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Optoelectronics, IEE Proceedings -

Issue 5 • Date 27 Oct. 2004

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Displaying Results 1 - 25 of 49
  • Optoelectronic properties of 2-D and 3-D-grown GaInNAs//GaAs QW light emitting diodes and laser diodes

    Publication Year: 2004 , Page(s): 421 - 425
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (313 KB)  

    The optoelectronic properties of light emitting diodes (LEDs) and laser diodes (LDs) based on GaInNAs/GaAs quantum wells (QWs) grown under two-dimensional (2-D) and three-dimensional (3-D) conditions are compared. In spite of the longer wavelength, the 2-D LED shows better luminescence properties than the one with island formation. The broadening of the photocurrent spectra and the Stokes shift are used to analyse carrier localisation effects. The emission from localised states is found to impact the electroluminescence of the 3-D device even at room temperature, while it does not significantly influence the high-temperature emission of the 2-D LED. From a detailed analysis of the low-temperature electroluminescence as a function of injected current, nonradiative recombination is found to be the main carrier recombination mechanism in a wide range of currents in the 3-D LED, while it is very small in the 2-D case. Consequently, defect formation seems to be closely related to the 3-D growth, limiting laser emission in these structures. These results suggest that 2-D-grown GaInNAs/GaAs QWs may be more desirable for achieving laser emission at long wavelengths. View full abstract»

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  • Growth of GaNxAs1-x atomic monolayers and their insertion in the vicinity of GaInAs quantum wells

    Publication Year: 2004 , Page(s): 254 - 258
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (394 KB)  

    The deposition of N-rich GaNAs atomic monolayers was investigated. Such layers were successfully grown while exposing a GaAs surface to a nitrogen plasma source during a growth interruption at 400°C in a molecular beam epitaxy reactor. N accumulation was confirmed and evaluated by secondary ion mass spectroscopy. This process is compatible with regrowth, as in situ monitored by reflection high-energy electron diffraction. The crystal shows good structural quality, as displayed by transmission electron microscopy, that reveals that the accumulation occurred within 1 nm. In a series of samples, two of these ultrathin GaNAs layers were inserted in GaAs barriers, on each side of a GaInAs quantum well (QW). A drastic effect of the N-rich layers on the QW photoluminescence (PL) intensity was observed, as well as on the carrier recombination dynamics, with a strong influence of the spacer thickness between the QW and the N-rich layers. A time-resolved PL analysis of these samples evidenced nonradiative relaxation times in the range of a few ps. This very short carrier lifetime is attributed to the presence of nonradiative centres related to the N-rich layers close to the QW, and can be used to design ultrafast optical devices. View full abstract»

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  • (Ga,In)(N,As)-based solar cells grown by molecular beam epitaxy

    Publication Year: 2004 , Page(s): 433 - 436
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (362 KB)  

    (Ga,In)(N,As) could be a promising material for use in monolithic four-junction solar cells since it can be grown lattice-matched to substrates such as GaAs and Ge, and its bandgap of 1 eV is complementary to that of the three other semiconductors Ge, GaAs and (Ga,In)P. The growth by molecular beam epitaxy of (Ga,In)(N,As)-based solar cells is reported. It was checked by high-resolution X-ray diffraction that the 1-μm-thick (Ga,In)(N,As) layers were lattice-matched to GaAs. The spectral responses of the solar cells provide evidence that (Ga,In)(N,As) converts photons with energy down to 0.9 eV. The comparison with reference GaAs solar cells indicates, however, a degradation of the short-circuit current, revealing short minority-carrier diffusion lengths. A (Ga,In)(N,As) 2 mm×2.5 mm solar cell with a p-i (Ga,In)(N,As) n-GaAs structure delivers a 2.1 mA/cm2 short-circuit current and has an open-circuit voltage of 0.264 V under natural solar illumination (air mass ∼1.5). View full abstract»

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  • Composition fluctuations in GaInNAs multi-quantum wells

    Publication Year: 2004 , Page(s): 271 - 274
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (318 KB)  

