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Role of ZnS buffer layers in growth of zincblende ZnO on GaAs substrates by metalorganic molecular-beam epitaxy
Journal of Crystal Growth 221 (2000) 435}439
Role of ZnS bu!er layers in growth of zincblende ZnO on GaAs substrates by metalorganic molecular-beam epitaxy A.A. Ashra"*, A. Ueta, H. Kumano, I. Suemune Research Institute for Electronic Science, Hokkaido University, Kita 12, Nishi 6, Kita-ku, Sapporo 060-0812, Japan
Abstract ZnS bu!er layers were used to grow ZnO "lms on GaAs(0 0 1) substrates. The role of ZnS bu!er layers in the growth of zincblende ZnO "lms on GaAs(0 0 1) substrates was investigated by atomic force microscopy, X-ray di!raction, and photoluminescence (PL) measurements. The optimization of the ZnS bu!er layer thickness resulted in improvements of the surface morphology and crystalline quality of ZnO "lms by homogeneous nucleation. With the optimized ZnS bu!er layer thickness of &72 nm the surface root-mean-square roughness of the grown ZnO "lm was minimized to &14 nm and the deep-level PL intensity was reduced to 1/76 of the near-band edge PL intensity at room temperature. 2000 Elsevier Science B.V. All rights reserved. PACS: 81.15.!z; 81.15.Hi; 61.82.Fk; 68.55.Jk Keywords: Zincblende ZnO; GaAs substrate; Bu!er layer; Atomic force microscopy; Photoluminescence
1. Introduction Research has been carried out for the development of short-wavelength semiconductor materials working in green, blue, or the shorter wavelength regions. Recently, an oxide material, ZnO, has attracted a great deal of attention due to its wide direct band gap energy of 3.37 eV at room temperature and a large exciton binding energy of 60 meV. Owing to the strong exciton binding energy, ZnO has been extensively studied considering their potential applications in electronic and photonic devices in the ultraviolet region. Notable develop-
* Corresponding author. Tel.: #81-11-706-2898; fax: #8111-706-4973. E-mail address: [email protected] (A.A. Ashra").
ments have been achieved in the "eld of fabrication of ZnO "lms grown on sapphire substrates [1}3] and a dominant wurtzite crystal structure has been observed. The wurtzite structure induces piezoelectric "eld e!ects by residual strain and this internal electric "eld tends to quench the exciton binding energy. On the other hand, zincblende ZnO is a metastable phase but is crystallographically isotropic. From the viewpoint of basic physics, zincblende ZnO will be attractive because the absence of giant internal electric "elds in this structure will allow the full exploitation of the strong excitonic e!ects. This will result in advantages in optical and electrical devices operations based on the excitonic e!ects. In device fabrication, for example, an optical cavity will be formed by cleavage through the 10 0 12 directions. Considering the future integration of monolithic devices, the
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A.A. Ashrax et al. / Journal of Crystal Growth 221 (2000) 435}439
ZnO-related light emitting devices grown on GaAs substrates will be preferred. Moreover, zincblende ZnO has the potential to be integrated with conventional Si and GaAs zincblende technology, to which attention will be paid in this era. Recently, we have reported a zincblende ZnO "lm on a GaAs(0 0 1) substrate grown at 6003C [4]. The crystalline quality of zincblende ZnO "lms grown on GaAs substrates will be a rational problem due to oxidation with substrates and the large lattice mismatch of about 19% between ZnO and GaAs, resulting in surface roughness and low crystalline quality. In the initial growth of ZnO "lms on GaAs substrates, the oxidation of GaAs surfaces produces amorphous oxide layers due to the volatile nature of the associated arsenic oxides [5], which prevents the growth of ZnO. The ZnS bu!er layer could prevent the formation of amorphous oxide layers on the GaAs surfaces and also the lattice mismatch between ZnO and ZnS/GaAs can be reduced to about 5%, resulting in the reduction of crystal structural defects. An understanding of the initial oxidation process on GaAs surfaces and the interface structures is quite important to improve the quality of the ZnO epilayers. The optimization of ZnS bu!er layers will improve the crystalline quality of ZnO "lms for a better surface morphology, crystalline quality, and optical properties. In this paper, we will demonstrate how a ZnS bu!er layer thickness contributes to improve the surface morphology and crystalline quality of ZnO epilayers grown on GaAs(0 0 1) substrates. The e!ect of a ZnS bu!er layer thickness on the surface morphology evolution of ZnO is investigated by atomic force microscopy (AFM), X-ray di!raction (XRD), and photoluminescence (PL) measurements.
