VERTICAL AND LATERAL MOBILITIES IN n-(Ga, Mn)N

 

Jihyun Kim and F. Ren

Department of Chemical Engineering

 University of Florida, Gainesville FL 32611

 

G. T. Thaler, R. Frazier, C. R. Abernathy, and S. J. Pearton

Department of Material Science and Engineering

University of Florida, Gainesville FL 32611

 

J. M. Zavada

US Army Research Office

Research Triangle Park, NC 27709

 

R. G. Wilson

Consultant, Stevenson Ranch, CA 91381

 

ABSTRACT

 Lateral electron mobilities in 0.2mm thick n-(Ga, Mn)N films were obtained from Hall measurements, producing values of 116~102 cm2/V×s in the temperature range 298~373K. These values are comparable, but slightly lower, to electron mobilities in n-GaN of the same electron concentration. By sharp contrast ,analysis of the reverse saturation current in mesa Schottky diodes fabricated in the n-(Ga, Mn)N show vertical electron mobilities of 840~336 cm2/V×s in the temperature range 298~373K. This is consistent with a reduction in electron scattering by charged dislocations for vertical transport geometries. [M. Misra, A. V. Sampath, and T. D. Moustakas, Appl. Phys. Lett. 76 1045(2000)].

    Recent theoretical (1-7) and experimental(8-18) results for (Ga, Mn)N and other transition metal-doped nitrides suggest that there are very promising for room temperature spintronics applications. A key requirement for successful realization of spin-based devices is an understanding of the transport properties within the dilute magnetic semiconductor ,since carriers need to be injected from contacts and cross heterointerfaces in order to be collected. Some potential early demonstration devices include spin light-emitting diodes, in which injection and recombination of spin-polarized carriers could lead to polarized light emission(19), and spin transistors in which electron field gating can be used to control the carrier-induced ferromagnetism. Most GaN is still grown heteroepitaxially on lattice-mismatched substrates such as sapphire and therefore contains high concentrations(usually³5´108cm-2 as measured by transmission electron microscopy, TEM) of threading dislocations and other extended defects. Numerous reports have demonstrated the deleterious effort of charged dislocations on the transverse carrier mobility in GaN(20-22). However, vertical devices are much less degraded by the repulsive band bending around dislocations and the directional dependence of the scattering due to the these dislocations because of the greater average distance between defects in the this geometry(20). This has been confirmed by an investigation of vertical and lateral transport in n-GaN films, which showed vertical electron mobilities of ~950 cm2/V×s compared to lateral mobilities of 150~200 cm2/V×s(23).

      In this letter we report an investigation of lateral and vertical electron mobilities in n-(Ga, Mn)N films, using Hall effect and Schottky barrier diodes. The vertical mobilities are factors of 3~8 higher than their lateral counterparts, which is consistent with the reduced effect of scattering by charged dislocations in the vertical direction(23).

      The starting samples were 3mm thick undoped(n~1016cm-3) GaN layers grown by Metal Organic Chemical Vapor Deposition at 1040°C on Al2O3 substrates. The Mn was introduced to a depth of ~0.2mm by direct implantation of 250keV Mn+ ions at a dose of 3´1016cm-2. The samples were held at ~350°C during the implant step in order to avoid amorphization(24), and subsequently annealed at 750°C for 5 mins. Magnetic and structural properties of these types of samples have been reported previously(25). Hall measurements were performed on blanket samples, using Ti/Al/Pt/Au contacts annealed at 500°C under N2. The Pt Schottky diode structures were fabricated using a similar procedure to that described in ref. 26. Capacitance-voltage measurements at 25°C showed an n-type carrier concentration of 3.5´1017cm-3 in the n-(Ga, Mn)N layer. A schematic of the diode structures is shown in Figure 1. Current-voltage(I-V) measurement were performed on the 100mm diameter diodes using a temperature-controlled stage and an HP4145B parameter analyzer.

    The Hall measurements showed (Ga, Mn)N electron mobilities in the range of 102 cm2/V×s at 373K to 116 cm2/V×s at 298K. We believe these values are  close to the true mobility in the (Ga, Mn)N, since measurements made on the GaN prior to Mn implantation showed much higher electron mobilities of ~600 cm2/V×s at 298K and thus if most of the current was flowing in the buffer layer,we expect to measure an effective mobility closer to this value. This latter value of 600 cm2/V.s is similar to that reported for high quality n-type GaN(27-30).

