Chemical Vapour Deposition of Gas Sensitive Metal Oxides

Abstract

This article presents a review of recent research efforts and developments for the fabrication of metal-oxide gas sensors using chemical vapour deposition (CVD), presenting its potential advantages as a materials synthesis technique for gas sensors along with a discussion of their sensing performance. Thin films typically have poorer gas sensing performance compared to traditional screen printed equivalents, attributed to reduced porosity, but the ability to integrate materials directly with the sensor platform provides important process benefits compared to competing synthetic techniques. We conclude that these advantages are likely to drive increased interest in the use of CVD for gas sensor materials over the next decade, whilst the ability to manipulate deposition conditions to alter microstructure can help mitigate the potentially reduced performance in thin films, hence the current prospects for use of CVD in this field look excellent.

1. Introduction

Gas detection, and determining the composition of gas mixtures, is necessary in many different fields, for example environmental monitoring, vehicle and industrial emission control and household security. Most studies have focused on the detection of H₂, CO₂, CO, O₂, O₃ or NH₃, because of their toxicity, their relation with atmospheric composition or the fact that they can be found at high levels in some environments. Detection of organic vapors such as methanol, ethanol, isopropanol, benzene and amines are also of great interest [1]. Metal oxides were some of the first materials used in chemoresistive gas sensors and are still the most widely used gas sensing materials. Numerous metal oxide semiconductor materials, including both single (e.g., ZnO, SnO₂, WO₃, TiO₂ and Fe₂O₃) and multi-component (BiFeO₃, MgAl₂O₄, SrTiO₃, and Sr_{1- y}Ca_yFeO_3-x) oxides, have been reported for use as the active layer [2,3]. The mechanism for gas detection in these materials is based on reactions that occur at the sensor surface, resulting in a change in the concentration of adsorbed oxygen. Oxygen ions adsorb onto the material’s surface, removing electrons from the bulk and creating a potential barrier that limits electron movement and conductivity. When reactive gases combine with this oxygen the height of the barrier is altered, changing conductivity. This change in conductivity is directly related to the composition of the gaseous environment allowing a quantitative determination of the gases present (under certain conditions) [4]. Metal oxide semiconductor sensors have proved to be sensitive to a large range of gases and studies have focused on understanding the relationship between sensor response and materials processing and chemistry, e.g., dopant level, synthesis and annealing temperature. These parameters can have a profound effect on the materials chemistry and structure, which in turn dramatically affect the gas sensing properties of the sensor device [2]. Currently the potential of metal oxide semiconductor sensors has not been fully realised, with other types of sensors (e.g., electrochemical or those based on optical or photo-ionisation principles) still favored for many industrial applications. However, new materials and techniques continue to be developed to improve the abilities and properties of metal oxide gas sensors, and with recent advances in understanding of materials chemistry and synthetic techniques their intrinsically favourable properties, coupled with their relative low cost and potential for miniaturization and portability, should mean they become ever more important tools in environmental monitoring.

Development of synthetic methods for producing materials for use in metal oxide gas sensors has been a major focus in the field and many routes have been investigated including hydrothermal [5], sol-gel [6], solid-state chemical reaction [7], thermal evaporation [8], vapor-phase transport [9], RF sputtering [10] and molecular beam epitaxy [11]. Two common preparative routes are solid-state and sol-gel; solid-state reactions allow for relatively simple synthesis but for complex materials can lead to poor chemical homogeneity, whilst sol-gel reactions provide very good homogeneity and small particle size dispersion but the process can be difficult to control reproducibly. An alternative synthesis technique is chemical vapour deposition (CVD), which is a process for the deposition of films of various materials via chemical reactions of gaseous reactants in an activated environment (e.g., temperature, light or plasma). In general, a CVD system consists of three main components:

• Precursor supply system
• CVD reactor
• Exhaust system.

The role of the precursor supply system is to generate precursors in the vapour phase and deliver them to the reactor, normally with the help of carrier gas, where the CVD reaction takes place. Typically, liquid precursors are used in order to generate sufficient vapour pressure when heated to intermediate temperature (<200 °C) with mixing of multiple precursor streams used to produce complex mixtures. Variants on this system such as liquid injection- and aerosol assisted-(AA)CVD [12] are typically used to introduce low volatility/solid precursors, with precursor evaporation only occurring inside the CVD reactor, although they also afford great opportunities in synthesizing doped or ternary/quaternary materials due to the relative ease of controlling stoichiometry through the precursor solution.

