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MBE-grown Zincblende MnSe1−xTex Thin Films on ZnTe
Journal of Crystal Growth 511 (2019) 19–24
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
MBE-grown Zincblende MnSe1−xTex Thin Films on ZnTe a,1
Man Kit Cheng , Jing Liang ⁎ Iam Keong Soua,d,
a,1
a,b
, Jian Xu
a
c
c
, Ying Hoi Lai , Sut Kam Ho , Kam Weng Tam ,
T
a
Department of Physics, The Hong Kong University of Science and Technology, Hong Kong, China Department of Electronic and Computer Engineering, The Hong Kong University of Science and Technology, Hong Kong, China Faculty of Science and Technology, University of Macau, Macau d William Mong Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Hong Kong, China b c
ARTICLE INFO
ABSTRACT
Communicated by K.H. Ploog
We report studies on the role of a ZnTe buffer in determining the crystalline phase of MBE-grown MnSe1−xTex alloy thin films. It was found that MBE growth of MnSe and MnTe directly onto a zincblende GaAs substrate usually result in their corresponding stable phases, which are rocksalt and hexagonal, respectively. A set of zincblende MnSe1−xTex alloy thin films with Te composition covering from 0.27 to 1 were fabricated on zincblende ZnTe buffer layers. We have addressed the lattice distortion issue due to thin film effect for these films. A combination of several structural characterizations demonstrates that a perfect lattice matched MnSe1−xTex/ ZnTe heterostructure can indeed be realized. The results of this study pave the way for realizing a double-barrier MnSe1−xTex/ZnTe/MnSe1−xTex resonant tunneling diode structure with perfect lattice match as a candidate for a promising THz emitter.
Keywords: A1. High resolution X-ray diffraction A3. Molecular beam epitaxy B1. Alloys B1. Tellurites
1. Introduction For various electronic and optoelectronic applications based on solid-state thin-film devices, the requirement for close or perfect lattice match limits [1] the utilization of a number of heterostructures. The use of a GaInAs/AlAs resonant tunneling diode (RTD) as a compact THz emitter shows great potential in THz wireless communication [2]. However, the fact that the potential of this device has to date not been exploited commercially is mainly because of its low output power. From reviewing the current III-V based RTD structure [2], there are large lattice mismatches among the heavily-doped GaInAs layer, the AlAs barriers, the GaInAs well and the InP substrate. Likely the lattice mismatching defects or strain of this system may be the key source that limits its output power. On the other hand, ZnTe crystals have been widely used as an efficient THz emitter via the optical rectification effect [3]. zincblende ZnTe with an energy bandgap of 2.25 eV has a lattice constant of 6.1034 Å, while zincblende MnSe with an energy bandgap of 3.4 eV has a lattice constant of 5.93 Å and zincblende MnTe with an energy gap of 3.18 eV has a lattice constant of 6.328 Å [4]. Since ZnTe’s lattice constant is in between those of MnSe and MnTe, one can in principle fine tune an x-value of MnSe1−xTex alloy to perfectly lattice match with ZnTe to form a double-barrier MnSe1−xTex/
ZnTe/MnSe1−xTex structure. With a ZnTe crystal as the substrate and heavily doped p-ZnTe layers as the top and bottom electrodes, one can realize a novel II-VI based RTD structure that could enjoy perfect lattice match. To the best of our knowledge, the experimental studies of MnSe1−xTex alloy have so far been limited to its magnetic properties on samples grown by solid state reaction approach with Te composition of 0 ≤ x ≤ 0.4 [5]. In this work, we investigated the growth conditions for fabricating zincblende MnSe1−xTex alloy thin films on zincblende ZnTe buffer layers with the aim to develop a novel ZnTe-based RTD structure with perfect lattice match. 2. Method All the samples were fabricated using a VG V80H molecular beam epitaxy (MBE) system. High-resolution transmission electron microscopy (HRTEM) images and electron diffraction patterns were captured by a JEOL 2010F TEM. Scanning electron microscope images were captured by a JEOL-6390 SEM. High resolution x-ray diffraction (HRXRD) measurements were carried out by a PANalytical multipurpose x-ray diffractometer. X-ray photoelectron spectroscopy (XPS) measurements were handled using a Kratos Axis Ultra DLD surface analysis instrument. Reflection high-energy electron diffraction
Corresponding author at: Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China. E-mail address: [email protected] (I.K. Sou). 1 Man Kit Cheng and Jing Liang contributed equally to this work. ⁎
https://doi.org/10.1016/j.jcrysgro.2019.01.029 Received 10 January 2019; Received in revised form 23 January 2019; Accepted 26 January 2019 Available online 28 January 2019 0022-0248/ © 2019 Elsevier B.V. All rights reserved.
