Papers

Download ZnO thin film deposition on sapphire substrates

Zhuo Chen¹, T. Salagaj², C. Jensen², K. Strobl², Mim Nakarmi¹, and Kai Shum1¹, ª

¹Physics Department, Brooklyn College of the City University of New York, 2900 Bedford
Avenue, Brooklyn, NY 11210, USA

²First Nano, a division of CVD Equipment Corporation, 1860 Smithtown Avenue, Ronkonkoma, NY 11779, USA

ªkshum@brooklyn.cuny.edu

ABSTRACT

ZnO thin films with thickness around 200 nm were deposited on a-plane sapphire
substrates by Chemical Vapor Deposition (CVD) method with a mixed ZnO-powder/C-powder
solid source. These films were characterized by Atomic Force Microscopy (AFM), Scanning
Electron Microscopy (SEM), and photoluminescence (PL) spectroscopy. The correlation
between surface structural properties of ZnO thin films and their optical signature measured by temperature dependence of PL is investigated for various growth conditions such as flow rate O2 injection gas and growth temperature. At room temperature, the columbic interaction enhanced absorption edge of 3.305 eV of these films was determined by optical absorption measurements.

INTRODUCTION

ZnO is very interesting and promising optical material because it has a room temperature direct band gap of 3.37 eV [1] and excitonic binding energy of 60 meV [2]. However, it remains very challenging to make it as a truly useful optoelectronic material. One of the challenges is to epitaxially grow ZnO on a suitable substrate although thin film deposition on various substrates such as silicon or sapphire wafers using different methods has been successful [3]. Another challenge is to understand and control the high level of non-intentionally doped residual electron density. This not-yet controllable residual electron density [4, 5] is the reason behind the inability of making ZnO a good p-type material necessary optoelectronic devices. It also limits the full potential of large exciton binding energy because of carrier screening effect.

In this work, we report on the experimental data resulting from our attempt to understand
how ZnO can be grown on a sapphire substrate by a solid-vapor-solid process in a commercially built chemical vapor deposition (CVD) apparatus using a solid source. Various growth parameters such as growth temperature and O2 gas injection rate were used to alter the ZnO nucleation process. Deposited films were characterized by scanning electron microscopy and photoluminescence. We found that ZnO films could be grown by the CVD method with the film quality comparable to these grown by more expensive apparatuses such as metal-organic chemical vapor deposition (MOCVD).

EXPERIMENTAL DETAILS

A commercially built vapor deposition (CVD) system (First Nano, ET2000) was used to
deposit thin film on sapphire substrates. It was equipped with a separated solid source heater, a three-zone (load, center, and end zone) furnace, a gas injector, a vacuum pump, and a quartz tube. The reaction tube was controlled by a three-zone furnace to obtain a uniform temperature across the substrate at the collecting area over 3 inches by 2 inches. The source material was mixed ZnO powder (Alfa Aesar, 99.99%) and graphite powder (Alfa Aesar, 99%) with mass ratio of 1:4. The solid source was placed at the load zone in the quartz tube and heated up to high temperature by an additional solid source heater to generate Zn vapor which was then carried into the center zone by the Ar carrying gas. The reacting gas (O2) was introduced into the system by gas injector at different locations to achieve the best uniformity over a given substrate size. Sapphire substrates with a-plane cut were used. Fig. 1a shows the schematic diagram for the CVD apparatus. Fig 1b displays typically measured temperature profiles across load to center zone. Two important parameters, growth (substrate) temperature (Tg) and O2 flow rate (FO2) will be discussed in this work. The surface morphology of deposited thin films was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Optical transmission/absorption spectra were measured from PerkinElmer Lambda950 optical spectrometer. Photoluminescence (PL) spectra were measured by Horiba NanoLog system coupled with an optical cryostat (4 to 350 K) from Advanced Research System.

