High power Led array for thinc-Si carrier lifetime

measurements

A Degree Thesis

Submitted to the Faculty of the

Escola Tecnica d’Enginyeria de Telecomunicacio de

Barcelona

Universitat Politecnica de Catalunya

by

Laura Gomez Ortega

In partial fulfilment of the requirements for the degree in

ELECTRONIC ENGINEERING

Advisor: Isidro Martın Garcıa

Barcelona, June 2016

Abstract

To measure crystalline silicon (c-Si) carrier lifetime, the Sinton WCT-100 with aflash lamp is available at the laboratories of the UPC’s Electronic Engineering De-partment. However, this setup is not accurate to measure thin c-Si samples. Theratio of light absorption for these samples strongly depends on the wavelength lead-ing to a difficult determination of the light absorption by the sample. We proposeto change the light source of the Sinton WCT-100 by a commercially-available highpower LED array emitting at a wavelength of 850 nm. In this project, a transcon-ductance amplifier is designed to control the ligth emission of the array through aperiodic waveform. In addition, a calibration procedure to measure thin c-Si samplesis defined. The single wavelength light simplifies the optical absorption calculationfor thin c-Si substrates improving their accuracy in the lifetime measurement. Themeasurements obtained with the new light source show reliable lifetime values forthin c-Si substrates at ranges where it is not possible to measure with the flash lamp.

1

Resum

Per mesurar el temps de vida de portadors en silici cristal·lı (c-Si), el Sinton WCT-100 amb un llum flash esta disponible als laboratoris del Departament d’ EnginyeriaElectronica de la UPC. No obstant, aquest sistema no es precıs a l’hora de mesurarmostres de c-Si fines. El ratio d’absorcio de llum per aquestes mostres depen forta-ment de la longitud d’ona, aixo condueix a una difıcil determinacio de l’absorcio dellum de la mostra. Proposem canviar la font de llum del Sinton WCT-100 per unconjunt de LEDs d’alta potencia disponible al mercat que emet a una longitud d’onade 850 nm. En aquest projecte es disenya un amplificador de transconductancia percontrolar l’emissio de llum del conjunt de LEDs mitjancant formes d’ona periodiques.A mes, es defineix un procediment de calibracio per mesurar mostres de c-Si fines.La llum de longitud d’ona unica simplifica el calcul d’absorcio optica per substratsde c-Si prims millorant la precisio en la mesura del temps de vida. Les mesuresobtingudes amb la nova font de llum mostren valors de temps de vida fiables persubstrats de c-Si prims a rangs en els quals no es possible mesurar amb la llum flash.

2

Resumen

Para medir el tiempo de vida de los portadores en silicio cristalino (c-Si), el SintonWCT-100 con una lampara flash esta disponible en los laboratorios del Departa-mento de Ingenieria Electronica de la UPC. No obstante, este sistema no es precisoa la hora de medir muestras de c-Si delgadas. El ratio de absorcion de luz para estasmuestras depende en gran medida de la longitud de onda, lo que lleva a una difıcildeterminacion de la absorcion de luz de la muestra. Proponemos cambiar la fuentede luz del Sinton WCT-100 por un conjunto de LEDs de alta potencia disponible enel mercado que emite a una longitud de onda de 850 nm. En este proyecto se disenaun amplificador de transconductancia para controlar la emision de luz del conjuntode LEDs mediante formas de onda periodicas. Ademas, se define un procedimientode calibracion para medir muestras de c-Si delgadas. La luz de longitud de ondaunica simplifica el calculo de absorcion optica para sustratos de c-Si delgados mejo-rando su precision en la medida del tiempo de vida. Las medidas obtenidas con lanueva fuente de luz muestran valores de tiempo de vida fiables para sustratos dec-Si delgados en rangos en los cuales no es posible medir con la lampara flash.

3

4

Acknowledgements

First of all, i would like to express my gratitude to my tutor Isidro Martin for beingalways available to answer my questions and helping me to go through this last halfyear. His help to improve this report was essential to conclude this thesis.

I would also like to thank my friends, without them all this years would havebeen impossible, and Ruben, he was a fundamental base to me and he encouragedme to never give up.

Last but not least, i want to thank my family for bring me here and supportingme all these years.

5

Revision history and approvalrecord

Revision Date Purpose0 02/06/2016 Document creation1 11/06/2016 Document revision2 20/06/2016 Document revision3 22/06/2016 Final version

DOCUMENT DISTRIBUTION LIST

Name e-mailLaura Gomez Ortega lauragomez.lgo@gmail.comIsidro Martın Garcıa isidro.martin@upc.edu

Written by: Reviewed and approved by:Date 20/06/2016 Date 22/06/2016Name Laura Gomez Name Isidro MartınPosition Project Author Position Project Supervisor

6

Contents

Abstract 1

Resum 2

Resumen 3

Acknowledgements 5

Revision history and approval record 6

Table of Contents 8

List of figures 9

List of tables 10

1 Introduction 111.1 Project background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.2 Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.3 Work Plan Packages, milestones and Gantt diagram . . . . . . . . . . 12

1.3.1 Work Packages . . . . . . . . . . . . . . . . . . . . . . . . . . 121.3.2 Milestones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.3.3 Gantt diagram . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2 State of the art 162.1 Transconductance amplifier . . . . . . . . . . . . . . . . . . . . . . . 162.2 Effective lifetime measurements . . . . . . . . . . . . . . . . . . . . . 17

