Recent advances in gate dielectrics and polarized light emission from GaN

 

S.J.Pearton(1),C.R.Abernathy(2),B.P.Gila(1),A.H.Onstine(1),M.E.Overberg(1),G.T.Thaler(1),

Jihyun Kim(2),B.Luo(2),R.Mehandru(2), F.Ren(2) and Y.D.Park(3)

 

1. Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA

2. Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA

3. Center for Strongly Correlated Materials Research, Seoul National University, Seoul, 151-747, Korea

KEYWORDS:GaN,magnetic semiconductors,gate dielectrics

Abstract

Inversion behavior has been demonstrated in gate-controlled p-GaN diodes using both MgO and Sc2O3 gate dielectrics and implanted n+ regions to provide a source of inversion charge.  The total surface state density was estimated from capacitance-voltage or charge pumping measurements to be in the range 3-8x1012 cm-2 after the implant activation annealing to form the source and drain regions. In addition, Mn doping of GaN during growth by Molecular Beam Epitaxy is found to produce room temperature ferromagnetism under conditions where the material remains single-phase. The layers can be used as injectors of spin-polarized carriers into light-emitting diode structures, with the potential for creating polarized optical output.

1.Introduction

      There is significant interest in developing gate dielectrics for GaN, which allow for metal-insulator-semiconductor(MIS) or metal-oxide-semiconductor(MOS) controlled transistors[1-13]. These would have numerous advantages over metal gate devices, including  typically better thermal stability and lower gate leakage. A further advantage of MOS or MIS GaN-based transistors would be the possibility of also using the gate oxide/insulator material as a surface passivation film.  Current state-of –the-art AlGaN/GaN HEMTs generally show a decrease in drain current at high drain-source voltage[14]. This collapse of the current produces a severe degradation of the rf output power. One of the main mechanisms for the phenomena is the presence of surface states in the region between the drain and source. The use of SiNx passivation layers on top of the AlGaN/GaN structure helps to partially mitigate the current collapse[14].

We have also found that oxides such as Sc2O3 and MgO are effective in reducing the effect of surface states on AlGaN/GaN HEMTs and, in addition, exhibit promise as gate dielectrics in GaN[15-17].In particular, the Sc2O3 produces more stable passivation of the GaN surface than MgO. The Sc2O3 had a bixbyite crystal structure, with a 9.2% lattice mismatch to GaN, a high dielectric constant(14) and  reasonable bandgap(6.3eV). In this letter we report on the capacitance-voltage(C-V) characteristics of gate-controlled Sc2O3/GaN diodes. The total number of surface states was estimated from the voltage shift of the C-V curves and found to be 8.2x1012 cm-2.

          The field of semiconductor spin transfer electronics (spintronics) seeks to exploit the spin of charge carriers in semiconductors. It is widely expected that new functionalities for electronics and photonics can be achieved if the injection, transfer and detection of carrier spin can be controlled above room temperature.  Since the magnetic properties of ferromagnetic semiconductors are a function of carrier concentration in the material in many cases, then it will be possible to have electrically or optically-controlled magnetism through field-gating of transistor structures or optical excitation to alter the carrier density. The development of magnetic semiconductors with practical ordering temperatures could lead to new classes of device and circuits, including spin transistors, ultra-dense non-volatile semiconductor memory and optical emitters with polarized output.In this paper we will also detail the achievement of room temperature ferromagnetism in (Ga,Mn)N and its utilization in light-emitting diode(LED) structures.

2.EXPERIMENTAL

The starting sample for the inversion experiments was a 1mm thick p-GaN (hole concentration is ~ 2x1017 cm-3 at 25oC) layer grown on a 2 mm undoped GaN buffer grown on a Al2O3 substrate by Metal Organic Chemical Vapor Deposition. The Sc2O3 was deposited by rf plasma-activated Molecular Beam Epitaxy(MBE) at 650oC using elemental Sc evaporated from a standard effusion cell at 1130 oC and O2 derived from an electron cyclotron plasma source operating at 200W forward power(2.45GHz) and 10-4 Torr. More details of the oxide growth and pre-cleaning of the GaN surface have been given previously[15,16]. The Sc2O3 layers were ~400Ĺ thick. N+ regions were created by selective implantation of Si+ at multiple energies and doses (70keV, 2x1013cm2, 195 keV, 6x1013cm-2 and 380 keV, 1.8x1014cm-2). The junction depth was ~0.4 mm from ion range simulations.  The samples were then annealed at 950oC under N2 to activate the Si-implanted regions. Windows were etched into the oxide and e-beam deposited p-ohmic(Ni/Au), n-ohmic(Ti/Al/Pt/Au) and gate metal(Pt/Au) were patterned by lithography. The separation of the n+ regions was ~ 60mm.

3.RESULTS AND DISCUSSION

3.1 Gate-controlled diodes

A Sc2O3/p-GaN MOS diode without the n+ regions to provide an external source of minority carriers cannot remain in thermal equilibrium as the applied gate bias is swept from negative to positive polarity because of the extremely low minority carrier generation rate in the wide bandgap (3.4eV) GaN.  Only deep-depletion characteristics were observed up to a measurement temperature of 300oC. Achievement of inversion was not possible due to the low concentration of thermally generated electrons.

