Introduction

Rare earth (RE) doped semiconductors have been of considerable interest for possible applications in light emitting devices and for their unique optical properties. The rare earth luminescence depends very little on the nature of the host and the ambient temperature. Recently CL and PL emission has been obtained over the visible and near infrared spectrum range from GaN grown on sapphire by MOCVD, and doped by implantation with Sm, Dy, Ho, Er, and Tm [1,2a,b]. The visible PL and EL emission have been obtained from Pr and Eu doped GaN grown by MBE on sapphire and Si substrates [3a, b].

Experimental Details

The GaN material used for this investigation was grown by MOCVD on the basal plane of 2 inch diameter sapphire substrates. The thicknesses of the epilayer was 1.8 :m, and electron concentrations 5×10¹⁶ cm⁻³. The implanting ion beam was inclined at 7⁰ to the normal of the GaN epilayer to prevent channeling. The GaN was high quality undoped n-type epilayer implanted at room temperature with praseodymium. Pr was implanted at three energies at doses chosen to give an approximation of a square concentration implant profile in the GaN epilayer (the projected range and peak concentration were ∼ 40 nm, and ∼ 3.1×10¹⁹ cm⁻³ respectively). Samples were given postimplant isochronal thermal annealing treatments (duration 0.5 h and 1 h) at temperature 1100⁰C in NH₃, N₂, Ar₂, and in forming gas (N₂+H₂). The emission spectra presented are obtained from samples annealed at 1100⁰ C, which seems to be the optimal annealing temperature for RE ions incorporated as the luminescent center. The photoluminescence spectra and kinetics measurements were performed using a He-Cd (325 nm) and N₂ (337 nm) lasers. The PL kinetics were measured using a double grating monochromator DIGIKROM CM112 assembled with Hamamatsu R928 (or R616) photomultipliers and a photon counting system with a turbo-multichannel scaler (Turbo-MCS, EG&G). The CL was excited by an electron beam incident upon the sample at a 45⁰ angel from an electron gun (Electron gun system EK-2035-R (500V and 20 kV) which was in a common vacuum ( of ∼5×10⁻⁷ torr) with the cryostat.

Energy Levels, and Excitation Processes

The very important question is where the trivalent rare earth ions are incorporated in GaN: at substitutional sites on the metal sublattice and/or interstitial sites. In a hexagonal GaN crystal the Ga atoms occupy sites of symmetry C_3v (similar to Zn atoms in ZnS wurtzite phase) and two distinct high-symmetry interstitial positions also with C_3v symmetry[4a,b]. Recently, using the emission channeling (EC) technique, the lattice site occupations of RE elements in GaN were determined as a relaxed substitutional Ga-sites with an average relaxation of about 0.025 nm [Reference Dalmer, Restle, Stotzler, Vetter, Hofsass, Bremser, Ronning and Davis5]. The rare earth ions can also aggregate especially at high concentration, as well as create complex centers in the presence of an anther ion e.g. oxygen . In this paper we studied the PL and CL and kinetics of GaN implanted with Pr. The Pr³⁺ free ion possesses 4f ² configuration which gives rise to a ³H₄ ground state and ³H_5,6, ³F_2,3,4, ¹G₄, ¹D₂, ³P_0,1, ¹I₆, ³P₂, and ¹S₀ excited states. If the crystal field symmetry at the Pr³⁺ site is known, then the number and symmetry of crystal field levels and the selection rules for transitions between these levels can be calculated. In C_3v crystal symmetry the states with J = 0, 1, 2, 3, 4, 5, 6, will split into 1(0), 1(1), 1(2), 3(2), 3(3), 3(4), 5(4), single(doubly) degenerate crystal field LSJ levels, respectively. The rare earth ions located at a specific crystal site are associated with characteristic optical transitions subject to the selection rules that are governed by the crystal symmetry of the site. It is generally accepted that rare earth impurities in III-V semiconductors create isoelectronic traps [Reference Lozykowski6]. The outer electron configurations of RE³⁺ ions are the same (5s²5p⁶). If the RE³⁺ ions replace the element from column III(Ga³⁺) in GaN semiconductors that are isovalent concerning outer electrons of RE³⁺ ions, we believed that they create isoelectronic traps in III-nitrides (REI-trap). That conclusion is supported by the fact that the atomic covalent radii (ionic RE³⁺) for all rare earths are bigger than atomic radii of Ga that they are replacing. Pauling’s electronegativity of RE elements (1.1-1.25) is smaller than Ga (1.81), for which it substitutes. We have evidence that the RE ion in III-V semiconductors can occupy different sites (not only substitutional). They can create more complex centers involving other impurities or native defects. The experimental data shows that RE ions introduce electron or hole traps in III-V semiconductors, and we do not have any evidence that RE ions act as a donor or acceptor. The nature of the RE isoelectronic trap (electron or hole trap) in III-nitrides must be determined. Knowledge about the microscopic structure of RE centers in III-nitrides is crucial for understanding the excitation processes of 4f-4f transitions which in turn can determine the future of the RE dopants in optoelectronic applications.