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Table 1 Growth conditions used for fabricating MnSe1−xTex thin film samples. Sample
#1 #2 #3 #4 #5 #6 #7 #8 #9 #10
ZnTe Layer
MnSe1−xTex Layer
Tsub(°C)
TZn(°C)
TTe(°C)
Growth time (min)
Tsub(°C)
TMn(°C)
TSe(°C)
TTe(°C)
Se:Te Flux Ratio
Growth time (min)
Thickness (nm)
/ / / 358 358 358 358 358 358 358
/ / / 245 245 245 245 245 245 245
/ / / 283 283 283 283 283 283 283
/ / / 60 40 40 40 40 40 40
405 300 310 310 310 310 310 310 310 310
730 730 770 770 770 770 770 770 770 770
162 155 / / 135 145 152 156 160 155.6
281 295 316 316 316 316 316 316 316 316
7.2:1 2.1:1 Te only Te only 1:15 1:5.8 1:3.0 1:1.2 1:1.0 1:1.3
90 80 60 60 60 60 60 60 60 60
∼240 ∼160 ∼190 ∼100 ∼100 ∼100 ∼100 ∼140 ∼160 ∼135
(RHEED) with a 13 keV electron gun was used for in-situ growth monitoring. The thicknesses of the thin films were obtained by crosssectional imaging using HRTEM and SEM. All samples were grown on epi-ready SI-GaAs (0 0 1) substrates and their growth conditions are given in Table 1. Our studies on the growth of MnSe1−xTex thin films were initiated with a purpose to test the possibility of their direct growth onto GaAs (0 0 1) substrates. The growth of Sample #1 to #10 was handled using source temperatures as listed in Table 1. Since a flux monitoring facility is not available in our MBE system, we estimated the Se:Te flux ratios used for the growth of the samples listed in Table 1 using the following approach. We fabricated two samples, one is a pure Se layer (growth time of 8 h 45 min) and the other is a pure Te layer (30 mins) on GaAs (0 0 1) substrates at substrate temperature of 30 °C (Such a low substrate temperature ensures that the sticking coefficients of Se and Te fluxes are close to 1). The elemental Se and Te cell temperatures used are TSe = 135 °C and TTe = 316 °C, respectively, the same as those used for the growth of Sample #5. Their film thicknesses were determined using cross-sectional SEM imaging to be hSe = 465 nm and hTe = 480 nm respectively (See Fig. S1(a) and S1(b) in supplementary data). Their fluxes (impingement rate) can then be estimated using the following formula:
=
dh NA dt M
presence of Te flux with significant amount, which could be attributed to the difference of the bonding strength between MnSe and MnTe (only the bonding strength of MnSe is available [10] but not that of MnTe, however MnTe is likely to have a lower bonding strength than MnSe as Te has a lower electron affinity (1.971 eV) as compared with Se (2.021 eV)) [11]. Our results are also consistent with previous findings that MnSe is stable in the octahedrally coordinated rocksalt structure [12] while the tetrahedrally coordinated zincblende polymorph is metastable [4,13]. We then investigated the growth of MnTe compound directly on GaAs (0 0 1) substrates by providing Mn and Te fluxes only. By tuning the Mn:Te flux ratio through a few trial runs, an optimized growth for single-phase MnTe was obtained in preparing Sample #3. Table 1 also lists its corresponding optimized source temperatures. The θ/2θ scan of Sample #3 is shown in Fig. S3, in which one can see that two layer peaks locate at 2θ = 28.2° and 58.4°, well matching with the reported 2θ values of (1 0 1) and (2 0 2) peaks of hexagonal MnTe [14]. Further evidence of the hexagonal phase of Sample #3 comes from its determined (1 0 1)/(2 0 2) integrated peak area ratio of 7.0, which is in good agreement with the reported value of 7.7 [13]. The resulting hexagonal phase of Sample #3 is not a surprise as it is well known that the stable phase of MnTe is a hexagonal NiAs structure [15]. Both studies described above indicate that MBE growth of MnSe and MnTe directly onto a zincblende GaAs substrate usually result in their corresponding stable phases, which are rocksalt and hexagonal, respectively. As the key objective of this research is to fine tune the composition of zincblende MnSe1−xTex to enjoy a lattice match with ZnTe, our next step is to try out the growth of MnSe1−xTex thin films on a ZnTe buffer grown on a GaAs substrate. Since a ZnTe layer grown onto a zincblende GaAs (0 0 1) substrate is in zincblende phase and it also enjoys a less lattice mismatch even for both zincblende MnSe and MnTe compound as compared with GaAs (the lattice constants for zincblende ZnTe, MnSe, MnTe and GaAs are 6.10 Å, 5.93 Å, 6.34 Å and 5.65 Å, respectively), one should expect that the MnSe1−xTex layers grown on top of zincblende ZnTe may crystallize in the same phase. As ZnTe is a II-VI material with a more ionic bonding nature similar to both MnSe and MnTe as compared to GaAs, which makes such an isophase growth using the MBE technique even more likely. This is attributed to the fact that MBE is a non-equilibrium growth technique that an epitaxial thin film grown by this technique can be crystallized in a metastable phase providing that the substrate (for direct growth) or a buffer layer can have a strong influence on the growth of the epilayer. In fact, MBE growth of zincblende MnTe and MnSe layers has been realized previously using either a CdTe substrate or a CdTe or ZnTe epilayer in quantum well or superlattice structures [16,17]. To test this idea, Sample #4, a MnTe layer, was grown with the same growth conditions as those used for the growth of Sample #3 except that the former growth was performed on a ZnTe buffer grown on a GaAs (0 0 1) substrate. The θ/2θ scan of Sample #4 is shown in Fig. S3. The two layer peaks of Sample #4 have 2θ values at 28.0° and 57.9°,
(1)
where is the linear deposition rate that can be calculated using the growth time and the measured layer thickness, ρ is the mass density of the layer that can be estimated using the data provided in Ref. [6,7], M is the molar mass of the layer, and NA is the Avogadro’s constant. For the fluxes of other cell temperatures used for the growth of other samples listed in Table 1, we carried out estimated calculation based on the temperature dependence of the vapor pressure of Se and Te [8]. The results for the Se:Te ratios estimated by this approach are provided in a column in Table 1. dh dt
3. Results and discussion As shown in Table 1, the estimated Se:Te flux ratios used for the growth of Sample #1 and #2 are 7.2:1 and 2.1:1, respectively. Fig. S2 of the supplementary data displays their HRXRD θ/2θ scans, which are basically identical, and their two layer peaks at 2θ = 32.8° and 68.8° (corresponding to a lattice constant of 5.45 Å) closely match with the reported angular positions of (0 0 2) and (0 0 4) peaks of rocksalt MnSe (corresponding to a lattice constant of 5.44 Å) [9]. Through performing high-resolution HRXRD scans on the layer peaks and corresponding data fitting, the peak area ratios of (0 0 2)/(0 0 4) of Sample #1 and #2 are determined to be 9.60 and 8.90, respectively, which are also in good agreement with the reported value of 10.7 for rocksalt MnSe [9]. These results indicate that the formation of rocksalt MnSe is more energetically favorable than forming a MnSe1−xTex alloy even at the 20
Journal of Crystal Growth 511 (2019) 19–24
M.K. Cheng et al.
