Engineering p–n junctions and bandgap tuning of InSe nanolayers by controlled oxidation

Exploitation of two-dimensional (2D) van der Waals (vdW) crystals can be hindered by the deterioration of the crystal surface over time due to oxidation. On the other hand, the existence of a stable oxide at room temperature can offer prospects for several applications. Here we report on the chemical reactivity of γ-InSe, a recent addition to the family of 2D vdW crystals. We demonstrate that, unlike other 2D materials, InSe nanolayers can be chemically stable under ambient conditions. However, both thermal- and photo-annealing in air induces the oxidation of the InSe surface, which converts a few surface layers of InSe into In2O3, thus forming an InSe/In2O3 heterostructure with distinct and interesting electronic properties. The oxidation can be activated in selected areas of the flake by laser writing or prevented by capping the InSe surface with an exfoliated flake of hexagonal boron nitride. We exploit the controlled oxidation of p-InSe to fabricate p-InSe/n-In2O3 junction diodes with room temperature electroluminescence and spectral response from the near-infrared to the visible and near-ultraviolet ranges. These findings reveal the limits and potential of thermal- and photo-oxidation of InSe in future technologies.


Introduction
Stacking different two-dimensional (2D) van der Waals (vdW) crystals to form heterostructures is a new route to the fabrication of electronic devices [1][2][3]. Due to the numerous materials in the family of 2D vdW crystals, such as graphene, hexagonal boron nitride (hBN), metal dichalcogenides (MoS 2 , MoSe 2 , WS 2 , etc), III-VI semiconductors (InSe, In 2 Se 3 , GaSe, GaTe, etc), and elemental semiconductors (black phosphorus, bP), a large and diverse variety of heterostructures are possible. This has already led to the successful fabrication of photodetectors, light emitting diodes, and high mobility field effect transistors [1,2,[4][5][6][7][8][9][10][11][12][13]. Among these vdW crystals, γ-InSe, a direct-band gap semiconductor, is attracting increasing interest. Strong quantum confinement effects with decreasing layer thickness and high room temperature electron mobility (>0.1 m 2 V −1 s −1 ) have been achieved in exfoliated InSe and/or films grown by physical vapour deposition [14][15][16][17][18][19]. Although the chemical stability of InSe has been questioned [20], recent research has shown that 2D InSe can be chemically inert under ambient conditions [21]. The chemical reactivity of the surface of a 2D material is very important as it can hinder the observation of novel phenomena and also impose constraints on device fabrication processes. For example, uncontrolled oxidation can compromise the use of multi-layer heterostructures in future technologies. On the other hand, as for silicon, the existence of stable oxides at room temperature can offer prospects for novel applications [22,23]. Thermal annealing of bulk In-Se compounds in air or in an oxygen-rich environment can produce native oxides [24][25][26][27]. The formation and control of such oxides in 2D flakes has not yet been examined and can offer opportunities to fabricate novel 2D hybrid heterostructures.
Here we demonstrate that unlike several other 2D vdW crystals, such as GaSe and bP [28,29], 2D InSe nanolayers can be chemically stable under ambient conditions over a period of several days. However, both 2 N Balakrishnan et al thermal-and photo-annealing in air can induce an oxidation of the InSe surface over a short period of time, thus converting a few surface layers of InSe into In 2 O 3 . The propensity of InSe crystals of nanoscale thickness to oxidize in air at temperatures approaching ~100 °C or when illuminated with intense laser light is at first sight disadvantageous to future device applications; however, it can be turned to advantage by exploiting the optical transparency and electrically conducting properties of In 2 O 3 . Different thermal-and photo-annealing conditions can be used to control the thickness of the oxide layer and tune the bandgap of the non-oxidized 2D InSe by means of quantum confinement. The oxidation can be activated in selected areas of the flake by a focused laser beam or prevented by capping the InSe surface with a film of hBN. We exploit oxidation to fabricate InSe/ In 2 O 3 p-n junctions with functional properties including rectification, electroluminescence, and photoresponsivity from the near-infrared (NIR) to the visible (VIS) and ultraviolet (UV) ranges at room temperature.
Our findings demonstrate the technological potential of InSe heterojunctions formed by surface oxidation. Oxidation of 2D InSe crystals could provide a platform for novel 2D heterostructures and also a route to the shielding of thin layers against contamination in air.

