xN Ternary Alloys Research Articles

Femtosecond time-resolved study in InxGa1−xN (0001) ultrathin epilayers: Effects of high indium mole fraction and thickness of the films

In view of promising full-solar-spectrum photovoltaic systems based on InxGa1−xN ternary alloys, femtosecond time-resolved study in ultrathin epilayers was employed in order to extract the fundamental properties of material. Two different thicknesses of epilayers were employed with relative high indium mole fractions. State filling effect at various probing energy states has been observed for both epilayers. Saturation of state filling as well as enhanced photoinduced absorption occurred at higher probing wavelengths. Furthermore, coherent acoustic phonon oscillations were also observed for both ultrathin epilayers with a thickness dependent oscillation frequency. Finally, absorption band edge of these alloys has been determined.

Growth of thick Al xGa 1−xN ternary alloy by hydride vapor-phase epitaxy

Growth of thick Al x Ga 1− x N ternary alloy using AlCl 3 and GaCl gases as group III precursors was performed by hydride vapor-phase epitaxy (HVPE). C-axis-oriented AlGaN layers could be grown at 1100 °C. It was found that the growth of Al x Ga 1− x N by HVPE was affected by the presence of H 2 in the carrier gas. Therefore, the solid composition x in Al x Ga 1− x N ternary alloy could be controlled by changing the input Al mole ratio of the group III precursor ( R Al) and/or the low range (<10%) of partial pressure of hydrogen (H 2) in the carrier gas ( F o). The growth rate of approximately 30 μm/h was obtained under inert carrier gas ( F o=0.0), while the growth rate decreased rapidly in the low R Al under a low partial pressure of H 2 in the carrier gas ( F o=0.1). These results are in good agreement with the thermodynamic prospect. Thus, the growth of Al x Ga 1− x N using HVPE is thermodynamically controlled.

Si-doped GaN∕AlN quantum dot superlattices for optoelectronics at telecommunication wavelengths

We report on the controlled growth by molecular beam epitaxy of 20-period Si-doped GaN∕AlN quantum dot (QD) superlattices, in order to tailor their intraband absorption within the 1.3–1.55μm telecommunication spectral range. The QD size can be tuned by modifying the amount of GaN in the QDs, the growth temperature, or the growth interruption time (Ostwald ripening). By adjusting the growth conditions, QDs with height (diameter) within the range of 1–1.5nm (10–40nm), and density between 1011 and 1012cm−2 can be synthesized, fully strained on the AlN pseudosubstrate. To populate the first electronic level, silicon can be incorporated into the QDs without significant perturbation of the QD morphology. All the samples exhibit strong p-polarized intraband absorption at room temperature. The broadening of the absorption peak remains below 150meV and can be as small as ∼80meV. This absorption line is attributed to transition from the s ground level of the QD to the first excited level along the growth axis, pz. The peak energies of both photoluminescence emission and intraband absorption are consistent with the QD structural characteristics, and with their evolution by changing the growth conditions. Tuning of the intraband absorption from 0.740eV (1.68μm)to0.896eV (1.38μm) is demonstrated. Finally, we show that the AlN buffer layer can be replaced by a conductive AlxGa1−xN (x=0.35 and 0.6) ternary alloy without significant modification of the intraband properties of the QD stack.

Effect of native defects on optical properties of InxGa1−xN alloys

The energy position of the optical-absorption edge and the free-carrier populations in InxGa1−xN ternary alloys can be controlled using high-energy He+4 irradiation. The blueshift of the absorption edge after irradiation in In-rich material (x&amp;gt;0.34) is attributed to the band-filling effect (Burstein-Moss shift) due to the native donors introduced by the irradiation. In Ga-rich material, optical-absorption measurements show that the irradiation-introduced native defects are inside the band gap, where they are incorporated as acceptors. The observed irradiation-produced changes in the optical-absorption edge and the carrier populations in InxGa1−xN are in excellent agreement with the predictions of the amphoteric defect model.

Molecular simulation study of miscibility in InxGa1−xN ternary alloys

Monte Carlo molecular simulation is applied to calculate miscibility behavior of a valence force-field model of InGaN alloy as a function of temperature. Calculations find that the upper critical solution temperature is 1550 K, in good agreement with previous studies based on regular solution theory. The simulations show that the excess entropy is small, and the excess enthalpy is insensitive to temperature, indicating that the regular-solution treatment is appropriate for this system.

Biexciton luminescence from AlxGa1−xN epitaxial layers

Excitonic optical properties of Ga-rich AlxGa1−xN ternary alloy epitaxial layers (x=0.019, 0.038, 0.057, 0.077, and 0.092) have been studied by means of photoluminescence (PL) and time-resolved PL spectroscopy. The luminescence line due to radiative recombination of biexcitons was clearly observed for all of the five ternary alloy epitaxial layers with different Al compositions. The energy separation between exciton luminescence and biexciton luminescence increased with increasing Al composition, which indicated the increase in the binding energy of biexcitons in ternary alloys.

