Nature of low-energy optical emission in doped AlGaN/GaN heterostructures (original) (raw)
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Optical characterization of AlN/GaN heterostructures
2003
AlN/GaN/sapphire heterostructures with AlN gate film thickness of 3-35 nm are characterized using photoreflectivity ͑PR͒ and photoluminescence ͑PL͒ spectroscopy. Under a critical AlN film thickness, the luminescence from the GaN channel layer near the interface proves to be excitonic. No luminescence related to the recombination of the two-dimensional electron gas ͑2DEG͒ is observed, in spite of high 2DEG parameters indicated by Hall-effect measurements. The increase of the AlN gate film thickness beyond a critical value leads to a sharp decrease in exciton resonance in PR and PL spectra as well as to the emergence of a PL band in the 3.40-3.45 eV spectral range. These findings are explained taking into account the formation of defects in the GaN channel layer as a result of strain-induced AlN film cracking. A model of electronic transitions responsible for the emission band involved is proposed.
Photoluminescence and persistent photoconductivity of AlxGa1-xN/GaN heterostructures
Applied Physics A, 2007
We have investigated the optical properties of Al x Ga 1−x N/GaN heterostructures (x = 0.08, 0.15, 0.33) grown by metal organic chemical vapor deposition on sapphire using photoluminescence (PL) and persistent photoconductivity (PPC) measurements. For the Al x Ga 1−x N/GaN heterostructures (HS) containing high Al composition, we observed an anomalous temperature-dependent photoluminescence and persistent photoconductivity effects. These results show a strong dependence of the physical properties of Al x Ga 1−x N/GaN HS on the Al content and layer thickness. The anomalous temperature-dependent PL is usually attributed to the presence of carrier localization states. These phenomena are explained based on the alloy compositional fluctuations in the Al x Ga 1−x N/GaN HS. From the PPC measurements, the photocurrent (PC) quenching was observed for Al x Ga 1−x N/GaN HS and it is explained by the metastable states formed in the underlying GaN layer. Also, the mechanisms behind the PC quenching and PPC phenomena are explained in detail.
Analysis of defect related optical transitions in biased AlGaN/GaN heterostructures
Materials Science in Semiconductor Processing, 2010
The optical transitions in AlGaN/GaN heterostructures that are grown by metalorganic chemical vapor deposition (MOCVD) have been investigated in detail by using Hall and room temperature (RT) photoluminescence (PL) measurements. The Hall measurements show that there is two-dimensional electron gas (2DEG) conduction at the AlGaN/GaN heterointerface. PL measurements show that in addition to the characteristic near-band edge (BE) transition, there are blue (BL) and yellow luminescence (YL) bands, freeexciton transition (FE), and a neighboring emission band (NEB). To analyze these transitions in detail, the PL measurements were taken under bias where the applied electric field changed from 0 to 50 V/cm. Due to the applied electric field, band bending occurs and NEB separates into two different peaks as an ultraviolet luminescence (UVL) and Y 4 band. Among these bands, only the yellow band is unaffected with the applied electric field. The luminescence intensity change of these bands with an electric field is investigated in detail. As a result, the most probable candidate of the intensity decrease with an increasing electric field is the reduction in the radiative lifetime.
Materials Science and …, 1997
We report on photoluminescence (PL) experiments performed on a n-type modulation doped GaN/A1,.,,Ga,,,5N heterojunction grown by low pressure metal organic chemical vapor deposition (MOCVD) technique. In the temperature range 7.4-80 K, we have investigated (i) the photoluminescence spectra from the degenerate 2D electron gas confined near the GaN/AlGaN interface> (ii) the impurity and band edge photoluminescence of the 3D undoped region of the GaN layer. The experiments were performed in the following manner: we excited the sample at 333.6 nm in two ways, on the AlGaN barrier (2D channel side) and on the substrate side. The comparison between the corresponding spectra and the temperature dependence of lines-intensity allowed us to identify without ambiguity the 2D and 3D character of the different lines. We find a very broad line which could come from the first E, subband transitions and at a higher ener,T two lines with which intensities increase with temperature. We attributed those lines to excitonic transitions associated to the second subband. Between E, and these 2D excitonic lines we observed a very intense line with which intensity decreased rapidly when temperature increased. The intensity of this line behave typically Iike the one of a transition at the Fermi energy of the 2D degenerate electron gas (Fermi edge singularity). 0 1997 Elsevier Science S.A.
