Structural and electrical properties of Fe films grown on InP substrates

Thin Solid Films 338 (1999) 161±164

Structural and electrical properties of Fe ®lms grown on InP substrates T.W. Kim a,*, D.U. Lee a, Y.S. Yoon b, Y.H. Shin c, C.O. Kim d a

Department of Physics, Kwangwoon University, 447-1 Wolgye-dong, Nowon-ku, Seoul 139-701, South Korea Division of Ceramics, Korea Institute of Science and Technology, P. O. Box 131, Cheongryang, Seoul, South Korea c Department of Electronic Engineering, Kyungwon University, Bokjung, Sujung Gu, Seongnam City, Kyung ki-do, South Korea d Department of Physics, Hanyang University, Seoul 100-715, South Korea b

Received 31 October 1997; accepted 10 July 1998

Abstract Fe thin ®lms were grown on p-InP (100) substrates by ion-beam-assisted deposition with the goal of producing sharp Fe/p-InP interfaces with enhanced rectifying properties. X-ray diffraction measurements showed that an Fe layer grown on an InP substrate was polycrystalline, with a relatively sharp interface as seen by Auger electron spectroscopy. Transmission electron microscopy also con®rmed the polycrystalline character of the Fe layer and showed that it consisted of small domains. Current±voltage (I±V) measurements performed on Fe/InP diodes revealed good recti®cation. The Schottky barrier height and the diode ideality factor obtained from these I±V measurements were 0.64 and 1.2 respectively. These results indicate that Fe ®lms grown on p-InP (100) at room temperature can be used for the fabrication of stable metal gates in new kinds of InP based metal±semiconductor ®eld-effect transistors. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Auger electron spectroscopy; Inidium phosphide; Iron; Transmission electron microscopy

1. Introduction The growth of magnetic thin ®lms on semiconductor substrates has attracted much attention for both scienti®c and technological reasons [1±6]. Since the interfacial properties of magnetic metal ®lms grown on III±V semiconductors play an important role in the inherent stability and reliability of the contacts, studies of the physical properties of metal/semiconductor heterointerfaces should be carried out in order to utilize these heterostructure more effectively. Fe/III±V semiconductor heterostructures present considerable interest owing to their many potential applications in new hybrid device structures such as metal±semiconductor ®eld-effect transistors [7±10]. Even though many works concerning Fe/GaAs heterostructures have been reported [7±10], to the best of our knowledge, Fe/InP heterostructures with high-quality interfaces have not yet been investigated. Furthermore, InP compound semiconductors are particularly interesting for applications in optoelectronic and high-speed electronic devices [11]. Fe/InP heterostructures with high-quality interfaces and good rectifying properties are necessary for the fabrication of these devices. Electronic devices using both Fe ®lms and InP substrates seem very promising but greater knowledge of the Fe/InP * Corresponding author. Tel.: +82-1-08291-05234; fax: +82-02-3089609

heterointerfaces is required. So, investigations of the structural and electrical properties of Fe ®lms grown on InP substrates are very important. This paper reports the structural and electrical properties of Fe ®lms grown on p-InP (100) substrates by ion-beam deposition (IBD) at room temperature. X-ray diffraction (XRD) measurements were performed to check the crystallinity of the grown layer, and Auger electron microscopy (AES) measurements were carried out in order to characterize the interface composition and also the quality of the grown ®lms. Transmission electron microscopy (TEM) measurements were performed to investigate the atomic structure of the Fe layer and the Fe/InP interface. Current±voltage (I±V) measurements were performed to investigate the possibility of producing Schottky diodes and to determine both their barrier height and ideality factor. 2. Experimental details Polycrystalline Fe with purity of 99.99% was used as a target material and was precleaned by repeated sublimation. The carrier concentration of the Zn-doped p-InP substrates with a (100) orientation used in this experiment was approximately 1 £ 1016 cm 23. The substrates obtained from Sumitomo were degreased alternately in warm acetone and trichloroethylene (TCE) three times, etched in a

0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(98)01063-3

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Fig. 1. An X-ray diffraction pattern of an Fe ®lm grown on an InP (100) substrate.

bromine±methanol solution mechanochemically, rinsed in deionized water thoroughly, etched in a mixture of H2SO4, H2O2, and H2O (4:1:1) at 408C for 10 min, and rinsed in TCE again. After the samples were cleaned chemically, they were mounted on a susceptor in the growth chamber. After the IBD chamber was evacuated to 1 £ 1026 Torr, the deposition was done at a substrate temperature of 300 K. In this case, a focused Ar 1 beam was used to sputter the Fe-metal target. The Fe deposition was done at a system pressure of 2 £ 1024 Torr, and the typical deposition rate Ê s 2 1. The discharge voltage was was approximately 1.8 A 400 V, and the ion gun was a cold hollow cathode. The bombarding ion energy and the ion-beam current measured by a Faraday cup were 1500 eV and 60 mA respectively. The XRD measurements were performed using a Rigaku

Fig. 3. Auger depth pro®le of the Fe/InP structure.

D/Max-B diffractometer with Cu Ka radiation. The AES measurements were performed on the as-grown ®lms using a Perkin-Elmer phi 400 scanning Auger microprobe. The TEM observations were performed in a JEOL 200CX transmission electron microscope operating at 400 kV. For the TEM measurements, the samples were thinned down to approximately 30 mm using diamond paper, and then milled by argon ions at liquid-nitrogen temperature until electron transparency was reached. The I±V characteristics were measured using a HP 4140B picoammeter with a ramp rate of 0.05 V s 2 1. The applied voltage was swept to current levels of 10 2 2 A during the d.c. measurements. Fe/InP Schottky barrier diodes were prepared by the mesa etching technique. Indium ohmic contacts were deposited and soldered onto the front and the rear surfaces of the samples. 3. Results and discussion

Fig. 2. Auger electron spectroscopy results obtained from the Fe/InP structure. The lower curve was obtained at the Fe surface and the middle Ê , respecand the upper curves were obtained at depths of 500 and 5000 A tively.

