Mise en page 1 - Laboratoire National des Champs Magnétiques ...

SEMICONDUCTORS AND NANOSTRUCTURES 2009Mobility spectrum analysis in InN:MgThe mechanism of electrical conductivity and the possibleoccurrence of superconductivity in InN remains the subjectof considerable controversy. All recent Hall measurementsof InN samples doped with Mg or Be acceptors give a signfor the Hall voltage appropriate for electrons. This anomalousresult is presumably due to the presence of an n-typesurface inversion layer. One of the method used to detectthe expected presence of multi-carrier conduction in theInN system is the measurement of the resistivity tensor as afunction of magnetic field to perform a quantitative mobilityspectrum analysis. In the case of InN:Mg such an analysisshould in particular give the signs of the contributionsoriginating from electrons and holes if both are present.However, in the case of InN:Mg this is a challenging measurementsince the mobility of holes is probably very lowand comparable with that of the surface electrons. In addition,the samples are inhomogenous and unfortunately, theycan be measured only in the van der Pauw configurationbecause etching of InN samples to produce Hall bars stillposes a problem. All this makes it difficult to obtain preciseabsolute values of the ρ xx and ρ xy resistivity components,which is indispensable for a reliable mobility spectrumanalysis. Moreover, even for strictly n-type samplesthere often appears in the mobility spectrum the so called“ghost holes” which can lead to errors in the interpretationof the results. Thus, an appearance of a hole peak in themobility spectrum is not necessarily a reliable signature ofthe real presence of holes in a sample.peak is higher than electron peak revealed a positive signfor the thermopower). It also agrees very well with the conclusionthat only for moderate Mg content can free holesreally be detected in InN:Mg, while for higher doping theformation of donor-like defects (in agreement with amphotericdefects model) makes the sample progressively moreand more n-type.Figure 56: Magnetic field dependence of a resistivity tensor componentsρ xx and ρ xy of the E1053 sampleWe have performed a systematic investigation of 11InN:Mg samples with Mg-doping from 6 × 10 18 cm −3 to7.3 × 10 20 cm −3 together with 3 undoped (n-type) InN referencesamples. Our previous preliminary measurementswith two van der Pauw configurations and magnetic fieldup to 19 T showed hole peaks in mobility spectrum analysisbut it was problematic to find a reasonable correlationbetween the mobility spectra and the nominal Mg-doping.Our recent measurements show that only all possible permutationsof configurations of current and voltage probesand both polarities of the magnetic field up to 28 T givesreliable results. Figure.56 presents typical ρ xx and ρ xytraces as a function of the magnetic field. In figure 57 wepresent the results of mobility spectrum analysis for a seriesof samples with increasing Mg content (from 6 × 10 18to 4.2×10 20 cm −3 ) for the samples E1003 to E1041 respectively).The obtained series of spectra correlates well withthe measurements of thermoelectric power performed onthe same set of samples (only sample E1053 for which holeFigure 57: Mobility spectrum analysis of a series of InN:Mg sampleswith increasing Mg content (from E1053 to E1041, respectively).D.K. MaudeL. Dmowski (Institute of High Pressure Physics, Warsaw), L.Konczewicz (GES-UM2, Montpellier), M.Baj (Institute ofExperimental Physics, University of Warsaw)44