The reactor is where external energy is added to the system, in the form of heat, light or plasma, to initiate the deposition reaction(s). Deposition involves two principal types of reactions, homogenous and heterogeneous; homogeneous reactions occur exclusively in the gas phase whilst heterogeneous reactions occur between gas phase species and a solid substrate (although frequently involving an initial gas phase reaction resulting in the formation of reactive intermediate species). In the case of homogeneous reaction the precursor/intermediate species undergo further gas phase decomposition resulting in the formation of a powder and by-products. This powder is typically non-adherent and these reactions are undesirable in CVD (although in pyrolysis reactions the powder may be the target and collected via a powder capture system), whilst the by-products are removed from the reaction chamber through the exhaust system. In the case of heterogeneous reactions diffusion of the precursor/intermediate species occurs at an interfacial gas/solid boundary layer, forming nucleation sites on the solid substrate. Subsequent nucleation processes take place on the surface of substrate resulting in the deposition of solid material, and manipulation of the reaction conditions can be used to promote the formation of either planar films or nanostructures [13]. It is also worth noting that the nature of the heterogeneous reactions means that the materials are atomically mixed, and under well-controlled conditions are homogeneous in composition across the deposition area. Hence CVD offers several potential advantages over other synthesis processes for the preparation of gas sensing materials and sensors, which include:

• A single step for gas sensor processing which combines both materials synthesis and integration of the material with the sensor platform.
• Production of atomically mixed homogenous materials, including complex stoichiometries, with good reproducibility.
• Ability to influence crystal structure and surface morphology.

This article presents a review of recent research efforts and developments for the fabrication of metal-oxide gas sensors using chemical vapour deposition (CVD), presenting its potential advantages as a materials synthesis technique for gas sensors along with a discussion of their sensing performance. In considering the literature we have compiled tables of sensing data, however we note that these are only a qualitative comparison as the response of metal oxide semiconductors in general is dependent not only on the material properties but also on the conditions used to test these materials towards the analytes.

2. Gas Sensing Materials

2.1. Tungsten Oxide

Tungsten oxide, WO₃, is a wide-bandgap n-type semiconductor, with bandgaps reported in the range of approximately 2.6–3.2 eV dependent on crystallinity and oxygen deficiency. WO₃ crystals are generally formed by corner and edge sharing of WO₆ octahedra, with various crystal phases dependent on temperature; monoclinic II (ε-WO₃, < −43 °C) → triclinic (δ-WO₃, −43 ° C to 17 °C) → monoclinic I (γ-WO₃, 17 °C to 330 °C) → orthorhombic (β-WO₃, 330 ° C to 740 °C) → tetragonal (α-WO₃, > 740 °C). Tungsten oxide can also possess non-stoichiometric properties because its rhenium oxide-like lattice can withstand a considerable amount of oxygen deficiency. Some of the better known non-stoichiometric tungsten oxides are W₂₀O₅₈, W₁₈O₄₉ and W₂₄O₆₈ [14].

Vapour deposited WO₃ and WO_x have both been used for gas sensing. These materials are typically monoclinic or tetragonal phases with a variety of morphologies reported including films, particles and low dimensional structures, with the formation of nanostructures (NS) demonstrated below 600 °C for AACVD [15] and at 800 °C for hot filament CVD. The starting materials reported in the production of gas sensitive tungsten oxide include metallic W [16,17], WO₃ (powder, pellet) [18,19], WCl₆ [20], W(OCl₄) [21], W(CO)₆ [22,23,24], or complexes such as [W(OPh)₆] [25,26], [NH₄][W₁₂O₃₉], [NH₄]₁₀H₂[W₂O₇]₆ or [ⁿBu₄N]₂[W₁₀O₃₂] [27]. Most commonly planar films of vapour deposited tungsten oxide have been employed and integrated directly into ceramic- [20,21,26,27], silicon- [16,22,23,25] or polymer-based [28] gas sensing devices. The localized CVD of tungsten oxide nanostructures on Si-based microhotplates (Figure 1) via heating provided from the sensor platform itself, rather than from the reactor chamber, has also been demonstrated as a viable method for the fabrication of gas sensors based on tungsten oxide [23], which provides interesting new possibilities for sensor processing.

As with other metal oxides, tungsten oxide deposited via CVD has been used in resistive mode, with demonstrated sensitivity to NO₂ [16,21,26], N₂O [17], C₂H₅OH [20,27], CO [23,25], NH₃ [18], H₂ [22], humidity [19] and aromatic compounds such as benzene [29] and toluene [30] (

Table 1. Summary of the features and sensing properties reported for chemical vapour deposited tungsten oxide.

