pss High-density remote plasma sputtering of high-dielectric-constant amorphous hafnium oxide films Editor s Choice solidi physica status

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1 Phy. Statu Solidi B 250, No. 5, (2013) / DOI /p Diorder in Order: A pecial iue on amorphou material honoring S. R. Elliott High-denity remote plama puttering of high-dielectric-contant amorphou hafnium oxide film phyica p tatu olidi aic olid tate phyic Editor Choice Flora M. Li 1, Bernhard C. Bayer 1, Stephan Hofmann 1, Stuart P. Speakman 1, Caterina Ducati 2, William I. Milne 1,3, and Andrew J. Flewitt *,1 1 Electrical Engineering Diviion, Engineering Department, Camridge Univerity, J J Thomon Avenue, Camridge CB3 0FA, UK 2 Department of Material Science and Metallurgy, Camridge Univerity, Pemroke Street, Camridge CB2 3QZ, UK 3 Kyung Hee Univerity, Diplay Reearch Laoratory, Department of Information Diplay, Seoul , South Korea Received 1 Novemer 2012, revied 4 January 2013, accepted 25 January 2013 Pulihed online 7 March 2013 Keyword amorphou material, hafnium oxide, high-k dielectric, thin film tranitor * Correponding author: ajf@eng.cam.ac.uk, Phone: þ , Fax: þ Hafnium oxide (HfO x ) i a high dielectric contant (k) oxide which ha een identified a eing uitale for ue a the gate dielectric in thin film tranitor (TFT). Amorphou material are preferred for a gate dielectric, ut it ha een an ongoing challenge to produce amorphou HfO x while maintaining a high dielectric contant. A technique called high target utilization puttering (HiTUS) i demontrated to e capale of depoiting high-k amorphou HfO x thin film at room temperature. The plama i generated in a remote chamer, allowing higher rate depoition of film with minimal ion damage. Compared to a conventional puttering ytem, the HiTUS technique allow finer control of the thin film microtructure. Uing a conventional reactive rf magnetron puttering technique, monoclinic nanocrytalline HfO x thin film have een depoited at a rate of 1.6 nm min 1 at room temperature, with a reitivity of V cm, a reakdown trength of 3.5 MV cm 1 and a dielectric contant of By comparion, uing the HiTUS proce, amorphou HfO x (x ¼ 2.1) thin film which appear to have a cuic-like hort-range order have een depoited at a high depoition rate of 25 nm min 1 with a high reitivity of V cm, a reakdown trength of 3 MV cm 1 and a high dielectric contant of 30. Two key condition mut e atified in the HiTUS ytem for high-k HfO x to e produced. Firtly, the correct oxygen flow rate i required for a given puttering rate from the metallic target. Secondly, there mut e an aence of energetic oxygen ion omardment to maintain an amorphou microtructure and a high flux of medium energy pecie emitted from the metallic puttering target to induce a cuiclike hort range order. Thi HfO x i very attractive a a dielectric material for large-area electronic application on flexile utrate. 1 Introduction Hafnium oxide (HfO x ) i a wide andgap material whoe high reitivity, high dielectric contant, k, and large conduction and valence and offet with repect to ilicon ha led to it adoption a a replacement for ilicon dioxide a the gate dielectric in CMOS device [1, 2]. In particular, the high value of k (typically 25) allow the thickne of the gate inulator to e increaed whilt maintaining the ame gate capacitance, reulting in a reduction in the gate leakage current due to electron tunnelling [3 5]. The indutrial relevance of HfO 2 i evident y Intel Corporation miletone of incorporating high-k hafnium-aed dielectric material into their 45-nm CMOS proce technology [6, 7]. Thin film tranitor (TFT) aed on metal oxide channel material uch a zinc oxide (ZnO), [8 14] indium zinc oxide (IZO) [15, 16] and indium gallium zinc oxide (IGZO) [17 19] may equally enefit from incorporation of HfO 2 a the gate dielectric. The motivation for thi i the drive toward flexile diplay on platic utrate uing organic (polymer or mall molecule) light emitting diode (OLED) [20]. In a imilar fahion to the commonly availale active matrix (AM) liquid crytal diplay (LCD), OLED diplay alo require an AM array of TFT to control the light emiion from each pixel. However, the performance required of the TFT i quite

2 phyica p tatu olidi 958 F. M. Li et al.: High-denity remote plama puttering of amorphou HfO x film different and, in particular, the TFT in AMOLED diplay require a much higher on-tate current and greater duty cycle compared to thoe ued for AMLCD. The TFT that are currently ued in AMLCD are aed upon an hydrogenated amorphou ilicon (a-si:h) channel and ilicon nitride (SiN x ) gate inulator. They uffer from a ignificant threhold voltage hift a a function of time with a gate ia applied, and depoition y tandard rf plama enhanced chemical vapor depoition (PECVD) typically require the utrate to e heated to temperature in exce Flora Li i Senior Scientit at Holt Centre/TNO, involved in the development of arrier and electrode for OLED/OPV device and R&D in flexile OLED diplay. She i alo the Project Manager for a European Commiion FP7 Project called Flex-o-Fa, which i aimed toward developing a pre-pilot manufacturing route for flexile OLED foil. Before joining Holt Centre, he worked at Polymer Viion a a Senior Scientit and Project Leader on the development of lowtemperature oxide TFT ackplane for integration in rollale OLED and e-paper diplay. She wa a potdoctoral fellow at the Univerity of Camridge, UK, with a reearch focu on tranparent metal oxide material and thin film device. She received the Ph.D. degree in Electrical and Computer Engineering from the Univerity of Waterloo, Canada, in 2008, where he invetigated the integration of organic thin film tranitor. Dr. Li i a coauthor of 20þ refereed pulication and two ook: CCD Image Senor in