Conduction band offset in InAs/GaAs self-organized quantum dots measured by deep level transient spectroscopy

Ghosh, Siddhartha Sankar; Kochman, B.; Singh, Jasprit; Bhattacharya, P.

doi:10.1063/1.126411

Cited by 35 publications

(16 citation statements)

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“…The obtained barrier height for the QD ground state is 171 meV, similar to the values reported by others. [15][16][17][18][19][20][21] Apart from the QD signal in the DLTS spectra in Fig. 3͑a͒, two pronounced peaks are located at about 115 and 250 K. In the DLTS spectra of Ref.…”

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confidence: 99%

“…The electrical space-charge technique, deep level transient spectroscopy ͑DLTS͒, 14 has recently been used to characterize QD structures, but most efforts have been focused on the intrinsic QD states. [15][16][17][18][19][20][21] The electrons can be thermally emitted out from the dots and then be detected by DLTS only when their electronic states are lifted above the bulk Fermi level. Therefore, by careful adjustment of the filling/reverse biases, electrons from the two intrinsic s states have been resolved successfully by this technique.…”

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Coexistence of deep levels with optically active InAs quantum dots

et al. 2005

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We present direct experimental evidence of the coexistence of deep levels with intrinsic quantum confinement states in large, self-assembled InAs quantum dots embedded in a GaAs matrix. The InAs quantum dots show very good optical properties, as evidenced by the strong photoluminescence at room temperature at ϳ1.3 m. Deep levels 160 and 484 meV below the GaAs conduction band edge have been identified at large reverse biases and high temperatures using deep level transient spectroscopy ͑DLTS͒ measurements. The reverse-bias dependence of the DLTS signal together with experimental results from the reference samples, containing thin InAs layers but no quantum dots, confirms that the deep levels coexist in the same layer as the InAs dots, and are most likely caused by the strain field during the lattice mismatched growth process. The densities of the deep levels in the structure are comparable to the density of the optically active quantum dots.Considerable efforts have been expended in the study of self-assembled semiconductor quantum dots ͑QDs͒, grown by the Stranski-Krastanov growth mode, because of their importance in device applications and in fundamental physics. 1,2 The excellent optical properties, such as strong photoluminescence ͑PL͒ intensity, are generally attributed to the coherent nature, i.e., defect free, of the QD islands. 1-6 In recent years, however, an increasing number of experiments have suggested the possibility that electronic deep levels might exist around or in what were regarded as coherent QDs that exhibit strong PL signals. For example, the presence of defects was suggested as a possible reason for the absence of the so-called phonon-bottleneck effect. 7,8 Other optical experiments, such as the quenching of PL signals at high temperatures, also suggested the possible existence of deep level defects around the QDs. 9-11 Dai et al. pointed out that the defect related centers existed at the InAs/ GaAs interface and played an important role in the PL quenching process. 12 They also indicated that the energy of the interface defects depended on the size of the quantum dots. By performing timeresolved optical characterizations of InAs QDs in GaAs, Fiore et al. speculated on the existence of nonradiative traps in the ͑In͒GaAs matrix in the close vicinity of the QDs, which would capture the carriers before they relaxed into the QDs. 13 Despite the evidence of the existence of unknown deep levels, the optical experiments could not provide confirmation and direct information, such as energy levels and concentrations, of possible deep levels due to their generally nonradiative nature. The electrical space-charge technique, deep level transient spectroscopy ͑DLTS͒, 14 has recently been used to characterize QD structures, but most efforts have been focused on the intrinsic QD states. 15-21 The electrons can be thermally emitted out from the dots and then be detected by DLTS only when their electronic states are lifted above the bulk Fermi level. Therefore, by careful adjustment of the filling/reverse b...

