| 1. |
- Arnaudov, B, et al.
(författare)
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Energy position of near-band-edge emission spectra of InN epitaxial layers with different doping levels
- 2004
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 69:11
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Tidskriftsartikel (refereegranskat)abstract
- We studied the shape and energy position of near-band-edge photoluminescence spectra of InN epitaxial layers with different doping levels. We found that the experimental spectra of InN layers with moderate doping level can be nicely interpreted in the frames of the "free-to-bound" recombination model in degenerate semiconductors. For carrier concentrations above n>5x10(18) cm(-3) the emission spectra can also be modeled satisfactorily, but a contribution due to a pushing up of nonequilibrium holes over the thermal delocalization level in the valence band tails should be considered in the model. The emission spectra of samples with low doping level were instead explained as a recombination from the bottom of the conduction band to a shallow acceptor assuming the same value of the acceptor binding energy estimated from the spectra of highly doped samples. Analyzing the shape and energy position of the free-electron recombination spectra we determined the carrier concentrations responsible for the emissions and found that the fundamental band gap energy of InN is E-g=692+/-2 meV for an effective mass at the conduction-band minimum m(n0)=0.042m(0).
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| 2. |
- Arnaudov, B., et al.
(författare)
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Free-to-bound radiative recombination in highly conducting InN epitaxial layers
- 2004
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Ingår i: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 36:4-6, s. 563-571
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Tidskriftsartikel (refereegranskat)abstract
- We present a theoretical simulation of near-band-edge emission spectra of highly conducting n-InN assuming the model of 'free-to-bound' radiative recombination (FBRR) of degenerate electrons from the conduction band with nonequilibrium holes located in the valence band tails. We also study experimental photoluminescence (PL) spectra of highly conducting InN epitaxial layers grown by MBE and MOVPE with electron concentrations in the range (7.7 × 1017-6 × 1018) cm-3 and find that the energy positions and shape of the spectra depend on the impurity concentration. By modeling the experimental PL spectra of the InN layers we show that spectra can be nicely interpreted in the framework of the FBRR model with specific peculiarities for different doping levels. Analyzing simultaneously the shape and energy position of the InN emission spectra we determine the fundamental bandgap energy of InN to vary between Eg = 692 meV for effective mass mn0 = 0.042m0 and Eg =710 meV for mn0 = 0.1m0. © 2004 Elsevier Ltd. All rights reserved.
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| 3. |
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| 4. |
- Bergman, Peder, et al.
(författare)
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Photoluminescence and electroluminescence characterization of InxGa1-x/InyGa1-yN multiple quantum well light emitting diodes
- 2002
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Ingår i: Materials Science Forum, Vols. 389-393. ; , s. 1493-1496
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Konferensbidrag (refereegranskat)abstract
- We report on a study of radiative recombination in In0.11Ga0.89N/In.0.01Ga0.99N multiple quantum wells (MQWs). The QWs were nominally undoped, while the InGaN barriers were Si doped. The MQW part is situated in the depletion field of a pn-junction structure with electrical contacts, so that both photoluminescence (PL) and electroluminescence (EL) can be studied as a function of bias. The PL and EL spectra are distinctly different, in particular at low temperatures. The spectral properties and related differences in PL decay times reflect different recombination conditions in the MQW region for the individual QWs.
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| 5. |
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| 6. |
- Buyanova, Irina, 1960-, et al.
(författare)
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Electronic structure of the 0.88-eV luminescence center in electron-irradiated gallium nitride
- 1999
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Ingår i: Physical review. B, Condensed matter and materials physics. ; 60:3, s. 1746-1751
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Tidskriftsartikel (refereegranskat)abstract
- Photoluminescence (PL) spectroscopy is employed to determine the nature of a near-infrared PL emission with a no-phonon line at ∼0.88 eV, commonly present in electron-irradiated GaN. This PL emission is suggested to originate from an internal transition between a moderately shallow excited state (with an ionization energy ∼21 meV) and the deep ground state (with an ionization energy ∼900 meV) of a deep defect. The existence of a higher-lying second excited state related to the 0.88-eV PL center is also shown from temperature-dependent studies. A different electronic character of the wave functions related to the first and second excited states has been revealed by PL polarization measurements. Since the PL emission has been observed with comparable intensity in all electron-irradiated GaN samples independent of doping on the starting material, it is proposed that either native defects, or common residual contaminants or their complexes are involved. The substitutional ON donor (or related complex) is considered as the most probable candidate, based on the observed striking similarity in the local vibrational properties between the 0.88-eV PL centers and the substitutional OP donor in GaP.
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| 7. |
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| 8. |
- Chen, Weimin, 1959-, et al.
(författare)
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Optical and Microwave Double Resonance of III-nitrides
- 1999
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Ingår i: Joint International Meeting the 196th Meeting of The Electrochemical Society ECS and the 1999 Fall Meeting of The Electrochemical Society of Japan ECSJ,1999. ; , s. 764-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
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| 9. |
- Chen, Weimin, 1959-, et al.
