Synthesis and Characterization of High c-axis ZnO Thin Film by Plasma Enhanced Chemical Vapor Deposition System and its UV Photodetector Application

Summary

We offered a method to directly synthesize high c-axis (0002) ZnO thin film by plasma enhanced chemical vapor deposition. The as-synthesized ZnO thin film combined with Pt interdigitated electrode was used as sensing layer for ultraviolet photodetector, showing a high performance through a combination of its good responsivity and reliability.

Abstract

In this study, zinc oxide (ZnO) thin films with high c-axis (0002) preferential orientation have been successfully and effectively synthesized onto silicon (Si) substrates via different synthesized temperatures by using plasma enhanced chemical vapor deposition (PECVD) system. The effects of different synthesized temperatures on the crystal structure, surface morphologies and optical properties have been investigated. The X-ray diffraction (XRD) patterns indicated that the intensity of (0002) diffraction peak became stronger with increasing synthesized temperature until 400 ^oC. The diffraction intensity of (0002) peak gradually became weaker accompanying with appearance of (10-10) diffraction peak as the synthesized temperature up to excess of 400 ^oC. The RT photoluminescence (PL) spectra exhibited a strong near-band-edge (NBE) emission observed at around 375 nm and a negligible deep-level (DL) emission located at around 575 nm under high c-axis ZnO thin films. Field emission scanning electron microscopy (FE-SEM) images revealed the homogeneous surface and with small grain size distribution. The ZnO thin films have also been synthesized onto glass substrates under the same parameters for measuring the transmittance.

For the purpose of ultraviolet (UV) photodetector application, the interdigitated platinum (Pt) thin film (thickness ~100 nm) fabricated via conventional optical lithography process and radio frequency (RF) magnetron sputtering. In order to reach Ohmic contact, the device was annealed in argon circumstances at 450 ^oC by rapid thermal annealing (RTA) system for 10 min. After the systematic measurements, the current-voltage (I-V) curve of photo and dark current and time-dependent photocurrent response results exhibited a good responsivity and reliability, indicating that the high c-axis ZnO thin film is a suitable sensing layer for UV photodetector application.

Introduction

ZnO is a promising wide-band-gap functional semiconductor material due to its unique properties such as high chemical stability, low cost, non-toxicity, low power threshold for optical pumping, wide direct band gap (3.37 eV) at RT and large exciton binding energy of ~60 meV ^1-2. Recently, ZnO thin films have been employed in many application fields including transparent conductive oxide (TCO) films, blue light emitting device, field-effect transistors, and gas sensor ^3-6. On the other hand, ZnO is a candidate material to replace indium tin oxide (ITO) owing to indium and tin being rare and expensive. Moreover, ZnO possesses high optical transmittance in the visible wavelength region and low resistivity compared with ITO films ^7-8. Accordingly, fabrication, characterization and application of ZnO has been extensively reported. This present study focuses on synthesizing high c-axis (0002) ZnO thin films by a simple and effectively method and its practical application towards a UV photodetector.

The recent research report findings indicate that the high quality ZnO thin film could be synthesized by various techniques such as sol-gel method, radio frequency magnetron sputtering, metal organic chemical vapor deposition (MOCVD), and so on ^9-14. Each technique has its advantages and disadvantages. For example, a principal advantage of sputtering deposition is that target materials with very high melting point are effortlessly sputtered onto the substrate. In contrast, the sputtering process is difficult to combine with a lift-off for structuring the film. In our study, the plasma enhanced chemical vapor deposition (PECVD) system was employed to synthesize high quality c-axis ZnO thin films. Plasma bombardment is a key factor in the synthesizing process that can increase the thin film density and enhance the ion decomposition reaction rate ¹⁵. In addition, the high growth rate and large-area uniform deposition are other distinctive advantages for PECVD technique.

Except for the synthesis technique, the good adhesion on the substrate is another considered issue. In many studies, the c-plane sapphire has been widely used as the substrate to synthesize high c-axis ZnO thin films because the ZnO and sapphire have the same hexagonal lattice structure. However, the ZnO was synthesized on sapphire substrate exhibiting rough surface morphology and high residual (defect-related) carrier concentrations due to the large lattice misfits between the ZnO and c-plane sapphire (18%) oriented in the in-plane direction ¹⁶. Compared with the sapphire substrate, a Si wafer is another widely used substrate for the ZnO synthesis. Si wafers have been extensively used in semiconductor industry; and thus, growth of high quality ZnO thin films on Si substrates is very important and necessary. Unfortunately, the crystal structure and thermal expansion coefficient between the ZnO and Si are obviously different leading to deterioration of crystal quality. Over past decade, great efforts have been made to improve the quality of ZnO thin films onto Si substrates by using various methods including ZnO buffer layers ¹⁷, annealing in various gas atmosphere ¹⁸, and passivation of the Si substrate surface ¹⁹. The present study successfully offered a simple and effectively method to synthesize high c-axis ZnO thin film onto Si substrates without any buffer layer or pre-treatment. The experiment results indicated that the ZnO thin films synthesized under the optimal growth temperature showed the good crystal and optical qualities. The crystalline structure, RF plasma composition, surface morphology, and optical properties of ZnO thin films were investigated by X-ray diffraction (XRD), optical emission spectroscopy (OES), field emission scanning electron microscopy (FE-SEM), and RT photoluminescence (PL) spectra, respectively. Moreover, the transmittance of ZnO thin films was also confirmed and reported.

