Kin Man Yu

On-Nanowire Axial Heterojunction Design for High-Performance Photodetectors

We report the growth of high-quality CdS/CdSxSe1-x axial heterostructure nanowires (NWHs) via a temperature-controlled chemical vapor deposition method. Microstructural characterizations revealed that these NWHs have a single-crystalline structure with abrupt heterojunctions. Local photoluminescence and mapping near the heterojunctions show only two separated narrow band-edge emission bands from the two different adjacent semiconductors, further demonstrating the high-quality of these heterostructures. Moreover, the photodetector based on the single NWH shows a performance (higher responsivity (1.18 × 10(2) A/W), faster response speed (rise ∼68 μs, decay ∼137 μs), higher Ion/Ioff ratio (10(5)), higher EQE (3.1 × 10(4) %), and broader detection range (350-650 nm)) at room temperature superior to that of photodetectors based on single band gap nanostructures. This work suggests a much simpler route to achieve superior NWHs for applications in optoelectronic devices.

Band structure of germanium carbides for direct bandgap silicon photonics

Compact optical interconnects require efficient lasers and modulators compatible with silicon. Ab initio modeling of Ge1−xCx (x = 0.78%) using density functional theory with HSE06 hybrid functionals predicts a splitting of the conduction band at Γ and a strongly direct bandgap, consistent with band anticrossing. Photoreflectance of Ge0.998C0.002 shows a bandgap reduction supporting these results. Growth of Ge0.998C0.002 using tetrakis(germyl)methane as the C source shows no signs of C-C bonds, C clusters, or extended defects, suggesting highly substitutional incorporation of C. Optical gain and modulation are predicted to rival III–V materials due to a larger electron population in the direct valley, reduced intervalley scattering, suppressed Auger recombination, and increased overlap integral for a stronger fundamental optical transition.

High mobility transparent amorphous CdO-In2O3 alloy films synthesized at room temperature

High mobility amorphous ionic oxide semiconductors (AIOSs) are ternary or quaternary heavy metal oxides which have been identified as technologically important materials for flexible transparent electronics because of their large area uniformity and low temperature processing compatibility. Here, we report on the room temperature synthesis of CdO-In2O3 alloy thin films in the full composition range using the magnetron sputtering technique on glass and plastic substrates. We found that alloys with a cation composition range of 10–55% Cd are amorphous with high mobility in the range of 30–45 cm2/Vs and an electron concentration of ∼3–4 × 1020 cm−3. The intrinsic and optical gap of these amorphous alloys varies from 2.7 to 3.2 eV and 3.2 to 3.4 eV, respectively. The room temperature processing, wide bandgap tunability, and low resistivity of ∼4–5 × 10−4 Ω cm make these amorphous films among the best AIOSs as transparent electrodes on flexible substrates.

Highly mismatched alloys for intermediate band solar cells

LBNL-57339. Preprint. 2005 MRS Spring Meeting, March 28-April 1, 2005, San Francisco, CA Symposium F: Thin-Film Compound Semiconductor Photovoltaics Highly Mismatched Alloys for Intermediate Band Solar Cells W. Walukiewicz 1 , K. M. Yu 1 , J Wu 2 , J. W. Ager III 1 , W. Shan 1 , M. A. Scrapulla 1,3 , and O. D. Dubon 1,3 , and P. Becla 4 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 Department of Chemistry and Chemical Biology, Harvard University, , Cambridge, MA 02138 Department of Materials Science and Engineering, University of California, Berkeley, CA Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 ABSTRACT It has long been recognized that the introduction of a narrow band of states in a semiconductor band gap could be used to achieve improved power conversion efficiency in semiconductor-based solar cells. The intermediate band would serve as a “stepping stone” for photons of different energy to excite electrons from the valence to the conduction band. An important advantage of this design is that it requires formation of only a single p-n junction, which is a crucial simplification in comparison to multijunction solar cells. A detailed balance analysis predicts a limiting efficiency of more than 50% for an optimized, single intermediate band solar cell. This is higher than the efficiency of an optimized two junction solar cell. Using ion beam implantation and pulsed laser melting we have synthesized Zn 1-y Mn y O x Te 1-x alloys with x<0.03. These highly mismatched alloys have a unique electronic structure with a narrow oxygen-derived intermediate band. The width and the location of the band is described by the Band Anticrossing model and can be varied by controlling the oxygen content. This provides a unique opportunity to optimize the absorption of solar photons for best solar cell performance. We have carried out systematic studies of the effects of the intermediate band on the optical and electrical properties of Zn 1-y Mn y O x Te 1-x alloys. We observe an extension of the photovoltaic response towards lower photon energies, which is a clear indication of optical transitions from the valence to the intermediate band. INTRODUCTION Efforts to improve the efficiency of solar cells have led to extensive experimental and theoretical studies of new materials and new cell designs. To date, the highest power conversion efficiencies have been achieved with multijunction solar cells based on standard semiconductor materials (~31%) [1,2]. It was recognized over thirty years ago that the introduction of states in a semiconductor band gap presents an alternative to multijunction designs for improving the power conversion efficiency of solar cells [3]. Detailed theoretical calculations indicated that a single junction cell with a properly located band of intermediate states could achieve power conversion efficiencies up to 62% - i.e. higher than those for optimized double-junction tandem cells [4-6]. Despite years of intense efforts no multi-band semiconductor material has been realized yet [6]. In this paper, we report the design and synthesis of a new type of material having a narrow band of extended states within a semiconductor band gap. The design of our material is based on the recently introduced band anticrossing (BAC) model of highly mismatched

