Zahra Vashaei

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application/pdf博士学位論文の要旨及び審査結果の要旨 (Summary of Thesis(DR))893688 bytesothe

Zahra, Vashaei

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Characterization of MgxZn1-xO Layers and TheirHeterostructures Grown by Plasma-assistedMolecular Beam Epitaxy著者 Zahra Vashaei号 50学位授与番号 3565URL http://hdl.handle.net/10097/37233if~ '~~~4 ~: ~~ ZahraVashaei i~: J~ ~~: 4~L*+' f~:~ (IC~~) ~~f,+'~Li~:~~~~jj] ~ Ifz;~184~3)~24~ ~~:f,*-lLi~:~O)~~:~~~~~~~~: ~~4,+-~L~:~lJ~~4~s~~ I ~~ ~l:~!~~~, ~~~:~~)~~;~i~ ~:~t;~C~~:;~~~:~j~II~~~:~F~~:~4 (1~:d~=~~~:) fi~~~~:~V~~~~~:~C ~~ 4~L ~~ ~ ~i~ ~ C.haracterization of Mg^Znl-*O Layers and Their Heterostructures Grown by Plasma-assisted Molecular Beam Epitaxy (7~ ~7~;l~~~A:~~~~~:c t~~~ ~ ~/~j~~: MgZnO ~~ ~ ~q)-~ ~z ~~i~~q)~~:~:'~'-'-~-"~~Hi}C f~~~~~l:~~) ~ sm ~C ~~ ~~ ~i ~: (E~~~) ~~tj~~~~~~i~~ /n+~ F~:~C ~:~tj~~~~~~~ ~~~~F~ ~lj ~;~, ~ ~* ~~ ,*~ ;f*~ *, ~~ ~~l ~a PFE ~C ;~~~'  ~I : ZnO is a II-VI wide band gap (3.37 eV) semiconductor with a large exciton binding energy of 60 meV, which is attracting attention for its application to UV Iightremitters, transparent, high power electronics, surface acoustic wave devices (SAWDS), piezoelectric transducers, gas-sensing and window material for display and solar cejls. Recent improvements in the quality and control of conductivity in bulk and epitaxial ZnO have increased interest in the -use of this material for short wavelength light emitters and transparent electronics. As a wide band gap semiconductor, ZnO is a candidate host for solid state blue to UV optoelectronics, including laser development. Moreover, by alloying ZnO with MgO and CdO the band gap can be tuned over wide range (3.3~ Eg~ 7.5 eV), which is essential for device fabrication. The strong desire to expand the semiconductor industy into wide band gap material and to explore exitonic properties for a new generation of optoelectrQnic and photonic devices emphasizes the need of detailed studies Qn ZnO and its related materials. Three essential subjects, which should be studied to develop ZnO-related devices, can be realized as. high crystal quality ZnO Iayers, conductivity contro] including n- and r~type ZnO, and tuning the band gap. It is well known that the electrical and optical properties in device applications are largely affected by lattice defects such as threading dislocations and stacking faults in a crystal. Therefore the growih of high crystal quality ZnO Iayers is significant. It is well known that the electrical and optical properties of ZnO films depend on their microstructure, and that the presence of defects and the structure of the film-substrate interface have a crucial role in the device performances. The low cost sapphire substrate has been most extensively used as substrate for ZnO. However the crystal quality of ZnO grown directly on (OOO I ) sapphire tends to be very poor due to the formation of rotated domains caused by the large lattice and thermal mismatch. In order to improve the crystal quality of ZnO Iayer, growih of ZnO/MgO double buffer layer has been explored using P-MBE, which can suppress the rotation domains and reduce screw dislocation density however the edge dislocation density is still high (5-lOxl09 cm~1). Therefore, further improve of the quality of ZnO is required. One of the conventional ways in order to reduce dislocation densities is applying graded buffer layer, which has not been investigated about P-MBE ZnO epitaxy so far. Therefore, we suggest Mg.Znl-.O alloys as a promising material apptoaching to this purpose. Mg~Znl-.O, which is realized by alloying MgO w~th ZnO, exhibits the same material advantages as pure ZnO. By varying the Mg composition, the band gap can be tuned from 3.3 to 5.4 eV, which is a significant property of Mg*Znl-*O for application as a proper barrier for ZnO in ZnO/MgZnO superlattice structures. Moreover, Mg.Znl-.O alloys are promising candidates in devices fabrication