Photoluminescence Related to the 2-Dimensional Electron Gas in Modulation Doped GaN/AlGaN Structures
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electrons normally have a very high mobility, since the carriers are separated from dopants in the barrier layer. This fact is frequently used for device applications, such as the high electron mobility transistor. The properties, electrical and optical, related to the 2DEG electrons have previously been extensively studied by several groups, mainly in the GaAs/AIGaAs [2-6] but also in the InGaAs/InP [7] material system. Single and double heterostructures, involving different nitride compounds, are also expected to be used for optoelectronic devices. The understanding of the recombination processes related to the modulation doped heterointerface and quantum wells is thus of vital importance. EXPERIMENTAL DETAILS The samples used in this study were grown by Metal Organic Vapour Phase Epitaxy (MOVPE) on optical grade polished sapphire substrates. An initial AIN buffer layer was introduced to accommodate the lattice mismatch with the substrate. For the single heterostructure sample this was followed by an intentionally undoped GaN layer and a 0.16 mm thick Si doped Al 0 .3 Ga0. 7 N barrier, see Fig. la. For the quantum well sample two additional AIGaN layers as well as a GaN buffer layer was added, see Fig lb. PL and time-resolved PL measurements were performed, in the temperature range from 2 K to room temperature, using pico second pulsed excitation from a frequency doubled mode locked dye laser. For
595 Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
spectral measurements the emitted PL was detected with a cooled CCD camera, while the time resolved measurements were performed with a syncroscan streak camera with a time resolution of about 20 ps. For longer time decays we used a time correlated photon counting system with a time resolution of about 150 ps.
EXPERIMENTAL RESULTS AND DISCUSSION
b) A1GaN :Si
0.1
AIGaN
0.003
a) A1GaN "Si
0.16
AlGaN
2
AIN
0.05
GaN AIGaN
1 0.07
Sapphire
[jm]
AIN
0.05
Sapphire
[I4m]
Figure 1. Schematic description of the two different modulation doped structures. a) shows the modulation doped single hetero interface, while b) shows the modulation doped quantum well sample. The 2DEG is, in both samples, formed in the shaded GaN layer.
The 2DEG in the single heterojunction sample is formed in the GaN layer at the interface to the modulation doped AlGaN barrier. The electron concentration in the 2DEG is about 1013 cm- 2 , as measured by electrical measurements as shown in Fig. 2. The electron concentration is almost independent of temperature. The mobility shows, on the other hand, a strong temperature dependence reaching about 4000 cm 2 /Vs at 77 K. This value of the mobility is larger than for bulk GaN, where the highest reported value at 77 K is 3000 cm 2 /Vs [9]. The time-integrated PL spectra of a modulation doped single heterostructure GaN/AlGaN sample are shown in Fig. 3 for different temperatures. The spectra are dominated by exciton emissions typical for bulk GaN, originating from the flat band region of the GaN epitaxial layer. At low temp
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