PREPARATION AND PHOTOLUMINESCENCE PROPERTIES OF RF-SPUTTERED ZnO FILMS

ZnO/Si films were prepared by radio frequency (RF) magnetron sputtering at room temperature. By optimizing the heat treatment conditions, we obtained a good quality film annealed at 700 oC for longer 60 minutes. This process was monitored carefully by Raman scattering spectroscopy, and X-ray diffraction. The photoluminescence study on this film revealed that only ultraviolet emissions due to donor-acceptor pair (DAP), neutral acceptor-bound exciton (AoX) and donor-bound exciton (DoX) were observed. The intensity and peak position of these emissions depend on the measurement temperature and excitation power density.


Introduction
ZnO is a transparently wide-band-gap semiconductor compound (E g = 3.37 eV) with the direct electronic transition, and has a large exciton binding energy of 60 meV that ensures high luminescence efficiency at room temperature.It is considered as one of the most promising candidates for development of low-voltage and ultraviolet optoelectronic devices, transparent ultra-violet protection films, gas sensors, and so forth [1,2].Recently, ZnO has attracted intensive interest because of new successes in synthesizing ZnO nanostructures that show many physically interesting properties, which are significant for applications in nanotechnology [3].
Accordingly, the quality of films obtained depends strongly on many experimental factors, and heat treatment conditions.This is also the main reason to explain why experimental results reported by author groups are inconsistent.Recently, Kim et al.
prepared ZnO films at room temperature using RFmagnetron sputtering.By varying RF power they obtained high-quality ZnO films with c-axis orientation [12].
In the present work, ZnO films were also grown on Si(100) substrates by RF-magnetron sputtering at room temperature.We based on the heat treatment to find out optimal conditions.This process was monitored carefully by Raman scattering spectroscopy and x-ray diffraction.Photoluminescence (PL) spectroscopy was used to investigate near-band-edge emissions of the film after obtained the optimal conditions.

Experimental Detail
ZnO films were grown on Si(100) substrates by RF-magnetron sputtering in an ambient Ar gas.The ZnO (4N) target was firstly sputtered without the substrates for 15 minutes to clean its surface, and then deposited on unheated Si substrates for 60 minutes.In order to stabilize the plasma source during the growing, the output power and working pressure were held at 150 W and 8.8 mTorr, respectively.The low-temperature prepared films were afterwards annealed in air (in a clean room) at temperatures 200-800 ºC for 15-180 minutes.Raman spectroscopy (LABRAM-1B, Jobin-Yvon) and x-ray diffraction (X'Pert Phillips diffractormeter) were employed to monitor the quality of the films.Their morphology and thickness were characterized by scanning electron microscopy (SEM) using a Hitachi-600 and atomic force microscopy (AFM) using a Digital Instruments Nanoscope-3.
For low-temperature PL measurements, the ZnO films were mounted on a cold finger of a closed-cycle helium cryostat (APD Cryonics Inc.) using copper paste.The measurement temperature was controlled by a Lakeshore-330 unit.A wavelength of 337.1 nm generated from a lowpressure nitrogen-gas laser (with a 10 Hz repetition rate and a 7-nm pulse width) was used as an excitation source.The excitation energy could be tuned by an optical filters system.

