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

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 water and easily ignites upon contact with air. DEZn must be very carefully injected into the steel cylinder and then pumped down to 6 Torr immediately. In step 3, make sure that each experimental parameter and step is accurately completed because even slightly different settings will lead to different results.

Step 4 must be carried out in a lithography laboratory and a clean room to avoid non-UV light illumination and dust-pollutionor particle-pollution effects. In step 5, power of the DC discharge power supply should be raised slowly; otherwise the Pt target will be broken. In step 6, the samples should be placed in the middle of the RTA chamber to ensure that the sample can be uniformly heated.

Data Analysis

The synthesized temperature is a crucial parameter for synthesizing high quality c-axis ZnO thin film because the thermal expansion coefficients and thermal mismatch strain effect with interfacial energy between the ZnO thin films and the Si substrate can significantly affect the crystallographic orientation. It can be clearly observed from XRD patterns (Figure 4). The strongest ZnO (0002) diffraction peak was obtained at 400 ^oC, indicating that the temperature is an optimal synthesized temperature for synthesizing c-axis ZnO thin film onto Si substrate by PECVD method. With a further increase in synthesized temperature up to 500 ^oC, leading to the deterioration of ZnO crystal quality. Furthermore, the ZnO thin film converted to amorphous phase when the synthesized temperature was set at 600 ^oC. It can be assumed that the high enough temperature (i.e., 400 ^oC) can provide sufficient energy, enhancing the ability of molecules to find the stable site and resulting in the ZnO thin film with high crystal quality. Simultaneously, the common defects including vacancies, interstitials, and dislocations in ZnO are also reduced. However, the ZnO will decompose into Zn and O₂ molecules when excessive high temperature (i.e., 500 and 600 ^oC) was employed ²⁵. Moreover, the different thermal expansion coefficients between ZnO (6.7 x 10^-6 K^-1) ²⁶ thin film and Si (4.18 x 10^-6 K^-1) ²⁷ substrate at 900 K will yield the thermal mismatch strain effect with interfacial energy. The above phenomena could deteriorate the crystal quality of ZnO thin film, resulting in the polycrystalline or amorphous phase.

For monitoring the plasma chemical composition during the ZnO synthesized process, the in–situ OES spectrum analysis of the RF plasma was carried (Figure 5). The OES analysis result illustrated that the three strong Zn emission peaks appeared around 475 nm and other distinct emission peaks were determined as O₂ and CO signals ²⁸. Moreover, some complicated emission peaks located in the range between 325 and 430 nm are determined as decomposition species of DEZn. The above beneficial information indicates that the OES is a useful instrument for in–situ monitoring of plasma chemical composition during the synthesized process.

The surface morphologies of the ZnO thin films were observed by FE-SEM (Figure 6A-E). It can be seen that the ZnO synthesized at 300 and 400 ^oC show densely packed with spherical grains and with small grain size distribution. Compared with that synthesized condition, other ZnO thin films exhibit disorderly and irregular surface, implying the polycrystalline or amorphous phase. It is worth noting that the SEM images are consistent with the previous XRD results.

The optical properties were determined by RT PL spectrum with 325 nm He-Cd laser. From the PL measurement (Figure 7), all ZnO thin films show a strong NBE emission peak located at around 375 nm, which is attributed to recombination of free-excitons through an exciton-exciton collision process ²⁹. Simultaneously, the broad defect-related DL emission peaks located at around 575 nm are obtained when the synthesized temperatures were set at 200 and 500 ^oC. It is also found that the NBE emission has a blue-shift to shorter wavelengths with increasing synthesized temperature. In general, the DL emission is referred to the impurities and various structural defects in the ZnO phase such as oxygen vacancy and zinc interstitial ³⁰, and the blueshift is due to the strong residual anisotropic strain in the ZnO thin film ³¹. Therefore, the PL results reflect that the synthesized temperature can significantly affect the optical properties of ZnO thin films. Among all ZnO samples, only two samples which were synthesized at 300 and 400 ^oC show a dominated NBE emission peak and negligible DL emission accompanying the high (0002) XRD diffraction peak.

The transparency for all synthesized ZnO thin films were measured by transmittance spectrum (Figure 8A) and the average transmittance value for each ZnO sample has been calculated (Figure 8B). The average transmittance value of ZnO samples synthesized at 200, 300 and 400 ^oC in the entire visible region is about 80%, but the samples synthesized at 500 and 600 ^oC show a relatively low transmittance, especially for the ZnO sample synthesized at 600 ^oC. The reason for the reduced transmittance of ZnO sample synthesized at higher temperature is still unclear until now. However, the high c-axis preferential orientation ZnO with high transparency has been obtained, indicating that the character could be used for future high-transmittance ZnO based optoelectronic devices.

