We have studied the capacitance-voltage (C- V) characteristics of Schottky barriers on inverted nGaAs/ N-AIGaAs and normal N-AIGaAs/n-GaAs heterojunctions. Impurities introduced during film growth produced a negative sheet charge of 6.0 X 10 II cm -2 at the interface of the inverted n-GaAs/N-AIGaAs heterojunction. The effectiveness of GaAs quantum wells in trapping these impurities was investigated. GaAs quantum wells 20 A wide were placed in intervals of 2500 A for the first 0.75 pm of the AIGaAs layer; in the last 0.25 pm, the periodicity of the quantum wells was progressively decreased by half with the last quantum well placed at about 160 A from the GaAs/ AIGaAs interface. The resulting measured interface charge concentration of 4.4 X 1010 cm -2 is more than a magnitude lower than measured before the use of the quantum wells and is essentially at the limit of the accuracy of the C-V technique for this structure

Lundstrom, Mark S

Melloch, Michael R

Tan, K. L.

English

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Purdue UniversityPurdue e-PubsDepartment of Electrical and ComputerEngineering Faculty PublicationsDepartment of Electrical and ComputerEngineering1986Effect of impurity trapping on thecapacitance‐voltage characteristics of n‐GaAs/N‐AlGaAs heterojunctionsK. L. TanPurdue UniversityMark S. LundstromPurdue University, lundstro@purdue.eduMichael R. MellochPurdue University, melloch@purdue.eduFollow this and additional works at: https://docs.lib.purdue.edu/ecepubsPart of the Electrical and Computer Engineering CommonsThis document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu foradditional information.Tan, K. L.; Lundstrom, Mark S.; and Melloch, Michael R., "Effect of impurity trapping on the capacitance‐voltage characteristics ofn‐GaAs/N‐AlGaAs heterojunctions" (1986). Department of Electrical and Computer Engineering Faculty Publications. Paper 76.http://dx.doi.org/10.1063/1.96520Effect of impurity trapping on the capacitance-voltage characteristics of n-GaAs/N-AlGaAs heterojunctionsK. L. Tan, M. S. Lundstrom, and M. R. MellochCitation:  48, 428 (1986); doi: 10.1063/1.96520View online: http://dx.doi.org/10.1063/1.96520View Table of Contents: http://aip.scitation.org/toc/apl/48/6Published by the American Institute of Physics... ;.;.;.;.;.;.;.;.;.;.;.;.;.;.;.;.;.;.;.;.;.;.;.;.;.;.;.;.;.:.:.:.:.:,:.:.:.: •.•.•.•.•........ : •.•.•.•.•.•.•.•.•.•.•.•.•.•.•.•.•.•.•.•.•.•.•. ~ .................•.•............................................................. ~ ...................................................•............•....................... Effect of impurity trapping on the capacitance-voltage characteristics of n-GaAs/ N-AIGaAs heterojunctions K. L. Tan,a) M. S. Lundstrom, and M. R. Melloch School of Electrical Engineering, Purdue University, West Lafayette, Indiana 47907 (Received 23 October 1985; accepted for publication 12 December 1985) We have studied the capacitance-voltage (C- V) characteristics of Schottky barriers on inverted n-GaAs/ N-AIGaAs and normal N-AIGaAs/n-GaAs heterojunctions. Impurities introduced during film growth produced a negative sheet charge of 6.0 X 10 II cm -2 at the interface of the inverted n-GaAs/N-AIGaAs heterojunction. The effectiveness of GaAs quantum wells in trapping these impurities was investigated. GaAs quantum wells 20 A wide were placed in intervals of2500 A for the first 0.75 pm of the AIGaAs layer; in the last 0.25 pm, the periodicity of the quantum wells was progressively decreased by half with the last quantum well placed at about 160 A from the GaAs/ AIGaAs interface. The resulting measured interface charge concentration of 4.4 X 1010 cm -2 is more than a magnitude lower than measured before the use of the quantum wells and is essentially at the limit of the accuracy of the C-V technique for this structure. Although the GaAs/ AIGaAs (so-called inverted) heterojunction has not achieved high performance in modu-lation-doped field-effect transistors, its properties are of con-siderable interest for other device applications. The inverted junction is also widely used for band discontinuity extraction by capacitance-voltage (C- V) analysis. I - 3 High-efficiency solar cells which make use of such heterojunction back-sur-face fields have been recently reported. 