Brief Review of Surface Passivation on III-V Semiconductor

crystals

Review Brief Review of Surface Passivation on III-V Semiconductor

Lu Zhou 1, Baoxue Bo 2, Xingzhen Yan 1, Chao Wang 1, Yaodan Chi 1 and Xiaotian Yang 1,*

1 Jilin Provincial Key Laboratory of Architectural Electricity & Comprehensive Energy Saving, School of Electrical Engineering and Computer, Jilin Jianzhu University, Changchun 130118, China; [email protected] (L.Z.); [email protected] (X.Y.); [email protected] (C.W.); [email protected] (Y.C.) 2 National Key Laboratory on High Power Semiconductor Lasers, Changchun University of Science and Technology, Changchun 130022, China; [email protected] * Correspondence: [email protected]

Received: 30 March 2018; Accepted: 9 May 2018; Published: 18 May 2018

Abstract: The III-V compound semiconductor, which has the advantage of wide bandgap and high electron mobility, has attracted increasing interest in the optoelectronics and microelectronics ﬁeld. The poor electronic properties of III-V semiconductor surfaces resulting from a high density of surface/interface states limit III-V device technology development. Various techniques have been applied to improve the surface and interface quality, which cover sulfur-passivation, plasmas-passivation, ultrathin ﬁlm deposition, and so on. In this paper, recent research of the surface passivation on III-V semiconductors was reviewed and compared. It was shown that several passivation methods can lead to a perfectly clean surface, but only a few methods can be considered for actual device integration due to their effectiveness and simplicity.

Keywords: III-V semiconductor; passivation; MOCAP; ALD; XPS

1. Introduction III-V semiconductor materials, such as GaAs, InGaAs, GaSb, InP, InAs, InSb, have high surface state density (>1013 cm−2) lacking a high quality intrinsic oxide passivation layer, which lead to Fermi level pinning, surface recombination accelerating, and therefore deteriorate the device performance in microelectronic and optoelectronic fields. For example, the surface state will cause light carrier loss and decrease the photoelectric conversion efficiency in solar cells [1]; In semiconductor lasers, the surface state in the cavity will cause nonradiative recombination of carriers, which reduce the laser emission efficiency and even lead to catastrophic optical mirror damage [2]; In metal oxide semiconductor field effect transistors (MOSFET) the surface state will cause C-V dispersion and hysteresis [3–5]. It is well known that the thermal SiO2 film on Si semiconductor has excellent interfacial properties and thermal stability. Unfortunately, the native surface oxides of III-V semiconductor induce unwanted interfacial defects due to the poor stability. Take GaAs for example, the native oxides of GaAs have a complicated chemistry where both As2O3 and Ga2O3 compounds will form when a clean GaAs surface is exposed to oxygen and light. These oxides, which act as nonradiative recombination centers, introduce a mass of surface energy levels in the band gap and pin the surface Fermi level within the band gap of the semiconductor [4,5]. These oxides also present water solubility with a pH dependency. The formation of Ga2O3 is thermodynamically favored, leaving bare arsenic atoms embedded within the oxide near the oxide/GaAs interface, and the As2O3 is also mobile at the grain boundaries [6]. So, it is necessary to passivate the III-V surface for good device performance. Various techniques have been applied to improve the surface and the interface quality, e.g., chemical etching, ion sputtering,

Crystals 2018, 8, 226; doi:10.3390/cryst8050226 www.mdpi.com/journal/crystals Crystals 2018, 8, 226 2 of 14 additional passivation film deposition, coupled with controlled annealing [7,8]. The ideal effect is that the complex surface reconstructions of semiconductors are broken up and return to a simpler state, and the passivation film can act as a diffusion barrier to prevent the re-oxidation of the III-V surface. The characteristics of passivation in various cleaning procedure were investigated by modern surface techniques, i.e., X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy (PL), and atomic force microscopy (AFM), Raman spectroscopy, combined with electrical characterization. Many reviews of passivation on III-V semiconductor surfaces were reported in the past, but most focused on particular techniques. Those such as [9] reviewed atomic layer deposition (ALD) methods, and [10,11] reviewed silicon interface control layer methods. In this paper, we reviewed the research process of the passivation on III-V semiconductor surface focusing mainly on sulfur-passivation, plasma-passivation and ultrathin film deposition, which are relatively novel and highly cited in recent years. It is useful for someone starting off in this field.

2. Preparation and Investigation Techniques

2.1. Sulfur Passivation Sulfur had been explored as electrical passivation agents for the III-V semiconductor. Take GaAs for example, in reactive S-containing solution, native oxide of GaAs could be extensively removed, and sulfides of Ga (GaxSy) and As (AsxSy) are future formed through chemical reactions. The S-bonds provide protection against further oxidation and reduce the density of surface states. Among various sulfurs, (NH4)2S solutions had been mostly used to produce S terminate III-V surfaces [12,13]. Using alcoholic instead of aqueous sulfide solutions had been reported to have a superior passivation effect. Lebedev et al. [14] reported a GaSb semiconductor which was passivated with (NH4)2S dissolved in water versus isopropanol, and found almost no residual oxygen was left on the GaSb passivated with (NH4)2S in isopropanol, while Ga2O3 was found using aqueous (NH4)2S solution. They also found the C contamination was reduced about 4 times using isopropanol; by contrast, it was increased 1.25 times using aqueous solution. (NH4)2S passivation had been able to reduce the interface states density markedly, but obvious drawbacks were the limited chemical stability of the (NH4)2S passivation in an ambient environment as well as air contamination. In contrast, the novel sulfur-passivation by long-chain alkylthiol self-assembled monolayers (SAMs) [6,15–19] promised much better stability under atmospheric ambient conditions and had been reported by various authors. Cuypers et al. reported using SAMs of 1-octadecanethiol (ODT, CH3[CH2]17SH) for passivating the GaAs(100) surfaces and interfaces [20]. They found ODT SAMs could provide a good protection of GaAs surfaces, although the contact angle and the PL intensity decreased overall slightly after a day of air exposure, the passivation by ODT was still much better than for the initial HCl cleaned surface. Figure1a shows a Synchrotron-radiation photoemission spectroscopy (SRPES) overview ◦ spectrum of the GaAs surface after the thermal desorption of ODT at 300 C and exposure to H2O vapor with a dose equivalent to 25 H2O pulses in an ALD device; we can see that the spectrum was dominated by As and Ga 3d peaks with only very small contributions from O 1s and C 1s. Figure1b,c results indicated that the ODT binds to the surface by the formation of both Ga– and As–thiolates, this meant the Ga–S bonds were at least thermally stable up to 300 ◦C. The SRPES also demonstrated both the absence of an interfacial oxide and of near-surface band bending with the SFL being close to the conduction band edge, as shown in Figure1d. Thus, sacrificial SAMs provided a highly promising GaAs surface preparation route that did not depend on the availability of in situ deposition to obtain clean GaAs surfaces. Crystals 2018, 8, 226 3 of 14 Crystals 2018, 8, x FOR PEER REVIEW 3 of 14 Crystals 2018, 8, x FOR PEER REVIEW 3 of 14

