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Arsenic doped p-type ZnO films are prepared by the photo-assisted metal organic chemical vapor deposition method. Using the photo-assisted technique, the acceptor activation process is simplified. The arsenic doping level, which decides the carrier distribution, could be controlled by changing the thickness of the pre-deposited GaAs layer. The crystal and optical quality of the ZnO films is good. The acceptor is AsZn–2VZn. Its ionization energy could be slightly reduced by increasing the arsenic doping level. This finding is very helpful to improve the hole concentration. Our experiments provide a new method to grow high performance p-type ZnO based photoelectric devices.
© 2017 Optical Society of America
1. Introduction
ZnO is an important II-VI compound semiconductor, which has a wide direct band gap of 3.37 eV at room temperature and large exciton binding energy of 60 meV. It has been recognized as a promising material for optoelectronic devices, such as ultraviolet light-emitting diodes (LEDs), laser diodes (LDs), photodetectors and solar cells [1–4]. In order to develop devices based on ZnO, complementary doping (both n-type and p-type doping) is necessary. The un-doped ZnO is n-type due to the formation of donors such as O vacancies (Vo) and Zn interstitials (Zni). And high quality n-type ZnO could been obtained by doping with Ga and Al elements [5,6]. Therefore, the growth of high-quality and stable p-type ZnO is essential. Many p-dopants, such as N, P, As, Sb, Co and dual-acceptor dopant [7–13], have been tried to grow p-type ZnO films. But it is still very difficult to grow stable p-type ZnO with reliable performance. The difficulty can be summarized for low solubility and deep level of the acceptor dopants [14]. Among these dopants, arsenic has been proposed to be a good acceptor impurity in the doping process of p-type ZnO thin films [15–17]. But the arsenic doped into ZnO is a complex acceptor (AsZn–2Vzn) with a relatively high ionization energy of 137 meV [18]. So in most of the literatures, which report the growth of arsenic doped p-type ZnO, have a same and necessary thermal annealing activation process in oxygen atmosphere.
Many methods have been used to grow arsenic doped p-type ZnO. The metal organic chemical vapor deposition (MOCVD) has advantages in doping and multilayer growth. It has already realized the industrialization application. In MOCVD system, researchers usually chose GaAs substrates to grow arsenic doped p-type ZnO, where arsenic acceptors could diffuse into the ZnO film by a subsequent thermal annealing process [17–20]. The suitable range of thermal annealing activation temperature and time are 500-550 °C and 30-60 min in oxygen atmosphere. However, this technical route is restricted to the GaAs substrate. And the GaAs substrate is also acted as an infinite impurity source, which is not conducive to control the arsenic doping level in the ZnO films. In previous report [17], we have grown arsenic doped p-type ZnO films using pre-deposited GaAs interlayer as finite surface doping source by MOCVD method with 60 min in situ thermal annealing process. Despite the p-type ZnO thin films is obtained, however, it is found that the thermal activation efficiency is not high and the controllable doping characteristics need further investigating. More importantly, beside the Hall measurement giving an overall result, another new method should be induced to carefully study the electrical properties of the p-type ZnO materials. In this letter, the arsenic doping controllable p-type ZnO films were grown by photo-assisted metal organic chemical vapor deposition method. Owing to the photo-assisted technique, the process of acceptor activation becomes much simplified. In order to carefully investigate the electrical characteristics of the ZnO films, the electrochemical capacitance-voltage profiling measurement was induced for its advantages in determining the uniformity of doping with depth. And then, the structure and the optical characteristics of p-type ZnO films with different arsenic doing level are systematically studied.
2. Experiments
On the first step, the GaAs layer, acting as finite surface doping source, was deposited on sapphire substrates by RF magnetron sputtering method at room temperature. High-purity polycrystalline GaAs and argon were used as target and sputtering gas. The sputtering power and working pressure were 80 W and 1 Pa, respectively. In order to control the arsenic doping level, we prepared two kinds of composite substrates (GaAs/Al2O3) that covered with about 10 and 20 nm GaAs layer, respectively.
