Nitrogen p-type doping in polycrystalline zinc selenide telluride films

Abstract

Nitrogen-doped zinc selenide telluride (N:ZnSe_xTe_1−x) is of interest because it is one of the widest-gap II–VI semiconductors that can still be doped p-type with reasonably high hole concentrations. We sputter deposit N:ZnSe_xTe_1−x films, varying Se/(Se + Te), or x, from 0 to 0.7, N₂ flow rate from 0.25 to 0.75 sccm, and substrate temperature from 250 to 370 °C. Increasing x from 0 to 0.39 at the optimal temperature of 370 °C and N₂ flow rate of 0.5 sccm leads to 1.3 atomic % nitrogen incorporation and wurtzite phase stabilization. Such doping and alloying increases hole concentration from 3 × 10¹⁸ cm⁻³ to 3 × 10¹⁹ cm⁻³, although mobility drops from 0.4 to 0.02 cm² V⁻¹ s⁻¹. Our sputtered N:ZnSe_0.38Te_0.62 has an absorption onset 0.1 eV greater than the ZnTe value of 1.87 eV. Increasing x from 0 to 0.51 enhances transmittance by moving absorption onset from 1.87 to 2.11 eV with diminished band gap bowing likely due to disorder, and increases the work function from 5.12 to 5.42 eV. This combination of tunable properties makes sputtered N:ZnSe_xTe_1−x desirable for transparent p-type contacts in polycrystalline Cd(Se,Te) optoelectronic devices.

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Introduction

Nitrogen-doped zinc selenide telluride (seleno–telluride; N:ZnSe_xTe_1−x) is interesting for optoelectronic applications because it is one of the widest-band gap II–VI semiconductors that can be doped p-type with reasonably high hole concentrations. Both ZnSe and ZnTe typically assume the zinc blende crystal structure, although the wurtzite polymorph has been reported for ZnTe,¹ nanocrystalline ZnSe,² and ZnSe_xTe_1−x alloys.³ Its potential applications include hole selective contacts for Cd(Se,Te) solar cells^4,5 and photoelectrochemical (PEC) carbon dioxide (CO₂) reduction cells.^6–10 It is also used in green and blue light-emitting diodes,^11–17 lasers,^18,19 visible and ultraviolet photodetectors,^7,10,20 spintronic devices,^21,22 and scintillators.^23,24

In particular for CdTe solar cells,^20,25 which is the thin-film photovoltaic (PV) absorber with the greatest deployment worldwide,²⁶ N:ZnTe is primarily used as the p-type contact. The CdTe has a high electron affinity (4.3 to 4.5 eV^27,28), a high work function (up to 5.7 eV²⁷) and defect chemistry that pins the Fermi level, which can lead to electrical barriers at its interfaces.^26,27 Resistance in the N:ZnTe (10³ Ω sq⁻¹), resistance in the CdTe (10⁴ Ω sq⁻¹), and recombination at their interface can also limit CdTe PV.²⁹ As the CdTe absorbers are alloyed with CdSe to tune the band gap and improve performance, the interface contact challenges can become even more difficult to address. Therefore, alloying ZnTe with ZnSe, and nitrogen doping the resulting zinc selenide telluride (N:ZnSe_xTe_1−x) would be attractive for Cd(Se,Te) devices, if it can improve on doping and band alignment relative to N:ZnTe.

ZnSe_xTe_1−x single crystals and epitaxial films have been successfully doped with nitrogen,^19,30–32 with hole concentrations demonstrated up to 10²⁰ cm⁻³ at Se/(Se + Te) (or x) of 0.6.³¹ Alloying ZnSe with Te has been reported to improve activation of nitrogen dopants.¹⁹ Phosphorus³³ and arsenic³⁴ dopants have also been used in epitaxial layers. Polycrystalline ZnSe_xTe_1−x films have been studied in several papers,^{3–5,21,23,35–44} but there are only three studies on p-type doping polycrystalline ZnSe_xTe_1−x.^3–5 Conductivities of 2 × 10⁻⁸ S cm^−1 4 and 10⁻³ S cm^−1 5 were reported in the initial publications. More recently the stabilization of the wurtzite polymorph was documented along with a 10¹⁹ cm⁻³ hole concentration in nitrogen doped ZnSe_0.5Te_0.5.³

