In-plane imperfections in GaN studied by x-ray diffraction

Abstract

We have studied a series of GaN films grown with a range of dislocation densities by atomic force microscopy (AFM), transmission electron microscopy (TEM) and high resolution x-ray diffraction (HRXRD). The (002), (004), (006), (105), (204), (302), (100), (110), (200) and (300) reflections were measured as reciprocal space maps (RSMs) or scans in ω and ω/2θ. The latter 4 in-plane reflections were measured using a low, or glancing, incident angle with respect to the film surface. We have used a variety of different methods to try and obtain reliable measurements for mosaic tilt, twist, crystallite size and microstrain both in- and out-of plane. From (hk0) data in-plane twist angles were measured ranging from 0.37° to 0.078° and in-plane microstrains from 3.5 × 10−4 to 1.8 × 10−4. The improvements in the quality of the GaN layers relate to the increased island coalescence time, which reduces in particular the number of edge-type threading dislocations. The first three samples had a much larger tilt ∼0.09° than the last three ∼0.04°. However, the latter samples were bowed, so results from a single measurement on the (002) peak are too large. Beam restriction on the (002) or an extrapolation from several (00l) reflections gives more reliable results. The values obtained for in-plane crystallite size are in general variable or unreliable. For some samples the sizes are considered to be too large to be accessible by XRD; in most cases the peak broadening is dominated by tilt or twist or microstrain and the results are sensitive to assumptions about the peak shape. For the samples with smaller measurable crystallite sizes, the (hk0) peaks were too weak to measure reliably. The cell parameters showed more compressive strain with fewer dislocations. The trends observed by HRXRD are consistent with AFM and TEM results.

The influence of nitridation time on the structural properties of GaN grown on Si (111) substrate

“…Large lattice mismatch with respect to the substrate commonly causes high dislocation densities in heteroepitaxial thin films [18][19][20][21]. These types of structures are often described by the model of mosaic crystals [20,22].…”

“…These types of structures are often described by the model of mosaic crystals [20,22]. The lateral and vertical dimensions of the crystallites are given by the lateral and vertical coherence length.…”

The influence of thickness and ammonia flow rate on the properties of AlN layers

“…In this model, a mosaic layer consists of single crystalline blocks with lateral and vertical coherence lengths. As was previously mentioned, XRD peaks are broadened due to the out-of-plane and in-plane rotation of the blocks (tilt and twist), limited crystallite size, and microstrain [11,12]. Detailed knowledge about this model can be obtained from Ref.…”

“…This result clearly shows that reducing the NH 3 flow rate improves the crystalline quality of the symmetric (0 0 0 2) plane. This is in agreement with the findings of Thapa et al [4] who investigated the effects of V-III ratio, N 2 -H 2 ratio, and growth temperature on the structural properties of AlN layers grown on c-plane sapphire substrates, and found that the low V/III ratio is an eminent growth condition for crystallographic perfection due to the The broadening of X-ray diffraction (XRD) peaks is mainly due to the limited crystallite size, misorientation of crystallites, and microstrain in epitaxial layers [11,12]. Additionally, the out-of-plane and in-plane misorientations of crystallites in the layers are related to the screw and edge threading dislocations (TDs), respectively [11].…”

Examination of the temperature related structural defects of InGaN/GaN solar cells

“…Twist is the rotation around the surface normal [15]. The XRD peaks for the mosaic GaN layer expand due to properties like twisting, tilting, limited crystal dimension and microstrain [16]. Symmetric x scans and azimuth / scans are analyzed in order to evaluate the mosaic structure of the GaN layer.…”