^{1,2}, K. M. Yu

^{1}, S. V. Novikov

^{3}, Z. Liliental-Weber

^{1}, C. T. Foxon

^{3}, O. D. Dubon

^{1,2}, J. Wu

^{1,2}and W. Walukiewicz

^{1,a)}

### Abstract

Typically only dilute (up to ∼10%) highly mismatched alloys can be grown due to the large differences in atomic size and electronegativity of the host and the alloying elements. We have overcome the miscibility gap of the GaN 1−xAsx system using low temperature molecular beam epitaxy. In the intermediate composition range (0.10 < x < 0.75), the resulting alloys are amorphous. To gain a better understanding of the amorphous structure, the local environment of the As and Ga atoms was investigated using extended x-ray absorption fine structure (EXAFS). The EXAFS analysis shows a high concentration of dangling bonds compared to the crystalline binary endpoint compounds of the alloy system. The disorder parameter was larger for amorphous films compared to crystalline references, but comparable with other amorphous semiconductors. By examining the Ga local environment, the dangling bond density and disorder associated with As-related and N-related bonds could be decoupled. The N-related bonds had a lower dangling bond density and lower disorder.

This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division of the U.S. DOE under Contract No. DE-AC02-05CH11231. The growth work at the University of Nottingham was supported by the EPSRC (Grant Nos. EP/I004203/1, EP/G046867/1, and EP/G030634/1). Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University. The SSRL Structural Molecular Biology Program was supported by the DOE Office of Biological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program (P41RR001209).

I. INTRODUCTION

II. EXPERIMENTAL DETAILS

A. Sample preparation

B. EXAFS measurements and data processing

III. RESULTS AND DISCUSSION

A. The As atom environment

B. The Ga atom environment

IV. SUMMARY AND CONCLUSION

### Key Topics

- Chemical bonds
- 28.0
- Dangling bonds
- 27.0
- Extended X-ray absorption fine structure spectroscopy
- 24.0
- III-V semiconductors
- 20.0
- Amorphous semiconductors
- 16.0

## Figures

Top: Raw absorption coefficient data collected in transmission mode for powder c-GaAs at the As K edge at T = 20 K. Bottom: Raw absorption coefficient data collected in fluorescence mode for a thin film of a-GaN0.55As0.45 on a glass substrate at the As K edge at T = 20 K.

Top: Raw absorption coefficient data collected in transmission mode for powder c-GaAs at the As K edge at T = 20 K. Bottom: Raw absorption coefficient data collected in fluorescence mode for a thin film of a-GaN0.55As0.45 on a glass substrate at the As K edge at T = 20 K.

EXAFS function data (points) extracted from absorption coefficient data weighted by k 2 along with fits (lines) for c-GaAs and a-GaN1 -x As x at different compositions from As K edge data. For amorphous samples, 50% of data points are shown to enhance clarity.

EXAFS function data (points) extracted from absorption coefficient data weighted by k 2 along with fits (lines) for c-GaAs and a-GaN1 -x As x at different compositions from As K edge data. For amorphous samples, 50% of data points are shown to enhance clarity.

Top: Magnitude of Fourier transform of k 2χ(k) for c-GaAs and a-GaN0.55As0.45 for data (points) and fits (lines). Fitted region in r-space was 1-4.8 Å for c-GaAs and 1.6–2.6 Å for a-GaN0.55As0.45. For the amorphous sample, no EXAFS is detected beyond the first coordination shell. Bottom: Magnitude and real part of the Fourier transform of k 2χ(k) for a-GaN0.55As0.45 to show quality of fit.

Top: Magnitude of Fourier transform of k 2χ(k) for c-GaAs and a-GaN0.55As0.45 for data (points) and fits (lines). Fitted region in r-space was 1-4.8 Å for c-GaAs and 1.6–2.6 Å for a-GaN0.55As0.45. For the amorphous sample, no EXAFS is detected beyond the first coordination shell. Bottom: Magnitude and real part of the Fourier transform of k 2χ(k) for a-GaN0.55As0.45 to show quality of fit.

