Kaigui Zhu

Lattice contraction and magnetic and electronic transport properties of Mn3Zn1−xGexN

The lattice and electronic and magnetic transport properties of the antiperovskite structure Mn3Zn1−xGexN compounds were investigated. For Mn3ZnN, there is a magnetic transition from antiferromagnetic to paramagnetic near 185K. Correspondingly, the resistivity shows an abrupt drop, but any sudden change of lattice parameters is not found. However, it is interesting that the partial substitution of Ge for Zn induces a lattice contraction near the magnetic transition temperature, where a drop of the resistivity remain, and the transition temperature point increases and the temperature range is broadened with increasing doped Ge contents. The thermodynamics properties were also investigated.

AlN/AlGaInN superlattice light-emitting diodes at 280 nm

Ultraviolet light-emitting diodes operating at 280 nm, grown by gas source molecular-beam epitaxy with ammonia, are described. The device is composed of n- and p-type superlattices of AlN(1.2 nm thick)/AlGaInN(0.5 nm thick) doped with Si and Mg, respectively. With these superlattices, and despite the high average Al content, we obtain hole concentrations of (0.7–1.1)×1018 cm−3, with the mobility of 3–4 cm2/V s and electron concentrations of 3×1019 cm−3, with the mobility of 10–20 cm2/V s, at room temperature. These carrier concentrations are sufficient to form effective p–n junctions needed in UV light sources.

Evolution of surface roughness of AlN and GaN induced by inductively coupled Cl2/Ar plasma etching

We study the effects of plasma etching on the evolution of surface roughness of GaN and AlN. The etch-induced roughness is investigated using atomic force microscopy by systematically varying plasma power, chamber pressure, and Cl2/Ar mixture gas composition. GaN etches three to four times more rapidly than AlN for identical plasma conditions. For both GaN and AlN, we find that the surface roughness is correlated to etch rate. Induced roughness remains comparable to the as-grown value provided etching is carried out below rates 400 (GaN) and 90 nm/min (AlN). Above these cutoff etch rates, the roughness increases in proportion to etch rate. This result is independent of plasma parameters varied to produce the higher etching rates. By analyzing the surface properties through the power spectral density (PSD), we correlate roughness with the formation of fine-scale features present as a consequence of more aggressive etching. The cutoff etch rates and spatial-frequency dependence of the PSD are interpreted us...

Plasma etching of AlN/AlGaInN superlattices for device fabrication

We report a study of plasma etching of GaN, AlN, and AlN/AlGaN superlattices for the processing of deep ultraviolet light emitting diodes. Etching was carried out using inductively coupled plasma of chlorine diluted with argon under reactive ion etching conditions. Using parameters selected for etch rate, anisotropy, and surface smoothness, we study etching of n- and p-type superlattices. The former etches at a rate of 250 nm/min, which is intermediate to that of AlN and GaN, while the latter exhibits a slower etch rate of 60 nm/min. Based on these studies, we prepare low-leakage p–n junctions and mesa light emitting diodes with peak emission at 280 nm.

Solar-blind ultraviolet photodetectors based on superlattices of AlN/AlGa(In)N

We describe solar-blind photodetectors based on superlattices of AlN/AlGa(In)N. The superlattices have a period of 1.4 nm, determined by x-ray diffraction, and an effective band gap of 260 nm measured by optical reflectivity. Using simple mesa diodes, without surface passivation, we obtain low dark leakage currents of 0.2–0.3 pA, corresponding to the leakage current density of ∼0.3 nA/cm2, and high zero-bias resistance of ∼1×1011 Ω. Excellent visible cutoff is obtained for these devices, with six orders of magnitude decrease in responsivity from 260 to 380 nm. These results demonstrate the potential of junctions formed by short-period superlattices in large-band-gap devices.

