Tomasz A. Krajewski

ZnO grown by atomic layer deposition: A material for transparent electronics and organic heterojunctions

We report on zinc oxide thin films grown by atomic layer deposition at a low temperature, which is compatible with a low thermal budget required for some novel electronic devices. By selecting appropriate precursors and process parameters, we were able to obtain films with controllable electrical parameters, from heavily n-type to the resistive ones. Optimization of the growth process together with the low temperature deposition led to ZnO thin films, in which no defect-related photoluminescence bands are observed. Such films show anticorrelation between mobility and free-electron concentration, which indicates that low n electron concentration is a result of lower number of defects rather than the self-compensation effect.

Zinc oxide for electronic, photovoltaic and optoelectronic applications

We show that the atomic layer deposition (ALD) technique has great potential for widespread use in the production of ZnO films for applications in electronic, photovoltaic (PV), and optoelectronic devices. The low growth temperature makes ALD-grown ZnO films suitable for fabrication of various semiconductor/organic hybrid structures. This opens up the possibility of novel devices based on very cheap organic materials, including organic light emitting diodes and third-generation PV cells.

The influence of growth temperature and precursors' doses on electrical parameters of ZnO thin films grown by atomic layer deposition technique

In this paper we report on the low-temperature growth (Ts=30-250^oC) of zinc oxide thin films by atomic layer deposition method using two different organic zinc precursors: diethylzinc and (for comparison) dimethylzinc, and deionized water as an oxygen precursor. An evident influence of growth temperature and precursors" doses on electron concentration and Hall mobility of obtained zinc oxide layers is presented. The lowest achieved room-temperature electron concentration was at the level of 10^1^6cm^-^3 with mobility up to 110cm^2/Vs.

Hafnium dioxide as a passivating layer and diffusive barrier in ZnO/Ag Schottky junctions obtained by atomic layer deposition

This paper reports on ZnO/Ag Schottky junctions obtained by the low temperature atomic layer deposition process. Introducing the thin (from 1.25 to 7.5 nm) layer of hafnium dioxide between the ZnO layer and evaporated Ag Schottky contact improves the rectification ratio to about 105 at 2V. For the ZnO/Ag junctions without the HfO2 interlayer, the rectification ratio is only 102. We assign this effect to the passivation of ZnO surface accumulation layer that is reported for ZnO thin films.

Nitrogen doped p-type ZnO films and p-n homojunction

We demonstrate the ZnO homojunction fully obtained by the atomic layer deposition technique at low temperature growth of 100?130 ?C. For the n-type partner of the junction we used undoped ZnO film obtained at 130 ?C, while nitrogen doped ZnO acted as the p-type partner of the junction. Nitrogen was introduced into the ZnO film during the ALD process by using ammonia water as an oxygen precursor and diethylzinc as a zinc precursor. The p-type conductivity of ZnO was activated by the subsequent annealing of the ZnO:N film in an oxygen or nitrogen atmosphere. The initial rectification ratio of 102 at ?2 V was raised to 4???104 by inserting an ultrathin Al2O3 layer between p- and n-type ZnO. The resulting rectification ratio is among the best parameters reported for a ZnO homojunction so far.

Electrical parameters of ZnO films and ZnO-based junctions obtained by atomic layer deposition

This work reports on the zinc oxide layers grown by atomic layer deposition (ALD) from dimethylzinc (Zn(CH3)2, DMZn) or diethylzinc (Zn(C2H5)2, DEZn) and deionized water precursors. These films are suitable for nanoelectronic applications, e.g. selecting elements in the new generation of non-volatile 3D memories constructed in the cross-bar architecture. This architecture imposes strict requirements on the parameters of obtained ZnO layers. Growth temperature must be below 200 °C, electron concentration not higher than 1017 cm−3 and mobility above 10 cm2 V−1 s−1. This is possible when the ALD growth method is used. We demonstrate the correlations between the structural, optical and electrical properties of ALD-ZnO layers. Their control allows us to obtain Schottky junctions with silver, whose parameters are suitable for the applications mentioned above. The ideality factor of about η ≈ 2.65 was calculated for the Schottky diodes based on the pure thermionic emission theory.

n-ZnO/p-4H-SiC diode: Structural, electrical, and photoresponse characteristics

Epitaxial n-type ZnO film has been grown, on a commercial 5 μm thick p-type 4H-SiC(00.1) Al doped epilayer, by atomic layer deposition. A full width at half maximum of the ZnO 00.2 diffraction peak rocking curve of 0.34° ± 0.02° has been measured. Diodes formed on the n-ZnO/p-4H-SiC heterostructure show rectifying behavior with a forward to reverse current ratio at the level of 109 at ±4 V, a leakage current density of ∼6 × 10−8 A/cm2, and a low ideality factor equal to 1.17 ± 0.04. In addition, the diodes exhibit selective photoresponse with a maximum at 367 nm, and with a current increase of ∼103 under illuminations with respect to the dark value, which makes such devices prospective candidates for ultraviolet light sensors.

Trap levels in the atomic layer deposition-ZnO/GaN heterojunction—Thermal admittance spectroscopy studies

In this work, a n-ZnO/p-GaN heterojunction is analyzed using admittance spectroscopy techniques. Capacitance transient measurements performed at 10 kHz reveal four majority-carrier deep levels, the most important one located at approximately 0.57 eV below the ZnO conduction band (CB) edge with a density about two orders of magnitude below the doping level (NT = 4 × 1015 cm−3). The others, located at 0.20 eV, 0.65 eV, and 0.73 eV, are about three orders of magnitude below the doping level (NT = 4–9 × 1014 cm−3).

ZnCoO Films by Atomic Layer Deposition - Influence of a Growth Temperature οn Uniformity of Cobalt Distribution

We report on the structural, electrical and magnetic properties of ZnCoO thin films grown by Atomic Layer Deposition (ALD) method using reactive organic precursors of zinc and cobalt. As a zinc precursor we applied either dimethylzinc or diethylzinc and cobalt (II) acetyloacetonate as a cobalt precursor. The use of these precursors allowed us the significant reduction of a growth temperature to 300oC and below, which proved to be very important for the growth of uniform films of ZnCoO. Structural, electrical and magnetic properties of the obtained ZnCoO layers will be discussed based on the results of SIMS, SEM, EDS, XRD, AFM, Hall effect and SQUID investigations.

2-D Finite-Element Modeling of ZnO Schottky Diodes With Large Ideality Factors

In this paper, we complement our previous work on the study of low-temperature rectifying junctions based on Ag/ZnO Schottky barriers. Diodes characterized by very high ION/IOFF ratio and ideality factors considerably higher than unity, in disagreement with the thermionic emission model, are modeled with a 2-D finite-element simulator. We could discard tunneling and inhomogeneous barrier-height distribution as sources for this anomalous value. A new interface charge layer model was therefore introduced, which is able to reproduce the electrical behavior in devices with large ideality factors without decreasing the rectifying properties.