such as photodetectors in ultravio]et (UV) and solar-blind (wavelength < 280 nm) regions, flame detection, fisrnace control, owing to their wide variation of band gap. - 475 -ZnO has wurtzie structure while MgO has rock salt structure. Therefore Mg*Znl-xO Iayers can possess two = different crystal structures, rock salt cubic or hexagonal wartzite depending on the Mg content. Dissimilarity ; between ZnO and MgO crystal structure causes metastable single phase, and phase separation happens within 0.4 <x< 0.6. It is well known that crystal structure and crystal quality of MgxZnl *O Iayers can be affected by the growih technique and growih condition. The structure evolution of MgxZnl xO~ Iayers from wurtzite to rock salt (and vice versa) has not been extensively studied yet. In addition, we introd~ce Mg*Znl *O alloys as a potential material to grade the buffer layer from MgO to ZnO and reduce the lattice mismatch as well as dislocation density. In current researeh we used P-MBE owing to its various advantages including precise controllability of composition, Iayer thickness, and growih rate for growing Mg*Znl-*O and ZnO Iayers. c-sapphire was used as a substrate because of its low cost, Iarge size, and high quality. However to overcome the large lattice mismatch between c-sapphire and Mg*Znl-*O and ZnO Iayers introducing a buffer layer is indispensable. Therefore, in order to investigate the effect of buffer layers on crystal structure of epilayers different kinds of templates were applied for Mg.Znl-'O and ZnO Iayers. Iiij; Characterization of the layers was performed using various methods including transmission electron microscopy (TEM), high resolution TEM (HRTEM), high resolution x-ray ,di~action (HRXRD), reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM), and Photoluminescence (PL). The first step toward growing Mg*Znl-*O heterostructures is the control of Mg . j,;"';:;;;:j 1 .O composition during the growih. For this reason, V-the dependency of composition as a function of O .9 flux ratio was investigated. Fig. I shows the Mg ,~ 0.8 .ef ~~{ e.' content. measured by EPMA as a function of '-f / ' ~ single phase(cubic)  Mg/(Zn+Mg) flux ratio determined by quartz i･5f 0.7 ----~~-e~-*---- -crystal thickness monitor at the sample position ~ 0.6 . ' ~le 0.5 l' (in-situ). It was found that Mg incorporation is much higher than that of Zn atoms. The sticking ll O eJO 0.4 --/iL----coefficient of Mg was estimated 4.8 times -~ e' :~ larger than the Zn sticking coefficient. The 0.3 /O single phase(hexagonal) stacking coefflcient ratio enables us to estimate ,' t,~ 0.2 , Mg content precisely during the growih. l 0.1 ,'O In order to investigate the effect of buffer layers on the crystal structure of the MgxZni_*O 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 layers, two series of MgxZnl-*O Iayers over a wide range composition (0<x<1) were grown JMg/(JMg+JZn) on MgO/c-sapphire and ZnO/MgO/c-sapphire templates. First we investigated the solid  solubility and crystal transition poi･nt of Fig. I Mg content in Mg.Znl-O films as a function of flux Mg*Znl-xO Iayers by XRD. The results indicated that the solid solubility of MgO in wurtzite-ZnO without any phase separation is 340/0 for Mg.Znl-.O Iayers grown on MgO/c-sapphire and it is 440/Q for Mg*Znl-*O Iayers grown on ZnO/MgO/c-sapphire. Whereas the solid solubility of ZnO in rock salt-MgO without any phase separation is 350/0 for Mg*Znl-*O Iayers grown on MgO/c-sapphire and it is 230/0 for Mg*Znl-*O layers grown on ZnO/MgO/c-sapphire. This specifies that the tendency ofMg*Znl-*O crystallization in wurtzite or rock salt structure depend on the template structure in addition to the growih condition. A model assuming a virtual crystal structure of Mg~Znl=0 and Vegard's law was used to interpret the bond-length variations with a good agreement with experimental results for 'both wartzite and rock salt structures. Low variation of bond length as a function of Mg composition in each