Selective growth of MOVPE on AlGaAs/GaAs patterned substrates for quantum nano-structures
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the device application, it is very hard to bury them because no crystal growth occurs on the masked region. In this paper, we propose and demonstrate selective area MOVPE using AIGaAs native oxide7' as a new mask material. Stable thin oxide layer can be formed on AlGaAs surface simply by exposing to air. After AlGaAs and GaAs thin layer growth, GaAs top layer is partially removed by lithography and etching processes as defined by the mask pattern. The underlying AlGaAs exposed area works as a mask for next GaAs/AlGaAs selective area growth because stable oxide are formed on AlGaAs surface. We also confirm the possibility of the regrowth on the masked region of the air-exposed AlGaAs surface after annealing under AsH 3/IH2 or H 2" ambient. Experimental procedure A low-pressure horizontal MOVPE system was used. The working pressure during crystal growth was 76 Torr. Purified hydrogen (H) was used as a carrier gas, and the total gas flow rate was 3.0 e/min. The source materials were trimethylgallium (TMGa), triethylaluminum (TEAl) and 20% arsine (AsH 3) in H 2. The partial pressure of TMGa and TEAl were 1.9 X 10 6atm and 4.5 X 10 7 atm, respectively. AsH 3 partial pressure and growth temperature were 6.7 X 10 5 atm, 650°C for selective area growth, and 5.0 X 10 atm and 650oC for regrowth on the masked region. Growth rate of GaAs was 1.4 A/sec. The preparation procedure of masked substrates is schematically shown in Fig. 1. First, GaAs buffer layer, 100nm A%Ga -,As and 1Onm GaAs top layer were grown on (001) GaAs substrate by MOVPE. Al compositions of the AlGaAs layer were 0.1, 0.2, 0.3 and 0.4. Next, line and space 259
Mat. Res. Soc. Symp. Proc. Vol. 448 ©1997 Materials Research Society
GaAs cap 10nm X AlxGal_xAs 100nm GaAs buffer
[110] 0aAs
su] (b)
(a)
['fl0]
Fig.1. Schematic view of preparation procedure of substrates. (a) First, GaAs/AlGaAs/GaAs layers were grown by MOVPE. (b) Next, native oxide mask with line & space patterns was formed on AlGaAs for selective area growth.
patterns were defined by electron beam lithography. Then, GaAs top layer was etched off using Citric acid/H 2O2 /H20 solution. The solution has high selectivity for etching rate between GaAs and AlxGalxAs.9). Figure 2 shows an atomic force microscope(AFM) image of AlGaAs native oxide mask. The widths of GaAs line and A1GaAs space were 400nm and 70nm, respectively, and the pattern was aligned along the [100] direction. Air-exposed AlGaAs areas (space) are expected to work as a mask in selective area MOVPE growth. In order to investigate the selectivity, GaAs layers were grown on these masked substrates. Before the growth, the substrates were annealed for 5 minutes within the furnace under AsH 3/H2 atmosphere, which was the standard thermal cleaning process.
500nm Fig.2. AFM image of AlGaAs native oxide masked substrate. Gray and black areas show GaAs and AlGaAs layer, respectively.
Results and discussion The dependence on Al composition of AlxGaj -As native oxide mask was investigated. Figure 3 shows the AFM images of GaAs grown on the
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