Quantum galvanomagnetic phenomena in a quasi-two-dimensional hole gas confined within ge/p Ge1-xSix heterosystem icon

Quantum galvanomagnetic phenomena in a quasi-two-dimensional hole gas confined within ge/p Ge1-xSix heterosystem




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НазваQuantum galvanomagnetic phenomena in a quasi-two-dimensional hole gas confined within ge/p Ge1-xSix heterosystem
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Ig. 8 for paper of M V Yakunin
Quantum galvanomagnetic phenomena in a quasi-two-dimensional hole gas confined within ge/p Ge1-xSix heterosystem

УДК 621.315



QUANTUM GALVANOMAGNETIC PHENOMENA IN A QUASI-TWO-DIMENSIONAL HOLE GAS CONFINED WITHIN Ge/p Ge1-xSix HETEROSYSTEM

M.V. Yakunin, G.A. Alshanskii, Yu.G. Arapov, V.N. Neverov, G.I. Harus, N.G. Shelushinina,


Institute for Metal Physics, Russian Academy of Sciences, Ekaterinburg 620219, Russia

E-mail: yakunin@imp.uran.ru, phone +7(3432) 499144

O.A. Kuznetsov


Physico-technical institute at Nizhnii Novgorod State University, Nizhnii Novgorod, Russia

A. de Visser, L. Ponomarenko


Van der Waals - Zeeman Institute, University of Amsterdam, The Netherlands


1. INTRODUCTION

A double quantum well (DQW), i.e. a system of two coupled two-dimensional (2D) layers, is an object, in which a number of unusual phenomena are realized. Since the distance between the layers is possible to make much less than the average distance between the carriers within a layer, interlayer correlated states may form in this system. These states cause a number of anomalies, which manifest especially brightly in the quantum Hall effect (QHE) regime: in the structure of the QHE experimental traces, in their temperature evolution, in the influence of the parallel component of magnetic field etc. [1].

To insert an artificial barrier between the layers is not the only way to create a DQW. A system of coupled 2D layers forms spontaneously in a quantum well within a heterostructure with modulation doped side barriers when the well is sufficiently wide and the carrier gas density in the well is high. The carriers in the well repulse each other since they have uncompensated charges, leading to a parabolic component of the potential, that results in the up-bending of the well bottom and formation of smaller scale triangular wells next to the sidewalls. The bottom bending amplitude increases with carrier gas density ps and the well width dw – see the inserts in fig.8. When the bending amplitude approaches the Fermi level the carrier gas separates into two 2D sublayers located in the triangular wells.

The separation of the electronic gas into sublayers has been observed in a number of experiments [1,2]. Our task is to explore such processes in the hole gas. The valence band differs from the conduction band in its more complex energy spectrum, which consists of two branches for the heavy holes and light holes. The interlayer tunneling is considerably depressed here due to the large heavy hole effective mass resulting in that the separation into sublayers is easier in a hole system.


2. EXPERIMENTAL

A series of p GexSix/Ge/p-Ge1-xSix (x = 0.070.1) quantum wells have been investigated, having central parts of Ge1-xSix barriers selectively doped with boron acceptors, different in the Ge layer width dw = 840 nm and hole gas density within the Ge layer ps = (15)·1015 m-2 (see Table 1). A good quality of the samples is seen in the hole mobilities  (4.2К)  1.7 m2/V·s, the highest among those achieved in the Ge-Si heterosystems. Measurements were performed in steady magnetic fields up to B = 12 T at temperatures T  0.1 K and in pulsed fields up to 40 T at T  1.6 K. Temperatures below 1.6 K were obtained in the He3/He4 dilution refrigerator.

Wide ranges of sample parameters studied yield a possibility to trace the qualitative changes in the hole gas from an almost ideal one, confined in a narrow rectangular well with the only ground subband populated, to a quasi-2D gas filling two subbands, and further on – to a system of two 2D sublayers in the self-formed DQW. These transformations manifest brightly in the observed picture of the quantum Hall effect, where the passing of the Fermi level through mobility gaps correspond to plateaus in the magnetic field dependence of the Hall magnetoresistivity (MR) at the values xy = h/ie2, with i = 1,2,… – the number of Landau levels under the Fermi level, and to minima in the longitudinal MR xx(B).


