AlAs Superlattices Research Articles

Growth of InGaAs/InAlAs superlattices for strain balanced quantum cascade lasers by molecular beam epitaxy

This study investigated the molecular beam epitaxy (MBE) growth conditions of strain-balanced (SB) In0.669GaAs/In0.362AlAs superlattices (SLs) for SB quantum cascade lasers (QCLs). The growth modes and properties of SB SLs are strongly affected by the growth conditions. The properties of the SB SLs were evaluated using atomic force microscopy (AFM) and high-resolution X-ray diffraction (HRXRD) analysis. Following the establishment of optimized conditions for SB SLs growth, SB QCL were grown. The HRXRD analysis and transmission electron microscopy (TEM) measurements showed that the grown samples exhibited abrupt interfaces and good structural quality. An epitaxial wafer was processed into a Fabry-Perot cavity with a ridge structure. The device operated in pulsed mode emitting ∼4.7 µm at room temperature with a peak power of 650 mW and a slope efficiency of 870 mW/A.

Electronic Properties of GaAs/AlAs Nanostructure Superlattice for Near Infrared Devices at Low Temperatures

We report here the electronic band structures of symmetric type I GaAs ( $$d_{1}= 2.83$$ nm)/AlAs ( $$d_{2}= 2.83$$ nm) superlattice as a function of the well thickness $$d_{1}$$ and the effect of the valence band offset $$\Lambda $$ , the ratio $$d_{2}$$ / $$d_{1}$$ , and the temperature on the band gap energy, performed in the envelop function formalism. These results are compared and discussed with the experimental measurements reported in the literature.

Intrinsic to extrinsic phonon lifetime transition in a GaAs–AlAs superlattice

We have measured the lifetimes of two zone-center longitudinal acoustic phonon modes, at 320 and 640 GHz, in a 14 nm GaAs/2 nm AlAs superlattice structure. By comparing measurements at 296 and 79 K we separate the intrinsic contribution to phonon lifetime determined by phonon–phonon scattering from the extrinsic contribution due to defects and interface roughness. At 296 K, the 320 GHz phonon lifetime has approximately equal contributions from intrinsic and extrinsic scattering, whilst at 640 GHz it is dominated by extrinsic effects. These measurements are compared with intrinsic and extrinsic scattering rates in the superlattice obtained from first-principles lattice dynamics calculations. The calculated room-temperature intrinsic lifetime of longitudinal phonons at 320 GHz is in agreement with the experimentally measured value of 0.9 ns. The model correctly predicts the transition from predominantly intrinsic to predominantly extrinsic scattering; however the predicted transition occurs at higher frequencies. Our analysis indicates that the ‘interfacial atomic disorder’ model is not entirely adequate and that the observed frequency dependence of the extrinsic scattering rate is likely to be determined by a finite correlation length of interface roughness.

Electric field effects on excitonic quantum beats in a single quantum well embedded in a GaAs/AlAs superlattice

We have investigated the characteristics of the quantum beat of excitons confined in a GaAs single quantum well (6.4 nm) embedded in the center of a GaAs (3.2 nm)/AlAs (0.9 nm) superlattice (SL) from the viewpoint of the resonant coupling between the subband states in the single quantum well (SQW) and the Wannier--Stark-localization states in the SL with the use of a reflection-type pump-probe technique. The resonant couplings, which cause anticrossing behaviors of relevant transition energies, were observed by electroreflectance spectroscopy and analyzed by calculating the eigenenergies as a function of electric field strength. It is found that the frequency and intensity of the quantum beat between the heavy-hole and light-hole excitons in the SQW significantly change under the resonant coupling conditions of the electron states. The changes of the quantum-beat properties result from the formation of bonding and antibonding electron states which modifies the excitonic transition properties in the SQW as a function of electric field strength. The experimental results of the quantum beat are reasonably explained by the electric-field-strength dependence of the energies and transition probabilities of the excitons in the SQW observed from electroreflectance spectra.

