All-optical modulation for semiconductor lasers by using threeenergy levels in n-doped quantum wells

An all-optical modulation method for semiconductor lasers using three energy levels in n-doped quantum wells is demonstrated. The modulation principle is based on the third-order interaction between interband-and intersubband-resonant light in the quantum-well structure. The modulation is demonstrated by a real-time single-shot experiment using a semiconductor laser for the interband-resonant light and a CO ₂ laser for the intersubband-resonant light. The dependences of the modulation depth on the intersubband-resonant light polarization and on the interband-resonant light wavelength indicate that the modulation is achieved by this principle. It is pointed out that the thermal effect appears when the power of the intersubband-resonant light becomes strong     

Enhanced interband-resonant light modulation byintersubband-resonant light in selectively n-doped quantum wells

The interband-resonant light modulation by the intersubband-resonant light in selectively n-doped quantum wells is investigated. The modulation efficiency depends greatly on the degree of nonlinear optical coupling between the interband and intersubband-resonant lights. It is shown theoretically and experimentally that the selective n-doping in the barrier layers of the quantum wells is very effective to increase the nonlinear coupling degree and thus the modulation efficiency. The thermal and the hot carrier effects on the modulation are also discussed     

Analysis on interband-resonant light modulation byintersubband-resonant light in n-doped quantum wells

The authors describe a theoretical analysis on interband-resonant light modulation by intersubband-resonant light in the n-doped quantum well. The modulation principle is briefly explained. The theoretical formulation for the modulation is derived by using density matrix theory. The modulation characteristics concerning the absorption coefficient and refractive index are calculated for various parameters. It is shown that the absorption coefficient for the interband-resonant light can be greatly changed by the incidence of intersubband-resonant light     

Low-bias-current direct modulation up to 33 GHz inInGaAs/GaAs/AlGaAs pseudomorphic MQW ridge-waveguide lasers

Modulation bandwidths of 24 GHz (I_bias=25 mA) and 33 GHz (I_bias=65 mA) are demonstrated for 3×100 μm² In_0.35Ga_0.65As/GaAs multiple quantum well ridge-waveguide lasers with undoped and p-doped active regions, respectively. These performance enhancements have been achieved both by lowering the growth temperature of the high-Al-mole-fraction cladding layers and by utilizing short-cavity devices, fabricated with dry-etched facets using chemically-assisted ion-beam etching. Both the undoped and p-doped lasers also demonstrate modulation current efficiency factors exceeding 5 GHz/mA^1/2, the best reported results for any semiconductor laser     

Observation and Modeling of a Room-Temperature Negative Characteristic Temperature 1.3-μm p-Type Modulation-Doped Quantum-Dot Laser

A room-temperature negative characteristic temperature (T₀ ) and ultralow threshold current density (J_th) of 48 Amiddotcm^-2 are demonstrated for a 1.3-mum InAs quantum dot laser. These characteristics are obtained by combining a high-growth-temperature GaAs spacer layer with p-type modulation doping of the quantum dots in multiple layer dot-in-a-well structures. Through a comparison of p-doped and undoped devices, a photon coupling mechanism is proposed to account for the different temperature dependences of Jth for the two devices. Numerical simulations based on a rate equation model, which includes photon coupling between ground and excited quantum dot states, are performed. The simulations are able to account for the very different temperature-dependent Jth behavior of the doped and undoped device     

DC and Dynamic Characteristics of P-Doped and Tunnel Injection 1.65-μm InAs Quantum-Dash Lasers Grown on InP (001)

