Yong Hang Zhang

Tunable optical gratings based on buckled nanoscale thin films on transparent elastomeric substrates

This letter reports a tunable optical grating based on buckled thin film with periodic sinusoidal patterns on a transparent elastomeric substrate. Submicron scale sinusoidal gratings have been fabricated with nanometer thick Gold/Palladium film coated on 30% pretensioned polydimethylsiloxane substrates. Due to competition between the soft elastomeric substrates and relatively stiff films, periodic wavy profiles are created upon releasing the pretension. The buckling profiles can be easily tuned by mechanically stretching or compressing. Optical transmittance diffraction testing has been conducted, and 85 nm peak wavelength shifts of the first order diffraction have been achieved by stretching the grating up to 30% of its original length.

Structural properties of Bi2Te3 and Bi2Se3 topological insulators grown by molecular beam epitaxy on GaAs(001) substrates

Thin films of Bi2Te3 and Bi2Se3 have been grown on deoxidized GaAs(001) substrates using molecular beam epitaxy. Cross-sectional transmission electron microscopy established the highly parallel nature of the Te(Se)-Bi-Te(Se)-Bi-Te(Se) quintuple layers deposited on the slightly wavy GaAs substrate surface and the different crystal symmetries of the two materials. Raman mapping confirmed the presence of the strong characteristic peaks reported previously for these materials in bulk form. The overall quality of these films reveals the potential of combining topological insulators with ferromagnetic semiconductors for future applications.

Highly conductive ZnO grown by pulsed laser deposition in pure Ar

Ga-doped ZnO was deposited by pulsed laser deposition at 200 °C on SiO2/Si, Al2O3, or quartz in 10 mTorr of pure Ar. The as-grown, bulk resistivity at 300 K is 1.8×10−4 Ω cm, three-times lower than that of films deposited at 200 °C in 10 mTorr of O2 followed by an anneal at 400 °C in forming gas. Furthermore, depth uniformity of the electrical properties is much improved. Mobility analysis shows that this excellent resistivity is mostly due to an increase in donor concentration, rather than a decrease in acceptor concentration. Optical transmittance is approximately 90% in the visible and near-IR spectral regions.

Simultaneous Enhancement of Electrical Conductivity and Thermopower of Bi2Te3 by Multifunctionality of Native Defects

Simultaneous increases in electrical conductivity (up to 200%) and thermopower (up to 70%) are demonstrated by introducing native defects in Bi2 Te3 films, leading to a high power factor of 3.4 × 10(-3) W m(-1) K(-2). The maximum enhancement of the power factor occurs when the native defects act beneficially both as electron donors and energy filters to mobile electrons. They also act as effective phonon scatterers.

Combined effects of shunt and luminescence coupling on external quantum efficiency measurements of multi-junction solar cells

The combined effects of shunt and luminescence coupling on the measurement artifacts of external quantum efficiency (EQE) are modeled and analyzed for the top, middle and bottom subcells of multi-junction solar cells. The measurement artifacts of a Ge bottom cell caused by the combined effects are explained with the models.

Long wavelength (1.3 and 1.5 μm) photoluminescence from InGaAs/GaPAsSb quantum wells grown on GaAs

Room temperature photoluminescence at wavelengths between 1.2 and 1.5 μm has been observed in samples consisting of InGaAs/GaPAsSb quantum well structures grown on GaAs. The emission wavelength is varied primarily by changing the composition within the GaPAsSb layer. It is proposed that such long wavelength emission results from a spatially indirect interband transition in the type-II quantum wells where the electron and hole wave functions have large spatial overlap.

Strained InGaAs/GaPAsSb heterostructures grown on GaAs (001) for optoelectronic applications in the 1100–1550 nm range

We present a novel semiconductor quantum well (QW) structure consisting of alternating (In,Ga)As and Ga(P,As,Sb) layers grown pseudomorphically on a GaAs substrate by all-solid-source molecular beam epitaxy. The band gap of the QW is determined by the thickness and composition of both types of layers and can be varied from 1.1 to 1.55 μm. Calculations show that the observed strong room-temperature photoluminescence in this wavelength range can be explained by a type-II transition in the QW. Structural investigations by reflection high-energy electron diffraction, transmission electron microscopy, and secondary ion mass spectroscopy confirm a triple layer structure with laterally modulated composition. Photoluminescence measurements reveal a linewidth of 50 meV at 1.3 μm and a luminescence decay time of 240 ps. Our investigations demonstrate the feasibility of this materials system for vertical cavity surface-emitting lasers and other optoelectronic devices on GaAs.

In situ temperature control of molecular beam epitaxy growth using band-edge thermometry

Band-edge thermometry is becoming an established noncontact method for determining substrate temperature during molecular beam epitaxy. However, with this technique thin-film interference and/or absorption in the growing epilayer can cause shape distortions of the spectrum that may be interpreted erroneously as real temperature shifts of the substrate. An algorithm is presented that uses the width of the spectrum to correct for apparent temperature errors caused by interference and absorption in the epilayer. This correction procedure is tested on substrate temperature data taken during the growth of a λ=930 nm resonant cavity, where the apparent substrate temperature oscillates ±5 °C during the growth of the mirror stacks. These oscillations are reduced to ±3 °C using the correction algorithm. A recently developed model for the substrate temperature dynamics in molecular beam epitaxy shows that roughly ±1 °C of the remaining ±3 °C temperature oscillations are real. Band-edge thermometry is also used to control the substrate temperature to within ±2 °C during the growth of near-lattice-matched InGaAs on InP, whereas the same growth under constant thermocouple temperature would result in a 50 °C rise in the actual substrate temperature.

Properties of the shallow O‐related acceptor level in ZnSe

Zinc selenide layers grown by molecular beam epitaxy (MBE) and doped with ZnO have been characterized using low temperature photoluminescence (PL) measurements as a function of excitation level, temperature, and laser energy (i.e., selectively excited donor‐acceptor pair luminescence or SPL), as well as reflectance measurements. An O‐related donor‐to‐acceptor (D0−A0) pair band is clearly observed in all of the ZnO‐doped layers, whose position varies from 2.7196 to 2.7304 eV, depending on the excitation level. The same peak occurs in a number of undoped, As‐doped, and Ga‐doped MBE samples, showing that O can occur as a residual impurity. Temperature‐dependent measurements reveal the existence of a corresponding conduction band‐to‐acceptor (e−A0) peak at 2.7372 eV (39.8 K), confirming the existence of the acceptor level. The binding energy of this acceptor is about 84±2 meV, which is 27 meV shallower than that of N. The SPL measurements reveal four excited states of the shallow acceptor level, separated fro...

Excitation dependent photoluminescence measurements of the nonradiative lifetime and quantum efficiency in GaAs

The nonradiative lifetime and spontaneous emission quantum efficiency in molecular-beam epitaxy grown bulk GaAs is determined using injection level dependent photoluminescence (PL) measurements. These measurements were performed at temperatures of 300, 230, 100, and 50K using a HeNe pump laser with powers ranging from 0.3to40mW. The quantum efficiency and lifetime is inferred from the power law relation linking pump power and integrated PL signal that is predicted by the rate equations. The nonradiative lifetime for bulk GaAs is determined to be 0.3μs, with an additional temperature dependent component attributed to the AlGaAs barriers that rapidly reduces the nonradiative lifetime at temperatures above 230K. The peak quantum efficiency is >0.96 at 300K and >0.99 at temperatures below 230K.