3. AN INTEGRATED HOLE GAS

In a relatively narrow QW, when population of the second subband is excluded, a regular structure of QHE is observed with even-numbered peculiarities [plateaus in xy(B) and minima in xx(B)] prevailing (fig.1).

Population of the second subband in samples with the wider wells or higher hole densities (more certainly – with the higher values of parameter psdw2) is reflected in changes of the structure of QHE experimental traces, particularly – the even-numbered peculiarities are changed for the odd-numbered ones (fig.2). This is due to that the Fermi level now moves within the energy range where the Landau levels form a complicated picture resulting from the superposition of the second subband levels on the ground subband levels.

The level positions may be extracted with high precision from an analysis of the temperature evolution of the xx(B) minima (fig.3). Since the minimum corresponds to the Fermi level position in the middle of a mobility gap, the activation energy obtained from the elevation of the minimum with temperature determines the gap between corresponding Landau levels. Especially interesting here are the gaps between the levels, which belong to different subbands. Since the smallest changes in the well width, as well as of the depth and shape of the well, lead to the changes in the intersubband distances, these activation gaps are highly sensitive to the QW parameters. This property is illustrated in fig.4, where depicted is how the differences in the well width were determined for two samples (1124 and 1125) with nominally the same parameters.


4. A SELF-ORGANIZED DOUBLE QUANTUM WELL

4.1. ALMOST A COMPLETE DIVISION INTO SUBLAYERS

In Ge layers of width dw  ~35 nm containing the density ps > ~5·1015 m-2 of holes, the hole gas is divided into two 2D sublayers. A bright indication of this is the disappearance of the QHE peculiarities for the filling factor i = 1 (as calculated for the whole width of a Ge layer) while they are quite distinct in the samples with more narrow wells [3] – fig.5.

In those samples (475/476) where the separation into sublayers is almost complete, a positive MR is observed in the weak field region (fig.6), while in the samples with more narrow layers the weak field negative MR is robustly observed originating from the quantum interference corrections to conductivity: fig.1. The simplest explanation of the positive MR is based on the existence of two kinds of carriers with different mobilities. Naturally is to connect them with the holes of the two self-formed sublayers located next to the normal and inverted interfaces of the Ge layer. Within the model of two types of carriers it was found for concrete samples that mobilities in the sublayers differ a factor of two or more (fig.6b). The results of modeling the QHE (B) curves in the vicinity of i = 2 plateau indicate that the hole densities in the sublayers differ no more than 20%. Therefore the differences in mobilities are due to different quality of the Ge layer interfaces, not to an asymmetry of the formed DQW. The former is because the normal and inverted interfaces are formed in different growth conditions. The inverted interface (which is on the substrate side) is formed on the layer of Ge1-xSix solid solution containing randomly distributed Ge and Si atoms among the lattice sites. Additionally, this layer is selectively doped, and the dopants have a property to float up approaching the interface. This leads to a reduction of mobility in the adjacent sublayer. Such problems are absent during formation of the opposite (normal) interface, which proceed on the layer of undoped elementary crystal matter – Ge. Thus, the QHE in wide layers, under conditions of a complete division of the hole gas into sublayers, acts as an instrument for control of the relative interface quality and of a symmetry of the potential profile.


4
a
.2. AN ITERMEDIATE STAGE OF DIVISION INTO SUBLAYERS.


A QUANTIZED HALL INSULATOR

In the layers of approximately the same width as in samples 475/476, dw  35 nm, but containing a considerably lower hole density, ps  1·1015 m-2, an unusual picture of QHE is discovered with anomalously wide plateau of the Hall resistance xy(B) = h/2e2, which extends into the magnetic fields where the longitudinal MR xx(B) already tends to large values reflecting a transition into the insulator phase [4] – fig.5. A similar picture of QHE has been revealed in the Princeton university group [5], where this phase has been titled a “quantized Hall insulator” (QHI). The nature of this hardly realizable state is tentatively associated with a specific character of the random potential in the heterosystem, leading to formation of the network of percolationaly linked 2D paddles of a certain size and small distribution in their effective diameter [6]. The low hole gas density in our wide layers, where the QHI phase have been observed, indicates according to our calculations of the potential profile and energy levels that the division into sublayers is in the intermediate stage. The fact that the QHI phase was observed just in these samples indicates a stabilization of this phase due to inter(sub)layer correlations.


