Local electronic and chemical structure at GaN, AlGaN and SiC heterointerfaces

Local electronic and chemical structure at GaN,AlGaN and SiC heterointerfaces

Leonard J. Brillsona,*, Shawn T. Bradleya, Sergey H. Tumakhaa,Stephen H. Gossa, Xiaoling L. Suna, Robert S. Okojieb,

J. Hwangc, William J. Schaffc

aThe Ohio State University, 205 Dreese Lab, 2015 Neil Ave., Columbus, OH 43210, USAbNASA Glenn Research Center, Cleveland, OH, USA

cDepartment of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853, USA

Received 16 June 2004; accepted 28 September 2004Available online 13 January 2005

Abstract

Defects and intermediate chemical phases at nanoscale heterointerfaces of GaN, AlGaN, and SiC can dominate theirmacroscopic electronic properties. We have used low energy electron-excited nanoscale luminescence spectroscopy incombination with secondary ion mass spectrometry and internal photoemission spectroscopy to correlate interface physicaland electronic properties for a variety of Schottky barrier and heterointerfaces involving these semiconductors. These resultsdemonstrate the key role of initial surface processing and subsequent chemical interaction on the heterointerface electronicstates, barriers, and carrier concentrations.# 2004 Published by Elsevier B.V.

PACS: 73.20.� r; 73.20.Dx; 73.30.+y; 78.60.Hk

Keywords: Schottky barriers; Interface states; Cathodoluminescence; GaN; AlGaN; SiC

1. Introduction

The wide band gap semiconductors GaN, AlGaN,and SiC are used in some of the most advanced micro-and optoelectronic devices today and rely on precise

control of electronic properties in multilayer filmstructures on a nanometer scale. Defects and newchemical phases inside these films and at theirheterointerfaces can affect Schottky barriers, ohmiccontacts, doping, free carrier confinement, andrecombination – with adverse consequences for thepower and speed of microelectronics. Yet measuringthe physical properties at these local interfaces andrelating them to electrical measurements has been a

www.elsevier.com/locate/apsuscApplied Surface Science 244 (2005) 257–263

* Corresponding author. Tel.: +1 614 292 8015;fax: +1 614 688 4688.

E-mail address: [email protected] (L.J. Brillson).

0169-4332/$ – see front matter # 2004 Published by Elsevier B.V.doi:10.1016/j.apsusc.2004.09.172

challenge for conventional techniques. We have usedlow energy electron-excited nanoscale luminescence(LEEN) spectroscopy [1] in combination withsecondary ion mass spectrometry (SIMS) and internalphotoemission spectroscopy (IPE) to correlate inter-face physical and electronic properties for a variety ofSchottky barrier and heterointerfaces involving thesesemiconductors. LEEN spectroscopy provides ameans to probe defects, near band edge (NBE)features indicative of semiconductor composition, andnew compound formation. Because of its capabilityfor both high surface sensitivity and nanoscale depthresolution, it is well suited to probe the local interfaceelectronic structure, even through thin metal over-layers. SIMS allows one to probe impurity andcompound fragments corresponding to either elec-trically-active dopants or interfacial dielectic layers.Coupled with depth profiling on a nanometer scale,this capability permits one to identify interfacialbonding features indicative of specific chemicalprocesses. IPE of diodes formed on the semiconductorsurfaces provide the most reliable method of Schottkybarrier measurement, free of artifacts due to, e.g.,other barriers (for instance, at AlGaN/GaN interfaces),interfacial recombination, and dopant assisted tunnel-ing. A particular advantage of IPE is, its ability togauge inhomogenous Schottky barrier formation.

We investigated a set of three representativeheterojunction types with the combination of LEEN,SIMS, and IPE: (1) Ni/AlGaN/GaN Schottky barriersprepared by a variety of common cleaning andprocessing techniques, (2) GaN/Al2O3 interfaces typicalof epitaxially-grown III-nitrides, and (3) Au, Ag, Ni, andTi reactive and unreactive interfaces with 4H-SiC(1 0 0). For metal–AlGaN junctions, pre-metalliza-tion processing conditions and post-metallizationannealing in ultrahigh vacuum annealing introduceSchottky barrier changes dominated by nanoscale alloycomposition. These heterogeneous interface featuresare revealed electronically through the metal overlayervia multiple near-band edge emissions, electrically viadual barrier heights, and spatially via secondary electronmicroscopy imaging on a nanoscale.

For GaN–Al2O3 interfaces, the formation of newchemical phases and diffusion during growth deter-mine local changes in doping and defects. Cross-diffusion of both oxygen and nitrogen and asubmicron-thick AlGaON interphase compound are

imaged in micro-cross section via both Auger electronspectroscopy (AES) and SIMS.