Figure 1 shows the C-VG characteristics for various biases on the n+p junction diode. There are several key features of the data. Firstly, a clear inversion behavior is observed as electrons supplied from the n+ regions are drawn into the gate. Secondly, when the n+p junction is reversed-biased, the onset of inversion is delayed, as demonstrated by the threshold voltage shift apparent in the C-V curves. This is the classical behavior experienced for a gate-controlled MOS diode[18]. Thirdly, the presence of a hook region in the C-V curve is caused by a surface-state-induced barrier against electron flow at the boundary of the MOS diode. This type of behavior has been reported previously for both SiC[19] and SiC[20] gate-controlled diodes and results from the nonequilibrium characteristics of deep level surface traps on the semiconductor. The total concentration of surface states, NSS can be obtained from the relation[21].

NSS ~ (ΔV.C0)/e

 where DV is the voltage difference between the two minima in the characteristic obtained with a grounded    n+ region, Co is the Sc2O3 capacitance under the gate and e is the electronic charge.  Future work will be able to establish the role of the implant activation anneal in this trap density for GaN MOS diodes.

Inversion was obtained at 25oC for a measurement frequency of 150 kHz, while the accumulation capacitance exhibits a strong dependence of frequency with roughly a factor of two difference when the frequency is decreased from 1 MHz to 150 kHz.  To explain the frequency-dependence of capacitance data, we need to invoke the presence of an interfacial layer between the stoichiometric Sc2O3 and p-GaN that produces an impedance contribution.  The resistance of the interfacial layer is assumed to decrease with frequency due to an increase in hopping conductance and this produces the observed decrease in accumulation capacitance with increasing measurement frequency[22]. Once again, we are investigating the effect of the implanted activation anneal on the resulting C-V characteristics and the extent of any interfacial layers.

     Figure 2 shows the measured charge-pumping current(ICP) as a function of gate pulse frequency.  The current increases linearly with this frequency beyond 100Hz where leakage effects become negligible.  The linear increase in ICP at higher frequencies confirms the so-called charge-pumping action in which a fixed charge is measured for each gate pulse.  This charge is injected across the n+p junctions in the gated diode.  The surface-state density, NSS, can be related to the pulse frequency u and charge-pumping current ICP through the relation

                                               

where AC is the diode channel area and e is the electronic charge.  From the frequency-dependence of the charge pumping current data, we obtain NSS = 3 ´ 1012 cm-2 at the Sc2O3/p-GaN interface.  This surface trap density is comparable to that reported previously for SiO2/SiC gated diodes that had been prepared with a similar implantation process cycle[20].  Note that no effort has been made in our structures to passivate surface states through forming gas anneals or plasma hydrogenation.  This is the first measurement of surface state density in GaN-based MOS structures that more fully approximate a transistor, i.e. the structures show inversion and have gone through an extensive device processing sequence[23,24].  Note that we achieved similar results for MgO/p-GaN diodes[25,26].

3.2 Ferromagnetism in GaN and application to spin-LEDs

             There are two major criteria for selecting the most promising materials for semiconductor spintronics.  First, the ferromagnetism should be retained to practical temperatures (i.e. >300 K).  Second, it would be a major advantage if there were already an existing technology base for the material in other applications.  Most of the work in the past has focused on (Ga,Mn)As and (In,Mn)As. However, the highest Curie temperatures reported for these materials (~110 K for (Ga,Mn)As[27] and ~35 for (In,Mn)As[28]) are too low for most practical applications. 

            The key breakthrough that focused attention on wide bandgap semiconductors as being the most promising for achieving practical ordering temperatures was the theoretical work of Dietl et al[29].  They predicted that cubic GaN doped with ~5at.% of Mn and containing a high concentration of holes (3.5x1020 cm-3) should exhibit a Curie temperature exceeding room temperature.

Two basic approaches to understand the magnetic properties of dilute magnetic semiconductors have emerged.  The first class of approaches is based on mean-field theory. The theories that fall into this general model implicitly assume that the dilute magnetic semiconductor is a more-or-less random alloy, e.g. (Ga,Mn)N, in which Mn substitutes for one of the lattice constituents.  The second class of approaches suggests that the magnetic atoms form small (a few atoms) clusters that produce the observed ferromagnetism[30].  A difficulty in experimentally verifying the mechanism responsible for the observed magnetic properties is that depending on the growth conditions employed for growing the DMS material.  It is likely that one could readily produce samples that span the entire spectrum of possibilities from single-phase random alloys to nanoclusters of the magnetic atoms to precipitates and second phase formation.  Therefore, it is necessary to decide on a case-by-case basis which mechanism is applicable.

