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Abstract： The present status of the Gd （gadolinium）Ⅲnitride semiconductor layers grown by plasmaassisted molecular beam epitaxy is described.No phase separation and substitutional incorporation are confirmed by Xray Diffraction and Xray Absorption Fine Structure measurements.Photoluminescence peak energy for the Gddoped InGaN is shifted with InN molar fraction.Clear hysteresis and saturation are observed in the magnetization versus magnetic field curves at room temperature.Si codoping as well as superlattice structures enhances the magnetization.Results are understood with the carriermediated ferromagnetism.Finally，examples of the spintronic semiconductor devices，where the relation between the spinpolarized carriers and the circularpolarized light is used，and the present status to realize such devices are described.
Key words： ⅢNitride Semiconductor； diluted magnetic semiconductors； Gadoliniumdoped； ferromagnetic characteristics； photoluminescence emission； sicodoping； superlattice structure； spintronic semiconductor device
1 Introduction
Diluted magnetic semiconductors （DMSs） have been gathering much interest from the industrial viewpoint because of their potentiality as a new functional material，which will open a way to fabricate novel functional semiconductor devices.For the device application，it is very important that the Curie temperature （Tc） of DMSs should be higher than room temperature.GaMnAs is best investigated and well established DMS[1].However，the Tc for the GaMnAs is still ～ 160 K[2] and is much lower than room temperature.
Theoretical calculations suggested that the transition metaldoped GaN will exhibit room temperature ferromagnetism[3-4].Mndoped GaN was grown by MBE and observed to have high temperature ferromagnetic （FM） characteristics with Tc as high as 940 K[5].Crdoped GaN was also grown by MBE and observed to have high temperature （>400 K） FM characteristics[6] as well as the photoluminescence （PL） emission[7].The observation of PL emission at room temperature is important to fabrication of practical spintronic devices that control charges，photons and spins； this is contrast to GaMnAs，where no PL emission was observed.High temperature FM characteristics were also reported by many other groups[8-9].The observation of FM characteristics was attributed to double exchange interaction by ab initio computations.However，the first principles Monte Carlo simulations suggested that the magnetic exchange interactions in wide bandgap DMSs are effectively short range and the calculated Tc should be low for the low concentrations of magnetic ions and in the absence of delocalized or weakly localized carriers[10].Considering the spinodal decomposition into onedimensional high concentration of magnetic ions （Konbuphase），high temperature FM characteristics were simulated[11]. Rareearth （RE） doped semiconductors are widely studied for the application to photonic devices.However，the first observation of high temperature FM characteristics was reported for the MBEgrown Gd （gadolinium）doped GaN with Tc >400 K similar to GaCrN[12].Since then，extensive studies have been conducted theoretically[13-17] and experimentally on the magnetic semiconductor GaGdN prepared by various techniques，which include MBE[18-20]，metalorganic vapor phase epitaxy （MOVPE）[21] as well as Gd ion implantation into GaN[22-23].In addition，ferromagnetism with incredibly high values of Curie temperature （TC～700 K） was reported in GaGdN films[24].The enhancement of magnetization by Si cocoping was reported[23，25].On the other hand，Dhar，et.al. reported the colossal magnetic moment in the low Gd concentration （1016-1018 cm-3） of GaGdN grown by MBE and explained within the framework of the phenomenological model[26].They also reported the colossal magnetic moment in the low Gd concentration （1016-1018 cm-3） of the Gd implanted GaGdN[27].The origin of the ferromagnetic order was attributed to the polarization of the surrounding host medium，interstitial nitrogen and oxygeninduced or electronmediated ferromagnetism[14].
Examples of the semiconductor spintonic devices are tunnel magnetoresistance （TMR） devices，circular polarized laser diodes （CPLDs） and spin field effect transistors （FETs）.CPLDs are one of important devices to realize the optical communication systems that are resistant to wiretapping.CPLDs can be fabricated by using the DMS as an active layer or a cladding layer.CPLDs are also important devices to construct the information processing systems by the combination of circularpolarized light controlled TMR devices.Bhattacharya，et.al.[28] reported the fabrication of CPLDs by using verticalcavity surfaceemitting laser （VCSEL） structure with GaMnAs DMS layer as spinpolarized carrier （hole） supplying layer and the observation of circularpolarized light at low temperature of 80 K because of low Tc of GaMnAs.To fabricate practical CPLDs，room temperature ferromagnetic DMS is requisite.The Gdand Dysprosium （Dy）doped Ⅲnitride DMSs exhibited the room temperature ferromagnetic characteristics as well as PL emission and conducting characteristics.Therefore，we can expect to fabricate the spintronic devices operating at room temperature.
