Time-resolved ultraviolet photoluminescence of ZnO/ZnGa₂O₄ composite layer

ZnO has attracted much attention as a promising material for ultraviolet optoelectronic devices. Growth and optical properties of high quality ZnO with various morphologies have been widely investigated to improving the efficiency of optoelectronic devices. However, because of the intrinsic ZnO with native defects acting as the luminescent centers in visible region, it is important to enhance the band-edge emission and suppress the defect-related emission.^1–5 

It has been proved that the intrinsic defects and crystalline quality of ZnO can be affected by extrinsic doping like group III elements (B, In, Al, Ga). Among these dopants, Ga shows its advantages over others such as lower reactivity with oxygen and smaller distortion of ZnO host lattice.⁶ The gallium related compounds such as triethylgallium,⁵ gallium nitrate,⁶ gallium oxide⁷ have been used as the Ga doping sources. In addition, the incorporation of gallium from solid substrates into ZnO crystals may occur through the diffusion during the growth process.^8–10 

Previously, we reported the photoluminescence (PL) properties of ZnO/ZnGa₂O₄ composite layer grown by the thermal oxidation of ZnS with gallium, which showed a single ultraviolet (UV) emission with long lifetime compared with the pure ZnO layer grown without gallium.¹¹ It is suggested that the gallium impurity incorporated into ZnO host crystals and improved the crystalline quality in the surface regions of ZnO/ZnGa₂O₄ composite layer.^1,5 However, the PL dynamics of ZnO/ZnGa₂O₄ composite layer is not clear. Therefore, in this research, the detailed ultraviolet PL with related carrier dynamic recombination process are investigated by the temperature dependent time-resolved PL.

The ZnO/ZnGa₂O₄ composite layer and pure ZnO layer were grown by the thermal oxidation of ZnS substrates at a temperature of 800 °C in the air with and without gallium, respectively. The ZnO/ZnGa₂O₄ composite structure consists of ZnO surface layer and ZnGa₂O₄ buffer layer on the substrate. The detailed growth procedure was reported previously.¹¹ The time-resolved PL measurement was performed within a temperature range of 10 ∼ 300 K. Frequency-tripled optical pulses with 100 fs pulse width generated from a Ti-sapphire laser were used for excitation. The excitation photon energy and power were 4.66 eV (266 nm) and 0.1 ∼ 5 mW, respectively.

The inset in Figure 1 shows the PL spectrum from pure ZnO layer at 10 K with excitation power of 1 mW. The PL peaks at 3.36 and 3.31 eV can be assigned to the neutral donor bound exciton (D⁰X) and donor-acceptor pair (DAP) emissions, respectively.¹² The additional features at 3.24 and 3.17 eV are attributed to the LO phonon replicas of DAP emission (DAP-1LO and DAP-2LO) with energy separation of 70 meV between them. It is clearly observed that the band edge emission from pure ZnO layer is dominated by the DAP excitons.

Fig. 1 shows the temperature dependent PL spectra from ZnO/ZnGa₂O₄ composite layer in the range of 10 ∼ 300 K with excitation power of 1 mW. The emission peaks of PL spectrum at 10 K at 3.35, 3.30, 3.23 and 3.16 eV correspond to the D⁰X, DAP, DAP-1LO and DAP-2LO emissions, respectively. The decreased PL intensity above 200 K indicates that there was a quenching process through non-radiative recombination.

By comparing the PL spectra from ZnO/ZnGa₂O₄ composite layer and pure ZnO layer at 10 K, it is clearly observed that the D⁰X emission from ZnO in ZnO/ZnGa₂O₄ composite layer was dramatically increased. The increase of D⁰X emission from Ga-doped ZnO nanowires⁸ or nanorods,⁹ and the suppression of DAP emission from In-doped submicron-sized ZnO crystals¹³ were observed as well at low temperatures. The enhanced D⁰X emission from ZnO in ZnO/ZnGa₂O₄ composite layer should be attributed to the incorporation of gallium impurity.

Figure 2 shows the time-evolutions of PL from ZnO/ZnGa₂O₄ composite layer and pure ZnO layer at two photon energies corresponding to the emission of D⁰X and DAP at 10 K with excitation power of 1 mW. Both D⁰X emissions from ZnO/ZnGa₂O₄ composite layer and pure ZnO layer showed a bi-exponetial decay process, which can be fitted by a bi-exponential decay function: A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂), where A₁ and A₂ are the corresponding amplitudes of the fast decay component (τ₁) and slow decay component (τ₂), which are generally attributed to the non-radiative relaxation and radiative recombination, respectively. The decay times of D⁰X emission from ZnO/ZnGa₂O₄ composite layer were τ₁ = 90 ps and τ₂ = 1293 ps while those from pure ZnO layer were τ₁ = 51 ps and τ₂ = 400 ps. However, it is found that both the amplitude ratios are A₁/A₂ = 99:1, which shows the main contribution of fast decay component to the D⁰X emission even the gallium was involved, though the non-radiative lifetime of D⁰X emission from ZnO/ZnGa₂O₄ composite layer is 1.76 times that from pure ZnO layer. On the other hand, the DAP emission from pure ZnO layer showed a single-exponential decay process with lifetime of 668 ps while that from ZnO/ZnGa₂O₄ composite layer showed a bi-exponential decay process with τ₁ = 187 and τ₂ = 1182 ps. The amplitude ratio A₁/A₂ = 60:40 shows the slightly larger contribution of fast decay component to the DAP emission from ZnO/ZnGa₂O₄ composite layer.