    GaInNAs/GaAs(001) multi-quantum wells grown by MBE at temperatures in the range 360-460°C have been studied by transmission electron microscopy and photoluminescence. The authors have observed the existence of periodic contrasts with 220BF reflection, which appear more pronounced when increasing the growth temperature. These strain contrasts have been associated to composition fluctuations in the wells and, therefore, it is suggested that an enhancement of the phase separation in the GaInNAs quantum wells occurs when varying the growth temperature from 360°C to 460°C. The photoluminescence results show a broadening of the emission peak over a similar growth temperature range. Thus, the degradation of the optical properties in the GaInNAs structures is suggested to be linked to the composition fluctuations. View full abstract»

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  • Effects of electron irradiation on photoluminescence from GaInNAs/GaAs multiple quantum wells subject to thermal annealing

    Publication Year: 2004 , Page(s): 290 - 292
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    Electron irradiation of a 1.3-μm GaInNAs/GaAs quantum-well heterostructure, grown by molecular beam epitaxy and subsequently subjected to rapid thermal annealing, is found to induce much stronger photoluminescence than that observed for an identical as-grown sample upon annealing. Annealing of the irradiated sample also causes an additional spectral blue-shift, reduces alloy potential energy fluctuations at the conduction band minimum, and narrows spectral linewidths. These irradiation-related phenomena are accompanied by a discernable change in X-ray diffraction features upon annealing, which indicate a change in quantum well alloy composition or structure. View full abstract»

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  • Physics of defects and hydrogen in dilute nitrides

    Publication Year: 2004 , Page(s): 369 - 377
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    First-principles theory is capable of unveiling physical properties of the various defects in dilute nitrides. The authors discuss some of their recent results for native defects, N-N pairs, as well as for hydrogen in GaAsN and GaPN. The studies have shown that defect physics of dilute nitrides is qualitatively different from that of conventional semiconductors owing to the involvement of nitrogen. This leads to a number of phenomena ranging from the existence of a new class of intrinsic traps, such as the N-N split interstitials, AsGa-N and VGa-N pairs, to a surprising modification of the fundamental bandgap by hydrogen. View full abstract»

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  • Ultrafast carrier dynamics in nitrogen-implanted GaAs

    Publication Year: 2004 , Page(s): 361 - 364
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    The comparative analysis of the ultrafast carrier dynamics of pure GaAs and ion-implanted GaNAs is reported. Different nitrogen concentrations (up to 4%) are implanted and subsequently annealed by rapid thermal annealing (RTA). Damage analysis by channelling Rutherford backscattering (RBS) reveals that the annealing step improves the crystal quality, but does not restore the original quality. From photoreflectance measurements it is concluded that the highest achieved active nitrogen content in the implanted samples is 0.5% for an equivalent implantation dose of 1%. Carrier dynamics are investigated by one-colour pump-probe measurements covering an excitation wavelength range of 730-860 nm (1.7-1.44 eV) with femtosecond time resolution. Comparison with non-implanted GaAs indicates that the carrier relaxation in the implanted samples is dominated by traps associated with implantation damage. View full abstract»

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  • N2-incorporation-induced blue shift in InGaAsN/GaAs quantum well during annealing

    Publication Year: 2004 , Page(s): 320 - 322
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (333 KB)  

    Three different kinds of quantum wells (QWs) were grown at the same growth temperature by molecular beam epitaxy (MBE): InGaAs/GaAs QW without any N2 background; InGaAs(N2)/GaAs double QWs with N2 flow to the sample but without plasma; and InGaAsN/GaAs QW with N-plasma. The latter two types of QW were grown on one sample. Post-growth rapid thermal annealing (RTA) was applied to them at 700°C. After 31.5 min of RTA, it was observed that the photoluminescence blue shift of the InGaAs(N2) QW was 40 meV, which was 9 meV more than that for the InGaAsN QW, and the InGaAs QW had only about 13 meV blue shift. However, for the as-grown case, the PL peak position of the InGaAs QW and InGaAs(N2) QWs were the same (1148 nm), and for the InGaAsN QW it was 220 nm longer. This indicates that there is N2 incorporation during InGaAsN growth, the N2 incorporation does not affect the band structure and the N2 incorporation dominates the blue shift during RTA. View full abstract»