In this paper，we will describe the experimental results for the Gddoped Ⅲnitride diluted magnetic semiconductors so far reported until now. 2 Growth and structural properties of Gddoped Ⅲnitride semiconductors
Rareearth （RE） doped Ⅲnitride semiconductors and their superlattice （SL） structures were grown on sapphire （0001） substrates and MOVPEgrown GaN （0001） templates after the growth of GaN buffer layer by plasmaassisted molecularbeam epitaxy （MBE）.Elemental Ga，In，Gd and Dy，and plasmaenhanced N2 were used as sources.Growth temperature was 600-700 ℃ for most samples，while the low growth temperature was also conducted to increase the RE atom concentration.Si codoping into （In）GaGdN layers and GaN barrier layers of SL structures were also carried out.
Xray diffraction （XRD） curves for the GaGdN layers with Gd concentration of 0～7.8% are measured[23].No diffraction peaks from secondary phases are observed for the samples with 0～6% Gd concentrations.Only sample with 7.8% Gd concentrations exhibits additional peaks assigned as NaCltype GdN （111） and GdN （222）.Expanded XRD curves near GaGdN （0004） diffraction are investigated.The peak position for the GaGdN shifts to lower angle as Gd concentration increases up to 5.8%，indicating that the lattice constant cin the cdirection increases with the increase of GdN content x，as can be seen in Fig.1.This tendency agrees with the larger atomic diameter of Gd than that of Ga.XRD data show that for the Gd with concentration higher than 6%，the diffraction peak and lattice constant approach again to those of GaN.This indicates that the lattice relaxation occurred for these Gd concentrations.
Xray absorption fine structure （XAFS） measurements were conducted to study the Gd atom substitution of Ga site in GaGdN samples.Radical distribution functions for GaGdN samples calculated with the XAFS data are shown in Fig.2.The first nearest （N） and second nearest （Ga） atom peaks from the Gd atom agree well with those of GaN （middle curve） and are clearly different from those of NaCltype GaN and Gd metal （lower two curves）.This clearly indicates that the most of Gd atoms substitute the Ga sites.The substitutional incorporation of Gd atoms into InGaGdN layer was also confirmed by XAFS.
3 Properties of Gddoped ⅢNitride semiconductors
3.1 Magnetic and optical properties of single layers
Magnetization （M） versus magnetic field （H） curves for the GaGdN sample at 7 K and 300 K are shown in Fig.3[12].The magnetic field is parallel to the sample plane.Hysteresis curves are observed at both temperatures.This indicates that the GaGdN is ferromagnetic at both temperatures.The saturation field was about 2 T and the coercivity Hc was about 0.07 T （70 Oe） at 300 K. Temperature dependence of magnetization is investigated at applied magnetic field of 100 G[12].The magnetization decreases slowly with increasing temperature and remains even at 400 K and there was no discontinuous change in the curve.This indicates that the Curie temperature of GaGdN is higher than 400 K.
To fabricate the long wavelength spincontrolled photonic devices，InGaGdN layers were also grown on MOVPEgrown GaN （0001） template substrates at 400 ℃.PL spectra for the InGaGdN （In：23%，Gd：～1%） sample as a function of temperature are shown in Fig.4.PL emission is clearly observed at all measuring temperatures （10-300 K）.The temperature variation of PL peak energy exhibits Sshaped behavior （redshiftblueshiftredshift） with increasing temperature，which is attributed to localized excitonic emission due to inhomogeneity and carrier localization in InGaGdN alloys with the consideration of the bandtail approach.This temperature variation is similar to those for the InGaN layers without Gddoping.