Table 2 XRD and XPS data of MnSe1−xTex samples. Sample
XRD
XPS
2θ Position
#5 #6 #7 #8 #9
(0 0 2)(°)
(0 0 4)(°)
Plane spacing along [0 0 1] (Å)
28.1 28.2 28.6 29.3 30.1
58.1 58.4 59.3 60.9 62.7
6.35 6.32 6.23 6.08 5.92
(0 0 2)/(0 0 4) Peak area ratio
Composition Se (%)
Te (%)
1.21:1 1.22:1 1.16:1 0.92:1 0.62:1
6 15 31 53 73
94 85 69 47 27
samples are found to be close to 1, indicating that they all are likely in zincblende phase, as if they are either in rocksalt or hexagonal phase, these ratios should be much higher, similar to the values of Sample #1, #2 and #3 mentioned earlier. Post-growth XPS measurements were performed on all five MnSe1−xTex samples. Ar sputtering was conducted prior to the XPS scans to remove their surface oxide layers to ensure their determined chemical compositions are reliable. The determined Se and Te compositions for Sample #5 – #9 can be found in Table 2. Fig. 2 displays a plot for the plane spacing along [0 0 1] versus the Te composition for these samples and the pure MnTe sample (Sample #4), in which the data were fitted with a linear behavior (the solid line) as expected from the Vegard’s law. It is worth mentioning that as shown in Table 2 the plane spacing along [0 0 1] of Sample #9 with a Te composition of 27% is 5.92 Å, which is even smaller than the lattice constant of 5.93 Å for pure zincblende MnSe reported in a previous experimental work conducted by Kolodziejski et al. [19]. We believe the difference is attributed to the fact that the thicknesses of the MnSe1−xTex samples used in this study range from ∼100 to 160 nm while those Zn1−xMnxSe samples used in Kolodziejski et al’s work are much thicker films, around 1–3 μm. It is well known that a thin film grown on a lattice mismatched layer may not be fully relaxed to reach its bulk lattice parameters [20] unless a certain thickness is reached, which depends on the magnitude of the lattice mismatch. Thus the MnSe1−xTex samples presented in this study are likely distorted slightly off its bulk cubic structure due to the thin film effect. In Fig. 2, the two data points for the lattice constants of pure MnSe and MnTe thick samples [4] are placed with a dashed line joining them (marked as “Reference”), which displays the predicted lattice constants for bulk MnSe1−xTex from Vegard’s law. A horizontal dashed line at lattice plane spacing of 6.1034 Å, which is the lattice constant of ZnTe, meets with the previous dashed line at x = 43%. This intercepting point corresponds to MnSe0.57Te0.43 that is perfectly lattice matched with ZnTe if Vegard’s law is strictly followed. One can see that the data points from the current work shows a characteristic behavior that samples with Te composition less than 43% (like Sample #9) have the
Fig. 1. (a) HRXRD θ/2θ scans of Sample #5 to #9 and (b) HRXRD θ/2θ scans of Sample #8 near ZnTe(0 0 4) peak with data fitting for extracting the lattice plane spacing of the MnSe0.53Te0.47 layer.
respectively, which well match with the reported values of the (0 0 2) and (0 0 4) peaks of zincblende MnTe [18]. The (0 0 2)/(0 0 4) integrated peak area ratio of Sample #4 is determined to be ∼1.2, in close agreement with the reported value of 1.7 for zincblende MnTe [18]. The growth of a set of MnSe1−xTex thin films was then carried out with fixed TMn, TTe and Tsub = 770 °C, 316 °C, and 310 °C, respectively, on to ZnTe (280 nm)/GaAs (0 0 1) substrates where the ZnTe buffer was grown using TZn, TTe and Tsub = 245 °C, 283 °C and 358 °C, respectively. In conducting these growths, TSe was varied ranging from 135 to 160 °C. The growth conditions used for the growth of the MnSe1−xTex samples from Sample #5 to #9 are listed in Table 1. Fig. 1(a) displays the θ/2θ scans of all five MnSe1−xTex samples, showing that the angular values of their two layer peaks increase with TSe. Sample #8 is a sample closely lattice-matched with the ZnTe buffer and its finer (0 0 4) scan is given in Fig. 1(b), which allows us to resolve the MnSe0.53Te0.47 layer peak from the neighboring ZnTe peak through data fitting, After performing data fitting on high-resolution scans of the layer peak regions of all the scans shown in Fig. 1(a), the plane spacing along [0 0 1] of all the MnSe1−xTex samples and their integrated (0 0 2)/(0 0 4) peak area ratios of the two layer peaks were obtained and they are displayed in Table 2. All the integrated peak area ratios of the five MnSe1−xTex
Fig. 2. Lattice plane spacing along [0 0 1] verses Te composition of MnSe1−xTex samples with a fitted line. The meaning of the two dash lines are given in the text. 21
Journal of Crystal Growth 511 (2019) 19–24
M.K. Cheng et al.
of 52% instead of 43%. These seemingly inconsistence may be explained by the uncertainties of the XPS measurements as indicated by the horizontal error bars of the data points of the current work as shown in Fig. 2. An investigation on whether a lattice distortion occurs in our MnSe1−xTex films was conducted on Sample #9 by performing an ω − 2θ scan for an asymmetric (2 2 4) Bragg reflection and the resulting broad scan profile is displayed in Fig. 3 with the 2θ values calibrated using the standard 2θ value of the GaAs (2 2 4) peak. Through performing data fitting on the overlapping ZnTe (2 2 4) buffer peak and MnSe0.73Te0.27 (2 2 4) layer peak as shown in the left of Fig. 3, the 2θ value of the MnSe0.73Te0.27 (2 2 4) layer peak was determined to be 78.4° corresponding to a [2 2 4] lattice spacing of 1.219 Å that is larger than the value of 1.209 Å as expected from a cubic unit cell, indicating that its lattice is indeed distorted in a way that the lattice planes along the [0 0 1] direction is compressed while that along the [2 2 4] direction is being stretched. We have performed a transformation from the rectangular cuboid unit cell of Sample #9 using the measured plane spacing values along [0 0 1] and [2 2 4] to a cubic unit cell assuming constant volume (this assumption is valid as the distortion due to nonfully relaxation for all the five MnSe1−xTex samples is in fact quite small) [20]. The analysis of this transformation yields a lattice constant for the corresponding cubic unit cell of Sample #9 to be 6.02 Å, which is quite close to the extrapolated value of 6.03 Å obtained from Vegard’s law based on the reported lattice constants of MnSe and MnTe [4], and also close to the value of 6.06 Å as estimated from the data of a previous
Fig. 3. HRXRD ω − 2θ asymmetric (2 2 4) scan of Sample #9 with data fitting performed for the overlapping ZnTe (2 2 4) and MnSe0.73Te0.27 (2 2 4) peaks.
