Investigation of As-doped ZnO films synthesized via thermal annealing of ZnSe/GaAs heterostructures
ARTICLE IN PRESS Journal of Crystal Growth 310 (2008) 3149– 3153 Contents lists available at ScienceDirect Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgroInvestigation of As-doped ZnO ﬁlms synthesized via thermal annealing ofZnSe/GaAs heterostructuresO. Maksimov Ã, B.Z. LiuMaterials Research Institute, Pennsylvania State University, University Park, PA 16802, USAa r t i c l e in fo abstractArticle history: We synthesized ZnO ﬁlms via oxidative annealing of ZnSe/GaAs heterostructures and investigated theirReceived 29 January 2008 structural and optical properties. Films were polycrystalline, c-axis oriented and exhibited superiorReceived in revised form optical properties. In addition, we detected nanometer-size As clusters into the ZnO ﬁlm and a GaxOy26 February 2008 layer at the ZnO/GaAs interface. Formation of an interfacial layer can prevent use of this technique forAccepted 14 March 2008 p-type doping and complicates identiﬁcation of the origin of p-type response in the annealed ZnO/GaAsCommunicated by R. FornariAvailable online 20 March 2008 heterostructures. & 2008 Elsevier B.V. All rights reserved.PACS:71.55.Gs81.40.Ef82.80.Pv87.64.BxKeywords:A1. Auger electron spectroscopyA1. p-Type dopingB1. ZnOB2. Semiconducting II–VI materials1. Introduction with transition metals  coupled with nanosecond-long spin coherence time measured at low temperatures  makes this There is a broad technological and scientiﬁc interest in zinc material extremely promising for future application in spinoxide (ZnO) due to its unique physical and chemical properties . electronics.It is a radiatively hard wide band gap semiconductor (EG$3.37 eV) Although high crystalline quality ZnO ﬁlms were grown usingthat can be easily doped n-type. Its band gap energy can be tuned molecular beam epitaxy (MBE), chemical vapor deposition (CVD),by alloying with MgO and CdO from 7.9 to 2.3 eV , covering and pulsed laser deposition (PLD), further progress in this area isdeep-UV to visible regions of the spectrum. It has a much higher slowed down by the difﬁculties associated with doping ZnOexciton binding energy, 60 meV, when compared with other wide p-type. It is generally acknowledged that high enough dopingband gap semiconductors like GaN or SiC, leading to the efﬁcient levels are difﬁcult to achieve both due to the background n-typeexcitonic transitions at room temperature. In addition, owing to doping originating from the presence of H impurities and pointthe availability of native substrates and amenability to wet defects, such as O vacancies and Zn interstitials [7–11], and due tochemical etching, ZnO is an extremely promising material for the large acceptor activation energies and/or low solubility ofthe development of optoelectronic devices, such as ultraviolet and commonly used group V (N, P, As) and group I (Li) dopants. Invisible light-emitting diodes (LEDs) and detectors. ZnO nanos- addition, a slow transition from p-type to n-type conductivity wastructures (nanoparticles, nanorods, nanobelts, etc.), which can be observed by a number of research groups. It was tentativelysynthesized using inexpensive physical vapor transport techni- assigned either to the acceptor migration from the substitutionalques, were shown to be extremely promising for application as gas to the interstitial position or to the hydrogen diffusion [12,13].sensors, due to the large surface to volume ratio , and In spite of these difﬁculties, there are reports of ZnO-dopedmicrolasers due to the superior optical properties . Further- p-type with group V (N [14,15], P [16,17], As [18–22], and Sbmore, reports of high-temperature ferromagnetism in ZnO doped [23,24]) and group I (Li ) elements. Co-doping with two potential acceptors (N and As)  or acceptor and donor (N and Al) [27,28] was also used. In the case of the acceptor–donor Ã Corresponding author. Tel.: +17242956624; fax: +17242956617. co-doping, the improvement is believed to be primarily due to the E-mail address: email@example.com (O. Maksimov). higher solubility of the forming N–Al–N complex. A temperature0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.jcrysgro.2008.03.027
