Conventional Quantum Wells Research Articles

Self-Consistent Analysis of Strain-Compensated InGaN–AlGaN Quantum Wells for Lasers and Light-Emitting Diodes

Strain-compensated InGaN-AlGaN quantum wells (QW) are investigated as improved active regions for lasers and light emitting diodes. The strain-compensated QW structure consists of thin tensile-strained AlGaN barriers surrounding the InGaN QW. The band structure was calculated by using a self-consistent 6-band kmiddotp formalism, taking into account valence band mixing, strain effect, spontaneous and piezoelectric polarizations, as well as the carrier screening effect. The spontaneous emission and gain properties were analyzed for strain-compensated InGaN-AlGaN QW structures with indium contents of 28%, 22%, and 15% for lasers (light-emitting diodes) emitting at 480 (500), 440 (450), and 405 nm (415 nm) spectral regimes, respectively. The spontaneous emission spectra show significant improvement of the radiative emission for strain-compensated QW for all three structures compared to the corresponding conventional InGaN QW, which indicates the enhanced radiative efficiency for light emitting diodes. Our studies show the improvement of the optical gain and reduction of the threshold current density from the use of strain-compensated InGaN-AlGaN QW as active regions for diode lasers.

Design of violet InGaN light-emitting diode with staggered quantum well structure

The staggered InGaN quantum well (QW) structure and the conventional InGaN QW structure for the emission at a particular wavelength of 400 nm are designed and theoretically investigated, including the distribution of the carriers’ concentration, the radiative recombination rate, the Shockley-Read-Hall (SRH) recombination rate as well as the output performance and the internal quantum efficiency. The theoretical result indicates that the staggered QW structure offers significant improvement of carriers’ concentration in the QW, especially the hole concentration. The output power and the internal quantum efficiency also show 32.6 % and 32.5 % enhancement, respectively, in comparison with that of the conventional InGaN QW structure. The reduction of the electron overflow can be the main factor for the improvement of the optical performance for novel staggered InGaN QW structure.

Self-consistent gain analysis of type-II ‘W’ InGaN–GaNAs quantum well lasers

Type-II InGaN–GaNAs quantum wells (QWs) with thin dilute-As (∼3%) GaNAs layer are analyzed self-consistently as improved III-nitride gain media for diode lasers. The band structure is calculated by using a six-band k⋅p formalism, taking into account valence band mixing, strain effect, spontaneous and piezoelectric polarizations, as well as the carrier screening effect. The type-II InGaN–GaNAs QW structure allows large electron-hole wave function overlap by confining the hole wave function in the GaNAs layer of the QW. The findings based on self-consistent analysis indicate that type-II InGaN-GaNAs QW active region results in superior performance for laser diodes, in comparison to that of conventional InGaN QW. Both the spontaneous emission radiative recombination rate and optical gain of type-II InGaN–GaNAs QW structure are significantly enhanced. Reduction in the threshold current density of InGaN–GaNAs QW lasers is also predicted.

Theoretical design of carrier injection rate and recombination rate in tunnel injection quantum well lasers

AbstractCarrier injection rate in tunnel injection quantum well structures used in semiconductor lasers was investigated through theoretical analysis. Carrier capture time from 3D‐state to 2D‐state in a conventional quantum well was several ten picoseconds and it is almost uncontrollable characteristics. The tunnel injection structure can be designed to control the carrier injection rate. The carrier transition time from carrier reservoir‐region of the tunnel injection structure to the active well was a few picoseconds when the structure was designed for high speed transition. As results, carrier injection to ground state of the active well takes less than 10 picoseconds. The change of the radiative recombination rate in the active well also defines the optimal design of the tunnel injection structure. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermal precipitation of self‐organized PbTe quantum dots in CdTe host matrix

