Thermal-activated carrier transfer in ZnCdO thin film grown by plasma-assisted molecular beam epitaxy

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Thermal-activated carrier transfer in ZnCdO thin ﬁlm grown by

plasma-assisted molecular beam epitaxy

K.F. Chien

a

, W.L. Hsu

a

, A.J. Tzou

a

, Y.C. Lin

a

, W.C. Chou

a,b,n

, L. Lee

c

, C.H. Chia

d

, C.S. Yang

e

a

Department of Electrophysics, National Chiao Tung University, HsinChu 30010, Taiwan

b_{NSC Taiwan Consortium of Emergent Crystalline Materials, Taiwan} c

Center of Nanoscience and Technology, Tunghai University, Taichung 40704, Taiwan

d

Department of Applied Physics, National University of Kaohsiung, Kaohsiung 81148, Taiwan

e

Graduate Program in Electro-Optical Engineering, Tatung University, Taipei 10452, Taiwan

a r t i c l e

i n f o

Available online 9 January 2013 Keywords:

A3. Molecular beam epitaxy B1. Oxides

B1. Zinc compounds

B2. Semiconducting II–VI materials

a b s t r a c t

The thermal-activated carrier transfer processes in a Zn0.98Cd0.02O thin ﬁlm grown by plasma-assisted molecular beam epitaxy were investigated using temperature-dependent and time-resolved photoluminescence (PL) spectroscopy. As the temperature increases from 50 to 220 K, the carriers transfer from shallow to deep localized states. Additionally, the carriers escape from the deep localized states above 220 K due to an activation energy of about 19 meV.

1. Introduction

Zinc oxide has received considerable attention due to its promising integration into optoelectronic devices. Its large band gap of about 3.37 eV at room temperature, high exciton binding energy (60 meV), and structural compatibility with GaN are some of the properties that make this material interesting for optoelec-tronic applications[1]. Recently, Zn1 xCdxO compound

semicon-ductors have been further utilized to tailor the band gap of ZnO from ultraviolet to visible spectral range owing to the small band gap of CdO of about 2.30 eV[2]. The fabrication of ZnO/Zn1 xCdxO

heterojunctions and multiple quantum wells would provide the key elements in ZnO-based light emitting diodes and laser diodes. Therefore, it is worthwhile to understand the light emission mechanism in the Zn1 xCdxO alloy system. However, the rock-salt

structure of CdO is dissimilar to the wurtzite structure of ZnO. It results in reducing crystalline quality and causing phase separation for Zn1 xCdxO alloys. The difﬁculty was solved by metal organic

chemical-vapor deposition growth technique, and Cd composition can be increased up to 2.0%[3]. However, the cathode-luminescence results showed that a low energy shoulder emerges. The sample quality was further improved by Sadofev et al. using molecular beam epitaxy (MBE) to eliminate the low energy shoulder[4]. In this article, high-quality Zn0.98Cd0.02O thin ﬁlm was grown by

plasma-assisted MBE. The temperature-dependent PL and time-resolved PL

(TRPL) were used to study the thermal-activated carrier transfer dynamics of Zn0.98Cd0.02O.

2. Experimental procedure

ZnO and Zn0.98Cd0.02O thin ﬁlms were grown on c-plane Al2O3

substrates by a SVT Associates MBE system equipped with conventional effusion cells for evaporation of elemental Zn (6N), Mg (5N), and Cd (6N). Oxygen (5N5) was supplied via an rf-plasma source after additional gas puriﬁcation. The substrates were degreased in acetone, methanol, and then chemically etched in H2SO4:H3PO4¼3:1 mixture at 160 1C for 15 min, followed by a

deionized water rinse. Prior to the growth, substrates were desorbed at 850 1C and treated with oxygen plasma to produce an oxygen terminated Al2O3surface. In order to reduce the lattice

mismatch between Zn1 xCdxO and Al2O3, the 300 nm thick

Zn0.98Cd0.02O ﬁlm was grown at 350 1C following a 70 nm thick

ZnO buffer layer grown at 650 1C. The oxygen ﬂow rate was 0.6 SCCM with plasma power 250 W. The Zn and Cd cell

temperatures for the growth of Zn0.98Cd0.02O thin ﬁlms were

290 and 180 1C, respectively. The optical characterization of the samples was analyzed by temperature-dependent PL and TRPL measurements. The 325 nm-line of a He–Cd laser was used as the excitation source for PL, and TRPL was excited using a pulsed laser (377 nm/40 MHz).