ARTICLE IN PRESS3150 O. Maksimov, B.Z. Liu / Journal of Crystal Growth 310 (2008) 3149–3153modulation epitaxy technique was also applied to achieve 002 GaAsN-doped p-type ZnO . Here, nitrogen-doped layers were Intensity (Arb. Units)grown at low temperature (400 1C) to increase solubility followedby the deposition of thin undoped layer at high temperature 002(950 1C) to improve crystalline quality. 3.05 In particular, p-type doping utilizing thermal As diffusion from Intensity (Arb. Units)GaAs substrates into the ZnO ﬁlms [18,21,30–34] and by oxidationof the ZnTe/GaAs heterostrucutres [35,36] was realized by a numberof research groups. In addition, both n- and p-type ZnO ﬁlms were 102synthesized through the annealing of undoped ZnSe crystals in the 12 16 20 24activated oxygen atmosphere (radical beam gettering epitaxy) ω (deg.)[37,38]. However, additional effects, such as formation of aninterfacial Zn2As2O7 layer , Ga diffusion into ZnO ﬁlm [40,41], 004GaAsand Zn diffusion into the GaAs substrate  were also reported. 110Also, isolated As atoms should act either as deep acceptors (As isincorporated substitutionally at the O position—AsO) and donors x100 101 004(As is incorporated substitutionally at the Zn position—AsZn) orremain amphoteric (As is incorporated interstitially—Asi). Thus,p-type conductivity is explained by the formation of a complex with 30 40 50 60 70two spontaneous Zn vacancies (AsZnÀ2VZn) . Clearly, data Θ -2Θ (deg.)interpretation is not straightforward and the origin of the p-type Fig. 1. XRD yÀ2y scan registered from the annealed ZnSe/GaAs heterostructure.response remains controversial. Hence, we synthesized ZnO ﬁlms by Inset is the XRD o scan registered for the /0 0 2S ZnO peak.thermal oxidation of MBE-grown ZnSe/GaAs heterostructures andinvestigated their structural and optical properties. A particular /0 0 1S or c-axis preferred orientation is usually observed dueemphasis was dedicated to the ﬁlm/substrate interface that was to its lowest surface free energy. Some misoriented ZnO grainsexamined using Auger electron spectroscopy (AES). (/1 0 1S, /1 0 2S and /11 0S) are also present. There were observations of Te and ZnO2 inclusions in the oxidized ZnTe and2. Experimental details ZnxNy ﬁlms [35,46]. Here, we do not detect any peaks that can be indexed as ZnSe, Se, or ZnO2 phases suggesting that ZnSe was High crystalline quality ZnSe ﬁlms were grown in the Veeco entirely transformed into ZnO by oxidative annealing. The /0 0 2SMBE system on the epi-ready semi-insulating (0 0 1) GaAs ZnO peak is relatively narrow, with a full-width at half-maximumsubstrates. Next, they were oxidized in a horizontal tube furnace (FWHM) of $0.31, which is comparable to the previous reportsby annealing in an oxygen ﬂow for 2 h at 500 1C. Our annealing time [31,49]. This corresponds to the mean grain size of $27 nmexceeds rapid annealing used for ZnTe ﬁlms (1–25 min) [35,36] and calculated using the Scherrer formulais comparable to the long annealing (1–5 h) applied toward ZnS[44,45] and ZnxNy [46–48] ﬁlms to assure full transformation of D ¼ 0:89l=b cos yZnSe into the ZnO. Thickness of ZnSe ﬁlms was varied between ˚ where l is the X-ray wavelength (1.5406 A), b is the FWHM of300 nm and 1 mm. Since similar results were obtained for all of the the diffraction peak in radians, and y is the Bragg diffractionﬁlms, only the data for a 700-nm thick ﬁlm is presented. angle . Structural and optical properties of the annealed ﬁlms were Inset of Fig. 1 is the o scan (rocking curve) of the /0 0 2S peakinvestigated using X-ray diffraction (XRD), Raman and photolumi- that shows out-of-plane mosaic spread and serves a goodnescence (PL) spectroscopies. XRD measurements were carried out indication of crystalline quality. Although it is broaderin yÀ2y and o modes (to determine out-of-plane orientation and (FWHM$3.051) than measured for the PLD-grown ZnO ﬁlmsmosaic spread) using a Scintag X2 diffractometer. Raman measure- (FWHM$1.451) , it is comparable to the value reported forments were performed in a backscattering geometry using a ZnO ﬁlm heavily doped with P (FWHM$3.211) . Thus, dopantconfocal Raman set up (CRM 200, WITec) equipped with an Ar+ -ion (As) incorporation could be the reason of the rocking curvelaser (a 488 nm line focused with a 40 Â objective was used for broadening.excitation), Actron spectrometer, and a charge-couple device Fig. 2 is the Raman spectrum collected from the annealedcamera (Andor DV401-BV CCD). PL measurements were performed ZnSe/GaAs heterostructure. As is expected for a highly texturedat room temperature. Luminescence was excited with a 266 nm line ﬁlm measured in a backscattering geometry, only EHigh (435 cmÀ1) 2of a pulsed Nd:YAG