We have studied the characteristics of 1.65-mum InAs self-organized quantum-dash lasers grown on InP (001) substrates, wherein special techniques of p-doping of quantum dashes and tunnel injection are incorporated for the first time. We measured a very large T₀ (196 K) in p-doped quantum-dash lasers, accompanied by an increase in threshold current density (J_th~1600 A/cm² ), compared to the undoped quantum-dash lasers (T₀=76 K and J_th~950 A/cm²). The p-doped lasers exhibit a maximum 3-dB bandwidth of 8 GHz, chirp ~1.0 Aring, and alpha-parameter ~1.0 (measured at subthreshold bias conditions) at a temperature of 278 K. Similar undoped quantum-dash lasers exhibit a 3-dB bandwidth of 6 GHz. A self-consistent model, that includes Auger recombination in quantum dashes, is developed to calculate the threshold current at various temperatures. A comparison of the calculated threshold current and T₀ with measured values reveals that Auger recombination in quantum dashes plays a major role in determining the values of threshold current and T₀ in both undoped and p-doped quantum-dash lasers. While p-doping increases the gain and differential gain, the presence of wetting layer states, the relatively large inhomogeneous broadening of quantum dashes, and the substantially increased Auger recombination upon p-doping severely limit the potential benefits. Superior characteristics, including large modulation bandwidth (f_{-3 dB}~12 GHz), near-zero alpha-parameter, and very low chirp (~0.3 Aring), are achieved when the technique of tunnel injection is also utilized     

Energy relaxation of light holes in InAs.15Sb.85/InSb multiple quantum wells

The energy relaxation rate of light in-plane holes in InAs_.15Sb_.85/InSb quantum wells has been measured using a Shubnikov-de Haas technique. In this Type II system, the holes reside in the InSb layers; strain reverses the heavy-light hole ordering and thus light holes are the charge carriers. The samples consist of 20 to 100 InAs_.15Sb_.85/InSb periods 100Å/200Å thick. The InAsSb barriers are doped with Be. The total carrier concentration p ≈ 1×10¹¹ cm⁻² is obtained from Hall data. Shubnikov-de Haas oscillation amplitudes are measured and used to determine the light hole temperature for a given applied power. The steady state power per carrier is equated to the energy relaxation rate to determine the carrier temperature T_H. The measured rate for low electric fields is proportional to T_Hⁿ-T_Lⁿ with n ≈ 3.2 and 2.5 for two different samples. These data are compared with theory and experiment for light holes in InGaAs/GaAs quantum wells.     

Yttrium- and Terbium-Based Interlayer on SiO2 and HfO2 Gate Dielectrics for Work Function Modulation of Nickel Fully Silicided Gate in nMOSFET

Novel yttrium- and terbium-based interlayers (Y_IL and Tb_IL, respectively) on SiO₂ and HfO₂ gate dielectrics were employed for NMOS work function Phi_m modulation of undoped nickel fully silicided (Ni-FUSI) gate. Bandedge Ni-FUSI gate Phi_m of ~4.11 and ~4.07 eV was obtained by insertion of ultrathin (~1 nm) Y_IL and Tb_IL, respectively, on the SiO₂ gate dielectric in a gate-first process (with 1000 degC anneal). NiSi Phi_m on SiO₂ could also be tuned between the Si midgap and the conduction bandedge E_C by varying the interlayer thickness. The achievement of NiSi Phi_m around 4.28 eV on the HfO₂ gate dielectric using interlayer insertion makes this an attractive Phi_m modulation technique for Ni-FUSI gates on SiO₂ and high-k dielectrics     

Analysis of transient interband light modulation by ultrashortintersubband resonant light pulses in semiconductor quantum wells

We explore the transient characteristics of an interband resonant light modulation process by ultrashort intersubband resonant light pulses in semiconductor quantum wells (QW's). The modulation characteristics in a three-level semiconductor QW system, including the effects due to in-plane momentum, have been investigated by a numerical analysis of the coupled Bloch equations using a density-matrix approach. We have studied the effect of the carrier density and the nature of the intersubband coupling light on the transient absorption features of the interband light. The modulation process has been compared in doped and undoped QW's. The switching behavior of the strong interband light field in presence of a train of intersubband light pulses has also been discussed     