The as-grown Fe ®lms prepared by IBD had mirror-like surfaces, as con®rmed using Normarski optical microscopy and scanning electron microscopy (SEM) measurements. Fig. 1 shows the XRD pattern for an Fe/InP heterostructure. The (110) and the (200) Ka1 diffraction peaks of the Fe together with the InP (200) peak are clearly observed. These results indicate that an Fe polycrystalline ®lm can be grown on an InP (100) substrate using the IBD technique. The results of the AES measurements showed that the asgrown ®lm consisted of Fe and carbon at the sample surface Ê , as shown in the lower and and of only Fe at a depth of 500 A the middle curves of Fig. 2 respectively. The Fe/InP sample Ê , as shown in the consisted of In and P at a depth of 5000 A upper curve of Fig. 2. The existence of the carbon and oxygen at the Fe ®lm surface could be due to typical contamination from the atmosphere before AES analysis. The Fe/InP compositional pro®le shows a thin region of excess In in the Fe ®lm layer near the InP substrate. Fig. 3

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Fig. 4. A bright-®eld transmission electron microscopy image of the Fe/InP structure.

shows that the compositional pro®le of the heterointerface between the Fe and the InP is relatively sharp. The few In and P atoms penetrate the Fe layer owing to partial dissolution in spite of the relatively ¯at heterojunction. Even though an interdiffusion or intermixing problem appears at the Fe/p-InP heterostructure interface, the interfacial reaction between the Fe and the InP due to thermal damage can be neglected [12-14]. The thickness of Fe is approximately Ê , and this value is in good agreement with those 3000 A determined from cross-sectional SEM and TEM measurements. The bright-®eld TEM image in Fig. 4 shows the interfacial layer between the top Fe layer and the bottom InP substrate. The Fe thin layer has a very smooth surface. The electron diffraction pattern from the Fe thin layer shows very weak spots and serials of rings, as shown in Fig. 5. These rings reveal the polycrystalline character of

Fig. 5. An electron diffraction pattern from transmission electron microscopy of the Fe layer.

Fig. 6. An electron diffraction pattern from transmission electron microscopy of the Fe/InP structure, with (hlk)F and (hlk)I corresponding to the Fe and InP indexes of the reciprocal vectors respectively.

the Fe layer. Also, these results are consistent with the very small particle scale of the ®lm as seen in the bright-®eld TEM image. Selected-area electron-diffraction patterns from TEM measurements at the Fe/InP heterointerface are shown in Fig. 6. The strong and regular spots originate from the InP substrate, and the weak irregular spots and the diffusion of the rings are related to the Fe polycrystalline ®lm and the amorphous interfacial layer [15] respectively. A threedimensional interfacial layer formed in the initial stage of ®lm growth prevents the growth of an Fe epitaxial ®lm [16]. A high-resolution TEM image of the Fe/p-InP structures is shown in Fig. 7. The results of the high-resolution TEM measurements indicate that an interfacial amorphous layer was formed at the Fe/InP interface and that the Fe thin ®lm was polycrystalline. The creation of the Fe polycrystalline layer might originate from the lattice mismatch between Fe and InP and from the existence of an amorphous interfacial layer prior to the growth of the Fe layer. However, during the IBD growth, the Fe atoms have a kinetic energy, so that formation of the amorphous layer and intermixing can take place, as con®rmed by the Auger depth pro®le. In addition to the X-ray, AES, and TEM measurements, I±V measurements were taken to investigate the possibility of a metal/semiconductor diode using the Fe layer. The I±V characteristics of the Fe/InP diode show good recti®cation at low current densities. The barrier height w B and the ideality factor n of the Fe/InP Schottky diodes were determined from the forward I±V characteristics using the conventional expression based on a thermionic emission mechanism [17]. The values of the Schottky barrier height and the diode

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produce high-quality Fe ®lms for application in electronic devices such as novel kinds of transistors.

Acknowledgements This work was supported by the Basic Science Research Institute Program, Ministry of Education, 1997, Project no. BSRI-97-2423. We would like to thank Dr. J.Y. Lee for TEM measurements.

References Fig. 7. High-resolution transmission electron microscopy image of the Fe/ InP structure.

ideality factor for zero bias at room temperature are 0.6 eV and 1.2 respectively. The value of barrier height obtained from the Fe/p-InP grown at room temperature is larger than that determined from similar Fe/n-InP structures [18]. Detailed electrical properties of the Fe/p-InP diodes will be published elsewhere [19]. 4. Summary and conclusions The results of XRD, AES, and TEM measurements showed that Fe thin ®lms grown on InP substrates by IBD at room temperature were polycrystalline. AES measurements showed that the Fe ®lms had uniform composition throughout the layers and that the Fe/InP heterointerface was relatively sharp. TEM measurements showed that an amorphous interfacial layer was formed between the polycrystalline Fe layer and the InP substrates. The results of the I±V measurements at room temperature showed a good recti®cation, and the barrier height and the ideality factor of the Fe/p-InP diode were 0.64 eV and 1.2 respectively. Even though detailed studies on electrical properties such as deep levels of Fe/InP heterostructures remain to be carried out, the present results indicate that it should be possible to

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