2.2.1 Measurement procedure . . . . . . . . . . . . . . . . . . . . . 21

3 Methodology / project development 223.1 LED array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.1.1 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . 233.2 LED driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.2.1 Technical characteristics . . . . . . . . . . . . . . . . . . . . . 263.2.2 Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . 273.2.3 Electronic characteristics . . . . . . . . . . . . . . . . . . . . . 293.2.4 Optical characteristics . . . . . . . . . . . . . . . . . . . . . . 30

3.2.4.1 Uniformity . . . . . . . . . . . . . . . . . . . . . . . 303.2.4.2 Optical linearity . . . . . . . . . . . . . . . . . . . . 31

7

4 Results 334.1 LED array vs. flash lamp . . . . . . . . . . . . . . . . . . . . . . . . . 334.2 Instrument calibration . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4.2.1 Calibration procedure . . . . . . . . . . . . . . . . . . . . . . 344.2.2 Example of thin c-Si sample measurement . . . . . . . . . . . 35

5 Budget 405.1 Hardware costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405.2 Development costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

6 Conclusions and Future development 426.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426.2 Future development . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Bibliography 43

Glossary 44

8

List of Figures

1.1 Gantt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.1 Transconductance amplifier with grounded load . . . . . . . . . . . . 162.2 Transconductance amplifier with floating load . . . . . . . . . . . . . 172.3 Measurements scenario . . . . . . . . . . . . . . . . . . . . . . . . . . 182.4 Sinton WCT-100 circuit diagram . . . . . . . . . . . . . . . . . . . . 182.5 Block diagram of a generic measurement system of τeff . . . . . . . . 192.6 Calibrated solar cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.1 INSTAR IN-905 spotlight . . . . . . . . . . . . . . . . . . . . . . . . 223.2 Circuit diagram of the commercial LED array . . . . . . . . . . . . . 233.3 Current-voltage characteristic of one LED . . . . . . . . . . . . . . . 243.4 Current-voltage characteristic of the LED array . . . . . . . . . . . . 243.5 Dependence of light intensity of one LED on the injected current . . . 253.6 LED driver and fan control . . . . . . . . . . . . . . . . . . . . . . . . 263.7 Schematic of the LED driver and the fan control . . . . . . . . . . . . 273.8 Layout of the LED driver and the fan control . . . . . . . . . . . . . 283.9 Final printed circuit board . . . . . . . . . . . . . . . . . . . . . . . . 293.10 Transconductance function . . . . . . . . . . . . . . . . . . . . . . . . 303.11 Uniformity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313.12 Optical linearity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.1 LED array vs flash lamp . . . . . . . . . . . . . . . . . . . . . . . . . 334.2 Calibration procedure description . . . . . . . . . . . . . . . . . . . . 354.3 PCD measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364.4 QSS-PC and PCD measurements . . . . . . . . . . . . . . . . . . . . 374.5 Reflection, transmission and absorption functions . . . . . . . . . . . 384.6 LED array vs flash lamp . . . . . . . . . . . . . . . . . . . . . . . . . 38

9

List of Tables

1.1 WP1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.2 WP2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.3 WP3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.4 WP4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.5 WP5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.6 Milestones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.1 Resistor values, calibration factor and light intensity . . . . . . . . . . 20

3.1 LED array structure characteristics . . . . . . . . . . . . . . . . . . . 23

5.1 Hardware costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405.2 Development costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

10

Chapter 1

Introduction

1.1 Project background

Silicon is the second most abundant element in the planet’s crust and the most com-mon material to manufacture photovoltaic cells. The final efficiency of photovoltaiccells is determined and characterized by the width of the energy gap, spectral re-sponse and other electronic properties as carrier lifetime. Commercial equipment isavailable in the laboratory of the UPC’s electronic department to realize the mea-surements of minority carrier lifetime on crystalline silicon (c-Si). The commercialsetup comes with a flash lamp typically used for photographic applications. Thislight source has a complex spectrum distribution of its light intensity. For thick c-Sisamples, all the light that the flash lamp emits is absorbed by the c-Si samples. As aconsequence, the determination of light absorption, which is a key parameter for life-time determination, can be done quite easily. When thin c-Si samples are measured,the ratio of light absorption strongly depends on the wavelength, which leads to adifficult determination of the light absorption at the sample. The main goal of thisthesis is to improve the commercial setup replacing the flash lamp by a LED array.The single wavelength light generation simplifies the optical absorption calculationfor thin c-Si substrates improving the accuracy in the lifetime measurement.

1.2 Goals

Project target is the development of electronics that control a commercially avail-able high power LED array in order to generate periodic light generation with thedesired waveforms. These electronics and the LED array replace the flash lamp,which is part of the commercial equipment that is at the UPC’s laboratory. Oncethe electronics and LED array has been built, next goal is the calibration of mea-surement setup for thin c-Si wafers. The final objective is to have reliable lifetimevalues for thin c-Si substrates for excess carrier density higher than 1014cm−3.

In order to simplify the LED array fabrication, it is planned to use a commercialLED array at a wavelength of 850 nm. These LED arrays are cheap and easy tofind for commercial applications like night vision for security cameras.

