New value of old knowledge: sulphur-based GaAs surface passivation and potential GaAs application in molecular electronics and spintronics

GaAs is well known for its extremely high electron mobility and direct band gap. Owing to the technological advances in silicon-based technology, GaAs has been limited to niche areas. This paper discusses the application of GaAs in molecular electronics and spintronics as a potential field for considering this amazing but challenging material. GaAs is challenging because its surface is characterized by a high density of surface states, which precludes the utilization of this semiconducting material in mainstream devices. Sulfur(S)-based passivation has been found to be significantly useful for reducing the effect of dangling bonds and was researched thoroughly. GaAs applications in molecular spintronics and electronics can benefit significantly from prior knowledge of GaAs and S interactions because S is a popular functional group for bonding molecular device elements with different semiconductors and metals. In this article, the problem associated with the GaAs surface is discussed in a tutorial form. A wide variety of surface passivation methods has been briefly introduced. We attempted to highlight the significant differences in the S-GaAs interactions for different S passivation methods. We also elaborate on the mechanisms and atomic-scale understanding of the variation in surface chemistry and reconstruction due to various S passivation methods. It is envisioned that GaAs and thiol-terminated molecule-based novel devices can exhibit innovative device characteristics and bring the added advantage of S-based passivation.


Introduction
GaAs is one of the most important candidates for electronic materials in the fabrication of lasers, the base material for quantum dots, and high-speed devices for optical and mobile communication [1][2][3]. Around the time when GaAs was relegated to niche areas owing to the advancement in already popular silicon devices, their application in the field of molecular devices has been explored. Molecular devices can benefit from GaAs magnetic properties in yielding next-generation spintronics devices. GaAs doped with magnetic impurities have shown excellent magnetic semiconductor properties [4,5]. The GaAs quantum dot with magnetic impurities has been considered the fast-evolving field of quantum computing and quantum cryptography [6]. The effect of the external magnetic field has been investigated to improve the GaAs-based quantum well's optical properties [7][8][9]. The role of GaAs as the substrate has been discussed in the context of improving the magnetotransport in other materials [10]. In summary, the targeted doping of GaAs has produced a wide variety of dilute ferromagnetic semiconductors [8,11]. GaAs-based dilute magnetic semiconductors have strong potential to produce a new field of spintronics or spin photonics research where electron spin plays a major role [12][13][14][15]. For example, connecting molecular nanostructures with GaAs dilute semiconductor and ferromagnetic electrodes can offshoot a new form of molecular spintronics. GaAs is promising for exploring photonics materials for specific types of wave emissions as explored in other materials [16].

Concept of combining surface passivation with tunnel junction-based molecular devices
Electrochemical sulfidation was effective in chemically bonding S to a GaAs electrode, as discussed extensively in this paper. We propose that GaAs can be employed in a tunnel junction-based molecular device scheme (figure 1) that has shown novel insights and advantages [21,54,55] beyond the conventional molecular device designs [17][18][19][20]. Under this approach, a tunnel junction with an exposed edge can be produced such that the thickness of the insulating gap (figure 1(a)) is less than the end-to-end length of the molecule possessing at least two thiol ends ( figure 1(b)) [55]. It is anticipated that the molecule bridging the insulating gap serves as a device element and governs the transport characteristics ( figure 1(c)). Excess molecules with thiol terminals will interact with the exposed GaAs surface and serve as passivants (figure 1(c)). One of the GaAs electrode can be replaced with a metallic electrode, for example, a NiFe ferromagnetic electrode (figure 1(d)), to produce molecular spintronics devices. Ferromagnetic electrodes in molecular devices can serve as sources of spin or spin detectors. In addition, if GaAs is doped to acquire magnetic properties, then the same molecule can also serve the additional role of establishing strong exchange coupling between the GaAs and ferromagnetic electrode, leading to highly correlated materials that can be termed molecule-based metamaterials with unique physical properties analogous to the system we demonstrated elsewhere [21,25,27]. In our prior study [21,25,27], paramagnetic molecules established an unprecedentedly strong exchange coupling between the two ferromagnetic electrodes. This exchange coupling resulted in intriguing current suppression and a spin-dependent photovoltaic effect [23]. Recent molecular spintronics studies have indicated that molecules' ability to covalently bond to the electrode can be considered a tool to yield novel device behaviors [26,29,30,[56][57][58][59]. The following sections describe the basics of the GaAs surface and its interactions with S-like passivants.

Problem with GaAs surface
It is well known that the oxidized GaAs surface, the starting point for most devices, has a high surface state density owing to the missing cations and anions from the surface [60]. Generally, Ga and As oxides do not form stoichiometric oxides, and hence Ga-O and As-O remain associated with atomic scale strains (figure 2(a)) and As and Ga point defects ( figure 2(b)). Hence GaAs oxides add more problems in the band gap (figure 2(c)) that trap charges and impact electrical and optical properties. The interface engineering to accounting for the surface states' impact on optical properties has been an active field in other III-V semiconductors [16].
It was observed that the ultra-high vacuum (UHV) cleaved (110) GaAs surface had no surface state-induced levels within the band gap. However, the UHV-cleaved (100) surface exhibited nominal surface state densities of [61,62]. This observation provides sufficient support for the concept that the very high surface state density is due to the interaction of foreign elements with the GaAs surface. Several models have defined the origin of the GaAs surface state. According to one group, oxidized GaAs produce a surface state in the entire bandgap [50]. However, it has also been suggested that the high density of midgap surface states, which are mainly responsible for the poor electrical properties of the GaAs surface, is due to the presence of antisite defects [63,64]. Spicer et al [60] proposed that the release of the adsorption energy of interacting oxygen (O) and various other metal atoms introduced a significant disorder at the GaAs surface layer. These disorders produce local strain at the oxide/GaAs interface. These defects also yield various types of defects, such as antisites and vacancies [60]. Deficits possessing an acceptor nature produce a high density of surface states and lead to defectinduced energy states ∼0.8 eV below the conduction band (CB). Moreover, Ga deficit defects produce significantly high donor nature surface states near the valence band (VB), but are relatively small in comparison with midgap states [60].
