Search

We report on a GaN/AlGaN quantum cascade detector operating in the terahertz spectral range. The device was grown by metal organic chemical vapor deposition on a c-sapphire substrate and relies on polar GaN/AlGaN step quantum wells. The active region thickness is in micrometer range. The structural, electrical, and optical investigations attest of high structural quality of the synthetized nitride material. The detector exhibits a peak photocurrent at 5.7 THz (23.6 meV) with a responsivity of 0.1 mA/W at 10 K under surface normal irradiation through a 10 μm period grating. The photocurrent persists up to 20 K.

We report on a prototype of piezogenerator based on vertically-aligned GaN nanowires grown by Plasma-Assisted Molecule Beam Epitaxy (PAMBE). Our device generates a peak-to-peak voltage from 20 mV to 60 mV depending on the applied force. The output power density reaches 0.76 mW/cm3 which is of the same order of magnitude as the state-of-art value. Furthermore, the pressure applied to deform the nanowires is of the order of kPa, much less than that used in previous reports (which were of the order of MPa) utilizing ZnO nanowires for the power generation. Thus, an enhanced mechanic-electrical conversion efficiency has been achieved. Our GaN nanowire based devices are promising both for pressure sensing applications and for mechanic energy harvesting.

We demonstrate intersubband polaritons in the short-infrared wavelength range (λ

Using the transfer matrix formalism, we have theoretically studied the vertical ballistic transport in GaN/AlGaN resonant tunneling diodes (RTDs) and superlattices with a small number of periods. We have calculated the transmission probability versus the longitudinal electron energy (T–E) and the current density–voltage (J–V) characteristics. Calculations of both T–E and J–V characteristics have been performed for different Al contents in the barriers. The asymmetry effects due to the internal electric field in the barriers are discussed. Applied to the RTD structure, our calculations demonstrate: (i) the increase of the peak-to-valley ratio of the negative differential resistance (NDR) with increasing Al content in the barriers, (ii) the dependence of the J–V resonance values on the current direction, and (iii) the asymmetry of the NDR with respect to the current direction due to the huge internal electric field in the structure. In the case of multiple quantum well structure (MQWS), the calculation results confirm the same trends as in the RTD case when the Al content is varied. In spite of the fact that it is more difficult to analyze the results in the case of MQWS, the obtained calculations demonstrate the applicability of the used model and of the numerical method to study GaN/AlGaN devices based on quantum well (QW) heterostructures. Furthermore, a design of an optimized 7QW structure operating symmetrically whatever the direction of the applied voltage is presented.

We have investigated intraband absorptions in GaN/AlN QDs for ultrafast all-optical switching applications at telecommunication wavelengths. Using time-resolved pump-probe experiments the electron lifetime in the excited state of the QDs is measured to be 165 fs. Intraband absorption saturation experiments reveal a record low switching energy. We then investigate quantum cascade ISB photodetectors based on GaN/AlGaN QWs and show that mesa devices with 17 × 17 μm 2 size provide a frequency response above 10 GHz at 1.5 μm wavelength.

In this paper we review the recent achievements in terms of GaN/ AlGaN-based intersubband (ISB) physics and devices. We first discuss the design issues and in particular the band bending effect and its influence on the ISB absorption. We then illustrate recent achievements in terms of III-nitride ISB devices with special emphasis on electro-optical modulator and a quantum cascade detector operating in the near-infrared. We show that these devices offer high frequency response at telecommunication wavelengths.

Spectra of interband light absorption in InAs/GaAs quantum dots covered by InGaAs quantum wells were investigated experimentally and theoretically. The main peaks in the spectra are connected with electron transitions between ground and excited quantum dot states. The value of an interband absorption cross section was determined. Interband light absorption was also studied under the conditions of interband optical pumping. The photoinduced change of interband absorption for polarized light was studied as a function of interband optical pump intensity and the light amplification was found. The change of absorption, including light amplification, is caused by the occupation of quantum dot states with non-equilibrium electrons and holes.

