W.K. Chim

Growth of Cu2O on Ga-doped ZnO and their interface energy alignment for thin film solar cells

Cu2O thin films are deposited by direct current reactive magnetron sputtering on borofloat glass and indium tin oxide (ITO) coated glass at room temperature. The effect of oxygen partial pressure on the structures and properties of Cu2O thin films are investigated. We show that oxygen partial pressure is a crucial parameter in achieving pure phases of CuO and Cu2O. Based on this finding, we fabricate heterojunctions of p-type Cu2O with n-type gallium doped ZnO (GZO) on ITO coated glass substrates by pulsed laser deposition for GZO thin films. The energy band alignment for thin films of Cu2O/GZO on ITO glass is characterized using high-resolution x-ray photoelectron spectroscopy. The energy band alignment for the Cu2O/GZO heterojunctions is determined to be type II with a valence band offset of 2.82 eV and shows negligible effects of variation with gallium doping. The higher conduction band of the Cu2O relative to that of GZO in the obtained band alignment shows that the heterojunctions are suitable for so...

Laser-induced nano-oxidation on hydrogen-passivated Ge (100) surfaces under a scanning tunneling microscope tip

Laser-induced nano-oxidation on hydrogen-passivated Ge (100) surfaces under a scanning tunneling microscope tip in air has been investigated. A 532 nm Nd:YAG pulsed laser with a pulse duration of 7 ns was used. A 2×2 oxide dot array with dot sizes between 20 and 30 nm and an oxide single line with a width less than 30 nm have been created using an electrochemical-etched tungsten tip under laser irradiation. The modified regions were characterized by atomic force microscope. The apparent depth of oxide layer as a function of laser intensity has been studied. The advantages and drawbacks of using a continuous wave laser and a pulsed laser will be discussed.

Role of oxygen for highly conducting and transparent gallium-doped zinc oxide electrode deposited at room temperature

In this work, we found that a desirable amount of oxygen can reduce defect related scattering in enhancing carrier mobility for pulsed laser deposited zinc oxide. However, excessive oxygen can lead to formation of oxygen interstitials that can act as compensation or scattering centers. At higher oxygen pressures, structural changes that increase grain boundary scattering prove to be very important. We introduce a simple transparency index to quantify the transmission of the thin films for usage as electrodes in photovoltaic devices. An excellent resistivity of ∼3.9×10−4u2002Ωu2009cm and an electron mobility of ∼19.2u2002cm2/Vu2009s with a transparency index of 0.84 (84% of total solar spectrum transmitted) were achieved at room temperature suggesting possible applications in plastic devices.

Conductance-voltage measurements on germanium nanocrystal memory structures and effect of gate electric field coupling

Conductance-voltage (G-V) analyses were performed on trilayer germanium (Ge) nanocrystal memory capacitor structures, consisting of a high dielectric constant (high-κ) layer (5nm thick) grown on silicon, a sputtered Ge middle layer (4nm thick), and a 20nm thick sputtered cap oxide layer (either SiO2 for moderate gate electric field coupling or HfAlOx for better electric field coupling). Comparisons of the G-V characteristics were performed with a control capacitor sample without nanocrystals. The distinctive characteristics due to nanocrystals could be separated and identified from the interface traps provided the memory structure has sufficiently high electric field coupling from the gate applied voltage, resulting in a large electric field across the tunnel dielectric layer. This work attempts to provide an explanation to the G-V characteristics under the following three conditions: (1) interface trap dominated, (2) nanocrystal dominated, and (3) a combination of effects from both interface traps and na...

Origin of charge trapping in germanium nanocrystal embedded SiO2 system: Role of interfacial traps?

Wet thermal oxidations of polycrystalline Si0.54Ge0.46 films at 600u200a°C for 30 and 50 min were carried out. A stable mixed oxide was obtained for films that were oxidized for 50 min. For film oxidized for 30 min, however, a mixed oxide with Ge nanocrystallites embedded in the oxide matrix was obtained. A trilayer gate stack structure that consisted of tunnel oxide/oxidized polycrystalline Si0.54Ge0.46/rf sputtered SiO2 layers was fabricated. We found that with a 30 min oxidized middle layer, annealing the structure in N2 ambient results in the formation of germanium nanocrystals and the annealed structure exhibits memory effect. For a trilayer structure with middle layer oxidized for 50 min, annealing in N2 showed no nanocrystal formation and also no memory effect. Annealing the structures with 30 or 50 min oxidized middle layer in forming gas ambient resulted in nanocrystals embedded in the oxide matrix but no memory effect. This suggests that the charge storage mechanism for the trilayer structure is clo...