Pump‐energy dependence of usual and unusual Bloch oscillations in a GaAs/AlAs superlattice

AbstractWe have systematically investigated the wave‐packet dynamics of Bloch oscillations (BOs) from the viewpoint of the characteristics of the Wannier‐Stark (WS) localization states in a GaAs (6.8 nm)/AlAs (0.9 nm) superlattice embedded in a p ‐i ‐n diode structure. The coherent oscillations were detected with a reflection‐type pump‐probe technique. It is clearly confirmed that the unusual BO with the frequency of 2eFD/h occurs in a weak localization regime in addition to the usual BO with eFD/h in a strong localization regime, where F is the applied electric field, and D is the superlattice period. The systematic results of the pump‐probe measurements as a function of pump energy indicate that the intensity of the unusual BO is drastically changed with pump energy, while a gradual change of that of the usual BO is observed. These results demonstrate that the quantum interference leading to the unusual BO with 2eFD/h is much sensitive to the pump energy compared with that of the usual BO. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of intermixing on bulk and interface Raman modes in GaAs:AlAs superlattice waveguide structures

Spatially resolved Raman spectroscopy at room temperature is used to study quantum well intermixing in GaAs:AlAs superlattice structures. Phonon modes are probed from the side facet along the [110] direction. The intermixing leads to the appearance of interfacial alloy modes and degraded the intensity of the superlattice interface (IF) modes, which can be used as a sensitive indicator of superlattice quality. These changes in the Raman spectra, along with spatially resolved photoluminescence, are used to distinguish the degree of intermixing in samples intermixed by impurity free vacancy diffusion at 850–950 °C, and to investigate the bandgap modulation in a periodically intermixed bandgap grating fabricated using ion implantation induced disordering. The shift of the GaAs-transverse optic mode to the GaAs-like transverse optic alloy mode and the degradation of the GaAs-IF mode are shown to provide the best indication of the extent of intermixing. Due to the large contrast in the GaAs-IF mode intensity between as-grown and intermixed superlattice, using IF modes is found to be a promising route for characterizing the resolution of the superlattice bandgap grating.

The self-sustained current oscillation and the dynamics in superlattices under the action of electric and magnetic fields

Self-sustained time-dependent current oscillations have been found in weakly coupled GaAs∕AlAs superlattices when the sequential resonant tunneling between adjacent quantum wells is the main electron transport mechanism. The oscillation regime was tunable by varying the doping densities and applied dc voltages. Based on the discrete sequential tunneling model, we theoretically studied the magnetic field dependence of the oscillation. The magnetic field B seems to be favorable for the formation of the static electric-field domains and to depress the current oscillation. Thus, the oscillation regime will be narrowed as the magnetic field strength increases. Driven by a transverse external microwave excitation, the system shows interesting nonlinear behaviors like quasiperiodicity, frequency locking, and periodicity.

Observation of the second‐nearest‐neighbor Bloch oscillation in a GaAs/AlAs superlattice

AbstractWe have systematically investigated the quantum beat of the miniband excitons and the Bloch oscillation in the Wannier‐Stark (WS) localization as a function of applied bias voltage in a GaAs (6.8 nm)/AlAs (0.9 nm) superlattice embedded in a p ‐i ‐n diode structure. The coherent dynamics behaviors were detected with a reflection‐type pump‐probe technique. We clearly observed the transformation process from the miniband‐exciton quantum beat to the Bloch oscillation with an increase in applied bias voltage producing an internal electric field. The noteworthy finding is the fact that the Bloch oscillation with the frequency of vBO = 2eFD /h appears in a weak localization regime in addition to the usual Bloch oscillation with vBO = eFD /h in a strong localization regime, where F is the electric field, and D is the superlattice period. The frequency of 2eFD /h indicates that the newly observed Bloch oscillation is due to the wave‐packet motion in the second‐nearest‐neighbor space range. The results described above are explained by the electric‐field‐strength dependence of the envelope‐function localization, which is estimated from the envelope‐function profile calculated by a transfer‐matrix method and the excitonic transitions observed by electroreflectance spectroscopy. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Excitonic quantum beat at the mini-Brillouin-zone boundary in a GaAs/AlAs superlattice