Graphical representation of key structural parameters determined by EXAFS analysis for As atom local environment. The data points for the amorphous alloys are offset in the x-axis for clarity.

Graphical representation of key structural parameters determined by EXAFS analysis for As atom local environment. The data points for the amorphous alloys are offset in the x-axis for clarity.

EXAFS function data (points) extracted from absorption coefficient data weighted by k 2 along with fits (lines) for c-GaAs, c-GaN, and a-GaN1 -x As x at different compositions from Ga K edge data. For amorphous samples, 30% of data points are shown to enhance clarity.

EXAFS function data (points) extracted from absorption coefficient data weighted by k 2 along with fits (lines) for c-GaAs, c-GaN, and a-GaN1 -x As x at different compositions from Ga K edge data. For amorphous samples, 30% of data points are shown to enhance clarity.

Magnitude and real part of Fourier transform of k 2χ(k) for a-GaN0.55As0.45 for data (points) and fits (lines). Fitted region in r-space was 0.8-3.1 Å. (Inset) Magnitude of Fourier transform of k 2χ(k) for c-GaN for data (points) and fit (line). Fitted region in r-space was 1.2–3.2 Å.

Magnitude and real part of Fourier transform of k 2χ(k) for a-GaN0.55As0.45 for data (points) and fits (lines). Fitted region in r-space was 0.8-3.1 Å. (Inset) Magnitude of Fourier transform of k 2χ(k) for c-GaN for data (points) and fit (line). Fitted region in r-space was 1.2–3.2 Å.

Top: Graphical representation of Ga coordination number determined by EXAFS analysis for first nitrogen shell (squares) and first arsenic shell (circles). The coordination number shown is multiplied by the alloying content to determine the actual number of nitrogen or arsenic atoms bonded to a given Ga atom. Bottom: Graphical representation of Ga σ 2 determined by EXAFS analysis for first nitrogen shell (squares) and first arsenic shell (circles). The results suggest that there are fewer dangling bonds and less bond length disorder due to nitrogen in the amorphous alloy. The data points for the amorphous alloys are offset in the x-axis for clarity.

Top: Graphical representation of Ga coordination number determined by EXAFS analysis for first nitrogen shell (squares) and first arsenic shell (circles). The coordination number shown is multiplied by the alloying content to determine the actual number of nitrogen or arsenic atoms bonded to a given Ga atom. Bottom: Graphical representation of Ga σ 2 determined by EXAFS analysis for first nitrogen shell (squares) and first arsenic shell (circles). The results suggest that there are fewer dangling bonds and less bond length disorder due to nitrogen in the amorphous alloy. The data points for the amorphous alloys are offset in the x-axis for clarity.

## Tables

Structural and statistical fit parameters from As K edge data for As-Ga distance in c-GaAs, and a-GaN1 − x As x at T = 20 K. = 1.01.

Structural and statistical fit parameters from As K edge data for As-Ga distance in c-GaAs, and a-GaN1 − x As x at T = 20 K. = 1.01.

Structural and statistical fit parameters from Ga K edge data for Ga-N distance in c-GaN, and a-GaN1 − x As x at T = 20 K. = 0.63.

Structural and statistical fit parameters from Ga K edge data for Ga-N distance in c-GaN, and a-GaN1 − x As x at T = 20 K. = 0.63.

Structural and statistical fit parameters from Ga K edge data for Ga-As distance in c-GaAs, and a-GaN1 -x As x at T = 20 K. Statistical parameters listed in Table II . = 0.63.

Structural and statistical fit parameters from Ga K edge data for Ga-As distance in c-GaAs, and a-GaN1 -x As x at T = 20 K. Statistical parameters listed in Table II . = 0.63.

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