Mid-infrared Intersubband Absorption Research Articles

Mid-infrared intersubband absorption in strain-balanced non-polar (In)AlGaN/InGaN multi-quantum wells

Mid-infrared intersubband transitions in strain-balanced m-plane (In)AlxGa1-xN/In0.16Ga0.84N (0.19≤x≤0.3) multi-quantum wells are reported for the first time in the range of 3.4–5.1 µm (244–360 meV). Direct and attenuated total-reflection infrared absorption measurements are interpreted using structural information revealed by high-resolution x-ray diffraction and transmission electron microanalysis. The experimental intersubband energies are better reproduced by calculations using the local-density approximation than the Hartree-Fock approximation for the exchange-correlation correction. The effect of charge density, quantum well width, and barrier alloy composition on the intersubband transition energy is also investigated to evaluate the potential of this material for practical device applications.

Near- and mid-infrared intersubband absorption in top-down GaN/AlN nano- and micro-pillars

We present a systematic study of top-down processed GaN/AlN heterostructures for intersubband optoelectronic applications. Samples containing quantum well superlattices that display either near- or mid-infrared intersubband absorption were etched into nano- and micro-pillar arrays in an inductively coupled plasma. We investigate the influence of this process on the structure and strain-state, on the interband emission and on the intersubband absorption. Notably, for pillar spacings significantly smaller (≤1/3) than the intersubband wavelength, the magnitude of the intersubband absorption is not reduced even when 90% of the material is etched away and a similar linewidth is obtained. The same holds for the interband emission. In contrast, for pillar spacings on the order of the intersubband absorption wavelength, the intersubband absorption is masked by refraction effects and photonic crystal modes. The presented results are a first step towards micro- and nano-structured group-III nitride devices relying on intersubband transitions.

Mid-Infrared Intersubband Absorption from p-Ge Quantum Wells Grown on Si Substrates

Mid-infrared intersubband absorption from p-Ge quantum wells with Si0.5Ge0.5 barriers grown on a relaxed Si0.2Ge0.8 buffer on a Si substrate is demonstrated from 6 to 9 μm wavelength at room temperature and can be tuned by adjusting the quantum well thickness. Fourier transform infra-red transmission demonstrate clear absorption peaks corresponding to intersubband transitions among confined hole states. The work indicates an approach that will allow quantum well intersubband photodetectors to be realized on Si substrates in the important atmospheric transmission window of 8-13 μm where many chemical and biological molecules have unique molecular absorption lines for spectroscopic identification.

8-band k·p modelling of mid-infrared intersubband absorption in Ge quantum wells

The 8-band k·p parameters which include the direct band coupling between the conduction and the valence bands are derived and used to model optical intersubband transitions in Ge quantum well heterostructure material grown on Si substrates. Whilst for Si rich quantum wells the coupling between the conduction bands and valence bands is not important for accurate modelling, the present work demonstrates that the inclusion of such coupling is essential to accurately determine intersubband transitions between hole states in Ge and Ge-rich Si1−xGex quantum wells. This is due to the direct bandgap being far smaller in energy in Ge compared to Si. Compositional bowing parameters for a range of the key modelling input parameters required for Ge/SiGe heterostructures, including the Kane matrix elements, the effective mass of the Γ2′ conduction band, and the Dresselhaus parameters for both 6- and 8-band k·p modelling, have been determined. These have been used to understand valence band intersubband transitions in a range of Ge quantum well intersubband photodetector devices in the mid-infrared wavelength range.

Mid-infrared intersubband absorption from p-Ge quantum wells grown on Si substrates

Mid-infrared intersubband absorption from p-Ge quantum wells with Si0.5Ge0.5 barriers grown on a Si substrate is demonstrated from 6 to 9 μm wavelength at room temperature and can be tuned by adjusting the quantum well thickness. Fourier transform infra-red transmission and photoluminescence measurements demonstrate clear absorption peaks corresponding to intersubband transitions among confined hole states. The work indicates an approach that will allow quantum well intersubband photodetectors to be realized on Si substrates in the important atmospheric transmission window of 8–13 μm.

Temperature dependence of mid-infrared intersubband absorption in AlGaN/GaN multiple quantum wells

The temperature dependence of the mid-infrared intersubband (ISB) absorption in non-polar (m-plane) and polar (c-plane) AlGaN/GaN quantum wells (QWs) is studied. The ISB absorption shifts to higher energy as the temperature is reduced from 300 K to below 10 K. Both m-plane and c-plane QWs show a small energy shift (1.6–2.6 meV) compared to AlGaAs/GaAs (3.5–5.2 meV) and AlSb/InAs (6.2 and 12 meV) QWs. Theoretical calculations considering the temperature induced material constant changes show good agreement with the experimental results. These results suggest that ISB transition energies in AlGaN/GaN QWs are more stable against temperature change mainly because of the heavy effective masses and small nonparabolicities.

