minority carrier lifetimes in molecular beam epitaxy grown alxga1-xas-gaas double hetero structures...

8/3/2019 Minority Carrier Lifetimes in Molecular Beam Epitaxy Grown AlxGa1-xAs-GaAs Double Hetero Structures Doped With

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Minority carrier lifetimes in molecular beam epitaxy grownAlxGs1_ xAs/GaAs double heterostructures doped with aluminumP. Sheldon, B. M. Keyes, A. K. Ahrenkiel, and S. E. AsherNational Renewable Energy Laboratory, Golden, Colorado 80401(Received 19 November 1992; accepted 4 January 1993)Both AI-treated pBN crucibles and Al-doped Ga melts have been used to reduce Ga-cell relatedoval defect densities in molecular beam epitaxy (MBE) grown epilayers. This practice, althougheffective in reducing the oval defect density, results in small amount s of Al contamination in thegrown films. In this work, we have grown diagnostic Al(UGao.7As/GaAs double heterostructure(DH) devices doped with Al in order to determine the effect of Al contamination on theminority-carrier lifetime. The active region of the DR structures was doped with Al at 5 X 1018atoms/cm3 to emulate the levels observed when Al-treated pBN crucibles are used. Al impuritylevels and distribution profiles were characterized by secondary ion mass spectrometry, andminority-carrier lifetimes of DH devices were characterized using time-resolvedphotoluminescence (PL) measurements. Device structures grown with various GaAs activelayer thicknesses were used to calculate the interface recombination velocity (S) and the bulklifetime ( rR) ' We found that Al doping of the GaAs active layer adversely affects the measuredPL decay time. GaAs active layers of DH devices doped with Al exhibited poor bulk lifetimes(r a< 120 ns) limited by Shockley-Read-Hall recombination. In contrast, DR devices grownwith AI-free active layers resulted in significantly improved PL decay times with bulk lifetimesas high as 'TB = 700 ns.

t INTRODUCTIONMany researchers have initiated studies to both identify

the origin and control the formation of oval defects inGa-based III-V materials grown by molecular beam epitaxy (MBE). The degree of attention this subject has attracted over the past decade clearly demonstrates the widespread nature of the problem. Oval defects adversely affectdevice performance and device yield. Therefore, they mustbe controlled if MB E is to become a productiontechnique. 1,2 In large-area devices, such as photovoltaic devices, oval defects can contribute to excessive leakage currents, which significantly inhibit device performance.3Three sources of oval defects have been identified; theseinclude 0) oval defects originating from the substrate orsubstrate preparation procedure; Oi) oval defects originating from particulates generated during sample loading orthe transfer process; and (iii) oval defects generated fromthe Ga source. Improvements in sample preparation procedures, clean room environments, and MBE wafer loading and transpor t designs have largely failed to reduce ovaldefect densities to acceptable levels. Therefore, in recentyears the Ga source related oval defects have received themost attention. These defects were carefully classified byFujiwara et al. 4 and are referred to as the a3 and a4 ovaldefects.

The origin of the a3 and a4 defects are still somewhatcontroversial; however, it is largely believed that they result from excess Ga nucleating en the substrate surface.There is significant evidence that Ga203 in the Ga melt isresponsible for these defects. It is believed that the Ga20)decomposes to gaseous Ga20, which is ejected from theeffusion cell and absorbed on the substrate surface. TheGa20 molecule then combines with two other Ga20 mol-1011 J. Vac. Sci. Technol. A 11(4}, Jui/Aug 1993

ecules reforming the more stable Ga203 species. This actsas a nucleation center for excess Ga, forming the basis ofthe oval defect. Some have suggested that Ga spitting mayalso playa role in oval defect generation; however, severalstudies indicate that this is not an important issue. 5,6

