Thin Solid Films 550 (2014) 715–722

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Effect of film growth rate and thickness on properties of Ge/GaAs(100)thin films

V.F. Mitin a,⁎, V.K. Lazarov b,c, L. Lari b,c, P.M. Lytvyn a, V.V. Kholevchuk a, L.A. Matveeva a,V.V. Mitin a, E.F. Venger a

a V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, 03028 Kyiv, Ukraineb Department of Physics, University of York, Heslington, York YO10 5DD, UKc York-JEOL Nanocentre, University of York, Heslington, YO10 5DD, UK

⁎ Corresponding author.E-mail addresses: mitin@isp.kiev.ua, mitin@microsens

0040-6090/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.tsf.2013.10.049

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 August 2012Received in revised form 10 October 2013Accepted 16 October 2013Available online 24 October 2013

Keywords:Ge/GaAsElectrical propertiesIntrinsic stressesSurface and interface morphologyPercolationPotential fluctuationsDoped and compensated semiconductorsStrongly compensated semiconductors

Wepresent a profound effect of film growth rate and thickness on the electrical properties, intrinsic stresses andsurface/interface morphology of thin (1 nm to 250 nm) Ge films grown on GaAs(100). The films studied in thiswork are epitaxially grown and have single crystal structure. The film–substrate interfaces are coherent, withoutmisfit dislocations. At high deposition rates, the film surface is smooth and does not depend on the film thicknessin contrast to low growth rate films that show pronounce surface roughness as the film thickness increases. Theintrinsic stresses in the films decreased with lower deposition rate. Moreover, the Ge films obtained at highdeposition rates are n-type, have low resistance and a high concentration of free charge carriers. The weaktemperature dependence of conductivity of high deposition rate specimens indicates that these films are heavilydoped degenerate semiconductors. The films preparedwith low deposition rates are p-type and highly resistant,with a low concentration of free charge carriers, and exhibit thermally activated conductivity of percolation-type.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

The Ge/GaAs heterostructure is promising for a number of practicalapplications such as high-performance solar cells [1–4], metal oxidesemiconductor field-effect transistors [5,6], microwave diodes [7],sensors, e.g. thermometers [8,9], and X-ray and optical radiationdetectors [8,10–13]. In addition the Ge/GaAs is an excellent modelsystem for IV/III–V heterostructures in general. This heterostructureis characterized by close values of the component lattice constantsas well as practical coincidence of their thermal expansion co-efficients [1]. These properties are favorable for epitaxial growthand fabrication of films and heterojunctions with almost perfectstructure.

The defect formation in the Ge/GaAs heterostructures depends onthe material which serves as the substrate (Ge or GaAs). In this work,we consider Ge film epitaxy on the GaAs(100) substrate. The Ge latticeconstant exceeds that of GaAs. Therefore, the Ge films grown on GaAssubstrates are compressed. It was experimentally shown [14] thatelastically compressed Ge films on GaAs may exist up to film thickness

or.com.ua (V.F. Mitin).

ghts reserved.

of 5 μm. In that case, there are no mismatch dislocations which couldimpair the electrical properties of the heterojunction. Thus, it is possibleto form a high-quality interface (without misfit dislocations) in theGe/GaAs heterostructure.

The distinguishing feature of Ge films on GaAs is the strongdependence of their structural, electrical and optical properties, onthe method of preparation. This is related to possible interdiffusionof the joining materials [15–23]. For Ge films, diffused Ga and Asatoms act as shallow acceptors and donors, respectively. Therefore,because of autodoping, the Ge films on GaAs may be p- or n-typewith different degrees of compensation including fully compensated[8,24–27]. Moreover the degree of structural perfection of the Gefilms, as well as the concentration of structural defects in them, isdetermined by the film preparation condition. The Ge films can beamorphous, polycrystal, or single crystal with different degrees ofstructural perfection.

