Post-growth band gap engineering for optoelectronic integration

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Post-growth band gap engineering for optoelectronic integration

Transcript Of Post-growth band gap engineering for optoelectronic integration

POST-GROWTH BAND GAP ENGINEERING FOR OPTOELECTRONIC INTEGRATION

A thesis by
Marco Umberto Paolo Ghisoni
submitted to the University ofLondon for the degree of PhD,

D epartm ent of Electronic and Electrical Engineering University College London

Septem ber 1992

ProQuest Number: 10045624
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ABSTRACT
This thesis presents results on a post-growth technique for selectively altering the shape of GaAs/AlGaAs quantum well (QW) m aterial, to produce a resultant blue shift in the optical spectra.
The technique used is known as im purity free vacancy diffusion (IFVD), and involves encapsulation with Si02 followed by rapid therm al annealing at elevated temperature. This is shown to give rise to surface Ga vacancies, that rapidly diffuse through the structure enhancing the interm ixing of the group III atoms at the w e ll/b a rrie r interface, and resulting in a blue shift of the lowest confined state.
W ork presented shows that IFVD allows large selective blue shifts w hile still retaining QW characteristics i.e. excitonic features and the optoelectronic response, namely the quantum confined Stark effect (QCSE). The diffused well can be successfully m odelled, to a first approxim ation by an error function solution. The depth dependence expected is not observed and this could be due to a gallium vacancy diffusion coefficient higher than previously reported.
IFVD is used to produce two different types of reflection m odulators from the same wafer, demonstrating that the technique has applications to surface-normal devices and also that it can be used in conjunction w ith multiple well structures.
The lateral resolution is found to be reasonable, and the change in well profile induced is shown to improve the carrier escape from the well and the saturation intensity. Importantly the use of dielectric caps of increasing thickness is shown to produce increased levels of interm ixing and hence blue shift. This last result is of particular im portance since the ability to produce selective area bandgap engineering has applications in the fabrication of optoelectronic integrated circuits (OEICs).
The thesis ends with a discussion of future directions which IFVD may take including applications in the field of w avelength division m ultiplexing (WDM).

LIST OF CONTENTS
Title p a g e .............................................................................................................. 1
Abstract................................................................................................................. 3
List of C ontents................................................................................................... 4
P r e fa c e ................................................................................................................... 8
Chapter 1 An Introduction to Q uantum Wells : Properties, Devices and Integration Techniques.
1.1 Introduction..............................................................................................12 1.2 Properties of Q uantum Well Materials
1.2.1 Quantised states and enhanced excitons................................13 1.2.2 QW response to an applied electric fie ld ..................................16 1.3 Status of Q uantum Well Technology 1.3.1 Introduction............................................................................. 19 1.3.2 Quantum well devices
1.3.2.1 Electrical conduction devices............................... 19 1.3.2.2 Lasers and optical amplifiers.............................. 20 1.3.2.3 Modulators, switches and detectors.................... 24 1.3.3 Integration................................................................................28 1.4 Selective Area Bandgap Engineering 1.4.1 Regrowth and Selective Epitaxial Growth (SEG).................. 34 1.4.2 Laser Induced Disordering (LID )............................................37 1.4.3 Impurity Induced Layer Disordering (IILD).......................... 37 1.4.4 Impurity Free Vacancy Dijfusion (IFVD).............................. 40
Chapter 2 The IFVD Process : From Encapsulation to Blue Shift
2.1 Introduction............................................................................................... 44 2.2 High Tem perature Annealing of Encapsulated GaAs

2.2.1 Vacancy production at GaAs!dielectric interfaces.................44 2.2.2 Diffusion of vacancies from the GaAs surface....................... 48 2.3 Vacancy Enhanced of GaAs/AlGaAs Structures 2.3.1 Intrinsic interdiffusion of GaAs!AlGaAs structures............... 52 2.3.2 Intermixing in encapsulated MQW systems............................ 56 2.4 Optical Blue Shift due to Interdiffusion................................................ 58 2.5 C onclusion..................................................................................................61
Chapter 3 Growth, Processing and Experimental Techniques
3.1 In tro d u ctio n ................................................................................................ 64 3.2 Grow th of MQW p-i-n Structures........................................................... 64 3.3 Deposition of Dielectric L ayers............................................................... 65
3.3.1 Plasma enhanced CVD (PECVD) of dielectric layers.............66 3.3.2 Chemical vapour deposition (CVD) of silicon dioxide............68 3.4 Rapid Thermal Processing........................................................................68 3.4.1 Sheffield RTP system................................................................ 69 3.4.2 UCL Hg/Xe arc lamp.................................................................71 3.5 Device Fabrication.....................................................................................73 3.6 Device Testing 3.6.1 Monochromator system............................................................ 75 3.6.2 Photocurrent measurements......................................................76 3.6.3 Reflection measurements.......................................................... 77 3.6.4 Ti.'Sapphire laser system...........................................................79 3.7 C onclusion.................................................................................................. 80
Chapter 4 Basic Properties of IFVD
4.1 In tro d u ctio n ................................................................................................ 83 4.2 M odelling Techniques
4.2.1 Calculation of sub-band transitions......................................... 83 4.2.2 Interdiffused well profile.......................................................... 85 4.3 Retention of QW Properties After IFVD 4.3.1 Enhanced intermixing................................................................86 4.3.2 Excitonic Retention................................................................... 88 4.3.3 The Quantum Confined Stark Effect (QCSE)........................... 89

