Measurement of induced surface charges, contact potentials, and surface states in GaN by electric force microscopy

We have studied molecular beam epitaxy grown GaN films of both polarities using electric force microscopy to detect sub 1 µm regions of charge density variations associated with GaN extended defects. The large piezoelectric coefficients of GaN together with strain introduced by crystalline imperfections produce variations in piezoelectrically induced electric fields around these defects. The consequent spatial rearrangement of charges can be detected by electrostatic force microscopy and was found to be on the order of the characteristic Debye length for GaN at our dopant concentration. The electric force microscope signal was also found to be a linear function of the contact potential between the metal coating on the tip and GaN. Electrostatic analysis yielded a surface state density of 9.4 ± 0.5 × 10^10 cm – 2 at an energy of 30 mV above the valence band indicating that the GaN surface is unpinned in this case

Electron diffusion length and lifetime in p-type GaN

We report on electron beam induced current and current–voltage (I–V) measurements on Schottky diodes on p-type doped GaN layers grown by metal organic chemical vapor deposition. A Schottky barrier height of 0.9 eV was measured for the Ti/Au Schottky contact from the I–V data. A minority carrier diffusion length for electrons of (0.2 ± 0.05) µm was measured for the first time in GaN. This diffusion length corresponds to an electron lifetime of approximately 0.1 ns. We attempted to correlate the measured electron diffusion length and lifetime with several possible recombination mechanisms in GaN and establish connection with electronic and structural properties of GaN

Solid phase recrystallization of ZnS thin films on sapphire

High quality ZnS thin films are important for light emitting diodes based on ZnS, which is a very efficient phosphor. To improve as grown, molecular beam epitaxial, (111)-oriented cubic ZnS films, where defects were introduced due to the large mismatch between ZnS and a sapphire substrate (~ 20%), the ZnS was recrystallized by annealing at temperatures in the 825–1000 °C range, and sulfur pressures of 10 atm. The films have been structurally characterized by high-resolution x-ray diffraction, and electron diffraction by electron channeling patterns. Structural properties of the films annealed at temperatures above 900° have improved significantly. Tilting in the recrystallized films has been reduced more than tenfold, with the recrystallized grains being defect-free. Most films were recrystallized in the as-grown, cubic form, as shown by electron channeling patterns. The surfaces of the films have been inspected with scanning electron microscope, and on most samples they have been found to remain smooth, although on some of the films annealed at elevated temperatures we have observed hexagonal pits. The role of sulfur gas overpressure in the recrystallization has been discussed, and possible effects on film evaporation, grain boundary migration and compliancy of sapphire substrate have been analyzed

Electric force microscopy of induced charges and surface potentials in GaN modified by light and strain

We have studied molecular beam epitaxy grown GaN films using electric force microscopy to detect sub-1 µm regions of electric field gradient and surface potential variations associated with GaN extended defects. The large piezoelectric coefficients of GaN together with strain introduced by crystalline imperfections produce variation in piezoelectrically induced electric fields around these defects. The consequent spatial rearrangement of charges can be detected by electrostatic force microscopy, and can be additionally modified by externally applied strain and illumination. The electron force microscopy signal was found to be a function of the applied tip bias, showed reversal under externally applied strain, and was sensitive to above band gap illumination

Microscopic processes during electron cyclotron resonance microwave nitrogen plasma-assisted molecular beam epitaxial growth of GaN/GaAs heterostructures: Experiments and kinetic modeling

A set of delta-GaNyAs1–y/GaAs strained-layer superlattices grown on GaAs (001) substrates by electron cyclotron resonance (ECR) microwave plasma-assisted molecular beam epitaxy (MBE) was characterized by ex situ high resolution X-ray diffraction (HRXRD) to determine nitrogen content y in the nitrided GaAs monolayers as a function of growth temperature T. A first order kinetic model is introduced to quantitatively explain this y(T) dependence in terms of an energetically favorable N for As anion exchange and thermally activated N-surface desorption and surface segregation processes. The nitrogen surface segregation process, with an estimated activation energy Es ~ 0.9 eV appears to be significant during the GaAs overgrowth of GaNyAs1–y layers, and is shown to be responsible for strong y(T) dependence

Kinetic modeling of microscopic processes during electron cyclotron resonance microwave plasma-assisted molecular beam epitaxial growth of GaN/GaAs-based heterostructures

Microscopic growth processes associated with GaN/GaAs molecular beam epitaxy (MBE) are examined through the introduction of a first-order kinetic model. The model is applied to the electron cyclotron resonance microwave plasma-assisted MBE (ECR-MBE) growth of a set of delta-GaNyAs1–y/GaAs strained-layer superlattices that consist of nitrided GaAs monolayers separated by GaAs spacers, and that exhibit a strong decrease of y with increasing T over the range 540–580 °C. This y(T) dependence is quantitatively explained in terms of microscopic anion exchange, and thermally activated N surface-desorption and surface-segregation processes. N surface segregation is found to be significant during GaAs overgrowth of GaNyAs1–y layers at typical GaN ECR-MBE growth temperatures, with an estimated activation energy Es ~ 0.9 eV. The observed y(T) dependence is shown to result from a combination of N surface segregation/desorption processes