Contribution to the Development of Electronic Devices Based on Zn3N2 Thin Films, and ZnO and GaAs Nanowires

The main scope of this thesis work is the design, fabrication and characterization of electronic devices based on advanced materials. Advanced materials refer to all materials that represent advances over traditional materials. On the other hand, novel structures such as nanowires (NWs) based on traditional materials are also considered advanced materials due to they have exceptional properties such as low dimensionality and special mechanisms that can improve the existing technology based on thin film and bulk materials.

Zinc nitride (Zn3N2) is an advanced material used in this work for the development of high performance thin film transistors (TFTs) due to its high electrical mobility and conductivity at large carrier concentrations. In addition, Zn3N2 based photo-transistors have been fabricated and characterized. The growth of Zn3N2 layers has been carried out by radio-frequency magnetron sputtering from a Zn target and using a N2/Ar plasma. Rutherford backscattering spectrometry (RBS) measurements performed on layers grown at high growth rates (rg) above 50 nm/min, show a stoichiometric Zn3N2 composition. X-ray diffraction and scanning electron microscopy analysis (SEM), showing the formation of polycrystalline Zn3N2 layers. The grain size and its preferential orientation depend on the substrate temperature (Ts), observing an improvement of the crystalline
orientation, and a reduction of the grain size as Ts decreases. In addition, the reduction of the rg (4.44 nm/min) leads to a less definition of grain boundaries, improving the electrical conduction between grains, and showing Hall mobility values of 100 cm2 /V·s for carrier concentrations of 3.2×1018 cm−3 . From SEM images and Hall results, one can conclude that the electrical transport through Zn3N2 is mainly limited by the grain boundaries and the ionized impurity scattering. Optical transmission measurements confirm a direct band gap semiconductor behaviour of Zn3N2, and an absorption edge blue-shift with Ts. On the other hand, Zn3N2 surface oxidation (metastability) has been demonstrated by spectroscopic ellipsometry and RBS. Surface oxidation rate is estimated between 20 and 36 nm/day, depending on the Ts; after sufficient time, Zn3N2 layers are completely transformed into transparent ZnO. In this work, it is shown a novel procedure to obtain sub-micron crystals of ZnO from the rapid transformation of Zn3N2 layers using electric arcs to accelerate the oxidation process. On the other hand, the oxidation of Zn3N2 can be prevented depositing a ZnO capping layer thereafter the deposition of Zn3N2 in vacuum conditions and in the same run. The research about Zn3N2 was part of works carried out in the AVANSENS (S2009/PPQ-1642) Spanish project, funded by the Comunidad Autónoma de Madrid. During that time, TFTs and photo-transistors were designed, fabricated and characterized using Zn3N2 as channel layer. The electrical characterization of TFTs shows transistor characteristics in n-channel enhancement-mode (threshold voltage of 6 V) without need of annealing process. Zn3N2 based TFTs and photo-transistors present high sensitivity to the visible (VIS) light, demonstrating the potential of Zn3N2 compound for VIS photodetectors (PDs).

Zinc oxide (ZnO) NWs based PDs present one of the highest photoconductive gain values among all the structures and materials with ultraviolet (UV) photosensitivity. In this work, UV PDs based on ZnO NWs have been designed, fabricated and characterized. Vertically aligned ZnO NWs have been grown by chemical vapor transport on Si(100) crystalline substrates, and using a thin film of Zn as seed layer. The optical characterization of these NWs shows high absorption in the UV spectral range. Optimizing growth conditions, ZnO NWs have been obtained with an excellent aspect ratio, high crystalline quality, radii below 50 nm, and lengths above 10 m, simplifying their integration in electronic devices. NW integration has been carried out by using alternating electric fields applied between conductive electrodes, being separated distances below the NW length. This cost-effective technique enables to trap and to align NWs at specific sites, controlling the
number of assembled NWs with some selective properties on the NW size, and allowing for their electrical characterization. Electric current measured through a single NW strongly depends on its diameter; O adsorption around ZnO NW surface leads to trap free charge along the NW surface, reducing the conductive volume in the NW and then lowering the current measured in dark ambient conditions. Since the NW is illuminated with UV light, photogenerated holes desorb surface O, releasing surface trapped electrons towards the conductive bulk. Then, the O adsorption/desorption mechanisms along NW surface modulate the space charge region of the NW. NW photoresponsivity (Rphoto) steadily increases as the NW radius reduces, confirming the important role of surface in the PD photoresponse, and yielding Rphoto of 108 A/W ( = 370 nm,
and 5 V) in NWs with a radius below 50 nm. In addition, transient photoresponse studies show that NWs with lower radii have longer rise times and shorter decay times mainly due to the mentioned surface trapping effects. ZnO NW surface properties and electrical characteristics have been also analyzed after different surface treatments, including hydroxylation and silanization.

GaAs NWs have been grown by Ga-assisted chemical beam epitaxy (CBE). SEM images demonstrate the formation of Ga droplets on oxidized Si(111) substrates at Ts of 580 ºC; Ga droplet formation is mainly due to: i) thermal cracking of the Ga metalorganic precursor on the oxide surface layer; ii) diffusion of Ga atoms along the oxide surface layer; iii) reaction of Ga with oxide at those regions where oxide is thinner (pinholes), enabling the nucleation in contact with the Si surface. The preparation of the Si(111) substrate surface is therefore crucial for the successful NW growth in CBE. For oxide thicknesses around 0.5 nm, Ga droplet formation is possible, allowing for the growth of GaAs NWs by means of the vapor-liquid-solid (VLS) mechanism. However, the use of thinner or thicker oxides promotes the growth of GaAs polycrystalline layer or inhibits the GaAs growth, respectively. The incubation time refers to the time at which NW growth starts, being ranged between 30 and 200 s, depending on the initial droplet size. For growths performed at Ts = 580 ºC and V/III = 0.8, a rg = 5.4 m/h is observed. Reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM) confirm the formation of vertically aligned GaAs NWs along the direction [111]B with pure zinc blende structure; TEM measurements also show that the whole NW body is free of twins. The RHEED spotty pattern characteristic of NW growth consists of horizontal bars whose length is related to the NW diameter, and which are formed from electrons scattered through the NW facets, demonstrating the NW hexagonal footprint in-situ. GaAs NWs have been also aligned between conductive electrodes using the same method utilized for ZnO NWs. Electrical characteristics of a single GaAs NW show dark currents in the nA range and sensitivity to the VIS light through a decrease of its resistivity. Nano-contacts formed between the Ga droplet and the GaAs NW facets during the VLS process are the main responsible for the device conductivity as demonstrated by the characterization of similar devices based on GaAs NWs without droplet. Most of the work has been carried out in the TEC2010-20796 project, funded by the Spanish Ministerio de Economía y Competitividad (MINECO). This project aims to design UV and VIS PDs based on ZnO and GaAs NWs, respectively. Results obtained during the research have confirmed the use of ZnO and GaAs NWs as active structures in the realization of UV and VIS PDs, improving the sensitivity, and enabling the integration of different semiconductor technologies at the microscale.