On the second step, the ZnO films were grown on the two composite substrates by photo-assisted MOCVD method. The tungsten–halogen lamps were used as a light source. Its continuous blackbody radiation spectra of the lamp cover the near-UV–vis–IR range [12] with cutoff wavelength of about 200 nm (6.2 eV). The long wavelength light is helpful to increase the growth temperature, and the short wavelength light (high-energy photons) is very helpful to promote the reaction with activation energy (such as acceptor activation). Here, we use five lamps in the reaction chamber. They are placed side by side in 5 cm away from the center of the sample holder. The lamps total input power is 600 W. In addition to the light, there was a thermal heater under the substrates. The total growth temperature near the substrate is 430 °C. The zinc source, oxygen source, chamber pressure and growth time were diethylzinc, oxygen, 200 Pa and 30 min, respectively. The diethylzinc is carried into the reaction chamber by high purity argon gas. The oxygen gas is directly put into the reaction chamber through different channel. Then, the ZnO films were in situ treated in oxygen atmosphere at 460 °C for 10 min to reduce the oxygen vacancy defects. The film thickness is ~400 nm. Here, we marked the ZnO films grown on composite substrates with 10 nm and 20 nm GaAs layer as less-doping sample A and more-doping sample B, respectively. The local chemical states and doping characteristics were investigated by X-ray photoelectron spectroscopy (XPS). Before testing, the sample surface were cleaned by Ar + -etching. The instrument model of the XPS system was ESCALAB 250xi, which could analyze the elements (except H and He) more than 0.1% atomic percentage. The structural quality of the ZnO films were characterized by X-ray diffraction (LabX XRD-6100, SHIMADZU) with Cukα radiation. The optical quality and the impurity levels were studied by temperature-dependent photoluminescence (PL) measurement. The PL spectra were measured with a He-Cd CW laser operating at 325 nm. The mode, power, beam diameter, beam divergence and incident angle were TEM00, 30 mW, 1.2 mm, 0.5 mrad and 45 degree, respectively. And the PL emission was detected using iHR-320 Jobin-Yvon monochromator with a charge-coupled device (CCD) detector. The spectral resolution of the spectrometer was 0.06nm. Hall-effect measurements were carried out to estimate an overall result about the film conductivity. And the changes of carrier concentration with measurement depth were carefully investigated by PN4400 electrochemical C–V profiler made by Accent Optical Technologies Inc. The 0.1 M ZnCl2 solution was used as an electrolyte.
3. Results and discussion
Figure 1 shows the XPS spectrum of As-3d core level of the less-doping sample A and more-doping sample B ZnO films. As seen, the peak located around 44.0 eV could be find in both samples. It indicates that arsenic atoms have already diffused into the ZnO films. Because the peak intensity of sample A is weaker than that of sample B, we could estimate that the amount of diffusing arsenic atoms in sample A is also much less than that of sample B. In other words, we could control the arsenic doping level by controlling the pre-deposited GaAs layer thickness. As expected, the gallium related signals are still not detected. It indicates that the gallium atoms have not diffused into the ZnO films because of the relatively lower growth temperature. This is consistent with our previous results and other references [17–22].
The XRD patterns are plotted in Fig. 2. The strongest diffraction peaks caused by ZnO (002) plane (for sample A and B) are located round 34.4°, which indicates the arsenic doped ZnO films have wurtzite-type structure with (002) preferred orientation. The other weak peaks located at 36.28° are caused by the diffraction of ZnO (101) plane [23]. The XRD results indicate that arsenic doped ZnO films has a good crystal quality. The inset of Fig. 3 shows room-temperature PL spectra. Both of the ZnO samples have a strong ultraviolet (UV) emission peak at 377-380 nm, which are caused by the radiative recombination between electrons and holes near the band edge. The intensity of visible region emission (from 430 nm to 550 nm) for less-doping sample A is much stronger than that for more-doping sample B. Because visible emission is ascribed to donor like defects (such as VO or Zni [14]), the PL results indicate that a large amount of donor like defects exists in the less-doping ZnO film. Furthermore, it is found that the UV peak positions of the two samples have a little difference. In order to investigate the internal mechanism, low-temperature PL measurement are implemented at 10K. As seen in Fig. 3, the neutral acceptor bound exciton (A0X) related emission peaks are located around 3.351 eV for both samples [24,25]. And the peaks at 3.305 eV (for less-doping sample A) and 3.314 eV (for more-doping sample B) are attributed to the recombination emission between free electrons and acceptor holes (FA) [24,25]. Comparing with sample A, the intensity of FA emission in sample B is much closer to that of A0X emission. This indicates the effect of the arsenic related acceptor would be much greater in more-doping sample. In addition, the emission peaks caused by recombination of donor acceptor pair (DAP) have different positions in the two samples. For sample A, the DAP emission is located at 3.221 eV, and the emission at 3.141 eV is attribute to the first-order longitudinal optical (LO) phonon replicas of the DAP emission [24,25]. For sample B, the DAP emission is located at 3.240 eV, and the emission at 3.166 eV is also attribute to the first-order longitudinal optical (LO) phonon replicas of the DAP emission [24,25]. The acceptor level can be calculated from the following equation: EA = Eg-EFA + kBT/2, where Eg is the intrinsic band gap, EFA is the transition between free electrons and acceptors. As seen from low temperature PL results, the value of EFA could be regarded as 3.305 eV and 3.314 eV for sample A and B. And Eg is evaluated as 3.437 eV [24]. So the EA is derived to be about 132 meV and 123 meV, respectively. They are consistent with the AsZn–2VZn acceptor model [18]. Moreover, the little difference of Ea between sample A and sample B indicates that the arsenic related acceptor level could become closer to the valence band by increasing the arsenic doping level. This is very helpful to improve the acceptor ionization efficiency, furthermore, obtaining good p-type conductivity. In addition, according to the low-temperature PL results, it could be inferred that the amount of the AsZn–2VZn acceptor is larger in more doping sample B. The appearance of VZn could reduce the amount of the Zni. So, in the inset of Fig. 3, the intensity of visible emission for more doping sample B is weaker than that in less doping sample A.
