Emre Gür

Molecular beam epitaxy of N-polar InGaN

We report on the growth of N-polar InxGa1−xN by N2 plasma-assisted molecular beam epitaxy. Ga-polar and N-polar InGaN films were grown at different growth temperatures and the composition was estimated by photoluminescence (PL) measurements. A growth model that incorporates the incoming and desorbing atomic fluxes is proposed to explain the compositional dependence of InGaN on the flux of incoming atomic species and growth temperature. The growth model is found to be in agreement with the experimental data. The peak PL intensity for N-face samples is found to exhibit a two order of magnitude increase for a 100 °C increase in growth temperature. Besides, at 600 nm, the N-face sample shows more than 100 times higher PL intensity than Ga-face sample at comparable wavelengths indicating its superior optical quality. The understanding of growth kinetics of InGaN presented here will guide the growth of N-polar InGaN in a wide range of growth temperatures.

High-temperature Schottky diode characteristics of bulk ZnO

Current?voltage (I?V) measurements of Ag/n-ZnO have been carried out at temperatures of 200?500?K in order to understand the temperature dependence of the diode characteristics. Forward-bias I?V analysis results in a Schottky barrier height of 0.82?eV and an ideality factor of 1.55 at room temperature. The barrier height of 0.74?eV and Richardson constant of 0.248?A?K?2?cm?2 were also calculated from the Richardson plot, which shows nearly linear characteristics in the temperature range 240?440?K. From the nkbT/q versus kbT/q graph, where n is ideality factor, kb the Boltzmann constant, T the temperature and q the electronic charge we deduce that thermionic field emission (TFE) is dominant in the charge transport mechanism. At higher sample temperatures (>440?K), a trap-assisted tunnelling mechanism is proposed due to the existence of a deep donor situated at Ec?0.62?eV with 3.3 ? 10?15?cm2 capture cross section observed by both deep-level transient spectroscopy (DLTS) and lnI0 versus 1/kbT plots. The ideality factor almost remains constant in the temperature range 240?400?K, which shows the stability of the Schottky contact in this temperature range.

Growth model for plasma-assisted molecular beam epitaxy of N-polar and Ga-polar InxGa1−xN

The authors have developed a comprehensive model for the growth of N-polar and Ga-polar InxGa1−xN by N2 plasma-assisted molecular beam epitaxy. GaN films of both polarities were coloaded and InxGa1−xN was grown in the composition range of 0.14<x<0.59 at different growth temperatures keeping all other conditions identical. The compositions were estimated by triple-axis ω-2θ x-ray diffraction scans as well as by room temperature photoluminescence measurements. The dependence of the In composition x in InxGa1−xN on growth temperature and the flux of incoming atomic species is explained using a comprehensive growth model which incorporates desorption of atomic fluxes as well as decomposition of InN component of InxGa1−xN. The model was found to be in good agreement with the experimental data for InxGa1−xN of both polarities. A N-polar In0.31Ga0.69N/In0.05Ga0.95N multi-quantum-well structure grown with conditions predicted by our growth model was found to match the compositions of the active layers well beside...

N-Polar III–Nitride Green (540 nm) Light Emitting Diode

We report the demonstration of a N-polar InGaN based green light emitting diode (LED) grown by N2 plasma-assisted molecular beam epitaxy (PAMBE). High quality multiple quantum well LEDs with In0.29Ga0.71N quantum wells were grown at a temperature of 600 °C by applying a new growth model. LED structures exhibited green emission, and electroluminescence measurements on the test structure showed peak emission wavelengths varying from 564.5 to 540 nm. The full width at half-maximum reduced from 74 to 63 nm as the drive current was increased to 180 A/cm2. This work is the first demonstration of an N-polar LED with emission in the green wavelength range.

Nanoporous ZnO photoelectrode for dye-sensitized solar cell

Nanoporous and macroporous structures were prepared by using self-assembled monolayer (SAM) onto ZnO thin films in order to investigate the efficiency of dye-sensitized solar cells (DSSCs) produced using these films. Using SAM on ZnO thin films, it is obtained successfully assembled large-area, highly ordered porous ZnO thin films. Varying nanoporous radius is observed between 20 and 50 nm sizes, while it is 500-800nm for macroporous radius. The solar conversion efficiency of 2.75% and IPCE of 29% was obtained using ZnO nanoporous/N719 dye/I-/I-3 electrolyte, while macroporous ZnO given solar conversion efficiency of 2.22% and IPCE of 18%.

Electrochemical growth of n-ZnO onto the p-type GaN substrate: p-n heterojunction characteristics

n-ZnO thin films were deposited electrochemically onto the p-GaN/Al2O3 substrate in order to form hetero p-n junction. X-ray diffraction measurement has been showed clearly (0002) c-axis orientation of grown ZnO thin film. Absorption measurements were carried out before and after growth process indicating both sharp absorption edges of GaN and ZnO thin films. Photoluminescence measurement shows n-ZnO film grown on p-GaN has a dominant emission at 2.8 eV. I-V characteristic of n-ZnO/p-GaN/Al2O3 heterojunction showed that almost five order of rectification has been achieved. Turn on voltages of the p-n heterojunction is found to be 1.12 V.n-ZnO thin films were deposited electrochemically onto the p-GaN/Al2O3 substrate in order to form hetero p-n junction. X-ray diffraction measurement has been showed clearly (0002) c-axis orientation of grown ZnO thin film. Absorption measurements were carried out before and after growth process indicating both sharp absorption edges of GaN and ZnO thin films. Photoluminescence measurement shows n-ZnO film grown on p-GaN has a dominant emission at 2.8 eV. I-V characteristic of n-ZnO/p-GaN/Al2O3 heterojunction showed that almost five order of rectification has been achieved. Turn on voltages of the p-n heterojunction is found to be 1.12 V.

