1. Introduction

Considerable interest in rare-earth (RE) doped nitride semiconductors has flourished due to their applications in optoelectronic devices, including optical fibers, solid-state lasers, LEDs and displays. RE ions have a partially filled inner shell (4f ⁿ) which is well-shielded by completely filled outer shells (5s ² and 5p ⁶), leading to sharp intra 4f ⁿ transitions which are almost independent of the host material. In particular, nitride semiconductors are promising host materials due to the strong ionic bonds, which increase the probability of the otherwise forbidden 4f intra-subshell transitions [1]. Of the III-nitrides, AlN has the widest bandgap of 6.2 eV, allowing for higher energy RE transitions as well as a decrease of thermal quenching of the emission intensity [2]. AlN also has the highest thermal conductivity (2.85 W/cm/°C at 300 K [3]) among the III-nitrides, which is >10 times higher than YAG at room temperature, to allow efficient heat removal from the active media, a requirement for maximizing performance of high power/high temperature devices. AlN doped with RE ions, such as Er [4] for emission at 1.54 µm and Eu and Tb for visible emission [5], have been reported. Another widely used RE ion, neodymium (Nd), has found great success in solid-state lasers such as Nd:YAG and Nd:YVO₄ lasers, and has a larger emission cross-section than Er. We therefore focus our studies on Nd doped AlN.

There are several different methods of incorporating RE ions into the host material, including ion-implantation, reactive ion sputtering and in situ doping. Usually, ion-implantation and sputtering produce a high density of defects, reducing the luminescence efficiency in the RE doped material. By in situ doping, on the other hand, we have recently observed emission associated with enhanced substitutional doping at the Ga-site in GaN:Nd [6,7], which has not been realized in sputtered [8] or ion-implanted GaN:Nd [9]. For an AlN host material, luminescence detected from in situ Er doped samples was orders of magnitude greater than from ion-implanted Er in AlN [4]. In this paper, we report in situ Nd doping of AlN grown by plasma-assisted molecular beam epitaxy (PA-MBE). Resolved Stark energy levels of Nd³⁺ ions in AlN are observed using temperature dependent photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy. Site-selective spectroscopy studies using combined excitation-emission (CEES) are also presented [10].

2. Material growth and experimental setup

The AlN:Nd layers are grown on single side polished c-plane sapphire by PA-MBE. After sapphire nitridation, a ~8 nm thick AlN buffer layer was grown followed by a ~400 nm thick Nd-doped AlN layer. The Nd cell temperature is fixed at a maximum temperature of 950 °C to avoid damage to the crucible containing the 99.98% pure Nd source material. Two effects dominate the incorporation of Nd into the AlN matrix at low concentrations. First, the arrival rate of Nd atoms must be sufficient relative to that of Al and nitrogen atoms to obtain the desired Nd concentration. Second, since Al preferentially incorporates over Nd in the crystal lattice, there must be enough unoccupied group III sites relative to the number of sites occupied by Al atoms. To observe the effects of the Al-N flux ratio on Nd incorporation, an Al beam equivalent pressure (BEP) ranging from 1.4 × 10⁻⁸ to 4.1 × 10⁻⁸ torr was used while holding the N flow rate constant at 0.60 sccm. The Al BEP range was chosen such that the Al to active N arrival rate ratio varied between 0.7 and 1.0. The purpose of this growth series was to study the effect of Al competition with Nd for group III sites on Nd incorporation. In addition to varying the Al BEP, a series of AlN:Nd films at substrate temperatures ranging from 800 °C to 950 °C was also grown. To characterize the effect of the AlN growth rate on Nd doping concentration, samples were grown with AlN growth rates of 54, 110 and 140 nm/hr. Secondary ion mass spectrometry (SIMS) was used to determine the Nd concentration in a total of six samples.

