Conduction band offset in InAsÕGaAs self-organized quantum dots measured by deep level transient spectroscopy

Quantum dots, realized by self-organization during strained layer heteroepitaxy, have recently found applications in microelectronics and optoelectronics. Selforganized quantum dots in the In~Ga!As/Ga~Al!As system are approximately pyramidal in shape with a base length of about 20 nm and a height of 6–8 nm. Typical molecularbeam epitaxial growth leads to an ordered array of 10– 10 dots/cm. However, because of the pyramidal shape and a complex strain profile within the dot, the band structure and the electronic states can only be calculated approximately. The band offsets, which are important for the design and understanding of a variety of devices, are also not known precisely. A technique that has been used with considerable success to measure heterostructure band offsets is to treat the potential of a quantum well grown with the two semiconductors analogous to that of a deep level defect and measure the temperature-dependent transient capacitance signal of a Schottky or p-n diode due to filling and emptying of the quantum well. The potential of a quantum dot, due to the three-dimensional confinement, is similar to that of an atom or a deep level trap. Transient capacitance measurements with InP and InAs/GaAs quantum dots have been reported. In this letter, we report on the determination of the band offsets in InAs/GaAs self-organized quantum dots by performing deep level transient spectroscopy ~DLTS! measurements on Au–AlGaAs Schottky diodes in which the quantum dot ~QD! layers are embedded in the AlGaAs layer. The heterostructure used for the DLTS measurements, grown by molecular-beam epitaxy ~MBE! on ~001! n-GaAs substrate, is schematically shown in Fig. 1. Growth is initiated with appropriate n-GaAs and graded buffer layers. Three layers of InAs quantum dots, with 30 Å GaAs barrier layers in between, are sandwiched in a n-type ~Si-doped! uniformly doped Al0.18Ga0.82As layer. This alloy composition is chosen since AlxGa12xAs with x<0.24 does not have the dominant trap known as the DX center. The quantum dot region is undoped and 250 Å undoped GaAs spacer layers are also inserted on both sides of the quantum dot layer. A n-type 100 Å GaAs layer serves as a capping layer. It is useful to note that multiple dot layers resemble a region with a three-dimensional ~3D! distribution of deep level defects in