Computer simulation of a-Si/c-Si heterojunction solar cell with high conversion efficiency

The p-type amorphous/ n-type crystalline silicon (P+ a-Si/N+ c-Si) heterojunction was simulated for developing the solar cells with high conversion efficiency and low cost. The characteristic of such cells with different work function of transparent conductive oxide (TCO) were calculated. The energy band structure, quantum efficiency and electric field are analyzed in detail to understand the mechanism of the heterojunction cell. Our results show that the a-Si/c-Si heterojunction is hypersensitive to the TCO work function, and the TCO work function should be large enough in order to achieve high conversion efficiency for P+ a-Si:H/N+ c-Si solar cell. With the optimized parameters set, the P+ a-Si:H/N+ c-Si solar cell reaches a high efficiency ({\eta}) up to 21.849% (FF: 0.866, VOC: 0.861 V, JSC: 29.32 mA/cm2).

Effect of n+-GaN subcontact layer on 4H–SiC high-power photoconductive switch

High-power photoconductive semiconductor switching devices were fabricated on 4H–SiC. In order to prevent current crowding, reduce the contact resistance, and avoid contact degradation, a highly n-doped GaN subcontact layer was inserted between the contact metal and the high resistivity SiC bulk. This method led to a two orders of magnitude reduction in the on-state resistance and, similarly, the photocurrent efficiency was increased by two orders of magnitude with the GaN subcontact layer following the initial high current operation. Both dry etching and wet etching were used to remove the GaN subcontact layer in the channel area. Wet etching was found to be more suitable than dry etching.

Electrical properties of p–n junctions based on superlattices of AlN/AlGa(In)N

Measurements of acceptor activation energy in p–n junctions based on superlattices of AlN (1.25 nm thick) and Al0.08Ga0.92(In)N (0.5 nm thick), with the average AlN content greater than 0.6, are reported. Structural characteristics of superlattices were determined using transmission electron microscopy and x-ray diffraction. p–n junctions in mesa-etched diodes exhibit low leakage current densities of 3×10−10 A/cm2 at near zero bias. Acceptor activation energy of 207±10 meV, obtained from the temperature dependence of the forward current, is very similar to that of uniform alloy of Al0.08Ga0.92N that constitutes the well material. The acceptor activation energy thus appears controlled by the well material and remains low despite high average AlN content and large band gap.

A new all-thin-film electrochromic device using LiBSO as the ion conducting layer

An all-thin-film electrochromic device glass/ITO/NiOx/LiBSO/WO3/ITO was fabricated by magnetron sputtering, in which LiBO2 + Li2SO4 (LiBSO) was used as the new ion conducting layer. The average visible light transmittances of bleached and coloured states reached 56.8% and 4.6%, respectively, and the optical transmittance modulation can reach 52.2%. The effect of substrate temperature on the device performance was investigated by comparing liquid nitrogen cooling and water cooling. The results showed that the device had a better electrochromic performance when fabricated at a low substrate temperature.

Development and Optimization of STEP—A Linear Plasma Device for Plasma-Material Interaction Studies

The linear plasma device Simulator for Tokamak Edge Plasma (STEP) has been constructed at Beihang University, Beijing, to study plasma-material interactions (PMIs) for fusion reactor applications. The device can produce versatile low-energy and high flux plasma in laboratory experiments and is highly cost-effective to replicate the fusion-relevant plasma environment to study PMI processes. The attractive feature of the device is its compact design with a main body dimension of 1.5 × 1.5 × 0.8 m3 including the plasma source, vacuum chamber, magnetic coils, and diagnostics. A longitudinal magnetic field of up to 0.26 T is used to confine the plasma onto the target in an ∼1-m-long vacuum tube. It can produce a steady-state plasma of low impinging ion energy of <100 eV, ion flux up to 1022 m−2 · s−1, and fluence of >1026 m−2 per exposure. Various plasma species such as hydrogen, deuterium, helium, and nitrogen can be produced to manipulate PMI processes for different target grades. The STEP device provides an experimental platform to improve the understanding of PMIs, validate computational simulation results, and build a database of fusion material performance and lifetime.