© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

p s scurrent topics in solid state physics

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caphys. stat. sol. (c) 5, No. 6, 1740–1742 (2008) / DOI 10.1002/pssc.200778615

Growth and magnetization study of transition metal doped GaN nanostructures

Shalini Gupta1, Hun Kang1, Matthew H. Kane1, 2, Eun-Hyun Park1, and Ian T. Ferguson*, 1, 2

1 Georgia Institute of Technology, School of Electrical and Computer Engineering, Atlanta, GA 30332, USA 2 Georgia Institute of Technology, School of Materials Science and Engineering, Atlanta, GA 30332, USA

Received 8 September 2007, revised 10 January 2008, accepted 12 January 2008

Published online 24 April 2008

PACS 68.55.A–, 75.75.+a, 81.05.Ea, 81.07.Ta, 81.15.Gh

* Corresponding author: e-mail ianf@ece.gatech.edu, Phone: +1 (404) 385 2885, Fax: +1 (404) 385 2886

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Diluted magnetic semiconductors (DMS) have at-

tracted considerable interest as materials that can support the transport and storage of spin. These materials have the added advantage that they can be integrated into existing electronic and optoelectronic devices to create multifunc-tional devices [1]. Several studies have been performed on bulk GaN doped with transition metals such as Manganese, as theoretical predictions and experimental data have shown that Ga1–xMnxN has a Curie temperature (TC) higher than room temperature (RT) [2]. Ferromagnetic nanostruc-tures are of interest as they are expected to have unique magnetic and magneto-optic properties due to quantum confinement [3]. Moreover, in the Arsenides it was deter-mined that ferromagnetic QDs increased the TC [4]. To this end, this study will focus on doping of optically active GaN nanostructures with transition metals (TM) Manga-nese and Iron.

2 Experimental The GaN nanostructures are grown by metal organic

chemical vapour deposition (MOCVD) on AlN template layers grown on sapphire substrates. This reactor has a

specially modified flow flange injection system that has been modified with dual injector blocks to minimize pre-reactions in the transport phase. Trimethyl gallium (TMGa), trimethyl aluminum (TMAl), and ammonia (NH3) were used as the source precursors for Ga, Al, and N, respectively. Bis-cyclopentadienyl manganese (Cp2Mn) and bis-cyclopentadienyl iron (Cp2Fe) was the TM source for manganese and iron respectively.

The AlN buffer layer is grown to introduce a 2.5 % lat-tice mismatch to promote GaN nanostructure growth [5]. In order to achieve high quality AlN buffer layers, a two step growth process was employed. First, a low-temperature (LT) AlN nucleation layer at 550 °C was grown on top of sapphire substrates. This was followed by a 0.7 µm thick AlN layer grown at 1100 °C. Atomic force microscopy (AFM) measurement on the surface of the AlN layer provided surface roughness of less than 0.6 nm. Thus, the surface was smooth enough to grow GaN nanostruc-tures. Furthermore, the rocking curve (Ω) scans of XRD confirmed that the AlN layer has a good crystalline quality.

To obtain a good baseline for doping of GaN nanos-tructures, nucleation studies of undoped GaN nanostruc-tures were performed first. A novel growth method has been developed to produce optically active GaN-based

This work presents the MOCVD growth and characterization

of optically active GaN nanostructures which have been

doped with the transition metals manganese and iron for po-

tential spintronic applications. Introduction of both these tran-

sition metals in GaN nanostructures enhanced the nucleation

of the nanostructures resulting in reduced lateral dimensions

and increased nanostructure density. Both Ga1–xMnxN and

Ga1–xFexN nanostructures showed hysteresis behaviour at

5 K. Further VSM measurements on Ga1–xFexN nanostruc-

tures at 300 K showed a hysteresis curve with a reduced coer-

cive filed and displayed superparamagnetic behaviour. These

magnetically active nanostructures are promising and provide

an incentive to further study them with the aim of eventually

utilizing them in spintronic applications and improving de-

vice efficiency.

phys. stat. sol. (c) 5, No. 6 (2008) 1741

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nanostructures using a two-step process. This first step is GaN deposition at low growth temperatures (<850 °C) and low V-III ratios (<30). This is followed by an in-situ acti-vation step involving a temperature ramp up to 970 °C in a nitrogen atmosphere to initialize a 3D growth process. Ga1–xTMxN nanostructures were obtained by introduc-ing the TM source during GaN deposition under the opti-mal conditions for the formation of nanostructures. The amount of TM incorporated was calibrated by secondary ion mass spectroscopy measurements (SIMS) of bulk lay-ers [6]. Mn (or Fe) was varied from 0% to 3%, as beyond this composition phase segregation effects have been ob-served in bulk Ga1–xTMxN layers [7]. In addition, all sam-ples for magnetization measurements had a 30 nm thick AlN cap layer. The surface morphology, size, and density of the nanostructures were analyzed using an ex - situ experimen-tal AFM in a PSIA XE 100 in both contact and non-contact mode. Raman spectroscopy measurements were performed to determine the crystalline quality using a Renishaw mi-cro-Raman system with a 488 nm excitation source. The optical property of the nanostructures was analyzed by photoluminescence (PL) measurements using a 325nm HeCd laser with a PIXIS100 CCD camera. SIMS was util-ized to analyze the composition of the TM. Vibrating sam-ple magnetometry (VSM) and superconducting quantum interference device (SQUID) were used for the magnetiza-tion measurements on the Ga1–xTMxN nanostructures.

3 Results and discussion AFM scans revealed that the GaN nanostructures had a lat-eral dimension of 40 nm and height of approximately 4 nm with island densities of 1x1010 cm–2 for a growth tempera-ture of 810 °C and a V-III ratio of 4.5. Furthermore, the AFM images also showed a Stranski-Krastanow like growth for these nanostructures [5].