11

1.3 Work Plan Packages, milestones and Gantt

diagram

1.3.1 Work Packages

Project: Documentation WP ref: (WP1)Major constituent: Reports Sheet 1 of 5

Short description: Writing reportssummarize the obtained resultsand help in the project management

Planned start date: 01/03/2016Planned end date: 27/06/2016Start event: Project beginningEnd event: Project end

Internal task T1:Project Proposaland Work Plan

Internal task T2: Project Critical ReviewInternal task T3: Final report

Deliverables:D1.1 ProjectproposalD1.2 Projectcritical reviewD1.3 Projectfinal report

Dates:01/03/2016

09/05/2016

27/06/2016

Table 1.1: WP1

Project: array characterization WP ref: (WP2)Major constituent: hardware characteriza-tion

Sheet 2 of 5

Short description: characterization of thecommercial LED array. We will start firstfor one LED and after that,the characterization of the whole structure

Planned start date: 18/02/2016Planned end date: 24/03/2016Start event: project beginningEnd event: characterizationend

Internal task T1: single LEDcharacterization

Internal task T2: LED arraycharacterization

Deliverables:D2.1Documentationof arraycharacterization

Dates:25/03/2016

Table 1.2: WP2

12

Project: design and fabrication of transcon-ductance amplifier

WP ref: (WP3)

Major constituent: hardware design and fab-rication

Sheet 3 of 5

Short description: design and assembly ofa voltage-to-current amplifier. The circuitmust fulfil the specified requirements

Planned start date: 25/03/2016Planned end date: 21/04/2016Start event: characterizationendEnd event: amplifier assembly

Internal task T1: design of the voltage-to-current amplifier and component selectioncharacterization

Internal task T2: assembly of a prototypeof the voltage-to-current amplifier andcharacterization

Deliverables:D3.1 Prototype oftransconductanceamplifier

Dates:22/04/2016

Table 1.3: WP3

Project: Characterization of the completelight source system

WP ref: (WP4)

Major constituent: Implementation and as-sembly

Sheet 4 of 5

Short description: implementation of thevoltage-to-current amplifier and theLED array, assembly at a printed boardand study of their characteristics

Planned start date: 22/04/2016Planned end date: 27/05/2016Start event: amplifier assemblyEnd event: Test1 beginningend

Internal task T1: assembly of thestructure at printed board

Internal task T2: characterizationand operation testing

Internal task T3: implementationof the complete structure

Internal task T4: characterizationand operation testing

Deliverables:D4.1 Completesystem LED arrayplus amplifierfulfiling therequirements

Dates:30/05/2016

Table 1.4: WP4

13

Project: calibration of lifetime measurementsetup

WP ref: (WP5)

Major constituent: study Sheet 5 of 5

Short description: replace the flash bythe light structure and realize ameasurement testing

Planned start date: 30/05/2016Planned end date: 17/06/2016Start event: test1 endEnd event: test2 end

Internal task T1: replace the lightstructure

Internal task T2: measurement testingInternal task T3: study of the results

Deliverables:D5.1 Report ofthe obtainedresults

Dates:20/06/2016

Table 1.5: WP5

1.3.2 Milestones

WP# Task# Short title Milestone / deliverable Date (week)1 1 Project Proposal and Work Plan D1.1 01/03/20161 2 Project Critical Review D1.2 09/05/20161 3 Final report D1.3 27/06/20162 1 Report on array characterization D2.1 25/03/20163 1 Design of transconductance am-

plifierM3.1 10/04/2016

3 2 Prototype of transconductanceamplifier

D3.1 22/04/2016

4 1 Printed board with transconduc-tance amplifier

M4.1 02/05/2016

4 2 Characterization of final amplifier M4.2 15/05/20164 3 System characterization report D4.3 30/05/20165 1 Setup calibration report D5.1 22/06/2016

Table 1.6: Milestones

14

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15

Chapter 2

State of the art

The state of the art of this project is a brief introduction to transconductance am-plifiers and a study of the main measurement methods applied to the recombinationeffective lifetime

2.1 Transconductance amplifier

The transconductance amplifier (gm amplifier) proportionally converts an inputvoltage to an output current. It is also called a voltage to current converter or V toI converter. It acts as a linear circuit with transfer ratio gm = Iout/Vin in A/V orSiemens (S). It is called transconductance because the efficiency of the amplifier ismeasured in units of conductance.

Transconductance amplifiers are classified into two types:

• Transconductance amplifier with grounded load [1]. In this circuit, the col-lector and emitter currents are approximately equal, then the load currentalso flows through the feedback resistor R which is responsible for the trans-formation between voltage and current. In addition, the input impedance ofOperational Amplifier seen by the signal generator is very high which makesit simpler to use. On account of this, there is still current feedback, whichmeans that the load current is stabilized.

Figure 2.1: Transconductance amplifier with grounded load

16

• Transconductance amplifier with floating load [1]. The input voltage is appliedto the non-inverting input terminal and the feedback voltage across R drivesthe inverting input terminal.

Figure 2.2: Transconductance amplifier with floating load

In this project we are going to use a variation of the basic circuit of the transcon-ductance amplifier with floating load. As it will be explained below, we need a highcurrent through the LED array to generate a light intensity as high as possible. Thisrequirement will be fulfilled by introducing a BJT transistor.

2.2 Effective lifetime measurements

The carrier lifetime (recombination lifetime) is defined as the average time it takesa carrier to recombine since it was photogenerated[2]. This parameter is crucial forsolar cell efficiency: the longer the lifetime, the more photogenerated carriers areable to reach the contacts and generate useful current at the output connectors ofthe device. The effective lifetime takes into account all the recombination processesthat simultaneously take place in the silicon sample, namely bulk recombination andfront and rear surface recombination.

The instruments used to carry out these lifetime measurements are the SintonWCT-100, a computer and a Tektronix TDS 210 oscilloscope. The disposition ofthe elements is showed in the next figure.

