Part of Topical Section onEmerging Semiconducting Oxides

Control of the conductivity of Si-dopedb-Ga2O3 thin films via growthtemperature and pressure

Stefan Müller*, Holger von Wenckstern, Daniel Splith, Florian Schmidt, and Marius Grundmann

Universität Leipzig, Institut für Experimentelle Physik II, Linnéstraße 5, 04103 Leipzig, Germany

Received 12 June 2013, revised 18 July 2013, accepted 30 July 2013Published online 11 September 2013

Keywords conductivity, Ga2O3, pulsed laser deposition, thin films

* Corresponding author: e-mail stefan.mueller@physik.uni-leipzig.de, Phone: þ49-341-9732604, Fax: þ49-341-9732668

Si-doped b-Ga2O3 thin films were grown at temperaturesbetween 400 and 650 8C and oxygen partial pressures rangingfrom 3� 10�4mbar to 2.4� 10�2mbar prepared by pulsedlaser deposition (PLD) on c-plane sapphire substrates. For highoxygen partial pressure the samples are composed of multiplecrystalline phases; for decreasing oxygen partial pressure thecrystallinity improves and single phase (�201)-oriented thinfilms are obtained for an oxygen partial pressure below10�3 mbar at a growth temperature of 650 8C. We find a

correlation between surface morphology of our thin films andtheir conductivity; an increasing root mean square surfaceroughness entails increased conductivity. Further we show thatthe oxygen partial pressure resulting in maximal conductivitydecreases with increasing growth temperature. The resultsprovide means to realize b-Ga2O3-based devices such asrectifiers, photodetectors or thin film transistors with optimizedsurface roughness, structural quality, and conductivity.

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1 Introduction In the past decade oxide semiconduc-tors with wide band gap around 3 eV were investigated withan increasing interest due to the manifold applicationcomprising transparent electrodes to thin film solar cells [1],rectifiers [2], detectors [3] (visible–blind UV-detectors, gassensors, pH-sensors, etc.), transparent electronics [4], e.g., inback planes of active-matrix displays, cheap, room-temperature processed, flexible circuitry [5], and light-emitting devices [3, 6]. The range of possible applications ofoxide semiconductors is even increased by exploitingmaterials with band gap far above 4 eV. b-Ga2O3 crystallizesin monoclinic structure with lattice parameters a¼ 12.23Å,b¼ 3.04Å, c¼ 5.8Å, and b¼ 103.78 (JCPDS 43-1012) andhas a room temperature band gap of about 4.7 eV [7] and cantherefore be utilized in solar-blind photodetectors [8–10]providing functionality not reachable with semiconductorshaving band gap below about 3.6 eV. Another field, in whichb-Ga2O3 allows substantial progress, is high-power elec-tronics. Due to its very large band gap, b-Ga2O3 has largerbreakdown field than GaN or SiC, being used in high-powerthin film electronics nowadays, while maintaining highconductivity [11]. The figure of merit for high-power

applications was introduced by Baliga [12] and is compiledfor Si, GaAs, 4H-SiC, GaN, diamond, and b-Ga2O3

in Table 1 of Ref. [11] showing that, besides diamond,b-Ga2O3 possesses the most favorable material propertiesconcerning high-power electronics among the materialscompared. In order to take advantage of this, the transportproperties and their dependence on the kind of dopantand its concentration have to be understood and must becontrolled.

For a long time the unintentional n-type conductivity ofb-Ga2O3 was attributed to oxygen vacancies (VO). Recently,density functional theory (DFT) calculations showed that VO

acts as deep donor [13, 14] and does not contribute to theconductivity. Therefore, doping with elements acting asshallow donors is necessary to enable growth of b-Ga2O3

having the electrical conductivity damanded by application.The DFT calculations of Varley et al. suggest that impuritieslike H, Si, Ge, Sn, F, and Cl acts as shallow donors inb-Ga2O3 [13]. For b-Ga2O3 bulk single crystals the effect ofSi doping on the conductivity and photoluminescence wasinvestigated in the last years [15, 16]. Villora et al. showed asignificant increase of the free carrier concentration, and

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hence an increase of the conductivity due to doping ofb-Ga2O3 single crystals with Si [15].

