Capacitance spectroscopy of InAs self-assembled quantum dots

Capacitance spectroscopy is used to study electronic properties of self-assembled InAs quantum dots. The capacitance-voltage, C(V), measurements in combination with the magneto-capacitance, C(B), results make it possible to investigate the electrostatic profile of a series of single-barrier n-i-n Ga...

Повний опис

Збережено в:

Бібліографічні деталі
Дата:	1998
Автори:	Martin, P.M., Belyaev, A.E., Eaves, L., Main, P.C., Sheard, F.W., Ihn, T., Henini, M.
Формат:	Стаття
Мова:	English
Опубліковано:	Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України 1998
Назва видання:	Semiconductor Physics Quantum Electronics & Optoelectronics
Онлайн доступ:	http://dspace.nbuv.gov.ua/handle/123456789/114663
Теги:	Додати тег Немає тегів, Будьте першим, хто поставить тег для цього запису!
Назва журналу:	Digital Library of Periodicals of National Academy of Sciences of Ukraine
Цитувати:	Capacitance spectroscopy of InAs self-assembled quantum dots / P.M. Martin, A.E. Belyaev, L. Eaves, P.C. Main, F.W. Sheard, T. Ihn, M. Henini // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1998. — Т. 1, № 1. — С. 7-12. — Бібліогр.: 16 назв. — англ.

Репозитарії

Digital Library of Periodicals of National Academy of Sciences of Ukraine

id	irk-123456789-114663
record_format	dspace
spelling	irk-123456789-1146632017-03-12T03:02:14Z Capacitance spectroscopy of InAs self-assembled quantum dots Martin, P.M. Belyaev, A.E. Eaves, L. Main, P.C. Sheard, F.W. Ihn, T. Henini, M. Capacitance spectroscopy is used to study electronic properties of self-assembled InAs quantum dots. The capacitance-voltage, C(V), measurements in combination with the magneto-capacitance, C(B), results make it possible to investigate the electrostatic profile of a series of single-barrier n-i-n GaAs/AlAs/GaAs heterostructures incorporating a layer of self-assembled InAs quantum dots in the AlAs barrier. We find that the negative charge associated with electron filling of the dots is closely compensated by a positive charge in the AlAs barrier, which we ascribe to ionised defects or impurities, possibly in association with the quantum dots. It is shown the compensation degree considerably depends on the growth conditions. Ємнiсна спектроскопiя використується для дослiдження електронних властивостей само-органiзованих InAs квантових точок. Вимiрювання вольт-фарадних характеристик, C(V), одночасно з даними по магнето-емностi, C(B), дозволяють дослiджувати електростатичний профiль однобар.єрних p-i-n GaAs/AlAs/GaAs гетероструктур, якi мiстять в AlAs бар.єрі шар само-органiзованих InAs квантових точок. Ми визначили, що негативний заряд, пов.язаний з заповненням квантових точок електронами, майже повнiстю компенсується позитивним зарядом в AlAs бар.єрi, що, на нашу думку, пов.язано з iонiзованими дефектами чи домiшками, якi обумовлені квантовими точками. Показано, що мiра компенсацiї суттєво залежить вiд ростових умов. Емкостная спектроскопия используется для исследования электронных свойств само-организованных InAs квантовых точек. Измерение вольт-фарадных характеристик, C(V), совместно с данными по магнето-емкости, C(B), позволяют исследовать электростатический профиль однобарьерных p-i-n GaAs/AlAs/GaAs гетероструктур, содержащих в AlAs барьере слой само-организованных InAs квантовых точек. Мы установили, что отрицательный заряд, связанный с заполнением квантовых точек электронами, почти полностью компенсируется положительным зарядом в AlAs барьере, который, по нашему мнению, связан с ионизованными дефектами или примесями, обусловленными квантовыми точками. Показано, что степень компенсации существенно зависит от ростовых условий. 1998 Article Capacitance spectroscopy of InAs self-assembled quantum dots / P.M. Martin, A.E. Belyaev, L. Eaves, P.C. Main, F.W. Sheard, T. Ihn, M. Henini // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1998. — Т. 1, № 1. — С. 7-12. — Бібліогр.: 16 назв. — англ. 1560-8034 PACS 73.20; 75.50 Pp http://dspace.nbuv.gov.ua/handle/123456789/114663 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
institution	Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection	DSpace DC
language	English
description	Capacitance spectroscopy is used to study electronic properties of self-assembled InAs quantum dots. The capacitance-voltage, C(V), measurements in combination with the magneto-capacitance, C(B), results make it possible to investigate the electrostatic profile of a series of single-barrier n-i-n GaAs/AlAs/GaAs heterostructures incorporating a layer of self-assembled InAs quantum dots in the AlAs barrier. We find that the negative charge associated with electron filling of the dots is closely compensated by a positive charge in the AlAs barrier, which we ascribe to ionised defects or impurities, possibly in association with the quantum dots. It is shown the compensation degree considerably depends on the growth conditions.
