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Near-field magneto-photoluminescence of quantum-dot-like composition fluctuations in GaAsN and InGaAsN alloys
Available online at www.sciencedirect.com
Physica E 21 (2004) 385 – 389 www.elsevier.com/locate/physe
Near-!eld magneto-photoluminescence of quantum-dot-like composition )uctuations in GaAsN and InGaAsN alloys A.M. Mintairova; b , P.A. Blagnovb , J.L. Merza;∗ , V.M. Ustinovb , A.S. Vlasovb , A.R. Kovshc , J.S. Wangc , L. Weic , J.Y. Chic a Department
of Electrical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA Physico-Technical Institute, RAS, St. Petersburg 194021, Russia c Industrial Technology Research Institute, Hsinchu, Taiwan
b Io e
Abstract Near-!eld magneto-photoluminescence scanning microscopy has been used to investigate the structural and optical properties of quantum-dot (QD)-like compositional )uctuations in GaAsN and InGaAsN alloys. Sharp spectral lines (halfwidth 0.5 –2 meV) from these QDs are observed at T ¡ 70 K, and their Zeeman splitting and diamagnetic shifts are used to determine the size (r ∼ 6–18 nm), density (∼ 100 m−3 ), and nitrogen composition. Near-!eld scanning images reveal phase separation eAects in the distribution of nitrogen, but little eAect appears from the presence or absence of indium. Indium does have a strong eAect on the exciton g-factor for observations in a magnetic !eld. ? 2003 Elsevier B.V. All rights reserved. PACS: 78.67.Hc; 61.46.+w Keywords: Near-!eld spectroscopy; Magnetoluminescence; Quantum dots; InGaAsN
1. Introduction Quaternary alloys of InGaAsN (also commonly referred to as GaInNAs) have in recent years attracted considerable attention as promising materials for laser diodes and eFcient solar cells at 1.3 and 1:5 m. This interest results from the fact that very small amounts of N added to GaAs results in a dramatic and unexpected shift of the band gap towards lower energy, which in turn results from the small size and large electronegativity of nitrogen compared with arsenic [1,2]. This eAect is usually described by a “giant bowing” ∗ Corresponding author. Tel.: +1-574-631-3111; fax: +1-574631-0651. E-mail address: [email protected] (J.L. Merz).
parameter (b ∼ 20 eV) of the band gap vs. composition diagram, so that, for example, the )uctuation of only 0.5% of N reduces the band gap of GaAsN by 100 meV [3,4]. The addition of N to GaAs also decreases the lattice constant signi!cantly, creating considerable strain. This strain can be reduced by adding In to substitute for Ga in the InGaAsN quaternary, which reduces the band gap further. For small amounts of N (up to 4% or 5%), it is possible to achieve lattice matching conditions when the In concentration is approximately three times that of the N concentration. However, many authors have observed the eAects of clustering or compositional )uctuations in these epitaxial layers rather than uniform growth. In the present paper we use near-!eld magneto-photoluminescence measurements as a function of temperature to study
1386-9477/$ - see front matter ? 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2003.11.081
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A.M. Mintairov et al. / Physica E 21 (2004) 385 – 389
the optical and structural properties of these compositional )uctuations in alloys with diAerent N and In content. It is shown that these )uctuations or clusters behave as quantum dots (QDs), whose size, density, and N concentration can be determined by the measurements reported here.
