SpringerOpen Newsletter

Receive periodic news and updates relating to SpringerOpen.

This article is part of the series International Conference on Superlattices, Nanostructures, and Nanodevices (ICSNN 2012).

Open Access Nano Express

The role of dislocation-induced scattering in electronic transport in GaxIn1-xN alloys

Omer Donmez1, Mustafa Gunes1, Ayse Erol1, Cetin M Arikan1*, Naci Balkan2 and William J Schaff3

Author Affiliations

1 Science Faculty, Department of Physics, Istanbul University, Vezneciler, Istanbul 34134, Turkey

2 School of Computer Science and Electronic Engineering, University of Essex, Colchester, Essex CO4 3SQ, United Kingdom

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

For all author emails, please log on.

Nanoscale Research Letters 2012, 7:490  doi:10.1186/1556-276X-7-490

The electronic version of this article is the complete one and can be found online at: http://www.nanoscalereslett.com/content/7/1/490


Received:16 July 2012
Accepted:21 August 2012
Published:31 August 2012

© 2012 Donmez et al.; licensee Springer.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Electronic transport in unintentionally doped GaxIn1-xN alloys with various Ga concentrations (x = 0.06, 0.32 and 0.52) is studied. Hall effect measurements are performed at temperatures between 77 and 300 K. Temperature dependence of carrier mobility is analysed by an analytical formula based on two-dimensional degenerate statistics by taking into account all major scattering mechanisms for a two-dimensional electron gas confined in a triangular quantum well between GaxIn1-xN epilayer and GaN buffer. Experimental results show that as the Ga concentration increases, mobility not only decreases drastically but also becomes less temperature dependent. Carrier density is almost temperature independent and tends to increase with increasing Ga concentration. The weak temperature dependence of the mobility may be attributed to screening of polar optical phonon scattering at high temperatures by the high free carrier concentration, which is at the order of 1014 cm−2. In our analytical model, the dislocation density is used as an adjustable parameter for the best fit to the experimental results. Our results reveal that in the samples with lower Ga compositions and carrier concentrations, alloy and interface roughness scattering are the dominant scattering mechanisms at low temperatures, while at high temperatures, optical phonon scattering is the dominant mechanism. In the samples with higher Ga compositions and carrier concentrations, however, dislocation scattering becomes more significant and suppresses the effect of longitudinal optical phonon scattering at high temperatures, leading to an almost temperature-independent behaviour.

Keywords:
GaxIn1-xN; In-rich GaxIn1-xN; Mobility; Electronic transport; 72.10.Fk; 72.20.Fr

Background

In the last decade, after the revision of the band gap energy from 1.9 to approximately 0.7 eV [1], intensive research has been carried out on InN and In-rich GaxIn1-xN alloys in order to re-determine the fundamental properties [2-4]. Despite much interest on the optical properties of InN and GaxIn1-xN [5,6], there has been a relatively small number of investigations to explain temperature-dependent electronic transport properties in GaxIn1-xN alloys [7,8].

In this article, we report the electronic transport properties of nominally undoped GaxIn1-xN alloys with different Ga concentrations (x = 0.06, 0.32 and 0.52). Hall effect results show that all the alloys are highly n-type, and the free carrier concentrations are independent of temperature.

Methods

Experimental details

The samples with different Ga concentrations (x = 0.06, 0.32 and 0.52) were grown by a Varian GEN-II gas source molecular beam epitaxy chamber on (0001) c-sapphire substrates with a 200-nm-thick GaN buffer layer. The growth temperature was varied from low to high with increasing Ga composition [9,10]. The thickness of the GaxIn1-xN layer was determined from the growth parameters and verified by backscattering spectrometry at nearly 500 nm. The GaxIn1-xN samples were fabricated in Hall-bar geometry, and ohmic contacts were formed by diffusing Au/Ni alloy. Hall effect measurements were carried out at temperatures between 77 and 300 K.