(författare)
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Role of the Substitutional Oxygen Donor in the Residual n-type Conductivity in GaN
- 1999
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Konferensbidrag (refereegranskat)abstract
- A detailed photoluminescence (PL) study reveals a striking similarity in local vibrational properties of a defect center in GaN as compared to that for the substitutional OP donor in GaP. This observation could be interpreted as if the center is in fact related to the substitutional oxygen donor in GaN. The deep-level nature experimentally determined for the defect center calls for caution of a commonly referred model that the substitutional oxygen donor is responsible for the residual n-type conductivity in GaN.
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| 10. |
- Darakchieva, Vanya, 1971-, et al.
(författare)
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Phonon mode behavior in strained wurtzite AlN/GaN superlattices
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 71:11, s. 115329-
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Tidskriftsartikel (refereegranskat)abstract
- We have studied phonons in AlN/GaN superlattices with different periods but a constant well-to-barrier ratio using a combination of infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The strain evolution in the superlattice structures is assessed by high-resolution x-ray diffraction and reciprocal space mapping. We have identified E1(TO), A 1(LO) and E2 localized, and E1(LO) and A 1(TO) delocalized superlattice modes. The dependencies of their frequencies on in-plane strain are analyzed and discussed, and the strain-free frequencies of the superlattice modes are estimated. A good agreement between theory and experiment is found in the case of GaN localized modes, while large deviations between theoretically estimated and experimentally determined frequency shifts are observed for the AlN localized modes. The delocalization effect on the A1(TO) and E1(LO) phonons, as well as the free-carrier effect on the E1(LO) phonon are also discussed. ©2005 The American Physical Society.
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| 11. |
- Esmaeili, M, et al.
(författare)
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Experimental and theoretical investigations of optical properties of GaN/AlGaN MQW nanostructures. Impact of built-in polarization fields
- 2009
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Ingår i: OPTO-ELECTRONICS REVIEW. - : Polish Academy of Sciences Chancellery. - 1230-3402 .- 1896-3757. ; 17:4, s. 293-299
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Tidskriftsartikel (refereegranskat)abstract
- We report the results from detailed optical spectroscopy from MOCVD grown GaN/AlGaN multiple quantum wells (MQWs), as opposed to most previous studies where MBE was employed by means of photoluminescence (PL) technique. In this paper we will present theoretical and experimental results demonstrating how polarization induced electric fields and bound interface charges in GaN/AlGaN MQWs affect the emission peak energy, PL line shape, as well as the emission line width. Theoretically estimated fields in this work are consistent with experimental data. Transition energy of the heavy hole and electron ground state Ee-hh in GaN/AlGaN MQWs were calculated and it is found that it stays in good agreement with the experimental data.
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| 12. |
- Esmaeili, M., et al.
(författare)
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Photoluminescence study of MOCVD-grown GaN/AlGaN MQW nanostructures : Influence of Al composition and Si doping
- 2007
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 18:2
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Tidskriftsartikel (refereegranskat)abstract
- A detailed study of low-temperature photoluminescence (PL) in GaN/AlGaN multiple quantum well (MQW) nanostructures has been reported. We have investigated the effect of Si doping and Al content on PL spectra and PL decay time of these structures. The temperature dependence of radiative as well as non-radiative lifetimes have been evaluated between 2K and room temperature for different Si doping. We found that radiative recombination at higher temperatures even up to RT is stronger in the doped sample, compared to the undoped one. Hole localization in GaN/AlGaN MQWs with different compositions of Al is demonstrated via PL transient decay times and LO phonon coupling. It is found that there is an increasing of the decay time at the PL peak emission with increasing Al composition. For the undoped sample, a non-exponential PL decay behaviour at 2K is attributed to localized exciton recombination. A slight upshift in QWs PL peak with increasing Al composition is observed, which is counteracted by the expected rise of the internal QW electric field with increasing Al. The localization energies have been evaluated by studying the variation of the QW emission versus temperature and we found out that the localization energy increases with increasing Al composition. © IOP Publishing Ltd.
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| 13. |
- Hai, P. N., et al.
(författare)
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Ga-related defect in as-grown Zn-doped GaN : An optically detected magnetic resonance study
- 2000
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 62:16, s. R10607-R10609
-
Tidskriftsartikel (refereegranskat)abstract
- A detailed study of as-grown Zn-doped GaN employing optically detected magnetic resonance (ODMR) spectroscopy is presented. Besides the well-known ODMR spectra of an effective-mass-like donor and Zn acceptor, a positive ODMR signal of an S=1 / 2 paramagnetic center was observed when monitoring the dominating blue luminescence band peaking at 2.8 eV. The involvement of a single Ga nucleus in the defect center is revealed from the rather well-resolved hyperfine interactions involving the isotopes 71Ga (39.9%) and 69Ga (60.1%), both with nuclear spin I=3 / 2. The C3v symmetry, the hyperfine interaction and the defect formation suggest a Ga-related complex nature of this center.