According to Besser et al [64], the surface states responsible for Fermi level pinning are introduced by arsenic (As) and gallium (Ga) antisite defects (figure 2). They proposed that the singly and doubly ionized As antisites produced double donor levels at 0.65 eV and 0.9 eV below conduction band (CB), respectively, and Ga antisites caused double acceptor levels near valance band (VB) [63]. The pinning position of the Fermi level was suggested to depend on the equilibrium concentrations of the Ga and As antisites. The As-rich surface, which appeared after GaAs etching in an acidic etchant, was energetically more favorable at room temperature and exhibited a very high density of As antisite defects. These As antisite defects were not compensated by the small amount of Ga antisites present, due to which the Fermi level was pinned by As antisite defects [64].
The unified energy-band model is based on a review of the issue of surface states (figure 2). There are two main sources of the surface states in the GaAs bandgap. The first source is the local strain present at the oxide/ GaAs interface, which exhibits a parabolic distribution of the surface states in the bandgap (figure 2). The second source of surface states are As-and Ga-deficit-induced defects such as antisites and vacancies [60] (figure 2), which create a high density of midgap surface states (due to As deficits or As antisites) and a low relative density of surface states near the VB (due to Ga deficits or antisites) [63,64]. Figure 2 shows the presence of intermediate energy states arising from local strain and these defects. The effect of the above-described surface states on the surface bandgap of GaAs on a metal-semiconductor junction with an insulator film in between is shown in figure 3. This clearly shows Fermi level pinning at approximately the midgap position. To eliminate these surface states from the energy gap, a passivation process should be capable of removing oxides and deactivating antisites, or As and Ga deficit defects. Moreover, the passivant should not create surface states because of its presence.
Gallium oxides were utilized as the passivants for GaAs surface. It was reported that the electron beam evaporation of Gd 3 Ga 5 O 12 garnet produced a mixed oxide film of (Ga 2 O 3 )1-x(Gd 2 O 3 )x with x% Gd 2 O 3 [65]. For x < 14%, this film exhibited low leakage current, high breakdown strength, and good surface passivation characteristics. The explanation adduced to define the passivation capabilities of (Ga 2 O 3 ) x (Gd 2 O 3 ) 1-x oxide layer was that Gd 2 O 3 , while exceeding the 14% limit in the oxide film, either reduced the oxide vacancies or stabilized the 3+oxidation state of gallium oxide. It is interesting that even though Gallium oxides did not succeed as passivants, but they have become very attractive as the substrate and major device elements due to the high-temperature compatibilities [80,81], and lateral field effect transistor [82].
Interestingly, air oxidation was studied for producing surface oxides. In a study, the Au/n-GaAs Schottky barrier diode (SBD) parameters were investigated with and without a thin native oxide layer fabricated on n-type GaAs. The native oxide of different thicknesses was obtained by exposing the GaAs surfaces to clean room air before metal evaporation. The barrier height value decreased with increasing the exposure time up to 10 days, and after 10 days, barrier height remained unaffected up to 45 days [83]. Barrier height was strong function of the existence of dipole in the oxides of Au/oxide/n-GaAs interfaces [83]. Capacitance-frequency characteristics of gold/n-type Gallium arsenide Schottky barrier diodes exposed to air was employed for evaluating the interface states energy distribution [84].
The nitrogen ion bombardment of GaAs (100) produced a strongly bonded GaN layer on the surface. The GaN film, whose thickness was a function of the bombarding-ion energy, exhibited a reduction in the surfacedepletion region. Bombarded nitrogen also reduces excess As and lattice disorder to yield improved surface properties [66]. However, no further advancement was reported in GaN based GaAs passivation method.
Chlorine was used for the GaAs surface passivation. Lu et al [67] reported the realization of an air-stable Clterminated GaAs surface, exhibiting improved surface electrical properties. GaAs was dipped in 10% HCl for a few minutes to produce a chloride-passivated surface. It was shown that the GaCl bond was significantly more thermodynamically stable than the As-Cl, Cl-H, and Ga-H bonds. The partial passivation was because of GaCl bonds that existed on the GaAs surface after treatment.
Hydrogen passivation of the GaAs surface, although extensively attempted in the late eighties and the early nineties from both experimental and theoretical viewpoints, could not produce effective and stable deactivation of surface states. The low-temperature stability of H-passivated GaAs precludes its application as a viable surface passivation method [68]. It is interesting to note that hydrogen passivation has led to industrially mature silicon products.
Manorama et al [69] have shown the passivation potential of plasma-deposited polymer films on GaAs surfaces. They observed that the polythiophene film was capable of significantly reducing the surface states and surface barrier height. The effectiveness of thiophene as a passivant was validated by PL, capacitance-voltage (C-V), and Raman scattering measurements [70].
Beaudry et al [71] reported phosphorous (P) base passivation of GaAs surfaces. It was carried out by two methods: actuating the exchange reaction on the GaAs surface in a tertiary butyl phosphine vapor ambiance and creating a direct thin GaP epitaxial layer. However, the growth temperature was significantly higher. Thus, the passivation was found to be stable over several months. These findings are in close agreement with results presented elsewhere [72].
Selenium (Se) has also been used as a passivant for realizing flat-band conditions for the surface band gap GaAs [73]. However, this process involves high-temperature annealing, which places a significant amount of Se on the GaAs surface after replacing As atoms with the bulk GaAs. It was believed that the overall change in atomic arrangement on/in the GaAs surface was the reason behind the unpinning of the surface Fermi level [73].
