Intrinsic Gap States Research Articles

Intrinsic Gap States in Semiconductors with Inverted Band Structure: Comparison of SnTe vs PbTe Nanocrystals

In this contribution we compare size-dependent optical properties of PbTe and SnTe nanocrystals (NCs). We demonstrate that the size dependence of the band edge absorption line and the photoluminescence (PL) of PbTe NCs can be quantitatively described by optical transitions between the lowest quantum confined states of conduction and valence bands. In contrast, the optical properties of SnTe NCs are strongly influenced by intrinsic gap states that arise from the inverted band structure that can be described by introducing a negative energy gap Eg < 0. In principle, these intrinsic gap states could be observed directly in PL and absorption of small size SnTe NCs, where the wave functions of these states are spread over the entire NC volume. In our samples, however, these transitions are not observed due to self-doping of SnTe NCs by holes created from negatively charged Sn vacancies. As a result, the absorption is blue-shifted due to filling of the gap states and confined valence band levels by holes (phenomenon known as Moss–Burstein effect), and PL is completely suppressed due to nonradiative Auger recombinations. The size-dependent Moss–Burstein effect in our NCs is quantitatively modeled by assuming a hole concentration of p ≈ 2.2 × 1020 cm–3 which corresponds to several tens of holes per NC.

Intrinsic localized gap states in IGZO and its parent single crystalline TCOs

We report on the X-ray absorption data for Indium–Gallium–Zink–Oxide thin films, amorphous ZnO films, amorphous SnOx films, and single crystalline In2O3, Ga2O3, ZnO, and SnO2 data. These absorption data probe the empty conduction band states explicitly. Also they allow for an elemental assignment using resonant excitation to derive the contributions of each metal ion. We find that the lowest states appear right at the Fermi energy and result from configuration interaction induced charge transfer states which we consider as intrinsic gap states.

Si passivation effects on atomic bonding and electronic properties at HfO2/GaAs interface: A first-principles study

A theoretical study on atomic structures and electronic properties of the interface between GaAs and HfO2 is reported. The intrinsic gap states are mainly originated from Ga dangling bonds, partial Ga-oxidation, and As−As dimers in the reconstructed interface structures. Si passivation interlayer can introduce two types of Si local bonding configuration of Si interstitial or substitutional defects (SiHf). SiHf–passivated interfaces are found to be energetically stable and can suppress the interfacial flat bandgap state stemming from partial Ga-oxidation into the valence band of bulk GaAs. Furthermore, gap states near the conduction bandedge are partially reduced. With the increase of Si concentration at the interface, the charge state of interfacial Ga decreases from +1.26 to between +0.73 and +0.80, and this change shows a Ga oxidation state transformation from Ga2O3 (+1.7) to Ga2O (+0.52) states. The metastable Si interstitials also eliminate Ga2O3-oxidation state and creates Ga2O-like Ga charge state at the interface. However, the gap states near the conduction bandedge cannot be passivated by substitutional (SiHf) nor by interstitial (Sii) silicon. The detailed nature of the gap states examined in this modeling study would facilitate further development of interface passivation and the optimization of Si-passivation layers.

Effects of thermal annealing on the band alignment of lanthanum aluminate on silicon investigated by x-ray photoelectron spectroscopy

In this work, we investigate the changes in the band offsets of lanthanum aluminate on silicon after postdeposition annealing at 600 and 800 °C by x-ray photoelectron spectroscopy (XPS). It is found that annealing at 800 °C reduces the conduction band offset from 2.31 to 1.39±0.2 eV. A detailed analysis is performed to ascertain the origin of the changes. We will show that the observed band offset changes are not a consequence of alterations in the bulk properties of the oxide film, but rather a true band alignment change between the two materials. After systematically considering “artefacts” of XPS measurements, including extra-atomic relaxation and differential charging, we conclude that the band offset changes originate mainly from an interfacial effect. While intrinsic gap states dipoles are not sufficient to account for the large band offset shifts, we turned our attention to examine the interface of the gate oxide stack. We show the existence of at least two types of dipoles. One of the dipoles exists at the silicon-silicon oxide interface, while the strength of the other dipole can be correlated with the thickness and the chemical stoichiometry of the interfacial silicate.

Control of efficiency of photon energy up-conversion in CdSe/ZnS quantum dots

Efficient photoluminescence (PL) up-conversion in CdSe/ZnS quantum dots prepared by an organometallic approach is reported. It is demonstrated that the efficiency of photon energy up-conversion and the magnitude of the spectral shift can be controlled by (i) the thickness of the ZnS layers, (ii) the temperature dependence of the excited-state absorption coefficient, and (iii) the dependence on the excitation intensity. From the analysis of the experimental data, it is proposed that intrinsic gap states are involved as intermediate states in the PL up-conversion, rather than nonlinear two-photon absorption or Auger processes.

Up-conversion luminescence via a below-gap state in CdSe/ZnS quantum dots

We report on the efficient photoluminescence up-conversion in colloidally-prepared core-shell CdSe/ZnS quantum dots. We demonstrate that the efficiency of photon energy up-conversion and the magnitude of the spectral shift can be controlled by the thickness of the ZnS shell, the temperature and the excitation intensity. Based on the analysis of the experimental data we propose that intrinsic gap states are involved in the photoluminescence up-conversion as intermediate states rather than nonlinear two-photon absorption or Auger processes.

Up-Conversion Luminescence in Colloidal CdTe Nanocrystals

ABSTRACTWe report on the efficient photoluminescence up-conversion in colloidally synthesized CdTe nanocrystals. We demonstrate that the efficiency of photon energy up-conversion and magnitude of spectral shift can be controlled: (i) by the size of the nanocrystals; (ii) by the temperature dependence of the excited state absorption coefficient; (iii) by the dependence on excitation intensity. From the analysis of the experimental data we can suggest that intrinsic gap states are involved as intermediate states in the photoluminescence up-conversion rather than nonlinear two-photon absorption or Auger processes.

Intrinsic Gap States in Semiconductor Nanocrystals

We demonstrate the existence of intrinsic gap states in bare and capped semiconductor nanocrystals within multiband effective mass theory. These states originate from Shockley-like surface states which, in small nanocrystals, extend over the entire crystal volume, facilitating their observation in absorption as well as in photoluminescence. The conditions under which such intrinsic states might be observed are discussed in light of the theory developed and analysis of the band parameters of direct gap semiconductors.