2. Experimental Four samples were grown by solid source MBE on (0 0 1) semi-insulating GaAs substrates at temperatures of 450 –520◦ C. Two of these samples were the ternary GaAs1−y Ny (y ∼ 0:01 and 0.03) and two were the quaternary Inx Ga1−x As1−y Ny (x ∼ 0:03 for y ∼ 0:01 and x ∼ 0:08 for y ∼ 0:03). A RF-plasma source was used to generate atomic nitrogen from N2 , and the layers had thickness 0.1–1 m. In one case (the Inx Ga1−x As1−y Ny sample with x = 0:08, y = 0:03), the layer was produced by growing an appropriate short-period superlattice (SPS), as described in Ref. [5], in order to minimize compositional )uctuations in the growth direction. The structure was capped by a 20 nm GaAs layer and annealed at 550◦ C for 10 min. No signi!cant diAerences were noted
between the SPS layer and the “uniform” layers, and TEM measurements showed evidence of the SPS after thermal anneal. Near-!eld photoluminescence (NPL) spectra [6] with spatial and spectral resolution of 300 nm and 0:5 meV, respectively, were taken in collection-illumination mode. The spectra were excited by 20 mW of 514:5 nm radiation, and were measured between 5 and 300 K and magnetic !eld strengths 0 –10 T. 3. Results and discussion Between 5 and 70 K the NPL spectra of all samples reveal structure, consisting of a series of multiple narrow lines (halfwidth = 0:05–2 meV) superimposed on several broad ( = 20–60 meV) bands located at 1–1:2 m. Two such spectra are shown in Fig. 1 for 3% N, strained (Fig. 1a) and lattice-matched with 8% In (Fig. 1b). We attribute the narrow lines (Ci ) to emission of excitons localized on QD-like compositional )uctuations (N-rich clusters), whereas the broad bands (resolved into several Gaussian peaks, Ai ) are regions of weak localization of excitons in the ternary or quaternary layer.
(a)
(c)
(b)
Fig. 1. (a) NPL of GaAs1−y Ny (y ∼ 0:03) and (b) Inx Ga1−x As1−y Ny (x ∼ 0:08 and y ∼ 0:03). Lines labeled Ci are quantum dots, bands Ai are areas of weak exciton localization. (c) Temperature dependence of the Ai bands.
A.M. Mintairov et al. / Physica E 21 (2004) 385 – 389
(a)
387
(b)
Fig. 3. Cluster radii calculated from the diamagnetic shifts observed for all quantum dot lines in the NPL spectra, as a function of the emission energy of the Ci lines. These results are for the 3% N samples shown in Figs. 1 and 2; solid circles are for InGaAsN, and the open circles are for GaAsN.
Fig. 2. Near-!eld magneto-PL spectra of GaAs1−y Ny (y ∼ 0:03) and Inx Ga1−x As1−y Ny (x ∼ 0:08 and y ∼ 0:03) at magnetic !elds B 6 10 T. The diamagnetic shift is shown for both samples by the dashed lines, whereas the dotted lines for the quaternary sample indicate Zeeman splitting. B is parallel to the growth direction.
At high temperature (cf. Fig. 1c) the bands Ai follow the usual band gap expansion of the lattice (Varshni model [7]). As T is reduced below ∼ 100 K, “freeze out” occurs into lower energy states of weak localization. Localization energies of 30 –40 meV can be read directly from the !gure. The sharp lines (Ci ) show relatively little energy shift at low temperature until their intensity vanishes at ∼ 60–70 K. Signi!cant is the fact that there is relatively little diAerence in behavior between the strained and lattice-matched cases, indicating that In plays little role in the weak and strong localization eAects observed in NPL. The QD spectral lines Ci are much narrower in the lattice-matched case, as expected. Magneto-photoluminescence measurements in the near !eld provide additional information concerning the size and composition of the QDs observed above. Fig. 2 shows the Zeeman splitting and diamagnetic shift observed for the same two samples shown in Fig. 1. Zeeman splitting was not observed for the sharp lines of the In-free samples (i.e., the exciton g-factor is zero), which can be attributed to the light-hole nature of the exciton in this strained layer. However, in lattice-matched InGaAsN these lines have a clear Zeeman splitting, whose energy increased with increasing In content and has values of 0.7 (g-factor ∼ 1) and
