Abstract
We report on temperature-dependent (10 K – 250 K) spectral and dynamical properties of free exciton–polariton and bound exciton emission in copper iodide (CuI) bulk single crystals analyzed by means of time-resolved photoluminescence spectroscopy. The characteristic line shape of the polariton emission at low temperatures is interpreted in terms of the “k-linear term effect” on the degenerate Z1,2 excitons in CuI. For free exciton–polaritons, an increase in the decay time with increasing temperature up to 360 ps at 160 K is observed. For bound exciton emission, decay times between 180 ps and 380 ps are observed at low temperatures, revealing the expected EB3/2 dependence of radiative lifetime on the localization energy. Based on the observed rise times of bound excitons at low temperatures, a defect density of shallow acceptors of 1 × 1017 cm−3 was estimated, in agreement with measured room temperature free hole density.
Highlights
Copper iodide in the thermodynamically stable zincblende phase (γ-CuI) is a promising wide-bandgap material[1] (EG ≈ 3.1 eV) for transparent optoelectronic and thermoelectric applications due to its large exciton binding energy[2] (EXB ≈ 62 meV), intrinsic p-type conductivity with a hole mobility of up to 43 cm2/Vs in bulk crystals,[3] and its high thermoelectric figure of merit.[4]
The observed emission features must be discussed in the exciton–polariton framework since the large oscillator strength of free excitons leads to a remarkable splitting of the polariton branches that is larger than the observed emission linewidth, in contrast to, e.g., GaAs, where polariton effects can be neglected due to the small exciton oscillator strength of 7 × 10−5.46 Here, we start the discussion based on the findings presented by Suga et al.[17,28]
The exciton binding energy was determined to be about 62 meV based on the energy position of the exciton states (n = 1, 2, and 3)
Summary
Copper iodide in the thermodynamically stable zincblende phase (γ-CuI) is a promising wide-bandgap material[1] (EG ≈ 3.1 eV) for transparent optoelectronic and thermoelectric applications due to its large exciton binding energy[2] (EXB ≈ 62 meV), intrinsic p-type conductivity with a hole mobility of up to 43 cm2/Vs in bulk crystals,[3] and its high thermoelectric figure of merit.[4] Until now, CuI has been successfully applied in transparent p–n heterojunctions,[5,6,7,8] thin film transistors,[9,10,11] solar cells,[12,13,14] and scintillators.[15] Published lasing emission in CuI-based microstructures[16] substantiates the suitability of CuI as a p-type component in compact integrated optoelectronic circuits and light-emitting devices. Large exciton oscillator strengths were obtained for CuI,[17] making this material interesting for the investigation of properties of bulk polaritons and for the investigation of polariton-related effects in CuI-based cavities. In the last few years, CuI crystals and films have been studied in terms of structural,[18,19,20] electrical,[21,22] and optical properties.[16,23,24,25] In particular, the excitonic emission lines of the near-band edge emission of CuI were investigated extensively at low temperatures.[2,16,17,26,27,28] The properties of the degenerate excitonic states were investigated by uniaxial-stress and magnetooptical measurements and explained in context of the polariton picture.[17,28]
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