Brief Review of Surface Passivation on III-V Semiconductor

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.


Introduction
III-V semiconductor materials, such as GaAs, InGaAs, GaSb, InP, InAs, InSb, have high surface state density (>10 13 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][4][5].
It is well known that the thermal SiO 2 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 As 2 O 3 and Ga 2 O 3 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 Ga 2 O 3 is thermodynamically favored, leaving bare arsenic atoms embedded within the oxide near the oxide/GaAs interface, and the As 2 O 3 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, 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.

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 (Ga x S y ) and As (As x S y ) are future formed through chemical reactions.The S-bonds provide protection against further oxidation and reduce the density of surface states.Among various sulfurs, (NH 4 ) 2 S 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 (NH 4 ) 2 S dissolved in water versus isopropanol, and found almost no residual oxygen was left on the GaSb passivated with (NH 4 ) 2 S in isopropanol, while Ga 2 O 3 was found using aqueous (NH 4 ) 2 S 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.(NH 4 ) 2 S passivation had been able to reduce the interface states density markedly, but obvious drawbacks were the limited chemical stability of the (NH 4 ) 2 S 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][16][17][18][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, CH 3 [CH 2 ] 17 SH) 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.Figure 1a 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 H 2 O vapor with a dose equivalent to 25 H 2 O 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. Figure 1b,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 Figure 1d.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.Pablo Mancheno-Posso et al. reported on the re-oxidation 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 Ga-2p and As-2p XPS spectra after passivation by six different alkanethiols and 3/30 min of ambient exposure.We can see that, the longer alkanethiol molecule, the better stability of sulfur-passivation (for lower Ga-O and As-O signal but stronger As-S signal).In his report, the ET-20C (1-eicosanethiol, C20H41-SH) had the best passivation stability for the formation of the most ordered and highestpacking density layer, and restrained the surface re-oxidation of ambient exposure for at least 30 min.Pablo Mancheno-Posso et al. reported on the re-oxidation of the GaAs surface in the atmosphere after passivation by six alkanethiols (C n H 2n+1 -SH where n = 3, 6, 8, 12, 18, 20) [21].Figure 2 shows the Ga-2p and As-2p XPS spectra after passivation by six different alkanethiols and 3/30 min of ambient exposure.We can see that, the longer alkanethiol molecule, the better stability of sulfur-passivation (for lower Ga-O and As-O signal but stronger As-S signal).In his report, the ET-20C (1-eicosanethiol, C 20 H 41 -SH) had the best passivation stability for the formation of the most ordered and highest-packing density layer, and restrained the surface re-oxidation of ambient exposure for at least 30 min.Pablo Mancheno-Posso et al. reported on the re-oxidation 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 Ga-2p and As-2p XPS spectra after passivation by six different alkanethiols and 3/30 min of ambient exposure.We can see that, the longer alkanethiol molecule, the better stability of sulfur-passivation (for lower Ga-O and As-O signal but stronger As-S signal).In his report, the ET-20C (1-eicosanethiol, C20H41-SH) had the best passivation stability for the formation of the most ordered and highestpacking density layer, and restrained the surface re-oxidation of ambient exposure for at least 30 min.Similar results was reported by Loredana Preda et al.They investigated the effects of the SAMs of 1,8-octanedithiol (ODT) and biphenyl 4,4 -dithiol (BPDT) on the electronic and chemical properties of p-GaAs with As terminated semiconductor surface [22].They verified that the hydrocarbon moiety had an important effect on tailoring the GaAs surface properties.The XPS spectra (Figure 3) showed that both ODT and BPDT bind to GaAs via the thiol group.Compared to the BPDT layer, the ODT layer provided better passivation protection against the reoxidation with ambient exposure.That was because the ODT molecule bound with both thiol groups to the GaAs surface, while the BPDT molecule bound with only one thiol group to the GaAs surface.
Similar results was reported by Loredana Preda et al.They investigated the effects of the SAMs of 1,8-octanedithiol (ODT) and biphenyl 4,4′-dithiol (BPDT) on the electronic and chemical properties of p-GaAs with As terminated semiconductor surface [22].They verified that the hydrocarbon moiety had an important effect on tailoring the GaAs surface properties.The XPS spectra (Figure 3) showed that both ODT and BPDT bind to GaAs via the thiol group.Compared to the BPDT layer, the ODT layer provided better passivation protection against the reoxidation with ambient exposure.That was because the ODT molecule bound with both thiol groups to the GaAs surface, while the BPDT molecule bound with only one thiol group to the GaAs surface.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 (for hours) to obtain clean surfaces and well-ordered SAM, which limit its practical application in device processing.