format	Article
author	Martin, P.M. Belyaev, A.E. Eaves, L. Main, P.C. Sheard, F.W. Ihn, T. Henini, M.
spellingShingle	Martin, P.M. Belyaev, A.E. Eaves, L. Main, P.C. Sheard, F.W. Ihn, T. Henini, M. Capacitance spectroscopy of InAs self-assembled quantum dots Semiconductor Physics Quantum Electronics & Optoelectronics
author_facet	Martin, P.M. Belyaev, A.E. Eaves, L. Main, P.C. Sheard, F.W. Ihn, T. Henini, M.
author_sort	Martin, P.M.
title	Capacitance spectroscopy of InAs self-assembled quantum dots
title_short	Capacitance spectroscopy of InAs self-assembled quantum dots
title_full	Capacitance spectroscopy of InAs self-assembled quantum dots
title_fullStr	Capacitance spectroscopy of InAs self-assembled quantum dots
title_full_unstemmed	Capacitance spectroscopy of InAs self-assembled quantum dots
title_sort	capacitance spectroscopy of inas self-assembled quantum dots
publisher	Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
publishDate	1998
url	http://dspace.nbuv.gov.ua/handle/123456789/114663
citation_txt	Capacitance spectroscopy of InAs self-assembled quantum dots / P.M. Martin, A.E. Belyaev, L. Eaves, P.C. Main, F.W. Sheard, T. Ihn, M. Henini // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1998. — Т. 1, № 1. — С. 7-12. — Бібліогр.: 16 назв. — англ.
series	Semiconductor Physics Quantum Electronics & Optoelectronics
work_keys_str_mv	AT martinpm capacitancespectroscopyofinasselfassembledquantumdots AT belyaevae capacitancespectroscopyofinasselfassembledquantumdots AT eavesl capacitancespectroscopyofinasselfassembledquantumdots AT mainpc capacitancespectroscopyofinasselfassembledquantumdots AT sheardfw capacitancespectroscopyofinasselfassembledquantumdots AT ihnt capacitancespectroscopyofinasselfassembledquantumdots AT heninim capacitancespectroscopyofinasselfassembledquantumdots
first_indexed	2025-07-08T07:46:04Z
last_indexed	2025-07-08T07:46:04Z
_version_	1837064025236570112
fulltext	7© 1998 ²íñòèòóò ô³çèêè íàï³âïðîâ³äíèê³â ÍÀÍ Óêðà¿íè Ô³çèêà íàï³âïðîâ³äíèê³â, êâàíòîâà òà îïòîåëåêòðîí³êà. 1998. Ò. 1, ¹ 1. Ñ. 7-12. Semiconductor Physics, Quantum Electronics & Optoelectronics. 1998. V. 1, N 1. P. 7-12. PACS 73.20; 75.50 Pp Capacitance spectroscopy of InAs self-assembled quantum dots P. M. Martin, A. E. Belyaev, L. Eaves, P. C. Main, F. W. Sheard, T. Ihn*, M. Henini Department of Physics, University of Nottingham, Nottingham, NG7 2RD, UK Present address: Dept. of Electronic & Electrical Engineering, University of Sheffield, Sheffield, S1 3JD, UK On leave from Institute of Semiconductor Physics, NASU, Kyiv, 252028, Ukraine Present address: Solid State Physics Lab., ETH Hoenggerberg, CH-8093 Zurich, Switzerland Abstract. Capacitance spectroscopy is used to study electronic properties of self-assembled InAs quan- tum dots. The capacitance-voltage, C(V), measurements in combination with the magneto-capacitance, C(B), results make it possible to investigate the electrostatic profile of a series of single-barrier n-i-n GaAs/AlAs/GaAs heterostructures incorporating a layer of self-assembled InAs quantum dots in the AlAs barrier. We find that the negative charge associated with electron filling of the dots is closely compensated by a positive charge in the AlAs barrier, which we ascribe to ionised defects or impurities, possibly in association with the quantum dots. It is shown the compensation degree considerably de- pends on the growth conditions. Keywords: MBE growth, InAs quantum dots, capacitance spectroscopy. Paper received 16.09.98; revised manuscript received 19.10.98; accepted for publication 28.10.98. Introduction A semiconductor quantum dot (QD) is the ultimate quan- tum-confined system. Its unique electronic properties rely on the delta-function-like energy dependence of the density of states due to the quantum confinement of carriers in all three dimensions [1]. At the same time, to exploit these prom- ising properties in a new generation of devices, the lateral dimensions of the structures have to be smaller than the prin- cipal characteristic length � the de Broglie wavelength of electrons. The second basic requirement is uniformity in the size and shape of a large ensemble of QDs densely packed in the active volume of the device. Among the existing tech- niques of QD fabrication, direct heteroepitaxial growth of highly strained structures by MBE and MOVPE (self-or- ganized Stranski-Krastanov heteroepitaxial growth [2]) has gained great interest because of the possibility to obtain struc- tures with a desired shape and, most important, narrow size distribution [3]. The success achieved by this technique in recent years attracts much interest to the optical and elec- tronic properties of QD arrays. A lot of works were made to characterise the energy and density of electronic states, elec- trical transport, and recombination processes in the QDs. This has been done using photoluminescence [4�6] and elec- trical techniques [7, 8]. Another method is using capacitance [9�11] or magneto-capacitance spectroscopy [12, 13]. Ca- pacitance studies are very promising for investigation of QD- contained structures and QD electronic states because of their high sensitivity to the charge transferred through the struc- ture. Note that most of capacitance measurements were made on Schottky barrier structures. Such structures have definite advantages, because by applying voltage we can scan through the energy spectrum of QDs. However, the band bending initially present in these structures does not allow observing the effects associated with piezoelectric fields and interface states. Besides, practically all experiments have been car- ried out on QDs embedded in GaAs matrix. In this paper we investigate the capacitance-voltage, C(V), and magneto-capacitance, C(B), characteristics of a series of single-barrier n-i-n GaAs/AlAs/GaAs hete- rostructures incorporating a layer of self-assembled InAs QDs in the AlAs barrier. The thicknesses of the layers of AlAs deposited before and after the InAs QD growth are varied. The C(V) and C(B) results are compared with those for a reference sample having no InAs and a sample in which InAs thickness is such that a continuous wetting layer (WL) of InAs is grown. The shape of the C(V) curve can be un- derstood in terms of a simple device model discussed in this paper. The net charge density in the QD-containing barrier is determined from analysis of the C(V) results in combina- tion with the C(B) results, and is confirmed using results from the reference sample. We find the net charge in all QD samples to be close to zero or positive at zero bias applied. The effect of this charge is to radically alter the picture of the band bending in the devices in comparison to the model derived from consideration of only the QD electronic states [13]. P. M. Martin et al.: Capacitance spectroscopy of InAs self-assembled quantum dots 8 ÔÊÎ, 1(1), 1998 SQO, 1(1), 1998 Experimental procedure The samples presented in this study were grown on a (100) n+GaAs substrate using MBE and all consist of n-type GaAs devices with an AlAs tunnel barrier region surrounded by 100 nm not intentionally doped spacers to minimise the effect of dopant diffusion into the central barrier region. These spacers are then enclosed by 100 nm of n-type lightly doped (1016 cm-3) GaAs layer, followed by 100 nm doped to 1017 cm-3, and then a heavily doped (1018 cm-3) contact layer. The reference sample consists of a 10 nm AlAs single bar- rier, while the WL sample consists of an AlAs double bar- rier structure enclosing 1.4 ML of InAs. The QD samples consist of an InAs QD layer enclosed in AlAs layers grown previously and subsequently to the QD growth. The QDs are