2. Experiment Film-growth experiments were carried out using an electron}cyclotron resonance (ECR)-assisted metalorganic molecular-beam epitaxy (MOMBE). In this experiment, semi-insulating GaAs(0 0 1) substrates were used by etching in chemical solution with H O : H O : H SO "1 : 1 : 4 at 603C. In the
growth chamber, the etched substrates were thermally cleaned by supplying trisdimethylaminoarsenic #ux of 1.0;10\ Torr at 5503C for 30 min prior to the growth of the ZnS bu!er layer. After the thermal cleaning, a ZnS bu!er layer with a thickness of 9}286 nm was grown at 4003C using diethyl zinc (DEZn) and ditertiarybutyl sul"de (DtBS) sources. The beam pressures of DEZn and DtBS were 0.5;10\ and 2.5;10\ Torr, respectively, which gives the VI/II ratio of 5. The typical growth rate of ZnS was 0.42 lm/h. A ZnO "lm was grown on a ZnS bu!er layer using DEZn with beam pressures of 0.5;10\ Torr and an oxygen plasma with an oxygen #ow rate of 1.0 sccm in which the VI/II ratio was 2.6. High-purity oxygen gas and 2.45 GHz microwave power up to 200 W were introduced into the plasma chamber under the 875 G magnetic "eld to satisfy the ECR condition. After the growth of ZnS the supply of DEZn was continued on the as-grown ZnS surface for about 5 min to exhaust the remaining DtBS from the chamber as it may be incorporated in the epilayer. The growth temperature of ZnO epilayers was also 4003C.
3. Results and discussion The measured surface roughness of ZnO as a function of the ZnS bu!er layer thickness is shown in Fig. 1. The root-mean-square (rms) of the surface roughness of ZnO and ZnS "lms were measured by AFM. In Fig. 1, the ZnS surface rms roughness gradually increased with increase of the bu!er layer thickness. In the initial stage of ZnS growth, the lattice mismatch of about 5% between ZnS and GaAs induces the tensile strain [6] parallel to the interface in ZnS. The critical thickness is known to be a few 10 As [7], up to which ZnS epilayers will grow coherently on GaAs substrates. When a ZnS "lm gets thicker than the critical thickness, the mis"t dislocations will be generated and the lattice relaxation occurs as shown in Fig. 2 up to the relaxation of the tensile strain. An example of the ZnS "lm surface morphology is shown in Fig. 3(a) and the surface rms roughness was 4.0 nm for the 3;3 lm AFM scan with a ZnS thickness of 72 nm. The increase of the bu!er layer
A.A. Ashrax et al. / Journal of Crystal Growth 221 (2000) 435}439
Fig. 1. ZnO and ZnS surface rms values as a function of the ZnS bu!er layer thickness. The ZnS bu!er layers were grown on GaAs(0 0 1) substrates to the thickness ranging from 9}286 nm, and 0.33 lm-thick ZnO was grown on top. The surface rms roughnesses of both ZnO and ZnS were measured in 3;3 lm AFM scan.
thickness roughens the ZnS surface and therefore the ZnO surface as shown in Fig. 1. The rms of the surface roughness of ZnO decreased once by the increase of the bu!er layer thickness and increased again as shown in Fig. 1. For the very thin ZnS "lm, the ZnS surface roughness is comparable to the "lm thickness. This will result in the incomplete surface coverage of the ZnS "lm over the GaAs surface. Therefore, a partial oxidation of the GaAs surfaces in the initial growth of the ZnO "lms will take place and the resultant amorphous oxide layers in the interface will prevent the growth of ZnO [4], resulting in the high rms value in this region of the thin ZnS bu!er layers. With the increase of the ZnS layer thickness, the ZnO rms roughness is reduced by the full surface coverage of GaAs with the bu!er layer. In Fig. 2, the ZnS vertical lattice parameter is increased to its bulk value for the thicker bu!er layers, suggesting the reduction of the lattice mismatch between ZnO and ZnS. This will result in the reduced rms roughness of ZnO and the minimum surface rms roughness observed at around the 72 nm thickness of the ZnS layer. On the 72 nm bu!er layer thickness the rms surface roughness of
437
Fig. 2. Lattice parameters for ZnS epilayers with various thickness. In the optimized area of 72-nm-thick ZnS layer, the ZnS out-of-plane lattice constant is 5.37 As , which is still a little smaller than the bulk value of 5.4093 As . The XRD-FWHM of ZnO "lms as a function of ZnS layer thickness is shown in the inset.
