简介概要

A study on the minority carrier diffusion length in n-type GaN films

来源期刊：Rare Metals2007年第3期

论文作者：WEN Cheng Paul

文章页码：271 - 275

摘要：The minority carrier diffusion length of n-type GaN films grown by metalorganic chemical vapor deposition （MOCVD） has been studied by measuring the surface photovoltaic （PV） spectra. It was found that the minority carrier dif- fusion length of undoped n-type GaN is considerably larger than that in lightly Si-doped GaN. However, the data suggested that the dislocation and electron concentration appear not to be responsible for the minority carrier diffusion length. It is suggested that Si doping plays an important role in decreasing the minority carrier diffusion length.

Rare Metals 2007,(03),271-275

A study on the minority carrier diffusion length in n-type GaN films

WEN Cheng Paul

Institute of Microelectronics Peking University Beijing 100871 China,

作者简介：DENG Dongmei E-mail: dmdeng@ime.pku.edu.cn;

收稿日期：2006-03-14

A study on the minority carrier diffusion length in n-type GaN films

Abstract：

The minority carrier diffusion length of n-type GaN films grown by metalorganic chemical vapor deposition (MOCVD) has been studied by measuring the surface photovoltaic (PV) spectra. It was found that the minority carrier dif- fusion length of undoped n-type GaN is considerably larger than that in lightly Si-doped GaN. However, the data suggested that the dislocation and electron concentration appear not to be responsible for the minority carrier diffusion length. It is suggested that Si doping plays an important role in decreasing the minority carrier diffusion length.

Keyword：

compound semiconductor material; minority carrier diffusion length; photovoltaic spectrum; GaN;

Received： 2006-03-14

1. Introduction

Owing to its wide bandgap,gallium nitride(GaN)has been focused on for several years for its extensive applications of optoelectronics,high power electronics,and high temperature electronics.Among the Ga N-based optoelectonic devices,Ga N ultraviolet photodetectors can be used in flame detection,secure space-to-space communication,and ozone layer monitoring.Thus,it is significant to fabricate high performance Ga N ultraviolet photodetectors.The structure design is necessary for realizing high performances of Ga N-based devices.Besides the improvement of the material quality and the device processing technology,the minority carrier diffusion length is a very important parameter for the devices design.In fact,some research work about the measurement of the minority carrier diffusion length of GaN films has been carried out[1-4].However,some problems still remain in doubt.For example,the essential factors influencing the minority carrier diffusion length in GaN need to be clarified.

Usually the minority carrier diffusion length can be obtained by the measurement of the electron beam induced current(EBIC)and the photovoltaic(PV)spectrum[1-3].However,the PV spectrum is a considerably simpler method.In this article,the minority carrier diffusion lengths of undoped and Si-doped n-type GaN are investigated.Through the measurement of the PV spectrum,it is deduced tha the minority carrier diffusion lengths in two undoped Ga N samples are about 1.05 and 1.18?m,which is nearly 4 times higher than that of two lightly Si-doped Ga N samples.However,the value of ful width at half maximum(FWHM)measured by double crystal X-ray diffraction(DCXRD)for Samples A-C is nearly the same,however,Sample D has considerably wider FWHM of DCXRD.It is implied that the minority carrier diffusion length is no significantly influenced by the dislocation density whereas it is influenced by the Si doping.

2. Experimental

The GaN samples in the experiments were grown on c-plane sapphire substrate by metalorganic chemical vapor deposition(MOCVD).The ammonia and trimethylgallium were used as N and Ga sources,respectively.The silane(SiH₄)was used as the dopant for Si-doped Ga N samples.Firstly,a Ga N buffer layer was deposited at 540°C,and then followed by the growth of about 4?m GaN epilayers at 1040°C.For the PV measurement,light from the 75 W Xenon lamp was chopped and focused into a monochromator.The sample was mounted on a sample holder with a quartz window coated with indium-tin-oxide(ITO)film.The Ga N surface contacts with the window,while the sapphire substrate contacts with a bottom electrode.The PV signal(?V)was coupled to and detected by a lock-in amplifier.Illuminating a Ga N surface with a UV-light source causes electron-hole pairs to be generated by absorbed photons.The electron-hole pairs were separated by the build-in electrical field across the space charge region of the near GaN surface.The optically excited electrons move toward the conduction band,whereas holes move toward the surface because the surface states are negatively charged acceptors in n-type Ga N[5].Consequently,the photogenerated holes are accumulated at the surface and produce a surface PV[6-7](?V).The value of?V is a function of the excess minority carrier density at the edge of the surface space-charge region.This density is in turn dependent upon the incident light flux,the optical absorption coefficient,the optical reflectance at the illuminated surface,the recombination velocity at the illuminated surface,as well as the diffusion length.The Hall measurement was adopted to obtain the carrier concentration,where indium was used for ohmic contact.All measurements were done at room temperature.