Magnetization versus magnetic field curves for the InGaGdN singlelayer samples with different Gd concentrations exhibit clear hysteresis and saturation magnetization （MS） at room temperature.Even clear hysteresis can be further observed from the expanded curve of the samples.It shows that the saturation magnetization increases with increased Gd concentration.With the increase of magnetic Gd atoms，the ferromagnetic interaction is increased.
3.2 Effect of low temperature growth
To increase the incorporated Gd concentration without secondary phase formation and to enhance the magnetization，GaGdN layers were grown at low temperature （LT） of 300 ℃ on sapphire （0001） substrates[23].XRD θ2θscan curves for the LTgrown GaGdN samples showed diffraction peaks from sapphire and GaGdN，but no obvious secondary phase such as GdN was detected.XRD peak from the GaGdN layer showed single crystalline characteristics，though the full width at half maximum is about twice of GaGdN layer grown at 700 ℃[23].By LTgrowth，GaGdN layers with Gd concentration as high as 12.5% and no secondary phases were obtained.When the growth was conducted at 700 ℃，the highest Gd concentration is about 5.8%.Radical distribution function obtained by the XAFS measurement clearly showed that the Gd atoms substitute the Ga sites[23].
The GaGdN layers grown at 300 ℃ with different Gd concentrations show room temperature ferromagnetism[23].With increased Gd concentration，magnetization per unit volume is increased.Theoretical calculations[13-14] showed that the spinglass state is stable for the GaGdN without carriers，but electron doping into GaGdN enhances the ferromagnetic interaction in GaGdN.It is possible that the carriers （electrons） coming from some types of defects such as that nitrogen vacancies enhance the ferromagnetism in GaGdN layers.From LTgrown GaGdN samples，PL emission was observed in the photon energy range 2-3.5 eV.Most of emission peaks are related to defects. 3.3 Effect of superlattice structures on magnetic structures
GaGdN/GaN superlattices （SLs） having various GaGdN and GaN layer thicknesses were grown on sapphire （0001） substrates[19，21].Magnetization （M） versus magnetic field （H） measurements for the 230 nmthick GaGdN bulk layer and the GaGdN/GaN SL samples having nearly the same total thickness of GaGdN （Gd concentration：1.2%） showed the clear hysteresis and saturation （ferromagnetic characteristics）.The magnetization for the SL samples is larger than that for bulk sample[19].Furthermore，the thinner each GaGdN QW layer thickness is or the thicker each GaN barrier layer thickness is，a larger magnetic moment per Gd atom was observed as shown in Fig.5[21].In Fig.5（a），the GaN layer thickness is kept constant at 3.8 nm and the GaGdN thickness is varied from 1.1 nm to 2.9 nm.With decreasing the GaGdN layer thickness the magnetic moment per Gd atom is increased.In Fig.5（b），the GaGdN layer thickness is kept constant at 1.3 nm and the GaN thickness is varied from 2.2 nm to 10.8 nm.With increasing the GaN layer thickness the magnetic moment per Gd atom is increased.
It is expected that electron carriers in GaN layers flow into and accumulate in the narrower bandgap GaGdN layers resulting the higher electron concentration （>1018 cm-3） in the thinner GaGdN layers.Especially，the distribution of Gd atoms is inhomogeneous in the layer[26]，so that local areas having higher Gd concentration would have very high electron concentration in the GaGdN layers.The stronger interaction between Gd spins through the electron spins is expected in the thinner GaGdN layer.Therefore，the observed enhanced magnetic moment can be understood with the carriermediated ferromagnetism.
Similar enhanced magnetization was also observed on the InGaGdN/GaN SL samples compared with that for the InGaGdN singlelayer （thickness：100 nm）.Both samples were grown under similar growth conditions except for the sample structure.The magnetic properties of these samples show clear saturation magnetization at about 16 emu/cm3 and 7 emu/cm3 for the SL sample and singlelayer sample，respectively.In the SLs，electron carriers can flow into narrow bandgap InGaGdN well layers from the wider bandgap GaN layers.These increased carrier concentrations improve the magnetization.