RESULTS AND DISCUSSIONS

Fig. 2a, 2b, and 2c display the SEM surface images for the 3 samples placed in the center
zone with different growth temperature of 720, 820, and 920 oC, respectively, for 1 hour
deposition time. It is clear that at 720 °C ZnO starts to nucleate on the sapphire surface, but the
speed the film growth is too slow, leading to the scattered ZnO islands on the sapphire surface
during 1 hour time period. At 820 °C, the speed of nucleation has dramatically increased and left
a thin layer of ~ 200 nm during 1 hour. Since the ZnO coverage on the sapphire surface is not
100%, therefore, the thickness of ZnO thin film was measured by AFM as shown in Fig. 3. At
920 °C, no visible ZnO film was deposited on the substrate.

Photoluminescence was measured from these 3 samples at room temperature with a Xelamp
set at 300 nm with band pass of 5 nm as a photoexcitation source. They are shown in Fig. 4a as green, blue, and red curves for the sample with the growth temperature of 720, 820, and
920 °C, respectively. All three spectra show a band edge emission at ~378 nm with negligible
green emission at 550 nm, normally attributed to defect-related emission. PL from the film with
growth temperature of 820 °C is certainly strongest among the three samples, consistent with the information provided by their SEM images. The origin of three weak PL peaks from 400 to 500 nm for the 720 oC sample is not clear.

In Fig. 4b, A Tauc plot [6] is displayed for the 820 °C sample. The optical band gap of 3.31 eV is obtained. This energy is consistent with other ZnO thin films obtained by other methods [7, 8] and with generally accepted ZnO band gap [1] at room temperature by taking into the excitonic effect on absorption edge.

With apparently optimal growth temperature of 820 °C on sapphire substrate, three ZnO
films were deposited with different O2 flow rate at 100, 500, and 1,000 standard cubic center
meter per minute (with a short notation of cc). The SEM images for these three samples are
shown in Fig. 5a, 5b, and 5c, respectively. For the 100 cc sample, it is clearly visible that there is
a large amount of surface defects (pits). However, the estimated number of pit density seems to
be on the order of 107 cm-2, comparable with these thin films deposited [9] by more expensive
apparatus MOCVD. Such defect density decreases as the O2 flow rate increases as shown in the
500 cc and 1000 cc samples.

PL spectra for the three samples with different O2 flow rates are displayed in Fig. 6. The
PL peak intensity seems to be consistent with the pit density as obtained from SEM images.
Although the data points are limited, apparently, the PL intensity (IPL) has a square root
relationship with the O2 flow rate, i.e., IPL ~ FO2 1/2.

CONCLUSIONS

In summary, we have presented our preliminary data on ZnO thin film deposition by CVD method using a solid source. It was found that the optimal growth temperature is 820 °C
and the film surface pit density decreases while photoluminescence intensity increases as the O2
flow rate increases. These films clearly show good poly-crystalline quality with low optically
active defect density as evidenced by the lack of so-called green emission around 550 nm.

ACKNOWLEDGMENTS

The authors would like to acknowledge the partial financial support from NYSTRA grant for this work through the CUNY center of advanced technology on photonic applications.

REFERENCES

1. E.Mollwo, in Semiconductors: Physics of II-VI and I-VII Compounds, Semimagnetic
Semiconductors, edited by O.Madelung, M. Schulz, and H. Weiss (Springer, Berlin,
1982), vol. 17 of Landolt-B¨ornstein New Series, p. 35.
2. E. O'Kane, Phys. Rev. B 18, 6849 (1978).
3. X. W. Sun and H. S. Kwok, J. Appl. Phys. 86, 408 (1999).
4. U. Ozgur et al., J. Appl. Phys. 98, 043101 (2005).
5. D. L. Look, J. Electronic Materials 35, 1299 (2006).
6. J. Tauc, Amorphous and Liquid Semiconductors (Plenum, London, 1974).
7. S. W. Whangbo et al., J. of Korean Physical Society 33, 456 (2000).
8. V. Cracium et al., Appl Phys. Lett. 65, 2963 (1994).
9. N. Oleynik, Ph.D. Dissertation, University of Magdeburg, July 2007.