The presence of the GaN A1(LO) mode in Raman measurements confirms the high crystalline quality of the nanostructures despite the extremely low V-III ratio and relatively low deposition temperatures. A shift towards higher energy was observed in the PL for smaller GaN nanostructures (Fig. 1) [8]. This shift is caused by a de-crease in the piezoelectric effect which has been verified by a numerical model. The optimized GaN nanostructures were then first doped with Mn. The Mn composition was varied from 0% to 2%. The surface morphology was strongly affected by

the presence of Mn atoms. The AFM characterization re-vealed the lateral dimension decreased to 30 nm and a height of 2 nm from the Mn deposition (Fig. 2). Further, is-land density increased to 3.0x1010 cm–2. Similar to the GaN nanostructures a post anneal was applied to the Ga1–xMnxN nanostructures and this resulted in ripening of the islands, deeming this step unnecessary. The observed nucleation behaviour of Ga1–xTMxN nanostructures at this time can be attributed to the increased metal concentration (i.e. de-crease in the V/III ratio), or to the role of Mn in enhancing nucleation, or both. SQUID measurements conducted on Ga1–xMnxN nanostructures showed a hysteresis behavior at 5 K as seen in Fig. 3, providing evidence of ferromagnetism in Ga1–xMnxN nanostructures. RT ferromagnetism in Ga1–xMnxN nanostructures was not observed due to the

(a) (b) (c)(a) (b) (c)

Figure 2 1 µm x 1 µm AFM images of (a) undoped GaN nanos-

tructures (without post-annealing). (b) 2% Mn incorporation en-

hances nucleation and results in 3-D growth, resulting in increased

island density and reduced lateral dimension. (c) Similarly, 3% Fe

doping of GaN nanostructures also results in enhanced nucleation.

AlN template

V/III = 7.5

Height = 10 nm

Diameter = 40 nm

Density = 1E1010 cm-2

V/III = 5.5

Height = 8 nm

Diameter= 30 nm

Density = 3E9 cm-2

GaN400 390 380 370 360

3.1 3.2 3.3 3.4 3.5

Wavelength (nm)

PL

In

ten

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arb

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Energy (eV)

T = 293 K

Eexc

= 325nm

Figure 1 PL measurement data for GaN nanos-tructures. A blue shift in seen with a decrease in the nanostructure size.

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0.0

40.0

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emu)

Applied Field (Oe)

T = 5K

2 % Mn doped

Figure 3 SQUID measurement of Ga0.98Mn0.02N nanostructures taken at 5 K.

1742 Shalini Gupta et al.: Growth and magnetization study of transition metal doped GaN nanostructures

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

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degradation of the crystalline quality caused by the intro-duction of Mn [9]. There are growth challenges that need to be tackled to improve the crystalline quality of these nanostructures. This study indicates that Ga1–xMnxN nanostructures are a promising ferromagnetic material. Additional studies need to be completed in order to under-stand the origin of the ferromagnetism in these structures whether it is due to Ga1–xMnxN nanostructures or due to some unidentified TM clusters. Similar to the affect of Mn the surface morphology of GaN nanostructures was strongly affected by the existence of Fe atoms. The AFM characterization revealed a de-crease in the lateral dimension to 30 nm and a height of 5 nm with 3% of Fe incorporation (Fig. 2c). The island density increased to approximately 1x1010 cm–2. It can be inferred that incorporating Fe enhances the nucleation of nanostructures and reduces the nanostructure size. The presence of Fe suppresses adatom migration due to altering the surface free energy [10, 11] and thereby enhances is-land formation. It can be speculated that the increased con-

centration of this TM is responsible for the observed nu-cleation behaviour of Ga1–xFexN nanostructures. Magnetization of GaN nanostructures doped with 2% Fe have been revealed by VSM and SQUID measurements at RT and LT respectively (Fig. 4). Magnetization studies by SQUID on Ga1–xFexN nanostructures revealed a hys-teresis curve with a diamagnetic contribution at 5 K. How-ever, VSM measurements on these structures at 300 K showed a hysteresis curve with a reduced coercive field and displayed superparamagnetic behaviour, thereby indi-cating the presence of clusters. Further growth, structural and magnetic investigations are underway to obtain a bet-ter understanding of the nanostructures formed and the ob-served ferromagnetism. This is the first report on observa-tion of magnetism in transition metal doped GaN nanos-tructures and provides an incentive to further study these nanostructures with the aim of eventually utilizing them in spintronic applications and improving device efficiency.

4 Conclusion

Transition metal doping of MOCVD grown GaN nanostructures with Mn and Fe were successfully com-pleted. It was determined that both Mn and Fe enhanced the nucleation of the nanostructures resulting in reduced lateral dimensions and increased nanostructure density. SQUID measurements on Ga1–xMnxN and Ga1–xFexN nanostructures showed hysteresis behaviour at 5K. Mag-netization data for Ga1–xFexN showed superparamagnetic behaviour indicating the presence of clusters. Optical char-acterization for these nanostructures are in progress. The overall observation that transition metal doping enhances nucleation and results in ferromagnetism makes transition metal doped GaN nanostructures promising for spintronic applications. These nanostructures could be used to en-hance the functionality and efficiency of semiconductor devices. Further investigations are needed to optimize these structures and to clarify the origin of ferromagnetism in them whether it is due to Ga1–xTMxN or ferromagnetic clusters.

Acknowledgements This research was funded by

AFOSR and monitored by Donald Silversmith. Contract #

FA9550-07-1-0141.

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Applied Field (Oe)

Fe QD: 5K

Fe QD: 300 K

Figure 4 SQUID and VSM measurement of

Ga0.98Fe0.02N nanostructures taken at 5 K and 300 K

respectively.