Controlling optical polarization of {11–22} semipolar multiple quantum wells using relaxed underlying InGaN buffer layers

Many spin-polarized-based devices utilize the optical polarization from semiconductor quantum wells (QWs) as a read out. Under ideal conditions at zero crystal momentum, 100% optical polarization is obtained from these QWs for fully polarized electrons. However, carrier populations typically extend over nonzero crystal momentum states, where band mixing results in nonideal optical polarization. We investigate a single InxGa1−xAs(x=0.2, and 0.08)QW in GaAs in a typical p-i-n spin injector structure, using eight band k.p theory including strain, electric field and quantum-confined Stark effects. By evaluating the carrier distribution and wave functions of the QW states, we find the resulting optical polarization is reduced to ∼60% at 10K, and further for high temperature and high fields. We show that under certain conditions we can flip the sign of the optical polarization, suggesting the possibility of an electric field controlled optical or spin polarization switch.

Emission wavelength red-shift by using “semi-bulk” InGaN buffer layer in InGaN/InGaN multiple-quantum-well

Band-edge photoluminescence of SiGe/strained-Si/SiGe type-II quantum wells on Si(100)

Extended X‐ray absorption fine structure above the Ga–K edge has been used to study the local structure of heterointerfaces in GaN/AlN multiple quantum wells (MQWs) grown by ammonia molecular beam epitaxy. The thickness of AlN and GaN layers in MQWs were evaluated using transmission electron microscopy, X‐ray diffraction and Raman spectroscopy. Two sets of MQWs structures with the number of period about of 20 and more than 100 were studied. The Ga‐Ga interatomic distance in MQWs is lower than that in bulk GaN due to elastic compression of GaN layers in MQWs. This effect is less pronounced in thick structures because of almost full relaxation of GaN layers. The Ga‐Al interatomic distance in thick MQWs is too long, about 0.1 Å, longer than typical values for AlGaN alloys. Then number of Ga cations in the second coordination shell of Ga atoms is lower than that predicted for abrupt heterointerfaces that evidences intermixing of heterointerfaces. The intermixing degree depends on the total thickness of MQWs (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Low energy plasma enhanced chemical vapour deposition (LEPECVD) is a deposition technique developed for the epitaxy of Si and SiGe at ultra-high deposition rates. Due to a high current plasma discharge composed of low energy particles, a high plasma enhancement can be obtained without any accompanying plasma induced damage of the wafer surface. The most important application of LEPECVD so far is for compositionally graded relaxed SiGe buffer layers. Such relaxed buffer layers are demonstrated with end composition up to pure Ge and with a growth time below 1 hour. A p-type hetero-MOSFET formed in a SiGe channel compressively strained to a Si0.5Ge0.5 relaxed buffer layer, is demonstrated as one example where the high growth rates of LEPECVD allows the synthesis of devices which cannot be produced with an acceptable throughput with conventional deposition methods. The room temperature effective hole mobility of 760 cm2/Vs obtained on such devices demonstrates a high structural and electrical quality of the LEPECVD material.

Strain relaxed graded SiGe buffer layers have been grown by low energy plasma enhanced chemical vapour deposition (LEPECVD). Due to the low ion energies involved in LEPECVD, exceptionally high plasma intensities can be applied without any ion damage of the epitaxial layers. Transmission electron microscopy and X-ray reciprocal space mapping show that relaxed buffer layers grown at rates exceeding 5 nm/s are of a comparable quality to buffer layers grown by standard techniques at much lower rates. Low temperature electric transport measurements on remotely doped tensilely strained Si quantum wells synthesized by LEPECVD show that also good electrical quality is achieved.

An ultraviolet light emitting diode with quaternary AlInGaN/AlInGaN multiple quantum wells and peak emission wavelength at 305 nanometers (nm) is reported for the first time. The peak emission wavelength can be tuned from 305 nm to 340 nm by varying the alloy compositions of the quaternary AlInGaN active layers using a pulse atomic layer epitaxy process. At 340 nm, for a 20 µm×1000 µm stripe geometry device an output power as high as 1 mW was measured from the sapphire substrate side.

