Development of CMOS-compatible processes of formation of germanium quantum dot (QD) dense arrays on the (001) silicon surface as well as multilayer Ge/Si epitaxial heterostructures on their basis is a challenging task of great practical significance [1
]. An important direction of applied researches in this area is the development of highly efficient monolithic far and mid infrared detector arrays which could be produced by a standard CMOS technology [9
]. Such detectors have to combine high perfection (uniformity, sensitivity, operating life, etc.) with high yield and low production price. A requirement of CMOS compatibility of technological processes imposes a hard constraint on conditions of all the phases of the QD array manufacturing starting from the stage of preparation of a clean Si surface for Ge/Si heterostructure deposition: on the one hand, formation of a photosensitive layer must be one of the latest operations of the whole device production cycle because otherwise the structure with QDs would be destroyed by further high-temperature annealings; on the other hand, high temperature processes during Ge/Si heterostructure formation on the late phase of the detector chip production would certainly wreck the readout circuit formed on the crystal. Therefore, lowering of the array formation temperature down to the values of
is strongly required [1
], and the Ge QD arrays meeting this requirement are referred to as CMOS-compatible ones.
In addition to the requirement of the low temperature of a Ge QD array formation, both high density of the germanium nanoclusters (>
) and high uniformity of the cluster shapes and sizes (dispersion <
10%) in the arrays are necessary for employment of such structures in CMOS IR detectors [12
]. The molecular beam epitaxy (MBE) is known to be the main technique of formation of Ge/Si heterostructures with QDs [2
]. A high density of the self-assembled hut clusters can be obtained in the MBE process of the Ge/Si(001) structure formation when depositing germanium on the Si(001) substrate heated to a temperature Tgr
550°C. In this case, the lower the temperature of the silicon substrate during the Ge deposition the higher the density of the clusters at the permanent quantity of the deposited Ge [16
]. For example, the density of the Ge clusters in the array was 6 × 1011
= 360°C, and the effective thickness of the deposited germanium layerb hGe
= 8 Å; the cluster density of only ~2 × 1011
was obtained at Tgr
= 530°C and the same value of hGe
There is another approach for obtaining dense cluster arrays. The authors of Refs. [4
] reached the cluster density of ~9 × 1011
using the pulsed irradiation of the substrate by a low-energy Ge+
ion beam during the MBE growth of the Ge/Si(001) heterostructures at Tgr
as high as 570°C.
Obtaining of the arrays of the densely packed Ge QDs on the Si(001) surface is an important task, but the problem of formation of uniform arrays of the Ge clusters is much more challenging one. The process of Ge/Si(001) heterostructure formation with the Ge QD dense arrays and predetermined electrophysical and photoelectric parameters cannot be developed until both of these tasks are solved. The uniformity of the cluster sizes and shapes in the arrays determines not only the widths of the energy spectra of the charge-carrier bound states in the QD arrays [4
], but in a number of cases the optical and electrical properties of both the arrays themselves and the device structures produced on their basis [22
]. To find an approach to the improvement of the Ge QD array uniformity on the Si(001) surface, it is necessary to carry out a detailed morphological investigation of them.
This article presents the results of our recent investigations of several important issues of the Ge dense array formation and growth. We have studied the array nucleation phase (the transition from 2D growth of the wetting layer (WL) to 3D formation of the QD array when the nuclei of both species of huts--pyramids and wedges [18
]--begin to arise on the (M
) patches of WL)[23
]. We have identified by STM the nuclei of both species, determined their atomic structures [18
] and observed the moment of appearance the first generation of the nuclei. We have investigated with high spatial resolution the peculiarities of each species of huts and their growth and derived their atomic structures [24
]. We have concluded that the wedge-like huts form because of a phase transition reconstructing the first atomic step of the growing cluster when dimer pairs of its second atomic layer stack up; the pyramids grow without such phase transitions. In addition, we have come to conclusion that wedges contain vacancy-type defects on the penultimate terraces of their triangular facets [24
] which may decrease the energy of addition of new atoms to these facets and stimulate the quicker growth on them than on the trapezoidal ones and rapid elongation of wedges. We have shown also comparing the structures and growth of pyramids and wedges that shape transitions between them are very unlikely [24
]. Finally, we have explored the array evolution during MBE right up to the end of its life when most of clusters coalesce and start forming a nanocrystalline 2D layer.
In the next sections, we present these results in detail.