structure reflects the substitution of Mg or Zn in each lattice sites without crystal strueture deformation. - 476 -*i{;,~ *,~~j Cross sectional HRTEM observations near the interface of three Mg.Znl-'O Iayers with wurtzie, rock salt and mixed crystal structure were done. A brief result will be discussed in following. Rock salt-MgxZnl-xO Iayers The HRTEM images show that the rock salt-MgoL76Zn0,240 Iayers grow along the [11l] direction with twinning on MgO/c-sa_pphir. The twin was identified as a coherent twin. The epitaxial orientation relationship from SADP was [1 lO]Mgz.oll[1 To]Mgo ll [10 To]=,pphi,e and (111) Mgz.oll(111) Mgo ll(OOOl) s'pphire, tvhich demonstrates the growih of cubic-Mg0.76Zn0.240 along the [1 1 1 J direction. Whereas the HRTEM image of rock salt-Mg0.77Zn0.230 Iayers grown on ZnO/MgO/c-sapphire shows that they grow along the [OOl] direction. From SADP the epitaxial relationship was defined as [110] Mgz.ollll ToO] z.oll[1 1 ~]Mgo ll [1 ~10]sapphi" and(OO1)Mgz.o ll(OO1)z.o ll(1 1 1)Mgo 11 (OOO1)sapphir' ,which demonstrates the growth of cubic-Mg(}.77Zn0.230 along the [OOl] direction. It can be concluded that MgxZnl-xO crystal orientation is sensitive to lattice misfit and crystal structure of buffer layer. Wurtzite-Mg*Znl-xO Iayers Based on XRD results the Mg0.2gZn0.720 Iayer has wurtzite structure however HRTEM observation demonstrated the growth of Mg0,28Zn0.720 Iayers with rock salt stucture on MgO/c-sapphire template for 5-25 nm and then changes to the more stab]e [OOO I J-oriented wurtzite phase. Rotation twin was observed in cubic phase. The epitaxial orientation reiationship of the film from the diffraetion pattern analysis was deterrnined as [1 ~10]Mgz.oll[1 To]M~o 11 [10 To],,pphi,. and (OOO1)M~z.o ll(111)Mgo ll(OOOl)sapphir" Therefore growih of rock salt-MgxZnl-xO with low Mg concentration is feasible using a thin MgO buffer layer o,n c-sapphire. HRTEM observation of wurtzite-Mg0.44Zh0.560 iay~r ~vas repres~ntative of high quality single wurtzite-MgxZnl-xO Iayer with low variation of lattice parameter due to alloying attributed to a relaxed layer. Mixed- Mg*Znl-*O Iayers Mgo.4 IZno.590 Iayer grown on MgO/c-sapphire has mixed structure of watzite and rock salt. The existence of a double layer with erystal structure transition from rock salt to wurtzite along the growih direction with a I O~ misorientation was observed by HRTEM for Mg_0.41Zno.590/ MgO/ c-sapphire. The epitaxial relationship was determined by SADP and RHEED as follow: [1 210J Mgzno (.,~) 11 [1 To] Mgzso(c) l] [1 To]Mgo fl [10 To] ="pphi,e and (OOO I ) Mgz,,o (,~) Il (1 1 l) Mgz*o(*) Ii (1 1 l) Mgoll (OOOl) sapphi** -The Mg0.69Zn0.310 Iayer grown on ZnO/MgO/c-sapphire also has mjxed structure of wurtzite and rock salt. However, cross sectional HRTEM observation of Mg0.69Zn0.310/ZnO/MgO/c-sapphire, revealed the growih of a 60nm wurtzite-Mgo.69Zne.310 Iayer, whieh changed to the rock salt-Mg0.69Zno.310 Iayer. The epitaxial relationship was determined by SADP and RHEED as follow: E110] Mgz.o ll[1 TOO]Mgznoll [1 TOO] zno I[[ll 2~]Mgo II [1 ~lO] sappl'i" and (OOl) Mgz.o ll(OOOl)Mgz.oll (OO I )zno il(1 1 1 )Mgo ll (OOO1)sapphire' This specifies that by introducing a ZnO buffer layer, the growih of wurtzite Mg.Znl-'O Iayers with high Mg concentration (x>0.5) is feasible , which can lead to the growih of wurtzite MgO on ZnO in the future and suggests the possible formation of wurtzite ZnOIMgO superlattices. As a result, it is concluded that phase separation occurs along the growih direction and none of the phases are embedded in the host matrix. The crystal quality of wurtzite MgxZnl ^O Iayers grown on ZnO/MgO/c-sapphire is better than that of grown on MgO/c-sapphire. However the crystal quality of cubic MgxZnl *O Iayers grown on MgO/c- sapphire is better than that grown on ZnO/MgO/c-sapphire. ZnO/MgZnO(c) & ZnO/MgZnO (w) As mentioned before, finding a proper buffer layer to overcome the lattice mismatch between ZnO and c-sapphire in order to improve