Prec.	CVD method	Tdep °C	Form	Features nm	Sensor type	Top °C	ppm	Gas	R	tres s	Ref.
W(OCl4)	PE	-	film	-	Ω	200	10	NO2	48	-	[21]
WCl6	AP	625	film	3600T	Ω	400	20	C2H5OH	8.5	-	[20]
W(CO)6	LP	500	NPs	140Ø	Ω	>450	5000	H2	***	-	[22]
W	HF	-	NPs	100Ø	Ω	50	1	NO2	4701	-	[16]
WO3	EB	-	NPs	9Ø 200T	NS*	100	10	NH3	-	-	[18]
WCMPLX	AA	500	P	1000Ø 30,000T	Ω	550	20	C2H5OH	5.1	-	[27]
W	HF	800	NWs	-	Ω	450	1	N2O	4.41	175	[17]
W(OPh)6	AAEF	-	NWs	−	Ω	250	0.8	NO2	120	-	[26]
W(CO)6	AAL°c	580	NWs	100–400Ø	Ω	375	80	CO	8	-	[23]
W(OPh)6	AA	400	NWs	60–120Ø 7000L	Ω	150	100	CO	5	-	[25]
W(OPh)6	AA	400	NWs	60–120Ø 7000L	Ω	200	1	C6H6	2	1114	[29]
W(CO)6	AA	500	NWs	50–100Ø 11000L	Ω	390	0.4	NO2	250	-	[24]
W(CO)6	AA	390	NWs	50–100Ø 10,000L	Ω	190 220	100 100	C7H8 C2H5OH	3 3.5	450 -	[30]
WO2.9	CVD	400	NRs	30–110Ø 1000T	O	RT	65**	H2O	2.16	-	[19]

Prec: precursors, T_dep: temperature of deposition, T_op: operating temperature, t_res: response time, ppm: parts per millon, R = Ra/Rg (oxidative gas), R = Rg/Ra (reductive gas), ^CMPLX: [ⁿBu₄N]₂[W₁₀O₃₂], PE: plasma enhanced, AP: atmospheric pressure, LP: low pressure, HF: Hot Filament, EB: Electron Beam, AA: Aerosol assisted, ^EF: Electric field, ^L°^c: Localized, O: optical, Ω: resistive, NS: noise spectroscopy, ^*: gas sensing assisted by Blue-LED, **: % Relative humidity, ***: low response and not stable, NPs: nanoparticles, NWs: nanowires, NRs: nanorods, ^Ø: diameter, ^T: film thickness, L: length.

). The relative sensor response (R) to ppm concentrations (C) of CO, C₂H₅OH and NO₂ (Table 1) are plotted in Figure 2. This relative value (i.e., Response/Concentration) was used as a quantitative factor to compare the sensitivity of the most common CVD-deposited tungsten oxide morphologies reported in the literature, although it is worth noting that strictly sensitivity is defined as the slope of the calibration curve (calibration curves were not available in most of the reports summarized in Table 1). It is apparent from Figure 2 that tungsten oxide has a notable sensitivity to NO₂ and this characteristic is generally observed for tungsten oxide making it a good candidate to selectively detect NO₂ in the presence of gases such as C₂H₅OH, CH₄, CO, NH₃, H₂, C₆H₆ and H₂S [21,24].

The properties of CVD have been used to enhance the performance of thin film tungsten oxide-based resistive sensors in several ways. For instance, the sensing properties to ethanol were improved by a change in microstructure that occurred by controlling film thickness in the range 6700 nm to 3600 nm. These changes produced a reduction in the baseline resistance of the sensors, with resistance decreasing with decreasing film thickness, with an attendant modification of the activation energy of conductance [20]. We note a similar study has demonstrated improved sensor performance in thicker films (30,000 nm) compared to thinner films (15,000 nm) [27], but it is likely that these results are influenced by the properties of the electrodes, specifically the electrode thickness, which need to have a similar thickness to that of the thin film sensor material in order to achieve optimum sensor performance [31]. For NP tungsten oxide, CVD has been used to control the size of NPs to improve sensor performance, with particles with sizes below 100 nm having better sensing properties to NO₂ regardless of processing parameters such as the oxygen pressure during CVD or the subsequent annealing temperature. This was attributed to the depletion layer extending throughout the material in small particles hence providing a larger conductivity change than for large particles that which are depleted only at the surface [16]. In addition, by manipulating CVD conditions to favour formation of networked nanowire mats rather than NPs the sensitivity of tungsten oxide to N₂O was improved by an order of magnitude compared to NP films (Figure 3). This was related to the higher surface-to-volume ratio of NS compared to NPs, although this study also highlights the need to use highly networked nanowires as opposed to (quasi) aligned nanowires for optimum gas sensing performance, as these behave similarly to single nanowire or parallel nanowire arrays [17]. Similar observations were also reported for gas microsensors based on tungsten oxide nanoparticles and nanowires grown via AACVD (Figure 3) [32].

