AACVD of Cu3N on Al2O3 Using CuCl2 and NH3

Cu3N has been grown on m-Al2O3 by aerosol-assisted chemical vapor deposition using 0.1 M CuCl2 in CH3CH2OH under an excess of NH3 at 600 °C, which led to the deposition of Cu that was subsequently converted into Cu3N under NH3: O2 at 400 °C in a two-step process without exposure to the ambient. The reaction of CuCl2 with an excess of NH3 did not lead to the growth of Cu3N, which is different to the case of halide vapor phase epitaxy of III-V semiconductors. The Cu3N layers obtained in this way had an anti-ReO3 cubic crystal structure with a lattice constant of 3.8 Å and were found to be persistently n-type, with a room temperature carrier density of n = 2 × 1016 cm−3 and mobility of µn = 32 cm2/Vs. The surface depletion, calculated in the effective mass approximation, was found to extend over ~0.15 µm by considering a surface barrier height of ϕB = 0.4 eV related to the formation of native Cu2O.


Introduction
Cu 3 N is a novel semiconductor in which crystal imperfections such as Cu interstitials (Cu i ) and nitrogen vacancies (V N ) give rise to states that are energetically located inside or very close to the conduction and valence bands, respectively [1], but do not give rise to any mid-gap states. Consequently, it has been suggested to be suitable as a solar cell absorber in view of the fact that it has an indirect energy band gap of~1.0 eV [2], but also due to the fact that n-and p-type doping are possible. However, despite the fact that Cu 3 N has been described as a defect-tolerant semiconductor, so far no one has fabricated a working p-n junction solar cell using Cu 3 N. In the past, Chen et al. [3] fabricated a Cu 3 N p-n homojunction on indium tin oxide, and Yee et al. [1] fabricated an Al: ZnO/ZnS/Cu 3 N p-n heterojunction, both of which exhibited rectifying behavior but no photogenerated current. This has been attributed to the large concentration of Cu i defects, which capture electrons and result into substantial Shockley-Read-Hall recombination and quenching of the steady-state minority carrier concentration under illumination. In other words, crystal imperfections such as V N and Cu i can still reduce the minority carrier lifetime and prevent the extraction of photogenerated electron-hole pairs in Cu 3 N. Nevertheless, Cu 3 N has been used successfully for energy storage as it has a cubic anti-ReO 3 crystal structure, belonging to the Pm3m space group (number 221) with a lattice constant of 3.8 Å [4], and a vacant body center that can readily act as a host for Li ions in batteries [5]. Cu 3 N has been obtained by many different methods including reactive sputtering [6], molecular beam epitaxy [7], atomic layer deposition [8,9] and pulsed laser deposition [10,11]. Recently, we converted Cu into Cu 3 N under NH 3 : O 2 between 400 • C and 600 • C, and observed distinct spectral features and maxima in differential transmission at 500 nm (≡2.48 eV), 550 nm (≡2.25 eV), 630 nm (≡1.97 eV) and 670 nm (≡1.85 eV) on a ps time scale by ultrafast pump-probe spectroscopy (UPPS) [12]. These correspond to the M and R direct energy band gaps of bulkrelaxed and strained Cu 3 N in excellent agreement with density functional theory (DFT) calculations of the electronic band structure [12]. This observation of the M and R direct energy band gaps in fact confirmed that Cu 3 N has a clean energy band gap. More recently, we also showed that iodine-doped Cu 3 N, i.e., I: Cu 3 N, is a p-type semiconductor and that at room temperature. CuCl 2 is soluble in water (75 g/100 mL H 2 O at 25 • C) and ethanol (53 g/100 mL CH 3 CH 2 OH at 25 • C) but less so in acetonitrile CH 3 CN (1.6 g/100 mL at 20 • C). In contrast, CuCl has a considerably lesser solubility than that of CuCl 2 . More specifically, CuCl is slightly soluble in water (0.0047 g/100 mL H 2 O at 20 • C) and insoluble in ethanol CH 3 CH 2 OH and acetone (CH 3 ) 2 CO. In order to obtain a satisfactory growth rate, a 0.1 M solution of CuCl 2 in CH 