3-1 The growth of ZnO films
3-1-1 Description of Laser-MBE system
The laser-molecular beam epitaxy (laser-MBE) growth system is also known as pulsed-laser deposition (PLD) and has been used to grow high quality films for more than a decade. The laser-MBE consists of two chambers, one is load lock chamber and the other is deposition chamber. The load lock chamber is a small spherical chamber with a Viton sealed quick access door for substrate loading. The substrate is mounted on a substrate holder. The substrate holder can be moved from the load lock into the deposition chamber by using a magnetically coupled transfer arm. By using the push-pull type pick-up manipulator equipped in the load lock module, substrates can be transferred from load lock to the deposition chamber. The deposition chamber, designed for pulsed laser deposition, is equipped with a target manipulator, a substrate holding and heating system, a pumping system, vacuum monitoring devices, necessary parts for laser inlet and a watching window.
3-1-22 The layouut of laser-MMBE systemm
3-1-3 Target preparation
The target was prepared by commercial hot-pressed stoichiometric ZnO target with purity of 99.999% and the target size is 1’’ × 0.118’’. Before the deposition, the laser beam was shone on the ZnO target to remove the contamination on the target surface.
3-1-4 Preparation of substrate
The composite γ-Al2O3/Si substrate was prepared by advanced nano epitaxy laboratory, NTHU, led by Prof. M. Hong and Prof. J. Kwo. The detailed preparation procedure of the substrate can be found in [29] and is also briefly described below.
Si wafers 2 inch in diameter with (111) as the normal to the wafer plane were put into a multi-chamber MBE/ultrahigh vacuum (UHV) system [30], after being cleaned with a Radio Corporation of America (RCA) method and an HF dip. Si wafers were heated to temperatures above ~550 °C, and then transferred under an UHV (to eliminate any
high-purity sapphire (purchased from Maintech, Huntingdon, PA, with a purity of 99.99%) was employed as the Al2O3 source in this work. During the oxide deposition, the vacuum in the chamber was maintained in the low 10−9 torr (even during the evaporation of sapphire) and substrate temperatures were maintained at about 700–750
°C. After the deposition of Al2O3, the substrates (γ-Al2O3/Si) were cut into pieces of an area of 1 × 1 cm2 for subsequent epitaxial growth of ZnO. Before transferring these substrates into laser-MBE load lock chamber, each substrate was put in acetone solution and cleaned in a supersonic oscillator for 5 minutes to remove the particles on the substrate surface. After the surface treatment, the substrates were mounted on the substrate holder and loaded into the ZnO growth chamber.
3-1-5 Operating process of laser-MBE
(1) Load the mounted substrate holder into the load lock chamber and turn on the scroll
pump.
(2) After the pressure of the chamber is pumped down to lower than 3 × 10-3 torr, then turn the turbo pump on.
(3) The pressure of the chamber should reach 10-9 torr in three hours. Only at this condition, we open the gate valve between the load lock and deposition chamber.
We use the magnetically coupled transfer arm to transfer the substrate holder from the load lock chamber into the deposition chamber.
(4) After substrate transferred, close the gate valve.
(5) Adjust the distance between the substrate and ZnO target to reach the appropriate
position.
(6) Make sure the water cooling system works properly.
(7) Turn on the thermal controller and power supply. Set the desired thermal program.
(8) Ensure the target substrate temperature is reached. Wait for a few minutes to let
the chamber pressure decline to 3 × 10-8.
(9) Turn on step motor to start the mirror scanner. Turn on target manipulator and
sample manipulator rotation to ensure the uniformed deposition.
(10) Turn on the laser and start the deposition.
(11) After thin film growth is completed, open the gate valve and use the magnetically
coupled transfer arm to retrieve the sample from the main chamber to the load lock chamber.
(12) Stop the turbo and scroll pump. When the blades of the turbo pump stop, we can
vent the load lock chamber by inserting N2 gas.
After surface treatment, the substrate was loaded in the chamber with the base pressure of 1 × 10-9 torr. We used a focused lens (f = 40 cm) to converge the laser beam through a viewpoint onto the target, which makes 45 degrees to the normal of the target. Inside the growth chamber, a ZnO ceramic target (99.999 % purity) was
located in front of the substrate holder at a distance of 5 cm and was ablated by a KrF excimer pulsed-laser with the wavelength, pulse duration, repetition rate, and laser fluence of 248 nm, 25 ns, 10 Hz, and 6 J/cm2, respectively. At the same time, the laser beam was scanned back and forth by moving a reflection mirror, which was driven by a stepping motor to prevent laser from hitting at the same spot on the target that leads to penetrate the target or non-uniform film growth. The substrate was heated with a halogen light bulb through the program temperature controller. The temperatures of the substrate holder were varied from 200oC to 500oC. The deposition of the ZnO thin films was carried out without oxygen flowing, under this circumstances, the pressure was maintained at 1 × 10-9 torr during deposition. The typical growth rate and sample thickness were 0.42 Å /s and ~300 nm, respectively.
3-2 Structural characterization of the ZnO films
3-2-1 X-ray diffraction
The crystal structure of ZnO films was inspected by XRD measurements which were performed with a four-circle diffractometer at the beamlines BL17A and BL13A of National Synchrotron Radiation Research Center, Taiwan, with incident wavelength 1.3344 and 1.023 Å, respectively. Two pairs of slits, located between the sample and a NaI scintillation detector, were employed and yielded a typical resolution of 4 × 10−3 Å−1.
3-2-2 Cross sectional TEM
Cross sectional TEM images and SAED patterns were taken using a field-emission-gun type TEM (Philips TECNAI-20) operated at 200 keV. For the preparation of cross-sectional TEM specimens, several steps of lapping and polishing are required. Figures 3.2(1) – (4) shows the specimen preparation processes. The first step of the preparation was a mechanical treatment of the materials, like sawing or punching to give the sample a square form with approximately 3 × 3 mm. Second, we glued the sample to a silicon substrate to form a formation of sandwich, as show in Fig.
3-2(2). Third, we polished the sandwich-like specimen by a polishing machine. We observed the specimen by an optical microscopy (OM) to see if there occurred a Newton’s ring in the specimen after polishing. If there existed Newton’s ring and showed no cracks in the OM image of the specimen, the thickness of specimen was proper for the cross sectional TEM measurement. Finally, the specimen was mounted on a copper ring, as shown in Fig. 3-2(4), and can be loaded into the TEM measurement holder.
3-3
preparation of a crosss-sectional TEM
rties
to calibrate spectral response of the spectrometer and detector. Each PL signal was collected for about 0.1 second at a step of 0.1 nm, and the data were transmitted through a GPIB card and recorded by a computer. The monochromator was a 32 cm long with three available resolutions of lines 600, 1200 and 1800 grooves/mm, respectively.
When the entrance and exit slits were both opened at about 50 μm, the resolution was about 0.1 nm in this system. Low temperature PL measurements were carried out by collecting the sample using a closed cycle cryogenic system.
325 nm He‐Cd Laser
Triax 320 Spectrometer
Cooling Compressor
Mirror
Mirror
Focal lens Focal lens
325 nm Long Pass Filter Sample
Holder
Sample
Fig. 3-3 Layout of PL system