17

Figure 2.3: Measurements scenario

The WCT-100 photoconductance instrument is developed by Sinton Instruments,originally established as Sinton Consulting in 1992, a company dedicated to devel-oping and applying new tools and analysis to R&D and manufacturing in siliconsolar cells and integrated circuits[3].

This instrument is used throughout the photovoltaic industry providing a rela-tively simple and inexpensive means to assess recombination parameters. Its mainuse is to obtain the effective lifetime τeff in function of the average minority carrierdensity ∆nav . The measurement consists of the illumination by the flash of thesilicon wafer and the calibrated solar cell. The instrument uses a coil to inductivelycouple the sample signal to an oscilloscope and a computer for data analyses andstorage. The conductance is measured through a calibrated RF bridge after zeroingthe signal of the wafer over the inductive coil and the light intensity by means of areference solar cell located near the sample.

Figure 2.4: Sinton WCT-100 circuit diagram

18

Since the RF bridge circuit, operating in a frequency of 13.56 MHz, is well de-signed, the output voltage and the increase in conductance of the c-Si wafer dueto the photogenerated carriers follow a linear relationship in a range that dependson the circuit topology. The variable resistor and capacitor are used to balance thebridge to zero, prior to the measurements.

In next figure, it can be observed the necessary elements to do the measurementswhere we can see that the oscilloscope simultaneously records the photoconductancesignal in (channel I) and the light intensity in (channel II). Once the signals arecaptured, they are transferred to the PC to perform the calculations needed to cal-culate the τeff (∆nav) curve.

Figure 2.5: Block diagram of a generic measurement system of τeff

As it has been mentioned, this system measures two magnitudes:

• Photoconductance of the sample (∆σ): A voltage signal proportional to thephotoconductance is obtained from the silicon wafer and provided to the os-cilloscope (channelI). From the following equation ∆nav can be estimated.

∆σ = W · q · (µn + µp) · ∆nav (2.1)

Where W is the sample thickness, q is the electron’s charge and µn and µp

are electrons and holes mobility respectively. This magnitude is measured inSiemens.

• Light intensity (Ilight): it is measured through the short-circuit current of acalibrated solar cell which is translated to voltage through a resistor and pro-vided to the oscilloscope (channel II). From this magnitude, the density ofthe emitted photons per unit of time that reach the sample and as a conse-quence, the photogeneration inside the sample Gext(t), is obtained. To obtainthe value of Gext(t) the next equation is used:

Gext(t) = Ilight(t) · fopt ·1

W(2.2)

19

Where Ilight(t) are light intensity detected by the calibrated solar cell in pho-tons per cm2 and second, W is the thickness of the sample and fopt is anoptical factor which consider how many photons are absorbed by the sample.This fopt is composed of two parts, a factor based on the difference of the lightintensity between the calibrated solar cell and the sample and a factor basedon its absorption of light, fint and fabs respectively.

Using the integrated solar cell, some problems of low signal-to-noise ratio appearwhen light intensity is very low. In order to increase the signal-to-noise ratio, thecalibrated solar cell integrated in the equipment has been replaced by an externalone which can be connected to five different resistors, which allow the measurementof the voltage by the oscilloscope, by a selector device. The user can vary the posi-tion of the selector and choose a different resistor depending on the light intensity. Ifa low light intensity is used, a higher value of the resistor increases the correspondingvoltage and thus the signal-to-noise ratio. In the table 2.1 we show the position ofthe resistor selector, the resistor values, the calibration factors and the typical lightintensity range used for each resistor.

Position Resistor value (Ω)Calibration factorfsuns(V/suns)

Light intensityrange (suns)

A 0.01 6.55x10-4 >10B 0.1 6.11x10-3 10-1C 1 6.55x10-2 1-0.1D 10 6.46x10-1 0.1-0.01E 100 6.5 <0.001

Table 2.1: Resistor values, calibration factor and light intensity

Figure 2.6: Calibrated solar cell

Once obtained the magnitudes involved in the calculation of τeff , the so-calledgeneral method is applied: a technique used for the determination of minority carrierlifetime based on a light pulse used to create excess carriers in the semiconductorsample. To calculate τeff , the next equation, proposed by Nagel (etal.) in 1999 [4],is evaluated at every illumination level.

τeff =∆nav

Gext(t) − ∂∆nav(t)∂t

(2.3)

20

It exists two limit cases of this general method:

• Quasi Steady State PhotoConductance method (QSS-PC): first introduced byR.A. Sinton and A. Cuevas in 1966 [5]. The light pulse varies very slow com-pared to the effective lifetime of the wafer. This offers a method for obtainingthe photoconductance under a large range of illumination intensities in a shorttime without significant sample heating. The value of fopt is needed to thiskind of measure since we need Gext. The equation to estimate τeff is:

τeff =∆nav

Gext(t)(2.4)

• PhotoConductance Decay (PCD): first introduced by D.T. Stevenson and R. J.Keyes in 1955 [6]. The light used is a very short pulse, that allows photogener-ated carriers to recombine when there is no light. The only magnitude neededis the photoconductance of the sample, since Gext(t) is 0, so the equation toestimate τeff is:

τeff =∆nav

−∂∆nav(t)∂t

(2.5)

2.2.1 Measurement procedure

1. The silicon wafer is placed over the inductive coupling coil. It is necessary toknow the width and doping of the sample.

2. Change the values of the resistor and the capacitor to balance the RF bridgeto zero.

3. Move the resistor control clockwise until obtaining an output voltage of 100mV.

4. Choose the value of the resistor depending on the selected light intensity level.

5. The light source is activated.

6. The oscilloscope shows the photoconductance signal and the light intensitysignal and they are recorded in the computer.

