The breakthrough occurred when techniques for self-organized growth of QD's allowed the fabrication of dense arrays of coherent islands, uniform in shape and size, and, simultaneously, free from undesirable defects. Recently, the figure of merit of QDHS lasers surpasses some of the key characteristics of QW devices in some of the most important applications. Article :. Date of Publication: May-June DOI: We present a model for the effect of various strain reducing layers on quantum dot heterostructures and study the corresponding variation in optoelectronic properties viz.
Three InAs QD 2. Low temperature 8 K PL spectra exhibits ground state peak at PLE measured at 8 K exhibited first and second excited state peaks at Highest absolute area measured using temperature dependent PL photocurrent was observed for sample B which can be justified by increment in quantum dots formation thus resulting higher quantum yield. Single pixel detectors were fabricated and sample B yielded lowest dark current density at 80 K. A multicolor spectral response was observed from sample B with corresponding peaks at 5. Some information concerning structures and methods is described in more detail in Ref.
The metamorphic QD emission spectrum shows a wide band at 0. The wide band of PV spectrum peaked at 1. It should be added that the In 0. Furthermore, a significant sharp fall above 1. The buffer layer is filled by numerous shallow levels and band non-uniformities originated from MBE growth defects and doping centers that redshift the interband absorption of GaAs [ 33 , 46 , 54 , 55 ].
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The feature after 1. A mechanism for this effect will be discussed in detail below.
The PV spectra measured through the si -GaAs substrate are inverted by sign of voltage below 1. Low-energy parts of the curves are given in the insets; the QD PL bands measured before [ 45 ] for both the structures are presented for the QD ground state energy positioning red. As it is mentioned above, the sharp fall of PV signal above 1.
Properties and applications of quantum dot heterostructures grown by molecular beam epitaxy
To clear effects of the layers beneath the bottom AuGeNi contact on the photoresponse, we have studied photoelectric properties of the structures using an indium contact at the substrate back. Unlike the previous Au and AuGeNi contact geometry that allows for the unipolar PV, the bipolar signal has been observed for the structures contacted to the sample top and substrate back Fig.
It is necessary to note that the PV sign changes along the photon energy axis, and in Fig.
Here, PV is considered to be positive when, as in the case of contact to the MBE layers, the positive potential is applied to the top Au contact while the negative one is applied to the bottom contact. All the optical transitions mentioned above contribute to the PV signal of the structures in the substrate-top contact geometry. However, when measuring PV through the substrate, the signal onset for the metamorphic and conventional structures occurs at about 0. The onset at 0. The aspects related to their location as well as the EL2 PC onset redshift have been discussed in detail elsewhere [ 10 , 45 ].
To understand the appearance of the PV signal in our samples, one should look at Fig. Summing up, the light-excited electrons holes drift predominantly toward the substrate surface , giving a positive potential at the Au contact and a negative one at the AuGeNi contact.
The band bending of the deeper layers beneath the AuGeNi contact is indicated in gray. The optical transitions observed in the PV spectra are indicated by vertical arrows; bold arrows show drift directions of the optically excited charge carriers under the internal field PV creation ; E F is Fermi energy.
Quantum dot heterostructures: Fabrication, properties, lasers (Review)
The calculations were carried out using Tibercad software [ 50 ]. Explaining the bipolar PV from the structures with the electrically active si -GaAs substrates, one can consider their calculated band structures in Fig. The Fermi level in the semi-insulating substrate is located much lower than the one in the n -doped MBE layers. The same applies to the excitation from EL2 defects above 0. This is clearly seen by comparing the QDs, WL, and buffer spectral bands on the PV curves of the structures contacted to MBE buffers with the valleys in spectra of the substrate-top-contacted samples.
For the higher energies, however, the excitation is absorbed closer to the sample surface not reaching the deeper MBE layers and substrate, which is the main source of negative signal.
Hence, the PV signal becomes positive at larger energies. Otherwise, a similar characteristic feature above 1.
The drastic fall of the PV amplitude has been explained, unlike in our case, by different charge carriers generated below and above 1. However, taking into account the drastic difference in the structures referred and present as well as specifics of the applied methods, we follow our interpretation of own results. A small feature near 1. These transitions obviously occur also in upper GaAs layers of our pseudomorphic structure, compensating mostly the negative effect of the near-substrate layers on the PV signal.
The reason for the small feature after 1. In our opinion, it is due to the slight fall of signal caused by the absorption edge of the upper MBE-grown nm thick GaAs buffer shading the QDs and WL which are more efficient contributors to PV at those photon energies. Indeed, electrons and holes generated in QDs and WL are carried to different sides and avoid recombination, unlike the volume generation, where recombination is much more probable.
This is the main reason of effective detection of photocarriers coming from even a single layer of QDs and WL. Photons of higher energies are band-to-band absorbed in near-surface n -GaAs buffer layer and electrons escape to the sample volume away from the holes, leading to the sharp rise of PV above 1.
Correctness of the suggested reason for the 1. Their PV spectra reveal the same feature without a barrier related to the MBE-layer interface to the substrate.
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Concerning the structures with the bottom contact on the s i -GaAs substrate, the PC spectra have thresholds near 0. Inset: electric scheme of connecting the sample for PC measurements.
Concerning the structures with the bottom contact on the s i -GaAs substrate with the EL2 center component, there is a competition between signal from absorption in the MBE layers and from EL2-related levels, as discussed above. Obviously, the electrons do not reach the bottom because of the high potential barrier see Fig.
The substrate has too high resistance, and the main drop of applied bias occurs on it, hence, no barrier lowering occurs. The main cause of the barrier below the AuGeNi contact is the si -GaAs substrate with a rather low positioning of the Fermi level resulting in the band bending opposite to that in the structure top. Color online calculated band profiles near In 0. Hence, for devices based on light absorption, a different structure design should be considered.
We believe, such an improvement is necessary to be suggested because many research groups consider si -GaAs substrate as a basis for novel p - n -type both QD infrared photodetectors [ 11 , 12 , 13 ] and solar cells [ 15 ]. Though such a buffer could absorb more excitation quanta above 1.