1.7 (g-factor ∼ 2) meV at 10 T for N = 1% and 3%, respectively. For both samples shown in Fig. 2 there is a clear diamagnetic shift increasing quadratically with magnetic !eld strength. This eAect provides the evidence that the Ci lines have QD character, i.e., result from regions of strong exciton localization. In the case of the quaternary sample, the diamagnetic shift is given by the center of gravity of the two Zeeman-split lines as a function of B. We found that the value of the diamagnetic coeFcient ( = SE=B2 ) increases with increasing N content but shows little dependence on the presence of In. has values 0.3–3 eV=T2 for various QDs in the low-nitrogen sample (y = 0:01) and 3–12 eV=T2 for higher N (y = 0:03). The cluster radius (r) can be deduced for each QD using the expression [8] = e2 x2 =8, where e is the electron charge, x2 describes the spatial extent of the ground state wave function (and therefore the cluster radius), and is the reduced mass of the exciton. It has been found [7,9] that this relationship underestimates the actual radius of a QD by a factor of 2. The calculated values of the cluster (QD) radii measured from the diamagnetic shifts of all lines observed for the 3% N samples are given in Fig. 3. Note that there is a clear tendency in the quaternary sample for the QD radii to increase with increasing emission energy. This suggests that the smaller dots have higher N concentration (hence emit at lower energy), or conversely, that the nitrogen tends to cluster densely into smaller regions. This effect was not observed for the In-free ternary sample. The average cluster radius for the 3% N quaternary
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A.M. Mintairov et al. / Physica E 21 (2004) 385 – 389
(a)
(b)
(c)
(d)
(e)
-3
Density ~ 100 m
Fig. 4. Monochromatic NPL intensity scans (a–e) of a 2 × 2 mm area of the GaAs0:97 N0:03 sample. Spectral scans are take at the positions (x; y) given for each spectrum, with the wavelengths indicated for which the NPL images were produced.
sample is measured to be ∼ 12 nm, and for the 3% N ternary it is ∼ 9 nm, as can be seen in the !gure. Not shown are the corresponding results for the lower N-containing samples (N ∼ 1%). For these samples (both ternary and quaternary) the cluster radii tend to be smaller, of order 6 –8 nm. This appears to be reasonable, as there is less N to form these clusters. We also !nd that the Ai -bands have a diamagnetic coef!cient of 30 –45 eV=T2 , corresponding to 2r ∼ 40– 60 nm, which is consistent with the weak localization regime assumed in the discussion above. We have also directly measured the density of clusters using near-!eld scanning techniques [5] to be ∼ 100 m−3 , approximately 10 orders of magnitude greater than that estimated for a random distribution (Stirling’s formula). Fig. 4 presents selected monochromatic NPL intensity images (scans a–e) of the 2 × 2 m area of the GaAs1−y Ny (y = 0:03) sample taken with x=y steps of 0:2 m. The NPL spectra are presented below the images. Simultaneous analysis of the images and the spectra shows that the spatial resolution of our experiments is 300 nm. The apparently larger spatial scale of the images (up to 600 nm) as well as their elon-
gated character seen in the scans a–d are due to the overlapping of diAerent lines. Scans a–d show locations for which at least !ve clusters are emitting at wavelengths within 1 nm of the detection wavelength. Our measurements reveal a strong inhomogeneity of the GaAsN layer on a length scale of 1 m. Indeed, in scan c the emission of the upper left 1 × 1 m section of the scan area is dominated by !ve clusters laterally separated by ∼ 0:4 m. The emission wavelengths of these clusters occupy a very small spectral range of ∼ 2 nm, indicating small diAerence (narrow distribution) in the cluster size and N content. In contrast, in the lower left section the cluster separation is much smaller (¡ 0:2 nm) but their wavelengths occupy a spectral range ∼ 30 nm (scans b–e), indicating a broad distribution of the sizes and N content. Similar long-scale inhomogeneity was observed in InGaAsN.