Passivation Film Depositing by Atomic Layer Deposition (ALD) Technique
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-MOSFETs, the high interface defect density between high-k gate and III-V semiconductor hampers the development of III-V MOSFETs.The Fermi level pinning phenomenon, resulting from high interface defect density at the high-k/semiconductor interface, could only produce MOSFETs with very poor control of carrier flow and various instabilities such as hysteresis and drain current drift.Therefore, interface passivation became very significant for realizing a high-performance of III-V MOSFETs.
ALD is the most advanced technique for high-k gate material deposition because it offers outstanding film thickness control, pinhole-free and conformal deposition around complex structure.Using ALD for interface passivation prior to gate film deposition has the advantage of in situ processing without any vacuum breaking, which further improve the interface quality, and the "selfcleaning" process [24][25][26][27][28] can reduce and/or removal of native oxides of III-V semiconductors by surface reactions with the ALD precursors during the initial stages of deposition.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 (for hours) to obtain clean surfaces and well-ordered SAM, which limit its practical application in device processing.

Passivation Film Depositing by Atomic Layer Deposition (ALD) Technique
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-MOSFETs, the high interface defect density between high-k gate and III-V semiconductor hampers the development of III-V MOSFETs.The Fermi level pinning phenomenon, resulting from high interface defect density at the high-k/semiconductor interface, could only produce MOSFETs with very poor control of carrier flow and various instabilities such as hysteresis and drain current drift.Therefore, interface passivation became very significant for realizing a high-performance of III-V MOSFETs.
ALD is the most advanced technique for high-k gate material deposition because it offers outstanding film thickness control, pinhole-free and conformal deposition around complex structure.Using ALD for interface passivation prior to gate film deposition has the advantage of in situ processing without any vacuum breaking, which further improve the interface quality, and the "self-cleaning" process [24][25][26][27][28] can reduce and/or removal of native oxides of III-V semiconductors by surface reactions with the ALD precursors during the initial stages of deposition.[29].
The interaction of Al (CH 3 ) 3 with native oxides of GaAs surfaces leads to most of the surface arsenic oxides and a significant portion the gallium oxide being consumed.It seems that just a half cycle of Al 2 O 3 could passivate the majority of the defects [24], but according to the research of Kent et al. [30], dissociative chemisorption of Al(CH 3 ) 3 was found to also lead to the formation of an ordered monolayer of dimethyl aluminum that not only passivated the As dangling bonds but also resulted in the formation of metal-metal bonds that pinned the Fermi level, so dosing with H 2 O or O 2 was required to passivate the surface and unpin the Fermi level by inserting O atoms in the metallic bond.
Generally, the arsenic oxides were energetically the easiest to remove and the indium oxides were hardest.Gang He et al. reported improving the interface quality in HfTiO/InGaAs metal oxide semiconductor capacitor (MOSCAP) by introducing ALD-Al 2 O 3 passivation film [31].XPS measurements shown in Figure 4  Surface passivation of III-V semiconductors using ALD-Al2O3 had been researched the most widely.As2O3 first interacts with Al (CH3)3 to be transformed to arsenic and Al2O3.The arsenic then reacts with H2O to become As2O5, which then reacts again with the next incoming pulse of Al (CH3)3 to become As and Al2O3.Most of the arsenic oxides evaporated since the melting point of As2O5 is low around 280 °C with a small amount of residual As2O5 left on the skin-depth top of Al2O3 [29].
The interaction of Al (CH3)3 with native oxides of GaAs surfaces leads to most of the surface arsenic oxides and a significant portion the gallium oxide being consumed.It seems that just a half cycle of Al2O3 could passivate the majority of the defects [24], but according to the research of Kent et al. [30], dissociative chemisorption of Al(CH3)3 was found to also lead to the formation of an ordered monolayer of dimethyl aluminum that not only passivated the As dangling bonds but also resulted in the formation of metal-metal bonds that pinned the Fermi level, so dosing with H2O or O2 was required to passivate the surface and unpin the Fermi level by inserting O atoms in the metallic bond.
