A delta-doped quantum well system with additional modulation doping
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A delta-doped quantum well with additional modulation doping may have potential applications. Utilizing such a hybrid system, it is possible to experimentally realize an extremely high two-dimensional electron gas (2DEG) density without suffering inter-electronic-subband scattering. In this article, the authors report on transport measurements on a delta-doped quantum well system with extra modulation doping. We have observed a 0-10 direct insulator-quantum Hall (I-QH) transition where the numbers 0 and 10 correspond to the insulator and Landau level filling factor ν = 10 QH state, respectively. In situ titled-magnetic field measurements reveal that the observed direct I-QH transition depends on the magnetic component perpendicular to the quantum well, and the electron system within this structure is 2D in nature. Furthermore, transport measurements on the 2DEG of this study show that carrier density, resistance and mobility are approximately temperature (T)-independent over a wide range of T. Such results could be an advantage for applications in T-insensitive devices.
KeywordsGaAs Transport Measurement Magnetic Field Component Total Magnetic Field Radiative Recombination Rate
two-dimensional electron gas
molecular beam epitaxy
Advances in growth technology have made it possible to introduce dopants which are confined in a single atomic layer . Such a technique, termed delta-doping, can be used to prepare structures which are of great potential applications. For example, many novel structures based on delta-doped structures [2, 3, 4, 5, 6, 7, 8, 9, 10] can be experimentally realized using very simple fabrication techniques. It is found that delta-doped quantum wells may suffer from surface depletion and carrier freeze-out, which compromise their performances, thereby limiting their potential applications. To this end, a delta-doped quantum well with additional modulation doping can be useful. The modulation doping provides extra electrons so as to avoid carrier freeze-out. On the other hand, it preserves the advantages of a delta-doped quantum well structure, such as an appreciable radiative recombination rate between the two-dimensional electron gas (2DEG) and the photo-generated holes , and an extremely high 2DEG density, suitable for high-power field effect transistor . It is worth mentioning that doped quantum wells with additional modulation doping [11, 12, 13, 14, 15, 16] have already been used to study the insulator-quantum Hall (I-QH) transition [17, 18, 19, 20, 21, 22, 23], a very fundamental issue in the fields of phase transition and Landau quantization. In order to fully realize its potential as a building block of future devices, it is highly desirable to obtain thorough understanding of the basic properties of a delta-doped quantum well with additional modulation doping. In this article, extensive resistance measurements on such a structure are described. At low temperatures (0.3 K ≤ T ≤ 4.2 K), the authors have observed a low-field direct I-QH transition. In situ tilted-field experiments demonstrate that the observed direct I-QH transition only depends on the magnetic field component applied perpendicular to the quantum well, and thus the electron system within our device is 2D in nature. Resistivity, carrier density, and hence mobility of the device developed are all weakly temperature dependent. These results may be useful for simplifying circuitry design for low-temperature amplifiers, and devices for space technology and satellite communications since extensive, costly and time-consuming tests both at room temperature and at low temperatures may not be required.
The sample that we used in these experiments was grown by molecular beam epitaxy (MBE). The layer sequence was grown on a semi-insulating (SI) GaAs (100) substrate as follows: 500 nm GaAs, 80 nm Al0.33Ga0.67As, 5 nm GaAs, Si delta-doping with a density of 5 × 1011 cm-2, 15 nm GaAs, 20 nm undoped Al0.33Ga0.67As, 40 nm Al0.33Ga0.67As layer with a Si-doping density of 1018 cm-3, and 10 nm GaAs cap layer. It is found that electrical contacts to a delta-doped quantum well with the same doping concentration do not show Ohmic behaviour at T < 30 K. Therefore, additional modulation doping is introduced in order to provide extra carriers so as to avoid this unwanted effect. As shown later, the carrier density of the 2DEG is indeed higher than the delta-doping concentration. Moreover, the electrical contacts to the 2DEG all show Ohmic behaviour over the whole temperature range (0.3 K ≤ T ≤ 290 K). Both results demonstrate the usefulness of additional modulation doping. The sample was processed into a Hall bar geometry using standard optical lithography. The sample studied in this study is different from that reported in Ref.  but was cut from the same wafer. Low-temperature magnetotransport measurements were performed in a He3 cryostat equipped with an in situ rotating insert. Transport measurements over a wide range of temperature were performed in a closed-cycle system equipped with a water-cooled electric magnet.
Both the strong and weak localization effects can compensate the reduced electron-phonon effect with decreasing T. To clarify the dominant mechanism leading to the compensation in this study, it is noted that the direct I-QH transition inconsistent with the global phase diagram of the quantum Hall effect reveals the absence of the strong localization [17, 18]. The magneto-oscillations following the semiclassical Shubnilkov-de Haas formula when B < 6T also indicates that the strong localization is not significant near B = 0 [14, 23]. Therefore, the weak localization effect should be responsible for the enhancement of the multiple scattering, compensating for the reduced electron-phonon effect .
In summary, electrical measurements of a delta-doped single quantum well with additional modulation doping have been presented. A direct I-QH transition in such a structure has been observed. In situ tilted-field measurements demonstrate that the observed 0-10 transition only depends on the magnetic field component applied perpendicular to the quantum well, and therefore the electron system within the sample studied is 2D in nature. Neither carrier freezeout nor second electronic subband at a high density of 6.5 × 1015 m-2 is observed in the system proposed. Transport measurements over a wide range of temperature reveal that ρ xx , n and μ all show very weak T dependencies. These results could be useful for devices which can maintain their characteristics over a wide range of temperature. Our results could also be useful for circuit design for low-temperature amplification, and devices for space technology and satellite communications.
This study was funded by the NSC, Taiwan.
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