Zn1−xMgxO multilayers grown by pulsed laser deposition

Journal of Luminescence 136 (2013) 285–290

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Temperature-dependent photoluminescence properties of ZnO/Zn1  xMgxO multilayers grown by pulsed laser deposition T. Rakshit a, I. Manna b,c, S.K. Ray d,n a

Advanced Technology Development Centre, IIT Kharagpur, Kharagpur 721 302, India Department of Metallurgical and Materials Engineering, IIT Kharagpur, Kharagpur 721 302, India c CSIR—Central Glass and Ceramic Research Institute, Kolkata 700 032, India d Department of Physics and Meteorology, IIT Kharagpur, Kharagpur 721 302, India b

a r t i c l e i n f o

abstract

Article history: Received 5 July 2012 Received in revised form 21 September 2012 Accepted 26 November 2012 Available online 5 December 2012

ZnO/Zn0.9Mg0.1O multilayer thin films were deposited on p-Si(1 0 0) substrates using pulsed laser deposition technique at varying substrate temperatures ranging from 300 1C to 700 1C. X-ray diffraction (XRD) studies reveal that the films possess a preferred (0 0 0 2) growth orientation with the intensity gradually increasing with substrate temperature. Temperature-dependent photoluminescence properties were studied in details to investigate the origin of near band edge emission and the quenching mechanism. The band gap at 10 K shows a blue-shift with the increase of substrate temperature, but is found to be higher for the sample grown at 500 1C than that of the 600 1C and 700 1C grown samples at all temperatures above 25 K. It appears that the dominant recombination mechanism changes from donor-bound to localized excitons with the increase in substrate temperature. & 2012 Elsevier B.V. All rights reserved.

Keywords: ZnO ZnMgO Multilayer Pulsed laser deposition (PLD) Photoluminescence (PL)

1. Introduction ZnO is an attractive material for application in optoelectronic devices like laser, LEDs, UV detector [1–4], due to its wide band gap (3.37 eV at room temperature) and large excitonic energy (60 meV). The wavelength for emission or detection can be tuned by using either ZnO quantum well or by alloying ZnO with a higher band gap material. MgO has a band gap of about 6.7 eV. Though the equilibrium solid solubility of MgO in ZnO in the bulk form is limited (less than 4 mol. %), but the ionic radii of Mg2 þ (0.057 nm) is similar to that of Zn2 þ (0.060 nm) [5]. Since physical vapor deposition is a non-equilibrium processing route, solid solubility of MgO in ZnO can be significantly enhanced (say upto 33 mol%) [6] in the metastable state after thin film deposition. Thus, when ZnO is alloyed with MgO, the band gap can be tuned and can be increased to above 4 eV [6]. When the higher band gap ternary Zn1 xMgxO alloy is grown in combination with a lower energy gap ZnO, they can provide a barrier/well heterostructure required for developing resonant tunneling diode (RTD) [7]. Double barrier resonant tunneling devices (DBRTD) are much attractive for ultrahigh frequency mixing and microwave–millimeter wave oscillation circuits, alternating to direct current converters, multi-valued logic, etc. It has been found that much higher optical efficiencies at room

n

Corresponding author. Tel.: þ91 3222 283838; fax: þ 91 3222 282700. E-mail address: [email protected] (S.K. Ray).

0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.11.037

temperature can be obtained from core-shell ZnO/ZnMgO multiquantum well heterostructures [8]. Tuning of optical band gap can also be done by varying the ZnO sublayer thickness in ZnO/MgO multilayers [9]. Besides band gap tuning, MgZnO thin film shows much higher sensor response to hydrogen than the undoped ZnO films [10]. There are several techniques for growing ZnO thin films, such as chemical vapor deposition [11], molecular beam epitaxy (MBE) [12], sputtering [13], pulsed laser deposition [14], and sol–gel technique [15]. Of these, pulsed laser deposition [16] and laser-induced MBE [17] have been successfully used to deposit high quality epitaxial layers of ZnO on sapphire and other substrates. These films are highly suitable for electronic and optoelectronic devices. In this paper, we report the growth of ZnO/ZnMgO multilayer thin films of same barrier and well thickness on p-Si(1 0 0) substrates at varying substrate temperatures using pulsed laser deposition technique. The effect of substrate temperature on the structural and optical properties of the films has been studied. Temperature-dependent photoluminescence properties of these films are also reported.