However，in the InGaN/GaGdN SL structures the magnetization was decreased.Such observation is indicated in the MHcurves，between the InGaGdN/GaN SL and InGaN/GaGdN SL set of samples measured at room temperature.In the InGaN/GaGdN SL structure，carriers （electrons） flow out of the Gdcontaining GaGdN layers and into the narrow bandgap InGaN layers.So the carrier concentrations in the GaGdN layers are decreased resulting in the suppression of the carriermediated magnetization. 3.4 Si codoping effect on magnetic and electric properties
Si codoped GaGdN layers were grown at 300℃.Figure 6 shows the room temperature MHcurves for the Sidoped and undoped GaGdN samples with Gd concentration of 8.9%[26].Clear hysteresis was observed for both samples.The saturation magnetization of Sidoped GaGdN，about 1046 emu/cm3，is 7 times as large as that of undoped GaGdN.Codoping with Gd and Si is expected to increase the shallow donor concentration and to enhance the ferromagnetic interaction.The enhancement of magnetization by Si codoping suggests the carriermediated ferromagnetism.Similar effect was also observed for the InGaGdN samples.The magnetization versus magnetic field （MH） curves of the InGaGdN sample and Si codoped InGaGdN sample show clear hysteresis and saturation characteristics measured at 300 K.With the increase of Si cell temperature （that is the increase of Si concentration），the increase of the saturation magnetization was observed.The increase in magnetization for the Si codoped sample is closely related to carriermediated ferromagnetism，that is，the Si codoping increases the electron concentrations in InGaGdN layers as also inferred in the Si codoped GaGdN samples and GaGdN/GaN superlattice samples.
Moreover，Si doping into wide bandgap GaN barrier layers in the InGaGdN/GaN SL structures was found to increase the magnetization as clearly observed from the room temperature MHcurves.One reason that could induce the enhancement of magnetization is also the carriermediated ferromagnetism.
4 Spintronic device application
CPLED （spin LED） was fabricated by using MOVPEgrown Gddoped GaN.InGaN/GaN multiple quantum well （MQW） active layer was sandwiched with Si and Mg codoped GaGdN cladding layers （inset of Fig.7（a））.The fabricated LED exhibited roomtemperature spinpolarized electroluminescence，controllable through an applied magnetic field in the Faraday configuration （Fig.7（a））.Additionally，polarization hysteresis was observed，with the spinLED retaining 9.3% spin polarization at zero applied field after being exposed to the magnetic field of 5 kg （Fig.7（b））.
With the interesting magnetic and optical properties observed on the Gddoped InGaNbased DMS layers，InGaNbased DMS material will enable the fabrication of longer wavelength spinbased electronic （spintronic） devices such as CPLEDs and CPLDs.MQW LED structures，3 nmInGaGdN （In：～3.9% and ～6%，Gd：～1%）） /9 nmGaN，sandwiched between Sidoped ntype and Mgdoped ptype GaN layers were grown on ntype GaN template[33].From these MQW LED structure samples，PL emission from the InGaGdN/GaN MQW layer was observed at around 400～500 nm at room temperature.Emission from GaN layers and defectrelated broadband emission are also observed.To realize the current injection CPLEDs and CPLDs by using MBEgrown GaGdN or GaDyN layers as an active layer or cladding layer，further studies are needed. 5 Summary
In this paper，the present status of the Gdand Dydoped Ⅲnitride semiconductor layers grown by plasmaassisted molecular beam epitaxy was described.XRD measurements indicated no phase separation and the Gd incorporation was verified with the XAFS measurement.PL emission was also observed and the PL peak energy for the InGaGdN was redshifted according to the InN molar fraction.Clear hysteresis and saturation were observed in the MHcurves at room temperature.Si codoping into （In）GaGdN layers enhanced the magnetization.In the InGaGdN/GaN SL samples，enhanced magnetization was also observed.Si doping into wide bandgap GaN layers in these SL structures further increased the magnetization.Such results can be understood with the carriermediated ferromagnetism.Finally，examples of the spintronic devices，where the relation between the spinpolarized carriers and the circularpolarized light is used，and the present status to realize such devices were described.