The as-synthesized ZnO thin film served as a sensing layer for UV photodetector application was also investigated in this study. The UV photodetector has great potential applications in UV monitoring, optical switch, flame alarm, and missile warming system ^20-21. There are many types of photodetectors which have been carried out such as positive intrinsic negative (p-i-n) mode and metal-semiconductor-metal (MSM) structures including Ohmic contact and Schottky contact. Each type has its own advantages and drawbacks. Currently, MSM photodetector structures have attracted intensive interest due to their outstanding performance in responsivity, reliability and response and recovery time ^22-24. The results presented here have shown that the MSM Ohmic contact mode was employed to fabricate ZnO thin film based UV photodetector. Such a kind of photodetector typically reveals a good responsivity and reliability, indicating that the high c-axis ZnO thin film is a suitable sensing layer for UV photodetector.

Protocol

1. Substrate Preparation and Cleaning

1. Cut 10 mm x 10 mm silicon substrates from Si(100) wafer.
2. Cut 10 mm x 10 mm glass substrates.
3. Use ultrasonic cleaner to clean the silicon and glass substrates with acetone for 10 min, alcohol for 10 min, and then isopropanol for 15 min.
4. Rinse the substrates with deionized (DI) water three times.
5. Blow-dry the substrates with a nitrogen gun.

2. DEZn Preparation and Preservation

Note: Diethylzinc (C₂H₅)₂Zn, also called DEZn, is a highly pyrophoric organozinc compound consisting of a zinc center bound to two ethyl groups. Never work alone when using DEZn. DEZn is very toxic and sensitive to the oxygen and water, be sure not to place the DEZn near the water. Always wear protective masks and eye protection; all procedures must be performed in the hood. Most importantly, unused DEZn must be stored in a 5 ^oC environment.

Note: For the first use of DEZn, follow step 2. If not, start the experiment from step 3.

1. Use syringe to draw out 30 ml DEZn from the bottle and then inject into a beaker placed in a steel cylinder.
2. Use a galvanized iron pipe to connect the steel cylinder with the reaction chamber.
3. Use mechanical pump and ball valve to pump down the steel cylinder in vacuum environment (to 6 Torr).
   Note: DEZn will severely react with oxygen, it must be maintained in the vacuum environment.
4. Store the unused DEZn in a 5 ^oC environment.

3. PECVD Chamber Preparation and Synthesis of ZnO Thin Films

Note: The schematic diagram of plasma enhanced chemical vapor deposition is depicted in Figure 1.

1. Set the working distance between showerhead electrode and sample stage at 30 mm.
2. Place the substrates on the sample stage of reaction chamber in proper location where there is a 3 cm distance from the DEZn inlet.
3. Open the rotary pump and gradually open the gate valves and butterfly valve.
4. Wait until the background pressure of the reactor chamber is lower than 30 mTorr.
5. Close the gate valves and butterfly valve, which connects to the rotary pump.
6. Then open the turbo pump and relative gate valves to reach high vacuum of 3 x 10^-6 Torr.
7. After reaching the necessary vacuum condition, open the heat controller and heat the sample stage to the synthesis temperature (200, 300, 400, 500, and 600 ^oC for different experiment parameters).
8. When the temperature and the pressure reach the necessary condition, close the turbo pump and then open the gate valves and butterfly valve which connects to the rotary pump simultaneously.
9. Next, open the gas inlet valves and turn on the argon gas flow controller simultaneously.
10. Flow the argon gas (0.167 ml/sec) into the chamber.
11. Set the chamber pressure to 500 mTorr.
12. Turn on the RF (13.56 MHz) generator and matching network, then set the RF power at 100 W for purging the samples surface for 15 min.
13. After finishing the purge of samples, turn the RF power down to 70 W.
14. Next, turn on the carbon dioxide gas controller and gas inlet valve.
15. Flow the carbon dioxide (0.5 ml/sec) into the chamber.
16. Set the working pressure at 6 Torr.
17. After the chamber pressure reaches 6 Torr, flow the high pure argon as carrier gas (0.167 ml/sec) for carrying diethylzinc (DEZn) into the chamber and open ball valve connected to the DEZn simultaneously. At the same time, start the synthesis of ZnO films.
18. Continue the plasma synthesis of ZnO films for 5 min.
19. After the ZnO films have been synthesized, seriatim turn off the RF generator, ball valve, heat controller and all of gas flow controllers along with gas inlet valves.
20. Take out the sample when the sample stage temperature cools down to RT. Note: The cooling rate is approximately 1.8 ^oC/min.

4. Preparation of Interdigitated-like Pattern onto As-synthesized ZnO Thin Film

Note: The schematic of lithography process is depicted in Figure 3.

