Photocapacitance Studies Of Light Induced Changes in the Density of Gap States of N-Type Doped and Undoped Hydrogenated
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PHOTOCAPACITANCE STUDIES OF LIGHT INDUCED CHANGES IN THE DENSITY OF GAP STATES OF N-TYPE DOPED AND UNDOPED HYDROGENATED AMORPHOUS SILICON FILMS Avgerinos V. Gelatos") and J. David Cohen, Physics Department, University of Oregon, Eugene, OR. 97403.
ABSTRACT We have studied the photoinduced changes of the conduction bandtail states and dangling bond defects in a series of n-type doped hydrogenated amorphous silicon samples of different doping levels, using the techniques of drive level capacitance profiling and voltage pulse transient photocapacitance. Each film was examined in a series of partially annealed states of progressively higher anneal temperature following light soaking. A general decrease in the number of dangling bonds and an increase in the number of occupied bandtail states was observed with increasing anneal temperature. The observed changes are discussed with respect to previously proposed defect reactions for photoinduced changes in n-type doped films, and we conclude that none of them can explain the observed changes in the lower doped film. Second, we discuss the application of transient voltage pulse photocapacitance and transient junction photocurrent to the study of the energy distribution and trapping properties of the photoinduced dangling bond defects in undoped hydrogenated amorphous silicon. We observe that the hole capture cross section of the photoinduced deep defects is larger by a factor of 3.3 compared to that of the intrinsic deep defects.
INTRODUCTION A common result in many of the Staebler-Wronski (S-W) effect studies in hydrogenated amorphous silicon (a-Si:H) films is the photoinduced increase of the deep defect density of states. These defects are assumed to be dangling bonds mainly because of ESR studies [1]. In the case of undoped a-Si:H, the more popular model explains this increase by proposing that the recombination of photoinduced carriers breaks weak Si-Si bonds and creates dangling bonds [2,3]. In the case of doped films however the situation appears to be more complicated. Together with a mechanism similar to the undoped films 0 2e- + 2Si°--P2Sij
(I)
we can also have activation or deactivation of dopant atoms. Reactions which have been suggested are
s0° + 0°-
+
14 + si-.siS + P4 0
-
-
Pi +Si3 -- S'4+ P4
(2)
0
(3)
both of which require either the annihilation or the creation of a dangling bond defect [4]. In the above three reactions Si3 represents a negatively charged dangling bond, Si4 represents a 4-fold coordinated Si atom in its bonding state, SiZ represents a 4-fold coordinated Si atom in its antibonding state, P3 represents a phosphorous atom in a non-doping configuration, and P 4 represents a phosphorous atom in a doping configuration. Thus a systematic study of the correlation between the occupied band tail (BT) states and dangling bond defects should disclose which reaction is dominant. In the first part of this paper we report such an investigation by applying the techniques of drive level capacitance profiling [51 and voltage pulse transient phot
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