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摘要：本研究利用分子束外延的方法生长了Gd掺杂氮化物半导体。根据X线衍射和XAFS测定未有发现第二相的析出。观察到了来自InGdGaN的光致发光，发光峰随着InN的摩尔份数的变化而变化。这些材料在室温明显地观测到了磁滞曲线。Si共掺杂的GaGdN超晶格显示出了超大的磁矩，其原因可归于载流子诱发铁磁。最后，说明了这一材料在自旋发光二极管中的应用。
关键词：稀磁半导体； 铁磁性； 光致发光； 硅共掺杂； 超晶格； 自旋半导体器件
（责任编辑：顾浩然，包震宇）
Key words： ⅢNitride Semiconductor； diluted magnetic semiconductors； Gadoliniumdoped； ferromagnetic characteristics； photoluminescence emission； sicodoping； superlattice structure； spintronic semiconductor device
1 Introduction
Diluted magnetic semiconductors （DMSs） have been gathering much interest from the industrial viewpoint because of their potentiality as a new functional material，which will open a way to fabricate novel functional semiconductor devices.For the device application，it is very important that the Curie temperature （Tc） of DMSs should be higher than room temperature.GaMnAs is best investigated and well established DMS[1].However，the Tc for the GaMnAs is still ～ 160 K[2] and is much lower than room temperature.
Theoretical calculations suggested that the transition metaldoped GaN will exhibit room temperature ferromagnetism[3-4].Mndoped GaN was grown by MBE and observed to have high temperature ferromagnetic （FM） characteristics with Tc as high as 940 K[5].Crdoped GaN was also grown by MBE and observed to have high temperature （>400 K） FM characteristics[6] as well as the photoluminescence （PL） emission[7].The observation of PL emission at room temperature is important to fabrication of practical spintronic devices that control charges，photons and spins； this is contrast to GaMnAs，where no PL emission was observed.High temperature FM characteristics were also reported by many other groups[8-9].The observation of FM characteristics was attributed to double exchange interaction by ab initio computations.However，the first principles Monte Carlo simulations suggested that the magnetic exchange interactions in wide bandgap DMSs are effectively short range and the calculated Tc should be low for the low concentrations of magnetic ions and in the absence of delocalized or weakly localized carriers[10].Considering the spinodal decomposition into onedimensional high concentration of magnetic ions （Konbuphase），high temperature FM characteristics were simulated[11]. Rareearth （RE） doped semiconductors are widely studied for the application to photonic devices.However，the first observation of high temperature FM characteristics was reported for the MBEgrown Gd （gadolinium）doped GaN with Tc >400 K similar to GaCrN[12].Since then，extensive studies have been conducted theoretically[13-17] and experimentally on the magnetic semiconductor GaGdN prepared by various techniques，which include MBE[18-20]，metalorganic vapor phase epitaxy （MOVPE）[21] as well as Gd ion implantation into GaN[22-23].In addition，ferromagnetism with incredibly high values of Curie temperature （TC～700 K） was reported in GaGdN films[24].The enhancement of magnetization by Si cocoping was reported[23，25].On the other hand，Dhar，et.al. reported the colossal magnetic moment in the low Gd concentration （1016-1018 cm-3） of GaGdN grown by MBE and explained within the framework of the phenomenological model[26].They also reported the colossal magnetic moment in the low Gd concentration （1016-1018 cm-3） of the Gd implanted GaGdN[27].The origin of the ferromagnetic order was attributed to the polarization of the surrounding host medium，interstitial nitrogen and oxygeninduced or electronmediated ferromagnetism[14].
Examples of the semiconductor spintonic devices are tunnel magnetoresistance （TMR） devices，circular polarized laser diodes （CPLDs） and spin field effect transistors （FETs）.CPLDs are one of important devices to realize the optical communication systems that are resistant to wiretapping.CPLDs can be fabricated by using the DMS as an active layer or a cladding layer.CPLDs are also important devices to construct the information processing systems by the combination of circularpolarized light controlled TMR devices.Bhattacharya，et.al.[28] reported the fabrication of CPLDs by using verticalcavity surfaceemitting laser （VCSEL） structure with GaMnAs DMS layer as spinpolarized carrier （hole） supplying layer and the observation of circularpolarized light at low temperature of 80 K because of low Tc of GaMnAs.To fabricate practical CPLDs，room temperature ferromagnetic DMS is requisite.The Gdand Dysprosium （Dy）doped Ⅲnitride DMSs exhibited the room temperature ferromagnetic characteristics as well as PL emission and conducting characteristics.Therefore，we can expect to fabricate the spintronic devices operating at room temperature.