Insulator passivation of In0.2Ga0.8As-GaAssurface quantum wells

The electronic passivation of (100) In_0.2Ga_0.8 As-GaAs surface quantum wells (QWs) using in situ deposition of an amorphous, insulating Ga₂O₃ film has been investigated and compared to standard Al_0.45Ga_0.55As passivation. Nonradiative lifetimes τ_r=1.1±0.2 and 1.2±0.2 ns have been inferred from the dependence of the internal quantum efficiency η on optical excitation density P₀' for the Ga₂O₃ and Al_0.45Ga_0.55As passivated In_0.02 Ga_0.8As-GaAs surface QW, respectively. Beyond identical internal quantum efficiency, the amorphous Ga₂O₃ insulator passivation simplifies device processing, eludes problems arising from lattice-mismatched interfaces, and virtually eliminates band bending in electronic and optoelectronic devices based on a low dimensional system such as quantum wells, wires, and dots     

Intersubband absorption of GaAs/AlGaAs quantum wells in MBE grownmid-infrared slab waveguides

A structure to enhance the absorbance due to intersubband transitions in GaAs/AlGaAs quantum wells is discussed. Mid-infrared slab waveguides including 30 quantum wells were grown using molecular-beam epitaxy (MBE). Photoluminescence experiments revealed an excellent uniformity of the samples. Absorption measurements over the whole 9-13.4-μm spectral range were performed for the first time using the combination of CO₂ and NH₃ lasers. Effective absorbance due to intersubband transitions as high as 14 dB were measured for 3-mm-long waveguides. The waveguide structure is expected to be a good candidate for optoelectronic devices in the 10-μm region     

Vertical multiple-quantum-well directional-coupler switch with lowswitching voltage

A novel device structure of the vertical directional-coupler optical switch with multiple quantum wells (MQWs) is proposed and fabricated. All the clad, guide, and separation layers are superlattices of Al_0.3Ga_0.7As and GaAs. This leads to a better control of the complete coupling length. The main feature of the proposed structure is that the refractive index modulation due to the quantum confined Stark effect (QCSE) is applied only to the undoped central region of the separation layer. It is found that this leads to reduction of the switching voltage. In an actual sample 138 μm long, a switching voltage of 5 V has been obtained     

Silicon–O–M–O–silicon superlattice

The success of heterojunction quantum wells and quantum dots in III–Vs has not been extended to silicon because the ideal barrier, SiO₂, is amorphous, preventing the formation of quantum structures with silicon. The possibility of a few monolayers of oxide inserted between adjacent silicon layers was proposed and realized with a superlattice (SL) structure consisting of Si–Si–O–Si–Si–Si, having a monolayer of oxygen in each period introduced by adsorption onto the 2×1 reconstructed surface along the Si(1 0 0). Reduction of the period leads to a slight up-shift of the energy of the emitted light, indicating that the essential objective of boosting the optical transition by promoting direct transitions has not been realized. Annealing in H₂+O₂ results in significant improvement in PL and EL, showing that specific defects, e.g., Si–O complexes may be responsible for the observed light emission. The role of Si–O complex being the origin of emission is further supported by the observation that the emission of visible light from polycrystalline Si and SiO₂ structure is similar to the epitaxial superlattice with oxygen. The computed strain in a new type of superlattices consisting of SiO₂, and GeO₂ is much lower than the Si–O SL. The EL in Si–O superlattice with the use of a Schottky barrier to provide electron–hole accumulation allows double injection into states higher in energy than the bandgap of Si, a prerequisite for injection laser without the need to use a wide-band pn-junction.     

1.3 μm lasers on GaAs(111)B employing ordered (InAs)1(GaAs)1 quantum wells for high frequency response applications

We propose a novel laser active region design that employs a strained and ordered ([nAs)₁(GaAs)₁ quantum well on a GaAs(111)B substrate for 1.31 μm high-speed applications. The increased Matthevvs-Blakeslee critical thickness for this orientation as compared to the (001) case allows for wider wells with higher indium compositions. In the In_0.5Ga_4.5As case, however, the bandgap is not significantly affected by the reduced quantum confinement because an increase in the hydrostatic strain component of the Hamiltonian for the (111)-orientation approximately negates any narrowing effects. By using an alternate monolayer superlattice active region to replace the alloy, we find that wavelengths well beyond 1.3 μm can be achieved. We also discuss some of the adBANtages of moving to the (111)-orientation that indicate higher modulation bandwidths are possible using this material system over conventional 1.3 μm laser diodes on InP substrates.     

