Photoluminescence studies of impurity transitions in AlGaN alloys

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AlN homoepilayers and heteroepilayers were grown on polar c-plane and nonpolar a-plane and m-plane orientations of AlN bulk and sapphire substrates by metal organic chemical vapor deposition. A systematic comparative study of photoluminescence properties of these samples revealed that all AlN homoepilayers ͑c, a and m planes͒ were strain free with an identical band gap of about 6.099 ͑6.035͒ eV at 10 ͑300͒ K, which is about 42 meV below the band gap of c-plane AlN heteroepilayers grown on sapphire. Also, nonpolar a-plane homoepilayers have the highest emission intensity over all other types of epilayers. We believe that a-plane AlN homoepilayers have the potential to provide orders of magnitude improvement in the performance of new generation deep UV photonic devices. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2965613͔ AlN has a complete solid solubility with GaN ͑Ref. 1͒ and emerged as an important semiconductor material for applications of light emitters down to 200 nm ͑Ref. 2͒ and detectors in the deep ultraviolet ͑DUV͒ and extreme UV spectral region. 3,4 Quantum wells ͑QWs͒ have been the device structure of choice for efficient III-nitride semiconductor based light emitters. 5,6 Conventional nitride c-plane multiple QW structures generate fixed sheet charges at the interfaces due to the spontaneous and piezoelectric polarization. 7-10 which induce internal electric fields, lead to carrier separation, and reduce the radiative recombination rate. Consequently, optoelectronic devices such as light emitting diodes and laser diodes based on heteroepitaxial c-plane oriented III-nitride materials possess reduced internal quantum efficiencies.There has been tremendous effort in the investigation of nonpolar a-plane and m-plane III-nitride epilayers 11 and optoelectronic devices with QW structures 9,10 to reduce the effects of internal electric fields. To achieve QW and other heterostructure based devices with improved performance, optical properties of epilayers grown on different orientations of available substrates have to be better understood. Furthermore, most previous studies on the fundamental band structures of AlN have been carried out for heteroepilayers grown on sapphires, which are plagued by lattice mismatch induced strain and high threading dislocation density, which significantly inhibited our ability for precisely determining the fundamental band structure parameters of AlN. In this work, we report a systematic comparative study of optical properties of both homoepitaxial and heteroepitaxial layers of AlN grown on polar c-plane as well as a-plane and m-plane orientations probed by DUV photoluminescence ͑PL͒.AlN bulk single crystal substrates were produced by sublimation crystal growth using polycrystalline AlN wafer as seeds and have a thickness of about 1 mm and an average grain size of about 2 ϫ 3 mm 2 . 12 The surface of AlN bulk crystal substrates was prepared by chemical mechanical polishing ͑done by NovaSiC͒ that provided a surface roughness of about 1 -4 nm. The dislocation density i...

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confidence: 77%

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confidence: 99%

Photoluminescence properties of AlN homoepilayers with different orientations

Sedhain

Nepal

Nakarmi

et al. 2008

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“…The PL emission at 3.2 eV is a DAP type transition involving a shallow donor and a V Al 3−/2− deep acceptor, which is observed as a 3.4 eV line in actual measurement due to some coulomb energy on the order of ϳ0.2 eV. 14,16 Note that the V Al 3− state is unable to capture any more electrons and is only able to participate in the where D 0 represents a shallow donor level. The presence of oxygen impurities tends to create more V Al as the formation energy ͑E form ͒ is reduced.…”

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confidence: 99%

The origin of 2.78 eV emission and yellow coloration in bulk AlN substrates

Sedhain

Du²,

Edgar³

et al. 2009

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The yellow color of bulk AlN crystals was found to be caused by the optical absorption of light with wavelengths shorter than that of yellow. This yellow impurity limits UV transparency and hence restricts the applications of AlN substrates for deep UV optoelectronic devices. Here, the optical properties of AlN epilayers, polycrystalline AlN, and bulk AlN single crystals have been investigated using photoluminescence ͑PL͒ spectroscopy to address the origin of this yellow appearance. An emission band with a linewidth of ϳ0.3 eV ͑at 10 K͒ was observed at ϳ2.78 eV. We propose that the origin of the yellow color in bulk AlN is due to a band-to-impurity absorption involving the excitation of electrons from the valence band to the doubly negative charged state, ͑V Al 2− ͒, of isolated aluminum vacancies, ͑V Al ͒ 3−/2− described by V Al 2− +h =V Al 3− +h + . In such a context, the reverse process is responsible for the 2.78 eV PL emission.