In an effort to reduce the density of oval defects, varioustechniques have been used to reduce the Ga20 emissionsfrom the Ga effusion cell. Two techniques that have successfully reduced oval defect densities require that a smallquantity of Al come in contact with the Ga melt. In oneapproach, Al (- 0.1 %) is added directly to the Ga melt.7,8Th e Al ties up the oxygen in the Ga20, forming the lessvolatile aluminum oxide (A120) . In the other approach, apE N crucible, which was previously used to deposit Al andhas a thin coating of Al near the orifice, is used for the Gaeffusion cell.9 In each case, a small amount of Al is unintentionally co evaporated and incorporated in the film.Chand has found that during the first few growths, the Alconcentration in the GaAs epilayer is x = 0.0016 (using anAl coated crucible for the Ga source).9 This correspondsto an Al concentration of 3.SX 1019 atoms/cm3, whichgradually decreases with time as the Ga coats the Al at theorifice of the crucible. Several researchers have suggestedthat a small percentage of Al contamination would notadversely affect the optical or electrical properties of thefilms. Chand has demonstrated that the majority carrierproperties are only negligibly affected. However, littlework has been done to explore the impact of Al doping onthe minority carrier material properties. In this work, wehave examined the effect Al doping has on the minoritycarrier lifetime of MBE grown GaAs epilayers. Al dopingof the GaAs layers was unintentional and resulted from areduced Al fiux emanating from the shuttered effusion cellduring GaAs growth. In this work, we have not attempted

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1012 Sheldon et al.: MBE grown AlxGa 1_..As/GaAs DH doped with AI 1012

GaAs Cap 100 AAlxGa1_xAs Barrier layer 100 A ( x ~ O . 3 )

GaAs Active L a ~ e r d=1600-5000 .

AlxGa1_,As Barrier Layer 100 A x=0.3) --GaAs Buffer Layer

3500 AAI,Ga1_xAs Layer (Composition Calibration)

Semi-Insulating GaAs Substrate(100)::f: OS'

FIG. 1. Schematic of the double heterostructure device used in this studyto measure the minority-carrier lifetimes by the time-resolved photoluminescence technique.

to qualify or quantify the oval defect density in the resulting film, but rather identify the affect of Al doping on theminority-carrier properties.II. EXPERIMENTAL

All epitaxial structures were grown by MBE in aPerkin-Elmer Physical Electronics MBE 400 system. TheGaAs and AIGaAs layers were undoped and deposited onsemi-insulating GaAs substrates [001]OS. Prior togrowth, the substrates were degreased with solvents andetched in a 2:1:10 (N H40H:H20 2:H20) solution. This wasfollowed by a de-ionized water oxidation, an isopropyl alcohol rinse, and a 2000 rpm spin dry. The wafers were thenmounted with In on a Mo block and transferred to theMBE system for low temperature outgassing. In this work,both the GaAs and AIGaAs layers were grown at a substrate temperature of -61O"C under As-stabilized conditions (near the transition to the Ga-stabilized regime). ALthough the best quality AIGaAs is typically achieved athigher temperatures, the single growth temperature wasused to minimize impurity accumulation at the heterointerfaces during temperature cycling and to ensure run torun reproducibility. The GaAs growth rate was 0.7 j lmlhand the AlxGal_ xAs growth rate was 1.0 j lm/h corresponding to an Al concentration of x = 0.3.

The generic AIGaAs/GaAs double heterostructures(DH) device used in this study is shown in Fig. 1. Thestructure consists of a thin GaAslAIGaAs layer used tocalibrate the group II I fluxes by reflection high energy electron diffraction (RHEED) intensity oscillation analysis.This was followed by a 3500 A GaAs buffer layer, and a1600-5000 A GaAs active layer sandwiched between two100 AAlo.3Gao.7As barrier layers. Finally, a 100 A GaAscapping layer was grown to protect the uppermost A I G a ~ s barrier from oxidation. Structures with the thin (100 A)barriers had significantly better lifetimes than those grownwith thicker barriers, and therefore,were used throughoutthis work. This result is similar to that reported by Dawsonet al. 10 Brief growth interrupts (10 s) were used at eachheterojunction in an attempt to improve the interface qual-J. Vac. Sci. Techno!. A, Vol. 11, No.4, JullAug 1993

w3

tn'E:;;J 1020?:'wcw.5 101....IIJ,.,

o 200 400 600 800Time (ns)

FIG. 2. Plot of the PL intensity vs time for DR devices with 0.47 ?tmthick GaAs active layers: Ca) with active layer unintentionally doped withAI; and (b) with AI-free active layer.

ity. AlGaAs surfaces were terminated with a single monolayer of GaAs prior to the growth interrupt to both reduceimpurity gettering at the highly reactive AIGaAs surfaceand promote surface smoothing.Time resolved photoluminescence (PL) measurementswere performed on a number of the DR structures described above. The PL decay measurement system uses thetime-correlated single-photon counting method describedelsewhere. I 1 Excitation of the cha rge carriers was accomplished using a Spectra Physics laser system consisting of acavity-dumped dye laser (model 3500) driven by a modelocked Ar + laser (model 2040). The output of the dyelaser was an 800 kHz pulse train tuned to 600 nm withpulse widths on the order of 8 ps full width at halfmaximum (FWHM). A neutral density filter was used tocontrol the intensity incident on the device, keeping the. . d . b 1 10 14 " 3excIted carner enslty e ow cm.