In this paper，we will describe the experimental results for the Gddoped Ⅲnitride diluted magnetic semiconductors so far reported until now. 2 Growth and structural properties of Gddoped Ⅲnitride semiconductors
Rareearth （RE） doped Ⅲnitride semiconductors and their superlattice （SL） structures were grown on sapphire （0001） substrates and MOVPEgrown GaN （0001） templates after the growth of GaN buffer layer by plasmaassisted molecularbeam epitaxy （MBE）.Elemental Ga，In，Gd and Dy，and plasmaenhanced N2 were used as sources.Growth temperature was 600-700 ℃ for most samples，while the low growth temperature was also conducted to increase the RE atom concentration.Si codoping into （In）GaGdN layers and GaN barrier layers of SL structures were also carried out.
Xray diffraction （XRD） curves for the GaGdN layers with Gd concentration of 0～7.8% are measured[23].No diffraction peaks from secondary phases are observed for the samples with 0～6% Gd concentrations.Only sample with 7.8% Gd concentrations exhibits additional peaks assigned as NaCltype GdN （111） and GdN （222）.Expanded XRD curves near GaGdN （0004） diffraction are investigated.The peak position for the GaGdN shifts to lower angle as Gd concentration increases up to 5.8%，indicating that the lattice constant cin the cdirection increases with the increase of GdN content x，as can be seen in Fig.1.This tendency agrees with the larger atomic diameter of Gd than that of Ga.XRD data show that for the Gd with concentration higher than 6%，the diffraction peak and lattice constant approach again to those of GaN.This indicates that the lattice relaxation occurred for these Gd concentrations.
Xray absorption fine structure （XAFS） measurements were conducted to study the Gd atom substitution of Ga site in GaGdN samples.Radical distribution functions for GaGdN samples calculated with the XAFS data are shown in Fig.2.The first nearest （N） and second nearest （Ga） atom peaks from the Gd atom agree well with those of GaN （middle curve） and are clearly different from those of NaCltype GaN and Gd metal （lower two curves）.This clearly indicates that the most of Gd atoms substitute the Ga sites.The substitutional incorporation of Gd atoms into InGaGdN layer was also confirmed by XAFS.
3 Properties of Gddoped ⅢNitride semiconductors
3.1 Magnetic and optical properties of single layers
Magnetization （M） versus magnetic field （H） curves for the GaGdN sample at 7 K and 300 K are shown in Fig.3[12].The magnetic field is parallel to the sample plane.Hysteresis curves are observed at both temperatures.This indicates that the GaGdN is ferromagnetic at both temperatures.The saturation field was about 2 T and the coercivity Hc was about 0.07 T （70 Oe） at 300 K. Temperature dependence of magnetization is investigated at applied magnetic field of 100 G[12].The magnetization decreases slowly with increasing temperature and remains even at 400 K and there was no discontinuous change in the curve.This indicates that the Curie temperature of GaGdN is higher than 400 K.
To fabricate the long wavelength spincontrolled photonic devices，InGaGdN layers were also grown on MOVPEgrown GaN （0001） template substrates at 400 ℃.PL spectra for the InGaGdN （In：23%，Gd：～1%） sample as a function of temperature are shown in Fig.4.PL emission is clearly observed at all measuring temperatures （10-300 K）.The temperature variation of PL peak energy exhibits Sshaped behavior （redshiftblueshiftredshift） with increasing temperature，which is attributed to localized excitonic emission due to inhomogeneity and carrier localization in InGaGdN alloys with the consideration of the bandtail approach.This temperature variation is similar to those for the InGaN layers without Gddoping.
Magnetization versus magnetic field curves for the InGaGdN singlelayer samples with different Gd concentrations exhibit clear hysteresis and saturation magnetization （MS） at room temperature.Even clear hysteresis can be further observed from the expanded curve of the samples.It shows that the saturation magnetization increases with increased Gd concentration.With the increase of magnetic Gd atoms，the ferromagnetic interaction is increased.
