2008-PSS-Thermal instability of electron traps in InAs-GaAs

Thermal instability of electron traps in InAs/GaAs quantum dot structures

M.Kaniewska ?O.Engstro

¨m ?M.Kaczmarczyk ?G.Zaremba

Received:26September 2007/Accepted:12March 2008/Published online:3April 2008óSpringer Science+Business Media,LLC 2008

Abstract Deep level transient spectroscopy (DLTS)in temperature and frequency scanned modes has been used to characterize deep-level defects present in samples with InAs/GaAs quantum dots (QDs).Two deep energy traps have been studied in details,a trap with thermal activation energy at 1.03eV,which has no correspondence to DLTS data in the literature to make any comparison and an accompanied trap at 0.79eV belonging to EL2family.The concentration of the 1.03eV trap exhibited signi?cant changes with temperature,whereas the trap at 0.79eV was stable on a reduced level.The results for the 1.03eV trap are discussed in terms of a metastable double-oscillator.

1Introduction

While there is a considerable amount of papers on quantum-con?ned energy levels related to QDs,much less work has been devoted to deep-level related defects existing in the presence of https://www.360docs.net/doc/fd11731744.html,monly acting as non-radiative centers,they are undesirable in QD-based lasers because they degrade luminescence ef?ciency [1].On the other hand,it has been demonstrated that a deep-level (M1)coupled to a QD energy level can provide a rapid energy-relaxation pathway through which electrons may thermalize and enhance luminescence ef?ciency of QDs [2].Also,utilizing

fast trapping into arsenic antisite defects in samples with low-temperature grown InAs/GaAs QDs,optical nonlin-earity and ultrafast response time have been demonstrated for use in all-optical switching [3].Thus,an insight into defect properties in QD structures is highly motivated.

Electrical measurement techniques based on depletion layer spectroscopy have been used to characterize deep-level defects present in InAs/GaAs QD samples.In the samples,when grown by molecular beam epitaxy (MBE)in the Stranski–Krastanov mode using the growth interruption technique,several deep levels were found depending on the growth conditions as studied by DLTS.Two levels with thermal activation energies of 0.14–0.16and 0.46–0.52eV from the conduction band edge,respectively were attributed to well-known grown-in defects,originally labelled M1and M4,which are characteristic of the growth interrupted interface occurring under an excess of As [4,5].Additional three levels A–C at 0.57–1.03eV were revealed in the samples when the substrate temperature was lowered from 580to 510°C for growing the GaAs cap layer [6].An interesting result of the study is that all these levels exhibited extremely non-homogenous spatial distributions and the increased concen-tration was observed in the vicinity of InAs insertion.

In the present continuation of the study,the 1.03eV electron trap has been investigated by DLTS in temperature scanned (TS)and frequency scanned (FS)modes.This trap,C,has no correspondence to DLTS data in the literature to make any comparison,it exhibits metastable properties and is accompanied by a trap,B,with electron activation energy at 0.79eV belonging to the EL2family.2Experimental

The investigated samples contained a single plane of self-assembled InAs QDs.It was located 0.5l m from the

M.Kaniewska (&)áM.Kaczmarczyk áG.Zaremba

Department of Analysis of Semiconductor Nanostructures,

Institute of Electron Technology,Al.Lotniko

′w 32/46,Warsaw 02-668,Poland e-mail:kaniew@ite.waw.pl

O.Engstro

¨m Department of Microtechnology and Nanoscience,

Chalmers University of Technology,Goteborg 41296,Sweden

J Mater Sci:Mater Electron (2008)19:S101–S106DOI 10.1007/s10854-008-9703-4

surface and surrounded by GaAs barrier layers.The structures were grown on (001)-oriented n +-GaAs sub-strates by MBE.The Stranski–Krastanov mode and the growth interruption technique were used to form the QDs.The buffer and cap GaAs layers were deposited at 580and 510°C,respectively,and were intentionally doped with Si to approximately 291016cm -3.A three-monolayer (ML)-thick InAs layer was grown at 510°C under a repeated sequence of 0.1ML InAs followed by a 2-s interruption under an excess of As 2.An average lateral size of the InAs QDs is 20nm with a height of 6nm,and the dot density is 3.591010cm -2.

Schottky contacts were fabricated by evaporating gold dots of 1mm diameter and AuGeNi ohmic contacts were prepared by annealing at 400°C/1min.

DLTS measurements were performed using a lock-in type spectrometer (DLS-83D system manufactured by Semilab,Hungary)operated at 1MHz.The ?lling pulse duration time during the measurements was ?xed and equal to 20l s.