Generally, the arsenic oxides were energetically the easiest to remove and the indium oxides were hardest.Gang He et al. reported improving the interface quality in HfTiO/InGaAs metal oxide semiconductor capacitor (MOSCAP) by introducing ALD-Al2O3 passivation film [31].XPS measurements shown in Figure 4 had confirmed that, after 20 cycles of ALD-Al2O3 passivation, none of the Ga-O and As-O bonds were detected, and significant suppressing the formation of In-O bond had been achieved.Electrical measurements of MOSCAP with HfTiO/4 nm-Al2O3/InGaAs structure indicated improved electrical characteristics were achieved.Although ALD of Al2O3 can suppress the formation of the As-O and Ga-O bonds, it seems that the self-cleaning ALD process was not sufficient to deposit a gate dielectric with a high-quality interface.For instance in Figure 6 the electrical properties of a stack consisting of 10 nm ALD Al2O3 on GaAs (100) were compared for two different surface preparations: HF etch directly and 1.2 nm Si passivation layer depositing by PECVD.In the latter case the frequency of the C-V curves was significantly reduced (as shown in Figure 6b,d).Si passivation layer at the interface resulted in gettering of oxygen from the substrate to result in a partially oxidized Si layer and the removal of all As-oxides (not shown here), as well as the higher 3+ oxidation state of Ga (as shown in Figure 6a,c).We also noted that the Ga2O bonding arrangement remains for the two interfaces suggesting that it was not the species primarily responsible for Fermi-level pinning.Although ALD of Al 2 O 3 can suppress the formation of the As-O and Ga-O bonds, it seems that the self-cleaning ALD process was not sufficient to deposit a gate dielectric with a high-quality interface.For instance in Figure 6 the electrical properties of a stack consisting of 10 nm ALD Al 2 O 3 on GaAs (100) were compared for two different surface preparations: HF etch directly and 1.2 nm Si passivation layer depositing by PECVD.In the latter case the frequency of the C-V curves was significantly reduced (as shown in Figure 6b,d).Si passivation layer at the interface resulted in gettering of oxygen from the substrate to result in a partially oxidized Si layer and the removal of all As-oxides (not shown here), as well as the higher 3+ oxidation state of Ga (as shown in Figure 6a,c).We also noted that the Ga 2 O bonding arrangement remains for the two interfaces suggesting that it was not the species primarily responsible for Fermi-level pinning.Although ALD of Al2O3 can suppress the formation of the As-O and Ga-O bonds, it seems that the self-cleaning ALD process was not sufficient to deposit a gate dielectric with a high-quality interface.For instance in Figure 6 the electrical properties of a stack consisting of 10 nm ALD Al2O3 on GaAs (100) were compared for two different surface preparations: HF etch directly and 1.2 nm Si passivation layer depositing by PECVD.In the latter case the frequency of the C-V curves was significantly reduced (as shown in Figure 6b,d).Si passivation layer at the interface resulted in gettering of oxygen from the substrate to result in a partially oxidized Si layer and the removal of all As-oxides (not shown here), as well as the higher 3+ oxidation state of Ga (as shown in Figure 6a,c).We also noted that the Ga2O bonding arrangement remains for the two interfaces suggesting that it was not the species primarily responsible for Fermi-level pinning.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 with 2 nm ALD ZnO interface passivation layer exhibited higher accumulation capacitance and a very small C-V dispersion and hysteresis.They explained the result that the presence of ZnO on n-GaAs can act as the oxygen reaction barrier and effectively reduces the Ga and As oxides, which suppresses the formation of low k interfacial layer on the GaAs surface, thus improving the interface quality and leading to higher accumulation capacitance.
ZnS has better thermochemical stability and higher band gap energy than ZnO [36], which makes it an attractive and promising candidate as an interface passivation layer (IPL) material in III-V based devices.Lucero et al. used ALD to format a ZnO/ZnS IPL by converting an (NH4)2S cleaned p-In0.53Ga0.47Aswith diethylzinc and water [37].Diethylzinc reacted with S and O on the InGaAs, ZnO wideband material has attracted increasing attention in the passivation field.Although O atoms exist in ZnO, it is nearly impossible for ZnO to react with Ga or As for the high exciting binding energy of Zn-O bonds [33].Young-Chul Byun et al. investigated ultrathin ZnO film by the ALD method as interface passivation layer between HfO 2 gate and GaAs substrate [34].The Ga-O bond/As segregation near the interface and the trapping of carriers near the valence band edge was significantly suppressed, greatly improving the C-V characteristics on p-type GaAs substrates, as shown in Figure 7. ZnO wideband material has attracted increasing attention in the passivation field.Although O atoms exist in ZnO, it is nearly impossible for ZnO to react with Ga or As for the high exciting binding energy of Zn-O bonds [33].Young-Chul Byun et al. investigated ultrathin ZnO film by the ALD method as interface passivation layer between HfO2 gate and GaAs substrate [34].The Ga-O bond/As segregation near the interface and the trapping of carriers near the valence band edge was significantly suppressed, greatly improving the C-V characteristics on p-type GaAs substrates, as shown in Figure 7. 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 with 2 nm ALD ZnO interface passivation layer exhibited higher accumulation capacitance and a very small C-V dispersion and hysteresis.They explained the result that the presence of ZnO on n-GaAs can act as the oxygen reaction barrier and effectively reduces the Ga and As oxides, which suppresses the formation of low k interfacial layer on the GaAs surface, thus improving the interface quality and leading to higher accumulation capacitance.
