Jianrong Dong

Annealing and oxidation of silicon oxide films prepared by plasma-enhanced chemical vapor deposition

We have investigated phase separation, silicon nanocrystal (Si NC) formation and optical properties of Si oxide (SiOx, 0<x<2) films by high-vacuum annealing and dry oxidation. The SiOx films were deposited by plasma-enhanced chemical vapor deposition at different nitrous–oxide/silane flow ratios. The physical and optical properties of the SiOx films were studied as a result of high-vacuum annealing and thermal oxidation. X-ray photoelectron spectroscopy (XPS) reveals that the as-deposited films have a random-bonding or continuous-random-network structure with different oxidation states. After annealing at temperatures above 1000 °C, the intermediate Si continuum in XPS spectra (referring to the suboxide) split to Si peaks corresponding to SiO2 and elemental Si. This change indicates the phase separation of the SiOx into more stable SiO2 and Si clusters. Raman, high-resolution transmission electron microscopy and optical absorption confirmed the phase separation and the formation of Si NCs in the films. Th...

Effects of AlAs interfacial layer on material and optical properties of GaAs∕Ge(100) epitaxy

GaAs∕AlAs∕Ge(100) samples grown at 650°C with AlAs interfacial layer thickness of 0, 10, 20, and 30nm were characterized using transmission electron microscopy, secondary ion mass spectrometry (SIMS), and photoluminescence (PL) techniques. SIMS results indicate that the presence of an ultrathin AlAs interfacial layer at the GaAs∕Ge interface has dramatically blocked the cross diffusion of Ge, Ga, and As atoms, attributed to the higher Al–As bonding energy. The optical quality of the GaAs epitaxy with a thin AlAs interfacial layer is found to be improved with complete elimination of PL originated from Ge-based complexes, in corroboration with SIMS results.

Group-V intermixing in InAs∕InP quantum dots

Postgrowth intermixing in InAs∕InP quantum dot (QD) structures have been investigated by rapid thermal annealing and laser irradiation techniques. In both cases, room-temperature photoluminescence (PL) measured from the QD structures after intermixing shows a substantial blueshift accompanied by an improvement in PL intensity and a reduction in linewidth. In the case of impurity free vacancy disordering, an energy shift of up to 350meV has been achieved. The maximum differential energy shift for samples capped with SiO2 and SiNx dielectrics was found to be 90meV. On the other hand, laser-induced intermixing allows differential energy shifts of more than 250meV in this material system. Micro-Raman measurement shows the appearance of InAs-type and InP-type optical phonon peaks from laser-annealed InAs∕InP QDs due to the exchange of As and P at the QD interfaces.

Absorption enhancement analysis of crystalline Si thin film solar cells based on broadband antireflection nanocone grating

1 mu m thick Si solar cells based on nanocone grating (NCG) with height of 100-800 nm and period of 100, 500, and 800 nm are numerically investigated through reflectivities, absorption enhancement factors, absorption spectra, optical generation rates, ultimate efficiencies, and diffraction angles. Compared with the planar Si solar cell, absorption enhancement are observed in any solar cells with NCG surface. Their absorption enhancement mechanism varies with the incident wavelength range. When incident wavelength lambda lambda > 500 nm, even though the absorption enhancement is still dominated by antireflection of the front surface, cavity-resonance effect and guided-mode excitation induced by high order diffraction start to make contribution. When lambda > 600 nm, the contribution of guided-mode excitation induced by lower-order diffraction becomes larger and larger once the diffraction angle is larger than its critical angle. For the structure with P = 100 nm, high-order diffraction cut-off at the longer wavelength range is the main reason of its lower absorption enhancement and ultimate conversion efficiency. For P = 800 nm, the lower absorption enhancement and ultimate efficiency is also observed due to the high reflection loss and mode leakage induced by 1(st) order diffraction where its diffraction angle is lower than its critical angle. Higher absorption and ultimate conversion efficiencies are achieved in P = 500 nm due to the good balance between antireflection performance and guide-mode excitation induced by the high order diffraction is achieved. Moreover, such absorption enhancement is closely related with its height of NCG gratings. Reflection loss reduction, the interaction volume reduction between the incident light and Si material, and higher photon density in NCG structure coexists with H increasing, which results in absorption enhancement in P = 500 nm and P = 800 nm, but absorption reduction in P = 100 nm where high order diffraction cut-off. Based on these analysis, we do believe that high absorption and ultimate conversion efficiency should be achieved in NCG-based solar cells where both the lower reflection in short wavelength domain and guide-mode excitation induced by 1(st) and 2(nd) diffraction in longer wavelength domain can be achieved. According to this rule, the optimized structure is NCG with P = 559 nm and H = 500 nm, by which, the highest optical generation rate of 536.57 x 10(4) W/cm(3) and ultimate efficiency of 28.132% are achieved. Such analysis should benefit the design of the thin film solar cells with nano-structured diffraction gratings.