AbstractThis paper describes a selective growth of PbTe/CdTe quantum wells (QWs) and quantum dots (QDs) in CdTe host matrix by molecular beam epitaxy. These two tellurides possess almost identical lattice constants, but differ fundamentally in their lattice structure. Owing to a strong phase separation by the lattice‐type mismatch, insertion of a PbTe thin layer in CdTe matrix at higher temperatures than 280 °C was resulted in a self‐organized growth of coherent and three‐dimensionally isotropic QDs. The same growth procedure at lower temperatures, on the other hand, yielded a conventional QW structure with sharp heterointerface. This QW structure, however, was found to precipitate well‐ordered QD‐array by a postgrowth annealing, indicating that the QD formation is induced by the minimization of interface energy between the inmiscible two tellurides. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spontaneous Emission and Characteristics of Staggered InGaN Quantum-Well Light-Emitting Diodes

A novel gain media based on staggered InGaN quantum wells (QWs) grown by metal-organic chemical vapor deposition was demonstrated as improved active region for visible light emitters. Fermi's golden rule indicates that InGaN QW with step-function like In content in the well leads to significantly improved radiative recombination rate and optical gain due to increased electron-hole wavefunction overlap, in comparison to that of conventional InGaN QW. Spontaneous emission spectra of both conventional and staggered InGaN QW were calculated based on energy dispersion and transition matrix element obtained by 6-band k middotp formalism for wurtzite semiconductor, taking into account valence-band-states mixing, strain effects, and polarization-induced electric fields. The calculated spectra for the staggered InGaN QW showed enhancement of radiative recombination rate, which is in good agreement with photoluminescence and cathodoluminescence measurements at emission wavelength regime of 425 and 500 nm. Experimental results of light-emitting diode (LED) structures utilizing staggered InGaN QW also show significant improvement in output power. Staggered InGaN QW allows polarization engineering leading to improved luminescence intensity and LED output power as a result of enhanced radiative recombination rate.

Type-II InGaN-GaNAs quantum wells for lasers applications

We present a visible III-nitride gain medium based on type-II InGaN-GaNAs quantum well (QW), employing thin dilute-As (∼3%) GaNAs layer. The utilization of GaNAs layer shifts the hole confinement to the center of the type-II QW, which significantly reduces the charge separation effect. The optical gain and spontaneous recombination rate of the type-II InGaN-GaNAs QW are analyzed and compared with those of conventional InGaN QW emitting in the blue regime (λ∼450nm), using six-band k.p formalism for energy dispersion of the III-nitride wurtzite semiconductor. The use of type-II QW leads to significant improvement in the optical gain and spontaneous recombination rate.

Nanostructure engineering of staggered InGaN quantum wells light emitting diodes emitting at 420–510 nm

AbstractWe demonstrated staggered InGaN quantum wells (QW) grown by metalorganic chemical vapor deposition (MOCVD) as improved active region for visible light emitters. Theoretical studies indicate that staggered InGaN QW with step‐function like In‐content in the well offers significantly improved radiative recombination rate and optical gain, in comparison to the conventional InGaN QW. Experimental results of light emitting diode (LED) structure utilizing staggered InGaN QW show good agreement with theory. Staggered InGaN QW allows polarization engineering leading to improvement of photoluminescence intensity and LEDs output power as a result of enhanced radiative recombination rate. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Simulation of quantum wells with ‘spikes’ and ‘dips’

The use of thin high-bandgap ‘spikes’ or thin low-bandgap ‘dips’ inside conventional rectangular quantum wells (QWs) gives supplementary flexibility in engineering intra- and inter-band energy level separation. The paper presents simulation and experimental studies on the effects of ‘spikes’ and ‘dips’ on the fundamental quantum well properties.

