Comment on “Some considerations concerning the detection of excitons by field ionization in a Schottky barrier” – [phys. stat. sol. (b) 234, No. 1, 23–35 (2002)]

Abstract

We revisit the detection of excitons in Cu 2 O by the exciton-mediated effect. It is shown that this type of detector is linear over a wide range of exciton densities, contrary to a recent claim that it should quickly saturate.A recent manuscript [1] addresses the issue of exciton detection by the voltaic effect in Cu 2 O. Based on theoretical considerations, the authors of reference [1] conclude that this type of detector, which was used by our group to report anomalous transport of high density excitons in Cu 2 O [2-4] is not adequate since it must necessarily be in a strong saturated regime. Drawing from this conclusion, the authors revisit our data and their interpretation, and then propose their own interpretation. We disagree with the conclusions of reference [1] and show in this paper that our detection method is indeed valid.For the sake of clarity, we start by briefly describing again the principle of the exciton detector itself, since the authors of [1] have apparently overlooked reference [5] where we had previously discussed its use at low particle densities. Paper [5] was triggered by the observation of a photovoltaic signal in Cu 2 O at low temperature. Through a semitransparent Au deposited electrode forming an ohmic contact, a weak, CW monochromatic light illuminated the front face of a single crystal sample of high quality cooled to low temperatures. From the back side, where a Cu electrode was deposited, a voltaic signal was detected in the absence of applied field. This result is surprising at first sight. A Cu/Cu 2 O interface is known to act as a Schottky barrier at room temperature, which under illumination gives rise to an external current in the absence of applied voltage. In a conventional Schottky detector, the presence of free carriers, holes in this case, is detected. However, the astonishing aspect was the fact that the results were obtained at low temperatures, even if excitons were produced in the first place. Thermal dissociation of excitons into free electron-hole pairs is highly unlikely in the bulk of Cu 2 O, in view of the large binding energy (150 meV) and small thermal energy, kT ~ 1 meV. We attributed the observed voltaic signal to the dissociation of excitons in the immediate vicinity of the metal/semiconductor contact by a built-in electric field, with subsequent collection of electrons and holes at the electrodes. Hence the name exciton-mediated voltaic effect. The voltaic signal was measured as a function of sample thickness, sample temperature, light frequency and intensity. All results, including complex reversal of spectral structures in the excitation spectrum, could be explained quantitatively by taking into account the migration of the

Response to “Comment on ‘Some considerations concerning the detection of excitons by field ionization in a Schottky barrier’” – [phys. stat. sol. (b)239, No. 1, 257–260 (2003)]

“…The comment [1] to the paper [2] has been expected by the authors. 1 In [2] it has been shown, that the Cu/Cu 2 O Schottky barrier which is used as an exciton detector via field ionisation of the excitons might well be in saturation for the conditions where excitonic superfluidity (ESF) and Bose-Einstein condensation (BEC) have been claimed to occur in [3][4][5][6] namely excitation intensities up to 4 × 10 6 W cm -2 , excitation photon energies in the excitonic ionisation continuum (excitation depth = 3 µm) and lattice temperatures around 2 to 4 K. These saturation effects will influence the excitation intensity dependence of the photo-current signal in the same way as a transition from diffusive to ballistic and presumably superfluid transport does. One of the main conclusions in [2] was therefore, that the data in [3][4][5][6] are not proof of ESF or BEC, though the occurrence of these effects cannot be excluded.…”

“…One of the main conclusions in [2] was therefore, that the data in [3][4][5][6] are not proof of ESF or BEC, though the occurrence of these effects cannot be excluded. The authors of this response appreciate, that even new experiments have been performed for the comment [1], which indeed help to clarify some open questions as shown below. As also shown below, they raise unfortunately more questions than they answer.…”

“…The spectra of Fig. 1 in [1] for resonant excitation in the 1s ortho exciton show, that absorption of light is necessary to observe the photo-current signal. It is not possible to create a signal by illuminating the contact directly through the sample in the transparent regions spectrally below and above the 1s ortho exciton resonance.…”

“…Without trying to be complete, we list some of the open questions and internal discrepancies in [1] or between [1] and earlier work of the groups of E. Fortin and A. Mysyrowicz [3][4][5][6] in the following sections 2 to 6 followed by a short conclusion.…”

Study of the 1s orthoexciton luminescence in Cu2O under two-photon excitation

“…Reports of 1s yellow paraexciton Bose-Einstein condensation (BEC) [4] and superfluidity [5], and 1s orthoexciton BEC [6] have attracted much attention. However, each of these experiments now has an alternative explanation [7][8][9][10][11][12] which leaves it an open question as to whether excitonic BEC and superfluidity has been, or can be, realized in Cu 2 O. Recent research has been directed toward finding different approaches to investigate this question.…”