3 CH 2 OH was prepared that has a dark green color due to the (CuCl 4 ) 2− ions that are yellow and Cu +2 ions that are blue. The 0.1 M CuCl 2 liquid precursor was turned into a mist using a Venturi nebulizer and Ar as a carrier gas. Square samples of~8 mm × 8 mm m-Al 2 O 3 with a thickness of~0.3 mm were cleaned sequentially in trichloroethylene, methanol, acetone and isopropanol at 80 • C, after which they were rinsed in deionized water at 20 • C and dried with nitrogen, followed by a dehydration bake at 120 • C. The clean m-Al 2 O 3 was loaded in a quartz boat that was positioned at the center of a 1" hot wall, single zone AACVD reactor, capable of reaching 1100 • C that was fed by a manifold consisting of four mass flow controllers connected to Ar, NH 3 , O 2 and H 2 and a separate side manifold for controlling the flow of Ar through the Venturi nebulizer, as shown in Figure 1a. The reactor was purged with 1000 mL/min of Ar for 10 min from the main manifold, after which the temperature was ramped at 30 • C/min under a flow of 90 mL/min Ar: 10 mL/min H 2 at one atmosphere. Upon reaching 600 • C, the flow of Ar: H 2 was interrupted and a flow of 800 mL/min NH 3 was initiated, while at the same time a flow of 1000 mL/min Ar was established through the nebulizer. A visible flow of the aerosol was observed on the upstream side that was maintained for 30 min. Subsequently, the flow of Ar through the Venturi nebulizer was interrupted and the reactor allowed to cool down to 400 • C under a flow of 300 mL min −1 NH 3 . Upon reaching 400 • C, the Cu was converted into Cu 3 N under a flow of 300 mL min −1 NH 3 and 15 mL min −1 O 2 for 30 min. At the end of the growth period, cool down took place under a flow of 300 mL/min NH 3 supplied from the main manifold until the temperature fell below 100 • C. A typical temperature-time profile is shown in Figure 1b. The Cu 3 N layers were removed after purging with 1000 mL min −1 of Ar at room temperature and were stored in a desiccator under vacuum.

Materials and Methods
Initially, 1.34 mg of CuCl2 (Aldrich 99.999%, 134.45 gmol −1 ), which has a rusty-brown color, was dissolved in 100 mL ethanol CH3CH2OH and stirred at 1000 rpm for 10 min at room temperature. CuCl2 is soluble in water (75 g/100 mL H2O at 25 °C) and ethanol (53 g/100 mL CH3CH2OH at 25 °C) but less so in acetonitrile CH3CN (1.6 g/100 mL at 20 °C). In contrast, CuCl has a considerably lesser solubility than that of CuCl2. More specifically, CuCl is slightly soluble in water (0.0047 g/100 mL H2O at 20 °C) and insoluble in ethanol CH3CH2OH and acetone (CH3)2CO. In order to obtain a satisfactory growth rate, a 0.1 M solution of CuCl2 in CH3CH2OH was prepared that has a dark green color due to the (CuCl4) 2− ions that are yellow and Cu +2 ions that are blue. The 0.1 M CuCl2 liquid precursor was turned into a mist using a Venturi nebulizer and Ar as a carrier gas. Square samples of 8 mm × 8 mm m-Al2O3 with a thickness of 0.3 mm were cleaned sequentially in trichloroethylene, methanol, acetone and isopropanol at 80 °C, after which they were rinsed in deionized water at 20 °C and dried with nitrogen, followed by a dehydration bake at 120 °C. The clean m-Al2O3 was loaded in a quartz boat that was positioned at the center of a 1" hot wall, single zone AACVD reactor, capable of reaching 1100 °C that was fed by a manifold consisting of four mass flow controllers connected to Ar, NH3, O2 and H2 and a separate side manifold for controlling the flow of Ar through the Venturi nebulizer, as shown in Figure 1a. The reactor was purged with 1000 mL/min of Ar for 10 min from the main manifold, after which the temperature was ramped at 30 °C/min under a flow of 90 mL/min Ar: 10 mL/min H2 at one atmosphere. Upon reaching 600 °C, the flow of Ar: H2 was interrupted and a flow of 800 mL/min NH3 was initiated, while at the same time a flow of 1000 mL/min Ar was established through the nebulizer. A visible flow of the aerosol was observed on the upstream side