Modelling of carrier mobility

The temperature dependence of carrier mobility is analysed using an analytic model where all possible scattering mechanisms are individually calculated using the material parameters given in Table 1. Experimental mobility curves are fitted with the theoretical mobility curves that are obtained using the analytical expressions for the major scattering mechanisms given in Table 2. Although GaxIn1-xN layer is thick enough (500 nm) not to be two-dimensional (2D), the analytic model considers transport in a 2D electron gas (2DEG). This is because the electronic transport takes place at the interface of GaxIn1-xN/GaN [11] and on 2D GaxIn1-xN surface layer [12].

Table 1. The material parameters used in scattering calculations (adopted from [[10],[13-15]])

Table 2. The formulas of major scattering mechanisms used in 2DEG mobility calculations

Results and discussions

Experimental results

Figure 1 shows the temperature dependence of the carrier concentration and the electron mobility between 77 and 300 K for all the samples investigated. Although the samples are not intentionally doped, the Hall effect results show that all the samples have n-type conductivity, and the free carrier densities are independent of the temperature; therefore, samples can be regarded as metallic-like over the whole temperature range as commonly reported by us and by other research groups [7,8,24-28]. It is clear from Figure 1a that the free carrier concentration increases by about a factor of 3 when the Ga composition increases from x = 0.06 to 0.52. Also, as seen in Figure 1b, when Ga concentration increases from x = 0.06 to 0.52, electron mobility has a sharp decrease from 1,035 cm2/Vs for Ga0.06In0.94 N to 30 cm2/Vs for Ga0.52In0.48 N at 77 K that may be associated with the contribution of both dislocations and point defects in the structure, which are acting as a source of donor-like defects, inducing high electron concentration. In the low-temperature region (≤100 K), the mobility is almost independent of temperature for all the samples. However, for the sample with the lowest Ga concentration, Ga0.06In0.94 N, it decreases from 1,035 to 890 cm2/Vs with increasing temperature from 100 to 300 K but does not show any significant change in the other two samples, which is a characteristic feature of metallic-like semiconductors [7,26,27]. The insensitivity of carrier mobility to temperature is commonly observed in polar materials with elevated carrier densities where the polar interactions are screened [19,25,29-33].

thumbnailFigure 1. Temperature dependence of (a) carrier density and (b) electron mobility.

Modelling of temperature dependence of mobility

In order to understand fully the temperature dependence of electron mobility, we compared the experimental mobility results with analytical theoretical models by taking into account all the possible scattering mechanisms. At low temperatures, the dominant scattering mechanism in bulk semiconductors is ionized impurity scattering that changes with temperature as T3/2. However, this kind of temperature dependence has not been observed in our samples. The samples have metallic-like characteristics, confirming the formation of a high-density 2DEG at both the GaN/GaxIn1-xN interface and on the GaxIn1-xN surface [26,27]. The dominant momentum relaxation mechanism is the electron-optical phonon scattering in GaxIn1-xN since it is a highly polar material above T > 150 K [34-36].

In the theoretical calculation, interface roughness, alloy, dislocation, optical and acoustic phonon scattering mechanisms with the appropriate expressions given in Table 2 were considered. The lateral size of the interface roughness Δ, correlation length Λ between interface fluctuations and the dislocation density are used as adjustable fitting parameters, and the values for the best fit are given in Table 3. The values that we used for the dislocation densities are in good agreement with the transmission electron microscopy (TEM) results taken from Ga0.34In0.66 N [9,25]. Look et al. [25] determined the dislocation density for both InN and Ga0.34In0.66 N using TEM and found that dislocation density in Ga0.34In0.66 N is actually higher than that of InN. It can be seen that the trend of the dislocation density depending on Ga concentration follows the carrier concentration, which means that there is a correlation between dislocation density and the corresponding carrier concentration.