Deep-Ultraviolet (Springer, 2005) and Organic Thin Film Tranitor Integration: A Hyrid Approach (John Wiley & Son, 2011). Andrew J. Flewitt received the B.Sc. degree in phyic from the Univerity of Birmingham, Birmingham, U.K., in 1994 and the Ph.D. degree in canning tunneling microcopy of amorphou ilicon from the Univerity of Camridge, Camridge, U.K., in Following thi, he wa a Reearch Aociate tudying the lowtemperature growth of ilicon-aed material in the Engineering Department, Univerity of Camridge. He wa appointed to a Lecturehip in the ame Department in Since 2009, he ha held the poition of Univerity Reader in Electronic Engineering. Hi reearch interet pan a road range of large area electronic and related field, including thin film tranitor and MEMS device. Dr. Flewitt i a Chartered Phyicit and a Memer of the Intitute of Phyic and the Intitution of Engineering and Technology. of 200 8C, which i incompatile with mot platic utrate [21]. Although other technique are eing developed to allow depoition at lower temperature [22 26] and compenation circuit are eing devied to counteract the threhold voltage hift [27, 28] to allow a-si:h TFT to e ued in AMOLED diplay, oth of thee approache add complexity to the device farication proce. Compared to the incument a-si:h TFT technology, metal oxide TFT have demontrated a much higher field effect moility (typically cm 2 V 1 1 ) and enhanced taility, making them an attractive alternative for AMOLED diplay ackplane application. Although much ha een made of the election of the metal oxide emiconductor in pulihed literature for metal oxide TFT, there ha een le focu on the equally important choice of gate dielectric. HfO x i one uch candidate material. Depoition of HfO x ha een achieved uing puled laer depoition (PLD) [29], atomic layer depoition (ALD) from metalorganic precuror [2, 30], and rf magnetron puttering from a HfO 2 target [31, 32] and a metallic Hf target [33, 34] in a ga mixture of argon and oxygen. However, in order to achieve good electrical characteritic, depoition at high temperature or a potdepoition anneal i required. In addition, PLD and ALD operate at relatively low growth rate of nm min 1. Thi limit the potential proce calaility for high volume manufacturing on an indutrial cale where film of ten to hundred of nanometre in thickne are required, epecially for thin film electronic application uch a TFT array for diplay ackplane. Amongt the variou depoition technique, ALD ha een the mot frequently tudied for depoiting the HfO 2 gate dielectric for CMOS device at elevated temperature. The reulting HfO 2 film tend to e polycrytalline, and may conit of variou crytalline phae: monoclinic, cuic, tetragonal, and orthorhomic [1, 3 5, 35, 36]. The monoclinic phae i the mot tale polymorph of HfO 2 under normal condition (i.e., the mot tale phae for HfO 2 ), ut it ha the lowet k value (20). The cuic phae, which i metatale, ut can exit indefinitely under normal condition, ha a higher k (30) whilt the tetragonal phae ha the highet k value (40 or higher) [36, 37]. While polycrytalline HfO 2 may contain phae with higher k value at local ite, the preence of grain and grain oundarie in polycrytalline film ha an undeirale influence on the device-to-device uniformity, leakage current and device taility. Therefore, amorphou gate dielectric are preferale. However, it ha remained an ongoing challenge to depoit amorphou HfO 2 at low temperature with high k value (>20). The author have previouly reported that a high-k (k 30) cuic-like amorphou HfO x (ca-hfo x, x ¼ 2.1) phae exit in thin film form [38]. The preent paper dicue the mechanim y which thi ca-hfo x may e depoited at low temperature uing a technique called high target utilization puttering (HiTUS). The advantage of the HiTUS technique are illutrated y comparing HfO x film

3 Editor Choice Phy. Statu Solidi B 250, No. 5 (2013) 959 depoited y thi method with thoe depoited y conventional MHz radio-frequency (rf) magnetron puttering. No intentional utrate heating and no pot-depoition annealing i employed in either cae, reulting in a proce that i compatile with the ue of platic utrate. Reactive puttering from a metallic target in the preence of argon and oxygen gae i ued in oth technique. It i demontrated that the ue of a metallic target afford control of the oxidation tate of the urface of the target and ample. An optimization tudy of HfO x thin film depoited y rf magnetron puttering and HiTUS i decried in the following ection. The electrical and phyical propertie of the optimized material i preented and correlated with the energetic of depoition. The material propertie are compared with the requirement of a gate dielectric in TFT application. 2 Experimental 2.1 Rf magnetron puttering A tandard rf magnetron puttering ytem (CCR Technology GmH) wa ued for HfO x depoition. The ytem contain three target. The vacuum chamer i pumped y a turo/rotary pump comination and a ae preure of 10 6 mar i achieved prior to depoition MHz rf radiation i applied to a 100 mm diameter metallic hafnium target of % purity (Tetourne Ltd.) in an atmophere of argon and oxygen gae, oth of which have a % purity (BOC Gae Ltd). Ga flow i regulated y ma flow controller. A tainle teel hroud i placed around the target that extend down to jut aove the ample, which i 100 mm elow the target. The hroud i only ued for metallic oxide depoition, and therefore erve to reduce the contamination of the puttered HfO x ample y other impuritie in the vacuum chamer. Furthermore, it confine the plama to a mall volume, therey increaing the depoition rate for a given rf power. 