produced by deposition of 1.8 ML of InAs at a growth temperature of 520°C. Layer parameters for the samples stud- ied in this work are shown in Table 1. The band diagram of the samples at zero voltage and under bias is schematically shown in fig. 1. Electrical contacts were formed on the doped GaAs layer by AuGe alloying, and devices were fabricated into mesas of 200 µm diameter. In this paper, the conven- tion of negative substrate for forward bias is used. All ca- pacitance measurements were made with a HP 4275A Multi- Frequency LCR Meter, in a variable temperature cryostat. The frequency range of the C(V) analyser was from 10 kHz to 10 MHz. No discernible difference in the functional form of the C(V) curve was found over this range of frequencies. Experimental results and discussion Fig. 2 shows the capacitance-voltage curves of the samples listed in Table 1. The capacitance values for the WL and QD samples have each been displaced by 4 pF for clarity. The capacitance data were obtained using the modulation amplitude of 2 mV at 1MHz with an LCR meter set in paral- lel RC measurement mode. This measurement treats the sam- ple as if it were a parallel combination of impedance, R, and single capacitance, C. Actually, our devices have a more complex equivalent circuit shown in fig. 1(c). The heavily doped regions of the device effectively act as the outer con- ducting plates of a capacitor. In addition, under a sufficient bias a two-dimensional electron gas (2DEG) forms in the QD states V = 0V λe s λd λf eV V =0.2V + + + R1 R2 R3 C1 C2 C3 2 D E G a) b) c) Fig. 1. Schematic band diagrams of the device at zero bias (a) and 0.2V (b), and the equivalent electric circuit of the device (c). Fig. 2. Capacitance-voltage curves for various devices measured at 4K and zero magnetic field. Top five curves are displaced verti- cally by values of ∆C = 4pF each for clarity. elpmaS elpmaS elpmaS elpmaS elpmaS ,reyalrednU ,reyalrednU ,reyalrednU ,reyalrednU ,reyalrednU mn ,reyalsAnI ,reyalsAnI ,reyalsAnI ,reyalsAnI ,reyalsAnI LM ,reyalrevO ,reyalrevO ,reyalrevO ,reyalrevO ,reyalrevO mn A 1.5 8.1 1.5 B 4.3 8.1 4.6 C 1.5 8.1 2.7 D 1.5 8.1 8.3 LW 1.5 4.1 1.5 lortnoC sAlAmn01 reirrabelgnis sDQon Table 1. The layer composition of InAs QD samples and the reference sample. -0,6 -0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 15 20 25 30 35 40 45 C B A D WL control T =4.2K C ( pF ) Voltage (V) P. M. Martin et al.: Capacitance spectroscopy of InAs self-assembled quantum dots 9ÔÊÎ, 1(1), 1998 SQO, 1(1), 1998 accumulation layer adjacent to the left-hand barrier, as shown in fig. 1(b). It acts as a third region of mobile electron charge. The parallel impedance combination C 1 and R 1 represents the slab of GaAs between the heavily doped layer and the 2DEG, C 2 and R 2 represent the barrier and depletion region between the 2DEG and the contact, while C 3 and R 3 ac- count for the effect of the impedance of the undepleted lightly doped region. It can be seen that there is no clear distinction between QD and non-QD samples apparent from the C(V) charac- teristics. All C(V) curves have a common shape for biases above ~200 mV, characterised by a declining capacitance. This is what should be expected, as the capacitance of all samples is simply dominated by that of the depleted contact region. Most variance seen in the samples is in the shape of C(V) around zero voltage. There are four samples that ex- hibit a pronounced dip in C(V), while two of QD samples show a pronounced peak at zero bias. To understand the origin of the form of the C(V) dependences, we will follow the arguments described below. If at zero bias there is no net charge in the barrier (or there is a small negative charge), then at low bias voltages we have accumulation on the left-hand side and depletion on the right-hand side. In this