Xiaolong Du

Nanostructure Formation and Passivation of Large-Area Black Silicon for Solar Cell Applications

Nanoscale textured silicon and its passivation are explored by simple low-cost metal-assisted chemical etching and thermal oxidation, and large-area black silicon was fabricated both on single-crystalline Si and multicrystalline Si for solar cell applications. When the Si surface was etched by HF/AgNO(3) solution for 4 or 5 min, nanopores formed in the Si surface, 50-100 nm in diameter and 200-300 nm deep. The nanoscale textured silicon surface turns into an effective medium with a gradually varying refractive index, which leads to the low reflectivity and black appearance of the samples. Mean reflectance was reduced to as low as 2% for crystalline Si and 4% for multicrystalline Si from 300 to 1000 nm, with no antireflective (AR) coating. A black-etched multicrystalline-Si of 156 mm × 156 mm was used to fabricate a primary solar cell with no surface passivation or AR coating. Its conversion efficiency (η) was 11.5%. The cell conversion efficiency was increased greatly by using surface passivation process, which proved very useful in suppressing excess carrier recombination on the nanostructured surface. Finally, a black m-Si cell with efficiency of 15.8% was achieved by using SiO(2) and SiN(X) bilayer passivation structure, indicating that passivation plays a key role in large-scale manufacture of black silicon solar cells.

Mg0.55Zn0.45O solar-blind ultraviolet detector with high photoresponse performance and large internal gain

A Schottky type metal-semiconductor-metal solar-blind ultraviolet detector was fabricated on high quality wurtzite Mg0.55Zn0.45O epitaxial film. Photoresponse spectra show a responsivity peak of 22 mA/W under 130 V bias. A sharp cutoff was recognized at a wavelength of 270 nm, and a temporal response measurement indicates a fast decay time of less than 500 ns. A large internal gain was observed and interpreted by a reduced Schottky barrier height model, which fits well with the experimental data.

Visible-blind ultraviolet photodetector based on double heterojunction of n-ZnO/insulator-MgO/p-Si

Exploiting a double heterojunction of n-ZnO/insulator-MgO∕p-Si grown by molecular beam epitaxy, a visible-blind ultraviolet (UV) photodetector has been fabricated. The photodetector shows a rectification ratio of ∼104 at ±2V and a dark current of 0.5nA at a reverse bias of −2V.The photoresponse spectrum indicates a visible-blind UV detectivity of our devices with a sharp cut off at the wavelength of 378nm and a high UV/visible rejection ratio. The key role of the middle insulating MgO layer, as a barrier layer for minority carrier transport, has been demonstrated.

Controlled growth of Zn-polar ZnO epitaxial film by nitridation of sapphire substrate

Surface nitridation is used to eliminate O-polar inversion domains and control the growth of single-domain Zn-polar ZnO film on sapphire (0001) substrate by rf-plasma-assisted molecular-beam epitaxy. It is found that the nitridation temperature is crucial for achieving quality AlN buffer layers and ZnO films with cation polarity, as demonstrated by ex situ transmission electron microscopy. Under optimal growth conditions, a 4×4 surface reconstruction was observed, which is confirmed to be a characteristic surface structure of the Zn-polar films, and can be used as a fingerprint to optimize the ZnO growth.

Probing Defects in Nitrogen-Doped Cu2O

Nitrogen doping is a promising method of engineering the electronic structure of a metal oxide to modify its optical and electrical properties; however, the doping effect strongly depends on the types of defects introduced. Herein, we report a comparative study of nitrogen-doping-induced defects in Cu2O. Even in the lightly doped samples, a considerable number of nitrogen interstitials (Ni) formed, accompanied by nitrogen substitutions (NO) and oxygen vacancies (VO). In the course of high-temperature annealing, these Ni atoms interacted with VO, resulting in an increase in NO and decreases in Ni and VO. The properties of the annealed sample were significantly modified as a result. Our results suggest that Ni is a significant defect type in nitrogen-doped Cu2O.

Semiconductor ultraviolet photodetectors based on ZnO and MgxZn1−xO

It is indispensable to develop wide-band-gap based ultraviolet (UV) photodetectors (PDs), which are one of the basic building blocks of solid state UV optoelectronic devices. In the last two decades, we have witnessed the renaissance of ZnO as a wide-band-gap semiconductor and an enormous development of ZnO-based UV PDs as a result of its superb optical and electronic properties. Since the first demonstration, a great variety of UV PDs based on ZnO and its related materials have been proposed and demonstrated. These PDs, with diverse device geometries, exhibit either high performance or multiple functions, reflecting a state-of-the-art technology of UV optoelectronics. In this review, we study the latest progress of UV PDs made on ZnO and MgxZn1−xO, which is a representative alloy of ZnO for band-gap engineering techniques. The discussion focuses on the device performance and the behind device physics according to the architecture of UV PDs.

Comparative study of n-MgZnO/p-Si ultraviolet-B photodetector performance with different device structures

A comparative study of n-MgZnO/p-Si UV-B photodetector performance was carried out with different device structures. The experimental results demonstrate superior photoresponse characteristics of the p-n heterojunction detector against the Schottky type metal-semiconductor-metal counterpart, including a sharper cutoff wavelength at 300 nm, a larger peak photoresponsivity of 1 A/W, and a faster response speed. The role of built-in field and low interface scattering in p-n heterojunction is explored, and the energy band diagram of n-MgZnO/p-Si is employed to interpret the efficient suppression of visible light photoresponse from Si substrate, revealing the applicability of this heterostructure in fabrication of deep ultraviolet detectors.

Cubic nitridation layers on sapphire substrate and their role in polarity selection of ZnO films

Well-defined cubic AlN ultrathin layers formed by nitridation of Al2O3 (0001) substrate at various temperatures were observed by high-resolution transmission electron microscopy. The polarity of the AlN layers strongly depends on the substrate pretreatment and nitridation temperature. The structure of the AlN layers plays a key role in polarity selection of subsequent ZnO films, and both Zn-polar and O-polar ZnO films could be steadily obtained by control of the cubic AlN layers.

Oxygen vacancies: The origin ofn-type conductivity in ZnO

Oxygen vacancy (VO) is a common native point defects that plays crucial roles in determining the physical and chemical properties of metal oxides such as ZnO. However, fundamental understanding of VO is still very sparse. Specifically, whether VO is mainly responsible for the n-type conductivity in ZnO has been still unsettled in the past fifty years. Here we report on a study of oxygen self-diffusion by conceiving and growing oxygen-isotope ZnO heterostructures with delicately-controlled chemical potential and Fermi level. The diffusion process is found to be predominantly mediated by VO. We further demonstrate that, in contrast to the general belief of their neutral attribute, the oxygen vacancies in ZnO are actually +2 charged and thus responsible for the unintentional n-type conductivity as well as the non-stoichiometry of ZnO. The methodology can be extended to study oxygen-related point defects and their energetics in other technologically important oxide materials.

Controlled growth of O-polar ZnO epitaxial film by oxygen radical preconditioning of sapphire substrate

Oxygen radicals pregrowth treatment and surface nitridation were used to eliminate Zn-polar inversion domains and control the growth of single-domain O-polar ZnO film on sapphire (0001) substrate by rf plasma-assisted molecular beam epitaxy. We found that the formation of oxygen-terminated sapphire surface prior to nitridation is crucial for achieving the anion polarity in subsequent AlN and ZnO layers, as demonstrated by formation of the 3×3 surface reconstruction during ZnO growth and ex situ polarity determination. This method, in general, can be applied to growth of other polar films, such as II-VI oxides and III-V nitrides, on sapphire (0001) substrates.