3.2 Effect of low temperature growth
To increase the incorporated Gd concentration without secondary phase formation and to enhance the magnetization，GaGdN layers were grown at low temperature （LT） of 300 ℃ on sapphire （0001） substrates[23].XRD θ2θscan curves for the LTgrown GaGdN samples showed diffraction peaks from sapphire and GaGdN，but no obvious secondary phase such as GdN was detected.XRD peak from the GaGdN layer showed single crystalline characteristics，though the full width at half maximum is about twice of GaGdN layer grown at 700 ℃[23].By LTgrowth，GaGdN layers with Gd concentration as high as 12.5% and no secondary phases were obtained.When the growth was conducted at 700 ℃，the highest Gd concentration is about 5.8%.Radical distribution function obtained by the XAFS measurement clearly showed that the Gd atoms substitute the Ga sites[23].
The GaGdN layers grown at 300 ℃ with different Gd concentrations show room temperature ferromagnetism[23].With increased Gd concentration，magnetization per unit volume is increased.Theoretical calculations[13-14] showed that the spinglass state is stable for the GaGdN without carriers，but electron doping into GaGdN enhances the ferromagnetic interaction in GaGdN.It is possible that the carriers （electrons） coming from some types of defects such as that nitrogen vacancies enhance the ferromagnetism in GaGdN layers.From LTgrown GaGdN samples，PL emission was observed in the photon energy range 2-3.5 eV.Most of emission peaks are related to defects. 3.3 Effect of superlattice structures on magnetic structures
GaGdN/GaN superlattices （SLs） having various GaGdN and GaN layer thicknesses were grown on sapphire （0001） substrates[19，21].Magnetization （M） versus magnetic field （H） measurements for the 230 nmthick GaGdN bulk layer and the GaGdN/GaN SL samples having nearly the same total thickness of GaGdN （Gd concentration：1.2%） showed the clear hysteresis and saturation （ferromagnetic characteristics）.The magnetization for the SL samples is larger than that for bulk sample[19].Furthermore，the thinner each GaGdN QW layer thickness is or the thicker each GaN barrier layer thickness is，a larger magnetic moment per Gd atom was observed as shown in Fig.5[21].In Fig.5（a），the GaN layer thickness is kept constant at 3.8 nm and the GaGdN thickness is varied from 1.1 nm to 2.9 nm.With decreasing the GaGdN layer thickness the magnetic moment per Gd atom is increased.In Fig.5（b），the GaGdN layer thickness is kept constant at 1.3 nm and the GaN thickness is varied from 2.2 nm to 10.8 nm.With increasing the GaN layer thickness the magnetic moment per Gd atom is increased.
It is expected that electron carriers in GaN layers flow into and accumulate in the narrower bandgap GaGdN layers resulting the higher electron concentration （>1018 cm-3） in the thinner GaGdN layers.Especially，the distribution of Gd atoms is inhomogeneous in the layer[26]，so that local areas having higher Gd concentration would have very high electron concentration in the GaGdN layers.The stronger interaction between Gd spins through the electron spins is expected in the thinner GaGdN layer.Therefore，the observed enhanced magnetic moment can be understood with the carriermediated ferromagnetism.
Similar enhanced magnetization was also observed on the InGaGdN/GaN SL samples compared with that for the InGaGdN singlelayer （thickness：100 nm）.Both samples were grown under similar growth conditions except for the sample structure.The magnetic properties of these samples show clear saturation magnetization at about 16 emu/cm3 and 7 emu/cm3 for the SL sample and singlelayer sample，respectively.In the SLs，electron carriers can flow into narrow bandgap InGaGdN well layers from the wider bandgap GaN layers.These increased carrier concentrations improve the magnetization.