case, resistances R 1 and R 3 more or less short circuit C 1 and C 3 . Also, R 2 ≅ ∞, since the AlAs barrier is thick and high. So, we are concerned mainly with the behaviour of C 2 . The areal capacitance C 2 is equal to ε r ε 0 /d. Here d = λ e + b + s + λ d , where λ e is the quantum stand-off distance of the electrons in the 2DEG, s is the undoped spacer layer thickness, λ d is the depletion layer width and λ f is the width of the lightly doped contact layer � see fig. 1(b). Thus, C 2 will strongly depend on the varia- tion of λ e and λ d . The Fang-Howard theory [14] gives λ e ~ n s -1/3 and λ d ~ n s , where n s e is the areal charge in the left- hand side 2DEG. So, at a low n s 1/C 2 is proportional to n s -1/3, but at a high n s 1/C 2 gradually increases with n s . This shows qualitatively that there is a maximum in C 2 , and con- sequently in the measured capacitance, at a finite n s (non- zero voltage), and minimum at zero voltage. The reference sample has a clear minimum in capacitance at 0 V, a feature that is shared by the wetting layer sample, and QD samples A and D. Therefore, the similarity in the capacitance values at low bias leads to the conclusion that samples A and D and the reference sample as well as the WL sample are very similar electrostatically, and that the QD-containing barrier in samples A and D is almost electrically neutral. If we assume that the samples have a small positive charge in the barrier (associated with QDs), then at zero bias both contacts are accumulated. Under these conditions, the middle capacitance C 2 is given by the expression: 1/C 2 ~λ e1 + b +λ e2 , where λ e1 and λ e2 are the quantum stand- off distances of electrons in the 2DEG formed in front and at rear of the barrier. Both λ e1 and λ e2 will be determined by sheet density of the 2DEG, i.e. λ e1 ~ n s1 -1/3 and λ e2 ~ n s2 -1/3, and n s1 + n s2 = N 0 , where N 0 is the net positive charge den- sity concentrated in the barrier. A simple algebra shows that in the case n s1 = n s2 the capacitance C 2 has maximum at zero bias. The behaviour of C(V) characteristics around zero volt- age for samples B and C is consistent with this interpreta- tion, thus confirming the presence of a net positive charge in the QD-containing barrier. In order to probe the structures further, magneto-capaci- tance measurements were performed at 4K. These included sweeping the magnetic field B, applied in parallel to the growth direction, up to ~10 T for a given bias voltage. This results in the well-known Shubnikov-de Haas-like magneto- oscillations of the capacitance with a periodicity in 1/B, due to the Landau levels in the accumulation layer passing through the Fermi energy [15]. The charge modulation aris- ing in this case affects the distribution of the electrical po- tential and screening length, and hence modulates the ca- pacitance of the device. The frequency of the oscillations of the fundamental field, B f = [∆(1/B)]-1, is thus related to the Fermi energy E F by B f = 2πmE F /eh, where m* and h are the effective mass and the Plank constant, respectively. The B f therefore measures the electronic sheet density of 2DEG in the accumulation layer n s = 2eB f /h. Typical traces of C(B) for the sample A are shown in fig. 3. The values of B f ex- tracted from the C(B) traces are shown for all the QD sam- ples in fig. 4. First of all, the magneto-oscillation series in C(B) are of interest because they prove the existence of a thermalised 2DEG and allow the electric field F across the barrier re- gion of the structure to be measured directly: F = n s e/ε r ε 0 . In a simple single-barrier structure, such as the reference sam- ple, the charge density in the electron accumulation layer can be calculated from the applied voltage given the doping Fig. 3. Magnetocapacitance oscillations measured at