(NH4)2S for the Inp MOS Capacitor with ALD Al2o3

materials

Communication Facile Process for Surface Passivation Using (NH4)2S for the InP MOS Capacitor with ALD Al2O3

Jung Sub Lee 1, Tae Young Ahn 1,* and Daewon Kim 2,*

1 Department of Orthopaedic Surgery and Medical Research Institute, Pusan National University Hospital, 179 Gudeok-ro, Seo-gu, Busan 49241, Korea; [email protected] 2 Department of Electronic Engineering, Institute for Wearable Convergence Electronics, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin 17104, Korea * Correspondence: [email protected] (T.Y.A.); [email protected] (D.K.)

Received: 9 June 2019; Accepted: 25 November 2019; Published: 27 November 2019

Abstract: Ammonium sulﬁde ((NH4)2S) was used for the passivation of an InP (100) substrate and its conditions were optimized. The capacitance–voltage (C–V) characteristics of InP metal-oxide-semiconductor (MOS) capacitors were analyzed by changing the concentration of and treatment time with (NH4)2S. It was found that a 10% (NH4)2S treatment for 10 min exhibits the best electrical properties in terms of hysteresis and frequency dispersions in the depletion or accumulation mode. After the InP substrate was passivated by the optimized (NH4)2S, the results of x-ray photoelectron spectroscopy (XPS) and the extracted interface trap density (Dit) proved that the growth of native oxide was suppressed.

Keywords: III–V semiconductor; indium phosphide (InP), Al2O3; (NH4)2S; sulfur passivation

1. Introduction As silicon-based, complementary metal-oxide-semiconductor (CMOS) technology has been approaching its fundamental limits, III–V semiconductors have been focused on as alternative channel materials due to their high electron mobility. In particular, indium phosphide (InP) is considered one of the promising materials due to its larger band gap than silicon (~1.34 eV), which results in low off-state leakage current and high breakdown field [1–3]. In addition, InP is preferred to a barrier layer between the gate oxide and the InxGa1 xAs channel [4]. Despite these advantages, the poor − interfacial quality between a high-k dielectric and an InP substrate compared to SiO2 and Si is still a major obstacle to overcome before high-performance MOS field-effect-transistors (MOSFETs) can be realized. Thus, every endeavor to keep the InP substrate clear of the native oxide growth and external contamination prior to deposition of the high-K dielectric is required for a reduction of Dit. For this purpose, ex situ cleaning methods using wet chemicals, such as HCl, H2SO4, NH4OH, and HF, have consistently been developed [5]. Various works have been done for the enhancement of the interfacial quality over the last few decades [6–8]. Recently, Cuypers et al. reported that sulfur passivation by (NH4)2S was useful for suppressing unwanted effects arising from pre-existing defects and the rapid re-oxidation of the surface after wet chemical cleaning [9]. As the group VI element, sulfur and oxygen have the same number of valence electrons; however, the electronegativity of sulfur is lower than that of oxygen. Hence, the interface of In–S become stable so that the formation of the native oxide can be effectively inhibited. To take advantage of this feature, (NH4)2S has widely been employed [1–3,10–13]. However, comprehensive analyses regarding the process parameters used in sulfur passivation which affect the InP MOSFETs remain insufficient. In this work, the conditions of (NH4)2S passivation were optimized to enhance the performance of the device. The electrical characteristics of an InP MOS capacitor (MOSCAP) were investigated

