Study of Ultraviolet Emission Spectra in ZnO Thin Films

Photoluminescence (PL) of ZnO thin �lms prepared on c-Al2O3 substrates by pulsed laser deposition (PLD) are investigated. For all samples, roomtemperature (RT) spectra show a strong band-edge ultraviolet (UV) emissionwith a pronounced low-energy band tail.e origin of this UV emission is analyzed by the temperature dependence of PL spectra.e result shows that theUV emission at RT contains different recombination processes. At low temperature donor-bound exciton (DX) emission plays a major role in PL spectra, while the free exciton transition (FX) gradually dominates the spectrum with increasing temperatures. It notes that at low temperature an emission band (FA) appears in low energy side of DX and FX and can survive up to RT. Further con�rmation shows that the origin of the band FA can be attributed to the transitions of conduction band electrons to acceptors (e, A), in which the acceptor binding energy is estimated to be approximately 121meV. It is concluded that at room temperature UV emission originates from the corporate contributions of the free exciton and free electrons-to-acceptor transitions.


Introduction
ZnO, with a direct band gap of 3.37 eV and a binding energy of exciton as high as 60 meV at room temperature (RT), has been extensively studied as a candidate material for ultraviolet (UV) light emitting diodes (LEDs) and laser diodes (LDs) [1][2][3].To realize the application of these devices, it is necessary to fabricate undoped ZnO thin �lms which avail to obtain a stable high-yield exciton emission at RT.However, it is well known that the fabrication of the high quality ZnO �lms is rather difficult.�ecause it is common to use Al 2 O 3 as the substrate for the growth of ZnO thin �lms, 18% lattice mismatch between ZnO and Al 2 O 3 results in the presence of various native defects in ZnO thin �lms [4][5][6][7].ese defects oen control directly or indirectly doping, compensation, minority carrier lifetime, and luminescence efficiency.Consequently, it is very important to understand behavior of these defects in ZnO-based materials.Photoluminescence (PL) emission spectroscopy is a useful method to examine the quality of the grown ZnO thin �lms, which may provide important information on understanding the carrier recombination processes and the role of defects in ZnO.
In the reported PL spectra, the origin of the room temperature UV emission was extensively studied.Most of the authors suggested that the UV emission at RT originates from free exciton recombination [8][9][10][11][12].However, Ohashi et al. reported that the most intense emission at RT for undoped crystals was not free-exciton recombination but was related to an unspeci�ed localized state [13].Zhao and Willander found that the room temperature UV emission contains two different transitions, in which one is related to the ZnO freeexciton and the other is related to the free-to-bound transition [14].Up to now, the room temperature UV emission is still in debate.In addition, the controversies on PL properties also present to some UV emission bands obtained at low temperature.For example, the 3.31 eV emission band observed in a great variety of ZnO materials has been interpreted controversially.Many authors have assigned this UV emission band to longitudinal-optical (LO) phonon replicas of the excitons (FX-LO) [15,16], acceptor-bound excitons (A 0 X) [17], electron-hole recombination from donor acceptor pairs (DAP) [18], free electron-to-acceptor transition (e, A 0 ) [19], and so forth.Noticeably, the 3.31 eV emission band is also frequently observed in intentionally p-doped ZnO [20][21][22][23][24][25].
Most of the works revealed that it is as a test criterion of p-type conductivity formed by substitutional acceptors.Recently, a remarkable work was reported in undoped ZnO epitaxial layers grown on a-Al 2 O 3 substrates, in which the 3.31 eV emission band observed at low temperature originates from a (e, A 0 ) transition.And they give clear evidence that the localized acceptor states causing the 3.31 eV luminescence should be associated with the stacking faults rather than the substitutional impurities [19].
On the other hand, the measurements of the electrical properties in undoped ZnO thin �lms grown on Al 2 O 3 substrate show n-type conductivities with low electron mobilities of <100 cm 2 V −1 s −1 , which is quite smaller than 300 cm 2 V −1 s −1 based on the reported value in ZnO �lms grown on lattice-matched ScAlMgO 4 substrates [2].Such low electron mobility implies the existence of the scattering mechanisms due to unknown localized states.In conclusion, until now the impact of these defects on the optical and electrical properties of ZnO is still a subject of much debate.e clarifying of the PL origin not only can deepen and enrich the research of the impurity and defect behaviors but also is more advantageous for the development of ZnO-based devices.
In this paper, we report near band edge UV luminescence in the ZnO thin �lms grown on c-Al 2 O 3 substrates by PLD method.e mechanism of the UV emission band is investigated by the temperature dependence of PL spectra.e 3.31 eV emission band observed at low temperature is assigned to the transitions of conduction band electrons to acceptors.It is suggested that at room temperature UV emission is composed of two recombination processes.One is the free-exciton emission (FX), another is the free electron -to-acceptor emission (e, A 0 ).