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| 14. |
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| 15. |
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| 16. |
- Harati Zadeh, Hamid, 1964-, et al.
(författare)
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Optical observation of discrete well width fluctuations in wide band gap III-nitride quantum wells
- 2007
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Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 244:5, s. 1727-1734
-
Tidskriftsartikel (refereegranskat)abstract
- A detailed observation of discrete well width fluctuations via localized excitons in the photoluminescence (PL) spectra of MOCVD-grown undoped GaN/Al0.07Ga0.93 N multiple quantum wells (MQWs) has been reported. Doublet excitonic features with a distance varying between 10 and 25 meV for different well widths (1.5 to 4.5 nm) are observed in the PL spectra. They are explained in terms of discrete well width variations by one c-lattice parameter, i.e. two GaN monolayers. By mapping the PL measurements across the samples with different excitation spot size, it is shown that the extension of areas with a constant well width is less than 1 μm2. TEM pictures give evidence of interface roughness, although the contrast is weak at this low Al composition. In addition we observe a long-range variation of the PL peak position across the sample, interpreted as a fluctuation in Al composition in the barriers. The residual broadening of an excitonic peak (apart from the splitting related to well width fluctuations) is about 10 meV, somewhat larger for larger well widths, and is mainly ascribed to hole localisation potentials in the QWs. Additional broadening occurs in the MQWs due to inequivalent properties of each QW within the excitation spot. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.
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| 17. |
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| 18. |
- Harati Zadeh, Hamid, 1964-, et al.
(författare)
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Photoluminescence study of Si-doped GaN/Al0.07Ga0.93N multiple quantum wells with different dopant position
- 2004
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 84:25, s. 5071-5073
-
Tidskriftsartikel (refereegranskat)abstract
- The Si-doped GaN/Al0.07Ga0.93N multiple quantum wells (MQW) were investigated, using photoluminescence (PL) and time-resolved (PL) measurements. The influence of Si doping on the emission energy and recombination dynamics of the MWQs were also investigated, with different dopant position in the wells. It was observed that the redshifted emission of the MQWs was attributed to the self-energy shift of the electron states due to the correlated motion of the electrons exposed to the fluctuating potential of the donor ions. It was also observed that the PL decay time of the sample was ∼760 ps, at low temperature.
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| 19. |
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| 20. |
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| 21. |
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| 22. |
- Haratizadeh, H., et al.
(författare)
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Time resolved photoluminescence study of Si modulation doped GaN/Al 0.07Ga0.93N multiple quantum wells
- 2004
-
Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 241:5, s. 1124-1133
-
Tidskriftsartikel (refereegranskat)abstract
- The effects of the Si doping level on the recombination dynamics and carrier (exciton) localization in modulation doped GaN/Al0.07Ga 0.93N multiple-quantum-well (MQW) structures were studied by means of photoluminescence (PL) and time-resolved PL measurements. All samples with different doping levels show a QW emission which is blue shifted with respect to the 3.48 eV PL peak from the GaN buffer layer. The decay time at the peak position remains nearly constant in the range of 320-420 ps at 2 K for all doping levels. For the undoped and low-doped samples (3 × 1018 cm-3), which have less free electrons in the QWs, a non-exponential PL decay behaviour at 2 K is attributed to localized exciton recombination. The more highly doped samples (5 × 1018 cm-3 to 10 20 cm-3) show almost exponential decay curves at 2 K, suggesting the recombination of free electrons and localized holes. This localization effect appears even at high electron concentrations to cancel the expected lowering of the radiative lifetime with doping at 2 K, such a lowering is clearly observed at elevated temperatures for the highly doped samples, however. The internal polarization-induced fields of the medium and highly-doped samples are partly screened by the electrons originating from the doping in the barriers. Only the PL peak of the undoped and low-doped samples shows a redshift with time delay, related to the photogenerated carriers. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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| 23. |
- Kamiyama, S., et al.
(författare)
-
Extremely high quantum efficiency of donor-acceptor-pair emission in N-and-B-doped 6H-SiC
- 2006
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 99:9
-
Tidskriftsartikel (refereegranskat)abstract
- High-efficiency visible light emission in N-and-B-doped 6H-SiC epilayers was observed in photoluminescence measurements at room temperature. The orange-yellow light emission due to the recombination of donor-acceptor pairs (DAPs) has a broad spectrum with a peak wavelength of 576 nm and a full width at half maximum of 110 nm at 250 K. The high B concentration of more than 1018 cm-3 improves the emission efficiency of the DAP recombination at a high temperature. Compared with the photoluminescence spectrum of GaN at 10 K, a high quantum efficiency of 95% was estimated for the highly B-doped sample. From time-resolved photoluminescence measurements, a DAP recombination time of 5.0 ms was obtained, which is in good agreement with the calculated value by the rate equation with the assumption of a 95% internal quantum efficiency. This is quite promising as a light-emitting medium by optical pumping, as well as monolithic light sources combined with nitride-based light-emitting diodes grown on the DA-doped SiC epilayer. © 2006 American Institute of Physics.