The pioneer work on fabrication and investigation of the Ge/GaAsheterostructure can be traced to the paper by Anderson [28]. A ratherlarge number of studies have been done to understand the effectof factors such as: preparation methods, substrate temperature, filmthickness and GaAs surface preparation on the growth mechanism[29–37], surface and heteroboundary morphology [15,23,36–43], in-trinsic stresses [12,33,39–42,44,45], and electrical [15,23,28,40,46–48]

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and optical [23,28,46,49] properties of Ge films on GaAs. However, theeffect of the deposition rate on the properties of Ge films on GaAsremains not fully explored, with few studies considering only highdeposition rates [50,51].

In our previous [8,24] and recent work [26,27] we show thatdeposition rate drastically affects the electrical and optical propertiesas well as surface morphology of thin (~100 nm) Ge films on GaAs. Inparticular we show that efficient control of the film properties can beachieved by varying their deposition rate.

The subject of this paper is a further in-depth study of the effect offilm growth rate with focus on thickness dependence, intrinsic stresses,surface/interface morphology and electrical properties of thin (1nm to250 nm) Ge films. We have found that at low deposition rates filmsurface morphology strongly depends on the film thickness. The filmsurfaces are rough, and the relief becomes more pronounced as thefilm thickness increases. In contrast, at high deposition rates, the filmsurface is smoother and does not depend on the film thickness. Theintrinsic stresses in the films decreased with lower deposition rate.The electrical properties of the films also vary dramatically dependingon the film deposition rate. These effects are essential and should betaken into account when developing and producing devices based onthe Ge/GaAs heterostructure. The variation and ability to tailor theelectrical properties of the Ge/GaAs heterostructure simply by varyingthe deposition rate open up the possibility for approaches in IV/III–Vsemiconductor devices.

2. Preparation conditions and measurement procedure

The Ge films were deposited using thermal evaporation of Ge in thevacuum (2 × 10−4 Pa) onto semi-insulating (107 Ω⋅cm) GaAs(100)substrates. The temperature of GaAs substrate during the filmdeposition was maintained at 500 °C. The film deposition rate wasmaintained constant over the course of each deposition; and varied(0.02–0.35) nm/s for different specimens. The evaporation sourcewas a glass–graphite crucible heated with electric current flowingthrough it. The deposition rate was set by the crucible currentthat determined the Ge evaporation temperature. The film thick-nesses varied from 1 nm to 250 nm. Before Ge film deposition, theGaAs substrates were treated with the H2SO4:H2O2:H2O = 4:1:1etchant for 3 min and thermal annealed in vacuum at 500 °С for15min.

The morphology of Ge film surfaces was studied with an atomicforce microscope (AFM) NanoScope IIIa. AFM measurements havebeen performed in tapping mode using silicon tip with nominal radiusof 10nm and cone angle of 16°.

Bright field (BF) transmission electron microscopy (TEM)imaging and selected area diffraction (SAD) have been performedby JEOL 2011. The high resolution TEM (HRTEM) and high angleannular dark field (HAADF) imaging have been done by the doubleaberration corrected JEOL 2200 FS microscope. Line-Scan energydispersive X-ray (EDX) analysis was performed using a Thermo-EDX system. TEM cross-section specimens were prepared by con-ventional methods that include mechanical thinning and polishingfollowed up by Ar ion milling to achieve electron transparence, asdescribed elsewhere [52].

The intrinsic stresses (σ) due to lattice mismatch between Ge andGaAs were determined from the radius of curvature of specimensurfaces using a profilograph and by application of the Stoney'sformula [53,54] σ ¼ Et2

6 1−νð ÞRd , where E, t and ν are Young's modulus,thickness and Poisson ratio of the substrate, respectively, d is the filmthickness and R is the heterosystem bending radius.

The electrical properties of the Ge films were determined by theconductivity, Hall effect and magnetoresistance, which were measuredin temperature range from 295K to 380K and magnetic field inductionof 2T.