Minimization of germanium penetration, nanocrystal formation, charge storage, and retention in a trilayer memory structure with silicon nitride/hafnium dioxide stack as the tunnel dielectric

Trilayer structures, consisting of a rapid thermal oxide (RTO) layer (2.5 or 5 nm thick) grown on silicon, a sputtered Ge middle layer (3–20 nm thick), and a 50-nm-thick sputtered silicon oxide capping layer, exhibit significant penetration of Ge atoms into the silicon substrate for devices with the smaller (2.5 nm) RTO thickness, resulting in negligible nanocrystal formation and hence no charge storage or memory effect. The Ge penetration is minimized by replacing the RTO layer with a high dielectric constant (high-κ) silicon nitride/hafnium dioxide stack (grown by metalorganic chemical vapor deposition) having a larger physical thickness but smaller equivalent oxide thickness of 1.9 nm. Results show that the high-κ trilayer structure exhibits better charge storage capability (in terms of a lower program voltage) and better charge retention performance as compared to the RTO trilayer structure.

Effects of annealing on the valence band offsets between hafnium aluminate and silicon

In this work, we examine the valence band offset of hafnium aluminate (HfAlO), a material of interest for use as a high dielectric constant (high-k) gate oxide, following postdeposition annealing. It is observed that annealing leads to a change in the band offset between the high-k oxide and the semiconductor. Our results conclusively show that the change is due to the existence of an electrostatic dipole field at the interface between HfAlO and the silicon substrate. This provides evidence to partly account for the observed flatband voltage shifts in high-k dielectric stack structures in the literature. The origin of the dipole field is also discussed in terms of the interfacial permittivity.

Theoretical model of interface trap density using the spread of the differential capacitance characteristics in scanning capacitance microscopy measurements

In this letter, we propose a theoretical model for the calculation of interface trap density (Dit) in a metal-oxide-semiconductor structure using data from scanning capacitance microscopy (SCM) measurements. The model is based on the correlation of Dit with the change in the full width at half maximum of the SCM differential capacitance (dC∕dV) characteristics. The good agreement between the calculated Dit values from the SCM theoretical model and the experimental midgap Dit values obtained from conductance measurements shows the validity of the proposed model. The model opens up possibilities for obtaining the spatial distribution (with nanometers resolution) of interfacial traps on a device using SCM measurements.

Clarifying the origin of near-infrared electroluminescence peaksfor nanocrystalline germanium in metal-insulator-silicon structures

The origin of room-temperature near-infrared electroluminescence (EL) of nanocrystalline germanium (Ge) embedded in oxide was investigated. The nanocrystals were synthesized by partial oxidation of silicon-germanium, Si0.54Ge0.46, films. Under constant current density bias in accumulation and inversion, Ge nanocrystals with diameters of 5 and 10nm exhibit strong luminescence at 1350nm. The 1350nm EL peak is only observed in the presence of elemental Ge and is enhanced with the formation of Ge nanocrystals. The introduction of hydrogen during annealing passivates the dangling bonds at the interfaces of nanocrystals, thus minimizing the energy spread in the 1350nm peak. The 1350nm peak intensity is a function of the injected carrier density, while the peak location remains constant in energy and independent of the applied bias. The results are a clear indication that the luminescence peak originates from radiative recombination of excitons confined in the nanocrystals.The origin of room-temperature near-infrared electroluminescence (EL) of nanocrystalline germanium (Ge) embedded in oxide was investigated. The nanocrystals were synthesized by partial oxidation of silicon-germanium, Si0.54Ge0.46, films. Under constant current density bias in accumulation and inversion, Ge nanocrystals with diameters of 5 and 10nm exhibit strong luminescence at 1350nm. The 1350nm EL peak is only observed in the presence of elemental Ge and is enhanced with the formation of Ge nanocrystals. The introduction of hydrogen during annealing passivates the dangling bonds at the interfaces of nanocrystals, thus minimizing the energy spread in the 1350nm peak. The 1350nm peak intensity is a function of the injected carrier density, while the peak location remains constant in energy and independent of the applied bias. The results are a clear indication that the luminescence peak originates from radiative recombination of excitons confined in the nanocrystals.

Electromagnetic Calculations of the Near Field of a Tip under Polarized Laser Irradiation

A general model of the near field emission for a tip under polarized laser irradiation is established for the case of imaging and processing by a laser-assisted scanning probe microscope (SPM). The near field emission of an actual SPM tip was calculated using the method of moment. The tip-sample geometry was modelled as a cone approaching a surface at the tip apex. Laser-induced current modes in the tip and sample were established in the form of trigonometric functions. This model can be used for rigorous calculations and has been applied to investigate the dependence of the near field on laser wavelength, tip aspect ratios, tip-sample distance and the dielectric properties of both tip and sample.