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| 24. |
- Kasic, A., et al.
(författare)
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Phonons and free-carrier properties of binary, ternary, and quaternary group-III nitride layers measured by infrared Spectroscopic Ellipsometry
- 2003
-
Ingår i: Physica Status Solidi. C, Current topics in solid state physics. - : Wiley. - 1610-1634 .- 1610-1642. ; 0:6 SPEC. ISS., s. 1750-1769
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- This work reviews recent ellipsometric investigations of the infrared dielectric functions of binary, ternary, and quaternary group-III nitride films. Spectroscopic Ellipsometry in the mid-infrared range is employed for the first time to determine phonon and free-carrier properties of individual group-III nitride heterostructure components, including layers of some ten nanometer thickness. Assuming the effective carrier mass, the free-carrier concentration and mobility parameters can be quantified upon model analysis of the infrared dielectric function. In combination with Hall-effect measurements, the effective carrier masses for wurtzite n- and p-type GaN and n-type InN are obtained. The mode behavior of both the E1(TO) and A1(LO) phonons are determined for ternary compounds. For strain-sensitive phonon modes, the composition and strain dependences of the phonon frequencies are differentiated and quantified. Information on the crystal quality and compositional homogeneity of the films can be extracted from the phonon mode broadening parameters. A comprehensive IR dielectric function database of group-III nitride materials has been established and can be used for the analysis of complex thin-film heterostructures designed for optoelectronic device applications. Information on concentration and mobility of free carriers, thickness, alloy composition, average strain state, and crystal quality of individual sample constituents can be derived. © 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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| 25. |
- Monemar, Bo, et al.
(författare)
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A hydrogen-related shallow donor in GaN?
- 2006
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Ingår i: Physica. B, Condensed matter. - : Elsevier BV. - 0921-4526 .- 1873-2135. ; 376, s. 460-463
-
Tidskriftsartikel (refereegranskat)abstract
- We present photoluminescence (PL) data for deliberately O-doped, high-resistive GaN samples where a new shallow donor-bound exciton (DBE) peak at about 3.4746 eV (corrected for strain shift) at 2 K appears. This DBE is strongly enhanced upon annealing in the entire range 450-900 degrees C. The possible relation of this DBE to a metastable H donor state is discussed. (c) 2006 Elsevier B.V. All rights reserved.
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| 26. |
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| 27. |
- Monemar, Bo, 1942-, et al.
(författare)
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Dominant shallow acceptor related to oxygen and hydrogen in GaN
- 2006
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 376-377, s. 440-443
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Tidskriftsartikel (refereegranskat)abstract
- We present new photoluminescence (PL) data of deliberately O-doped and Mg-doped GaN layers grown by MOCVD. The combination of these data with positron annihilation spectroscopy (PAS) and SIMS results obtained on the same samples shows a clear correlation of the PL intensity of the acceptor related emissions at 3.466 and 3.27 eV (at 2 K) with O doping. The acceptor is stable upon annealing in N-2 in our highly resistive samples, while it is known be unstable in p-GaN. Our tentative conclusion is that this very commonly occurring acceptor is either a V-Ga-O-H complex or a second configuration of the Mg acceptor containing H.
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| 28. |
- Monemar, Bo, et al.
(författare)
-
Evidence for Two Mg Related Acceptors in GaN
- 2009
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Ingår i: PHYSICAL REVIEW LETTERS. - 0031-9007 .- 1079-7114. ; 102:23, s. 235501-
-
Tidskriftsartikel (refereegranskat)abstract
- The optical signatures of Mg-related acceptors in GaN have been revisited in samples specifically grown on bulk GaN templates, to avoid strain broadening of the optical spectra. Bound-exciton spectra can be studied in these samples for Mg concentrations up to [Mg]approximate to 2x10(19) cm(-3). Contrary to previous work it is found that instabilities in the photoluminescence spectra are not due to unstable shallow donors, but to unstable Mg-related acceptors. Our data show that there are two Mg-related acceptors simultaneously present: the regular (stable) substitutional Mg acceptor, and a complex acceptor which is unstable in p-GaN.
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| 29. |
- Monemar, Bo, et al.