Here we sputter deposit N:ZnSe_xTe_1−x films and study their optoelectronic properties. We alter N₂ flow during sputtering and co-deposit from ZnTe and ZnSe targets to make a compositional gradient. We find hole concentration optima at N₂ flow of 0.5 sccm and Se/(Se + Te) (x) composition of 0.39. High temperature growth (370 °C) yields hole concentrations up to 3 × 10¹⁹ cm⁻³ at x of 0.39 and nitrogen concentration of 1.3% in the wurtzite structure, which is a tenfold improvement relative to x of 0. Increasing x from 0 to 0.51 monotonically increases work function from 5.12 to 5.42 eV and absorption onset energy from 1.87 to 2.11 eV, as the disorder in polycrystalline thin films likely reduces band gap bowing. These semiconductor properties make N:ZnSe_xTe_1−x interesting for transparent p-type contact applications in Cd(Se,Te) and other optoelectronic devices.

Results and discussion

We examine ZnSe_0.36Te_0.64 films sputtered with different N₂ flow rates by x-ray photoelectron spectroscopy (XPS) in Fig. 1a. The N 1s peak intensity shows that higher N₂ flow during sputtering leads to more N in the film. The N 1s binding energy of 396.15 eV is consistent with nitrogen present as N³⁻ with zinc coordination, the desired chemical state for a Group V acceptor in ZnSe_xTe_1−x.⁴⁵ The measured N concentration increases from 0.6 to 2.5 total atomic % as the flow rate increases from 0.25 to 0.75 sccm, as shown in Fig. 1b. XPS survey scans and Zn 2p_3/2, Te 4d, Se 3d, N 1s, and O 1s high resolution core levels are in Fig. S1. For a constant 0.5 sccm N₂ flow rate, increasing x from 0 to 0.5 decreases N incorporation. XPS shows 10 to 34% greater x than X-ray fluorescence (XRF) in Table S1 and Fig. S2, which compare XRF and XPS compositions. Se surface enrichment is unlikely, as there is little peak shift with grazing incidence XRD (GIXRD) incidence angle (Fig. S3). Therefore, our use of pure element XPS sensitivity factors may be incorrectly shifting x higher in Fig. S2.

Fig. 1 Composition measurements: (a) XPS N 1s spectra and (b) N composition in total atomic % of ZnSe_0.36Te_0.64 films grown at 370 °C with varied N₂ flow, showing that N₂ flow during sputtering correlates with N in the film.

Hall effect shows that increasing growth temperature from 250 to 370 °C increases hole concentration. Increasing N₂ from 0.25 to 0.5 sccm slightly increases hole concentration, but increasing N₂ from 0.5 to 0.75 sccm apparently deactivates N (Fig. 2a). The hole concentration—mobility tradeoff is also best for N₂ flow of 0.5 sccm. Similar to N₂ flow, increasing x from 0 to 0.4 increases hole concentration, but p sharply drops at x > 0.4 (Fig. 2b). Increasing N₂ flow is also known to favor the wurtzite structure relative to sphalerite (zinc blende),³ but that should not influence p-type doping because we find no differences in doping or band edge between the different polytypes.

Fig. 2 Hole concentration versus hole mobility for all x compositions (a) and x or Se/(Se + Te) (b) for films with x from 0 to 0.49 grown at varied substrate temperature and N₂ flow.

At N₂ of 0.5 sccm, films with x of 0 and 0.34 have the sphalerite structure, but increasing x from 0.38 to 0.47 increases the wurtzite x-ray diffraction (XRD) peak intensities (Fig. S4). XRD in Fig. S4 further shows that at x of 0.47, increasing N₂ flow favors the wurtzite even more than at lower x. ZnSe (x of 1) has the sphalerite structure again (Fig. 3a), in agreement with density functional theory (DFT) predicting wurtzite stability only at moderate x and N compositions.³ Fig. 3b depicts the wurtzite stabilization at moderate x composition and N₂ flow, while Fig. S5 has the sphalerite and wurtzite lattice parameter variation with x composition. Fig. S5 shows that our films have larger lattice parameters than the bulk ZnTe and ZnSe values, a possible indicator of tensile strain. Fig. S6 has magnified (111) XRD reflections near 25.3° 2θ for x of 0, showing that increasing N₂ flow from 0.25 to 0.5 sccm decreases full width at half maximum and lattice parameter. Thus, the optimal N₂ flow of 0.5 sccm may form more nitrogen-on-anion (N_Te and N_Se) acceptors, which add disorder and shrink the lattice (Fig. S6) while enhancing p-type doping (Fig. 2). The lattice parameter stops shifting around x of 0.67 in Fig. S5, which could relate to the Zn–Se bond lengths staying relatively constant as ZnSe is alloyed with Te.^22,46–48

