Solid-state electronics

Doping-related behavior divergence of some crystalline materials has raised doubts about Urbach tailing universality. In this paper, we infer the atomic scale interpretation of this divergence on the basis of experimental records and comparative studies. We focus on a widely known wurtzite structured crystal and find that Urbach tailing was inhibited once doping charged loci succeeded to establish local coordinated donors networks or so-called topological filaments. The case of doping-free and saturated ZnO networks unexpected Urbach tailing features has been discussed. All the observed behaviors confirm the Urbach tailing universality despite their controversial aspect.

A physical explanation to the controversial Urbach tailing universality

In this paper, we try to give an explanation to the spectacular Urbach tailing associated to ytterbium. This rare earth element performs odd spectra due to its 5f-electron shell configuration and complexity of its transition dynamics within band structure. Ytterbium particular forbidden band alteration has been associated to two major inconsistencies: calculating atomic single-particle wave functions and spectra without taking into account the non-spherical effects and neglecting the high parity non conserving (PNC) amplitudes in ytterbium absorption-emission spectra. The actual band structure calculation has been achieved within the Hartree-Fock-Slater approximation. It led to a noticeable blue shift resulting in narrowing the forbidden band from upper edge. Parity non conserving (PNC) amplitudes were also proposed as an explanation to width reduction from both upper and lower band edges. The relative extent of these reductions seems to coincide with Urbach tailing dimensions recorded in recent studies.

Atomic structures beyond the spherical approximation along with PNC as conjectured explanations to Urbach tailing in neutral isolated ytterbium

Approximately 4-μm-thick GaN epitaxial films were directly grown onto a GaN/sapphire template, sapphire, Si(111), and Si(100) substrates by high-temperature pulsed laser deposition (PLD). The influence of the substrate type on the crystalline quality, surface morphology, microstructure, and stress states was investigated by X-ray diffraction (XRD), photoluminescence (PL), atomic force microscopy (AFM), transmission electron microscopy (TEM), and Raman spectroscopy. Raman scattering spectral analysis showed a compressive film stress of −0.468 GPa for the GaN/sapphire template, whereas the GaN films on sapphire, Si(111), and Si(100) exhibited a tensile stress of 0.21, 0.177, and 0.081 GPa, respectively. Comparative analysis indicated the growth of very close to stress-free GaN on the Si(100) substrate due to the highly directional energetic precursor migration on the substrate’s surface and the release of stress in the nucleation of GaN films during growth by the high-temperature (1000 °C) operation of PLD. Moreover, TEM images revealed that no significant GaN meltback (Ga–Si) etching process was found in the GaN/Si sample surface. These results indicate that PLD has great potential for developing stress-free GaN templates on different substrates and using them for further application in optoelectronic devices.

Optoelectronic Properties and Structural Characterization of GaN Thick Films on Different Substrates through Pulsed Laser Deposition

We present an analytical model that combines the complementary experimental evidence of spatial dispersion (DAP recombination) and energetic dispersion (band tails). The model describes the competition between thermalization and recombination of excess carriers trapped in exponentially distributed (in energy), discrete localized (in space) states. We use the energy dependence of the relaxation rates to derive the energy and time dependence of sub gap photoluminescence. The model predicts that the yellow luminescence band (YLB) and blue luminescence band (BLB) commonly observed in GaN are not separate entities, but reflect the competition of thermalization and recombination. A distinct kink is observed in transient PL in the microsecond range, in the limiting cases of strong tailing and/or low temperatures, indicating the transition from thermalization-limited to (radiative) recombination-limited excess carrier relaxation. Both prediction are in line with experiment, and able to resolve interpretational difficulties. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermalization and recombination in exponential band tail states

We have applied short laser pulses with a minimal pulse width of 5 ps to study the non-radiative and radiative carrier density decay in polycrystalline GaN. The GaN films are deposited by a cyclic pulsed laser deposition (PLD) technique at a relatively low substrate temperature of 600 °C on sapphire substrates. Results from photoluminescence decay and transient photocurrents are compared with respect to trapping, recombination, and transport of photogenerated carriers in polycrystalline GaN thin films.

Non-radiative and radiative properties of PLD-deposited polycrystalline GaN studied by UV ps-to-ns laser pulses

Morphology and composition of GaN films grown by cyclic-pulsed laser deposition

Photoinduced excess carrier dynamics in PLD-grown ZnO

Optical and transport data on GaN samples grown by low-temperature pulsed laser deposition are presented. Large below-gap band tails are observed in optical absorption spectroscopy. The most intense photoluminescence lines of medium crystalline quality samples can be attributed to excitons bounded to stacking faults. The samples of the highest quality show, besides the ubiquitous yellow band, a large near band emission (NBE) peaking at 3.47 eV. The large width of the NBE is attributed to the joint effects of the band tails and an elevated carrier concentration. Secondary photocurrent transients show a slow decay reaching the millisecond time region for a variety of excitation wavelengths and intensities, with a power law decay exponent of around —0.3. The long times are attributed to the trapping of excess carriers in extended band tail states. From the comparison of photocurrent decay with excess carrier-induced microwave reflection it is concluded that the decay dynamics at the air/film and film/substrate interfaces are identical.

Low temperature photoluminescence, transient photoconductivity and microwave reflection for optical properties and transport in PLD-GaN

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Papers

Non-radiative and radiative properties of PLD-deposited polycrystalline GaN studied by UV ps-to-ns laser pulses

Morphology and composition of GaN films grown by cyclic-pulsed laser deposition

Photoinduced excess carrier dynamics in PLD-grown ZnO

Low temperature photoluminescence, transient photoconductivity and microwave reflection for optical properties and transport in PLD-GaN