Calculated Band Alignment Research Articles

Band alignments between SmTiO3, GdTiO3, and SrTiO3

The generation of a two-dimensional electron gas (2DEG) with unprecedented high density at the interface between two complex oxides has spurred interest in the growth and characterization of these materials. Interfaces between SrTiO3 and the rare-earth titanates SmTiO3 and GdTiO3 exhibit 2DEG densities of 3 × 1014 cm−2. Band alignments are key descriptors of these interfaces, and the authors report a joint experimental/computational investigation. Photoemission spectroscopy was used to measure the band alignments at the SmTiO3/GdTiO3 (110)o interface. In parallel, hybrid density functional calculations were performed. The measured and calculated band alignments for both the top of the O 2p band and the Ti 3d lower Hubbard band agree to within 0.13 eV. Our results also shed light on the position of the lower Hubbard band with respect to the O 2p valence band.

Engineering Solar Cell Absorbers by Exploring the Band Alignment and Defect Disparity: The Case of Cu‐ and Ag‐Based Kesterite Compounds

The development of kesterite Cu2ZnSn(S,Se)4 thin‐film solar cells is currently hindered by the large deficit of open‐circuit voltage (Voc), which results from the easy formation of CuZn antisite acceptor defects. Suppressing the formation of CuZn defects, especially near the absorber/buffer interface, is thus critical for the further improvement of kesterite solar cells. In this paper, it is shown that there is a large disparity between the defects in Cu‐ and Ag‐based kesterite semiconductors, i.e., the CuZn or CuCd acceptor defects have high concentration and are the dominant defects in Cu2ZnSn(S,Se)4 or Cu2CdSnS4, but the AgZn acceptor has only a low concentration and the dominant defects are donors in Ag2ZnSnS4. Therefore, the Cu‐based kesterites always show p‐type conductivity, while the Ag‐based kesterites show either intrinsic or weak n‐type conductivity. Based on this defect disparity and calculated band alignment, it is proposed that the Voc limit of the kesterite solar cells can be overcome by alloying Cu2ZnSn(S,Se)4 with Ag2ZnSn(S,Se)4, and the composition‐graded (Cu,Ag)2ZnSn(S,Se)4 alloys should be ideal light‐absorber materials for achieving higher efficiency kesterite solar cells.

Electronic transport in strained p-modulation Be-doped Ga0.8In0.2As/GaAs single quantum well

We report on the electronic transport properties of p-modulation Be-doped Ga0.8In0.2As/GaAs single quantum well. The experiments included the spectral photoluminescence between 8 and 300 K, and Hall effect measurements at temperatures between 14 and 300 K. The effect of strain which induces splitting of the valence band as light and heavy hole bands on transport is discussed. The calculated band alignment of the GaInAs sample using model-solid theory including strain effects indicates large conduction band discontinuities and a much smaller valance band discontinuity in GaInAs. The effect of the conduction and valance band discontinuity on the electronic transport properties is also discussed.

Hydrogen Passivation of Impurities in Al2O3

Carbon and nitrogen are contaminant impurities in Al2O3 dielectrics grown by atomic layer deposition, leading to deleterious effects in device performance. We investigate whether these impurities can be passivated using hydrogen. The role of atomic hydrogen in the electronic properties is addressed by examining formation energies and charge-state transition levels of C-H and N-H complexes. Combined with calculated band alignment, we then assess the impact on Al2O3/semiconductor interfaces. We find that hydrogen is indeed an effective passivating agent: it removes carbon-related carrier traps and passivates negative fixed charge associated with nitrogen.

Native defects in Al2O3 and their impact on III-V/Al2O3 metal-oxide-semiconductor-based devices

Al 2 O 3 is a promising material for use as a dielectric in metal-oxide-semiconductor devices based on III-V compound semiconductors. However, the presence of deep levels and fixed charge in the Al2O3 layer is still a concern, with native defects being a possible cause of traps, leakage, and fixed charge. We report hybrid density functional calculations for vacancies, self-interstitials, and antisites in Al2O3. The energetic positions of defect levels are discussed in terms of the calculated band alignment at the interface between the oxide and relevant III-V materials. We find that oxygen vacancies are the defects most likely to introduce gap levels that may induce border traps or leakage current in a gate stack. In addition, both self-interstitials and aluminum vacancies introduce fixed charge that leads to increased carrier scattering in the channel and shifts the threshold voltage of the device.

Modeling HfO 2/SiO 2/Si interface

We present ab inito calculations of a realistic HfO 2/SiO 2/Si interface and discuss its structural and electronic properties. Calculations reveal a variety of possible non-epitaxial atomic arrangements at the interface, associated with a substantial atomic disorder in the SiO 2 and HfO 2 region. Calculated band alignment, although predictably smaller than experimental values, allows for instructive analysis of band gap variation, dipole formation and defects near the interface.

Resonant tunneling of two-dimensional electrons into one-dimensional subbands of a quantum wire

Liquid phase epitaxy regrowth on the edge of in situ cleaved substrates is employed to create a vertical two-dimensional electron gas in a double-barrier tunneling potential. Resonant tunneling of two-dimensional electrons through one-dimensional quantum wire subbands is unambiguously identified by negative differential resistance features in the transport characteristics. The bias positions of these features agree with simple tunneling theory estimates based on conservation laws and the calculated band alignment in the structure under bias.