Optical investigation of type II GaSb∕GaAs self-assembled quantum dots

Abstract

We have studied the emission and absorption properties of type II GaSb/ GaAs quantum dots embedded in a p-i-n photodiode. The excitation power evolution provides clear signatures of the spatially separated confinement of electrons and holes in these nanostructures. We have estimated the confinement potential for the holes to be ϳ500 meV, leading to an intense room temperature emission assisted by recapture processes from the wetting layer. Photocurrent measurements show strong absorption in the wetting layer and in the quantum dots at room temperature which are important for photodetection applications based in this system. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2827582͔In recent years, GaSb/ GaAs structures have aroused great interest due to their type II band alignment and intrinsically different behavior compared to the well known InAs/ GaAs system. Fundamental issues regarding their growth process, energy level structure, and optical properties in addition to their technological applications in photodetection and photovoltaics have been already investigated in different configurations such as quantum dots ͑QDs͒, 1-5 quantum wells 6 ͑QWs͒ or ternary compounds. 7 In this work, we present various results regarding the GaSb/ GaAs QDs system, which extend and complete previous works.The QDs studied here were grown by solid source molecular beam epitaxy on a n-type GaAs͑001͒ substrate after deposition of a n-type GaAs buffer layer ͑Si: 1 ϫ 10 18 cm −2 ͒. The QDs were nucleated at 480°C, using a growth rate of 0.1 ML/ s. The formation of the GaSb QDs was detected by the change of the reflection high energy electron diffraction pattern after the deposition of 1.3 ML of GaSb. The GaSb layer, with a nominal thickness of 2 ML, was then exposed to Sb flux for 20 s and then annealed for 20 s without an Sb flux to limit the amount of Sb segregated during capping. The GaAs capping was done at 0.4 ML/ s in two steps. In the first step, a 10 nm thick GaAs layer was grown at the temperature of QD nucleation to avoid their destabilization. In the second one, a 40 nm thick GaAs layer was deposited at 570°C. During growth, the As and Sb beam equivalent pressures were 1.0ϫ 10 −5 and 1.9ϫ 10 −6 mbar, respectively. This scheme was repeated six times, with a 3 min growth interruption under an As 4 flux to lower the substrate temperature before the nucleation of the next QD layer. On top, a p-type 300 nm thick GaAs layer ͑Be: 1 ϫ 10 18 cm −2 ͒ was grown at 580°C. Finally, standard optical lithography and wet etching techniques were used to define mesas and metal Ohmic contacts. Figure 1͑a͒ shows the photoluminescence ͑PL͒ spectra recorded at 20 K as a function of excitation power at 532 nm. We can clearly identify two bands centered at 1.32 and 1.05 eV. The narrow high energy band corresponds to the wetting layer ͑WL͒ recombination and dominates the spectrum at low temperatures. Its peak energy position is compatible with a GaSb WL thickness of 0.7 nm, 2 which is larger than the total amount of GaSb deposited ͑0.57 nm͒...

Review of Experimental Results Related to the Operation of Intermediate Band Solar Cells

“…PL measurements have also revealed IB -> VB recombination in GaAs/AlGaAs QDs outside [70] and within [49], [66] the framework on IBSC research. Emission related to the presence of the IB has been reported for the GaSb/GaAs system [44], [45], [71]. Note that, contrarily to the rest of QD materials presented herein, this system has a type II alignment in the CB, and the IB is formed out of hole confined states in the VB of the QD [72], with the IB sitting closer to the VB than to the CB.…”

Height stabilization of GaSb/GaAs quantum dots by Al-rich capping

“…Similar to other GaSb/GaAs QD research, the wetting layer is strong relative to the QD peak. [20][21][22] The energy positions of the wetting layer peak for the Al-containing capping are blue-shifted to between 1.35 and 1.36 eV, with respect to the GaAs capped sample that emits at 1.32 eV. Since the Al capped layers retained the QD height better than the GaAs capped layer, it is expected that less Sb out-diffused from the nanostructures in these samples.…”

Hole migration and optically induced charge depletion in GaSb/GaAs wetting layers and quantum rings

“…14 Strong blueshifts of emission energy with increasing excitation power are regularly seen in type-II systems. Capacitive charging, [15][16][17] band bending, [18][19][20][21][22][23] and state filling 24 are commonly used to explain the blueshift of the type-II QD/QR emission energy. But capacitive charging is believed to be dominant in the sample investigated here.…”