Universal alignment of hydrogen levels in semiconductors, insulators and solutions

Abstract

Hydrogen strongly affects the electronic and structural properties of many materials. It can bind to defects or to other impurities, often eliminating their electrical activity: this effect of defect passivation is crucial to the performance of many photovoltaic and electronic devices. A fuller understanding of hydrogen in solids is required to support development of improved hydrogen-storage systems, proton-exchange membranes for fuel cells, and high-permittivity dielectrics for integrated circuits. In chemistry and in biological systems, there have also been many efforts to correlate proton affinity and deprotonation with host properties. Here we report a systematic theoretical study (based on ab initio methods) of hydrogen in a wide range of hosts, which reveals the existence of a universal alignment for the electronic transition level of hydrogen in semiconductors, insulators and even aqueous solutions. This alignment allows the prediction of the electrical activity of hydrogen in any host material once some basic information about the band structure of that host is known. We present a physical explanation that connects the behaviour of hydrogen to the line-up of electronic band structures at heterojunctions.

Significant Carrier Concentration Changes in Native Electrodeposited ZnO

“…21 The refined lattice constants were consistent for samples deposited over a range of pH values (5.5-7.5) and deposition potentials (-1.3 V to -0.85 V), consistent with previous results from our group over a more acidic pH range (4-5.5). 5 Qualitative comparisons of relative peak heights ( Figure 1) also confirmed that, while our samples are polycrystalline, there is a preferred (002) orientation in the deposit, which is typical for electrodeposited ZnO. 4,15,22 This (002) texture is moderately accentuated for samples deposited at -1.1 V (pH 6.5), coinciding with a morphological change from hexagonal crystallites to rice-like structures, as shown in Figure 2, and reported previously.…”

“…This may be due to variations in the electrodeposit coverage (parameter f in Eq. 4), since earlier studies 5 have shown that small pinholes can occur in our electrodeposits. Fabregat-Santiago et al have also shown that sample-dependent capacitance can occur when there are sample/electrolyte interactions.…”

“…10 However, hydrogen doping in ZnO has been observed to go against this trend. 5,6 Hydrogen-donated electrons can fill empty states in the conduction band of ZnO when doping levels are high enough to create a degenerate semiconductor, thereby leading to increased band gaps with higher doping levels (Moss-Burstein effect). 4,8 Our data are consistent with this Moss-Burstein doping, whether for a single pH (Figure 6a for samples deposited at pH 6.5 from a Cl − -free electrolyte) or over our whole range of deposition conditions (Figure 6b).…”

“…2,4 Considerable scientific effort has been devoted to understanding how and why hydrogen is the cause of this n-type doping. 5,6 Hydrogen is expected to be an amphoteric dopant in most materials by compensating existing defects. In ZnO, however, electronically active interstitial H + serves as a donor 7 and increases the band gap energy when doping levels are high enough to lead to degenerate semiconducting behavior.…”

Surprising stability of neutral interstitial hydrogen in diamond and cubic BN

“…In the majority of cases, H 0 i is never the most stable charge state at any Fermi level (E F ). This is referred to as H i exhibiting "negative-U " character [2]. The position of the (+/−) transition level shows universal behavior across many different materials, when aligned on an absolute energy scale, and can serve as a reference for estimating band alignments [2].…”