type-II Multiple Quantum Wells Research Articles

Nonequilibrium Hot-Carrier Transport in Type-II Multiple Quantum Wells for Solar-Cell Applications

Prototypes for hot-carrier solar cells based on type-II $\mathrm{InAs}/{\mathrm{AlAs}}_{0.16}{\mathrm{Sb}}_{0.84}$ multiple quantum wells are examined with ac photoconductivity as a function of lattice temperature and photoexcitation energy to determine the photoexcited charge-carrier transport. These samples previously exhibited an excitation energy onset of a metastable regime in their short-time charge-carrier dynamics that potentially improves their applicability for hot-carrier photovoltaic cells. The transport results illustrate that the ac photoconductivity is larger in the regime corresponding to the metastability as a result of higher excitation photocarrier densities. In this excitation regime, the ac photoconductivity is accompanied by slightly lower carrier mobility, arising from the plasmalike nature of carriers scattered by Auger recombination. Outside of this regime, higher mobility is observed as a result of a lower excitation density that is more readily achievable by solar concentration. Additionally, at ambient temperatures, more scattering events are accompanied by slightly lower mobility, but the excitation dependence indicates that this is accompanied by an ambipolar diffusion length that is greater than half a micron. These transport properties are consistent with good quality in-organic elemental and III-V semiconductor solar cells and far exceed those of organic and perovskite materials. The transport results complement the dynamics observed in type-II $\mathrm{InAs}/{\mathrm{AlAs}}_{0.16}{\mathrm{Sb}}_{0.84}$ and can guide the design of hot-carrier solar cells based on these and related materials.

Nonlinear Characteristics of Uni-Traveling Carrier Photodiode With InGaAs/GaAsSb Type-II Multiple Quantum Wells Absorber

The linearity of high power photodetectors is the limiting factor to the dynamic range performance of analog optical links and microwave photonics applications. In this work, we characterized the third order intermodulation distortion (IMD3) of a uni-traveling carrier photodiode with InGaAs/GaAsSb type-II multiple quantum wells absorber. The 10 μm diameter photodiode shows a third order output intercept point of 36.6 dBm at 200 MHz and 12.9 dBm at 25 GHz, under reverse bias of −5 V and photo current of 15 mA. Combining with the measured device parameters, an equivalent circuit model based simulation is carried out, and the general trend of the simulated IMD3 is consistent with the measurement results. By separately involving the nonlinear mechanisms, the source of the nonlinearity is studied in detail.

High-speed uni-traveling carrier photodiode for 2 μm wavelength application

Current optical communication systems operating at the 1.55 μm wavelength band may not be able to continually satisfy the growing demand on data capacity within the next few years. Opening a new spectral window around the 2 μm wavelength with recently developed hollow-core photonic bandgap fiber and a thulium-doped fiber amplifier is a promising solution to increase transmission capacity due to the low-loss and wide-bandwidth properties of these components at this wavelength band. However, as a key component, the performance of current high-speed photodetectors at the 2 μm wavelength is still not comparable with those at the 1.55 μm wavelength band, which chokes the feasibility of the new spectral window. In this work, we demonstrate, for the first time to our knowledge, a high-speed uni-traveling carrier photodiode for 2 μm applications with InGaAs/GaAsSb type-II multiple quantum wells as the absorption region, which is lattice-matched to InP. The devices have the responsivity of 0.07 A/W at 2 μm wavelength, and the device with a 10 μm diameter shows a 3 dB bandwidth of 25 GHz at −3 V bias voltage. To the best of our knowledge, this device is the fastest photodiode among all group III-V and group IV photodetectors working in the 2 μm wavelength range.

Design of InP-Based High-Speed Photodiode for 2-<inline-formula> <tex-math notation="LaTeX">$\mu$ </tex-math> </inline-formula>m Wavelength Application

In this paper, we proposed and designed the high speed uni-traveling carrier photodiodes operating beyond 2 μm. InGaAs/GaAsSb type-II multiple quantum wells (MQWs) were used as the absorption region of uni-traveling carrier photodiode to have optical response beyond 2 μm. A rate equation model was developed to study the bandwidth characteristics of this photodiode. The carrier dynamics of the carrier sweeping out process was discussed, and the structure parameters of the quantum wells were optimized. Result shows a 3-dB bandwidth over 40 GHz can be achieved with the optimized design. We also studied the feasibility of dual depletion PIN (DD-PIN) photodiode with MQWs absorber. The optimized DD-PIN photodiode could achieve similar bandwidth under relatively high-bias condition.

Dynamic model and bandwidth characterization of InGaAs/GaAsSb type-II quantum wells PIN photodiodes.

In this work, we demonstrated a normal incident PIN InGaAs/GaAsSb type-II multiple quantum wells (MQW) photodiode on InP substrate for 2 μm wavelength high-speed operation. The photodiode has a responsivity of 0.35 A/W at room temperature at 2 μm, and a 3 dB bandwidth of 3.7 GHz. A carrier dynamic model is developed to study the bandwidth of the multiple quantum wells photodiode. Simulation results match the experimental data well, and analysis shows that hole transport limits the 3 dB bandwidth performance. By optimizing the MQW design, higher bandwidth performance (>10 GHz) can be achieved.