2.2 High target utilization puttering (HiTUS) A chematic diagram of the HiTUS depoition ytem i illutrated in an earlier pulication [39]. The HiTUS depoition ytem generate a remote, high-denity rf argon plama in the idearm, which i then amplified and directed onto the target uing two electromagnet. Although the plama ha a high denity cm 3, the ion energy i <50 ev, and hence puttering i only achieved with the application of an additional target ia. Thi enale a high level of independent control of the putter depoition parameter and decouple the ion denity (which i controlled y the rf antenna power upply) from the ion energy (which i controlled y the ia power upply). The utrate to target ditance i 250 mm, and o the ample i removed from the magnetically confined puttering plama. A reactive depoition proce i ued to generate the oxide film required. Thi i when a metal target i ued with an argon/oxygen ga mixture reulting in very high depoition rate. The argon and oxygen are injected into the chamer eparately uing ga ditriution ring, with the argon next to the target and the oxygen in cloe proximity to the utrate. The remote generation of the plama enale nearly full target eroion; thi utantially reduce the level of target poioning oerved compared to magnetron procee [40], and no feedack mechanim i required. Depoition of HfO x i performed uing the HiTUS ytem without intentional utrate heating from a 4 00 diameter metallic hafnium target of % purity and 6 mm thickne (Tetourne Ltd) in an atmophere of argon and oxygen gae, oth of which have a % purity (BOC Gae Ltd). The chamer i pumped to a ae preure of mar. Argon ga i then admitted at a flow of 50 ccm which raie the preure to mar. The rf launch power i 1 kw, the DC ia power i varied from 600 to 1200 W, and the oxygen flow i varied etween 10 and 20 ccm. 2.3 Characterization method HfO x ample, with film thickne in the range of nm, were depoited onto clean, n-type Si(100) wafer. Their refractive index and thickne were determined uing a Gaertner He Ne (633 nm) ellipometer. The thickne of elected film wa crochecked y cro-ectional tranmiion electron microcopy (TEM) and X-ray photoelectron pectrocopy (XPS) depth profiling uing Ar-ion puttering. Metal-inulator-emiconductor (MIS) capacitor tructure were formed y thermally evaporating chromium/ aluminum ilayer contact of 100 nm thickne onto the HfO x ample through a hadow mak with hole of 0.5, 0.7, and 1.0 mm diameter. Electrical characterization wa performed uing an Agilent B1500A Semiconductor Parameter Analyer or a Hewlett Packard HP4140B picoammeter/voltage ource, and a Hewlett Packard HP4192A LF impedance analyzer. A delay time of everal econd wa ued etween the application of a voltage and meaurement of the current to allow the reading to tailize. The reitivity wa calculated from current voltage (I V) meaurement at an applied field trength of 1 MV cm 1,a thi i a typical operating field in a TFT. The reakdown field wa defined to have occurred when the current denity reached 0.02 A m 2 [41]. The dielectric contant wa extracted from capacitance voltage (C V) characteritic, meaured at a frequency etween 1 khz and 1 MHz, uing the formula C ¼ ke 0A d ; (1) where C i the meaured capacitance, e 0 i the permittivity of free pace, A the capacitor area, and d i the film thickne. Sample for plan view TEM were depoited onto ilicon nitride memrane and analyzed uing a Tecnai F20 intrument operating at 200 kv. X-ray diffraction (XRD) meaurement (theta theta can) were performed uing a Bruker D8 with Cu Ka (0.154 nm) radiation. Sample were tilted out of plane y 38 to upre the intene (400) reflection of the ingle-crytalline Si wafer. The experimental diffrac-

4 phyica p tatu olidi 960 F. M. Li et al.: High-denity remote plama puttering of amorphou HfO x film tion pattern were matched with reference pattern from the International Centre for Diffraction Data (ICDD; powder diffraction file and ) and with [35]. XPS wa performed uing an Amicu Photoelectron Spectrometer. The optical tranmiion pectrum wa meaured uing a Perkin Elmer LAMBDA 950 UV/Vi/NIR Spectrophotometer. 3 Reult and dicuion 3.1 Rf magnetron puttered film Rf magnetron puttering of HfO x take place at ga preure mar. At thi preure, the mean free path of pecie in an O 2 /Ar ga mixture i 5.5 cm, and thi i relatively independent of the ga mixture due to the very imilar colliion diameter of thee two gae [42]. Therefore, it i clear that the likelihood of ga phae reaction etween Hf atom that have een puttered from the target and oxygen pecie in the plama i very low. On the other hand, Hf i one of the mot eaily oxidized metal indeed, it i often ued a a getter for oxygen ga. The time for complete expoure of a urface to a ga at a preure of 10 3 mar i It follow that thi i alo a good meaure of the time required to oxidize the Hf target urface. Hence, the mechanim y which HfO x i puttered from a metallic target i a two-tage proce: firtly the target urface i oxidized and uequently HfO x pecie are puttered from the target, primarily y the high ma argon ion in the plama. The oervation of a colored HfO x thin film on the target urface at the end of a depoition i indicative of thi oxidation proce. High quality HfO x i depoited under condition where the oxidation rate i ufficiently high relative to the puttering rate to enure that metallic Hf i not puttered. Under low preure condition, an optimum flow ratio of O 2 /Ar i oerved. Thi can e een in the variation in refractive index and depoition rate a a function of flow ratio hown in Fig. 1. For low O 2 /Ar flow ratio, there i inufficient oxygen preent for oxidation of the Hf target urface, whilt the high concentration of Ar mean that the puttering rate (and hence the depoition rate) i high. For very high O 2 /Ar ratio, there i inufficient Ar preent to maintain an effective puttering proce a the target ecome heavily poioned, and the depoition rate fall apprecialy, a doe the refractive index. Baed