The samples were placed in a closed-loop helium cryostat system for temperature dependent PL and PLE studies. For above bandgap excitation, a Nd:YVO₄-pumped continuous wave Ti:sapphire laser tunable between 750 and 950 nm with ~400 mW of power was used. The resulting fluorescence was collected into a 1 m focal length spectrometer and focused onto a liquid nitrogen cooled Ge detector. CEES measurements were performed at 4 K using an argon laser-pumped dye laser. The emission spectra were collected using a liquid nitrogen cooled-CCD array mounted onto a 0.25 m focal length spectrometer.

3. Results and discussion

Figure 1 shows that an increase in the Nd incorporation coincides with an increase in the PL peak intensity normalized by AlN:Nd layer thickness and a decrease in Al BEP. A lower Al BEP is expected to increase Nd incorporation due to competition between the Al and Nd atoms for the group III-site. The Nd concentration was also observed to increase with reduced growth rate (data not shown), indicating that the concentration of Nd may be limited by the arrival rate of Nd from the effusion cell. At a fixed Al BEP of 2.83 × 10⁻⁸ torr, a set of samples were grown at various substrate temperatures (800, 850, 900, and 950 °C, circled data points in Fig. 1). Although effects of growth temperature have been shown in GaN:Er [11] and AlN:Er [4], no significant effect from the substrate temperature on PL intensity or Nd incorporation is observed over our temperature range. The highest Nd concentration incorporation obtained is 0.08 at. % at an Al BEP of 1.4 × 10⁻⁸ torr, an AlN growth rate of 54 nm/hr, and substrate temperature of 850 °C.

 

Fig. 1 The incorporated Nd concentration plotted as a function of peak PL intensity normalized by AlN:Nd layer thickness. The Al BEP is noted for each of the data points. The circled data points represent a fixed Al BEP while varying the substrate temperature from 800 to 950 °C.

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The PL spectra at room temperature and low temperature (~13 K) in Fig. 2(a) shows transitions from the ⁴ F _3/2 to the ⁴ I _9/2, ⁴ I _11/2, and ⁴ I _13/2 manifolds in Nd³⁺ ions in AlN between 1.30 and 1.36 eV (910–950 nm), 1.08 and 1.13 eV (1100–1150 nm), and 0.84 and 0.89 eV (1400–1480 nm), respectively. The AlN:Nd film under investigation has a Nd concentration of 0.08 at. % and is optically excited using 837 nm. The strongest emission peak occurs at ~1.12 eV (1108 nm). In addition to the ⁴ F _3/2 → ⁴ I _9/2, ⁴ I _11/2, and ⁴ I _13/2 transitions, phonon replicas which appear as weak peaks shifted lower in energy from the major peaks are alsoobserved. The crystal-field split energy levels of the Nd³⁺ ions can be clearly resolved, even at room temperature, and are similar for various AlN:Nd samples (data not shown). The center energy of each manifold in AlN:Nd is similar to that for in situ Nd doped GaN, but with slight shifts in the Stark energy levels due to the different crystal-fields in AlN and GaN. An example is shown in Fig. 2(b) of the PL spectra for the ⁴ F _3/2 → ⁴ I _11/2 transition for both AlN:Nd and GaN:Nd. A splitting of the ⁴ F _3/2 doublet is measured to be ~5 meV.

 

Fig. 2 (a) Room temperature and low temperature (~13 K) PL spectra in logarithmic scale from AlN:Nd optically excited at 837 nm showing transitions from the ⁴ F _3/2 level to the ⁴ I _13/2, ⁴ I _11/2, and ⁴ I _9/2 levels. (b) Low temperature PL spectra from AlN:Nd and GaN:Nd showing emission peaks corresponding to the ⁴ F _3/2 → ⁴ I _11/2 transition.