5. A PARALLEL MAGNETIC FIELD

In the measurements under magnetic fields configured parallel to the layers, a strong negative MR is revealed in a series of samples with wide layers, dw > ~20 nm, reaching 30-40 % of its zero field value with minimum at the fields higher than 10 T, while in samples with narrow layers, dw < ~10 nm, the MR is no higher than 1 % and is positive – fig.7.

In the inserts to fig.8 presented are the QW potential profiles, energy levels and wave functions calculated from the self-consistent solution of the Schrodinger and Poison equations. The exact quantum mechanical solution for the valence band should be obtained on the basis of the Luttinger Hamiltonian. But because of its complicated matrix structure this is a rather cumbersome task [7]. To make it easier, we solved the Schrodinger equation with the scalar Hamiltonian, but considered an important feature of the valence band energy spectrum in a QW – its strong anisotropy [8]. The confinement energy levels in the valence band are predetermined only by the bulk effective hole mass. In our case this is a Ge heavy hole mass for the <111> direction: mhhv111/m0 = 0.50 [9]. Contrary, the motion along the layer is characterized by the strong intermixture of the heavy and light hole states. Therefore the hole mass at the bottom of the first confinement subband Ehh1(k||) is essentially smaller than the bulk one and, according to estimations [8], is about mhh1/m0 = 0.09 in Ge. It may be even smaller considering the crystallographic anisotropy and effects of a mechanical stress, approaching the bulk light hole mass along <111> direction mlhv111/m0 = 0.04. On the other hand, the heavy hole confinement subbands are essentially nonparabolic [8]. In the first subband the mass mhh quickly grows up with energy. The hole effective mass in our samples was determined from the temperature damping of the Shubnikov – de Haas oscillations in the weak field region to be mhh1/m0 = 0.10  0.12 [10]. The results in fig.8 are in fact calculated for an electron with anisotropic mass: m/m0 = 0.5 и m|| /m0 = 0.1. Comparison for certain QWs of the calculations based on the Luttinger Hamiltonian and those for anisotropic scalar Hamiltonian indicate the correctness of our estimations.

According to the calculated energy level picture (fig.8), in samples 1123 and 1125 with relatively wide layers but with undivided hole gas the Fermi level in zero magnetic field is inside the second subband. Therefore the observed strong negative MR in these samples is caused by the depopulation of the second subband due to its diamagnetic shift in energy, that extinguish the intersubband scattering (fig.8b,c). As is seen in fig.8d, in samples 475/476 (with divided hole gas) the Fermi level enters the third subband E3 while the two lowest subbands – E1 и E2 – almost coincide in the DQW profile. This result is just due to the combination of the small heavy hole mass for motion along the layer mhh|| and the large value of the mass that determine the spatial quantization; it cannot be achieved in calculations for isotropic hole mass mhhv/m0 = 0.50, and in this property the peculiar nature of the valence band is brightly manifested. Thus, the strong negative MR in samples 475/476 may be explained in terms of the same mechanism that works in samples 1123 and 1125 with the second subband populated. Namely, as a consequence of the upper subband depopulation due to its diamagnetic shift in energy under parallel magnetic field. The only difference is that in samples 475/476 the third subband is depopulated, not the second one. Although in zero field the third subband contained no more than 10 % of all holes in the sample, a possibility for holes in the first and second subbands to scatter elastically into the third subband, when the Fermi level is higher than the third subband edge, reduces considerably the average hole mobility. Thus the extinguishing of this scattering channel, as in the samples 1123 and 1125, yields a strong negative MR.

At the same time the smallness of the portion of holes residing in the third subband leads to that the conclusions made earlier for samples 476/476 still hold. Indeed, considering the holes in the third subband as the second type of holes with different mobility doesn’t make it possible to describe the positive MR in fig.6. Therefore the conclusion about the different mobilities in the self-formed sublayers remains valid. Under perpendicular magnetic fields the third subband depopulates at much lower fields (~2 T) than due to its diamagnetic shift under parallel field. Therefore the initial population of this subband doesn’t change the conclusion about the coincident Landau level fans corresponding to two lowest subbands, which only remain populated at fields B > ~2 T.