Several n-type Ga N samples were prepared and investigated in this experiment.There are two unintentionally doped and two Si doped Ga N samples.Sample A is undoped Ga N sample with corresponding background electron concentrations of5.0×10¹⁵ cm^-3.The other two samples,B and C,were intentionally lightly Si-doped GaN grown with different SiH₄ fluxes of 0.05 and 0.45 nmol/min,respectively.The corresponding electron concentrations were 1.5×10¹⁶ and 1.8×10¹⁷ cm^-3,respectively.Except the flux of Si H₄,the other growth conditions were held constant during the growth of these three samples.In the PV spectra measurements,the photogenerated electron-hole pairs under the illumination of UV light beam were separated by the inherent electron field near the surface of GaN,and then?V appeared across the sample.The?V values of the three Ga N samples varying with the photon energy of the incident light are shown in Fig.1.A first,a strong PV effect above the energy band gap of these GaN samples appears starting from around3.4 eV.However,the?V decreases with increasing incident photon energy,which can be attributed to the decrease of the light penetration depth and a higher electron-hole recombination rate at the surface of the Ga N samples[8].The intensity of?V decreases with the increase of SiH₄ flux and the undoped Ga N shows the strongest?V.It is probably related to the fact that the Si-doped Ga N samples have higher electron concentrations.In other words the surface accumulated holes can be recombined by these electrons,and then lead to a small PV signal[9-10].In addition,the intensity of?V probably results from different minority carrier diffusion lengths.

Fig.1.PV spectra of the undoped and Si-doped GaN samples.

3. Discussion

It has been reported that the minority carrier diffusion length of semiconductor materials can be determined by the PV spectrum measurement[9-10]The dependence of?V on the absorption coefficien obeys the rule below[7]:

whereΦ_eff is the incident photon flux,αis the absorption coefficient,S is the surface recombination velocity near the surface,D is the minority carrier diffusion coefficient,and L is the minority carrier diffusion length.A=qn₀/kTexp(qV/kT)for a depletion layer,where q V is the barrier height,KT is the thermal energy,and n₀ is the equilibrium electron concentration.As a direct gap semiconductor,the absorption coefficient of GaN can be expressed as follows[11]:

where hv is the incident photo energy,and E_g is the bandgap of hexagonal GaN,about 3.4 eV.

Also,according to Eq.(1),under the condition that the light penetration depth and the diffusion length are considerably smaller than the sample thickness but considerably larger than the surface depletion width,the diffusion length can be derived from a linear plot of 1/?V versusα^-1 by the intercept of the line with the abscissa axis.Fig.2 shows the plot of 1/?V versusα^-1 for the undoped GaN Sample A.From a linear fit to the experimental data,the intercept on the abscissa is calculated,which is about1.18?m,i.e.,the minority carrier diffusion length of the undoped GaN Sample A is about 1.18?m.Fig.3shows the plot of 1/?V versusα^-1 for three Ga N samples.From a linear fit to the experimental data,the minority carrier diffusion lengths are calculated to be about 1.05,0.25,and 0.27?m for Samples A,B,and C,respectively.These values indicate that the undoped Ga N sample has a longer minority diffusion length than the Si-doped samples.Besides,these diffusion length values of GaN are comparable with the reported values measured by EBIC[1-2].As far as the diffusion lengths of these two light Si-doped GaN samples are concerned,there is only very slight change in their minority carrier diffusion lengths,although their electron concentration increases dramatically with the increase of SiH₄ flux.Since the nearly same full width at half maximum(FWHM)of theω-scan rocking curve measured by double crystal X-ray diffraction(DCXRD)for the three samples is as narrow as 180 arcsec for both(002)and(102)planes,?ωis the angle variety.It is assumed that the smaller minority carrier diffusion length in light Si-doped GaN samples is not a result caused by the effect of the dislocation density,as is indirectly represented by the FWHM of DCXRD Regarding the undoped Ga N Sample A having a considerably lower electron concentration,it will be a question whether the higher electron concentration because of the Si doping leads to the lower minority carrier diffusion length of the Si-doped GaN sample