Figure 1. GaAsGaAs surface surface after after thermal thermal desorption desorption of of the sacrificial sacriﬁcial SAM. ( a) The overview SRPES Figure 1. GaAs surface after thermal desorption of the sacrificial SAM. (a) The overview SRPES spectrum indicates low O and C surface contamination. The The ( (bb)) As As 3d 3d and and ( c) Ga 3d spectra also spectrum indicates low O and C surface contamination. The (b) As 3d and (c) Ga 3d spectra also demonstrate the absence of surface oxides above the detection limit. ( d) The valence band spectrum demonstrate the absence of surface oxides above the detection limit. (d) The valence band spectrum indicates the absence of strong strong band band bending bending near near the the surface. surface. Reprod Reproduceduced with with permission permission from from [20], [20], indicates the absence of strong band bending near the surface. Reproduced with permission from [20], copyright American Chemical Society (2016). copyright American Chemical Society (2016). Pablo Mancheno-Posso et al. reported on the re-oxidation of the GaAs surface in the atmosphere Pablo Mancheno-Posso et al. reported on the re-o re-oxidationxidation of the GaAs surface in the atmosphere after passivation by six alkanethiols (CnH2n+1-SH where n = 3, 6, 8, 12, 18, 20) [21]. Figure 2 shows the after passivation byby sixsix alkanethiolsalkanethiols (C(CnnHH2n+12n+1-SH-SH where where n n = = 3, 3, 6, 6, 8, 8, 12, 12, 18, 18, 20) 20) [21]. [21]. Figure Figure 22 showsshows thethe Ga-2p and As-2p XPS spectra after passivation by six different alkanethiols and 3/30 min of ambient Ga-2p and As-2p XPS spectra after passivationpassivation byby sixsix differentdifferent alkanethiolsalkanethiols andand 3/303/30 min of ambient exposure. We can see that, the longer alkanethiol molecule, the better stability of sulfur-passivation exposure. We can see that,that, thethe longerlonger alkanethiolalkanethiol molecule,molecule, the betterbetter stabilitystability of sulfur-passivationsulfur-passivation (for lower Ga–O and As–O signal but stronger As–S signal). In his report, the ET-20C (1-eicosanethiol, (for lower Ga–O and As–O signal but stronger stronger As–S As–S signal). signal). In In his his report, report, the the ET-20C ET-20C (1-eicosanethiol, (1-eicosanethiol, C20H41-SH) had the best passivation stability for the formation of the most ordered and highest- C20HH4141-SH)-SH) had had thethe bestbest passivation passivation stability stability for for the the formation formation of the of most the orderedmost ordered and highest-packing and highest- packing density layer, and restrained the surface re-oxidation of ambient exposure for at least 30 min. densitypacking layer,density and layer, restrained and restrained the surface the re-oxidationsurface re-oxi ofdation ambient of ambient exposure exposure for at least for 30at min.least 30 min.

Figure 2. The Ga-2p and As-2p X-ray photoelectron spectroscopy (XPS) spectra for GaAs passivated Figure 2. The Ga-2p and As-2p X-ray photoelectron spectroscopy (XPS) spectra for GaAs passivated byFigure six alkanethiols 2. The Ga-2p after and ( As-2pa) 3 min X-ray and photoelectron(b) 30 min of ambient spectroscopy exposure. (XPS) Reproduced spectra for GaAswith permission passivated by six alkanethiols after (a) 3 min and (b) 30 min of ambient exposure. Reproduced with permission fromby six [21], alkanethiols copyright after Elsevier (a) 3 B. min V. and(2016). (b) 30 min of ambient exposure. Reproduced with permission from [[21],21], copyright ElsevierElsevier B.B. V.V. (2016).(2016).

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Crystals 2018, 8, x FOR PEER REVIEW 4 of 14 Similar results was reported by Loredana Preda et al. They investigated the effects of the SAMs of Similar results was reported by Loredana0 Preda et al. They investigated the effects of the SAMs 1,8-octanedithiolof 1,8-octanedithiol (ODT) (ODT) and and biphenyl biphenyl 4,4 4,4-dithiol′-dithiol (BPDT) (BPDT) on on the the electronic electronic and and chemical chemical propertiesproperties of p-GaAsof p-GaAs with with As terminatedAs terminated semiconductor semiconductor surface surface [ 22[22].]. TheyThey veriﬁedverified that that the the hydrocarbon hydrocarbon moiety moiety hadhad an an important important effect effect on on tailoring tailoring the the GaAsGaAs surfacesurface properties. The The XPS XPS spectra spectra (Figure (Figure 3)3 showed) showed thatthat both both ODT ODT and and BPDT BPDT bind bind to to GaAs GaAs viavia thethe thiolthiol group. Compared Compared to to the the BPDT BPDT layer, layer, the the ODT ODT layerlayer provided provided better better passivation passivation protection protection againstagainst the reoxidation reoxidation with with ambient ambient exposure. exposure. That That was was becausebecause the the ODT ODT molecule molecule bound bound with with both both thiol thiol groups groups to the to GaAs the GaAs surface, surface, while thewhile BPDT the moleculeBPDT boundmolecule with bound only one with thiol only group one thiol to the group GaAs to surface.the GaAs surface.

FigureFigure 3. 3.XPS XPS spectra spectra of of Ga-2p Ga-2pand and As-2pAs-2p for p-GaAs substrate substrate exposed exposed to to ambient ambient after after etching etching (a,d (a),, d), BPDTBPDT passivation passivation (b (,be,)e and) and ODT ODT passivation passivation ((cc,,ff). Reproduced with with permi permissionssion from from [22], [22 copyright], copyright ElsevierElsevier B. B. V. V. (2016). (2016).