In the parallel magnetic field induced MR of the samples with divided hole gas, some additional weak structure is observed (fig.7). These MR singularities may be due to the complex structure of the upper hole subband. As it follows from calculations [8] (for the simple model of an infinite rectangular well), the second and the third heavy hole subbands in Ge, contrary to the first one, have additional lateral extrema in their energy dispersion Ei(k||). On the other hand, the existence of these singularities only in samples with the self-formed DQW while they are absent in samples 1123 and 1125, in which the upper subband is also populated but the DQW profile has not formed yet, may witness the existence of some additional peculiarities in the hole energy spectrum. For example, – those due to the relative lateral shift in parallel magnetic field of the energy dispersion paraboloids E(k||) connected with each of the self-formed sublayers [11].

In summary, the main results are as follows. Shown is that information may be obtained concerning the relative position of confinement subbands and the quantum well parameters from analysis of the changes in the structure of QHE experimental traces with the well width and hole gas density. Investigations in parallel magnetic fields, where the strong negative MR for populated upper subbands is revealed, yield analogous information. For wells wider than ~35 nm the hole gas is found to divide in two 2D sublayers. Analysis of MR in this situation yields the relative characteristics of the normal and inverted interfaces of the Ge layer. In the intermediate stage of division of the hole gas into sublayers the new phase, a quantized Hall insulator, is discovered.

Support of Russian Foundation for Basic Researches, projects 02-02-16401 and 01-02-17685, is greatly acknowledged.


References for paper of M V Yakunin et al.. «QUANTUM GALVANOMAGNETIC PHENOMENA IN A QUASI-TWO-DIMENSIONAL HOLE GAS CONFINED WITHIN Ge/p Ge1-xSix HETEROSYSTEM»


[1] J P Eisenstein, Experimental studies of multicomponent quantum Hall systems, Chap 2, in Perspectives in Quantum Hall Effect, Eds S Das Sarma, A Pinczuk, pp 37-70, John Wiley, NY (1997); S M Girvin and A H MacDonald, Multicomponent quantum Hall systems, Chap 5, ibid., pp 161-224.

[2] Y W Suen, J Jo, M B Santos, L W Engel, S W Hwang, M Shayegan, Missing integral quantum Hall effect in a wide quantum well // Phys Rev B 44(11), pp 5947-5950 (1991).

[3] Yu G Arapov, G I Harus, V N Neverov, N G Shelushinina, M V Yakunin, G A Alshanskii, O A Kuznetsov, Probing the p Ge1 xSix/Ge/p-Ge1-xSix quantum well by means of the quantum Hall effect // Nanotechnology, 11(4), pp 351-358 (2000).

[4] M V Yakunin, Yu G Arapov, O A Kuznetsov, V N Neverov, Bistability of quantum magnetotransport in multilayered Ge/p-Ge1-xSix heterostructure with wide potential wells // JETP Letters, 70(4), pp 301-308 (1999).

[5] M Hilke, D Shahar, S H Song, D C Tsui, Y H Xie, Don Monroe, Experimental evidence for a two-dimensional quantized Hall insulator // Nature, 395, pp 675-677 (1998).

[6] E Shimshoni, A Auerbach, Quantized Hall insulator: transverse and longitudinal transport // Phys Rev B, 55(15), pp 9817-9823 (1997).

[7] R Winkler, M Merkler, T Darnhofer, U Rossler, Theory for the cyclotron resonance of holes in strained asymmetric Ge-SiGe quantum wells // Phys Rev B, 53(16), pp 10858-10865 (1996); M Kubisa, L Bryja, K Ryczko, J Misiewicz et al. Photoluminescence investigations of 2D hole levels in p-type single AlxGaxAs/GaAs heterostructures // Preprint Cond-mat 0211594.

[8] M I Dyakonov, A V Haetskii, Spatial quantization of holes in a semiconductor with complicated valence band and of carriers in a gapless semiconductor // ZhETF, 82(5), pp 1584-1590 (1982).

[9] J C Hensel, K Suzuki, Quantum resonances in the valence bands of germanium. II. Cyclotron resonances in uniaxially stressed crystals // Phys Rev B, 9(10), pp 4219-4257 (1974).

[10] Yu G Arapov, N A Gorodilov, O A Kuznetsov et al., Magnetoresistance oscillations in Ge/p-Ge1-xSix stressed superlattices under tilted magnetic fields // Fizika i Tehnika Poluprovodnikov, 27(7), pp 1165-1174 (1993).

[11] T Jungwirth, T S Lay, L Smrcka, M Shayegan, Resistance oscillation in wide single quantum wells subject to in-plane magnetic fields // Phys Rev B, 56(3), pp 1029-1032 (1997); S K Lyo, Giant field-induced variation of the cyclotron mass in coupled two-dimensional electron gases // Phys Rev B, 51(16), pp 11160-11163 (1995).

