Hole range in compensated hydrogenated amorphous silicon

Journalof Non-CrystallineSolids66 (1984)71-76 North-Holland,Amsterdam

71

HOLE RANGE IN COMPENSATED HYDROGENATED AMORPHOUS SILICON

S. GRAMMATICA, F. JANSEN, J. MORT, and M. MORGAN Xerox Webster Research Center Webster, NY 14580

Xerographic discharge measurements are reported on 10 tim thick compensated hydrogenated amorphous silicon samples for doping levels as high as 5000 ppm by weight. Excellent xerographic characteristics including photosensitivity, charge acceptance and hole range are mainlaincd up to doping levels ~ 100 ppm by weight. For higher doping levels both charge acceptance and hole range degrade due to associated increases in mid gap state densities.

Allan et al.[11 first reported the production of compensated amorphous hydrogenated silicon. They used the high resistivity of such a material to study electron hopping in the phosphorus donor band by the time of flight technique.

In fact they showed that

reasonable compensation, indicated by a mid-mobility gap Fermi level position, could be achieved at gaseous doping levels of phosphine and diborane as high as 2000 ppm by volume.

The measurements made on samples ~ 1 btm thick indicated that electron

transport at room temperature occurred partially via the conduction band (including shallow trapping) and partially via the donor band. In this article, we wish to report measurements of the range of holes in much thicker compensated amorphous silicon samples ~ 10 #m. The samples were prepared at 230°C on the cathode of a d.c. glow discharge reactor. The precursor silane gas was doped with diborane and phosphine in the range of 100 to 5000 ppmw (i.e., ppm by weight) where the diborane/phosphine ratio was ~ 1.0. Figure 1 shows the photoinduced discharge curve which is the surface potential for positive corona charge as a function of (log) exposure for a number of samples in which the compensation ratio has been optimized for charge acceptance commenstlrate with good hole range. The monochromatic light of 5500 ,~ was strongly absorbed so the photodischarge shown was determined by the transport of holes through the bulk of the sample.

At very low light exposures the surface potential is

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S. Grammatica et al. / Compensated hydrogenated amorphous silicon

72

constant and measures the initial charge acceptance of the sample.

As the exposure

increases the surface potential decreases as photodischarge occurs. At large exposures the surface potential should be zero if the photogenerated carriers at all fields have a range comparable or larger than the sample thickness.

Where this condition is not met the

photoreceptor does not discharge completely and a residual voltage related to the carder range remains.[ 2] ~......~.~.~...~..~''I

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Photoinduced discharge curves for a series of samples compensated with a boron/phosphorus ratio ~ 1.0 where the absolute doping levels in the gas phase are as indicated. The dashed line is for a sample doped with phosphorus which had no charge acceptance. The monochromatic light of 5500 A is strongly absorbed in the positively charged surface so that the photodischarge is due to the drift of holes.

The data in Figure 1 shows two systematic trends as the dopant concentrations increase. First the charge acceptance decreases. This is not associated with increasing difficulty in maintaining precise compensation at high doping levels which could cause the Fermi level to significantly depart from the mid-gap position. From calculations by Chen and Jansen[3], the Fermi level position in an amorphous compensated semiconductor is a rather weak function of the dopant ratio only and experimentally this ratio is easily adjusted and maintained. This degree of control allows good optimization of the charge acceptance and the hole range particularly as xerographic measurements are sensitive to changes in both

73

s. Grammatica et al. / Compensated hydrogenated amorphous silicon

parameters. We feel that it is more likely that the decrease in charge acceptance arises from an increased bulk thermal generation rate associated with the increase of deep lying gap states in compensated material, Such gap states arise from the tailing of the states associated with boron and phosphorus incorporation in amorphous silicon and are possibly due to the formation of new complexes in the material due to the simultaneous presence of boron and phosphorous. It is a remarkable fact, nonetheless, that significant charge acceptance occurs for dopant concentrations as high as ~ 5000 ppmw whereas with phosphorus or boron alone no significant charge acceptance is obtained with only ~ 10 ppmw doping levels. The second important feature displayed in Figure 1 is that material compensated at 100 ppmw is essentially indistinguishable in its xerographic electrical characteristics from the material which is compensated against intrinsic donor states with only 4 ppmw of boron. In Fig. 2 the optimum exposure for maximum contrast potential (input density of unity) is shown for a few of the compensated samples. The relative insensitivity of the optimum exposure for dopant levels of < 800 ppmw shows that the high 250 ~ ,