Passivation by the long-chain alkylthiol SAM have the advantage of environmental friendliness, Passivation by the long-chain alkylthiol SAM have the advantage of environmental friendliness, simple process, low cost, and good treatment effect, but at the same time, needs a relatively long time simple process, low cost, and good treatment effect, but at the same time, needs a relatively long time (for hours) to obtain clean surfaces and well-ordered SAM, which limit its practical application in (for hours) to obtain clean surfaces and well-ordered SAM, which limit its practical application in device processing. device processing. 2.2. Passivation Film Depositing by Atomic Layer Deposition (ALD) Technique 2.2. Passivation Film Depositing by Atomic Layer Deposition (ALD) Technique III-V compound semiconductors are regarded as promising candidates to replace the silicon III-V compound semiconductors are regarded as promising candidates to replace the silicon channel for future MOSFETs because of their high mobility [23]. However, compared with Si- channelMOSFETs, for future the high MOSFETs interface because defect of density their high betw mobilityeen high-k [23 gate]. However, and III-V compared semiconductor with Si-MOSFETs, hampers thethe high development interface of defect III-V densityMOSFETs. between The Fermi high-k level gate pinning and phenomenon, III-V semiconductor resulting hampersfrom high the developmentinterface defect of III-V density MOSFETs. at the high-k/semiconductor The Fermi level pinning interface, phenomenon, could only resulting produce from MOSFETs high interface with defectvery densitypoor control at the of high-k/semiconductor carrier flow and various interface, instabilities could such only as hysteresis produce MOSFETsand drain withcurrent very drift. poor controlTherefore, of carrier interface flow passivation and various became instabilities very sign suchificant as hysteresis for realizing and draina high-performance current drift. Therefore,of III-V interfaceMOSFETs. passivation became very significant for realizing a high-performance of III-V MOSFETs. ALDALD is is the the most most advanced advanced techniquetechnique forfor high-k gate gate material material deposition deposition because because it itoffers offers outstandingoutstanding film film thickness thickness control, control, pinhole-freepinhole-free and conformal conformal deposition deposition around around complex complex structure. structure. UsingUsing ALD ALD for interfacefor interface passivation passivation prior prior to gate to filmgate deposition film deposition has the has advantage the advantage of in situ of processing in situ withoutprocessing any vacuumwithout any breaking, vacuum which breaking, further which improve further theimprove interface the interface quality, andquality, the and “self-cleaning” the “self- processcleaning” [24– process28] can [24–28] reduce can and/or reduce removal and/or ofremova nativel of oxides native ofoxides III-V of semiconductors III-V semiconductors by surface by reactionssurface withreactions the ALDwith precursorsthe ALD precursors during the during initial the stages initial of stages deposition. of deposition.

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SurfaceSurface passivation passivation of of III-V semiconductors using ALD-Al2OO33 hadhad been been researched researched the the most most widely.widely. As 2OO33 firstfirst interacts interacts with with Al (CH 3))33 toto be be transformed transformed to to arsenic arsenic and and Al 2OO33.. The The arsenic arsenic then then reactsreacts with with H H22OO to to become become As As22OO55, ,which which then then reacts reacts again again with with the the next next incoming incoming pulse of Al (CH3)3 toto become become As As and Al2OO33. .Most Most of of the the arsenic arsenic oxides oxides evaporated evaporated since since the melting pointpoint ofof AsAs22O5 isis ◦ lowlow around around 280 280 °CC with with a a small small amount amount of of residual residual As As22OO55 leftleft on on the the skin-depth skin-depth top top of of Al Al2O2O3 3[29].[29 ]. TheThe interaction interaction of of Al Al (CH (CH33))33 withwith native native oxides oxides of of GaAs GaAs surfaces surfaces leads leads to to most most of of the the surface surface arsenicarsenic oxides oxides and and a asignificant significant portion portion the the gallium gallium oxide oxide being being consumed. consumed. It seems It seems that thatjust a just half a cyclehalf cycleof Al of2O3 Al could2O3 could passivate passivate the majority the majority of the ofdefects the defects [24], but [24 ],according but according to the toresearch the research of Kent of etKent al. et[30], al. dissociative [30], dissociative chemisorption chemisorption of Al(CH of Al(CH3)3 was3)3 foundwas found to also to alsolead leadto the to formation the formation of an of orderedan ordered monolayer monolayer of dimethyl of dimethyl aluminum aluminum that thatnot only not onlypassivated passivated the As the dangling As dangling bonds bonds but also but resultedalso resulted in the in formation the formation of metal–metal of metal–metal bonds bonds that thatpinned pinned the theFermi Fermi level, level, so dosing so dosing with with H2O H 2orO Oor2 was O2 was required required to passivate to passivate the surface the surface and unpin and unpinthe Fermi the level Fermi by level inserting by inserting O atoms O in atoms the metallic in the bond.metallic bond. Generally,Generally, the the arsenic arsenic oxides oxides were were energeticall energeticallyy the easiest the easiest to remove to remove and the and indium the indiumoxides wereoxides hardest. were hardest.Gang He Ganget al. reported He et al. improving reported improving the interface the quality interface in HfTiO/InGaAs quality in HfTiO/InGaAs metal oxide semiconductormetal oxide semiconductor capacitor (MOSCAP) capacitor (MOSCAP)by introducing by introducing ALD-Al2 ALD-AlO3 passivation2O3 passivation film [31]. film XPS [31]. measurementsXPS measurements shown shown in Figure in Figure 4 had4 confirmedhad confirmed that, that,after after20 cycles 20 cycles of ALD-Al of ALD-Al2O3 passivation,2O3 passivation, none ofnone the ofGa–O the Ga–Oand As–O and As–Obonds bondswere detected, were detected, and significant and significant suppressing suppressing the formation the formation of In–O of bond In–O hadbond been had achieved. been achieved. Electrical Electrical measurements measurements of MOSCAP of MOSCAP with HfTiO/4 with HfTiO/4 nm-Al2O nm-Al3/InGaAs2O3 structure/InGaAs indicatedstructure improved indicated improvedelectrical characteristics electrical characteristics were achieved. were achieved.

Figure 4. XPS spectra of As 3d (a), Ga 2p (b), In 3d (c) and O 1s (d) with the influence of ALD-Al2O3 Figure 4. XPS spectra of As 3d (a), Ga 2p (b), In 3d (c) and O 1s (d) with the inﬂuence of ALD-Al2O3 growthgrowth cycles. cycles. Reproduced Reproduced with with permission permission from from [31], [31], copyright copyright American Chemical Society (2016).

C. Mukherjee et al. put forward the bilayer of ALD-TiO2/Al2O3 structure to passivate the InGaAs C. Mukherjee et al. put forward the bilayer of ALD-TiO2/Al2O3 structure to passivate the InGaAs MOSFETs surface and used as the gate oxide in the meanwhile [8]. TiO2 had high dielectric constant MOSFETs surface and used as the gate oxide in the meanwhile [8]. TiO2 had high dielectric constant and Al2O3 was beneficial to reduce the interface defect density. It was found that the TiO2/Al2O3 and Al O was beneﬁcial to reduce the interface defect density. It was found that the TiO /Al O structure2 helped3 to control the As out-diffusion and reduced the As–O and In–O bond on the2 InGaAs2 3 surface. It was shown in Figure 5 that with the action of VA (vacuum annealing) at 350 °C, all native oxides were effectively removed.