4. Conclusion Using near-!eld magneto-PL scanning microscopy we have observed QD-like compositional )uctuations in InGaAsN and GaAsN alloys having N
A.M. Mintairov et al. / Physica E 21 (2004) 385 – 389
compositions of 1% and 3%. These )uctuations manifest themselves by the appearance of narrow lines (halfwidth 0.2–2 meV) at temperatures below 70 K. These represent regions of strong excitonic localization. Their size, density, and nitrogen excess of these regions have been determined, and phase separation eAects have been observed in the distribution of N. Regions of weak localization have also been observed, and their spatial extent determined. The presence or absence of indium had no observable eAect on the size and density of these clusters, but a strong eAect of indium on the exciton g-factor was observed. Acknowledgements The authors wish to thank the W.M. Keck Foundation, the NATO Science for Peace Program
389
(Grant SFP-972484) and DARPA/ONR (Grant N00014-01-1-0658) for support of this research. References [1] S. Sakai, et al., Jpn. J. Appl. Phys. 32 (1993) 4413. [2] S.-H. Wei, A. Zunger, Phys. Rev. Lett. 76 (1996) 664. [3] S. Frankoeur, et al., Appl. Phys. Lett. 72 (1998) 1857; J.F. Geiz, et al., J. Cryst. Growth 195 (1998) 401. [4] E.D. Jones, et al., Phys. Rev. B 62 (2000) 7144. [5] A.M. Mintairov, et al., Phys. Rev. Lett. 87 (2001) 277401. [6] K. Matsuda, et al., Appl. Phys. Lett. 78 (2001) 1508. [7] Y.P. Varshni, Physica 34 (1967) 149. [8] P.D. Wang, et al., Phys. Rev. B 53 (1996) 16458. [9] I.E. Itskevich, et al., Appl. Phys. Lett. 70 (1997) 505; L.R. Wilson, et al., Phys. Rev. B 57 (1998) R2073; M. Bayer, et al., Phys. Rev. B 57 (1998) 6584; B. Kowalski, et al., Phys. Rev. B 58 (1998) 2026.
Physica E 21 (2004) 385 – 389 www.elsevier.com/locate/physe
Near-!eld magneto-photoluminescence of quantum-dot-like composition )uctuations in GaAsN and InGaAsN alloys A.M. Mintairova; b , P.A. Blagnovb , J.L. Merza;∗ , V.M. Ustinovb , A.S. Vlasovb , A.R. Kovshc , J.S. Wangc , L. Weic , J.Y. Chic a Department
of Electrical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA Physico-Technical Institute, RAS, St. Petersburg 194021, Russia c Industrial Technology Research Institute, Hsinchu, Taiwan
b Io e
Abstract Near-!eld magneto-photoluminescence scanning microscopy has been used to investigate the structural and optical properties of quantum-dot (QD)-like compositional )uctuations in GaAsN and InGaAsN alloys. Sharp spectral lines (halfwidth 0.5 –2 meV) from these QDs are observed at T ¡ 70 K, and their Zeeman splitting and diamagnetic shifts are used to determine the size (r ∼ 6–18 nm), density (∼ 100 m−3 ), and nitrogen composition. Near-!eld scanning images reveal phase separation eAects in the distribution of nitrogen, but little eAect appears from the presence or absence of indium. Indium does have a strong eAect on the exciton g-factor for observations in a magnetic !eld. ? 2003 Elsevier B.V. All rights reserved. PACS: 78.67.Hc; 61.46.+w Keywords: Near-!eld spectroscopy; Magnetoluminescence; Quantum dots; InGaAsN
1. Introduction Quaternary alloys of InGaAsN (also commonly referred to as GaInNAs) have in recent years attracted considerable attention as promising materials for laser diodes and eFcient solar cells at 1.3 and 1:5 m. This interest results from the fact that very small amounts of N added to GaAs results in a dramatic and unexpected shift of the band gap towards lower energy, which in turn results from the small size and large electronegativity of nitrogen compared with arsenic [1,2]. This eAect is usually described by a “giant bowing” ∗ Corresponding author. Tel.: +1-574-631-3111; fax: +1-574631-0651. E-mail address: [email protected] (J.L. Merz).
