Electrical characterization of dislocations in gallium nitride using advanced scanning probe techniques

GaN-based materials are promising for high speed and power applications such as amplifier and communications circuits. Ga, In, and AIN-based alloys span a wide optical range (2–6.1 eV) and exhibit strong polarizations making them useful in many devices; however, films are highly defective (∼10 8 dislocations cm−2) due to lack of suitable substrates. Thus, nanoscale electronic characterization of these dislocations is critical for device and growth optimization. Scanning probe techniques enable characterization at length-scales unattainable by conventional techniques.; First, scanning Kelvin probe microscopy (SKPM) was used to image surface potential variations due to charged dislocations in HVPE-grown GaN. The film's structural evolution «with thickness was monitored showing a decrease in dislocation density, likely through dislocation reaction. Numerical simulations were used to investigate tip-size effects when imaging highly localized (tens of nm) potential variations indicating that measured dislocation induced potential features in GaN can be much smaller (∼80%) than true variations. Next, capacitance variations in MBE-grown HFETs, due to dislocations-induced carrier depletion, were imaged with scanning capacitance microscopy (SCM). The distribution of these charged centers was correlated with buffer schemes showing that an AIN buffer leads to pseudomorphic (2D) nucleation and randomly distributed misfit dislocations while deposition directly on SiC results in island (3D) nucleation and a domain structure with dislocations grouped at domain boundaries. Hall measurements and numerical simulations were also carried out to further study the implications of these microstructures. Numerical results indicated that randomly distributed dislocations deplete a larger fraction of free carriers than the same density of grouped dislocations and correlated favorably with Hall results. Correlated SKPM and conductive AFM (C-AFM) measurements were then used to study negatively charged and highly conductive dislocations in MBE-grown GaN and established that edge component dislocations are negatively charged while pure screw dislocations are uncharged and highly conductive. Finally, C-AFM and surface photovoltage (SPV) microscopy were used to evaluate local electronic behavior of MOCVD-grown Mg-doped GaN showing regions of reduced conductivity and increased SPV correlated with dislocations. Mg segregation to dislocation cores, leaving Mg-poor regions surrounding the dislocations, was sited as the cause for the observed local electronic properties.