that was maintained for 30 min. Subsequently, the flow of Ar through the Venturi nebulizer was interrupted and the reactor allowed to cool down to 400 °C under a flow of 300 mL min −1 NH3. Upon reaching 400 °C, the Cu was converted into Cu3N under a flow of 300 mL min −1 NH3 and 15 mL min −1 O2 for 30 min. At the end of the growth period, cool down took place under a flow of 300 mL/min NH3 supplied from the main manifold until the temperature fell below 100 °C. A typical temperature-time profile is shown in Figure 1b. The Cu3N layers were removed after purging with 1000 mL min −1 of Ar at room temperature and were stored in a desiccator under vacuum.  The morphology and crystal structure of the Cu 3 N layers were determined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The carrier density and mobility of the Cu 3 N layers were determined by the Hall effect in the van der Pauw configuration

Results and Discussion
The reaction of CuCl 2 in CH 3 CH 2 OH with an excess of NH 3 did not lead to the direct deposition of Cu 3 N, as in the case of HVPE of III-V semiconductors such as In x Ga 1−x N, but resulted into the deposition of metallic Cu on m-Al 2 O 3 that had a shiny, reflective surface and metallic conductivity. A typical SEM image of the Cu layer obtained on m-Al 2 O 3 at 600 • C is shown in Figure 1c, from which one may observe that the Cu layer is polycrystalline and consists of grains oriented along a single direction. A higher magnification image is also shown in Figure 1d, from which it is evident that the grains have sizes of~5 µm, while a side view of the Cu on m-Al 2 O 3 is shown in Figure 1e, showing that columnar growth occurs. The epitaxial growth of Cu on c-Al 2 O 3 and a-Al 2 O 3 has been investigated extensively for the growth of high-quality graphene [24,25], but only a few have considered the growth of Cu on m-Al 2 O 3 [26]. The deposition of Cu on m-Al 2 O 3 , which contains grooves or steps along specific crystallographic directions, as shown in Figure 2a, will lead to instabilities and ruptures of the Cu layer at elevated temperatures [27]. These ruptures occur at high curvature sites, i.e., peaks and ridges, which act as retracting edges leading to a net flux of atoms away from the high positive curvature regions. For sufficiently thin layers, this process will lead to a self-assembly of the Cu grains along a specific direction [28]. The morphology and crystal structure of the Cu3N layers were determined by scanning electron microscopy (SEM) and x-ray diffraction (XRD). The carrier density and mobility of the Cu3N layers were determined by the Hall effect in the van der Pauw configuration by using a Keithley 2635A constant current source in conjunction with a Keithley 2182 voltmeter controlled by LabView.

Results and Discussion
The reaction of CuCl2 in CH3CH2OH with an excess of NH3 did not lead to the direct deposition of Cu3N, as in the case of HVPE of III-V semiconductors such as InxGa1−xN, but resulted into the deposition of metallic Cu on m-Al2O3 that had a shiny, reflective surface and metallic conductivity. A typical SEM image of the Cu layer obtained on m-Al2O3 at 600 °C is shown in Figure 1c, from which one may observe that the Cu layer is polycrystalline and consists of grains oriented along a single direction. A higher magnification image is also shown in Figure 1d, from which it is evident that the grains have sizes of 5 µm, while a side view of the Cu on m-Al2O3 is shown in Figure 1e, showing that columnar growth occurs. The epitaxial growth of Cu on c-Al2O3 and a-Al2O3 has been investigated extensively for the growth of high-quality graphene [24,25], but only a few have considered the growth of Cu on m-Al2O3 [26]. The deposition of Cu on m-Al2O3, which contains grooves or steps along specific crystallographic directions, as shown in Figure 2a, will lead to instabilities and ruptures of the Cu layer at elevated temperatures [27]. These ruptures occur at high curvature sites, i.e., peaks and ridges, which act as retracting edges leading to a net flux of atoms away from the high positive curvature regions. For sufficiently thin layers, this process will lead to a self-assembly of the Cu grains along a specific direction [28].  