   To obtain the vertical mobilities, temperature dependent I-V measurements were performed in the Schottky diode structures. The I-V characteristics measured from 298~373K are shown in Figure 2. The barrier heights(fB) extracted from the forward part of the I-V characteristics were obtained as reported previously(26), with values ranging from 0.91eV at 298K to 0.88eV at 373K.The saturation current density, JS, can be represented as (31)

 

where A** is the Richardson’s constant for (Ga, Mn)N(26), T is the absolute measurement temperature and k is Boltzmann’s constant. This can also be written in the form(23,32)

where NC is the effective density of states in the conduction band, mVERT is the electron mobility in the perpendicular direction, e is the electron charge, Vbi is the built-in voltage, e is the dielectric constant and Nd is the doping density in the (Ga, Mn)N. From the data in Figure 2, we extracted the values for vertical mobility in the (Ga, Mn)N shown in Table I. Note that these values are factors of ~3-8 higher at a given temperature than the lateral mobilities obtained from the Hall data. As pointed out by Misra et. al., if the active area of the diodes is less than the geometric area then the effective vertical mobilities will be even higher than calculated here(23).

    To give a visual representation of why the lateral mobility is more degraded by threading scattering than is the vertical mobility, Figure 3 shows a cross-sectional TEM micrograph of the (Ga, Mn)N/GaN structure. Threading dislocations originating from the GaN/Al2O3 interface can reach the surface and therefore electrons traveling laterally through the structure encounter scattering from all of these defects. By contrast, for vertical transport, there is a relatively large fraction of undefective material through which electrons can pass with undegraded mobility. The defects remaining in the (Ga, Mn)N are mostly loops, which have only a second-order effect on the electrical properties, as reported previously for implanted GaAs(33).

    To place the experimental data in context, Figure 4, shows these results along with the calculated electron drift mobility of undoped GaN in both lateral and vertical directions and the individual components from the scattering processes (acoustic, polar phonon, and piezoelectric) present(30). While no quantitative conclusions may be drawn, it is clear that the vertical and lateral mobilities are of comparable magnitude to those in material with minimal scattering. In the existing(Ga, Mn)N the vertical electron mobility is relatively unaffected by charged dislocation scattering and gives an indication of the values that it will be possible to achieve for lateral electron mobilites in material synthesized on low defect GaN such as free-standing quasi-substrates(34).

     In summary, the effect of dislocation scattering on electron mobility in (Ga, Mn)N has been examined through a comparison of vertical and lateral transport properties. The vertical electron mobilities are found to be a factor of 3~8 higher than the corresponding lateral mobilities at the same temperature. (Ga, Mn)N-based spintronics devices with vertical geometries will be at an advantage relative to lateral devices.

 

ACKNOWLEDGEMENTS

 The work at UF was partially supported by the US Army Research Office under grants nos. ARO DAAD 19-01-1-0701 and DAAD 19-02-1-0420 and by NSF CTS 991173,ECS-0224203  and DMR 0101458.

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Table I .Transport parameters measured for (Ga, Mn)N

T(K)

fB(eV)

JS(A×cm-2)

mVERT(cm2/Vs)

mLAT(cm2/Vs)

298

0.910

4.227E-8

840

116

323

0.899

6.595E-7

507

108

348

0.881

1.078E-5

401

104

373

0.878

8.429E-5

336

102

 

Figure Captions

Figure 1. Schematic of Schottky diode structures used for vertical transport measurements.

Figure 2. I-V characteristics as a function of temperature from n-(Ga, Mn)N Schottky diodes.

Figure 3. TEM cross-section of (Ga, Mn)N layer formed in GaN by high dose Mn implantation.

Figure 4. Theoretical drift mobilities in pure GaN, along with contribution from the various scattering processes present and the experimentally determined vertical and lateral mobilities for n-(Ga, Mn)N.(after ref. 30)

Fig. 1



Fig. 2

Fig. 3

Fig. 4