            The mean field approach basically assumes that the ferromagnetism occurs through interactions between the local moments of the Mn atoms, which are mediated by free holes in the material.  The spin-spin coupling is also assumed to be a long-range interaction, allowing use of a mean-field approximation{31-36]. In its basic form, this model employs a virtual-crystal approximation to calculate the effective spin-density due to the Mn ion distribution.  The direct Mn-Mn interactions are antiferromagnetic so that the Curie temperature, TC, for a given material with a specific Mn concentration and hole density (derived from Mn acceptors and/or intentional shallow level acceptor doping), is determined by a competition between the ferromagnetic and anti-ferromagnetic interactions. 

Initial reports of the energy level of Mn in GaN show it is very deep in the gap, EV + 1.4 eV[37], and thus would be an ineffective dopant under most conditions.  Some strategies for enhancing the hole concentration do exist, such as co-doping both acceptors and donors to reduce self-compensation effects[38] or the use of selectively-doped AlGaN/GaN superlattices in which there is transfer of free holes from Mg acceptors in the AlGaN barriers to the GaN quantum wells.

            A further issue that needs additional exploration in the theories is the role of electrons, rather than holes, in stabilizing the ferromagnetism in DMS materials.  All of the reports of ferromagnetism in (Ga,Mn)N, for example, occur for material that is actually n-type.  Since the material has to be grown at relatively low temperatures to avoid Mn precipitation and therefore only Molecular Beam Epitaxy (MBE) can be used, there is always the possibility of unintentional n-type doping from nitrogen vacancies, residual lattice defects or impurities such as oxygen.  Therefore stoichiometry effects, crystal defects or unintentional impurities may control the final conductivity, rather than Mn or the intentionally-introduced acceptor dopants. 

      While most of the theoretical work for DMS materials has focused on the use of Mn as the magnetic dopant, there has been some progress on identifying other transition metal atoms that may be effective. The predicted stability of ferromagnetic states in GaN doped with different 3d transition metal atoms found that (Ga,V)N and (Ga,Cr)N showed stable ferromagnetism for all transition metal concentrations whereas Fe, Co or Ni doping produced spin-glass ground states[39].  For the case of Mn, the ferromagnetic state was the lowest energy state for concentrations up to ~20%, whereas the spin-glass state became the most stable at higher Mn concentrations.

In epitaxial GaN layers grown on sapphire substrates and then subjected to solid state diffusion of Mn at temperatures from 250-800°C for various periods, clear signatures of room temperature ferromagnetism were observed[40,41]. The Curie temperature was found to be in the range 220-370K, depending on the diffusion conditions.  The use of ion implantation to introduce the Mn produced lower magnetic ordering temperatures[42].

            In (Ga,Mn)N films grown by MBE at temperatures between 580-720°C with Mn contents of 6-9 at.%, magnetization (M) versus magnetic field (H) curves showed clear hysteresis at 300K, with coercivities of 52-85 Oe and residual magnetizations of 0.08-0.77 emu/g at this temperature[43]. The temperature dependence of the magnetization for a sample with 9 at.% Mn, yielded an estimated TC of 940K using a mean field approximation.  Note that while the electrical properties of the samples were not measured, they were almost certainly n-type..  Room temperature ferromagnetism in n-type (Ga,Mn)N grown by MBE has also been reported by Thaler et al.[44]. Figure 3 shows M-T and M-H data as a function of Mn content in the GaMnN. In that case, strenuous efforts were made to exclude any possible contribution from the sample holder in the superconducting quantum interference device (SQUID) magnetometer or other spurious effects.  It is also worthwhile to point out that for the studies of (Ga,Mn)N showing ferromagnetic ordering by magnetization measurements, a number of materials characterization techniques did not show the presence of any second ferromagnetic phases within detectable limits.  In addition, the values of the measured coercivities are relatively small. Figure 4 (top) shows a spin-LED structure, in which the top n-contact layer is ferromagnetic GaMnN.  When bias is applied to the device, it operates as an efficient blue LED(bottom).  We are currently determining the temperature dependence of the spin injection efficiency in these structures.

ACKNOWLEDGEMENTS

 The work at UF is partially supported by ONR(H. B. Dietrich, N00014-1-02-04) and NSF(DMR0101438 and CTS 9901173). The work at SNU was partially supported by KOSEF and Samsung Electronics Endowment through CSCMR.

 

 

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Figure Captions

Figure 1.           C-V characteristics of Sc2O3/p-GaN gated diodes as a function of drain bias at 25oC in the dark. The voltage ramp rate was 0.05 V×s-1 and measurement frequency 150kHz.

Figure 2.        Charge-pumping current versus gate pulse frequency.

Figure 3.     (a) B-H at 300 K from MBE-grown (Ga,Mn)N with ~9 at.% Mn (closed circles).  The data from the sapphire substrate is shown as open circles.  (b) Magnetization as a function of temperature for the MBE-grown (Ga,Mn)N with ~9 at.% Mn.  (c)  B-H at 300 K from MBE-grown (Ga,Mn)N with various Mn concentrations.

Figure 4.  (Ga,Mn)N/InGaN/GaN light-emitting diode structure (top) and spectral output from the device (bottom).