Lateral AlGaAs- superlattice

Introduction. Nanomaterials with 2D-nanostructures (nanolayers)

Text 2A

Read text 2A with its introduction and answer the questions.

Memorize the following basic vocabulary and terminology to text 2A

LESSON 2

1. abundant new physics – многочисленные новые физические процессы, явления;

2. Molecular Beam Epitaxy – молекулярно-пучковая эпитаксия

3. atomic precision – атомарная точность;

4. adjacent quantum wells – смежные квантовые ямы;

5. lateral modulation – поперечная модуляция;

6. band structure – зонная струтура;

7. lithographically defined top gates – верхний затвор, полученный литографическим способом;

8. Brillouin zone[4] – зона Бриллюэна;

9. Fermi energy[5] – энергия Ферми;

10. inherent inadequacy of lateral modulation schemes – присущее несоответствие поперечной схемы модуляции;

11. produce concurrently – изготавливать параллельно, согласовано;

12. cleaved edge overgrowth technique – метод выращивания на сколотой грани;

13. unprecedented precision – беспрецедентная точность;

14. in situ – непосредственно, в момент образования, по месту;

15. heterointerface – граница раздела в гетеропереходе;

16. finite overlap – полное перекрытие, наложение;

2D-nanostructures are structures like layers, having large (near 1 μm) sizes on two coordinate axes, and nanometer size on third axe.

Abundant new physics was brought about by the invention of the superlattice (SL) concept and its subsequent realization through molecular beam epitaxy (MBE) of layered semiconductor structures with atomic precision. The formation of minibands isolated by minigaps in the vertical SL direction ensues from the coupling between adjacent quantum wells. In order to reduce the dimensionality of the system, electrons are confined in one direction to a quantum well, and a lateral periodic potential modulation may additionally be imposed from the surface of the sample with, for example, lithographically defined top gates. As in the conventional vertical SL geometry, an artificial band structure derives from the reduced width of the Brillouin zone and zone folding. Magnetotransport offers an excellent tool for the study of the resulting band structure in these laterally modulated two-dimensional systems (2DES), since oscillations in the magnetoresistance provide immediate information on the area encircled by closed electron orbits at the Fermi energy E _F. Such experimental evidence for an artificial band structure is sparse. Only very recently, using two-dimensional modulation, unambiguous proof of two different closed electron orbits was achieved. This lack of evidence may be related to the inherent inadequacy of lateral modulation schemes in producing concurrently a high quality 2DES and a sufficiently short period and large amplitude modulation to guarantee the occupation of only few, well-isolated minibands.

We use a new concept to fabricate lateral SLs based on the cleaved edge overgrowth technique, that overcomes the limitations of previous geometries by periodically modulating the material composition directly adjacent to the 2DES. In this way, both the period and the modulation strength can be tailored with unprecedented precision by MBE growth. In a first MBE step, an undoped SL with lattice constant d =100 nm of 30 periods of 50 nm GaAs and 50 nm Al_0.32Ga_0.68As is grown between two n -GaAs contacts, that act as source and drain. In a second MBE step, the sample is cleaved in situ and immediately thereafter overgrown by a 30 nm undoped GaAs layer, a 100 nm AlAs barrier, and an n- GaAs gate contact. By applying a positive gate voltage with respect to source and drain a 2DES is induced at the GaAs/AlAs heterointerface. The finite overlap of the electron wave function with the SL causes a modulation of the electron density n _s in x direction with a strength that depends on the GaAs layer thickness. For our sample this variation of the density, integrated over the z direction, exceeds 10%, as determined by a self-consistent 2D-Poisson/Schrödinger calculation. (2474)

Answer the following questions:

1) What is a 2-D nanostructure? 2) How was the new physics brought about? 3) In what way was the dimensionality of the system reduced? 4) Why does magnetotransport offer an excellent tool for the study of the resulting band structure in laterally modulated 2DES? 5) What was the difficulty in understanding of the resulting band structure in laterally modulated two-dimensional systems? 6) Why is a new concept to fabricate lateral SLs based on the cleaved edge overgrowth technique?

Task 1. Look through the text again carefully and explain the difficulty in understanding of the resulting band structure in laterally modulated two-dimensional systems?

Task 2. Describe the process of the lateral SL fabrication in detail and draw the picture of the process in a stepwise manner.

Task 3. Use your background knowledge of physics and the information given in the text to explain the terms Brillouin zone and Fermi energy.

Task 4. Use Internet to find more information about the new concept on super lattice fabrication and present it.

Task 5. Give the summary of the text mentioning the process of the lateral SF fabrication.