Materials 2019, 12, 3917; doi:10.3390/ma12233917 www.mdpi.com/journal/materials Materials 2019, 12, 3917 2 of 7 Materials 2018, 11, x FOR PEER REVIEW 2 of 7 whileThen, varyingthe chemical the passivation properties conditions of the sulfur-passiva of (NH4)2S,ted in thisinterface case thewith concentration the optimized and conditions treatment were time. Then,compared the chemicalwith those properties of a non-passiv of the sulfur-passivatedated interface. Furthermore, interface with the the Dit optimizeddistribution conditions across the were InP comparedenergy bandgap with those was of extracted a non-passivated using a interface.conductance Furthermore, method [14–17] the Dit distributionin order to acrossevaluate the InPthe energyinterfacial bandgap quality, was which extracted affects using the performances a conductance of method InP MOSFETs. [14–17] in order to evaluate the interfacial quality, which affects the performances of InP MOSFETs. 2. Experimental 2. Experimental MOSCAPs were fabricated on an undoped InP (100) wafer with an n-type carrier concentration of 5 ×MOSCAPs 1015 cm−3. wereInitially, fabricated the substrates on an undoped were cleaned InP (100) with wafer a 1% with HF an solution n-type carrierfor 5 min, concentration after which of 15 3 5they10 werecm treated− . Initially, with the(NH substrates4)2S at concentrations were cleaned of with 1%, a 5%, 1% HF10%, solution and 22% for which 5 min, were after whichdiluted they by × weredeionized treated H2 withO for (NH 10 min4)2S at concentrationsroom temperature of 1%, (300 5%, K). 10%, Other and substrates 22% which were were treated diluted with by deionized the fixed Hconcentration2O for 10 min of at 10% room (NH temperature4)2S for 5 (300min, K).10 Othermin, and substrates 30 min. were As treatedthe gate with oxide, the 7 fixed nm concentrationof Al2O3 was ofdeposited 10% (NH on4) 2theS for substrate 5 min, 10 at min, a temperature and 30 min. of As 150 the °C gate by oxide, means 7 nmof atomic of Al2 Olayer3 was deposition deposited (ALD) on the substrateafter a rinse at aby temperature water was perf of 150ormed.◦C by In means the ALD of atomic process, layer trimethylaluminum deposition (ALD) (TMA) after a and rinse H by2O waterwere wassequentially performed. supplied In the under ALD process, purging trimethylaluminum with N2 during each (TMA) deposition and H 2cycle.O were For sequentially a gate electrode, supplied 10 undernm of purgingNi and 100 with nm N2 ofduring Au were each deposited deposition using cycle. a For thermal a gate evaporator electrode, 10through nm of Nia shadow and 100 mask. nm of AuAfterwards, were deposited the MOSCAPs using a thermalwere annealed evaporator with throughambient a gas shadow (4% H mask.2/96% Afterwards,N2) at 400 °C the for MOSCAPs30 min. were annealed with ambient gas (4% H2/96% N2) at 400 ◦C for 30 min. 3. Discussion and Results 3. Discussion and Results The structural properties of the Au/Ni/Al2O3/InP samples were characterized by high-resolution transmissionThe structural electron properties microscopy of the Au(HR-TEM)/Ni/Al2O 3us/InPing samples a field-emission were characterized transmission by high-resolution microscope transmissionoperated at 300 electron kV (Tecnai microscopy G2 F30). (HR-TEM) In order using to study a field-emission the bonding transmission status at the microscope Al2O3/InP interface, operated atex 300situ kV X-ray (Tecnai photoelectron G2 F30). In spectroscopy order to study (X thePS) bonding was analyzed status using at the Ala monochromated2O3/InP interface, Al ex Κα situ (1486.7 X-ray photoelectroneV) source with spectroscopy a base pressure (XPS) was in analyzedthe mid-10 using−10 Torr a monochromated range. Capacitance-voltage Al Kα (1486.7 (C–V) eV) source and 10 withconductance–voltage a base pressure in (G–V) the mid-10 measurements− Torr range. were Capacitance-voltagecarried out using an (C–V)E4980A and precision conductance–voltage LCR meter at (G–V)room temperature. measurements were carried out using an E4980A precision LCR meter at room temperature. A schematic of thethe MOSCAPMOSCAP isis shownshown inin FigureFigure1 a.1a. AlAl22O3 was deposited over the entire InP substrate. Afterwards, patterned Ni and Au were deposited through a shadow mask which had holes with a diameter of 300 µμm. Figure 1 1bb showsshows aa cross-sectionalcross-sectional HR-TEMHR-TEM imageimage ofof thethe MOSCAPMOSCAP onon thethe InP substrate, whichwhich waswas treatedtreated withwith thethe 10%10% (NH(NH44))22S solution for 10 min. The TEM image indicates that thethe ALDALD AlAl22O3 has a uniform thickness andand anan amorphousamorphous structure.structure. Moreover, an abrupt interface was created withoutwithout thethe growthgrowth ofof thethe nativenative oxide.oxide.