Energies
An electroluminescence (EL) phenomenon in unipolar-doped GaN/AlN/GaN double-barrier heterostructures—without any p-type contacts—was investigated from 4.2 K to 300 K. In the range of 200–300 K, the extracted peak photon energies agree with the Monemar formula. In the range of 30 to 200 K, the photon energies are consistent with A-exciton emission. At 4.2 K, the exciton type likely transforms into B-exciton. These studies confirm that the EL emission comes from a cross-bandgap (or band-to-band) electron-hole radiative recombination and is excitonic. The excitons are formed by the holes generated through interband tunneling and the electrons injected into the GaN emitter region of the GaN/AlN heterostructure devices.
Deep ultraviolet photoluminescence of Tm-doped AlGaN alloys
Applied Physics Letters, 2009
The ultraviolet ͑UV͒ photoluminescence ͑PL͒ properties of Tm-doped Al x Ga 1−x N ͑0.39Յ x Յ 1͒ alloys grown by solid-source molecular beam epitaxy were probed using above-bandgap excitation from a laser source at 197 nm. The PL spectra show dominant UV emissions at 298 and 358 nm only for samples with x = 1 and 0.81. Temperature dependence of the PL intensities of these emission lines reveals exciton binding energies of 150 and 57 meV, respectively. The quenching of these UV emissions appears related to the thermal activation of the excitons bound to rare-earth structured isovalent ͑RESI͒ charge traps, which transfer excitonic energy to Tm 3+ ions resulting in the UV emissions. A model of the RESI trap levels in AlGaN alloys is presented.
Photoluminescence properties of in situ Tm-doped AlxGa1−xN
Applied Physics Letters, 2003
We report on the photoluminescence ͑PL͒ properties of in situ Tm-doped Al x Ga 1Ϫx N films (0рx р1) grown by solid-source molecular-beam epitaxy. It was found that the blue PL properties of Al x Ga 1Ϫx N:Tm greatly change as a function of Al content. Under above-gap pumping, GaN:Tm exhibited a weak blue emission at ϳ478 nm from the 1 G 4 → 3 H 6 transition of Tm 3ϩ. Upon increasing Al content, an enhancement of the blue PL at 478 nm was observed. In addition, an intense blue PL line appeared at ϳ465 nm, which is assigned to the 1 D 2 → 3 F 4 transition of Tm 3ϩ. The overall blue PL intensity reached a maximum for xϭ0.62, with the 465 nm line dominating the visible PL spectrum. Under below-gap pumping, AlN:Tm also exhibited intense blue PL at 465 and 478 nm, as well as several other PL lines ranging from the ultraviolet to near-infrared. The Tm 3ϩ PL from AlN:Tm was most likely excited through defect-related complexes in the AlN host.
Two-Dimensional Electron Gas Recombination in Undoped AlGaN/GaN Heterostructures
Japanese Journal of Applied Physics, 2004
The radiative recombination of a two-dimensional electron gas (2DEG) was investigated in Al0.30Ga0.70N/GaN single heterostructures (SHs) without intentionally doping the barrier material, i.e., where the 2DEG appears at the interface due only to polarization effects. In addition to the typical excitonic transitions and the LO-phonon replicas originating from the GaN flat-band region, the photoluminescence spectra displayed three well-defined transitions. Their small binding energies and the observed blue shift with the excitation density suggested the association of these new emissions to quasi-2D excitons. On the basis of the thermal and excitation power dependences, the transitions were assigned to interface excitonic lines. Applying a weak electric field parallel to the growth direction, which depletes the triangular well, corroborated the 2DEG nature.