2. Experimental details ZnO/ZnMgO multilayer thin films were deposited on p-Si(1 0 0) substrates at varying substrate temperatures (Ts) by the pulsed laser deposition (PLD) technique. Two ceramic targets of ZnO and Zn0.9Mg0.1O (both of purity 99.999%) were used for this purpose.

T. Rakshit et al. / Journal of Luminescence 136 (2013) 285–290

layer i.e. layer 1 is ZnO buffer layer. Layers 2, 4, 6, and 8 as denoted by arrows, are that of ZnMgO. The thin layers in between them, i.e. 3, 5, and 7 are that of ZnO. XRD spectra of the multilayers grown at different substrate temperatures are shown in Fig. 2(a). Intensity distribution of characteristic peaks suggests that the multilayer films possess preferred (0 0 0 2) growth orientation for all samples i.e. they are c-axis oriented. It is interesting to note that the (0 0 0 2) peak tends to enhance in intensity as the substrate temperature increases from 300 1C to 700 1C. Thus, the intensity ratio of (0 0 0 2) to (1 0 1 3) peak also increases gradually with rise of substrate temperature, as shown in Fig. 2(b). This is attributed to the increase of the adatom mobility in the film at higher substrate temperature, resulting in improved crystallinity and preferred growth orientation. Temperature dependent photoluminescence (PL) measurements were carried out in order to have insight on the emissions related to the variation of the substrate temperature. Fig. 3 shows the normalized PL properties of the samples recorded at 10 K. The broad peak around 2.1–2.5 eV is related to defects, such as oxygen

Substrate temperature of: 300 °C (a) 400 °C (b) 500 °C (c) (d) 600 °C 700 °C (e)

(0002)

(1013)

For the preparation of Zn1 xMgxO target (x¼0.1), the desired amount of MgO powder was mixed homogeneously with ZnO powder, and disk-shaped specimens of 20 mm in diameter and 5 mm in thickness were obtained by the uniaxial pressing at 6  106 kg/m2. The specimens were sintered at 800 1C for 2 h and at 1200 1C for 4 h to obtain high density targets. Before loading into the growth chamber, the substrates were degreased in acetone and ethanol followed by etching in 1% HF solution for 2 min to remove the native oxide layer from the surface of the substrate, and then rinsed in de-ionized water. A KrF excimer laser (l ¼248 nm, t ¼25 ns) at an energy density of 2 J/cm2 was used for the ablation of the targets. The substrate was placed at a distance of 4 cm from the target. The growth chamber was evacuated to a base pressure of 1  10  6 mbar using a combination of rotary and turbo pumps before deposition. To reduce the lattice mismatch-induced strain effect, a ZnO buffer layer of about 145 nm thick was grown on the p-Si(1 0 0) substrate at 100 1C in vacuum. Then, a series of 3 period ZnO/ZnMgO multilayers were grown at different substrate temperatures varying from 300 1C to 700 1C. During the multilayer film deposition, the oxygen pressure inside the chamber was kept at 1  10  1 mbar and the pulse repetition rate was maintained at 10 Hz. The ZnMgO barrier and ZnO well layer thickness were kept fixed at 100 nm and 10 nm, respectively, for all the samples. The cross-section of the deposited films were studied using a field emission scanning electron microscope (ZEISS SUPRA 40) equipped with energy-dispersive X-ray analyzer. The phase aggregate of the multilayer was characterized by X-ray diffraction (XRD) (Philips X-Pert MRD) pattern using CuKa radiation (45 kV, 40 mA) of wavelength 0.15418 nm in grazing incidence mode. Photoluminescence (PL) measurements in the temperature ranging from 10 to 300 K were carried out using a He–Cd laser as an excitation source which is operating at 325 nm with an output power of 45 mW, and a TRIAX 320 monochromator which is fitted with a cooled Hamamatsu R928 photomultiplier detector.