However，in the InGaN/GaGdN SL structures the magnetization was decreased.Such observation is indicated in the MHcurves，between the InGaGdN/GaN SL and InGaN/GaGdN SL set of samples measured at room temperature.In the InGaN/GaGdN SL structure，carriers （electrons） flow out of the Gdcontaining GaGdN layers and into the narrow bandgap InGaN layers.So the carrier concentrations in the GaGdN layers are decreased resulting in the suppression of the carriermediated magnetization. 3.4 Si codoping effect on magnetic and electric properties
Si codoped GaGdN layers were grown at 300℃.Figure 6 shows the room temperature MHcurves for the Sidoped and undoped GaGdN samples with Gd concentration of 8.9%[26].Clear hysteresis was observed for both samples.The saturation magnetization of Sidoped GaGdN，about 1046 emu/cm3，is 7 times as large as that of undoped GaGdN.Codoping with Gd and Si is expected to increase the shallow donor concentration and to enhance the ferromagnetic interaction.The enhancement of magnetization by Si codoping suggests the carriermediated ferromagnetism.Similar effect was also observed for the InGaGdN samples.The magnetization versus magnetic field （MH） curves of the InGaGdN sample and Si codoped InGaGdN sample show clear hysteresis and saturation characteristics measured at 300 K.With the increase of Si cell temperature （that is the increase of Si concentration），the increase of the saturation magnetization was observed.The increase in magnetization for the Si codoped sample is closely related to carriermediated ferromagnetism，that is，the Si codoping increases the electron concentrations in InGaGdN layers as also inferred in the Si codoped GaGdN samples and GaGdN/GaN superlattice samples.
Moreover，Si doping into wide bandgap GaN barrier layers in the InGaGdN/GaN SL structures was found to increase the magnetization as clearly observed from the room temperature MHcurves.One reason that could induce the enhancement of magnetization is also the carriermediated ferromagnetism.
4 Spintronic device application
CPLED （spin LED） was fabricated by using MOVPEgrown Gddoped GaN.InGaN/GaN multiple quantum well （MQW） active layer was sandwiched with Si and Mg codoped GaGdN cladding layers （inset of Fig.7（a））.The fabricated LED exhibited roomtemperature spinpolarized electroluminescence，controllable through an applied magnetic field in the Faraday configuration （Fig.7（a））.Additionally，polarization hysteresis was observed，with the spinLED retaining 9.3% spin polarization at zero applied field after being exposed to the magnetic field of 5 kg （Fig.7（b））.
With the interesting magnetic and optical properties observed on the Gddoped InGaNbased DMS layers，InGaNbased DMS material will enable the fabrication of longer wavelength spinbased electronic （spintronic） devices such as CPLEDs and CPLDs.MQW LED structures，3 nmInGaGdN （In：～3.9% and ～6%，Gd：～1%）） /9 nmGaN，sandwiched between Sidoped ntype and Mgdoped ptype GaN layers were grown on ntype GaN template[33].From these MQW LED structure samples，PL emission from the InGaGdN/GaN MQW layer was observed at around 400～500 nm at room temperature.Emission from GaN layers and defectrelated broadband emission are also observed.To realize the current injection CPLEDs and CPLDs by using MBEgrown GaGdN or GaDyN layers as an active layer or cladding layer，further studies are needed. 5 Summary
In this paper，the present status of the Gdand Dydoped Ⅲnitride semiconductor layers grown by plasmaassisted molecular beam epitaxy was described.XRD measurements indicated no phase separation and the Gd incorporation was verified with the XAFS measurement.PL emission was also observed and the PL peak energy for the InGaGdN was redshifted according to the InN molar fraction.Clear hysteresis and saturation were observed in the MHcurves at room temperature.Si codoping into （In）GaGdN layers enhanced the magnetization.In the InGaGdN/GaN SL samples，enhanced magnetization was also observed.Si doping into wide bandgap GaN layers in these SL structures further increased the magnetization.Such results can be understood with the carriermediated ferromagnetism.Finally，examples of the spintronic devices，where the relation between the spinpolarized carriers and the circularpolarized light is used，and the present status to realize such devices were described.
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摘要：本研究利用分子束外延的方法生长了Gd掺杂氮化物半导体。根据X线衍射和XAFS测定未有发现第二相的析出。观察到了来自InGdGaN的光致发光，发光峰随着InN的摩尔份数的变化而变化。这些材料在室温明显地观测到了磁滞曲线。Si共掺杂的GaGdN超晶格显示出了超大的磁矩，其原因可归于载流子诱发铁磁。最后，说明了这一材料在自旋发光二极管中的应用。
关键词：稀磁半导体； 铁磁性； 光致发光； 硅共掺杂； 超晶格； 自旋半导体器件
（责任编辑：顾浩然，包震宇）