3. Experimental results

3.1. Surface morphology

Fig. 1 shows AFM images of film surfaces at different thicknessesobtained at low (0.02 nm/s) and high (0.35 nm/s) deposition rates.Fig. 1 also shows line-profile (height variations) of the correspondingfilm surfaces presented in Fig. 1. The morphology of film surfacesstrongly depends on film thickness and deposition rate. The filmsprepared by low deposition rate have surfaces with pronouncedmorphology of grains of irregular shape and substructure. At a filmthickness of about 1 nm, a nucleating layer with characteristic graindiameter of 12nm is formed. The character of surface grain distributionchanges from log-normal to normal and bimodal in a 1 nm, 10 nmand 20 nm thick layers, respectively. The root-mean-square (RMS)roughness and surface area increase (SAI) (defined as the difference ofthe surface area and its projection area divided by the projection area)of the films increasemonotonically with thickness;when film thicknessis 250nm, their values are 27nm and 25%, respectively (Fig. 2a). At filmthickness over 50nm, the grain size increases abruptly with broadeningof the grain diameter histogram (Fig. 2b). The grain size histogramshave been determined using the conventional watershed algorithm;we determined the grain boundaries, calculated their area and ex-pressed the diameter in terms of that area. Then we chose the optimalhistogram bin for all distributions.

In contrast to low deposition rate film surfaces, the surfaces of highdeposition rate films (0.35 nm/s) are smoother, with the exception oflow film thicknesses during the initial growth stages. The growth beginsfrom a nanograin nucleating layer (at a thickness of ~1 nm) with thesame log-normal size distribution as low growth rate (not presentedhere). Typical grain diameter of the ~1 nm nucleating layer is 9 nm;these grains proceed the growth with pits. The density and depth ofpits decrease as the film thickness increases. At film thickness of10 nm, the pit depth is about 7–9 nm — see Fig. 1(k); the pits arepractically not observed at film thickness over 50 nm. RMS and SAIpeaks at epitaxial layer thickness of d = 20 nm (RMS and SAI valuesare 2.8 nm and 3%, respectively). At film thickness d≥50nm, RMS andSAI are 0.7 nm and 0.15%, respectively, and show no further filmthickness dependence (Fig. 2a).

The corrugated surface of Ge epitaxial films grown at low depositionrate demonstrates anisotropy of spatial orientation of relief elements.This anisotropy coincides well with characteristic crystallographicsymmetry of surface faceting of crystalline Ge (Fig. 3). The polar surfacenormal orientation histograms indicate presence of four maximumsclose to thepositions ofGe crystallographic planes {113} (25°).Moreover,one can observe facet orientation dependence on the Ge film thickness.At thickness of 20 nm, only fourfold symmetry of facet orientationdistribution is observed, without explicit maximums. As thicknessincreases to 50 nm, clear maximums are formed whose arrangement iscloser to the position of crystallographic planes {227} (22°). At filmthickness of 100 nm (Fig. 3c), the arrangement of maximums coincideswith orientation of germanium crystallographic planes {113}; there arealso weakly pronounced maximums symmetrically arranged near{159}. As film thickness increases to 250 nm, the character of spatialdistribution of surface fragment slopes does not change. However,there is a tendency of a decrease of their angle from 25° to 22° (deviationfrom {113} to {227}), and the surface roughness reaches 27nm.

Fig. 4 shows HAADF image of the Ge film near the surface regionwith labeled surface facets for films deposited at low rate. The dominantsurface facets are of {113} (25°) type, also seen in previous studies[55,56]. This is a good agreement to AFM data with Ge crystallographicplanes as discussed above.

At the same time, no similar anisotropy is observed in surfacefragment orientation for the whole set of thicknesses of Ge filmsgrown at high deposition rate. Those films had a smooth surface withroughness within the 0.5–0.9nm range.