Intensity (arb. unit)

286

(e) (d) (c)

3. Results and discussion

(b) (a)

Fig. 1 shows the cross-sectional field emission scanning electron microscopy (FESEM) of the multilayer grown at 500 1C. The bottom

20

30

40

50 2θ (degree)

60

70

80

7 6 5 4 3 2

1

Intensity ratio of (0002):(1013) peak

7

8

6

5

4

3

2 300 Fig. 1. Cross-sectional FESEM micrograph of the multilayers grown at 500 1C. Bottom layer is ZnO buffer layer, followed by 3 periods of ZnO (thin layer)/ZnMgO layers, i.e. layer 1 is ZnO buffer layer, layers 2, 4, 6, and 8 are that of ZnMgO, and layers 3, 5, and 7 are that of ZnO.

400 500 600 Substrate temperature (°C)

700

Fig. 2. (a) Grazing incidence X-ray diffraction pattern of the multilayers grown at different substrate temperatures. (b) Variation of peak intensity ratio of (0 0 0 2) to (1 0 1 3) planes with substrate temperature.

T. Rakshit et al. / Journal of Luminescence 136 (2013) 285–290

0.8

QW

Substrate temperature of:

(a)

(a) (b) (c) (d) (e)

(b)

0.6

300 °C 400 °C 500 °C 600 °C 700 °C

Normalized Intensity (arb. unit)

Normalized Intensity (arb. unit)

1.0

0.4 (e) 0.2

(d)

2.1

ZnO buffer

ZnMgO barrier

700 °C 600 °C 500 °C 400 °C 300 °C

(c)

0.0

287

2.4

2.7 3.0 Energy (eV)

3.3

3.2

3.6

3.3

3.4 3.5 Energy (eV)

3.6

3.7

Fig. 3. Normalized photoluminescence spectra at 10 K of the multilayers grown at different substrate temperatures.

100

ZnMgO-MgMg þZnZn þ VO þ

ZnMgO-Mgi þZni þ

1 O2 m 2

1 O2 m 2

ð1Þ

ð2Þ

The generated oxygen vacancies and Zn and Mg interstitials slightly degrade the quality of the films leading to an increase in FWHM for the 600 1C grown sample. But the lowering of FWHM for the 700 1C grown sample indicates the origin of photoluminescence broadening to some other mechanism in addition to the defect concentrations in the multilayer thin films. While the oxygen vacancy and Zn and Mg interstitial concentrations

80 FWHM (meV)

vacancies, zinc and magnesium interstitials in the films [18,19]. Most samples show a sharp near band edge emission (NBE) peak in the UV range, except for those grown at 300 1C and 400 1C, which have much larger defects states. This difference indicates that the samples grown at higher temperatures have low defect density. Fig. 4(a) shows the normalized PL spectra at 10 K of only the NBE peak in the UV range. The spectra exhibit a strong emission from ZnO quantum well (QW), which is found to be blue-shifted from 3.487 to 3.503 eV with the increase of substrate temperature from 300 1C to 700 1C. The weak broad shoulder around 3.41 or 3.44 eV is attributed to ZnO buffer layer and that around 3.66 eV is due to ZnMgO barrier layer. The full-width-at-half-maximum (FWHM) of the QW emission peak as a function of substrate temperature is shown in Fig. 4(b). With the increase of growth temperature up to 500 1C, there is a decrease in the FWHM. However, the FWHM increases for 600 1C grown sample and decreases again for the sample grown at 700 1C. It is known that desorption of zinc-related species occurs easily at higher growth temperatures [20]. As a result, the interstitial magnesium ions, which are abundantly present in the samples grown at 300 1C and 400 1C, tend to occupy the position of zinc ions as the substrate temperature is increased [19]. This enhances the magnesium content of the films with a resultant reduction of the defect states (as observed from Fig. 3). Thus it leads to an increase in band gap and decrease in FWHM, as the substrate temperature rises from 300 1C to 500 1C. But as the substrate temperature is increased above 500 1C, the emission from defect states becomes important. This is due to the reevaporation of oxygen at higher temperatures following the reactions [20].