3Results and discussion

DLTS spectra from the studied samples for identifying the characteristics of the QDs have been reported in Ref.[7]and interpreted on the basis of a model presented in Refs.[8,9].For the present work,spectra are presented in Fig.1from a high temperature range measured by means of double correlation DLTS (DDLTS)in temperature scanned mode (TS-DDLTS),which reveal deep energy

traps B and C.The data were obtained with a sample,which was not subjected to any additional thermal treat-ment after the MBE and Schottky diode preparation.The spectra in Fig.1were measured at low emission rate window,e w =5.4s -1,when scanning temperature up and down covering the range between 250and 470K in a sequence indicated by numbers 1–5.Spectrum 1reveals trap B as a shoulder on the main peak C when increasing temperature between 250and 470K.From spectrum 2one notices that the DDLTS peak height related to trap C decreases by about 50%when reproducing the spectrum with decreasing temperature.Spectra 3and 4show that subsequent heating the sample up to 470K in the step 3results in further but much smaller reduction of peak C.A very small increase of the peak height is observed when heating the sample up in step 5.Finally,the DDLTS spectrum was found to be stable after several scans in the measurement temperature range when going up and down and no changes were observed in the spectrum when measured after keeping the sample several months at room temperature.However,the amplitude of trap C signi?-cantly increases when TS-DDLTS spectra are shifted to higher temperature when increasing the emission rate window,e w ,as compared to the amplitude after steps 1–5at e w =5.4s -1and evidenced in Fig.2.

The temperature evolution of the amplitude was also studied by means of DDLTS in frequency scanned mode (FS-DDLTS).This is an alternative way of using DLTS,which allows to keep the temperature constant during the measurement cycle.A ‘‘fresh sample’’was annealed at 520K during 1.5h and cooled down at a rate of 0.06

K/s.

Fig.1Typical TS-DDLTS spectra of the studied samples.Num-bers 1–5indicate subsequently registered spectra when sweeping temperature up and down.Voltage levels of two excitation pulses were,V 1=1.5V and V 2=2.5V,respectively.Reverse bias volt-age,V R =4.8V,emission rate window,e w =5.4s -1.Temperature sweeping rate,a =0.06

K/s

Beginning measurements at 360K and increasing tem-perature in 20K increments the spectra obtained in a wide temperature range are shown in Fig.3.The measurement temperature makes the amplitude behaviour similar to that observed in TS-DDLTS spectra in Fig.2.

A deconvolution into two curves was used to separate peaks

B and

C and to determine their locations on the frequency scale as a function of temperature.In the lock-in system used,a pulse repetition frequency,f ,corresponds to an emission rate window e w =2.179f .The thermal emission rates versus temperature determined for the sample before and after annealing are shown as an Arrhenius plot in Fig.4.Fits to the emission data yield activation energies and apparent capture cross-sections,

D

E B =0.79eV,r B 9X B =3.5910-14cm 2and D E C =1.03eV,r c 9X C =9.6910-13cm 2for traps B and C,respectively.The product r i 9X i includes the limiting value of the capture cross-section,r i ,when T ??and a factor,X i ,which includes changes in entropy;D E i is the sum of an energy barrier for multiphonon capture and a changes in enthalpy connected with electron emission [10].The values agree well with those determined in an earlier investigation of the traps where a very good separation of trap B from trap was obtained by means of applying var-ious exciting pulse parameters [6].The coincidence of the data allows to use the peak separation procedure for con-trolling DLTS peak amplitudes.TS-DDLTS signal heights in Fig.2were converted into trap concentrations using the standard formula given by Lefevre and Schultz [11]and the data obtained are presented in Fig.5.Initial concentrations,as determined from peak heights in spectrum in Fig.1,are equal to about 291015and 591015cm -3for traps B

and C,respectively.Due to a thermal history in steps 1–5the concentrations were reduced to a level of 1.391015and 1.291015cm -3,https://www.360docs.net/doc/fd11731744.html,ually,decreasing the trap concentration as a result of heat treatment is inter-preted in terms of out-annealing or out-diffusion of defects from a space observation window.However,we did not observe any change with respect to shape of the depth concentration pro?le after reducing the trap signals as checked by DDLTS pro?ling.Therefore,the latter case in terms of out-diffusion of the defects can be excluded.As seen in Fig.5,the concentration of trap C recovers on a level of about 75%of its initial concentration.From the inset in Fig.5,where the concentration of trap C is shown in an Arrhenius plot,it is found that the process of the trap recovery is thermally activated with an activation energy of 0.28eV.