Improved photovoltaic properties of ZnTeO-based intermediate band solar cells

Highly mismatched ZnTe1-xOx (ZnTeO) alloy is one of the potential candidates for an absorber material in a bulk intermediate band solar cell (IBSC) because a narrow, O-derived intermediate band IB (E-) is formed well below the conduction band CB (E+) edge of the ZnTe. We have previously demonstrated the generation of photocurrent induced by two-step photon absorption (TSPA) in ZnTeO IBSCs using n-ZnO window layer. However, because of the large conduction band offset (CBO) between ZnTe and ZnO, only a small open circuit voltage (Voc) was observed in this structure. Here, we report our recent progress on the development of ZnTeO IBSCs with n-ZnS window layer. ZnS has a large direct band gap of 3.7 eV with an electron affinity of 3.9 eV that can realize a smaller CBO with ZnTe. We have grown n-type ZnS thin films on ZnTe substrates by molecular beam epitaxy (MBE), and demonstrated ZnTe solar cells and ZnTeO IBSCs using n-ZnS window layer with an improved VOC. Especially, a n-ZnS/i-ZnTe/p-ZnTe solar cell showed an improved Voc of 0.77 V, a large short-circuit current density of 6.7 mA/cm2 with a fill factor of 0.60, yielding the power conversion efficiency of 3.1 %, under 1 sun illumination.

Oxygen vibrational modes in ZnS1−xOx alloys

ZnS1−xOx alloy films were studied via resonant Raman spectroscopy. Films with a low oxygen content exhibit ZnS longitudinal optical modes and additional modes attributed to O local vibrational modes (LVMs). The frequencies of these modes are explained by a simple mass-defect model. As the O content increases, pairs and larger clusters form, causing the O mode to transition from an LVM to a delocalized phonon. The composition dependence of the modes shows agreement with the modified random element isodisplacement model. Low-temperature measurements show that the O-related mode is composed of multiple features, attributed to zincblende and wurtzite structural regions.

Room-Temperature-Synthesized High-Mobility Transparent Amorphous CdO–Ga2O3 Alloys with Widely Tunable Electronic Bands

In this work, we have synthesized Cd1-xGaxO1+δ alloy thin films at room temperature over the entire composition range by radio frequency magnetron sputtering. We found that alloy films with high Ga contents of x > 0.3 are amorphous. Amorphous Cd1-xGaxO1+δ alloys in the composition range of 0.3 < x < 0.5 exhibit a high electron mobility of 10-20 cm2 V-1 s-1 with a resistivity in the range of 10-2 to high 10-4 Ω cm range. The resistivity of the amorphous alloys can also be controlled over 5 orders of magnitude from 7 × 10-4 to 77 Ω cm by controlling the oxygen stoichiometry. Over the entire composition range, these crystalline and amorphous alloys have a large tunable intrinsic band gap range of 2.2-4.8 eV as well as a conduction band minimum range of 5.8-4.5 eV below the vacuum level. Our results suggest that amorphous Cd1-xGaxO1+δ alloy films with 0.3 < x < 0.4 have favorable optoelectronic properties as transparent conductors on flexible and/or organic substrates, whereas the band edges and electrical conductivity of films with 0.3 < x < 0.7 can be manipulated for transparent thin-film transistors as well as electron transport layers.

Multicolor emission from intermediate band semiconductor ZnO 1â 'x Se x

Photoluminescence and photomodulated reflectivity measurements of ZnOSe alloys are used to demonstrate a splitting of the valence band due to the band anticrossing interaction between localized Se states and the extended valence band states of the host ZnO matrix. A strong multiband emission associated with optical transitions from the conduction band to lower E− and upper E+ valence subbands has been observed at room temperature. The composition dependence of the optical transition energies is well explained by the electronic band structure calculated using the kp method combined with the band anticrossing model. The observation of the multiband emission is possible because of relatively long recombination lifetimes. Longer than 1 ns lifetimes for holes photoexcited to the lower valence subband offer a potential of using the alloy as an intermediate band semiconductor for solar power conversion applications.

Multiband modification of III-V dilute nitrides for IBSC application

The subband features E‒ and E+ for the conduction band of III-V dilute nitride alloys make them promising for intermediate band solar cell application. However, presence of bandgap states can limit the two-step photon absorption activity, a necessary requirement for IBSC functionality. A model analysis is performed to characterize the density of states. The sub-band tails states are characterized using a temperature-dependent map of photo-modulated reflectance spectroscopy for GaNAs thin films grown on GaAs substrates using molecular beam epitaxy. The effect of indium and antimony incorporation on the subband features were investigated. Marked improvements in the thin films were observed both for the lower (E‒) and the upper (E+) conduction bands (CB) when In was introduced with marginal enhancement by Sb. These improvements are associated with suppression of tail states below both the E‒ and E+ bands. Sb rather mainly plays a surfactant role improving the abruptness of the GaNAs/GaAs hetero-interface.

Intermixing studies in GaN1−xSbx highly mismatched alloys

GaN1-xSbx with x∼5%-7% is a highly mismatched alloy predicted to have favorable properties for application as an electrode in a photoelectrochemical cell for solar water splitting. In this study, we grew GaN1-xSbx under conditions intended to induce phase segregation. Prior experiments with the similar alloy GaN1-xAsx, the tendency of Sb to surfact, and the low growth temperatures needed to incorporate Sb all suggested that GaN1-xSbx alloys would likely exhibit phase segregation. We found that, except for very high Sb compositions, this was not the case and that instead interdiffusion dominated. Characteristics measured by optical absorption were similar to intentionally grown bulk alloys for the same composition. Furthermore, the alloys produced by this method maintained crystallinity for very high Sb compositions and allowed higher overall Sb compositions. This method may allow higher temperature growth while still achieving needed Sb compositions for solar water splitting applications.