A deltadoped quantum well system with additional modulation doping
 DongSheng Luo^{},
 LiHung Lin^{2}Email author,
 YiChun Su^{3},
 YiTing Wang^{3},
 Zai Fong Peng^{2},
 ShunTsung Lo^{3},
 Kuang Yao Chen^{3},
 YuanHuei Chang^{3},
 JauYang Wu^{4},
 Yiping Lin^{1}Email author,
 ShengDi Lin^{4},
 JengChung Chen^{1},
 ChunFeng Huang^{5} and
 ChiTe Liang^{3}Email author
DOI: 10.1186/1556276X6139
© Luo et al; licensee Springer. 2011
Received: 31 July 2010
Accepted: 14 February 2011
Published: 14 February 2011
Abstract
A deltadoped quantum well with additional modulation doping may have potential applications. Utilizing such a hybrid system, it is possible to experimentally realize an extremely high twodimensional electron gas (2DEG) density without suffering interelectronicsubband scattering. In this article, the authors report on transport measurements on a deltadoped quantum well system with extra modulation doping. We have observed a 010 direct insulatorquantum Hall (IQH) transition where the numbers 0 and 10 correspond to the insulator and Landau level filling factor ν = 10 QH state, respectively. In situ titledmagnetic field measurements reveal that the observed direct IQH 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 Tinsensitive devices.
Introduction
Advances in growth technology have made it possible to introduce dopants which are confined in a single atomic layer [1]. Such a technique, termed deltadoping, can be used to prepare structures which are of great potential applications. For example, many novel structures based on deltadoped structures [2–10] can be experimentally realized using very simple fabrication techniques. It is found that deltadoped quantum wells may suffer from surface depletion and carrier freezeout, which compromise their performances, thereby limiting their potential applications. To this end, a deltadoped quantum well with additional modulation doping can be useful. The modulation doping provides extra electrons so as to avoid carrier freezeout. On the other hand, it preserves the advantages of a deltadoped quantum well structure, such as an appreciable radiative recombination rate between the twodimensional electron gas (2DEG) and the photogenerated holes [9], and an extremely high 2DEG density, suitable for highpower field effect transistor [8]. It is worth mentioning that doped quantum wells with additional modulation doping [11–16] have already been used to study the insulatorquantum Hall (IQH) transition [17–23], 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 deltadoped 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 lowfield direct IQH transition. In situ tiltedfield experiments demonstrate that the observed direct IQH 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 lowtemperature amplifiers, and devices for space technology and satellite communications since extensive, costly and timeconsuming tests both at room temperature and at low temperatures may not be required.
Experimental details
The sample that we used in these experiments was grown by molecular beam epitaxy (MBE). The layer sequence was grown on a semiinsulating (SI) GaAs (100) substrate as follows: 500 nm GaAs, 80 nm Al_{0.33}Ga_{0.67}As, 5 nm GaAs, Si deltadoping with a density of 5 × 10^{11} cm^{2}, 15 nm GaAs, 20 nm undoped Al_{0.33}Ga_{0.67}As, 40 nm Al_{0.33}Ga_{0.67}As layer with a Sidoping density of 10^{18} cm^{3}, and 10 nm GaAs cap layer. It is found that electrical contacts to a deltadoped 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 deltadoping 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. [14] but was cut from the same wafer. Lowtemperature magnetotransport measurements were performed in a He^{3} cryostat equipped with an in situ rotating insert. Transport measurements over a wide range of temperature were performed in a closedcycle system equipped with a watercooled electric magnet.
Results
Both the strong and weak localization effects can compensate the reduced electronphonon effect with decreasing T. To clarify the dominant mechanism leading to the compensation in this study, it is noted that the direct IQH transition inconsistent with the global phase diagram of the quantum Hall effect reveals the absence of the strong localization [17, 18]. The magnetooscillations following the semiclassical Shubnilkovde 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 electronphonon effect [25].
Conclusions
In summary, electrical measurements of a deltadoped single quantum well with additional modulation doping have been presented. A direct IQH transition in such a structure has been observed. In situ tiltedfield measurements demonstrate that the observed 010 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 × 10^{15} 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 lowtemperature amplification, and devices for space technology and satellite communications.
Abbreviations
 2DEG:

twodimensional electron gas
 IQH:

insulatorquantum Hall
 MBE:

molecular beam epitaxy
 RT:

room temperature
 SI:

semiinsulating
 T :

temperature.
Declarations
Acknowledgements
This study was funded by the NSC, Taiwan.
Authors’ Affiliations
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