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Combinatorial material science and strain engineering enabled by pulsed laser deposition using radially segmented targets Max Kneiß, Philipp Storm, Gabriele Benndorf, Marius Grundmann, and Holger von Wenckstern ACS Comb. Sci., Just Accepted Manuscript • DOI: 10.1021/acscombsci.8b00100 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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ACS Combinatorial Science

Combinatorial material science and strain engineering enabled by pulsed laser deposition using radially segmented targets Max Kneiß,∗ Philipp Storm, Gabriele Benndorf, Marius Grundmann, and Holger von Wenckstern Felix-Bloch-Institute for Solid State Physics, Universität Leipzig, Leipzig, Germany E-mail: [email protected] Phone: +49 (0)341 9732641

Abstract Vertical composition gradients of ternary alloy thin films find applications in numerous device structures. Up to now such gradients along the growth direction have not been realized by standard pulsed laser deposition (PLD) systems. In this study, we propose an approach based on a single elliptically-segmented PLD target suited for the epitaxial growth of vertically graded layers. The composition of the thin films can be varied by a simple adjustment of the position of the PLD laser spot on the target surface. We demonstrate this principle for the Mgx Zn1−x O alloy system. Such vertically composition-graded Mgx Zn1−x O thin films exhibit high optical quality and a well-defined Mg-content for each layer. No signs of interdiffusion of Mg-atoms between the layers have been found. Further, this method is capable to deposit homogeneous thin films with any desired, well-defined cation composition having the same high optical and structural quality as films grown by conventional PLD.

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ACS Combinatorial Science 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Keywords step grading, composition control, pulsed laser deposition, ternary alloy system, thin films

Introduction The ability to control the chemical composition of ternary alloy thin films in growth direction has been shown to be beneficial to a variety of systems and devices. The step-grading of Mgx Zn1−x O barriers for Mgx Zn1−x O/ZnO quantum well systems grown by molecular beam epitaxy (MBE) and the corresponding engineering of the bandstructure and wavefunctions has been shown to be an adequate tool to manipulate emission energies and decay times of the excitonic quantum well emission. 1 A grading of Mgx Zn1−x O buffer layers enabled the MBE growth of Mgx Zn1−x O/ZnO quantum well systems beyond the solubility limit of Mg in ZnO. 2 Further, graded buffer layers between the active regions of GaInP/GaInAs highefficiency solar cells improved their crystalline quality leading to enhanced performance. 3 Graded AlGaN buffer structures in AlGaN/GaN field-effect transistors using metal-organic chemical vapor deposition (MOCVD) on cost-effective substrates caused a reduced dislocation density in the active region by strain engineering as well as lower leakage currents due to polarization engineering. 4 Such polarization engineering can also be utilized as polarization doping, where high p-type doping for enhanced hole injection currents in AlGaN LED structures can be realized by composition grading in growth direction. 5 Graded structures have further been shown to improve the carrier capture efficiency in graded index separate confinement heterostructure (GRINSCH) laser diodes while simultaneously acting as waveguides for the emitted laser radiation due to the grading of the refractive index. 6–8 Additionally, a theoretical study predicted an enhanced emission efficiency for ZnO-based UV LEDs by using step-graded MgZnO multiple quantum barriers. 9 Such composition grading is wellestablished for deposition methods such as MBE or MOCVD, where the flux of the source materials can be adjusted during growth. However, in the case of standard pulsed laser depo2


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sition (PLD) setups a continuous variation of the chemical composition of ternary alloy thin films in growth direction has been impossible until now. A standard PLD system possesses one ablation laser. 10 Therefore, the number of available alloy compositions is limited by the amount of single composition targets which can be mounted into the vacuum chamber of the setup. The only alternatives until now would be rather cost-intensive multi-beam setups 11 or movable-mirror geometries, 12,13 which need to be exactly calibrated. To the best of the authors knowledge, there is only one study in literature, where a single wedge-segmented target was used to control the composition of KTa1−x Nbx O3 thin films. 14 This was achieved by a fixed radial position of the PLD laser spot on the target surface in combination with an off-axis rotation of the target. However, we did not find further reports where this approach was used nor were the resulting thin films structurally or optically characterized. Additionally, no composition grading of thin films in growth direction was observed. We have already reported on a PLD approach using a single, azimuthally-segmented target for the creation of lateral continuous composition spreads. 15 In this study, we propose a novel PLD technique for standard PLD systems enabling composition gradients in growth direction using a single elliptically-segmented target, where the chemical composition differs between inner and outer segment of the ellipse. With this approach, the chemical composition of the deposited layer can be tuned by the position of the PLD laser spot on the target. We demonstrate here this technique on the Mgx Zn1−x O alloy system. Mgx Zn1−x O is a well-investigated crystalline system which can be grown phase pure in the wurtzite structure up to alloy compositions of x ≈ 0.5. 16 The deposition under various conditions and methods including optical, electrical and structural characterizations has been intensively described in literature, 15–38 especially also for PLD. 15,17–23 It further has a wide range of applications in electronic or optoelectronic devices. 16,39–48 Due to the well known variation of the bandgap and emission energy the Mg-content in thin films can be directly deduced from these properties. 20,49,50 Interface charges and an internal electric field due to high spontaneous and piezoelectric polarizations can lead to effects such as electron accumulation at interfaces 51 (2DEG) or the quantum-