3. Results and discussion

Fig. 1shows the PL spectra of ZnO and Zn0.98Cd0.02O ﬁlms at 10 K.

The emission peaks at 3.361 and 3.366 eV were assigned to the

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Journal of Crystal Growth

n

Corresponding author at: Department of Electrophysics, National Chiao Tung University, HsinChu 30010, Taiwan. Tel.: þ886 3 571 2121;

fax: þ 886 3 572 5230.

E-mail address: [email protected] (W.C. Chou).

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excitons bound to neutral donors (DoX)[5]. The peak at 3.377 eV is

attributed to the free A excitons (FXA)[6]. When Cd atoms were

introduced into ZnO, the PL band becomes broad and the spectral position of the PL peak shifts to 3.185 eV. According to the experi-mental results of Gruber et al.[7], the Cd composition herein can be estimated as 2%. Additionally, the PL emission proﬁle of Zn0.98Cd0.02O

is not symmetry due to the existence of localized states at low energy

side. The transmittance spectrum at 10 K shown in the inset shows a strong absorption near the band edge. It can be seen that the Cd incorporation reduces the band gap energy and causes a Stokes’ shift of about 100 meV. The Stokes’ shift is attributed to the localization of excitons due to the Cd compositional ﬂuctuation[8].

In order to further investigate and compare the optical properties of Zn0.98Cd0.02O with ZnO, the temperature-dependent PL

measure-ments of ZnO and Zn0.98Cd0.02O were carried out. InFig. 2(a), both

DoX and FXA of ZnO shift to lower energies, and the line-width

broadens with increasing temperature. The PL linewidth broadening is related to the carrier–phonon interaction. Additionally, because of thermally activated processes of carriers, FX dominates at high temperature. InFig. 2(b), three PL peaks of Zn0.98Cd0.02O labeled as

P1, P2, and P3 were observed at different temperatures. The P1 emission was observed at low temperature. When T4140 K, addi-tional peaks, P2 and P3, become visible. The intensity of P2 increases and exceeds that of P1 when T4220 K. Similar optical properties

were also observed by Yang et al. [9] in ZnSe1 xTex (x¼0.01)

epilayers. In the case of ZnSe1 xTex, at low temperature of 10 K,

X/Te is not observed and the PL of the Ten-bound excitons (X/Ten) is

very pronounced. The energy states of X/Te and X/Te clusters become observable as the temperature was increased. Thus, the PL peaks of P1, P2, and P3 can be attributed to the emissions from X/Cdn, X/Cd,

and X/Cd cluster, respectively. Fig. 2(c) shows the dependence of

X/Cdn intensity on temperature with a double-channels activation

energy function:[10] IPLð Þ ¼T I0 1þ C1exp Ea1=kBT þC2exp Ea2=kBT ð1Þ

Fig. 1. (a) PL spectra of ZnO and Zn0.98Cd0.02O ﬁlms at 10 K. The inset shows the PL

and transmittance spectra of Zn0.98Cd0.02O at 10 K.

Fig. 2. PL spectra of (a) ZnO and (b) Zn0.98Cd0.02O thin ﬁlms at various temperatures. (c) Integrated PL intensity of X/Cdnas a function of temperature for ZnCdO.

The solid line is ﬁtted by Eq.(1).

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where IPL(T) is the integrated PL intensity at temperature T, kBis the

Boltzmann constant, Ea1and Ea2denote the activation energy, C is a

tunneling factor, and I0 is the integrated intensity at the low

temperature limit. In this study, Ea1¼4 meV and Ea2¼19 meV were

extracted. The activation energy Ea1 is attributed to the average

localization energy of the X/Cdn. On the other hand, the activation

energy Ea2corresponds to thermal energy of 220 K, which is similar to

the quenching temperature of X/Cdnintensity. Therefore, we consider

that the activation energy Ea2is the energy for carriers transferred

from X/Cdnto X/Cd.

