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A-plane GaN epilayers doped with erbium (GaN:Er) have been grown on r-plane sapphire substrates by metal organic chemical vapor deposition. The 1.54 µm emission properties were probed by photoluminescence (PL) emission spectroscopy and compared with those of c-plane GaN:Er. It was found that the emission intensity from a-plane GaN:Er is 4 times higher than that of c-plane GaN:Er. The intensity of the 1.54 µm emission was found to increase with increasing Er molar flux. A-plane Er-doped GaN epilayers exhibit a small thermal quenching effect, with only a 12% decrease in the integrated intensity of the 1.54 µm PL emission, occurred between 10 and 300 K.
© 2015 Optical Society of America
1. Introduction
Doped in a solid host, Er3+ ion has allowable intra-4f shell transition from its first excited state 4I13/2 to the ground state 4I15/2 and the transition corresponds to a wavelength of minimum optical loss in silica based optical fibers (1.54µm) [1–7]. Thus, Er-doped materials are ideal candidates to make amplifiers for optical communications. There has been considerable effort devoted to the development of Er doped semiconductors aimed at achieving emitters, optical amplifiers, and photonic integrated circuits with the combined functionalities of Er optical fibers and narrow bandgap semiconductors such as InGaAsP. While the Er3+ related emission energy is not affected by the host, its emission intensity decreases as the temperature is raised. This thermal quenching effect limits the light emission efficiency at room temperature and, as a consequence, prevents the employment of the existing Er-based semiconductor devices in broad applications. The fact that the thermal stability of Er emissions increases with an increase in the energy gap of the semiconductor host material [8,9] has motivated research efforts to use wide bandgap semiconductors as host materials for Er. GaN has also been studied [10–13] for its optoelectronic application of infrared emission at 1.54 µm using Er doping. GaN has advantages over other semiconductors such as a direct bandgap transition which is very important in optical applications, a large energy bandgap (3.4 eV) which results in very low thermal quenching [14], and thermal and chemical robustness. GaN has proven to be an excellent host for Er ions [15–17]. C-plane Er doped GaN grown by metal organic vapor deposition (MOCVD) showed a low degree of thermal quenching (20%) between 10 and 300 K [18,19]. However, further improvements in quantum efficiency at 1.54 µm through the reduction of thermal quenching and increasing of Er incorporation in the optically active sites are still needed.
So far, all reported works on GaN:Er are for epilayers grown on c-plane and there have been no reports on the impact of the crystal orientation on Er incorporation and Er related emission in non-c-plane GaN films. Strong effects of the crystal orientation on dopant and impurity incorporation have been observed in GaN, GaAs, and InP [20]. In this work, we investigate MOCVD growth of non-polar a-plane GaN doped with Er and the emission properties of the Er3+ related emission at 1.54 µm. The results showed that the 1.54 µm emission intensity from a-plane GaN:Er is about 4 times higher than that from c-plane GaN:Er and the thermal quenching in the temperature range of 10 - 300 K for a-plane GaN:Er is about 12% compared to about 20% for c-plane GaN:Er.
2. Experimental details
GaN:Er epilayers were deposited on r- and c-plane sapphire substrates using MOCVD. For comparison, both r-plane and c-plane sapphire substrates were put side by side in the reactor. Hence, both a-plane and c-plane GaN:Er epilayers were grown under identical conditions. Both samples have approximately the same thickness, indicating equal growth rates. The gallium source was trimethylgallium (TMG) and the nitrogen source was ammonia (NH3). Trisisopropylcyclopentadienylerbium (TRIPEr) was used for the in situ Er doping. Hydrogen was the carrier gas and was kept constant at 2 standard liter per minute(SLM). The growth started with a thin (50 nm) GaN buffer layer grown at 550 °C and 300 Torr, followed by the growth of ~1.4 µm GaN template grown at 1060 °C and 300 Torr and a 0.5µm Er-doped GaN epilayers grown at 1040 °C and 30 Torr. The inset of Fig. 1 shows the layer structure of Er doped a-plane GaN epilayer grown on r-plane sapphire substrate. X-ray diffraction (XRD) was used to determine crystalline quality. Photoluminescence (PL) spectroscopy was employed to study the optical properties of the Er-doped GaN epilayers. The PL system consists of a frequency doubled, tripled, and quadruple 100 femtosecond Ti: Sapphire laser with an average power of 150 mW at 263 and 395 nm, 1–3mW at 196 nm, and a repetition rate of 76 MHz. Infrared detection was accomplished by an InGaAs detector.
Fig. 1 X-ray diffraction θ-2θ scan of a-plane Er doped GaN epilayer grown on r-plane sapphire substrate. Inset: Schematic layer structure of a-plane Er doped GaN epilayer (GaN:Er).
3. Results and discussion
The growth surface of GaN:Er epilayers grown on r-plane sapphire was determined to be a-plane using x-ray diffraction (XRD) θ-2θ scan, which detected sapphire (10-12), (20-24), and a-plane GaN (11-20) reflections, as illustrated in Fig. 1. Only the a-plane reflection peak at 2θ = 57.7° was observed. Since the GaN (0002) reflection at 2θ = 34.56° was not detected, demonstrating that the films were uniformly a-plane oriented. The measured full-width at half-maximum (FWHM) of the (11–20) peak in the ω-scan (x-ray rocking curve) is about 600 arcsec. For comparison, the XRD rocking curve FWHM of the (0002) peak of c-plane GaN:Er is about 450 arcsec.
