Characterization for materials and devices

The scanning electron microscopy (SEM) was used the secondary electron mode to observe the morphology of ZnO materials and electronic device. The model of the SEM used here is Hitachi 4700.

3.2.2 Transmission electron microscopy (TEM)

One of the typical characters of nano-phase materials is the small object size.

Although some structural features can be revealed by x-ray and neutron diffraction, direct imaging of nanomaterials is only possible using high resolution transmission

electron microscopy (HRTEM, FEI / Philip Tecnai F20). TEM is a unique technique because it can produce a real space image on the atom distribution in the nanocrystal surface. With a finely focused electron probe, the structural characteristic of a single nanomaterial can be fully understood. Normally, the chemical analysis system, the energy disperse X-ray spectrometer (EDX), was attached on TEM system.

3.2.3 X-ray diffraction spectroscopy

X-ray diffraction analysis of the ZnO specimens was carried out by using an X-ray diffractometry (XRD) (Philips PW3710 or MAC Science MAXP3) with conventional θ/2θ scans. The X-ray was generated by a Cu target (Cu Kα) operated at 50kV and 60mA, and the scanning speed was 0.02 deg/step, 1deg/min from 20° to 80°.

3.2.4 UV absorption

UV absorption was done in UV-VIS-NIR scanning spectrophotometer (SHIMADZU UV-3101PC) at wavelengths from 350 to 800 nm by employing both a tungsten-iodide (WI) lamp for the visible region and a deuterium (D2) lapmp for the ultraviolet region.

3.2.5 Thermal gravimetric analysis

Thermal weight loss was measures by thermal Gravimetric Analysis (TGA;

Perkin-Elmer, thermal gravimetric analyzer 7), which is a simple analytical technique that measures the weight loss (or weight gain) of a material as a function of temperature. As materials are heated, they can loose weight from a simple process

such as drying, or from chemical reactions that liberate gasses. Some materials can gain weight by reacting with the atmosphere in the testing environment. Since weight loss and gain are disruptive processes to the sample material or batch, knowledge of the magnitude and temperature range of those reactions are necessary in order to design adequate thermal ramps and holds during those critical reaction periods.

A sample of the test material is placed into a platinum cup that is supported on, or suspended from an analytical balance located outside the furnace chamber. The balance is zeroed, and the sample cup is heated according to a predetermined thermal cycle. The balance sends the weight signal to the computer for storage, along with the sample temperature and the elapsed time. The TGA curve plots the TGA signal, converted to percent weight change on the Y-axis against the reference material temperature on the X-axis.

3.2.6 Hall effect measurement

The importance of the Hall-effect [125-127] is underscored by the need to determine accurately carrier density, electrical resistivity, and the mobility of carriers in semiconductors. The Hall-effect provides a relatively simple method for doing this.

Because of its simplicity, low cost, and fast turnaround time, it is an indispensable characterization technique in the semiconductor industry and in research laboratories.

It is listed as one of the most-commonly used characterization tools.

Fig. 3.2 Schematic of the Hall effect in a long, thin bar of semiconductor with four ohm contacts. The direction of the magnetic field B is along the z-axis and the sample has a finite thickness d.

The basic physical principle underlying the Hall-effect is the Lorentz force.

When an electron moves along a direction perpendicular to an applied magnetic field, it experiences a force acting normal to both directions and moves in response to this force and the force affected by the internal electric field. For an n-type, bar-shaped semiconductors shown in Fig. 3.2, the carriers are predominately electrons of bulk density n. Assume that a constant current I flow along the x-axis from left to right in the presence of a z-directed magnetic field. Electrons subject to the Lorentz force initially drift away from the current line toward the negative y-axis, resulting in an excess surface electrical charge on the side of the sample. This charge results in the Hall voltage, a potential drop across the two sides of the sample. (Note that the force

on holes is toward the same side because of their opposite velocity and positive charge.) This transverse voltage is the Hall voltage VH and its magnitude is equal to IB/qnd, where I is the current, B is the magnetic field, d is the sample thickness, and q

(1.602 x 10^-19 C) is the elementary charge. In some cases, it is convenient to use layer or sheet density (ns = nd) instead of bulk density. One then obtains the equation

ns = IB/q|VH| (3.3)

Thus, by measuring the Hall voltage VH and from the known values of I, B, and q, one can determine the sheet density ns of charge carriers in semiconductors. If the measurement apparatus is set up as described later in Section III, the Hall voltage is negative for n-type semiconductors and positive for p-type semiconductors. The sheet resistance RS of the semiconductor can be conveniently determined by use of the van der Pauw resistivity measurement technique. Since sheet resistance involves both sheet density and mobility, one can determine the Hall mobility from the equation

µ = |VH|/RSIB = 1/(qnSRS) (3.4)

If the conducting layer thickness d is known, one can determine the bulk resistivity (ρ

= RSd) and the bulk density (n = nS/d)

3.2.7 Atomic force microscope (AFM)

The atomic force microscope (AFM) is a very high-resolution type of scanning probe microscope, with demonstrated resolution of fractions of a nanometer, more than 1000 times better than the optical diffraction limit. The AFM was invented by Binnig, Quate and Gerber in 1986, and is one of the foremost tools for imaging,

measuring and manipulating matter at the nanoscale. The term 'microscope' in the name is actually a misnomer because it implies looking, while in fact the information is gathered by feeling out the surface with a mechanical feeler.

3.2.8 X-ray photoelectron spectroscopy

The XPS was taken to investigate the bonding energy between each element of thin film. By absorbing a photo, an atom gains an energy amount equal to hν. It then releases an electron to regain its original stable energy state. The released electron retains all the energy from the striking photon, which can escape from the atom and keep it moving. The incident photons usually carry an energy range from 1 to 2keV by XPS analysis.