High-Speed InP-Based p-i-n Photodiodes With InGaAs/GaAsSb Type-II Quantum Wells

We present and discuss the performance characteristics of InP-based p-i-n photodiodes (PDs) with InGaAs/GaAsSb type-II multiple quantum wells absorption regions designed to absorb light at mid-infrared wavelengths. Top-illuminated and waveguide-integrated PDs are fabricated with dark currents as low as 100 nA at −2 V, an external responsivity as high as 0.27 A/W at 2 $\mu \text{m}$ and 0.3 A/W at 1.55 $\mu \text{m}$ , and a 3-dB bandwidth of 3.5 GHz at 2 $\mu \text{m}$ .

MOVPE growth of antimonide-containing alloy materials for long wavelength applications

GaAs-based heterostructures comprised of GaAs 1− x N x –GaAs 1− y Sb y ( x<0.03, y<0.35) multiple quantum wells (MQW) that utilize ‘W’-shaped type-II transitions have potential for realizing high-performance monolithic VCSELs and edge-emitting lasers with low temperature sensitivity in the 1.55 μm wavelength region. Metal-organic vapor-phase epitaxy (MOVPE) growth of GaAsSb is complicated by both thermodynamically driven phase separation and kinetic effects that arise from incomplete thermal decomposition of methyl- and hydride precursors at typical growth temperatures. The impact of growth chemistry on the formation of strained and pseudomorphic films was studied through the growth of relaxed GaAsSb films and multi-period pseudomorphic GaAsSb/GaAs superlattices. Trimethyl- and triethyl-gallium and trimethyl- and triethyl-antimony were used in a variety of combinations. The observed variations of the Sb incorporation efficiency for relaxed and strained films with growth conditions are not predicted by the existing thermodynamic models of the growth, indicating a coupling of the surface growth chemistry and the strain-induced changes in the surface stoichiometry. Through modification of the growth chemistry and process conditions, an extended range of Sb incorporation was realized as well as enhanced control over the alloy composition in strained layers. These achievements lead directly to an extended wavelength range in type-II MQW structures.

Scanning-tunneling-microscopy observation of heterojunctions with a type-II band alignment in ZnSe∕BeTe multiple quantum wells

Heterojunctions of ZnSe∕BeTe multiple quantum wells (MQW) with a type-II band alignment have been investigated by cross-sectional scanning tunneling microscopy (STM). The brightness of the ZnSe and BeTe layers in the cross-sectional STM image is inverted between filled- and empty-state images, taken by switching the bias polarity of the sample bias voltage in constant current mode. Such inversion of the brightness indicates changes in the band offsets of the conduction and valence bands between the ZnSe and BeTe layers of the type-II MQW. The roughness of interfaces in the filled state images has also been investigated on an atomic scale. It is found that the roughness amplitude Δ, and the correlation length Λ, which characterize the observed interfacial roughness, are comparable to the values observed for III-V heterostructures.

Strong near-infrared photoluminescence and absorption from Si/Si1−xGex type-II multiple quantum wells on bulk crystal SiGe substrates

We report a study of photoluminescence (PL) and optical absorption in high-quality tensile-strained Si type-II multiple quantum wells (QWs) grown on bulk crystal SiGe substrates by using low-temperature ultrahigh-vacuum chemical vapor deposition. Detailed PL experiments, as a function of excitation density together with applied uniaxial stress, were performed to study the band alignment and to help elucidate the origin of the observed PL peaks. Spatially direct and indirect transitions of the Si QWs and an intense broad subgap PL were observed. With increasing excitation power, the PL lines of Si QWs shift to lower energies, whereas the broad PL shifts to higher energy. The applied [110] compressive stress results in a redshift of the substrate PL lines, as expected, but a blueshift of both the Si QW PL and the broad PL lines. The optical absorption data also show strong absorption in the near-infrared region where the broad PL was observed. Near the absorption edge (Eg), the energy dependence obeys a square law rather than a square root law, suggesting that Eg is associated with a quasi-direct transition. The Eg value deduced according to the square law agrees well with the broad PL peak energy provided that temperature-dependent Eg, excitonic binding energy, and exciton localization energy, as well as quantum confinement are considered.

Type-II quantum-well lasers for the mid-wavelength infrared

We discuss an improved mid-wave infrared diode laser structure based on InAs-Ga1−xInxSb- InAs-Ga1−xAlxSb Type-II multiple quantum wells. The proposed design combines strong optical coupling, 2D dispersion for both electrons and holes, suppression of the Auger recombination rate, and excellent electrical and optical confinement.

Spin-relaxation process of holes in type-II Al0.34Ga0.66As/AlAs multiple quantum wells.

We have measured the spin memory and the spin-relaxation times of holes in type-II quantum wells by means of pump-and-probe methods. The fast \ensuremath{\Gamma}-X interlayer scattering which is characteristic of type-II multiple quantum wells allows us to observe directly the hole spin-relaxation process. The hole spin relaxation is described by two decay components. The faster decay time ranges from 20 to 100 ps and depends on the number density of ``antiparallel-spin'' holes. This indicates that the scattering between up-spin holes and down-spin holes is the major spin-relaxation process of holes. The slower time component is about 20 ns, which is probably ascribed to the spin relaxation of localized holes in the well layers.