(författare)
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Influence of depletion fields on photoluminescence of n-doped InGaN/GaN multiple quantum well structures
- 2002
-
Ingår i: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 192:1, s. 21-26
-
Tidskriftsartikel (refereegranskat)abstract
- We report on a detailed study of low temperature photoluminescence (PL) in InxGa1-xN multiple quantum wells (MQWs) with x in the range 0.1 and highly Si-doped barriers of In0.01Ga0.99N. The entire MQW structure was grown at 800 degreesC. One sample with 3 QWs of width 3.5 nm and barriers of width 10.5 nm had the MQW in the depletion region of the outer surface. Two PL peaks were observed, one QW exciton from the OW closest to the GaN buffer, one lower energy peak related to a 2DEG at the interface to the GaN buffer layer. In a second similar sample, 5 QWs of width 3 nm and with 6 nm highly Si-doped In0.01Ga0.99N barriers the MQW was placed in the n-side depletion region of a pn-junction, i.e. a complete LED structure with semitransparent top metallisation. Two PL peaks are observed also in this case, of similar origin as described above. With forward bias, this structure shows lower-energy PL emission, indicating the gradual activation of the other QWs closer to the pn-junction. At high forward bias the low-energy part of the PL spectrum becomes similar to the electroluminescence (EL) spectrum.
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| 30. |
- Monemar, Bo, 1942-, et al.
(författare)
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Influence of polarization fields and depletion fields on photoluminescence of AlGaN/GaN multiple quantum well structures
- 2003
-
Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 237:1, s. 353-364
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Tidskriftsartikel (refereegranskat)abstract
- We report on a detailed study of low temperature photoluminescence (PL) in Al0.07Ga0.93N/GaN multiple quantum wells (MQWs). The structures were grown on sapphire with the conventional low temperature AlN nucleation layer and thick GaN buffer layer. Several sets of 5 QW MQW samples were studied, one set with Si doping in the barriers up to or above the metallic limit. Nominally undoped MQW samples were also studied. The spectral behaviour of the doped samples was strongly affected by the near surface depletion field, causing overlap of different spectra from non-equivalent QWs. The QWs closest to the surface are presumably inactive in some samples, due to a very high depletion field. For the case of undoped samples, on the other hand, the near surface QWs are active and most prominent in the PL spectra. The structure from discrete well width variations is here resolved in the PL spectra. The results demonstrate that for structures with no additional capping layer both the depletion field and the polarisation fields need to be considered in the interpretation of experimental data. The theoretically estimated fields in this work are consistent with the experimental spectra. The presence of localisation even in the case of metallic samples, as observed by a constant PL decay time independent of doping, is discussed in terms of penetration of the hole wave functions into the AlGaN barriers. This localisation is also manifested in a sizeable LO phonon coupling strength in all samples studied.
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| 31. |
- Monemar, Bo, et al.
(författare)
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Mg related acceptors in GaN
- 2010
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Ingår i: Phys. Status Solidi C 7. ; , s. 1850-
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Konferensbidrag (refereegranskat)
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| 32. |
- Monemar, Bo, et al.
(författare)
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Optical characterization of III-nitrides
- 2002
-
Konferensbidrag (refereegranskat)abstract
- Recent developments in material properties of GaN and related heterostructure combinations are reviewed, with emphasis on optical data. We discuss recent polarized photoluminescence (PL) data on the free excitons in GaN, obtained from thick HVPE grown layers. The exchange splitting constant is found to be about 0.6 meV, a more accurate value than previous suggestions. The PL signatures of shallow donors and acceptors, i.e. the bound excitons, are discussed and tentatively identified. Intrinsic point defects are discussed in terms of stability and experimental signatures. Quantum well structures in the InGaN/GaN and GaN/AlGaN systems are briefly discussed, with emphasis on localization of carriers and excitons. © 2002 Published by Elsevier Science B.V.
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| 33. |
- Monemar, Bo, et al.
(författare)
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Optical characterization of InGaN/GaN MQW structures without in phase separation
- 2001
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Ingår i: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 228:1, s. 157-160
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Tidskriftsartikel (refereegranskat)abstract
- Photoluminescence and cathodoluminescence spectroscopies are used to investigate the properties of the band edge emission of InGaN/(In)GaN multiple quantum well (MQW) structures which do not show evidence of phase separation in high resolution electron microscopy. The data still show a clear low energy peak in the spectra. about 0.1 eV below the main exciton peak. Possible interpretations of this second peak are discussed.
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| 34. |
- Monemar, Bo, et al.
(författare)
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Optical investigation of AlGaN/GaN quantum wells and superlattices
- 2004
-
Ingår i: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 201:10, s. 2251-2258
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Tidskriftsartikel (refereegranskat)abstract
- We report a detailed optical study of several sets of multiple quantum wells (MQWs) in the AlGaN/GaN system, as well as AlN/GaN superlattice (SL) structures. In this study all materials were grown by MOCVD, as opposed to most previous studies where MBE was employed. In undoped MQWs discrete photoluminescence (PL) peaks related to discrete well width fluctuations by one full c lattice parameter are clearly observed. In doped samples this effect appears to be screened. While the recombination process in undoped samples is excitonic, in MQWs doped with Si above about 5 x 10(18) cm(-3) free electrons (a 2DEG) are dominant, and the PL process is a free electrons-localized hole transition at low temperatures. The hole localization prevails up to very high n-doping, as was previously observed in bulk GaN. The hole localization is demonstrated via several experiments, including results on PL transient decay times and LO phonon coupling. Near surface band bending, due mainly to dopant depletion in doped structures or interaction with surface states in case of higher Al content in barriers, influences the distribution of electron filling among the QWs, making a detailed modeling of the spectral shape somewhat ambiguous. It is found that AlN barriers promote a strong room temperature PL signal from the QWs, as opposed to the case with AlGaN barriers.