A low-temperature surface passivation scheme was suggested for the fabrication of a surface-passivating insulator film through the electrodeposition of sodium diodecyldithiocarbamate DODTC [74]. PL and Raman measurements validated the effectiveness of this novel method and confirmed the significant reduction in the charge depletion region and unpinning of the surface Fermi level. X-ray photoelectron spectroscopy (XPS) study revealed that reactive sulfur present in DODTC chemically bonded with surface As atoms to deactivate the surface state [74].
Surface passivation was also achieved by oxidizing the GaAs surface through photooxidation [75,76]. Oxygen dissolved in flowing deionized water (DI) was charged on the GaAs surface in the presence of white light. It was found that the Fermi level was still pinned when the photoluminescence intensity (PLI) increased significantly. Based on experimental and theoretical defect models, it was suggested that PLI enhancement was possible without decreasing the surface state density. Further research on the mechanism of PLI was thoroughly conducted, and the above concept was discarded. A discussion on this subject is provided in the mechanism section of this thesis.
Kim et al [77] probed the passivation capabilities of gallium fluoride (GaF 3 ) on GaAs surface [77]. The GaF 3 film was directly deposited using a molecular beam epitaxy (MBE) system. The electrical characterization of the GaF 3 /GaAs interface revealed a reduction in the surface state density. Willston et al [76] showed that fluoride ions (F + ) bombarded on a GaAs surface depleted the surface As due to the formation of volatile arsenic fluorides. However, a gamut of fluorides, comprising GaF, GaF 2 , and GaF 3 , was present on the GaAs surface. The stabilities of these three fluorides differ with respect to temperature. GaF 3 was dominant at low temperatures, whereas GaF was the major fluoride at high temperatures. Interestingly, the GaAs surface was completely devoid of fluorides at approximately 300°C. These studies are very helpful in understanding recent developments in the field of GaAs surface passivation utilizing F elements in conjunction with other passivants.
Unfortunately, none of the above-listed passivation approaches were successful in producing GaAs-based transistors or using devices that could compete with silicon. Among the passivants, sulfur(S) is the most successful element in ameliorating the GaAs surface quality. S significantly decreased the surface recombination velocity [63] and increased the PLI [85]. In addition, the surface barrier height becomes more sensitive to the metal work function [86,87] after quenching a major part of the active surface states by S on the GaAs surface [88]. The science and methodology of S passivation have improved significantly in the past few years because of extensive theoretical [89,90] and experimental studies [39,47,[91][92][93][94]. A complete realization of the flat band energy gap, free from surface states, has been reported using improved S-passivation methods under specific conditions [87]. However, these improved S passivation techniques grossly depend on processing at a high temperature (∼400°C) under ultra-high vacuum (UHV) conditions [87,94,95]. The succeeding section is devoted to covering the important aspects of S passivation methodologies, mechanisms, and other related factors from various viewpoints.

Sulphur passivation of GaAs
Initially, S passivation was carried out using aqueous (aq.) solutions of sodium sulfide (Na 2 S) [49,79] and ammonium sulfide ((NH 4 ) 2 S) (with or without dilution) [91,92,94,96]. Further improvement in the S passivation effectiveness was noticed by using a nonaqueous solvent to prepare a sulfide solution for passivation [39,47,95]. A change in the S charging method from sample dipping to electrochemical S charging also showed a definite improvement in the electrical properties of the GaAs surface [97][98][99][100]. To achieve stable S passivation and improved surface properties, many theoretical and experimental studies have been carried out to determine the high-temperature characteristics of S-passivated GaAs [40,87]. The results of these studies showed that Ga-S bonds on GaAs provided very stable S passivation without providing any extra surface state in the energy bandgap [90]. However, the As-S bonds were remarkably less stable and inferior in terms of their electrical properties. These facts prompted researchers to provide only Ga-S bonds on GaAs surfaces either by direct deposition [101][102][103] or by adding annealing treatment in addition to S passivation treatments [87].

Sulfur passivation methodologies (a) Passivation by aqueous sulfide solution
The solution utilized were Na 2 S (aq.) and (NH 4 ) 2 S (with and without dilution), respectively. The solution treatment to bring about surface passivation invariably starts with surface cleaning (degreasing and deoxidation) of sample [49,79]. The solution treatment leading to surface cleaning (degreasing and deoxidation) of sample caused the removal of some sources of defects present at the GaAs/oxide interfaces [49,79]. The cleaned sample was dipped in solution at room temperature [104] or slightly higher for several minutes to several hours. Sometimes, the cleaned GaAs were spin-coated with sulfide solution. Two main drying operations have been reported for sulfide-solution-treated samples: spin-drying and N 2 drying. The effects of the processing parameters and solution characteristics of the passivation properties are discussed in this paper. Here, we briefly discuss the effects of the surface chemistry and temperature maintained during sulfidation. These two factors have a major impact on the final bonding types on the S GaAs surface.
Deoxidation of the GaAs surface is an important treatment before sulfide passivation because it determines the surface chemistry of the GaAs sample before S passivation. It is well known that Ga atoms on the GaAs surface are highly dissolvable in acids such as HCl and H 3 PO 4 , leaving deoxidized As-rich surfaces after dipping the GaAs sample in these acids [48]. However, the ammonium hydroxide solution produced a near-stoichiometric GaAs surface free from elemental As (As°) [105]. Depending on the relative amount of As and Ga, the quantity of As-S and Ga-S bonds changes in the S-treated samples [106,107]. It is clearly shown that Na 2 S (aq.) solution produced a higher number of As-S bonds for a given As/Ga ratio, while (NH 4 ) 2 S has a greater tendency to yield more Ga-S bonds than the Na 2 S (aq.) solution for As/Ga ratios of [106,108].