on Fig. 1, the optimal O 2 /Ar flow ratio for the rf magnetron puttering ytem i near 1:1. Thi i higher than might e expected due to the preence of the hroud which limit the ingre of ga into the plama region, where the oxygen i effectively conumed in the oxidation proce. The optimum flow ratio i reduced when no target hroud i employed. Figure 2 how how the current denity varie a a function of applied electric field for an optimized HfO x ample (aed on an O 2 /Ar flow ratio of 1:1). Very low current denitie are oerved under the application of moderate electric field which correpond to a reitivity of ( ) V cm. Thi i comparale with the reitivity of ilicon nitride ued in a-si:h TFT. Under thee moderate electric field (1MVcm 1 ), Poole-Frenkel conduction dominate, a can e een y the linear form of the graph hown a the inet to Fig. 2. The Poole-Frenkel current i governed y the relation J ¼ E exp 2a p ffiffiffi E f ; (2) k B T where J i the current denity, E the electric field trength, f the arrier height, k B the Boltzmann contant, T the aolute temperature, and a and are contant. Such conduction i caued y the excitation of trapped electron into the conduction and of the inulator where the field reduce the arrier [43]. The material i found to reak down under the application of an electric field of 3.5 MV cm 1 for a ample of 100 nm thickne. The capacitance voltage curve for MIS tructure incorporating the optimized HfO x are hown in Fig. 3. Very little hyterei i oerved in thee curve. The fixed charge denity i etimated to e cm 2 and the flat and voltage i 1.5 V. The dielectric contant of the HfO x i Figure 1 Variation in the depoition rate and refractive index of rf magnetronputteredhfo x ampleaafunctionofo 2 /Arflowratio. Figure 2 Leakage current denity a a function of electric field for an optimized rf magnetron puttered HfO x dielectric film (100 nm thick). Inet how a Poole-Frenkel plot.

5 Editor Choice Phy. Statu Solidi B 250, No. 5 (2013) 961 Figure 3 Capacitance voltage characteritic for an optimized rf magnetron puttered HfO x dielectric film (100 nm thick). Meaurement were performed uing MIS capacitor tructure. Inet i a TEM micrograph of a typical film with aociated electron diffraction pattern howing the preence of nanocrytal of HfO x emedded in an amorphou matrix. The lattice pacing oerved in the crytalline region i conitent with a monoclinic tructure. found to e 18.2, which i comparale with meaurement performed on material depoited y PLD and ALD [2, 44]. The microtructure of the optimized rf magnetron puttered HfO x ha een invetigated (inet to Fig. 3) y TEM. The reult revealed that the material i nanocrytalline in nature with randomly oriented, mall crytal 10 nm in diameter eing emedded within an amorphou matrix. Thi i conitent with the electron diffraction pattern which conit of road, amorphou ring together with right pot that are caued y the preence of the crytal (ee inet to Fig. 3). The fact that the crytal are very mall and are urrounded y amorphou material mean that there are no grain oundarie which extend through the whole thickne of the inulating layer. Conequently, the reitivity i high and relatively uniform acro the ample. It i likely that the urface of the crytal i highly defective, and thee may act a trap for electron, which i conitent with the Poole-Frenkel tunnelling mechanim that dominate the conduction proce. 3.2 HiTUS film The growth mechanim of thin film depoited y the HiTUS ytem i very different to that depoited y the rf magnetron puttering ytem. In the latter cae, oxidation take place near the target, and the primary pecie puttered from the target i likely to e HfO x. In uch a configuration, target poioning (i.e., over-oxidation of the target) i a major concern. In contrat, for the HiTUS ytem, the oxygen ga i injected cloe to the ample where it i effectively gettered y the depoiting metal, and o oxidation occur at or cloe to the utrate. Thi ha two important conequence: firt, the plama eam i dominated y a flux of argon ion which omard the metallic hafnium target, and econd, the aence of oxygen in the plama mean that there i no oxygen ion omardment of the ample. For the purpoe of depoiting high quality HfO x, it i therefore clearly critical that there i ufficient oxygen in the ytem relative to the metallic puttering rate for full oxidation of the ample, ut not exce oxygen o that oxygen tart to enter the puttering plama leading to oth oxygen ion omardment and target poioning. Therefore, initial development of HiTUS HfO x thin film focued on examining the variation in material propertie a a function of target power and oxygen flow rate, which largely control the puttering rate of metallic content and the oxidation rate of the metallic pecie, repectively. A hown in Fig. 4a, the depoition rate increae with target power (with all other parameter fixed), which can e explained y an increae in puttering rate of the metal target. Thi increae in metallic content of the HfO x thin film i reflected in the increae in the refractive index (due to increaing denity [45]) with increae in target power in Fig. 4, and in the decreae in the electrical reitivity a target power increae aove 900 W in Fig. 4c. Exceive target power alo caue a drop in the reakdown field, a hown in Fig. 4d, which i mot likely due to a reduction in the inulating propertie with increaed metallic content. Figure 4e reveal an increae in the dielectric contant from 23 to 32 a the target power increae from 600 to 1200 W. Both the increae in refractive index and in dielectric contant ugget denification of the thin film with increaing target Figure 4 Variation in the (a) depoition rate, () refractive index, (c) reitivity, (d) reakdown field and (e) dielectric contant of HiTUS HfO x a a function of target power.

