Intersubband Absorption Energy Research Articles

Tuning of the intersubband absorption in a shallow InAs/InP quantum wire by a transverse electric field

Intersubband absorption in a shallow InAs/InP quantum wire (QWR) under a transverse electric field perpendicular to the wire axis is investigated theoretically using the plane wave method in detail. The transverse field is oriented either parallel or perpendicular to the height direction of QWR. The intersubband absorption energy increases with increasing the tilt angle of field along the height direction. The large cross-sectional ratio of QWR leads to the strong polarization anisotropy of the intersubband absorption, which is sensitive to one polarization of light along the width direction while it is insensitive to one polarization of light along the height direction. The intersubband oscillator strength of polarized light along the width direction decreases as the tilt angle of field increases whereas that of polarized light along the height direction first increases until it reaches a maximum, and then decreases. A transverse applied field is shown to provide an efficient tool to tune the wavelength and output of the intersubband device of shallow QWR.

Tuning intersubband absorption in a shallow InAs/InP quantum wire via a transverse tilted magnetic field

Intersubband absorption in a shallow InAs/InP rectangular quantum wire (QWR) in a transverse tilted magnetic field is investigated theoretically within the effective mass approximation. The results show that the intersubband absorption energy increases monotonously with increasing the tilt angle and magnetic field strength. Intersubband optical absorption favors the incident light polarized along the wire width direction, while it is not allowed for the incident light polarized along the wire height direction, showing the absorption anisotropy due to the QWR geometric shape. Therefore, it provides additional freedom to tune the wavelength of intersubband devices by varying the tilted angle of transverse magnetic field in a shallow QWR.

Near-Infrared Absorption in Lattice-Matched AlInN/GaN and Strained AlGaN/GaN Heterostructures Grown by MBE on Low-Defect GaN Substrates

We have investigated near-infrared absorption and photocurrent in lattice-matched AlInN/GaN and strained AlGaN/GaN heterostructures grown by molecular-beam epitaxy (MBE) on low-defect GaN substrates for infrared device applications. The AlGaN/GaN heterostructures were grown under Ga-rich conditions at 745°C. Material characterization via atomic force microscopy and high-resolution x-ray diffraction indicates that the AlGaN/GaN heterostructures have smooth and well-defined interfaces. A minimum full-width at half-maximum of 92 meV was obtained for the width of the intersubband absorption peak at 675 meV of a 13.7 A GaN/27.5 A Al0.47Ga0.53N superlattice. The variation of the intersubband absorption energy across a 1 cm × 1 cm wafer was ±1%. An AlGaN/GaN-based electromodulated absorption device and a quantum well infrared detector were also fabricated. Using electromodulated absorption spectroscopy, the full-width at half-maximum of the absorption peak was reduced by 33% compared with the direct absorption measurement. This demonstrates the suitability of the electromodulated absorption technique for determining the intrinsic width of intersubband transitions. The detector displayed a peak responsivity of 195 μA/W at 614 meV (2.02 μm) without bias. Optimal MBE growth conditions for lattice-matched AlInN on low-defect GaN substrates were also studied as a function of total metal flux and growth temperature. A maximum growth rate of 3.8 nm/min was achieved while maintaining a high level of material quality. Intersubband absorption in AlInN/GaN superlattices was observed at 430 meV with full-width at half-maximum of 142 meV. Theoretical calculations of the intersubband absorption energies were found to be in agreement with the experimental results for both AlGaN/GaN and AlInN/GaN heterostructures.

Intersubband absorption energy shifts in 3-level system for asymmetric quantum well terahertz emitters

Intersubband absorption energy shifts in 3-level system stemming from depolarization and excitonlike effects are investigated. Analytically, the expressions we derive present good explanations to the conventional 2-level results and bare potential transition energy results; and numerical results show that they are more exact than the previous studies to describe the 3-level system depolarization and excitonlike shift (DES) character especially for higher carrier density (more than 8×1011 cm−2). One interesting detail we find is that the “large blue” DES becomes “slight redshift” in the low doping limit (less than 1.9×1011 cm−2), which may be neglected by the previous studies of intersubband transitions. Temperature character of DES in the step well structure is also numerically studied. Finally the above are applied to calculate asymmetric step quantum well structures. The two main functional aspects of terahertz (THz) emitters are discussed and several basic optimizing conditions are considered. By adjusting the well geometry parameters and material composition systematically, some optimized structures which satisfy all of the six conditions are recommended in tables. These optimizations may provide useful references to the design of 3-level-based optically pumping THz emitters.

Temperature stability of intersubband transitions in AlN/GaN quantum wells

Temperature dependence of intersubband transitions in AlN/GaN multiple quantum wells grown with molecular beam epitaxy is investigated both by absorption studies at different temperatures and modeling of conduction-band electrons. For the absorption study, the sample is heated in increments up to 400 °C. The self-consistent Schrödinger–Poisson modeling includes temperature effects of the band gap and the influence of thermal expansion on the piezoelectric field. We find that the intersubband absorption energy decreases only by ∼6 meV at 400 °C relative to its room temperature value.