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confined Stark effect in MgZnO/ZnO quantum wells. 52–55 Graded Mgx Zn1−x O structures so far have been rare in literature apart from graded quantum well structures 1 and graded buffer layers 2 already mentioned above. We present in this study homogeneous Mgx Zn1−x O thin films with different chemical compositions grown by the novel approach using a single elliptically-segmented ZnO/Mg0.4 Zn0.6 O target. The method will be described in detail and the theoretically expected composition of the thin films in dependence on the position of the PLD laser spot on the target is calculated. Further, we will show that our approach is suitable for the growth of high-quality composition-graded Mgx Zn1−x O thin films that can also be incorporated in heterostructures for device applications.

Novel PLD Method and Experimental Procedures For the growth of homogeneous Mgx Zn1−x O thin films with varying Mg-content and thin films with a step-grading of the Mg-content in growth direction we used a novel PLD approach employing one single elliptically-segmented target. In our standard PLD setup, a circular ceramic target with a homogeneous chemical composition is used. This target is rotated during ablation and the radial position of the laser spot on the target is varied continuously. With this, a homogeneous ablation of the target material is ensured. In our novel approach an elliptically-segmented ternary alloy target with different chemical composition of the alloy system inside and outside of the ellipse is employed (see Figure 1). The composition of the inner segment shall be denoted as Ax B1−x O, the one of the outer segment as Ay B1−y O. As in the standard PLD approach the target is rotated during ablation, the radial position of the laser spot on the target however remains fixed to a certain value r. The ablation track of an ideal point-like laser spot on the target surface is then a circle with radius r running both through the inner and outer segment of the target, see Figure 1a. The composition of the flux of ablated particles in the plasma moving towards the substrate correspondingly changes

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constantly between x and y when the laser ablates in the inner or outer segment, respectively. The time average of the particle flux composition and therefore also the composition of the growing thin film is then directly related to the ratio of the path lengths of the laser spot in the outer and inner segment. If the PLD system employs substrate rotation then the rotational frequency of the substrate should be much lower than the one of the target. For identical frequencies, the target composition is mapped laterally onto the substrate instead of an average cation flux. This effect is exploited for lateral composition spreads, 15 but needs to be avoided for our approach. The path length ratio is given by the angle ϕ(r), which is the intersection angle of the circular path of the laser spot and the elliptical boundary of the inner target segment. In the upper panel of Figure 1a a radial position of r1 corresponds to an intersection angle ϕ1 (r1 ) and a resulting thin film composition of χ = χ(r1 ). An increase of the radial position r to a value of r2 results in a decrease of the intersection angle to ϕ2 (r2 ) < ϕ1 (r1 ) corresponding to an increased path length ratio as is shown in the lower panel of Figure 1a. The resulting average composition of the flux of particles and therefore also the thin film composition changes as well. The thin film composition from this particular value of r = r2 is then denoted as γ = γ(r2 ) with γ(r2 ) > χ(r1 ) for an initial composition of the target segments y > x. The expected thin film composition can be quantified by simple geometrical considerations under assumption of an ideal point-like shape of the focused laser spot and a stochiometric transfer of the composition of the target segments. The intersection angle ϕ(r) can be directly calculated from the equation of the ellipse given as: √

r(ϕ) = p

b 1 − ε2 cos2 ϕ

,ε=

a2 − b2 . a

(1)

a is the longer side of the ellipse and b the shorter side, see also the inset in Figure 1b. The path length in the inner segment is then given by 4ϕr and the one in the outer segment by 2πr − 4ϕr. The resulting thin film composition χ(r) is then the sum of the compositionweighted ratio of these path lengths to the total circumference 2πr of the circle of the laser path: 5



