Molecular beam epitaxy

Brief explanation of the Molecular Beam Epitaxy technique, materials, benefits and problems and applications.

1. Molecular Beam Epitaxy Enrique Flores Fontau IEE3030-Microelectronics Tallinn University of Technology

2. Introduction What is Epitaxy ? Epitaxy types Growth modes Molecular Beam Epitaxy Working principle MBE Growth process MBE features In situ monitoring Materials SS-MBE Benefits and Problems Applications Conclusions Contents

3. Introduction What is Epitaxy ?

4. Epitaxy is the process of growing a thin crystalline layer on a crystalline substrate. Epitaxial layer is always thinner than the substrate Epitaxial grow techniques : What is epitaxy ? Vapor- Phase Epitaxy Liquid Phase-Epitaxy Molecular Beam Epitaxy VPE is a modification of chemical vapor deposition LPE is a method to grow semiconductor crystal layers from the melt on solid substrates. MBE is based on an UHV(Ultra High Vacuum ) technique . Chemical reactions involved Chemical reactions involved No chemical reactions involved .

5. Homoepitaxy The deposition substrate is the same material as we are depositing from the beam. ( e.g Si on Si) Heteroepitaxy Substrate and material are of different composition in order to fabricate integrated crystalline layers of different materials . ( e.g GaAs on Si) Epitaxy types

6. There are three main growth modes that can occur depending upon the substrate temperature, the deposition rate and available surface energy Growth modes

7. Molecular Beam Epitaxy What is Molecular Beam Epitaxy ?

8. Pumping Systems Growth Chamber , epitaxial growth happens . Load lock facilitates the introduction and removal of samples Auxiliary chamber host analytical and process equipment MBE system

9. Gas sources are heated in separate k- cells or electron beam evaporators to achieve molecular or atom beams . No interaction with each other until they reach the Surface. During the deposition , the interactions of the atoms produce the epitaxial growth . MBE Growth process

10. Controlling , via shutters and the temperature of the source, will control the rate of impinging materials. The temperature of the substrate will control the rate of diffusion and desorption . Background gases help to avoid monolayr contamination . MBE Growth process

11. Deposition rate ( ) : 1-5 s Growth temperature ( ) : 550 Thickness control ( Å ) : 5 Interface width ( Å ) : 5 Shuttering control: 0.1 s   MBE features

12. Reflection High Energy Electron Diffraction (RHEED) Observe removal of contaminants from the substrate surface Calibrate growth rates Estimate the substrate temperature Determine the stoichiometry Analyze surface morphology – RHEED pattern Study growth kinetics – RHEED intensity oscillations In Situ Monitoring

13. Materials What kind of materials are used ?

14. Materials used on MBE Different materials are used depending the type of MBE, but we will focus on Solid Source MBE type . Molecular Beams Substrate target Group III – V molecular beams III-V Semiconductors SS- MBE Group II – VI molecular beams II-VI Semiconductors Others IV-VI Semiconductors , Heusler alloys , silicides , metals ... Typically, the substrate target is a semiconductor material with useful electronic properties. The molecular beam quite often is composed of evaporated elemental substances such as gallium and arsenic

15. Materials used on MBE III-V semiconductors offer high electron mobility and a direct high band gap. II-VI semiconductors exhibit direct large band gaps , but have some problems with conductivity. IV – VI Semiconductors also offer a narrow band gap.

16. Benefits and Problems Is it worth to use ?

17. Clean surfaces. Monitoring in situ. Independent vaporization of each material. Multiple sources are used to grow alloy films and heterostructures . Deposition is controlled at submonolayer level. Extremely flexible technique since growth parameters are varied independently. Benefits / Problems of MBE Very low deposition rates: 1um – 100nm per hour are used. High equipment cost and long set up time. Extreme Flexibility (uncontrolled flexibility = chaos!). Many Boring Evenings! Mostly Broken Equipment! Mega-Buck Evaporation!

18. Applications Are there any electronic applications ?

19. Applications The driving force today is the fabrication of advanced electronic and optoelectronic devices . Transistors (HEMT,HBT ) : Microwave devices (IMPATT) Optoelectronic devices (MQW) laser

20. Conclusions Key points of the topic

21. Very well controlled and clean result. High equipment cost and long setup time In situ monitoring High Speed electronic and optoelectronic applications III-V semiconductors as GaAs are the most common used in Electronic and OptoElectronics devices . Conclusions

22. Any Questions ? Ah, I get it now !