Fig.2.Dependence of 1/?V onα^-1 for the undoped Ga N Sample A.

Fig.3.Dependence of 1/?V onα^-1 for the samples.

To investigate this problem,another undoped Sample D is prepared.By comparing with undoped Sample A,different growth parameters are adopted to grow Sample D;consequently,the undoped Sample D has a higher background electron concentration of 3.6×10¹⁶ cm^-3.At the same time,the FWHM of theω-scan rocking curve measured by DCXRD is240 arcsec and 295 arcsec for the(002)and(102)planes,respectively(Fig.4(a)).The minority carrier diffusion length of Sample D is also obtained by the PV measurement.The 1/?V versusα^-1 of undoped Ga N Sample D is plotted in Fig.5,where the inset of the figure is the PV spectrum.The intercept on the abscissa is about 1.18?m through a linear fitting to the experimental data,i.e.,the minority carrier diffusion length of undoped GaN Sample D is about1.18?m.The undoped Sample D almost has the same minority carrier diffusion length as another undoped Sample A.

Fig.4.DCXRDω-scan rocking curves at(002)and(102)planes for undoped Samples D(a)and A(b).

Fig.5.Dependence of 1/?V onα^-1 for Sample D,which is an undoped sample with a higher electron concentration than Sample A.The insert is the PV spectrum of Sample D.

To display the result clearly,the growth condition and the characteristic result of Samples A-D are listed in Table 1.There is nearly no distinct difference between the minority carrier diffusion lengths of the undoped Ga N Sample A and the undoped Sample D,although Sample D has considerably wider FWHM of DCXRD(Fig.4).It is demonstrated again that the diffusion length value is no significantly influenced by the dislocation density[12].By comparing the electron concentration between the undoped Sample D and the Si-doped Sample B,it is suggested that the electron concentration is also not responsible for the difference in the minority carrier diffusion length.As a whole,i seems that neither the dislocation density nor the electron concentration has any essential influence on the minority carrier diffusion length of Ga N in the experiments.However,an obvious conclusion can be observed,i.e.,the undoped Ga N samples have considerably larger minority carrier diffusion lengths than that of the light Si-doped GaN samples.It implies that a Si doping,even if it is light,may have a strong influence on the properties of GaN samples i.e.on the minority diffusion length.This probably indicates that some kind of point defects in Si-doped Ga N can influence the minority carrier diffusion length of the GaN samples.

Theoretical calculation suggests that the formation of Ga vacancies is enhanced by the complex formation with Si impurities[13],i.e.,the concentration of Ga vacancies in undoped GaN is considerably lower than that in Si-doped Ga N.Usually,the behavior of Ga vacancies is regarded as deep levels in the forbidden gap.When the incident light illuminates the GaN sample,the photo-generated holes in n-type GaN are easily captured by the Ga vacancies,and consequently the minority carrier diffusion length will be decreased.It is also concluded that S doping has considerably more complicated behavior in GaN than expected.

4. Conclusion

The minority carrier diffusion length of n-type Ga N was investigated by the measurement of PV spectra.It was found that the undoped GaN samples have considerably larger minority carrier diffusion lengths than the Si-doped Ga N.However,the data suggest that the dislocation and electron concentration appear not to be responsible for the minority carrier diffusion length.It is thus suggested that the Si doping plays an important role in decreasing the minority carrier diffusion length.

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Table 1.Growth conditions and characteristic parameters of the n-type GaN samples