χ(r) =

(4ϕr)x + (2πr − 4ϕr)y . 2πr

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(2)

Solving equation (1) for ϕ and using the result in equation (2) gives an analytical result for the expected thin film composition:  s  2 1 b  2 1− . χ(r) = y − (y − x) arccos  π ε r

(3)

The result is plotted in Figure 1b. For laser spot positions smaller than the shorter axis of the ellipse b, the composition x of the inner segment is reproduced as well as the composition y of the outer segment for r > a. Between these two values of r a smooth variation of composition between x and y can be realized. The consequence is that, using this approach, we are able to vary the thin film composition by a simple geometric change of the radial position of the laser spot on the target. Doing so in-situ and stepwise during ablation results in a thin film whose composition varies (quasi-)continuously in growth direction. How continuous the gradient will be thereby depends on the stepsize and number of steps chosen. With this, we are able to control the chemical composition of thin films in growth direction for strain engineering, bandgap engineering or dopant profiles, as only some examples. In analogy to our segmented target approach for lateral continuous composition spreads, 15 we call this method vertical (quasi-)continuous composition spread (VCCS). The Mgx Zn1−x O thin films in this study were grown using a elliptically-segmented target consisting of binary ZnO inside of the ellipse and an mixture of ZnO and MgO in the outside. The ratio of MgO to ZnO was chosen such that the mixture has a cation ratio of Mg/(Mg + Zn) = 0.4. The powders were ball-milled prior to use and filled into the target press with an already elliptical segmentation using an elliptically-shaped metal tube. After pressing, the resulting pellet was sintered in air at 1150◦ C for 12 h. An image of the used ceramic target prior to deposition is shown as inset in Figure 1b. The inner ellipticallyshaped segment has the dimensions of a ≈ 7 mm and b ≈ 3.5 mm. The Mg-content in the

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AxB1-xO

AyB1-yO

A B1- O thin film

substrate 1

r1

r1

variation of

variation of

laser spot position

composition

r2

2

A B1- O thin film

(a)

r1

Figure 1: (a) Working principle of the novel PLD technique for an elliptically-segmented target with composition x in the inner segment and composition y in the outer segment. When the radial position of the laser spot r on the target is varied, also the path length ratio of the laser track between inner and outer segment changes. This is emphasized as a change in the intersection angle φ between the circular track of the PLD laser spot and the elliptical boundary of the two segments. This corresponds to a variation in the thin film composition. (b) Expected thin film composition in dependence on the radial position of the laser spot on the target according to equation (3). The inset shows a photographic image of the target used in this study prior to deposition.

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segments was confirmed using energy dispersive X-ray spectroscopy (EDX) using a FEI Nova Nanolab 200 system. The geometrical elongations of the ellipse were further validated by X-ray diffraction (XRD) 2θ-ω line scans over the target surface along the shorter and longer axis of the ellipse. For these measurements a Panalytical Pro MRD system was employed. The reflections corresponding to cubic MgO were not observed in the binary ZnO segment as expected (linescans can be found in Figure S1). Therefore, no significant interdiffusion of Mg-atoms between the segments occurred. All thin films in this study were grown on a-sapphire substrates (Crystec). The samples with a single (vertically and laterally) homogeneous Mgx Zn1−x O layer were deposited either on an Al-doped ZnO (AZO) buffer layer or without any buffer layer. The Al-doped ZnO buffer layers were grown using a conventional PLD process employing a KrF excimer laser (Lambda Physik LPX 305) operating at 248 nm wavelength with a pulse energy of 600 mJ. The laser was focused to an area of ≈ 2 × 6 mm2 resulting in an energy density of ≈ 2 Jcm−2 on the target surface. The setup is designed such that the laser beam is incident on the target at an angle of ≈ 45◦ with respect to the target normal. The target to substrate distance was kept at 10 cm. The center of the target is shifted by about 0.5 cm with respect to the center of the substrate for a slight off-axis ablation to reduce particulates on the substrates and increase thin film homogeneity. A complete description of the PLD setup can be found in Refs. 10,56,57 and 58. The target consisted of ZnO and 1 wt. % Al2 O3 . The substrate temperature during growth of the buffer layers was kept at ≈ 670◦ C and an oxygen partial pressure of p(O2 ) = 0.02 mbar was used. The laser pulses for the ≈ 160 nm thick buffer layers were applied at a frequency of 15 Hz. The Mgx Zn1−x O layers were grown at a slightly lower substrate temperature of ≈ 620◦ C and the same oxygen partial pressure as for the AZO buffer layers. The radial position r was fixed for these layers such that only one homogeneous Mg-content per sample is expected. Seven samples were deposited for each buffer with the fixed r ranging from 2 − 8 mm in 1 mm steps. The radial position of the laser spot on the target is adjusted by a rotation of the target carousel. Therefore, the