For a variety of metals on 4H–SiC, depth-resolvedstudies through the metal contact reveal the formationof mid-gap defect states extending only nanometersaway from the junction. These states vary in theirranges of depth and depend sensitively on the degreeof interface reactivity, as well as, on subsequentannealing in ultrahigh vacuum (UHV). The pervasiveappearance of emissions related to point defects andthe absence of new gap states due to chemical reactionindicates that native defects rather than extrinsicmetal-induced states dominate the SiC–metal inter-face state behavior.

2. Effects of cleaning and annealing on atomiccomposition and Schottky bar r ier s at Ni/AlGaNinter faces

Schottky barriers are central to many GaN andAlGaN alloy devices and researchers have examinedvarious surface processing methods to control andoptimize Schottky barrier heights (SBHs) for GaN [2].While several studies have recently dealt with SBHsfor metals on various Al mole fractions of AlGaNalloys, relatively little is known about the effect ofprocesses used for AlGaN cleaning and annealing ontheir SBHs. Here we show that pre-metallizationsurface treatments affect the IPE SBH’s of Ni onAlGaN/GaN heterostructures, and that they correlatewith alloy band gap measured by LEEN and withinterfacial impurities by SIMS and Auger electronspectroscopy. IPE is commonly used to determineSBHs of metal–semiconductor systems includingAlGaN. For Ni/AlGaN diodes, IPE also reveals dualSchottky barriers at the same diode caused byinterfacial changes in alloy composition.

We investigated three common processes used toclean III-nitrides prior to metallization: organicsolvents + HCL + HF etching (process A), the sameprocess followed by UV–ozone cleaning to removecarbon (process B), and the latter process followed byan HF strip to remove the oxide formed in B [3]. Airexposure between cleaning and UHV introduction formetallization was uniformly limited to less than 30 s.UHV metallization consisted of 25 nm thick circulardiodes of diameter 0.5 and 1 mm on 25 nm thick

L.J. Brillson et al. / Applied Surface Science 244 (2005) 257–263258

AlGaN grown epitaxially on GaN. All three processesresulted in IPE curves with threshold values corre-sponding to single Schottky barriers. After annealing

in ultrahigh vacuum, Fig. 1 shows that all diodesexhibited IPE slopes corresponding to two distinctSBHs, f b1 and f b2, see Fig. 1 inset. For process A(HCL + HF), a second, higher SBH appears, whichwas attributed to Ni penetrating the monolayeradventitious carbon and oxygen monolayers andmaking intimate contact with the AlGaN. Process Balso produced dual barriers after annealing as well.Here f B

b2 > f Ab2 due to the oxide formed by the UV–

ozone. SIMS reveals the primary constituents of thisoxide to be AlO and GaO, with AlO exceeding GaO by� 30% due in principle to the much stronger Al–Obond strength. HF etching decreases, both oxide levelsby 50% according to AES, removing proportionallymore Al than Ga in the process. IPE shows apronounced decrease in f B

b2. For diodes formed onsurfaces cleaned with process C (UV–ozone + HF),f B

b2 decreases by almost 0.2 eV relative to diodesprepared on the same surface by process B (UV–ozonealone). We acquired LEEN spectra with 3 keVincident beam energy to probe the region just beneaththe Ni layer. Fig. 2 presents LEEN results showingtwo qualitatively different emission spectra fordiodes cleaned by the two processes. For process Bdiodes, LEEN spectra exhibit a single NBE peak at3.9 eV corresponding to the expected 25% Al alloy

L.J. Brillson et al. / Applied Surface Science 244 (2005) 257–263 259

Fig. 1. Schottky barrier heights f b1 and f b2 from IPE curves. Afterannealing at 325 8C and above, a second barrier onset appears forboth processes A and C (see inset). Process C decreases f b2

significantly.

Fig. 2. LEEN spectra of diodes treated with HCL + HF process A (diodes 1–4) vs. diodes treated with process A, UV–ozone oxidation + HFstrip process C (diodes 5–11). A 0.2 eV decrease occurs with process C diodes, indicating a lower band gap and Al alloy composition.

composition based on a unity bowing parameter.For diodes on the same wafer involving surfacestreated with process C, the NBE exhibits a pronounceddecrease to 3.7 eV, corresponding to an alloy composi-tion of 16 to � 35% Al/Ga ratio decrease. Note thatdetection of this optical band gap change would bedifficult as best to obtain by other spectroscopictechniques, given that the altered region of AlGaNwas only � 5 nm. Nevertheless, this ultrathin alloyphase change decreased the Ni SBH by 0.2 eV,consistent with the 0.2 eV decrease in band gap andextrapolated change in electron affinity x according to aclassical Schottky model, i.e., f B = f M � x, wheref M = metal work function. Besides the presence ofmultiple barriers at these commonly prepared diodes,these results also demonstrate the role that interfacial Oplays in promoting these alloy changes. The initialreaction between O and AlGaN at elevated temperaturesresults in preferential formation of AlO versus GaO.Subsequent etching of these oxides leads to apreferential depletion of Al versus Ga in AlGaN anda corresponding reduction in SBH. Thus, surfacecleaning critically affects the interface chemistry thatdetermines SB uniformity.