Figure 2a. Likewise shown are the peaks corresponding to the underlying m-Al2O3, which has an oxygen-terminated surface with tetragonal crystal symmetry that is suitable for the epitaxial growth of semiconductors with a cubic crystal structure. It is worthwhile to point out that the deposition of Cu on n-type Si (001) resulted in columnar growth, as shown in Figure 3a,b. The Cu pillars have a height of 20 µm, but they are not ordered in any way. No Cu3N was obtained under an excess of NH3 by varying the temperature between 300 °C and 800  Figure 2a. Likewise shown are the peaks corresponding to the underlying m-Al 2 O 3 , which has an oxygen-terminated surface with tetragonal crystal symmetry that is suitable for the epitaxial growth of semiconductors with a cubic crystal structure. It is worthwhile to point out that the deposition of Cu on n-type Si (001) resulted in columnar growth, as shown in Figure 3a,b. The Cu pillars have a height of~20 µm, but they are not ordered in any way. No Cu 3 N was obtained under an excess of NH 3 by varying the temperature between 300 • C and 800 • C. Instead, the reaction of CuCl 2 in CH 3 CH 2 OH with NH 3 always led to the deposition of Cu on m-Al 2 O 3 , which occurs via the reduction of CuCl 2 to CuCl and then into Cu by the H 2 evolving from NH 3 .
°C. Instead, the reaction of CuCl2 in CH3CH2OH with NH3 always led to the deposition of Cu on m-Al2O3, which occurs via the reduction of CuCl2 to CuCl and then into Cu by the H2 evolving from NH3. More specifically, the 0.1 M solution of CuCl2 is initially converted into a mist of liquid drops and mixed with NH3, which is soluble in CH3CH2OH [29]. Subsequently, the liquid drops are vaporized at an elevated temperature, and the CH3CH2OH gives C2H4 and H2O according to the reaction C2H5OH  C2H4 + H2O. No carbon is released from the pyrolysis of C2H4 between 500 °C and 800 °C [30]. Upon vaporization, CuCl2, which has a melting point of 498 °C, will be reduced to CuCl, which has an even lower melting point of 423 °C [31], and finally into metallic Cu by the H2 evolving from the breakdown of NH3. Before elaborating further, it is useful to note that the thermal breakdown of NH3 into N2 and H2 was investigated as early as 1905 by White et al. [32], who showed that it depends on the gas flow, i.e., residence time as well as the temperature. In particular, White et al. [32] showed that a flow of 200 mL/min NH3 resulted in a dissociation of 5% NH3 at 600 °C and 10% at 700 °C. However, the breakdown of NH3 is also promoted catalytically by the deposited Cu at elevated temperatures [33]. In other words, the Cu deposited on the m-Al2O3 will participate actively in the dissociation of NH3 near the surface, thereby further promoting the reduction of CuCl2 and deposition of Cu, which has a melting point of 1085 °C. A schematic representation of the proposed reaction mechanism is shown in Figure 3c. For completeness, it must also be pointed out that the NH3 will react with CH3CH2OH and give ethylamine (CH3CH2NH2) and acetonitrile (CH3CN), which have boiling points of 20 °C and 82 °C, respectively. CH3CH2NH2 and CH3CN will dissociate into HCN and CH4 depending on the temperature and residence time, but they are not expected to influence the overall reaction governing the deposition of Cu. It is also important to mention that the Cu will tend to react with H2O supplied from the CH3CH2OH and give CuO and Cu2O. However, no oxides are detected in Figure 2a, so it is very likely that they are reduced to metallic Cu due to the H2 evolving from the NH3 over the Cu. This is consistent with the findings of Kim et al. [34], who showed that CuO is converted into metallic Cu under an excess of H2 without the formation of intermediate Cu4O3 or Cu2O.
