Applied Surface Science 190 (2002) 222–225
Current instabilities in GaAs/InAs self-aggregated quantum dot structures Zs.J. Horva´tha,*, P. Frigerib, S. Franchib, Vo. Van Tuyena, E. Gombiab, R. Moscab, L. Do´zsaa a
Hungarian Academy of Sciences, Research Institute for Technical Physics and Materials Science, P.O. Box 49, Budapest 114, H-1525 Hungary b MASPEC Institute-CNR, Parco Area delle Scienze 37a, 43010 Fontanini-Parma, Italy
Abstract Excess current was obtained in GaAs/InAs quantum dot structures at low temperatures and low current levels. This excess current exhibited instabilities with changing the bias, and over the time. It has been concluded that the excess current is a minority injection current connected with recombination through defects originated from the formation of QDs. The instabilities are connected with unstable occupation of energy levels induced by the above defects, which depend on temperature and on the current level. # 2002 Elsevier Science B.V. All rights reserved. Keywords: GaAs/InAs; Quantum dots; Current instability; Defects; Minority injection; Recombination
1. Introduction Recently, the electrical characteristics of InAs quantum dot (QD) and quantum well (QW) structures embedded in GaAs confining layers were studied using Au Schottky junctions as test devices [1,2]. It was obtained that—in comparison with a reference Schottky junction without any InAs QW or QD layer—QDs affected the electrical behaviour of the structures more significantly, than QW. Further on, an unexpected excess current was obtained in the QD structures at low temperatures and low current levels. This excess current exhibited instabilities with changing the bias, and over the time. In this paper, a model
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is proposed for the interpretation of the excess current instabilities. According to our model, minority injection current enhanced by recombination via defects connected with QDs, is responsible for the above features.
2. Experimental The InAs QD and QW structures embedded in GaAs confining layers and the reference Schottky structure drawn in Fig. 1, were grown by MBE and atomic layer MBE (ALMBE). As the coverages of the highly mismatched InAs layers increase beyond a critical value Ytr of 1.6 monolayer (ML) for InAs on GaAs, the growth of continuous two-dimensional InAs layers evolves in the formation of self-aggregated three-dimensional QDs, according to the Stranski– Krastanov growth mode. The structures consist of:
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 8 7 2 - 8
Zs.J. Horva´th et al. / Applied Surface Science 190 (2002) 222–225
Fig. 1. The studied reference Schottky (a), QW (b), and QD (c) structures.
(i) GaAs buffer layers with electron concentration of 2 1016 cm3 and thickness of 1 mm, (ii) QDs or QWs obtained using InAs coverages of 3.0 and 1.0 ML, respectively, and (iii) GaAs upper confining layers, with electron concentration of 2 1016 cm3. The buffer layers were grown by MBE at 580 8C; then the growth was interrupted for 210 s to lower the growth temperature to that used for the deposition
of InAs QW or QDs by ALMBE (at 460 8C). The upper confining layers were grown after interruptions of 210 s by ALMBE (at 400 8C) for 0.4 mm and by MBE (at 580 8C) for the subsequent 0.6 mm. This growth procedure was determined so as to optimise the photoluminescence properties of the GaAs/InAs QDs. As4/Ga and As4/In beam equivalent pressure ratios of 16 and 28 were used; the In and Ga fluxes were adjusted for InAs and GaAs growth rates of 0.16 and 1.0 ML/s, respectively; during ALMBE, the growth rate was set at 0.2 ML per cycle and the As supply time was chosen so as to give a sharp ð2 4Þ surface reconstruction at the end of the As cycle. The substrates were radiatively heated at growth temperatures Tg, which was measured by a suitable optical pyrometer for Tg of 450 8C and by a thermocouple (TC) not in direct contact with the substrate for T g < 450 C. For T g < 450 C, the TC readings were corrected by the difference (200 8C) between the TC and the optical pyrometer values measured at T g > 450 C. The InAs coverages were determined by using the growth rate measured by observing the 2D–3D growth transition on a calibration substrate just before the preparation of the QDs or of the QW, and by assuming that the transition takes place at the generally accepted value Ytr of 1.6 ML. This value is very close to that (1.57 ML) determined by very careful experiments , under slightly different conditions. It should be recalled, however, that the values of Ytr are not very sensitive to growth conditions. The QD density was about 1011 cm2. Back ohmic contacts were prepared by evaporation and annealing of AuGeNi alloy. Au Schottky contacts with a diameter of 400 mm were then formed by evaporation and conventional photolithographic techniques. The I–V measurements were performed in the temperature range 80–360 K by steps of 20 K in dark. The temperature of samples was stabilised during the measurements with an accuracy of 0.1 K, while the accuracy of the absolute temperature was within 0.5 K. Special care was taken studying current instabilities as a function of bias and time. The same measurements were carried out on all three wafers studied. The obtained I–V characteristics were evaluated for thermionic emission using an effective Richardson constant of 8.2 A cm2 K2.