(0 0 1) lattice plane spacing smaller than the lattice constants predicted by the Vegard’s law, while samples with Te composition more than 43% (like Samples #4, #5, #6 and #7) display an opposite characteristic. This in fact is consistent with the expectation from the thin film effect that a MnSe1−xTex thin film grown on a ZnTe buffer will be compressed if x < 0.43 and will be stretched out if x > 0.43 as their lateral lattice parameters will be the same as that of ZnTe before lattice relaxation is initiated and will not reach a cubic structure until fully relaxation is achieved. It should be pointed out that the data point of Sample #8 doesn’t seem to agree with this characteristic behavior and the horizontal dashed line meets with the fitted solid line at a Te composition
Fig. 4. Results of structural characterization of Sample #10: (a) HRXRD θ/2θ scan of the (0 0 4) overlapping peak; (b) cross-sectional TEM image of this sample; (c) high-resolution cross-sectional TEM image of the junction between the MnSe0.49Te0.51 layer and ZnTe buffer layer; and (d) the corresponding electron diffraction pattern of (c). 22
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M.K. Cheng et al.
data, we present a direct comparison between the cross-sectional TEM image of Sample #10 as shown in Fig. 4(c) with the simulated TEM lattice images for zincblende and rocksalt lattice structures given by Sanchez et al. [22], which offers another evidence that the MnSe1−xTex layer of Sample #10 is indeed in zincblende phase. The Te composition, x, of Sample #10 has been determined by XPS measurements to be around 51%. The MnSe0.49Te0.51 layer of Sample #10 offers the dream barrier layer for constructing a double-barrier MnSe0.49Te0.51/ZnTe/ MnSe0.49Te0.51 RTD device that enjoys perfect lattice match. We have fabricated such a device with an active area (S) around 7.5 mm2. Its structure is shown in Fig. 5(a). The p-type doping of the four ZnTe:N layers in this structure was handled using an Oxford Applied Research nitrogen atom/radical beam source (Model: MPD21) with the control of the nitrogen flow rate to reach two different doping levels (p+ and p−). Fig. 5(b) displays the observed room-temperature I-V characteristics of this device, showing a negative differential resistance (NDR) signature. The device displays a peak current density (Jp) about 0.097A/cm2, a peak-to-valley current ratio of 3.30 and a negative differential resistance (ΔV/ΔI) of about 29.5 Ω. The appearance of NDR characteristics shows its potential as a THz emitter device. Detailed studies on the transport properties of this RTD device are underway in our laboratory and the results will be reported elsewhere. 4. Conclusion In summary, with the aim of fabricating a zincblende MnSe1−xTex/ ZnTe heterostructure with perfect lattice match, the factors that determine the crystalline phase of MBE-grown MnSe1−xTex thin films were investigated. Our studies have found that direct growth of MnSe and MnTe on GaAs (0 0 1) substrates only results in their stable rocksalt and hexagonal phase, respectively. With the assistance of a ZnTe buffer, MnSe1−xTex thin films in zincblende phase were successfully fabricated with Te composition covering a range from 0.27 to 1. The thin film effect on the distortion of the alloy lattice has been addressed and in particular we have provided detailed studies on the MnSe0.73Te0.27 thin film. The results from HRXRD, HRTEM and electron diffraction demonstrate that a MnSe1−xTex/ZnTe heterostructure with perfect lattice match is indeed achievable. We have fabricated a double-barrier MnSe0.49Te0.51/ZnTe/MnSe0.49Te0.51 RTD device with a perfect lattice match and it indeed shows an NDR signature, which shines light on the realization of a novel THz emitter device.
Fig. 5. Double-barrier MnSe0.49Te0.51/ZnTe/MnSe0.49Te0.51 RTD device: (a) device structure and (b) its room-temperature I-V characteristics showing the NDR characteristics.
theoretical study [21]. These results confirm that the lattice plane spacing values along the [0 0 1] direction versus the Te composition of MnSe1−xTex determined in this study are in fact consistent with the corresponding lattice constants reported previously [5] after considering the distortion due to thin film effect. In order to obtain a MnSe1−xTex sample with even better lattice matching with ZnTe, Sample #10 was grown using a Se cell temperature of 155.6 °C, which is just 0.4 °C lower than that used for Sample #8, while keeping all other growth conditions the same as those of Sample #8. A complete HRXRD θ/2θ scan for Sample #10 is displayed in Fig. S4 in supplementary data. Fig. 4(a) displays a high-resolution θ/ 2θ scan of the (0 0 4) overlapping peak of Sample #10, in which only a single peak is identified for the combination of the ZnTe buffer peak and the epitaxial alloy layer peak, indicating that indeed a perfect lattice matching has been realized. Fig. 4(b) shows a cross-sectional TEM image of Sample #10. Further confirmation of the perfect lattice matching of Sample #10 comes from the cross-sectional high-resolution TEM image of Sample #10 and its corresponding electron diffraction pattern as shown in Fig. 4(c) and (d), respectively. As shown in Fig. 4(c), besides some regions in the image are out of focused due to the limitation of our TEM system, the two lattices of MnSe1−xTex and ZnTe are indistinguishable. This is further confirmed from the electron diffraction pattern taken across both lattices as displayed in Fig. 4(d), in which only a single pattern is present. In Fig. S5 of the supplementary
Acknowledgments This work was mainly supported by the Science and Technology Development Fund of Macao Special Administrative Region, People’s Republic of China (Project No. 015/2013/A1) and partially supported by the Department of Physics, The Hong Kong University of Science and Technology, Hong Kong SAR, China. Appendix A. Supplementary material See supplementary data for cross-sectional SEM images of a pure Se and a pure Te layer grown on GaAs ( 0 0 1 ) substrates, HRXRD θ/2θ scans of Sample #1 – #4, a complete HRXRD θ/2θ scan for Sample #10, and evidence of the zincblende phase of Sample #10. Supplementary data to this article can be found online at https://doi.org/10.1016/j. jcrysgro.2019.01.029. References [1] R.D. Vispute, V. Talyansky, S. Choopun, R.P. Sharma, T. Venkatesan, M. He, X. Tang, J.B. Halpern, M.G. Spencer, Y.X. Li, L.G. Salamanca-Riba, Appl. Phys. Lett. 73 (3) (1998) 348, https://doi.org/10.1063/1.121830. [2] M. Asada, S. Suzuki, N. Kishimoto, Jpn. J. Appl. Phys. 47 (2008) 4375, https://doi.