In short, CuCl2 is reduced to CuCl and then into Cu by the H2 evolving from NH3, according to: CuCl2 + H2 → CuCl + 2HCl and 2CuCl + H2 → 2Cu + 2HCl. The HCl reacted in turn with the More specifically, the 0.1 M solution of CuCl 2 is initially converted into a mist of liquid drops and mixed with NH 3 , which is soluble in CH 3 CH 2 OH [29]. Subsequently, the liquid drops are vaporized at an elevated temperature, and the CH 3 CH 2 OH gives C 2 H 4 and H 2 O according to the reaction C 2 H 5 OH → C 2 H 4 + H 2 O. No carbon is released from the pyrolysis of C 2 H 4 between 500 • C and 800 • C [30]. Upon vaporization, CuCl 2 , which has a melting point of 498 • C, will be reduced to CuCl, which has an even lower melting point of 423 • C [31], and finally into metallic Cu by the H 2 evolving from the breakdown of NH 3 . Before elaborating further, it is useful to note that the thermal breakdown of NH 3 into N 2 and H 2 was investigated as early as 1905 by White et al. [32], who showed that it depends on the gas flow, i.e., residence time as well as the temperature. In particular, White et al. [32] showed that a flow of 200 mL/min NH 3 resulted in a dissociation of 5% NH 3 at 600 • C and 10% at 700 • C. However, the breakdown of NH 3 is also promoted catalytically by the deposited Cu at elevated temperatures [33]. In other words, the Cu deposited on the m-Al 2 O 3 will participate actively in the dissociation of NH 3 near the surface, thereby further promoting the reduction of CuCl 2 and deposition of Cu, which has a melting point of 1085 • C. A schematic representation of the proposed reaction mechanism is shown in Figure 3c. For completeness, it must also be pointed out that the NH 3 will react with CH 3 CH 2 OH and give ethylamine (CH 3 CH 2 NH 2 ) and acetonitrile (CH 3 CN), which have boiling points of 20 • C and 82 • C, respectively. CH 3 CH 2 NH 2 and CH 3 CN will dissociate into HCN and CH 4 depending on the temperature and residence time, but they are not expected to influence the overall reaction governing the deposition of Cu. It is also important to mention that the Cu will tend to react with H 2 O supplied from the CH 3 CH 2 OH and give CuO and Cu 2 O. However, no oxides are detected in Figure 2a, so it is very likely that they are reduced to metallic Cu due to the H 2 evolving from the NH 3 over the Cu. This is consistent with the findings of Kim et al. [34], who showed that CuO is converted into metallic Cu under an excess of H 2 without the formation of intermediate In short, CuCl 2 is reduced to CuCl and then into Cu by the H 2 evolving from NH 3 , according to: CuCl 2 + H 2 → CuCl + 2HCl and 2CuCl + H 2 → 2Cu + 2HCl. The HCl reacted in turn with the excess NH 3 , giving NH 4 Cl, i.e., NH 3 + HCl → NH 4 Cl, which solidified below its sublimation temperature, i.e.,~340 • C near the cool end of the reactor, very similar to what occurs during conventional HVPE of III-Vs. The reduction of CuCl 2 into Cu may also be achieved by using H 2 as opposed to NH 3 . In order to show this, the 0.1 M solution of CuCl 2 in CH 3 CH 2 OH was used to deposit a layer of CuCl 2 on 15 mm × 30 mm soda lime glass (SLG) slides by drop-casting, as shown in Figure 4a. The CuCl 2 layer had a light green color and good uniformity, and a typical SEM image is shown in Figure 4b. This was converted into Cu under a flow of (i) 10 and (ii) 50 mL.min −1 pure H 2 at 400 • C for 30 min, as shown schematically in Figure 4c. The CuCl 2 as-deposited on SLG displayed a crystalline structure and multiple peaks in the XRD, as shown in Figure 4d, but all the peaks were eliminated after the reduction of the CuCl 2 into Cu.