ZnS has better thermochemical stability and higher band gap energy than ZnO [36], which makes it an attractive and promising candidate as an interface passivation layer (IPL) material in III-V based devices.Lucero et al. used ALD to format a ZnO/ZnS IPL by converting an (NH4)2S cleaned p-In0.53Ga0.47Aswith diethylzinc and water [37].Diethylzinc reacted with S and O on the InGaAs, Similar results were reported by Liu Chen et al. [35], who found deposition of ZnO prior to Al 2 O 3 gate materials deposition directly can further reduce the interface trap density.The n-GaAs MOSCAP with 2 nm ALD ZnO interface passivation layer exhibited higher accumulation capacitance and a very small C-V dispersion and hysteresis.They explained the result that the presence of ZnO on n-GaAs can act as the oxygen reaction barrier and effectively reduces the Ga and As oxides, which suppresses the formation of low k interfacial layer on the GaAs surface, thus improving the interface quality and leading to higher accumulation capacitance.
ZnS has better thermochemical stability and higher band gap energy than ZnO [36], which makes it an attractive and promising candidate as an interface passivation layer (IPL) material in III-V based devices.Lucero et al. used ALD to format a ZnO/ZnS IPL by converting an (NH 4 ) 2 S cleaned p-In 0.53 Ga 0.47 As with diethylzinc and water [37].Diethylzinc reacted with S and O on the InGaAs, reducing Ga 3+ and As 3+ to lower oxidation states.Measurements of C-V characteristics of HfO 2 -InGaAs MOSCAP in Figure 8 showed that the ZnO/ZnS bilayer nearly eliminated frequency dispersion and hysteresis.
reducing Ga 3+ and As 3+ 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.Henegar et al. reported a novel method to transport the native oxides away from the GaAs surface by ALD TiO2 film [38].In his study, a 4 nm TiO2 film were deposited on native oxide GaAs at 100 °C firstly.The low temperature were used to limit the native oxide mobility and "clean-up" reactions.This layer protected the interfacial layer from precursor and air exposure and created a native oxide concentration gradient in the sample.Secondly, select films were heated for hours at 250 °C in the ALD reactor under N2; this thermal activation step was aimed at promoting native oxide migration into the film.Lastly, a 3 nm TiO2 were deposited at a higher temperature to remove any native oxide that may have reached the surface.The XPS confirmed the removal of the arsenic (as shown in Figure 9) and gallium oxides during the second layer of 3 nm TiO2 deposition at 150-250 °C when a large enough thermal budget was applied before and/or during the deposition.They also found that the gallium oxides were less mobile and harder to remove than arsenic oxides.It turns out that passivation film depositing by ALD technology provides a quick and reliable way to obtain low oxygen-contaminate III-V surfaces making these cleaning methods preferable for device processing.Henegar et al. reported a novel method to transport the native oxides away from the GaAs surface by ALD TiO 2 film [38].In his study, a 4 nm TiO 2 film were deposited on native oxide GaAs at 100 • C firstly.The low temperature were used to limit the native oxide mobility and "clean-up" reactions.This layer protected the interfacial layer from precursor and air exposure and created a native oxide concentration gradient in the sample.Secondly, select films were heated for hours at 250 • C in the ALD reactor under N 2 ; this thermal activation step was aimed at promoting native oxide migration into the film.Lastly, a 3 nm TiO 2 were deposited at a higher temperature to remove any native oxide that may have reached the surface.The XPS confirmed the removal of the arsenic (as shown in Figure 9) and gallium oxides during the second layer of 3 nm TiO 2 deposition at 150-250 • C when a large enough thermal budget was applied before and/or during the deposition.They also found that the gallium oxides were less mobile and harder to remove than arsenic oxides.reducing Ga 3+ and As 3+ 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.Henegar et al. reported a novel method to transport the native oxides away from the GaAs surface by ALD TiO2 film [38].In his study, a 4 nm TiO2 film were deposited on native oxide GaAs at 100 °C firstly.The low temperature were used to limit the native oxide mobility and "clean-up" reactions.This layer protected the interfacial layer from precursor and air exposure and created a native oxide concentration gradient in the sample.Secondly, select films were heated for hours at 250 °C in the ALD reactor under N2; this thermal activation step was aimed at promoting native oxide migration into the film.Lastly, a 3 nm TiO2 were deposited at a higher temperature to remove any native oxide that may have reached the surface.The XPS confirmed the removal of the arsenic (as shown in Figure 9) and gallium oxides during the second layer of 3 nm TiO2 deposition at 150-250 °C when a large enough thermal budget was applied before and/or during the deposition.They also found that the gallium oxides were less mobile and harder to remove than arsenic oxides.It turns out that passivation film depositing by ALD technology provides a quick and reliable way to obtain low oxygen-contaminate III-V surfaces making these cleaning methods preferable for device processing.It turns out that passivation film depositing by ALD technology provides a quick and reliable way to obtain low oxygen-contaminate III-V surfaces making these cleaning methods preferable for device processing.

