High resolution EL2 and resistivity topography of SI GaAs wafers

Abstract

The mesoscopic inhomogeneity of LEC grown semi-insulating (SI) GaAs wafers has been investigated with EL2' absorption topography (EAT), photoluminescence topography (PLT), point contact topography (PCT) and contactless resistivity mapping (COREMA). Significant progress with respect to sensitivity of EAT and lateral resolution of COREMA has been achieved. High resolution topograms of wafers cut from ingots subject to standard and modified annealing procedures are presented. Direct comparison of EL2' and resistivity topograms reveals significant differences in the mesoscopic contrast and a contrast reversal for modified annealing. These observations can be explained very satisfactorily by assuming mesoscopic inhomogeneity of the intrinsic acceptor concentration which is modified during annealing. A model involving generation of Ga vacancies by dissolution of As,, antisites and gettering of the interstitial As at precipitates is presented and discussed. A. IntroductionThe macro-and mesoscopic homogeneity of liquid-encapsulated Czochralski (LEC) SI GaAs wafers has been improved in recent years by systematic optimization of ingot annealing processes. But mesoscopic fluctuations of resistivity, related to the cellular distribution of dislocations, continue to stimulate research-and application-oriented investigations, requiring topographic optical and electrical characterization techniques with high sensitivity and lateral resolution. PLT has been widely used for qualitative homogeneity assessment, although the correlation between PL intensity and resistivity appears to be incidental and often is not observed at all. Hence, high resolution, high sensitivity topograms of the resistivity itself (obtained with PCT and COREMA) and of the EL2 concentration (obtained with EAT) are needed. Evaluation of these topograms not only qualifies the material, but also allows elucidating details in the defect redistributions and the resulting changes of the compensation processes. B. Experimental DetailsThe EL2' absorption topography [l] has been enhanced in sensitivity to obtain precise mapping of concentration variations down to 1% with a lateral resolution of 30 pm. Mesoscopic resistivity mappings with comparable lateral resolution can be achieved by PCT [2]. We have implemented this technique and corroborated its reliability by direct comparison with measurements of the Bergakademie Freiberg group [3] and with COREMA measurements. The non-contacting resistivity technique COREMA [4] thus far offered a lateral resolution of about 2.5 mm, insufficient for mesoscopic investigations. We have achieved an improvement of three orders of magnitude by designing a 80 pm diameter capacitive probe with upgraded measurement circuitry. Using the same step size of 30 pm as PCT, the topograms can be directly compared. For large cell structures in the mm range, the relative variation of conductivity is comparable for PCT and p-COREMA. As will be shown below, mesosocopic variations on a smaller scale encountered in LEC grown material are somewhat u...

Structural Defect Studies of Semiconductor Crystals with Laue Topography

“…Structural defects, especially dislocations, give rise to a locally enhanced concentration of dopants and the electrical active EL2 lattice defect [6] affecting the material's mesoscopic homogeneity. This can be studied ex-situ by infrared absorption topography (EL2 absorption topography, EAT) [7]. The low thermal conductivity (λ GaAs = 0.46 Wcm -1 K -1 @ T = 300K) of GaAs and its low critical shear stress near the melting temperature lead to the material's liability to thermoplastic relaxation of thermally induced stress by generation and multiplication of dislocations during crystal growth and subsequent thermal treatments, making it impossible to grow large diameter crystals free of dislocations [8].…”