The electrical properties of lattice mismatched InAs/GaP heterojunctions are examined. In spite of a high dislocation density at the heterointerface, the current versus voltage characteristics show nearly ideal behavior with low reverse leakage currents and high breakdown voltages. The forward currentvaried exponentially with bias displaying ideal factors of 1.10 or less. Band offsets estimated from current–voltage and capacitance–voltage analysis are consistent with previous estimates based on differences in Schottky barrier heights

Chen, E H

Chin, T. P.

Lundstrom, Mark S

Woodall, J. M.

English

Purdue E-Pubs

Purdue UniversityPurdue e-PubsDepartment of Electrical and ComputerEngineering Faculty PublicationsDepartment of Electrical and ComputerEngineering1997Electrical characteristics of nearly relaxed InAs/GaP heterojunctionsE H. ChenPurdue UniversityT. P. ChinPurdue UniversityJ. M. WoodallPurdue UniversityMark S. LundstromPurdue University, lundstro@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.Chen, E H.; Chin, T. P.; Woodall, J. M.; and Lundstrom, Mark S., "Electrical characteristics of nearly relaxed InAs/GaPheterojunctions" (1997). Department of Electrical and Computer Engineering Faculty Publications. Paper 115.https://docs.lib.purdue.edu/ecepubs/115Electrical characteristics of nearly relaxed InAs/GaP heterojunctionsE. H. Chen, T. P. Chin, J. M. Woodall, and M. S. LundstromCitation:  70, (1997); doi: 10.1063/1.118614View online: http://dx.doi.org/10.1063/1.118614View Table of Contents: http://aip.scitation.org/toc/apl/70/12Published by the American Institute of PhysicsElectrical characteristics of nearly relaxed InAs/GaP heterojunctionsE. H. Chen, T. P. Chin, J. M. Woodall,a) and M. S. LundstromSchool of Electrical Engineering and NSF-MRSEC for Technology-Enabling Heterostructure Materials,Purdue University, West Lafayette, Indiana 47907-1285~Received 18 November 1996; accepted for publication 20 January 1997!The electrical properties of lattice mismatched InAs/GaP heterojunctions are examined. In spite ofa high dislocation density at the heterointerface, the current versus voltage characteristics shownearly ideal behavior with low reverse leakage currents and high breakdown voltages. The forwardcurrent varied exponentially with bias displaying ideal factors of 1.10 or less. Band offsets estimatedfrom current–voltage and capacitance–voltage analysis are consistent with previous estimates basedon differences in Schottky barrier heights. ©1997 American Institute of Physics.@S0003-6951~97!00112-5#Compound semiconductor devices are typically grownon substrates with a matching lattice constant. For latticemismatched systems, various kinds of strain relaxation layersare used to reduce the density of threading dislocationswhich propagate into the active layers. These 60°-type,threading dislocations typically pin the Fermi level and pro-vide nonradiative recombination centers that degrade the op-tical and electrical performance of devices. Recently, Changet al. reported that by growing InAs directly on GaP~100! bymolecular beam epitaxy~MBE!, the epilayer relaxes via theformation of an array of 90°-type, edge dislocations at theheterointerface.1 The density of the threading dislocations isrelatively low. This nearly relaxed InAs/GaP material systemwith low threading dislocation densities shows promise fordevice applications. For example, the nearly relaxed InAslayer ~typically 2–20 nm thick! is lattice matched to theIn0.8Ga0.2As/In0.8Al0.2As system, which could provide a lat-tice matched heterostructure system for long-wavelengthemitters and detectors and other small band gap devices. Toassess the device potential for this technology, we examinethe electrical characteristics of nearly relaxed InAs/GaP het-erojunctions. In spite of the large lattice mismatch~'11%!and high density of 90°-type dislocations, the electrical char-acteristics are surprisingly good, showing nearly ideal for-ward and reverse bias behavior.The growth of InAs on GaP was performed in a solidsource GEN-II MBE system equipped with valved phosphor-ous and arsenic crackers. Valved phosphorus crackers havebeen demonstrated to be a viable growth technique for phos-phides in solid-source MBE.2–4 Both InGaP and InP baseddevices have been achieved. In this letter, InAs was grownon GaP forming an abrupt heterojunction with an 11%mismatch.1 The GaP~100! substrate was heated to 660 °C foroxide desorption, then GaP was grown at 600 °C. For somesamples, a 20 period GaP/AlGaP~5 nm/5 nm! superlatticewas incorporated as a barrier for impurities diffusing fromthe substrate.~Low temperature photoluminescence mea-surements showed that the intensity of the sulfur-bonded exi-ton was greatly reduced by this technique.! After growth ofN- or P-type GaP, the substrate temperature was lowered to350 °C for the growth of InAs. A thin~2–20 nm! InAs layerwas deposited two dimensionally under slightly In-rich con-ditions followed by a thicker~200 nm! In0.8Ga0.2As layerwhich is closely lattice matched to the nearly relaxed InAs.Subsequent atomic force spectroscopy~AFM! studieshowed a high density of defects thought to be In dropletscaused by the In-rich growth conditions necessary for two-dimensional growth.