Observation of Deep Defect Relaxation Processes in Hydrogenated Amorphous Silicon-Germanium Alloy
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OBSERVATION OF DEEP DEFECT RELAXATION PROCESSES IN HYDROGENATED AMORPHOUS SILICON-GERMANIUM ALLOY FAN ZHONG and J.DAVID COHEN Department of Physics and Materials Science Institute, University of Oregon, Eugene OR 97403
ABSTRACT Modulated photocurrent (MPC) spectroscopy has been used to investigate the energy distribution of deep defect states in photo-CVD grown a-SilxGex:H alloys. We observe a distinct electron trapping feature at a thermal energy of 0.55eV below Ec for higher Ge concentration (x = 0.62) alloys. For those samples with Ge concentration between 30-50 at.%, an anomalously large phase shift was observed within a temperature window between 150K to 240K. The modulated photocurrent exhibits strong quenching at the edges of this window. We suggest that these MPC spectra are controlled by deep defect relaxation processes in this temperature region, which causes electron thermal emission processes to be greatly suppressed. This effect can be reduced by increasing the reverse bias. Under such conditions, the energy position of these defect states can be roughly estimated to lie near Ec - 0.33eV. This result supports the recent result obtained from transient photocapacitance and photocurrent measurement.
INTRODUCTION Deep defect states in the mobility gap play a central role in deten-nining the transport and recombination properties of amorphous semiconductors. It is of great importance to obtain infonnation about their dynamics properties and energy distribution. Recent capacitance transient and ESR transient studies on lightly n-type doped a-Si:H samples reveal that deep defect relaxation processes may be the dominant factor detennining the thermal transition energy of those defects for different charge states1 which is further verified by the results from modulated photocurrent and steady state capacitance measurements on the same samples. 2 , 3 For hydrogenated amorphous silicon-genrnanium alloys, these defect relaxation processes could play a more important role than in a-Si:H since there are more network degrees of freedom in the alloys. Previous transient photocapacitance and photocurrent studies on high quality photoCVD grown a-Sil-xGex:H (0.25 O'Nce-E/kBT and the steady state photo-electron capture rate Cn =< v > ofl0h. (Nc is the effective density of state at conduction band mobility edge); namely R I(cjE)
R 2(o),T, E) =
e1 C°2 +(Cn + en)2
0°2e,(c,, + e,,(3 ((02 +(Cn +eE) 22
(3)
3 Under low illumination level conditions (average density of photocarrier no < 109 cm" ) the ph thermal emission rate en is larger than capture rate cn for defect energy above the midgap, and a 8 function approximation can be applied to obtain thennal emission energy scale:
R, (coTE) = (R2 (moTE))l/
with the energy scale
2
-
C02 +e
2
= kBT7c 8(E- E(oT)) 2
E(co,T) = kBTLn(< v > aNc)
(4)
(5)
(0
Thus, Eq.(l) and Eq.(2) both yield an energy distribution for the defect states, g(E(co,T)), by changing modulation frequency co = 2nf or measurement temperature T. Eq.(2) further improves the energy resolution of the
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