Effect of doping on the mid-infrared intersubband absorption in GaN/AlGaN superlattices grown on Si(111) templates

We report on the effect of Si doping on the mid-infrared intersubband absorption in GaN/AlGaN superlattices. For increasing doping levels, interband luminescence displays a blueshift and a broadening of the band edge caused by the screening of the internal electric field and band-filling effects. The intersubband absorption energy is mainly governed by many-body effects like exchange interaction and depolarization shift, which increase the e1–e2 subband separation. The ISB blueshift induced by many-body effects can be more than 50% of the e1–e2 transition energy.

Effects of well width and growth temperature on optical and structural characteristics of AlN/GaN superlattices grown by metal-organic chemical vapor deposition

AlN/GaN superlattices (SLs) employing various well widths (from 1.5 to 7.0 nm) are grown by metal-organic chemical vapor deposition technique at various growth temperatures (TS) (from 900 to 1035 °C). The photoluminescence (PL), x-ray diffraction, and intersubband (ISB) absorption characteristics of these SLs and their dependency on well width and growth temperature are investigated. Superlattices with thinner wells (grown at the same TS) or grown at lower TS (employing the same well width) are shown to demonstrate higher strain effects leading to a higher PL energy and ISB absorption energy. Simulations are employed to explain the experimental observations. ISB absorptions from 1.04 to 2.15 μm are demonstrated via controlling well width and growth temperature.

Midinfrared intersubband absorption in GaN/AlGaN superlattices on Si(111) templates

We report on the observation of midinfrared intersubband absorption in Si-doped GaN/AlGaN superlattices grown by plasma-assisted molecular-beam epitaxy on semi-insulating GaN-on-Si(111) templates. TM-polarized absorption attributed to transition between the first two electronic levels in the quantum wells peaked in the range from 2 to 9 μm. The relative spectral width remains around 20% in the whole midinfrared spectral range. Doping is predicted to have a large influence on the intersubband absorption energy due to screening of polarization-induced internal electric field.

Systematic experimental and theoretical investigation of intersubband absorption inGaN∕AlNquantum wells

We have studied the electronic confinement in hexagonal (0001) $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{N}$ multiple quantum wells by means of structural (high-resolution x-ray diffraction and transmission electron microscopy) as well as optical characterizations, namely intersubband absorption and interband photoluminescence spectroscopies. Intense intersubband absorptions covering the $1.33--1.91\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ wavelength range have been measured on a series of samples with well thicknesses varying from $1\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}2.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The absorption line shape exhibits either a pure Lorentzian shape or multiple peaks. In the first case the broadening is homogeneous with a state-of-the-art low value of $67\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. We deduce a dephasing time of the electrons in the excited subband ${T}_{2}$ of about $20\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$. For structured spectra the absorption can be perfectly reproduced with a sum of several Lorentzian curves; the individual peaks originate from absorption in quantum well regions with thickness equal to an integer number of monolayers. We have also carried out simulations of the electronic structure which point out the relevance of the nonparabolicity and many-body corrections on the intersubband absorption energy. The intersubband absorption exhibits a blue shift with doping as a result of many-body effects dominated by the exchange interaction. An excellent agreement with the experimental data is demonstrated. The best fit is achieved using a conduction band offset at the $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{N}$ heterointerfaces of $1.7\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and a polarization discontinuity $\ensuremath{\Delta}P∕({ϵ}_{0}{ϵ}_{r})$ of $10\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.3em}{0ex}}\mathrm{M}\mathrm{V}∕\mathrm{cm}$.

Photoluminescence, intersubband absorption, and double crystal x-ray diffraction in p-doped InGaAs/AlGaAs strained multiple quantum wells

We report the correlation of photoluminescence (PL), infrared intersubband absorption, and double crystal x-ray diffraction (DCXRD) data for a p-doped InGaAs/AlGaAs strained multi- ple-quantum-well structure grown by molecular beam epitaxy. A PL doublet at 1.476 and 1.563 eV involves two confined holes states and their 87 meV separation is in good agreement with the measured intersubband absorption of about 14.5 μm (85 meV). Furthermore, when the well width obtained from DCXRD measurement is included excellent agreement with an envelope function calculation is found for the energy levels determined by PL and intersubband absorption energy.

Intersubband absorption in n-type Si⧸Si 1−xGe x multiple quantum well structures formed by Sb segregant-assisted growth

n-Si/Si 1− x Ge x multiple quantum wells with “abrupt” heterointerfaces have been created by segregant-assisted growth (SAG) using Sb. Distinct well-width dependence of intersubband absorption energy in thinner quantum wells ( L z = 14−26 A ̊ ) has been successfully observed. The well-width dependence of the absorption peak energy was in good agreement with a calculation based on the Kronig-Penney model where interfacial transience was taken to be shorter than than 0.4 nm. Present results show the potential power of SAG for creating “abrupt” Si/Si 1− x Ge x interfaces.