Table 3. The values of the parameters used in the calculations

It is clear from Figure 2 that at low temperatures, electron mobilities in Ga0.06In0.94 N and Ga0.32In0.68 N are determined by alloy potential-induced scattering, interface roughness scattering and dislocation scattering mechanisms. Optical phonon scatterings become significant at high temperatures, as described above. Figure 3 shows experimental and calculated temperature-dependent mobility of the Ga0.52In0.48 N. The dislocation density increases with Ga concentration; therefore, its effect on the mobility becomes more pronounced in this sample. At low temperatures, mobility is limited by the same scattering mechanisms as in the other samples. At high temperatures, however, interface roughness and alloy potential restrict the mobility, but effect of the dislocation scattering becomes less dominant as a result of shortening Debye screening length due to higher carrier density. Furthermore, in the high-carrier-concentration regime, electron–phonon scattering is heavily screened, as described above and in references [19,25,29-33].

thumbnailFigure 2. Experimental and calculated temperature dependence of mobility curves for (a) Ga0.06In0.94N and (b) Ga0.32In0.68N.

thumbnailFigure 3. Measured and calculated mobility versus temperature Ga0.52In0.48N.

Conclusions

In this paper, we have investigated electronic transport properties of nominally undoped In-rich GaxIn1-xN structures with different Ga concentrations. Hall effect results show that 2DEG mobility in GaxIn1-xN decreases and becomes temperature insensitive with increasing Ga concentrations. The samples are not intentionally doped, but they all have n-type conductivity. Electron density increases with increasing Ga composition. The temperature dependence of electron mobility is determined by taking into account all the major scattering mechanisms. The decrease of the electron mobility with Ga concentration is explained in terms of increased dislocation scattering. The weak temperature dependence of the mobility at high temperatures might be associated with reduced electron-optical phonon scatterings. Alloy and interface roughness scattering mechanisms are dominant at low temperatures. In samples with higher Ga fractions, dislocation scattering becomes more significant, and at high temperatures, phonon scattering is restricted due to increase of dislocation density. At high temperatures, phonon scattering is only pronounced in the samples with low electron densities.

Abbreviations

LO-phonon: longitudinal optical phonon; LA-phonon: longitudinal acoustic phonon; 2DEG: two-dimensional electron gas; TEM: transmission electron microscopy; IFR: interface roughness.

Competing interests

The authors declare that they have no competing interest.

Authors' contributions

OD and MG carried out the experiments and fitted the Hall mobility data with AE and MCA. OD, MG, AE and MCA wrote the manuscript in conjunction with NB. WJS grew the investigated samples. All authors read and approved the final manuscript.

Acknowledgments

This work was supported by Scientific Projects Coordination Unit of Istanbul University with Project Number BYP 25027. We also acknowledge the partial support from Republic of Turkey, Ministry of Development. (Project Number: 2010 K121050).

References

  1. Wu J, Walukiewicz W, Yu KM, Ager JW III, Aller EE, Lu H, Schaff WJ, Saito Y, Nanishi N: Unusual properties of the fundamental band gap of InN.

    Appl Phys Lett 2002, 80:3967-3969. Publisher Full Text OpenURL

  2. Wu J, Walukiewicz W: Band gaps of InN and group III nitride alloys.

    Superlattices Microstruct 2003, 34:63-75. Publisher Full Text OpenURL

  3. Bechstedt F, Furthmüller J, Ferhat M, Teles LK, Scolfaro LMR, Leite JR, Davydov VY, Ambacher O, Goldhahn R: Energy gap and optical properties of InxGa1 –xN.

    Phys Status Solidi A 2003, 195:628-633. Publisher Full Text OpenURL

  4. Monemar B, Paskova PP, Kasic A: Optical properties of InN—the bandgap question.

    Superlattices Microstruct 2005, 38:38-56. Publisher Full Text OpenURL

  5. Walukiewicz W, Li SX, Wu J, Yu KM, Ager JW III, Haller EE, Lu H, Schaff WJ: Optical properties and electronic structure of InN and In-rich group III-nitride alloys.