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Figure 3(a) shows the PLE spectra in logarithmic scale at room temperature and low temperature (~13 K) collected at the strongest emission wavelength, 1108 nm. Transitions from the ⁴ I _9/2 ground state to the upper states ⁴ F _5/2 and ² H _9/2, and ⁴ F _7/2 and ⁴ S _3/2 between 1.46 and 1.54 eV (805–850 nm) and 1.58 and 1.62 eV (765–785 nm), respectively, as well as phonon replicas shifted higher in energy are observed. The strongest emission occurs at an excitation energy of ~1.48 eV (837 nm). As in the PL data, the crystal-field split levels can be clearly resolved at room temperature and slight shifts in the Stark energy levels between Nd in AlN and GaN are observed. Figure 3(b) illustrates the PLE spectra for the ⁴ I _9/2 → ⁴ F _5/2, ² H _9/2 transition for AlN:Nd and GaN:Nd.

 

Fig. 3 (a) Room temperature and low temperature (~13 K) PLE spectra in logarithmic scale from AlN:Nd collected at 1108 nm showing transitions from the ⁴ I _9/2 ground state to the ⁴ F _15/2, ² H _9/2, ⁴ F _7/2, and ⁴ S _3/2 upper levels. (b) Low temperature PLE spectra from AlN:Nd and GaN:Nd corresponding to the ⁴ I _9/2 → ⁴ F _15/2, ² H _9/2 transition.

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A closer examination of the crystal-field split levels using CEES reveals three Nd incorporation sites. Figure 4 shows CEES data corresponding to emission from the ⁴ F _3/2 upper state to the ⁴ I _9/2 ground state after excitation in the spectral region of the ⁴ G _7/2 and ⁴ G _5/2 excited states. The three incorporation sites (a, b, c) are indicated by the rectangles connecting peaks with different excitation and emission energies. All observed transitions can be attributed to these three sites, taking into account emission from excited sublevels of the ⁴ F _3/2 as well as coupling to a localized phonon (12.4eV). Using the strongest emission line for each site as a measure, we obtained a relative abundance of a:b:c = 95.6%:4.0%:0.4%. The dominance of the main site is less pronounced compared to Nd in GaN, for which essentially a single incorporation site (i.e unperturbed Nd on the Ga site) was observed [6,7]. However, a similar level of perturbed sites was observed for Eu in GaN and was attributed to perturbed Eu ions on the Ga site. In this case, a distinct difference in the excitation pathways (after excitation of bulk electron-pairs) has also been observed [12,13]. To test if a similar behavior occurs for Nd in AlN, we excited the material with an electron beam and detected the resulting cathodoluminscence (CL). Taking the ratio of CL strengths of the strongest peak associated with sites a, b, and c, we find a similar ratio as seen in the CEES data, indicating no difference in excitation efficiency for these sites. The crystal field-split level energies of Nd³⁺ ions in AlN for sites a, b, and c as determined by PL, PLE, and CEES measurements are summarized in Table 1 .

 

Fig. 4 CEES data at 4 K of emission intensity as a function of excitation and emission energies for a AlN:Nd film. The three identified incorporation sites are indicated by lines in different colors and lines styles. For site a, phonon-coupled excitation transitions are indicated with arrows.

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Table 1. Energy of the Crystal Field-Split Levels of Nd³⁺ Ions in AlN for the Main Incorporation Site a and Two Minority Sites b and c As Determined by PL, PLE, and CEES

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4. Conclusions

In summary, we have demonstrated Nd incorporation with a concentration as high as 0.08 at. % into AlN using in situ doping with PA-MBE. PL, PLE, and CEES data show resolved crystal-field split levels of the ⁴ I _9/2, ⁴ I _11/2, ⁴ I _13/2, ⁴ F _3/2, ⁴ F _5/2, ² H _9/2, ⁴ F _7/2, ⁴ S _3/2, ⁴ G _5/2, and ⁴ G _7/2 manifolds of the Nd-ion. Site-selective spectroscopy using CEES shows one dominating site, but also clearly reveals two minority incorporation sites. These minority sites are much more pronounced compared to in situ Nd doped GaN.