~Subsequent optimization of growth pa-rameters has greatly reduced the density of these defects.!Four different samples, which represent the four possibleInAs/GaP isotype and anisotype heterojunctions, weregrown. These samples are denoted asn1N, p1P, n1P, andp1N, where the capital letter refers to the large band gapside ~GaP! and the small letter to the small band gap side~nearly relaxed InAs!. The film structures, targeted thick-nesses, and doping densities are summarized in Fig. 1. Di-odes were fabricated by first patterning with photoresist thendepositing a 200/100/10 nm Au/Ti/Au metallization layer forOhmic contacts~the same metallization was used for bothn1- and p1-In0.8Ga0.2As top layers!. Mesas were thenformed by selectively wet etching the exposed In0.8Ga0.2Asand the underlying InAs until the GaP interface was reached.The diode sizes ranged from 3040 to 4803750 mm2.Backside ohmic contacts for bothN1- and P1-GaP sub-strates consisted of pure In.Current–voltage measurements were performed using anHP-4145 parameter analyzer in a dark environment, and thecapacitance–voltage measurements were performed at fre-quencies between 10 kHz and 1 MHz using an HP-4275Aimpedance analyzer. To extract activation energies, sampleswere tested under vacuum using an HP-4142 while varyingthe temperature from 200 to 300 K.a!Electronic mail: woodall@ecn.purdue.eduFIG. 1. Illustration of the film structures used for the In0.8Ga0.2As/InAs/GaP diodes.1551Appl. Phys. Lett. 70 (12), 24 March 1997 0003-6951/97/70(12)/1551/3/$10.00 © 1997 American Institute of PhysicsFigures 2 and 3 show the measured I–V characteristicsof typical diodes. As shown in Fig. 2, the forward biasedcurrents displayed a ‘‘turn-on’’ voltage that increased in theorder p1P, n1P, n1N, and p1N, which suggests thatDEC.DEV, as expected for this material system.5 In reversebias, all diodes showed low leakage currents and breakdownvoltages consistent with avalanche breakdown in the GaP.The differences can be attributed to differences in the dopingdensity of the GaP layers. Critical fields of 83105 and13106 V/cm were estimated for then1N and p1Pstructures, respectively; these values are about 80% of thetheoretical values.6 Edge effects at the corners of the rectan-gular diodes or field enhancements at the defects revealed byAFM are possible causes for the lowered critical field. The‘‘soft’’ breakdown characteristics displayed in Fig. 2 mayindicate thermionic field emission.7Figure 3 shows the forward-biased I–V characteristics oftypical diodes. All devices displayed an exponential charac-teristic with an ideality factor,n, between 1.04 and 1.10 overat least three orders of magnitude. At a given bias, the mag-nitude of the forward current decreased in the order,p1P,n1P, n1N, and p1N, which reflected the differences inturn-on voltages observed in the linear plots. We note thatsome of thep1N diodes displayed a small shunt leakagecurrent, but no such leakage was observed in the other di-odes. This may indicate that thep1-InAs layer in the aniso-type heterojunctions is somewhat more susceptible to defectspossibly because of Fermi-level pinning near the conductionband.The forward biased currents can all be fit to a Schottky-barrierlike expression of the form,J5A** T2e2qfb /kBT@eqV/nkBT21#, ~1!whereA** is an effective Richardson contact,n is the ide-ality factor, andfb the barrier height. A form like this mightbe expected for the isotype heterojunctions which operatemuch like Schottky barriers, but Schottky-barrierlike charac-teristics also occur in anisotype heterojunctions when thebarrier height is large.8 By performing temperature-dependent measurements from'200 to 300 K and plottingln(J/T2) versus 1/T at a fixed bias, the barrier heights werededuced, and the results are listed in Table I.Reverse biased C–V measurements were also per-formed. No hysteresis was observed as the bias voltage wasswept, and the results were independent of frequency be-tween 10 kHz and 1 MHz. Plots of 1/C2 versusVrev werelinear, so the doping density of the lightly doped GaP couldeasily be deduced. The results, along with the built-in volt-ages as obtained from the intercept voltages, are also listed inTable I.The measured I–V and C–V characteristics are gener-ally consistent with the expected band offsets for this mate-rial system,5 but a precise determination is clouded by sev-eral uncertainties. One is the nature of transport across theheterointerface, which involves an indirect semiconductor,GaP, and a direct one, InAs.9 Other uncertainties includeFermi level pinning which may occur at the interface as wellas possible interface charges and dipoles. As a first orderanalysis, we ignore these complications and interpret the re-sults in terms of ideal band line ups. Forn1N (p1P) diodes,the barrier height is the conduction~valence! band disconti-nuity minus the position of the Fermi level within theFIG. 