ZnO was &14 nm for the 3;3 lm AFM scan as shown in Fig. 3(b). The h}2h x-ray di!raction (XRD) spectrum was recorded for the ZnO "lm grown with the ZnS bu!er layer thickness of 72 nm and is shown in Fig. 4. The XRD di!raction peak at 44.723 is identi"ed to be the ZnO(0 0 4) with the corresponding out-of-plane lattice constant of 4.37 As , which is far di!erent from that of wurtzite ZnO of 5.20 As along the c-axis and 3.25 As along the a-axis. The transmission electron microscope (TEM) measurements showed that the ZnO "lm is tetragonally distorted with the in-plane lattice parameter of 4.58 As and the out-of-plane lattice parameter of 4.36 As which gives a bulk lattice constant of 4.47 As [4]. The calculated value of the zincblende ZnO lattice constant is 4.60 As [8] which is close to 4.47 As , indicates that the grown ZnO "lm is zincblende. The crystalline quality of the grown ZnO is improved by the optimization of the bu!er layer thickness as is shown in the inset of Fig. 2 by the reduced fullwidth at half-maximum (FWHM) of the di!raction peaks. Also, the di!raction peak intensity of ZnO relative to that of GaAs is much improved by two orders in Fig. 4 for the "lm grown with the
438
A.A. Ashrax et al. / Journal of Crystal Growth 221 (2000) 435}439
Fig. 4. XRD spectrum of ZnO "lm grown on the optimized ZnS bu!er layer thickness. The vertical lattice constant of 4.37 As was deduced. The ZnO "lm thickness was 0.33 lm and typical growth rate was 0.22 lm/h.
Fig. 3. (a) AFM image of a ZnS "lm grown on GaAs surface with the "lm thickness of 72 nm and (b) AFM image of ZnO grown on ZnS/GaAs layer with the ZnS layer thickness of 72 nm. ZnO "lm thickness is 0.33 lm and the VI/II ratio was 2.6. The surface rms roughness was measured in 3;3 lm AFM scan for both "lms.
72-nm-thick bu!er layer compared with the reported one [4] in which the ZnS layer thickness was 143 nm. PL measurements were performed at room temperature (RT) on the ZnO "lms. The dominant ultraviolet emission peak was observed near the band edge at energies of 3.36 eV at 14 K and of
Fig. 5. Photoluminescence spectrum measured at room temperature by using a He}Cd laser excitation source with the wavelength of 325 nm. The band edge emission was observed at the photon energy of 3.27 eV with the FWHM of &113 meV. The ZnS bu!er layer and ZnO "lm thickness were 36 nm and 0.33 lm, respectively.
3.27 eV at RT as shown in Fig. 5. The PL peak at 14 K is very close to the re#ectance peak and the lowest-energy peak in the PL excitation spectrum measured at 14 K both at 3.368 eV, and the Stokes shift was within 1 meV [9]. This small Stokes shift
A.A. Ashrax et al. / Journal of Crystal Growth 221 (2000) 435}439
implies that the ZnO "lm has a very high purity, and the origin of luminescence is mainly attributed to the free-exciton transitions. The PL-FWHM at RT and LT, however, for the 72-nm-thick bu!er layer were &148 and &18 meV, respectively, which is large compared with the reported one [4,9]. But the ratio of the peak intensities of near-band edge emission (I ) to that of weakly observed deep, # level emission (I ) at around 2.50 eV was im"* proved for the samples of 72 nm thick bu!er layer and the luminescence emission intensity ratio of I /I was as high as 76 even at room temper, # "* ature. This value is also high compared to the reported value of 60 [9] observed in ZnO samples grown on 286-nm-thick ZnS bu!er layer. These comparisons also show that ZnO "lms have high optical properties under the optimization of ZnS "lm thickness. 4. Conclusion In conclusion, we demonstrated the optimization of the ZnS bu!er layer thickness for the growth of ZnO "lms on GaAs(0 0 1) substrates. From the investigation of surface morphology, crystalline quality, and optical properties studied by AFM, XRD, and PL measurements, the optimized ZnS bu!er layer thickness was &72 nm for the growth of zincblende ZnO "lms on GaAs substrates. In addi-
439
tion, the bulk zincblende ZnO lattice constant measured with the XRD and TEM observations is 4.47 As .