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
MBE-grown Zincblende MnSe1−xTex Thin Films on ZnTe a,1
Man Kit Cheng , Jing Liang ⁎ Iam Keong Soua,d,
a,1
a,b
, Jian Xu
a
c
c
, Ying Hoi Lai , Sut Kam Ho , Kam Weng Tam ,
T
a
Department of Physics, The Hong Kong University of Science and Technology, Hong Kong, China Department of Electronic and Computer Engineering, The Hong Kong University of Science and Technology, Hong Kong, China Faculty of Science and Technology, University of Macau, Macau d William Mong Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Hong Kong, China b c
ARTICLE INFO
ABSTRACT
Communicated by K.H. Ploog
We report studies on the role of a ZnTe buffer in determining the crystalline phase of MBE-grown MnSe1−xTex alloy thin films. It was found that MBE growth of MnSe and MnTe directly onto a zincblende GaAs substrate usually result in their corresponding stable phases, which are rocksalt and hexagonal, respectively. A set of zincblende MnSe1−xTex alloy thin films with Te composition covering from 0.27 to 1 were fabricated on zincblende ZnTe buffer layers. We have addressed the lattice distortion issue due to thin film effect for these films. A combination of several structural characterizations demonstrates that a perfect lattice matched MnSe1−xTex/ ZnTe heterostructure can indeed be realized. The results of this study pave the way for realizing a double-barrier MnSe1−xTex/ZnTe/MnSe1−xTex resonant tunneling diode structure with perfect lattice match as a candidate for a promising THz emitter.
Keywords: A1. High resolution X-ray diffraction A3. Molecular beam epitaxy B1. Alloys B1. Tellurites
1. Introduction For various electronic and optoelectronic applications based on solid-state thin-film devices, the requirement for close or perfect lattice match limits [1] the utilization of a number of heterostructures. The use of a GaInAs/AlAs resonant tunneling diode (RTD) as a compact THz emitter shows great potential in THz wireless communication [2]. However, the fact that the potential of this device has to date not been exploited commercially is mainly because of its low output power. From reviewing the current III-V based RTD structure [2], there are large lattice mismatches among the heavily-doped GaInAs layer, the AlAs barriers, the GaInAs well and the InP substrate. Likely the lattice mismatching defects or strain of this system may be the key source that limits its output power. On the other hand, ZnTe crystals have been widely used as an efficient THz emitter via the optical rectification effect [3]. zincblende ZnTe with an energy bandgap of 2.25 eV has a lattice constant of 6.1034 Å, while zincblende MnSe with an energy bandgap of 3.4 eV has a lattice constant of 5.93 Å and zincblende MnTe with an energy gap of 3.18 eV has a lattice constant of 6.328 Å [4]. Since ZnTe’s lattice constant is in between those of MnSe and MnTe, one can in principle fine tune an x-value of MnSe1−xTex alloy to perfectly lattice match with ZnTe to form a double-barrier MnSe1−xTex/
ZnTe/MnSe1−xTex structure. With a ZnTe crystal as the substrate and heavily doped p-ZnTe layers as the top and bottom electrodes, one can realize a novel II-VI based RTD structure that could enjoy perfect lattice match. To the best of our knowledge, the experimental studies of MnSe1−xTex alloy have so far been limited to its magnetic properties on samples grown by solid state reaction approach with Te composition of 0 ≤ x ≤ 0.4 [5]. In this work, we investigated the growth conditions for fabricating zincblende MnSe1−xTex alloy thin films on zincblende ZnTe buffer layers with the aim to develop a novel ZnTe-based RTD structure with perfect lattice match. 2. Method All the samples were fabricated using a VG V80H molecular beam epitaxy (MBE) system. High-resolution transmission electron microscopy (HRTEM) images and electron diffraction patterns were captured by a JEOL 2010F TEM. Scanning electron microscope images were captured by a JEOL-6390 SEM. High resolution x-ray diffraction (HRXRD) measurements were carried out by a PANalytical multipurpose x-ray diffractometer. X-ray photoelectron spectroscopy (XPS) measurements were handled using a Kratos Axis Ultra DLD surface analysis instrument. Reflection high-energy electron diffraction
Corresponding author at: Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China. E-mail address: [email protected] (I.K. Sou). 1 Man Kit Cheng and Jing Liang contributed equally to this work. ⁎
https://doi.org/10.1016/j.jcrysgro.2019.01.029 Received 10 January 2019; Received in revised form 23 January 2019; Accepted 26 January 2019 Available online 28 January 2019 0022-0248/ © 2019 Elsevier B.V. All rights reserved.