Crystals 2018, 8, 226 6 of 14 structure helped to control the As out-diffusion and reduced the As–O and In–O bond on the InGaAs surface. It was shown in Figure5 that with the action of VA (vacuum annealing) at 350 ◦C, all native oxidesCrystals were 2018, effectively8, x FOR PEER removed. REVIEW 6 of 14

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Figure 5. XPS spectra of Ga 3d (a,b) and As 3d (c,d) from TiO2/Al2O3/In0.53Ga0.47As structure without Figure 5. XPS spectra of Ga 3d (a,b) and As 3d (c,d) from TiO2/Al2O3/In0.53Ga0.47As structure without and with vacuum annealing (VA). Reproduced with permission from [8], copyright American andFigure with vacuum5. XPS spectra annealing of Ga (VA). 3d (a Reproduced,b) and As 3d with (c,d permission) from TiO2/Al from2O3 [/In8],0.53 copyrightGa0.47As Americanstructure without Chemical Chemical Society (2016). Societyand with (2016). vacuum annealing (VA). Reproduced with permission from [8], copyright American Chemical Society (2016). Although ALD of Al2O3 can suppress the formation of the As–O and Ga–O bonds, it seems that theAlthough self-cleaning ALD ALD of Al process2O3 can was suppress not sufficient the formation to deposit of the a As–Ogate dielectric and Ga–O with bonds, a high-quality it seems that Although ALD of Al2O3 can suppress the formation of the As–O and Ga–O bonds, it seems that theinterface. self-cleaning For instance ALD process in Figure was 6 not the sufﬁcient electrical to properties deposit a of gate a stack dielectric consisting with aof high-quality 10 nm ALD interface.Al2O3 the self-cleaning ALD process was not sufficient to deposit a gate dielectric with a high-quality Foron instance GaAs (100) in Figure were compared6 the electrical for two properties different surf of aace stack preparations: consisting HF of 10etch nm directly ALD AlandO 1.2on nm GaAs Si interface. For instance in Figure 6 the electrical properties of a stack consisting of 10 nm 2ALD3 Al2O3 passivation layer depositing by PECVD. In the latter case the frequency of the C-V curves was (100)on wereGaAs compared(100) were for compared two different for two surface different preparations: surface preparations: HF etch directly HF etch and directly 1.2 nm and Si 1.2 passivation nm Si significantly reduced (as shown in Figure 6b,d). Si passivation layer at the interface resulted in layerpassivation depositing layer by PECVD.depositing In theby latterPECVD. case In the the frequency latter case of thethe C-Vfrequency curves of was the signiﬁcantly C-V curves reduced was gettering of oxygen from the substrate to result in a partially oxidized Si layer and the removal of all (assignificantly shown in Figure reduced6b,d). (as Si shown passivation in Figure layer 6b,d). at the Si interface passivation resulted layer in at gettering the interface of oxygen resulted from in the As-oxides (not shown here), as well as the higher 3+ oxidation state of Ga (as shown in Figure 6a,c). substrategettering to of result oxygen in afrom partially the substrate oxidized to Siresult layer in anda partially the removal oxidized of allSi layer As-oxides and the (not removal shown of here),all We also noted that the Ga2O bonding arrangement remains for the two interfaces suggesting that it asAs-oxides well as the (not higher shown 3+ here), oxidation as well state as ofthe Ga higher (as shown 3+ oxidation in Figure state6a,c). of Ga We (as also shown noted in thatFigure the 6a,c). Ga 2 O was not the species primarily responsible for Fermi-level pinning. bondingWe also arrangement noted that the remains Ga2O bonding for the twoarrangement interfaces remains suggesting for the that two it wasinterfaces not the suggesting species primarily that it responsiblewas not the for species Fermi-level primarily pinning. responsible for Fermi-level pinning.

Figure 6. Cont.

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Figure 6. (a) Ex situ XPS analysis after HF-last GaAs with overlying 1.0 nm Al2O3 layer by ALD, (b) Figure 6. (a) Ex situ XPS analysis after HF-last GaAs with overlying 1.0 nm Al2O3 layer by ALD, (b) C-V C-V characteristics of (a) after an additional 9.0 nm Al2O3 deposition, (c) ex situ XPS analysis after HF- Figure 6. (a) Ex situ XPS analysis after HF-last GaAs with overlying 1.0 nm Al2O3 layer by ALD, (b) characteristics of (a) after an additional 9.0 nm Al2O3 deposition, (c) ex situ XPS analysis after HF-last 2 3 lastC-V GaAs characteristics with overlying of (a) after 1.2 annm additional Si by PECVD 9.0 nm Aland2O 3 1.0deposition, nm Al (Oc) exlayer situ XPSby ALD,analysis and after (d HF-) C-V GaAs with overlying 1.2 nm Si by PECVD and 1.0 nm Al2O3 layer by ALD, and (d) C-V characteristics characteristicslast GaAs with of ( coverlying) after an 1.2additional nm Si by9.0 PECVDnm Al2 Oand3 deposition. 1.0 nm Al Reproduced2O3 layer by withALD, permission and (d) C-V from of (c) after an additional 9.0 nm Al2O3 deposition. Reproduced with permission from [32], copyright [32],characteristics copyright Elsevier of (c) after B. V. an (2009). additional 9.0 nm Al2O3 deposition. Reproduced with permission from Elsevier B. V. (2009). [32], copyright Elsevier B. V. (2009). ZnO wideband material has attracted increasing attention in the passivation field. Although O atomsZnO existZnO wideband in wideband ZnO, materialit is material nearly has impossiblehas attracted attracted for increasingincreasing ZnO to react attention attention with Gain in the or the Aspassivation passivation for the high field. exciting field. Although Although binding O O atomsenergyatoms exist of exist inZn–O ZnO, in ZnO, bonds it is it nearlyis [33]. nearly Young- impossible impossibleChul for forByun ZnOZnO et to al. react react investigated with with Ga Ga or or ultrathinAs As forfor the thehighZnO high excitingfilm exciting by binding the bindingALD energymethodenergy of as Zn–O of interface Zn–O bonds bonds passivation [33 [33].]. Young-Chul Young- layer betweenChul ByunByun HfO et2 al.gate investigated investigated and GaAs substrateultrathin ultrathin ZnO[34]. ZnO Thefilm filmGa–Oby the by bond/As ALD the ALD method as interface passivation layer between HfO2 gate and GaAs substrate [34]. The Ga–O bond/As methodsegregation as interface near passivationthe interface layer and between the trapping HfO2 gateof carriers and GaAs near substrate the valence [34]. Theband Ga–O edge bond/As was segregation near the interface and the trapping of carriers near the valence band edge was segregationsignificantly near suppressed, the interface greatly and the improving trapping the of carriersC-V characteristics near the valence on p-type band GaAs edge wassubstrates, significantly as significantly suppressed, greatly improving the C-V characteristics on p-type GaAs substrates, as suppressed,shown in Figure greatly 7. improving the C-V characteristics on p-type GaAs substrates, as shown in Figure7. shown in Figure 7.