The excitation processes of RE ions can be generally divided into two categories, direct and indirect excitation processes. The direct excitation process occurs in selective excitation of 4fⁿ electrons by photons (PL selective excitation) or in CL and EL by collision with hot electrons. Indirect excitation process occurs via transfer of energy to the 4fⁿ electron system from electron-hole pairs generated by photons with higher energy than the bandgap (PL excited above bandgap), injected in forward bias p-n junction, or generated by hot carriers in CL and EL. An excitation mechanism in CL and EL involves direct impact excitation of RE³⁺ ions by hot electrons, as well as an energy transfer from the generated electron-hole pairs or by impact excitation (or ionization) involving impurity states outside the 4f shell, with subsequent energy transfer to this shell. The most important excitation mechanism, from applications point of view, is the excitation of the RE ions by energy transfer process from electron-hole pairs generated in conduction and valence bands (by photons, hot electrons-CL) or injected in a forward bias p-n junction. There are three possible mechanisms of energy transfer. The first is the energy transfer process from excitons bound to structured isoelectronic centers to the core electrons.The second mechanism is the transfer of energy to the core electrons involving the structured isoelectronic trap occupied by electron (hole) and free holes (electrons) in the valence (conduction) band. The third mechanism is the transfer through an inelastic scattering process in which the energy of a free exciton near a RE structured trap is given to the localized core excited states. If the initial and final states are not resonant, the energy mismatch must be distributed in some way, e.g., by phonon emission or absorption.

Luminescence

In this paper, we present PL and CL spectra of Pr³⁺ in GaN in the spectral region of 350 to 1150nm. The luminescence of Pr³⁺ was excited indirectly, generating electron-hole pairs in GaN hosts by He-Cd, or N₂ lasers, and by electron beam excitation during CL measurement. Figure 1 shows two spectra CL (a) and PL (b) normalized to unity at peak (3) taken at 13 K and 330 K. The observed spectra shown in Fig. 1, undoubtedly exhibit all the features of a trivalent rare earth ion.

Fig.1 CL (a) and PL (b) spectra of GaN: Pr³⁺ recorded at 13 K and 330 K Inserts show magnified lower intensity emission lines of CL (a) and PL (b) spectra respectively. Numbers refer to transitions assignment from Fig. 1a.

Very weak luminescence was observed from ³P₂,¹I₆, ³P₁ levels (not all marked in Fig.1), and strong luminescence from ³P₀, and ¹D₂ levels. To show the weaker manifolds lines we enlarged the CL and PL spectra recorded at 13 K and 330 K as show in inset of Fig.1 a and b. A sharp CL emission line assigned to ³P₁→³H₅ transition (inset Fig.1 a) is only weakly seen on the short wavelength side of the interference modulated PL wide band (in spectral region ∼ 400 – 600 nm) observed at 330 K in inset Fig.1 b. The nature of this wide band is not clear. The origin of it can be due to commonly observed “yellow band” overlapped partially with unresolved transition lines starting from ³P₂, ¹I₆ ³P₁ ³P₀ levels. The assignments for the Pr³⁺ transitions in GaN have been made by comparison with data from other papers for the trivalent praseodymium ions in different crystals [7a,b]. Figure 2 shows PL spectra of GaN: Pr annealed at 1100⁰ C during 1h in NH₃, N₂, Ar₂, and in forming gas (N₂+H₂), at atmospheric pressure. The temperature dependence of intensity (area under curve) for dominant transition ³P₀→³F₂ plotted in Fig. 3 shows that the Pr³⁺ luminescence quenching is weak. The sample annealed during 1 h exhibits about two times stronger emission than the sample annealed during .5 h.