In order to conﬁrm the origin of the emissions from X/Cd, X/Cdn,

and X/Cd clusters, the PL peak positions of the free-exciton (FX), X/Cd, X/Cdn, and X/Cd clusters as a function of temperature were plotted in

Fig. 3(a). The FX peak positions obtained from transmittance are well ﬁtted by the Vashini’s prediction[11]which is written as

EgðTÞ ¼ Egð0Þ

a

T2=ð

b

þTÞ ð2Þ

where Eg(0) is the band-gap energy at T¼ 0 K, and

a

and

b

are the

corresponding thermal coefﬁcients. The ﬁtting is labeled by solid

lines in Fig. 3(a). The X/Cd peaks exhibit a similar trend. The

emission energy of X/Cd is 80 meV lower than that of FX. It implies that there is not enough energy to offer the carriers transferred from X/Cd states to FX states. Therefore, X/Cd dominates the carrier

recombination at 300 K. However, the X/Cdnpeak demonstrates a

fast redshift at temperatures above 50 K. This phenomenon could be

attributed to continuous localized states formed with the X/Cdn

states. We proposed that excitons assisted by the scattering of LO phonons would transfer to the deep localized states, and the possibility of this re-localization process is promoted as the tem-perature increases because the spreading of exciton wave-function enhances the interaction with LO phonon ﬁelds. In this condition, the increased temperature only causes small parts of the carriers to delocalize from the deep localized states to the shallow ones.

The X/Cdn peak redshifts dramatically until the carriers were

relocalized at the X/Cd cluster states (the deepest localized states). On the contrary, as the temperature exceeds 220 K, the thermal energy overthrows the activation barrier of 19 meV, which pro-motes large amount of the carriers to escape from the deep

localization and the X/Cd emission dominates.Fig. 3(b) shows the

power dependent PL spectra at 220 K for Zn0.98Cd0.02O. The intensity

of PL emission is enhanced with increasing excitation power. Under

an excitation power of 65 W/cm2_{, the emission energies of X/Cd}

cluster and X/Cd are at 2.782 and 3.160 eV, respectively. As the excitation power is increased, the X/Cd cluster exhibits energy

blueshift and the X/Cdn state dominates the spectrum. This result

implies that the density of state for the X/Cdnis much higher than

that for the X/Cd cluster. The increasing excitation density saturates the lower energy states. As a result, the X/Cdnstate dominates the

emission.

In order to provide further evidence to support the existence of localized states and to demonstrate the origin of the radiative

Fig. 3. (a) PL peak energy trace of X/Cd, X/Cdn, X/Cd cluster and FX at various temperatures. The solid lines are Varshni’s ﬁts. (b) Power dependent spectra of

Zn0.98Cd0.02O at 220 K.

Fig. 4. Temporal evolution of the PL spectra at (a) 10 K, (b) 100 K, and (c) 150 K. K.F. Chien et al. / Journal of Crystal Growth 378 (2013) 208–211 210

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recombination, we performed TRPL measurements at 10 K, 100 K and 150 K as shown inFig. 4. The carrier recombination time decreases with increasing temperature. It implies that the thermalized carriers could relax easily over a long distance and ﬁnd a lower local-energy minimum. Besides, the main PL peak positions under-goes no obvious energy shift over time at 10 K, reﬂecting the main recombination of the localized state with the same concentration of Cd. However, the main peak shifts 39 meV toward the low energy side with time at 100 K. It reveals that the thermal-activated carriers transfer to the deeper localized states via the phonon scattering. As the temperature increases to 150 K, a larger redshift of 100 meV with the delay time was found, indicating more carriers can transfer to the deeper localized states. In combination with temperature-dependent PL and TRPL, we demonstrate the carrier transfer dynamics in Zn0.98Cd0.02O. The

strong redshift of the X/Cdnpeak energy is caused by the carrier

transfers to the deep localized states as To220 K. Moreover, a

delocalization process occurs as the temperature exceeds 220 K.

4. Conclusion

Emissions of X/Cd, X/Cdn, and X/Cd cluster from Zn0.98Cd0.02O

thin ﬁlm were investigated by PL and TRPL spectroscopy. Two emission peaks, which are attributed to the carrier recombinations of X/Cd states and X/Cd cluster states, were observed at elevated temperatures. From 50 to 220 K, the carriers transfer from shallow X/Cdnstates to deep localized X/Cdnstates. Above 220 K, the carriers

escape from the deep localized X/Cdnstates to X/Cd states.

Acknowledgments

This work was supported by the National Science Council and the Ministry of Education under Grant numbers NSC100-2119-M-009-003 and MOE-ATU 101W961, respectively.

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