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  • Influence of growth temperature on carrier recombination in GaInNAs-based lasers

    Publication Year: 2004 , Page(s): 447 - 451
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (378 KB)  

    Relatively small changes in growth parameters, such as the active layer growth temperature, have been shown to have dramatic effects on the threshold current density Jth for GaInNAs-based lasers. The authors consider the influence of the growth temperature on the contribution of the defect-related current to Jth in structures with nominally identical quantum wells. The two sets of devices used in this study are 1.27-μm GaInNAs SQW edge-emitting lasers which are nominally identical except that the active regions were grown at 422°C and 456°C. At room temperature it was found that Jth for the high growth temperature lasers (∼330 A/cm2) is reduced by a factor of ∼2.8 when compared to Jth of the low growth temperature devices (∼970 A/cm2). To determine the processes responsible for this change the light emitted from a window in the substrate contact of the devices was measured as a function of current and temperature, which yields the absolute magnitudes of the defect-, radiative- and Auger-related recombination processes at threshold. The results show that the strong increase in Jth with changing growth conditions can be explained almost entirely by an increase in the monomolecular recombination coefficient A, yielding a larger defect-related current contribution. Furthermore, the extracted radiative- and Auger-related current paths in the two devices are of very similar magnitude. This is strong evidence that the change in growth conditions does not affect the internal losses of the devices, as the threshold carrier density nth appears not to have changed significantly. By increasing the number of wells to three, Jth per well of the high growth temperature devices could be further reduced (∼190 A/cm2) at room temperature. The results suggest that increasing the number of wells not only decreases the carrier density at threshold but additionally reduces the average defect-related current density in these devices. View full abstract»

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  • Optimisation of GaAs-based (GaIn)(NAs)//GaAs vertical-cavity surface-emitting diode lasers for high-temperature operation in 1.3-μm optical-fibre communication systems

    Publication Year: 2004 , Page(s): 417 - 420
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (335 KB)  

    Performance of GaAs-based (GaIn)(NAs)/GaAs vertical-cavity surface-emitting diode lasers (VCSELs) at higher temperatures for second-generation optical-fibre communication systems is examined with the aid of the comprehensive threshold fully self-consistent optical-electrical-thermal-gain model. As expected, in standard double-oxide-confined VCSELs, an increase in the active-region diameter is followed by an increase in the lowest-threshold transverse mode order, especially at higher temperatures. It has been found that reduction of a diameter of the bottom aperture may ensure fundamental LP01 mode operation even at higher temperatures. View full abstract»

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  • Effect of nitrogen ions on the properties of InGaAsN quantum wells grown by plasma-assisted molecular beam epitaxy

    Publication Year: 2004 , Page(s): 305 - 308
    Cited by:  Papers (2)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (239 KB)  

    The advantages of the InGaAsN/GaAs system comprising the active layers of single- and multi-quantum well laser diodes (emission wavelengths in the second and third optical windows, AlGaAs reflectors etc.), in comparison with other materials, make this quaternary material an interesting field for study. The monoatomic nitrogen species required for the growth of InGaAsN layers by molecular beam epitaxy, using a radiofrequency power source, are mixed in the plasma with ionised species, among others. The authors present a detailed characterisation of the plasma in the vicinity of the growing surface by measuring its I-V characteristics. A magnetic field was used to deflect ions and their effect on the properties of InGaAsN quantum wells was observed. These ionised species were observed to damage the surface, introducing nonradiative centres. As observed by photoluminescence experiments, the optical quality is improved as the density of ions impinging on the surface is reduced. Rapid thermal annealing experiments were also carried out, showing that the observed PL intensity improvement is related to the ion concentration in the quantum wells. View full abstract»

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  • Epitaxy and characterisation of dilute III-As1-yNy on GaAs and InP

