Yuanjie Li

Measurement of Zn0.95Cd0.05O∕ZnO (0001) heterojunction band offsets by x-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy was used to measure the energy discontinuity in the valence band (ΔEv) of Zn0.95Cd0.05O∕ZnO heterostructures grown by rf plasma-enhanced molecular-beam epitaxy. A value of ΔEv=0.17±0.03eV was obtained by using the Zn 2p energy level as a reference. Given the experimental band gap of 2.9 eV for the Zn0.95Cd0.05O, this would indicate a conduction band offset ΔEC of 0.30 eV in this system.

Low specific contact resistance Ti∕Au contacts on ZnO

Ti∕Au Ohmic contacts on heavily Al-doped (n∼1019cm−3) n-ZnO produce low specific contact resistivity of 2.4×10−7Ωcm2 in the as-deposited condition and extremely low minimum values of 6×10−8Ωcm2 after annealing at 300°C.The contact resistance is independent of measurement temperature after low temperature anneals, suggesting that tunneling is the dominant transport mechanism in the contacts. The contact morphology roughens after annealing at 150°C and Auger electron spectroscopy depth profiling shows Zn outdiffusion through the metal and intermixing of Au and Ti. However, the morphology does not significantly worsen after anneals at 450°C. This metallization scheme looks very attractive for the n-electrode of ZnO-based light-emitting diode structures.

Electronic properties of H and D doped ZnO epitaxial films

ZnO epitaxial films grown by pulsed laser deposition in an ambient of H2 or D2 exhibit qualitatively different electronic properties compared to films grown in vacuum or O2 or bulk single crystals annealed in H2. These include temperature-independent resistivities of ∼0.1Ωcm, carrier (electron) concentrations in the 1018cm−3 range, mobilities of 20–40cm2∕Vs, and negligible (a few meV) activation energies for conduction. These transport properties are consistent with H (D) forming an ultrashallow donor or conduction band states not achievable by postgrowth annealing in H2.

A study of H and D doped ZnO epitaxial films grown by pulsed laser deposition

We examine the crystal structure and electrical and optical properties of ZnO epitaxial films grown by pulsed laser deposition in a H2 or D2 ambient. n-type electrical conductivity is enhanced by three orders of magnitude as a result of growing in H2 (D2) compared to ZnO films grown in O2. Hall effect measurements reveal very small carrier activation energies and carrier concentrations in the mid-1018 cm−3 range. Optical absorption measurements show that the enhanced conductivity is not a result of ZnO reduction and interstitial Zn formation. Photoluminescence spectra suggest excitonic emission associated with exciton-hydrogen donor complex formation and show no evidence for midgap emission resulting from defects. We have modeled the transport properties of H (D) doped ZnO films using variable range hopping and surface layer conductivity models, but our data do not fit well with these models. Rather, it appears that growth in H2 (D2) promotes the formation of an exceedingly shallow donor state not seen in...

Low-resistance Au and Au∕Ni∕Au Ohmic contacts to p-ZnMgO

Both electron-beam-deposited Au and Au∕Ni∕Au are found to produce low-resistance Ohmic contacts (ρC in the range 2.5×10−5–7.6×10−6Ωcm2) to p-type ZnMgO (p∼1016cm−3) on sapphire substrates after annealing at 600°C. The as-deposited contacts are rectifying in both cases. For the Au contact, the annealing produces a small amount of outdiffusion of Zn to the surface of the contact, whereas with the Au∕Ni∕Au, both Ni and Zn are found on the surface. In both cases, the formation of Zn vacancies may play a role in increasing the near-surface hole concentration in the ZnMgO.

Properties of phosphorus-doped zinc oxide films grown by pulsed laser deposition

Electrical and chemical bonding properties of P-doped ZnO thin films grown by pulsed laser deposition on sapphire substrates were systematically characterized utilizing the Hall effect and x-ray photoelectron spectroscopy (XPS) measurements. Oxygen growth pressure and postannealing processing play a great role in the properties of these films. Increasing oxygen growth pressure from 5 to 20 Pa enhanced the resistivity of P-doped ZnO films by three orders of magnitude. P-doped ZnO films grown at 700 °C under 20 Pa O2 exhibited p-type conductivity with hole concentration of 5×1017 cm−3 and hole mobility of 0.3 cm2/V s. Rapid thermal annealing processing decreased the electron density in the P-doped ZnO films. XPS binding energies of P 2s and 2p peaks showed formation of P–O bonds which increased with oxygen pressure in the films. This indicates formation of defect complexes of P dopants occupying zinc sites PZn and zinc vacancies VZn in the P-doped ZnO films.

