Bulk photovoltaic effect in carbon-doped gallium nitride revealed by anomalous surface photovoltage spectroscopy

I. Levine, I. Gamov, M. Rusu, K. Irmscher, C. Merschjann, E. Richter, M. Weyers, and Th. Dittrich

Phys. Rev. B 101, 245205 – Published 24 June 2020

Abstract

A bulk photovoltaic effect was observed in insulating carbon-doped gallium nitride crystals by investigating the light-induced change of the contact potential difference with a Kelvin probe as a function of the carbon concentration in the crystal. Bulk polarization was caused by excitation of defects setting in at photon energies of 0.95–1.05 eV (C1) and 2.5–2.6 eV (C3). From the dependence on the carbon concentration, C1 can be assigned to isolated carbon defects and C3 to tricarbon defects.

Received 10 March 2020
Revised 18 May 2020
Accepted 3 June 2020

DOI:https://doi.org/10.1103/PhysRevB.101.245205

Physics Subject Headings (PhySH)

Ballistic transport Band gap Color centers Crystal defects Dopants Electric polarization Optoelectronics Photoconductivity Photovoltaic effect

Photoexcitation

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

I. Levine¹, I. Gamov ², M. Rusu ³, K. Irmscher ², C. Merschjann ⁴, E. Richter⁵, M. Weyers ⁵, and Th. Dittrich¹

¹Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, 12489 Berlin, Kekuléstrasse 5, Germany
²Leibniz-Institut für Kristallzüchtung, 12489 Berlin, Max-Born-Strasse 2, Germany
³Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Abteilung Struktur und Dynamik von Energiematerialien, 14109 Berlin, Hahn-Meitner-Platz 1, Germany
⁴Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institut für Angewandte Materialforschung, 14109 Berlin, Hahn-Meitner-Platz 1, Germany
⁵Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, 12489 Berlin, Gustav-Kirchhoff-Strasse 4, Germany

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Vol. 101, Iss. 24 — 15 June 2020

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Images

Figure 1
Spectra of the CPD for GaN with c-plane orientation (Ga-polar side on the surface) unintentionally doped (a) and doped with carbon with concentrations of $5 \times 10^{17}, 1.9 \times 10^{18}, 9 \times 10^{18}$ , and $1.4 \times 10^{19} c m^{- 3}$ [(b)–(e), respectively]. For comparison, the thin line in (c) shows the spectrum for GaN:C with $a$ -plane orientation doped with carbon concentration of $1.9 \times 10^{18} c m^{- 3}$ . The relative intensity spectrum of the Xe lamp used for illumination is shown as a dotted black line in (b).
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Figure 2
Sequences of CPD of six on-off cycles (illumination at 440 nm and dark, respectively) measured from the Ga-polar side (blue line) and from the N-polar side (red line) of the GaN:C ( $[C] = 9 \times 10^{18} c m^{- 3}$ ) crystal. The dark times $(t_{off})$ were 800 s and the illumination times $(t_{on})$ were 100 (1–4), 50 (5), and 25 (6) s. The insets show the corresponding measurement configurations with the vibrating Kelvin probe. Due to experimental limitations, the measurement range was limited to between −14.0 and +13.6 V.
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Figure 3
Temperature-dependent resistivity $ρ$ (measured in the dark) for GaN:C crystals with $[C] = 9 \times 10^{18} c m^{- 3}$ (black) and $[C] = 1.4 \times 10^{19}$ (red). The dashed lines are extrapolations of linear least-squares fittings to the experimental data points for an estimation of the resistivity at room temperature. The blue circle indicates the resistivity of the sample with $[C] = 9 \times 10^{18} c m^{- 3}$ at 300 K derived from the dielectric relaxation time of the on-off cycling measurement using the Kelvin probe (Fig. 2).
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Figure 4
(a) Log-log plot of the light intensity dependence (relative to the intensity used in Figs. 1 and 2) of the open circuit voltage in C1 and C3 (difference between the maxima of CPD and CPD at the onset of the features). For C1 (red), the sample with $[C] = 1.9 \times 10^{18} c m^{- 3}$ was used, under illumination wavelength $λ = 940 nm$ . For C3 (blue) the sample with $[C] = 9 \times 10^{18} c m^{- 3}$ was used, under illumination wavelength $λ = 440 nm$ . The lines shown in the low light intensity regime correspond to linear dependence. (b) Dependence of the open circuit voltage in C1 and C3 on [C], normalized to the carbon concentration.
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Figure 5
Suggested band diagram for the C1 and C3 transitions leading to BPVE in GaN:C. (a) An electron is excited from the VB to a single-carbon defect $(C^{i / i - 1})$ . The polar axial symmetry of the defect and crystal structure promotes a directed ballistic transport of the hole in the VB along the $c$ axis, eventually terminated by trapping in a relatively shallow defect. The electron on the single-carbon defect and the hole trapped in the shallow level near the VB (dashed black lines) form an electric dipole along the polar $c$ axis with an average distance of $δ_{da}$ ; (b) In an analogous manner to (a), an electron is excited from the tricarbon defect $(C^{i / i + 1})$ into the band (leaving a hole on the tricarbon defect) and eventually gets trapped in a relatively shallow level near the CB. Due to the superposition of the local dipoles, a net photovoltaic electric field is formed along the $c$ axis, which is measured as an anomalous SPV.
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