X-ray reciprocal lattice mapping and photoluminescence of GaN/GaAlN Multiple Quantum Wells; strain induced phenomena.
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Internet Journal Nitride Semiconductor Research
X-ray reciprocal lattice mapping and photoluminescence of GaN/GaAlN Multiple Quantum Wells; strain induced phenomena. R. Langer1, J Simon1, O. Konovalov1, N. Pelekanos1, A. Barski1 and M. Leszczy˜ski2 1CEA/Grenoble, 2High
Département de Recherche Fondamentale sur la Matière Condensée/SP2M, Pressure Research Center,
(Received Tuesday, July 21, 1998; accepted Monday, October 26, 1998)
Structural properties of GaN/GaAlN multiple quantum wells (MQW) grown by nitrogen plasma assisted MBE on MOCVD-grown GaN/sapphire (GaN pseudosubstrates) have been characterised by X-ray reciprocal lattice mapping to determine the strain and composition of ternary alloys. The results clearly demonstrate that the barriers of GaAlN with up to 17% of aluminium content grown by plasma assisted MBE on GaN are fully strained. Optical properties have been characterised by low temperature photoluminescence. Photoluminescence emission peaks corresponding to the GaN/ GaAlN MQW structures revealed strong red-shift with respect to the GaN energy gap. This can be explained by a strong internal electric field present in the QW’s which is attributed to a transfer of piezoelectric field due to Fermi-level alignment.
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Introduction
GaAlN/GaN quantum wells (QW’s) are subject of current investigations due to their potential for electronic and optoelectronic applications. Many publications report on the characterisation of MOCVD or MBE grown GaN/AlN and GaN/GaAlN superlattices and/or multi-quantum well (MQW) structures [1] [2] [3] [4]. Recently, a strong piezoelectric field has been evidenced in GaN/GaAlN heterostructures [5] [6] and attributed to strain related phenomena. In this work, we carefully characterise the strain configuration in GaN/GaAlN MQW’s and based on that we analyse their optical emission spectra which seem to be dominated by piezoelectric effects. 2
Experimental
Growth of nitride films was performed in a RIBER MBE 2300 chamber. Standard Knudsen cells have been used for gallium and aluminium evaporation and the active nitrogen was generated by a commercial (EPI Unibulb) radio frequency (RF) cell. Samples were fixed with indium on a molybdenum sample holder. Growth was performed at a relatively low substrate temperature of 650°C, controlled by an infrared pyrometer. The composition of the GaAlN films was adjusted by varying the beam equivalent pressure of the gallium and aluminium
fluxes. The thickness of the layers was controlled by RHEED oscillations. MBE growth was performed on 1.5 µm thick GaN layers grown by MOCVD on sapphire. The oxide on this MOCVD grown GaN surface can easily be removed by acid etching and heating prior to growth. A well defined RHEED pattern and intense RHEED oscillations show that the treated MOCVD surface is smooth on an atomic scale [7]. To completely get rid of the eventual influence from surface oxide or from a contamination layer, we interpose a few 1000 Å thick GaN layer. Finally, high resolution X-ray diffraction was performed on a 4-circle goniomete
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