Experiment
ZnO thin �lms were fabricated on c-Al 2 O 3 substrates by using a KrF excimer laser (Lamda Physics Compexpro 205,   24 nm,   20 ns pulse duration).e laser beam was focused onto a rotating target at a 45 ∘ angle of incidence, and the energy density of the laser beam at the target surface was maintained at about 2 J/cm 2 .A 99.99% purity ZnO ceramic target with the thickness of 4 mm and the diameter of 60 mm was used as source materials.
Al 2 O 3 substrates degreased in acetone and methanol for 10 min, respectively, and then etched in a hot (160 ∘ C) solution of H 2 SO 4 : H 3 PO 4 = 3 : 1 for 15 min., followed by a rinse in deionized water and dried by the high-pure nitrogen gas before being loaded into the growth chamber.Prior to growth, the chemical cleaned substrates were thermally treated at 800 ∘ C in high vacuum atmosphere (∼ 6 × 10 −4 Pa) for about 30 min to remove the surface contaminants.Sequentially, ZnO was deposited on this treated substrate at 700 ∘ C for 120 min.In the growth process, O 2 partial pressure in the growth chamber was varied from 0.2 to 5 Pa.e repetition frequency of the laser was 5 Hz, and the targetsubstrate distance was 8.5 cm.
A�er growth, the sample quality was con�rmed by a Rigaku O/max-RA X-ray diffractometer with Cu   radiation (  0142 nm).Photoluminescence spectra were measured at different temperatures.e sample was attached to the cold �nger of an optical cryostat in conjunction with a cryogenic refrigerator and cooled down to ∼10 K. e 325 nm line of a He-Cd laser with a power of 20 mW was used as the excitation source.e photoluminescence from the sample was dispersed through a monochromator (ZLX-FS Omni-3005) and detected by a photomultiplier tube (Hamamatsu R928) followed by a photon counter (Zolix DCS200PC).e carrier concentration and Hall mobility were measured by ET-9007 Hall measurement system through the Van de Pauw method.

Results and Discussion
Figure 1 shows the patterns of X-ray diffraction (XRD) for four samples of ZnO thin �lms grown on c-Al 2 O 3 substrates at different O 2 partial pressures in the growth chamber, which are 5, 3, 1, 0.5 Pa for the samples A, B, C, and D, respectively.It is noted that besides the Al 2 O 3 (006) peak, only ZnO (002) and (004) diffraction peaks can be observed for all samples.is indicates that the grown ZnO thin �lms have the wurtzite structure with a high c-axis orientation.To further con�rm the crystal quality of the ZnO �lms, XRD (103) -scan measurements were performed.e inset of Figure 1 shows the measured result for the sample A. It is clearly seen the six peaks separated by 60 ∘ with almost same intensities, indicating the formation of a sixfold symmetric single-crystal ZnO.
Figure 2 shows photoluminescence (PL) spectra in UV region range for the above samples at RT excited by a He-Cd laser with 325 nm line.As seen in Figure 2, one UV emission band with a central wavelength of 377.5 nm (3.284 eV) can be observed for the four samples.It is obvious that this UV emission band has a large linewidth (>100 meV) and one shoulder (arrow in the �gure) can be clearly seen at lower-energy side of this peak.e ZnO RT UV emission is extensive reported as a characteristic excitonic emission in the literatures [8][9][10][11][12].In addition, Most of the works indicated that low-energy band tail of UV peak at RT is associated to LO-phonon replica of free exciton [16,21].
In order to study the origin of the room temperature UV emission band, the temperature dependence of the PL spectrum from the ZnO thin �lms grown on a sapphire substrate has been measured.Figure 3 shows the PL spectra at various temperatures for the sample A. At 10 K, the spectrum mainly composed of a strong emission of D 0 X band and a weak band labeled FA located at 3.355 and 3.309 eV, respectively.As temperature increases to 50 K, the FA-LO band appears in low energy side of the band FA.In addition, one peak (FX) can be clearly observed in high energy side of D 0 X band at 50 K and becomes stronger and stronger with temperature increasing to 130 K.At 90 K, the FX at 3.370 eV is comparable in intensity to the remaining D 0 X emission at 3.350 eV.As the temperature increases from 130 to 260 K, the intensity of D 0 X band decreases rapidly and the FX emission band becomes increasingly important in spite of its intensity decreases with increasing of temperature.It notes that at 110 K an emission band labeled FA-2LO appears in low energy side of the FA-LO band, and the bands of FA, FA-LO, and FA-2LO can survive up to 260 K. Signi�cantly, a careful study of these bands should be necessary to consider because of their contribution to the room temperature UV emission.
