Magnetodielectric coupling in nonmagnetic Au/GaAs:Si Schottky barriers

We report on a heretofore unnoted giant negative magnetocapacitance $\left(>20\%\right)$ in nonmagnetic Au/GaAs:Si Schottky barriers that we attribute to a magnetic field induced increase in the binding energy of the shallow donor Si impurity atoms. Depletion capacitance $\left(C_{\text{dep}}\right)$ dispersion identifies the impurity ionization and capture processes that give rise to a magnetic field dependent density of ionized impurities. Internal photoemission experiments confirm that the large field-induced shifts in the built-in potential, inferred from $1/C_{\text{dep}}^2$ vs voltage measurements, are not due to a field-dependent Schottky barrier height, thus requiring a modification of the abrupt junction approximation that accounts for the observed magnetodielectric coupling.

Figure 1

(Color online) Current-voltage characteristics show a temperature- and field-dependent forward-bias threshold. The top panel shows the rectifying (diode) characteristics on a semilogarithmic current-voltage scale for forward $(+)$ and reverse $(-)$ bias voltages at 300 K (red circles) and 20 K (blue squares). The bottom panel shows on a linear current-voltage scale the forward bias onsets of conduction at the indicated temperatures for $H=0$ (closed symbols) and $H=70\text{kOe}$ (open symbols).

Figure 2

(Color online) Capacitance of a Au/GaAs:Si Schottky junction decreases with increasing applied magnetic field $H$. The relative change in capacitance $\Delta C_{\text{dep}}/C_{\text{dep}}$ becomes increasingly more negative with increasing field. The solid curves represent data taken at the decreasing temperatures (top to bottom) indicated in the legend. The solid orange squares associated with the 20 K isotherm are calculated using Eq. (1) with the M-S parameters extracted from $1/C^2$ versus voltage plots such as shown in Fig. 3. Inset: schematic of band bending with parameters defined in the text. Capture processes (red arrow) tend to dominate near the interface whereas ionization processes are distributed over the depletion width $W$. The large capacitance associated with a high density of polarized bonds at the interface is in series with the much smaller depletion capacitance.

Figure 3

(Color online) Linearity of $1/C^2$ vs $V_R$ plots enable determination of M-S parameters. Data at 20 K are shown at 1 MHz for the indicated fields $H$ (top panel) and at $H=0$ for the indicated frequencies $f$ (bottom panel). The vertical arrows mark selected extrapolated intercepts with the abscissa corresponding to the built-in potential $V_{bi}\left(f,H\right)$ at 20 K. The inset to the lower panel shows the frequency dependence of $V_{bi}$ (right-hand axis) for $H=0$ (70 kOe) for solid (open) triangles and the frequency dependence of $N_D$ (left-hand axis, see text) for $H=0$ (70 kOe) for solid (open) squares. The inset of the upper panel shows IPE data in which $H$-independent extrapolations (dotted lines) to the abscissa at the indicated temperatures imply that there is no dependence of the Schottky barrier height on $H$ up to 70 kOe.

Figure 4

(Color online) Temperature- and field-dependent frequency dispersion of the capacitance reveals two separate loss processes. In the three panels the frequency-dependent capacitance (left-hand axis, top curves) and loss (right-hand axis, bottom curves) are shown as a function of frequency for each of the three indicated temperatures. Each group of curves contains data for the five different fields indicated in the legend of the middle panel. For fixed fields the loss peaks shift to lower frequency as the temperature is reduced. The horizontal arrow in each panel marks the isothermal shift to lower frequency of the low-frequency loss peak (ionization) as the field is increased from 0 to 70 kOe.

Figure 5

(Color online) Activated binding energy $E_a$ of Si impurity donated electrons increases with increasing field. The dependence of $E_a$ on $H$ is shown for both the low-frequency (red squares, ionization) and high-frequency (blue circles, capture) loss peaks. Each point is determined by fixing the field and plotting the frequencies of the loss peaks versus $1/T$ on a semilogarithmic plot. The slopes of the resulting linear plots (e.g., inset for $H=70\text{kOe}$) determine the $H$-dependent binding energies.