Figure 1. (a) Device schematic for the InP metal-oxide-semiconductormetal-oxide-semiconductor (MOS) capacitor. (b) HR-TEM

image of 7 nm thick AlAl2O3 onon the the InP InP (100) (100) substrate. substrate.

Figure 2a–e shows the C–V characteristics of the devices treated with diluted (NH4)2S solutions at different concentrations. Three issues exist in all of the curves. First, the accumulation capacitance cannot reach the theoretical value of the oxide capacitance, which is calculated according to the actual thickness of the gate dielectric. This phenomenon originates from the low density of states (DOS) of the InP substrate above the conduction band. Secondly, there is frequency dispersion in the depletion

Materials 2019, 12, 3917 3 of 7

Figure2a–e shows the C–V characteristics of the devices treated with diluted (NH 4)2S solutions at different concentrations. Three issues exist in all of the curves. First, the accumulation capacitance cannot reach the theoretical value of the oxide capacitance, which is calculated according to the actual thicknessMaterials 2018 of, 11 the, x gateFOR PEER dielectric. REVIEW This phenomenon originates from the low density of states (DOS)3 of of 7 the InP substrate above the conduction band. Secondly, there is frequency dispersion in the depletion mode,mode, an indicator of of the the level level of of D Dit,it which, which is isan an important important characteristic characteristic in terms in terms of enhancing of enhancing the theperformance performance of MOSFETs. of MOSFETs. Lastly, Lastly, there there is also is alsofrequency frequency dispersion dispersion in the in accumulation the accumulation mode. mode. This Thisis associated is associated with with conductive conductive losses, losses, which which are are mostly mostly caused caused by by border border traps traps located located in in the gategate dielectricdielectric near the interface [16,18,19]. [16,18,19]. As As shown shown in in Figure Figure 2f,2f, hysteresis hysteresis and and frequency frequency dispersion dispersion in inthe the accumulation accumulation are are effectively effectively suppressed suppressed in in the the sample sample treated treated with the 10%10% (NH(NH44)2S solution. Consequentially,Consequentially, thethe smallestsmallest DDitit andand borderborder traptrap distributiondistribution werewere achievedachieved byby thethe 10%10%(NH (NH4)4)22SS treatment,treatment, whereaswhereas (NH(NH44))22S concentrations of 1% and 5% were not sufficient sufficient for substrate passivation, andand aa concentrationconcentration ofof 22%22% resultedresulted inin excessiveexcessive surfacesurface roughnessroughness [[20].20].

FigureFigure 2.2. The capacitance–voltage (C–V)(C–V) characteristics ofof ((aa)) 0%,0%, ((bb)) 1%,1%, ((cc)) 5%,5%, ((d)) 10%,10%, andand ((e)) 22%22%

