Characterization of 3C-SiC/6H-SiC Heterostructures Grown by Vacuum Sublimation

In I-V, EL characterization it was concluded that during the growt h on n-type 6H-SiC substrate of 3C-SiC layer the not intentionally doped regions of p-t ype conductivity formed. It thus appears that two types of pn structures are being formed identifi ed by the polarity of voltage applied to Ni contact on the surface of the epitaxial layer and di splaying different spectra of injection EL. In the EL spectra of the first structure type a peak at h νmax= 2.3 eV is present which, similar to the peak due to free exciton annihilation in 3C-SiC. In the EL spectra of the second structure type two peaks are observed of the same origin as the peaks identified in 6H-SiC pn structures and one more, longer wavelength peak at h νmax between 2.35 and 2.5 eV. TEM revolved that the intermediate layer belongs to 6H polytype. According to EBIC characterization the intermediate layer in studied sample was p-type conductivity. Introduction. Cubic silicon carbide (3C-SiC) is at present a topic of considera ble interest for semiconductor electronics due to the highest charge mobility among all SiC poly types. At certain conditions a growth of α-SiC epilayers on basal orientated substrates of 6H-SiC is a ccompanied by growth of islands of 3C-SiC or transformation of the whole 6H-SiC epilayer into 3C-SiC epilayer. Deposition of 3C layer corresponds to the island growth resulting in double positi on boundaries (DPBs) and stacking fault (SF) formation. Modifying growth conditions in the grow th zone it is possible to obtain various density of DPBs and SF. In recent years, considerable progress has been achieved in the sublimation epitaxy technology. High-quality epitaxial structures of the (n)3C-SiC-(n)6H-SiC type with low densities of DPBs and SF were obtained [1]. The area of n-3C-SiC twin was up to 25 mm. Electrical characteristics of the pn structures were close to those of high perfection pn homostructures based on bulk 3C-SiC [2]. Within the framework of this paper the questions to be discussed will be concerned with epilayer having high density of DPB’s to understand the connection of this defect with electrical characteristics of the structure. Experimental. Epitaxial 3C-SiC layer was grown on 6H-SiC substrate by typical for n-type epitaxy sublimation process in vacuum. Very high density of DPB’s was confirmed by X-ray topography (Fig.1, see also [3]). After that a Ni film was deposited by magnetron sputtering on the surface of the epitaxial layer at 300 C. Ni spots 100microns in diameter were formed by photolitography. Backside ohmic contact (to the Materials Science Forum Online: 2003-09-15 ISSN: 1662-9752, Vols. 433-436, pp 293-296 doi:10.4028/www.scientific.net/MSF.433-436.293 © 2003 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-12/03/20,10:56:47) substrate) was formed by capacitor discharge and using a In-Ga alloy. Current-voltage (I-V) and electroluminescence (EL) characteristics of the diodes obtained by this way were investigated. Then the sample was cut into a few pieces. One piece was used f or forming mesa-structures by reactive ion-plasma etching. I-V and EL characteristics of the diode mesa -structures of height 4 and 16 microns were taken. The 3C-SiC6H-SiC structure cross sect ions were studied on other pieces by methods of electron beam induced current (EBIC) and secondary e lectrons (SE) in a JSM-50A scanning electron microscope and by transmission electron microscopy (TEM). Results of characterization. I-V characterization. Before mesa-structure formation and after the first etching (t o the depth of 4 microns) I-V characteristics for both polarities of voltage appl ied to the Ni contact are similar and ohmic (with resistance of about 1 MOhm) up to voltages of about 1V. At higher voltages, up to about 100 V (Fig.2, left), dependence is power-law and close to quadratic one. Second etching (to the depth of 16 microns) affected the I-V characteristics (the ohmic-law dependence remained but the resistance increased to about 200 MOhm) and the I-V characteri sti s measured at different voltage polarities became different. It is necessary to emphasi ze the appearance of exponential dependence of the current on voltage I=I oexp(qU/(nkT)) (with n=2 and I o=5*10 -16 A at T=296K) at negative voltage polarity (minus at the Ni contact; region (a) at Fig.2, left). At the highest currents (30-100 mA) it is possible to observe S-type switching. Fig.2. Current-voltage characteristics of structur e N16-11 before etching (curves 1 and 2), after etc hing to a depth of 4 microns (curves 3 and 4), after etching to a depth of 16 microns (curves 5 and 6); curves 1, 3 and 5 ( empty symbols) are the positive voltage polarity (plus to Ni conta ct), curves 2, 4 and 6 (full symbols) are the negat ive voltage polarity (minus to Ni contact) (left picture); EL spectra of structures at positive voltage polarity (I=40mA, U =127V (structure N21-13, curve 1) and I=15mA, U=100V (structure N1611, curve 3) and at negative voltage polarity (I=-2 6mA, U=104V (structure N21-13, curve 2); injection EL spec trum of 3C-SiC pn structure (curve 4 [2], J=50A/cm ); injection EL spectrum of 6H-SiC ion implanted pn structure (c urve 5, J=14A/cm) (right picture). From analysis of the I-V characteristics it is possible to conclude that: a) exist at least 2 diode structures connected in series in opposite directions; b) Ni contact to SiC probably forms a Schottky diode to the layer of p-type conductivity; this conclusion is supported by the shape of the I-V characteristics f or Schottky diode which was formed by a point contact (provided with a needle) to the surface layer; c) current spreading to the area outside of Ni-metallization is observed before e tching. EL characterization. For either polarity of the voltage applied to the Ni contact an electroluminescence was observed. All structures are character ized by non-uniform EL over the diode area. Moreover, before etching EL could be observed outside the s tructure area, sometimes spreading over a distance of about the structure diameter (100 micr ons); this EL more often 1.0E-3 1.0E-2 1.0E-1 1.0E+0 1.0E+1 1.0E+2 Voltage, V 1.0E-9 1.0E-8 1.0E-7 1.0E-6 1.0E-5 1.0E-4 1.0E-3 1.0E-2 1.0E-1 C ur re nt , A 1 (+)