T = 4K and different bias voltages applied to sample A. 0 1 2 3 4 5 17,8 17,9 18,0 18,1 V=0.2V B (T) 16,6 16,7 16,8 16,9 V=0.3V C ( pF ) 15,7 15,8 15,9 16,0 16,1 V=0.4V P. M. Martin et al.: Capacitance spectroscopy of InAs self-assembled quantum dots 10 ÔÊÎ, 1(1), 1998 SQO, 1(1), 1998 profile in the depletion region. This is essentially a result of being able to equate the charge densities in the accumulated and depleted regions. In a structure where there is poten- tially an additional charge density with an unknown magni- tude and bias dependence, such as QD structures, this is not strictly possible because an additional degree of freedom has been introduced, i. e. F can have a contribution from QDs. In this case, the direct measurement of the electronic charge density in the accumulation layer can be used to cal- culate the unknown charge density at a given bias, in ex- actly the same manner as described for the single barrier. Simply put, the addition of a degree of freedom requires another one to be ascertained. The dependencies of B f on the applied voltage shown in fig. 4 for all samples are very close to linear ones over the entire voltage range and look very similar, though they ap- pear to be offset to form a family. The curves for samples B and C have the largest offset in the set and extrapolate to a significant positive value at 0 V. This offset points to a posi- tive net charge in the barrier. Explanation of the C(V) and C(B) results is that a net positive charge exists in the single barrier, at least equal in magnitude to the electronic charge density in the QD layer. The positive charge attracts electrons to the accumulation layers adjacent to the barrier and thus induces the devia- tions from the capacitance of the reference sample, exhib- ited most clearly by samples C and D. The electrons accu- mulated in the collector under a finite bias will rapidly de- plete, and the differential capacitance will return to values very similar to the reference sample, as is seen. Although the simple model of equivalent circuit used by us can reproduce the general features of the C(V) and C(B) curves, the model is insufficiently realistic to predict the re- sults quantitatively, and hence cannot be used for accurate calculations of any charge densities in the QDs. Instead, the difference in the C(V) and C(B) types of measurement can be exploited. The C(V) trace in principle can be integrated to give the total transferred charge as func- tion of voltage, though this requires the effective area to be given very accurately. The B f values from the C(B) traces Fig. 5. Integrated C(V) curves (solid lines) and the n s values taken from C(B) traces (circles). Upper � sample C, lower � reference sample. -0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 0,0 0,5 1,0 1,5 2,0 control sample b) n sx1 0- 11 ( cm -2 ) Voltage (V) 0,0 0,5 1,0 1,5 2,0 sample C (51/72) a) Fig. 4. Fundamental field values, B f , (left axis) and charge sheet density, n s , (right axis) plotted as a func- tion of the applied voltage. -0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 0,25 0,50 0,75 1,00 1,25 1,50 1,75 2,00 n sx1 0- 11 ( cm -2 ) control WL A B C D B f ( T ) Voltage (V) P. M. Martin et al.: Capacitance spectroscopy of InAs self-assembled quantum dots 11ÔÊÎ, 1(1), 1998 SQO, 1(1), 1998 Fig. 6. The values of capacitance at zero voltage versus ∆n s . Solid line presents the least-squares fit. yield directly the charge sheet density, n s , in a particular region, the 2DEG. Fig. 5(a) shows the sheet density calcu- lated using the B f data and integrated C(V) (scaled to give n s ) data for the reference sample. The agreement is excel- lent, given only the mesa radius has been adjusted (within several percents) to bring the curves into agreement. It should be noted that the effect of processing