Exciton–polariton behaviour in bulk and polycrystalline ZnO

Abstract

We report detailed reflectance studies of the exciton-polariton structure of thin film polycrystalline ZnO and comparison with bulk crystal behaviour. Near-normal incidence reflectance spectra of these samples are fitted using a two-band dielectric response function. Our data show that the reflectance data in polycrystalline ZnO differ substantially from the bulk material, with Fabry-Perot oscillations at energies below the transverse A exciton and above the longitudinal B exciton in the films. In the strong interaction regime between these energies no evidence is seen of the normally rapid oscillations associated with the anomalous waves. We demonstrate that the strong interaction of the damped exciton with the photon leads to polaritons in this region with substantial damping such that the Fabry-Perot modes are eliminated. Good qualitative agreement is achieved between the model and data. The importance of the polariton model in understanding the reflectance data of polycrystalline material is clearly seen.Keywords: exciton, polariton, thin film, ZnO.Author to whom correspondence should be addressed: ZnO is receiving renewed attention for wide band-gap device applications due to a number of advantages it presents in comparison with GaN for example, e.g. larger exciton binding energy (60 meV), availability of large area substrates for homo-epitaxy etc. [1][2][3]. ZnO received a great deal of attention in the 1950's-70's, when the fundamental optical and electronic properties were studied by various authors [4][5][6]. In many respects it was an important system for studying the exciton-polariton concept for a two-band system [6,7].Recent studies have concentrated on the properties of thin films, given the deviceoriented motivation for these investigations. We have studied the reflectance spectra from polycrystalline ZnO samples grown on sapphire using pulsed laser deposition (PLD) and those from bulk single crystal material. We have used a two band polariton model with suitable boundary conditions (both electro-magnetic (EM) and additional boundary conditions (ABC)) in a thin layer to fit the experimental data [8]. Experimental details and resultsA ceramic polycrystalline ZnO target (99.99%) and a KrF excimer laser (λ=248 nm) with laser energy density of 1.7 J/cm 2 at a pulsed repetition rate of 10 Hz and pulse duration of 26 ns were used for the ablation process. The target to substrate distance was ~ 4 cm. The ZnO films were deposited on (0001) sapphire substrates at an oxygen oscillations due to multiple reflections in the thin film. Previous Raman and AFM measurements on these samples indicate that the unannealed sample has the smallest grain size and the largest surface roughness (standard deviation of ~10 nm) and that the grain size increases and the roughness decreases (to a value of ~3 nm) in the annealed samples [9]. The decrease in the intensity of excitonic features with decreasing grain size is a consequence of electric fields due to charge trapping at grain boundaries [11].We believe that the varia...

Study of exciton-polariton modes in nanocrystalline thin films of CuCl using reflectance spectroscopy

“…At photon energies below and between the exciton resonance energy positions, the L value for photon-like modes (with a small exciton-like character) will greatly increase and these fringes will become prominent, as has been reported previously for ZnO thin films. 15,16 This trend is clearly shown in Figure 3, with the Fabry-Perot fringes increasing in prominence below and between the exciton resonance energy positions.…”

“…Equations (13) and (14) above correspond to the boundary conditions from Maxwell's equations and Eqs. (15) and (16) to the Pekar ABCs for the A and B exciton bands, respectively, showing that the total material polarization due to each exciton band (itself caused by the electric fields of the associated propagating exciton-polariton mode) vanishes, in this case at the surface. Equation (13) is represented visually in Figure 2(a).…”

“…The most notable point in terms of the damping coefficients for both excitons, however, is that they are of the same order of magnitude as those of the bulk crystal, unlike the case of ZnO thin films where the thin film damping coefficients are an order of magnitude larger than those of the bulk ZnO crystal. 15,16 This is due to the much larger excitonic binding energies in CuCl compared to ZnO (190 meV versus 60 meV) which means the excitonic complex in CuCl is much more stable in a poorer quality nanocrystalline thin film environment.…”

Study of exciton–polariton modes in nanocrystalline thin films of ZnO using reflectance spectroscopy

“…The single-crystal material was purchased from the EaglePicher Corporation, and was grown using the Seeded Vapor Phase Transport (SVPT) method. More details of the reflectance measurements are given in [10,13]. No observable change is seen with different polarisations of the incident radiation.…”

Effects of excitonic diffusion on stimulated emission in nanocrystalline ZnO

“…3,12 Reflectance data on our samples, however, show a reduction in the oscillator strengths ͑i.e., a reduction in the longitudinal-transverse splitting, ⌬ LT ͒ compared to bulk material, in addition to increased exciton damping. 8,13 Additionally, the nanocrystalline films show mediocre PL quality compared to bulk material. Other reported linear optical measurements ͑e.g., PL, absorption, and reflectance͒ on poly-or nanocrystalline ZnO which show lasing at room temperature also reveal no evidence of giant oscillator strengths, 3-6,14 similar to our data.…”