The enhancement of conductivity of b-Ga2O3 thin filmgrown by pulsed laser deposition (PLD) was demonstratedby doping of Sn [17–19]. First, Orita et al. reported onpolycrystalline b-Ga2O3 thin films doped with 1% SnO2 onc-plane sapphire and silica glass substrates having amaximum conductivity of 1 S cm�1 [17]. The maximumconductivity of Sn-doped b-Ga2O3 thin films was improvedby Orita et al. by optimization of the growth conditions to8.2 S cm�1 [18].

In this paper, we report systematic investigation oftransport properties of Si-doped b-Ga2O3 thin films grownby PLD. The growth temperature and oxygen backgroundpressure both have tremendous influence on crystallinity,surface topology and the electrical sample properties.

2 Experimental details For the present investigationwe used b-Ga2O3 thin films grown by PLD. The target usedconsists of Ga2O3 with a fraction of 1wt% SiO2. The thinfilms were grown at temperatures (TG) between 400 and650 8C. Additionally, the oxygen partial pressure (p(O2))was varied between 3� 10�4 and 2.4� 10�2mbar. The laserused is a KrF excimer laser (248 nm) with a energy density of2 J cm�2 at the target. The target-substrate distance of thesetup used is 10 cm. For each sample grown 30 000 laserpulses were applied, however, the sample thickness rangedbetween 300 and 1400 nm. X-ray diffraction (XRD)measurements were done with a Philips X’pert with aBragg–Brentano goniometer. The surface topography wasrecorded with a Park XE-150 atomic force microscope(AFM) in dynamic non-contact mode. Conductivitymeasurements were performed in van der Pauw geometryat room temperature. Evaporated Al/Ti contacts of 50 nmoverall thickness are used as Ohmic contacts [20]. After anannealing for 10min at 500 8C under nitrogen ambient(800mbar) [10] all contacts exhibit linear current–voltagecharacteristic and negligible contact resistance.

3 Results and discussion Figure 1 depicts the XRDpatterns of b-Ga2O3 thin films on c-plane sapphiresubstrates. The thin films were deposited at TG¼ 650 8Cand p(O2) ranging from 3� 10�4 to 1.6� 10�2mbar. Forp(O2) up to 1� 10�3mbar the b-Ga2O3 thin films are singlephase and (�201)-oriented. The orientation is indicated by theoccurrence of the peaks at 18.98, 38.48, and 59.28 whichcorrespond to the (�201), (�402), and (�603) planes of b-Ga2O3,respectively. The (�201)-orientation of b-Ga2O3 was reportedseveral times for thinfilms on c-plane sapphire substrates [18,19, 21, 22], GaN substrates [23] and a-plane sapphiresubstrates [22] in literature. Recently, the growth of(�201)-oriented b-Ga2O3 on c-plane sapphire substrateswas explained by Nakagomi and Kokubun due to the samearrangement of the oxygen atoms of c-plane sapphiresubstrate and in the (�201) equivalent plane of b-Ga2O3 [22].The gallium atoms bind to the oxygen atom layer of thesapphire substrate such that only a minor mismatch between

the (001) planes of the sapphire and the (�201) plane ofb-Ga2O3 occurs. The peaks visible at 41.78 and 90.88 are the(006) and (0012) diffraction peaks of the c-plane sapphiresubstrate, respectively. At p(O2) above 10�3mbar the thinfilms have several not randomly distributed orientations,referred to as polycrystalline from now on. The XRD patternof the thin film grown at 2� 10�3mbar in Fig. 1 shows twoadditional reflections at 30.38 and 57.78 corresponding to(�110) and (�313) lattice planes of b-Ga2O3, respectively.With further increasing p(O2) several other orientationsoccur. In the XRD pattern of the thin film deposited at1.6� 10�2mbar shown in Fig. 1 additional peaks with highintensities at 44.78, 59.98, and 64.28 are visible. These peakscorrespond to the (�601), (113), and (�204) planes of b-Ga2O3.These peaks were also observed for polycrystalline thin filmson c-plane sapphire substrates reported by Orita et al. [24].For TG below 650 8C the crystalline quality decreases.Tangible, for p(O2)¼ 10�3mbar the thin film is single phaseand (�201)-oriented for TG¼ 650 8C. At growth temperaturesof 580 and 510 8C additional reflections at 30.38 and 57.78corresponding to the (�110) and (�313) lattice planes,respectively, are visible which were also observablefor TG¼ 650 8C, but higher p(O2)¼ 6�10�3mbar. For580 8C (510 8C) the thin films are polycrystalline forp(O2)¼ 1� 10�3mbar (6� 10�4mbar). At a growth tem-perature of 400 8C single crystalline thin films were notobtained within the range of p(O2) investigated.