Comparison of Ti∕Al∕Pt∕Au and Ti∕Au Ohmic contacts on n-type ZnCdO

A comparison of Ti∕Au and Ti∕Al∕Pt∕Au Ohmic contacts on n-type Zn0.05Cd0.95O layers grown on ZnO buffer layers on GaN/sapphire templates showed a minimum contact resistivity of 2.3×10−4Ωcm2 at 500°C anneal temperature for Ti∕Al∕Pt∕Au and 1.6×10−4Ωcm2 at 450°C for Ti∕Al. The morphology of the Ti∕Al∕Pt∕Au contacts showed much better thermal stability and remained smooth until at least 450°C, whereas the Ti∕Au contacts show a reacted appearance after 350°C anneals. Auger electron spectroscopy depth profiling of the contact schemes as a function of anneal temperature suggests that the formation of TiOx phases that induce oxygen vacancies in the ZnCdO are responsible for the improved contact resistance after annealing in both types of metal schemes.

Quantitative Determination of Deuterium Atom Concentration in Zinc Oxide Thin Films by Time‐of‐Flight Secondary Ion Mass Spectrometry

Time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) was used to quantitatively determine deuterium (D) concentration in pulsed‐laser deposited (PLD) zinc oxide (ZnO) thin films on Al2O3 substrates. Dual‐beam analysis technique was applied, where the sputtering ion source was Cs+ and the analysis ion source was Bi3+. Standard samples were prepared by implanting D+ ions into a high purity ZnO single crystal. Detection limit of ToF‐SIMS instrument for D atoms in ZnO film was studied, and it was found to be about 5×1016 atoms/cm3. In addition, our data show that it is very difficult to characterize low concentration of H atoms by normal ToF‐SIMS due to H background interference.

Self-assembled transparent a-IGZO based TFTs for flexible sensing applications

In this work, transparent amorphous InGaZnO (a-IGZO) thin films have been deposited on quartz glass substrates using RF magnetron sputtering at room temperature at the O2 flow rate from 0 to 4 sccm. Electrical transport properties and optical transmittance of the a-IGZO films strongly depend on O2/Ar flow rate ratio. Electrical conductivity of the a- IGZO films dramatically decreases from semiconducting to semi-insulating with increasing the O2 flow rate above 1 sccm. Optical transmittance of the a-IGZO films, however, has been improved up to 85% by increasing O2 flow rate. Amorphous IGZO-based thin film transistors have been developed on indium-tin-oxide glass substrates with the phosphorus doped nanogranular SiO2 film of ~800 nm deposited by plasma-enhanced chemical vapor deposition at room temperature as the gate dielectric layer. The accumulation of protons at the IGZO/SiO2 interface induces a large electricdouble- layer capacitance. Enhancement-mode IGZO/SiO2/ITO TFT devices with field effect mobility of 10.5 cm2/Vs and low threshold voltage of 0.7 V have been achieved. Furthermore, a-IGZO TFT devices exhibit subthreshold swing of 70 mV/dec and Ion/off of 1.5x106. Optical transmission spectra show that the a-IGZO thin film transistors arrays on glass substrates is highly transparent with optical transmittance above 80%.

Effect of argon annealing of phosphorus-doped ZnO and (Zn,Mg)O thin-films grown pulsed laser deposition

The transport and annealing properties of phosphorus-doped (Zn,Mg)O thin films grown via pulsed laser deposition (PLD) are studied. The electron carrier concentration for (Zn,Mg)O:P films decreases with increasing deposition and Ar annealing temperature. All the films exhibit good crystallinity with c-axis orientation. This result indicates the importance of activation of the P dopant in (Zn,Mg)O:P films. The as-deposited ZnO:P film properties show less dependence on the deposition growth temperatures. The resistivity of the (Zn,Mg)O:P films is significantly higher than the ZnO:P films grown under similar conditions, indicating separation of the conduction band edge relative to the defect donor state. The annealed ZnO:P films are n-type with resistivity dependent on annealing temperature.