Photoluminescence properties of AlN homoepilayers with different orientations

Abstract

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...

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

“…This explanation is consistent with the previously reported results of transparent AlN, in which a ͑V Al -2O N ͒ 1− related absorption band around 4.7 eV emerged, while the 2.8 eV band completely disappeared in samples containing the highest oxygen concentrations ͑1.2ϫ 10 21 cm −3 ͒. 8,9 In summary, we have carried out optical studies on different AlN samples by PL measurement to address the question of why bulk AlN crystals appear yellow in color. We propose that the absorption band centered around 2.8 eV, which corresponds to the excitation of electrons from the valence band to the V Al 2− state of ͑V Al ͒ 3−/2− , is responsible for the yellowish color of bulk AlN substrates.…”

“…The AlN epilayer and polycrystalline AlN exhibit dominant free exciton ͑FX͒ emission peaks at 5.98 and 5.96 eV, respectively. 8,12,13 However, the FX transition at 5.95 eV and broad emission lines at 4.0 and 2.76 eV have comparable emission intensities in bulk m-AlN. The impurity band around 4.0 eV was previously identified as a donor-acceptor-pair type transition involving a shallow donor and an aluminum vacancy ͑V Al ͒ complex with 2/1 negative charges, either 14 Figure 1͑b͒ compares the 10 K PL spectra of the same set of samples, which shows an obvious blue shift of the FX line to 6.06 ͑6.04͒ eV in the AlN epilayer sample ͑polycrystalline AlN͒.…”

“…Samples studied included c-AlN epilayers grown on sapphire by metal organic chemical vapor deposition, polycrystalline c-textured AlN grown by physical vapor transport, and bulk m-AlN single crystals produced by sublimation crystal growth using polycrystalline AlN wafers as seeds. 8 The epilayer, polycrystalline, and bulk AlN samples appear in transparent, light yellow, and amber colors under room light. These samples were determined to contain about 1.5ϫ 10 17 , 1.0ϫ 10 18 , and 2 ϫ 10 18 cm −3 of oxygen impurities, respectively, via SIMS measurements.…”

“…Currently, several groups are developing various techniques for the growth of bulk AlN crystals. [1][2][3][4][5][6][7][8] Recent reports of homoepitaxial layer growth and device fabrication using bulk AlN substrates have already demonstrated potential applications. [6][7][8] Despite recent progress made in nitride epilayers and devices on AlN substrates, critical issues remain.…”

Negative spin-exchange splitting in the exciton fine structure of AlN

“…From samples with non-and semipolar surfaces, however, energies at or below 6.035 eV are found. [16][17][18] These differences cannot be explained by changes in strain and/or impurity concentrations alone. In the following, we will show that a natural and physical interpretation is possible in terms of spin-exchange splitting by distinguishing between C 1 and C 5 excitons.…”