3. Interdiffusion and reaction at GaN/Al2O3

inter faces

The heterojunction between GaN and its Al2O3

growth template represents a second interface withimportant chemical and electronic changes. Ofparticular concern has been the formation of a highlydegenerate interface layer in GaN grown on sapphireby hydride vapor phase epitaxy (HVPE). Micro-cathodoluminescence spectroscopy (CLS) in crosssection demonstrates that O diffusing out of Al2O3

into GaN creates shallow donor levels responsible forthis n-type doping. Fig. 3 shows CLS spectra versusposition normal to the GaN/Al2O3 interface for ahighly degenerate (6 � 1016 cm� 2) interface [4,5].The increased emission below the NBE energycorresponds to a new donor bound exciton with� 34 meV binding energy. SIMS of this interfacereveals an O profile whose intensity matches closelythat of the donor impurity feature in CLS. Similaremission near the interface of several HVPE GaNheterojunctions corresponds monotonically with the

dopant concentration measured using electrochemicalC–V profiling [4].

Micro-CLS, AES, and SIMS reveal other signifi-cant chemical and electronic changes. Micro-AESmaps for O show extensive diffusion of this HVPEGaN/Al2O3 interface over a micron or more.Significantly, these maps also indicate increased Alnear the GaN interface and N diffusion more than amicron into the Al2O3 [5]. This diffusion dependsstrongly on initial growth conditions. For sapphiresurfaces pre-treated with Zn, AES maps appear abrupton a nanoscale. Such pretreatment may form a ceramicthat acts as a diffusion barrier [6]. Without suchbarriers, extensive interdiffusion can occur at the high(1100 8C) HVPE growth temperatures. This inter-diffusion can produce major electronic changes at theGaN/Al2O3 interface. Rather than a discrete transitionbetween GaN and Al2O3 emission, Fig. 3 spectrareveal intermediate regions unlike either constituent.NBE emission at 3.4 eV is dominant in the GaNwhereas a well-known defect emission at 3.8 eV isevident in the Al2O3 associated with a Al–O–Ncomplex. Spectra right at the interface exhibit


Fig. 3. Micro-CL spectra of GaN/Al2O3 interface in cross sectionshowing 3.40 eV GaN NBE, 3.88 eV Al2O3 defect, and interface-specific 3.56 eV feature vs. distance dint from the junction. The latterindicates GaAlN alloy formation and band tailing/filling due todegenerate doping by O impurity donors.

emissions characteristic of AlGaN alloy formationwith NBE energies corresponding to � 10% Al,assuming a unity bowing parameter. Broadening ofthese features indicates variations in this compositionas well as both band tailing and filling due to thedegenerate O doping. AES profiles of each elementacross the interface match those obtained by a moreinvolved SIMS experiment. Again, initial surfacepreparation has a major effect on the interfacechemical and electronic features.

4. 4H–SiC Schottky bar r ier formation

LEEN studiesofmetal–SiC interfacesprovide a thirdtype of information about interface chemical andelectronic changes on a nanoscale. While SBs at variousSiC polytype–metal junctions have been studiedextensively, little is known about the nature of interfacestates believed to affect the charge transfer process. Ourprevious LEEN studies of metals on 6H and 4H–SiCreveal optical transitions involving states due tochemical bonding [7], stacking faults [8,9], anddislocations [10]. These gap states can extend through-out the SiC or be localized within nanometers of aninterface. We have now examined the states at themetal–SiC(1 0 0) Si face for a representative set ofmetals. Previously, we showed that thermodynamicallyreactive or unreactive metals on a variety of III–V andII–VI compound semiconductors exhibited qualitative

differences in SBH [11]. Furthermore, these micro-scopic chemical changes could have macroscopiceffects [11]. Previous studies have shown that metalson SiC(1 0 0) Si exhibit a moderate SB dependence onmetal work function, [12] indicative of only a weakinterface effects on Fermi level stabilization and bandbending. Yet the nature and spatial extent of these statesis not yet established. Fig. 4a illustrates LEEN spectrataken from 5 nm Au overlayers on 4H–SiC cleaned withacetone, methanol, and rinsed in DI water. These spectrashow both 3.2 eV NBE emission and broad � 2.4 eVemission due to 3C polytype inclusions or impurities.Again, spectra are obtained with incident energies thatexcite electron–hole pair creation, just below the metalinterface. Comparison of spectra before and after metaldeposition reveals new emission in the range 1.7� 2 eV.Significantly, 500 8C annealing reduces this emissionalong with intensity around 2.3 eV. No such changesoccur at the bare surface.