Figure 4 shows the �tted spectra by multipeaks of Lorentzian line shape at the four typical temperatures.According to their energy values, the FX and D 0 X peaks were attributed to the emission of free exciton and the recombination of excitons bound to neutral donors, respectively.As reported in [18], D 0 X assigned to donor-bound excitons with 10-15 meV binding energy dominates in low temperature spectra and the free-exciton recombination plays a major role in high temperature.By comparing with the peak positions of the FA and FX, two peaks have the energy spacing of about 47 meV, which is less than the energy of ZnO LO phonon [17].erefore, it can be believed that the FA peak should be associated with the impurity or defects rather than the �rst LO phonon replica of the free exciton recombination (FX).
In our previous work, PL spectrum at 80 K of undoped ZnO thin �lm grown by plasma-assisted molecular beam epitaxy (P-�B�) clearly shows the �rst and the second LO phonon replica of FX (FX-LO and FX-2LO) [21].In this paper, the fabrication of the samples was used by PLD method on a deviation from the stoichiometric ratio condition.e measure of the �lm thicknesses shows the increase of the growth rate with increasing O 2 partial pressure.is indicates that the growth of the �lms is on a rich-Zn condition, resulting in the observation of FA emission related to the defects.For FA-LO and FA-2LO bands, we note that the energy differences between the FA-LO and FA-2LO bands to the FA band are close to one and two LO phonon energies of ZnO.is implies that FA-LO and FA-2LO bands should correspond to the �rst and the second LO phonon replica of the FA band. Figure 5(a) exhibits the temperature () dependence of integral PL intensities () for FX band.One can clearly see that at high temperature, the emission intensity represents the decrease with temperature increasing due to the thermal quenching.e dependence of  on  for FX band can be �tted by the following formula where  0 is the peak intensity at temperature   0 K,  is a parameter,   is the activation energy in the thermal quenching process, and   is the Boltzmann constant.From the plots (solid line), the thermal activation energy is estimated to be 59 meV for FX band.is value agrees well with the free exciton binding energy of ZnO (∼60 meV) [1][2][3][4].Figure 5(b) shows the intensity ratio  0 of FA to FA-LO (FA/FA-LO) as a function of temperature.It can be seen that the  0 value presents a downtrend as the temperature increases.In the same temperature range, the phonon replicas could be much stronger than the no-phonon recombination due to self-absorption, but the intensity ratio of the �rst to the second LO-phonon replica should increase linearly with temperature [26,27].As shown in Figure 5(b), it is found that  0 is decreased with the temperature from 90 to 260 K. us, FA-LO band is not the second LO replica of FX, but rather is the �rst LO replica of FA, that is, FA band cannot be the �rst LO replica of FX.
Although we excluded that the FA band is from the �rst LO replica of the free exciton, the luminescence band still exist many other controversial luminescent mechanisms [14,15,17,18,28].e comparison with literature [14,19,28] strongly suggests that the observed FA band originates from free-to-acceptor transition, that is, the recombination of an electron from the conduction band with a hole bound to an acceptor state, labeled (e, A 0 ).A further con�rmation of this assignment will be demonstrated below.In Figure 3, PL spectra exhibit that the FA band at low energy side of D 0 X and FX can be clearly observed in whole temperature range from 11 to 260 K.At lower temperature (<130 K), the intensity of the FA, and FA-LO bands gradually becomes strong with increase temperature.At 130 K, four evident emission peaks labeled FX, D 0 X, FA, and FA-LO are located at 3.360 eV, 3.346 eV, 3.301 eV, and 3.227 eV, respectively.With further increases in ZnO sample temperature, the FA band deceases in relative intensity and becomes more pronounced at the high-energy tail.is is a typical feature of the freeto-bound transition [19].In undoped ZnO, typical donors have binding energies in the range of 46-63 meV [29], while acceptor binding energy is larger (>100 meV) [30].Due to the release of the electrons from donors with smaller binding energy, the electron concentration in the conduction band increases with increasing temperature (<130 K), resulting in the FA emission intensity increases.For >130 K, the observed thermal quenching of the FA band is related to hole release from acceptors.
