S.-H. Wei

RESONANT HOLE LOCALIZATION AND ANOMALOUS OPTICAL BOWING IN INGAN ALLOYS

Using large supercell empirical pseudopotential calculations, we show that alloying of GaN with In induces localization in the hole wave function, resonating within the valence band. This occurs even with perfectly homogeneous In distribution (i.e., no clustering). This unusual effect can explain simultaneously exciton localization and a large, composition-dependent band gap bowing coefficient in InGaN alloys. This is in contrast to conventional alloys such as InGaAs that show a small and nearly composition-independent bowing coefficient. We further predict that (i) the hole wave function localization dramatically affects the photoluminescence intensity in InGaN alloys and (ii) the optical properties of InGaN alloys depend strongly on the microscopic arrangement of In atoms.

Band gaps of GaPN and GaAsN alloys

The importance of atomic relaxations, chemical disorder, and epitaxial constraints on the band gap of random, anion-mixed nitride alloys GaPN and GaAsN have been investigated, via pseudopotentials calculation. It has been demonstrated that simple approximations such as the virtual crystal approximation, or the use of high-symmetry ordered structure to mimic a random alloy, or the neglect of atomic displacements, are inadequate. It is found that a fully relaxed, large supercell calculation reproduces well the experimental band gaps of GaPN and GaAsN films.

Electronic, structural, and magnetic effects of 3d transition metals in hematite

We present a density-functional theory study on the electronic structure of pure and 3d transition metal (TM) (Sc, Ti, Cr, Mn, and Ni) incorporated α-Fe2O3. We find that the incorporation of 3d TMs in α-Fe2O3 has two main effects such as: (1) the valence and conduction band edges are modified. In particular, the incorporation of Ti provides electron carriers and reduces the electron effective mass, which will improve the electrical conductivity of α-Fe2O3. (2) The unit cell volume changes systematically such as: the incorporation of Sc increases the volume, whereas the incorporation of Ti, Cr, Mn, and Ni reduces the volume monotonically, which can affect the hopping probability of localized charge carriers (polarons). We discuss the importance of these results in terms of the utilization of hematite as a visible-light photocatalyst.

Van der Waals metal-semiconductor junction: Weak Fermi level pinning enables effective tuning of Schottky barrier

The Schottky barrier for carrier injection into 2D semiconductors can be effectively tuned by using 2D metals. Two-dimensional (2D) semiconductors have shown great potential for electronic and optoelectronic applications. However, their development is limited by a large Schottky barrier (SB) at the metal-semiconductor junction (MSJ), which is difficult to tune by using conventional metals because of the effect of strong Fermi level pinning (FLP). We show that this problem can be overcome by using 2D metals, which are bounded with 2D semiconductors through van der Waals (vdW) interactions. This success relies on a weak FLP at the vdW MSJ, which is attributed to the suppression of metal-induced gap states. Consequently, the SB becomes tunable and can vanish with proper 2D metals (for example, H-NbS2). This work not only offers new insights into the fundamental properties of heterojunctions but also uncovers the great potential of 2D metals for device applications.

Kesterite Successes, Ongoing Work, and Challenges: A Perspective From Vacuum Deposition

Recent years have seen dramatic improvements in the performance of kesterite devices. The existence of devices of comparable performance, made by a number of different techniques, provides some new perspective on what characteristics are likely fundamental to the material. Here, we review progress in kesterite device fabrication, aspects of the film characteristics that have yet to be understood, and challenges in device development that remain for kesterites to contribute significantly to photovoltaic manufacturing. Performance goals, as well as characteristics of midgap defect density, free carrier density, surfaces, grain boundaries, grain-to-grain uniformity, and bandgap alloying are discussed.

