Transport and Recombination Channels in Undoped Microcrystalline Silicon Studied by ESR and EDMR

Abstract

We present a detailed study of ESR and spin-dependent transport (EDMR) on μc-Si. We identify to different types of defects at g=2.0055(±3) and g=2.0044(±5) and study their influence on transport and recombination by stepwise annealing the samples. We find that transport is not controlled by defects if ND<1018 cm−3. For ND>1018cm−3 a dramatic decrease of the conductivity is found and we identify a hopping contribution in transport. To explain our ESR and EDMR results we propose a simple model where most defects are distributed at the surface of the columns and transport is along percolation paths. We also observe minor metastable changes of the defect density which are assigned to adsorption of atmospheric oxygen.

Electron spin resonance studies of microcrystalline and amorphous silicon irradiated with high energy electrons

“…As important results it can be stated: (1) the signal shape after irradiation depends on the material structure, (2) the electron bombardment creates paramagnetic defects with ESR line parameters close to those already present in the material, (3) additional features observed occasionally at the final annealing step in lc-Si:H ( Fig. 3(a) and (b)) are in agreement with earlier reports [4,10], (4) the changes upon irradiation in the lc-Si:H material are not compatible with a simple increase of just one resonance, neither at g = 2.0043 nor at g = 2.0052, (5) fully amorphous material shows a distinct line shape variation on the high magnetic field side upon irradiation (Fig. 3(d)).…”

“…In a-Si:H an ESR line is observed with a g-value of 2.0055 and a line width of DH pp = 7 G. Identification of this resonance with Si dangling bonds (db) is widely accepted. ESR studies on lc-Si:H do show signals at g = 2.0040 À 2.0053, significantly shifted from the db g-value in a-Si:H and which are considerably affected by preparation conditions, structure composition and post-preparation treatment [1][2][3][4][5][6][7][8][9][10][11][12][13]. The density of these states is closely related to the electronic quality of the material [3].…”

Stability of microcrystalline silicon for thin film solar cell applications

“…The results show similarities with the findings reported earlier [13,17,20]. Differences in the observations could be due to the different material preparation and different experimental conditions and should not be over interpreted at this point.…”

“…There have only been a few investigations on recently prepared material [16][17][18][19][20]. In the present paper we will discuss the results of some of the phenomena for material and solar cells prepared by PECVD or HWCVD and compare these results with earlier investigations.…”

Some controversial points related to transport in microcrystalline silicon

“…On the basis of atomic force microscopy (AFM) combined with the local current mapping [2], electron spin resonance (ESR) [3,4] and many other techniques it is generally believed that µc-Si:H is actually composed of at least two characteristic sizes of crystalline Si grains: the small ones (10 -30 nm) and the large ones (100 -500 nm), formed as aggregates (columns) from the small ones.…”