Zs.J. Horva´ th et al. / Applied Surface Science 190 (2002) 222–225
3. Results and discussion Although QW and QD layers were relatively far from the Au/GaAs interface out of the equilibrium depletion depth of the Schottky junction, it has been observed that QDs affected the electrical behaviour of the structures much more, than QWs [1,2]. Concerning the I–V characteristics, the structures with QDs showed higher current levels for the same forward bias (consequently lower apparent Schottky barrier height) and much larger series resistance, than the reference Schottky structure and the structures with QW, as presented in Fig. 2 and Table 1. Further on, an excess current was observed in all the studied QD structures at low temperatures and low current levels (i.e. an additional current over the current attributed to the Schottky junction yielding linear log I–V relation at
Fig. 3. I–V characteristics of five GaAs/InAs QD structures measured at 100, 180 and 260 K, and with voltage steps of 10 mV.
low current levels), as can be seen in Fig. 2. Much lower excess currents were observed for the reference sample, and no excess current was present for QW structures, as also seen in Fig. 2. The excess current in QD structures was different for each individual diodes (see Fig. 3) and unstable exhibiting instabilities with changing the bias, as it is presented in Fig. 4, and over the time, as presented in Fig. 5. No current instabilities
Fig. 2. I–V characteristics of GaAs/InAs QD (solid lines), QW (dotted lines) and reference Schottky structures (dashed lines) measured at 100, 180 and 260 K, and with voltage steps of 10 mV.
Table 1 The parameters obtained from the I–V measurements at 300 K for the reference, the QW and QD structures: apparent barrier height, ideality factor, and series resistance Sample
Reference QW QD
0:86 0:01 0:81 0:01 0:75 0:01
1:03 0:01 1:03 0:01 1:03 0:01
Rs (O) 50 10 <100 950 5
Fig. 4. Excess currents in a GaAs/InAs QD structure measured in the temperature range 100–260 K with temperature steps of 20 K and voltage steps of 2 mV.
Zs.J. Horva´ th et al. / Applied Surface Science 190 (2002) 222–225
paths. This current component is limited by the recombination velocity through the defect levels. So, it exhibits a considerable contribution at low currents only. The occupation of these levels is unstable, therefore the depletion depth around these defects and the recombination velocity through them change with changing the bias and over the time, yielding instability of this current component.
Fig. 5. Excess currents in a QD structure measured in the temperature range 140–240 K with temperature steps of 20 K at forward bias of 0.25 V as a function of time with a period of 0.6 s.
were obtained for the reference and for the QW structures. The presence of excess currents indicates an additional path for current flow parallel with the Schottky junction. As QDs are out of the depletion layer, it is not likely that this additional current flows through them. Further on, the instability of current was observed below room temperature only, which indicates that its activation energy is about or below 20 meV. However, the energy levels attributed to InAs QDs between GaAs confining layers are much deeper, in the range of a few 100 meV [3,4]. This also indicates that QDs themselves are not responsible for the excess current. We propose the following model for the explanation of the excess current. The current flowing through the structures is limited mainly by the Schottky junction. This is indicated by the ideality factors of 1.03 (see Table 1). In QD structures, there is a parallel current mechanism due to recombination probably via defect levels which can be connected with mechanical stresses originated from the formation of QDs. These defects are probably located close to the metal/semiconductor interface yielding additional minority injection current
Unexpected excess current has been obtained in GaAs/InAs QD structures at low temperatures and low current levels. This excess current exhibited instabilities with changing the bias, and over the time. It has been concluded that the excess current is a minority injection current connected with recombination through defects originated from the formation of QDs. The instabilities are connected with unstable occupation of energy levels in defects depending on temperature and on the current level.
Acknowledgements The work at MFA and CNR-MASPEC was supported by the (Hungarian) National Scientific Research Fund (OTKA) under Grant Nos. T020315 and T035272, and by the CNR-PF MADESS, respectively. References ´.  Zs.J. Horva´ th, L. Do´ zsa, Vo. Van Tuyen, B. Podo¨ r, A Nemcsics, P. Frigeri, E. Gombia, R. Mosca, S. Franchi, Thin Solid Films 367 (2000) 89.  L. Do´ zsa, Zs.J. Horva´ th, Vo. Van Tuyen, B. Podo¨ r, T. Moha´ csy, S. Franchi, P. Frigeri, E. Gombia, R. Mosca, Microelectron. Eng. 51–52 (2000) 85.  S. Fafard, Z.R. Wasilewski, C.Ni. Allen, D. Picard, M. Spanner, J.P. McCaffrey, P.G. Piva, Phys. Rev. B 59 (1999) 15368.  H.L. Wang, D. Ning, H.J. Zhu, F. Chen, H. Wang, X.D. Wang, S.L. Feng, J. Cryst. Growth 208 (2000) 107.