Acknowledgements The authors would like to thank Mr. Mitsuo Hoshiyama for his enormous technical support in developing the MOMBE system.
References [1] A. Ohtomo, M. Kawasaki, T. Koida, K. Masubuchi, H. Koinuma, Y. Sakurai, Y. Yoshida, T. Yasuda, Y. Segawa, Appl. Phys. Lett. 72 (1998) 2466. [2] Y. Chen, D.M. Bagnall, H. Koh, K. Park, K. Hiraga, Z. Zhu, T. Yao, J. Appl. Phys. 84 (1998) 3912. [3] Y. Chen, H.-J. Ko, S.-K. Hong, T. Yao, Appl. Phys. Lett. 76 (2000) 559. [4] A.A. Ashra", A. Ueta, A. Avramescu, H. Kumano, I. Suemune, Appl. Phys. Lett. 76 (2000) 550. [5] E.J. Tarsa, M. De Graef, D.R. Clarke, A.C. Gossard, J.S. Speck, J. Appl. Phys. 73 (1993) 3276. [6] S. Nam, B. O, K.-S. Lee, Y.D. Choi, J. Crystal Growth 194 (1998) 61. [7] J.W. MaCamy, D.H. Lowndes, J.D. Budai, R.A. Zuhr, X. Zhang, J. Appl. Phys. 73 (1993) 7818. [8] J.E. Ja!e, A.C. Hess, Phys. Rev. B 48 (1993) 7903. [9] H. Kumano, A. A. Ashra", A. Ueta, A. Avramescu, I. Suemune, J. Crystal Growth, 214/215 (2000) 280.
Role of ZnS bu!er layers in growth of zincblende ZnO on GaAs substrates by metalorganic molecular-beam epitaxy A.A. Ashra"*, A. Ueta, H. Kumano, I. Suemune Research Institute for Electronic Science, Hokkaido University, Kita 12, Nishi 6, Kita-ku, Sapporo 060-0812, Japan
Abstract ZnS bu!er layers were used to grow ZnO "lms on GaAs(0 0 1) substrates. The role of ZnS bu!er layers in the growth of zincblende ZnO "lms on GaAs(0 0 1) substrates was investigated by atomic force microscopy, X-ray di!raction, and photoluminescence (PL) measurements. The optimization of the ZnS bu!er layer thickness resulted in improvements of the surface morphology and crystalline quality of ZnO "lms by homogeneous nucleation. With the optimized ZnS bu!er layer thickness of &72 nm the surface root-mean-square roughness of the grown ZnO "lm was minimized to &14 nm and the deep-level PL intensity was reduced to 1/76 of the near-band edge PL intensity at room temperature. 2000 Elsevier Science B.V. All rights reserved. PACS: 81.15.!z; 81.15.Hi; 61.82.Fk; 68.55.Jk Keywords: Zincblende ZnO; GaAs substrate; Bu!er layer; Atomic force microscopy; Photoluminescence
1. Introduction Research has been carried out for the development of short-wavelength semiconductor materials working in green, blue, or the shorter wavelength regions. Recently, an oxide material, ZnO, has attracted a great deal of attention due to its wide direct band gap energy of 3.37 eV at room temperature and a large exciton binding energy of 60 meV. Owing to the strong exciton binding energy, ZnO has been extensively studied considering their potential applications in electronic and photonic devices in the ultraviolet region. Notable develop-
* Corresponding author. Tel.: #81-11-706-2898; fax: #8111-706-4973. E-mail address: [email protected] (A.A. Ashra").