Fig.2 PL spectra of GaN: Pr³⁺ annealed at 11000° C during 1h in NH₃, N₂, Ar₂, and in forming gas

Fig.3 The PL intensity of GaN: Pr³⁺ annealed at 1100⁰ C during 1h in NH₃, N₂, Ar₂, monitored <$>

Decay Time Measurements

The quenching of luminescence from rare earth ions doped semiconductors is very important for optoelectronic applications, and has been the subject of many investigations. That controls the quantum efficiencies and the luminescence lifetimes of rare earth ions. In our study we concentrate on measurements of lifetimes of the RE fluorescent levels as a function of temperature. The study of PL quenching of the singly doped (Pr³⁺) GaN with temperature can provide information on interaction between like ions, ions with native defects and ions with unintentionally incorporated impurities. At low concentrations of praseodymium in GaN, the Pr³⁺ ions form a disordered system with wide range of inter-ion separations. For a resonant process an excitation can be transferred from the excited donor D^* to the neighboring donor ion in ground state (D) such that the second ion ends in the identical excited state D^*. We called that process a donor-donor (D-D) transfer. This migration of the excitation over the donors changes the trapping efficiency since all excited donors, including those which are initially far away from any trap, can now transfer energy to donors which have traps as near neighbors. Figure 4 shows an example of CL and PL decay (shown as semilog plots) of ³P₀→³F₂(653 nm) transitions at 12 K, with best fitting to double exponential decay for CL (solid line). Generally the decay times for transition ³P₀→³F₂ for CL and PL are 42 :s and 38.9 :s respectively, with little change with temperature as can be seen from inset in Fig. 4. The CL and PL decay kinetics of the ³P₁ → ³H₅ at 522 nm, ³P₀ → ³F₂ at 653 nm,³P₁ → ³F₃ at 673 nm, ³P₀ → ³F₄, at 755nm, ³P₀ → ¹G₄ at 913nm and ¹D₂ → ³F₃ at 956 nm emissions have been measured and the experimental results of the decay time data are summarizes in table I for temperatures 12 K and 300 K. The early time part of PL decay curve in Fig.4 from (and seems to be true for all investigated PL lines) is one exponential while the late- time luminescence decay is exponential with radiative decay time τ_o = 38.9 μs suggesting that some of the Pr³⁺ ions are suggesting that some of the Pr³⁺ ions are in very isolated sites, or otherwise the cross relaxation between them is not allowed. The exponential component extracted from the experimental PL decay data of emission line at 653 nm portrayed for selected tempratures in inset Fig.4 is a radiative decay time τ₀ attributed to Pr³⁺ ions from isolated sites. The decay times τ of Pr³⁺ luminescence at 653 nm, is found to obey the activation formula in temperature range from 12 K to 270 K:

(1)

Fig. 4 Luminescence decay observed from GaN: Pr³⁺ after N₂ laser pulse exc. (PL) and electron pulse exc. (CL) at 653 nm. Inset shows temperature dependence of ³P₀ decay time (■ ●) fitted to Eq.1 (doted, solid lines).

TABLE 1. CL and PL decay times of GaN:Pr³⁺

where τ₀ is assumed to be the low temperature decay time and ΔE is the activation energy. The values obtained from the best fit (solid line in inset of Fig. 4) to the PL data are: τ₀ = 37.56 μs, β = 2.39×10⁵ s⁻¹ and ΔE = 533 cm⁻¹, and for CL τ₀ = 42.4 μs, β = 1.2×10⁵ s⁻¹ and ΔE = 540 cm⁻¹ respectively (the ΔE is close to GaN TO phonon). The difference between CL and PL decay kinetics are probably related to different excitation processes and different centers involved in CL and PL emission (more experimental data and detailed analysis will be published elsewhere).

Conclusions

In summary, it was demonstrated, that rare earth Pr³⁺ ions implanted into GaN after post-implantation isochronal annealing at 1100⁰ C in N₂, at atmospheric pressure can be activated as luminescent centers emitting in the near UV, visible, and near infrared regions. The sharp characteristic emission lines corresponding to Pr³⁺ intra-4f ⁿ - shell transitions are resolved in the spectral range from 350 nm to 1150nm, and observed over the temperature range of 12 K – 335K. The fluorescence decay curves of ³P₁, ³P₀ and ¹D₂ levels emission were studied as a function of temperature. From this the characteristic time of the exponential decay of ³P₀ → ³F₂ transition was determined Strong luminescence observed at low and room temperature using above bandgap photo excitation and electron beam, suggested that Pr in GaN can be effectively excited by forward bias p-n junction end utilized in LED and semiconductors lasers.