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Physica E 35 (2006) 207–211

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One-step preparation of ultra-wide b-Ga2O3 microbelts and theirphotoluminescence study

Aurangzeb Khana,�, Wojciech M. Jadwisienczakb, Martin E. Kordescha

aDepartment of Physics and Astronomy and CMSS program, Ohio University, Athens, OH 45701, USAbSchool of Electrical Engineering and Computer Science, Ohio University, Athens, OH 45701, USA

Received 31 May 2006; accepted 26 July 2006

Available online 7 September 2006

Abstract

Ultra-wide b-Ga2O3 microbelts are synthesized in one-step process and their photoluminescence (PL) emission is studied. The width

and thickness of the ultra-wide b-Ga2O3 sheets are in the ranges 40–80mm and 50–200 nm, respectively, while the lengths of these sheets

are in the range of several hundreds of micrometers. The microbelts have been investigated with scanning and transmission electron

microscopes (SEM, TEM, HRTEM), X-rays diffraction (XRD) and energy-dispersive X-rays spectroscope (EDX). Room-temperature

PL emission spectra of the as-grown microstructures show a broad blue peak centered at around 480 nm. At temperatures below 140K,

another band centered at 715 nm emerges. This band is mainly attributed to nitrogen impurities in b-Ga2O3 material. It is believed that

the nitrogen impurities act as acceptors in b-Ga2O3 inducing hole traps which recombine with electrons trapped at oxygen vacancy

donors generating red-light emission.

r 2006 Elsevier B.V. All rights reserved.

PACS: 81.05.Hd; 81.07.Bc; 81.10.Bk

Keywords: Gallium oxide; Gallium oxide luminescence; Microbelts

1. Introduction

Nanoscale building blocks such as quantum dots,nanowires and nanotubes have received considerableattention due to their size-dependent properties andpotential applications in electronic and optoelectronicdevices [1–7]. However, the alignment and assembly ofthese building blocks still make it difficult to constructuseful nanoscale devices such as integrated circuits. Wideband-gap semiconductors such as GaN, AlN, ZnO, andIn2O3, have been synthesized with focus on their nanoscalestructures and opto-electrical properties. Monoclinic gal-lium oxide (b-Ga2O3), a wide band-gap material(Eg ¼ 4.8 eV) [8], has also been studied for both electricaland ultraviolet (UV) optical properties and gas sensorapplications [9–13].

e front matter r 2006 Elsevier B.V. All rights reserved.

yse.2006.07.019

ing author. Tel.: +1740 597 1259; fax: +1 740 593 0433.

ess: khan@helios.phy.ohiou.edu (A. Khan).

Over the last several years attempts have also been madeto synthesis nanosheets and nanobelts of the wide band gapmaterials including Ga2O3. Low-dimensional nanobelts ornanosheets exhibit different morphologies than nanotubesor nanowires and have high ratio of area to volume [14,15].Nanobelts or nanosheets could be potentially a usefulsystem for investigating and achieving a full understandingof dimensionally confined transport phenomena in semi-conductors. These structures were also suggested asbuilding platforms for functional devices [15]. The low-dimensional b-Ga2O3 materials have been synthesizedthrough various methods such as laser ablation, acarbon-assisted route, arc discharge, thermal evaporationand physical evaporation, metal organic chemical vapordeposition [7,16–25]. However, relatively few studies arereported for b-Ga2O3 nanobelts and nanosheets. Here, wereport a simple one step preparation routine of ultra-wideand ultra-large b-Ga2O3 microbelts by heating ingots of Gametal at 950 1C in a conventional tube furnace in Argongas flow. Song et al. [8] and Xiang et al. [26] have

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synthesized b-Ga2O3 nanostructures using similar ap-proach; however, the belts we synthesize are wider andlarger and have well-defined edges. These ultra-wideb-Ga2O3 microbelts could easily be manipulated and usedin optoelectronics.

2. Experimental

Small pieces of Ga metal were loaded into a quartz boatand then kept in a quartz tube (4 ft long and 1 in diameter)placed horizontally in a furnace. Downstream of Ar gas(99.9% purity) at flow of 20–30 sccm was used towards theGa ignots. The furnace was heated and kept at 950 1C for30min. After heating the system was cooled down to roomtemperature. The Ga pieces have whiskers coming out ofthem in all directions. The samples were then characterizedvia X-ray diffraction (XRD) (Rigaku Geigerflex, 2000Watts) with CuKa (1.54 A), scanning electron microscope(SEM, JEOL JSM 5300), energy dispersive X-ray (EDX)analyzer attached to SEM and transmission electronmicroscope (TEM, JEOL 1010) and HRTEM PhilipsCM300 UT instrument operated at 300 kV and providinga point-to-point resolution of 1.72 A. Photoluminescence(PL) spectra were studied with a He–Cd laser withexcitation wavelength 325 nm in temperature range from10 to 300K. For the PL measurements, the samples weremounted on a cold-finger cooled by a closed-cycle heliumcryostat. The PL excited by a He–Cd laser (325 nm) wascollected by a quartz lens on the entrance slit of thespectrograph monochrometer (ISA model HR-320) oper-ated in Czerny–Turner configuration with different holo-graphic gratings. The optical signal was detected by a

Fig. 1. SEM images (a–d) of the ultrawide b-Ga2O3 m

Princeton Instruments back-illuminated charge-coupleddevice camera model TEA-CCD-512TK with a UV/ARcoating and controlled by a computer.