We report an optically-pumped alternative nitride-based laser with room-temperature emission at 353 nm. The active region comprised of non-polar a-plane Al0.04Ga0.96N/Al0.08Ga0.92N multiple quantum wells whereas the lasing cavity consisted of Al0.15Ga0.85N clad and Al0.10Ga0.85N waveguide layers and naturally cleaved facet mirrors. The layers were grown over r-plane sapphire substrates by metalorganic chemical vapor deposition technique. A room temperature lasing threshold for N2-laser photoexcitation of 110 kW/cm2 and a modal optical gain of 215 cm-1 was measured at the peak emission wavelength.

There is increasing interest in III-nitride films and multiple quantum well structures grown in non-polar or semi-polar orientations for application in light-emitting devices. We describe a method to obtain the compositions and the thicknesses of layers within III-nitride quantum well or superlattice structures grown in non-polar or semi-polar orientations, based on X-ray scattering. For each new crystallographic orientation considered, new axes were obtained and both the lengths and angles of these new axes calculated relative to the original conventional reference axes. These angles provide the coefficients of the matrix to transform the elastic constants published in the conventional setting (as used for polar c-plane oriented III-nitrides) into the appropriate new values. The new characteristic lengths and new elastic constants are then put into the general equation that relates the composition of a fully strained layer to the experimentally measured out-of-plane alloy d-spacing. Thus we have (a) determined the alloy composition from the difference between the experimentally measured alloy d-spacing and that of the substrate and (b) calculated the strained d-spacing for a given alloy composition for input to simple kinematical simulation software. In addition for quantum well structures the thickness ratio of well-to-barrier must be determined. Here we use the minima in the low angle reflectivity data. The repeat thickness and thus the thicknesses of the well and barrier layers, can be obtained from either the low or the high-angle data. We then cross-check by comparing the experimental and the simulated high-angle diffraction data. This method has been applied successfully to heteroepitaxial non-polar and semi-polar GaN/AlGaN and InGaN/GaN multiple quantum well structures and may also be used to find the composition of epilayers. The method works even in the presence of tilt between the superlattice and the GaN “template’, although in this case additional high-angle diffraction data at different settings must be collected.

Resonant tunneling of variously strained Si/Ge xSi 1−x/Si heterostructures

In this thesis we report the studies of optical properties of ZnMnSe/ZnSeTe multiple quantum wells. Quite interesting results have been obtained from our studies which should be very useful for our understanding and application of these materials. Ⅰ. Novel properties of photoluminescence in type-II ZnMnSe/ZnSeTe multiple quantum wells The optical properties of type-II ZnMnSe/ZnSeTe multiple quantum wells were investigated by photoluminescence. From the photoluminescence (PL) spectra, a giant blueshift of the peak energy under moderate excitation power has been observed. It can be well interpreted in terms of the band-bending effect due to spatially separated photoexcited carriers in a type-II band alignment. In addition, a large optical anisotropy of ZnMnSe/ZnSeTe multiple quantum wells (MQWs) has been discovered. Based on the studies of the dependences of excitation intensity and temperature, we point out that the observed anisotropy is an inherent property of the type-II band alignment in ZnMnSe/ZnSeTe MQWs. Ⅱ. Thickness dependence of optical properties of ZnMnSe/ZnSeTe multiple quantum wells Photoluminescence spectra and Raman scattering were performed to study thickness dependence of optical properties of ZnMnSe/ZnSeTe multiple quantum wells. Strong photoluminescence associated with iso-electronic Te traps was observed. As the thickness of the ZnSeTe layer decreases to 1 nm the mechanism of luminescence changes. In Raman spectra, the LO and TO phonons of ZnSeTe layer were observed. A blueshift of optical phonon modes was observed as the thickness of the ZnSeTe decreases. This phenomenon can be well interpreted in terms of the compressive strain effect.

We review the occurrence of periodic or near periodic electric-field domains stimulated by bound-to-continuum transitions under the influence of infrared radiation in multiple uncoupled quantum wells. The conditions of formation of such domains and their stability are studied theoretically using the proposed phenomenological model.

