Yinzhu Zhang

Carrier concentration dependence of band gap shift in n-type ZnO:Al films

Al-doped ZnO (AZO) thin films have been prepared by mist chemical vapor deposition and magnetron sputtering. The band gap shift as a function of carrier concentration in n-type zinc oxide (ZnO) was systematically studied considering the available theoretical models. The shift in energy gap, evaluated from optical absorption spectra, did not depend on sample preparations; it was mainly related to the carrier concentrations and so intrinsic to AZO. The optical gap increased with the electron concentration approximately as ne2∕3 for ne≤4.2×1019 cm−3, which could be fully interpreted by a modified Burstein–Moss (BM) shift with the nonparabolicity of the conduction band. A sudden decrease in energy gap occurred at 5.4−8.4×1019 cm−3, consistent with the Mott criterion for a semiconductor-metal transition. Above the critical values, the band gap increased again at a different rate, which was presumably due to the competing BM band-filling and band gap renormalization effects, the former inducing a band gap widen...

Low-resistivity, stable p-type ZnO thin films realized using a Li–N dual-acceptor doping method

A Li–N dual-acceptor doping method has been developed to prepare p-type ZnO thin films by pulsed laser deposition. The lowest room-temperature resistivity is found to be ∼0.93Ωcm, much lower than that of Li or N monodoped ZnO films. The p-type conductivity of ZnO:(Li,N) films is very reproducible and stable, with acceptable crystal quality. The acceptor activation energy in ZnO:(Li,N) is about 95meV. ZnO-based homostructural p-n junctions were fabricated by depositing an n-type ZnO:Al layer on a p-type ZnO:(Li,N) layer, confirmed by secondary ion mass spectroscopy. The current-voltage characteristics exhibit their inherent rectifying behaviors.

Control of p- and n-type conductivities in Li-doped ZnO thin films

Li-doped ZnO films were prepared by pulsed laser deposition. The carrier type could be controlled by adjusting the growth conditions. In an ionized oxygen atmosphere, p-type ZnO was achieved, with the hole concentration of 6.04×1017cm−3 at an optimal Li content of 0.6at.%, whereas ZnO exhibited n-type conductivity in a conventional O2 growth atmosphere. At a Li content of more than 1.2at.% only high-resistivity ZnO was obtained. The amount of Li introduced into ZnO and the relative concentrations of such defects as Li substitutions and interstitials could play an important role in determining the conductivity of films.

p-type ZnO films deposited by DC reactive magnetron sputtering at different ammonia concentrations

Abstract Films were prepared on α-Al 2 O 3 (0001) substrates by DC reactive magnetron sputtering in NH 3 –O 2 ambient at different ammonia concentrations (from 0% to 100%). Results showed that N-doped, p-type ZnO films with c -axis orientation were achieved at the ammonia concentrations of 25%, 50% and 75%. The carrier density was typically 10 17 cm −3 , the resistivity was around 30 Ω cm and the transmittance was about 90% in visible region. At 0% ammonia concentration, intrinsic ZnO films with c -axis orientation were obtained. At 100% ammonia concentration, however, the films deposited are zinc polycrystal films.

Self-assembled ZnO quantum dots with tunable optical properties

Self-assembled ZnO quantum dots (QDs) were achieved by a vapor phase transport process. ZnO nanodots were naturally formed on solid substrates in the Volmer-Weber growth mode. Size control of nanodots could be readily realized by varying the growth time. The as-prepared ZnO QDs are of high quality and very stable after formation. The blueshift of band gap energies derived from quantum confinement effects was confirmed by optical absorption spectra. Photoluminescence spectra revealed the tunable behavior of ultraviolet luminescence due to exciton localization. The realization of size-tuned color from ZnO QDs makes them more promising for practical applications.