It is similar to RS spectra of ZnO materials, conventional vibration modes 2-E 2 (M), E 2 (H)
and A 1 &E 1 (LO) centered at 331, 437 and 575 cm -1 , respectively, are observed [13][14][15].When the annealing temperature and time increase, the spectral lines become narrower and shift towards higher frequencies, revealing the change in microstructure of the films.This is more visible as considering the variation of a cusp around 564 cm -1 , which is superimposed by two peaks.The first one is A 1 &E 1 (LO) centered at 575 cm -1 characterizing the crystalline phase and the other centered at 560 cm -1 characterizing the amorphous phase, see Fig. 1 [2,13-15].Their relative Raman intensity changes strongly under the heat treatment conditions.Since the RS intensity is proportional to the scattering efficiency and a material amount contributed to the Raman scattering process, the crystalline volume fraction can be quantified from integrated intensities of the amorphous and crystalline peaks, I a and I c , respectively [16].With d > 3 nm, the dependences of the crystalline volume fraction on the annealing temperature and time obtained are shown in Fig. 2(a,b).We can see that for the films annealed at temperatures 200-500 ºC the crystalline volume fraction increases when the annealing temperature increases, see Fig. 2(a).Meanwhile, at a given annealing temperature of 500 ºC this fraction reaches above 85% as the annealing time is about 30-60 minutes.It becomes unchanged with longer annealing times, see  To achieve a larger crystalline volume fraction, the as-prepared films need to be annealed at higher temperatures.By extrapolating to the 100% value, as graphed in Fig. 2(a), the annealing temperature is deduced to be about 600-700 ºC.In an attempt to verify the extrapolated temperature range, we annealed the films at higher temperatures up to 800 ºC for 60 minutes.Fig. 2(c) shows the RS spectra in the frequency range of 550-650 cm -1 for these films compared with the RS spectrum of a ZnO single crystal.Clearly, for the film annealed at 700 ºC the feature of the A 1 /E 1 (LO) mode is similar to that of the crystalline bulk.When the film is annealed at 800 ºC, however, this peak shifts slightly towards a lower frequency, revealing the change in its structure.
To further understand the structural change taking place in the films as annealed at different temperatures for 60 minutes, x-ray diffraction was employed.As graphed in Fig. 3, the x-ray diffraction (XRD) patterns of the films annealed at temperatures below 600 ºC appear a sharp peak centered at a diffraction angle of 34.4 o characterizing the crystalline phase.Its background is a broad peak, characteristic of the amorphous phase.However, at the annealing temperatures 600-700 ºC only the peak coming from the crystalline structure is dominant, indicating the reduction of the amorphous phase [12].If increasing the annealing temperature up to 800 ºC, the XRD pattern appears two additional peaks, see Fig. 3, i.e. the film has a polycrystalline structure.These observations are in good agreement with the RS data as mentioned above.Herein, by optimizing the heat treatment conditions we also obtained highquality ZnO films.
Physical processes happened to the films can be explained briefly as follows: during the film deposition by RF-magnetron sputtering in the ambient Ar gas, Zn and O atoms escaped from the ZnO target surface could react with each other to form stoichiometric ZnO crystals.However, there were also many other Zn atoms reacted with O atoms to form ZnO δ (δ << 1) with an amorphous structure.Both these phases deposited randomly onto the Si substrates without rearranging in order or further reaction, since the substrates were not heated.This accounted for the appearance of the sharp XRD peak at 34.4º backgrounded by the broaden line for the as-prepared films, see Fig. 3. Furthermore, the RS spectra in this case emerged the cusp around 564 cm -1 overlapped by the two modes as mentioned above.When the films were annealed at different temperatures, the ZnO crystals would rearrange in order, and ZnO δ reacted with oxygen in air to become stoichiometric ZnO crystals (i.e.δ → 1).Thus, the crystalline volume fraction of the films increased with increasing the annealing temperature (T an ) and time (t an ), regarding the increase in the (002)-XRD peak intensity and the narrowing of the cusp around 564 cm -1 in the RS spectra.At the optimal condition (T an = 700 ºC and t an ≥ 60 minutes), ZnO δ transformed completely to the stoichiometric formation (i.e.δ = 1), and the ZnO crystals with a good orientation were observed.Accordingly, the XRD pattern appeared only a sharp peak, and the RS-peaks intensity increased, no RS mode from the amorphous phase was observed.
Fig. 4 shows SEM and AFM images for the film annealed at 700 ºC for 60 minutes.As a result, its morphology is uniform.The thickness and surface roughness of the film were determined to be about 1.5 µm and 20 nm, respectively.These characteristics make sure that PL spectra recorded on different sample areas are similar.
As an additional evidence that further confirmed the quality of the ZnO film annealed at 700 ºC for 60 minutes, Fig. 5 shows roomtemperature PL spectra for the as-prepared, T an = 700 and 800 ºC films.There is no PL signal to from the as-prepared film, meaning that the amorphous phase is not favorable for PL emission.However, the PL signal is improved rapidly as increasing T an .For the film annealed at 700 ºC, only two peak emissions in the ultraviolet (uv) region are dominant over the whole spectrum range studied, showing that the film has a good quality [6,7].If increasing T an above 700 ºC, for example T an = 800 ºC as graphed in Fig. 5, besides the uv emissions there is the appearance of a visible emission centered at 2.4 eV.The origin of this emission is still a controversial matter, which needs to be further studied [17,18].In our work, we only concentrate on investigating PL spectra of the good-quality film with T an = 700 ºC and t an = 60 minutes.In an attempt to clarify the origin of the uv emissions, we have studied dependence of the emissions on the measurement temperature and excitation power density for the T an = 700 ºC sample.Fig. 6 (a) shows the PL spectra of the film at some measurement temperatures with an excitation power density of 13 kW/cm 2 .