4 For both applica-tions the properties of the interface are of considerable con-cern; interface traps may enhance minority-carrier recombination in solar cells and may adversely affect the accuracy of band discontinuity measurements. A compre-hensive study of conduction-band discontinuities in GaAsl AIGaAs heterojunctions was recently reported by Okumura et al. I Using Kroemer's C- V technique,2,3 they determined both the band discontinuity and interface charge density, These authors reported that the extracted band discontinui-ties were in error when the measured interface charge den-si ty exceeded 10 II cm - 2. Deep level transient spectroscopy measurements confirmed that the traps were localized at the GaAsl AIGaAs interface. 5 While the source of traps was not identified, Okumura et al. 1.5 believed the traps to be extrinsic since a larger trap concentration was obtained when growth was interrupted at the interface. When extracting band discontinuities by C- V analysis, the inverted GaAs on AIGaAs (binary on top of ternary) heterojunction should be used in order to accurately judge the position of the interface from the measured apparent carrier profile, 2 The molecular beam epitaxy (MBE) growth of inverted modulation-doped GaAs on AIGaAs hetero-junctions has exhibited inferior carrier mobilities in com-parison to the normal AIGaAs on GaAs heterojunction. 6 Several possible causes for the poor mobility of the inverted heterostructure have been advanced. The poor mobility may be the result of a rough surface formed due to the difficulty of growing thick layers of AIGaAs with high crystalline quali-ty.7 A second possible explanation is the tensile strain per-pendicular to the interface caused by the larger lattice con-.) Present address: Honeywell Inc., Physical Sciences Center, Bloomington, MN 55420, stant of the AIGaAs.H Third, inferior mobilities may be due to impurities trapped in a thin layer of GaAs at the interface when the Al flux is terminated. 9 Drummond et aP have shown that a 150-A-thick AIGaAs/GaAs three period superlattice, in place of the undoped AlGaAs spacer layer, can improve the carrier mobilities by 6.5 times in the invert-ed heterostructure. In this letter we investigate the role of impurity trapping on the C- V characteristics of n-GaAs/ N-AIGaAs (inverted) and N-AIGaAsln-GaAs (normal) heterojunctions. The MBE system used in this work was a Perkin-Elmer model PHI400. The inverted heterojunctions were grown with a 0.5-,um GaAs buffer layer at a substrate temperature of 580·C. The AIGaAs layer was typically about 1 pm thick and grown at a substrate temperature of 700·C. The first 200-300 A of the 0,2-,um-thick top GaAs layer was also grown at 700·C before the substrate temperature was re-duced to 620 ·C for the remainder of the growth. Growing the first 200-300 A of GaAs at a temperature of 700 ·C has been shown to reduce the sensitivity of interface sharpness to growth conditions.? The doping density of the GaAs and AIGaAs was approximately equal; both were doped at about 1 X 1017 cm- 3 • The heterojunctions grown were C-V profiled by the use of a MSI Electronics Inc. Model Hg-2 mercury probe, (When properly calibrated, this mercury probe is capable of reproducibility of better than 10%.10) Our initial hetero-junctions exhibited the expected accumulation of electrons in the GaAs side and depletion of electrons in the AIGaAs side. During the course of this study, gas source MBE, using nitrogen taken from a liquid nitrogen dewar, was also being performed in the same system. The apparent carrier profile of a sample grown immediately following gas introduction (sample Sl) showed no accumulation in the GaAs side. Heterojunction sample S2 grown the following day dis-played a similar depleted interface as shown in Fig. 1. The cause of this depletion at the interface is thought to be a negative sheet charge at the interface of the heterojunction due to a layer of impurities. Miller et ai, 11,12 have postulated the cause of such impurity layers as due to the impurities segregating at the surface during growth of the AIGaAs. 