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laser spot moves in an arc over the target surface during a change of r. The pulse frequency was kept at 15 Hz to ensure a sufficient point density on the laser track. However, during one rotation of the target . 1 nm of material is deposited onto the substrate, such that a thermal diffusion of Mg-atoms and a homogeneous distribution throughout the layer is still provided. The thickness of these layers calculated from the pulse number is ≈ 40 nm. Reference samples from targets with homogeneous Mg-content were grown under the same deposition conditions using conventional PLD (c-PLD). The vertically composition-graded Mgx Zn1−x O layers were grown on similar AZO buffer layers and with the same deposition conditions as the homogeneous layers. Here, r was varied stepwise during deposition such that layers with defined Mg-content were stacked on top of each other. Between the deposition of each of these layers a growth interruption of ≈ 30 s occurred during the change of r. The thickness of each layer is ≈ 20 nm. The optical properties of the thin films were characterized by low temperature photoluminescence (PL) spectroscopy at T = 2 K using a He-bath cryostat. The samples were excited either by a cw-HeCd laser (λ = 325 nm, samples with r ≤ 4 mm) or a frequency-doubled solid state 532 nm cw-laser (λ = 266 nm, frequency doubled by a Coherent MBD 266 system, samples with r > 4 mm and step-graded layers). The luminescence was dispersed in a f = 320 mm monochromator (Jobin Yvon HR320) using a 600 lines/mm grating and was detected by a liquid nitrogen cooled Jobin Yvon back-illuminated CCD with 2048 × 512 pixels. The PL spectra were deconvoluted using the program fityk by M. Wojdyr. 59 The crystal structure of the samples was investigated by XRD 2θ-ω scans using a modified wide-angle Philips X’Pert Bragg-Brentano diffractometer and Cu-Kα radiation (λ = 1.54 Å). The Mgcontent of the unbuffered samples was determined by EDX with the setup already described above. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) was employed to investigate the surface morphology of the layers, where a Park Systems XE-150 and a FEI Nova Nanolab 200 was used, respectively.

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Results and Discussion Homogeneous Mgx Zn1−x O Thin Films: Optical Properties Figure 2a shows the low temperature PL spectra of a series of Mgx Zn1−x O thin films grown on ZnO:Al (AZO) buffer layers using the novel approach. Excitonic emission with several phonon replica in the distance of the LO-phonon energy in ZnO of ≈ 70 meV can be observed. The emission energy of the layers is clearly larger than any emission expected from binary ZnO layers 60 (gray shaded area in Figure 2a) and is especially larger than the free excitonic emission in ZnO at 3.375 eV. 49,60 Remaining small peaks in the shaded area are due to emission from the AZO buffer layers. Moreover, the emission energy and peak broadening clearly increases with increasing radial position r of the laser spot on the target. This behavior is expected for the Mgx Zn1−x O alloy system, 19,20 since an Mg-incorporation into the lattice causes an increase in bandgap energy of the alloy crystal 50,61 and therefore an increase in the energy of the excitonic emission. An increase of the line width of the emission is also expected with increasing Mg-content due to alloy broadening. 19,28,62–64 As proposed, the Mg-content in the thin films therefore increases with increasing r. Further, only one peak is observed for each sample. We therefore conclude that in our layers the distribution of Mg-atoms is homogeneous in growth direction. Figure 2b shows the PL-spectra of one representative layer grown at r = 4 mm (red) in comparison to a Mgx Zn1−x O thin film grown by conventional PLD (c-PLD, black). The line shape, peak position and broadening of the emission are almost identical and emphasize the clearly homogeneous Mg-distribution and optical quality of our thin films, which is therefore similar to films grown by c-PLD. To determine the emission energies and energetic broadening of the excitonic emission, the PL-spectra were deconvoluted using several Gaussians. A Gaussian line shape is generally expected for the luminescence of alloy systems due to inhomogeneous broadening caused by the statistical distribution of the alloy constituents. 62–64 The parameters for the Gaussian of the maximum of the emission were free parameters, while the other Gaussians were separated