Materials 2022, 15, x FOR PEER REVIEW 6 of 11 excess NH3, giving NH4Cl, i.e., NH3 + HCl → NH4Cl, which solidified below its sublimation temperature, i.e., 340 °C near the cool end of the reactor, very similar to what occurs during conventional HVPE of III-Vs. The reduction of CuCl2 into Cu may also be achieved by using H2 as opposed to NH3. In order to show this, the 0.1 M solution of CuCl2 in CH3CH2OH was used to deposit a layer of CuCl2 on 15 mm × 30 mm soda lime glass (SLG) slides by drop-casting, as shown in Figure  4a. The CuCl2 layer had a light green color and good uniformity, and a typical SEM image is shown in Figure 4b. This was converted into Cu under a flow of (i) 10 and (ii) 50 mL.min −1 pure H2 at 400 °C for 30 min, as shown schematically in Figure 4c. The CuCl2 asdeposited on SLG displayed a crystalline structure and multiple peaks in the XRD, as shown in Figure 4d, but all the peaks were eliminated after the reduction of the CuCl2 into Cu. The Cu deposited on m-Al2O3 at 600 °C by AACVD using CuCl2 and NH3 has a higher crystal quality compared to the Cu obtained by sputtering, which was nonetheless successfully converted into crystalline Cu3N under a flow of 300 mL/min NH3 and 15 mL/min O2 between 400 °C and 600 °C, as shown previously [12]. The Cu3N obtained in this way had an anti-ReO3 cubic crystal structure, and we observed the M and R direct energy band gaps of Cu3N by UPPS in excellent agreement with DFT calculations of the electronic structure, confirming that it has a clean energy gap [12]. Consequently, the polycrystalline Cu layer that was obtained by AACVD on m-Al2O3 at 600 °C was converted into Cu3N under a flow of 300 mL/min NH3 and 15 mL/min O2 at 400 °C. The Cu3N had an olive-green-like color, and a typical SEM image of the Cu3N layer on m-Al2O3 is shown in Figure 1f. This exhibited peaks in the XRD, as shown in Figure 2b, corresponding to the anti-ReO3 cubic crystal structure of Cu3N with a lattice constant of 3.8 Å.
The reaction of Cu with NH3 containing O2 and the formation of Cu3N can be understood by considering the catalytic oxidation of NH3 by O2 in the presence of a catalyst, e.g., Cu, Pt, etc., at elevated temperatures, as described by Carley et al. [35], who investigated the catalytic reactivity of Cu (110) metal surfaces with coadsorbed NH3 and O2. More specifically, Carley et al. [35] proposed that the oxidation of NH3 leads to the formation of a stabilized N monolayer on the Cu metal surface, which in turn is The Cu deposited on m-Al 2 O 3 at 600 • C by AACVD using CuCl 2 and NH 3 has a higher crystal quality compared to the Cu obtained by sputtering, which was nonetheless successfully converted into crystalline Cu 3 N under a flow of 300 mL/min NH 3 and 15 mL/min O 2 between 400 • C and 600 • C, as shown previously [12]. The Cu 3 N obtained in this way had an anti-ReO 3 cubic crystal structure, and we observed the M and R direct energy band gaps of Cu 3 N by UPPS in excellent agreement with DFT calculations of the electronic structure, confirming that it has a clean energy gap [12]. Consequently, the polycrystalline Cu layer that was obtained by AACVD on m-Al 2 O 3 at 600 • C was converted into Cu 3 N under a flow of 300 mL/min NH 3 and 15 mL/min O 2 at 400 • C. The Cu 3 N had an olive-green-like color, and a typical SEM image of the Cu 3 N layer on m-Al 2 O 3 is shown in Figure 1f. This exhibited peaks in the XRD, as shown in Figure 2b, corresponding to the anti-ReO 3 cubic crystal structure of Cu 3 N with a lattice constant of 3.8 Å.
The reaction of Cu with NH 3 containing O 2 and the formation of Cu 3 N can be understood by considering the catalytic oxidation of NH 3 by O 2 in the presence of a catalyst, e.g., Cu, Pt, etc., at elevated temperatures, as described by Carley et al. [35], who investigated the catalytic reactivity of Cu (110) metal surfaces with coadsorbed NH 3 and O 2 . More specifically, Carley et al. [35] proposed that the oxidation of NH 3 leads to the formation of a stabilized N monolayer on the Cu metal surface, which in turn is responsible for the conversion of the bulk Cu layer into Cu 3 N. It should be noted that the reaction of NH 3 with O 2 also gives H 2 O according to the reaction NH 3 + O 2 → NO + H 2 O, which was observed to condense near the cool end of the reactor upon increasing the gas flow of O 2 . The reaction mechanism of the conversion of Cu into Cu 3 N is depicted schematically in Figure 3d. No Cu 3 N was obtained from Cu by using only NH 3 , in accordance with Matsuzaki et al. [15]. Moreover, no CuO or Cu 2 O peaks are detected in Figure 2b, but Cu 2 O will nevertheless form as native oxide on the surface of the Cu 3 N upon exposure to the ambient, as we have shown previously by using Raman spectroscopy [36]. Before considering the electrical properties of the Cu 3 N layers, it is useful to point out that the reaction of CuCl 2 with a smaller flow of 100 mL/min NH 3 at 600 • C mainly led to the deposition of Cu 2 O, not Cu 3 N.