FigureFigure 7. 7.The The C-V C-V characteristics characteristics of of HF+ HF+ (NH(NH44))22S cleaned HfOHfO22/p-GaAs/p-GaAs MOSCAP MOSCAP with with and and without without Figure 7. The C-V characteristics of HF+ (NH4)2S cleaned HfO2/p-GaAs MOSCAP with and without 10 10cycle cycle ALD–ZnO ALD–ZnO passivation passivation layer. layer. ReproducedReproduced with permissionpermission from from [34], [34], copyright copyright American American 10 cycle ALD–ZnO passivation layer. Reproduced with permission from [34], copyright American ChemicalChemical Society Society (2014). (2014). Chemical Society (2014).

Similar results were reported by Liu Chen et al. [35], who found deposition of ZnO prior to Al2O3 Similar results were reported by Liu Chen et al. [35], who found deposition of ZnO prior to Al2O3 gate materials deposition directly can further reduce the interface trap density. The n-GaAs MOSCAP gateSimilar materials results deposition were reported directly bycan Liu further Chen redu et al.ce [the35], interface who found trap deposition density. The of n-GaAs ZnO prior MOSCAP to Al2O 3 with 2 nm ALD ZnO interface passivation layer exhibited higher accumulation capacitance and a gatewith materials 2 nm ALD deposition ZnO interface directly passivation can further layer reduce exhibited the interface higher trap accumulation density. The capacitance n-GaAs MOSCAP and a very small C–V dispersion and hysteresis. They explained the result that the presence of ZnO on n- withvery 2 nmsmall ALD C–V ZnO dispersion interface and passivation hysteresis. layer They exhibited explained higher the result accumulation that the presence capacitance of ZnO and on a n- very GaAs can act as the oxygen reaction barrier and effectively reduces the Ga and As oxides, which smallGaAs C–V can dispersion act as the andoxygen hysteresis. reaction Theybarrier explained and effectively the result reduces that thethe presenceGa and As of ZnOoxides, on which n-GaAs suppresses the formation of low k interfacial layer on the GaAs surface, thus improving the interface suppresses the formation of low k interfacial layer on the GaAs surface, thus improving the interface can actquality as the and oxygen leading reaction to higher barrier accumulation and effectively capacitance. reduces the Ga and As oxides, which suppresses thequality formation ZnSand hasleading of lowbetter kto interfacial thermochemicalhigher accumulation layer onstability the capacitance. GaAs and higher surface, band thus gap improving energy than the interfaceZnO [36], qualitywhich and leadingmakesZnS to higherithas an attractivebetter accumulation thermochemical and promising capacitance. stabilitycandidate and as an higher interface band passivation gap energy layer than (IPL) ZnO material [36], in which III- makesZnSV based it has an devices. betterattractive thermochemical Lucero and etpromising al. used stability ALD candidate to andformat as higher an a ZnO/ZnS interface band gap IPLpassivation by energy converting than layer ZnOan (IPL) (NH [36 material4)],2S which cleaned in makesIII- it anV basedp-In attractive0.53 devices.Ga0.47As and Lucerowith promising diethylzinc et al. used candidate and ALD water to as format [37]. an interfaceDiethylzinc a ZnO/ZnS passivation reacted IPL by with converting layer S and (IPL) O an on (NH materialthe4 )InGaAs,2S cleaned in III-V p-In0.53Ga0.47As with diethylzinc and water [37]. Diethylzinc reacted with S and O on the InGaAs, based devices. Lucero et al. used ALD to format a ZnO/ZnS IPL by converting an (NH4)2S cleaned p-In0.53Ga0.47As with diethylzinc and water [37]. Diethylzinc reacted with S and O on the InGaAs,

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Crystals 2018, 8, x FOR PEER REVIEW 8 of 14 3+ 3+ reducing Ga and As to lower oxidation states. Measurements of C-V characteristics of HfO2-InGaAs reducing Ga3+ and As3+ to lower oxidation states. Measurements of C-V characteristics of HfO2- MOSCAP in Figure8 showed that the ZnO/ZnS bilayer nearly eliminated frequency dispersion InGaAsCrystals 2018MOSCAP, 8, x FOR PEERin Figure REVIEW 8 showed that the ZnO/ZnS bilayer nearly eliminated frequency8 of 14 and hysteresis.dispersion and hysteresis. reducing Ga3+ and As3+ to lower oxidation states. Measurements of C-V characteristics of HfO2- InGaAs MOSCAP in Figure 8 showed that the ZnO/ZnS bilayer nearly eliminated frequency dispersion and hysteresis.