Because the ZnO phase can adsorb oxygen molecules in the general environment and desorb oxygen molecules under UV illumination, it can be used as UV photodetector. Oxygen molecules can adsorb onto a ZnO surface by capturing free electrons from the conduction band of the ZnO [O₂ +e^–→ O₂^–], which yields a depletion layer near the surface, leading to greatly decrease the conductivity of the ZnO. When ZnO is illuminated by photon energy more than band gap of the ZnO (i.e., UV light), electron-hole pairs will be generated [hʋ→ e^– + h⁺]. These carriers from electron-hole pairs will move toward the surface and neutralize the adsorbed oxygen molecules [h⁺ + O₂^–→ O₂]. The neutralization oxygen molecules can effortlessly desorb form the surface of ZnO, which makes the conductivity increase significantly. Therefore, according to this theory, the high c-axis ZnO thin film was used as a sensing layer to fabricate UV photodetector. The UV detection characteristics of the fabricated Pt/ZnO photodetector were investigated and confirmed by measuring the I–V curve with and without UV light illumination and time-dependent photocurrent response (Figure 9 and 10). An obvious symmetric behavior could be observed, indicating that the high quality Ohmic contact has been achieved at the Pt/ZnO interface after post-annealed in argon at 450 ^oC by RTA. According to the measured I–V curves, the dark current is about 0.36 mA and the photo-current is about 4.3 mA at 5 V, indicating the current difference between the UV illuminated and dark condition. Because the performance of UV photodetector is critical dependent on its response and recovery times, the time-dependent photo-current response was implemented. The response time is usually defined as the time to approach 90 % of the maximum photo-current, and recovery time is the time to decay to 10 % of the maximum photo-current. From the time-dependent photo-current response measurement, the response and recovery time are about 3 and 9 s, respectively. Moreover, the UV light was switched on and off five times for testing the reliability. According to the above measurements, the ZnO based UV photodetector shows a fast responsivity and high reliability, which could be used in potential development for commercial UV photodetector application.

In summary, this present study provides a method to synthesize high c-axis preferential orientation ZnO thin film onto Si substrates by PECVD with a fine control of the synthesized temperature. The optimal c-axis ZnO thin film was synthesized at 400 ^oC showing a competitive and functional characteristic in terms of crystal structure, optical property, and transparency to the visible light. The ZnO based UV photodetector combined with Pt interdigitated electrode with Ohmic contact exhibits a fast responsivity and high reliability under UV light (38 mW cm^-2) at the bias of 5 V. This present work could be a valuable direction and application both in research and industry.

Potential Benefits and Drawbacks of PECVD Technique

Plasma enhanced chemical vapor deposition (PECVD) is a useful technique which has been employed to synthesize thin films from vapor state to solid state on the substrates. The plasma is generally originated by DC or RF power supply between showerhead electrode ordinarily used as top electrode and sample stage usually used as bottom electrode, respectively. The space between these two electrodes is filled with the chemical reactions from reactant gases. The crystal orientation and composition of the sample is dependent on the synthesis condition and the ratio of reactant precursors. For instance, the DEZn and CO₂ as precursors were used to provide zinc and oxygen sources for synthesizing ZnO thin film, respectively. Obviously, the plasma is a main feature in this technique. Through the plasma enhanced assistance, ionized reactant atoms or molecules that effectively diffuse to the substrate surface, and then readily react with neighboring ionized atoms or molecules to form high dense configuration thin film. As a consequence, all the samples can be exposed to energetic ion bombardment during the synthesis process.

Because the plasma reaction is consecutively occurring during the synthesis process, the main benefits of PECVD technique include: (a) the lower synthesis temperature, almost all samples can be synthesized at low temperature (from 100 to 450 ^oC), (b) high aspect ratio condition is available (if use high density plasma), (c) high deposition rate, (d) RF powered showerhead with optimized gas delivery, provides uniform plasma processing, (e) good thin film surface uniformity, and (f) chemical composition structure can be finely controlled.

However, the PECVD technique involves chemical reaction and high density plasma bombardment, and it has some limitations and drawbacks. The whole process will produce a large number of chemical and particulate pollutions, required careful and appropriate handling. High pure gases are necessary in this technique, so the limitation of a specific precursor such as DEZn that can be processed in solution. If the source does not have the ability to dissolve in some solvent to prepare volatile precursor, the carrier gas will not deliver the reagent gas into the chamber to do synthesis. Moreover, although high density plasma bombardment can improve uniformity and quality of thin film, high residual stress will be created resulting in break of the thin film. Also, this technique needs an extra DC or RF power supply system leading to a little higher in cost compared with conventional chemical vapor deposition (CVD).

Future Directions and Applications

Many process parameters such as working pressure, gas composition, gas flow rate, RF power, reactant precursor and others are all interdependent for synthesizing thin films in the PECVD technique. Only the change of synthesis temperature has been studied in this presented research. There is still a lot of work requiring for further investigating the limitations and capabilities in this technique. For example, the gas composition could vary the stoichiometry of thin films and working pressure could affect the mean free path of ionized gases, which will yield diverse thin film configuration. Therefore, future work will require for further understanding how to manipulate the crystal quality or properties through above mentioned parameters.

The PECVD technique has been widely used in the manufacturing of the synthesis of thin films on the semiconductors which usually requires low process temperature. The PECVD method has been allowed and used to synthesize thin film on the polymer substrates, which also can be employed in flexible devices application. In addition to the low temperature application, we have already fabricated non-polar ZnO thin film by increasing the synthesis temperature, which could be used in high-efficiency Light-Emitting Diode (LED) or environmental sensor technology.