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3.2. Crystallinity and interface morphology

The electron diffraction investigation of the film structure showedthat all films are single crystal, as illustrated by the SAD from the Ge

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film grown at low deposition rate (Fig. 4). The epitaxial relationbetween the film and the substrate is a simple cube on cube relation:Ge(001)||GaAs(001). The interface betweenGe andGaAs iswell defined(Fig. 5a), with no presence of misfit dislocations, as shown bycontinuous (111) atomic planes across the Ge/GaAs interface (Fig. 5b).Interface roughness (up to 1 nm), most likely as a result of substratepreparation or atomic mixing, can be seen from the HRTEM images(Fig. 5a). The high resolution (HR) HAADF imaging supports thisobservation (Fig. 6a). Possible atomic mixing at the interface regionwas studied by EDX. Line-scan EDX across the interface indicates thatmixing could have occurred, though due to the beam spread and thesize of interface region (up to 1 nm) this should be examined byadditional methods in order to fully verify the presence of atomicmixing at the Ge/GaAs interface.

At high film growth rate, the BF-TEM (not shown here) as well asHAADF (Fig. 6a) show the presence of linearly shaped defects thatstart at the interface and extend in the Ge film for several tens of nm.The HR-HAADF (Fig. 6b) and the HRTEM (not shown here) clearlyshow that these defects are twin stacking faults (SF). The SF occurs onthe {111} type of planes, and varies in width from several to tens ofatomic planes. All the SFs originate at the interface and at small numberof SF we observe the presence of dislocations, either at the Ge/GaAsinterface or along the twin mirror plane of the SF.

At low film growth rate some regions appear in the bulk of the filmswhich are clearly seen in BF-TEM (Fig. 4) and HAADF (Fig. 6c). Regionswith dark contrast are observed along the hollows of surface relief inHAADF images. The HR-HAADF (Fig. 6c) shows that these dark contrastregions are fully crystal; the darker contrast most likely originates fromthe presence of voids along the grain boundaries, thus these regionsappear darker in the HAADF imaging since this technique is Z-contrastsensitive. The void formation is a result of the 3D columnar growth ofthe Ge film. Besides the presence of the voids when grains coalesce noother defects or crystalline misorientation has been observed.

3.3. Intrinsic stresses

The Ge crystal lattice constant (0.56579 nm) is bigger than that ofGaAs (0.56536 nm); the misfit is about 0.076% [57]. Therefore, the Gefilms on GaAs have to be compressed. Indeed, the investigations ofradius of curvature of the Ge/GaAs heterostructure showed that Gefilms are compressed. The intrinsic stresses in the Ge films about100 nm thick calculated using the Stoney's formula are at the level ofseveral tenth parts of GPa. They depend on the film growth rate,decreasing from 6.7 × 108 Pa to 2 × 108 Pa (i.e. by more than threetimes) as the deposition rate decreased from 0.35 nm/s to 0.02 nm/s(Fig. 7).

3.4. Electrical transport properties

The transport phenomena also differed drastically in films obtainedat high and low deposition rates. Our investigations showed that Gefilms with thickness of about 100 nm obtained at low deposition rateare p-type and high-resistant (10–20 Ω⋅cm), with low concentration(1015–1016 cm−3) and mobility (20–100 cm−2/V⋅s) of free chargecarriers (determined from Hall effect measurements) at room tem-perature. Their conductivity is thermally activated and is described bythe expression ρ(T)= ρ1exp(ε1 / kT) with activation energy ε1 close orequal to Eg / 2 for Ge (ε1=0.34eV for specimen presented in Fig. 8). In

Fig. 1. AFM images from (1×1) μm area of Ge films of various thicknesses grown on GaAsat the deposition rates of 0.35 nm/s (a–e) and 0.02 nm/s (f–j). Thicknesses are: 1 nm (a,f),10nm (b,g), 50nm (c,h), 120nm (d,i) and 250nm (e,j). Typical surface height profiles areshown in (k) and (l) for corresponding AFM images. The film thickness value indicated atprofile line. Curves shifted vertically for better visualization. Z-scale color coding shown atthe right side of AFM image.

Fig. 2.Dependencies of RMSand surface area increasing on thickness (a) and graindiameter histograms (b) for theGefilms of different thicknesses grownat 0.02nm/s (open symbols) and0.35 nm/s (filled symbols) rates (V). The film thickness (d) is indicated in histograms. The lines in panel (a) are just to guide the eyes.