The electron-hole pair generation Gext(t) and the average excess carrier den-sity ∆nav are calculated from the curves obtained by this procedure with the nextequations:

Gext(t) = Ilight(t) · fopt ·1

W(2.6)

∆nav(t) =∆σ(t)

W · q · (µn + µp)(2.7)

Finally the τeff can be estimated with the equation 2.3.

21

Chapter 3

Methodology / projectdevelopment

After the brief description of the background, this chapter explains the physical im-plementation of the LED array and its control electronics. First the LED array willbe described which is commercially available. A detailed knowledge of this item iscrucial for the correct design of the LED driver which will control the current thatwill pass through the LED array.

3.1 LED array

The chosen element to replace the flash lamp that comes with the commercial equip-ment used in this thesis, the Sinton WCT-100, is a commercial high power LEDarray INSTAR IN-905 shown in figure 3.1. This LED array comes with an externalstructure also used in this project.

Figure 3.1: INSTAR IN-905 spotlight

The next table shows the specifications of the acquired commercial device:

22

Iluminationangle

60 degrees

Distance 20-25 mLEDs 60Wavelength 850nm (infrared)Output/LED 0.2WPower supply 110-230VMax. power consumption 10WWeight 400gStructure measurements in cm (H/W/D) 17/15.5/12LED array measurements in cm (H/W/D) 14/10/1.5

Table 3.1: LED array structure characteristics

The next figures show the internal connexions of the LED array and the externalview of the LED array:

Figure 3.2: Circuit diagram of the commercial LED array

3.1.1 Characterization

A study of this commercial acquisition has been realized. In the next figure, it canbe observed the I-V of a single LED.

23

Figure 3.3: Current-voltage characteristic of one LED

As it can be seen, a LED shows a voltage drop in the range of 1.5-2 V dependingon the current that flows through it. Then, with the configuration of the LED arrayshown in figure 3.2, the expected voltage drop in the array is 9-12 V which is ingood agreement with the experimental measurement shown in figure 3.4 .

Figure 3.4: Current-voltage characteristic of the LED array

24

Additionally, we measure the light intensity emitted by a LED for different for-ward currents. To do so, we use a calibrated solar cell whose External QuantumEfficiency at 850 nm is known (EQE = 66.0 %) . Then, the measured current inthis cell can be translated into received photons using the following expression:

I(phot/s) = I(A) · 1

EQE· 1

q(3.1)

Using this procedure, we obtain the dependence of the number of radiated pho-tons on the forward current which is shown in figure 3.5. As it can be observed inthis figure, the light intensity is linear with the forward current up to about 150 mA.Beyond this point the radiation efficiency decreases due to an increase in the LEDtemperature. Despite of this effect can be seen at 150 mA, we consider that a reason-able maximum current for the LED is 200 mA taking into account the following tworeasons. Firstly, the measured LED is isolated from the array which increases thedifficulty in heat dissipation. Secondly, periodic waveforms are going to be used, liketriangle waveforms. Then, the time at which the maximum current is going to flowthrough the LED is strongly reduced relaxing the thermal dissipation requirements.Considering the scheme of the array, with 10 rows of LEDs and a maximum currentof 200 mA per LED, the maximum current that passes through the LED array is 2A.

It must be mentioned that in figure 3.4 it can be observed that the maximumcurrent showed is 1A, but this limit is due to a restriction of the I-V measurementinstrument.

Figure 3.5: Dependence of light intensity of one LED on the injected current

Combining the information about the maximum current through the array (2A)and its voltage drop (9-12V), the characteristics of the power supply needed for theLED array and the current driver are ± 15 V with a maximum current of 2 A.

25

3.2 LED driver

The LED driver will fulfill the following specifications:

• Supply voltage of ±15.

• Maximum current of 2A.

• Light intensity must change less than 5% in a region of 2 cm diameter wherethe sensor is located.

• - The transconductance of the voltage-to-current amplifier should be of 1 Swith a 10% tolerance.

• - Deviation from linear behavior of the transference function of the transcon-ductance amplifier lower than 5%.

3.2.1 Technical characteristics

The necessity to control the input voltage and the output current, which will passthrough the LED array, led to the decision to use a voltage-to-current convertercircuit. To prevent the circuit components to overheat, it was decided to add a fanand the necessary electronic to supply the fan. In the next figures are shown thementioned circuits.

Figure 3.6: LED driver and fan control

The transistor must be in active forward region in order to keep the amplifierin its linear range. This means that the collector to emitter voltage Vce must behigher than Vce|sat = 0.2 V. This parameter can be easily calculated by applyingthe following expression where we have used that the emitter voltage is equal to thesignal voltage Vg by virtual short-circuit:

Vce = Vc − Ve = Vcc − Varray − Vg (3.2)

From the previous analysis, we know that Vcc is 15V and the voltage drop at theLED array is 9-12V. Then, the maximum values for Vg is about 3V. In order to keepa security margin, the input values of Vg will vary between 0V and 2V.

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0V < Vg < 2V (3.3)

This voltage sweep must be translated to current through the resistor R wherethe following expression applies:

I =VgR

= gm · Vg (3.4)

The combination of the current range through the LED array ( 0 < I < 2 A) andthe signal range (0 < Vg < 2 V) results in a resistor R = 1Ω. As it can be deducedfrom equation 3.4 the transconductance of the amplifier will be 1 S, as mentionedin the requirements.