3. Results and discussions

Fig. 1 shows SEM micrographs of as synthesizedb-Ga2O3 micrometer size sheets and belts. It can be seenin the SEM micrographs that the width and length of thesheets are in the range of 40–100 mm and several hundredsof micrometers, respectively, while the thickness of thesesheets varies in the range of 50–200 nm. It is noteworthythat these sheets have straight edges and relatively regularrectangular geometrical appearance and have greater widthto thickness ratio as well as longer length than reported inthe literature [26–31]. Fig. 1 (a) Shows a low magnificationimage of ‘‘shredded’’ paper like microbelts. Some of themicrobelts are grown in stacks and stripes appearing onsome of them are seen in Fig. 1(b) and (c). Apart from theb-Ga2O3 microbelts, nanowires and nanorods are alsoobserved in the synthesized material. These nanowires andnanorods are seen in Fig. 1(c), but majority of thestructures are microbelts. An example of a well-isolatedmicrobelt collected on a Si substrate which is 80 mm wideand 600 mm long is shown in Fig. 1(d).Fig. 2 illustrates XRD and EDX of the as-grown

microbelts. The diffraction peaks in Fig. 2(a) are similarto those of bulk polycrystalline b-Ga2O3 which areindexed as the monoclinic structure with a ¼ 1.223 nm,b ¼ 0.304 nm, c ¼ 0.581 nm and are in good agreementwith literature [32]. All peaks in the XRD spectraare related to b-Ga2O3 and no other peak from other

icrobelts. The bar scale is 100mm for all images.

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Fig. 2. XRD (a) and EDX (b) spectra of b-Ga2O3 microbelts. (a) Shows XRD spectra of the microbelts placed on plastic while (b) shows EDX spectra of

the microbelts placed on Si substrate.

Fig. 3. TEM and HRTEM images of as made b-Ga2O3 sheets. The inset in (b) is the selected area electron diffraction pattern of microbelts.

A. Khan et al. / Physica E 35 (2006) 207–211 209

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Fig. 5. PL spectra of the b-Ga2O3 microbelts excited with He–Cd laser

(325 nm) and measured in 10–300K temperature range.

A. Khan et al. / Physica E 35 (2006) 207–211210

crystalline forms of gallium oxide were observed. The XRDresults revealed that the end products after oxidizingmetallic Ga ingots were indeed b-Ga2O3 and other forms ofcrystalline gallium oxide (if any) were below the detectionlevel. Fig. 2(b) depicts an EDX spectrum of the microbelts.The O and Ga peaks are clearly visible while the Si peakcomes from the substrate. No other peaks from any othermaterial were observed from the sample.

Fig. 3 illustrates low magnification TEM images ofb-Ga2O3 microbelts. The waving shapes are apparent in thesheets. The contrast observed over the sheets is due to abending of the sheets with respect to the electron beam.This is an electron diffraction phenomenon and is mostfrequently observed in metal foils due to deformation andbending [33]. A high resolution TEM image of a portion ofmicrobelt is shown Fig. 3(c) which shows that the belts aresingle crystalline and free from dislocations. The insets inFig. 3(b) are their selected area electron diffraction (SAED)patterns.

The widths of the sheets vary from 2 to 50 mm as isshown in Fig. 3(a) and (b). By analyzing more than twohundred b-Ga2O3 microbelts from various images notshown here, it was determined that the average width of themicrobelts is in the 40–60 mm range. A representativediagram of the width profile of the b-Ga2O3 sheets is givenin Fig. 4.

Fig. 5 shows evolution of the PL spectra of as-grownb-Ga2O3 microbelts in the visible region as a function oftemperature. We believe that observed emission resultsfrom excitation of defects in the material. The dominantemission band for the b-Ga2O3 microbelts is spanning from350 to 675 nm and peaks at 482 nm. There was no bandedge UV peak observed in PL spectra. The maximum peakposition of blue–green band is in agreement with thepreviously recorded PL for b-Ga2O3 [8,17,34]. The blue–

Fig. 4. Width profile of as grown b-Ga2O3 microbelts. The dashed line is

the Gaussian fit to the width distribution profile.

green band emission (480 nm) may originate from therecombination of an electron on a donor formed by oxygenvacancies and a hole on an acceptor formed by galliumvacancies or by gallium–oxygen vacancy pairs [35–38]. Inour experiment, the gallium ingots were oxidized inrelatively oxygen deficient environment, what might resultin generation of oxygen vacancies in the as synthesizedmicrobelts. The temperature dependence of the PL emis-sion was studied at temperatures between 10 and 300K,respectively. The green band peak position does not shiftnoticeably in the investigated temperature range. Thefull width at half maximum of the PL blue–green peak isabout 120 nm and does not change in the investigatedtemperature range. At temperatures below 140K,another band centered at 715 nm emerges gaining itsintensity at lower temperatures [8]. This band is attributedto nitrogen impurities in b-Ga2O3 material [8,25]. It wasshown that nitrogen impurities act as acceptors in b-Ga2O3

inducing hole traps which recombine with electronstrapped at oxygen vacancy donors generating red-lightemission [8,25].

4. Conclusion

We have demonstrated the synthesis of monoclinicgallium oxide (b-Ga2O3) microbelts with well-definedshapes and edges by heating small gallium metal ingots inthe Ar gas flow. The XRD and TEM analysis revealed thatthese nanostructures are pure b-Ga2O3 and are highlycrystalline. These microbelts have extremely large width tothickness and length to thickness ratios. Furthermore,fabricated structures could be manipulated into specificarchitectures without expensive lithographic approach atthe nanoscale level. Due to their good crystallinity,semiconducting properties and large specific surface area,

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these structures might be used in device making applica-tions such as sensors, light emitters and building blocks forelectrochemical nanodevices.

Acknowledgement

We thank Dr. Saule Aldaberganova and the Universityof Erlangen Nuremberg, Institute, for assistance with theHRTEM.

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