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Table 1 Growth conditions used for fabricating MnSe1−xTex thin film samples. Sample
#1 #2 #3 #4 #5 #6 #7 #8 #9 #10
ZnTe Layer
MnSe1−xTex Layer
Tsub(°C)
TZn(°C)
TTe(°C)
Growth time (min)
Tsub(°C)
TMn(°C)
TSe(°C)
TTe(°C)
Se:Te Flux Ratio
Growth time (min)
Thickness (nm)
/ / / 358 358 358 358 358 358 358
/ / / 245 245 245 245 245 245 245
/ / / 283 283 283 283 283 283 283
/ / / 60 40 40 40 40 40 40
405 300 310 310 310 310 310 310 310 310
730 730 770 770 770 770 770 770 770 770
162 155 / / 135 145 152 156 160 155.6
281 295 316 316 316 316 316 316 316 316
7.2:1 2.1:1 Te only Te only 1:15 1:5.8 1:3.0 1:1.2 1:1.0 1:1.3
90 80 60 60 60 60 60 60 60 60
∼240 ∼160 ∼190 ∼100 ∼100 ∼100 ∼100 ∼140 ∼160 ∼135
(RHEED) with a 13 keV electron gun was used for in-situ growth monitoring. The thicknesses of the thin films were obtained by crosssectional imaging using HRTEM and SEM. All samples were grown on epi-ready SI-GaAs (0 0 1) substrates and their growth conditions are given in Table 1. Our studies on the growth of MnSe1−xTex thin films were initiated with a purpose to test the possibility of their direct growth onto GaAs (0 0 1) substrates. The growth of Sample #1 to #10 was handled using source temperatures as listed in Table 1. Since a flux monitoring facility is not available in our MBE system, we estimated the Se:Te flux ratios used for the growth of the samples listed in Table 1 using the following approach. We fabricated two samples, one is a pure Se layer (growth time of 8 h 45 min) and the other is a pure Te layer (30 mins) on GaAs (0 0 1) substrates at substrate temperature of 30 °C (Such a low substrate temperature ensures that the sticking coefficients of Se and Te fluxes are close to 1). The elemental Se and Te cell temperatures used are TSe = 135 °C and TTe = 316 °C, respectively, the same as those used for the growth of Sample #5. Their film thicknesses were determined using cross-sectional SEM imaging to be hSe = 465 nm and hTe = 480 nm respectively (See Fig. S1(a) and S1(b) in supplementary data). Their fluxes (impingement rate) can then be estimated using the following formula:
=
dh NA dt M
presence of Te flux with significant amount, which could be attributed to the difference of the bonding strength between MnSe and MnTe (only the bonding strength of MnSe is available [10] but not that of MnTe, however MnTe is likely to have a lower bonding strength than MnSe as Te has a lower electron affinity (1.971 eV) as compared with Se (2.021 eV)) [11]. Our results are also consistent with previous findings that MnSe is stable in the octahedrally coordinated rocksalt structure [12] while the tetrahedrally coordinated zincblende polymorph is metastable [4,13]. We then investigated the growth of MnTe compound directly on GaAs (0 0 1) substrates by providing Mn and Te fluxes only. By tuning the Mn:Te flux ratio through a few trial runs, an optimized growth for single-phase MnTe was obtained in preparing Sample #3. Table 1 also lists its corresponding optimized source temperatures. The θ/2θ scan of Sample #3 is shown in Fig. S3, in which one can see that two layer peaks locate at 2θ = 28.2° and 58.4°, well matching with the reported 2θ values of (1 0 1) and (2 0 2) peaks of hexagonal MnTe [14]. Further evidence of the hexagonal phase of Sample #3 comes from its determined (1 0 1)/(2 0 2) integrated peak area ratio of 7.0, which is in good agreement with the reported value of 7.7 [13]. The resulting hexagonal phase of Sample #3 is not a surprise as it is well known that the stable phase of MnTe is a hexagonal NiAs structure [15]. Both studies described above indicate that MBE growth of MnSe and MnTe directly onto a zincblende GaAs substrate usually result in their corresponding stable phases, which are rocksalt and hexagonal, respectively. As the key objective of this research is to fine tune the composition of zincblende MnSe1−xTex to enjoy a lattice match with ZnTe, our next step is to try out the growth of MnSe1−xTex thin films on a ZnTe buffer grown on a GaAs substrate. Since a ZnTe layer grown onto a zincblende GaAs (0 0 1) substrate is in zincblende phase and it also enjoys a less lattice mismatch even for both zincblende MnSe and MnTe compound as compared with GaAs (the lattice constants for zincblende ZnTe, MnSe, MnTe and GaAs are 6.10 Å, 5.93 Å, 6.34 Å and 5.65 Å, respectively), one should expect that the MnSe1−xTex layers grown on top of zincblende ZnTe may crystallize in the same phase. As ZnTe is a II-VI material with a more ionic bonding nature similar to both MnSe and MnTe as compared to GaAs, which makes such an isophase growth using the MBE technique even more likely. This is attributed to the fact that MBE is a non-equilibrium growth technique that an epitaxial thin film grown by this technique can be crystallized in a metastable phase providing that the substrate (for direct growth) or a buffer layer can have a strong influence on the growth of the epilayer. In fact, MBE growth of zincblende MnTe and MnSe layers has been realized previously using either a CdTe substrate or a CdTe or ZnTe epilayer in quantum well or superlattice structures [16,17]. To test this idea, Sample #4, a MnTe layer, was grown with the same growth conditions as those used for the growth of Sample #3 except that the former growth was performed on a ZnTe buffer grown on a GaAs (0 0 1) substrate. The θ/2θ scan of Sample #4 is shown in Fig. S3. The two layer peaks of Sample #4 have 2θ values at 28.0° and 57.9°,
(1)
where is the linear deposition rate that can be calculated using the growth time and the measured layer thickness, ρ is the mass density of the layer that can be estimated using the data provided in Ref. [6,7], M is the molar mass of the layer, and NA is the Avogadro’s constant. For the fluxes of other cell temperatures used for the growth of other samples listed in Table 1, we carried out estimated calculation based on the temperature dependence of the vapor pressure of Se and Te [8]. The results for the Se:Te ratios estimated by this approach are provided in a column in Table 1. dh dt
3. Results and discussion As shown in Table 1, the estimated Se:Te flux ratios used for the growth of Sample #1 and #2 are 7.2:1 and 2.1:1, respectively. Fig. S2 of the supplementary data displays their HRXRD θ/2θ scans, which are basically identical, and their two layer peaks at 2θ = 32.8° and 68.8° (corresponding to a lattice constant of 5.45 Å) closely match with the reported angular positions of (0 0 2) and (0 0 4) peaks of rocksalt MnSe (corresponding to a lattice constant of 5.44 Å) [9]. Through performing high-resolution HRXRD scans on the layer peaks and corresponding data fitting, the peak area ratios of (0 0 2)/(0 0 4) of Sample #1 and #2 are determined to be 9.60 and 8.90, respectively, which are also in good agreement with the reported value of 10.7 for rocksalt MnSe [9]. These results indicate that the formation of rocksalt MnSe is more energetically favorable than forming a MnSe1−xTex alloy even at the 20
