Two dimensional hole gases (2DHG) form in uncapped GaN/InGaN struc-tures (see Section 3.1). The hole mobility in InGaN is however much lower than the electron mobility [90]. Two dimensional electron gases (2DEG) thus
+σ
Figure 6.16: Schematic of InGaN and InGaN/AlGaN based heterostructures. The polarization chargesσ and the band profiles are indicated. In both structures a 2DEG and a 2DHG form within InGaN. Additional po-larization charges introduced by an AlGaN layer allow only electron confinement for thin InGaN barriers.
0
Figure 6.17: a) Profile of CBM and VBM with a In0.2Ga0.8N layer with varying thickness, without doping. b) sheet carrier density over In0.1Ga0.9N thickness for a structure with a 15 nm and a structure with a 50 nm GaN cap layer.nsH andnsE increases comparably with InGaN thickness.
For thicker capping layers, however, more electrons than hole can be obtained.
promise better results for low temperature magneto transport measurements.
Under certain conditions, 2DEGs form in addition to 2DHGs in GaN/In-GaN/GaN heterostructures. The coexistence of p- and n-conductive channels is however not desired as it results in ambiguous transport results (see Section 3.3). This section therefore studies the impact of the heterostructure design on the sheet carrier densities of 2DEGs and 2DHGs.
Two types of InGaN based heterostructures were grown during this work as depicted in Figure 6.16. First GaN/InGaN/GaN and then GaN/InGaN/Al-GaN/GaN structures will be discussed. For both types of structures, the electron channel lies within the InGaN layer.
InFigure 6.17 a), the profile of the conduction band minimum (CBM) and the valence band maximum (VBM) in a GaN/InGaN/GaN structures with different InGaN thicknesses is shown. No doping is considered for better illustration. The direction of the polarization field in InGaN results in an upward tilt of the bands with respect to the surface. For thicker layers, the voltage difference between the two interfaces of the InGaN layer increases.
Above a critical thickness, VBM becomes larger than EF and a 2DHG forms with a sheet hole densitynsH. The critical thickness depends on the strength of the polarization field and thus on the In content.
The accumulated holes represent a charge which induces an additional field between the lower GaN/InGaN interface and the surface. The direction of this field is opposite to the polarization field. It reduces the effective field within InGaN and results in a downward tilt of the bands in the GaN cap layer. The strength of this field depends onnsH, and for large nsH, the CBM drops belowEF at the upper InGaN/GaN interface. Electrons accumulate and form a 2DEG with a sheet electron densitynsE. The formation of the 2DEG requires sufficiently highnsH. Therefore, a 2DEG forms at thicker InGaN barriers compared to the 2DHG. In addition,nsE<nsH for all InGaN
1.0E13
320 340 360 380 400
no doping
no doping
a) b)
Figure 6.18: a) Profile of CBM and VBM for a 4 nm In0.2Ga0.8N barrier with varying GaN cap layer thickness. An undoped structure is given for comparison.
b) Sheet carrier density over GaN cap layer thickness for a 20 nm In0.2Ga0.8N barrier. The electron density increases with cap layer thickness, with the hole density remaining almost unaffected.
layer thicknesses. This is shown for the thin cap layer in Figure 6.17 b), where now doping is taken into account.
The situation changes for thicker GaN cap layers because of screening of the surface potential. As shown in Figure 6.18 a)the screening results in a downward band bending below the surface. For thicker GaN cap layers, CBM drops belowEF and a 2DEG forms with no 2DHG existing. Therefore, 2DEGs may form at InGaN thicknesses lower than the critical thickness for 2DHG formation. This is shown in Figure 6.17 b) for the thicker cap layer. With increasing cap layer thickness,nsE increases while nsH remains constant as shown in Figure 6.18 b). Thick cap layers are thus needed
0.2 0.4 0.6 0.8 1.0
Figure 6.19: Electron concentration over indium content and InGaN thickness with 50 nm GaN cap layer. Four areas are indicated: 2DEG depletion, nsE>nsH= 0,nsE>nsH andnsE<nsH. Structures with only 2DEGs are limited to a very narrow range at low indium content.
0.2 0.4 0.6 0.8 1.0 5
10 15 20 25 30 35
InGan thickness (nm)
indium content (x)
nsE (cm-2)
1E12
1E13
2E13
3E13
4E13
nsE < nsH
nsE > nsH
nsH = 0
Figure 6.20: Electron concentration over indium content and InGaN thickness with 20 nm Al0.2Ga0.8N and 2 nm GaN cap layer. Three areas are indicated:
nsE>nsH= 0,nsE>nsH andnsE<nsH. Structures with only 2DEGs are achieved for various InGaN thicknesses.
for nsE>nsH, and structures with 50 nm cap layers are considered in the following. However, the number of holes increases stronger with the InGaN thickness thannsE, and nsE<nsH is obtained for thick InGaN barriers with higher indium content even for thick cap layers.
A contour plot of nsE depending on InGaN thickness and In content for a 50 nm capped GaN/InGaN/GaN structure is shown inFigure 6.19. For thin layers with low indium content neither a 2DEG nor a 2DHG forms. At thicker barriers with high In content, mainly p-conductivity is expected. A range of structures with nsE >nsH can be found. Structures with nsE>nsH= 0 are, however, limited to low In concentrations.
By introducing an AlGaN layer as shown in Figure 6.16, the range of structures withnsE>nsH= 0 can be extended as shown inFigure 6.20. The strained AlGaN layer induces an additional positive polarization charge at the InGaN/AlGaN interface while the negative charge at the GaN/InGaN interface remains unchanged (see Figure 6.16). As a result, nsE increases stronger by introducing the AlGaN layer compared tonsH, but the critical thickness for 2DHG formation remains unchanged. Therefore, structures with only two dimensional electron accumulation are obtained at thinner InGaN barriers. The growth of GaN/InGaN/AlGaN/GaN structures with thin InGaN layers therefore reduces the contribution of a p-conductive channel.