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PL-intensity (arb. units)

r (mm)

2

3

4

5

6

7/8

ZnO

ZnO:Al buffer

T=2K

3.25

3.50

(a)

PL-intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.75

4.00

4.25

4.50

Photon energy (eV)

c-PLD radial position r = 4 mm

ZnO:Al buffer

T=2K 3.4

(b)

3.5

3.6

3.7

3.8

3.9

Photon energy (eV)

Figure 2: (a) Low temperature photoluminescence (PL) spectra for a series of Mgx Zn1−x O thin films grown with different radial positions r on AZO buffer layers. (b) Comparison of normalized low temperature PL spectra of Mgx Zn1−x O thin films with similar composition grown via c-PLD (black) and the novel technique (red) for r = 4 mm. Lineshape and broadening of the emission are almost identical, corresponding to similar optical properties of the two thin films.

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by the fixed LO phonon energy of 70 meV to account for the phonon replicas of the emission during fitting. A deconvoluted spectrum including the single Gaussians can be found in Figure S3. The Mg-content in the layers was then determined from the emission energy of the maximum of the excitonic emission using an empirical formula for similar PLD-grown layers: 20

EPL (x)(eV) = 3.38 + 1.35x + 2.4x2 ,

(4)

where EPL is the emission energy and x the Mg-content. The Mg-content in dependence on r is shown in Figure 3a. It clearly increases with the radial position until r = 7 mm and is constant for r > a at a value of x ≈ 0.39. This corresponds to the composition of the outer target segment (x = 0.4, see blue dashed line in Figure 3a). A comparison to the Mg-content expected from theory for an ideal model system (equation (3)) is shown as black dashed line in Figure 3a. It reproduces the data fairly well for larger r, but deviates for small r. This is due to the shape of the laser spot, which is not point-like shaped, but rather elongated (see Figure S2). The laser therefore ablates also the outer segment for r < b. This increases the Mg-content for lower r in comparison to the model. Further, the inner segment of the target does not have an ideal elliptical shape. Nevertheless, the model reproduces the Mg-content fairly well with only small adjustments of ε, b and y in equation (3) to account for non-idealities of the geometry. Figure 3b shows the energetic broadening of the emission as the full width at half maximum (FWHM) in dependence on the Mg-content in the layers. The FWHM increases with Mg-content as expected for alloy systems. Dashed lines are theoretical curves for the alloy broadening in the Mgx Zn1−x O system: 64 √ FWHM(x) = 2 2 ln 2 Here,

dEg dx

dEg dx

s

x(N − x)V0 . Vexc

(5)

is the derivative of the bandgap energy with respect to the alloy composition.

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target:

0.4

outer

x

segment 0.3

Mg-content

elliptical target model

0.2

0.1

ZnO:Al buffer 0.0

(a) 140

1

2

3

4

5

6

7

8

9

Radial position r (mm) this work elliptical target c-PLD

120

alloy broadening (N = 1)

FWHM (meV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


cluster broadening (N = 3)

100

cluster broadening (N = 4)

80

60 Lorenz et al. (c-PLD)

40

Dietrich et al. (c-PLD) Zippel et al. (c-PLD) Müller et al. (c-PLD)

20

Kubota et al. (L-MBE) Heitsch et al. (c-PLD) Takagi et al. (MBE)

0

(b)

0.0

0.1

0.2

0.3

0.4

0.5

Mg-content x

Figure 3: (a) Mg content of the Mgx Zn1−x O thin films in dependence on the radial position r of the laser spot on the target. The Mg-content was calculated from the PL-peak energy of the excitonic emission determined by a deconvolution of the corresponding PL-spectra. (b) Full width at half maximum (FWHM) of the excitonic emission of the thin films in dependence on the Mg-content x. Dashed lines are theoretical broadenings for the MgZnO alloy system assuming a statistical distribution of Mg atoms (equation (5)). N corresponds to the average number of Mg-atoms within a cluster, where (MgO)N clusters are considered fairly stable. Literature values for several different techniques are given as open symbols (Refs. 16,19,21,30,57,65,66). Slightly lower FWHM values for the literature samples are due to much higher film thicknesses of these thin films.