With a maximum Vg of 2V and a resistor of 1 Ω, the maximum power this resistorwill support can be calculated with the next equation:

P =V 2

R(3.5)

The maximum power is 4W. Then, we use a resistor able to stand a power of 10W.

The next component is the regulator, the selected one is the 7805 (Appendix B),which has an output voltage of 5V that is connected to the input of the fan.

The last components of the circuit are three decoupling capacitors; a capacitor of330µF connected between Vee and ground, a capacitor of 330µF connected betweenVcc and ground and a capacitor of 100nF connected between the output signal ofthe regulator and ground.

3.2.2 Implementation

Once specified all the circuit components, the next step is to realize the schematicdesign of the printed circuit board (PCB) by using the Eagle 7.5.0, a diagram andPCB design program.

Figure 3.7: Schematic of the LED driver and the fan control

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Figure 3.8: Layout of the LED driver and the fan control

In figure 3.7, the electrical design of the board is shown while in figure 3.8 weshow the conductive tracks layout, where the real size and position of the compo-nents can be also observed. The PCB will only have one printed side; it is enoughfor the circuit’s size. Another aspect to mention is the board connectors, the LEDconnectors and the input signal connectors are placed on the left side of the boardand the fan connectors are placed on the bottom right side.

Now that components have been specified and the board printed the implemen-tation can start. The necessary elements are:

1. PCB

2. An operational amplifier LF353. The maximum voltage supply is ±18V(Appendix C).

3. A BU406 NPN transistor. The important factors are the ICmax=7 A anda Pmax=60 W (Appendix D) because this is a high power circuit and thetransistor will have to withstand currents of 2A.

4. A resistor of 1Ω and 10W.

5. Three capacitors. Two of them with a capacity of 300 µF and one with acapacity of 100 nF.

6. An eight pack baseboard for the OA, if it is needed a replacement of thecomponent it will be easier to change it.

7. To connect the LED array, the input signals and the fan would be required 8connectors.

8. The PCB will be mounted inside the commercial structure mentioned above.To make more accessible the connection of the input signals to the PCB, 4connection clamps will be assembled on the commercial structure surface

9. Wire to connect the LED array and the fan to the board and the input signalsconnectors to the connection clamps on the structure surface.

10. A 7805 regulator to supply the fan with 5V.

11. Fan

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Once collected all the necessary elements, it is time to solder the componentsto the PCB and assemble the connection clamp and the fan in the commercialstructure. After soldering some tests are realized to verify the proper function ofthe circuit. In next figure is shown the final state of the board.

Figure 3.9: Final printed circuit board

3.2.3 Electronic characteristics

The value of the transconductance gm of the circuit can be estimated by measuringthe relation between the output current Iout and the input voltage Vin. The inputsignal is a constant voltage; the output current is indirectly measured through thevoltage drop in the 1 Ω resistor and converted to current with the next equation:

V = I ·R (3.6)

For an accurate measurement, the real value of the resistance was measuredbefore soldering it with a multimeter leading to R = 0.988 Ω. In the next graph itcan be observed the relation mentioned above. Additionally, the experimental valuesare fitted by a linear trend line with excellent agreement. The linear behaviour ofthe amplifier is maintained up to 2.6 V when the transistor saturates. This voltagecorresponds to a maximum current through the array of 2.47 A, which fulfils the2 A requirement. The gm value coincides with the value of the trend line’s sloperesulting in a gm of 0.947 S. This value also satisfies the requirement of a gm valueof 1±0.1 Ω.

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Figure 3.10: Transconductance function

3.2.4 Optical characteristics

3.2.4.1 Uniformity

One of the most important aspects to examine is the height where is placed theLED array structure. The further the array is placed, the greater uniformity willbe because the light beam is more opened, but there will be less light power. If thesilicon sample is nearer the array, uniformity will be worse, but it will be possibleto illuminate with a higher light power.

In the next graph is shown the uniformity distribution at a height of 8 cm.Measures were realized with a photodiode. The measured area is 3x3 cm with thecentre of the measures at the centre of the wafer position. Additionally, a sketch ofthe sensor coil is also drawn indicating where the c-Si sample conductance will bemeasured. In this area, a difference below 2 % can be measured which achieves thementioned requirement.

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Figure 3.11: Uniformity

3.2.4.2 Optical linearity

This characteristic allows testing the impact of the temperature and the input volt-age on the LED array. To perform this test, the emitted light of the LED array wasmeasured in relation to a continuous input voltage. The measurement was realizedin total darkness to achieve more accurate values.

In the next figure can be observed the experimental values measured after 5minutes at 2 A to ensure LEDs were heated up. Two measurements were realized,one with the fan turned on and one with the fan turned off.

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Figure 3.12: Optical linearity

As it can be observed in the figure 3.12, the emitted light of the LED array isproportional to the input voltage for currents up to 1 A. For higher currents, theLED temperature increases reducing the light emission efficiency. For this reason,the last measured points do not continue the linear trend. Notice that the pointsresulting from the study with the fan turned on (circles) present a better responsethan the points resulting from the study with the fan turned off (square). This re-sponse reaches higher light intensity values, as expected, because the LED’s coolingdue to the fan.

It must be mentioned that a similar measurement was presented for one LEDin figure 3.5. In that case, a linear relationship between current and light intensitywas measured up to 150 mA corresponding to 1.5 A for the array (ten LED rows).The difference could be attribute to the more heat generated by the full array whichis encapsulated inside the structure. In any case, in the common operation of theLED array, a periodic waveform will be used which will keep current beyond 1.5 Afor very short times.

