# Presentation: 568345

## TEM Study of InGaAs/GaAs Structural Evolution Near the Stranski-Krastanow Transformation

**TEM Study of InGaAs/GaAs Structural Evolution Near the Stranski-Krastanow Transformation**
**J. Wellman*********, T. George*********, R. Leon*********, S. Fafard**********, J. Zou***********, and D. J. H. Cockayne*********
*******JPL, Pasadena, CA, **********NRC Ottawa, Canada, ***********Australian Key Center for Microscopy and Microanalysis, Sydney, Australia**

**MOCVD horizontal reactor cell operating at 1/10 atmosphere **
**(CH****3****)****3****Ga - 5.36 x10****-6****, (CH****3****)****3****In - 5.18 x10****-6****, AsH****3**** - 2.5 x 10****-4**
**H****2**** flow rate: 5.0 slm for graded growth**
**                      *****[17.5 slm for uniform growth]***
**Growth rate: 0.5 to 0.75 ml/sec **
**Growth temperature: 550****o****C**
**Nominal composition: In****0.6****Ga****0.4****As**
**All substrates semi-insulating GaAs (001).**

**2.  Graded Growth**

***Reducing hydrogen flow results in a  spatial gradient in growth rate.   In our study the growth rate  ranged from 0.5 - 0.75 ml/sec.***

**Uncapped**

**Capped**

**GaAs**

**InGaAs**

**GaAs**

**InGaAs**

**GaAs**

***Schematic of specimens used in this study.  Some were capped with 30 nm (ungraded) GaAs.***

**QD Concentration**

***Once islands begin to form, island concentration increases exponentially to saturation in a fraction of a monolayer  (a and b correspond to different AsH******3****** partial pressure).  We can study the continuous stages in island evolution on a single growth specimen.***

**3.  Photoluminescence**

***See 4 distinct regimes in the luminescence as deposition increases:***
***1. Narrow, intense peak from only the wetting layer (WL) redshifts as the WL  gets thicker.***
***2.  Broader, lower energy QD peak emerges.  Simultaneously, the WL luminescence intensity drops sharply and no longer shifts, indicating that there is no further growth or depletion of the WL.***
***3.  QD luminescence redshifts as the concentration approaches saturation.  No detectable emission from the WL.***
***4.  QD peak intensity falls off drastically at island coalescence (intensity of green spectra scaled up by a factor of two (I X 2).  ***

**Study *****uncapped***** InGaAs/GaAs graded growth specimens**
**Digital Instruments Nanoscope III**
**Deflection Images shown**

**4.  Force Microscopy**

***Atomic Force Microscopy images show island morphology from formation to coalescence.  Note the presence of large, irregular islands at coalescence (e) - a possible explanation for the drop in PL intensity. ***

**Deflection images of surface evolution for InGaAs/GaAs islands at the points indicated by the arrows.  Each image is 1 ******** 0.5 ********m.**

**R. Leon and S. Fafard, *****PRB***** 58, R1726 (1998).**

***As  island concentration increases to saturation, average island diameter decreases.******  ***

**AFM images of uncapped InGaAs/GaAs Quantum Dots  (250nm X 250 nm).  Above each frame is a magnified (200%) representative dot to illustrate size evolution.**

**Studied capped and uncapped InGaAs/GaAs specimens.**
**Specimen preparation:**
**Mechanical thinning**
**Dimpler/grinder**
**Bromine/methanol etch**
**Philips 430 (300keV) and CM12 (120keV).**
**On-zone bright field imaging.**
**Each frame below  is 280 nm ******** 280 nm.**

**5.  TEM**
**(Transmission Electron Microscopy)**

**For a discussion of the strain contrast, see **
**J. Zou, X. Z. Liao, D. J. H. Cockayne,  and **
**R. Leon, *****PRB***** **_**59**_**, May 15, 1999 (to be published).**

***TEM images of uncapped  islands show  larger average island diameter at low concentration.****** ***

***TEM images of capped islands show no significant change in island diameter with concentration.***

**6.  Size Evolution**
**Quantitative Analysis**

***Plot of island concentration versus average island diameter clearly shows a significant increase in uncapped island diameter with decreasing concentration (red circles and blue squares).   The capped islands (green diamonds), on the other hand, do not appear to follow this same trend, as we see no significant change in island size from low to high concentrations.***

- The layer-by-layer growth mode is observed for lattice-matched materials of identical crystal structure.  Examples:  **Au on Ag** and **AlGaAs on GaAs.**

- Direct island growth is seen in materials with large lattice mismatch, high interfacial energy or different crystal structures.  Examples: **GaN on saphire** and **InAs on GaP.  **

- This type of growth occurs for crystals of  dissimilar lattice parameters  but low interfacial energy, like **Ge on Si** and **InAs on GaAs**.  After an initial layer-by-layer growth, islands form spontaneously, leaving a thin “wetting layer” underneath.

**Van de Merwe**

**Volmer-Weber**

**Stranski-Krastanow**

**1.  Quantum Dots**

***In this experiment, we study InGaAs islands grown on GaAs.  The InGaAs islands, or Quantum Dots (QDs), are of the Stranski-Krastanow type.  This system has been the subject of several other studies, including:***
***Structural and radiative evolution in quantum dots near the InGaAs/GaAs Stranski-Krastanow transformation, R. Leon and S. Fafard, PRB ***_***58***_***, R1726 (1998).***
***Transmission-electron microscopy study of the shape of buried InGaAs/GaAs quantum dots, X. Z. Liao et. al., PRB ***_***58***_***, R4235 (1998).***
***Different paths to tunability in III-V quantum dots, R. Leon et. al., JAP ***_***84***_***, 248 (1998).***
***Indium segregation and enrichment in coherent InGaAs quantum dots on (001) GaAs, X. Z. Liao, et. al., to be published.***