Secondary ion mass spectrometry (SIMS) depth profiles were obtained using a Cameca IMS 3/ ion microprobe. A 12.5 keY O 2+ primary beam was used with detection of positive secondary ions. The analyzed area was-22 jlffi in diameter. The aluminum concentration wascalculated from tabulated relative sensitivity factors, whichlimit the accuracy of the absolute concentration to a factorof 2. This uncertainty does not effect the comparison ofprofiles from different samples.

III. RESULTS AND DISCUSSIONFigure 2(a) shows the time resolved PL decay data

measured from an AIGaAs/GaAs DH structure with anactive layer thickness of ~ 0 . 4 7 f.1m (see Fig. 1). This sample was undoped and had a measured lifetime of 94 ns. Themeasured PL lifetime is obtained by fitting the decay dataover the lower injection, single exponential region. The PLlifetime for a DH device can be approximated by


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1013 Sheldon fi t 1iI1.: MBE grown AlxGsj_.As/GaAs DH doped with AI 10131 1 2S---+' ipL - 'in d ' (1 )

where 1'pL is the measured PL decay time, l'B is the bulkminority carrier lifetime in the GaAs active layer, S is theinterface recombination velocity between the GaAs activelayer and each of the AIGaAs barriers, and d is the thickness of the active layer. In this work, for simplicity, weassume that both GaAs-AIGaAs interfaces have equivalent recombination velocities (Le., S = SI = S2). However,it is unlikely that the inverted interface (GaAs depositedon top of AIGaAs) has an interface recombination velocityas low as the better quality normal interface. 12 Th e bulklifetime Tn has both a nonradiative or Shockley-Read-Hallrecombination component (7nr), 13 and a radiative recombination component (1'r), where

1 1 1- = - + - .78 Tn r 7 r

(2 )In low injection, the radiative lifetime is inversely proportional to the majority carrier density N

(d)Tr= BN ' (3 )

where B is the radiative recombination coefficient and(d) is the photon recycling factor.

Fo r GaAs, B is 2 X 10 - to cm 3/s , as calculated byCasey et 01. 14 Assuming a majority carrier density of1 X 1014 cm -- 3 (a typical value for material grown in ourMB E system), and a (d) = 1, the upper limit on theradiative lifetime calculated using Eq. (3) is - 50 f1S. Th eeffect of self-absorption is to increase the effective radiativelifetime. Thus, (d) is always greater than 1, and the effective radiative lifetime would be greater than 50 f1s. Thisis orders of magnitude larger than any of the measuredlifetimes reported in this article. Consequently. theminority-carrier lifetime is not radiatively limited, and theonly thickness dependence of TpL is that associated withthe interface recombination velocity. By disregarding theradiative component, 7 fi r and S can be determined by measuring 7P L for films with different active layer thicknesses[see Eqs. (1 ) and (2)]. Figure 3 is a plot of l/TPL as afunction of 2/d for a series of DH structures with activeregion thicknesses of 0.16, 0.24, and 0.47 f,Lm. A fi t to thedata. using Eq. (1), yields a T B = 7 nr = 118 ns and anS = 53 em/s. The low interface recombination velocity isindicative of high quality interfaces. Interface recombination velocities as low as 30 em/s have been measured forMBE grown DR devices; however, these were grown atmuch higher substrate temperatures where better qualityinterfaces are expected. 1O The bulk lifetime (T B = 118 ns)is significantly lower than the expected radiative limit(7,>50.0 f,Ls) and is the dominant recombination mechanism in these DH devices. This result indicates that theremust be a high density of nonradiative "killer" centers inthe GaAs active layer.