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Table 2 XRD and XPS data of MnSe1−xTex samples. Sample
XRD
XPS
2θ Position
#5 #6 #7 #8 #9
(0 0 2)(°)
(0 0 4)(°)
Plane spacing along [0 0 1] (Å)
28.1 28.2 28.6 29.3 30.1
58.1 58.4 59.3 60.9 62.7
6.35 6.32 6.23 6.08 5.92
(0 0 2)/(0 0 4) Peak area ratio
Composition Se (%)
Te (%)
1.21:1 1.22:1 1.16:1 0.92:1 0.62:1
6 15 31 53 73
94 85 69 47 27
samples are found to be close to 1, indicating that they all are likely in zincblende phase, as if they are either in rocksalt or hexagonal phase, these ratios should be much higher, similar to the values of Sample #1, #2 and #3 mentioned earlier. Post-growth XPS measurements were performed on all five MnSe1−xTex samples. Ar sputtering was conducted prior to the XPS scans to remove their surface oxide layers to ensure their determined chemical compositions are reliable. The determined Se and Te compositions for Sample #5 – #9 can be found in Table 2. Fig. 2 displays a plot for the plane spacing along [0 0 1] versus the Te composition for these samples and the pure MnTe sample (Sample #4), in which the data were fitted with a linear behavior (the solid line) as expected from the Vegard’s law. It is worth mentioning that as shown in Table 2 the plane spacing along [0 0 1] of Sample #9 with a Te composition of 27% is 5.92 Å, which is even smaller than the lattice constant of 5.93 Å for pure zincblende MnSe reported in a previous experimental work conducted by Kolodziejski et al. [19]. We believe the difference is attributed to the fact that the thicknesses of the MnSe1−xTex samples used in this study range from ∼100 to 160 nm while those Zn1−xMnxSe samples used in Kolodziejski et al’s work are much thicker films, around 1–3 μm. It is well known that a thin film grown on a lattice mismatched layer may not be fully relaxed to reach its bulk lattice parameters [20] unless a certain thickness is reached, which depends on the magnitude of the lattice mismatch. Thus the MnSe1−xTex samples presented in this study are likely distorted slightly off its bulk cubic structure due to the thin film effect. In Fig. 2, the two data points for the lattice constants of pure MnSe and MnTe thick samples [4] are placed with a dashed line joining them (marked as “Reference”), which displays the predicted lattice constants for bulk MnSe1−xTex from Vegard’s law. A horizontal dashed line at lattice plane spacing of 6.1034 Å, which is the lattice constant of ZnTe, meets with the previous dashed line at x = 43%. This intercepting point corresponds to MnSe0.57Te0.43 that is perfectly lattice matched with ZnTe if Vegard’s law is strictly followed. One can see that the data points from the current work shows a characteristic behavior that samples with Te composition less than 43% (like Sample #9) have the
Fig. 1. (a) HRXRD θ/2θ scans of Sample #5 to #9 and (b) HRXRD θ/2θ scans of Sample #8 near ZnTe(0 0 4) peak with data fitting for extracting the lattice plane spacing of the MnSe0.53Te0.47 layer.
respectively, which well match with the reported values of the (0 0 2) and (0 0 4) peaks of zincblende MnTe [18]. The (0 0 2)/(0 0 4) integrated peak area ratio of Sample #4 is determined to be ∼1.2, in close agreement with the reported value of 1.7 for zincblende MnTe [18]. The growth of a set of MnSe1−xTex thin films was then carried out with fixed TMn, TTe and Tsub = 770 °C, 316 °C, and 310 °C, respectively, on to ZnTe (280 nm)/GaAs (0 0 1) substrates where the ZnTe buffer was grown using TZn, TTe and Tsub = 245 °C, 283 °C and 358 °C, respectively. In conducting these growths, TSe was varied ranging from 135 to 160 °C. The growth conditions used for the growth of the MnSe1−xTex samples from Sample #5 to #9 are listed in Table 1. Fig. 1(a) displays the θ/2θ scans of all five MnSe1−xTex samples, showing that the angular values of their two layer peaks increase with TSe. Sample #8 is a sample closely lattice-matched with the ZnTe buffer and its finer (0 0 4) scan is given in Fig. 1(b), which allows us to resolve the MnSe0.53Te0.47 layer peak from the neighboring ZnTe peak through data fitting, After performing data fitting on high-resolution scans of the layer peak regions of all the scans shown in Fig. 1(a), the plane spacing along [0 0 1] of all the MnSe1−xTex samples and their integrated (0 0 2)/(0 0 4) peak area ratios of the two layer peaks were obtained and they are displayed in Table 2. All the integrated peak area ratios of the five MnSe1−xTex
Fig. 2. Lattice plane spacing along [0 0 1] verses Te composition of MnSe1−xTex samples with a fitted line. The meaning of the two dash lines are given in the text. 21
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of 52% instead of 43%. These seemingly inconsistence may be explained by the uncertainties of the XPS measurements as indicated by the horizontal error bars of the data points of the current work as shown in Fig. 2. An investigation on whether a lattice distortion occurs in our MnSe1−xTex films was conducted on Sample #9 by performing an ω − 2θ scan for an asymmetric (2 2 4) Bragg reflection and the resulting broad scan profile is displayed in Fig. 3 with the 2θ values calibrated using the standard 2θ value of the GaAs (2 2 4) peak. Through performing data fitting on the overlapping ZnTe (2 2 4) buffer peak and MnSe0.73Te0.27 (2 2 4) layer peak as shown in the left of Fig. 3, the 2θ value of the MnSe0.73Te0.27 (2 2 4) layer peak was determined to be 78.4° corresponding to a [2 2 4] lattice spacing of 1.219 Å that is larger than the value of 1.209 Å as expected from a cubic unit cell, indicating that its lattice is indeed distorted in a way that the lattice planes along the [0 0 1] direction is compressed while that along the [2 2 4] direction is being stretched. We have performed a transformation from the rectangular cuboid unit cell of Sample #9 using the measured plane spacing values along [0 0 1] and [2 2 4] to a cubic unit cell assuming constant volume (this assumption is valid as the distortion due to nonfully relaxation for all the five MnSe1−xTex samples is in fact quite small) [20]. The analysis of this transformation yields a lattice constant for the corresponding cubic unit cell of Sample #9 to be 6.02 Å, which is quite close to the extrapolated value of 6.03 Å obtained from Vegard’s law based on the reported lattice constants of MnSe and MnTe [4], and also close to the value of 6.06 Å as estimated from the data of a previous
Fig. 3. HRXRD ω − 2θ asymmetric (2 2 4) scan of Sample #9 with data fitting performed for the overlapping ZnTe (2 2 4) and MnSe0.73Te0.27 (2 2 4) peaks.