Reduction of threading dislocations in crack-free AlGaN by using multiple thin SixAl1−xN interlayers

Crack-free AlGaN thin films were directly grown on SiC substrates by metalorganic vapor phase epitaxy, and their threading dislocation density was reduced by one order of magnitude using 1–2 nm thick, heavily Si-doped AlN multiple interlayers. The interlayers form SixAl1−xN ternary alloys, where the Si molar fraction ranges typically from 0.07 to 0.17. This technique enables us to grow crack-free AlGaN films, since the film thickness of about 1 μm is much smaller than that required in conventional epitaxial lateral overgrowth techniques. Both termination and looping of threading dislocations were observed near the interlayers using cross-sectional transmission electron microscopy. Light emitting devices with the SixAl1−xN multiple interlayers showed a remarkable improvement in the intensity and spectral width of electroluminescence and the series resistance.

Localized biexcitons in AlxGa1−xN ternary alloy epitaxial layers

AbstractExcitonic optical properties of AlxGa1−xN ternary alloy epitaxial layers have been studied by means of photoluminescence (PL), time‐resolved PL, and PL excitation (PLE) spectroscopy. The luminescence line due to radiative recombination of biexcitons was clearly observed in an Al0.08Ga0.92N epitaxial layer. The PL decay time of excitons and biexcitons was estimated to be 350 and 210 ps, respectively. PLE spectroscopy of biexcitons enabled us to observe a two‐photon absorption process of biexcitons. On the basis of the energy separation between exciton resonance and two‐photon biexciton resonance, the binding energy of biexcitons in Al0.08Ga0.92N was estimated to be 15 ± 2 meV. This value was approximately 2.5 times as large as the binding energy of biexcitons in GaN. This value was also found to be comparable to the observed energy separation between the exciton luminescence and the biexciton luminescence, which indicated the strong localization of biexcitons. (© 2003 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Temperature dependence of Stokes shift in InxGa1−xN epitaxial layers

Optical properties of InxGa1−xN epitaxial layers with various indium compositions (x=0.02, 0.03, 0.05, 0.06, and 0.09) have been studied by means of temperature-dependent optical absorption and photoluminescence spectroscopy. A clear peak due to the absorption of InxGa1−xN ternary alloys was observed up to 300 K, which enabled us to investigate the temperature dependence of the Stokes shift. The Stokes shift at 4 K increased with an increase in the indium composition, and was estimated to be 22 and 45 meV for the samples with x=0.02 and 0.09, respectively. With an increase in temperature up to about 50 K, the Stokes shift increased slightly. With a further increase in temperature from 50 to 100 K, the Stokes shift decreased. Above 100 K, the Stokes shift was independent of the temperature and showed an almost constant value up to 300 K. The Stokes shift at 300 K was estimated to be 19 and 34 meV for the samples with x=0.02 and 0.09, respectively. This temperature dependence of the Stokes shift was characteristically common to all of the samples used in the present work, and was observed to be more prominent for the samples with higher indium compositions.

Temperature Dependence of Optical Absorption Spectra in In x Ga 1— x N Epitaxial Layers

Optical properties of InxGa1—xN epitaxial layers with indium compositions of x = 0.02, 0.05, and 0.09 have been studied by means of optical absorption (OA) and photoluminescence (PL) spectroscopy. The clear peak due to the absorption of InxGa1—xN ternary alloys was observed up to 300 K for the first time. As a result, temperature dependence of Stokes shift was investigated. The Stokes shift was estimated to be 51 meV for the sample with x = 0.05. With increasing temperature up to about 50 K, the Stokes shift slightly increased. With further increasing temperature from 50 to 100 K, the Stokes shift conversely decreased. Above 100 K, the Stokes shift was independent of temperature and indicated almost a constant value up to 300 K. The Stokes shift at 300 K was estimated to be 44 meV. This temperature dependence of Stokes shift was a characteristic common to all of the samples used in the present work.

Effects of Si-doping on luminescence properties of InxGa1−xN epitaxial layers

Recombination dynamics of intense near-UV luminescence from InxGa1−xN (x⩽0.1) ternary alloy epitaxial layers, especially from Si-doped ternary alloy layers, has been studied by means of time-resolved spectroscopy. The intensity of the near-UV luminescence from Si-doped layers was approximately one order of magnitude stronger than that from undoped layers. Furthermore, an additional sharp luminescence line was observed under high-density excitation only from the Si-doped layers. The effects of Si-doping on the optical properties of InxGa1−xN layers are discussed.