Plasma Treatment and Nitridation
Plasma treatment is the removal of surface oxide through the impact of highly energetic ions (e.g., Ar + , N + [39][40][41]) or chemical reaction (e.g., NH 3 + , H + , HS + ), then the dangling bonds are further saturated.This led to a great reduction of the surface state density.Among various plasma treatments, N-plasma passivation has gained the most extensive research and been considered a very effective way to get clean GaAs surface by terminating GaAs bonds with GaN [42][43][44].
In the report of H. H. Lu et al. [45,46], they treated GaAs MOS Capacitor with N 2 and NH 3 respectively at the gas flow of 4 sccm, 350 • C and power of 120 W in PECVD chamber.Both of the two plasma-passivated MOSCAP exhibited improved interfacial and electrical properties compared to the untreated one, and NH 3 -plasma passivated sample had the better effect than N 2 -plasma, which showed the lower leakage current density and interface-state density.That may be for the NH 3 -plasma passivation can provide additional reactive species, such as H atoms and NH radicals, than N 2 -plasma passivation which provided nitrogen atoms only, so leading to better removal of the elemental As, Ga-O and As-O bonds.
Seung-Hwan Kim et al. applied the novel SF 6 plasma passivation technique with inserting a ultrathin ZnO IPL for the metal interlayer semiconductor (MIS) structure to alleviate the interface trap states [47].The optimized plasma conditions had a process time of 10 s, gas flow of 10 sccm, pressure of 5 mTorr, source power of 50 W, and the ALD-ZnO thickness of 1.3 nm.The source/drain contact resistance testing in Figure 10 showed that the current density of the SF 6 -treated MIS contact were about 4 times and 15 times larger than that of the (NH 4 ) 2 S-passivated MIS contact and non-passivated MIS contact.The result proposed a promising non-alloyed S/D ohmic contact for III-V MOS.