60

40

300

400 500 600 Substrate temperature (°C)

700

Fig. 4. (a) Normalized photoluminescence spectra of near-band edge emission peak at 10 K of the multilayers grown at different substrate temperatures. The spectrum of the sample grown at 300 1C has been fitted with Gaussian curves (dash lines). (b) Variation of the full-width-at-half-maximum of ZnO QW peak at 10 K with substrate temperature.

dominate the defect related PL in the region 2.1–2.5 eV, the broadening of band edge emission also depends on the quantum well/barrier interface roughness [21] and the composition fluctuation in ZnMgO barrier. It may be possible that the higher adatom mobility of the atomic and molecular species on the substrate at a high temperature of 700 1C produces a much smoother interface, which compensates the degradation in the film caused by the defect states. This may reduce the FWHM of the 700 1C grown sample. Additionally, the enhancement in band gap with the increase in substrate temperature from 500 1C to 700 1C indicates the role of substitutional magnesium ions in the lattice. Fig. 5(a), (b), and (c) shows the temperature dependent PL spectra of the samples grown at 300 1C, 500 1C, and 700 1C, respectively. The corresponding spectra in the inset show the normalized NBE peak. In all the spectra, an increase in temperature causes lowering of the intensity of the NBE peaks as well as the defect state emissions (except at 25 K for the sample grown at 300 1C). For the sample grown at 300 1C, the defect state emission intensity is relatively larger compared to the NBE peak one

T. Rakshit et al. / Journal of Luminescence 136 (2013) 285–290

10 K 25 K 50 K 75 K 100 K 150 K 200 K 250 K 300 K

50 K 75 K 100 K 150 K 200 K 250 K 300 K

3.0

3.1

3.2

3.3

3.4

3.5

3.6

3.7

Energy (eV)

Normalized Intensity (arb. unit)

10 K 25 K

Intensity (arb. unit)

Intensity (arb. unit)

10 K 25 K 50 K 75 K 100 K 150 K 200 K 250 K 300 K

Normalized Intensity (arb. unit)

288

10 K 25 K 50 K 75 K 100 K 150 K 200 K 250 K 300 K

3.0

3.1

3.2

3.3

Ts = 300 °C

3.4

3.5

3.6

3.7

Energy (eV)

Ts = 500 °C

2.7 3.0 Energy (eV)

3.3

Normalized Intensity (arb. unit)

2.4

Intensity (arb. unit)

2.1

3.6

2.1

2.4

2.7 3.0 Energy (eV)

3.3

3.6

10 K 25 K 50 K 75 K 100 K 150 K 200 K 250 K 300 K

10 K 25 K 50 K 75 K 100 K 150 K 200 K 250 K 300 K

3.0

3.1

3.2

3.3

3.4

3.5

3.6

3.7

Energy (eV)

Ts = 700 °C

2.1

2.4

2.7 3.0 Energy (eV)

3.3

3.6

Fig. 5. Temperature dependent PL spectra of the samples grown at (a) 300 1C, (b) 500 1C, and (c) 700 1C. Inset shows normalized PL spectra of only the corresponding NBE peak, with the base lines shifted in vertical direction for clarity.

(Fig. 5(a)). Intensity of the NBE peak increases slightly with the rise of temperature from 10 to 25 K, and then decreases afterwards. It can be seen from the corresponding inset that the emission from the ZnO QW is red-shifted with the increase of temperature from 10 to 300 K. For the sample grown at 500 1C, strong NBE emission peak is observed at 10 K with very low defects, but the intensity decreases very rapidly with increase of temperature (Fig. 5(b)). Corresponding inset reveals that the emission from the ZnO QW is blue-shifted with increase of temperature from 10 to 100 K, and then red-shifted up to 300 K. The sample grown at 700 1C exhibits a strong NBE peak, with its intensity gradually decreasing with temperature (Fig. 5(c)). The emission from the ZnO QW is slightly blue-shifted with increase of temperature from 10 to 50 K, and then red-shifted up to 300 K, as observed from the corresponding inset. This blue-red shift observed in the 500 1C and 700 1C grown sample is usually attributed to the radiative emission of localized excitons [22]. Initially, the excitons are localized in the local potential minima caused by the roughness or defects in the interface and/or