parameter (b ∼ 20 eV) of the band gap vs. composition diagram, so that, for example, the )uctuation of only 0.5% of N reduces the band gap of GaAsN by 100 meV [3,4]. The addition of N to GaAs also decreases the lattice constant signi!cantly, creating considerable strain. This strain can be reduced by adding In to substitute for Ga in the InGaAsN quaternary, which reduces the band gap further. For small amounts of N (up to 4% or 5%), it is possible to achieve lattice matching conditions when the In concentration is approximately three times that of the N concentration. However, many authors have observed the eAects of clustering or compositional )uctuations in these epitaxial layers rather than uniform growth. In the present paper we use near-!eld magneto-photoluminescence measurements as a function of temperature to study
1386-9477/$ - see front matter ? 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2003.11.081
386
A.M. Mintairov et al. / Physica E 21 (2004) 385 – 389
the optical and structural properties of these compositional )uctuations in alloys with diAerent N and In content. It is shown that these )uctuations or clusters behave as quantum dots (QDs), whose size, density, and N concentration can be determined by the measurements reported here.
2. Experimental Four samples were grown by solid source MBE on (0 0 1) semi-insulating GaAs substrates at temperatures of 450 –520◦ C. Two of these samples were the ternary GaAs1−y Ny (y ∼ 0:01 and 0.03) and two were the quaternary Inx Ga1−x As1−y Ny (x ∼ 0:03 for y ∼ 0:01 and x ∼ 0:08 for y ∼ 0:03). A RF-plasma source was used to generate atomic nitrogen from N2 , and the layers had thickness 0.1–1 m. In one case (the Inx Ga1−x As1−y Ny sample with x = 0:08, y = 0:03), the layer was produced by growing an appropriate short-period superlattice (SPS), as described in Ref. [5], in order to minimize compositional )uctuations in the growth direction. The structure was capped by a 20 nm GaAs layer and annealed at 550◦ C for 10 min. No signi!cant diAerences were noted
between the SPS layer and the “uniform” layers, and TEM measurements showed evidence of the SPS after thermal anneal. Near-!eld photoluminescence (NPL) spectra [6] with spatial and spectral resolution of 300 nm and 0:5 meV, respectively, were taken in collection-illumination mode. The spectra were excited by 20 mW of 514:5 nm radiation, and were measured between 5 and 300 K and magnetic !eld strengths 0 –10 T. 3. Results and discussion Between 5 and 70 K the NPL spectra of all samples reveal structure, consisting of a series of multiple narrow lines (halfwidth = 0:05–2 meV) superimposed on several broad ( = 20–60 meV) bands located at 1–1:2 m. Two such spectra are shown in Fig. 1 for 3% N, strained (Fig. 1a) and lattice-matched with 8% In (Fig. 1b). We attribute the narrow lines (Ci ) to emission of excitons localized on QD-like compositional )uctuations (N-rich clusters), whereas the broad bands (resolved into several Gaussian peaks, Ai ) are regions of weak localization of excitons in the ternary or quaternary layer.
(a)
(c)
(b)
Fig. 1. (a) NPL of GaAs1−y Ny (y ∼ 0:03) and (b) Inx Ga1−x As1−y Ny (x ∼ 0:08 and y ∼ 0:03). Lines labeled Ci are quantum dots, bands Ai are areas of weak exciton localization. (c) Temperature dependence of the Ai bands.
A.M. Mintairov et al. / Physica E 21 (2004) 385 – 389
(a)
387
(b)
Fig. 3. Cluster radii calculated from the diamagnetic shifts observed for all quantum dot lines in the NPL spectra, as a function of the emission energy of the Ci lines. These results are for the 3% N samples shown in Figs. 1 and 2; solid circles are for InGaAsN, and the open circles are for GaAsN.