Results and discussion
Our samples were prepared from bulk Bridgmangrown rhombohedral γ-InSe. The primitive unit cell of γ-InSe has a lattice parameter c = 24.961 Å (along the c-axis) and contains three layers, each consisting of four closely-packed, covalently bonded, atomic sheets in the sequence Se-In-In-Se [30], see figure 1(a). Within each a-b plane, atoms form hexagons with lattice parameter a = b = 4.002 Å. The layers interact by weak van der Waals forces, resulting in anisotropic electronic properties. The InSe bulk layers are Cddoped and have p-type conductivity with hole density p = (8 ± 3) × 10 13 cm −3 at T = 300 K, as derived from Hall effect measurements [31,32]. Flakes with areas from 1 to 10 3 µm 2 and thickness L from 4 to 100 nm were prepared from as-grown crystals by mechanical exfoliation using adhesive tape and deposited on SiO 2 / Si substrates.

Thermal-and photo-oxidation of 2D γ-InSe
Room temperature (T = 300 K) PL spectra of exfoliated p-InSe flakes with different layer thickness L are shown in figure 1(b). Each PL spectrum was measured following a 1 h thermal annealing step in air at temperatures T a ranging from 100 °C to 175 °C. The PL emission energy of freshly exfoliated flakes exhibits a large blue-shift with decreasing L, consistent with the quantum confinement of photo-excited carriers by the external surface of the flakes [14]. For flakes with L > 15 nm, the PL peak energy position is not affected by the annealing, although the PL intensity decreases significantly at T a > 150 °C ( figure 1(b)). In contrast, the PL emission of the thinner flakes exhibit an energy blue-shift, ΔE, with increasing T a (figures 1(b) and (c)) and/or with increasing annealing time t a (see supporting information S1 (stacks.iop.org/TDM/4/025043/mmedia)). For example, for flakes with L = 5 nm, the PL emission blue-shifts by up to ΔE = 35 meV at T a = 175 °C and t a = 1 h ( figure 1(b)). As shown in figures 2(a), a blueshift of the PL emission energy can be also induced by a focused laser beam (beam diameter, d ≈ 1 µm) with power P a > 1 mW. The optical images of the flakes before and after laser exposure reveal a clear change of contrast around the laser exposed areas of the flake (see circled areas in figure 2(b)). This is accompanied by an increase of the flake thickness, as measured by AFM ( figure 2(b)). We attribute the changes in the optical and structural properties of the annealed InSe layers to oxidation. As discussed below, the oxidation partially converts InSe into In 2 O 3 , thus modifying the confinement of the photo-generated carriers in the non-oxidized InSe layer. For an effective oxidation of the flakes to occur in air, the layers should be annealed to high temperatures (T a > 100 °C) and/ or relatively high laser power (P a > 0.5 mW on a 1 µm diameter spot). In particular, at the highest laser powers examined in this work (>5 mW), oxidation can occur within a few seconds and fully quenches the PL emission. This deterioration of the optical properties at high laser powers is likely to arise from crystal defects in the oxide layer and/or at the interface with the InSe layer.
To explore these effects in detail, we use x-ray photoelectron spectroscopy (XPS), a sensitive technique that can probe the elemental and chemical composition of surfaces. High resolution XPS was performed on freshly exfoliated InSe layers, on InSe layers exposed to air under ambient conditions for up to 72 h, and on InSe layers annealed at T a = 175 °C for t a = 1 h. The high resolution spectra of the In 3d and Se 3d core levels are shown in figures 3(a) and (b), respectively. A peak model using pseudo-Voigt functions (sum of Figure 2. (a) Room temperature PL spectra of an InSe flake before and after laser exposure at different powers, P a . (b) Optical image of the flake before (left) and after (right) laser exposure at powers P a = 1 mW, 3 mW and 6 mW (λ a = 532 nm, t a = 30 mins). The circles indicate the laser-exposed areas of the flake. The image on the right is an AFM image of the flake after the laser exposure. The inset shows the AFM z-profiles of a laser-exposed areas.