6 phyica p tatu olidi 962 F. M. Li et al.: High-denity remote plama puttering of amorphou HfO x film power [45, 46]. In general, to e uitale a a gate dielectric in TFT, we want to elect a film with the highet reitivity and the highet reakdown trength. From thi tudy of the impact of target power, it i concluded that, for the depoition condition conidered here (15 ccm of oxygen), a target power of W appear optimal. Thi reult in a depoition rate of 25 nm min 1, an average refractive index of 2.09, an average reitivity of V cm, an average reakdown field of 2.5 MV cm 1 and a dielectric contant of 30. The impact of oxygen flow rate on the depoition rate of HfO x i illutrated in Fig. 5a. A the oxygen flow rate increae, the depoition rate decreae a a reult of increaed oxidation at the target urface. When the oxygen flow rate i too low, there i inufficient oxidation near the utrate and puttering of metal atom dominate the depoition proce to yield a higher depoition rate. However, thi lead to HfO x film with a relatively high refractive index and an undeiraly low reitivity a hown in Fig. 5 and c, repectively, uggeting too much metallic content. On the other hand, when the oxygen flow rate i ufficiently high, the oxidation proce dominate over the puttering proce, leading to a more reitive film depoited at a lower rate. A high oxygen flow rate i neceary to trigger ufficient oxidation to yield HfO x film with a higher reakdown field (ee Fig. 5d). To e applicale a a gate dielectric material, HfO x need to e a reitive a poile with a high reakdown trength. Thu, a high oxygen flow rate i deirale, ut at the expene of a lower depoition rate. The importance of an adequate quantity of oxygen pecie in metal oxide i key a oxygen vacancie are normally the dominant ource of charge trap in HfO x film. It ha een reported that depoition and/or pot-proceing conditioning in an oxygen-rich amient i deirale for removal or paivation of thee defect [47]. Thu, it appear that a high oxygen flow rate i required for reducing the defect denity in HfO x. Figure 5e reveal a decreae in the dielectric contant from 34 to 24 a the oxygen flow rate increae from 12 to 20 ccm. The concurrent decreae in refractive index and in dielectric contant ugget a reduction in the film denity with oxygen flow rate, which can e explained y a reduction in the relative metallic content in the film [45]. From thi tudy of the impact of the oxygen flow rate, it i concluded that, for the depoition condition conidered here (800 W of target power), an oxygen flow rate of ccm appear optimal, reulting in a depoition rate of 25 nm min 1,an average refractive index of 2.1, an average reitivity of V cm, an average reakdown field of 3MVcm 1 and a dielectric contant of up to 32. Figure 6 (adapted from the author previou report on thi material [38]) plot the leakage current denity a a function of electric field for an optimized HfO x thin film (depoited at target power of 800 W and oxygen flow rate of 15 ccm). A leakage current denity in the range of 1 10 na cm 2 i oerved for electric field <1MVcm 1. Under moderately high electric field (1.5 2 MV cm 1 ), Schottky Emiion conduction dominate, a can e een y the linear form of the graph hown a the inet to Fig. 6. In thee condition, the arrier f etween the Fermi energy in a metal and the conduction and in an inulator i ufficiently lowered to allow thermally excited carrier to make thi Figure 5 Variation in the (a) depoition rate, () refractive index, (c) reitivity, (d) reakdown field, and (e) dielectric contant of HiTUS HfO x a a function of oxygen flow rate. Figure 6 Leakage current denity a a function of electric field characteritic for an optimized HiTUS HfO x thin film (adapted from Ref. [38]). The film thickne i approximately 100 nm. Inet how a Schottky Emiion plot in the high field regime. Meaurement were performed uing an MIS capacitor tructure.