This paper reports on GaN/AIN light-emitting devices relying on intersubband transitions. All samples have been grown by molecular beam epitaxy. We first present a systematic investigation of single or coupled GaN/AIN quantum wells grown by MBE. We then present the recent observation of strong ISB resonant enhancement of the second-harmonic generation of 1 μm radiation. We finally report on the first observation of ISB luminescence in GaN QWs, which opens new prospects for the realization of nitride unipolar lasers based on current injection or optical pumping.

We have demonstrated GaN/AlN quantum dots (QD) photodetectors, relying on intraband absorption and in-plane carrier transport in the wetting layer. The devices operate at room temperature in the wavelength range 1.3–1.5 μm. Samples with 20 periods of Si-doped GaN QD layers, separated by 3 nm-thick AlN barriers, have been grown by plasma-assisted molecular-beam epitaxy on an AlN buffer on a c -sapphire substrate. Self-organized dots are formed by the deposition of 5 monolayers of GaN under nitrogen-rich conditions. The dot height is 1.2±0.6 to 1.3±0.6 nm and the dot density is in the range 1011–1012 cm−2. Two ohmic contacts were deposited on the sample surface and annealed in order to contact the buried QD layers. The dots exhibit TM polarized absorption linked to the s–pz transition. The photocurrent at 300 K is slightly blue-shifted with respect to the s–pz intraband absorption. The responsivity increases exponentially with temperature and reaches a record value of 10 mA/W at 300 K for detectors with interdigitated contacts.

In this work, we present the plasma-assisted molecular-beam epitaxial growth of quantum well infrared photodetector (QWIP) structures, including the Si-doped GaN/AlN short-period superlattice of the active region, conductive AlGaN claddings and integration of the final device. The growth of Si-doped GaN/AlN multiple quantum well (QW) structures is optimized by controlling substrate temperature, metal excess and growth interruptions. Structural characterization confirms a reduction of the interface roughness to the monolayer scale. P-polarized intersubband absorption peaks covering the 1.33–1.91 μm wavelength range are measured on samples with QW thickness varying from 1 to 2.5 nm. The absorption exhibits Lorentzian shape with a line width around 100 meV in QWs doped 5×1019 cm−3. To prevent partial depletion of the QWs owing to the internal electric field, we have developed highly-conductive Si-doped AlGaN cladding layers using In as a surfactant during growth. Complete ISB photodetectors with 40 periods of 1 nm-thick Si-doped GaN QWs with 2 nm-thick AlN barriers have been grown on conductive AlGaN claddings, the Al mole fraction of the cladding matching the average Al content of the active region. Temperature-dependent photovoltage measurements reveal a narrow (90 meV) detection peak at 1.39 μm.

We present a study of the effect of Si doping localization on the optical and structural properties of GaN/AlN multiple-quantum-well structures for intersubband (ISB) absorption at 1.55 μm. Samples were either undoped or Si doped in different regions (barrier, quantum well (QW), middle of barrier or middle of QW). Structural characterization by atomic force microscopy and X-ray diffraction does not show significant differences in the crystalline quality. All doped samples present room-temperature p-polarized ISB absorption of about 1%–2% per pass, with a line width of 80–90 meV. In contrast, undoped samples present a weaker ISB absorption with a record line width of 40 meV. Both photoluminescence (PL) and ISB absorption display structured shapes whose main peaks correspond to monolayer fluctuations of the well thickness. The emission and absorption line widths depend on the Si doping concentration, but not on the Si location.

We present a systematic investigation of ultrathin single and coupled GaN/AlN quantum wells grown by molecular beam epitaxy for applications to unipolar devices. Narrow Lorentzian-shaped intersubband absorptions are demonstrated tunable at telecommunication wavelengths. The linewidth is as small as 40 meV which sets a new state-of-the-art. Simulation of the intersubband wavelength and comparison with measurements leads us to revise the value of the conduction band offset at the GaN/AlN interface (1.7 ± 0.05 eV). We also present the observation of strong electron state coupling between two GaN wells separated by an ultra-thin 2 monolayer thick AlN barrier. This study allows us to refine the potential description at the GaN/AlN interface at the atomic monolayer scale. Excellent agreement with measurements is achieved assuming a potential drop over 1 ML.