(NH(NH44))22S treatedtreated Au Au/Ni/Al/Ni/Al2O23O/InP.3/InP. They They were were measured measured at room at temperature room temperature under various under frequencies various offrequencies 100 Hz, 1 of kHz, 100 10Hz, kHz, 1 kHz, 100 10 kHz, kHz, and 100 1 kHz, MHz. and (f) 1 Measured MHz. (f) Measured hysteresis hysteresis at 100 kHz at for 100 all kHz samples. for all Frequencysamples. Frequency dispersion dispersion of the accumulation of the accumulation capacitance capacitance ranging from ranging 100Hz from to 1100 MHz Hz (fourto 1 MHz decades). (four decades). Subsequently, based on a concentration of 10%, a short-term treatment time such as 5 or 10 min is proper for passivation, while a longer treatment time such as 30 min could attack the surface akin to Subsequently, based on a concentration of 10%, a short-term treatment time such as 5 or 10 min the process with the 22% solution at 10 min (refer to the Supplementary Figure S1). On the other hand, is proper for passivation, while a longer treatment time such as 30 min could attack the surface akin other concentrations are also able to passivate the substrate, but they are not appropriate, because the to the process with the 22% solution at 10 min (refer to the Supplementary Figure S1). On the other 1% and 5% solutions require longer treatment times and 22% causes excessive surface roughening; hand, other concentrations are also able to passivate the substrate, but they are not appropriate, there are no additional advantages compared with 10%. In this regard, another substrate, such as because the 1% and 5% solutions require longer treatment times and 22% causes excessive surface In Ga As [8,21] or InSb [20], shows the same trend depending on the concentration and treatment roughening;0.53 0.47 there are no additional advantages compared with 10%. In this regard, another substrate, time of the solution. Therefore, the 10% (NH4)2S solution for 10 min is an optimal passivation condition such as In0.53Ga0.47As [8,21] or InSb [20], shows the same trend depending on the concentration and for the InP (100) as well as other III–V substrates to attain the best electrical characteristics. treatment time of the solution. Therefore, the 10% (NH4)2S solution for 10 min is an optimal Using the 10% (NH ) S treatment as the optimum condition for InP substrates, the interface passivation condition for4 the2 InP (100) as well as other III–V substrates to attain the best electrical characteristics of Al O /InP were further studied through an analysis of the chemical properties and characteristics. 2 3 Dit distributions beyond the electrical properties. Using the 10% (NH4)2S treatment as the optimum condition for InP substrates, the interface Figure3a,b shows the In 3d core level spectra of InP substrates with or without sulfur passivation, characteristics of Al2O3/InP were further studied through an analysis of the chemical properties and respectively. There are three deconvoluted peaks caused by the native oxides of indium oxides—InO, Dit distributions beyond the electrical properties. InPO /In(PO ) , and InPO with binding energy levels of 444.9 eV, 445.5 eV, and 446.1 eV, apart Figure4 3a,b3 3 shows thex In 3d core level spectra of InP substrates with or without sulfur passivation, respectively. There are three deconvoluted peaks caused by the native oxides of indium oxides—InO, InPO4/In(PO3)3, and InPOx with binding energy levels of 444.9 eV, 445.5 eV, and 446.1 eV, apart from another peak caused by the InP substrate with a binding energy of 444.4eV [9,22]. Importantly, according to a comparison of Figure 3a,b, the peaks related to InPO4/In(PO3)3 and InPOx were reduced due to sulfur passivation, which prevented the native oxide from growing. A similar trend was also observed at P 2p core level spectra of the InP substrate, as shown in Figure 3c,d. There

Materials 2019, 12, 3917 4 of 7 from another peak caused by the InP substrate with a binding energy of 444.4eV [9,22]. Importantly, Materials 2018, 11, x FOR PEER REVIEW 4 of 7 according to a comparison of Figure3a,b, the peaks related to InPO 4/In(PO3)3 and InPOx were reduced due to sulfur passivation, which prevented the native oxide from growing. A similar trend was also observedwere two at peaks P 2p corecaused level by spectra In(PO3 of)3 and the InPthe substrate,InP substrate as shown with binding in Figure energies3c,d. There of 134.1 were and two 129 peaks eV [9,22]. In addition to the In 3d spectra, the P 2p spectra also indicated that the native oxide was caused by In(PO3)3 and the InP substrate with binding energies of 134.1 and 129 eV [9,22]. In addition reduced due to sulfur. to the In 3d spectra, the P 2p spectra also indicated that the native oxide was reduced due to sulfur.