inaccuracies, leading to different device sizes than expected, and stray capaci- tance will both be subsumed in this single correction. The variation in the device capacitance is sufficiently small, so that a small series stray capacitance will only lead to an approximately constant reduction in the measured value, and thus it mimics the change in the device area. However, the main point to be noted is that the curves are in agreement for all bias values. Fig. 5(b) shows a similar graph for the integrated C(V) and n s values for sample C. In this case, the mesa area and a 0 V offset (vertical shift of the curve on the whole) added to the integrated C(V) are used to bring the curves to a fit. The offset used corresponds to a net charge density in the barrier of 1.5⋅1010 cm-2. The fit is again excel- lent over the entire bias range. Note that the ∆n s value ob- tained is similar to the value one could obtain from inspec- tion of fig. 4. Effectively, the alignment of the curves over the entire bias range allows making a deduction about the behaviour at 0 V. As the C(V) around 0 V and the n s (or B f ) series are close to one another, there appears to be a finite density in the accumulation layers at 0 V. The same procedure has been performed for all QD sam- ples, and the results are shown in fig. 6, where the extracted ∆n s values are plotted versus C(0). Extrapolating the points linearly to zero, ∆n s value, which effectively assumes the electrical neutrality in the barrier of the reference sample, gives the capacitance value very close to that of the refer- ence sample. The result confirms our explanation of the zero- field C(V) at low temperatures. Summary In conclusion, we investigate the C(V) and C(B) character- istics of a series of single-barrier n-i-n GaAs/AlAs/GaAs heterostructures incorporating a layer of self-assembled InAs quantum dots in the AlAs barrier. Some of the samples show minima and some maxima in the zero-field C(V) at low tem- peratures. This provides information about the degree of accumulation in the GaAs layers adjacent to the barrier at zero bias. We have shown that the reference sample exhibits the same behaviour as the WL sample, implying electrical neutrality in the barrier of these samples. The QD-contain- ing samples are seen to have a slight excess of positive charge, leading to electron accumulation at zero bias. This positive charge in the AlAs barrier, which we ascribe to ion- ised defects or impurities, possibly in association with the quantum dots, compensates the negative charge associated with electron filling of the dots, and the compensation de- gree considerably depends on the growth conditions, in par- ticular on the overlayer thickness. There are two interesting consequences of this arrangement. The first is that there are electrons brought into close contact with the QDs at zero bias applied. The coupling of the electrons to the QDs may be enhanced by this at low biases, and may even lead to interesting zero-bias tunneling effects. Another consequence is that positive charge in the QD-containing barrier radi- cally alters the picture of the band bending in the devices in comparison to the model taking into account only charging of the QD states with electrons. Acknowledgements This work is supported by EPSRC (U.K.). A.E.Belyaev and L.Eaves acknowledge the financial support of the Royal Society and EPSRC, respectively. A.E.B. was partly sup- ported by STCU under Project No.464. References 1. D. V. Averin, K. K. Likharev. In: Mesoscopic Phenomena in Solids, Edited by B. L. Altshuler, P. A. Lee, R. A. Webb, Elsevier, Amster- dam, 1991. 2. I. N. Stranski, L. Krastanow. Akad. Wiss. Lit. Mainz Math.-Natur Kl. 11b, 146, 797 (1939). 3. R. Notzel. Semicond. Sci. Technol., 11, 1365 (1996). 4. S. Fafard, D. Leonard, J. L. Merz, P. M. Petroff. Appl. Phys. Lett., 65, 1388 (1994). 