Hyuck Soo Yang

Fabrication of Hybrid n-ZnMgO/n-ZnO/p-AlGaN/p-GaN Light-Emitting Diodes

We report on the fabrication of UV light-emitting diodes (LEDs) based on a p–n junction n-ZnMgO/n-ZnO/ p-AlGaN/ p-GaN semiconductor triple-heterostructure. Radio-frequency plasma-assisted molecular beam epitaxy was used to grow the complete heterostructure on p-AlGaN/ p-GaN c-plane sapphire templates. Cross-sectional transmission electron microscopy showed single-crystal quality of the pseudomorphically grown ZnO active region of the device. The LEDs were fabricated by a process involving both wet and dry etching. Electroluminescence emission most likely associated with ZnO excitonic transition was observed up to 370°C. The results show the potential of ZnO-based materials for UV emitters of potentially lower cost and with comparable or higher emission intensity than comparable AlGaN/GaN devices.

Ti∕Au n-type Ohmic contacts to bulk ZnO substrates

Electron-beam-deposited Ti∕Au ohmic contacts on undoped (n∼1017cm−3) bulk ZnO substrates exhibited as-deposited specific contact resistivity of 3×10−4Ωcm2, regardless of the polarity (Zn face or O face) of the ZnO substrate. The annealing environment (air or N2) also had no significant effect on contact properties. The specific contact resistivity slightly decreased after annealing at 300 °C but started to increase above 350 °C. The measurement temperature dependence of specific contact resistivity revealed that the dominant current transport mechanism is field emission even in the moderately doped ZnO. As the annealing temperature increased, some voids were observed on the metal surface, possibly due to reaction of Ti∕Au metallization and the evaporation of the oxygen from the ZnO substrate.

Low-resistance ohmic contacts to p-ZnMgO grown by pulsed-laser deposition

Electron-beam deposited Ni∕Au is found to produce ohmic contacts to p-type ZnMgO (p∼1016cm−3) after annealing in the range 300–450°C. A minimum specific contact resistance of ∼10−2Ωcm2 at room temperature and of 4×10−5Ωcm2 at 473K from circular transmission line measurements was obtained after annealing at 350°C for 1min in air. Higher anneal temperatures produced higher contact resistances and reaction of the contact metallurgy with diffusion of Ni to the surface of the Au layer where it became oxidized. The effective barrier height obtained from the measurement temperature dependence of the specific contact resistance, assuming thermionic emission, showed a value of 0.39±0.01eV, much lower than the theoretical value of ∼2.4eV. The difference in theoretical and experimental values is a clear indication of the strong role played by surface states, as reported earlier for Au Schottky contacts on ZnO.

Effect of inductively coupled plasma damage on performance of GaN–InGaN multiquantum-well light-emitting diodes

InGaN multiquantum-well light-emitting diodes (LEDs) in the form of unpackaged die with emission wavelengths from 420to505nm were exposed to either Ar or H2 inductively coupled plasmas as a function of both rf chuck power (controlling incident ion energy) and source power (controlling ion flux). The forward turn-on voltage is increased by both types of plasma exposure and is a function of both the incident ion energy and flux. The reverse bias current in the LEDs is much larger in the case of H2 plasma exposure, indicating that preferential loss of nitrogen leads to increased surface leakage. The current transport in the LEDs is dominated by generation-recombination (ideality factor ∼2) both before and after the plasma exposures.

Role of Ion Energy and Flux on Inductively Coupled Plasma Etch Damage in InGaN/GaN Multi Quantum Well Light Emitting Diodes

The effect of inductive coupled plasma (ICP) etching of GaN light-emitting diodes (LEDs) on the device performance was investigated. InGaN/GaN multi-quantum well LEDs with emission wavelengths 420 nm were fabricated using Cl2/Ar ICP etching to form the mesa for the n-contact. Different rf chuck powers (controlling incident ion energy) and source powers (controlling ion flux) were used to examine their effect on junction leakage current. At high ion fluxes (source powers > 500 W), the junctions were very heavily damaged. The forward turn-on voltage was increased by using higher ion energies from 4.55 V for 120 eV ion energy to 5.1 V for 300 eV. The reverse bias current was much less sensitive to the ion energy during plasma etching and increased only above an average ion energy of ~300 eV. The current transport in the LEDs is dominated by generation-recombination and surface leakage components (ideality factor ≥2) in all cases.

