Yu-Ping Zeng

Three-dimensional hole gas induced by polarization in (0001)-oriented metal-face III-nitride structure

Polarization-doping via graded AlGaN layer on N-face (0001¯) GaN has been demonstrated as an inspiring p-type doping method for wide-band-gap nitrides. However, the polarity of III-nitrides grown by metal organic chemical vapor deposition is metal-face typically. In this paper, we show that three-dimensional mobile hole gas induced by polarization can be formed in (0001)-oriented metal-face III-nitride structure. The hole concentration of a Mg-doped AlxGa1−xN layer with graded Al composition from x=0.3 to 0 grown on AlN buffer layer is remarkably enhanced, compared with that of a Mg-doped GaN layer grown under the same conditions. In addition, the hole concentration in the graded AlGaN layer is absence of freezeout as the temperature decreases, indicating that the hole is induced by polarization. This p-type doping method paves a way for achieving high-efficiency in wide-band-gap semiconductor light-emitting devices with p-type doping problem.

Theoretical study of polarization-doped GaN-based light-emitting diodes

Insufficient hole injection is a major impediment to the luminescence efficiency of III-nitride light-emitting diodes (LEDs). In our previous work by Zhang et al. [Appl. Phys. Lett. 97, 062103 (2010)], high-density mobile three-dimensional hole gas is obtained in Mg-doped Al composition graded AlGaN layer with Al composition linearly decreasing from a certain value to 0. In this paper, it is revealed by a theoretical study that the hole injection efficiency in blue-light GaN-based LEDs can be greatly enhanced by using this polarization-doped method. An increase in the electroluminescence intensity and the internal quantum efficiency in polarization-doped GaN-based LEDs is observed, in comparison with a conventional LED.

Effect of polyvinyl alcohol additive on the pore structure and morphology of the freeze-cast hydroxyapatite ceramics

Porous hydroxyapatite (HAP) ceramics with different morphologies were fabricated by the freeze casting method. The morphologies of HAP ceramics were modified by adjusting the concentration of polyvinyl alcohol (PVA) additive in the HAP slurries. HAP ceramics without PVA additive were composed of non-interconnected macroscopic lamellar pores and porous ceramic walls. With PVA additive, the HAP ceramics were made up of small lamellar pores or three-dimensional reticulate pores and porous ceramic walls. PVA additive had no effect on the phase composition of HAP ceramics. The open porosity and pore connectivity were improved because of the addition of PVA.

Plasma spray coatings in different nanosize alumina

Three kinds of nanostructured alumina coatings were prepared by air plasma spraying (APS). The microstructure of these coatings was characterized by SEM and X-ray diffraction (XRD). The surface roughness, porosity and microhardness of as-sprayed coatings were measured. The results indicated that the smaller the size of starting powder was, the better properties the coating possessed. The differences among phase analysis of these as-sprayed coatings could explain it.

Tuning the phase and morphology of In2O3 nanocrystals via simple solution routes

By decomposing In(OH)3 under different reaction conditions, In2O3 nanocrystals with controllable phases and morphologies were synthesized. Metastable hexagonal In2O3 nanocrystals were obtained by an alcoholysis process in ethanol, and followed by decomposition in air. The well-defined cubic In2O3 nanocubes can be fabricated by treating In(OH)3 in methanol at 250??C for 25?h, whereas the replacement of the solvent by ethanol yields nanorods. The results indicated that the solvent, dopants, pressure, and reaction temperature are responsible for the variations of phases and morphologies. In addition, the photoluminescence properties of the products are strongly dependent on their phases, crystallinity, and morphologies.

Intraband absorption in the 8–12 μm band from Si-doped vertically aligned InGaAs/GaAs quantum-dot superlattice

Normal-incident infrared absorption in the 8–12-μm-atmospheric spectral window in the InGaAs/GaAs quantum-dot superlattice is observed. Using cross-sectional transmission electron microscopy, we find that the InGaAs quantum dots are perfectly vertically aligned in the growth direction (100). Under the normal incident radiation, a distinct absorption peaked at 9.9 μm is observed. This work indicates the potential of this quantum-dot superlattice structure for use as normal-incident infrared imaging focal arrays application without fabricating grating structures.

Improvement of efficiency of GaN-based polarization-doped light-emitting diodes grown by metalorganic chemical vapor deposition

Our simulated results [Appl. Phys. Lett. 98, 101110 (2011)] reveal that polarization-doped light-emitting diodes (LEDs) have improved internal quantum efficiency due to the enhanced hole injection caused by the improved hole concentration and smooth valence band. In this letter, in order to verify these calculated results, polarization-doped LEDs grown by metalorganic chemical vapor deposition are investigated. The results show that the polarization-doped LED has the improved electroluminescence intensity and external quantum efficiency (EQE) compared with the conventional LED. The influence of the degree of AlGaN gradation on polarization-doped LEDs is also studied. It is found that the polarization-doped LED has the highest EQE when the Al composition of the graded AlGaN is linearly decreased from 0.2 to 0.

Tuning the crystal structure and magnetic properties of Fe doped In2O3 nanocrystals

Fe doped In2O3 nanocrystals were synthesized by a coprecipitation method under external magnetic fields. X-ray diffraction data and high-resolution transmission electron microscopy analyses indicated the formation of single phase without any parasitic phases. The crystal structure of the nanocrystals is tuned by changing Fe concentrations and intensities of magnetic fields. Room temperature ferromagnetism in Fe doped In2O3 nanocrystals can be activated by suitable magnetic fields.

Abnormal phase transition and magnetic properties in Cu, Fe co-doped In2O3 nanocrystals

Fe, Cu co-doped In2O3 nanocrystals were synthesized by a coprecipitation method. Phase analyses revealed that Fe ions have high solubility (up to 15.4at.%) in the In2O3 matrix, while the Cu ions strongly restrain In2O3 phase transition from cubic to hexagonal. Raman spectroscopy shows that by adding Cu ions, the defect concentration increases. The samples show no evidence of ferromagnetism by additional Cu doping, indicating that Cu content might be a key point to realize room temperature ferromagnetism in Fe doped In2O3.

Rapid thermal annealing effects on step-graded InAlAs buffer layer and In0.52Al0.48As/In0.53Ga0.47As metamorphic high electron mobility transistor structures on GaAs substrates

A step-graded InAlAs buffer layer and an In0.52Al0.48As/In0.53Ga0.47As metamorphic high electron mobility transistor (MM-HEMT) structures were grown by molecular beam epitaxy on GaAs (001) substrates, and rapid thermal annealing was performed on them in the temperature range 500-800 degreesC for 30 s. The as-grown and annealed samples were investigated with Hall measurements, and 77 K photoluminescence. After rapid thermal annealing, the resistivities of step-graded InAlAs buffer layer structures became high. This can avoid leaky characteristics and parasitic capacitance for MM-HEMT devices. The highest sheet carrier density n(s) and mobility mu for MM-HEMT structures were achieved by annealing at 600 and 650degreesC, respectively. The relative intensities of the transitions between the second electron subband to the first heavy-hole subband and the first electron subband to the first heavy-hole subband in the MM-HEMT InGaAs well layer were compared under different annealing temperatures