The spectral and polarization dependences of intraband light absorption in n- and p-doped structures with InAs/GaAs quantum dots covered by InGaAs layers were studied experimentally and theoretically. The transmittance of p- and s-polarized mid-infrared radiation was studied in a multi-pass sample geometry. Elastic strain, piezoelectric fields and a complex valence band structure were taken into account in calculations of the energy spectrum, the carrier wavefunctions and intraband optical matrix elements. The experimentally determined value of the absorption cross section corresponding to |0 0 0→|0 0 1 intraband electron transitions is 4.1 × 10−15 cm2. Light absorption related to intraband hole transitions was found to be significantly smaller. The dispersion of the dot sizes was evaluated using the experimentally determined spectral widths of intraband absorption peaks.

The electronic confinement in GaN quantum wells coupled by an ultra-thin AlN barrier is investigated both experimentally and theoretically. The strong coupling between the wells is evidenced by the observation of two pronounced intersubband absorptions peaked at around 0.6 and 0.95 eV for 2 monolayer thick AlN coupling barrier. In agreement with calculations, these absorptions are attributed respectively to the transitions between the ground states of the two coupled wells and between the ground state and the excited state delocalized between the two wells. The experimental results provide clear evidence that the potential drop at the GaN/AIN interfaces is not abrupt, but spread over 1 monolayer.

In this work we discuss the optical properties of a GaN/Al 0.11 Ga 0.89 N multiple-quantum-well structure grown on 4H-SiC by plasma-assisted molecular-beam epitaxy. Photoluminescence lines related to the fundamental and excited electronic levels are identified, in good agreement with simulations of the electronic structure. Temperature dependence studies reveal an anomalous behaviour with quenching of the e 1 -hh 1 while the e 2 -hh 1 line becomes dominant at room temperature. This behaviour could be explained by the increasing thermal population of the e 2 state and the higher radiative efficiency of the e 2 -hh 1 transition compared to the e 1 -hh 1 transition due to the intense electric field in the well.

We report on the controlled growth of Si doped GaN/AIN quantum dot (QD) superlattices, in order to tailor their intersubband absorption within the 1.3-1.5 μm telecommunication wavelengths. The QD size is tuned by modifying the amount of GaN in the QDs and the growth temperature. Silicon can be incorporated in the QDs to populate the first electronic level, without significant perturbation of the QD morphology. As a proof of the capability of these structures for infrared detection, a quantum-dot intersubband photodetector at 1.38 μm with lateral carrier transport is demonstrated.

We investigated the energy states in InAs-GaAs quantum dot infrared photodetector (QDIP) structure utilizing near-infrared transmission spectroscopy and photoluminescence (PL) spectroscopy, and correlated to the results of intersubband transitions observed in photocurrent (PC) spectrum. The transmission spectrum at room temperature (RT) shows inflections in the 0.9-1.15 eV region due to the interband absorption in the InAs QDs with peaks at 0.96, 1.04, and 1.11 eV. The peak at 0.96 eV, in agreement with the PL data at RT, is clearly the fundamental hhl-el absorption of the dot. The two other peaks can be attributed to the hh2-e2 and hh3-e3 inter-band dot absorption. These results show that there are at least two (likely three) bound states in the conduction and valence band of the InAs QDs, respectively. The PC spectrum was observed at 11 K, between 100 and 400meV of transition energies with a peak at 163meV, which corresponds to el-e well intersubband transition.

We present detailed analysis of inter- and intraband transitions in GaN/AlN self-organized quantum dots grown on sapphire, silicon (111) and 6H-SiC substrates. Samples containing quantum dots of different size were characterized by means of transmission electron microscopy, photoluminescence and photoinduced absorption spectroscopy. Interlevel transitions in the conduction band were identified in the 0.52-0.98 eV range covering the telecommunication wavelength band. The s-p(z), transition ranges from 0.52 eV to 0.8 eV for dots with height of 6 down to 1.5 nm, respectively. Experimental results are compared with theoretical calculations showing that in larger dots the transition energy is governed by the value of the internal field.