Fig. 2. Near-!eld magneto-PL spectra of GaAs1−y Ny (y ∼ 0:03) and Inx Ga1−x As1−y Ny (x ∼ 0:08 and y ∼ 0:03) at magnetic !elds B 6 10 T. The diamagnetic shift is shown for both samples by the dashed lines, whereas the dotted lines for the quaternary sample indicate Zeeman splitting. B is parallel to the growth direction.
At high temperature (cf. Fig. 1c) the bands Ai follow the usual band gap expansion of the lattice (Varshni model [7]). As T is reduced below ∼ 100 K, “freeze out” occurs into lower energy states of weak localization. Localization energies of 30 –40 meV can be read directly from the !gure. The sharp lines (Ci ) show relatively little energy shift at low temperature until their intensity vanishes at ∼ 60–70 K. Signi!cant is the fact that there is relatively little diAerence in behavior between the strained and lattice-matched cases, indicating that In plays little role in the weak and strong localization eAects observed in NPL. The QD spectral lines Ci are much narrower in the lattice-matched case, as expected. Magneto-photoluminescence measurements in the near !eld provide additional information concerning the size and composition of the QDs observed above. Fig. 2 shows the Zeeman splitting and diamagnetic shift observed for the same two samples shown in Fig. 1. Zeeman splitting was not observed for the sharp lines of the In-free samples (i.e., the exciton g-factor is zero), which can be attributed to the light-hole nature of the exciton in this strained layer. However, in lattice-matched InGaAsN these lines have a clear Zeeman splitting, whose energy increased with increasing In content and has values of 0.7 (g-factor ∼ 1) and
1.7 (g-factor ∼ 2) meV at 10 T for N = 1% and 3%, respectively. For both samples shown in Fig. 2 there is a clear diamagnetic shift increasing quadratically with magnetic !eld strength. This eAect provides the evidence that the Ci lines have QD character, i.e., result from regions of strong exciton localization. In the case of the quaternary sample, the diamagnetic shift is given by the center of gravity of the two Zeeman-split lines as a function of B. We found that the value of the diamagnetic coeFcient ( = SE=B2 ) increases with increasing N content but shows little dependence on the presence of In. has values 0.3–3 eV=T2 for various QDs in the low-nitrogen sample (y = 0:01) and 3–12 eV=T2 for higher N (y = 0:03). The cluster radius (r) can be deduced for each QD using the expression [8] = e2 x2 =8, where e is the electron charge, x2 describes the spatial extent of the ground state wave function (and therefore the cluster radius), and is the reduced mass of the exciton. It has been found [7,9] that this relationship underestimates the actual radius of a QD by a factor of 2. The calculated values of the cluster (QD) radii measured from the diamagnetic shifts of all lines observed for the 3% N samples are given in Fig. 3. Note that there is a clear tendency in the quaternary sample for the QD radii to increase with increasing emission energy. This suggests that the smaller dots have higher N concentration (hence emit at lower energy), or conversely, that the nitrogen tends to cluster densely into smaller regions. This effect was not observed for the In-free ternary sample. The average cluster radius for the 3% N quaternary
388
A.M. Mintairov et al. / Physica E 21 (2004) 385 – 389
(a)
(b)
(c)
(d)
(e)
-3
Density ~ 100 m
Fig. 4. Monochromatic NPL intensity scans (a–e) of a 2 × 2 mm area of the GaAs0:97 N0:03 sample. Spectral scans are take at the positions (x; y) given for each spectrum, with the wavelengths indicated for which the NPL images were produced.