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| 35. |
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| 36. |
- Monemar, Bo, et al.
(författare)
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Optical properties of InGaN/GaN and AlGaN/GaN multiple quantum well structures
- 2003
-
Ingår i: Proceedings of SPIE, the International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. ; 5023, s. 63-67
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We report on low temperature photoluminescence (PL) in In xGa1-xN multiple quantum wells (MQWs) with x in the range 0.1 and highly Si doped barriers of In0.01Ga0.99N. One sample with 3 QWs of width 3.5 nm and barriers of width 10.5 nm had the MQW in the depletion region of the outer surface. Two PL peaks were observed, one QW exciton from the QW closest to the GaN buffer, one lower energy peak related to a 2DEG at the interface to the GaN buffer layer. In a second similar sample 5 QWs of width 3 nm and with 6 nm highly Si doped In0.01Ga 0.99N barriers the MQW was placed in the n-side depletion region of a pn-junction. At low temperatures the PL and electroluminescence (EL) spectra are quite different at no, low, or reverse bias, the PL appearing at higher energy. At high forward bias a spectral component at the EL position appears. This proves a strong influence of the depletion field on the optical spectra. Preliminary results are also reported for n-doped Al0.07Ga 0.93N/GaN structures, with near surface MQWs.
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| 37. |
- Monemar, Bo, 1942-, et al.
(författare)
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Optical signatures of dopants in GaN
- 2006
-
Ingår i: Materials Science in Semiconductor Processing. - : Elsevier BV. - 1369-8001 .- 1873-4081. ; 9:1-3, s. 168-174
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Tidskriftsartikel (refereegranskat)abstract
- The characteristic optical spectra for shallow donors and acceptors in GaN are discussed. The most accurate photoluminescence (PL) data are obtained from samples grown on freestanding GaN substrates, where strain shifts are absent and a low spectroscopic line width is obtained. Recent PL data for excitons bound to the O and Si donors are discussed in some detail, giving accurate values for the binding energies and excited bound donor states. The Mg-acceptor is the most important one for p-doping, but the related optical spectra are controversial. We show that there are two acceptors present in Mg-doped GaN, with two different acceptor bound exciton peaks, and also two corresponding lower energy donor-acceptor pair spectra. We give tentative evidence for their interpretation. © 2006 Elsevier Ltd. All rights reserved.
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| 38. |
- Monemar, Bo, 1942-, et al.
(författare)
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Oxygen related shallow acceptor in GaN
- 2005
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Ingår i: MRS Fall Meeting,2004. - : Materials Research Society. ; , s. E5.10.11-E5.10.11
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 39. |
- Monemar, Bo, et al.
(författare)
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Photoluminescence in n-doped In0.1Ga0.9N/In0.01Ga0.99N multiple quantum wells
- 2002
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Ingår i: MRS Internet Journal of Nitride Semiconductor Research. - 1092-5783. ; 7:7, s. 1-
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Tidskriftsartikel (refereegranskat)abstract
- In0.1Ga0.9N/In0.01Ga0.99N multiple quantum wells (MQWs) with heavily Si-doped barriers, grown with Metal Organic Vapor Phase Epitaxy (MOVPE) at about 800(0)C, have been studied in detail with optical spectroscopy. Such structures are shown to be very sensitive to a near surface depletion field, and if no additional layer is grown on top of the MQW structure the optical spectra from the individual QWs are expected to be drastically different. For a sample with 3 near surface QWs and Si-doped barriers, only the QW most distant from the surface is observed in photoluminescence (PL). The strong surface depletion field is suggested to explain these results, so that the QWs closer to the surface cannot hold the photo-excited carriers. A similar effect of the strong depletion field is found in an LED structure where the MQW is positioned at the highly doped n-side of the pn-junction. The internal polarization induced electric field in the QWs is also rather strong, and incompletely screened by carriers transferred from the doped barriers. The observed PL emission for this QW is of localized exciton character, consistent with the temperature dependence of peak position and PL decay time. The excitonic lineshape of 35-40 meV in the QW PL is explained as caused by a combination of random alloy fluctuations and interface roughness, the corresponding localization potentials are also responsible for the localization of the excitons in the low temperature range (
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| 40. |
- Monemar, Bo, et al.