We have investigated excitonic quantum beats (QBs) at the mini-Brillouin-zone boundary (π-point) in a GaAs (6.8 nm)/AlAs (1.1 nm) superlattice by using a femtosecond reflection-type electro-optic sampling technique. Photoreflectance spectroscopy was adopted to characterize the optical transition energies of the heavy-hole (HH) and light-hole (LH) excitons at the mini-Brillouin-zone center (Γ-point) and the π-point. By systematically changing the pump-pulse energy in the energy region from the Γ-point exciton to the π-point exciton, we have succeeded to observe the QB of the HH and LH excitons at the π-point in addition to the ordinary QB at the Γ-point. The observed frequencies agree with the values estimated from the energy difference between the relevant two excitons. The decay time of the QB signal at the π-point is much shorter than that at the Γ-point. This behavior reflects a very fast relaxation process of the π-point excitons in the miniband dispersion.

Characterizing Bandgap Gratings in GaAs : AlAs Superlattice Structures Using Interface Phonons

Interface Raman modes were used to study quantum-well intermixing in GaAs : AlAs superlattice (SL) structures using room-temperature spatially resolved Raman spectroscopy. The intermixing was observed to degrade the interface mode intensity, which can be used as a sensitive indicator of the SL quality. This feature, along with spatially resolved photoluminescence, was used to investigate bandgap modulation in periodically intermixed bandgap gratings, fabricated using implantation induced disordering. Using interface modes instead of bulk-like modes is a promising avenue for characterizing SL structures that rely on intricate bandgap features

Dephasing of Bloch oscillating electrons in GaAs-based superlattices due to interface roughness scattering

We have investigated dephasing mechanisms of Bloch oscillating electrons in GaAs-based superlattices (SLs) by time-domain terahertz electro-optic sampling method. It was found that dephasing time τr for GaAs∕AlAs SLs in the Wannier-Stark regime rapidly becomes shorter with decreasing well width Lw as τr∝Lw3−5. The observed strong Lw dependence of τr indicates that the dominant dephasing mechanism is interface roughness scattering, which was indeed confirmed by quantitative comparison between theory and experiment. It was also found that alloy disorder scattering is negligibly weak even in the case of GaAs∕Al0.3Ga0.7As SLs.

Modifying the electronic properties of GaAs∕AlAs superlattices with low-density nitrogen doping

We investigate how low concentrations of N impurities (x<1%) modify the electrical and optical properties of a GaAs∕AlAs superlattice positioned in the intrinsic i region of a p-i-n diode. Increasing x from 0% to 0.9% enhances the integrated intensity of the low temperature electroluminescence emission from the superlattice by more than a factor of 104. We attribute this behavior to reduced transmission of electrons through the superlattice due to the N-induced carrier localization.

New Editorial Board member of physica status solidi (b)