Fig. 3 Structural properties: (a) XRD of films with x varied from 0 to 1 and N₂ of 0.5 sccm grown at 370 °C, along with calculated sphalerite-ZnTe, wurtzite-ZnSe_0.47Te_0.53 and sphalerite-ZnSe peaks, and (b) a phase map of polytype XRD peaks, showing that sphalerite is stable at low and high x, and N₂ stabilizes wurtzite relative to sphalerite at moderate x.

We find that increasing x monotonically increases absorption onset energy (Fig. 4a), showing not much band gap bowing (Fig. 4b). This is surprising because former reports on single crystal,^49–53 epitaxial,^54–58 and DFT^59–63 ZnSe_xTe_1−x find large band gap bowing, usually a decrease of 0.1 to 0.2 eV with a minimum near x of 0.3 to 0.35. On the other hand, evaporated and electrodeposited films have linearly increasing band gaps with x (that is, no band gap bowing).^{21,23,37,39,41,42,44} From x of 0 to 0.34, the Tauc plot in Fig. S7 shows band gap widening by 0.12 eV instead of the 0.05 eV narrowing in Fig. 4. Thus, the Tauc plot has more band gap bowing and better quantitative agreement with reported single crystal ZnSe_xTe_1−x band gaps, so disorder may diminish bowing in the absorption onset energy (Fig. 4), where we choose an absorption onset threshold of 10⁴ cm⁻¹ to minimize the effect of film smoothness-caused interference fringes. DFT finds a connection between disorder and band gap bowing,^55,60–64 and some DFT studies have predicted far less band gap bowing in ZnSe_xTe_1−x.^{18,61,63,65–68} In Te-rich ZnSe_xTe_1−x alloys the Zn–Te bonds are the same length as in ZnTe and in Se-rich ZnSe_xTe_1−x the Zn–Se bonds are the same length as in ZnSe,^22,46–48 which may relate to the link between order and band gap bowing. We also note that in the literature introducing N into ZnTe films decreases the apparent band gap,^69–72 so more work is needed to separate band gap bowing, disorder from sputtering and N incorporation, x shifts, and defect energies such as N_Se and N_Te acceptors.

Fig. 4 Optical properties: (a) UV-visible spectroscopy log10 absorption coefficient plot for films with x of 0 to 1 and N₂ of 0.5 sccm grown at 370 °C. (b) The gray line at 1 × 10⁴ cm⁻¹ in (a) intercepts the data to show a monotonic absorption onset energy increase with x.

Kelvin probe measurements show a nearly linear increase in work function with x for N₂ of 0.25 sccm (Fig. 5a). Increasing N₂ flow appears to separate more highly doped material at low x < 0.6 from less doped material at x > 0.6, the latter of which greatly increases work function by lowering the valence band. Fig. S8 has XPS secondary electron cutoff (SECO) for different x composition and N₂ flow rates, showing similar work function trends at greater magnitudes, relative to Kelvin probe (Fig. 5). Extrapolating E_F–E_V from ultraviolet photoelectron spectroscopy (UPS) on a logarithmic scale (Fig. S9) shows similar E_F–E_V as Hall effect data for the different N₂ flow rates (Table 1).

Fig. 5 (a) Kelvin probe (rings) and XPS-extracted work functions as a function of x for films grown at 370 °C and varied N₂ flow. (b) Energy band alignment for N:ZnSe_xTe_1−x with varied x and N₂ of 0.5 sccm grown at 370 °C, incorporating XPS SECO work function, Hall E_F–E_V, and UV-visible spectroscopy absorption onset energy.