    Publication Year: 2004 , Page(s): 247 - 253
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    Epitaxial growth and characterisation of Ga1-xInxAs1-yNy films and quantum wells are presented. Starting with the epitaxy on GaAs, recent results on the local bonding of nitrogen in Ga1-xInxAs1-yNy are reviewed, revealing that bonding of nitrogen is controlled by an interplay between bond cohesive energy and reduction of local strain. Thus, III-N bonding can be changed from Ga-N to In-N by post-growth thermal annealing. For high In-content Ga1-xInxAs1-yNy on InP it is demonstrated that only small amounts of Ga are necessary to cause the bonding of the nitrogen atoms to at least one Ga neighbour. The epitaxy on InP substrates, equivalent to a drastic increase in indium content, allows an extension of optical transitions to longer wavelengths. The feasibility of high In-content Ga1-xInxAs1-yNy pseudomorphic quantum wells on InP is shown. The deterioration of the photoluminescence properties with increasing nitrogen incorporation can be partially compensated by thermal annealing. Within the resolution limits of the secondary ion mass spectrometry experiments, no annealing-induced loss of nitrogen was observed. The indium-rich strained Ga0.22In0.78As0.99N0.01 quantum wells are shown to exhibit room-temperature photoluminescence at wavelengths up to 2.3 μm. Finally quantum well lasers emitting at wavelengths beyond 2 μm are demonstrated. View full abstract»

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  • Blue shift in InGaAsN/GaAs quantum wells with different width

    Publication Year: 2004 , Page(s): 317 - 319
    Cited by:  Papers (1)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (286 KB)  

    Three InGaAsN/GaAs QWs with well widths of 3, 5 and 9 nm were grown in one single sample with a GaAs barrier width of 35 nm under the same growth conditions. Post-growth rapid thermal annealing (RTA) was applied at 700°C on this sample and photoluminescence (PL) was measured after different RTA times. It was observed that the blue shifts (BS) were the same, up to ∼15 meV in the first 30 s, for all three QWs; after this the BS of the 9-nm QW saturated very soon at 24 meV and the BSs of 5-nm and 3-nm QWs were saturated much more slowly at more than 45 meV and 57 meV, respectively. There are at least two factors that affect the BS: interdiffusion and short-range order (SRO). SRO is started and saturated much faster than interdiffusion during annealing. View full abstract»

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  • Boundary conditions for the electron wavefunction in GaInNAs-based quantum wells and modelling of the temperature-dependent effective bandgap

    Publication Year: 2004 , Page(s): 393 - 396
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (253 KB)  

    The boundary conditions were investigated for the electron wavefunction in Ga1-xInxNyAs1-y-based heterostructures described by the band anticrossing model. Among other advantages, the utilisation of these boundary conditions simplifies the calculation of, for example, transition energies in quantum wells. The derived equations were applied to model the temperature-dependence of the effective bandgap in Ga1-xInxNyAs1-y/GaAs quantum well structures with high indium concentration. From a fit to the experimental photoreflectance data, evidence was found to show that the nitrogen level EN in the band anticrossing Hamiltonian, measured with respect to the valence band edge, shifts to higher energies with decreasing temperature. This extends similar results reported in the literature for low indium content epilayers. View full abstract»

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  • Effects of rapid thermal annealing on optical quality of GaNP alloys

    Publication Year: 2004 , Page(s): 335 - 337
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (228 KB)  

    Significant improvements in radiative efficiency of GaNP epilayers grown on GaP substrates by solid-source molecular beam epitaxy (MBE) are achieved by post-growth rapid thermal annealing (RTA). From temperature-dependent CW and time-resolved photoluminescence (PL) spectroscopies combined with PL excitation measurements, the observed improvements are attributed to annealing out of competing nonradiative centres. This conclusion is supported by the following experimental evidence: reduced thermal quenching of the PL intensity resulting in a substantial (up to 18 times) increase at room temperature (RT) after RTA, and simultaneous improvements in carrier lifetime at RT deduced from time-resolved PL measurements. View full abstract»

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  • Percolation-based multimode Ga-N behaviour in the Raman spectra of GaInAsN