Figure 8. C-V characteristics of (NH4)2S cleaned HfO2/p-InGaAs MOSCAP with (a) and without (b) Figure 8. C-V characteristics of (NH4)2S cleaned HfO2/p-InGaAs MOSCAP with (a) and without ZnO/ZnS IPL. Reproduced with permission from [37], copyright The Japan Society of Applied Physics (b) ZnO/ZnS IPL. Reproduced with permission from [37], copyright The Japan Society of Applied (2016). PhysicsFigure (2016). 8. C-V characteristics of (NH4)2S cleaned HfO2/p-InGaAs MOSCAP with (a) and without (b) HenegarZnO/ZnS etIPL. al. Reproduced reported witha novel permission method from to [37],transport copyright the The native Japan oxidesSociety ofaway Applied from Physics the GaAs (2016). Henegarsurface by et ALD al. reportedTiO2 film a[38]. novel In his method study,to a 4 transport nm TiO2 film the nativewere deposited oxides away on native from oxide the GaAs GaAs surfaceat 100 °C firstly. The low temperature were used to limit the native oxide mobility and “clean-up” ◦ by ALD TiOHenegar2 film [et38 al.]. Inreported his study, a novel a 4 nmmethod TiO 2tofilm transport were depositedthe native oxides on native away oxide from GaAsthe GaAs at 100 C reactions. This layer protected the interfacial layer from precursor and air exposure and created a firstly.surface The low by ALD temperature TiO2 film [38]. were In usedhis study, to limit a 4 nm the TiO native2 film oxidewere deposited mobility on and native “clean-up” oxide GaAs reactions. at native oxide concentration gradient in the sample. Secondly, select films were heated for hours at 250 This layer100 °C protected firstly. The the low interfacial temperature layer were from used precursor to limit andthe native air exposure oxide mobility and created and “clean-up” a native oxide °C in the ALD reactor under N2; this thermal activation step was aimed at promoting native oxide concentrationreactions. gradient This layer in protected the sample. the Secondly,interfacial selectlayer from films precursor were heated and air for exposure hours at and 250 created◦C in the a ALD migration into the film. Lastly, a 3 nm TiO2 were deposited at a higher temperature to remove any native oxide concentration gradient in the sample. Secondly, select films were heated for hours at 250 reactornative under oxide N2 ;that this may thermal have activationreached the step surface. was aimedThe XPS at confirmed promoting the native removal oxide of the migration arsenic (as into the °C in the ALD reactor under N2; this thermal activation step was aimed at promoting native oxide film.shown Lastly, in a 3Figure nm TiO 9) and2 were gallium deposited oxides atduring a higher the second temperature layer of to 3 removenm TiO2 any deposition native oxideat 150–250 that may migration into the film. Lastly, a 3 nm TiO2 were deposited at a higher temperature to remove any have°C reachednative when oxide a the large that surface. enough may Thehave thermal XPSreached budget confirmed the wassurface. applied the The removal beforeXPS confirmed ofand/or the arsenicduring the removal the (as deposition. shown of the inarsenic They Figure also(as9 ) and found that the gallium oxides were less mobile and harder to remove than arsenic◦ oxides. galliumshown oxides in Figure during 9) theandsecond gallium layeroxides of during 3 nm the TiO second2 deposition layer of at3 nm 150–250 TiO2 depositionC when at a large150–250 enough thermal°C budgetwhen a large was enough applied thermal before budget and/or was during applied the before deposition. and/or during They the also deposition. found that They the also gallium oxidesfound were that less the mobile gallium and oxides harder were to less remove mobile than and arsenicharder to oxides. remove than arsenic oxides.

Figure 9. Schematic illustrating the experimental steps used to decouple the native oxide transport and removal processes. Reproduced with permission from [38], copyright American Chemical Society (2016).Figure 9. Schematic illustrating the experimental steps used to decouple the native oxide transport Figure 9. Schematic illustrating the experimental steps used to decouple the native oxide transport and removal processes. Reproduced with permission from [38], copyright American Chemical Society and removal processes. Reproduced with permission from [38], copyright American Chemical It(2016). turns out that passivation film depositing by ALD technology provides a quick and reliable Societyway to obtain (2016). low oxygen-contaminate III-V surfaces making these cleaning methods preferable for deviceIt processing. turns out that passivation film depositing by ALD technology provides a quick and reliable Itway turns to obtain out that low passivation oxygen-contaminate ﬁlm depositing III-V surfaces by ALDmaking technology these cleaning provides methods a quickpreferable and for reliable device processing. way to obtain low oxygen-contaminate III-V surfaces making these cleaning methods preferable for device processing. Crystals 2018, 8, 226 9 of 14

Crystals 2018, 8, x FOR PEER REVIEW 9 of 14 2.3. Plasma Treatment and Nitridation 2.3. Plasma Treatment and Nitridation Plasma treatment is the removal of surface oxide through the impact of highly energetic ions +Plasma+ treatment is the removal of surface oxid+e through+ + the impact of highly energetic ions (e.g., Ar ,N [39–41]) or chemical reaction (e.g., NH3 ,H , HS ), then the dangling bonds are further (e.g., Ar+, N+ [39–41]) or chemical reaction (e.g., NH3+, H+, HS+), then the dangling bonds are further saturated. This led to a great reduction of the surface state density. Among various plasma treatments, saturated. This led to a great reduction of the surface state density. Among various plasma N-plasma passivation has gained the most extensive research and been considered a very effective treatments, N-plasma passivation has gained the most extensive research and been considered a very way to get clean GaAs surface by terminating GaAs bonds with GaN [42–44]. effective way to get clean GaAs surface by terminating GaAs bonds with GaN [42–44]. In the report of H. H. Lu et al. [45,46], they treated GaAs MOS Capacitor with N and NH In the report of H. H. Lu et al. [45,46], they treated GaAs MOS Capacitor with N2 and2 NH3 3 ◦ respectivelyrespectively at theat the gas gas ﬂow flow of of 4 4 sccm, sccm, 350 350 °CC andand power of of 120 120 W W in in PECVD PECVD chamber. chamber. Both Both of the of the twotwo plasma-passivated plasma-passivated MOSCAP MOSCAP exhibited exhibited improvedimproved interfacial interfacial and and electrical electrical properties properties compared compared to theto the untreated untreated one, one, andand NH NH3-plasma3-plasma passivated passivated sample sample had the had better the effect better than effect N2-plasma, than N 2which-plasma, whichshowed showed the lower the lower leakage leakage current current density density and in andterface-state interface-state density.density. That may That be for may the be NH for3- the NHplasma3-plasma passivation passivation can canprovide provide additional additional reactive reactive species, species, such as such H atoms as H and atoms NH and radicals, NH radicals,than thanN N2-plasma2-plasma passivation passivation which which provided provided nitrogen nitrogen atoms atoms only, only, so leading so leading to better to better removal removal of the of the elementalelemental As, As, Ga–O Ga–O and and As–O As–O bonds. bonds. Seung-Hwan Kim et al. applied the novel SF6 plasma passivation technique with inserting a Seung-Hwan Kim et al. applied the novel SF6 plasma passivation technique with inserting a ultrathinultrathin ZnO ZnO IPL IPL for for the the metal metal interlayer interlayer semiconductor semiconductor (MIS) (MIS) structure structure to to alleviate alleviate the the interface interface trap trap states [47]. The optimized plasma conditions had a process time of 10 s, gas flow of 10 sccm, states [47]. The optimized plasma conditions had a process time of 10 s, gas ﬂow of 10 sccm, pressure pressure of 5 mTorr, source power of 50 W, and the ALD-ZnO thickness of 1.3 nm. The source/drain of 5 mTorr, source power of 50 W, and the ALD-ZnO thickness of 1.3 nm. The source/drain contact contact resistance testing in Figure 10 showed that the current density of the SF6-treated MIS contact resistance testing in Figure 10 showed that the current density of the SF6-treated MIS contact were were about 4 times and 15 times larger than that of the (NH4)2S-passivated MIS contact and non- aboutpassivated 4 times andMIS 15contact. times largerThe resu thanlt proposed that of the a promising (NH4)2S-passivated non-alloyed MIS S/D contact ohmic andcontact non-passivated for III-V MISMOS. contact. The result proposed a promising non-alloyed S/D ohmic contact for III-V MOS.

