Photoluminescence spectroscopy of erbium implanted gallium nitride Myo Thaik and U. Ho¨mmericha) Department of Physics, Research Center for Optical Physics, Hampton University, Hampton, Virginia 23668
R. N. Schwartz and R. G. Wilson Hughes Research Laboratories, Malibu, California 90265
J. M. Zavada U.S. Army Research Office, Research Triangle Park, North Carolina 27709
~Received 13 June 1997; accepted for publication 4 September 1997! Results of a photoluminescence ~PL! and photoluminescence excitation ~PLE! study of Er implanted GaN are presented. Upon optical excitation at 325 and 488 nm, we observed strong 1.54 mm Er31 PL which remained temperature stable from 15 to 550 K. At 550 K, the integrated PL intensity decreased by ;10% for above gap excitation (l ex5325 nm) and ;50% for below gap excitation (l ex5488 nm) relative to its value at 15 K. The excellent temperature stability makes GaN:Er very attractive for high temperature optoelectronic device applications. PLE measurements were conducted to gain insight into the Er31 excitation mechanisms in the GaN host. The PLE results show that Er31 can be excited continuously over a broad wavelength region spanning from 425 to 680 nm. In addition, sharp PLE features were observed at approximately 495, 525, 553, 651, and 980 nm. The PLE spectrum suggests that optically active Er31 ions can be excited either through carrier-mediated processes involving defects in the host or through resonant pumping into Er31 4 f energy levels. With respect to these two excitation schemes, distinct Er31 PL properties were observed for resonant and off-resonant Er31 excitation indicating the presence of different subsets of Er31 ions in GaN. © 1997 American Institute of Physics. @S0003-6951~97!01744-0#
Rare earth doped semiconductors have received world wide attention because of possible applications in optoelectronics.1,2 These systems exhibit a temperature stable luminescence wavelength which is nearly independent of the specific semiconductor host. The continuing interest in rare earth doped semiconductors arises from the prospect of developing novel electroluminescence devices which combine the electronic properties of semiconductors with the unique luminescence features of rare earth ions.1–3 A significant amount of work has been devoted to the study of Er doped semiconductors because Er31 exhibits luminescence at 1.54 mm which overlaps the minimum loss region of silica-based fibers used in optical communications. The main problems limiting the performance of current Er doped semiconductor devices are poor luminescence efficiency and low incorporation of optically active Er31 ions. Recent studies have shown that the Er31 luminescence intensity at room temperature is strongly related to the band gap energy of the semiconductor host. It was found that semiconductors with larger band gap, exhibit less temperature quenching of Er31 luminescence.4 Consequently, research efforts have shifted towards studying Er31 doped into wide gap semiconductors.2–6 We are currently engaged in a comprehensive study of the optical properties of Er31 doped III-nitride semiconductors.7–9 Because of their wide band gap, IIInitride semiconductors are expected to be ideal host for Er31 ions to emit strong 1.54 mm photoluminescence ~PL!. Results on the observation of 1.54 mm photoluminescence of Er implanted GaN have been reported by Wilson et al.7 and a!
