Study of Deep Levels in LT-GaAs Materials and SI-GaAs Wafers by an Improved Thermoelectric Effect Spectroscopy
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STUDY OF DEEP LEVELS IN LT-GaAs MATERIALS AND SIGaAs WAFERS BY AN IMPROVED THERMOELECTRIC EFFECT SPECTROSCOPY Z.C. Huang and C.R. Wie Department of Electrical and Computer Engineering, State University of New York at Buffalo, Bonner Hall, Buffalo, NY 14260
ABSTRACT We have developed a simpler and more reliable method of thermoelectric effect spectroscopy (TEES) by eliminating the second heater in the technique. We have applied this method to the deep level studies in semi-insulating(SI) GaAs epitaxial layers grown at a low temperature by molecular beam epitaxy (LT-GaAs) and SI-undoped GaAs, Cr-doped GaAs. We have found that the electrical contacts on front and back surfaces of the sample are more reliable for the TEES measurement than both contacts made on the same surface. In this contact arrange"ment,the temperature difference of about 1-2K between the back and front surfaces was enough to produce a clear and reliable TEES data, without the need for a second heater. The results obtained by TEES are consistent with the results obtained by photo-induced current transient spectroscopy (PICTS) and by thermally stimulated current (TSC) measurements. The TEES results clearly distinguish between the electron traps and the hole traps. We will discuss the results on the various semi-insulating GaAs samples and the advantages and limitations of the TEES technique.
I. INTRODUCTION There exist several spectroscopic techniques for studying deep levels in semi-insulating (SI) III-V semiconductors. Thermally stimulated current (TSC)['] has a relatively small temperature range due to its large dark current when temperature is increased beyond a certain value (typically 250K). Photoinduced current transient spectroscopy (PICTS)t 2J is effective in determining the trap energy levels and the capture cross sections. These techniques, however, do not give a clear distinction between the electron and hole traps. Although it was claimed that under certiain conditions, the type of a given trap could be determined by PICTS and/or TSC data, there was always some ambiguity and not all traps could be determined. Thermoelectric effect spectroscopy (TEES) was recently developed by Santic and Desnica13 ]. They added a second heater on the sample to create temperature gradient without any external applied bias. The thermal gradient induced enough electrical current with its sign corresponding to the type of dominant free carriers, and allowing the determination of the trap types.
II. IMPROVED METHOD OF THERMOELECTRIC EFFECT SPECTROSCOPY The TEES arrangement is shown in Fig. I(a). We have found that a simple electrical connection between the front and back surfaces of the semi-insulating sample allowed the flow of thermoelectric current on the order of several picoamperes due to the temperature difference between the two surfaces on the order of IK, as shown in Fig. 1(b) for a 500g.tm-thick undoped SI-GaAs wafer. With this arrangement we could eliminate the need for a second heater and the possibility of an accidental reversal of temperature
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