Nitrogen is a deep acceptor in ZnO

Zinc oxide is a promising material for blue and UV solid-state lighting devices, among other applications. Nitrogen has been regarded as a potential p-type dopant for ZnO. However, recent calculations [Lyons, Janotti, and Van de Walle, Appl. Phys. Lett. 95, 252105 (2009)] indicate that nitrogen is a deep acceptor. This paper presents experimental evidence that nitrogen is, in fact, a deep acceptor and therefore cannot produce p-type ZnO. A broad photoluminescence (PL) emission band near 1.7 eV, with an excitation onset of ∼2.2 eV, was observed, in agreement with the deep-acceptor model of the nitrogen defect. The deep-acceptor behavior can be explained by the low energy of the ZnO valence band relative to the vacuum level. C © 2011 Author(s). This article is distributed under a Creative Commons Attribution Non-Commercial Share Alike 3.0 Unported License. [doi:10.1063/1.3582819]

Zinc oxide, a direct-gap semiconductor with a wide band gap of 3.4 eV, 1 is considered a promising material for a variety of electronic and optoelectronic applications, including blue and UV solid-state lighting devices.A suitable p-type dopant is, however, an outstanding issue for realizing its potential for practical device applications. 24][5] Experiments [6][7][8] and some theoretical investigations 9,10 suggest N O to produce a shallow acceptor in ZnO, with an estimated hole ionization energy of ∼200 meV.
As reviewed in Ref. 2, the scientific literature is full of reports that nitrogen doping can produce p-type ZnO.Lyons, Janotti, and Van de Walle 11 revisited the properties of nitrogen in ZnO following a first-principles computation using hybrid functionals.Their calculations show that N O is a deep acceptor, with the (0/-) acceptor level 1.3 eV above the valence band maximum.Optical absorption and emission energies of 2.4 eV and 1.7 eV, respectively, were estimated from a configurationcoordinate diagram analysis.The Franck-Condon shift of 0.7 eV is due to large lattice relaxation of the deep acceptor.These theoretical results suggest that the reports of p-type doping need to be reexamined.
In this paper, we present an experimental investigation of bulk N-doped ZnO samples.Photoluminescence (PL) measurements were carried out at room temperature using JY-Horiba FluoroLog-3 spectrofluorometer equipped with double-grating excitation and emission monochomators (1200 grooves/mm, resolution 5 nm) and an R928P photomultiplier tube (PMT).The excitation source was a 450-W xenon CW lamp.An instrumental correction was performed on all PL spectra, to correct for the wavelength-dependent PMT response, grating efficiencies, and the variation in output intensity from the lamp.Infrared (IR) absorption spectra of nitrogen-related complexes were obtained with a Bomem DA8 vacuum Fourier transform infrared (FTIR) spectrometer equipped with a globar light source, a KBr beamsplitter, and a liquid-nitrogen-cooled indium antimonide (InSb) detector.A Janis closed-cycle helium cryostat system was used to maintain the samples at low temperatures.
Bulk single-crystal samples were grown via a seeded chemical vapor transport method in an ammonia ambient, which provided the nitrogen, as well as hydrogen, dopants. 12An IR absorption peak at 3148 cm -1 at room temperature (3151 cm -1 at 10 K) was observed.This peak has been unambiguously assigned to a N-H complex. 13Thermal anneals were carried out in an evacuated silica ampoule filled with 0.5 atm oxygen before sealing.Annealing at 675 o C drives out hydrogen donors 14 while the N-H bond remains intact.To activate nitrogen acceptors, the sample was annealed at 775 o C, causing the N-H pairs to dissociate, leading to a reduction in the N-H absorption peak intensity (Fig. 1).The decrease in free carrier absorption after the 775 o C anneal indicates that the nitrogen acceptors compensate a fraction of the donors.This decrease in n-type conductivity was previously observed by Hall-effect measurements on similarly grown samples annealed in oxygen. 13igure 2 shows PL emission spectra at an excitation wavelength of 490 nm (2.53 eV) for the sample annealed in O 2 at (a) 675 o C and subsequently at (b) 775 o C. A broad "red" emission band is detected at ∼730 nm (1.70 eV), in good agreement with the deep-acceptor model. 11To further increase the concentration of N acceptors, the sample was subsequently annealed at 775 o C for 8.5 h.The sample shows a decrease in N-H peak intensity and corresponding increase in the red emission band (Fig. 2(c)).