    Publication Year: 2004 , Page(s): 338 - 341
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    The transverse optical (TO) and longitudinal optical (LO) phonons of bulk (∼1-μm-thick) as-grown Ga1-yInyAs1-xNx/GaAs [001] layers with x∼0.03-0.04 and y up to 0.30 are investigated by Raman scattering. Three-mode behaviour is observed which discriminates between Ga-N modes from isolated N atoms (∼460 cm-1), clustered N in a Ga environment (∼425 cm-1) and N involved in Ga3InN complexes (∼480 cm-1). The local N-bonding in GaInAsN is quantitatively estimated via full contour modelling of the TO multimode Ga-N Raman lineshapes. In particular, it is found that the incorporation of In can significantly reduce GaN segregation, but only for a very narrow range of N compositions just above the theoretical N-solubility limit xs∼0.02 in GaAs. View full abstract»

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  • Synthesis and properties of highly mismatched II-O-VI alloys

    Publication Year: 2004 , Page(s): 452 - 459
    Cited by:  Patents (1)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (710 KB)  

    Ternary and quaternary dilute II-VI oxides were synthesised using a highly non-equilibrium method: the combination of O ion implantation and pulsed-laser melting. CdOxTe1-x thin films have been produced with x up to 0.015 and with the energy gap reduced by 0.15 eV. Optical transitions corresponding to both the lower (E-) and upper (E+) conduction sub-bands, resulting from the anticrossing interaction between the localised O states and the extended conduction states of the matrix, are clearly observed in quaternary Cd0.6Mn0.4OxTe1-x and Zn0.88Mn0.12OxTe1-x layers. These results have important implications for the existing theoretical models of the electronic structure of the highly mismatched alloys. In Zn1-xMnxTe, where the O level lies below the conduction band edge, it was demonstrated that incorporation of a small amount of oxygen leads to the formation of a narrow, oxygen-derived band of extended states located well below the conduction band edge of the ZnMnTe matrix. The three absorption edges of this material (∼0.73, 1.83 and 2.56 eV) cover the entire solar spectrum providing a material envisioned for multiband, single-junction, high-efficiency photovoltaic devices. View full abstract»

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  • Influence of cluster states on band dispersion in bulk and quantum well (ultra-)dilute nitride semiconductors

    Publication Year: 2004 , Page(s): 397 - 401
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (445 KB)  

    The band-anti-crossing (BAC) model successfully describes many of the electronic properties of GaNxAs1-x. Experimental and theoretical studies show a range of resonant defect levels close to the conduction band edge in GaNxAs1-x, due to the formation of N complexes which are ignored in the conventional BAC model. The consequences of these resonant levels for the band dispersion are investigated. The rapid increase in N-N pairs with N composition (∝x2) is shown to have little effect on the calculated room-temperature band-edge dispersion, but modifies the low-temperature band dispersion with increasing x. For low temperatures, it is shown that at low N composition (0.001≤x≤0.01) the band dispersion is best described using a modified BAC model, which explicitly includes the effects of N-N pairs, while at higher compositions (x>0.01) the effects of longer-range N-N interactions need also to be considered. The consequences of this are analysed for the predicted evolution of band dispersion with x in magneto-tunnelling experiments. View full abstract»

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  • GaInNAs SESAMs passively mode-locking 1.3-μm solid-state lasers

    Publication Year: 2004 , Page(s): 437 - 441
    Cited by:  Patents (1)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (540 KB)  

    A GaInNAs semiconductor saturable absorber mirror (SESAM) is demonstrated that is able to passively mode-lock a 1.3-μm Nd:YLF laser. The mirror was grown by molecular beam epitaxy and consists mainly of a GaInNAs single quantum well on an AlAs/GaAs distributed Bragg reflector. Two designs were grown: resonant and antiresonant. The first favoured optical characterisation, while the second one was more fitted to self-start passive mode-locking in the laser. Rapid thermal annealing was used to tune this wavelength closer to the desired laser wavelength of 1314 nm. Nonlinear characterisation of both designs allowed calculation of the saturation fluence of a 10-nm GaInNAs QW with photoluminescence around 1330 nm independently of design. It was measured to be 3.5±0.5 μJ/cm2. The incorporation of about 2% of N in In0.36Ga0.64As red-shifted the as-grown photoluminescence to about 1370 nm. The nonlinear optical characterisation of the antiresonant SESAM showed low saturation fluence (11.2 μJ/cm2), low nonsaturable losses and a recovery time suitable for ps-pulse generation. With this SESAM we obtained clean self-starting mode-locking with 6.7-ps pulses at a repetition rate of 117 MHz and a maximum average output power of 580 mW. View full abstract»