Figure 2(a) depicts the dependence of the growth rate onp(O2) for TG¼ 650 8C. The growth rate (thickness) of thethin films was calculated from thin layer interferencesmeasured by transmission measurements. The refractiveindex nwas determined by spectroscopic ellipsometry and isin the visible range between 1.9 and 2 depending on thewavelength and growth properties. For p(O2)< 10�3mbarwe observe a slight increase of the thin film thickness withdecreasing p(O2). This behavior is explainable by thedecreasing interaction between the plasma plume and thebackground gas ambient resulting in a higher net flux of

6 ×10

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3 ×10-4

1.6 ×10

-3

p(O ) (mbar)2

Figure 1 XRD pattern of b-Ga2O3 thin films grown on c-planesapphire substrates at a growth temperature of 650 8C and oxygenpartial pressures as labeled. The XRD intensity is shown on alogarithmic scale.

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particles impinging on the substrate. In the pressure rangebetween 10�3mbar and 1.2� 10�2mbar the thicknessincreases rapidly with increasing p(O2). The increase ofthe thickness of the thin films with higher p(O2) cannot be

explained by the interaction between the Ga2O3 plasma andthe background gas ambient, therefore, a second mechanisminduces this behavior. In Fig. 1 the tendency of increasingpolycrystallinity for p(O2)� 2� 10�3mbar is obvious.Therefore, the increasing growth rate of these thin films islikely due to the increasing polycrystalline nature and theincreasing number of bonding sites compared to singlephase epitaxial growth. To illustrate this behavior in moredetail, we plotted in Fig. 2(b) the maximal intensity of the(�110) (left axis) and the (�201) (right axis) b-Ga2O3 peakversus the growth rate for all growth temperaturesinvestigated. The intensity of the (�201)-reflection is inprinciple constant for growth rates up to 0.02 nm pulse�1.This corresponds to p(O2) of �10�3mbar and hence to agrowth regime for which single phase thin films are obtained.For higher growth rates (corresponding to higher p(O2)), thetotal film thickness increases (note, the compactness of thethin films does not change). On the contrary, the intensity ofthe (�110) reflection is absent for low growth rates (low p(O2)and single-phase thin films) but increases significantly forhigher growth rates for which our thin films exhibit multiplephases. This implies that more bonding sites are availableresulting in a higher sticking coefficient for this growthregime.

The surface morphology of the thin films (TG¼ 650 8C)measured by AFM is visualized in Fig. 3. The sample grownat the highest p(O2)¼ 2.4� 10�2mbar exhibits a structurecomposed of many elongated grains. For lower p(O2) theshape of grains is more symmetric. For p(O2)¼ 10�2mbarall grains have circular shape. For p(O2)< 10�2mbar thegrain size decreases. The roughness of the thin film dependsstrongly on p(O2). The root mean square surface roughness(Rq) of the different thin films is depicted in Fig. 4. For the

TG = 650°C

(a)

(b)

650°C580°C510°C

Figure 2 (a) Growth rate of the thin films in dependence on theoxygen partial pressure for a growth temperature of 650 8C.(b) Maximal intensity of the (�110) (left axis) and the (�201) (rightaxis) peak versus the growth rate of the thin film.

Figure 3 AFM images of b-Ga2O3 thin films grown on c-plane sapphire at a growth temperature of 650 8C and oxygen partial pressures aslabeled. Each image depicts an area of 5� 5mm2.