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It corresponds to the derivative of equation (4). Vexc is the excitonic volume, which is taken 3 as the statistically relevant excitonic volume Vexc = 8π aXB , 62,63 where aXB is the excitonic Bohr radius given as aXB (x) = aHB εr (x) m0 /µ. Here, εr (x) = 7.46 + 2.24x is the compositiondependent dielectric constant of Mgx Zn1−x O, 19,64,67,68 m0 is the free electron mass and µ = me mh / (me + mh ) is the reduced effective mass of the exciton, where me = 0.28m0 and mh = 0.95m0 for the effective polaron masses in binary ZnO was used 69,70 (corresponding to band effective masses of 0.24m0 and 0.78m0 , respectively). Both values are varying heavily from publication to publication 69–80 (Experimental band effective masses without polaronic effects: me = (0.23 − 0.5)m0 and mh = (0.45 − 0.78)m0 , see also Ref. 74 and references therein) and are also expected to change with Mg-content. 23,37,81,82 The used value for the electron effective polaron mass was measured directly using cyclotron resonance, 69 while both electron and hole effective polaron masses used here have been found to lead to correct exciton transition energies for bulk ZnO as well as quantum well heterostructures. 70 V0 is the inverse cation density or the minimal volume where potential fluctuations can occur and is given by half √

the unit cell volume for wurtzite MgZnO, which is V0 =

3 2 ac 4

√

=

3 (0.325 4

nm)2 0.52 nm ≈

23.8 × 10−3 nm3 . 83 The constant N takes possible clustering of Mg-atoms into account and is the average number of Mg-atoms within such a cluster. This is motivated by the fact that (MgO)N clusters are fairly stable. 84 The experimental broadenings, both for thin films grown by the novel approach as well as for the reference samples grown by c-PLD, are indeed lying close to the theoretical lines of the cluster broadening for N = 3 or 4. Thin films from literature are mostly following these lines alike, regardless of deposition method, such as for MBE, 16 Laser-assisted MBE 30 or PLD, 19,21,57,65,66 as also shown in Figure 3b. Slightly lower FWHM for some literature samples could be due to much higher film thicknesses. This further emphasizes the equivalent optical properties of the thin films grown by the novel approach as compared to high-quality layers from literature. The importance of this method lies in the fact that the whole composition range of the alloy can be grown with suitable quality from only one single target up to the concentration of the outer segment.

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Homogeneous Mgx Zn1−x O Thin Films: Structural Properties Figure 4a shows the XRD 2θ-ω scans obtained from a series of unbuffered thin films. Apart from the substrate peaks labeled as Al2 O3 , only reflections corresponding to the MgZnO (0001) plane can be indexed according to JPCDS card no. 36-1451 (binary ZnO). The thin films therefore grow purely in the wurtzite phase with a well-defined c-plane orientation, as expected for wurtzite MgZnO thin films on a-sapphire. 15,85 In Figure 4b the MgZnO (0002) reflection is displayed with higher resolution. It shifts to higher angular positions with increasing r, i.e. increasing Mg-content in the layers. This corresponds to a decrease in c-lattice constant typical for MgZnO thin films. 15,17 Further, only a single reflection per layer without any shoulders is observed emphasizing the homogeneity of the layers and the absence of any crystallographic phase separation. In Figure 5a, the c-lattice constant of the thin films is plotted against their Mg-content determined by EDX. The lattice constant was calculated from the position of the MgZnO (0002) reflection. It decreases almost linearly with Mg-content as expected from literature. 15,17 Comparable thin films grown by c-PLD 17,86 show the same trend and similar lattice constants such that no additional strain or defects are present in the layers. 87–89 A direct comparison of the XRD 2θ-ω scans of the MgZnO (0002) reflection for the reference sample grown by c-PLD and a thin film with similar composition grown by the novel technique is shown in Figure 5b. Similar to the luminescence peaks, the line shape, 2θ position and the broadening is almost identical confirming equivalent crystalline quality of the thin films. For the growth of step-graded heterostructures, the surface morphology of the homogeneous thin films is of great importance, since it is the growth template of the following layers. Further, it defines the roughness of the interfaces of the layer stack. Figure 6 shows the 2 × 1 µm2 AFM images of four selected and representative Mgx Zn1−x O thin films grown on AZO buffer layers in comparison to a thin film grown by c-PLD on an AZO buffer layer. One can observe a smooth but granular surface structure without droplets for this image size. A low droplet density is also indicated in large area SEM images shown in Figure S4. The 15


(a)

10

20

30

40

50

60

2

70

Al2O3 (2 2 -4 0)