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Coexistence of deep levels with optically active InAs quantum dots

et al. 2005

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“…The band offsets for InAs/ GaAs nanostructures obtained for the strain distribution simulated within the Keating and anharmonic models are compared with the available experimental data [3][4][5] in Table III. The local band structure was obtained within the sp 3 d 5 s * empirical tight-binding model where the Hamiltonian matrix elements depend on the distance between the atoms.…”

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confidence: 99%

“…2(c)] were computed for the quantum dot crystal (QDC) reported in Ref. 5. The structure consists of three layers of regimented vertically stacked dome-shaped quantum dot arrays (with a 20 nm base diameter and a 7 nm height) on top of the 0.7 nm wetting layer, with a small ͑3 nm͒ vertical separation between the QD layers.…”

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Effect of anharmonicity of the strain energy on band offsets in semiconductor nanostructures

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Allmen

Oyafuso

et al. 2004

Applied Physics Letters

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Anharmonicity of the interatomic potential is taken into account for the quantitative simulation of the conduction and valence band offsets for strained semiconductor heterostructures. The anharmonicity leads to a weaker compressive hydrostatic strain than that obtained with the commonly used quasiharmonic approximation of the Keating model. Compared to experiment, inclusion of the anharmonicity in the simulation of strained InAs/ GaAs nanostructures results in an improvement of the electron band offset computed on an atomistic level by up to 100 meV. The accurate simulation of the electronic structure is of utmost importance for the design of nanoelectronic and optoelectronic device structures. It has been shown both theoretically 1,2 and experimentally, 1,3-5 that the energy spectrum in semiconductor nanostructures is extremely sensitive to the built-in strain. The continuum elasticity method fails to adequately describe the strain profile in InAs/ GaAs heterostructures with a 7% lattice mismatch between the constituent materials. 2 The two-parameter valence-force-field (VFF) Keating model 6,7 is a commonly used approximation for atomistic-level calculations of the equilibrium atomic positions in realistic-size nanostructures. 8 In this letter the quasiharmonic Keating model is shown to be insufficient to describe highly strained InAs/ GaAs nanostructures due to the anharmonicity of the strain energy.The Keating model treats atoms as spring-connected points in a crystal lattice. The strain energy depends only on nearest-neighbor interactions 6,7The coefficient ␣ corresponds to the spring constant for the bond length distortion, while ␤ corresponds to the change of the angle between the bonds ("bond-bending"). The summation is over all atoms m of the crystal and their nearest neighbors n and k. r mn and d mn are the vectors connecting the mth atom with its nth neighbor in the strained and unstrained material, respectively. The Keating potential in Fig. 1 (dashed line) fails to reproduce the weakening of the realistic interatomic interaction (solid line with circles) with increasing distance between atoms and it underestimates the repulsive forces at close atomic separation. Therefore Eq. (1) can adequately describe the strain energy only at small deformations. In InAs/ GaAs heterostructures, the lattice mismatch is as large as 7% and anharmonicity of the interatomic potential is expected to become important.The anharmonicity is included directly into the VFF constants ␣ and ␤ of the Keating model ͑3͒with ␤ 0 nmk ϵ ͱ ␤ 0 mn ␤ 0 mk , B nmk ϵ ͱ B mn B mk , and C nmk ϵ ͱ C mn C mk .nmk and 0 nmk are the actual and the unstrained angles between mn and mk bonds, respectively. In homogeneous materials all bonds are the same and the indexes m, n, and k can be dropped. ␣ 0 and ␤ 0 are the VFF constants in the unstrained crystal. The anharmonicity corrections A and C describe the dependence of ␣ and ␤ on hydrostatic strain, while B is responsible for the change of the bond-bending term with the angle between bonds. The d...

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“…However it should be pointed out that the results of both C -V and space charge spectroscopy techniques on QD samples have been observed to strongly depend on the growth parameters of the GaAs and InAs layers as well as on the structure design. Indeed the experimental results reported for GaAs/InAs(QD)/GaAs structures grown in different laboratories [6][7][8] show marked differences probably due to the different techniques used to grow the QDs and the difficulties in reproducing the same growth conditions. As a consequence carrier accumulation peaks and quantum levels associated to QD in some cases are not clearly detectable by the above space charge spectroscopy techniques.…”

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confidence: 99%