Characterization of the sulfide solution-passivated GaAs surface revealed the following. Na 2 S (aq.) and (NH 4 ) 2 S solution increased the PLI [79]. However, it was reported that sodium sulfide treatment did not unpin the surface Fermi level [64] while (NH 4 ) 2 S did not noticeably unpin the surface Fermi level. Sodium sulfide solution passivation was observed to be very sensitive to the concentration of sulfide salt [109], whereas the (NH 4 ) 2 S solution effectiveness was indifferent to the concentration factor [86]. The S passivation effect vanished after thorough rinsing with both types of sulfide solution. Na 2 S (aq.) and (NH 4 ) 2 S solutions were effective in reducing surface oxides, gallium oxide, and arsenic oxides [48,79]. However, the dissolution rate of As°was significantly higher in the (NH 4 ) 2 S solution than in Na 2 S (aq.) solution [92]. Sandroff et al [78] noticed a clear distinction between the nature of the bonding S and the GaAs surface for both solutions. It has been suggested that Na 2 S (aq.) solution provided As 2 S 3 type compound on the GaAs surface, where the S atom bridged between two As atoms, while the (NH 4 ) 2 S solution-treated GaAs exhibited two S atoms staying between two As atoms [46,47]. The mechanism of surface passivation is discussed in detail later in this paper. The aq. The (NH 4 ) 2 S solution passivation produced superior electrical properties compared with those of aq. Na 2 S passivation on the GaAs surface.

(b) Passivation by nonaqueous sulfide solution
Sulfide solution treatment was more effective when sodium sulfide and ammonium sulfides were dissolved in a solvent with a lower dielectric constant [39,47,110]. However, the passivation methodology remains almost akin to aqueous sulfide solution passivation of the GaAs surface. The properly degreased and deoxidized GaAs samples were immersed in a nonaqueous sulfide solution at room temperature [39,47,110]. The residual solution was removed from the sample surface by spinning it at 1000 rpm all the time. The effectiveness of the application of nonaqueous solvents (alcohols with low dielectric constants such as ethyl alcohol (C 2 H 5 OH) 20.18], and tertiary butanol (t-C 4 H 9 OH) [ε = 12.47]) was confirmed by PL, XPS, and Raman scattering studies [111]. The results of the PL and Raman analyses are presented in figure 4 that is based on data from the prior [39,47,110].
These  The simultaneous improvement in both electrical properties indicated an overall change in the surface state spectrum in the energy band gap of [85]. The performance of the (NH 4 ) 2 S nonaqueous solution was inferior to that of the Na 2 S nonaqueous solution (alcoholic solution) when a solvent with the same dielectric constant was used, which has been explained by the solution chemistry principle discussed elsewhere [47,51]. Remarkable changes in the nature of the surface bonds were also observed during the XPS study of the nonaqueous Na 2 S solution-treated GaAs surface (figure 5) [111].
The XPS data in figure 5 clearly show that oxide removal and sulfur coverage were significantly better with the nonaqueous Na 2 S solution than with the aqueous solution [111]. Excess As (As°) removal was poor in the case of nonaqueous Na 2 S but helped in justifying that elemental As°does not play any role in S passivation [105]. It was also observed that the formation of Ga-S bonds was possible by the use of an alcoholic solvent [46]. As discussed earlier, Ga-S bonds on GaAs provided very stable S passivation without providing any extra surface state in the energy bandgap [90]. However, the As-S bonds were inferior in terms of their electrical properties because some intermediate charge trapping states persisted within the GaAs bandgap. Nonaqueous Na 2 S provides more Ga-S bonds than aqueous Na 2 S solution (figure 5) [111]. As a result, the nonaqueous Na 2 S solution clears up the band gap more effectively from the defect-induced energy starts that impact SRV. The aqueous solution leaves As-S bonds on the surface that also produce energy states within the energy band gap, impacting SRV adversely.
(c) Electrochemical S charging Electrochemically charged S produces superior quality surface passivation over normal sulfide solution passivation [97][98][99][100]. The following methodology was adopted to achieve surface passivation: After degreasing and deoxidation, as discussed for the aqueous sulfide solution passivation methodology, the GaAs sample was placed in a Teflon holder [97] or in a wax case, such that only one face of the GaAs sample was exposed to the S-containing electrolyte. An ohmic contact was established with the surface to be passivated prior to fixing the sample in the electrochemical cell. Different types of S-containing electrolytes were used: Na 2 S-ethylene glycol [99] and (NH 4 ) 2 S solution, a solution obtained by sequential mixing of propylene glycol with ammonia and hydrogen sulfide (H 2 S) [98]. Supplying anodic current in the milliamps [97] or microamps range for a few minutes charged sulfide ions. The counter electrodes used in the electrochemical cell were a metal plate [97] and a Pt [98]. The electrochemically charged GaAs surface was rinsed in ethanol and then blown in dry nitrogen gas [98].
XPS studies of electrochemically charged GaAs surfaces showed the presence of Ga sulfides (Ga 2 S 3 and GaS) and arsenic sulfide (As 2 S 5 ) [98]. The passivation film thus produced was capable of providing a more stable deactivated surface compared to the passivation film produced by ordinary sulfide solution treatment [97]. Moreover, electrochemical S passivation was significantly stable against water rinsing [99]. The presence of stable GaS and As 2 S 5 (As 5+ state) was mainly responsible for the improvement in the properties of the electrochemically S-passivated surface [99]. Additionally, it is well established that As in a higher oxidation state provides better interface properties than As 3+ state [97].

(d) Deposition of GaS
The superior properties of Ga-S bonds on GaAs surfaces prompted researchers to deposit GaS directly on a suitably reconstructed surface. Realization of this process necessitates the availability of highly controlled systems and high substrate temperatures to maintain the required surface reconstruction and surface chemistry.