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  • Surface rate-limiting steps and modelling of the nitrogen growth kinetics of GaAs1-xNx/GaAs

    Publication Year: 2004 , Page(s): 259 - 262
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (320 KB)  

    The growth features of the nitrogen incorporation into GaAs grown by metalorganic vapour phase epitaxy (MOVPE) are presented, with a comparison between experimental results and a kinetic model. The so-called 'distribution coefficient' of nitrogen varies in the range 180-1000 depending essentially on growth temperature. Experimental results were analysed with a schematic process of surface reaction kinetics for controlling the mass transfer. Some kinetic rate constants are proposed. The modelling of N incorporation at 520-560°C allows an identification of species coming from the DMHy thermal decomposition. Surprisingly, this study shows that the rate of N incorporation weakly depends on the arsenic precursor at low growth temperatures. View full abstract»

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  • Mutual passivation effects in highly mismatched group III-V-N alloys

    Publication Year: 2004 , Page(s): 460 - 464
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (453 KB)  

    The authors have recently discovered a new effect in the dilute InGaAsN alloy system in which electrically active substitutional Si donors and isoelectronic N atoms passivate each other's activity. This is manifested in a drastic reduction of the free electron concentration and, simultaneously, an increase in the fundamental bandgap in Si-doped InGaAsN alloys. Systematic studies of the mutual passivation effect in Si-doped InGaAsN alloys show that the passivation process is controlled by the diffusion of Si to the nearest neighbours of N-occupied sites, forming nearest neighbour Si-N pairs. Analytical calculations of the passivation process based on Ga-vacancy-mediated diffusion show good agreement with the experimental results. It is shown that the mutual passivation provides a method to precisely control electrical properties of InGaAsN:Si. Investigations were made of scattering mechanisms limiting electron mobility in InGaAsN through a judicious choice of the annealing conditions to vary the electron concentration by more than two orders of magnitude. The mobility of free electrons in InGaAsN is quantitatively explained on the basis of the band anticrossing model. View full abstract»

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  • Intrinsic limits on electron mobility in disordered dilute nitride semiconductor alloys

    Publication Year: 2004 , Page(s): 352 - 356
    Cited by:  Papers (1)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (258 KB)  

    The authors have previously shown that there is a fundamental connection between the composition-dependence of the conduction-band-edge energy and the n-type carrier scattering cross-section in dilute alloys, imposing general limits on the electron mobility in Ga(In)NAs alloys and heterostructures. A simple general expression is derived for scattering in the ultra-dilute regime, showing that the scattering rate is proportional to |dEc/dx|2, the square of the initial variation of the conduction-band-edge energy with alloy composition x. The mobility estimated in GaNxAs1-x using the two-level band-anti-crossing (BAC) model is of the right magnitude (∼1000 cm2/Vs), but still larger than typical experimental values. The effects of N clusters and inhomogeneous broadening of energy levels are considered, including the formation of N-N pairs (where a single Ga atom has two N neighbours) and more complex clusters. It is shown that such complexes play a major role in further limiting the mobility, giving values of 200-400 cm2/Vs, in close agreement with experiment. It is concluded that random alloy scattering, rather than film quality or other factors, dominates the carrier mobility in these materials. View full abstract»

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  • Exciton localisation in InGaAsN and GaAsSbN observed by near-field magnetoluminescence and scanning optical microscopy (NSOM)

    Publication Year: 2004 , Page(s): 346 - 351
    Cited by:  Patents (1)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (520 KB)  