physica status solidi (b)Volume 243, Issue 5 p. 969-969 Editorial BoardFree Access New Editorial Board member of physica status solidi (b) First published: 27 March 2006 https://doi.org/10.1002/pssb.200690011AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract physica status solidi is proud to announce the nomination of a new member of the Editorial Board of pss (b), Bernard Gil, Montpellier, France. Information on other Editorial Board members can be found in phys. stat. sol. (a) 201, 826, 2865 (2004); 202, 347 (2005) and phys. stat. sol. (b) 241, 974, 2873 (2004); 242, 495 (2005). Bernard Gil, Montpellier Born 1957, Bernard Gil received his Ph.D. in 1981 and joined the “Centre National de la Recherche Scientifique” (CNRS) the year after. He obtained his “Doctorat d'Etat” in 1985. He has authored more than 200 scientific papers and contributed to organize some 30 international conferences. He is also directing the Groupe d'Etude des Semiconducteurs (GES), a joint research laboratory between the CNRS and the University of Montpellier II. Among the scientific contributions of Gil are specific properties of excitons bound to non-cubic centres in GaP, the symmetry of the conduction band of short-period GaAs–AlAs superlattices, the effect of strain fields on phonons and on the valence band physics in GaN epitaxial films, excitonic recombination properties in GaN and InGaN quantum wells and dots, and the excitonic fine structure in ZnO films and ZnO–ZnMgO quantum wells. Bernard Gil has been member of the Editorial Board of physica status solidi (b) since March 2006. e-mail: gil@GES.univ-montp2.fr Volume243, Issue5Special Issue: Current Trends in Electronic StructureApril 2006Pages 969-969 RelatedInformation

Generation and detection of terahertz coherent transverse-polarized acoustic phonons by ultrafast optical excitation of GaAs∕AlAs superlattices

We have used femtosecond pump-probe techniques to generate and detect coherent transverse and quasitransverse polarized acoustic phonons in GaAs∕AlAs superlattices. Direct generation of transverse phonons is achieved using superlattices grown on the low-symmetry, (311) and (211), crystal planes. The frequency of the generated phonons is determined by the superlattice period and is in the region of 0.4THz. The dependence of the mode structure on the pump polarization suggests that a Raman scattering process is responsible for coherent phonon generation. Using bolometers on the back surface of the substrate, we show that the transverse phonons leak out of the superlattice and propagate over macroscopic distances at low temperature.

New Editorial Board member of physica status solidi (b)

Abstractphysica status solidi is proud to announce the nomination of a new member of the Editorial Board of pss (b), Bernard Gil, Montpellier, France.Information on other Editorial Board members can be found in phys. stat. sol. (a) 201, 826, 2865 (2004); 202, 347 (2005) and phys. stat. sol. (b) 241, 974, 2873 (2004); 242, 495 (2005).Bernard Gil, MontpellierBorn 1957, Bernard Gil received his Ph.D. in 1981 and joined the “Centre National de la Recherche Scientifique” (CNRS) the year after. He obtained his “Doctorat d'Etat” in 1985. He has authored more than 200 scientific papers and contributed to organize some 30 international conferences. He is also directing the Groupe d'Etude des Semiconducteurs (GES), a joint research laboratory between the CNRS and the University of Montpellier II. Among the scientific contributions of Gil are specific properties of excitons bound to non‐cubic centres in GaP, the symmetry of the conduction band of short‐period GaAs–AlAs superlattices, the effect of strain fields on phonons and on the valence band physics in GaN epitaxial films, excitonic recombination properties in GaN and InGaN quantum wells and dots, and the excitonic fine structure in ZnO films and ZnO–ZnMgO quantum wells.Bernard Gil has been member of the Editorial Board of physica status solidi (b) since March 2006.e‐mail: gil@GES.univ‐montp2.fr

Miniband-width effects on Wannier–Stark localization of the first and second quantized states in a GaAs/AlAs superlattice

We performed electroreflectance (ER) measurements in order to reveal the transformation processes from the minibands to the Wannier–Stark (WS) localization states in a GaAs (6.8 nm)/AlAs (0.9 nm) superlattice. The high sensitivity of ER spectroscopy enabled us to observe the interband optical transitions associated with the second ( n=2) minibands in addition to those associated with the first ( n=1) minibands. It is found that the electric field strengths for the formation of the WS-localization states of the n=2 minibands are considerably higher than those of the n=1 minibands. The experimental results are quantitatively analyzed on the basis of the theoretical calculation of the electric-field-strength dependence of the miniband states using a transfer-matrix method. It is concluded that the critical electric field strengths for the formation of the WS-localization states clearly correlate with the miniband widths.