Figure 3. The In 3d core level XPS spectra of the sample (a) with or (b) without 10% (NH4)2S treatment. Figure 3. The In 3d core level XPS spectra of the sample (a) with or (b) without 10% (NH4)2S treatment. The P 2p core level spectra of the sample (c) with or (d) without 10% (NH4)2S treatment. The P 2p core level spectra of the sample (c) with or (d) without 10% (NH4)2S treatment.

Figure4 illustrates G–V contour maps and the D it extracted from the InP MOSCAPs with or withoutFigure sulfur 4 illustrates passivation. G–V These contour contour maps maps and the exhibit Dit extracted the magnitude from the of theInP normalized MOSCAPs parallelwith or without sulfur passivation. These contour maps exhibit the magnitude of the normalized parallel conductance, (Gp/ω)/Aq, versus both the AC frequency and the gate voltage, where ω is the frequency, Aconductance, is the area of(G thep/ω MOSCAP,)/Aq, versus and both q is the elementaryAC frequency charge. and Fromthe gate the contourvoltage, maps,where the ω lineis the to frequency, A is the area of the MOSCAP, and q is the elementary charge. From the contour maps, the connect the maximum value of (Gp/ω)/Aq represents the band bending eﬃciency and the Fermi level line to connect the maximum value of (Gp/ω)/Aq represents the band bending efficiency and the movement with gate bias [15,23,24]. The maximum value of (Gp/ω)/Aq increases with a steeper slope Fermi level movement with gate bias [15,23,24]. The maximum value of (Gp/ω)/Aq increases with a steeper slope with the 10% (NH4)2S compared to the case with no sulfur passivation, as shown in Figure 4a,b. Therefore, it should be noted that the sulfur passivation leads to the depinning of the Fermi level, making the band bending more efficient. Moreover, border traps were validated again. In the case with no passivation, the (Gp/ω)/Aq peak spreads over the entire frequency range in the

Materials 2019, 12, 3917 5 of 7

with the 10% (NH4)2S compared to the case with no sulfur passivation, as shown in Figure4a,b. Therefore,Materials 2018 it, 11 should, x FOR PEER be noted REVIEW that the sulfur passivation leads to the depinning of the Fermi level,5 of 7 making the band bending more eﬃcient. Moreover, border traps were validated again. In the case accumulation, compared to the case with 10% (NH4)2S passivation. This implies that the conductive with no passivation, the (Gp/ω)/Aq peak spreads over the entire frequency range in the accumulation, loss is caused by two factors: border traps and series resistance. The maximum value of (Gp/ω)/Aq compared to the case with 10% (NH4)2S passivation. This implies that the conductive loss is caused byextracted two factors: by the border measurement traps and equipment series resistance. was used The as maximum the level of value the ofDit (Gp distribution/ω)/Aq extracted across the by themaps. measurement Figure 4c indicates equipment that was the used MOSCAP as the levelwith ofa sulfur the Dit treatment distribution shows across a lower the maps. Dit distribution Figure4c indicatesthan that thatwithout the MOSCAP a sulfur treatment. with a sulfur This treatment result is consistent shows a lower with D theit distribution C–V characteristics than that shown without in aFigure sulfur 2a,d. treatment. This result is consistent with the C–V characteristics shown in Figure2a,d.

FigureFigure 4.4. Contour map map of of normalized normalized conductance conductance (G (Gp/ωp/)/Aqω)/Aq as asa function a function of frequency of frequency andand gate gate bias biasfor MOSCAPs for MOSCAPs (a) without (a) without or (b) orwith (b )10% with (NH 10%4)2S (NH treatment.4)2S treatment. (c) Dit distribution (c)Dit distribution across the InP across energy the InPbandgap energy extracted bandgap from extracted the conductance from the conductance method for method MOSCAPs for MOSCAPs with or without with or without10% (NH 10%4)2S

(NHtreatment.4)2S treatment.