In order to verify that the recombination of free-toacceptor is responsible for the observed FA band, the temperature dependent peak position is analyzed, as shown in Figure 6.e open circles in Figure 6 are the data generated from the FA band.Because the temperature dependence of the free-to-bound transition energy differs from the bandgap energy by   2, a curve-�tting analysis of the temperature dependence of the FA transition energy by using the following formula [31]: where    and  FA  are the temperature-dependent band gap energy and FA band energy, respectively,   is the acceptor binding energy,   is the Boltzmann constant,  and  are constants,   0 is the band gap energy at  = 0 .e blue curves in Figure 6 represent the results of the best �t according to (2).e energy   is obtained to be 121 meV.e �tted values of   0,  and  are equal to 3.440 eV, 8.6 × 10 −4 eV/K and 800 K, respectively, which are in good agreement with those reported by Wang and Giles [31].is fact provides evidence that the observed FA transition has the characteristic of the free-to-bound transition.In Figure 6, the FX and D 0 X peak energies from PL spectra are also plotted with solid square and solid circle symbols.As can be seen, although the FX, the D 0 X, and the FA transition energies are reduced with increasing temperature, the change of their transition energies is different.e transition energies of the free and bound excitons show similar temperature dependence as the band-gap energy (red lines).Hence, the redshi� of the FA emission peak is signi�cantly smaller than the FX and D 0 X. is further identi�es the FA band as a freeto-bound transition.ough we identify the FA band as a free-to-bound transition, a further investigation is required to clarify this bound state is donor-like or acceptor-like.Because this band around 3.310 eV always appears in p-type ZnO, not only in N-doped [20,21] but also in P-doped [22,23], and Asdoped [24,25] samples, it has been assigned to (e, A 0 ) transition related to these substitutional acceptors.However, this luminescence band has indeed frequently been observed in undoped ZnO samples, especially in ZnO nanostructure materials [14,15,17,18].In our previous works, the 3.31 eV luminescence assigned (e, A 0 ) transition is observed in Ndoped p-type ZnO thin �lms [21] and ZnO nanowalls [28].If these reported results in undoped ZnO are consistent with p-type ZnO, that is, the observed luminescence band around 3.31 eV is ralated to acceptors, it is necessary to discuss the origin of the acceptors in undoped n-type ZnO.
Recently, some research results con�rm the existence of certain acceptor states in n-type ZnO with relatively high concentrations [19,32].Janotti and Van De Walle given the acceptor/donor concentration ratio of 0.41 in ZnO : Ga samples and suggested the dominant acceptors are likely zinc vacancies ( Zn ) and/or neutral complexes related to  Zn .Indeed, the theoretical study shows that  Zn is deep acceptor with a low formation energy, and it can act as compensating centers in n-type ZnO [5].Simultaneously on the experimental,  Zn has been directly identi�ed as the dominant acceptor in as-grown ZnO [7].On the other hand, Schirra and Schneider reported that the acceptor states related to stacking fault in ZnO �lms grown on a-Al 2 O 3 substrates [19], in which the 3.31-eV luminescence assigned to (e, A 0 ) transition is found to be related to a high local density of acceptors in conjunction with crystallographic defects.e existence of these acceptors with the estimated concentration of 10 18 ∼ 10 20 cm −3 , which might exceed the donor concentration, will play a vital role for the electrical properties [19,32].From this consideration, the room temperature electrical properties of ZnO �lms were measured by the four-probe van der Pauw method.Based on these measurements, the grown samples show n-type characteristics with a resistivity of the order of 10 Ω⋅cm.In addition, obtaining such high resistivity corresponds to a mobility of 10 cm 2 (V s) −1 and a carrier concentration of 10 15 cm −3 .It is well known that undoped ZnO �lms has a nature of the residual n-type conductivity due to donor-like intrinsic defects, such as oxygen vacancies (  ) and interstitial zinc atoms (Zn  ).In the majority of the pertinent works [2,3], the obtained carrier concentration in undoped ZnO �lms is the order of 10 16 -10 18 cm −3 .Obviously, these values are much higher than that of our sample.us, high resistivity in our work is suggested to be due to the compensating effect formed by large numbers of acceptor states.Here, the acceptors likely arising from the native defects will cause the electrical properties degradation.Not only such, these acceptor states will bring important in�uence on room temperature UV emission.To identify the acceptor origin need further investigation in detail.

Conclusion
In summary, we have performed a detailed study about photoluminescence properties of ZnO thin �lms grown on c-Al 2 O 3 substrates by pulsed laser deposition.e origin of UV emissions at RT is studied carefully by measuring different temperature spectra of ZnO thin �lms.e result shows at low temperature donor-bound exciton emission plays a major role in PL spectra, while the free-exciton transition gradually dominates the spectrum with increasing temperatures.e room temperature UV emission contains two different transitions.One is related to the ZnO free-exciton and the other is related to the free-to-bound transition.e focus is put on the con�rmation of this the free-tobound transition observed at 3.309 eV at low temperature.It is strongly suggested that the 3.309 eV band originates from free-electrons-to-acceptor recombination.e acceptor binding energy is estimated to be about 121 meV.

F 1 : 3 F 2 :
X�D spectra of ZnO thin �lms grown on c-Al 2 O 3 substrate.e inset is -scan curve of the (103) re�ection of sample A. UV PL spectra of the grown ZnO thin �lms at room temperatures.