The photoluminescence excitation (PLE) spectrum (Fig. 3) was obtained by monitoring the red emission band (at 730 nm) as a function of excitation wavelength.An excitation onset of ∼2.2 eV was observed, consistent with the configuration coordinate diagram for optical transitions based on the deep-acceptor model (Fig. 3 inset).The main features of the PLE spectrum -an excitation onset followed by a monotonic increase with photon energy -are similar to PLE spectra observed for deep native acceptors in ZnSe. 15The PLE spectrum concurs with the observed optical transmission spectrum of the sample (not shown).Absorption of photon energies > 2.2 eV gives the sample a reddish appearance.A similar absorption profile has been correlated with nitrogen acceptors detected by electron paramagnetic resonance. 16,17 frared absorption spectra were obtained at 10 K while the sample was exposed to 475 nm (2.61 eV) laser light. 18An increase in free-carrier absorption was observed (Fig. 4).This provides additional evidence of a deep level, since a photon energy of 2.61 eV cannot cause a transition of an electron from a shallow acceptor level to the conduction band in ZnO, which has a band gap of 3.4 eV.The photoconductivity persists for several hours when the sample is left in the dark.The observation of free carrier absorption is consistent with the optical excitation of an electron from the N O − deep acceptor to the conduction band.The persistence of the free carriers following illumination is consistent with large lattice relaxation of a deep-level impurity. 19o further test the deep-acceptor model, an N-doped sample was annealed in hydrogen at 700 o C.An increase in free carrier density and N-H peak intensity was observed after the hydrogen anneal, indicating passivation of N acceptors by hydrogen.The PL emission shows a dramatic disappearance of the red emission band after the hydrogen anneal (Fig. 5), manifestly consistent with hydrogen passivation of deep N acceptors.
Native defects are often invoked to account for the observed optical and electrical properties of ZnO. 20To verify that the red emission that we observe and associate with N acceptors is neither due to oxygen vacancies nor zinc vacancies, nominally undoped single-crystal ZnO samples from Cermet, Inc. 21 were annealed in zinc vapor and oxygen, respectively.The zinc-annealed sample turned red, in agreement with prior work that attributed the red color to oxygen vacancies, 20 while the oxygenannealed sample did not show any color change.Neither of the samples exhibits the red PL emission band (Figs.2(d) and 2(e)).When excited by above-band-gap light, the Cermet samples and our N-doped samples all exhibit the well-known "green luminescence" band centered at ∼2.5 eV. 2 The green PL emission increased upon annealing in zinc or oxygen vapor.Green luminescence has been attributed to various defects, including the zinc vacancy, oxygen vacancy, and copper impurity. 2The observation that red luminescence occurs only in N-doped samples provides additional support for our assignment.
The deep acceptor behavior of nitrogen in ZnO appears to be at odds with results in other II-VI semiconductors.In ZnTe, 22 ZnSe, 23 and ZnS, 24 nitrogen is a shallow acceptor and can be used to obtain low-resistivity p-type material.The fundamental difference between ZnO and these other materials can be seen by examining the positions of their valence-band maxima, relative to the vacuum level.Figure 6 shows such a band alignment diagram, 25 where the nitrogen (0/-) acceptor level is assumed to have a constant absolute energy.In ZnO, the (0/-) FIG. 4. IR absorption spectra obtained at 10 K during 475 nm light exposure and in the dark after exposure, using the spectrum before light exposure (not shown) as a reference.The increase in absorption is attributed to enhanced free-carrier absorption due to illumination.level is 1.3 eV above the valence-band maximum, making nitrogen a deep acceptor.For the other II-VI semiconductors shown in Fig. 6, the (0/-) level lies in the valence band.In those cases, the nitrogen atom is negatively charged and weakly binds a hole; i.e., it acts as a shallow acceptor.
In summary, N-doped ZnO samples exhibit a broad red PL emission band, peaking near 1.7 eV, with an excitation onset of ∼2.2 eV, in good agreement with the deep-acceptor model 11 for the nitrogen impurity.The intensity of this red emission band increases with the concentration of activated nitrogen acceptors.Observation of an increase in free-carrier concentration when the sample is exposed to 475 nm (2.  25 The constant (0/-) acceptor level (dashed line) results in nitrogen acting as a deep acceptor in ZnO.In the other semiconductors, this level is resonant with the valence band, causing nitrogen to act as a shallow acceptor.

FIG. 1 .
FIG. 1. (Color online) Room-temperature IR absorption of N-doped ZnO after annealing in O 2 at 675 o C and 775 o C. The reduction in background free-carrier absorption corresponds to the dissociation of a fraction of the N-H complexes.Inset: Baseline-corrected spectra of N-H absorption peaks.

FIG. 2 .
FIG. 2. (Color online) Room-temperature PL emission spectra (excitation wavelength: 490 nm) for N-doped ZnO after annealing in O 2 at (a) 675 o C for 3.5 h, and subsequently at (b) 775 o C for 3.5 h and (c) 775 o C for 8.5 h.Also shown are the emission spectra of Cermet samples annealed in (d) Zn vapor and in (e) O 2 .

FIG. 3 .
FIG. 3. (Color online) Room-temperature PLE spectrum for the red emission band at 730 nm wavelength, from the N-doped ZnO sample.Inset: Configuration coordinate diagram for the deep-acceptor model. 11 FIG. 5. (Color online) Room-temperature PL emission of the sample before and after hydrogen annealing.The disappearance of the "red" PL emission is attributed to hydrogen passivation of deep nitrogen acceptors.