Plasma Treatment and Nitridation
Plasma treatment is the removal of surface oxide through the impact of highly energetic ions (e.g., Ar + , N + [39][40][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, N-plasma passivation has gained the most extensive research and been considered a very effective way to get clean GaAs surface by terminating GaAs bonds with GaN [42][43][44].
In the report of H. H. Lu et al. [45,46], they treated GaAs MOS Capacitor with N2 and NH3 respectively at the gas flow of 4 sccm, 350 °C and power of 120 W in PECVD chamber.Both of the two plasma-passivated MOSCAP exhibited improved interfacial and electrical properties compared to the untreated one, and NH3-plasma passivated sample had the better effect than N2-plasma, which showed the lower leakage current density and interface-state density.That may be for the NH3plasma passivation can provide additional reactive species, such as H atoms and NH radicals, than N2-plasma passivation which provided nitrogen atoms only, so leading to better removal of the elemental As, Ga-O and As-O bonds.
Seung-Hwan Kim et al. applied the novel SF6 plasma passivation technique with inserting a ultrathin ZnO IPL for the metal interlayer semiconductor (MIS) structure to alleviate the interface trap states [47].The optimized plasma conditions had a process time of 10 s, gas flow of 10 sccm, pressure of 5 mTorr, source power of 50 W, and the ALD-ZnO thickness of 1.3 nm.The source/drain contact resistance testing in Figure 10 showed that the current density of the SF6-treated MIS contact were about 4 times and 15 times larger than that of the (NH4)2S-passivated MIS contact and nonpassivated MIS contact.The result proposed a promising non-alloyed S/D ohmic contact for III-V MOS.In the research of Alekseev et al. [48], surface nitridation by hydrazine-sulfide solution were used for GaAs nanowires because of the high stability of Ga-N bonds.To avoid micro-etching and prevent possible damage to the NW surface morphology, they used a low alkaline (pH ~8.5) hydrazine sulfide solution by adding anhydrous hydrazine dihydrochloride (N2H4 × 2HCl) into hydrazine hydrate and then introducing Na2S into the solution up to a concentration of 0.01 M. Figure 11 shows typical μ-PL spectra measured from unpassivated, sulfided, and nitrided NWs.They found nitridation producing an essential increase in the NW conductivity and the μ-PL intensity as well as evidence of surface passivation.Estimations showed that the nitride passivation reduced the surface state density by a factor of 6, the effects of the nitride passivation were also stable under atmospheric ambient conditions for six months.In the research of Alekseev et al. [48], surface nitridation by hydrazine-sulfide solution were used for GaAs nanowires because of the high stability of Ga-N bonds.To avoid micro-etching and prevent possible damage to the NW surface morphology, they used a low alkaline (pH ~8.5) hydrazine sulfide solution by adding anhydrous hydrazine dihydrochloride (N 2 H 4 × 2HCl) into hydrazine hydrate and then introducing Na 2 S into the solution up to a concentration of 0.01 M. Figure 11 shows typical µ-PL spectra measured from unpassivated, sulfided, and nitrided NWs.They found nitridation producing an essential increase in the NW conductivity and the µ-PL intensity as well as evidence of surface passivation.Estimations showed that the nitride passivation reduced the surface state density by a factor of 6, the effects of the nitride passivation were also stable under atmospheric ambient conditions for six months.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 thicknesses.AlN and Al2O3 films were deposited by plasma enhanced (2800 W) and thermal activated ALD techniques respectively.Frequency-dependent capacitance voltage measurements shown in Figure 12 demonstrated that the insertion of the AlN layer significantly reduced the midgap interface states density, and the indium out-diffusion phenomenon observed for Al2O3/InGaAs structures was prevented with the insertion of AlN on the top of InGaAs.High temperature annealing of 500 °C was required for interface quality improvement when thick AlN layers were used; however, stacks with ultrathin AlN layers (~1 nm) can achieve the same interface quality while using only a mild anneal of 400 °C.AlN film deposited by ALD [50], sputtering [51] or MOCVD [52] all produced the effective passivation effect.Mary Edmonds et al. [53] reported a saturated Si-Hx capping layer deposited on InGaAs(001) surface in HV-ALD chamber by using a single Si3H8 ALD precursor at 250 °C or cyclically dosing 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.C was required for interface quality improvement when thick AlN layers were used; however, stacks with ultrathin AlN layers (~1 nm) can achieve the same interface quality while using only a mild anneal of 400 • C. AlN film deposited by ALD [50], sputtering [51] or MOCVD [52] all produced the effective passivation effect.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 thicknesses.AlN and Al2O3 films were deposited by plasma enhanced (2800 W) and thermal activated ALD techniques respectively.Frequency-dependent capacitance voltage measurements shown in Figure 12 demonstrated that the insertion of the AlN layer significantly reduced the midgap interface states density, and the indium out-diffusion phenomenon observed for Al2O3/InGaAs structures was prevented with the insertion of AlN on the top of InGaAs.High temperature annealing of 500 °C was required for interface quality improvement when thick AlN layers were used; however, stacks with ultrathin AlN layers (~1 nm) can achieve the same interface quality while using only a mild anneal of 400 °C.AlN film deposited by ALD [50], sputtering [51] or MOCVD [52] all produced the effective passivation effect.Mary Edmonds et al. [53] reported a saturated Si-Hx capping layer deposited on InGaAs(001) surface in HV-ALD chamber by using a single Si3H8 ALD precursor at 250 °C or cyclically dosing 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.Mary Edmonds et al. [53] reported a saturated Si-Hx capping layer deposited on InGaAs(001) surface in HV-ALD chamber by using a single Si 3 H 8 ALD precursor at 250 • C or cyclically dosing Si 2 Cl 6 and atomic hydrogen at 350 • C.They proved that a Si-H x 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 Table 1.

