In recent years, ZnMgO semiconductor alloys, with a direct bandgap tunable between 3.37 eV and 7.8 eV, become
one of the most suitable materials for the fabrication of ultraviolet detectors. In this paper, we have fabricated
metal-semiconductor-metal photodetectors on 1-μm thick Zn0.8Mg0.2O films. The interdigital metal electrodes are
500 μm long and 5 μm wide with an interelectrode spacing 2 μm, 5 μm and 10 μm, respectively. Zn0.8Mg0.2O films
were grown on quartz by ratio frequency magnetron sputtering at 500°C. Dark current, spectral responsivity and
pulse response were carried out for the devices with different finger pitches. All the photodetectors showed the
peak responsivity at 330 nm and the ultraviolet-visible rejection ratio (R330 nm/R400 nm) is more than four orders
of magnitude at 3 V bias. For the device with 2 μm finger pitch, the detectivity was calculated as 4.2×1011 cm
Hz1/2/W at 330 nm. Furthermore, the transient response measurement for all devices revealed similar rise time of
10 ns. The 90%-10% fall times are 130 ns, 170 ns and 230 ns for the devices with different finger pitches of 2 μm, 5 μm
and 10 μm, respectively.
ZnO nanowires were obtained by using a simple vapor solid process. The length of the nanowires is about 1 μm, with
diameters ranging from 40 nm to 150 nm. X-ray diffraction (XRD) pattern confirms the nanowires are wurtzite structure
with c-axis preferred orientation. Photoluminescence (PL) measurement shows a strong UV emission and a weak visible
emission. Field emission (FE) results proved the nanowires have low turn-on voltage and high field enhancement factor,
which indicates ZnO nanowire is a suitable candidate for field emission display.
In this paper, we prepared carbon doped nanocrystalline ZnO by pyrolyzed zinc stearate at 250°C and 300°C respectively. The XRD curves indicate the sample has polycrystalline hexagonal wurtzite structure. The XRD data of the sample prepared at 250°C and 300°C has a bigger angle shift about 0.05°and 0.3°respectively. That indicate the structure of the sample has some changes. The EDS indicate the sample contains Zn, O and C. So the XRD shift may attribute to the C. The XPS indicate the C doped in the crystal lattice of ZnO of the sample prepared at 300°C, and the sample prepared at 250°C may be only a few of C doped in the crystal lattice of ZnO. The PL of the sample prepared at 300°C only has a weak ultraviolet emission, which indicates C modified the nanocrystalline ZnO surface as a non-radiative recombination center. In this process C could non-radiatively recombine the carries on the nanocrystallin ZnO surface. The sample prepared at 250°C has a strong visible emission at about 530 nm. This emission band could be attributed to oxygen vacancy because C schlepped some oxygen on the nanocrystalline ZnO surface.
CdSe and ZnCdSe quantum dots (QDs) were grown under Stranski-Krastanow (S-K) mode by low-pressure metalorganic chemical vapor deposition (LP-MOCVD). The formation process of CdSe QDs below critical thickness was observed by atomic force microscopy (AFM). The formation mechanism of CdSe QDs below the critical thickness was due to the effect of surface diffusion and strain release. ZnCdSe QDs were grown based on the calculated critical thickness. Two kinds of variations in the ZnCdSe QDs appeared over time, the Ostwald ripening process and dot formation process. ZnSeS dots were grown under Volmer-Weber (V-W) mode. With increasing the growth duration, the size of dots becomes larger and the density decreases, which is explained by virtue of the surface free energy.
CdS and ZnCdS epilayers are grown on GaAs by LP-MOCVD. It is found that the thickness of CdS epilayer is greatly influenced by the growth temperature. In our case, the sticking coefficient of CdS is nearly zero at temperatures greater than or equal to 500°C. The wide values of FWHMs of CdS emission band are thought as a result of the poor crystal quality caused due to the deviation of stoichiometry ratio of Cd/S. The crystal quality can be imprved with increasing the thickness of CdS epilayers when the flow rate of H2S is selected. The growth conditions of Zn0.76Cd0.24S epilayers aer investigated and the deep center emission band is attributed to sulfer-related defect.
ZnO thin films have been grown on a (400) Si substrate by plasma-molecular beam epitaxy (P-MBE). The sample was characterized by X-ray diffraction (XRD) and photoluminescence (PL). X-ray diffraction result exhibits a strong (002) diffraction peak of ZnO thin film. In PL spectra, a dominant ultraviolet light (UL) emission at 3.265eV is observed at room temperature (RT). According to the energy position of the UL emission, this luminescence at RT was considered to be related to exciton recombination. The samples were annealed in oxygen for two hours at different temperatures, XRD shows the improvement of crystal quality with increasing annealing temperature.
At high excitation intensity the photoluminescence (PL) spectra of ZnCdSe/ZnSe multiple quantum wells were studied, which showed strong excitonic emission and a broad emission band at low energy side. The dependence of the broad emission band on excitation intensity shoed obviously that the broad emission band is related to impurity. In time resolved luminescence spectra, with increasing the delay times (ns), the broad emission band shifts to low energy side and full width at half maximum decreased, which showed the typical characteristic of donor-acceptor pairs (DAP) emission. And then, the reason that the excitonic emission peak and the DAP band decay with same speed was discussed and it was attributed to the free carriers relax effect.
CdS-CdS1-xSex SLSs were grown on (111) GaAs substrates by MOCVD, and their characteristics were analyzed by PL measurements. Luminescence results show that the band-gap of superlattice can be controlled by changing compositions, and crystalline quality of SLSs can be improved by controlling composition and inducing buffer layer. This indicates that the PL spectrum is more sensitive to investigate the structure of SLSs.
The picosecond optical bistability of Zn1-xCdxSe/ZnSe (x equals 0.23) multiple quantum well (MQW) has been observed by a Streak Camera at room temperature for the first time. In the absorption spectra of ZnCdSe/ZnSe MQW, there are two peaks at 515 nm and 530 nm. We discussed the origin of the two peaks and physical mechanism of the bistability.
Excitonic properties of ZnSe1-xSx epilayer fabricated on GaAs substrate by a Ap-MOCVD have been investigated. Luminescence, excitation, and time-resolved spectroscopy have been employed to study the interaction between excitons and electrons at 77 and 300 K. Under pulsed N2 laser excitation it is found that the peak shift of the near band edge emission is large, especially at room temperature. This is because the bottoms of the conduction bands are filled at higher temperature and the carriers from the exciton-exciton scattering must occupy higher energy levels. Therefore a shift of the peak toward lower energy is expected.