Figure 2 compares the room temperature (300 K) PL spectra of a-plane and c-plane GaN:Er epilayers covering a spectral range from 1.40 to 1.65 µm, using excitation wavelength λexc = 375 nm. It is observed that the 1.54 µm emission intensity from a-plane GaN:Er is about 4 times higher than that of c-plane GaN:Er. SIMS results provide a concentration of about 2 x 1020 cm−3 in both samples, which suggest that the Er doping concentration is predominantly controlled by the Er flow rate and GaN growth rate. However, a small impact of the surface orientation on the Mg, Si, and Fe dopant incorporation was observed by Cruz et al [20]. The Er dopant’s environment and strain are not identical in a-plane and c-plane GaN:Er epilayers, which could contribute to the observed difference in the 1.54 µm emission intensities [21].
Fig. 2 Comparison of room temperature (300 K) Er-related PL spectra near1.54 µm between a-plane and c-plane Er doped GaN epilayers. The excitation wavelength used was 375 nm.
It is well known that the intensity of the Er-related PL spectra is a strong function of the Er concentration in GaN epilayer. Hence the Er concentration was varied to determine the optimal Er concentration. The Er concentration was controlled by changing the Er molar flux from 20.2 - 31.5 µmol/min. Figure 3 shows the dependence of the 1.54 µm PL spectra measured at 300 K for λexc = 375 nm on Er molar flux. It can be seen that the 1.54 µm emission line shape is independent of the Er flux. The inset of Fig. 3 shows the PL intensity at 1.54 µm as a function of Er molar flux and demonstrates a direct correlation between PL intensity at 1.54 µm and Er molar flux in the studied range. The results indicate that the number of optical active centers increases almost linearly with an increase of the Er molar flux.
Fig. 3 Room temperature PL spectra of a-plane Er doped GaN epilayers showing the variation in the emission intensity of the Er related emission at 1.54 µm with the Er molar flux employed during the growth. The excitation wavelength used was 375 nm. Inset: The PL intensity at 1.54 µm as a function of Er molar flux.
Figure 4 shows the temperature evolution of the PL spectra near 1.54 µm of a-plane GaN:Er measured in the spectral range of 10 - 400 K. The 1.54 µm emission peak due to the radiative intra-4f Er3+ transition from 4I13/2 to 4I15/2 is dominant at low temperatures with its emission intensity decreases with an increase in temperature. The transition located at 1.51µm is not visible at 10 K and its emission intensity increases with temperature following a different trend compared to the 1.54 µm peak.
Fig. 4 PL spectra of the 1.54 µm emission of a-plane Er doped GaN measured at different temperatures from 10 to 400 K. The excitation wavelength used was 195 nm.
Knowledge on the thermal-quenching properties of the Er-related luminescence at 1.54 µm can be obtained from temperature evolution of the PL spectra. Figure 5 shows the integrated PL emission intensity (Iint) of the 1.54 µm emission line (integrated over a range of 1.45 - 1.62 µm) for a-plane GaN:Er as a function of temperature between 10 and 400 K. The temperature dependence of the integrated PL shows a 12% decrease in Iint between 10 and 300 K. This small degree of thermal quenching is even lower than the reported thermal quenching (20%) for Er doped c-plane GaN [18,19].
Fig. 5 Integrated PL emission intensity of the 1.54 µm emission of a-plane Er doped GaN vs sample temperature. Inset: Arrhenius plot of the integrated PL intensity of the 1.54 µm emission. The solid line in the plot is the least squares fit of the measured data to Eq. (1). The excitation wavelength used was 195 nm.
The inset of Fig. 5 shows the Arrhenius plot of the integrated PL intensity near 1.54 µm between 10 and 400 K for a-plane GaN:Er. The solid line is the least squares fit of the measured data to the equation
where Iint (T) and I0 are, respectively, the integrated PL intensities at finite temperature T and 0 K, while E0 is the activation energy of the thermal quenching, and k is Boltzmann's constant. The PL intensity is thermally activated with an activation energy of about 258 ± 15 meV, which is larger than a value of 191 meV observed in c-plane GaN:Er [19] and explains the reduced thermal quenching of the 1.54 µm emission in a-plane GaN:Er. This again suggests that the Er dopant’s environment is not identical in a-plane and c-plane GaN:Er epilayers.4. Conclusion
In summary, a-plane Er doped GaN epilayers were grown by MOCVD. It was found that the room temperature intensity of Er-related PL emission at 1.54 µm for a-plane Er-doped GaN is four times stronger than that of c-plane Er-doped GaN. The emission intensity at 1.54 µm was found to increase almost linearly with an increase of Er molar flux employed during the growth. Thermal quenching of the emission at 1.54 µm in a-plane GaN:Er from 10 to 300 K is 1.6 times smaller than that measured in c-plane GaN:Er. The results imply that the use of a-plane GaN as a host crystal for Er doping represents a promising route for realizing 1.54 µm optoelectronic and photonic devices with improved emission efficiency.
Acknowledgments
This research was supported by JTO/ARO (W911NF-12-1-0330). Talal Mohammed Al tahtamouni is grateful to the William J. Fulbright Commission of the United States and the Fulbright Committee in Jordanfor the support of the Fulbright Visiting Scholarship at Texas Tech University. H. X. Jiang and J. Y. Lin are grateful to the AT&T Foundation for the support of Ed Whitacre and Linda Whitacre Endowed chairs.
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