Size-dependent persistent photocurrent and surface band bending inm-axial GaN nanowires

Abstract

The size-dependent persistent photocurrent (PPC), which refers to a photocurrent persisting for a long time after the excitation light source is terminated, has been investigated in m-axial GaN nanowires (NWs) with diameters of 20-130 nm. These NWs possess polar side surfaces and thus exhibit strong surface band bending (SBB). With different diameters, a different rise time in photoconductivity (PC) upon excitation light illumination and a different decay time in the PPC are observed; the latter is attributed to a long carrier lifetime caused by a frustrated recombination process. The intensity (I)-dependent photocurrent-gain () measurement displays a-I dependence that follows a power-law relationship with fitting indices of ∼0.85-0.89, indicating that the long carrier lifetime-induced PPC of GaN NWs is caused by an SBB effect instead of a bulk trap effect. In addition, size-dependent decay times reveal two regimes for the different sizes of NWs. The decay time of the NW above critical diameter (d crt , 30-40 nm) is found to be ∼13 000 s, while the smaller NW (<d crt) is <800 s. Herein, we propose that the surface-induced effective barrier height for different sizes of GaN NWs is the dominant factor that explains the apparent size dependence. The temperature-dependent decay time measurements determine an effective barrier height of 226 meV for a 65-nm NW, whereas the 20-nm NW has an effective barrier height of 32 meV, confirming that SBB effects of different sizes are responsible for the size-dependent PPC in m-axial GaN NWs.

Studies of photoconductivity and field effect transistor behavior in examining drift mobility, surface depletion, and transient effects in Si-doped GaN nanowires in vacuum and air

“…7,8,11,13,38 However, these comparisons require further context since the NWs studied encompassed a wide range of n b as follows:…”

“…(c) Studies of m-axis NWs with N % 10 18 cm À3 reportedly yielded 1 ¼ 0.226 eV 38. (d) As just described in (a-c) above, such similar values of 1 over a wide range of n b suggest a distribution of surface traps with energy in the vicinity of 0.3 eV below E c at the surface.…”

Semiconductor nanowires studied by photocurrent spectroscopy

“…(ii) The photogenerated electrons Ahn et al 2007;Calarco et al 2005;) (holes) Thunich et al 2009) increase the density of free charge carriers in the nanowire core which is called photodoping effect. The photogating effect was reported for GaAs- Thunich et al 2009) and Ge-based nanowires, the photodoping effect for GaAs- Thunich et al 2009), Ge- (Ahn et al 2007;Kim et al 2010), GaN-(Calarco et al 2005;Chen et al 2011;González-Posada et al 2012;Polenta et al 2008;Sanford et al 2010), InSe- (Zhai et al 2010) and Si-based nanowires. However, photogating and photodoping are interconnected to each other, because an interband excitation always generates electrons and holes in the nanowire.…”

Fabrication of photonic crystal circuits based on GaN ultrathin membranes by maskless lithography

“…In contrast to this, PPC and OQ effects in nanoperforated membranes are determined by surface states. The large number of holes in these membranes leads to extended depletion regions around them, making the surface-to-volume ration similar to that for GaN nanowires [24][25][26]. It is recognized that the long carrier lifetime and PPC in GaN nanowires are caused by surface band bending (SBB) rather than by bulk trap effects.…”