Abstract
In this study we present a method for measuring bulk traps using deep-level spectroscopy techniques in metal–insulator–semiconductor (MIS) structures. We will focus on deep-level transient spectroscopy (DLTS), although this can be extended to deep-level optical spectroscopy (DLOS) and similar techniques. These methods require the modulation of a depletion region either from a Schottky junction or from a highly asymmetric p–n junction, junctions that may not be realizable in many current material systems. This is the case of wide-bandgap semiconductor families that present a doping asymmetry or have a high residual carrier concentration or surface carrier accumulation, such as InGaN or ZnO. By adding a thin insulating layer and forming an MIS structure this problem can be circumvented and DLTS/DLOS can be performed under certain conditions. A model for the measurement of bulk traps in MIS structures is thus presented, focusing on the similarities with standard DLTS, maintaining when possible links to existing knowledge on DLTS and related techniques. The model will be presented from an equivalent circuit point of view. The effect of the insulating layer on DLTS is evaluated by a combination of simulations and experiments, developing methods for the measurement of these type of devices. As a validation, highly doped ZnO:Ga MIS devices have been successfully characterized and compared with a reference undoped sample using the methods described in this work, obtaining the same intrinsic levels previously reported in the literature but in material doped as high as cm−3.
Highlights
Deep-level spectroscopy techniques are powerful tools for assessing the presence of electrically active defects in semiconductor materials [1, 2]
Its principle of operation was extended later to deep-level optical spectroscopy (DLOS) [2], where the emission of carriers from the traps is obtained by optical excitation, but where the physical processes involved are generally the same
While with deeplevel transient spectroscopy (DLTS) only the bandgap region below ∼1 eV from the conduction band can be probed in n-type material, with DLOS the rest of the bandgap down to the minority carrier band can be analyzed
Summary
Deep-level spectroscopy techniques are powerful tools for assessing the presence of electrically active defects in semiconductor materials [1, 2]. 50 (2017) 065104 of acceptors in n-type material (or donors in p-type material) [3,4,5,6,7] Both DLTS and DLOS rely on the use of specific devices in which a depletion region is well defined, such as in highly asymmetric p–n junctions (where the depletion region drops on the lightly doped side of the junction) or Schottky junctions. By extending the knowledge of DLTS/ DLOS for bulk deep-level analysis to MIS devices in a controlled way, and by properly quantifying the new effects this insulating layer generates on the techniques, a wider range of materials could be analyzed. The processes of carrier emission from bulk states were analyzed, obtaining very similar expressions to those from DLTS They failed to point out the similarities between the measurement processes for MIS and Schottky/ p–n diodes, despite the presence of an insulating layer. Our work further expands and simplifies the analysis of DLTS in MIS devices, comparing the obtained spectra with those measured in a non-insulator-affected sample, which is of great importance in materials with native insulating layers that are difficult to characterize and for the measurement of semiconductors with a high carrier concentration (intrinsic or from doping), in which the design of the experiment is crucial to maximize the sensitivity of the measurement
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