fluctuation in the ZnMgO barrier composition. As temperature rises (10–100 K and 10–50 K for 500 1C and 700 1C grown sample, respectively), these excitons are thermalized to occupy the higher energy states. This results in the blue-shift of the peak. But above 100 K, the lifetime of the carriers decreases, so thermalized excitons have reduced chances to reach the lower energy states before recombination. Therefore, a red-shift is observed, which is mainly because of the temperature-induced band gap shrinkage [23]. It may also be noted that the decrease of intensity of the ZnO QW peak with temperature is much rapid for the sample grown at 500 1C resulting in a very low emission at 300 K, than that grown at 700 1C. This indicates the presence of relatively higher nonradiative recombination centers in the 500 1C grown sample. Since the nature of the temperature dependent PL spectra of the sample grown at 400 1C and 600 1C is almost similar to that grown at 300 1C and 500 1C, respectively, the spectra of these two samples have not been shown separately. In order to obtain a better understanding of this result, the ZnO quantum well peak energy has been plotted as a function of

T. Rakshit et al. / Journal of Luminescence 136 (2013) 285–290

Eg ðT Þ ¼ Eg ð0Þ

K
exp yE =T 1

ð3Þ

where Eg(T) is the band gap energy, K represents the electronphonon coupling strength and yE is the Einstein temperature. The plots are fitted with Eq. (3) taking K ¼0.09 eV from reference [25]. Table 1 presents the extracted yE from the fittings. The values of Einstein temperature lie within those obtained from ZnO bulk crystals (240 K) [25] and ZnO/ZnMgO multiple quantum wells (417 K) [26]. The integrated intensity of the ZnO quantum well peak as a function of reciprocal temperature is plotted in Fig. 7. The plot can be well described by the following expression [27]: I0

I¼ 1 þ a1 exp Ea1 =kT þa2 exp Ea2 =kT

ð4Þ

3.52

Peak Energy (eV)

3.50

3.48

3.44

3.42

Substrate temperature of: 300 °C 400 °C 500 °C 600 °C 700 °C 0

50

100

150 200 Temperature (K)

250

300

Fig. 6. Variation of ZnO quantum well peak energy with temperature (open symbols) of the multilayers grown at different substrate temperatures. The solid lines are the fit to the graphs with Bose–Einstein-type expression.

Table 1 Einstein temperature and activation energy for the multilayers grown at different substrate temperatures. Substrate temperature (in 1C)

300 400 500 600 700

Einstein temperature (in K)

2937 5 3117 3 3777 50 3617 26 2867 3

Ea2 = 6.8 meV

Ea1 = 36 meV

Ea2 = 6.6 meV Ea1 = 33 meV Ea2 = 5.2 meV Ea1 = 59 meV

Ea2 = 4.2 meV Ea1 = 69 meV

0

20

40

60

80

(e) 700 °C

(d) 600 °C

(c) 500 °C

(b) 400 °C

(a) 300 °C

100

1000/T (K-1)

where I0 is the intensity at T¼0 K, a1 and a2 are coefficients measuring the efficiency of the quenching within the quantum well, and k is Boltzmann’s constant. The presence of two different activation energy values (Ea1 and Ea2), as extracted by fitting Eq. (4), is indicative that there are two competing nonradiative processes. The parameters extracted from the fitting are summarized in Table 1. The activation energy Ea1 and Ea2 describes the mechanism dominant in the higher ( 4100 K) and lower ( o100 K) temperature regions, respectively. For the sample grown at 300 1C and 400 1C, it is found that the intensity increases

3.46

Ea2 = 3.9 meV Ea1 = 42 meV

Integrated PL Intensity (arb. unit)

temperature. The plot is shown in Fig. 6. All these plots can be well described by the familiar Bose–Einstein-type expression [24] for temperature dependent band gap shrinkage

289

Activation energy (in meV) Ea1

Ea2

69 59 33 36 42

4.2 5.2 6.6 6.8 3.9

Fig. 7. Plot of the integrated PL intensities as a function of reciprocal temperature (solid squares) for the samples grown at (a) 300 1C, (b) 400 1C, (c) 500 1C, (d) 600 1C, and (e) 700 1C. The solid lines are the fit to the graphs.