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 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.

Figure 1 .
Figure 1.GaAs surface after thermal desorption of the sacrificial SAM.(a) The overview SRPES 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 indicates the absence of strong band bending near the surface.Reproduced with permission from [20], copyright American Chemical Society (2016).

Figure 1 .
Figure 1.GaAs surface after thermal desorption of the sacrificial SAM.(a) The overview SRPES 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 indicates the absence of strong band bending near the surface.Reproduced with permission from [20], copyright American Chemical Society (2016).

Figure 1 .
Figure 1.GaAs surface after thermal desorption of the sacrificial SAM.(a) The overview SRPES 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 indicates the absence of strong band bending near the surface.Reproduced with permission from [20], copyright American Chemical Society (2016).
Surface passivation of III-V semiconductors using ALD-Al 2 O 3 had been researched the most widely.As 2 O 3 first interacts with Al (CH 3 ) 3 to be transformed to arsenic and Al 2 O 3 .The arsenic then reacts with H 2 O to become As 2 O 5 , which then reacts again with the next incoming pulse of Al (CH 3 ) 3 to become As and Al 2 O 3 .Most of the arsenic oxides evaporated since the melting point of As 2 O 5 is low around 280 • C with a small amount of residual As 2 O 5 left on the skin-depth top of Al 2 O 3 had confirmed that, after 20 cycles of ALD-Al 2 O 3 passivation, none of the Ga-O and As-O bonds were detected, and significant suppressing the formation of In-O bond had been achieved.Electrical measurements of MOSCAP with HfTiO/4 nm-Al 2 O 3 /InGaAs structure indicated improved electrical characteristics were achieved.Crystals 2018, 8, x FOR PEER REVIEW 5 of 14