Similar features are induced by Ag deposition andannealing. Since neither Au nor Ag form compoundswith Si and C, these induced features may beassociated with interdiffusion and defect formation.A SIMS depth profile of a Au–SiC contact annealedfor 1 h each at 500, 800, and 900 8C in UHV shows Audiffusion only 10–15 nm into the SiC [13]. Corre-spondingly, the LEEN features decrease with increas-ing energies and excitation depths with almost nochange detectable for 2 keV, equivalent to � 20 nmmaximum excitation depth.


Fig. 4. LEEN spectra of 4H–SiC: (a) before and after 4 nm Au deposition. Difference spectrum shows metal-induced feature at 1.7–2.0 eV; (b)before and at after 800 8C anneal of 3 nm Ti. Difference spectrum shows induced feature at � 2.85 eV.

For the more reactive Ni and Ti, Fig. 4b showsqualitatively different behavior. Both induce newemission at 2.7–2.9 eV and only with 800–900 8Cannealing. Similar 2.7–2.9 eV emission has beenreported for single stacking faults induced byelectrical stress [10]. Likewise, 1.7–1.9 eV emissionwas identified with partial dislocations bounding suchstacking faults [10]. We have also observed newemission for these diodes at � 2.5 eV with 800–900 8Cannealing, previously identified as double stackingfaults induced by thermal stress at � 900 8C and high(> 1019 cm� 3 N) doping, as occurs here. There appearto be no new sub-band gap emissions at these metal–SiC interfaces besides those already linked tomorphological defects.

Diffusion and point defect formation may accountfor the 1.7–2 eV emission induced by Au depositionand decreased by annealing. Such diffusion is likelygiven that both Au and Ag form eutectics with Si attemperatures within the annealing ranges reportedhere. Furthermore, the point defects formed by suchdiffusion are also present in the bulk SiC and could beresponsible for the dislocation-related emissions.Rather than localized states associated with partialdislocations per se, these extended defects may attractSi vacancies or other point defects that ‘‘dress’’ thedislocation. Indeed photoluminescence studies ofirradiated 4H–SiC associate Vsi with emission at1.85 eV [14]. Interdiffusion at the Au–Si interfacewould increase Vsi locally and decrease away fromthe interface. Subsequent annealing could promoteAu–Vsi complexes or accumulation of Si at thesurface once it is depleted of Au. Hence, 1.7–2 eVemission may indicate movement of point defectsrather extrinsic surface states.

It is significant that the range of Schottky barriersreported for metals on 4H–SiC extends from 1.1 to1.8 eV with these minimum and maximum valuesnearly coinciding with the energies of inclusions ordefects (� 2.3 eV above the valence band) and statesassociated with partial dislocations (� 1.8 eV) belowthe conduction band. Hence Schottky barrier forma-tion at 4H–SiC interfaces may well be dominated bynative defects rather than extrinsic states induced byspecific metal interactions. The absence of new phasesor defects with chemically-reactive metals contrastswith the effects measured for III–V and II–VIcompounds [11]. This qualitative difference may be

due to the Group IV bonding of SiC. Since bothcarbide and silicide reaction products can form withNi or Ti, neither Si nor C is expected to be consumedpreferentially, limiting any defect formation asso-ciated with stoichiometry changes. In all cases,pronounced gap state changes that can affect macro-scopic electronic properties take place within nan-ometers of the junction.

5. Conclusions

Overall, the electronic structure at AlGaN, GaNand SiC interfaces can arise by both local chemicaland defect mechanisms. Ni/AlGaN junctions exhibitSchottky barriers that depend on local AlGaNcomposition changes. Chemical diffusion and reactionat GaN/sapphire interfaces alter both the localcomposition and free carrier concentration. Deeplevels associated with extended defects dominate thelocalized states at metal–SiC interfaces. These resultsdemonstrate the key role of surface processing andchemical interaction on the electronic properties ofwide gap semiconductor heterojunctions.

Acknowledgements

This work was supported by the Office of NavalResearch (Colin Wood), the National Science Foun-dation (Verne Hess), the Department of Energy (JaneZhu), the Air Force Office of Scientific Research, andNASA.

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Local electronic and chemical structure at GaN, AlGaN and SiC heterointerfaces

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