In this paper we present experimental and theoretical study of intersubband transitions at telecommunication wavelengths in GaN/AlN quantum wells grown by molecular beam epitaxy on sapphire substrates. Crossed Rutherford back-scattering and photoluminescence experiments show that strong in-plane carrier localization occurs due to thickness fluctuations at GaN/AlN interfaces. Fourier transform infrared spectroscopy and photo-induced absorption spectroscopy performed on doped and undoped samples reveal a systematic blue-shift of the e1–e2 transition due to many body interactions. A good agreement is achieved between experiment and self-consistent Schrodinger–Poisson calculations. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High-resolution spectroscopy in the mid-infrared spectral range is used to study electronic transitions between size-quantization subbands in stepped quantum wells under picosecond interband excitation. The contributions from intersubband and intrasubband absorption of light are separated by using the difference in time profiles of the absorption coefficient for these cases. For stepped quantum wells, spontaneous interband luminescence and superluminescence are studied for different excitation levels. For structures with quantum dots, the intraband absorption spectra for n-and p-type structures and the spectra of photoinduced intraband absorption and emission (for polarized radiation) for undoped structures are studied.

International audience

GaN/AlN quantum dots grown by molecular beam epitaxy either on silicon (111), sapphire or 6H-SiC (0001) substrate have been investigated using photoluminescence, cathodoluminescence, Fourier transform infrared spectroscopy and attenuated total reflection spectroscopy. In-plane polarized intraband absorption is observed at energies of similar to 150 and 310 meV in GaN dots with 22 and 7.5 nm base diameter, respectively. For bigger dots, we observe three interlevel absorptions with a polarization component along the c-axis at energies ranging from 0.52 to 0.97 eV. Based on a simple 2D modelling of the confinement energies we show that the resonant absorptions involve conduction-band interlevel transitions between the electron ground state and states with one or two nodes along the c-axis and that in large dots, the internal field governs the transition energies. (C) 2002 Elsevier Science B.V. All rights reserved.

Results for interband and intraband absorption and emission in quantum dot structures are presented The observed peaks correspond to interband electron transitions between ground and excited states of QDs. Near infrared (λ ≃ 1 μm) lasing was obtained at high intensities of excitation. Surface mid infrared (λ > 10 μm) photoluminescence connected with the intraband electron transitions between the states of wetting layer and the ground states in QDs was also studied.

We report on molecular beam epitaxial growth and structural characterisations of unintentionally-doped InAs/AlSb strained multiple quantum wells grown on GaSb substrates. The crystalline quality and the interface roughness obtained using different cell shutter sequences at the arsenide/antimonide interfaces are compared by high resolution X-ray diffraction and small angle X-ray reflectivity measurements. Optical characterisations by photo-induced absorption spectroscopy are then carried out. A strong e1→e2 p-polarised intersubband absorption is observed in the mid infrared with a narrow broadening factor, revealing the good material quality of the samples.

International audience; We present pump-mid-infrared-probe measurements in InAs/InA1As self-organized quantum dots allowing to isolate the electron dynamics. By tuning the probe to an intraband transition in the conduction band at 13 mu m, we measured a characteristic time of 3 ps for the electron capture from the barrier to the ground state of the dots at low excitation densities. At higher densities, this capture time decreases to 1.5 ps indicating the contribution of Auger-like processes. (C) 2000 Elsevier Science B.V. All rights reserved.

We report on high-power GaAs/AlGaAs quantum fountain unipolar lasers emitting at 14– 15 μm under optical pumping by a pulsed CO2 laser. Two kinds of devices were investigated: edge lasers with side-facet pumping and broad-area lasers with a patterned top grating for coupling the pump at normal incidence. Collected optical powers as high as 4.16 W corresponding to an estimated power per facet of 17.6 W have been achieved in TM00 mode for broad-area lasers. The lasing wavelength is shown to be tunable (Δλ/λ≈2.5%) by changing the pump wavelength. Broad-area lasers exhibit excellent beam quality with very low in-plane divergence in TM00 mode. Mode hopping is observed under intense pumping.