In an effort to identify the source of the killer centerresponsible for degrading the bulk lifetime, SIMS analysiswas performed on all the DH device structures. The onlyJ. Vac. Sci. TechnoL ft., Vol. 11, No" 4, Jul/Aug 1993

1 I I I

lI'LPL = 52.9 (2/d) + 8,45x106

60000 80000 H)(lOOO 120000

FIG. 3. Plot of inverse photoluminescence lifetime vs 21d. where d is thethickness of the GaAs active layer and equals 0.16, 0.24, and 0.47 fl-m. Bygrowing double heterostructure devices under identical conditions andvarying only the thickness of the active layer we can calculate 1"Band S.

impurity discovered common to all device structures wasan unusually high background level of Al in the GaAslayers. A SIMS depth profile of the sample measured inFig. 2(a) is shown in Fig. 4. The Ai concentration in theactive layer and buffer layer of the DR device is - 5 X 1018atoms/em3 (approximately the Al concentration expectedfrom the oval defect density reduction techniques of Chandand Kirchner et al. ). In the substrate, the Ai concentrationdecreases by more than an order of magnitude. The concentration measured in the substrate is the Ai backgroundlimit of the SIMS system. The Al background limit variesfrom day to day based on the amount of Ai containingmaterial analyzed previously. The presence of Al in the

0.0 0.2 0.4 0.6 O.S 1.0 1.2 1.4Depth (11m)

FIG. 4. SIMS depth profile of Ii double heterostru cture device with a 0.47J.Lm active layer. The Ai effusion cell shutter was closed during the deposition of both the GaAs buffer and active layers and the cell was maintained at the growth temperature.


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1014 Sheldon et al.: MBE grown AlxGa1_xAs/GaAs DH doped with AI 1014r-; A I G a ~ s B ~ r r i e ; s ~ ,- 10 22 r GaAs GaAsl t ' Active Layer \ Butfer Layer5 1021 ' \

GaAsSubstrate

\\

" " f t \ l I \ \ & I I ~ M M - - AI Background -Limit

jJ0.0 0.2 0.4 0.6 O.S 1.0

Depth ( ~ m ) 1.2 1.4

FIG. 5. SIMS depth profile of a double heterostructure device with a 0.47/-lID active layer. The AI effusion cell shutter was closed during the deposition of both the GaAs buffer and active layers. The effusion cell wasmaintained at the growth temperature during the growth of the GaAsbuffer layer and was reduced by 400 'C during the growth of theactive layer.

devices raises two important questions: (i) what effect does5 X 1018 cm - 3 Al have on the minority carrier propertiesof the epilayer; and (ii) what is the source of the AI?

Al has a relatively low vapor pressure, and therefore,must originate from a hot source during growth. There aretwo likely candidates that could explain this phenomenon:(i) the Ga melt could be contaminated with a smallamount of AI, which would coevaporate with the Ga during GaAs growth; or (iO there could be some Al leakagearound the shuttered hot Ai effusion cell. One way to identify the origin of the Al would be to grow a DR device,identical to that depicted in Fig. 1, and reduce the temperature of the shuttered Al effusion cell during the growth ofthe GaAs active layer. If the Ga melt were contaminatedwith AI, the AI concentration in the active layer should beunaffected by reducing the Ai effusion cell temperatureduring this period. Conversely, if the Al is leaking from theshuttered source, the reduced vapor pressure should translate to a lower Al concentration in the active layer.

Figure 5 shows the SIMS profile obtained from a samplewhere the Al effusion cell temperature was reduced by400 C during the growth of the active layer. Reducing theAl effusion cell temperature by 400C will cause a decreasein the Al flux of more than three orders of magnitude.Prior to the completion of the active layer, the Al effusioncell temperature was ramped back up to growth temperature so that the second AIGaAs barrier could be grown. InFig. 5, we see that the Al concentration in the buffer layeris very similar to that observed in the SIMS profile of theDH device shown in Fig. 4. However, the AI concentrationin the active layer, where the Al effusion cell temperaturewas reduced, is about an order of magnitude lower and issimilar to the background level detected in the substrate(the SIMS background limit). Note that the Al profiles ofthe two AIGaAs barriers in Fig. 5 are not identical. TheJ. Vac. Sci. Techno!. A, Vol. 11, No.4, Jul/Aug 1993

GaAslAIGaAs interface of the first barrier is quite sharpbecause the Al effusion cell' temperature was allowed todrop rapidly after the barrier layer was completed. Conversely, the AIGaAs/GaAs interface of the second barrieris poorly defined. This is because the Al oven was rampedup to growth temperature prior to the completion of theactive layer, to allow the Al flux to stabilize. The Al concentration profile in Fig. 5 indicates that the Al effusioncell is contributing to the doping of the GaAs layers. Thisis surprising because when the shutter is closed the substrate does not have a Hne-of-sight view of the effusion cell,and therefore, should not see any part of the Al flux. Onepossible explanation could be that the Al is re-evaporatedfrom an adjacent surface that does have a line-of-sight viewof the substrate. Work investigating this problem is ongoing and will be published elsewhere.