The effect of changing concentration discloses an interesting metastability of trap.Models for different kinds of metastabilities have been widely discussed in the liter-ature on the basis of con?gurational coordinate diagrams [12–14].In order to interpret the data we assume that the defect exists in two stable con?guration positions,C and F,and switches between them with rates e C and e F for tran-sitions from C to F (CF)and from F to C (FC),respectively.The thermal transition rates are given by

e C eF T?m C eF Texp àU C eF T

kT a

e1T

where T a is the annealing temperature,v i are the oscillator attempt frequencies,U i are energy barriers for

each

Fig.3FS-DDLTS spectra from the sample,which was thermally treated at 520K/1.5h and cooled down up to 360K.Temperature sweeping rate,a =0.06K/s.The spectra were measured in temper-ature range 360–500K beginning with the lowest temperature and then with increasing

temperature

transition,and k is Boltzmann’s constant.We ?nd that the experimental data can be described in a model assuming that con?guration F has an activation energy for electron emission high enough to exclude this process in the temperature range investigated.This means that all electron emission from the trap con?guration comes from position C,while the emission is switched off for traps occupying position F.The total trap concentration,N T ,is shared between concentrations N C and N F ,so that N T =N C +N F .At thermal equilibrium,the concentrations N C (F )are given by the following expression

N C eF T?e F eC T

e C te F N T e2T

N C and N F are plotted as a function of T a in Fig.6.Data used for these and further calculated results are given in Table 1.The ratio m C /m F =400used in the calculations may seem unreasonably high,but corresponds to an entropy change of 6k .From corresponding theoretical estimates,this value is within practical limits [15].

When starting the measurement on a ‘‘fresh’’sample at about 300K along curve in Fig.1,the thermal transition rates e C are too low in the lower temperature range to follow the temperature changes during the DLTS scan and no change in the trap concentration is expected.Therefore,for temperatures below about 380K,curve 1in Fig.1with the peak position at T =400K re?ects a non-equilibrium concentration of trap,the most probably at 500K as shown in Fig.6.However,e C increases with increasing tempera-ture and above a critical temperature becomes comparable

with the temperature sweeping rate,a .In this case,the trap concentration decreases and it is only a matter of time to reach an equilibrium concentration at T =400K as it is seen in Fig.1.Concentration levels of trap C,indicated in Fig.6,re?ect a reduction of the magnitude by a factor of about 3between 400and 500K,which was ?tted from our experimental data.On the other hand,an increase in the trap concentration with increasing rate window and con-sequently,with increasing temperature,seen in Fig.2,involves the reverse transitions FC,which contribute at higher temperatures as a result of the higher barrier U F .In order to consider the effects in a quantitative way,DLTS spectra were simulated using the model proposed above.Thermal emission rate for electrons was expressed [10]

e C ?r C X C N CB m exp àD E C

kT

e3T

where N CB is the effective density of states in the conduction band,and m is the electron average thermal velocity.Equation 3was also used to obtain the

defect

Fig.5Temperature dependences of trap concentrations.The con-centrations for traps B and C were calculated on the basis of data in Fig.2and the formula given in Ref.[11].Empty (full)marks indicate data obtained from scans recorded with decreasing (increasing)temperature.The lines are only a guide for eye.The inset shows semi-logarithmic plot of the concentration versus inverse temperature for trap C.A slope of the straight line yields an energy barrier 0.28

eV

Fig.6Equilibrium concentrations versus annealing temperature,T a ,for the trap in con?gurations C and F.Two dashed lines indicates that the ratio N C /N F was frozen in at 500K and the trap concentration in con?guration C decreased by a factor of about 3between 400and 500K.Calculations were done on the basis of Eq.2and using data in Table 1

Table 1Data used in calcula-tions for determining quantities presented in Figs.6,7,and 8

U C 0.1eV U F 0.36eV m C /m F 1/400D E B 0.79eV D E C 1.0eV

r B X B 3.5910-14cm 2r C X C

9.6910-13cm 2

energy levels from the variation of e i with T presented in Fig.4.Modi?ed by N C (T a )as given by Eq.2,the time dependence of the trap population with carriers,n C ,n C et T?N C eT a Texp eàe C t T

e4T

was used for DLTS signal calculations.Results of simu-lations of TS-DLTS spectra in two limiting cases when e C (F ))a and e C (a ,respectively,are presented in Fig.7for a comparison with the experimental data in Fig.1.Theoretical graphs were also calculated for the comparison with the FS-DDLTS curves in Fig.3.The temperature

changes between subsequent frequency scans were assumed to be slow enough to keep the concentrations of N C and N F close to the thermal equilibrium values.The similarities between these experimental results and the corresponding theoretical data,shown in Figs.7and 8,con?rm the observation that the con?guration C–F is metastable.

4Conclusions

The results presented demonstrate that QD growth induces two very deep energy levels:trap B with an activation energy of 0.79eV for electron emission,consistent with the EL2-family,and trap C with a value of 1.03eV for the corresponding quantity.To our knowledge,this is the largest activation energy for electron emission so far reported in GaAs.The most striking property of trap C is its metastability.Taking into account that the concentration of trap C can be shared between two con?gurations,its concentration can be as high as 191016cm -3.The presence of both defects may strengthen the physical effects related to fast trapping carriers as described in Ref.[3]as the spatial location of the defects almost correlates with the position of the QD plane.