ments have been achieved in the "eld of fabrication of ZnO "lms grown on sapphire substrates [1}3] and a dominant wurtzite crystal structure has been observed. The wurtzite structure induces piezoelectric "eld e!ects by residual strain and this internal electric "eld tends to quench the exciton binding energy. On the other hand, zincblende ZnO is a metastable phase but is crystallographically isotropic. From the viewpoint of basic physics, zincblende ZnO will be attractive because the absence of giant internal electric "elds in this structure will allow the full exploitation of the strong excitonic e!ects. This will result in advantages in optical and electrical devices operations based on the excitonic e!ects. In device fabrication, for example, an optical cavity will be formed by cleavage through the 10 0 12 directions. Considering the future integration of monolithic devices, the
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 7 3 2 - 6
436
A.A. Ashrax et al. / Journal of Crystal Growth 221 (2000) 435}439
ZnO-related light emitting devices grown on GaAs substrates will be preferred. Moreover, zincblende ZnO has the potential to be integrated with conventional Si and GaAs zincblende technology, to which attention will be paid in this era. Recently, we have reported a zincblende ZnO "lm on a GaAs(0 0 1) substrate grown at 6003C [4]. The crystalline quality of zincblende ZnO "lms grown on GaAs substrates will be a rational problem due to oxidation with substrates and the large lattice mismatch of about 19% between ZnO and GaAs, resulting in surface roughness and low crystalline quality. In the initial growth of ZnO "lms on GaAs substrates, the oxidation of GaAs surfaces produces amorphous oxide layers due to the volatile nature of the associated arsenic oxides [5], which prevents the growth of ZnO. The ZnS bu!er layer could prevent the formation of amorphous oxide layers on the GaAs surfaces and also the lattice mismatch between ZnO and ZnS/GaAs can be reduced to about 5%, resulting in the reduction of crystal structural defects. An understanding of the initial oxidation process on GaAs surfaces and the interface structures is quite important to improve the quality of the ZnO epilayers. The optimization of ZnS bu!er layers will improve the crystalline quality of ZnO "lms for a better surface morphology, crystalline quality, and optical properties. In this paper, we will demonstrate how a ZnS bu!er layer thickness contributes to improve the surface morphology and crystalline quality of ZnO epilayers grown on GaAs(0 0 1) substrates. The e!ect of a ZnS bu!er layer thickness on the surface morphology evolution of ZnO is investigated by atomic force microscopy (AFM), X-ray di!raction (XRD), and photoluminescence (PL) measurements.
2. Experiment Film-growth experiments were carried out using an electron}cyclotron resonance (ECR)-assisted metalorganic molecular-beam epitaxy (MOMBE). In this experiment, semi-insulating GaAs(0 0 1) substrates were used by etching in chemical solution with H O : H O : H SO "1 : 1 : 4 at 603C. In the
growth chamber, the etched substrates were thermally cleaned by supplying trisdimethylaminoarsenic #ux of 1.0;10\ Torr at 5503C for 30 min prior to the growth of the ZnS bu!er layer. After the thermal cleaning, a ZnS bu!er layer with a thickness of 9}286 nm was grown at 4003C using diethyl zinc (DEZn) and ditertiarybutyl sul"de (DtBS) sources. The beam pressures of DEZn and DtBS were 0.5;10\ and 2.5;10\ Torr, respectively, which gives the VI/II ratio of 5. The typical growth rate of ZnS was 0.42 lm/h. A ZnO "lm was grown on a ZnS bu!er layer using DEZn with beam pressures of 0.5;10\ Torr and an oxygen plasma with an oxygen #ow rate of 1.0 sccm in which the VI/II ratio was 2.6. High-purity oxygen gas and 2.45 GHz microwave power up to 200 W were introduced into the plasma chamber under the 875 G magnetic "eld to satisfy the ECR condition. After the growth of ZnS the supply of DEZn was continued on the as-grown ZnS surface for about 5 min to exhaust the remaining DtBS from the chamber as it may be incorporated in the epilayer. The growth temperature of ZnO epilayers was also 4003C.