FigureFigure 10. 10.(a )(a I-V) I-V characteristics characteristics and and ((bb)) speciﬁcspecific contact resistivity resistivity for for metal metal semiconductor semiconductor (MS) (MS) contactcontact and and MIS MIS contacts. contacts. The The ZnO ZnO is isused used asas interlayerinterlayer and and 1.3 1.3 nm nm ZnO ZnO is isconsidered considered the the optimal optimal thicknessthickness in bothin both MIS MIS contacts. contacts.Insets Insets inin ((aa)) showshow a a band band diagram diagram of of the the MIS MIS contact contact (left) (left) and and a a schematicschematic of of the the electrical electrical measurementsmeasurements of of the the MIS MIS contact contact (right). (right). Reproduced Reproduced with permission with permission from from[47], [47 copyright], copyright IEEE IEEE (2016). (2016).

In the research of Alekseev et al. [48], surface nitridation by hydrazine-sulfide solution were usedIn the for research GaAs nanowires of Alekseev because et al. of [48 the], surfacehigh stability nitridation of Ga–N by hydrazine-sulfidebonds. To avoid micro-etching solution were and used for GaAsprevent nanowires possible becausedamage ofto thethe highNW stabilitysurface ofmorphology, Ga–N bonds. they To used avoid a micro-etchinglow alkaline (pH and ~8.5) prevent possiblehydrazine damage sulfide to the solution NW surface by adding morphology, anhydrous they hydrazine used a low dihydrochloride alkaline (pH ~8.5)(N2H hydrazine4 × 2HCl) into sulfide solutionhydrazine by adding hydrate anhydrous and then introducing hydrazine dihydrochlorideNa2S into the solution (N2H up4 ×to 2HCl)a concentration into hydrazine of 0.01 M. hydrate Figure and then11 introducing shows typical Na μ2-PLS into spectra the solutionmeasured up from to a unpassivated, concentration sulfided, of 0.01 M.and Figure nitrided 11 NWs. shows They typical foundµ-PL spectranitridation measured producing from unpassivated, an essential increase sulfided, in the and NW nitrided conductivity NWs. Theyand the found μ-PL nitridationintensity as producingwell as an essentialevidence of increase surface inpassivation. the NW conductivity Estimations showed and the thatµ-PL the intensitynitride passivation as well as reduced evidence the surface of surface passivation.state density Estimations by a factor showedof 6, the effects that the of nitridethe nitride passivation passivation reduced were also the stable surface under state atmospheric density by a factorambient of 6, the conditions effects of for the six nitride months. passivation were also stable under atmospheric ambient conditions for six months.

Crystals 2018, 8, 226 10 of 14 Crystals 2018, 8, x FOR PEER REVIEW 10 of 14

Crystals 2018, 8, x FOR PEER REVIEW 10 of 14

FigureFigure 11. 11.(a )(aµ) -PLμ-PL intensity intensity distribution distribution overover thethe surface of of nitrided nitrided n-GaAs n-GaAs NW; NW; (b ()b μ) -PLµ-PL spectra spectra taken from the middle section of the nitrided, sulfided, and unpassivated NWs. µA μ-PL spectrum takenFigure from 11. the (a middle) μ-PL intensity section of distribution the nitrided, over sulfided, the surface and unpassivatedof nitrided n-GaAs NWs. NW; A -PL (b) spectrumμ-PL spectra from from a nitrided NW measured after six months of storage in ambient air is also presented. Reproduced a nitridedtaken from NW measuredthe middle after section six monthsof the nitrided, of storage sulfided, in ambient and unpassivated air is also presented. NWs. A Reproduced μ-PL spectrum with with permission from [48], copyright American Chemical Society (2015). permissionfrom a nitrided from [ 48NW],copyright measured Americanafter six months Chemical of storage Society in ambient (2015). air is also presented. Reproduced Krylovwith permission et al. [49] from used [48], oxygen-free copyright American AlN dielec Chemicaltric Societyto reduce (2015). the interface state density and Krylov et al. [49] used oxygen-free AlN dielectric to reduce the interface state density and systematically studied the interface properties of Al2O3/AlN/InGaAs structures with different AlN Krylov et al. [49] used oxygen-free AlN dielectric to reduce the interface state density and systematicallythicknesses. AlN studied and Al the2O interface3 films were properties deposited of by Al plasma2O3/AlN/InGaAs enhanced (2800 structures W) and thermal with different activated AlN systematically studied the interface properties of Al2O3/AlN/InGaAs structures with different AlN thicknesses.ALD techniques AlN and respectively. Al2O3 films Fr wereequency-dependent deposited by plasma capacitance enhanced voltage (2800 measurements W) and thermal shown activated in thicknesses. AlN and Al2O3 films were deposited by plasma enhanced (2800 W) and thermal activated ALDFigure techniques 12 demonstrated respectively. that the Frequency-dependent insertion of the AlN layer capacitance significantly voltage reduced measurements the midgap interface shown in ALD techniques respectively. Frequency-dependent capacitance voltage measurements shown in Figurestates 12 density, demonstrated and the indium that the out- insertiondiffusion of thephenomenon AlN layer observed significantly for Al reduced2O3/InGaAs the midgap structures interface was Figure 12 demonstrated that the insertion of the AlN layer significantly reduced the midgap interface statesprevented density, with and the the insertion indium of out-diffusion AlN on the top phenomenon of InGaAs. observedHigh temperature for Al2O annealing3/InGaAs of structures 500 °C was was states density, and the indium out-diffusion phenomenon observed for Al2O3/InGaAs structures was preventedrequired with for interface the insertion quality of improvement AlN on the top when of InGaAs. thick AlN High layers temperature were used; annealing however, ofstacks 500 ◦withC was prevented with the insertion of AlN on the top of InGaAs. High temperature annealing of 500 °C was requiredultrathin for AlN interface layers quality(~1 nm) improvement can achieve the when same thick interface AlN quality layers werewhile used; using however,only a mild stacks anneal with required for interface quality improvement when thick AlN layers were used; however, stacks with ultrathinof 400 °C. AlN AlN layers film (~1deposited nm) can by achieve ALD[50], the sputtering same interface [51] or quality MOCVD while [52] usingall produced only a mildthe effective anneal of ultrathin AlN layers (~1 nm) can achieve the same interface quality while using only a mild anneal passivation◦ effect. 400ofC. 400 AlN °C. filmAlN depositedfilm deposited by ALDby ALD[50], [50], sputtering sputtering [51 [51]] or or MOCVD MOCVD [[52]52] all produced produced the the effective effective passivationpassivation effect. effect.