Acknowledgements The work was ?nancially supported by the Swedish Foundation for Strategic Research (NanoDev-project)and by Polish Ministry of Science and Higher Education (Project Nos.3T11B00729and 1.12.053).

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表示“需要”(v)的单词:need, require, involve ,entail 表示“意味着”的单词:mean, involve, entail 表示“宿舍”的单词:dormitory, quarters, lodgings 表示“住宿”的单词:accommodations, (a)lodging 表示“欺骗”(v)的单词:cheat, deceive, trick, fool 表示“渴望”(v)的单词和词组:desire, yearn for/after, long for, thirst for,aspire to do sth, look forward to doing sth, keen on/about doing sth, keen to do sth, keen that, have an urge to do sth (具有做某事的强烈欲望,非常想做某事,形容极度渴望) 表示“同情”(v)的词组:sympathize with, yearn for, feel sympathy for 表示“重要的”的单词:important, crucial, vital, essential, integral, indispensable, significant 表示“参加,从事”的词组:take part in, join(in),participate in, involve sb in(主动, 其被动形式为sb be involved in), sb be engaged in=sb be employed in 表示“能力”的单词:ability和capability(主要用于人)talent(只用于人), competence, capacity(可用于人或物), faculty 表示“想象”(v)的单词和词组:imagine, fancy, fantasy,envisage(英)=envision(美) conceive of 表示“包含(v)”的单词和词组:include, involve, comprehend, inclusive of, embrace, embody, comprise, contain(实际包含的量),encompass 表示“能容纳”的词组:can hold/accommodate 表示“表现,体现”(v)的单词:render, reflect, display, indicate, manifest, embody 表示“导致,引起(v)”的单词和词组:render, entail, lead to, result in, bring about, cause, trigger=prompt(诱发),pose(本义为造成问题或危机) 表示“强大的,强有力的”的单词和词组:powerful, mighty, potent, strong 表示“(感觉等)敏锐的,灵敏的”的单词:keen, acute(首选这两个词)perceptive 表示“提供”的单词和词组:provide/supply sb with sth=provide/supply sth for sb=render/offer sb sth, contribute sth to sth 表示“忙于……,埋头于……”的单词和词组:be buried in, be employed in, be engaged in, sb be immersed in=immerse oneself in 表示“致力于”的词组:devote oneself to =dedicate oneself to, sb be devoted to=sb be dedicated to 表示“表现,体现”(v)的单词:render, reflect, manifest, characterize 表示“显示,表明”(v)的单词:show, suggest, indicate, reflect, reveal,(首选这三个词)emerge 表示“证明”(v)的单词:validate, demonstrate, verify, confirm,prove 表示“证实(v)”的单词:affirm,confirm, prove 表示“说明,阐释(v)”的单词:explain, illustrate, elaborate 表示“持续不断的”的单词:continual,continuous, incessant 表示“(计划,规定等)被实施,生效”的词组有come into effect/force, be implemented, be put into effect/execution, be carried into execution=be put in/into execution 表示“意识到+从句”的词组:know+that从句realize+that从句be/become aware of the fact+ that从句be conscious+that 从句 表示“阻止,阻碍”(v)的单词或词组:prevent/inhibit/hinder sb from doing sth, impede sb to do sth, hamper sb in doing sth 表示“强烈的,剧烈的”的单词:severe, intense, violent, poignant, acute,keen 表示“严重的”的单词:serious, severe, acute, grave 表示“著名的,闻名的”的单词:famous(一般用于褒义),eminent,

关于时间管理的英语演讲

Dear teacher and colleagues: my topic is on “spare time”. It is a huge blessing that we can work 996. Jack Ma said at an Ali's internal communication activity, That means we should work at 9am to 9pm, 6 days a week .I question the entire premise of this piece. but I'm always interested in hearing what successful and especially rich people come up with time .So I finally found out Jack Ma also had said :”i f you don’t put out more time and energy than others ,how can you achieve the success you want? If you do not do 996 when you are young ,when will you ?”I quite agree with the idea that young people should fight for success .But there are a lot of survival activities to do in a day ,I want to focus on how much time they take from us and what can we do with the rest of the time. As all we known ,There are 168 hours in a week .We sleep roughly seven-and-a-half and eight hours a day .so around 56 hours a week . maybe it is slightly different for someone . We do our personal things like eating and bathing and maybe looking after kids -about three hours a day .so around 21 hours a week .And if you are working a full time job ,so 40 hours a week , Oh! Maybe it is impossible for us at

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