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Chapter 4

Results

4.1 LED array vs. flash lamp

Some measurements are made to compare the results between the flash that comeswith the equipment and the LED array performed in this project. As it will beexplained below, a calibration procedure is needed to determine the optical factorfor both light sources. The objective of this first section is to demonstrate that theflash lamp can be successfully replaced by the fabricated LED array. Then, in theresults presented in figure 4.1, the optical factor has been calibrated following theprocedure mentioned below and both measurements are plotted together for a directcomparison.

Figure 4.1: LED array vs flash lamp

As can be observed in the figure above, both measurements overlap in the 1015-1016 cm−3 range. This result demonstrates that the LED array can effectively workas a light source for lifetime measurements. Additionally, some differences can beseen. On the one hand, the LED array incident power on the wafer is lower thanthe one obtained using the flash lamp.

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This result is expected since the LED array emits at a very narrow wavelengthrange while the flash lamp covers the visible and near infra-red band. On the otherhand, the result using the LED array allows us to measure at lower excess carrierdensities. The sensitivity of Sinton instrument is limited by the signal-to-noise ratioof the photoconductance channel. Then, a periodic waveform emitted by the LEDarray can be captured by the oscilloscope and through its averaging function thesignal-to-noise ratio is improved. In summary, we can say that the LED array haslower power but a better control over the measurements.

Henceforth, all the measurements were made with the LED array.

4.2 Instrument calibration

4.2.1 Calibration procedure

This section is a crucial part of the project because it is essential to achieve the finalobjective of accurate measurements in thin c-Si samples. As it was mentioned insection 2.2, the optical factor fopt can be divided in two terms: a factor related tothe light intensity difference between the sample and the calibrated solar cell thatwe use to measure this parameter fint; and a factor linked to the light absorptionby the sample fabs. Then:

fopt = fint · fabs (4.1)

For thin c-Si samples, fabs can be measured or simulated based on the well-knownoptical properties of the c-Si. Then, the problem is reduced to the determination offint for a certain height of the LED array over the sample. To do so, we use a longlifetime sample. For this type of samples, we can measure under two conditions:short and long light pulse. The former leads to measure under PCD conditionswhere equation 2.5 applies. Notice that in this equation, no optical factor is neededsince the lifetime measurement only depends on the excess carrier density ∆nav andits evolution on time. Then, the obtained lifetime values are very robust. Theiraccuracy only depends on the linear relation between the measured voltage and thereal ∆nav, which is fulfilled by the equipment design. Once we know lifetime valuesfor a certain ∆nav range, we can now perform the QSS-PC measurement. In thiscase, fopt can be determined by overlapping both measurements. With fopt and fabs,we can calculate fint by applying equation 4.2. In figure 4.2, a graphical descriptionof the calibration procedure is shown.

fint =foptfabs

(4.2)

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Figure 4.2: Calibration procedure description

In the next section, an example of this measurement procedure is explained.

4.2.2 Example of thin c-Si sample measurement

Following the procedure explained above, as long lifetime sample we use a 280 µmwidth c-Si wafer whose surfaces have been passivated by 50 nm Al2O3 film. Thisfilm drastically reduces surface recombination. Bulk recombination is also low dueto the high quality c-Si material used in this sample. Then we start by measuringthis sample under PCD and QSS-PC conditions.

For PCD measurements, a short pulse, for example 10 ms, is used. A plotof the photoconductance and light intensity channel is shown in figure 4.3a. Weapply equation 2.5 to the photoconductnace values once the light is off obtaining areference value of τeff independent of the optical factor shown in figure 4.3b.

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(a) Photoconductance and lightintensity with short pulse

(b) Lifetime data from PCD signal

Figure 4.3: PCD measurements

In the next measurement, a waveform with a much longer decay than the signalmentioned above, like a triangular signal (see figure 4.4a), is used. In this case, thelifetime determination is achieved using the QSSPC method which needs informationfrom the two oscilloscope channels, the photoconductance and the calibrated solarcell’s light intensity. Once finished the previous steps, the QSSPC signal is plottedtogether with the PCD signal, as it is shown in figure 4.4b. First these signalswill not overlap, but varying the optical factor the QSSPC signal will move untilit coincides with the PCD signal. The right fopt in this case is 1.9 because theQSS-PC perfectly matches the PCD signal. For a direct comparison, the QSS-PCmeasurements with fopt equal to 1.8 and 2.0 are also shown in the graph. As it canbe seen, in both cases a worse matching is obtained. As a consequence, introducingthe accuracy in the determination of this parameter, we can say that fopt is equalto 1.9±0.05.

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(a) Photoconductance and lightintensity

(b) Comparison between QSS-PC andPCD signal

Figure 4.4: QSS-PC and PCD measurements

As explained in a previous section, the optical factor can be divided in two parts,the fabs based on the absorption of light of the sample and the fint based on thedifference of the light intensity between the calibrated solar cell and the sample.The fabs can be obtained from two sources, simulations of the examined sample andmeasurements performed on the wafer. The fabs of the 280 µm sample at 850nm is0.87, obtained in this case by simulations [7]. Then applying equation 4.2, a fint of2.18 is deduced.

Now, we measure the thin c-Si sample, in particular a 30µm thick c-Si wafer pas-sivated by 50 nm Al2O3 film. For this sample, the fabs must be known. We measurethe reflection (R) and transmission (T) of this sample for wavelengths ranging from300 to 1200 nm resulting in the curves shown in figure 4.5. Then, the absorption(A) of the sample can be calculated as A=1-R-T. As a conclusion, the fabs of the30µm sample is 0.63 (indicated by the arrow in figure 4.5. This new fabs must bemultiplied by the obtained fint (fint=2.18) resulting in a fopt value for the new mea-surements of 1.38.

