Abstract
Composition fluctuations in GaN:Mg have been investigated by optical transmission spectroscopy, near-field scanning optical microscopy (NSOM), and scanning Kelvin probe microscopy (SKPM). An Urbach tail is observed in the optical transmission spectrum of heavily Mg-doped GaN, with the magnitude of Urbach energy increasing with increasing compositional/potential fluctuations. From the relationship between the Urbach energy and the potential fluctuation magnitude, the spatial size of the compositional fluctuations in GaN:Mg (with the average Mg concentration of ∼4× 1019 cm-3) is calculated to be 220 nm. The nanoscale compositional fluctuations are determined by NSOM and SKPM. A spatial variation of the transmission mode NSOM intensity is observed with a characteristic wavelength of ∼300 nm, which is attributed to the inhomogeneous distribution of Mg. The normalized root-mean-square (rms) fluctuation of the transmission mode NSOM intensity for heavily Mg-doped GaN is a factor of 5 greater than that for undoped GaN. Similar microstructure patterns are observed in the NSOM and surface potential images of GaN:Mg as probed by SKPM. The variation of local surface potential (on the order of 100 mV) is attributed to the same compositional fluctuations of Mg, which changes the local surface work function. The local Mg concentrations are calculated from the SKPM image. For GaN:Mg with the Mg concentration of 4× 1019 cm-3, a rms value of 5.4× 1019 cm-3 of the compositional fluctuations has been observed.
| Original language | English (US) |
|---|---|
| Article number | 023513 |
| Journal | Journal of Applied Physics |
| Volume | 98 |
| Issue number | 2 |
| DOIs | |
| State | Published - Jul 15 2005 |
Funding
The authors would like to thank Dr. Shekhawat at the Nanoscale Integrated Fabrication, Testing and Instrumentation Center for technical help with the NSOM and SKPM measurements. This work is supported by NASA Grant No. NAG5-1147. Table I. Mg doping densities, hole concentrations, Urbach energies, and potential fluctuation magnitudes of undoped and Mg-doped samples. Sample no. Mg ( 10 19 ∕ cm − 3 ) p ( × 10 17 cm − 3 ) Γ (meV) E URB (meV) I 0 − 2.3 a 0 17.8 II 2 1.6 82 25.2 III 4 4.5 45 24.5 IV 5 2.6 96 26.4 a Indicates n -type material. FIG. 1. (a) shows the optical transmission spectra of GaN:Mg for samples with different average doping densities. The transmission at λ = 488 nm decreases with increasing Mg concentrations. (b) shows the absorption coefficients of different samples just below the band-edge energy. In the inset of Fig. 1(b) the Urbach energies ( E URB ) of different films are plotted as a function of the potential fluctuation magnitude Γ . FIG. 2. A schematic of the potential fluctuations in heavily Mg-doped, compensated GaN. FIG. 3. (a) and (b) show the simultaneously acquired topographic and transmission mode NSOM images of sample III. (c) shows the transmission mode NSOM image of sample I (undoped GaN). FIG. 4. A schematic band diagram for a p -type GaN:Mg sample and the probe tip used in SKPM measurements. FIG. 5. (a) shows the surface potential image of a 1 × 1 - μ m 2 region of sample III probed by SKPM. (b) shows the surface potential image of a 0.78 × 0.78 - μ m 2 region of undoped GaN. (c) shows the mapping of Mg concentrations calculated from Fig. 5(a) using Eqs. (5) and (7) . (d) shows the histogram of Mg concentrations.
ASJC Scopus subject areas
- General Physics and Astronomy