Nickel-doped zinc oxide (ZnNiO) was grown on sapphire by metal organic chemical vapor deposition (MOCVD) with varying Ni content under two growth conditions of 400°C/100 Torr and 450°C/30 Torr. Elemental composition indicated that Ni could occupy Zn and O/interstitial sites in ZnNiO. Ni-doping in ZnO resulted in shifts in X-ray diffraction (002) peak, and introduced a (111) phase. Absorption spectrum showed a reduction in near band edge with Ni content in both the samples’ sets. Samples grown at 400°C/100 Torr had a band gap reduction from 3.276 eV to 3.269 eV, while those synthesized at 450°C/30 Torr showed reduction from 3.287 eV to 3.260 eV. The bandgap reduction rate was influenced by growth conditions, and sites activated for Ni incorporation during the growth. Nickel could introduce shallow energy states near the valence band in ZnNiO, and result in a reduction in the bandgap. A potential for bandgap tunability, and controllable introduction of energy states in zinc oxide with transition metal doping by MOCVD, could widen the application range of zinc oxide-based materials for energy harvesting and electronics.
Zinc oxide (ZnO) is an earth abundant wide bandgap semiconductor of great interest in the recent years. ZnO has many unique properties, such as non-toxic, large direct bandgap, high exciton binding energy, high transparency in visible and infrared spectrum, large Seebeck coefficient, high thermal stability, high electron diffusivity, high electron mobility, and availability of various nanostructures, making it a promising material for many applications. The growth techniques of ZnO is reviewed in this work, including sputtering, PLD, MOCVD and MBE techniques, focusing on the crystalline quality, electrical and optical properties. The problem with p-type doping ZnO is also discussed, and the method to improve p-type doping efficiency is reviewed. This paper also summarizes the current state of art of ZnO in thermoelectric and photovoltaic applications, including the key parameters, different device structures, and future development.
ZnO-based materials show promise in energy harvesting applications, such as piezoelectric, photovoltaic and thermoelectric. In this work, ZnO-based vertical Schottky barrier solar cells were fabricated by MOCVD de- position of ZnO thin films on ITO back ohmic contact, while Ag served as the top Schottky contact. Various rapid thermal annealing conditions were studied to modify the carrier density and crystal quality. Greater than 200 nm thick ZnO films formed polycrystalline crystal structure, and were used to demonstrate Schottky solar cells. I-V characterizations of the devices showed photovoltaic performance, but but need further development. This is the first demonstration of vertical Schottky barrier solar cell based on wide bandgap ZnO film. Thin film and bulk ZnO grown by MOCVD or melt growth were also investigated in regards to their room- temperature thermoelectric properties. The Seebeck coefficient of bulk ZnO was found to be much larger than that of thin film ZnO at room temperature due to the higher crystal quality in bulk materials. The Seebeck coefficients decrease while the carrier concentration increases due to the crystal defects caused by the charge carriers. The co-doped bulk Zn0:96Ga0:02Al0:02O showed enhanced power factors, lower thermal conductivities and promising ZT values in the whole temperature range (300-1300 K).
This work describes a band-engineered transverse thermoelectric with p-type Seebeck in one direction and ntype orthogonal, with off-diagonal terms that drive heat flow transverse to electrical current. Such materials are named p × n type transverse thermoelectrics. Whereas thermoelectric performance is normally limited by the figure of merit ZT, p × n type materials can be more easily geometrically shaped and integrated for devices, leading to more compact, longer lifetime, enhanced efficiency coolers for infrared detectors or photovoltaic generators.
We used the two-wire 3ω method to measure the in-plane and out-of-plane thermal conductivity of thin
films and analyzed the error for all fitting parameters. We find the heater half-width, the insulating layer
thickness and the out-of-plane thermal conductivity of the insulating layer the most sensitive parameters in
an accurate fitting. The data of a 2.5 μm GaAs thin film suggests that the phonon mean free path in the
film is limited to the film thickness, far shorter than that in the bulk material at low temperatures.
We introduce a power-law approximation to model non-linear ranges of the thermal conductivity, and under
this approximation derive a simple analytical expression for calculating the temperature profile in high power
quantum cascade lasers and light emitting diodes. The thermal conductivity of a type II InAs/GaSb superlattice
(T2SL) is used as an example, having negative or positive power-law exponents depending on the thermal range
of interest. The result is an increase or decrease in the temperature, respectively, relative to the uniform thermal