80%-Gaussian and 20%-Lorentzian) was fitted to the XPS spectra to distinguish contributions from different spin-orbit states and to identify the presence of metal oxides. The data and analysis confirm that detectable quantities of neither indium nor selenium oxides form when InSe is exposed to atmosphere. The binding energies of In 3d 5/2 (E In = 444.7 ± 0.1 eV) and Se 3d 5/2 (E Se = 54.1 ± 0.1 eV) are in good agreement with those in the literature for non-oxidized In-Se compounds [33][34][35]. They are not affected by exposure to air for over 72 h, indicating that the InSe nanolayers are stable under ambient conditions for several days. Moreover, the PL emission and Raman peaks of the exfoliated nanolayers persist for months when the flakes are stored in ambient conditions. We attribute the chemical stability of InSe to its high crystalline quality and low density of surface defects. Recent work has also shown that most ambient gases (e.g. H 2 O, CO, CO 2 , N 2 , O 2 ) tend to interact weakly with InSe and that their bonding requires the presence of In and Se vacancies and/or Stone-Wales defects [21]. However, this behavior changes when the flakes are exposed to high temperatures. Following the annealing at T a = 175 °C (t a = 1 h), the high resolution XPS In 3d spectrum shows two additional peaks at E In = 445.4 eV and 452.8 eV ( figure 3(a)). These values correspond to the binding energy of In 3d reported in the literature for [33,36]. Similarly, In 4d spectra shows two additional peaks at E In = 18.3 eV and 19.2 eV, which correspond to In 2 O 3 (supporting information S2) [37]. We note that the XPS spectra show no evidence of selenium oxide ( figure 3(b)). This suggests that Se tends to desorb from the surface, leaving behind nucleation sites for O-absorption. The oxidation process is also evident from additional complementary studies. As shown in figure 3(c), elemental energy dispersive x-ray (EDX) maps of thermally annealed (T a = 175 °C and t a = 1 h) InSe reveal a homogenous in-plane distribution of In, Se and O. Furthermore, transmission electron microscopy (TEM) studies of the heterostructures reveal the formation of the oxide layer and its crystal structure (supplementary information S4 and section 2.3).

Quantum confinement in InSe/In 2 O 3 heterostructures
The formation of In 2 O 3 at the expense of InSe changes the quantum confinement of the photogenerated carriers. To model the electronic properties of the InSe/In 2 O 3 heterostructures, we examined the PL and AFM data for several flakes annealed in air under different conditions. As shown in figure 4(a), the room temperature PL peak energy, E 2D , of the freshly exfoliated p-InSe flakes is well described by a simple infinite square quantum well potential model, i.e.
2635 eV and E b = 0.015 eV are the band gap energy and exciton binding energy for bulk InSe [38], µ c ∥ = 0.054 m e is the electron-hole reduced mass for motion along the c-axis [14], and m e is the electron mass in vacuum. The E 2D (L) dependence for p-type InSe is similar to that reported previously for n-type InSe [14,15]. Following  annealing, the thickness of the InSe layer is reduced and the air/InSe interface is replaced by In 2 O 3 /InSe, which we model as a half-infinite quantum well using the band alignment between In 2 O 3 and InSe from [39] (dashed line in figure 4(a)). The conduction band (CB) minimum of In 2 O 3 lies above that of InSe by ΔE c = 0.29 eV, whereas the valence band (VB) edge of In 2 O 3 lies below the VB of InSe by ΔE v = 2.05 eV ( figure 4(b)). Thus the oxide layer provides a lower potential barrier for electrons compared to that for non-oxidized InSe. By comparing the measured PL peak energy before and after annealing with the modeled curves, we can estimate the thickness, L * < L, of the non-oxidized InSe. Figure 5(a) shows the dependence of L * on L for two different annealing conditions. It can be seen that L * is reduced by about half the initial thickness L over a wide range of L from 4 nm to 14 nm (see also supporting information S3). From our estimate of L * and the thickness L a of the flake measured by AFM after annealing ( figure 5(b)), we estimate the thickness L o of the oxide layer, i.e. L o = L a − L * ( figure 5(c)). For example, for the freshly exfoliated and annealed flake shown in figure 5(d), following annealing the thickness of the flake increases from L ~ 8 nm to L a ~ 12 nm. The latter comprises the contribution of the non-oxidized InSe (L * ~ 4 nm) and In 2 O 3 (L a ~ 8 nm) layers. Thus the oxide layer grows both down into the InSe flake and up out of it, with ~50% of the oxide thickness lying below the original surface and ~50% above it. Correspondingly, carriers experience a different confinement in the InSe/ In 2 O 3 heterostructure compared to the freshly exfoliated InSe, leading to an increase of the effective band gap energy by about 10 meV.