Figure captions for paper of M V Yakunin et al.. «QUANTUM GALVANOMAGNETIC PHENOMENA IN A QUASI-TWO-DIMENSIONAL HOLE GAS CONFINED WITHIN Ge/p Ge1-xSix HETEROSYSTEM»


Fig.1. A typical QHE picture in a sample with undivided hole gas. T = 1.7 K.

In the insert: the enhanced weak field area, where the negative MR is seen.

Fig.2. Changes with population of the second subband in the structure of the longitudinal MR traces under the QHE regime (the psdw2 parameter increases from the lower to upper traces – see the table 1).

Fig.3. Evolution with temperature of the MR minimum in the vicinity of the filling factor i = 2.

Fig.4. The activation energies for sample 1125 in comparison with the data for sample 1124 (lower). The differences are explained in terms of different population of the second subband due to small differences in the well width (in the upper picture – the magnetically quantized energy spectrum: solid lines for sample 1125, the dashed lines – for sample1124; the energy axis is directed into the valence band). The gap is the double the activation energy.

Fig.5. The QHE in samples of three kinds containing a hole gas …

  • integrated over the whole width of the relatively narrow layer – in samples 1006 and 1124;

  • almost a completely divided into two sublayers in samples 475/476, and

  • being in the intermediate stage of separation – in samples 451.

Note the absence of the = 1 state in samples with divided hole gas and an anomalously wide plateau in case of intermediate separation. = 1.6-4.2 K.

Fig.6. Positive MR in samples with the self-formed DQW.

  1. A general view. The traces are positioned in the same order as in the inserted description.

  2. Results of fitting the curves calculated in the model of two kinds of carriers (lines) with the experimental traces for sample 476a4 (circles).

Fig.7. MR under parallel magnetic fields. There is no dependence for the sample with the narrowest well (#578), but the negative, up to ~40%, MR is observed, when the upper subbands are populated, both in case of integrated hole gas (samples 1123, 1125) and for the divided gas (samples 475 and 476).

Fig.8. Parallel magnetic field induced MR of samples 578 (a), 1123 (b), 1125 (c) and 475/476 (d). The well width dw [nm] and the hole gas density ps [1015 m-2] are indicated. In the inserts: the calculated potential profiles, energy levels and the wave functions. In the left inserts are the general views of potential and in the right ones – for enhanced energy. The energy axis direction is into the valence band.



Table 1. The sample parameters.

Sample

,

m2/V·s

ps ,

1015 m-2

dw*, nm

psdw2

578б2

0.3

1.4

8

0.09

1006-1

1.4

4.9

12.5

0.77

1124b3

1.0

2.8

20(21.4)

1.28

1125a7

1.7

2.8

20(22)

1.36

1123a6

1.4

3.4

20(23.5)

1.88

1003-2

1.5

4.8

22

2.32

451б4

0.7

1.4

36

1.81

475/476

0.6-1.3

5

38

7.22


* In the brackets are the results obtained from the data analysis performed in the paper.



Fig.1 for paper of M V Yakunin et al.. «QUANTUM GALVANOMAGNETIC PHENOMENA IN A QUASI-TWO-DIMENSIONAL HOLE GAS CONFINED WITHIN Ge/p Ge1-xSix HETEROSYSTEM»


Fig.2 for paper of M V Yakunin et al.. «QUANTUM GALVANOMAGNETIC PHENOMENA IN A QUASI-TWO-DIMENSIONAL HOLE GAS CONFINED WITHIN Ge/p Ge1-xSix HETEROSYSTEM»

»

Fig.3 for paper of M V Yakunin et al.. «QUANTUM GALVANOMAGNETIC PHENOMENA IN A QUASI-TWO-DIMENSIONAL HOLE GAS CONFINED WITHIN Ge/p Ge1-xSix HETEROSYSTEM»


Fig.4 for paper of M V Yakunin et al.. «QUANTUM GALVANOMAGNETIC PHENOMENA IN A QUASI-TWO-DIMENSIONAL HOLE GAS CONFINED WITHIN Ge/p Ge1-xSix HETEROSYSTEM»