Multi-component transparent conducting oxides: progress in materials modelling

Transparent conducting oxides (TCOs) play an essential role in modern optoelectronic devices through their combination of electrical conductivity and optical transparency. We review recent progress in our understanding of multi-component TCOs formed from solid solutions of ZnO, In(2)O(3), Ga(2)O(3) and Al(2)O(3), with a particular emphasis on the contributions of materials modelling, primarily based on density functional theory. In particular, we highlight three major results from our work: (i) the fundamental principles governing the crystal structures of multi-component oxide structures including (In(2)O(3))(ZnO)(n) and (In(2)O(3))(m)(Ga(2)O(3))(l)(ZnO)(n); (ii) the relationship between elemental composition and optical and electrical behaviour, including valence band alignments; (iii) the high performance of amorphous oxide semiconductors. On the basis of these advances, the challenge of the rational design of novel electroceramic materials is discussed.

Indications of short minority-carrier lifetime in kesterite solar cells

Solar cells based on kesterite absorbers consistently show lower voltages than those based on chalcopyrites with the same bandgap. We use three different experimental methods and associated data analysis to determine minority-carrier lifetime in a 9.4%-efficient Cu2ZnSnSe4 device. The methods are cross-sectional electron-beam induced current, quantum efficiency, and time-resolved photoluminescence. These methods independently indicate minority-carrier lifetimes of a few nanoseconds. A comparison of current-voltage measurements and device modeling suggests that these short minority-carrier lifetimes cause a significant limitation on the voltage produced by the device. The comparison also implies that low minority-carrier lifetime alone does not account for all voltage loss in these devices.

Observation and first-principles calculation of buried wurtzite phases in zinc-blende CdTe thin films

We report direct observation of the existence of buried thin wurtzite CdTe layers in nominally pure zinc-blende CdTe thin films using high-resolution transmission electron microscopy. The formation of the buried wurtzite layers is a result of the formation of high density of planar defects in the zinc-blende films—the wurtzite layers are formed by closely spaced lamellar twins. First-principles calculations reveal that the presence of the buried wurtzite layers may be responsible for the poor electrical properties of the polycrystalline zinc-blende CdTe films.

The thermodynamics of codoping: how does it work?

Abstract Using first-principle total energy calculations, we have studied the energetics of codoping [Katayama-Yoshida, Yamamoto, Phys. Stat. Sol. (b) 202 (1997) 763.] in II–VI semiconductors. We demonstrate that (i) for Cd-based II-VI materials such as CdTe, the recently proposed codoping method (e.g., 2 acceptors+1 donor) may neither increase the solubility of the desired (p-type) dopants nor improve shallowness of the acceptor level. To increase solubility, one needs to suppress the formation of secondary phases involving the dopant by low-T quasi-equilibrium growth/doping processes. To improve the dopant shallowness, one needs to avoid the interaction between the acceptors using combinations such as (1 double acceptor+1 single donor): for example V Cd 2− +Cl Te + . (ii) We further demonstrate that the recent experimental results on p-type ZnO [Joseph et al., Japan J. Appl. Phys. 38 (1999) L1205.] may have little to do with codoping. Instead, the data on n + -, p- and p + -samples can be consistently understood in terms of increased solubility by incorporating molecular dopants: N 2 O and NO. The NO doping source is present due to plasma decomposition of N 2 O.

Overcoming doping bottlenecks in semiconductors and wide-gap materials

Abstract There often exist strong doping bottlenecks that may severely restrict potential applications of semiconductors, especially in wide-band-gap materials where bipolar doping is impossible. Recent rapid progress in semiconductor research has reached a point where these doping limitations must be overcome in order to tune semiconductors for precisely required properties. Here, we discuss how to find out what causes the doping bottlenecks. We based our discussion on a set of recent, novel developments regarding the doping limitations: the “doping limit rule” distilled from both phenomenological studies and from first-principles calculations. The thermodynamic doping bottlenecks are identified as due mainly to the formation of intrinsic defects whose formation enthalpies depend on the Fermi energy, and always act to negate the effect of doping.