Magnetic properties of Cu-doped GaN grown by molecular beam epitaxy

Cu-doped GaN films were epitaxially grown by molecular beam epitaxy on $C$-plane sapphire substrates with an AlN buffer layer. Growth under metal-rich and nitrogen-rich conditions was investigated for different Cu-to-Ga beam-equivalent-pressure ratios $x_{\mathrm{BEP}}$. The samples were characterized by scanning electron microscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction. Films within a narrow range of $x_{\mathrm{BEP}}$ around 1$\%$ exhibit ferromagnetic behavior with a Curie temperature higher than 400 K. For higher $x_{\mathrm{BEP}}>1\%$, islands of a Cu-Ga compound are predominantly formed at the surface as nonferromagnetic precipitates. Our detailed study shows that the saturation magnetization of ferromagnetic films with $x_{\mathrm{BEP}}\approx 1\%$ decreases with increasing film thickness. This suggests that the ferromagnetism arises from defects, such as threading dislocations, created by the incorporation of Cu into the GaN.

Figure 1

SEM images of the samples with varying BEP ratio. The insets show images with a higher magnification.

Figure 2

EDX spectra metal-rich grown GaN:Cu ($x_{\mathrm{BEP}}=1.2\%$, sample N0121) taken with the electron beam focused on (a) an island and (b) the uncovered epitaxial film. The point of focus is marked by a circle in the corresponding SEM images (insets).

Figure 3

X-ray diffraction diagrams ($\theta -2\theta$ scans) for metal-rich grown Cu-doped GaN ($x_{\mathrm{BEP}}=1.2\%$, sample N0121) (a) before and (b) after etching with HNO${}_3$ for 5 min. (c) X-ray diffraction diagram of undoped GaN film N0300. The insets show rocking curves ($\omega$  scans) of the GaN(0002) reflection. Intensity is always in logarithmic scale.

Figure 4

N-rich grown Cu-doped GaN ($x_{\mathrm{BEP}}=1.1\%$, sample N0335). (a) SEM image. (b),(c) EDX spectra taken with the electron beam focused on (b) an island and (c) the uncovered GaN surface. The insets show SEM images where the point of focus is marked by a circle.

Figure 5

X-ray diffraction diagram ($\theta -2\theta$ scan) of N-rich grown Cu-doped GaN ($x_{\mathrm{BEP}}=1.1\%$, sample N0335). The inset shows the rocking curve ($\omega$  scan) of the GaN(0002) reflection.

Figure 6

(a) Magnetic moment $m$ vs magnetic field $H$ of a sapphire substrate at $T=10$ K. (b) Magnetization curve of GaN:Cu ($x_{\mathrm{BEP}}=0.9\%$, sample N0332) at $T=10$ K. Open symbols, magnetic moment $m\left(H\right)$ of GaN:Cu and substrate (raw data). Dashed line indicates the diamagnetic contribution of the sapphire substrate. Closed symbols, magnetization $M\left(H\right)$ of GaN:Cu after subtraction of the substrate contribution to $m$.

Figure 7

Magnetization curves of GaN:Cu films with (a) $x_{\mathrm{BEP}}=1.2\%$ and (b) $x_{\mathrm{BEP}}=4.8\%$ measured at $T=10$ K (solid symbols) and at 290 K (open symbols). Bottom-right insets show the temperature dependence $M\left(T\right)$ measured in a field ${\mu }_{0}H=0.05$ T. Top left inset in (a) shows $M\left(H\right)$ close to zero field.

Figure 8

(a) Saturation magnetization of GaN:Cu at $T=10$ K vs BEP ratio $x_{\mathrm{BEP}}$ for samples grown under different conditions (see Table ). The gray area indicates the range $x_{\mathrm{BEP}}\approx 1\%$, where $M_{\mathrm{S}}$ is enhanced. (b) Same data as (a) but classified into three thickness ranges. Solid lines serve as guides to the eye.

Figure 9

Saturation magnetization of GaN:Cu films with $x_{\mathrm{BEP}}=1.1$–1.2$\%$ at $T=10$ K vs thickness $d_{\mathrm{GaN}}$. The open circle indicates sample N0364. The solid line serves as a guide to the eye. The inset shows $M\left(H\right)$ of two 60-nm-thick samples with $x_{\mathrm{BEP}}=1.1\%$ grown on 20-nm Al buffer layer (solid triangles, N0362) and on a 5-$\mu$m-thick GaN template (open triangles, N0364).

Figure 10

$M\left(H\right)$ at $T=10$ K of GaN:Cu films of different BEP ratios (a) $x_{\mathrm{BEP}}=0$, (b) $x_{\mathrm{BEP}}=1.1\%$, and (c) $x_{\mathrm{BEP}}=2.1\%$ as grown (solid squares) and after etching in HCl (open circles).