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other groups.10–14 A systematic study by Torvik et al.13 showed that similar to Er31 doped in narrow gap host, oxygen co-implantation plays an important role in the optical activation of Er31 ions in GaN. These authors also presented the first demonstration of cathodoluminescence11 and electroluminescence12 from Er, O implanted GaN. The exact nature of the Er luminescence center and its excitation mechanisms in the GaN host, however, have not yet been fully explored. In this article, we present new spectroscopic results of Er implanted GaN including high temperature PL intensity measurements and photoluminescence excitation studies ~PLE!. Our results demonstrate that the 1.54 mm Er31 PL from Er implanted GaN is extremely stable up to 550 K ~temperature limit of our heating device!. Moreover, our PLE studies reveal that Er31 can be excited indirectly through carrier-mediated processes or directly through intra4 f Er31 transitions. Similar PLE results have been recently reported by us for Er: AlN doped during ~MOMBE! growth9 and seem to be an inherent characteristic of Er doped IIInitride semiconductors. The 1-mm-thick GaN film was grown on a sapphire substrate using reactive ion-beam molecular beam epitaxy ~MBE!.7 Er was implanted at an energy of 300 keV and a fluence of 231014 cm22. Oxygen was co-implanted at an energy of 40 keV and a fluence of 1015 cm22. The 40 keV oxygen energy was chosen to overlap the oxygen and erbium implantation profiles. After implantation the sample was annealed at ;650 °C for approximately 60 mins. As previous secondary ion mass spectroscopy ~SIMS! data have shown, most of the Er is located within 200 nm of the sample surface and the Er depth distribution does not change signifi-
Appl. Phys. Lett. 71 (18), 3 November 1997 0003-6951/97/71(18)/2641/3/$10.00 © 1997 American Institute of Physics 2641 Downloaded 07 Jun 2001 to 169.226.115.84. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
FIG. 1. Photoluminescence spectra at 300 and 550 K from Er implanted GaN ~a! for above ~325 nm! band gap excitation, ~b! for below ~488 nm! band gap excitation.
cantly with annealing up to 900 °C.7 Integrated PL intensity measurements were carried out using either the 325 nm line ~10 mW! of a HeCd laser or the 488 nm line ~50 mW! of an argon ion laser. The pump beam diameter was ;1 mm. The presented PL spectra are not corrected for the system response of the luminescence setup. For time-resolved PL and PLE measurements, an optical parametric oscillator ~Surelite OPO, Continuum! pumped by a Q-switched Nd:YAG laser ~5–10 ns pulses! was used as excitation source. The resolution in the PLE studies was limited by the bandwidth of the OPO system which changed from ;40 cm21 at 425 nm to ;200 cm21 at 680 nm. In the near infrared region (;1000 nm) the laser linewidth was ;60 cm21. The OPO output power was monitored during the PLE studies using a pyroelectric power meter to normalize the PLE signal. The photoluminescence was dispersed with a single grating 1 m monochromator and detected with a liquid nitrogen ~LN! cooled Ge detector ~response time of 0.5–1 ms!. A 850 nm long-pass filter was placed in front of the entrance slit of the monochromator to minimize stray laser light. A lock-in technique ~cw PL experiments! or a boxcar averager ~pulsed PL experiments! was used to process the data. Temperature dependent PL measurements were conducted using a closedcycle helium refrigerator for the range from 15 to 300 K. Above 300 K, a home-made heating element was employed which had a temperature accuracy of 65 °C. Figure 1 shows Er31 PL spectra from Er implanted GaN at 300 and 550 K excited above ~325 nm! and below ~488 nm! the band gap (;3.4 eV) of GaN. Above gap excitation is important for future GaN:Er devices because it simulates electron-hole pair mediated pumping occurring in forward biased p – n junctions. For both excitation wavelengths we observed strong Er31 emission peaking at ;1.535 m m and a linewidth of 50 nm full width half maximum ~FWHM!. The large linewidth suggests that Er31 ions occupy a range of sites and the PL spectra are inhomogeneously broadened.10–14 The temperature dependent integrated Er31 PL intensity up to 550 K is depicted in Fig. 2. At 550 K, the integrated intensity decreased relative to its value at 15 K by only ;10% for above gap excitation and by ;50% for below gap excitation, respectively. These results provide
FIG. 2. Temperature dependence of the integrated Er31 PL intensity for above and below gap excitation. For above gap excitation the integrated PL intensity changed only by 10% for the temperature range 15–550 K.
further evidence that thermal quenching of Er31-related PL is less in wide gap semiconductors.4–7 To our knowledge, our GaN:Er sample exhibits the weakest PL temperature quenching observed from any Er31 doped III-V semiconductor to date, even less than Er31 doped SiC.6 It is interesting to note that above and below excitation lead to slightly different Er31 PL spectra and PL temperature dependencies. The PL spectrum for above gap excitation shows additional features located at 1620 nm. This indicates the existence of different subsets of Er31 ions with distinct excitation and de-excitation schemes.14 In order to gain more insight in the incorporation of Er31 ions in GaN as well as their excitation mechanisms, we performed PLE measurements at room temperature. The PLE spectrum of Er implanted GaN is depicted in Fig. 3. The Er31 luminescence was monitored at 1.535 mm while vary-
FIG. 3. Photoluminescence excitation spectrum of Er implanted GaN. The Er31 PL was monitored at 1.535 mm. The broad PLE band spanning from 425 to 680 nm is attributed to carrier-mediated Er31 excitation. The sharp features at 495, 525, 553, 651, and 980 nm are due to resonant intra-4 f Er31 excitation.