CVD has also been demonstrated to simplify sensor processing by providing direct integration of tungsten oxide sensor materials with the sensor platform. This has been demonstrated particularly for aerosol assisted CVD, which has shown the selective deposition of networked or quasi-aligned tungsten oxide NS on ceramic- [33], silicon- [32] and polymer-based platforms [28].

2.2. Zinc Oxide

Zinc oxide (ZnO) is a II-VI semiconductor with a wide direct band gap (3.37 eV), large exciton binding energy (60 meV), spontaneous polarization and piezoelectric constants which make it an attractive material for electronic, optoelectronic, energy generator and photocatalytic applications [34]. This material has been widely used for gas detection since the early 1960s, and it is still among the most reported metal oxide materials used for gas sensing. Most of the interesting functionalities of ZnO originate from its wurtzite crystal structure, which can be described as a number of alternating planes composed of tetrahedrally coordinated O²⁻ and Zn²⁺ ions, stacked along the c-axis. This crystal does not possess inversion symmetry, having a large spontaneous polarization along the [0001] crystalline direction [34,35].

Metallic Zn [36,37,38,39,40,41,42,43] or diethylzinc (Et₂Zn) [44,45,46,47], both with O₂ as carrier (reactive) gas, have been used as precursors for the formation of ZnO via CVD, although other simple precursors such as zinc nitrate (Zn(NO₃)₂) [48] and organometallic complexes (e.g., Zn(II) ketoiminate) [49,50], have also been reported. ZnO films or NPs have been typically achieved at deposition temperatures between 350 and 450 °C, whereas NS of this material have been reported at deposition temperatures exceeding 450°C, either with or without the use of gold (catalytic) seeds to encourage NS formation. Similarly to tungsten oxide, CVD has been used to facilitate processing of sensors with the deposited zinc oxide often integrated directly onto planar ceramic- or silicon-based gas sensing devices, with the exception of structures in the form of wires which apparently have been exclusively used as single structures integrated into the device after a post-transfer process (i.e., single wire sensors) (

Table 2. Summary of the features and sensing properties reported for chemical vapour deposited zinc oxide.

Prec.	CVD method	Tdep °C	Form	Features nm	Sensor Type	Top °C	ppm	gas	R	tres s	Ref.
Et2Zn	MO	-	Film	130FT	Ω	300	1660	CO	1.6	-	[44]
Et2Zn	PE	-	Film	500 FT 38 CZ	Ω +O	RT	200000	O2	1.8	-	[45]
ZnCMPLX	AA	450	Film	25CZ	Ω	60	500	C2H5OH	2	10	[51]
Et2Zn	MO	450	NPs	-	Ω	300	1000	CO	1.53	-	[46]
Zn(OAc)2	AA	350	NPs	12CZ	Ω	300	10000	DMA	1.7	240	[52]
Zn	CVD	550	NWs	30000Ø	Ω+OSNW	RT	200	C2H5OH	2	-	[36]
ZnO	CVD	-	NWs	130Ø 4000L	Ω+OSNW	200	1×106	CO	4	-	[53]
Zn	CVD	-	NWs	80Ø 3500L	FETSNW	-	400	CO	3	-	[37]
Zn	UHV	650	NWs	100Ø	Ω+OSNW	RT	100	H2	1.35	3	[38]
Et2Zn	MO	500	NRs	100Ø	Ω	300	500	O2	3.5	-	[47]
Zn	CVD	600	NRs	-	Ω+O	RT	2.5	O3	1300	45	[39]
Zn	PE	-	NRs	100Ø 2000L	Ω	400		CH2O	100	-	[40]
Zn	CVD	700	HS	-	Ω	RT	250	CO	1.8	-	[41]
Zn	VT	700	HS	800TB 150TC	FETSNW	200	1	NO2	2	-	[42]
ZnCMPLX	PE	300	HS		Ω	100	0.28	O3	1000	-	[49]
Zn	VT	410	HS	5000Ø	Ω	400	205	CH2O	38	-	[43]
ZnCMPLX	PE	300	HS		Ω	400	5000	H2	14	-	[50]
Zn(NO3)2	C	1100	F	20000	Ω SF	400	500	C2H5OH	15.3	-	[48]

Prec: precursors, T_dep: temperature of deposition, T_op: operating temperature, t_res: response time, ppm: parts per millon, R = Ra/Rg (oxidative gas), R = Rg/Ra (reductive gas), Et2Zn: Diethylzincm, ^CMPLX: complex Ω: resistive, O: optical, ^SNW: single nanowire configuration, ^SF: single flake, ^FT: film thickness, ^Ø: diameter, L: length, CZ: Crystallite size, ^TC: thickness of the combs, ^TB: thickness of the NBs, NWs: nanowires, NPs: nanoparticles, NRs: nanorods, HS: hierarchical structures, F: flake, DMA: Dimethylamine.