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Figure 4.5: Reflection, transmission and absorption functions

Using this value, a QSS-PC measurement is performed leading to curve shown infigure 4.6. As it can be seen in this figure, the effective lifetime is measured for ∆nav

up to 5 ·1014 cm−3. If τeff values beyond this excess carrier density are needed, nowthe flash lamp can be used. The optical factor with the flash lamp is unknown, buttaking advantage that τeff is unique and known for a certain ∆nav range, we canchange this parameter value to overlap the flash measurement with the one obtainedwith the LED array, as it is shown in figure 4.6. Notice that the combination of flashmeasurement at high excess carrier densities with LED array at low ones allows usto extend the measurement range compared to the one available only with the flashlamp.

Figure 4.6: LED array vs flash lamp

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Finally, it must be mentioned that these measurements have been repeated withthe LED array at a different height. The one explained in this section was carriedout at a height of 20 cm and a second measurement was done at a height of 8 cm.For this second measurement we applied the same calibration procedure. In thiscase, a fint of 4.59 was deduced. As expected, the obtained lifetime curves are thesame for the two heights. This result validates the correct operation of the usedmeasurement methodology.

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Chapter 5

Budget

In this chapter the project costs are detailed. It should be mentioned that thesoftware used to do the lifetime measurements has no additional licenses costs.

5.1 Hardware costs

Components Qantity Cost (e) Total cost(e)7805 regulator 1 0.22 0.22BU406 transistor 2 0.64 1.28Capacitor 100 nF 1 0.21 0.21Capacitor 300 uF 2 0.48 0.96Connection clamps 4 0.79 3.16Connectors serie 1 0.43 0.43Eight pack baseboard 1 0.1 0.1Fan 1 7.63 7.63LED array INSTAR IN-905 2 85 170LF353 OA 1 0.34 0.34PCB 1 30 30Resistor 1ohm 10W 2 1.42 2.84Wire 1 1.63 1.63Total - - 218.8

Table 5.1: Hardware costs

5.2 Development costs

In table 5.2 are separated the development costs in the different phases that theproject has.

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ConceptHour by

ECTS CreditCredits Hours Price/hour Cost (e)

Array characterization 140 1120Design and fabrication

of transconductance amplifier30 24 200 8 e 1600

Characterization of the completelight source system

180 1440

Calibration of lifetimemeasurement setup

200 1600

Total 720 5760

Table 5.2: Development costs

5.3 Summary

• Hardware costs: 218.8 e

• Development costs: 5760 e

The approximately cost -only keeping in mind the hardware and the cost todevelop the system- is 5978.8 e

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Chapter 6

Conclusions and Futuredevelopment

6.1 Conclusions

The premise of this project was the development of electronics that control a com-mercially available high power LED array in order to generate periodic illumina-tion with specific waveforms and the calibration of measurement setup for thin c-Siwafers. The main goals has been fulfilled since the electronics used to performthe measurements and the calibration of the measurement setup were successfullyachieved leading to reliable lifetime values for thin c-Si substrates for excess carrierdensity higher than 1014cm−3.

6.2 Future development

There are some lines to follow the development of this project.

On the one hand, there are improvements that affect the software used to processthe data obtained from the oscilloscope. This software can be adapted to the newLED array. Since the fopt is divided in two factors, as explained above, this divisioncould be reflected in the software where only one fopt is introduced.

On the other hand, the setup result of this project can also be used with otherinvestigation purposes. To have a specific waveform with a certain level of powercan be very useful to other c-Si characterization techniques, like photoluminescencemeasurements, among others.

42

Bibliography

[1] Prof. Pramod Agarwal. Analog circuits. url: http://nptel.ac.in/courses/117107094//lecturers/lecture 13/lecture13 page2.htm.

[2] S.M. Sze and M.K. Lee. Semiconductor Devices: Physics and Technology. Ed.John Wiley and Sons. 2013.

[3] Sinton Consultin Inc. url: http : / / www . sintoninstruments . com / index .html/.

[4] A.G. Aberle A H. Nagel C. Berge. Generalized analysis of quasi-steady-state andquasi-transient measurements of carrier lifetimes in semiconductors. J. Appl.Phys. 86, 6218. 1999.

[5] R.A. Sinton and A. Cuevas. Contactless deetermination of current-voltage char-acteristics and minority-carrier lifetimes in semiconductors from quasi-steady-state photoconductance data. Apl. Phys. Lett. 69. 1996.

[6] D.T. Stevenson and R.J. Keyes. Measurement of short carrier lifetimes in ger-manium and silicon. J. Appl. Phys. 26, 190. 1955.

[7] PVLighthouse. url: https://www.pvlighthouse.com.au/.

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Glossary

BBJT: Bipolar Junction Transistor

Cc-Si: crystalline Silicon

DD: Depth

EEQE: External Quantum Efficiency

HH: Height

LLED: Light Emitting Diode

NNPN: Negative-Positive-Negative

OOA: Operational Amplifier

PPC: Personal ComputerPCB: Printed Circuit BoardPCD: PhotoConductance Decay

QQSS-PC: Quasi Steady State Photo-

ConductanceR

RF: Radio FrequencyR&D: Research & Development

UUPC: Universidad Politecnica de

CatalunaW

W: Width

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