Spatially Resolved Cathodoluminescence Studyof As Doped GaN

A. Bell1) (a), F. A. Ponce (a), S. V. Novikov (b), C. T. Foxon (b),

and I. Harrison (c)

(a) Department of Physics and Astronomy, Bateman Physical Sciences Center F-wing,Arizona State University, Tempe, AZ, 85287-1504, USA

(b) School of Physics and Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK

(c) School of Electrical and Electronic Engineering, University of Nottingham,Nottingham, NG7 2RD, UK

(Received June 21, 2001; accepted August 2, 2001)

Subject classification: 73.20.Hb; 78.55.Cr; 78.60.Hk; S7.14

The introduction of arsenic (As) into GaN to produce a group-V ternary alloy has been of muchrecent interest, mostly because of the prospect of reducing the GaN bandgap. We have performeda systematic study of the role of As in GaN grown by molecular beam epitaxy (MBE). The Ascontent of this series of samples varies from 3.4 � 1017 to 4.2 � 1018 cm––3. The data are presentedto show how As effects the optical properties of GaN. Our focus is on the nature of the strongluminescence band found at �475 nm. The intensity of the GaN near bandedge emission is shownto decrease and the 475 nm emission to increase with As content. This is attributed to the largeAs atoms disrupting the GaN lattice and creating defects or stacking faults that act as non-radia-tive centers. We have used scanning electron microscopy (SEM) and cathodoluminescence (CL) toinvestigate the spatial uniformity of the �475 nm emission in these materials and show that theluminescence is inhomogeneous indicating arsenic segregation.

1. Introduction There has been a growing interest in the development of the GaNAsalloy system in recent years [1–7]. The prospect of reducing the bandgap of GaN bythe addition of As has a major influence on the research being conducted. The GaNAsalloy system has a bandgap range stretching from the UV into the IR and the largemismatch, of more than 20%, in the lattice constants of GaN and GaAs has beenshown theoretically to lead to a limited miscibility and a strong, composition-dependentbowing of the bandgap [1]. The limited miscibility makes the incorporation of As intoGaN difficult, however, the large bowing parameter will lead to a substantial bandgapchange when relatively small amounts of As are incorporated into GaN.In 1976, Pankove and Hutchby [2] conducted a study of implanted GaN. One of the

implant elements that were studied was As. A characteristic luminescence band at2.58 eV was observed only in the As implanted samples. This band has been observedin GaNAs layers grown by metal organic chemical vapor deposition (MOCVD) at 2.58eV [3] and 2.5 eV [4] and layers grown by MBE at �2.6 eV [5]. In a theoretical paper,the nature of this luminescence band has been attributed to As occupying a Ga site(AsGa) in the GaN lattice [1]. It was shown that the large difference in the covalentradii of N (at 0.75 �A) and As (at 1.20 �A) compared to Ga (at 1.26 �A), makes it ispossible for As to occupy a Ga site. The transition level for the AsGa (2+/+) donor was

1) Corresponding author; Phone: 480 965 0138; Fax: 480 965 7954; e-mail: abigail.bell@asu.edu

phys. stat. sol. (b) 228, No. 1, 207–211 (2001)

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shown to be 2.7 eV and for the (+/0) donor state, 2.2 eV. Since As on a nitrogen site(AsN) creates donor levels that occur at 0.11 eV and 0.31 eV above the valence band,the �2.6 eV luminescence was attributed to AsGa.Arsenic doped GaN layers have also been shown to enhance the �3.28 eV donor-to-

acceptor pair (DAP) luminescence [6], which was attributed to As-associated isoelectro-nic centers.

2. Experimental Details In an effort to further understand the luminescence processesthat occur in As-doped GaN, spatially resolved cathodoluminescence (CL) was carriedout on As-doped GaN. The As-doped GaN samples were grown on sapphire substrates,at 800 oC, in a home-made MBE reactor, which has been described in detail elsewhere[7]. The active nitrogen species were generated using an Oxford Applied ResearchCARS25 rf activated plasma source. The As2 flux was generated using a purpose builtAs source and was varied for different samples. Prior to growth, the sapphire substratewas nitrided at 800 oC for 30 min. The thickness of the samples is estimated to bebetween 1.0 and 1.1 mm.The As concentration of the samples was found using secondary ion mass spectro-

scopy (SIMS) with an As implanted standard. The error in the As concentration wasestimated to be approximately 100%, however, the relative As concentrations is moreaccurate than that. The As flux used during growth and As concentration in the filmsare displayed in Table 1.