Ferromagnetism of anatase Ti1−XCoXO2−δ films grown by ultraviolet-assisted pulsed laser deposition

We report the effect of ultraviolet irradiation during pulsed laser deposition on the growth of cobalt-doped TiO2 films in terms of the crystalline and magnetic properties. Due to more active oxygen gas species created by photodissociation and enhanced atomic migration on the substrate surface, highly crystalline films could be obtained at a relatively low deposition temperature of 400 °C. The full width at half maximum of the (004) peak rocking curve was 0.46° for the UV-assisted film deposited at 400 °C on the LaAlO3(001) substrate. Fourfold symmetry was observed in phi scans of the Ti0.93Co0.07O2−δ film, which indicated that this film was grown in cube-on-cube epitaxial orientation with the [001] normal to the LaAlO3(001) substrate surface. The temperature dependence of the zero-field-cooled/field-cooled magnetization was examined and the hysteresis was clearly identified by the nonzero difference between them. The Curie temperature was estimated to be well over 350 K, and at room temperature the coerc...

Growth and properties of Co-doped TiO2 thin films grown on buffered Si substrate

Epitaxial growth of a Co-doped anatase TiO2 thin film on silicon has been achieved by using SrTiO3/TiN bilayers as a buffer. All layers were sequentially formed by pulsed laser deposition. X-ray φ scan and selective area electron diffraction patterns revealed that SrTiO3/TiN buffer layers were grown with a cube-on-cube orientation relationship with silicon. Ferromagnetism was observed at room temperature with a coercivity of around 607 Oe. However, the ferromagnetism is attributed to the highly cobalt enriched TiO2 clusters which nucleate after a certain growth of film, as observed by atomic/magnetic force microscopy as well as cross-sectional transmission electron microscopy.

Enhanced Crystalline and Magnetic Properties of Co-Doped TiO2 Films Grown by Ultraviolet-Assisted Pulsed Laser Deposition

Using in situ ultraviolet irradiation during pulsed laser deposition (UVPLD), enhanced magnetic properties (larger remanence and coercivity) were obtained in Co-doped anatase TiO 2 films. The UVPLD grown films exhibited smaller full width at half-maximum value for the (004) peak of the Co-doped TiO 2 layer, indicating higher crystallinity. No superparamagnetic behavior was examined in the zero field cooled/field cooled measurements, suggesting an intrinsic diluted magnetic semiconductor characteristic. X-ray photoelectron spectroscopy results indicated that the cobalt in the films was in the +2 formal oxidation state and a much stronger binding coordination between oxygen and cations (Co and Ti) was inferred in the UVPLD grown films.

Structural and magnetic properties of cobalt-doped rutile thin films deposited on TiN–Si

Highly oriented Co-doped rutile TiO2 thin film on silicon has been achieved by using a TiN buffer layer. The direct deposition of TiO2:Co film on TiN–Si yields (110)-oriented film structure having two equivalent structural domains which are perpendicular to each other. Interestingly, no ferromagnetism is observed in this film structure down to 10K, the reason for which is attributed to the formation of cobalt silicide in the silicon. On the other hand, when the TiO2:Co film is deposited on TiO2, formed by in situ oxidation of TiN–Si, cobalt appears highly mobile leading to segregation at the TiO2–Si interface along with the formation of cobalt clusters in the TiO2:Co film region. This is responsible for the ferromagnetism in this case.

Fabrication of p-n junctions in as-grown ZnMgO/ZnO films

We achieved p-(Zn,Mg)O by doping with phosphorous and the conduction type was confirmed by capacitance-voltage properties of metal/insulator/p-(Zn,Mg)O:P diode structures as well as Hall measurements. The p-(Zn,Mg)O:P/n-ZnO junction was grown by pulsed laser deposition on bulk ZnO doped with Sn. Without post-growth annealing, the phosphorous-doped ZnMgO showed p-type conductivity (hole density ~1016 cm-3, mobility ~6 cm2V-1s-1) in the as-grown state. The metal contacts in top-to-bottom p-n junctions were made with Ni/Au as the p-ohmic and Ti/Au as the backside n-ohmic contact. The p-contacts showed improved characteristics after annealing up to 350 - 400 °C, but the n-contacts were ohmic as-deposited. The simple, low temperature growth (≤500 °C) and processing sequence (≤400 °C) shows the promise of ZnO for applications such as low-cost UV light emitters and transparent electronics.