Zheng Xinhe

Strain relaxation and optical properties of etched In0.19Ga0.81N nanorod arrays on the GaN template

InGaN/GaN epilayers, which are grown on sapphire substrates by the metal—organic chemical-vapour deposition (MOCVD) method, are formed into nanorod arrays using inductively coupled plasma etching via self-assembled Ni nanomasks. The formation of nanorod arrays eliminates the tilt of the InGaN (0002) crystallographic plane with respect to its GaN bulk layer. Photoluminescence results show an apparent S-shaped dependence on temperature. The light extraction efficiency and intensity of photoluminescence emission at low temperature of less than 30 K for the nanorod arrays are enhanced by the large surface area, which increases the quenching effect because of the high density of surface states for the temperature above 30 K. Additionally, a red-shift for the InGaN/GaN nanorod arrays is observed due to the strain relaxation, which is confirmed by reciprocal space mapping measurements.

High Concentration InGaN/GaN Multi-Quantum Well Solar Cells with a Peak Open-Circuit Voltage of 2.45 V

We report InGaN/GaN multi-quantum well (MQW) solar cells with a comparatively high open-circuit voltage and good concentration properties. The open circuit voltage ( V oc ) keeps increasing logarithmically with concentration ratio until 60 suns. The peak V oc of InGaN/GaN MQW solar cells, which has a predominant peak wavelength of 456 nm from electroluminescence measurements, is found to be 2.45 V when the concentration ratio reaches 333×. Furthermore, the dependence of conversion efficiency and fill factor on concentration ratio are analyzed.

Designing of 1 eV GaNAs/GaInAs superlattice subcell in current-matched four-junction solar cell*

A reasonably-thick GaNAs/GaInAs superlattice could be an option as a roughly 1 eV subcell to achieve high-efficiency multi-junction solar cells on a lattice-matched Ge substrate. A detailed consideration of a high-efficiency design for a GaInP/GaAs/1 eV/Ge device is presented. Calculations have been done for this structure to obtain the confined energies of the electrons and holes by utilizing the Kronig-Penney model, as well as the absorption coefficient and thereby the external quantum efficiency. The effect of well layers, GaNAs or GaInAs, on the absorption and photocurrent density under the AM 1.5 condition is discussed in order to realize a requirement of current matching in the four-junction solar cells. The management of these considerations implies the feasibility of the GaNAs/GaInAs superlattice subcell design to improve the overall conversion efficiency of lattice matched GaInP/GaAs/1 eV/Ge cells.

Effects of growth temperature on high-quality In0:2Ga0:8N layers by plasma-assisted molecular beam epitaxy

High-quality In0.2Ga0.8N epilayers were grown on a GaN template at temperatures of 520 and 580 °C via plasma-assisted molecular beam epitaxy. The X-ray rocking curve full widths at half maximum (FWHM) of (10.2) reflections is 936 arcsec for the 50-nm-thick InGaN layers at the lower temperature. When the growth temperature increases to 580 °C, the FWHM of (00.2) reflections for these samples is very narrow and keeps similar, while significant improvement of (10.2) reflections with an FWHM value of 612 arcsec has been observed. This improved quality in InGaN layers grown at 580 °C is also reflected by the much larger size of the crystalline column from the AFM results, stronger emission intensity as well as a decreased FWHM of room temperature PL from 136 to 93.9 meV.

Growth of a-Plane GaN Films on r-Plane Sapphire Substrates by Metalorganic Chemical Vapour Deposition

Nonpolar a-plane GaN films were grown on r-plane sapphire substrates by metalorganic chemical vapour deposition (MOCVD) under various conditions. The surface morphologies of epitaxial films are studied by atomic force microscopy. The pit density and size both decrease with the increasing growth temperature, decreasing growth pressure or V/III ratio, while the roughness of the surface increases. Formation mechanisms of the pits in the films are discussed.

Crystallographic tilt in GaN layers grown by epitaxial lateral overgrowth

The crystallographic tilt in GaN layers grown by epitaxial lateral overgrowth (ELO) on sapphire (0001) substrates was investigated by using double crystal X-ray diffraction (DC-XRD). It was found that ELO GaN stripes bent towards the SiNx mask in the direction perpendicular to seeding lines. Each side of GaN (0002) peak in DC-XRD rocking curves was a broad peak related with the crystallographic tilt. This broad peak split into two peaks (denoted as A and B), and peak B disappeared gradually when the mask began to be removed by selective etching. Only narrow peak A remained when the SiNx mask was removed completely. A model based on these results has been developed to show that there are two factors responsible for the crystallographic tilt: One is the non-uniformity elastic deformation caused by the interphase force between the ELO GaN layer and the SiNx mask. The other is the plastic deformation, which is attributed to the change of the threading dislocations (TDs)—from vertical in the window regions to the lateral in the regions over the mask.

The design of 1 eV band-gap of GaNAs/InGaAs short-period super-lattice solar cell

The GaNAs/InGaAs short-period super-lattice(SPSL) with a feature of space separation in In and N constituent and equivalent 1 eV band-gap is one of important structures as an active region to achieve high efficiency of GaInNAs- based solar cell in the future. To experimentally realize the required band-gap for high conversion efficiency in multi- junction solar cells on Ge substrate, we demonstrate a propagation matrix method to calculate some dependences of GaNAs/InGaAs SPSLs on their structural parameters. The results show that the GaNAs/InGaAs SPSL structures can be flexible to obtain the 1 eV energy gap by a reasonable choice of the period number, barrier thickness as well as concentrations of In and N. Additionally, the calculation illustrates a relationship of respective modulation from those super-lattice structures for the SPSL active region which emits or absorbs light at 1 eV in energy.