“…3 Although photoluminescence ͑PL͒ or cathodoluminescence ͑CL͒ investigation has shown some characteristic luminescence peaks related to deep electronic levels in AlGaN and AlN, [3][4][5][6][7] electrical characterization on deep levels in AlGaN or AlGaN/GaN heterostructure has been reported only a few times. [8][9][10][11] In particular, the deep-level transient spectroscopy ͑DLTS͒ method only detected deep levels with activation energies under 0.9 eV, 10 in spite of the fact that a near-midgap level can act as a dominant recombination center rather than a level near conduction or valence band.…”

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confidence: 99%

“…Thus, the present study indicates that the AlGaN layer includes, at least, a nearmidgap level with an energy range between 1.5 and 2.3 eV from the bottom of the conduction band. Several groups have reported deep-level peaks in PL and CL spectra for AlGaN and AlN, [3][4][5][6][7] which are related to complex defects based on the III-column vacancies ͑V Al and V Ga ͒. In particular, V Al -oxygen defects have been considered to be the most possible origin for a deep-level-related luminescence observed in AlN.…”

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confidence: 99%

Near-midgap deep levels in Al0.26Ga0.74N grown by metal-organic chemical vapor deposition

Sugawara

Kotani

Hashizume

2009

A deep level with an activation energy of 1.0 eV in n-type Al 0.26 Ga 0.74 N grown by metal-organic chemical vapor deposition was detected by deep-level transient spectroscopy ͑DLTS͒ with a sampling time window of several seconds. The deep-level density was 6 ϫ 10 15 cm −3 . At the temperatures around which the DLTS peaks were observed, capacitance transient was measured. Under the dark condition, a capacitance increase was observed, corresponding to the thermal emission of electrons from the level with 1.0 eV activation energy. After that, we observed a large capacitance increase under illumination with 2.3 eV photon energy. On the basis of potential simulation taking account of deep levels, we found that the photoinduced capacitance change arose from electron emission from additional near-midgap levels in energy ranging from E C − 1.5 to E C − 2. Deep levels in AlGaN induce not only serious degradation, such as drain current instability and lack of long-term reliability, in AlGaN/GaN-based transistors 1,2 but also significant reduction in light-emitting efficiency in ultraviolet light-emitting diodes and laser diodes using AlGaN materials. 3 Although photoluminescence ͑PL͒ or cathodoluminescence ͑CL͒ investigation has shown some characteristic luminescence peaks related to deep electronic levels in AlGaN and AlN, 3-7 electrical characterization on deep levels in AlGaN or AlGaN/GaN heterostructure has been reported only a few times. [8][9][10][11] In particular, the deep-level transient spectroscopy ͑DLTS͒ method only detected deep levels with activation energies under 0.9 eV, 10 in spite of the fact that a near-midgap level can act as a dominant recombination center rather than a level near conduction or valence band. To detect near-midgap levels in AlGaN, a temperature range should extend to 800 K in a typical DLTS measurement condition. In such very high temperatures, a Schottky or a p-n junction suffers from serious leakage and thermal junction breakdown. In this letter, we characterize near-midgap deep levels in AlGaN grown by metal-organic chemical vapor deposition ͑MOCVD͒ using a combination of DLTS and photoassisted capacitance transient methods.We used a Si-doped n-Al 0.26 Ga 0.74 N layer with a thickness of 1.0 m grown at 1000°C on a sapphire substrate by MOCVD. Hall measurements determined the typical values of electron concentration and mobility of the AlGaN layer at room temperature ͑RT͒ to be 2 ϫ 10 17 cm −3 and 100 cm 2 / V s, respectively. As a ring-shaped Ohmic contact, a Ti/Al/Ti/Au multilayer structure ͑20/50/20/150 nm͒ was deposited on the AlGaN surface, followed by an annealing at 800°C for 1 min in N 2 ambient. A circular Schottky electrode with a diameter of 200 m was fabricated at the center of the Ohmic ring by an electron-beam deposition of Ni.To try to detect deep levels with large activation energies in a temperature range as low as possible in the DLTS measurement, a time window of up to several seconds was used with an optimized signal sampling system and a temperaturesweeping rate...