Po-Wei Liu

Mid-infrared electroluminescence at room temperature from InAsSb multi-quantum-well light-emitting diodes

Room-temperature electroluminescence is reported from InAsSb multiple-quantum-well light-emitting diodes. The diodes exhibited emission in the mid-infrared peaking near 4μm. The spectral dependence on injection current at 4K was investigated and two transitions were identified, centered at 4.05 and 3.50μm, which are associated with the eigenstates of the confined holes inside the quantum well. The use of an Sb predeposition and As flux surface exposure during epitaxial growth was observed to have a major effect on the electroluminescence output.

Photoluminescence and bowing parameters of InAsSb∕InAs multiple quantum wells grown by molecular beam epitaxy

Detailed studies are reported on the photoluminescence of InAsSb∕InAs multiple quantum wells grown by molecular beam epitaxy on InAs substrates with the Sb mole fraction ranging from 0.06 to 0.13. From 4K photoluminescence the band alignment was determined to be staggered type II. By comparing the emission peak energies with a transition energy calculation it was found that both the conduction and valence bands of InAsSb alloy exhibit some bowing. The bowing parameters were determined to be in the ratio of 4:6. For a sample with Sb composition ∼0.12 in the quantum well the photoluminescence emission band covers the CO2 absorption peak making it suitable for use in sources for CO2 detection.

Optical studies of strained type II GaAs0.7Sb0.3/GaAs multiple quantum wells

We report a detailed investigation on the optical transitions of strained type II GaAs0.7Sb0.3/GaAs (100) multiple quantum wells. For the theoretical calculations, both of the elastic deformational potential of intrinsic compressive biaxial strain, and quantum confinement effects are included. The asymmetric photoluminescence spectra reveal the features of excited state transition and quantum confinement Stark effect at high and low temperatures, respectively. The asymmetry features have also been investigated and confirmed by low-temperature photoluminescence experiments under different excitation power. From polarized photoluminescence excitation and photoconductivity spectra, both of the type I and type II optical transitions can also be clearly identified.

Nature of persistent photoconductivity in GaAs0.7Sb0.3∕GaAs multiple quantum wells

The optoelectronic properties of undoped type-II GaAs0.7Sb0.3∕GaAs (100) multiple quantum wells have been investigated by photoluminescence (PL), and photoconductivity measurements. Quite interestingly, persistent photoconductivity (PPC) has been discovered in this material. The decay kinetics of the PPC effect can be well described by the stretched-exponential function IPPC(t)=IPPC(0)exp[−(t∕τ)β], (0<β<1), which is similar to the behavior observed in many disorder systems. Through the study of the PPC effect under various conditions, and combining with the characteristics of the PL spectra, we identify that the origin of the PPC effect arises from the spatial separation of photoexcited electrons and holes. Here, the photoexcited electrons fall into the GaAs layer, and holes are trapped by local potential minima due to alloy fluctuations in the GaAsSb layer. This process prevents the recombination of electrons and holes, and thus the PPC occurs. In order to return to the initial states, photoexcited elect...

Modulation spectroscopy study of the effects of growth interruptions on the interfaces of GaAsSb/GaAs multiple quantum wells

The effects of growth interruption times combined with Sb exposure of GaAsSb/GaAs multiple quantum wells (MQWs) have been investigated by using phototransmittance (PT), contactless electroreflectance (CER) and wavelength modulated surface photovoltage spectroscopy (WMSPS). The features originated from different portions of the samples, including interband transitions of MQWs, interfaces and GaAs, are observed and identified through a detailed comparison of the obtained spectra and theoretical calculation. A red-shift of the interband transitions and a broader lineshape of the fundamental transition are observed from samples grown under Sb exposure compared to the reference sample grown without interruption. The results can be interpreted in terms of both increases in Sb content and mixing of Sb in the GaAs interface layers. An additional feature has been observed below the GaAs region in the samples with Sb treatment. The probable origin of this additional feature is discussed.

Study on the Band Line-up of GaAsSb/GaAs Quantum Wells

Because of the band-bending effect, the blue-shift on the emission wavelength of GaAsSb/GaAs QW laser was observed as the cavity length was shortened. We utilized this effect to investigate the band line-up of the GaAsSb/GaAs QW. Through a simulation based on solving the Poisson and Schrodinger equations simultaneously for the band structure and optical gain of GaAsSb/GaAs QW, we found that the valence band offset ratio (Q) of the unstrained GaAs0.64Sb0.36/GaAs is 1.02, and the unstrained band-gap bowing parameter of GaAsSb is 1.31. In the calculation, band filling effect and the band-gap shrinkage induced by many-body effect were considered.

GaAsSb/GaAs type-II quantum well and its application on /spl sim/1.3 /spl mu/m laser

The band offset of the type-II GaAs/sub 0.7/Sb/sub 0.3//GaAs quantum well (QW) is studied. We propose an extrapolation method to remove the band-bending effect and determine the flat-band transition energy of the type-II QW from photoluminescence (PL) measurement. Then, we compare the PL peak energies of the type-II GaAs/sub 0.7/Sb/sub 0.3//GaAs QW and the type-I Al/sub 0.3/Ga/sub 0.7/As/GaAs/sub 0.7/Sb/sub 0.3/ QW to obtain the strained band gap energy of GaAs/sub 0.7/Sb/sub 0.3/ and the valence-band-offset ratio of the type-II QW. The obtained band gap energy and valence-band-offset ratio are 1.01 eV and 1.15. GaAsSb/GaAs double-quantum-well lasers were also grown and fabricated. The laser demonstrates a very low threshold current density of 210 A/cm/sup 2/ with an emission wavelength of 1.28 /spl mu/m.