    J Cryst Growth 2004, 269:119-127. Publisher Full Text OpenURL

  6. Hsu L, Jones RE, Li SX, Yu KM, Walukiewicz W: Electron mobility in InN and III-N alloys.

    J Appl Phys 2007, 102:073705-073710. Publisher Full Text OpenURL

  7. Lin SK, Wu KT, Huang CP, Liang CT, Chang YH, Chen YF, Chang PH, Chen NC, Chang CA, Peng HC, Shih CF, Liu KS, Lin TY: Electron transport in In-rich InxGa1 –xN films.

    J Appl Phys 2005, 97:046101. Publisher Full Text OpenURL

  8. Gunes M, Balkan N, Zanato D, Schaff WJ: A comparative study of electrical and optical properties of InN and In0.48 Ga0.52N.

    Microelectron J 2009, 40:872-874. Publisher Full Text OpenURL

  9. Liliental-Weber Z, Zakharov DN, Yu KM, Ager JW III, Walukiewicz W, Haller EE, Lu H, Schaff WJ: Compositional modulation in InxGa1−xN: TEM and X-ray studies.

    J Electron Microsc 2005, 54:243-250. Publisher Full Text OpenURL

  10. Tiras E, Gunes M, Balkan N, Schaff WJ: In rich In1−xGaxN: composition dependence of longitudinal optical phonon energy.

    Phys Status Solidi B 2010, 247:189-193. Publisher Full Text OpenURL

  11. Zanato D, Gokden S, Balkan N, Ridley BK, Schaff WJ: The effect of interface-roughness and dislocation scattering on low temperature mobility of 2D electron gas in GaN/AlGaN.

    Semicond Sci Technol 2004, 19:427-432. Publisher Full Text OpenURL

  12. Veal TD, Piper LFJ, Phillips MR, Zareie MH, Lu H, Schaff WJ, McConville CF: Scanning tunnelling spectroscopy of quantized electron accumulation at InxGa1−xN surfaces.

    Phys Status Solidi A 2006, 203:85-92. Publisher Full Text OpenURL

  13. Morkoc H: Carrier Transport. Handbook of Nitride Semiconductors and Devices. Weinheim: Wiley; 2008:165-395. OpenURL

  14. Levinshtein M, Rumyantsev S, Shur M: Properties of Advanced SemiconductorMaterials: GaN, AlN, InN, BN, SiC, SiGe. Canada: Wiley; 2001. OpenURL

  15. Ridley BK, Foutz BE, Eastman LF: Mobility of electrons in bulk GaN and AlxGa1 −xN/GaN heterostructures.

    Phys Rev B 2000, 61:16862-16869. Publisher Full Text OpenURL

  16. Hutson AR: Piezoelectric scattering and phonon drag in ZnO and CdS.

    J Appl Phys 1961, 32:2287-2292. Publisher Full Text OpenURL

  17. Ridley BK: The electron–phonon interaction in quasi-two-dimensional semiconductor quantum-well structures.

    J Phys C: Solid State Phys 1982, 15:5899-5917. Publisher Full Text OpenURL

  18. Hirakawa K, Sakaki H: Mobility of the two-dimensional electron gas at selectively doped n -type AlxGa1-xAs/GaAs heterojunctions with controlled electron concentrations.

    Phy Rev B 1986, 33:8291-8303. Publisher Full Text OpenURL

  19. Sun Y, Balkan N, Aslan M, Lisesivdin SB, Carrere H, Arikan MC, Marie X: Electronic transport in n- and p-type modulation doped GaxIn1−xNyAs1−y/GaAs quantum wells.