Direct deposition of GaS was successfully performed using a gallium cluster ([(t-Bu) GaS] 4 ) as a singlesource precursor under ultra-high vacuum conditions of the MBE system [102,103,112]. The substrates were cleaned using trisodium ethylaminoarsinic (TDMAAS) and bisdimethylaminochloroarsinic (BDMAASCl) [102]. During the deposition of pure GaS, the precursor cell was maintained at 120°C [102,112]. The substrate temperature was maintained in the 350°C to 500°C temperature range. The substrate was at a high temperature because it was essential to produce the required surface reconstruction, which played an important role in determining the extent of the improvement in the electrical properties of the GaS/GaAs interface. For instance, As rich (4×4) surface reconstruction produced 5×10 10 eV cm −2 surface states [102] while As rich (2×4) surface reconstruction yielded 1.8×10 11 eV cm −2 density of surface states [112] at GaS/GaAs interface. The deposited GaS film was reported to be amorphous and exhibited excellent surface morphology [102]. The GaS/GaAs interface exhibited excellent electrical properties only when the GaS film thickness was on the lower side because the excess GaS produced a high interface strain. The high lattice strain present at the interface facilitated the conversion of the film from amorphous to polycrystalline, with a high dislocation density [102]. Moreover, the GaS film behaved like an insulator because it had a bandgap of 3.5 eV [112]. The deposition of GaS produced an impressive enhancement in the PLI and almost flattened the energy band gap near the surface. The passivation was also found to be stable for at least two years [102].
(e) S passivation followed by annealing.
Moriety et al [85] used the following scheme to achieve an almost complete unpinning of the surface Fermi level in the energy gap. First, the as-capped GaAs (100) samples were indium bonded to a tantalum or molybdenum holder before being inserted into the sample in the UHV system. In the UHV system, As decapping was performed at 350°C. The As decapped samples were heated in 400°C-570°C range temperature to produce an As-rich c(2×8) surface reconstruction and near stoichiometric Ga rich (4×1) surface reconstruction as the starting stage. Thoroughly cleaned samples with c(2 × 8) and (4 × 1) starting surface reconstructions were charged with molecular beam sulfur in an electrochemical cell. First, an anodic current of 0.5 mA was maintained for 5 min at room temperature to produce a 1 ML coverage of the GaAs surface.
The top view of the Ga-rich (1 × 1) atomic arrangement after 1 ML S charging identifies the positions of different atom locations along the thickness via various legends [88]. The atomic arrangement after 1 ML of S charging on (1×1) surface reconstruction is shown in figure 6(a). The sulfur charging step was followed by an annealing treatment at 450°C-500°C, which yielded a (2 × 1) surface reconstruction. The XPS analysis of these samples confirmed the unpinning of the surface Fermi level for the sample with (4 × 1) surface reconstruction [85]. The corresponding surface reconstruction was significantly different after annealing, as shown in figure 6(b). Partial S charging (1/2 ML) produced a surface with unreplaced Ga atoms (figure 6(c)) [88].
The unpinning of the (4 × 1) surface reconstruction was due to the optimum atomic arrangement caused by S passivation and annealing. It was also shown that the S atom successfully replaced the As atoms near the topmost surface layer of GaAs, which is in agreement with the findings presented elsewhere [40,113]. As atoms were observed in the 3rd layer of GaAs. More importantly, the resultant atomic arrangement contained Ga-S bonds on the surface layer attached with partially or completely replaced As from the top layer [41,87]. This experimental finding is in good accordance with the atomic arrangement predicted for the (2 × 1) reconstructed S-passivated surface for engendering complete unpinning of the surface Fermi level. In the case of c(2 × 8), which is an As-rich GaAs surface, it was found that the replacement of As at the surface with S did not occur significantly after annealing the S-passivated sample [87,91]. This method was again energy-intensive and required a high degree of control.

Atomic arrangement and surface reconstruction of passivated surface
The atomic arrangement of the S-passivated surface mainly depended on the S coverage and the availability of activation energy [88]. Three types of surface bonds, As-S, Ga-S, and S-S, affect the surface density of states in the energy bandgap. The As-S bond formation requires less activation energy than Ga-S. The number of As-S, Ga-S, and S-S bonds increased with S coverage. More importantly, the existence of S-S bonds is strongly dependent on the availability of S on GaAs, compared to As-S and Ga-S [49].
It was observed that S attached to the As-S bonds released itself upon increasing the temperature and did not leave the GaAs surface until the temperature was high [98]. The atomic arrangement along different atomic layers of GaAs charged with S at low temperatures is shown in figure 7(a) [91,92,94,96]. Upon heating the freed S atoms, they formed bonds with surface Ga atoms and simultaneously started replacing bulk-associated As atoms (As at the 3rd layer) [40,113]. According to the theoretical calculation, the replacement of alternate bulkassociated atoms by S on the surface with (2 × 1) reconstruction and Ga-S surface bonds is the optimal atomic arrangement ( figure 7(b)) for the realization of the flat band condition [102]. This atomic arrangement significantly matches that conceived based on experimental studies [87]. The main difference is in the extent of removal of bulk-associated As from the surface layer.
The impact of the presence of the above-described bond on the energy-band diagram is as follows. The first principal study suggested that the As-S bond contained 2.25 electrons, while for strong bonding between As and S, two electrons are needed ( figure 7(a)) [91,92,94,96]. The presence of excess 0.25 electrons present in the antibonding level weakens this bond. However, the Ga-S bond contained exactly 2.0 electrons and had completely unfilled antibonding levels ( figure 7(b)). Figure 8 shows the electronic level formation due to the As-S and Ga-S bonds. After the S passivation of the Ga-terminated GaAs surface bonding and antibonding energy levels were out of the GaAs bandgap region, they did not have any impact on the GaAs direct bandgap (figure 8) [90]. However, the antibonding state arising after the As-S interaction on the As-terminated GaAs surface left an antibonding state within the GaAs energy bandgap (figure 8) [90]. This energy band model clearly showed that the appearance of a new electronic level was due to the partially filled antibonding states of the As-S bond. In contrast, no level appeared in the energy band gap due to the Ga-S bond possessing a filled bonding state and a completely vacant antibonding state [90]. The electrical properties of an S-passivated GaAs surface depend on the final surface chemistry and atomic arrangement on the surface [38,42,43,87,114].