1. Use a hot plate to bake the as-synthesized ZnO sample at 150 ^oC for 10 min.
2. Place the sample on the spin coater, and then dispense the liquid solution of photoresist (S1813) with 100 µl onto the ZnO sample.
3. Run the spin coater at 800 rpm for 10 sec and then accelerate to 3,000 rpm for 30 sec to produce a uniformly thin layer.
4. Soft bake the photoresist-coated ZnO sample at 105 ^oC for 90 sec.
5. After the soft-baking, use UV light to expose the photoresist-coated sample trough a photomask by mask aligner. The exposure time is 2 sec and the power is 400 W.
   Note: The pattern of photomask is designed as interdigitated-like, which is 0.03 mm wide and 4 mm long (14 pairs) and has an inter-electrode spacing of 0.15 mm as depicted in Figure 2. It is worth noting that the total photosensitive area is 84.32 mm² for the detector.
6. After the exposure procedure, use tweezers to clip the sample, and then immerse into the diluted developer (mix 50 ml of developer and 150 ml of deionized water) through actions of swinging from side to side for 35 s to obtain the developed sample.
7. Rinse the developed sample with DI water and dry with nitrogen gas.
8. Use the optical microscope to check the pattern intact. If not, use acetone to remove the photoresist and repeat steps 4.2 to 4.7 until the perfect pattern has been obtained.
9. Hard bake the sample at 120 ^oC for 20 min.

5. Deposition of Pt Top Electrode and Chemical Lift-off

1. Use the RF magnetron sputtering system to deposit a thin conductive Pt layer (100 nm) on the top of the developed sample before proceeding to chemical lift-off procedure.
2. Set the distance between target and substrate at 13 mm.
3. Use the mechanical pump to reach a rough vacuum of 5 mTorr.
4. Then, use the turbo pump to obtain a high vacuum of 7 x 10^-7 Torr.
5. Wait until the chamber reaches the high vacuum, close the turbo pump and open the mechanical pump subsequently.
6. Flow the argon gas at 0.3 ml/sec into the chamber by mas flow controller until the chamber pressure reach the working pressure of 100 mTorr.
7. Turn on the direct current (DC) discharge power supply and set the DC power at 15 W for sputtering the Pt thin film electrode onto the sample for 25 min.
8. After the Pt electrode layer has been deposited by magnetron sputtering method, take out the sample from the chamber.
9. Immerse the sample into the acetone liquid for chemical lift-off process by ultrasonic cleaner to remove the photoresist.
10. Set the cleaning time at 1 min to thoroughly remove photoresist, and then obtain the interdigitated-like Pt electrode onto the ZnO thin film.

6. RTA process

1. Place the as-fabricated Pt/ZnO sample into the RTA system.
2. Use the mechanical pump and gate valve to pump down the RTA chamber pressure to 20 mTorr.
3. Wait until the chamber pressure reaches 20 mTorr, flow argon gas at 0.3 ml/sec into the chamber and set the working pressure of 5 Torr.
4. Next, set the heating rate as 100 ^oC/min.
5. Then, anneal the sample at 450 ^oC for 10 min.
6. Once annealed, wait until the sample cools to RT, then take out the sample.

Results

The ZnO (0002) thin films with high c-axis preferred orientation have been successfully synthesized onto the Si substrates by using the PECVD system. The carbon dioxide (CO₂) and the diethylzinc (DEZn) were used as oxygen and zinc precursors, respectively. The crystal structure of ZnO thin films was characterized by X-ray diffraction (Figure 4), indicating that the ZnO thin film synthesized at 400 ^oC with the strongest (0002) diffraction peak. When the synthesized temperatu...

Discussion

Critical steps and modifications

In step 1, the substrates should be thoroughly cleaned and steps 1.3 to 1.5 followed to make sure that there is no grease or organic and inorganic contaminations on the substrates. Any grease or organic and inorganic contaminations on the substrate surface will significantly reduce the adhesion of the film.

Step 2 is the most important procedure before the ZnO film preparation process. DEZn is very toxic and violently reacts with wat...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
RF power supply	ADVANCED ENERGY	RFX-600
Butterfly valve	MKS	253B-1-40-1
Mass flow controller	PROTEC INSTRUMENTS	PC-540
Pressure controller	MKS	600 series
Heater	UPGRADE INSTRUMENT CO.	UI-TC 3001
Sputter gun	AJA INTERNATIONAL	A320-HA
DEZn 1.5M	ACROS ORGANIC USA, New Jersey		also called Diethylzinc (C2H5)2Zn
Spin coater	SWIENCO	PW - 490
I-V measurement	Keithley	Model: 2400
Photocondutive measurement	Home-built
UV light sourse	Panasonic	ANUJ 6160
Mask aligner	Karl Suss	MJB4
Photoresist	Shipley a Rohm & Haas company	S1813
Developer	Shipley a Rohm & Haas company	MF319
Silicon wafer	E-Light Technology Inc	12/0801
Glass substrate	CORNING	1737	P-type / Boron