    J Phys Condens Matter 2009, 21:174210-174217. PubMed Abstract | Publisher Full Text OpenURL

  20. Kearney MJ, Horrell AI: The effect of alloy scattering on the mobility of holes in a quantum well.

    Semicond Sci Technol 1998, 13:174-180. Publisher Full Text OpenURL

  21. Ng HM, Doppalapudi D, Moustakas TD, Weimann NG, Eastman LF: The role of dislocation scattering in n-type GaN films.

    Appl Phys Lett 1998, 73:821-823. Publisher Full Text OpenURL

  22. Abdel-Motaleb IM, Korotkov RY: Modeling of electron mobility in GaN materials.

    J Appl Phys 2005, 97:093715-093721. Publisher Full Text OpenURL

  23. Kundu J, Sarkar CK, Mallick PS: Calculation of electron mobility and effect of dislocation scattering in GaN.

    Semicond Phys, Quantum Elect & Optoelect 2007, 10:1-3. OpenURL

  24. Donmez O, Yilmaz M, Erol A, Ulug B, Arikan MC, Ulug A, Ajagunna AO, Iliopoulos E, Georgakilas A: Influence of high electron concentration on band gap and effective electron mass of InN.

    Phys Status Solidi B 2011, 248:1172-1175. Publisher Full Text OpenURL

  25. Look DC, Lu H, Schaff WJ, Jasinski J, Liliental-Weber Z: Donor and acceptor concentrations in degenerate InN.

    Appl Phys Lett 2002, 80:258-261. Publisher Full Text OpenURL

  26. Wang CX, Tsubaki K, Kobayashi N, Makimoto T, Maeda N: Electron transport properties in AlGaN/InGaN/GaN double heterostructures grown by metalorganic vapor phase epitaxy.

    Appl Phys Lett 2004, 84:2313-2315. Publisher Full Text OpenURL

  27. Thakur JS, Naik R, Naik VM, Haddad D, Auner GW, Lu H, Schaff WJ: Electron transport properties in AlGaN/InGaN/GaN double heterostructures grown by metalorganic vapor phase epitaxy.

    J Appl Phys 2006, 99:023504-023508. Publisher Full Text OpenURL

  28. Donmez O, Gunes M, Erol A, Arikan MC, Balkan N: High carrier concentration induced effects on the bowing parameter and the temperature dependence of the band gap of GaxIn1−xN.

    J Appl Phys 2011, 110:103506-103511. Publisher Full Text OpenURL

  29. Zanato D, Tiras E, Balkan N, Boland-Thoms A, Wah JY, Hill G: Momentum relaxation of electrons in InN.

    Phys Status Solidi C 2005, 2:3077-3081. Publisher Full Text OpenURL

  30. Ridley BK: Quantum Processes in Semiconductors. New York: Oxford University Press; 1999. OpenURL

  31. Sun Y, Vaughan M, Agarwal A, Yilmaz M, Ulug B, Ulug A, Balkan N, Sopanen M, Reentilä O, Mattila M, Fontaine C, Arnoult A: Inhibition of negative differential resistance in modulation-doped n-type GaxIn1−xNyAs1−y/GaAs quantum wells.

    Phys Rev B 2007, 75:205306-205316. OpenURL

  32. Su Y, Wen Y, Hong Y, Lee HM, Gwo S, Lin YT, Tu LW, Lui HL, Sun CK: Using hole screening effect on hole–phonon interaction to estimate hole density in Mg-doped InN.

    Appl Phys Lett 2011, 98:252106-252108. Publisher Full Text OpenURL

  33. Kirillov D, Lee H, Harris JS: Raman scattering study of GaN films.

    J Appl Phys 1996, 80:4058-4062. Publisher Full Text OpenURL

  34. Thomsen M, Jönen H, Rossow U, Hangleiter A: Spontaneous polarization field in polar and nonpolar GaInN/GaN quantum well structures.

    Phys Status Solidi B 2001, 248:627-631. OpenURL

  35. Feneberg M, Thonke K, Wunderer T, Lipski F, Scholz F: Piezoelectric polarization of semipolar and polar GaInN quantum wells grown on strained GaN templates.

    J Appl Phys 2010, 107:103517-103522. Publisher Full Text OpenURL

  36. Lu CJ, Bendersky LA, Lu H, Schaff WJ: Threading dislocations in epitaxial InN thin films grown on (0001) sapphire with a GaN buffer layer.

    Appl Phys Lett 2003, 83:2817-2819. Publisher Full Text OpenURL