Yu-Jia Zeng

Structural, optical, and electrical properties of (Zn,Al)O films over a wide range of compositions

(Zn,Al)O thin films have been prepared by a dc reactive magnetron sputtering system with the Al contents in a wide range of 0–50at.%. The structural, optical, and electrical properties of (Zn,Al)O films were detailedly and systematically studied. The amount of Al in the film was nearly the same as, but often lower than, that in the sputtering target. The growth rate of films monotonically decreased as the Al content increased. In a low Al content region (<10at.%), Al-doped ZnO (AZO) thin films could be obtained at 400°C in an Ar–O2 ambient with good properties. The optimal results of n-type AZO films were obtained at an Al content of 4at.%, with low resistivity ∼10−4Ωcm, high transmittance ∼90% in the visible region, and acceptable crystal quality with a high c-axis orientation. The band gap could be widened to 3.52eV at 4at.% Al due to the Burstein-Moss shift [E. Burstein, Phys. Rev. 93, 632 (1954)] modulated by many-body effects. An appropriate Al-doping concentration served effectively to release the r...

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...

ZnO light-emitting diode grown by plasma-assisted metal organic chemical vapor deposition

We report a breakthrough in fabricating ZnO homojunction light-emitting diode by metal organic chemical vapor deposition. Using NO plasma, we are able to grow p-type ZnO thin films on n-type bulk ZnO substrates. The as-grown films on glass substrates show hole concentration of 1016–1017cm−3 and mobility of 1–10cm2V−1s−1. Room-temperature photoluminescence spectra reveal nitrogen-related emissions. A typical ZnO homojunction shows rectifying behavior with a turn-on voltage of about 2.3V. Electroluminescence at room temperature has been demonstrated with band-to-band emission at I=40mA and defect-related emissions in the blue-yellow spectrum range.

p-type conduction in N–Al co-doped ZnO thin films

p-type ZnO thin films have been realized by the N–Al co-doping method. Secondary ion mass spectroscopy demonstrated that the N incorporation was enhanced evidently by the presence of Al in ZnO. The lowest room-temperature resistivity was found to be 57.3Ωcm with a Hall mobility of 0.43cm2∕Vs and carrier concentration of 2.25×1017cm−3 for the N–Al co-doped p-type ZnO film deposited on glass substrate. The results were much better than those for the N-doped p-type ZnO. Moreover, the co-doped film possesses a good crystallinity with c-axis orientation and a high transmittance (90%) in the visible region.

Dopant source choice for formation of p-type ZnO: Li acceptor

Li-doped, p-type ZnO thin films have been realized via dc reactive magnetron sputtering. An optimized result with a resistivity of 16.4Ωcm, Hall mobility of 2.65cm2∕Vs, and hole concentration of 1.44×1017cm−3 was achieved, and electrically stable over a month. Hall-effect measurements supported by secondary ion mass spectroscopy indicated that the substrate temperature played a key role in optimizing the p-type conduction of Li-doped ZnO thin films. Furthermore, ZnO-based p-n homojunction was fabricated by deposition of a Li-doped p-type ZnO layer on an Al-doped n-type ZnO layer.

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.

Identification of acceptor states in Li-doped p-type ZnO thin films

We investigate photoluminescence from reproducible Li-doped p-type ZnO thin films prepared by dc reactive magnetron sputtering. The LiZn acceptor state, with an energy level located at 150meV above the valence band maximum, is identified from free-to-neutral-acceptor transitions. Another deeper acceptor state located at 250meV emerges with the increased Li concentration. A broad emission centered at 2.96eV is attributed to a donor-acceptor pair recombination involving zinc vacancy. In addition, two chemical bonding states of Li, evident in x-ray photoelectron spectroscopy, are probably associated with the two acceptor states observed.

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.

Electrical characterization of ZnO-based homojunctions

Electrical characteristics have been studied for ZnO p-n and p-i-n homojunctions, with optimization of device structures for improved performance. Capacitance-voltage measurements confirm the formation of abrupt junctions. The current-voltage characteristics exhibit their inherent electrical rectification behavior. The p-ZnO:(N,Al)∕n-ZnO:Al homojunctions fabricated on sapphire substrates combining with the intrinsic ZnO buffer layer have acceptable p-n diode characteristics, with the forward turn-on voltage of 1.4V and the reverse breakdown voltage of 5.3V. By introduction of an intrinsic (Zn,Cd)O layer, the resultant p-ZnO:(N,Al)∕i-(Zn,Cd)O∕n-ZnO:Al homojunction exhibits a high reverse breakdown voltage of ∼18V.

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.