Dongzhi Hu

Semiconductor heterostructures and optimization of light-trapping structures for efficient thin-film solar cells

Sub-wavelength photonic structures and nanoscale materials have the potential to greatly improve the efficiencies of solar cells by enabling maximum absorption of sunlight. Semiconductor heterostructures provide versatile opportunities for improving absorption of infrared radiation in photovoltaic devices, which accounts for half of the power in the solar spectrum. These ideas can be combined in quantum-well solar cells and related structures in which sub-wavelength metal and dielectric scattering elements are integrated for light trapping. Measurements and simulations of GaAs solar cells with less than one micron of active material demonstrate the benefits of incorporating In(Ga)As quantum-wells and quantum-dots to improve their performance. Simulations that incorporate a realistic model of absorption in quantum-wells show that the use of broadband photonic structures with such devices can substantially improve the benefit of incorporating heterostructures, enabling meaningful improvements in their performance.

Improvement of performance of InAs quantum dot solar cell by inserting thin AlAs layers

A new measure to enhance the performance of InAs quantum dot solar cell is proposed and measured. One monolayer AlAs is deposited on top of InAs quantum dots (QDs) in multistack solar cells. The devices were fabricated by molecular beam epitaxy. In situ annealing was intended to tune the QD density. A set of four samples were compared: InAs QDs without in situ annealing with and without AlAs cap layer and InAs QDs in situ annealed with and without AlAs cap layer. Atomic force microscopy measurements show that when in situ annealing of QDs without AlAs capping layers is investigated, holes and dashes are present on the device surface, while capping with one monolayer AlAs improves the device surface. On unannealed samples, capping the QDs with one monolayer of AlAs improves the spectral response, the open-circuit voltage and the fill factor. On annealed samples, capping has little effect on the spectral response but reduces the short-circuit current, while increasing the open-circuit voltage, the fill factor and power conversion efficiency.

GaAs micro-pyramids serving as optical micro-cavities

An efficient light‐matter coupling requires high‐quality (Q) micro‐cavities with small mode volume. We suggest GaAs micro‐pyramids placed on top of AlAs/GaAs distributed Bragg reflectors to be promising candidates. The pyramids were fabricated by molecular‐beam epitaxy, electron‐beam lithography and a subsequent wet‐chemical etching process using a sacrificial AlAs layer. Measured Q‐factors of optical modes in single pyramids reach values up to 650. A finite‐difference time‐domain simulation assuming a simplified cone‐shaped geometry suggests possible Q‐factors up to 3600. To enhance the light confinement in the micro‐pyramids we intend to overgrow the pyramidal facets with a Bragg mirror—results of preliminary tests are given.

Thermal annealing of InAs quantum dots on patterned GaAs substrates

We investigated the effect of in‐situ thermal annealing on InAs quantum dots (QDs) grown site‐selectively on pre‐patterned GaAs substrates. We compare as grown and annealed samples. A morphological transition is observed where originally two QDs merge into one larger dot.

TEM 3-beam study of annealing effects in InGaNAs using ab-initio structure factors for strain-relaxed supercells

We report on a Transmission Electron Microscopy 3-beam technique based on the interference of 000, 200 and 220. Nonlinear imaging artefacts are eliminated by Fourier filtering, yielding 200 and 220 lattice fringe images, from which chemically sensitive contrast and strain are measured, respectively. In this way, In and N composition can be mapped at atomic scale in quaternary InGaNAs by comparison with simulated reference data. Our Bloch wave simulations are based on structure factors derived from supercells with 106 atoms, which have been strain-relaxed by valence force field methods. Additionally, the influence of electron redistributions due to chemical bonding is accounted for by modified atomic scattering amplitudes derived from density functional theory. By comparing local compositions in an annealed In0.28Ga0.72N0.025As0.975 sample with its as-grown counterpart, we find homogenisation of InGaNAs layer thickness and –stoichiometry upon annealing.

Pulsed Electrical Spin Injection into InGaAs Quantum Dots: Studies of the Electroluminescence Polarization Dynamics

We present time‐resolved studies of the spin polarization dynamics during and after initialization through pulsed electrical spin injection into InGaAs quantum dots embedded in a p‐i‐n‐type spin‐injection light‐emitting diode. Experiments are performed with pulse widths in the nanosecond range and a time‐resolved single photon counting setup is used to detect the subsequent electroluminescence. We find evidence that the achieved spin polarization shows an unexpected temporal behavior, attributed mainly to many‐carrier and non‐equilibrium effects in the device.

Toward high-efficiency quantum dot solar cells: optimized gratings for ultrathin waveguide devices

We report progress in developing optimized diffraction gratings for coupling solar radiation from the airmass 0 spectrum into waveguide modes of ultrathin quantum dot solar cells (QDSCs). Electromagnetic simulations have been used to optimize the grating geometry and to analyze the nature of diffraction within the device structure. These results suggest that increases in photocurrent of over 100% at wavelengths of QD absorption, corresponding to over 10% improvement in short-circuit current, can be achieved in optimal ultrathin devices by incorporating gratings in the rear contact.

Time-resolved studies of pulsed electrical spin injection into single InGaAs quantum dots

Spin light emitting diodes (spin-LEDs) are prominent for electrically injecting electronic spin states into semiconductor quantum dots (QDs) and are highly suitable for spin storage [1]. For an efficient quantum information processing with these QDs, the initialization of the electronic spin states has to be accomplished repeatably and reliably with short electrical pulses. Here, we report on an anomaly of the circular polarization degree (CPD) emitted by InGaAs QDs within a spin-LED with a ZnMnSe spin aligner, if the device is excited with pulses of some nanoseconds width.

Optical microcavities with pyramidal shape

Recently, GaAs pyramids standing on top of distributed Bragg reflectors (DBRs) have been studied as candidates for high-quality (Q) and low mode-volume optical cavities [1, 2]. Their fabrication through a wetchemical etching process allows an easy geometrical tuning of pyramid size and facet angles. Q factors reaching 700 have been observed by micro-photoluminescence measurements [2]. While these previously investigated pyramid structures rely on total internal reflection at the facets and a DBR at the base plane, two novel designs of pyramidal structures are suggested in this contribution in order to enhance light confinement. On the one hand, freestanding reversed pyramids have been realized as shown in Fig. 1a. In such structures, light confinement is based exclusively on total internal reflection. Calculations based on finite-element methods suggest potentially very high Q factors for such geometries, especially for pyramids with an octagonal base (feasible with the same method). On the other hand, truncated pyramids standing on top of a DBR have been metallized or overgrown with another DBR as proposed in Ref. [2]. In contrast to freestanding pyramids the latter approach implies that both vertical as well as lateral light confinement are achieved by DBRs (see cross-section in Fig. 1c).

Optical cavity modes in micro-pyramids

We report on the fabrication and investigation of pyramidal GaAs micro-cavities on top of a Bragg mirror. A finite-difference time-domain simulation supports the experimentally found optical mode structure for such a cavity shape.