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the most resistant (compensated) Ge films the Hall coefficient reversessign at elevated temperatures, and the transverse magnetoresistancegreatly increases with temperature (Fig. 8).

Fig. 3. 3D polar surface normal orientation histograms (facet orientation) for specimens grown a50 nm, 100 nm and 250 nm, respectively.

Contrary to the above, the Ge films obtained at low deposition rateare n-type and low-resistant (0.1–0.01Ω⋅cm), with high concentrationsof free charge carriers (1018–1019 cm−3). Thermal dependence of

t deposition rate of 0.02nm/s; (a), (b), (c) and (d) correspond to film thicknesses of 20nm,

Fig. 4. (a) BF-TEM cross-section of the Ge film at low deposition rate. (b) HAADF image ofthe Ge film near the surface region with labeled surface facets (c). SAD of the film–

substrate showing the cube on cube epitaxy between the Ge/GaAs(100).

Fig. 5. (a) HRTEM of the Ge/GaAs interface. (b) Bragg filtered HRTEM, from the region in(a), showing the continuity of atomic planes (111) across the interface with no presenceof interface misfit dislocations.

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conductivity, Hall coefficient and magnetoresistance is weak (Fig. 8)characteristic of heavily doped degenerated semiconductors.

4. Discussion of results

The Ge films under consideration have single crystal structure andcoherent dislocation-free film–substrate interfaces. Deposition ratestrongly affects the electrical properties, intrinsic stresses and surfacemorphology of the Ge films. Besides, the film surface morphologydepends on film thickness.

4.1. Surface and interface evolution, and intrinsic stress relaxation

The early stages of film growth (up to thickness of 20 nm) appearsimilar for both low and high deposition rates. The growth beginsfrom a nanograin nucleating layer. Then for high deposition rate pitsare formed. As the film thickness increases pits heal and disappear atfilm thicknesses over 50 nm; in this case, surface roughness is below1 nm (Figs. 1a and 2). Appearance of pits at high deposition rates canbe explained by the effect of phase decomposition and formation ofAs atom clusters that prevent the acquisition of Ge adatoms, aphenomenon observed and discussed in [43,58]. This correlateswell with the electrical investigations that showed that such filmswere n-type.

At low deposition rate, no pits are formed. Grains of irregular shapedetermine the film surface morphology. This can be clearly seen at filmthicknesses over 50 nm. Our investigations showed that both thecontinuous film and protruding grain structure were single crystal andperfect, and the interface was coherent and dislocation-free. The filmscould not be considered as “highly oriented polycrystal films”. The

only statement that could be made was that some regions appear inthe bulk of the films grown at low deposition rates. These regions (thatare of lower density) might be related to the presence of voids. Therewas no off-orientation between most of the bulk and the above regionsof lower densitywhose disposition coincidedwith the hollows of surfacerelief. This could be related to 3D columnar growth of theGefilm. Besidesthe presence of the voids when grains coalesce no other defects orcrystalline misorientation has been observed. At the same time, thefilms grown at high deposition rates have no regions of lower densityand are also single crystal, with perfect structure and smooth surface.

The pronounced surface morphology of these films could lead torelaxation of intrinsic stresses in the films. It was noted in [59] thatappearance of compositional and morphological inhomogeneities instressed dislocation-free single crystal films may be one of the mainways of structural relaxation. Indeed, by comparing the intrinsicstresses σ (Fig. 7) with the data given in Fig. 2 for films of comparablethickness one can see that intrinsic stresses in the 103 nm film(deposited at a rate of 0.02 nm/s) with 12.3% increase of SAI are lessby three times than in a film of close thickness (deposited at a rate of0.35 nm/s) with as low as 0.1% increase of SAI. This indicates that themechanism of intrinsic stress relaxation in Ge/GaAs films grown atlow deposition rate is surface corrugation of the epitaxial layer. Thecrystallographic planes {113} and {227} (Fig. 3) are predominant forcorrugated surfaces of Ge epitaxial films, depending on their thickness.