4.4. ConclusionsConclusions

TheThe eeffectsﬀects ofof (NH(NH44))22SS passivationpassivation withwith variousvarious concentrationsconcentrations andand treatmenttreatment timestimes werewere investigated.investigated. Two Two characterization characterization methods, methods, an an electrical electrical method method and anda chemical a chemical method, method, were wereemployed employed for comprehensive for comprehensive analyses analyses of ofthe the su surfacerface properties. properties. In In terms terms of of the electricalelectrical characteristics,characteristics, thethe 10%10% (NH(NH44))2S treatment for 10 min waswas thethe optimaloptimal conditioncondition toto improveimprove thethe electricalelectrical properties of devices devices built built on on InP InP (100) (100) substrates. substrates. This This was was supported supported by both by both the C–V the C–V plot plotand andthe theG–V G–V contour contour map map in interms terms of ofthe the frequenc frequencyy dispersions dispersions and and hysteresis. hysteresis. In In terms ofof thethe chemicalchemical characteristics,characteristics, thethe devicedevice withwith the the 10%10% (NH(NH44))22S treatment forfor 1010 minmin showed relativelyrelatively lowlow nativenative oxideoxide peaks,peaks, particularlyparticularly arisingarising fromfrom thethe phosphorus-relatedphosphorus-related oxideoxide comparedcompared toto thatthat withoutwithout sulfursulfur passivation.passivation. This waswas supportedsupported byby XPSXPS data.data. Finally, itit waswas alsoalso foundfound thatthat thethe levellevel ofof DDitit waswas lowerlower inin thethe devicedevice withwith thethe 10%10% (NH(NH44))22S treatment than in the device with no sulfur passivation. TheThe proposedproposed strategystrategy toto optimizeoptimizethe the conditions conditions of of diluted diluted (NH (NH44))22SS providesprovides aa guidelineguideline forfor furtherfurther improvementsimprovements ofof InP-basedInP-based high-performancehigh-performance CMOS CMOS technology. technology.

Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/12/23/3917/s1, Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: The Figure S1: The capacitance-voltage characteristics of the Au/Ni/Al2O3/InP which was treated on 10% (NH4)2S for (a)capacitance-voltage 5, and (b) 10, and (c)characteristics 30 min. Measurement of the Au/Ni/Al frequencies2O3/InP are which 100 Hz,was 1 treated kHz, 10 on kHz, 10% 100 (NH kHz,4)2S andfor (a) 1 MHz. 5, and (b) 10, and (c) 30 min. Measurement frequencies are 100 Hz, 1 kHz, 10 kHz, 100 kHz, and 1 MHz. Author Contributions: Conceptualization, D.K.; Methodology, D.K.; Investigation, T.Y.A.; Writing—Original DraftAuthor Preparation, Contributions: D.K.; Conceptualization, Writing—Review &D.K.; Editing, Methodology, D.K.; Supervision, D.K.; Investigation, J.S.L.; Project T.Y.A.; Administration, Writing—Original J.S.L.; FundingDraft Preparation, Acquisition, D.K.; J.S.L. Writing—Review & Editing, D.K.; Supervision, J.S.L.; Project Administration, J.S.L.; Funding:Funding Acquisition,This research J.S.L. received no external funding. Acknowledgments:Funding: This researchThis received study was no supportedexternal funding. by Biomedical Research Institute Grant 2018B000, Pusan National University Hospital. Acknowledgements: This study was supported by Biomedical Research Institute Grant 2018B000, Pusan Conﬂicts of Interest: The authors declare no conﬂict of interest. National University Hospital.

Conflicts of Interest: The authors declare no conflict of interest.

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