Intersubband (lSB) optical absorption in different nitride-based heterostructures grown by metal-organic chemical vapour deposition (MOCVD) is reported. The role of indium in AlInN/GaN multi-quantum wells (MQWs) is investigated. At high concentration (15%) AlInN is quasi lattice-matched to GaN and no cracks appear in the structure. At very low indium concentration (similar to 2%) the material quality is improved without decreasing the ISB transition wavelength with respect to the case of indium-free structures. Different mechanisms of strain relaxation in pure and 2% indium-doped AlN/GaN MQW structures are also investigated. ISB transition wavelengths of 2 urn for AlN/GaN MQWs, and 3 mu n for AlInN/GaN MQWs with 15% of In are achieved. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report on the observation of intersubband luminescence at room temperature from GaN/AlN quantum wells and quantum dots. The quantum wells are designed to exhibit three bound states in the conduction band. Optical pumping resonant with the e1-e3 transition (λ = 0.98 μm) is used to populate the e3 excited state. Emission corresponding to the e3-e2 transition is observed peaked at 2.13 (2.3) μm for heavily (lightly) doped quantum wells. We also report on the observation of room temperature emission at λ ≈ 1.5 μm from a stack of GaN/AlN QDs through resonantly-enhanced Raman scattering involving GaN LO-phonons. The quantum dot emission is ascribed to the pz-s intraband transition of the dots. We show that this process provides room for population inversion. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the demonstration of an intersubband (ISB) coupled quantum-well modulator operating at room temperature and telecommunication wavelength using a GaN-AIN quantum-well structure. The optical modulation is shown to result from electron tunneling between the wells. Stark shifting of the ISB absorption is observed. The maximum modulation depth is 0.79% at lambda= 2.3 mum and 0.18% at lambda= 1.37 mum for a mesa device with only 151-nm interaction length. We show that by reducing the mesa size down to 15 times 15 mum2, optical modulation bandwidth as large as 3 GHz can be obtained.

We report on the demonstration of low-loss, single-mode GaN-InAlN ridge waveguides (WGs) at fiber-optics telecommunication wavelengths. The structure grown by metal-organic vapor phase epitaxy contains AlInN cladding layers lattice-matched to GaN. For slab-like WGs propagation losses are below 3 dB/mm and independent of light polarization. For 2.6-mum-wide WGs the propagation losses in the 1.5- to 1.58-mum spectral region are as low as 1.8 and 4.9 dB/mm for transverse-electric- and transverse-magnetic-polarization, respectively. The losses are attributed to the sidewall roughness and can be further reduced by the optimization of the etching process.

Recent achievements on quantum fountain infrared sources relying on intersubband emission under optical pumping of coupled quantum well structures are reviewed. Population inversion can be achieved at mid-infrared wavelengths using a careful engineenng of the scattering rates between subbands. Observations of intersubband luminescences under either intersubband or interband optical pumping is reported. At low temperatures, the spontaneous emission is shown to result from direct optical excitation of the electrons in the upper subband. However, under interband excitation at room-temperature, the luminescence is the manifestation of the black-body emission of the two-dimensional electron gas. Population inversion at 77 K is then demonstrated for the first time using optical pumping by a free-electron laser. Large stimulated gains at 12.5 μm wavelength are reported. Lasing action appears to be feasible under infrared optical pumping.

Advanced concepts in intersubband optical emission for mid-infrared (MIR) and far-infrared (FIR) lasers in nanostructures are presented. Electronic tunability of low dimensional systems, and new simulation techniques are implemented for engineering electron scattering time with phonons and improving laser performances. We successively describe the operation of an optically pumped "quantum fountain laser" made of asymmetric coupled quantum well structures, a FIR quantum wire laser pumped by optic phonons and an electrically tunable, mode locked FIR quantum dot laser.

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...

Mid-infrared optical emission due to intersubband transitions between excited conduction subbands of a coupled quantum well structure is studied. The emission process is based on optical pumping of free carriers from the ground subband into the third subband followed by a radiative transition from the third subband into the second subband and a fast phonon assisted relaxation into the ground subband. We have observed spontaneous emission at 14 μm that persists up to room temperature. Our results indicate that population inversion between conduction subbands and large stimulated gain can be achieved.