Fig. 6: (a) The PL spectra of the T an = 700 ºC film at different measurement temperatures under an excitation power density of 13 kW/cm 2 ; (b) the emission peaks of AºX, DºX and FX; (c) the temperature dependence of the AºX-and DºX-peak emissions compared with that of the FX emission in a ZnO bulk crystal, solid line (X A ).
It is different from the room-temperature spectrum, the PL spectra recorded at low temperatures below 100 K appear three peaks centered at 3.137, 3.317 and 3.360 eV (determined at 13 K).Among them, the 3.137 eV peak is attributed to the donor-acceptor pair (DAP) transition, its physical nature is presented afterwards [19].Besides these emissions, a shoulder at the high-energy side centered at 3.373 eV is also observed, which is associated with the free-exciton (FX) transition, see Fig. 6(b) [6,7].According to earlier reports, the distance between FX and neutral donor-bound exciton (D o X) emissions obtained is about 12 -15 meV [6,7,10,18].This suggests that the 3.360 eV peak in our case belongs to D o X.For the 3.137 eV peak, its origin is not specified [10,[19][20][21][22].We believe that it comes from the acceptorbound exciton (AºX) [10,19,22] rather than from the DAP one [20,21].This assertion is based on studying the excitation-power-density dependence of the DAP and AºX emissions.
As shown in Fig. 6(a), with increasing temperature from 13 K the intensity of the DºX and AºX emissions decreases gradually while their peak position shifts towards lower energies, which has a similar tendency for the temperature dependence of the free-exciton (FX) emission (X A ) in the ZnO bulk crystal, see Fig. 6(c) [5].At temperatures above 100 K, however, besides the persisting existence of the DAP emission the DºX and AºX peaks are overlapped each other.Their sum peak also shifts towards lower energies, denoted as BE in Fig. 6(c).
To further understand the intrinsic of DAP, D o X, and AºX emissions, we have studied the PL spectra of the film at 13 K when the excitation power density varied from 5 to about 10 6 kW/cm 2 .The PL spectra recorded are shown in Fig. 7.At low excitation power densities of below 367 kW/cm 2 , we can see the clear separation of DºX and AºX emissions.With increasing the excitation density from 5 kW/cm 2 , the intensity of all the emissions (DAP, DºX, and AºX) increases.However, the AºX-emission intensity becomes stronger and dominant as the excitation power density is above 37 kW/cm 2 .This reveals that the acceptor concentration in the T an = 700 ºC film is quite large, probably since nitrogen in air diffused in the ZnO host lattice during the heat treatment [6,19,23].At excitation power densities above 367 kW/cm 2 the DºX and AºX emissions become overlapped.This situation is similar to the case of the high-temperature PL spectra, see Fig. 6(a).Having analyzed variation of the emissions versus the excitation density, the following features are observed: (i) the DºX and AºX peaks seem unchanged in their position; this result is in good agreement with identifying AºX as discussed above.(ii) The DAP peak shifts to higher energies, which could be described by an exponential function [19,22] where I exc and ω p are the excitation density and the peak position, respectively, and β is a parameter dependence of the compensation ratio (β = 4.2 meV, in our case), see Fig. where E D and E A are the binding energies of donor and acceptor states, respectively.And, e, e, and r are the electric charge, dielectric constant, and the distance of the donor-acceptor pair, respectively.At low temperatures, the distance of the donor-acceptor pair decreases when the excitation power density increases, resulting in the high-energies shift of the DAP peak.It is used this feature to identify the DAP emission in ZnO.

Conclusions
A good quality ZnO thin film was grown by RF-magnetron sputtering.Influence of temperature and excitation power density on the near-band-edge emissions of the sample was studied.At low temperatures and excitation power densities, the emissions coming from DAP, AºX and DºX centered at 3.137, 3.317 and 3.360 eV, respectively, were observed.However, at temperatures above 100 K and/or high excitation densities the A o X and D o X emissions became overlapped.The change in their peaks position with respect to temperature and the excitation power density was analyzed and discussed.
ρ is the ratio of the Raman efficiencies for crystalline and amorphous ZnO.Assuming that ZnO crystals have a spherical shape with the crystalline size d, ρ is given by 250 Fig 2(b).

Fig. 2 .
Fig. 2. The dependences of the crystalline volume fraction on (a) the annealing temperature and (b) time; (c) the RS spectra in the frequency range of 550 -650 cm -1 for the annealed films compared with the RS spectrum of a ZnO single crystal..

Fig. 3 .
Fig. 3.The representative XRD patterns for the films annealed at temperatures 480 -800 o C for 60 minutes.

Fig. 4 .
Fig. 4. SEM and AFM images show the morphology for the film annealed at 700 ºC for 60 minutes.

Fig. 5 .
Fig. 5.The room-temperature PL spectra for the films annealed at 700 and 800 ºC for 60 minutes compared to the PL spectrum of the as-prepared one.

Fig. 7 (
Fig. 7 (a) The PL spectra of the T an = 700 ºC film measured at 13 K with the excitation power density varying from 5 to about 10 6 kW/cm 2 ; (b) the variation of the DAP, DºX, and AºX peaks with the excitation power density, the dotted lines for DºX and AºX guide the eye only, and the solid line for DAP is the fitting curve to Eq. (3).
7(b).The high-energy shift of the DAP peak (E ) can be explained by means of an expression[24]