Transport Measurements in LPCVD Amorphous Silicon Obtained from Disilane
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TRANSPORT MEASUREMENTS IN LPCVD AMORPHOUS SILICON OBTAINED FROM DISILANE.
C. MANFREDOTTI* G.GERVINOt L.MONTALDIt U. NASTASIt R.MURRI* AND L. SCHIAVULLI* * Istituto di Fisica Superiore, C.so M. D'Azeglio 46, 10125 Torino, Italy and Gruppo Nazionale di Struttura della Materia, C.N.R.,Italy ** Dipartimento di Fisica, Via Amendola 173, 70126 Bari, Italy
ABSTRACT Measurements of high temperature conductivity on a-Si:H obtained by LPCVD from Si 2 H6 at temperatures between 450 and 500 CC indicate clearly a change on the activation energy from 0.9 - 1.0 eV to 0.55 - 0.6 eV around 600 °K . The results are not strongly different from those obtained in GD aSiH4 :however, the coefficent of the energy shift of the mobility edge with temperature is greater by a factor of 2, while the average extension of state distribution at T= 0 °K is roughly a factor 1.5 larger. the tail which is one The main discrepancy concerns the pre-exponential factor, giving a product (NcukT)of the order or two orders of magnitude larger, 1 By assuming a conduction mobility two orders of of 1021 cm V" s" . magnitude larger than the maesured Hall mobility one obtains a value for N which is only a factor four times larger than what currently assumed for a-Si:H.
INTRODUCTION silicon deposited both from SiH and from Si H LPCVD amorphous displays several features, which are not strongly different from GD abut with an hydrogen content at least one order of magnitude lower Si:H, For [1] and practically equal to the background content for a-Si. instance the possibility of achieving E = 1.7 eV with 0.6% hydrogen has been demonstrated, and values of DOS density at Fermi level content 3 at least as measured by SCLC below 1016 cm eVlhave been reached [2], technique. In the present work it is indicated that transport properties change only to a small extent by using Si 2 H6 . In particular only the energy of conductivity is ±arger indicating a cleaner activation conduction band tail. Moreover, the lower activation energies at room temperature are no more present, with the consequence that conduction in tail states is not likely in this case.
EXPERIMENTAL The a-Si:H films have been grown on 3" ceramic substrates (Rosenthal) While the on a 5" quartz tube with a 60 cm long deposition region. temperature of the hot-wall furnace has been varied on the range 450°C the Si 2 H6 flux was kept at 50 SCCM and the pressure at 0.18 Torr. 500 °C, is No carrier gas has been used. Hydrogen content as measured by SIMS, 0.6% . The behaviour of growth velocity vg as a function of deposition is reported in Fig. 1 both for SiH4 and Si 2 H6 :by using temperature disilane, an increase in vg by an order of magnitude is noticed and the activation energy is more constant with a value E A =1.68 eV compatibly with a process of hydrogen evolution at these tempeatures [3 The conductivity as a function of temperature has been measured by
Mat. Res. Soc. Symp- Proc. Vol. 70. , 1986 Materials Research Society
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