Limits and Properties of Size Quantization Effects in InAs Self Assembled Quantum Dots
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the energetic position as well as the line shape of the QD spectra is no longer affected by a change in InAs coverage but the intensity of the QD signal decreases. We attribute this effect to the onset of dislocated island formation accompanied by an increase of non radiative recombination centers. EXPERIMENT We have grown two samples by MBE under an As pressure of 7xl0-5 Torr. All material except the InAs was deposited at 600'C with rotating wafers. The InAs was grown at 530'C. Due to an asymmetric position of the In cell in the MBE chamber and a fixed wafer position, we achieved a change of the InAs coverage across the wafer. Sample 1 was designed for capacitance spectroscopy. 40 periods of a AlAs/GaAs (2nm/2nm) superlattice and 200nm intrinsic GaAs compensate the surface roughness of the semiinsulating (100) substrate. It follows a 20nm thick Si-doped GaAs layer (n-10 18cm-3 ). 30nm intrinsic GaAs separates the InAs system (WL and QDs) from the back contact. The deposition of InAs was stopped immediately after the RHEED pattern changed from streaky to spotty indicating the onset of the island formation. The QDs were covered with 5nm GaAs, 7 periods of an AlAs/GaAs (2nm/2nm) short period superlattice and finally with a 10nm thick GaAs cap layer. The bias is applied between a circular Schottky gate (1004tm diameter, Au on top of NiCr) and a Ni/AuGe ohmic back contact. Details of the growth are described elsewhere [19]. Sample 2 is a pin-structure grown on a (100) semiinsulated GaAs substrate. On top of a 1.51im n-doped GaAs (n;10 1Scm-3) buffer layer, we have grown 40 periods of a n-doped GaAs/AlAs (1.5nm/1.5nm) short period superlattice followed by 30nm i-GaAs and the InAs QD system. In order to observe the dislocated island formation, we stopped the InAs deposition at ;2ML. 100nm intrinsic GaAs cap the dot system. The p-region consists of 40 periods of a GaAs/AlAs (1.5nm/1.5nm) short period superlattice and a cap layer of 10 nm GaAs. At the edge of a 1004m square mesa structure we defined a ohmic Zn/Au p-contact. A Au/Ge pad forms the ohmic ncontact at the bottom of the mesa. We used a 5210 EG&G dual phase Lock-In amplifier in our C-V experiments. In order to measure the capacitance at T=4.2K, a small AC bias of 5mV was added to a variable DC bias. The modulation frequency was low enough (;30Hz) to ensure a carrier population equilibrium during each cycle. For our PL studies we used an Ar' laser. The luminescence signal was detected by a cooled Ge detector. In our PV and PC experiments we illuminated our sample with a Tungsten Halogen lamp (100W) dispersed by a monochromator. The chop frequency was &200Hz. The modulated PV and the PC signal was measured with a SRS 530 Lock-In amplifier. RESULTS Coherent island regime The coherent island formation is studied on sample 1 with C-V, PL and PV spectroscopy. Fig. 1 depicts typical C-V and PL spectra measured at different positions on the wafer. In both figures the measured data show a strong dependence on the InAs coverage (i.e. position on the wafer). In C-V trace A
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