    The authors present measurements of photoluminescence spectra and imaging of the sharp emission lines which result from the radiative recombination of excitons trapped in compositional fluctuations in dilute nitride epitaxial layers and quantum wells, using low-temperature near-field scanning optical microscopy (NSOM). Both weak and strong localisation (i.e. quantum-dot-like emission) are observed for samples of GaAsN and InGaAsN, with little difference depending on the presence or absence of In. Localisation energies are in the range 10-60 meV. Zeeman splitting and diamagnetic shifts are observed in the near-field magneto-photoluminescence, and from these effects the size (r=3-17 nm), composition fluctuations (Δy ∼0.5%) and density (100-1000 μm-3) of the compositional clusters are determined. Using two-dimensional NSOM images, the authors observe a strong lateral inhomogeneity for the GaAsN epilayers resulting from nitrogen clustering. A one-dimensional compositional modulation along the [110] direction is also observed for a GaAsSbN quantum well on the length scale of 100 nm. Transmission electron micrographs confirm the existence of these spatial nonuniformities. Time-resolved spectroscopy has also been carried out on one of the samples reported here (In0.08GaAsN0.03). Pump-and-probe photoluminescence experiments provide the following time constants: the exciton relaxation time to the ground states of the localisation regions is found to be ∼40-70 ps as the emission varies from high to low energy, and the time for depopulation of these localised states is 2-4 ns. View full abstract»

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  • GaInNAs(Sb) long wavelength communications lasers

    Publication Year: 2004 , Page(s): 407 - 416
    Cited by:  Papers (5)
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (704 KB)  

    Dilute nitride GaInNAs and GaInNAsSb alloys grown on GaAs have quickly become excellent candidates for a variety of lower cost 1.2-1.6 μm lasers, optical amplifiers and high-power Raman pump lasers that will be required to power the Internet and advanced communications systems capable of delivering multi-Gbit/s data rates to the desktop. Two particularly critical devices are vertical-cavity surface-emitting lasers (VCSELs) which must operate at high data rates (≥10 Gbit/s), uncooled over a broad thermal operating range and high power (≥500 mW) edge-emitting lasers for Raman amplifier pumps. Despite the fact that these materials are grown in a metastable regime, and there are still many remaining challenges, progress has been both rapid and very promising. Some of the material challenges include the limited solubility of N in GaAs, nonradiative defects that are caused by either or a combination of N incorporation, low growth temperature, and ion damage from the N plasma source. N and Sb add a unique set of properties to this metastable alloy; however, this significantly increases the complexity of its characterisation. The addition of Sb significantly improves the epitaxial growth and optical properties of the material at wavelengths longer than 1.3 μm and broadens the range of In and N composition alloys that can be grown. By adding Sb to the alloy, luminescence has been greatly enhanced between 1.3 and 1.6 μm where normally poor quality material results. Progress in overcoming some of the material challenges is described, particularly GaAsNSb against GaNAs QW barriers, plasma-source ion damage and progress in realising record-setting edge-emitting lasers and the first VCSELs operating at 1.5 μm based on GaInNAsSb QWs grown by solid-source MBE on GaAs. View full abstract»

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  • Effect of lattice ionicity on hydrogen activity in II-VI materials containing isoelectronic oxygen impurities

    Publication Year: 2004 , Page(s): 465 - 468
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (251 KB)  

    The authors studied the effect of hydrogen irradiation on the optical properties of ZnTe, ZnTe1-ySy (y=0.015) and ZnSe1-xOx (x=0.0023, 0.0057 and 0.009) epilayers. Photoluminescence measurements show the full passivation of O-related recombination bands in ZnTe and ZnTe0.985S0.015 samples unintentionally doped with oxygen. However, hydrogen irradiation does not affect the bandgap reduction following O incorporation in ZnSe1-xOx alloys. This lack of interaction between O and H in ZnSe1-xOx points toward a scarce localised character in the ZnSe1-xOx band edges, as supported by the study of the temperature dependence of the ZnSe1-xOx bandgap. View full abstract»

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