Intersubband Emission Research Articles

Towards a Si/SiGe Quantum Cascade Laser for Terahertz Applications

ABSTRACTWhile electroluminescence has been demonstrated at terahertz frequencies from Si/SiGe quantum cascade emitters, to date no laser has been achieved due to poor vertical confinement of the optical mode. A method of increasing the vertical confinement of the optical mode for a Si/SiGe quantum cascade laser is demonstrated using silicon-on-silicide technology. Such technology is used with epitaxial growth to demonstrate a strain-symmetrised 600 period Si/SiGe quantum cascade interwell emission and the polarisation is used to demonstrate the optical confinement. Electroluminescence is demonstrated at ∼3 THz (∼100 μm) from an interwell quantum cascade emitter structure. Calculated model overlap and waveguide losses for ridge waveguides are comparable to values from GaAs quantum cascade lasers demonstrated at terahertz frequencies. The effects of high doping levels in Si/SiGe quantum cascade structures is also investigated with impurity emission demonstrated rather than intersubband emission for the highest doping levels used in the cascade active regions.

Interwell intersubband electroluminescence from Si/SiGe quantum cascade emitters

The quantum cascade laser provides one potential method for the efficient generation of light from indirect materials such as silicon. While to date electroluminescence results from THz Si/SiGe quantum cascade emitters have shown higher output powers than equivalent III–V emitters, the absence of population inversion within these structures has undermined their potential use for the creation of a laser. Electroluminescence results from Si/SiGe quantum cascade emitters are presented demonstrating intersubband emission from heavy to light holes interwell (diagonal) transitions between 1.2 THz (250 μm) and 1.9 THz (156 μm). Theoretical modeling of the transitions suggests the existence of population inversion within the system.

Monte Carlo simulation of tunable mid-infrared emission from coupled Wannier–Stark ladders in semiconductor superlattices

We present a theoretical and experimental study on the mid-infrared electroluminescence associated with transitions between electric-field-induced conduction states, forming the Wannier–Stark ladder, in strongly coupled GaAs/AlAs superlattices. The interwell and intrawell radiative transitions in the whole range of electric fields from the moderate localization to the resonance-induced delocalization regimes have been experimentally investigated. Monte Carlo simulations show a very good agreement between experimental and theoretical electroluminescence spectra. Results show that the application of an electric field in the range from 100 to 250 kV/cm shifts the emission peak, related with interwell diagonal transitions between Stark-localized ground states of two adjacent wells, up to the limit corresponding to the merging of this electroluminescence peak with the intersubband emission between excited and ground state of the same well. The theoretical investigation indicates that interwell scattering via LO phonons is responsible for the population of the excited state of the ladder.

Strain compensated Si/SiGe quantum well and quantum cascade on Si 0.5Ge 0.5 pseudosubstrate

This study follows up our previous investigation of the valence band (VB) intersubband emission from quantum cascade structures grown lattice matched on Si substrates. Here, Si/Si 1−x Ge x (x=80%) heterostructures are investigated which are deposited by MBE on a virtual substrate of relaxed SiGe containing 50% of Ge. TEM analysis reveal flat and abrupt interfaces for structures grown at temperatures T growth≈300°C. Intersubband absorption and photoluminescence emission manifest well-defined interfaces and good material quality. The observed intersubband line positions are found to be in good agreement with k· p model calculations for the VB. This is in contrast to the observed type II no phonons recombination which is found at consistently lower energy than expected. Finally, electrically excited intersubband emission from a strain compensated cascade structure containing three periods is presented.

Phonon engineered quantum cascade terahertz emission

The observation of terahertz (THz) intersubband emission from GaAs/AlGaAs quantum cascade structures employing higher energy interface and barrier confined AlAs-like phonons for depopulation is reported. Emission is observed at 12.0 meV (2.9 THz) and 17.5 meV (4.2 THz) with full-width at half-maximum of 0.7 and 1.6 meV , respectively, at T=10 K from two different structures. The structures consisted of 40 periods of the quantum cascade module and relied on spatially diagonal (interwell) transition for the terahertz emission. The possibility of tuning the emission frequency using quantum confined Stark effect is also demonstrated.

Intersubband electroluminescence from Si/SiGe cascade emitters at terahertz frequencies

The quantum cascade laser provides one possible method of realizing high efficiency light emitters in indirect band gap materials such as silicon. Electroluminescence results from Si/SiGe quantum cascade emitters are presented demonstrating edge emission from heavy-hole to heavy-hole transitions and light-hole to heavy-hole transitions. In surface-normal emission, only light-hole to heavy-hole electroluminescence is observed as predicted by theory. Intersubband emission is demonstrated at 2.9 THz (103 μm wavelength), 8.9 THz (33.7 μm), and 16.2 THz (18.5 μm) from the Si/SiGe quantum cascade heterostructures.