The energy shift of the PL emission depends on the annealing conditions and it is larger for thin flakes due to the stronger sensitivity of their band gap energy to the layer thickness ( figure 4(a)). In general, the oxidation is accompanied by an increase of the surface roughness. Furthermore, crystalline defects that are present in the oxide and/or at the interface with InSe, can quench the PL signal. This effect is more pronounced as the electronic wave function spreads out into the In 2 O 3 potential barrier, e.g. for thin layers and for annealing at high T a and/or P a .
We also investigated the effect of capping the InSe flakes with a layer of hexagonal boron nitride (hBN). As shown in figure 6, the oxidation of the InSe surface can be prevented by capping the flakes with a thin (~10 nm) hBN film: following thermal annealing, an energy shift of the PL emission is visible only on those areas of the InSe flakes that are not capped with hBN. Moreover, the PL intensity of the uncapped InSe decreases by a factor of 8. On the other hand, the PL intensity of the capped InSe decreases by only a factor of 2. We note that although hBN is strongly resistant to oxidation up to high temperatures (~800 °C) [40], following thermal-or photo-annealing the regions of InSe encapsulated by hBN show a reduction in PL intensity; this is likely caused by the formation of crystal defects at the InSe/hBN interface. Thus although hBN prevents the oxidation of InSe, it does not stop the degradation of the optical properties of InSe.
These results show that we can exploit the thermaland photo-induced oxidation of InSe flakes in air to form InSe/In 2 O 3 heterostructures (figures 1 and 2). By choosing appropriate annealing conditions, we can vary the thickness of the oxide and hence tune the bandgap of the non-oxidized InSe layer by means of quantum confinement (figures 4 and 5). The oxidation can be performed on selected areas of the InSe flakes by partially capping with a film of hBN or by a focused laser beam, which provides a tool to modify the band gap energy in the layer plane with micrometer spatial resolution (figure 2).

Oxidation of p-type InSe for functional devices
The existence of a stable oxide on InSe and the selective oxidation of InSe offer the prospect for several technological applications. For the stable cubic (bixbyite) phase of In 2 O 3 , the primitive unit cell has a lattice parameter a = 10.077 Å (see figure 1(a)) [41]. In 2 O 3 has n-type conductivity due to the excess of In-atoms and oxygen vacancies, both of which act as donors. It is also transparent over a wide spectral range with a band gap energy E g = 3.6 eV at T = 300 K [39]. These properties, in combination with those of p-type InSe, can be used to design and fabricate different types of heterostructures, including rectifying p-n junctions and light emitting diodes. To fabricate the p-n junctions, bulk flakes of p-InSe with area of ~5 × 5 mm 2 and thickness of ~1 mm were annealed in air at T a = 450 °C for t a = 96 h. These annealing conditions were chosen to form a thick conducting oxide layer.  absorption in bulk γ-InSe at T = 300 K. We attribute the cut-off at hv ~ 3 eV to the photon absorption in the oxide: the high energy incident photons are mostly absorbed near the surface where the recombination time is short and photocarriers recombine before being collected at the junction interface. For white light excitation of power 100 mW cm −2 , the junctions produce an open-circuit voltage V oc ~ 0.6 V and a short-circuit current density J sc ~ 30 mA cm −2 at room temperature, corresponding to a conversion efficiency of 6%. The p-n junction also operates as light-emitting diode at room temperature. Figure 7(e) shows typical room temper ature electroluminescence (EL) emission spectra. The EL emission is peaked close to the band gap energy of γ-InSe and redshifts with increasing current due to lattice heating. These results demonstrate the potential of the bipolar character of these heterostructures for technological applications.