Fig.5 for paper of M V Yakunin et al.. «QUANTUM GALVANOMAGNETIC PHENOMENA IN A QUASI-TWO-DIMENSIONAL HOLE GAS CONFINED WITHIN Ge/p Ge1-xSix HETEROSYSTEM»





Fig.6 for paper of M V Yakunin et al.. «QUANTUM GALVANOMAGNETIC PHENOMENA IN A
(a)
QUASI-TWO-DIMENSIONAL HOLE GAS …»




(b)


xx, k


B, T

Fig.7 for paper of M V Yakunin et al.. «QUANTUM GALVANOMAGNETIC PHENOMENA IN A QUASI-TWO-DIMENSIONAL HOLE GAS CONFINED WITHIN Ge/p Ge1-xSix HETEROSYSTEM»






Fig.8 for paper of M V Yakunin et al.. «QUANTUM GALVANOMAGNETIC PHENOMENA IN A QUASI-TWO-DIMENSIONAL HOLE GAS CONFINED WITHIN Ge/p Ge1-xSix HETEROSYSTEM»





Квантовые гальваномагнитные явления в квазидвумерном дырочном газе гетеросистемы Ge/p Ge1-xSix

М. В. Якунин, Г. А. Альшанский, Ю. Г. Арапов, В. Н. Неверов, Г.И. Харус, Н.Г. Шелушинина, О. А. Кузнецов, A. de Visser, L. Ponomarenko




Исследованы магнитосопротивление и квантовый эффект Холла (КЭХ) дырочного газа в квантовой яме (КЯ) гетеросистемы p GexSix/Ge/p GexSix при толщине слоя Ge от 8 до 38 нм и плотности дырочного газа в слое Ge от 1·1015 до 6·1015 м-2 в магнитных полях до 40 Тл при температурах до 0.1 К. Обнаружены сильные изменения структуры кривых КЭХ при заселении верхних подзон размерного квантования дырок и показано, что из анализа таких изменений можно извлекать детальную информацию о параметрах КЯ. Обнаружено, что при ширине слоя Ge более 35 нм заключенный в слое дырочный газ разделяется на два двумерных подслоя, сосредоточенных у гетерограниц слоя. Анализ магнитосопротивления в таких условиях позволяет получить сравнительные характеристики прямой и обратной гетерограниц слоя. На промежуточной стадии разделения на подслои в данной самоорганизующейся двойной КЯ выявлено существование фазы квантованного холловского диэлектрика. При параллельной слоям конфигурации магнитного поля обнаружено сильное отрицательное магнитосопротивление, обусловленное опустошением верхних подзон вследствие их диамагнитного сдвига по энергии. В параллельном магнитном поле в магнитосопротивлении образцов с разделенным на подслои дырочным газом выявлены дополнительные особенности, отражающие сложную структуру поверхности энергетической дисперсии в двойной КЯ, сформированной в валентной зоне.


QUANTUM GALVANOMAGNETIC PHENOMENA IN A QUASI-TWO-DIMENSIONAL HOLE GAS CONFINED WITHIN Ge/p Ge1-xSix HETEROSYSTEM


M.V. Yakunin, G.A. Alshanskii, Yu.G. Arapov, V.N. Neverov, G.I. Harus, N.G. Shelushinina, O.A. Kuznetsov, A. de Visser, L. Ponomarenko


Magnetoresistance and the quantum Hall effect (QHE) are studied of the hole gas confined in the p GexSix/Ge/p GexSix quantum well (QW) within ranges of the QW widths 838 nm and hole gas densities (16)·1015 m-2 under magnetic fields up to 40 T and temperatures down to 0.1 K. Strong changes in the structure of the QHE experimental traces are revealed when the upper subbands become populated and shown is that detailed information may be extracted from the analysis of these changes. It was found that in the Ge layers wider than 35 nm the hole gas is separated into two 2D sublayers located next to the interfaces. In this case the analysis of weak field magnetoresistivity yields relative characteristics of the normal and inverted heterojunctions. At the intermediate stage of the self-organized formation of the double QW, the quantized Hall insulator phase is discovered. For magnetic fields configured parallel to the layers, a strong negative magnetoresistance is found caused by the depopulation of the upper subbands due to their diamagnetic shift in energy. Under parallel magnetic fields some additional peculiarities in the magnetoresistivity of the samples with divided hole gas have been found, which are tentatively explained as being due to the complicated structure of the energy dispersion surface of the double QW formed in the valence band.




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