4.4 Analysis of Interdiffusion Data 4.4.1 Comparison o f interdiffusion coefficient results................... 94 4.4.2 Depth Dependence...................................................................97
4.5 Possible Problems Arising from IFVD 4.5.1 Excitonic resolution................................................................ 108 4.5.2 Doping Considerations........................................................... 110 4.5.3 Reproducibility........................................................................I l l
4.6 C onclusion................................................................................................112
Chapter 5 IFVD Applied to Surface-Normal Modulators
5.1 Introduction.............................................................................................. 115 5.2 AFPM Operation
5.2.1 Basic Principles...................................................................... 115 5.2.2 Operating modes of an AFPM ................................................117 5.2.3 Integrated reflector and cavity tolerances.............................118 5.3 Tailoring of the AFPM Operating Characteristics 5.3.1 Aims and original wafer reflection spectra............................119 5.3.2 Normally-off operation........................................................... 122 5.3.3 Normally-on operation............................................................124 5.4 A pplications............................................................................................. 127 5.5 C onclusion................................................................................................130
Chapter 6 Im portant Parameters for Applications
6.1 In troduction............................................................................................. 133 6.2 Lateral Resolution
6.2.1 Theoretical evaluation............................................................ 133 6.2.2 Experimental evaluation.........................................................136 6.3 M onolithically Integrated M ulti-W avelength O peration 6.3.1 Introduction............................................................................. 140 6.3.2 Experimental procedure.......................................................... 141 6.3.3 Results......................................................................................144 6.3.4 Conclusions..............................................................................148 6.4 Carrier Escape and Saturation Intensity ..............................................148 6.5 C onclusions.............................................................................................. 155

Chapter 7 Future Directions for IFVD Research
7.1 In tro d u ctio n ..............................................................................................158 7.2 Future Analysis of the IFVD Process
7.2.7 Depth dependence.................................................................158 7.2.2 Doping effects....................................................................... 160 7.2.5 The influence of the encapsulant..........................................160 72.4 Investigative techniques....................................................... 161 7.3 Structures for IFV D ................................................................................. 163 7.4 A pplications 7.4.1 QW configurations...................................................................171 7.4.2 Device configurations..............................................................173
7.4.2.1 Transmitter applications..................................... 174 7.4.2.2 Receiver applications...........................................176 7.5 Other Material System s.......................................................................... 179 7.6 C onclusions..............................................................................................180
C o n c lu sio n ............................................................................................................. 182
A ppend ices A ppendix A:- Full description of stru ctu res............................................. 185 A ppendix B:- Error function solution to interdiffusion problem . . .188 A ppendix C;- Model & param eters for subband energy calculation . 192
R eferen ces...............................................................................................................196
A ck n o w led g em en ts..............................................................................................225
List of P u b lic a tio n s............................................................................................... 226

PREFACE
The development of high quality quantum well (QW) m aterial in the last decade, has allowed the dem onstration of m any different optoelectronic devices, with attractive properties. Though research interest in QW m aterial is high, if they are to see a wide-scale comm ercial breakthrough monolithic integration of such devices is required, i.e. the form ation of optoelectronic integrated circuits (OEICs).
The work of this thesis came about after reading results published by Ralston and co-workers at Cornell University concerning a technique know n as im purity free vacancy diffusion (IFVD), which appeared an attractive tool for the fabrication of QW based OEICs. Prelim inary investigations proved encouraging and from those studies the work presented in this thesis evolved.
After giving an introduction to QW properties and devices. Chapter 1 goes on to discuss the characteristics that make the monolithic integration of different devices desirable. The desire for selective area bandgap engineering in order to control the optical properties across a chip is highlighted as being an absolute necessity for successful integration of optical components. In order to achieve this area selectivity it is show n that growth and post-growth techniques can be used, and that IFVD offers certain advantages.
In Chapter 2 the IFVD process is analysed step-by-step using published work. The encapsulation of GaAs with Si02, followed by high temperature annealing is show n to cause preferential Ga outdiffusion from the surface. The resulting Ga vacancies formed at the surface, on diffusing to the QW region, can enhance interdiffusion of the w ell/barrier, and im portantly interm ixing in general is show n to lead to a blue shift in the optical spectra.
A fter a description, in C hapter 3, of the experim ental set-ups used C hapters 4-6 present the results obtained during this work. C hapter 4 shows the selectivity of the IFVD process, and the large blue shifts it can produce, and also that the excitonic and optoelectronic properties associated w ith QW material are retained. Data is also presented showing an absence of the expected depth dependence; a surprising result which should allow the use of m ultiple QW (MQW) structures in association
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w ith IFVD. In Chapter 5 such an MQW is used to produce, via IFVD, surface normal reflection modulators, from the same wafer, which operate in completely different modes. The lateral resolution of the technique, along w ith its effect on the carrier escape from the well is exam ined in C hapter 6. In this chapter we also consider the attractive possibility of creating m ultiple w avelength operating areas w ith IFVD. Use is m ade throughout the thesis of computer modelling to find the well profile after interdiffusion, by comparing the calculated lowest energy transition to the experimental observed value.
The final chapter puts forward some suggestions for directions which future IFVD research will need to pursue. W ays of exam ining the fundam ental mechanisms of the process, and also dem onstrating the applicability of the technique, using wavelength division m ultiplexing (WDM) systems as an example, are given.
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