2642 Appl. Phys. Lett., Vol. 71, No. 18, 3 November 1997 Thaik et al. Downloaded 07 Jun 2001 to 169.226.115.84. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
FIG. 4. Room temperature PL spectra excited resonant ~525 and 651 nm! and off-resonant ~440, 488, and 570 nm! with an intra-4 f Er31 transition. Different spectra are observed for direct and indirect Er31 excitation which indicates the existence of distinct subsets of Er31 ions.
ing the excitation wavelength from 425 to 1020 nm. Striking features of the PLE spectrum are a broad band and sharp peaks located at 495, 525, 553, 651, and 980 nm. These sharp peaks coincide with the following Er31 intra-4 f tran4 4 4 sitions: I 15/2→ 4 F 7/2 , I 15/2→ 2 H 11/2 , I 15/2→ 4 S 3/2 , 4 4 4 4 I 15/2→ F 9/2 , and I 15/2→ I 11/2 , respectively, and are assigned to resonant Er31 excitation. The broadband PLE spanning from 425 to 680 nm is attributed to Er31 excitation processes involving defects in the GaN host.14,15 The PLE result observed for GaN:Er is similar to our previous report of below gap excitation of Er31 ions in AlN.9 This suggests that direct and indirect Er31 excitation schemes are an inherent feature of Er31 ions in III-nitride semiconductors. We are currently extending our PLE measurements to GaN:Er doped during MOMBE growth. Preliminary results indicate that this sample also shows an overlap of broadband carriermediated Er31 excitation and sharp intra-4 f Er31 excitation. Figure 4 shows PL spectra of GaN:Er excited at wavelengths resonant and off-resonant with an intra-4 f Er31 transition. It can be noted, that the Er31 PL spectra are significantly different for direct and indirect below gap excitation which provides further support for the existence of distinct classes of Er31 ions. Furthermore, it was observed that the Er31 PL, excited resonantly with an Er31 transition, is longer lived (;3.0 ms) than that excited using off-resonant excitation (;2.5 ms). This observation is consistent with recent
results reported for Er implanted in GaN ~MOCVD! using resonant excitation at 983 nm and above gap excitation at 351.1 nm.13 In summary, results of a high temperature photoluminescence and photoluminescence excitation study of Er implanted GaN are presented. Strong Er31 PL at 1.535 mm was observed for below and above gap excitation. The integrated Er31 PL intensity remained nearly constant up to 550 K. It was found that the Er31 PL spectra, as well as the PL quenching behavior, changed with excitation wavelength, indicating the existence of different classes of Er31 ions with distinct excitation and de-excitation schemes. This observation was further supported by PLE measurements which showed that 1.535 mm Er31 PL can be excited either through direct optical excitation of intra-4 f Er31 levels or by an indirect carrier-mediated process. Further studies are currently being undertaken to elucidate the nature of the different Er31 sites in GaN in terms of PL efficiency, saturation behavior, as well as excitation schemes. Gaining deeper insight into the incorporation and excitation mechanisms of optically active Er31 ions will be crucial in the advancement of Er doped III-nitride electroluminescence devices. The authors from Hampton University acknowledge financial support by NASA through Grant No. NCC-1-251 and the Army Research Office through Grant No. DAAH0496-1-0089.
1
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Appl. Phys. Lett., Vol. 71, No. 18, 3 November 1997 Thaik et al. 2643 Downloaded 07 Jun 2001 to 169.226.115.84. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp