Gate-Dependent Carrier Diffusion Length in Lead Selenide
Quantum Dot Field-Effect Transistors
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Abstract
We
report a scanning photocurrent microscopy (SPCM) study of colloidal
lead selenide (PbSe) quantum dot (QD) thin film field-effect transistors
(FETs). PbSe QDs are chemically treated with sodium sulfide (Na<sub>2</sub>S) and coated with amorphous alumina (a-Al<sub>2</sub>O<sub>3</sub>) by atomic layer deposition (ALD) to obtain high mobility,
air-stable FETs with a strongly gate-dependent conductivity. SPCM
reveals a long photocurrent decay length of 1.7 μm at moderately
positive gate bias that decreases to below 0.5 μm at large positive
gate voltage and all negative gate voltages. After excluding other
possible mechanisms including thermoelectric effects, a thick depletion
width, and fringing electric fields, we conclude from photocurrent
lifetime measurements that the diffusion of a small fraction of long-lived
carriers accounts for the long photocurrent decay length. The long
minority carrier lifetime is attributed to charge traps for majority
carriers