Dispersion and Modulation of the Linear Optical Properties of GaAs–AlAs Superlattice Waveguides Using Quantum-Well Intermixing

We report on the large modulation of the optical properties of a 14:14 monolayer GaAs-AlAs superlattice waveguide following quantum-well intermixing. Low-temperature photoluminescence measurements illustrate a large 169-meV differential blue-shift obtained between the disordering-suppressed and disordering-enhanced materials. Effective index measurements are presented as a function of polarization, for both the as-grown and disordered material for near-bandedge and half-bandedge wavelengths, which is the wavelength range 775-1550 nm. The largest effective refractive index shift observed was 9/spl times/10/sup -2/ which exceeds that previously reported for disordered AlGaAs ternary multilayer structures, and illustrates the potential of the superlattice for the fabrication of etch-free, planar optical components with large index contrast.

Electroreflectance Observation of Transformation Processes of the First and Second Minibands to Wannier–Stark Localization States in a GaAs/AlAs Superlattice

We have investigated the transformation processes of the first (n=1) and second (n=2) minibands to the Wannier–Stark (WS) localization states in a GaAs (6.8 nm)/AlAs (0.9 nm) superlattice embedded in a p–i–n structure by electroreflectance spectroscopy. The high sensitivity of electroreflectance enabled us to observe the interband optical transitions associated with the n=2 minibands in addition to those associated with the n=1 minibands. The systematic results of the electroreflectance spectra as a function of electric field strength demonstrate that the electric field strength for the formation of the WS-localization state of the n=2 miniband is higher than that of the n=1 miniband, which reflects a difference in the energy widths of the n=1 and n=2 minibands. The experimental results are quantitatively discussed on the basis of the electric-field-strength dependence of the energies and envelope functions of the n=1 and n=2 minibands calculated using a transfer-matrix method. We find a clear correlation between the miniband widths and the critical electric field strengths for the formation of the WS-localization states.

Angle dependence of acoustic phonon-assisted tunneling in a weakly coupled superlattice: Evidence for terahertz phonon amplification

We investigate nonequilibrium acoustic phonon-assisted tunneling in a weakly coupled GaAs∕AlAs superlattice (SL). Previously it had been predicted that, in such structures, the conditions for terahertz acoustic phonon amplification can be achieved [B. A. Glavin, V. A. Kochelap, and T. L. Linnik, Appl. Phys. Lett. 74, 3525 (1999)]. We present experimental measurements of the change in current ΔI due to an incident nonequilibrium phonon pulse generated by thermalizing a laser pulse in a metal film. In particular, we measure the dependence on the angle of incidence θ of the phonons relative to the SL growth direction. The results are compared with numerical calculations made using the two-well model. It is found that good agreement between the measurements and calculations can be obtained for all θ if we assume that amplification is occurring for phonons propagating in a direction near θ=0. Due to the amplification of phonons with energy ℏωq≲Δ, where Δ is the energy drop per period of the SL, the nonequilibrium phonon distribution in the vicinity of the SL is different from the expected Planck spectrum of the phonon source. We show that this can explain the experimental measurements of ΔI at θ=0. We present these results as evidence for phonon amplification in the 400-GHz–1-THz frequency range.

Generation of terahertz monochromatic acoustic phonon pulses by femtosecond optical excitation of a gallium nitride/aluminium nitride superlattice

We describe the generation of pulses of monochromatic acoustic phonons by ultrafast laser excitation of a gallium nitride/aluminium nitride superlattice (SL). The SL response to the optical excitation was studied using femtosecond pump-probe measurements. Superconducting bolometers were used to detect the phonons that leak out of the SL and propagate across the substrate. We see that, for excitation wavelengths around 430 nm, a pronounced enhancement in the longitudinal acoustic phonon signal occurs. The results are consistent with similar measurements using GaAs∕AlAs SL’s, where propagating monochromatic phonons were generated under resonant photoexcitation [Appl. Phys. Lett. 81, 3497 (2002)].