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Title: Influence of Thermal Annealing on Free Carrier Concentration in (GaN) 1–x(ZnO) x Semiconductors

It has been previously demonstrated that the efficiency of (GaN) 1–x(ZnO) x semiconductors for solar water splitting can be improved by thermal annealing, though the origin of this improvement was not resolved. In the present work, it is shown that annealing reduces the free carrier (electron) concentration of (GaN) 1–x(ZnO) x. The time-, temperature-, and atmosphere-dependent changes were followed through two simple techniques: indirect diffuse reflectance measurements from 0.5 to 3.0 eV which show very high sensitivity to the free carrier response at the lowest energies and EPR measurements which directly probe the number of unpaired electrons. For the thermal annealing of investigated compositions, it is found that temperatures of 250 °C and below do not measurably change the free carrier concentration, a gradual reduction of the free carrier concentration occurs over a time period of many hours at 350 °C, and the complete elimination of free carriers happens within an hour at 550 °C. These changes are driven by an oxidative process which is effectively suppressed under actively reducing atmospheres (H 2, NH 3) but which can still occur under nominally inert gases (N 2, Ar). Surprisingly, it is found that the N 2 gas released during thermal oxidationmore » of (GaN) 1–x(ZnO) x samples remains trapped within the solid matrix and is not expelled until temperatures of about 900 °C, a result directly confirmed through neutron pair-distribution fuction (PDF) measurements which show a new peak at the 1.1 Å bond length of molecular nitrogen after annealing. Preliminary comparative photoelectrochemical (PEC) measurements of the influence of free carrier concentration on photoactivity for water oxidation were carried out for a sample with x = 0.64. Samples annealed to eliminate free carriers exhibited no photoactivity for water oxidation, while a complex dependence on carrier concentration was observed for samples with intermediate free carrier concentrations. The methods demonstrated here provide an important approach for quantifying (and controlling) the carrier concentrations of semiconductors which are only available in the form of loose powders, as is commonly the case for oxynitride compounds.« less
Authors:
 [1] ;  [1] ;  [2] ; ORCiD logo [2] ;  [1] ; ORCiD logo [3] ; ORCiD logo [4]
  1. Stony Brook Univ., NY (United States)
  2. Univ. of Massachusetts, Amherst, MA (United States)
  3. Brookhaven National Lab. (BNL), Upton, NY (United States)
  4. Brookhaven National Lab. (BNL), Upton, NY (United States); Stony Brook Univ., NY (United States)
Publication Date:
Report Number(s):
BNL-200016-2018-JAAM
Journal ID: ISSN 1932-7447; TRN: US1802007
Grant/Contract Number:
SC0012704
Type:
Accepted Manuscript
Journal Name:
Journal of Physical Chemistry. C
Additional Journal Information:
Journal Volume: 121; Journal Issue: 42; Journal ID: ISSN 1932-7447
Publisher:
American Chemical Society
Research Org:
Brookhaven National Laboratory (BNL), Upton, NY (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Semiconductor; free carrier concentration; annealing
OSTI Identifier:
1425025

Huang, Huafeng, Sklute, Elizabeth C., Lehuta, Keith A., Kittilstved, Kevin R., Glotch, Timothy D., Liu, Mingzhao, and Khalifah, Peter G.. Influence of Thermal Annealing on Free Carrier Concentration in (GaN)1–x(ZnO)x Semiconductors. United States: N. p., Web. doi:10.1021/acs.jpcc.7b06455.
Huang, Huafeng, Sklute, Elizabeth C., Lehuta, Keith A., Kittilstved, Kevin R., Glotch, Timothy D., Liu, Mingzhao, & Khalifah, Peter G.. Influence of Thermal Annealing on Free Carrier Concentration in (GaN)1–x(ZnO)x Semiconductors. United States. doi:10.1021/acs.jpcc.7b06455.
Huang, Huafeng, Sklute, Elizabeth C., Lehuta, Keith A., Kittilstved, Kevin R., Glotch, Timothy D., Liu, Mingzhao, and Khalifah, Peter G.. 2017. "Influence of Thermal Annealing on Free Carrier Concentration in (GaN)1–x(ZnO)x Semiconductors". United States. doi:10.1021/acs.jpcc.7b06455. https://www.osti.gov/servlets/purl/1425025.
@article{osti_1425025,
title = {Influence of Thermal Annealing on Free Carrier Concentration in (GaN)1–x(ZnO)x Semiconductors},
author = {Huang, Huafeng and Sklute, Elizabeth C. and Lehuta, Keith A. and Kittilstved, Kevin R. and Glotch, Timothy D. and Liu, Mingzhao and Khalifah, Peter G.},
abstractNote = {It has been previously demonstrated that the efficiency of (GaN)1–x(ZnO)x semiconductors for solar water splitting can be improved by thermal annealing, though the origin of this improvement was not resolved. In the present work, it is shown that annealing reduces the free carrier (electron) concentration of (GaN)1–x(ZnO)x. The time-, temperature-, and atmosphere-dependent changes were followed through two simple techniques: indirect diffuse reflectance measurements from 0.5 to 3.0 eV which show very high sensitivity to the free carrier response at the lowest energies and EPR measurements which directly probe the number of unpaired electrons. For the thermal annealing of investigated compositions, it is found that temperatures of 250 °C and below do not measurably change the free carrier concentration, a gradual reduction of the free carrier concentration occurs over a time period of many hours at 350 °C, and the complete elimination of free carriers happens within an hour at 550 °C. These changes are driven by an oxidative process which is effectively suppressed under actively reducing atmospheres (H2, NH3) but which can still occur under nominally inert gases (N2, Ar). Surprisingly, it is found that the N2 gas released during thermal oxidation of (GaN)1–x(ZnO)x samples remains trapped within the solid matrix and is not expelled until temperatures of about 900 °C, a result directly confirmed through neutron pair-distribution fuction (PDF) measurements which show a new peak at the 1.1 Å bond length of molecular nitrogen after annealing. Preliminary comparative photoelectrochemical (PEC) measurements of the influence of free carrier concentration on photoactivity for water oxidation were carried out for a sample with x = 0.64. Samples annealed to eliminate free carriers exhibited no photoactivity for water oxidation, while a complex dependence on carrier concentration was observed for samples with intermediate free carrier concentrations. The methods demonstrated here provide an important approach for quantifying (and controlling) the carrier concentrations of semiconductors which are only available in the form of loose powders, as is commonly the case for oxynitride compounds.},
doi = {10.1021/acs.jpcc.7b06455},
journal = {Journal of Physical Chemistry. C},
number = 42,
volume = 121,
place = {United States},
year = {2017},
month = {9}
}