MgZnO (0 0 0 4)

layers

MgZnO (0 0 0 2)

unbuffered

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Al2O3 (1 1 -2 0)


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radial position r 2 mm 3 mm 4 mm 5 mm 6 mm 7 mm 8 mm

80

90

100

110

(°)

β

Figure 4: (a) XRD 2θ-ω scans of unbuffered Mgx Zn1−x O thin films grown with the novel technique. Besides reflections of the a-sapphire substrates only reflections of the thin films corresponding to c-oriented growth in the wurtzite phase can be indexed. (b) Zoom onto the wurtzite MgZnO (0002) reflection. β denotes reflections of the substrate due to Kβ radiation. The peaks shift to higher angular positions with increasing r (increasing Mgcontent) corresponding to a decrease of the c-lattice constant.

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Lattice constant c (Å)

5.24

5.22 ZnO 5.20

5.18 elliptical target

5.16

conventional PLD reference sample A. Rahm

5.14

Ohtomo et al.

0.0

0.1

0.2

(a)

0.3

MgZnO (0 0 0 2)

c-PLD,

(b)

0.4

0.5

Mg-content x

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Al2O3 (1 1 -2 0)

x = 0.27

radial position r = 5 mm,

x = 0.28

unbuffered layers

30

31

32

33

34

35

2

36

37

38

39

40

(°)

Figure 5: (a) c-lattice constant of Mgx Zn1−x O thin films grown by c-PLD and VCCS-PLD, respectively. The data is compared to literature results. 17,86 The reference sample shown in (b) also exhibits a similar lattice constant. The Mg-content of the samples was determined by EDX. (b) XRD 2θ-ω scan of the MgZnO (0002) reflection for a thin film grown with the novel technique (red) in comparison to a thin film with similar Mg-content grown with c-PLD (black). Lineshape and broadening are essentially identical corresponding to similar structural qualities.

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Figure 6: AFM images of the surface of AZO buffered Mgx Zn1−x O thin films for different values of r (and x) in comparison to a thin film grown by c-PLD. Smooth surfaces can be observed for all Mg-contents. The surface morphology is analogous to thin films grown by c-PLD.

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droplet densities are further similar to or even lower than those of MgZnO thin films grown by conventional PLD, also shown in Figure S4. A high density of droplets can possibly occur using segmented targets, when the laser ablates at the borders of the segments. 15,90 However, pressing and sintering the segmented target as a whole seems to diminish this effect, since the borders are pronounced to a significantly smaller extent in comparison to a target consisting of sawn and piecewise reassembled segments. This had already been investigated for amorphous zinc-tin-oxide thin films grown by the lateral continuous composition spread technique. 90 The surface roughness Rq is well below 1 nm regardless of Mg-content in the layers. Further, the surface morphology is similar to the film grown by c-PLD, the thin films grown with the novel technique are even slightly smoother. The surface quality of the layers grown with the VCCS technique is therefore suitable for the growth of heterostructures.

Mgx Zn1−x O Thin Films with Vertical Composition Spread For the demonstration of the growth of step-graded Mgx Zn1−x O heterostructures, we have deposited several Mgx Zn1−x O layers with different composition on top of each other on a AZO buffer layer. For this, we used a fixed r during the deposition of each layer which was varied with the deposition of each subsequent layer in-situ. We have grown and characterized three different sample structures. One where the Mg-content increases in growth direction with seven Mgx Zn1−x O layers deposited using r = 2 − 8 mm in steps of ∆r = 1 mm (x = 0.08 − 0.39). The second sample is an inverted structure of the first layer, where the Mgcontent decreases in growth direction. Finally, the last layer also exhibits an increasing Mg-content in growth direction, but with a narrower composition grading of r = 1.5 − 5 mm in ∆r = 0.5 mm steps (eight Mgx Zn1−x O layers, x = 0.05 − 0.28). Each Mgx Zn1−x O layer of these stacks had a thickness of ≈ 20 nm. The corresponding structures are also schematically depicted in Figure 7a. Figure 7a further shows the low temperature PL spectra of the step-graded Mgx Zn1−x O thin films. One can observe separate emission peaks for each layer up to r = 5 mm. For 19



graded structure ~ 20 nm per layer r (mm)

Figure 7: (a) Low temperature PL-spectra for step-graded Mgx Zn1−x O thin films. The radial position of the laser spot on the target has been changed stepwise during growth according to the schematics next to the spectra. Slightly higher energies for the inversed grading (red) are within the error of the radial position of the PLD setup and can be due to a small slip depending on whether a position is approached from higher or lower r. (b) AFM image of the step-graded thin film with ∆r = 0.5 mm. A smooth surface morphology of the complete stack is still observable.