III-nitride wide bandgap semiconductors have been at the center stage of the semiconductor R & D effort over the last decade. Photonic/electronic devices based on III-nitrides, including UV/blue/green/white LEDs, violet/blue LDs, UV detectors, and high power/temperature transistors, have reached high performance level. This talk will provide a brief overview on recent advances made by our group and others in the area of III-nitrides for energy and hydrogen generation. The presentation will concentrate on topics of material and device challenges of III-nitride solar cells and photoelectrochemical cells (PECs). The bandgap of InGaN expands from about 0.65 to 3.4 eV, which covers the entire solar spectrum. In principle, a multi-junction solar cell based on multiple layers of InGaN with different In-contents is capable of capturing the entire spectrum of the sunlight and can thus be highly efficient. To count for the relatively low materials quality for high In content InGaN alloys, InGaN/GaN multiple quantum wells (MQWs) have been adopted for the construction of nitride solar cells. We have grown, fabricated, and characterized InGaN based solar cells utilizing the InGaN/GaN MQWs p-i-n structures.1-3The fabricated solar cells exhibited open circuit voltage of about 2 V, a fill factor of about 60% and overall efficiency of about 3% under AM 1.5 irradiation. The performance of these nitride solar cells under concentrated sunlight has also been investigated. In additional to its bandgap match to the solar spectrum, the bandgap of InGaN alloys can also be engineered to optimally match with conditions necessary for direct hydrogen generation by water splitting using sunlight.4-6 The results on hydrogen gas generation via solar water splitting accomplished using n- and p-type InGaN epilayer as working electrodes will be discussed. Direct solar water splitting and hydrogen gas generation under zero bias has been realized using an InGaN/GaN multiple quantum well solar cell as a photoelectrochemical cell. Under the irradiation by a simulated sunlight (AM1.5G solar simulator), a 1.5% solar-to-fuel conversion efficiency has been achieved under zero bias, setting a new benchmark of employing III-nitrides as photoreactive electrodes for artificial photosynthesis. Time dependent hydrogen gas production photocurrent measured over a prolonged period revealed an excellent chemical stability of InGaN in aqueous solution. Our results indicate that InGaN is an excellent artificial photosynthesis material which is capable to provide usable clean fuel (hydrogen gas) with the sunlight being the only energy input. InGaN materials growth effort was supported by NSF (DMR-1206652) and the energy device effort was by DOE (FG02-09ER46552). Jiang & Lin are grateful to AT&T Foundation for the support of Ed & Linda Whitacre endowed chairs. R. Dahal, B. Pantha, J. Li, J. Y. Lin, and H. X. Jiang, “InGaN/GaN multiple quantum well solar cells with long operating wavelengths,” Appl. Phys. Lett. 94, 063505 (2009). R. Dahal, J. Li, K. Aryal, J. Y. Lin, and H. X. Jiang, "InGaN/GaN multiple quantum well concentrator solar cells," Appl. Phys. Lett. 97, 073115 (2010). B N. Pantha, J. Y. Lin, and H. X. Jiang, “III-nitride nanostructures for energy generation,” Proc. SPIE 7608, 76081I (2010). J. Li, J. Y. Lin, and H. X. Jiang, “Direct hydrogen gas generation by using InGaN epilayers as working electrodes,” Appl. Phys. Lett. 93, 162107 (2008). K. Aryal, B. N. Pantha, J. Li, J. Y. Lin, and H. X. Jiang, "Hydrogen generation by solar water splitting using p-InGaN photoelectrochemical cells," Appl. Phys. Lett. 96, 052110 (2010). R. Dahal, B. N. Pantha, J. Li, J. Y. Lin, and H. X. Jiang, “Realizing InGaN monolithic solar-photoelectrochemical cells for artificial photosynthesis,” Appl. Phys. Lett. 104, 143901 (2014)

Investigation of defect structures in InGaN/GaN multiple quantum wells grown on Si(1 1 1) substrate by metalorganic vapor phase epitaxy

Compressively strained Ge channels on relaxed SiGe buffer layers