Figure 12. Typical capacitance-voltage characteristics at 400 Hz–1 MHz frequency range of (a,b) Au/Cr/Al2O3 (18 nm)/InGaAs; (c,d) Au/Cr/Al2O3 (17 nm)/AlN (1 nm)/InGaAs and (e,f) Au/Cr/Al2O3 (4 Figure 12. Typical capacitance-voltage characteristics at 400 Hz–1 MHz frequency range of (a,b) Figurenm)/AlN 12. (14Typical nm)/InGaAs capacitance-voltage gate stacks measured characteristics after annealing at 400 at 400 Hz–1 °C (a MHz,c,e) and frequency at 500 °C range(b,d,e) of Au/Cr/Al2O3 (18 nm)/InGaAs; (c,d) Au/Cr/Al2O3 (17 nm)/AlN (1 nm)/InGaAs and (e,f) Au/Cr/Al2O3 (4 a bin N2 for 5 min. Reproduced with permissicond from [49], copyright AIP (2017). ( , nm)/AlN) Au/Cr/Al (14 nm)/InGaAs2O3 (18 nm)/InGaAs; gate stacks measured ( , ) Au/Cr/Al after annealing2O3 (17 at 400 nm)/AlN °C (a,c,e) (1and nm)/InGaAs at 500 °C (b,d, ande) (e,f) Au/Cr/Al O (4 nm)/AlN (14 nm)/InGaAs gate stacks measured after annealing at 400 ◦C(a,c,e) in N2 for 5 min.2 3 Reproduced with permission from [49], copyright AIP (2017). Mary Edmonds◦ et al. [53] reported a saturated Si–Hx capping layer deposited on InGaAs(001) and at 500 C(b,d,e) in N2 for 5 min. Reproduced with permission from [49], copyright AIP (2017). surface in HV-ALD chamber by using a single Si3H8 ALD precursor at 250 °C or cyclically dosing Mary Edmonds et al. [53] reported a saturated Si–Hx capping layer deposited on InGaAs(001) Si2Cl6 and atomic hydrogen at 350 °C. They proved that a Si–Hx monolayer can remove all the surfaceMary in Edmonds HV-ALD et chamber al. [53] reportedby using aa saturatedsingle Si3H Si–Hx8 ALD capping precursor layer at 250 deposited °C or cyclically on InGaAs(001) dosing dangling bonds and leave a charge balanced InGaAs surface. ◦ surfaceSi2Cl6 in and HV-ALD atomic chamberhydrogen by at using 350 °C. asingle They Siproved3H8 ALD that precursora Si–Hx monolayer at 250 C can or cyclicallyremove all dosing the dangling bonds and leave a charge balanced InGaAs surface.

Crystals 2018, 8, 226 11 of 14

◦ Si2Cl6 and atomic hydrogen at 350 C. They proved that a Si–Hx monolayer can remove all the dangling bonds and leave a charge balanced InGaAs surface. Ultrathin passivation film especially nitride film deposition with plasma pretreatment can provide excellent contamination removal efficiency and a stable protection, but the plasma energy and film thickness should be carefully optimized to gain less roughness and a less stressed surface. Previous technology focused on III-V semiconductor surface passivation and the testing methods are listed in Table1.

Table 1. Summary of all reviewed passivation methods. (×: does not apply).

Material Passivation Method Testing Advantages Disadvantages Reference XPS, AFM [16,22] ODT XPS, PL [17,20] Clean and smooth HDT ATR-FTIR, PLLong processing time [18] surfaces; Simple ODT+(NH4)2S SEM, XPS [6]

CnH2n+1, n ≥ 12 XPS [19,21]

C-V, AFM Possible suboxides [26,27] ALD-Al2O3 In situ processing Drain Currentresidues [25] Environmental Sputter C-V × [51] friendly AlN MOCVD SIMS, C-V [52] GaAs Well Reliability Complex equipment MOCVD C-V, XPS, FIRTEM [33] ALD XPS, SIMS, C-V In situ processing [34] ZnO × +ZnS/ZnO C-V, XPS Clean surfaces [37]

+Annealing C-V, XPS Simple AsOx residues [7]

+LaON Well removal of [46] NH3 Plasma C-V, I-V, XPS × +YON Ga/As oxides. [45]

N2-H2 plasma XPS Lower As traps Need to be carefully [39] optimized N2 plasma XPS Well stability [40–44] Well thermal SF +ZnO XPS, C-V Possible GaF residues [47] 6 stability 3

ALD-TiO2+Anneal FTIR, XRD Extremely clean Complex [38]

N2H4+Na2S µ-PL Stable effects × [48] Inefﬁcient passivate the (NH ) S+Annealing C-V Simple [12] 4 2 states close to VBM High temperature -AlN TEM, C-V, XPS In situ processing [49,50] annealing is required InGaAs High surface -SiHx, Si XPS STM × [32,53] ALD uniformity

-Al2O3 C-V, XPS [24,29,31] -TiO /Al O XPS, C-V, SIMSIn situ processing Possible suboxides [8] 2 2 3 residues ALD-Al2O3 AFM, XPS, C-V [28] GaSb (NH4)2S+2-propanol XPS, PL Low C signal Possible oxide residues [14] ALD-ZnS XPS, TEM, C-V [36] In situ processing × InP N2 Plasma C-V [3] ODT XPS, AFM [15] Simple Long processing time InSb (NH4)2S XPS, AFM [13]

3. Conclusions This paper reviewed the present status of surface passivation technology for III-V semiconductors, which mainly focused on sulfur passivation by long-chain alkylthiol SAM, passivation film deposition by ALD technology, plasma treatment and nitridation. The passivation methods discussed in this paper Crystals 2018, 8, 226 12 of 14 were primarily about the microelectronics field, but these methods were also used in optoelectronic devices, which led to a low contamination III-V surface. Long-chain alkylthiol SAM was the simplest and least expensive method, but had the drawback of being time-consuming which limit its piratical application in device processing. Plasma treatment and nitridation need to optimize the plasma energy and film thickness carefully to gain less roughness and a less stressed surface. By contrast, ALD passivation technology was efficient and easier to use, possibly making the integration of ALD passivation into the device processing easier. Many of these passivation techniques worked in the laboratory, and to improve the repeatability in the field will be the major problem.

Acknowledgments: This research was funded by the Jilin Province Science and Technology Development Plan (No. 20170520169JH, 20160204069GX and 20170101111JC), National Natural Science Foundation of China (No. 51672103 and 61774024) and National Key R&D Project (No. 2016YFB0401103 and 2017YFB0405100). Thank Selma Xin for editing this manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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