the luminescent properties has not been fully understood. In this article, impacts of point defects on the luminescence quantum efficiency of NBE emissions and on the intensity of deep emission bands will be described, based on the results of steady-state and time-resolved photoluminescence (TRPL) and positron annihilation measurements. The room temperature nonradiative lifetime (τNR) of the NBE excitonic photoluminescence (PL) peak in polar (0001) and (000-1) , nonpolar (11-20) and (10-10), and zincblende (001) GaN layers prepared by various growth techniques was shown to increase with the decrease in concentration or size of Ga vacancies (VGa) and with the decrease in gross concentration of point defects including complexes, leading to an increase in the NBE PL intensity. As the edge TD density decreased, the concentration or size of VGa tended to decrease and τNR tended to increase. However, there existed remarkable exceptions. The results indicate that the nonradiative recombination process is governed not by single point defects, but by certain defects introduced with the incorporation of VGa, such as VGa-defect complexes. Similar relations were found in AlxGa1-xN alloy films grown by metalorganic vapor phase epitaxy: i. e. τNR at room temperature increased with the decrease in the concentration of cation vacancies (VIII) and with the decrease in gross concentration of point defects. In addition to nonradiative processes, the VIII concentration was found to correlate with the intensity ratio of characteristic deep emission band to the NBE emission (Ideep/INBE). For example, Ideep/INBE at low temperature for the deep emission bands at 4.6, 3.8, and 3.1 eV of AlN epilayers grown by NH3-source molecular beam epitaxy had a linear correlation with the concentration or size of Al vacancies (VAl). Since the relative intensities of 3.1 eV and 3.8 eV bands increased remarkably with lowering the supply ratio of NH3 to Al (V/III ratio) and growth temperature (Tg), they were assigned to originate from VAl-O as well as VAl-shallow donor complexes. The VAl concentration could be decreased by adjusting the V/III ratio and Tg. In the case of AlxGa1-xN alloys, the concentration or size of VIII and Ideep/INBE at 300 K increased simultaneously with the increase in x up to approximately 0.7. Similar to the case for GaN and AlN, the deep emission band was assigned as being due to the emission involving VIII-O complexes.

Correlation between the violet luminescence intensity and defect density in AlN epilayers grown by ammonia‐source molecular beam epitaxy

AbstractIntensity ratios of deep cathodoluminescence (CL) bands at 4.6, 3.8 and 3.1 eV to the near‐band‐edge emission of AlN epilayers grown by NH3‐source molecular beam epitaxy were correlated with the change in S parameter of positron annihilation measurement, which represents the concentration or size of Al vacancies (VAl). The origins of the deep emission bands were assigned to donor‐acceptor pairs associated with VAl and/or VAl‐defect complexes. The VAl concentration was decreased by adjusting the flux ratio of NH3 to Al and growth temperature, resulting in observation of fine excitonic features in the CL spectra. From the energy separation between the ground and first excited states, the binding energy of free A ‐exciton was estimated to be 48 meV. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Relation between Al vacancies and deep emission bands in AlN epitaxial films grown by NH3-source molecular beam epitaxy

Intensity ratios of characteristic deep cathodoluminescence (CL) bands at 4.6, 3.8, and 3.1eV to the near-band-edge emissions at 11K of AlN epilayers grown by NH3-source molecular beam epitaxy were correlated with the change in the S parameter of positron annihilation measurement, which represents the concentration or size of Al vacancies (VAl). Since the relative intensities of 3.1 and 3.8eV bands increased remarkably with lowering supply ratio of NH3 to Al (V/III ratio) and growth temperature (Tg), they were assigned to originate from VAl-O complexes. The VAl concentration could be decreased by adjusting V/III ratio and Tg, resulting in observation of fine excitonic features in the CL spectra. From the energy separation between the ground and first excited states, the binding energy of A exciton was determined to be 48meV.

