Title: Influence of Thermal Annealing on Free Carrier Concentration in (GaN)_1–x(ZnO)_x Semiconductors

Abstract

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 solidmore » 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:

Huang, Huafeng ^[1]; Sklute, Elizabeth C. ^[1]; Lehuta, Keith A. ^[2]; Kittilstved, Kevin R.  ^[2];

• Search DOE PAGES for author "Kittilstved, Kevin R."
• Search DOE PAGES for ORCID "0000-0002-9852-7454"

Glotch, Timothy D. ^[1]; Liu, Mingzhao  ^[3];

• Search DOE PAGES for author "Liu, Mingzhao"
• Search DOE PAGES for ORCID "0000-0002-0999-5214"

Khalifah, Peter G.  ^[4]

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• Search DOE PAGES for ORCID "0000-0002-2216-0377"

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+ Show Author Affiliations

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:

Wed Sep 13 00:00:00 EDT 2017

Research Org.:

Brookhaven National Laboratory (BNL), Upton, NY (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:

1425025

Report Number(s):

BNL-200016-2018-JAAM
Journal ID: ISSN 1932-7447; TRN: US1802007

Grant/Contract Number:  

SC0012704

Resource 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

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Semiconductor; free carrier concentration; annealing