Strain-balanced Quantum Wells Research Articles

GaAs-based strain-balanced GaNxAs1-x-yBiy type-I and -II multi-quantum wells for near-infrared photodetection range

Electronic band structure of t2-GaNx2As1-x2-y2Biy2/t1-GaNx1As1-x1-y1Biy1 strain-balanced quantum wells (QWs) elaborated on GaAs substrate was theoretically studied. Our study is based on the band anti-crossing model combined with the envelope function formalism. Using the zero stress balance model, the strain compensation criteria (N and Bi compositions, and thickness ratio (t1/t2) for the strain-balanced QWs is investigated. The critical thickness for each strained GaNAsBi layer is also discussed. Through the results, we found that the control of the compositions (x1,y1) and (x2,y2) for the tensile and compressive strained layers respectively, could modulate the band alignment of the strain-balanced QWs. The use of strain-balanced GaNx2As1-x2-y2Biy2/GaNx1As1-x1-y1Biy1type-II ”W” QWs is quite promising candidate for GaAs-based near-infrared photodetection range as compared to strain-balanced type-I QWs counterparts.

Can a Hot-Carrier Solar Cell also be an Efficient Up-converter?

The hot-carrier solar cell is a very ambitious device concept, which has a thermodynamic efficiency limit of around 84% when operated at the maximum power point. However, if the same device is instead operated at open circuit, then it becomes an efficient radiator of blackbody radiation at the temperature of the hot carriers. Such a configuration is similar to a thermal photovoltaic converter, but one in which the thermal gradient is maintained by a hot electron-hole gas rather than by a physically hot lattice. In this scheme (see Fig. 1), a low-bandgap hot-carrier material is placed behind a conventional solar cell and absorbs sub-bandgap photons, generating a hot-carrier distribution which re-radiates this energy, some of which can be collected by the solar cell located above. The additional photons have been thermally up-converted by a “hot-carrier radiator.” We will discuss the thermodynamic efficiency limit of a hot-carrier radiator placed behind a conventional single-junction solar cell, and present some experimental results toward developing a proof-of-concept device using strain-balanced quantum wells.

Experimental characterization of high-speed 155 μm buried heterostructure InGaAsP/InGaAlAs quantum-well lasers

Detailed experimental characterization is performed for 1550 nm semi-insulating regrown buried heterostructure Fabry-Perot (FP) lasers having 20 InGaAsP/InGaAlAs strain-balanced quantum wells (QWs) in the active region. Light-current-voltage performance, electrical impedance, small-signal response below and above threshold, amplified spontaneous emission spectrum below threshold and relative intensity noise spectrum are measured. Different laser parameters such as external differential quantum efficiency ηd, background optical loss αi, K-factor, D-factor, characteristic temperature T0, differential gain dg/dn, gain-compression factor ϵ, carrier density versus current, differential carrier lifetime τd, optical gain spectrum below threshold, and chirp parameter α are extracted from these measurements. The FP lasers exhibited a high T0(78-86.5°C) and very high-resonance frequency (23.7 GHz). The results indicate that appropriately designed lasers having a large number of InGaAsP well/InGaAlAs barrier QWs with shallow valence-band discontinuity can be useful for uncooled high-speed direct-modulated laser applications.

Quantum confined Stark effect near 1.5 μm wavelength in InAs0.53P0.47/GayIn1−yP strain-balanced quantum wells

We report that high-quality InAsP/GaInP strain-balanced multiple quantum wells (MQWs) at 1.5 μm wavelength can be grown by gas-source molecular beam epitaxy. In order to obtain a large quantum-confined Stark effect, relatively thick (∼100 Å) InAs0.53P0.47 layers (lattice mismatch 1.8%) have been grown without strain relaxation. Our theoretical calculation shows that, while fulfilling the strain balance condition, the GaInP barriers should have a large thickness and small strain to reduce strain relaxation in these layers. Large Stark shift (32 meV at 180 kV/cm field) has been observed in a p-i-n diode structure with 20-period InAs0.53P0.47/Ga0.16In0.84P MQWs as the intrinsic region. The large change of absorption coefficient under electric field (∼3800 cm−1 at 80 kV/cm) indicates that this material is very promising for optical modulators near 1.5 μm.