There exist significant differences in the nature of surface reconstruction and type of bonding after lowtemperature sulfide solution passivation (T < 100°C) and high-temperature sulfide passivation schemes (figure 9) [85]. The clean GaAs band gap at low temperature was impacted by the dangling bonds, and the Fermi energy level was pinned between the conduction and valence band around 0.8 eV (figure 9) [85]. After S passivation at RT, the impact was the same for the As-rich c(2 × 8) reconstruction and Ga-rich (4 × 1) reconstruction. However, upon increasing the annealing temperature, the Ga-rich GaAs surface started showing a full band gap of GaAs (figure 9), while the As-rich GaAs never reached the full band gap level. Upon further heating, S started to leave the surface and produced a near-pristine GaAs surface with a smaller energy bandgap. This study provides important insights into the fabrication of GaAs molecular spintronic devices regarding the importance of the initial GaAs state and temperature. Because it is extremely difficult to consider individual cases, one may need to specifically study the GaAs interaction with different molecular device elements.
Interestingly, the near-room-temperature sulfide passivation surface chemistry significantly depends on the methodology used [49,105]. For example, Na 2 S passivation mainly produced As-S-type bonds on the Gaterminated surface ( figure 10(a)) [78]. On the other hand, (NH 4 ) 2 S based sulfide treatment yielded Ga-S, As-S, and S-S bonds [49]. Interestingly, (NH 4 ) 2 S produced two S atoms between the adjacent As atoms ( figure 10(b))  [78]. The model presented in figure 10(b) is based on the one discussed for the (NH 4 ) 2 S treated GaAs that did not incorporate the presence of Ga-S bonds [49]. The presence of Ga-S bonds, along with As-S bonds, was validated by the same group via an XPS study conducted on a nonaqueous (NH 4 ) 2 S solution-treated GaAs surface [47,78]. It is apparent that there is a need for a complete atomic model that includes As-S and Ga-S.
The reconstruction of the (NH 4 ) 2 S treated GaAs surface was theoretically estimated to be (1 × 1). Although it matched the LEED results recorded on a room-temperature (RT) H 2 S passivated GaAs surface [113], it significantly differed from the actual surface reconstruction (2 × 2) observed on the (NH 4 ) 2 S treated surface. The reason for this difference is the amount of S released by H 2 S and (NH 4 ) 2 S during passivation.
Electrochemical S passivation of GaAs surfaces produces gallium sulfide (Ga 2 S 3 ) and arsenic sulfide (As 2 S 5 ), which are in a higher oxidation state than gallium sulfide and As 2 S 3 formed during simple sulfide solution treatment [97,98]. Electrochemically passivated GaAs exhibits superior electrical properties and higher stability than normal sulfide passivation [97]. No appropriate atomic model has been suggested for the electrochemically passivated GaAs surface.
Close control over the S passivation parameters is necessary when efforts are made to realize optimum atomic arrangements for complete unpinning of the surface Fermi level. A controlled environment of the MBE system and high substrate temperature were employed to grow a thick GaS film on a suitably reconstructed GaAs surface [91]. By maintaining the substrate temperature in the 350°C-500°C range, As-rich surface reconstructions, such as c(4×4) [102] and (2 × 4) [94] were established. GaS was deposited on these surfaces to impart excellent electrical properties at interface [101]. The choice of depositing a GaS film, and not an arsenic sulfide film, on the GaAs surface, was based on Ga-S's superior electrical properties and stability over As-S, as discussed earlier [61]. GaS bonds were found to be stable up to ∼530°C [45], while the As-S bonds vanished before reaching 300°C [45,115].
Another energy-intensive passivation method, in which S passivation is followed by annealing, successfully produces a surface energy gap with an unpinned Fermi level of [87]. This method, apart from producing GaS on the surface, also replaced a significant amount of the substrate associated with As atoms with S. This treatment yielded a (2 × 1) surface reconstruction on the S-passivated surface [87]. The atomic model of the S-passivated and annealed surfaces was in close agreement with the results of the theoretical study. The significant difference in the atomic models conceived based on the experimental and theoretical study was in the extent of substrateassociated As replaced by S. Therefore, suitable experimental studies are required to address this issue.
Moriarty et al [85] clearly showed the importance of starting the surface chemistry and surface reconstruction. It was reported that an As-rich c(2 × 8) GaAs surface Fermi level remains unpinned owing to inadequate Ga-S bond formation and paucity of surface As atom replacement with S. Moreover, As dimers, which themselves create extra electronic states in the energy gap, were also present after sulfidation and annealing treatment on surface c(2 × 8) reconstruction. Figure 10 shows that a near-stoichiometric GaAs surface with (4 × 1) surface reconstruction was observed with a completely unpinned surface Fermi level after the same S passivation followed by annealing treatment [85]. Another surface reconstruction (2×6), which is also considered to be stable, was reported to have five S dimers in a unit [116,117]. On heating, this dimmer row breaks out from the middle to yield a (2 × 3) surface recombination. Several researchers have also shown that S-passivated and annealed GaAs also tend to acquire S dimers attached to Ga atoms [41,118]. According to different studies, the S dimer gives one electron to the Ga atom to provide a stable and neutral atomic arrangement on a passivated surface. Another critical aspect of the S-passivated GaAs surface is the direction of the dimers present [119,120]. S and As dimers were observed to be oriented in related directions on S passivated and annealed surface around 450°C [85]. Heating beyond 550°C eliminated S and As dimers and gave rise to Ga dimers oriented in the direction [85].