Investigation of injector doping density on intersubband terahertz electro-luminescence from GaAs–AlGaAs quantum cascade structures

We report a comprehensive study of GaAs–Al 0.15Ga 0.85As quantum cascade electro-luminescence devices designed to emit in the far-infrared. Strong intersubband electro-luminescence is observed at 3.9 THz with a FWHM of typically 0.21 THz . We investigate the intersubband emission as a function of the doping density in the injector regions of the cascade structures. A systematic increase in intersubband emission intensity with doping density is observed owing to the enhanced current flow through the devices, with powers of up to 55 pW detected.

Terahertz interminiband emission and magneto-transport measurements from a quantum cascade chirped superlattice

Graded superlattice quantum cascade lasers have shown high operation performance in the mid-infrared. A similar design has been used for terahertz intersubband emission. Narrow band ( Δν=3 meV ) waveguide emission was observed at an energy of 18.8 meV . The energy spectrum was confirmed by magneto-transport measurements.

Widely tunable mid-infrared emission from coupled Wannier–Stark ladders in semiconductor superlattices

We report on the observation of mid-infrared electroluminescence associated with transitions between electric-field-induced conduction states forming the Wannier–Stark ladder in strongly coupled GaAs/AlAs superlattices. Both inter-well and intra-well radiative transitions have been studied in the whole range of electric fields from the moderate localization to the resonance-induced delocalization regimes. The application of average electric fields in the range 100–250 kV/cm, allowed to continuously shift the emission peak related with inter-well 1→1 diagonal transitions between Stark-localized ground states up to the limit corresponding to the merging of the 1→1 electroluminescence peak with the 2→1 intersubband emission.

Valence band intersubband electroluminescence from Si/SiGe quantum cascade structures

The principle of intersubband emission is applied to the Si/SiGe material system, using hole intersubband transitions in structures grown pseudo-morphically on Si substrate by molecular beam epitaxy. Cascade structures consisting of three times four repetitions of a five quantum well sequence are investigated. The design constraints are found to be imposed mainly by the total amount of strain, giving limitations to the number of wells per cascade and the total number of cascade periods. Despite the close approach to the critical thickness for misfit dislocations, requiring low temperature growth, these structures reveal intersubband electro-luminescence with a linewidth as narrow as 22meV. Peak energies between 125 and 154meV are obtained by tuning the well width and Ge content of the single active quantum well. By comparison with the emission from a III–V cascade structure, the non-radiative lifetime of the upper emission state is determined. It is found to depend strongly on the structure's design, but can reach values comparable to those in similar III–V cascade structures. A discussion of the importance of carrier escape to the continuum and the injection efficiency, as well as the relaxation via the light hole state is given.

A dual-color injection laser based on intra- and inter-band carrier transitions in semiconductor quantum wells or quantum dots

A new type of semiconductor injection laser capable of simultaneously generating radiation in the mid-infrared (MIR) (/spl lambda//spl sim/10 /spl mu/m) and near-infrared (NIR) (/spl lambda//spl sim/0.9 /spl mu/m) spectral regions is proposed. The MIR emission is a result of intersubband (intraband) electron transitions within a three-level conduction band in a quantum well or a quantum dot. The NIR emission, on the other hand, is due to conventional interband recombination of injected electrons and holes into the conduction and valence bands, respectively. The conditions for population inversion in the intersubband emission process are determined by an appropriately engineered energy structure for a three-level system in the conduction band of a quantum well or dot structure: for the quantum-well-based system, the structure has an asymmetric funnel shape to provide long electron-phonon lifetime at the third (top) energy level. Under high carrier injection, NIR interband emission depopulates the conduction ground level of the quantum well, thereby stabilizing the electron concentration at this level-a necessary condition fur the operation of the MIR laser. This paper discusses the calculation of the population inversion conditions, the requisite gain, and threshold current for MIR laser operation. We also present a preliminary design of the laser structure with a composite waveguide that accommodates both mid- and NIR stimulated emission.

Far-infrared electroluminescence from parabolic quantum well superlattices excited by resonant tunneling injection

We present an investigation of intersubband emission at far-infrared wavelengths from semiconductor superlattices with parabolically graded quantum wells. Light emission is produced by sequential resonant tunneling injection of electrons into higher energy levels of the quantum wells and subsequent radiative decay. The current versus voltage curves of these devices exhibit negative differential resistances characteristic of the sequential resonant tunneling injection. A single, narrow emission peak is observed from the superlattices with parabolic quantum wells demonstrating radiative decay through multiple evenly spaced energy levels. When a chirped superlattice acting as an electron energy filter replaces the barrier in each period of the structure, clearer resonances are observed in the current–voltage characteristics and more efficient injection is achieved.