In order to measure the Hall effect, Ag ohmic contacts were deposited at the four corners of the Cu 3 N layers on m-Al 2 O 3 . It has been shown that Au, Ag and Cu in Cu 3 N give rise to a semiconductor-to-metal transition and remarkably constant electrical resistivity over a very broad range of temperatures [37]. Consequently Ag, Au and Cu may be used for the formation of ohmic contacts on Cu 3 N, and in the past, we have shown that Au and Ag deposited on n-type Cu 3 N results in the formation of contacts with linear IVs [13]. The Cu 3 N layers on m-Al 2 O 3 were found to be n-type and had room temperature carrier densities of 2 × 10 16 cm −3 with a maximum mobility of 32 cm 2 /Vs. The Cu 3 N layers are n-type as they are Cu-rich, but also due to the fact that oxygen may be included in the Cu 3 N by the preferential formation of interstitial oxygen (O i ) that acts as donors, not as acceptors [36]. Furthermore, the Cu 3 N layers obtained here were found to be persistently n-type, and the carrier density and mobility did not exhibit any changes upon illumination with light of λ = 450 nm under ambient conditions. In other words, the n-type Cu 3 N layers did not exhibit any photoconductivity, which may be attributed to recombination via Cu i and V N states, in accordance with Yee et al. [1].
It is worthwhile pointing out here that Matsuzaki et al. [15] showed that epitaxial Cu 3 N layers with a thickness of 25 nm on SrTiO 3 were p-type, due to the upward surface band bending mediated by the chemisorption of O 2 − , but switched to n-type upon exposure to ultraviolet (UV) light and reverted back to p-type after terminating the irradiation. In contrast, they observed that the Cu 3 N layers remained n-type after exposure to UV light under vacuum, confirming that the adsorbed O 2 − is responsible for the surface inversion observed under ambient conditions in air. However, the epitaxial Cu 3 N layers of Matsuzaki et al. [15] were found to be persistently n-type under ambient conditions, with a carrier density of the order of 10 14 cm −3 and mobility of 100 cm 2 /Vs after annealing under NH 3 between 125 and 350 • C, suggesting a change in the composition of the surface and overall band bending. The Cu 3 N layers obtained here were found to be persistently n-type and had a room temperature carrier density of 2 × 10 16 cm −3 , perhaps due to the fact that after the conversion of Cu into Cu 3 N under NH 3 : O 2 , the flow of NH 3 was maintained for at least 30 min until the temperature fell well below 100 • C.
However, the properties of Cu 3 N layers with a thickness of a few tens of nm will depend strongly on the properties of the surface but also the properties of the underlying substrate that is often overlooked. The Cu 3 N layers obtained here are persistently n-type with a carrier density of 2 × 10 16 cm −3 , most likely due to the fact that the thickness of the Cu 3 N layers is greater than 1 µm, so it is bulk-like and will not be strongly influenced by properties of the surface or underlying m-Al 2 O 3 . In thermodynamic equilibrium, the Fermi level (E F ) with respect to the conduction band minimum (E C ) away from the surface and deep in the bulk is determined from: where N C is the conduction band effective density of states, k is Boltzmann's constant and T the temperature taken to be equal T = 300 K. The electron density is equal to n = 2 × 10 16 cm −3 , and the conduction band effective density of states in Cu 3 N is given by: where m n is the electron effective mass in Cu 3 N taken to be m n = 0.16 m o [6], m o is the freeelectron mass and h is Planck's constant. This gives N C = 1.6 × 10 24 m −3 or 1.6 × 10 18 cm −3 , so E C − E F = 0.11 eV in the bulk where a flat band condition exists. On the other hand, the energetic position of the Fermi level with respect to the conduction band edge, i.e., E C − E F , at the surface is dependent on the local density and energetic position of any surface states that will be occupied by electrons, which in turn may pin the Fermi level at the surface. According to Navío et al. [38], the Fermi level at the surface of ultrathin Cu 3 N layers is pinned at the middle of the gap, which will give rise to a barrier height of φ b = 0.5 eV. The surface depletion region will extend into the Cu 3 N, and the depletion width is: where ε S = ε R ε o , ε R is the static dielectric constant of Cu 3 N, ε o the permittivity of free space, e the electron charge and N D the donor density taken to be equal to n = 2 × 10 16 cm −3 .