Improved surface morphology in GaN homoepitaxy by NH3-source molecular-beam epitaxy

GaN homoepitaxial layers of improved surface morphology were obtained by a NH3-source molecular-beam epitaxy method supplying a proper reactive NH3-to-Ga flux ratio (V/III ratio) on the growing surface, combined with a procedure to prevent the surface roughening of the GaN/(0001) Al2O3 epitaxial templates, which were prepared by metalorganic vapor-phase epitaxy. In situ monitoring on the heated templates revealed their thermal decomposition above 700 °C in ultrahigh vacuum, which gave rise to surface unevenness of both initially atomically flat templates and the homoepitaxial layers. The surface flatness was maintained by depositing the “flux-modulated” GaN prior to the high-temperature growth, where Ga flux was supplied intermittently during heating the template under continuous NH3 flow. The increase in V/III ratio greatly improved the surface flatness. As a result of the epilayer/template interface control and growth optimization, the epilayers exhibited higher mobility, smaller full width at half-maximum value (34 meV) of excitonic photoluminescence (PL) peak, and longer PL lifetime of 180 ps at 293 K.

GaN Epitaxial Growth Process at High Growth Temperature by NH3 Source Molecular Beam Epitaxy

ABSTRACTWe have investigated in detail dependence of annealing GaN buffer layer and GaN growth processes on a sapphire substrate at a high temperature of 1000 degree C. The GaN layers are grown by NH3 gas source molecular beam epitaxy. The behavior of GaN buffer and epitaxial layer has been observed by in-situ reflection high-energy electron diffraction and the surface morphologies of as-grown and chemically etched GaN layers by atomic force microscopy. It is found that there is distinct difference in the surface morphology of epitaxial GaN layer between at growth temperatures of below 950 degree C and that of 1000 degree C. It has been considered that the growth kinetics of GaN epitaxial layer extremely depends on the growth temperature.

Hydrogen-dependent lattice dilation in GaN

Using Raman spectroscopy we have analysed the strain status ofGaN films grown on sapphire substrates by NH3 source molecularbeam epitaxy (MBE). In addition to the expected compressive biaxial strain,in some cases GaN films grown on c-face sapphire substrates suffer fromserious tensile biaxial strain. This anomalous behaviour has been wellinterpreted in terms of interstitial hydrogen-dependent lattice dilation.The hydrogen concentration in the films is measured by nuclear reactionanalysis (NRA). With increasing hydrogen incorporation, the residualcompressive biaxial strain is first further relaxed, and then turns intotensile strain when the hydrogen contaminant exceeds a criticalconcentration. The hydrogen incorporation during the growth process isfound to be growth-rate dependent, and is supposed to be strain driven. Webelieve that the strain-induced interstitial incorporation is another wayfor strain relaxation during heteroepitaxy, besides the two currently wellknown mechanisms: formation of dislocations and growth front roughening.

Electrical properties of GaN deposited on nitridated sapphire by molecular beam epitaxy using NH 3 cracked on the growing surface

We have found that GaN epilayers grown by NH 3-source molecular beam epitaxy (MBE) contain hydrogen. Dependent on the hydrogen concentration, GaN on (0 0 0 1) sapphire can be either under biaxially compressive strain or under biaxially tensile strain. Furthermore, we notice that background electrons in GaN increase with hydrogen incorporation. X-ray photoelectron spectroscopy (XPS) measurements of the N1s region indicate that hydrogen is bound to nitrogen. So, the microdefect Ga…H–N is an effective nitrogen vacancy in GaN, and it may be a donor partly answering for the background electrons.

Fermi-edge singularity observed in a modulation-doped AlGaN/GaN heterostructure

In this letter, we report on the observation of Fermi-edge singularity in a modulation-doped AlGaN/GaN heterostructure grown on a c-face sapphire substrate by NH3 source molecular beam epitaxy. The two-dimensional electron gas (2DEG) characteristic of the structure is manifested by variable temperature Hall effect measurements down to 7 K. Low-temperature photoluminescence (PL) spectra show a broad emission band originating from the recombination of the 2DEG and localized holes. The enhancement in PL intensity in the high-energy side approaching Fermi level was observed at temperatures below 20 K. At higher temperatures, the enhancement disappears because of the thermal broadening of the Fermi edge.