In addition, compositional inhomogeneities in Ge films can alsoreduce the elastic strain energy in the heterosystem. It is also known

Fig. 6. (a) HAADF from the Ge/GaAs interface showing the formation of stacking fault defects that originate at the interface. (b) HR-HAADF from a staking fault region in Ge film showingthe twin nature of the faults. (c) Lowmagnification HAADF fromGe/GaAswith dark contrast regions along the hollows of surface relief. (d) HR-HAADF fromGewith dark contrast regions,darker contrast indicates void formations.

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that for layers grown on mismatched substrates, there is a couplingbetween the local composition and the surface morphology, so that arough film will decompose, while a flat layer will be homogeneous[58,59].

4.2. Conductivity, Hall effect and magnetoresistance

The presence of compositional inhomogeneities in the Ge films isconfirmed by electrical transport measurements. Indeed the filmsgrown at low deposition rate have higher resistivity, low concentrationsand mobility of the free charge carriers. Their conductivity is thermallyactivated. To give consistent explanation for the electrical properties

Fig. 7.Dependence of intrinsic stresses (σ) inGefilms onGaAs on thefilmgrowth rate (V).The film thickness was constant (about 130 nm).

of such films, we assume the presence of fluctuations of electrostaticpotential in the films related to random nonuniform distribution ofimpurities and compensation, i.e. compositional disorder. The modelof potential fluctuations in doped and compensated semiconductorswas developed in [60]. Such potential fluctuations modulate semi-conductor energy bands leading to appearance of potential relief anddeep tails of density of states in the semiconductor band gap (Fig. 9).The main characteristics of potential relief are amplitude γ and spatial

Fig. 8. Typical dependencies of resistivity (ρ), Hall coefficient (RH) andmagnetoresistance(Δρ / ρ) on inverse temperature for 100 nm Ge films obtained at deposition rates of0.02 nm/s (1) and 0.35 nm/s (2).

Fig. 9. Energy diagram of a heavily doped and fully compensated semiconductor. r is thesize of a typical potential fluctuation; γ the LSFEP amplitude; εp the percolation level(dotted line); εF the Fermi level.

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scale r that depend on the characteristics of random electrostatic field.In heavily doped and strongly compensated semiconductors γ and rdepend on the type of spatial distribution of impurities (correlated oruncorrelated), doping level and degree of compensation [60]. In heavilydoped and fully compensated semiconductors with uncorrelateddistribution of impurities the amplitude γ of potential relief is equal toEg/2 (Fig. 9). Potential relief leads to appearance of regions with freecharge carriers in the potential relief valleys and dielectric regionscontaining no charge carriers. In this case, conductivity is percolation-type and is characterized by percolation level εр [60].

At high temperatures (Т N 250 K) conductivity of Ge films (that isrealized by thermionic emission of charge carriers from the Fermilevel εF to the percolation level εр) is characterized by activation energyε1=|εp−εF|. It should also be taken into account that, in the presence ofa symmetric random potential, the percolation threshold εp depends onthe dimensionality of space [61–65]. This is a general property of allpercolation systems. A three-dimensional (3D) potential relief (d N r)reduces εp (Fig. 9). Only for a two-dimensional (2D) potential reliefd≤ r the percolation threshold εp corresponds to the average value ofE(r) (i.e. EC and EV) and ε1 = |εp − εF| can reach up to half of the Geband gap (Eg / 2).

Indeed for thin Ge films (thickness of about 100nm) potential reliefcharacteristic size r can be equal or greater than the film thickness[25–27]. In this case the potential relief is two-dimensional and 2Dpercolation is observed. In such films, that are fully compensated,temperature dependence of conductivity is exponential with activationenergy close or equal to Eg / 2.

