XPS for probing the dynamics of surface voltage and photovoltage in GaN

Abstract

a b s t r a c tWe describe application of two different data gathering techniques of XPS for probing the dynamics of surface voltage and surface photovoltage (SPV) developed in microseconds to seconds time-domain, in addition to the conventional steady-state measurements. For the longer (seconds to milliseconds) regime, capturing the data in the snapshot fashion is used, but for the faster one (down to microseconds), square wave (SQW) electrical pulses at different frequencies are utilized to induce and probe the dynamics of various processes causing the surface voltage, including the SPV, via the changes in the peak positions. The frequency range covers anywhere from 10 −3 to 10 5 Hz for probing changes due to charging (slow), dipolar (intermediate), and electronic (fast) processes associated with the external stresses imposed. We demonstrate its power by application to n-and p-GaN, and discuss the chemical/physical information derived thereof. In addition, the method allows us to decompose and identify the peaks with respect to their charging nature for a composite sample containing both n-and p-GaN moieties.

Chemical Visualization of a GaN p-n junction by XPS

“…Figure 1b depicts the Ga 2p 3/2 peak of the p- and n- regions of the middle device recorded under +2 V bias as well as when grounded. For an undoped GaN (un-GaN), the tabulated binding energy of the Ga2p 3/2 is 1117.9 eV, there is already a small binding energy difference of 0.4 eV between p- and n- regions when the XPS data is recorded in the conventional (both electrodes grounded) fashion, due to differences in their Femi-level pinning at the surface, in accordance with published data 51 52 53 54 55 , which was discussed in detail in our previous work 56 57 . The more dramatic change between the p- and n- regions materializes when recorded under +2 V reverse bias, upon which the peaks in the n- and p-regions exhibit 2.4 eV relative binding energy difference, due to charge built-up across the junction.…”

“…In principle both the green 532 nm (2.3 eV) and the violet 405 nm light (3.1 eV) should not have contributed to the band to band photoconductivity of GaN with a band-gap of 3.4 eV 54 . Hence, the presence of deep traps and defects must be causing such changes, which was also reported in our previous studies 56 57 .…”

Vertical current-flow enhancement via fabrication of GaN nanorod p–n junction diode on graphene

“…The shift in the binding energy values of Ga 3d and N 1s due to Mg doping is as recorded in Fig. 4(a) and (b) respectively [22,23]. This evidences the doping of Mg in the GaN nanorods array.…”

Location and Visualization of Working p-n and/or n-p Junctions by XPS

“…Another advantage of such an application is, in addition to saving time, its parallel detection instead of a serial one, allowing to cancel out many unavoidable experimental imperfections. Furthermore, although not applicable to the present device, by varying the frequency of the SQW, the time-dependent response of certain peaks can also be obtained for certain materials and devices, as was demonstrated in our previous publications for devices made out of photosensitive materials like CdS54 and GaN55.…”