3. Results and discussion The measured surface roughness of ZnO as a function of the ZnS bu!er layer thickness is shown in Fig. 1. The root-mean-square (rms) of the surface roughness of ZnO and ZnS "lms were measured by AFM. In Fig. 1, the ZnS surface rms roughness gradually increased with increase of the bu!er layer thickness. In the initial stage of ZnS growth, the lattice mismatch of about 5% between ZnS and GaAs induces the tensile strain [6] parallel to the interface in ZnS. The critical thickness is known to be a few 10 As [7], up to which ZnS epilayers will grow coherently on GaAs substrates. When a ZnS "lm gets thicker than the critical thickness, the mis"t dislocations will be generated and the lattice relaxation occurs as shown in Fig. 2 up to the relaxation of the tensile strain. An example of the ZnS "lm surface morphology is shown in Fig. 3(a) and the surface rms roughness was 4.0 nm for the 3;3 lm AFM scan with a ZnS thickness of 72 nm. The increase of the bu!er layer
A.A. Ashrax et al. / Journal of Crystal Growth 221 (2000) 435}439
Fig. 1. ZnO and ZnS surface rms values as a function of the ZnS bu!er layer thickness. The ZnS bu!er layers were grown on GaAs(0 0 1) substrates to the thickness ranging from 9}286 nm, and 0.33 lm-thick ZnO was grown on top. The surface rms roughnesses of both ZnO and ZnS were measured in 3;3 lm AFM scan.
thickness roughens the ZnS surface and therefore the ZnO surface as shown in Fig. 1. The rms of the surface roughness of ZnO decreased once by the increase of the bu!er layer thickness and increased again as shown in Fig. 1. For the very thin ZnS "lm, the ZnS surface roughness is comparable to the "lm thickness. This will result in the incomplete surface coverage of the ZnS "lm over the GaAs surface. Therefore, a partial oxidation of the GaAs surfaces in the initial growth of the ZnO "lms will take place and the resultant amorphous oxide layers in the interface will prevent the growth of ZnO [4], resulting in the high rms value in this region of the thin ZnS bu!er layers. With the increase of the ZnS layer thickness, the ZnO rms roughness is reduced by the full surface coverage of GaAs with the bu!er layer. In Fig. 2, the ZnS vertical lattice parameter is increased to its bulk value for the thicker bu!er layers, suggesting the reduction of the lattice mismatch between ZnO and ZnS. This will result in the reduced rms roughness of ZnO and the minimum surface rms roughness observed at around the 72 nm thickness of the ZnS layer. On the 72 nm bu!er layer thickness the rms surface roughness of
437
Fig. 2. Lattice parameters for ZnS epilayers with various thickness. In the optimized area of 72-nm-thick ZnS layer, the ZnS out-of-plane lattice constant is 5.37 As , which is still a little smaller than the bulk value of 5.4093 As . The XRD-FWHM of ZnO "lms as a function of ZnS layer thickness is shown in the inset.
ZnO was &14 nm for the 3;3 lm AFM scan as shown in Fig. 3(b). The h}2h x-ray di!raction (XRD) spectrum was recorded for the ZnO "lm grown with the ZnS bu!er layer thickness of 72 nm and is shown in Fig. 4. The XRD di!raction peak at 44.723 is identi"ed to be the ZnO(0 0 4) with the corresponding out-of-plane lattice constant of 4.37 As , which is far di!erent from that of wurtzite ZnO of 5.20 As along the c-axis and 3.25 As along the a-axis. The transmission electron microscope (TEM) measurements showed that the ZnO "lm is tetragonally distorted with the in-plane lattice parameter of 4.58 As and the out-of-plane lattice parameter of 4.36 As which gives a bulk lattice constant of 4.47 As [4]. The calculated value of the zincblende ZnO lattice constant is 4.60 As [8] which is close to 4.47 As , indicates that the grown ZnO "lm is zincblende. The crystalline quality of the grown ZnO is improved by the optimization of the bu!er layer thickness as is shown in the inset of Fig. 2 by the reduced fullwidth at half-maximum (FWHM) of the di!raction peaks. Also, the di!raction peak intensity of ZnO relative to that of GaAs is much improved by two orders in Fig. 4 for the "lm grown with the
438
A.A. Ashrax et al. / Journal of Crystal Growth 221 (2000) 435}439
Fig. 4. XRD spectrum of ZnO "lm grown on the optimized ZnS bu!er layer thickness. The vertical lattice constant of 4.37 As was deduced. The ZnO "lm thickness was 0.33 lm and typical growth rate was 0.22 lm/h.