Copyright (c) 1997 Elsevier Science S.A. All rights reserved. Strain- and composition-engineered multi-quantum wells and superlattices of IV−IV compounds have numerous potential applications in electronic, optoelectronic and optical devices in Si-compatible technology. Rapid thermal chemical vapor deposition (RTCVD) at low pressure and low temperature is a particularly promising technique for growing such heterostructures. Binary and ternary SiGeC multilayers were grown on (001)-Si by computer-controlled RTCVD. Hydrogen-diluted SiH 4 , GeH 4 and SiH 3 CH 3 were used as precursors. Transmission electron microscopy and X-ray diffraction demonstrated that these structures had abrupt interfaces and were free of extended crystal defects. Secondary ion mass spectroscopy was used to check their chemical composition. Fourier transform infrared spectrometry showed that carbon was only substitutionally incorporated under optimized growth conditions. The band structure and electron−hole recombination were investigated by photoluminescence and infrared absorption measurements. The infrared absorption spectra showed intersubband transitions in Si/SiGeC multi-quantum wells for the first time.

A survey is given of a variety of IR spectroscopy studies on p-type Si/SiGe quantum wells. Valence-band intersubband absorption experiments were performed on a large number of samples with different parameters and explained quantitatively by a self-consistent Luttinger–Kohn-type calculation. The dependence on polarization of the absorption was analyzed. On the basis of these quantum wells, mid-IR detectors were fabricated and characterized in terms of responsivity, dark current and detectivity. In asymmetric, compositionally stepped quantum wells, second-harmonic generation of CO2 laser radiation was observed.

We investigated intersubband transitions in the valence band of Si1−xGex quantum wells. After a brief review of the recent achievements related to intersubband transitions, we present an original technique, photo-induced IR absorption spectroscopy, to study intersubband transitions in both doped and undoped quantum wells. A comparison with photo-induced intersubband absorption in GaAs quantum wells as well as the variation of the intersubband absorption with Ge content, layer thickness and doping and the temperature dependence of photo-induced absorption are reported. The normal incidence IR modulation is presented in a second part. It is shown that the index variations associated with free carrier and intersubband absorptions significantly affect the IR modulation. The thermal stability of narrow Si–Si1−xGex quantum wells is investigated in a third section. The interdiffusion leads to a red shift of the intersubband transitions. It is shown that the local variation of the effective mass can enhance the normal incidence intersubband absorption. Finally, intersubband relaxation in doped quantum wells is investigated using a picosecond free electron laser. The subband lifetime is first deduced from the saturation of intersubband absorption vs. pump intensity. In a second step, the subband lifetime is measured directly by time-resolved pump and probe experiments. Both techniques demonstrate that the subband lifetime T1 is shorter than 1 ps.

The optical properties of wurtzite GaN nanowires containing single Al0.14Ga0.86N/GaN quantum discs of different thickness have been investigated. The dependence of the photoluminescence (PL) transition energy on the quantum disc thickness and the thickness of a lateral AlGaN shell has been simulated in the framework of a three-dimensional effective mass model, accounting for the presence of a lateral AlGaN shell, strain state and the piezoelectric and spontaneous polarization. The predicted transition energies are in good agreement with the statistics realized on more than 40 single nanowire emission spectra and PL spectra of ensembles of nanowires. The emission spectra of the single quantum discs exhibit a Lorentzian shape with a homogeneous line width as low as 3 meV. Finally, we discuss the dependence of the interband transition energy on diameter.

A theoretical investigation of an optically-pumped mid-infrared intersubband semiconductor laser is presented. The influence of electrons and dopant ions on the conduction band structure is simulated with a self-consistent Poisson–Schrodinger solver. Electron-polar optical phonon interactions are calculated by using a macroscopic phonon model with electromagnetic boundary conditions. In order to assess the influence of the electronic temperature on the device optical performances, electron dynamics under optical pumping are investigated within a rate equation model where particle and energy flow equations are derived from Boltzmann's equation with Fermi statistics. Our calculations show that population inversion between the first and second excited states can occur at 77 K under intersubband optical excitation.