The ZnO/ZnGa₂O₄ composite layer showed the longer radiative lifetime than the pure ZnO layer, which means that the photo-generated carriers from ZnO in ZnO/ZnGa₂O₄ composite layer can diffuse to a distance before the radiative recombination. It was suggested that the carriers have a possibility to be trapped to the surface states, depending on the defect concentration, they may either tunnel to defect levels within the band gap and recombine through the radiative process or remain to the surface states and recombine through the non-radiative process.¹⁴ It is considered that the carriers diffusing in pure ZnO layer mainly tunneled to the defect levels resulting in the visible emission, while those trapped in surface states in ZnO/ZnGa₂O₄ composite layer recombined through the non-radiative process due to the suppression of defect-related emission.¹¹ Therefore, it is suggested that the fast decay of D⁰X and DAP emissions from ZnO in ZnO/ZnGa₂O₄ composite layer mainly originated in the surface recombination.

It has been reported that the surface recombination of pure ZnO nanorods was suppressed by the thermal treatment and the contribution of fast decay was reduced consequently.¹⁵ In contrast, the fast decay component of ZnO/ZnGa₂O₄ composite layer still showed the larger contribution to the combination process of D⁰X and DAP emissions. It is considered that the additional energy level induced by the Ga doping influenced the recombination dynamics as well. It has been reported that an energy or charge transfer process from the host to the dopant is generally associated with the luminescent transitions of Cu doped ZnO.^16–18 Differing from the Cu acceptor, the substitution of Ga donor at the Zn sites suppressed the visible emission of ZnO.¹¹ Besides the surface recombination, it is assumed that the transient charge transfer from the ZnO host to the Ga dopant probably occurred during the non-radiative recombination process, although the fast decay time of D⁰X and DAP emissions from ZnO/ZnGa₂O₄ composite layer changed reversely as compared to the pure ZnO layer.

Figure 3 shows the PL decay times as functions of temperature for ZnO/ZnGa₂O₄ composite layer with excitation power of 1 mW. Both of the D⁰X and DAP emissions showed the similar temperature dependence of decay time. The radiative lifetime reached a maximum value at 50 K, which is longer than that at 10 K. The increase of radiative lifetime below 115 K has been observed in the pure ZnO nanowires.¹⁹ It is suggested that there exists a competition between the radiative and nonradiative recombination process, the radiative recombination process dominates during the recombination process below the threshold temperature.¹⁹ 

In particular, the abrupt decrease is clearly observed for the radiative lifetime. The drastic drop of decay time with temperature has been also observed in the pure ZnO epilayer, which was attributed to the strong temperature dependence of the bound exciton circular polarization as a consequence of the quasi-equilibrium population distribution between the free and bound states.²⁰ In the case of ZnO/ZnGa₂O₄ composite layer, it is considered that the bound exciton circular polarization is probably responsible for the sudden decrease of radiative lifetime. On the other hand, the increase trend of fast decay became obvious at higher temperatures as well as the slow decay. At this stage, it is considered that the of residual non-radiative channels were thermally activated and the non-radiative process was enhanced.^8,21,22 However, the slow decay was still much longer lifetime than the fast decay even at room temperature, which suggests that the slow PL decay of ZnO/ZnGa₂O₄ composite layer was mainly determined by the radiative lifetime and the radiative recombination process controlled the carrier recombination even at room temperature.

Figure 4 shows the PL spectra from ZnO/ZnGa₂O₄ composite layer at 10 K with excitation powers in the range of 0.1 ∼ 5 mW. It is observed that the relative intensity of DAP/D⁰X emission was dramatically increased with the excitation power. It indicates that the D⁰X emission was more easily saturated than the DAP emission. The similar phenomenon was observed in N-doped ZnO powder previously.²³ In addition, the slight red-shift of D⁰X emission with the excitation power may be related to the increase of DAP transition.²⁴ The broadened emission bands of D⁰X and DAP under higher excitation power may be due to the electric field screening effect.²⁴ 

The ultraviolet photoluminescence of ZnO/ZnGa₂O₄ composite layer was investigated by the time-resolved PL as a function of measuring temperature and excitation power. The ZnO in ZnO/ZnGa₂O₄ composite layer showed the D⁰X and DAP emissions around 3.35 and 3.30 eV, respectively. The D⁰X emission was easily saturated than the DAP emission with the excitation power. The D⁰X and DAP emissions from ZnO/ZnGa₂O₄ composite layer showed a bi-exponential decay process, however, the distinct contribution of slow decay component to the D⁰X and DAP emissions was observed. The radiative recombination process with ultra-long lifetime controlled the carrier recombination of ZnO/ZnGa₂O₄ composite layer even at room temperature though the fast decay increased and the slow decay decreased with temperature. In addition, the D⁰X emission from ZnO/ZnGa₂O₄ composite layer was dramatically enhanced as compared with that from pure ZnO layer grown without gallium.

This work was supported by National Natural Science Foundation of China (No. 51202191), Natural Science Basic Research Plan in Shaanxi Province of China (No. 2012JQ6002), and Scientific Research Program Funded by Shaanxi Provincial Education Department of China (No. 12JK0427). The authors would like to thank N. Asaka and T. Aritake of Waseda University for their assistance with data analysis.