F. H. Julien

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...

Intraband absorption in n-doped InAs/GaAs quantum dots

We have investigated the intraband absorption within the conduction band of InAs/GaAs quantum dots. The islands obtained by self-organized epitaxy are modulation doped with a silicon planar doping 2 nm below the dot layer plane. The dots exhibit infrared absorption polarized along the growth axis in the midinfrared spectral range. The absorption is maximum around 150 meV with a large broadening around 130 meV. This broadening is attributed to size fluctuations within the one dot layer plane and the consequent variation of the electron confinement energy with the dot size. The magnitude of the absorption along the growth axis for the one dot layer plane is ≈2.5×10−2% which corresponds to an equivalent absorption cross section σz≈3.1×10−15 cm2. We show that the intraband absorption can also be clearly observed using a photoinduced infrared absorption technique with the doped quantum dots.

Nanometer Scale Spectral Imaging of Quantum Emitters in Nanowires and Its Correlation to Their Atomically Resolved Structure

We report the spectral imaging in the UV to visible range with nanometer scale resolution of closely packed GaN/AlN quantum disks in individual nanowires using an improved custom-made cathodoluminescence system. We demonstrate the possibility to measure full spectral features of individual quantum emitters as small as 1 nm and separated from each other by only a few nanometers and the ability to correlate their optical properties to their size, measured with atomic resolution. The direct correlation between the quantum disk size and emission wavelength provides evidence of the quantum confined Stark effect leading to an emission below the bulk GaN band gap for disks thicker than 2.6 nm. With the help of simulations, we show that the internal electric field in the studied quantum disks is smaller than what is expected in the quantum well case. We show evidence of a clear dispersion of the emission wavelengths of different quantum disks of identical size but different positions along the wire. This dispersion is systematically correlated to a change of the diameter of the AlN shell coating the wire and is thus attributed to the related strain variations along the wire. The present work opens the way both to fundamental studies of quantum confinement in closely packed quantum emitters and to characterizations of optoelectronic devices presenting carrier localization on the nanometer scale.

Near infrared quantum cascade detector in GaN∕AlGaN∕AlN heterostructures

A quantum cascade detector in the GaN/AlGaN/AlN material system was implemented. The design takes advantage of the large internal field existing in the nitrides in order to generate the essential saw tooth energy level structure. The device operates in the near IR spectral range with a room temperature responsivity at λ=1.7μm of 10mA∕W (1000V∕W) at zero bias. The spectroscopic measurements are in good agreement with simulations.

Band‐edge and deep level photoluminescence of pseudomorphic Si1−x−yGexCy alloys

Photoluminescence of strained Si1−x−yGexCy alloys grown by rapid thermal chemical vapor deposition on Si(100) is investigated. Two dominant features are reported: At low pump intensities, the photoluminescence is dominated by a deep level broad luminescence peak around 800 meV whereas at high pump intensities, a well‐resolved band‐edge luminescence (no phonon and transverse optic replica) is observed. At 77 K, we attribute this band‐edge feature to an electron‐hole plasma luminescence of the ternary alloy. The dependences of the deep level and band‐edge peaks versus the excitation power density are, respectively square‐root‐like or superlinear. A blue shift of the energy gap of Si1−x−yGexCy alloys with respect to Si1−xGex alloy is observed. The blue shift increase with carbon content corresponds to what is expected for the bulk alloy. An eventual influence of the strain relaxation cannot be excluded.

Detailed analysis of second‐harmonic generation near 10.6 μm in GaAs/AlGaAs asymmetric quantum wells

We report on the observation of resonant intersubband second‐harmonic generation in asymmetric GaAs/AlGaAs quantum wells using a cw or Q‐switched tunable CO2 laser as the pumping source. The dependence of the second‐harmonic intensity with the pump photon wavelength is presented for the first time. A Lorentzian‐like second‐harmonic line shape is found with a maximum at 10.9 μm and a linewidth of 0.4 μm (4.1 meV). These results are in good agreement with theoretical predictions. The expected quadratic dependence of the second‐harmonic conversion efficiency with pump intensity is well verified for intensities up to 150 kW/cm2. The calibrated second‐harmonic power reaches 0.13 μW for a cw pump power of 0.8 W. The value of 7.2×10−7 m/V deduced for the second‐order nonlinear susceptibility is about 1900 times greater than that found in bulk GaAs.

Infrared spectroscopy of intraband transitions in self-organized InAs/GaAs quantum dots

We have investigated the midinfrared absorption between confined levels of undoped InAs/GaAs quantum dots obtained by self-organized growth. The infrared absorption is measured by a photoinduced infrared spectroscopy. Quantum dots with different sizes are analyzed as a function of temperature, interband pump photon energy, intensity, and infrared polarization. We show that in the 90–250 meV energy range the quantum dots exhibit intraband absorption between confined levels, which are polarized along the growth axis as for usual conduction intersubband transitions in quantum wells. Intraband absorption is observed for either selective excitation of the dots or excitation via absorption in the wetting and GaAs layers. Based on the energy position and the temperature dependence, the infrared resonances are attributed to intraband transitions between confined holes and to bound-to-continuum transitions of electrons, which, respectively, shift to high and low energy as the dot size is decreased. The reported fe...

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.

Intersubband spectroscopy of doped and undoped GaN/AlN quantum wells grown by molecular-beam epitaxy

We report experimental and theoretical results on interband and intersubband transitions in GaN quantum wells with strained AlN barriers. All of the samples are grown by molecular-beam epitaxy on sapphire (0001) substrates. The results show that even at room temperature, strong electron localization occurs in the plane of the quantum wells due to the combined effect of monolayer thickness fluctuations and the high internal field in the GaN layers. We also demonstrate that the intersubband absorption is systematically blueshifted in n-doped quantum wells with respect to nominally undoped samples as a result of strong many-body effects, namely the exchange interaction. The results for both undoped and doped quantum wells are in good agreement with simulations.