Mechanism of S passivation
There are several opinions regarding the S-passivation mechanism. Sandroff et al [78,79] claimed that the formation of the protective As 2 S 3 phase was the basis for S passivation. Another group of researchers suggested that the amelioration of surface electrical properties was due to band bending resulting from sulfide solution treatment [63,79]. According to Hasegawa et al, sulfide treatment creates a fixed negative charge near the surface, which improves the surface properties [75]. The most popular concept of S passivation is based on the reduction in the surface state density [47,51].
The idea that As 2 S 3 was the basis of S passivation was refuted by the findings presented in [105]. Two different S-containing solutions gave equal enhancement to PLI, even when the As 2 S 3 phase was absent in one case. Besser et al [64] claimed that band bending was induced by a sulfide solution on the GaAs surface, which pushed the electrons away from the surface or the surface states affected the region. An increase in PLI was Figure 10. Atomic arrangement of (a) Na 2 S and (b) (NH 4 ) 2 S passivated GaAs surface. Reprinted figure with permission from [78], Copyright (1989) by the American Physical Society. observed even when the Fermi level was still unpinned for this reason [63]. Many studies have shown that the increase in PLI and Fermi level unpinning observed after sulfide solution treatment forms the basis for the denial of the latter theory [47]. Hasegawa et al [75] claimed that an increase in PLI was possible without decreasing the surface state by the negative charge created by the sulfide treatment on the GaAs surface. Experimental studies on S passivation show that the source of the surface states always decreases to a small or large extent after passivation treatment [92,103]. On this basis, the concept of Hasegawa et al [75] was found to be inconsistent with existing experimental results.
The aqueous Na 2 S solution improved the PLI; however, the Fermi level remained unpinned [64]. However, when a nonaqueous Na 2 S solution was used for S passivation, unpinning of the Fermi level was observed along with an increase in PLI [47]. This suggests that in the former case, only a small part of the surface state was reduced, whereas in the latter case, the surface state reduction was significantly large. The XPS results confirmed that the sources of the surface states were greatly reduced by the nonaqueous sulfide treatment [47].
Low-temperature sulfidation is limited by the formation of an As-S bond, which creates surface states within the bandgap. The Ga-S bond is more stable and does not produce extra surface states after passivation [121]. To harness the superior properties of Ga-S bonds, the following two schemes were employed. First, direct deposition of GaS on a suitably reconstructed As-rich surface under controlled UHV conditions was performed [102]. In another case, annealing was performed with S passivation of GaAs [87].
Direct deposition of GaS from a suitable precursor on a substrate, maintained at a high temperature in UHV, yielded an extremely low surface state density at the interface. As there was no oxygen in the system, only the termination of the dangling bond on a reasonably good surface with an already low surface state density was left. The deposition of GaS effectively passivated the remaining surface states to the extent that almost complete band bending was eliminated [102]. Sulfidation followed by annealing also yielded a nearly flat band condition because of the following reasons. Annealing treatment, apart from producing GaS bonds on the surface, also partially replaced the surface-associated As atoms with S [40] to produce a very stable and electrically inert atomic arrangement. For this passivation treatment, the final surface chemistry was determined by XPS, which was in close agreement with the theoretically calculated surface chemistry for complete unpinning of the surface Fermi level from the mid-gap states. The reduction in the surface state density depends on the methodology used. S passivation relies on high-temperature treatment in a highly controlled environment to completely unpin the Fermi level.
Insights from the S passivation of GaAs suggest that any futuristic molecular device utilizing GaAs as a source or drain should strive to form Ga-S-molecule-S-rather than As-S-molecule-S-. The Ga-S bond produces an empty antibonding state that remains out of the main band gap, whereas As-S produces an antibonding state that compromises the GaAs direct band gap. GaAs-based molecular devices targeting Ga-S bonds are expected to be highly stable and exhibit superior electrical properties.

Multi reagent GaAs passivation
Sulfur passivation methods require a very controlled environment, and annealing is an essential step in the passivation scheme. Therefore, it is challenging to assimilate this method into the device fabrication routine. Low-temperature surface passivation produces a deactivated and stable GaAs surface. Recently, more than one reagent has been explored for producing better surface electrical properties than single-reagent sulfur passivation. More than a single-reagent sulfide passivation showed a clear improvement in the barrier height. According to Jeng et al [122], a drastic improvement in the electrical properties of the GaAs surface was observed after applying a joint passivation scheme involving S and F passivants. GaAs treated with phosphorous sulfide/ ammonium sulfide [P 2 S 5 /(NH 4 ) 2 S] and HF solution, followed by annealing at 300°C for 18 h, showed a high degree of improvement in the electrical properties of the GaAs surface. The effect of the combined reagents was greater than that of the individual treatments in P 2 S 5 /(NH 4 ) 2 S and HF solutions. It was expected that the formation of S fluoride, such as SF 6 (with a high binding energy of -291.8 kcal), was the most probable reason for this improvement in the electrical properties. However, further research is required to understand the mechanism of combined passivation. This finding suggests that opportunities exist for improving the surface properties of GaAs by using more than one passivant.
We also attempted a combined passivation approach by employing sulfide and fluoride ions from the solution, which treated GaAs surfaces with sulfide and fluoride ion-bearing solutions in a different order and evaluated the difference in surface properties via photoluminescence [53].
Research has also shown that sulfide-passivated GaAs exhibits an additional PLI enhancement after hydrogen annealing. Furthermore, GaAs treated with sulfide solution followed by metal salt solution produced air-stable surface passivation, which remained stable for more than a year. However, all steps of this combined passivation were executed at room temperature under ambient conditions. Combined surface passivation can produce superior surface electrical properties and more stable surface passivation. This relatively new surface passivation stream promises to yield a more efficient, cost-effective, and less time-consuming method for surface passivation; however, most two-step passivations are not very well characterized by a wide range of surface-sensitive techniques such as XPS and REED.