Hot-electron magnetophonon resonances in doped and undoped GaAs/(Ga,Al)As quantum wells studied using photoluminescence

Hot-electron magnetophonon resonances in the effective temperature T_eff of photo-excited carriers in GaAs/Al_xGa_1-xAs quantum wells have been observed using photoluminescence for the first time. The technique has been demonstrated to be applicable to both doped and undoped structures, allowing the hot-electron magnetophonon effect in the latter to be studied. The experimental data are in good agreement with values of T_eff=T_eff(B) calculated using a modell or the energy relaxation of hot two-dimensional carriers in a strong magnetic field.     

SiGe／Si多量子阱中垂直方向红外吸收及共振色散效应

用垂直入射的中红外光束调制非掺杂SiGe/Si量子阱中光致子带间吸收，氩离子激光器作为子带间跃迁的光泵浦源在阱中产生载流子，红外调制光谱用步进式傅立叶变换光谱仪记录，实验中观察到明显的层间干涉效应与子带间跃迁有关的色散效应，理论和实验分析认为样品折射率变化造成的位相调制可以补偿吸收所造成的幅度调制。     

Postgrowth control of GaAs/AlGaAs quantum well shapes byimpurity-free vacancy diffusion

The control of quantum well shapes in GaAs/AlGaAs material after growth has been investigated both theoretically and experimentally. Double quantum well samples capped either by SiO₂ or fluorides of the group IIA elements were annealed, and energy gap shifts were measured by photoluminescence. These experimental energy shifts were compared to a theoretical model to obtain the diffusion coefficient of aluminum into the quantum wells. Fluorides were found to inhibit the intermixing process almost completely, whereas SiO₂ is known to enhance it. The aluminum diffusion coefficients for samples annealed at 920°C for 30 s are 4.0×10^-17 cm²/s and 2.1×10^-15 cm²/s for SrF₂ and SiO₂ caps, respectively. The activation energies found were 4.09 and 6.40 eV for the same two caps     

Low threshold current density 1.3-μm strained-layer quantum-welllasers using n-type modulation doping

We have developed 1.3 μm n-type modulation-doped strained-layer quantum-well lasers. Modulation-doped lasers with long cavities (low threshold gain) exhibit much lower threshold current densities than conventional lasers with undoped barrier layers. The lowest threshold current density we obtained was 250 A/cm² for 1500 μm long lasers with five quantum wells. The estimated threshold current density for an infinite cavity length was 38 A/m²/well. This is the lowest value for InGaAsP-InGaAsP and InGaAs-InGaAsP quantum well lasers to our knowledge     

In-well ambipolar diffusion in room-temperature InGaAsP multiplequantum wells

Three-pulse transient amplitude grating experiments were performed using picosecond pulses at 1.525 μm on room-temperature InGaAsP multiple quantum wells using a periodically poled LiNbO₃ (PPLN) optical parametric oscillator. An ambipolar diffusion coefficient of 7.2 cm²/s was measured from the diffraction efficiency decay rates. We deduce the presence of alloy scattering or an increase in interface scattering when comparing these results with those of similar experiments on GaAs-AlGaAs multiple quantum wells     

Refractive index modulation based on excitonic effects inGaInAs-InP coupled asymmetric quantum wells

The effect of excitons in GaInAs-InP coupled asymmetric quantum wells on the refractive index modulation, is analyzed numerically using a model based on the effective mass approximation. It is shown that two coupled quantum wells brought in resonance by an applied electric field will, due to the reduction in the exciton oscillator strengths, have a modulation of the refractive index which is more than one order of magnitude larger than in a similar quantum well structure based on the quantum confined Stark effect, but with no coupling between the quantum wells. Calculations show that combining this strong electrorefractive effect with self-photo-induced modulation in a biased-pin-diode modulator configuration, results in an optical nonlinearity with a figure of merit of 20 cm³/J at a wavelength of 1.55 μm. This value is large compared to optical nonlinearities originating from band edge resonance effects in III-V semiconductor materials