L. Nevou

GaN/AlN short-period superlattices for intersubband optoelectronics: A systematic study of their epitaxial growth, design, and performance

We have studied the effect of growth and design parameters on the performance of Si-doped GaN/AlN multiquantum-well (MQW) structures for intersubband optoelectronics in the near infrared. The samples under study display infrared absorption in the 1.3–1.9 μm wavelength range, originating from the photoexcitation of electrons from the first to the second electronic level in the QWs. A commonly observed feature is the presence of multiple peaks in both intersubband absorption and interband emission spectra, which are attributed to monolayer thickness fluctuations in the quantum wells. These thickness fluctuations are induced by dislocations and eventually by cracks or metal accumulation during growth. The best optical performance is attained in samples synthesized with a moderate Ga excess during the growth of both the GaN QWs and the AlN barriers without growth interruptions. The optical properties are degraded at high growth temperatures (>720 °C) due to the thermal activation of the AlN etching of GaN. Fr...

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. ...

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.

Ultrastrong coupling regime and plasmon polaritons in parabolic semiconductor quantum wells.

Ultrastrong coupling is studied in a modulation-doped parabolic potential well coupled to an inductance-capacitance resonant circuit. In this system, in accordance to Kohns theorem, strong reduction of the energy level separation caused by the electron-electron interaction compensates the depolarization shift. As a result, a very large ratio of 27% of the Rabi frequency to the center resonance frequency as well as a polariton gap of width 2π × 670  GHz are observed, suggesting parabolic quantum wells as the system of choice in order to explore the ultrastrong coupling regime.

GaN/AlGaN intersubband optoelectronic devices

This paper reviews recent progress toward intersubband (ISB) devices based on III-nitride quantum wells (QWs). First, we discuss the specific features of ISB active region design using GaN/AlGaN materials, and show that the ISB wavelength can be tailored in a wide spectral range from near- to long infrared wavelengths by engineering the internal electric field and layer thicknesses. We then describe recent results for electro-optical waveguide modulator devices exhibiting a modulation depth as large as 14 dB at telecommunication wavelengths. Finally, we address a new concept of III-nitride QW detectors based on the quantum cascade scheme, and show that these photodetectors offer the prospect of high-speed devices at telecommunication wavelengths.

Room temperature demonstration of GaN∕AlN quantum dot intraband infrared photodetector at fiber-optics communication wavelength

We fabricated a communication wavelength photodetector based on intraband transition in GaN∕AlN self-assembled quantum dot heterostructures. The quantum dot photodetector is based on in-plane transport and has a room temperature spectral peak responsivity of 8mA∕W at wavelength of 1.41μm. We use multipass waveguide geometry to show that the polarization sensitive optical absorption spectrum of the heterostructure is nearly the same as its photocurrent spectral response. This establishes that the detector’s response is due to the presence of quantum dots in its active layer. We use photoluminescence, transmission, and intraband photocurrent spectroscopy to consistently describe the alignment between the energy levels of the quantum dots and that of the wetting layer.

Short-wavelength intersubband electroabsorption modulation based on electron tunneling between GaN∕AlN coupled quantum wells

Room temperature intersubband electroabsorption modulation in GaN∕AlN coupled quantum wells is demonstrated at short infrared wavelengths, covering the fiber-optics telecommunication wavelength range. Electroabsorption modulation with opposite sign is observed at λ=1.2–1.67μm and λ=2.1–2.4μm. The electromodulation originates from electron tunneling between a wide well (reservoir) and a narrow well separated by an ultrathin AlN barrier. Both the intersubband absorption and the modulation spectroscopic measurements are in good agreement with the simulations. The maximum modulation depth is ∼44% at λ=2.2μm. The −3dB cutoff frequency limited by the RC time constant is 11.5MHz for 700×700μm2 mesas.Room temperature intersubband electroabsorption modulation in GaN∕AlN coupled quantum wells is demonstrated at short infrared wavelengths, covering the fiber-optics telecommunication wavelength range. Electroabsorption modulation with opposite sign is observed at λ=1.2–1.67μm and λ=2.1–2.4μm. The electromodulation originates from electron tunneling between a wide well (reservoir) and a narrow well separated by an ultrathin AlN barrier. Both the intersubband absorption and the modulation spectroscopic measurements are in good agreement with the simulations. The maximum modulation depth is ∼44% at λ=2.2μm. The −3dB cutoff frequency limited by the RC time constant is 11.5MHz for 700×700μm2 mesas.

Short wavelength (λ=2.13μm) intersubband luminescence from GaN∕AlN quantum wells at room temperature

The authors report the observation of room-temperature intersubband luminescence at λ=2.13μm from GaN∕AlN quantum wells under optical pumping at λ=0.98μm. The quantum wells are designed to exhibit three bound states in the conduction band. The emission arises from the e3e2 intersubband transition. Photoluminescence excitation spectroscopy shows that the emission is only observed for p-polarized excitation at wavelengths corresponding to the e1e3 intersubband transition. The measured external quantum efficiency is 10pW∕W.

Indium surfactant effect on AlN/GaN heterostructures grown by metal-organic vapor-phase epitaxy: Applications to intersubband transitions

We report on a dramatic improvement of the optical and structural properties of AlN∕GaN multiple quantum wells (MQWs) grown by metal-organic vapor-phase epitaxy using indium as a surfactant. This improvement is observed using photoluminescence as well as x-ray diffraction. Atomic force microscopy shows different surface morphologies between samples grown with and without In. This is ascribed to a modified relaxation mechanism induced by different surface kinetics. These improved MQWs exhibit intersubband absorption at short wavelength (2μm). The absorption linewidth is as low as 65meV and the absorption coefficient is increased by 85%.

Electron confinement in strongly coupled GaN∕AlN quantum wells

The electron confinement in double GaN∕AlN quantum wells coupled by an ultrathin AlN barrier has been investigated by means of structural and optical measurements. The intersubband absorption spectra present two peaks attributed to the e1-e2 and e1-e3 transitions, respectively. The results of photoluminescence and intersubband spectroscopies are compared with simulations of the electronic structure based on the envelope function formalism. A good agreement is obtained for all investigated samples. These results provide clear evidence that the potential drop at the GaN∕AlN heterointerfaces is not abrupt, but is spread over one monolayer.