Growth, Spectroscopy, and Laser Application of Self-Ordered Ill-V Quantum Dots D. Bimberg, M. Grundmann, and N.N. Ledentsov Introduction The development and application of semiconductor light-emitting and laser diodes has been a huge success during the last 30 years in key areas of modern technology like communications, record- ing, and printing. Still there is ample room for improvement through combi- nation of the atomlike properties for zero-dimensionally localized carriers in quantum dots (QDs) with state-of-the- art semiconductor-laser technology. Low, temperature-insensitive threshold current; high gain; and differential gain have been predicted 1 ' 2 since the early 1980s. In the past two decades, the fabrication of QDs has been attempted using colloidal techniques (see the article by Nozik and Micic in this issue), patterning, etching, and layer fluctuations (see the article by Gammon in this issue). However a break- through occurred recently through the employment of self-ordering mechanisms during epitaxy of lattice-mismatched materials (see the next section) for the creation of high-density arrays of QDs that exhibit excellent optical properties, particularly high quantum efficiency, up to room temperature. The zero- dimensional carrier confinement and subsequent atomlike electronic properties have a drastic impact on optical proper- ties (see the section on Spectroscopy). Also intimately connected is the appli- cability of QDs as a novel gain medium in state-of-the-art laser diodes with supe- rior properties (see the section on Lasers). Growth Several lattice-mismatched material combinations such as (In,Ga)As/GaAs, 3 ~ 7 InP/InGaP, 8 ' 9 and Ge/Si 10 exhibit the coherent-island Stranski-Krastanow 11 growth mode. Typically a thin wetting layer forms when the amount of deposited material does not exceed a critical value of a few lattice constants. Deposition of additional material results in the forma- tion of islands that allow for greater re- laxation of elastic energy than does a two-dimensional (flat) layer. The density, size, and shape of the islands depend largely on the growth conditions. If growth conditions are inappropriate or if too much material is deposited, large dis- located islands appear locally or islands coalesce—creating defects as well. Epitaxial control of the InGaAs/GaAs material system is currently quite ad- vanced. Several authors have reported a critical thickness of 1.6 monolayers for the onset of formation of three- dimensional (3D) islands. However de- tails on the formation of the wetting layer are complex; wirelike features de- velop for submonolayer deposition, 1213 and 3D features appear and disappear before true 3D islands evolve. 14 Defect- free and dense arrays of InAs/GaAs QDs can be grown with both major growth techniques—molecular-beam epitaxy (MBE) 6 ' 15 and metalorganic chemical va- por deposition (MOCVD) (Figure l) 16 ' 18 — in a similar fashion. Typical dot sizes vary between a 7- and 20-nm base size with a height of several nanometers. The area density reaches values of several 10" cm 2 . For specially chosen growth condi- tions, islands having a uniform optimal "equilibrium" size are formed; 13 the is- land size does not depend on the amount of material deposited, and more material deposited results solely in a higher den- sity of the same size dots. A hierarchy of ordering phenomena of the dot size and shape and the lateral position (into a two-dimensional square Bravais lattice) was observed 15 ' 9 ' 211 and explained using strain-energy arguments. 21 Further re- search is dedicated to distinguishing the relative roles of the kinetic and thermo- dynamic aspects because temperature and arsenic pressure in MBE 15 or arsine supply in MOCVD 18 play a crucial role in the dot formation. Figure 1. Plan-view transmission- electron-microscopy (TEM) image of a single sheet of metalorganic- chemical-vapor-deposition (MOCVD)- grown InAs/GaAs quantum dots (QDs) (Technische Universitat Berlin). TEM: Max-Planck-lnstitut in Halle. Vertically stacked QDs (Figure 2) de- velop upon deposition of multiple dot layers. 22 24 The barrier thickness deter- mines the growth morphology. For thin barriers (a few nanometers), island-shape transformation occurs. 24 For intermedi- ate thickness (—10 nm), vertical correla- tion of island positions appears'"' 22 ' 23 ' 25 that gets lost at a larger barrier thickness (>20 nm). The interplay of surface strain fields due to buried dots and growth energetics has been discussed and is pre- dicted to lead to improved lateral order- ing. 26 Such ordering in vertical stacks has MRS BULLETIN/FEBRUARY 1998 31