7 Editor Choice Phy. Statu Solidi B 250, No. 5 (2013) 963 tranition, reulting in a Schottky Emiion Current, which i decried y J ¼ ct 2 exp a p ffiffiffi E f (3) k B T where J i the current denity, E the electric field trength, f the arrier height, k B the Boltzmann contant, T the aolute temperature, and a,, and c are contant. In comparion to the trap-aited Poole-Frenkel conduction oerved in rf magnetron puttered HfO x film, the oervation of Schottky emiion for HiTUS puttered HfO x i an indication of an apparent reduction in the ulk defect denity in the film. Schottky emiion ha alo een reported in rf puttered HfO 2 after annealing at 500 8CinaN 2 atmophere [48], and in PLD HfO 2 annealed at 500 8C inn 2 [49]. By comparion, an advantage of HiTUS HfO x i the aility to otain equivalently good quality film (controlled defect denity) without utrate heating during growth or potdepoition annealing. Temperature-dependent current voltage meaurement are eing examined to develop a etter undertanding of the underlying current conduction mechanim. The capacitance voltage curve for the MIS tructure incorporating the optimized HfO x have een previouly reported y the author [38]. The dielectric contant of thi HfO x i found to e Very little hyterei i oerved; Figure 7 X-ray diffraction (XRD) can of typical a-depoited (a) and vacuum annealed () rf-magnetron puttered HfO x film a well a a-depoited (c) and vacuum annealed (d) HiTUS HfO x film. The laeled peak in (a) and () can e aigned to the tronget reflection of monoclinic HfO 2 (laelled m, ICDD-PDF file ). The road diffraction feature around 328 in(c) i acried to amorphou HfO x, while (d) matche the diffraction pattern of polycrytalline cuic HfO 2 (laelled c, ICDD-PDF file ; Deignate two reflection from a minor contriution from another phae that i amiguou to aign). the fixed charge denity i etimated to e cm 2 and the flat and voltage i 2.5 V. The microtructure of typical HiTUS HfO x ample ha een tudied y XRD and compared to typical rf-magnetron puttered film in Fig. 7. To eek more in-depth inight in the film microtructure, we alo examined the XRD can of rf magnetron puttered and HiTUS film after vacuum annealing (10 7 mar) at 640 8C for 35 min. The adepoited rf magnetron puttered film how a diffraction peak correponding to the mot intene reflection of monoclinic HfO 2 (Fig. 7a). Thi i commonly aigned to monoclinic nanocrytallite [50] and i in agreement with our TEM reult (Fig. 3inet). The XRD can for the rf magnetron puttered film doe not change ignificantly after annealing (Fig. 7), indicating the film doe not undergo further crytallization. In contrat, the a-depoited HiTUS HfO x film exhiit only a very diffue reflection centered around 328 (Fig. 7c) which i commonly acried to an amorphou tructure [51], which, y definition, refer to a olid with no long-range order ut with local hort-range order of 1 2 ond length ditance. We have previouly confirmed the amorphou tructure of thee HiTUS film y TEM (ee Fig. 3 in Ref. [38]). The annealed HiTUS film exhiit a clear diffraction pattern of predominantly polycrytalline cuic HfO 2 (with a mall contriution of another amiguou phae), a hown in Fig. 7d. The crytallographic evolution from an amorphou to a predominantly cuic phae i rare for HfO 2 film, a the appearance of a cuic phae i commonly accompanied y other polymorph [52, 53]. Baed on thi unuual crytallization ehavior we ugget that the a-depoited amorphou HiTUS HfO x poee a diordered cuic-like hort-range order (i.e., the Hf atom tend to e eightfold coordinated and the O atom tend to e fourfold coordinated a oppoed to the monoclinic coordination of even for Hf and either three or four for O) [54]. Thi hort-range order act a nucleation ite for topotactic rearrangement to yield a crytalline cuic phae upon high temperature annealing. Thi i conitent with the high k of 30 meaured in HiTUS film, ince k value in the range of are often linked to cuic polymorph of HfO 2 [36, 37]. Such a connection etween a high k in amorphou HfO 2 and a cuiclike hort-range order wa alo uggeted y recent imulation of the tructure and dielectric propertie of amorphou HfO x [55]. Thi cuic-like amorphou hafnium oxide (ca-hfo x ) i in contrat to the rf magnetron puttered HfO x, which i monoclinic and poee a ignificantly lower k 18. Thi i in good agreement with previouly pulihed literature where the monoclinic phae (incl. monoclinic-like local coordination in amorphou film) i often linked to a lower k value (k 20) [36, 37, 56]. Further characterization how that the HiTUS ca-hfo x ha an optical (Tauc) gap of 5.4 ev. Depth reolved XPS meaurement demontrate only ignal from Hf and O in the film (not hown) and thu etalih that the HiTUS film are free from any elemental impuritie. Thi i important