).

Chemical vapour deposited ZnO has shown sensitivity to gases such as CO [37,41,44,46,53], H₂ [38,46,50], C₂H₅OH [36,48], CH₂O [40,43], NO₂ [42,49], O₃ [39] and O₂ [45,47], working in resistive mode, although a few papers also report the use of ZnO in optical sensing [36,39,40,45,53]. The multifunctionality of ZnO has also allowed for the operation of sensors in an optical-resistive mode, in which adsorption of the gaseous molecules is induced via the use of UV light, which has been shown to favour the room temperature detection of O₃ [39] and CH₂O [40].

A comparison of the sensor response as a function of the gas concentrations in ppm (see Section 2.1 for details) for various CVD-deposited ZnO morphologies (Figure 4) suggests that ZnO NS provide improved sensor responses compared to thin films or particle-like films, which is consistent with the enhanced sensing properties attributed to high surface-to-volume-ratio materials. Whilst single wire sensors based on ZnO have shown good response at room temperature, ZnO films comprised of NS such as flakes, rods, or belts grouped as mats, films or agglomerates may ultimately be more advantageous due to easier integration with sensor devices, whilst still possessing greater responses compared to particle-like planar films.

The ability to influence morphology in CVD deposited material has been exploited in a number of studies to improve the gas sensing performance of zinc oxide thin films, for instance by altering reaction conditions wurtzite structure films textured along the [001] direction were prepared with differing morphology. Optimal conditions for CO sensing were found when using columnar-like ZnO grains 130 nm thick, as opposed to grains growing laterally to the substrate sized 100, 110, and 160 nm [44].

Similarly improved sensor response to H₂ was recorded for single ZnO wires 100 nm in diameter as opposed to wires with diameters of 200 nm or 600 nm which showed lower responses (less than 10%) [38]. These results were attributed to the higher concentration of structural defects when the diameter of ZnO wires are decreased, and hence by targeting wires with smaller diameters sensor performance can be optimized. In addition, hexagonal ZnO wires with curved sides demonstrated superior ethanol sensing performance than similar wires with straight sides (Figure 5), which was attributed to higher surface-to-volume ratio of the curved side wires [36].

A further demonstration of the use of CVD to manipulate morphology to improve sensor performance was obtained for hollow microspheres with nanorods grown on their outer surface formed in-situ during CVD reaction, which were found to possess more oxygen vacancies and surface sites compared to non-hierarchical structures due to structure-determined residual stress that promoted the adsorption of oxygen and electron trapping [43].

2.3. Tin Oxide

Tin oxide is an intrinsic n-type wide-bandgap (3.6–4.0 eV) semiconductor with applications in transparent conducting electrodes, antireflective coatings and gas sensors. The dual valence of tin, with tin preferably attaining oxidation states of +2 or +4, facilitates the variation of the surface oxygen composition and in turn the gas sensing properties of this material [54,55]. SnO₂ in its thick film form is one of the most used materials in current commercial resistive gas sensors, and is one of the most studied materials for gas sensing in the literature. Despite this the literature related to chemical vapour deposited SnO₂ for gas sensing is less reported than for other metal oxides.

Similarly to zinc and tungsten oxides, SnO₂ has been synthetized in the form of planar films, nanoparticles, nanowires and nanorods, with a strong dependence of the microstructure on the deposition temperature (

Table 3. Summary of the features and sensing properties reported for chemical vapour deposited tin oxide.