Fig. 3 PL spectra of the ZnO films at 10K, the inset shows the normalized results at room temperature.
The electrochemical capacitance-voltage (ECV) profiling measurement is induced to investigate the electrical characteristics of the ZnO films for its advantages in determining the uniformity of doping with depth. The suitable electrolyte is 0.1 M ZnCl2 [26], which could make a good rectifying contact with ZnO films as well as properly remove it. Figure 4 shows the capacitance C versus voltage V plots for the less-doping and more-doping ZnO films. The results of C-V measurement indicates the ideal schottky barrier have been formed between ZnCl2 electrolyte and p-type ZnO films. As seen from Fig. 5, the slope of C−2-V curve is small for the more-doping ZnO film. This indicate that the hole concentration will be larger in it. Figure 6 shows the carrier concentration profile versus depth measured for the different ZnO films. For less-doping sample A, the hole concentration is 2.5 × 1016 cm–3 near the film surface, and it slowly increases to a maximum value of 3.4 × 1016 cm–3 at depth of 0.2 μm. Then, it decreases slightly to 3.0 × 1016 cm–3 on approaching the interface. This is caused by the non-uniformity distribution of the arsenic atoms from the relatively thinner pre-deposited GaAs layer, for which the arsenic atoms reaching the ZnO surface is very likely smaller than that in the film. For more-doping ZnO film, the hole concentration is 2.1 × 1017 cm–3 near the film surface, it slowly decreases to a minimum value of 1.8 × 1017 cm–3 at depth of 0.4 μm. As seen, the distribution of the hole concentration is relatively uniform. It indicates that the 20 nm pre-deposited GaAs layer is suitable under our selected experiment conditions. In addition, Hall measurement is carried out to give an overall electrical properties of the samples. The hole concentration of sample A and sample B are 3.1 × 1016 cm–3 and 2.4 × 1017 cm–3, respectively. The above results shows that the electrical characteristics of the arsenic doped p-type ZnO films could be controlled by changing the thickness of pre-deposited GaAs layer.
Fig. 4 Capacitance versus measurement voltage for different ZnO samples with 0.1 M ZnCl2 electrolyte.
4. Conclusion
In conclusion, doping controllable p-type ZnO films were prepared using photo-assisted metal organic chemical vapor deposition without conventional long-time and high-temperature thermal annealing process. Under suitable growth conditions, the diffusion of arsenic atoms would not greatly affect the crystal structure and the room-temperature optical quality of the ZnO films. Based on the low-temperature photoluminescence measurement results, the arsenic related acceptor is confirmed as AsZn–2VZn. The results of electrochemical capacitance-voltage profiling and X-ray photoelectron spectroscopy measurements indicate that the arsenic doping level could be controlled by changing the thickness of the pre-deposited GaAs layer, which would decide the level and the distribution of the hole concentration in the p-type ZnO films. Mover, we found the acceptor ionization energy could be slight reduced by increasing the arsenic doping level. This is very helpful to further improve the hole concentration. Our experiments provide a new method to grow high performance p-type ZnO, which could be widely used in fabricating ZnO related photoelectric devices.
Funding
The authors gratefully acknowledge the support of the National Natural Science Foundation of China (NSFC) (11601069) and the Scientific Research Fund of Liaoning Provincial Education Department (L2014457).
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