(författare)
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Photoluminescence of excitons in InxGa1-xN/InyGa1-yN multiple quantum wells
- 2002
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Ingår i: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 190:1, s. 161-166
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Tidskriftsartikel (refereegranskat)abstract
- We report on a detailed study of excitons in InxGa1-xN/InyGa1-yN multiple quantum wells (MQWs) with an In composition x in the QWs of about 0.1, and a small In composition y in the barrier of 0.01-0.02. The MOVPE growth procedure was optimized to allow growth without In segregation. The InyGa1-yN barriers had a Si doping of about 5 x 10(18) cm(-3) . The low temperature photoluminescence spectra show two sets of exciton-like spectra with quite different properties. The lower energy emission has a small thermal activation energy (about 5 meV), and thus disappears at elevated temperatures, it is not observed at room temperature. The higher energy exciton state has a decay time of about 5 ns, while the lower energy process is much slower. We have also done preliminary studies on samples where the MQW region is situated in a p-n junction field, with semi-transparent contacts, to study the effects of varying the bias across the MQW structure. The combination of optical data can e interpreted in terms of a substantial potential gradient across the MQW region for both samples. The conclusion is that probably only one QW is emitting at low T (and no bias), and the second lower energy PL peak originates from a shallow notch in the conduction band at the interface between the thick GaN buffer layer and the first Ga(In)N barrier.
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| 41. |
- Monemar, Bo, et al.
(författare)
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Photoluminescence of InGaN/GaN and AlGaN/GaN multiple quantum well structures : Role of depletion fields and polarization fields
- 2003
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Ingår i: Physica status solidi. A, Applied research. - Weinheim, Germany : Wiley-VCH Verlagsgesellschaft. - 0031-8965 .- 1521-396X. ; 195:3, s. 523-527
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Tidskriftsartikel (refereegranskat)abstract
- We report on low temperature photoluminescence (PL) in InxGa1-xN multiple quantum wells (MQWs) with x in the range 0.1 and highly Si doped barriers of In0.01Ga0.99N. The MQW was placed either in the near surface depletion field or at the n-side depletion field of a pn-junction. At low temperatures the PL and electroluminescence (EL) spectra are quite different at no, low, or reverse bias, the PL appearing at higher energy. At high forward bias a spectral component at the EL position appears in PL. This proves a strong influence of the depletion field on the optical spectra. Preliminary results are also reported for Al0.07Ga0.93N/GaN structures, with near surface MQWs with 5 QWs, both Si-doped and nominally undoped. These structures show clear spectral features related to nonequivalent QWs in a graded depletion field.
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| 42. |
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| 43. |
- Monemar, Bo, et al.
(författare)
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Properties of the main Mg-related acceptors in GaN from optical and structural studies
- 2014
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Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 115:5, s. 053507-
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Tidskriftsartikel (refereegranskat)abstract
- The luminescent properties of Mg-doped GaN have recently received particular attention, e. g., in the light of new theoretical calculations, where the deep 2.9 eV luminescence band was suggested to be the main optical signature of the substitutional Mg-Ga acceptor, thus, having a rather large binding energy and a strong phonon coupling in optical transitions. We present new experimental data on homoepitaxial Mg-doped layers, which together with the previous collection of data give an improved experimental picture of the various luminescence features in Mg-doped GaN. In n-type GaN with moderate Mg doping (less than10(18) cm(-3)), the 3.466 eV ABE1 acceptor bound exciton and the associated 3.27eV donor-acceptor pair (DAP) band are the only strong photoluminescence (PL) signals at 2 K, and are identified as related to the substitutional Mg acceptor with a binding energy of 0.225 +/- 0.005 eV, and with a moderate phonon coupling strength. Interaction between basal plane stacking faults (BSFs) and Mg acceptors is suggested to give rise to a second deeper Mg acceptor species, with optical signatures ABE2 at 3.455 eV and a corresponding weak and broad DAP peak at about 3.15 eV. The 2.9 eV PL band has been ascribed to many different processes in the literature. It might be correlated with another deep level having a low concentration, only prominent at high Mg doping in material grown by the Metal Organic Chemical Vapor Deposition technique. The origin of the low temperature metastability of the Mg-related luminescence observed by many authors is here reinterpreted and explained as related to a separate non-radiative metastable deep level defect, i.e., not the Mg-Ga acceptor. (C) 2014 AIP Publishing LLC.
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| 44. |
- Monemar, Bo, et al.
