Showing papers by "Youdou Zheng published in 2003"

Raman and photoluminescence of ZnO films deposited on Si (111) using low-pressure metalorganic chemical vapor deposition

Abstract: The highly c-oriented ZnO films were epitaxially grown on n-type Si (111) substrate at the temperature range of 340–460 °C using the low-pressure metalorganic chemical vapor deposition method. All films exhibit a pronounced (002) peak for ZnO, indicative of the strong c-axis oriented characteristic. The ZnO film grown at 400 °C shows the best structural quality along with the largest lateral grain size, well supported by the narrowest full width at half maximum of ZnO (002) peak about 0.19° in x-ray diffraction. However, the temperature dependence of the vibrational modes at 436 and ∼563 cm−1 in Raman spectra revealed a low density of oxygen vacancies in the films grown at low temperatures, which is supposed to determine the photoluminescence (PL) properties. At low temperatures, the narrow ultraviolet (UV) near band emission dominated the PL spectrum with a very weak low energy tail near the band. High temperature (up to 460 °C) would cause serious oxygen deficiency, resulting in the weak broad UV band w...

Magnetoresistance oscillations induced by intersubband scattering of two-dimensional electron gas in Al0.22Ga0.78N/GaN heterostructures

Abstract: Magnetointersubband scattering (MIS) oscillations of two-dimensional electron gas (2DEG) in Al0.22Ga0.78N/GaN heterostructures have been investigated by means of magnetotransport measurements at low temperatures and high magnetic fields. Double periodic Shubnikov–de Haas oscillations modulated by MIS oscillations have been observed due to the intersubband scattering of the 2DEG at the two lowest subbands in the triangular quantum well at the heterointerface. By using the fast Fourier transform analysis, it is found that the MIS oscillations become slightly weaker with an increase in temperature. From the MIS frequency, the energy separation between the first and the second subbands is determined to be 80 meV. The observation of the MIS effect indicates that the effective masses of the electrons in the first and second subbands are the same in Al0.22Ga0.78N/GaN heterostructures.

Synthesis of Zinc Aluminate Spinel Film through the Solid‐Phase Reaction between Zinc Oxide Film and α‐Alumina Substrate

Abstract: We synthesized spinel ZnAl2O4 film on α-Al2O3 substrate using a solid-phase reaction between the pulsed-laser-deposited ZnO film and α-Al2O3 substrate. Auger electron spectroscopy showed that the atomic distribution in the spinel ZnAl2O4 was inhomogeneous, which indicated that the reaction was diffusion controlled. Based on X-ray fluorescence measurements, the apparent growth activation energy of ZnAl2O4 was determined as 504 kJ/mol. X-ray diffractometry spectra showed that, as the growth temperature increased, the ZnAl2O4 film became disoriented from the single (111) orientation. The ZnAl2O4 (333) diffraction peak shifted toward a small angle, and its full-width at half-maximum decreased from 1.30° to 0.37°. At the growth temperature of 1100°C, the morphology of the ZnAl2O4 was initially transformed from islands to stick structures, then to bulgy-line structures with increased growth time. X-ray diffractometry spectra showed that these transformations were correlated with changes of ZnAl2O4 orientation.

Multisubband transport of the two-dimensional electron gas in AlxGa1-xN/GaN heterostructures

Abstract: Multisubband transport of the two-dimensional electron gas (2DEG) in modulation-doped Al0.22Ga0.78N/GaN heterostructures has been investigated by means of magnetotransport measurements at low temperatures and high magnetic fields. It is found that the mobility of the 2DEG in the first subband in a triangular quantum well at the heterointerface decreases significantly, while the mobility of the 2DEG in the second subband increases, when the Al0.22Ga0.78N barrier is partially relaxed. Such behavior of the 2DEG mobility is explained by the nonuniformity of the piezoelectric polarization field at the heterointerface induced by the Al0.22Ga0.78N relaxation and strong interface scattering. Meanwhile, it is concluded that the scattering from the remote ionized donors is the main mechanism contributing to the quantum scattering time and responsible for the intersubband scattering in the quantum well at the heterointerface.