Table 1 Summary of key results. Hole concentration is p, hole mobility is μ_p, conductivity is σ, and work function is φ. An asterisk (*) means the E_F–E_V is estimated assuming resistivity is proportional to p. A double asterisk (**) means the band gap is interpolated from other x compositions

XRF x or Se/(Se + Te)	0	0.35	0.36	0.38	0.51	0.68
N2 (sccm)	0.5	0.25	0.5	0.75	0.5	0.5
p (cm^−3)	3.1 × 10^18	1.5 × 10^17	1.2 × 10^19	6.5 × 10^16	5.5 × 10^17	—
μp (cm^2 V^−1 s^−1)	0.40	4.3	0.062	0.99	0.14	—
σ (S cm^−1)	0.20	0.10	0.12	0.010	0.012	4 × 10^−3
Hall EF–EV (eV)	0.05	0.12	0.01	0.15	0.09	0.28*
UPS EF–EV (eV)	0.23	0.21	0.29	0.33	0.40	—
Kelvin ϕ (eV)	4.83	5.00	4.95	4.91	4.97	5.21
XPS ϕ (eV)	5.12	5.23	5.20	5.25	5.42	—
Eg (eV)	1.87	1.94**	1.95**	1.98**	2.11**	2.38

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The complete picture of band positions for 0.5 sccm N₂ is shown in Fig. 5b. We use the XPS work functions because unlike Kelvin probe data, XPS data yield absolute functions. Perhaps for this reason, they are in better agreement with a former report.²⁸ As x is increased from 0 to 0.36, the band gap widens 0.08 eV, the Fermi energy decreases 0.08 eV, and the valence band decreases by 0.04 eV. As x is increased from 0.36 to 0.51, the band gap widens 0.16 eV, the Fermi energy drops another 0.22 eV, and the valence band drops another 0.3 eV. Thus, hole concentrations are optimal for x of 0.3 to 0.4 (Fig. 2), while lower valence band alignment is achieved at x of 0.5 to 0.7 (Fig. 5). Since alloying N:ZnTe with Se increases transmittance, increases hole concentration, and lowers the valence band (Table 1), we conclude that N:ZnSe_xTe_1−x is an attractive alternative to N:ZnTe for p-type contacts in optoelectronics.

Conclusions

For sputter deposited polycrystalline N:ZnSe_xTe_1−x at optimal temperature of 370 °C and N₂ flow of 0.5 sccm resulting in 1.3% N incorporation, increasing x from 0 to 0.39 improves hole concentration from 3 × 10¹⁸ cm⁻³ to 3 × 10¹⁹ cm⁻³, although mobility drops from 0.4 to 0.02 cm² V⁻¹ s⁻¹. Increasing x from 0 to 0.38 increases absorption onset energy by 0.1 eV because band gap bowing is diminished, likely due to disorder. Increasing x from 0 to 0.51 increases work function from 5.12 to 5.42 eV and increases transmittance by shifting the absorption onset energy from 1.87 to 2.11 eV. The increased work function, shifted absorption onset and higher hole concentration together make N:ZnSe_xTe_1−x attractive for transparent p-type contact application in Cd(Se,Te) and other optoelectronic devices.

Methods

We clean 2 in. × 2 in. soda-lime glass (SLG) substrates with a thickness of 2.3 mm by ultrasonication in hot de-ionized (DI) water with Liquinox soap for 20 min. We then rinse thoroughly with DI water and blow with N₂ until completely dry. Starting at a base pressure of 10⁻⁷ Torr, we radio frequency (RF) co-sputter in 2.7 mTorr Ar (16 sccm) from ZnTe and ZnSe targets (99.99%; 2 in. diameter; 0.125 in. thick) at 20 W and 40 W, respectively. Depositing for 60 min leads to 300–500 nm thick films. The ZnTe and ZnSe targets are on opposite sides of the chamber, 37° off-normal to the substrate, and we co-deposit on stationary substrates to grow a compositional gradient across the substrates. We ramp substrate temperature to 370, 310, or 250 °C over the course of 50 min and pre-sputter for 10 min before opening the target shutters. We flow 0.25, 0.5 or 0.75 sccm N₂ through a gas distributor near the substrates and do not use an N₂ cracker.