sample is measured to be ∼ 12 nm, and for the 3% N ternary it is ∼ 9 nm, as can be seen in the !gure. Not shown are the corresponding results for the lower N-containing samples (N ∼ 1%). For these samples (both ternary and quaternary) the cluster radii tend to be smaller, of order 6 –8 nm. This appears to be reasonable, as there is less N to form these clusters. We also !nd that the Ai -bands have a diamagnetic coef!cient of 30 –45 eV=T2 , corresponding to 2r ∼ 40– 60 nm, which is consistent with the weak localization regime assumed in the discussion above. We have also directly measured the density of clusters using near-!eld scanning techniques [5] to be ∼ 100 m−3 , approximately 10 orders of magnitude greater than that estimated for a random distribution (Stirling’s formula). Fig. 4 presents selected monochromatic NPL intensity images (scans a–e) of the 2 × 2 m area of the GaAs1−y Ny (y = 0:03) sample taken with x=y steps of 0:2 m. The NPL spectra are presented below the images. Simultaneous analysis of the images and the spectra shows that the spatial resolution of our experiments is 300 nm. The apparently larger spatial scale of the images (up to 600 nm) as well as their elon-
gated character seen in the scans a–d are due to the overlapping of diAerent lines. Scans a–d show locations for which at least !ve clusters are emitting at wavelengths within 1 nm of the detection wavelength. Our measurements reveal a strong inhomogeneity of the GaAsN layer on a length scale of 1 m. Indeed, in scan c the emission of the upper left 1 × 1 m section of the scan area is dominated by !ve clusters laterally separated by ∼ 0:4 m. The emission wavelengths of these clusters occupy a very small spectral range of ∼ 2 nm, indicating small diAerence (narrow distribution) in the cluster size and N content. In contrast, in the lower left section the cluster separation is much smaller (¡ 0:2 nm) but their wavelengths occupy a spectral range ∼ 30 nm (scans b–e), indicating a broad distribution of the sizes and N content. Similar long-scale inhomogeneity was observed in InGaAsN.
4. Conclusion Using near-!eld magneto-PL scanning microscopy we have observed QD-like compositional )uctuations in InGaAsN and GaAsN alloys having N
A.M. Mintairov et al. / Physica E 21 (2004) 385 – 389
compositions of 1% and 3%. These )uctuations manifest themselves by the appearance of narrow lines (halfwidth 0.2–2 meV) at temperatures below 70 K. These represent regions of strong excitonic localization. Their size, density, and nitrogen excess of these regions have been determined, and phase separation eAects have been observed in the distribution of N. Regions of weak localization have also been observed, and their spatial extent determined. The presence or absence of indium had no observable eAect on the size and density of these clusters, but a strong eAect of indium on the exciton g-factor was observed. Acknowledgements The authors wish to thank the W.M. Keck Foundation, the NATO Science for Peace Program
389
(Grant SFP-972484) and DARPA/ONR (Grant N00014-01-1-0658) for support of this research. References [1] S. Sakai, et al., Jpn. J. Appl. Phys. 32 (1993) 4413. [2] S.-H. Wei, A. Zunger, Phys. Rev. Lett. 76 (1996) 664. [3] S. Frankoeur, et al., Appl. Phys. Lett. 72 (1998) 1857; J.F. Geiz, et al., J. Cryst. Growth 195 (1998) 401. [4] E.D. Jones, et al., Phys. Rev. B 62 (2000) 7144. [5] A.M. Mintairov, et al., Phys. Rev. Lett. 87 (2001) 277401. [6] K. Matsuda, et al., Appl. Phys. Lett. 78 (2001) 1508. [7] Y.P. Varshni, Physica 34 (1967) 149. [8] P.D. Wang, et al., Phys. Rev. B 53 (1996) 16458. [9] I.E. Itskevich, et al., Appl. Phys. Lett. 70 (1997) 505; L.R. Wilson, et al., Phys. Rev. B 57 (1998) R2073; M. Bayer, et al., Phys. Rev. B 57 (1998) 6584; B. Kowalski, et al., Phys. Rev. B 58 (1998) 2026.