Considering that the static dielectric constant of Cu 3 N is ε R~1 0 [39], the depletion width is found to be w = 0.16 µm, taking into account that the Fermi level at the surface of Cu 3 N layers is pinned at the middle of the gap, according to Navío et al. [38]. However, despite the fact that we did not detect any CuO or Cu 2 O in the XRD, a thin layer of Cu 2 O will exist on the surface of Cu 3 N. According to Hodby et al. [40], the Fermi level at the surface of Cu 2 O is pinned at states residing energetically in the upper half of the band gap~0.4 eV below the conduction band edge. The native Cu 2 O layer of Cu 3 N is expected to have a thickness of only a few nm and will be completely depleted, so the depletion width taking φ b = 0.4 eV is found to be w = 0.15 µm. The conduction band potential profile of the Cu 3 N layer including the native Cu 2 O layer at its surface is shown in Figure 5a, where the work function and electron affinity of Cu 3 N i.e., φ(Cu 3 N) = 5.0 eV and χ(Cu 3 N) = 3.5 eV [41] have been considered as well as the work function and electron affinity of Cu 2 O, i.e., φ(Cu 2 O) = 4.8 eV and χ(Cu 2 O) = 3.2 eV [42]. The formation of p-type Cu 2 O over the n-type Cu 3 N will lead to the confinement of photogenerated electron-hole pairs at the Cu 2 O/Cu 3 N heterojunction, which will inadvertently result into recombination via states at the interface, thereby suppressing the photoconductivity. This mechanism is different to that put forward by Yee et al. [1], who fabricated an Al: ZnO/ZnS/Cu 3 N p-n heterojunction that exhibited rectifying behavior but no photogenerated current, which was attributed to the large concentration of Cu i defects that capture electrons and result in substantial Shockley-Read-Hall recombination and quenching of the steady-state minority carrier concentration under illumination. While it is possible that both mechanisms are responsible for the suppression of the photocurrent and photoconductivity in Cu 3 N, it is imperative that the surface recombination should be suppressed via the deposition of suitable layers that prevent the formation of Cu 2 O that was originally suggested to act as a suitable passivation layer for Cu 3 N, similar to that of SiO 2 for Si p-n junction solar cells [2].

Conclusions
Cu 3 N layers have been grown on m-Al 2 O 3 by aerosol-assisted chemical vapor deposition using 0.1 M CuCl 2 in CH 3 CH 2 OH under an excess of NH 3 at 600 • C, which resulted in the deposition of epitaxial Cu layers consisting of oriented grains with a face-centered cubic crystal structure that were subsequently converted into Cu 3 N under NH 3 : O 2 at 400 • C in a two-step process without exposure to the ambient. The reaction of CuCl 2 with an excess of NH 3 did not give Cu 3 N, which is different to halide vapor phase epitaxy of III-V semiconductors such as In x Ga 1−x N. The Cu 3 N layers obtained in this way have an anti-ReO 3 cubic crystal structure and persistent room temperature carrier density of n = 2 × 10 16 cm −3 and mobility of µ n = 32 cm 2 /Vs, but they did not exhibit any photoconductivity due to recombination via surface states in the Cu 2 O or via indirect recombination via Cu i defects, which capture electrons and result into substantial Shockley-Read-Hall recombination Funding: This research received no external funding.