36 S. Müller et al.: Control of the conductivity of Si-doped b-Ga2O3 thin films

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thin film grown at p(O2)¼ 2.4� 10�2mbar, Rq¼ 6.24 nm.Rq has a maximum value of 11.9 nm for p(O2)¼ 6� 10�3

mbar. With a further decrease of p(O2), Rq decreases againand reaches 3.7 nm for p(O2)¼ 10�3mbar.

The logarithm of the thin film conductivity is depicted inFig. 4 as well. From the figure it is evident, that thedependence of log(s) and Rq on the p(O2) is alike. Theconductivity of the thin film grown with the highest p(O2) of2.4� 10�2mbar is 8� 10�4 S cm�1. The highest conduc-tivity is observable at p(O2)¼ 6� 10�3mbar with0.2 S cm�1. With decreasing p(O2) the conductivitydecreases strongly to a value of 3.5� 10�5 S cm�1 forp(O2)¼ 10�3mbar. The dependence of the conductivity canin parts be explained by kinetic and thermodynamicprocesses involved during growth. With increasing p(O2)the interaction between ablated particles forming the plasmaplume and the background oxygen molecules increases.However, the weight of the different constituents within theplasma differs. Therefore, scattering of ablated silicon andoxygen ions is more effective compared to the much heaviergallium ions [25], resulting in a decreased net flux of theselighter particles impinging on the substrate for higher p(O2).This oxygen and silicon deficient growth for high p(O2)causes the decrease of conductivity. A tentative explanationof the decrease of conductivity for low p(O2) is a decreasedincorporation of Si into the thin film. Villora et al. explainedthe doping of b-Ga2O3 single crystals grown by a floatingzone technique with Si by the comparable low partial vaporpressures of the different Si–O species, which are similar tothat of the Ga–O species [15]. For higher partial vaporpressures the dopant atoms evaporate to a higher extent fromthe Ga2O3 melt. A similar mechanism possibly causes adecreased incorporation of Si from the Ga2O3 plasma intothe thin film at low p(O2) during growth. EDXmeasurementsof the thin films grown at 650 8C substantiates this reasoning.The normalized Si content of the samples measured withEDX is shown in Fig. 4 (right black axis). We see an increase

of the Si content in the thin films with decreasing p(O2) by afactor of 3. The Si content reaches its maximumat 1� 10�3mbar. With a further decrease of p(O2) the Sicontent decreases slightly. However, the conductivity differsby three orders of magnitude. Therefore, a secondmechanism is also responsible for the dependence of theconductivity on the p(O2). This mechanism is probablyconnected to different incorporation sites of the Si atoms inthe b-Ga2O3 host lattice for different p(O2). For afurther understanding of these processes additional inves-tigations of the structural properties by transmission electronmicroscopy, the bindings states of Si in the thin films asdetermined by XPS and temperature dependent Hall effectmeasurements for clarification of the temperature activatedconduction mechanism are necessary, but are beyond thescope of this paper.

In previous reports concerning optimized growthcondition of SnO2-doped b-Ga2O3 thin films [17–19] onlya single pair of TG and p(O2) resulting in optimalconductivity were found. Here, we report a set of suchpairs. Figure 5 depicts the logarithm of conductivity independence on p(O2) for three different growth temper-atures. It is clearly visible that the p(O2), for which thehighest conductivity is obtained decreases with decreasinggrowth temperature. For TG¼ 650 8C, the maximal conduc-tivity (0.2 S cm�1) is reached for p(O2)¼ 6� 10�3mbar.The highest conductivity of 0.065 S cm�1 (0.185 S cm�1) forTG¼ 580 8C (510 8C) is obtained for p(O2)¼ 2� 10�3mbar(1�10�3mbar). As TG is reduced, the maximal conductivityis observed at lower p(O2). For a growth temperature of400 8C all thin films grown at a p(O2) above 6� 10�3mbarhave a low conductivity (smaller than 10�5 S cm�1). Forpressures below 4� 10�3mbar the as grown thin films had abrownish color and turned black after the annealingnecessary for the fabrication of the Ohmic contacts.The conductivity of these sample is distinct from thebehavior already described for the samples grown at highertemperatures and is therefore not included in Fig. 5. Theelectron mobility of the thin films is between 0.05 and

