Microscopic Basis for the Mechanism of Carrier Dynamics in an Operating $p\mathrm{\text{−}}n$ Junction Examined by Using Light-Modulated Scanning Tunneling Spectroscopy

The doping characteristics and carrier transport in a GaAs $p\mathrm{\text{−}}n$ junction were visualized with a $\sim 10\mathrm{nm}$ spatial resolution, using light-modulated scanning tunneling spectroscopy. The dynamics of minority carriers under operating conditions, such as recombination, diffusion, and electric field induced drift, which had previously been analyzed on the basis of empirical electric properties, were successfully examined on the nanoscale. These results provide a solid basis for elucidating the mechanism of the carrier transport properties predicted by using the macroscopic analysis.

Figure 1

(a) Experimental setup. (b) $I\mathrm{\text{−}}V$ curve (left axis) and SPV spectrum (right axis) obtained on the $n$-type area (set point: $V_S=+3.0\mathrm{V}$, $I=0.10\mathrm{nA}$). (c) 1D energy diagrams of the tunnel junction for an $n$-type semiconductor with a positive sample bias voltage under dark (left) and illuminated (right) conditions. When electron (blue circles)-hole (red circles) pairs are produced by super-band-gap illumination, holes flow towards the surface due to the electric field affecting the area, resulting in the reduction of band bending. (d) Light-intensity dependence of SPV measured on an $n$-type area ($V_S=+2.5\mathrm{V}$).

Figure 2

(a) SPV mapping at $V_S=+2.5\mathrm{V}$ ($1000\mathrm{nm}\times 1000\mathrm{nm}$, $80(\mathrm{\text{lateral}})\times 20(\mathrm{\text{vertical}})$ LM-STS spectra). (b) A plot of the cross-section along the blue line marked in (a). (c), (d) are SPV mappings of the same area at $V_S=+2.5\mathrm{V}$ and $V_S=-2.5\mathrm{V}$, respectively ($500\mathrm{nm}\times 500\mathrm{nm}$, $67\times 17$ LM-STS spectra). (e) Cross sections of the SPV images along the red lines in (c)—red dots, (d)—blue dots. Green bar represents the space charge region (SCR) at the interface. (f) SPV spectra obtained by averaging all data in each region (blue: $n$-type region, green: space charge region, red: $p$-type region).

Figure 3

(a) Experimental setup. (b) A series of SPV mappings as a function of the applied forward-bias voltage ($K_S=+2.5\mathrm{V}$). (c) A current-voltage relation macroscopically obtained for the $p\mathrm{\text{−}}n$ junction. (d) (upper) Band diagram of a $p\mathrm{\text{−}}n$ junction with the distribution of minority electrons (blue circles) and holes (red circles) under a forward-biased voltage condition. $F_h$ and $F_e$ denote the Fermi levels for holes and electrons; (lower) 1D band diagrams of the STM tunnel junction with the STM tip placed at the positions indicated by the two green dashed lines. (e) Cross sections of SPV images in (b) across the interface of the $p\mathrm{\text{−}}n$ junction.