We use XRF to measure film composition with a Rh anode at 50 kV for 60 s at 22 locations across each 2 in. × 2 in. sample, quantifying relative Se and Te content with Fischer XDV-SDD WinFTM software. We cut 5 mm × 5 mm squares and use silver paint contacts for Hall effect measurements at 1 T. We convert hole concentration to E_F–E_V assuming a valence band density of states (N_V) of 1.8 × 10¹⁹ cm⁻³.⁷³ We perform symmetric θ–2θ XRD with a Bruker D8 using monochromated Cu Kα radiation with a 1 mm spot and a 2-dimensional detector, integrating over χ. We do not correct for the 2D detector pixels’ angular coverage,⁷⁴ so the lower intensity at 2θ of 20°–23° is related to the diffractometer. We perform GIXRD on a Rigaku Smartlab diffractometer with a 2 mm spot in parallel beam geometry. We simulate diffraction patterns of ZnSe_xTe_1−x by interpolating the lattice parameters of the ZnTe sphalerite (a = 6.102 Å),⁷⁵ ZnTe wurtzite (a = 4.31 Å; c = 7.09 Å),⁷⁶ ZnSe wurtzite (a = 4.003 Å; c = 6.54 Å),⁷⁷ and ZnSe sphalerite (a = 5.62 Å)⁷⁸ end-members (Vegard's law). We fit lattice parameters to experimental patterns by matching multiple simulated XRD peak positions. We perform UV-visible spectroscopy with a Cary 6000 spetrophotometer and a diffuse reflectance integrating sphere. We combine transmissivity (T), reflectivity (R) and thickness (t; from stylus profilometry) to construct Tauc plots, where absorption coefficient, α = ln[(1 − R)²/T]/t.⁷⁹ We square the product of α, Planck's constant (h), and photon frequency (ν) and use a least-squares fit on the linear region, extrapolating to α = 0 for band gap. We use a Kelvin probe to measure work function by calibrating to Au and Al.

For films used in XPS measurements, instead of SLG we use Tec-15 substrates (SLG/diffusion barrier/SnO₂:F). We make electrical contact to the SnO₂:F layer using a molybdenum mask. We perform XPS measurements at near-normal electron-take off angle using monochromatic Al Kα radiation in a customized Physical Electronics VersaProbe III. High energy resolution XPS spectra are quantified into “equivalent homogeneous compositions”⁸⁰ using literature sensitivity factors and Shirley backgrounds. We collect wide range “survey” spectra using 280 eV pass energy to assure sample cleanliness. We clean air-exposed films using a gas cluster ion source (15 kV Ar₂₀₀₀⁺) until no extrinsic impurities can be observed. We perform work function measurements using low power X-ray excitation with a −5 V bias, and a pass energy of 6.5 eV. Under these conditions and using XPS analyzer settings, with He I radiation (21.22 eV) the Fermi edge of clean gold is located at 1460.5 eV. We use X-rays rather than UPS in work function determinations to avoid a helium lamp induced artifact observed on some materials.⁸¹ We obtain valence band edges from log10 plots of UPS data, a procedure that captures low density of states (DOS) edges that can evade detection by linearly scaled XPS or UPS spectra.^82,83 We take core level spectra (pass energy 55 eV) with and without exposure to our polychromatic helium lamp to test for possible source-induced photovoltages during UPS: we observe none. We numerically subtract satellites in UPS spectra after measuring the relative intensities of the satellites from spectra of the Fermi edge feature of clean gold foil. We calibrate the binding energy scale of the XPS and UPS experiments using the Fermi edge feature of clean metal foils and the literature values of copper 2p_3/2 (932.67 eV) and gold 4f_7/2 (83.98 eV).⁸⁴ We work up data using a combination of Physical Electronics MultiPak v9.9.1.1 and Wavemetrics Igor Pro 9.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: XPS survey scans and high resolution core levels; XRF and XPS compositions; GIXRD; XRD for different N₂ flow rates and x compositions; sphalerite and wurtzite lattice parameters; magnified XRD peaks showing full width at half maximum; UV-visible spectroscopy Tauc plot; XPS SECO data and fits; log UPS data and fits. See DOI: https://doi.org/10.1039/d6tc00099a.

Acknowledgements

This work was authored by NLR for the U.S. Department of Energy (DOE), operated under contract No. DE-AC36-08GO28308. Funding for structural and optoelectronic characterization was provided by the Liquid Sunlight Alliance, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Fuels from Sunlight Hub, under award number DE-SC0021266. Support for the thin film growth was provided by First Solar Inc. under Agreement 10543 CRD13507. The authors gratefully acknowledge the help of Jocienne Nelson. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government.

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