Ultrawide band quantum dot light emitting device by postfabrication laser annealing

An ultrawide band quantum dot (QD) light emitting device (LED) with bandwidth of 360nm covering 1284–1644nm spectral range has been demonstrated by postfabrication laser-irradiation technique. The integrated light output of the QD LED was found to increase by four times after laser annealing, attributed to the improved homogeneity of the QDs and enhanced lateral electrical and optical confinements at the active region after intermixing. Large wavelength blueshift of 315nm has been obtained at the laser annealed region and an overall increase in bandwidth of 22% has been obtained in the QD LED after postfabrication laser annealing.An ultrawide band quantum dot (QD) light emitting device (LED) with bandwidth of 360nm covering 1284–1644nm spectral range has been demonstrated by postfabrication laser-irradiation technique. The integrated light output of the QD LED was found to increase by four times after laser annealing, attributed to the improved homogeneity of the QDs and enhanced lateral electrical and optical confinements at the active region after intermixing. Large wavelength blueshift of 315nm has been obtained at the laser annealed region and an overall increase in bandwidth of 22% has been obtained in the QD LED after postfabrication laser annealing.

Structural and optical properties of GaInP grown on germanium by metal-organic chemical vapor deposition

Structural and optical properties of Si-doped as well as nominally undoped GaInP epilayers grown on Germanium substrates by metal-organic chemical vapor deposition have been investigated by high resolution transmission electron microscope and photoluminescence (PL). Si incorporation results in an increased inner band PL transition and a blue shift of PL energy with increasing temperature, which arises from the trapping states around Ge-GaInP interface due to Ge diffusion to GaInP epilayer as well as Si doping. For the inter band PL transition, a competition between the emission processes near the band edge and in the ordered GaInP domains is responsible for the inverted S shape temperature dependence of PL peaks. By analyzing the time-resolved PL results, we attribute this emission near the ordered states to the localized states due to the potential fluctuation, which is induced by the compositional inhomogeneity of Ga and In in the partially ordered GaInP.

Effective recombination velocity of textured surfaces

Surface texturization is an effective way to enhance the absorption of light for optoelectronic devices but it also aggravates the surface recombination by enlarging the surface area. In order to evaluate the influence of texture structures on the surface recombination, an effective surface recombination velocity is defined which is assumed to have an equivalent recombination effect on a flat surface. Based on numerical and analytical calculation, the dependences of effective surface recombination on the pattern geometry, the surface recombination velocity, and the diffusion length are analyzed.

Continuous-wave operation of AlGaInP/GaInP quantum-well lasers grown by metalorganic chemical vapor deposition using tertiarybutylphosphine

Strained AlGaInP/GaInP multiple-quantum-well laser structures have been grown by metalorganic chemical vapor deposition using teriarybutylphosphine as the phosphorus precursor and ridge waveguide lasers of 4 μm wide have been fabricated. Room temperature continuous-wave lasing has been obtained with an emission wavelength of about 670 nm. A single-facet output power of more than 18 mW has been achieved for an as-cleaved laser chip. It can be concluded that it is feasible to fabricate AlGaInP red lasers using less toxic metalorganic source tertiarybutylphosphine in parallel with conventionally used highly toxic PH3.

Interpretation of anomalous temperature dependence of anti-Stokes photoluminescence at GaInP2/GaAs interface

In this letter, we report on temperature-dependent anti-Stokes photoluminescence (ASPL) at an interface between partially ordered GaInP2 epilayer and GaAs substrate. It is found that the intensity of the ASPL depends strongly on temperature accompanying with a clear blueshift in energy. A localized-state luminescence model was employed to quantitatively interpret temperature dependence of the ASPL. Excellent agreement between the theory and experiment was obtained. Radiative recombination mechanism of the up-converted carriers was discussed.

Broadband quasi-omnidirectional antireflection AlGaInP window for III-V multi-junction solar cells through thermally dewetted Au nanotemplate

Al(Ga)InP subwavelength structures (SWS) were fabricated and optimized through thermally dewetted Au nanotemplate and ICP pattern-transfer. When λ< 900 nm, most AlGaInP nanostructures exhibit the reflectivity of less than 2% and insensitive to the incident angle up to 45°. When λ extends to 1800 nm, the reflectivity of less than 5% over 0°-45° is achieved in the optimized nanostructure, which benefits III-V multi-junction solar cells to improve their efficiency. Moreover, not only is such cost-effective nano-fabrication process completely compatible with the other processing of III-V solar cells, but their defined disordered SWS benefit the antireflection performance over broadband and wide view according to the comparison between the measurement and simulation results from AlGaInP SWS.