Fig. 3. (a) AFM image of a ZnS "lm grown on GaAs surface with the "lm thickness of 72 nm and (b) AFM image of ZnO grown on ZnS/GaAs layer with the ZnS layer thickness of 72 nm. ZnO "lm thickness is 0.33 lm and the VI/II ratio was 2.6. The surface rms roughness was measured in 3;3 lm AFM scan for both "lms.
72-nm-thick bu!er layer compared with the reported one [4] in which the ZnS layer thickness was 143 nm. PL measurements were performed at room temperature (RT) on the ZnO "lms. The dominant ultraviolet emission peak was observed near the band edge at energies of 3.36 eV at 14 K and of
Fig. 5. Photoluminescence spectrum measured at room temperature by using a He}Cd laser excitation source with the wavelength of 325 nm. The band edge emission was observed at the photon energy of 3.27 eV with the FWHM of &113 meV. The ZnS bu!er layer and ZnO "lm thickness were 36 nm and 0.33 lm, respectively.
3.27 eV at RT as shown in Fig. 5. The PL peak at 14 K is very close to the re#ectance peak and the lowest-energy peak in the PL excitation spectrum measured at 14 K both at 3.368 eV, and the Stokes shift was within 1 meV [9]. This small Stokes shift
A.A. Ashrax et al. / Journal of Crystal Growth 221 (2000) 435}439
implies that the ZnO "lm has a very high purity, and the origin of luminescence is mainly attributed to the free-exciton transitions. The PL-FWHM at RT and LT, however, for the 72-nm-thick bu!er layer were &148 and &18 meV, respectively, which is large compared with the reported one [4,9]. But the ratio of the peak intensities of near-band edge emission (I ) to that of weakly observed deep, # level emission (I ) at around 2.50 eV was im"* proved for the samples of 72 nm thick bu!er layer and the luminescence emission intensity ratio of I /I was as high as 76 even at room temper, # "* ature. This value is also high compared to the reported value of 60 [9] observed in ZnO samples grown on 286-nm-thick ZnS bu!er layer. These comparisons also show that ZnO "lms have high optical properties under the optimization of ZnS "lm thickness. 4. Conclusion In conclusion, we demonstrated the optimization of the ZnS bu!er layer thickness for the growth of ZnO "lms on GaAs(0 0 1) substrates. From the investigation of surface morphology, crystalline quality, and optical properties studied by AFM, XRD, and PL measurements, the optimized ZnS bu!er layer thickness was &72 nm for the growth of zincblende ZnO "lms on GaAs substrates. In addi-
439
tion, the bulk zincblende ZnO lattice constant measured with the XRD and TEM observations is 4.47 As .
Acknowledgements The authors would like to thank Mr. Mitsuo Hoshiyama for his enormous technical support in developing the MOMBE system.
References [1] A. Ohtomo, M. Kawasaki, T. Koida, K. Masubuchi, H. Koinuma, Y. Sakurai, Y. Yoshida, T. Yasuda, Y. Segawa, Appl. Phys. Lett. 72 (1998) 2466. [2] Y. Chen, D.M. Bagnall, H. Koh, K. Park, K. Hiraga, Z. Zhu, T. Yao, J. Appl. Phys. 84 (1998) 3912. [3] Y. Chen, H.-J. Ko, S.-K. Hong, T. Yao, Appl. Phys. Lett. 76 (2000) 559. [4] A.A. Ashra", A. Ueta, A. Avramescu, H. Kumano, I. Suemune, Appl. Phys. Lett. 76 (2000) 550. [5] E.J. Tarsa, M. De Graef, D.R. Clarke, A.C. Gossard, J.S. Speck, J. Appl. Phys. 73 (1993) 3276. [6] S. Nam, B. O, K.-S. Lee, Y.D. Choi, J. Crystal Growth 194 (1998) 61. [7] J.W. MaCamy, D.H. Lowndes, J.D. Budai, R.A. Zuhr, X. Zhang, J. Appl. Phys. 73 (1993) 7818. [8] J.E. Ja!e, A.C. Hess, Phys. Rev. B 48 (1993) 7903. [9] H. Kumano, A. A. Ashra", A. Ueta, A. Avramescu, I. Suemune, J. Crystal Growth, 214/215 (2000) 280.