laser (Microchip NanoUV-266, JDS Uniphase), and ALO (569 cmÀ1) modes are observed . ALO peak is much 1 1spectrally resolved through a spectrometer (ISA, Edison), and more intense, when compared with EHigh and has a characteristic 2detected with a photomultiplier tube. asymmetric shape with a low-energy tail. An increase in intensity The morphology was investigated with scanning electron of an ALO peak was reported for the N-doped ZnO  while 1microscopy (SEM) in the ﬁeld emission SEM (JEOL 6700F). The similar asymmetry was recorded for a Sb-doped ZnO . Thus,surface and in-depth composition analysis was performed by AES shape and intensity of ALO mode can serve an indication of dopant 1in the Physical Electronics 670 ﬁled emission scanning Auger incorporation.nanoprobe using a 10 KeV and 10 nA electron beam. The samples In addition to the two lines originating from the ZnO ﬁlm, threewere 301 tilted with respect to the electron beam during analysis. more are present at a low-energy side of the spectrum. An intenseThree KV Ar ion sputtering was used for depth proﬁling. peak at 292 cmÀ1 is the LO mode from the GaAs substrate. Two other peaks recorded at 199 and 257 cmÀ1 require more attention.3. Results and discussion They can be assigned as Eg and A1g modes of the crystalline As  indicating there are nanometer-size As clusters dispersed into the Fig. 1 is a yÀ2y scan collected from the annealed ZnSe ﬁlm. ZnO layer. Absence of the As peaks in the XRD spectrum can beIt reveals that the ﬁlm is a highly textured /0 0 2S ZnO. Such explained by their small volume fraction.
ARTICLE IN PRESS O. Maksimov, B.Z. Liu / Journal of Crystal Growth 310 (2008) 3149–3153 3151 A1g (As) 3.262 eV 7000 Intensity (Arb. Units) LO (GaAs) Eg (As) 165 meV LO (ZnO)Intensity (Arb. Units) A1 6000 E2high (ZnO) 3.5 3.0 2.5 2.0 Energy (eV) Fig. 4. PL spectrum collected at room temperature from the annealed ZnSe/GaAs heterostructure. 5000 Si KLL O KLL 200 400 600 Zn LMM Raman Shift (cm-1) Intensity (Arb. Units)Fig. 2. Raman spectrum collected from the annealed ZnSe/GaAs heterostructure. As LMM C LMM 200 400 600 800 1000 1200 1400 1600 Kinetic Energy (eV) Fig. 5. AES survery spectrum from the surface of the annealed ZnSe/GaAs heterostructure. Fig. 5 shows a surface spectrum acquired prior to sputtering. In addition to the expected Zn, O, and As, Si and C are found. While C is from the hydrocarbon contamination unavoidable during the Fig. 3. SEM image of the annealed ZnSe/GaAs heterostructure. sample handling, Si contamination may come from the wall of the quartz tube during annealing. Both C and Si are present only on Fig. 3 is the SEM micrograph showing the surface of the ZnO the surface. Se or Ga are not observed, indicating these elementsﬁlm. It is composed of dense grains with uniform size distribution are either not present or below the detection limit of theas is expected for a polycrystalline ﬁlm. technique. The presence of As on the surface indicates possible Fig. 4 is the PL spectrum collected at room temperature. It is diffusion of As into ZnO, which is conﬁrmed by the depth proﬁle,dominated by an intense, narrow (FWHM$165 meV) band edge as shown in Fig. 6. It can be seen that As is uniformly distributedemission line at $3.262 eV. No signiﬁcant deep-level emission, within the ZnO ﬁlm (Region I), with a concentration of roughly 8.5usually originated from the point defects such as Zn vacancies atomic percents. Thermally activated As diffusion from the GaAs(VZn), Zn interstitials (Zni), and oxygen vacancies (VO),  is substrate into the ZnO ﬁlm was previously detected usingpresent at $2.4 eV. Thus, although the ﬁlm is polycrystalline, secondary ion mass spectroscopy (SIMS) [18,30,35,41,57]. Arsenicindividual ZnO grains are close to stoichiometry and of high content depended on the deposition/annealing conditions with upoptical quality. We should note that superior optical properties to 1021 cmÀ3 As atoms usually incorporated into ZnO. Bywere reported for the ZnO ﬁlms obtained via oxidative annealing comparison, our As concentration is higher, probably, due to theof ZnS [44,45] and Zn3N2 . long annealing time. Since it is above the solubility limit,
