Z. F. Li

Raman study of N+-implanted ZnO

Raman scattering has been used to study the influence of nitrogen, a potential acceptor in ZnO, on the lattice dynamics of ZnO. It is found that N+ implantation increased the lattice disorder and induced some vibration modes to be Raman active at 275, 504, and 644cm−1, respectively. Based on theoretical and experimental study, the origin of the additional Raman peak at about 275cm−1 is attributed to the vibration of Zn atoms, where part of its first nearest neighbor O atoms are replaced by N atoms in the crystal lattice.

Raman study for E2 phonon of ZnO in Zn1−xMnxO nanoparticles

Raman scattering at room temperature is reported in Zn1−xMnxO nanoparticles for the range of 0⩽x⩽0.15. The effect of compositional disorder is obtained by analyzing the broadening and asymmetry of the first-order E2(high) phonon mode. It is found that the Raman line shapes for the ZnO E2(high) phonon in Zn1−xMnxO alloys can be well described by the spatial correlation model. It is shown that the substitutional disorder can introduce changes in the linewidth, line center position, and asymmetry of the first-order E2(high) phonon mode in Zn1−xMnxO nanoparticles.

Low-temperature interface engineering for high-quality ZnO epitaxy on Si(111) substrate

ZnO(0001)∕Si(111) interface is engineered by using a three-step technique, involving low-temperature Mg deposition, oxidation, and MgO homoepitaxy. The double heterostructure of MgO(111)∕Mg(0001)∕Si(111) formed at −10°C prevents the Si surface from oxidation and serves as an excellent template for single-domain ZnO epitaxy, which is confirmed with in situ reflection high-energy electron diffraction observation and ex situ characterization by transmission electron microscopy, x-ray diffraction, and photoluminescence. The low-temperature interface engineering method can also be applied to control other reactive metal/Si interfaces and obtain high-quality oxide templates accordingly.

Closed-air induced composite wetting on hydrophilic ordered nanoporous anodic alumina

We investigate wetting behaviors of two kinds of hydrophilic ordered nanoporous anodic alumina (PAA). The water contact angle (CA) increases at first and subsequently decreases with increasing pore diameter of PAA with closed-pore structure, while the water CA decreases monotonously on PAA with open-pore structure. These interesting wetting behaviors are mainly due to the presence/absence of closed-air in the PAA. The closed-air could prevent water from entering into the nanopores. This work offers insight to control wetting by tailoring the surface nanostructure and will be significant for applications in printing, coating, etc.

Parameter determination from resistance-voltage curve for long-wavelength HgCdTe photodiode

A data-processing approach has been developed to obtain device parameters from resistance-voltage (R-V) curves measured on long-wavelength HgCdTe n-on-p photodiodes. The physical model used for R-V curve fitting includes the dark current mechanisms induced by diffusion, generation recombination, trap-assisted tunneling, and band-to-band tunneling. Moreover, the series resistance effect is also taken into account. Six parameters, which include the dopant density Nd in the n region, the ratio of mobility to lifetime of electrons μn∕τn in the p region, the effective lifetime τ0 in the depletion region, the relative energy position of trap level Et∕Eg and its density Nt in the depletion region, and the series resistance Rs, can be extracted from measured R-V curves. The fitting procedure has been presented in detail and the error ranges of the extracted parameters have been discussed. By fitting to the R-V characteristics of three long-wavelength devices with different Cd compositions, the applicability of ou...

Microphotoluminescence mapping on CdZnTe: Zn distribution

We report microphotoluminescence (μ-PL) mappings of CdZnTe wafers on micrometer and millimeter scales. The acquired PL spectra have been fitted to a model based on band-to-band transition including the contribution of localized states in the energy gap. The fitting yields energy gap Eg, which is correlated to the Zn fraction x in Cd1−xZnxTe. The statistics of large numbers of fitted Eg reveal the inhomogeneity of the Zn composition while the map of Eg gives the distribution of Zn atoms. The comparison between the PL mappings before and after epi-ready chemomechanical processing show a great improvement in homogeneity due to the removal of surface defects and damage by the processing. Our results demonstrate the feasibility of PL mapping in determining the Zn composition homogeneity and distribution in a CdZnTe wafer.

Extended mode in blocked impurity band detectors for terahertz radiation detection

We demonstrate the existence of an interfacial barrier in blocked impurity band (BIB) detectors using temperature-dependent dark current and corresponding theoretical calculations. Considering the effects of the interfacial barrier, the calculated photoresponse is in good agreement with the experimental results. A dual-excitation model, including the direct excitation over the full barrier and excitation to the band minimum with subsequent tunneling into the blocking layer, is proposed to quantitatively explain the observed photoresponse extension. A concept of extended-mode detection is developed to suggest the option for some selective photoresponse in the terahertz region and open the possibility of extending BIB photoresponse to lower frequency.

Evolution of infrared photoreflectance lineshape with temperature in narrow-gap HgCdTe epilayers

Temperature-dependent (11–290K) infrared photoreflectance (PR) measurements are performed on as-grown arsenic-doped HgCdTe epilayers in a midinfrared spectral region. Main PR features near bandedge manifest clear evolution of lineshape with temperature, of which the fittings identify besides a band-band process several below-gap processes. Analyses show that these features are due to photomodulation-induced screening of donor-acceptor pairs and photomodulation of band- impurity and band-band reflectance, their intensities correlate to the joint concentration of the involved energetic states. Temperature-dependent infrared PR may be a right optical spectroscopy for identifying impurity levels in semiconductors such as HgCdTe with high-density impurities.

Dimensionality of photoluminescence spectrum of GaAs/AlGaAs system

We have theoretically studied the radiative recombination process between a conduction-band electron and a valence-band hole to analyze the photoluminescence (PL) spectrum. It is shown that due to the characteristics of the energy density of states, the line shape of the PL peak depends strongly on the dimensionality of the system under investigation, thus indicating the importance of the PL line shape in the identification of the corresponding optical transition process. Increasing the quantum confinement of a system from three-dimensional (3D bulk material) to two-dimensional (quantum well), one-dimensional (quantum wire) and zero-dimensional (quantum dot) results in a transition from a highly nonsymmetric PL peak to a symmetric Lorentzian one centered at the optical transition energy.

Effects of bias and temperature on the intersubband absorption in very long wavelength GaAs/AlGaAs quantum well infrared photodetectors

The temperature-and bias-dependent photocurrent spectra of very long wavelength GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) are studied using spectroscopic measurements and corresponding theoretical calculations. It is found that the peak response wavelength will shift as the bias and temperature change. Aided by band structure calculations, we propose a model of the double excited states and explain the experimental observations very well. In addition, the working mechanisms of the quasi-bound state confined in the quantum well, including the processes of tunneling and thermionic emission, are also investigated in detail. We confirm that the first excited state, which belongs to the quasi-bound state, can be converted into a quasi-continuum state induced by bias and temperature. These obtained results provide a full understanding of the bound-to-quasi-bound state and the bound-to-quasi-continuum state transition, and thus allow for a better optimization of QWIPs performance