Observation of mid-infrared intersubband absorption in non-polar m-plane AlGaN/GaN multiple quantum wells

Mid-infrared (4.20–4.84 μm) intersubband absorption in non-polar m-plane Al0.5Ga0.5N/GaN multiple-quantum wells is observed at room temperature. 10 period Al0.5Ga0.5N/GaN multiple-quantum wells were grown on free-standing m-plane GaN substrates by metalorganic chemical vapor deposition (MOCVD), and the high-quality structural and optical properties are revealed by x-ray diffraction and photoluminescence studies. Through this we have demonstrated that MOCVD grown non-polar m-plane AlGaN/GaN quantum wells are a promising material for mid-infrared intersubband devices.

Effect of delta doping on mid-infrared intersubband absorption in AlGaN/GaN step quantum well structures

We present an ensemble Monte Carlo simulation to investigate the dynamics of electrons in 3-level AlGaN/GaN step quantum wells (SQW), which are potentially interesting for the development of mid-infrared optically pumped intersubband (ISB) terahertz lasers. Through comparison, delta-doping is found holding advantage over uniform-doping in mid-infrared ISB absorption. We demonstrate that ISB absorption widths and peak values significantly depend on the doping concentration, but not on the doping location. The broadening of spectral lineshapes with higher doping density is attributed to the higher electron–electron scattering rate.

Characterization of thin AlSb/AlAs barriers on InAs by mid-infrared intersubband absorption measurements

We present mid-infrared intersubband absorption measurements on InAs/AlSb coupled-quantum-wells systems with thin AlSb barriers. The AlSb barrier width in our samples is varied between 1 and 4 monolayers, with a strained AlAs boundary layer used for strain compensation. The optical absorption energy difference between the 1–4 and 2–3 transitions is used as a measure of the barrier coupling strength and modeled by a one-band Schroedinger solver. Our results let us conclude that the composite AlSb/AlAs barrier behaves like an effective AlSb barrier with an effective thickness that does not include the last As layer. This observation must be taken into account when designing complex heterostructures relying on very thin AlSb, like in InAs/AlSb quantum cascade lasers.

Strain relaxation in GaN/Al0.1Ga0.9N superlattices for mid-infrared intersubband absorption

We have studied the strain relaxation in GaN/Al0.1Ga0.9N superlattices grown by plasma-assisted molecular-beam epitaxy on different buffer layers. A periodic relaxation detected in situ was identified as an elastic phenomenon related to the stress induced by the Ga excess adlayer. Plastic relaxation results in an increase of the density of type a dislocations, which is minimum in the case of growth on GaN. There is no evidence of stacking fault formation or crack propagation in any of the samples.

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.

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.

Midinfrared intersubband absorption in InGaAs/GaAsSb multiple quantum wells

We report on the observation of midinfrared intersubband (ISB) absorption in InGaAs/GaAsSb multiple quantum wells grown lattice-matched to InP substrates by molecular beam epitaxy. ISB absorption in a broad wavelength region (5.8–11.6 μm) is observed in quantum wells with well widths ranging between 4.5 and 12 nm. The conduction band offset at the InGaAs/GaAsSb heterointerface is measured to be 360 meV, giving an excellent agreement between the theoretically calculated ISB transition energies and the Fourier-transform infrared spectroscopy measurements over the whole range of well widths under investigation.

InGaAs/GaAsSb Heterostructures: Aluminum-Free Intersubband Devices

AbstractAn experimental study on mid-infrared intersubband absorption in InGaAs/GaAsSb multiple quantum wells grown lattice-matched to InP substrates by molecular beam epitaxy is presented. Intersubband absorption in a broad wavelength region (5.8 - 11.6 μm) is observed in multiple quantum well samples with well widths ranging between 4.5 and 12 nm. A conduction band offset at the InGaAs/GaAsSb heterointerface of 360 meV gives an excellent agreement between the theoretically calculated ISB transition energies and the Fourier-transform infrared spectroscopy measurements over the whole range of well widths under investigation. Two kinds of intersubband devices based on the InGaAs/GaAsSb material system are presented: a quantum well infrared photodetector operating at a wavelength of 5.6μm and an aluminum-free quantum cascade laser. The presented quantum cascade laser emits at a wavelength of 11.3 μm, with a threshold current density of 1.7 kA/cm2 at 78 K.