Deep levels in a-plane, high Mg-content MgxZn1-xO epitaxial layers grown by molecular beam epitaxy

Deep level defects in n-type unintentionally doped a-plane MgxZn1−xO, grown by molecular beam epitaxy on r-plane sapphire were fully characterized using deep level optical spectroscopy (DLOS) and related methods. Four compositions of MgxZn1−xO were examined with x = 0.31, 0.44, 0.52, and 0.56 together with a control ZnO sample. DLOS measurements revealed the presence of five deep levels in each Mg-containing sample, having energy levels of Ec − 1.4 eV, 2.1 eV, 2.6 V, and Ev + 0.3 eV and 0.6 eV. For all Mg compositions, the activation energies of the first three states were constant with respect to the conduction band edge, whereas the latter two revealed constant activation energies with respect to the valence band edge. In contrast to the ternary materials, only three levels, at Ec − 2.1 eV, Ev + 0.3 eV, and 0.6 eV, were observed for the ZnO control sample in this systematically grown series of samples. Substantially higher concentrations of the deep levels at Ev + 0.3 eV and Ec − 2.1 eV were observed in ZnO compared to the Mg alloyed samples. Moreover, there is a general invariance of trap concentration of the Ev + 0.3 eV and 0.6 eV levels on Mg content, while at least and order of magnitude dependency of the Ec − 1.4 eV and Ec − 2.6 eV levels in Mg alloyed samples.

Detailed characterization of deep level defects in InGaN Schottky diodes by optical and thermal deep level spectroscopies

Schottky diode properties of semitransparent Ag(4 nm)/Au(4 nm) metal stack on In0.2Ga0.8N were investigated and defect characterization was performed using capacitance deep level transient (DLTS) and optical spectroscopy (DLOS). DLTS measurements made on the In0.2Ga0.8N Schottky diodes, which displayed a barrier height of 0.66 eV, revealed the presence of two deep levels located at Ec-0.39 eV and Ec-0.89 eV with nearly identical concentrations of ∼1.2 × 1015 cm−3. Three deeper defect levels were observed by DLOS at Ec-1.45 eV, Ec-1.76 eV, and Ec-2.50 eV with concentrations of 1.3 × 1015cm−3, 3.2 × 1015cm−3, and 6.1 × 1016 cm−3, respectively. The latter, with its high trap concentration and energy position lying 0.4 eV above the valance band, suggests a possible role in compensation of carrier concentration, whereas the mid-gap positions of the other two levels imply that they will be important recombination-generation centers

High energy electron irradiation effects on electrical properties of Au/n-ZnO Schottky diodes

High energy electron irradiation (HEEI) was performed on Au/n-ZnO Schottky diodes (SDs) and the effects of irradiation were compared with a reference SD. Current–voltage and capacitance–voltage measurements revealed that the barrier height and donor concentration decrease from 0.746 to 0.665 eV and from 4.55 × 1014 cm−3 to 1.76 × 1014 cm−3, respectively, while the ideality factor increases from 1.61 to 3.95 after irradiation. Ionization temperatures of traps were observed by means of thermally stimulated capacitance measurements at temperatures 307 K, 365 K and 332 K, 385 K and 477 K for the irradiated and the reference SDs, respectively. Deep level transient spectroscopy measurements revealed a defect level at 870 meV and capture cross sections of 0.88 × 1012 cm2 for the reference SD and two HEEI induced defects at energies 670 and 780 meV and capture cross sections of 29.6 × 10−12 cm2 and 3.08 × 10−12 cm2 for the irradiated SD, respectively.

Temperature-dependent electrical characterization of nitrogen-doped ZnO thin film: vacuum annealing effect

Temperature-dependent Hall effect measurements were carried out at an N-doped ZnO thin film grown by the reactive sputtering method onto (001) Si substrate before and after being vacuum annealed at 900 °C. p-Type ZnO thin film was obtained with a relatively high mobility of ~60 cm2 V−1 s−1, a high carrier concentration of 2.5×1017 cm−3 and a low resistivity of 0.4 Ω cm. After vacuum annealing, the temperature dependence of electrical parameters such as mobility and carrier concentration showed highly different characteristics. Time-resolved photoluminescence (TRPL), PL and x-ray diffraction measurements (XRD) were performed after the annealing process to check whether the high-temperature annealing can remove the ZnO film on Si or not. The PL measurement shows band-to-band recombination at 360 nm and TRPL shows the exciton recombination lifetime to be 571.7 ps. The XRD measurement reveals highly preferred c-axis (0002) orientation. Activation energies were calculated using the ln σ versus 1000 T−1 plot to be 20 meV for the as-grown and 24 and 6.8 meV after the vacuum annealing process.