slightly with the rise of temperature from 10 to 25 K. This ‘‘negative thermal quenching’’ occurs when the carriers are thermally activated with energies lower than that of the initial state of PL emission [28]. This quenching has been observed previously in the emission from donor-bound excitons [29]. There may be some deep levels, which become activated at temperatures below 25 K and give rise to enhanced intensity with temperature. Thus, the emission in the low temperature region appears to be due to recombination of donor-bound excitons. Moreover, activation energy Ea1 for the sample grown at 300 1C and 400 1C has been found to be 69 meV and 59 meV, respectively. These values are close to the binding energy of donors in ZnO (  60 meV) [30]. This emission is, therefore, due to donorbound excitons with very low quantum confinement effects. In temperature dependent PL spectra presented in Fig. 6, blueshift of band gap has been observed in the range 10–100 K for the samples grown at 500 1C and 600 1C, and 10–50 K for that grown at 700 1C. Thus, the activation energy Ea2 is likely to be due to excitons thermalizing from potential minima caused by fluctuation in the interface or alloy composition. Ea2 obtained for the samples grown at 500 1C and 600 1C is  7 meV and for that grown at 700 1C is  4 meV. The 500 1C and 600 1C grown samples exhibited a higher blue-shift, which means they require a higher thermal energy (activation energy) to occupy the higher energy states than that of the 700 1C grown sample. Activation energy Ea1 for the samples grown at 500 1C, 600 1C, and 700 1C has been found to be 33, 36, and 42 meV, respectively. The enhancement of activation energy implies that the influence of defect-related nonradiative recombination becomes weaker with the increase of substrate temperature from 500 1C to 700 1C. Presence of relatively larger nonradiative recombination centers has already been observed for the 500 1C grown sample in the temperature dependent PL spectra of Fig. 5(b) and (c). It is interesting to note that the samples grown at 500 1C and 600 1C exhibits higher activation energy than the 700 1C one at low temperatures (o100 K), but the case becomes reversed at higher temperatures (4100 K). It has been reported that the magnitude of blue-red shift with temperature lessens with the decrease of localized sites [31]. The presence of relatively higher nonradiative recombination centers in the sample grown at 500 1C may be due to the high density of localized sites. This

290

T. Rakshit et al. / Journal of Luminescence 136 (2013) 285–290

may result in higher blue-shift change with temperature (in the region To100 K) and thus increases Ea2. But with the further increase of temperature, nonradiative recombination becomes more dominant leading to a rapid quenching of the PL intensity and thus decreasing Ea1. The sample grown at 700 1C has relatively lower nonradiative recombination centers, so density of localized sites are also fewer, resulting in less blue-red shift change with increase of temperature. Thus, it is found that the nonradiative recombination centers play a significant role in the emission from ZnO/ZnMgO MQW structure. It has been found previously that dislocations may act as non-radiactive recombination centers [8,27]. The increase of activation energy Ea1 with the increase of substrate temperature from 500 1C to 700 1C implies that the nonradiative recombination has a much stronger influence over the emission.

4. Conclusion ZnO/Zn0.9Mg0.1O multilayer thin films were grown on p-Si(1 0 0) substrates using pulsed laser deposition technique at varying substrate temperatures ranging from 300 1C to 700 1C. Xray diffraction analysis reveal that the films have a preferred caxis orientation. Temperature-dependent photoluminescence properties show that the excitonic emission has an enhanced intensity for the samples grown at higher temperatures than that grown at relatively lower temperature of 300 1C. The samples grown at 300 1C and 400 1C exhibits only red-shift with increasing temperature, whereas a characteristic blue-red shift has been observed for the samples grown at 500 1C, 600 1C, and 700 1C due to the localization of excitons in the potential minima because of interface or alloy fluctuations. The presence of nonradiative recombination centers plays an important role in the emission characteristics from ZnO QW. The dominant recombination changes from donor-bound to localized excitons with the increase in the substrate temperature.

Acknowledgments This work was partially supported by FIR project grant from DRDO, Government of India.

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