Properties Of Epitaxial Cdte On Si(lll) With a (Ca,Ba)F 2 Buffer Layer
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PROPERTIES OF EPITAXIAL CdTe ON Si(lll) WITH A (Ca,Ba)F
2
BUFFER LAYER
H. Zogg, S. Blunier AFIF at Swiss Federal Institute of Technology, ETH-H6nggerberg HPT, CH-8093 ZUrich, Switzerland. ABSTRACT Epitaxial CdTe has been grown onto Si(lll) wafers by MBE with the aid of a composition graded (Ca,Ba)F 2 buffer layer to surmount the large misfit of 19%. Untwinned CdTe layers with smooth surfaces, narrow X-ray lines and strong photoluminescence with a narrow near band edge peak were obtained. The results indicate a comparable structural quality to well known CdTe layers on sapphire, InSb or GaAs used as buffers to grow (Hg, Cd)Te for IR-device applications. In addition, the CdTe layers are near strain free despite a large thermal expansion mismatch. This is most probably due to dislocations which are able to move along the fluoride/Si interface even after growth and down to near room temperature. T
NTRODUCTION
Single crystalline CdTe layers are desired for different device applications including their use as buffers to grow narrow gap Hgl_xCdxTe for IR-sensor array fabrication. Due to the difficulty of obtaining suitable bulk CdTe substrates, epitaxial CdTe layers have been grown on foreign substrates like InSb, sapphire or GaAs by different groups [1-5]. It was found that the structural quality of such layers was superior to bulk material. Strong photoluminescence with narrow near band-edge peaks were observed, and X-ray rocking curves indicated that the layers were free of small angle boundaries. However, the most interesting substrate for, device applications would be Si because of costs, mechanical strength and possibility to fabricate VLSI for signal processing. So far, CdTe layers grown by molecular beam epitaxy (MBE) on Si showed a nonspecular surface reflectance and no observable photoluminescence at 77K [6]. This is at least partly due to the large lattice mismatch of almost 20% between CdTe and Si. Using graded CaF 2 -SrF 2 -BaF 2 buffer layers, we found that rather large mismatches (up to 141-) can be surmounted by depositing (near) lattice matched CaF 2 onto the Si substrate and by changing the composition gradually or in steps toward BaF 2 [7,8]. We have already applied this technique to grow PbTe and PbSnSe on Si [9], and have used the layers to fabricate the first intrinsic infrared sensors in epitaxial narrow gap materials on Si. These sensors were sensitive up to -10 um and operated near the room temperature photon background noise limit [9]. In the following, we will describe the properties of epitaxial CdTe on Si we have grown using the same fluoride buffer layer technique. Inital results of these experiments have already been published [10] elsewhere. GROWTH About 70 A CaF 2 were deposited onto precleaned Si(lll) substrates at about 7000C, followed by BaF 2 of about 2000 A thickness and gradually reduced growth temperatures down to approx. 4000C. Typical growth rates were in the range 0.5-2 A/sec. More details of this growth sequence are described in refs. [8] and [10]. CdTe was then deposited f
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