Rq(nm)

-3

-4

(S/cm)

Figure 4 Logarithmic conductivity versus the oxygen partialpressure in the PLD-chamber during the deposition on the thin filmsgrown at 650 8C (left axis). Root mean square surface roughness independence on the oxygen partial pressure (right blue axis). Sicontent normalized to the sum of Ga and Si content (right blackaxis).

650°C580°C510°C

(S/cm)

-4

-5

Figure 5 Logarithmic conductivity versus the oxygen partialpressure in the PLD-chamber during the deposition of the thin filmsfor different growth temperatures. The solid lines are guides to theeye, respectively.

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0.15 cm2V�1 s�1 at room temperature, the smaller mobilityis observed for lower growth temperature. The increase ofthe conductivity is mainly caused by the increase of freecarrier concentration. For the sample with the highestconductivity the free carrier density is about 1� 1019 cm�3.For a b-Ga2O3 thin film doped with 1% SnO2 Orita et al.reported a free carrier density of 1.44� 1019 cm�3 [17].Thus we conclude a high doping efficiency of Si asshallow donor in b-Ga2O3 thin films. The highest mobilityof our thin films is 0.15 cm2V�1 s�1 and about afactor of three lower than that reported by Orita et al.(0.44 cm2V�1 s�1) [17] for b-Ga2O3 thin films on c-planesapphire substrats. This behavior can be caused by differ-ences in growth parameters such as target-substrate distanceor energy density of the laser used.

For the preparation of unipolar devices like Schottkycontacts or field effect transistors the net doping densityshould be below the conduction-band edge density of states.For an effective electron mass of 0.342me [26] for b-Ga2O3

the maximum net doping density is in the range of5� 1018 cm�3 at 300K. Therefore, the mobility of hetero-epitaxial b-Ga2O3 must be increased, e.g., by introduction ofadequate buffer layers or annealing processes, for a furtherincrease of conductivity.

4 Conclusions In summary our results show that theconductivity of the 1% SiO2-doped PLD grown b-Ga2O3

thin films on c-sapphire can be controlled over severalorders of magnitude in dependence on the growthtemperature and oxygen partial pressure. In contrast toformer reports on SnO2-doped b-Ga2O3 we demonstratedthat there exists a set of pairs of growth temperature andoxygen partial pressure resulting in maximal conductivityinstead of just a single pair reported for Ga2O3:Sn inliterature [17–19]. This allows, if demanded by the desiredapplication, to vary the growth temperature between510 and 650 8C yet maintaining high conductivity.Further, we showed a correlation between the surfacemorphology of the thin film grown at different pressuresand the conductivity. The root mean square surfaceroughness of thin films with high conductivity is largercompared to samples with lower conductivities. Wealso showed that crystalline structure varies betweenpolycrystalline and single crystalline (�201)-oriented thinfilms in dependence on the growth temperature and oxygenpartial pressure. The growth rate of our thin films is alsohighly dependent on the oxygen partial pressure duringgrowth. These results demonstrate, that the control of easilyadjustable PLD growth parameters such as the backgroundpressure and growth temperature is sufficient to indepen-dently tune sample crystallinity, surface morphology,and conductivity of b-Ga2O3:Si and provides means toexploit b-Ga2O3 as functional material within heteroepitax-ial thin films.

Acknowledgements This work has been supported by theDeutsche Forschungsgemeinschaft in the framework of Sonder-

forschungsbereich 762 “Functionality of Oxidic Interfaces” and theEFRE (SAB 100132251). We thank M. Lorenz for critical readingthe manuscript, H. Hochmuth for PLD growth of the investigatedsamples, G. Ramm for the preparation of PLD targets, M. Hahnfor the preparation of the Ohmic contacts, J. Lenzner for theEDX measurements and T. Böntgen for the determination of therefractive index.

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