8 phyica p tatu olidi 964 F. M. Li et al.: High-denity remote plama puttering of amorphou HfO x film Tale 1 Summary of phyical propertie of rf magnetron puttered HfO x and HiTUS ca-hfo x thin film. Propertie of high quality rf-pecvd ilicon nitride dielectric (from Ref. [59]) are alo included for comparion. The value quoted for HfO x correpond to the optimized recipe. parameter rf magnetron puttered HfO x HiTUS ca-hfo x rf-pecvd ilicon nitride utrate temperature (8C) depoition rate (nm min 1 ) refractive index reitivity (V cm) at 1 MV cm reakdown field (MV cm 1 ) dielectric contant tructure nano-crytalline amorphou amorphou optimized recipe 1:1 O 2 :Ar flow ratio 15 ccm O 2 flow, 800 W target power ee Ref. [59] ince impuritie have previouly een reported to tailize metatale HfO 2 polymorph [30, 57, 58], and thu the purity of the ample (XPS enitivity limit <0.1 1%) enure that the unique film propertie are only related to the HiTUS depoition. A Hf:O toichiometry ratio of 1:2.1 i etimated from XPS. The HiTUS film exhiit an average reitivity of V cm and a reakdown trength in exce of 3MVcm 1. Thee propertie are comparale to thoe of PECVD ilicon nitride for a-si:h TFT (ee Tale 1) [59] and fulfill the requirement for uch a gate dielectric material. Baed on the XRD reult, it i oerved that the HiTUS depoition ytem can produce ca-hfo x without intentional utrate heating (heating from momentum tranfer and econdary electron i ignificantly le than 100 8C). Crytallization during room temperature magnetron putter depoition can e attriuted to omardment of the ample urface y oxygen ion [46, 60]. A dicued previouly, uch oxygen ion are not normally preent in HiTUS puttering, allowing the direct formation of an amorphou material. However, there i a econd iue a to the reaon why the HiTUS depoition permit the formation of a cuic-like hort-range order while rf magnetron puttering favor monoclinic tructure. In general, the monoclinic phae of hafnium oxide i favored at room temperature and preure (RTP) a it ha the lowet free energy of formation and the lowet denity [1]. Therefore, it wa expected that the amorphou tate of thi material would alo favor a monoclinic hort-range order [56]. However, it i known that other material which can exit in a numer of crytalline phae do not alway adopt the hort range order of the lowet free energy phae in their amorphou tate at RTP. Perhap the mot well-known of thee i caron. In thi cae, the graphitic phae ha a lower free energy than the diamond phae, ut under the correct growth condition, amorphou thin film may e depoited with predominantly diamond-like (p 3 ) hort-range order rather than graphiticlike (p 2 ) hort-range order [61]. Diamond-like caron (DLC) material with a high p 3 content i frequently depoited y ion-aited depoition technique, uch a filtered cathodic vacuum arc (FCVA) [62]. In thee cae, the p 3 content i highly dependent on the energy of the caron pecie arriving at the growth urface, with the highet p 3 content achieved for ion energie of 100 ev [63]. Thi i explained y a uplantation model [61]. If the incoming caron ion ha an energy, E i, that i le than a penetration threhold energy, E p, then it will only e ale to ind to the growth urface without penetration. In the aence of any contraint, the low energy p 2 onding reult. However, if E i > E p, then the ion will penetrate elow the urface, increaing the denity in thi uurface growth zone, with the reult that the ion take up the more dene p 3 onding configuration. Provided that there i not a large quantity of exce energy to allow relaxation of the ytem ack to the low energy p 2 tate, then a DLC material will reult. In thi work, it i potulated that the HiTUS putter condition allow a imilar uplantation proce to occur, leading to the formation of ca-hfo x, rather than a monoclinic-like amorphou material. The key material parameter in aeing thi i E p. Thi i not a imple parameter to ae a it i known that the urface of the ca- HfO x i only partially oxidized at any moment in time during growth. For implicity, the cae of metallic Hf will e conidered in the firt intance. When an atom or ion arrive at the growing urface, there i an energy gain aociated with urface inding, E, a the urface appear a an attractive potential. Therefore, the total energy of the incoming atom or ion i E i þ E. Thi urface inding energy i 6.85 ev for Hf [64]. For penetration to occur, the total energy of the incoming atom, E i þ E, mut exceed the diplacement threhold energy E d,o E p ¼ E d E : (4) There are no report of the value of E d value for Hf, ut it i known empirically that E d i etween four and five time greater than the coheive energy for mot metal [65], and o 29 ev would e a good etimation, uggeting a penetration energy of 24 ev. Therefore, for the higher denity ca-hfo x to e formed, the Hf atom arriving at the growth urface mut have an energy greater than thi, otherwie a monoclinic microtructure will reult.