Prec.	CVD method	Tdep, °C	Form	Features, nm	Sensor type	Top °C	ppm	gas	R	tres s	Ref.
SnCl2.2H2O	CVD	-	Film	100T	Ω	-	6	NO2	1.2	-	[59]
TTB	MO	350	Film	50T	Ω	RT	5	H2S	1.1	-	[63]
SnCl4	IBI	RT	Film	400T	Ω	500	-	H2	-	-	[60]
SnCl4	ALD	250	Film	2.6T	Ω	300	-	CO	43	-	[61]
TMH	LE	-	NPs	~15Ø	Ω	200	20	NO2	77	160	[66,67]
Sn(NO3)4	Loc	375	NPs	-	Ω	-	200	CH4O	5	-	[64]
T-crown	AA	400	NPs	18–36Ø	Ω	300	10	NO2	1.7	-	[65]
TEH	C	850	NPs	1000Ø	Ω	300	500	C2H5OH	1075	31	[68]
SnCl2.2H2O	CVD	375	P	-	Ω	240	300	H2	1.03	-	[62]
Sn	CVD	750	NWs	-	Ω		400	CO	3.9	10	[56]
Sn	CVD	800	NWs	41Ø	ΩSNW	300	500	NO2	17	3	[57]
DBTA	PE	-	NRs	1200L 45Øb-10Ø	ΩSNW	250	100	H2	13	-	[69]
Sn	CVD	800	NWs	60Ø 20,000L	Ω	200	1	NO2	90	8	[58]
TTB	CVD	700	Plates	30–40T	Ω	250	100	C2H5OH	1.5	10	[55]

Prec: precursors, T_dep: temperature of deposition, T_op: operating temperature, t_res: response time, ppm: parts per millon, R=Ra/Rg (oxidative gas), R=Rg/Ra (reductive gas), TTB: Tin(IV)tert-butoxide, TEH: Tin(II)ethylhexanoate, T-crown: Sn(18-crown-6)Cl₄, TMH: Tetramethyltin, DBTA: dibutyltin diacetate, LE: laser enhanced, Loc: Localized, PE: plasma enhanced, C: combustion, IBI: ion beam induced, P: particles, NPs: nanoparticles, ^Ø: diameter, ^Øb: diameter at the base of nanostructure, ^T: film thickness, L: length, NWs: nanowires, NRs: nanorods. Ω: resistive, ^SNW: single nanowire configuration.

), with films and particles reported at deposition temperatures below 400 °C and nanostructured SnO₂ obtained at temperatures exceeding 700 °C. The most common precursors for the synthesis of SnO₂ via CVD include metallic tin [56,57,58], salts (SnCl₂ and SnCl₄) [59,60,61,62], [Sn(O^tBu)₄] [55,63], and other less common precursors such as Sn(NO₃)₄ [64] and the complexes [(CH₃(CH₂)₃CH(C₂H₅)CO₂)₂Sn] and [Sn(18-crown-6)Cl₄] [65], with O₂ often used as a reactive carrier gas. SnO₂ films have often been tested without being integrated into traditional gas sensing devices whereas SnO₂ in the form of NPs and NS have been directly integrated with planar ceramic-platforms. In common with tungsten oxide the localized CVD of SnO₂ nanoparticles on Si-based microhotplates using the platforms microheaters has been shown to be a viable method for the integration of the sensing metal oxide with the sensor platform [64], whilst SnO₂ single nanowire sensors are typically integrated by means of a post-transfer process.

In general SnO₂ films deposited via CVD show sensitivity to NO₂ [57,58,59,65,66], CO [56,61], C₂H₅OH [55,68], H₂ [60,62,69], H₂S [63] and CH₃OH [64]. The relative sensor response (see Section 2.1 for details) in Figure 6 indicates similar performance for planar films and particles with a notable difference in sensors based on nanowires which show a high relative response to NO₂, although we note this could be related to the particular microtrenched transducing platform used or the use of Au catalyst seeds for the formation of the nanowires [58].

Laser-induced CVD (L-CVD) SnO₂ films with a grain-like (20 nm) surface showed enhanced sensing properties to NO₂ compared to SnO₂ films grown via an alternative rheotaxial growth and thermal oxidation (RGTO) method, with the magnitude of response doubled and the response time reduced. This was attributed to the smaller grains produced via L-CVD which were considered to improve gas diffusion through grains [66,67]. Similar observations were also noticed for SnO₂ grown via AACVD, indicating that grains with smaller size elongated in one direction increase the sensor response [65]. The positive influence of small grain-like SnO₂ surfaces for sensing reductive gases as H₂ compared to compact films was also noticed, and this was attributed to the higher degree of reduction to Sn²⁺ or Sn° species in grain-like SnO₂ surfaces, likely at the outermost surface layer of the grains where oxygen vacancies can be stabilized [60].

