Fig.1: Schematic layer scheme of a "classical" nipi-structure. Vo denotes the potential modulation and Eg is the energy gap of the bulk material.
The resulting nearly parabolic band edge modulation depends on the thickness of the individual layers and their doping level. In this case the intrinsic regions consist of the space charge regions between the different layers. In order to increase the length of the intrinsic zone without changing the curvature of the potential in the minima and maxima, undoped layers can be grown between the two doped layers. An extreme form of this concept is to grow intrinsic layers separated by monolayers which carry all the dopants [3,4]. Such a delta- doping SL or sawtooth SL is shown schematically in Fig 2.
Fig.2: Schematic layer scheme of a delta-doping structure.
The constant electric field between the delta- layers leads to the typical linear dependence of the potential profile in the structure. This structures are more effective in creating sufficient potential modulation even for very short period length. Random distribution of dopants cause band edge fluctuations which are especially important for small periods. However, in delta- doped systems, these fluctuations are much smaller in comparison to the conventional doping scheme (Fig.1) [5]. For the same two dimensional doping level in the delta- doped nipi's the mean distance between the dopants is much smaller than in the homogeneously doped layers. The concept of growing intrinsic layers requires some flexibility in the growth technique which is possible with MBE growth. The only way of growing intrinsic layers with HWE is to use a p-i-n-i-p- layer scheme.
The simplest structures which is based on the nipi-concept consists of 3 layers and is a p-n-p- or a n-p-n-structure.