9 Editor Choice Phy. Statu Solidi B 250, No. 5 (2013) 965 Uing the work of Drüedau et al. [66], it i poile to etimate oth the average and maximum energie, he at i and E max, repectively, of incoming Hf atom from rf magnetron and HiTUS puttering. E max i given y E max ¼ kge 0 E ; (5) where k i the energy diipation in the colliion cacade (0.42 for Hf from Ref. [66]), G i the puttering efficiency, which i 0.60 for Hf puttering with Ar ion and E 0 i the energy of the incoming Ar ion. The average kinetic energy i then given y he at i ¼ E 2=3 Emax 1=3 : (6) Therefore, given that for oth rf magnetron puttering and HiTUS puttering, low preure regime are employed with low proailitie of ga phae interaction etween the target and ample, E 0 i the key parameter controlling the energetic of the arriving Hf pecie which i different etween the two ytem. For the rf magnetron puttering ytem, E 0 will e limited to the dc offet potential of the target, which i meaured to e 145 V. Sutituting 145 ev for E 0 into Eq. (5) give 36.5 ev for E max and hence 12.0 ev for he at i. Hence, very few pecie are likely to have an energy greater than E p. In the cae of HiTUS depoition, however, E 0 i very well defined y the target potential, which i meaured to e 520 ev. Thi reult in ignificantly higher value for oth E max and he at i of 124 and 18 ev, repectively. In thi cae, a ignificant proportion of the depoiting Hf pecie will have ufficient energy to uplant elow the growing urface, leading to material denification, and the formation of the cuic-like hort range order, a oerved experimentally, and illutrated in Fig. 8. The propoed mechanim for ion omardment induced growth of cuic-like amorphou (hort-range ordered) hafnium oxide at low temperature uing the HiTUS proce, a decried aove, i implitic in that it doe not account for (i) variale colliion angle incident on complex ond and Figure 8 Schematic illutration of the growth of ca-hfo x uing the HiTUS proce highlighting the preence of well-defined growth zone. The top urface i dominated y chemiored pecie (mot notaly molecular oxygen). Beneath thi there i a low-denity growth front due to depoition y low energy preuror, with a further uurface compaction zone elow thi where uplantation i occurring. Both the growth front and compaction zone are likely to e only a few monolayer think, and therefore chemiored oxygen will diffue into thi region. unound pecie preent within the lower denity evolving growth front and (ii) aociated colliion cacade event a they penetrate into the altered layer (denifying tranition thickne) with concomitant atomic movement that promote the formation of the ulk cuic-like amorphou oxide film propertie oerved. The pecific nature and propertie of the omarding pecie, the importance of the atom-to-ion arrival rate ratio and energy range, and the role played y the chemiored oxygen at the onet of the coating growth need further invetigation in order to develop a etter undertanding of the mechanim() y which the ion omardment, a provided y the HiTUS proce, impart a window of energy to promote appropriate molecular diociation, lattice diplacement, and atomic rearrangement under minimal damage and tre relaxation condition that are elieved to e at the heart of the low temperature production of ca-hfo x. 3.3 Material comparion for large-area electronic application Tale 1 compare the propertie of HfO x thin film depoited at room temperature y rf magnetron puttering with ca-hfo x depoited y the HiTUS ytem and ilicon nitride depoited at 300 8C y rf plama enhanced chemical vapor depoition (rf-pecvd), a ued in a-si:h TFT. Generically, it i clear that the reitivity and reakdown trength of the HfO x material depoited y either technique i of the ame order a for ilicon nitride, and thi make the application of HfO x a the gate dielectric in TFT viale. However, there i the advantage that the dielectric contant of the ca-hfo x i ignificantly higher than for ilicon nitride, which allow a greater on-tate current for the ame applied gate voltage, and the depoition temperature i compatile with the ue of platic utrate. Thi will e of particular relevance in TFT for AMOLED diplay. However, the HiTUS ca-hfo x ha further key advantage over the rf puttered material. The elimination of oxygen ion omardment in the HiTUS ytem mean that material can e depoited with an amorphou microtructure. Thi ha a profound influence upon electrical propertie a conduction under the application of moderate field i now dominated y urface effect (Schottky emiion) rather than ulk defect aociated with grain oundarie (Poole- Frenkel tunneling). It i for thi reaon that the HiTUS ca-hfo x ha a ignificantly enhanced reitivity and dielectric contant compared with rf magnetron puttered HfO x. The HiTUS material i alo more dene, a can e een y comparing oth the refractive index and dielectric contant (ee Tale 1) [45, 46]. Alo, from a manufacturaility perpective, the ignificantly higher growth rate achieved y HiTUS puttering make thi technique intrinically more cot effective for large-area electronic. 4 Concluion It ha een demontrated that high-k amorphou HfO x dielectric layer can e grown without utrate heating y the HiTUS ytem from a metallic hafnium target. The thin film produced uing the HiTUS proce demontrate a high reitivity of up to V cm,

10 phyica p tatu olidi 966 F. M. Li et al.: High-denity remote plama puttering of amorphou HfO x film high reakdown trength of 3 MV cm 1, and a high dielectric contant of 30. Thee propertie are uperior to rf magnetron puttered HfO x thin film, which are characterized y a lower electrical reitivity, a lower dielectric contant, a higher defect denity and a nanocrytalline microtructure. The amorphou phae i facilitated in the HiTUS proce y the aence of oxygen ion omardment of the utrate during growth. The amorphou HfO x produced y the HiTUS ytem i characterized y a much higher k and formed at a higher depoition rate than other previouly reported amorphou HfO 2 film. XRD reult of HiTUS HfO x indicate that the film poee a hort-range order dominated y cuic-like atomic coordination which lead to the high k value meaured. It i potulated that the HiTUS proce allow growth of thi cuic-like amorphou (ca-)hfo x material through a higher energy of depoiting Hf pecie compared with rf magnetron puttering, which exceed the threhold penetration energy leading to material denification through uplantation. The ca-hfo x produced i compatile with ue a the gate dielectric layer in TFT for AMOLED diplay on platic utrate, and i an attractive dielectric option for other large-area electronic application. Acknowledgement The author acknowledge the financial upport of thi project provided y the Centre for Advanced Photonic and Electronic (CAPE) through the HiMo Project, y the EPSRC through the Camridge Integrated Knowledge Centre HiPZOT Project (grant no. EP/E023614/1) and the FlexIC Project (grant no. TS/I001158/1), and y the European Communitie Seventh Framework Programme ORAMA project (grant agreement CP-IP ). Reference [1] J. H. Choi, Y. Mao, and J. P. Chang, Mater. Sci. Eng. R 72,97 (2011). [2] Y.-S. Lin, R. 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