3. Logarithmic forward current vs voltage characteristic of the fourdiodes. The size of the diodes was 2703360 mm2 and the measurementtemperature was 23 °C.FIG. 2. Linear current vs voltage characteristics of four typical diodes. Thesize of the diodes was 903120 mm2 and the measurement temperature was23 °C.TABLE I. Summary of the I–V and C–V measurements and the conduction and valence band discontinuitiesdeduced from the I–V barrier heights and C–V intercept voltages.I–V C–VDiode n fb ~eV! DE ~eV! NB(cm23) Vbi ~V! DE ~eV!p1P '1.10 0.59 DEV50.59 3.3 1017 0.79 DEV50.88n1N '1.07 0.98 DEC51.01 1.131017 1.19 DEC51.35p1N '1.08 1.40 DEC50.90 1.631017 1.44 DEC51.06n1P '1.04 1.13 DEV50.60 1.3 1017 1.05 DEV50.631552 Appl. Phys. Lett., Vol. 70, No. 12, 24 March 1997 Chen et al.n1(p1) InAs. Forn1-InAs, we estimate that the Fermi levellies about 0.23 eV within the nonparabolic conduction band.At the same time, however, heavy doping effects in then1-InAs will shrink the band gap by roughly 0.2 eV therebylowering the Fermi level.10 Accounting for these effects, weextracted barrier heights of the isotype heterojunctions fromthe I–V and C–V data and report the results in Table I.For thep1N (n1P) anisotype heterojunctions, the bar-rier height is the conduction~valence! band discontinuityplus the band gap of the nearly relaxed InAs, plus the Fermi-level penetration into the valence~conduction! band of therelaxed InAs. We estimate the band gap of the nearly relaxedInAs to be'0.5 eV, the band gap of In0.8Ga0.2As, which hasthe same lattice constant. Again, we have the same uncer-tainties regarding the position of the Fermi level and theband gap narrowing effects. Accounting for them, we esti-mate the band offsets for the anisotype heterojunctions andreport the results in Table I.As estimated from Schottky barrier height differences,and band line ups for this material system are expected to beDEC'1.4 eV andDEV'0.4 eV.5 Although there is somespread in the results, Table I shows that the observed con-duction band discontinuities were much larger than those forthe valence band. For ideal heterojunctions, we expectDEC1DEV1EG ~nearly relaxed InAs!5EG~GaP!52.27 eV.This is roughly true for the C–V results of the anisotypeheterojunctions and the I–V results, but the C–V results forthe isotype heterojunctions give anomalously high values.The smallerDEC obtained from thep1N I–V data as com-pared to then1N data could be explained by Fermi-levelpinning of the relaxedp1-InAs/N-GaP interface, and thehigh values obtained by C–V analysis for the isotype hetero-junctions may indicate the presence of interfacial charges.In summary, the vertical transport properties of isotypeand anisotype nearly related InAs/GaP heterojunctions wereexamined. The resulting I–V characteristics have nearlyideal, Schottky-barrierlike, forward bias characteristics, andlow leakage in reverse bias. The conduction band offsets arelarger than the valence band offsets and their magnitudes arein general agreement with expectations. This near-ideal be-havior is obtained in the presence of a high density of dislo-cations in the plane of the heterointerface. These results sug-gest a number of potential device possibilities withp1P1InAs/GaP heterojunctions suitable for providing Ohmic backcontacts and then1N and anisotype junctions providing rec-tification.This work was supported by NSF through the MaterialsResearch Science and Engineering Center for Technology-Enabling Heterostructure Materials, under Grant No. DMR-9400415. The authors acknowledge helpful discussion withJ. C. P. Chang and W. Wright concerning AFM studies ofthe films.1J. C. P. Chang, T. P. Chin, and J. M. Woodall, Appl. Phys. Lett.69, 981~1996!.2G. W. Wicks, M. W. Koch, J. A. Varriano, F. G. Johnson, C. R. Wie, H.M. Kim, and P. Colombo, Appl. Phys. Lett.59, 342 ~1991!.3J. N. Baillargeon, A. Y. Cho, F. A. Thiel, R. J. Fischer, P. J. Pearah, andK. Y. Cheng, Appl. Phys. Lett.65, 207 ~1994!.4T. P. Chin, W. L. Chen, G. I. Haddad, J. C. P. Chang, and J. M. Woodall,J. Vac. Sci. Technol. B14, 2225~1996!.5S. Tiwari and D. J. Frank, Appl. Phys. Lett.60, 630 ~1992!.6S. M. Sze,Physics of Semiconductor Devices~Wiley, New York, 1981!.7E. H. Rhoderick and R. H. Williams,Metal-Semiconductor Contacts, 2nded. ~Clarendon, Oxford, 1988!, pp. 89–140.8M. S. Lundstrom, Solid-State Electron.29, 1173~1986!.9A. A. Grinberg, Phys. Rev. B33, 7256~1986!.10S. C. Jain, J. M. McGregor, and D. J. Roulston, J. Appl. Phys.68, 3747~1990!.1553Appl. Phys. Lett., Vol. 70, No. 12, 24 March 1997 Chen et al.

Electrical characteristics of nearly relaxed InAs/GaP heterojunctions

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Electrical characteristics of nearly relaxed InAs/GaP heterojunctions

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