Influence of the polarization on interfacial properties in Al/SiO2/GaN/Al0.4Ga0.6N/GaN heterojunction metal–insulator–semiconductor structures

Abstract: GaN-based metal–insulator–semiconductor (MIS) structures were fabricated by depositing a SiO2 film on a metalorganic chemical vapor deposition-grown GaN/Al0.4Ga0.6N/GaN double heterojunction. Various-frequency capacitance–voltage (C−V) measurements were carried out on the MIS structures. The measured C−V curves show a notable flatband shift of up to about 12.5 V with a typical polarization hysteresis window (9.4 V in width). Moreover, the capacitance of the heterojunction MIS structure reaches a minimum value under 4.1 V bias (forward scan) or −6.5 V bias (reverse scan). Due to the strong polarization and piezoelectric effects existing in AlGaN/GaN heterostructures, these results are deemed to be due to the influence of the piezoelectricity and polarization effects in the structure. Conductance–voltage (G−V) measurements were also performed. Various-frequency C−V and the G−V behaviors indicate that interface states have little influence on the sample.

Growth of Si1-x-yGexCy Alloy Layers on Si by Chemical Vapor Deposition Using Ethylene

Abstract: High quality Si 1-x-yGexCy alloy layers were grown on Si (100) substrate by rapid thermal processing very low pressure chemical vapor deposition (RTP/VLP -CVD) using ethylene(C 2H4) as C source. The composition of Si 1-x-yGexCy allow layers were measured by Auger Electron Spectroscopy (AES). The C incorporation mode was characterized by Fourier transform infrared spectroscopy (FTIR). The content of substitutional C in Si 1-x-yGexCy alloy layers was calculated from the local vibration mode (LVM) of Si-C bond in FTIR spectrosco py. It was found that the lower temperature and higher SiH 4/C2H4 flow ratio are helpful in forming the substitutional C and improving the crystal quality. The mechanism responsible for C inc orporation in Si 1-x-yGexCy alloy layer grown by RTP/VLP-CVD using C 2H4 was explained by the reaction between SiH 4 and C2H4 at different growth temperature.

Synthesis of ZnAl2O4/α-Al2O3 complex substrates and growth of GaN films

Abstract: With the solid phase reaction between pulsed-laser-deposited (PLD) ZnO film and α-Al2O3 substrate, ZnAl2O4/α-Al2O3 complex substrates were synthesized. X-ray diffraction (XRD) spectra show that as the reaction proceeds, ZnAl2O4 changes from the initial (111)-oriented single crystal to poly-crystal, and then to inadequate (111) orientation. Corresponding scanning electron microscope (SEM) images indicate that the surface morphology of ZnAl2O4 transforms from uniform islands to stick structures, and then to bulgy-line structures. In addition, XRD spectra present that ZnAl2O4 prepared at low temperature is unstable at the environment of higher temperature. On the as-obtained ZnAl2O4/α-Al2O3 substrates, GaN films were grown without any nitride buffer using light-radiation heating low-pressure MOCVD (LRH-LP-MOCVD). XRD spectra indicate that GaN film on this kind of complex substrate changes fromc-axis single crystal to poly-crystal as ZnAl2O4 layer is thickened. For the single crystal GaN, its full width at half maximum (FWHM) of X-ray rocking curve is 0.4°. Results indicate that islands on thin ZnAl2O4 layer can promote nucleation at initial stage of GaN growth, which leads to the (0001)-oriented GaN film.

MOCVD growth and properties of ZnO and Zn/sub 1-x/Mg/sub x/O films

Abstract: In this work, the high-quality ZnO based films were epitaxially deposited by low-pressure metal organic chemical vapor deposition (LP-MOCVD) technique. The characteristics of ZnO films were studied as a function of the growth conditions such as the growth temperatures, and the flow ratio of II/VI, which dramatically affect the structure and the associated optical properties of the epilayer. The spontaneous emission spectrum measured at the low temperature of 4 K shows an emission peak related to donor-bound exciton at the full width of 2 nm, indicating rather high-quality of the ZnO epilayer obtained by optimizing the above growth conditions. The Zn/sub 1-x/Mg/sub x/O alloy thin film has been grown with the incorporation of Mg at a maximum value of x=0.22. The structure and optical properties of Zn/sub 1-x/Mg/sub x/O alloy thin film has then been discussed briefly.