“Spiked” GaInP Quantum Wells for Shorter Red Wavelength Emission

Compositional and strain limitations are often restricting the emission wavelength from quantum-well (QW) lasers. The letter presents simulation and experimental results on the effects of including thin higher bandgap layers into GalnP QWs emitting in the short red wavelength range. These were considered "spiked" single QWs since the thin higher bandgap layers used in our studies varied in composition and led to carrier wavefunctions that are like perturbed single QW wavefunctions rather than wavefunctions of coupled QWs. The edge-emitting lasers having up to four-monolayer AlGalnP "spikes" in the QWs had an emission blueshift of up to 25 nm, whereas the degradation of other laser characteristics was in line with the degradation observed when similar emission blueshift was generated by conventional QW modification.

Polarization engineering via staggered InGaN quantum wells for radiative efficiency enhancement of light emitting diodes

Staggered InGaN quantum wells (QWs) grown by metal-organic chemical vapor deposition are demonstrated as improved active region for visible light emitters. Theoretical studies indicate that InGaN QW with step-function-like In content in the quantum well offers significantly improved radiative recombination rate and optical gain in comparison to the conventional type-I InGaN QW. Experimental results of light emitting diode (LED) structure utilizing staggered InGaN QW show good agreement with theory. Polarization band engineering via staggered InGaN quantum well allows enhancement of radiative recombination rate, leading to the improvement of photoluminescence intensity and LED output power.

Reduction in Escape Times of Photogenerated Charge Carriers with Asymmetric Intrastep Quantum Wells and Subsequent Improvement in Saturation Optical Intensity

Escape times of photogenerated charge carriers from an InGaAsP intrastep quantum well (IQW) are calculated and compared with those from a conventional quantum well (QW). Since the red shift of the IQW is initiated at a higher electric field, the escape times from the IQW are smaller than those from the conventional QW at given values in modulator transmission. From the reduction in escape times, improvements in the exciton saturation intensity are estimated to be a factor of ∼5 for both high and low modulator-transmission points.

Strong exciton-photon coupling in a microcavity containing layered perovskite semiconductors

The authors have realized a Pérot-Fabry microcavity containing a two-dimensional layered perovskite-type semiconductor, (C6H5C2H4–NH3)2PbI4, between a dielectric mirror and a metallic mirror. A strong coupling regime between the perovskite exciton and the confined photon mode has been evidenced at room temperature from angular-resolved reflectivity experiments. An anticrossing of 140meV is observed between the excitonic and cavity modes. (C6H5C2H4–NH3)2PbI4 having a self-organized multiple quantum well structure, they have determined the oscillator strength per quantum well of this structure: 4×1013cm−2, which is one order of magnitude higher than in conventional inorganic quantum wells.

Origin of high oscillator strength in green-emitting InGaN∕GaN nanocolumns

Optical characterization has been performed on an InGaN∕GaN nanocolumn structure grown by nitrogen plasma assisted molecular beam epitaxy not only in macroscopic configuration but also in a microscopic one that can be assessed to a single nanocolumn. The photoluminescence (PL) decay monitored at 500nm is fitted with a double exponential curve, which has lifetimes of 0.67 and 4.33ns at 13K. These values are two orders of magnitude smaller than those taken at the same wavelength in conventional InGaN∕GaN quantum wells (QWs) grown toward the C orientation. PL detection of each single nanocolumn was achieved using a mechanical lift-off technique. The results indicate that the very broad, macroscopically observed PL spectrum is due to the sum of the sharp PL spectrum from each nanocolumn, the peak energy of which fluctuates. Moreover, unlike conventional QWs, the blueshift of a single nanocolumn is negligibly small under higher photoexcitation. These findings suggest that carrier localization as well as the piezoelectric polarization field is suppressed in InGaN∕GaN nanocolumns.

Electron mobility enhancement in AlN∕GaN∕AlN heterostructures with InGaN nanogrooves

The authors show that the electron mobility can be strongly enhanced in AlN∕GaN∕AlN heterostructures with the shallow InxGa1−xN channel—nanogroove—in the middle of the potential well. The modified heterostructure has the room-temperature electron mobility, which is five times larger than that in conventional quantum wells. The maximum mobility enhancement is achieved for In content x≈0.05, which is sufficient to weaken the intersubband electron scattering without leading to the substantial electron—interface-phonon scattering. The mobility enhancement is pronounced for a wide range of the carrier densities (1011–1013cm−2), which is important for GaN technology.