ARTICLE IN PRESS3152 O. Maksimov, B.Z. Liu / Journal of Crystal Growth 310 (2008) 3149–3153 80 this layer should have a nanocrystalline or amorphous structure. Region I Region II Region III Thus, oxidation of ZnSe is not self-limiting process at this ﬁlmApprox. concentration (atom%) thickness (o1 mm) and further oxidation of GaAs substrate occurs. 60 Furthermore, As diffusion into the ZnO layer can be facilitated by O the oxidation process through the anion exchange observed in the Zn Ga ZnSe/GaAs system [59,60]. We should also mention that Ga2O3 40 As layer was detected at the ZnO/(Cu, In)GaSe2  and ZnO/GaN interfaces [62,63]. Since much higher temperatures (41000 1C) are required to promote reaction between ZnO and Ga2O3 , we do not expect formation of more complex phases like ZnGa2O4. 20 Still, GaxOy interfacial layer can prevent use of oxidative annealing for p-type doping and complicates identiﬁcation of the origin of p-type response in the annealed ZnO/GaAs heterostructures. For 0 example, Zn-doped Ga2O3 is a p-type wide band gap semicon- Sputtering time (s) ductor (EG$4.8 eV) [65,66]. Then, electroluminescence may beFig. 6. Depth proﬁle of the annealed ZnSe(700 nm)/GaAs heterostructure. due to the hole injection from the p-type Ga2O3 into the ZnO.Quantiﬁcation is performed by applying instrument-default relative sensitivity While GaxOy interfacial layer does not form in many cases, forfactors to the integrated peak areas. The slight off-stoichiometry may arise either example when a few mm thick ZnTe is used [35,36], some of thefrom the deviation of the instrument-default relative sensitivity factors from thereal values, or from other factors like preferential sputtering. previous reports of successful p-type doping  and device fabrication [31–34] using As diffusion into the ZnO/GaAs hetero- structures have to be revisited since very limited structural characterization is reported. Clearly, SIMS data cannot be used as the proof of substitutional or interstitial As incorporation into the ZnO alloy. Instead, As can be present in the nanometer-size clusters. Finally, detailed structural characterization should be performed in each case with particular attention being dedicateddN (E)/dE (a.u.) toward the ZnO/GaAs interface to exclude interfacial layer Ga in formation. Region III Ga in Region II 4. Conclusions We synthesized ZnO ﬁlms via oxidative annealing of ZnSe/ GaAs heterostructures and investigated their optical and structur- al properties using a wide range of techniques. Films were highly textured and exhibited sharp band edge PL at room temperature. 1065 1070 1075 1080 1085 We observed that As diffusion into ZnO layer is accompanied by Kinetic Energy (eV) the formation of nanometer-size As clusters within the ZnO ﬁlm and a GaxOy layer at the ZnO/GaAs interface. Such a layerFig. 7. The Ga LMM spectra acquired at different depths from the annealed complicates identiﬁcation of the origin of p-type response in theZnSe/GaAs heterostructure. annealed ZnO/GaAs heterostructures and can prevent use of oxidative annealing for p-type doping.formation of As clusters, detected using Raman spectroscopy, isnot surprising. Due to the presence of clusters, we cannot judge on Acknowledgmentsthe amount of As incorporated into ZnO alloy at the substitutional(AsO and AsZn) and interstitial (Asi) positions and on the efﬁciencyof thermal activated diffusion as a doping technique. This work was partially supported by the Department of the Navy, Ofﬁce of Naval Research under Grant N00014-07-1-0460. Fig. 6 also shows that the zinc/oxygen ratio remains constant($0.65) within the ZnO ﬁlm in Region I, indicating uniform Any opinions, ﬁndings and conclusions or recommendationsoxidation of ZnSe ﬁlm during annealing. The slight off-stoichio- expressed in this material are those of the authors and do notmetry may arise either from the deviation of the instrument- necessarily reﬂect the views of the Ofﬁce of Naval Research.default relative sensitivity factors from the real values, or from I would also like to thank Dr. Nitin Samarth (Pennsylvania Stateother factors like preferential sputtering. Notice that Ga is not University) for providing ZnSe ﬁlms. The authors acknowledge usepresent within the ZnO ﬁlm, and low concentration of Ga in of facilities at the PSU Site of the NSF NNIN under Agree-Region I is from the background noise. ment#0335765. It is interesting to see an interfacial layer (Region II) formedbetween the ZnO ﬁlm and the GaAs substrate, as shown in Fig. 6. ReferencesRegion II contains Ga, O, and a trace amount of As. In addition, asshown in Fig. 7, the Ga LMM peak in this region has a 3 eV shift  U. Ozgur, Y.I. Alivov, C. Liu, A. Teke, M.A. Reschikov, S. Dogan, V. 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