Atomic layer deposition (ALD), a technique which is related to CVD but allows atomic level control of film thickness, has also been used to examine the influence of SnO₂ film thickness on sensor performance. The response to CO was found to increase when increasing SnO_x ALD film thickness from 1.6 nm to 2.6 nm, whereas it decreased on further increasing film thickness from 2.6 nm to 5.9 nm [61]. The results were interpreted in terms of the Debye length and resistance for the films. The Debye length was comparable with the film thickness of 2.6 nm corresponding to the maximum responsivity for CO gas sensing. For film thicknesses >2.6 nm, the decrease in response was explained by a larger fraction of the film with thickness greater than the Debye length that was not affected by the O₂ and CO chemisorption. For film thicknesses <2.6 nm, the response decrease was attributed to the increasing resistance of the SnO_x ALD film. Similar observations were found for single nanowire sensors with diameter 40 nm, close to the depletion zone depth (13.4 nm) calculated for NO₂ adsorbed on SnO₂, which had higher response to NO₂ compared to wires with larger diameters (between 62 and 117 nm) [57]. This is similar to the results found previously for ZnO nanowires [38], although the rationale provided is different (increased defect density for ZnO and Debye length for SnO₂). Thinner nanowires also showed an improved detection limit to NO₂ (Figure 7).

2.4. Complex Oxides

The p-type semiconductor titanium-doped chromium oxide (CTO) Cr_2-xTi_xO_3+y (0≤x≤0.4) shows very good selectivity and sensitivity towards NH₃ and H₂S [70,71], with several techniques having been used to synthesize CTO gas sensing material including sol-gel [72,73,74,75,76], solid-state [77,78,79], solution [80,81] and CVD [82,83]. The synthesis of CTO powders, prepared via solid-state or sol-gel, and screen-printed onto an alumina sensor platform are the most widely used techniques but CTO has also been deposited using atmospheric pressure CVD (APCVD) using [CrO₂Cl₂] and [TiCl₄] or [Ti(Oⁱpr)₄] as metal precursors [83,84]. For APCVD deposited material the gas response against 80 pm of ethanol increased with reducing film thickness (

Table 4. Summary of the features and sensing properties reported for chemical vapour deposited complex metal oxides.

Material	Prec.	CVD Method	Tdep °C	FT nm	Top °C	ppm	gas	R	Ref.
Co3O4:F	Co(dpm)2 Co(hfa)2·TMEDA	PE	200 300 400		200	100	Acetone		[90]
Cr2O3:Ti	Cr(acac)3 Ti-butoxide	AA	550	150-1000					[82]
Cr2O3:Ti	CrO2Cl2 TiCl4	AP	400 475 550	500 1000 1500	400	80	CH3CH2OH	3.1 1.7 1.1	[83,84]
Cr2O3:Ti	Chromium acetate Ti(acac)2OiPr2	ESAVD	650		200 300400500	500	NH3	1.05 1.18 1.22 1.46	[85,86]
In2O3:Ta	InMe3 Ta(NMe2)5	AA	550	650	500	100 0.08	CH3CH2OH NO2	16.9 3.01	[95]
In2O3:Ti	InMe3 Ti(NMe2)4	AA	550	790	500	100 0.08	CH3CH2OH NO2	2.62 1.80	[95]
In2O3:Zn	ZnO In2O3	CVD	400-550		RT	1-5	CO		[94]
SnO2:In	SnCl4 InCl3	CVD	400	200	50-250	1000	H2 Methanol CO	1.14 1.23 1.20	[96,97]

Prec: precursors, T_dep: temperature of deposition, T_op: operating temperature, ppm: parts per millon, R=Ra/Rg (oxidative gas), R=Rg/Ra (reductive gas), FT: film thickness, ES: Electrostatic spray assisted vapour deposition, PE: plasma enhanced, TMA: Trimethylamine, acac: cetylacetone, Me: methyl, TMEDA: tetramethylethylenediamine.

), with thinner films also having higher ideal operating temperature, increasing from 500 to 575 °C on passing from 1500 to 500 nm film thickness. Comparison to 1500 nm thick screen-printed CTO sensors, using powder with an identical composition formed via solid-state synthesis, showed the screen-printed sensors had a better R/R₀ response towards ethanol (ca. 3-4) than an equivalent APCVD sensor (1.5), although film adhesion was better for the CVD material than the screen-printed one which was fragile and readily delaminated. The difference in sensitivity was attributed to microstructure, with the lower response against ethanol due to the high density/lack of microporosity in the microstructure. Subsequently electrostatic spray assisted vapour deposition (ESACVD) was used to manipulate microporosity when depositing Cr_1.8T_0.2O₃ onto silicon wafers [85,86]. An increase in film porosity was obtained by adding a low amount of polyvinyl alcohol and ethylene glycol to the precursor solution; Figure 11a,b shows the microstructure of CTO films obtained using 0.05 and 0.005 M respectively of precursor solution without addition of polymer whilst Figure 8c and 11d show the equivalent microstructure with addition of polymer. The sensors prepared with the addition of polymer exhibited an enhancement in the gas response towards 500 ppm of ammonia compared to those prepared without additive which was attributed to the increased porosity (30-40% with polymer compared to 0-20% without).