428 Appl. Phys, Lett. 48 (6), 10 February 1986 0003-6951/86/060428-03$01,00 © 1986 American Institute of Physics 428 u 10 16 '--_-'-__ '--_-'-__ '--_-'-_--' 0.126 0198 0.270 0341 x(microns) FIG. 1. Apparent carrier profile obtained from C- V measurements of sam-ple S2 (inverted heterojunction). When the top GaAs layer is grown, the impurities are trapped at the heterojunction interface. The impurities, be-lieved to be introduced during the gas source MBE run, have not been identified at this time. If the depletion was indeed due to impurities incorporat-ing and segregating at the surface of the AlGaAs, the deple-tion at the interface should not have been observed if a nor-mal heterojunction had been grown instead (i.e., ternary on top of binary). In the normal heterojunction the impurities would be trapped at the surface of the AlGaAs and the Schottky barrier would deplete through this thin layer of impurities. A normal heterojunction (sample 83) was grown using similar conditions as before and the resultant apparent carrier profile in Fig. 2 showed an accumulation layer on the smaller band-gap side and a depletion on the wider band-gap side as expected. This carrier profile sup-ports the suspicion that the depletion at the interface was due to impurity contamination and then segregation at the sur-face of the AIGaAs. These observations support those re-cently reported by Okumura et a/.5 An inverted heterojunc-tion grown after sample S3 still exhibited a depleted interface similar to sample S2. During the course of this work we found that after the first introduction of impurities the sub-sequent growth of inverted heterojunctions resulted in ap-parent carrier concentrations similar to sample 82. Since it has been demonstrated that GaAs quantum wells trap impurities, 9.11.12 GaAs quantum wells placed peri-odically in the thick AIGaAs may be capable of trapping '" , E u 10 17 '--_-'-__ L-_-'---_----'L-_--'--_---' 0.085 0128 0.171 0214 X (microns) FIG. 2. Apparent carrier profile obtained from C- V measurements of sam-ple S3 (normal heterojunction). 429 Appl. Phys. Lett., Vol. 48, No.6, 10 February 1986 schottky barrier r:::lo. 2}-Lml GaAs I t= :~~i 1---_____ -rV~~=· 312 A ~:~~1 =====-=======I~ 625A f----------- GaAs/AIGaAs 2500A superlaltice 2500A -----------n + substrate 11/lllllllzzzJ ohmic contact FIG. 3. Inverted heterojunction with GaAs quantum wells inserted in the AIGaAs layer. Thickness of the GaAs quantum wells is 20 A with decreas-ing periodicity as it approaches the top heterojunction. impurities away from the top heterojunction interface and hence reduce the interface charge. The GaAs quantum wells were chosen to be 20 A wide and were placed in intervals of 2500 A for the first 0.75 pm of the AIGaAs layer. In the last 0.25 11m, the periodicity of the quantum wells was progres-sively decreased by half as shown in Fig_ 3, with the last GaAs quantum well placed at about 160 A away from the interface. Heterojunction sample S4 was grown with conditions similar to that of samples S 1 and S2 except with the addition of the above described GaAs quantum wells placed in the AlGaAs layer. The resultant apparent carrier profile in Fig. 4, extracted from the C- V measurements, did indeed show an accumulation of carriers on the GaAs side of the heterojunc-tion and a depletion on the AIGaAs side. These results sug-gest that the GaAs quantum wells trapped impurities and prevented them from accumulating on the top surface of the growing AIGaAs. As mentioned earlier, a C- V technique introduced by Kroemer2.J can be used to extract the interface charge den-sity and conduction-band discontinuity of a heterojunction. Using Kroemer's C- V technique, a negative interface charge of6.0X 1011 cm- 2 was measured for heterojunction sample S2. The measured interface charge for sample S4 was ';' 10 17 1\---------E u ~ c:: 0174 0234 0293 X (microns) FIG. 4. Apparent carrier profile obtained from C-Vmeasurements of sam-pie S4 (inverted heterojunction with quantum wells). Tan, Lundstrom, and Melloch 429 ..