Prospective GaAs-based molecular electronics and spintronics can greatly benefit from the multiple-reagent approach. Notably, some alkane-like molecules [123] can be sleeked and provide a high density of coverage. However, DNA [124] single-molecule magnets (SMM) [125] and porphyrin [126], like different candidate molecules, possess very different shapes and volumes. Based on the molecules used for molecular device fabrication, additional complementary small molecules can be chosen to passivate the exposed GaAs surface. In addition, to the difference in molecules of interest, GaAs may also be doped with different types of deposits like Mn to give them magnetic characteristics [127,128].

Related preliminary work
Interest in GaAs-based molecular devices are an active field of research. Similar to the study of the interaction of S with GaAs for the purpose of surface passivation, researchers have started studying the potential molecular device interaction with GaAs. Preda et al [129] have investigated the thiol(S) functional group of a large molecule with GaAs. In their study, Ga and As competed for thiolate formation on p-GaAs(1 1 1) surfaces when interacting with 4,4' -thio-bis-benzene-thiolate. Contrary to the interaction of atomic S with GaAs, they concluded that a more stable thiolate layer formed on the As-terminated surface. The stability of the thiolate layer on the As-terminated GaAs was significantly greater than that formed on the Ga-terminated surface because of the stronger self-assembling effects developed between the adsorbed species. This study enables researchers to consider other factors that can impact the nature of the molecular interaction with GaAs. Researchers have not reported the impact of differences in various possible methods of self-assembly of molecules on GaAs [129]. For example, the molecule can be charged electrochemically and by using various solvents, which makes a remarkable difference in the S passivation reviewed here. Will the conclusions of this study [129] still hold good?.
We also attempted to test the tunnel-junction-based molecular device approach in the case where one electrode is a semiconductor. We focused on readily available p-Si and produced a Si/AlOx(2 nm)/NiFe (10 nm) tunnel junction using a previously method published elsewhere [130,131]. For this study, we utilized a ∼3 nm long paramagnetic octametallic molecular cluster(OMC) with four alkane tethers terminated with thioacetate groups [132,133]. A tunnel junction was fabricated in the cross-junction form with exposed side edges to allow molecular bridging between Si and NiFe ( figure 11(a)). OMC molecules typically increase the current absorber at the bare tunnel junction level, proving that electron transport channels can be established via these molecules ( figure 11(b)). Interestingly, we observed that a strong interaction of the OMC paramagnetic molecule core with Si and NiFe via S linkage produced an intriguing impact. In our preliminary study, we observed that the S-based OMC bridge produces a transient current state at room temperature. After the application of the magnetic field, the OMC-treated Si/AlOx/NiFe returned to a current level similar to that of the bare junction current state ( figure 11(c)). During the transient state, the Si/AlOx/NiFe junction with the OMC molecule exhibited current suppression more than three orders of magnitude lower at room temperature ( figure 11(d)). It is noteworthy that OMC interaction with two ferromagnetic electrodes has also produced current suppression [21,22] and many other intriguing phenomena, such as the spin-photovoltaic effect [23]. Erve et al [134] also worked on Fe/ AlOx/Si systems and were able to inject a pure spin current into silicon via the AlOx tunnel barrier. In our case, the current is influenced by the OMC molecular channels, which are gateways for spin and charge transport; OMCs are also capable of spin filtering and may assume various spin states. The exact impact of the OMC channels on the Si/AlOx/NiFe magnetic and electrical properties is not clear.
It is noteworthy that, unlike GaAs, Si has an indirect bandgap and has the tendency to strongly adhering oxides. Making an S-Si bond is good, but the real advantage of this approach is that it does not produce strong oxides and has a tendency to form bonds with S. This study provides a foundation for exploring a new paradigm of devices involving molecules as spin channels between magnetic electrodes and high-potential semiconductors such as GaAs.

Conclusion
In this review, various features of surface-associated problems and surface passivation methods and mechanisms are discussed. The following are the main points emanating from the discussion of the active GaAs surface. The oxidized GaAs surface was characterized by two types of surface states. The first type of surface state originates from the lattice strain present at the oxide-semiconductor interface, and it possesses a parabolic distribution in the band gap. The second type of surface state was produced by As and Ga deficits, which were produced by the release of the adsorption energy of foreign atoms on the GaAs surface. The effectiveness of a surface passivation method is determined by its ability to passivate both types of surface states to the maximum extent possible. For GaAs-based molecular devices, these energy levels may decrease to some extent owing to S passivation produced by the thiol functional groups of the molecules. Interestingly, other sulfur-bearing chemicals or small molecules can self-assemble on GaAs, increasing the degree of surface passivation.
The efficacy of sulfide passivation was found to be highly sensitive to the type of methodology used to affect the surface state spectrum in the energy gap. The resultant surface chemistry of the passivated GaAs surface depends on the initial surface chemistry, type of passivant, passivate charging method, and post-passivation treatments such as annealing. For example, aqueous Na 2 S solution was only capable of reducing the SRV without unpinning the surface Fermi level, whereas the alcoholic Na 2 S sulfide solution not only reduced the SRV but also freed the surface Fermi level from the mid-gap states. It appears that in the former case, the passivant was only able to interact with surface states other than As deficits created mid-gap states, whereas in the latter case, sulfur atoms interacted with all the surface states and affected all the electrical properties of the GaAs surface. This result suggests that GaAs-based molecular device fabrication will require an understanding of the molecular treatment process, solvent, and temperature.
It is noteworthy that many molecules were considered for quantum computations of [135][136][137]. The specifications of the target molecule for quantum computation must also consider the electrode materials. The superior transport and bandgap characteristics of GaAs are highly promising for molecule-based quantum computation devices.