5. J. Y. Marzin, J. M. Gerard, A. Izrael, D. Barrier, G. Bastard. Phys. Rev. Lett., 73, 716 (1994). 6. M. Grundman, N. Ledentsov, O. Stier, D. Bimberg, V. M. Ustinov, P. S. Kop�ev, Zh. I. Alferov. Appl. Phys. Lett., 68, 979 (1996). 7. M. Narihiro, G. Yusa, Y. Nakamura, T. Noda, H. Sakaki. Appl. Phys. Lett., 70, 105 (1997). 8. I. E. Itskevich, T. Ihn, A. Thornton, M. Henini, H. A. Carmona, L. Eaves, P. C. Main, D. K. Maude, J. C. Portal. Jpn. Journ. Appl. Phys., 36, 4073 (1997). 9. R. C. Ashoori, H. L. Stormer, J. S. Weiner, L. N. Pfeiffer, S. J. Pearton, K. W. Baldwin, K. W. West. Phys. Rev. Lett., 68, 3088 (1992). 10. H. Drexler, D. Leonard, W. Hansen, J. P. Kotthaus, P. M. Petroff. Phys. Rev. Lett., 73, 2252 (1994). 0,00 0,05 0,10 0,15 0,20 14 16 18 20 22 24 26 control D WL A B C C (0 ) (p F ) ∆n s (0) (1011, cm-2) P. M. Martin et al.: Capacitance spectroscopy of InAs self-assembled quantum dots 12 ÔÊÎ, 1(1), 1998 SQO, 1(1), 1998 11. G. Medeiros-Ribeiro, D Leonard, P. M. Petroff. Appl. Phys. Lett., 66, 1767 (1995). 12. M. Fricke, A. Lorke, J. P. Kotthaus, G. Medeiros-Ribeiro, P. M. Petroff. Europhys. Lett., 36, 197 (1996). 13. B. T. Miller, W. Hansen, S. Manus, R. J. Luyken, A. Lorke, J. P. Kotthaus, S. Huant, G. Medeiros-Ribeiro, P. M. Petroff. Phys. Rev. B, 56, 6764 (1997). 14. I. E. Itskevich, T. Ihn, A. Thornton, M. Henini, T. J. Foster, P. Moriarty, A. Nogaret, P. H. Beton, L. Eaves, P. C. Main. Phys. Rev. B, 54, 16401 (1996). 15. F. F. Fang, W. E. Howard. Phys. Rev. Lett., 16, 797 (1966). 16. H. L. Leadbeater, E. S. Alves, F. W. Sheard, L. Eaves, M. Henini, O. H. Hughes, G. A. Toombs. J. Phys.: Condens. Matter, 1, 10605 (1989). ÅÌÍIÑÍÀ ÑÏÅÊÒÐÎÑÊÎÏIß InAs ÑÀÌÎ-ÎÐÃÀÍIÇÎÂÀÍÈÕ ÊÂÀÍÒÎÂÈÕ ÒÎ×ÎÊ Ï. Ì. Ìàðòií Øåôôiëäñüêèé Óíiâåðñèòåò, Âåëèêîáðèòàíiÿ Î. Å. Áåëÿåâ Iíñòèòóò ôiçèêè íàïiâïðîâiäíèêiâ ÍÀÍ Óêðà¿íè Ë. Iâç, Ï. Ê. Ìåéí, Ô. Â. Øiðä, Ì. Õåíiíi Íîòòiíãåìñüêèé Óíiâåðñèòåò, Âåëèêîáðèòàíiÿ Ò. Ií Ëàáîðàòîðiÿ ôiçèêè òâåðäîãî òiëà, Øâåéöàðiÿ ªìíiñíà ñïåêòðîñêîïiÿ âèêîðèñòóºòüñÿ äëÿ äîñëiäæåííÿ åëåêòðîííèõ âëàñòèâîñòåé ñàìî-îðãàíiçîâàíèõ InAs êâàíòîâèõ òî÷îê. Âèìiðþâàííÿ âîëüò-ôàðàäíèõ õàðàêòåðèñòèê, C(V), îäíî÷àñíî ç äàíèìè ïî ìàãíåòî-åìíîñòi, C(B), äîçâîëÿþòü äîñëiäæóâàòè åëåêòðîñòàòè÷íèé ïðîôiëü îäíîáàð�ºðíèõ p-i-n GaAs/AlAs/GaAs ãåòåðîñòðóêòóð, ÿêi ìiñòÿòü â AlAs áàð�ºð³ øàð ñàìî- îðãàíiçîâàíèõ InAs êâàíòîâèõ òî÷îê. Ìè âèçíà÷èëè, ùî íåãàòèâíèé çàðÿä, ïîâ�ÿçàíèé ç çàïîâíåííÿì êâàíòîâèõ òî÷îê åëåêòðîíàìè, ìàéæå ïîâíiñòþ êîìïåíñóºòüñÿ ïîçèòèâíèì çàðÿäîì â AlAs áàð�ºði, ùî, íà íàøó äóìêó, ïîâ�ÿçàíî ç iîíiçîâàíèìè äåôåêòàìè ÷è äîìiøêàìè, ÿêi îáóìîâëåí³ êâàíòîâèìè òî÷êàìè. Ïîêàçàíî, ùî ìiðà êîìïåíñàöi¿ ñóòòºâî çàëåæèòü âiä ðîñòîâèõ óìîâ. ÅÌÊÎÑÒÍÀß ÑÏÅÊÒÐÎÑÊÎÏÈß InAs ÑÀÌÎ-ÎÐÃÀÍÈÇÎÂÀÍÍÛÕ ÊÂÀÍÒÎÂÛÕ ÒÎ×ÅÊ Ï. Ì. Ìàðòèí Øåôôèëäñêèé Óíèâåðñèòåò, Âåëèêîáðèòàíèÿ À. Å. Áåëÿåâ Èíñòèòóò ôèçèêè ïîëóïðîâîäíèêîâ ÍÀÍ Óêðàèíû Ë. Èâç, Ï. Ê. Ìýéí, Ô. Â. Øèðä, Ì. Õåíèíè Íîòòèíãåìñêèé Óíèâåðñèòåò, Âåëèêîáðèòàíèÿ Ò. Èí Ëàáîðàòîðèÿ ôèçèêè òâåðäîãî òåëà, Øâåéöàðèÿ Åìêîñòíàÿ ñïåêòðîñêîïèÿ èñïîëüçóåòñÿ äëÿ èññëåäîâàíèÿ ýëåêòðîííûõ ñâîéñòâ ñàìî-îðãàíèçîâàííûõ InAs êâàíòîâûõ òî÷åê. Èçìåðåíèå âîëüò-ôàðàäíûõ õàðàêòåðèñòèê, C(V), ñîâìåñòíî ñ äàííûìè ïî ìàãíåòî-åìêîñòè, C(B), ïîçâîëÿþò èññëåäîâàòü ýëåêòðîñòàòè÷åñêèé ïðîôèëü îäíîáàðüåðíûõ p-i-n GaAs/AlAs/GaAs ãåòåðîñòðóêòóð, ñîäåðæàùèõ â AlAs áàðüåðå ñëîé ñàìî- îðãàíèçîâàííûõ InAs êâàíòîâûõ òî÷åê. Ìû óñòàíîâèëè, ÷òî îòðèöàòåëüíûé çàðÿä, ñâÿçàííûé ñ çàïîëíåíèåì êâàíòîâûõ òî÷åê ýëåêòðîíàìè, ïî÷òè ïîëíîñòüþ êîìïåíñèðóåòñÿ ïîëîæèòåëüíûì çàðÿäîì â AlAs áàðüåðå, êîòîðûé, ïî íàøåìó ìíåíèþ, ñâÿçàí ñ èîíèçîâàííûìè äåôåêòàìè èëè ïðèìåñÿìè, îáóñëîâëåííûìè êâàíòîâûìè òî÷êàìè. Ïîêàçàíî, ÷òî ñòåïåíü êîìïåíñàöèè ñóùåñòâåííî çàâèñèò îò ðîñòîâûõ óñëîâèé.

Capacitance spectroscopy of InAs self-assembled quantum dots

Репозитарії

Схожі ресурси