A theoretical self‐consistent investigation of optically pumped mid‐infrared intersubband semiconductor laser with hot electron effects is presented. Electron dynamics under optical pumping are investigated within a rate equation formulation where particle and energy flow equations are derived from Boltzmann’s equation using Fermi statistics. Electron‐polar optical phonon interactions with suitable screening are calculated by using a macroscopic model with slab and interface phonon modes. Our calculations show that despite hot electron effects, population inversion between the first and second excited states can occur at low temperatures under intersubband optical excitation. It is anticipated that lasing in the mid‐infrared can be achieved with asymmetric quantum well structures optimized for electron concentrations exceeding 1011/cm2.

The interdiffusion and thermal stability of narrow Si/SiGe multi‐quantum wells is investigated by photoluminescence and intersubband spectroscopy. The photoluminescence exhibits a blueshift as a function of the temperature of annealing. The activation energy of the intermixing process and the interdiffusion coefficient are deduced from the photoluminescence shift versus temperature of the anneal. The intersubband absorption is measured by photoinduced infrared spectroscopy on the interdiffused samples for light polarized perpendicular (z polarization) or parallel (x polarization) to the layer plane. In z polarization, the absorption of annealed samples exhibits a redshift respective to the as‐grown sample which is enhanced as more levels are confined in the well. The magnitude of this shift is in good agreement with simulations based on the data obtained by photoluminescence experiments. The redshift of the intersubband absorption in x polarization is lower than in z polarization due to the lower dependen...

We have investigated photo-induced intersubband absorption in the valence band of Si/SiGe quantum wells. Carriers are optically generated in the quantum wells using interband optical pumping. Both p- and s-polarized intersubband absorptions of 3 nm 30% SiGe multi-quantum wells are measured using a waveguide geometry. At normal incidence, the infrared modulation is surprisingly quenched around 10 μm. We show that this result can be interpreted by taking into account the Fabry-Perot reflections in the multi-quantum well stack and the anomalous dispersion associated with intersubband transitions. The intersubband absorption is found to be dependent on the interband excitation wavelength. This indicates that the s-polarized photo-induced absorptions which involve bound-to-bound and bound-to-continuum transitions are probably assisted by potential fluctuations linked to interface roughness.

Electroluminescence spectroscopy of short gate high‐electron‐mobility transistors (HEMTs) on InP substrates is performed at cryogenic temperatures. Electroluminescence is a reliable tool to investigate impact ionization as compared to studies based on gate current which depend on the weakness of the intrinsic gate current intensity. In on‐state biased devices, a low energy (0.7–0.9 eV) recombination band is observed which is related to radiative recombination of carriers created by impact ionization in the low band gap InGaAs channel. The evolution of the luminescence intensity versus bias applied to the device shows that the electroluminescence intensity and impact ionization depend on two competing parameters: the electric field in the gate–drain access area and the drain current intensity. We show that the so‐called ‘‘kink’’ effect, which is a noticeable increase of the output conductance and which is observed at relatively moderate drain bias (600–750 mV) in our devices, is not correlated with impact ...

We have studied the optical properties and growth by rapid thermal chemical vapour deposition of multi-quantum well heterostructures based on the group IV elements Si, Ge and C. Si 1 − x Ge x Si , Si 1 − x − y Ge x C y Si , Si 1 − x Ge x Si 1 − y C y and Si 1 − y C y Si heterostructures were investigated by X-ray diffraction (XRD), secondary ion mass spectroscopy (SIMS), photoluminescence and Raman spectroscopy. Different features are observed depending on the growth temperature. At 575 °C, carbon and germanium are incorporated as expected, whereas at lower temperatures (550 °C), strong segregation occurs, as indicated by the decrease in the number of periods of the heterostructure observed by SIMS and XRD. Photoluminescence identifies a defect line associated with carbon and oxygen. Raman spectroscopy shows that, although carbon compensates for the external strain, the energy of the Si-Ge vibration mode is only weakly modified.