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74

S. Grammatica et al. / Compensated hydrogenated amorphous silicon

photosensitivity of the material is maintained, indicating little, if any, decrease in recombination lifetimes in the photoexcitation region. When the dopant concentration is ~ 800 ppmw, in addition to loss of charge acceptance, it is seen that a significant residual voltage remains indicating a loss in hole range. A decrease in hole range can either be associated with a decrease in microscopic carrier mobility, /zo, or deep trapping lifetime, ~'d" Since #oZd = P.drifta'eff (where P'drift and ref f also include effects of shallow traps), then a decrease in /tdrift is usually due to changes in shallow traps rather than #o and is always offset by a corresponding increase in the effective lifetime. As a result, changes in shallow trap densities will result in a constant # r product. A decrease in carrier range is most likely to be associated with a change in deep traps thus decreasing r d since the microscopic mobility is less defect sensitive than ~'d. The deep trapping lifetime can decrease either due to an increase in the density of deep traps and/or their capture cross-section.

Although boron and phosphorus are usually

thought of as substitutional acceptors and donors some fraction can be expected to be incorporated as deeper lying non-substitutional donor and acceptor-like states.

This

appears consistent with the relative insensitivity of the hole range for total dopant concentrations < 100 ppmw. Such states could also serve as thermal generation sites and thus also account for the observed increase in dark conductivity reflected in the decreasing charge acceptance. Certainly the data suggests a correlation between the loss of charge acceptance and carrier range. It is this combined loss of charge acceptance and carrier range which leads to the decrease in the value of the maximum contrast potential seen in Fig. 2. Additional evidence which supports the picture of increased deep gap state density upon compensation is qualitatively derived from field effect measurements on this material. Even at relatively low dopant concentrations of ~100 ppmw the field effect is rapidly quenched as indicated by several orders of magnitude decrease in the maximum achievable on-currents.[4] This gives rise to an apparent anomaly in that the absence of a field effect implies a pinned Fermi level and yet the loss of charge acceptance is attributed to an increased dark conductivity. Recent workl3] has shown that this paradox is resolved by the calculation of Fermi statistics taking into account previously neglected non-substitutional donor and acceptor-like states. Similar conclusions regarding the introduction of new states above the valence band edge in compensated material has also been postulated from luminescence, esr and conductivity studies by Street et al.[5]

S. Grammatica et al. / Compensated hydrogenated amorphous silicon

75

The present results suggest that compensated hydrogenated amorphous silicon up to doping levels of several hundred ppmw (with a boron/phosphorus ratio ~ 1.0) has potential use as a xerographic photoreceptor since both adequate charge acceptance and hole range are maintained. Such compensated material has an additional virtue in that the associated introduction of deep lying gaip states quenches the field effect and this has benefit since recent work[6] has shown that field induced shifts in Fermi level can result in severe degradation of images in the xerographic imaging step due to lateral motion of charges. REFERENCES 1)

D. Allan, P.G. LeComber and W.E. Spear, Proc. 7th Intl. Conf. on Amorph. and Liq. Semiconductors, Edinburgh, 323 (1977).

2)

J. Mort and I. Chert, Appl. Solid State Sci. 5, 69 (1975).

3)

1. Chen and F. Jansen, Phys. Rev. B29, Mar.15(1984).

4)

K. Okumura, F. Jansen and J. Mort (unpublished).

5)

R.A. Street, D.K. Biegelsen and J.C. Knights, Phys. Rev. B24, 969 (1981).

6)

J. Mort, F. Jansen, S. Grammatica, M. Morgan and I. Chen, J. Appl. Phys. (Manuscript # Lc4314).