(författare)
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Radiative recombination in In0.15Ga0.85N/GaN multiple quantum well structures
- 1999
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Ingår i: MRS Internet Journal of Nitride Semiconductor Research. - 1092-5783. ; 4:16
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Tidskriftsartikel (refereegranskat)abstract
- We present a study of the radiative recombination in In0.15Ga0.85N/GaN multiple quantum well samples, where the conditions of growth of the InGaN quantum layers were varied in terms of growth temperature (< 800 degrees C) and donor doping. The photoluminescence peak position varies strongly (over a range as large as 0.3 eV) with delay time after pulsed excitation, but also with donor doping and with excitation intensity. The peak position is mainly determined by the Stark effect induced by the piezoelectric field. In addition potential fluctuations, originating from segregation effects in the InGaN material, from interface roughness, and the strain fluctuations related to these phenomena, play an important role, and largely determine the width of the emission. These potential fluctuations may be as large as 0.2 eV in the present samples, and appear to be important for all studied growth temperatures for the InGaN layers. Screening effects from donor electrons and excited electron-hole pairs are important, and account for a large part of the spectral shift with donor doping (an upward shift of the photoluminescence peak up to 0.2 eV is observed for a Si donor density of 2 x 10(18) cm(-3) in the well), with excitation intensity and with delay time after pulsed excitation (also shifts up to 0.2 eV). We suggest a two-dimensional model for electron- and donor screening in this case, which is in reasonable agreement with the observed data, if rather strong localization potentials of short range (of the order 100 Angstrom) are present. The possibility that excitons as well as shallow donors are impact ionized by electrons in the rather strong lateral potential fluctuations present at this In composition is discussed.
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| 45. |
- Paskov, Plamen, 1959-, et al.
(författare)
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Nonpolar GaN layers grown by sidewall epitaxial lateral overgrowth : optical evidences for a reduced stacking fault density
- 2008
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Ingår i: Proc. of the 7th International Conference on Nitride Semiconductors,2007. - Linköping : Department of Physics, Chemistry and Biology, Linköping University. ; , s. 1768-1770
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Konferensbidrag (refereegranskat)abstract
- Nonpolar a -plane and m -plane GaN layers grown by MOCVD employing sidewall epitaxial lateral overgrowth (SELO) are studied by photoluminescence (PL) and spatially resolved micro-PL. The effects of the groove orientations and the groove/terrace width ratio on the emission spectra, particularly on the stacking fault (SF) related emission bands in the 3.29–3.42 eV spectral region, are examined. The PL spectra of both types of nonpolar layers reveal a significant reduction of the defect related emissions when the grooves are oriented perpendicular to the c-axis of GaN and the groove/terrace width ratio is smaller than one. The suppression of SF formation in the areas where a lateral overgrowth along the [0001] GaN direction occurs is confirmed by micro-PL measurements showing no SF related emissions over the terrace regions. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
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| 46. |
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| 47. |
- Paskov, Plamen, 1959-, et al.
(författare)
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Phonon-assisted photoluminescence in InGaN/GaN multiple quantum wells
- 2002
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Ingår i: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 234:3, s. 755-758
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Tidskriftsartikel (refereegranskat)abstract
- The LO-phonon sidebands of the photoluminescence in InGaN/GaN multiple quantum wells has been investigated in the temperature range from 20 to 300 K. Analysing the intensity distribution among the phonon replicas, the strength of the exciton-phonon interaction has been estimated. The Huang-Rhys factor was found to be ÿ0.3, much larger than in GaN. The enhancement has been attributed to the exciton localization on a length scale smaller than the exciton Bohr radius and to the large internal electric field, which increases the spatial separation of the electron and hole charge densities along the growth axis.
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| 48. |
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| 49. |
- Paskov, Plamen, 1959-, et al.
(författare)
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Photoluminescence study of near-surface GaN/AlN superlattices
- 2008
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. ; , s. 68940G1-
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Konferensbidrag (refereegranskat)abstract
- We report on the emission properties of GaN/AlN superlattices (SLs) grown by metalorganic chemical vapor deposition on a thick GaN layer. Nominally undoped and Si-doped SL structures with the well/barrier thickness ratio 3:1 and different SL periods are investigated. It is found that in these SLs without capping layer the energy position, intensity and linewidth of the emission are determined by the interplay of the built-in polarization field, the depletion field arising from the pinning of the Fermi level at the surface, and the screening of the electric field in the quantum well due to the both the polarization-induced two-dimension electron gas (2DEG) and the photo-generated carriers. A non-uniform equilibrium electron distribution and an electron accumulation at the bottom AlN/GaN interface are evidenced by the observed recombination of the 2DEG with the photo-excited holes occurring below the GaN bandgap.
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| 50. |
- Pozina, Galia, et al.
(författare)
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Effect of annealing on metastable shallow acceptors in Mg-doped GaN layers grown on GaN substrates
- 2008
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Ingår i: Applied Physics Letters. - : Institutionen för fysik, kemi och biologi. - 0003-6951 .- 1077-3118. ; 92:15, s. 151904-
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Tidskriftsartikel (refereegranskat)abstract
- Mg-doped GaN layers grown by metal-organic vapor phase epitaxy on GaN substrates produced by the halide vapor phase technique demonstrate metastability of the near-band-gap photoluminescence (PL). The acceptor bound exciton (ABE) line possibly related to the C acceptor vanishes in as-grown samples within a few minutes under UV laser illumination. Annealing activates the more stable Mg acceptors and passivates C acceptors. Consequently, only the ABE line related to Mg is dominant in PL spectra for the annealed samples. The temporal changes in PL are permanent at low temperatures; however, they can be recovered after heating to 100 K or higher.
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