208 A. Bell et al.: Spatially Resolved CL Study of As Doped GaN

Tab l e 1The As flux used during growth and As concentration found using SIMS are shown foreach sample

sample ID run # As flux(mbar)

As conc.(1017 cm––3)

1 MS-115 1 � 10––9 5.42 MS-117 1 � 10––7 9.33 MS-120 5 � 10––7 7.14 MS-119 1 � 10––6 9.05 MS-122 3.5 � 10––6 18.56 MS-223 7.9 � 10––6 16.47 MS-118 1.3 � 10––5 42.4

Fig. 1. Room temperature CL spectra of As-doped GaN with Vaccel ¼ 3 keV, Ibeam ¼ 170 pAand magnification �10000. The �475 nm emis-sion appears to compete with the near band-edge emission

Scanning electron microscopy (SEM)and CL measurements were carried outusing a JSM 6300 microscope. The sam-ple was cooled to 4.2 K using liquid he-lium. The room temperature CL spectrawere taken and the results were relatedto the As concentration. An SEM im-

age was taken at a high magnification and spatially resolved CL was performed on thefeatures found.

3. Results and Discussion Figure 1 shows the room temperature CL spectra of samples1–5 and 7. The acceleration voltage (Vaccel) was 3 kV, the beam current (Ibeam) was170 pA and the magnification (mag) was �10000 (i.e. scan width �20 mm). The GaNbandedge emission occurs at 362 nm (3.43 eV) and the As defect related emission band isobserved at �475 nm (2.61 eV). Since the As concentration in these samples is in therange of parts per million, we observe no emission related to the GaNAs alloy bandedge.The intensity of the bandedge emission appears to get stronger as the �475 nm emissiondecreases. This would suggest that the two are competing processes. The position of thenear 475 nm emission band is approximately the same for all samples. The intensities ofthe GaN bandedge emission and the �475 nm emission are plotted versus As concentra-tion in Fig. 2. The trend shown in this figure is that the bandedge emission intensity de-creases with increasing As concentration and that the �475 nm emission intensity in-creases with As concentration. The increase in the As emission correlates with the theorythat this emission is somehow related to As. A possible explanation for the decrease inthe GaN bandedge emission is that when As is introduced into the GaN lattice, even insmall concentrations, the large As atoms distort the GaN lattice, creating dislocations orstacking faults that act as non-radiative centers. As the As concentration is increased, thenumber of non-radiative centers is increased and hence the GaN bandedge emission in-tensity will decrease.SEM images and CL spectra of sample 6 were taken at 4.2 K, with Vaccel ¼ 5 keV,

Ibeam ¼ 120 nA at a magnification of �40 K (i.e. the horizontal width is 5 mm) anda slit width of 1 mm. Figure 3 shows the SEM image of the sample. Light contrastfeatures, measuring �1 mm across were observed. Fig. 4 shows the CL spectra takenover the whole region shown in the SEM image and at a magnification of �300 Kin the positions marked 1–4. The spectra taken over the whole area exhibits emis-sion at 379 nm (3.27 eV) and a broad band at �475 nm. The 379 nm emissioncould be due to bandedge emission from zinc blende GaN. This seems unlikely sincethe room temperature emission of sample 6 is consistent with wurtzite GaN. It ismore likely to be the donor–acceptor pair (DAP) commonly found in GaN [6]. The

phys. stat. sol. (b) 228, No. 1 (2001) 209

Fig. 2. Peak intensities versus As concentra-tion, taken from the room temperature CLspectra

4 K spectra of the other samples exhibit the wurtzite bandedge and the 379 nmemission which is often accompanied by the first and second LO phonon replicaswhich are a common feature of DAP emission. Thus, we attribute the emission to aDAP.The CL spectra taken from the light feature at position 1, exhibits strong emission at

391 nm (3.17 eV) and the intensity of the broad emission �475 nm is reduced. The391 nm emission is redshifted in comparison to the spectra taken over the whole re-gion, which would suggest this is a region of high As concentration in which the Ga-NAs alloy is forming. The spectra taken from the dark region at position 2 exhibitsweak emission in the region of the GaN bandedge and strong 475 nm emission. Similarresults were obtained for positions 3 and 4. This inhomogeneous emission would sug-gest that the As is segregated across the sample.

4. Conclusions In conclusion, as the As concentration in GaN is increased, the GaNnear bandedge emission intensity decreases. This is explained as being caused by non-radiative centers, which are created by disruption to the GaN lattice by the large Asatoms. The introduction of As into GaN increases the intensity of the �475 nm emis-sion. This adds strength to the theory that the �475 nm emission is As related. AnSEM/CL study showed that the CL emission is inhomogeneous and that in �1 mmregions, the GaNAs alloy is formed.

Acknowledgements We are grateful for support from the Office of Naval Research(contract No. N00014-00-1-0133) and from Durel Corporation. The growth of the mate-rial was undertaken with support from UK EPSRC (GR/M67438).

210 A. Bell et al.: Spatially Resolved CL Study of As Doped GaN

Fig. 3. SEM image of sample 6 with Vaccel ¼ 5 keV, Ibeam ¼ 120 nA and at a magnification of x40K. The horizontal width is 5 mm

Fig. 4. CL spectra of sample 6 taken at a magnification of 40000� over the whole region shown inFig. 3. and at a magnification of 300000� at positions 1 and 2 shown in Fig. 3. Vaccel ¼ 5 keV andIbeam ¼ 120 nA

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