Cobalt(II,III) oxide, Co₃O₄, is a magnetic p-type semiconductor most often used as a heterogeneous catalyst, in Li-ion batteries or as a solid-state sensor [80,87,88,89]. CVD has been used to improve the sensor performance by homogenous doping with fluorine, with F-doped Co₃O₄ successfully grown at temperatures between 200 and 400 °C by plasma enhanced-chemical vapour deposition using single-source precursors, Co(dbm)₂ (where dbm = 1,3-Diphenyl-1,3-propanedione) and Co(hfa)₂TMEDA (where hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate and TMEDA = N,N,N’,N’-tetramethylethylenediamine) respectively [90]. Sensors were tested against 100 ppm of acetone at different operating temperatures and whilst undoped films exhibited a higher sensitivity when the operating temperature was 300 or 400 °C, F-doped Co₃O₄ operating at 200 °C showed the best response of all the tested sensors (Table 4). The addition of fluorine, which is reported to increase the carrier concentrations/mobility of n-type oxide semiconductors [91,92], unexpectedly produced a higher current response in the p-type Co₃O₄ at an operating temperature of 200 °C. The presence of fluorine was thought to increase the number of holes (h⁺), the main p-type semiconductor charge carriers, by saturating dangling bonds at the surface of Co₃O₄ which otherwise would have trapped h⁺ carriers and hence reduced conductivity [93].

In₂O₃ is a widely used material in transparent conductors, in display panels and solar cell windows, in optical-antistatic coatings and it has also been used in gas sensing, with the typically low conductivity of In₂O₃ improved by doping it with zinc, titanium or tantalum [94,95]. CVD has been used to deposit mat-like Zn-doped In₂O₃ nanowires (NWs), at substrate temperatures between 400 and 550 °C, with the deposited Zn-In₂O₃ nanowires being 10–30 μm long with diameters between 50 and 300 nm (Figure 9). ZnO and In₂O₃ with graphite powder were used as sources.

Gas response to CO was enhanced by addition of zinc as a dopant, with Ra/Rg increasing from 1.2 to 2.5 (Table 4). The response and recovery times were also dramatically improved (to 20 and 10 seconds respectively) with sensors based on undoped In₂O₃ not saturating within the period of test (500 seconds). Zinc-doped indium oxide proved to be selective against CO, being relatively more sensitive to CO than to NO₂ or NO. In₂O₃ has also been doped with tantalum or titanium via AACVD at 450°C using InMe₃ (where Me = methyl) and M(NMe₂)_n (where M = Ti, n = 4; M = Ta, n = 5) as precursors [95]. Undoped indium oxide films were comprised of nanoparticles ~ 100 nm in diameter, with tantalum doping reducing particle size to ~80 nm but titanium doping increasing particle size to ~150 ± 10 nm (Figure 10).

Ta-In₂O₃ showed a much higher response to ethanol than either Ti-doped (~six times greater) or undoped (~sixteen times greater) In₂O₃ (Table 4), with the enhanced sensitivity ascribed to the decreased grain size and hence increased surface area. The response of Ta-In₂O₃ towards ethanol was particularly enhanced relative to the increase in sensitivity towards NO₂, NH₃, CO or butane where the greatest enhancement compared to an undoped sensor was double (Figure 11), suggesting there is also a chemical enhancement for ethanol sensing over and above the increase due to surface area. These results are in contrast to those obtained using Zn as the dopant which displayed particular enhancement towards CO (over NO₂) [94], indicating the potential to provide selectivity in In₂O₃ via use of different of dopant atoms.

3. Conclusions

Chemical vapour deposition has been used for synthesis of a wide variety of gas sensitive metal oxides. Planar thin films typically have poorer gas sensing performance compared to traditional screen printed equivalent, attributed to reduced porosity, but the ability to manipulate deposition conditions to alter microstructure, and/or promote formation of nanostructured materials, can mitigate reduced sensitivity. CVD is a highly promising technique for new materials synthesis due to its ability to homogenously form the complex doped and ternary/quaternary compositions which are likely to be at the heart of future advances in the field. However the real benefit of CVD is realised when considering lower power microsensor platforms, either alumina, silicon or polymer, where the ability to reproducibly integrate materials directly with the sensor platform provides an important process benefit compared to competing synthetic techniques. This advantage is likely to drive increased interest in the use of CVD for gas sensor materials over the next decade, and hence the current prospects for use of CVD in this field look excellent.