Discovering a Defect that Imposes a Limit to Mg Doping in p-TypeGaN
Gallium nitride (GaN) is the III-V semiconductor used to produce blue light-emitting diodes (LEDs) and blue and ultraviolet solid-state lasers. To be useful in electronic devices, GaN must be doped with elements that function either as electron donors or as acceptors to turn it into either an n-type semiconductor or a p-type semiconductor. It has been found that GaN can easily be grown with n-conductivity, even up to large concentrations of donors--in the few 10{sup 19}cm{sup -3} range. However, p-doping, the doping of the structure with atoms that provide electron sinks or holes, is not well understood and remains extremely difficult. The only efficient p-type dopant is Mg, but it is found that the free hole concentration is limited to 2 x 10{sup 18}cm{sup -3}, even when Mg concentrations are pushed into the low 10{sup 19}cm{sup -3} range. This saturation effect could place a limit on further development of GaN based devices. Further increase of the Mg concentration, up to 1 x 10{sup 20}cm{sup -3} leads to a decrease of the free hole concentration and an increase in defects. While low- to medium-brightness GaN light-emitting diodes (LEDs) are remarkably tolerant of crystal defects, blue and UV GaN lasers are much less so. We used electron microscopy to investigate Mg doping in GaN. Our transmission electron microscopy (TEM) studies revealed the formation of different types of Mg-rich defects [1,2]. In particular, high-resolution TEM allowed us to characterize a completely new type of defect in Mg-rich GaN. We found that the type of defect depended strongly on crystal growth polarity. For crystals grown with N-polarity, planar defects are distributed at equal distances (20 unit cells of GaN); these defects can be described as inversion domains [1]. For growth with Ga-polarity, we found a different type of defect [2]. These defects turn out to be three-dimensional Mg-rich hexagonal pyramids (or trapezoids) with their base on the (0001) plane and their six walls formed on {l_brace}1123{r_brace} planes (Fig. 1a). In [1120] and [1100] cross-section TEM micrographs the defects appear as triangular (Fig. 1b) and trapezoidal (Fig. 1c). In projection, the sides of these defects are inclined at 43{sup o} and 47{sup o} to the base depending on the observation direction. The pyramid size varies from 50{angstrom}-1000{angstrom} depending on the growth method, but the angle between the base and sides remain the same. The direction from the tip of the pyramid to its base (and from the shorter to the longer base for trapezoidal defects) is along the Ga to N matrix bond direction (Fig. 1a-d).
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE Director. Office of Science. Basic EnergySciences
- DOE Contract Number:
- DE-AC02-05CH11231
- OSTI ID:
- 928231
- Report Number(s):
- LBNL-60791-Ext.-Abs.; R&D Project: 513340; BnR: KC0201030; TRN: US0804305
- Resource Relation:
- Conference: Microscopy and Microanalysis, Chicago, IL,07/31/2006-08/02/2006
- Country of Publication:
- United States
- Language:
- English
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