Midinfrared intersubband absorption in wide band gap II-VI ZnxCd1−xSe multiple quantum wells with metastable zincblende MgSe barriers

The authors report the observation of midinfrared intersubband (ISB) absorption in MgSe∕ZnxCd1−xSe multiple quantum wells (MQWs) grown on (001) InP substrates by molecular beam epitaxy. The in situ reflection high energy electron diffraction shows that zincblende MgSe∕ZnxCd1−xSe MQWs can be grown with the introduction of ZnxCd1−xSe spacer layers. For quantum wells with the nominal well width of 11–15 ML (3.2–4.4nm) ISB absorption in the wavelength range of 3.3–4.9μm is observed. The conduction band offset (CBO) of the MgSe∕ZnxCd1−xSe heterostructure is estimated to be 1.2eV. Using the estimated CBO, the theoretically calculated interband and ISB transition energies agree well with photoluminescence and Fourier-transform infrared spectroscopy measurements. With such a large CBO, ISB transitions in this material system could be extended to the near infrared region.

Midinfrared intersubband absorption in lattice-matched AlInN∕GaN multiple quantum wells

We report the observation of midinfrared intersubband (ISB) absorption in nearly lattice-matched AlInN∕GaN multiple-quantum-wells. A clear absorption peak is observed around 3μm involving transitions from the conduction band ground state to the first excited state. In addition to ISB absorption, photoluminescence experiments were carried out on lattice- matched AlInN∕GaN single quantum wells in order to determine the spontaneous polarization discontinuity between GaN and Al0.82In0.18N compounds. The experimental value is in good agreement with theoretical predictions. Our results demonstrate that the AlInN∕GaN system is very promising to achieve crack-free and low dislocation density structures dedicated to intersubband devices in the 2–4μm wavelength range.

Bound-to-bound midinfrared intersubband absorption in carbon-doped GaAs∕AlGaAs quantum wells

Bound-to-bound intersubband absorption in the valence band of modulation-doped GaAs quantum wells with digitally alloyed AlGaAs barriers was studied in the midinfrared wavelength range. A high-purity solid carbon source was used for the p-type doping. Strong narrow absorption peaks due to heavy-to-heavy hole transitions are observed with out-of-plane polarized light, and weaker broader features with in-plane polarized light. The heavy-to-heavy hole transition energy spans the spectral range between 206 to 126 meV as the quantum well width is increased from 25 to 45 Å. The experimental results are found to be in agreement with calculations of a six-band k∙p model taking into account the full band structure of the digital alloy.

Midinfrared intersubband absorption in strain-compensated InGaP/InGaAs superlattices on (001) GaAs

Intersubband optical transitions in strain-compensated In0.32Ga0.68As–In0.32Ga0.68P superlattices grown using gas-source molecular-beam epitaxy on (001)GaAs are investigated by means of midinfrared absorption and low-temperature photoluminescence. Strong absorption corresponding to the transition from the first to second electronic subband is measured at wavelengths between 5.6 and 10.5 μm. The data indicate that the conduction band offset between the strained In0.32Ga0.68As and the strained In0.32Ga0.68P is 370 meV and the electron effective mass in the strained In0.32Ga0.68As well is 0.060m0. This material system is an interesting GaAs-based candidate for applications in midinfrared intersubband emitters and detectors.

Midinfrared intersubband absorption on AlGaN/GaN-based high-electron-mobility transistors

Intersubband absorption measurements on two nominally undoped AlGaN/GaN-based high-electron-mobility transistors with different Al compositions in the barrier layer are presented. The first transistor with a barrier consisting of Al0.6Ga0.4N showed an absorption peak at 247 meV (1973 cm−1) with a full width at half maximum (FWHM) of 126 meV, while the second device utilizing an Al0.8Ga0.2N barrier had its peak at 306 meV (2447 cm−1) with a FWHM of 86 meV. Self-consistently computed potentials and intersubband transition energies showed good agreement with the experimental findings, and therefore confirmed previously published values for the internal piezoelectric field in such structures.

Optimization of lamellar gratings for quantum-well infrared photodetectors

This article presents the results of a self-consistent model of midinfrared intersubband absorption in n-doped quantum wells, which are optically coupled by one-dimensional reflective gratings. In the vicinity of the cutoff of the grating, multiple reflections at the well-barrier interfaces play an important role. With an optimized grating, a near-unity absorption quantum efficiency is predicted for the transverse-magnetic polarization. The influence of the top contact on the design of the grating is studied. Performances of standard detectors are compared to those using an additional aluminum-arsenide confinement layer below the bottom contact. Finally, the influence of the number of wells on the performance of the device is investigated.