the ZnO quality is the main subject of interest. For this reason, Mg Znl 'O buffer layers with different crystal structure of wurtzite and rock salt were applied as buffer layers for Zndxlayers. In situ RHEED observation revealed that the critical thickness to obtain a relaxed layer is 2 nm for cubic-MgZnO Iayers grown on MgO, while the critical thickness is 8 nm for wartzite-MgZnO Iayers grown on MgO. ZnO Iayers grown on cubic-MgZnO buffer layers are relax afier 4 nm and become immediately relaxed when grown on wurtzite-MgZnO buffer layers. The defect structure of the ZnO films was characterized by TEM and XRD. It was found that ZnO Iayers - 477 -grown on wurtzite MgZnO buffer layers have much lower screw-type threading dislocation density than those grown on roek salt MgZnO. Whereas, ZnO Iayers grown on wurtzite MgZnO buffer layers have slightly larger edge-type threading dislocation density than ZnO Iayers grown on rock salt MgZnO buffer layers. Mixed-type threading dislocations and dislocations loops were observed in ZnO layer grown on rock salt MgZnO. (Fig. 2) ~~) +*""~""'*,< ~';{~ji;&  ;'+'.･,;;*･} ~"'~'~+*".~;;,~'*=>*;.~' ** .~~ "'~~:~"+~"'**"*' ~{~ (c) (d) 'i~='-Is' ~ =_ ';~*if~i'~+';' Fig. 2 XTEM images of ZnO Iayers grown on Mg0.75Zn0.250(c)/ c-sapphire (a), (b) and Mg0.25Zno.750 (w)/ c-sapphire (c), (d). Bright fie]d condition with g=[O002] was used for (a), (c) and weak-beam condition with g=[lO I O] was used for (b), (d). An interface study was done by HRTEM observation. The results demonstrated that stacking faults form near the ZnO/ MgZnO(c) heterointerface, while there was not any evidence of stacking fault formation in ZnO/MgZnO (w). ZnO grown on rock salt MgZnO showed higher PL emission intensity than that grown on wurtzite MgZnO attributed to the lower edge disiocation density. In conclusion, we have widely studied the microstructure ofMg*Znj_*O Iayers, which are of interest for ZnO band gap engineering and as a buffer layer for ZnO Iayers to improve its crystal quality. Based on our results a rock salt Mg*Znl-*O buffer iayer has some advantages such as decreasing the edge dislocation density and increasing the PL intensity. However a wurtzite Mg*Znl *O buffer layer decreases the screw dislocation density. Therefore we suggest a graded buffer layer consists of Zfo/ Mg*Znl *O(w)/ Mg Zn O(c)/MgO as a prospective - * I -*method to improve the ZnO crystal quality. We hope that the present study will contribute to the development ZnO-based optoelectronic devices. - 478 -I ~ 'iil;i ,:"ii  論文審査結果の要旨 紫外発光材料として注目を集める酸化亜鉛(ZnO)は、デバイス応用へ向けて現在さかん に研究が行われている。著者はZnO薄膜の高品質化に向け、MgZnO薄膜を緩衝層とし て利用することを提案している。本論文ではMgZnO薄膜の結晶構造とMg組成との関係 を明らかにし、また緩衝層としての有効性を議論している。本論文はこれらの研究成果 をまとめたもので、全6章からなる。 第1章は序論である。 第2章では、研究に用いた実験装置とその原理について述べている。 第3章では、MgOIA1203(0001)上へのMgZnO薄膜のプラズマ援用分子線エピタキシ成 長について述べている。まず、MgZnO薄膜においてはMg組成34%以下ではウルツ鉱構 造をとり、Mg組成65%以上では岩塩構造をとり、その中間組成では両者が混在している ことが明らかにされている。その成長機構については2つの事項が示されている。まっ はMg原子の薄膜への取込確率がZn原子に比べて著しく高いこと、もう1っはウルツ鉱 構造MgZnO薄膜の初期成長過程では岩塩構造からウルツ鉱構造への遷移が見られるこ とである。最後に、岩塩構造MgZnO薄膜は11111配向の双晶となっていることが示され ている。 第4章では、ZnOIMgO/A1203(0001)上へのMgZnO薄膜のプラズマ援用分子線エピタ キシ成長について述べている。まず、ZnO緩衝層を追加することで、ウルツ鉱構造MgZnO 薄膜がMg組成44%についてまで作成可能となることが示されている。また、岩塩構造 MgZnO薄膜を形成した場合には10011配向となることが示されている。これら事項は明ら かにZnO緩衝層による影響であり、緩衝層を利用したMgZnO薄膜の配向制御の可能性 を示す重要な知見である。 第5章では、MgZnO薄膜をZnO薄膜成長時の緩衝層として利用する場合の有用性を 議論している。著者はウルツ鉱構造MgZnO薄膜を利用した場合と、岩塩構造MgZnO薄 膜を利用した場合の2者について検討を行っている。得られるZnO薄膜中の欠陥密度に ついて述べられており、らせん転位密度はウルツ鉱構造MgZnO薄膜を利用した場合に低 く、刃状転位密度は両者共にほぼ同等であるものの若干ウルツ鉱構造MgZnO薄膜を利用 した場合の方が高いことが明らかにされている。著者は総合的な観点から、ウルツ鉱構 造MgZnO薄膜が緩衝層としては有効であることを結論している。 第6章は結論である。 以上、本論文ではMgZnO薄膜の結晶成長とZnO薄膜形成時の緩衝層としての有効性 について詳細に検討を行っており、得られた知見は応用物理学に寄与するところが少な くない。 よって、本論文は博士(工学)の学位論文として合格と認める。 一479一

Characterization of MgxZn1-xO Layers and Their Heterostructures Grown by Plasma-assisted Molecular Beam Epitaxy

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プラズマ援用分子線エピタキシ成長MgZnO層とそのヘテロ構造の構造評価に関する研究

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