Figure 6 .
Figure 6.(a) Ex situ XPS analysis after HF-last GaAs with overlying 1.0 nm Al2O3 layer by ALD, (b) C-V characteristics of (a) after an additional 9.0 nm Al2O3 deposition, (c) ex situ XPS analysis after HFlast GaAs with overlying 1.2 nm Si by PECVD and 1.0 nm Al2O3 layer by ALD, and (d) C-V characteristics of (c) after an additional 9.0 nm Al2O3 deposition.Reproduced with permission from [32], copyright Elsevier B. V. (2009).ZnO wideband material has attracted increasing attention in the passivation field.Although O atoms exist in ZnO, it is nearly impossible for ZnO to react with Ga or As for the high exciting binding energy of Zn-O bonds[33].Young-Chul Byun et al. investigated ultrathin ZnO film by the ALD method as interface passivation layer between HfO2 gate and GaAs substrate[34].The Ga-O bond/As segregation near the interface and the trapping of carriers near the valence band edge was significantly suppressed, greatly improving the C-V characteristics on p-type GaAs substrates, as shown in Figure7.

Figure 6 .
Figure 6.(a) Ex situ XPS analysis after HF-last GaAs with overlying 1.0 nm Al 2 O 3 layer by ALD, (b) C-V characteristics of (a) after an additional 9.0 nm Al 2 O 3 deposition, (c) ex situ XPS analysis after HF-last GaAs with overlying 1.2 nm Si by PECVD and 1.0 nm Al 2 O 3 layer by ALD, and (d) C-V characteristics of (c) after an additional 9.0 nm Al 2 O 3 deposition.Reproduced with permission from [32], copyright Elsevier B. V. (2009).

Crystals 2018, 8 , 14 Figure 6 .
Figure 6.(a) Ex situ XPS analysis after HF-last GaAs with overlying 1.0 nm Al2O3 layer by ALD, (b) C-V characteristics of (a) after an additional 9.0 nm Al2O3 deposition, (c) ex situ XPS analysis after HFlast GaAs with overlying 1.2 nm Si by PECVD and 1.0 nm Al2O3 layer by ALD, and (d) C-V characteristics of (c) after an additional 9.0 nm Al2O3 deposition.Reproduced with permission from [32], copyright Elsevier B. V. (2009).

Figure 9 .
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 .
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 .
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 10 .
Figure 10.(a) I-V characteristics and (b) specific contact resistivity for metal semiconductor (MS) contact and MIS contacts.The ZnO is used as interlayer and 1.3 nm ZnO is considered the optimal thickness in both MIS contacts.Insets in (a) show a band diagram of the MIS contact (left) and a schematic of the electrical measurements of the MIS contact (right).Reproduced with permission from [47], copyright IEEE (2016).

Figure 10 .
Figure 10.(a) I-V characteristics and (b) specific contact resistivity for metal semiconductor (MS) contact and MIS contacts.The ZnO is used as interlayer and 1.3 nm ZnO is considered the optimal thickness in both MIS contacts.Insets in (a) show a band diagram of the MIS contact (left) and a schematic of the electrical measurements of the MIS contact (right).Reproduced with permission from [47], copyright IEEE (2016).

Figure 11 .
Figure 11.(a) μ-PL intensity distribution over the surface of nitrided n-GaAs NW; (b) μ-PL spectra taken from the middle section of the nitrided, sulfided, and unpassivated NWs.A μ-PL spectrum from a nitrided NW measured after six months of storage in ambient air is also presented.Reproduced with permission from [48], copyright American Chemical Society (2015).

Figure 11 .
Figure 11.(a) µ-PL intensity distribution over the surface of nitrided n-GaAs NW; (b) µ-PL spectra taken from the middle section of the nitrided, sulfided, and unpassivated NWs.A µ-PL spectrum from a nitrided NW measured after six months of storage in ambient air is also presented.Reproduced with permission from [48], copyright American Chemical Society (2015).

Crystals 2018, 8 , 14 Figure 11 .
Figure 11.(a) μ-PL intensity distribution over the surface of nitrided n-GaAs NW; (b) μ-PL spectra taken from the middle section of the nitrided, sulfided, and unpassivated NWs.A μ-PL spectrum from a nitrided NW measured after six months of storage in ambient air is also presented.Reproduced with permission from [48], copyright American Chemical Society (2015).

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