Intersubband transitions in coupled wells with disorder

Disorder due to doping or interface roughness in quasi-two-dimensional systems causes in-plane localization of the electrons and strongly affects their optical properties. We present numerical calculations of intersubband (IS) emission spectra for the case of a doped double-quantum-well, including disorder localization by an exact diagonalization method. It is demonstrated that the inhomogeneous line broadening of the IS peak is then determined by an interplay of correlation-effects in the single-particle spectrum and the spatially indirect nature of the IS inter-well transitions. The resulting line width is larger than in the case of intra-well transitions, but still considerably smaller than the typical fluctuations of the effective random potential. We further show how the energy selective tunneling injection of electrons into the upper subband of a coupled-well structure and the observation of the IS luminescence spectrum can yield valuable information about the physics of localization in multi-subband systems.

Narrow-linewidth terahertz intersubband emission from three-level systems

Narrow-linewidth terahertz spontaneous emission resulting from interwell (or diagonal) intersubband transition from an electrically pumped multiple-quantum-well structure was observed. The center frequency of the emission peak is at 2.57 THz, and its full width at half maximum is 0.47 THz. The emission frequency is in good agreement with the calculated intersubband transition energy of 11.3 meV (corresponding to 2.7 THz) in a three-level system, which was designed to achieve population inversion between two radiative levels.

High-power GaAs/AlGaAs quantum fountain unipolar laser emitting at 14.5 μm with 2.5% tunability

We demonstrate operation of a high-power quantum fountain unipolar laser relying on intersubband emission in optically pumped GaAs/AlGaAs quantum wells. The collected power per facet is as large as 2.3 W at 20 K and 1.5 W at 120 K, which translates into 6.6 W optical power per facet at low temperature accounting for collection efficiency. The maximum operating temperature is 135 K. We also demonstrate that the lasing wavelength can be tuned by as much as Δλ/λ≈2.5% simply by tuning the pump wavelength.

Quantum fountain infrared light sources based on intersubband emissions in quantum wells

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.

Intersubband terahertz lasers using four-level asymmetric quantum wells

We demonstrate the potential for laser operation at far-infrared wavelengths (30–300 μm, 1–10 THz) by using intersubband emission in four-level GaAs/AlGaAs asymmetric (stepped) quantum wells. Achieving population inversion in these devices depends critically on the lifetimes of the nonradiative intersubband transitions, and so we have performed detailed calculations of electron–electron and electron–phonon scattering rates. Our four-subband structures show potential for the realization of room temperature lasing, unlike previously considered three-subband structures which did not give population inversions except at impractically low electron densities and temperatures. Auger-type electron–electron interactions involving the highly populated ground subband effectively destroyed the population inversion in three level systems, but in these four subband structures the inversion is maintained by strong phonon-mediated depopulation of the lower laser level. The largest population inversions are calculated at low temperatures (<30 K), but for the structures with higher emission energies, room temperature (300 K) operation is also predicted.

Efficient photoluminescence from triangular quantum wells at the interface of an InP/In0.53Ga0.47As heterostructure

Efficient photoluminescence (PL) with quantum yield close to 1 from InP/In0.53Ga0.47As heterostructures (HSs) at temperatures 77–300 K and low excitation levels is observed and investigated. The PL is due to a quasi-triangular quantum well (TQW) located at the HS interface and consists of two spectrally similar lines: InGaAs interband emission and emission from the bottom level of the TQW. It is found that as the temperature increases, the intersubband emission rises, while the TQW radiation is quenched.

Mid-infrared intersubband electroluminescence from a single-period GaAs/AlGaAs triple barrier structure

This letter reports the observation of intersubband electroluminescence from a single-period resonant tunneling structure. Intersubband emission (λ≈8.4 μm), with a full width at half maximum of 7 meV, was observed from a GaAs/AlGaAs triple barrier structure with quantum well widths of 66 and 33 Å. The emission was coupled out of the sample by a metallic grating with a period of 5 μm deposited on the surface. The intensity of emission follows the resonance behavior in the I–V characteristics. As the temperature increases from 10 to 200 K, the emission intensity decreases monotonically by a factor of ∼2 and the emission energy shifts down slightly (ΔE≈2.7 meV). The temperature dependence of the emission energy is explained by a combination of thermal broadening of the electron distribution and the nonparabolicity of the conduction bands.

Dual-wavelength emission from optically cascaded intersubband transitions

Dual-wavelength intersubband emission at 8 and 10 microm is reported in a three-level quantum-well system in which one electronic state is at the same time the lower level of the first optical transition and the upper level of the second. Results are presented for two different AlInAs/GaInAs quantum cascade structures featuring single-well active regions with two vertical transitions or double-well active regions with one diagonal and one vertical transition. Laser action has been achieved between the excited states of the single-well device and on the diagonal transition of the double-well structure. In the latter case the wavelength can be electric-field tuned by means of the Stark effect also above threshold.