We observed p→ n inversion of Hall coefficient RH in the films astemperature increased (Fig. 8). This shows that low deposition ratefilms have high degree of compensation. The inversion temperature Tiis ~310K. The sign of RH corresponded to n-type (p-type) conductivityat T N Ti (T b Ti) (Fig. 8). Such inversion indicates strong compensationand bipolar conductivity in the films. Indeed, a high degree of com-pensation (the Fermi level is in the midgap) and fluctuations of theelectrostatic potential lead to two independent conductivity channels.At relatively high temperatures, conductivity is realized by bothelectrons and holes thermally transferred to their percolation levelswhich are spaced by potential barriers, i.e. conductivity is bipolar. Inthis case, one should take into account contribution to kinetic effectsfrom both electrons and holes. It is known that different charge carrierscontribute differently to the kinetic coefficients.

Inversion of the Hall coefficient in Ge films indicates predominanceof electrons over holes as temperature increases, i.e. transition tointrinsic conductivity. Low inversion temperature (Ti=310K) indicatesstrong compensation of the films. A relatively high concentration ofintrinsic charge carriers ni (1015–1016 cm−3) as compared with that inpure intrinsic i-Ge (ni=1014cm−3, Ref. [66]) results from the presenceof deep tails of the density of states in the semiconductor band gap

which leads to an increase in the number of intrinsic charge carriers inthe semiconductor.

It should also be noted that transversemagnetoresistance in the filmsincreases strongly with temperature (Fig. 8). The magnetoresistanceeffect may reflect the behavior of mobility of the free charge carriers.The increase of the magnetoresistance with temperature could beexplained by reduction of free charge carriers scattering on fluctuationsof random electrostatic potential as well as by reduction of probabilityof their capture to localized states near the percolation level. This alsofollows from the presence of a potential relief and compositionaldisorder in the single crystal Ge films under investigation.

As to theGefilms grownat high deposition rate, there is practically nocompositional disorder in them, the amplitude of electrostatic potentialfluctuations is low (γ b kT) and the films are heavily doped and slightlycompensated. Therefore, their conductivity and magnetoresistance donot depend on temperature.

5. Conclusion

We showed that growth rate strongly affects the properties of Gefilms on GaAs. This effect is essential and should be taken into accountwhen developing and producing devices based on the Ge/GaAsheterostructure.

All the Ge films studied in this work have single crystal structuresand coherent, dislocation free film–substrate interface. The filmsprepared at high deposition rate have smooth surfaces (with RMS ofabout 0.5–0.9 nm) and this roughness does not depend on the filmthickness (above film thickness of about 50 nm). In contrast, at lowgrowth rate film roughness is large and thickness dependent. The RMSincreases monotonously with film thickness and can reach up to27nm in the film of 250nm thick.

We suggest that such surface corrugation of the layer provides amechanism for intrinsic stress relaxation in the stressed dislocation-free single crystal Ge-on-GaAs films grown at low deposition rates.Indeed, we also found that the intrinsic stresses strongly depend onthe rate of film growth; they decrease as deposition rate is lowered.Depending on thickness of Ge epitaxial films, the crystallographicplanes {113} and {227} are predominant for their corrugated surfaces.

The transport phenomena in Ge films obtained at low and highdeposition rate differ drastically. The Ge films obtained at highdeposition rate are n-type, low-resistant (0.1–0.01 Ω⋅cm), heavilydoped and slightly compensated. Temperature dependence of conduc-tivity in these films is weak or practically absent. The films obtained atlow deposition rate are p-type, highly-resistant (~100Ω⋅cm), heavilydoped, and strongly compensated; in the limiting case fully com-pensated. Their conductivity is thermally activated. The activationenergy of conductivity can reach up to half of the Ge band gap. Theproperties of such films have been explained in the context of large-scale potential fluctuations. Although such films are single crystal,their conductivity is of percolation-type. The assumption of 2D potentialrelief and 2D percolation in the extremely compensated thin (~100nm)films enabled us to explain consistently the experimental resultsobtained.

Acknowledgments

This work was partially funded under the Ukrainian State Program“Nanotechnologies and nanomaterials”, Project Nos. 2.2.6.15 and3.6.1.10. V.K.L. was supported by a Royal Academy of Engineeringfellowship grant.

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