Structural and magnetic properties of molecular beam epitaxy grown chromium selenide thin films

Abstract

Chromium selenide thin films were grown epitaxially on Al2O3(0001) and Si(111)-(7×7) substrates using molecular beam epitaxy (MBE). Sharp streaks in reflection high-energy electron diffraction and triangular structures in scanning tunneling microscopy indicate a flat smooth film growth along the c-axis, and is very similar to that from a hexagonal surface. X-ray diffraction pattern confirms the growth along the c-axis with c-axis lattice constant of 17.39 Å. The grown film is semiconducting, having a small band gap of about 0.034 eV, as calculated from the temperature dependent resistivity. Antiferromagnetic nature of the film with a Néel temperature of about 40 K is estimated from the magnetic exchange bias measurements. A larger out-of-plane exchange bias, along with a smaller in-plane exchange bias is observed below 40 K. Exchange bias training effects are analyzed based on different models and are observed to be following a modified power-law decay behavior. as observed from neutron diffraction studies [6][7][8][9]. Because of Cr vacancies in alternate layers, the moment associated with Cr atoms located on two different layers are different due to different neighboring environment and this leads to the complexity in the magnetic structure below TN.Previously, chromium selenide systems have been studied to investigate their suitability as thermoelectric material for intermediate-temperature applications [10][11][12][13][14], intermediate temperature power generation [13], electrochemical sensors [15], etc. Several groups have studied the structural, magnetic, electrical and thermoelectric properties of single crystal Cr2+xSe3-x compounds grown using solid state reaction method [10][11][12][13]16], soft chemical and hydrothermal synthesis [15,17,18] and chemical vapor transport method [4,14,[19][20][21]. However, the studies focus mostly on the improvement in thermoelectric properties of transition-metal-doped bulk samples of Cr2Se3. The epitaxial growth and different physical properties of Cr2Se3 thin films are yet to be explored in detail. Molecular beam epitaxy (MBE) is a highly specialized technique used to grow ultra-high purity large-area epitaxial thin films with abrupt interfaces and with precise control over their thicknesses. Compared to other growth techniques, MBE offers greater control to incorporate dopants in thin films. This makes it even more suitable growth method, as the electrical, magnetic and thermoelectric properties of this material system can be largely varied with addition of transition-metal/chalcogen dopants [1, 9-13, 21,22].In this work, we report the epitaxial growth of Cr2Se3 thin films under ultra-high vacuum (UHV) directly on Al2O3(0001) and Si(111)-(7×7) surfaces using MBE. Interestingly, we show that the growth occurs along (001) direction (c-axis). We present the details of growth, structural, electrical and magnetic properties characterized by several in situ and ex situ techniques, e.g., reflection high energy electron diffraction (RHEED), x-ray diffraction (XRD), scanning...

Growth of wafer-scale chromium sulphide and selenide semiconductor films

“…It demonstrates that the resistance of Cr 2 S 3 film increases with the decreasing temperature, presenting a typical semiconductor behaviour (figure 5 This result is in keeping with those of CVD-grown Cr 2 S 3 samples [27,45,46]. Although some reported results exhibit metallic behaviour of Cr 2 Se 3 [38], the measurement of the temperature-dependent resistance is consistent with the semiconductor characteristics [43,48].…”

First-Principles Calculations of the Spin-Dependent Electronic Structure and Strain Tunability in 2D Non-van der Waals Chromium Chalcogenides Cr2X3 (X = S, Se, Te): Implications for Spintronics Applications

“…Cr 2 X 3 (X = S, Se, Te) is in a family of metal-deficient NiAs-type structures. The crystal structure of both Cr 2 S 3 and Cr 2 Se 3 are rhombohedral (the space group R 3̅, 148), while Cr 2 Te 3 is trigonal (the space group P 31 c , 163). ,,,− While a Cr layer is sandwiched between X layers, which makes a backbone of the whole structure, a deficient Cr layer connects the backbone layers, as shown in Figure a,b. The Cr atoms are hexagonally coordinated in the ab plane, as shown in Figure c.…”

“…The narrow band gap of Cr 2 S 3 ( E g = 0.45 eV) agrees with previous experimental ( E g = 0.15 eV) , and computational results ( E g = 0.44 eV). Cr 2 Se 3 has been reported both as metallic , and semiconducting ( E g = 0.034 eV). Such discrepancy in conductivity might be attributed to the thickness, as shown in thickness dependence on the band gap in Cr 2 S 3 .…”

“…Cr 2 X 3 behaves as ferrimagnetic, antiferromagnetic, or fully compensated ferrimagnetic (FCFiM) and ferromagnetic, with X varying from S, Se, and Te, respectively. − Cr 2 S 3 and Cr 2 Se 3 have been studied for their low thermal conductivity as thermoelectric materials. , However, experimental studies on Cr 2 S 3 showed its ferrimagnetic and narrow gap semiconducting property with its thickness-dependent carrier-type (electrons and holes) transition. ,− Cr 2 Se 3 exhibits AFM and metallic property. ,− However, there has been a report of semiconducting Cr 2 Se 3 and ferri-like behaviors because of the noncollinear spin behavior of the magnetic structure. , Cr 2 Te 3 is metallic with FM order. − Recent studies have shown noncollinear spin order, ultrahigh coercivity, and tunable critical temperature. , Although there have been some efforts made in the computational study of these materials, ,,,− a systematic approach for this unique covalently layered structure of Cr 2 X 3 is still lacking.…”

“…26,31−34 However, there has been a report of semiconducting Cr 2 Se 3 and ferri-like behaviors because of the noncollinear spin behavior of the magnetic structure. 33,35 Cr 2 Te 3 is metallic with FM order. 36−38 Recent studies have shown noncollinear spin order, ultrahigh coercivity, and tunable critical temperature.…”