Effects of Dry Etching and Hydrogen Passivation on Transport Properties and Photoluminescence of GaAs/AlGaAs Heterostruc
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ABSTRACT We present an investigation of electron cyclotron resonance plasma etching induced damage of the 2-dimensional electron gas (2DEG) in GaAs/A1GaAs heterostructures using low temperature photoluminescence (PL), electron paramagnetic resonance (EPR) and the Shubnikov-de Haas (SdH) effect. Dry etching of half of the 20 nm top layer of GaAs results in a decrease of the single-particle relaxation time by 20 - 50%, while the concentration of a surface-related paramagnetic defect increases by about one order of magnitude. At the same time, the PL intensity decreases by a factor of 5 - 10. Plasma hydrogenation experiments, annealing and wet etching experiments have been carried out, and the different characterization results are related to each other. We find that passivation in a hydrogen plasma, which leads to a strong increase of the PL intensity of etched as well as unetched samples, causes damage to the transport properties of the 2DEG. The defect concentration is not related in a simple way to the changes of optical and transport properties.
INTRODUCTION The use of an electron cyclotron resonance (ECR) plasma together with metalorganic reactive ion etching (MORIE) has provided a new technique to fabricate low-dimensional structures. MORIE has been demonstrated to be a good alternative to Cl-based etching chemistry [1-3]. Methane and hydrogen which are usually used for MORIE are non-toxic and easy to handle. Good surface morphology, controllability and low etching rates make MORIE a suitable process for etching of GaAs/AIGaAs heterostructures without the need to form an etch-stop layer. It is also expected that low energy ECR plasma will introduce less damage in the structures. Devices based on the high mobility of a 2-dimensional electron gas (2DEG) at GaAs/AlGaAs interfaces are often manufactured using dry etching. It is known that dry etching introduces radiation damage to the structures and drastically reduces the photoluminescence (PL) of quantum wells and 2DEGs by reducing the lifetime of free carriers [4-6]. However, very little experimental data is available on the effects of MORIE on transport properties in 2DEGs [1]. In this paper we present a combination of transport and PL experiments to study radiation damage in GaAs/AlGaAs 2DEG structures induced by ECR-MORIE. Microwave detected Shubnikov-de Haas (SdH) oscillations were used to determine the single-particle relaxation time ts and the sheet 85 Mat. Res. Soc. Symp. Proc. Vol. 326. &1994 Materials Research Society
carrier concentration ns. Electron paramagnetic resonance (EPR) was used to monitor the density of paramagnetic defects in the sample. The samples were characterized before and after MORIE and, in different experiments, the role of hydrogen passivation, heat treatments and wet etching was investigated.
EXPERIMENTAL The samples used in these experiments were modulation-doped 2DEG GaAs/AlxGalxAs (x=0.3) structures grown by metalorganic vapor phase epitaxy (MOVPE). The heterostructure consisted of a 800 nm thick undoped GaAs layer, a 15 n
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