If the Al doping in the GaAs active layer was the sourceof the nonradiative recombination center, the measured PLlifetime should increase as the Al concentration is decreased. The PL decay for the 0.47 fLm DH device with thereduced Al active layer concentration (see Fig. 5) is shownin Fig. 2(b). The measured PL lifetime is 271 ns; wellbelow the radiative limit, but almost three times largerthan that measured for the DR device with the AI-dopedactive layer [see Fig. 2(a)]. If we assume that the interfacerecombination velocity is the same as that measured fromthe Al doped samples (S = 53 cm/s), the bulk lifetime hasincreased to 697 ns with the reduction in Al content. Asecond film with a different active layer thickness wouldallow us to determine if S has also changed, but one wasnot available. If S increases, the corresponding bulk lifetime would also have to increase beyond 697 ns to fit themeasured data. If S decreases, the corresponding bulk lifetime must also decrease. In the most extreme case of Simproving to 0 cm/s, the bulk lifetime would equal the PLlifetime of 271 ns. This is still an improvement of morethan a factor of 2. In the more realistic extreme case of Sdecreasing to 30 em/s,lO the bulk lifetime would become414 ns. Thus, Al doping at a level of 5 X 1018 em - 3 appearsto increase the density of nonradiative recombination centers, thereby degrading the bulk minority-carrier lifetime.

IV. SUMMARY AND CONCLUSIONSIn summary, we have grown diagnostic AIGaAs/GaAs

DR devices by MBE and analyzed them using timeresolved PL measurements. We find that when the GaAsactive layer of the DH device is doped with Al at a level of-5X 1018 atoms/em3, the minority-carrier properties ofGaAs epitaxial thin films degrade significantly. I t appearsthat the Al produces nonradiative recombination centersthat reduce the bUlk lifetime from 700 ns to less than 120ns. This result is important because Al doping levels of5 X 10 18 atoms/cm3 are similar to those that result fromusing the oval defect density reduction techniques that require AI to come in contact with the Ga melt describedearlier. This work suggests that these techniques shouldnot be considered if poor minority-carrier properties willadversely affect device performance.


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1015 Sheldon et til.: MSE grown AlxGa 1 _xAs/GaAs DH doped with AI 1015ACKNOWLEDGMENTS

This work was performed under Contract No. DEAC02-83CHlOO93 to the U.S. Department of Energy.1M. Shinohara, T. Ito, K. Wada, and y, Imamura, lpn. 1. App!. Phys.23, L371 (1984).

2N. Susa and H. Okamoto, Jpn. J. Appl. Phys. 23 317 (1984).3M. R. Melloch, S. P. Tobin, T. B. Stellwag, C. Bajgar, A. Keshavarzi,M. S. Lundstrom and K. Emery, l. Vac. Sci. Techno!. B 8,379 (1990).4K. Fujiwara, K. Kanamoto, Y. N. Ohta, Y. Tokuda, and T. Nakayama, J. Cryst. Growth 80, 104 (1987).5y. H. Wang, W. C. Liu, C. Y. Chang, and S. A. Liao, J. Vac. Sci.Techno!. B 4, 30 (1986).

J. Vae. Sci. Technol. A, Vol. 11, No.4, Jul/Aug 1993

"Shang-Lin Weng, App!. Phys. Lett. 49, 345 (1986).7p. D. Kirchner, J. M. Woodall, J. L. Freehouf, and O. D. Pettit, Appl.Phys. Lett. 38, 427 (1981).

8D. G. Schlom, W. S. Lee, T. Ma, and J. S. Harris, Jr., J. Vac. Sci.TechnoL B 7,296 (1989).

9N. Chand, App!. Phys. Lett. 56, 466 (1990).!Op. Dawson and K. Woodbridge, Apr!. Phys. Lett. 45, 1227 (1984).11R. K. Ahrenkiel, D. J. Dunlavy, and T. Hanak, Solar Cells 24, 339(1988).

IZH. Morkoc, L. C. Witkowski, T. J. Drummond, C. M. Stanchak, A. Y.Cho, and B. G. Streetman, Electron Lett. 16, 753 (1980).

!3W. Shockley and W. T. Read, Jr., Phys. Rev. 87,835 (1952).14H. C. Casey, Jr. and F. Stern, J. App!. Phys. 47, 631 (1976).

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minority carrier lifetimes in molecular beam epitaxy grown alxga1-xas-gaas double hetero structures...

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