Robert Heinhold

Method of choice for fabrication of high-quality ZnO-based Schottky diodes

We present a comprehensive comparison of electrical properties of differently fabricated high quality Schottky contacts on ZnO thin films grown by pulsed laser deposition. Thermally evaporated Pd/ZnO Schottky contacts exhibit ideality factors as low as 1.06 due to their high lateral homogeneity. The effective Richardson constant determined using these homogeneous contacts is (7.7±4.8)A cm−2 K−2 close to the theoretical value of 32 A cm−2 K−2. However, their rectification ratio is at most five orders of magnitude due to their comparably small barrier height ( ≈0.7eV). The largest effective barrier height ( 1.11 eV) and rectification ratio ( 7×1010) was obtained for reactively sputtered PdOx/ZnO Schottky contacts. Eclipse pulsed laser deposited IrOx/ZnO Schottky contacts were found to combine very good lateral homogeneity ( n≈1.1), with a reasonably large barrier height ( 0.96 eV) and large rectification ratio ( ≈9 orders of magnitude). Our results for differently fabricated Schottky contacts suggest that t...

Optical and defect properties of hydrothermal ZnO with low lithium contamination

The removal of lithium and other group I contaminants from hydrothermally grown ZnO results in significant changes in its electrical, optical, and device characteristics. A significant reduction in donor compensation allows the fabrication of low series resistance Schottky contacts with extremely high rectification ratios and also quenches 4 K photoluminescence emission from excitons bound to ionized donors. Three new electron traps with activation energies of 115, 160, and 190 meV are created along with an increase in the activation energy of the dominant shallow donor from 50 to 70 eV which is associated with an accompanying removal of aluminum.

Comparative study of deep defects in ZnO microwires, thin films and bulk single crystals

Electrical properties of a ZnO microwire grown by carbo-thermal evaporation, a ZnO thin film grown by pulsed-laser deposition and a hydrothermally grown ZnO bulk crystal are compared. Deep defects were investigated by means of deep-level transient spectroscopy. The defect level E3 was observed in all samples investigated. Additionally, a defect labelled T2 that preferentially forms under Zn-rich condition was detected in the microwire, the thin film and the bulk sample. Our results indicate that V Zn is likely involved in this defect.

Polarity effects in the optical properties of hydrothermal ZnO

Significant polarity-related differences in the near-band-edge photoluminescence from the Zn-polar and O-polar faces of hydrothermally grown ZnO single crystals, particularly in the ionized donor bound and free exciton recombination regions, were initially enhanced and then extinguished on annealing in oxygen at 400 °C and 600 °C, respectively. Polarity effects were also observed in the defect band emission with a structured green band associated with deep copper acceptor impurities appearing at lower annealing temperatures on the O-polar face. The loss of hydrogen is implicated in both these behaviors and in a sudden semiconductor-to-insulator transition between 200–300 °C.

The presence of a (1×1) oxygen overlayer on ZnO(0001) surfaces and at Schottky interfaces

The atomic surface and interface structures of uncoated and metal-coated epi-polished ZnO(0001) Zn-polar wafers were investigated via surface x-ray diffraction. All uncoated samples showed the presence of a fully occupied (1 × 1) overlayer of oxygen atoms located at the on-top position above the terminating Zn atom, a structure predicted to be unstable by several density functional theory calculations. The same oxygen overlayer was clearly seen at the interface of ZnO with both elemental and oxidized metal Schottky contact layers. No significant atomic relaxations were observed at surfaces and interfaces processed under typical device fabrication conditions.

Mobility of indium on the ZnO(0001) surface

The mobility of indium on the Zn-polar (0001) surface of single crystal ZnO wafers was investigated using real-time x-ray photoelectron spectroscopy. A sudden transition in the wettability of the ZnO(0001) surface was observed at ∼520 °C, with indium migrating from the ( 0001¯) underside of the wafer, around the non-polar ( 11¯00) and ( 112¯0) sidewalls, to form a uniform self-organized (∼20 A) adlayer. The In adlayer was oxidized, in agreement with the first principles calculations of Northrup and Neugebauer that In2O3 precipitation can only be avoided under a combination of In-rich and Zn-rich conditions. These findings suggest that unintentional In adlayers may form during the epitaxial growth of ZnO on indium-bonded substrates.

Characterisation and device applications of Zn x Mg 1−x O films grown by pulsed laser deposition

Zn<inf>x</inf>Mg<inf>1−x</inf>O films have been deposited using pulsed laser deposition (PLD) onto a-plane sapphire substrates and incorporated into ultraviolet sensors based on IrO/ Zn<inf>x</inf>Mg<inf>1−x</inf>O Schottky barrier contacts. Electron diffraction showed that the films were highly crystalline with low surface roughness. Atomic force micros-copy showed this roughness to be &#60;2% of the film thickness. The IrO<inf>x</inf>/Zn<inf>x</inf>Mg<inf>1−x</inf>O Schottky devices exhibited good rectification (barrier heights ∼0.88 eV and ideality factors ∼1.7) and UV sensing capability.