Wavelength Extension Effect on Lasing Characteristics of Highly-Strained GaInAs/GaAs Vertical-Cavity Surface-Emitting Lasers with Cavity Detuning

Cavity detuned highly-strained GaInAs/GaAs vertical cavity surface emitting lasers (VCSELs) were investigated. Devices emitting beyond 1.2 µm of the wavelength were realized by combining conventional quantum wells emitting at 1.18 µm and large wavelength detuned cavities. Various detuning conditions from -35 to 80 nm were compared for clarifying the detuning effect. The longest wavelength was 1.27 µm for an 80 nm detuned device. The highest operating temperature of a 55 nm detuned device was 140 °C under cw operation. It was found that approximately 50 nm detuning was necessary to reach the threshold gain level for a large detuned device and the condition was produced by self-heating. Relaxation resonance frequencies were characterized and over 2 GHz was observed even for a 1.27 µm VCSEL.

The Optical Properties of Heterostructures with Quantum-Confined InGaAsN Layers on a GaAs Substrate and Emitting at 1.3–1.55 μm

Under study is the photoluminescence of two types of MBE-grown heterostructures with quantum-confined InGaAsN/GaAs layers: (1) conventional InGaAsN quantum wells (QWs) in GaAs, and (2) heterostructures with an active region consisting of a short-period GaAsN/InGaAsN superlattice that has an InGaAsN QW with a submonolayer InAs insertion at its center. At room temperature, the structures under study emit light in the range from ∼1.3 to ∼1.55 μm. In the second type of heterostructure, emission with a wavelength larger than 1.5 μm can be obtained at lower nitrogen and indium concentrations than in a conventional QW. This leads to a significant depression of the effects related to decomposition of an InGaAsN solid solution, thus improving the radiative efficiency of the InGaAsN QWs.

Optimal design of gan-algan bragg-confined structures for intersubband absorption in the near-infrared spectral range

A method is proposed for the design and optimization of structural parameters of GaN-AlGaN Bragg-confined structures with respect to peak intersubband absorption from the ground to the first excited state,1 /spl rarr/ 2 electronic transition, in the near infrared spectral range. An above-the-barrier bound state was used to extend the range of transition energies above the values available in conventional quantum wells. Intrinsic polarization fields and nonparabolicity effects were taken into account. The selection of optimal parameters, maximizing the absorption at wavelengths of 1.55 and 1.3 μm, was performed by using a simulated annealing algorithm, and optimal structures with infinite superlattices as confinement regions were thus designed. These optimal parameters were then used to set realistic, finite structures with a small number of layers, the performance of which was re-evaluated by solving the Schrodinger-Poisson equation self-consistently for a few different levels and profiles of doping.

Internal optical loss measurements in InGaAs-InAlGaAs quantum-well lasers operating around 1550 nm

A multisection device technique is employed to carry out internal optical loss measurements in two types of InGaAs-InAlGaAs quantum-well structures. One structure consists of conventional identical-width quantum wells and the other, a broader spectral-width material, consists of multiple-width quantum wells in the active region. The temperature dependence of the internal optical losses is also investigated for both structures.

Efficient blue light-emitting diodes with InGaN/GaN triangular shaped multiple quantum wells

InGaN/GaN triangular shaped multiple quantum wells (QWs) grown by grading In composition with time were adopted as an active layer of blue light-emitting diodes (LEDs). Compared to the LEDs with conventional rectangular QW structures, the triangular QW LEDs showed a higher intensity and a narrower linewidth of electrical luminescence (EL), a lower operation voltage, and a stronger light-output power. EL spectra of the triangular-QW-based LEDs also showed that the peak energy is nearly independent of the injection current and temperature, indicating that the triangular QW LED is more efficient and stable than the rectangular one.