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larger r the peaks are already strongly broadened for the single layers and merge into each other for the graded layers. The absence of the r = 5 mm emission peak for the inverse graded layer with decreasing Mg-content shown in red is possibly due to self-absorption processes. Further, excited electron hole pairs from these layers can diffuse into regions with lower Mg-content and recombine there, thus further increasing the emission intensity for layers with lower Mg-content. The absence of emission of the r = 5 mm layer is therefore no sign of lower quality of this layer. Nevertheless, due to the occurrence of separate peaks in the PL spectra even for the narrow graded thin film shown in blue, we conclude that significant interdiffusion of Mg-atoms during growth does not take place. Therefore, each layer exhibits a well-defined and homogeneous Mg-content. Further, the peak positions of the single layers of the three investigated graded thin film samples coincide very well with each other. Slightly higher energies for the inversed graded layer are within the error range of the radial position of the PLD setup. It can be caused by a slight slip in the position of the target whether r is approached from an initially higher or lower value. A high reproducibility of the layer composition is therefore given for the VCCS technique. The broadening of the peaks is similar as well, a comparison of emission energies and broadenings for step-graded layers in dependence on radial positions or the respective Mg-content can be found in Figure S5. The emission properties of our graded thin films are further similar to graded MBE-grown Mgx Zn1−x O layer structures from literature. 2 Figure 7b shows an example of the surface morphology of a complete layer stack, where the AFM image of the narrow graded sample is shown. The surface is still smooth and granular. The surface roughness is of the order of one nm, even with eight layers of different composition grown on top of each other. Our approach should therefore be suitable for the growth of high-quality ternary alloy heterostructures with a well-defined and controllable composition gradient or even graded quantum well structures.

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Conclusion In summary, we have demonstrated a novel PLD technique for the control of the chemical composition of ternary alloy thin films in growth direction suitable for the growth of high-quality composition graded heterostructures. The technique, which we call vertical continuous composition spread (VCCS), employs a single elliptically-segmented PLD target with different chemical composition inside and outside of the ellipse. We have shown that we are able to control the composition of MgZnO thin films by the radial position of the PLD laser spot on the target. Homogeneous Mgx Zn1−x O thin films were grown spanning almost the complete composition range between inner and outer segment of the used target (binary ZnO inside and Mg0.4 Zn0.6 O outside of the ellipse). The Mg-content in the layers in dependence on the radial position r of the laser spot on the target is in reasonable agreement with the theoretically calculated composition assuming an ideal point-like PLD laser spot. The optical and structural quality of the layers is similar to comparable thin films grown by conventional PLD, such that the layers are suitable for the growth of composition-graded heterostructures. As a consequence, the growth of step-graded Mgx Zn1−x O thin films with highly reproducible and well-defined Mg-content of the single layers has been demonstrated. Correspondingly, no signs of a significant interdiffusion of Mg between the single layers could have been found. This approach is of course transferable to any other material system, as long as an elliptically-shaped target can be produced from the two involved materials. Therefore, it is promising for future work on composition-graded thin films for strain engineering, bandgap engineering or dopant profiling which had been so far out of reach for standard PLD systems.

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Author Information Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgement We are indebted to Gabriele Ramm and Monika Hahn for target preparation and to Jörg Lenzner for SEM and EDX measurements. We also would further like to thank Holger Hochmuth for the necessary changes in the control program for the PLD setup and Laurenz Thyen for the help in the construction of filling tubes for the target powders. This work was supported by European Social Fund within the Young Investigator Group "Oxide Heterostructures" (SAB 100310460) and Deutsche Forschungsgemeinschaft within the framework of Sonderforschungsbereich 762 "Functionality of Oxide Interfaces". M.K. also acknowledges the Leipzig School for Natural Sciences BuildMoNa.

Supporting Information Available The following files are available free of charge. • XRD 2θ-ω linescans of the elliptically-segmented MgZnO/ZnO PLD target; schematics of the ablation area on the target for different PLD laser spot positions; deconvoluted PL spectrum of one MgZnO layer; Large area SEM images for several MgZnO thin films grown with the novel technique in comparison to films grown by conventional

23



PLD; and Mg-contents and PL peak broadenings for all layers in the graded structures (PDF) This material is available free of charge via the Internet at http://pubs.acs.org/.

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