ZnO light-emitting diodes fabricated on Si substrates with homobuffer layers

ZnO homojunction light-emitting diodes (LEDs), comprised of N–Al codoped p-type ZnO and Al-doped n-type ZnO layers, were fabricated on Si substrates with homobuffer layers. The current-voltage measurements showed typical diode characteristic with a threshold voltage of about 3.3V. The electroluminescence (EL) bands at 110K consisted of a near-band-edge emission at 3.18eV and a deep level emission at 2.58eV. The EL emissions were assigned as radiative recombinations, presumably of donor-acceptor pairs, in the p-type layer of the LED. The quenching of EL with temperature was attributed to the degradation of p-type conducting of the ZnO:(N,Al) layer.

Room-temperature photoluminescence from ZnO∕ZnMgO multiple quantum wells grown on Si(111) substrates

A set of ten-period ZnO∕Zn0.85Mg0.15O multiple quantum wells with well thickness varying from 2.5to5nm has been grown on Si(111) substrates by pulsed laser deposition. A periodic structure with sharp interfaces was observed by cross-sectional transmission electron microscopy. The room-temperature photoluminescence resulting from the well regions exhibits a significant blueshift with respect to the ZnO single layer. The well layer thickness dependence of the emission energy from the well regions was investigated and compared with a simple theoretical model. The results suggest that the quantum confinement effects in the quantum wells can be observed up to room temperature.

Improved p-type conductivity and acceptor states in N-doped ZnO thin films

N-doped, p-type ZnO (ZnO : N) thin films were prepared by magnetron sputtering using NO as the N-doping source. The introduction of Ar in the growth ambient could evidently improve the p-type conductivity and crystal quality of the ZnO : N films, with the lowest room-temperature resistivity of 3.51 Ω cm obtained at an optimal Ar partial pressure of 30%. The p-type ZnO : N films have high optical quality, as suggested by temperature-dependent photoluminescence spectra. The NO substitution acceptor state with an energy level of 180 meV was identified from the free-to-neutral-acceptor (e, A 0) transition. The NO-acceptor bound-exciton binding energy was derived to be 17 meV from the neutral-acceptor-bound-exciton and free exciton emissions. The Haynes factor was about 0.094 for the NO acceptor in ZnO. Besides the NO acceptor, a zinc vacancy (VZn) acceptor state with an energy level of 255 meV was also identified in ZnO : N from the (e, A 0) transition, which might also contribute to the observed p-type conductivity.

Rational synthesis and tunable optical properties of quasialigned Zn1−xMgxO nanorods

Quasialigned, single-crystal Zn1−xMgxO (x=0–0.32) nanorods were synthesized on Si substrates by thermal evaporation. Zn1−xMgxO nanorods grew along the [0001] crystal direction and had uniform hexagonal planes with diameters of 420–120nm. The predominant ultraviolet luminescence could be tuned from 379 (x=0) to 305nm (x=0.32) at room temperature. This blueshift indicated the band gap engineering in Zn1−xMgxO nanorods. Temperature-dependent photoluminescence was used to illustrate the free-exciton emission from Zn1−xMgxO nanorods. The exciton binding energy decreased from 59 (x=0) to 49meV (x=0.18) and then increased to 54meV (x=0.32).

The effects of group-I elements co-doping with Mn in ZnO dilute magnetic semiconductor

Mn-Li codoped ZnO (Zn(Mn,Li)O), Mn-Na codoped ZnO (Zn(Mn,Na)O), and Mn-K codoped ZnO (Zn(Mn,K)O) thin films were deposited on quartz substrates by pulsed laser deposition. The doping effects of group-I elements (e.g., Li, Na, and K) on the structural, magnetic, and optical properties of the Mn doped ZnO (ZnMnO) films were discussed. X-ray diffraction and K-edge x-ray absorption near-edge structure measurements revealed that all the films showed a hexagonal wurtzite ZnO structure, and no other clusters, precipitates, or second phases were detected. Zn(Mn,Na)O and Zn(Mn,Li)O films showed a weak p-type conductivity, while the Zn(Mn,K)O film appeared a highly resistivity. The saturation magnetization of Zn(Mn,Na)O and Zn(Mn,Li)O films was 1.2 and 0.18 μB/Mn, respectively. The hole-related defects, induced by doping with a low content of Li or Na, contributed to the room temperature ferromagnetism in the ZnMnO system.