•.•.•.•. ;.;.; .•............... ;.;.;.;.;.;.;.;.;.;.;.;.;.;.;.:.:.:.: •.•.•.•.•.•.•.•................................ ;.;.;-;.:.:.:.:.: ......... -•.•.•............ :.: ....................... :.:.:.:.: •.•.•........ : ................•.•................ 4.4X 1010 cm- 2, more than a magnitude lower than that measured before the use of the GaAs quantum wells. (This measured interface charge of 4.4 X 1010 cm - 2 is essentially at the limit of the accuracy of the C-V technique for this struc-ture. 13 ) The measured conduction-band discontinuity for sample S4 (GaAsl Alo.2 Gao.s As heterojunction) was 0.121 eV with a deviation ofless than 4 meV across the substrate. This measured discontinuity of 0.121 eV corresponds to a l1Ec = O.Sl1EG for x = 0.2. In conclusion, we have shown that impurity trapping at the interface of the inverted GaAsl AlGaAs heterojunction produces high interface charge. These results support the recent observations of Okumura et aJ. 5 We also demonstrat-ed that the use of a few GaAs quantum wells placed periodi-cally in the AIGaAs is an effective means for producing low interface charge inverted heterojunctions even in the pres-ence of impurity contamination. Capacitance versus voltage analysis indicates that the technique is capable of an inter-face charge reduction of an order of magnitude. The tech-nique may be of use in device applications that require in-verted GaAsl AlGaAs heterojunctions with thick AlGaAs layers. This work was supported by the National Science Foun-dation MRL grant No. DMR 83-16988 and the Solar Ener-gy Research Institute contract No. XL-S-OSO 18-1. M. R. Melloch would like to thank Dr. Timothy J. Drummond of 430 Appl. Phys. Lett., Vol. 48, No.6, 10 February 1986 Sandia National Laboratories for discussions concerning the role of impurities in AlGaAs. We would also like to thank T. Dungan and R. Noren for contributions to this work. 'H. Okumura, S. Misawa, S. Yoshida, and S. Gonda, App!. Phys. Lett. 46, 377 (1985). 2H. Kroemer, W. Y. Chien, 1. S. Harris, Jr., and D. D. Edwall, App!. Phys. Lett. 36, 295 (1980). ]H. Kraemer, Appl. Phys. Lett. 46, 504 (1985). 4R. P. Gale, John C. C. Fan, G. W. Turner, and R. L. Chapman, Proceed. ings of the 1984 Phorovoltaics Specialist Conference (IEEE, New York, 1984), p. 1422. 'H. Okumura, S. Misawa, and S. Yoshida, The Second International Con· ference (Yamada Conference) on Modulated Semiconductor Structures, Kyoto, Japan, Sept. 9-13, 1985. 6H. Morko«, L. C. Witkowski, T. J. Drummond, C. M. Stanchak, A. Y. Cho, and B. G. Streetman, Electron. Lett. 16, 753 (1980). 7W. Kopp, S. L. Su, R. Fisher, W. G. Lyons, R. E. Thorne, T. J. Drum. mond, H. Morko«, and A. Y. Cho, App!. Phys. Lett. 42, 615 (1983). "T. J. Drummond, J. Klem, D. Arnold, R. Fisher, R. E. Thorne, W. G. Lyons, and H. Morko«, App/. Phys. Lett. 42, 615 (1983). 9S. R. McAfee, D. V. Lang, and W. T. Tsang, App!. Phys. Lett. 40, 520 (1982). ,op. S. Schaffer and T. R. Lally, Solid State Techno\. 26, 229 (1983). "R. C. Miller, W. T. Tsang, and O. Munteanu, Appl. Phys. Lett. 41, 374 ( 1982). "P. M. Petroff, R. C. Miller, A. C. Gossard, and W. Wiegmann, App!. Phys. Lett. 44, 217 (1984). "K. L. Tan, MSEE thesis, Purdue University, August 1985. Tan, Lundstrom, and Melloch 430 

Effect of impurity trapping on the capacitance‐voltage characteristics of n‐GaAs/N‐AlGaAs heterojunctions

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Effect of impurity trapping on the capacitance‐voltage characteristics of n‐GaAs/N‐AlGaAs heterojunctions

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