Conclusion
Our data and analysis demonstrate that InSe nanolayers can be chemically stable under ambient conditions for several days. The high chemical stability of InSe differs from the behavior reported for numerous 2D van der Waals crystals, including other metal chalcogenide layered compounds. Furthermore, thermal-and photoannealing in air can be used to induce the oxidation of the InSe surface, resulting in a reduction of the InSe layer thickness. Correspondingly, carriers experience a different quantum confinement in the InSe/In 2 O 3 heterostructure from the case of simple InSe nanolayers. This leads to a change of the band gap energy relative to that of freshly exfoliated InSe. However, crystalline defects that are present in the oxide and/or at the interface with InSe, can quench the optical signals. The measured reduction of the PL intensity is an unwanted side effect of the oxidation, which nevertheless does not prevent the fabrication of working devices. The oxidation can be activated in selected areas of the flake by a focused laser beam or prevented by capping the InSe surface with hexagonal boron nitride. To test the potential of oxidation for applications, we have fabricated p-InSe/n-In 2 O 3 junctions with functional properties including rectification, electroluminescence emission, and broad band photoresponsivity at room temperature. These findings reveal the limits and potential of thermal-and photo-oxidation of InSe nanolayers, and are relevant to exploitation of InSe in future technologies.

Methods
The bulk InSe crystals were grown by the Bridgman method at the Institute for Problems of Materials Science, The National Academy of Sciences of Ukraine, Chernivtsi Branch, Ukraine. The as-grown InSe crystals were studied by x-ray diffraction (XRD) using a DRON-3 x-ray diffractometer with a monochromatic Cu-Kα radiation of wavelength λ = 1.5418 Å. The XRD data revealed that the InSe crystal had γ-phase structure. For the XPS studies InSe flakes (areas from 1 to 10 3 µm 2 ) were exfoliated on 7 × 7 mm 2 SiO 2 /Si substrates. The XPS measurements were performed using a Kratos AXIS ULTRA with a monochromatic Al Kα x-ray source (hν = 1486.6 eV) operated at 10 mA emission current and 12 kV anode potential (P = 120 W), and the data processing was performed using CASAXPS version 2.3.17PR1.1 software with Kratos sensitivity factors (RSFs) to determine atomic% values from the peak areas. The electron collection spot size is ~ 700 × 300 µm 2 . All XPS binding energies were calibrated with respect to the C 1s peak at a binding energy of 284.8 eV. For the transmission electron microscopy (TEM), convergent beam electron diffraction (CBED), and energy-dispersive x-ray (EDX) studies, a cross-sectional sample of the junction was prepared by Ga ion beam thinning, with a protective Pt layer deposited on the region of interest to prevent damage of the structure during ion beam milling and lift-out in an FEI Quanta 3D FIB-SEM equipped with an Omniprobe micromanipulator system. TEM analysis was performed in a JEOL JEM-2100Plus equipped with an Oxford Instruments XMax 100TLE EDX detector and a JEOL JEM 2100F equipped with an Oxford Instruments XMax 80 EDX detector.
The experimental set-up for µPL and µEL measurements comprised a He-Ne laser (λ = 633 nm) and a frequency-doubled Nd:YVO 4 laser (λ = 532 nm), an XY linear positioning stage, an optical confocal microscope system, a spectrometer with 150 and 1200 groves mm −1 gratings, equipped with a charge-coupled device and a liquid-nitrogen cooled (InGa)As array photodetector. The laser beam was focused to a diameter d ≈ 1 µm using a 100 × objective and the µPL spectra were measured at low power (P ~ 0.1 mW) to avoid lattice heating. For the photoconductivity studies, light from a quartz halogen lamp, dispersed through a MDR-23 diffraction grating monochromator, and modulated with a mechanical chopper, was focused onto the junction. The photocurrent signal was measured using a standard lock-in amplification technique. Images of the InSe layer topography were acquired by AFM in tapping mode under ambient conditions. spectroscopy (XPS) of InSe nanolayers, time-dependent thermal annealing studies at constant temperature, high resolution transmission electron microscopic (HRTEM) and convergent beam electron diffraction (CBED) images of p-InSe/n-In 2 O 3 junction device and a table of electron and hole effective masses of InSe and In 2 O 3 for motion along the c-axis. This material is available free of charge via the Internet at http:// iopscience.iop.org/. The data, including images and spectroscopic measurements, on which this manuscript was based are available as an online resource with digital object identifier 10.17639/nott.73.