(0 0 1) lattice plane spacing smaller than the lattice constants predicted by the Vegard’s law, while samples with Te composition more than 43% (like Samples #4, #5, #6 and #7) display an opposite characteristic. This in fact is consistent with the expectation from the thin film effect that a MnSe1−xTex thin film grown on a ZnTe buffer will be compressed if x < 0.43 and will be stretched out if x > 0.43 as their lateral lattice parameters will be the same as that of ZnTe before lattice relaxation is initiated and will not reach a cubic structure until fully relaxation is achieved. It should be pointed out that the data point of Sample #8 doesn’t seem to agree with this characteristic behavior and the horizontal dashed line meets with the fitted solid line at a Te composition
Fig. 4. Results of structural characterization of Sample #10: (a) HRXRD θ/2θ scan of the (0 0 4) overlapping peak; (b) cross-sectional TEM image of this sample; (c) high-resolution cross-sectional TEM image of the junction between the MnSe0.49Te0.51 layer and ZnTe buffer layer; and (d) the corresponding electron diffraction pattern of (c). 22
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data, we present a direct comparison between the cross-sectional TEM image of Sample #10 as shown in Fig. 4(c) with the simulated TEM lattice images for zincblende and rocksalt lattice structures given by Sanchez et al. [22], which offers another evidence that the MnSe1−xTex layer of Sample #10 is indeed in zincblende phase. The Te composition, x, of Sample #10 has been determined by XPS measurements to be around 51%. The MnSe0.49Te0.51 layer of Sample #10 offers the dream barrier layer for constructing a double-barrier MnSe0.49Te0.51/ZnTe/ MnSe0.49Te0.51 RTD device that enjoys perfect lattice match. We have fabricated such a device with an active area (S) around 7.5 mm2. Its structure is shown in Fig. 5(a). The p-type doping of the four ZnTe:N layers in this structure was handled using an Oxford Applied Research nitrogen atom/radical beam source (Model: MPD21) with the control of the nitrogen flow rate to reach two different doping levels (p+ and p−). Fig. 5(b) displays the observed room-temperature I-V characteristics of this device, showing a negative differential resistance (NDR) signature. The device displays a peak current density (Jp) about 0.097A/cm2, a peak-to-valley current ratio of 3.30 and a negative differential resistance (ΔV/ΔI) of about 29.5 Ω. The appearance of NDR characteristics shows its potential as a THz emitter device. Detailed studies on the transport properties of this RTD device are underway in our laboratory and the results will be reported elsewhere. 4. Conclusion In summary, with the aim of fabricating a zincblende MnSe1−xTex/ ZnTe heterostructure with perfect lattice match, the factors that determine the crystalline phase of MBE-grown MnSe1−xTex thin films were investigated. Our studies have found that direct growth of MnSe and MnTe on GaAs (0 0 1) substrates only results in their stable rocksalt and hexagonal phase, respectively. With the assistance of a ZnTe buffer, MnSe1−xTex thin films in zincblende phase were successfully fabricated with Te composition covering a range from 0.27 to 1. The thin film effect on the distortion of the alloy lattice has been addressed and in particular we have provided detailed studies on the MnSe0.73Te0.27 thin film. The results from HRXRD, HRTEM and electron diffraction demonstrate that a MnSe1−xTex/ZnTe heterostructure with perfect lattice match is indeed achievable. We have fabricated a double-barrier MnSe0.49Te0.51/ZnTe/MnSe0.49Te0.51 RTD device with a perfect lattice match and it indeed shows an NDR signature, which shines light on the realization of a novel THz emitter device.
Fig. 5. Double-barrier MnSe0.49Te0.51/ZnTe/MnSe0.49Te0.51 RTD device: (a) device structure and (b) its room-temperature I-V characteristics showing the NDR characteristics.
theoretical study [21]. These results confirm that the lattice plane spacing values along the [0 0 1] direction versus the Te composition of MnSe1−xTex determined in this study are in fact consistent with the corresponding lattice constants reported previously [5] after considering the distortion due to thin film effect. In order to obtain a MnSe1−xTex sample with even better lattice matching with ZnTe, Sample #10 was grown using a Se cell temperature of 155.6 °C, which is just 0.4 °C lower than that used for Sample #8, while keeping all other growth conditions the same as those of Sample #8. A complete HRXRD θ/2θ scan for Sample #10 is displayed in Fig. S4 in supplementary data. Fig. 4(a) displays a high-resolution θ/ 2θ scan of the (0 0 4) overlapping peak of Sample #10, in which only a single peak is identified for the combination of the ZnTe buffer peak and the epitaxial alloy layer peak, indicating that indeed a perfect lattice matching has been realized. Fig. 4(b) shows a cross-sectional TEM image of Sample #10. Further confirmation of the perfect lattice matching of Sample #10 comes from the cross-sectional high-resolution TEM image of Sample #10 and its corresponding electron diffraction pattern as shown in Fig. 4(c) and (d), respectively. As shown in Fig. 4(c), besides some regions in the image are out of focused due to the limitation of our TEM system, the two lattices of MnSe1−xTex and ZnTe are indistinguishable. This is further confirmed from the electron diffraction pattern taken across both lattices as displayed in Fig. 4(d), in which only a single pattern is present. In Fig. S5 of the supplementary
Acknowledgments This work was mainly supported by the Science and Technology Development Fund of Macao Special Administrative Region, People’s Republic of China (Project No. 015/2013/A1) and partially supported by the Department of Physics, The Hong Kong University of Science and Technology, Hong Kong SAR, China. Appendix A. Supplementary material See supplementary data for cross-sectional SEM images of a pure Se and a pure Te layer grown on GaAs ( 0 0 1 ) substrates, HRXRD θ/2θ scans of Sample #1 – #4, a complete HRXRD θ/2θ scan for Sample #10, and evidence of the zincblende phase of Sample #10. Supplementary data to this article can be found online at https://doi.org/10.1016/j. jcrysgro.2019.01.029. References [1] R.D. Vispute, V. Talyansky, S. Choopun, R.P. Sharma, T. Venkatesan, M. He, X. Tang, J.B. Halpern, M.G. Spencer, Y.X. Li, L.G. Salamanca-Riba, Appl. Phys. Lett. 73 (3) (1998) 348, https://doi.org/10.1063/1.121830. [2] M. Asada, S. Suzuki, N. Kishimoto, Jpn. J. Appl. Phys. 47 (2008) 4375, https://doi.
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